Plasma-Assisted One-Step Direct Methanol Conversion to Ethylene Glycol and Hydrogen: Process Intensification

Abstract

This perspective reports a process intensification strategy that converts methanol into ethylene glycol (MeOH-2-EG) in a single step to circumvent multi-step naphtha cracking into ethylene followed by ethylene epoxidation to ethylene oxide (EO) and the subsequent hydrolysis of EO to ethylene glycol (EG). Due to the thermodynamic restriction for the direct MeOH-2-EG, plasma-assisted catalysis was introduced, and platinum group metals were identified as prospective transition metal catalysts that can achieve the formation of strong metal hydride bonds and guarantee the controlled C–C coupling of two plasma-activated hydroxymethyl radicals (*CH₂OH) from methanol, both of which are essential for the single-step MeOH-2-EG.

1. Introduction

The negative impact of industrial petroleum processes on the environment, especially in terms of global warming, and aquatic and terrestrial pollution has increased the search for alternative nonpetroleum processes to keep existing petrochemical and allied businesses running. Ethylene glycol (EG) production is one process that is currently being affected and will be severely affected in the near future as the global reliance on fossil fuels declines. EG is an important petrochemical product that is widely used for the manufacture of antifreeze and coolant in automobiles, de-icing fluid for windshields of aircraft, desiccant for natural gas production, and a precursor for the manufacture of polyester fibers and resins: predominantly poly(ethylene terephthalate) (PET) [1]. EG has an annual production of over 25 million metric tons, and its demand has been estimated at a rate of 5% per year [2]. At present, EG is primarily produced at the industrial scale in a multistep process via the cracking of petroleum-derived naphtha to produce ethylene, followed by ethylene epoxidation to ethylene oxide (EO), and the subsequent hydrolysis of EO to EG (Figure 1a). As the future reliance on crude oil resources shrinks, the synthesis of EG using other alternative approaches such as the conversion of syngas to EG through dimethyl oxalate (DMO) attracts more and more interest [1]. As expected, these two multistep processes suffer from both high energy demand and a long production chain, thus reducing efficiency and increasing production costs. A robust and economically attractive route to produce EG must reduce the number of processing steps, utilities, and production costs and improve safety, i.e., process intensification (PI). In the course of the author’s search for appropriate feedstock and thermodynamic process simulation studies, process-intensified, one-step EG production using cheap and highly abundant methanol (MeOH) as the feedstock, i.e., the direct conversion of MeOH to EG (MeOH-2-EG), was developed. This process can be achieved in a single step, unlike the multistep status quo; in addition, hydrogen (H₂), which is clean energy, is obtained as the by-product (Figure 1b). Furthermore, this proposed route will reduce the pressure on the ethylene market, which is presently being gradually overstretched by the recent increase in polyethylene (made from ethylene monomers) demands from the automobile and aircraft industries for replacing several replaceable noncritical metallic components/parts with polymers. Recently, automobile and aircraft manufacturers have opted to replace several replaceable automobile and airplane metallic parts with polymers to reduce their static weight and eventually reduce fuel consumption in order to reduce the carbon dioxide (CO₂) emissions released from exhausts into the atmosphere.

In recent times, due to the global clamor for a green and sustainable environment, there are growing interests in the utilization of nonpetroleum carbon resources, in particular, shale gas (methane, CH₄) and CO₂ for the sustainable production of chemicals, therefore, increasing the importance of MeOH synthesis within the C1 chemistry domain [3,4]. According to the Market.us [5], the MeOH market is witnessing substantial growth, with a projected value of USD 66.2 billion at a CAGR of 5.83%. Presently, this increment in MeOH production has led to a significant reduction in prices due to a surplus in supply beyond the demand in the international market. For example, available data showed that between March 2022 and July 2023, in Europe and the US, FOB Rdam and FOB USG methanol prices have dropped from an average of USD 460/mt and EUR 455 to about USD 230/mt and EUR 190, respectively [6]. This trend in methanol prices is also seen in China, Southeast Asia, and the Middle East. Thus, there is a strong incentive to develop new methods or routes for the effective valorization of MeOH via C–C coupling with high selectivity within the framework of process intensification and circular economy [7]. The typical conversion of MeOH usually involves the activation of its C–H, C–O, and/or O–H bonds. The system that can selectively activate the relatively stable and poorly reactive C–H bond of MeOH while preserving the C–O and O–H bonds to guarantee a C–C coupling is challenging. Unfortunately, this is the sole route for achieving MeOH-2-EG (Figure 1b). Furthermore, the preliminary thermodynamic simulation results established that the reaction is not feasible at any temperature according to the minimization of Gibb’s free energy based on the chemical reaction equilibria theory (Figure 2) for different reaction phases (liquid or gas) for MeOH and EG, viz.:

Case 1: CH₃OH_(l) → C₂H₆O_2(l) + H_2(g)

Case 2: CH₃OH_(g) → C₂H₆O_2(l) + H_2(g)

Case 3: CH₃OH_(g) → C₂H₆O_2(g) + H_2(g)

The only case with seeming propensity is Case 1 with ΔG < 0 between 250 and 450 °C. However, maintaining MeOH at the liquid phase within 250 and 450 °C is not thermodynamically feasible since the critical temperature (T_c) of MeOH is ~240 °C (Case ⸎, Figure 2). The non-feasibility of Cases 2–3, despite the seamless conversion of MeOH to other C1 products like formaldehyde (CH₂O) over supported silver catalysts, can be due to the atomic bond strengths since the C–H bond’s energy is much higher than that of the C–O bond, although it is lower than that of O–H bonds in a MeOH molecule (

Table 1. Methanol atom bond energy and length.

	Bond Atom	Atom Number	Bond Energy (kJ/mol)	* Bond Length (Å)
	C(1)	O(2)	358	1.402
	C(1)	H(3)	413	1.113
	C(1)	H(4)	413	1.113
	C(1)	H(5)	413	1.113
	O(2)	H(6)	467	0.924

* Calculated using Chem Office software package (ChemDraw Ultra version 12.0).

). Another possibility is the relatively high energy demand of MeOH selective dehydrogenation (ΔH = +84 kJ mol⁻¹) compared to the dehydrogenation of higher alcohols, such as ethanol (ΔH = +68 kJ mol⁻¹) [8,9,10]. Thus, it appears that the use of MeOH as a C1-feedstock for catalytic C–C coupling is restricted to dehydrative oligomerization processes like the MeOH-to-gasoline (MTG), MeOH-to-olefins (MTO), and the Monsanto and Cativa processes of methanol carbonylation, which are the second-largest volume applications of homogenous catalysis [9]. This implied that the selective activation of the stable sp³ α-C–H bond in MeOH without breaking the C–O bonds at elevated reaction temperatures is challenging in synthetic chemistry; hence, it is of high academic significance [4].

So far, among the few findings in the literature that have attempted direct MeOH-2-EG, there are the works of Xie et al. [4], wherein a visible light-driven dehydrogenative coupling of MeOH-2-EG was reported. They showed that over a molybdenum disulfide nanofoam-modified cadmium sulfide nanorod catalyst, EG was formed at a high efficiency, with 90% selectivity and hydrogen as the side product. They showed that the photoexcited holes on a cadmium sulfide nanorod led to preferential C–H bond activations instead of the O–H bond in methanol via a concerted proton–electron transfer mechanism, thus forming hydroxymethyl radicals (*CH₂OH) that readily desorb from catalyst surfaces for subsequent coupling. Considering process economics, facile process design, and more importantly, EG yields, the intervention of the visible light-driven dehydrogenative coupling of MeOH-2-EG will be problematic due to low throughput.

To surmount the challenge of selective C–H bond scission while preserving O–H and C–O bonds, the idea of dielectric barrier discharge (DBD) plasma becomes attractive due to its capacity for selective high-energy scission based on the Siemens reports of its first experimental investigation on simple barrier discharges in 1857. DBD plasma can initiate a series of ionization and chemical processes that are far from thermodynamic constraints under ambient operating conditions [6]. Previously, DBD was employed for methanol conversion with the original intention of obtaining hydrogen-rich syngas; incidentally, a trace amount of EG was found in the product effluent. This serendipity prompted another deliberate effort by Zhang et al. [11] in a systematic study exploring the direct synthesis of EG from the plasma of methanol vapor; interestingly, encouraging results were obtained using hydrogen as a carrier gas in a non-catalytic double dielectric barrier discharge (DDBD) reactor. Their results demonstrated that as the input power was increased from 8.5 W to 28.6 W, the conversion of MeOH increased from 6.1% to 30.2%, while EG selectivity decreased from 69.8% to 38.0%. The elevation of input power mainly increased undesired CO and CH₄ by-products. Poor EG selectivity can be explained in terms of the atom bond strengths mentioned above and the bond length in Table 1 wherein the C–H bonds have intermediate bond lengths. Since the literature has indicated that chemical reactions in DBD are governed by the electron’s temperature instead of the thermal processes or gas temperature, several drawbacks such as the low conversion of reactants, poor selectivity of desired products, and low energy efficiency are thus inevitable under DBD plasma alone. Thus, to tailor the product distribution, i.e., to improve the desired selectivity, the combination of DBD plasma and catalysis becomes attractive.

Recently, the author explored similar plasma-assisted catalysis (PAC) to convert CH₄ into C₂H₄ and H₂ in a single step, which hitherto is not thermodynamically feasible due to the symmetricity of the four high-energy C–H bonds of CH₄ [12]. Serendipity offers a future option for CH₄ conversion (X_CH4) to light olefins to replace the state-of-the-art steam reforming of CH₄ relative to syngas at elevated temperatures (700–1100 °C) and the subsequent conversion of the syngas to olefins (at 200–280 °C), with a large attendant volume of wastewater. The findings showed that X_CH4 increased from 36.7% under plasma conditions without any catalysts to an average of above 55% over a series of Ni catalysts that were functionalized with oxalate ligands to downsize the Ni particle size and was supported on either HCl- or NaOH-treated kaolin supports, and it was calcined at either 700 or 800 °C. Furthermore, Ni catalysts calcined at 800 °C, and those supported on NaOH-treated kaolin showed a higher X_CH4, ethylene–ethane ratio (C₂=/C₂–) and hydrogen yield (Y_H2), which are all attributed to the catalyst’s strong metal–support interaction. When the Ni catalysts were replaced with a ceria alloyed platinum catalyst, X_CH4 increased to 73.5% with a high Y_H2 of 33.8% and a C₂=/C₂– ratio of ca. 12.3 compared to an average Y_H2 of ~20% and C₂=/C₂– ratio ca. 0.032 observed over the Ni-based catalysts. Conclusively, while only C₂H₆ and minuscule H₂ without any C₂H₄ formation were observed under a non-catalytic plasma system, substantially, larger amounts of C₂H₄ and H₂ were obtained under the PAC using alloyed Pt catalysts than compared to Ni catalysts from CH₄, which otherwise was not thermodynamically feasible.

2. Perspective

Based on the success of PAC for the single-step conversion of CH₄ into C₂H₄ and H₂, the concept of direct MeOH-2-EG becomes an attractive perspective for massively leveraging the current low price of MeOH with respect to valorization into EG. This perspective offers a fascinating nonpetroleum route for the sustainable production of EG with clean H₂ as the by-product. This can be achieved via rational design and the synthesis of transition metal-based catalysts that can form a reasonably stable metal-hydride bond to mimic a recent successful report on homogeneous catalysis for MeOH C–C coupling to provide a discrete product of hydro-hydroxymethylation such as 1,1-disubstituted allenes in order to form homoallylic neopentyl alcohols [9]. To achieve this ambition, platinum group metals are highly promising, specifically those that are third-row transition metals, such as iridium and platinum, which generally form stronger metal–hydrogen bonds. Thus, the formation of a strong M–H bond (M = Ir or Pt) will compensate for the loss of a strong MeOH C–H bond, which introduces C–H bond scission in the MeOH-2-EG less endothermic in a somewhat Hammond’s postulate phenomenon [9]. The well-tailored Ir or Pt catalyst particle size must be significantly downsized to a narrow sub-nanometer cluster range to exhibit a strong metal–support interaction that can guarantee the controlled C–C coupling of two plasma-activated *CH₂OH radicals (from MeOH) to EG and H₂. To further enhance our fundamental understanding, density functional theory (DFT) and density functional-based tight binding (DFTB), including first principle molecular dynamics with machine learning approaches, are highly invaluable, especially for investigating ground and excited states pathways for catalysis to optimize C–C couplings from generated *CH₂OH under plasma. To date, reports regarding this integrated strategy have not been sighted in the literature; hence, this perspective is proposed.