As a study for toluene disproportionation reaction to benzene and xylene, the properties of natural zeolite mined in Kuryongpo, Kyungsang-Namdo and its property changes after acid treatment, the effects of the operating conditions on toluene conversion and selectivity in a packed-bed reactor, and the reaction mechanism and kinetics were investigated.
The chemical composition analysis and X-ray diffraction spectrum of the original zeolite sample showed that the sample contained 30 - 40% of the mordenite structure, and other impurities such as clinoptiolite, quartz, feldspar, and amorphous clays. The surface area measured by nitrogen adsorption in the BET system was 75$M^2$/gr for original sample and increased to a maximum value of 320 $M^2$/gr after treatment with 2N hydrochloric acid solution. This results from the rearrangement of the pore structure and the enlargement of the pore openings by decationization and extraction of aluminum and other impurities.
The sample treated with higher acid concentration caused the collapse of the structure which was observed from the X-ray diffraction spectrum. Toluene conversion for toluene disproportionation reaction in a packed-bed reactor reached a maximum value at 2N acid treated catalyst because of strong acidities on catalyst surface and enough pore size for reactants and products to diffuse. The deactivation rate of such a catalyst that treated with 1 - 3N hydrochloric acid solution was much faster than with other concentration because of coke deposition by the strong acid sites. The selectivity of xylene over benzene decreased with increasing acid concentration indicating increased Lewis acid-sites on the catalyst surface which correspond to cracking rather than isomerization.
The reaction was carried out at various operating conditions to study the catalytic actibity and optimum reaction conditions. Large catalyst to feed rate ratio which means longer contact time resulted in higher conversion, but the selectivity of xylene over benzene became lower because of the side reactions of reacted products. The conversion increased as the reaction temperature increased, but higher temperature caused faster deactivation. Below the temperature of 400℃, the deactivation phenomena almost disappeared during the experimental run time. To minize the catalyst deactivation rate, the hydrogen as an inert gas was introduced. It was shown that a high hydrogen to toluene mole ratio could diminish the coke deposition to some extent even at high temperature. The conversion decrease caused by repeated regeneration was relatively small during early stage of the run time(8% reduction after third regeneration), indicating that the deactivation was mainly due to the coke deposition on the catalyst surface. The catalyst activity became more stable as the number of regeneration increased, indicating that the strong acid sites gradually disappeared and relatively mild and weak acid sites increased.
The reaction kinetics for toluene disproportionation reaction was studied within the range of negligible pore diffusion and plug flow assumption for the packed-bed reactor. The reaction data were analyzed with integral method. A reaction kinetics based on the second roder Langmuir-Hinshelwood mechanism was developed and confirmed by the experiment. At the reaction temperature of 450℃, for example, the reaction rate constant was 0.202 grmole/grcat. hr and the adsorption equilibrium constant was 9.8 $atm^{-1}$. From the Arrhenius plot of the rate constant and the adsorption equilibrium constant, the activation energy of 26.4 Kcal/mole and the heat of adsorption of 3.8Kcal/mole were obtained.
톨루엔으로 부터 벤젠과 크실렌을 얻기위한 톨루엔 불균화반응에 대한 연구로써, 경남 구룡포에서 채광되는 천연 제올라이트의 물리적, 화학적 성질과 산처리 후의 성질의 변화, 고정층 반응기를 통한 촉매활성, 반응조건, 반응속도등을 규명하고자 하였다.
화학조성분석과 X-선 회절 실험결과 원시료는 30 - 40% mordenite 성분을 포함하고 있고 그외 clinoptiolite, 석영, 장석과 무정형 점토물질의 혼재가 예상된다. 질소 흡착실험에 의하면 원시료는 $75M^2/gr$ 의 표면적 밖에 가지지 못하나 산처리가 진행될수록 급격한 증가를 보여 2 N 염산으로 처리시 $320M^2/gr$의 최대치가 나타났다. 이것은 산처리에 의한 양이온 교환, 알루미늄 용출등에 따른 세공의 확대와 구조의 재배열에 기인하는 것으로 고농도의 산에 의해서는 구조 자체의 파괴로 감소현상을 나타내며 X-선 회절 스펙트럼으로 부터 확인될 수 있었다.
톨루엔 불균화반응에 대한 전화율은 2 N 염산농도를 처리한 촉매를 사용시 최대를 나타내나 촉매표면의 강산점으로 인하여 이 농도 근처에서는 심한 활성저하를 나타낸다. 처리한 산농도가 높아지면 활성저하 현상은 덜하나 Lewis 산점의 생성에 의해 벤젠 대한 크실렌의 선택성은 감소하는 걸로 나타난다.
촉매에 대한 반응물의 접촉시간이 길어지면 전화율은 커지나 부반응으로 인하여 크실렌 선택성은 감소하며 반응온도가 증가하면 전화율은 증가하지만 촉매 활성저하가 심하다. 약 400℃ 이하의 온도에서는 전화율은 보잘것 없지만 활성저하는 거의 없는 걸로 나타났다.
들어가는 수소의 양이 많아지면 어느정도 활성저하를 방지할 수 있었으며 사용한 촉매의 재생 실험결과 재생 횟수에 따른 초기 전화율 감소는 비교적 적은걸로 보아 촉매활성저하는 주로 촉 매표면의 탄소석출에 기인하는 같으며 재생횟수가 증가할수록 보다 안정한 상태로 변해감을 알 수 있었다.
불균화 반응에 대한 반응속도는 세공확산이 무시되고 피스톤 유체흐름의 범위내에서 Langmuir-Hinshelwood 메카니즘의 2차식으로 나타낼수 있었으며 26.4Kcal/mole 의 활성화 에너지와 3.8Kcal/mole 의 흡착엽을 측정할 수 있었다.