Accelerated Synthesis Of Zeolites Via Hydroxyl Radicals Generated By Different Methods

Zeolites are microporous crystalline aluminosilicates that are widely used as ion-exchangers in the detergent industry, catalysts in the petrochemical and chemical industry, and adsorbents in massive air separation. So far, zeolites have been the most important solid catalysts in the chemical industry. Since their introduction as a new class of industrial materials, the annual market for synthetic zeolites has grown immensely, to several million tons worldwide per year.In the hydrothermal crystallization of zeolites from basic media, it has been well accepted that the hydroxide ions(OH–) catalyze the depolymerization of the aluminosilicate gel by breaking the Si,Al–O–Si,Al bonds and catalyze the polymerization of the aluminosilicate anions around the hydrated cation species by remaking the Si,Al–O–Si,Al bonds. Recently, it has been discovered that the hydroxyl free radicals(?OH) are involved in the zeolite crystallization under hydrothermal conditions. The hydroxyl free radicals generated by ultraviolet irradiation can significantly accelerate the crystallization processes of zeolites—such as Na–A, Na–X and Na Z–21. The discovery that the zeolite synthesis mechanism can be promoted through free radicals sheds a new light on zeolite crystallization and will create new routes for the synthesis of zeolite materials that are largely demanded in the chemical industry.In this thesis, several methods were adopted to introduce hydroxyl free radicals to the synthetic systems of zeolites of Silicalite-1 and Na A, by means of, for example, UV irradiation, addition of Fenton’s reagent, addition of persulfate, and utilization of radicalized seeds. We investigated the influence of radicals on the crystallization of these zeolites, characterized the radicals by electron paramagnetic resonance(EPR) technique. The radicals introduced by above methods accelerated the crystallization of zeolites remarkably. Our work will provide novel approaches for efficient synthesis of zeolites by utilizing hydroxyl free radicals. The main results are summarized as follows:1. Hydroxyl radicals generated by ultraviolet irradiation were introduced to Silicalite-1 synthetic system, in which tetrapropylammonium hydroxide was used as the organic template. The synthesis was conducted at 70 ?C. The hydroxyl, silicon-based, and ethanol radicals were detected by electron paramagnetic resonance(EPR). The hydroxyl free radicals involved crystallization process was shortened for 20 hours than the normal crystallization process.Fenton’s reagent, which can generate large amounts of hydroxyl radicals, was also added to the synthetic system of Silicalite-1. The Fenton’s reagent had a much better acceleration effect than the UV irradiation.Inhibiting ?OH may slow down the zeolite crystallization. Because ethanol is an effective ?OH scavenger, we crystallized Silicalite-1 under UV conditions by evaporating the ethanol in the mixture. The crystallization process was accelerated when the ethanol was removed.2. Persulfate can produce sulfate radicals under heating, which would react with H2 O easily to generate hydroxyl radicals and sulfate at any p Hs. The crystallization of Silicalite-1 was remarkably accelerated by adding sodium persulfate. The EPR signal of ethanol radicals was observed during the synthesis and the intensity of the signal was increased with the increase of the concentration of sodium persulfate, indicating the increase of the concentration of hydroxyl free radicals. The crystallinity of the products at the reaction time of 16 hours is the same as that under the normal condition at the reaction time of 24 hours. We also found that sodium sulfate solution can also produce hydroxyl radicals. Increasing the concentration of sodium persulfate can enhance the acceleration effect on the crystallization process. The introduction of the sodium persulfate can decrease the usage of the organic template, thus reduce the cost of synthesizing zeolites and the pollution of environment. In addition, the crystallization process of zeolite Na A and sodalite can also be accelerated by the sodium persulfate. The hydroxyl free radicals generated from the sodium persulfate may also affect the phase selectivity of the synthetic systems.3. A new seeding-synthesis-related strategy has been developed to accelerate the crystallization of zeolites. By means of milling and heating, the Silicalite-1 zeolite crystals can be radicalized forming surface nonbridging oxygen hole center(NBOHC, ≡Si-O?). The corresponding g values of 2.0043 and 2.0048 unambiguously confirmed the existence of the ≡Si-O? radicals in the milled and heated crystals by EPR. And the signal of hydroxyl free radicals was detected by EPR in aqueous solution of radicalized seeds. Radicalized crystals were used as seeds that showed obvious accelerating effect in the crystallization of nanosized Silicalite-1 than the non-radicalized seeds. It can be expected that the application of this strategy will further improve the efficiency of the zeolite synthesis in industry.