**5. Zeolites for CFP**

Zeolites are microporous crystalline materials of SiO2 and Al2O3 that have Brønsted and Lewis acidic sites [17, 46]. HZSM-5 also have ordered structure microporous structures. In the refining process, zeolites are integral to crude oil conversions in the gas phase. The feed for these refining processes are bereft of minerals, oxygenates, and water. Under these conditions, zeolite lifetimes are quite long. However, conversion of biomass occurs in a condensed, aqueous phase, which are highly challenging conditions for zeolites. Oxygenate conversions are difficult to achieve because of their tendency to coke. In addition to BTX, CFP produces a very large volume of wide variety of hydrocarbons, valuable for fuel (fuel gas, gasoline and diesel blendstock). Therefore, the literature on CFP catalyst development is quite voluminous due to interests in bio-fuels.

A comparison of HZSM-5, HY, H-BEA, and H-mordenite for pyrolysis oil upgrading has been made [37, 47]. A method to evaluate catalysts for ex situ vapor upgrading is the pyroprobe analytical pyrolyzer. This important tool provides high heating rates and means to analyze complex mixtures of products [48]. An examination of feeds ranging from glucose, xylitol, cellulose, to cellobiose was explored with catalyst candidates of HZSM-5, silicalite, H-BEA, HY, and SiO2/Al2O3. Based on this study, HZSM-5 has been found to be the highest performing catalyst. Pore characteristics and shape selectivity for monoaromatics of HZSM-5 are superior to other zeolites. Another comparison study was conducted on HZSM-5, MCM-41, and HY [49]. HZSM-5 was again the most effective zeolite among this group to produce up to 33.1% BTX and 86.4% aromatics selectivity.

The silica-alumina ratio (SAR) of HZSM-5 is also an important parameter for deoxygenation reactions [50]. As the SAR is reduced, the density of Brønsted acids increases. Other zeolites of similar SAR were inferior to HZSM-5 because they were not microporous. Mesoporous zeolites were not selective for aromatics. For example, sulfated ZrO2 is a superacid, a property important for aromatics formation, but this metal oxide was ineffective because of the absence of shape-selectivity. Coupling ZrO2 to MCM-41 resulted in higher levels of pyrolysis vapor deoxygenation and was found to be stable against coking.
