Challenging the deactivation resistance of Co/Cu-dendritic ZSM-5 zeolites in methane pyrolysis for clean hydrogen

ZSM-5 zeolites with dendritic nanoarchitecture has been recently developed, showing outstanding accessibility due to its singular multilevel porosity, high connectivity and preferential location of the acid sites on the external surface. The present work provides insights on how these features impact also positively on their resistance against deactivation by coke deposition. To that end, a variety of dendritic ZSM-5 samples, incorporating Co and Cu species, have been tested as catalysts in methane pyrolysis, being compared with several reference samples. This process is an interesting route for the production of clean (COx-free) hydrogen but also a strongly demanding reaction due to the huge amounts of carbon being formed and deposited over the catalyst. Using a thermobalance at different temperatures, the Co/Cu-containing dendritic zeolites exhibited very short induction times, fast kinetics, and high hydrogen selectivity, due to the good dispersion achieved of the metallic species, as demonstrated by XPS measurements. In addition, the dendritic nanoarchitecture promotes the outward growth of large carbon deposits, thus attenuating deactivation. Consequently, the dendritic catalysts retained significant activity even after carbon/catalyst ratios as high as 5.7 w/w for the Co-Al-MFI (d) sample. Moreover, in fixed-bed tests, they showed strong stability and maintained a rather constant methane conversion and high selectivity for hydrogen over time on stream.