TiO2 Nanocatalysts in Oxidation

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Synergistic Effects of Bimetallic PtPd/TiO2 Nanocatalysts in Oxidation of Glucose to Glucaric Acid: Structure Dependent Activity and Selectivity Xin Jin, Meng Zhao, Muzzammil Vora, Jian Shen, Chun Zeng, Wenjuan Yan, Prem S. Thapa, Bala Subramaniam, and Raghunath V. Chaudhari Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b04841 • Publication Date (Web): 23 Feb 2016 Downloaded from http://pubs.acs.org on February 27, 2016

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Industrial & Engineering Chemistry Research

Synergistic Effects of Bimetallic PtPd/TiO2 Nanocatalysts in Oxidation of Glucose to Glucaric Acid: Structure Dependent Activity and Selectivity Xin Jin,1$ Meng Zhao,1$ Muzzammil Vora,1,2 Jian Shen,3 Chun Zeng,1 Wenjuan Yan,1,2 Prem S. Thapa,4 Bala Subramaniam,1,2 Raghunath V. Chaudhari1,2* 1

Center for Environmentally Beneficial Catalysis, University of Kansas, 1501 Wakarusa

Drive, Lawrence, Kansas 66047, USA 2

Department of Chemical and Petroleum Engineering, University of Kansas, 1530 W 15th

St., Lawrence, Kansas 66045, USA 3

Department of Chemistry, University of Kansas, 1251 Wescoe Hall Drive, Lawrence,

Kansas 66045, USA 4

Microscopy and Analytical Imaging Laboratory, Haworth Hall, 1200 Sunnyside Ave,

University of Kansas, Lawrence, Kansas 66045, USA

$

Equally contributing authors

*

Corresponding author: [email protected]

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Abstract Direct oxidation of glucose with enhanced selectivity to glucaric acid with tartronic and oxalic acids as co-products is reported using bimetallic PtPd/TiO2 catalysts under mild conditions. Bimetallic PtPd catalysts display significantly enhanced catalytic activity (TOF: 2,404 h-1) and improved selectivity to glucaric acid (S: 44%) in glucose oxidation compared to monometallic catalysts (TOF: 248 h-1, S: 4%). Oxidation of glucose follows a consecutive reaction with gluconic acid as an intermediate with inhibition of second step (to glucaric acid and C-C cleavage reactions) by presence of glucose. Surface characterization using TEM, SEM, chemisorption, UV−vis and XRD distinguished the particle morphologies and provided insights into structure-activity relations. Reaction pathway for glucose oxidation is proposed based on product distribution. These results provide new insights into the design of bimetallic catalysts for oxidation of glucose to glucaric acid.

Key words: glucose; oxidation; glucaric acid; bimetallic catalyst; substrate inhibition


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1. Introduction Glucaric acid is one of the top twelve platform chemicals from biomass and a key intermediate to provide a renewable alternative for adipic acid synthesis required for nylon 6-6 and other products in plastics and textile industries.1 At present, adipic acid is produced from petroleum feedstock (e.g. benzene/cyclohexane) and manufactured via several energy intensive processes with serious environmental issues.2 Therefore, development of alternative, safer and environmentally benign route for adipic acid has great significance. Direct oxidation of glucose to glucaric acid is the key step in one of the potential routes for adipic acid synthesis from renewables (Scheme 1).3 At present, glucaric acid is mainly synthesized via oxidation of glucose using concentrated nitric acid or bleaching agents, a process generating significant amounts of toxic substances and waste products.4,

5

Replacing these oxidants with more

environmentally friendly and inexpensive ones will provide both economic and environmental benefits for this emerging technology. We recently reported catalytic oxidation of glucose and gluconic acid to glucaric acid over hybrid PtCu catalysts using molecular oxygen at mild conditions with significantly improved activity and selectivity.3 It was found that gluconic acid could be converted to a variety of C1−C6 mono and dicarboxylic acids under very mild conditions (45 °C, 0.1 MPa O2). The major products were glucaric (C6), tartronic (C3), oxalic (C2), as well as glyceric (C3), lactic (C3), glycolic (C2) and formic (C1) acids. During direct oxidation of glucose, it was observed that C-C cleavage of glucose was significant, which led to large amounts of C1−C3 carboxylic acids (S ~ 60%) limiting the selectivity of glucaric acid to 25 %. It was 3 ACS Paragon Plus Environment


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evident from this work that the mechanisms of C−O, C−C and C−H activation on surface metal atoms play important roles in controlling the selectivity, but such structureperformance relationships are not yet clearly understood for glucose oxidation to glucaric acid. In another aspect, the oxidation reactions are known to be very sensitive to surface morphologies of nanoparticles.6-8 In particular, catalysts based on Pt nanoparticles, have been extensively used in petrochemical upgrading, biomass conversion, fuel cells and other energy related applications.9-16 Bimetallic Pt nanocatalysts including PtCu,17,

18

PtNi19 and PtSn20 often exhibit unique features such as synergistic activity or shape controlled selectivity. Among all bimetallic Pt combinations, PtPd is a very special one because Pt and Pd have very similar lattice parameters.21,

22

This feature potentially

favors the formation of uniform PtPd nanoparticles with controlled structures. Although the mechanism of PtPd nanoparticle formation in homogeneous medium has been intensively investigated in the last decades,6,

23, 24

their catalytic activity in biomass

conversion reactions such as glucose oxidation is much less known.7, 25, 26 In this manuscript, bimetallic PtPd catalysts supported on TiO2 are reported for direct single step oxidation of glucose to glucaric acid for the first time (35−45 °C, 0.1 MPa O2 pressure). PtPd nanoparticles with alloy, core-shell and cluster-in-cluster structures were synthesized and immobilized on TiO2 support via one-pot aqueous chemistry. Detailed concentration-time profiles and reaction rates for C−O, C−C and C−H formation/cleavage reactions were measured experimentally on various PtPd catalysts. On the basis of surface characterizations by TEM, SEM, XRD and UV−vis, structure-performance correlation was established for various reactions including the formation of gluconic, 4 ACS Paragon Plus Environment

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glucaric (C−O and C−H activation), tartronic, oxalic (C−C cleavage, C−H and C−O activation), and other mono carboxylic acids (C−C cleavage) on different PtPd catalyst structures. Selected PtPd/TiO2 catalysts were further investigated to understand the possible reaction pathways in glucose oxidation. A detailed reaction network for glucose conversion to glucaric acid is also proposed consistent with the co-products formed during oxidation.

2. Experimental 2.1 Chemicals Glucose (99.5%), NaOH (97%), sodium gluconate (97%), potassium glucarate (98%), lactic acid (98%), glycolic acid (99%), formic acid (95%) and NaBH4 (98%) were purchased from Sigma Aldrich. Glyceric (2 mol/L aqueous solution) and tartronic (98%) acids were obtained from Fisher Scientific. Metal precursors such as H2PtCl6 (37.5% Pt basis) and Pd(NO3)2 (dihydrate) as well as rutile (< 100 nm) and anatase (< 25 nm) TiO2 nanopowders were also purchased from Sigma Aldrich. 2.2 Catalyst preparation Catalysts consisting of PtPd nanoparticles supported on TiO2 were prepared via a simple in situ reduction method in aqueous medium. This method was found to be effective for immobilizing Pt-based nanoparticles on solid supports.27 In general, known amounts of H2PtCl6 and Pd(NO3)2 were mixed with deionized (DI) water added dropwise into an aqueous phase slurry of TiO2. Depending on how H2PtCl6 and Pd(NO3)2 were added, PtPd nanoparticles with alloy, core-shell and cluster-in-cluster configurations were formed on rutile or anatase TiO2 supports (TiO2-r, TiO2-a). Detailed catalyst preparation procedures followed are described here. (1) For PtPda catalyst, predetermined amounts of metal precursors, H2PtCl6.6H2O and Pd(NO3)2 were dissolved in 150 mL DI water, 5 ACS Paragon Plus Environment


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followed by addition of this solution dropwise to a 300 mL aqueous slurry of TiO2 containing acetonitrile. The concentration of precursors in the slurry was in the range of 1.36×10-4−1.44×10-3 kmol/m3.28 After stirring for 2 h, a solution of 0.2 g of NaBH4 in 50 mL water was introduced into the slurry dropwise, after which the whole mixture was stirred overnight (16−20 h). The Pt and Pd metal contents in solid supports were in the range of 0.88−1.12 wt% as determined by ICP. (2) For Pd-Pt sample, Pd(NO3)2 dissolved in 75 mL DI water was first added dropwise to the TiO2 slurry in 300 mL water. Then 0.1 g of NaBH4 in 25 mL was slowly added to this slurry. After 2 h, 75 mL of H2PtCl6.6H2O solution was added dropwise, after which another 25 mL of 0.1 g NaBH4 was added. The whole mixture was stirred for an additional 2 h before the solid catalysts were filtered. (3) For Pt-Pd sample, the order of Pd and Pt precursor addition was changed. (4) For PtPdc catalyst sample, 75 mL of H2PtCl6.6H2O and 75 mL of Pd(NO3)2 solutions were prepared separately. Then 60 mL of H2PtCl6.6H2O and 15 mL of Pd(NO3)2 solutions were added to the TiO2 slurry in water dropwise before 25 mL of 0.1 g NaBH4 was charged. After 2 h reduction time, 15 mL of H2PtCl6.6H2O and 60 mL of Pd(NO3)2 solution were added followed by addition of another 25 mL of 0.1 g NaBH4 . The catalyst was filtered, washed with DI water and dried in a vacuum oven after additional 2 h of reduction time. (5) For PtPda7 sample, the experimental procedure was identical to (1) except that the metal precursors were reduced at pH=7 (tuned with NaOH addition). 2.3 Activity tests The activity tests were carried out in a three-neck flask with controlled heating hot plate under magnetic stirring at 1000 RPM. Similar operating procedures have already been discussed previously and hence only discussed briefly here.3 For oxidation experiments,


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NaOH solution was slowly introduced at a rate of 0.04 mL/min to glucose solution and catalyst slurry. The addition rate was controlled by a HPLC pump. The catalyst and glucose solution mixture was heated at a desired reaction temperature before both NaOH solution and O2 (at a rate of ~ 60 mL/min) were introduced. The total pressure is maintained at 0.1 MPa throughout each experiment. A liquid condenser was used to condense the vapor. During reaction, small amounts of samples (0.5−2 mL) were withdrawn from the reaction mixture and analyzed by HPLC, for which analytical conditions and chromatograph used were similar to those described previously.29 We observed that the maximum solvent loss during experiments is approximately 0.6 mL out of 50 mL. The significance of gas-to-liquid, liquid-to-solid and intra-particle mass transfer limitation was evaluated using the criteria proposed previously and the mass transfer and solubility parameters calculated using literature correlations.31 Detailed calculations have been shown in supporting information in order to confirm the insignificance of both external and internal mass transfer limitation. In addition, the pH values of reaction medium were measured using a pH meter purchased from Vernier. From the concentration values, conversion (X), selectivity (S), carbon balance (C%) and turnover frequency (TOF based on surface Pt and Pd atoms, obtained from chemisorption and particle size measurement, in h-1) were calculated as defined below. (1) Conversion is defined as the ratio of moles of carbon in converted substrate (e.g. glucose) after reaction to that in substrate which is charged initially.



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(2) Selectivity is defined as the ratio of amounts of carbon in a specific product (e.g. glucaric acid) formed, to the moles of carbon in converted substrate after reaction. (3) Carbon balance is the ratio of total carbons in all formed products to that in converted substrate. (4) Catalyst activity is defined as the ratio of moles of converted substrate to moles of both Pt and Pd sites on surface (determined from chemisorption and TEM characterization), and reaction time. 2.4 Catalyst Characterization Brunauer–Emmett–Teller (BET) measurement, chemisorption, UV−vis spectroscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM) and x-ray diffraction (XRD) were carried out as described previously.27, 29 BET: N2 adsorption studies were carried out using NOVA 2200e Instrument. Detailed measurement procedures were similar to that described previously.32 Chemisorption: H2 adsorption was carried out in Autochem 2910 Instrument. Temperature programmed desorption (TPD) of H2 was carried out in the same pot after chemisorption study of a sample. The ratios of both Pt and Pd metal species on surface to bulk were used to calculate surface activity of mono and bimetallic catalysts. The results from chemisorption were also compared with particle size measurement from TEM characterization. UV−vis spectroscopy: Surface absorbance under UV−vis was carried out using Shimadzu UV−3600 UV−VIS−NIR Spectrophotometer. The samples were dispersed in


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hexane solution and the solvents were dried on a quartz plate before optic spectrum data was recorded. Transmission electron microscopy (TEM): Sample preparation and detailed procedures are similar to that previously described.29 Samples were prepared by suspending the solid catalyst sample in ethanol and agitating in an ultrasonic bath. 10 µL of catalyst sample was placed onto a copper mesh grid. The wet grid was allowed to air-dry for several minutes prior to examination under TEM.33 Around 200 particles were measured and average particle size as well as standard deviation were calculated. Energy-dispersive xray spectroscopy was used to map the dispersion of Pt and Pd elements within selected nanoparticles. The particle size measurement of mono and bimetallic Pt and Pd catalysts was carried out using ImageJ software. Scanning electron microscopy (SEM): A Versa 3D dual beam Scanning Electron Microscope/Focused Ion Beam (FEI, Hillsboro, OR, USA) with a silicon drift EDX detector (Oxford Instruments, X-Max, UK) was used to measure the surface morphology, elemental composition and distribution of metals. All the SEM data reported were obtained at an acceleration voltage of 15kV, spot size 3.0 and the images were collected with an ET (Everhart Thornley) detector. The elemental mapping and energy spectrums were acquired with Aztec tools (Oxford Instruments, UK). X-ray diffraction (XRD): This measurement was performed on a Bruker D8 powder diffractometer with a copper target (CuKα radiation) operating at 40 kV and a current of 40 mA to analyze the crystal structures of materials.



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3. Results and Discussion 3.1 Characterization of mono Pt/TiO2 and bimetallic PtPd/TiO2 catalysts 3.1.1 Physical Properties and Chemisorption Surface area and pore size analysis were carried out for solid catalyst samples (Table 1). Pt and Pd metal contents in catalyst samples were measured by ICP. The Pt and Pd metal contents are in the range of 0.88−1.12 wt% and 0.94−1.12 wt%, respectively. The total surface area of TiO2 supported catalysts is very low, in the range of 19−30 m2/g. The overall pore volume of TiO2 materials is also low. Pt and Pd nanoparticles are therefore believed to be predominantly deposited on the surface of the support (confirmed by TEM). Chemisorption using H2 were also carried out for all solid catalyst samples to estimate the active metal dispersion (Table 1). Metal dispersion measured from chemisorption was also compared with the information from TEM measurement. It is found that metal dispersion values from both measurements agree very well. 3.1.2 TEM TEM images for Pt/TiO2-r, Pt/TiO2-a, PtPda/TiO2-r, PtPda/TiO2-a, and EDX mapping of Pt-Pd/TiO2-a and PtPda7/TiO2-a catalysts are shown in Figures 1 (a)−(e) respectively. In particular, we find that monometallic Pt on both rutile [TiO2-r, (i)−(ii) in Figure 1 (a)] and anatase [TiO2-a, (iii)−(iv) in Figure 1 (a)] support display similar particle sizes (3.2−4.0 nm), suggesting that the different crystalline forms of TiO2 support have negligible effect on the Pt particle formation during NaBH4 reduction. However, when Pd species were added to the monometallic Pt system [Figures 1 (b) and (c)], larger nanoparticles (6.5−7.5 nm) were observed on both TiO2 supports. EDX spectroscopy was carried out under STEM (scanning TEM) mode, in order to confirm the morphologies of bimetallic nanoparticles. In particular, element mapping was 10 ACS Paragon Plus Environment

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conducted on selected nanoparticles, where both Pt and Pd signals were collected and mapped by EDX detector, and then presented with different colors. Take Figure 1 (b) as a representative example [the right part of Figure 1 (b)]. It is found that both Pt and Pd signals are completely overlap with each other within the selected nanoparticle. Such observations were made on other nanoparticle in Figure 1 (b). This information confirms the alloy form of PtPd nanoparticles shown in Figure 1 (b). Similarly, we also confirm the alloy structures in Figure 1 (c) for PtPda/TiO2-a sample. Take Figure (d) as another important example, where Ptcore-Pdshell structures were observed. Specifically, both Pt and Pd signals were collected and mapped. It is found in the selected particle that, Pd element is well dispersed through the whole region while Pt is only found to be concentrated in the small core area. Clearly, by overlapping the two signals, one would see the PtcorePdshell morphologies in this sample. Following exactly the same methodology, STEM and EDX images presented in Figures 1 (e) and S2 further confirm the structures of bimetallic Ptcore-Pdshell and cluster-in-cluster structures. We also used line scan as an alternative approach to confirm the bimetallic structures. Take Figure S3 as an example. Line scan for Pt and Pd show that the two elements, although co-exist in the scanned region, are not uniformly distributed in the selected particles. This information confirms the cluster-incluster morphologies in PtPdc/TiO2-a catalyst. Furthermore, used PtPda/TiO2-a catalyst was also characterized using TEM technique. We observe a particle size increase from 4−9 nm to 6.7−16.4 nm after three recycles. But the alloy structures of PtPda/TiO2-a catalyst remains stable.



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3.1.3 SEM SEM images of selected bimetallic PtPd catalysts were also collected for the purpose of investigating the surface morphologies of TiO2 supported catalysts. As seen in Figure 2, (a) PtPda/TiO2-a, (b) PtPdc/TiO2-a and (c) PtPda7/TiO2-a images along with surface elemental analysis are presented. Pt and Pd are found to be well dispersed on TiO2 support, which is consistent with information obtained from TEM characterization. We also plotted the bulk and surface Pt metal composition in PtPda/TiO2-a, PtPdc/TiO2-a and PtPda7/TiO2-a catalyst samples and compared with literature data.34 Literature reports [solid and dash lines in Figure 2 (d)] show that Pd species tend to migrate towards the surface of nanoparticles, which were prepared by impregnation method. However, it is interesting to find that the surface composition of Pt (atomic ratio of Pt to Pd) of the three samples in this study is higher than literature values, indicating that the aqueous wet chemical reduction of Pd2+ is faster than Pt4+ resulting in relatively uniform bimetallic nanoparticles. 3.1.4 UV−vis Optical absorption characteristics of various bimetallic PtPd/TiO2 catalysts was carried out in order to probe the possible metal-metal and metal-support interactions on catalyst surface (Figure 3). Only PtPda7 sample displays an absorption onset at 310 nm, while all other samples show a shift of onset wavelength towards 330 nm. The bandgap on PtPda7 (Pd-shell) sample is similar to monometallic Pt/TiO2 absorption, suggesting its plasmon absorbance is insignificant.3 The red shift on other bimetallic samples indicates a strong plasmon absorbance of these bimetallic PtPd nanoparticles on TiO2 support. More specifically, we find that both PtPda/TiO2-r and PtPda/TiO2-a catalysts display almost identical optical behavior. Hence, it is possible that the interactions between PtPd alloy 12 ACS Paragon Plus Environment

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particles with different TiO2 supports are similar. But other bimetallic samples, including Pd−Pt and PtPdc ones, displaying cluster-in-cluster morphologies with different extents, show relatively weak absorbance signals compared with PtPda/TiO2-r and PtPda/TiO2-a catalysts. 3.1.5 XRD XRD analysis was further conducted on selected catalyst samples. XRD patterns of rutile and anatase TiO2 supported monometallic Pt and bimetallic PtPd catalysts are shown in Figure 4. For PtPda/TiO2-r sample, strong diffraction peaks at 27o, 36o and 55o indicate [110], [101] and [211] lattices in rutile phase, while 25o and 48o peaks found on TiO2-a supported samples suggest the presence of [101] and [220] phases. Due to the lower loading of Pt and Pd metals, it is difficult to detect their characteristic peaks. We find that Pt [111] peak overlaps with one of the peaks on TiO2-r support around 40o, while a small peak at 82o might indicate existence of Pt [311] on the surface. A similar peak is also found on TiO2-a samples but shifted slightly towards 85o, which indicates a possible interaction with Pd species on the support. Pd [111] exhibits a similar diffraction peak around 40o, which is also weak. The small peak at 47o in bimetallic PtPd samples suggests that Pd [200] might exist on the catalyst surface. 3.2 Activity and selectivity of mono and bimetallic catalysts For the purpose of comparing the activity and selectivity of various mono and bimetallic catalysts, external and intraparticle mass transfer limitation have been evaluated (see supporting information). It is confirmed that the reaction was carried out at kinetic controlled regime. In the benchmark studies on direct oxidation of glucose, we found that bimetallic PtPda/TiO2 catalysts display a synergistic effect in enhancing both the catalytic activity and glucaric acid selectivity compared to monometallic Pt/TiO2 and Pd/TiO2 13 ACS Paragon Plus Environment


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catalysts. In particular, the preliminary studies were carried out on mono and bimetallic Pd/TiO2-a, Pt/TiO2-a and PtPda/TiO2-a catalysts at 45 °C (Figure 5 and Table S1). Glucose conversion on Pd/TiO2-a and Pt/TiO2-a catalysts was found to be 30% and 56% respectively in 12 h, while with a bimetallic PtPda/TiO2-a catalyst complete conversion of glucose was achieved in 10 h. The glucose oxidation activities measured as TOF on Pd/TiO2-a, Pt/TiO2-a and PtPda/TiO2-a catalysts are 50, 248 and 2,404 h-1, respectively. With respect to product distribution, both monometallic Pd/TiO2-a and Pt/TiO2-a catalysts give high selectivity towards gluconic acid (S = 57−76%) with negligible glucaric acid formation (S ~ 4%) during 12 h reaction time. In sharp contrast, glucaric acid selectivity is found to be 31% on bimetallic PtPda/TiO2-a catalyst. Besides, 5-ketogluconic acid (4 in Scheme 2) selectivity is about 19−33% on monometallic catalysts,35 while the formation of this product is negligible on the bimetallic catalyst. These differences in activity and selectivity indicate that the bimetallic PtPd catalyst has higher oxidation activity for gluconic (2) to glucaric acid (5) while this reaction is very weak on monometallic Pt and Pd catalysts. In addition, other products, tartronic, oxalic, glyceric, glycolic and lactic acids were also detected on the bimetallic catalyst while the selectivity towards these products is low on monometallic catalysts. Support effects on activity and selectivity of monometallic Pt and bimetallic PtPd catalysts were also studied (Tables 2 and S1). We found that TiO2-a outperformed TiO2-r in terms of oxidation activity (Entries 1 and 2 in Table 2, Entries 1−6 in Table S1). Further experimental results using PtPda/TiO2-a catalyst (Entry 3) showed that glucose was actually completely converted even before 6 h at 45 °C. The combined selectivity of C6 (gluconic and glucaric acid) products is higher than 82%, implying that occurrence of 14 ACS Paragon Plus Environment

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C-C cleavage reactions was negligible within 6 h reaction time. When the reaction was prolonged to 12 h, the selectivity of gluconic acid decreases from 58% to 43% while that of glucaric, tartronic and oxalic acids increases from 34.7% to 45.7%. After 24 h reaction, the selectivity to glucaric, tartronic and oxalic acids is 40.4%, 15.4% and 5.5% respectively. These results suggest that gluconic acid is a key intermediate for the formation of these aldaric acids (glucaric, tartronic and oxalic acids). Although C−C cleavage occurs simultaneously with glucaric acid formation, most of the products were aldaric rather than aldonic acids (gluconic, glyceric, glycolic, lactic and formic acids), indicating that secondary oxidation reactions were significant on the bimetallic PtPda/TiO2-a catalyst surface. The oxidation performance of bimetallic PtPd catalysts on TiO2-a was therefore the main focus of further studies in this work. 3.3 Structure-activity relationship Various bimetallic PtPda, Pt−Pd, Pd−Pt, PtPda7 and PtPdc catalysts supported on TiO2-a were prepared and evaluated for glucose oxidation at 45 oC and 0.1 MPa O2 pressure. The results during 6 h reaction time are shown in Figure 6. Glucose conversion was total on all PtPd catalysts. With regard to the selectivity towards oxidation products, PtPda catalyst exhibits 25.3%, 6.9% and 2.5% selectivity to glucaric, tartronic and oxalic acids respectively, while only 10.4%, 3.9% and 2.6% of these acids were observed on Pd−Pt catalyst. Furthermore, the combined selectivity to C1-3 acids is relatively higher on Pd−Pt catalyst (S > 17%) compared with PtPda (S = 12%). PtPda7 catalyst exhibits dominant gluconic acid selectivity after 6 h, suggesting its poor secondary oxidation activity. As shown in TEM characterization of these bimetallic PtPd catalysts (Figures 1, S1 and S2), we find that PtPda, PtPdc, Pt-Pd are alloy, cluster−in−cluster and Pd−shell structures, 15 ACS Paragon Plus Environment


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respectively. Clearly, PtPda alloy structure displays better oxidation performance than other morphologies while cluster and Pd−shell structures exhibit relatively poor activity and selectivity. In terms of possible metal−support and metal-metal interactions, as already shown in Figure 3, we observed that both PtPda and Pt−Pd samples show strong metal-TiO2 interaction. For PtPda7 catalyst, the bandgap is similar to monometallic Pt/TiO2, suggesting that Pt and TiO2 interaction is insignificant. This phenomenon is consistent with its poor secondary oxidation activity. In addition, although PtPdc catalyst also showed bandgap shift (~ 330 nm), the absorbance is lower than the alloy and random alloy structures (PtPda and Pt−Pd), indicating that the metal-metal interaction in cluster−in−cluster structure is relatively weak. In order to understand the dependence of global oxidation reaction rates, including primary and secondary oxidation as well as C−C cleavage reactions on PtPd structures, reaction profiles of glucose oxidation were experimentally measured on selected PtPd catalysts. We chose PtPda, PtPdc and PtPda7 catalysts for this study. The corresponding concentration-time profiles are shown Figures 7−10. As shown in Figure 7, concentration profiles on PtPda catalyst at 45 °C show that glucose concentration decreased from 0.28 kmol/m3 to zero within only 3 h. Notably, gluconic acid was the dominant product before glucose was completely consumed. The formation of glucaric acid as well as other carboxylic acids with lower C numbers was detectable only after 3 h, with gluconic acid reaching a peak and then decreasing from 0.27 kmol/m3 to < 0.05 kmol/m3 within 24 h. The concentrations of glucaric, tartronic and oxalic acids increased from almost zero before 3 h to approximately 0.11, 0.095 and 0.07 kmol/m3 respectively within 24 h. 16 ACS Paragon Plus Environment

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Glucose conversion profiles for PtPdc catalyst at 45 °C are shown in Figure 8. It is found that glucose concentration decreased from 0.28 kmol/m3 to zero within 6 h reaction, suggesting a relatively lower glucose oxidation rate on PtPdc compared to PtPda catalyst. Interestingly, before glucose was completely consumed, the concentration of aldaric acids was negligible, suggesting a similar glucose inhibition effect in secondary oxidation reactions as observed for both PtPda and PtPdc catalysts. With regard to the concentration of secondary oxidation products, the formation of glucaric, tartronic and oxalic acids was only slightly lower on PtPdc catalyst at 24 h compared to PtPda catalyst. The observed inhibition effect on PtPda and PtPdc catalysts indicates that the C=O bond in glucose might interact/adsorb strongly on the bimetallic PtPd surface. This intriguing possibility might provide insights into the plausible reaction pathways. Hence, additional experiments were specifically designed. (1) We carried out a control experiment with both glucose and sodium gluconate (molar ratio: 4/6) as the starting materials, simulating conditions for 60% conversion of glucose (Figure 9). It is observed that glucose was completely converted < 4 h, while the concentration of gluconic acid also reached the peak value of 0.27 kmol/m3 before starting to decline. Importantly, before glucose was consumed completely, the concentrations of glucaric, tartronic and oxalic acids were again found to be negligible. (2) In addition, reaction profiles on bimetallic PtPda7 were also measured (see Figure 10). For this experiment, the glucose concentration was found to decrease from initial 0.28 kmol/m3 to zero within 6 h reaction, suggesting primary oxidation rate is similar to PtPdc catalyst. However, the gluconic acid concentration only decreased from 0.26 to 0.16 kmol/m3 within 24 h time. And the concentration of glucaric acid as well as tartronic and 17 ACS Paragon Plus Environment


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oxalic acid is only less than half of that obtained on both PtPda and PtPdc catalysts within 24 h reaction, indicating a significantly lower secondary oxidation rates on PtPda7 catalyst at 45 °C. Results in both (1) and (2) agree very well with the experimental findings in Figures 7 and 8, further confirming our hypothesis of the possible glucose inhibition effect on bimetallic PtPd catalysts. This effect actually favorably prevents the side reactions such as C−C cleavage of glucose in the reaction medium in the initial phase of the reaction. Thus, it is highly possible that C−C cleavage occurs significantly with gluconic acid in the reaction medium. This observed inhibition effect was not well understood in previous studies, where researchers found that the primary oxidation and C−C cleavage reactions occurred simultaneously as parallel reactions on metal catalyst surface.36-39 Considering the fact that monometallic Pt and Pd catalysts studied in the benchmark experiments displayed poor selectivity for glucaric acid, it is clear that the combination of two metal species accelerates the formation of carboxylic RCO−OH groups, not only promoting both primary and secondary oxidation reactions but also restraining C-C cleavage during the conversion of glucose. Furthermore, the formation rates of gluconic acid (r1, primary oxidation), glucaric acid (r2, secondary oxidation), tartronic and oxalic acids (r3, C−C cleavage and secondary oxidation) as well as monocarboxylic acids (r4, glyceric, lactic, glycolic and formic acids, C−C cleavage reactions) are plotted for bimetallic PtPda (alloy), PtPdc (cluster−in−cluster) and PtPda7 (Pd shell) and monometallic Pt catalysts (Figures 7−10, Table S1). The reaction rates were measured when a particular species formation started in the reaction medium (initial formation rate). For example, r2 on the alloy structure (PtPda catalyst) 18 ACS Paragon Plus Environment

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was calculated when glucaric acid formation started in reaction medium (from Figure 7). As seen from Figure 11, the formation rate of gluconic acid (r1, orange bar) is significantly higher on alloy structure compared with others, suggesting that PtPd alloy nanoparticles promote the primary oxidation reaction of glucose in the reaction medium. As expected, the measured rate for r2 (green bar) is much lower than r1 on all investigated catalysts. In particular, r2 is approximately two times higher on PtPd alloy catalyst (PtPda) compared to other structures. The alloy catalyst also outperformed others in terms of r3 and r4. 3.4 Effects of reaction conditions Two additional experiments, one at 35 oC, and another one with lower glucose concentration (0.167 kmol/m3) at 45 oC were carried out on PtPda catalyst in order to understand the effects of reaction conditions on glucose oxidation. As shown in Entry 1 of Table 3, small amounts of 5-keto-gluconic acid were detected in the reaction mixture at 35 oC whereas its concentration was negligible at 45 oC under otherwise similar reaction conditions (Table 2). The selectivity towards glucaric acid is only 8.4% after 12 h, which is significantly lower than that at 45 °C (Entry 4 in Table 3). This observation implies that as reaction temperature decreases, the secondary oxidation rate (oxidation of gluconic acid) is slowed down considerably, suggesting relatively higher second step oxidation barriers compared with the first step oxidation. When the reaction was prolonged to 24 h (Entry 2 of Table 3), 5-keto-gluconic acid disappeared with increasing selectivity to glucaric acid (22%) and tartronic acid (10.8%). After 72 h, gluconic acid selectivity decreased to approximately 21% while that towards glucaric and tartronic acids increased to 44.3% and 17.4%, respectively. The results at 35 19 ACS Paragon Plus Environment


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o

C and 45 oC indicate that the temperature, while clearly influencing the reaction rates,

has negligible effects on glucaric acid and tartronic acid selectivity on the PtPda alloy catalyst. Experimental results with lower initial glucose concentration are shown in Entries 4−6 of Table 3. When we compare the results on PtPda alloy catalyst in Entry 4 with Figure 6, it is found that although glucose conversion was completed within 6 h in both cases, the selectivities towards primary and secondary products are very different. Specifically, the selectivity of gluconic acid is 69% with 0.167 kmol/m3 while this value is lower (55%) with 0.277 kmol/m3 of initial glucose concentration. The difference indicates that the secondary reactions are more significant when glucose concentration is higher. In terms of secondary products, glucaric, tartronic and oxalic acid selectivities were 11%, 10% and 3.6% (see Table 3), while these were 25%, 7% and 4% respectively in the case shown in Figure 6. When the reaction was prolonged to 24 h (Entry 6), glucaric acid selectivity was increased to 41% with large amounts of tartronic and oxalic acids formation as co-products. This observation means that higher alkali to substrate ratio actually promotes further conversion of gluconic acid to lower aldaric acids via C-C cleavage. In addition, we also recorded the pH values in the reaction medium during batch experiments. An example is shown in Figure 12 (using PtPda catalyst), it is found that pH value of liquid phase is 11.5±0.3 at the beginning of glucose oxidation reaction (in the presence of PtPda catalyst), after which the value quickly drops to 10.2±0.3 and 9.3±0.2 after 0.5 h and 1 h. pH values of the reaction medium further drops to 6.5 after 24 h reaction. It is well established that glucose tends to undergo decomposition reactions at 20 ACS Paragon Plus Environment

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pH > 12.4,5,29,37,38 In this case however, we observed that C-C cleavage and side reactions of glucose were completely restrained at the beginning (from 0 to 3 h), indicating that catalytic oxidation of glucose is dominant on bimetallic PtPda catalyst. From 3 to 6 h (pH ~ 7.5), gluconic acid is found to undergo both further oxidation and C-C cleavage to a variety of secondary products. This observation confirms that PtPda catalyzed oxidation and C-C cleavage was significant. Stability studies were also carried out on PtPta/TiO2 catalyst at 45 oC and 0.1 MPa O2. As shown in Table 4, after three recycles, the catalyst still displays complete glucose conversion with similar glucaric and tartronic acid selectivities. Metal leaching was measured for Pt and Pd after three recycles. It is found that the total Pt and Pd leaching are < 4.2% and < 4% respectively after three recycles and at this level the catalytic performance is not affected significantly. These results suggest that PtPta/TiO2 catalyst is very stable under our reaction conditions. 3.5 Reaction pathways We already noted based on concentration-time profiles that glucose displays a substrate inhibition effect for second step oxidation reactions as well as C−C cleavage (retroaldolization) thus also affecting the selectivity pattern. In order to further understand this interesting behavior, we carried out another set of control experiments with fructose, an isomer of glucose and obtained temporal conversion and selectivity data at 45 °C (Figure 13). The top two pie graphs in Figure 13 present the conversion and product distribution data of glucose oxidation at 6 h and 10 h. Although glucaric acid formation was negligible at



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the beginning, due to the glucose inhibition effect, glucaric acid selectivity was enhanced from almost zero to 25%, from 3 h to 6 h (Figure 6), and further to 37% within 10 h. Interestingly, we did not observe a similar substrate inhibition effect during fructose oxidation, as shown in the bottom two pie graphs in Figure 13. Within 6 h reaction, fructose only displays 53% conversion on PtPda alloy catalyst at 45 °C. Even before fructose was completely converted, other reactions such as C−C cleavage already occur. Notably, we did not detect glucose in the reaction solution, indicating that once fructose slowly isomerized to glucose, oxidation occurred to consume it. As seen from the results during a 6 h run, gluconic acid was still the dominant product during fructose conversion. Only 12% glucaric acid selectivity was observed, suggesting that isomerization, primary oxidation as well as secondary oxidation reactions compete on PtPda catalyst surface. In sharp contrast to results with glucose as substrate, we observed appreciable glyceric acid formation during fructose conversion. This suggests that both retro-aldolization of fructose (C3−C3 cleavage) as well as further oxidation of the C3 intermediates are significant. These results support two of our hypotheses: (a) Inhibition induced by terminal C=O (aldose) surface species is more significant than secondary C=O (ketose) species during oxidation reactions. (2) Existence of secondary C=O bond in fructose favors facile C-C cleavage, although overall reaction rate is lower than glucose oxidation. We also compared the glucose oxidation results of this work with reported glycerol (a C3 sugar polyol) oxidation results over Pt−based catalysts.40-42 In the cited studies on glycerol oxidation, poor liquid-phase carbon balance (67%−85%) is reported when glycerol conversion was high (> 30%, 50−90 °C).43, 44 This is because once C=O bond (i.e. carbonyl species) was formed from glycerol, decarbonylation catalyzed by noble 22 ACS Paragon Plus Environment

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metals led to a side reaction generating CO, which either deactivated the catalysts or formed CO2 (in the form of carbonate salt) in the reaction medium. These products could not be quantitatively assessed by HPLC. However, in our glucose oxidation study, improved liquid phase carbon balance (88−96%, see Tables 2 and 3, Figure 13) was observed in most of the experiments, suggesting that decarbonylation of glucose is not significant compared with that of glycerol. The difference might be possibly caused by the different nature of C3 and C6 substrates. Based on the conversion data shown in Figures 7−12, we propose here plausible reaction pathway for glucose and fructose conversion (see Schemes 3 and 4). (a) Primary reaction(s). Glucose (1) conversion to gluconic acid (2) is clearly the primary oxidation reaction on PtPd catalysts. As observed from Figures 7−10, C-C cleavage of glucose is almost negligible in this case and hence not shown.. (b) Oxidation of gluconic to glucaric acid. Further oxidation of (2) will lead to the formation of glucuronic acid (3) in presence of metal catalysts. We observed that (3) easily isomerizes to 5-keto-gluconic acid (4) in the reaction medium, as already shown on monometallic Pt catalysts (Tables S1 and S2). The combination of Pt and Pd catalysts enhances the oxidation activity such that (3) can be further oxidized to glucaric acid (5). (c) C-C cleavage and consecutive oxidation. Our results confirm that (2) is the dominant product from glucose conversion, while C1−C3 products are generated in significant amounts (S > 54%) when sodium gluconate is used as the substrate for oxidation experiments (Table 2). Therefore it is clear that C-C cleavage reactions occur after (2) is formed. Direct C−C cleavage of (4) will generate precursors for tartronic acid



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(6).45 C−C cleavage of (3) will generate C2 and C4 intermediates and quickly give C2 intermediates, which will eventually form oxalic acid (7) through further oxidation. Further, direct C−C cleavage of gluconic acid can generate several monocarboxylic acid precursors such as formaldehyde, glycolic aldehyde and glyceraldehyde, which undergo further reaction to form lactic (8), formic (10), glycolic (9) and glyceric acids (11). Of course, (9) and (11) can be further oxidized to (7) and (6) in presence of O2. (d) C-C cleavage of fructose (12). It is known that existence of C=O easily induces C−C cleavage in a sugar molecule.27, 29, 45-51 Thus, it is interesting to know if fructose (12, a ketose and isomer of glucose) will undergo instantaneous C−C cleavage or isomerization to (1) for further oxidation. Our experimental results show that the reaction of (12) in presence of PtPda alloy catalyst with O2 is very slow. Only < 60% conversion of (12) was observed in 6 h (Figure 13) as compared to total conversion of (1) in 4 h on the same catalyst at otherwise identical conditions. The different reaction rates suggest that (12) is not as reactive as (1). The detection of (2), (5), (6−11) (see Figure 13) clearly suggests that isomerization of (12) is also significant in the reaction medium. As hypothesized, (12) is expected to undergo facile C-C cleavage reactions. The 22% selectivity of (11) supports our proposed reaction pathway, which C3-C3 cleavage occurs for β-hex-ketose (12) in the alkaline medium. Therefore, it is clear that C-C cleavage, isomerization and oxidation are major competing reactions in the reaction system with fructose as a substrate.

4. Conclusion The direct one-pot oxidation of glucose to glucaric acid (S = 44%) with tartronic (S = 15%), oxalic (S = 6%) acids as co-products using a bimetallic PtPd alloy catalyst is 24 ACS Paragon Plus Environment

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reported here for the first time. In particular, the structure-dependent oxidation activity of bimetallic PtPd nanoparticles has been studied for glucose oxidation. TiO2 supported PtPd catalysts exhibited synergistic activity compared with monometallic Pt and Pd ones for both primary and secondary oxidation of glucose at 45 °C and 0.1 MPa O2. Surface characterization using TEM, SEM, XRD and UV−vis of various bimetallic PtPd nanocatalysts further confirmed that the alloy structure showed the best oxidation activity among all the bimetallic structures. Concentration-time profiles on different bimetallic PtPd catalysts showed glucose inhibited the second step oxidation of gluconic acid in the reaction medium, with the secondary oxidation and C-C cleavage occurring after glucose was consumed. Based on temporal concentration-time profiles of glucose and fructose oxidations, plausible reaction pathways are proposed. The results presented here on the structural effects of bimetallic catalysts and the observed inhibition effect of glucose provides guideline for rational design of improved catalysts and process optimization to maximize the yield of desired products.

Acknowledgement X. J. acknowledges a postdoctoral fellowship from Center for Environmentally Beneficial Catalysis at the University of Kansas. This work is supported by NSF/EPA grant CHE1339661. We want to thank Dr. Victor Day for carrying our XRD analysis.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. Additional information on reaction results, analytical conditions and TEM characterization.



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Figures, Tables and Schemes

(a)

(b)

(c)

(d)

(e)



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(f) Figure 1. TEM images of (a) Pt/TiO2-r (i, ii) and Pt/TiO2-a (iii, iv) samples, (b) PtPda/TiO2-r, (c) PtPda/TiO2-a, STEM and EDX mapping of (d) Pt-Pd/TiO2-a, (e) PtPda7/TiO2-a and (f) used PtPda/TiO2-a catalysts, (white bars indicate 10 nm scale)


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(a)

(b)



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(c)

(d) Figure 2. SEM images of (a) PtPda/TiO2-a, (b) PtPdc/TiO2-a and (c) PtPda7/TiO2-a and (d) surface-bulk relationship of PtPd alloy particles


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Figure 3. UV-vis spectra of bimetallic PtPd/TiO2 catalysts



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Figure 4. X-ray diffraction patterns of different TiO2 supported PtPd catalysts


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Figure 5. Conversion and selectivity of glucose oxidation on Pd/TiO2-a, Pt/TiO2-a and PtPda/TiO2-a catalysts (Reaction conditions. T: 45 °C, substrate concentration: 0.28 kmol/m3, 1.50 kmol/m3 of NaOH solution was added to glucose solution at 0.04 mL/min rate, catalyst loading: 0.1 g, solvent: DI H2O, total liquid volume: 50 mL, O2 bubbling rate: 60 mL/min at 0.1 MPa. Glu: gluconic acid, Glu-k: 5-keto-gluconic acid, Gla: glucaric acid, Tar: tartronic acid, others: oxalic glyceric, lactic, glycolic and formic acids. Reaction time: 12 h for Pd/TiO2-a and Pt/TiO2-a, 10 h for PtPda/TiO2-a).



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Figure 6. Glucose conversion and selectivity on different PtPd/TiO2 catalysts at 45 °C in 6 h. Reaction conditions: 1.50 kmol/m3 of NaOH solution was added to glucose solution at 0.04 mL/min rate, catalyst loading: 0.1 g, solvent: DI H2O, O2 bubbling rate: 60 mL/min at 0.1 MPa. Glu: gluconic acid, Gla: glucaric acid, Tar: tartronic acid, Oxa: oxalic acid. *Others: glyceric, lactic, glycolic and formic acids.


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Figure 7. Glucose concentration-time profiles on PtPda/TiO2-a catalyst at 45 °C. Reaction conditions: 1.50 kmol/m3 of NaOH solution was added to glucose solution at 0.04 mL/min rate. catalyst loading: 0.1 g, solvent: DI H2O, O2 bubbling rate: 60 mL/min at 0.1 MPa. Glu: gluconic acid, Gla: glucaric acid, Tar: tartronic acid, Oxa: oxalic acid. *Others: glyceric, lactic, glycolic and formic acids.



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Figure 8. Glucose concentration-time profiles on PtPdc/TiO2-a catalyst at 45 °C (see Figure 7 for explanation of abbreviations and reaction conditions)


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Figure 9. Concentration-time profiles on PtPdc/TiO2-a catalyst at 45 °C with glucose and sodium gluconate (molar ratio: 4/6) as starting materials (see Figure 7 for abbreviation and reaction conditions).



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Figure 10. Glucose concentration-time profiles on PtPda7/TiO2-a catalyst at 45 °C (see Figure 7 for abbreviation and reaction conditions)


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Figure 11. Formation rates of gluconic acid (r1), glucaric acid (r2), tartronic and oxalic acids (r3) as well as monocarboxylic acids (r4, glyceric, lactic, glycolic and formic acids) at 45 °C on different Pt-based catalysts (catalyst loading: 2 kg/m3).



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Figure 12. pH values along with reaction time at 45 °C (catalyst: PtPda/TiO2-a, refer to Figure 7 for experimental details).


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Figure 13. Conversion and product distribution of glucose and fructose oxidation at 45 °C on PtPda/TiO2-a catalyst (Glu: gluconic acid; Gla: glucaric acid; Tar: tartronic acid; Oxa: oxalic acid; Other acids: glyceric, glycolic, lactic and formic acids; C def%: carbon deficit in the system)



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Table 1. Physical properties and chemisorption analysis of solid catalyst samples

SBET (m2/g)

Vpore (10-2 m3/g)

Pt/TiO2-a

29.4

PdTiO2-a

Catalyst

Metal dispersion (%)

Metal content (wt%) by ICP

Chemisorption

TEM

Pt

Pd

2.99

46

58

0.89

0

19.1

5.67

59

51

0

1.12

PtPda/TiO2-r

19.9

4.56

42

56

1.00

1.11

PtPda/TiO2-a

23.3

6.16

51

58

0.91

0.95

PtPda7/TiO2-a

21.6

5.09

44

49

0.88

0.94

30

24

1.12

1.10

used 21.5 6.13 PtPda/TiO2-a Catalyst particle density: 3900 kg/m3.


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Table 2. Benchmark data for oxidation reactions on PtPd/TiO2 catalysts

no

Catalyst

Substrate

Selectivity (%)

Time (h)

X (%) Glu

Gla

Tar

Oxa

Others*

1

PtPda/TiO2-r

Gluconate

6

41.4

-

41.2

28.7

6.8

17.0

2

PtPda/TiO2-a

Gluconate

6

78.5

-

37.8

28.8

8.0

16.5

3

PtPda/TiO2-a

Glucose**

6

100

57.7

25.3

6.9

2.5

2.9

4

PtPda/TiO2-a

Glucose**

10

100

42.8

31.1

10.7

3.9

4.0

5

PtPda/TiO2-a

Glucose**

24

100

28.0

40.4

15.4

5.5

6.7

Experimental conditions. T: 45 °C, substrate concentration: 0.28 kmol/m3, NaOH concentration: 0.75 kmol/m3, catalyst loading: 0.1 g, solvent: DI H2O, total liquid volume: 50 mL, O2 bubbling rate: 60 mL/min at 0.1 MPa. Glu: gluconic acid, Gla: glucaric acid, Tar: tartronic acid, Oxa: oxalic acid. *Others: glyceric, lactic, glycolic and formic acids. **1.50 kmol/m3 of NaOH solution was added to glucose solution at 0.04 mL/min rate.



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Table 3. Glucose oxidation on PtPda alloy catalyst under different initial conditions

Concentration (kmol/m3)

T (°C)

Time (h)

X (%)

1

0.277

35

12

2

0.277

35

3

0.277

4

no

Selectivity (%) Glu

5-k-Glu

Gla

Tar

Oxa Others*

100

77.2

3.1

8.4

3.2

1.2

0.9

24

100

55.6

0.1

22.2 10.8

3.1

6.1

35

72

100

21.1

-

44.3 17.4

5.3

9.1

0.167

45

6

100

69.2

-

11.0

9.9

3.6

5.9

5

0.167

45

12

100

33.6

-

22.1 14.4

6.6

7.8

6

0.167

45

24

100

8.9

-

41.2 19.9

6.3

8.8

Experimental conditions same as Table 2.


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Table 4. Stability study on PtPta/TiO2 catalyst at 45 °C and 0.1 MPa O2 Selectivity (%) Recycle no

Time (h)

X (%) Glu

5-k-Glu

Gla

Tar

Oxa

Others*

1

24

100

28.7

-

40.9

15.6

4.5

7.8

2

24

100

27.9

-

42.1

14.3

3.6

6.7

3

24

100

29.1

-

39.5

16.4

2.2

6.0

Experimental conditions same as Table 2. Solid catalyst was recovered by centrifuge and washed with deionized water for six times prior to use in subsequent batch studies.



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Scheme 1. Conversion of glucose to adipic acid

Scheme 2. Glucose oxidation to gluconic and glucaric acids


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Scheme 3. Plausible reaction pathways of glucose oxidation



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Scheme 4. Plausible reaction pathways of C-C cleavage of fructose


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Graphical Abstract Glucose inhibition enhances selectivity towards glucaric acid and prevents side reactions on bimetallic PtPd/TiO2 catalysts during glucose oxidation


TiO2 Nanocatalysts in Oxidation

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