Brief Overview of the Partial Oxidation of Methane to ... - Springer Link

Topics in Catalysis Vol. 22, Nos. 3–4, April 2003 (# 2003)

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Brief overview of the partial oxidation of methane to synthesis gas Andrew P.E. York, Tiancun Xiao, and Malcolm L.H. Green The Wolfson Catalysis Centre, Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QR, UK

A review of the main developments in the partial oxidation of methane to synthesis gas since the first paper in 1929 to the present day is given. The reaction is discussed from the view of the thermodynamics; the main catalysts studied for the reaction are summarised, and the reaction mechanism is discussed. The review is not comprehensive, but it is designed to provide a general background to the most important developments in the field. KEY WORDS: methane; catalytic partial oxidation; synthesis gas; catalysts

Introduction Methane is the predominant component of natural gas, which is forecast to outlast oil by a significant margin (around 60 years) [1] despite the relatively underdeveloped state of the gas industry. Furthermore, natural gas is found plentifully in many locations around the world [2]. Therefore, most studies on the utilisation of natural gas concentrate almost exclusively on methane activation; but it also normally contains small quantities of higher hydrocarbons (e.g. ethane, propane, etc.), and sometimes other gases such as hydrogen sulfide, carbon dioxide and nitrogen [3]. Currently, the use of natural gas as a feedstock for chemical synthesis or for the synthesis of fuels is uneconomical due to the low cost of oil coupled with the high cost of natural gas storage and transportation from the remote reservoirs where it is most abundant. Methods to enhance the value of natural gas, either by synthesising more valuable chemicals or more readily transportable products, have been investigated, particularly in the last 20 years, but yields tend to be too low to compete with oil because the products are more reactive than methane. For example, in the oxidative coupling of methane to ethane and ethane there is an inherent limit to the maximum achievable yield of around 30% [4–6], since an important part of the reaction mechanism involves gas-phase kinetics, while in the continuous direct oxidation of methane to methanol [7,8] or formaldehyde [9] the maximum yields so far obtained are around 8% and 4%, respectively. A batch process giving methanol yields > 50% was recently described, but this is also less than ideal, due to the need for a mercury catalyst and the consumption of sulfuric acid, leading to sulfur dioxide which then needs to be converted back to sulfuric acid [10]. Industrial processes for the production of hydrogen cyanide [11] in the

To whom correspondence should be addressed. E-mail: [email protected]

reaction of methane with ammonia (Degussa) or ammonia and oxygen (Andrussow), and ethyne by pyrolysis [12], are available but both have drawbacks due to the extremely high temperatures required for reactions ð> 1300 KÞ: At this time, the only economically available route for the conversion of methane into more valuable chemicals is via synthesis gas. Several synthesis gas production methods are available, depending on the purpose of industrial application. As shown in table 1, synthesis gas can come from steam reforming, oxyreforming or decomposition of methanol (mainly used in hydrogen production for fuel cells [13,14], because methanol is easy to transport and has a high energy density); methanol is synthesised from synthesis gas produced from coal or natural gas. Using methane to prepare synthesis gas can be realised through three reactions, i.e., steaming reforming, dry reforming and partial oxidation. To date, the only large-scale process for natural gas conversion is the endothermic reaction known as steam reforming to synthesis gas [15]: CH4 þ H2 O ¼ CO þ 3H2

H298 ¼ þ206 kJ mol1 :

The first description of a process for the conversion of hydrocarbons with steam was published in 1868 using CaO as a medium, resulting in the formation of CaCO3 and hydrogen [16]. In 1890 Mond and Langer improved the process by using a nickel catalyst [17], and it was subsequently used, in combination with Fischer– Tropsch technology, by Germany in World War II and South Africa during the Apartheid era for the synthesis of chemicals such as fuels and alcohols. The process is highly endothermic and current industrial catalysts are usually based on nickel. However, nickel also promotes carbon formation, which leads to catalyst deactivation and reactor plugging. To overcome this problem, and render carbon formation thermodynamically unfavorable, industrial steam reformers add excess 1022-5528/03/0400–0345/0 # 2003 Plenum Publishing Corporation

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Process name

Reaction

H298 (kJ

Steam reforming

CH4 þ H2O ¼ CO þ 3H2

CO2 reforming (dry reforming)

Industrial application

Advantages

Associated problems

206

H2 production, synthesis gas production

Low carbon deposition, suitable for highpressure processes, easy separation of the products

High H2/CO ratio, need separation for follow-up F-T or methanol synthesis, energyintensive process

CO2 þ CH4 ¼ 2CO þ 2H2

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Synthesis gas or H2 production

Use of two greenhouse gases, i.e. CO2 as the feedstock, high availability in some gas fields

Energy-intensive process, low H2/CO ratio, more H2 is needed for follow-up F-T or methanol process, easy carbon deposition

Partial oxidation (oxy-reforming)

CH4 þ 12 O2 ¼ CO þ 2H2

36

Synthesis gas or H2 production

Mild exothermic reaction, energy saving, H2 to CO ratio 2 (suitable for methanol or F-T synthesis)

Hot-spot may occur in the catalyst bed

Methanol steam reforming

CH3 OH þ H2O ¼ CO2 þ 3H2 CH OH þ 1 O ¼ CO þ 2H

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H2 production

High yield of H2

Energy-intensive process

192

H2 production

Exothermic reaction, save energy

Hot-spot may occur in the catalyst bed

Methanol oxy-reforming

3

2

2

2

2

mol)

A.P.E. York et al./Partial oxidation of methane to synthesis gas

Table 1 Comparison of synthesis gas production from different routes


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the thermodynamic advantages this process has over steam reforming.

Figure 1. Summary of methane conversion routes.

quantities of steam to the feedstock. While suppressing carbon formation, this practice creates a new problem, namely an increase in the H2 =CO and/or CO2 =CO ratios, where low ratios are desirable for optimal downstream processes [15]. An alternative process for synthesis gas formation is the exothermic methane partial oxidation reaction: CH4 þ 12 O2 ¼ CO þ 2H2

H298 ¼ 36 kJ mol1 :

This process, like steam reforming, has a long history, but has attracted much less attention until the last decade. A summary of some of the routes that have been investigated for methane utilisation and valorisation is shown in figure 1.

2. Thermodynamic analysis of methane partial oxidation Figure 2 shows the partial oxidation of methane schematically and also gives some thermodynamic information. This process is likely to become more important in the future of methane conversion due to

(1) Partial oxidation is mildly exothermic, while steam reforming is highly endothermic. Thus, a partial oxidation reactor would be more economical to heat. In addition it can be combined with endothermic reactions, such as steam reforming or dry reforming with carbon dioxide to make these processes more energy efficient. (2) The H2 =CO ratio produced in stoichiometric partial oxidation is around 2, and this ratio is ideal for downstream processes, in particular methanol synthesis. This avoids the need to remove valuable hydrogen, which is produced in excess in steam reforming. (3) The product gases from methane partial oxidation can be extremely low in carbon dioxide content, which must often be removed before synthesis gas can be used downstream. (4) Partial oxidation technology avoids the need for large amounts of expensive superheated steam. However, an oxygen separation plant, which is also costly, may be required in cases where nitrogen (from air) is undesirable in high-pressure downstream processes. The first papers detailing the catalytic partial oxidation of methane to synthesis gas were published in 1929 by Liander [18], in 1933 by Padovani and Franchetti [19] and in 1946 by Prettre et al. [20]. However, high yields of synthesis gas were only obtained at temperatures in excess of 850 8C. The latter studies showed that below this temperature non-equilibrium product distributions were observed. In addition, carbon formation over the supported nickel catalysts used was not studied in any detail. Because of these factors, as well as the success of the steam reforming process, partial oxidation was left alone for decades.

Figure 2. Thermodynamic representation of the partial oxidation of methane.

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The effect of reaction conditions on the product distribution of the methane partial oxidation reaction is discussed theoretically from the view of thermodynamics below. It is proposed that methane partial oxidation to synthesis gas proceeds in two steps. First methane is combusted by oxygen to give CO2 and H2 O: Then the remaining unreacted methane is reformed with the H2 O and CO2 to give CO and H2 : The overall calculation was carried out on the basis of the following reactions: 2 CH4 þ O2 ! 1:5 CH4 þ 0:5 CO2 þ H2 O 0:5 CO2 þ 0:5CH4 ! CO þ H2

Figure 3. Thermodynamic calculation of partial oxidation of methane to synthesis gas, changes of CH4 conversion and CO and H2 selectivity with temperature and pressure.

In the late 1980s Green and co-workers began a renaissance in the study of methane partial oxidation. While investigating trends in the behavior of the lanthanides for oxidative coupling using pyrochlores containing noble metals and rare earth metals, they observed high yields of synthesis gas [21]. Studies revealed reduction of the noble metal ruthenium in a lanthanum pyrochlore ðLn2 Ru2 O7 Þ resulted in a lanthanide oxide-supported ruthenium catalyst which had excellent activity for methane partial oxidation. This time no carbon could be seen using the naked eye on the post-reaction samples, and confirmation of this was obtained by high-resolution electron microscopy. This observation prompted a detailed investigation of stoichiometric methane partial oxidation over noble metals, and other catalysts, by a very substantial number of research groups.

CH4 þ H2 O ! CO þ 3H2 2 CH4 þ O2 ! 2 CO þ 4 H2 : On the basis of this mechanism, thermodynamic calculations were performed using the ‘‘Thermo Chemical Calculator’’ (Chemkin, incorporating Stanford’s STANJAN equilibrium solver), for a stoichiometric feedstock and assuming no carbon formation. The data plots are presented in a three-dimensional form (figure 3). For methane partial oxidation to syngas, the thermodynamic calculation results suggest a high temperature is advantageous for high methane conversion and selectivity to CO and H2 : However, increasing the pressure in the reactor is unfavorable for CH4 conversion and CO and H2 selectivity. The prediction of the CH4 conversion and product selectivity under specific conditions is shown in two-dimensional forms (figures 4(a) and (b)). Apparently, under 1 bar at 1073 K, theoretical CH4 conversion should be up to 90%, and selectivities to CO and H2 are 97%. At 8 bar and 1073 K, CH4 conversion is only 70%, and CO and H2 selectivities are around 85%.

Figure 4. Thermodynamic equilibrium calculations at: (a) atmospheric pressure and (b) at 8 bar); (&) XCH4 ; (.) S[CO], (~) S½CO2 ; (!) S½H2 :


3. Catalysts for partial oxidation of methane to synthesis gas After nearly 100 years of development, three main types of catalysts have been investigated for the partial oxidation of methane to synthesis gas. They are supported nickel, cobalt or iron catalysts; supported noble metal catalysts; and transition metal carbide catalysts. Some of these catalyst systems will now be discussed in greater detail.

3.1. Supported nickel, cobalt or iron catalysts The earliest work on catalytic partial oxidation was performed by Liander [18], Padovani and Franchetti [19], and Prettre et al. [20] who found that synthesis gas with the product ratio of H2 =CO ¼ 2 could be produced at 1000–1200 K and 1 atm over supported nickel catalysts. Most researchers since have come to similar conclusions to these early researchers; nickel is highly active for synthesis gas production, but that it also catalyses carbon formation. Lunsford and co-workers studied a nickel supported on alumina partial oxidation system in the temperature range 720–1173 K. They reported that CO selectivity approaching 95% and virtually complete conversion of methane could be achieved at temperatures higher than 973 K. It was found, however, that stable operation is not sustainable unless an amount of oxygen larger than the stoichiometric requirement (i.e. O2 =CH4 > 0:5) is supplied. They also found that three regions existed in the catalyst bed; (i) NiAl2 O4 ; (ii) NiO=Al2 O3 ; which is active for complete combustion of methane to CO2 and H2 O; and (iii) supported nickel metal particles, which are active for reforming of methane with CO2 and H2 O to synthesis gas [22]. It is now generally agreed that it is the Ni metal which is the active component for synthesis production via methane partial oxidation. However, under conditions where synthesis gas is produced, carbon deposition also occurs. To decrease the carbon deposition and increase the stability of the support and thus to extend the catalyst lifetime, much work has been done on the modification of the support. Choudhary and co-workers [23–32] have studied nickel catalysts supported over ytterbium oxide, CaO, TiO2 ; ZrO2 ; ThO2 ; UO2 and rare earth oxide-modified alumina supports. They found that the NiO containing MgO, CaO, rare earth oxides or alumina catalysts showed high catalytic activity in the process at a very low contact time. The order of the performance for nickel containing ZrO2 ; ThO2 and UO2 is NiO=ThO2 > Ni=UO2 > NiO=ZrO2 :SiO2 and TiO2 are not good supports for the reaction, because sintering of Ni gives inactive binary metal oxide phases under the high-temperature reaction conditions. Although modification of the supports can improve the catalyst stability to some extent, catalyst deactivation is still unavoidable

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due to both carbon deposition and to loss of the nickel metal surface area. On a Ni/MgO system a substantial amount of carbon was produced [22–25]. The Ni/MgO catalyst system has been widely studied by Ruckenstein [32,33] and Santos et al. [34]. It is believed that the observed high catalyst stability results from the formation a the solid solution, e.g., Ni occupies sites in the MgO lattice and is distributed evenly in the catalyst. The Ni is present as very small particles and the weak basicity of the MgO somewhat suppresses carbon deposition [35– 37]. However, the formation of the solid solution in the catalyst only extends the catalyst life and deactivation eventually occurs due to carbon deposition. Addition of rare earth metal oxide or alkaline metal oxide to alumina or the use of rare-earth metal oxide as support can restrict carbon deposition [38–45]. Nickel catalysts supported with rare earth oxide- and alkaline metal oxide-modified alumina have been tested for 500 hours without observable decrease of XCH4 and SCO and SH2 : [46] The promotion of the rare earth oxide addition on the catalyst support is probably due to its capability for oxygen storage, which can help by oxidising the surface carbon deposited. It is also believed that the presence of a rare earth oxide such as CeO2 can stablise the support and prevent it from sintering during the high-temperature reaction. Other supports such as CaAl2 O4 [42] and AIPO4 -5 [47] have also been investigated as supports for nickel catalysts for POM to synthesis gas. It seems that CaAl2 O4 has a high ability to resist sintering and carbon deposition, hence giving good CH4 conversion and CO and H2 selectivity. AIPO4 5 supported catalysts show high performance for the POM reaction to synthesis gas. However, with the time on stream, AIPO4 5 is transformed into tridymites, and the surface area of the support decreases dramatically. A similar promoting effect of the support on the suppression of carbon deposition with nickel catalysts is found with perovskite catalysts (Ni=Ca0:8 Sr0:2 TiO3 ). These were prepared using the citrate method, and could catalyse methane partial oxidation to thermodynamic equilibrium with only a minute amount of carbon detectable after 150 h reaction [48,51]. The authors proposed the support was able to control the size of the metal crystallites, maintaining them below the threshold size at which carbon formation becomes a problem [50]. They also suggested that oxygen atoms in the support may be able to react with surface carbon, thereby keeping the nickel surface carbon free. Another way to control nickel metal crystallite particles in a supported catalyst is to prepare a catalyst by reduction of Ni containing Ni/ Mg/Al hydrotalcite-type precursors [51]. The catalyst activity and selectivity is related to the reductivity of nickel metal in the catalyst, and the nickel metal content and contact time of the reactants have a significant influence on the catalyst performance. Further studies on this catalyst system are currently ongoing.

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In addition to the modification of the supports to improve the stability of nickel catalyst for POM reaction, other active components such as Co, Fe or noble metal such as Ru, Pt and Pd were added to the catalysts to assist in reduction of carbon deposition. Provendier [52] found that addition of iron can stabilise nickel catalysts. They prepared mixed LaNix Feð1xÞ O3 perovskites ð0 x 1Þ using a sol–gel related method. These systems are the precursors of highly efficient catalysts in partial oxidation of methane to synthesis gas. Nickel can be stabilised by increasing the amount of iron. These systems permit control of the reversible migration of nickel from the structure to the surface. Choudhary [30] pointed out that addition of cobalt to NiO=Yb2 O3 ; NiO=ZrO2 and NiO=ThO2 catalysts causes a drastic reduction in the rate of carbon formation and also resulted in a large decrease in the onset temperature (or the catalyst activation) of the oxidative conversion of methane to synthesis gas, because the presence of Co promotes the reduction of Ni and thus improves the catalyst activity. There have been some studies on supported Co or Fe catalysts for the partial oxidation of methane to synthesis gas. But it is thought that Co and Fe catalysts have a much lower performance for POM reaction, because CoO and Fe2 O3 have higher activity for complete oxidation of methane [31,44,53]. The order for POM activity of the supported catalysts is Ni Co > Fe: It is pointed out that cobalt catalysts are active for POM to synthesis gas only when promoted with elements, which favored Co reducibility [54]. This is why cobalt catalysts with high Co content often show high performance for the POM reaction [28,55]. It is proposed that the active component of cobalt catalysts for POM is metallic Co, and cobalt metal is much easier to oxidize. However, the stability of the cobalt catalyst depends on the method of preparation [56,57]. The choice of support has a significant effect on the performance of the cobalt

catalyst for POM. The deactivation of an aluminasupported cobalt catalyst may be due to the sintering of the active component and the formation of CoAl2 O4 : High methane conversion to synthesis gas has been achieved over Co=ZrO2 ; while conversion decreased dramatically when Co=La2 O3 was used [58]. In summary, supported Ni and Co catalysts have been widely studied for methane partial oxidation to synthesis gas. Very limited attention has been paid to iron catalysts. The deactivation of nickel catalyst for partial oxidation of methane is due to carbon deposition, and the loss of nickel due to the high flow rate. Co and Fe have higher melting and vaporising points than nickel, which may be alternatives for nickel catalysts if a higher performance can be obtained. Further clues to techniques for modifying nickel catalysts to suppress carbon formation may be obtained from the literature on steam reforming, where methods for controlling crystallite size or the addition of dopants are well established [15].

3.2. Group VIII transition metals Green and co-workers showed that high yields of synthesis gas can be obtained over nearly all the noble metal catalysts used, as well as over the rare earth ruthenium pyrochlores. For example, under reaction conditions of 1050 K and 1 bar, for stoichiometric partial oxidation with air, they obtained a methane conversion of 94%, with carbon monoxide and hydrogen selectivities of 97% and 99%, respectively, at total oxygen conversion [59–62]. Further, all the catalysts were catalysing the partial oxidation reaction to thermodynamic equilibrium, ignoring elemental carbon formation. No carbon deposition was seen on these catalysts and a study by Claridge et al. has shown that the relative order of carbon formation is Ni > Pd Rh, Ru, Ir, Pt (figure 5) [63].

Figure 5. Carbon formation over catalysts after stoichiometric methane partial oxidation at 1 atm and 1050 K (Ni ¼ Ni (Harshaw) catalyst; all experiments were run for 24 h, except the Ni catalysts: CRG ‘‘F’’, CRG ‘‘H’’ and Ni were run for 150, 80 and 60 mins, respectively).


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The effect of different reaction conditions on the catalyst performance in POM reaction is shown in figure 6. The experimental product distributions appear to match closely the theoretical data, which we have reported above.

3.3. Effect of temperature The effect of temperature on the product distribution on the partial oxidation of methane can be predicted from thermodynamic equilibrium calculations for most of the range of temperatures used. At lower temperatures ð< 450KÞ no conversion is achieved as the kinetic barrier to the reaction has not been reached. For temperatures between around 500 and 1120 K the products can be predicted from thermodynamics. As the temperature is increased the selectivity to carbon monoxide and hydrogen increases and the conversion of methane also increases. This results in extremely high yields of synthesis gas at temperatures around 1050 K, as shown in figure 6(a).

3.4. Effect of pressure The effect of pressure on the partial oxidation reaction is shown in figure 3. Increasing the pressure in the system leads to a decrease in both the methane conversion, and the selectivities to carbon monoxide and hydrogen, as predicted by the thermodynamics. Entropically, at elevated pressure, the reforming reactions are less favourable, since they result in gas expansion, and are driven away from synthesis gas formation, whilst the total combustion reaction is much less affected, so that carbon dioxide and water formation becomes increasingly favorable with increasing pressure. The experimental results of POM over Ln2 Ru2 O7 are shown in figure 6(b), which are very close to the thermodynamic predictions.

3.5. Effect of reactant ratio The effect of the CH4 =O2 ratio on the product distribution from the partial oxidation reaction is shown in figure 6(c). At a CH4 =O2 ratio of 2 the stoichiometry is ideal for the partial oxidation reaction, and high yields of synthesis gas are produced. However, if this ratio is decreased, the stoichiometry becomes closer to that needed for the total combustion reaction. Therefore, the formation of carbon dioxide and water becomes more favorable, at the expense of synthesis gas selectivity, while the methane conversion increases. If the methane/ oxygen ratio is increased, an excess of methane is present leading to a decrease in the methane conversion. However, the stoichiometry is now further from that

Figure 6. Effect of reaction conditions on the partial oxidation of methane over Ln2 Ru2 O7 ((&) XCH4 ; (.) S[CO], (~) S½H2 ). All conditions the same unless stated otherwise ðCH4 =O2 ¼ 2; T ¼ 1050 K; p ¼ 1 atm; GHSV ¼ 4 104 h1 Þ: (a) Temperature ðLn ¼ YbÞ; (b) Pressure ðLn ¼ DyÞ; (c) Reactant ratio ðLn ¼ PrÞ; (d) Gas hourly space velocity ðLn ¼ PrÞ:

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required by the total combustion reaction, and so the synthesis gas selectivity increases.

3.6. Effect of reactant flow rate At low space velocities, i.e. less than 4 104 h1 ; the thermodynamic equilibrium product distribution is reached. However, at higher space velocities the kinetics of the reaction become limiting, and some of the reactions, that may contribute to the mechanism for the partial oxidation reaction do not reach equilibrium. Therefore, under these conditions the thermodynamic equilibrium is not reached. Another important consideration at these higher flow rates is the exothermicity of the reaction, potentially leading to hot-spots in the reactor. This is a problem encountered by some research groups and potentially affecting the conclusions of mechanistic studies. The effect of the reactant space velocity on the partial oxidation reaction is shown in figure 6(d). Because of the high performance of the noble metal catalysts for the POM reaction to synthesis gas, these catalysts have attracted great attention and have been widely studied in the last three decades. Poirier et al. observed the much higher activity of ruthenium catalysts than nickel catalysts at extremely high flow rates ð0:893 mol CH4 kg1 s1 ; CH4 =O2 =He ¼ 8=4=3Þ; under kinetic control of the products. Even a ruthenium catalyst with very low metal loading, 0:015%Ru= Al2 O3 ; was found to be more active and selective than 5%Ni=SiO2 :[64] Hochmuth [65] and Hickman and Schmidt and coworkers [66–70] have investigated methane partial oxidation over monolith-supported noble metal catalysts. The former study, carried out on a pilot-plant scale at high reactant flow rates, revealed that a Pt–Pd catalyst was extremely active for synthesis gas production, and concluded that exothermic total oxidation occurred at the front of the catalyst bed, followed by reforming reactions. The latter studies, by Hickman and Schmidt, again demonstrated that high synthesis gas yields ð> 90%Þ could be obtained over Rh and Pt catalysts at very high flow rates ð104 and 102 sÞ: At this point it is worth noting that the relative order of activity for steam reforming catalysts has been determined as: Ru Rh > Ni > Ir > Pd Pt Co Fe [15,71]; while for dry reforming the order is: Ru > Rh; Ni; Ir > Pt > Pd [72,73]. This is very similar to the activities of the partial oxidation catalysts, demonstrating the similarities between the three processes. The activity order of the noble metal catalyst also depends on the preparation method and support properties. Basini [74] and co-workers prepared anionic clays as precursors of noble metal-based catalysts for methane activation and found that the synthesis gas production activity increased according to the order Rh > Ru Ir Pt > Pd: The best catalytic perfor-

mances were observed for a 1% Rh content (at. ratio) and Rh contents above 1% did not increase the activity, unlike that observed for Ru based catalysts. For a supported Ir catalyst in the POM reaction [75], the support activity order was as follows: TiO2 ZrO2 Y2 O3 > La2 O3 > MgO Al2 O3 > SiO2 : Choudhary et al. [76]. compared a series of rare-earth metal oxidesupported noble metal catalysts, and found that among the Pt- and Pd-containing catalysts, the best performance was shown by Pt=Gd2 O3 and Pd=Sm2 O3 ; respectively. These catalysts showed high selectivity for CO but low selectivity for H2 due to the reverse watergas shift reaction. They thought that the basic nature of alkaline and rare earth oxides not only acted as a support for dispersing noble metals but also played a significant role in deciding the activity/selectivity of the Pt- or Pd-containing catalysts. Ruckenstein et al. [77–79] studied the effect of different forms of MgO precursors, rare earth oxide supports and other stable supports on the Rh catalyst performance for POM to synthesis gas. They found that the formation of a compound between the rhodium and support oxide is strongly dependent on the nature of the support and the calcination temperature. No such compounds were formed over Al2 O3 and SiO2 supported Rh catalysts, but LaRhO3 ; MgRh2 O4 ; YRhO3 and RhTaO4 could be formed over LaO2 O3 -; MgO-; Y2 O3 - and Ta2 O5 -supported catalysts after their calcination at suitable temperatures. La2 O3 and MgO provided more stable catalytic activities and selectivities than Y2 O3 - and Ta2 O5 -supported catalysts at suitable temperature. The stability of the catalyst with strong metal and support interaction could be notably improved at a higher reaction temperature. Among the irreducible metal oxides, Al2 O3 ; La2 O3 and MgO provided stable catalytic activities and selectivities during 100 h of reaction, and the activity decreased in the sequence La2 O3 < Al2 O3 MgO: In the MgOsupported Rh catalysts with different forms of MgO precursors, no notable effect of the precursor of MgO was noted, although it is found that there are three kinds of rhodium-containing species, Rh2 O3 ; Rh2 O3 interacting with the support and MgRh2 O4 ; in the oxidized MgO-supported Rh catalysts. The strong interactions between rhodium and magnesium oxides are suggested to be responsible for the high stability of the catalyst. They [80] also studied the effect of the Rh content on the Al2 O3 -supported catalyst performance, and it has been pointed out that the activity and selectivities for loadings between 0.5 and 5.0 wt% are almost identical. Precious metal membrane catalysts and reactors have attracted considerable attention recently. These catalysts and reactors can lead to POM reaction at low temperature with a high selectivity to CO and H2 : It has been pointed out [81] that H2 production can be enhanced by Pd-based membranes for the conversion of natural gas to synthesis gas and liquid fuels. Kikuchi


et al. [82] found that POM occurred at a low temperature of 773 K by reaction of an oxygen-deficient CH4 =O2 mixture over supported precious metal membrane catalysts: XCH4 and SCO and SH2 were promoted by removing H2 from the reaction system. In a membrane reactor, it was found that deposition of coke began after H2 O consumption was complete. Addition of steam to the reactant flow could effectively depress coke deposition and improve the yield of H2 via both steam reforming and the water-gas shift reaction. Similar to the membrane reactor, monolithic reactors or monolite-supported noble metal catalysts have been studied for the POM reaction to synthesis gas [83,84]. Over 90% selectivities to synthesis gas (a 2:1 H2 : CO mixture) and > 90% XCH4 and complete conversion of O2 were obtained on Rh-coated ceramic monoliths with a contact time of 103 s: With Pt catalysts under the same conditions, SH2 drops to 70%; while with Pd, the catalyst rapidly gives carbon deposition. With higher alkanes, synthesis gas is produced on Rh with comparable selectivities and conversions on metal-coated monoliths. Another very interesting noble metal catalyst system is ruthenium perovskite oxide, Ba3 NiRuTaO9 :[85] At 1173 K, it can facilitate 95%XCH4 with a 98% selectivity for synthesis gas formation, and at 1070 K, it catalyses the total conversion of C2 H6 with a 94% selectivity for synthesis gas formation. The bulk structure of the perovskite catalyst remained unchanged, and no carbon deposition was observed during the course of these reactions. Finally, it should be noted that the studies on carbon-free partial oxidation have been performed in reactor tubes with ‘‘inert’’ linings, e.g. quartz, since the walls of the reactor itself will catalyse carbon formation; for further large-scale industrial application of partial oxidation technology this problem will need to be overcome or truly inert reactor linings are required.

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3.7. Early transition metal carbides and other catalysts Recently, York et al. were studying the performance of supported molybdenum oxide catalysts for the oxidation of methane at elevated temperature and pressure when they observed high synthesis gas yields. Subsequent studies showed that molybdenum carbide Mo2 C (figure 7(a)) was formed in situ, and it was the carbide that was active for the methane oxidation. Molybdenum carbide catalysts were found to be extremely stable at elevated pressures (figure 7(b)), while at atmospheric pressure oxidation of the carbide occurred, resulting in MoO2 : Again under stoichiometric conditions no carbon deposition was observed on the post-reaction catalyst samples, while a study of the relative activity of molybdenum carbide to the noble metals demonstrated that it had an activity similar to iridium, both per active site and per gram [86–88]. High space velocity was detrimental to the carbide catalyst stability. The deactivation of the catalyst may result from the oxidation of the carbide into oxide, followed by vaporisation of MoO3 under atmospheric pressure. However, under elevated-pressure conditions, the vaporisation of molybdenum oxide does not occur, since the molybdenum carbide phase is stabilised by synthesis gas and residual methane. A recent study shows that addition of transition metals or dopants can increase the catalyst activity and stability significantly [89,90]. With the addition of transition metal promoters, the catalysts activity can be as high as the noble metal catalyst even at very high space velocity and pressure conditions, but there is much less carbon deposition over the carbide catalysts.

4. Methane partial oxidation mechanisms A considerable amount of work concerning the elucidation of the methane partial oxidation mechanism has been carried out, and here the most cogent studies

Figure 7. (a) TEM image of bulk Mo2 C as prepared and used for methane partial oxidation. (b) Stability of Mo2 C (1170 K, 8.7 bar, 5:25 103 h1 ; CH4 =air ¼ 2=5): (*) XCH4 ; (&) S[CO], (^) H2 =CO:

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will be discussed. Two general mechanisms have been proposed to account for the oxidative conversion of methane to synthesis gas over metal catalysts: (i) an indirect mechanism involving methane total combustion, followed by steam and dry reforming reactions, which is often referred to as the ‘‘combustion and reforming reactions mechanism’’ (CRR); (ii) a direct oxidation mechanism in which surface carbon and oxygen species can combine to form primary products, known as the ‘‘direct partial oxidation mechanism’’ (DPO). First mention of the DPO mechanism was made by Prettre et al. [20]. Their experiments, later repeated by Vermeiren et al. [91], indicated that the longitudinal temperature profile of the catalyst bed was not uniform. In fact, at the front of the catalyst bed the temperature was significantly higher than that of the latter part of the catalyst bed and the furnace temperature, as shown in figure 8(a). In addition, York et al. [92] repeated the work initially reported by Choudary and co-workers [23], who were using high reactant flow rates over a Ni/ MgO catalyst in an attempt to achieve non-equilibrium product distributions. Figure 8(b) clearly shows the presence of a hot-spot at the front of the catalyst bed. These studies point to an initial exothermic reaction, followed by endothermic processes, and a reaction mechanism involving the reactions shown in figure 2 has been proposed to account for these observations. Initially, part of the methane reacts with oxygen to form

carbon dioxide and water, and then these products reform the remainder of the methane to give the product synthesis gas. Indeed, it should be noted that the noble metals and nickel have all been shown to catalyse methane steam and dry reforming to synthesis gas very efficiently. In this reaction mechanism synthesis gas is a secondary product. Later, Green and co-workers studied the effect of reaction conditions on the product distribution for methane partial oxidation [60]. Again the CRR mechanism was able to account for the fact that at higher space velocities or oxygen/methane ratios the selectivity to carbon dioxide and water increases at the expense of synthesis gas, again indicating that synthesis gas is a secondary product. The second partial oxidation mechanism, or DPO mechanism, has been proposed by Hickman and Schmidt to explain their results using rhodium- and platinum-coated monolith catalysts, under adiabatic conditions at very short residence times [66–70]. In this proposed mechanism, the synthesis gas is produced as a primary product: CH4 ¼ CðadsÞ þ 4HðadsÞ CðadsÞ þ ½Os ¼ COðadsÞ ¼ COðgÞ 2HðadsÞ ¼ H2ðgÞ They constructed a model incorporating the elementary adsorption, desorption and surface reaction steps involved in a mechanism, of which some of the most important steps are shown in the equations above. All

Figure 8. (a) Schematic representation of the heat distribution in a partial oxidation catalyst bed. (b) Photograph of the catalyst exotherm for a Ni/MgO catalyst at high reactant flow rate ð5:2 105 mL g1 h1 Þ:


the experimental results obtained over the rhodium and platinum catalysts fitted their model within a few percent, while the changes in carbon monoxide and hydrogen selectivity were also closely reproduced by the model. For example, a higher H2 selectivity was observed over the rhodium catalyst than the platinum catalyst (86% and 60%, respectively), while the CO selectivities were almost the same (92% and 95%, respectively). The difference in H2 selectivity over Rh and Pt was explained by the relatively higher activation energy for HðadsÞ þ OðadsÞ ¼ HOðadsÞ on a Rh surface, and this was borne out by the predictions of their model. They also found that the catalyst bed temperature was higher for Pt than Rh, indicating the Rh catalyst is more selective for the formation of synthesis gas as a primary product, since this is less exothermic than the indirect route involving total oxidation that may be more favorable over Pt. Finally, they showed that on increasing the reactant flow rate, an increase in the synthesis gas selectivity was seen, contrary to the results presented elsewhere, mentioned earlier in this section, and shown in figure 6(d). These observations cannot be readily explained using the CRR mechanism. Recently there have been studies of the partial oxidation mechanism over several specific catalyst and under specific conditions. Weng et al. [93–95] studied supported Rh and Ru catalysts using in situ timeresolved FTIR spectroscopy and found that CO is the primary product of POM reaction over reduced and ‘‘working state’’ Rh=SiO2 catalyst. Direct oxidation of CH4 is the main pathway of synthesis gas formation over Rh=SiO2 catalysts. CO2 is the primary product of POM over Ru=Al2 O3 and Ru=SiO2 catalysts. The dominant reaction pathway for synthesis gas formation over Ru=Al2 O3 catalysts is CRR. On the Rh=SiO2 catalyst, the main reaction is apparently DPO, and also the reaction mechanism appears to be related to the concentration of O2 in the feedstock. Transient response and isotope exchange techniques have been used to study the POM mechanism over several catalyst systems. Nakagawa [96,97] pointed out that with Ir=TiO2 and Rh=SiO2 catalyst, the synthesis gas production proceeded via the CRR path. However, with Rh=TiO2 and Rh=Al2 O3 catalysts, at first, endothermic decomposition of CH4 to H2 and the deposited carbon or CHx probably took place at the front edge of the catalyst bed and then deposited carbon or generated CHx species would be oxidised to COx : The reaction pathway of partial oxidation of methane with Rh-loaded catalysts depends strongly on the support materials. Ruckenstein [98,99] showed that over Ni=SiO2 ; CH; CH2 and CH3 species are formed during the reaction, i.e., that CH4 is activated via its dissociation before oxidation. The amount of methane involved in the isotopic exchange reaction was larger than that converted to CO and CO2 : Consequently, the dissociation of methane is not ratedetermining. Over an unreduced NiO=SiO2 catalyst,

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methane reacts directly with oxygen without its predissociation. Jin et al. [100], obtained similar results over alumina-supported nickel catalysts. It appears that the reaction temperature can significantly change the reaction mechanism. Over NiO/MgO the pyrolysis mechanism was dominant at 973 and 1023 K, but at 123 K the combustion reforming mechanism also played a role [101]. Li et al. [102] pointed out that keeping the catalyst surface in the reduced state is the precondition of high conversion of CH4 and high selectivities to CO and H2 : The surface state of the catalyst decides the reaction mechanism and plays a very important role in the conversions and selectivities of partial oxidation of CH4 : Surface Ni–C and Ni–O bonds are proposed in the partial oxidation of methane to synthesis gas Summarising the current evidence for the partial oxidation mechanism over supported metal catalysts, it appears that both mechanisms can occur over noble metal and transition metal catalysts. The reaction conditions can modify the reaction routes. Two possible reaction mechanisms have been discussed [87] for the transition metal carbides catalysts for the methane partial oxidation to synthesis gas. (i)

DPO-type mechanism: this involves surface species similar to those shown for the DPO mechanism above. However, it is likely that synthesis gas is not a primary product over the carbide catalysts, and that CO2 and H2 O are important reaction intermediates. (ii) A redox mechanism: the O2 ; CO2 or H2 O in the reactor can react with surface carbide carbon species, generating vacancies and oxide species. These can then react with carbon from methane, returning the site back to a carbide. This is shown below for the reaction of CH4 and CO2 : Mo2 C þ 5CO2 ¼ 2MoO2 þ 6CO 2MoO2 þ 5CH4 ¼ Mo2 C þ 4CO þ 10H2 : A study using in situ Raman and pulse techniques showed that the most probable mechanism over carbide catalysts is the redox mechanism: a possible model for the reaction is shown in figure 9. The oxygen first reacts with the carbide, producing CO; the oxide surface is then reduced by methane to produce CO and H2 : Because the re-carburisation of the oxide surface by methane is a slow process and an endothermic reaction, the carbide catalyst is only stable under pressure conditions and at a lower space velocity [90].

5. Problems and future study Non-catalytic homogeneous partial oxidation for synthesis gas production is well established. For example, in Sarawak, Malaysia, Shell have been successfully operating a highly selective process for production of

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synthesis gas at high temperatures, typically > 1400K; and pressures of around 50–70 atm. as part of the middle distillate synthesis process (SMDS) [103]. The use of a catalyst would significantly reduce the operating temperature required for the reaction, making the process more economically attractive [104]. However, more work is required to solve the following main problems for the practical application of this process. 1. Carbon deposition over the reactor and catalyst bed. There are two possible routes for the formation of carbon, namely methane decomposition and the Boudouard reaction: CH4 ¼ CðsÞ þ 2H2 ðmethane decomposition reactionÞ 2CO ¼ CðsÞ þ CO2 ðBoudouard reactionÞ: Studies by Claridge et al. demonstrated that both reactions are thermodynamically favorable under reaction conditions typical for methane partial oxidation, but that the source of most of the carbon is likely to be methane via the former reaction [63]. Evidence for this was given by observing the amount of carbon deposited on a nickel catalyst under pure methane or carbon dioxide; at high temperatures the pure methane gave significantly more deposited carbon than pure carbon monoxide, while supporting evidence arose from the fact that carbon built up from the front of the catalyst bed, where methane partial pressure was at its highest. Two types of carbon are formed on the partial oxidation catalysts as shown in figure 10: i) encapsulate carbon, which envelops the nickel particles resulting in deactivation, and ii) whisker carbon, which grows from the face

Figure 9. Model of 2 CH4 þ O2 reaction to synthesis gas over molybdenum carbide catalysts.

Figure 10. Micrograph showing carbon deposited on a nickel catalyst after partial oxidation.

of the nickel particles and does not alter the rate of synthesis gas formation, but is likely to eventually result in reactor clogging. Detailed studies on the carbon formation mechanism have been carried out for the related steam reforming reaction [15,105–107]. To suppress the carbon deposition, more work needs to be done on the catalyst preparation and reaction conditions optimisation. For example, some steam may be added to the feedstock to eliminate the hotspot in the catalyst bed, and also may be helpful to suppress the carbon deposition. 2. Active component loss during the reaction, particularly nickel catalyst. The overall POM reaction is mildly exothermic, while it may occur by two steps, first combustion and then dry and wet reforming. In the combustion step, a large amount of heat is evolved. This melts the supported metal and leads to metal separation from the support. Because nickel has a lower melting point than other active components such as noble metal or Co and Fe, it may be easy to deactivate. This may be improved by increasing the support and metal interaction, and carrying out the reaction under a milder temperature. 3. Group VIII metal catalysts have shown superior advantages to nickel metal catalyst in carbon deposition resistance, but the carbon deposition is still unavoidable at high temperature, because of the acidic nature of the support. Also a high noble metal loading is required to sustain the high activity; thus the cost of the catalyst is very high. The combination of a small amount of group VIII metal with transition metals such as Co, Ni or Fe may be a way to decrease the catalyst cost and improve catalyst activity. The development of new alternative catalysts such as transition metal carbide catalysts is expected to decrease the catalyst cost and improve the catalyst stability. Because of the thermodynamic restriction, the POM reaction under high pressure often gives rise to more CO2 and H2 O; and XCH4 is very limited. To change the product distribution, the feedstock needs to be optimised. New technology such as membrane catalyst and reactor are expected to lead the reaction to a dynamic state, and thus to decrease the effect of the thermodynamic restriction. The combination of POM and steam reform-


ing as an alternative, to increase CH4 conversion and CO and H2 selectivity, is also possible. In addition, a more stable catalyst system, which can resist carbon deposition under an excessive of methane feedstock would be very desirable to increase the selectivity to CO and H2 : A high CH4 =O2 ratio is also desired from the view of safety, because a lower CH4 =O2 ð 1:5Þ ratio increases danger of explosions [108], and this is of particular importance when pressure is employed.

6. Conclusions The partial oxidation of methane to synthesis gas is likely to become more important as it is more energy efficient, in the future. Early work showing that nickel is an active catalyst for this reaction has now been followed up, particularly in the last 10 years. Now a number of potential alternatives have been discovered for carbon-free methane partial oxidation, including the noble metals and molybdenum and tungsten carbides. As a final comment, it is worth noting that in 1933 Padovani and Franchetti stated that partial oxidation may become of interest for the product of hydrogen, ammonia and alcohols if the cost of oxygen production could be reduced [19]. Without more detailed engineering studies and economic evaluation, the future of widespread partial oxidation for synthesis gas production may still be some way off. However, Korchnak and Dunster have concluded that methanol plants based on partial oxidation technology for synthesis gas generation may be significantly more attractive than current processes [109]. In addition, partial oxidation is ideal for use in fuel cells, providing both synthesis gas and electricity at the same time [110]. Therefore, methane partial oxidation would seem to merit further investigation.

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