Zeolite-Catalyzed Conversion of Ethene to Value-Added Chemicals

Abstract

The use of coal as fuel made the first industrial revolution in the 1780s possible with the operation of steam engines, which became the cornerstone of the development of human society. The full-fledged development of oil wells and related chemical processes, triggered by the first commercial drilling of the Drake oil well in 1859, accelerated the pace of human society evolution and achieved the present modern society. To this day, these fossil resources are indispensable for the production of crucial energy and products. However, disastrous climate change due to greenhouse gas emissions resulting from the perpetual use of fossil resources is now threatening human life and imparted importance to sustainable development. Much attention, in this regard, has been paid to the environmentally friendly production of various chemical products to sustain human life. On the other hand, the so-called shale gas revolution leading to the lightening of cracking feed has triggered a deficit of C3 – C4 olefins, which were mainly produced by naphtha cracking. This has urged researchers to efficient alternatives for the production of various light olefins (i.e., C2 – C4), indispensable chemical building blocks in the petrochemical industry, using renewable and sustainable feedstocks, which is of particular significance in both academic and industrial perspectives. The use of ethene as the alternative feedstock was proposed due to the increasing production capacity of it from renewable and alternative sources (i.e., methane oxidative coupling reaction, methanol-to-olefins (MTO), bio-ethanol dehydration, and ethane cracking). Moreover, its identical structure to today’s bulk chemicals increases the usability of ethene as a key intermediate for the production of value-added chemicals. In this thesis, I present the zeolite-catalyzed ethene conversions for the production of value-added olefins (i.e., propene and 1-butene). The reaction mechanism and catalytic active site governing the catalytic behavior of the ethene conversions over zeolite catalysts has been investigated. Since the first recognition by Dahl and Kolboe in the mid 1990s, the hydrocarbon pool mechanism has been long regarded as relevant to the methanol to olefins reaction only. However, the zeolite-catalyzed conversion of ethene to propene (ETP) has recently alluded to the possibility of its reaction mechanism as another example of the hydrocarbon pool mechanism. While knowledge of the reaction mechanisms in zeolite catalysis is essential for achieving selectivity control, on the other hand, the exact identification and roles of bicyclic aromatic hydrocarbon pool species in ETP remain to be elucidated, which is far from the case of monocyclic ones in MTO. In order to detail the hydrocarbon pool mechanism of the ETP, ethene conversion was conducted over H-UZM-35 zeolite in the presence of a small concentration (2 mol% of ethene) of ten different aromatic hydrocarbons. Among the co-feeds employed, only C-2 substituted bicyclic aromatic species (i.e., 2-methynaphthalene, 2-ethylnaphthalene, and 2-isopropylnaphthalene) were found to have beneficial effects on the selective propene production. The overall experimental results of this study led us to propose a bicyclic aromatic-based mechanism of the zeolite-catalyzed ETP reaction where naphthalene, C-2 substituted bicyclic aromatics, and 2-isopropyl-7-methynaphthalene serve as hydrocarbon pool species. The theoretical results demonstrate that the Gibbs free energy barriers of the transition states for the ethylation of key bicyclic hydrocarbon pool species (naphthalene, 2-methylnaphthalene, and 2-ethylnaphthalene) are significantly lower compared with the transition states for the same elementary step of the corresponding monocyclic ones (benzene, toluene, and ethylbenzene). Intuitively, this can be understood given that the transition state for the ethylation of each of the former aromatic species has a larger number (5 vs. 3) of resonance structures than that of each of the latter ones and is thus thermodynamically more stable. Since the commercialization of the Shell Higher Olefins Process (SHOP) in 1977, ethene oligomerization, using homogeneous transition-metal catalysts containing polydentate ligands in the presence of an alkylaluminum co-catalyst, has become an important catalytic process for the production of key intermediates in polymer, lubricant, and surfactant industries. However, although many organonickel complexes have been explored for this reaction so far, the environmental and technological problems of the homogeneous catalysts have been assessed as severe disadvantages. To solve these problems, developing efficient heterogeneous catalysts for ethene dimerization is of industrial importance. Up to now, nickel-exchanged beta (Ni-beta) has been reported to show high durability in ethene dimerization under industrially relevant conditions, rendering it attractive as a potential replacement for commercial homogeneous catalysts. In this thesis, I prepare a series of nickel-containing zeolite catalysts with various preparation methods and perform their catalytic tests in ethene dimerization to 1-butene at the high-pressure condition. Among the catalysts employed, Ni-beta catalyst shows a considerably better 1-butene yield when prepared by solid-state ion exchange compared to liquid-phase ion exchange and incipient wetness impregnation. The CO IR results reveal the presence of a new type of Ni site in the former catalyst, which is not detectable in the latter two catalysts. Successful Ni K-edge XANES and EXAFS analyses demonstrate that the new Ni sites is bent mono(μ-oxo)dinickel ([Ni-O-Ni]2+) complex, of which formation was further corroborated by UV-Visible spectroscopy and DFT calculation results. Based on the overall results, I conclude that solid-state ion exchange is an efficient method to form the intrazeolitic [Ni-O-Ni]2+ species which are more active for ethene dimerization than the widely accepted Ni2+ ions.