Preparation Of Microstructure Controlled Silicalite-1 Zeolite Membranes And Their Application In Ethanol-Water Separation

Silicalite-1 zeolite membrane possesses uniform pore size of-0.55 nm with hydrophobic framework,excellent chemical and thermal stability.It has a good application prospect for recovery of fuel ethanol by integrated fermentaion-pervaporation process.The structure-directing agent(SDA)is critical for the growth,structure and separation performance of Silicalite-1 zeolite membranes which are prepared mostly under the single SDA.The growth and separation performance of Silicalite-1 membrane under binary SDAs(TEA+,TPA+and TBA+)is not clear yet.Moreover,it is generally known that the microstructure directly affects the separation performance of Silicalite-1 membrane,but the relationship between them still needs to be further clarified.Herein,performance of Silicalite-1 membrane was manipulated by F-as mineralizer,the preparation route using TBA++TEA+as binary SDAs was developed and the structure and separation performance of membrane were optimized by TBA+in the presence of TPA+.The main research contents and results are as follows:(1)Manipulation of ethanol-selective Silicalite-1 membranes by NaF as mineralizerSilicalite-1 membranes were prepared by in-situ and seed-induced wet gel transformation using NaF as mineralizer on traditional tubular alumina support without prefabrication of intermediate layer.The effects of H2O/Si ratio,crystallization time,TPA+/Si ratio and water at the bottom of the autoclave on the membrane were investigated under seed-induced wet gel conversion method.The Silicalite-1 membrane with separation factor of 43 and flux of 2.7 kg·m-2·h-1 for 5 wt.%ethanol/water under 60? was prepared under the molar ratio of precursor of 1SiO2:0.8NaF:0.08TPABr:60H2O,crystallization time of 8 h and 4 g deionized water at the bottom of the autoclave.The well intergrown Silicalite-1 membrane with good surface hydrophobicity was prepared by in-situ wet gel conversion method.The maximal surface water contact angle was 105°.This method has the advantages of traditional steamed assisted conversion method,such as less solvents,high material utilization,low self-generated pressure and safety,etc.,as well as strong adaptability towards the support,which is expected to be extended to the preparation of other membranes.However,the separation performance for ethanol recovery of the Silicalite-1 membrane by wet gel conversion method has yet to be improved.(2)Structure-directing behavior and influence on the growth,structure and separation performance of Silicalite-1 membranes of quaternary ammonium(TEA+,TPA+and TBA+)with simple structure as SDAs.Silicalite-1 membrane was prepared by TBA++TEA+as binary SDAs.The impact of TBABr/Si ratio,TEABr/Si ratio,TBAOH/Si ratio,content of TEOS,H2O/Si ratio and crystallization time on the growth and separation performance of membrane was investigated.The Silicalite-1 membrane with separation factor of 64 and flux of 1.4 kg·m-2·h-1 for 5 wt.%ethanol/water under 60? was prepared under the molar ratio of synthetic liquid of 1:0.2TBAOH:0.2TEABr:180H2O and crystallization time of 48 h.13C NMR confirmed that both TBA+and TEA+were incorporated into pore of Silicalite-1 zeolite as SDAs.This work expanded the alternative preparation approachs of alcohol-selective Silicalite-1 membranes.The growth,structure and separation performance of Silicalite-1 membrane were regulated by TBA+under constant TPA+/Si ratio.It was found that a small amount of TBA+could accelerate the growth,optimize the structure and improve the separation performance of Silicalite-1 membrane,but too much TBA+would inhibit the growth of Silicalite-1 membrane,thus reducing its separation performance.The separation factor for 5 wt.%ethanol/water under 60? increased from 35 to 59(with flux of 1.85 kg·m-2·h-1)with the increase of TBABr/Si ratio from 0 to 0.01 under TPAOH/Si=0.09.It was found that a small amount of TBABr could accelerate membrane crystallization,and promote its evolution along the "101-orientation>c-orientation".The membrane at 101-orientation stage showed the optimal separation factor due to synergy of good surface hydrophobicity,compactness and less impurity aluminum.In addition,a distribution model of silanol groups on the Silicalite-1 crystal and membrane surface was proposed.The silanol groups is dense on the crystal edge(the intersection of crystal faces),but sparse in the crystal surface(<1 01>,<020>and<100>).Therefore,the silanol density on the membrane surface with all the crystals arranged on the support along the 101-orientation is less than that along the 002-orientation.The model can explain the observed phenomenon that all of the membranes with the highest surface water contact angle under different TPAOH/Si ratio were preferentially 101-orientated.This model was preliminarily verified by the obvious correlation between the surface water contact angle,the membrane surface O/Si ratio observed by XPS and relative peak intensity I101/I(102+103+104+002).The strategy using TBA++TPA+as binary SDAs was further adopted to optimize the separation performance of Silicalite-1 membrane under constant(TBA++TPA+)/Si ratio.It was found that the separation factor was improved due to that the promotion effect of favorable compactness overcame the inhibition effect of adverse surface hydrophobicity.The separation factor for 5 wt.%ethanol/water under 60? could be improved from 37 to 56 for the membrane under crystallization time of 24 h,and from 43 to 71 under crystallization time of 48 h.In addition,the influence of feed concentration(from 1 wt.%to 50 wt.%)on membranes with favorable separation performance was investigated.The separation factor first increased and then decreased with the increase of concentration,and the highest separation factor was related to the crystallization time.The highest separation factor was 79 with flux of 1.40 kg·m-2·h-1 at 3 wt.%for the membrane under crystallization time of 24 h and it was 86 with flux of 1.52 kg·m-2·h-1 at 8 wt.%for the membrane under crystallization time of 48 h.Unlike the case under presence of TEA+,TBA+did not enter the Silicalite-1 zeolite pore in the presence of TPA+according to 13C NMR observation.We infer that TBA+may facilitate the formation of silica secondary units with Silicalite-1 framework formed by the hydrolysis of silicon source in the presence of TPA+.