The work presented in this thesis addressed the synthesis, characterization and application of new type of zeolite materials possessing micro-/mesoporous structural hierarchy.
Zeolite is a crystalline material possessing uniform micropores (0.3 < diameter < 2 nm) of molecular dimension. The molecular sieving ability of zeolite has enabled the creation of selective separation (ion exchange, sorption, etc.) and catalytic processes. In particular, zeolites with various structures constitute one of the most widely used classes of acid catalyst in industry. However, in many applications, slow diffusion into the microporous structure imposes a diffusion limitation on reaction rate. The diffusion limitation also accelerates coke formation at pore entrances, which significantly reduces catalytic activity and recyclibility of zeolite catalysts. To solve these problems, several attempts to synthesize zeolite with a large external surface area, hence with facile molecular diffusion into zeolite structure, were made in the past. In the present work, zeolites possessing tunable mesoporous structure were synthesized by adding a rationally designed amphiphilic organosilane surfactant into conventional alkaline zeolite synthesis mixtures. The zeolite products were rigorously characterized by a complementary combination of X-ray diffraction (XRD), $N_2$ adsorption, scanning and transmission electron microscopy. The analysis results reveal that the present method is suitable as a direct synthesis route to ‘hierarchical’ zeolites that contain secondary mesoporosity as well as the intrinsic zeolite microporosity. The mesopore diameters can be finely tailored, similar to the ordered mesoporous silicas having amorphous frameworks. The hierarchical zeolite exhibited a narrow, small-angle XRD peak, which is characteristic of the short-range correlation between mesopores, similar to disordered wormhole-like mesoporous materials. The synthesis of the crystalline aluminosilicate materials with tunable mesoporosity and strong acidity has potentially important technological implications for catalytic reactions of large molecules, whereas conventional mesoporous materials lack hydrothermal stability and acidity. Facile diffusion through mesopores improves the access of reactant molecules to the strong acid sites and also minimizes the diffusion length of coke precursors out of the micropore. This effect can lead to a significant change in the product selectivity, increase in the catalytic activity, and increase the catalysts lifetime. The increasing lifetime will give a remarkable advantage in many catalytic processes, particularly in continuous flow processes, because of the cost-down effect due to less frequent interruption for catalyst regeneration or replacement.
본 연구에서는 합리적인 분자설계를 토대로 유기화학 작용기와 무기작용기를 결합시킨 유기실란 계면활성제를 합성하였다. 이러한 ‘유기-무기 복합 계면활성제’를 제올라이트의 합성 조성에 첨가함으로써 10년 동안 메조기공물질 연구에서 숙원 과제이었던 ‘마이크로 결정성 제올라이트 골격으로 이루어진 메조다공성 물질’을 합성하는데 성공하였다. 이러한 물질은 제올라이트의 강산성과 동시에 메조다공구조의 빠른 분자 확산성을 동시에 보이는 장점이 있다. 유기-무기 복합 계면활성제를 이용한 본 합성 방법은 알루미노실리케이트 제올라이트 뿐만 아니라 순수 실리케이트, 타이타노실리케이트, 알루미노포스페이트의 다양한 조성을 갖는 마이크로다공성 분자체의 합성에 모두 적용할 수 있었다.
이렇게 합성된 메조다공성 제올라이트 물질은 강산성과 빠른 분자 확산성 때문에 기존의 알루미노실리케이트 촉매들에 비해 증진된 촉매 활성과 선택성을 나타냈으며, 특히 큰 분자들이 포함된 촉매 반응의 경우에는 보다 비약적으로 증진된 촉매 기능성을 나타냈다. 또한 이들 메조다공성 제올라이트 물질은 코크(coke)의 형성이 억제되기 때문에 여러 유기 촉매 반응에서 비약적으로 증진된 촉매 수명을 보였다. 이러한 장점 때문에 메조다공성 제올라이트 물질은 폐플라스틱의 분해 공정, 중질유의 개질 반응 등 산업적으로 매우 중요하고 큰 분자들이 포함된 촉매 반응에서 미래의 고성능 촉매로서 널리 이용될 수 있을 것으로 기대된다.
