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Journal of Membrane Science 198 (2002) 119–128
Development of a membrane reactor for the partial oxidation of
hydrocarbons: direct oxidation of propane to acrolein
Peter Kölsch
a
,
∗
, M. Noack
a
, R. Schäfer
a
, G. Georgi
a
, R. Omorjan
b
, J. Caro
a
a
Institut für Angewandte Chemie Berlin-Adlershof e.V., Richard-Willstätter-Str. 12, D-12489 Berlin, Germany
b
Faculty of Technology, University of Novi Sad, Bul. Cara Lazara 1, 21000 Novi Sad, Yugoslavia
Received 2 May 2001; received in revised form 15 August 2001; accepted 10 September 2001
Abstract
The one-step synthesis of acrolein from propane with the catalyst (Ag
0
.
01
Bi
0
.
85
V
0
.
54
Mo
0
.
45
O
4
) was studied in a tubular
membrane reactor (MR) in the temperature region 450–550
◦
C. Oxygenate-selective membranes were prepared by in situ
silica modification of a porous ceramic by the controlled hydrolysis of tetraethylorthosilicate (TEOS). The catalysts were
arranged as granulated layer inside the ceramic membrane tube. Oxygen was dosed through the porous reactor wall to the
tube side of the MR containing the catalyst and the feed propane. The oxygenate-selective membrane separates the reaction
products acrolein and water from the other products and feed gas mixture with a separation factor of 2–3. Compared with
the co-feed packed bed reactor, the yields and the selectivities of acrolein could be increased by a factor of 2–3 in the MR at
equal propane conversions. © 2002 Elsevier Science B.V. All rights reserved.
Keywords:
Ceramic membrane; Membrane reactor; Composite membranes; Catalytic membrane reactor; Acrolein; Propane
1. Introduction
brane reactor (MR) in comparison with the co-feed
reactor (CFR). Especially, the potential of the MR
on increasing the selectivity and yield will be
studied using the well-known multi-oxide catalyst
(Ag
0
.
01
Bi
0
.
85
V
0
.
54
Mo
0
.
45
O
4
). The experimental data
shown in this paper will be compared with data of the
modeling of this reaction in a following paper.
The technical production of acrolein from propane
is carried out in two separate steps with propylene as
intermediate. Propylene is mainly produced by steam
cracking from propane. In a second process, propy-
lene is oxidized to acrolein or acrylic acid. Most of
the acrolein produced is processed to acrylic acid. For
economic and ecological reasons, the direct synthesis
of acrolein from propane in a single-stage process at-
tracts much attention. Such a technical process could
not be realized up to now.
The aim of this work was to study the one-step
partial oxidation of propane to acrolein in a mem-
2. Membrane supported partial oxidation of
hydrocarbons
Membrane assisted catalysis is considered as an al-
ternative to conventional catalytic reactors, especially
in dehydrogenation, hydrogenation and partial oxi-
dation. Miguel et al. [1] showed in a comparison of
fixed bed reactors and MR, that in MR higher yields
and fluxes, higher space-time-yields, more stable
∗
Corresponding author. Tel.:
+
49-30-6392-4338;
fax:
+
49-30-6393-4350.
E-mail address:
koelsch@aca-berlin.de (P. Kölsch).
0376-7388/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S0376-7388(01)00656-1
120
P. Kölsch et al. / Journal of Membrane Science 198 (2002) 119–128
reaction conditions and a higher operation safety can
be reached. However, the use of polymer membranes
is only possible up to a maximum temperature of
130
◦
C because of their limited thermal stability. Fur-
thermore, polymer membranes are restricted in their
use due to swelling and dissolution effects by organic
solvents. In the partial oxidation of propane with
H
2
O
2
on Nafion membranes at 110–130
◦
C, Frustreri
et al. [2] could obtain improved selectivities but due
to the maximum possible temperatures of 130
◦
C,
the conversion was low. On the contrary, inorganic
catalytic MR can work at higher temperatures.
In technical partial oxidation, the hydrocarbon con-
centration in the reaction mixture is limited to 1–2% in
order to avoid the explosion region. The explosion ten-
dency can also be reduced by diluting with N
2
,CO
2
,
noble gases or steam. The space-time-yields of the re-
actor decrease with increasing dilution of the feed. The
oxidation of
n
-butane with the oxygen dosage across
membrane for the maleic anhydride synthesis is de-
scribed by Xue and Ross [3]. The butane concentra-
tion in the feed could be increased from initially 1–2
to 15% by using a membrane as oxygen distributor.
The use of a butane-rich feed gave a higher maleic an-
hydride yield. Both the
By fine tuning of the pore size and by chemical
functionalization of the pore wall, membranes can
be prepared which are adapted to both a catalytic
reaction and to a separation problem. Disadvantages
of inorganic membranes in comparison with polymer
membranes are their higher production costs. One can
calculate at present with 1250 Euro/m
2
membrane
area plus module costs of about 750 Euro/m
2
. With
increasing production volume these costs will sink
strongly. Porous ceramic membranes are produced at
present commercially up to pore diameters of about
1 nm (nanofiltration (NF) membranes). However,
membranes with even smaller pores are necessary for
the molecular separation of gases. Such membranes
are under development worldwide on a laboratory
scale. Usually one or several thin functional layers
with pores lower than 1 nm are brought on suitable
meso- and macroporous supports. The separating
layers can be made by the sol–gel technique [9],
CVD-technique, CVI-technique [10], crystallization
of zeolites [11] or chemical modification of the surface
of the supports pores. So far, there exists no industrial
MR.
p
values between shell and
tube side and the way of O
2
distribution had a signif-
icant effect on the reaction rate.
Weinlei et al. [4] have tested four different types
of flow arrangements of feed and sweep in a tubu-
lar MR in the propane/propylene to acrolein reaction
using the pelletized catalyst (Ag–Bi–V–Mo–O). The
highest selectivities of ca. 18% were obtained for dos-
ing the oxygen from the shell to the tube side of the
MR. Mallada et al. [5] have examined two types of
dosing oxygen and butane in the oxidation of butane
to maleic anhydride in the MR. The best results were
found if the butane was dosed from the tube side to the
shell side of the tubular membrane where the oxygen
and the pelletized catalyst were located. In the MR,
favorable radial flow conditions as well as a better
temperature constancy was obtained. For the reduction
of by-products and a higher selectivity to acrolein or
acrylic acid, Arnold et al. [6] suggested to arrange the
catalyst as a thin layer direct on the wall of the tubu-
lar membrane thus, avoiding hot spots and balancing
the temperature profile in the reactor.
The development of an inorganic MR on a labo-
ratory scale was amplified in the last 10 years [7,8].
3. Partial oxidation of propane to
acrolein—state of art
Partial oxidation of propane to acrolein or acrylic
acid are carried out on a laboratory scale with mul-
tifunctional crystalline mixed-oxide catalysts contain-
ing Mo, Nb, Te, Bi, V, P, Sb, alkali and earth alkali
metals, Ag, Au, Si, elements of the 8–10th group and
others [12,13]. Bettahar et al. [12] report the possible
influences of the different components of the catalysts
in the propane to acrolein reaction. A Mars and van
Krevelen mechanism is proposed where lattice oxygen
anions are assumed to facilitate dehydrogenation while
metallic cations ensure the redox mechanism of differ-
ent oxidation states. Various types of framework po-
sitions influence the reaction path. The selectivity and
the conversion of propane to acrolein are controlled
by the following competitive catalytic reactions:
1. Allyl-intermediate formation in the propane to
propylene transformation.
2. Acrolein or acrylic acid formation from propylene
radicals.
P. Kölsch et al. / Journal of Membrane Science 198 (2002) 119–128
121
The neighbourhood of the different O-species,
which causes either the allyl or radical reaction path,
can influence each other.
The oxidative dehydrogenation of propane to propy-
lene requires higher temperatures than the partial oxi-
dation of propylene to acrolein. It would be favorable,
therefore, to carry out the reaction in separate reactors
with optimized catalysts under optimized pressure and
temperature conditions. However, an analysis of the
patent literature shows that there exists a high inter-
est in a single-stage synthesis of acrolein/acrylic acid
from propane in one reactor configuration. Bifunc-
tional reactors are described in which the reaction is
carried out in two subsequent spatially separated re-
actor parts [14]. In a first part of the reactor the oxida-
tive dehydrogenation of the propane is carried out and
in the second part the partial oxidation takes place.
With the multi metal oxide catalysts (Mo, W, Bi, Co,
Fe, Si, K)/(Mo, W, Co) the attainable acrolein/acrylic
acid yields are usually 5.9/1.4% and the selectivities
for both products are up to 67%. Pure bismuth molyb-
date oxides are inactive in the propylene oxidation to
acrolein. Acrolein and small amounts of acrylic acid
are formed if silver and vanadium are added to a bis-
muth molybdate catalyst.
Kim et al. [15] obtained acrolein yields of 12% and
selectivities of up to 44% with these catalysts. It was
found that yields of this order can be obtained only
if in the first part of the reactor the dehydrogenation
of propane to propylene in a (pre-catalytic) homoge-
neous gas phase reaction takes place and then in the
second part of reactor the catalytic partial oxidation of
the propylene to acrolein follows. That is to say, the
reaction engineering is similar to the present two-stage
technical process.
Without the pre-catalytic reaction the acrolein the
maximum yields so far reported do not exceed 3–5%
[16]. Yields of 3% and selectivities up to 34% were
reached using metal oxide catalysts of the composi-
tion Me
12
−
x
version were reached. The literature shows that the
catalysts known up to now give unsatisfactory results
for the acrolein selectivities and yields with exception
of [19]. However, it seems to be extremely difficult to
reproduce this type of catalyst [14].
Stern and Graselli [18] have described the reac-
tion kinetics and suggested a Mars and van Krevelen
mechanism for the oxydehydrogenation of propane
and the further oxidation to acrolein at the catalyst
(Ni
0
.
5
Co
0
.
5
MoO
4
/SiO
2
)at500
◦
C. It was shown that
the relative rate of acrolein formation from propy-
lene is 3.5 times higher than the formation of propy-
lene from propane. The rate of CO
x
formation from
acrolein was found to be 13 times higher than the
acrolein formation from propylene and the rate of
CO
x
formation was 46 times higher than the propy-
lene formation from propane. This is a typical inter-
mediate problem which is solved classically by short
residence times, fast isolation of intermediate prod-
ucts by quenching or oxygen limitation. Another so-
lution could be the selective removal of acrolein from
the reactor across an oxygenate-selective membrane.
As shown in this paper, the preparation of such mem-
brane with a sufficient acrolein-selectivity at reaction
temperature is possible. Furthermore, it was shown by
Stern and Graselli that the reaction order of oxygen
in the selective oxidation of propane to acrolein with
oxygen is near zero, whereas, it was found to be 0.5
for the non-selective total oxidation of acrolein to car-
bon dioxide and water. It should by beneficial, there-
fore, to work at low oxygen partial pressure since low
oxygen pressure supports the desired reaction path of
selective oxidation. Oxidation at low oxygen partial
pressure could be realized when we dose the oxygen
to the reactor via a porous membrane.
These two concepts namely, (i) to drain off the
acrolein as a reactive intermediate from the reactor
by an oxygenate-selective membrane, and (ii) to dose
the oxygen to the reactor via a porous membrane, are
Ag, Ca, Mg, Zn)
supported by Al
2
O
3
,TiO
2
, SiO
2
[12,13]. Possible re-
action mechanisms of the catalytic partial oxidation of
propane are given by Savary et al. [17] as mentioned
by Stern and Graselli [18].
Ushikubo et al. [19] describe a catalyst with a sub-
stantial higher performance. With the pelletized cata-
lyst (MoV
0
.
3
Te
0
.
23
Nb
0
.
12
O
=
) yields of 48.5% acrylic
acid with a selectivity of 60.5% at 80% propane con-
Fig. 1. Reaction products of the partial oxidation of propane.
Bi
x
VMo
12
O
24
(Me
x
122
P. Kölsch et al. / Journal of Membrane Science 198 (2002) 119–128
two promising techniques followed in this paper to in-
crease the selectivity of the direct partial oxidation of
propane to acrolein.
Fig. 1 shows the reaction scheme for the direct oxi-
dation of propane to the reactive intermediate acrolein.
4. Experimental
4.1. Preparation of the oxygenate-selective
membrane
Commercial ceramic tubes based on Al
2
O
3
with an
asymmetrical structure were used (producer: Herms-
dorfer Institute for Technical Ceramic (hitk)/inocermic
GmbH). The asymmetric membrane support con-
sists of four consecutive
-Al
2
O
3
layers and a final
-Al
2
O
3
layer (ultrafiltration layer) with increasingly
smaller pore diameters of 3000, 600, 200, 60 and 6 nm,
respectively. Pore narrowing and a hydrophilic func-
tionalization for hydrophilic and oxygenate-selective
separations were carried out by in situ-hydrolysis of
tetraethylorthosilicate (TEOS) after the patent [20].
By gas phase adsorption the pore system of the starting
ceramic was first saturated at 23
◦
C with water (water
loading of
1 mg of water/g of ceramic). Then the
ceramic tube was saturated with 50% TEOS–ethanol
solution and brought into an autoclave and heated un-
der autogeneous pressure up to 250
◦
C. First the TEOS
reacts with reactive accessible
<
Fig. 2. (a) FE-SEM of the cross-section of the SiO
x
modified
membrane with the
-Al
2
O
3
and
-Al
2
O
3
layer; (b) EDX line
scan of the Al K
and Si K
intensities of the about 2
m thick
-Al
2
O
3
and parts of the following 10
m
-Al
2
O
3
layer.
-Al–OH groups of
the pore wall surface under formation of Al–O–Si–O.
Simultaneously, SiO
precipitation by condensation
reactions of the partly hydrolyzed TEOS molecules
is assumed. The pore wall surface of the support
ceramic gets a hydrophilic character by formation
of a network of finely distributed SiO
x
with high
concentrations of Si–OH groups. Finally, the tubular
membrane was calcined in air at 600
◦
C.
The membrane so prepared was characterized by
means of phase interference microscopy, field emis-
sion secondary electron spectroscopy/energy disper-
sive X-ray spectroscopy (FE-SEM/EDX), nuclear
magnetic resonance (NMR), electron spectroscopy
for chemical analysis (ESCA) and thermal analy-
sis [21,22]. NMR and ESCA examinations yielded
that SiO
x
is bound chemically to surface Al–OH
groups. Thermal analysis and permeation measure-
ments showed that the SiO
x
-Al
2
O
3
layer. The
line scan FE-SEM/EDX (Fig. 2b) represents the
length-resolved distributions of the aluminum and
silicon across the membrane. It follows from Fig. 2
that the SiO
x
network obtained by the TEOS hydrol-
ysis is mainly deposited within the
-Al
2
O
3
layer.
Correspondingly, in phase interference microscopy
no noticeable changes of the surface roughness of
the membrane after SiO
x
functionalization could be
observed which is different to the results of SiO
x
coating by the sol–gel technique where a smoother
surface was obtained.
The SiO
x
membrane obtained by TEOS hydrolysis
shows in general a selectivity for polar components
and can be used, therefore, as a hydrophilic membrane
for the separations of water from organic solvents (fine
drying) or for the separation of polar compounds from
purge gases [23].
x
networks were stable for
a long time at 600
◦
C.
The FE-SEM (Fig. 2a) shows the cross section
of a
-Al
2
O
3
layer and the first
P. Kölsch et al. / Journal of Membrane Science 198 (2002) 119–128
123
diameter. By a centric corundum bar in the middle of
the ceramic tube (diameter of 6 mm) the volume of
the tube side is reduced. The endings of the tube are
glazed by a low melting glass for a gas-tight sealing
of the membrane with viton O-rings. The head ends of
the reactor are water-cooled enabling measurements
up to 600
◦
C. All inlet and outlet gas streams of the
MR could be switched with a multifunction valve to
the on line-coupled gas chromatograph. The C, H and
O balances had a precision >95%.
The measurements were carried out with the cat-
alysts Ag
0
.
01
Bi
0
.
85
V
0
.
54
Mo
0
.
45
O
4
at temperatures
400–550
◦
C. The granulated catalyst (ca. 0.74 g) di-
luted with Al
2
O
3
was placed in the circular gap
(0.5 mm) between the corundum bar and the ceramic
tube.
5. Results and discussion
5.1. Permeation studies on the oxygenate-selective
membrane
Fig. 3. Schema of the MR.
Permeation measurements showed that oxygenates
permeate faster through this silica membrane than
hydrocarbons and inert gases. Single gas permeation
measurements on this membrane at 500
◦
C showed
that acrolein and water permeate 2 times faster
than the other feed/product components participat-
ing in the reaction (Fig. 4). This oxygenate-selective
membrane can be used, therefore, for the separa-
tion of the reaction products (acrolein and water) in
4.2. Membrane reactor
Fig. 3 shows schematically the MR. The feed and
the sweep gas were regulated by mass flow controllers.
Neon was used as an internal standard gas.
The heart of the MR is the ceramic membrane
tube of 30 cm length, 10 mm outer and 7 mm inner
Fig. 4. Permeation fluxes of key components involved in the direct oxidation of propane to acrolein on the SiO
x
modified membrane
obtained by in situ-hydrolysis of TEOS under autoclave conditions followed by calcination in air at 600
◦
C.
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