The catalytic hydrogenation of furfural to furfuryl alcohol

peer-reviewedThe production of energy and chemicals from renewable resources has gained significant attention as the global effort and legislated requirements to transition from fossil fuels to biofuels intensifies. The synthesis of furfural from biomass has resulted in extensive fine chemical production. The main hydrogenation product of furfural is furfuryl alcohol which has been produced industrially with the use of a copper chromite catalyst. Elimination of the environmentally toxic Cu-Cr catalyst was the focus of this research culminating in the synthesis of a reusable bimetallic catalyst with high selectivity to furfuryl alcohol. This work initially focused on the synthesis of monometallic catalysts by wet impregnation and concentrated on metals such as platinum, palladium, copper and nickel. Platinum displayed higher selectivity to furfuryl alcohol while palladium showed higher furfural conversion under the conditions studied. Experiments conducted using ethanol as the solvent had a negative effect on the selectivity to the desired product, furfuryl alcohol, with high quantities of 2-furaldehyde diethyl acetal and difurfuryl ether also formed. Commercial catalysts were employed which confirmed the involvement of ethanol in the formation of additional products. Consequently, toluene was selected as an alternative solvent facilitating selectivity to furfuryl alcohol only under all conditions studied. A 0.9%Pt/SiO2 catalyst was selected as the most suitable catalyst for furfural hydrogenation to furfuryl alcohol following characterisation which highlighted that increased metal loading resulted in a larger particle size and lower metal dispersion. This catalyst became the focus of subsequent research. Subsequently, the focus of the research was the production of bimetallic catalysts using the 0.9%Pt/SiO2 catalyst as a base for the selection of promoter metals. The bimetallic catalysts were synthesised using the controlled surface reactions technique. It was found that, while the selectivity of all catalysts to furfuryl alcohol was close to 100%, the conversion was influenced significantly by the promoter metal and followed the order tin>molybdenum>manganese>barium>iron>nickel. Furfural conversion of 47% and close to 100% selectivity to furfuryl alcohol was achieved using a 0.6%Pt0.4%Sn/SiO2 catalyst at 100°C and 20 bar hydrogen pressure. Synthesis techniques including sequential impregnation and co-impregnation were investigated and compared to the controlled surface reactions technique. Coimpregnation was the best technique giving a furfural conversion of 62 % compared to the controlled surface reactions catalyst with 47% conversion. Detailed characterisation of the controlled surface reactions catalyst 0.6%Pt0.3%Sn/SiO2(CSR) and the co-impregnation catalyst 0.7%Pt- 0.3%Sn/SiO2(Co-I) demonstrated electronic modifications of the bimetallic catalysts caused by charge transfer from platinum to tin. TEM analysis showed that the particle size ranged from 1.5–3 nm for the co-impregnated catalyst to 1.5–5 nm for the controlled surface reactions catalyst indicating that furfural adsorption was favoured by a smaller particle size. The TOF was 0.70 s-1 and 1.40 s-1 for the coimpregnated and controlled surface reactions catalysts respectively. The loading of the promoter metal tin synthesised by co-impregnation was also investigated in the range of 0.0–0.7wt% which led to the selection of 0.7%Pt-0.3%Sn/SiO2 as the most active catalyst providing a compromise between promoting effect and dilution of the platinum active sites. A thorough investigation of the hydrogenation reaction conditions was also conducted. The investigation found that calcination is necessary and the most active catalyst was calcined at 450°C. A range of reaction temperatures from 25–150°C showed conversion increased with temperature but furfural desorption was promoted at temperatures above 100°C while increased hydrogen pressure facilitated an increase in furfural conversion. A kinetic model was established which could estimate the kinetic parameters, activation energy and rate constant, allowing the prediction of the experimental data. Regeneration and reuse of the catalyst showed that reuse of the catalyst is as efficient as regeneration. The influence of solvent on the reaction was also investigated using protic solvents 1-propanol and 2-propanol together with aprotic solvents toluene and propanone. The aprotic solvents displayed higher selectivity to furfuryl alcohol but furfural conversion was lower. Although furfuryl alcohol was the main product of both protic solvents additional undesirable products were also formed. These included 2- methyl furan, 2-propoxymethyl furan and 2-furaldehyde dipropyl acetal using 1- propanol and difurfuryl ether and 2-furaldehyde diethyl acetal using 2-propanol from interactions between furfural and/or furfuryl alcohol with the protic solvent