Prussian blue analogues (PBAs) are low-cost porous materials with promising potential in gas separation, owing to their abundance of open metal sites. To access these open metal sites PBAs need to be activated through removal of water from the porous structure. However, the conditions required for effective activation and their structural consequences remain poorly understood, with existing effective methods being costly. Therefore, to gain insights in the activation process and its effects on structure, we systematically investigated the effect of activation temperature on three different ferricyanide-based PBAs (FePBA, CoPBA, and CuPBA) using a simple nitrogen flow activation procedure. We successfully synthesised these structures and optimised the activation procedure for micropore capacity and crystallinity. Infrared spectroscopy as well as CO2 and CO adsorption measurements revealed that, even under optimised conditions, water remained within the PBAs, with only a limited number of open metal sites accessible. At higher activation temperatures, micropore capacity and crystallinity decrease. This hydrophilicity, however, also showed positives, for example in the application of PBAs as desiccants. CuPBA, specifically, boasts a water adsorption of 355 mg g􀀀 1 at a relative humidity of ≤10%, competitive with zeolites and silica per weight unit, but higher per volume unit. Overall, however, while PBAs offer promise for gas adsorption owing to their high surface area and low cost, practical utilisation of their open metal sites remains challenging due to their strong affinity for water.

A key challenge in capturing CO2 from postcombustion gases is humidity due to competitive adsorption between CO2 and H2O. Multivariate (MTV) metal-organic frameworks (MOFs) have been considered a promising option to address this problem, e.g., combining CO2-affinitive and hydrophobic groups. Here, we synthesized a series of amine and methyl cofunctionalized MTV MIL-53(Al)-xNH2(1 - x)CH3 and their parent materials. All the mixed linker MIL-53(Al)-xNH2(1 - x)CH3 showed amino linker enrichment compared to the synthesis ratio, yet the linkers were distributed relatively homogeneously from the bulk to the surface. Material hydrophobicity or hydrophilicity varied with methyl or amino group content, respectively. The single-component adsorption indicated that certain mixed linker MIL-53(Al)-xNH2(1 - x)CH3 might outcompete the parent materials. In CO2-H2O competitive adsorption, however, the hydrophobic parental MIL-53(Al)-CH3 outperformed the mixed linker MOFs. CO2 adsorption capacities of 5.4, 4.9, and 3.6 wt % were found for 0.3 bar of CO2 under 0, 5, and 10% RH, respectively. The results highlight that materials with enhanced hydrophobicity and tight-fitting pores can outperform groups with high CO2 affinity in the CO2 capture under humid conditions.

Covalent organic frameworks (COFs) are ideal platforms to spatially control the integration of multiple molecular motifs throughout a single nanoporous framework. Despite this design flexibility, COFs are typically synthesized using only two monomers. One bears the functional motif for the envisioned application, while the other is used as an inert connecting building block. Integrating more than one functional motif extends the functionality of COFs immensely, which is particularly useful for multistep reactions such as electrochemical reduction of CO2. In this systematic study, we synthesized five Ni(II)- and Zn(II)-porphyrin-based COFs, including two pure component COFs (Ni100 and Zn100) and three mixed Ni/Zn-COFs (Ni75/Zn25, Ni50/Zn50, and Ni25/Zn75). Among these, the Ni50/Zn50-COF exhibited the highest catalytic performance for the electroreduction of CO2 to CO and formate at −0.6 V vs RHE, as was observed in an H-cell. The catalytic performance of the COF catalysts was further extended to a zero-gap membrane electrode assembly (MEA) operation where, utilizing Ni50/Zn50, CH4 was detected along with CO and formate at a high current density of 150 mA cm–2. In contrast, under these conditions predominantly H2 and CO were detected at Ni100 and Zn100 respectively, indicating a clear synergistic effect between the Ni- and Zn-porphyrin units.

Large amounts of carbon monoxide are produced by industrial processes such as biomass gasification and steel manufacturing. The CO present in vent streams is often burnt, this produces a large amount of CO₂, e.g., oxidation of CO from metallurgic flue gasses is solely responsible for 2.7% of manmade CO₂ emissions. The separation of N₂ from CO due to their very similar physical properties is very challenging, meaning that numerous energy-intensive steps are required for CO separation, making the CO separation from many process streams uneconomical in spite of CO being a valuable building block in the production of major chemicals through C1 chemistry and the production of linear hydrocarbons by the Fischer-Tropsch process. The development of suitable processes for the separation of carbon monoxide has both industrial and environmental significance. Especially since CO is a main product of electrocatalytic CO₂ reduction, an emerging sustainable technology to enable carbon neutrality. This technology also requires an energy-efficient separation process. Therefore, there is a great need to develop energy efficient CO separation processes adequate for these different process streams. As such the urgency of separating carbon monoxide is gaining greater recognition, with research in the field becoming more and more crucial. This review details the principles on which CO separation is based and provides an overview of currently commercialised CO separation processes and their limitations. Adsorption is identified as a technology with the potential for CO separation with high selectivity and energy efficiency. We review the research efforts, mainly seen in the last decades, in developing new materials for CO separation via ad/bsorption and membrane technology. We have geared our review to both traditional CO sources and emerging CO sources, including CO production from CO₂ conversion. To that end, a variety of emerging processes as potential CO₂-to-CO technologies are discussed and, specifically, the need for CO capture after electrochemical CO₂ reduction is highlighted, which is still underexposed in the available literature. Altogether, we aim to highlight the knowledge gaps that could guide future research to improve CO separation performance for industrial implementation.