The crystalline nature of zeolites offers the opportunity to obtain membranes with regular tridimensional network at molecular scale and for this reason are able to separate mixture of substances on the basis of different molecular size an d shape, such as for example isomers [ ], and also on the basis of different adsorption properties [ ]. Besides, for their thermal and chemical stability are also used in membrane reactors [ ]. For example, a MFI zeolite membrane is used in the catalytic de-hydrogenation of i-butane and the experimental results were described by [ , ]. However, zeolite membrane application at industrial level is strongly limited by cost of the support (e.g ceramic support is responsible for at least 70 % of the zeolite membrane price [ ]) and the reproducibility problem. Today, industrial applications are only relative to LTA zeolite membranes for organic solvent dehydration by means of pervaporation and vapour permeation processes [ ]. Zeolite membrane are deposited on support (ceramic and stainless steel) because self-standing zeolite layer are very fragile. The most used support material is the alumina probably owing to the availability of high quality micro-, nano- and ultra-filtration support and for their smooth top surface. In fact, a smooth surface is an important requirement in order to have continuous zeolite layer. Stainless steel (SS) supports have rougher surface and larger pore and also higher thermal coefficient expansion with respect to ceramic ones [ ]. Therefore, SS supported zeolite membranes are more susceptible to cracks during calcination step carried-out after the hydrothermal synthesis in order to remove the template. An important requirement of any membrane to be industrially used is a high permeability with a high selectivity. However, there are technical problems in the development of supported zeolitic very thin zeolite layer defect free. The traditional one step method [ ] tends to produce almost defect free membranes with high selectivity, but the flux are very low due to the high tortuosity of the membranes because a significant part grows into the pores of the support. The most promising preparation method for zeolite membranes is the secondary growth [ ], mainly aimed to cover the surface of the support with a layer of zeolite seeds. Subsequently, a hydrothermal treatment is carried out on the seeded support to favour the crystal growth. Therefore, decoupling zeolite nucleation from crystal growth, the optimization of the conditions of each step independently is allowed. This method has potential advantages in terms of reproducibility and control of membrane structure if compared with the traditional method [ ].Seeding is a very critical step since it influences the membrane quality. The following seeding procedures : rubbing [ ], dip-coating [ , ] , spin coating [ ] improve the quality of the membranes with respect to the in situ synthesis method but they also present some limitations [ ]. More controllable seeding procedures described in literature involve the filtration [ , ] of a water suspension of zeolite crystals through a porous support . The cross-flow filtration allows to prepare more uniform and compact zeolite layer then dead-end filtration. However, when tubular support is used during the filtration there is the zeolite layer formation only on the bottom part of the support. A new seeding procedure for tubular membranes was designed [16, ] and combines: ocross-flow filtration of a water suspension of zeolite seeds through a porous support; o support tilting with respect to the horizontal plane; osupport rotation along its longitudinal axis. This novel procedure gives the possibility to prepare membranes of good quality and with high reproducibility. Tubular zeolite membranes having different permeation properties were prepared by using the innovative secondary growth method [16, 19] on ceramic supports. The catalytic Pt/Na-Y membranes were obtained by ion-exchange, calcination and reduction. and their catalytic performance for the CO Selox. was tested in a flow-through MR. Experimental results were collected at 200°C, with different feed compositions and pressures (up to 6 bar). CO content was ca. 1%, the feed O2/CO molar ratio was 1-1.5. The catalytic membranes effectively removed CO from 10,000 ppm down to 10-50 ppm operating the single stage MR with O2/CO=1 in the Feed. The membranes with lower permeance succeeded in bringing the CO amount less than 10 ppm even at low reaction pressure, and were not affected by changes in the feed composition and feed flow rate [ ]. The experimental results suggest the practical application of these catalytic MRs into a fuel processor producing CO-free hydrogen.
Preparation and Characterization of Zeolite Membranes
Catia Algieri
2009
Abstract
The crystalline nature of zeolites offers the opportunity to obtain membranes with regular tridimensional network at molecular scale and for this reason are able to separate mixture of substances on the basis of different molecular size an d shape, such as for example isomers [ ], and also on the basis of different adsorption properties [ ]. Besides, for their thermal and chemical stability are also used in membrane reactors [ ]. For example, a MFI zeolite membrane is used in the catalytic de-hydrogenation of i-butane and the experimental results were described by [ , ]. However, zeolite membrane application at industrial level is strongly limited by cost of the support (e.g ceramic support is responsible for at least 70 % of the zeolite membrane price [ ]) and the reproducibility problem. Today, industrial applications are only relative to LTA zeolite membranes for organic solvent dehydration by means of pervaporation and vapour permeation processes [ ]. Zeolite membrane are deposited on support (ceramic and stainless steel) because self-standing zeolite layer are very fragile. The most used support material is the alumina probably owing to the availability of high quality micro-, nano- and ultra-filtration support and for their smooth top surface. In fact, a smooth surface is an important requirement in order to have continuous zeolite layer. Stainless steel (SS) supports have rougher surface and larger pore and also higher thermal coefficient expansion with respect to ceramic ones [ ]. Therefore, SS supported zeolite membranes are more susceptible to cracks during calcination step carried-out after the hydrothermal synthesis in order to remove the template. An important requirement of any membrane to be industrially used is a high permeability with a high selectivity. However, there are technical problems in the development of supported zeolitic very thin zeolite layer defect free. The traditional one step method [ ] tends to produce almost defect free membranes with high selectivity, but the flux are very low due to the high tortuosity of the membranes because a significant part grows into the pores of the support. The most promising preparation method for zeolite membranes is the secondary growth [ ], mainly aimed to cover the surface of the support with a layer of zeolite seeds. Subsequently, a hydrothermal treatment is carried out on the seeded support to favour the crystal growth. Therefore, decoupling zeolite nucleation from crystal growth, the optimization of the conditions of each step independently is allowed. This method has potential advantages in terms of reproducibility and control of membrane structure if compared with the traditional method [ ].Seeding is a very critical step since it influences the membrane quality. The following seeding procedures : rubbing [ ], dip-coating [ , ] , spin coating [ ] improve the quality of the membranes with respect to the in situ synthesis method but they also present some limitations [ ]. More controllable seeding procedures described in literature involve the filtration [ , ] of a water suspension of zeolite crystals through a porous support . The cross-flow filtration allows to prepare more uniform and compact zeolite layer then dead-end filtration. However, when tubular support is used during the filtration there is the zeolite layer formation only on the bottom part of the support. A new seeding procedure for tubular membranes was designed [16, ] and combines: ocross-flow filtration of a water suspension of zeolite seeds through a porous support; o support tilting with respect to the horizontal plane; osupport rotation along its longitudinal axis. This novel procedure gives the possibility to prepare membranes of good quality and with high reproducibility. Tubular zeolite membranes having different permeation properties were prepared by using the innovative secondary growth method [16, 19] on ceramic supports. The catalytic Pt/Na-Y membranes were obtained by ion-exchange, calcination and reduction. and their catalytic performance for the CO Selox. was tested in a flow-through MR. Experimental results were collected at 200°C, with different feed compositions and pressures (up to 6 bar). CO content was ca. 1%, the feed O2/CO molar ratio was 1-1.5. The catalytic membranes effectively removed CO from 10,000 ppm down to 10-50 ppm operating the single stage MR with O2/CO=1 in the Feed. The membranes with lower permeance succeeded in bringing the CO amount less than 10 ppm even at low reaction pressure, and were not affected by changes in the feed composition and feed flow rate [ ]. The experimental results suggest the practical application of these catalytic MRs into a fuel processor producing CO-free hydrogen.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
