Study On Reaction Mechanism For Isobutane Dehydrogenation Over Ni-based Catalyst

With the rapidly increasing demand of isobutene,the development of alkane dehydrogenation is more and more important.Pt and Cr O_x-based catalysts have been widely used in alkane dehydrogenation for industrial production,which exhibit good dehydrogenation performance.Due to their cost and environment issues,the researches on eco-friendly non-noble-metal catalysts for alkane dehydrogenation have been widely received attention.Ni and Pt are in the same group of the periodic table,and Ni-based catalysts are also active for alkane dehydrogenation.The studies of Ni-based catalysts and the alkane dehydrogenation paths are of great importance to get a comprehensive understanding of alkane dehydrogenation.In this thesis,reaction mechanism of isobutane dehydrogenation was studied over Ni-based catalyst.The main contents are as follows.After the improvement of Ni/Si O₂ catalysts,the catalyst with optimum reaction behaviors of isobutane dehydrogenation was chosed to analyze the improved mechanism by in-situ FTIR and transient response adsorption.Meanwhile,the reaction mechanism had been explored.As for isobutane reacting over Ni/Si O₂ catalyst with Ni crystallites average diameter of 6.7 nm at 600oC,isobutane can completely convert into CH₄,H₂ and coke.In order to promote isobutane dehydrogenation,the improvement of the Ni/Si O₂ catalyst is essential,including the modification of preparation method and the addition of the second component.Both of decreasing the loading of Ni and the col-gel method are beneficial to dispersing Ni crystallites,facilitating isobutene formation.For example,the selectivity to isobutene is 57.5 wt%as the average diameter of Ni grains decreases to 2.2 nm.Unfortunately,the catalyst is almost completely deactivated within 120 min due to the high selectivity to coke.The addition of the second components to Ni/Si O₂ also can increase isobutene selectivity.As the addition of Cu and Sn to Pt catalyst as well as the addition of P to single-component Ni catalyst promote olefin production for alkane dehydrogenation in reported literatures,Cu,P and Sn are chosed.Due to the weak interaction between Ni and Cu in Ni-Cu alloy,Ni clusters consisting in Ni-Cu/Si O₂ catalyst lead to the low isobutene selectivity(about 10 wt%).As for Ni-P/Si O₂ catalyst,the reaction results for isobutane dehydrogenation changes with Ni:P ratio.Ni-P/Si O₂ catalyst with Ni:P ratio of 1:1 exhibits the optimum dehydrogenation performance with isobutane conversion of 22 wt%and isobutene selectivity of 69.9 wt%.Nevertheless,the loss of P species and coke formation lead to the deactivation of Ni-P/Si O₂ catalyst.Fortunately,the addition of Sn to Ni/Si O₂ catalyst(Ni:Sn=1:1)to form Ni_2.67Sn₂ can substantially increase isobutene selectivity to 90.5 wt%with the conversion of 20.7 wt%;in addition,the conversion of isobutane and the selectivity to isobutene remained stable for 300 h.Thus,Ni-Sn/Si O₂ catalyst with the molar ratio of Ni to Sn of 1:1 was used to the further study.Based on the results of XRD and XPS,et al.,the structural and electronic properties of Ni crystallites before and after the addition of Sn are totally different,which change the adsorption behaviors of isobutane and isobutene to promote isobutane dehydrogenation.The adsorption modes and energies of isobutane and isobutene on Ni/Si O₂ catalyst before and after the addition of Sn were determined by in-situ FTIR and a novel transient response adsorption approach.In the absence of Sn,isobutane is adsorbed in a multiple-points adsorption with hydrogen atoms in different alkyl groups of isobutane,facilitating C-C bonds rupture of isobutane.After Sn introduced,a double-sites adsorption mode with the hydrogen atoms in methylidyne and methyl groups,is speculated instead,benefit for isobutene formation.Moreover,double-site adsorption mode of isobutene with C=C bond and the hydrogen atom in a methyl group is turned into single-site mode with C=C bond by Sn.Finally,the adsorption energy of isobutene is obviously reduced from 74 k J mol^-1 to 50 k J mol^-1,increasing repulsive interaction between isobutene and catalyst surface to reduce the secondary reactions.And then the reaction mechanism of isobutane dehydrogenation over Ni-Sn/Si O₂ catalyst was studied.With regard to isobutane dehydrogenation under GHSV above 150 h^-1,the selectivity to isobutene is above90 wt%with tiny amount of coke formation.Compared with the significantly higher amount of coke formed during isobutene reacting on Ni-Sn surface under the same reaction conditions,gaseous isobutene in the product seems to be generated directly instead of through the desorption of adsorbed molecule.Based on the above conclusion,it can be inferred that two C-H bonds respective in the methylidyne and a methyl are simultaneously broken without C atoms interacting with Ni-Sn surface.Thus,the possible reaction mechanism of isobutane dehydrogenation over Ni-Sn/Si O₂ catalyst is described as follows.The two H atoms respectively located in the methylidyne and methyl are adsorbed on Ni-Sn surface,and then these two C-H bonds are activated and broken to produce gaseous isobutene and adsorbed H atoms.The last step is the formation of H₂ molecule.At last but not least,the kinetic experiments were performed.For the adsorption of isobutane on Ni-Sn/Si O₂ catalyst,the adsorption capacity(q_t)at contact time t was determined by transient response pulse adsorption,and they are consistent with pseudo-first-order kinetic equation for adsorption.The adsorption activation energy was 7.4 k J mol^-1 calculated by Arrenhius equation with adsorption rate constant in the range of 280-370°C.After eliminating the effects of external and internal diffusion,reaction rates of isobutane were determined under different reaction conditions with the conversion of isobutane lower than 5%.Moreover,the rate equations for different reaction mechanism had been selected,and the parameters were fitting.The elementary reactions for the appropriate dehydrogenation model are the adsorption isobutane on Ni-Sn surface,adsorbed isobutane converted to adsorbed H atom and gaseous isobutene,and the desorption of H₂.The reaction activation energy is 137 k J mol^-1.