The Influence of Varying Si/Al Ratio (SAR) of Beta Zeolite in the Methanol to Hydrocarbons (MTH) Reaction

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Excessive greenhouse gas emissions, like carbon dioxide, contribute to global warming and climate change. Methanol is hydrogenated from syngas and can react to produce hydrocarbons in a reaction known as methanol to hydrocarbons (MTH). Catalysts are vital in this reaction and are largely of zeolite origin. The zeolite typology, acidity, and reaction conditions donate the products produced and catalytic stability. Further, previous work shows increased catalytic stability and higher desired product selectivity when metal is incorporated onto the zeolite’s framework. We study the role of varying silica/alumina ratio (SAR) of beta zeolite via dealumination and incorporating titanium to understand their effect on product distribution, catalytic lifetime, and deactivation in the MTH reaction The samples maintained their structural integrity following the dealumination and metal incorporation. Techniques like XRD, N2 physisorption, ICP–OES, FTIR, and Raman spectroscopy are shown and discussed. They confirm the preservation of the zeolite structure following dealumination and metal incorporation. Pyridine-FTIR and ammonia TPD are used to understand the acidity character of the samples. Both show decreased acidity as the SAR increases. 27Al NMR and 1H NMR show the removal of extra framework 27Al as SAR increases and the presence of silanol nests in the dealuminated samples, respectively. A packed bed reactor in a PID setup with a UV-vis probe is used to test the catalytic activity and study the neutral and charged species formation, respectively. The catalytic activity results show a decrease in conversion as the SAR increases for the dealuminated samples. High propylene/ethylene ratio reaching up to 41.5 is observed for the 13M sample. Further, the UV-vis analysis shows the higher formation of bulkier hydrocarbons, like polyaromatics, as the reaction progresses. It is found that the parent sample deactivates quicker than the dealuminated samples as it presents stagnant UV-vis bands at the end of the reaction. The higher accumulation of polyaromatics and lower product formation of ethylene, in higher SARs, is related to the aromatic cycle hindrance and the dominance of the olefinic cycle products.

Bokhari, M. (2022). The Influence of Varying Si/Al Ratio (SAR) of Beta Zeolite in the Methanol to Hydrocarbons (MTH) Reaction [KAUST Research Repository].


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