**4. Mechanism**

Fast pyrolysis rapidly depolymerizes lignocellulose to its constituents: hemicellulose, cellulose, and lignin. Under these conditions, cellulose is depolymerized to glucose, hemicellulose to C5 sugars, and lignin to phenolic compounds [22, 38, 39]. Once glucose is made, it is quickly dehydrated to anhydrosugars which are then converted to furanics. Furanics are diffused into zeolite pores where it is combined with in situ olefins and undergoe Diels Alder cyclization and dehydration to aromatics. Zeolite imparts a "confinement effect" for chemicals undergoing dehydrocyclization within the catalyst interior and once formed, monoaromatics exit the catalyst [40]. At a molecular level, at 450–600 °C, the furanic oxygen coordinates to Al of the zeolite. Deoxygenation of organics result in elimination of CO2, CO, and H2O. As a test of this hypothesis, furan and olefins were used as models in HZSM-5 catalyzed upgrading. HZSM-5 has a three-dimensional pore structure with intersecting systems of 10-membered ring pores. The dimension of ring pores is nearly ideal for production of pX: 5.1 x 5.5 Å (sinusoidal pores), and 5.3 x 5.6 Å (straight pores) [41]. HZM-5's utility in conversion of oxygenates to aromatics was demonstrated by Mobil's commercial methanol-to-gasoline process. Within HZSM-5, ethylene and furan did not yield benzene, but replacement of ethylene by propylene led to toluene [42]. Likewise, 2-methylfuran and propylene produced xylenes. In the absence of olefins, Diels Alder dimerization and dehydration of furan results in benzofuran (Eq. (1)). Graphitic coke was also deposited onto the catalyst, which contributed to catalyst deactivation in about 30 minutes. When the upgrading temperature was increased to 650 °C, the selectivity was shifted to olefins.

Other investigators have examined conversion of methylated furan over Ga/ HZSM-5 [43]. Even though model compounds were used, the reaction network was extremely complex due to numerous secondary reactions at 300–500 °C. Coking became a dominant reaction at temperature above 500 °C.

A "technical catalyst" such as extruded Al2O3 -HZSM-5 was explored in CFP of pine wood and cellulose at 500 °C [44]. The focus of the study was on effect of pyrolysis temperature. The type of coke made was highly dependent on the feedstock. Yet, even by 2020, the mechanism of CFP remained elusive. Gaps still exist between theoretical models and applied pyrolysis. Thus far, there is still insufficient information generated from a single biomass source [45]. High aromatics selectivity and strong resistance to catalyst deactivation are keys to the success of CFP for biopX. Therefore, extensive efforts have been directed toward zeolite modifications.
