Zeolite-like molecular sieves

Zeolite-like Molecular Sieves with Intersecting (or Interconnected) Channels... 

Zeolite and Zeolite-Like Molecular Sieve Synthesis... 

Another drawback of oil-sealed pumps is the back-streaming of oil vapour into the roughing line, which may occur at low pressure. Contamination by back-streaming oil can be drastically reduced by using proper traps like molecular sieve traps with zeolite (see Section 1.6.4). 

The issues and challenges in molecular sieve science have dramatically changed in recent years and will continue to be redefined. As new and clever approaches to nanoporous materials are developed, the pore size and structural limitations that presently exist will no doubt be vanquished. As bolder endeavors expand the scope of this field, many new applications should also emerge. It seems the progress and innovations described in this chapter reflect a growing interdisciplinary nature in molecular sieve research. It is likely that the importance of large pore zeolites and molecular sieves will evolve in many nontraditional areas. 

A surprisingly large number of important industrial-scale separations can be accomplished with the relatively small number of zeolites that are commercially available. The discovery, characterization, and commercial availability of new zeolites and molecular sieves are likely to multiply the number of potential solutions to separation problems. A wider variety7 of pore diameters, pore geometries, and hy7drophobicity in new zeolites and molecular sieves as well as more precise control of composition and crystallinity in existing zeolites will help to broaden the applications for adsorptive separations and likely lead to improvements in separations that are currently in commercial practice. 

CAS-1 is the first microporous calcosilicate zeolite-like crystal material and can be easily synthesized with or without organics. The reversible cation-exchangeability and the selectively adsorptive properties of CAS-1 are analogous to those for zeolites and molecular sieves. CAS-1 can withstand high temperature calcination indicating that it has a good thermal stability that is common for zeolites. 

The incorporation of Ti into various framework zeolite structures has been a very active research area, particularly during the last 6 years, because it leads to potentially useful catalysts in the oxidation of various organic substrates with diluted hydrogen peroxide [1-7]. The zeolite structures, where Ti incorporation has been achieved are ZSM-5 (TS-1) [1], ZSM-11 (TS-2) [2] ZSM-48 [3] and beta [4]. Recently, mesoporous titanium silicates Ti-MCM-41 and Ti-HMS have also been reported [5]. TS-1 and TS-2 were found to be highly active and selective catalysts in various oxidation reactions [6,7]. All other Ti-modified zeolites and molecular sieves had limited but interesting catalytic activities. For example, Ti-ZSM-48 was found to be inactive in the hydroxylation of phenol [8]. Ti-MCM-41 and Ti-HMS catalyzed the oxidation of very bulky substrates like 2,6-di-tert-butylphenol, norbomylene and a-terpineol [5], but they were found to be inactive in the oxidation of alkanes [9a], primary amines [9b] and the ammoximation of carbonyl compounds [9a]. As for Ti-P, it was found to be active in the epoxidation of alkenes and the oxidation of alkanes and alcohols [10], even though the conversion of alkanes was very low. Davis et al. [11,12] also reported that Ti-P had limited oxidation and epoxidation activities. In a recent investigation, we found that Ti-P had a turnover number in the oxidation of propyl amine equal to one third that of TS-1 and TS-2 [9b]. As seen, often the difference in catalytic behaviors is not attributable to Ti sites accessibility. 

Zeolite beta has been widely used in the petroleum refining and fine chemical industries due to its high thermal stability and strong acidity. Like LTA-, FAU-, MOR-, and MFI-type molecular sieves, zeolite beta molecular sieve has been industrially produced for a long time. The laboratory synthetic procedure described here was developed by M.A. Camblor and J. Perez-Pariente.[128] Tetraethylammonium hydroxide (Alfa 40 wt% TEAOH), sodium chloride, potassium chloride, silica, sodium hydroxide, sodium aluminate, and deionized water were used to prepare the reaction gel with a composition of 1.97 Na20 1.00 K20 12.5 (TEA)20 A1203 50 Si02 750 H20 2.9 HC1. The detailed procedure for the preparation of 20 g of anhydrous product is described below ... 

Since 1980, the applications zeolites and molecular sieves in the speciality and fine chemicals increased enormously. Zeolites are being used in the various types of reactions like cyclization, amination, rearrangement, alkylation, acylations, ammoxidation, vapour and liquid phase oxidation reactions. Zeolites and molecular sieves have also been used to encapsulate catalytically active co-ordination complexes like ship-in-bottle and as a support for photocatalytic materials and chiral ligands. Redox molecular sieves have been developed as an important class of liquid and vapour phase oxidation and ammoxidation reactions. We have discussed few typical recent examples of various types of reactions. 

Schunk S A and Schuth F 1998 Synthesis of zeolite-like inorganic compounds Molecular Sieves Science and Technology vo 1, ed H G Karge and J Weitkamp (Berlin Springer) pp 229-63... 

The separation factors are relatively low and consequently the MR is not able to approach full conversion. With a molecular sieve silica (MSS) or a supported palladium film membrane, an (almost) absolute separation can be obtained (Table 10.1). The MSS membranes however, suffer from a flux/selectivity trade-off meaning that a high separation factor is combined with a relative low flux. Pd membranes do not suffer from this trade-off and can combine an absolute separation factor with very high fluxes. A favorable aspect for zeoHte membranes is their thermal and chemical stability. Pd membranes can become unstable due to impurities like CO, H2S, and carbonaceous deposits, and for the MSS membrane, hydrothermal stability is a major concern [62]. But the performance of the currently used zeolite membranes is insufficient to compete with other inorganic membranes, as was also concluded by Caro et al. [63] for the use of zeolite membranes for hydrogen purification. 

The use of zeolites can also be very helpful in removing a reaction product that unfavourably influences the yield of the desired product. Thus, in the manufacture of antibiotic cefoxitin, the amide acylation results in the generation of HCI, which can be removed by the addition of molecular sieve 3 A or 4 A, which has a large capacity for HCI (Weinstock, 1986). Other examples are reactions in which products like methanol or water retard the rate and prevent the reaction to reach the desired degree of completion. Molecular sieves capture methanol or water very well. 

In the above discussion, we have presumed that the tortuosity factor t is characteristic of the pore structure but not of the diffusing molecules. However, when the size of the diffusing molecule begins to approach the dimensions of the pore, one expects the solid to exert a retarding influence on the flux and this effect may also be incorporated in the tortuosity factor. This situation is likely to be significant in dealing with catalysis by zeolites (molecular sieves). 

Barter, R.M. (1948) Synthesis of a zeolitic mineral with chabazite-like sorptive properties. /. Chem. Soc., 127 Barter, R.M. and Riley, D.W. (1948) Sorptive and molecular sieve properties of a new zeolitic mineral. /. Chem. Soc., 133. 

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