Integrated Conversion of Hemicellulose and Furfural into γ

Research Article pubs.acs.org/acscatalysis

Integrated Conversion of Hemicellulose and Furfural into γ‑Valerolactone over Au/ZrO2 Catalyst Combined with ZSM‑5 Shanhui Zhu,*,† Yanfeng Xue,†,‡ Jing Guo,†,‡ Youliang Cen,†,‡ Jianguo Wang,† and Weibin Fan*,† †

State Key Laboratory of Coal Conversion Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100039, People’s Republic of China S Supporting Information *

ABSTRACT: The high-yield synthesis of the biofuel γ-valerolactone (GVL) is a challenging task, which currently stems from the depolymerization of cellulose to levulinic acid, followed by its hydrogenation. We have developed a novel integrated process for the production of GVL from hemicellulose without using liquid acids and external hydrogen. The hemicellulose feed underwent hydrolysis and consecutive dehydration to produce furfural over ZSM-5 catalyst. Subsequently, the formed furfural with 2-propanol performed tandem conversion to GVL over Au/ZrO2 catalyst combined with ZSM-5. This process gave a high yield of GVL under mild conditions: up to 61.5% based on hemicellulose. The outstanding performance was mainly ascribed to the strong interface interaction of Au with ZrO2 species, large amounts of medium-strength acid sites over ZSM-5, and efficient synergy between active metal and acid sites. KEYWORDS: γ-valerolactone, hemicellulose, furfural, Au, ZSM-5

1. INTRODUCTION The utilization of renewable biomass for the chemical industry is becoming increasingly attractive in light of decreasing fossil fuel resources and increasing emission of greenhouse gases. Hemicellulose is a heterogeneous biopolymer and nonedible biomass that accounts for about 30% of the organic carbon resources on Earth.1,2 As one of the three most important components (cellulose, hemicellulose, and lignin) in lignocellulosic biomass, hemicellulose is currently used as animal feed, burned to supply energy in paper and pulp industries, or directly treated as agricultural waste.2 Although many valuable chemicals such as γvalerolactone (GVL),3−5 2,5-dimethylfuran,6,7 polyol,8−11 and alkane12−15 have been prepared from cellulose, few available processes have been reported for the potential utilization of hemicellulose.1,15−18 Thus, it is imperative to develop effective strategies to transform renewable hemicellulose into value-added chemicals and fuels. GVL has been identified as one of the most promising platform molecules for the production of transportation fuels.19−22 Additionally, GVL facilitates the dissolution of cellulose and humins, thus being considered as a potential solvent for biomass transformation.23−26 Currently, GVL is primarily synthesized from cellulose via hydrolysis, dehydration, and successive rehydration to levulinic acid (LA) followed by its hydrogenation. Wettstein et al.3 adopted 0.1−1.25 M HCl to decompose © XXXX American Chemical Society

cellulose into LA in a biphasic system. The extracted LA was then selectively hydrogenated to GVL over Ru−Sn/C. Heeres et al.27 also investigated this process by trifluoroacetic acid and Ru/C mixed catalysts at 180 °C and 6.0 MPa of H2, in which a 29% yield of GVL was obtained. Obviously, this tandem process usually suffers from liquid acid catalysts for the efficient hydrolysis of cellulose, and high-pressure H2 is necessary for the reduction of LA. Although formic acid has been used as an H-donor in some reports,28−30 its decomposition requires harsh reaction conditions and still releases high-pressure H2 as well as unwanted CO2. Deng et al.29 reported that the pressure of the reactor increased rapidly from 0.1 to 8 MPa due to the Ru-catalyzed decomposition of formic acid. Recently, Román-Leshkov et al.31 have developed a domino reaction process for the production of GVL, in which the initial xylose proceeds to dehydration to furfural over H2SO4 in a NaCl−H2O−2-butanol mixed system, and then the separated furfural converts to GVL over Zr-Beta and Al-MFI-ns by a Meerwein−Ponndorf−Verley reaction. Despite these great advances, the invention of a safer, more economical, and more environmentally friendly process is highly desirable for Received: December 17, 2015 Revised: February 12, 2016

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ACS Catalysis Scheme 1. Process for the Production of GVL from Hemicellulose

HAuCl4·4H2O aqueous solution. After 30 min of sonication, 5 mL of NaBH4 (0.25 mol/L) aqueous solution was dripped into the above mixture. After it was kept in sonication for another 30 min, the suspension was separated by centrifugation, and the remaining solid was dried overnight at 80 °C. The Au loading was 1.13 wt % from an ICP test. Other supported metal catalysts (Au/CeO2, Au/C, Au/GO, Pt/ZrO2, Pd/ZrO2, and Ru/ZrO2) were also prepared by similar methods, and the active metal contents were controlled at ca. 1 wt %. The textural properties, metal content, and XRD patterns of ZrO2 supported samples are displayed in Table S1 and Figure S1 in the Supporting Information. H 4 SiW 12 O 40 /ZrO 2 (HSiW/ZrO 2 ), H 3 PW 12 O 40 /ZrO 2 (HPW/ZrO2), and H3PMo12O40/ZrO2 (HPMo/ZrO2) were prepared by an incipient wet impregnation method. For example, HSiW aqueous solution was impregnated into ZrO2 followed by drying at 80 °C overnight. Then, the sample was calcined at 250 °C for 4 h in flowing air. The active acid content was fixed at 15 wt % in the three catalysts. Al-exchanged HPW (AlPW) catalyst was synthesized by an ion-exchange method.35 A 10 g portion of HPW was dispersed into 20 mL of water with vigorous stirring. Subsequently, the desired amount of Al(NO3)3·9H2O (0.1 mol/L) solution was dripped into the above mixture. After continuous stirring for 2 h, the excess water was evaporated, and the remaining solid powder was dried at 80 °C overnight. The sample was then calcined at 250 °C for 4 h. The molar ratio of Al3+ to HPW was 1:3; namely, one hydrogen atom was replaced by Al3+ ion in one AlPW molecule. ZSM-5 (Si/Al = 100), ZSM-48 (Si/Al = 150), Hβ (Si/Al = 20), and Hmordenite (Si/Al = 15) were synthesized by a hydrothermal method.33 In a typical synthesis of ZSM-5, sodium aluminate (NaAlO2), sodium hydroxide (NaOH), tetrapropylammonium hydroxide (TPAOH), and silica sol (40 wt % SiO2, 4 wt % Na2O) were dripped into the calculated amount of water with vigorous stirring. This mixture was aged at 25 °C for 4 h to form a gel, and then the gel was transferred into a Teflon-lined stainless steel autoclave to crystallize at 170 °C for 48 h. The solid product was extensively washed with water, recovered by filtration, dried at 100 °C overnight, and calcined at 550 °C for 10 h. Finally, the obtained Na-ZSM-5 zeolite was changed into the H-form by ion exchange with NH4NO3 solution twice at 80 °C for 10 h. 2.3. Catalyst Characterization. N2 adsorption−desorption isotherms (mesopore analysis) were measured at −196 °C on a Micromeritics TriStar 3000 instrument. The samples were first degassed under vacuum at 300 °C for 8 h before the measurements were taken. Micropore analysis was conducted on a Micromeritics ASAP 2020 apparatus. The microporous

the conversion of renewable biomass into GVL under mild conditions. On the basis of this background, we report a new approach for the transformation of hemicellulose into GVL without using liquid acid and an external H2 supply. As shown in Scheme 1, after hemicellulose is decomposed by efficient acid sites, the formed furfural undergoes a tandem process to GVL, in which active metal and acid sites are required to work cooperatively. This tandem reaction includes the following steps: (1) the reduction of furfural into furfuryl alcohol (FAL) via a transfer hydrogenation reaction (THR) using 2-propanol as an H-donor, (2) the acid-catalyzed alcoholysis of FAL with 2-propanol to 2propyl levulinate (PL), and (3) the reduction of PL into corresponding 2-propyl 4-hydroxypentanoates (PHP) followed by its lactonization to yield GVL over metal and acid sites. In comparison with the cellulose protocol,27−29 this route improves the C atom economy, achieves 100% C atom utilization in theory, and avoids the energy-costly separation of byproduct formic acid. In addition, the 2-propanol transfer hydrogenation system has remarkable advantages in comparison to pressurized H2, including an improvement in process safety and the recycling of formed acetone by facile hydrogenation over commercial Cubased catalyst.32

2. EXPERIMENTAL SECTION 2.1. Chemicals. The following were obtained from the indicated suppliers: H4SiW12O40 (HSiW, Sinopharm Chemical Reagent Co., Ltd., China (SCRC)), H3PW12O40 (HPW, SCRC), H3PMo12O40 (HPMo, SCRC), Al(NO3)3 (SCRC), Ce(NO3)3· 6H2O (SCRC), H2AuCl4·4H2O (SCRC), H2PtCl6·6H2O (SCRC), PdCl2 (SCRC), RuCl3·3H2O (SCRC), hemicellulose (xylan from corn cob, Aladdin, xylose content ≥90% with a molecular weight of ca. 30000), xylose (SCRC), furfural (SCRC), 2-propanol (SCRC), ZrO2 (Jiangsu Qianye Co., Ltd., China), activated carbon (C, Liyang Zhuxi Carbon Co., Ltd., China), and NaBH4 (SCRC). 2.2. Catalyst Preparation. Graphene oxide (GO) was prepared by a modified Hummer’s method described in our previous work.33 Rod CeO2 was synthesized by a hydrothermal method.34 A 4.5 mmol amount of Ce(NO3)3·6H2O was first added to 90 mL of a NaOH solution (6 mol/L), and then the mixed solution was vigorously stirred for 10 min at room temperature. The mixture was hydrothermally treated in an autoclave at 100 °C for 24 h. Finally, the solid sample was collected by centrifugation, washed with water, dried at 80 °C overnight, and calcined in air at 400 °C for 4 h. Au/ZrO2 was prepared by an ultrasound-assisted deposition method. First, 0.5 g of ZrO2 was dispersed into 20 mL of water in a beaker by ultrasound sonication, followed by the addition of 2036

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ACS Catalysis Table 1. Transfer Hydrogenation Reactions of Furfural with 2-Propanol to FAL over Different Catalystsa entry

catalyst

t (h)

conversion (%)

selectivity (%)

TOF (molfurfural molmetal−1 h−1)b

production rate (molFAL molmetal−1 h−1)

1 2 3 4 5 6 7 8

Pt/ZrO2 Pd/ZrO2 Ru/ZrO2 Au/ZrO2 Au/ZrO2 Au/CeO2 Au/C Au/GO

3 3 3 3 1 6 6 6

18.7 31.1 89.1 100 92.6 26.8 0.4 1.6

95.9 92.5 98.9 99.0 100 97.6 100 100

2.6 3.7 18.3 124.1 124.1 3.9 0.03 0.1

2.5 2.4 7.5 14.4 40.4 2.0 0.03 0.1

Reaction conditions: 0.2 g of catalyst, 0.5 mM furfural, 10 g of 2-propanol, 120 °C. bTOF values were calculated in the range of initial kinetic behavior, and the conversion was below 20%. a

conveniently sampled online at 12 h, 18 h, 24 h, and 30 h during the reaction, respectively. For the reusability test, the spent catalyst was repeatedly washed, carefully separated by centrifugation, and dried overnight at 80 °C. Following that, the spent catalyst was reused according to the procedure of section 2.4.1. 2.4.3. Integrated Conversion of Xylose and Hemicellulose. A 0.5 mmol portion of xylose or hemicellulose (based on the molar amount of anhydroxylan unit), 0.2 g of γ-butyrolactone (GBL), 10 g of 2-propanol (0.5 g of H2O was added in the conversion of hemicellulose), and 0.2 g of ZSM-5 were put into the autoclave. The reactor was pressurized to 0.1 MPa with N2 and then heated to 160 °C (170 °C for the conversion of hemicellulose). After 6 h, the reactor system was quickly cooled to room temperature in an ice−water bath. After the separation of ZSM-5, 0.2 g of fresh ZSM-5 and 0.2 g of Au/ZrO2 were added to the above mixture and continued to react at 120 °C for 30 h. The following procedure was the same as section 2.4.1. Liquid products such as furfural were analyzed by the same GC as above. The content of xylose was determined with an external standard method by HPLC (Waters 2695) using a refractive index detector (Waters 2414) and X Bridge Amide column. Because the used hemicellulose (xylan) is soluble in 2-propanol, the conversion of hemicellulose cannot be determined according to the changed mass during the reaction. The analysis conditions for HPLC are as follows: mobile phase acetonitrile/water/ ammonium hydroxide (80/20/0.1 in volume ratio), flow rate 0.4 mL/min, column temperature 35 °C, and detector temperature 40 °C.

volume was estimated by the t-plot method. The pore size was calculated using nonlocal density functional theory (NLDFT) by assuming a slit pore model. XRD was recorded on a Rigaku MiniFlex II desktop X-ray diffractometer operating with Cu Kα radiation at 40 kV and 40 mA at a scanning speed (2θ) of 4°/min. ICP optical emission spectroscopy (Optima 2100DV, PerkinElmer) was measured to analyze element loading and leached species. NH3-TPD was performed in an Auto Chem II 2920 apparatus (Micromeritics, USA). Typically, 0.1 g of the sample was first pretreated in flowing Ar at 400 °C for 20 min. Once the system was cooled to 100 °C, the sample was flushed with NH3 in order to reach its saturation state. Subsequently, the sample was heated to 600 °C at a ramp of 10 °C/min, and the desorbed NH3 was monitored by a TCD detector. CO2-TPD was conducted with the same apparatus as above. The sample was pretreated at 300 °C for 20 min in Ar, cooled to 50 °C, and then saturated with CO2 for 30 min. The sample was heated to 600 °C at 10 °C/min, and the desorbed CO2 was recorded by TCD. TEM was tested on a field-emission transmission electron microscopy (FETEM, JEM-2011F) operating at 200 kV voltage. The sample was dispersed in ethanol by ultrasonication and deposited on carboncoated copper grids. XPS was conducted under ultrahigh vacuum on a Kratos AXIS ULTRA DLD spectrometer using Al Kα radiation and a multichannel detector. The binding energy was calibrated by the C 1s peak at 284.6 eV within ±0.1 eV error. 2.4. Activity Tests. 2.4.1. Transfer Hydrogenation of Furfural with 2-Propanol. All of the reactions were performed in a 50 mL Teflon-lined stainless steel autoclave. Typically, 0.5 mmol of furfural, 10 g of 2-propanol, and 0.2 g of Au/ZrO2 (or 0.2 g ZSM-5) were loaded into the autoclave. After the reactor was sealed, the remaining air was purged by flushing with N2 five times. The reactor was pressured to 0.1 MPa with N2 and then heated to 120 °C. After the desired reaction time, the reactor system was quickly cooled to room temperature in an ice−water bath. The products and catalysts were separated by centrifugation. The products were analyzed by gas chromatography (Shimadzu GC-2010) with a flame ionization detector using a DB-1 capillary column (60 m × 0.25 mm × 0.25 μm). Benzyl alcohol was used as an internal standard to calibrate the liquid product concentrations and carbon balances. The calibration factors are given in the Supporting Information. The conversions and yields of products were expressed as mole percent, on the basis of the total furfural amount. The assignments of products were determined by GC-MS. 2.4.2. Leaching and Reusability Test. For the leaching test, the catalysts were removed from the reaction system after furfural reacted with 2-propanol for 6 h. Subsequently, the reaction was continued for another 18 h at 120 °C. The products can be

3. RESULTS AND DISCUSSION 3.1. Transfer Hydrogenation of Furfural with 2Propanol. In the initial experiment to select the superior THR catalyst, a series of ZrO2-supported metal catalysts (Pt, Pd, Ru, and Au) were explored using furfural as the substrate (Table 1). All of the catalysts gave high FAL selectivity (>92%), suggesting that THR facilitated the suppression of FAL overhydrogenation. Nevertheless, the use of pressurized H2 led to the formation of large amounts of the byproducts furan, 2methylfuran, and tetrahydrofurfuryl alcohol.36,37 Unexpectedly, Au/ZrO2 exhibited excellent reduction reactivity and showed 100% furfural conversion over 3 h. Even for 1 h, Au/ZrO2 gave 92.6% conversion and was much more active than Pt/ZrO2, Pd/ ZrO2, and Ru/ZrO2. The turnover frequency (TOF) was explored to reflect intrinsic catalytic activity, and the results are given in Table 1. In comparison with other metal counterparts, Au/ZrO2 gave a 6.8−47.7-fold increase in TOF and also substantially improved the production rate of FAL. Although Au nanoparticles have emerged as active and extremely selective 2037

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Figure 1. TEM images and Au particle size distributions of (A) Au/ZrO2, (B) Au/CeO2, (C) Au/C, (D) Au/GO, and (E) spent Au/ZrO2.

catalysts in oxidation reactions,38−40 their applications are rare in H2-mediated reduction reactions because the Au surface has difficulty in adsorbing and activating H2.41,42 However, the in situ generated active H* species from 2-propanol can promote the hydrogenation of furfural on Au surface sites during the THR

process. Additionally, Lu et al.41 have adopted computations and experiments to reveal that the gold surface serves as a Lewis acid site and couples with Lewis base to construct effective frustrated Lewis pairs to active H* species. In our Au/ZrO2 system, the support ZrO2 could behave as a Lewis base and form effective 2038

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metal catalyst, a series of typical solid acid catalysts were deliberately chosen and synthesized, including heteropolyacids, ion exchange resin, and zeolites. These solid acids were characterized by BET, XRD, and NH3-TPD. As shown in Table S3 in the Supporting Information, ZrO2 supported heteropolyacids displayed ca. 50 m2/g BET surface area, while AlPW was a nonporous material. Micropore analysis reflected that Hmordenite had 0.842 nm (Table S4 in the Supporting Information) average pore size and 409.1 m2/g BET surface area. ZSM-5 possessed a high surface area and 0.562 nm pore size. As indicated by XRD results (Figure S3A in the Supporting Information), the Keggin structures of supported heteropolyacids and AlPW were intact and were completely preserved. Each zeolite catalyst showed typical XRD peaks (Figure S3B) representing its inherent structure. The NH3-TPD profiles and acid amounts of these catalysts are depicted in Figure 3 and Table

frustrated Lewis pairs with a Lewis acidic Au surface (Figure S2 in the Supporting Information), which enhanced the reactivity of furfural hydrogenation. Similar to the case for oxidation reactions,38 Au-based catalysts also have strong support effects in furfural reduction by 2propanol. In comparison to other supports, ZrO2-supported Au catalyst was 1−3 orders of magnitude more active, and the TOF was up to 124.1 molfurfural molAu−1 h−1. In the case of Au/CeO2, the conversion of furfural was low: only 26.8% over 6 h. Au/C was not effective for this TPR reaction. Recently, graphene oxide (GO) has been widely reported for immobilizing active metal species owing to the abundant amount of oxygen-containing groups and unique nanosheet structure.33,43,44 Unfortunately, Au nanoparticles caused severe agglomeration on the GO surface, and the average Au particle size was up to 21.4 nm (Figure 1D). Indeed, Au/GO showed very low catalytic activity. XPS results (Figure 2) were indicative of the weak interaction between Au

Figure 3. NH3-TPD profiles of various solid acid catalysts. Figure 2. Au 4f XPS photoemission peaks of various supported Aubased catalysts.

2, respectively. The supported heteropolyacids showed broad and weak peaks in low-temperature regions, suggesting that they possessed low acidity. In contrast, AlPW displayed an extremely strong and symmetrical peak centered at 550 °C, and the total acidity was as high as 1.22 mmol/g. Of the catalysts, Hmordenite had the highest acidity (1.52 mmol/g), in which two broad and large NH3-TPD peaks were centered at 185 and 490 °C, respectively. A similar profile was observed in the case of Hβ. ZSM-48 showed the lowest acidity, and the amount was only 0.15 mmol/g. ZSM-5 mainly possessed medium-strength acid sites, while NaZSM-5 primarily contained weak acid sites. Next, the direct synthesis of GVL from furfural with 2propanol was conducted over Au/ZrO2 combined with various solid acid catalysts at 120 °C for 24 h. The reaction results are given in Table 2. Without the aid of solid acid, Au/ZrO2 could not produce any GVL. Three ZrO2-supported heteropolyacid catalysts (HSiW/ZrO2, HPW/ZrO2, and HPMo/ZrO2) gave low GVL yields, and the main products were the intermediates PL and 2-propyl furfuryl ether (FE), indicating their weak acid strength. As mentioned previously,33 the catalytic conversion of FE into PL required only weak acid sites. Thus, the presence of much residual FE also confirmed the low acid strength of supported heteropolyacids. The nonporous heteropolyacid salt AlPW contained abundant strong acid sites, but it was inefficient for the integrated conversion of furfural. Nevertheless, large amounts of isopropyl ether were detected due to the catalysis of AlPW. This was indicative of the bad cooperation of AlPW with

and C or GO that led to the significant agglomeration of Au nanoparticles. The surface Au content from XPS (Table S2 in the Supporting Information) was lower than that of bulk Au from ICP over Au/C and Au/GO, probably due to the serious sintering of Au nanoparticles. Conversely, the obvious shift of the Au 4f signal toward low binding energy reflected the strong electronic interaction between Au and ZrO2 species. Au/ZrO2 contained a higher surface Au concentration, which verified the presence of highly dispersed Au species. HRTEM (Figure 1A) showed that Au nanoparticles directly connected to m-ZrO2 without any transition layers and formed a strongly anchoring interface. This interfacial regions were rather robust, as evidenced by the clear distinction of Au {111} and m-ZrO2 {111} lattice fringes.45 As shown in the size distribution histogram (Figure 1), more than 90% of Au nanoparticles were in the range of 3.5−5.5 nm and were close to a monodisperse state. Obviously, the unprecedented performance of Au/ZrO2 in furfural hydrogenation could be tentatively ascribed to the strong interface interaction, well-dispersed Au nanoparticles, and the formation of effective frustrated Lewis pairs. 3.2. Conversion of Furfural to GVL. According to the above analysis on the conversion of furfural to GVL with 2propanol, three tandem steps are involved, wherein active metal and acid sites are required to work cooperatively to synthesize GVL smoothly. After Au/ZrO2 was determined to be an active 2039

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illustrated in Figure 4, furfural was consumed quickly in the initial stage. The conversion of furfural reached 100% for 1 h, being

Table 2. Integrated Conversion of Furfural with 2-Propanol to GVL over Au/ZrO2 and Different Acid Catalystsa yield (%)c entry 1 2 3 4 5 6 7 8 9 10 11e 12 13f 14g,h 15g,i

catalyst HSiW/ZrO2 HPW/ZrO2 HPMo/ZrO2 AlPW Amberlyst Hmordenite Hβ ZSM-48 ZSM-5 ZSM-5 NaZSM-5 ZSM-5 ZSM-5 ZSM-5

acidity (mmol/g)b 0.25 0.27 0.31 1.22 1.52 1.19 0.15 0.43 0.43 0.35 0.43 0.43 0.43

GVL

FE

PL

MF

0 27.1 24.7 19.3 10.8 1.5 41.6 47.5 13.1 77.5 0 7.3 80.4 54.9 61.5

0.9d 20.8 29.7 6.9 2.4 0 1.4 0 61.3 0.7 0 1.9 0.3 0.2 0.1

0 48.6 39.5 63.2 78.7 9.9 34.9 32.8 3.0 6.9 0 3.3 4.2 1.3 1.1

0 1.2 0 4.0 0 0 0 0 15.6 8.5 0 0 5.3 2.6 3.9

Figure 4. Kinetic behavior of integrated conversion of furfural with 2propanol to GVL over Au/ZrO2 and ZSM-5. Reaction conditions: 0.2 g of Au/ZrO2, 0.2 g of ZSM-5, 0.5 mM furfural, 10 g of 2-propanol, 120 °C.

a Reaction conditions unless stated otherwise: 0.2 g of Au/ZrO2, 0.2 g of solid acid, 0.5 mM furfural, 10 g of 2-propanol, 120 °C, 24 h. bThe total acidity was determined by NH3-TPD. cAbbreviations: GVL, γvalerolactone; FE, 2-propyl furfuryl ether; PL, 2-propyl levulinate; MF, 5-methyl-2(5H)-furanone. dThe other product was FAL with 99.0% yield. eWithout Au/ZrO2. fAt 30 h, the mass balance was 93%. gThe detailed reaction conditions are summarized in section 2.4.3. hXylose, the conversion was 100% and mass balance was 73%. iHemicellulose, the mass balance was 79%.

higher than that obtained from Au/ZrO2 alone (92.6%). The formed FAL could immediately process alcoholysis to produce FE and PL on the acidic sites of ZSM-5, which promoted the conversion of furfural according to the law of chemical equilibrium. The GVL yield gradually increased with reaction time, and the maximum yield reached up to 80.4% over 30 h. The yields of FE and PL exhibited volcano trends, implying that FE and PL were intermediates in the transformation of FAL. PHP was formed by PL hydrogenation and was only detected in a trace amount, because it was highly active and underwent lactonization to produce GVL rapidly. Additionally, 5-methyl-2(5H)-furanone (MF) was also formed via the elimination of 2-propanol from PL over ZSM-5. Although the yield of MF behaved according to a volcano trend, MF gradually became the primary byproduct with reaction time. This indicated that MF could transfer hydrogenate to GVL catalyzed by Au/ZrO2, but the reaction rate was very low. The combined results demonstrate that the formation of GVL from PL proceeds predominantly via a PHP-mediated pathway, consistent with the hydrogenation of levulinic acid or methyl levulinate.47,48 Accordingly, the possible pathway for the tandem conversion of furfural is proposed in Scheme 2. A leaching test was performed to exclude the contribution of leached Au species. When Au/ZrO2 and ZSM-5 were removed from the reaction mixture after 6 h, no extra products were detected by continuing this reaction for another 18 h (Figure 5). The remaining solution was checked by ICP, and indeed there was no leaching of Au. In addition, the reusability tests showed that the reactivity did not obviously decline upon four reaction cycles (Figure 6). To our surprise, even after four cycles over 96 h, the structure and morphology of Au nanoparticles were largely maintained, and the average Au particle size only slightly increased from 4.5 to 5.4 nm (Figure 1E). Moreover, no obvious diffraction patterns of Au were detected from XRD, because the weak pattern of Au {111} at 38.3° overlapped with ZrO2. An XPS test also indicated the stable electronic structure of Au (Figure 2). These results unequivocally demonstrated a strong interface interaction between Au and ZrO2 species. However, a slight decrease in GVL yield was still observed, most probably due to the mild aggregation of Au nanoparticles.

Au/ZrO2. In the case of strongly acidic Amberlyst, the yield of GVL was only 1.5%, but abundant isopropyl ether was formed. Au/ZrO2 was probably poisoned by leached S species, as evidenced by an ICP test (up to 187 ppm). Regarding zeolites, strongly acidic Hmordenite and Hβ gave moderate GVL yields of ca. 45%, while only a 13.1% yield was obtained over the weakly acidic ZSM-48. Of the examined catalysts, medium-strength acidic ZSM-5 exhibited the highest yield of GVL, up to 77.5%, suggesting that ZSM-5 can synergistically work with Au/ZrO2 in the conversion of furfural to GVL. This synergistic effect can be better verified at low conversion by controlling the reaction time. As shown in Table S5 in the Supporting Information, the conversion of furfural over Au/ZrO2 + ZSM-5 was much higher than the sum of conversions with Au/ZrO2 and ZSM-5 for 0.5 h. In addition, GVL could not be produced over Au/ZrO2 or ZSM-5 alone (entries 1 and 11). The analogous Na form of ZSM-5, which did not exchange with NH4NO3 and retained large amounts of weakly acidic sites, was made to probe the intrinsic acidic sites. Reaction result showed that GVL yield sharply deceased to 7.3% over NaZSM-5. This was mainly ascribed to the lack of medium-strength acid sites. Apparently, medium-strength acid sites are the most active for the integrated conversion of furfural. In addition, the large surface area and perpendicularity of intersecting channels for ZSM-5 may facilitate the diffusion of reactants and products46 and thus favor the formation of GVL. Notably, 2-propanol also plays an important role in this process. In addition to acting as a reactant, 2-propanol is an excellent hydrogen source and solvent to suppress overhydrogenation and the formation of humins.33,36,37 To explore the reaction pathways of furfural conversion with 2propanol, the product distribution as a function of reaction time was investigated over Au/ZrO2 and ZSM-5 at 120 °C. As 2040

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ACS Catalysis Scheme 2. Reaction Pathway for the Production of GVL from Hemicellulose

Figure 6. Reusability test performed for Au/ZrO2 + ZSM-5 in the integrated conversion of furfural with 2-propanol into GVL. Reaction conditions: 0.2 g of Au/ZrO2, 0.2 g of ZSM-5, 0.5 mM furfural, 10 g of 2propanol, 120 °C, 24 h.

Figure 5. GVL yield as a function of reaction time in the leaching test. Reaction conditions: 0.2 g of Au/ZrO2, 0.2 g of ZSM-5, 0.5 mM furfural, 10 g of 2-propanol, 120 °C. The black profile denotes the normal reaction in furfural conversion, while the red profile represents the leaching test in which the catalysts have been removed from the system.

comparable to the best result.15 The unprecedented formation of furfural from hemicellulose can be mainly ascribed to the highly efficient ZSM-5 and inhibition of condensation reactions between furfural and intermediates owing to the solvent effect of GBL. Subsequently, the formed furfural was easily converted to GVL with a yield of 75.2% over Au/ZrO2 combined with ZSM-5 at 120 °C. The overall molar yield of GVL on the basis of the hemicellulose content was up to 61.5% (Table 2, entry 15), which was higher than that from a cellulose pathway.4,27−30,50 In comparison to previous methods, this process was totally heterogeneous and did not require high-pressure hydrogen or the neutralization of liquid acid.

3.3. Integrated Conversion of Hemicellulose to GVL. Inspired by the promising results obtained from furfural, we applied this methodology to the conversion of carbohydrate and biomass: that is, xylose and hemicellulose. An integrated two-step process was required for the high-yield production of GVL. In the first step of the conversion of xylose or hemicellulose, the addition of small amounts of bioderived γ-butyrolactone (GBL)49,50 could solubilize robust humins, restrain furfural degradation reactions, and greatly improve furfural yields (Table S6 in the Supporting Information). After the introduction of GBL, the furfural yield was remarkably increased from 47.7% to 75.3% in xylose dehydration over ZSM-5 at 160 °C. In a consecutive step, furfural underwent tandem conversion to produce GVL with 2-propanol over Au/ZrO2 and ZSM-5 hybrid catalysts. Thus, a total GVL yield of 54.9% was achieved from xylose (Table 2, entry 14). Regarding the conversion of hemicellulose (xylan from corn cob), ZSM-5 initially gave an 81.8% yield of furfural in a GBL− water−2-propanol mixed solvent at 170 °C, which was

4. CONCLUSION An 80.4% yield of GVL was successfully synthesized from the tandem conversion of furfural with 2-propanol over Au/ZrO2 combined with ZSM-5. Au/ZrO2 possessed strong interface interaction, along with well-dispersed Au nanoparticles, and ZSM-5 showed large amounts of medium-strength acid sites and intersecting channels, which worked synergistically to enhance 2041

DOI: 10.1021/acscatal.5b02882 ACS Catal. 2016, 6, 2035−2042

Research Article

ACS Catalysis

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the reactivity. Additionally, the hybrid catalyst was rather robust upon four reaction cycles, and no leached species were detected. More importantly, this catalytic system can be unitized to integrate conversion of hemicellulose into GVL via a two-step process. A 61.5% overall yield of GVL was obtained from hemicellulose. This provides a facile route to synthesize GVL under mild conditions without using liquid acid and external H2 supply.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.5b02882. Catalyst characterization (ICP, BET, XRD, CO2-TPD, NH3-TPD, XPS, and N2 adsorption/desorption isotherms) and partial reaction results (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for S.Z.: [email protected]. *E-mail for W.F.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Basic Research Development Program (973 Program) of China (2011CB201403), the National Natural Science Foundation of China (21403269, 21273264), and the Youth Innovation Promotion Association CAS (2015140).

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DOI: 10.1021/acscatal.5b02882 ACS Catal. 2016, 6, 2035−2042