Oxidation of methane to methanol and formaldehyde over Co-ZSM-5 molecular sieves: Tuning the reactivity and selectivity by alkaline and acid treatments of the zeolite ZSM-5 agglomerates

摘要

Alkaline treatment of templated Na-ZSM-5 zeolites with NaOH solutions resulted in the creation of inter-crystalline mesopores within the zeolite agglomerates, while preserving the micropore volume and crys-tallinity. It was found that the zeolite external surface area increased with increasing NaOH concentration and pretreatment time. These alkaline-treated samples were loaded with cobalt and a linear relationship between the number of cobalt oxidic species and the external surface area of the zeolite could be established. This in turn leads to a linear relationship between the ZSM-5 surface area and the amount of methanol produced over Co-ZSM-5 from methane and oxygen at 423 K. Attempts to remove extra framework alumina species by an acid treatment were successful. However, this acid treatment increased, after Co-deposition, the amount of highly dispersed Co~(2+) inside the ZSM-5 channels, which resulted in a higher selectivity towards formaldehyde. The nature of the charge-compensating cation also determined the cobalt speciation. Co-H-ZSM-5 samples contained more cobalt inside the channels and were more selective towards formaldehyde formation than Co-Na-ZSM-5 samples.