Post Modification And Catalytic Properties Of Layered Zeolites

For designing and synthesizing new structures with more opened channel system and more exposed external surface area, this article focused on the structural modification of layered zeolites to prepare catalysts with different structural characteristics. Firstly, we obtained lamellar precursors PLS-3 and HUS-2 through direct hydrothermal synthesis. Then, we carried out six parts of researches aiming at these two precursors through various methods, like calcination, acid treatment, intercalation and interlayer silylation, etc.Part one, layered aluminosilicates of PLS-3 were hydrothermally synthesized and employed as the catalysts for the skeletal isomerization of 1-butene. PLS-3 lamellar precursors were successfully prepared at different Si/Al ratios (50-∞) using layered silicate H-kanemite as silica source and tetramethylammonium hydroxide as structure directing agent. A direct calcination caused an interlayer dehydration condensation, converting the precursors into 3D FER structure with an intersecting micropore system of 10 x 8-MR. The physicochemical properties of the Al-PLS-3 materials thus prepared were characterized by various techniques. Their catalytic properties in the skeletal isomerization of 1-butene has been investigated and compared with conventional ferrierite zeolite catalyst. Al-PLS-3 synthesized at Si/Al = 50 exhibited the same catalytic reactivity as a commercial ferrierite rpoduced by Tosho Co. Ltd.. However, possessing a much smaller crystal size, A1-PLS-3 exhibited a much longer catalytic duration against coke formation.Part two, an interlayer expanded zeolite IEZ-PLS-3 with 12 x 10-MR pore system was postsynthesized from the lamellar precursor of PLS-3 aluminosilicate by interlayer silylation with diethoxydimethylsilane (DEDMS) in HCl-EtOH solution at 443 K for 20 h. The resulting material was characterized by various techniques such as XRD, SEM, adsorption of N2, water, and benzene, IR and NMR spectroscopies, and its catalytic properties were investigated by comparison to those of other zeolites with similar Si/Al ratios in m-xylene isomerization/disproportionation, Friedel-Crafts acylation of anisole with acetic anhydride. In the m-xylene isomerization/djsproportionation reaction, IEZ-PLS-3 showed a higher conversion than PLS-3 and gave an isomerization to disproportionation ratio close to that of Beta zeolite, characteristic of the shape-selective properties of typical 12-MR zeolites. IEZ-PLS-3 was more active than Beta in Friedel-Crafts alkylation and acylation reactions, implying that it was a promising solid acid catalyst for processing bulky molecules.Part three, a simple HCl-EtOH solution treatment has been conducted to modify the structure of Al-PLS-3, the FER-type aluminosilicate lamellar precursors with various Si/Al ratios (60-∞). With prolonging the treatment time from 15 min to 24 h at 443 K, the precursors underwent a variety of structural changes, including removal of occluded organic species, loss of layer stacking order and restoration of an ordered structure by interlayer pillaring. The post structural modification gave rise to a sub-zeolite of FER topology within a short time of 40 min, which possessed disordered stacking of FER sheets, named as ECNU-8. Containing the FER laminates and tetrahedral Al in framework, but mitigating the spatial restrictions in particular for the bulk molecule reactions, ECNU-8 proved to be a promising solid acid catalyst for processing larger substrates like 1,3,5-triisopropylbenene.Part four, a novel extra-large pore zeolite with pore system consisting of 14× 12-MR channels was post-synthesized by silylation of derivative of ECNU-8 using 1,3,5,7-tetramethylcyclosilone as silane. Firstly, we intercalated ECNU-8 in 4-amion-2,2,6,6-tetramentylniperidine aqueous to get a new lamellar precursor with similar structure to PREFER but a smaller crystal size, named as PREFER-S. Then interlayer expansion was taken on PREFER-S using the above mentioned silylating agent to get the final product D4h-PLS-3. The structure of D4h-PLS-3 was confirmed by Rietveld refinement of the PXRD. The solid-liquid reaction was taken to introduce Ti as active sites to D4h-PLS-3, the resulting catalyst exhibited obviously superior catalytic performance in epoxidation of bulk substrate cyclohexene than the conventional titanosilicates.Part five, developing new metallosilicates with open pore apertures was a ubiquitous challenge in the fields of catalysis and porous materials. Herein, we disclosed a facile and cheap way to post-synthesize large-pore titanosilicates from zeolitic layered precursors, in which the isomorphous substitution of Ti for Si and the interlayer expansion with movable Si debris was realized simultaneously in aqueous H2TiF6 solution at room temperature. By using this versatile one-pot method without external silane source, various lamellar precursors, PLS-3, PLS-4, RUB-39 and MCM-47, have been converted successfully into corresponding interlayer-expanded zeolite (IEZ) structures that contained tetrahedrally coordinated Ti in the framework. In particular, we have traced the structural transformation and Ti-incorporation process of IEZ-Ti-PLS-3, resolved its crystalline structure by Rietveld refinement of the synchrotron PXRD, and further investigated its catalytic properties in the epoxidation of alkenes with hydrogen peroxide. IEZ-Ti-PLS-3 not only catalyzed actively the epoxidation of linear alkenes, but also exhibited advantages in the reactions involving bulky molecules like cyclic alkenes.Part six, starting from pure silica layered precursor HUS-2, we got new structure with 12 x 8-MR channels through the similar way for preparing D4h-PLS-3. Under room temperature, we conducted acid treatment and intercalation using (6R,10S)-6,10-dimethyl-5-azoniaspiro[4.5]decane successively. The structure of the resulting material was analyzed by Rietveld refinement of the synchrotron PXRD.