Shaolong Wan, Christopher Waters, Abhishek Gumidyala, Rolf Jentoft, Lance Lobban, Steven Crossley, Daniel Resasco, and Richard Mallinson
Since the mid 1980s, HZSM-5 has been shown to be a superior catalyst for upgrading the vapors from biomass pyrolysis to produce aromatic hydrocarbons. However, this comes with relatively low liquid yields, significant coke and gas formation and rapid deactivation. We have studied HZSM-5 catalysis of biomass pyrolysis vapors in a custom CDS pyroprobe with a separate reactor in order to understand how to increase liquid yield and improve catalyst stability. Compared with pyrolyzing the catalyst mixed with the biomass, different product selectivities are observed with the separate reactor due primarily to differences in residence time. The pyroprobe pyrolysis of a biomass-catalyst mixture lasts for seconds and results in high conversion of a primary cellulose decomposition product, levoglucosan, as well as light oxygenates including acetic acid. When configured in a separate reactor with much shorter residence time, high conversion of the light oxygenates is obtained, but low conversion of levoglucosan is observed. In this separate configuration, multiple pulses of pyrolysis vapors may be passed over the catalyst, thus allowing the deactivation characteristics to be studied. For example, at a constant number of acid sites in the catalyst bed, the deactivation rate was found to be much faster with a catalyst containing a higher acid site density. Different HZSM-5 materials have been tested, with different separate reactor temperatures. These have included variation in the Si/Al ratio from 40 to 11.5 and the use of core-shell materials with a core of silicalite, an outer layer of HZSM-5, and also a material with a further outer layer of silicalite. Additionally, the effect of the passivation of surface acidity has been examined.
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