On the Controllable Soft-Templating Approach to Mesoporous Silicates

Ordered mesoporous materials in catalysis

Pore size determination in modified micro- and mesoporous materials. Pitfalls and limitations in gas adsorption data analysis

The Hydrothermal Synthesis of Zeolites: History and Development from the Earliest Days to the Present Time

The introduction of synthetic zeolites has led to a paradigm shift in catalysis, separations, and adsorption processes, due to their unique properties such as crystallinity, high-surface area, acidity, ion-exchange capacity, and shape-selective character. However, the sole presence of micropores in these materials often imposes intracrystalline diffusion limitations, rendering low utilisation of the zeolite active volume in catalysed reactions. This critical review examines recent advances in the rapidly evolving area of zeolites with improved accessibility and molecular transport. Strategies to enhance catalyst effectiveness essentially comprise the synthesis of zeolites with wide pores and/or with short diffusion length. Available approaches are reviewed according to the principle, versatility, effectiveness, and degree of reality for practical implementation, establishing a firm link between the properties of the resulting materials and the catalytic function. We particularly dwell on the exciting field of hierarchical zeolites, which couple in a single material the catalytic power of micropores and the facilitated access and improved transport consequence of a complementary mesopore network. The carbon templating and desilication routes as examples of bottom-up and top-down methods, respectively, are reviewed in more detail to illustrate the benefits of hierarchical zeolites. Despite encircling the zeolite field, this review stimulates intuition into the design of related porous solids (116 references).

Hierarchical zeolites: enhanced utilisation of microporous crystals in catalysis by advances in materials design.

Steam-Stable Aluminosilicate Mesostructures Assembled from Zeolite Type Y Seeds

The structural integrity of Al-MCM-41 and related mesoporous aluminosilicate molecular sieves has been significantly improved in recent years through direct assembly and postsynthesis treatment methods.[1] Nevertheless, the hydrothermal instability and mild acidity remain inferior to conventional zeolites and limit potential applications in petroleum refining and fine chemicals synthesis.[2] One might expect to improve both the stability and acidity of these materials if zeolite-like order could be introduced into the pore walls. One approach, first introduced by van Bekkum and co-workers,[3] is to transform the preassembled walls of Al-MCM-41 into zeolitic structures by post-assembly treatment with a microporous zeolite structure director, such as tetrapropylammonium cations. More recent studies have shown that the walls of the mesostructure can indeed be converted to a zeolitic product, but the microporous zeolite phase (ZSM-5) is segregated from the mesostructure, giving rise to ZSM-5/ MCM-41 composites.[4] These composites exhibited an enhancement in acidity for hydrocarbon cracking in comparison to mechanical mixtures of ZSM-5 and MCM-41 and an improvement in steam stability for purely siliceous composites.[4c] We recently reported an alternative approach to more acidic and hydrothermally stable mesostructures based on the direct assembly of nanoclustered aluminosilicate precursors that normally nucleate zeolite type Y crystallization.[5] These protozeolitic species, known as azeolite seedso, promote zeolite nucleation by adopting AlO4 and SiO4 connectivities that resemble the secondary building units of a crystalline zeolite.[6] The assembly of the Na-nucleated zeolite type Y (faujasitic) seeds under hydrothermal conditions in the presence of cetyltrimethylammonium ions afforded hexagonal MSU-S mesostructures with Si/Al ratios in the range 1.6:1 to 10:1. The replacement of Na by NH4 ions in the as-made mesostructure, followed by calcination in the presence of the surfactant, afforded exceptionally acidic and steam-stable mesostructures. However, the steam stability was enhanced by structure-stabilizing occlusions of carbon that formed during the calcination process. That is, the steam stability at 800 8C was in part a consequence of the exceptional acidity of a framework that formed structure-stabilizing carbon, and not entirely a result of an intrinsically stable framework. Here we also make use of protozeolitic nanoclusters to form exceptionally acidic and steam-stable aluminosilicate dichloromethane (10/1). Phosphirene complex 4 was isolated as yellow crystals (2 g, 66 %). 31P NMR (81 MHz, CDCl3): dy201.0 (1J(P,W) 294.9 Hz); 13C NMR (50 MHz, CDCl3): d 89.3 (d, 1J(C,P) 18.8 Hz; P-C C), 93.9 (d, 2J(C,P) 5.8 Hz; P-C C), 120.9 (s; C C-Ph, Cipso), 195.9 (d, 2J(C,P) 9.1 Hz; cis-CO), 198.4 (d, 2J(C,P) 34.8 Hz; trans-CO); MS: m/z (%): 634 (6) [M], 494 (100) [My 5 CO]; elemental analysis (%) calcd for C27H15O5PW: C 51.10, H 2.36; found: C 51.48, H, 2.42. 6 : Biphosphirene 6 was isolated as light yellow crystals by chromatography with hexane/dichloromethane (4/1). 13C NMR (50 MHz, CDCl3): d 137.4 (d, 1J(C,P) 5.4 Hz; Cipso, Ph-P), 143.2 (pseudo t, 1J(C,P) 2J(C,P) 13.6 Hz; Ph-C(P)C-P); MS: highest mass 785 [My 10 COyH].

Steam‐Stable MSU‐S Aluminosilicate Mesostructures Assembled from Zeolite ZSM‐5 and Zeolite Beta Seeds

FDU-1 silicas with large cage-like pores (diameter about 10 nm) were synthesized under acidic conditions from tetraethyl orthosilicate in the presence of a poly(ethylene oxide)-poly(butylene oxide)-poly(ethylene oxide) triblock copolymer template B50−6600 (EO39BO47EO39). High-resolution transmission electron microscopy and small-angle X-ray scattering provided strong evidence that FDU-1 silica synthesized under typical conditions is a face-centered cubic Fm3m structure with 3-dimensional hexagonal intergrowth and is not a body-centered cubic Im3m structure, as originally reported. Samples synthesized in a wide range of conditions (initial temperatures from 298 to 353 K; hydrothermal treatment at 333−393 K) exhibited similar XRD patterns and their nitrogen adsorption isotherms indicated a good-quality cage-like pore structure. The examination of low-pressure nitrogen adsorption isotherms for FDU-1 samples, whose pore entrance diameters were evaluated using an independent method, allowed us to conclude that...

Ordered mesoporous silica with large cage-like pores: structural identification and pore connectivity design by controlling the synthesis temperature and time.

Significant improvements in both the hydrothermal stability and the acidity of mesostructured aluminosilcates have been reported recently. New assembly pathways, along with post-synthesis treatment methods, have made it possible to form structures with thick and more highly crosslinked framework walls. The resulting structures are slow to degrade under hydrothermal conditions in comparison to conventional analogs. Also, improved methodologies for grafting Al centers into the walls of pre-assembled frameworks have afforded aluminosilicate mesostructures with enhanced acidity. The most promising strategy for improving the hydrothermal stability and acidity of aluminosilicate mesostructures, however, is based on the use of protozeolitic nanoclusters. These so-called “zeolite seeds” can be directly assembled into hexagonal, cubic, wormhole, and foamlike framework structures under a variety of assembly conditions. They also can be grafted into the walls of pre-assembled frameworks to form more stable acidic derivatives.

Aluminosilicate mesostructures with improved acidity and hydrothermal stability

Although the assembly of aluminosilicate foams and large-pore hexagonal mesostructures from zeolite BEA, MFI, and FAU seeds requires a reduction in pH from initially very basic values (pH > 12) to ...

Yu Liu

Papers

Steam-Stable Aluminosilicate Mesostructures Assembled from Zeolite Type Y Seeds

Steam‐Stable MSU‐S Aluminosilicate Mesostructures Assembled from Zeolite ZSM‐5 and Zeolite Beta Seeds

Ordered mesoporous silica with large cage-like pores: structural identification and pore connectivity design by controlling the synthesis temperature and time.

Aluminosilicate mesostructures with improved acidity and hydrothermal stability

Assembly of Hydrothermally Stable Aluminosilicate Foams and Large-Pore Hexagonal Mesostructures from Zeolite Seeds under Strongly Acidic Conditions