Tianjun Sun

Composites of metal–organic frameworks and carbon-based materials: preparations, functionalities and applications

Metal–organic frameworks (MOFs), assembled by metal ions or their clusters and organic linkers, are one of the state-of-the-art crystalline materials. Their features such as ultra-high porosity, synthetic tailorability and relative ease of synthesis make them promising candidates for diversified applications. Controllable integration of MOFs and carbon-based materials not only leads to further enhancement of single-phase MOFs in terms of stability and electrical conductivity, but also surprisingly brings about a number of new functionalities like formation of new pores and template effects. These benefits allow the resultant MOF–carbon composites to be applied beyond the fields of single-phase MOFs. Increasing research interests have been aroused in this rapidly developing interdisciplinary area. This review aims to specifically group together the important reports focused on MOF–carbon composites till now. The methods used for composite synthesis and applications of the composites are investigated and categorized. The review also exclusively discusses the functionalities stemming from the synergistic effects of the two intriguing materials and pictures the future prospects at the end.

Effects of precipitation aging time on the performance of CuO/ZnO/CeO2-ZrO2 for methanol steam reforming

Abstract CuO/ZnO/CeO 2 -ZrO 2 catalysts for methanol steam reforming (MSR) were prepared by a co-precipitation procedure, and the effects of precipitation aging time on the catalytic performance were investigated. It was found that the prolonged precipitation aging time increased the surface Cu atoms and improved the reducibility of catalyst, but decreased the oxygen storage capacity. A nearly linear increase between the surface Cu atoms and H 2 production rate was obtained in prepared CuO/ZnO/CeO 2 -ZrO 2 catalysts with prolonged precipitation aging time. However, CO concentration increased with the decrease of the oxygen storage capacity. Considering the H 2 production rate and CO level, the optimal precipitation aging time was 2 h. CuO/ZnO/CeO 2 -ZrO 2 prepared using this aging time exhibited the best activity with suppressed CO formation.

Synthesis optimization of the ultra-microporous [Ni3(HCOO)6] framework to improve its CH4/N2 separation selectivity

Separation of methane and nitrogen is an important issue in upgrading low-quality natural gas, and non-cryogenic, adsorption-based separation of CH4/N2 is particularly challenging. In this report, a metal–organic framework (MOF) adsorbent, namely a [Ni3(HCOO)6] framework, is comprehensively investigated for the separation of CH4–N2 mixture via pure gas adsorption and binary gas breakthrough experiments. All the prepared samples synthesized using different routes were also studied in detail by powder XRD, FT-IR, SEM, TGA/DSC and argon adsorption. The results show that the adsorptive separation performances can be improved significantly by optimizing the synthesis of the framework. The precursors play crucial roles in the crystallization of [Ni3(HCOO)6] frameworks, giving rise to a variability in ultra-micropore volume, surface area and pore size. Good crystallization can result in large ultra-micropore volume and furthermore brings about high separation selectivity. The [Ni3(HCOO)6] framework synthesized from nickel nitrate and methyl formate exhibits the best crystallization and the largest micropore volume, leading to the highest CH4/N2 separation selectivity of up to 7.5 in the pressure range of 2.0–10 bar, which is the highest value reported for MOFs. Moreover, this adsorbent presents uniform nanosized crystals (∼140 nm), permanent porosity and consistent separation performances, making the [Ni3(HCOO)6] framework a promising candidate for natural gas upgrading.

Effects of the oxidation extent of the SiC surface on the performance of Ni/SiC methanation catalysts

Ni/SiC methanation catalysts were prepared by an incipient wetness impregnation method. Effects of the oxidation extent of the SiC surface on low-temperature activity and high-temperature stability of the catalysts were investigated. Samples were characterized by thermogravimetry and differential scanning calorimetry, N-2 adsorption-desorption, Fourier transform infrared spectra, temperature-programmed desorption of NH3, X-ray diffraction, temperature-programmed reduction of H-2 and H-2 chemisorption. The surface area and nickel dispersion of the catalysts decreased with increasing oxidation temperature of the SiC supports, while both reducibility and stability of the catalysts increased. The Ni/SiC catalyst with the unoxidized SiC support showed the poorest high-temperature stability probably because of the weak anchorage of Ni particles to the support. The Ni/SiC samples prepared on the SiC supports oxidized at 500 and 700 degrees C had better low-temperature activity and high-temperature stability, which was because Ni particles were well dispersed on and strongly anchored to these properly oxidized supports. The Ni/SiC catalyst with the SiC support oxidized at 900 degrees C showed the worst low-temperature activity because of the larger Ni particles caused by the less active oxide layer due to the overoxidation of the support

Separation of CH4/N2 mixtures in metal–organic frameworks with 1D micro-channels

This work aims to study the features of CH4 and N2 adsorption inside 1D micro-channels and develop the best suitable MOF candidates for the adsorptive separation of CH4 against N2. For this purpose, four MOFs ([Ni3(HCOO)6], [Cu(INA)2], Al-BDC and Ni-MOF-74) with similar network topology and single 1D micro-channel have been systematically investigated via structure characterization and selective gas adsorption and separation. The selected MOFs are classified into three groups. Thereinto, Ni-MOF-74, with coordinatively unsaturated metal sites, is considered as strong polar adsorbent, whilst Al-BDC is treated as moderate polar adsorbent owing to the polar linkers. However, [Ni3(HCOO)6] and [Cu(INA)2] are regarded as apolar (weak polarity) adsorbents because of lack of any polar functional groups inside the frameworks. The adsorption potential of CH4 follows the trend Ni-MOF-74 > [Ni3(HCOO)6] > [Cu(INA)2] > Al-BDC, while Ni-MOF-74 > Al-BDC > [Ni3(HCOO)6] > [Cu(INA)2] for the potential of N2. This implies that pore size and electrostatic interactions have different contributions to the CH4 or N2–MOFs interactions, resulting in excellent CH4/N2 selectivity more than 6 on [Ni3(HCOO)6] and [Cu(INA)2].

Experimental Evaluation of the Adsorption, Diffusion, and Separation of CH4/N2 and CH4/CO2 Mixtures on Al-BDC MOF

Adsorption equilibrium, thermodynamics, and kinetics of CH4, N2, and CO2 were investigated by volumetric-chromatographic and inverse gas chromatographic (IGC) methods on the Al-BDC MOF. The binary adsorption data from the volumetric-chromatographic experiments represents that the Al-BDC MOF has a high CO2/CH4 selectivity ca. 11 and a CH4/N2 selectivity ca. 4.3 at 303 K, and appears to be a good candidate for the CH4 separation. The initial adsorption heats of CH4, N2, and CO2 on the Al-BDC MOF were determined to be 15.3, 11.5, and 32.2 kJmol−1 by IGC method, respectively. Moreover, the micropore diffusivities of N2, CH4 and CO2 in the Al-BDC MOF at 303 K were also estimated to be 1.58 × 10−7 cm2/s, 7.04 × 10−8 cm2/s, and 3.95 × 10−9 cm2/s, respectively. The results indicate that micropores play a crucial role in the adsorptive separation of the CH4/N2 and CH4/CO2 mixtures, and the IGC method is a validity manner to estimate the thermodynamic and kinetic parameters of MOF adsorbents.

Template-based Synthesis of a Formate Metal-Organic Framework/Activated Carbon Fiber Composite for High-performance Methane Adsorptive Separation.

Simultaneous improvement in adsorption selectivity and capacity for single adsorbents is challenging but counting for much in adsorptive separations. To this end, a formate metal-organic framework and activated carbon fiber composite was synthesized in our work by a simple two-step process, involving homogeneous precipitation of a MOF precursor on an activated carbon fiber and subsequent template replication. The resultant core-shell composite, ACF@[Ni3 (HCOO)6 ], exhibited optimized adsorption performance both in selectivity and capacity for the separation of CH4 /N2 to most of state-of-the-art adsorbents.

Targeted Synthesis of Ultrastable High-Silica RHO Zeolite Through Alkali Metal–Crown Ether Interaction

High-silica RHO zeolite was directly synthesized using an alkali metal-crown ether (AMCE) complex as organic structure-directing agent (OSDA). Derived from the UV-vis spectra and zeolite patterns, the crown ether-cesium cation interaction was found to have crucial effect on the enhancement of silica content within the zeolite framework. The synthesized RHO zeolites possess up to four times larger silica/alumina ratio (SAR) values than that in their conventional form, which gives them extraordinarily rigid frameworks even after hydrothermal aging under 800 °C. Compared to commercial zeolites, copper-exchanged high-silica RHO zeolites demonstrate considerably high reaction activity in NOX removal, making them promising candidates for diesel exhaust treatment.

Enhanced Trace Carbon Dioxide Capture on Heteroatom-Substituted RHO Zeolites under Humid Conditions

Boron and copper heteroatoms were successfully incorporated into the frameworks of high-silica RHO zeolite by adopting a bulky alkali-metal-crown ether (AMCE) complex as the template. These heteroatom-doped zeolites show both larger micropore surface areas and volumes than those of their aluminosilicate analogue. Proton-type RHO zeolites were then applied for the separation of CO2 /CH4 /N2 mixtures, as these zeolites have weaker electric fields and, thus, lower heats of adsorption. The adsorption results showed that a balance between working capacity and adsorption heat could be achieved for these heteroatom-doped zeolites. Ideal adsorbed solution theory predictions indicate that these zeolites should have high selectivities even for remarkably dilute sources of CO2 . Finally, the heteroatom-substituted zeolites, especially the boron-substituted one, could be thermally regenerated rapidly at 150 °C after full hydration and maintained high regenerability for up to 30 cycles; therefore, they are potential candidates for trace CO2 removal under humid conditions.

Organic‐Free, ZnO‐Assisted Synthesis of Zeolite FAU with Tunable SiO2/Al2O3 Molar Ratio

Zeolite FAU with tunable SiO2 /Al2 O3 molar ratio has been successfully synthesized in the absence of organic structure-directing agents (OSDA). Specifically, the addition of zinc species contributes to the feasible and effective adjustment of the framework SiO2 /Al2 O3 molar ratio between about 4 and 6 depending on the amount of zinc species added in the batch composition. In contrast, a typical OSDA such as tetramethylammonium hydroxide (TMAOH) has a limited effect on the SiO2 /Al2 O3 molar ratio of the zeolite. The role of zinc species is essential for the crystallization of zeolite FAU with a higher SiO2 /Al2 O3 molar ratio under the particular synthesis conditions. It is speculated that zinc species may suppress the incorporation of aluminum into the aluminosilicate framework, which is due to the Coulombic repulsive interaction. A higher SiO2 /Al2 O3 molar ratio is also found to be accompanied by a lower CO2 adsorption heat for CO2 /CH4 separation.