Synthesis And Adsorption Performance Of Mesoporous Materials From Diatomite

With the rapid development of economy, the heavy metal pollution has become a serious environmental problem in recent years, which represents a significant threat to human health and aquatic ecosystem. Mesoporous molecular sieves have become excellent adsorbents to remove heavy metal ions due to their unique large surface area, controllable pore diameters and easily incorporated bonding groups. However, the mesoporous molecular sieves are commonly synthesized using expensive chemical reagents. The application of mesoporous molecular sieves is limited because of the high cost. Therefore, the development of a low-cost and environmentally friendly method for synthesis of mesoporous molecular sieves has important scientific significance and application prospect. In this dissertation, MCM-41 mesoporous molecular sieves and ZSM-5/MCM-41 composites were synthesized from diatomite. Moreover, amine-functionalized mesoporous molecular sieves NH₂-MCM-41 were prepared through surface modification of MCM-41 and used to remove heavy metal ions in aqueous solution. The main results are listed as follows:（1） MCM-41 mesoporous molecular sieves were prepared from diatomite using 1-hexadecyl-3-methylimidazolium bromide ([C₁₆mim]Br) as the template. The effects of various synthetic parameters including molar ratio of [C₁₆mim]Br/SiO₂, pH, crystallization temperature and time were discussed. MCM-41 samples with the large pore size （3.3 nm-4.9 nm） were obtained by adjusting the dosage of [C₁₆mim]Br during the synthesis process. The resultant samples had good hydrothermal stability. The mesoporous structure was retained in boiling water for 5 days. The experimental results indicated the feasibility of a synthetic route to MCM-41 with high hydrothermal stability from diatomite.（2） MCM-41 mesoporous molecular sieves were prepared from diatomite using hexadecyltrimethylammonium bromide （CTAB） as the template. The effects of various synthetic parameters including molar ratio of CTAB/SiO₂, pH, crystallization temperature and time were discussed. The resultant samples possessed large surface area, pore volume and good thermal stability. The mesoporous structure was retained up to 900 ℃. The method realized the effectively use of diatomite and greatly reduced the synthetic cost of the mesoporous molecular sieves.（3） ZSM-5/MCM-41 composites were synthesized using diatomite as silica source and using CTAB and tetrapropylammonium bromide （TPABr） as the templates via a two-step crystallization method. Various parameters such as crystallization time, crystallization temperature, molar ratio of CTAB/SiO₂ and TPABr/SiO₂ were discussed. The optimized experimental conditions were:the reactant molar ratio SiO₂:0.33 Na₂O:0.04 Al₂O₃:0.18 CTAB:0.20 TPABr:56 H₂O, pH= 11.0, crystallization at 100 ℃ for 2 days; re-crystallization was done at pH 9.5 and 125 ℃ for 7 days. In addition, the composite ZSM-5/MCM-41 and the mechanical mixture of ZSM-5 and MCM-41 were characterized by XRD, SEM and N2 adsorption-desorption measurements. The results indicated the structure of ZSM-5/MCM-41 was different from the mechanical mixture. ZSM-5/MCM-41 composite possessed a well-ordered mesoporous phase as well as microporous phase.（4） NH₂-MCM-4I was synthesized by post-synthesis grafting of 3-aminopropyltrimethoxysilane （APTMS） onto the mesoporous MCM-41 surface. The content of total aminopropyl groups in the resultant sample 0.3NH₂-MCM-41 was 3.09 mmol/g by TG/DTA analysis. Batch experiments were carried out with sample 0.3NH₂-MCM-41 for the removal of Cd²⁺. The effects of various parameters including the amount of APTMS, pH, contact time and adsorbent dose were discussed. The rate-limiting step was determined by film diffusion and the intra-particle diffusion. The adsorption was fitted the pseudo-first-order kinetics model and Langmuir adsorption model. The maximum adsorption capacity of Cd²⁺ was 102.9 mg/g. The thermodynamic data indicated the adsorption of Cd²⁺ on the sample 0.3NH₂-MCM-41 was spontaneous, endothermic and entropy increment. Moreover, the adsorption behavior of 0.3NH₂-MCM-41 was studied in Cd²⁺-Ni²⁺ binary solution. The high selectivity coefficient aCd/Ni showed the adsorbent displayed extremely good selectivity to Cd²⁺.