Direct Synthesis of Hierarchical ZSM-5 Zeolite and Its Performance in Catalyzing Methanol to Gasoline Conversion

Abstract

A simple method was developed and tested to synthesize hierarchical ZSM-5 zeolites using tetrapropylammonium hydroxide as a sole structure directing agent to generate intercrystalline mesopores. XRD analysis confirmed the formation of ZSM-5 zeolites. SEM and TEM imaging analyses revealed the presence of hierarchical structure, formed by self-assembly of highly crystalline, nanosized primary particles. Solid-state MAS NMR analysis showed the vast majority of Al atoms were tetrahedrally incorporated with Si atoms in the zeolite framework. N2 physisorption studies indicated that the mesopores of an average pore size of 30 nm were created in the hierarchical ZSM-5 zeolites with an abundance of micropores. The performance of these zeolites in catalyzing methanol to gasoline conversion was evaluated using a fixed-bed reactor operating at 350 °C with ca. 16.5% methanol in nitrogen at a total pressure of 1.1 MPa and WHSV of 1.2 h–1. The hierarchical ZSM-5 zeolites exhibited almost 100% methanol conversion and 59% selectivity toward the gasoline range hydrocarbons. The coke formation on the used catalyst was studied using a N2 physisorption technique and a thermogravimetric analyzer. The hierarchical ZSM-5 zeolite showed less than 2% coke formation and deposition over 24 h on stream, much less compared to the conventional ZSM-5 zeolite with nearly 8% carbon deposition under the same reaction conditions.

Selective production of propylene from methanol over high-silica mesoporous ZSM-5 zeolites treated with NaOH and NaOH/tetrapropylammonium hydroxide

“…The larger spices would be more likely to become trapped and thereby form a coke precursor. So, the solely microporous ZSM-5 catalyst is progressively deactivated as the micropores are filled with coke deposits, or their entrances become blocked [54]. In contrast, in the case of hierarchical structured zeolites (alkali-treated) with intra-and/or intercrystalline mesoporosity, coke is formed mainly on the external surface and/or mesopores.…”

Fluoride-mediated nano-sized high-silica ZSM-5 as an ultrastable catalyst for methanol conversion to propylene

“…Zeolites could be used more efficiently if the diffusion path length of the micropore could be reduced by synthesis of nanocrystals or by mesopore formation in larger zeolite crystals, improving transport of molecules to and from the active sites [4,8,10] . Numerous studies have been reported to synthesize hierarchical structured zeolites [11][12][13][14][15][16] , though these morphologies lead to lower crystallinity and more framework defects [17,18] . Framework defects are prone to coke formation, as several studies have demonstrated a direct correlation between coke formation and silanol defects [19][20][21][22][23][24] .…”

Strategies to Enhance the Catalytic Performance of ZSM-5 Zeolite in Hydrocarbon Cracking: A Review

“…The mesoporous ZSM-5 zeolites with improved diffusion efficiency and acids accessibility showed better performance than the conventional ZSM-5 zeolite. TPAOH, a conventional microporous template, can be used as the single SDA to prepare hierarchical ZSM-5 zeolite [75,76]. Hua et al [76] successfully prepared the hierarchical ZSM-5 zeolite using only TPAOH as the SDA through one-step hydrothermal process at a relatively low temperature of 100 • C, and they proposed a "nucleation/growth-demetallation/recrystallization" mechanism during the hydrothermal synthesis process.…”