Chromium terephthalate metal–organic framework MIL-101: synthesis, functionalization, and applications for adsorption and catalysis

Abstract

The chromium terephthalate metal–organic framework, MIL-101 (MIL, Matérial Institut Lavoisier), is comprised of trimeric chromium(III) octahedral clusters interconnected by 1,4-benzenedicarboxylates, resulting in a highly porous 3-dimentional structure. The large pores (29 and 34 Å) and high BET surface area (>3000 m² g⁻¹) with a huge cell volume (≈702 000 ų) together with the coordinatively unsaturated open metal sites that can be subjected to diverse post-synthesis functionalization or guest encapsulation, and excellent hydrothermal/chemical stability, make MIL-101 particularly attractive for applications, such as selective gas adsorption/separation, energy storage and heterogeneous catalysis. This paper reviews the current status of research and development on the synthesis, functionalization and applications of MIL-101 for adsorption/catalytic reactions.

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1. Introduction

Metal–organic frameworks (MOFs) are a class of crystalline porous hybrid materials assembled by the bonding of metal ions or clusters linked with polyfunctional organic molecules to form infinite one-, two or three dimensional frameworks. These porous materials are generally constructed from metal ions or clusters of zinc, copper, chromium, aluminium, zirconium and other metals, and organic components of carboxylates or N-containing aromatic groups.^1–3 The most interesting properties of MOFs are their well-ordered tunable porous structures with a wide range of pore sizes and exceptional textural properties, such as high surface areas and high pore volumes, which enable a range of applications for gas adsorption/separation,^4–9 catalysis,¹⁰ and drug delivery.¹¹ In recent years, several extensive review articles dealing with the synthesis, structural description, surface modification, and adsorption/catalytic application of various MOF materials have appeared.^12–24

In 2004, Férey et al. prepared MIL-100 (MIL, Matérial Institut Lavoisier) hydrothermally using combined computer simulations and targeted chemistry. The framework of MIL-100 was composed of trimeric chromium(III) interconnected by benzene-1,3,5-tricarboxylate (BTC) anions (Fig. 1), which was the first example of MOFs with crystalline walls having a unique hierarchical pore system and high Langmuir surface area.²⁵ MIL-101 with a chemical composition of {Cr₃F(H₂O)₂O(BDC)₃·nH₂O} (n ∼ 25; 1,4-benzenedicarboxylate (BDC)) and superior physicochemical properties emerged soon after.²⁶ The robust framework of MIL-101 was comprised of similar trimeric chromium(III) octahedral clusters interconnected by BDC molecules resulting in an augmented MTN zeotype structure (Fig. 1).

Fig. 1 Structural representation of MIL-100 and MIL-101.^25,26

This structure was comprised of two types of mesoporous cages with diameters of ∼29 and 34 Å accessible through two types of microporous windows (the smaller cages have pentagonal windows with a free opening of ∼12 Å, while the larger cages possess both pentagonal and hexagonal windows with a ∼14.7 Å by 16 Å free aperture) (Fig. 2).

Fig. 2 Schematic diagram of the pentagonal and hexagonal windows of MIL-101.²⁶

The material exhibits excellent stability against moisture and other chemicals, and the terminal water molecules in MIL-101 can be removed by heating in air or under vacuum at 423 K, which generates two coordinatively unsaturated open metal sites (CUS) per trimeric Cr(III) octahedral cluster. In addition to its highly porous nature, MIL-101 has attracted considerable attention because functional modifications on MIL-101 can be achieved easily either directly using a functionalized ligand during the synthesis or indirectly via the diverse post-synthesis chemical treatment on the CUS or on the organic linkers.

Among the MOFs known, MIL-101 is one of the most promising porous materials for future energy and environmental applications, surpassing MOF-5 or HKUST-1, owing to its superior physicochemical properties including high hydrothermal/chemical stability and desirable textural properties. This paper reviews the recent progress on the synthesis, functionalization and applications in adsorption/catalysis of the chromium terephthalate MIL-101 since Hong et al. first reported the versatility of MIL-101 in their comprehensive experiments in 2009.²⁷ Readers from engineering disciplines, particularly newcomers to MOFs science, will gain a clearer perspective on the potential of the MOF materials for future sustainability by concentrating on the synthesis and applications of a specific model compound out of the myriad of MOF materials available.

2. Synthesis and characterization

2.1 Hydrothermal synthesis

Férey et al.²⁶ reported the preparation of a porous chromium terephthalate metal–organic framework, MIL-101, and a large number of synthesis studies^28–37 were followed afterwards under different synthesis conditions (additive, temperature, time, and with support materials etc.), as listed in Table 1. MIL-101 can also be prepared by microwave heating^38–41 or via a drygel conversion method.⁴²

Table 1 MIL-101 synthesis studies^a

Method	Synthesis conditions			Textural properties		Comments	Ref.
	Synthesis medium	Temp. (K)	Time (h)	SBET (m^2 g^−1)	Vpore (cm^3 g^−1)
a TMAOH (tetramethylammonium hydroxide), CTAB (cetytrimethylammonium bromide), EG (expanded graphite), 4-NTm (4-nitroimidazole).b Langmuir surface area.
Hydrothermal	H2O/HF	493	8	4100	2.0	Synthesis and structure analysis	26
						Incorporation of Keggin polyanions
	H2O/TMAOH	453	24	3197	1.73	Synthesis in an alkaline medium	28
						H2 storage
	H2O/HF	493	8	3115	1.58	Synthesis with temperature-programming	29
	H2O/CH3COOH	473	8	2736	1.50	Synthesis in acetic acid instead of HF	30
						Volatile organic compounds (VOC) adsorption
	H2O	491	18	3460	—	HF-free synthesis and comparison (conventional vs. microwave), phosphotungstic acid composite	31
						Baeyer condensation and epoxidation reaction
	H2O/stearic acid	453	4	2691	2.95	Synthesis of MIL-101 (19–84 nm in size) using a monocarboxylic acids	32
						CO2 adsorption studies
	H2O/HF/CTAB	493	8	846		Synthesis of hierarchically mesostructured MIL-101 with different morphologies using cationic surfactant CTAB	33
						Dye removal
	H2O/HF/EG	493	2	3751	1.8	Synthesis of nano-sized MIL-101 crystals (∼200 nm) using expanded graphite as a structure-directing template	34
	H2O/4-NTm	423	24	2542	1.59	Synthesis using 4-nitroimidazole	35
	H2O	493	8	—	—	Thin films on alumina via in situ seeding of nanoparticles by the use of dimethylacetamide	36
	H2O	453	4	—	—	Thin films on alumina via dip-coating in the presence of polyethyleneimine with controlled thickness (100–545 μm)	37
Microwave	H2O/HF	483	0.67	3900	2.3	Synthesis	38
						Benzene sorption
	H2O	473	0.02	4200^b	—	HF-free synthesis and thin films on silicon wafer by a dip-coating route	39
						Water and alcohol sorption properties
	H2O/HF	493	1	3054	2.01	Synthesis	40
						Studies on adsorption and diffusion of benzene
	H2O/NaOH	483	0.25	3223	1.57	Comparison (conventional vs. microwave)	41
Dry-gel conversion	H2O/HF	493	12	4164	2.77	Detailed synthesis studies	42
						Synthesis of MIL-101-NO2 and incorporation of phosphotungstic acid

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MIL-101 can be prepared hydrothermally using a chromium salt and H₂BDC with the aid of a small amount of HF in an autoclave under autogenous pressure conditions.²⁶ In a typical synthesis, Cr(NO₃)₃·9H₂O (0.400 g, 1 mmol), H₂BDC (0.166 g, 1 mmol) and HF (0.2 mL, 1 mmol) in water (4.8 mL, 265 mmol) are heated at 493 K for 8 h, which forms a crystalline green powder. The solid product contains impurities of unreacted or recrystallized H₂BDC present both outside and within the pores of MIL-101. The formation of impurity phases normally result in a low product yield. The removal of impurities is difficult, and rigorous purification steps are normally required. Initially, the green solid is filtered through a large pore fritted glass filter (no. 2) and then through small pore (no. 5) paper filter to remove the large H₂BDC crystals. This is followed by multi-step solvent treatments using water, hot ethanol and DMF, and finally aqueous NH₄F solutions.²⁷ Several attempts have been made to increase the purity of the MIL-101 product by temperature-programmed synthesis to induce the formation of large residual carboxylic acid impurity crystals that can be isolated more easily using a large pore fritted glass filter²⁹ or the addition of tetramethylammonium hydroxide (TMAOH) to promote the dissolution of H₂BDC.²⁸

Thermogravimetric analysis (TGA) in air showed that MIL-101 is stable up to 548 K. The N₂ sorption isotherm was of type I with secondary uptakes at P/P₀ ∼ 0.1 and at P/P₀ ∼ 0.2, which reflects the two types of microporous windows in MIL-101. The BET surface area and pore volume of the sample as high as ∼4100 m² g⁻¹ and 2.0 cm³ g⁻¹, respectively, were reported.²⁶ On the other hand, a more typical BET surface area of MIL-101 determined by other researchers was in the range, ca. 2500–3200 m² g⁻¹.

Several new synthetic strategies have been suggested to prepare MIL-101 with higher product purity without the use of toxic and highly corrosive HF. The material was synthesized in an alkali medium with a reaction mixture of Cr(NO₃)₃·9H₂O : H₂BDC (1 : 1) and TMAOH (0.05 mol L⁻¹) at 453 K for 24 h.²⁸ The resulting MIL-101 had a BET surface area of 3197 m² g⁻¹ after washing with distilled water in high product yield (∼88%). MIL-101 with a BET surface area and pore volume of 2736 m² g⁻¹ and 1.50 cm³ g⁻¹, respectively, was also prepared using acetic acid instead of HF as an additive at 473 K, followed by a solvent treatment with DMF and ethanol.³⁰ Bromberg et al. synthesized MIL-101 from a reaction mixture of Cr(NO₃)₃·9H₂O and H₂BDC in an aqueous medium without acid or alkali at 491 K for 18 h with a 64% product yield.³¹ The BET surface area of the sample was found to be 3460 m² g⁻¹ after a DMF treatment at 343 K for 18 h.

Jiang et al. reported the size-controlled synthesis of MIL-101 (19 nm to 84 nm in size) using a variety of monocarboxylic acids, such as stearic acid, benzoic acid, 4-methoxybenzoic acid, 4-nitrobenzoic acid, and perfluorobenzoic acid, as a mediator in a reaction mixture of Cr(NO₃)₃·9H₂O : H₂BDC : H₂O (1 : 1 : 1680) at 453 K in ∼49% yield.³² The BET surface area and total pore volume of the samples were found to be 2646–2920 m² g⁻¹ and 2.33–2.95 cm³ g⁻¹, respectively. Nano-sized MIL-101 crystals (∼200 nm in size) with a BET surface area of 3751 m² g⁻¹ and a total pore volume of 1.8 cm³ g⁻¹ was also prepared using an expanded graphite-template method at a shorter reaction time of 2 h at 493 K.³⁴ Huang et al. reported the preparation of hierarchically mesostructured MIL-101 using the cationic surfactant, cetyltrimethylammonium bromide (CTAB) as a supramolecular template to produce nanoparticles (120–200 nm) with different morphologies, such as octahedral microcrystals, nanospheres, and nanoflowers.³³ The cationic surfactant played a key role in controlling the morphology of the resulting nanoparticles.

MIL-101 in film form was also prepared. Férey et al. immersed a silicon wafer into a nanoparticle suspension of MIL-101 in ethanol to form a thin film with thicknesses up to 160 nm via a dip-coating method at room temperature.³⁹ Jiang et al. reported the synthesis of 2.5 μm thick MIL-101 films on an alumina support by immersing adimethylacetamide (DMA)-wetted alumina plate into a reaction mixture of Cr(NO₃)₃·9H₂O and H₂BDC in water at 493 K for 8 h.³⁶ The authors expanded the concept and reported the synthesis of MIL-101 and MIL-101-NH₂ films on alumina substrates by immersing alumina into a nanoparticle suspension of MIL-101 in ethanol in the presence of polyethylenimine (PEI) via dip-coating to furnish crack-free films of MIL-101-PEI-n and MIL-101-NH₂-PEI-n with a controlled thickness.³⁷ Scanning electron microscopy (SEM) showed that the film thickness was in the range, 100 to 545 nm.

2.2 Microwave synthesis

Microwave-assisted synthesis techniques for the preparation of nanoporous materials under hydrothermal conditions have the advantages of (1) fast crystallization, (2) phase selectivity, (3) narrow particle size distribution, and (4) facile morphology control.^43–46

Jhung et al. evaluated the microwave synthesis of MIL-101, and observed a significant decrease in crystal size (40–90 vs. 250–600 nm) and synthesis time compared to hydrothermal electric heating with a corresponding BET surface area and pore volume of 3900 m² g⁻¹ and 2.3 cm³ g⁻¹, respectively.³⁸ MIL-101 was synthesized within 60 min at 483 K under microwave irradiation at 600 W, in which the molar composition of the substrate mixture was Cr(NO₃)₃·9H₂O : H₂BDC : HF : H₂O (1 : 1 : 1 : 280). The material was also synthesized by microwave heating in an aqueous medium without HF at 473 K to form non-aggregated nanoparticles, 22 ± 5 nm in size, with a Langmuir surface area of ∼4200 m² g⁻¹.³⁹ The nano-sized MIL-101 was synthesized using aqueous NaOH without HF, producing crystals, ∼50 nm in size, with a 37% product yield at 483 K.⁴¹

2.3 Dry gel conversion synthesis

Dry gel conversion (DGC) applied to the synthesis of inorganic materials, such as zeolites normally have the following advantages: (1) minimization of waste disposal; (2) reduction in reaction volume; and (3) complete conversion of gel to a uniform crystalline product in high yield. Recently, Ahn et al. reported the drygel conversion synthesis of MIL-101.⁴² Initially a Cr(NO₃)₃·9H₂O granular precursor was ball-milled, which was then mixed with H₂BDC and ground again for 15 min. The finely ground mixture of the substrates (140 mg) was placed on a holed Teflon-plate inside a Teflon-lined stainless steel autoclave. H₂O (6.25 mL) and HF (1.0 mg, 47% aq.) were added to the bottom of the autoclave and the entire assembly was heated to 493 K for 12 h. The DGC synthesis produced MIL-101 with a high BET surface area (4164 m² g⁻¹) after a single wash with distilled water in high product yield (∼90%). SEM, X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) analysis confirmed that no recrystallized H₂BDC formed during the DGC process. The authors also conducted the synthesis of a functionalized MIL-101 using 2-nitroterephthalic acid as an organic ligand and the encapsulation of phosphotungstic acid (H₃PW) into MIL-101 under DGC reaction conditions without using HF. The method might be useful for the large scale production of MIL-101 for industrial use due to the high product yield and simple washing procedure.

3. Direct and post-synthesis organic functionalization

The functionalization of MOF structures is often necessary to create active sites for a range of catalytic/adsorption applications. Table 2 lists the cases of organic-functionalization carried out on MIL-101.

Table 2 Organic functionalization and post-synthesis modification studies of MIL-101^a

Organic linker	Reaction conditions		Textural property		Comments	Ref.
	Temp. (K)	Time (h)	SBET (m^2 g^−1)	Vpore (cm^3 g^−1)
a PSM (post-synthesis modification), TMAOH (tetramethylammonium hydroxide), PEI (polyethyleneimine), NCSs (nitrogen-containing compounds), SCCs (sulphur-containing compounds).
2-Nitroterephthalic acid	453	96	1307–1425	—	Direct synthesis of MIL-101-NO2 in aqueous medium, PSM of MIL-101 to MIL-101-NO2 using nitrating acid and converted to MIL-101-NH2 and urea derivative	47
Monosodium 2-sulfoterephthalic acid	453	144	1915	—	Direct synthesis of MIL-101-SO3H in HCl	48
					Cellulose hydrolysis
2-Nitroterephthalic acid	453	144	2146	1.19	Direct synthesis of MIL-101-NO2 in HCl	49
					Water sorption properties
2-Aminoterephthalic acid	423	12	1675	—	Direct synthesis of MIL-101-NH2 in aqueous NaOH	50
					CO2, CH4 adsorption studies
Substituted terephthalic acid	453	96	991–2282	—	Synthesis of single- and mixed-linker derivatives using high-throughput methods	51
					PSM of MIL-101-Br-NO2, MIL-101-Br-NH2
Terephthalic acid and 2-(diphenylphosphino)terephthalic acid	483	24	3020	—	Synthesis of MIL-101-PPh2 using mixed-linkers in the presence of TMAOH	52
Monosodium 2-sulfoterephthalic acid	453	24	1754	0.91	Direct synthesis of MIL-101-SO3H	53
					Acetalization of aldehydes with diols
2-Aminoterephthalic acid	403	24	2070	2.26	Direct synthesis of MIL-101-NH2 and PSM of MIL-101-NH2 via tandem diazotisation process to produce azo dye-, iodo- and fluoro-functionalized MIL-101	54
					CO2 sorption studies
Terephthalic acid	493	9	638	0.18	Grafting NH2 and SO3H groups on organic ligand and CUS by three-step PSM of MIL-101	55
					One-pot deacetalization-nitroaldol reaction
Monosodium 2-sulfoterephthalic acid and 2-nitroterephthalic acid	453	144	2190	1.37	Direct synthesis of MIL-101-NO2-SO3H and PSM by the reduction of the NO2 groups to MIL-101-NH2-SO3H	56
					One-pot deacetalization-nitroaldol reaction
Terephthalic acid	493	8	3555	2.07	Grafting of amines (ethylenediamine, diethylenetriamine and 3-aminopropyltrialkoxysilane) onto CUS Knoevenagel condensation, Heck coupling reaction	57
Terephthalic acid	493	8	3125–183	1.63–0.10	Incorporation of PEI into MIL-101	58
					CO2 sorption studies
2-Aminoterephthalic acid	423	6–16	2297–96	2.12–0.37	Direct synthesis of MIL-101-NH2 and post-synthesis PEI incorporation	59
					Separation of CO2 from CO2/CH4 mixture
Terephthalic acid	493	8	3555	1.75	In situ formation of MIL-101 on activated carbon	60
					Hydrogen storage studies
Terephthalic acid	493	6	—	—	Incorporation of H2SO4 and H3PO4 into as-prepared MIL-101 in an aqueous medium	61
					High proton-conducting properties at moderate temperature
Terephthalic acid	493	10	3858	2.09	In situ formation of MIL-101 on graphite oxide	62
					Adsorption properties of NCSs and SCCs

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Nitro-functionalized MIL-101 (MIL-101-NO₂) was synthesized using 2-nitroterephthalic acid (H₂BDC-NO₂) and a mixture of CrCl₃ : H₂BDC-NO₂ : H₂O (1.6 : 1 : 278) at 453 K in 96 h.⁴⁷ The BET surface area of the sample was 1425 m² g⁻¹. Akiyama et al. reported the preparation of sulfonic acid functionalized MIL-101 (MIL-101-SO₃H) using monosodium 2-sulfoterephthlicacid (H₂BDC-SO₃Na), in which CrO₃, H₂BDC-SO₃Na, HCl, and water were treated hydrothermally at 453 K for 6 days.⁴⁸ The same group also reported the synthesis of MIL-101-NO₂ under similar reaction conditions, as applied for MIL-101-SO₃H.⁴⁹ The BET surface area of MIL-101-SO₃H and MIL-101-NO₂ were 1915 and 2146 m² g⁻¹, respectively. Lin et al. reported the synthesis of amine functionalized MIL-101 (MIL-101-NH₂) using 2-aminoterephthalic acid (H₂BDC-NH₂) in an aqueous sodium hydroxide solution at a lower temperature of 423 K.⁵⁰ The material showed a mean particle size of ca. 50 nm and a BET surface area of 1675 m² g⁻¹.

Lammert et al. reported the synthesis of single- and mixed-linker functionalized MIL-101 derivatives using a high-throughput synthesis technique.⁵¹ Fig. 3 shows the linkers used in high-throughput screening. They described the effects of three different metal sources (CrO₃, CrCl₃ and Cr(NO₃)₃·9H₂O); highly crystalline MIL-101 materials were produced using CrCl₃ as the metal source and 2-bromoterephthalic or 2-nitroterephthalic acid as the mixed-linker components. Recently, Morel et al. synthesized phosphine-functionalized MIL-101 in an aqueous solution of TMAOH using 2-(diphenylphosphino)terephthalic acid and H₂BDC at a molar ratio of 1 : 6 at 483 K with high crystallinity and phase purity.⁵² The BET surface area of the sample was 3020 m² g⁻¹.

Fig. 3 Structure of the organic linkers used in the high-throughput synthesis technique.⁵¹

Table 2 also lists the results of the investigations into the post-synthesis modification (PSM) of MIL-101. Bernt et al. converted MIL-101 to MIL-101-NO₂ using an acid mixture (conc. HNO₃ and conc. H₂SO₄) with ice cooling.⁴⁷ The author reported the PSM of MIL-101-NO₂ through the reduction of the NO₂ group with SnCl₂·2H₂O to NH₂, which was reacted further with ethyl isocyanate to produce a urea derivative (MIL-101-UR2), as described in Fig. 4.

Fig. 4 Schematic representation of PSM of MIL-101 to MIL-101-NO₂, MIL-101-NH₂ and urea derivative.⁴⁷

The PSM of MIL-101-NH₂ via a tandem diazotization process to furnish azo dye-, iodo- and fluoro-functionalized MIL-101 were also reported (Fig. 5).⁵⁴

Fig. 5 Schematic representation of PSM of MIL-101-NH₂ through a tandem diazotization process.⁵⁴

MIL-101-NH₂ was reacted with NaNO₂ in the presence of HCl to form an arenediazonium chloride salt as an intermediate, which was then converted to azo dye- and iodo-functionalized MOF (MIL-101-azo and MIL-101-I) by the addition of an aqueous solution of C₆H₅OH and KI, respectively, and a diazonium tetrafluoroborate salt was formed from a reaction mixture of NaNO₂ and HBF₄, which was then heated to 373 K to form a fluoro-functionalized MIL-101 (MIL-101-F) by eliminating nitrogen.

A site-isolated acid–base bifunctional MIL-101 was obtained by the three-step PSM of MIL-101: (i) BOC (N-tert-butoxycarbonyl)-protected amino group grafting on CUS, (ii) direct sulfonation of the phenylene backbone with chlorosulfonic acid, and (iii) BOC de-protection of the amino groups by a thermal treatment.⁵⁵ The material, MIL-101-NH₂-SO₃H, showed a lower BET surface area (638 m² g⁻¹) than pure MIL-101 (1333 m² g⁻¹; suspected of incomplete purification). Recently, Ahn et al. reported the synthesis of MIL-101-NH₂-SO₃H using a simple two-step process: (i) direct hydrothermal synthesis of MIL-101-NO₂–SO₃H using H₂BDC-SO₃Na and H₂BDC-NO₂ organic linkers and (ii) one-step PSM by reduction of the NO₂ groups to NH₂ using SnCl₂, as described in Fig. 6,⁵⁶ which achieved significant improvement in the surface area (2190 m² g⁻¹).

Fig. 6 Schematic representation of the synthesis of bifunctional MIL-101-NH₂–SO₃H.⁵⁶

Goesten et al. reported the PSM of MIL-101 through the chloromethylation of aromatic rings with methoxyacetal chloride in the presence of aluminium chloride to produce CM-MIL-101, followed by the introduction of diphenylphosphine (PPh₂) to form a multifunctional material, PPh₂-MIL-101 (Fig. 7).⁶³

Fig. 7 Schematic representation of the chloromethylation of the aromatic rings of MIL-101 and the introduction of diphenylphosphine.⁶³

The chloromethylene functional group in CM-MIL-101 was confirmed by ¹H magic angle spinning nuclear magnetic resonance (MAS NMR) and FT-IR spectroscopy. The PPh₂-MIL-101 was highly porous with a BET surface area of 1800 m² g⁻¹. The CM-MIL-101 was treated with catalytically active tetra(4-pyridyl)porphyrinatomanganese(III) acetate (Mn(TPyP)OAc) to form a hybrid material, Mn(TPyP)OAc/CM-MIL-101 by the elimination of a chlorine atom (Fig. 8).⁶⁴ FT-IR, diffuse reflectance UV-Vis spectroscopy (UV-Vis DRS), inductively coupled plasma (ICP) and SEM confirmed the post-synthesis metalation. Nguyen et al.⁶⁵ reported the binding of vanadyl acetylacetonate to MIL-101 via a two-step PSM route: (i) the incorporation of dopamine (dop) to the unsaturated Cr(III) sites and (ii) reaction with VO (acetylacetonate)₂ to form a mono-catecholate vanadium(IV) oxo compound, V(dop)-MIL-101, which had a BET surface area of ∼1770 m² g⁻¹.

Fig. 8 Post-synthetic metalation of CM-MIL-101.⁶⁴

Several nitrogen-containing organic precursors have been used to prepare amine-grafted MIL-101 composites.^{57–59,66–68} Hwang et al. reported the grafting of amine molecules onto CUS in MIL-101, as described in Fig. 9.⁵⁷

Fig. 9 Schematic representation of ethylenediamine ligand grafting onto CUS.⁵⁷

The materials were prepared by the activation of MIL-101 at 423 K for 12 h under vacuum, followed by heating with amine precursors, such as ethylenediamine (ED), diethylenetriamine (DETA) or 3-aminopropyltrialkoxysilane (APS), in toluene under reflux. FT-IR, CO adsorption, BET surface area measurements, and elemental analysis confirmed the successful grafting of the amines onto CUS in MIL-101. Ahn et al. similarly reported the grafting of DETA onto the open metal sites in MIL-101.⁶⁷ Dialkylaminopyridine ligands (Fig. 10) including 4-dimethylaminopyridine, 4-pyrrolidinopyridine, 4-morpholinopyridine, and 4-(3-aminopropyl)-methylaniline were also incorporated onto the open metal sites in MIL-101 by heating MIL-101 with the precursors in toluene under reflux.⁶⁸ The polyethyleneimine (PEI)-incorporated MIL-101-NH₂ was obtained using a two-step process: (i) direct hydrothermal synthesis of MIL-101-NH₂ using 2-aminoterephthalic acid followed by (ii) refluxing it with PEI in methanol at 383 K for 12 h under vacuum conditions.⁵⁹

Fig. 10 Structure of the dialkylaminopyridine ligands.⁶⁸

4. Guest encapsulation

4.1 Encapsulation of polyoxometalate (POM) and metal phthalocyanine

MIL-101 was subjected to post-synthesis guest encapsulation by making use of its high thermal and chemical stability against water and other common organic solvents, as well as meso-sized cages for the incorporation of catalytically active guest molecules. POM are polyatomic compounds that are composed of bulky clusters of transition metal oxide anions. These bifunctional redox materials have a wide range of applications in material science and catalysis. Metal phthalocyanine (MPc) compounds also show good catalytic activity for the oxidation of organic compounds.

Férey et al. first reported the incorporation of a large Keggin polyanion, PW₁₁O₄₀⁷⁻, into MIL-101 from an aqueous solution at room temperature, and estimated that approximately five highly charged Keggin moieties can be incorporated within the large cages of MIL-101 (volume of the Keggin unit and large cages of MIL-101 were 2250 and 20 600 A³, respectively).²⁶ The encapsulated material showed a significantly lower surface area than pure MIL-101. Moreover, XRD, N₂ sorption measurements, TGA, ³¹P solid state NMR, and FT-IR spectroscopy confirmed the incorporation of Keggin ions within the pores. Several other studies also reported the encapsulation of polyoxotungstates,^31,69–74 Ti/Co-monosubstituted Keggin heteropolyanions,⁷⁵ polyoxovanadate,⁷⁶ lanthanopolyoxometalates (Ln = Eu³⁺ and Sm³⁺),⁷⁷ zinc-monosubstituted Keggin heteropolyanion,⁷⁸ and boronperoxotungstate⁷⁹ into MIL-101 using impregnation or one-pot hydrothermal synthesis routes.

The encapsulation of large metal phthalocyanine (MPc), (MPcF₁₆, M = Fe, Ru) and (FePc^tBu₄)₂N complexes into MIL-101 was conducted using a wet infiltration method.⁸⁰ N₂ physisorption measurements and energy-dispersive X-ray spectroscopy (EDS) analysis confirmed the homogeneous distribution of the FePcF₁₆ or RuPcF₁₆ complexes inside the MIL-101 matrix, whereas the bulky (FePc^tBu₄)₂N complex was found adsorbed on the outer surfaces. FePcS/MIL-101 (FePcS = irontetrasulfophthalocyanine) was prepared by the adsorption of FePcS in an aqueous medium,⁸¹ in which FePcS was deposited on both the internal and external surface of MIL-101 crystals.

Anbia et al. reported the in situ synthesis of MWCNT@MIL-101 (MWCNT = multi-walled carbon nanotube) from a reaction mixture of Cr(NO₃)₃·9H₂O and H₂BDC in HF/H₂O at 493 K.⁸² The composite material, AC@MIL-101 (AC = activated carbon) was also synthesized using similar route by adding AC at different loadings in situ during the synthesis of MIL-101.⁶⁰ The hybrid proton-conducting materials, H₂SO₄@MIL-101 and H₃PO₄@MIL-101, were synthesized by introducing the as-prepared MIL-101 to an aqueous solution of H₂SO₄ (2.7 M) or H₃PO₄ (2.6 M) at room temperature.⁶¹ The composite material, GO/MIL-101 (GO = graphite oxide), was prepared by the in situ formation of MIL-101 on GO by introducing the as-prepared GO into a mixture of Cr(NO₃)₃·9H₂O and H₂BDC in HF/H₂O at 493 K for 10 h.^62,83 The surface area of the GO/MIL-101 was found to be dependent on the amount of GO used. A small amount of GO (ca. 0.25%) exhibited a higher value than MIL-101, but larger amounts of GO (>0.5%) reduced the porosity.⁶²

4.2 Encapsulation of metals nanoparticles (NPs)

The encapsulation of metal NPs into the interior cavities of MOF crystals has attracted significant attention in recent years because of its potential applications in energy storage and catalysis. In most cases, NPs are formed both inside the pores and on the external surface on the MOFs. In metal NPs-based catalysts, it appears that the catalytic activity and selectivity can be controlled by the size of the catalytically active phase and by the nature of the supports. Considerable effort has been made to improve the catalytic performance of MIL-101 through the immobilization of noble metals or non-noble metal NPs, as listed in Table 3.

Table 3 Encapsulation of metal nanoparticles over MIL-101^a

Preparation method	Synthesis conditions				NP size (nm)	Comments	Ref.
	Metal precursor	Reducing agent	Temp. (K)	Time (h)
a Amines (ED, DETA, APS), DMF (dimethyl formamide), acac (acetylacetonate), ac (acetate), AB (ammonia borane).
Anion exchange	H2PdCl4, H2PtCl6	NaHB4	273	2	2–4	M-Amine-MIL-101 (M = Pd, Pt, Au)	57
						Knoevenagel condensation, Heck reaction
	HAuCl4
Incipient wetness impregnation (CHCl3)	Pd(acac)2	H2	473	1	∼1.5	Pd/MIL-101	84
						Cyanosilylation reaction, and hydrogenation of styrene and cyclooctene
Impregnation (DMF)	Pd(NO3)2	H2	473	2	1.9 ± 0.7	Pd/MIL-101	85
						Suzuki–Miyaura and Ullmann coupling reaction
Incipient wetness impregnation (DMF)	Pd(NO3)2	H2	473	2	2.5 ± 0.5	Pd@MIL-101	29
					4.2 ± 1.1	One-pot methyl isobutyl ketone synthesis
Colloidal deposition	HAuCl4	H2	473	2	9.8 ± 3.4	Au/MIL-101	86
					2.3 ± 1.1	Aerobic oxidation of alcohol
Impregnation (CHCl3)	Pd(acac)2	H2	473	2	2.1	Pd/MIL-101	87
						One-pot indole synthesis in water
Solution infiltration	Pd(NO3)2	H2	473	4	2.6 ± 0.5	Pd/MIL-101	88
						Direct C2-arylation of indoles
Impregnation (H2O)	HAuCl4	H2	473	3	2–8	Au–Pd/MIL-101 and Au–Pd/ED-MIL-101	89
	H2PdCl4
						Dehydrogenation of HCOOH for chemical H2 storage
					2–15
Impregnation (H2O)	Pd(NO3)2	H2	473	2	∼3	Pd@MIL-101	90
						One-pot synthesis of menthol
Chemical vapour deposition	[(C5H5)Pd(C3H5)] [(C5H5)2Ni]	H2	343	20	2–3	Ni/Pd@MIL-101	91
						Hydrogenation of dialkyl ketones
Impregnation (hexane–H2O)	H2PtCl6	H2	473	5	1.8 ± 0.2	Pt@MIL-101	92
						Liquid-phase AB hydrolysis, solid-phase AB thermal dehydrogenation, gas-phase CO oxidation
Deposition-precipitation (H2O)	HAuCl4	H2	473	2	1.8 ± 0.5	Au/MIL-101	93
						Oxidation of cyclohexane
Colloidal deposition (H2O)	PdCl2	NaHB4	273	—	2.5 ± 0.5	Pd/MIL-101	94
						Aerobic oxidation of alcohol
Colloidal deposition (H2O)	H2PdCl4	NaHB4	273	—	2–3	Pd/MIL-101	95
						Phenol hydrogenation
Incipient wetness impregnation (acetone–DMF)	Pd(acac)2	CO + H2	473	1	3–9	Pt/MIL-101, Pd/MIL-101, PtPd/MIL-101, Pt@Pd/MIL-101	96
	Pt(acac)2
	H2PtCl6					CO oxidation
Impregnation (ethanol)	H2PtCl6	HCOONa	368	2	1.5–2.5	Pt/MIL-101	97
						Asymmetric hydrogenation of α-ketoesters
Colloidal deposition (H2O)	H2PtCl6	NaHB4	273	—	150–350	Pt@MIL-101	98
						Hydrogenation of nitroarenes
Impregnation (hexane–H2O)	H2PtCl6, H2PdCl4, HAuCl4	H2	473	5	∼2.5	M@MIL-101 (M = Pt, Pd, Au or Rh)	99
	RhCl3					Reduction of Cr(VI) to Cr(III)
Impregnation (hexane–H2O)	HAuCl4	NaHB4	273	—	1.8 ± 0.2	AuNi@MIL-101	100
	NiCl2					AB hydrolysis
Impregnation (H2O)	Cu(NO3)2	NH2NH2	—	—	2–3	Cu/MIL-101	101
						Reduction of aromatic nitro compounds
					∼100
Impregnation (acetone–DMF)	Pd(ac)2	CH3COCH3	318	24	2–10	Pd/MIL-101	102
						Oxidation of alcohol
						Hydrogenation of alkenes and aldehyde
Infiltration (H2O)	H2PdCl4	NaHB4	273	3	1.8 ± 0.4	Pd@MIL-101	103
						Hydrolysis of AB
Incipient wetness impregnation (CHCl3)	Pd(acac)2	H2	473	2	2–3	Pd/MIL-101	104
						Hydrogenation of 2,3,5-trimethylbenzoquinone
Impregnation (H2O)	K2PtCl6	NaHB4 and NH3BH3	273	—	1.9 ± 0.4	Ni–Pt@MIL-101	105
						H2 generation from aqueous alkaline solution of hydrazine
	NiCl2
Impregnation (ethanol–H2O)	K2PtCl6	H2	373	5	5.0 ± 0.5	Pt-MIL-101	106
						Hydrogenation of olefin mixture, benzonitrile and linoleic acid
Impregnation (CH3CN–toluene)	C13H19O2Rh	—	343	2.5	150–250	Rh@MIL-101	107
						Hydroformylation of olefin

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Férey et al. reported the encapsulation of noble metals, such as Pd, Pt and Au into the amine-grafted MIL-101 by anion exchange through the three-step PSM process (i) treatment of surface amines with aqueous HCl to form positively charged surface ammonium groups, (ii) reaction of the cationic surface NH₃⁺ with [PdCl₄]²⁻, [PtCl₆]²⁻, and [AuCl₄]⁻, and (iii) reduction of noble metal ions using NaBH₄ at low temperature to produce NPs in the range of 2–4 nm. Some NPs (>20 nm) were also found outside the pores.⁵⁷ Pd NPs supported on MIL-101 can also be prepared from either of the following: (i) chloroform solution of Pd(acac)₂ added to an activated MIL-101 and reduction by H₂ at 473 K to form Pd particles of ∼1.5 nm,⁸⁴ (ii) DMF solution of Pd(NO₃)₂·2H₂O and reduction by H₂ at 473 K to form Pd particles 1.9 ± 0.7 nm,⁸⁵ (iii) aqueous solution of PdCl₂ using polyvinyl alcohol as a protecting agent and NaBH₄ as a reducing agent to produce NPs, 2.5 ± 0.5 nm in size,⁹⁴ (iv) Pd(CH₃COO)₂ infused into MIL-101 in an acetonitrile solution and reduction by H₂ at 473 K to produce NPs in the range, 2–3 nm,¹⁰⁸ or (v) an acetone solution of Pd(CH₃COO)₂ added to MOF and heated at 318 K for 24 h, followed by a treatment with DMF at 373 K to form NPs in the range, 2–10 nm, dispersed over the external surface and inside the pore system.¹⁰²

Xu et al. reported the immobilization of Pt NPs into MIL-101 nanopores using a double solvent method.⁹² In a typical synthesis, metal precursor H₂PtCl₆ dissolved in a hydrophilic solvent (water) with a volume less than or equal to the pore volume of MIL-101 was added slowly to the suspension of MIL-101 in a large excess of n-hexane as the hydrophobic solvent with stirring, followed by reduction using H₂ at 473 K. Transmission electron microscopy (TEM) and electron tomography showed that the encapsulated Pt NPs in Pt@MIL-101 contained very small particles, 1.2–3.0 nm in size, distributed homogeneously throughout the interior cavities of the MIL-101 crystals with no Pt NPs on the external surface. The same group reported the immobilization of AuNi NPs using the combined double solvent method and liquid-phase concentration-controlled reduction route by reduction using NaBH₄.¹⁰⁰ A high-concentration of NaBH₄ produced well dispersed AuNi NPs, 1.8 ± 0.2 nm size, within the pores without any deposition on the external surface of the framework, whereas a low-concentration of reducing agent resulted in NPs aggregation, >5.0 nm in size, on the external surface.

The mono- and bimetallic polyhedral metal nanocrystals supported on MIL-101, Pt/MIL-101, Pd/MIL-101, and PtPd/MIL-101 were obtained by incipient wetness using M(acac)₂ salt (M = Pt, Pd or Pt/Pd), followed by reduction using CO/H₂ at 473 K.⁹⁶ The high-angle annular dark field and bright field scanning transmission electron microscopy images showed that cubic Pt, tetrahedral Pd, and octahedral PtPd nanocrystals were deposited on MIL-101. The authors also reported the preparation of core–shell Pt@Pd/MIL-101 (spherical Pt was used as core for the growth of Pd shell) by seed-mediated two-step reduction from H₂PtCl₆ and Pd(acac)₂ salts in an acetone/DMF solvent, followed by the same reduction process.

The encapsulation of Au–Pd, Au–Pt, Ru–Pd, Au, Pd, Pt or Ru NPs into MIL-101, or amine-grafted MIL-101 were obtained by impregnation and subsequent reduction by H₂ at 473 K.⁸⁹ The Au–Pd NPs in Au–Pd/MIL-101 and Au–Pd/ED-MIL-101 were 2–8 and 2–15 nm in size, respectively. Hermannsdörfer et al. reported the preparation of bimetallic Ni/Pd@MIL-101 by the chemical vapour deposition of [(C₅H₅)Pd(C₃H₅)] and [(C₅H₅)₂Ni], followed by hydrogenation using H₂ at 343 K to produce nanoparticles in the range, 2–3 nm.⁹¹ Recently, Cao et al. reported the preparation of bimetallic NiPt@MIL-101 by impregnation using aqueous K₂PtCl₆ and NiCl₂ solutions, followed by reduction using NaBH₄ and NH₃BH₃.¹⁰⁵ TEM and EDS showed that the encapsulated NiPt NPs in NiPt@MIL-101 contained particles 1.9 ± 0.4 nm in size, which were well-dispersed within the mesoporous cavities in MIL-101.

Pt (20 wt%)/C doped MIL-101 was obtained by the simple ball mill mixing of MIL-101 and Pt (20 wt%)/C under a 0.2–0.3 MPa argon atmosphere.¹⁰⁹ A carbon bridge between MIL-101 and Pt (20 wt%)/C was built by adding sucrose to the mixture, and heating the mixture in a tubular reactor protected by flowing nitrogen at 483 K after ball milling.

5. Applications

5.1 Adsorption

The large surface area and pore volume, high concentration of CUS, excellent thermal stability, as well as high resistance to moisture make MIL-101 an attractive candidate material for the adsorption of gases, water vapour and organic compounds. To enhance its adsorption performance, diverse functional groups or additives have been introduced to the MIL-101 structure to form versatile hybrid materials with supplementary functionalities. Tables 4 and 5 list the reported adsorption studies on MIL-101.

Table 4 Gas adsorption over MIL-101

Adsorbent^a	Adsorbate	Temp. (K)	Pressure (bar)	Uptake (mmol g^−1)	ΔHadsorption (kJ mol^−1)	Comments	Ref.
a CB (carbon bridge), MWCNT (multi-walled carbon nanotubes), SWCNT (single-walled carbon nanotubes), AC (activated carbon), PEI (polyethylenimine), DETA (diethylenetriamine), TEPA (tetraethylenepentamine).b mmol cm^−3.
MIL-101	H2	77	80	30.5	10–9.3	Adsorption capacity and heat of adsorption	110
MIL-101	H2	77	20	22.5	4.3	Heat of adsorption	111
MIL-101	H2	293	1900	36	—	H2 storage by ionic clusters-doped MIL-101	112
MIL-101 (pellet)	H2	77.3	80	43.5	—	Adsorption by MIL-101 pellets	113
MIL-101 (pellet)	H2	19.5	0.9	52.9	—	Adsorption by MIL-101 pellets	114
CB-Pt/C-MIL-101	H2	293	50	5.7	—	Enhance H2 storage by spillover effect	109
CB-Pt/AC-MIL-101	H2	298	100	7.2	21–12	Enhance H2 storage by spillover effect	115
CB-Pd/AC-MIL-101	H2	298	32	2.3	—	Enhance H2 storage by spillover effect	116
CB-Pt/CMK-3-MIL-101	H2	298	20	6.7	—	Enhance H2 storage by spillover effect	117
Pd@MIL-101	H2	298	45	1.2	—	H2 chemisorption	108
Mo6Br8F6-MIL-101	H2	77	60	9.7	—	H2 storage by [Mo6Br8F6]^2− cluster loaded MIL-101	118
Li^+-MIL-101	H2	77	1	17	—	Enhance H2 storage by lithium-doping on MIL-101	119
n-Hexane-MIL-101	H2	298	63	6.3^b	—	Enhanced H2 uptake by n-hexane confined in MIL-101	120
SWCNT@MIL-101	H2	298	60	3.2	—	Enhanced H2 sorption by SWNT incorporated MIL-101	121
AC@MIL-101	H2	77	60	50.5	—	Enhanced H2 sorption in AC incorporated MIL-101	60
MIL-101	CO2	298	0.1	0.49	—	Regeneration conditions and influence of flue gas contaminants	122
MIL-101	CO2	303	50	40	44–24	Adsorption capacity and heat of adsorption	123
	CH4		60	13.6	18–8
MIL-101	CO2	298	30	22.9	4.0–28.6	Adsorption equilibrium and kinetics	124
MIL-101	CO2	298	15	16.8	31	HF-free-synthesis of MIL-101 for CO2 adsorption; water stability	125
MIL-101	CO2	273	1	3.35	—	Size-controlled synthesis of MIL-101 nanoparticles, enhanced CO2/N2 selectivity	32
MIL-101	CO2	288	1.1	3.8	38.81	Equilibrium and dynamics gas adsorption studies	126
MIL-101	CO2	295	36	21.3	32.5–21	Effect of open metal sites and adsorbate polarity	127
MIL-101-NH2-PEI film	CO2	273	1	3.7	—	Facile synthesis of MIL-101 and MIL-101-NH2 films; enhanced CO2/N2 selectivity	37
MIL-101-NH2	CO2	289	25	15	51–19	CO2 adsorption by directly synthesized amine-functionalized MIL-101	50
DETA-MIL-101	CO2	298	1	0.7	69–28	CO2 adsorption by amine grafted MIL-101	67
TEPA-MIL-101	CO2	298	0.2	1.4	42.5–24	Selective adsorption of CO2 over CO by amine grafted MIL-101	66
PEI-MIL-101	CO2	298	1	5	—	Amine-impregnated MIL-101 for selective CO2 capture	58
PEI-MIL-101-NH2	CO2	323	2.1	3.6	—	Amine-impregnated MIL-101 for selective CO2 capture	59
MIL-101-NH2	CO2	273	0.15	2	43–23	Selective and enhanced CO2 adsorption by NH2 functionalization	128
MWCNT@MIL-101	CO2	298	10	1.35	—	Enhanced CO2 adsorption by a MSWNT-incorporated MIL-101	82
MIL-101	Ar	283	5.3	0.9	15–12.5	Polarizability and quadrupole moment effect	129
	CH4			2.2	19–14.2
	CO2			8.0	24–20
	SF6			9.1	26–23.5
	C3H8			13.4	37–26
MIL-101	C2H2	313	1	6.4	—	Effect of purification conditions on gas storage and separations	130
	CO2	298	1	4.6
MIL-101	SF6	298	18	12.3	—	High pressure adsorption isotherms	131
	CF4		35	7.86
MIL-101	H2S	303.1	20	38.4	—	High adsorption capacity	132
MIL-101	n-Butane	293	0.774	11.2		Adsorption capacity and heat of adsorption	133

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Table 5 Water vapour and organic species adsorption over MIL-101

Adsorbent^a	Adsorbate	Temp. (K)	Pressure	Uptake (mg g^−1)	ΔHadsorption (kJ mol^−1)	Comments	Ref.
a GO (graphite oxide), N-SRGO (nitrogen in straight run gas oil), N-LCO (nitrogen in light cycle oil), IL (ionic liquid), H3PW (phosphotungstic acid), HS (hierarchically structured), PED (protonated ethylenediamine), ED (ethylenediamine).b Vwater/Vfilm.
MIL-101	H2O	298	P/P0 = 0.67	428	—	Adsorption and H2O stability	115
MIL-101	H2O	298	P/P0 = 0.9	1276	45.13	Adsorption and H2O stability	134
MIL-101	H2O	298	P/P0 = 0.92	1010	—	Adsorption and H2O stability	135
MIL-101	H2O	303	P/P0 = 0.96	1700	78–44	Studies of H2O sorption isotherm, kinetics, and hydrothermal stability	136
MIL-101	H2O	298	P/P0 = 0.85	1360	70–19	Effect of functional groups in MIL-101 on H2O sorption	49
MIL-101 film	H2O	—	P/P0 = 1	0.79^b	—	Hierarchically-structured thin films of MIL-101	39
MIL-101-NH2	H2O	293	P/P0 = 0.9	1060	43	Cyclic stability of functionalized MIL-101	137
MIL-101	Benzene	303	P/P0 = 0.5	1303	—	Microwave synthesis of MIL-101; high and rapid adsorption	38
MIL-101	Benzene	298	55 mbar	1172	—	Adsorption and diffusion of benzene	40
MIL-101	Benzene	305	P/P0 > 0.7	1404	—	High uptake	27
	n-Hexane		P/P0 > 0.7	1084
MIL-101	Benzene	313	P/P0 = 0.85	1443	68–34	Co-adsorption of n-hexane and benzene vapour	138
	n-Hexane		P/P0 = 0.63	1041	61–38
MIL-101	n-Heptane	313	P/P0 = 1	795	40–29	Adsorption of C5–C9 hydrocarbons	139
MIL-101	Acetone	298	P/P0 = 0.55	1291	—	Adsorption of volatile organic compounds with different molecular size and shape	140
	Benzene			1291
	Toluene			1096
	Ethylbenzene			1105
	m-Xylene			727
	o-Xylene			758
	p-Xylene			1067
MIL-101	p-Xylene	298	5.82 mbar	1009	24.8–44.2	Adsorption equilibrium and kinetics	141
MIL-101	p-Xylene	313	P/P0 = 0.8	1405	67–39	Adsorption and separation of xylene isomer vapours	142
	o-Xylene			1315	76–40
	m-Xylene			1320	70–39
MIL-101	Ethyl acetate	288	54 mbar	924	36.48–42.25	Adsorption and diffusion	143
MIL-101	n-Butylamine	298	—	934	75–125	Adsorption of VOC	30
MIL-101	1,2-Dichloroethane	288	8 mbar	1880	42.0–61.6	Adsorption isotherms, kinetics, and desorption	144
GO@MIL-101	Acetone	288	160 mbar	1195	49–58	MOF/graphite oxide composite; high uptake	83
GO@MIL-101	n-Hexane	298	180 mbar	1042	—	MOF/graphite oxide composite; high uptake	145
MIL-101	N-SRGO	—	—	9	—	Sorption capacities and selectivity towards N-containing compounds	146
	N-LCO			20
MIL-101	Benzothiophene	298	—	0.37	—	Experiment/computational study; adsorption of organosulfur compounds	147
GO@MIL-101	Benzothiophene	298	—	28	—	MOF/graphite oxide composite; adsorption of N-and S-containing compounds	62
	Quinoline			549
	Indole			319
IL-MIL-101	Benzothiophene	298	—	87	—	Ionic liquid supported on MIL-101 for adsorptive desulfurization	148
MIL-101 (H3PW)	Quinoline	298	—	274	—	H3PW impregnated MIL-101; adsorptive denitrogenation	74
MIL-101	Xylenol orange	298	—	312	11	Kinetic and thermodynamic studies	149
HS-MIL-101	Methylene blue	—	—	23	—	Hierarchically mesostructured MIL-101 synthesis; accelerated adsorption kinetics	33
PED-MIL-101	Methyl orange	308	—	219	29.5	Surface modification; adsorption equilibrium and kinetics	150
MIL-101	Uranine	298	—	127	25.3	Adsorption kinetics, mechanism, and thermodynamics	151
MIL-101	Pyridine	293	—	950	—	Experimental and theoretical study; high uptake	152
MIL-101	C70	303	—	198	18.1	Selective adsorption and extraction of C70 and higher fullerenes	153
MIL-101	Bisphenol A	303	—	253	—	Adsorption equilibrium and kinetics	154
MIL-101	Naproxen	298	—	119	—	Adsorption equilibrium and kinetics	155
	Clofibric acid			312
ED-MIL-101	Naproxen	298	—	154	—	Surface modification; adsorption equilibrium and kinetics	156
	Clofibric acid			347

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5.1.1 H₂ adsorption. As a clean and efficient energy carrier, H₂ storage has attracted enormous attention. One approach involves the sorption of H₂ on the surface of porous solids.¹⁵⁷ Latroche et al. reported H₂ adsorption on MIL-101, which exhibited approximately 6.1 wt% H₂ uptake under 8 MPa and 77 K (Fig. 11) with a heat of adsorption of 10 kJ mol⁻¹ at low surface coverage conditions.¹¹⁰ Small pores in MIL-101 were believed to play a major role in H₂ adsorption. H₂ adsorption by MIL-101 in pellet form was also reported.¹¹³ The specific surface area and micropore volume decreased with increasing pelletizing pressure increases and optimal volumetric adsorption capacity of 40 g L⁻¹ (under 8 MPa, 77.3 K) was obtained.

Fig. 11 Adsorption (point-up triangles) and desorption (point-down triangles) pressure–composition isotherm (PCI) curves for MIL-101 at 298 (empty symbols) and 77 K (full symbols).¹¹⁰

MIL-101 has been functionalized via a variety of methods to enhance the H₂ adsorption capacity, typically via a spillover effect. Liu et al. studied the H₂ storage behaviour of Pt/C doped and carbon bridged Pt/C-modified MIL-101.¹⁰⁹ Both greatly enhanced the H₂ storage capacity of MIL-101 at ambient temperatures with the latter exhibiting greater H₂ uptake. Doping and modification in MIL-101 influenced H₂ storage by assisting in H₂ dissociation and their effective spillover to the MOF surface. Similarly, the H₂ storage capacity was enhanced by a factor of 2.8 for MIL-101 by carbon bridged spillover.¹¹⁵ Szilágyi et al. introduced Pd nanoparticles into MIL-101 without a carbon bridge,¹⁰⁸ and also observed an increase in the ambient-temperature H₂ uptake, which was attributed to palladium hydride formation rather than H₂ spillover. Klyamkin et al. examined the ultra-high-pressure H₂ storage performance (up to 1900 bar) over MIL-101 and MIL-101 doped with ionic clusters ([Re₄S₄F₁₂]⁴⁻, [SiW₁₁O₃₉]⁷⁻).¹¹² Low temperature (81 K) sorption measurements indicated that doping had no effect, while doping at high temperatures provided an increase in the number of H₂ binding sites such that doping of MIL-101 by [Re₄S₄F₁₂]⁴⁻ clusters increased the number of H₂ binding sites by 2.5 fold at 293 K. Dybtsev et al. studied the influence of [Mo₆Br₈F₆]²⁻ cluster unit inclusion within MIL-101 on H₂ storage performance.¹¹⁸ At room temperature and 8 MPa, the H₂ storage capacity of the Mo₆Br₈F₆-MIL-101 was over twice that of MIL-101, which may be assigned to higher heat of adsorption for H₂ in Mo₆Br₈F₆-MIL-101. Xiang et al. examined the doping of Li⁺ to the MIL-101 frameworks for H₂ storage.¹¹⁹ At 77 K and 1 bar, the H₂ uptake in MIL-101 was increased by 43% (from 2.37 to 3.39 wt%) after lithium doping, which was attributed to the strong affinity of Li⁺ for H₂ molecules. Clauzier et al. reported enhanced H₂ uptake in hybrid sorbents consisting of n-hexane confined in MIL-101.¹²⁰ When n-hexane was confined in MIL-101, the H₂ solubility increased 22.2 fold compared to that of n-hexane alone. Such enhanced solubility effect was much larger than that observed with either silica aerogel or MCM-41. The important role of surface chemistry and accessible surface area in enhancing the gas solubility was proposed. Prasanth et al. reported H₂ adsorption by single-walled carbon nanotube (SWCNT)-incorporated MIL-101 composites.¹²¹ The H₂ sorption capacities of MIL-101 increased from 6.37 to 9.18 wt% at 77 K up to 60 bar and from 0.23 to 0.64 wt% at 298 K up to 60 bar, which was attributed to the decrease in pore size and increase in micropore volume in MIL-101 by SWCNT incorporation. Similar results were reported with an activated carbon-incorporated MIL-101.⁶⁰

5.1.2 CO₂ adsorption. CO₂ is a major anthropogenic greenhouse gas. In recent years, intensive efforts have been made to develop new technologies/processes for effective CO₂ capture and storage. Among them, the majority of research activities focused on examining sorption-based technologies and processes, involving solid CO₂ adsorbents.¹⁵⁸ Llewellyn et al. examined CO₂ adsorption on MIL-101, and reported a record capacity of 40 mmol g⁻¹ or 390 cm³_STPcm⁻³ at 5 MPa and 303 K.¹²³ The heat of adsorption of CO₂ at low surface coverage (−44 kJ mol⁻¹) was higher than that reported for other MOFs, and was of the same order of magnitude as that measured by zeolites due to the coordination of CO₂ molecules onto the chromium open metal sites in MIL-101. Jiang et al. reported higher CO₂ adsorption selectivity over N₂ by nano-sized MIL-101 (19–84 nm).³² Liu et al. examined the influence of flue gas contaminants on the adsorption of CO₂ by MIL-101, as well as the regeneration conditions.¹²² Three trace flue gas contaminants (H₂O, NO, SO₂) were each added to a 10 vol% CO₂/N₂ feed flow but they had minimal impact on the adsorption capacity of CO₂ by MIL-101. In addition, complete regeneration could be achieved within 10 min at 328 K in temperature swing adsorption with N₂-stripping, or at 348 K for vacuum-temperature swing adsorption at 20 kPa. Almost 99% of the pre-regeneration adsorption capacity was preserved after 5 adsorption–desorption cycles under a gas flow of 10 vol% CO₂, 100 ppm SO₂, 100 ppm NO, and 10% RH, respectively.

Frequently, amine species have been introduced to MIL-101 to increase its CO₂ adsorption capacity and selectivity against other gases. Tetraethylenepentamine (TEPA) grafted on the Cr(III) sites in MIL-101 for the selective adsorption of CO₂ over CO was reported by Wang et al.⁶⁶ The selectivity for CO₂ over CO was improved from 1.77 to 70.20 at 298 K and a total pressure of 40 kPa. DETA grafted on the Cr(III) sites of MIL-101 led to better CO₂ adsorption capacity at low pressures as well as a higher heat of adsorption compared to the original MIL-101.⁶⁷ The post-synthetically prepared MIL-101 with amino groups also showed higher selectivity over N₂ and CH₄, compared to the original MIL-101.¹²⁸ Chen et al. reported PEI-incorporated MIL-101 adsorbents prepared by a wet impregnation method, which exhibited dramatically enhanced CO₂ adsorption capacity at low pressures (4.2 mmol g⁻¹ at 298 K, 0.15 bar with a 100 wt% PEI loading), rapid adsorption kinetics, and ultrahigh selectivity for CO₂ over N₂ in the flue gas with 0.15 bar CO₂ and 0.75 bar N₂ (CO₂ over N₂ selectivity was 770 at 298 K, and 1200 at 323 K).⁵⁸ In addition, they achieved improved CO₂/CH₄ selectivity and CO₂ adsorption capacity at low pressure by a dual amine functionalized MIL-101 adsorbent prepared by incorporating PEI into the framework of a directly synthesized amine-functionalized MIL-101 (Fig. 12).⁵⁹ Enhanced CO₂ adsorption selectivity over N₂ by MIL-101 and MIL-101-NH₂ films that were fabricated through a PEI-assisted dip-coating method was also reported.³⁷

Fig. 12 CO₂ and CH₄ adsorption isotherms for amine-MIL-101 (circles), 50PEI-amine-MIL-101 (triangles), 75PEI-amine-MIL-101 (squares) and 100PEI-amine-MIL-101 (stars) at 298 K. Symbols: filled, CO₂ adsorption; hollow, CH₄ adsorption.⁵⁹

Zhang et al. published the results of a multi-scale modelling study to examine the effects of functional groups (–CH₃, –Cl, –NO₂, –CN and –NH₂) on the adsorption and separation of CO₂/N₂ over MIL-101.¹⁵⁹ The CO₂ uptake in the low-pressure regime increased in the order of MIL-101 < MIL-101-CN < MIL-101-NO₂ < MIL-101-Cl < MIL-101-CH₃ < MIL-101-NH₂, which followed the sequence of strength of the binding energies between CO₂ and the functional groups. The CO₂/N₂ selectivity was similarly enhanced after functionalization, and the predicted breakthrough time was increased with the longest breakthrough time in MIL-101-NH₂ being approximately 2 times longer that in MIL-101.

Anbia et al. reported CO₂ adsorption on a hybrid composite of acid-treated multi-walled carbon nanotubes (MWCNTs) and MIL-101.⁸² The composite had the same crystal structure and morphology as that of virgin MIL-101, but the CO₂ adsorption capacity was increased by approximately 60% (from 0.84 to 1.35 mmol g⁻¹) at 298 K and 10 bar. The increase in CO₂ adsorption capacity by MWCNT@MIL-101 was attributed to the increase in micropore volume of MIL-101 by MWCNT incorporation.

5.1.3 H₂O vapour adsorption. Water sorption technologies are widely used commercially in industrial or indoor desiccant applications, fresh water production and adsorption heat transformation. Silica gel and zeolites currently have commercial applications, and tests using MOFs are also being carried out. Li et al. reported H₂O vapour adsorption on MIL-101.¹¹⁵ A H₂O adsorption amount of 42.8 wt% was measured at 298 K and P/P₀ = 0.67, which was much higher than that by HKUST-1 or COF-1. This larger amount was attributed to the high pore volume and metal-oxide clusters present in the framework of MIL-101. In addition, MIL-101 was stable in moist air after exposing the sample to ambient air for 7 days with a relative humidity ∼40% at 298 K. Ehrenmann et al. reported a high H₂O uptake of 1.01 g g⁻¹ (at 298 K, P/P₀ = 0.92) by MIL-101 with an S-shaped isotherm.¹³⁵ Only a slight decrease in performance was detected with a H₂O uptake capacity of 98.1% and 96.8% after 20 and 40 cycles, respectively, compared to the initial load, indicating the excellent stability of MIL-101. Seo et al. reported the exhaustive analysis of the H₂O sorption properties on MIL-101.¹³⁶ High equilibrium uptakes of 1.50–1.70 g g⁻¹ in dehydrated MIL-101 at 303–313 K and P/P₀ > 0.5 were achieved. MIL-101 showed both high adsorption and desorption rates less than 353 K, and excellent stability under cyclic operation of H₂O adsorption–desorption. Highly porous thin films of MIL-101 on a silicon wafer with a dual hierarchical porous structure showed double-step H₂O adsorption associated with both different sizes of pores in MIL-101 coupled with the inter-particle mesoporosity.³⁹ Akiyama et al. examined the H₂O sorption properties of MIL-101 containing different substituents (–H, –NO₂, –NH₂, –SO₃H) in the ligand (Fig. 13).⁴⁹

Fig. 13 Water adsorption isotherms at 298 K for desolvated MIL-101 (a), MIL-101-NH₂ (b), MIL-101-SO₃H (c), and MIL-101-NO₂ (d). P/P₀ is the relative pressure of water, where the saturation vapour pressure (P₀) is 3.16 kPa. A is the amount adsorbed (g g⁻¹).⁴⁹

All the materials exhibited a H₂O uptake of ca. 0.8–1.2 g g⁻¹, and the adsorbed H₂O could be released at ca. 353 K without high evacuation. The sorption profile could be tuned by changing the substituents in the ligand, such that the isotherm lines of MIL-101-NH₂ and MIL-101-SO₃H moved to lower P/P₀ values compared to those of MIL-101, which was attributed to the highly hydrophilic groups on their pore surfaces. Khutia et al. reported H₂O sorption cycle measurements on NH₂- or NO₂-functionalized MIL-101 synthesized by post-synthetic modification.¹³⁷ The MIL-101-NH₂ showed excellent H₂O loading (1.06 g g⁻¹ at 293 K, P/P₀ = 0.9) as well as H₂O stability over the 40 adsorption–desorption cycles.

5.1.4 Other adsorption for energy/environmental applications. As listed in Tables 4 and 5, other adsorption processes by MIL-101 for energy/environmental applications, such as methane storage, adsorptive removal of VOC, dyes, and nitrogen-(or sulphur) containing compounds have also been reported. Llewellyn et al. reported a CH₄ uptake of 13.6 mmol g⁻¹ or a volume capacity of 135 cm³_STPcm⁻³ by MIL-101 at 6 MPa and 303 K, which was below the DOE target for methane storage on a volume basis (180 cm³_STPcm⁻³).¹²³ For CH₄ storage, MIL-101 itself is unsatisfactory, whereas several other MOF materials meeting the DOE target have emerged recently.¹⁶⁰ A high n-butane adsorption capacity (11.2 mmol g⁻¹ at 293 K, 0.774 bar) on MIL-101 was reported by Klein et al., which was approximately three times higher than that on than Cu₃(BTC)₂, and twice as much as that with activated carbon.¹³⁴ The CO uptakes of 1.13 mmol g⁻¹ (at 850 mm Hg, 288 K) and 6.5 mmol g⁻¹ (at 66 bar, 295 K) by MIL-101 were reported with a high heat of adsorption (ca. 42 kJ mol⁻¹).^126,127 Tanh Jeazet et al. reported mixed-matrix membranes made from water-stable MIL-101 and polysulfone (PSF) for O₂/N₂ separation.¹⁶¹ MIL-101 microcrystals could adhere well to PSF and yield a very robust mixed-matrix MIL-101-PSF membrane. At 24 wt% MIL-101, the permeability of the membrane increased almost four fold with respect to the pure polymer tested under the same conditions, while keeping the high selectivity for O₂ over N₂ of 5–6. Hamon et al. reported a high H₂S adsorption capacity of 38.4 mmol g⁻¹ on MIL-101 at 2 MPa and 303 K, which was significantly higher than that of MIL-53 (Al, Cr, Fe), MIL-47 (V) and MIL-100 (Cr).¹³²

Jhung et al. reported benzene adsorption by MIL-101 synthesized under microwave irradiation.³⁸ The MIL-101 exhibited a significantly larger sorption capacity for benzene (16.7 mmol g⁻¹ at 303 K and P/P₀ = 0.5) than those by SBA-15, HZSM-5, or commercial active carbon (Fig. 14) with rapid sorption kinetics.

Fig. 14 Sorption isotherms for benzene at 303 K on MIL-101, a commercial active carbon, HZSM-5 zeolite, and mesoporous silica SBA-15.³⁸

Hong et al. also reported a large sorption uptake of benzene by MIL-101, i.e., 19.5 mmol g⁻¹ at P/P₀ > 0.7 and 305 K,²⁷ and observed that MIL-101 also has a high uptake of n-hexane; the equilibrium sorption capacity of n-hexane at 303 K was 12.6 mmol g⁻¹ at P/P₀ > 0.7. Trens et al. reported the co-adsorption of n-hexane and benzene vapours onto MIL-101,¹³⁸ which showed that MIL-101 had a better affinity for benzene than n-hexane; benzene was adsorbed preferentially at low coverage at the expense of n-hexane. Yang et al. revealed the adsorption performance of MIL-101 towards several VOCs, such as acetone, benzene, toluene, ethylbenzene, m-xylene, o-xylene, and p-xylene, with different molecular properties.¹⁴⁰ MIL-101 was found to exhibit higher adsorption capacities than zeolites, activated carbons, or other reported adsorbents. The adsorption of VOCs on MIL-101 takes place by a pore filling mechanism, and the size- and shape-selectivity of VOCs molecules were observed. Huang et al. studied the adsorption characteristics of six VOCs with different functional groups and polarities (n-hexane, toluene, methanol, butanone, dichloromethane, and n-butylamine) on MIL-101.³⁰ Again, MIL-101 had significantly higher affinity and higher adsorption capacity towards VOCs than activated carbons, which showed higher affinity towards the VOCs containing a heteroatom or aromatic ring, particularly amines; MIL-101 exhibited the strongest affinity to n-butylamine with an adsorption capacity of 12.8 mmol g⁻¹. CUS in MIL-101 are vital in the sorption process.

Kim et al. reported a large pyridine adsorption capacity (950 mg g⁻¹ at 293 K) on MIL-101.¹⁵² Pyridine could be adsorbed either on the carbon atoms of the carboxylic groups of the BDC (benzene-1,4-dicarboxylate) units or on the CUS in MIL-101. The retention of pyridine under evacuation at 423 K, as well as the calculated stabilization energy suggested the strong adsorption of pyridine on the CUS of MIL-101. Ahmed et al. reported the adsorptive removal of nitrogen-containing compounds (NCCs) and sulfur-containing compounds (SCCs) in model fuels by a graphite oxide/MIL-101 composite (GO/MIL-101).⁶² The composite exhibited the highest adsorption capacity for NCCs among the adsorbents reported, which was attributed to the higher surface area of GO/MIL-101 over MIL-101 (Fig. 15). The adsorbent could be used several times after regeneration without any detectable degradation in performance. The same type of GO/MIL-101 was also shown to have a high acetone adsorption capacity (20.1 mmol g⁻¹ at 288 K, 161.8 mbar), which was much higher than that of bare MIL-101.⁸³

Fig. 15 Adsorption isotherms for (a) quinoline and (b) indole on MIL-101 and 0.25% GO/MIL-101.⁶²

Khan et al. produced acidic ionic liquids (ILs)-supported MIL-101 for adsorptive desulfurization.¹⁴⁸ A remarkable improvement in the benzothiophene adsorption capacity was observed for ILs supported on MIL-101 compared to bare MIL-101, which was explained by the acid–base interactions between the acidic IL and basic benzothiophene molecules. Similarly, adsorptive denitrogenation with MIL-101 impregnated with H₃PW was reported;⁷⁴ a 1% loading of H₃PW on MIL-101 increased the adsorption of quinoline by approximately 20%, owing to the favourable interactions between the acid and base sites.

Haque et al. reported the adsorptive removal of methyl orange (MO) in an aqueous solution by MIL-101.¹⁵⁰ The porosity and pore size in MIL-101 were found to play important roles in the adsorption, and a specific electrostatic interaction between MO and the adsorbent was responsible for the rapid and high uptake of the dye. The adsorption of MO was a spontaneous and endothermic process. Similar results regarding xylenol orange¹⁴⁹ and uranine adsorption¹⁵¹ have been reported. Huang et al. reported remarkably accelerated adsorption kinetics for methylene blue removal compared to the bulk MIL-101 by a hierarchically-structured MIL-101 prepared using CTAB as a supramolecular template.³³ Huo et al. reported the use of MIL-101 for the rapid magnetic solid phase extraction of polycyclic aromatic hydrocarbons (PAHs) from environmental water samples.¹⁶² The simple mixing of MIL-101 with silica-coated Fe₃O₄ micro-particles in solution with sonication was applied to the adsorbent preparation. Hydrophobic and π–π interactions between the PAHs and framework terephthalic acid molecules, and π-complexation between PAHs and the Lewis acid Cr(III) sites in the pores of MIL-101 played a significant role in the adsorption of PAHs. Hasan et al. investigated the liquid-phase adsorption of naproxen and clofibric acid over MIL-101.¹⁵⁵ The adsorption was attributed to an electrostatic interaction between the chemicals and adsorbent. The adsorption rate and adsorption capacity were improved using MIL-101 modified with basic NH₂ groups, which can adsorb naproxen and clofibric acid through an acid–base interaction.¹⁵⁹ Yang et al. examined the selective adsorption and extraction of C₇₀ and higher fullerenes on MIL-101.¹⁵³ MIL-101 showed the preferential adsorption of C₇₀ and higher fullerenes over C₆₀ with high selectivity (α_C70/C60 = 24). Therefore, the selective extraction of C₇₀ and higher fullerenes from crude carbon soot could be achieved easily via a simple adsorption–desorption process. The used MIL-101 could be regenerated by washing with o-dichlorobenzene under ultra-sonication.

5.2 Heterogeneous catalysis

MIL-101 and its organic-functionalized or guest-incorporated forms have been applied as efficient catalysts for a range of reactions involving different active sites. In this section, the performance of the MIL-101 in the Lewis acid, Lewis base, acid–base concerted, tandem, oxidation, hydrogenation, C–C coupling, and other reactions will be presented.

5.2.1 Cyanosilylation of benzaldehyde. MIL-101 was applied to Lewis acid-catalysed cyanosilylation reaction (Scheme 1), and benzaldehyde and trimethylsilylcyanide were converted to the corresponding product in 98.5% yield at 313 K after 3 h, which showed higher catalytic activity of MIL-101 than other MOFs, such as Cu₃(BTC)₂.⁸⁴ The heterogeneity of the catalysts was confirmed in a hot filtration test and recycle runs. XRD confirmed that the structure of MIL-101 had been maintained after the recycle runs.

Scheme 1 Cyanosilylation of benzaldehyde.

5.2.2 Knoevenagel condensation. Amine-functionalized MIL-101 was applied to the Knoevenagel condensation reaction as a base- and acid-catalysed reaction (Scheme 2).^57,67 Férey et al. reported the Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate using amine-grafted MIL-101, which showed good activity (96.3–97.7% conversion and ∼99% product selectivity) in cyclohexane at 353 K.⁵⁷ ED-MIL-101 exhibited superior activity (10 times higher) than APS-grafted SBA-15 mesoporous silica. No catalytic activity was observed in the Knoevenagel condensation of benzophenone and malonitrile over ED-MIL-101. This dependence of the catalytic activity on the reactant/product size suggests that the reaction occurred mostly inside of the pores of MIL-101, where the active amine groups were available. Juan-Alcañiz et al. also reported the Knoevenagel condensation reaction of benzaldehyde with ethyl cyanoacetate using H₃PW/MIL-101 prepared by the direct encapsulation of phosphotungstic acid into MIL-101, and ∼99% conversion was obtained in toluene at 313 K after 200 min.⁷⁰ The high catalytic performance originated from the partial substitution of tungsten by Cr³⁺ to create the so-called lacunary structure. The heterogeneity of the catalysis in the liquid phase reaction was confirmed by a hot filtration experiment and recycle runs, and no leaching of the guest was observed in the filtrate after recycling.

Scheme 2 Knoevenagel condensation reaction.

5.2.3 CO₂ cycloaddition of epoxide. MIL-101 was applied to the Lewis acid/base-catalysed cycloaddition of CO₂ to alkene oxides under high pressure CO₂ (8–100 bar) conditions over the temperature range, 298–393 K (Scheme 3)^163,164 in the presence of tetrabutylammonium bromide (TBAB) as a co-catalyst under solvent-free conditions.¹⁶³ Styrene oxide and propylene oxide converted to a cyclic carbonate using CO₂ (8 bar) in the presence of TBAB at 298 K with an 82% yield (after 24 h) and 95% yield (after 48 h), respectively, whereas cyclohexene oxide exhibited only a 5% yield after 64 h. The recycled catalyst showed significantly lower activity than the fresh one and the BET surface area decreased substantially, suggesting pore blocking by the carbonaceous deposits formed during the reaction. Recently, Ahn et al. reported the cycloaddition reaction of CO₂ and styrene oxides in chlorobenzene using different MOFs with different acid–base functionalities.¹⁶⁴ MIL-101 showed a 63% yield of styrene carbonate using CO₂ (20 bar) at 373 K after 4 h. On the other hand, UiO-66-NH₂ exhibited a significantly higher yield of styrene carbonate (∼94%) than MIL-101 under identical reaction conditions. The authors suggested that the high catalytic activity of UiO-66-NH₂ originated from high populations of both Lewis acid and base sites, which suggests that the amine-functionalized MIL-101, MIL-101-NH₂, would be equally effective in the same reaction. The heterogeneity of the catalyst was confirmed in a hot filtration test and recycle runs. XRD confirmed that the structure of MIL-101 had been retained after the recycle runs.

Scheme 3 CO₂ cycloaddition of epoxide.

5.2.4 One pot deacetalization-nitroaldol reaction. A cascade or tandem reaction is a consecutive series of intramolecular organic reactions, which often proceed in the presence of highly reactive chemical intermediates. The main advantages of the reactions are (i) the reaction rate can be accelerated because of its intramolecular nature, (ii) displays high atomic utilization efficiency, and (iii) does not involve workup and isolation of many intermediates. Li et al. reported the acid–base-catalysed one-pot tandem reaction using MIL-101-NH₂-SO₃H possessing both Bronsted acid and base sites,⁵⁵ which showed good activity in the hydrolysis of benzaldehyde dimethyl acetal and the subsequent Henry reaction with nitromethane to form 2-nitrovinylbenzene (Scheme 4). Ahn et al. synthesized bifunctional MIL-101-NH₂-SO₃H in a different but much simpler route, as described earlier,⁵⁶ and the material exhibited excellent catalytic activity in the deacetalisation–nitroaldol reaction with 100% conversion and 99.3% selectivity to 2-nitrovinylbenzene at 323 K after 6 h. The catalytic performance was superior to other bifunctional catalysts, such as periodic mesoporous organosilicas containing organic amines and sulfonic acid groups or heteropoly acid and amine-grafted SBA-15. The catalyst could be reused several times without any loss of its initial catalytic activity. XRD and FT-IR confirmed that the crystallinity of the catalyst had been retained during the reaction with no leaching observed.

Scheme 4 One-pot deacetalization-nitroaldol reaction.

5.2.5 Organophosphorous ester degradation. Dialkylaminopyridine (DAAP)-incorporated MIL-101 was applied to the Lewis acid–base catalysed hydrolysis degradation of diethyl 4-nitrophenyl phosphate, which is a biocide applied widely for crop protection and as a structural analogue for chemical warfare agents (Scheme 5).⁶⁸ The material induced the efficient degradation of organophosphorous ester paraoxon in a water–acetonitrile mixture at room temperature to form diethyl phosphate and p-nitrophenol (79–100% conversion after 24 h at pH 10), showing 7 times and 47 times higher activity than the parent MIL-101 and free dialkylaminopyridine, respectively, because of the synergistic effects of Lewis acidic Cr(III) and DAAP as a Lewis base.

Scheme 5 Organophosphorous ester degradation.

5.2.6 Baeyer condensation of benzaldehyde. Bromberg et al. reported the Baeyer condensation of benzaldehyde and 2-naphthol, and in the three-component condensation of benzaldehyde, 2-naphthol and acetamide under solvent-free microwave irradiation using phosphotungstic acid-incorporated MIL-101, H₃PW/MIL-101.³¹ Benzaldehyde and 2-naphthol were converted to dibenzoxanthene in high yield (96%) within a short reaction time (2 min) at 363 K (Scheme 6), and benzaldehyde, 2-naphthol and acetamide formed 1-amidoalkyl-2-naphthol in high yield (95%) at 403 K after 5 min (Scheme 7). The heterogeneous nature of the catalysis was confirmed. H₃PW/MIL-101 showed higher catalytic activities in both condensation reactions compared to those by the parent MIL-101.

Scheme 6 Baeyer condensation of benzaldehyde and 2-naphthol.

Scheme 7 Baeyer condensation of benzaldehyde, 2-naphthol and acetamide.

5.2.7 Oxidation of alkanes. Ahn et al. reported the liquid-phase oxidation of tetralin using tert-butyl hydroperoxide (t-BuOOH) or O₂ and trimethylacetaldehyde as an oxidant,¹⁶⁵ making direct use of Cr(III) in MIL-101 for oxidation. MIL-101 exhibited higher catalytic activity (68% conversion and 86% selectivity to 1-tetralone) using t-BuOOH at 353 K after 8 h than a chromium-containing aluminophosphate, CrAPO-5 (47% conversion and 82% selectivity) under identical reaction conditions. The conversion and selectivity were strongly dependent on the reaction temperature, time, catalyst amount, and concentration of t-BuOOH. This study also demonstrated that the nature of the solvents and the sources of the oxidant play important roles. The heterogeneity of the catalyst was confirmed in a hot filtration test and recycling runs. XRD confirmed that the structure of the catalyst was unchanged during the reaction.

The hybrid materials, FePcF₁₆@MIL-101, RuPcF₁₆@MIL-101 and N(FePc^tBu₄)₂@MIL-101 were similarly applied to the aerobic oxidation of tetralin using molecular oxygen at 383 K.⁸⁰ FePcF₁₆@MIL-101 and RuPcF₁₆@MIL-101 showed high activity with a turnover number (TON) of 48 200 and 46 300, respectively, after 24 h. MPcF₁₆ (M = Fe or Ru) species were located inside the MIL-101 matrix and high catalytic performance was observed. On the other hand, dimeric N(FePc^tBu₄)₂ species deposited on the external surface of MIL101 were formed as the reaction proceeded and the catalytic activity declined. Cyclohexane oxidation over MIL-101 using t-BuOOH at 343 K produced cyclohexanone and cyclohexanol with 36% conversion and 75% selectivity to cyclohexanone.¹⁶⁶ XRD and FT-IR spectroscopy confirmed that the catalyst was reusable and stable under the reaction conditions. The post-synthetic covalently attached tetra(4-pyridyl)porphyrinatomanganese(III) acetate to CM-MIL-101, Mn(TPyP)OAc/CM-MIL-101, as shown in Fig. 8, was found to be an efficient heterogeneous catalyst for the hydroxylation of a variety of alkanes, such as tetralin, ethylbenzene, propylbenzene, cyclohexane, and cyclooctane, using sodium periodate as the oxidant and imidazole as a co-catalyst.⁶⁴ The hybrid material exhibited high activity (52–72% conversion) and selectivity (58–100%) to the corresponding alcohols and ketones at 298 K.

5.2.8 Oxidation of alkenes and sulfoxidation of aryl sulphide. The POM encapsulated in MIL-101 was applied to the epoxidation of alkenes using H₂O₂ as the oxidant.^31,69,75 Bromberg et al. reported the epoxidation of p-caryophyllene using H₃PW/MIL-101.³¹ The material exhibited high activity (95% yield to caryophyllene oxide) using 30% aqueous H₂O₂ as the oxidant in acetonitrile at 328 K after 5 min under microwave-assisted heating. In this case, deprotonation of the H₃PW anion immobilized within the MIL-101 cages prevented epoxide ring-opening during the reaction. The heterogeneous nature of the catalyst was confirmed by a hot filtering experiment. XRD confirmed that the structure of MIL-101 had been retained after the recycle runs. Titanium-and cobalt-monosubstituted Keggin heteropolyanions in MIL-101 exhibited high activity (88% conversion) and selectivity (100% to oxide) in the epoxidation of caryophyllene using H₂O₂ (0.2 M) at 323 K after 4 h, whereas α-pinene and cyclohexene were converted to allyllic products (verbenol/verbenone or cyclohexene-2-ol/cyclohexene-2-one) with good activity (∼40% conversion).⁷⁵ The same group reported H₂O₂-based alkene epoxidation using [(PW₄O₂₄)³⁻]/MIL-101 and [(PW₁₂O₄₀)³⁻]/MIL-101.⁶⁹ The materials were found to be efficient heterogeneous catalysts for the epoxidation of a variety of alkenes (3-carene, limonene, α-pinene, cyclohexene, cyclooctene, 1-octene) using aqueous H₂O₂ in acetonitrile at 323 K. The heterogeneity of the catalyst in the liquid phase reaction was confirmed. They also revealed the oxidation of cyclohexene and α-pinene using molecular oxygen (1 bar) in the presence of t-BuOOH over MIL-101 under solvent-free conditions, which produced allyllic products.¹⁶⁷ The lanthanide-containing Keggin-type POM and zinc-monosubstituted Keggin heteropolyanions encapsulated into MIL-101 were used as a heterogeneous catalyst for the oxidation of alkenes, such as styrene, cyclohexene, cyclopentene, cyclooctene, indene, 1-octene, 1-decene, and 1-dodecene using H₂O₂ as an oxidant in acetonitrile with good activity.^77,78 The epoxidation of a variety of alkenes (cyclohexene, cyclooctene, styrene, 1-octene and 1-decene) over the Mn-complex-immobilized MIL-101, Mn(TPyP)OAc/CM-MIL-101, were also reported.⁶⁴ The material exhibited good activity in the epoxidation of alkenes with high yield (77–96%). The heterogeneous nature of the catalyst was provided by XRD and UV-Vis DRS, but the leaching of Mn (0.22%) was observed in the filtrate after the first run.

Hwang et al. reported the catalytic activity of MIL-101 for the liquid-phase sulfoxidation of aryl sulfides to sulfoxides using 35% H₂O₂ in acetonitrile with high conversion (88–99%) and selectivity (100%) at 298 K after 12 h.¹⁶⁸ The oxidation of thioanisole using t-BuOOH at 298 K over the vanadyl(monocatecholate)-decorated MIL-101 catalyst was reported.⁶⁵ The material showed good activity in the oxidation of thioanisole with 57% and 26% yield to sulfoxide and sulfone, respectively.

5.2.9 Oxidation of aromatic alcohol. The selective oxidation of an alcohol to an aldehyde, particularly benzaldehyde from benzyl alcohol, is one of the most versatile and important classes of organic reactions for laboratories and industries. The development of ecologically acceptable catalytic systems using molecular oxygen or air as an oxidant, which only produces water as a by-product, has attracted considerable attention. The Pd nanoparticle-supported on MIL-101 were found to be an efficient heterogeneous catalyst for the oxidation of a range of alcohols, such as benzyl, allylic, aliphatic, and heterocyclic alcohol to aldehydes using air or O₂.^94,102 Chen et al. reported the aerobic oxidation of alcohols using pure O₂ at 353 K, and the catalyst showed high conversion (95–99%) and selectivity (96–99%) in toluene.⁹⁴ The authors also reported the oxidation of benzyl alcohol under solvent-free conditions and showed high product selectivity (>99%) with a high turnover frequency (16 900 h⁻¹). Recycling runs over the spent catalysts, measurements of the Pd content after the catalytic reaction by atomic absorption spectroscopy (AAS) analysis, and XRD before and after the reaction confirmed the heterogeneity of the reaction. Recently, Ahn et al. reported the oxidation of benzyl alcohol to benzaldehyde over Pd-MIL-101 with 85% conversion and 97% selectivity in o-xylene in air at 358 K after 14 h.¹⁰² The oxidation of various alcohols using molecular oxygen over Au MPs supported on MIL-101 was also reported.⁸⁶ The catalyst exhibited high activity (∼99% conversion and ∼99% product selectivity) in toluene at 353 K, and showed high turnover frequency (TOF) (29 300 h⁻¹) due to the well dispersed Au NPs and the electron donation effect of the aryl rings to the Au NPs within the cages of MIL-101. Au NPs supported on PMA-MIL-101 [PMA = phenylene-mono(oxamate)] were also applied to the oxidation of benzyl alcohol using air. In addition, benzyl alcohol was converted to benzaldehyde with TOF ≈ 7 min⁻¹ with high selectivity (100%) in toluene at 353 K, in which Au NPs of ∼2 nm in size were encapsulated inside the MOF cages.¹⁶⁹

5.2.10 Hydrogenation reaction. Pd, Pt and Ni NPs supported on MIL-101 were employed in the hydrogenation of alkenes, aldehydes, ketones, phenol, ketoester, nitroarene, benzonitrile, and linoleic acid.^{84,91,95,97,98,102,104,106} The Pd NPs supported on MIL-101 exhibited good catalytic activity in the hydrogenation of the C=C bond in alkenes to the corresponding alkanes and benzaldehyde to benzyl alcohol.^84,102 The catalyst was reusable several times without loss of initial high catalytic activity, as confirmed by the repeated reaction cycles and a hot filtration experiment. TEM indicated that the original spherical Pd nanoparticles had a similar morphology to that of the fresh catalyst and the good crystalline MIL-101 structure was retained after the recycle runs.¹⁰²

The Pd nanoparticles-supported MIL-101 was used in the hydrogenation of phenol in an aqueous solution using molecular H₂ (5 bar) at 323 K, and showed high activity (85% conversion and 98.8% selectivity to cyclohexanone).⁹⁵ The Pd NPs were located inside the MIL-101 cavities. Zhao et al. reported the hydrogenation of 2,3,5-trimethylbenzoquinone in the liquid phase using Pd/MIL-101 (2 wt% Pd), and 99% conversion was obtained at 353 K after 100 min using H₂ (6.0 bar), in which the Pd nanoparticles, 2–3 nm in size, were located in the mesoporous cages of MIL-101.¹⁰⁴ Pd/MIL-101 (2 wt% Pd) showed better activity (TOF of 675 min⁻¹) than 2.0 wt% Pd on activated carbon (TOF = 519 min⁻¹). The heterogeneity of the catalytic reaction was confirmed. Hermannsdörfer et al. reported the hydrogenation of 3-heptanone using a PdNi NPs-supported MIL-101 catalyst.⁹¹ The catalytic performance of the bimetallic PdNi@MIL-101 was superior to the monometallic Pd@MIL-101 or Ni@MIL-101, supporting the bimetallic synergistic effect of the Pd–Ni NPs. Pan et al. reported the asymmetric hydrogenation of ethyl pyruvate and ethyl-2-oxo-4-phenylbutyrate using a Pt nanoparticle-supported MIL-101 catalyst.⁹⁷ The catalyst exhibited high activity (92–97% conversion and ∼77% ee) after modification with cinchonidine. The Pt nanoparticles-supported on MIL-101 were also found to be an efficient heterogeneous catalyst for the hydrogenation of nitroarenes, 1-octene, benzonitrile, and linoleic acid.^98,106

5.2.11 Suzuki–Miyaura, Ullmann and Heck coupling reaction. Pd NPs-supported MIL-101 were applied to the Suzuki–Miyaura, Ullmann and Heck coupling reactions.^57,67,85,169 Yuan et al. conducted rigorous catalytic tests to evaluate the catalytic properties of Pd/MIL-101 for the Suzuki–Miyaura and Ullmann coupling reaction of aryl chlorides in aqueous media.⁸⁵ The product yield was dependent on the reaction time, molar ratio of the reactants, and catalyst concentration. NaOMe was an excellent base for the Suzuki–Miyaura coupling reaction (Scheme 8) in the presence of TBAB, and the product yield ranged from 81 to 97%. The efficiency of the Suzuki–Miyaura coupling reaction was dependent on the reaction atmosphere such that the reaction under nitrogen gave a much higher yield (∼96%) of the cross-coupling product in the coupling reaction of 4-chloroanisole with phenylboronic acid than the reaction in open air (∼33%). In the Ullmann homo-coupling reaction (Scheme 9), 4-chloroanisole was converted to 4,4-dimethoxy biphenyl (97% yield) under a nitrogen atmosphere using TBAB and NaOMe as a base. In open air, the reaction also proceeded to give a similar product yield under identical reaction conditions. The catalyst was reusable five times without any loss of its catalytic efficiency, but a small amount of Pd (<0.2% of the total) was observed in the solution at the end of the reaction. The Pd-loaded onto amine-grafted MIL-101 was active in the Heck coupling reaction of acrylic acid and iodobenzene in N,N-dimethylacetamide (DMA) in the presence of triethylamine at 393 K (Scheme 10).^57,67 The hot filtration test and recycling experiments supported its heterogeneous nature.⁶⁷

Scheme 8 Pd NPs supported MIL-101 catalyzed Suzuki–Miyaura coupling reaction.

Scheme 9 Pd NPs supported MIL-101 catalyzed Ullmann coupling reaction.

Scheme 10 Amine grafted Pd NPs supported MIL-101 catalyzed Heck coupling reaction.

5.2.12 One-step synthesis of methyl isobutyl ketone. Pd@MIL-101 was applied to a multifunctional catalytic reaction involving condensation, dehydration and hydrogenation steps for the liquid-phase synthesis of methyl isobutyl ketone (MIBK), as described in Scheme 11.²⁹ The material showed better performance (60% conversion and 90.2% selectivity) in the one-pot synthesis of MIBK in acetone and H₂ (7.5 bar) at 423 K than palladium on Zn–Cr mixed oxide or on the MCM-22 catalysts under the same reaction conditions. The catalyst could be reused several times without any significant loss of its initial catalytic activity, and XRD and TEM confirmed that the crystallinity of Pd@MIL-101 had been retained during the reaction with no metal leaching or agglomeration.

Scheme 11 One-step synthesis of methyl isobutyl ketone.

5.2.13 One-pot indole synthesis. Pd/MIL-101 (1.0 mol% Pd) was applied as a heterogeneous catalyst for the synthesis of indole in an aqueous medium, and 2-iodoaniline and phenylacetylene were converted to the product with ∼95% yield at 363 K after 15 h (Scheme 12).⁸⁷ The catalyst showed higher catalytic efficiency than the Pd/MCM-41 catalyst, owing to the enhanced surface hydrophobicity and the presence of Lewis acid sites on MIL-101. In addition, the catalyst showed higher activity than commercial Pd/C, which might be due to the highly dispersed Pd active sites and the confinement effect of Pd NPs inside the cages of MIL-101. The catalyst could be reused ten times without any loss of its initial catalytic activity, and XRD and TEM confirmed that the crystallinity of Pd/MIL-101 had been retained during the reaction with no metal leaching or agglomeration.

Scheme 12 One-pot synthesis of indole.

5.2.14 Direct C₂ arylation of indoles. The Pd nanoparticles supported on MIL-101 (0.1 mol% Pd) was an efficient heterogeneous catalyst for the direct C₂ arylation of the substituted indoles using caesium acetate in DMF at 393 K for 24 h (Scheme 13).⁸⁸ Pd/MIL-101 exhibited higher activity (∼85% yield) than commercial Pd/C or Pd-MIL-53 (Al)–NH₂ (∼25% yield). The higher catalytic performance of Pd/MIL-101 originated from the homogeneously-distributed Pd NPs within the accessible mesoporous cages of MIL-101. The heterogeneity of the Pd/MIL-101 catalyst in the liquid phase reaction was confirmed by a hot filtration experiment, but the minor leaching of Pd (0.4 ppm) was observed in the filtrate solution after recycling.

Scheme 13 C₂-arylation of substituted indoles.

5.2.15 Chemical H₂ storage reaction. The hydrogen-rich material, such as ammonia borane (NH₃BH₃), hydrous hydrazine (N₂H₄·H₂O) and formic acid (HCOOH), are commonly used as chemical hydrogen storage materials that release H₂ through the hydrolysis, dehydrogenation and pyrolysis reactions. Au, Pd, Pt, Ru and Ni nanoparticles supported on MIL-101 were tested for chemical H₂ storage.^{89,92,100,103,105} Gu et al. reported the dehydrogenation of formic acid using a range of bimetallic and monometallic Au, Pd, Pt, and Ru nanoparticles immobilized on MIL-101 and ED-MIL-101 (Scheme 14).⁸⁹ Both Au–Pd/ED-MIL-101 and Au–Pd/MIL-101 exhibited higher activity in water at 363 K for the generation of high quality H₂ than monometallic Pd/ED-MIL-101, owing to the bimetallic synergistic effect of Au–Pd NPs. No activity was observed when Au/ED-MIL-101 or Ru/MIL-101 was used under identical reaction conditions. The stability/durability was confirmed by recycle runs. Recently, Cao et al. reported hydrogen generation from an aqueous alkaline solution (0.5 M NaOH) of hydrazine using NiPt@MIL-101 at 323 K (Scheme 15).¹⁰⁵ The material Ni₈₈Pt₁₂@MIL-101 exhibited excellent catalytic activity with a high TOF of 350 mol H₂ mol_metal⁻¹ h⁻¹. On the other hand, no activity was observed in the dehydrogenation of the aqueous solution of hydrazine using the parent MIL-101. The catalyst exhibited higher catalytic activity when compared to other NiPt NPs on ZIF-8 or Al₂O₃, Ce₂O₃, or dendrimer under identical reaction conditions.

Scheme 14 Hydrogen storage from formic acid.

Scheme 15 Hydrogen storage from aqueous alkaline solution of hydrazine.

Pt@MIL-101 was tested in liquid-phase NH₃BH₃ hydrolysis and solid-phase NH₃BH₃ thermal dehydrogenation.⁹² In the liquid-phase using Pt/NH₃BH₃ (molar ratio of 0.0029), the material Pt@MIL-101 (2 wt% Pt) showed two times higher activity to release H₂ from NH₃BH₃ at room temperature than 2 wt% Pt/γ-Al₂O₃, indicating the small size of the Pt NPs (1.8 ± 0.2 nm) within MIL-101, where the active sites located inside the MOF matrix led to higher activity (Scheme 16). When NH₃BH₃ loaded either into MIL-101 or Pt@MIL-101 was applied to solid phase thermal dehydrogenation (Scheme 17), the latter released pure H₂, whereas the former evolved H₂ and NH₃ but no borazine was formed in either case. XRD and TEM clearly confirmed that the crystallinity and Pt particle size of Pt@MIL-101 were retained after the completion of the reaction. The Pd nanoparticles supported on MIL-101 (4 wt% Pd) were applied to the chemical storage of H₂ and exhibited high activity in the hydrolysis of NH₃BH₃ to release H₂ at room temperature with a TOF of 45 mol H₂ mol_Pd⁻¹ min⁻¹, which was higher than that reported for other Pd-based nano-catalysts.¹⁰³ The stability of the catalyst was confirmed by performing recycle runs. The bimetallic catalyst, AuNi@MIL-101, with an Au/Ni atomic ratio of 7 : 93 was also an efficient catalyst for the generation of H₂ in liquid-phase NH₃BH₃ hydrolysis at room temperature with a TOF of 66.2 mol H₂ mol_cat⁻¹ min⁻¹.¹⁰⁰ The catalyst could be reused five times without significant loss of the initial catalytic activity, and XRD and TEM confirmed that the crystallinity of AuNi@MIL-101 had been retained during the reaction with no metal leaching or agglomeration.

Scheme 16 Hydrogen release from ammonia borane.

Scheme 17 Hydrogen release from ammonia borane loaded on Pt@MIL-101.

5.2.16 Other catalytic reactions. H₃PW/MIL-101 was tested for two acid-catalysed reactions of esterification of n-butanol with acetic acid in the liquid phase to butyl acetate and the gas phase dehydration of methanol to dimethyl ether in a fixed bed in good yield.⁷⁰ The sulfonic acid functionalized MIL-101 was an efficient catalyst for the liquid-phase acetalization of benzaldehyde and glycol to form benzaldehyde glycol acetate with high conversion (97.3%) and selectivity (100%) at 353 K, which are higher in activity than USY (50.3% conversion).⁵³ Pd@MIL-101 was applied to the multifunctional catalytic reaction involving the isomerization and hydrogenation steps for the liquid-phase synthesis of menthol.⁹⁰ The material exhibited high activity (>99% conversion, 86% selectivity to menthol and 81% diasteroselectivity to (−)-menthol) in the one-pot synthesis of menthol from citronellal at 453 K for 18 h.

Yadav et al. reported the reduction of highly water-soluble and toxic Cr(VI) to the much less toxic Cr(III) using Pt or Pd immobilized MIL-101 in the presence of excess formic acid.⁹⁹ Pt (2 wt%)@MIL-101 exhibited higher activity (∼100% after 40 min) in the reduction of an aqueous solution of potassium dichromate at 323 K than Pd (2 wt%)@MIL-101 (∼100% after 210 min), as measured by UV-Vis spectroscopy. MIL-101, Au@MIL-101, and Rh@MIL-101 were catalytically inactive for this reaction under identical reaction conditions.

Copper NPs immobilized onto MIL-101 were applied to the catalytic reduction of 4-nitrophenol and 4-nitroaniline using NaBH₄ in the aqueous phase, which were converted to 4-aminophenol and p-phenylenediamine, respectively, in good yield, as measured by UV-Vis spectroscopy.¹⁰¹ The catalytic performance of the Cu nanoparticles on MIL-101 was superior to that of the pure CuNPs. Pd- and Pt-containing MIL-101 were found to be efficient catalysts for the one-pot nitroarene reduction and reductive amination of carbonyl compounds.¹⁷⁰ The catalysts exhibited better activity than the commercially available Pd and Pt metal catalysts under identical reaction conditions.

6. Other applications

6.1 Drug delivery and biomedical analysis

Horcajada et al. examined the adsorption and delivery of an anti-inflammatory drug, ibuprofen, by MIL-101.¹⁷¹ MIL-101 allowed a high dose of the drug and long controlled delivery, providing advantages for holding large pharmacological molecules; MIL-101 showed an adsorption capacity of 1.4 g of ibuprofen per gram of dehydrated MIL-101 (Fig. 16) and the delivery rate was significantly slower than that with MCM-41. Čendak et al. examined the interactions of indomethacin and tetrahydrofuran solvent molecules within MIL-101.¹⁷² Loading a MIL-101 with indomethacin proved to be very efficient (0.9–1.1 g of indomethacin per gram of MIL-101). Solid-state NMR measurements, however, showed that indomethacin did not attach to the MOF, whereas the THF solvent molecules attached to the framework metallic trimeric units through hydrogen bonding and were not removed sufficiently during drying.

Fig. 16 Ibuprofen delivery (mg IBU per g dehydrated material vs. t) from MIL-101 in comparison to MIL-100 and MCM-41.¹⁷¹

Gu et al. reported the effective enrichment of peptides with the simultaneous exclusion of proteins from complex biological samples in MIL-101, which was attributed to the molecular sieving effect by the highly ordered micropores in MIL-101 for the selective inclusion and exclusion of large biomolecules.¹⁷³ MIL-101 was stable throughout the enrichment procedure, and the peptides were unaffected.

6.2 Gas/liquid chromatographic separation

Yan et al. reported a series of studies on the utilization of MIL-101 as a stationary phase for gas/liquid chromatographic separation.^174–177 They achieved the baseline separation of p-xylene, o-xylene, m-xylene, and ethylbenzene (EB) on the MIL-101 coated capillary column by GC within 1.6 min without the need for temperature programming (Fig. 17).¹⁷⁶

Fig. 17 GC separation of xylene isomers and EB on a MIL-101 coated capillary column (15 m long × 0.53 mm i.d.) at 160 °C under a N₂ flow rate of 3 mL min⁻¹. The mass of each isomer was 350 ng.¹⁷⁶

The excellent selectivity of the MIL-101 coated capillary column for the separation of xylene isomers and EB was attributed to the host–guest interactions and the CUS and suitable polarity of MIL-101. In addition, columns packed with MIL-101 allowed the excellent separation of not only C₆₀ and C₇₀ (achieved within 3 min with a selectivity of α_C70/C60 = 17), but also other high fullerenes, such as C₇₆, C₇₈, C₈₂, C₈₄, C₈₆, and C₉₆,¹⁷⁴ which was attributed to the differences in fullerene solubility in the mobile phase, and the π–π and van der Waals interactions between the fullerenes and the MIL-101 frameworks. The MIL-101 packed column also gave baseline separation of dichlorobenzene (DCB) and chlorotoluene isomers, and EB and styrene with a high column efficiency and good precision.¹⁷⁵ MIL-101 offered high affinity for the ortho-isomers, allowing rapid and selective separation from other isomers within 3 min using dichloromethane as the mobile phase. They also attempted the high performance liquid chromatographic separation of nitroaniline, aminophenol and naphthol isomers as well as a sulfadimidine/sulfanilamide mixture on the MIL-101 packed column.¹⁷⁷ The interaction between the open metal sites of MIL-101 and the polar analytes was adjusted by adding the appropriate amount of MeOH to the mobile phase to achieve the effective separation of the polar analytes by making use of the competition of MeOH with the analytes for open metal sites. Huang et al. reported MOF-organic polymer monoliths prepared by the microwave-assisted polymerization of ethylene dimethacrylate (EDMA), butyl methacrylate (BMA) and 2-acrylamido-2-methylpropane sulfonic acid (AMPS) with MIL-101 as the stationary phases for capillary electrochromatography (CEC) and nano-liquid chromatography (nano-LC).¹⁷⁸ The hybrid column showed high permeability and efficient separation of various analytes (xylene, chlorotoluene, cymene, aromatic acids, polycyclic aromatic hydrocarbons, and trypsin digested BSA peptides) either in CEC or nano-LC. These merits were attributed to the inclusion of MIL-101 in the column material, which provided high surface areas, nano-sized pores, and aromatic terephthalate moieties in the MOF–polymer monolith. Hu et al. used a MIL-101 packed micro-column for the on-line sorptive extraction and direct analysis of naproxen and its metabolites from urine samples.¹⁷⁹ Through the MIL-101 based in-tube sorptive extraction method coupled with high-performance liquid chromatography (HPLC) and fluorescence detection, naproxen and 6-O-desmethylnaproxen in urine samples could be analysed directly without an additional sample pre-treatment. Hu et al. examined the separation of two liquid mixtures (arginine, phenylalanine, and tryptophan in water, and three xylene isomers in hexane) using MIL-101 as the stationary phase by molecular simulation.¹⁸⁰ For the first mixtures, the elution order was found to be arginine > phenylalanine > tryptophan, whereas the elution order was p-xylene > m-xylene > o-xylene for the second mixture. A separation mechanism based on the cooperative solute–solvent and solute–framework interactions was proposed.

6.3 Proton conduction

Ponomareva et al. impregnated MIL-101 with H₂SO₄ or H₃PO₄ to afford solid materials with proton-conducting properties at moderate temperatures.⁶¹ The proton conductivities of the H₂SO₄@MIL-101 and H₃PO₄@MIL-101 at 423 K under low humidity conditions outperformed those by other MOF-based materials, and were comparable to Nafion®. The acid molecules residing inside the pores of MIL-101 were not removed by either heating or by hydration in a humid atmosphere. Recently, Dybtsev et al. reported the incorporation of triflic and toluene sulphinic acids into MIL-101 to produce hybrid proton-conducting solid electrolytes.¹⁸¹ The hybrid material, triflic acid@MIL-101 showed high proton conductivity (0.08 S cm⁻¹) at 15% relative humidity and a temperature of 333 K. The material was found to be stable during the multiple measurements or prolonged heating.

6.4 Sensors

Hu et al. reported the surface-enhanced Raman scattering (SERS) detection by Au nanoparticles-embedded MIL-101.¹⁸² The as-synthesized AuNPs/MIL-101 nanocomposites were highly sensitive SERS substrates by effectively pre-concentrating the analytes in close proximity to the electromagnetic fields at the SERS-active metal surface owing to a combination of the localized surface plasmon resonance properties of the AuNPs and the high adsorption ability of MIL-101.

7. Conclusions

MIL-101 can be synthesized hydrothermally using acid additives as well as in water in acid-free synthesis under high pressure conditions. The textural properties of MIL-101, however, differ widely among research groups, and a more reproducible synthesis with more effective purification protocols should be established. The MIL-101 materials produced in aqueous reaction media without acid or alkali additives are more environmentally benign, but slight deterioration in textural properties occurs. Alternative synthesis methods demanding less energy and practical synthesis scale-up will be needed in the future. In this regard, the dry gel conversion synthesis of MIL-101 allowing a high product yield with a simple purification/activation procedure can be promising. Organic-functionalization or the encapsulation of various inorganic/organic guest molecules in MIL-101 have been investigated widely for adsorption/catalytic applications, and the scope of research will definitely grow for practical applications.

MIL-101 has high thermal stability and strong resistance to moisture and organic molecules, which makes it a good candidate material for the adsorption of gases, water and volatile organic compounds in an industrial perspective. Large amounts of H₂ and CO₂ can be adsorbed by MIL-101, both with a high heat of adsorption under high pressure conditions. H₂ storage capacity can be enhanced through a variety of methods, such as noble metal spillover and cation doping. The enhanced CO₂ adsorption performance by amine-functionalized MIL-101 is also well-established, which has already shown the efficient separation of CO₂ and high selectivity over CH₄, N₂ and CO. The functionalization of MIL-101, particularly through post synthetic methods, normally leads to a hybrid material with a lower surface area, resulting in decreased CO₂ uptake under higher pressure conditions. MIL-101 shows high equilibrium uptake, rapid sorption/desorption rates at moderate temperatures and excellent hydrothermal stability for H₂O adsorption, making it a promising material for heat transformation applications or energy-efficient H₂O sorption applications. MIL-101 also shows high adsorption performance for volatile organic compounds, nitrogen- or sulfur-containing compounds, and dyes. Specific functional groups have been introduced depending on the specific adsorbates to improve the adsorption performance.

Coordinatively unsaturated Cr sites in MIL-101 are active sites for the catalysis of Lewis acid-catalysed and sometimes oxidation reactions. MIL-101 can also be synthesized or modified as a bifunctional catalyst bearing both Brønsted acid and base functionalities. In addition, the immobilization of noble metal nanoparticles on MIL-101 can also produce a wide range of nanoparticle-supported MIL-101, which are useful for cascade or tandem reactions that display high atomic utilization efficiency and do not involve workup and the isolation of many intermediates. Heteropolyacid-impregnated MIL-101 is also promising for a range of liquid phase acid/base and oxidation reactions. In some cases, Cr or impregnated metals have been detected in the filtrate of the liquid phase reactions, and stability of the catalytic system needs to be monitored closely in some cases. The noble metal or non-noble metal nanoparticles deposited on MIL-101 also show promising catalytic activities for chemical hydrogen storage. The number of such hybrid materials is expected to grow rapidly in the near future, which will not only be applicable to clean energy technology, but also to the preparation of materials for the release of drugs and pharmaceutical chemicals.

MIL-101 in chromatography/membrane separation, proton conduction, and surface-enhanced Raman scattering detection is also under investigation.

There is a large potential of MIL-101 for sustainability, and further research on the introduction of new functional groups to the MIL-101 will be useful to produce versatile hybrid materials for future industrial applications.

Acknowledgements

This study was supported a NRF grant (2013005862) funded by Korean government (MEST).

Notes and references

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