Mesoporous Aluminosilicates with Ordered Hexagonal Structure, Strong Acidity, and Extraordinary Hydrothermal Stability at High Temperatures

Abstract

Highly ordered hexagonal mesoporous aluminosilicates (MAS-5) with uniform pore sizes have been successfully synthesized from assembly of preformed aluminosilcate precursors with cetyltrimethylammonium bromide (CTAB) surfactant. The aluminosilicate precursors were obtained by heating, at 100--140 degrees C for 2--10 h, aluminasilica gels at the Al(2)O(3)/SiO(2)/TEAOH/H(2)O molar ratios of 1.0/7.0--350/10.0--33.0/500--2000. Mesoporous MAS-5 shows extraordinary stability both in boiling water (over 300 h) and in steam (800 degrees C for 2 h). Temperature-programmed desorption of ammonia shows that the acidic strength of MAS-5 is much higher than that of MCM-41 and is comparable to that of microporous Beta zeolite. In catalytic cracking of 1,3,5-triisopropylbenzene and alkylation of isobutane with butene, MAS-5 exhibits greater catalytic activity and selectivity, as compared with MCM-41 and HZSM-5. The MAS-5 samples were characterized with infrared, UV--Raman, and NMR spectroscopy and numerous other techniques. The results suggest that MAS-5 consists of both mesopores and micropores and that the pore walls of MAS-5 contain primary and secondary structural building units, similar to those of microporous zeolites. Such unique structural features might be responsible for the observed strong acidity and high thermal stability of the mesoporous aluminosilicates with well-ordered hexagonal symmetry.

Identifying the isolated transition metal ions/oxides in molecular sieves and on oxide supports by UV resonance Raman spectroscopy

“…4). We also found that this band for Ti-MCM-41 and Ti-SBA is between 1100 and 1110 cm -1 [50][51][52], always lower than 1125 cm -1 . This difference in frequency essentially reflects the different coordination of titanium ions.…”

Hierarchically Nanoporous Zeolites and Their Heterogeneous Catalysis: Current Status and Future Perspectives

“…After then, multi-step synthesis strategies for assembling preformed zeolite seeds into a mesoporous structure were proposed in order to prevent the separate formation of zeolite and mesoporous silica [61][62][63][64]. In a modified method, Naik et al performed steam-assisted crystallization (i.e., dry-gel synthesis) of zeolite seeds that were preassembled by surfactants [65].…”

Isomorphous Substitution of Zn2+ in aluminosilicate framework creates two types of mesopores without templates

“…Mesoporous materials have good surface area, high thermal stability, good acid sites and uniform pore size 6 . Aluminium incorporated mesoporous materials were initially reported by Corma et al 3 .…”