Study On N-Butane Isomerization Over Sulfated Zirconia Catalyst

Isomerization of n-butane is important in improving the comprehensive utilization level of C₄ resources.Environmentally friendly sulfated zirconia catalyst is a promising candidate to replace the conventional chlorinated alumina catalyst due to its high activity for n-butane isomerization at low temperature.In this dissertation,the properties of active sites over noble metal-free alumina-promoted sulfated zirconia?SZA?in n-butane isomerization have been studied and the role of strong interaction between sulfate species and zirconia support in the formation of active sites has been elucidated.Then,we investigated the activation route and reaction mechanism of n-butane and deactivation reason of SZA catalyst during isomerization reaction.Based on the above investigations,an attempt on n-butane isomerization in a circulating fluidized bed?CFB?pilot unit has been made to achieve continuous reaction-regeneration process.The highly active SZA catalyst can be prepared by impregnating amorphous Zr?OH?₄-Al?OH?₃ using sulfuric acid solution and subsequently calcining at high temperature.Calcination at 650 ^oC leads to a maximal isomerization activity,which is contributed by the pure tetragonal phase of zirconia support,the monolayer dispersion of sulfate species,and the highest density of Br?nsted acid sites.The sulfation treatment prevents the grain growth and phase transformation of zirconia crystal,increases the catalyst surface area and pore volume,and provides the indispensable Br?nsted acid sites for isomerization reaction.Although showing high activity for n-butane isomerization,SZA catalyst suffers from fast deactivation.The change of the sulfate species during the deactivation process was determined quantitatively by a temperature-programmed evolution-mass spectrometer.Two active sites,reducible sites and acid sites,exist on the catalyst surface and show a synergistic effect on n-butane isomerization reaction.The properties and interactions of two kinds of active sites has been investigated by in situ FTIR experiments and DFT calculations.In situ FTIR measurements show that the dehydration strikingly enhances the strength of the catalytic Br?nsted acid sites and the covalency of S=O bonds in sulfate species.The bridging Zr-OH groups have strong acidity via the strong electron-withdrawing effect of the S=O bond,which is also supported by DFT calculations.Universally,the strong adsorption of ammonia and n-butane on SZA surface leads to the loss of superacidity and oxidizability by irreversible changes in surface properties.A coexistence and interaction mechanism of oxidative and protonative activation of n-butane is proposed.The protonative route is efficient but deactivates quickly,while the oxidative route is limited by reaction equilibrium and occurs during the whole deactivation stage.The water and sulfite species formed in the oxidative route decrease the strength of the acid sites and the oxidizing ability of the sulfate species.Three strategies for improving isomerization selectivity over SZA catalyst and corresponding mechanisms are proposed:?1?The incomplete removal of coke deposition leads to a decrease in overall activity without the reduction of the rate of isobutane formation,which results from the inhibition of“dimerization-cracking”reactions;?2?The thermal treatment protocols strongly affect the surface properties of the catalyst.Thermal treatment at 600 ^oC only selectively removes the unsteady sulfate species on the catalyst surface,which decreases density of active sites and suppresses the side reactions;?3?The addition of La/Ni into SZA with co-precipitation method improves the stability of tetragonal ZrO₂.The interaction between sulfate species and La₂O₃ and/or NiO is weaker than that between sulfate species and tetragonal zirconia,leading to a lower density of sulfate species on catalyst surface and a higher isomerization selectivity.The SZA catalyst,prepared by spray granulation in large-scale,was used in a circulating fluidized bed?CFB?unit to achieve the isomerization of n-butane.The pilot-scale evaluation showed high stability of the SZA catalyst and that the catalytic activity was dominated by the residence time of n-butane rather than its linear velocity.Increases in the reaction and regeneration temperature both led to an increase in the conversion of n-butane and a decrease in the selectivity to isobutane,caused by increasing side reactions.Although the regeneration was conducted in air,a trace of SO₂ evolved during the regeneration,which could be minimized at the appropriate gas stripping temperature,low regeneration temperature,and high space time of the feed.The pilot-scale evaluation in CFB unit lasted for 300 h,and the results showed that the overall catalytic behavior fluctuated steadily on the whole.During 300 h of stability test,the conversion of n-butane and selectivity to isobutane maintained at 57.6 wt.%.In general,44.0 wt.%isobutane was constantly yielded,implying an outstanding potential for commercial application of the CFB process over SZA catalysts.The average isomerization indices?50.4%?were close to the equilibrium conversion of n-butane under test conditions.