Studies On Surface Hydrophobic Modification Of Carbon Membrane

Carbon molecular sieving membrane is a novel carbon-based membrane material prepared by pyrolysis of carbonaceous materials under vacuum or inert atmosphere. Carbon membrane has an excellent gas separation performance, good thermal and chemical stability. Carbon membrane has a great application potential in the gas separation field, especially the small molecular gas separation like air separation and H2recovery. However, the weak mechanical strength of homogeneous carbon membrane limits the application of carbon membrane. In order to improve the mechanical strength of carbon membrane, a great deal of researchers has been focused on the exploration of supported carbon membrane with good mechanical strength and great gas separation performance. Carbon membrane are susceptible to the effects of oxygen and humidity in the process of preparation, application and storage, which results in the decrease of gas flux. It is widely accepted that the key factors in H2O sorption are the porosity and the oxygen-containing groups of carbon membrane. While carbon surfaces are generally hydrophobic, oxygen-containing surface groups act as primary sites for the sorption of H2O. The molecules sorbed on the primary sites attract additional H2O through hydrogen bonding, leading to the eventual formation of water clusters. Since this process ultimately reduces the pore capacity available to other permeating species, significant membrane function is lost. In order to delay the aging process, the surface hydrophobic modification of carbon membrane is of important significance.Methylated SiO2colloidal sol solutions were prepared by hydrolysis and condensation reactions of mixed tetraethoxysilane (TEOS) and methyltriethoxysilane (MTES) using EtOH as a solvent with NH3as a catalyst. Hydrophobic modified carbon membranes were prepared by coating carbon membrane with the methylated SiO2solutions and calcinating at400-600℃in an N2atmosphere. In order to determine the optimal preparation conditions of Silica sol, the ratio of TEOS to MTES, the adding amount of H2O and EtOH, the reaction temperature and the reaction time were studied. The thermostability, surface chemical structure, surface roughness and surface morphology of the hydrophobic carbon membrane were characterized by TG, FTIR, AFM and SEM.(1) The hydrophobicity of carbon membranes is greatly improved by introducing the methylated silica structure onto the carbon membrane via silica sol. The preparation conditions of silica sol, especially the reaction temperature, have great influence on the surface chemical structure, morphology and roughness of hydrophobic carbon membrane. The contents of Methyl group on the membrane surface play a decisive role in the improvement of hydrophobic properties of carbon membranes. The hydrophobicity of carbon membranes can effectively tuned by controlling the preparation condition of silica sol and membrane calcination condition. The hydrophobic carbon membrane prepared under the optimum conditions exhibits an excellent hydrophobicity. Its surface roughness is34.263nm and water contact angle is138°.(2) Hydrophobic modification greatly reduces water activity on the surface of the carbon membrane, improves the aging problem of carbon membrane and prolonges the service life of the carbon membrane. O2permeation flux of hydrophobic carbon membrane can still keep around65%of the original flux after30days under the condition of60%relative humidity, while O2permeation flux of original carbon membrane dropped to10%of the original flux under the same condition.(3) Hydrophobic modified carbon membrane exhibits good pervaporation performance. Total flux of pervaporation increases obviously and separation factor decreases slightly as the temperature rises. As the ethanol and isopropyl alcohol concentration increase, total flux increases obviously and separation factor decline significantly. When the temperature was75"C and ethanol content was10%, total flux reached1.18kg/(m2·h), separation factor reached25.26. When the temperature was80℃and isopropyl alcohol content was10%, total flux reached1.023kg/(m2·h), separation factor reached20.62.