Hydrothermal stability is one of the most important technical challenges for hydrogen separation silica membranes. Metal doping into silica matrix was proposed, and it was found that Ni and Co were effective to improve the hydrothermal stability. Co-doped silica membranes showed a high permeance of 1.8 × 10−7 mol/(m2 · s · Pa) and H2/N2 permeance ratio of 730 with hydrothermal stability (500 °C, steam partial pressure 300 kPa). Using hydrothermally stable silica membranes, permeation properties of helium, hydrogen and water vapor were examined based on the activation energy of permeation. The activation energy of H2 permeation correlated well with the permeance ratio of He/H2. The permeance ratios of H2/H2O of silica membranes are always larger than unity, although the kinetic diameter of hydrogen (0.289 nm) is larger than that of water (0.265 nm). Silica membranes were applied to bimodal catalytic membranes, which consisted of microporous silica top layer for selective permeation of hydrogen and a bimodal catalytic support layer where catalysts such as Ni were impregnated inside a bimodal support consisting of mesopores (γ-alumina) and macropores (α-alumina). Increased performance for production of hydrogen was confirmed by catalytic membranes for steam reforming of methane.
This review describes recent developments in the field of membrane reactors for obtaining high-purity hydrogen from organic chemical hydrides using hydrogen-selective amorphous silica membranes prepared by chemical vapor deposition. The application of such membrane reactors for hydrogen production reactions enables us to achieve higher conversion because of equilibrium shifts and to obtain high-purity hydrogen in one step, thus accomplishing an effective hydrogen supply. The most important issue in developing membrane reactors is the selection and preparation of appropriate hydrogen-selective silica membranes for each dehydrogenation reaction. Initially, the pore-size control method was developed by changing chemical structures of silica precursors in the chemical vapor deposition step. Using these membranes, membrane reactors for dehydrogenating cyclohexane or methylcyclohexane were developed and equilibrium shifts were achieved under various reaction conditions. High-purity hydrogen (> 99.9 %) was stably attained by operating the membrane reactors with neither carrier gas nor sweep gas.
The major mechanisms of gas permeation through solid membranes are described and the applicable equations describing the permeance are presented. The mechanisms depend on the relative size of the permeating molecules and the diameter of the pores. As pore size decreases the operable mechanisms are Hagen-Pouiselle flow, Knudsen diffusion, surface diffusion, gas-translation, and finally solid-state diffusion. The Hagen-Pouiselle mechanism involves flow through large pores, while the Knudsen mechanism involves collision of molecules with the walls of pores of intermediate size. Surface diffusion deals with movement of molecules trapped in the potential field of the walls of pores of relatively small size, while gas-translation involves molecules that can escape the field, but are constrained by the small pores. Finally, solid-state transport comprises dissolution and transport by diffusion within the solid. These mechanisms are illustrated for hydrogen permeance with the use of two membranes, an alumina membrane with intermediate sized pores and a silica on alumina membrane of dense structure.
Integration of catalytic reaction and separation operations is important for process intensification (PI). Experimental studies on promotion of the dehydrogenation reaction using hydrogen adsorbent during catalytic dehydrogenation reaction were reviewed. The promotion effect was lost after saturation of the adsorbent, so that regeneration under reduced pressure was essential to start the reaction again. The developed reactor was the so-called pressure swing reactor (PSR) or pressure swing adsorption type reactor (PSA reactor) after the cyclic operations of pressurization and evacuation. Dehydrogenation of cyclohexane, and dehydroaromatization of n-hexane, propane and methane were investigated. Pt/Al2O3–CaNi5, Zn/H-ZSM-5–Mg51Zn20, Zn/H-ZSM-5–Ti fine powder, and Mo/H-ZSM-5–Ti fine powder catalysts showed high promotion effects. Because of the differences in reaction temperature, the recommended combination of catalyst and adsorbent varies depending on the reaction. Swing of partial pressure of hydrogen by inert purge gas regenerated the promotion effect showing the potential of the PSR for PI.
An efficient catalyst is desired to produce lighter isoparaffins from heavy n-paraffins by isomerization and hydrocracking, which are environmentally friendly processes for producing high-octane fuel. This paper assesses the suitability of a catalyst consisting of beta zeolite and nanosized (i.e., 5-50 nm; hereafter ns) alumina particles for isomerization and hydrocracking of large n-paraffins. The catalytic performances of three catalysts, namely an alumina-supported metal catalyst (NiMo/γ-Al2O3), a two-component catalyst (ns Al2O3–beta zeolite), and a three-component catalyst (NiMo/γ-Al2O3–ns Al2O3–beta zeolite) for an isothermal reaction with n-hexadecane were evaluated. The results reveal that the conversion and selectivity are improved by increasing the number of components in a catalyst. Specifically, the three-component catalyst exhibits superior catalytic performance between 225 and 350 °C due to concerted effect of the three components. We investigated the effect of the SiO2/Al2O3 molar ratio of beta zeolite on the performance of three-component catalysts and found that beta zeolite with a SiO2/Al2O3 molar ratio of 25 has a higher activity and cracking selectivity with high isoparaffin selectivity of the cracked products (hereafter iso-selectivity) than three-component catalysts with SiO2/Al2O3 molar ratios of 16, 50, and 150; this indicates that there is an optimum SiO2/Al2O3 molar ratio of zeolite in the three-component catalyst.The three-component catalyst composed of NiMo/γ-Al2O3, ns Al2O3, and dealuminated beta zeolite had a high conversion and iso-selectivity. Its catalytic performance is more suitable for producing isoparaffins of gasoline fraction than these of three three-component catalysts composed of ns Al2O3 and non-dealuminated beta zeolite, ns SiO2 and non-dealuminated beta zeolite, or ns SiO2 and dealuminated beta zeolite. Acid treating was found to remove extra-framework aluminum and amorphous alumina from the beta zeolite surface to make Si–OH. It produced reformed acid sites between the Si–OH of the external zeolite surface and ns Al2O3 surfaces, which consequently improved the isomerization and cracking activities due to the acid sites existing at nanopores.
Triggering oxidative steam reforming of ethanol was examined at low temperature over different supported Ni catalysts. Ni/Ce0.5Zr0.5O2, Ni/CeO2, and Ni/MgAl2O4 reduced at 800 °C enabled triggering the reaction at 100 °C without external energy input. This behavior was rationalized by internal heat supply by heat evolution from oxidation of Ce3+ and metallic Ni0. The generated heat increased catalyst bed temperature up to autoignition temperature and thus the reaction was initiated. Especially, Ni/Ce0.5Zr0.5O2 triggered the reaction repeatedly without H2 treatment from the second cycle. This is related to superior redox property and low autoignition temperature of ethanol oxidative steam reforming over Ni/Ce0.5Zr0.5O2 comparing with other supported Ni catalysts.
The effect of adding Mo in the low K range of composition of Fe–Ce–K and Fe–Ce–Mo–K mixed-oxide catalysts on dehydrogenation of ethylbenzene was studied. Fe–Ce–K formed considerable amounts of by-products in the low K range. Addition of Mo not only made the catalyst inactive in the low K range, but also prevented the formation of by-products because adding Mo decreased the surface area considerably in the low K range. X-ray diffraction (XRD) showed Mo formed three molybdates (K2MoO4, KCe(MoO4)2, and Fe2(MoO4)3), which were inactive for the reaction. Therefore, the effect of adding Mo resulted from the formation of molybdates on the surface, which covered the active sites for by-product formation.
During the preparation of Cu–Zn–O catalyst, addition of small amount of Polyethylene Glycol (PEG) could increase the BET surface area and make Cu crystalline more finely dispersed, by stopping the sintering or agglomeration of the catalyst precursors. The PEG-added catalysts exhibited the enhanced activity for the low-temperature methanol synthesis from CO2-containing syngas, compared to their conventional analogue.