The work presented in this dissertation addresses the catalytic effect of hierarchical zeolite in toluene methylation with methanol. Benzene, toluene, and xylene are the basic raw materials for various petrochemical products. Industrially, surplus toluene is usually converted into a more valuable xylene via either transalkylation or methylation process. Among toluene-to-xylene processes, toluene methylation using methanol has been extensively studied owing to its high catalytic selectivity towards xylene. Nevertheless, the toluene conversion is ca. 40% over various conventional zeolite catalysts and further improvement is demanded for the prevalent commercialization of this process. The present work was able to determine that the diffusion path-length of a zeolite catalyst could strongly affect the toluene conversion. Thus, the effect of the zeolite catalyst’s crystal thickness on the catalytic performance has been investigated in detail. Several MFI zeolites with different crystal thickness in the range of 2.5–200 nm were hydrothermally synthesized using appropriate structure-directing agents and characterized using various techniques. The zeolites were evaluated as the catalysts for toluene methylation using methanol. The catalytic performances of the MFI zeolite catalysts were observably enhanced progressively as the zeolite’s thickness decreased. A toluene conversion as high as 72% could be obtained over the MFI nanosheet catalyst with a crystal thickness of 2.5 nm, which was the best result reported thus far.
The catalytic performance was greatly dependent on the zeolite crystal thickness according to the catalytic results because the toluene methylation using methanol often competed with the methanol-to-hydrocarbon (MTH) conversion, which was an undesirable side reaction. Thus, the remarkable high catalytic performance of the nanosheet catalyst was attributable to the low selectivity of the side products via the MTH route. Therefore, regulation of the crystal thickness in the zeolite could be considered a viable means of enhancing the catalytic performance for toluene methylation using methanol.
벤젠, 톨루엔, 자일렌은 석유화학 제품의 기본 원료로, 중질 납사의 접촉개질공정을 통해 생산되는데 시장 수요보다 잉여의 톨루엔이 생성되기 때문에, 이를 고부가 자원인 자일렌으로 변환하는 노력이 계속되어왔다. 톨루엔 메틸화 반응은 주로 MFI 제올라이트를 촉매로 사용하여 연구되었는데, 자일렌 생산 효율이 높아서 주목을 받아왔다. 하지만, 반응물인 메탄올이 제올라이트 내부산점에 의해 올레핀으로 소모되는 부반응으로 인해 높은 생산효율에도 불구하고 톨루엔 전환율은 40% 수준에 못 미치고 있다. 이 학위논문은 내부산점 대비 외부산점이 많은 위계나노다공성 제올라이트를 촉매로 적용하여 연구한 내용을 다루고 있다. 외부산점이 증가할 경우, 부반응을 상당히 억제할 수 있었고, 메조세공을 통한 원활한 물질 확산으로 인해 코크의 생성을 억제하여 촉매수명이 상당히 개선되었다. 구체적으로, 2.5 ~ 200 nm 결정두께를 갖는 MFI 제올라이트를 합성하여 톨루엔 메틸화 촉매 성능을 평가하였다. 그 결과 결정두께가 감소함에 따라 촉매 성능이 향상되었고, 2.5-nm MFI 나노스펀지의 경우 문헌상 가장 높은 72% 톨루엔 전환율을 보고할 수 있었다. 이외에도, MFI 제올라이트의 표면 개질 효과 및 BEA 제올라이트의 촉매 특성에 관해서도 연구하였다.