Mechanistic and kinetic aspects of pentose dehydration towards furfural in aqueous media employing homogeneous catalysis

In this paper both the mechanistic and kinetic aspects of furfural formation from pentoses in aqueous acidic media have been reviewed. Based on the reviewed literature, a comprehensive reaction mechanism has been proposed consisting of more than one route, all starting from acyclic xylose, and involving alternately 1,2-enolization, beta-elimination or isomerization via 1,2-hydride shift as key steps. Those studies that employ combined acid-base catalysts, soluble halide salts and trivalent cations in aqueous solutions appear to be most promising. Next, a detailed overview is presented of the results of kinetic studies on furfural formation from pentoses and furfural disappearance in aqueous acidic media. Although these results span over a very wide range of both experimental conditions and different kinetic models employed, an attempt has been made to present the published kinetic data in such a manner that it allows a global comparison. Since even in those cases where the reaction conditions seemed to be comparable, the reported kinetic constants often agree merely in the order of magnitude, thus, the validity of most of the data presented here is restricted to the specific conditions as used by each author. Additionally, a very concise overview is included of research on direct furfural production from lignocellulosic materials. In conclusion, the intricate set of reactions accompanying furfural formation from pentoses, although appearing well established in some aspects, is yet to be fully unraveled, especially with regard to the complex set of side and loss reactions seemingly involving largely unknown reaction intermediates. Such uncertainties are reflected in the contradictory kinetic models exploited and kinetic data presented in the literature, which still prevent a common and coherent interpretation.