CATALYTIC MEMBRANE REACTORS INVOLVING
INORGANIC MEMBRANES
– A short overview –
Anne JULBE and André AYRAL
Institut Européen des Membranes (ENSCM, UM2, CNRS), Université Montpellier 2 (cc047) Place
Eugène Bataillon, 34095 Montpellier cedex 5 FRANCE
Introduction
Membrane reactors (MRs) as a concept, dates back to 1960s and a large number of papers
have been published on this multidisciplinary vibrant subject at the frontier between catalysis,
membrane science and chemical engineering [1-14]. In such an integrated process, the membrane is
used as an active participant in a chemical transformation for increasing the reaction rate, selectivity
and yield. The membrane does not only play the role as a separator but also as part of the reactor.
Membrane reactors, combining in the same unit a conversion effect (catalyst) and a separation effect
(membrane), already showed various potential benefits (increased reaction rate, selectivity and yield)
for a range of reactions involving the membrane as extractor, distributor or fluid-solid contactor. Due
to the generally severe conditions of heterogeneous catalysis, most MR applications use inorganic
membranes, which can be dense or porous, inert or catalytically active.
The interest of MRs has been largely demonstrated at the laboratory scale, namely for
hydrogenation, dehydrogenation, decomposition and oxidation reactions including partial oxidation
and oxidative coupling of methane. Though some small industrial installations already exist, the
concept has yet to find widespread industrial applications. One of the important factors hindering
further commercial development of MRs are the membrane themselves, their support and the
surrounding modules (performance, stability, cost, sealing,..) which still need optimization and new
developments. After a rapid overview of the working concepts of MRs, several examples of current
research and developments in the field of inorganic membrane reactors will be described.
General considerations on inorganic membranes for MRs
Inorganic membranes for MRs can be inert or catalytically active, they can be either dense
or porous, made from metals, carbon, glass or ceramics. They can be uniform in composition or
composite, with a homogeneous or asymmetric porous structure. Membranes can be supported on
porous glass, sintered metal, granular carbon or ceramics such as alumina.
Different membrane shapes can be used such as flat discs, plates, corrugated systems, tubes
(dead-end or not), capillaries, hollow fibers, or monolithic multi-channel elements for ceramic
membranes, but also foils, spirals or helix for metallic membranes. The shape of the separative
element induces a specific surface/volume ratio for the reactor, which needs to be maximized,
typically above 500 m2/m3, for industrial applications [14]. Apart from the evident need for low cost,
resistant and efficient membranes for the process, highly permeable membranes are required for all
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applications. This parameter is directly related to the membrane structure which can be dense or
porous, and which defines the transport mechanisms.
Dense membranes have been largely and successfully investigated in MRs, for reactions
consuming or generating H2 or O2 [4]. Indeed these membranes, exclusively permselective to either
H2 [15] or O2 [16,17], are generally used either as efficient H2 extractors or as O2 distributors. H2
permselective dense membranes include Pd and Pd alloys membranes, which are commercially
available (Johnson–Matthey). Thin supported films and new alloyed compositions have been
recently developed in order to reduce membrane cost, sensitivity to sulfur species and embrittlement
upon aging. Dense ceramic membranes are also considered for H2 separation, e.g. with proton
conducting membranes based on zirconate and cerate perovskite systems [18].
O2 permselective dense membranes include metallic (Ag) or ceramic membranes (e.g.
Yttrium-stabilized zirconia (YSZ), BiMeVOx, La2NiO4+ or (La-Sr)(Fe-Co)O3- perovskites and related
oxides). Gas transport in dense metallic membranes occurs via a solution/ diffusion mechanism. In
stabilized zirconia, ionic conduction is involved at high temperature (800-1000°C); an electronic current
is needed in such electrochemical reactors (Fig 1a). The mixed ionic and electronic conductivity of
perovskites avoids the need of an external electrode for these materials (Fig. 1b). The gradient of partial
pressure PO2 on both sides of the membranes is the driving force for ion transport. Composite materials
involving a mixture of an ion conducting oxide with an electronic conducting phase (e.g. metal) is also
considered (Fig. 1c). One of the common drawbacks to these attractive dense ceramic membranes is
their limited permeability and thermo-chemical stability upon aging in operating conditions. Dense
membranes performance has been improved namely by decreasing membrane thickness (supported
membranes), by increasing the surface roughness and by developing new materials [16].
a
b
c
Figure 1. Different
membrane concepts
incorporating an oxygen
ion conductor: (a) mixed
conducting oxide, (b) solid
electrolyte cell (oxygen
pump), and (c) dual-phase
membrane [17].
Although being less permselective than dense membranes, porous membranes offer a higher
permeability and have been extensively used in catalytic reactors. The gas transport mechanisms in
porous membranes can be related to the ratio between the pore sizes and the mean free path length of
gas molecules [19]. The typical gas transport mechanisms in porous membranes are: molecular
diffusion and viscous flow (macropores and mesopores), capillary condensation (mesopores), Knudsen
diffusion (mesopores), surface diffusion (mesopores and micropores) and micropore activated diffusion.
The contribution of the different mechanisms is dependent on the properties of the membranes and the
gases as well on the operating conditions of temperature and pressure. At high temperature, when
adsorption is no more effective, capillary condensation and surface diffusion are no more involved. In
these conditions, the permselectivity increases when pore sizes decrease and pressure can be used to
control the transport through membranes with macropores or big mesopores. As far as the
permselectivity is not always a key factor in membrane reactors, membrane research for MRs focus on
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both microporous and mesoporous materials, with a large range of porous texture and compositions
adaptable to a large number of applications.
The main membrane functions in MRs
The concept of combining membranes and reactors is being explored in various
configurations, which can be classified in three groups, related to the role of the membrane in the
process [1]. As shown in figure 2, the membrane can act as:
(a) an extractor, where the removal of the product(s) increases the reaction conversion by
shifting the reaction equilibrium;
(b) a distributor, where the controlled addition of reactant(s) limits side reactions; and
(c) an active contactor, where the controlled diffusion of reactants to the catalyst can lead
to an engineered catalytic reaction zone.
In the first two cases, the membrane is usually catalytically inert and is coupled with a
conventional fixed bed of catalyst placed on one of the membrane sides.
1-EXTRACTOR
A, B
C
Equilibrium shift
A + B
Dense or ultramicroporous membranes
D
D
Increased conversion
sweep
2-DISTRIBUTOR
Dense or porous
membranes
3-ACTIVE CONTACTOR
A
B
A
C
B
Controlled addition of a reactant.
Limitation of side reactions
B
B
A
D)
C (
Increased selectivity
A+B
A
B
C
A
B
C
Catalytically
active membranes
C + D
or
C
Controlled diffusion of reactants
to the catalyst
Cat.
Cat
A+ B
C
Increased conversion
(& selectivity)
Figure 2. The three main membrane functions in membrane reactors [5].
The extractor mode corresponds to the earlier applications of MRs and has been applied to
increase the conversion of a number of equilibrium limited reactions, such as alkane dehydrogenation,
by selectively extracting the hydrogen produced. Other H2 producing reactions such as the water gas
shift, the steam reforming of methane and the decomposition of H2S and HI, have been also
successfully investigated with the MR extractor mode. The H2 permselectivity of the membrane and
its permeability are two important factors controlling the efficiency of the process. Although most
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extractor applications feature H2 removal, several decomposition reactions in which O2 is removed
have been also considered [6].
An example of a compact and economical on-site H2 production unit based on steam
methane reforming coupled with membrane technology is shown in figure 3. This type of membrane
reformer has been developed in 2002 in Japan by the New Energy and Industrial Technology
Development Organization (NEDO) & Japan Gas association. This system typically works at 500550°C (instead of 800°C in classical methane reformers) and is able to produce 20-40 Nm3/h [20].
The Pd-based membrane selectively extracts the H2, then shifting the equilibrium to the production
side (CH4 + H2O  CO + 3 H2). The residual CO separated from H2 is burned to CO2 and the
generated heat is utilized for the reforming reaction. In newly developed systems, the CO is used in
the water gas shift reaction (CO + H2O  CO2 + H2) in order to increase the H2 yield.
In H2 producing reaction, substantial conversion improvements can be obtained with H2
permselective porous membrane extractors such as Pd-based membranes, or almost dense silica
membranes prepared by chemical vapor deposition/ infiltration or sol-gel process [21]. Carbon
molecular sieves [22] are also considered as H2 extractors, except cannot in oxidative atmospheres. We
have to note that silica membranes have a limited stability upon aging above 400°C in the presence of
steam, although this problem has been largely improved recently with composite sol-gel derived
membranes such as Ni-SiO2 or C-SiO2 [21]. Dense proton conducting membranes (as ceramic-metal
composite materials) are under also under study as infinitively selective H2 extractors [4].
Pdbased
membra
ne
Figure 3. Membrane reformer developed in 2002 by the NEDO & Japan Gas Association
(including researchers from Tokyo Gas Co., Ltd and Osaka Gas Co., Ltd).
The distributor mode is typically adapted to consecutive parallel reaction systems such as
partial oxidation or oxy-dehydrogenation of hydrocarbons or oxidative coupling of methane. For these
applications the membrane, separating the alkane from O2, is generally used to control the supply of O2
in a fixed bed of catalyst in order to by-pass the flammability area, to optimize the O2 profile
concentration along the reactor, and to maximize the selectivity in the desired oxygenate product [2,5].
This concept has also a beneficial role in mitigating the temperature rise in exothermic reactions. In
such reactors, the O2 permselectivity of the membrane is an important economic factor because air can
be used instead of pure O2. However, the limited permeability of dense O2 permselective ceramic
membranes below 800°C and problems of long-term stability has limited their commercial
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development. In spite of their poor permselectivity, meso and macroporous membranes remain
attractive oxygen distributors for oxidative reactions below 700°C [2,5].
An example of membrane distributor concept used for converting methane to syngas by
partial oxidation (CH4 + 1/2O2  CO + 2H2) is shown in figure 4 [23]. The dense ion conducting
membrane is able to selectively transport oxygen above 700°C. They are exclusively permselective
to oxygen ions and deliver highly reactive oxygen species (O*) on the reaction side. This type of
reformer, resulting in significant cost savings, is currently considered worldwide in a number of
industrially driven consortia and namely by Air Products and Chemicals, Inc.
Figure 4. Syngas production with a ion transport membrane combining air separation and
methane partial oxidation into a single unit operation [23].
In the active contactor mode, the membrane acts as a diffusion barrier and does not need to
be permselective but catalytically active. The concept can be used with a forced flow-mode or with
an opposing reactant mode. The forced flow contactor mode, largely investigated for enzymecatalyzed reactions, has been also applied to the total oxidation of volatile organic compounds [2].
The opposing reactant contactor mode applies to both equilibrium and irreversible reactions [2,5], if
the reaction is sufficiently fast compared to transport resistance (diffusion rate of reactants in the
membrane). In such case a small reaction zone forms in the membrane (if sufficiently thick and
symmetric) in which reactants are in stoechiometric ratio. Triphasic (gas/liquid/solid) reactions,
which are limited by the diffusion of the volatile reactant (e.g. olefin hydrogenation), can also be
improved by using this concept. Indeed the volatile reactant does not have to diffuse through a liquid
film, as far as a gas/liquid interface is created inside the pores, in direct contact with the catalyst [3,5].
An example of a catalytic contactor process developed for the oxidation of waste water is
shown in figure 5. This process has been recently developed and patented by SINTEF within the
frame of a European project called Watercatox [24]. An overpressure of air or oxygen is applied to
the outside wall of the tube. The overpressure (5-15 bar) forces the gas-liquid interface to a position
close to the catalytic layer.
34
Oxidised
products
Figure 5. Principle of
the WATERCATOX
process developed for
waste water
treatment [24]
Other strategies are considered in our group for the continuous catalytic degradation of
VOCs in water. For example, a mesoporous photocatalytic TiO2-based membrane can be used to
reject the macromolecules and colloids contained in the aqueous effluent, although small organic
molecules go across the membrane. They are degraded to CO2 and H2O thank to UV-visible
illumination of the membrane back-side (Fig. 6) [25].
Figure 6. The photocatalytic membrane reactor concept developed for the treatment of aqueous
effluents (from [25])
The different types of membranes and membrane/catalyst
arrangements used in MRs
The different types of MR configurations can be also classified according to the relative
placement of the two most important elements of this technology: the membrane and the catalyst.
Three main configurations can be considered (Fig. 7): the catalyst is physically separated from the
membrane; the catalyst is dispersed in the membrane; or the membrane is inherently catalytic. The
first configuration is often called ‘inert membrane reactor’ (IMR) by opposition to the two other ones
which are ‘catalytic membrane reactors’ (CMRs) [5].
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Figure 7. The main membrane/catalyst combinations: (a) bed of catalyst on an inert membrane;
(b) catalyst dispersed in an inert membrane; and (c) inherently catalytic membrane .
Catalyst physically separated from an inert membrane
In most cases an inert membrane compartmentalizes the reactor but is not directly involved
in the catalytic reaction. The catalyst pellets are usually packed or fluidized on the membrane (Fig.
7a) which acts as an extractor and/or as a distributor (fractionation of products or controlled addition
of a reactant). This type of configuration is probably one of the most promised to development as far
as only the separative function of the membrane has to be controlled. Inert membrane reactors have
been largely studied in the literature with both dense permselective membranes and porous
membranes.
Other important field of application for inert porous membranes concerned their use as
oxygen distributors in partial oxidation or oxy-dehydrogenation of alkanes, or in the oxidative
coupling of methane. The membranes used for this type of application need to play the role as a
barrier achieving the desired transmembrane flux while avoiding the back-diffusion of the second
reactant in the oxygen rich compartment. When using meso or macro-porous membranes, this latter
function can be achieved by an imposed pressure gradient [5].
Catalyst dispersed in an inert porous membrane
When the catalyst is immobilized within the pores of an inert membrane (Fig. 7b), the
catalytic and separation functions are engineered in a very compact fashion. In classical reactors, the
reaction conversion is often limited by the diffusion of reactants into the pores of the catalyst or
catalyst carrier pellets. If the catalyst is inside the pores of the membrane the combination of the open
pore path and transmembrane pressure provides easier access of the reactants to the catalyst (Fig. 8).
Two contactor configurations: forced flow-mode or opposing reactant mode, can be used with these
catalytic membranes, which does not necessarily need to be permselective. It is estimated that a
membrane catalyst could be 10 times more active than in the form of pellets, provided that the
membrane thickness and porous texture, as well as the quantity and location of the catalyst in the
membrane, are adapted to the reaction kinetics.
36
Figure 8. Comparison of the contact reactant/catalyst situation in: a) a classical bed reactor
configuration and b) an active membrane contactor configuration (from [5]).
In biphasic applications (gas/catalyst) the porous texture of the membrane must favor gaswall (catalyst) interactions to ensure a maximum contact of the reactant with the catalyst surface. In
the case of catalytic consecutive-parallel reaction systems, such as the selective oxidation of
hydrocarbons, the gas-gas molecular interactions must be limited because they are non-selective and
lead to a total oxidation of reactants and products. Because of these reasons, small pores mesoporous
or microporous membranes, in which the dominant gas transport is Knudsen or micropore activated
diffusion, are typically favored for contactor applications in biphasic reactions. Greater pore sizes
(10-25 nm) are preferred for triphasic contactor applications (e.g. hydrogenation of liquid alkenes or
VOCs oxidation in water as shown in Fig. 5) with an opposing reactant mode. The gas phase
combustion of VOCs has been successfully investigated with a Pt--Al2O3 membrane with a flowthrough contactor mode [2].
Inherently catalytic membranes
The last types of membranes used in MR applications are called inherently catalytic
membranes (Fig. 7c). In this highly challenging case, the membrane material serves as both separator
and catalyst, and controls the two most important functions of the reactor. As in the previous case
such porous catalytic membranes are used as active contactors to improve the access of the reactants
to the catalyst. A number of meso- and micro-porous inorganic membrane materials have been
investigated for their intrinsic catalytic properties such as alumina, titania, zeolites with acid sites,
rhenium oxide, LaOCl, RuO2-TiO2 and RuO2-SiO2, VMgO, or La-based perovskites [5].nThe
mesoporous TiO2 photocatalytic membranes currently studied in our group for the continuous
degradation of VOCs in water is a typical example of an inherently catalytic membrane. The
morphology of the nanophase photocatalytic TiO2 mesoporous membranes prepared by the sol-gel
process [25] is shown in figure 9.
In most contactors, the catalytic membrane does not need to be permselective but needs to
be highly active for the considered reaction, to contain a sufficient quantity of active sites, to have a
sufficiently low overall permeability and to operate in the diffusion controlled regime. In most cases,
new synthesis methods have to be developed for preparing these catalytically active membranes,
namely when the optimum catalyst composition is complex. The catalytic membrane composition,
activity and porous texture have to be optimized for each considered reaction and keep stable upon
37
use. This challenge explains the limited number of examples given in the literature for the
development of inherently catalytic membranes.
150 nm
Figure 9. Typical morphology of the photocatalytic TiO 2 mesoporous membranes prepared
by the sol-gel process and observed by Field Emission-SEM (from [25]).
Other types of membrane reactors
A number of additional examples which can be considered as inorganic membrane reactors
with attractive properties can be found in the current literature:
* Catalytic particle traps for car exhaust treatment
* Trifunctional membrane reactor for water treatment by ozonation
* Zeolite encapsulated catalyst
* Solid oxide fuel cells and electrolysis cells
Catalytic particle traps for car exhaust treatment
When separation and catalytic functions have not to be performed by the same membrane
material, the catalytic material may only cover the grains of the support which may be covered or not
with a permselective top-layer membrane. Starting from formulated sols and macroporous supports
with adapted tortuosity, porosity and pore size, this method yields catalytic contactors with low
pressure drop and high reactivity. This concept is typically used in catalytic particle traps and in fourways catalysts. Such ceramic contactors can be designed to perform only the oxidation of soot, CO
and CxHy whereas NOx are treated separately. In the four-ways catalyst, the ceramic contactor should
perform continuously the oxidation of fly-ash within its pores, the removal or reduction of noxious
gases, and the oxidation of CO and hydrocarbons. The abatement of NOx can be performed either by
storage in, e. g. barium oxide, or by simultaneous reduction of NOx and soot oxidation over specific
catalysts and through a series of complex reactions. This type of catalytic contactor has reached an
advanced state of technological development with the perspective of large scale industrial
applications in the coming years. An example of four-ways catalytic contactor developed in our
38
group in collaboration with CTI (Céramiques Techniques Industrielles, Salindres, France) is shown
in figure 10 [26].
Figure 10. The four-ways catalyst concept. (a) Ceramic SiC monolith with alternatively
occluded channels, (b) Dead-end channel, c) SEM micrographs of the channel porous
structure, with the different possible thickness for the catalytic layer covering the SiC grains
(from [26]).
Trifunctional membrane reactor for water treatment by ozonation
When a selective extraction is also required, the deposited catalytic material can be
coupled with a permselective top-layer membrane. This concept, leading to compact efficient multifunctional systems, has been developed recently in several groups for both liquid and gas phase
applications. A typical example of composite catalytic reactor for the steam reforming of methane is
reported in [27]. The reforming catalyst covers the grains of a macroporous support and a H2
selective silica-based top-layer ensures the H2 extraction. Another example of such an integrated
membrane reactor approach has been developed in relation between Hong-Kong University of
Science and Technology and our Institute, by coupling the deposition of a high specific surface area
ozonation catalyst on the grains of a macroporous -alumina support, with a zeolite pervaporation
top-layer membrane for the pure water extraction [28].
39
Zeolite encapsulated catalyst
Zeolite Membrane Encapsulated Catalyst (ZMEC) is an original concept of microdesigned membrane reactor. In such a reactor the permselective membrane is coated on the catalyst
grains. The permselective membrane controls the traffic of both reactants and products to and from
the catalyst. The concept revealed an original way to use zeolite membranes in catalytic reactors
while limiting the influence of defects (non-zeolite pores) on large-scale membranes [29]. Coating
catalyst particles with a selective zeolite layer can reveal useful for either increasing reactant
selectivity (e.g. selective hydrogenation of linear molecules with silicalite-1 coated Pt/TiO2 as shown
in figure 11 [29]) or product selectivity (e.g. alkylation of toluene with methanol, or isomerization of
xylene with silicalite-1 coated ZSM-5). This strategy can also be applied to protect the catalyst from
poisoning.
Figure 11. The Zeolite Membrane
Encapsulated Catalyst concept applied to
the selective hydrogenation of olefins on
Pt/TiO2 catalyst, with traffic control over
reactants and products [29].
Straight
olefin
Branched
olefin
Pt/TiO2
Straight
paraffin
Solid oxide fuel cells and Solid oxide
electrolysis cells
Both solid oxide fuel cells (H2 + ½O2  H2O + electricity) and solid oxide electrolysis
(H2O  H2 + ½O2, H = 285.83 kJ/mol) can also be considered as inorganic electrochemical
membrane reactors as far as they contain both porous electrodes with a catalytic/electrocatalytic
activity and a dense ion conducting electrolyte (oxygen or proton conducting ceramic membrane).
High temperature electrolysis is a process which could increase conversion efficiency of
classical electrolysis to the range of 45 to 50 %. The efficiency increase is achieved because high
temperature electrolysis utilizes a significant amount of heat, for example from a nuclear reactor. The
added heat decreases the amount of electricity required to separate H2O into H2 and O2. In France,
both EDF and AREVA are currently examining the use of high temperature electrolysis powered by
nuclear technologies. Two options are currently under study, either with O= conducting electrolytes
above 800 °C or with proton conducting electrolytes below 700°C. The Department of Energy in the
US and China are also highly active in this field. The development of reversible electrode materials,
efficient in both SOFC and SOEC, is an attractive but highly challenging option (Fig. 12).
As envisioned by Jules VERNE in 1873, “water is the coal of the future (Cyrus Smith)” [31].
Effectively 1 liter of water can theoretically produce 2kWh (actually energy losses reduce the real yield)
and a number of renewable energy sources, including the solar one, are potentially attractive to perform
the complete “green” cycle shown in figure 12.
40
O2 + 4ē  2O=
2O=  O2 + 4ē
H2 + O=  H2O + 2ē
2H2O + 4ē  2H2 + 2O2
Figure 12. Example of a DOE program to develop reversible electrode materials for SOFC
and solid oxide high temperature steam electrolysis (GE Global Research, Northwestern
University, and Functionnal Coating Technology).
SUN
H2O
H2
Figure 13. Non-polluting energy cycle based on water and solar energy.
Conclusion
The selection of the ideal membrane composition and supporting structure has to be
derived from an application-based design. Furthermore, a concerted effort is needed for
understanding membrane transport, developing reproducible low cost synthesis routes for modules,
improving operational stability and developing relevant quality estimator methods for membrane
41
units. On this basis, catalytic membrane reactors will obviously play a key role in future advanced
water cleaning and power production systems as well as in industrial chemical production processes.
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[31] Jules Verne, “L'île mystérieuse”, 1873.
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