Syntheses and applications of meso-microporous zeolites in vapor phase upgrading of biomass

Abstract

Despite the fact that zeolites have been applied to biomass upgrading for decades, there are still various obstacles that need to be overcome. In applications of using zeolites to catalyze biomass pyrolysis vapors, the yields of hydrocarbons are relatively low and catalysts frequently suffer from fast deactivation. Diffusion limitation is widely accepted to be the dominant reason for the two challenges above. Researchers have been intensively engaged in developing hierarchical zeolites which possess both introduced mesopores and original micropores to accelerate the diffusion of complex intermediates and products from biomass pyrolysis vapors. A number of hierarchical zeolites have been created in recent years. However, most of these novel structures are only involved in simplified model compounds to demonstrate potential in promising applications in real biomass. It has become increasingly important to investigate the catalytic activity of hierarchical zeolites in biomass, due to the raised economic and environmental requirements to commercialize biofuels upgraded from biomass. I synthesized a multilayered MFI nanosheet zeolite, a category of hierarchical zeolites which performed well in model reactions (methanol to gasoline). The nanosheet zeolite was thoroughly characterized. It possessed uniform mesopores on the external surface and micropores on the internal surface. The catalytic activity of MFI nanosheet was investigated via two complementary instruments, micropyrolyzer-GCMS and molecular beam mass spectrometer (MBMS). Hydrocarbon yields of biomass over the nanosheet were found to be competitive to the yields over conventional ZSM-5 catalyst which possessed similar acid sites but lacked hierarchical structure. The species of hydrocarbon products were the same over the two catalysts, implying that mesopores did not change the distribution of products. Meanwhile, the completed deactivation process of MFI nanosheet was studied. It was found that MFI nanosheet achieved a lifetime approximately three times of conventional ZSM-5. However, coke (carbonaceous by-products) contents on MFI nanosheet were higher than ZSM-5. Coked catalysts were collected at various biomass-to-catalyst ratios and characterized to track the change of pore volumes and active acid sites in the process of deactivation. Based on the results of catalytic activities and characterizations, it is suggested that mesoporous MFI nanosheet has much better accessibility to active acid sites, which enables the longer catalyst lifetime while resulting in more coke formation.