| Increasingly stringent environmental requirements and upgrades of commercial gasoline have limited the content of olefins and aromatics in gasoline,which results in lower octane number.Alkylate is an ideal high-octane blending component in the gasoline.Conventional industrial installations commonly use hydrofluoric acid and sulfuric acid as catalysts,which have the disadvantages of high toxicity,high corrosiveness and difficult separation.In contrast,the solid acid alkylation process is safer and more friendly to the environment.However,the rapid deactivation of solid acid catalysts has prevented its largescale industrial application.The I/O ratio in the pore is an important factor governing the catalytic stability of the catalyst.In this work,the competitive adsorption mechanism of isobutane/butene on Beta zeolite catalyst was studied firstly through theoretical calculation.And then the Beta zeolite was modified according to the calculated conclusions to improve the catalytic stability of the catalyst.The results will provide theoretical guidance for the development of high efficiency alkylation catalyst.The main research results are as follows:(1)Firstly,the adsorption properties of isobutane and 1-butene over the Beta zeolites with different Si/Al ratios were investigated by the CB-GCMC method.The results showed that there were two types of adsorption sites for 1-butene and only one for isobutane in single-component adsorption,and the adsorption amount was mainly affected by the size of adsorbent molecules.In the two-component competitive adsorption,the content of butene in the acidic zeolite is much higher than that of isobutane,and the isobutane/butene ratio(I/O)in the pore of the zeolite gradually decreases as the Si/Al ratio decreases.From the perspective of the intrapore I/O ratio,the decrease of Si/Al ratio is unfavorable to the catalytic stability of the catalyst.Therefore,under the condition of relatively low Si/Al,increasing the I/O ratio in the pore by surface modification will be an important means to improve the catalytic stability of catalyst.(2)Based on the effect of I/O ratio in the pore on C4 alkylation reaction,in this paper,octadecyltrichlorosilane(OTS)was used to silanized the outer surface of Beta zeolite.The effect of loading amount and reaction temperature on the catalytic performance was investigated.The results suggested that the catalytic performance was firstly enhanced and then weakened with the increase of loading amount of OTS.Moderate silylation can reduce the outer surface polarity of the catalyst and improve the adsorption selectivity of isobutane on the catalyst surface,which in turn increases the I/O ratio in the pore and reduces the polymerization of olefins at the active center.However,excessive silylation can lead to pore plugging.When the OTS-Beta catalyst was loaded with 5% silylation and the reaction temperature was 80 °C.The selectivity of C8 reached 76.5%.The selectivity of TMP reached 58.6%.The stability of the catalyst was improved by 27% compared to the unmodified catalyst.(3)The zeolite was further modified by boric acid to investigate the effect of acid modification on the structure and catalytic activity of the zeolite.It was found that the acidity of the catalyst was weakened after the boric acid modification.Thus,the polarity of the catalyst surface was reduced and the adsorption selectivity of olefins in the active center was decreased,which was beneficial to reduce the polymerization of olefins and prolong the catalyst life.In addition,the acid modification could increase the specific surface area and pore volume of the catalyst.The B-Beta catalyst showed better catalytic activity and stability at a boric acid concentration of 0.2 mol/L and a reaction temperature of 85 °C.The C8 selectivity reached 62.1%.The TMP selectivity reached 56.2%.The catalyst stability increased by 27% compared to the unmodified catalyst. |
PDF Full Text Request