Steam reforming of alcohols and carboxylic acids: Importance of carboxyl and alcoholic hydroxyl groups on coke properties

Abstract

Alcohols and carboxylic acids have been used intensively as feedstock for production of hydrogen via steam reforming. They have different functionalities and might exhibit the distinct coking behaviors. In this study, ethanol, n-propanol, acetic acid and propionic acid were chosen as the representatives of alcohols/carboxylic acids and their reaction characteristics in steam reforming over Ni/SBA-15 catalyst were comparatively investigated with the focus on understanding the difference of the properties of the coke formed. The results indicated that although the steam reforming of the alcohols generated more coke than that of the carboxylic acids, the catalyst showed the higher catalytic stabilities, due to different properties of the coke formed. The steam reforming of the alcohols produced the coke of higher aromaticity with a higher content of carbon, more large defective aromatic rings, higher crystallinity, higher thermal stability and resistivity towards the oxidation than that from steam reforming of the carboxylic acids. The coke in the carbon nanotube form in steam reforming of the alcohols and the carboxylic acids also differed in terms of wall thickness and length of the carbon nanotubes, as the reaction intermediates were different. The encapsulating coke was more abundant in steam reforming of carboxylic acids, as carboxylic acids are prone to form the amorphous thermal coke via cracking, which led to rapid deactivation of the catalyst. The types or properties of the coke, not only the amount, play important roles in determining stability of the catalyst.