Synthesis of bimetallic Zr(Ti)-naphthalendicarboxylate MOFs and their properties as Lewis acid catalysis

Abstract

Bimetallic Zr(Ti)-NDC based metal–organic frameworks (MOFs) have been prepared by incorporation of titanium(IV) into zirconium(IV)-NDC-MOFs (UiO family). The resulting materials maintain thermal (up to 500 °C), chemical and structural stability with respect to parent Zr-MOFs as can be deduced from XRD, N₂ adsorption, FTIR and thermal analysis. The materials have been studied in Lewis acid catalyzed reactions, such as, domino Meerwein–Ponndorf–Verley (MPV) reduction–etherification of p-methoxybenzaldehyde with butanol, isomerization of α-pinene oxide and cyclization of citronellal.

---

Introduction

Over the past few years, metal–organic frameworks (MOFs) have attracted tremendous attention owing to their intriguing structural topologies and potential applications for gas adsorption and separation, catalysis, and sensing. The metathesis of metal ions or ligands from MOFs has been reported.^1,2 This fact has important implications for the stability of these materials and easier preparation and has been named as post synthetic exchange (PSE). Cation and anion exchange reactions have been observed with nanoparticles³ and other materials,^4–6 but observation of such phenomena in MOFs is relatively recent. For example, the exchange of cations in MOF occurs^7–12 by exposure to solutions containing metal ions, and it was found that ion exchange of the metals at the secondary building units occurs without significant changes in the framework structure.

A titanium MOF is one of the most interesting candidates due to a large abundance of the metal, cheap price, low toxicity and photocatalytic properties. However, to date, scarce porous titanium MOFs have been reported, except MIL-125 and NTU-9. The difficulties to synthesize titanium MOFs directly prompted us to search for new synthetic strategies and PSE could be an alternative to obtain certain Ti-MOFs that cannot or are difficult to be synthesized directly.

Recently, it has been reported that a series of porous titanium MOFs can be successfully synthesized by the PSE strategy.¹³ The incorporation of the smaller Ti atom in the water and high temperature stable Zr-based UiO-66, enhanced adsorption capacity¹⁴ and maintains the very high, thermal, chemical¹⁵ and structural stability of UiO-66 during water adsorption/desorption cycles.¹⁶ Ti-UiO-66 also shows exceptional gas permeability in mixed matrix membranes.¹⁷ Moreover Cohen et al. have reported that a mixed-ligand (terephthalic acid/2-aminoterephthalic acid), mixed-metal (Zr–Ti)-UiO-66-derivative results an effective photocatalyst for CO₂ reduction under visible light irradiation.¹⁸ Ti-substituted UiO-66(Ti)-NH₂ also showed enhanced photocatalytic performance via a Ti-mediated electron transfer mechanism.^19,20 In the present manuscript, we described a facile preparation of bimetallic Zr(Ti)-NDC-MOFs (NDC = 2,6-naphthalendicarboxylate, NDC-NH₂ = 4-aminonaphthalen-2,6-dicarboxylic acid) by introduction of Ti(IV) ions into the preformed Zr-NDC framework. The resultant materials are good Lewis acid catalysts for cascade reactions involving the Meerwein–Ponndorf–Verley (MPV) reduction followed by etherification, reactions of cyclization of citronellal, and isomerization of α-pinene oxide.²¹

Results and discussion

Synthesis and characterization of the Zr(Ti) MOFs

Stable Zr(Ti)-NDCs were prepared following a similar procedure of Cohen et al.¹³ by exposure of the parent Zr-NDC-MOF²² to dimethylformamide (DMF) solutions of TiCl₄(THF)₂ (THF: tetrahydrofurane) for 5 days at 85 °C, after removing the solid by centrifugation and washing with fresh DMF and THF (Scheme 1). Mixed NDC/NDC-NH₂-MOFs hereafter will be referred Zr(Ti)-NDC-NH₂. The absence of DMF into materials could be confirmed by the absence of their well-defined ν(C=O) band at 1660 cm⁻¹ (Fig. S6†).

Scheme 1 Synthesis of Zr(Ti)NDC-MOFs.

The statistical incorporated Ti(IV) (Zr_6−xTi_xO₄(OH)₄(NDC)₆) was quantified by total X-ray fluorescence (TXRF) spectroscopy being the Zr/Ti ratio 2.6 for Zr(Ti)-NDC and 2.5 for Zr(Ti)-NDC-NH₂ (Table S1†) and the final metal compositions were Zr_4.4Ti_1.6 for Zr(Ti)-NDC and Zr_4.3Ti_1.7 for Zr(Ti)-NDC-NH₂.

For mixed NDC-NH₂-MOFs the ratio between ligands was obtained by elemental analysis of nitrogen and confirmed by UV-visible spectroscopy and corresponds to a 20% of amino ligand content. ¹H NMR of digested Zr-NDC-MOFs confirmed the presence of both NDC and NDC-NH₂ linkers (Fig. S13†). Hydrofluoric acid was employed to digest the materials, because of the high affinity of Zr for fluoride. Electrospray ionization-mass spectrometry (ESI-MS) of digested Zr-MOFs was also performed (Fig. S14†).

The Zr(Ti)-NDC-MOFs show excellent crystallinity, as evidenced by the powder X-ray diffraction (PXRD) (Fig. 1), the good agreement between the XRD patterns of Zr(Ti)-derivative and parent-Zr indicates that the framework of parent Zr-MOF is not collapsed and rules out the possibility of the formation of Ti-based impurities; moreover, no diffraction peaks belonged to other titanium species could be observed. UiO66(Ti)-NH₂ and UiO67(Ti)-NH₂ were also prepared following the same procedure for comparative purposes. As observed, peaks in XRD patterns of Zr(Ti)-NDC shifted to higher reflection angles (2θ = 6.55) as compared to original Zr-NDC (2θ = 6.43) (from 6.30 to 6.36 for Zr(Ti)-NDC-NH₂), the 2θ value in UiO66(Ti)-NH₂ (from 7.28 to 7.41)¹⁹ and UiO67(Ti)-NH₂ shifts from 5.77 to 5.80 (Fig. S2†). This fact is also observed over inorganic solid solutions indicating that Ti(IV) ion (0.605 Å) substitutes a larger Zr(IV) ion (0.72 Å) in Zr–O oxo-clusters.^23,24

Fig. 1 PXRD patterns of UiO- and UiO-Ti-MOFs.

The X-ray photoelectron spectroscopy (XPS) of the elements of interest, Zr 3d and Ti 2p, of Zr(Ti)NDC-MOF are shown in Fig. 2 (general survey can be observed in Fig. S12†). Fig. 2a presents the Ti 2p region showing two peaks at 458.7 eV and 464.5 eV, corresponding to Ti 2p_3/2 and Ti 2p_1/2 respectively, providing another confirmation of the successful incorporation of the Ti moiety in Zr-NDC. The spin energy separation is 5.8 eV. The peak position of Ti⁴⁺ is consistent with Ti⁴⁺ in an octahedral coordination environment.^25–27 The Zr 3d was shown in Fig. 2b with two peaks at 182.7 and 185 eV, due to Zr 3d_5/2 and Zr 3d_3/2. These data are ver similar to that previously described for UiO66(Ti)-NH₂ (ref. 19) and UiO66(Ti).²⁰

Fig. 2 XPS spectra of Zr(Ti)NDC-MOF: (a) Ti 2p and (b) Zr 3d regions.

The thermogravimetric analysis (TGA) of Zr(Ti)-NDC showed a high thermal stability for these compounds with a thermal decomposition temperature similar to that of Zr-NDC (>400 °C, Fig. S3†).

The porosity was investigated by the BET analysis (N₂, 77k) of Zr(Ti)-NDC (1068 m² g⁻¹ which is comparable to that of the parent Zr-NDC) (∼1062 m² g⁻¹) (Table S2, Fig. S1†). PXRD patterns collected after the N₂ sorption experiments indicate that all other samples remain intact after activation; although some peak broadening was observed (Fig. S4†).

Similar to parent Zr-NDC and Zr-NDC-NH₂, the UV-visible diffuse-reflectance spectra of Zr(Ti)-NDC and Zr(Ti)-NDC-NH₂ also show two main absorption peaks at around 290 and 350 nm (Fig. 3). However, the peak at 350 nm, corresponding to the absorption of Zr–O clusters is broader,^19,20 that could be attributed to the absorption of new Ti–O oxo-groups (Fig. S5†). In addition to the peak at 350 nm, a shoulder to ca. 400 nm also appears in Zr-NDC-NH₂ and Zr(Ti)-NDC-NH₂ which can be assigned to the amino group. A new band at 650 nm is also observed for Zr(Ti)-NDC-NH₂ probably due to a charge transfer. These results indicate that Ti(IV) has been successfully incorporated in Zr-NDC-MOF.

Fig. 3 The solid-state UV-Visible absorption and emission spectra of Zr-NDC-compounds.

Zr(Ti)-NDCs show in the FTIR spectra the characteristic ν(Zr–O) bands at 682, 642 cm⁻¹ and ν(Ti–O) at 665 cm⁻¹ (Fig. S6†).

The solid-state photoluminescence spectra of the Zr-NDC compounds were studied at room temperature. As shown in Fig. 3, with excitation at 350 nm, the emission wavelength of the Zr-NDC²⁸ is red-shifted to 405 nm (475 nm for Zr-NDC-NH₂), while in the case of Zr(Ti)-NDC the emission wavelength appears at 424 nm (504 nm for Zr(Ti)-NDC-NH₂). Thus, the introduction of a functional group (–NH₂) on the organic ligand, and other metal as Ti improved the luminescent behavior.

¹³C CP/MAS-NMR spectra of evacuated Zr(Ti)-NDC samples (Fig. 4 and S8†) showed clearly all signals corresponding to the linkers at 172–171 (COO), 134.8–134.6 (CCOO), 132.8–132.5, 130.2–130.0, 127.4–127.1, and 125.7–125.4 ppm which are characteristic of the unique carbon atoms of NDC and NDC-NH₂ linkers, respectively, C–NH₂ resonance (for the mixed linker compounds) do not appears possibly due to their poor relaxation.

Fig. 4 ¹³C NMR of (1) Zr-NDC; (2) Zr(Ti)-NDC; (3) Zr-NDC-NH₂; (4) Zr(Ti)-NDC-NH₂.

The textural morphology of Zr-NDC and Zr-NDC-NH₂ reveal some changes on the surface of crystals (from the defects by replacing a 8-coordinated Zr^IV ion for an 6-coordinated Ti^IV ion) although the reaction was performed at a temperature below 85 °C (Fig. S7†).

Lewis acid catalyzed reactions with Zr(Ti)-MOFs

MOF catalysts show several potential advantages for catalysis, such the high surface area that facilitates the contact between substrates and catalytic active sites, the regular pores and the facile functionalization and incorporation of catalytic active sites into frameworks can be beneficial for catalysis.^22,29–33 A recent work,³⁴ presents an extensive study in which the activity of MOFs has been compared, under the same reaction conditions, with their simple homogeneous representatives and other common inorganic–organic hybrid materials and inorganic solid catalysts to put into perspective the activity and selectivity of MOFs. This study shows that the preparation of mixed-multivariant-MOFs could be an alternative to the conventional Lewis acids catalysts. Based on this concept, now we study the influence of Ti on Zr-NDC-MOFs for catalyzing the cascade reaction involving the Meerwein–Ponndorf–Verley reduction of p-methoxybenzaldehyde with 2-butanol as hydrogen source (Scheme 2). Moreover they have also been studied in two acid-catalyzed model reactions, the carbon–carbon bond forming – Prins reaction (Scheme 3) and the isomerization of α-pinene oxide (Scheme 4).

Scheme 2 Domino reaction of aldehydes with 2-butanol: MPV reduction of the aldehyde to alcohol with successive etherification.

Scheme 3 Cyclization of citronellal to isopulegol and its isomers.

Scheme 4 Isomerization of α-pinene oxide.

To explore the Lewis acid sites, we investigated the ammonia temperature-programmed desorption (NH₃-TPD) (commonly used to determine the acidity of zeolites)³⁵ of the Zr-NDC and Zr(Ti)-NDC, indicating the presence of acid sites in the catalysts (Fig. 5). Zr-NDC and Zr(Ti)-NDC showed weak to medium strength acidity.^36,37

Fig. 5 NH₃-TPD profiles of Zr-NDC and Zr(Ti)-NDC.

Cascade Meerwein–Ponndorf–Verley (MPV) reduction and etherification. Etherification of oxygenated organic compounds is a fundamental organic transformation for the synthesis of fine chemicals.^38,39 4-Methoxybenzyl 1-methylpropyl ether (Scheme 2) has a fruity pear odor and is a potential fragrance compound, which is commercially prepared by etherification of 4-methoxybenzyl alcohol (which is obtained industrially by reduction of 4-methoxybenzaldehyde) with 2-butanol. Thus, the process requires two steps: (a) the reduction of 4-methoxybenzaldehyde to the corresponding alcohol and (b) the etherification reaction.⁴⁰ It would therefore be of interest to design a process that could carry out the aldehyde reduction and etherification in a one-pot process to produce the title ether in high yield and selectivity.⁴¹

Brønsted acids can catalyze the formation of ethers when starting from two alcohol molecules,⁴² and sulfuric acid has been applied in homogeneous phase;⁴³ but are limited to primary ethers. In contrast, water stable Lewis acid catalysts as Sn- and Zr-containing silicate molecular sieves with framework single, isolated metal sites are active and selective for catalyzing MPV reduction of aldehydes as well as etherification of alcohols.⁴⁴ Thus, considering that Zr-NDC-MOFs show Lewis acidity, it could be of interest to find if they are able to promote efficiently the tandem reaction involving the MPV reduction of p-methoxybenzaldehyde with 2-butanol to p-methoxybenzyl alcohol, followed by its etherification with an excess of 2-butanol (see Scheme 2), and how they compare with other solid Lewis acids such as Sn-, Zr-, Ta-beta zeolites^44,45 Results in Table 1 show that the desired fragrance was obtained with Zr-NDC and Zr(Ti)-NDC, being the Zr(Ti) catalyst more active for the global process (yield > 90%). The linker NDC-NDC-NH₂ also has a beneficial effect on the final activity. The reaction proceeds under mild reaction conditions and all catalysts proved to be active for yielding the ether compound with good yield, and easy. The selectivity for the one-pot process is very high, and the desired ether was the only product observed. In this case, the intermediary alcohol rapidly reacts on the catalyst to give the corresponding ether. Substrate scope for the etherification reaction was explored by using Zr(Ti)-NDC as catalyst. Benzaldehyde readily reacted with 2-butanol to form the ether product 2, while aldehydes with electron withdrawing groups at the aromatic ring (see Table 1, entry 7) can be more difficult to react with Zr(Ti)-NDC.

Table 1 Domino reaction of aldehydes with 2-butanol in the presence of Zr-NDC-catalysts^a

	Catalyst	Aldehyde R	Conv.^b (h)	Selectivity^c (%)
				Alcohol	Ether
a Reaction conditions: aldehyde (0.16 mmol), 2-butanol (1 mL), catalyst (5 mg), MW oven (150 °C).b Analysed by GC-MS analysis.c Normalized to 100%.d Conventional reflux.
1	Zr(Ti)-NDC	OCH3	90 (4)	10	90
2	Zr(Ti)-NDC-NH2	OCH3	95 (4)	—	100
3	Zr(Ti)-NDC-NH2^d	OCH3	65 (4), 98 (21)	1.5	98.5
4	Zr-NDC	OCH3	19 (4)	—	100
5	Zr-NDC-NH2	OCH3	30 (4)	7	93
6	Zr(Ti)-NDC	H	97 (4)	—	100
7	Zr(Ti)-NDC	F	85 (4)	58	42
8	Zr-beta^44	OCH3	100 (8)	—	100
9	Sn-beta^44	OCH3	71 (8)	—	100
10	TiO2	OCH3	0 (4)	—	—

---

Zr(Ti) is the most active catalyst for the both isolated reactions MPV reaction and etherification. When Zr-NDC was tested for their catalytic activity in the MPV reduction of cyclohexanone with 2-butanol it shows practically no activity and conversions of 5% were obtained after 1 h of reaction time. On the other hand, conversions of 20 and 60% were obtained with Zr(Ti) after 1 and 4 h of reaction time. When the etherification of 1-butanol with p-methoxybenzyl alcohol is carried out, 15% conversion was observed for the Zr-NDC-catalyzed reaction and 83% for Zr(Ti)-NDC. These results together with our previous work,^44,46 indicate that in the case of domino reactions catalyzed by Lewis acid sites, MOFs offer the possibility to maximize the final yield by introducing more than one type of Lewis acid site in the framework. Indeed, a catalyst with Zr and Ti in the MOF maximize each one of two steps of the process to give the highest yield of the desired ether (see Table 1). A blank experiment with TiO₂ shows that it is inactive for this reaction and our experimental conditions (Table 1, entry 10).

Recycling experiments with Zr(Ti)-NDC-NH₂ catalyst show that the activity decreases after three cycles, and longer reaction times were necessary to obtain a better selectivity to the ether (Fig. S11†).

Selective cyclization of citronellal to isopulegol. After it was established that the incorporation of titanium sites into the Zr-NDC-MOF network generates new Lewis acid catalysts, the performance of these materials was explored for the cyclization of citronellal to isopulegol (Scheme 3). For this reaction the diastereoselectivity is an important issue, since it is relevant for the preparation of menthol.⁴⁷ With homogeneous Lewis acid catalysts high diastereoselectivities may be obtained especially by using ZnBr₂ or aluminum tris(2,6-diphenylphenoxide).⁴⁸ Several solid acid materials for the above reaction have been also explored, such as zeolites containing isolated and well-defined Lewis acid centers, such as Sn and Zr atoms.^47,49,50 More recently MOF materials as Cu₃(BTC)₂ (ref. 51 and 52) [BTC = benzene-1,3,5-tricarboxylate] or UiO-66(Zr)^53,54 yield also high diastereoselectivities to isopulegol.

Here, the citronellal cyclization to isopulegol has been studied with Zr-NDC and Zr(Ti)-NDC catalysts and the results compared with those obtained with other MOFs and inorganic solid acids as MCM-41(Ti), and Sn-beta zeolite. (±) Citronellal was submitted to a Zr-NDCs-catalyzed Prins reaction at different temperatures and catalyst loadings in toluene (each catalyst was previously activated at 493 K). Results in Table 2, show that in all cases, the selectivity for the four diastereomeric pulegols with respect to other products was >98% and selectivity for the desired isopulegol diastereomer was in the order of 65–75% with respect to the other three diastereomers.

Table 2 Selectivities to isopulegol obtained with different Zr-NDC-catalysts samples in toluene^a

Entry	Catalyst	T (°C)	Cat. (mg)	Conv. (%)	Sel.^b (%)	Diast.^c (%)
a Reactions were carried out in toluene with citronellal (0.3 mmol, 58 μL) added to 5, 10, 20 mg of catalyst preactivated at 200 °C, reaction time: 24 h.b Selectivity for the four diastereomeric pulegols with respect to other products.c Selectivity for the isopulegol diastereomer with respect to the other three diastereomers.d This work.
1	Zr-NDC	100	10	47	99	68
2	Zr(Ti)-NDC	100	10	48	99	75
3	Zr-NDC	100	20	60	99	62
4	Zr(Ti)-NDC	100	20	90	99	69
5	Zr-NDC	150	5	58	98	69
6	Zr(Ti)-NDC	150	5	82	98	71
7	Zr-NDC-NH2	150	5	56	99	60
8	Zr(Ti)-NDC-NH2	150	5	65	99	62
9	Zr-NDC (untreated)	150	5	45	99	74
10	Zr(Ti)-NDC (untreated)	150	5	57	99	76
11	UiO-66-NH2	150	5	15^d	99	72
12	Cu3(BTC)2	150	5	50^d	99	73
13	MCM-41(Ti)	150	5	90^d	99	70
14	Sn-beta^49	80	50	>99	>98	83
15	Ti-beta^49	80	50	35	>98	56
16	Cu3(BTC)2 (ref. 51)	110	100	80	—	65
17	UiO-66 (ref. 53)	100	—	20	—	75
18	UiO-66-NH2 (ref. 53)	100	—	20	—	75
19	MIL-101(Cr)^55	80	—	>99	>99	74
20	MOF-808-1.3SO4 (ref. 56)	60	—	97	—	67
21	Zr-TUD-1 (ref. 57)	80	—	99.6	99.6	65

---

Zr(Ti)-NDC shows better performance than Zr-NDC at different temperatures and catalyst loading. For these heterogeneous catalysts, the diastereoselectivity for (±)-isopulegol decreased slightly at higher temperatures, although the overall selectivity to cyclization improved (Table 2). Zr-MOFs with NDC-NH₂ linker are less active than NDC-derivatives. The cyclization of citronellal also proceeds smoothly without any solvent being the rate of reaction higher without solvent, which could be explained by a dilution effect. The diastereoselectivity for (±)-isopulegol was similar without solvent and in toluene, 63 and 66%, respectively. Under the same reaction conditions Cu₃(BTC)₂ shows a similar selectivity than Zr(Ti)-NDC catalysts. A comparison with data from literature for other MOF catalysts (Cu₃(BTC)₂, UiO-66, MIL-101(Cr), MOF-808-SO₄) reveals that Zr(Ti)-NDC displays similar rates and selectivity (Table 2). A comparison with other solid molecular sieve inorganic catalyst such as MCM-41(Ti) and Sn-beta shows that the inorganic solids are more active and Sn-beta presents better diastereoselectivity (83%).

Finally, for preparative purposes, an experiment was scaled up by a factor of 10 and a conversion of 80% and diastereoselectivity of 60% were reached.

Cycle times of 24 h were chosen in order to ensure that a possible loss of activity of Zr(Ti)-NDC could be detected. Although the conversion decreased by about 15% in each consecutive batch experiment, the diastereoselectivity for (±)-isopulegol remained constant. The loss of activity might be partly attributed to successive treatments or blockage of pores. The catalyst was calcined at 300 °C (heating rate 1 °C min⁻¹) for 2 h to remove any possible inhibitors. Activity of the Zr(Ti)-NDC was partially recovered up to 90–95%. Moreover, no leaching of zirconium or titanium could be detected (ICP-OES, detection limit 0.002 ppm). A hot filtration experiment was also performed and no reaction was observed in the filtrate upon removal of the catalyst after 8 h of reaction.

Comparison of X-ray diffraction patterns and SEM images of Zr-NDC and Zr(Ti)-NDC catalysts before and after several reaction runs did not reveal significant differences (Fig. S10†).

Isomerization of α-pinene oxide. The third test reaction for Lewis acid catalysis of Zr(Ti)-NDCs is the isomerization of α-pinene oxide, a highly sensitive substrate towards acids, which reacts to give a mixture of campholenic aldehyde (5), isopinocamphone (6), pinocarveol (7) and others products (Scheme 4). Campholenic aldehyde is a fragrance compound prepared in high yield when a suitable Lewis acid is used.^58–61 It has been seen that MOFs as Cu₃(BTC)₂,^51,52,62 UiO-66,⁶³ MIL-100(Fe),^64,65 Fe(BTC)⁶² with well-defined Lewis acid exhibited moderate activity and selectivity for this reaction.

The acid-catalyzed isomerization of α-pinene oxide was investigated using here Zr-NDC and Zr(Ti)-NDCs as catalysts and 1,2-dichloroethane (DCE) as solvent, at 70 °C (data in other solvents and temperatures were included in ESI, Table S3†). The added-value campholenic aldehyde product was formed in 50% yield at 80% conversion. The by-products included isopinocamphone, pinocarveol and traces of trans-carveol (Scheme 4). Without catalyst the reaction is slow and selective to campholenic aldehyde is poor, indicating that Zr(Ti)-NDCs promote the reaction. All reported yields and selectivities were reproducible for at least three experiments and were constant until complete conversion. The catalytic performance of Zr(Ti)s for the reaction is strongly dependent on the type of solvent used as shown in Table S3.†

Reactions were also carried out with other MOFs (Cu₃(BTC)₂, Fe-MIL-101) under the same conditions to compare the results with those obtained with Zr-NDC-MOFs. According to Table 3, the conversion and selectivity towards campholenic aldehyde depends on the type of Zr-MOF showing Zr(Ti)-NDC the best activity and selectivity (entry 2). It is well known that Lewis acid sites favor the formation of campholenic aldehyde; therefore, it is reasonable to suggest that the better catalytic properties of Zr(Ti)-catalysts are determined by titanium. Another aspect for the catalytic properties may be the difference in the structure of Zr(Ti)-MOFs, which can affect the reagents accessibility to active sites. This could explain the better activity and selectivity observed, in the presence of Zr(Ti)-NDC with pores of 2.06 nm (1.48 nm for Zr-NDC). Zr(Ti)-NDC give better results than MIL101 (Fe) (entry 10), MIL100 (Fe) (entries 15, 16) but better selectivity was found when Cu₂(BTC)₃ was the catalyst (entries 9, 14). A comparison with inorganic solids as Ti-beta shows that Zr(Ti)-NDC has a better catalytic activity but a lower selectivity towards campholenic aldehyde. MCM-41(Si/Al = 15) shows a similar behavior than Zr(Ti)-MOF.

Table 3 Zr-NDCs-catalyzed isomerization of α-pinene oxide^a

Entry	Catalyst	Conv. (%) (h)	Selectiv. (%)
			5	6	7
a Reaction conditions: 5 mg cat, α-pinene oxide (0.13 mmol, 20 mg), 70 °C in 1,2-dichloroethane; yields and selectivity correspond to and overage value of three experiments.b This work.c Reactions were carried out at room temperature with 0.1 g of α-pinene oxide in 5 mL of solvent added to 0.1 g of Cu3(BTC)2.d Reaction conditions: 0.5 mL of α-pinene oxide, 50 mg catalyst activated at 150 °C for 2 h under vacuum before use, 70 °C without solvent.e Reaction conditions: 1,2-dichloroethane, 5 mg catalyst, 30 °C.
1	Zr-NDC	74 (24)	51	19	30
2	Zr(Ti)-NDC	88 (24)	58	20	22
3	Zr-NDC-NH2	56 (24)	43	28	29
4	Zr(Ti)-NDC-NH2	55 (24)	50	17	33
5	UiO67-NH2	68 (24)	44	26	30
6	UiO67(Ti)-NH2	75 (24)	50	22	28
7	UiO66-NH2	46 (24)	42	24	34
8	UiO66(Ti)-NH2	74 (24)	31	27	42
9	Cu2(BTC)3^b	73 (24)	82	3.5	7
10	MIL101(Fe)^b	62 (24)	48	10	34
11	MCM-41(Si/Al = 15)^b	94 (21)	60	—	—
12	UiO66 (ref. 63)	100	45	—	—
13	Cu2(BTC)3 (ref. 62)	8 (6)	48	41	—
14	Cu2(BTC)3 (ref. 51)	70 (40)^c	80	—	—
15	MIL100(Fe)^62	22 (6)^d	45	40	—
16	MIL100(Fe)^64	96 (0.5)^e	56	—	—
17	Ti-beta^61	29 (24)	81	—	—

---

Recycling experiments show that the activity of Zr(Ti)-NDC decreases about 30% upon recycling, probably due to absorption of subproducts on the active sites; regeneration by washing with ethanol or calcination of catalyst at 200 °C lead to a slightly recovering of catalytic activity (80% than original ones).

Experimental

Detailed experimental procedures and characterization of the parent Zr-NDC-MOFs, intermediates and final materials can be found in ESI.†

Evaluation of Lewis acid catalysis

The catalytic performance of Zr(Ti)-MOFs was evaluated by using three reactions: a cascade etherification reaction, the cyclization of citronellal to isopulegol and the isomerization of α-pinene oxide.

General procedure for the domino etherification reaction starting from an aldehyde. MOF catalyst (5 mg) was added to a solution of 4-methoxybenzaldehyde (22 mg, 0.16 mmol) in 2-butanol (1 mL) in a microwave sealed-vessel equipped with a magnetic stirring bar, the reaction mixture was heated under reflux. This flask was irradiated in a Microwave Synthesis Reactor Anton Paar Monowave 300 for a total of 4 h according to the following heating program: step (1) 18 °C to 120 °C (1 min), step (2) hold at 120 °C (2 h), step (3) cool to 55 °C, step (4) 55 °C to 120 °C (1 min), step (5) hold at 120 °C (2 h), step (6) cool to 55 °C.

Cyclization of citronellal. In a Schlenck under N₂, 58 μL (0.3 mmol) of citronellal was added to the catalyst suspension (5 mg, preactivated at 220 °C for 2 h under vacuum) in toluene (0.5 mL). The reaction mixture was heated at 150 °C under continuous stirring for 24 h. At the end, the catalyst was separated by filtration and washed with toluene and then reused. The progress of the reaction was monitored by GC-MS.

Isomerization of α-pinene oxide. The reaction was performed in a glass microreactor (2.0 mL, Supelco Analytical). The catalyst (preactivated for 2 h at 220 °C under vacuum) was suspended (5 mg) in 0.5 mL of dichloroethane, and then 20 μL (0.13 mmol) of α-pinene oxide was added. This mixture was magnetically stirred at 70 °C for 24 h. The products were identified by GC-MS. After the completion of the reaction, the catalytic mixture was separated by filtration and washed with dichloroethane. It was then reused for the above reaction at least 4 times.

Conclusions

In this paper, we have prepared bimetallic Zr(Ti)-NDC-MOFs by introduction of titanium atoms into Zr-MOFs and evaluated the effect of Ti on their Lewis acid catalytic properties. Zr(Ti)-NDC-MOFs are active and selective catalysts for producing ethers of interest as fine chemicals starting from one aldehyde and one alcohol, through a domino reaction that involves a Meerwein–Ponndorf–Verley reduction of the aldehyde followed by etherification of the alcohol. No metal leaching has been detected during the reaction. They result also, effective catalysts in the cyclization of citronellal and for the isomerization of α-pinene oxide with higher activity and selectivity than parent Zr-NDC material.

Acknowledgements

This work was supported by MINECO through the MAT2014-52085-C2-2 P project. A. M. R. A. is grateful to the MINECO for FPI fellowship.

Notes and references

1. C. K. Brozek and M. Dinca, Chem. Soc. Rev., 2014, 43, 5456.
2. M. Kim, J. F. Cahill, Y. Su, K. A. Prather and S. M. Cohen, Chem. Sci., 2012, 3, 126.
3. D. H. Son, S. M. Hughes, Y. Yin and A. Paul Alivisatos, Science, 2004, 306, 1009.
4. J. a. Zhao, L. Mi, J. Hu, H. Hou and Y. Fan, J. Am. Chem. Soc., 2008, 130, 15222.
5. H. Fei, M. R. Bresler and S. R. J. Oliver, J. Am. Chem. Soc., 2011, 133, 11110.
6. H. Fei, C. H. Pham and S. R. J. Oliver, J. Am. Chem. Soc., 2012, 134, 10729.
7. S. Das, H. Kim and K. Kim, J. Am. Chem. Soc., 2009, 131, 3814.
8. T. K. Prasad, D. H. Hong and M. P. Suh, Chem.–Eur. J., 2010, 16, 14043.
9. Z. Zhang, L. Zhang, L. Wojtas, P. Nugent, M. Eddaoudi and M. J. Zaworotko, J. Am. Chem. Soc., 2012, 134, 924.
10. G. Mukherjee and K. Biradha, Chem. Commun., 2012, 48, 4293.
11. Q. Yao, J. Sun, K. Li, J. Su, M. V. Peskov and X. Zou, Dalton Trans., 2012, 41, 3953.
12. H. Wang, W. Meng, J. Wu, J. Ding, H. Hou and Y. Fan, Coord. Chem. Rev., 2016, 307(2), 130.
13. M. Kim, J. F. Cahill, H. Fei, K. A. Prather and S. M. Cohen, J. Am. Chem. Soc., 2012, 134, 18082.
14. C. Hon Lau, R. Babarao and M. R. Hill, Chem. Commun., 2013, 49, 3634.
15. V. Bon, V. Senkovskyy, I. Senkovska and S. Kaskel, Chem. Commun., 2012, 48, 8407.
16. Q. Yang, H. Jobic, F. Salles, D. Kolokolov, V. Guillerm, C. Serre and G. Maurin, Chem.–Eur. J., 2011, 17, 8882.
17. S. J. D. Smith, B. P. Ladewig, A. J. Hill, C. H. Lau and M. R. Hill, Sci. Rep., 2015, 5, 7823.
18. Y. Lee, S. Kim, J. K. Kang and S. M. Cohen, Chem. Commun., 2015, 51, 5735.
19. D. Sun, W. Liu, M. Qiu, Y. Zhang and Z. Li, Chem. Commun., 2015, 51, 2056.
20. A. Wang, Y. Zhou, Z. Wang, M. Chen, L. Sun and X. Liu, RSC. Adv., 2016, 6, 3671.
21. DOI: 10.1039/c3ta11182b, Previous results were presented on the 1st EUROMOF congress.
22. A. M. Rasero-Almansa, A. Corma, M. Iglesias and F. Sánchez, ChemCatChem, 2014, 6, 3426.
23. Y.-F. Li, D. Xu, J. I. Oh, W. Shen, X. Li and Y. Yu, ACS Catal., 2012, 2, 391.
24. P. Jaiban, A. Rachakom, S. Jiansirisomboon and A. Watcharapasorn, Nanoscale Res. Lett., 2012, 7, 1.
25. H. G. T. Nguyen, L. Mao, A. W. Peters, C. O. Audu, Z. J. Brown, O. K. Farha, J. T. Hupp and S. T. Nguyen, Catal. Sci. Technol., 2015, 5, 4444.
26. F. Hayati, S. Chandren, H. Hamdan and H. Nur, Bull. Chem. React. Eng. Catal., 2014, 9, 28.
27. L. Xu, D.-D. Huang, C.-G. Li, X. Ji, S. Jin, Z. Feng, F. Xia, X. Li, F. Fan, C. Li and P. Wu, Chem. Commun., 2015, 51, 9010.
28. W. Zhang, H. Huang, D. Liu, Q. Yang, Y. Xiao, Q. Ma and C. Zhong, Microporous Mesoporous Mater., 2013, 171, 118.
29. B. Li, M. Chrzanowski, Y. Zhang and S. Ma, Coord. Chem. Rev., 2016, 307(2), 106.
30. F. Vermoortele, R. Ameloot, A. Vimont, C. Serre and D. De Vos, Chem. Commun., 2011, 47, 1521.
31. R. Srirambalaji, S. Hong, R. Natarajan, M. Yoon, R. Hota, Y. Kim, Y. Ho Ko and K. Kim, Chem. Commun., 2012, 48, 11650.
32. T. Toyao, M. Fujiwaki, Y. Horiuchi and M. Matsuoka, RSC. Adv., 2013, 3, 21582.
33. A. M. Rasero-Almansa, A. Corma, M. Iglesias and F. Sánchez, ChemCatChem, 2013, 5, 3092.
34. P. Garcia-Garcia, M. Muller and A. Corma, Chem. Sci., 2014, 5, 2979.
35. N. Katada, H. Igi and J.-H. Kim, J. Phys. Chem. B, 1997, 101, 5969.
36. J. Kim, S.-N. Kim, H.-G. Jang, G. Seo and W.-S. Ahn, Appl. Catal., A, 2013, 453, 175.
37. T. Kajiwara, M. Higuchi, D. Watanabe, H. Higashimura, T. Yamada and H. Kitagawa, Chem.–Eur. J., 2014, 20, 15611.
38. O. Mitsunobu, in Comprehensive Organic Synthesis, Pergamon, Oxford, 1991, p. 1,  DOI:10.1016/B978-0-08-052349-1.00151-7.
39. C. Lee and R. Matunas, in Comprehensive Organometallic Chemistry III, Elsevier, Oxford, 2007, p. 649,  DOI:10.1016/B0-08-045047-4/00136-9.
40. K. Bauer, D. Garbe and H. Surburg, Common Fragrance and Flavor Materials: Preparation, Properties and Uses, Wiley, 2008.
41. M. J. Climent, A. Corma, S. Iborra and M. J. Sabater, ACS Catal., 2014, 4, 870.
42. F. F. Roca, L. De Mourgues and Y. Trambouze, J. Catal., 1969, 14, 107.
43. L. Brandsma and J. F. Arens, in The Ether Linkage (1967), John Wiley & Sons, Ltd, 2010, p. 553,  DOI:10.1002/9780470771075.ch13.
44. A. Corma and M. Renz, Angew. Chem., Int. Ed., 2007, 46, 298.
45. A. Corma, F. X. Llabrés i Xamena, C. Prestipino, M. Renz and S. Valencia, J. Phys. Chem. C, 2009, 113, 11306.
46. A. Corma, M. a. T. Navarro and M. Renz, J. Catal., 2003, 219, 242.
47. M. Boronat, A. Corma and M. Renz, in Turning Points in Solid-State, Materials and Surface Science: A Book in Celebration of the Life and Work of Sir John Meurig Thomas, The Royal Society of Chemistry, 2008, p. 639,  10.1039/9781847558183-00639.
48. G. Heydrich, G. Gralla, M. Rauls, J. Schmidt-Leithoff, K. Ebel, W. Krause, S. Oehlenschläger, C. Jäkal, M. Friedrich, E. J. Bergner, N. Kashani-Shirazi and R. Paciello, US Pat., 8,318,985 B2, 2012.
49. A. Corma and M. Renz, Chem. Commun., 2004, 550,  10.1039/b313738d.
50. M. Boronat, P. Concepcion, A. Corma, M. T. Navarro, M. Renz and S. Valencia, Phys. Chem. Chem. Phys., 2009, 11, 2876.
51. L. Alaerts, E. Séguin, H. Poelman, F. Thibault-Starzyk, P. A. Jacobs and D. E. De Vos, Chem.–Eur. J., 2006, 12, 7353.
52. M. Vandichel, F. Vermoortele, S. Cottenie, D. E. De Vos, M. Waroquier and V. Van Speybroeck, J. Catal., 2013, 305, 118.
53. F. Vermoortele, M. Vandichel, B. Van de Voorde, R. Ameloot, M. Waroquier, V. Van Speybroeck and D. E. De Vos, Angew. Chem., Int. Ed., 2012, 51, 4887.
54. F. Vermoortele, B. Bueken, G. Le Bars, B. Van de Voorde, M. Vandichel, K. Houthoofd, A. Vimont, M. Daturi, M. Waroquier, V. Van Speybroeck, C. Kirschhock and D. E. De Vos, J. Am. Chem. Soc., 2013, 135, 11465.
55. F. G. Cirujano, F. X. Llabres i Xamena and A. Corma, Dalton Trans., 2012, 41, 4249.
56. J. Jiang, F. Gándara, Y.-B. Zhang, K. Na, O. M. Yaghi and W. G. Klemperer, J. Am. Chem. Soc., 2014, 136, 12844.
57. A. Ramanathan, M. C. Castro Villalobos, C. Kwakernaak, S. Telalovic and U. Hanefeld, Chem.–Eur. J., 2008, 14, 961.
58. A. Corma and H. García, Chem. Rev., 2003, 103, 4307.
59. J. Kaminska, M. A. Schwegler, A. J. Hoefnagel and H. van Bekkum, Recl. Trav. Chim. Pays-Bas, 1992, 111, 432.
60. K. Wilson, A. Rénson and J. H. Clark, Catal. Lett., 1999, 61, 51.
61. P. J. Kunkeler, J. C. van der Waal, J. Bremmer, B. J. Zuurdeeg, R. S. Downing and H. van Bekkum, Catal. Lett., 1998, 53, 135.
62. A. Dhakshinamoorthy, M. Alvaro, H. Chevreau, P. Horcajada, T. Devic, C. Serre and H. Garcia, Catal. Sci. Technol., 2012, 2, 324.
63. F.-G. Xi, Y. Yang, H. Liu, H.-F. Yao and E.-Q. Gao, RSC. Adv., 2015, 5, 79216.
64. M. N. Timofeeva, V. N. Panchenko, A. A. Abel, N. A. Khan, I. Ahmed, A. B. Ayupov, K. P. Volcho and S. H. Jhung, J. Catal., 2014, 311, 114.
65. F. Vermoortele, R. Ameloot, L. Alaerts, R. Matthessen, B. Carlier, E. V. R. Fernandez, J. Gascon, F. Kapteijn and D. E. De Vos, J. Mater. Chem., 2012, 22, 10313.

---

Footnote

† Electronic supplementary information (ESI) available: Experimental details on the preparation and characterization of materials, additional tables and figures are included. See DOI: 10.1039/c6ra23143h

---