Thermocatalytic cleavage of C–C and C–O bonds in model compounds and kraft lignin by NiMoS2/C nanocatalysts

Abstract

The effective utilization of biomass derived lignin as a source of chemicals and fuels involves chemical transformation such as depolymerization and deoxygenation reactions. Since lignin macromolecules are predominantly made up of both C–C and C–O bond linkages, the simultaneous cleavage of these linkages requires the design of a highly efficient catalytic system. Therefore, this paper reports an innovative technique for preparing NiMoS₂ nanoparticles as an upgrading catalyst for lignin via a hydrodeoxygenation (HDO) route. The key aspect of this strategy is to produce single layered, short MoS₂ slabs (∼4 nm) that possess increasingly exposed edge active sites in contrast to the bulk MoS₂ materials. A microemulsion synthesis technique was utilized to prepare the NiMoS₂ nanocatalyst, which was dispersed on an activated carbon support giving a weak metal-support interaction that facilitated a “metal-like character” of the catalyst as well as the formation of type-II NiMo phases. Initial catalyst screening for the HDO of lignin phenolic model compounds (phenol, guaiacol, veratrole and syringol) showed that partial deoxygenation accompanied by hydrogenation was the favoured reaction pathway. Moreover, rapid cleavage of C–C and C–O bonds of recalcitrant α-O-4, β-O-4 and 4-O-5 linkages was achieved in a relatively short reaction time (i.e. <3 h). Eventually, the catalyst was demonstrated to be capable of depolymerizing kraft lignin by hydrogenolysis to produce monomeric and dimeric phenolic compounds as indicated by the GPC and GC-MS results. The reusability of the catalyst for guaiacol conversion, as a prototype for other phenolic compounds, showed that catalytic activity is stable and maintained even after three reaction cycles. Catalytic stability was ascribed primarily to the preservation of the catalytic active edges containing short MoS₂ layers and high stacking (up to 7 layers) and as a result, leached sulphur and coke formed were minimized. Thus, synthesising amorphous NiMoS₂/C nanocatalysts by the microemulsion technique represents a promising alternative for designing cheap and efficient catalysts for lignin upgrading.