Studies On The Catalysts For Dehydrogenation Of Isobutane To Isobutylene

Recently, the dehydrogenation of isobutane has been attracting extensive attention due to the growing demand of isobutylene. Pt-Sn/Al₂O₃ and Cr₂O₃/Al₂O₃ catalysts are widely used in the industrial processes due to their good performance. However, both of them have their own disadvantages. For Pt-based catalysts, high investment and operating costs are inevitable; while for Cr-based catalysts, Cr⁶⁺ is harmful to human body as well as environment and the quick deactivation of this catalysts lead to frequent regeneration. As a result, it is important to develop relatively inexpensive and environment-friendly catalysts.This thesis focuses on the catalytic dehydrogenation of isobutane over vanadia- and nickel-based catalysts. X-ray diffraction（XRD）, nitrogen adsorption-desorption, Fourier transform infrared（FT-IR） spectroscopy, hydrogen-temperature programmed reduction（H₂-TPR）, and thermogravimetric analysis（TGA） were used to characterize the structure and reducibility of vanadia or nickel species on the surface of supports.The catalytic activity of VO_x/MgAl₂O₄ was correlated to the different surface vanadia species and their reducibility. Experimental results showed that both isolated and polymeric VO4 species on the surface were present at low vanadium loading（<5%） which can be reduced more easily than samples with higer vanadium loading. As V₂O₅ loadings increased, magnesium vanadate compounds（Mg₃V₂O₈、Mg---₂V₂O₇） appeared, which were difficult to be reduced by hydrogen. 5VO_x/MgAl₂O₄ exhibited the highest activity. Based on the experimental results, it is suggested that low valence V³⁺ species on the surface are the active species for this reaction.The catalytic activity of nickel nitrate-derived NiO/Al₂O₃ was related to the status of the nickel species on the catalysts surface. The Ni²⁺ ions in the tetrahedral and octahedral sites of γ-Al₂O₃ presented at lower nickel loading（<15%） favored the dehydrogenation reaction, which led to high selectivity of isobutylene. However, when NiO loading was high, the reduction of NiO crystallites led to agglomeration of metallic nickel species, which resulted in cracking prevailing over dehydrogenation. It is worth noting that the catalytic activity of the nickel sulfate-derived NiO/Al₂O₃ catalyst was higher than that of the nickel nitrate-derived catalyst for dehydrogenation of isobutane. And the catalysts also exhibited good selectivity towards isobutylene regardless of the NiO loading. Based on the catalytic performance of nickel sulfate-derived NiO/Al₂O₃ calcined at different temperatures, it was found that the sulfur species retained after calcination can significantly improve selectivity towards isobutylene. And the activity of Ni₃S₂ formed during reaction was higher than that of tetrahedrally and octahedrally coordinated nickel ions.NiO/Al₂O₃ with high nickel loading can catalyze the rapid rupture of C-C bond in isobutane which led to abundant formation of CH₄ and coke. Treatment of catalysts with Bi, Sn, P, S can significantly suppress the formation of coke and improve the distribution of products. Combined with the catalytic results on pre-reduced and presulfided catalysts, it is revealed that the Ni₃S₂ is the active species for this reaction. And the obvious loss of sulfur could be the main reason for irreversible deactivation of the catalysts.