Partial Oxidation of Methane in a Catalytic Ruthenium Membrane

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Partial Oxidation of Methane in a Catalytic Ruthenium Membrane Reactor Luca Paturzo,† Fausto Gallucci,† Angelo Basile,*,† Paolo Pertici,‡ Nico Scalera,§ and Giovanni Vitulli‡ Institute on Membrane Technology, ITM-CNR, c/o University of Calabria, via P. Bucci, cubo 17/C, I-87030 Rende (CS), Italy, Istituto per la Chimica dei Composti OrganoMetallici, ICCOM-CNR, sezione di Pisa, c/o Dipartimento di Chimica e Chimica Industriale, Universita` di Pisa, via Risorgimento 35, I-50126 Pisa, Italy, and Dipartimento di Chimica e Chimica Industriale, Universita` di Pisa, via Risorgimento 35, I-50126 Pisa, Italy

A catalytic ruthenium membrane reactor (CRMR) is analyzed with regard to the methane partial oxidation reaction for synthesis gas production. The catalytic membrane consists of a commercial ceramic tube, where two different layers of ruthenium nanoparticles have been deposited on the inner surface. The metal deposition was performed by decomposition of a Ru complex under mild conditions. Different operating modes are analyzed for CRMR to establish the potentiality of the Ru catalyst deposited on the tubular ceramic support. A comparison in terms of methane conversion at various temperatures is presented. Experimental runs of CRMR containing a oneRu-layer membrane and a two-Ru-layer membrane have also been compared. 1. Introduction The major part of hydrogen production from natural gas in the world corresponds to 45 Mt/year and is realized by the steam reforming reaction1

CH4 + H2O ) CO + 3H2

∆H°298 K ) +206 kJ/mol (1)

This is a highly endothermic reaction, so that several engineering problems are involved because of the high thermal resistance as well as the high mechanical strength of the materials employed. Because of the well-known disadvantages of this reaction,2 other reactions have recently been considered. Partial oxidation of methane (POM) to syngas is one of these:

CH4 + 0.5O2 ) CO + 2H2

∆H°298 K ) -36 kJ/mol (2)

From a thermodynamic viewpoint, reaction (2) is a mildly exothermic reaction. However, it has been demonstrated in the literature that the overall reaction system is endothermic. In fact, the reaction mechanism involves the total combustion of methane (which produces carbon dioxide and water) followed by reforming and water gas shift reactions, as well as methane cracking and gasification reactions:3-5

CH4 + 2O2 S CO2 + 2H2O (total combustion of methane) (3) CH4 + H2O S CO + 3H2 (steam reforming reaction) (4) * To whom correspondence should be addressed. Tel.: (+39) 0984 492011. Fax: (+39) 0984 402103. E-mail: a.basile@ itm.cnr.it. † ITM-CNR. ‡ ICCOM-CNR. § Universita` di Pisa.

CH4 + 2H2O S CO2 + 4H2 (steam reforming reaction) (5) CO + H2O S CO2 + H2 (water gas shift reaction) (6) CH4 + CO2 S 2CO + 2H2 (dry reforming of methane) (7) 2CO S C + CO2 (Boudouard reaction)

(8)

CH4 S C + 2H2 (methane cracking)

(9)

C + H2O S CO + H2 (carbon gasification by steam) (10) C + O2 S CO2 (carbon gasification by O2)

(11)

In conclusion, the reaction system presents lower problems in terms of thermal management than methane steam reforming. With a H2/CO mole ratio of 2.0, POM would be a viable alternative reaction to the commonly adopted methane steam reforming reaction for syngas production. This reaction system has been largely studied using traditional reactors over, for example, Nibased catalysts,6 NiO/MgO catalysts,7 Ru monoliths,8 Ru/TiO2 catalyst,9,10 and Co/MgO catalysts.11 Different papers regarding the application of a membrane reactor concept are also present in the recent literature.12-16 In principle, a membrane reactor permits the selective removal of a certain product from the reaction system: with respect to a traditional process (i.e., a tubular packed-bed reactor), the membrane reactor gives the possibility of achieving either higher methane conversion operating at the same fixed temperature or the same methane conversion but operating at lower temperature, with a corresponding energy savings.17-19 From the viewpoint of this paper, however, a catalytic membrane seems to be an interesting way to fix a catalyst for a reaction system: the aim is to get a good catalyst distribution as well as an optimized catalyst

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activity. In fact, even though the Ru catalyst is already used for POM reactions,8-10 the application of a tubular support for the catalyst distribution is not present in the literature. With this regard, in this investigation, the membrane reactor performance for POM is evaluated by using a porous commercial membrane, having a tubular shape and an internal deposit of Ru nanoparticles. In particular, two Ru membranes have been produced, having one Ru layer and two Ru layers, respectively. The metal was deposited following a synthetic procedure previously used to deposit ruthenium nanoparticles on solid supports.20,21 The main focus of this experimental work is to realize a catalytic membrane rather than a highly selective one. In other words, the interest is mainly devoted to the preparation and characterization of a catalytic membrane rather than to a selective one, although the coupling of both good catalytic activity and high gas permselectivity could be a hazardous and successive ambition. Experimental results in terms of gas permeation and reaction tests are reported. Hydrogen and nitrogen permeance data, as well as methane conversion versus temperature plots, are presented.

Figure 1. Synthesis of the complex Ru(η6-cycloocta-1,3,5-triene)(η4-cycloocta-1,5-diene) and its reaction with hydrogen with the formation of metallic ruthenium.

Figure 2. Scheme of the membrane reactor module.

2. Description of the Experimental Apparatus The CRMR consists of a stainless steel module containing the tubular Ru-based composite membrane. The starting ceramic support is an alumina ceramic mesoporous tube furnished by SCT (France) and vitrified at the ends. Geometrical dimensions of the ceramic support are length ) 25 cm, i.d. ) 0.67 cm, and o.d. ) 1.02 cm. Two Ru layers were deposited on the internal surface by decomposition of the complex Ru(η6-cycloocta1,3,5-triene)(η4-cycloocta-1,5-diene), hereinafter referred to as Ru-complex. Both one-layer and two-layer membranes were housed in the stainless steel module. Both membranes were studied in terms of gas permeation and methane conversion for the POM reaction. 2.1. Membrane Preparation Procedure. The starting tubular support was filled with a yellow solution of 0.1 g (3.2 × 10-4 mol) of Ru-complex in 10 mL of dry n-octane. Afterward, the support was plugged at both ends. The system was placed in a Schlenk tube (a glass box) equipped with a side tape. In this box, a pressure of 0.1 mmHg was realized, and hydrogen was fed until the atmospheric pressure was reached. The box was left to rotate for 1 night. Then, the resulting uncolored solution was removed, and the composite membrane achieved (having ruthenium deposited on the internal side) was washed with a solution of pentane. Afterward, it was dried under an Ar stream. Also pentane was used as the solvent instead of n-octane. The described procedure was followed for making the first Ru deposition. The second layer of Ru was similarly deposited using 0.072 g (2.3 × 10-4 mol) of Ru-complex. The simple synthetic procedure to prepare Ru-complex is shown in Figure 1. The reaction with hydrogen, involved in the deposition of the metal on the support, is also shown in this figure. 2.2. Experimental Setup. The membrane was placed inside the stainless steel module and sealed by means of graphite O rings. Figure 2 shows the scheme of the membrane reactor module, consisting of a stainless steel tube housing the tubular membrane. In principle, two inlet streams are present: feed and sweep gas streams. Also, there are two outlet streams: a permeate stream

Figure 3. Scheme of the experimental plant adopted for carrying out the POM reaction.

and a retentate (i.e., not permeated) one. Two graphite gaskets ensure that the tube side (lumen) and the permeate side (shell) do not communicate during experimental tests. They are placed at both ends of the module. In this way, gases in the lumen side do not get in contact with gases in the shell side. The schematic diagram of the experimental reaction system is shown in Figure 3. The catalytic membrane was pretreated using H2 for 3 h under a flow rate of 1.12 × 10-3 mol/min at 500 °C. For reaction runs, gas flow rates of CH4, O2, and N2 were stabilized by means of mass-flow controllers. Reaction tests were performed using the following feed gas mixtures: CH4/O2/N2 ) 2/1/ 14; CH4 ) 2 × 10-3 mol/min. The permeate stream pressure (shell side) was continually kept at 1 atm, while the feed gas pressure was 1.2 bar abs. To establish which one gives the best performance in terms of methane conversion, CRMR was run in three different modes (Figure 4). In particular, Figure 4a shows the permeate closed configuration; i.e., the permeate outlet tube is kept closed and only one outlet stream is present. This configuration is similar to a traditional tubular reactor. Figure 4b is referred to as the retentate closed configuration; i.e. the retentate outlet tube is kept closed and the gaseous mixture exits from the permeate side. This configuration should give the maximum contact between the gaseous mixture and the dispersed Ru catalyst. Figure 4c shows the membrane reactor configuration; i.e., the membrane reactor is executed in the usual way. During the reaction, the maximum temperature variation along the module was 1 °C between the feed gas stream and the retentate gas stream. The maximum pressure difference was 0.02 bar between the

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temperature started from 350 °C and was increased up to 500 °C by 50 °C steps. Each temperature was kept for 1 h before sending the outlet streams to the gas chromatograph. Almost three chromatograms were made for each experimental point, and an average value was plotted in terms of methane conversion versus temperature. A similar procedure was followed for the two-layer CRMR, but the temperature started from 300 °C. Permeation tests using pure gases (hydrogen and nitrogen) were performed with the same experimental apparatus. Output results are expressed in terms of pure gas permeance, defined as

Pe )

J [mol/(s‚m2)] permeating flux ) pressure gradient ∆p [Pa]

All gases used were >99.995% pure. Two thermocouples were located at both ends of the module. The first one was located in the feed side; in particular, the measuring point was 1 cm inside the module and used to keep the temperature at the set-point value. The other thermocouple, a multiple (four points) one, was used in order to verify the flat temperature profile along the reactor. The reactor was heated using a thermolyne heating tape connected to a temperature controller (set point). 3. Results and Discussion

Figure 4. Operating modes for the CRMR: (a) permeate closed configuration; (b) retentate closed configuration; (c) membrane reactor configuration.

feed inlet and the retentate outlet. The reaction temperature range was 300-500 °C. No sweep gas was used. A gas chromatograph (Carlo Erba 4200) having a packed-column Carboxen 1000 for analyzing gas mixtures was used, with argon as the carrier gas (25 mL/ min). The oven temperature was fixed at 140 °C and the thermal conductivity detector at 250 °C. Methane conversion and hydrogen selectivity were calculated considering all of the involved streams, depending on the kind of configuration adopted. In particular, the following general definition of both the methane conversion and hydrogen reaction selectivity is given:

CH4 conversion (%) ) CH4,feed - (CH4,retentate + CH4,permeate) × 100 CH4,feed H2 selectivity (%) ) H2,retentate + H2,permeate

× (H2,retentate + H2,permeate) + (CO2,retentate + CO2,permeate) 100

During reaction tests, outlet gas streams were dried using a glass column containing H2O vapor adsorbent (drierite, furnished by the W. A. Hammond Drierite Co.). Then, the dry stream was fed to the gas chromatograph. The atom mass balances generally closed to within (8% in all experimental data reported in this work. With regard to experimental tests of the one-layer CRMR, the

Figure 5 shows the SEM images (cross section and top view) of the two-layer Ru membrane. In particular, Figure 5a shows the cross section of the membrane. The homogeneous Ru thickness (32 µm) is observable in Figure 5b, and the zoom (Figure 5d) shows the fine particles of Ru dispersed in the alumina matrix. A quite homogeneous metallic surface is observed in Figure 5c, although the Ru particles do not cover all of the ceramic surface. The first Ru layer consists of 32 mg of catalytic material, while the second one consists of 23 mg of additional metal. It should be mentioned that, using pentane as the solvent instead of n-octane, the diffusion of the solution from the internal side (lumen) to the external side (shell) of the membrane was observed, with a resulting external deposition of the metal on the tubular support. For this reason, we tested only the membranes treated with n-octane. With regard to both the one- and two-layer Ru membranes, Table 1 summarizes both the temperature and average pressure effects on hydrogen permeance, as well as the nitrogen one. Permeation tests were performed with pure gases, in the retentate closed configuration (Figure 4b). Permeance is defined as the gaseous flux [expressed in mol/(cm2‚s)] which permeates from the lumen side to the shell side divided by the average pressure value [pav ) (plumen + pshell)/2, expressed in Pa]. At room temperature (25 °C), the H2/N2 selectivity remains almost constant at 2.3-2.4 for both membranes. The H2/N2 selectivity is defined as the H2 pure gas permeance divided by the N2 pure gas permeance. When the temperature is changed from room temperature to higher temperature (500 °C), permeances increase for both gases in each membrane considered. For example, H2 permeance was 2.53 × 10-5 mol/(s‚m2‚ Pa) at 0.02 bar rel and 25 °C versus 3.49 × 10-5 mol/ (s‚m2‚Pa) at 0.02 bar rel and 500 °C. Permeation tests demonstrate that the permeation mechanism is due to the combined effects of both Knudsen and Poiseuille mechanisms. In fact, the ideal separation factor (i.e., the

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Figure 5. SEM photographs (cross section and top view) of the two-layer Ru membrane: (a) overview of the tubular section; (b) magnification of the cross section shown in part a; (c) top view; (d) zoom of a particular part of the cross section shown in part b. Table 1. Comparison between the One- and Two-Layer Ru Membranes in Terms of H2 and N2 Permeance versus the Average Pressure Value, at 25 and 500 °Ca first Ru layer

a

second Ru layer

Pe(H2) Pe(N2) H2/N2 pav

T ) 25 °C 2.53 1.05 2.4 0.02 bar rel

0.079 0.034 2.3 0.76 bar rel

Pe(H2) Pe(N2) H2/N2 pav

T ) 500 °C 3.49 1.25 2.8 0.02 bar rel

1.8 1 2.4 0.05 bar rel

Pe [)] 10-5 mol/(s‚m2‚Pa).

Knudsen mechanism) of H2/N2 is 3.74, higher than the experimental results of this work (2.3-2.8). Moreover, when the Knudsen mechanism is present alone, gas permeance decreases if the temperature increases; despite this, gas permeances increase if the temperature increases when the Poiseuille mechanism is present alone. The combination of the two cited effects is such that both H2 and N2 permeances increase with increasing temperature in our experimental conditions. Furthermore, at 25 °C, considering the one- and two-layer Ru membranes, both permeances decrease, so that the single gas H2/N2 selectivity keeps almost the same value (2.3-2.4). The permeance decreasing from the one- to

Figure 6. Methane conversion versus temperature for CRMR containing one layer of deposited Ru membrane, three membrane reactor configurations, p ) 0.2 bar rel, CH4/O2/N2 ) 2/1/14, and CH4,feed ) 2 × 10-3 mol/min.

two-layer membrane is probably due to the pore size reduction after the second Ru deposition: in this condition, the permeate flux decreases and, consequently, so does the permeance. Figure 6 shows the CRMR performance in terms of methane conversion versus temperature. In particular, these experimental results are related to the one-layer Ru membrane. The membrane reactor was run in the three different modes described above (and illustrated

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Ind. Eng. Chem. Res., Vol. 42, No. 13, 2003 Scheme 1. Effect of the Second Ru Deposition on the Methane Conversion in CRMR, Retentate Closed Configuration, under a Constant Mass Flow Rate of Reactants

Figure 7. Methane conversion versus temperature for CRMR containing one and two layers of deposited Ru membranes, retentate closed configuration, p ) 0.2 bar rel, CH4/O2/N2 ) 2/1/ 14, and CH4,feed ) 2 × 10-3 mol/min.

in Figure 4): permeate closed (i.e., a traditional reactorlike configuration), retentate closed (i.e., maximum contact between the Ru catalyst and the gaseous reactants), and membrane reactor configuration. This comparison demonstrates that similar methane conversion values have been achieved for all of the configurations studied. In principle, it seems that the best performance could be obtained by operating the membrane reactor in a traditional-like configuration (i.e., by keeping the shell side closed and considering the catalyst distributed along the reactor on the ceramic support; Figure 4a). However, the experimental evidence was such that no significant difference has been observed in the range of temperature investigated. For example, at 300 °C methane conversion was about 23% for both the retentate closed and the permeate closed configurations, and it was about 24% for the membrane reactor configuration. At 450 °C, methane conversion was about 37% for both the permeate closed and the membrane reactor configurations, and it was about 40% for the retentate closed configuration. The thermodynamic equilibrium curve is also plotted in this figure. According to the endothermicity of the overall reaction system, methane conversion always increases with an increase in the temperature. All of the experimental curves are below the thermodynamic equilibrium, except for 500 °C: at this temperature, both the retentate closed and the permeate closed configurations approach the thermodynamic value (54%), while the membrane reactor gives about 59% of methane conversion. Considering the retentate closed configuration (Figure 4b), it should be noted that all reactants are able to get in contact with the deposited Ru particles within the alumina matrix (Figure 5d). Nevertheless, the limited selectivity of the membrane is such that also reactants permeate, and an immediate benefit of the membrane reactor is not present. However, a little benefit is observed at 500 °C. Anyway, the effectiveness of the dispersed Ru catalyst on the ceramic membrane is completely demonstrated because the methane conversion approaches the thermodynamic equilibrium prediction. Figure 7 plots the behavior of CRMR in terms of methane conversion versus temperature. In particular, a comparison of the experimental results between oneand two-layer Ru membrane reactors is shown. The retentate closed configuration is considered (Figure 4b) so that only the permeate outlet stream was present.

The thermodynamic equilibrium curve is also plotted in this figure. As was already mentioned above, in the retentate closed configuration, the reactant gases keep full contact with the Ru catalyst deposited inside the membrane. For both kinds of membranes, an increasing trend was observed, according to the endothermicity of the overall reaction system. In particular, considering the one-layer Ru membrane, methane conversion was about 23% at 350 °C and 54% at 500 °C. Nevertheless, methane conversion was about 15% at 300 °C and about 55% at 500 °C for the two-layer Ru membrane. The twolayer Ru membrane seems to give a lower methane conversion with respect to the one-layer Ru membrane, and the maximum difference is achieved at 450 °C. At this temperature, the maximum methane conversion is about 40% for the one-layer Ru membrane versus a methane conversion of about 23% related to the twolayer Ru membrane. This experimental evidence can be explained considering two opposite effects on methane conversion, due to the second Ru deposition. The situation can be discussed by following Scheme 1. In particular, with regard to the catalytic effect, an increase in the Ru amount should give an increase in methane conversion. On the contrary, the second Ru deposition should reduce the pore size, increasing the gas velocity throughout the membrane (permeation effect): in this condition, the contact time with the Ru catalyst decreases, so methane conversion is negatively affected. In our experimental condition, the second effect seems to prevail. Considering the atom mass balance closure, the same conversion is approached when the temperature reaches 500 °C. With regard to the hydrogen reaction selectivity, the one-layer Ru membrane gives a value of 50% at 500 °C versus a value of 71% for the two-layer Ru membrane, at the same temperature, operating in the retentate closed configuration (Figure 4b). The hydrogen reaction selectivity (regarding the hydrogen produced during the chemical reaction) is defined as the ratio between the hydrogen produced over the CO2 byproduct in the outlet stream(s) of the membrane reactor (see section 2.2). Although at 500 °C methane conversion is similar for both one- and two-layer Ru membranes, the hydrogen

Ind. Eng. Chem. Res., Vol. 42, No. 13, 2003 2973 Table 2. Methane Conversion (%) for Different Kinds of Membrane Reactors, p ) 0.2 bar rel, CH4/O2/N2 ) 2/1/14, CH4,feed ) 2 × 10-3 mol/min, N2,sweep ) 1.56 × 10-3 mol/min (for PMR)a T (°C) CRMR (n ) 2) CPMR22 (n ) 2) PMR5 400 450

18 24

10 28

45.3 56.2

thermodynamic equilibrium 34 43

a CRMR ) catalytic ruthenium membrane reactor. CPMR ) composite palladium membrane reactor.22 PMR ) dense palladium membrane reactor.5

selectivity is higher for CRMR housing the two-layer Ru membrane. In other words, a higher hydrogen yield was achieved after the second Ru deposition. This aspect could be explained by considering that the partial oxidation reaction results from the sum of many reactions (see the Introduction), so probably changing from the first to the second Ru layer, a different contribution of each single reaction is present. Finally, 100% of O2 conversion was achieved in all of the experimental conditions adopted. Table 2 shows comparisons with the literature data5,22 in terms of methane conversion for three different kinds of membrane reactors. The aim of these comparisons is to show our membrane reactor performance in terms of methane conversion achieved using a relatively low amount of the Ru catalyst. The comparisons are indeed qualitative and point out the interesting methane conversion values with respect to other membrane reaction systems present in recent literature. CRMR housing the two-layer Ru membrane is considered in the retentate closed configuration. At 400 °C, CRMR gave 18% in terms of methane conversion versus 10% for the composite palladium membrane reactor (CPMR22) and 45.3% for the dense palladium membrane reactor (PMR5). A total of 34% of methane conversion for thermodynamic equilibrium is predicted. At 450 °C, the trend is similar: 24% in terms of methane conversion for CRMR versus 28% for CPMR22 and 56.2% for PMR.5 A total of 43% of methane conversion for thermodynamic equilibrium is predicted. It should be noted that CRMR operating at 400 °C overcomes the performance of CPMR,22 which contains a tubular composite palladium membrane packed with about 3 g of a commercial Ni-based catalyst: this comparison confirms the potentiality of the catalytic Ru-based membrane in terms of a good catalyst activity per unit of catalyst weight, due to the particular catalyst distribution over the ceramic commercial tubular support. Moreover, with regard to CRMR housing the one-layer Ru membrane (n ) 1) and considering the retentate closed configuration (Figure 7), methane conversion approaches the thermodynamic equilibrium. In fact, it reaches about 30% at 400 °C and about 40% at 450 °C versus 34% and 43% predicted from thermodynamic equilibrium, respectively. Another comparison can be made by considering a recent paper dealing with POM carried out in a tubular packed-bed reactor over a Ru-based catalyst.10 At 500 °C, methane conversion reached 20% in their experimental conditions versus 54% achieved in this work for both the permeate closed and the retentate closed configurations and 59% for the membrane reactor configuration (all of these values are referred to as the one-layer Ru membrane; see Figure 6). The main reason is the difference in the experimental conditions, in particular the catalyst load in the reactor. In fact, in their work the catalyst load was 5 mg of dispersed Ru

at a metal loading of 0.5 wt %10 versus 32 mg for the one-layer Ru membrane of this work. Furthermore, those authors adopted a feed ratio CH4/O2/N2 ) 6/3/91 (total feed rate ) 250 mL/min)10 versus a feed ratio of CH4/O2/N2 ) 2/1/14 (total feed rate ) 381.5 mL/min) in this work. These differences (in particular the difference in the catalyst load) well explain why methane conversion in our work approaches the equilibrium while methane conversion in the cited paper10 is lower than equilibrium. However, it should be stressed that there is a substantial difference in the preparation of the active Ru catalyst because in the mentioned paper10 Ru metal is dispersed over a doped TiO2 carrier. In principle, the Ru catalyst needs such doped supports in order to exhibit high catalytic activity. In our work, however, the starting tubular support did not suffer any pretreatment, and nevertheless the deposited Ru exhibited an interesting catalytic activity, such that thermodynamic equilibrium conversion was approached. On these bases, the reader could conclude that the first Ru deposition (32 mg of Ru) makes the commercial ceramic tube a very interesting support for achieving high methane conversion values. 4. Conclusions Methane conversion into syngas via a partial oxidation reaction was studied in a catalytic membrane reactor containing one- and two-layer Ru membranes. Ru was deposited in a very simple way by decomposition of a Ru complex under a hydrogen stream. The consecutive Ru deposition inside a commercial ceramic support affects methane conversion at each temperature investigated. For what concerns gas permeation, the composite membrane shows H2/N2 selectivity lower than the Knudsen one. Despite this, successful reaction tests were achieved: the maximum methane conversion was about 59% at 500 °C for the membrane reactor configuration, time factor ) 27.5 gRu‚min/molCH4, and p ) 1.2 bar versus a thermodynamic equilibrium value of 54%. Moreover, 100% of O2 conversion was achieved in the experimental conditions adopted. However, a deeper investigation is needed to understand the effect of consecutive Ru deposition inside the tubular ceramic membrane. Two conflicting effects arise on the methane conversion as a result of the consecutive Ru deposition: this aspect could be deeply analyzed by performing other Ru depositions and checking their effects on the membrane reactor performance. The examination of the catalyst surface could be helpful, but this is not the aim of this paper. Membrane reactors equipped with a catalytic membrane could be interesting for their easier and cheaper preparation. In fact, although the H2/N2 selectivity seems low (2.3-2.8), particular benefits toward methane conversion are also due to the fine catalyst distribution, as well as a good catalyst activity (per unit of catalyst weight) over a ceramic commercial tubular support. In conclusion, the Ru deposition performed with the described technique produced a catalytic membrane that gave methane conversion values near the thermodynamic predictions, at each temperature investigated. Although, in principle, the Ru catalyst needs doped supports in order to exhibit high catalytic activity, in our work the starting tubular support did not suffer any pretreatment, and nevertheless the deposited Ru exhibited an interesting catalytic activity, such that thermodynamic equilibrium conversion was approached.

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Acknowledgment The authors thank Mr. Giuseppe Calabro` (University of Calabria) and Mr. Angelo Fama` (CNR-ITM) for the execution of some experimental tests. Also thanks to Ms. Enrica Fontananova for the SEM photographs of the catalytic membrane. Special thanks goes to Mrs. America Oliva for improving the English of the paper. Literature Cited (1) Gaudernack, B.; Lynum, S. Hydrogen from natural gas without release of CO2 to the atmosphere. Int. J. Hydrogen Energy 1998, 23 (12), 1087. (2) Basile, A.; Paturzo, L. An experimental study of multilayered composite palladium membrane reactors for partial oxidation of methane to syngas. Catal. Today 2001, 67 (1-3), 55. (3) Dissanayake, D.; Rosynek, M. P.; Kharas, K. C. C.; Lunsford, J. H. Partial oxidation of methane to carbon-monoxide and hydrogen over a Ni/Al2O3 catalyst. J. Catal. 1991, 132, 117. (4) De Groote, A. M.; Froment, G. F. Simulation of the catalytic partial oxidation of methane to synthesis gas. Appl. Catal. A 1996, 138, 245. (5) Basile, A.; Paturzo, L.; Lagana`, F. The partial oxidation of methane to syngas in a palladium membrane reactor: simulation and experimental studies. Catal. Today 2002, 67 (1-3), 65. (6) Chu, Y.; Li, S.; Lin, J.; Gu, J.; Yang, J. Partial oxidation of methane to carbon monoxide and hydrogen over NiO/La2O3/γAl2O3 catalyst. Appl. Catal. A 1996, 134, 67. (7) Ruckenstein, E.; Hu, Y. H. Methane partial oxidation over NiO/MgO solid solution catalysts. Appl. Catal. A 1999, 183, 85. (8) Hickman, D. A.; Haupfear, E. A.; Schmidt, L. D. Synthesis gas-formation by direct oxidation of methane over Rh monoliths. Catal. Lett. 1993, 17, 223. (9) Boucouvalas, Y.; Zhang, Z.; Verykios, X. E. Partial oxidation of methane to synthesis gas via the direct reaction scheme over Ru/TiO2 catalyst. Catal. Lett. 1996, 40, 189. (10) Elmasides, C.; Verykios, X. E. Mechanistic study of partial oxidation of methane to synthesis gas over modified Ru/TiO2 catalyst. J. Catal. 2001, 203, 477. (11) Chang, Y.-F.; Heinemann, H. Partial oxidation of methane to syngas over Co/MgO catalystssis it low-temperature? Catal. Lett. 1993, 21, 215.

(12) Kikuchi, E.; Nemoto, Y.; Kajiwara, M.; Uemiya, S.; Kojima, T. Steam reforming of methane in membrane reactors: comparison of electroless-plating and CVD membranes and catalyst packing modes. Catal. Today 2000, 56 (1-3), 75. (13) Nam, S. W.; Yoon, S. P.; Ha, H. Y.; Hong, S.-A.; Maganyuk, A. P. Methane steam reforming in a Pd-Ru membrane reactor. Korean J. Chem. Eng. 2000, 17 (3), 288. (14) Jarosch, K.; de Lasa, H. I. Novel Riser Simulator for Methane Reforming using High-Temperature Membranes. Chem. Eng. Sci. 1999, 54, 1455. (15) Oklany, J. S.; Hou, K.; Hughes, R. A simulative comparison of dense and microporous membrane reactors for the steam reforming of methane. Appl. Catal. A 1998, 170 (1), 13. (16) Shu, J.; Grandjean, B. P. A.; Kaliaguine, S. Methane steam reforming in asymmetric Pd- and Pd-Ag/porous SS membrane reactors. Appl. Catal. A 1994, 119, 305. (17) Coronas, J.; Santamaria, J. Catalytic reactors based on porous ceramic membranes. Catal. Today 1999, 51, 377. (18) Saracco, G.; Neomagus, H. W. J. P.; Versteeg, G. F.; Van Swaaij, W. P. M. High-temperature membrane reactors: potential and problems. Chem. Eng. Sci. 1999, 54, 1997. (19) Paturzo, L.; Basile, A.; Drioli, E. High-Temperature Membrane Reactors and Integrated Membrane Operations. Rev. Chem. Eng. 2002, 18 (6), 511. (20) Pertici, P.; Vitulli, G. (Cycloocta-1,3,5-triene)(cycloocta-1,5diene)ruthenium(0) in the development of ruthenium chemistry. Comments Inorg. Chem. 1991, 11, 175. (21) Pertici, P.; Scalera, N.; Vitulli, G.; Salvadori, P.; Hoang, M.; Turney, T. W. Polyorganophopshazene-ruthenium systems: preparation, structural study and catalytic activity. 9th International Symposium on Macromolecule-Metal Complexes, Brooklyn, New York, Aug 19-23, 2001; p 37. (22) Paturzo, L.; Basile, A. Methane Conversion to Syngas in a Composite Palladium Membrane Reactor with Increasing Number of Pd Layers. Ind. Eng. Chem. Res. 2002, 41 (7), 1703.

Received for review November 7, 2002 Revised manuscript received March 5, 2003 Accepted March 6, 2003 IE020873X