Hydrodeoxygenation of 2,5-dimethyltetrahydrofuran over bifunctional Pt–Cs2.5H0.5PW12O40 catalyst in the gas phase: enhancing effect of gold

2,5-Dimethyltetrahydrofuran (DMTHF) is deoxygenated to n-hexane with >99% selectivity at mild conditions (90 °C, 1 bar H2 pressure, fixed-bed reactor) in the presence of the bifunctional metal-acid catalyst Pt–CsPW comprising Pt and Cs2.5H0.5PW12O40 (CsPW), an acidic Cs salt of Keggin-type heteropoly acid H3PW12O40. Addition of gold to the Pt–CsPW catalyst increases the turnover rate at Pt sites more than twofold, whereas the Au alone without Pt is not active. The enhancement of catalyst activity is attributed to PtAu alloying, which is supported by STEM-EDX and XRD analysis.

Biomass-derived furanic compounds are of interest as a renewable feedstock, which can be processed into a range of valueadded chemicals and green fuels via catalytic hydroconversion. 1-8 Hydrodeoxygenation (HDO) of furanic compounds using bifunctional metal-acid catalysis has been demonstrated to be an effective strategy to produce green fuels under mild conditions 3,4,6,[8][9][10][11][12] and references therein. The HDO over bifunctional metal-acid catalysts is much more efficient compared to the reaction over monofunctional metal catalysts. 11,12 Previously, we have reported HDO of a wide range of oxygenates in the gas phase to produce alkanes in the presence of bifunctional catalysts comprising Pt, Ru, Ni and Cu as metal components and Keggin-type heteropoly acids, with their activity decreasing in the order Pt > Ru > Ni > Cu. 13,14 Pt-CsPW comprising Pt and strongly acidic heteropoly salt Cs 2.5 H 0.5 -PW 12 O 40 (CsPW) has been reported to be a highly efficient catalyst for the HDO of 2,5-dimethylfuran (DMF) and 2,5dimethyltetrahydrofuran (DMTHF) to produce n-hexane with 100% yield at 90-120 C and ambient pressure. 11,12 The HDO of DMTHF over Pt-CsPW occurs through a sequence of hydrogenolysis, dehydration and hydrogenation steps catalysed by Pt and proton sites of the bifunctional catalyst (Scheme 1). These include the ring opening of DMTHF to form 2-hexanol on Pt sites followed by its dehydration on proton sites of CsPW to hexene, which is nally hydrogenated to n-hexane on Pt sites. 12 It is the facile dehydration of the secondary alcohol intermediate that drives the HDO process forward. 11,12 The ratelimiting step is either the ring hydrogenolysis or 2-hexanol dehydration depending on the ratio of accessible surface metal and acid sites Pt/H + . 12 Other platinum group metals such as Pd, Ru and Rh, that have high selectivity to ring hydrogenation rather than ring hydrogenolysis, 2,7 have low activities in HDO of DMF and DMTHF. 11 Bimetallic PtAu and PdAu catalysts have been reported to have an enhanced performance in comparison to monometallic Pt and Pd catalysts, 15-31 for example, in hydrogenation, 16,21,29 hydrodesulphurisation, 27,28 oxidation, 22,24 isomerisation 15,19,30,31 and other reactions. 17,18,20,25,26 The enhancement of catalyst performance by addition of gold can be attributed to geometric (ensemble) and electronic (ligand) effects of the constituent elements in PtAu and PdAu bimetallic species. 25,26 Here we looked at the effect of Au on the performance of Pt-CsPW catalysts in the HDO of DMTHF in the gas phase (see the ESI for experimental details †). The CsPW heteropoly salt is a well-known solid acid catalyst; it possesses strong proton sites, large surface area and high thermal stability ($500 C decomposition temperature). 9,32-34 Supported bimetallic catalysts PtAu/SiO 2 and PtAu/CsPW were prepared by coimpregnation of H 2 PtCl 6 and HAuCl 3 onto SiO 2 and CsPW followed by reduction with H 2 at 250 C (ESI †). This method gives supported bimetallic PtAu nanoparticles of a random composition together with various Pt and Au nanoparticles. 15,16,31 Information about the catalysts studied is given in Table 1.
Powder X-ray diffraction (XRD) has been widely used for the characterization of supported Au alloy catalysts. 26 The XRD patterns for the silica-supported catalysts 6.4% Pt/SiO 2 , 6.5% Au/ SiO 2 and 6.6% Pt/5.9% Au/SiO 2 are shown in Fig. 1. As expected, the 6.4% Pt/SiO 2 and 6.5% Au/SiO 2 catalysts display the fcc pattern of Pt and Au metal nanoparticles. The Pt peaks are broader than the Au peaks, which indicates a higher dispersion of Pt particles, with an average particle size of 8.0 nm for Pt and 46 nm for Au, which is in agreement with the STEM values (  2 shows the high-angle annular dark eld (HAADF) STEM images of the three silica-supported catalysts 6.4% Pt/ SiO 2 , 6.5% Au/SiO 2 and 6.6% Pt/5.9% Au/SiO 2 with metal nanoparticles indicated as bright spots on the darker background. In the Pt/SiO 2 catalyst, there are two populations: small Pt particles of 5 nm size and coalesced Pt particles of a larger size. The Au/SiO 2 catalyst displays Au particles of spherical, rectangular and triangular morphology, with an average size of 38 nm. The bimetallic PtAu/SiO 2 catalyst shows a high agglomeration and different kinds of morphology of metal particles.
The energy dispersive X-ray spectroscopic analysis (EDX) of metal particles in the PtAu/SiO 2 catalyst shows that these particles contain both platinum and gold. EDX elemental mapping clearly demonstrates that Pt and Au maps cover the same areas of PtAu/SiO 2 catalyst (Fig. 3), indicating PtAu alloying with formation of a non-uniform bimetallic PtAu particles. More EDX mapping is presented in the ESI (Fig. S1 †).
STEM-EDX for CsPW-supported Pt, Au and PtAu catalysts has been reported elsewhere. 16 These STEM images are difficult to analyse due to W, Pt and Au having similar large atomic  numbers Z (74, 78, and 79, respectively). Crystalline CsPW containing 70 wt% of W displays a strong background which makes it difficult to discern smaller Pt and Au particles from the Z-contrast HAADF images and determine accurately metal particle size. Nevertheless, the STEM-EDX analysis indicates the presence of bimetallic PtAu particles in the PtAu/CsPW catalyst with a wide range of Pt/Au atomic ratios. 16 Representative results for HDO of DMTHF in the presence of bifunctional metal-acid catalysts Pt-CsPW and PtAu-CsPW, which were used as physical mixtures of metal and acid components at similar Pt loadings, are shown in Table 2. The HDO reaction was carried out in owing hydrogen at 90 C and ambient pressure in a xed-bed reactor (ESI †). The molar ratio of surface metal and proton sites in the catalysts was chosen low enough (Pt/H + ¼ 0.03-0.1) to ensure the reaction being limited by the DMTHF ring opening step. 12 The density of surface Pt sites was estimated from the Pt dispersion (Table 1), the density of surface proton sites in CsPW was calculated assuming a cross section of the PW 12 O 40 3À Keggin unit of 144Å 32,33 and the CsPW surface area of 135 cm 2 g À1 (Table 1).
In the absence of Pt, the CsPW alone (entry 1) and Au-CsPW (entries 2 and 5) showed a negligible activity (1.9-2.2% DMTHF conversion with practically no 2-hexanol and n-hexane formed). Physically mixed Pt-CsPW catalysts, Pt/CsPW + CsPW and Pt/ SiO 2 + CsPW (1 : 9 w/w), exhibited a high activity giving >99% n-hexane selectivity at 8.0 to 85% DMTHF conversion depending on the catalyst and reaction conditions, in agreement with the previous report. 12 It should be noted that the catalyst based on Pt/SiO 2 had almost 6-fold greater activity than the one based on Pt/CsPW in terms of turnover frequency (TOF) per surface Pt site (cf. entries 3 and 6), thus demonstrating a strong effect of Pt support.
As can be seen from Table 2, the addition of gold to Pt/CsPW and Pt/SiO 2 caused a signicant enhancement of catalyst activity, with DMTHF conversion increasing 1.3-2 times compared to the corresponding Pt only catalysts (cf. entries 3 and 4, 6 and 7, 8 and 9). As the gold alone was practically inactive, the increase in catalyst activity can be attributed to the enhancement of activity of the Pt sites. For the bimetallic PtAu catalysts, the TOF values at Pt sites increased 1.3-2.4 times as compared to the monometallic Pt catalysts ( Table 2). The results at lower DMTHF conversions of 8-17% give a more accurate estimate of the TOF enhancement reaching 1.7-2.4 times (cf. entry 3 with 4 and 8 with 9). Previously, the gold enhancement on a similar scale has been reported for alkane isomerisation on PtAu-CsPW catalysts. 15,31 The enhancement of catalyst activity by addition of gold has been attributed to geometric (ensemble) and electronic (ligand) effects of the constituent metals in PtAu bimetallic nanoparticles. 26 The XRD and STEM-EDX data shown above clearly  demonstrate PtAu alloying in the PtAu/SiO 2 catalyst leading to the formation of bimetallic PtAu species. The same has also been reported for the PtAu/CsPW catalyst. 16 Previously, it has been shown that the HDO of DMTHF on Pt-CsPW is a structuresensitive reaction, 12 hence the geometric effects may be expected to contribute to the gold enhancement. However, in order to prove the role of geometric and electronic effects as the cause of the gold enhancement, more accurate metal dispersion measurements complemented by spectroscopic characterisation will be required. We also tested the performance of bifunctional PdAu/SiO 2 + CsPW and PtPd/SiO 2 + CsPW bimetallic catalysts under similar conditions in comparison to the corresponding monometallic Pd and Pt catalysts. However, no enhancement of activity was observed. This is in agreement with XRD analysis, which showed no distinct PdAu alloying in PdAu/SiO 2 (Fig. S2 in the ESI †).
In conclusion, we have demonstrated that the addition of gold to the Pt-CsPW catalyst has an enhancing effect on the HDO of DMTHF, increasing the turnover rate at Pt sites more than twofold. The enhancing effect is attributed to PtAu alloying. The formation of bimetallic PtAu nanoparticles in the PtAu-CsPW catalyst is conrmed by STEM-EDX and XRD.

Author contributions
The manuscript was written through contributions of all authors.

Conflicts of interest
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