Designing ordered mesoporous aluminosilicates under acidic conditions via an intrinsic hydrolysis method

Abstract

Acid-mediated synthesis of ordered mesoporous aluminosilicates (OMAS) with medium-to-strong Brønsted acid sites and trivalent aluminium exclusively in a tetrahedral framework structure is realized by a newly devised intrinsic hydrolysis method. In this way, we have synthesized a series of well-ordered OMAS, e.g., H-AlSBA-15 and H-AlIITM-56, which are otherwise difficult to distinguish under acidic conditions owing to very different hydrolysis rates of both silicon and aluminium sources used for preparation as well as easy dissociation of thus formed Al-O-Si bonds. This novel intrinsic hydrolysis approach, however, relies mainly on similar hydrolysis rates of constituent inorganic species leading to efficient condensation. Thus, the innovative methodology using tetraethyl orthosilicate and aluminium citrate, respectively, as tetravalent silicon and trivalent aluminium as precursors facilitates the formation of high quality OMAS with a narrow pore size distribution, thicker walls, and trivalent aluminium in a tetrahedral framework structure with a high aluminium content, as evidenced by a battery of characterization techniques, viz., XRD, XRF, SEM, TEM and BET. The resulting materials, in turn, generate Brønsted acid sites in the aluminosilicate matrix, with the absence of the usually encountered Lewis acid sites, viz., extra-framework and/or non-framework species, as confirmed by both Al MAS-NMR and NH-TPD studies. All the prepared catalysts exhibit excellent activity towards the tertiary butylation of phenol, and the high activity of the catalysts is attributed to the unique and exclusive presence of medium-to-strong acid sites in the OMAS matrix.

Nanostructured zeolite with brain-coral morphology and tailored acidity: a self-organized hierarchical porous material with MFI topology

“…ordered mesoporous aluminosilicates. [17][18][19][20] In the present work, we report the successful synthesis of nano-zeolite, designated as n-ZSM-5, in the absence of mesoporogens under the designed synthesis conditions (Fig. 1).…”

“…S3 ‡) using 500 mg of the catalyst. [17][18][19][20]27 The reactor set-up was preheated to 350°C in a flow of air for 2 h followed by cooling to the desired reaction temperature, i.e., 140°C, in a nitrogen atmosphere. The ratio of reactants, weight hour space velocity, and Si/Al ratio of the catalysts has been varied to study the activity of the catalyst.…”

“…9,45,46 Nonetheless, the broadness of the chemical shift around '35 ppm' indicates the presence of small amounts of penta-coordinated aluminium (Al P ) as structural Lewis acid sites. 19…”

“…Numerous efforts have also been made to alleviate the diffusion limitations by inducing mesoporosity in the zeolite matrix, however, these approaches use various expensive templating strategies and tedious post-synthetic modifications. [14][15][16][17][18][19] In this regard, the development of novel nanostructured materials with unique morphology that exhibit inherent mesoporosity, high external surface area and enhanced diffusivity has gained significant interest owing to their exceptional performance in catalysis. [20][21][22] However, tailoring the crystallization process to achieve the desired morphology and porosity without employing expensive mesoporogens is a major challenge.…”

Catalytic nanosponges of acidic aluminosilicates for plastic degradation and CO2 to fuel conversion

“…We aim to achieve an efficient hetero-condensation between tetraethylorthosilicate (TEOS) and the Al precursor by choosing suitable reactants with comparable hydrolysis rates 24 – 27 , and avoid homo-condensation and the formation of two distinct phases. To realize this, we have studied the effect of two different Al precursors having different reactivity, aluminum acetylacetonate (Al-AC) and aluminum isopropoxide (Al-IP) 24 , on their condensation with Si precursor TEOS. Five different materials are synthesized with varying the Si/Al ratio to achieve tunable acidity.…”