- Email: [email protected]
Natural gas conversion
90
Natural gas conversion Robert Burch* and 8hik C Tsangt Partial conversion of natural gas is central to the production of hydrogen and carbon monoxide and its total conversion is important in combustion processes. The former is now a relatively mature area and so although there have recently been a massive number of publications, very few provide any significant new information. Catalytic combustion continues to attract attention, and there is still the prospect that, even for large gas turbine applications, a successful process can be developed.
Addresses Catalysis Research Centre, Department of Chemistry, University of Reading, Whiteknights, Reading, RG6 6AD, UK *e-mail: [email protected] +e-mail: [email protected] Current Opinion in Solid State & Materials Science 1997, 2:90-93 Electronic identifier: 1359-0286-002-00090 © Current Chemistry Ltd ISSN 1359-0286
Introduction Natural gas is found in many different parts of the world and current reserves are estimated to exceed 3500 trillion cubic feet. Its principal c o m p o n e n t is m e t h a n e and so natural gas represents one of the most c o n v e n i e n t sources of relatively pure hydrocarbon feedstocks. U n d o u b t e d l y natural gas will continue to grow in relative importance both as a fuel and also in the manufacture of chemicals. Recently, there has been much research aimed at finding ways to utilize natural gas and some advances have been made [1",2"]. This review describes results, published in the open literature during the past 18 months, concerned with the d e v e l o p m e n t of catalytic processes for the oxidative coupling of methane, methane reforming and partial oxidation reactions, and the catalytic combustion of methane. N o a t t e m p t has been made to review the voluminous patent literature.
Oxidative coupling of methane M e t h a n e is difficult to transport over long distances and since the seminal work of Keller and Bhasin in 1982 [3] (who reported that m e t h a n e could be dimerized to form ethane and ethene) there has b e e n an explosion of publications on this methane coupling reaction (over 300 since 1st January 1995). It has to be said that industrial assessments of the economic feasibility of m e t h a n e coupling have all concluded that the process is not viable [2"], so the vast n u m b e r of papers which still appear on this subject are, at best, of passing academic interest.
Many, indeed, are in the ' s t a m p collecting' category! Some published work, however, does merit attention. To date, the m a x i m u m C z yields under industrial conditions are around 20-25%. T h e r e has been no improvement on this in the past five years, in spite of the amount of research undertaken, and one wonders if this is indeed the limit for conventional steady state reactors. Possibly, there are opportunities to perform this reaction using novel engineering concepts to overcome the thermodynamic yields. For example, using m e m b r a n e technology/chromatography techniques may allow in situ separation of products from reactant molecules which may further shift the equilibrium towards the product side. Recently, Wang and Lin [4"] have discussed the use of oxide membranes to separate hydrocarbons from oxygen, so that the C a products can be quickly removed. Based on their model, calculation suggests that yields of over 70% C 2 are possible; however, this would only be true under conditions where the oxygen permeation flux, methane flow rate and reaction rates all matched each other exactly. It is also assumed that all the reactions have intrinsically very fast rates. In reality, reports on developing m e m b r a n e reactors for this reaction are far from optimistic [5-18]. Tonkovich et al. [5] have studied the reaction using inorganic m e m b r a n e reactors and compared the results with fixed-bed reactors. T h e i r best membrane reactor, based on samarium oxide d o p e d magnesium oxide, produced only marginal i m p r o v e m e n t s in the yield. T h e magnitude of the yield increase was much less than reported for other partial oxidation reactions. T e n e l s h o f et al. [6] have investigated a dense m e m b r a n e of mixed-conducting p e r o v s k i t e - t y p e oxides and found that conversions were only 1-3%. By studying LaOCI membranes, Mirodatos and co-workers [7,8] concluded that the severe operating conditions (high temperature and high water concentration [a side product]) of this reaction caused textural instability which restricted any transport effects to the macroporous domain. It seems we have already reached a plateau in our learning curve for this process. T h e d e v e l o p m e n t of a counter-current chromatography reactor is interesting. T h i s novel reactor is claimed to be able to rapidly separate products from reactants, allowing high levels of conversion in equilibrium limited reactions [19"]. Significant e n h a n c e m e n t in C 2 selectivity for the methane coupling reaction was reported [20"]. This seems a plausible approach and further d e v e l o p m e n t of the methane coupling reaction may be possible if suitable methods are found to separate the C 2 products rapidly from the reaction mixture and also if the reaction can be
Natural gas conversionBurchand Tsang
combined with other chemical processes to yield more valuable products.
M e t h a n e r e f o r m i n g and partial o x i d a t i o n reactions Hydrogen and synthesis gas (CO + H 2) production are major processes which use natural gas as a feedstock and there are three conversion reactions that attract industrial interest, namely, steam reforming, methane dry reforming with carbon dioxide, and methane partial oxidation with oxygen or air. Although research into the reforming reactions first started before 1940, they have recently attracted much renewed interest [1°,2 °] to find a cheaper means to manufacture synthesis gas. Carbon deposition leading to catalyst deactivation is still the major problem in these processes. To avoid this, the conventional approach is to use excess H 2 0 to render the carbon formation reaction thermodynamically unfavourable. This is the basis of most commercial steam reforming processes and of the dry reforming (Calcor) process where, respectively, excess steam or CO 2 is employed with relatively low cost nickel-based catalysts. There are drawbacks to these processes. Higher than stoichiometric ratios of steam or CO 2 will make the operations much more costly (it is expensive to generate superheated steam in the case of steam reforming), and the quality of the synthesis gas will be affected (undesirable CO/H 2 ratios at high dilution levels, making further processing and recycling necessary). In the partial oxidation reaction, carbon formation may be avoided by increasing the oxygen to methane ratio, but this also increases the explosion hazards. Very recently, it was found that some catalysts, particularly supported noble metals such as Ru, Pt, Ir, are very effective (despite their high cost), giving no significant coke formation during the partial oxidation of methane [1 °] and also during the dry reforming reaction [1°,2°]. Under the same conditions, Ni/AI203 catalysts were rapidly deactivated with deposition of substantial amounts of carbon. It seems that the current major challenge in this area is to develop a low cost, robust catalyst with high reforming activity which is also resistant to carbon deposition. Ross and co-workers [2° ] have been exploring such possibilities. T h e y have studied the stability of Pt on various supports and the minimum Pt loading for the dry reforming reaction. T h e y claim that carbon deposition can still occur over most of the supported Pt catalysts under reaction conditions close to those likely to be encountered under industrial conditions. Interestingly, they reported that 0.5wt% Pt/ZrO 2, one of their most active catalysts, showed no extensive carbon deposition for 1000h at ~700"C, even though it would appear that the working catalyst had approximately a monolayer of carbon on its surface. This would seem to suggest that the effect of the support could be very important in reducing carbon formation, a point that seems to have been largely ignored over the past 60 years.
91
Zhang and Verykios [21 °'] have described a promotion effect on Ni, using La203 as a support material, which can enhance the dry reforming activity during the initial 2 - 5 h and maintain the high activity for a further 100h with no apparent deactivation. This result was in sharp contrast to their findings with other supports, which exhibited continuous deactivation with time on stream due to heavy carbon deposition. T h e beneficial effect of adding lanthanum for the dry reforming reaction was also recognized independently by two Chinese groups at Lanzhou [22] and Beijing [23]. It is interesting to recall the fact that lanthanum ions have long been known as effective promoters for methanation and steam reforming reactions. This support effect in reducing carbon formation might be due to the strong absorption properties of the support towards carbon dioxide, where the adsorbed carbon dioxide reacts with any surface carbon to give CO; this reaction is also likely to occur at the interface between the support and the metal (the metal surface is thought to produce adsorbed surface carbon). It should also be noted that La203 indeed has a strong tendency for trapping carbon dioxide to form an oxycarbonate phase that has been detected by XRD during the reaction [21°°]. Ni/AI•O 3 catalysts passivated by sulfur are reported to deposit carbon at a very low rate in the steam reforming reaction [1°], but the sulphur severely attenuates the activity. In contrast, high reforming activity has been found on Pt/ZrOz [2 °] and Ni/LazO3 [21°°1 or Ni/AI203/La203 [22] without loss of activity due to carbon deposition. In the partial oxidation reaction, a recent claim of carbon-free operation was made using some thermally stable nickel-based mixed oxides [24°,25]. This could be important since nickel is much cheaper and is a readily available material. It is possible that nickel ions may be stabilized against reduction in the mixed oxides, thus avoiding carbon formation. One should be very cautious about the time scale and reactor size used for evaluating carbon deposition over catalysts. Results have previously indicated that slower carbon deposition rates are associated with smaller metal particles. Thus, the rate of carbon deposition can increase dramatically for samples with longer thermal histories (as the metal sinters). Furthermore, stability in the rate of synthesis gas production does not necessarily prove that no macroscopic carbon deposition occurs since considerable amounts of 'whisker' carbon can still be built up without altering the rate of synthesis gas formation. Ideally, the rates of carbon deposition should be compared using conditions as close as possible to those used in industrial practice. An ingenious demonstration of partial oxidation using a small adiabatic reactor for converting methane and oxygen to synthesis gas, and also for the conversion of light alkanes in natural gas to their partially oxidized products, at very high space velocities (volumetric flow of reactants per unit volume of catalyst bed per unit time), has been reported by Goetsch and Schmidt [26°°]. T h e y proposed
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Solid catalysts and porous solids
that carrying out the partial oxidation at extremely short residence times would allow direct catalytic processes on the surface to dominate. This has triggered some recent mechanistic studies [27",28"] and modelling [29] of the partial oxidation of methane at different space velocities in order to address the relative importance of the direct and indirect mechanisms. By using temperature programmed reaction and isotope transient techniques to investigate the partial oxidation and steam-COz mixed reforming of methane, Qin eta/. [27"] concluded that partial oxidation can proceed via both direct and indirect mechanisms, the ratio for each mechanism being dependent on the relative concentration and kinetics of adsorbed atomic oxygen and gaseous oxygen species. Wang e t a / . [28°] came to a similar conclusion after performing a detailed transient kinetic (TAP [Temporal Analysis of Products]) study, using very small pulses of reactants at short du:ation. They found that at high concentration of adsorbed oxygen the dominant products were water and carbon dioxide, and methane adsorption was inhibited, but at low concentrations of adsorbed oxygen methane adsorption was fast and in that case the direct products were CO and H e. Combination of the exothermic partial oxidation reaction with the endothermic steam reforming reaction (autothermal reactor) has been explored because it can provide important economic and operational benefits [30-32].
been reported by Rodriguez et al. [40"]. Modelling of the combustion reactions in monoliths is important since full scale testing is exceedingly expensive; Kolaczkowski and colleagues have published good work in this area [41",42].
Catalytic
1. •
combustion
of natural
gas
Catalytic combustion of hydrocarbon fuels provides a means of eliminating the formation of nitrogen oxides and acid rain. Environmental pressures have focused attention on the catalytic combustion of natural gas both for electrical power generation (for large gas turbines) and for heat generation. The latter, as exemplified by the replacement of a conventional heat exchanger with a catalytic burner, represents a relatively easy problem, whereas the size constraints and exceedingly high space velocities required for gas turbine applications may present insurmountable problems for catalytic combustion. For natural gas combustion it is well known that palladium is the most active catalyst and will almost certainly be used in any practical application [33",34]. The lack of thermal stability above about 900°C requires a second stage catalyst to be used and this is likely to be based on the mixed oxide-type catalysts reported by Arai and colleagues [35,36]. The proceedings of the 1994 International Workshop on Catalytic Combustion provides an excellent overview of the field of catalytic combustion, from basic studies and materials, through modelling, to applications [37"]. Additional publications of note concerning mixed oxide catalysts include a structural study of hexaaluminate microcrystals by Arai and colleagues [38°] and the use of lanthanum-stabilized alumina as a support for mixed oxide catalysts by Marti et a/. [39]. Very elegant structural studies on palladium catalysts have
Conclusions
The catalytic conversion of natural gas is now quite a mature subject and although there have been over 700 publications since January 1995, none provide any indication that a technological breakthrough is imminent, and only a few offer new insight into the mechanism of the conversion reactions. Advances, if they are to come, will require a much higher level of innovation than has been recently displayed. Novel approaches to reactor design may provide a way forward but it is not clear from the literature that any truly exciting concepts are being developed. Furthermore, it is surely time to move on from a semi-empirical evaluation of 'novel' catalyst formulations. In catalytic combustion there are still many important challenges for materials scientists and mechanical engineers.
References
and
recommended
reading
Papers of particular interest, published within the annual period of review, have been highlighted as: • •°
of special interest of outstanding interest
Tsang SC, Claridge JB, Green MLH: Recent advances in the conversion of methane to synthesis gas. Catal Today 1995, 23:3-15. A review mainly covering the formation of synthesis gas. 2. •
Ross JRH, van Keulen AN J, Hegarty MES, Seshan K: The catalytic conversion of natural gas to useful products. Catal Today 1996, 30:193-199. A review covering methane coupling, steam reforming and dry reforming. 3.
Keller GE, Bhasin MM: Synthesis of ethylene via oxidative coupling of methane. 1. Determination of active catalysts. J Cata/1982, 73:9-19.
4. •
Wang W, Lin YS: Analysis of oxidative coupling of methane in dense oxide membrane reactors. J Membrane Sci 1995, 103:219-233. Discusses the capability of membranes for methane conversion. 5.
Tonkovich ALY, Jimenez DM, Zilka JL, Roberts GL: Inorganic membrane reactors for the oxidative coupling of methane. Chem Eng Sci 1996, 51:3051-3056.
6.
Tenelshof JE, Bouwmeester HJM, Verweij H: Oxidative coupling of methane in a mixed-conducting perovskite membrane reactor. Appl Cata/A 1995, 130:195-212.
7.
Chanaud P, Julbe A, Larbot A, Guizard C, Cot L, Borges H, Fendler AG, Mirodatos C: Catalytic membrane reactor for oxidative coupling of methane. 1. Preparation and characterization of LaOCI membranes. Catal Today 1995, 25:225-230.
8.
Borges H, Giroirfendler A, Mirodatos C, Chanadu P, Julbe A: Catalytic membrane reactor for oxidative coupling of methane. 2. Catalytic properties of LaOCl membranes. Catal Today 1995, 25:37?-383.
9.
Mleczko L, Pannek U, Niemi VM, Hiltunen J: Oxidative coupling of methane in a fluidized-bed reactor over a highly-active and selective catalysL Ind Eng Chem Res 1996, 35:54-61.
10.
Ramachandra AM, Lu Y, Ma YH, Moser WR, Dixon AG: Oxidative coupling of methane in porous vycor membrane reactors. J Membrane Sci 1996, 116:253-264.
Natural gas conversion Burch and Tsang
93
11.
Meng GY, Wang T, Pang DK: Simulation of methane coupling conversion in a hydrogen permeable membrane reactor [abstract]. Abs ACS 1995, 209:112-PETR.
12.
Herguido J, Lafagra D, Menengez M, Santamaria J, Guimon C: Characterization of porous ceramic membranes for their use in catalytic reactors for oxidative coupling. Catal Today 1995, 25:263-269.
13.
TenelshofJE, Vanhassel BA, Bouwmeester HJM: Activation of methane using solid oxide membranes. Catal Today 1995, 25:397-402.
29.
Degroote AM, Froment GF: Simulation of the catalytic partial oxidation of methane to synthesis gas. Appl Cata/A 1996, 138:245-264.
14.
Hibino T, Sato T, Ushiki K, Kuwahara Y: Membrane reactor for oxidative coupling of CH4 with an oxide ion-electron hole mixed conductor. J Chem Soc Faraday Trans 1995,
30.
Christensen TS: Adiabatic pre-reforming of hydrocarbons an important step in syngas production. Appl Cata/A 1996, 138:285-309.
31.
Ma L, Trimm DL, Jiang C: The design and testing of an autothermal reactor for the conversion of light-hydrocarbons to hydrogen. 1. The kinetics of the catalytic-oxidation of lighthydrocarbons. App/Cata/A 1996, 138:275-283.
32.
Ma L, Trimm DL: Alternative catalyst bed configurations for the autothermic conversion of methane to hydrogen. Appl Cata/A 1996, 138:265-273.
91:4419-4422.
15.
Pugaley TS, Berruti F: The circulating fluidized-bed catalytic reactor - reactor model validation end simulation of the oxidative coupling of methane. Chem Eng Sci 1996, 51:2751-2756.
16.
SuzukiS, Sasaki T, Kojima T, Yamamura M, Yoshinari T: New procass-development of natural-gas conversion technology to liquid fuels via OCM reaction. Energ Fuel 1996, 10:531-536.
17.
Guczi L, Vansantan RA, Sarma KV: Low-temperature coupling of methane. Catal Rev-Sci Eng 1996, 38:249-296.
18.
Babe P, Li YD, Marquaire PM, Come GM, Baronnet F: A new catalytic jet-stirred reactor application to the study of the oxidative coupling of methane. Oxidat Commun 1996, 19:173-185.
Use of isotopic techniques to elucidate the mechanism for the reforming reaction. 28. -
Wang DZ, Dewaele O, Degroote AM, Froment GF: Reactionmechanism and role of the support in the partial oxidation of methane on R h / A I 2 0 3 . J Cata/1996, 1 5 9 : 4 1 8 - 4 2 6 . TAP reactor investigation of partial oxidation on rhodium catalysts.
33. Burch R, Hayes MJ: C-H bond activation in hydrocarbon A oxidation on solid catalysts. J Mol Catal 1995, 100:13-33. n overview of the mechanism of C-H bond activation on various types of metallic and oxidic catalysts. 34.
19. ..
Butch R, Urbano FJ: Investigation of the active state of supported palladium catalysts in the combustion of methane. Appl Catal A 1995, 124:121-138.
35.
Inoue H, Sekizawa K, Eguchi K, Arai H: Changes of crystalline phase and catalytic properties by cation substitution in mirror plane of hexaaluminate compounds. J Solid State Chem 1996, 121:190-196.
20.
36,
Eguchi K, Arai H: Recent advances in high-temperature catalytic combustion. Catal Today 1996, 29:379-386.
Bjorklund MC, Carr RW: The simulated counter-current movingbed chromatographic reactor - a catalytic and separation reactor. Cata/Today 1995, 25:159-168. An interesting novel reactor concept, which is especially good for reactions at low single pass conversion or for processes with low selectivities. Kruglov AV, Bjorklund MC, Carr RW: Optimization of the simulated countercurrent moving-bed chromatographic reactor for the oxidative coupling of methane. Chem Eng Sci 1996, 51:2945-2950. Practical demonstration of a novel reactor concept. •
21. •-
Zhang ZL, Verykios XE: Carbon-dioxide reforming of methane to synthesis gas over Ni/La203 catalysts. App/Catal A 1996, 138:109-133. Interesting demonstration of a nickel catalyst which is resistant to carbon laydown and simultaneously shows an enhancement in activity. 22.
Chu YL, Li SB, Lin JZ, GU JF, Yang YL: Partial oxidation of methane to carbon-monoxide and hydrogen over NiO/La203/gamma-AI203 catalyst. Appl Catal A 1996, 134:67-80.
23.
Cheng ZX, Wu QL, Li JL, Zhu QM: Effects of promoters and preparation procedures on reforming of methane with carbon dioxide over Ni/AI203 catalyst. Catal Today 1996, 30:147-155.
Hayakawa T, Harihara H, Andersen AG, York APE, Suzuki K, Yasuda H, Takehira K: A sustainable catalyst for the partial oxidation of methane to syngas. Angew Chem Int Ed 1996, 35:192-195. Details of a complex oxide catalyst, containing nickel, which is resistant to carbon deposition.
37. •-
Arai H, Fukuzawa H (Eds): Selected papers presented at the international workshop on catalytic combustion held in Tokyo, Japan April 18-20, 1994. Catal Today 1995, 26:217-363. A series of papers (presented at the 1994 International Workshop on Catalytic Combustion) which give an excellent update on the status of catalytic combustion from both a fundamental and an industrial perspective. 38. •
Machida M, Sato A, Murakami M, Kijima T, Aral H: Structure and property of coherent spinel surface-layers on hexasluminate microcrystals. J Catal 1995, 157:713-720. The Arai group have been at the forefront in developing high temperature catalysts and this paper provides a detailed structural analysis of typical systems. 39.
24. •
25.
Battle PD, Claridge JB, Copplestone FA, SW Carr, Tsang SC: Partial oxidation of natural gas to synthesis gas over ruthenium perovskite oxides. Appl Catal A 1994, 118:217-227.
26. Goetsch DA, Schmidt LD: Microsecond catalytic partial -. oxidation of alkanes. Science 1996, 271:1560-1562. A summary of work from this group on the use of very short contact times for partial oxidation of hydrocarbons. Also see other references within this summary. 27. •
Qin DY, Lapszewicz J, Jiang XZ: Comparison of partial oxidation and steam-CO2 mixed reforming of CH 4 to syngas on MgOsupported metals. J Cata/1996, 159:140-149.
Marti PE, Maciejewski M, Balker A: La0.sSr0.2MnO3+x supported on LaAIO3 and LaAIllO18 prepared by different methods - influence of preparation method on morphological and catalytic properties in methane combustion. Stud Surf Sci Catal 1995, 91:617-626.
Rodriguez NM, Oh SG, Dalla Betta RA, Baker RTK: In situ electron-microscopy studies of palladium supported on AI203, SiO 2 and ZrO 2 in oxygen. J Cata11995, 157:676-686. There has been much debate about the structure of palladium catalysts under oxidizing conditions and this paper provides some excellent direct structural information. 40. •
41. •
Kolaczkowski ST, Serbetcioglu S: Development of combustion catalysts for monolith reactors - a consideration of transport limitations. Appl Catal A 1996, 138:199-214. An assessment of the role of modelling in catalyst and system design. 42.
Hayes RE, Kolaczkowski ST, Thomas TJ, Titiloye J: Transient experiments and modeling of the catalytic combustion of methane in a monolith reactor. Ind Eng Chem Res 1996, 35:406-414.
Natural gas conversion Robert Burch* and 8hik C Tsangt Partial conversion of natural gas is central to the production of hydrogen and carbon monoxide and its total conversion is important in combustion processes. The former is now a relatively mature area and so although there have recently been a massive number of publications, very few provide any significant new information. Catalytic combustion continues to attract attention, and there is still the prospect that, even for large gas turbine applications, a successful process can be developed.
Addresses Catalysis Research Centre, Department of Chemistry, University of Reading, Whiteknights, Reading, RG6 6AD, UK *e-mail: [email protected] +e-mail: [email protected] Current Opinion in Solid State & Materials Science 1997, 2:90-93 Electronic identifier: 1359-0286-002-00090 © Current Chemistry Ltd ISSN 1359-0286
Introduction Natural gas is found in many different parts of the world and current reserves are estimated to exceed 3500 trillion cubic feet. Its principal c o m p o n e n t is m e t h a n e and so natural gas represents one of the most c o n v e n i e n t sources of relatively pure hydrocarbon feedstocks. U n d o u b t e d l y natural gas will continue to grow in relative importance both as a fuel and also in the manufacture of chemicals. Recently, there has been much research aimed at finding ways to utilize natural gas and some advances have been made [1",2"]. This review describes results, published in the open literature during the past 18 months, concerned with the d e v e l o p m e n t of catalytic processes for the oxidative coupling of methane, methane reforming and partial oxidation reactions, and the catalytic combustion of methane. N o a t t e m p t has been made to review the voluminous patent literature.
Oxidative coupling of methane M e t h a n e is difficult to transport over long distances and since the seminal work of Keller and Bhasin in 1982 [3] (who reported that m e t h a n e could be dimerized to form ethane and ethene) there has b e e n an explosion of publications on this methane coupling reaction (over 300 since 1st January 1995). It has to be said that industrial assessments of the economic feasibility of m e t h a n e coupling have all concluded that the process is not viable [2"], so the vast n u m b e r of papers which still appear on this subject are, at best, of passing academic interest.
Many, indeed, are in the ' s t a m p collecting' category! Some published work, however, does merit attention. To date, the m a x i m u m C z yields under industrial conditions are around 20-25%. T h e r e has been no improvement on this in the past five years, in spite of the amount of research undertaken, and one wonders if this is indeed the limit for conventional steady state reactors. Possibly, there are opportunities to perform this reaction using novel engineering concepts to overcome the thermodynamic yields. For example, using m e m b r a n e technology/chromatography techniques may allow in situ separation of products from reactant molecules which may further shift the equilibrium towards the product side. Recently, Wang and Lin [4"] have discussed the use of oxide membranes to separate hydrocarbons from oxygen, so that the C a products can be quickly removed. Based on their model, calculation suggests that yields of over 70% C 2 are possible; however, this would only be true under conditions where the oxygen permeation flux, methane flow rate and reaction rates all matched each other exactly. It is also assumed that all the reactions have intrinsically very fast rates. In reality, reports on developing m e m b r a n e reactors for this reaction are far from optimistic [5-18]. Tonkovich et al. [5] have studied the reaction using inorganic m e m b r a n e reactors and compared the results with fixed-bed reactors. T h e i r best membrane reactor, based on samarium oxide d o p e d magnesium oxide, produced only marginal i m p r o v e m e n t s in the yield. T h e magnitude of the yield increase was much less than reported for other partial oxidation reactions. T e n e l s h o f et al. [6] have investigated a dense m e m b r a n e of mixed-conducting p e r o v s k i t e - t y p e oxides and found that conversions were only 1-3%. By studying LaOCI membranes, Mirodatos and co-workers [7,8] concluded that the severe operating conditions (high temperature and high water concentration [a side product]) of this reaction caused textural instability which restricted any transport effects to the macroporous domain. It seems we have already reached a plateau in our learning curve for this process. T h e d e v e l o p m e n t of a counter-current chromatography reactor is interesting. T h i s novel reactor is claimed to be able to rapidly separate products from reactants, allowing high levels of conversion in equilibrium limited reactions [19"]. Significant e n h a n c e m e n t in C 2 selectivity for the methane coupling reaction was reported [20"]. This seems a plausible approach and further d e v e l o p m e n t of the methane coupling reaction may be possible if suitable methods are found to separate the C 2 products rapidly from the reaction mixture and also if the reaction can be
Natural gas conversionBurchand Tsang
combined with other chemical processes to yield more valuable products.
M e t h a n e r e f o r m i n g and partial o x i d a t i o n reactions Hydrogen and synthesis gas (CO + H 2) production are major processes which use natural gas as a feedstock and there are three conversion reactions that attract industrial interest, namely, steam reforming, methane dry reforming with carbon dioxide, and methane partial oxidation with oxygen or air. Although research into the reforming reactions first started before 1940, they have recently attracted much renewed interest [1°,2 °] to find a cheaper means to manufacture synthesis gas. Carbon deposition leading to catalyst deactivation is still the major problem in these processes. To avoid this, the conventional approach is to use excess H 2 0 to render the carbon formation reaction thermodynamically unfavourable. This is the basis of most commercial steam reforming processes and of the dry reforming (Calcor) process where, respectively, excess steam or CO 2 is employed with relatively low cost nickel-based catalysts. There are drawbacks to these processes. Higher than stoichiometric ratios of steam or CO 2 will make the operations much more costly (it is expensive to generate superheated steam in the case of steam reforming), and the quality of the synthesis gas will be affected (undesirable CO/H 2 ratios at high dilution levels, making further processing and recycling necessary). In the partial oxidation reaction, carbon formation may be avoided by increasing the oxygen to methane ratio, but this also increases the explosion hazards. Very recently, it was found that some catalysts, particularly supported noble metals such as Ru, Pt, Ir, are very effective (despite their high cost), giving no significant coke formation during the partial oxidation of methane [1 °] and also during the dry reforming reaction [1°,2°]. Under the same conditions, Ni/AI203 catalysts were rapidly deactivated with deposition of substantial amounts of carbon. It seems that the current major challenge in this area is to develop a low cost, robust catalyst with high reforming activity which is also resistant to carbon deposition. Ross and co-workers [2° ] have been exploring such possibilities. T h e y have studied the stability of Pt on various supports and the minimum Pt loading for the dry reforming reaction. T h e y claim that carbon deposition can still occur over most of the supported Pt catalysts under reaction conditions close to those likely to be encountered under industrial conditions. Interestingly, they reported that 0.5wt% Pt/ZrO 2, one of their most active catalysts, showed no extensive carbon deposition for 1000h at ~700"C, even though it would appear that the working catalyst had approximately a monolayer of carbon on its surface. This would seem to suggest that the effect of the support could be very important in reducing carbon formation, a point that seems to have been largely ignored over the past 60 years.
91
Zhang and Verykios [21 °'] have described a promotion effect on Ni, using La203 as a support material, which can enhance the dry reforming activity during the initial 2 - 5 h and maintain the high activity for a further 100h with no apparent deactivation. This result was in sharp contrast to their findings with other supports, which exhibited continuous deactivation with time on stream due to heavy carbon deposition. T h e beneficial effect of adding lanthanum for the dry reforming reaction was also recognized independently by two Chinese groups at Lanzhou [22] and Beijing [23]. It is interesting to recall the fact that lanthanum ions have long been known as effective promoters for methanation and steam reforming reactions. This support effect in reducing carbon formation might be due to the strong absorption properties of the support towards carbon dioxide, where the adsorbed carbon dioxide reacts with any surface carbon to give CO; this reaction is also likely to occur at the interface between the support and the metal (the metal surface is thought to produce adsorbed surface carbon). It should also be noted that La203 indeed has a strong tendency for trapping carbon dioxide to form an oxycarbonate phase that has been detected by XRD during the reaction [21°°]. Ni/AI•O 3 catalysts passivated by sulfur are reported to deposit carbon at a very low rate in the steam reforming reaction [1°], but the sulphur severely attenuates the activity. In contrast, high reforming activity has been found on Pt/ZrOz [2 °] and Ni/LazO3 [21°°1 or Ni/AI203/La203 [22] without loss of activity due to carbon deposition. In the partial oxidation reaction, a recent claim of carbon-free operation was made using some thermally stable nickel-based mixed oxides [24°,25]. This could be important since nickel is much cheaper and is a readily available material. It is possible that nickel ions may be stabilized against reduction in the mixed oxides, thus avoiding carbon formation. One should be very cautious about the time scale and reactor size used for evaluating carbon deposition over catalysts. Results have previously indicated that slower carbon deposition rates are associated with smaller metal particles. Thus, the rate of carbon deposition can increase dramatically for samples with longer thermal histories (as the metal sinters). Furthermore, stability in the rate of synthesis gas production does not necessarily prove that no macroscopic carbon deposition occurs since considerable amounts of 'whisker' carbon can still be built up without altering the rate of synthesis gas formation. Ideally, the rates of carbon deposition should be compared using conditions as close as possible to those used in industrial practice. An ingenious demonstration of partial oxidation using a small adiabatic reactor for converting methane and oxygen to synthesis gas, and also for the conversion of light alkanes in natural gas to their partially oxidized products, at very high space velocities (volumetric flow of reactants per unit volume of catalyst bed per unit time), has been reported by Goetsch and Schmidt [26°°]. T h e y proposed
92
Solid catalysts and porous solids
that carrying out the partial oxidation at extremely short residence times would allow direct catalytic processes on the surface to dominate. This has triggered some recent mechanistic studies [27",28"] and modelling [29] of the partial oxidation of methane at different space velocities in order to address the relative importance of the direct and indirect mechanisms. By using temperature programmed reaction and isotope transient techniques to investigate the partial oxidation and steam-COz mixed reforming of methane, Qin eta/. [27"] concluded that partial oxidation can proceed via both direct and indirect mechanisms, the ratio for each mechanism being dependent on the relative concentration and kinetics of adsorbed atomic oxygen and gaseous oxygen species. Wang e t a / . [28°] came to a similar conclusion after performing a detailed transient kinetic (TAP [Temporal Analysis of Products]) study, using very small pulses of reactants at short du:ation. They found that at high concentration of adsorbed oxygen the dominant products were water and carbon dioxide, and methane adsorption was inhibited, but at low concentrations of adsorbed oxygen methane adsorption was fast and in that case the direct products were CO and H e. Combination of the exothermic partial oxidation reaction with the endothermic steam reforming reaction (autothermal reactor) has been explored because it can provide important economic and operational benefits [30-32].
been reported by Rodriguez et al. [40"]. Modelling of the combustion reactions in monoliths is important since full scale testing is exceedingly expensive; Kolaczkowski and colleagues have published good work in this area [41",42].
Catalytic
1. •
combustion
of natural
gas
Catalytic combustion of hydrocarbon fuels provides a means of eliminating the formation of nitrogen oxides and acid rain. Environmental pressures have focused attention on the catalytic combustion of natural gas both for electrical power generation (for large gas turbines) and for heat generation. The latter, as exemplified by the replacement of a conventional heat exchanger with a catalytic burner, represents a relatively easy problem, whereas the size constraints and exceedingly high space velocities required for gas turbine applications may present insurmountable problems for catalytic combustion. For natural gas combustion it is well known that palladium is the most active catalyst and will almost certainly be used in any practical application [33",34]. The lack of thermal stability above about 900°C requires a second stage catalyst to be used and this is likely to be based on the mixed oxide-type catalysts reported by Arai and colleagues [35,36]. The proceedings of the 1994 International Workshop on Catalytic Combustion provides an excellent overview of the field of catalytic combustion, from basic studies and materials, through modelling, to applications [37"]. Additional publications of note concerning mixed oxide catalysts include a structural study of hexaaluminate microcrystals by Arai and colleagues [38°] and the use of lanthanum-stabilized alumina as a support for mixed oxide catalysts by Marti et a/. [39]. Very elegant structural studies on palladium catalysts have
Conclusions
The catalytic conversion of natural gas is now quite a mature subject and although there have been over 700 publications since January 1995, none provide any indication that a technological breakthrough is imminent, and only a few offer new insight into the mechanism of the conversion reactions. Advances, if they are to come, will require a much higher level of innovation than has been recently displayed. Novel approaches to reactor design may provide a way forward but it is not clear from the literature that any truly exciting concepts are being developed. Furthermore, it is surely time to move on from a semi-empirical evaluation of 'novel' catalyst formulations. In catalytic combustion there are still many important challenges for materials scientists and mechanical engineers.
References
and
recommended
reading
Papers of particular interest, published within the annual period of review, have been highlighted as: • •°
of special interest of outstanding interest
Tsang SC, Claridge JB, Green MLH: Recent advances in the conversion of methane to synthesis gas. Catal Today 1995, 23:3-15. A review mainly covering the formation of synthesis gas. 2. •
Ross JRH, van Keulen AN J, Hegarty MES, Seshan K: The catalytic conversion of natural gas to useful products. Catal Today 1996, 30:193-199. A review covering methane coupling, steam reforming and dry reforming. 3.
Keller GE, Bhasin MM: Synthesis of ethylene via oxidative coupling of methane. 1. Determination of active catalysts. J Cata/1982, 73:9-19.
4. •
Wang W, Lin YS: Analysis of oxidative coupling of methane in dense oxide membrane reactors. J Membrane Sci 1995, 103:219-233. Discusses the capability of membranes for methane conversion. 5.
Tonkovich ALY, Jimenez DM, Zilka JL, Roberts GL: Inorganic membrane reactors for the oxidative coupling of methane. Chem Eng Sci 1996, 51:3051-3056.
6.
Tenelshof JE, Bouwmeester HJM, Verweij H: Oxidative coupling of methane in a mixed-conducting perovskite membrane reactor. Appl Cata/A 1995, 130:195-212.
7.
Chanaud P, Julbe A, Larbot A, Guizard C, Cot L, Borges H, Fendler AG, Mirodatos C: Catalytic membrane reactor for oxidative coupling of methane. 1. Preparation and characterization of LaOCI membranes. Catal Today 1995, 25:225-230.
8.
Borges H, Giroirfendler A, Mirodatos C, Chanadu P, Julbe A: Catalytic membrane reactor for oxidative coupling of methane. 2. Catalytic properties of LaOCl membranes. Catal Today 1995, 25:37?-383.
9.
Mleczko L, Pannek U, Niemi VM, Hiltunen J: Oxidative coupling of methane in a fluidized-bed reactor over a highly-active and selective catalysL Ind Eng Chem Res 1996, 35:54-61.
10.
Ramachandra AM, Lu Y, Ma YH, Moser WR, Dixon AG: Oxidative coupling of methane in porous vycor membrane reactors. J Membrane Sci 1996, 116:253-264.
Natural gas conversion Burch and Tsang
93
11.
Meng GY, Wang T, Pang DK: Simulation of methane coupling conversion in a hydrogen permeable membrane reactor [abstract]. Abs ACS 1995, 209:112-PETR.
12.
Herguido J, Lafagra D, Menengez M, Santamaria J, Guimon C: Characterization of porous ceramic membranes for their use in catalytic reactors for oxidative coupling. Catal Today 1995, 25:263-269.
13.
TenelshofJE, Vanhassel BA, Bouwmeester HJM: Activation of methane using solid oxide membranes. Catal Today 1995, 25:397-402.
29.
Degroote AM, Froment GF: Simulation of the catalytic partial oxidation of methane to synthesis gas. Appl Cata/A 1996, 138:245-264.
14.
Hibino T, Sato T, Ushiki K, Kuwahara Y: Membrane reactor for oxidative coupling of CH4 with an oxide ion-electron hole mixed conductor. J Chem Soc Faraday Trans 1995,
30.
Christensen TS: Adiabatic pre-reforming of hydrocarbons an important step in syngas production. Appl Cata/A 1996, 138:285-309.
31.
Ma L, Trimm DL, Jiang C: The design and testing of an autothermal reactor for the conversion of light-hydrocarbons to hydrogen. 1. The kinetics of the catalytic-oxidation of lighthydrocarbons. App/Cata/A 1996, 138:275-283.
32.
Ma L, Trimm DL: Alternative catalyst bed configurations for the autothermic conversion of methane to hydrogen. Appl Cata/A 1996, 138:265-273.
91:4419-4422.
15.
Pugaley TS, Berruti F: The circulating fluidized-bed catalytic reactor - reactor model validation end simulation of the oxidative coupling of methane. Chem Eng Sci 1996, 51:2751-2756.
16.
SuzukiS, Sasaki T, Kojima T, Yamamura M, Yoshinari T: New procass-development of natural-gas conversion technology to liquid fuels via OCM reaction. Energ Fuel 1996, 10:531-536.
17.
Guczi L, Vansantan RA, Sarma KV: Low-temperature coupling of methane. Catal Rev-Sci Eng 1996, 38:249-296.
18.
Babe P, Li YD, Marquaire PM, Come GM, Baronnet F: A new catalytic jet-stirred reactor application to the study of the oxidative coupling of methane. Oxidat Commun 1996, 19:173-185.
Use of isotopic techniques to elucidate the mechanism for the reforming reaction. 28. -
Wang DZ, Dewaele O, Degroote AM, Froment GF: Reactionmechanism and role of the support in the partial oxidation of methane on R h / A I 2 0 3 . J Cata/1996, 1 5 9 : 4 1 8 - 4 2 6 . TAP reactor investigation of partial oxidation on rhodium catalysts.
33. Burch R, Hayes MJ: C-H bond activation in hydrocarbon A oxidation on solid catalysts. J Mol Catal 1995, 100:13-33. n overview of the mechanism of C-H bond activation on various types of metallic and oxidic catalysts. 34.
19. ..
Butch R, Urbano FJ: Investigation of the active state of supported palladium catalysts in the combustion of methane. Appl Catal A 1995, 124:121-138.
35.
Inoue H, Sekizawa K, Eguchi K, Arai H: Changes of crystalline phase and catalytic properties by cation substitution in mirror plane of hexaaluminate compounds. J Solid State Chem 1996, 121:190-196.
20.
36,
Eguchi K, Arai H: Recent advances in high-temperature catalytic combustion. Catal Today 1996, 29:379-386.
Bjorklund MC, Carr RW: The simulated counter-current movingbed chromatographic reactor - a catalytic and separation reactor. Cata/Today 1995, 25:159-168. An interesting novel reactor concept, which is especially good for reactions at low single pass conversion or for processes with low selectivities. Kruglov AV, Bjorklund MC, Carr RW: Optimization of the simulated countercurrent moving-bed chromatographic reactor for the oxidative coupling of methane. Chem Eng Sci 1996, 51:2945-2950. Practical demonstration of a novel reactor concept. •
21. •-
Zhang ZL, Verykios XE: Carbon-dioxide reforming of methane to synthesis gas over Ni/La203 catalysts. App/Catal A 1996, 138:109-133. Interesting demonstration of a nickel catalyst which is resistant to carbon laydown and simultaneously shows an enhancement in activity. 22.
Chu YL, Li SB, Lin JZ, GU JF, Yang YL: Partial oxidation of methane to carbon-monoxide and hydrogen over NiO/La203/gamma-AI203 catalyst. Appl Catal A 1996, 134:67-80.
23.
Cheng ZX, Wu QL, Li JL, Zhu QM: Effects of promoters and preparation procedures on reforming of methane with carbon dioxide over Ni/AI203 catalyst. Catal Today 1996, 30:147-155.
Hayakawa T, Harihara H, Andersen AG, York APE, Suzuki K, Yasuda H, Takehira K: A sustainable catalyst for the partial oxidation of methane to syngas. Angew Chem Int Ed 1996, 35:192-195. Details of a complex oxide catalyst, containing nickel, which is resistant to carbon deposition.
37. •-
Arai H, Fukuzawa H (Eds): Selected papers presented at the international workshop on catalytic combustion held in Tokyo, Japan April 18-20, 1994. Catal Today 1995, 26:217-363. A series of papers (presented at the 1994 International Workshop on Catalytic Combustion) which give an excellent update on the status of catalytic combustion from both a fundamental and an industrial perspective. 38. •
Machida M, Sato A, Murakami M, Kijima T, Aral H: Structure and property of coherent spinel surface-layers on hexasluminate microcrystals. J Catal 1995, 157:713-720. The Arai group have been at the forefront in developing high temperature catalysts and this paper provides a detailed structural analysis of typical systems. 39.
24. •
25.
Battle PD, Claridge JB, Copplestone FA, SW Carr, Tsang SC: Partial oxidation of natural gas to synthesis gas over ruthenium perovskite oxides. Appl Catal A 1994, 118:217-227.
26. Goetsch DA, Schmidt LD: Microsecond catalytic partial -. oxidation of alkanes. Science 1996, 271:1560-1562. A summary of work from this group on the use of very short contact times for partial oxidation of hydrocarbons. Also see other references within this summary. 27. •
Qin DY, Lapszewicz J, Jiang XZ: Comparison of partial oxidation and steam-CO2 mixed reforming of CH 4 to syngas on MgOsupported metals. J Cata/1996, 159:140-149.
Marti PE, Maciejewski M, Balker A: La0.sSr0.2MnO3+x supported on LaAIO3 and LaAIllO18 prepared by different methods - influence of preparation method on morphological and catalytic properties in methane combustion. Stud Surf Sci Catal 1995, 91:617-626.
Rodriguez NM, Oh SG, Dalla Betta RA, Baker RTK: In situ electron-microscopy studies of palladium supported on AI203, SiO 2 and ZrO 2 in oxygen. J Cata11995, 157:676-686. There has been much debate about the structure of palladium catalysts under oxidizing conditions and this paper provides some excellent direct structural information. 40. •
41. •
Kolaczkowski ST, Serbetcioglu S: Development of combustion catalysts for monolith reactors - a consideration of transport limitations. Appl Catal A 1996, 138:199-214. An assessment of the role of modelling in catalyst and system design. 42.
Hayes RE, Kolaczkowski ST, Thomas TJ, Titiloye J: Transient experiments and modeling of the catalytic combustion of methane in a monolith reactor. Ind Eng Chem Res 1996, 35:406-414.