변형된 Zn/ZSM-5를 이용한 2-methyl-2-butene의 방향족화 반응 연구

Abstract

The business circumstances surrounding petroleum industry have been going worse. The rising prices of raw materials due to depletion of crude oil and political unset in Middle East have deteriorated refining profitability. Accordingly, the importance of FCC process, which mainly produces gasoline and diesel from vacuum gas oil and vacuum residue, has been increased. Moreover, environmental regulations for hazardous substance, e.g., sulfur, olefins, aromatics, etc, in the petroleum products are also being strengthened. In particular, MOE of Korea recommends reduction of contents of olefin in the fuel by half of the present level in near future. To meet this new regulation, the present olefin contents in the FCC fuel should be reduced by half. Thus, it is expected that the amount of olefin in the FCC by-products significantly increases. FCC by-products are normally olefin rich hydrocarbon mixture and consists of about 50% of olefinic C5 branched hydrocarbons. Yet, their use is extremely limited in petroleum and petrochemical industry. In the difficult situations, aromatization, which converts low valuable FCC by-products to high valuable products, e.g., benzene, toluene, and xylene isomers becomes one of the most promising solutions. Nevertheless, commercialization of manufacturing aromatics from these feedstocks still remained a great challenge due to the serious problem of fast deactivation of catalysts by the high reactivity of olefinic hydrocarbons, leading to the formation of coke. Two solutions exit for commercialization. One is development of new catalysts, highly restraint to the coking, and the other is the introduction of various process equipment and systems. Here, we selected the former to solve the fundamental problem of aromatization.At first, a new method to modify the acidity of zeolite was attempted to improve the activity and stability of Zn/ZSM-5 in aromatization of 2-methyl-2-butene. Pre-coking and regeneration in the presence of O2 at 773K generated a large exothermic heat in a short time, which resulted in the thermal dealumination of framework aluminum that acted as a Brønsted acid site. The treatment had a profound effect on the acid properties of the catalyst as probed by 27Al MAS NMR, temperature-programmed desorption of ammonia, and pyridine FTIR. The results showed that concentration of oxygen in regeneration step had critical effects on the acidity and stability of Zn/ZSM-5. Thus improvement of lifetime of Zn/ZSM-5 in aromatization of 2-methyl-2-butene was achieved without deteriorating activity and selectivity by the pre-coking and controlled regeneration by oxygen.Secondly, three Zn/ZSM-5 catalysts with Si/Al ratios of 17, 33 and 42 were treated by a weak base of Na2CO3 to create a hierarchical micro-meso pore system and to improve the catalyst stability in aromatization of 2-methly-2-butene. The changes in physical properties of ZSM-5 by the treatment were observed by ICP, XRD, 27Al MAS NMR, N2 ads orption-desorption, SEM and TEM. In addition, the acidity changes were measured by TPD of ammonia and pyridine FTIR. Among the three catalysts, Zn/ZSM-5 with Si/Al of 42 was most affected by the Na2CO3 treatment. Thus, a hierarchical micro-meso pore system was formed with intraparticle bimodal mesopores in the forms of cavity and cylinder that connected internal micropores to external surface of the catalyst. This pore structure was most suitable to enhance the tolerance to coking and bring a dramatic increase in catalyst stability in aromatization of 2-methyl-2-butene.