Desilicated ZSM-5 Catalysts: Properties and Ethanol to Aromatics (ETA) Performance

Herein, desilication in increasingly harsh conditions was used to introduce mesopores into two different industrial ZSM-5 catalysts (Si/Al ratio 11 or 29). For desilicated samples, increasing BET surface areas, mesopore volumes, and Si(OH) densities were noted. Bronsted acid site (BAS) densities increased upon desilication, as formerly inaccessible BAS in blocked pores became available, while the strength of the BAS was maintained upon desilication. Using KOH instead of NaOH as desilication agent can increase the mesopore volume generated per mass loss. The correlations between desilication strength and properties were largely determined by the parent Si/Al ratio. In general the introduced mesopores increased lifetimes in the ETA conversion, while additional Si(OH) groups introduced by desilication reduce the lifetime again. The lifetime is thus determined by a complex interplay of BAS density, improved reactant transport by introduced mesopores and Si(OH) density. There were no additional aromatics formed in desilicated samples during the conversion of ethanol and the samples were, in terms of aromatic yield, outperformed by a microporous parent. However, as result of longer lifetimes less ethanol was lost due to coke formation. It is concluded that desilication should be combined with other post-modifications to increase aromatic production and lifetime. Desilication is identified as valuable tool for increasing the lifetime of ZSM-5 catalysts used in the ethanol-to-aromatic conversion. A thorough solid-state NMR characterization enables disentangling the influence of changes in Si(OH) group density, Bronsted acidity, and mesopore volume on the catalyst's performance in terms of products and lifetime.image