Abstract
A family of tungstated zirconia solid acid catalysts were synthesised via wet impregnation and subsequent thermochemical processing for the transformation of glucose to 5-hydroxymethylfurfural (HMF). Acid strength increased with tungsten loading and calcination temperature, associated with stabilisation of tetragonal zirconia. High tungsten dispersions of between 2 and 7 W atoms·nm−2 were obtained in all cases, equating to sub-monolayer coverages. Glucose isomerisation and subsequent dehydration via fructose to HMF increased with W loading and calcination temperature up to 600 °C, indicating that glucose conversion to fructose was favoured over weak Lewis acid and/or base sites associated with the zirconia support, while fructose dehydration and HMF formation was favoured over Brönsted acidic WOx clusters. Aqueous phase reforming of steam exploded rice straw hydrolysate and condensate was explored heterogeneously for the first time over a 10 wt% WZ catalyst, resulting in excellent HMF yields as high as 15% under mild reaction conditions.
Highlights
We report the first application of a solid acid catalyst for the production of HMF from hydrolysate and condensate streams produced via the steam explosion of rice straw
Tungstated zirconia catalysts having 5 and 10 wt% W were prepared via wet impregnation and calcined at 500–700 °C to investigate the optimum WOx surface density and Lewis:Brönsted ratio for the direct conversion of glucose to HMF under the mild conditions
Glucose conversion over WZ catalysts ranged between 5% and 20%, with HMF selectivity increasing with W loading and calcination temperature up to 600 °C
Summary
The energy and atom efficient transformation of biomass waste feedstocks such as rice, corn and coconut husks, rice and wheat straw, corn cobs and palm kernels into sustainable gasoline and/or diesel drop-in biofuels, offers new routes to environmentally-benign renewable energy resources [1,2]. Glucose or fructose derived from the cellulose component of biomass can be converted into HMF through its stepwise dehydration under mild aqueous conditions. A number of solid acid catalysts have shown potential for this transformation [11,12,13], with a recent systematic investigation of sulfated zirconia highlighting a bifunctional pathway involving Lewis acid catalysed glucose isomerisation to fructose over the parent zirconia support, and subsequent Brönsted acid catalysed dehydration to HMF, conferring a yield of 2%–3% at 100 °C [14]. We demonstrate the utility of tungstated zirconia for the aqueous phase transformation of hydrolysate sugar (obtained from rice straw lignocellulose hydrolysis) to HMF as a function of tungsten oxide loading and calcination temperature, achieving HMF yields as high as 10%–15% from steam exploded hydrosylate and liquid condensate
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