Abstract
This study evaluated Al-ZSM-5 nanocrystals grown from silicalite-1 seed crystals as catalysts for the methane dehydroaromatization (MDA) reaction. Silicalite-1 seed crystals sized between 30 and 40 nm were used to grow Al-ZSM-5 under various synthesis conditions. The size of Al-ZSM-5 was significantly affected by the Si/Al ratio (SAR), synthesis time, and silica nutrients/seed crystal ratio (NSR). Larger crystals were obtained with an increased SAR in the synthesis sols. Gradual growth of Al-ZSM-5 occurred with synthesis time, although the growth in crystal size ceased at 5 h of synthesis at 120 °C, indicating the rapid growth of Al-ZSM-5 aided by the silicalite-1 seeds. Precise tuning of Al-ZSM-5 size was possible by changing the nutrient/silicalite-1 seed ratio; a higher NSR led to larger crystals. Two representative Al-ZSM-5 crystals with SARs of 35 and 140 were prepared for catalyst testing, and the crystal sizes were tailored to <100 nm by controlling NSR. The MDA reaction was conducted in the presence of the prepared Al-ZSM-5. The catalyst size exhibited distinct differences in catalyst stability, while the SAR of catalysts did not produce noticeable changes in the catalyst stability of the Al-ZSM-5 crystals and commercial zeolites in this reaction system.
Highlights
Zeolites are microporous inorganic materials that are characterized by their unique shape selectivity and interconnected acidic pore channels [1,2]
We attempted to prepare Al-ZSM-5 catalysts with sub-100 nm diffusion channels to evaluate the impacts of nanoscopic change in diffusion channels on catalyst stability
It was important that these crystals had a sufficiently high crystallinity to function as an acid catalyst for the methane dehydroaromatization (MDA) reaction
Summary
Zeolites are microporous inorganic materials that are characterized by their unique shape selectivity and interconnected acidic pore channels [1,2]. For improved access to abundant natural gas resources, zeolites have been tested as catalysts to convert methane into syngas, liquid chemicals, and fuels via catalytic acid reactions [3]. The methane dehydroaromatization (MDA) reaction in the presence of the ZSM-5 catalyst is an efficient means of converting methane into benzene and hydrogen [4]. The facile isomorphous substitution [5,6,7,8], aromatic selective pores [9,10,11], and strong Brønsted acid sites [12] of ZSM-5 improve its conversion kinetics. Commercialization of the MDA reaction over ZSM-5 is yet to occur due to its rapid catalyst deactivation, low equilibrium conversion, and complex reaction pathways [13]. There has been extensive research directed at addressing these issues through catalyst design [10,12], reaction system engineering [14,15], and elucidation of the reaction mechanism [16,17]
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