MESOPOROGEN-FREE HIERARCHICALLY POROUS ZSM-5 ZEOLITES: SYNTHESES, MECHANISMS AND HIERARCHY ENHANCEMENT

• Main Author: THOMRYES MARTH KADJA (NIM: 30515005), GRANDPRIX
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/22306
• Institution: Institut Teknologi Bandung

Zeolite is a class of microporous (<2 nm) crytalline aluminosilicate materials. Among 232 types of zeolite frameworks, including natural and synthetic zeolites, ZSM-5 is one of the most utlized zeolite in the industrial fields, especially petrochemical industry. ZSM-5 is an important component of FCC (fluid catalytic cracking) catalysts which has a specific role to boost the octane number of the fuels. However, the main issue in the utilization of ZSM-5 as catalysts is the solely presence of micropores (0,55 nm) which hinder the diffusion of reactant and product molecules to and out of the active sites, respectively. Thus, a large porosity, i.e. mesopores (2-50 nm), is strictly needed for enhancing the molecular diffusion within zeolite frameworks. Zeolite which possesses, at least, two levels of porosities (micropores and mesopores) is called hierarchically porous zeolite. In general, hierarchically porous zeolite is synthesized in the presence of mesopores-directing agents (mesoporogen) such as, surfactants, polymers or carbon nanoparticles under hydrothermal treatment at high temperature above 100 °C, which is, unfortunately, not environmentally and economically benign. <br /> <br /> <br /> In this report, hierarchically porous ZSM-5 was synthesized in the absence of mesoporogen at low-temperature (90 °C). Several factors which govern the crystallization were investigated in detail, including the amount of Al, NaOH, H2O and TPABr. The increase of Al content will decelerate the crystallization due to the expansion of ZSM-5 unit cell. Alkalinity which is determined by the amount of NaOH play a significant role since it catalyzes the breaking and formation of T–O–T bonds through hydrolysis and condensation reactions, respectively. The higher amount of NaOH will lead to an accelerated crystallization. However, in an excessive amount NaOH, the condensation will proceed much faster resulting the in the formation of hard, dense aluminosilica. The success of low-temperature synthesis is also determined by the amount of H2O. A lower amount of H2O will increase the concentration of each reactant, thus compensating the rate deceleration due to the decrease of temperature. Nevertheless, H2O is indispensable since it acts as dissolution and transport media. TPABr is the structure-directing agent of ZSM-5 which should participate in the synthesis of ZSM-5. Herein, the amount of TPABr can be supressed to TPABr/SiO2 = 0.035, while the common synthesis of ZSM-5 requires the amount of TPABr around TPABr/SiO2 = 0.2. N2 adsorption-desorption isotherms, SEM, FE-SEM, TEM and HR-TEM show that the obtained ZSM-5 possesses intercrystalline mesopores around 3.5 nm. <br /> <br /> <br /> Several characterizations such as, XRD, spectroscopy (ATR-IR, Raman, 29Si dan 27Al MAS NMR), and electron microscopy (SEM, FE-SEM, TEM dan HR-TEM) were employed for ex-situ investigation of ZSM-5 crystallization at low-temperature. At the onset of crystallization, most of TPA+ and Al species resides in the liquid phase, followed by the dissolution of Si species simultaneously with the incorporation of TPA+ into the gel phase. With prolonged synthesis time, the amorphous phase undergoes structural rearrangement towards ordered structure following solid gel transformations mechanisms. Low-temperature induce the formation of numerous nuclei which further grown as smaller crystallites. These crystallites can form aggregates which enable the for <br /> <br /> <br /> mation of intercrystalline mesopores. <br /> <br /> <br /> Desilication-controlled hydrocarbons content method is applied to enhance the hierarchy of ZSM-5 synthesized at low-temperature. Hydrocarbons within zeolite frameworks, either as TPA+ or degraded TPA+, can inhibit the attack of hydroxyl ions to Si–O–Si bonds. Hence, the formation of mesopores can be controlled. Desilication leads to the formation of additional mesopores ranging from 7 to 20 nm whose the intensity increases with decreased amount of hydrocarbons. This leads to the enhancement of hierarchy descripted by an indexed hierarchy factor (IHF). Catalytic tests on liquid phase (Claisen-Schmidt condensation of benzaldehyde and acetophenone) and gas phase (LPDE cracking) reactions show the proportional relationship between hierarchy and catalytic activitu on both reactions. <br />