ZrO2 mixture

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Total hydrogenation of furfural over Pd/Al2O3 and Ru/ ZrO2 mixture under mild conditions: essential role of tetrahydrofurfural as an intermediate and support effect Renjie Huang, Qianqian Cui, Qingqing Yuan, Haihong Wu, Yejun Guan, and Peng Wu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00801 • Publication Date (Web): 04 Apr 2018 Downloaded from http://pubs.acs.org on April 10, 2018

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ACS Sustainable Chemistry & Engineering

Total hydrogenation of furfural over Pd/Al2O3 and Ru/ZrO2 mixture under mild conditions: essential role of tetrahydrofurfural as an intermediate and support effect Renjie Huang, Qianqian Cui, Qingqing Yuan, Haihong Wu, Yejun Guan*, Peng Wu Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering, East China Normal University, North Zhongshan Road 3663, Shanghai, China *Corresponding author. Email: [email protected]

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Abstract The total hydrogenation of furfural (FAL) in aqueous solution was investigated under mild conditions over a series of physical mixture of Pd and Ru supported catalysts. Monometallic catalysts showed unfulfilled selectivity toward total hydrogenation: supported Pd catalysts tends to give a mixture of tetrahydrofufural (THFAL), furfuryl alcohol (FA), and tetrahydrofurfuryl alcohol (THFA) while supported Ru catalysts is apt to hydrogenate C=O bond. When supported Pd and Ru catalysts were mixed, the yield of THFA was significantly improved. A support-dependent promotion effect was noticed, in particular, Pd/Al2O3 and Ru/ZrO2 mixture gave 99% yield of THFA at 30 °C and 0.5 MPa H2. Detailed kinetics studies suggest that the reaction pathway with THFAL as the intermediate could be involved in this process. The apparent activation energy of furan hydrogenation over Pd/Al2O3 and the subsequent C=O bond hydrogenation over Ru/ZrO2 is about 21 and 30 kJ/mol, respectively. The low activation energies may well explain the observed activity of total hydrogenation of FAL over bimetallic mixtures. This two-step reaction process was also investigated in a fixed-bed reactor with two-catalyst bed, which gave stable THFA yield and shows promising industrial application.

Keywords: bimetal catalyst, synergy, biomass, furanic compound, tetrahydrofurfuryl alcohol

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Introduction Global warming and depletion of fossil resources are two major issues concerning the sustainable development.1 Biomass and its derivatives, as an abundant and inexpensive carbon source for the production of renewable bio-fuels and high value-added compounds, thus attract tremendous attentions.2-5 As a biomass-derived chemical, furfural (FAL) is predominantly produced by the hydrolysis and dehydration of xylan in lignocellulose.6-11 It has been considered to be a platform chemical for a series of chemical intermediates such as furfuryl alcohol (FA), furan, tetrahydrofuran, tetrahydrofurfuryl alcohol (THFA), and etc.12-21 These processes generally involve hydrogenation, hydrogenolysis, and decarbonylation reactions. THFA has been widely used as green solvent in agricultural application, printing ink, industrial and electronics cleaner due to its low toxicity and biodegradable nature.22 More recently, THFA has been also regarded as a precursor of diols, i.e. 1,5-pentanediol, which can be used as a monomer for polyester or polyurethane production.23-25 Therefore the preparation of THFA has attracted much attention.26-31 Table 1 summarize the recent findings on hydrogenation of furfural over mono or bimetal catalysts containing Cu, Ni, Pd, Ru or their alloys in batch reactor.22 THFA is conventionally produced through two steps. In the first step, FAL is hydrogenated to FA over Cu-Cr or Ni catalysts.26-28 Then the FA is hydrogenated to THFA over Ni or noble metal catalysts.29-31 These processes gave THFA yield ranging from 90% to 99%, whereas high temperature about 100-150 oC and H2 pressure ranging from 1-4 MPa were required.32-38 The high temperature employed in this process may lead to 3



coke deposition and catalyst deactivation due to the polymerization of FA. Therefore, a few studies have been devoted in the direct hydrogenation of FAL into THFA under mild conditions.39-42 Recently Pd based catalysts and alloys have been found to be able to catalyze the total hydrogenation at or close to room temperature. As an example, hydroxyapatite-supported Pd catalyst could catalyze the total hydrogenation of FAL in 2-propanol with 100% conversion and 100% THFA yield.41 The hydroxyapatite support may contribute to the dispersion of Pd nanoparticles and thus leading to better activation of hydrogen. Pd-Pt nanoalloys could even provide 95% yield of THFA under 30 °C, 3 bar H2 in 4 h in 2-propanol.42 The outstanding catalytic activity of PdPt alloy was a result of an electronic promotional effect from the synergistic combination of Pd and Pt. Herein, we systematically studied the total hydrogenation of FAL over a series of physically mixed Pd and Ru supported catalysts in aqueous solution under mild conditions (30 °C, 0.5 MPa H2). The results showed that a mixture of Pd/Al2O3 and Ru/ZrO2 could lead to THFA yield with 99% under the tested reaction conditions. By carefully investigating the reaction mechanisms of FAL to THFA, we propose that the reaction pathway with tetrahydrofurfural (THFAL) as the intermediate (pathway 2 in Scheme 1) could be more promising than pathway 1 that through FA as intermediate. This conclusion is based on the finding that the activation energy of THFAL formation over Pd/Al2O3 is much lower than FA formation under the reaction conditions. Moreover the subsequent hydrogenation of C=O bond in THFAL over Ru/ZrO2 also has lower energy barrier than that of FA, thus leading to excellent total 4


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hydrogenation of FAL under mild conditions. This two-step reaction process has also been investigated in a fixed-bed reactor with two-catalysts bed, which gives stable THFA yield and shows promising industrial application. Experimental Material synthesis and catalyst preparation SiO2, ZrO2 and Al2O3 were used in the purchased forms. Titanate nanotubes (TNT) and UiO-66 were prepared according to the references.43,44 For TNT, commercial anatase-type TiO2 (2 g) was added to 100 mL of NaOH aqueous solution (10 M). After being stirred for 10 min, the mixture was transferred into a Teflon-lined (120 mL) stainless steel autoclave and statically heated in an oven at 130 ◦C for 72 h. The white product was filtered and washed with large amount of deionized water until the pH was 7. The final products were subsequently dried at 110 ◦C overnight. For UiO-66 (Zr), ZrCl4 (0.265 g) and 1, 4-benzenedicarboxylic acid (H2BDC) (0.264 g) were dissolved in DMF (35 mL) at room temperature. The resulting mixture was placed in a Teflon-lined autoclave in a preheated oven at 120 °C for 3 days. After the solution was cooled to room temperature, the precipitate was filtered and repeatedly washed with absolute ethanol for 3 days while heated at 60 °C in an oil bath. The resulting powder was filtered, and dried under vacuum at 50 °C. The supported Pd catalysts were prepared by a deposition-reduction method. Initially, the support (0.5 g) was diluted in H2O (60 mL) with stirring. A specified amount of H2PdCl4 aqueous solution (21.512 gPd/L) was added to the mixture and stirred for 3 h. The final pH value of suspension was adjusted to 10 by adding NaOH aqueous 5



solution (1 M). The Pd precursor was reduced with NaBH4 (NaBH4/Pd = 10, molar ratio) and followed with stirring for another 30 min for the full reduction of Pd2+ species. The resulting powder was filtered, and dried at 80 °C overnight. Ru nanoparticles were loaded with the same method with RuCl3 as the precursor. Catalysts characterization The powder X-ray diffraction (XRD) patterns were collected on a Rigaku Ultima IV X-ray diffractometer using Cu Kα radiation (λ = 1.5405 Å) operated at 35 kV and 25 mA. Liquid nitrogen adsorption was used to determine the BET surface areas and pore volumes on the BELSORP-Max equipment. Prior to the adsorption measurements, the samples were degassed in situ under vacuum at 300 °C for 6 h. Ru/Metal–organic frameworks were degassed in situ under vacuum at 150 °C for 6 h. Transmission electron microscopy (TEM) images were taken on a FEI Tecnai G2 F30 microscope operated at 300 kV. Pulse CO chemisorption was performed on a Micromeritics AutoChem II 2920 to determine the metal dispersion of the catalysts. Prior to measurement, the catalyst was reduced in a flow of 80 mL/min 10 vol.% H2 in Ar at 200 °C for 2 h. After cooled to 40 °C in He, the CO gas pluses (5 vol.% in He) were introduced in a flow of 110 mL/min. The CO gas phase concentration was monitored by thermal conductivity detector (TCD). The Pd and Ru loading were quantified by inductively coupled plasma (ICP) on the Thermo IRIS Intrepid II XSP atomic emission spectrometer. The supported Pd catalysts were digested using aqua regia while supported Ru catalysts were digested by hydrochloric acid with a trace of hydrogen peroxide, respectively. The obtained solutions were diluted with deionized 6


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water before measurement. Catalytic activity measurements Reactions in the batch reactor were carried out in a Teflon-lined (120 mL) steel batch reactor. As-prepared catalysts were loaded into the reactor with 9.9 mL of deionized water and 100 µL of substrate. No pretreatment on the catalysts was conducted prior to reaction. After purged five times with H2, the reactor was pressurized with 0.5 MPa of H2. The mixture was stirred in an oil bath at 30 °C. The catalytic hydrogenation reaction on a fixed-bed reactor was conducted with a continuous flow of H2 (30 mL/min) and H2 pressure of 1 MPa. In a typical experiment, 1.5 g catalysts (sieved to 40 to 60 mesh) were packed in the central portion of the reactor tube to avoid any temperature gradients, while the top and bottom portions were filled with silica beads. When the reactor temperature is stable the aqueous solution of FAL (1 vol.%) was introduced into the reactor with Series pump at a certain flow rate. The liquid phase product was collected in the gas-liquid separator at room temperature. It should be noted that no significant formation of volatile species occurs since the reaction is carried out under mild conditions. The hydrogenation products were periodically withdrawn from the gas-liquid separator. All products were diluted with ethanol and analyzed with a Tianmei 7900 GC equipped with a DM-FFAP capillary column (30 m length, 0.25 µL film thickness and 0.25 mm internal diameter). N2 was used as carrier gas. Conversion of FAL and product selectivity are defined as below. Conversion = (total mol products/introduced mol reactant)×100%

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(1)


Selectivity = (mol product/total mol pruducts)×100%,

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(2)

where total products include THFAL, THFA and FA. We have checked some of the reactions by using cyclohexanol as internal standard. The carbon balances ranged between 95% to 105%. The apparent activation energies (Ea) of the hydrogenation reactions over Pd or Ru catalysts were calculated based on the TOFs at different temperatures. TOF = (n×X)/(N×t ),

(3)

where n is the initial mole of substrate, X is the conversion of substrate, N is the mole of supported metal, and t is the reaction time. Results and discussion Catalyst characterization The structural properties, including the BET specific surface areas (SSA) and total pore volume (Vtotal) were determined by N2 physisorption. The results are listed in Table 1. The specific surface area of Pd/Al2O3, Pd/SiO2, Pd/ZrO2 and Pd/TNT is 222, 188, 16 and 159 m2/g, respectively. The specific surface of Ru/Al2O3, Ru/SiO2, Ru/ZrO2 and Ru/UiO-66 is 219, 208, 13 and 510 m2/g. Pd/TNT and Ru/UiO-66 were selected because they showed good activity in hydrogenation of FA and FAL, respectively, according to our previous studies17, 43. The Pd and Ru loading on all catalysts were close to 1.0 and 2.0 wt.%, respectively. In the case of Ru/UiO-66, substantially higher Ru loading (2.7 wt.%) was noticed probably because some UiO-66 materials were decomposed during the preparation. Fig. 1 displays the XRD patterns of supported Pd (Fig. 1A) and supported Ru (Fig. 1B) catalysts. The 8


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diffraction peaks of each catalyst are assigned to its corresponding supports and no visible diffractions due to Pd or Ru were observed, meaning that both Pd and Ru are highly dispersed on supports. The metal dispersion and particle size were further characterized by CO chemisorption and TEM. Pd/Al2O3 gave the highest dispersion of 38% and the Pd particle size was estimated to be about 3 nm. Pd supported on other oxides showed similar dispersion around 8%, with particle size about 13 nm. The high Pd dispersion on alumina is likely due to the interaction between PdCl4- and surface Al-OH groups.45 The promotion role of Al2O3 in supporting metal nanoparticle is also applied for Ru catalyst. Ru on Al2O3 gave dispersion of about 61%, which is 3 times higher than that on SiO2 (18%), ZrO2 (14%) or UiO-66 (20%). The TEM images shown in Figure 2 suggest that highly dispersed Pd or Ru nanoparticles are clearly observed on these supports. Hydrogenation of furfural over mono-metal catalysts We firstly investigated the performance of supported mono-metal catalysts in the hydrogenation of FAL. Table 2 shows the FAL conversion and product distribution over 50 mg of supported Pd catalysts at 30 oC and 0.5 MPa H2. After reaction for 3 h, the FAL conversion increases with the order of Pd/ZrO2 (34%) < Pd/SiO2 (63%) < Pd/TNT (85%) < Pd/Al2O3 (100%). The main product on these catalysts were THFAL, with selectivity ranging from 40% to 84% depending on the nature of support. For Pd/ZrO2 the THFAL selectivity was about 59%, and the rest compounds are THFA and FA, with selectivity being 16% and 25%, respectively (Table 2, Entry 1). Very similar product distribution to that over Pd/ZrO2 was obtained in case of 9



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Pd/TNT (Table 2, entry 3), which gave 56% selectivity to THFAL. Though Pd/TNT is highly active in total hydrogenation of FA43, it is not selective for the total hydrogenation of FAL. Pd/SiO2 catalyst gave equivalent selectivity of THFAL (40%) and FA (39%). Among these catalysts, Pd/Al2O3 gave the highest selectivity of 84% to THFAL while no FA was formed. It should be noted that on monometallic Pd catalysts, the selectivity of FA and THFA attributed to C=O reduction is rather low. By contrast, they showed very high activity in furan ring hydrogenation, which is consistent with its unique performance in C=C bond reduction.40, 42, 46 Table 3 compares the catalytic performance of supported Ru catalysts in FAL hydrogenation. 100 mg of Ru catalyst was used while keeping the temperature and H2 pressure similar to that for Pd catalyst. After reaction for 4 h, the FAL conversion follows the trend of Ru/Al2O3 (23%) < Ru/ZrO2 (43%) ≈ Ru/UiO-66 (45%) < Ru/SiO2 (50%). Compared with Pd, Ru is a well-known good catalyst in the hydrogenation of C=O group.47,48 In consistent with this observation, all catalysts tested in this study gave FA selectivity higher than 95%. It is worthy of mentioning that Ru/ZrO2 shows good performance in FAL hydrogenation irrespective of its low Ru dispersion. This behavior is likely due to the unique property of ZrO2 in enhancing the hydrogenation activity of Ru nanoparticles toward C=O bond.49 The above results suggest that saturation of furan ring or C=O bond can be selectively catalyzed by supported Pd or Ru nanoparticles under mild conditions, respectively. The selective hydrogenation of FAL has been extensively studied, in particularly the production of FA, where selective reduction of C=O bond is preferred.50, 10


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By contrast, total

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hydrogenation of FAL to THFA still remains a challenging task under mild reaction conditions for monometallic catalysts. Hydrogenation activity over physically mixed supported Pd and Ru catalysts Because of the poor selectivity towards THFA over monometallic nanoparticles, several studies tried to increase the THFA yield by using multi-metal catalysts.39,46 The results suggested that catalysts containing multi-metal alloy showed enhanced activity in THFA production. By contrast, the physically mixed monometallic catalysts were not very efficient in total hydrogenation. We herein also tested the catalytic performance of mixtures of a series of Ru catalysts and Pd catalysts under mild conditions. Table 4 shows the catalytic performance of Pd/Al2O3 and different Ru catalysts. Almost full conversion of FAL was achieved for all mixtures. The presence of Ru catalysts did not result in noticeable formation of FA as observed for monometallic Ru catalyst. Interestingly, the yield of THFA was remarkably increased and the THFA yield over Pd/Al2O3 and Ru/SiO2 was 41%, which is about 2.5 times of that over monometallic Pd/Al2O3 catalyst. This improvement was even more pronounced for the mixture of Pd/Al2O3 and Ru/ZrO2, in which case the THFA yield of 80% was achieved. This result is different to the previous finding that the Pd/SiO2 and Ir/SiO2 mixture did not show significant enhancement as reported by Nakagawa et al.39 We surmise that the nature of the support plays a determining role in this synergetic effect, which may affect the metal-support interaction and chemoselective hydrogenation activity51. Next, we investigated the catalytic performance of Ru/ZrO2 catalyst mixed with Pd 11



nanoparticles supported on various oxides, namely, ZrO2, SiO2, TNT, and Al2O3. The results in Table 5 show that four combinations all gave FAL conversion higher than 87% after reaction for 3 h. The selectivity of THFA follows the trend of Pd/ZrO2

ZrO2 mixture

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