Zhenzhen Lin,
Xiaoxiong Cai,
Yanghe Fu,
Weidong Zhu
and
Fumin Zhang
*
Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Institute of Advanced Fluorine-Containing Materials, Zhejiang Normal University, 321004 Jinhua, People's Republic of China. E-mail: zhangfumin@zjnu.edu.cn
First published on 12th September 2017
We herein report a high-yielding one-pot upgrade strategy for converting biomass-derived methyl levulinate (ML) into γ-valerolactone (GVL) over a dual-functional catalyst prepared by depositing Ru nanoparticles on a sulfonic acid-functionalized Zr-based metal–organic framework (SO3H-UiO-66). Under the mild conditions of 80 °C and 0.5 MPa H2 for 4 h in aqueous solution, a quantitative (100%) yield of GVL was obtained over the prepared Ru/SO3H-UiO-66 catalyst. In contrast, a very limited yield of GVL was achieved in the control experiment by first hydrogenating the reactant ML over a metal catalyst without any acidity (e.g. Ru/C) to produce the 4-hydroxypentanoic acid methyl ester (4-HPME) intermediate, followed by treatment of this intermediate over the acidic SO3H-UiO-66 support in the absence of metal. We also found that the catalytic activity and selectivity of Ru/SO3H-UiO-66 were significantly suppressed upon neutralization of its acidic sites, thereby confirming the indispensable role of the sulfonic acid groups in promoting the intramolecular dealcoholation of the 4-HPME intermediate. Furthermore, the Ru/SO3H-UiO-66 catalyst was recyclable over five cycles without any significant loss in its catalytic activity, thus rendering this precious metal/acid dual-functional catalyst a potential candidate for efficient GVL production under mild conditions.
To date, GVL has mainly been obtained from levulinic acid (LA), a platform molecule derived from lignocellulosic biomass, via a two-step catalytic process involving selective hydrogenation under a high pressure of H2 and subsequent intramolecular dehydration.5–9 Indeed, many reports exist into the catalytic conversion of LA using both homogeneous and heterogeneous noble and non-noble metal catalysts in liquid and vapor phase systems.5–9,11–18 Among the various metals investigated, ruthenium appears to be the most active and selective metal for the conversion of LA to GVL, mainly due to its selective hydrogenation of carbonyl groups without altering other unsaturated functionalities.11–18 However, traditional catalysts, such as commercial Ru/C, often exhibit leaching of the ruthenium species through the formation of metal–carboxylate complexes with LA, which is likely caused by the weak acidity of LA (pKa = 4.59),19–21 results in a low catalytic activity and poor catalyst reusability.
As an alternative substrate, levulinic ester, which is produced by the acid-catalyzed alcoholysis of various carbohydrate fractions of lignocellulose, is acid-free, and can also be converted to GVL.5–9 Recently, several strategies based on the use of heterogeneous catalysts have been reported for the liquid-phase upgrade of methyl levulinate (ML) to GVL.22–24 For example, Hengne and co-workers reported that a 5 wt% Ru/C catalyst was active in the hydrogenation of ML, giving a conversion of 95% and a GVL selectivity of 91% under a H2 pressure of 3.4 MPa at 130 °C over 2 h in methanol.22 In addition, Nadgeri et al. reported that a 100% conversion of ML with 32% GVL selectivity could be achieved over a 1.0 wt% Ru/graphite catalyst under 3 MPa of H2 at 70 °C over 10 min in water.23 However, no apparent increase in the GVL yield was observed when the reaction time was extended further to 4 h. Generally, the conversion of ML to GVL involves a two-step reaction sequence, namely hydrogenation of the carbonyl groups over metal sites to give the 4-hydroxypentanoic acid methyl ester (4-HPME) intermediate, and subsequent dealcoholization of 4-HPME to yield GVL catalyzed by the acidic sites (Scheme 1). Therefore, the prerequisite for establishing an efficient ML to GVL conversion process is to develop high performance dual-functional solid catalysts containing metal nanoparticles and acidic sites to enhance the process efficiency (i.e., catalytic activity and selectivity) and process simplicity.14,15
As a relative new class of crystalline ordered materials, metal–organic frameworks (MOFs), have attached significant attention in several areas, in particular in the field of catalysis, because of their controllable pores characteristics, large internal surface areas, and tunable chemical properties.25–27 For example, the typical Zr-based MOF, UiO-66, is comprised of 12-coordinated Zr6O4(OH)4 clusters that are connected three-dimensionally with terephthalic acid (BDC) linkers.28–30 In this case, the cavities present in the UiO-66 structure provide the possibility to encapsulate metal nanoparticles within its frameworks to ultimately prevent nanoparticles agglomeration.31–33 Moreover, the BDC linker of UiO-66 can be tuned to introduce functionality by partially or integrally replacing it with analogous organic linkers via direct or post-modification.35,36 In combination with its high thermal and chemical stabilities, the above properties render UiO-66 an ideal candidate for application in heterogeneous catalysis.31–39
Thus, we herein report the catalytic cascade hydrogenation–cyclization of ML with the aim of producing GVL in high-yields over Ru nanoparticles supported on a functionalized UiO-66 catalyst. Through the appropriate tuning of the molar ratio of the mixed organic linkers of 2-sulfonylterephthalic acid monosodium salt (2-NaSO3-H2BDC) and BDC, we expect that a highly stable sulfonic acid-functionalized UiO-66 (SO3H-UiO-66) catalyst can be synthesized easily via a direct solvothermal approach.40 Subsequently, Ru nanoparticles will be introduced into the cavities of SO3H-UiO-66 using a wet impregnation technique and the resulting Ru/SO3H-UiO-66 catalyst will be employed in the aqueous-phase conversion of ML to GVL. We expect that the SO3H-UiO-66 frameworks will distribute homogeneously in the aqueous medium due to their hydrophilic properties and low density,41 thereby facilitating the adsorption/diffusion of the reactants. Furthermore, the abundant cavities present in the SO3H-UiO-66 host will be expected to allow the facile dispersal and stabilization of the imbedded Ru nanoparticles, which can then adsorb and activate H2 and promote the hydrogenation of ML to yield 4-HPME. Finally, we propose that the acidic –SO3H groups tethered on the MOF frameworks will accelerate the subsequent intramolecular dealcoholization of 4-HPME to afford GVL in high yields. In this context, our aim is to construct a multifunctional catalyst coupled with precious metal nanoparticles and acidic sites in a single support to enhance catalytic activity and give a reproducible performance in the upgrade of ML.
For preparation of the 5.0 wt% Ru/SO3H-UiO-66 catalyst, a sample the pre-dried SO3H-UiO-66 (0.2 g) was dispersed in deionized water (20 mL) at 25 °C. An aqueous RuCl3 solution (2.08 mL, 0.05 mol L−1) was then added to the above suspension under vigorous stirring and the resulting slurry was stirred at 25 °C for 24 h. Subsequently, the reduction of Ru3+ was conducted by the dropwise addition of aqueous NaBH4 (3.6 mL, 28 mg; [NaBH4]/[Ru] = 7) into the above suspension upon cooling using an ice-bath. After allowing the reduction to proceed for 30 min, the resulting solid was collected by centrifugation prior to treatment in a solution of diluted HCl (3.6 mL, 0.2 mol L−1) in methanol, and rinsed with methanol and water to remove any excess additional HCl. Finally, the obtained solid was dried overnight at 120 °C under reduced pressure prior to use. For comparison, the Ru nanoparticles supported on UiO-66 and NH2-UiO-66 (referred to as 5.0 wt% Ru/UiO-66 and 5.0 wt% Ru/NH2-UiO-66, respectively) were also prepared using above method with BDC and NH2-BDC as the starting organic ligand.
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Fig. 1 (A) XRD patterns, (B) N2 adsorption isotherms, (C) FTIR spectra and (D) TGA curves of the SO3H-UiO-66 support and the prepared Ru/SO3H-UiO-66 catalyst. |
Analysis of the various samples by inductively coupled plasma atomic emission spectroscopy (ICP-AES) indicated that the measured Ru loadings were close to the nominal amounts present in the precursor (Table S1 in ESI†). In addition, no sodium ions were detected, indicating that all the Na+ ions of the 2-NaSO3-H2BDC ligand had been exchanged with H+ (from HCl) during preparation. In addition, as shown in the sulfur X-ray photoelectron spectrum (XPS) in Fig. 2A, a single S 2p peak attributable to the sulfonic acid groups was present at 168.3 eV,43 which indicated that almost all S was present in the forms of sulfonic acid, thereby confirming the FTIR observations. Furthermore, the presence of Ru0 in the catalyst was confirmed by XPS analysis (Fig. 2B), as clearly demonstrated by the presence of a band at 461.8 eV, which is characteristic of the zerovalent Ru species.16,43 Moreover, acid–base titrations indicated that the acidity of Ru/SO3H-UiO-66 was 0.35 mmol g−1.36,44–46 It should be considered here that the missing-linker defect sites (consisting of Zr–OH sites) in SO3H-UiO-66 also contributed to the total Brønsted acid content, although their acidity strength is significantly weaker than that of the –SO3H sites.28,36 We could therefore conclude that the quantified acidity originated primarily from the Brønsted acidity –SO3H groups tethered on the UiO-66 frameworks.
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Fig. 2 XPS spectra of (A) S spectrum and (B) Ru 3p for the prepared 5.0 wt% Ru/SO3H-UiO-66 catalyst. |
Furthermore, energy-dispersive X-ray spectroscopy (EDX) mapping confirmed that the Ru nanoparticles were evenly distributed within SO3H-UiO-66 (Fig. 3), while transmission electron microscopy (TEM) observations (Fig. 4) confirmed the uniformity of the 2–4 nm diameter Ru nanoparticles that were homogeneously dispersed within the SO3H-UiO-66 matrix. A characteristic interlayer spacing of 0.23 nm was also determined from the high-resolution TEM image (Fig. 4B), which was ascribed to the (100) plane of the hcp-structured Ru nanoparticles.
Following successful characterization, the 5.0 wt% Ru/SO3H-UiO-66 catalyst was tested in the aqueous-phase conversion of biomass-derived ML to GVL in an autoclave at 80 °C and at a hydrogen pressure of 0.5 MPa (Fig. 5). In the absence of catalyst or in the presence of only pristine SO3H-UiO-66, no reaction took place. However, upon the addition of 5.0 wt% Ru/SO3H-UiO-66 to the reaction system, the concentration of 4-HPME increased dramatically within the first 60 min, accompanied by a sharp decrease in the ML concentration, thereby demonstrating that ML is mainly hydrogenated to yield 4-HPME in the initial step. Interestingly, the production of GVL was also observed in the initial 5 min of the reaction, implying that either intramolecular dealcoholation of 4-HPME occurred immediately, or that ML could be converted directly into GVL.22–24 As the reaction progressed, the intramolecular dealcoholation of 4-HPME continued smoothly, with almost complete conversion of 4-HPME to GVL being observed after 400 min. Remarkably, GVL was found to be stable in the aqueous reaction system, as increasing the reaction time further to 540 min did not generate any additional hydrogenation or hydrogenolysis products.
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Fig. 5 Evolution of reactant and product distributions as a function of reaction time. Reaction conditions: ML (0.5 g), catalyst (0.05 g), H2O (15 mL), 80 °C, 0.5 MPa H2. |
Interestingly, the one-pot conversion of ML into GVL over heterogeneous catalysts has been reported previously (see Table 1).23,24,47 For example, the use of a commercially available 5.0 wt% Ru/C in methanol gave an ML consumption of 97.8% over 160 min at 130 °C and 1.2 MPa H2 with 89.4% selectivity towards GVL (Table 1, entry 1).47 Indeed, the use of alcohols as the reaction medium for levulinic ester hydrogenation appears advantageous, as such solvents can be derived from lignocellulosic biomass and so have a low environmental impact.5–9 However, from the viewpoint of sustainable chemistry, the use of water as reaction medium is more desirable, since it is environmentally benign. We therefore attempted the conversion using water as the reaction solvent, and were surprised to find that superior results were obtained. More specifically, using the 5.0 wt% Ru/SO3H-UiO-66 catalyst, a 74.5% yield of GVL was obtained under relatively mild reaction conditions (entry 4), which was a higher conversion than that obtained over a 4.5 wt% Ru/Zr5SMS catalyst (entry 3).23 Moreover, upon comparison of our 5.0 wt% Ru/SO3H-UiO-66 catalyst with Ru/C and with the UiO-66- and NH2-UiO-66- supported Ru nanoparticles (entries 5–8), it was apparent the Ru/SO3H-UiO-66 catalyst gave a significantly higher ML conversion and GVL selectivity than the Ru/C. In contrast, Ru/UiO-66 and Ru/NH2-UiO-66 exhibited only moderate activities, producing GVL in lower yields than Ru/SO3H-UiO-66 (see Table S1 and Fig. S2–S6, ESI† for characterization of the control catalysts). Of course, it should be admitted that the metal dispersions may be different in the above different cases, which could also affect the reactivity. These results therefore suggest that the nature of the support plays a key role in determining the activity and selectivity of the Ru nanoparticles for the hydrogenation–cyclization of ML. Furthermore, it has been reported that the SO3H-UiO-66-based catalyst is easily dispersed in water,44 thereby enhancing contact between the catalyst and the substrate, and increasing the catalytic performance of Ru/SO3H-UiO-66.
Entry | Catalyst | s/ca | Solvent | Temp. (°C) | pH2b (MPa) | t (min) | Con.c (%) | Sel.d (%) |
---|---|---|---|---|---|---|---|---|
a Molar ratio of ML to Pd in the catalyst used.b H2 pressure.c Conversion of ML.d Selectivity for GVL. | ||||||||
1 | 5.0 wt% Ru/C47 | 348 | CH3OH | 130 | 1.2 | 160 | 97.8 | 89.4 |
2 | 5.0 wt% Ru/C23 | 118 | CH3OH | 130 | 3.5 | 120 | 95 | 91 |
3 | 4.5 wt% Ru/Zr5SMS24 | 199 | CH3OH | 70 | 0.5 | 240 | >99.9 | 67.1 |
4 | 5.0 wt% Ru/SO3H-UiO-66 | 200 | H2O | 70 | 0.5 | 240 | >99.9 | 74.5 |
5 | 5.0 wt% Ru/SO3H-UiO-66 | 200 | H2O | 70 | 0.5 | 45 | 95.5 | 20.7 |
6 | 5.0 wt% Ru/UiO-66 | 200 | H2O | 70 | 0.5 | 45 | 90.3 | 13.8 |
7 | 5.0 wt% Ru/NH2-UiO-66 | 200 | H2O | 70 | 0.5 | 45 | 73.7 | 14.6 |
8 | 5.0 wt% Ru/C | 155 | H2O | 70 | 0.5 | 45 | 44.6 | 10.3 |
It was also expected that the acidic properties of the catalyst may play a key role in this reaction, and so we examined the pH of the reaction system in the presence of the 5.0 wt% Ru/SO3H-UiO-66 catalyst, and found that it ranged from 4.8 to 5.1. As discussed above, the acidic sites of SO3H-UiO-66 originate mainly from the sulfonic acid groups tethered to the UiO-66 frameworks. Thus, the high catalytic activity and selectivity of Ru/SO3H-UiO-66 in this cascade reaction are likely due to the synergistic effects between the “host” and the “guest”, where the Ru nanoparticles guests provide the H2 activation/hydrogenation activity and the SO3H-UiO-66 host offers the acidity, thereby triggering the subsequent intramolecular dealcoholation.22–24
To further explore the influence of the acid sites present in SO3H-UiO-66 on the ML conversion, a control hydrogenation–cyclization experiment was performed over the 5.0 wt% Ru/SO3H-UiO-66 catalyst and in the presence of a calculated amount of NaOH with respect to the acidic sites available on the catalyst (Fig. 6). As indicated, upon neutralization of the acidic sites by NaOH, the activity and selectivity of the catalyst decreased significantly, thereby confirming poisoning/neutralization of the acidic sites through the strong interaction with NaOH.44 This result clearly demonstrates the importance of acidic sites in the catalytic intramolecular dealcoholation reaction.
In addition, another control experiment was conducted in order to clarify the possible reaction mechanism. As shown in the case (a) of Fig. 7, a small portion of GVL was formed over 5.0 wt% Ru/C along with the major HPME product by the catalytic hydrogenation reaction. Subsequently, we treated this intermediate with the SO3H/UiO-66 support (0.05 g) at 70 °C upon removal of Ru/C by hot filtration. It was observed that the generated HPME was partially converted into GVL in the following 4 h (in the case (b) of Fig. 7), while the concentration of ML in this intermediate had no any change. These results consistently confirmed the importance of acidic sites in the catalytic intramolecular dealcoholation reaction. In our opinion, the formation of the small amount GVL in the presence of 5.0 wt% Ru/C could be probably ascribed to the following two reasons: HPME might be self-catalyzed to yield GVL under the reaction conditions; Ru clusters with a diameter size smaller than 1 nm (even single ruthenium site), which could not be observed by our conventional TEM or HRTEM techniques, were partially responsible for the direct conversion of ML to GVL. In the further work, we will investigate these factors in detail. Nevertheless, based on the above discussion, we can deduce that the mechanism to produce GVL from HPME apparently requires the Ru/SO3H-UiO-66 rather than just the SO3H-UiO-66 support. The metal–acid interfacial sites of the developed Ru/SO3H-UiO-66 may be relevant to this transformation.
To allow the catalyst to be recycled, isolation from the reaction solution by facile filtration is preferable. In this case, filtration and washing of the spent 5.0 wt% Ru/SO3H-UiO-66 catalyst allowed it to be recycled five times without any significant loss in catalytic activity (Fig. 8). Notably, XRD, N2 adsorption, FTIR, EDX mapping, and ICP-AES measurements (Fig. S7–S10, ESI†) confirmed that the reused catalyst exhibited a comparable porous structure, Ru nanoparticle dispersion, and chemical composition as the fresh one. These results demonstrate that the prepared 5.0 wt% Ru/SO3H-UiO-66 catalyst is sufficiently stable to be employed in the hydrogenation–cyclization of ML to yield GVL.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra06293a |
This journal is © The Royal Society of Chemistry 2017 |