Comparative study of the transformation of n-butane, n-hexane and n-heptane over H-MOR zeolites with various Si/Al ratios Original Research Article

The transformations of n-butane, n-hexane and n-heptane were carried out in a flow reactor at 523 K, palkane=0.1 bar, pnitrogen=0.9 bar over a series of H-mordenite samples with framework Si/Al ratios from 6.6 to 80. Because of the rapid deactivation of the samples (mostly during n-hexane and n-heptane transformations), a series of product analyses was performed at a very short time-on-stream in order to obtain, with good accuracy, the activity and selectivity of the fresh samples. With all the samples, n-heptane is slightly more reactive than n-hexane and much more reactive than n-butane (15–100 times). The effect of the acid-site density on the mordenite activity is different for n-butane, n-hexane and n-heptane transformations, which suggests that these reactions occur through different mechanisms: bimolecular with n-butane; monomolecular with n-hexane and n-heptane. The bimolecular mechanism of n-butane transformation is confirmed by simultaneous formation of isobutane, propane and pentanes as primary products. With all the H-mordenite samples, isomers and C3–C5 alkanes appear as primary products of n-hexane transformation. From n-heptane, C3–C5 alkenes are observed as primary products as well as isomers and C3–C6 alkanes. The isomer/light products ratio is approximately equal to 2 from n-hexane and 0.2 from n-heptane, as is expected from the relative difficulty in the modes of cracking: difficult C mode (involving two secondary carbenium–ion intermediates) from n-hexane and relatively easy B mode (one tertiary and one secondary carbenium–ion intermediates) from n-heptane. However, most of the light products do not result from direct cracking of C6 and C7 compounds. Whatever the reactant, the product distribution is practically identical for all the dealuminated samples. Very different distributions of the C3–C6 products are observed with the non-dealuminated sample: faster formation of C3 at the expense of C4–C6, in particular C4. This large change in selectivity should be due to the presence of mesopores in the dealuminated samples rather than the larger density of acid sites in the non-dealuminated one.