Study On Synthesis Law Of Bimodal Mesopores Molecular Sieves

The micropore structures (pore diameter <l nm) of microporous molecularsieves limit their wide spread applications. However, in recent years, mesoporousmolecular sieves were widely used for petrochemical industry, industrial catalysis,biomedicine, environmental conservation and preparation of new functionalitynano-materia, due to their high specific surface areas and controlled pore structures.This thesis takes the synthesis and characterization of bimodal mesoporous materials(BMMs) as the main research contents, and finds out structure parameters andtopographical variation rules as the research targets. BMMs is a new kind ofmesoporous materials which consists of worm-like mesopores of3nm and largeaccumulated holes around10~30nm. This thesis firstly reviews the developmenthistory and formation mechanisms of mesoporous materials, outlines the developmentof porous materials, summarizes some important synthesis mechanisms of porousmaterials, and points out the problems of mesoporous molecular sieve. This thesissystematically investigates various influence factors (such as ammonia content, alkalicategory, cosolvent) on the structure parameters and morphologies of BMMs particles.The resultant series of BMMs are well characterized by means of XRD, BET, SEM,TEM, FT-IR and TG, and the corresponding synthesis mechanism is proposed.Research contents and results are as follows:1. BMMs were synthesized in sol-gel process by using cetyltrimethylammoniurnbromide (CTAB) as template, tetraethyl orthosilicate (TEOS) as silica source andammonia as catalyst. Effects of the ammonia content on BMMs were discussed. Itshows that the structure parameters and morphologies of BMMs can be “tailored” bythe changes of amount of ammonia content. With an increasing ammonia additive inthe sol-gel process, the hydrolysis rate of TEOS will be accelerated, leading the dvalue of (100) peak and pore volume gradullly minished, and the particle sizesgradually increase. The specific areas firstly increase, and then decrease.2. Based on above research, the third charpter inspects the synthesizing rule ofBMMs in different alkali medium (inorganic base Sodium hydroxide, organic alkaliEthylenediamine (EDA), and alkali salt Dipotassium hydrogenphosphate (DKP)). Theresults show that the d values of (100) peak and the wall thickness of mesopore arereduced with the increase in alkali content, but more reduced in NaOH medium. Theaccumulated holes can only be formed in a narrow pH range in NaOH medium, so itis not easy to form accumulated holes in strong electrolytes solution. While in EDAmedium, it is very easy to form accumulated holes at a wide pH range. In DKPmedium, it can form two accumulated holes. The resulting BMMs particles tend toform agglometion in NaOH medium, but to form uniform particles in EDA and DKP medium.3. The fourth charpter of this thesis studies the influence ofN,N-Dimethylformamide (DMF) on the structure parameters and morphologies ofBMMs. Simultaneously, the BMMs containing different amount of DMF are treatedin hydrothermal conditions, and the corresponding structure parameters andtopographical variation rule are summarized. The results show that the order degree ofBMMs is reduced after addition of DMF. Both overhigh crystallization temperatureand long crystallization time can completely destroy the mesoporous pore. Comparedwith crystilization time, crystallization temperature shows much bigger effects onpore structure. The secondary pore sizes gradully grow large with increasingcrystallization temperature, and when the temperature reaching at150℃, the primarypore will disappear. However, when the crystallization temperature is100℃,crystallization time is24h, and mole ratio of DMF in the mixture is10, it willgenerate mesoporous materials with a well dispersion and clear pore sturture.