Sorption Properties Of Volatile Organic Compounds (VOCs) On Mesoporous SBA-15 Molecular Sieve

Volatile organic compounds (VOCs) are one of the main air pollutants, which can be a threaten to environment and human health. Its pollution control has attracted more and more attention. Adsorption technologies has been recognized as an efficient way and the thermal swing rotor adsorption technology could condense the low concentration VOCs continuously. The key issue of this technology is the adsorbents. Mesoporous SBA-15 molecular sieve has high specific surface area, large uniform main pore sizes, thick pore wall and good stability, which is potential for catalysis, adsorption, separation etc. In this thesis, a series of mesoporous SBA-15 molecular sieves were prepared and evaluated for its applicability as a sorbent for adsorption of VOCs, using toluene and ethyl acetate as the model compounds. The main contents in this paper are summarized as follows:1. SBA-15 mesoporous molecular sieves were synthesized under acidic condition, using Pluronic P123 (EO₂₀PO₇₀EO₂₀, EO=CH₂CH₂O- and PO = -CH₂(CH₃)₃CHO-) as the template and TEOS (Tetraethylorthosilicate) as the silica source. MCM-41 material was synthesized under alkaline condition, using CTAB (Cetyltrimethylammonium bromide) as the template. SA-XRD (Small-angle X-ray diffraction) and N₂-sorption isotherms results for the two kinds of samples exhibit the P6mm space group mesophases structure properties, with similar BET (Brunauer-Emmett-Teller) surface areas. The adsorption/desorption properties of toluene and ethyl acetate on SBA-15 and MCM-41 have been evaluated using breakthrough curves and TPD (temperature programmed desorption) techniques. A larger amount of toluene adsorption and a higher desorption temperature were observed on the SBA-15; while both samples exhibited similar properties for ethyl acetate sorption. The interaction between organic molecules and adsorbents were evaluated by affinity index. It was found that SBA-15 and MCM-41 are philic to ethyl acetate but phobic to toluene. The larger amount toluene retained and higher desorption temperature of toluene on SBA-15 were also ascribed to the higher adsorption potential existed in the micropores. In contrast to microporous adsorbents such like TS-1 and Silicalite-1, desorption of organic gases from mesoporous molecular sieves could be achieved at lower temperature.2. The pore structures of SBA-15 mesoporous molecular sieves were manipulated by synthesizing at different temperatures (80℃, 100℃, 135℃) and aging at different time (0 h, 12 h, Id, 2 d, 5 d). All materials were investigated by SA-XRD and N₂-sorption isotherm techniques. The results show that the surface area declined at the higher aging temperature and the mesoporous volume of samples increased with aging time, which was probably due to the fact that a gradual transition from micropore to mesopore under the higher aging temperature and the longer aging time. The sorption properties of toluene and ethyl acetate on the SBA-15 samples were tested by breakthrough curves and TPD techniques. The interactions processes may exist between organic template and inorganic silica species are discussed on basis of sorption results. Less adsorption capacities and easier desorption were observed with increasing aging temperature. When it came to the different aging time, the adsorption capacities of organic gases first increased then decreased. Experiments confirmed that the dynamic adsorption capacities of toluene was proportional to its micropore volume, and the ethyl acetate adsorption capacity on SBA-15 is much higher than that of toluene.3. The sorption properties of methanol and ethyl acetate on SBA-15 materials were investigated by TG-DTG analysis, and three classical dynamics methods (ASTM E698,Friedman and OFW) were used to further analysis. The ethyl acetate desorption properties was evaluated in terms of activation energy for desorption, pre-exponential factor and variation curve with desorption processes. Air purge was operated after the breakthrough curves reached equilibrium. Diffusion coefficients of toluene and ethyl acetate were obtained, based on the gas concentration and desorption time under operating condition. It was found that the diffusion was limited by the pore structure.