The goal of this research is to develop zeolite membranes that can simultaneously provide high H2 selectivity and high H2 permeance while possessing adequate hydrothermal and chemical resistances for hydrogen separation from fuel gases and WGS reaction at high temperature of 400 to 550°C. This dissertation work consists of three main parts of research including (1) the development of microwave-assisted method for synthesis of MFI zeolite membranes from template free precursors by seeded secondary growth; (2) controlled modification of MFI zeolite channels by chemical deposition and understanding the mechanism of high H2-selectivity against slightly bigger gases with high H2 permeance at elevated temperatures; and (3) the demonstration of high temperature water gas shift reaction (WGS) in the modified MFI zeolite membrane for enhanced CO conversion.
Membranes have been synthesized by seeded secondary growth method from a template-free precursor solution containing NaOH, SiO2, H2O, and Al2(SO4)3 under conventional and microwave heating methods. The effects of precursor composition and synthesis conditions on the membrane formation and membrane quality have been investigated. The MFI zeolite membranes obtained by seeded secondary from the template-free precursor tend to develop oblique orientation. Gas permeation tests on small molecules, such as H2 and CO2, and large molecules, such as SF6 have shown that the membranes were free of pinholes and contain minimal amount of non-zeolitic intercrystal spaces. The zeolite membranes obtained from template-free precursors do not need a firing for template removal that avoids the risk of defect formation during the template removal process. The microwave synthesis method was found to dramatically reduce the hydrothermal synthesis duration as compared to the traditional heating method, and hence is more energy-efficient.
According to the molecular dynamic diffusion theory, the MFI-type zeolite pore size is incapable of obtaining H2/CO2 selectivity much high than the Knudsen factor (i.e. 4.7) at high temperature. A new method of on-stream catalytic cracking deposition (CCD) has been developed for MFI zeolite membrane modification to reduce the effective pore opening so that a size discrimination effect can be achieved for H2 against other small gases like CO2. The methyldiethoxysilane (MDES) was used as the silane precursor for CCD modification and the MFI zeolite pore size was narrowed from 0.56 nm to ~ 0.33 nm after the modification. The modified membrane obtained a H2/CO2 permselectivity as high as 147 at 450°C and the modified structure also showed good thermal stability in the presence of steam. Theoretical analysis based on the molecular dynamic diffusion model has revealed that the dramatically enhanced H2 selectivity in the modified is due primarily to the reduction of the molecular load of large molecules in the zeolite cavity.
A porous α-alumina tube supported MFI-type zeolite membrane with a small membrane area of 1.1 cm2 has been modified by the CCD method. The modified zeolite membrane tube was combined with a Cr-doped ferrite catalyst for WGS membrane reaction in a temperature range of 400-550 °C. The WGS reaction results demonstrate that the zeolite membrane reactor is effective for enhancing CO-conversion (χco) at kinetically favorable high temperatures especially when high space velocity (WHSV) and low steam-to-CO ratios (RH2O/CO) are used. At 550 °C with a high WHSV of 60,000 h-1 and a low RH2O/CO of 1.0, the modified zeolite MR achieved χco of 81.7%, which was much higher then that in the traditional packed-bed reactor (χco =62.5%) and well above the equilibrium limit (χco =65%) as well.