Operando Molecular Spectroscopy during Catalytic Biomass Pyrolysis

Abstract With the declining amount of fossil fuel resources, the need to develop sustainable processes that produce liquid hydrocarbon fuels has stimulated an explosion of research in biomass conversion technologies. One of the simplest biomass conversion methods that results in a liquid hydrocarbon fuel is catalytic biomass pyrolysis. Catalytic biomass pyrolysis has been examined from process technology and catalyst discovery points of view, but the lack of a well-defined catalyst with known molecular structure(s) has hampered the development of catalyst structure-activity relationships for value-added chemicals and fuels. Supported Al2O3/SiO2 catalysts were synthesized and extensively characterized using ex situ high field 27Al NMR spectroscopy, in situ Raman spectroscopy, in situ IR spectroscopy, and adsorption of probe molecules (CO and NH3) to determine the precise molecular structures and chemical nature of the surface AlOx acid sites. The supported Al2O3/SiO2 catalysts underwent significant structural changes upon dehydration and possessed three distinct surface aluminum oxide sites, surface AlO4, AlO5 and AlO6 sites. Additionally, two surface Lewis acid sites (LAS) and four Brønsted acid sites (BAS) were identified. Medium strength LAS are from NMR-invisible surface AlO6 sites that convert into surface AlO4 sites upon dehydration. The strongest LAS and strongest BAS both correlate with the presence of NMR-visible AlO5 sites and the second strongest BAS is associated with NMR-visible surface AlO4 sites.Changes in the catalyst/biomass structure and the evolution of pyrolysis products were monitored under real pyrolysis reaction conditions using in situ IR spectroscopy and operando Raman spectroscopy-Mass spectrometry. For the first time, the evolution of key pyrolysis products from biomass and its constituents was correlated with acid site nature and surface AlOx molecular structure(s), establishing the first structure-activity relationships for catalytic biomass pyrolysis. Several surface reaction intermediates exist on the catalyst surface for the cellulose and hemicellulose fractions of biomass, and their monomers; including furan, conjugated alkenes, alkenes conjugated to aromatic rings, and small ring alkenes. No reaction intermediates were detected during catalytic lignin pyrolysis and only a minor amount of surface furan was detected during catalytic wood pyrolysis.Based on real-time mass spectrometry results, many reaction pathways were identified. The dehydration of xylose into furfural was shown to be catalytically enhanced at low temperatures (~170oC), demethylation of toluene/xylene into benzene was demonstrated to be a viable reaction pathway, aromatic polymerization was found to be the main catalyst deactivation mechanism, and aromatic (de)polymerization is responsible for the release of high molecular weight aromatics (naphthalene, phenanthrene, pyrene, etc.) at high reaction temperatures (> 400oC). The monomer sugars glucose and xylose utilized different catalytic active sites, usually of weaker acid strength, for biomass pyrolysis than their polymers cellulose and xylan (hemicellulose). Notably, for cellulose and lignin catalytic pyrolysis nearly all products correlated with strong BAS associated with AlO5 or AlO4 sites. Additionally, supported Al2O3/SiO2 catalysts were found to selectively break lignin propenyl group and methoxy group bonds to produce enhanced amounts of phenol, 2-methoxyphenol, and 2,6-dimethoxyphenol relative to non-catalytic lignin pyrolysis and lignin pyrolysis with zeolites.The cellulose fraction of biomass produced more anhydrosugar products (levoglucosan and levoglucosenone), hemicellulose (xylan) produced more furans, and lignin produced more phenolics. Correlations between catalytic active sites and catalytic wood pyrolysis products could not be established. This is because wood is a sum of its constituents and the resulting wood pyrolysis product trends were non-linear combinations of the trends observed for the individual components of biomass. In general, supported Al2O3/SiO2 catalysts produced more furan-based compounds than reference HZSM-5 catalysts, which produced more 6-membered ring aromatics.