Cyclohexene skeletal isomerization

The conversion of cyclohexanol on acid sites in zeolites and boralites is composed of two steps dehydration (to cyclohexene and water) and consecutive reactions of cyclohexene skeletal isomerization and disproportionation. Our IR and catalytic studies have shown that the dehydration occurs on both strong and weak Bronsted sites. On the other hand, only the strong Bronsted acid sites are required for isomerization and disproportionation. This observation may be used to propose a new ipethod for investigation of heterogeneity of acid sites in zeolites by a simple catalytic test. 

Moreover, although the rate constant depends on the precipitation medium, the most important influence is that of catalyst composition. Thus, we observe a strong decrease in c talytic activity as the TiOj content increases however this decrease is higher at 60 wt% TiOj than at 76 wtM Ti02 so that the kKox values (at 673 K) on AJT -P-31, APTi-P-11 and APTi-P-13 are, respectively, 22.4, 7.3 and 4.0. The same effect was found in cyclohexene skeletal isomerization [13,14] and in the all lation of phenol with methanol [21,22]. So, the results can be well interpreted in terms of the number of more accessible strong acid sites (measured vs. AN and DTBMPY). 

The analysis of volatile products of the conversion of cyclohexene over Al-MCM-41 with various A1 contents has revealed that the reaction runs mainly according to two mechanisms known as cyclohexene skeletal isomerization (CSl) and cyclohexene hydrogen transfer (HT). The results show which of the two schemes predominates, depending on the reaction temperature and the A1 content of the catalyst. The processes of CSI and HT are accompanied by cracking and alkylation, which are proved by the presence of products with 1 to 9 carbon atoms even though the Ce compounds strongly prevail in all cases. Composition of the volatile products of the conversion depends on both concentration and strength of Bronsted and Lewis acid centres. 

The acid strength of aluminum phosphorous oxide is enhanced by the addition of SO4 up to 3 wt% Enhancement of acid strength is suggested by high catalytic activities for 1-butanol dehydration and cyclohexene skeletal isomerization. However, addition of excess S04 ions reduces the activity for the reactions. The catalytic activity of the oxide prepared from aluminum sulfate is different from those prepared from chloride and nitrate for 1-butanol dehydration equilibrium mixture of butene isomers is produced over the oxide from sulfate whereas the ratios of l-butene/2-butenes and cis/trans art larger than the equilibrium ratios over the oxides from chloride and nitrate. The catalytic behavior of the oxide from sulfate different from the other oxides is caused by the presence of a small amount of SO4 ions generating strong acid sites. 

Skeletal ring contraction steps of primary C7 and Cg rings are more probable than bicyclic intermediates (132b). Aromatization of methylcyclo-pentane indicated no carbonium mechanism with a nonacidic catalyst. Instead, Pines and Chen (132b) proposed a mechanism similar to that defined later as bond shift. This is a methyl shift. Two additional isomerization pathways characteristic of chromia have also been demonstrated vinyl shift (94) and isomerization via C3 and C4 cyclic intermediates (90a). These were discussed in Section III. 1,1-Dimethylcyclohexane and 4,4-dimethyl-cyclohexene gave mainly toluene over various chromia catalysts. Thus, both skeletal isomerization and demethylation activities of chromia have been verified. The presence of an acidic almnina support enhances isomerization dual function effects are thus also possible. 

The detailed synthesis procedure and textural properties (surface area, Sggy in m2 g-1 pore volume, V in ml g"1 and main pore diameter, d in nm), determined by nitrogen adsorption from 8.E.T. method have been published elsewhere (refs. 13-18) and are summarized in Table 1, where the surface acidity and basicity of supports are also collected. These values were determined by a spectro-photometric method described elsewhere (ref. 19), that allows titration of the amount (in tunol g 1) of irreversibly adsorbed benzoic acid (BA, pKa> 4.19), pyridine (PY, pka= 5.25) or 2,6-diterbutyl-4-methylpyridine (DTMPY, pKa 7.5) employed as titrant agents of basic and acid sites, respectively. Furthermore, the apparent rate constant values of different supports in the gas-phase skeletal isomerization of cyclohexene (CHSI), in Mmol atm"1 g"1 s-1, at 673 K, are also collected in Table 1, because these values are another way of measuring the stronger acid sites of supports (ref. 19). 

Poulet et al (79) reported that the isomerization activity for transforming irans- into dv-pentadiene on MoP/Al catalysts decreases as a result of phosphorus addition. Gishti et al (59) noted that the activity for skeletal isomerization of cyclohexene into methylcyclopentenes on reduced MoP/Al was reduced as a result of phosphorus addition. Iwamoto and Grimblot (85) reported, however, that cyclohexene isomerization on sulfided MoP/Al sol-gel catalysts increases with increasing phosphorus content. Since the active sites for hydrocracking and isomerization reactions are predominantly associated with the catalyst acidity, the results should depend strongly on both the nature and surface properties of the catalysts and on the reactants being converted. 

The acid-catalyzed isomerization of cycloalkenes usually involves skeletal rearrangement if strong acids are used. The conditions and the catalysts are very similar to those for the isomerization of acyclic alkenes. Many alkylcyclohexenes undergo reversible isomerization to alkylcyclopentenes. In some cases the isomerization consists of shift of the double bond without ring contraction. Side reactions, in this case, involve hydrogen transfer (disproportionation) to yield cycloalkanes and aromatics. In the presence of activated alumina cyclohexene is converted to a mixture of 1-methyl- and 3-methyl-1-cyclopentene 103... 

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