| Microcalorimetric and reaction kinetic studies are combined to investigate the reaction pathways for butane conversion over several solid acid catalysts, particularly, to correlate surface acidity with the rate constants of the reaction steps.; Olefins in the butane feed act as initiators and promotors of butane isomerization. Feed olefins are required to initiate butane isomerization over H-mordenite, H-Y zeolites, and amorphous silica alumina catalysts. Sulfated-zirconia catalysts are active without the presence of feed olefins. The rate of catalyst deactivation is slower during isobutane isomerization than during n-butane isomerization. More rapid catalyst deactivation during n-butane isomerization appears to be caused primarily by coke formation. Because of slow deactivation, isobutane isomerization is an effective probe to study solid acid catalysts at pseudo-steady state reaction conditions.; Isobutane isomerization over solid acid catalysts at about 450 K can be viewed as a surface chain reaction comprised of initiation, propagation, and termination steps. The surface chain reaction can be initiated via several pathways, e.g., adsorption of feed olefins onto acid sites, and in-situ generation of olefins from alkane dehydrogenation processes and subsequent adsorption of the olefins onto acid sites. The propagation steps involve oligomerization of C{dollar}sb4{dollar} species to form C{dollar}sb8{dollar} species, rearrangement and {dollar}beta{dollar}-scission of C{dollar}sb8{dollar} species, and hydride transfer from gas phase isobutane to the reactive intermediates, leading to isomerization products and isobutyl reactive intermediates. The termination steps involve coke formation, as well as hydride transfer between isobutane and higher molecular weight reactive intermediates to form, for example, C{dollar}sb6{dollar} and C{dollar}sb8{dollar} alkanes.; A kinetic model based on this reaction scheme was developed to describe isobutane isomerization over solid acid catalysts at temperature near about 450 K. The general agreement of model predictions with experimental observations suggests that the proposed reaction mechanism adequately describes the main surface hydrocarbon reactions for isobutane isomerization over solid acid catalysts. Kinetic analyses show that stronger Bronsted acid sites exhibit higher rate constants for {dollar}beta{dollar}-scission steps, thereby increasing the rate of isobutane conversion. Sulfated-zirconia catalysts, which exhibit a higher rate of isobutane isomerization, have higher rate constants for oligomerization. |
