Postsynthetic Exchange Method Research Articles

Mixed-Metal Cu-BTC Metal-Organic Frameworks as a Strong Adsorbent for Molecular Hydrogen at Low Temperatures.

The advancement of hydrogen and fuel cell technologies hinges on the development of hydrogen storage methods. Metal–organic frameworks (MOFs) are one of the most favorable materials for hydrogen storage. In this study, we synthesized a series of isostructural mixed-metal metal–organic frameworks (MM-MOFs) of 1,3,5-benzenetricarboxylate (BTC), M-Cu-BTC, where M = Zn2+, Ni2+, Co2+, and Fe2+ using the post-synthetic exchange (PSE) method with metal ions. The powder X-ray diffraction patterns of MM-MOFs were similar with those of single-metal Cu-BTC. Scanning electron microscopy indicates the absence of amorphous phases. Inductively coupled plasma mass spectroscopy of the MM-MOFs shows successful metal exchanges using the PSE method. The N2 adsorption measurements confirmed the successful synthesis of porous MM-MOFs. The metal exchanged materials Ni-Cu-BTC, Zn-Cu-BTC, Fe-Cu-BTC, and Co-Cu-BTC were studied for hydrogen storage and showed a gravimetric uptake of 1.6, 1.63, 1.63, and 1.12 wt %; respectively. The increase in hydrogen adsorption capacity for the three metal exchanged materials is about 60% relative to that of the parent MOF (Cu-BTC). The improvement of gravimetric uptake in M-Cu-BTC (where M = Ni2+, Zn2+, and Fe2+) is probably due to the increase in binding enthalpy of H2 with the unsaturated metal sites after the partial exchange from Cu2+ to other metal ions. The higher charge density of metal ions strongly polarizes hydrogen and provides the primary binding sites inside the pores of Cu-BTC and subsequently enhances the gravimetric uptake of hydrogen.

Titanium incorporated and g-C3N4-coated NH2-UiO-66 for enhanced photocatalytic hydrogen evolution

A g-C3N4 coated and titanium cooperated with NH2-UiO-66 prepared via a post-synthetic exchange method showed enhanced photocatalytic performance for hydrogen generation under visible light. The structure of NH2-UiO-66 was not destroyed and the optical performance of MOF composite was significantly improved than NH2-UiO-66. Based on the TEOA-ErB system and Pt cocatalyst, the highest H2 production efficiency of 0.1 g-C3N4/NH2-UiO-66(0.1Ti) could amount to 43.5 mmol h−1 g−1, which is about 2.6 times higher compared to parent NH2-UiO-66. These findings suggest that the two modification methods could effectively inhibit the recombination of electron–hole pairs and improve the photocatalytic performance of MOF.

Platinum nanoparticles decorated and titanium incorporated with NH2-UiO-66 for photocatalytic hydrogen production

Skeleton modification and inside decoration are considered as two important routes for the enhancement of MOF-based photocatalytic properties. Herein, we report a platinum nanoparticles decorated in NH2-UiO-66 and titanium cooperated with Pt@NH2-UiO-66 prepared via a post-synthetic exchange method showed enhanced photocatalytic performance for hydrogen generation under visible light. Based on the TEOA-ErB system, the highest H2 production efficiency of Pt@NH2-UiO-66(0.1Ti) could amount to 0.6 mmol h−1 g−1, which is about 6 times higher compared to parent NH2-UiO-66.

Fabrication and Characteristics of Mn@ Cu3(BTC)2 for Low-Temperature Catalytic Reduction of NOx with NH3

Novel catalysts with high activity for the selective catalytic reduction of NO with NH3 (NH3-SCR) at low temperatures are highly demanded. In this study, mixed-node metal-organic frameworks (MOFs), e.g. Mn@CuBTC with controlled Mn composition in Cu3(BTC)2, were fabricated using postsynthetic exchange method and their structural characteristics and catalytic performances for NH3-SCR reaction were assessed. A series of analyses in terms of structure, surface morphology, texture, and chemical state determined that Mn ions were successfully incorporated into the Cu3(BTC)2 crystal lattice as well as adsorbed on the walls of nanopores in the framework. The pore sizes can be finely tuned in the presence of Mn ions in the cages, which significantly suppressed water adsorption. The NH3-SCR activity of Mn@CuBTC exhibited nearly 100% NOx conversion rate in the temperature range (230–260°C). The superior NH3-SCR performance is attributed to the proper pore sizes, reduced water content, and the synergistic effect between manganese and copper ions in the MOF structure, which enhanced NH3 bound to the active Lewis sites.

Mixed-Metal Metal–Organic Frameworks as Catalysts for Liquid-Phase Oxidation of Toluene and Cycloalkanes

A series of isostructural mixed-metal metal–organic frameworks (MM-MOFs) of 1,3,5-benzenetricarboxylate (BTC), M–Zn–BTC, where M = Cu(II), Co(II) and Fe(II) have been synthesized using the post-synthetic exchange method. The catalytic activities of the M–Zn–BTC have been investigated for the oxidation of toluene and cycloalkanes in the presence of hydrogen peroxide as oxidant. The M–Zn–BTC catalysts show improved activity and selectivity for toluene oxidation. The introduction of the second metal to the node of the MOFs improved the toluene conversion from 22 to 70% and the selectivity of benzaldehyde improved from 22 to 57%. The Fe–Zn–BTC showed the best activity in terms of benzaldehyde selectivity for the oxidation of toluene. However, Cu–BTC gave the highest conversion for cyclohexane and methylcyclohexane oxidation.

Ag–Cu–BTC prepared by postsynthetic exchange as effective catalyst for selective oxidation of toluene to benzaldehyde

A mixed-node MOF catalyst Ag–Cu–BTC was prepared by postsynthetic exchange (PSE) method. It is believed that PSE method can realize isomorphous replacement of Ag ion to framework Cu ion in Cu–BTC successfully. The catalytic performance of Ag–Cu–BTC was investigated via selective oxidation of toluene to benzaldehyde by molecular oxygen in the absence of solvent and initiator. This catalyst exhibits good catalytic performance: on the premise of keeping highly selective catalysis of Cu–BTC for toluene oxidizing to benzaldehyde, the introduction of Ag (Ag content is 2.76wt.%) can promote toluene conversion from 6.5% to 12.7%.