**6. Zeolite modifications**

Because HZSM-5 plays such a vital role in CFP, numerous investigations have been launched to determine whether modifications of the physical structure can improve aromatics selectivity and minimize coking. Modification of zeolite's pore mouth sizes can improve the catalyst for improving of BTX selectivities [51, 52]. Specifically, narrowing the pore mouth openings by chemical deposition techniques was explored [53]. The deposited chemicals also covered external acid sites, which can improve pX selectivity [5]. Phosphorous-modified HZSM-5 promoted greater space confinement and increased pX selectivity. A comparison of various deposited silicon materials was conducted by KH550, TEOS, and methyl silicone oil [54, 55]. KH550 modification of HZSM-5 increased furan conversion to 24.5% aromatics vs. 18.8% unmodified HZSM-5 (see reference [5]). Coking of the modified catalyst was also reduced from 44.1 to 26.7%. A decrease in benzene and increase in toluene and naphthalenes selectivities were observed. Chemical liquid depositions of TiO2 to HZSM-5 also resulted in improvements to BTX yields from 17.0 to 23.5%, and reduced levels of coking [56].

The other direction to zeolite modifications is to create mesopores. Unilamellar mesoporous MFI nanosheets (UMN) were tested against another mesoporous material, Al-SBA-15 [57]. UMN has stronger acid sites which result in greater cracking

*Catalytic Fast Pyrolysis of Lignocellulosic Biomass to Benzene, Toluene, and Xylenes DOI: http://dx.doi.org/10.5772/intechopen.99946*

and deoxygenation activities. Ketones and alcohols were converted to furanics and aromatics. Another method to produce mesopores is NaOH desilication of HZSM-5 [58]. This method was successful to create intraparticle mesoporous materials which then improved diffusion. By coupling ZrO2 with desilicated zeolites, a more active zeolite was accompanied with longer catalyst lifetimes [19]. The resulting coke was "softer" (i.e. less aromatic) and was easier to remove. A two-stage microporous and mesoporous vapor upgrading system was examined [59]. In this case, HZSM-5 and MCM-41 (3:1) were evaluated. By increasing the MCM-41 loading, more xylenes were made than toluene. A comparison of chemical liquid deposition (CLD) and acid dealumination (AD) of HZSM-5 was conducted [60]. CLD decreased pore size while AD increased pore size. In both cases, decrease of strong acid sites occurred. CLD boosted BTX yield to 37.2% while AD produced 30.4%. In both cases there was a decrease in indene, naphthalenes, and coke precursors.

Most recently, Li et al. reported development of a core-shell catalyst of HZSM-5@ MCM-41 made from an external recrystallization method [61]. HZSM-5@MCM-41 required a lower temperature of 500 °C to produce hydrocarbons, versus 550 °C for unmodified HZSM-5. Monoaromatic hydrocarbons produced from rape straw pyrolysis was as high as 11.43 wt%, which is 2.5 times higher than obtained from HZSM-5. Furthermore, HZSM-5@MCM-41 produced less graphitic coke.
