Role of MoO3 on a Rhodium Catalyst in the Selective Hydrogenolysis

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Role of MoO3 on a Rhodium Catalyst in the Selective Hydrogenolysis of Biomass-Derived Tetrahydrofurfuryl Alcohol into 1,5-Pentanediol Jing Guan,† Gongming Peng,† Quan Cao,†,‡ and Xindong Mu*,† †

CAS Key Laboratory of Bio-Based Materials, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, Shandong China ‡ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: Selective hydrogenolysis of biomass-derived tetrahydrofurfuryl alcohol (THFA) to produce 1,5-pentanediol (1,5-PeD) is accomplished by a binary catalyst consisting of MoO3 and supported Rh nanoparticles; a 1,5-PeD selectivity up to 80% is achieved in the present work. Moreover, a very interesting phase-transfer behavior for MoO3 during the reaction is observed with the assistance of different characterization techniques. In this process, MoO3 dissolves partially in the liquid phase under the reaction conditions and is transformed into the soluble hydrogen molybdenum oxide bronzes (HxMoO3) in the presence of H2, which are recognized as the genuinely active sites for the C−O bond breaking of THFA. Density functional theory (DFT) calculations were then carried out to simulate the plausible mechanisms and highlight the role of Mo in the ring-opening process of THFA in more detail. We propose that the formation of 1,5-PeD takes place in a two consecutive reactions. THFA first undergoes acid-catalyzed ring-opening process to form the key intermediate 5-hydroxypentanal with the homogeneous catalysis of dissolved HxMoO3. The intermediate is then quickly hydrogenated into 1,5-PeD under the heterogeneous catalysis of Rh. The concerted “hydrogen-transfer−ring-opening” mechanism plausibly explains the high reaction selectivity toward 1,5-PeD in the hydrogenolysis of THFA and is verified by the reactivity trends of related substrates.

1. INTRODUCTION The production of fuels and chemicals from renewable biomass resources is a promising approach for meeting the energy and environmental challenges caused by diminishing fossil resources and increasing greenhouse gas emissions.1−3 However, the overfunctionalized biomass-derived molecules with usually 20−50 wt % oxygen content4 require deoxygenation for further use to obtain conventional chemicals, such as hydrocarbons or partially oxygenated hydrocarbons. Hydrodeoxygenation (HDO) is one of the most potentially valuable processing routes to selectively cleave C−O bonds in oxygen-containing compounds without reducing the number of carbon atoms. The key challenge faced by the development of HDO catalysts is consuming less H2 while maintaining high stability under HDO conditions. In this respect, two reducible transition-metal oxides, in particular MoO35−8 and WO3,9,10 have been recently reported to be attractive and effective catalysts for the selective scission of the C−O and CO bonds in various biomass-derived oxygenates, including aliphatic and cyclic ketones, esters, furanics, and phenolic compounds. When the traditional transition-metal catalysts such as Pt, Pd, Ru, Rh, Ni, Cu,5,11−15 metal sulfides,16,17 and metal carbides18−20 are compared to MoO3 and WO3, it becomes clear that MoO3 and WO3 are unique catalysts possessing all of the desirable properties toward C−O bond scission while leaving the carbon chain intact in the HDO products. © XXXX American Chemical Society

Of various hydrodeoxygenation reactions, selective C−O hydrogenolysis of polyols and cyclic ethers to their corresponding α,ω-diols represents an important class of reactions. Diols, especially α,ω-diols, are widely used as monomers to produce polyesters and polyurethanes. An excellent review by Schlaf covers both hetero- and homogeneous hydrogenolysis of C3−C6 polyols, with a focus on the methods for the manufacture of α,ωdiols from selective cleavage of secondary OH groups.21 The first efforts toward the direct hydrogenolysis of tetrahydrofurfuryl alcohol (THFA) to 1,5-pentanediol (1,5-PeD) with high chemoselectivity were made by Tomishige et al. in 2009.22,23 In their pioneering research, Rh/SiO2 modified with oxophilic promoter ReOx or MoOx was used, with the maximum yield of 1,5-PeD reaching 86% on Rh−ReOx/SiO2.22 In the following years, other systems, such as Re and Mo promoters combined with highly reducible Rh/C as well as Ir/SiO2, have also been applied for this process,24−32 in which etheric bond cleavage occurred primarily at more substituted C−O junctures. For example, the hydrogenolysis of THFA over Rh−ReOx/C and Rh−MoOx/C catalysts was studied by Dumesic et al., and the selectivity to the 1,5-PeD product was up to 97%.31 Even more Received: August 18, 2014 Revised: September 30, 2014

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dx.doi.org/10.1021/jp508313y | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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composition and acidity. The likely catalytically active species is the dissolved H0.9MoO3 in the liquid phase, and a cascade reaction pathway under the cooperation between the homogeneous HxMoO3 and heterogeneous Rh/C catalysts was proposed. DFT calculations were performed herein to examine the C−O bond breaking of THFA, which is likely to take place through a concerted “hydrogen-transfer−ring-opening” process that stabilizes 5-hydroxypentanal formation. The mechanistic insights from this work confirmed the bifunctional role of metal catalysts integrated with Mo additives for hydrogenolysis reactions and may be extended to guide the design of more efficient heterogeneous catalysts for the direct deoxygenation of biomass to diols.

recently, Ir-MoOx was reported to be an effective catalyst for C−O cleavage of THFA, producing 1, 5-PeD with selectivities exceeding 70%.32 Despite the growing body of experimental studies in the hydrogenolysis of THFA over metal catalyst promoted with highly oxophilic additives (MoOx), the attempts aimed at clarifying the structure of MoOx catalyst and the mechanistic insights devoted to explaining the high selectivity of the C−O bond hydrogenolysis are still limited. Tomishige’s group proposed the formation of low-valent MoOx species in synergy with the Rh metal surface and postulated a direct C−O bond hydrogenolysis mechanism based on the reactivity of related substrate, kinetic analyses, and deuterium-label experiments. The substrate is first adsorbed on the electron-deficient rhenium atom through the −CH2OH group to form terminal alkoxide. Then, a hydride species activated on the Rh or Ir metal reacts directly with the 2-position of the alkoxide to break the C−O bond via a SN2 reaction. Hydrolysis of the reduced alkoxide releases the product.25 On the other hand, the research group of Dumesic investigated the catalysis of Rh−ReOx/C and Rh−MoOx/C for THFA hydrogenolysis and proposed another mechanism.31 It involves the protonation of the less-accessible oxygen atom of the THFA ring by an acidic OH group of ReOx or MoOx, followed by a concerted hydride transfer from the neighboring primary alcoholic carbon atom to form a stable RCH2−CHOH+ oxocarbenium intermediate, which leads to C−O cleavage to open the ring. This mechanism plausibly explains the reaction selectivity and was also supported by density functional theory (DFT) calculations.31,33 In addition, a few experimental and theoretical studies have been conducted to address the nature of the active Mo species as well as the mechanisms underlying the catalytic effect of MoO3 in the HDO of biomass-derived model oxygenates. Different conclusions have been drawn, and different mechanistic explanations have been described. For example, Prasomsri and Román-Leshkov6,7 reported that the oxygen vacancies on the hydrogen molybdenum bronze HxMoO3 (where x is nonstoichiometric) are assumed to play a key role in breaking the C−O bond of various lignin-derived model compounds and converting phenolics to aromatic hydrocarbons without saturating the aromatic ring. Other reports showed that a molybdenum suboxide MoO3−x is responsible for the active site in the HDO of acetaldehyde.34,35 In spite of these advances in the study of the function of Mo species, the hydrogenolysis reaction for biomass derivatives with oxygenated ligands is pretty complex, and more than one type of reaction pathway can be involved simultaneously even on the same catalyst, given the presence of multiple oxygen-containing functional groups.28,36,37 In this context, the key catalytic imperative is to clarify unambiguously the structure of the active site and further the reaction network of C−O bond hydrogenolysis. In this contribution we focus our efforts toward the direct catalytic transformation of THFA to 1,5-PeD. We first present a screening of catalysts for obtaining a high activity and stability consisting of a molybdenum-based component integrated with a Rh component. Among various binary catalysts, a combination of Rh/SiO2 and MoO3 gives the highest selectivity (80%) in the hydrogenolysis of C−O bond. Mechanistic concepts are then formulated regarding the nature of the active site for this catalyst system. In this respect, NH3-temperature-programmed desorption (TPD) profiles, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and infrared (IR) examinations have been performed on both fresh and H2-pretreated MoO3 catalysts to elucidate their

2. METHODS 2.1. Catalyst Preparation. Rh/SiO2 catalyst was prepared by impregnation of 10 g of SiO2 support (Qingdao Haiyang Chemical Co., Ltd., 80−120 mesh, 346 m2 g−1) with an aqueous solution of 1.02 g of RhCl3·3H2O (Shanghai Jiuling Chemical Co., Ltd.). After the impregnation procedure, the catalyst was dried and then calcined at 500 °C. The loading amount of Rh was 4 wt %. Rh−MoOx/SiO2 catalysts were prepared according to the procedure described by Koso et al:21 10 g of silica support was sequentially impregnated with 100 mL of aqueous solutions containing 1.02 g of RhCl3·3H2O and 0.017 g of (NH4)6Mo7O24·4H2O (Sinopharm Chemical Reagent Co., Ltd.), followed by a drying procedure. The catalysts were calcined in air at 500 °C after drying. The loading amount of Rh was 4 wt %, and the ratio of Mo to Rh was 0.13 in molar bases. Commercial Rh/C catalyst with 5 wt % loading amount of Rh was provided by the Shanxi Kaida Chemical Engineering Co., Ltd. Molybdenum trioxide (MoO3) was obtained from Tianjin Kemiou Chemical Reagent Co., Ltd. Heteropolyacids (H3PMo12O40, H3PW12O40, H4SiW12O40) and H2MoO4 samples were supplied by the Sinopharm Chemical Reagent Co., Ltd. In the binary catalyst, the two functional components of supported Rh catalyst and Mo compounds were combined just by physical mixing. 2.2. Catalytic Reaction. All hydrogenolysis experiments of THFA (Zibo Huaao Chemical. Co., Ltd.) were carried out in a 100 mL stainless-steel autoclave. For each reaction, THFA, water, and an appropriate amount of catalyst were charged into the 100 mL autoclave together, which was purged three times and then heated to the reaction temperature (120 °C) and pressurized to 6 MPa with H2. After an appropriate reaction time, the reactor was cooled to room temperature. The liquid phase was diluted with 5 mL of methanol, and the products were further used for GC analyses, which were carried out on a VARIAN 450-GC equipped with a DB-FFAP capillary column. The internal standard (1,4-butanediol) method was used for the quantitative determination of the aqueous phase. The used catalysts were recovered after each run by centrifugation, washed with excess water, dried in air, and then put into the reactor for the next run. 2.3. Catalyst Characterization. The powder X-ray diffraction patterns of the MoO3 catalyst and reduced MoO3 at 120 °C by flowing H2 with pressure of 6 MPa were performed by using a Bruker D8 ADVANCE diffractometer under Ni-filtered CuKα (λ= 0.154 nm) radiation. A Thermo Nicolet FTIR Spectrometer was used to obtain the Fourier transformed infrared (FTIR) spectra of the parent and reduced MoO3 samples. Temperature-programmed desorption of preadsorbed NH3 was carried out with a Micromeritics Autochem II chemisorption B



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Rh−HxMoO3 catalyst, we have added a Rh atom in proximity of the −OH2 group on the HxMoO3 surface, which allows adsorption of hydrogen (Figure 1b). The first-principles calculations have been carried out within the generalized gradient approximation (GGA) with the Perdew−Burke−Ernzerhof (PBE)44 functional for the exchange−correlation term. The electron−ion interaction was described by the projector augmented wave (PAW) method.45,46 A (3 × 3 × 1) Monkhorst−Pack47 k-point sampling was used; the energy cutoff was 400 eV. Spin-polarization was considered for all calculations. The convergences of energy and forces were set to 1 × 10−4 eV and 0.03 eV/Å, respectively. The transition states were searched using the nudged-elastic band (NEB)48 method to calculate the minimum energy profiles along the prescribed reaction pathways with the initial and final states chosen based on the optimized structures. The theoretical method employed in the present study was demonstrated to be accurate in describing the structural and energetic properties for HxMoO3.49,50

analyzer. X-ray photoelectron spectroscopy (XPS) was used to study the chemical composition and the oxidation state of the elements on the catalyst surface. The data was obtained using a Thermo ESCALAB250 instrument with a monochromatized Al Kα line source (Al Kα = 1486.6 eV). All the binding energies were referenced to the C 1s peak at 284.6 eV. 2.4. Periodic DFT Calculations. Periodic DFT calculations were performed using the Vienna Ab-initio simulation package (VASP).38,39 As demonstrated further below, our powder XRD investigation indicates that reduction of MoO3 occurs upon exposure to H2 at 120 °C to form a H0.9MoO3 phase serving as the possible active species. To date, the exact structure of hydrogen molybdenum bronzes has not been definitively established. In particular, the locations of the inserted hydrogen atoms and their interaction with the host lattice oxygens are not completely understood.40−42 The previous structural determination of H0.9MoO3 phase indicates that the protons mainly form OH2 groups with the terminal oxygen atoms of the MoO6 octahedron around Mo. In the present study, the most stable orthorhombic crystalline structure of bulk MoO3 was adopted.43 The surface was described using a slab model that consists of two bilayers of MoO3(010), with the bottom layer fixed and the top layer and the adsorbates allowed to relax. A supercell containing one (3 × 2) MoO3(010) slab, which is large enough to accommodate surface reactive species, and 15 Å vacuum space between adjacent slabs was employed. After initial geometry optimization of the MoO3(010) slab, subsequent calculations were performed on the surface structures of the metastable bronze compounds. To reproduce the experimental structure of H0.9MoO3 phase, the intercalated hydrogen atoms were adsorbed on each terminal oxygen (Ot) atom in the middle of the MoO3 first layer, and no hydrogen was placed in the remaining bottom layers (Figure 1a). We assumed that the

3. RESULTS AND DISCUSSION 3.1. Effect of Water on Catalytic Performance of Rh−MoOx/SiO2. As previously documented,22−30 modification of Rh catalyst with MoOx species has greatly promoted the selective cleavage of C−O bond in THFA in terms of good combinations of activity and selectivity. Although water is proved to be the most suitable solvent, and the catalyst worked effectively over a wide range of reactant concentrations from 5 wt % THFA aqueous solution up to 60%,23 much remains to be learned about the role of water in this one-pot reaction. In this respect, the influences of mass ratios of reactant to water on the selectivity of 1,5-PeD in the catalytic conversion of THFA over Rh−MoOx/SiO2 were evaluated and are summarized in Table 1. In this experiment, the total amount of THFA and water was fixed. With various high THFA:H2O ratios, it was found that the THFA conversion and product selectivity are sensitive to the ratio of water. The catalysts work in low content of water not as efficiently as those of high content. With the increase of the THFA:water ratio from 1:1 to 3:1, the conversions of the reactant, the yields of 1,5-PeD and 1-pentanol (1-PeOH), are reduced gradually, but the selectivity of 1,5-PeD does not change significantly, which reaches a maximum of 80% at 1:1 THFA aqueous solution. However, in the case of the hydrogenolysis of neat THFA without solvent (THFA:H2O = 1:0), the selectivity of 1,5-PeD drops rapidly to 50%, whereas a clear increase is observed for the 1-PeOH yield. These observations allow us to speculate that an appropriate source of water is needed to enhance the conversions of THFA in the selective production of 1,5-PeD. Additionally, the conversion rate of THFA indicates that high ratio of water accelerates the hydrogenolysis reactions in the solutions of different THFA concentrations.

Figure 1. DFT-calculated model structure of HxMoO3 and bifunctional Rh−HxMoO3 surface.

absence of protons in the bottom oxygen layers has a negligible effect on the interaction between the surface reactive species and the substrate. To model the bifunctional role of the binary

Table 1. Rh−MoOx/SiO2 Catalyzed Hydrogenolysis of THFA into 1,5-PeDa sel. (%) H2 (MPa)

THFA/H2O (wt %/wt %)

conv. (%)

1,5-PeD

1-PeOH

conv. rateb (gTHFA gcat−1 h−1)

4 6 6 6

1:1 1:1 3:1 1:0

12.6 22.5 13.5 5.8

80.2 80.0 79.2 50

11.1 8.0 8.9 29.3

0.23 0.41 0.36 0.21

a Conv. = conversion; sel. = selectivity. Reaction conditions: 120 °C; Rh−MoOx/SiO2 (Rh, 4 wt %), 0.7 g (nMo/nRh = 0.13); reaction time, 20 h; total mass of THFA solution, 50 g. bRate is calculated from the mass of tetrahydrofurfuryl alcohol converted per gram of catalyst per hour. The same method is used in the following calculations.

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Table 2. Hydrogenolysis of THFA over the Binary Catalyst Composed of MoO3 with Rh/SiO2a sel. (%)

a

entry

catalyst

conv. (%)

1,5-PeD

1-PeOH

conv. rate (gTHFA gcat−1 h−1)

1 2 3

4% Rh/SiO2 (2 g) + MoO3 (0.02 g) 4% Rh/SiO2 (2 g) + MoO3 (0.2 g) 4% Rh/SiO2 (2 g) + MoO3 (0.5 g)

2.6 27.9 33.0

80.6 80.3 78.3

11.5 12.5 11.2

0.016 0.16 0.17

120 °C; 6 MPa H2; reaction time, 20 h; total mass of THFA solution, 50 g; THFA concentration, 50 wt %.

Table 3. Recycling of Rh/SiO2 and MoO3 Binary Catalysta

Mechanistically, a possible explanation would be that water can contribute to construct and maintain the active species, such as Mo−OH, that act as Brønsted sites, and helps to stabilize the transition state via hydrogen bonding. Similarly, Dumesic et al. observed a decreased activation barrier in the modeling of ringopening process of THFA with water as a result of waterstabilized oxocarbenium ion transition state.25 Tomishige has also investigated the solvent effect in the hydrogenolysis activity of C−O bond over Ir-ReOx/SiO2 catalyst and proposed that the direct mechanism which is driven by the active hydride-like species works better in water than in alkane solvent.51 Considering H2O is a byproduct of the THFA hydrogenolysis, this property is promising. 3.2. Catalytic Performance of Rh/SiO2 and MoO3 Binary System. However, the limitations of the MoOx-promoted Rh/SiO2 catalysts for practical applications are also recognized regarding the stability of the catalyst because potential leaching of Mo species from the catalyst under aqueous reaction environments could take place. To achieve a simpler method of catalyst preparation and to optimize the catalyst stability caused by catalyst deactivation, a binary catalyst combining MoO3 and Rh/SiO2 was synthesized and tested in the hydrogenolysis reaction of THFA. This approach is advantageous over previous protocol from this laboratory and others in many ways: (1) easy preparation of the catalyst, (2) a decay in the hydrogenolysis activity caused by leaching Mo species could be overcome by adding MoO3 into the reactor directly, and (3) the two functional sites can be tunable in the binary catalyst. We investigated a series of binary catalysts composed of MoO3 and Rh/SiO2 in various relative amounts. As shown in Table 2, the conversion of THFA and the selectivity of 1,5-PeD are strongly dependent on the relative amounts of Rh and MoO3. The THFA conversion and the 1,5-PeD yield are gradually increased with the increasing amount of MoO3 from 0.02 to 0.5 g if the amount of Rh/SiO2 is fixed at 2 g. The yield of 1,5-PeD reaches the highest value of 25.8% at 0.5 g MoO3. However, the selectivity of 1,5-PeD goes down slightly, and the trends can be explained by the progress of overhydrogenolysis to give 1-pentanol accompanied by the higher conversion. It is also noted that when the MoO3 amount is increased from 0.02 to 0.2 g, the conversion of THFA increases dramatically from 2.6% to 27.9%, indicating that MoO3 could promote significantly the hydrogenolysis of THFA. Furthermore, the combination catalyst of Rh/SiO2 and MoO3 manifests excellent reusability; it could be used more than four times without remarkable loss in the 1,5-PeD yield, and the average 1,5-PeD yield is over 26% in the fourth run (Table 3). Further evidence that the reducible metal oxides can promote the catalytic properties of metal sites has been reported in the hydrogenolysis of cellulose to produce ethylene glycol (EG) over tungsten-based catalysts. Zhang and co-workers found that the combination of WO3 with Rh/C catalyst led to the preferential conversion of cellulose to EG compared to its coreduction bimetal counterpart, more likely involving the dissolved tungsten bronze (HxWO3) active species.52,53 By taking advantage of the

sel. (%) times

conv. (%)

1,5-PeD

1-PeOH

1 2 3 4

32.5 32.6 32.8 33.0

79.3 80.1 78.1 79.5

11.4 10.7 12.2 11.8

Reaction conditions: 120 °C; reaction time, 20 h; Rh/SiO2, 2 g (Rh, 4 wt %); MoO3, 0.5 g; H2, 6 MPa; THFA concentration, 50 wt %; total mass of THFA solution, 50 g. a

unique properties of WO3, Liu et al. also reported that the presence of WO3 in the Ru/C catalyst can largely accelerate the hydrolysis of cellulose to sugar intermediates and more significantly improve the selective cleavage of the C−C bonds in these sugars to target product, which possibly takes place by complexation of the sugar intermediates with WO3 crystallite and subsequent rearrangement of C−C bonds of the sugars.54 3.3. Catalytic Performance of Rh/C Combined with Various Mo Species. We subsequently investigate the catalytic performance of Rh/C integrated with various Mo or W species, which included heteropolyacids (H3PMo12O40, H3PW12O40, H4SiW12O40), MoO3, WO3, and H2MoO4, with the preliminary results listed in Table 4. It can be seen that 1,5-PeD remains the predominant product regardless of which Mo (or W) compounds is used in the binary catalyst, except for H2MoO4 additives which shows 1-PeOH as major product. Interestingly, the 1,5-PeD yields diminish in the following trend: H3PMo12O40 > H3PW12O40 > H4SiW12O40 > H2MoO4. H3PMo12O40 gives rise to a slightly higher 1,5-PeD yield and selectivity comparative to that on MoO3. In contrast, addition of H2MoO4 to Rh/C catalyst has inhibiting effects for the conversion of THFA. The selectivity of 1,5-PeD decreases, but concurrently the formation of 1-PeOH increases significantly. However, upon addition of a small amount of acids to the H2MoO4 and Rh/C system, both the overall activity and the selectivity to the target product increase, which strongly suggests that the acid function is highly desirable for achieving improved hydrogenolysis activity in the catalyst compared to metal catalysts alone. 3.4. Catalyst Characterization. We have shown that MoO3 is unique in catalyzing the selective hydrogenolysis of THFA to 1,5-PeD combined with the hydrogenation catalyst such as Rh. This brings about a fundamental question: what is the genuinely catalytically active species in the ring opening of THFA in the reaction? To elucidate the phase changes of MoO3 during the reaction, we performed XRD experiments on both fresh MoO3 samples and MoO3 reduced with H2 under typical reaction conditions (Figure 2). These analyses show the MoO3 after the reaction with H2 displays an XRD pattern different than that of fresh MoO3, with the appearance of the characteristic XRD pattern of HxMoO3 phase according to the literature.41 Most of the peaks for the HxMoO3 samples are indexed to H0.9MoO3 (H0.9MoO3, JCPDS 53-1024). The strong diffraction peaks D



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Table 4. Hydrogenolysis of THFA over the Binary Catalyst Composed of Mo (or W) Compounds with Rh/C sel. (%) entry

catalysta

conv. (%)

1,5-PeD

1,2-PeD

1-PeOH

2-PeOH

conv. rate (gTHFA gcat−1 h−1)

1 2 3 4 5 6 7 8 9 10 11

5% Rh/C (0.3 g)b 5% Rh/C(0.3 g) + WO3 (0.03 g)c 5%Rh/C (0.3 g) + MoO3 (0.03 g)c 5% Rh/C (0.3 g) + MoO3 (0.3 g)c 5% Rh/C (0.3 g) + MoO3 (1.0 g)b 5% Rh/C (0.3 g) + H2MoO4 (1.0 g)b pH = 2.64d 5% Rh/C (0.3 g) + H2MoO4(1.0 g)b pH = 5.0 5%Rh/C (0.3 g) + H2MoO4 (1.0 g)b pH = 6.26e 5% Rh/C (0.3 g) + HPMo (1.0 g)b 5% Rh/C(0.3 g) + HPW (1.0 g)b 5% Rh/C (0.3 g) + HSiW (1.0 g)b

3.6 1.1 7.7 21.0 27.9 3.8 2.8 2.9 26.9 16.3 12.0

56.8 − 57.4 70.9 67.6 45.6 27.8 33.4 71.1 69.1 81.7

− − 9.0 4.9 − −

30.4 − 6.5 6.0 6.5 54.4 72.2 66.6 4.8 7.0 3.5

12.8 − 27.1 18.2 25.9 − − − 19.7 23.9 14.8

5 × 10−3 1.4 × 10−3 9.7 × 10−3 1.5 × 10−2 8.9 × 10−3 1.2 × 10−3 9 × 10−4 9.3 × 10−4 8.6 × 10−3 5.2 × 10−3 3.8 × 10−3

− 4.4 − −

HPW = H3PW12O40, HPMo = H3PMo12O40, HSiW = H4SiW12O40. b120 °C; 6 MPa H2; reaction time, 24 h; THFA concentration, 2 wt %; total mass of THFA solution, 50 g. c120 °C; 8 MPa H2; reaction time, 24 h. dAdditive HNO3. eAdditive NaOH.

a

Figure 2. XRD patterns of MoO3 catalyst (a) before and (b) after H2 treatment at 120 °C and 6 MPa H2.

appear at 12.2°, 24.2°, and 37.2°, which correspond to the (200), (110), and (600) planes, respectively. Many experimental studies on the formation mechanism of HxMoO3 have been carried out, and a so-called “hydrogen spillover” mechanism was proposed.55,56 In this process, the hydrogen molecules first undergo dissociative chemisorption upon interacting with precious metal catalysts, and subsequently hydrogen atoms are spilled onto the bulk lattice of MoO3 via forming strong covalent bonds with the terminal oxygen atoms. The protons can readily migrate into the lattice to form hydrogen molybdenum bronze. The IR spectra of MoO3 catalyst before and after H2 exposure are presented in Figure 3. As can be seen, all peaks in the curve of MoO3 are in good agreement with the polycrystalline MoO3 powders in the literature.57 Specifically, the peak at 990 cm−1 is assigned to the terminal oxygen (Mo6+O) stretching mode. The peaks at 876 and 819 cm−1 come from the stretching vibrations of the oxygen atoms in Mo2−O entity, and the broad band centered at about 599 cm−1 represents the stretching mode for the triply coordinated oxygen (Mo3−O) vibration, which results from edge-shared oxygen atoms in common to three adjacent octahedra. By closely investigating the IR spectra after H2 exposure, it is seen that all peaks in the curve of HxMoO3 resemble those of MoO3. However, the most notable change after H2 exposure is observed for the intensities of IR peaks at 990, 876, 819, and 599 cm−1, which reduced greatly in HxMoO3. At the same time, the IR peak at 485 cm−1 vanishes. In addition, the peak at 990 cm−1 is red-shifted to 994 cm−1 upon H2 intercalation, while the peaks at 876 and 599 cm−1 blue shift to

Figure 3. IR patterns of MoO3 catalyst before and after H2 treatment at 120 °C and 6 MPa H2.

866 and 587 cm−1, respectively. According to the literature,58 these results all provide evidence that HxMoO3 phase has been formed during H2 interaction with the MoO3 crystallites. The XPS spectra recorded on the two samples are demonstrated in Figure 4, with molybdenum element under consideration. In MoO3, the doublets of 236.1 and 232.9 eV are attributed to the binding energies of the 3d3/2 and 3d5/2 of Mo6+, respectively, which are in agreement with the standard data.59 E



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under typical reaction conditions, a transparent solution was observed together with dark precipitates upon cooling to room temperature (as shown in Figure S1 of Supporting Information). The dark solid is proven to be H0.9MoO3 on the basis of XRD examinations (see Figure 2). On the other hand, another important phenomenon observed is the leaching of molybdenum in the solution. The amount of the eluted Rh and Mo species in the liquid phase after each run over binary 4% Rh/SiO2 (2 g) and MoO3 (0.5 g) catalyst was measured by using inductively coupled plasma (ICP), and the results are shown in Table 5. We found Table 5. Concentration of Mo in the Liquid Phase during Reuse Experimentsa

Figure 4. Mo 3d XPS spectra of MoO3 catalyst before and after H2 treatment at 120 °C and 6 MPa H2.

run number

Mo concentration in liquid phase (ppm)b

Mo leached from the MoO3 catalyst (wt %)c

1 2

51.0 13.7

0.77 0.20

a

The reactions were performed with 50 wt % THFA and on 4% Rh/SiO2 (2 g) and MoO3 (0.5 g) at 120 °C and 6 MPa. bThe Mo concentration in the liquid phase after each run was determined by ICP. c The Mo leached from the catalyst was calculated by the following equation: Mo leached (wt %) = (weight of Mo in the liquid phase)/ (weight of Mo on the catalyst) ×100%.

In the case of HxMoO3, the two peaks shift to the low energy of 234.3 and 231.2 eV compared to MoO3, corresponding to 3d3/2 and 3d5/2 of Mo5+. We inferred that the molybdenum has been reduced to Mo5+ in the hydrogen intercalation process. Acidity of the reduced MoO3 was determined by means of NH3 temperature-programmed desorption technique. The catalyst sample was pretreated in flowing 6 MPa H2 at 120 °C prior to the adsorption of NH3. The results are illustrated in Figure 5. The amount of NH3 desorbed from the parent MoO3 is

that the leaching amount of Rh is below the detection limit of ICP analysis, indicating that the leaching of Rh is negligible. In contrast, the molybdenum species is detected in a concentration of ∼51.0 ppm in the liquid phase after the first run and 13.7 ppm after the second run during the reuse experiments, implying that MoO3 dissolves slightly in water at room temperature and the dissolving ability of MoO3 may increase greatly at elevated temperatures. On the basis of this evidence, it is reasonable to speculate that MoO3 first dissolves partially in high-temperature water and transforms into soluble molybdenum bronze (HxMoO3) in the presence of hydrogen atmosphere. Upon cooling to room temperature, the dissolved HxMoO3 species is precipitated from the solution and transformed into a dark solid of HxMoO3. However, with the exposure time to air, the dissolved HxMoO3 is finally oxidized to MoO3 over the binary catalyst after the reaction, as indicated by Figure S2 of Supporting Information. Such an attractive property is an indicator of temperature-dependent phase-transfer behavior of MoO3. A similar phenomenon is observed for a temperature-controlled phase-transfer catalyst-H2WO4 in combined with Ru/C, which reflects the formation of dissolved H xWO 3 by reduction of H2WO4 in the catalytic conversion of cellulose to ethylene glycol.52,53 Thus, we propose that this dissolved HxMoO3 species, although in a very low concentration, is the genuinely catalytically active species promoting the C−O bond-breaking reaction of THFA through a homogeneous catalysis pathway. It should be mentioned that although MoO3 dissolved slightly in water, the Mo leached from the MoO3 catalyst over the repetitive runs was calculated at 0.77% and 0.20%, respectively. In other words, the recovery of MoO3 should be, theoretically, close to 99% without considering the loss in handing such as filtration and washing. In this respect, the reusability of the binary catalyst is acceptable. 3.6. Reaction Mechanisms Studied by DFT. 3.6.1. HxMoO3 Surface. DFT calculations were performed in order to reveal the origin of the high selectivity of the binary Rh/C and MoO3 catalyst. As discussed above, dissolved HxMoO3 has been

Figure 5. NH3 temperature-programmed desorption profiles for MoO3 catalyst before and after H2 treatment at 120 °C and 6 MPa H2.

negligible. The NH3-TPD profile is greatly altered by H2 reduction. The peak maximum appears at 536 K, indicating that acid sites are present on the HxMoO3. The superstructure analysis of hydrogen distribution in H0.9MoO3 from a combined theoretical−experimental study shows that the intercalated protons predominantly reside in the van der Waals (vdW) gap forming OH2 groups with the terminal oxygen atom of the MoO6 octahedron.40−42 Thus, it should be expected that the increased acidity for the reduced phase of HxMoO3 originates from the OH2 groups at the double protonated axial oxygen atoms. 3.5. Active Site for C−O Cleavage of THFA. As described above, upon treatment of yellow solid of MoO3 with hot water F



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Scheme 1. Possible Reaction Pathway for the Selective C−O Bond Cleavage from THFA to Produce 1,5-PeD and 1,2-PeD over HxMoO3 Catalyst

are shown in Figure 7. For 1,5-PeD, the initial configuration for the adsorbed state of THFA corresponds to the formation of hydrogen bonding between the −OH group at C(1) and O(4) atom on the catalyst surface, as well as that between −OH2 group on the HxMoO3 surface and the O(2) atom in the ring. At this stage, the adsorption energy is calculated to be −0.8 eV. Once the adsorbed state is obtained, C−O bond splitting has to be considered to continue the hydrogenolysis route. The acidcatalyzed C−O activation is thought to proceed through the onestep deprotonation of primary −CH2OH group, hydrogen transfer from the primary −CH2OH group to the neighboring C(2) atom in the ring, the ring opening, and the protonation of the O(2) atom in the ring, thus resulting in the formation of a 5-hydroxypentanal intermediate which can be stabilized via hydrogen bonding with the catalyst surface. In the transition state, the C(2)−H(2) bond is shortened from 2.162 Å in the initial state to 2.076 Å in the transition state (TS), while the secondary C(2)−O(2) bond in the ring is elongated from 1.462 to 1.879 Å. The H(3) atom originally residing on the acidic Mo−OH group transfers to the O(2) atom in the THFA ring, forming the O(2)−H(3) bond. The O(1)−H(1) bond rotates to optimize the O(1)···H(1)···O(4) structure in the TS, with the distance at 1.749 Å. The formation of this transition state presents a relatively high activation barrier of 1.26 eV. Nevertheless, this barrier remains attainable and close to the DFT-calculated value for the acid-catalyzed ring opening of THFA on a model Rh200Re1OH cluster.25 In the case of 5-hydroxypentanal, the C(2)−O(2) bond is further increased to 3.146 Å and the migration of H(1) atom to the surface O(4) generates the aldehyde group in 5-hydroxypentanal. This reaction is slightly exothermic with −0.74 eV. The next step consists of a rapid Rh-catalyzed hydrogenation of 5-hydroxypentanal to give the final product 1,5-PeD, which will be described in the upcoming section.

suggested as the active phase for the hydrogenolysis of THFA to 1,5-PeD. In the case of H0.9MoO3, the intercalated protons predominantly reside in the vdW gap forming OH2 groups with the terminal oxygen atoms, and half of the outer axial oxygens are engaged in two O−H bonds.40−42 As depicted in Figure 1a, we have considered the typical structure of intercalated hydrogen in H0.9MoO3, which shows the terminal oxygen atom is the most favorable site for accommodating two H atoms. According to our results, the distance of two Ot−H bonds is 0.98 Å with the H−Ot−H angle being 106.7°. 3.6.2. Proposed Mechanism for C−O Bond Cleavage on HxMoO3 Surface. Several possible reaction mechanisms have been proposed for hydrogenolysis reactions of C−O bond over Mo-based catalyst. For Rh−ReOx/C and Rh−MoOx/C catalyst, Dumesic et al. reveals that the surface acidic Re−OH group plays a central role in the hydrogenolysis of secondary C−O bonds for a broad range of cyclic ethers and polyols.31 The reaction proceeds via a concerted protonation, hydride transfer, and ringopening mechanism involving oxocarbenium ion intermediates. In this case, it seems reasonable to hypothesize a similar acidassisted ring-opening process of THFA considering the presence of acidic Mo−OH group on dissolved HxMoO3. As shown in Scheme 1, starting from the adsorbed state of THFA on the HxMoO3 surface, the reactants can undergo C−O bond cleavage to form stable hydroxypentanal intermediate through concerted hydrogen transfer from −CH2OH group to the neighboring carbon in the ring. Accordingly, we performed density functional calculations to elucidate the energetics involved in this process. As a preliminary study for the reactivity, DFT studies were performed on the THFA molecule to elucidate the C−O cleavage mechanism catalyzed by HxMoO3, and the optimized structures of initial, transition, and final states on the routes leading to the hydroxypentanal intermediates are represented in Figure 6. The reaction energetics and the corresponding energy barriers G



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Figure 6. DFT-calculated initial, transition, and final states for the concerted hydrogen transfer and ring opening of THFA over a H0.9MoO3 model structure to yield (a) 1,5-PeD and (b)1,2-PeD.

can subsequently hydrogenate to produce 1,2-PeD. Therefore, the adsorption of transition state on the 1,5-PeD reaction pathway is found more favorable than that on the 1,2-PeD pathway by 0.2 eV, and the overall process for the 5-hydroxypentanal formation is predicted to be more exothermic (0.62 eV) than that for 2-hydroxypentanal. The energetics indicate that the formation of 1,5-PeD in THFA hydrogenolysis is more favored. This is consistent with the experimentally observed high selectivities of 1,5-PeD and low selectivities for 1,2-PeD formation. 3.6.3. Proposed Mechanisms for C−O Bond Cleavage over Bifunctional Rh−HxMoO3 Catalyst. The reaction profiles of THFA hydrogenolysis over the Rh−HxMoO3 catalyst were simulated with DFT. For such a process, we proposed that the formation of 1,5-PeD takes place in a two consecutive reactions. First, THFA undergoes C−O bond cleavage via a concerted hydrogen-transfer mechanism to form the key intermediate 5-hydroxypentanal with the homogeneous catalysis of dissolved HxMoO3. The unsaturated intermediate is then quickly hydrogenated into 1,5-PeD under the heterogeneous catalysis of transition metals like Rh. In this respect, the solid Rh catalyst prefers to interact with the dissolved HxMoO3 species in the liquid phase via surface oxygen atoms, leading to the formation of direct Rh−O bond at the liquid−solid interface, whereas the direct interaction between Rh and Mo atoms is impossible in our bifunctional model. This is further confirmed by the XRD examinations of the recycled catalyst, as shown in Figure S2 of Supporting Information, which found that the Mo species was dominated by MoO3 after the reaction. This conclusion is quite different from that of the heterogeneously modified Rh−MoOx/C catalyst with the observation of direct Rh−Mo bond. The bifunctional Rh−HxMoO3 structure is shown in Figure 2b, in which the modeling of carbon support was

Figure 7. DFT-calculated potential energy profiles for the H0.9MoO3catalyzed ring opening of THFA to form 1,5-PeD and 1,2-PeD.

In addition to this breaking of secondary C(2)−O(2) bond of THFA to produce 1,5-PeD, Figure 6b depicts the transition and final states on the possible 1,2-PeD reaction pathway where THFA undergoes the hydrogenolysis of the primary C(5)−O(2) bond, which also occurs in a single concerted step. The transition state for this ring-opening step shows H(2) atom is abstracted from C(1) and coordinated to C(5). Simultaneously, the C(5)− O(2) separation distance in the TS is 1.277 Å longer than that in the initial state. The activation energy barrier associated with step is estimated to be 1.46 eV. Following the breaking of the primary C(5)−O(2) bond, the resultant 2-hydroxypentanal intermediate forms with the aldehyde group attached on the surface and the C(5)−O(2) distance is enlarged to 3.126 Å. The step is slightly exothermic with −0.12 eV. The unsaturated 2-hydroxypentanal H



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Figure 8. DFT-calculated initial, transition, and final states for the concerted hydrogen transfer and ring opening of THFA over bifunctional Rh−H0.9MoO3 catalyst to yield 1,5-PeD.

concerted mechanism facilitated by the surface Mo−OH group. It is thermodynamically favored with an exothermic energy of −1.4 eV. From Figure 8b, we see that the transition state is very similar in character to the species formed at the HxMoO3 surface. This process requires an activation free energy barrier of 1.96 eV. In the next phase of the catalytic cycle, hydrogenation of 5-hydroxypentanal to 1,5-PeD is found to be, as expected, exothermic and kinetically less challenging than the ring-opening process. The preferred pathway of 5-hydroxpentanal hydrogenation to 1,5-PeD occurs first via hydrogenation of the carbonyl oxygen O(1) followed by the hydrogenation of the carbon C(1) and has the highest barrier of 1.77 eV in the first reaction step of converting −CHO to −CHOH, while the last step of converting −CHOH to −CH2OH is more feasible with an energy barrier of 1.05 eV. The formation of the 1,5-PeD product is favored with a large exothermicity (−2.64 eV), according to our DFT calculations. Recovery of the Rh−H0.9MoO3 surface to its initial state would then complete the catalytic cycle. This process is expected to be exothermic by −1.8 eV. We conclude that the unique properties of acidic Mo−OH on HxMoO3 render it a promising hydrogenolysis catalyst with excellent selectivity toward the production of α,ω-diols from cyclic ethers. According to our extended studies on heteropoly acids and molybdic acid integrated with Rh/C catalyst (Table 4), the high hydrogenolysis activity of H3PMo12O40 can be attributed to its intrinsically strong acidic Mo−O(H)−Mo group and high dissolving ability in water. In contrast, the weak Brønsted acidity of H2MoO4 leads to a much lower 1,5-PeD yield in the catalytic transformation of THFA. It is also noted that the solubility of H2MoO4 in water is much lower than that for MoO3. Accordingly, the lower concentration of dissolved Mo species under reaction conditions for H2MoO4 may be another reason for the low 1,5-PeD yield. Our results confirm the acidic Mo−OH in dissolved HxMoO3 acts as the active species to catalyze the C−O bond cleavage, and the two different functional sites, Mo-based component and a solid metal catalyst, are tunable in these binary catalysts, which would benefit the design of more efficient catalytic conversion systems.

neglected in our present calculations. It can be seen that the atom of Rh is strongly bound to four oxygen sites on the bronze support with the average Rh−O bond at 2.083 Å. Computational studies were next performed to map the energy landscape of the conversion of THFA to 1,5-PeD over bifunctional Rh−HxMoO3 catalyst, as shown in Scheme S1 of Supporting Information. The mostly likely DFT-calculated paths for the C−O bond cleavage are presented in Figure 8. The reaction energetics with their corresponding activation barriers are given in Figure 9. In the first part of the catalytic cycle, the key

Figure 9. DFT-calculated potential energy profiles for the ring opening of THFA to form 1,5-PeD over bifunctional Rh−H0.9MoO3 catalyst.

reaction is acid-catalyzed ring opening of THFA via transition state TS1 upon THFA binding and activation of secondary etheric oxygen. This pathway is similar to the HxMoO3-catalyzed C−O bond cleavage of THFA. In the initial state, THFA is preferably held to the surface −OH2 group and H2 readily adsorbs onto the Rh site, which exhibits an adsorption energy of −1.22 eV. Subsequent hydrogen shift from −CH2OH unit to the vicinal C(2) atom results in the formation of a stable 5-hydroxypentanal intermediate. This step also occurs in a I



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and the mechanism toward ring opening can assist future studies on more efficient catalytic conversion systems.

To confirm the acid-catalyzed hydrogen-transfer mechanism proposed in our present work, we have tried the experiments regarding the hydrogenolysis of glycerol and ethylene glycol. The hydrogenolysis reactivity profiles for glycerol and ethylene glycol over binary Rh/C and MoO3 catalyst were examined under similar conditions as specified in entry 5, Table 4, and these results are shown in Table S2 of Supporting Information. It is apparent that glycerol is more reactive than ethylene glycol. The hydrogenolysis of glycerol leads to the formation of 1,2-propanediol as the predominant product at 3% conversion. The explanation of the significantly higher selectivity toward hydrogenolysis of the hydroxyl group at the primary carbon atom is consistent with the acid-catalyzed oxocarbenium ion route. The HO+CH2CH2CH2OH oxocarbenium ion formed upon the hydride transferring to the secondary hydroxyl group is less stable because this ion is delocalized over the primary carbon atom in the structure. In contrast, the attack of hydride at the primary hydroxyl group allows for the formation of a more stable HOCH2−CHO+H−CH3 oxocarbenium ion, with the ion delocalized over the secondary carbon atom and further stabilized by the vicinal OH group. The greater stability of the secondary HOCH2−CHO+H−CH3 oxocarbenium ion is further proved by the energy calculations performed within Gaussian 09 package (see Supporting Information). In the case of ethylene glycol, unfortunately, almost zero conversion of ethylene glycol is observed, which may have originated directly from the low stability of primary carbenium ions HO+CH2CH3 formed upon dehydration of this reactant. The poor hydrogenolysis activity of ethylene glycol has been reported previously by using Rh/C catalyst.60 Therefore, the above experimental trend for glycerol and ethylene glycol is consistent with the acidcatalyzed oxocarbenium ion chemistry with the homogeneous catalysis of dissolved HxMoO3.

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ASSOCIATED CONTENT

S Supporting Information *

Photographs of MoO3 before and after hydrothermal treatment at 120 °C and 6 MPa H2, XRD patterns of binary Rh/C and MoO3 catalyst after the reaction and then exposure to air with time, hydrogenolysis of various substrates over binary Rh/C and MoO3 catalyst, the optimized structures of primary and secondary oxocarbenium ion intermediates in the glycerol hydrogenolysis reaction catalyzed homogeneously over HxMoO3, and the proposed reaction mechanism for the hydrogenolysis of THFA to produce 1,5-PeD under the cooperation between homogeneous HxMoO3 and heterogeneous Rh/C catalysts. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86 (532) 80662724. Tel: +86 (532) 80662723. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledged the financial support by the National Natural Science Foundation of China (Grants 21303238, 21273260, and 21433001), Qingdao Applied Basic Research Program (14-2-474-jch), the Shandong Provincial Natural Science Foundation for Distinguished Young Scholar, China (JQ201305), and the Foundation of State Key Laboratory of Coal Conversion (J13-14-603). The authors also thank Supercomputing Center, Computer Network Information Center of Chinese Academy of Sciences, and the super computational center of CAS-QIBEBT for providing computational resources.

4. CONCLUSION We developed a highly efficient and green catalytic system comprising MoO3 and a supported Rh catalyst for the hydrogenolysis of THFA. This binary system is stable and easily recyclable, and importantly, the decay in the hydrogenolysis activity caused by leaching Mo species could be overcome by adding MoO3 into the reactor directly. 1,5-PeD is obtained as the major product with negligible formation of competitive 1,2-PeD product and overhydrogenolysis product 1-pentanol. The characterization results from XRD, IR, XPS, NH3-TPD, and ICP support a temperature-controlled phase-transfer behavior of MoO3 in which dissolved hydrogen molybdenum oxide bronzes (H0.9MoO3) in the liquid phase is generated by reduction of MoO3 under the reaction conditions and acts as a highly active and selective hydrogenolysis catalyst for C−O bond breaking of THFA. To understand the role of Mo in the ring-opening process of THFA in more detail, DFT calculations were then performed on the plausible mechanisms and reaction energetics of THFA conversion to pentanediol on both H0.9MoO3 and Rh−H0.9MoO3 catalysts. Our results indicate a cascade reaction under the cooperation between homogeneous H0.9MoO3 and heterogeneous Rh catalyst. In this process, the acidic Mo−OH group on the H0.9MoO3 surface is likely responsible for the C−O bond cleavage of THFA, which takes places through a concerted “hydrogen-transfer−ring-opening” process that stabilizes 5-hydroxypentanal formation, followed by the hydrogenation of unsaturated intermediate at a neighboring Rh site to afford 1,5-PeD. Insights gained into the nature of the active site

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REFERENCES

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Role of MoO3 on a Rhodium Catalyst in the Selective Hydrogenolysis

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