No abstract

This review appraises the chemical conversion processes recently reported for the production of hydroxylmethylfurfural (HMF), a key biorefining intermediate, from carbohydrate biomass feedstocks. Catalytic sites or groups required for the efficient and selective conversion of hexose substrates to HMF are examined. The principle of concerted catalysis was used to rationalise the dehydration of fructose and glucose to HMF in non-aqueous media. A survey of reported reaction routes to diesel-range biofuel intermediates from HMF or furfural is presented and self-condensation reaction routes for linking two or more HMF and furfural units together toward obtaining kerosene and diesel-range biofuel intermediates are highlighted. The reaction routes include: benzoin condensation, condensation of furfuryl alcohols, hetero Diels-Alder reaction and ketonisation reaction. These reaction routes are yet to be exploited despite their potential for obtaining kerosene and diesel-range biofuel intermediates exclusively from furfural or hydroxylmethylfurfural.

Fischer-Tropsch synthesis (FTS) occupies a key position in the search for alternatives to petroleum for obtaining liquid hydrocarbons. Hydrocarbons can be produced from alternative carbonaceous resources (natural gas, coal, biomass and waste) through FTS and the use of biomass is particularly attractive from a carbon footprint point of view. However, the nature of biomass resources dictates a different exploration approach compared to fossil fuel resources. Compared to coal and natural gas based FTS processes where economics of scale is an advantage, a Biomass-to-Liquid (BTL) plant is more suited for smaller scale operation. Thus, the BTL process economy will benefit from a condensed FTS processes. Moreover, considering the expected role of FTS in regional and global hydrocarbon supply in the near future, it becomes pertinent to strive towards improving the process economy. This requires molecular and process engineering and detailed knowledge of the reaction is a pre-requisite to engineering the reaction at molecular level. Syngas to hydrocarbon involves consecutive steps of CO activation, C-C coupling, hydrogenation and desorption of the hydrocarbon product. Atomic details of the dynamics of these steps are still unclear. Recently, clearer pictures about activation are now available. However, over the course of ninety years since the first report on an FTS, proposed pathway of C-C coupling has come full cycle from oxygenate to nonoxygenate and back to oxygenate intermediates. To this end, we attempted an X-ray of progress made at providing answers to issues in the chemistry of FTS. The review focuses on product distribution, macro kinetics and the mechanism of FTS. We compare FTS with other C-C coupling reactions of CO, identify catalytic entities for controlling product selectivity and finally we offer an outlook on future directions of fundamental research towards resolving the lingering questions on the mechanism of C-C coupling in FTS.