Large-pore Molecular Sieves

Casci J L 1994 The preparation and potential applications of ultra-large pore molecular sieves a review Stud. Surf. Sc/. Catai. 85 329-56... 

The foregoing discussion has focused on the most important commercial molecular sieves, zeolites. New directions in the preparation of framework stmctures of different chemical composition and of large-pore molecular sieves have also appeared. 

M. E. Davis (ed.). Large Pore Molecular Sieves, Catal. Today, 19 (1994). 

CHA (-34, -44, -47), ERI (-17), GIS (-43), LEV (-35), LTA (-42), FAU (-37) and SOD (-20). Also shown is the pore size and saturation water pore volume for each structure type. The structures include the first very large pore molecular sieve, VPl-5, with an 18-ring one-dimensional channel with a free pore opening of 1.25 nm [29], large pore (0.7-0.8nm), intermediate pore (0.6nm), small pore (0.4 nm) and very small pore (0.3 nm) materials. Saturation water pore volumes vary from 0.16 to 0.35cm /g, comparable to the pore volume range observed in zeolites (see Chapter 2 for detailed structures). 

Derouane, E.G., Maistriau, L, Gabelica, Z., Tuel, A., Nagy, J.B., and Von Ballmoos, R. (1989) Synthesis and charaderization of the very large pore molecular sieve MCM-9. Appl. Catal., 51, L13-L20. 

Large-pore molecular sieves (like VPI-5) (54), amorphous... 

Since the first synthesis of TS-1 in 1983 [1], considerable efforts have been devoted to the synthesis of titanium-containing zeolites [2, 3]. Recently, Ti-beta, a large-pore molecular sieve, has been extensively studied [4, 5]. Owing to its unique large-pore channel system, Ti-beta seems to be more active than the medium-pore TS-1 catalyst for the oxidation of cyclic and branched alkenes with aqueous hydrogen peroxide. Under the usual synthesis conditions, however, Ti-beta crystallizes with some Al as a framework constituent [4], This leads to the presence of acid centers, which may have a detrimental effect on the activity or selectivity of this type of catalyst. Since 1992, the discovery of a new family of mesoporous molecular sieves has received much attention [6,7], Because of their mesoporous nature (20-100A), the Ti-MCM-41 zeolites may be useful as oxidation catalysts for larger molecules [8], In this... 

Unfortunately, the use of TS1 (as well as TS2 discovered in 1990 by the group of Ratnasamy (27)) in catalytic oxidations is restricted to the relatively small substrates able to enter the pores of these zeolites (apertures 0.55 nm). Therefore, many research groups attempted to incorporate titanium in large pore molecular sieves BEA zeolites, mesoporous molecular sieves MCM41 and MCM48. Other transition metal zeolites were also synthesized and tested in oxidation one of the main problems of these systems is the release of redox cations in liquid phase (24). Progress remains to be made to develop molecular sieves catalyzing the oxidation... 

With respect to other large pore molecular sieve supports like ALPOs very little is known about the electronic nature of the supported metals. Preliminary experiments in our laboratory indicate that the reducibility of the metal catalyst precursor and the dispersion of the metal in the final material differs significantly from that in conventional zeolites. 

The major activities in the science and application of zeolite catalysts are still observed in the field of (shape selective) acid catalysis. However, additional thrust areas can be clearly identified today, viz. zeolites in oxidation or base catalysis, applications in environmental protection, catalysis by ship-in-the-bottle complexes, to enumerate just a few. Many aspects of zeolite catalysis have been covered in a number of recent review articles [e g., 1-6] including the potential catalytic applications of ultra-large pore molecular sieves [7]. Hence there is no real need, nor would it be feasible on the limited number of pages allotted to this review, to cover every aspect fi om the huge amount of work done recently in the field. Rather, the authors restricted themselves to selected topics in catalysis by zeolites which, in their own view, deserve particular attention in the years to come. 

As stated some years ago [2, 4], shape selective catalysis involving bulky molecules continues to be a thrust area in zeolite catalysis. Consequently, test reactions have been developed which are particularly suited to characterize large and super-large pore molecular sieves [34]. In view of possible commercial applications, recent work focussed on the shape selective synthesis of substituted dinuclear aromatics, i.e., 4,4 -diisopropylbiphenyl and 2,6-dialkylnaphthalenes, due to their potential as components in high-temperature resistent polyesters or as liquid crystals. Recent advances in this field are covered in two excellent review articles [35, 36]. 

M. Yoshikawa, P. Wagner, M. Lovallo, K. Tsuji, T. Takewaki, C. Chen, L.W. Beck, C. Jones, M. Tsapatsis, S.I. Zones, andM.E. Davis, Synthesis, Characterization, and Structure Solution of CIT-5, a New, High-silica, Extra-large-pore Molecular Sieve. J. Phys. Chem. B, 1998,102, 7139-7147. 

E.G. Derouane, L. Maistreiau, Z. Gabelica, A. Tuel, J.B. Nagy, and R. von Ballmoos, Synthesis and Characterization of the Very Large Pore Molecular Sieve MCM-9. Appl. Catal., 1989, 51, 13-20. 

M.S. Rigutto, R. de Ruiter, J.P.M. Niederer, and H. Van Bekkum, Titanium-Containing Large Pore Molecular Sieves from Boron-Beta Preparation, Characterization and Catalysis. Stud. Surf. Sci. Catal., 1994, 84C, 2245-2252. 

Thus under the chosen screening conditions, SAPO-5, MAPO-5 and MAF0-36, all large pore molecular sieves with approximately 8 angstrom pore sizes, catalyze from 12 to 22 % disproportionation while SAFO-11, MAF0-11 and SAPO-41 with approximately 6 angstrom pores suffer only 2 to 7 I xylene losses. These results are consistent with the concept of transition state shape... 

Principal Characteristics. - Large-pore molecular sieves like mordenite, X and Y zeolite, SAPO-5, MeAPO-5, and MAPO-5 have also been used as catalysts for methanol conversion into olefins. 

Deactivation bv Coke. - The channels of large-pore molecular sieves can accommodate carbonaceous residues that lead to loss of catalytic activity. The deactivation by coke is rather rapid with mordenite, due to the blockage of the uniform noninterconnecting channels by coke. SAPO-5, MAPO-5, and MeAPO-5 have also a onedimensional porous structure and can be deactivated in a similar way. Faujasite-like zeolites, such as zeolite X and Y, can initially accommodate some coke in the large cavities without blockage of the pore structure. 

Table 2. Sorption of Indole isomers 4 and 5 onto large-pore molecular sieves...

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