Acid‐Mediated Synthesis of Ordered Mesoporous Aluminosilicates: The Challenge and the Promise

Abstract

A new intrinsic hydrolysis method was employed, for the first-time, to synthesize well-ordered H-AlSBA-15 with trivalent aluminium exclusively in the tetrahedral framework structure of SBA-15. Unlike other methods, which involve incorporation of aluminium ions in both the framework (Brønsted) and non-framework (Lewis) sites of the silicate matrix, the intrinsic hydrolysis method isomorphously substitutes aluminium ions in the tetrahedral network even at high aluminium content. This unique approach relies mainly on the hydrolysis rates of the inorganic (silicon and aluminium) precursors used for the preparation in such a way that the condensation occurs simultaneously so as to overcome the usually encountered difficulties in stabilizing aluminium ions in the silicate matrix. In this way, we could successfully synthesize high quality Brønsted acidic H-AlSBA-15, hitherto not reported. The synthesized materials were systematically characterized by various analytical, spectroscopic, and imaging techniques, including XRD, Brunauer-Emmett-Teller (BET) surface area measurements, TEM, SEM, Si and Al magic angle spinning NMR spectroscopy, X-ray fluorescence (XRF), and NH temperature-programmed desorption (TPD). The characterization results reveal the presence of a highly porous structure (with narrow pores) and tetrahedrally coordinated trivalent aluminium in the silicate matrix with more medium to strong Brønsted acid sites. The resulting high quality catalysts exhibit excellent activity for tert-butylation of phenol with high selectivity towards para-tert-butyl phenol and 2,4-di-tert-butyl phenol.

Synthesis of Well-Ordered Mesoporous Aluminosilicates with High Aluminum Contents: The Challenge and the Promise

“…For example, a 0 increased from 10.63 nm (Al–SBA-AM) to 12.44 nm (Al–SBA-50) due to the introduction of Al. This result is probably because the bond length of Al–O (0.175 nm) is longer than that of Si–O (0.161 nm) . In this investigation, it is interesting that the n (SiO 2 )/ n (Al 2 O 3 ) ratio in the final Al–SBA-15 products is higher than that of the initial gel mixtures, suggesting the high solubility of aluminum species even in a neutral medium.…”

“…Compared with sample Al–SBA-AM, samples Al–SBA-10 to Al–SBA-50 exhibited three well-resolved diffraction peaks, which corresponded to diffractions of the (100), (110), and (200) lattice planes. These results indicated that the ordering of the mesostructure of SBA-15 is maintained during the process of implanting Al species into the silica pore walls by the SPS method . However, a gradual decrease in the diffraction peak intensity was observed with a decrease in the n (SiO 2 )/ n (Al 2 O 3 ) ratio.…”

“…Notably, as seen from the results of Table , Al–SBA-15 exhibited much thicker mesopore walls of 5.4–6.76 nm (Al-SBA-50 to Al-SBA-20) compared with that of sample Al-SBA-AM (4.43 nm). Zhao’s investigation indicated that the grafting of Al species could improve the condensation of silica species, resulting in the thicker and denser mesopore walls. However, the cell parameters of Al-SBA-50 to Al-SBA-20 changed slightly.…”

A Remarkable Catalytic Activity of Hierarchical Zeolite (ZH‐5) for Tertiary Butylation of Phenol with Enhanced 2,4‐Di‐t‐Butylphenol Selectivity

“…Subsequently, the aluminium content of the synthesized samples have been varied in order to study the interplay between the acidity and catalytic activity of the zeolite in the selective formation of 2,4‐di‐tertiary butylphenol, an industrially important chemical intermediate . Although, various catalysts viz., polymer resins, zeolites, heteropoly acids, sulfated zirconia and ionic liquids have been studied for the t‐butylation of phenol, most of them have yielded distributed products owing to the diffusion limitations and high acidities . In this regard, the hierarchical zeolites with strong Brønsted acid sites in the mesoporous channels can be promising catalysts for the selective formation of 2,4‐di‐ t ‐butyl phenols.…”

Rational design, synthesis, characterization and catalytic properties of high-quality low-silica hierarchical FAU- and LTA-type zeolites

“…In particular, zeolites with FAU (Zeolites X and Y) and LTA (Zeolite-A) topologies have potential applications in various industrial processes 5 , 6 . Despite such remarkable success, these materials suffer from severe mass-transfer limitations owing to sluggish diffusion characteristics in the narrow pore-channels 7 – 9 . From the perspective of modern crude oil reserves with high boiling fractions and feedstock from unconventional resources, it is important to develop effective zeolite catalysts for the efficient utilization of existing resources.…”