Abstract
This paper evaluated the effect of calcination temperature and the use of Mg/Al2O3 on the decomposition of HFC-134a. Two commercialized catalysts, Al2O3 and Mg/Al2O3, were calcined at two different temperatures (500 and 650 °C) and their physicochemical characteristics were examined by X-ray diffraction, Brunauer–Emmett–Teller analysis, and the temperature-programed desorption of ammonia and carbon dioxide analysis. The results show that, in comparison to Al2O3, 5% Mg/Al2O3 exhibited a larger Brunauer–Emmett–Teller surface area and higher acidity. The relative amount of strong acid sites of the catalysts decreased with increasing calcination temperature. Although a more than 90% decomposition rate of HFC-134a was achieved over all catalysts during the sequential decomposition test of HFC-134a using a vertical plug flow reactor connected directly to a gas chromatography/mass spectrometry system, the lifetime of the catalyst differed according to the catalyst type. Compared to Al2O3, Mg/Al2O3 revealed a longer lifetime and less coke formation due to the increased Brunauer–Emmett–Teller surface area and weak Lewis acid sites and basic sites arising from Mg impregnation. Higher temperature calcination extended the catalyst lifetime with the formation of less coke due to the smaller number of strong acid sites, which can lead to severe coke formation. A valuable by-product, trifluoroethylene, was formed as a result of the decomposition. Based on the experimental results, a reaction is proposed which reasonably explains the decomposition reaction.
Highlights
Rapid global warming and climate change in recent decades and the increased frequency and impact of environmental disasters, such as global warming, sea level rises, heat, drought, and floods, have raised global interest in greenhouse gases [1,2]
The seriousness of HFC-134a was highlighted in the Kyoto Protocol [8] and a decision was made to reduce its usage in the Kigali Amendment to the Montreal Protocol [9]
The decreased coke formation over Mg/Al2O3-650 compared with Mg/Al2O3500 confirmed that the relative number of strong acid sites is strongly related to catalyst deactivation confirmed that the relative number of strong acid sites is strongly related to catalyst deactivation during the catalytic decomposition of HFC-134a
Summary
Rapid global warming and climate change in recent decades and the increased frequency and impact of environmental disasters, such as global warming, sea level rises, heat, drought, and floods, have raised global interest in greenhouse gases [1,2]. Many studies have applied metal-impregnated Al2 O3 to increase the overall lifetime of the catalyst for the decomposition of fluorinated hydrocarbons. Li et al reported the use of a metal-supported catalyst for the catalytic decomposition of HFC-143a [21]. They explained that metal phosphates can provide a more stable decomposition efficiency of fluorinated hydrocarbons due to the presence of weak acidic sites and dehydrofluorination proceeds via a carbonium-ion mechanism. Previous studies on the use of metal-supported catalysts for the decomposition of other fluorinated hydrocarbons suggested that the catalytic efficiency of Al2 O3 can be increased and become more stable by metal impregnation, but there has been little systematic research on its use for HFC-134a decomposition.
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