Urea solution treatment: A facile and moderate approach to achieve hierarchical zeolite beta

Catalytic Upgrading of Lignite Pyrolysis Volatiles to Aromatics over Urea-Modified HZSM-5: Study on Desilication of Preserved Micropores

“…In comparison of these peaks, it can be found that there were small differences in the crystallinity of catalysts prepared with different intensity treatments (Figure ). Overall, the relative crystallinity (RC) of some modified catalysts, as shown in Table , even showed an increase compared to the traditional inorganic alkali treatments, which was consistent with the results of previous studies . It is generally assumed that alkali or alkaline solution treatment predominantly removed the framework Si species while nevertheless accompanied by the removal of part of the inherent and newly generated non-crystal amorphous skeleton Al .…”

“…The schematic diagram of reforming of lignite pyrolysis volatiles by urea-modified HZ5 is shown in Figure . The possible reaction pathway to promote LA formation based on the product distribution reflected by the physicochemical properties of the urea-modified HZ5 was proposed: (1) The formation pathway of LAs basically followed the mechanisms of “hydrocarbon pool” and “phenolic pool” as the final precursor of LAs, which were mainly formed in the pore structure of the catalysts, especially micropores. − (2) The upgrading optimization of pyrolysis volatiles, however, mainly strengthened the different stages of reaction through synergistic regulation of the catalyst pore size (micropores, aromatization center; mesopores, “highways” for molecular diffusion mass transfer), acid sites (Brønsted acid sites, cracking active center; Lewis acid site, hydrogenation and dehydrogenation), and specific surface area (active site control center). ,,, (3) Urea hydrolysis can promote the desilication hierarchy of the catalysts by providing homogeneous and stable alkaline media that introduced mesopores while well-preserving micropores, which facilitated the diffusion of more macromolecules and their accessibility to the active sites. Removed Al during desilication may be reintroduced into the framework as a new Al source that provided more new Brønsted acid sites, thus cracking macromolecules into LA precursor fragments. , Finally and most importantly, the preserved micropores ensure subsequent aromatization and, thus, can facilitate the generation of LAs.…”

“…Overall, the relative crystallinity (RC) of some modified catalysts, as shown in Table 1, even showed an increase compared to the traditional inorganic alkali treatments, which was consistent with the results of previous studies. 18 It is generally assumed that alkali or alkaline solution treatment predominantly removed the framework Si species while nevertheless accompanied by the removal of part of the inherent and newly generated non-crystal amorphous skeleton Al. 16 It is worth noting, however, that the SAR of modified HZ5 had no corresponding change, only slightly decreasing with increasing the urea concentration and treatment time.…”

“…31−34 (2) The upgrading optimization of pyrolysis volatiles, however, mainly strengthened the different stages of reaction through synergistic regulation of the catalyst pore size (micropores, aromatization center; mesopores, "highways" for molecular diffusion mass transfer), acid sites (Brønsted acid sites, cracking active center; Lewis acid site, hydrogenation and dehydrogenation), and specific surface area (active site control center). 2,31,34,36 (3) Urea hydrolysis can promote the desilication hierarchy of the catalysts by providing homogeneous and stable alkaline media that introduced mesopores while well-preserving micropores, 18 which facilitated the diffusion of more macromolecules and their accessibility to the active sites. Removed Al during desilication may be reintroduced into the framework as a new Al source that provided more new Brønsted acid sites, thus cracking macromolecules into LA precursor fragments.…”

“…Therefore, it is very important to find a mild, effective, economical, and environmentally friendly reprocessing desilication reagent. Tian et al 18 reported a facile and moderate preparation method of hierarchical zeolite treated with urea solution. The results showed that the method could effectively form mesopores for β zeolites with a Si/Al ratio of 12.5−75 while basically maintaining the crystal structure, and the acidity of the catalyst can be adjusted to a certain extent.…”