Structure and Mechanism of Titania-supported Platinum-molybdenum

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Structure and Mechanism of Titania-supported Platinum-molybdenum Catalyst for Hydrodeoxygenation of 2-Furancarboxylic Acid to Valeric Acid Takehiro Asano, Yoshinao Nakagawa, Masazumi Tamura, and Keiichi Tomishige ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b01104 • Publication Date (Web): 30 Apr 2019 Downloaded from http://pubs.acs.org on May 4, 2019

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ACS Sustainable Chemistry & Engineering

Structure and Mechanism of Titania-supported Platinum-molybdenum Catalyst for Hydrodeoxygenation of 2-Furancarboxylic Acid to Valeric Acid

Takehiro Asano,a Yoshinao Nakagawa,a,b,* Masazumi Tamuraa,b and Keiichi Tomishigea,b,*

a

Department of Applied Chemistry, School of Engineering, Tohoku University, 6-6-07 Aoba,

Aramaki, Aoba-ku, Sendai, 980-8579, Japan b

Research Center for Rare Metal and Green Innovation, Tohoku University, 468-1, Aoba,

Aramaki, Aoba-ku, Sendai 980-0845, Japan

* Corresponding authors. E-mail: [email protected], Tel/Fax: +81-22-795-7215; [email protected], Tel&Fax: +81-22-795-7214.

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Abstract Selective hydrodeoxygenation of 2-furancarboxylic acid, which can be a relatively stable platform chemical from hemicellulose, to valeric acid was investigated in detail, which reaction has been reported to be specifically promoted by supported Pt-MoOx catalysts. The dependence of catalytic performance of Pt-MoOx/TiO2 on loading amounts of Pt and Mo showed that the highest activity is obtained at around Mo ~0.5 wt% when Pt loading amount is fixed. With enough amount of catalyst that can minimize the effect of deactivation, ~60% yield of valeric acid was obtained over all Pt-MoOx/TiO2 catalysts with Pt ≥1 wt% and Mo 0.5 wt% loadings. Characterization results of Pt-MoOx/TiO2 catalysts with XRD and CO adsorption showed that the Pt particles (3~5 nm, depending on Pt loading amount) were not covered with MoOx species, suggesting that MoOx species were mainly located on TiO2 support surface. Mo K-edge XAFS results suggest that the MoOx species in Pt-MoOx/TiO2 have Mo(IV) valence state and some of the MoIVOx species have direct bonds with Pt atom on Pt metal particles. The number of the direct Pt-Mo bond became smaller after catalytic use, which can be related to the deactivation. Therefore, the Pt-Mo bimetallic site can be the catalytically active site. Based on the solvent effect, reactivity trends of related substrates and reaction orders in kinetics, a reaction mechanism is proposed where the ring is opened after addition of one hydrogen atom to the 2-position of the furan ring.

Keywords furan, carboxylic acid, hydrodeoxygenation, platinum, molybdenum


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INTRODUCTION Conversion of lignocellulosic biomass to useful chemicals has been attracting more and more attention. Because of the complex nature of lignocellulose, the use of lignocellulose-derived pure compounds as intermediates is a widely used approach.1-4 Such lignocellulose-derived compounds are called “platform chemicals” and they include carboxylic

acids,

furans

and

sugar

alcohols.

Among

them,

furfural

and

5-hydroxymethylfurfral are relatively easily produced by dehydration of carbohydrates and both of them have great potential as sources of bio-based products.5-8 However, furfural and 5-hydroxymethylfurfural are unstable unsaturated aldehydes with difficulty in storage and transport. On the other hand, oxidization products of furfural and 5-hydroxymethylfurfural, namely 2-furancarboxylic acid (FCA) and 2,5-furandicarboxylic acid (FDCA), are stable crystalline solids. These productions have been hot topics in furfural/5-hydroxymethylfurfural conversions,9-11 and several catalyst systems have been developed which give excellent yields even without the use of additive (base).12-20 However, the use of FCA and FDCA as sources of bio-based compounds is very limited except the use of FDCA as a monomer of polyester.21 Since the carboxylic acid moiety is produced by oxidation of reactive sources, reductive conversions of this group are not attractive in view of biomass conversions. The reduction of furan ring with retention of carboxylic acid moiety is more attractive. The production of adipic acid from FDCA was first reported in a patent,22 and recently Vlachos and Xu et al. have reported improved systems.23,24 These systems are multistep conversion ones of FDCA into adipic acid via hydrogenation of the furan ring and subsequent C-O dissociations in tetrahydrofuran ring, and HI is a key component in the latter C-O dissociation steps. We have recently reported a direct hydrodeoxygenation system of FCA to valeric acid (pentanoic acid) using Pt-MoOx/TiO2 catalyst.25 The reported highest yield in this Letter article was 51%, and 3 ACS Paragon Plus Environment


direct conversion of FDCA to adipic acid is also possible, with 21% yield. In contrast to the indirect systems, this Pt-MoOx/TiO2 catalyst cannot produce valeric acid from tetrahydrofuran-2-carboxylic acid, which is the ring-hydrogenated product of FCA. This indicates that the C-O bonds in the furan ring of FCA are dissociated before total hydrogenation of furan ring. Higher reactivity in C-O dissociation of furan ring than tetrahydrofuran ring is also known for furfuryl alcohol over noble metal catalysts including Pt, in which systems pentanediols are produced;26-32 however, the nature of -CH2OH group (an electron-donating group) is much different from that of -COOH group (an electron-withdrawing group). The reaction conditions are also much different between furfuryl alcohol reduction to pentanediols (typically alcohol solvent, basic catalyst) and FCA reduction to valeric acid (water solvent, acidic media). The dependence on the catalyst composition is also unique. While many Pt-based catalysts including simple Pt/-Al2O3 have some activity in furfuryl alcohol reduction to pentanediols,27,29,30 the catalyst survey for FCA reduction to valeric acid in the previous report25 showed that both Pt and Mo are indispensable: other combinations of noble metal + Mo or those of Pt + group 5-7 metal (V, W and Re) showed almost no activity in valeric acid formation from FCA. Physical mixture of Pt/TiO2 and Mo/TiO2 was also not active, indicating the necessity of the presence of both Pt and Mo on the same support particle. The effect of support is less remarkable; however, TiO2 support showed the best valeric acid selectivity. Literature shows that bimetallic Pt-Mo catalysts are active in other reactions such as water-gas-shift reaction,33-35 carbonyl hydrogenation36-41 and C-O hydrogenolysis of tetrahydropyran-2-methanol.42,43 Shimizu et al. have intensively investigated Pt-MoOx/TiO2 catalysts for reductive formations of C-N bond and sulfoxide reduction to sulfone.44-47 However, these reported systems are not specific to Pt-Mo catalysts: other combinations show 4 ACS Paragon Plus Environment

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some activity; for example, Rh-Mo catalyst is also active in carboxylic acid hydrogenation,36-38 C-O hydrogenolysis,42,43 reductive formations of C-N bond47 and sulfoxide reduction.45 In addition, Pt-MoOx/TiO2 catalyst of Shimizu’s group40,43-47 has relatively large loading amount of Mo (7 wt% on 50 m2/g TiO2; Pt 5 wt%), while we confirmed that the catalyst with 0.5 wt% Mo loading (on 30 m2/g TiO2; Pt 4 wt%) has higher activity than those with higher loading amount of Mo. Keeping in mind these differences from our Pt-MoOx catalyst for FCA reduction and Pt-Mo catalysts in the literature, in this report, we investigated the structure and mechanism of this unique catalytic system.

EXPERIMENTAL SECTION Catalyst preparation Pt-MoOx/TiO2 catalysts were prepared by sequential impregnation method as the previous report:25 TiO2 support (AEROXIDE P25, AEROSIL, calcined at 973 K for 1 h, 30 m2/g) was impregnated with Pt(NO2)2(NH3)2 in nitric acid (Tanaka Kikinzoku Kogyo). After drying at 383 K for overnight, the Pt/TiO2 was impregnated with aqueous solution of (NH4)6Mo7O24·4H2O (Wako Pure Chemical Industries, Ltd.), and then it was dried at 383 K for overnight again. The dried catalysts were calcined at 773 K for 3 h under air. The loading amount of Pt and Mo was 0-20 wt% and 0-2 wt%, respectively, and the amount was described in each result.

Activity test Catalytic reactions were performed in a 190 ml stainless-steel autoclave with an inserted glass vessel. Substrates are commercially available and were used as received (2-furancarboxylic acid (FCA), 3-furancarboxylic acid and DL-lactic acid: Wako Pure 5 ACS Paragon Plus Environment


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Chemical Industries, Ltd; furan, 2,3-dihydrofuran and 2,5-dihydrofuran: Tokyo Chemical Industry Co., Ltd.). The catalyst (typically 0.10 g), solvent (19 g), and substrate (10 mmol) were loaded in the inserted glass vessel with a spinner. The glass vessel was set to the reactor, and the air content in the reactor was purged by flushing three times with 1 MPa of hydrogen (Showa Denko K.K.). The autoclave was then heated to 413 K. When the temperature reached 413 K, the H2 pressure became 1.5 MPa, and then the H2 pressure was set to the desired value when necessary. The reaction time was defined as 0 h at that time. The heating to 413 K took about 30 min. The reaction mixture in the reactor was magnetically stirred at 500 rpm at all times. After an appropriate reaction time, the reactor was cooled with water bath. The gas was collected in a gas bag. The liquid and solid were collected together with washing solvent (1,4-dioxane 20 g, Wako Pure Chemical Industries, Ltd.). The catalyst was separated by filtration. The liquid was analyzed by FID-GC (Shimadzu GC-2025 or GC-2014) equipped with an HP-FFAP capillary column, and the gas was analyzed by FID-GCs equipped with an Rtx-1 PONA capillary column and with a Porapak N packed column and methanator. Products were also identified by GC-MS (QP-2010, Shimadzu). DL-lactic acid was analyzed by HPLC (Prominence, Shimadzu) with RID and UV-vis detector. Aminex HPX-87H column and 0.01 M H2SO4 aq as mobile phase were used for separation. Conversion, yield, and carbon balance were calculated by the following equations (1)-(3). In almost all cases, the carbon balance values were significantly lower than 100% (typically 70-90%). In the previous report,25 we measured total organic carbon (TOC) amount in aqueous phase, and the result suggested that heavy water-soluble compounds (e.g. polymers) that cannot be detected by GC were present in the reaction solution. conversion [%] = (1 ― yield [%] =

𝑎𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝑑𝑒𝑡𝑒𝑐𝑡𝑒𝑑 𝑠𝑢𝑏𝑠𝑡𝑟𝑎𝑡𝑒 [𝑚𝑚𝑜𝑙] 𝑎𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝑙𝑜𝑎𝑑𝑒𝑑 𝑠𝑢𝑏𝑠𝑡𝑟𝑎𝑡𝑒 [𝑚𝑚𝑜𝑙] )

× 100

(1)

𝑎𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝑝𝑟𝑜𝑑𝑢𝑐𝑡 [𝑚𝑚𝑜𝑙] × 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑐𝑎𝑟𝑏𝑜𝑛 𝑎𝑡𝑜𝑚𝑠 𝑖𝑛 𝑡ℎ𝑒 𝑝𝑟𝑜𝑑𝑢𝑐𝑡 𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒 𝑎𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝑙𝑜𝑎𝑑𝑒𝑑 𝑠𝑢𝑏𝑠𝑡𝑟𝑎𝑡𝑒 [𝑚𝑚𝑜𝑙] × 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑐𝑎𝑟𝑏𝑜𝑛 𝑎𝑡𝑜𝑚𝑠 𝑖𝑛 𝑎 𝑠𝑢𝑏𝑠𝑡𝑟𝑎𝑡𝑒 𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒


× 100

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(2) carbon balance [% ] = ∑(𝑎𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝑑𝑒𝑡𝑒𝑐𝑡𝑒𝑑 𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒 [𝑚𝑚𝑜𝑙] × 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑐𝑎𝑟𝑏𝑜𝑛 𝑎𝑡𝑜𝑚𝑠 𝑖𝑛 𝑡ℎ𝑒 𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒) 𝑎𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝑙𝑜𝑎𝑑𝑒𝑑 𝑠𝑢𝑏𝑠𝑡𝑟𝑎𝑡𝑒 [𝑚𝑚𝑜𝑙] × 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑐𝑎𝑟𝑏𝑜𝑛 𝑎𝑡𝑜𝑚𝑠 𝑖𝑛 𝑎 𝑠𝑢𝑏𝑠𝑡𝑟𝑎𝑡𝑒 𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒

× 100 (3)

Catalyst characterization The “reduced” catalyst samples were prepared by treating the catalyst added to water solvent with 1.5 MPa H2 at 413 K for 30 min. The “used” catalyst samples were collected after the reaction with the conditions of 0.1 g catalyst, 10 mmol 2-furancarboxylic acid, 19 g water, 1.5 MPa H2, 413 K and 4 h. XRD patterns were recorded with Rigaku MiniFlex600 diffractometer with Cu Kα radiation (λ = 0.154 nm, 45 kV, 40 mA). The Pt crystallite size was determined with the Scherrer’s equation and the width of Pt(111) peak after subtracting the pattern of support, similarly to our previous report.25 Field emission scanning transmission electron microscope (FE-STEM)

images were obtained with a Hitachi HD-2700 instrument operated at 200 kV. The samples were dispersed in ethanol, and they were dropped on Cu grids under air. Average Pt particle size (d) was calculated by the following equation: d = (nidi3)/(nidi2) (di: size of each particle, ni: number of particle with size di). The amount of CO chemisorption was measured with MicrotracBEL BELCAT II using a pulse adsorption method. The calcined catalyst (about 50 mg) was pre-reduced with H2 at 473 K for 30 min. After reduction, the sample tube was purged with He for 30 min at 473 K. Subsequently the adsorption was performed at 308 K. Adsorption gas was 10 vol.% CO/He, and gas volume was 0.888 cm3/pulse. Adsorption was measured until the difference of last three times became within 1%. The XAFS spectra were measured at the BL14B2 station of SPring-8. Detectors (ion 7 ACS Paragon Plus Environment


chambers) for I0 were filled with 85% N2 + 15% Ar and 80% Ar + 20% Kr for Pt L3-edge and Mo K-edge measurement, respectively. Detectors for I were filled with 50% N2 + 50% Ar and 50% Ar + 50% Kr for Pt L3-edge and Mo K-edge measurement, respectively. The catalyst sample after reduction or reaction was transferred to a UV/vis cell (UV-Cuvette micro 8.5 mm, BRAND GMBH + CO KG; optical path length: 2 mm) in a glove bag filled with N2. The data for Pt L3-edge and Mo K-edge were collected in a transmission mode (edge jumps: 0.33-1.0) and a fluorescence mode (edge jumps: 0.30-1.0), respectively. For extended X-ray absorption fine structure (EXAFS) analysis, the oscillation was first extracted from the raw spectrum using a spline smoothing method, and Fourier transformation of the k3-weighted oscillation was performed. The range of Fourier transformation and that of filtering in inverse Fourier transformation for curve fitting are shown in each result. In the curve fitting, the curves and the empirical phase shifts for Pt-Pt and Mo-O bonds were determined from the data of Pt foil and Na2MoO4, respectively. Theoretical curves for the Mo-Mo, Mo-Pt, and Pt-Mo bonds were calculated with FEFF 8.20 program. Analyses of EXAFS and X-ray adsorption near edge spectra (XANES) data were performed using a computer program (REX2000, ver. 2.6.0; Rigaku Corp.).

RESULTS AND DISCUSSION Performance of catalyst with different loading amounts In our previous report,25 the performance of Pt-MoOx/TiO2 with different Mo loading amount was compared at constant Pt amount (4 wt%), and the catalyst with Mo/Pt=0.25, which corresponds to 0.5 wt% Mo, showed the highest activity and selectivity to valeric acid. Here, we changed the loading amount of both Pt and Mo of Pt-MoOx/TiO2 catalyst in the range of 1-20 wt% and 0.125-2 wt%, respectively, and compared the performances in FCA 8 ACS Paragon Plus Environment

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reduction. The reaction results over a constant amount of catalyst are summarized in Table 1. In the cases of Pt 4 wt%, 10 wt% and 20 wt%, the highest yield of valeric acid was obtained over the catalyst with Mo 0.5 wt%. We have already confirmed that the over-reaction of valeric acid is negligible at this temperature (413 K) over Pt-MoOx/TiO2 catalyst.25 Therefore, decrease of valeric acid yield with increase of Mo amount at high loading amounts (Pt ≥ 4 wt%, Mo ≥ 0.5 wt%) means lower selectivity of such catalysts. Lowering the Pt loading amount below 4 wt% decreased the valeric acid yield significantly, even based on Pt amount. However, we have already reported that the Pt-MoOx/TiO2 catalyst is deactivated during the reaction:25 Pt-MoOx/TiO2 (Pt 4 wt%, Mo 0.5 wt%) catalyst after use showed lower activity (conversion: 97% → 64%; valeric acid yield: 51% → 24%) even when the catalyst was collected in wet form without exposure to air. The decrease of activity is neither due to the sintering of Pt particles, which was confirmed by XRD and TEM, nor to the loss of active metal by leaching ( 2-propanol (4%) > methanol ( isopropyl alcohol > t-butyl alcohol.68-70 The adsorption of methanol on MoOx species in Pt-MoOx/TiO2 catalyst can explain the decrease of valeric acid yield in the presence of methanol solvent. The effect of acetic acid can be also explained by the adsorption on the catalyst surface. Considering that FCA substrate has also a carboxyl group, competitive adsorption of acetic acid and FCA can decrease the reactivity of FCA in the presence of acetic acid solvent. Table 6 shows the reaction results of compounds related to FCA. The reactivity of tetrahydrofuran-2-carboxylic acid and -valerolactone has been already tested in the previous paper,25 and they do not give valeric acid by reduction over Pt-MoOx/TiO2. In order to check whether 2-hydroxycarboxylic acid (2-hydroxyvaleric acid) is an intermediate of valeric acid formation, reaction of lactic acid (2-hydroxypropionic acid) was tested (Entry 4). The reactivity of lactic acid was much lower than that of FCA. The main products were 1,2-propanediol and 1-propanol, and the deoxygenated product (propionic acid) was hardly formed.

Further

considering

that

1-pentanol

formation

was

negligible

in

FCA

hydrodeoxygenation to valeric acid, 2-hydroxycarboxylic acid is not an intermediate of valeric acid formation. As furan derivatives, 3-furancarboxylic acid and furan were tested. The reactivity of 3-furancarboxylic acid was much lower than FCA (2-furancarboxylic acid), and a small amount of the deoxygenation product (2-methylbutyric acid) was formed (Entry 5). Simple hydrogenation of 3-furancarboxylic acid to tetrahydrofuran-3-carboxylic acid was also very slow. In the case of furan (Entry 6), the reactivity was high, and the main products were THF, 1-butanol and butane. These products are hydrogenation product, ring-opening product without deoxygenation, and deoxygenated product, respectively, and they correspond to tetrahydrofuran-2-carboxylic acid, -valerolactone (or 5-hydroxyvaleric acid) and valeric 15 ACS Paragon Plus Environment


acid in FCA reduction, respectively. Therefore, same reactions proceed for both substrates, furan and FCA, although the selectivity is different. The low carbon balance in the reaction of furan might be due to the low boiling point of furan (304 K). Next, the reactions of 2,3-dihydrofuran and 2,5-dihydrofuran, which are possible intermediates in furan hydrodeoxygenation, were tested, since such dihydrofuran derivatives of FCA are not easily available. The main products were THF, 1-butanol and 1,4-butanediol, and the selectivity patterns were similar from both dihydrofurans but different from that from furan. Butane, which corresponds to the target deoxygenated product, was hardly formed, suggesting that free dihydrofuran derivatives are not intermediates in furan ring deoxygenation. 1,4-Butanediol can be produced by hydration of 2,3-dihydrofuran and subsequent hydrogenation.71 The negligible formation of 2,5-dihydroxyvaleric acid in FCA reduction also suggests that FCA hydrodeoxygenation does not involve free dihydrofuran derivatives as intermediates. Based on the overall data we propose a reaction mechanism as shown in Figure 8, although there is still large uncertainty. The FCA molecule is adsorbed on the catalyst surface with the carboxyl group. The reaction starts with the addition of one hydrogen atom to the furan ring as a pre-equilibrium reaction step, considering the moderate reaction order with respect to H2 (~0.5) and the low reactivity of dihydrofurans toward hydrodeoxygenation. Considering that main by-products are 5-functionalized valeric acid derivatives, the C-O bond at 1,2-position of the furan ring can be first activated and dissociated. Therefore, we think that the first hydrogen addition occurs at the carbon atom at 2-position. However, the hydrogen addition at 2-position is not thermodynamically favorable: the addition breaks the conjugation between the furan ring and the carboxylate group. Rather, the carboxyl group directs the position of the hydrogen addition, i.e. the carbon atom which is bonded to the carboxyl group is attacked by 16 ACS Paragon Plus Environment

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hydrogen. This idea can explain the very different reactivity between 2- and 3-furancarboxylic acids: hydrogen addition to the 3-position of 3-furancarboxylic acid cannot activate the C-O bond, and the addition itself is more difficult than that to the 2-position of FCA, leading to very low yield of both ring-opening and hydrogenation products. The energetics of hydrogenation of furan ring was calculated by DFT, and the results are shown and discussed in Scheme S1, Supporting Information. The next step is the ring-opening of the 2-monohydrofuran intermediate, which is the rate-determining step. Considering the large increase of conversion rate by the presence of Mo in catalyst and polar water solvent, we think that the ring-opening step involves Mo-O bond formation and charged surface species. The electron to charge the surface species should be supplied from the free electrons in Pt metal particle, and the consumed electron is replenished from H2 (1/2H2 → H+ + e-). The C-O bond at 1,2-position of the furan ring is dissociated, and the surface 5-oxypentadienoic acid species is produced. Desorption of this species

and

successive

hydrogenation

produces

5-hydroxyvaleric

acid

and

then

-valerolactone, which are by-products of valeric acid formation. In order to avoid such side reactions, production of valeric acid requires dissociation of C-O bond at 5-position very quickly before desorption from the catalyst surface. Here, we think that MoIV works as reducing agent for the C-O dissociation, since the C-O bond is located distant from surface Pt atom which supplies active hydrogen species. The C-O dissociation requires 2 electrons, and the MoIV species is converted into MoVI=O species, which is re-transformed into MoIV species by reduction with H2 and Pt catalyst. Two-electron reduction of coordinated organic molecule by MoIV active center has been already known in the deoxydehydration reaction system where ReV-ReVII or MoIV-MoVI redox pairs reduces vicinal diols to alkenes with an external reducing agent: M(n-2)+ + R-CHOH-CHOH-R’ → Mn+=O + R-CH=CH-R’ + H2O; Mn+=O + 2(H) 17 ACS Paragon Plus Environment


→M(n-2)+ + H2O.72,73 MoOx/TiO2 has been actually reported to have some activity in deoxydehydration74,75 even with H2 as the reducing agent.75 The specific performance of Mo addition in comparison with other group 5-7 metals can be explained by the role of additive as two-electron redox catalyst: when Re is added to Pt metal, the Re species are known to become low-valent species in reductive conditions.76 Re species are only active in deoxydehydration when the valence is kept high.77 In contrast, W and V species on TiO2 are difficult to reduce by two-electrons from the maximum valence even in the presence of noble metal.78-80 The surface adsorbed organic species becomes pentadienoic acid by the reduction with MoIV, and the Pt-catalyzed hydrogenation gives final product valeric acid. The reduction step produces MoVI species. The re-transformation of MoVI species to MoIV as described above and adsorption of substrate (FCA) completes the catalytic cycle (it is not clear which proceeds earlier between Mo reduction and FCA adsorption). The involvement of MoVI species in the catalytic cycle agrees with the short catalyst life of Pt-MoOx/TiO2 catalyst. While reduced MoIV species can have direct bond to noble metal including Pt,48 the most oxidized MoVI species has weaker interaction with noble metal surface. In addition, the solubility of MoVI species to water solvent is higher than that of reduced Mo species. The weak interaction with noble metal and higher solubility of MoVI species can lead to gradual breakdown of Pt-Mo interface site, which decreases the activity and selectivity of bimetallic Pt-MoOx/TiO2 catalyst. Re-construction of Pt-Mo interface site is a key for regeneration of Pt-MoOx/TiO2 catalyst; possible regeneration methods include dissolution and re-deposition of Pt and/or MoOx species.

CONCLUSIONS 18 ACS Paragon Plus Environment

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The activity of Pt-MoOx/TiO2 catalyst for 2-furancarboxylic acid (FCA) reduction to valeric acid depends on the loading amounts. The dependence of activity on the Mo amount is similar for catalysts with various Pt loading amount: the activity increases with increase in Mo amount up to around 0.5 wt%-Mo, and then the activity is almost unchanged or slightly decreased above 0.5 wt%-Mo. The catalyst is gradually deactivated, and both activity and selectivity are decreased during turnovers. High yield of valeric acid is only obtained with enough amount of catalyst; on the other hand, with enough amount of catalyst, the final yield of valeric acid (~60%) is similar over Pt-MoOx/TiO2 (Mo 0.5 wt%) catalysts with various Pt loading amounts. These activity trends and characterization results by XRD, CO adsorption and XAFS suggest that most Mo species are located on the surface of TiO2 support as sub-monolayer MoIV oxide. Some Mo species are located between TiO2 and Pt metal surfaces, and the Pt-Mo bimetallic site can be the catalytically active site. The amount of Pt-Mo bimetallic site is decreased during the reaction, which can be connected to the deactivation. Water solvent is by far the best solvent in the valeric acid production. Addition of methanol or acetic acid decreases the activity of Pt-MoOx/TiO2 catalyst, probably via covering of Mo site and Pt site, respectively. Based on the characterization results, kinetics (reaction order of 0.5 with respect to H2 pressure) and the reactivity of related substrates such as furan and dihydrofuran, we propose a reaction mechanism involving the ring-opening at C-O bond at 1,2-position after addition of one hydrogen atom to the 2-position of the ring and the dissociation of the other C-O bond on MoIV center which works as a 2-electron reducing agent.

ACKNOWLEDGEMENTS This work is supported by JSPS KAKENHI grant number 18H05247. XAFS experiments 19 ACS Paragon Plus Environment


at the BL14B2 station of SPring-8 were performed with the approval of the Japan Synchrotron Radiation Research Institute (JASRI; proposal nos. 2017B1839, 2018A1754 and 2018B1805). We thank Technical Division of School of Engineering, Tohoku University for TEM measurement.

ASSOCIATED CONTENTS Supporting Information XRD patterns of Pt/TiO2, raw data of EXAFS, assignment of long Mo-metal signal in Mo K-edge EXAFS, raw data of kinetics, and DFT calculation results for hydrogenated furans.

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Table 1. Hydrodeoxygenation of 2-furancarboxylic acid (FCA) over Pt-MoOx/TiO2 catalysts with various loading amountsa O

O O FCA

OH

OH

H2 O VA

+

O

O OH +

O +

HO O

DVL

THFCA

O

OH HVA

OH

+

O OVA + C4s + Others

Loading FCA Yield [%-C] Carbon amount [%] conv. balance b c d e f g [%] Pt Mo [%] VA THFCA DVL HVA OVA C4s Others 1 0 14

Structure and Mechanism of Titania-supported Platinum-molybdenum

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