Synthesis of nanosized HZSM-5 zeolites isomorphously substituted by gallium and their catalytic performance in the aromatization

Abstract

In this article, we describe a novel strategy for the seed-induced in-situ hydrothermal synthesis of nanosized HZSM-5 zeolites that are partially or completely isomorphously substituted by gallium, denoted as GaAlHNZ5 and GaHNZ5, respectively. The most noteworthy features of this strategy are that it significantly reduces organic templates in the synthesis process and effectively prevents the loss of gallium species compared to the traditional impregnation method. Thus, this new strategy can be considered an efficient green route for the synthesis of heteroatom HZSM-5 zeolites. In a preliminary attempt to correlate the structure, acidity and catalytic behavior of 1-hexene and FCC gasoline aromatization, various techniques, including XRD, SEM, N2 adsorption, Py-IR, 27Al MAS NMR, 29Si MAS NMR and 71Ga MAS NMR as well as 13C CP-MAS NMR, were used in the characterization of the catalysts. The results show that the samples prepared by Ga isomorphous substitution can facilitate the formation of both Brønsted and Lewis acid sites. In addition to this finding, the aromatics yield was observably increased with the increase of the Ga/Al ratio after Ga partial or complete substitution for Al in the zeolite framework. GaHNZ5 exhibited the highest aromatic yield of ca. 64.6% due to the larger amount of the weakened Brønsted acid sites and the enhanced Lewis acid sites, as well as the most appropriate B/L ratio of 4.4.