Carbon Monoxide Separation

Introduction

Carbon monoxide represents a key feedstock in the petrochemical industry, which is mainly produced by the partial oxidation of carbon-containing com pounds, appearing in the mixtures including hydrogen, carbon dioxide, nitrogen and hydrocarbons. The application of carbon monoxide in industrial must be in sufficient purity, so the separation carbon monoxide from the mixed gaseous before any further applications is necessary and important. The current industrial route towards carbon monoxide purification involves a combination of cryogenic distillation, acid gas removal and dehydration^[1] (as shown in Fig.1). But these methods associated high-energy consumption and/or poor stability of the absorption solvents remain key barriers in large-scale deployment. By contrast, a more energy-economic carbon monoxide purification at ambient temperature can be attained with the assistance of MOFs adsorbents, which has become the mainstream direction.

Fig. 1 Schematic of carbon monoxide process and purification steps.
(The colour code distinguishes between chemicals to be recovered and used (in green) and impurities (red))

Importance of Carbon Monoxide Separation

• Methanol production: Methanol is primarily produced from carbon monoxide as raw materials.
• Hydroformylation: The process of hydroformylation requires pure carbon monoxide, which uses a homogenous catalytic reaction to convert carbon monoxide and alkenes into aldehydes. Since the introduction of commercial scale hydroformylation in 1948, the total consumption of products based on this process has risen to millions of metric tonnes per year.
• Phosgene production: Carbon monoxide is also utilized as a reagent during the industrial synthesis of phosgene, which is an important industrial reagent used in the production of isocyanates, a precursor to numerous polymers.
• Acetic acid production: The production of acetic acid also uses carbon monoxide as a reagent. Acetic acid is widely used as a solvent and as a precursor for numerous useful products.

After separation and purification, carbon monoxide as a key material can be used in numerous large-scale industrial processes for synthesis of higher value compounds. The above industrial processes emphasize further the requirement for large-scale carbon monoxide separation and purification in industry.

Carbon Monoxide Separation Using MOFs

The molecular size and properties of carbon monoxide make it very challenging to separate from other small gas molecules such as N₂, CO₂, H₂, and CH₄. The versatile nature of MOFs allows maximisation of carbon monoxide uptake by manipulating the carbon monoxide-metal interactions through appropriately choosing metal nodes having the strongest binding, and then combination of virtue of some unique structural features of MOFs, relevant separations can be realized easily. For example, Sato et al. reported a soft nanoporous MOF, [Cu(aip)(H₂O)] (aip = 5-azidoisophthalate), for CO/N₂ separation. [Cu(aip)(H₂O)] can undergo a global structural transformation during the binding interaction of carbon monoxide with open Cu²⁺ sites, which collaboratively enables the implementation of effective selective carbon monoxide capture from N₂^[2].

Carbon Monoxide Separation Mechanism

The carbon monoxide separation mechanism occurring by MOFs mainly depend on the selective adsorption of MOFs towards to carbon monoxide, and then separating carbon monoxide from gaseous mixture. Adsorption can be classified depending on the strength of the surface interactions into physisorption and chemisorption, in which the physisorption occurred in sites of MOFs linkers and the chemisorption occurred in unsaturated metal sites of the MOFs (as shown in Fig.2). In addition, another separation mechanism is size exclusion, according to the carbon monoxide molecules to be similar to the pore sizes of the MOFs, and then separating and purifying carbon monoxide from other small molecules (as shown in Fig.3).

Fig.2 (A) CO-metal interactions (chemisorption); (B) surface interactions through ligand functional groups(physisorption)

Fig.3 The scheme of size exclusion separation mechanism

What Can Alfa Chemistry Do

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