Preliminary Study on Syngas Production from a CO₂ and CH₄ Mixture via Non-Thermal Dielectric Barrier Discharge Plasma Incorporated with Metal–Organic Frameworks

Abstract

Dry reforming has gained widespread attention among CO₂ utilization approaches, as it is able to convert both CO₂ and CH₄ into syngas, thus mitigating global warming. Moreover, dielectric barrier discharge (DBD) non-thermal catalytic plasma reactors are potential technologies for CO₂ and CH₄ conversion, due to their low energy consumption and ease of operation. Catalysts also play an important role in ensuring optimal performance. For instance, metal–organic frameworks (MOFs) such as ZIF-8, NH₂-UiO-66(Zr), and NH₂-MIL-53(Al) are rarely reported in the literature for plasma technologies in dry reforming, despite their strong attributes such as high surface area and charge characteristics. In this work, these MOF catalysts were synthesized and characterized to evaluate their internal morphology, crystallinity, and surface area. Characterization studies showed that ZIF-8, NH₂-UiO-66(Zr), and NH₂-MIL-53(Al) generally showed similar properties to those results reported in the literature. Additionally, based on DBD catalytic plasma testing, NH₂-UiO-66(Zr) with an input power of 30 W recorded the highest H₂ and CO yields of 3.20% and 2.34%, respectively, at a CO₂:CH₄ molar ratio of 7:3. These values could be referred to for future studies on the improvement of MOF catalysts performance in dry reforming under the plasma processes prior to upscaling.

1. Introduction

CO₂ and CH₄ are greenhouse gases (GHGs) that require mitigation to reduce their impact on the environment. In fact, these gases can possibly be used as high-quality feed sources for useful products. As of 2022, it has been reported that GHG emissions have exceeded 36.8 Gt for global CO₂ emissions from energy industries [1]. Therefore, conversion of GHGs can be seen as a potential chemical source to synthesize helpful chemical products and simultaneously mitigate the dangerous effects of GHGs to the environment. When compared to alternative solutions such as storage, utilization has seen better preference, particularly because storage is demanding in terms of cost, long-term ambiguity, and transportation issues [2].

CO₂ and CH₄ can be used for the synthesis of syngas, which is an intermediate to produce fuel and useful chemical products. From syngas, processes such as Fischer–Tropsch can convert syngas into various types of chemicals, depending on the carbon monoxide (CO) to hydrogen (H₂) ratio. Moreover, through these methods, environmentally friendly products such as methanol, petrol, diesel, and chemical products can be manufactured. There are various methods to perform CO₂ and CH₄ conversion, such as CO₂ and CH₄ splitting, steam reforming of methane (SRM), membrane reforming, autothermal reforming, and dry reforming of methane (DRM). Among those technologies, DRM has been gaining greater attention, as it requires only CO₂ and CH₄ gas to synthesize syngas without further addition of gas, thus making it highly sought after for environmental and industrial benefits. Furthermore, DRM tends to be more preferable due to its higher energy efficiency and higher conversion rate compared to various CO₂ and CH₄ conversion methods [3,4]. Equation (1) shows the CO₂ and CH₄ conversion reaction that occurs in DRM processes for CO and H₂ formation [5,6]:

$${\mathrm{C}\mathrm{H}}_4\left(\mathrm{g}\right)+{\mathrm{C}\mathrm{O}}_2\left(\mathrm{g}\right)\leftrightarrow 2\mathrm{C}\mathrm{O}\left(\mathrm{g}\right)+{2\mathrm{H}}_2\left(\mathrm{g}\right) ∆\mathrm{H}=+247 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$$

(1)

Plasma technology, particularly non-thermal (cold) plasma, is among the various technologies which can be used in DRM processes. Non-thermal plasma exists as a mixture of completely or partially charged particles and electrons that can be generated through the supply of an electrical current through the gases [7]. Non-thermal plasma essentially generates highly energetic electrons in an electrical field. These electrons then proceed to collide with the bonds of the CO₂ and CH₄ gas molecules, which results in the breaking of the bonds to form ions and free electrons. The highly reactive species are then able to form product gas molecules such as CO and H₂ [8,9]. Since the electron mass is very low, non-thermal plasma does not cause a significant temperature rise for bulk gas, and the temperature will remain close to ambient [8]. The method of utilizing non-thermal plasma provides advantages, as the temperature required can be maintained close to ambient conditions. Among them, DBD non-thermal plasma offers greater advantages because it is easier to be generated and controlled. However, the selectivity of the non-catalytic DBD plasma towards the syngas is usually low and thus, catalysts usually will be incorporated in the process to improve syngas production efficiency [9].

Therefore, catalysts are essential in DRM reactions to facilitate the reforming of CO₂ and CH₄ by overcoming the thermodynamic barriers, which leads to improved CO₂ and CH₄ conversions, as well as CO and H₂ yields [10]. There are various heterogeneous catalysts that have been utilized for DRM processes, such as non-noble and noble metals and transition metal oxides. Non-noble and noble metals, such as nickel and platinum, respectively, are among the most commonly studied catalysts in conventional DRM processes. However, most catalysts reported for catalytic plasma reactors for dry reforming have faced several drawbacks, such as coke formation, which can impede performance, has low catalytic activity, and has a high cost [11]. Hence, to address these drawbacks, metal–organic frameworks (MOFs) have the potential to be explored as new catalysts for improving CO₂ and CH₄ conversion and syngas yield due to their large surface area and surface charge states. For instance, Vakili et al. in 2020 [8] studied a DBD plasma reactor for DRM using synthesized ZrO₂, UiO-67, and PtNP@UiO-67 MOFs. They found that UiO-67 was able to improve plasma generation and increase CO₂ and CH₄ conversion by 10% and 18%, respectively. MOFs such as ZIF-8, NH₂-UiO-66(Zr), and NH₂-MIL-53(Al) are relatively new catalysts with great potential, despite their limited usage in DRM processes.

Therefore, this research work aims to explore the potentiality of ZIF-8, NH₂-UiO-66(Zr), and NH₂-MIL-53(Al) as catalysts in the DRM process using DBD cold plasma for the formation of CO and H₂ from CO₂ and CH₄. Characterization of the MOF catalysts will also be conducted using a field emission scanning electron microscope (FESEM), transmission electron microscope (TEM), X-ray diffraction (XRD), and Brunauer–Emmett–Teller (BET) to evaluate its morphology, particle size, crystallinity, and surface area.

2. Materials and Methods

2.1. Materials

The materials included in this work were predominately for the synthesis of the ZIF-8, NH₂-UiO-66, and NH₂-MIL-53(Al) catalysts, which included zirconium(IV) chloride (purity > 98%), aluminum chloride hexahydrate (purity > 99%), zinc nitrate hexahydrate (purity > 98%), and 2-methylimidazole (purity > 99%), which were acquired from ACROS Organics, while 2-aminoterephthalic acid (purity > 99%, Mw: 181.15 g/mol, NH₂-BDC) and pivalic acid (purity > 99%) were purchased from Sigma Aldrich (Petaling Jaya, Malaysia). In addition, acetonitrile (purity > 99.5%), formic acid (purity = 98–100%) and tetraisopropyl orthotitanate (purity > 97%) were attained from Merck (Petaling Jaya, Malaysia). Solvents such as methanol (purity > 99.9%) and dimethylformamide (purity > 99%, DMF) were obtained from Merck.

2.2. Synthesis of ZIF-8, NH₂-UiO-66, and NH₂-MIL-53(Al) Catalysts

ZIF-8 was synthesized by following the method reported by Lai et al. (2014) [12]. Firstly, zinc nitrate hexahydrate and 2-methylimidazole were dissolved in methanol under stirring for 1 h at room temperature. The particles were recovered by centrifugation at 7800 rpm for 5 min. The solid particles were then washed with fresh methanol several times and dried in the oven at 60 °C for 24 h prior to use.

Next, NH₂-UiO-66 was synthesized by following the method reported by Shen et al. (2016) [13]. Zirconium (IV) chloride, 2-aminoterepthalic acid, and formic acid were dissolved in 50 mL of dimethylformamide (DMF) solvent. The solution was placed in a Teflon-lined stainless-steel autoclave and heated at 120 °C for 24 h in an oven. Then, the autoclave was removed from the oven and cooled at room temperature. The solution was then centrifuged at 7800 rpm for 10 min to separate the nanocrystals and the residual solution. Subsequently, the solution was placed into a glass bottle and immersed in methanol for solvent exchange. After 3 days, the methanol was removed, and the remaining yellow powder was activated under a vacuum oven at 100 °C for 20 h.

Finally, NH₂-MIL-53(Al) was synthesized by following the method reported by Mubashir et al. (2019) [14]. DMF solvent was added into 7.5 mL of di-ionized water and stirred for 5 min. After that, aluminum chloride solution was prepared by mixing aluminum chloride hexahydrate into the prepared solution. Subsequently NH₂-BDC acid was stirred into the remaining prepared solution. Then, both aluminum chloride and acid solutions were mixed under vigorous stirring for 10 min. Next, the mixture was transferred into the pressure vessel and heated in the oven at 150 °C for 24 h. The resultant particles were recovered and washed with a diluted DMF-water solution three times. To remove the amino acid, solid particles were further activated with DMF solvent via reflux at 150 °C for 24 h.

2.3. Characterization of ZIF-8, NH₂-UiO-66, and NH₂-MIL-53(Al) Catalysts

The resultant synthesized ZIF-8, NH₂-UiO-66, and NH₂-MIL-53(Al) MOF catalysts underwent various characterizations. Firstly, the morphologies of the synthesized MOF catalysts were determined using a field emission scanning electron microscope (FESEM, Tescan Brand, Brno, Czech Republic, model Clara) and a transmission electron microscope (TEM, Hitachi model HT7830, Tokyo, Japan). In addition, the crystallinities of the synthesized MOF catalysts were investigated using X-ray diffraction (XRD, Panalytical brand, Melvern, England, model Xpert3 Powder). Lastly, through the BET theory, the specific surface area and pore characteristics were analyzed using a surface area analyzer (SAP, Micromeritics, Norcross, GA, USA, Model ASAP 2020).

2.4. Experimental Runs for Non-Thermal DBD Plasma DRM

To conduct the non-thermal dielectric barrier discharge (DBD) plasma experiment, the reactor system was set up as shown in Figure 1. The dielectric layer was produced in a quartz tube with an outer diameter of 5 mm and a wall thickness of 1 mm. A stainless-steel rod was inserted into the middle of the tube, which would act as a high-voltage electrode, while a stainless-steel gauze was wrapped around the quartz tube, which acted as the ground electrode. The discharge length was set to 2 cm, with a discharge gap of 2.5 mm. The electrodes were connected to a high-voltage power supply with a peak voltage of 30 kV through a set-up transformer. The discharge power was manipulated from 5 W to 30 W, with a frequency set to 15 kHz. The CO₂ and CH₄ gas mixture was fed into the DBD reactor using a volumetric flow controller with a mixture total flowrate of 50 mL/min. The CO₂:CH₄ molar ratio was also varied from 3:7 to 7:3. The product gas stream then flowed into a custom-built gas analyzer equipped with a thermal conductivity sensor to measure the outlet gas. Each test was carried out for approximately 10 min to ensure a steady-state condition to obtain a consistent result. The outlet temperature of the product gas stream was close to ambient due to the low power. The ZIF-8, NH₂-UiO-66, and NH₂-MIL-53(Al) MOF catalysts were each inserted into the DBD plasma reactor via the quartz tube and packed with glass wool as the top and bottom layer to create the catalyst bed. The catalyst weight was fixed at 100 mg per catalyst.

3. Results and Discussion

3.1. Characterization Study of ZIF-8, NH₂-UiO-66, and NH₂-MIL-53(Al) Catalysts

The synthesized ZIF-8, NH₂-UiO-66, and NH₂-MIL-53(Al) MOF particles were characterized with FESEM, TEM, XRD, and BET to evaluate their morphologies, crystal structures, and surface areas, respectively. The morphologies of the synthesized catalysts, including ZIF-8, NH₂-MIL-53(Al), and NH₂-UiO-66(Zr), were characterized by FESEM and TEM, as shown in Figure 2. Firstly, FESEM images of ZIF-8 were shown to have a rhombic dodecahedron shape and an average size of 30 nm to 55 nm, which is in compliance with the results obtained from the literature [12]. Additionally, TEM images of ZIF-8 indicated that the particles contained a highly porous structure [15]. Next, for NH₂-MIL-53(Al), FESEM and TEM images showed an ellipsoidal shape with an average size of around 50 nm to 70 nm, similar to the results reported in the literature [14]. Meanwhile, FESEM and TEM images of NH₂-UiO-66(Zr), as shown in Figure 2e,f, demonstrated small, octahedrally cubic shapes with an average size of 60 nm to 100 nm [13]. Overall, the particle sizes of all MOFs catalysts synthesized in this work were below 100 nm.

The XRD patterns of the synthesized catalysts are shown in Figure 3. For ZIF-8, several characteristic peaks were identified at 2θ values of 8.5°, 10.5°, 16°, and 18°. In addition, the main peak found at the 2θ value of 8.5° was reported to contribute to the stable rhombic dodecahedron shape observed from Figure 2a [12]. Meanwhile, the XRD pattern obtained for the synthesized NH₂-UiO-66(Zr) showed good agreement with the literature in regard to the obtained peaks at around 2θ values of 7° and 8° [13,16,17]. The XRD characteristic peaks at 2θ values of 6.5°, 10.5°, 12°, 15.5°, and 17.5° were obtained for NH₂-MIL-53(Al) samples synthesized in this work, which corresponded to the NH₂-MIL-53(Al) structure, as reported in the literature [18]. Generally, the XRD results obtained for all samples confirmed the successful synthesis of the samples.

Table 1. BET surface area analysis of ZIF-8, NH₂-UiO-66(Zr), and NH₂-MIL-53(Al) obtained in this work.

Catalysts	BET Surface Area (m2/g)
ZIF-8	1157.92
NH2-UiO-66(Zr)	527.74
NH2-MIL-53(Al)	239.96

shows the BET surface area analysis of the MOF catalysts. The results showed that ZIF-8, NH₂-UiO-66(Zr), and NH₂-MIL-53(Al) obtained BET surface areas of 1157.92 m²/g, 527.74 m²/g, and 239.96 m²/g, respectively, which are in close compliance with the results reported in the literature [12,13,18].

3.2. Effect of CO₂:CH₄ Molar Ratio on Syngas Production

The H₂ and CO yields obtained from the non-thermal catalytic DBD plasma reactor incorporated with the MOF catalyst are shown in Figure 4 and Figure 5. The overall syngas production was presented with varying CO₂:CH₄ molar ratios from 7:3 to 3:7 and input powers from 5 W to 30 W. Then, 100 mg of ZIF-8, NH₂-UiO-66(Zr), and NH₂-MIL-53(Al) each was inserted into the DBD plasma reactor to analyze their performance in CO and H₂ formation.

Figure 4 shows the results for the H₂ yield obtained by using different MOF catalysts. Generally, it can be observed that with increasing input power, the H₂ yield for all MOF catalysts at varying CO₂:CH₄ molar ratios showed an increasing trend. This was likely due to the increase in plasma power and density with increasing input power, which led to improved electron collision and greater CO₂ and CH₄ conversions, therefore increasing the H₂ yield with increasing input power [19]. In terms of the CO₂:CH₄ molar ratio, 7:3 was recorded to produce a greater H₂ yield, compared to CO₂:CH₄ molar ratios of 5:5 and 3:7. It could be due to the higher CO₂ feed concentration, which improved the catalytic performance of MOF catalysts and allowed for greater interaction between the MOF and plasma irradiation [20].

Among the MOF catalysts, NH₂-UiO-66(Zr) stood out as the best-performing catalyst by producing higher H₂ yields for all CO₂:CH₄ molar ratios by at least 50% to 70% when compared at 30 W. Both ZIF-8 and NH₂-MIL-53 showed nearly similar H₂ yield results for CO₂:CH₄ molar ratios of 5:5 and 7:3. However, at a CO₂:CH₄ molar ratio of 3:7, NH₂-MIL-53(Al) showed greater performance for H₂ yield production, compared to ZIF-8. Overall, NH₂-UiO-66(Zr) with a CO₂:CH₄ molar ratio of 7:3 and an input power of 30 W recorded the highest H₂ yield at 3.20%.

On the other hand, based on the CO yield obtained, as shown in Figure 5, it can be similarly observed that with increasing input power, all MOF catalysts showed increasing CO yields. By increasing the discharge power, it increased the density and the strength of the plasma, which would increase the conversion and thus increased the yield performance for both CO and H₂. In terms of the CO₂:CH₄ molar ratio, 3:7 was recorded to produce a greater CO yield, compared to CO₂:CH₄ molar ratios of 5:5 and 7:3. ZIF-8 and NH₂-MIL-53(Al) once again produced nearly identical results for the CO yield, while NH₂-UiO-66(Zr) demonstrated greater CO yield performance at all CO₂:CH₄ molar ratios. Overall, NH₂-UiO-66(Zr) with a CO₂:CH₄ molar ratio of 7:3 recorded the highest CO yield at 2.34%. In addition, CO₂:CH₄ molar ratios of 5:5 and 3:7 for CO yields for NH₂-UiO-66(Zr) were nearly equivalent.

From the results obtained for the H₂ and CO yields shown in Figure 4 and Figure 5, it can be seen that the overall performances of the ZIF-8, NH₂-UiO-66(Zr), and NH₂-MIL-53(Al) MOF catalysts were relatively low in comparison to various literature studies focusing on DRM performance using DBD plasma. For instance, Vakili et al. utilized the UiO-67 MOF catalyst to study the performance of MOF in a plasma–catalyst system, in which they obtained H₂ and CO yields of around 30% and 34%, respectively, with an output power of 11 W and a CO₂/CH₄ molar feed ratio of 1. Nevertheless, there could be several potential reasons for the obtained low performance in this study. Firstly, as shown in the FESEM images in Figure 2, the particle sizes of the synthesized ZIF-8, NH₂-UiO-66(Zr), and NH₂-MIL-53(Al) MOF catalysts were relatively small (<100 nm). The small size of the catalyst could hinder the plasma radiation in the DBD plasma reactor and therefore affect the performance of the DRM process [21]. Besides that, the current DBD plasma setup utilized pure CH₄ and CO₂ gas feeds without the presence of any inert gas, such as helium or argon. It has been reported in past literature that without the presence of dilation of the CH₄ and CO₂ gas feeds with an inert gas, the syngas yield will be relatively low, as dilation is required to stabilize the DBD plasma and enhance the ionization of the reactive species, which could improve the conversion of CH₄ and CO₂ [22,23].

Therefore, for future works, the modification of the particle size of MOFs and the DBD plasma can be recommended to potentially increase the performance of MOF catalysts in syngas formation by utilizing a DBD plasma system. By synthesizing MOFs with a larger particle size, any hinderance for the generated plasma could be reduced. In addition, metal nanoparticles such as platinum can also be incorporated into MOFs, which may result in greater active sites and thus improve the syngas yield. For the DBD plasma setup, it is recommended that the CO₂ and CH₄ feed gas be dilated with an inert gas, as it will make the plasma radiation more stable and potentially allow for greater ionization of the inlet gas feed, which may improve the overall syngas performance.

4. Conclusions

For the present work, NH₂-UiO-66(Zr) produced the better results, compared to ZIF-8 and NH₂-MIL-53(Al), at an input power of 30 W, which recorded H₂ and CO yields of 3.20% and 2.34% at a CO₂:CH₄ molar ratio of 7:3, respectively. However, it should be noted that the overall performances of the MOF catalysts were relatively low in comparison to various literature studies for DRM, possibly due to their small particle sizes and the absence of any inert gas in the feed. Therefore, it is recommended that modifications should be performed for future works, such as synthesizing MOFs with larger particle and pore sizes, incorporating metal nanoparticles into the MOF particles, and utilizing inert gases, such as argon or helium, to increase the generated DBD plasma stability and irradiation. Overall, this study provided valuable insights that could be used for future studies with the MOF catalysts to further improve performance for future upscaled plasma technologies in DRM processes.