Recent development in catalyst and reactor design for CO₂ reforming of alcohols to syngas: A review

Highlights

• A review on the dry reforming of alcohols over Ni-based catalyst is presented.

• Alcohols as the potential feedstock for production of syngas.

• Recent review on the catalyst and reactor design in dry reforming.

• Thermodynamic analysis and future potential catalyst development are reported.

Abstract

Global energy demand has had a significant impact on the Green House Gases (GHGs) emission, which have harmful effects on the environment and human health. Catalytic CO₂ reforming of biomass-derived alcohols converted into syngas is regarded as a viable option for reducing GHGs. Even though this is sustainable and ecologically beneficial for utilizing value-added biomass, little research has been done on the topic. Due to the high reaction temperature, metal sintering and coke formation have been widely reported as the major challenges in the CO₂ reforming process. These conditions could reduce the catalyst efficiency and cause the reactor system to shut down. To address these issues, this article provides a review of the current catalyst and reactor design development for CO₂ reforming of methanol, ethanol, glycerol, and butanol to generate synthetic gas. Thermodynamic analysis, catalyst performance, optimizing reaction parameters, and future potential catalyst design are also covered.

Introduction

In 2020, the major greenhouse gases (GHGs) of carbon dioxide (CO₂) were reported to decrease by 5.7% from 2019. This intermittent trend occurred as a result of the current coronavirus disease (COVID-19) pandemic around the world. Unfortunately, the CO₂ emission is predicted to rebound nearly 5% in the year 2021, which is in parallel with the global economic recovery, especially for energy-related industries (IEA, 2021). China produces the most CO₂ emissions, with 11.535 gigatons produced in 2019, followed by the United States with 5.243 gigatons. Meanwhile, Malaysia produces almost 30 billion CO₂ emissions, which come from various sectors, including daily human activities, residential areas, transportation, electricity generation, and industrial. Persistent greenhouse gas emissions continuously raise global temperatures, leading to abnormal natural disasters like droughts, floods, and storms (Roslan et al., 2020). According to the report by Asian Pacific Energy Centre (APEC), the CO₂ emission in Malaysia is projected to grow by 4.2% per year and possibly increase to 414 million tonnes in 2030 for the commercial sector, followed by the transport sector (400 million tonnes), industry sector (270 million tonnes), other transformation sectors (172 million tonnes) and electrical generation (150 million tonnes) (Hosseini et al., 2013).

According to Le et al. (2021), carbon capture, utilization, and storage (CCUS) became one of the significant parts to mitigate the climate change caused by anthropogenic CO₂ emissions while meeting the growing global energy demand (Le et al., 2021). Thus, minimizing carbon releases is an essential attempt to reduce global warming hazards and climate changes affected by GHGs emissions. Among them, CO₂ utilization is becoming a promising technology in dealing with world climate change In general, CO₂ can mainly be utilized either as a direct solvent, refrigerant, and extracting agent, or reactant gas to produce beneficial chemicals, for instance, light alcohol, urea, and syngas (Chung and Chang, 2016; Zhao et al., 2020).

Various established processes have been reported for syngas production, including hydrocarbon reforming (Ismagilov et al., 2019; Abdullah et al., 2020a,b; Abdullah et al., 2021), electrolysis (Kaddami and Mikou, 2017), and biochemical (photosynthesis and fermentation) processes (Suffredini et al., 2017). Most of the energy industries generate syngas (H₂ and CO) from the process of fossil fuels reforming, including liquid petroleum, coal, natural gas, and naphtha in which unavoidably produced a high amount of CO₂ that caused an environmental contaminants issue due to the releasing of the hazardous and chemicals substances to the atmosphere (Kaddami and Mikou, 2017). The global energy crisis is currently concern with the problem of depletion of fossil-based fuels as primary resources. This issue demand on other alternative energy sources for the production of a cleaner and sustainable form of fuel energy to replace current fossil fuels.

Recently, many researchers focused on synthesis syngas using dry reforming of biomass-derived feedstocks of alcohols like ethanol, butanol, methanol, and glycerol (Siew et al., 2014; Aouad et al., 2018; Yu et al., 2019; Abidin et al., 2020), as this renewable resource being carbon neutral process to future hydrogen economy (Muradov and Vezirog, 2008) and less effect to the global climate (Zakaria et al., 2015). CO₂ reforming with oxygenates (short-chain alcohol) as the feedstock has become one of the most attractive routes for producing CO and H₂ due to low reactant costs, promote cleaner fuels and a desired to reduce CO₂ emissions in the environment while producing a valuable product (Li et al., 2016; Azizan et al., 2020). Among four types of alcohols, ethanol and glycerol became the popular feedstocks for dry reforming in syngas production due to their unique characteristics, as tabulated in Table 1.

Methanol reforming has been considered a promising technology for producing syngas compared to gasoline and diesel fuel because of its free sulfur content, high hydrogen density, readily available, easily transported, and stored (Iulianelli et al., 2014). Methanol can basically be operated at low temperatures because it consists of one carbon atom (Aouad et al., 2016). Besides that, biomass-derived ethanol, which came from renewable biomass such as sugar-based feedstock (sugarcane and corn), starch, and cellulosic feedstocks, became one of the promising alternatives for syngas production due to its benefits in terms of non-toxic, biodegradable, sulfur-free, accessible storage, and high availability (Aouad et al., 2018; Arku et al., 2018). The ethanol production is expected to reach almost 200 billion litres in 2023, with an annual expansion rate of 2% (Voegele, 2018). In addition, butanol has become one of the interesting renewable feedstock compared to ethanol and methanol because it possesses a higher hydrogen composition (Kumar et al., 2018). It can be manufactured via the fermentation process of several feedstocks such as corn, wheat, lignocellulosic biomass, sugarcane, and microalgae. It also became one of the essential raw materials in various industries such as chemical and petrochemical because it has excellent tolerance capacity towards water contamination, inflammable and has a lower vapour pressure that able to minimize the risk of vapour lock (Kumar et al., 2018). Moreover, butanol has a low vapour pressure point, making it a potential and safe fuel for high temperature reforming processes (Kumar et al., 2018).

Additionally, glycerol by-products from biodiesel production as sustainable raw material for syngas production were reported among researchers worldwide. Dang et al. (2020) claimed that the glycerol reactant would act as a promising hydrogen donor (Dang et al., 2020) in which helps for stabilizing the decomposition products, thus preventing the coking. Glycerol is chosen as one of the reactants in the dry reforming process because it is non-toxicity, has lower pollutant emissions, and is biodegradable than conventional diesel (Dahdah et al., 2020). The United Nations, Food and Agriculture Organization reported that the amount of glycerol by-products from biodiesel production is continuously increased up to 4 billion litres in 2020 with 36 billion litres of biodiesel production. While the crude glycerol price was decreased as low as 18 cents/L (Fig. 1) due to global biodiesel demands, it increases at about 0.7 million tons per year annually (Lam et al., 2010). This makes this abundant glycerol become a significant issue that needs to be highlighted as its purification process is also costly. This makes this abundant glycerol become a significant issue that needs to be highlighted as its purification process is also costly. Purifying crude glycerol via the distillation process is pricy and not economically feasible, specifically for a small-scale biodiesel factory. On the contrary, various biodiesel factories have use crude glycerol to generate energy via direct burning process, in addition to the purification process. However, the process is not easy because the high glycerol viscosities impede the flow, pumping processes and flame spray. Moreover, high ignition temperature of crude glycerol lessens the effectiveness of the combustion process, resulting in the creation of very toxic acrolein (Bac et al., 2019; Roslan et al., 2021). To tackle the issue, the reuse of glycerol as raw sources in the catalytic process for syngas production had highly received attention. This alternative is beneficial as it can reduce environmental pollution and the syngas product can be applied in alcohol-based synthesis and Fischer–Tropsch (FT).

Recently, several review papers discussing on dry catalytic reforming has been published. Yu et al. (2019) reported on the CO₂ reforming of ethanol and glycerol in which focusing on the thermodynamic pathway for both alcohols to produce syngas (Yu et al., 2019). Roslan et al. (2020) reported the review article on the application of Ni-based catalyst in glycerol reforming processes for productions of syngas. This research group also focused on the dry glycerol reforming that highlighted on development of the catalyst and catalyst deactivation phenomena (Roslan et al., 2020). However, no review articles were devoted to the CO₂ reforming of alcohols. Thus, the present article offers an overview of the application of the methanol, ethanol, glycerol, and butanol for CO₂ catalytic reforming to generate syngas. The detailed thermodynamic analysis and reforming catalyst development, including active metal, promoters, and support, were discussed. Additionally, the design of the reactor and a possible future catalyst for improved catalytic reforming activity were also being discussed.