Depletion of fossil fuel reserves and an increase in greenhouse gases in the atmosphere has accelerated the search for a renewable and cleaner source of energy. Cellulose present in lignocellulosic biomass can be selectively converted to C6 sugar alcohols (hexitol) which are stable platform chemical and can be used as an intermediate for production of fuels from biomass. The conversion of cellulose into hexitol proceeds through sequential hydrolysis and hydrogenation reaction in aqueous phase using a metal catalyst. At present, the conversion of cellulose is limited by its low reactivity and insolubility in water. In the current study, factors influencing this reaction are investigated with an objective of maximising the reactivity of cellulose and developing a process that can be easily upgraded for large-scale production of sugar alcohols.
Commercial heterogeneous catalytic process to produce sorbitol from cellulose faces many challenges including low yield of products and high cost of noble metals required for the conversion. In this study, cellulose conversion to sugar alcohols was first investigated using a cheap Ni based bimetallic catalyst. Ni-Pt bimetallic catalyst was proven more effective in sugar alcohols synthesis when compared with Pt catalyst alone. Monometallic Ni catalysts showed little activity for the reaction, but with the addition of a small amount of Pt to the Ni catalyst (Ni:Pt = 22:1 atom ratio), the activity was greatly enhanced. Among the various supports used, Ni–Pt supported on a mesoporous beta zeolite support provided highest yield of 36.6%. The presence of a small amount of Pt promotes the protonation of water and hydrogen molecules, which spill over to Ni sites increasing its hydrogenation activity. Through these results, it was established that cheaper Ni catalyst are suitable for comparable yield of sugar alcohols. However, the reactivity of untreated cellulose was found to be very slow under the reaction conditions used. This issue was tackled by investigating the depolymerisation of cellulose using mechanocatalytic pretreatment. Rapid depolymerisation of cellulose was observed during ball milling in the presence of H2SO4. This resulted in formation of oligomers, which were completely soluble in water at ambient condition. Soluble oligomers were found to have an average degree of polymerisation of 7 monomer units. Using high-resolution NMR spectroscopy it was determined that cellulose depolymerisation was accompanied by catalytic repolymerisation reaction, which led to formation of α (1→6) linkages producing branched oligomers that were soluble despite their high degree of polymerisation. The soluble oligomers showed excellent reactivity towards hydrolysis–hydrogenation in the presence of bi-metallic Ni–Pt/alumina catalyst. High yield (90%) of hexitol was obtained with only 1 h of reaction time.
To successfully implement this process at larger scale it is imperative to perform the reaction in a continuous reactor. This was achieved by replacing conventional hydrogenation reaction with transfer hydrogenation using iso-propanol as hydrogen donor. Ru supported on activated carbon was found to be active for transfer hydrogenation of cellulose oligomers, which were produced by the milling of acidulated microcrystalline cellulose. C6 sugar alcohol yield of 85 % was obtained in less than 1 h reaction time in a batch reactor. Transfer of hydrogen from iso-propanol to glucose using Ru catalyst was found to be through direct di-hydride transfer without formation of gaseous H2. This was beneficial for implementation of a fixed bed reactor for conversion of soluble cellulose oligomers to sugar alcohols. Conversion of cellulose oligomers into hexitol was successfully achieved in a fixed bed continuous flow reactor with 36.4% yield at LHSV of 4.7 h-1. Finally, an attempt was made to upgrade the process to use raw lignocellulosic biomass instead of pure cellulose. Mechanical depolymerisation of acidulated biomass was found to be sensitive towards the milling atmosphere. In presence of air the solubility in water decreased after reaching a maximum as the acid loading was increased. Milling in presence of Ar resulted in higher solubility at low acid amounts, which did not decrease under higher acid loading. Hydrolysis of carbohydrate fraction in depolymerised biomass was very fast but yield of sugar alcohols was very low due to rapid deactivation of the catalyst. Nevertheless very high yield of sugars such as glucose and xylose were obtained in very short time.