Alkenes mixtures, shape selective

TABLE 4. Shape-selective hydrogenation of alkene mixtures in the presence of 1% Pt-ZSM-5 catalyst reduced in a mixture of alkenes and H2... 

The neutral molecular sieve, aluminum phosphate (ALPO, Chapter 10) can also serve as a shape selective support for metal catalysts. These catalysts are normally prepared by the incipient wetness process followed by appropriate drying and reduction procedures. Fig. 13.19 illustrates the selectivity observed using a Ni/ALPO catalyst to hydrogenate a mixture of styrene and a methyl styrene (Eqn. 13.13). The rates of hydrogenation of cyclic alkenes over a Rh/ALPO catalyst decreased in the order cyclopentene cyclohexene > cycloheptene > cyclooctene but no selectivity was observed in a competitive hydrogenation of a mixture of cyclohexene and cyclooctene. 75... 

Solid state ion exchange is a versatile tool for the fast and easy preparation of metal containing small pore (i. e., 8-membered ring) zeolites. Therefore it offers a valuable alternative to the crystallization inclusion method with its limited applicability. The introduction of noble metals into small pore zeolites via solid state ion exchange results in highly shape selective catalysts over which the hydrogenation of the linear alkene out of an equimolar mixture of hexene-(l) and 2,4,4-trimethylpentene-(l) is strongly preferred. This indicates that the major part of the metal is located in the intracrystalline voids of the zeolites. Preliminary fUrther experiments in our laboratory surest that the new method is not restricted to noble metal chlorides, but also works with other salts, e. g., oxides and nitrates. 

In conclusion, it is demonstrated in this work that the hydrogenation behaviour of cycloalkenes can differ quite significantly depending upon whether the alkene is present in pure form or as a mixture. The results suggest that factors other than size/shape selectivity of catalyst can impart differences in yields of hydrogenation of cycloalkenes even though such differences in yields may not he apparent for the olefins in their individual forms. 

Zeolites are also excellent catalysts in the selective dehydration of tertiary alcohols. Shape selectivity, i. e. selective dehydration of 1-butanol to 1-butene over CaA zeolite in the presence of 2-butanol has also been reported [36]. The bulky 2-methylcyclohexanol, in turn, undergoes dehydration over zeolite Y, although resulting in a mixture of alkenes [37]. 

Methanol and Wood Conversion Product Classes. Methanol has been used in this screening work to ascertain catalyst activity. The methanol relative product distribution on an active, pure catalyst is shown in Figure lA. (Table II gives the identification of the ions observed). No methanol (m/z 31 and 32) breakthrough was observed, and the first formed product, dimethyl ether (m/z 45 and 46), has been consumed to form a mixture of C2 to Cg olefins and toluene, xylene, and trimethyl-benzene. Note the lack of benzene and alkanes. With lower space velocities and higher methanol partial pressure, the alkenes are known to disproportionate to branched alkanes and to form more aromatics (11). The absence of products above m/z 120 indicates the well-known shape selectivity of the catalyst. 

Zeolites can facilitate the shape-selective bromination of olefins. When a mixture of cyclohexene and oct-2-ene are reacted with bromine the two alkenes are brominated to a similar extent. However, when the zeolite catalyst silicalite-1 is present the reaction becomes selective [143]. This selectivity depends upon the order in which the reactants are introduced to the catalyst. If the alkene mixture is stirred with the zeolite prior to the addition of bromine then the straight chain octene enters the zeolite pores. The more bulky cyclohexene remains in solution and is halogenated in preference to the octene when the bromine is introduced. If the bromine is pre-absorbed on the zeolite before the alkene mixture is added then the selectivity of the process is reversed [144]. 

Nowadays we dispose of a large number of zeolites which can separate mixtures of alkanes, alkenes and aromatics based on shape selectivity or specific interactions with cations. Unfortunately, many of these materials have very small... 

Successful separation of alkanes and alkenes has been documented when microporous membranes have been used [79,138]. The physiochemical properties, size, and shape of the molecules will play an important role for the separation, hence critical temperatures and gas molecule configurations should be carefully evaluated for the gases in mixture. On the basis of gas properties and process conditions, the separation may be performed according to selective surface flow or molecular sieving (refer to Section 4.2 on transport). The transport may also be enhanced by having a Ag compound in the membrane. The Ag ion will form a reversible complex with the alkene, and facilitated transport results. Selectivities in the range of 200-300 have been reported for separation of ethene-ethane and propene-propane [138]. Successful separation of alkanes and alkenes will be important for the petrochemical industry. Today the surplus hydrocarbons in the purge gas are usually flared. Membranes which should be suitable for this application are the carbon molecular sieves (see Section 4.3.2) and nanostructured materials (Section 4.3.3). 

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