Literature review has been performed in order to identify recent publications on hydrogen separation methods, hydrogen solubility, materials and concepts in research institutes and laboratories. The aim of the performed literature survey was to monitor recent worldwide literature and find out whether some of the developed and reported solutions might possibly help to improve existing hydrogen separation concept in PDh system, enabling efficient complete separation of hydrogen from all unwanted hydrocarbons.
Literature survey on hydrogen separation technique
Basically there are four important methods applied to the separation of gases in the industry: absorption, adsorption, cryogenic and membranes.
Pressure swing adsorption (PSA) is a gas purification process consisting of the removal of impurities on adsorbent beds. The usual adsorbents and gases adsorbed are molecular sieves for carbon monoxide, activated carbon for CO2, activated alumina or silica gel. Industrial PSA plants consist of up to 12 adsorbers and along with the number of valves required this makes the system rather complicated and complex. The PSA process is usually a repeating sequence of the following steps: adsorption at feed pressure, co-current depressurisation to intermediate pressure, counter-current depressurisation to atmospheric pressure usually starting at 10 % to 70 % of the feed pressure, counter-current purge with hydrogen enriched or product gas at ambient pressure, co-current pressure equalisation and finally, co-current pressurisation with feed or secondary process gas. For hydrogen purification by PSA hydrogen purity is high but the amount of rejected hydrogen is also relatively high (10 – 35 %). It seems also that cryogenic technology might not be applicable for PDh process gas separation. Cooling down the mixture will finally end in a solid jet fuel and a gas phase. Handling the solid is more difficult when compared with liquid. During the survey it became evident that membrane technology is the most popular, used and still investigating for the improvement process for hydrogen separation therefore the focus of the study is mainly on this technique.
The membrane separation process involves several elementary steps, which include the solution of hydrogen and its diffusion as atomic hydrogen through the membrane bulk material. Nowadays, membrane technologies are becoming more frequently used for separation of wide varying mixtures in the petrochemical related industries. According to Sutherland it is estimated that bulk chemicals and petrochemicals applications represented about 40% of the membrane market in the whole chemicals industry or about $ 1.5 billions, growing over 5 % per year. Membrane gas separation is attractive because of its simplicity and low energy cost.
The advantages of using membrane gas separation technologies could be summarized as following:
- Continuous and clean process, membranes do not require regeneration, unlike the adsorption or the absorption processes, which require regeneration step leading to the use of two solid beds or a solvent regeneration unit. Required filtration system is simple and inexpensive.
- Compared with conventional techniques, membranes can offer a simple, easy-to-operate, low-maintenance process.
- Membrane process is simple, generally carried out at atmospheric conditions which, besides being energy efficient, can be important for sensitive applications in pharmaceutical and food industry.
- The recovery of components from a main stream using membranes can be done without substantial additional energy costs.
Membrane is defined essentially as a barrier, which separates two phases and restricts transport of various chemicals in a selective manner. A membrane can be homogenous or heterogeneous, symmetric or asymmetric in structure, solid or liquid; can carry a positive or negative charge or be neutral or bipolar. Transport through a membrane can be affected by convection or by diffusion of individual molecules, induced by an electric field or concentration, pressure or temperature gradient. It takes place when a driving force is applied to the components in the feed. In most of the membrane processes, the driving force is a pressure difference or a concentration (or activity) difference across the membrane. Another driving force in membrane separations is the electrical potential difference. This driving force only influences the transport of charged particles or molecules.
The hydrogen separation factor is sometimes used to specify membrane quality. It is defined as following:
where ni stands for moles of species i transferred through the membrane and ?pi stands for the partial pressure difference of species i through the membrane.
The membrane thickness may vary from as small as 10 microns to few hundred micrometers. Basic types of membranes are presented in Figure 4.
Membranes in petrochemical industry are mainly used for concentration, purification and fractionation however they may be coupled to a chemical reaction to shift the chemical equilibrium in a combination defined as a membrane reactor. Using a membrane is adding costs to any process, therefore in order to overcome the cost issue another advantages must overcome the added expenses like material with a very good separation factor, high flux, high quality membrane materials (stable during many months of operation). In a membrane separation reactor both organic and inorganic membranes can be used. Many industrial catalytic processes involve the combination of high temperature and chemically harsh environments favouring therefore inorganic membranes due to their thermal stability, resistance to organic solvents, chlorine and other chemicals. Some promising applications using inorganic membranes include certain dehydrogenation, hydrogenation and oxidation reactions like formation of butane from dehydrogenation of ethyl benzene, styrene production from dehydrogenation of ethyl benzene, dehydrogenation of ethane to ethane, oxidative coupling of methane etc. In membrane reactor two basic concepts can be distinguished as can be seen in Figure 5.
- reaction and separation combined in one reactor (catalytic membrane reactor)
- reaction and separation are not combined and the reactants are recycled along a membrane system (membrane recycle reactor)
Catalytic membrane reactor concept is used especially with inorganic membranes (ceramics, metals) and polymeric membranes where the catalyst is coupled to the membrane. Membrane recycle reactor can be applied with any membrane process and type of membranes. Most of the chemical reactions need catalyst to enhance the reaction kinetics. The catalyst must be combined with the membrane system and various arrangements are possible, as can be seen in Figure 6. The advantage of the catalyst located inside the bore of the tube is simplicity in preparation and operation. When needed the catalyst could be easily replaced. In case of top layer filled with catalyst and membrane wall, the catalyst is immobilized onto the membrane.
Palladium has been known to be a highly hydrogen permeable and selective material since the 19th century. The existing Pd-based membranes can be mainly classified into two types according to the structure of the membrane as (i) self-supporting Pd-based membranes and (ii) composite structures composed of thin Pd-based layers on porous materials. Most self-supporting Pd-based membranes are commercially available in the forms that are easily integrated into a separation setup. However these membranes are relatively thick (50 mm or more) and therefore the hydrogen flux through them is limited. Thick palladium membranes are expensive and rather suitable for use in large scale chemical production. For practical use it is necessary to develop separation units with reduced thickness of the layer. An additional problem is that in order to have adequate mechanical strength, relatively thick porous supports have to be used. In the last decade a significant research has been carried out to achieve higher fluxes by depositing thin layers of Pd or Pd alloys on porous supports like ceramics or stainless steel. A submicron thick and defect-free palladium-silver (Pd-Ag) alloy membrane was fabricated on a supportive microsieve by using microfabrication technique and tested by Tong et al. The technique also allowed production of a robust wafer-scale membrane module which could be easily inserted into a membrane holder to have gas-tight connections to outside. Fabricated membrane had a great potential for hydrogen purification and in application like dehydrogenation industry. One membrane module was investigated for a period of ca. 1000 hours during which the membrane experienced a change in gas type and its concentration as well as temperature cycling between 20 – 450 °C. The measured results showed no significant reduction in flux or selectivity, suggesting thus very good membrane stability. The authors carried out experiments with varying hydrogen concentration in the feed from 18 to 83 kPa at 450 °C to determine the steps limiting H2 transport rate. It is assumed that the fabricated membrane may be used as a membrane reactor for dehydrogenation reactions to synthesize high value products although its use may be limited due to high pressures of tens of bars. Schematic drawing of the hydrogen separation setup is presented in Figure 7. The membrane module was placed in a stainless steel holder installed in a temperature controlled oven to ensure isothermal operation. The H2/He feed (from 300 to 100 ml/mol) was preheated in spirals placed in the same oven. The setup was running automatically for 24 h/day and could handle 100 recipes without user intervention.
Tucho et al. performed microstructural studies of self-supported Pd / 23 wt. % Ag hydrogen separation membranes subjected to different heat treatments (300/400/450 °C for 4 days) and then tested for hydrogen permeation. It was noted that changes in permeability were dependent on the treatment atmosphere and temperature as well as membrane thickness. At higher temperatures significant grain growth was observed and stress relaxation occurred. Nam et al. were able to fabricate a highly stable palladium alloy composite membrane for hydrogen separation on a porous stainless steel support by the vacuum electrodeposition and laminating procedure. The membrane was manufactured without microstructural change therefore it was possible to obtain both high performance (above 3 months of operation) and physical and morphological stability of the membrane. It was observed that the composite membrane had a capability to separate hydrogen from gas mixture with complete hydrogen selectivity and could be used to produce ultra-pure hydrogen for applications in membrane reactor. Tanaka et al. aimed at the improved thermal stability of mesoporous Pd-YSZ-g-Al2O3 composite membrane. The improved thermal stability allowed operation at elevated temperature (> 500 °C for 200 hours). This was probably the result of improved fracture toughness of YSZ-g-Al2O3 layer and matching thermal expansion coefficient between palladium and YSZ. Kuraoka, Zhao and Yazawa demonstrated that pore-filled palladium glass composite membranes for hydrogen separation prepared by electroless plating technique have both higher hydrogen permeance, and better mechanical properties than unsupported Pd films. The same technique was applied by Paglieri et al. for plating a layer of Pd and then copper onto porous ?-substrate. Zahedi et al. developed a thin palladium membrane by depositing Pd onto a tungsten oxide WO3 modified porous stainless steel disc and reported that permeability measurements at 723, 773 and 823 K showed high permeability and selectivity for hydrogen. The membrane was stable with regards to hydrogen for about 25 days. Certain effort has been performed for improving hydrothermal stability and application to hydrogen separation membranes at high temperatures. Igi et al. prepared a hydrogen separation microporous membranes with enhanced hydrothermal stability at 500 °C under a steam pressure of 300 kPa. Co-doped silica sol solutions with varying Co composition (Co / (Si + Co) from 10 to 50 mol. %) were prepared and used for manufacturing the membranes. The membranes showed increased hydrothermal stability and high selectivity and permeability towards hydrogen when compared with pure silica membranes. The Co-doped silica membranes with a Co composition of 33 mol. % showed the highest selectivity for hydrogen, with a H2 permeance of 4.00 x 10-6 (m3 (STP) × (m × s × kPa)-1) and a H2/N2 permeance ratio of 730. It was observed that as the Co composition increased as high as 33 %, the activation energy of hydrogen permeation decreased and the H2 permeance increased. Additional increase in Co concentration resulted in increased H2 activation energy and decreased H2 permeance. Due to high permselectivity of Pd membranes, high purity of hydrogen can be obtained directly from hydrogen containing mixture at high temperatures without further purification providing if sufficient pressure gradient is applied. Therefore it is possible to integrate the reforming reaction and the separation step in a single unit. A membrane reformer system is simpler, more compact and more efficient than the conventional PSA system (Pressure Swing Adsorption) because stem reforming reaction of hydrocarbon fuels and hydrogen separation process take place in a single reactor simultaneously and without a separate shift converter and a purification system. Gepert et al. have aimed at development of heat-integrated compact membrane reformer for decentralized hydrogen production and worked on composite ceramic capillaries (made of ?-Al2O3) coated with thin palladium membranes for production of CO-free hydrogen for PEM fuel cells by alcohol reforming. The membranes were tested for pure hydrogen and N2 as well as for synthetic reformate gas. The process steps comprised the evaporation and overheating of the water/alcohol feed, water gas shift combined with highly selective hydrogen separation. The authors have focused on the step concerned with the membrane separation of hydrogen from the reforming mixture and on the challenges and requirements of that process. The challenges encountered with the development of capillary Pd membranes were as following: long term temperature and pressure cycling stability in a reformate gas atmosphere, the ability to withstand frequent heating up and cooling down to room temperature, avoidance of the formation of pin-holes during operation and the integration of the membranes into reactor housing. It was observed that palladium membranes should not be operated at temperatures below 300 °C and pressures lower than 20 bar, while the upper operating range is between 500 and 900 °C. Alloying the membrane with copper and silver extend their operating temperature down to a room temperature. The introduction of silver into palladium membrane increases the lifetime, but also the costs when compared with copper. Detailed procedure of membrane manufacturing, integration into reformer unit and testing is described by the authors. Schematic of the concept of the integrated reformer is shown in Figure 8. The membrane was integrated in a metal tube embedded in electrically heated copper plates. Before entering the test tube, the gases were preheated to avoid local cooling of the membrane. Single gas measurements with pure N2 and H2 allowed the testing of the general performance of the membrane and the permselectivity for the respective gases to be reached. Synthetic reformate gas consisting of 75 % H2, 23.5 % CO2 and 1.5 % CO was used to get information about the performance. The membranes were tested between 370 – 450 °C and pressures up to 8 bar. The authors concluded that in general the membranes have shown good performance in terms of permeance and permselectivity including operation under reformate gas conditions. However, several problems were indicated concerning long-term stability under real reforming conditions, mainly related to structural nature (combination of different materials: ceramic, glaze, palladium resulted on incoherent potential for causing membrane failure). At operation times up to four weeks the continuous Pd layer remained essentially free from defects and pinholes.
Han et al. have developed a membrane separation module for a power equivalent of 10 kWel. A palladium membrane containing 40 wt. % copper and of 25 mm thickness was bonded into a metal frame. The separation module for a capacity of 10 Nm3 h-1 of hydrogen had a diameter of 10.8 cm and a length of 56 cm. Reformate fed to the modules contained 65 vol. % of hydrogen and the hydrogen recovery through the membrane was in the range of 75 %. Stable operation of the membrane separation was achieved for 750 pressure swing tests at 350 °C. The membrane separation device was integrated into a methanol fuel processor. Pientka et al. have utilized a closed-cell polystyrene foam (Ursa XPS NIII, porosity 97 %) as a membrane buffer for separation of (bio)hydrogen. In the foam the cell walls formed a structured complex of membranes. The cells served as pressure containers of separated gases. The foam membrane was able to buffer the difference between the feed injection rate and the rate of consumption of the product. Using the difference in time-lags of different gases in polymeric foam, efficient gas separation was achieved during transient state and high purity hydrogen was obtained. Argonne National Laboratory (ANL) is involved in developing dense hydrogen-permeable membranes for separating hydrogen from mixed gases, particularly product streams during coal gasification and/or methane reforming. Novel cermet (ceramic-metal composite) membranes have been developed. Hydrogen separation with these membranes is non-galvanic (does not use electrodes or external power supply to drive the separation and hydrogen selectivity is nearly 100 % because the membrane contain no interconnected porosity). The membrane development at ANL initially concentrated on a mixed proton/electron conductor based on BaCe0.8Y0.2O3-d (BCY), but it turned to be insufficient to allow high non-galvanic hydrogen flux. To increase the electronic conductivity and thereby to increase the hydrogen flux the development focused on various cermet membranes with 40-50 vol. % of metal or alloy dispersed in the ceramic matrix. Balachandran et al., described the development performed at ANL. The powder mixture for fabricating cermet membranes was prepared by mechanical mixing Pd (50 vol. %) with YSZ, after that the powder mixture was pressed into discs. Polished cermet membranes were affixed to one end of alumina tube using a gold casket for a seal (as can be seen in Figure 9). In order to measure the hydrogen permeation rate, the alumina tube was inserted into a furnace with a sealed membrane and the associated gas flow tubes.
Hydrogen permeation rate for Pd/YSZ membranes has been measured as a function of temperature (500-900 °C), partial pressure of hydrogen in the feed stream (0.04-1.0 atm.) and membrane thickness (» 22-210 mm) as well as versus time during exposure to feed gases containing H2, CO, CO2, CH4 and H2S. The highest hydrogen flux was » 20.0 cm3 (STP)/min cm2 for » 22- mm thick membrane at 900 °C using 100 % hydrogen as the feed gas. These results suggested that membranes with thickness < 22 mm should give a higher hydrogen flux. The hydrogen flux of Pd/YSZ membranes showed no degradation in a simulated synthesis gas atmosphere and was stable for » 270 hours in atmospheres containing up to ca. 400 ppm of hydrogen sulphide. Hardy et al.have focused on developing cermet membranes made from nanoscale precursor powders for which both barium cerate-based proton conducting ceramic and the nickel oxide were co-sythesized in a single combustion reaction. Among the barium cerate composition investigated, the 30 % Zr and 15 % Nd-doped material exhibited the best combination of chemical stability in CO2 and conductivity in hydrogen environments.
In the last decade Matrimid 5218 (Polyimide of 3,3′,4,4′-benzophenone tetracarboxylic dianhydride and diamino-phenylindane) has attracted a lot of attention as a material for gas separation membranes due to the combination of relatively high gas permeability coefficients and separation factors combined with excellent mechanical properties, solubility in non-hazard organic solvents and commercial availability. Shishatskiy et al. have developed asymmetric flat sheet membranes for hydrogen separation from its mixtures with other gases. The composition and conditions of membrane preparation were optimized for pilot scale membrane production. The resulting membrane had a high hydrogen flux (1 m3 (STP)/m2h*bar) and selectivity of H2/CH4 at least 100, close to the selectivity of Matrimid 5218, material used for asymmetric structure formation. The hydrogen flux through the membranes increased with the decrease of polymer concentration and increase of non-solvent concentration. In addition, the influence of N2 blowing over the membrane surface (0, 2, 3, 4 Nm3 h-1 flow rate) was studied and it was proved that the selectivity of the membrane decreased with increase of the gas flow. The SEM image of the membrane supported by Matrimid 5218 is shown in Figure 10.
The stability against hydrocarbons was tested by immersion of the membrane into the mixture of n-pentane/n-hexane/toluene in 1:1:1 ratio. Stability tests showed that the developed membrane was stable against mixtures of liquid hydrocarbons and could withstand continuous heating up to 200 °C for 24 and 120 hours and did not lose gas separation properties after exposure to a mixture of liquid hydrocarbons. The polyester non-woven fabric used as a support for the asymmetric membrane gave to the membrane excellent mechanical properties and allowed to use the membrane in gas separation modules.
Interesting report on development of compact hydrogen separation module called MOC (Membrane On Catalyst) with structured Ni-based catalyst for use in the membrane reactor was presented by Kurokawa et al. In the MOC concept a porous support itself had a function of reforming catalyst in addition to the role of membrane support. The integrated structure of support and catalyst made the membrane reformer more compact because the separate catalysts placed around the membrane modules in the conventional membrane reformers could be eliminated. In that idea first a porous catalytic structure 8YSZ (mixture of NiO and 8 mol. % Y2O3-ZrO2 at the weight ratio 60:40) was prepared as the support structure of the hydrogen membrane. The mixture was pressed into a tube closed at one end and sintered then in air. Slurry of 8YSZ was coated on the external surface of the porous support and heat-treated for alloying. Obtained module of size 10 mm outside and 8 mm inside diameter, 100 ~ 300 mm length and the membrane thickness was 7 ~ 20 mm were heated in flowing hydrogen at 600 °C for 3 hours to reduce NiO in the support structure into Ni before use (the porosity of the support after reduction was 43 %). A stainless steel cap and pipe were bonded to the module to introduce H2 into the inside of the tubular module. Figure 11 presents the conceptual structure design of the MOC module as compared with the structure of the conventional membrane reformer.
The sample module in the reaction chamber was placed in the furnace and heated at 600 °C, pre-heated hydrogen (or humidified methane) was supplied inside MOC at the pressure of 0.1 MPa and the permeated hydrogen was collected from the outside chamber around the module at ambient pressure. The 100 ~ 300 mm long modules with 10 mm membrane showed hydrogen flux of 30 cm3 per minute per cm2 which was two times higher than the permeability of the conventional modules with palladium based alloy films. Membrane On Catalyst modules have a great potential to be applied to membrane reformer systems. In this concept a porous support itself has a function of reforming catalyst in addition to the role of membrane support. It seems that Membrane On Catalyst modules have a great potential to be applied to membrane reformer systems.
Amorphous alloy membranes composed primarily of Ni and early transition metals (ETM) are an inexpensive alternative to Pd-based alloy membranes, and these materials are therefore of particular interest for the large-scale production of hydrogen from carbon-based fuels. Catalytic membrane reactors can produce hydrogen directly from coal-derived synthesis gas at 400°C, by combining a commercial water-gas shift (WGS) catalyst with a hydrogen-selective membrane. Three main classes of membrane are capable of operating at the high temperatures demanded by existing WGS catalysts: ceramic membranes producing pure hydrogen via ion-transfer mechanism at ³ 600 °C, alloy membranes which produce pure hydrogen via a solution-diffusion mechanism between 300 – 500 °C and microporous membranes, typically silica or carbon, whose purity depends on the pore size of the membrane and which operate over a wide temperature range dependent on the membrane material. In order to explore the suitability of Ni-based amorphous alloys for this application, the thermal stability and hydrogen permeation characteristics of Ni-ETM amorphous alloy membranes has been examined by Dolan et al. Fundamental limitation of these materials is that hydrogen permeability is inversely proportional to the thermal stability of the alloy. Alloy design is therefore a compromise between hydrogen production rate and durability. Amorphous Ni60Nb(40-x)Zr(x) membranes have been tested at 400°C in pure hydrogen, and in simulated coal-derived gas streams with high steam, CO and CO2 levels, without severe degradation or corrosion-induced failure. The authors have concluded that Ni-Nb-Zr amorphous alloys are therefore prospective materials for use in a catalytic membrane reactor for coal-derived syngas. Much attention has been given to inorganic materials such as zeolite, silica, zirconia and titania for development of gas- and liquid- separation membranes because they can be utilized under harsh conditions where organic polymer membranes cannot be applied. Silica membranes have been studied extensively for the preparation of various kinds of separation membranes: hydrogen, CO2 and C3 isomers.
Kanezeashi have proposed silica networks using an organo-inorganic hybrid alkoxide structure containing the organic groups between two silicon atoms, such as bis(triethoxysilyl)ethane (BTESE) for development of highly permeable hydrogen separation membranes with hydrothermal stability. The concept for improvement of hydrogen permeability of silica membrane was to design a loose-organic-inorganic hybrid silica network using mentioned BTESE (to shift the silica networks to a larger pore size for an increase in H2 permeability). A hybrid silica layer was prepared by coating a silica-zirconia intermediate layer with a BTESE polymer sol followed by drying and calcination at 300°C in nitrogen. A thin, continuous separation layer of hybrid silica for selective H2 permeation was observed on top of the SiO2-ZrO2 intermediate layer as presented in Figure 12. Hybrid silica membranes showed a very high H2 permeance, ~ 1 order of magnitude higher (~ 10-5 mol m-2 s-1 Pa-1) than previously reported silica membranes using TEOS (Tetraethoxysilane). The hydrothermal stability of the hybrid silica membranes due to the presence of Si-C-C-Si bonds in the silica networks was also confirmed.
Nitodas et al. for the development of composite silica membranes have used the method of chemical vapour deposition (CVD) in the counter current configuration from TEOS and ozone mixtures. The experiments were conducted in a horizontal hot-wall CVD quartz reactor (Figure 13) under controlled temperature conditions (523 – 543 K) and at various reaction times (0 -15 hours) and differential pressures across the substrate sides using two types of substrates: a porous Vycor tube and alumina (g-Al2O3) nanofiltration (NF) tube. The permeance of hydrogen and other gases (He, N2, Ar, CO2) were measured in a home-made apparatus (able to operate under high vacuum conditions 10-3 Torr, feed pressure up to 70 bar) and the separation capability of the composite membranes was determined by calculating the selectivity of hydrogen over He, N2, Ar, CO2. The in-situ monitoring of gas permeance during the CVD development of nanoporous membranes created a tool to detect pore size alterations in the micro to nanometer scale of thickness. The highest permeance values in both modified and unmodified membranes are observed for H2 and the lowest for CO2. This indicated that the developed membranes were ideal candidates for H2/CO2 separations, like for example in reforming units of natural gas and biogas (H2/CO2/CO/CH4).
Moon et al. have studied the separation characteristics and dynamics of hydrogen mixture produced from natural gas reformer on tubular type methyltriethoxysilane (MTES) silica / ?-alumina composite membranes. The permeation and separation of CO pure gas, H2/CO (50/50 vol. %) binary mixture and H2/CH4/CO/CO2 (69/3/2/26 vol. %) quaternary mixture was investigated. The authors developed a membrane process suitable for separating H2 from CO and other reformate gases (CO2 or CH4) that showed a molecular sieving effect. Since the permeance of pure CO on the MTES membrane was very low (CO » 4.79 – 6.46 x 10-11 mol m-2 s-1 Pa-1), comparatively high hydrogen selectivity could be obtained from the H2/CO mixture (separation factor: 93 – 110). This meant that CO (which shall be eliminated before entering fuel cell) can be separated from hydrogen mixtures using MTES membranes. The permeance of the hydrogen quaternary mixture on MTES membrane was 2.07 – 3.37 x 10-9 mol m-2 s-1 Pa-1 and the separation factor of H2 / (CO + CH4 + CO2) was 2.61 – 10.33 at 323 – 473 K (Figure 14). The permeation and selectivity of hydrogen were increased with temperature because of activation of H2 molecules and unfavourable conditions for CO2 adsorption. Compared to other impurities, CO was most successfully removed from the H2 mixture.
The MTES membranes showed great potential for hydrogen separation from reforming gas with high selectivity and high permeance and therefore they have good potential for fuel cell systems and for use in hydrogen stations. According to the authors, the silica membranes are expected to be used for separating hydrogen in reforming environment at high temperatures.
Silica membranes prepared by the CVD or sol-gel methods on mesoporous support are effective for selective hydrogen permeation, however it is known that hydrogen-selective silica materials are not thermally stable at high temperatures. Most researchers reported a loss of permeability of silica membranes even 50 % or greater in the first 12 hours on exposure to moisture at high temperature. Much effort has been spent on the improvement of the stability of silica membranes. Gu et al. have investigated a hydrothermally stable and hydrogen-selective membrane composed of silica and alumina prepared on a macroporous alumina support by CVD in an inert atmosphere at high temperature. Before the deposition of the silica-alumina composite multiple graded layers of alumina were coated on the alumina support with three sols of decreasing particle sizes. The resulting supported composite silica-alumina membrane had high permeability for hydrogen (in the order of 10-7 mol m-2 s-1 Pa-1) at 873 K. Significantly the composite membrane exhibited much higher stability to water vapour at the high temperature of 873 K in comparison to pure silica membranes. The introduction of alumina into silica made the silica structure more stable and slowed down the silica disintegration process. As mentioned, silica membranes produced by sol-gel technique or by CVD applied for gas separation, especially for H2 production are quite stable in dry gases and exhibit high separation ratio, but lose the permeability when used in the steamed gases because of sintering or tightening. Thi