A redox cycle approach for the production of H2 by two-step methanol reforming

During recent years, many processes for the production of hydrogen from renewable sources have been proposed, comprising thermal, catalytic, electrolytic and photolytic processes. Catalytic steam-reforming is also an option that can be used for the transformation of bioalcohols into hydrogen. Because of the fascinating challenge of obtaining an inherent separation of hydrogen from the C-containing products, during last years various alternative approaches to conventional SR + WGS have been proposed. In the so-called chemical-loop approach, the SR reaction is decoupled into two spatially and temporarily separated steps: during the first step a reductant (usually methane) is first contacted with a metal oxide, which oxidizes the former into carbon oxides and water. The reduced metal oxide is then reoxidized with water, to produce hydrogen and restore the original oxidation state and the O2- content of the metal oxide.In the present work, we report about a study on the feasibility of a two-step cycle approach for the catalytic production of hydrogen from methanol and water, using the spinel-type oxide CoFe2O4 as the electrons carrier.Contacting the spinel with methanol at T> 300°C led to its progressive reduction, with development of a metallic Co-Fe phase. At high temperature, the formation of metal carbide was also observed. The reoxidation of the reduced spinel with water led to the formation of CoFe2O4 and to the generation of H2; however, some residual CoFe alloy hinted for an incomplete bulk oxidation of the material by steam. Reduction-reoxidation cycles were repeated, confirming the reversibility of the redox process. The major drawback was the accumulation of coke during the reduction step; however, the amount of coke was a function of the main reduction parameters, i.e., temperature and reaction time. Carrying out the methanol oxidation step at temperature no higher than 370°C and the hydrogen-production step at 420°C, allowed minimizing the accumulation of coke during the first step and obtaining nil CO formation during the second step. On the other hand, the overall process efficiency is also related to the degree of spinel reduction, that required optimisation of the reaction time during the two steps. The modifications of the spinel chemical-physical features occurring during repeated redox cycles were investigated. The initial reactivity was a function of the degree of spinel crystallinity; however, sintering phenomena rapidly led to a collapse of the surface area, with a final behaviour that was no longer a function of the initial features of the fresh spinel.