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