Zhongzheng Zhang

Facile one-pot synthesis of mesoporous carbon and N-doped carbon for CO2 capture by a novel melting-assisted solvent-free method

A facile and efficient one-pot melting-assisted solvent-free method was successfully developed for the first time to produce hierarchically mesoporous carbon and nitrogen-enriched mesoporous carbon materials. This method used a very simple thermal treatment process instead of normally reported solvent-based preparations, thus making it potentially very applicable for fast and large scale production of mesoporous carbons. The obtained carbon materials were comprehensively characterized by X-ray diffraction, Raman spectroscopy, N2 sorption, scanning electron microscopy, transmission electron microscopy, CHN analysis, X-ray photoelectron spectroscopy, and elemental mapping. The results show that the as-synthesized carbon materials possess well-developed hierarchical porous structures, uniform pore sizes, and high surface areas, and the specific structures can be adjusted by changing the temperature and duration of the thermal treatment process. Moreover, the resultant carbon material with a high surface area of 748 m2 g−1 exhibits excellent CO2 capture properties with a capacity of 2.73 mmol g−1 at 298 K and 1 bar, and a CO2 selectivity of 21.6 under flue gas conditions. More importantly, due to the successful incorporation of large amounts of highly dispersed N in the carbon matrix (11.67%), the as-synthesized NMC sample exhibits a significantly enhanced CO2 capacity of 1.66 mmol g−1 with an excellent CO2 selectivity of 240.7 at 348 K and 1 bar, revealing great promise for practical CO2 capture applications.

Chromium-based metal–organic framework/mesoporous carbon composite: synthesis, characterization and CO2 adsorption

New composites of a water-stable chromium-based metal organic framework MIL-101 and mesoporous carbon CMK-3 were in situ synthesized with different ratios of MIL-101 and CMK-3 using the hydrothermal method. The composites as well as the parent materials were characterized by X-ray diffraction, thermo gravimetric analysis, scanning electron microscope, transmission electron microscope and nitrogen/carbon dioxide adsorption isotherms. The hybrid material possesses the same crystal structure and morphology as its parent MIL-101, and exhibits an enhancement in CO2 adsorption uptakes when compared to MIL-101 and CMK-3. The increase in CO2 uptakes was attributed to the combined effect of the formation of additional micropores, the enhancement of micropore volume and the activation of unsaturated metal sites by CMK-3 incorporation.

Experimental Measurement of the Adsorption Equilibrium and Kinetics of CO2 in Chromium-Based Metal-Organic Framework MIL-101

A series of MIL-101 samples with hierarchical pore structures and different surface areas (2000–4800 m2/g) was synthesized by hydrothermal method and characterized by X-ray diffraction, thermogravimetric analysis and 77-K N2 adsorption isotherm. The adsorption equilibrium and kinetics of CO2 were studied at 288, 298 and 308 K within a pressure range of 0–5 MPa by a volumetric method. The adsorption heat, mass-transfer constant, diffusion coefficient and diffusion activation energy were also investigated in this work. The results showed that the chromium-based MIL-101 adsorbent exhibited an impressive selectivity for CO2 over N2 and had an adsorption capacity of 20 mmol/g of CO2 at 298 K and 5.0 MPa, which was much better than that of other conventional adsorbents (e.g. SBA-15, MCM-41, SG-A). The adsorption heat of CO2 on MIL-101 was in the range of 21–45 kJ/mol, which decreased with the loading of CO2. The mass-transfer constants and diffusion coefficients increased with the temperature and decreased with the pressure, whereas the diffusion activation energy decreased with the increased pressure, indicating that adsorption of CO2 at high pressures was easier. In addition, a linear correlation was found between CO2 uptake and surface areas at low pressure, which showed that the adsorption capacity of CO2 could be controlled by adjusting the surface area of the prepared adsorbents in this condition.

Enhancing low pressure CO2 adsorption of solvent-free derived mesoporous carbon by highly dispersed potassium species

Highly dispersed potassium species were introduced on a mesoporous carbon surface following an oxidation and subsequent ion exchange protocol. The samples were characterized and their CO2 adsorption performance was systematically evaluated by both static and dynamic adsorption tests. It was found that the generated surface functionality can be tuned by controlling the reaction temperature and/or using different oxidant(s), and thus potassium species can be introduced in an adjustable way without significant alteration on the textural properties of the samples. Although adsorption at atmospheric pressure was not influenced, low pressure CO2 uptake and adsorption selectivity were considerably enhanced by potassium introduction owing to the highly dispersed potassium species. A high CO2 adsorption capacity of 5.9 wt% was achieved at 25 °C and 0.15 bar with excellent cyclic stability, and the adsorbents can be readily regenerated at 115 °C under N2 purging.

Facilely controlled synthesis of a core-shell structured MOF composite and its derived N-doped hierarchical porous carbon for CO2 adsorption

A new strategy for controlled synthesis of a MOF composite with a core–shell structure, ZIF-8@resorcinol–urea–formaldehyde resin (ZIF@RUF), is reported for the first time through in situ growth of RUF on the surface of ZIF-8 nanoparticles via an organic–organic self-assembly process by using hexamethylenetetramine as a formaldehyde-releasing source to effectively control the formation rate of RUF, providing the best opportunity for RUF to selectively grow around the nucleation seeds ZIF-8. Compared with the widely reported method for MOF composite synthesis, our strategy not only avoids the difficulty of incorporating MOF crystals into small pore sized materials because of pore limitation, but also effectively guarantees the formation of a MOF composite with a MOF as the core. After carbonization, a morphology-retaining N-doped hierarchical porous carbon characterized by its highly developed microporosity in conjunction with ordered mesoporosity was obtained. Thanks to this unique microporous core–mesoporous shell structure and significantly enhanced porosity, simultaneous improvements of CO2 adsorption capacity and kinetics were achieved. This strategy not only paves a way to the design of other core–shell structured MOF composites, but also provides a promising method to prepare capacity- and kinetics-increased carbon materials for CO2 capture.

Potassium Tethered Carbons with Unparalleled Adsorption Capacity and Selectivity for Low-Cost Carbon Dioxide Capture from Flue Gas

Carbons are considered less favorable for postcombustion CO2 capture because of their low affinity toward CO2, and nitrogen doping was widely studied to enhance CO2 adsorption, but the results are still unsatisfactory. Herein, we report a simple, scalable, and controllable strategy of tethering potassium to a carbon matrix, which can enhance carbon-CO2 interaction effectively, and a remarkable working capacity of ca. 4.5 wt % under flue gas conditions was achieved, which is among the highest for carbon-based materials. More interestingly, a high CO2/N2 selectivity of 404 was obtained. Density functional theory calculations evidenced that the introduced potassium carboxylate moieties are responsible for such excellent performances. We also show the effectiveness of this strategy to be universal, and thus, cheaper precursors can be used, holding great promise for low-cost carbon capture from flue gas.