VERY LARGE PORES MOLECULAR SIEVE

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. 

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. 

Synthesis and charaeterization of the very large pore molecular sieve MCM-9 , AppL CataL 51, L13-L20. 

Rudolph PR, Crowder CE (1990) Structure refinement and water location in the very large-pore molecular sieve VPI-5 by X-ray lUetveld techniques. Zeolites 10 163-168 Sanders MJ, Leslie M, Catlow CRA (1984) Interatomic potentials for Si02. J Chem Soc Chemical Commun 1271-1273... 

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. 

Principal Characteristics. - Molecular sieves with pore openings of about 0.45 nm show very interesting shape-selectivity properties for the conversion of methanol to olefins (MTO process). The small-pore molecular sieves studied in the MTO process are chabazite, erionite, zeolite T, ZK-5, ZSM-34, zeolite A, SAPO-17, SAPO-34, and SAPO-44. All of them can sorb only straight chain molecules, e.g. primary alcohols and linear paraffins and olefins, but no branched isomers and aromatics the pore opening is smaller than the kinetic diameter of branched and aromatic molecules, but large enough to permit the access of linear molecules. 

The available intrazeolitic space should be sufficient to accomodate a complex with a diameter of approximately 1.5 nm. This seems possible in faujasite type zeolites as weli as in very large pore AlPO -n molecular sieves or substituted anaiogs. VPI-5 is a candidate (23). For AIPO4-8 molecular graphics analysis predicts - as is confirmed by synthesis attempts - that accomodation of the complex for sterical reasons in this molecular sieve is impossible. The Cloverite material, the structure of which has been described elsewhere in this volume, also is a potentiai support. 

To ensure a better separation, molecular sieving will act much better This size exclusion effect will require an ultramicroporous (i.e pore size D < 0.7 nm) membrane Such materials should be of course not only defect-free, but also present a very narrow pore size distribution. Indeed if it is not the case, the large (less separative and even non separative, if Poiseuille flow occurs) pores will play a major role in the transmembrane flux (Poiseuille and Knudsen fluxes vary as and D respectively). The presence of large pores will therefore cancel any sieving effect... 

Whereas the acetylation of phenyl ethers over zeolite catalysts leads to the desired products, acetylation of 2-MN occurs generally at the very activated C-l position with formation of l-acetyl-2-methoxynaphthalene (l-AMN). A selectivity for l-AMN close to 100% can be obtained over silicoaluminate MCM-41 mesoporous molecular sieves[22] and FAU zeolites,133 341 whereas with other large pore zeolites with smaller pore size (BEA, MTW, ITQ-7), 2-AMN (and a small amount of l-acetyl-7-methoxynaphthalene, 3-AMN) also appears as a primary product. Average pore size zeolites, such as MFI, are much less active than large pore zeolites. These differences were related to shape selectivity effects and a great deal of research work was carried out over BEA zeolites in order to specify the origin of this shape selectivity the difference is either in the location for the formation of the bulkier (l-AMN) and linear (2-AMN) isomers (only on the outer surface for l-AMN, preferentially within the micropores for 2-AMN)[19 21 24 28 381 or more simply in the rates of desorption from the zeolite micropores.126 32 33 351... 

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