Catalytic Dense Membrane Reactor Research Articles

Selection of mixed conducting oxides for oxidative dehydrogenation of propane with pulse experiments

In this study, propane pulse experiments at 550 °C are used as a method to select suitable oxides for further operation of catalytic dense membrane reactor (CDMR) for oxidative dehydrogenation of propane. Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3− δ (BSCF), La 2NiO 4+ δ (LN) and PrBaCo 2O 5+ δ (PBC) powders were used as model catalysts to explore the catalytic properties of membrane surfaces in terms of activity and selectivity. Furthermore, as propane pulses induce removal of oxygen from the sample, which is therefore depleted in oxygen by reducing the oxide, crucial information on the effect of the oxygen content in the samples ( i.e. the oxidation degree) on reactivity and selectivity is obtained. It will be shown that LN is the most promising material for membrane application, provided that it is operated in the optimal window of reduction degree, to ensure high selectivity towards C 3H 6. Contrary, PBC and BSCF are not suitable for CDMR. In fact, PBC showed low selectivity to C 3H 6 due to significant formation of CO 2, independent of the oxidation degree of the material and BSCF appears to adsorb CO 2 by forming carbonates, which might be detrimental for long term operation. However, pulse experiments revealed the remarkable stability of BSCF catalyst's activity towards CO 2 adsorption. Despite the presence of carbonate, the material preserved the ability to act as an oxygen source for propane and can be completely regenerated via oxidation. Additionally, the onset temperature in TPR appears to correlate well with the reactivity of lattice oxygen ions with propane, for the three materials studied.

Electrochemical modification of the catalytic properties of composite membranes of substituted Bi 4V 2O 11 (BIMEVOX) for propane oxidation in a catalytic dense membrane reactor

Dense BICOVOX membrane, bulk BICOVOX/Au cermet membrane and BICOVOX membrane with a BICOVOX/Au cermet on its surfaces are investigated in the partial oxidation of propane under open circuit voltage (OCV) and under electrical bias at 700 °C. The propane conversion remains in the range 10–12% whatever the conditions of polarisation. Mostly cracking products are observed. Hydrogen is the main product with a selectivity around 55–60%. At OCV, no product of oxidation is detected except water which can not be quantified. On the membrane with BICOVOX/Au cermet electrodes on surface, traces of CO are observed as well as a small increase of propylene content under anodic polarisation. This can be explained by partial oxidation and oxidative dehydrogenation of propane, respectively. An anodic polarisation leads to a decrease of hydrogen due to its oxidation into water. In contrast, an increase of the hydrogen content is observed under cathodic polarisation. The effects on the membrane are modest, but they show the possibility that such a system offers for modifying the catalytic properties of membrane materials in a CDMR.

Selective oxidation of hydrocarbons in a catalytic dense membrane reactor: Catalytic properties of BIMEVOX (Me = Ta)

Used as dense membranes in a catalytic dense membrane reactor, Bi 2V 0.8Ta 0.2O 5.5 (BITAVOX) is more or less active in the oxidation of propane, ethane and propene according to its surface roughness. When the surface roughness is low, membranes are poorly active in the oxidation of propane (conversion <2%). CO and propene are the main products and the stability is high over long experimental periods. BITAVOX membranes with increased surface roughness exhibit high conversions in the oxidation of propane, ethane and propene. Activities and selectivities evolve with time. In a first short period (<250 s), the products are similar to those of polished samples, then mainly H 2 and CO are produced and subsequently H 2 and coke. These evolutions are probably related to surface and sub-surface restructuring due to an insufficient oxygen supply through the membrane to face the high catalytic activity induced by the increase in surface area. The results suggest that the O 2− diffusion is limited by the low electronic conductivity of BIMEVOX materials. Nevertheless the membrane is not reduced irreversibly, even in coking conditions. This is confirmed by ex situ characterisations carried out on the materials (XRD, SEM, XPS). An improvement of the electronic conductivity of this material is necessary in order to stabilize the intermediate catalytic behaviour which is interesting for syngas or H 2 production from light hydrocarbons.

Catalytic dense membranes of doped Bi4V2O11 (BIMEVOX) for selective partial oxidation: chemistry of defects versus catalysis

A catalytic dense membrane reactor (CDMR) is used to physically separate the reaction step from the reoxidation of the catalyst. By decoupling the redox mechanism prevailing in mild oxidation of hydrocarbons, the operating conditions may be optimized resulting in an increase of selectivity. The membranes are made up of BIMEVOX oxides, obtained by partial substitution of V in γ-Bi4V2O11 by ME (Co, Cu, Ta). Experiments performed on BIMEVOX dense membranes using propene and propane are described in terms of, (i) active sites on polished or unpolished surfaces, (ii) operating conditions (T, pO2 in the high oxygen partial pressure compartment), which determine the selectivity, either to mild oxidation products (acrolein, hexadiene, CO), or to partial oxidation products (CO, H2), and, (iii) nature of ME cations and relative properties. The discussion deals with the respective role of electronic versus oxide ion conductivities which depend on defects in the structure as well as on the redox properties of cations.

Bi 2V 1− xMe xO 5.5− δ (Me = Ta, Ni) membranes for selective oxidation of C 1–C 3 alkanes in a catalytic dense membrane reactor

Pure and Me-doped γ-Bi 2V 1− x Me x O 5.5− δ (BIMEVOX) (Me = Ta, Ni) materials exhibit a transient behaviour during the oxidation of C 1–C 3 alkanes when shaped as dense membranes separating two compartments fed with diluted hydrocarbon and with air, respectively (catalytic dense membrane reactor). Alkenes are first produced at low conversion and CO and H 2 follow at higher conversion. The H 2/CO ratio increases up to values close to the stoichiometric ones and continues to increase after a peak of activity during which coke builds up. The same transient phenomena as initially observed happen after burning the coke by replacing diluted hydrocarbon by air. The fact that the membrane can be in situ regenerated means that the structure and integrity of the membranes have not been modified, as indeed shown by physicochemical analyses (XRD, SEM and XPS). The catalytic properties are mainly related to the presence of ME which affects the electronic conductivity of the materials. At variance with quite neutral BITAVOX, the behaviour of BINIVOX is close to that of BICOVOX which is also a p-type semiconductor. Only BINIVOX is able to oxidise methane quite steadily (conversion ca. 22–25 mol%) to syngas at low temperature (650 °C).

BIMEVOX as dense membrane in catalytic reactor (ME = Co, Cu, Ta)

A catalytic dense membrane reactor allows to physically separate the oxygen feed from the reactant (hydrocarbon) feed with a catalytic membrane chosen among oxide ion conducting materials. The membrane plays a double role, it provides the oxygen needed for selective oxidation and acts as a catalyst. The catalytic properties of BIMEVOX (ME = Co, Cu, Ta) membranes were examined in the mild oxidation of propene and of propane. During the complex transient state observed when the surface is rough, the nature and distribution of products are different from those obtained with mirror-polished membranes in a former work. In particular, syngas is formed with propene as well as with propane, and it precedes the production of hydrogen and coke. The complex behaviour differs according to ME and seems to be related to the different nature of electron semi-conduction induced by each dopant.

Transient behaviour of dense catalytic membranes based on Cu- and Co-doped Bi 4V 2O 11 (BIMEVOX) in the oxidation of propene and propane

ME-doped γ-Bi 4V 2O 11 (BIMEVOX) oxides are highly oxide ion conducting materials and this property may be profitably used in selective oxidation of hydrocarbons. The catalytic properties of BICUVOX and BICOVOX when shaped as dense membranes displayed in catalytic dense membrane reactor are examined in the oxidation of propene and of propane. Mirror-polished BICUVOX and BICOVOX membranes studied previously were poorly active for propene oxidation because of a small number of active sites but showed an excellent stability and reproducibility (lasting more than 1 month) during which products of mild oxidation (acrolein, hexadiene) and CO were formed. Membranes with depolished surfaces exhibit high conversions of propene (up to 60 mol%), and also of propane (up to 20 mol%) but – contrary to mirror-polished membranes – a complex transient behaviour is observed during which syngas production occurs. The membrane polarisation followed by in situ solid electrolyte potentiometry shows that the oxygen reservoir is far higher than expected on the reaction side which is separated (by the membrane) from the oxidising side where (diluted) oxygen is reduced to O 2− specie. The influence of oxygen partial pressure on the catalytic performance suggests that the electronic conductivity of the material is limiting the oxygen flux through the membrane, and thus is determining the catalytic properties and transient behaviours of depolished membranes.

Redox stability of phases based on bismuth oxide studied by voltammetry on microsamples

Cyclic voltammetry on microsamples was applied to bismuth tungsten oxides and bismuth molybdenum oxides to check their stability to reduction and define their limit of use. In this study, different mechanisms of reduction of bismuth were observed. Aurivillius Bi 2WO 6 derivatives exhibit a two-step mechanism (Bi 0→Bi 2+ and Bi 2+→Bi 3+), similar to pure bismuth oxide. In contrast, the investigated bismuth molybdenum oxides (Bi 26Mo 10O 69-related phases and Bi 2Mo 3O 12) revealed a one-step mechanism (Bi 0→Bi 3+). A better stability was therefore revealed for bismuth–molybdenum oxides, which are well known for their catalytic properties. This makes these materials attractive for use as membrane for catalytic dense membrane reactor.

Oxygen permeation versus catalytic properties of bismuth-based oxide ion conductors used for propene oxidation in a catalytic dense membrane reactor

Dense membranes of Bi2O3–25% Er2O3 (BE25), BE25–Ag cermet, and Bi2V0.9Co0.1O5.35 (BICOVOX.10) were used to separate the two compartments of the so-called catalytic dense membrane reactor. The setup allows to measure the rate of permeation of dioxygen as well as the catalytic properties in oxidation of hydrocarbons. Higher oxygen permeation rates are observed on BE25 and BE25–Ag cermet than on BICOVOX.10 whereas the latter is more active for propene oxidation. The polarization of the membrane measured in situ during catalytic reaction shows that the overall oxygen transfer in BICOVOX.10 is enhanced and that permeation of dioxygen is not a key property in selective oxidation of hydrocarbons.

Oxygen permeation versus catalytic properties of bismuth-based oxide ion conductors used for propene oxidation in a catalytic dense membrane reactor

Dense membranes of Bi 2 O 3 –25% Er 2 O 3 (BE25), BE25–Ag cermet, and Bi 2 V 0.9 Co 0.1 O 5.35 (BICOVOX.10) were used to separate the two compartments of the so-called catalytic dense membrane reactor. The setup allows to measure the rate of permeation of dioxygen as well as the catalytic properties in oxidation of hydrocarbons. Higher oxygen permeation rates are observed on BE25 and BE25–Ag cermet than on BICOVOX.10 whereas the latter is more active for propene oxidation. The polarization of the membrane measured in situ during catalytic reaction shows that the overall oxygen transfer in BICOVOX.10 is enhanced and that permeation of dioxygen is not a key property in selective oxidation of hydrocarbons.