Catalytic Conversion of Alkanes to Olefins by Carbon Dioxide

1126

Energy & Fuels 2004, 18, 1126-1139

Catalytic Conversion of Alkanes to Olefins by Carbon Dioxide Oxidative DehydrogenationsA Review Shaobin Wang* and Z. H. Zhu Department of Chemical Engineering, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia Received October 16, 2003. Revised Manuscript Received April 15, 2004

The utilization of carbon dioxide (CO2), which is one of the main contributors to the greenhouse effect, has been a topic of global interest, from both fundamental and practical viewpoints. In addition to be the sources of carbon, CO2 might also be utilized as an oxygen source or oxidant, because it can be considered to be a nontraditional (mild) oxidant and oxygen transfer agent. In this paper, CO2 as an oxidant for the selective oxidative conversions of alkanes to alkenes has been reviewed, including methane coupling to ethylene, C2-C4 alkanes dehydrogenation to their corresponding olefins, dehydroaromerization of lower hydrocarbons to benzene, and dehydrogenation of ethylbenzene to styrene. It has been shown that CO2 has the potential to offer a promising alternative to oxygen for selective oxidation, whereas the catalyst development is the key to the applications.

1. Introduction Carbon dioxide (CO2) is one of the most important greenhouse gases. Mitigation of CO2 emission from various sources has been a worldwide objective. It has been recognized that enhanced capacities in the area of chemical catalysis could have a significant role in addressing the global atmospheric CO2 problem. Conversion and utilization of CO2 (as well as methane, CH4) are important subjects in the field of C1 chemistry. Several technologies have been proposed for improving the efficiency of energy conversion and utilization of CO2. Generally, CO2 recovered from flue stacks or atmosphere can be sequestered in oceans or spent gas and oil wells. Alternatively, recovered CO2 can be used to produce chemicals, fuels, and other useful products. There are several motivations for producing chemicals from CO2 whenever possible: (1) CO2 is an inexpensive, nontoxic feedstock that can frequently replace toxic chemicals such as phosgene or isocyanates. (2) CO2 is a renewable feedstock compared to oil or coal. (3) The production of chemicals from CO2 can lead to new materials such as polymers. (4) New routes to existing chemical intermediates and products could be more efficient and economical than current methods. (5) The production of chemicals from CO2 could have a small but significant positive impact on the global carbon balance.1 In the past decades, for catalytic chemistry, most efforts have been concentrated on the utilization of CO2 as a source of carbon. Only recently it has been proposed * Author to whom correspondence should be addressed. E-mail address: [email protected].

that CO2 might also be utilized as an oxygen source or oxidant, because it can be considered to be a nontraditional (mild) oxidant and oxygen transfer agent. The use of CO2 as an oxidant for the partial oxidation of lower alkanes may become an important route for the utilization of natural gas. It is known that the natural gas in many areas contains CO2 in a large quantity, in addition to CH4 and other lower alkanes. It would be highly desirable to utilize such low-valued natural gas without emission of CO2 via the simultaneous transformation of CH4 and other lower alkanes to valuable chemicals or fuels. The wide availability and low price of light alkanes make them among the least reactive and chemically most stable natural resources. These hydrocarbons are mostly used as a fuel; however, their potential as feedstock for more-useful chemical products is of great practical interest. A notable example of the utilization of alkanes is their conversion to unsaturated hydrocarbons, because the present-day chemical industry is heavily dependent on the use of olefins as starting materials. In the past decades, oxidative dehydrogenation (ODH) of alkanes by oxygen has been proposed as an alternative to the high-temperature operation of dehydrogenation processes and much effort has been exerted for effective catalyst development and process control. However, because of the exothermic characteristic of these reactions, it is necessary to remove heat and avoid the over-oxidation of olefins to carbon oxides to (1) Arakawa, H.; Aresta, M.; Armor, J. N.; Barteau, M. A.; Beckman, E. J.; Bell, A. T.; Bercaw, E.; Creutz, C.; Dinjus, E.; Dixon, D. A.; Domen, K.; DuBois, D. L.; Eckert, J.; Fujita, E.; Gibson, D. H.; Goddard, W. A.; Goodman, D. W.; Keller, J.; Kubas, G. J.; Kung, H. H.; Lyons, J. E.; Manzer, L. E.; Marks, T. J.; Morokuma, K.; Nicholas, K. M.; Periana, R.; Que, L.; Rostrup-Nielson, J.; Sachtler, W. M. H.; Schmidt, L. D.; Sen, A.; Somorjai, G. A.; Stair, P. C.; Stults, B. R.; Tumas, W. Chem. Rev. 2001, 101, 953-996.

10.1021/ef0340716 CCC: $27.50 © 2004 American Chemical Society Published on Web 06/24/2004

Catalytic Conversion of Alkanes to Olefins

Energy & Fuels, Vol. 18, No. 4, 2004 1127

give high selectivity toward olefins. Therefore, it is expected that the use of the weak oxidant CO2, replacing oxygen in the reactions, will suppress the deep oxidation. In this paper, we will review the progress of the conversion of various alkanes to olefins in ODH in the presence of CO2. These reactions include CH4 coupling to ethylene (C2H4), C2-C4 alkanes dehydrogenation to their corresponding olefins, dehydroaromerization of lower hydrocarbons to benzene (C6H6), and dehydrogenation of ethylbenzene (EB) to styrene. It is shown that the use of CO2 instead of oxygen provides an effective method of CO2 utilization in selective oxidation. 2. Oxidative Coupling of Methane (OCM) with Carbon Dioxide to Ethane and Ethylene Methane (CH4) is an important natural resource in the world, with an abundance of C2-C4 hydrocarbons. Basically, there are three routes for the chemical use of CH4: (i) an indirect route, through conversion to syngas; (ii) direct coupling in the presence of O2 or H2; and (iii) direct conversion to oxygenates. In the past two decades, a great number of papers have been published on the direct conversion of CH4, particularly on the oxidative coupling of methane (OCM) to C2 hydrocarbons (ethane (C2H6) and ethylene (C2H4)) by O2.2-5 The reactions involved are listed as follows:

Figure 1. Equilibrium conversions of methane (CH4) to ethane (C2H6) (dotted lines) and ethylene (C2H4) (solid lines) using carbon dioxide (CO2): curves a and c, CO2:CH4 ) 1; curves b and d, CO2:CH4 ) 2. (From ref 15.)

2.1. Thermodynamics. The overall reactions for CH4 coupling to C2H6 and C2H4 under CO2 can be expressed as follows:

2CH4 + CO2 f C2H6 + CO + H2O (∆H° ) 62.1 kJ/mol) (7) 2CH4 + 2CO2 f C2H4 + 2CO + 2H2O (∆H° ) 196.2 kJ/mol) (8) At the same time, the following side reactions that produce CO are possible:

2CH4 + O2 f C2H4 + 2H2O

(1)

CH4 + CO2 f 2CO + 2H2

(9)

1 2CH4 + O2 f C2H6 + H2O 2

(2)

CH4 + 3CO2 f 4CO + 2H2O

(10)

3 CH4 + O2 f CO + 2H2O 2

(3)

C2H4 + 2O2 f 2CO + 2H2O

(4)

5 C2H6 + O2 f 2CO + 3H2O 2

(5)

2CO + O2 f 2CO2

(6)

The inevitable formation of carbon monoxide (CO) and CO2 from the reactant (CH4) and products (C2H4 and C2H6) (reactions 3-5), however, seems to be one of the most serious problems, from a practical point of view. CO2, as an oxidant, has several advantages over O2. First, CO will be the only byproduct in this case (see reactions 7 and 8 later in this work). Second, unlike O2, CO2 will not induce gas-phase radical reactions. In other words, the reactions of CH4 and CO2 to produce C2 hydrocarbons will mainly be controlled by heterogeneous catalysis. It is thus expected that the development of active catalysts achieve high selectivity to C2 hydrocarbons. (2) Lunsford, J. H. Angew. Chem., Int. Ed. Engl. 1995, 34, 970980. (3) Martin, G. A.; Mirodatos, C. Fuel Process. Technol. 1995, 42, 179-215. (4) Mleczko, L.; Baerns, M. Fuel Process. Technol. 1995, 42, 217248. (5) Voskresenskaya, E. N.; Roguleva, V. G.; Anshits, A. G. Catal. Rev.sSci. Eng. 1995, 37, 101-143.

Equilibrium conversions of CH4 to C2H6 (reaction 7) and C2H4 (reaction 8) from thermodynamic calculations are shown in Figure 1. Conversion increases as the temperature and CO2:CH4 ratio each increase. An enhancement in the CO2:CH4 ratio increases the CH4 conversions to C2H6 and C2H4 and their yields exceed 15% and 25%, respectively, at 800 °C for the reactant with a CO2: CH4 ratio of 2. These values, if achieved, would meet the target C2 yield (ca. 30%) estimated from economic evaluations.6 The key point for the realization is to develop an efficient catalyst that is capable not only of activating both CH4 and CO2 but also of producing C2 hydrocarbons selectively. Some investigations have been conducted in the past few years, and progress is being achieved in catalyst development. 2.2. Catalyst System. Enhancement of C2 formation by CO2 was first observed in the oxidative coupling of CH4 over a PbO/MgO catalyst,7 but it could not be sustained in the absence of O2.8 Asami et al. systematically investigated the catalytic activities of more than 30 metal oxides for the conversion of CH4 by CO2 in the absence of O2 and found that lanthanide oxides generally showed higher activities (Figure 2). Among them, praseodymium oxide or terbium oxide exhibited relatively good catalytic performance; a C2 yield of 1.5% with (6) Kuo, J. C. W.; Kresge, C. T.; Palermo, R. E. Catal. Today 1989, 4, 463-470. (7) Aika, K. N.; Takahito, J. Chem. Soc., Chem. Commun. 1988, 7071. (8) Nishiyama, T. A.; Kenichi, J. Catal. 1990, 122, 346-351.

1128 Energy & Fuels, Vol. 18, No. 4, 2004

Wang and Zhu

Figure 3. Selectivity (top panel) and yield (bottom panel) of C2 hydrocarbons over lanthanide oxides at 850 °C. (From ref 10.)

Figure 2. Selectivity (top panel) and yield (bottom panel) of C2 hydrocarbons as a function of CH4 conversion for the oxidative coupling of methane (OCM) in CO2. (From ref 9.)

a selectivity of ca. 50% was obtained at 850 °C (Figure 3).9-11 Wang et al. further investigated the conversion of CH4 to C2H6 over praseodymium oxide and reported that praseodymium oxide could be effective in the presence of CO2 at temperatures as low as ca. 500-650 °C to form C2 hydrocarbons.12 In addition to the monoxide, binary oxide catalysts were also investigated by several research groups for the oxidative coupling of methane by CO2. A binary oxide (La2O3-ZnO) was observed to have high C2 selectivity and good stability, giving a C2 yield of 2.8%.13 Although this yield is better than that of any monoxide system reported previously, it is still quite low. Wang et al. investigated ceria that had been modified with alkali-metal and alkaline-earth-metal oxides for the reaction and reported that CaO-CeO2 systems are (9) Asami, K.; Fujita, T.; Kusakabe, K.; Nishiyama, Y.; Ohtsuka, Y. Appl. Catal. A 1995, 126, 245-255. (10) Asami, K.; Kusakabe, K.; Ashi, N.; Ohtsuka, Y. Appl. Catal. A 1997, 156, 43-56. (11) Asami, K.; Senba, H.; Yano, M.; Yonezawa, Y.; Ohtsuka, Y. J. Jpn Pet. Inst. 1999, 42, 165-169. (12) Wang, Y.; Zhuang, Q. L.; Takahashi, Y.; Ohtsuka, Y. Catal. Lett. 1998, 56, 203-206. (13) Chen, C. L.; Xu, Y. D.; Li, G. J.; Guo, X. X. Catal. Lett. 1996, 42, 149-153.

effective and that the C2 yield could reach 6.1% at 900 °C.14,15 Characterization of the system indicated that the redox of Ce4+/Ce3+ relates to the activation of CO2, i.e., the dissociation to CO and oxygen species, which accounts for CH4 conversion, and the basic Ca2+ ion in the catalyst greatly enhances the selectivity to C2 hydrocarbons.15 In the meantime, they also reported that CaO-Cr2O3 catalysts are effective for this reaction.16 A binary catalyst of CaO-ZnO was also tested by the same group for the reaction. A C2 selectivity approaching 80% with a C2 yield of 4.3% was achieved over the catalyst with a Ca:Zn ratio of 0.5 at 875 °C. The lattice oxygen of the CaO-ZnO could convert CH4 mainly to H2 and CO. The presence of CO2 formed a new oxygen species, which was active and selective for the conversion of CH4 to C2 hydrocarbons. The CaO component in the catalyst enhanced the adsorption of CO2 and thus suppressed the reaction involving the lattice oxygen. The reduced Zn site was suggested to activate CO2.17 Recently, Wang et al. further reported their investigations of catalytic performance and kinetics over other binary oxides, mainly CaO-MnO2, SrO-MnO2, and BaO-MnO2, for the CO2 coupling of CH4 to C2H6 and C2H4. At temperatures of g840 °C, the CaO-MnO2 catalyst exhibited performance similar to that of other calcium-containing binary oxide catalysts (CaO-CeO2, CaO-Cr2O3, and CaO-ZnO); C2 selectivity and yield at 850 °C increased remarkably with increasing partial pressure of CO2, and apparent activation energies observed over these catalysts were approximately the same (190-220 kJ/mol). When the temperature was decreased from 840 °C to 825 °C, CH4 conversion and C2 selectivity over the CaO-MnO2 catalyst abruptly (14) Wang, Y.; Takahashi, Y.; Ohtsuka, Y. Appl. Catal. A 1998, 172, L203-L206. (15) Wang, Y.; Takahashi, Y.; Ohtsuka, Y. J. Catal. 1999, 186, 160168. (16) Wang, Y.; Takahashi, Y.; Ohtsuka, Y. Chem. Lett. 1998, 12091210. (17) Wang, Y.; Ohtsuka, Y. J. Catal. 2000, 192, 252-255.



Table 1. Catalyst Systems for Methane (CH4) Coupling with Carbon Dioxide (CO2) to C2 Hydrocarbons catalyst

reference

temp. (°C)

CO2:CH4 ratio

CH4 conversion (%)

C2 selectivity (%)

C2 yield (%)

La2O3-ZnO ZnO-CaO CeO2-CaO Cr2O3-CaO MnO2-CaO Cr2O3-SrO ZnO-SrO CeO2-SrO MnO2-SrO ZnO-BaO CeO2-BaO Cr2O3-BaO MnO2-BaO MnO2-SrCO3 Na2WO4-Mn/SiO2

Chen et al.13 Wang and Ohtsuka17 Wang et al.15 Wang et al.16 Wang et al.16 Wang and Ohtsuka18 Wang and Ohtsuka18 Wang and Ohtsuka18 Wang and Ohtsuka18 Wang and Ohtsuka18 Wang and Ohtsuka18 Wang and Ohtsuka18 Wang and Ohtsuka18 Cai et al.19 Yu et al.21

850 875 850 850 850 850 850 850 850 850 850 850 850 875 820

1.0 2.3 1.0 2.3 2.3 2.3 2.3 2.3 2.3 2.3 2.3 2.3 2.3 2.3 2.0

3.1 5.4 5.0 6.3 3.9 2.4 2.8 1.5 3.9 0.6 0.4 0.8 3.8 5.7 4.7

91 80 62 63 68 37 79 64 85 74 55 42 67 51 94

2.8 4.3 3.2 4.0 2.7 1.3 2.2 1.0 3.3 0.4 0.2 0.3 2.6 4.5 4.5

decreased and a discontinuous change was also observed in the Arrhenius plots. On the other hand, the SrOMnO2 and BaO-MnO2 catalysts exhibited kinetic features that were different from the CaO-MnO2 system; C2 selectivity at 850 °C changed only slightly with the partial pressure of CO2, and the activation energies were constant over the entire temperature range examined and notably lower. Characterizations revealed that a solid solution of Ca0.48Mn0.52O was the main phase for the CaO-MnO2 catalyst after reaction at 850 °C, whereas, at 800 °C, some Ca2+ ions were separated from the solid solution to form CaCO3, which covered the catalyst surface. Such a difference accounted for the discontinuous change in the catalytic behavior with temperature. With the SrO-MnO2 and BaO-MnO2 catalysts, SrCO3 and BaCO3 were formed, along with MnO2, after reaction, and the carbonates were suggested to react with MnO2 to form SrMnO2.5 and BaMnO2.5 in the conversion process of CH4 with CO2.18 Cai et al. further investigated the system of Mn/SrCO3 for the selective conversion of CH4 to C2 hydrocarbons, using CO2 as an oxidant and achieved C2 selectivities of 88% and 79.1%, with a C2 yield of 4.3% and 4.5% over catalysts with an Mn:Sr ratio of 0.1 and 0.2, respectively.19 Zhou and his colleagues investigated a series of lead/ calcium catalysts, promoted with lithium, potassium, and lanthanum, for OCM and found that CO2 added to the feed resulted in a decreased CH4 conversion and an increased C2 selectivity for the lithium/lead/calcium catalyst.20 Na2WO4-Mn/SiO2, which is a better catalyst for the oxidative coupling of CH4 with O2, was further investigated in the conversion of CH4 with CO2 to C2 hydrocarbons. A CH4 yield of ∼5% and a selectivity to C2 of 95% were obtained at 820 °C. The reaction temperature was observed to favor CH4 conversion but not the selectivity to C2. This is due to surface lattice oxygen, which is responsible for the selective oxidation of CH4 to C2, whereas the bulk lattice oxygen is responsible for deep oxidation.21 Table 1 presents a summary of the catalytic activity of the various catalysts that have been investigated. The (18) Wang, Y.; Ohtsuka, Y. Appl. Catal. A 2001, 219, 183-193. (19) Cai, Y. C.; Chou, L. J.; Li, S. B.; Zhang, B.; Zhao, J. Catal. Lett. 2003, 86, 191-195. (20) Zhou, B.; McFarlane, R. A.; Smith, K. J. Can. J. Chem. Eng. 1995, 73, 103-109. (21) Yu, L.; Hou, R.; Liu, X.; Xue, J.; Li, S. Stud. Surf. Sci. Catal. 1998, 119, 307-311.

MnO2- and ZnO-based catalysts are observed to exhibit higher C2 yield. A strong basic oxide (e.g., BaO) will not favor the activation of CH4. It is also observed that the results reported previously still show a low C2 yield, compared to the thermodynamic calculations. The highest CH4 conversion is 6%, at ∼850 °C, which suggests that these catalyst systems are not effective in activating CH4. Therefore, it is necessary to develop new catalyst systems or to find a new method for the activation of CH4, thus improving CH4 conversion. Zhang et al. recently reported using pulse corona plasma as an activation method and applied it for the reaction of CH4 and CO2 over some catalysts. The products were C2 hydrocarbons and the byproducts were CO and molecular hydrogen (H2). CH4 conversion and the yield of C2 hydrocarbons were affected by the CO2 concentration in the feed. The conversion of CH4 increased as the CO2 concentration in the feed increased, while the yield of C2 hydrocarbons decreased. The synergism of La2O3/ γ-Al2O3 and plasma gave a CH4 conversion of 24.9% and C2 hydrocarbons yield of 18.1% were obtained at a plasma power input of 30 W. The distribution of C2 hydrocarbons was changed through the use of a PdLa2O3/γ-Al2O3 catalyst, and the major C2 product was C2H4.22 2.3. The Role of Carbon Dioxide and the Reaction Mechanism. In the reaction of CH4 coupling, it is generally believed that the activation of CH4 is the most important step. The activation of CH4 involves cleavage of the C-H bond; however, it is not clear whether that cleavage is (i) heterolytic, resulting in the formation of methyl ions (CH3+) or CH3•, or (ii) homolytic, leading to the direct formation of a methyl radical released into the gas phase.3 Currently, it is generally accepted that the active sites for the activation of CH4 can be ascribed to O-(s) or O2-(s) via the reactions

CH4 + O-(s) f CH3• + OH-

(11)

CH4 + O2-(s) f CH3- + OH-

(12)

In the presence of O2, CH3- can convert to the CH3 radicals required for dimerization to C2H6 via reaction with molecular oxygen (O2). (22) Zhang, X. L.; Dai, B.; Zhu, A. M.; Gong, W. M.; Liu, C. H. Catal. Today 2002, 72, 223-227.


CH3- + O2 f CH3• + O2-

Wang and Zhu

(13)

The O2- superoxide species is known to be very reactive and capable of promoting deep oxidation of the hydrocarbons to COx. Therefore, it is necessary to suppress the production of O2- in OCM. Several investigations have found that a co-feed of CO2 in OCM could have several effects, including poisoning of the active sites, stabilization of the active sites, an increase in the activation energy for CH4 conversion, and enhancement of the C2 yield or C2 selectivity, depending on the catalyst systems and process conditions.23,24 Lunsford and co-workers25 presented a plausible explanation of the CO2 effect on CH4 conversion and C2 selectivity during OCM on Li/MgO. The enhancement of C2 selectivity was attributed to the poisoning effect that CO2 had on the secondary reactions between methyl radicals and the surface modified by the presence of CO2. For CO2 OCM, Asami et al. suggested two possible reaction mechanisms, depending on the types of metal oxides.10 One possible mechanism is that CO2 is first adsorbed over the metal oxide and decomposed to form an active oxygen species, and CO is released. The active species oxidize CH4 to form C2H6 through the methyl radical, and C2H4 would be formed through the pyrolysis and ODH of C2H6.

CO2 + * f CO + O*

(14)

O* + CH4 f CH3• + HO

(15)

2CH3• f C2H6

(16)

The other mechanism would involve the reaction of methane with lattice oxygen in metal oxides to form a methyl radical, which, in turn, is converted to C2 hydrocarbons, and the partly reduced oxides are then oxidized by CO2.

O (surface) + 2CH4 f 2CH3• + H2O + 0 (17) 2CH3• f C2H6

(18)

0 + CO2 f CO + O (surface)

(19)

Liu et al. used O2-TPD and CO2 pulse reaction and observed that the mechanism of CH4 with CO2 to form C2 hydrocarbons was via the first reaction mechanism over Na2WO4-Mn/SiO2,21 whereas for the La2O3/ZnO system, the second mechanism was proposed for CO2 OCM.13 Wang et al. reported that the oxygen vacancies in praseodymium oxide have key roles in the low-temperature reaction of CH4 coupling with CO2. CO2 adsorbs on the oxygen vacancy to form surface oxygen species, which then activate CH4.12 In their work on binary oxide catalysts based on CaO (CaO-ZnO, CaO-Cr2O3, CaOCeO2, CaO-MnO2), they found similar observations, that the oxygen vacancies connected to oxides account for CO2 activation in producing active oxygen species. CH4 would be converted by the active oxygen to a CH3 (23) Smith, K. J.; Galuszka, J. Ind. Eng. Chem. Res. 1994, 33, 1420. (24) Galuszka, J. Catal. Today 1994, 21, 321-331. (25) Wang, D.; Xu, M.; Shi, C.; Lunsford, J. H. Catal. Lett. 1993, 18, 323-328.

radical that is an intermediate for C2 formation. The lattice O atoms are responsible for the conversion of CH4 to CO.15-17 For strontium- and barium-based catalysts, they found that the kinetic features for the Sr-Mn and Ba-Mn catalysts are different from those of the CaMn catalyst.18 In the presence of CO2, these oxides may be converted to the corresponding carbonates, which may subsequently be transformed partly to SrMnO2.5 and BaMnO2.5. These oxides activate CH4 to form the CH3 radicals and underwent coupling reactions to yield C2H6. The mechanism is listed below. Cai et al. investigated the role of CO2 in the Mn-SrCO3 catalyst, and their results indicate that the oxygen species responsible for selective C2 formation does not originate from lattice oxygen but is generated from CO2 during the reaction.19

2SrCO3 + 2MnOf 2SrMnO2.5 + CO + CO2 (20) 2BaCO3 + 2MnOf 2BaMnO2.5 + CO + CO2 (21) 2SrMnO2.5 + CH4 f 2SrO + 2MnO + CH3• + OH(ad) (22) 2BaMnO2.5 + CH4 f 2BaO + 2MnO + CH3• + OH(ad) (23) Therefore, it is concluded that CO2 could have two important roles in CH4 coupling to C2H6 and C2H4. One role is to be activated by the oxide surface to form active oxygen species, inducing CH4 cleavage and giving methyl radicals, and the other role is to act as an oxidized agent in the cycle of CH4 activation. 3. CO2 Oxidative Dehydrogenation of C2-C4 Alkanes to Olefins In the past two decades, there has been a strong interest to study the ODH of light alkanes (ethane, propane, and butane), rather than the direct dehydrogenation of hydrocarbons, because of the potential commercial interest to utilize alkanes effectively. In fact, the dehydrogenation of paraffins (eq 24) to give the corresponding olefins and H2 is a strongly endothermic process that must be performed at temperatures >600 °C. At the high reaction temperatures, other unwanted reactions that lead to coke also occur, and the catalyst requires frequent regeneration.

CnH2n+2 f CnH2n + H2

(24)

For all reactions of alkanes with oxygen (reaction 25), the abstracted hydrogen is oxidized, releasing the heat of reaction, and the conversion becomes significant at a much lower reaction temperature. However, because of the exothermic characteristics of these reactions in the presence of oxygen, it is necessary to remove heat and avoid the over-oxidation of olefins to carbon oxides, to give high selectivity toward the olefins. Reaction 26 shows the ODH of alkanes in the presence of CO2. Other than the products of alkenes and water, CO is the main byproduct.

1 CnH2n+2 + O2 f CnH2n + H2O 2

(25)

CnH2n+2 + CO2 f CnH2n + CO + H2O

(26)


In the past twenty years, many papers have been published on the development of catalyst systems for the ODH of alkanes using oxygen, and their results can be found in several reviews.26-30 Here, we will review the progress of CO2 ODH. 3.1. Ethane Oxidative Dehydrogenation to Ethylene by CO2. 3.1.1. Catalyst System. The overall reaction can be described as follows:

C2H6 + CO2 f C2H4 + CO + H2O (∆H° ) 134 kJ/mol) (27) Other side reactions can also occur:

C2H6 f C2H4 + H2

(∆H° ) 137 kJ/mol) (28)

C2H6 + 2CO2 f 4CO + 3H2 H2 + CO2 f CO + H2O

(∆H° ) 431 kJ/mol) (29) (∆H° ) 7 kJ/mol) (30)

All these reactions are observed to be endothermic reactions, although, in comparison with the side reactions, CO2 ODH is more favorable, in regard to thermodynamics. The promoting effect of CO2 in the ODH of C2H6 was first reported by Wang and his colleagues.25 They found that CO2, either formed or added to the system, increased the selectivity for the desired hydrocarbon products during the oxidative coupling of CH4 and the ODH of C2H6 by oxygen over Li+/MgO catalysts. The improved selectivities were attributed to the poisoning effect that CO2 had on the secondary reactions of alkyl radicals with the surface. Krylov et al. conducted investigations on the catalytic ODHs of C1-C7 alkanes by CO2 on several oxide catalysts, Fe2O3, Cr2O3, MnO2, and on multicomponent systems.31,32 For the ODH of C2H6 with CO2 at 800 °C, MnO2-based catalysts exhibited high activities. Further improvement could be achieved when the catalyst was modified by promoters Cr and K. Nakagawa et al.33 studied the dehydrogenation of C2H6 by CO2 over several oxides and reported that gallium oxide (Ga2O3) is an effective catalyst for this reaction, giving a C2H4 yield of 18.6% with a selectivity of 94.5% at 650 °C. They also tested various oxidesupported Ga2O3 catalysts for this reaction and found that Ga2O3/TiO2 is the most active. C2H4 yields reached ca. 20%-25% and the selectivity was ca. 70%-90% at 650 °C in the 17% C2H6/83% CO2 feed.34 Using a pulsed reaction technique, they found that the use of CO2 in the dehydrogenation of C2H6 could reduce carbon depo(26) Bhasin, M. M.; McCain, J. H.; Vora, B. V.; Imai, T.; Pujado, P. R. Appl. Catal. A 2001, 221, 397-419. (27) Blasco, T.; Nieto, J. M. L. Appl. Catal. A 1997, 157, 117-142. (28) Mamedov, E. A.; Corberan, V. C. Appl. Catal. A 1995, 127, 1-40. (29) Kung, H. H.; Kung, M. C. Appl. Catal. A 1997, 157, 105-116. (30) Cavani, F.; Trifiro, F. Catal. Today 1995, 24, 307-313. (31) Krylov, O. V.; Mamedov, A. K.; Mirzabekova, S. R. Catal. Today 1995, 24, 371-375. (32) Krylov, O. V.; Mamedov, A. K.; Mirzabekova, S. R. Ind. Eng. Chem. Res. 1995, 34, 474-482. (33) Nakagawa, K.; Okamura, M.; Ikenaga, N.; Suzuki, T.; Kobayashi, T. Chem. Commun. 1998, 1025-1026. (34) Nakagawa, K.; Kajita, C.; Ide, Y.; Okamura, M.; Kato, S.; Kasuya, H.; Ikenaga, N.; Kobayashi, T.; Suzuki, T. Catal. Lett. 2000, 64, 215-221.


sition over the catalyst and assist in the rapid desorption of C2H4 from the catalyst’s surface.35 They further explored an oxidized-diamond-supported Cr2O3 catalyst and found that the oxidized diamond had a significant role in improving the catalyst performance of the Cr2O3 catalyst in the dehydrogenation of C2H6 to C2H4 by CO2, giving a C2H4 yield of 22.5% with a C2H4 selectivity of 87.7% at 650 °C. The activity of the oxidized-diamondsupported Cr2O3 catalyst increased as the CO2 partial pressures increased. X-ray photoelectron spectroscopy (XPS) analyses of the fresh and reacted catalysts, in the presence and absence of CO2, revealed that CO2 maintained the surface Cr2O3 at a higher oxidation state than the reaction in the absence of CO2. On the other hand, the bulk of the Cr2O3 was not reduced in either atmosphere.36,37 Valenzuela et al.38,39 investigated the selective ODH of C2H6 with CO2 over ceria (CeO2)-based catalysts. The presence of Ca2+ ions in solid solution in the CeO2 crystalline network significantly increased the selectivity to C2H4 and the efficiency of CO2 as an oxidant in the heterogeneous reaction. They believed that the reaction was rather complicated, because of the coexistence of homogeneous dehydrogenation in the gas phase and heterogeneous catalysis on the catalyst surface. CeO2 acted as a redox catalyst. The actual contribution for C2H4 formation from heterogeneous catalysis is 75%-55% in the temperature range of 680-720 °C. The reaction routes can be expressed in the following reactions:

2CeO2 + C2H6 f Ce2O3 + C2H4 + H2O

(31)

Ce2O3 + CO2 f 2CeO2 + CO

(32)

Wang et al. investigated the effect of support on the catalytic ODH of C2H6 into C2H4 by CO2 over an unsupported Cr2O3 catalyst and several supported Cr2O3 catalysts on metal oxides such as Al2O3, SiO2, TiO2, and ZrO2.40 The unsupported Cr2O3 showed medium catalytic activity in this reaction; the support would exert a very different effect on the catalytic behavior. The catalytic activity varied with the nature of supports. Cr2O3/SiO2 catalysts exhibited excellent performance in this reaction. Cr2O3 loading also affected the catalytic activity; 8 wt % Cr2O3/SiO2 catalysts could produce a C2H4 yield of 55.5% at 61% C2H6 conversion at 650 °C. Characterization indicated that the distribution of Cr2O3 on the supports and surface chromium species structure were influenced by the nature of supports. The acidity/basicity and redox property of the catalysts determined the catalytic activity in the dehydrogenation of C2H6 by CO2. On the basis of this (35) Nakagawa, K.; Kajita, C.; Okumura, K.; Ikenaga, N.; NishitaniGamo, M.; Ando, T.; Kobayashi, T.; Suzuki, T. J. Catal. 2001, 203, 87-93. (36) Nakagawa, K.; Kajita, C.; Ikenaga, N.; Suzuki, T.; Kobayashi, T.; Nishitani-Gamo, M.; Ando, T. J. Phys. Chem. B 2003, 107, 40484056. (37) Nakagawa, K.; Kajita, C.; Ikenaga, N. O.; Kobayashi, T.; Nishitani-Gamo, M.; Ando, T.; Suzuki, T. Chem. Lett. 2000, 11001101. (38) Valenzuela, R. X.; Bueno, G.; Corberan, V. C.; Xu, Y. D.; Chen, C. L. Catal. Today 2000, 61, 43-48. (39) Valenzuela, R. X.; Bueno, G.; Solbes, A.; Sapina, F.; Martinez, E.; Corberan, V. C. Top. Catal. 2001, 15, 181-188. (40) Wang, S. B.; Murata, K.; Hayakawa, T.; Hamakawa, S.; Suzuki, K. Appl. Catal. A 2000, 196, 1-8.


Wang and Zhu

Figure 4. Catalytic performance of Cr/SiO2, Cr/SO4-SiO2, and Cr-K/SO4-SiO2 catalysts as a function of time at 650 °C for the oxidative dehydrogenation (ODH) of propane (C3H8) by CO2. (From ref 42.)

characterization, they proposed the following mechanism:

C2H6 f C2H4 + H2

(33)

2C2H6 + 2CrO3 f 2C2H4 + Cr2O3 + 3H2O (34) Cr2O3 + 2CO2 f 2CO + 2CrO3

(35)

H2 + CO2 f CO + H2O

(36)

C2H6 + 2CO2 f 4CO + 3H2

(37)

C2H6 + H2 f 2CH4

(38)

They further investigated a series of Cr2O3/SiO2 catalysts that were modified by some acidic and basic oxides and found that the sulfation of silica presented a positive effect on C2H6 conversion and C2H4 yield while tungstation and addition of strong basic promoters (alkali-metal oxides) suppressed the catalytic activity. A CrK/SO4-SiO2 catalyst exhibited the best performance, giving high activity and stability (Figure 4).41,42 Characterizations indicate that the addition of sulfate changes the bulk and surface properties of Cr2O3/SiO2, promoting the reduction of Cr6+ to Cr3+ and favoring the catalytic conversion. The ODH of C2H6 to C2H4 by CO2 over a series of silica-supported chromium oxide catalysts was also investigated by another research group. The results showed that the catalysts were effective for the reaction and CO2 in the feed promoted the catalytic activity. The 5 wt % Cr/SiO2 catalyst exhibited excellent performance, with 30.7% C2H6 conversion and 96.5% C2H4 selectivity at 700 °C. Electron spin resonance (ESR) and ultraviolet-diffuse reflectance spectroscopy (UV-DRS) were used to probe the active sites and the species with high-valency states (41) Wang, S. B.; Murata, K.; Hayakawa, T.; Hamakawa, S.; Suzuki, K. Catal. Lett. 1999, 63, 59-64. (42) Wang, S. B.; Murata, K.; Hayakawa, T.; Hamakawa, S.; Suzuki, K. Catal. Lett. 2001, 73, 107-111.

(Cr5+ and/or Cr6+) were determined to be important for the reaction.43 A Cr/H-ZSM-5 catalyst was also tested for the reaction. It is reported that the maximum rate of formation of C2H4 at 650 °C was 0.516 mmol gcat/min and the selectivity was >90%. CO2 maintained the activity of the catalyst by removing coke from the surface of the catalyst. Temperature-programmed reduction (TPR) spectrometry profiles of the Cr/H-ZSM-5 catalysts suggested that high-oxidation-state chromium species such as Cr6+ were present on the catalyst, which was considered the key to higher activity.44 Recently, Wang et al. investigated the performance of a mesoporous molecular sieve (Si-MCM41) supported chromium catalyst prepared by different methods in selective dehydrogenation of lower alkanes with CO2 and reported 99.7% selectivity to C2H4 at 11.5% C2H6 at 550 °C for C2H6 dehydrogenation.45 Activated-carbon-supported oxide catalyst systems were investigated by Yang et al.46 Cr2O3 was the best catalyst among the activated-carbon-supported iron, manganese, molybdenum, tungsten, and chromium oxide catalysts. At 550-650 °C, an C2H4 selectivity of 69.6%-87.5% was achieved for a C2H6 conversion of 8.5%-29.2%. CO2 facilitates the dehydrogenation of C2H6 and enhances C2H6 conversion and C2H4 yield. Moreover, coke deposition on the catalyst was significantly retarded.46 A Cr/Si-2 catalyst was developed to possess good reactivity for the formation of C2H4 by the ODH of C2H6 with CO2 by Xu et al.47 The addition of manganese and lanthanum oxides could improve the reactivity of the catalyst. They also found that increasing the CO2:C2H6 ratio of the feed gas could increase the C2H6 conversion and C2H4 selectivity of the Cr/Si-2 catalyst. Moreover, the addition of steam to the feed gas could diminish coke formation during the reaction process. The main reactions for the dehydrogenation of C2H6 with CO2 to C2H4 have been deduced to consist of the cracking of C2H6 to C2H4 and CO2 hydrogenation to CO.47 The catalytic performance of Fe/Si-2 and Fe-Mn/Si-2 catalysts for the conversion of C2H6 with CO2 to C2H4 was further examined in a continuous-flow and fixed-bed reactor. The results showed that the Fe-Mn/Si-2 catalyst exhibits much better reaction activity and selectivity to C2H4 than those of the Fe/Si-2 catalyst, achieving a C2H4 selectivity of 93% and a C2H6 conversion of 66%. The promotion of Mn to the Fe/Si-2 catalyst lies not only in increasing C2H4 selectivity but also in improving the catalytic stability greatly. The promotion of Mn to the improvement of the catalytic stability of the Fe/silicalite-2 catalyst results from the great prohibition of coke formation on the Fe-Mn/silicalite-2 catalyst surfaces. The coking-decoking behaviors of these catalysts were studied through thermogravimetry (TG). The catalytic performances of the catalysts after regeneration for conversion of C2H6 or dilute C2H6 in fluidized (43) Ge, X.; Zhu, M. M.; Shen, J. Y. React. Kinet. Catal. Lett. 2002, 77, 103-108. (44) Mimura, N.; Takahara, I.; Inaba, M.; Okamoto, M.; Murata, K. Catal. Commun. 2002, 257-262. (45) Wang, Y.; Ohishi, Y.; Shishido, T.; Zhang, Q.; Takehira, K. Stud. Surf. Sci. Catal. 2003, 146, 725-728. (46) Yang, H.; Lin, L.; Wang, Q.; Xu, L.; Xie, S.; Liu, S. Stud. Surf. Sci. Catal. 2001, 136, 87-92. (47) Xu, L.; Lin, L.; Wang, Q.; Li, Y.; Wang, D.; Liu, W. Stud. Surf. Sci. Catal. 1998, 119, 605-610.



Table 2. Catalyst Systems for CO2 Oxidative Dehydrogenation (ODH) of Ethane (C2H6) to Ethylene (C2H4) catalyst

reference

temp. (°C)

CO2:C2H6 ratio

Mn/SiO2 Mn/Al2O3 K-Cr-Mn/SiO2 Ga2O3 Ga2O3/TiO2 Ga2O3/Al2O3 Ga2O3/ZrO2 Ga2O3/ZnO Ga2O3/SiO2 Cr2O3/diamond V2O3/diamond CeO2 CaO-CeO2 Mo2C/SiO2 Cr/SO4-SiO2 K-Cr/SO4-SiO2 Cr/H-ZSM5 Cr/Si-MCM41 Cr/AC Fe/AC Mn/AC Na2WO4-Mn/SiO2 Cr/Si-2 Cr-Mn/Si-2 Cr-Mn-Ni/Si-2 Cr-Mn-La/Si-2 Fe-Mn/Si-2

Krylov et al.32 Krylov et al.32 Krylov et al.31 Nakagawa et al.33 Nakagawa et al.35 Nakagawa et al.35 Nakagawa et al.35 Nakagawa et al.35 Nakagawa et al.35 Nakagawa et al.36 Nakagawa et al.36 Valenzuela et al.38 Valenzuela et al.39 Solymosi and Nemeth50 Wang et al.41 Wang et al.42 Mimura et al.44 Wang et al.45 Xu et al.47 Xu et al.47 Xu et al.47 Liu et al.51 Xu et al.47 Xu et al.47 Xu et al.47 Xu et al.47 Xu et al.48

800 800 830 650 650 650 650 650 650 650 650 750 750 600 650 650 650 550 650 650 650 800 800 800 800 800 800

1.5 1.5 1 5 5 5 5 5 5 5 5 2 2 1 5 5 9 5.6 1 1 1 1 1 1 1 1 1

Conversion (%) CO2 C2H6 49.0 50.3 52.3

21.9 21.0 23.5 13.8 11.8 43.8 19.5 23.5 26.7 20.7 39.1

73.1 78.4 82.6 19.6 20.2 13.1 14.8 11.1 9.5 27.4 9.3 42.4 25.0 16.0 67.2 68.0 68.2 11.5 28.9 9.9 10.0 53.3 60.6 63.1 69.7 63.6 68.6

C2H4 selectivity (%)

C2H4 yield (%)

61.0 46.6 76.8 94.5 70.8 71.6 72.6 89.8 97.9 86.7 89.2 71.4 90.5 87.0 81.8 82.5 69.5 99.7 70.5 76.0 75.2 97.0 79.6 81.1 80.6 85.8 92.3

44.6 36.5 63.4 18.5 14.3 9.4 10.7 10.0 9.3 23.8 8.3 30.3 22.6 13.9 55.0 56.1 47.4 11.5 20.4 7.5 7.5 51.7 48.2 51.2 56.2 54.6 63.3

catalytic cracking (FCC) off-gas with CO2 to C2H4 were also examined. The results showed that both the activity and selectivity of the Fe-Mn/Si-2 catalyst after regeneration reached the same level as those of the fresh catalyst, whereas it is difficult for the Fe/Si-2 catalyst to refresh its reaction behavior after regeneration.48,49 Other non-oxide catalyst systems were also developed. Solymosi and Nemeth50 prepared Mo2C on SiO2 and found it to be an effective catalyst for the dehydrogenation of C2H6 to produce C2H4 in the presence of CO2. The selectivity to C2H4 at 575-650 °C was 90%-95% at a C2H6 conversion of 8%-30%.50 The performance of the Na2WO4-Mn/SiO2 catalyst for the ODH of C2H6, under oxygen and CO2, was investigated and compared. C2H6 conversion and selectivity to C2H4 values of >70% could be achieved at 700 °C, with a space velocity of >30 000 h-1. The stability test of the catalyst for the ODH of C2H6 showed that C2H6 conversion and selectivity remained constant for 100 h. CO2 could be used as an oxidant instead of O2 for ODH of C2H6. A C2H6 conversion of 53.3% and 97% selectivity to C2H4 could be obtained at 800 °C. The selectivity decreased when temperature was >800 °C. The surface lattice oxygen was proposed to be responsible for selective ODH of C2H6, whereas the bulk lattice oxygen was responsible for the deep oxidation of C2H6.51 Table 2 summarizes the catalytic activity of previously investigated catalyst systems. A comparison of the relative C2H4 yields suggests that the catalyst systems that contain radical promoting metals (chromium, manganese, and iron) are the most effective. The type of support also influences the catalyst activity. The best

C2H4 yields can be achieved on the catalysts that use acidic supports such as SiO2, Si-2, and H-ZSM5. 3.1.2. Kinetics and Mechanism. CO2 ODH of C2H6 to C2H4 is a new process. Most of the recent investigations have focused on the catalyst development. Few papers have been concerned with the kinetics. Krylov and his colleagues31,32 proposed a kinetic equation for a Cr-Mn/ SiO2 catalyst. The rate of C2H6 dehydrogenation by CO2 is described by the equation

(48) Xu, L. Y.; Liu, J. X.; Yang, H.; Xu, Y.; Wang, Q. X.; Lin, L. W. Catal. Lett. 1999, 62, 185-189. (49) Xu, L. Y.; Liu, J. X.; Xu, Y. D.; Yang, H.; Wang, Q. X.; Lin, L. W. Appl. Catal. A 2000, 193, 95-101. (50) Solymosi, F.; Nemeth, R. Catal. Lett. 1999, 62, 197-200. (51) Liu, Y.; Xue, J.; Liu, X.; Hou, R.; Li, S. Stud. Surf. Sci. Catal. 1998, 119, 593-597.

Figure 5 shows the effect of temperature on the equilibrium conversion of C3H8 with and without CO2 and effect of the ratio of CO2 to C3H8 on the equilibrium conversion of C3H8. The equilibrium conversion of C3H8 in the presence of CO2 is much higher than that without

rC2H4 )

k1pC2H6kOXpCO2 kOXpCO2 + kREDpCO

Valenzuela et al. investigated the kinetics of the CO2 dehydrogenation of C2H6 over CeO2 and CaO-CeO2 systems and found that the activation energies are 3040 kcal/mol and the reaction orders are 0.55-0.95, with respect to C2H6.38,39 Xu et al. investigated the kinetics of CO2 ODH of C2H6 over Cr/Si-2 catalysts and obtained an activation energy of 55.5 kJ/mol.47 3.2. Oxidative Dehydrogenation of Propane by CO2. For the CO2 ODH of propane (C3H8), some typical reactions, including dehydrogenation, ODH, and reforming reactions, might proceed on catalysts.

C3H8 f C3H6 + H2

(39)

C3H8 + CO2 f C3H6 + CO + H2O

(40)

C3H8 + 3CO2 f 6CO + 4H2

(41)

H2 + CO2 f CO + H2O

(42)


Wang and Zhu

Figure 5. Equilibrium conversion of C3H8 as a function of (a) temperature (P ) 1 atm, CO2:C3H8 ) 5:1) and (b) CO2:C3H8 ratio. (From ref 58.)

CO2. For example, at 600 °C and with a CO2:C3H8 molar ratio of 5:1, the equilibrium conversion of C3H8 reaches 92%, whereas, in the absence of CO2, only 50% conversion is achieved under the same conditions. In other words, in the presence of CO2 and with a CO2:C3H8 molar ratio of 5, the temperature of dehydrogenation can be ∼100 °C lower than that for simple dehydrogenation without dilution. As the CO2:C3H8 ratio increases, the equilibrium conversion of C3H8 will also increase. Zhaorigetu and his colleagues reported the promoting effect of CO2 in the dehydrogenation of C3H8 when they investigated the ODH of C3H8 by O2.52 They found that the addition of CO2 into the feed resulted in an increased C3H6 selectivity, together with a decreased CO formation through the total oxidation of the alkane or the formed alkene on rare-earth vanadates as well as the niobium-promoted catalysts.52 Takahara and coworkers53,54 then investigated the CO2 dehydrogenation of C3H8 over various supported Cr2O3 catalysts. Their results showed that CO2 exerted a promoting effect only on SiO2-supported Cr2O3 catalysts. Zou et al.55 investi(52) Zhaorigetu, B.; Kieffer, R.; Hindermann, J. P. Stud. Surf. Sci. Catal. 1996, 101, 1049-1058. (53) Takahara, I.; Saito, M. Chem. Lett. 1996, 973-974. (54) Takahara, I.; Chang, W. C.; Mimura, N.; Saito, M. Catal. Today 1998, 45, 55-59.

gated the effect of Cr2O3 loading in CrOx/SiO2 systems on the C3H8 dehydrogenation reaction under a CO2 atmosphere. The order of C3H8 conversion and selectivity to propylene (C3H6) was observed to be as follows: 2.5 wt %CrOx/SiO2 > 5 wt %CrOx/SiO2 > 10 wt %CrOx/ SiO2. The results of ESR and UV-vis DRS (measured during the reaction process) indicated that the active centers of the CrOx/SiO2 catalysts for the reaction is the Cr5+ species, which is probably generated from Cr3+ species that oxidizes during the calcination and chromate reduction during the reaction. The role of CO2 is to reoxidize the Cr3+ species to Cr5+ species in the reaction. The differences in the catalytic behavior of other various supported Cr2O3 catalysts in the reactions were also investigated. It was reported that Cr2O3/SBA15 and Cr2O3/ZrO2/SBA-15 were more selective to C3H6 and more resistant to coking, in comparison to Cr2O3/ ZrO2 and Cr2O3/γ-Al2O3 for the non-oxidative dehydrogenation of C3H8. In the ODH of C3H8 by O2 and CO2, Cr2O3/SBA-15 also displayed better activity, selectivity, and stability than the other two supported catalysts. The C3H8 conversion and C3H6 yield on the Cr2O3/SBA15 catalyst for the ODH of C3H8 by CO2 at 550 °C (55) Zou, H.; Ge, X.; Li, M. S.; Shangguan, R. C.; Shen, J. Y. Chin. J. Inorg. Chem. 2000, 16, 775-782.


reached 24.2% and 20.3%, respectively.56 In addition, Cr/MCM-41 catalysts were investigated and a C3H8 conversion of 17% and C3H6 selectivity of 93%-95% were obtained.45 Several other types of catalysts (except chromiumbased systems) were also tested. Solymosi et al.57 investigated the ODH of C3H8 with CO2 over supported Mo2C catalysts. The selectivity to C3H6 at 500-600 °C is 85%-90%; the yield is ∼11% at 670 °C. The advantages of CO2 use for the dehydrogenation of C3H8 to C3H6, on the basis of thermodynamic considerations, were reported and several metal oxides (e.g., Ga2O3, Cr2O3, and Fe2O3, unsupported and supported on γ-Al2O3 and SiO2) were tested by Michorczyk and Ogonowski.58 The Ga2O3 catalyst was determined to be an effective agent for the dehydrogenation of C3H8 to C3H6. The yield of C3H6 at 600 °C was 30.1%.58 The effects of CO2 added in the gas feed for the ODH of C3H8 to C3H6 are investigated on NiMoO4 catalysts. Despite the low reaction temperature range (400-480 °C), it turned out that CO2 was not inert during the reaction. CO2 behaved as a strong oxidant, with respect to the catalyst. CO2 helped to maintain the catalytic surface in a high oxidation state under conditions for which NiMoO4 underwent reduction. CO2 clearly promoted the nonselectivity of the catalyst and could inhibit deactivation. Under particular conditions, it could improve C3H6 formation.59 Nakagawa et al.60 investigated several oxides on oxidized diamond for the dehydrogenation of C3H8 in the presence of CO2 and found that only supported Cr2O3 and V2O5 catalysts showed effective catalytic activity, whereas other metal oxides (e.g., ZrO2, ZnO, Fe2O3, and Ga2O3) showed little effects. They also found that the catalyst activity was dependent on the type of support and that the support activity order was as follows: oxidized diamond > Al2O3 > activated carbon > SiO2. 3.3. Oxidative Dehydrogenation of Butane. NButenes (C4H8) and isobutene (i-C4H8) productions are also of considerable industrial importance. Much progress has been made in the development of catalysts for ODH of n-butane (C4H10) and isobutane (i-C4H10) to C4H8 and i-C4H8 in the past several years. More-detailed information on the ODH of C4H10 by oxygen can be found in a recent review.61 However, little work has been done for these reactions when CO2 is used as an oxidant. Krylov et al.31,32 reported that a Cr-Mn-La-V/Al2O3 catalyst for CO2 dehydrogenation of C4H10 showed 81%84% C4H10 conversion and 76%-78% selectivity to C2C4 olefins at 630-650 °C. They also found that various Cr-Mn/SiO2 and Cr-Mn-K/Al2O3 catalysts were effective for the CO2 dehydrogenation of i-C4H10. The most effective catalyst, Cr-Mn/Al2O3, produced an i-C4H10 conversion of 61%-66% with 78-81% selectivity to i-C4H8. The kinetic studies gave a form similar to that (56) Zhang, X. Z.; Yue, Y. H.; Gao, Z. Catal. Lett. 2002, 83, 19-25. (57) Solymosi, F.; Nemeth, R.; Oszko, A. Stud. Surf. Sci. Catal. 2001, 136, 339-344. (58) Michorczyk, P.; Ogonowski, J. React. Kinet. Catal. Lett. 2003, 78, 41-47. (59) Dury, F.; Gaigneaux, E. M.; Ruiz, P. Appl. Catal. A 2003, 242, 187-203. (60) Nakagawa, K.; Kajita, C.; Ikenaga, N.-O.; Nishitani-Gamo, M.; Ando, T.; Suzuki, T. Catal. Today 2003, 84, 149-157. (61) Madeira, L. M.; Portela, M. F. Catal. Rev. Sci. Eng. 2002, 44, 247-286.


of the Mars-van Krevelen equation:

ri-C4H8 )

k1pi-C4H10k2pCO2 k1pCO2 + k2pi-C4H10

Wang et al. reported, for i-C4H10 dehydrogenation with Cr/MCM41 catalyst, a selectivity of 90.4% at an i-C4H10 conversion of 18.3%.45 Shimada et al.62 found that iron loaded on activated carbon produced an i-C4H10 conversion of 48% with 80% i-C4H8 selectivity at 600 °C in the ODH of i-C4H10 by CO2. The co-feeding of CO2 promoted dehydrogenation, as compared to feeding bulk i-C4H10 or an i-C4H10 and argon mixture. A redox cycle of Fe3O4 and metallic iron was suggested for the catalytic cycle in the CO2 ODH by analyzing XRD and XPS results. Carbon deposition was reduced under a CO2 stream, as compared with the reaction under an argon atmosphere. Bi et al. investigated the influence of the addition of CO2 in the ODH of i-C4H10 over La/ Ba/Sm oxide catalysts in the temperature range of 450600 °C.63 That study showed that La/Ba/Sm oxide was an active and stable catalyst for the ODH of i-C4H8, achieving 40% i-C4H8 conversion and 50% selectivity to i-C4H8.63 The kinetic investigations indicated that the activation energies for CO2 ODH are 12-16 kcal/mol, which are lower than those of dehydrogenation (32-34 kcal/mol). Nakagawa et al.60 recently extended their work on the oxidized-diamond-supported catalysts for C2H6 dehydrogenation to C4H10 and i-C4H10 dehydrogenation in the presence or absence of CO2. They found a very small promoting effect of CO2 on the C4H10 and i-C4H10 conversion over a V2O5 catalyst. The weak promoting effect of a CO2 atmosphere in the cases of C3 and C4 alkanes might be ascribed to the fact that lower reaction temperatures were applied to obtain considerable conversions, because the bond dissociation energies of CH2 or CH groups in C2H6, C4H10, and i-C4H10 are lower than that of CH3 in C2H6. In the dehydrogenation of a higher alkane, the produced alkenes contain allylic hydrogen, which has a much lower bond dissociation energy than that of starting alkanes. Thus, the abstraction of hydrogen from the allylic position may proceed more rapidly to produce coke precursors. Deactivation of the catalyst could not be avoided, consequently decreasing the alkene yields. 4. Aromatization of Lower Hydrocarbons in the Presence of CO2 BTX hydrocarbons (benzene, toluene, and xylenes) are important sources of petrochemicals for gasoline and other feedstocks. Aromatization of lower alkanes is an interest in industry, and many efforts have been made in this area. The transformation of CH4 to aromatics is thermodynamically more favorable than the transformation of CH4 to C2H6, and extensive efforts have also been devoted to the direct conversion along this line in heterogeneous catalysis. In 1993, Wang et al.64 reported (62) Shimada, H.; Akazawa, T.; Ikenaga, N.; Suzuki, T. Appl. Catal. A 1998, 168, 243-250. (63) Bi, Y. L.; Zhen, K. J.; Valenzuela, R. X.; Jia, M. J.; Corberan, V. C. Catal. Today 2000, 61, 369-375. (64) Wang, L.; Tao, L.; Xie, M.; Xu, G.; Huang, J.; Xu, Y. Catal. Lett. 1993, 21, 35.


Wang and Zhu

Figure 6. Effect of CO2 addition on CH4 feed by varying the CO2 concentration (1.6%, 4.1%, 12%) for the formation rate (nmol s-1 gcat-1) on a carbon base of (a) hydrocarbons, (b) ethane + ethylene (C2H6 + C2H4), (c) benzene (C6H6), and (d) naphthalence versus time on stream (given in units of min). (From ref 68.)

on the dehydroaromatization of CH4 under non-oxidizing conditions in a flow reactor mode on HZSM-5 zeolite catalysts modified with transition-metal ions. Since then, many researchers have paid more attention to this process and made encouraging progress toward catalyst development.65,66 Because of coking, this reaction suffers severely from its rapid catalyst deactivation. Without doubt, how to stabilize the catalyst activity would be a primary problem to be solved in realizing its future application. However, neither new active and selective catalysts nor a thorough understanding of the mechanism of the reaction has been achieved. To achieve the high activity and stability in methane dehydroaromatization, novel approaches to reduce carbon deposition are being made. The co-feeding of some oxidants (NO, O2, CO, and CO2) with CH4 has been proposed.67 Here, we discuss some investigations using CO or CO2 as an oxidant. Ohnishi et al.68 reported their investigation on the catalytic dehydroaromatization of CH4 via the addition of CO and CO2 at 1 atm and 700 °C on Mo/HZSM-5 and Fe/Co-modified Mo/HZSM-5 catalysts. With pure CH4 as the feed gas, molybdenum-supported catalysts showed rapid deactivation, because of significant coke formation. In contrast, the addition of a few percent of CO and CO2 to the CH4 feed promoted benzene (C6H6) production and significantly improved the stability of the Mo/HZSM-5 catalyst at prolonged times-on-stream. For iron- and cobalt-modified Mo/HZSM-5 catalysts, the (65) Xu, Y. D.; Lin, L. W. Appl. Catal. A 1999, 188, 53-67. (66) Shu, Y. Y.; Ichikawa, M. Catal. Today 2001, 71, 55-67. (67) Tan, P. L.; Leung, Y. L.; Lai, S. Y.; Au, C. T. Catal. Lett. 2002, 78, 251-258. (68) Ohnishi, R.; Liu, S. T.; Dong, Q.; Wang, L.; Ichikawa, M. J. Catal. 1999, 182, 92-103.

CH4 reaction with CO yielded higher activities of C6H6 production with good catalytic stability for more than 100 h, because of minimization of the coke formation to 10%) to the CH4 feed gas largely inhibited the formation of aromatic products such as C6H6, in a manner similar to complete poisoning by the addition of a few percent of O2 (Figure 6). For other catalysts (e.g., zeolite-supported rhenium (Re) catalysts), stable catalytic activity could be obtained only when CO2 or CO was added into the CH4 feed.69 Under pressurized conditions, the catalytic dehydroaromatization of CH4 by the addition of CO and CO2 was also investigated by the same group. Under a CH4 pressure of >0.2 MPa and a suitable CO2 concentration in the CH4 feed, the high activity and stability were achieved at elevated reaction temperatures of >750 °C and CH4 space velocities of >5400 mL g-1 h-1, because of minimized coke formation on the Mo/HZSM-5 catalyst. It was suggested that the added CO2 reacted with coke before its transformation to inactive graphitic carbon, blocking the microporous channels of the HZSM-5 zeolite, which is more detrimental to the CH4 dehydrocondensation reaction.70 In addition to the CH4 dehydroaromization described above, few investigations have been reported on dehydroaromization of C2H6 and C3H8 using CO2. Nakagawa et al. reported the promoting effect of CO2 on the aromatization of C2H6 and C2H4 over a Ga2O3/HZSM-5 (69) Ohnishi, R.; Ichikawa, M. Catal. Surv. Jpn. 2002, 5, 103-110. (70) Shu, Y. Y.; Ohnishi, R.; Ichikawa, M. J. Catal. 2002, 206, 134142.



portant application is in styrene-butadiene latex. The most important commercial production routes for styrene are the catalytic dehydrogenation of ethylbenzene (EB) in the presence of a large quantity of steam at high temperatures of 600-700 °C and the oxidation of EB to ethylbenzenehydroperoxide, which is subsequently reacted with propylene to give styrene and propylene oxide.72 In commercial practice, steam is co-fed with the EB and serves several purposes. First, it delivers the required heat and acts as a diluent, resulting in a shift to higher equilibrium conversions. Second, it limits the buildup of carbonaceous deposits by gasification. Finally, as an oxidation agent, it renders the iron oxide in an appropriate oxidation state. However, this process suffers from energy loss and catalyst deactivation, because of coking. The catalyst slowly deactivates and typically needs to be replaced every 1-2 years. Therefore, much effort has been exerted to improve the practice. CO2 as an oxidant can suppress the deactivation of the catalyst and increase energy savings. Matsui et al.73 first reported a beneficial effect of CO2, with respect to catalyst deactivation, in 1991. The addition of small amounts of CO2 (0.1-0.5 mol %) to the feed over a commercial catalyst resulted in a suppression of the catalyst activity decay, albeit at the cost of a slight decrease in the rate of styrene formation. Recently, much interest have been raised in the use of CO2 instead of steam and, thus, several catalyst systems have been developed. Figure 7. (a) Yield of styrene at equilibrium in the dehydrogenation of ethylbenzene (EB) in the presence of CO2 or H2O. Simple dehydrogenation uses a H2O(CO2):EB ratio of 9. (b) Effects of the CO2:EB or H2O:EB ratio on the yield of styrene at equilibrium. (From ref 82.)

catalyst at >650 °C.34 Aromatics yields were higher than those without CO2. HZSM-5 and metal ions, such as Zn2+, Cr3+, Fe3+ and Ni2+, in HZSM-5 catalysts have been tested for the aromatization of C3H8, and a 15% selectivity to aromatics at 50% conversion has been achieved over HZSM-5. The incorporation of metal ions enhanced the catalytic activity for CO2 reduction. The addition of CO2 was observed to suppress the coke deposition during the aromatization of C3H8.71 The schematics of the reaction routes are described as follows:

C3H8 f C3H6 + H2

(43)

CO2 + C3H8 f C3H6 + CO + H2O

(44)

2C3H6 f C6H6 + 3H2

(45)

CO2 + H2 f CO + H2O

(46)

5. Dehydrogenation of Ethylbenzene to Styrene Styrene is one of the most important monomers in the modern petrochemical industry. It is mainly used for the production of many different polymeric materials, such as polystyrene, styrene-acrylonitrile, and acrylonitrile-butadiene-styrene (ABS). Another im(71) Ihm, S. K.; Park, Y. K.; Lee, S. W. Appl. Organomet. Chem. 2000, 14, 778-782.

C6H5-C2H5 + CO2 f C6H5-CHdCH2 + CO + H2O (47) C6H5-C2H5 f C6H5-CHdCH2 + H2

(48)

CO2 + H2 f CO + H2O

(49)

Mimura and Saito proposed two possible reaction pathways for the dehydrogenation in the presence of CO2: a one-step pathway, shown in reaction 47, and a two-step pathway, via reaction 48 followed by reaction 49. The equilibrium yields of styrene at different temperatures for the two reaction pathways, and the simple dehydrogenation, were calculated and are shown in Figure 7a. The equilibrium yield of styrene in the presence of CO2 either via the one-step pathway or the two-step pathway is greater than that in the presence of steam. Higher CO2 content in the co-feed favors the yield of styrene. In the case of a CO2:ethylbenzene ratio of >8 for a one-step pathway, or in the case of a CO2: ethylbenzene ratio of >3 for a two-step pathway, the equilibrium yield of styrene in dehydrogenation in the presence of CO2 at 550°C is higher than that in the presence of steam at 600 °C (see Figure 7b). An early attempt to use CO2 as a diluent and oxidant in the dehydrogenation of EB to styrene was performed over an activated-carbon-supported iron catalyst (17% Fe) at 500-700 °C. An addition of 20-30 mol % of lithium nitrate to iron resulted in a significant increase in the catalytic activity. The highest yield of styrene (72) Meima, G. R.; Menon, P. G. Appl. Catal. A 2001, 212, 239245. (73) Matsui, J.; Sodesawa, T.; Nozaki, F. Appl. Catal. 1991, 67, 179188.


(40%-45%) with >90% selectivity was obtained at an Li:Fe ratio of 0.1-0.2 (mol/mol). In addition to styrene, CO and water (H2O) were formed as products. This indicated that the reaction proceeded via an ODH mechanism.74 Chang and co-workers75,76 investigated the dehydrogenation of EB with CO2 over a ZSM-5-supported iron oxide catalyst, a commercial catalyst (K-Fe2O3), and unsupported iron oxide (Fe3O4), for comparison. They found that EB was predominantly converted to styrene via an oxidative manner. CO2 in this reaction played the role of an oxidant to greatly improve catalytic activity as well as coke resistance of the catalyst. On the other hand, both the commercial catalyst and the unsupported Fe3O4 exhibited a considerable decrease in catalytic activity under the same conditions. They suggested that an active phase for the dehydrogenation with CO2 over ZSM-5-supported iron oxide catalyst would, instead, be a reduced and isolated magnetite (Fe3O4)-like phase that has a oxygen deficiency in the zeolite matrix.77,78 In the follow-up research, they investigated a series of zirconia (ZrO2)-based catalysts for this reaction. ZrO2 itself was observed to be active for the dehydrogenation of EB, especially in the presence of CO2. This positive effect of CO2 was highly dependent on the crystalline phases of ZrO2. The higher the tetragonal phase contained in ZrO2, the higher the EB conversion and styrene selectivity that were obtained. Highly tetragonal ZrO2 was more active in ODH than monoclinic ZrO2. The differences of catalytic activities could be ascribed to the differences of the surface area and CO2 affinity, relative to surface basicity.79 The addition of a CeO2 promoter to ZrO2 improved the catalytic activity significantly, which was attributed to the increasing basicity, as well as oxygen vancancies. The loading of iron oxide onto only ZrO2 among ZrO2-related materials is effective for improving the activity.80 Vislovskiy et al.81 reported high activity and selectivity over various vanadia-alumina (V2O5-Al2O3) catalysts in dehydrogenation of EB to styrene with CO2. Redox behavior of V2O5 played a key role in the dehydrogenation. Among several additives, antimony oxide has been observed to improve catalyst stability as well as catalytic activity to produce styrene. Characterization revealed that the addition of the antimony oxide led to an easier redox cycle between fully oxidized and reduced vanadium species. Mimura et al. estimated the energy required for the dehydrogenation of EB to produce styrene using CO2 and the commercial process using steam to be 6.3 × 108 cal/t-styrene and 1.5 × 109 cal/t-styrene, respectively (74) Sugino, M.-O.; Shimada, H.; Turuda, T.; Miura, H.; Ikenaga, N.; Suzuki, T. Appl. Catal. A 1995, 121, 125-137. (75) Chang, J. S.; Park, S. E.; Park, M. S. Chem. Lett. 1997, 11231124. (76) Chang, J.-S.; Park, S.-E.; Kim, W. Y.; Anpo, M.; Yamashita, H. Stud. Surf. Sci. Catal. 1998, 114, 387-390. (77) Chang, J. S.; Noh, J.; Park, S. E.; Kim, W. Y.; Lee, C. W. Bull. Korean Chem. Soc. 1998, 19, 1342-1346. (78) Park, M. S.; Chang, J. S.; Kim, D. S.; Park, S. E. Res. Chem. Intermed. 2002, 28, 461-469. (79) Park, J. N.; Noh, J.; Chang, J. S.; Park, S. E. Catal. Lett. 2000, 65, 75-78. (80) Noh, J.; Chang, J. S.; Park, J. N.; Lee, K. Y.; Park, S. E. Appl. Organomet. Chem. 2000, 14, 815-818. (81) Vislovskiy, V. P.; Chang, J. S.; Park, M. S.; Park, S. E. Catal. Commun. 2002, 3, 227-231.

Wang and Zhu

Figure 8. Flow diagrams of a present commercial process and a new process. (From ref 82.)

(Figure 8), which indicates that the new process using CO2 should be an “energy-saving process”. They thus developed an Fe/Ca/Al oxide catalyst, which exhibited high performance in the dehydrogenation of EB with CO2 and less deactivation.82 The dehydrogenation of EB over the catalyst in the presence of CO2 was considered to proceed via 45% of a one-step pathway and via 55% of a two-step pathway.83 They also prepared some calcined hydrotalcite-like compounds as catalysts for the dehydrogenation of EB in the presence of CO2. The activity of the FeAl2Zn6 oxide catalyst was the highest among the catalysts tested.84 Badstube et al. also investigated the catalytic behavior of iron supported on activated carbon in the ODH of EB with CO2 as an oxidant. High EB conversion (>70%) and selectivity toward styrene (>90%) were observed at 550 °C.85 They further studied the influence of the type and concentration of transition-metal and alkalimetal oxides, and the type of the catalyst support, as well as the pretreatment method, on catalytic activity and stability of catalysts in the dehydrogenation of EB with CO2. Activated-carbon-supported iron oxide promoted by an alkali-metal oxide (Fe2O3-K2O/C) exhibited the highest performance in the reaction.86 However, this catalyst underwent deactivation, because of the formation of an inactive carbonaceous deposit. It was found that the activity depended almost linearly on the amount of (and decrease in) surface area. An excess of CO in relation to styrene in the product distribution suggested that the deposit was formed as a result of styrene polymerization, and further dehydrogenation, on the catalyst surface.87 Kustrowski et al. synthesized some other catalysts that were based on hydrotalcitelike precursors (Mg-Fe and Mg-Al-Fe) and applied them in the dehydrogenation of EB with CO2. During the EB dehydrogenation, the catalysts gradually lost their activity, because of carbonaceous deposits that were created on the surface. Nevertheless, the losses in (82) Mimura, N.; Saito, M. Catal. Today 2000, 55, 173-178. (83) Mimura, N.; Saito, M. Appl. Organomet. Chem. 2000, 14, 773777. (84) Mimura, N.; Takahara, I.; Saito, M.; Sasaki, Y.; Murata, K. Catal. Lett. 2002, 78, 125-128. (85) Badstube, T.; Papp, H.; Kustrowski, P.; Dziembaj, R. Catal. Lett. 1998, 55, 169-172. (86) Badstube, T.; Papp, H.; Dziembaj, R.; Kustrowski, P. Appl. Catal. A 2000, 204, 153-165. (87) Dziembaj, R.; Kustrowski, P.; Badstube, T.; Papp, H. Top. Catal. 2000, 11, 317-326.


activity, relative to the time-on-stream, could be completely restored by oxygen pulses.88 Activated-carbon-supported vanadium catalysts were investigated by Sakurai and his colleagues for the dehydrogenation of EB in an argon or CO2 atmosphere.89 The catalyst, which can be described as 1.0 mmol of vanadium loaded onto 1 g of activated carbon, afforded the highest EB conversion of 67.1%, a styrene yield of 54.2%, and a styrene selectivity of 80.8% at 550 K with a W/F of 70 gcat h/mol of EB in CO2. EB conversion and styrene yield in the presence of CO2 were 14.0% higher than those in argon. They also found that MgO-supported vanadium gave a markedly high yield and selectivity of styrene during the dehydrogenation of EB to styrene under CO2. CO2 could oxidize the reduced vanadium oxide species on MgO and keep the vanadium species at a high oxidation state.90 At 600 °C, the catalyst afforded the highest styrene yield, 73.8% with a selectivity of 90.1%, in the presence of CO2. Active phases of vanadium in the dehydrogenation reaction were believed to be V5+ species in V2O5 or Mg3V2O8 on highly dispersed MgO. The reduced species, V4+ and V3+, were less reactive for the dehydrogenation.91 In addition, the dehydrogenation of isopropylbenzene to alpha-methylstyrene was performed using various supported metal oxide catalysts in the presence of CO2. An activated-carbon-supported vanadium oxide catalyst afforded a high activity in a CO2 atmosphere: the alpha-methylstyrene yield in a CO2 atmosphere was twice as great as that obtained in an argon atmosphere at 450 °C.92 Recently, the dehydrogenation of EB in the presence of CO2 over an Al2O3-supported iron oxide catalyst was investigated at temperatures in the range of 525-600 °C under atmospheric pressure. The yield of styrene increased as W/F increased and reached its highest value (74%) at a W/F value of ∼40 gcat min/mmol EB, where the selectivity to styrene was 96%. An increase (88) Kustrowski, P.; Rafalska-Lasocha, A.; Majda, D.; Tomaszewska, D.; Dziembaj, R. Solid State Ionics 2001, 141, 237-242. (89) Sakurai, Y.; Suzaki, T.; Ikenaga, N.; Suzuki, T. Appl. Catal. A 2000, 192, 281-288. (90) Sakurai, Y.; Suzaki, T.; Nakagawa, K.; Ikenaga, N.-O.; Aota, H.; Suzuki, T. Chem. Lett. 2000, 526-527. (91) Sakurai, Y.; Suzaki, T.; Nakagawa, K.; Ikenaga, N.; Aota, H.; Suzuki, T. J. Catal. 2002, 209, 16-24. (92) Sakurai, Y.; Suzaki, T.; Nakagawa, K.; Ikenaga, N.; Suzuki, T. Catal. Lett. 2000, 69, 59-64.


in reaction temperature, from 575 °C to 600 °C, improved the yield of styrene but decreased the selectivity to styrene slightly. The yield of styrene was the highest at CO2:EB ratios of 5-10. The selectivity to styrene increased slightly as the CO2:EB ratio increased from 15 to 30. However, an increase in either the reaction temperature or the CO2:EB ratio could produce more carbonaceous deposits on the catalyst during the dehydrogenation in the presence of CO2.93 6. Conclusions Carbon dioxide (CO2), which is a major greenhouse gas, can be utilized as a mild oxidant for the selective oxidation of hydrocarbons. Dehydrogenation of alkanes by CO2 to olefins is a promising alternative to dehydrogenation and oxidative dehydrogenation (ODH) by oxygen, which suffer from either coking problems or over-oxidation. CO2 has also been observed to be an effective oxidant in the oxidative coupling of methane (OCM) to ethane (C2H6) and ethylene (C2H4), the ODH of C2-C4 alkanes to alkenes, the dehydroaromerization of lower hydrocarbons (methane (CH4), C2H6, and propane (C3H8)) to benzene (C6H6), and the ODH of ethylbenzene (EB) to styrene. For all these reactions, several efficient catalyst systems have been developed, and their redox property dominates the catalytic activity. In those reactions, CO2 can have different roles, depending on the catalyst systems and reactions: (i) CO2 can produce active oxygen species; (ii) CO2 reoxidizes the reduced oxides, forming the redox cycle; and (iii) CO2 oxidizes the carbon species, reducing coking. However, few investigations have been conducted in the past to explore the reaction mechanism for those reactions, and the active sites for hydrocarbon activation are still under debate. Therefore, it is recommended that the future research should be more focused on reaction mechanisms and kinetics, to provide more information on the catalyst design. For CH4 coupling with CO2, moreefficient catalyst systems should be developed to meet the theoretical prediction. In CO2 dehydrogenation and dehydroaromerization of lower alkanes to alkenes, deactivation of the catalyst is a problem for developing new catalyst systems with long-term stable performance. EF0340716 (93) Saito, M.; Kimura, H.; Mimura, N.; Wu, J.; Murata, K. Appl. Catal. A 2003, 239, 71-77.

Catalytic Conversion of Alkanes to Olefins by Carbon Dioxide

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