(500d) Catalytic Fast Pyrolysis of Cellulose to Aromatics with Nanocrystalline ZSM-5: Role of Extra-Framework Aluminum
The pyrolysis of lignocellulosic biomass represents a simple, cheap, and efficient approach to produce bio-based fuels and chemicals from renewable feedstocks. In this process, solid biomass is heated to high temperature (500 – 700 °C) in order to thermochemically convert it to light gases (CO, CO2), solid char, and organic vapours, which can be further condensed to obtain the desired liquid bio-oil. However, the high oxygen content of the bio-oil makes it unsuitable for direct use. Additional processes that involve one or several heterogeneous catalysts are required to decrease the oxygen concentration from approximately 45% to less than 7% and achieve stable blends with petroleum that allow refining. Integrated approaches where the catalyst is directly mixed with the biomass are particularly appealing as pyrolysis and deoxygenation occur simultaneously in the same reactor. Notably, catalytic fast pyrolysis (CFP) using ZSM-5 zeolite as a catalyst produces benzene, toluene, xylene, and naphthalene, which can be used as building blocks by the petrochemical industry or further converted to gasoline-range hydrocarbons using hydrogenation processes already employed in refining. Previous investigations using ZSM-5 led to aromatics yields of 27-30% and no clear correlation with mesoporosity, crystal size, or crystallinity was observed, most probably because of the complexity of the system. Here, we synthesized a series of nanocrystalline ZSM-5 samples and characterized them systematically using nitrogen physisorption, XRD (with internal standard), FE-SEM, 27Al MAS-NMR, NH3-TPD, and FTIR of adsorbed pyridine. Extra-framework aluminum species were found to be detrimental and favor coke formation. The yield to desired products was increased by up to 135% by controlling crystallinity and aluminum insertion in the zeolitic framework.