Fundamentals of Inorganic Membrane Science and Technology Edited by A.J. B urggraafand L. Cot

�9 1996, Elsevier Science B.V. All rights reserved

Chapter 2

Important characteristics of inorganic membranes

A.J. Burggraaf

Department of Chemical Technology, Laboratory of Inorganic Materials Science, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands

2.1 INTRODUCTION

The transport properties (i.e. permeation and separation efficiency) of inor- ganic membrane systems depend, to a large extent, on the microstructural features of the membrane and the architecture of membranes and modules. The microstructural features, such as pore shape and morphology, pore size (distri- bution), interconnectivity/tortuosity, as well as the architecture of the mem- brane and membrane-support combinations will be briefly described. Here, architecture means the way the different parts of the membrane system or module are shaped and combined.

The aim of this chapter is to serve as a background and guide to the variety of membrane systems and module types which are used in subsequent chapters.

Very briefly, the state of the art concerning some materials' properties will be treated.

Economically the most important group is that of ceramic membranes and therefore a certain focus is given to it.

2.2 TYPES OF I N O R G A N I C MEMBRANES

A membrane can be described as a semipermeable barrier between two phases which prevents intimate contact. The barrier must be permselective.

22 2 ~ IMPORTANT CHARACTERISTICS OF INORGANIC MEMBRANES

Usual ly a n u m b e r of m e m b r a n e s are combined in a module , wh ich is the smalles t practical uni t conta ining a set of m e m b r a n e s and any s u p p o r t i n g s tructures . Two ma in classes of m e m b r a n e can be d is t inguished: dense (non- porous) and porous ones.

Dense m e m b r a n e s are m a d e f rom solid layers of meta ls (e.g. Pd alloys) for h y d r o g e n separat ion, or of mixed (electronic, ionic) conduc t ing oxides for oxygen separat ion. A special form are the LIMs (liquid immobi l i sed m e m - branes) wh ich consist of a po rous suppor t filled wi th a l iquid or mol ten salt which is semipermeable .

Porous m e m b r a n e s consist of a porous wall or po rous top layers (metal, oxide, glass) on a porous (metal-oxide) suppor t . A var ie ty of pore shapes and archi tectures exist, as s h o w n in Table 2.1

The mos t s imple form is a single, un i fo rmly s t ruc tu red wall of a certain material , the so-called symmetr ic , s tand-a lone membranes . Examples are dense meta l or oxide tubes and porous hol low fibres. To obtain sufficient mechanica l s trength, single-walled symmetr ic systems usual ly have a considerable thickness.

TABLE 2.1

Types of inorganic membranes

Type ( c l a s s ) Material/process Architecture

Shlgle wall (symmetric)

Single wall (asymmetric)

Supported, multilayered (asymmetric)

Modified structures

Support

- dense oxide or metal -LIM - porous glass or carbon - track etch - porous alumina, anodic

oxidation of A1

- porous alumina (anodic oxidation)

- d e n s e oxide or metal - porous ceramic membranes:

alumina, zirconia, titania, carbon

- composite ceramic-metal, ceramic-ceramic

- ceramic

- ceramic-organic

- porous alumina - porous carbon - porous metal

tubes, plate

tube, hollow fibre

(thin) sheets

layers on porous support tube, disk multflayers on porous support plate, disk, tube, monolith

(partially) plugged pores, intra pore deposits intra pore coating

plate, disc, tube, monolith plate, tube, hollow fibre woven structures, disc, tube

2 - - I M P O R T A N T C H A R A C T E R I S T I C S O F I N O R G A N I C M E M B R A N E S 23

This is a disadvantage for obtaining large fluxes, which require thin separation layers. The solution to this problem is asymmetric structures. Usually these consist of a support ing system with large pores (low flow resistance) of suffi- cient mechanical strength on top of which are layers with gradually decreasing pore size. This is shown in Fig. 2.6 for asymmetric ceramic membranes. The top layer contacting the liquid or gaseous phase to be separated or reacted has the smallest pore diameter. The pore diameter as well as the physico-chemical nature of the pore walls can be changed by additional treatments.

In subsequent sections a brief discussion will be given of pore characteristics and architectural aspects of modules as well as details of some systems.

2 . 3 M I C R O S T R U C T U R A L P O R E A N D P O R E N E T W O R K C H A R A C T E R I S T I C S

Porous materials have a very complex structure. Many studies have been devoted to describing and characterising them (see Chapter 9, Refs. [1-3,6,18]).

Typical pore shapes are schematically represented in Fig. 2.1 and summa- rised in Table 2.2

The most simple pore morphologies (Figs. 2.1A and B) are those of more or less straight cylindrical or conical-shaped pores. This type of pore is formed in so-called track etch and in "anopore" membranes. The latter is obtained by anodic oxidation of A1 metal foils and results in porous (amorphous) alumina (rnesoporous) membranes. A detailed discussion is given by Burggraaf and Keizer in Ref. [1]. These types of membrane are useful for fundamental trans-

TABLE 2.2

Microstructural pore (network) characteristics

Pore morphology Process, material

Straight channels -cylindrical, conical, (a)symmetric

Interconnected voids/pores - Spongy structures - Packing of particles

- spherical particles plates - fibrilles, inorganic polymers

- zeolite structures

Modified structures

Anodic oxidation (alumina) track etch

- porous glass, carbon - ceramic membranes -macro-/mesoporous (x-A1203, zirconia,

titania, ~'-A1203 microporous silica

- MFI (ZSM5, Silicalite)

Sol-gel, CVD e.g. silica, metals, many catalytic materials

24 2 ~ IMPORTANT CHARACTERISTICS OF INORGANIC MEMBRANES

J

1 G

Fig. 2.1. Schematic picture of pore shapes. A and B are single wall, symmetric and asymmetric membranes respectively with straight cylindrical (a) or conical (b) pore shape; (c) represents a ceramic

asymmetric multilayered membrane with interconnected pores.

port studies but are not important for practical process applications due to low porosity (track etch) and mechanical weakness and vulnerability.

Pore systems found in useful membranes are characterised by intercon- nected voids (pores). Spongy microstructures are observed by phase decompo- sition and leaching [1] or in porous carbon, obtained by controlled pyrolysis of polymeric precursors [1].

The economically most important ceramic membranes are characterised by a pore network obtained by a packing of particles in each layer of the multilayered system (Fig. 2.1c). In the case of a packing of spherical particles there is a relation between the particle diameter, the pore diameter and the packing symmetry (coordination number). The sharper the particle size distribution and the more ideal the packing, the sharper the pore size distribution will be. In this type of packing there exist two characteristic pore sizes: (a) the pore diameter of the large inscribed sphere determined by the cavities in the packing and (b) the inscribed diameter of the "windows" between these cavities. The first deter- mines the porosity, the second represents the narrow passages in the network and is thus important for the separation properties.

Packings can also be obtained by a packing of plates as shown in Fig 2.2 [2]. Note that here the pores have a slit-shaped structure with a limiting pore diameter in only one direction. Because thermostable particles with diameters below 5-6 nm are very difficult to make, microporous membranes with a pore diameter below 2 nm cannot be produced by packings of spherical or plate- shaped particles. Packings of fibrillous particles can result in microporous membranes as observed by de Lange et al. [3] with polymeric silica particles (see also the Chapter 8). Finally, zeolite membranes are formed by intergrown

2 - - IMPORTANT CHARACTERISTICS OF INORGANIC MEMBRANES 25

I !

s

p

I

�9 I !

. . - - A - S r j - ..

I . J _~ �9 I - S s _ j

J S S

m ~ , m o

..d |

7 / � 9 O~lm~ 4.... ,a

Fig. 2.2. Idealised model of a packing of plates as formed in the boehmite membrane structure; d is the distance between two boehmite crystals A and B, ~ is the thickness of the boehmite plates

(Leenaars and Burggraaf [2]).

@ ) ',-

__

Fig. 2.3. Schematic picture of pore types in a porous membrane, a: Isolated pore; b,f: dead end pore; c,d: tortuous and/or rough pores (d) with constrictions (c); e: conical pore.

zeolite particles with interparticle pores filled with another material. The in- tracrystalline pores are part of the crystallographic structure and have a very uniform diameter.

After the initial packing of the particles into layers from suspens ions or colloidal sols, the layers are dried and heat-treated to produce the stable structure. During heat treatment, sintering occurs resulting in loss of porosity, increase of density and changes in pore size and yielding the final pore network structure. Again several types of pore can be dist inguished as shown in Fig. 2.3.

26 2 m I M P O R T A N T C H A R A C T E R I S T I C S O F I N O R G A N I C M E M B R A N E S

The interconnectivity and the related tortuosity as shown by pore d in Fig. 2.3 is important. Systems with interconnected pore systems usually have tortu- ous pore networks with many constrictions and dead-end pores. This is not favourable for their transport properties and becomes more serious the lower is their porosity.

2.3.1 Modified Structures

Pore structures of membranes formed in a pr imary process can be modified by subsequent modification processesto change properties and to obtain tailor- made systems for specific applications. Figure 2.4 shows schematically some typical microstructures.

Figure 2.4a shows a mono- or multi-layer coating on the internal surface of the pore network. This type of coating can be used to decrease the pore size or to change the chemical nature of the pore structure. An example of a metal-or- ganic coating on the inside of a porous glass membrane is given in Fig. 2.5. This makes the membrane useful for desalting applications in reverse osmosis processes [4]. Figure 2.4b shows deposits of particles within the porous struc- ture. This is useful in catalytic membrane reactors [5]. If particles are concen- trated on certain sites, they can partially plug the pore system (Fig. 2.4d). If the plugs are concentrated near the pore entrance this is usually accompanied by the formation of a thin layer and vice versa. Samples of such a structure (Fig. 2.4c) are given by de Lange et al. [6] for microporous silica membranes made by a sol-gel process with a layer thickness of about 50 nm and plugs penetrat ing the mesopores of the 7-A1203 support by about 50 nm. Another example (Fig. 2.4d) has been reported by Tsapatsis and Gavalas [7] for silica membranes made with

A B

G D

Fig. 2.4. Schematic representation of modified membrane top-layer structures. (A) monolayer or multilayer deposit; (B) nanoparticles within pores; (C) thin film on top of the membrane and plugs

penetrating the pore entrance; (C) plugs or constrictions at a certain site in the top layer.

2 - - I M P O R T A N T CHARACTERISTICS OF I N O R G A N I C M E M B R A N E S 27

.03 H H 1.1

c-c-c- SChH pc ~I

I~ H H C-C-C- S0)H H H H

Fig. 2.5. Metal-organic coating on a modified porous glass membrane [4].

a CVD process with porous plugs of 10 ~tm just below the pore entrances of a porous glass support.

2.3.2 Supports

The quality of the support (or supporting layers, see Figs. 2.1 and 2.6) beneath the separation layer is critical for the quality of the membrane itself. Defects and irregularities in the support usually produce defects in the layer applied on it. Defects are, for example, pores much larger than the average pore diameter of the support as well as grains broken out of the support surface. They will give rise to so-called "pinholes" in the layer on top of the support. Surface irregulari- ties causing rough surfaces exclude the formation of defect-free thin and smooth layers in a single step. Finattythe wetting behaviour of the surface is ~mportant in layer formation processes. Severe local changes in wettability result in pin- hole formation. Consequently, high quality supports should be smooth, have constant and homogeneous surface characteristics (wettability) and preferably have a relatively narrow pore size distribution. They should have sufficient mechanical strength which does not age with time.

2.4 ARCHITECTURE OF MEMBRANE SYSTEMS

Membrane systems and modules are produced in a variety of architectural types, as shown in Table 2.3

Multilayered asymmetric (ceramic) membranes with an architecture as shown schematically in Fig. 2.6 are economically the most important.

The basic concept behind such asymmetric (composite) structures is to min- imise the overall hydraulic resistance of the permeate flow through the mem- brane structure. This requires a defect-free separation layer with small pores and as thin as possible on top of a support with large pores.

TABLE 2.3

Architecture of membrane systems and modules

1 Multilayered asymmetric membranes 2 Hollow fibre

porous glass 3 Multichannel (ceramic) monolithic element

with 6-18 channels. a-alumina, cordierite

4a

4

3

2

I

4b Multilayered asymmetric membranes sheets of porous plates

tubes, disc, plate modules ensemble of parallel fibres module of elements

Multilayered asymmetric membranes (tubes) modules alumina, alumina-zirconia ensemble of tube bundles

modules stack of flat sheets (SOFC type)

2 8 2 ~ IMPORTANT CHARACTERISTICS OF INORGANIC MEMBRANES

Fig. 2.6. Schematic representation of an asymmetric (composite) ceramic membrane. 1. Porous support (1-15 lxm pores); 2. intermediate layer(s) (pore diameter dp = 100-1500 nm); 3. mesoporous separation layer (dp = 3-100 nrn); 4. Modification of 3 to microporous separation layer (dp = 0.5-2 nm).

In most synthesis processes it is not possible to produce the thin separation layer directly on top of a support with large pores because the precursor system from which the separation layer is made will significantly penetrate the sup- port ing pores (e.g. small particles from which small-pore membranes are made will penetrate much larger pores). This will result in a strongly increasing flow resistance. Furthermore, thin layers covering wide pores are mechanically unstable and will crack or peel off easily.

A practical solution is to produce a graded system by adding one or more intermediate layers with gradually decreasing layer thickness and pore size between bulk support and separation layer. The larger the difference is between the pore size of the support and of the separation layer, the larger is the number of intermediate layers. Sometimes the separation layer a n d / o r its support ing layer is produced not in a single step but in two or more steps, which means that it is a bi- or multi layer system itself. This is done to enhance the layer quality by decreasing the number and size of defects by covering or filling the defects in the underlying layer with the final (top) layer.

2 m IMPORTANT CHARACTERISTICS OF INORGANIC MEMBRANES 29

A

- " N .

= 99 "~ Membrane ! \ support ! r - layer First & ~ . ~ I -1

e> 9 7 - layer Sec/nd ~ ~

9e- .,y=' ,I .... -~ r~ 9 5 . . . . . . I , , , l . . . . . I ,, ,

1 0 1 0 0 1 , 0 0 0 1 0 , 0 0 0 1 0 0 , 0 0 0

P o r e d l a m e t e r ( A )

Fig. 2.7. Pore size distribution of a four-layered alumina membrane (Hsieh et al. [8]).

An example of the pore characteristics of a four-layer alumina system is shown in Fig. 2.7 in which the top layer is a mesoporous (alumina) membrane with an average pore diameter of about 6 nm and a thickness of 5--6 ~tm. The intermediate layers have thicknesses of 15-20 ~tm and pore diameters of 0.5-0.7 ~tm, respectively. A microporous system has been reported by de Lange et al. [6] based on a n 0c-A1203 support (pore diameter =0.2 mm) with t w o 7-A]203

intermediate layers (total thickness =7 ~tm, pore diameter ~4 nm) and a final silica separation layer with a thickness of about 100 nm (see Fig. 2.4c) and a pore diameter of about 0.5 nm. The intermediate T-A1203 layers are very smooth the average roughness is about 40 nm B which is a necessary requirement to obtain high-quality silica top layers. The two-step synthesis of the intermediate layers improves the defect quality of the system. A two-step synthesis of the microporous silica (titania) layer further improves the quality of the micropor- ous system (as determined by its gas separation properties). This type of multilayered system is produced today in disc, plate and tube form. In the case of tubes these can be assembled in a module containing a number of tubes connected to a single manifold system.

To increase the mechanical robustness and the surface area-to-volume ratio, which gives more filtration area per unit volume of membrane element, alu- mina multichannel monolithic elements have been developed, as shown in Fig. 2.8. These monolithic elements can again be combined into modules. Surface area-to-volume ratios of 30-250 m 3 / m 2 for tubes, 130-400 m3/m 2 for multichan- nel monolithics and up to 800 m 3 / m 2 for honeycomb multichannel monolithics are reported by Hsieh [9].

A further increase of the packing density (surface area-to-volume ratio) is obtained with experimental hollow-fibre modules, as shown in Fig. 2.9. They consist of long, hollow fibres with an internal diameter ranging from 40 to 300 ~tm and wall thicknesses of 10-100 ~tm. Surface area-to-volume ratios of more

30 2 -- IMPORTANT CHARACTERISTICS OF INORGANIC MEMBRANES

ulk a ,mnnd

Channel

S

~ a t e m ~ h s y ~

" / / / hlyer / '

Permeate /

Fig. 2.8. Schematic picture of a porous mulfichannel monolithic membrane element.

FIBER B1 PLU

HOLLOW

,UE

PERMEA~ MODUL

H IGH-PRESSU]~ GAS M IXTUR

SHELL

PERMEATE OUTLET

Fig. 2.9. Hollow-fibre module.

than 1000 m 3 / m 2 have been repor ted [9]. The small size of the internal bore can

often p resen t a p rob lem w h e n large quant i t ies of gas are p e r m e a t i n g (high p ressu re d rop d o w n the fibre bore). Hol low fibres are m a d e of h igh - t empera - ture po lymers , po rous glass or carbon, a l though some exper imenta l w o r k on ceramic fibres is r epor ted in l i terature.

2 - - I M P O R T A N T C H A R A C T E R I S T I C S O F I N O R G A N I C M E M B R A N E S 31

PERMEATI

E

FEED

Fig. 2.10. Flow path in a stack of flat membranes.

A very interesting development would be the production of flat stacks of composite sheets (plates) similar to those developed in solid oxygen fuel cell (SOFC) technology. In this way a very high packing density can be obtained in a robust module configuration with modest pressure drops. The principle is shown in Fig. 2.10 where the feed gas flows through channels in a stack of semipermeable porous sheets, gas permeates through the walls of the sheet and the permeate flows out of the system through a separator space.

2.5 SOME GENERAL CHARACTERISTICS

Porosities of membrane components vary widely and values are reported ranging from 20 to 60%. Commonly, values of 30-40% are used. Pore sizes range from macropores (>500 nm) via mesopores (20-500 nm) to micropores (<2 nm). A great problem is the lack of reliable measurement methods to measure the porosity and pore size distribution of supported membranes (see Chapter 4).

The thickness of the separation membrane layer and of other layers in asymmetric membranesrepresents a trade-off between high flux requirements (requiring thin layers) on the one hand, and physical integrity and defect requirements (requiring thick layers) on the other hand. Current commercial products generally show layer thicknesses of the separation layer in the 10-20 ~tm range, but values of approximately 5 ~tm have also been reported. For microporous layers, thinner layers should be developed. The bulk support and intermediate layers have thickness values ranging from I to 2 mm, to provide sufficient mechanical strength, and 10-100 ~tm, respectively. In the case of single wall, stand-alone systems thickness values are similar to that of supports in the case of, e.g., porous glass and are in the range of 50-100 ~tm for hollow fibres.

2.5.1 Commercially available inorganic membranes

A variety of membrane materials has been investigated and reported [1,9] and an overview of commercially available systems has been given by Hsieh [9]. Alumina, zirconia and, more recently, titania membranes are used in large- scale applications. The more complex shapes, i.e. monolith and honeycomb, are almost exclusively based on (z-alumina or cordierite.

32 2 ~ I M P O R T A N T C H A R A C T E R I S T I C S O F I N O R G A N I C M E M B R A N E S

Some examples of different systems and new developments are given below with their trade name and producer.

Carbosep membranes (Tech-Sep, France) are made of a zirconia layer at- tached to a porous carbon supporting tube assembled into modules containing up to 252 tubes. The same company produces Kerasep membranes of alumina or titania on a monolithic alumina-titania support containing 7-19 channels.

Membralox membranes produced by US Filter/SCT (USA) is the name of a group of tubular and monolithic (multichannel) alumina membranes. The supporting system is formed by high-purity c~-alumina multilayers with a final coating of alumina or zirconia. This system has now been developed further to obtain smaller pore diameters for nanofiltration and gas separation.

Ceramem membranes (CeraMem, USA) produces honeycomb-shaped monolithic supports of cordierite, the channels of which are coated with zir- conia, silica, y-alumina or c~-alumina separation layers.

Le Carbone Lorraine Company (France) produces all carbon asymmetric membranes. The support consists of a porous fibre composite tube which is coated on the inner face with one or more porous carbon films obtained by hydrolysis of polymer precursor films.

Dedest Corporation (USA) has announced the production of porous inox steel membrane systems (ultrafiltration applications).

Mott Corporation (USA) produces porous metal filters (inox, nickel, monel, inconel, silver, platinum) and has announced the development of porous me- tallic membranes in disc and tube form.

Membrane systems with pore diameters in the micropore range (gas separa- tion, nanofiltration) are not yet commercially available but are produced for development and marketing purposes by, e.g., Velterop B.V. (Enschede, Neth- erlands) and Media and Process Technology Inc. (Pittsburgh, USA). These systems have an c~-alumina support combined with multilayered y-alumina (mesoporous) layers and a silica (microporous) separation layer.

Experimental zeolite membranes are reported to be grown directly on c~-alu- mina or stainless steel (disc or tubular) supports.

It should be noted that most of the systems discussed have been developed for use in liquid separations at low ambient temperatures. For high-tempera- ture applications, such as in gas phase processing and membrane reactors, the sealing of the membrane system elements in modules is critical. Several solu- tions have been tried to connect ceramic membranes to housing or header plates of the module, such as local cooling, the use of high temperature polymers (up to 230~ the use of graphite/carbon filament and graphite packings (up to 300~ in oxidizing and to 1000~ in reducing environments) and ceramic-to- metal connections. Further development to obtain reliable and less expensive solutions for high-temperature applications is necessary.

2 m IMPORTANT CHARACTERISTICS OF INORGANIC MEMBRANES 3 3

2.6 CONSIDERATIONS ON CHEMICAL RESISTANCE

Inorganic membranes are inherently more stable than organic membranes at temperatures over 200~ and with various chemicals such as aggressive organic compounds and liquids with extreme pH values. Nevertheless, for applications in environments which call for long-term contact between corrosive chemicals (e.g. strong acids or bases, hot gases) the corrosion behaviour of the membrane components should be quantitatively known.

It is surprising that there are very limited data on the chemical resistance of various oxide materials. Most of the data are obtained on solid, non-porous materials and with simple dissolution test methods. In the few cases where porous materials are used, hydrodynamic conditions are not or are inade- quately taken into account and flow of the aggressive media through the pore network does not occur during the tests.

The problem has been analyzed by Z~iter and Burggraaf [10,11] and a strat- egy and test method has been proposed to measure adequately corrosion behaviour of supported membranes in liquid media under conditions ap- proaching those in applications. Their conclusion was that reliable, quantitative data on chemical resistance of ceramic membranes has not appeared in the published literature. The procedure proposed by Z~iter et al. is a combination of (acid) corrosion under conditions where the liquid flows through the mem- brane, as in applications, measurement of the change in pore-size distribution with permporometry and of the water permeation as a function of corrosion time, and measurement of the amount of corroded material in the aggressive liquid. Some of the more interesting observations made on corrosion of y-alu- mina and composite materials of alumina-zirco~a inHNO3 solutions are:

(a) Corrosion of mesoporous membranes might differ from bulk material due to small particle size (and consequently severe curvature of the pore surface).

(b) Due to differences in curvature, corrosion is sometimes strongly localised. Without observable changes in average pore diameter quite severe changes in water permeation rate occur, indicating changes in the internal microstructure of the pore network.

(c) Impurities have a great effect on the corrosion rate which can initially be high and then after some time levels off.

(d) Even o~-A1203 supports were slightly affected by nitric acid solutions of pH = 1 or 2. In a control experiment with high purity (99.99%) Sumitomo powders (grain size 0.6-0.8 ~tm) a similar result was found indicating that indeed small particles (high surface area) and/or very small residual impurity levels concentrated on grain boundaries ~ necks between grains and surface regions ~ affect the corrosion behaviour.

These results might explain the experimental observation of a slow deterio- ration of mechanical properties (decrease of burst pressure) which sometimes

34 2 -- IMPORTANT CHARACTERISTICS OF INORGANIC MEMBRANES

occurs after p ro longed use of the m e m b r a n e sys tem in l iquid fil tration applica-

tions. Probably local corrosion of the necks be tween grains in the p o r o u s ceramic s t ructure ( formed du r ing sintering in the p roduc t i on process) weakens the necks and so the s t rength of the membrane .

REFERENCES

1. A.J. Burggraaf and K. Keizer, Synthesis of Inorganic membranes, in: R.R. Bhave (Ed.), Inorganic Membranes, Synthesis, Characterisation and Applications. van Nostrand Rein- hold, New York, 1991, pp. 10-63.

2. A.F.M. Leenaars, K. Keier and A.J. Burggraaf, The preparation and characterisation of alumina membranes with ultrafine pores. ]. Coll. Interface Sci., 105 (1985) 27-40.

3. (a) R.S.A. de Lange (1994). Microporous sol-gel derived ceramic membranes for gas separation. PhD Thesis, University of Twente, Enschede, The Netherlands. (b) R.S.A. de Lange, J.H.A. Hekkink, K. Keizer and A.J. Burggraaf, Polymeric silica based sols for membrane modification applications. J. Non-Cryst. Solids, 191 (1995) 1-16.

4. R. Schnabel and W. Vaulont, High pressure techniques with porous glass membranes. Desalination, 24 (1978) 249-272.

5. V.T. Zaspalis and A.J. Burggraaf, Inorganic membrane reactors to enhance the produc- tivity of chemical processes, in: R.R. Bhave (Ed.), Inorganic Membranes Characterisation and Applications. von Nostrand Reinhold, New York, 1991, pp. 177-208.

6. R.S.A. de Lange, J.H.A. Hekkink, K. Keizer and A.J. Burggraaf, Formation and charac- terisation of supported microporous ceramic membranes prepared by sol-gel modifica- tion techniques. J. Membr. Sci., 99 (1995) 57-75.

7. M. Tsapatsis and G. Gavalas, Structure and aging characteristics of the permselective SiO2-Vycor membranes. J. Membr. Sci., 87 (1994) 281-296.

8. H.P. Hsieh, R.R. Bhave and H.L. Flemming, Microporous alumina membranes. J. Membr. Sci., 39 (1988) 221-241.

9. H.P. Hsieh, General characteristics of inorganic membranes, in: R.R. Bhave (Ed.), Inorganic Membranes Characterisation and Applications. van Nostrand Reinhold, New York, 1991, pp. 64-94.

10. J. Z6ter, Chemical and thermal stability of (modified) mesoporous ceramic membranes. PhD Thesis, University of Twente, Enschede, The Netherlands, 1995.

11. J. Z6ter, W. Boer, K. Keizer, H. Verweij and A.J. Burggraaf, The thermal and chemical stability of classical and modified mesoporous membranes. In: Y.H. Ma (Ed.), Proc. of the 3rd. Int. Conference on Inorganic Membranes (ICIM 94), July 10-14, 1994, Worcester, Mass, USA, pp. 381-390.