Organotemplate-free And Seed-directed Synthesis Of Zeolite

Among the porous materials, aluminosilicate zeolites have been attracted muchattention due to their uniform pore size, high surface area, high thermal stability, highhydrothermal stability and controllable acidic sites in the framework, which ensuretheir extensive applications in petrochemistry, fine chemical synthesis, adsorption,separation, and ion-exchange. Zeolites developed from natural zeolites to artificialsynthesis zeolites. Initially, the zeolites such as LTA, FAU were hydrothermallysynthesized from alkaline aqueous solution in the presence of alkali metal ions oralkali earth metal ions. After that, by introduction of organic structure directing agents(SDAs), synthesis of zeolites enters the rapid development period and numerouszeolites with various structures and composition have been successfully synthesized.Up to now, more than200kinds of zeolites have been developed and most of themwere prepared in the presence of organic SDAs. On the other hand, to resolve thelimiting dispersing of guest molecules in zeolites, the researchers are attempting topreparation of zeolites with hierarchical structure, which were also prepared by usingorganic templates. It is to say that the use of organic templates increases the types ofzeolites, promoting their applications in industry. However, with the development ofeconomy, people are aware of the use of organic template against today’s society ofenergy saving and environmental protection theme. For example, high cost of organictemplates increases the cost of synthesizing zeolites; the open micropores are obtainedby calcination at high temperature, which not only wastes a lot of energy and destroys the crystalline structures such as excluding the aluminium from the frameworks, butalso products lots of environmentally undesirable gases such as nitrogen oxides andcarbon oxides. Although the researchers have developed some novel methods such asextraction, decomposition by oxidation, to remove organic templates, these methodsneed complicated process and the organic templates can’t be removed completely.Therefore, development low cost, environment-friendly and green synthesis routestoward zeolites has become one of the research focuses in the field of zeolitesynthesis.No use of organic templates is the best way for avoiding the problems providedby organic templates, and some important zeolites have been synthesized in theabsence of organic templates. For example, ZSM-5and ECR-1were prepared bytuning the ratios of precursors in the starting gel; ZSM-34was prepared by using Lzeolite seeds solution as SDAs; Beta, RTH and MTW zeolites were prepared by solidseeds-directed method. Thus, seed-directed synthesis (SDS) method for preparation ofaluminosilicate zeolites is a high-performanced route, compared to other template-freemethods. In the present study, based on the method for preparation of Beta zeolite, theSDS method was successfully used for preparation of zeolites with other heteroatom,zeolites with various structures and zeolites with hierarchical structure.Fe-containing zeolites play an important role in decomposition of nitrogen oxides,and particularly Fe-Beta zeolite with unique three-dimensional network of large pores(12MR) has been widely studied due to its excellent catalytic activities. Normally,Fe-Beta zeolites were prepared by post-treatments of Beta zeolites with FeCl3andFe2(SO4)3, or direct synthesis of Fe-Beta in the presence of organic templates(tetraethyl ammonium, TEA~+) In the chapter2, we have demonstrated that theorganotemplate-free and seed-directed route has been successfully applied forsynthesizing Fe-Beta zeolite (Si/Al=4.9、Si/Fe=50) using calcined Beta as seeds. TheFe-Beta thus obtained has good crystallinity, high surface area, uniform crystals, andtetrahedral Al3+and Fe3+species. It is seen that the Fe-Beta zeolites have openmicropores due to the absence of organic template TEA+, and the BET surface areaand microporous volume of Fe-Beta are610m2/g and,0.24cm3/g respectively. Furthermore, steam treatment of Fe-Beta zeolites at high temperature results inredistribution of the iron species, which ensure Fe-Beta zeolites exhibit excellentcatalytic activity for direct decomposition of N2O.People always pay attention to the preparation of zeolites with relatively largepore size due to their application for bulky molecules catalysis. However, lessattention has been paid on preparation of zeolites with relatively small pore size,which also play an important role in shape selectivity of catalytic reaction.Particularly, much attention should be paid on the preparation of the small porezeolites with low framework density and relatively large pore volume, such as LEVzeolites (8MR). In Chapter3, LEV zeolites have been successfully prepared by theSDS method using RUB-50as seeds in the presence of a small amount of ethanol.Physicochemical characterizations for the crystallization process and crystal productsexhibit that the seeds are stable and the crystallization process can be concluded as thealuminosilicate gel crystallizing around the seeds. We also examine the factors on thesynthesis of zeolites, such as gel composition, amount of seeds, kinds of alkalinemetal ions and synthesis temperature, revealing that the crystallization area of LEV isvery narrow. Furthermore, the introduction of ethanol can habit the formation of MORzeolite. Finally, Catalytic tests for the conversion of methanol-to-olefins (MTO) showthat H-form of LEV-SDS zeolite exhibits good conversion of methanol and highselectivities for ethylene and propylene.For a long time, the people think the zeolite seeds should have the same structureto the final products. In the following part, we will prepare zeolites using the zeoliteseeds with similar structure to the final products. It is well known that the buildingunits of FER and CDO are the same, and their difference is only a shift of layers inthe horizontal direction. In Chapter4, FER zeolites have been successfully preparedby SDS method using RUB-37zeolites as seeds, which is a zeolite containing CDOstructure. Although the FER zeolites with ratio of silica to aluminum lower than10have been prepared in the absence of template, but it is usually necessary to useorganic templates for synthesizing high silica FER zeolite. The ratio of silica toaluminum in FER synthesized by SDS method is as high as14.5in the present work. The crystallization mechanism can be explained as follow: XRD pattern and UV-VisRaman spectra exhibit the seeds dissolved into small secondary building units, and theFER zeolites were formed inducing by these building units. By the same way, HEUzeolites were successfully prepared using RRO zeolites as seeds.ZSM-5plays an important role in industrial catalysis. Normally, the ratio of silicato aluminum in ZSM-5is larger than12by the conventional method and this ratiodetermine the amount of acid sites. Thus, decrease the ratio of silica to aluminum isan effective way for increasing the amount of acid sites. Furthermore, the guestmolecules dispersing limit is relatively weak in hierarchical ZSM-5, which is a goodcatalyst for bulky molecules catalysis. The conventional methods for preparation ofhierarchical ZSM-5are divided into two kinds: first, preparation of hierarchicalZSM-5using mesostructure templates in the presence or in the absence of SDAs;Second, formation of mesostructure by post-treatment of ZSM-5. The direct synthesismethod using at least one kind of template, and the post-treatment method can’tensure the high crystallization of ZSM-5. In Chapter5, Al-rich hierarchical ZSM-5zeolites have been successfully prepared by SDS method in the absence of anyorganic templates. The ration of Si to Al is9.6in the final samples with tetrahedralAl3+species in framework. The crystallization process can be concluded as thealuminosilicate gel crystallizing around the seeds, and the seeds play an importantrole in final morphology of product. By using nanosize ZSM-5as seeds, hierarchicalZSM-5assembled by nanorods is obtained, and some mesopores were also formedbetween the nanorods. The hierarchical ZSM-5thus obtained exhibit excellentcatalytic performance for cumene cracking, such as high conversion of cumene andhigh stability, which can be attributed to large amount of acid sites and the presence ofmesostructure.