Study On The Structure-directing Effect Of Zeolite Beta Seeds

Zeolite Beta, which possesses three-dimensional and intersected12-membered ring pore channels, is a high-silica zeolite that has important industrial applications. In recent years, the seed-directed synthesis of zeolite Beta in the absence of organic structure-directing agent (OSDA) has attracted much attention. This dissertation focused on the structure-directing effect and behavior of seeds with different SiO2/Al2O3ratios and crystal sizes in the OSDA-free synthesis of zeolite Beta. In addition, the synergistic effect of seed and organic template on the crystallization of low TEA+/SiO2ratio gels, and the seed pass-on synthesis of zeolite Beta had also been studied. The main results and conclusions obtained are as following:Firstly, ten zeolite Beta seeds, which were synthesized with different raw materials, organic templates and methods, were characterized by XRD, SEM, XRF, etc. Results show that, the seeds differed mainly in SiO2/Al2O3ratio (11.4～∞) and crystal size (<100nm-9μm). Then they were used to direct the crystallization of gels prepared with fumed silica, NaAlO2and NaOH. Results show that, not only the literature recommended nano-agglomerated seeds (SiO2/Al2O3<52), but also the microsized and pure silica seeds can direct the OSDA-free synthesis of zeolite Beta. This means that, more seeds are available for the OSDA-free synthesis of zeolite Beta. However, seed with higher SiO2/Al2O3ratio and smaller crystal size is better for the synthesis of high-crystallinity zeolite Beta.Furthermore, the structure-directing behavior of different seeds was investigated on the basis of solid-phase change during the crystallization of the OSDA-free gel, which was characterized by XRD, SEM and other techniques. Results show that, the seeds all underwent dissolution at the early stage of crystallization. The dissolution rate and extent of the seeds, which were closely related to their structure-directing behavior, were found to depend on the SiO2/Al2O3ratio, crystal size, and calcination pre-treatment of the seeds, as well as the state of gel. Fortunately, two kinds of crystals with distinct size and appearance were found in the microsized seeds-directed products. The bigger ones came from the residues of the seeds. The surface of the Al-containing seed residues was covered by new crystals, while that of the pure silica seed residues were bare. These phenomena show that the seed surface growth mechanism is not the only way for the seed to work. The secondary nucleation path induced by the dissolved fragments of the seeds also played a role. This is probably the most important way for bigger seeds and the only way for pure silica seeds to work.We also investigated:(1) the minimum amount of TEAOH for the spontaneous nucleation of gel;(2) the spontaneous nucleation of the gel by supplementing TE AX (X=F, Cl, Br) into the gel, which had a TEAOH content less than the minimum content; and (3) the feasibility of overcoming the problems in the OSDA-free synthesis (i.e., low SiO2/Al2O3ratio and low yield of the product) by the aid of the synergy of seed and a small amount of organic template in the presence of NaOH or ammonium solution as supplemental alkali source.Finally, the seed pass-en synthesis of zeolite Beta, that is, to use the product of No. n generation as seed to direct the No. n+1generation synthesis, in both OSDA-free and low TEA+/SiO2ratio gels was studied. Results show that, the seed pass-on synthesis only work for one generation in the OSDA-free gel, but for three generations in the low TEA+/SiO2ratio gels (TEAOH/SiO2=0.03-0.10), Obviously, the increase in TEAOH content is helpful for the seed pass-on synthesis. The characterization of products with XRD, XRF, N2physisorption and SEM suggest that the decrease in SiO2/Al2O3ratio and the increase in crystal size weakened the structure-directing effect of seed in the seed pass-on synthesis of zeolite Beta.