Dry Reforming of Ethanol and Glycerol: Mini-Review

Abstract

Dry reforming of ethanol and glycerol using CO₂ are promising technologies for H₂ production while mitigating CO₂ emission. Current studies mainly focused on steam reforming technology, while dry reforming has been typically less studied. Nevertheless, the urgent problem of CO₂ emissions directly linked to global warming has sparked a renewed interest on the catalysis community to pursue dry reforming routes. Indeed, dry reforming represents a straightforward route to utilize CO₂ while producing added value products such as syngas or hydrogen. In the absence of catalysts, the direct decomposition for H₂ production is less efficient. In this mini-review, ethanol and glycerol dry reforming processes have been discussed including their mechanistic aspects and strategies for catalysts successful design. The effect of support and promoters is addressed for better elucidating the catalytic mechanism of dry reforming of ethanol and glycerol. Activity and stability of state-of-the-art catalysts are comprehensively discussed in this review along with challenges and future opportunities to further develop the dry reforming routes as viable CO₂ utilization alternatives.

1. Introduction

Carbon dioxide is one of the key greenhouse gases causing the observed global warming. Therefore, more attention has been given to the utilization of renewable energy and CO₂ recycling technologies. It is mainly from the combustion of fossil fuels since the Industrial Revolution age. In 1750, the CO₂ concentration in the atmosphere was 277 ppm according to Joos et al. and reached concentrations as high as 406.7 ppm in July 2017 [1]. However, CO₂ emissions have been found to be constant for the last three years as shown Figure 1a according to the International Energy Agency, proving the effect of the global effort regarding environmental policies.

Carbon capture and storage is regarded as a main method to limit CO₂ release in the atmosphere; nevertheless, this technology has not demonstrated commercial viability so far. A more challenging alternative from the chemical and engineering perspective is the carbon capture and recycling (CCR) approach, where CO₂ is converted and recycled back to fuels and added value chemicals.

CO₂ can react with methane following the reaction of dry reforming of methane reaction (DRM) forming syngas. Syngas contained a mixture of carbon monoxide and hydrogen, which is a crucial raw compound in the chemical industry. Applications of synthesis gas are summarized in Figure 2. It can be used as base material for methanol synthesis (Equation (1)) that can later be upgraded to higher commercial value chemicals such as dimethyl ether (DME), formaldehyde or acetic acid. When introduced into a Fisher–Tropsch unit, syngas can produce fuels or waxes (Equation (2)). Finally, it can be rearranged into methane (Equation (3)) or used as a hydrogen source:

CO + 2H₂ ⇌ CH₃OH,

(1)

nCO + (2n + 1) H₂ ⇌ C_nH_2n+2 + H₂O,

(2)

CO + 3H₂ ⇌ CH₄ + H₂O.

(3)

Bio-ethanol, as a renewable biofuel, has been proposed as a substitute for fossil fuels. First generation of bio-ethanol was produced from food and was also heavily criticized since it opened the dilemma “food or fuel”. Currently, the second generation of bio-ethanol is mainly from lignocellulosic biomass residues and organic municipal solid waste with more complicated methods in order to avoid the consumption of food as shown in Figure 3. Therefore, bio-ethanol can be regarded as nearly CO₂ neutral. In 2018, the total production of bio-ethanol reached 28,700 million gallons and 83.7% is from USA and Brazil [3].

Biodiesel, the second most used biofuels after bio-ethanol, also attracted a wide amount of attention, which is mainly from the transesterification reaction of triglycerides. During the process, 1 kg of glycerol, the main by-product, was produced for 9 kg of biodiesel [4]. It was estimated that the production of glycerol each year can reach 4.2 million tons up to 2020 globally, while less than 3.5 million tons is needed each year [5]. Therefore, it is necessary to explore new market opportunities for glycerol.

A number of processes, such as pyrolysis and gasification with and without catalysts, have been developed to utilize bio-ethanol and glycerol [6,7,8,9]. To effectively close the carbon cycle, dry reforming of bio-ethanol and glycerol to produce syngas or H₂, which can be used directly or converted into high value chemical compounds were proposed [10,11,12,13]. Dry reforming normally occurs at relatively high temperatures (900–1200 K) in the existence of CO₂. The process involved several parallel reactions, including direct cracking, water gas shift reaction (WGSR), and methanation reactions, which may affect the final products. Therefore, extensive studies have been conducted to understand the dry reforming pathways of bio-ethanol and glycerol and try to maximize the production of H₂ [14,15]. Various catalysts, such as Ni, Rh, and Cu-based, were also proposed to optimize the reaction pathways to avoid the formation of undesired by-products and hinder the formation of carbon on the catalyst.

2. Dry Reforming of Ethanol

The simulation of dry reforming of ethanol by thermodynamic analysis has been studied to determine optimum conditions for H₂ production [16,17,18]. Figure 4 showed the variation of gas composition with temperature. For the studied temperature ranges, H₂ and CO increase linearly with temperature from 823 to 1123 K reflecting the endothermic character of the reforming reaction. According to Equation (4), an equal amount of H₂ and CO can be formed from the ethanol dry reforming. However, the thermodynamic analysis predicted that ethanol can be completely converted to H₂, CO, CH₄, CO₂, H₂O, and C. In addition, CH₃CHO may be formed by a dehydrogenation reaction following Equation (5). The formed CH₃CHO can be further decomposed into CH₄ and CO (Equation (6)) or reformed into H₂ and CO (Equation (7)). The CH₃CHO can also be dehydrated into C₂H₄ (Equation (8)) or ether Equation (9). Carbon deposition was predicted to happen especially in the low temperature window with a slight decrease of solid carbon formation when temperature is increased. Above 1100 K, ethanol was completely transformed into H₂, CO, and C. According to Equation (10), ethylene polymerization can partially form the carbon. The ethanol can also decompose into CO, H₂, CH₄, and acetone directly (Equations (11)–(14)). The formed CO and H₂ can be transformed into methane according to Equations (15) and (16). The DRM is strongly endothermic and can produce hydrogen and carbon monoxide (Equation (17)). At lower temperatures, WGSR, an exothermic and reversible reaction, can favor the production of hydrogen and carbon oxide (Equation (18)). The carbon residues can also be formed following decomposition of methane, Boudouard reaction, and reduction of CO (Equations (19)–(22)). According to the thermodynamics, a two-step reaction mechanism was then proposed for the dry reforming of ethanol. Initially, ethanol was directly decomposed into methane and acetaldehyde intermediates, which can be subsequently reformed to syngas [19,20]. Moreover, H₂O was predicted due to the reversed WGSR. Therefore, it is evident that ethanol reforming by CO₂ forming dominated for the whole temperatures ranges and other side reactions may also occur such as direct decomposition into methane, H₂/CO/C mixtures and C₂H₄O, methane reforming, and reverse WGSR:

$${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}+{\mathrm{CO}}_2\to 3{\mathrm{H}}_2+3\mathrm{CO}\Delta H_{298}^{\mathrm{o}}=296.7{\mathrm{KJ}\mathrm{mol}}^{-1},$$

(4)

$${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}\to {\mathrm{CH}}_3\mathrm{CHO}+{\mathrm{H}}_2\Delta H_{298}^{\mathrm{o}}=68.4{\mathrm{KJ}\mathrm{mol}}^{-1},$$

(5)

$${\mathrm{CH}}_3\mathrm{CHO}\to {\mathrm{CH}}_4+\mathrm{CO}\Delta H_{298}^{\mathrm{o}}=-18.785{\mathrm{KJ}\mathrm{mol}}^{-1},$$

(6)

$${\mathrm{CH}}_3\mathrm{CHO}+{\mathrm{CO}}_2\to 3\mathrm{CO}+2{\mathrm{H}}_2 \Delta H_{298}^{\mathrm{o}}=228.2{\mathrm{KJ}\mathrm{mol}}^{-1},$$

(7)

$${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}\to {\mathrm{H}}_2\mathrm{O}+{\mathrm{C}}_2{\mathrm{H}}_4 \Delta H_{298}^{\mathrm{o}}=45.4{\mathrm{KJ}\mathrm{mol}}^{-1},$$

(8)

$${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}\to 1/2{\mathrm{H}}_2\mathrm{O}+1/2{\mathrm{C}}_2{\mathrm{H}}_5{\mathrm{OC}}_2{\mathrm{H}}_5 \Delta H_{298}^{\mathrm{o}}=-12.0{\mathrm{KJ}\mathrm{mol}}^{-1},$$

(9)

$${\mathrm{C}}_2{\mathrm{H}}_4\to \mathrm{polymer}\to 2\mathrm{C}+2{\mathrm{H}}_2,$$

(10)

$${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}\to 3{\mathrm{H}}_2+\mathrm{CO}+\mathrm{C}\Delta H_{298}^{\mathrm{o}}=124.3{\mathrm{KJ}\mathrm{mol}}^{-1},$$

(11)

$${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}\to {\mathrm{CH}}_4+{\mathrm{H}}_2+\mathrm{CO}\Delta H_{298}^{\mathrm{o}}=49.7{\mathrm{KJ}\mathrm{mol}}^{-1},$$

(12)

$${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}\to 3/2{\mathrm{CH}}_4+1/2{\mathrm{CO}}_2 \Delta H_{298}^{\mathrm{o}}=-73.9{\mathrm{KJ}\mathrm{mol}}^{-1},$$

(13)

$${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}\to 0.6{\mathrm{CH}}_3{\mathrm{COCH}}_3+1.2{\mathrm{H}}_2+0.2{\mathrm{CO}}_2 \Delta H_{298}^{\mathrm{o}}=1.2{\mathrm{KJ}\mathrm{mol}}^{-1},$$

(14)

$$2{\mathrm{H}}_2+2\mathrm{CO}\to {\mathrm{CH}}_4+{\mathrm{CO}}_2\Delta H_{298}^{\mathrm{o}}=-247.0{\mathrm{KJ}\mathrm{mol}}^{-1},$$

(15)

$$4{\mathrm{H}}_2+{\mathrm{CO}}_2\to {\mathrm{CH}}_4+2{\mathrm{H}}_2\mathrm{O}\Delta H_{298}^{\mathrm{o}}=-146.7{\mathrm{KJ}\mathrm{mol}}^{-1},$$

(16)

$${\mathrm{CH}}_4+{\mathrm{CO}}_2\to 2{\mathrm{H}}_2+2\mathrm{CO}\Delta H_{298}^{\mathrm{o}}=247.0{\mathrm{KJ}\mathrm{mol}}^{-1},$$

(17)

$${\mathrm{H}}_2\mathrm{O}+\mathrm{CO}\to {\mathrm{H}}_2+{\mathrm{CO}}_2\Delta H_{298}^{\mathrm{o}}=-41.1{\mathrm{KJ}\mathrm{mol}}^{-1},$$

(18)

$${\mathrm{CH}}_4\to \mathrm{C}+2{\mathrm{H}}_2\Delta H_{298}^{\mathrm{o}}=74.6{\mathrm{KJ}\mathrm{mol}}^{-1},$$

(19)

$$\mathrm{CO}\to 1/2\mathrm{C}+1/2{\mathrm{CO}}_2\Delta H_{298}^{\mathrm{o}}=-86.2{\mathrm{KJ}\mathrm{mol}}^{-1},$$

(20)

$${\mathrm{H}}_2+\mathrm{CO}\to \mathrm{C}+{\mathrm{H}}_2\mathrm{O}\Delta H_{298}^{\mathrm{o}}=-131.3{\mathrm{KJ}\mathrm{mol}}^{-1},$$

(21)

$$2{\mathrm{H}}_2+{\mathrm{CO}}_2\to \mathrm{C}+2{\mathrm{H}}_2\mathrm{O}\Delta H_{298}^{\mathrm{o}}=-90.1{\mathrm{KJ}\mathrm{mol}}^{-1}.$$

(22)

Thermodynamic analysis showed that the optimal conditions for dry reforming of ethanol are: reaction temperatures between 1200–1300 K, CO₂/ethanol ratios of 1.2–1.3. At these conditions, ~45% H₂, ~50% CO, and complete conversion of ethanol can be realized without remarkable solid carbon formation [21]. Ethanol conversion increased with the CO₂/ethanol ratio increasing, but kept almost constant at higher CO₂/ethanol ratios. A recent experimental study showed that ethanol conversion stabilized from CO₂/ethanol ratio of 1.4 [22]. Moreover, the H₂ in syngas also decreased at CO₂/ethanol higher than 1.4, while CO increased. This trend showcases the direct impact of the reversed WGSR during the dry reforming processes as reported elsewhere [23,24,25].

In order to avoid the carbon formation and undesired gas components, different catalysts were synthesized and utilized for the dry reforming of ethanol, such as Ni, Cu, Rh, and Co based catalysts. The Ni based catalyst is the most studied one. Al supported nano-NiO-SiO₂ catalysts with Ni amount of 5, 7, 10, and 15% have been synthesized and used for the dry reforming of ethanol [26]. Catalysts containing 10% Ni showed the best activity partially due to the clustering of Ni particles at higher Ni amounts. Less CO and higher CH₄ were also observed due to higher rate of WGSR and direct decomposition of ethanol into CH₄ promoted by the catalyst, respectively. The desired condition for dry reforming of ethanol was found at 1023 K with the ratio of CO₂/Ethanol being 1.4. At this condition, the conversion of CO₂ was observed to be 75%, while 100% conversion of ethanol was nearly obtained. Wei et al. used the Ni/KIT-6 (mesoporous silica) and Ni/SiO₂ for the dry reforming of ethanol [27]. A comparatively better activity and stability was shown for Ni/KIT-6 catalyst; however, Ni/SiO₂ was deactivated quickly. Complete ethanol conversion was obtained at 823 K for Ni/KIT-6 without observing clear deactivation of the catalyst after 40 h tests. Smaller particle size of Ni was observed for Ni/KIT-6, which can be stabilized by the confinement effect of catalyst channels.

The smaller particle size of Ni markedly hindered the carbon formation. The stronger interaction of metal-support also inhibited the sintering of active metals. Therefore, the catalyst of Ni/KIT-6 greatly inhibited the coke deposition and contributed to the superior activity. Catalysts of Ni/Y₂O₃–ZrO₂ using two different preparation methods of polymerization and wet impregnation were proposed for the dry reforming of ethanol at 600, 700 and 800 °C [10]. At 600 °C, the highest CH₄ was obtained using a 5%Ni/Y₂O₃–ZrO₂ catalyst, due to the direct cracking of ethanol into CH₄ and slow dry reforming rate of CH₄ at 600 °C. Increasing temperature to 700 °C accelerated the dry reforming rate of CH₄, which was then enhanced further at 800 °C. It was showed that stronger interaction between NiO and oxygen vacancies of support was responsible for its superior reforming ability. The preparation of Ni/Y₂O₃–ZrO₂ catalyst by polymerization in one step was proven as an easier and superior way of loading Ni onto Y₂O₃–ZrO₂ support than by wet impregnation. In order to prevent the deactivation of Ni catalysts, Ni has also been incorporated into different supports of Al₂O₃, CeO₂, MgO and ZrO₂ to study their activity in the dry reforming of ethanol [28]. Ni-CeO₂ catalyst showed the best performance at 1023 K than at 973 K, suggesting that CO₂ conversion can be enhanced at higher temperatures, while carbon deposition was hindered. It was found that the reducibility of Ni, which was influenced by the interaction between support and Ni, along with the redox property of CeO₂ can promote the selectivity for H₂ and inhibit the unfavored side reactions. The results revealed that specific surface area of supports can affect the particle size and reducibility of active metals. It can be observed that interaction between Ni and the supports affected the particle size strongly, reducibility of Ni. Oxygen vacancy of support also can inhibit the formation of carbon.

To optimize the performance of Ni based catalysts, promoters such as La and Ce were also incorporated [11,19,29]. The advanced redox properties of Ce resulted in great benefits for a Ce-Ni/Al₂O₃ tested in the dry reforming of ethanol. Compared with unpromoted ones, Ce-promoted catalyst can hinder the formation of solid coke due to its oxygen mobility ascribed to CeO₂ as a promoter. The proportion of amorphous carbon as opposed to graphitic carbon was also observed with Ce-Ni/Al₂O₃. Upon incorporation of Ce, the interaction between Ni and the supports was strengthened, which can promote the formation of smaller NiO clusters facilitating the reducibility of Ni. The incorporation of Ce lowered reduction temperatures by around 315 K. NiO and NiAl₂O₄ were observed on the surface of the supports during calcination while CeO₂ was observed on the promoted one. The derivative weight of calcination and H₂-temperature program reduction (H₂-TPR) of two catalysts was shown in Figure 5, respectively. As shown in Figure 5a, the first peak, P1 (at 501–509 K), was caused by the direct decomposition of metal nitrates forming the corresponding metal oxides (Equations (23) and (24)). The second peak (P2) (591–598 K) was ascribed to NiAl₂O₄ formation (Equation (25)). The third peaks (P3) of Ce-promoted catalyst (661 K) was due to the transformation of Ce³⁺ to Ce⁴⁺ during calcination (Equation (26)). In Figure 5b, two distinct peaks were observed for the H₂-TPR analysis of catalysts. The P1 (557–599 K) was due to the formation of metallic Ni from the reduction of NiO (Equation (27)), while P2 peak (668–671 K) was due to the transformation of NiAl₂O₄ to Ni (Equation (28)):

$$\mathrm{Ni}{\left({\mathrm{NO}}_3\right)}_2\to \mathrm{NiO}+{\mathrm{N}}_2{\mathrm{O}}_5,$$

(23)

$$2\mathrm{Ce}{\left({\mathrm{NO}}_3\right)}_3\to 3{\mathrm{N}}_2{\mathrm{O}}_5+{\mathrm{Ce}}_2{\mathrm{O}}_3,$$

(24)

$$\mathrm{NiO}+{\mathrm{Al}}_2{\mathrm{O}}_3\to {\mathrm{NiAl}}_2{\mathrm{O}}_4,$$

(25)

$${\mathrm{Ce}}_2{\mathrm{O}}_3+0.5{\mathrm{O}}_2\to 2{\mathrm{CeO}}_2,$$

(26)

$$\mathrm{NiO}+{\mathrm{H}}_2\to \mathrm{Ni}+{\mathrm{H}}_2\mathrm{O},$$

(27)

$${\mathrm{NiAl}}_2{\mathrm{O}}_4+{\mathrm{H}}_2\to \mathrm{Ni}+{\mathrm{Al}}_2{\mathrm{O}}_3+{\mathrm{H}}_2\mathrm{O}.$$

(28)

Similarly, La has also been incorporated into Ni/Al₂O₃ [11,20]. La can enhance the dispersion of nickel particle and hinder the agglomeration of active phases. The best performance of dry reforming of ethanol was observed at 3% La loading due to the benefits of La₂O₃ as promoter. The H₂-TPR analysis of three catalysts were shown in Figure 6. Different from Ce promoted Ni/Al₂O₃, three peaks were observed for 3%La-10%Ni/Al₂O₃. The first one, P1, was due to the reduction of NiO to metallic Ni form. The second peak P2 was due to the reduction of NiO particles strongly absorbed by supports. The third peak P3 was due to the reduction of spinel NiLa₂O₄ phase to elementary Ni. For 10%Ni/Al₂O₃ and 3%La-10%Ni/Al₂O₃, the temperature for NiO reduction (P1 and P2) decreased to a lower value after La loading, which may be ascribed to the transferring electrons from La₂O₃ to NiO particles. As a result, the enhanced electron density facilitated reduction of NiO. It was also found that dry reforming of ethanol may occur through direct decomposition of ethanol to methane intermediates. The formed intermediated were then reformed to a mixture of CO and H₂ following DRM. Similar to Ce promoted catalyst, amorphous carbon dominated on the surface of used La-Ni/Al₂O₃ catalyst. This is may be due to the enhanced basicity of the promoted catalysts and the multiple oxidation states of La, which can provide a shortcut of coke conversion by redox reactions (Equations (29) and (30)).

$$L{a}_{\alpha }{O}_{\beta }+C_x{\mathrm{H}}_{1-x}\to L{a}_{\alpha }{O}_{\beta -x}+\mathrm{xCO}+\left(1-\mathrm{x}\right)/2{\mathrm{H}}_2,$$

(29)

$$L{a}_{\alpha }{O}_{\beta -x}+{\mathrm{CO}}_2\to L{a}_{\alpha }{O}_{\beta }+\mathrm{xCO}.$$

(30)

Co based catalysts promoted by Ce and La have also been proposed for the dry reforming of ethanol [30,31,32]. The addition of Ce and La facilitated the reduction of Co₃O₄ to CoO, which can be further transformed into Co. The reducibility of active metals in catalysts were analyzed using H₂-TPR. The reduction ability followed the order of 10%Co/Al₂O₃ < 3%Ce-10%Co/Al₂O₃ < 3%La-10%Co/Al₂O₃. Three peaks were observed. The first peak for all catalysts was due to the reduction of Co₃O₄ to CoO, while the second peak was ascribed to the formation of Co from CoO reduction. The third small shoulder was most likely ascribed to the reduction of pinel CoAl₂O₄ species strongly absorbed by support. Consistent with reducibility of catalysts, the dry reforming activity of ethanol followed the order of 3%La-10%Co/Al₂O₃ > 10%Co/Al₂O₃ > 3%Ce-10%Co/Al₂O₃. The relationship indicated that the La and Ce can promote the amount of Co active sites by accelerating the reduction of Co₃O₄ demonstrating a clear correlation redox properties—catalytic performance.

The dry reforming of ethanol was proposed using Cu-based catalysts [33,34,35,36]. A catalyst of Cu/Ce_0.8Zr_0.2O₂ was prepared using the co-precipitation method. The synergistic interaction between metal and support along with the redox characteristic of Ce caused a promising activity. A new preparation method of the ball-milling technique was then proposed by the same group for dry reforming of ethanol. The catalyst synthesized by the ball-milling technique showed a better performance than the co-precipitation method, which can be ascribed to the better dispersion of Cu and optimized interaction between metal and support. Moreover, no encapsulated graphite-type carbon was found for this catalyst. In a further study, Cu-based catalysts on various supports (CeO₂, ZrO₂, CeO₂-ZrO₂) were synthesized. As revealed, the reducibility of Cu and oxygen vacancies decreased in the order of CuCeZr > CuCe > Cu/Zr. As expected, the CuCeZr catalyst showed the best performance due to higher metal dispersion, stronger interaction between metal and support, and greater concentration oxygen vacancies that are envisaged as reactive sites for molecules’ activation. To prove the assumption, all catalysts were exposed to H₂-TPR analysis as shown in Figure 7. For the supports, the incorporation of Zr⁴⁺ accelerated mobility of O and increased reducible Ce⁴⁺ cations. For Cu-based catalysts, the reduction started at about 100 °C with the highest peaks at 142 °C, 154 °C and 162 °C for CuCe, CuCeZr and CuZr, respectively. This is due to the stronger interaction between Cu and CeZr supports that can accelerate oxygen mobility. As a result, catalyst reduction temperature can be reduced.

Due to the high activity for C–C bond cleavage and low coke formation, Rh has been extensively used for dry reforming of methane [37]. To validate its applicability on the dry reforming of ethanol by Rh based catalysts, various Rh based catalysts have been prepared. Figure 8 shows that ethanol decomposition may occur by two major routes [38]: (1) forming an oxametallacycle intermediate on Rh(1,1,1), which was directly decomposed to methane and CO; and (2) dehydrogenated intermediates (C₂H₄O and acetyl species), which can be further decomposed to CO and CHx species.

Due to the proven redox property of CeO₂, Rh/CeO₂ has been synthesized for the dry reforming of ethanol [14,39]. A relatively good activity and stability were observed for this catalyst due to the interaction between Rh and CeO₂ and again the advanced redox features of CeO₂, which can accelerate CO₂ activation and prevent carbon deposition. The mechanism was illustrated in Figure 9.

In further research, Rh/Ce-SBA-15 has been synthesized for the dry reforming of ethanol [40]. The proportion of Ce relative to Si affected markedly the H₂ production and selectivity. The introduction of Ce into the 1%Rh/SBA-15 increased reforming activity due to the strengthened mobility of oxygen species on the surface of catalysts. Rh-based catalysts were also prepared for the dry reforming of ethanol using MxOy–Al₂O₃ oxides (M = Zr, Mg, Ni, Ce, La) as supports [41]. As shown in Figure 10, the production of H₂ followed the order: Rh/NiO–Al₂O₃ >> Rh/Al₂O₃ ≈ Rh/MgO–Al₂O₃ ≈ Rh/CeO₂–Al₂O₃ > Rh/ZrO₂–Al₂O₃ ≈ Rh/La₂O₃–Al₂O₃. The superior performance of Rh/NiO–Al₂O₃ may be due to the existence of NiAl₂O₄ spinel phase, hindering the rhodium deactivation, and the high dispersion of Rh favored by co-existence of nickel particles.

The 316 steel (Fe: 61%; Cr: 16–18.5%; Ni: 10–14%; Mo: 2–3%; C: 0.03%) and carbon steel also proved the dry reforming activity of ethanol [42,43,44]. Compared with the conditions without catalyst, steel can promote the catalytic cracking and catalytic reforming of ethanol forming H₂ and CO. In this process, carbon nanofilament was formed as the desired product. It was proposed that nanoparticles of magnetite can be formed from the oxidation treatment prior to dry reforming reaction, which can be reduced into Fe₃C during the reforming process. The formed Fe₃C is responsible for the formation of H₂, CO, and nanofilament. Hence, a take-home message is to check the potential catalytic activity of the reactor itself and always run a blank test to rule out contributions beyond our catalytic materials.

In this part, various catalysts used for dry reforming of ethanol have been summarized. The most used catalyst is Ni based one due to its low cost even though other catalysts of Co, Cu, and Rh based have also been proposed. The most common problem is the deactivation of catalyst caused by coke formation. In order to minimize this effect, promoters of Ce and La were introduced, which showed a promising effect due to the redox properties of Ce and enhanced dispersion of Ni by Ce and La. Different catalyst supports were also discussed. The effect of support on performance is attributed to the interaction between support and active metals. Strong interaction between NiO and oxygen vacancies of catalyst can enhance the catalyst performance. In the ideal conditions, complete conversion of ethanol can be achieved with a theoretical amount of H₂ obtained. Therefore, dry reforming of ethanol is a promising technology to produce syngas.

3. Dry Reforming of Glycerol

The dry reforming of glycerol is expected to follow a similar pattern to that of ethanol. Theoretically, 3 mol of H₂ and 4 mol of CO can be extracted from 1 mol of dry reforming of glycerol according to Equation (31). However, direct decomposition of glycerol may also occur following the overall reaction as shown in Equation (32) along with methanation and (reverse) WGSRs. Thermodynamic analysis has been used to study the effect temperature, pressure, and CO₂/glycerol ratios on glycerol conversion [45,46]. It showed that atmospheric pressure is preferred for the glycerol conversion [47]:

$${\mathrm{C}}_3{\mathrm{H}}_8{\mathrm{O}}_3+{\mathrm{CO}}_2\to 3{\mathrm{H}}_2+4\mathrm{CO}+{\mathrm{H}}_2\mathrm{O}\Delta H_{298}^{\mathrm{o}}=292.3{\mathrm{KJ}\mathrm{mol}}^{-1},$$

(31)

$${\mathrm{C}}_3{\mathrm{H}}_8{\mathrm{O}}_3\to 4{\mathrm{H}}_2+3\mathrm{CO}\Delta H_{298}^{\mathrm{o}}=251.2{\mathrm{KJ}\mathrm{mol}}^{-1}.$$

(32)

As shown in Figure 11, temperature showed a positive effect on H₂ production, which reached a maximum conversion at CO₂/glycerol ratios higher than 1 due to the promoted reverse WGSR at higher CO₂ pressure. At a CO₂/glycerol ratio between 0 and 1 over 975 K, more than three moles of H₂ can be obtained, while more syngas (H₂ and CO) can be produced at higher temperatures and CO₂/glycerol ratios. The optimum condition for syngas production occurred at 1000 K with CO₂/glycerol ratio of 1, at which 6.4 mol of syngas (H₂/CO = 1:1) can be obtained for each mole of glycerol.

In the absence of catalysts, the glycerol conversion is low. To optimize H₂ production and reaction conditions, various catalysts have been synthesized for the dry reforming of glycerol. Mesoporous catalysts of Ni/γ-Al₂O₃ with different amounts of Ni were prepared for the dry reforming of glycerol [12]. With Ni amount increasing from 5 to 15%, the glycerol conversion increased, while it decreased at higher Ni content. The H₂-TPR analysis showed that the higher Ni amount promoted the reducibility of the Ni catalyst, which can be related to lower dispersion of Ni at higher content of Ni. Other catalysts, including Rh, Ni, and Co based catalysts (Al₂O₃-ZrO₂-TiO₂ support) have been synthesized for the dry reforming of glycerol [4], with the particle size of Rh, Ni and Co being 3–4, 10–15, 40–50 nm, respectively. The highest activity was observed for Rh, while the activity of Co was the lowest. Even though the activity of Ni based catalyst was below that of Rh based catalyst, it can still render 95% of the thermodynamic CO₂ conversion at maximum. The high performance of Ni based catalyst may be due to its ability of gasifying deposited carbon and metallic nature during the reaction. Comparatively, Co based catalyst showed an inferior performance to Rh and Ni due to the sintering, carbon deposition and Co oxidation. To understand the mechanism, all catalysts were exposed to H₂-TPR analysis as shown in Figure 12. Al₂O₃-ZrO₂-TiO₂ support showed a broad H₂ consumption in the high temperature region (i.e., 893–953 K), which is likely due to reduction of the Al₂O₃-ZrO₂-TiO₂ support. In addition to this high temperature peak, Rh/Al₂O₃-ZrO₂-TiO₂ revealed two more low-temperature H₂ consumption features at 160 and 200 °C, which can be attributed to the reduction of Rh³⁺ to Rh⁺ and Rh⁺ to metallic Rh species, respectively. Co/Al₂O₃-ZrO₂-TiO₂ also revealed two low temperature features with relatively lower H₂ consumption around 220–300 °C which can be ascribed to two-step reduction of Co₃O₄ to CoO and CoO to metallic Co. In addition, there exists a reduction feature at 420 °C, which can be ascribed to the reduction of CoO_x species that are strongly interacting with the Al₂O₃-ZrO₂-TiO₂ support material. Ni/Al₂O₃-ZrO₂-TiO₂ exhibited a similar H₂ consumption profile to that of Al₂O₃-ZrO₂-TiO₂ support and lacks a low temperature reduction peak (e.g., for Ni⁺² → Ni⁰), indicating the presence of predominantly metallic Ni species on fresh samples with a negligible contribution from NiO and Ni (OH)₂. For all catalysts, a negative effect of CO₂/glycerol ratios between 1 and 4 on glycerol conversion was observed mainly due to the reverse WGSR. For CO₂ conversion, the catalysts showed an increase in activity for CO₂/glycerol ratio increasing from 1 to 2. However, CO₂ conversions decreased at higher ratios. This phenomenon has been observed by other researchers [13,48].

A similar study on dry reforming of glycerol by La-Ni/Al₂O₃ showed that glycerol conversion and H₂ generation rate reached a peak at CO₂/glycerol ratio of 1.67, at which CH₄ reached the minimum. The existence of CH₄ in the presence of glycerol only, along with the declining trend at higher CO₂/glycerol (>1.67), indicated that CH₄ may be formed from direct decomposition of glycerol, which can be then reformed by CO₂ to CO and H₂. Regarding the H₂/CO ratio, the direct decomposition of glycerol produced the highest value, but still below 2, which can be used in the Fischer–Tropsch process in some cases depending on the targeted end products. The declining trend with CO₂/glycerol ratio increasing may be caused by the reverse WGSR. The noble metals (Rh, Ru, Ir, Pd, Pt) were also supported on MgAl₂O₄ and compared for dry reforming of glycerol [49]. Similarly, Rh showed highest activity and stability, which may be ascribed to high active metal areas. The catalytic activity followed the order: Rh > Ru > Ir > Pd > Pt. Cement clinker-supported Ni catalysts were also synthesized for the dry reforming of glycerol [50,51]. It was found that direct decomposition of glycerol was responsible for the H₂ production as opposed to a direct interaction between glycerol and CO₂ during reforming reaction. Therefore, even though the cement clinker can facilitate the adsorption of CO₂, it did not favor the production of H₂.

To optimize the behavior of Ni based catalyst, La has been incorporated as a promoter for the dry reforming of glycerol [13,48,52,53]. After evaluating the effect of different content of La loaded, it was found that the glycerol conversion decreased in the order of 2%La-Ni/Al₂O₃ > 3%La-Ni/Al₂O₃ > 4%La-Ni/Al₂O₃ > 1%La-Al₂O₃ > Ni/Al₂O₃ > 5%La-Ni/Al₂O₃ > Al₂O₃. The main reason is that Ni metal can be better dispersed by La. However, when La content was above 2%, the clogging of pores may occur even though there is reduced particle size of Ni metals. Moreover, compared with the non-promoted catalyst, the La promoted catalysts showed a better carbon resistance due to its redox property to remove carbon and less acidity of Ni catalyst decreased by La. The promoters Re and Ag were also incorporated to optimize the performance of Ni based catalysts [54,55]. Similar to La, Ag can also improve the dispersion of Ni metals. However, at a high content of 5% Ag in catalyst, some active metal surface may be covered by Ag, which can decrease the catalyst performance. Differently, the Re can increase the glycerol conversion by enhancing the surface adsorption of OH group of the glycerol.

Rh based catalysts (Rh/ZrO₂ and Rh/CeO₂) were also synthesized for dry reforming of glycerol [15]. A declining trend of glycerol conversion and H₂/CO ratios with CO₂/glycerol increasing were also observed for the studied catalysts. Rh/ZrO₂ deactivated faster than Rh/CeO₂, due to the sintering of Rh metals and coke formation.

The dry reforming of glycerol with and without catalysts were summarized in this part. Without a catalyst, the production of H₂ from glycerol was low. For the various catalysts, including Ni, Rh, and Co, the highest activity was observed for Rh, while the activity of Co was the lowest. For the noble metals, the catalytic activity followed the order: Rh > Ru > Ir > Pd > Pt. Therefore, it can be concluded that Rh based catalysts showed a superior performance, especially on CeO₂ support. The La was also reported to promote the activity of active metals due to its redox property to remove carbon and less acidity of the Ni catalyst. Even though the conversion of glycerol is lower than ethanol generally, the glycerol conversion can still reach 90% in the presence of Rh/Al₂O₃-ZrO₂-TiO₂. The summarization showed the promising way of dry reforming of glycerol in the well-designed catalysts for H_2.

The dry reforming of ethanol and glycerol were summarized in

Table 1. Summary of dry reforming of ethanol and glycerol using different catalysts.

Active Metals	Supports	Promoters	Sample	Comments
Ni	SiO2		Ethanol	Clustering of Ni may occur at high Ni loading [26].
	SiO2, KIT-6			Stronger interaction of Ni-support inhibited the sintering of Ni [27].
	Y2O3–ZrO2			Stronger interaction between Ni and oxygen vacancies of support accelerated the activity.
	Al2O3, CeO2, MgO and ZrO2			Ni-CeO2 catalyst showed the best performance. Redox property of CeO2 can promote the selectivity for H2 and inhibit the unfavored side reactions [10].
	Al2O3	Ce		Ce-promoted catalyst can inhibit the formation of carbon due to its oxygen mobility of CeO2 [28].
	Al2O3	La		La can enhance the dispersion of Ni particle and hinder the sintering of metal oxides [29].
Co	Al2O3	Ce, La		La and Ce can increase the number of Co active sites by accelerating the reduction of Co3O4 [30,31,32].
Cu	CeO2, ZrO2	Ce		CuCeZr catalyst showed the best activity and stability due to higher Cu dispersion, strong metal support interaction and more oxygen vacancies [33,34,35,36].
Rh	SBA-15	Ce		The interaction between Rh and CeO2 and strong redox capacity of CeO2 can accelerate of CO2 activation and prevent carbon deposition [40].
	Al2O3, MgO–Al2O3, ZrO2–Al2O3	Ce, La		The superior performance of Rh/NiO–Al2O3 is due to the presence of NiAl2O4 spinel phase, and high dispersion of Rh [41].
Ir	Ce0.75Zr0.25O2			The dispersion, reducibility of Ir, oxygen vacancies of supports and interaction between Ir andsupport decreased with the calcination temperature increasing. IrCeZr550 catalyst (calcination at 823 K) exhibited a satisfactory performance with high activity and stability. Increasing calcination temperature up to 850 °C decreased markedly the interaction between Ir and support, dispersion and reducibility of Ir, and the dissociation capacity of the C–C bond [56].
Ni	Al2O3		Glycerol	The glycerol conversion increased for Ni from 5 to 15%, while decreased at higher Ni, which may be related to lower Ni dispersion at higher content of Ni [12].
	Al2O3	La		Glycerol conversion and H2 generation rate reached a peak at CO2/glycerol ratio of 1.67 [13].
	Al2O3	La		Ni metal can be better dispersed by La [48].
	Al2O3	Ag		The best glycerol conversion (40.7%) and high yield of H2 (32%) were observed for Ag-Ni/Al2O3. This result can ascribed to the smaller crystallite size of Ag in Ag-Ni/Al2O3, which was beneficial for metal dispersion [57].
	Al2O3	Re		Approximately 61% and 56% of glycerol conversion and hydrogen yield were observed for 5% Re-Ni/CaO, respectively. For 15% Ni/CaO, the conversion of glycerol and H2 yield are lower, being 35 and 30%, respectively. The addition of Re increases the acidic sites of the catalyst and promoted the adsorption of OH group of the glycerol onto the surface of catalyst [54].
Rh, Ni and Co	ZrO2-TiO2			Rh showed the highest activity, while the activity of Co was the lowest [4].
Rh, Ru, Ir, Pd, Pt	MgO–Al2O3			The catalytic activity followed the order: Rh > Ru > Ir > Pd > Pt [49].
Rh	CeO2, and ZrO2			Rh/ZrO2 deactivated faster than Rh/CeO2, due to the sintering of Rh metals and coke formation [15].

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4. Conclusions and Future Perspectives

This review summarized the recent advance of dry reforming of ethanol and glycerol in the presence of various catalysts. Temperatures showed a positive effect on conversion, while CO₂/(ethanol or glycerol) ratio showed a different trend. The highest glycerol conversion can be maintained at CO₂/glycerol ratios of 1–2, while, for the case of ethanol reforming, the CO₂/ethanol ratios around 1.4 seem to be the optimum. The H₂/CO ratio also decreased with CO₂/glycerol ratio increasing due to the reverse WGSR taking place simultaneously. For dry reforming of both compounds, the intermediate CH₄ was regarded as the main primary intermediate from the direct decomposition of ethanol and glycerol, which can be then reformed into H₂ and CO. The active metals, promoters, and supports were summarized in Figure 13. Ni based catalysts have shown their possible application in dry reforming of ethanol and glycerol, while deactivation issues due to the formation of carbon on the surface of catalyst and sintering of active metals hindered its wide application. Various promoters such as La and Ce have been proven to enhance Ni performance. Briefly, the boosted performance of the promoted catalysts is ascribed to: (i) Strong metal-support interaction: encapsulation of active metal nanoparticles by supports, (ii) Mobile oxygen species and creation of oxygen vacancies introduced by promoters mitigating coke formation.

The future foresees a great deal of possibilities for CO₂ reforming reactions as a direct route to tackle CO₂ emissions. Perhaps the industrial application of dry reforming remains as the biggest challenge for this technology nowadays. In fact, very few attempts of commercializing dry reforming plants have been conducted so far with the CALCOR process (a reforming process combined with CO₂ recovery and CO purification) patented by caloric being one of the first successful stories [58]. The development of advanced catalysts with high activity, selectivity, and, more importantly, stability constitute the principal bottleneck for the implementation of dry reforming reaction unit in realistic applications. Herein, the directions seem to go through the combination of engineered catalysts with the addition of promoters, multiple active phases, and sacrificial agents to avoid deactivation with cleverly designed regeneration strategies. At lab-scale, we have identified the need to conduct rigorous studies using realistic reforming mixtures (i.e., containing sulphur impurities)—this an effort that the academic community should embrace to ensure a solid design of scalable high-performing catalysts. In any case, there is a lot of room for research in this promising area of catalysis for CO₂ valorization, and this mini-review just intends to be an introductory quick-guide to selected dry reforming reactions showcasing their potential contribution in the next generation of zero-carbon catalytic processes.