Highly Efficient Transfer Hydrogenation of Levulinate Esters to Î³

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Kinetics, Catalysis, and Reaction Engineering

Highly Efficient Transfer Hydrogenation of Levulinate Esters to #-Valerolactone over Basic Zirconium Carbonate Fukun Li, Zhangmin Li, Liam John France, Jiali Mu, Changhua Song, Yuan Chen, Lilong Jiang, Jinxing Long, and Xuehui Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00712 • Publication Date (Web): 04 Jul 2018 Downloaded from http://pubs.acs.org on July 5, 2018

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Industrial & Engineering Chemistry Research

Highly Efficient Transfer Hydrogenation of Levulinate Esters to γ-Valerolactone over Basic Zirconium Carbonate Fukun Li,† Zhangmin Li,† Liam John France,† Jiali Mu,† Changhua Song,† Yuan Chen,† Lilong Jiang,‡ Jinxing Long*,† and Xuehui Li*,† †

School of Chemistry and Chemical Engineering, State Key Laboratory of Pulp and Paper

Engineering, South China University of Technology, Guangzhou, 510640, China ‡

National Engineering Research Center of Chemical Fertilizer Catalyst, Fuzhou University, Fuzhou

350002, P. R. China. *Corresponding Author; E-mail: [email protected] (X. Li) and [email protected] (J. Long). Tel: 0086 20 8711 4707. Fax: 0086 20 8711 4707.

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Novel and selective strategies for platform chemical production from renewable biomass are highly attractive in respects to value-added utilization of sustainable resources. In this study, a series of low-cost, commercially available transition metal carbonates (Zr, Ni, Mg, Zn and Pb) were investigated for catalytic transfer hydrogenation of levulinate esters to γ-valerolactone (GVL) via the cascade process of Meerwein-Ponndorf-Verley (MPV) reduction and lactonization reaction. Amongst the selected catalysts, basic zirconium carbonate is the most active, with the highest turnover frequency (TOF) of 3.1 h-1 and a surface reaction rate of 0.21 mol m-2 h-1. At 453 K, 3.0 h and 1.0 MPa N2, 100% ethyl levulinate conversion, 96.3% GVL yield and 91.9% hydrogen donor utilization are observed due to the cooperative effect between acid (Mn+) and base (-OH) sites. Furthermore, this catalyst shows high recyclability under the optimized conditions, where a satisfactory catalytic activity is shown even after six consecutive runs. KEYWORDS: levulinate esters; γ-valerolactone; transfer hydrogenation; basic zirconium carbonate; cooperative effect

1. INTRODUCTION Efficient conversion of natural biomass to fuels or platform chemicals, such as, 5-hydroxymethyl furfural (5-HMF), levulinic acid (LA) and lactic acid, is a valid alternative to reduce the increasingly serious environmental problems caused by the excessive consumption of fossil resources.1-3 Amongst various green chemicals, γ-valerolactone (GVL) is versatile and has been widely used in many fields.4,5 It is an 2


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important intermediate as it can be further converted to value-added chemicals and high quality bio-fuels such as 1, 4-pentanediol, pentenoic acid and long chain liquid alkenes.6-8 It can also be used as a green solvent for organic synthesis and biomass utilization due to its outstanding physiochemical properties (boiling point: 480 K, flash point: 363 K) and low toxicity. For example, the water/GVL mixed solvent was employed for raw biomass pre-treatment, yielding extraction of 70-90% soluble carbohydrates.9,10 In addition, GVL itself is an excellent oxygen-containing biofuel and a high quality additive for gasoline and diesel.11,12 Generally, this platform chemical is produced from LA and levulinate esters (LEs) via a cascade hydrogenation-cyclization reaction process in the presence of molecular hydrogen.4 In this process, the reduction of the carbonyl group is generally identified as the rate-determining step and can be promoted under a hydrogen-containing atmosphere over various metal catalysts, such as Pt, Au and Cu etc.13-16 However, traditional hydrogenation technologies lead to a number of issues, namely, process safety problems arising from hydrogen production and storage in addition to unselective substrate hydrogenation, limiting the application of this simple strategy. In recent years, the development of transfer hydrogenation is considered more attractive and safe due to the absence of molecular hydrogen. Formic acid (FA) and alcohols are promising alternatives as active hydrogen donors for the carbonyl reduction reaction.17 Noble metal catalysts such as Ru-based materials18,19 and Au/ZrO2-VS20 have been employed, where, FA is used as the hydrogen source, but, high cost of catalyst and a corrosive liquid phase restrict its potential large-scale application. Replacement of the 3



hydrogen donor with alcohol can remove the corrosive nature of the liquid phase and warrants increasing attention as a viable alternative, while the use of non-noble metal catalysts is more economically desirable. For example, Raney Ni has been found to be efficient for room temperature GVL production using i-propanol as the hydrogen donor and reaction media.21,22 Lin et al.23,24 found that GVL can be produced from methyl levulinate (ML) using methanol as the hydrogen donor in the presence of CuO or Cu-Cr oxide. While these systems are a step in the right direction towards realizing industrial biomass transfer hydrogenation, they still suffer from numerous drawbacks, particularly, high catalyst dosage, low stability and relatively harsh synthesis/reaction conditions. Zirconium-containing materials had been verified as viable transfer hydrogenation catalysts as well. In 2011, Dumesic et al.25 first reported that ZrO2, amongst a number of different metal oxides, possessed significant catalytic activity for the transfer hydrogenation of butyl levulinate (BL) when 2-butanol was used as the hydrogen donor. Where, the catalyst was shown to possess both acid and base characteristics (Zr4+ and O2-, respectively), while exhibiting high thermochemical stability, which played key roles in the Meerwein-Ponndorf-Verley (MPV) reduction. Based on the above findings, a number of zirconium-containing materials have been developed for the synthesis of GVL via transfer hydrogenation, of particular note, Zr-Beta,26,27 Zr(OH)4,28 Zr-HBA29, Zr-Phy,30 Zr-CA31 and Zr-MOF.32,33 However, it is well known that the synthesis of these catalysts usually employs complex procedures and in some cases make use of toxic reagents, hampering its potential industrial application. For example, hydrofluoric acid and N, 4


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N-dimethylformamide (DMF) were used in the preparation of Zr-beta and Zr-Phy catalysts, respectively. As an important commercially available solid base, basic zirconium carbonate has been widely employed in textile, paint, and paper industries.34 It is also an important precursor for the preparation of ZrO2,35,36 zirconium based catalyst supports37,38 and a suitable solid base used in biomass refinery operations.39,40 It has been demonstrated previously that Lewis acid sites are responsible for processing LA and its esters to alkyl 4-hydroxypentanoate,27,41 which is then converted to GVL in the presence of Brønsted acid sites.42 Zirconium phosphate, another important and environmentally friendly material, has found application in the biomass refinery and features both adjustable Brønsted and Lewis acid sites.43-47 In our previous study, we found that the cooperation between Lewis and Brønsted acid sites is responsible for highly efficient conversion of BL to GVL over adjustable zirconium phosphates.48 As such, basic zirconium carbonate, comprised of both Lewis acid sites (zirconium ions) and base sites (carbonates and hydroxides), has the potential to act as a transfer hydrogenation catalyst for the conversion of LA and LEs to GVL. Based on this concept, the catalytic performance of low-cost, commercially available basic zirconium carbonate was investigated in comparison with other typical basic metal carbonates (Ni, Mg, Zn and Pb). Numerous parameters (reaction temperature, time, catalyst dosage and donor choice) were investigated to probe their influence upon reaction. Catalyst deactivation was examined via the application of multiple characterization techniques, in an effort to interpret the 5



changes in the catalyst after successive reactions. In addition, the hydrogen donor utilization was systematically investigated, as it is a key parameter for transfer hydrogenation, but has not yet received enough attention in current studies.

2. EXPERIMENTAL SECTION 2.1. Materials Zirconium and other typical basic metal carbonates (Ni, Mg, Zn, Pb) were purchased from Aladdin (Shanghai, China), and pressed and sieved to 200 mesh prior to use. Naphthalene, i-propanol, LA, ML, ethyl levulinate (EL), BL and GVL were obtained from J&K Scientific Ltd (Beijing, China). Other reagents were supplied by Guanghua Chemical Factory Co. Ltd (Guangdong, China). All reagents were of analytical grade and used without further purification. Propyl levulinate (n-PL) and isopropyl levulinate (i-PL) were synthesized according to our previous report.48 2.2. Catalyst Characterization The composition of basic zirconium carbonate was determined by thermogravimetric analysis according to the method provided in previous work.49 X-ray diffraction (XRD) patterns were recorded on a Rigaku D/MAX-3A Auto X-ray diffractometer with Cu Kα radiation (λ= 0.15418 nm) operated at 40 kV and 40 mA over a 2θ range from 5 to 90o with a 0.05o step size and a counting time of 1 s per step. N2 adsorption-desorption isotherms were conducted on a Micromeritics ASAP 2020 micropore size analyzer at 77 K. Samples were degassed at 373 K for 12 h under vacuum prior to analysis. Sample 6


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acidity and basicity with temperature desorption of NH3 and CO2 respectively were measured using a Micromeritics AutoChem ІІ 2920 chemisorption analyzer with a thermal conductivity detector. Samples (100 mg) were degassed at 393 K for 1.0 h and cooled to 373 K under 20 mL min-1 He. NH3 or CO2 was absorbed for 0.5 h and then He (20 mL min-1) was introduced to remove physically absorbed NH3 or CO2 for 1.0 h. The resulting sample was heated to 745 K at a rate of 10 K min-1. The morphologies of materials were examined on a Zeiss Merlin instrument (Bruker, Multimode 8, Germany) operated at 5.0 kV. Fourier transform infrared spectroscopy (FT-IR) was undertaken on a Bruker VERTEX 33 spectrometer with KBr pellets between 4000 and 400 cm-1. Thermal analysis was performed on a NETZSCH STA 449C, samples were dried at 333 K for 12 h under vacuum prior to analysis. Approximately 4.0 mg of sample was heated from 308 to 1073 K at a rate of 10 K min-1 with an air feed of 20 mL min-1. Elemental analysis was conducted on a Vario EL III elemental analyzer. Samples were dried at 333 K for 12 h under vacuum prior to analysis. 2.3. Transfer Hydrogenation Procedures Transfer hydrogenation of LA and LEs to GVL was carried out in a 30 mL Teflon-lined stainless-steel autoclave. For example, 2.0 mmol EL, 0.30 mmol basic zirconium carbonate and 10 mL i-propanol were placed into the reactor, which was then sealed, purged with N2 three times and pressurized to 1.0 MPa with N2. The reactor was then placed into a pre-heated oil-bath at the desired reaction temperature and stirred at 800 rpm. After the required reaction time had elapsed, the reactor was removed and cooled to room temperature with flowing water. The catalyst was separated by centrifugation 7



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(10000 rpm, 10 min), filtration, washed with i-propanol thoroughly (3.0 mL × 3) and dried at 333 K for 6.0 h for catalyst recycle tests. The resulting solution was diluted to 25 mL with pure i-propanol for quantitative product analysis. Catalyst leaching tests were conducted under optimized reaction conditions except for reaction time. The autoclave was cooled to room temperature with flowing water immediately after 2.0 h. After removal of the solid catalyst, the liquid phase was heated to the desired temperature again for a further 3.0 h. 2.4. Product Analysis Qualitative identification of products was conducted on gas chromatography-mass spectrometer apparatus (GC-MS; Agilent 7890B/5977A; Agilent MS library) equipped with a HP-5 MS capillary column (30 m × 250 µm × 0.25 µm). The initial temperature (323 K) was held for 1.0 min and then heated at 20 K min-1 to 473 K (1.0 min). Quantitative analysis of products, using naphthalene as the internal standard, was performed on an Agilent GC-7890B equipped with a flame ionization detector (GC-FID) employed as the same temperature program and capillary column as described for GC-MS. LEs conversion (CLEs), GVL yield (YGVL), i-PL yield (Yi-PL), GVL selectivity (SGVL) and utilization of hydrogen donor (UHD) were calculated according to equations 1-5 respectively. CLEs mol %= 1-

MLEs MF

YGVL mol %=

MGVL

Yi-PL mol %=

Mi-PL

MF MF

×100%

(1)

×100%

(2)

×100%

(3)

8


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SGVL mol %=

YGVL

UHD mol %=

MGVL

CLEs Mac

×100%

(4)

×100%

(5)

Where MF stands for the initial molar amount of substrate (2.0 mmol), MLEs, MGVL, Mi-PL and Mac stand for the molar amount of LEs, GVL, i-PL and acetone after reaction respectively.

3. RESULTS AND DISCUSSION 3.1. Catalyst Screening The catalytic activities of zirconium and other basic metal carbonates were investigated for transfer hydrogenation of EL to GVL using i-propanol as hydrogen donor and solvent at 453 K for 3.0 h. The results exhibited in Table 1 reveal that the EL conversion and GVL yield are negligible in the absence of catalyst (Table 1, entry 1), however, they are enhanced to varying degrees by the basic metal carbonate employed with 0.60 mmol hydroxyl groups (The structural information of various basic metal carbonates are listed in Table S1). Basic zirconium carbonate exhibits the best catalytic activity, giving 100% EL conversion, a 96.3% GVL yield and the largest turnover frequency (TOF) of 3.1 h-1 (Table 1, entry 2). Whereas, only 30.2% GVL yield is found with basic lead carbonate, although it possesses the highest surface reaction rate (0.43 mol m-2 h-1) due to its low surface area (Table 1, entry 3). GC-MS results demonstrate that no alkyl 4-hydroxypentanoate was detected in these basic metal carbonate catalyst systems. This shows clearly that the intermediate alkyl 4-hydroxypentanoate is readily converted to GVL and that the EL reduction process is the rate-limiting step in this cascade 9



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hydrogenation-cyclization reaction. It is noteworthy that this process has high hydrogen donor utilization (84.7 to 96.3%), which can be attributed to the dehydrogenation of i-propanol in the presence of amphoteric catalysts.50 Besides, only one by-product, i-PL, was detected, which was generated from the trans-esterification reaction between EL and i-propanol. This finding is significantly different to other studies, where several other kinds of by-products were simultaneously generated, such as, ethyl 4-ethoxypentanote,27 5-methyl-3-vinyldihydrofuran-2(3H)-one,28 5-methyl-5-vinyldihydrofuran-2(3H)- one,28 3-(2-hydroxypropan-2-yl)-5-methyldihydrofuran-2-(3H)-one51

and

3-(2-hydroxypropyl)-5-methyldihydrofuran-2(3H)-one.51 In addition, the influence of hydroxyl group (M-OH) amount on the transfer hydrogenation of EL to GVL was investigated. As shown in Figure 1, the EL conversion and GVL yield can be promoted by increasing the amount of M-OH for all basic metal carbonates, achieving about 90% hydrogen donor utilization for each run. EL conversion and GVL yield increases with a corresponding decrease in i-PL are observed over basic zirconium carbonate with increasing M-OH dosage between 0.15 and 0.60 mmol. Then, a slight change is found at the continuous increase of M-OH amount due to thorough consumption of EL at the M-OH dosage of 0.60 mmol. In contrast, the GVL yield increases with increasing M-OH quantity over basic lead carbonate, which can be attributed to its low surface area. This result also explains well with the fact that low GVL yield is obtained over basic lead carbonate, though the highest surface reaction rate is shown (Table 1, entry 3). 10


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Many previous studies claimed that the acid and base sites of catalyst show an intensive synergistic effect for transfer hydrogenation of EL to GVL.28-31 Therefore, in an effort to probe the influences of acid and base sites, two probe molecules (piperdine and benzoic acid as alkali and acid respectively) were chosen to selectively deactivate the accessibility of acid/base sites located on basic zirconium carbonate with reference to the method provided by Tang and co-workers.28 As shown in Table 1, a sharp decrease in catalytic activity was observed upon addition of either probe molecule. In the presence of 1.0 mmol piperidine or benzoic acid, EL conversion decreases from 100% to 53.8 and 43.6%, and GVL yield decreases from 96.3% to 40.5 and 35.1% (Table 1, entries 2, 7 and 8) respectively, demonstrating the importance of acid and base sites in this process. NaOH and Zr(OH)4 were used as model hydroxyl compounds to probe the role of OHfunctionality. When NaOH was employed, no GVL was obtained while i-PL was the main product (Table 1, entry 9). However, the GVL yield (42.1%) exhibited a distinct increase in the presence of Zr(OH)4 (Table 1, entry 10). Meanwhile, Na2CO3 and zirconium carbonate were used as model carbonate compounds to explore the role of CO32-. The results listed in Table 1 demonstrated that either Na2CO3 or zirconium carbonate showed no activity on the transfer hydrogenation with i-PL from the transesterification as the single product (Table, entries 11-12). According to previous study

27

, the conversion of LEs to GVL is comprised of systematic carbonyl reduction

and lactonization. In essence, the latter, which facilitates the conversion of alkyl 4-hydroxypentanoate to GVL, is an intramolecular transesterification process. Therefore, 11



it is believed that OH- plays a dominant role for the whole cascade reactions, while CO32promotes the transesterification of the alkyl 4-hydroxypentanoate to GVL. Previous work showed that hydride transfer (H-) plays a key role in MPV reduction processes.28 Hence, in a control test, basic zirconium carbonate was chosen for the detection of H- as it showed the best activity among the basic metal carbonates explored in this study. To this end, the N2 atmosphere was substituted for O2, as a consequence the EL conversion and GVL yield decreased to 64.0% and 54.2% respectively (Table 1, entry 13). These findings suggest competitive scavenging of the generated H- species by superoxide radical anions generated by O2,52 leading to the observed decreases in EL conversion and GVL selectivity. To further explore the potential of zirconium-containing material, a series of catalysts were prepared (ZrO2, Zr(OH)4 etc.) and compared to the basic carbonate, which outperformed any other materials generated in this study (Table 1, entries 2, 10, 14, 16). Interestingly, this relatively simple and environmentally benign catalyst exhibited similar performance to the above mentioned works29,30 and more complex layered Zr-benzylphosphonate nanohybrids (Table 1, entry 17).53 3.2. Influence of Catalyst Dosage The concentration profiles of EL, GVL and i-PL during EL transfer hydrogenation catalyzed by different amounts of basic zirconium carbonate are presented in Figure 2. At the end of all reactions, the concentration of EL decreases with increasing catalyst amount, while an inverse trend is observed for GVL concentration. Considering hydrogen 12


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donor utilization, it exhibits a typical volcano curve, demonstrating a maximum of 91.9% at 0.300 mmol catalyst. A significant impact on i-PL yield is observed as a function of catalyst dosage, for example, 0.054 mol L-1 EL (26.7%) remains unconverted when the amount of basic zirconium carbonate is 0.075 mmol, similarly, a large amount of i-PL (0.044 mol L-1, 21.9% yield) was obtained. These findings suggest that intensive competition exists between transfer hydrogenation and transesterification at low catalyst concentration. Incrementally increasing the amount of catalyst from 0.075 to 0.300 mmol, resulted in both EL and i-PL concentrations systematically decreasing (0.054 to 0 mol L-1 and 0.044 to 0.008 mol L-1 respectively). Subsequently GVL concentration and therefore yield, increases across this range, which coincides with the extent of hydrogen donor utilization. However, no significant changes in these values are observed when increasing dosage to 0.450 mmol, suggesting that the optimized catalyst value is achieved at around 0.300 mmol. 3.3. Influence of Reaction Temperature and Time The changes in the concentration of EL and products with various reaction temperature and time using basic zirconium carbonate catalyst are presented in Figure 3. Catalytic efficiency is comparatively poor at low reaction temperature, for instance, 0.178 mol L-1 EL (89.1% conversion) is converted to 0.137 mol L-1 GVL (68.5% yield) after 5.0 h at 423 K. The concentration of i-PL remains approximately constant (~20.5% yield) from 2.0 to 5.0 h, indicating a chemical equilibrium for this by-product is achieved under these reaction conditions. When the temperature is elevated, the rate of reaction and process 13



efficiency is substantially enhanced, for example, 100% EL conversion with 0.193 mol L-1 GVL (96.3% yield) can be realized after 3.0 h at 453 K. The hydrogen utilization degree can also be used as a means of assessing the influence of temperature upon reaction, elevation of the latter was found to yield a substantial improvement in the former at all reactions times. This clearly demonstrates the significance of reaction temperature in promoting the reduction of the carbonyl bond. It is observed that the concentration of i-PL increases initially before decreasing at longer reaction times (Figure 3b-d). Interestingly, almost the same maximum i-PL concentration is found before 2.0 hours of reaction for all temperatures at or below 443 K, while a notable decrease is observed at 453 K (0.040 to 0.019 mol L-1). At the latter temperature, 0.027 and 0.150 mol L-1 EL and GVL concentrations can be determined respectively for the first 1.0 h, but, these values change to 0 and 0.193 mol L-1 respectively when the reaction time is prolonged to 3.0 h (Figure 3d). It is found that the hydrogen donor utilization decreases as reaction time increases, this can be explained by a near maximization of EL conversion at 3.0 h, after which it remains relatively constant, while the solvent/hydrogen donor is continually transformed. These findings help to illustrate the importance of reaction temperature and time for optimizing conversion of EL and i-PL to GVL, while maximizing hydrogen donor utilization efficiency. 3.4. Influence of Hydrogen Donor The influence of hydrogen donor on the transfer hydrogenation of EL catalyzed by basic zirconium carbonate was investigated under optimized reaction conditions determined 14


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above. It has been considered that the reduction potential obtained from the difference between the standard molar enthalpy of formation for the alcohol and carbonyl compound, is a suitable means of assessing the relative ease of hydrogen abstraction. The lower the value, the stronger the hydrogen donating ability of the alcohol.54 As listed in Table 2, the primary alcohols which have high reduction potential lowers the activity of the catalyst significantly, for example, 33.7 and 46.3% EL conversion, 1.2 and 39.3% GVL yield were achieved for methanol and ethanol respectively (Table 2, entries 1-2). Comparatively, secondary alcohols give better results than those obtained with primary alcohols, which can be attributed to their lower reduction potentials.54 For instance, complete EL conversion and 96.3% GVL yield were obtained in i-propanol media, but these values decrease to 58.7 and 33.8% respectively when n-propanol was employed (Table 2, entries 3 and 4). It was also found that the catalytic efficiency decreases a little with increasing carbon number of the secondary alcohol (Table 2, entries 4, 6 and 8). This phenomenon can be explained by a more significant negative steric effect caused by the longer side chains of the secondary alcohol, which hinders the formation of the transition state between alcohol and active sites.22 3.5. Influence of Substrate on the Transfer Hydrogenation Reaction The influence of substrate choice was investigated over the basic zirconium carbonate catalyst using a number of different LEs and LA with variable temperature and optimized conditions determined previously (catalyst dosage of 0.30 mmol and reaction time of 3.0 h). Figure 4 demonstrates that the transfer hydrogenation performance of LEs 15



substantially depends on the length of the carbon-chain in the ester fraction. Higher reaction temperature is required for the complete conversion of LEs with longer carbon-chain lengths. For example, 100% ML conversion and 95.2% GVL yield are obtained when the reaction temperature is 433 K. However, 463 K is required for BL to obtain the same level of conversion and GVL yield. Compared with LEs, relatively high temperature (473 K) is necessary for LA transfer hydrogenation, which can be attributed to the strong interaction between the basic site of catalyst and the acidic group found on LA. As discussed above, i-PL is the sole by-product generated over these basic metal carbonates, interestingly 73.4% i-PL conversion with a GVL yield of 69.2% can be achieved at 463 K, indicating that i-PL can be converted to a reasonable extent over the commercial basic zirconium carbonate catalyst under the optimized reaction conditions. This finding accords well with results presented previously in Figure 2 and 3, where the i-PL concentration decreases with the increase of catalyst dosage, reaction time and temperature. However, to obtain 100% conversion, the reaction temperature must be elevated to 493 K, to overcome the intensive steric hindrance of this molecule, which also helps to explain the fact that i-PL is the only by-product for LEs transfer hydrogenation processes. 3.6. Leaching and Reusability of the Catalyst Catalyst stability and recyclability are key parameters for industrial application, hence, leaching was examined under optimized reaction conditions. It has been reported 16


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previously that active components can be dissolved in hot solution and re-adsorbed on the catalyst surface upon cooling,55 so both hot and cold filtration tests had been conducted. As presented in Figure 5, the GVL yield shows no obvious change after the catalyst was removed after 2.0 h for both tests, indicating that catalyst leaching is insignificant. ICP-AES analysis demonstrates that no zirconium ions are detected in the filtrate. These findings demonstrate that the catalyst used in this study has a high chemical stability. Subsequently, the reusability of the catalyst was investigated. As displayed in the Figure 6, the catalytic activity shows no obvious change even after six runs when the reaction time is 1.0 h. With the reaction time set at 3.0 h, the loss in conversion is negligible during the first four runs, while a slight decrease is observed during the fifth and sixth runs, meanwhile a gradual decrease for the GVL selectivity was exhibited with the increased cycle number. It should be noted that 91.0% EL conversion with 81.1% GVL yield could still be obtained after six runs, showing reasonable reusability of this catalyst. To examine the possible reason for the slight activity loss, a series of characterizations (XRD, SEM, N2 adsorption-desorption isotherms, FT-IR, TG-DTG and elemental analysis) were used to compare the fresh and spent (after the sixth cycle at 3.0 h) catalysts. The surface morphologies and XRD patterns of fresh and spent catalysts show no obvious change (Figure S2 and S3 respectively). But the surface area decreases from 31.0 to 21.4 m3 g-1 after the sixth cycle (Figure S4). In the FTIR spectra (Figure 7), the peak observed at 3450 cm-1 is ascribed to the stretching vibration of surface hydroxyl groups (including water and Zr-OH). Bands at 1578 and 1350 cm-1 could be assigned to 17



asymmetric stretching mode of CO32-, meanwhile, peaks at 1066 and 860 cm-1 are designated to the symmetric stretching mode of CO32-.40 The band at 660 cm-1 is attributed to the stretching vibration of Zr-O.56 However, the intensities of peaks located at 1350, 1066 and 860 cm-1 decrease sharply over the spent catalyst, while five new signals are observed ca. 2947 (C-H stretch ), 1430 (C-H bend), 1232 and 1150 (C-O stretch), and 950 cm-1 (RCH=CHR bend). Therefore, this FTIR spectrum demonstrates the existence of nonvolatile organic compounds on the surface of the spent catalyst. Previous studies57,58 suggest that these unidentified compounds form from self-aldol condensation of levulinate ester in the presence of base catalysts, lowering the carbon balances listed in Table 1 and 2. This deposition of non-volatile species on the catalyst surface can be further verified by elemental analysis. It is found that the fresh catalyst possesses a carbon content of 3.51 wt.%, close to the theoretical value (3.49 wt.%), but after 6 intensive reaction cycles, this increases to 13.13 wt.%. TG-DTG curves (Figure 8) shows that both fresh and spent catalysts give the same weight losses of 11.4 wt.% before 483 K, this can be attributed to the removal of absorbed and structural water. Interestingly, the maximum rate of weight loss observed for this feature is found at lower temperature in the spent catalyst. This may be associated with the deposited carbonaceous species, which can reduce the hydrophilicity of the catalysts surface.59 At higher temperatures (483-745 K), an obvious difference is found between the weight losses for fresh and spent catalysts. The former exhibits a weight loss of 6.3 wt.% due to the removal of hydroxyl groups,39 while, the latter shows much higher 18


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weight loss (17.3 wt.%), attributed to the removal of the aforementioned non-volatile species deposited on the catalyst surface. The weight loss in the range from 745 to 937 K was caused by the decomposition of carbonate,40 having removals of 4.2 and 3.9 wt.% for the fresh and spent catalyst, respectively. Between 937 and 1070 K weight losses are associated with dehydroxylation of the incomplete ZrO2 lattice,39 the fresh and spent catalyst exhibit losses of approximately 1.8 and 0.6 wt.%, respectively. It is noteworthy that the ZrO2/CO32- molar ratio of basic zirconium carbonate increases from 2.0 to 2.2 after the sixth run. This change can be ascribed to part decomposition of CO32- and in situ generation of OH group according to previous study on the UIO-66 catalyst, which has a similar functional group as basic zirconium carbonate.32 It is well known that Zr-OH is beneficial for MPV reduction of EL to GVL,26-28 hence, basic zirconium carbonate shows excellent reusability despite the drop in BET surface area and nonvolatile organic chemical deposition. As discussed above, the acid and base sites present on basic zirconium carbonate are responsible for its high catalytic activity, so the acid-base properties of the fresh and spent catalysts were further investigated using NH3-TPD and CO2-TPD, respectively (Figure 9, Table 3). It clearly demonstrates that basic zirconium carbonate has regular acid and base centers. The resulting NH3-TPD profile of the fresh catalyst shows the coexistence of weak and moderate acid sites according to peak fitting deconvolution (Figure 9a-b). Both acid sites show a lower concentration for the spent catalyst without significant change of their relative proportion. While the base sites of the fresh and spent 19



catalysts are deconvoluted into three peaks at 431-442, 537-550 and 606-635 K, indicating the coexistence of weak, moderate and strong base sites (Figure 9c-d). The concentration of both weak and strong base sites decreases over spent catalyst compared to the fresh, while an increase in the proportion of moderate base sites increase. Hence, accompanying with the nonvolatile organic chemical deposition and surface area decrease, the change in acid-base properties also contributes to the apparent decrease in catalytic activity after six consecutive reaction cycles.

4. CONCLUSIONS Basic metal carbonates are found to be efficient catalysts for the transfer hydrogenation of LA and LEs to GVL, a versatile platform chemical from renewable lignocellulosic biomass, using i-propanol as both solvent and hydrogen donor. The cooperation effect between acid (Mn+) and base (-OH) sites (basic zirconium carbonate) facilitates the highly selective formation of GVL under relatively mild conditions. 100% EL conversion and 96.3% GVL yield were obtained with a TOF value of 3.1 h-1 and a hydrogen donor utilization of 91.9% at 453 K for 3.0 h. Furthermore, basic zirconium carbonate has good recyclability, being readily reusable for six cycles, while maintaining appropriate catalytic activity and selectivity. Thus, this catalytic system has a significant potential for GVL production from sustainable lignocellulosic biomass due to its high process efficiency, simple technology, cost-effectiveness and catalyst reusability. Supporting Information The Supporting Information is available free of charge on the ACS Publication website: 20


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including characterization of various catalysts (Figure S1-S4), and structural information of various catalysts (Table S1). Notes The authors declare no competing financial interest. ACKNOWLEDGEMENT The authors gratefully acknowledge the financial support of the Natural Science Foundation of China (Grant No. 21736003, 21336002 and 21690083), the Natural Science Foundation of Guangdong Province, China (No. 2015A030311048), and the Science and Technology Program of Guangzhou, China (201804020014 and 201804010337). REFERENCES (1) Mika, L. T.; Cséfalvay, E.; Németh. Á. Catalytic conversion of carbohydrates to initial platform chemicals: Chemistry and sustainability. Chem. Rev. 2018, 118, 505-613. (2) Shi, N.; Liu, Q. Y.; Zhang, Q.; Wang, T. J.; Ma, L. L. High yield production of 5-hydroxymethylfurfural from cellulose by high concentration of sulfates in biphasic system. Green Chem. 2013, 15, 1967-1974. (3) Liao, Y. H.; Liu, Q. Y.; Wang, T. J.; Long, J. X.; Ma, L. L.; Zhang, Q. Zirconium phosphate combined with Ru/C as a highly efficient catalyst for the direct transformation of cellulose to C-6 alditols. Green Chem. 2014. 16, 3305-3312. (4) Liguori, F.; Moreno-Marrodan, C.; Barbaro, P. Environmentally friendly synthesis of 21



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Figure Captions Figure 1. Influence of M-OH amount upon various basic metal carbonates on transfer hydrogenation of EL to GVL. Reaction conditions: 2.0 mmol EL, 10 mL i-propanol, 453 K, 3.0 h, 1.0 MPa N2. Figure 2. Influence of catalyst dosage on transfer hydrogenation of EL to GVL. Reaction conditions: 2.0 mmol EL, 10 mL i-propanol, 453 K, 3.0 h, 1.0 MPa N2. Figure 3. Influence of reaction time and temperature for conversion of EL to GVL (a) 423 K, (b) 433 K, (c) 443 K, (d) 453 K. Reaction conditions: 2.0 mmol EL, 0.30 mmol basic zirconium carbonate, 10 mL i-propanol, 1.0 MPa N2. Figure 4. Influence of substrate on transfer hydrogenation for (a) ML, (b) EL, (c) PL, (d) i-PL, (e) BL, (f) LA. Reaction conditions: 2.0 mmol substrate, 0.30 mmol basic zirconium carbonate, 10 mL i-propanol, 3.0 h, 1.0 MPa N2. Figure 5. Leaching test for basic zirconium carbonate for the transfer hydrogenation EL to GVL. Reaction conditions: 2.0 mmol EL, 10 mL i-propanol, 453 K, 1.0 MPa N2. Figure 6. Reusability of basic zirconium carbonate (a) 1.0 h and (b) 3.0 h. Reaction conditions: 2.0 mmol EL, 0.30 mmol basic zirconium carbonate, 10 mL i-propanol, 453 K, 1.0 MPa N2. Figure 7. FT-IR profiles of fresh and spent catalyst after six runs. Figure 8. TG and DTG profiles of the fresh and spent catalyst. Figure 9. NH3-TPD and CO2-TPD profiles of the fresh (a, c) and spent (b, d) catalyst. Signals of CO2-TPD have been magnified 300 times. 31


Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11Entry 12 13 14 15 1 16 17 2 18 19 20 3 21 22 4 23 24 25 5 26 27 28 6 29 30 7 31 32 33 8 34 35 9 36 37 38 10 39 40 41 11 42 43 12 44 45 46 13 47 48 14 49 50 51 15 52 53 54 16 55 56 57 58 59 60

Table

Catalytic

1.

Performance

of

Various

Basic

Metal

Page 32 of 45

Carbonates

and

Zirconium-Containing Catalystsa SBET

Yield (%)

Conv.

C.B.

UHD

S.R. (mol

TOF

(%)b

(%)

m-2 h-1)c

(h-1)d

Sel. (%)

Catalyst 2

-1

m g

(%)

GVL

Blank

-

Highly Efficient Transfer Hydrogenation of Levulinate Esters to Î³

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