Other Molecular Sieves

The titanium silicalites are composed of Ti04 and Si04 tetrahedra joined through shared oxygens. They are structurally isomorphous to the high silica 

The primary use for the titanium silicalites is as shape selective catalysts for hydrogen peroxide oxidations. Propylene is converted to propylene oxide at greater than 98% selectivity and 99% peroxide conversion at 50°C over TS-1. 2,97 Butadiene is oxidized to the monoepoxide (Eqn. 10.26), also in high selectivity, and primary alcohols are oxidized to the aldehydes in all cases with selectivites greater than 80%.97 

Montmorillonite supported zinc chloride, called Clayzic, has become an important solid acid catalyst. This material has both the Bronsted acidity associated with the clay as well as Lewis acidity from the zinc ions. Thermal activation at 275°C gave a catalyst that promoted the benzylation of cumene at 40°C in near quantitative yield.  

Generally speaking, however, heating exchanged clays results in a considerable loss of activity caused by the loss of water and subsequent collapse 

Hattori, Stud. Surf. Sci. Catal, 78 (Heterog. Catal. Fine Chem., Ill), 35 (1993). 

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See Nitromethane Molecular sieve See other MOLECULAR SIEVE INCIDENTS... 

See other molecular sieve incidents, solvated oxosalt incidents... 

Synthetic zeolites and other molecular sieves are important products to a number of companies in the catalysis and adsorption areas and numerous applications, both emerging and well-established, are encouraging the industrial synthesis of the materials. There are currently no more than a few dozen crystalline microporous structures that are widely manufactured for commercial use, in comparison to the hundreds of structures that have been made in the laboratory. See Chapter 2 for details on zeolite structures. The highest volume zeolites manufactured are two of the earliest-discovered materials zeolite A (used extensively as ion exchangers in powdered laundry detergents) and zeolite Y (used in catalytic cracking of gas oil). 

Thanks to these positive aspects, it is evident that the time has come to update replace traditional stoichiometric and homogeneously catalysed Fine Chemical syntheses, which are often highly polluting and economically undesirable, with sustainable processes involving catalysis over zeolites and other molecular sieves. 

Molecular sieve science is growing rapidly. The uses of molecular sieves as shape selective catalysts for various chemical reactions continue to increase. Below, we illustrate several new trends in the use of zeolites and other molecular sieves as shape selective catalysts. 

Chemical analysis and AAS data of well crystalline, pure forms of CAS-1, K-CAS-1 and Na-CAS-1 were summarized in the Table 2. The Ca and Si had incorporated in the framework of CAS-1 because they were unexchangeable during the cation exchange reaction. But the K and Na cations present in the as-synthesized CAS-1 are reversibly exchangeable, like in other molecular sieves, for such cations as Li, Na or NH/. The Na" present in the as-synthesized CAS-1 is introduced by the addition of the colloidal silica. 

It seems that fluid-bed cracking reactor (thermal or catalytic) is the best solution for industrial scale. However, regeneration and circulation of so-called equilibrium cracking catalyst is possible for relatively pure feeds, for instance crude oil derived from vacuum gas oils. Municipal waste plastics contain different mineral impurities, trace of products and additives that can quickly deactivate the catalyst. In many cases regeneration of catalyst can be impossible. Therefore in waste plastics cracking cheap, disposable catalysts should be preferably applied. Expensive and sophisticated zeolite and other molecular sieves or noble-metal-based catalysts will find presumably limited application in this kind of process. The other solution is thermal process, with inert fluidization agent and a coke removal section or multi-tube reactor with internal mixers for smaller plants. 

Other molecular sieve membranes are prime candidates as well. Molecular sieve carbon membranes exhibits very high separation factors in the laboratory. Their microstnictuies can be tailored by adjusting the synthesis and calcination conditions however, the issues of their mechanical, chemical and thermal stabilities under various potential application environments have not been addressed. 

In many studies the separation factor, which is indicative of the membrane s ability to separate two gases in a mixture, is predominantly governed by Knudsen diffusion. Knudsen diffusion is useful in gas separation mostly when two gases are significantly different in their molecular weights. In other cases, more effective uansport mechanisms are required. The pore size of the membrane needs to be smaller so that molecular sieving effects become operative. Some new membrane materials such as zeolites and other molecular sieve materials and membrane modifications by the sol-gel and chemical vapor deposition techniques are all in the horizon. Alternatively, it is desirable to tailor the gas-membrane interaction for promoting such transport mechanisms as surface diffusion or capillary condensation. 

What has to be noted first is that the number of applications for a few molecular sieves is high, however there exist many more stmctures that currently have no application. The widely employed molecular sieves include various forms of faujasites, mordenites, zeolite BETA, ZSM5, TS-1, zeolite L and to a lesser degree S(Me)AP05. For the other molecular sieves examples of utilization are quite scattered and are mainly confined to comparative studies. This suggests that a move to more catalytic chemistry and less material oriented approaches is required. It has to be critically noted that the quality of the materials used often varies quite substantially and this makes it difficult to derive genuine structure-activity correlations. 

Aluminophosphates, silicoalumiuopbosphates and related other molecular sieves... 

The enthalpy exchange is accomplished through the use of typical adsorbent materials such as silica gel, zeolites or other molecular sieves which transfer water molecules by the difference in the partial pressure of vapor within the opposing air streams. The operation of a FCS that includes this component requires specific... 

Olefins can be produced from methanol, in varying concentrations, over many catalysts. Again, the patent literature is extensive. The most promising ones seem to be zeolites and other molecular sieve catalysts. Some of the most prominent ones are listed in Table 1. Experimental data for many of these catalysts were summarized before (refs. 8-10). MTO catalysts can be divided into two major groups "small pore", such as erionite, and ZSM-34, and "medium pore", such as ZSM-5. Various combinations and modifications of catalysts are also known. A particularly interesting example is phosphorous modified ZSM-5 (ref. 11). ... 

The authors recognize that some of the results presented in this paper parallel published findings based on other types of infrared spectroscopic techniques, but feel that their presentation in the context of this paper serves to establish the credibility of the DRIFT/GC/MS procedure in advance of the publication of similar studies currently under way on other molecular sieve systems that have not been investigated in detail by such methods. 

For SAPO-11 type materials it was observed (29) that the as synthesized SYN (with di-n propylamine template) the calcined (CAL), hydrated (WAT) and cyclohexane (CYC) saturated samples exhibit different XRD patterns as shown in fig.5. As for other molecular sieves it was important to determine if silicon was incorporated into the AIPO4 framework or not and subsequently if basic or acidic properties were induced depending if Si " was substituting Al or P ", respectively. The X-ray diffraction pattern analysis allowed us to show that A1 and P atoms are strictly alternate and that the material crystallizes in the non centro symmetric Ima2 space group with a = 1.867 (2), b= 1.3373 (2) and c = 0.84220 (9) nm (33). The channels along c axis are elliptical and are occupied by the di-n propylamine molecules. Disordered domains were also evidenced presumably... 

Proposed mechanisms for C-C bond formation can be organized into the following classifications carbene [70,71], carbocation [72], oxonium ylide [73,74], and free radical [75]. Some of these mechanisms [72-74] invoke a framework-bound methoxy species as a methylating agent or intermediate in the reaction. Dybowski and coworkers reported a C NMR study that supported the formation of methoxy groups in HZSM-5 [76], but the existence of these species is still controversial [77]. Framework-bound alkoxy species clearly do form on other molecular sieve catalysts. For example, Anderson and Klinowski have shown that when methanol is heated to 573 K on the silicoaluminophosphate catalyst SAPO-5, a framework-bound methoxy species forms, which is readily seen in a C MAS spectrum obtained after heating [77]. 

Comparison with other molecular sieves (Fig. 11) shows that the yields obtained with FER are very high indeed. As elaborated upon elsewhere [6-8], we have proposed that the isomerisation involves a bi-molecular mechanism in which e.g. di-methylhexene isomers crack selectively to isobutene and n-butene (Fig. 12). The mono-molecular mechanism requires the energetically unfavourable primary carbenium ions. Molecular modelling [7] has provided support for this mechanism in that the branched octenes can be formed in the intra-crystalline voids of FER but their diffusion out of the pores is hindered. 

Sulfate-activated Ti02 and Zr02 or a molecular sieve (LZ-Y52, 4A, AW-500, and AW-300) were used in the oligomerization of Cio-20 alkenes. Olefin conversion was 64% and dimer/trimer ratio 5.57, and 86% and 1.80 for sulfated Ti02 and Zr02, respectively. The AW-sulfate activated molecular sieve showed higher activity in comparison with the other molecular sieves. 

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