Functional Defect-patching And Microstructural Optimization Of Zeolite Membranes For The Dehydration Of Acetic Acid

Acetic acid is one of the most important chemical intermediates in the chemical industry, often requiring dehydration and refining in its production and application. Distillation, as an energy-intensive and high-cost process has being used in conventional process for the dehydration of highly concentrated acetic acid aqueous solution. In recent years, zeolite membrane pervaporation, as an efficient, energy-saving and polluteless technology, have stimulated much scientific and technological interest, being regarded as an alternative method to the distillation. However, Al-rich surface and grain boundary defects have inevitably formed during hydrothermal synthesis of hydrophilic zeolite membranes for the dehydration of acetic acid, seriously degrading their selectivity and acid stability. In this dissertation, high PV performance and acid stability have been carried out on new synthetic and post-treatment strategies of zeolite membranes based on the functional defect-patching (FDP) and microstructural optimization method. The main points can be summarized as follows:(1) Based on catalytic transport separation mechanism, functional defect-patching method has been developed to the elimination of zeolite membranes. The PVI & P4VP groups, involving electronegative imine, have grafted onto the grain boundaries of MOR zeolite membrane, conjointly formed a compact PVI/MOR & P4VP/MOR hybrid selective barrier. It is effectively to have patched the membrane defects and protected the zeolite phase from acid corrosion. When dehydration of the 83 wt.% acetic acid at 80℃, the separation factor of the membranes reached∞. Furthermore, over the 225 d acid stability test, the membrane still kept the excellent PV-performance.(2) Contrary to the complexity of FDP process, direct crystallization of defect-free, acid-proof film is consistently pursued for making hydrophilic PV membranes popular in process industries. A novel microstructural optimization method has been developed to synthesis of MOR zeolite membrane. Based on the knowledge of nucleation and crystallization for zeolite membranes, a novel two-step deposition technique has been demonstrated for successful fabrication of a thin and uniform seed layer directly on a macroporous support with pore size of about by using nano-sized seeds instead of previously provided a smooth surface by covering a dense transition layer. The MOR zeolite membranes, with the thickness 3～4μm and the Si/Al ratios of 5.6～20.1, have been successfully hydrothermal synthesized for 24 h at a relatively low crystallization temperature of 150℃ using F-as a structure modifier or mineralizer. The formation of the acid nonresistant film microstructures in conventional synthesis, such as Al-rich film surface and grain boundaries, have been effectively avoided by the accurate regulation and control of the Si/Al ratios in the wider scale and the optimization of the distribution of Si, Al atoms in zeolite film. When dehydration of the 83 wt.% acetic acid at 80℃, the separation factor of the membranes reached oo. Furthermore, over the 225 d acid stability test, the membrane still kept the excellent PV-performance.(3) Microwave hydrothermal technique has also been used as a better heating way instead of conventional hydrothermal technique for microstructural optimization growth MOR zeolite membranes. The microwave-assisted membranes have been more productive for the dehydration of acetic acid than the membranes by using conventional hydrothermal technique, due to the nearly no zeolite crystallized in the support pores during 4 h microwave synthesis and the thinner film layer with the thickness of only-2μm.(4) For the sake of superior PV performance, microstructural optimization growth super-hydrophilic ZSM-5 zeolite membranes has also carried out at 175℃for 48 h with a low gel Si/Al ratio of 10 using F- as a structure modifier or template. The membrane has been with the thickness of 5-6μm and the Si/Al ratio of 13.5. It has broken through the restrictiveness that the gel Si/Al ratios for synthesis of ZSM-5 zeolite membrane could not below 20 while not using organic template, efficaciously increased the hydrophilicity of the membrane. The formation of the acid nonresistant film microstructures in conventional synthesis, such as Al-rich film surface and grain boundaries, has also been avoided. So the membrane had excellent PV performance for the dehydration of concentrated acetic acid and high acid stability. When dehydration of the 83 wt.% acetic acid at 80℃, the total flux of the membrane reached 2.637 kg m-2 h-1, and the corresponding separation factor was oo. Furthermore, over the 225 d acid stability test, the membrane still kept the excellent PV-performance.