Soft-templating Synthesis Of Hierarchical Zeolitic Materials:Mechanism And Structure Manipulation

Zeolitic materials are widespread heterogeneous catalysts in oil refining and petrochemistry. However, the sole presence of micropores in the material causes limited diffusion of reactants and products and/or the accessibility of intrazeolite acid sites towards large molecules, leading to unsatisfactory catalytic performance. Zeolitic materials with hierarchical pore structure have improved molecular transport and accessibility and are considered as promising materials for industry. In this thesis, hierarchical zeolitic materials were synthesized by dual-templating method using various soft templates.Carboxyl-ended organosilane (IDAA silane) was explored to enhance the interactions between hexadecyltrimethylammonium bromide (CTAB) and zeolite seeds during hydrothermal synthesis and the mechanism was verified by further characterizations. Short-chain organosilanes with different organic moiety were used to explore the templating effect on the mesopore generation in ZSM-5, and the structural evolution of the representative sample during crystallization was tracked to elucidate the mechanism for this method. A rational selection for easily available cationic polymers to synthesize hierarchical ZSM-5was taken, which was explained from the structual point of view. The templating effect of selected cationic polymer was studied by varying synthetic condition. Low amount of tetraethylammonium hydroxide (TEAOH) was used to synthesize small-sized SAPO-34by vapor-phase transport method and the effect of tetraethylammonium ion (TEA+) was studied by changing the anion group of TEAOH. The main results of this work are as follows:(1) The ester form of IDAA silane can silanize the pre-crystallized zeolite seeds and simultaneously transform its end group into carboxyl anion by base catalysis in the alkaline sol (pH=10-11) at90℃for6h. The carboxyl-capped zeolite seeds have enhanced interaction with CTAB micelles and thus can form hierarchical ZSM-5zeolite with CTAB-templated mesopores during the elevated-temperature hydrothermal synthesis. The CTAB involved zeolite samples synthesized with APTES and without organosilane can obtain mesopore-free zeolite aggregates and a mixture of hierarchical zeolite and amorphous mesophase, respectively. Infrared spectroscopy (FT-IR) spectra verify the hydrolysis of carboxyl ester, and29Si MAS NMR confirms the attachment of IDAA silane onto the zeolite. However, the content of IDAA silane in the zeolite structure is verified to be low. (2)3-aminopropyltrimethoxy-silane (APTMS),[3-(2-aminoethyl)aminopropyl]-trimethoxysilane (AEAPTMS) and phenylaminopropyltrimethoxysilane (PHAPTMS) were explored to synthesize hierarchically porous ZSM-5zeolites by steam-assisted crystallization (SAC) of silanized dry gel. N2adsorption/desorption measurement and transmission electron microscopy (TEM) indicate that the three short-chain organosilanes can generate mesopores to different degrees and PHAPTMS with bulky cross-section is the most excellent mesopore directing agent. Structural evolution of PHAPTMS-ZSM-5characterized by X-ray diffraction (XRD), N2adsorption/desorption measurement and scanning electron microscopy (SEM) shows that a large number of mesopores are produced in the initial steaming, but they are not related to the presence of PHAPTMS. The initially formed mesoporosity is mostly preserved by the PHAPTMS molecules, which inhibit the crystal growth and suppress the high mobility of aluminosilicate species under the steam atmosphere.When Beta zeolite was synthesized by the SAC method, its successful crystallization is closely related to good dispersion of sodium cations (Na+) in ethanol solution and dry gel. Between the two employed organosilanes, short-chain PHAPTMS can not generate appreciable mesopores, whereas long-chain hexadecyltrimethoxysilane (HDTMS) can successfully create abundant mesopores of3-10nm in diameter, which indicates that the long-chain moiety has a greater spatial separation effect than the short-chain counterpart.(3) By using small-sized nanoblocks as the zeolite precursor, epichlorohydrin-N,N-dimethyl-1,3-diaminopropane copolymer (PCA) can successfully serve as a meso-template to synthesize hierarchical ZSM-5zeolite (PCA-ZSM-5) with mesopores ranging from7to50nm. However, when its structural analogue epichlorohydrin-dimethylamine polyamine (PCS) was used as the meso-template, the obtained PCS-ZSM-5has lower mesoporosity than ZSM-5nanocrystallite aggregates (NCA-ZSM-5) synthesized without meso-template.13C MAS NMR and thermal gravimetric analysis (TGA) demonstrate that PCA can be entrapped within zeolite and partially retain its cationic charges and macromolecular structure after hydrothermal treatment, consequently exerting a significant templating effect on the synthesis of hierarchical ZSM-5. However, PCS decomposes severely into small amine molecules. In term of the structual characterisitics, PCA has cationic centers widely separated by more than3carbons, but PCS has short separation of cationic centers. Therefore, PCA has lower decomposition rate and higher stability than PCS during zeolite crystallization. Further investigation into the templating effect of PCA shows that the small-sized and negatively charged nanoblocks can easily wrap and assemble with PCA and transform into hierarchical ZSM-5templated by PCA. However, when using large-sized zeolite seeds to synthesize ZSM-5, PCA shows a negligible templating effect because PCA possesses limited charge density and low accessibility of cationic charges and thus has insufficient interactions with zeolite seeds. The catalytic activities of PCA-ZSM-5and NCA-ZSM-5for acetalization of cyclohexanone with methanol are inferior to that of PCS-ZSM-5, which has the highest number of acid sites, but the catalytic activities for aldol condensation of benzaldehyde with n-butyl alcohol follow the order of PCA-ZSM-5> NCA-ZSM-5> PCS-ZSM-5, in agreement with the order of mesoporosity.(4) Vapor containing morpholine (MOR) has assisted the crystallization of dry gel with low amount of TEAOH to obtain SAPO-34with size of several hundred nm (TM-SAPO-34(H)). When additional step of ultrasonic treatment of wet gel before being heated into dry gel was added, the crystal size of obtained TM-SAPO-34(US+H) reduces to below100nm and the crystals are in the form of aggregates. The results indicate that TEAOH is capable of generating many nucleation sites and ultrasonic treatment can strengthen the capability. TM-SAPO-34(H) and TM-SAPO-34(US+H) have long lifetime (270vs.310min) on methanol-to-olefin (MTO) reaction. The role of TEA+in the crystallization of SAPO-34is further explored by using TEABr instead of TEAOH. Without the presence of OH-, TEA+can not induce small-sized crystals through increasing nucleation sites, but can inhibit the crystal growth to some extent.