The role of catalyst acidity and shape selectivity on products from the catalytic fast pyrolysis of beech wood

The catalytic fast pyrolysis (CFP) of biomass represents an efficient integrated process to produce deoxygenated stable liquid fuels and valuable chemical products from lignocellulosic biomass. The zeolite ZSM-5 is a widely studied catalyst for the CFP process. However, its microporous structure may limit the diffusion of high molecular weight pyrolysis intermediates to its active sites. Mesoporous aluminosilicates such as Al-SBA-15 are promising materials with larger pore sizes that can overcome these diffusional limitations. Previous comparisons between mesoporous aluminosilicates and ZSM-5 for the CFP process have neglected the disproportionately high acidity of ZSM-5. In this study, an Al-SBA-15 catalyst has been synthesised with high acidity, comparable to that of a ZSM-5 catalyst with a Si:Al ratio of 15:1. The synthesised Al-SBA-15 catalyst was characterised by N2 physisorption, XRD and propylamine-TPD, and was compared to a ZSM-5 catalyst and a typical industrial equilibrium fluid catalytic cracking catalyst (e-FCC). All three catalysts were used at three different catalyst to biomass (C/B) ratios, to investigate the effect of varying concentrations of acid sites on the product distribution from the catalytic fast pyrolysis of beech wood. Interestingly, despite their dissimilar structural architectures, all three solid acid catalysts displayed similar reaction pathways towards the cracking of high molecular weight products such as levoglucosan and formation of intermediates including phenolics and furans. However, the selectivity towards the final catalytic products was dictated mainly by the structure of the catalysts. Despite their very similar surface area and acidity, the ZSM-5 exhibited high selectivity for the formation of desirable aromatic hydrocarbon products due to its shape-selective micropore structure, while Al-SBA-15 instead shifted the selectivity towards the formation of undesirable coke. The results highlighted the importance of catalyst shape-selectivity in the catalytic fast pyrolysis of biomass for the conversion of pyrolysis vapours into desirable products and the suppression of undesirable solid byproduct formation

Catalytic co-pyrolysis of biomass and waste plastics as a route to upgraded bio-oil

A two-stage reactor system consisting of co-pyrolysis of biomass and plastic in the 1st stage and catalytic upgrading (zeolite ZSM-5 catalyst) of the derived pyrolysis gases in the 2nd stage was used to investigate the yield and composition of the product gases and bio-oil. Biomass waste wood and waste plastics in the form of high density polyethylene, low density polyethylene, polypropylene, polystyrene and polyethylene terephthalate were used as feedstock. The addition of the plastics to the biomass with co-pyrolysis-catalysis, produced a higher CnHm gas yield compared with what would be expected by calculation, suggesting some interaction of the biomass and plastic. The presence of waste plastic resulted in a decrease in the relative proportion of oxygenated compounds in the product oil compared to pyrolysis of biomass alone; for example a reduction of >65% for biomass with polyethylene and polypropylene and >95% reduction for biomass with polystyrene. The fuel properties of the co-pyrolysis upgraded oil were improved compared to biomass alone; for example, the co-pyrolysis of polystyrene and biomass showed an improved relative proportion of compounds in the C5 ― C12 fuel range (76%). In terms of the ratio of biomass to plastic, even low quantities of plastic (9:1 biomass:plastic ratio) produced a lower relative proportion of oxygenated bio-oil compounds, for example biomass:polystyrene at a ratio of 9:1 reduced the relative proportion of oxygenated compounds in the product bio-oil by >55%