Production of Green Aromatics From Catalytic Fast Pyrolysis of Lignocellulosic Biomass

Catalytic Fast Pyrolysis (CFP) is a promising technology for the direct production of gasoline range aromatics from lignocellulosic biomass in a single catalytic reactor. The most challenging part of this process is controlling the complex homogeneous and heterogeneous reaction chemistry to maximize aromatic yield and suppress the production of coke. To gain fundamental understanding of this process, we studied reaction chemistry for CFP of glucose (i.e. cellulose model compound) with isotopic labeling of ¹³C glucose and ex-situ FT-IR technique in a pyroprobe reactor coupled with GC-MS. It was found that glucose is completely decomposed in one second and converted into dehydrated products, including anhydrosugars and furans. The dehydrated products then enter into the zeolite catalyst pore where they are converted into aromatics, CO, CO₂, and water through a series of dehydration, decarbonylation, decarboxylation, oligomerization, and dehydrogenation reactions. The isotopic labeling studies revealed that aromatic formation reaction proceeds through a common intermediate or “hydrocarbon pool” composed of these dehydrated products. This is a shape selective reaction where the product selectivity is related to the catalyst properties. We have systematically studied CFP with different zeolites including small pore zeolites (ZK-5 and SAPO-34), medium pore zeolites (Ferrierite, ZSM-23, MCM-22, SSZ-20, ZSM-11, ZSM-5, IM-5, and TNU-9), and large pore zeolites (SSZ-55, beta, Y zeolite). The aromatic yield is a function of the pore size and internal pore volume of the zeolite catalyst. Medium pore zeolites with pore sizes in the range of 5.2 to 5.9 Å and moderate pore intersection size such as ZSM-5 and ZSM-11 produced the highest aromatic yield and least amount of coke. The kinetic diameter estimation of aromatic products and the reactants revealed that the majority of these molecules can fit inside the zeolite pores of most of the medium and large pore zeolites, while the polycyclic aromatics form by secondary reactions on the exterior surface of zeolite catalyst. ZSM-5 catalyst, the best catalyst for aromatic production, can be modified further to improve its catalytic performance.  These modifications include: (1) adjusting the concentration of acid sites inside the zeolites catalyst; (2) incorporation of mesoporosity into the ZSM-5 framework to enhance its diffusion characteristics, and (3) adding metal oxides promoters to the ZSM-5. Mesoporous ZSM-5 showed high selectivity for monofunctional aromatics, such as benzene, toluene, xylenes, and less for naphthalene. Metal oxide promotion of ZSM-5 increased the aromatic yield by over 40%. A pilot scale reactor was designed and built where CFP was demonstrated that it could produce liter quantities of aromatic products directly from solid woody biomass feeds.