Research progress of CO2 oxidative dehydrogenation of propane to propylene over Cr-free metal catalysts

CO2-assisted oxidative dehydrogenation of propane (CO2-ODHP) is an attractive strategy to offset the demand gap of propylene due to its potentiality of reducing CO2 emissions, especially under the demands of peaking CO2 emissions and carbon neutrality. The introduction of CO2 as a soft oxidant into the reaction not only averts the over-oxidation of products, but also maintains the high oxidation state of the redox-active sites. Furthermore, the presence of CO2 increases the conversion of propane by coupling the dehydrogenation of propane (DHP) with the reverse water gas reaction (RWGS) and inhibits the coking formation to prolong the lifetime of catalysts via the reverse Boudouard reaction. An effective catalyst should selectively activate the C–H bond but suppress the C–C cleavage. However, to prepare such a catalyst remains challenging. Chromium-based catalysts are always applied in industrial application of DHP; however, their toxic properties are harmful to the environment. In this aspect, exploring environment-friendly and sustainable catalytic systems with Cr-free is an important issue. In this review, we outline the development of the CO2-ODHP especially in the last ten years, including the structural information, catalytic performances, and mechanisms of chromium-free metal-based catalyst systems, and the role of CO2 in the reaction. We also present perspectives for future progress in the CO2-ODHP. Graphical abstract


Introduction
Propylene is a mainstay in the C3 industry, which can be used to prepare basic organic chemicals through a variety of reaction pathways, such as polypropylene (PP) [1], propylene oxide (PO) [2], acrylic acid (AA) [3], acetone [4], acrylonitrile [5], isopropanol [6], and butanol [7] (Fig. 1). The high-abundance downstream products of propylene are widely used in the terminal fields of home appliances, textiles, building materials, automobiles, electronic appliances, and health care, etc., which support the increasing demand for propylene. Especially, the global demand for the medical mask prepared by melt-blown fabric which is a material manufactured from polypropylene had risen sharply even suffered a shortage for the COVID-19 since 2020. Thus, the great demand for PP has further promoted the production of propylene.
The conventional synthetic methods of propylene involve fluid catalytic cracking (FCC) and steam cracking of naphtha, light diesel, and other oil by-products, in which propylene is obtained as one of the co-products [8]. More than 75% of propylene were produced by FCC and steam cracking technologies in 2020 (Fig. 1). However, these methods cannot meet the fast-increasing demand of propylene for the low selectivity to propylene and the rapid consumption of fossil resources [9]. Therefore, to develop more efficient technologies for fabricating of propylene such as dehydrogenation of propane (DHP) [10], methanolto-olefins (MTO) [11], coal-to-olefins (CTO) [12], Fischer-Tropsch to Olefins (FTO) [13], and C4 olefin disproportionation technology [14] is highly important. Among them, the DHP technology, as an on-purpose route to obtain propylene, has gradually become the main force to produce propylene in recent years ( Figs. 1 and 2).
Indeed, the DHP technology was greatly promoted in recent years, which is benefited from a blowout of propane yield due to the success of the shale gas revolution in the USA [15][16][17]. In 2019, shale gas production reached 6321.2 billion cubic meters, accounting for 30% of the total natural gas production in the USA, and the compound annual growth rate of shale gas in the past ten years reached 16.5% (Fig. 3). Propane, relying on shale gas, has also flooded into the global energy market to lower the price of propane and makes the DHP technology profitable. Thus, dehydrogenation technologies including FBD-4, Catofin, Oleflex, PDH, STAR, ADHO, FCDh, and K-PROTM have been widely established, whose catalysts mostly involve Pt-based and CrO x -based materials (Table 1) [9,18,19]. Nonetheless, the DHP is a strong endothermic (DH = ? 124 kJÁmol -1 ) and pressurized reaction and is not only limited by thermodynamic equilibrium but also by kinetics. The high-temperature condition is essential for the reaction [20]; however, the cleavage of the C-C bond in hydrocarbons under high-temperature conditions, which impels the occurrence of side reactions such as deep dehydrogenation, oligomerization, and coking, results in a significant decrease in propylene selectivity and formation of coke deposits [21]. Besides, under harsh conditions, the catalysts are liable to sintering and irreversible deactivation [20][21][22]. In addition, the precious Pt metal and highly toxic Cr catalysts are employed in the DHP reaction, and this increases investment cost and environmental burden, which goes against sustainable development.
Thus, the oxidative dehydrogenation of propane (ODHP) is an attractive route to substitute the DHP route due to its exothermicity, and it can break away from thermodynamic limitation even at low temperatures [23]. Nowadays, the traditional ODHP with O 2 as an oxidizing agent (O 2 -ODHP) has been extensively studied by employing transition metal oxide catalysts with redox characters, such as chromium [24], vanadium [25][26][27], and molybdenum [28,29]. In addition, the hexagonal boron nitride (h-BN) has been discovered as a potential catalyst for the O 2 -ODHP reaction, which exhibited a high catalytic activity (14% conversion of propane and 79% selectivity to propylene at 490°C) and well-suppressed overoxidation [23,[30][31][32]. However, the inevitable overoxidation of hydrocarbons or propylene to carbon oxides (CO x ) is an urgent reason for lowering the selectivity of olefins  (propylene and ethylene). Along with the exploitation of efficient catalysts for improving the propylene selectivity, the use of a soft oxidant such as CO 2 to replace O 2 has become another way to solve the problems in the O 2 -ODHP reaction [33]. On the other hand, after the industrial revolution, the amount of CO 2 emissions has increased sharply, which causes serious harm to the environment and the global ecosystem [34]. In this background, many countries have pledged to reduce CO 2 emission, and among them, in 2020, China proposed a timetable for the peaking CO 2 emissions before 2030 and carbon neutrality before 2060 (Fig. 4). After that, many protocols on reducing CO 2 concentration were proposed, especially converting CO 2 into valuable chemicals. CO 2 is a high oxidation state of carbon with a high enthalpy of formation (* 394 kJÁmol -1 ) and a low chemical potential molecule. Presently, CO 2 can be used as a C1 resource to synthesize many valuable chemicals via different pathways, such as reforming of CO 2 with methane to syngas [35], CO 2 hydrogenation to methanol, dimethyl ether, and hydrocarbon fuels [36][37][38], CO 2 polymerization to dimethyl carbonate [39][40][41][42], CO 2 carbonation conversion [43], CO 2 oxidative dehydrogenation of propane (CO 2 -ODHP) [44,45].
Indeed, the CO 2 -ODHP is endothermic in nature (Reaction (1), DH 298 K = ? 164 kJÁmol -1 ) as opposed to the O 2 -ODHP reaction (DH 298 K = -118 kJÁmol -1 ). However, CO 2 can consume the H 2 in situ produced in the DHP (Reaction (2)) via the reverse water gas reaction (RWGS, Reaction (3)) further shifts the reaction equilibrium to produce more propylene. In addition, the coke deposition can be removed via the reverse Boudouard reaction (Reaction (4)), and thus, the lifetime of catalyst will be prolonged. Furthermore, for a redox metal catalyst, CO 2 can re-oxidize the reduced catalyst to promote its recovery and further achieve the reaction. However, the CO 2 -ODHP reaction is very complicated and can be proceeded via various pathways, some of which associate with side reactions such as CO 2 dry reforming of propane (Reaction (5)) and propane cracking (Reaction (6)). Through thermodynamic calculations, Michorczyk et al. [46] found that the equilibrium yield of propylene could be improved by coupling propane dehydrogenation with RWGS reaction. Subsequently, Liu et al. found that propane conversion and propylene selectivity were immediately enhanced after the introduction of a suitable amount of CO 2 but suppressed by adding excessive CO 2 due to the competitive adsorption of propane and CO 2 on the catalyst surface [47]. Therefore, improving the selectivity of CO 2 -ODHP by inhibiting the side reactions is crucial to obtaining a high propylene yield. Meanwhile, the desorption of propylene in time not only improves the yield of product but also inhibits the formation of coke deposition.
Even though the CO 2 -ODHP reaction is an extremely challenging issue, many efforts have been devoted to the reaction, especially in developing highly efficient catalysts. To date (Nov. 10th, 2021), the number of published papers gradually increased and has reached up to 2702 based on the Web of Science from 2001 (Fig. 5). And there are some excellent reviews of the dehydrogenation of alkanes to alkenes elaborating the catalysts development, mechanism exploration, and energy analysis of the reaction, etc. For example, Weckhuysen et al. reviewed the dehydrogenation of light alkanes to summarize the nature of active sites, supports and promotes, reaction mechanism, and deactivation pathway of different catalytic systems [19]. Gong et al. described emerging technologies and catalyst development in DHP [9]. Yang et al. outlined the most promising catalysts in the selective dehydrogenation of light alkanes [8]. Hermans et al. reviewed the development of boron-based catalysts for alkane oxidative dehydrogenation [23]. There are also some reviews on CO 2 as a soft oxidant and promoter in oxidative dehydrogenation of light alkanes [35,45,48,49]. In addition, Rownaghi et al. summarized the roles of promoter and support in CO 2 oxidative C-H bond scission in the absence and presence of CO 2 [50]. Chen et al. outlined relationship between the structure and performances in achieving the reaction via diversified characterization methods [44]. Shi et al. also reviewed the nature of active sites and the roles of modifiers and supports for different types of catalysts in CO 2 oxidative dehydrogenation of light alkanes [51]. Besides, Wu et al. described the DHP with different soft oxidants (e.g., CO 2 , N 2 O, S-containing compounds, and halogens/ halides) [33].
As mentioned above, the Cr-based catalysts are widely used in the reaction. However, they are toxic and harmful to the environment. The Cr-free catalysts play important roles in the development of CO 2 -ODHP, and some of them have great potential applications in industry. However, no relative reviews are systematically generalized on the Crfree catalysts. Based on our recent works on CO 2 -ODHP, in this review, we focused on the Cr-free catalysts especially in the CO 2 -ODHP reaction mainly reported within the last decade, including the structural information and the nature of active sites, catalytic performances, mechanisms of Cr-free catalytic systems, as well as the role of CO 2 in the reaction.

Catalysts in CO -assisted dehydrogenation of propane to propylene
For the CO 2 -ODHP reaction, an effective catalyst not only needs to selectively activate the C-H bond with suppression of the C-C cleavage, but also has an affinity for CO 2 activation. As a typical acidic molecule, CO 2 is preferentially adsorbed on the basic sites of catalysts, while propane prefers to be adsorbed on the acidic sites [44,50]. Thus, a catalyst with suitable acidity and basicity is also a key factor to obtain satisfactory catalytic performance. To date, CrO x -based materials have been pointed out as the most promising catalysts in the CO 2 -ODHP reaction for the high activity. It has been widely accepted that during the reaction, propane is dehydrogenated to propylene over tetrahedra Cr 6? sites, which is simultaneously reduced to octahedra Cr 3? . The reduced Cr 3? can be re-oxidized to Cr 6? by CO 2 , and thus complete a redox cycle [52].  [53]. However, the inherent properties of Cr such as short lifetime and toxicity force researchers to seek alternative and sustainable catalytic systems. Takehira et al. [54] investigated the catalytic performances over different M-MCM-41 catalysts (M: V, Cr, Mn, Fe, Co, Ni and Ga) in the CO 2 -ODHP and found that the activity followed the order of Cr (Fig. 6). The results greatly encouraged researchers to further explore other catalytic systems with Cr-free. Nowadays, the typical catalysts with Cr-free for the CO 2 -ODHP include the redox-type transition metal oxides (V, Ce, Mo, etc.), main group metals (Ga, In, etc.), and precious metals (Pt, Pd, Au, Rh, etc.). The expansion of catalytic systems ignites the hope of realizing the industrialization of CO 2 -ODHP to offset the demand gap of propylene.

Cerium-based rare metal catalysts
Generally, Ce-based material is a crucial component in three-way catalysts (TWCs) applied in the automobile exhaust gas due to its high oxygen-storage capacity (OSC) [55]. CeO 2 has oxidizing properties under an oxygen-poor or anoxic condition. Recently, it has been reported that Ce could effectively activate the dry reforming of alkanes and the oxidative dehydrogenation of ethylbenzene to styrene in the presence of CO 2 . The catalytic activity was related to the facile realization of the Ce 4? /Ce 3? redox cycle, which could also induce direct C=O bond scission of CO 2 to provide available lattice oxygen for selective oxidation and coke suppression. In addition, the results of in situ DRIFTS tests and the density functional theory (DFT) calculations revealed that CO 2 could reactively adsorb on ceria to form carbonate and bicarbonate at low temperatures. Subsequently, they could be decomposed and desorbed at high temperatures to release available surface sites for ODH reaction [56,57]. Therefore, the high OSC performance, excellent redox properties, and natural activation ability for CO 2 on Ce-based catalysts verify that they have a potential application in the CO 2 -ODHP reaction.
Ji et al. employed SBA-15 as a support to prepare Cebased monolithic catalyst for the CO 2 -ODHE (E means ethane), which exhibited excellent performance with 63.9% conversion of ethane and 87.2% selectivity to ethylene at 750°C [58]. They also revealed that Ce 4? species with a high oxidation state were the active sites, which could be reduced to Ce 3? species in dehydrogenation process and the reduced Ce 3? species were further reoxidized to Ce 4? species by CO 2 at 750°C. Subsequently, Djinović et al. [59] utilized an electron-rich active carbon (AC) to adjust the electron environment of CeO 2 and prepared a CeO 2 /AC catalyst, which could achieve the CO 2 -ODHP. The catalyst was found to be susceptible to the dehydrogenation of propane, and especially a catalyst with large CeO 2 size was beneficial for re-oxidation with CO 2 . It is difficult to improve the performance of the sole CeO 2 in heterogeneous catalysis probably for the low specific surface area (S BET ) and single pore structure. Thus, the introduction of foreign metals into CeO 2 is an important method to adjust the electronic properties of CeO 2 , improve the stability, and enrich the surface oxygen vacancies for achieving excellent catalytic activity [60][61][62][63]. Corberán et al. improved the catalytic performances of CO 2 -ODHE via incorporating Ca species into the CeO 2 framework [64]. Wang et al. [65] dispersed Ni on the surface of CeO 2 to improve the amount of oxygen vacancies of CeO 2 , resulting in excellent catalytic performances in RWGS reaction.
Even though the lattice oxygen plays an important role in catalysis, its strong oxidizing properties in CeO 2 can cause cracking and dry reforming of propane. In order to appropriately reduce the oxidizing capacity of the lattice oxygen in CeO 2 , Wang and Tsilomelekis [66] introduced Fe into CeO 2 to obtain FeCeO x solid solution catalysts for the CO 2 -ODHP and found that the selectivity of ODHP was enhanced at a high Fe/Ce ratio and cracking was suppressed. A 5FeCeO 2 catalyst exhibited the most stable catalytic behavior in a 20-h continuous test with 9.5% conversion of propane and 45% selectivity to propylene. In addition, the results of ex-situ Raman, XPS, and TEM revealed that the coke deposition was the main reason for deactivation of the catalyst with a low Fe/Ce ratio. For a catalyst with a high Fe content, the sintering of CeO 2 and the migration of Fe to form nanocrystals were the main reasons for the irreversible deactivation. The instability of the catalyst structure limits the further improvement of catalytic activity.
Nowicka et al. [67] introduced Zr and Al into CeO 2 to form a CeZrAlO x catalyst, which could increase the amount of potential oxygen vacancies and improve durability of the catalyst. Subsequently, Pd was impregnated onto the CeZrAlO x support (denoted as Pd/CeZrAlO x ) to improve the redox characteristic of Ce species, which achieved the CO 2 -ODHP reaction with 9.5% of propane conversion and 93% of propylene selectivity at 500°C with a long-term stability. The DRIFTS analysis suggested that Pd could induce the dissociation of CO 2 on surface of the spent catalyst to form CO (Fig. 7b). Although the byproducts of CO and H 2 O were generated in Mars-van Krevelen (MvK) route and RWGS route, the former route did not require the stepwise formation of H 2 as an intermediate. Based on this point, they proposed that the MvK and RWGS mechanisms could be linked with Ce-based catalysts, namely lattice oxygen adsorbed hydrogen of alkane molecules to form alkene and H 2 O, while CO 2 replenished these oxygen species through Pd, releasing CO into the gas phase. At the same time, the in situ H 2 generated from the dehydrogenation of propane was consumed by CO 2 via the RWGS reaction, resulting in shifting the equilibrium of alkane dehydrogenation (Fig. 7a).
Besides, the oxygen vacancy of CeO 2 possesses the ability of dissociation of CO 2 to CO and lattice oxygen. The lattice oxygen of CeO 2 can maintain precious metals in a highly oxidized state, resulting in improving the dehydrogenation performance of alkane. In addition, the bimetallic alloys can modify the surface d-band center and change the adsorption energies of reactants and intermediates, resulting in different catalytic performances. For example, Chen et al. prepared several bimetallic catalysts supported on CeO 2 (such as CoPt/CeO 2 , CoMo/CeO 2 , NiMo/CeO 2 , and NiFe/CeO 2 ) for the reactions of reforming and CO 2 -ODHE [68]. The DFT calculated energy profiles on CoPt/CeO 2 and NiFe/CeO 2 confirmed that the C-C bond cleavage of ethane to CH 3 *, CO*, and H 2 O(g) was exothermic on Pt-terminated CoPt(111) leading to enhancement for the reforming reaction to produce syngas (Fig. 8a). Contrarily, the pathway for selective C-H bond cleavage was more energetically favorable on the mixed NiFe (111) surface, leading to the improvement of selectivity of ODHE (Fig. 8b). Subsequently, the same group investigated the effect of Ni/Fe molar ratios of NiFe/ CeO 2 catalysts for the CO 2 -ODHP and the CO 2 -ODHE [21,[69][70][71]. They found that the product distribution could be adjusted by changing the amount of Fe. A small amount of Fe (Ni 3 Fe 1 ) was beneficial to forming syngas, while an optimal amount of Fe (Ni 1 Fe 3 ) greatly increased the selectivity to alkene. The in situ characterization and DFT calculations revealed that the NiFe 3 was composed of oxidized Fe and metallic Ni, and the Ni-FeO x interfacial sites could selectively break the C-H bonds but preserve the C-C bonds of alkene to improve the selectivity to alkene. Different from the Ni-FeO x catalyst, the Ni-CeO x interfacial sites could efficiently cleave all of the C-H and C-C bonds to produce syngas. Thus, controlled synthesis of the two distinct active sites could enable rational enhancement of the alkene selectivity in the CO 2 -ODHP and the CO 2 -ODHE.
In summary, the catalytic performance of the CO 2 -ODHP over Ce-based catalysts is generally determined by the rate of the Ce 4? /Ce 3? redox cycle, which is related to the amount and stability of lattice oxygen in the catalysts. Generally, the construction of a solid solution can improve the stability of Ce-based catalysts. Besides, introducing a foreign metal on the surface of CeO 2 to create an interfacial effect can also improve the Ce 4? /Ce 3? redox cycle, resulting in an improvement of catalytic performance. Although Ce-based catalysts have exhibited promising results, there are still many difficulties must be overcome for obtaining a higher catalytic activity than that of Crbased catalysts.

Vanadium-based catalysts
V-based material is one of the most promising alternatives to Pt-and CrO x -based catalysts in ODHP. Kondratenko et al. compared the catalytic performances of a VO x /MCM-41 catalyst possessing highly dispersed VO x species with industrially relevant CrO x /MCM-41 and Pt-Sn/Al 2 O 3 catalysts [72]. They found that the former catalyst exhibited a higher propylene selectivity and better stability during several cycles of the dehydrogenation of propane and oxidative regeneration. And the formation of carbon   (111). Reproduced with permission from Ref. [68]. Copyright 2016, Elsevier deposits was the main reason for the deactivation of the VO x /MCM-41. In addition, according to the type of supports, VO x could appear in multiple forms including monomeric, oligovanadate, polymeric, and crystalline V 2 O 5 . Among them, the monomeric and the oligovanadate were beneficial to the formation of C 3 H 6 , while the crystalline V 2 O 5 caused over-oxidation of hydrocarbon [25,73]. Besides, similar to Cr-based catalysts, the interconversion among V 3? , V 4? , and V 5? species could also achieve the redox cycle with the assistance of CO 2 [67]. Suzuki et al. proved that the activity of the oxidized diamond-supported V 2 O 5 catalyst in the CO 2 -ODHP afforded a near twofold higher activity than that in the DHP without CO 2 [74]. The reason was that CO 2 could maintain the surface VO x species in a high state of oxidation.
Generally, the activity of V species mainly depends on the type and structure, particle size, loading amount, and vanadium dispersion on support. Lemonidou et al. [75] investigated the catalytic performance of V-based catalysts supported on Al 2 O 3 , TiO 2 , ZrO 2 , and MgO for the ODHP and revealed that the reducibility and the reduction extent of V species were affected by the acid-base character of the support, resulting in that V 2 O 5 /TiO 2 was the most active catalyst but V 2 O 5 /Al 2 O 3 showed the highest selectivity to propylene. The Al 2 O 3 , ZrO 2 , and TiO 2 with high acid properties were beneficial to the formation of highly dispersed monolayer V species, while MgO with high base property contributed to the formation of mixed magnesium metavanadate phase [76,77]. Liu [73]. They found that the V 3? species were closely related to TOF persite of the catalyst, which were considered as more active for propane dehydrogenation than the other V 5? and V 4? species. Additionally, the in situ DRIFT spectroscopy revealed that propylene was formed from the desorption of the intermediates of propyl-vanadium (C 3 H 7 -V) and propenyl-vanadium (C 3 H 5 -V). Meanwhile, the coke deposition formed and led to the deactivation of catalyst. These findings were also proved by the work of Liu's group [26]. However, the dispersion of V species is difficult to control in conventional impregnation method, which makes it difficult to identify the V species on the catalyst surface.
Guo et al. successfully synthesized nV-MSNSs catalysts in an oil-water biphasic stratification reaction ( Fig. 9) [79]. They found that the vanadium components could be introduced into the mesoporous walls during the growth and assembly processes. The obtained catalyst possessed high dispersibility and showed excellent catalytic performances even after eight reaction regeneration cycles. Besides, they proposed the reaction mechanism for the reaction. Initially, the O of vanadyl (V=O) group abstracts an H atom from a secondary carbon atom of propane to form HO-V IV species, while the oxygen in V-O-Si species abstracts another H atom to yield H 2 OÁV III . In this way, propylene is formed, and the V III species is formed by oxidation of CO 2 . A similar mechanism was also proposed by Sauer et al. via DFT calculations and microkinetic simulations [80]. The dimeric and oligomeric VO x species have a similar catalytic process below the monolayer coverage. In addition, it is proved that H-abstraction is the rate-determining step, which is caused by the carrier's kinetic preference for V dimers. Subsequently, Lang et al. examined the catalytic performance of CO 2 -ODHP over nV/MCM-41 catalysts prepared by a ''one step'' method [81]. They confirmed that the catalysts with high dispersion of isolated VO 4 3were highly active and stable in the reaction. Besides, they confirmed that CO 2 promoted the reaction remarkably via the coupling reactions of DHP and RWGS.
Besides, the structural and catalytic performances of V-based catalysts are also improved by employing a promoter. It has been proved that there is no interaction between dissimilar metal (Cr, Mo, W, etc.) oxides and VO x , but they only act as physical barriers to prevent the polymerization of VO x [82]. For example, Galvita et al. found that the agglomeration of the VO x species in a WO x -VO x /SiO 2 catalyst could be hindered since WO x acted as a physical barrier to the VO x species [83]. Moreover, a control experiment of adding D 2 into the C 3 H 8 and CO 2 feed gases confirmed that the reaction followed ODHP and was accompanied by the RWGS (Fig. 10). Combined with DFT calculations, they confirmed that the CO 2 -ODHP occurred at the isolated O=V 5? -O-V 3? sites. However, the reduced V 4? active sites also played an important role in activating and extracting the second C-H bond of C 3 H 8 . However, they proposed two reaction pathways that were probably involved in the reaction over the O=V 5? -O-V 3? sites, i.e., the direct ODHP pathway and the coupling DHP and the RWGS pathway. In the former pathway, C 3 H 8 could be oxidized by the O=V 5? -O-V 3? sites to produce C 3 H 6 and H 2 O, and O-V 3? -O-V 3? sites were simultaneously generated as well, which was immediately re-oxidized by CO 2 to O=V 5? -O-V 3? sites. CO 2 was converted to CO. However, the coupling of DHP and RWGS pathway followed two steps. At first, DHP occurred over the O=V 5? -O-V 3? sites to produce C 3 H 6 and H 2 , and then, the partial H 2 could reduce the O=V 5? -O-V 3? sites to the O=V 3? -O-V 3? sites, which were re-oxidized by CO 2 to the O=V 5? -O-V 3? sites and CO. A similar pathway was confirmed by Hess et al. using operando UV Raman spectroscopy [84,85]. They observed distinct changes of the surface V species involving the breakage of V-O-V bonds, a partial reduction in vanadyl, as well as the formation of support hydroxyl groups.
Generally, vanadium oxide can form vanadate with alkali metals. Guliants et al. varied the surface coverage of VO x species by using sodium on the silica surface [86]. They found that Na ? could significantly improve the dispersion of surface VO x species and resulted in the formation of surface Na-metavanadate. Subsequently, Hossain et al. introduced La 2 O 3 into Al 2 O 3 supports to obtain a VO x /La 2 O 3 -Al 2 O 3 catalyst for the CO 2 -ODHP [87]. They found that the introduction of La decreased the reducibility Fig. 9 Method of preparation of nV-MSNSs, and reaction and regeneration cycle mechanism for CO 2 -ODHP of monomeric VO x species. Reproduced with permission from Ref. [79]. Copyright 2017, American Chemical Society Fig. 10 Proposed mechanism of CO 2 -ODHP reaction over V-based catalysts based on DFT calculations. Reproduced with permission from Ref. [83]. Copyright 2016, Elsevier and total acidity of the catalyst, resulting in an improvement of propylene selectivity. CO 2 can act as a mild oxidant in direct interaction with the hydrocarbons and reoxidize the reduced catalysts while masking the non-selective sites of the catalysts to enhance olefins selectivity. In addition, the anti-coke formation performance of V-based catalysts can be enhanced by 3D printing which can considerably modify the surface properties and bulk oxide phase dispersion, thus leading to enhanced metal oxide reducibility and exceptional CO 2 -ODHP performance [88][89][90].
Compared with Cr-based catalysts, V-based catalysts possess excellent catalytic performances and suppressed deactivation rates. However, the redox ability of V species depends primarily on the dispersion which can be adjusted by promoter, support, and the preparation method, further resulting in different performances. In addition, the rich V species including V 5? , V 4? , V 3? , isolated V, oligomeric V n? , and crystalline V 2 O 5 make the identification of active species very difficult. Thus, the design strategy should consider many factors simultaneously to obtain an effectively V-based catalyst for industrial application of CO 2 -ODHP.

Gallium-based catalysts
Along with Cr-and V-based catalysts, Ga-based catalysts have been recognized as another important catalytic system for the CO 2 -ODHP since the Ga 2 O 3 /TiO 2 catalyst was established in achieving the CO 2 -ODHE [50,91]. Subsequently, Michorczyk and Ogonowski [92] found that the bulk Ga 2 O 3 catalyst was more effective for the CO 2 -ODHP than the bulk Cr 2 O 3 and Fe 2 O 3 catalysts. The bulk Ga 2 O 3 provided a propylene yield of 30.1% at 600°C, which was greater than the 10% yield obtained over the bulk Cr 2 O 3 and Fe 2 O 3 . Subsequently, they successfully enhanced the initial activity and stability of the catalyst by adding K to Ga 2 O 3 catalyst [93]. There are five polymorphs of a-, b-, c-, d-, and e-Ga 2 O 3 , and all of them were widely investigated in the CO 2 -ODHP [94]. Among them, the b-Ga 2 O 3 exhibited the highest catalytic activity for the abundant of coordinatively unsaturated Ga 3? cations and the conjugated effect of H and O on surface [94]. Meanwhile, they also revealed that the reaction occurred via the heterolytic dissociation of propane over Ga 2 O 3 and CO 2 promoted the reaction via the RWGS and the reverse Boudouard reaction. However, the low surface area lowered catalytic performance over the b-Ga 2 O 3 . Cao et al. successfully prepared a mesoporous b-Ga 2 O 3 with a high surface area by employing eco-friend sucrose as the nonsurfactant template and applied it in catalyzing the CO 2 -ODHP [95]. The conversion of propane was approximately twofold higher than that over the conventional mesoporousfree b-Ga 2 O 3 , attributed to the improved textural structure providing a high abundance of coordinative unsaturated surface Ga sites. Meanwhile, Michorczyk et al. [96] synthesized a thermally stable mesoporous Ga 2 O 3 with the pore size and surface areas in the ranges of 6.2-6.5 nm and 231-322 m 2 Ág -1 by the nanocasting approach. Compared with the Ga 2 O 3 prepared without a hard template, the mesoporous Ga 2 O 3 exerted a higher resistance on deactivation during the CO 2 -ODHP reaction.
Normally, the activity of Ga-based catalysts is determined by the coordination state which is affected by the nature of support. The acidity and basicity also affect the catalytic performances remarkably. Yue et al. investigated the effects of support on the catalytic performances in the CO 2 -ODHP over the Ga 2 O 3 loaded on differential supports such as TiO 2 , Al 2 O 3 , ZrO 2 , SiO 2 , and MgO [97,98]. The Ga 2 O 3 /TiO 2 , Ga 2 O 3 /Al 2 O 3 , and Ga 2 O 3 /ZrO 2 catalysts offered high catalytic activities for their abundant mediumstrong acidic sites (Fig. 11). Especially, the Ga 2 O 3 /TiO 2 catalyst with a strong interaction between TiO 2 and Ga 2 O 3 possessed abundant reduced Ga d? (d \ 2) offered the highest conversion of propane (32%). Due to un-reduction of Ga 2 O 3 under reaction temperature, the dehydrogenation of propane proceeded via a heterolytic dissociation step, namely propane was activated on Ga 2 O 3 active sites via the heterolytically dissociative adsorption, forming Ga-H and Ga-OR species, and the latter was further decomposed to products [97,99]. Subsequently, Cao et al. found that the structure of spinel-type Ga 2 O 3 -Al 2 O 3 solid solutions was beneficial to the dispersion of surface Ga sites which formed the abundant weak acidic sites and tetrahedral Ga 3? cations, resulting in a high activity and stability in the CO 2 -ODHP [100,101]. To improve the stability of Ga 2 O 3 -Al 2 O 3 catalysts, Michorczyk et al. [96] prepared mesoporous Ga 2 O 3 -Al 2 O 3 oxides with pore architecture similar to SBA-15, which exhibited a promising catalytic performance and stability in the CO 2 -ODHP. Tan et al. prepared Ga 2 O 3 -Al 2 O 3 catalysts by a hydrothermal method, which gave 35.2% conversion of propane and 95.0% selectivity to propylene in the CO 2 -ODHP [102]. The characterization results revealed that the high surface area of Ga 2 O 3 -Al 2 O 3 possessed a large amount of tetrahedral Ga ions (e.g., Ga 3? and Ga d? , d \ 2) related to medium-strong Lewis acidic sites, resulting in a superior catalytic activity.
Furthermore, zeolites are usually employed as supports for building bifunctional Ga-based catalysts applied in propane aromatization, in which the Ga sites catalyze dehydrogenation of propane and the Brønsted acidic sites are responsible for the aromatization of propylene [103][104][105]. Thus, it is appropriate to reduce the amount of Brønsted acid sites to adjust the product distribution in the CO 2 -ODHP. Yue et al. adjusted the acidity of the Ga 2 O 3 / HZSM-5 catalysts through dealumination of the HZSM-5 and achieved a 22% yield of propylene without observable deactivation in 100 h [106,107]. After that, the same group introduced phosphorus into HZSM-5 to decrease the acid strength and strong acid sites of the Ga 2 O 3 /HZSM-5 catalyst, resulting in an improvement of stability [108]. Subsequently, they employed ZSM-48 as a support to prepare Ga 2 O 3 /ZSM-48 catalysts, which exhibited a higher propylene selectivity in the CO 2 -ODHP than over the Ga 2 O 3 /HZSM-5. The improved selectivity resulted from the high Si-containing zeolite with a weaker acid strength than ZSM-5 [109]. At the same time, they also improved the propylene selectivity by replacing the ZSM-5 with MWW-type zeolites of ITQ-2 and MCM-22. The ITQ-2 showed a more open pore structure but a lower amount of strong Brønsted acid sites. After loading Ga 2 O 3 , Ga 2 O 3 / ITQ-2 provided a higher propylene selectivity and better stability for the CO 2 -ODHP than Ga 2 O 3 /ZSM-5 catalysts [110]. Furtherly, Orlyk et al. [111] used dealuminated BEA zeolite (SiBEA) as support with adjustable acidic-basic and redox properties to obtain Ga x SiBEA catalysts. They could achieve the CO 2 -ODHP with a 40% yield of propylene due to the formation of Lewis acid sites by isolated Ga III sites. Agafonov et al. [112] found that the silanol nests of SiO 2 could enhance the dispersion of active Ga species and the SiO 2 with low acidity could prevent coke deposition to improve the stability of the catalyst in the CO 2 -ODHP. Yue et al. also found that in Ga 2 O 3 /NaZSM-5 catalysts, silanol nests were helpful in dispersing active species of GaO x and could act as Brønsted acidic sites to produce conjugation effects with Ga species to promote the dehydrogenation reaction [113]. Lercher et al. verified the synergetic interaction between Ga ? and Brønsted acid sites, namely the protons were able to exchange with Ga ? to improve the activity of dehydrogenation [114]. However, a high Ga content could promote the formation of Ga x O y clusters, resulting in low catalytic activity. In addition, the highest reaction rates in propane dehydrogenation could be achieved at equal concentrations of Brønsted and Lewis acid Ga ? sites for the synergy effect between them [115].
In most Ga-zeolite systems, it has been proved that there are many types of Ga species such as isolated Ga 3? , Ga d?   [116]. It was found that compared to other Ga species, [GaH] 2? was the most active site for the dehydrogenation of light alkane. Under the catalyzing of [GaH] 2? , the DH value for transition states (TSs) formation decreased with increasing the carbon number in light alkene (Fig. 12).
Besides the common metal oxides, GaN constitutes another potential heterogeneous catalyst for oxidative dehydrogenation of propane [30,32,[117][118][119][120]. Inspired by the special strong local electrostatic polarization of GaN [121][122][123][124][125], recently, our group developed several GaNbased catalysts for C-H bond activation in the CO 2 -ODHP reaction [47,[126][127][128]. For example, Q-x (a type of SiO 2 supports with varying pore size) as supports to prepare GaN/Q-x catalysts which exhibited good to excellent catalytic activity and reusability in CO 2 -ODHP. It was also found that CO 2 played a positive role in inhibiting carbon deposition on surface of the catalyst during the reaction (Fig. 13a, b). However, an obviously long induction period was required. Subsequently, we replaced Q-x with commercial NaZSM-5 to synthesize GaN/NaZSM-5 catalysts with an initial propane conversion of 45.0% and propylene yield of 28.3% at 600°C and found that the strong acidic sites of NaZSM-5 were beneficial to the dispersion of GaN. And the strong interaction between GaN and the NaZSM-5 support contributed to the formation of Si-O-GaN species, which reacted with propane to form Si-O-GaN-H species, and CO 2 further reacted with Si-O-GaN-H to form the *COOH intermediate and then release propylene, CO, and the Si-O-GaN active site (Fig. 13c-g). Based on the experimental results, we proposed the coupling of DHP and RWGS mechanism, namely DHP path occurred over Ga species to form C 3 H 6 and H 2 , and then, the in situ Fig. 12 Ethane dehydrogenation via different mechanisms over different Ga species at 823 K and 1.01 9 10 5 Pa, a alkyl mechanism (cyclic route) on [GaH] 2? ; b stepwise alkyl mechanism on [GaH 2 ] ? ; c alkyl mechanism on Ga ? ; d effects of increasing chain length and substitution on light alkene formation TSs in alkyl light alkane dehydrogenation mechanism on [GaH] 2? . Reproduced with permission from Ref. [116]. Copyright 2018, American Chemical Society produced H 2 was consumed via RWGS to produce CO and H 2 O, resulting in the equilibrium of DHP shifting toward propylene formation (Fig. 13h).
For the non-redox Ga-based catalysts, the CO 2 -ODHP reaction follows the mechanism of two-step non-oxidative routes, namely the step of direct dehydrogenation of propane and the step of RWGS. The combination of these two steps results in the reaction equilibrium shifts to the production of propylene. Therefore, it is practicable to employ zeolite to build bifunctional catalysts with Ga for CO 2 -ODHP. In terms of C-H bond activation, propane is usually heterolytically dissociative on Ga species to form Gahydride and Ga-alkoxy species. Thus, it is necessary to select promoters or heteroatom to adjust the electronic structure of Ga to achieve excellent catalytic performances. In addition, the thermal stability and hydrophobicity of support such as zeolite also need to be considered for industrial application.

Indium-based catalysts
Indium, as an element similar to Ga in the IIIA group, has also been widely investigated in the CO 2 -ODHP. In practical terms, the catalysts of indium oxide in combination with zeolites or Al 2 O 3 have been applied in de-NO x reactions due to the good redox characteristics of indium species [129,130]. In terms of the CO 2 -ODHP reaction, Cao et al. studied the effects of foreign metals in the metal Fig. 13 a TG curves and b Raman spectra of used 5 wt% GaN/Q-3 catalysts with and without CO 2 . Reproduced with permission from Ref. [47]. Copyright 2020, Elsevier. c-f XPS spectra of fresh and used catalysts of 5 wt% GaN/NaZSM-5; g TPSR results at different reaction temperatures over 5 wt% GaN/NaZSM-5 catalyst; h reaction mechanism for CO 2 -ODHP over GaN/NaZSM-5 catalysts. Reproduced with permission from Ref. [126]. Copyright 2021, American Chemical Society oxide of In 2 O 3 -MO x (M = Al, Zn, Zr, Ti, Fe, Mg, Si and Ce) on the catalytic performances of the CO 2 -ODHP [131]. They found that In 2 O 3 -Al 2 O 3 catalyst showed the highest activity and superior long-term stability for the higher redox property derived from the interaction between In 2 O 3 and Al 2 O 3 . Then, they investigated the effects of acid-base properties of Al 2 O 3 , ZrO 2 , and SiO 2 supports and the interactions between these supports with In 2 O 3 on the performances for the reaction. It was found that the highintensity basic sites on the In 2 O 3 -ZrO 2 catalyst remarkably promoted the RWGS reaction. The high dispersion of In 2 O 3 and balanced acid/base properties in the In 2 O 3 -Al 2 O 3 catalyst resulted in superior catalytic performance in the CO 2 -ODHP [132]. Unlike Ga-based catalysts, the excellent redox characteristics of In 2 O 3 make it possible to be reduced to In 0 , which cannot be re-oxidized by CO 2 [132,133]. Thus, the CO 2 -ODHP reaction over In-based catalysts also follows the mechanism of two-step non-oxidative routes. Cao et al. investigated the In 2 O 3 -Al 2 O 3 bimetallic oxide in the CO 2 -ODHP and confirmed that the combination of the two components was beneficial for the dispersion of In 2 O 3 and formation of In-Al-O species with high reducibility. They proposed that the metallic In 0 species was crucial for the dehydrogenation, and basic sites of Al 2 O 3 were responsible for the activation of CO 2 to promote the RWGS reaction [133,134]. In addition, Ma et al. found that there was no induction period over In/ HZSM-5 catalyst in the CO 2 -ODHP reaction, at the same time, the formation of In 0 would lead to a decrease in activity of In/HZSM-5 [135]. Thus, they considered that In 3? was the main active species, which promoted not only the DHP but also RWGS in the CO 2 -ODHP reaction (Fig. 14).
The CO 2 -ODHP reaction follows the mechanism of the two-step non-oxidative routes over In-based catalysts despite the fact that In shows excellent redox characteristics. And the dispersion that In 2 O 3 and acid-base properties of supports are crucial to achieving the CO 2 -ODHP. Nevertheless, the identification of active species is still an important issue in In-based catalysts.

Precious metal (Pt, Rh, Ru, and Pd) catalysts
Precious metals such as Pt, Ru, Rh, Pd, and Au are also applied in the DHP reaction [8]. Among these reported precious metals, Pt-based catalysts have been successfully applied in the industrial Oleflex technology of the DHP. Many strategies were adopted to reduce Pt loading, improve stability, and resist sintering and carbon deposition [10,[136][137][138][139][140].
For the CO 2 -ODHP reaction, it is worth noting that the Pt-based catalyst can cleave both of the C-H and C-C bonds of propane. Thus, the products contain a large amount of syngas (CO and H 2 ) [71,141]. To improve the selectivity of propane dehydrogenation to propylene, tandem catalysts could be applied. For example, recently, Notestein et al. successfully created nanoscale tandem catalysts by growing a 2-nm shell of In 2 O 3 over Pt/Al 2 O 3 for the ODHP, in which Pt NPs were used for the propane dehydrogenation and the interface between In 2 O 3 and Pt was responsible for oxidizing surface H atoms to water  [135]. Copyright 2020, Wiley-VCH [142]. This approach minimized coking and improved the yield of propylene up to 30%.
Simultaneously, McFarland et al. compared the catalytic activity and stability of Ru loaded on CeO 2 and ZrO 2 catalysts for the CO 2 partial oxidation of propane [60]. They found that the excellent activity for the C-C cleavage was conducted over Ru even neither catalyst showed measurable activity for the CO 2 -ODHP to propylene. O'Brien et al. enhanced the yield of propylene in the CO 2 -ODHP for twofold over 1 wt% Ru modified the conventional Cr 2 O 3 /SiO 2 catalyst than over the ruthenium-free catalyst [143]. They found that a small amount of Ru promoted the activation of CO 2 and removed the adsorbed H 2 on the catalyst surface via RWGS to shift the equilibrium of reaction to produce propylene, resulting in an increase in the activity without breaking the C-C bonds.
Solymosi and Tolmacsov [141] have proved that the activities of Pt-, Ru-, Rh-, and Pd-based catalysts for the CO 2 reforming of propane followed the order of Rh [ Ru [ Pt [ Pd. Subsequently, they investigated the adsorption, decomposition, and reaction of propane with CO 2 over Rh loaded on various supports (Al 2 O 3 , SiO 2 , TiO 2 , and MgO). The propane could interact with Rh sites to form propylene with selectivity ranges of 50%-60% at 824-923 K [144].
In addition, Pd-based catalysts are also positive for the CO 2 -ODHP, which can promote both the reduction and reoxidation of the support and activate CO 2 [67]. Recently, the dehydrogenation of light alkanes could be conducted via the photocatalysis route under mild conditions (Fig. 15). Liang et al. reported a photocatalytic CO 2 -ODHE over Pd/TiO 2 catalysts at room temperature [145]. CO 2 could be activated over Pd NPs to significantly promote the production of C 2 H 4 and syngas, and a 1 wt% Pd/TiO 2 catalyst exhibited C 2 H 4 and syngas production rates of 230.5 and 282.6 lmolÁg cat -1 Áh -1 , respectively. The intermediate energy level at the 3d orbital of Pd and the electron channel at the Pd-O covalent bonding over the Pd/TiO 2 catalyst facilitated the electron transfer, excitation, and separation, and finally achieved the reaction to form ethylene, CO, and H 2 .
Besides, Solymosi et al. also studied the product distribution of the CO 2 -ODHP over Au deposited on ZnO, MgO, and Al 2 O 3 , respectively [146]. The propane conversion and propylene selectivity over the Au/ZnO were approximately 17% and 56%, respectively. CO 2 had a significant promoting effect for the reaction over the Au/ZnO catalyst due to the electronic interaction between the n-type ZnO and Au particles. Besides the oxidative dehydrogenation and dry reforming reaction, the decomposition of C 3 H 8 and surface carbon deposition could also be observed.
Despite these reported precious metal catalysts being active for the CO 2 -ODHP reaction, their defects of expensive, sensitive, easy to be deposited with carbon and sinter, etc., make them rarely applied in the reaction. Importantly, the high activity of precious metals for the C-H and C-C bonds of propane and C=C bonds of propylene make them exhibit low propylene selectivity in the reaction.

Other catalysts
In addition to the catalytic system mentioned above, there are other catalysts such as Mo-, Zn-, Fe-, and Zr-based catalysts used in the CO 2 -ODHP with differential catalytic performances. Fig. 15 Possible reaction mechanism for photocatalytic ODE with CO 2 over Pd/TiO 2 catalyst. Reproduced with permission from Ref. [145]. Copyright 2018, American Chemical Society Similar to Cr-, V-, and Ce-based catalysts, Mo-based catalysts possess multiple oxidation states and are also potential in the CO 2 -ODHP. For instance, Gaigneaux et al. confirmed that a NiMoO 4 catalyst showed interesting catalytic performances, and CO 2 could be dissociated over the catalyst to form adsorbed oxygen species, which could participate in the reaction to enhance the conversion of propane [147,148]. Generally, the terminal M=O bonds of molybdenum oxide can abstract H atom from the methylene group to form a surface hydroxyl group, and an adsorbed iso-propyl can be eliminated as a hydride to generate propylene and a second hydroxyl group. Meanwhile, the molybdenum oxide species were recovered and the by-product of water was formed [29]. Hossain et al. studied the CO 2 -ODHP reaction catalyzed by MoO 3 / La 2 O 3 -c-Al 2 O 3 catalysts. The incorporation of La 2 O 3 into c-Al 2 O 3 could improve the selectivity to olefins via moderating the surface acidity of the catalyst. Moreover, CO 2 could partially re-oxidize the reduced active sites, resulting in an enhancement of overall performances in the CO 2 -ODHP [149]. In short, the special redox properties of Mobased catalysts are conducive to the proceeding of the CO 2 -ODHP, implying they possess potential application in the reaction. However, there are still more works to explore the catalytic mechanism.
The coke formation on surface of catalyst is the main reason for the catalyst deactivation. As we know, iron species possess inherent anti-coking properties, and thus, they are wildly applied as a promoter to protect the catalyst against carbon deposits. For example, Michorczyk et al. [150] investigated the catalytic performance of Fe-based catalysts (e.g., pure Fe 2 O 3 , Fe 2 O 3 /AC, Fe 2 O 3 /MgO, and Fe 2 O 3 /c-Al 2 O 3 ) in the CO 2 -ODHP. They found that the reduced Fe 2? can be stabilized on alkaline supports of Al 2 O 3 and MgO, which would be re-oxidized by CO 2 . Subsequently, Wang and Tsilomelekis [66] combined Fe with CeO 2 to catalyze the CO 2 -ODHP and found that a high amount of Fe could minimize cracking. All the tested Fe-based catalysts exhibited comparable stability over 20 h continuously. However, the reduction in FeO x nanoparticles/clusters in supported Fe-based catalysts could be carbonated to FeC x in light alkane dehydrogenation, which further catalyzed the coke formation, resulting in deactivation [151]. Thus, the stability of FeO x was the key for the reaction. Xiao et al. introduced Fe into the framework of MFI siliceous zeolite to avoid the deep reduction and carbonization of Fe species, which exhibited extremely high performances and superior coke resistance in a 200-h run [152]. Besides, the modified zinc cations in Zn-modified zeolites were similar to Ga species in Ga-modified zeolites. ZnO dissociated the adsorbed propane to form C 3 H 6 , which was strongly dependent on the relative position of [Zn-C 3 H 7 ] ? and framework attached H ? [153]. Yue et al. reported that a high Si/Al ratio in a ZnO/HZSM-5 catalyst could decrease the acidity of the catalyst to suppress the side reactions, such as cracking, oligomerization and aromatization [154]. The ZnO/HZSM-5(160) (160 is the Si/Al molar ratio) catalyst exhibited the best performances with a propane conversion of 41.5% and a propene yield of 25.8% at a steady state for the CO 2 -ODHP. Afterward, they treated HZSM-5 by steam to decrease the amount of Brønsted acid sites on the ZnO/HZSM-5 catalysts for suppressing coke formation. By this way, the catalyst offered a high propylene yield of 29.7% and possessed superior stability [155].
The catalytic performances of various Cr-free catalysts used in the CO 2 -ODHP reaction are collected in Table 2.
3 Roles of CO 2 in CO 2 -ODHP reaction For a long time, CO 2 has been extensively studied because of its special advantages in the ODHP. Under normal circumstances, for variable-valence metal catalysts such as Cr and V metals, the metal will be reduced to low valences by the reactant molecules, and CO 2 as a weak oxidant can effectively recover these metals. By using CO 2 , the deep oxidation of propane can be avoided, and relatively high propylene selectivity can be ensured. In addition, CO 2 can react with H 2 generated from the DHP reaction to produce CO and H 2 O via the RWGS, promoting the dehydrogenation reaction to proceed and improving the activity of the catalyst. Besides, CO 2 can react with the coke formed in the reaction to produce CO via the reverse Boudouard reaction and further avoid the catalyst deactivation, as well as effectively prolong the lifetime of a catalyst.

Promote redox cycle of redox-active metal oxides
In terms of redox metal catalysts (e.g., Cr-, V-, Ce-, and Mo-based catalysts), CO 2 can be used as a soft oxidant to maintain the high valence state of the active sites at reaction atmosphere to benefit the dehydrogenation of propane to propylene and enhance the stability of catalysts [67,83,156]. It is generally believed that the redox metal catalysts follow the MvK mechanism in the CO 2 -ODHP reaction. The MO x species are simultaneously reduced to MO x-1 by propane to form propylene, and then, the reduced MO  (Fig. 16).
Recently, Liu et al. reported that the DHP and the CO 2 -ODHP coexisted over Cr-based catalysts and further proved that the polymeric Cr 6? oxides were more active but less selective than the isolated Cr 6? oxides for the CO 2 -ODHP, the amount of which was increased with the increasing degree of the polymerization for the polymeric Cr 6? oxides [53]. And the polymeric Cr 6? oxides with a high degree of polymerization were the high propane conversion but the least selective sites for the CO 2 -ODHP. Similar to the Cr-based catalysts, the high valence active sites in other catalysts such as V-, Mo, Ce-based catalysts can be also reduced by propane to form propylene and low valence active sites, which are immediately re-oxidized by CO 2 to form H 2 O and active sites. Especially, in the CO 2 -ODHP reaction catalyzed by a VO x /c-Al 2 O 3 , the whole reaction involved ODH of propane to propylene, RWGS, reverse Boudouard reaction, oxidation reaction, and catalyst re-oxidation with CO 2 , respectively. CO 2 could also reoxidize the VO x-1 to VO x [87].

Promote RWGS reaction
Even the RWGS reaction is thermodynamically limited, it is a way to proceed under the conditions of the PDH (500-600°C), which can shift the equilibrium to produce more C 3 H 6 [161][162][163]. Cao et al. demonstrated that the promotion of CO 2 on the yield of propylene was attributed to the promotion of simple dehydrogenation by coupling with the In-based catalysts for RWGS reaction [132,133]. Through continuous H 2 -CO 2 mixed gas pulse experiments, Liu et al. proved that the in situ generated H 2 in the reaction was also consumed by the RWGS reaction on the GaN-based catalyst, thereby improving the activity of the catalyst [47,126]. Galvita et al. carried out the reaction in the feed of D 2 /C 3 H 8 /CO 2 with a ratio of 1:1:1, only 45% of the resulting water-contained D 2 O, confirming that the reaction followed both of the CO 2 oxidative dehydrogenation route and the RWGS in combination with the nonoxidative dehydrogenation route [83]. This broke the previous understanding that the two reaction pathways were mutually exclusive and provided a new idea for a clearer understanding of the reaction mechanism of the CO 2 -ODHP.

Promote reverse Boudouard reaction
The coke deposition is the main reason for the deactivation of catalysts, which is composed of graphitized and amorphous carbons resulting from cracking, oligomerization, and aromatization of propylene [164]. Liu et al. compared the amount of coke deposition over a 5 wt% GaN/Q-3 catalyst after use in the dehydrogenation of propane with or without CO 2 [47]. They found that the catalyst possessed more amount of coke deposition in the absence of CO 2 , indicating that CO 2 could effectively restrict the carbon deposition. In addition, Raman spectroscopy analysis revealed that a higher proportion of graphitic carbon was observed for the 5 wt% GaN/Q-3 catalyst used in the CO 2 -ODHP, indicating that CO 2 could consume the amorphous carbon on surface of catalyst via reverse Boudouard reaction, resulting in the corresponding deposited coke with a higher graphitization degree.

Summary and outlook
In this review, the opportunities, catalyst development, mechanism, and role of CO 2 in the CO 2 -ODHP have been summarized. CO 2 -ODHP, as a potential alternative route to the DHP, not only has the potential to remedy the demand gap of propylene, but also can reduce the CO 2 concentration in the environment. CO 2 used as a soft oxidant can Fig. 16 Proposed mechanism of ODHP reaction over a 0.5 wt% Ni-Cr/Si catalyst. Reproduced with permission from Ref. [160]. Copyright 2012, Wiley-VCH avoid the over-oxidation of propane and propylene in the O 2 -ODHP. It has at least the following three functions: (1) CO 2 can maintain the high oxidation state of the catalytic active sites of some redox catalysts; (2) CO 2 can remove the in situ produced H 2 to value-added CO via the RWGS reaction; (3) CO 2 can also delay the formation of coke over the catalyst surface via the reverse Boudouard reaction to prolong the lifetime of a catalyst. To date, there are several catalytic systems such as redox-type metal oxide catalysts (Ce and V), main group metal oxide catalysts (Ga and In), and precious metal (Pt, Rh, Ru, and Pd) in the CO 2 -ODHP, igniting the hope of commercialization of this process. Despite these achievements, there are still lots of works to be done before the industrial application of the CO 2 -ODHP.
In terms of catalyst design, an efficient catalyst should possess the abilities of selective activation of C-H bond of propane and prohibition of C-C cleavage. It is also efficient for activation of C=O bonds of CO 2 to improve the yield of propylene and the efficiency of CO 2 utilization. One possible way is to construct some bifunctional catalytic systems, namely the catalytic active center is responsible for dehydrogenation and the support is responsible for activation of C=O bonds of CO 2 . Thus, it is critical to accurately identify the active sites and in-depth understand the interaction between the metal and the support. Besides, the competitive adsorptions among C 3 H 8 , C 3 H 6 and CO 2 cannot be ignored, which have influences on the propane conversion, propylene yield, and CO 2 utilization. That may be controlled via adjusting the texture structure and acid-base properties of catalyst.
Besides, the accurate establishment of the reaction mechanism for the CO 2 -ODHP can help to understand the reaction path, which in turn guides the design of catalyst. Thus, it is critical to capture the intermediates in the reaction. Fortunately, it has greatly facilitated the capture and analysis of reaction intermediates with the development of various in situ characterization techniques and theoretical calculations in recent years.
Then, the catalyst deactivation, resulting from coke deposition and sintering of active components on catalyst surface, seriously troubles the industrial application of catalysts. That may be avoided via optimizing reaction conditions such as temperature, partial pressure, and the type of reactor. In addition, it is also an effective method to introduce promoters into catalysts.