Abstract
Carbon monoxide (CO) purification from syngas impurities is a highly energy and cost intensive process. Adsorption separation using metal–organic frameworks (MOFs) is being explored as an alternative technology for CO/nitrogen (N2) and CO/carbon dioxide (CO2) separation. Currently, MOFs' uptake and selectivity levels do not justify displacement of the current commercially available technologies. Herein, we have impregnated a leading MOF candidate for CO purification, i.e. M-MOF-74 (M = Co or Ni), with Cu+ sites. Cu+ allows strong π-complexation from the 3d electrons with CO, potentially enhancing the separation performance. We have optimised the Cu loading procedure and confirmed the presence of the Cu+ sites using X-ray absorption fine structure analysis (XAFS). In situ XAFS and diffuse reflectance infrared Fourier Transform spectroscopy analyses have demonstrated Cu+–CO binding. The dynamic breakthrough measurements showed an improvement in CO/N2 and CO/CO2 separations upon Cu impregnation. This is because Cu sites do not block the MOF metal sites but rather increase the number of sites available for interactions with CO, and decrease the surface area/porosity available for adsorption of the lighter component.
Highlights
The production of carbon monoxide (CO) as an industrial reagent requires downstream separation processes to achieve the high purity level required for CO to be used as a chemical feedstock (i.e. >99 mol%).[1,2] During the CO puri cation process, by-product impurities such as CH4, CO2 and N2 must be removed
The optimisation has focused on identifying the type of Cu salt, the Cu loading as well as the metal–organic frameworks (MOFs) activation and Cu reduction process required to maximise CO uptake
In terms of activation and Cu reduction, we have established that heating at 250 C with 10% CO for 6 h allowed the best compromise between minimising the time of CO used at elevated temperature while maximising the amount of Cu+ generated
Summary
The production of carbon monoxide (CO) as an industrial reagent requires downstream separation processes to achieve the high purity level required for CO to be used as a chemical feedstock (i.e. >99 mol%).[1,2] During the CO puri cation process, by-product impurities such as CH4, CO2 and N2 must be removed. While current CO capture and gas separation technologies exist for puri cation,[2,3] numerous energy-intensive steps are required which contribute to large capital and operating costs.[4] The design of a single adsorption process to purify CO from all impurities would bring energy and cost savings Their adsorption performance, especially ACs' and zeolites', fall below the CO capacity and selectivity levels required to justify the displacement of the current benchmark technology, i.e. cryogenic distillation.[2] MOFs have shown the most promising performance to date,[6] recording the highest CO adsorption uptakes and theoretical CO/X selectivities (where X 1⁄4 H2, N2 and CH4).[5,7]. We have performed a systematic Cu impregnation study on Ni-MOF-74 and Co-MOF-74.5,7 We have: (i) synthesized Cu+-impregnated MOF-74 structures to simultaneously use the metal–CO binding strength of the framework M2+ and introduce additional Cu+ ions, (ii) optimised the Cu impregnation and monitored in situ the Cu2+ to Cu+ reduction process using X-ray absorption near edge structure (XANES) analysis, (iii) veri ed Cu+–CO and M2+–CO binding using in situ DRIFTS analysis and (iv) tested Cu@Ni-MOF-74 for dynamic CO/N2 and CO/CO2 separation
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