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Palladium-Rhenium Catalysts for Selective Hydrogenation of Furfural: Evidence for an Optimum Surface Composition Simon T. Thompson, and H. Henry Lamb ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01398 • Publication Date (Web): 23 Sep 2016 Downloaded from http://pubs.acs.org on September 23, 2016
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Palladium-Rhenium Catalysts for Selective Hydrogenation of Furfural: Evidence for an Optimum Surface Composition Simon T. Thompson and H. Henry Lamb* Department of Chemical and Biomolecular Engineering, North Carolina State University, Campus Box 7905, Raleigh, North Carolina 27695, USA
[email protected], (919)-515-6395 ______________________________________________________________________________ ABSTRACT: Bimetallic catalysts comprising a platinum-group metal (e.g., Pt, Pd, Ru) and rhenium (Re) have important applications in petroleum refining, industrial chemicals production, and biomass conversion. In this work, a series of PdRe/Al2O3 catalysts was investigated for selective hydrogenation of furfural to furfuryl alcohol (FAL) at 150°C and 1 atm in a differential reactor. The results demonstrate that PdRe/Al2O3 catalysts have greater FAL selectivity and activity than Pd/Al2O3 catalysts. Over the bimetallic catalysts, decreased furan production is accompanied by a marked increase in hydrogenation activity. PdRe (1:1) catalysts prepared using [Pd(NH3)4](NO3)2 are significantly more active than catalysts prepared using Pd(NO3)2. PdRe (1:2) catalysts are more selective to FAL but less active than (1:1) catalysts prepared using the same precursors. The superior activity of PdRe/Al2O3 catalysts for selective hydrogenation of furfural is inferred to result from Re surface modification of Pd nanoparticles—disrupting Pd ensembles and creating new highly active Pd-Re sites. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) indicates an enhanced linear-to-bridging ratio for CO chemisorbed on surface Pd atoms, supporting this hypothesis. H/CO chemisorption ratio at 35°C varies inversely with Re surface coverage and correlates with furfural turnover frequency (TOF) and FAL selectivity. Thus, the observed TOF maximum at H/CO ~0.25 suggests an optimum surface composition of approximately 75% Re and 25% Pd. High-angle annular dark field (HAADF) scanning-transmission electron microscopy (STEM) with energy-dispersive X-ray (EDX) analysis shows intimate contact of Re clusters with Pd nanoparticles in the most active catalysts.
Keywords: furfuryl alcohol; CO chemisorption; H2 chemisorption; TPR; CO DRIFTS; HAADFSTEM; EDX 1 ACS Paragon Plus Environment
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1 Introduction Production of fuels and chemicals from biomass has been an area of intense research in recent years. Hemicellulose pyrolysis leads to furfural and related compounds, and selective hydrogenation of furfural produces furfuryl alcohol (FAL) that is used as a monomer for foundry resins and in other industrial applications [1]. Furfural hydrogenation over Pd poses the challenge of selective aldehyde hydrogenation without decarbonylation to furan. Several groups have investigated furfural conversion over supported Pd [2-6]. Higher reaction temperatures (≥ 200°C) typically favor furfural decarbonylation, and conversely, lower temperatures favor furfural hydrogenation to FAL. Previous investigations of platinum group metal (PGM)-Re catalysts found that Re addition increases hydrogenation/hydrogenolysis chemoselectivity as compared to the Group VIII metal alone [7-11]. Recent models attribute the increased activity and selectivity of PGMRe catalysts to the interaction of ReOx species with the surfaces of Group VIII metal particles. Tomishige and coworkers studied several silica-supported PGM-Re catalysts, including Rh-Re [11, 12], Ir-Re [13], Pt-Re [14], and Pd-Re [9]. Using extended X-ray absorption fine structure (EXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS), they inferred that the active sites are at the interface of ReOx clusters with the PGM surface (e.g., for selective hydrogenation of fatty acids to alcohols over PdRe/SiO2 [9]). Hydrogenolysis and ring-opening of tetrahydrofurfuryl alcohol (THFAL) and the related compound tetrahydropyran-2-methanol were investigated over RhRe/SiO2 catalysts [11, 15]. Experiments and density functional theory (DFT) indicate that the RhRe/SiO2 catalysts are bifunctional in aqueous reaction media. Rh0 surface sites provide a hydrogenation-dehydrogenation function, and adjacent hydroxylated Re
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cations function as acid sites. PdRe/Al2O3 catalysts have received less attention for selective conversion of biomass-derived compounds. Strong interactions between Re7+ oxo anions and γ-Al2O3 complicate catalyst preparation. Formation of Re-O-Al bonds between perrhenate species and the support has been demonstrated using Raman spectroscopy, EXAFS spectroscopy, and density functional theory (DFT) [16-21]. The coexistence of Re species in several different oxidation states in Re/Al2O3 catalysts after reduction has been reported [21, 22]. In addition, Re oxidation state may change during catalysis; therefore, it is difficult to determine whether the active sites comprise Re0 or ReOx species. Meitzner, et al. [23] presented evidence from EXAFS spectroscopy for alloying in PdRe/Al2O3 catalysts prepared by incipient wetness co-impregnation using Pd(NO3)2 and HReO4. Pd and Re only form bulk solid solutions over a very limited range of compositions [24], but it is well known that in small bimetallic clusters, there is the possibility for interaction of Pd and a second metal [25, 26]. Ziemecki, et al. [27, 28] prepared PdRe/Al2O3 catalysts using a sequential method in which a solution of the Re precursor was added to a reduced and passivated Pd/Al2O3 catalyst. Patents for PdRe/Al2O3 and PdRe/C catalysts also reference this or very similar preparation methods [29, 30]. Suppression of β-PdHx formation has been attributed to alloy formation in PdRe/Al2O3 catalysts and bulk samples made by impregnating Pd black with Re2O7 and KReO4 [28]. In situ X-ray diffraction of the bulk Pd + KReO4 sample during reduction in 8.5% H2 indicated that two PdRe alloy phases were formed [28]. More recently, PdRe/Al2O3 catalysts for hydrodechlorination and alkane reforming were prepared [31-33] and characterized by temperature-programmed hydride decomposition; this work also relied heavily on suppression of β-PdHx formation as evidence for Pd-Re interactions.
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Catalyst preparation methods aimed at achieving selective deposition of either Pd or Re species on supported monometallic particles of the other metal have been devised [34-37]. In one such method, termed "catalytic reduction," a perrhenate species is reduced by H adsorbed on the surface of Pd particles, leading to selective Re deposition. Bimetallic catalysts prepared by catalytic reduction are effective at low Re loadings for selective hydrogenation of succinic acid to 1,4-butanediol [34]. Conversely, the "overlayer" method targets selective deposition of Pd atoms on the surface of metallic Re particles. Hydrogen chemisorption has been employed to characterize the Pd sites in these catalysts [36, 37]. Accordingly, it has been demonstrated that the H‒surface bond energy is diminished with respect to Pd [36, 37], in agreement with DFT calculations for a pseudomorphic Pd layer on Re(0001) [38, 39]. In this work, a series of PdRe/Al2O3 catalysts prepared using different Pd and Re precursors and impregnation sequences was investigated for selective hydrogenation of furfural to FAL at 150°C and 1 atm in a differential reactor. Furan selectivity is less than 10% over the bimetallic catalysts, as compared to 35-45% over Pd/Al2O3. The high activity of PdRe/Al2O3 catalysts for selective hydrogenation is inferred to result from Re surface modification of Pd nanoparticles—disrupting Pd ensembles and creating new highly active Pd-Re sites. 2 Experimental 2.1 Catalyst preparation Two Pd precursors, [Pd(NH3)4](NO3)2 (tetraammine, TA) (10 wt% solution, Aldrich, 99%) and Pd(NO3)2•x H2O (nitrate, NO3) (99.9% Pd basis, Strem), and two Re precursors, NH4ReO4 (ammonium perrhenate, N) (99+%, Alfa Aesar) and HReO4 (perrhenic acid, H) [76.5 wt% aqueous solution, 99.99% Re, Acros Organics], were purchased and used as received. A 4 ACS Paragon Plus Environment
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high-purity γ-Al2O3 (Grace-Davison MI-209, 183 m2/g BET surface area, 0.80 cm3/g total pore volume) was used as the support. Inductively coupled plasma-optical emission spectrometry (ICP-OES) analysis found low levels of alkali and alkaline earth metal impurities: 3.3 ppm Na and Pd3-TA > Pd3-N. H2 uptake decreases for Pd/Al2O3 catalysts when chemisorption measurements are made at higher temperatures (70 and 100°C), as expected. Specific CO uptake and CO/Re are very low for Re/Al2O3 catalysts. Low CO uptake may result from poor metal dispersion, incomplete Re reduction, and/or re-oxidation of Re0 by γ-Al2O3 surface hydroxyl groups during extended evacuation at 400°C [40, 45, 46]. H/Re ratios for Re/Al2O3 at 35°C are even lower (by nearly an order of magnitude) in agreement with the literature [32, 36, 40, 43]. H/Re ratio increases with measurement temperature consistent with activated dissociative chemisorption [43, 47]. H2 chemisorption on PdRe/Al2O3 catalysts is also suppressed relative to CO chemisorption at 35°C. Pd3Re5-CI and Pd3Re5-SI chemisorb more CO but less H2 than Pd3-N. Similarly, the specific CO uptake of Re5Pd1.5-SI is greater than that of Pd1.5-TA, but its H2 12 ACS Paragon Plus Environment
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uptake is much lower. The specific CO uptakes for Re5Pd3-SI(350), Re5Pd3-SI(400) and Re10Pd3-SI are approximately equivalent to that of Pd3-TA; however, the corresponding H2 uptake values are all significantly lower. For PdRe catalysts, specific H uptake is nearly independent of specific CO uptake at 35°C (Figure S2a). Specific H uptake increases from 7.8 µmol/g for Pd3Re5-SI to 25-30 µmol/g for the remaining PdRe (1:1) catalysts. Even greater suppression of H2 chemisorption at 35°C is observed for PdRe (1:2) catalysts. Moreover, H2 uptake by PdRe/Al2O3 catalysts increases with temperature suggesting activated dissociative chemisorption. A linear correlation of specific H uptake at 100°C with CO uptake at 35°C was found for Re/Al2O3 and PdRe/Al2O3 catalysts irrespective of Pd:Re ratio; the slope of the correlation is ~0.5 (Figure S2b). An analogous linear correlation for Pd/Al2O3 catalysts has a slope of ~0.8 (Figure S2b). 3.3 Furfural hydrogenation The furfural reaction pathways of interest are shown in Scheme 1. Selective hydrogenation of the aldehyde moiety yielding FAL is desired. An undesired parallel reaction, decarbonylation, produces furan and CO. Secondary hydrogenation of furan yields tetrahydrofuran (THF). Hydrogenation of FAL to 2-methyl-furan (2-MF) may occur in competition with FAL desorption from the catalyst surface (primary product) or via readsorption and reaction of FAL (secondary product). Formation of THFAL also may occur via primary and secondary (as shown) hydrogenation pathways. The formation of 2-methyltetrahydrofuran (2-MTHF) via ring saturation of 2-MF is also indicated, but this product is unlikely to be detected at the low furfural conversions investigated in this work.
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Pd/Al2O3 catalysts are highly active for furfural conversion at 150°C and 1 atm requiring weight hourly space velocities (WHSVs) >20 h-1 in order to limit conversions to
