The catalytic fast pyrolysis of biomass

Abstract

One of the main challenges for the catalytic fast pyrolysis of biomass waste is the development of active and stable catalysts that can deal with the large variety of thermal decomposition intermediates from lignocellulosic biomass waste. The microporous zeolite ZSM-5 is one of the most extensively studied catalysts in this area due to its strong acidity and shape selectivity. However, a major limitation of ZSM-5 is its micropore structure, which limits the diffusion of large molecules produced from the pyrolysis of lignocellulose substrates. For this reason, mesoporous aluminosilicate materials, such as Al-SBA-15 have attracted attention as alternative catalysts. However, these catalysts usually suffer from low acid site loadings and low acid strength compared to zeolites. Therefore, the performance of mesoporous aluminosilicate materials was inferior to ZSM-5 zeolites when previously tested for the catalytic fast pyrolysis of biomass waste. A range of high acidity Al-SBA-15 catalysts have been synthesised using a novel synthesis method which achieved acidities comparable to ZSM-5. Aluminium incorporation was shown to be positively correlational to acidity. The Al-SBA-15 catalyst with the greatest aluminium to silicon ratio was AlSBA15(5) with a Si:Al= 5 and exhibited a high acidity of 564 μmol g-1. This is one of the highest acidities achieved by an Al-SBA-15 catalyst in the literature currently. AlSBA15(5) was subsequently tested for the catalytic fast pyrolysis of biomass components (model compounds of cellulose, lignin and hemicellulose) and beech wood, compared to industrially relevant solid acid catalysts (ZSM-5, SAPO-34, Al2O3, K10 and e-FCC). The findings suggest that all acidic catalysts initially follow a similar reaction pathway, cracking high molecular weight compounds in the pyrolysis vapours into smaller organic molecules. As the ratio of catalyst to biomass was increased, AlSBA15(5) did not increase the yield of valuable compounds and instead favoured the production of coke. In contrast, at increased catalyst to biomass ratios the shape selectivity of ZSM-5 became more significant and increased the yield of valuable aromatic products such as xylene and toluene, while suppressing coke formation. Although the in situ location of the catalyst had a small positive effect on the yield of products in the micro-reactor work, ex situ catalytic fast pyrolysis should be considered for commercial applications due to the independent control of both fast pyrolysis and catalytic temperature and vapour residence times, and the prevention of catalyst deactivation due to AAEMs.