Synthesis And Characterization Of Novel Catalytic Molecular Sieves

Due to the relatively high stability, the uniform pore distribution, high surface areas and high catalytic activity, microporous zeolites have been one of the molecular sieve catalysis material most studied at present. In order to improve the existing process and establish new catalytic reactions, continuous work should be done to develop and study the new structure and properties of zeolites, which will help the synthesis of novel catalytic molecular sieves with the special functions in catalysis. In this sense, new multi-dimensional zeolites have been synthesized to expand the zeolitic pore structures and shape-selective catalytic properties. This is often achieved with certain types of organic templates. Recently, the diquaterany alkylammonium ions show some advantages in synthesizing novel zeolites with 10/12MR intercommunicated pores, which is valuable to explore novel shape-selective catalysts. Such advantages can profit from the rich flexibilities and higher charge density of the skeleton of diquaterany alkylammonium ions when compared with organic templates with small and rigid structures. Up to now, these advantages have been considered as the important aspects in the synthesis of zeolites with new characteristics both in pore structures and framework properties.Meanwhile, the restrictions from the micropores of zeolites have been met when they are used in catalysis and diffusion related to the macromolecules. Thus, the hierarchical zeolites have been synthesized by introducing framework or aggregated mesopore in zeolites through different routes. Moreover, considering the mesoporous sieves with even larger pore size and higher surface areas but with lower catalytic activity than zeolites, the micro-mesoporous composite molecular sieves have been synthesized to combine both advantages of mesoprous materials and microporous zeolites. From the point of view, the successful synthesis of both novel porous materials not only provides an important reference to resolve the restrictions from microporous zeolites, but also gives a wider catalytic application for the outstanding properties of the according zeolites and a new direction of catalytic development for mesoprous materials.In current thesis, the synthesis of novel molecular sieves with catalytic properties has been studied from two points of view based on microporous zeolites, hierarchical zeolites and micro-mesoporous composite molecular sieves. First, the synthesis of multi-dimensional microporous aluminosilicates has been systematically studied, and the synthesis of different zeolites including conventional microporous aluminosilicate, titanosilicate and aluminophosphates has also been attempt, both of which are based on the template effect of diquaterany alkylammonium ions and controlling the composition of gel systems. Second, different kinds of micro-mesoporous composite molecular sieves and hierarchical zeolites have been synthesized by using different routes based on the utilization of the preformed precursors of microporous zeolites and hard templating of porous aerogels and microprous zeolites.In Chapter 3, the synthesis, characterization and catalytic properties of COK-5 with different Si/Al ratios are described. Various key parameters (including the ratios of Si/Al and/or Na/Si, the type/concentration of alkali cations, and the structure of dialkylammonium) are investigated in detail for the successful synthesis of COK-5. Under our synthesis conditions, COK-5 zeolites are found to be synthesized only in a narrow range of Si/Al ratios and Na/Si ratios. The concentration of Na+ in the initial gel should be carefully controlled for the successful synthesis of COK-5 with lower Si/Al ratios. Rising the crystallization temperature (<180°C) or adding seed (including heterogenous seeds ZSM-57 and ZSM-22) can shorten the crystallization time of COK-5. N2 adsorption-desorption results suggest that the COK-5 samples synthesized from gels with different Si/Al ratios show different texture properties. Some relatively strong acid sites present on COK-5 zeolite are detected by NH3-TPD. The catalytic performances of H-form COK-5 zeolites have been studied for the liquid-phase alkylation of phenol with tert-butanol. All the samples are catalytically active, which behave similarly in the conversion of phenol tested on other zeolites with 10MR channels, and the different product distributions are observed on the COK-5 samples with different Si/Al ratios. We speculated that the existence of a certain amount of acidic sites and the unusual pore systems (e.g., distorted 12MR pore) of COK-5 should be highly correlated with their catalytic properties. In addition, it is found that COK-5 shows good catalytic performance in the etherification of alcohols when testing on the catalytic dehydration of ethanol. Furthermore, in the hydrocracking of heavy aromatic hydrocarbon mixtures, some better catalytic shape-selective properties have been observed on COK-5 when compared with MOR, one of most suitable zeolitic catalysts for the title reaction. Continuously, the synthesis of other kinds of zeolites, including convention aluminosilicates, titanosilicates and aluminophosphates, has been investigated to expand the influence of different diquaternary alkylammonium ions as organic templates, which has brought about some novel characteristics and surface properties to the corresponding zeolites. By using N-methylimidazolium-based diquaternary alkylammonium ions as structure-directing agents(SDAs), and carefully controlling the Na/Al ratios, one-dimensional 10MR ZSM-22 zeolites have been obtained with high purities and a wide range of Si/Al ratios from 30~150. Combined with the mineralization of fluoride added, Et6-diquat-6 can direct the synthesis of MOR, and more, a high value of Si/Al ratio (>30) can be obtained under high content of sodium in the gel system. Different types of microporous titanosilicates have been directed depending on the different structures of diquaternary alkylammonium ions and synthesis conditions, which promise a high potential possibility to synthesize larger pore titaosilicates by this route. The effective synthesis of APO-5 using Et6-diquat-n as a new kind of SDA, has also been studied in detail in Chapter 4. Under proper conditions, MgAPO-5 with relatively Mg content can be successfully synthesized, and their acid and base properties should be influenced by different SDAs. In the vapor-phase O-methylation of catechol, the metal-doped MeAPO-5 solvothermally synthesized give higher catalytic performance than those hydrothermally synthesized, which indicates a good candidate of catalytic molecular sieves with bi-functionally weak acid-base properties.In Chapter 5, the synthesis of micro-mesoporous composite and hierarchical zeolitic materials has been expanded, which based on aluminophosphate and aluminosilicate sieves, respecitively. First, the synthesis of mesoporous SBA-15 coated with the preformed precursor of microporous aluminophosphate (AlPO) has been described, and physicochemical properties of the coated samples were investigated in detail. The characterization results suggest that the nanometer-scaled zeolite units are present on the wall surface of SBA-15, thus bringing the weak acid-base characteristics to the resulting mesoporous materials, and that the acid-base properties of these materials can be modified by the aging treatment in glycol. Moreover, vapor-phase O-methylation of catechol with methanol has been studied on these coated samples as catalysts. It is found that the coated sample bearing suitable weak acid-base sites exhibits the relatively high activity and selectivity to guaiacol. Second, hierarchical aluminophosphates have been obtained by using the organic aerogel and carbon aerogel as hard templates, respectively. Characterizations indicate the presence of mesopores and the different templating effect of two kinds of aerogels. Third, by modifying the process of crystallization the controllable synthesis of amorphous micro-mesoporous composite materials containing zeolitic precursors has also been attempted. It is found that changing the structures of diquaternary SDAs has a big effect on the framework structures and pore distributions of the precursors of zeolitesAs an application of microporous zeolitic sieves used in the synthesis of porous materials, some different kinds of zeolites are used to hard template the porous carbon nitrides with very high surface areas. By using COK-5 as a hard template, the porous carbon nitride can keep part of COK-5 frameworks under the liquid-phase polymerization of carbon and nitrogen and meanwhile show high surface areas. When introducing a vapor-phase polymerization process more like DGC, porous carbon nitride with small part of MFI framework can be obtained by using ZSM-5 as a hard template. Further studies have shown that the synthesis of micro/mesoporous carbon nitride can be improved by properly introducing C/N sources and selecting zeolites.