Interconnected Hierarchical ZSM-5 with Tunable Acidity Prepared by a Dealumination–Realumination Process: A Superior MTP Catalyst

Abstract

ZSM-5 that uses TPAOH as a template has an Al-rich exterior and defective Si-rich interior; thus, a simple base leaching selectively removed the Si-rich interior while the Al-rich exterior was protected. This catalyst showed no change in stability comparing with parent ZSM-5 during the MTP reaction that was attributed to the enclosed hollow structure and richly acidic outer shell. A preliminary fluorination, however, both removed defective Si-sites and caused distortion in tetrahedral aluminum that made the outer shell susceptible to alkaline treatment. These distorted tetrahedral Al were mostly leached out by NaOH in 1 min. Furthermore, aluminum in the filtrate was slowly redeposited onto the zeolite, serving as external pore-directing agents to control silicon dissolution from the Si-rich interior. This dealumination-realumination alkaline treatment process led to a higher solid yield and a uniform opened-mesopore structure with mesopores around 13 nm in diameter. This material was characterized by SEM, TEM, N adsorption, and mercury porosimetry. In addition, NH-TPD, OH-IR, Al MAS NMR, andH MAS NMR results demonstrated that the reinserted Al were unlike the framework Al, contributing less to acidity. The dealumination-realumination process, therefore, was also capable of tuning the acidity of the mesoporous ZSM-5. This mesoporous catalyst exhibited a longer lifetime and a higher propylene selectivity than other catalysts with an enclosed mesopore structure.

State of the Art and Perspectives of Hierarchical Zeolites: Practical Overview of Synthesis Methods and Use in Catalysis

“…This catalyst showed a long lifetime and high propylene formation activity in the MTP process, when compared to the parent zeolite. [65] Figure 3. The effect of different treatments and the Al-coordination in the zeolite structure.…”

“…This catalyst showed a long lifetime and high propylene formation activity in the MTP process, when compared to the parent zeolite. [ 65 ]…”

Synthesis of ZSM‐5 by hydrothermal method with pre‐mixing in a stirred‐tank reactor

“…It presents a well‐defined porosity, which enables reactants and products to cross channels easily . Therefore, ZSM‐5 has been widely used in many processes including methanol to gasoline (MTG), methanol to propylene (MTP), aromatic selective transformation, alkanes selective cracking, etc.…”

Facile Solvent‐free Synthesis of Hollow Fiber Catalyst Assembled by c–axis Oriented ZSM‐5 Crystals

“…Song et al. also synthesized hollow ZSM‐5 through a dealumination‐realumination process and the resultant product exhibited superior performance on methanol to propylene owing to its fast mass transformation . Wang et al.…”

“…Song et al also synthesized hollow ZSM-5 through a dealumination-realumination process and the resultant product exhibited superior performance on methanol to propylene owing to its fast mass transformation. [22] Wang et al developed MFI zeolite with unique bayberry-like hollow sphere and hollow fiber morphology, providing higher catalytic efficiency in methanol to aromatics reactions. [23][24] ZSM-5 zeolites with hollow structures could improve catalytic performance greatly with the help of hierarchical architectures.…”