Novel hierarchical HZSM-5 zeolites prepared by combining desilication and steaming modification for converting methanol to propylene process

The effect of mesoporosity formation and acidity modification have been investigated by desilication and combined desilication-dealumination treatments over highly siliceous zeolite HZSM-5 (Si/Al = 200) and its catalytic performance has been studied in the conversion of methanol to propylene (MTP) reaction. Desilication of a conventional microporous HZSM-5 catalyst was performed using NaOH, mixtures of NaOH and tetrapropylammonium hydroxide (TPAOH), and mixtures of NaOH and tetrabutylammonium hydroxide (TBAOH) with different ratios. Subsequent mild steaming treatment has been used to modify acidity of the selected samples. The physicochemical properties of all samples were characterized by XRD, FE-SEM, BET and NH3-TPD methods. Textural and acidity properties confirmed that TBAOH is more effective than TPAOH in the mesoporosity formation, micropore volume preservation, and acidity modification. Steaming treatment after desilication over the sample with TPAOH/(NaOH + TPAOH) ratio of 0.4, led to increase in selectivities to propylene from 38.4 to 41.3%, and total light olefins from 69.4 to 76.6%, while it led to decrease in C-5(+) components selectivity from 14.8 to 10.1%. The combined alkaline-steam treatment over the sample with TBAOH/(NaOH + TBAOH) ratio of 0.2 compared to the parent one led to considerable higher selectivities to propylene (44.8 vs. 30.7%), total light olefins (84.1 vs. 57.9%), as well as lower selectivities to C-5(+) components (7.4% vs. 27.1%). Moreover, this sample showed double lifetime (830 h) in MTP reaction compared to the conventional micropore ZSM-5 catalyst (425 h). The results showed that desilication led to a remarkable mesoporosity development, while steaming treatment generally influenced on the HZSM-5 acidity. Therefore, the combined alkaline-steam treatment leads to HZSM-5 zeolite formation with tailored pore architecture and surface acidic properties.