Nanocomposite MFI Zeolite Membranes: Synthesis And Application To Molecular Sieving Separation And Extractor-type Catalytic Membrane Reactor

Membrane technology has been forseen for long as a better option for carrying out gas se-paration and catalytic reaction in the same operating unit, which improves the catalytic per-formance by shifting the thermodynamic equilibrium. Unlike polymeric membranes, inorgan-ic membranes have gained increasing interest in the last years for such applications due to their higher resistance to high temperatures, high pressures and harsh environments. Among the inorganic membranes, zeolite membranes, with well-defined microporous frameworks, constitute a promising option for carrying out better separations relying on adsorption and diffusion difference and molecular sieving. The first and foremost challenge of membrane technology is how to develop reproducible synthesis protocols providing high-quality mem-branes in terms of higher gas permeances and higher separation factors.This thesis focuses on the preparation of nanocomposite MFI zeolite membranes via pore-plugging hydrothermal synthesis and their applications for separation of hydrocarbon isomers and m-xylene isomeri-zation.In this study, hollow fibres offer the great advantage of higher suface-to-volume ratios compared to conventional tubular supports. The synthesis of MFI-alumina hollow fibre mem-brane, however, is more intricate than the tubular membrane. As a matter of fact, the reprodu-cibility of the synthesis protocols for MFI zeolite membrane can be greatly affected by the support quality of hollow fiber. To overcome this shortcoming, prior to synthesis, the hollow fibres have been subjected to a quality test using a gas-liquid displacement technique to cha-racterize their pore size distributions. Only the supports with the maximum pore sizes lower than 0.6μm have been found to allow efficient pore plugging after synthesis, inferred from their CO2/N2 and p-xylene/m-xylene separation performance, in which the best zeolite mem-brane prepared on hollow fiber support with pore sizes centred at 0.2μm has the separation factors of CO2/N2 of 8 and PX/MX up to 100.Nanocomposite B-MFI hollow fibre membranes have also been successfully synthesized in one pot by isomorphous substitution of Si with B during hydrothermal synthesis, boron being localized exclusively in the MFI framework. B-MFI hollow fibre membranes show improved separation performance of the separation factors of PX/MX of 110 and n-hexane/2,2-dimethylbutane of 200 and low permeance, compared to MFI-alumina counterparts on the vapour separation of xylene and n-hexane isomers. When nanocomposite B-MFI hollow fibre membrane is used for xylene separation, the temperature of the maximum separation factor is shifted to lower temperatures (473K), and the membrane keeps its better separation factor at higher feed xylene pressures (4.3kPa). Pre-adsorption of n-hexane or 1,3,5-TMB has little effect on ternary xylene separation, suggesting absence of the intercrystalline defects in the MFI material. A modelling study on n-hexane/2,2-dimethylbutane separation reflects that hexane separation within both MFI and B-MFI-alumina membranes is mainly driven by dif-fusion differences and configurational entropy effects. The pure n-C6 permeation performance can be described using the Maxwell-Stefan formalism with preferential diffusion of n-C6, in which the computed Maxwell-Stefan diffusivities at zero coverage for n-C6 show values about 2 orders of magnitude higher than those measured for 2,2-DMB.Highly reproducible MFI tubular membranes with nanocomposite architecture have been synthesized via the so-called pore-plugging method. The tubular zeolite membrane, showing optimal p-xylene/m-xylene separation factor of 55, has been packed with a Pt-HZSM-5 cata-lyst and used as extractor-type catalytic membrane reactor for m-xylene isomerization. Effects of the location of catalyst packing, reaction temperature, gas hourly space velocity, reaction time and reactor configuration on the performance of membrane reactor was investigated. The membrane reactor displayed a maximum p-xylene yield of ca.28% and a maximum selectivi-ty of ca.42% at 523K when computed at combined mode, showing an increase about 10% compared to an equivalent fixed-bed reactor. Higher catalytic performance was obtained when the catalyst packing was close to the inner top layer of the membrane support. The gas hourly space velocity and reaction time exert only a slight effect on the catalytic performance.