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Using periodic density functional theory calculations, the aldol condensation of acetaldehyde to 3-hydroxybutanal over dehydroxylated MgO surfaces with and without structure defects was investigated. Compared with the C-C coupling step, the enolization step via proton transfer of the α-hydrogen of acetaldehyde to the MgO surface or the proton back-transfer step to form the desired 3-hydroxybutanal has a higher energy barrier, indicating that the proton transfer process is the key step for the aldol condensation on MgO. To highlight the effect of water, we also calculated the proton transfer steps in the presence of water and studied the reaction pathways over the partially hydroxylated MgO surface. The results show that water can participate in the proton back-transfer step by donating a proton to the alkoxide anion to form the 3-hydroxybutanal, thus reducing the activation energy; the surface OH groups induce a lowering of the activation energy barriers for the overall reaction. The results of the electronic structure analysis indicate that a strong Lewis acid-weak/medium base pair may have the best performance for aldol condensation, such as Mg-O-D produced by divacancy defects and Mg-OH produced by the dissociative adsorption of water. A strong Lewis acid generated by low-coordinated Mg can adsorb and stabilize the acetaldehyde molecule near the catalyst surface which is beneficial for the abstraction of an α-proton from an acetaldehyde molecule, and a medium or weak Brønsted base is favorable for the proton back-transfer step.

In this study, Mn-MOF-74 was successfully synthesized and further modified via two paths for enhanced water resistance. The structure and morphology of the modified samples were investigated by a series of characterization methods. The results of selective catalytic reduction (SCR) performance tests showed that polyethylene oxide-polypropylene-polyethylene oxide (P123)-modified Mn-MOF-74 exhibited outstanding NO conversion of up to 92.1% in the presence of 5 vol.% water at 250 °C, compared to 52% for Mn-MOF-74 under the same conditions. It was concluded that the water resistance of Mn-MOF-74 was significantly promoted after the introduction of P123 and that the unmodified P123-Mn-MOF-74 was proven to be a potential low-temperature SCR catalyst.