Platinum-Catalyzed aqueous-Phase hydrogenation of d?Glucose to d?Sorbitol

Article abstract of DOI:10.1021/acscatal.6b02369

Aqueous-phase hydrogenation of D-glucose to D-sorbitol was systematically investigated over silica-supported Pt nanoparticles to elucidate structure?reactivity relations and mechanistic insight. D-Glucose hydrogenation over large Pt particles competes with its isomerization to D-fructose over low-coordination (electron-deficient) Pt sites; D-sorbitol production by the former process was structure insensitive for nanoparticles spanning 3?17 nm, whereas isomerization was favored by smaller particles, with both pathways independent of the choice of fumed silica or mesoporous SBA-15 support. While D-fructose was readily hydrogenated to D-mannitol under the same reaction conditions, the latter underwent minimal isomerization to D-sorbitol, which is, therefore, a direct product of D-glucose ring opening and subsequent hydrogenation of the aldose conformer. D-Sorbitol production was favored by low D-glucose concentrations (<10 wt %), high H₂ pressures (>40 bar), and low reaction temperatures (<140 °C), which suppressed undesired polymerization side reactions.

Full text of DOI:10.1021/acscatal.6b02369

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Article
Pt catalyzed aqueous-phase hydrogenation of D-glucose to D-sorbitol
Xingguang Zhang, Lee J Durndell, Mark A. Isaacs, Christopher M. A. Parlett, Adam F. Lee, and Karen Wilson
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02369 • Publication Date (Web): 26 Sep 2016
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Pt catalyzed aqueousꢀphase hydrogenation of Dꢀ
glucose to Dꢀsorbitol
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Xingguang Zhang,† Lee J. Durndell,† Mark A. Isaacs,† Christopher M.A. Parlett, † Adam F.
Lee,†* and Karen Wilson†*
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†European Bioenergy Research Institute, Aston University, Birmingham B4 7ET, UK.
ABSTRACT. Aqueous phase hydrogenation of Dꢀglucose to Dꢀsorbitol was systematically
investigated over silica supported Pt nanoparticles to elucidate structureꢀreactivity relations and
mechanistic insight. Dꢀglucose hydrogenation over large Pt particles competes with its
isomerization to Dꢀfructose over low coordination (electron deficient) Pt sites; Dꢀsorbitol
production by the former process was structure insensitive for nanoparticles spanning 3ꢀ17 nm,
whereas isomerization was favored by smaller particles, with both pathways independent of the
choice of fumed silica or mesoporous SBAꢀ15 support. While Dꢀfructose was readily
hydrogenated to Dꢀmannitol under the same reaction conditions, the latter underwent minimal
isomerization to Dꢀsorbitol which is thus a direct product of Dꢀglucose ringꢀopening and
subsequent hydrogenation of the aldose conformer. Dꢀsorbitol production was favored by low Dꢀ
glucose concentrations <10 wt%, high H2 pressures >40 bar and low reaction temperatures <140
°C, which suppressed undesired polymerization side reactions.
KEYWORDS: Glucose; Hydrogenation; Platinum; Sorbitol; SBAꢀ15.
1. INTRODUCTION
Carbohydrates derived from lignocellulosic biomass waste, such as agricultural or forestry
residues and the organic fraction of municipal solid waste, are an attractive source of biogenic
carbon from which to derive sustainable biofuels and platform chemicals.1ꢀ5 In 2004, Dꢀsorbitol
was identified as one of the 12 most important value added chemicals obtainable from biomass
by the US Department of Energy, as a potential source of alkanes for liquid biofuels, or
“outstanding building block for commodity chemicals”, notably through its dehydration to
isosorbide.6,7 Esters and derivatives of Dꢀsorbitol also represent important additives and
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intermediates in the cosmetics and pharmaceuticals sectors, with the molecule a key sugar
substitute in the food and beverage industry, with an annual global production ~800,000 tons.6,7
DꢀSorbitol is commonly obtained through the transition metal catalyzed hydrogenation of Dꢀ
glucose,8 notably over Ni9,10 11,12, Ru,13ꢀ16,17 Pt18 and Rh10 supported on oxides, carbons,
polymers, hydrotalcites and zeolites. Commercial processes typically employ either Raney Ni or
carbon supported Ru catalysts, however the former offers relatively poor selectivity and are
prone to progressive onꢀstream deactivation and product contamination,12 while the latter is
expensive and susceptible to poisoning by strongly bound organic impurities or sulfur
compounds.19 Obviously the existence of a commercial catalytic process must not preclude the
pursuit of further process improvements and advancement of fundamental scientific knowledge,
and hence studies have explored the role of basic supports, or poorly reduced active phases, in
promoting the Canizzaro side reaction and concomitant gluconic acid formation,20 a possible
contributor to the deactivation of Ni and Ru catalysts.21 The influence of dopants to improve
activity and selectivity,11 such as RuꢀB,22 NiꢀP23 and CoꢀB,24 has also been exploited, however
the resulting catalysts possess low surface areas and poor thermal stability, underperforming
Raney Ni in industrial application. Pt nanocatalysts are generally more reducible and offer
greater stability under hydrothermal reaction conditions than Ni or Ru, and exhibit excellent
performance in hydrogenation and hydroisomerization reactions.25,26,27 Gallezot et al
demonstrated continuous glucose hydrogenation to sorbitol in high yields over Ru/C,17 but
employed high H2 pressures (80 bar) and a 40 wt% glucose solution more concentrated than
likely to be available from biorefineries through e.g. steam explosion wherein the resulting
hydrolysate and condensate comprise ~1ꢀ10 wt% glucose,28 and 8ꢀ15 g catalyst charges in the
fixed bed. Sorbitol yields from Ru supported on alumina and titania are far lower in batch,
typically 20ꢀ60 % even at 120 bar hydrogen.12 In contrast, Perrard and coꢀworkers18 reported 10
wt% Pt/C catalysts operating at 100 °C and 80 bar H2 pressure with a specific activity of 1800
mmolˑgPtꢀ1ˑhꢀ1 and selectivity >99.5 % to Dꢀsorbitol. Hydrotalcite addition to the reaction
medium dramatically enhanced both glucose hydrogenation and concomitant sorbitol production
over a 3.5 wt% Pt/Al2O3 catalyst, attributed to enhancement of open chain form glucose under
alkaline conditions.29 The only direct comparative studies of glucose hydrogenation over low
loading Ru and Pt on activated carbon reported identical activities between the two metals in
both batch and flow operation, but superior sorbitol selectivity for platinum in all cases.30,31
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Unfortunately, none of the preceding studies employed reaction conditions free from mass
transport limitations or reported carbon mass balances, prohibiting reliable and quantitative
assessment of the influence of either catalyst properties or operating conditions, or comparisons
between catalysts.
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Despite the potential significance of Pt catalyzed carbohydrate hydrogenation32ꢀ35 for
biorefining, the aqueous phase hydrogenation of Dꢀglucose to Dꢀsorbitol therefore remains
poorly understood, with no systematic studies into the reaction network or structure sensitivity,
or performance benchmarking of intrinsic reaction rates. Here we report the first fundamental
investigation into structureꢀactivity relationships for aqueous phase glucose hydrogenation by
any family of wellꢀdefined and systematicallyꢀrelated catalysts, identifying strong particle size
and support effects for silica supported Pt catalysts. Such insight will prove valuable for the
development of more robust, active and selective catalysts for the overall hydrolytic
hydrogenation of cellulose to sugar alcohols,36 an important route to improve the efficiency of
biomass conversion to platform chemicals and fuels.37,38
2. EXPERIMENTAL
Catalyst synthesis
Two families of fumed silica and SBAꢀ15 supported Pt catalysts were prepared by wet
impregnation.39 Typically, 2.5 g of the support (SBAꢀ15 or fumed silica) was contacted with 25
mL aqueous ammonium tetrachloroplatinate(II) solution with the metal precursor concentration
adjusted to achieve nominal Pt loadings spanning 0.05ꢀ2 wt%. Slurries were stirred for 18 h
overnight at room temperature, and subsequently evaporated to dryness at 80 °C under agitation.
The resulting powders were calcined at 550 °C for 2 h under static air in a muffle furnace (ramp
rate 1 °C.minꢀ1), followed by reduction at 400 °C for 2 h (ramp rate 10 °C.minꢀ1) under flowing
H2 (10 mL.minꢀ1). Catalysts were stored under ambient conditions and used for reactions without
further treatment.
Catalyst characterization
Nitrogen porosimetry was undertaken on a Quantachrome Nova 4000e porosimeter and
analyzed with NovaWin software version 11. Samples were degassed in vacuo at 120 °C for 2 h
prior to measurement of nitrogen adsorption/desorption isotherms at ꢀ196 °C. BET (Brunauerꢀ
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