Mechanism Of Free Radicals On Accelerating The Crystallization Of Zeolites

Zeolites, as a class of inorganic microporous crystalline materials, were widely used in catalysis, adsorption–separation, and ion exchange. Because of the complex synthetic chemistry, there is still no clear understanding of the crystallization mechanism so far. Generally, a typical synthesis of zeolite involves the following steps: i) formation of an amorphous gel, ii) depolymerization of the amorphous gel forming the soluble aluminosilicates and silicates anions via breaking Si,Al-O-Si,Al bonds, and iii) re-polymerization of the anions via remaking the Si,Al-O-Si,Al bonds around the hydrated cation species, that is, the structure-directing agents. All of these three steps were catalyzed by hydroxide ions(OH–). In this thesis, we found that hydroxyl free radicals(?OH) were involved in the zeolite crystallization under hydrothermal conditions. The crystallization processes of zeolites—such as Na–A, Na–X, Na Z–21, and SBA-15—can be accelerated with hydroxyl free radicals generated by ultraviolet irradiation or Fenton’s reagent. The detailed results are summarized below:I: We studied the reaction system of Na A(Si O2/0.46Al2O3/4.4Na2O/60H2O)at 298 K for 24 h under either UV conditions or dark conditions. Under UV conditions, 24 h of crytallization results based on the XRD and SEM analysis indicate that the products were already crystallized. The initial gel, however,presented an amorphous state under the dark condition. We also studied the reaction system of Na X(Si O2/0.21Al2O3/9.36Na2O/85H2O) and Na Z-21(Si O2/0.32Al2O3/10.05Na2O/85H2O) either UV conditions or dark conditions, and the results were similar to that of Na A. To understand the reason of the experimental phenomena, the electron paramagnetic resonance(EPR) technique was used to characterize the ?OH and the derived radical species formed in the zeolitereaction system. The EPR signals of ?OH and Si? were recorded in situ after the reaction mixture under UV radiation. For comparison, the initial reaction mixture and the pure water were also characterized under dark conditions and none of these signals were observed. Considering that the concentration of radicals in the non-UV irradiated initial reaction mixture and in the pure water might be too low to be detected by DMPO, a recently developed spin-trapping agent,5-tert-butoxycarbonyl-5-methyl-1-pyrroline-N-oxide(BMPO) was used, that allows a long time accumulation of the EPR signals. Strikingly, ?OH was also observed in the initial synthesis mixture under dark conditions for 10 h.We further investigated the synthesis Na A(Si O2/0.46Al2O3/4.4Na2O/60H2O)under the direct sunlight conditions. The XRD and SEM results indicate that the products were already well crystallized in 10 h. The Na A can be synthesized by using the sunlight resource that will provide a new research direction for energy-saving synthesis of zeolites.II: We investigated the crystallization process of zeolite Na-A with the starting molar composition of Si O2/0.46Al2O3/4.4Na2O/60H2 O at 298 K under dark and UV conditions with different irradiances(2.0, 4.0, and 8.0 m W/cm2).The results show that the rate of radical production was influenced by the density of UV radiation. With the increase of the UV irradicance density, the crytallization rate of zeolites become faster.To further confirm which stage was accelerated by ?OH, we investigated the synthesis of zeolite Na-A with the starting molar composition of Si O2: 0.46Al2O3:4.4Na2O: 60H2 O at 298 K by UV pretreatment in the induction period for 18 h followed by crystallization under dark conditions. The results show that the crystallization of the synthetic system under such conditions is similar to that under UV conditions, indicating that ?OH mostly influences the nucleation stage.On the other hand, reducing the OH- concentration may slow down the crystallization of zeolites. In the above studies, the Na2O/Si O2 molar ratio was of4.4. We further studied the crystallization behavior of the initial reaction mixture with a reduced Na2O/Si O2 molar ratio of [3.08 vs 4.4] at 298 K. The results clearly demonstrate that reducing OH- can decrease the crystallization rate of zeolite; whereas ?OH can promote the crystallization at the same time. Notably,the crystallization rate of sample under UV radiation was faster than that of sample without UV radiation, even than that of sample with Na2O/Si O2 molar ratio of 4.4 without UV radiation.III. Theoretical calculations provide more insight into how ?OH accelerates the nucleation stage. Geometries were fully optimized without restrictions, and the nature of the stationary points was characterized by means of frequency calculations. We considered a dimeric Si(OH)3-O-Si(OH)3 species, a mono-deprotonated [Si(OH)2-O-Si(OH)3]Na system, and a highly deprotonated[Si O2(OH)-O-Si O3]Na5 system. The calculation results indicate that the presence of ?OH in the reaction medium will considerably increase the rate of Si-O bond breaking and depolymerization of the gel and accelerate the rate of nucleation.IV: Taking advantage of the catalytic activity of ?OH radicals in breaking the Si-O-Si bonds, without additing any inorganic acid mesoporous SBA-15 and Fe-SBA-15 were successfully synthesized in the TEOS-CTAB-H2 O system under the UV conditions with the addition of sodium persulfate or Fenton reagent. The products show highly ordered hexagonal mesostructure and the two iron species including isolated 4-coordinated iron and free of iron oligomer were coexisted in the Fe-SBA-15 with medium strong acidity.The discovery that hydroxyl free radicals(?OH) involve in the zeolite crystallization and can accelerate the crystallization process provides new insight into the zeolite crystallization mechanism, which will facilitate a more efficient production process in zeolite synthesis.