Preparation and evaluation of catalysts for the production of ethylene via steam cracking

Various types of catalyst samples selective for the production of ethylene via steam cracking were prepared. Controlled ratios of two or more oxides and high calcination temperatures were used in order to obtain the desired crystal phases. BET surface-area measurement and X-ray diffraction techniques were used for the examination of the samples. The prepared samples were tested in an experimental pyrolysis unit using n-hexane as a feed. It was found that a catalyst with the formula 12CaO-7Al2O3 has the highest selectivity ratios. The catalyst activity varied widely depending on the operating conditions. At short residence times the catalyst increases the conversion of the feed and hence the olefin yields compared with “inert” α-alumina and thermal cracking (empty reactor). At relatively long residence times and high hydrocarbon partial pressure the catalyst promotes the formation of carbon oxides. Therefore, with a judicious choice of the operating conditions, the catalyst 12CaO-7Al2O3 can act as a steam cracking and/or a steam reforming catalyst. Testing of a sample following treatment with hydrogen at high temperature showed no catalytic activity. It is believed that the increased catalyst activity is due to the excess of oxygen that exists in the crystal phase Ca12Al14O33 of the 12CaO-7Al2O3 catalyst.