Investigation On The Preparation Of Diatomite-based Porous Mineral Materials And Their Adsorption/catalysis Performance On The Organic Pollutants

Diatomite, also known as diatomaceous earth or kieselgur, is a fossil assemblage of diatom shells composed of amorphous hydrated silica(SiO2·x H2O), which is classified as opal-A in mineralogy. As one of the preponderant mineral resources in China with the world’s second ascertained storage, diatomite possesses various characteristics such as unique macroporosity and mesoporosity, high porosity, stable framework and high thermal stability, which contribute to its extensive use in the environmental area. However, the monomodal porosity and low specific surface area of diatomite constrain its application. The modification of diatomite using physical or chemical methods for the preparation of novel diatomite-based composite is a direct and effective method to expand the applicable range of diatomite. Yet there still exists immaturity in the research and development technology for the fabrication of such novel composite and quantities of problems need to be resolved. Specifically, in the aspect of organic pollutants adsorption, the surface modification procedures of diatomite are laborious with a low loading capacity and high cost. In addition, the composite adsorbent powder is easily disturbed by the airflow, leading to the very few report on its gas adsorption property. In the aspect of organic pollutants catalysis, the low surface area and adsorption capacity of diatomite cause the low loading amount of the catalyst, resulting in a low catalytic activity of the novel composite catalyst.Considering the above mentioned problems, this work focused on improving the porous structure and specific surface area of diatomite through surface modification or recombination, for the preparation of various diatomite-based composite adsorbents/catalysts with hierarchically porous structures, based on expanding applicable range of diatomite in the environmental pollutants processing field. The adsorption/catalysis performance and the related mechanism of the obtained diatomite-based composites were simultaneously systematically investigated. This work mainly includes:1. Preparation of two types of diatomite-based composite adsorbents through surface modification and aperture extension of diatomtie. And the adsorption-desorption performance of the composites was systematically evaluated. 1) Diatomite was pre-treated using modification agent, followed by the in situ reflux synthesization of silicalite-1 nanoparticles on its surface, for the fabrication of diatomite/silicalite-1 composite. 2) Diatomite was used as raw materials and polymeric sponge method was selected for the fabrication of diatomite-based porous ceramic supports. Then silicalite-1 nanoparticles in situ coated on the surface such supports under mild conditions for the preparation of diatomite-based ceramic/silicalite-1 monolith. Several techniques were used to investigate the morphology, texture and loading mechanism of the obtained composites. 3) Benzene was used as the model organic compound to systematically evaluate the adsorption-desorption performance of the composites.2. Fabrication of diatomite/titanium silicalite(TS-1) composites with hierarchically porous structure and high dispersity through aperture extension and ion doping. The structural diversity of the obtained composites synthesized under various crystallization time and loading mechanism were analyzed in detail, and cationic dyes such as methylene blue(MB) and crystal violet(CV) were used as the model molecule to evaluate the performance of the diatomite/TS-1 composites and the related dynamic mechanisms.The main works and specific conclusions of this thesis are listed as follows:(1) A novel method for the preparation of diatomite-based composite adsorbent with hierarchically porous structure was proposed, in which surface pre-modification of the diatomite was combined with the in situ mild reflux synthesization of silicalite-1 nanoparticles. Moreover, the benzene adsorption and the related dynamic mechanisms were investigated as well.1) Diatomite was pre-treated using modification agent, followed by the in situ reflux synthesization of silicalite-1 nanoparticles on its surface, for the fabrication of diatomite/silicalite-1 composite. The obtained composite possessed a hierarchically porous structure. And the specific surface area and micropore volume of the composite were 348.7 m2/g and 0.127 cm3/g, respectively, with a zeolite loading amount of up to 60.2%. Its high zeolite loading amount was ascribed to the in situ loading mechanism: compared with silicalite-1 nanoparticles, the silicalite-1 nuclei with a smaller particle size can be adsorbed onto diatomite more easily under the same electrostatic attraction for further crystallization. Thus the novel synthesized composite possessed higher zeolite loading amount than that synthesized via the secondary growth method.2) The static(45.6 mg/g) and dynamic benzene adsorption capacities of the composite(148.1 mg/g) were much higher than raw diatomite. Moreover, the obtained composite exhibited considerably higher static(94.9 mg/g(Sil-1nano) and dynamic benzene adsorption capacities(246.0 mg/g(Sil-1nano)) per unit mass of zeolite than did the commercial ZSM-5(66.5 mg/g(Sil-1nano) and 173.9 mg/g(Sil-1nano) respectively). Firstly, benzene adsorption in the macropore of diatomite mainly relys on free diffusion, and the in situ coating process introduced micropores of the silicalite-1 nanoparticles, in which benzene adsorption mainly relys on significantly configurational diffusion. The composite captured benzene molecules through both free and configurational diffusion, leading to much higher benzene adsorption capacities than diatomite. Secondly, the support improved the dispersity of silicalite-1 nanoparticles and provided more possible entryways for benzene molecules, resulting in a higher untilization efficiency of zeolite.3) The diatomite/silicalite-1 composite exhibited better benzene diffusion and mass transfer rate than Sil-1nano and commercial ZSM-5, which is due to the introduced macroporosity of the diatomite that improved the degree of freedom for benzene diffusion and contributed to the better benzene mass transfer process.(2) A new method for the preparation of diatomite-based monolithic composite with hierarchically porous structure was proposed. Moreover, the benzene adsorption and diffusion performance and the dynamic mechanisms of the composite with three-dimensional structure were investigated as well.1) Porous ceramic supports with three-dimensional reticulated structures were first prepared using the polymeric sponge method in which diatomite was used as the ceramic framework and polyurethane foam was used as the sacrificial template. The optimum synthetic formula was inverstigated. This process was followed by facile in situ homogeneous coating of silicalite-1 nanoparticles on the surface of the ceramic under mild conditions to form diatomite-based ceramic/silicalite-1 composite. The well-distributed coating mechanism is as follows: surface of the porous ceramic were positively charged after pre-modification, and the silicalite-1 nuclei formed from the prehydrolysis of the zeolite precursor solution were negatively charged. Under the action of the electrostatic attration, silicalite-1 nuclei were adsorbed on the surface of porous ceramic, developing to 80 nm nanoparticles through crystallization.2) Specific area and micropore volume of the composites were 122.9 m2/g and 0.07 cm3/g, respectively, with a high zeolite loading of 32.4%. The obtained composite possessed reticulated structure with hierarchically porosity. The in situ coating process improved the dispersity of silicalite-1 nanoparticles, leading to a more extensive mesoporous distribution range of the composite than that of pure silicalite-1 nanoparticles.3) The composites exhibited a much higher benzene adsorption capacity(133.3 mg/g(Sil-1nano)) compared with that of a commercial micron-sized ZSM-5 product(66.5 mg/g(Sil-1nano)) and a synthesized silicalite-1 nanoparticles(94.7 mg/g(Sil-1nano)). This is due to that the exitence of the diatomite support in the composite, which improved the the dispersity of silicalite-1 nanoparticles, increased the contact area between silicalite-1 and benzene molecules, leading to a higher benzene adsorption capacity. In addition, the adsorption-desorption kinetic equations of the composites fit well with the LDF model, indicating that the benzene diffusion through a barrier to enter the micropore and stacking mesopores of silicalite-1 nanoparticles is the rate-determining step. Moreover, the composites also displayed high benzene adsorption-desorption kinetic rate constants, which indicates a superior adsorption and desorption mass transfer rate due to the improved degree of freedom for benzene diffusion in the reticulated pore walls of the monoliths.(3) Diatomite/TS-1 composites prepared through in situ hydrothermal and reflux methods respectively. And their photocatalytic performance and the related dynamic mechanisms of the cationic dyes were investigated as well.1) Diatomite was pre-treated using modification agent, followed by the in situ hydrothermal synthesization of TS-1 nanoparticles on its surface, for the fabrication of diatomite/TS-1 composite. The obtained diatomite/TS-1 composites possessed hierarchically porous structure. And the highest specific surface area and micropore volume of the obtained composites were 521.3 m2/g and 0.254 cm3/g, respectively. The zeolite loading amount(TS-1 with a particle size of 200 nm) varied with the change of hydrothermal time and could be adjusted to 96.8%. There was a positive correlation between the zeolite loading amount and the composites’ removal efficiency for MB, which is due to that with the increase of zeolite loading amount, the corporation of the catalytic active component Ti in the composite increased. The maxium MB removal efficiency of the composites reached 99.1%. This value was even higher than that observed using TiO2 nanoparticles, which is due to the synergistic effect of efficient adsorption and photocatalysis resulting from the newly formed hierarchically porous structure and improved dispersion of TS-1 nanoparticles onto diatomite. Moreover, the MB removal rate constant of the composites was more than twice as high as that of pure TS-1 nanoparticles, which is due to that the diatomite support improved the the dispersity of TS-1 nanoparticles, increased the contact chances between catalytic active component Ti and dye molecules, leading to an increased catalytic rate.2) The in situ reflux route was proposed for the preparation of the hierarchically porous diatomite/TS-1 composites. The obtained diatomite/TS-1 composites also possessed hierarchically porous structure. As the reflux reaction lasted a long time, the diatom shell was partly dissolved and some macropores of diatomite disappeared. The zeolite loading amount varied with the change of reflux time and could be adjusted to 54.1%(TS-1 with a particle size of 100 nm). This value and the MB removal efficiency were both lower than those of hydrothermally synthesized composites, which was attributed to the mild reaction condition of the reflux route that constrained the growth of TS-1 nanoparticles. There was also a positive correlation between the zeolite loading amount and the composites’ removal efficiency for CV. And the maxium CV removal efficiency of the composites reached 90.4%. The mechanism lies, diatomite support improved the dispersity of TS-1 nanoparticles and the composite provided hierarchical pores, which increased the contact chances and areas between TS-1 CV dyes, leading to a much higher CV photocatalytic efficiency and kinetic rate of the composites than those of TS-1 nanoparticles.