Alkali metal cation doping of metal-organic framework for enhancing carbon dioxide adsorption capacity

Metal-organic frameworks （MOFs） have attracted much attention as adsorbents for the separation of CO2 from flue gas or natural gas. Here, a typical metal-organic framework HKUST-I（also named Cu-BTC or MOF-199） was chemically reduced by doping it with alkali metals （Li, Na and K） and they were further used to investigate their CO2 adsorption capacities. The structural information, surface chemistry and thermal behavior of the prepared adsorbent samples were characterized by X-ray powder diffraction （XRD）, thermo-gravimetric analysis （TGA） and nitrogen adsorption-desorption isotherm analysis. The results showed that the CO2 storage capacity of HKUST-1 doped with moderate quantities of Li＋, Na＋ and K＋, individually, was greater than that of unmodified HKUST-1. The highest CO2 adsorption uptake of 8.64 mmol/g was obtained with 1K-HKUST-1, and it was ca. 11% increase in adsorption capacity at 298 K and 18 bar as compared with HKUST- 1. Moreover, adsorption tests showed that HKUST-1 and 1K-HKUST-1 displayed much higher adsorption capacities of CO2 than those of N2. Finally, the adsorption/desorption cycle experiment revealed that the adsorption performance of 1K-HKUST-1 was fairly stable, without obvious deterioration in the adsorption capacity of CO2 after 10 cycles.

Adsorption behavior of carbon dioxide and methane in bituminous coal:A molecular simulation study

The adsorption behavior of CO_2, CH_4 and their mixtures in bituminous coal was investigated in this study. First, a bituminous coal model was built through molecular dynamic(MD) simulations, and it was confirmed to be reasonable by comparing the simulated results with the experimental data. Grand Canonical Monte Carlo(GCMC)simulations were then carried out to investigate the single and binary component adsorption of CO_2 and CH_4with the built bituminous coal model. For the single component adsorption, the isosteric heat of CO_2 adsorption is greater than that of CH_4 adsorption. CO_2 also exhibits stronger electrostatic interactions with the heteroatom groups in the bituminous coal model compared with CH_4, which can account for the larger adsorption capacity of CO_2 in the bituminous coal model. In the case of binary adsorption of CO_2 and CH_4mixtures, CO_2 exhibits the preferential adsorption compared with CH_4 under the studied conditions. The adsorption selectivity of CO_2 exhibited obvious change with increasing pressure. At lower pressure, the adsorption selectivity of CO_2 shows a rapid decrease with increasing the temperature, whereas it becomes insensitive to temperature at higher pressure. Additionally, the adsorption selectivity of CO_2 decreases gradually with the increase of the bulk CO_2 mole fraction and the depth of CO_2 injection site.

Parametric study and effect of calcination and carbonation conditions on the CO_2 capture performance of lithium orthosilicate sorbent

The world is currently facing the challenges of global warming and climate change. Numerous efforts have been taken to mitigate CO2 emission, among which is the use of solid sorbents for CO2 capture. In this work, Li4SiO4 was synthesised via a sol-gel method using lithium nitrate （LiNO3） and tetraethylorthosilicate （SiC8H20O4） as precursors. A parametric study of Li：Si molar ratio （1-5）, calcination temperature （600-800℃） and calcination time （1-8 h） were conducted during sorbent synthesis. Calcination temperature （700-800℃） and carbonation temperature （500-700℃） during CO2 sorption activity were also varied to confirm the optimum operating temperature. Sorbent with the highest CO2 sorption capacity was finally introduced to several cyclic tests to study the durability of the sorbent through 10 cycles of CO2 sorption-desorption test. The results showed that the calcination temperature of 800℃ and carbonation temperature of 700℃ were the best operating temperatures, with CO2 sorption capacity of 7.95 mmol CO2·（g sorbent）^-1 （93% of the theoretical yield）. Throughout the ten cyclic processes, CO2 sorption capacity of the sorbent had dropped approximately 16.2% from the first to the tenth cycle, which was a reasonable decline. Thus, it was concluded that Li4SiO4 is a potential CO2 solid sorbent for high temperature CO2 capture activity.

Capture of carbon dioxide from flue gas on TEPA-grafted metal-organic framework Mg_2 (dobdc)

Carbon dioxide （CO2） adsorption on a standard metal-organic framework Mg2（dobdc） （Mg/DOBDC or Mg-MOF-74） and a tetraethylenepentamine （TEPA） modified Mgz（dobdc） （TEPA-Mg/DOBDC） were investigated and compared. The structural information, surface chemistry and thermal behavior of the adsorbent samples were characterized by X-ray powder diffraction （XRD）, infrared spectroscopy （IR）, thermogravimetric analysis （TGA） and nitrogen adsorption-desorption isotherm analysis. CO2 adsorption capacity was measured by dynamic adsorption experiments with N2-CO2 mixed gases at 60℃. Results showed that the CO2 adsorption capacity of Mg/DOBDC was significantly improved after amine modification, with an increase from 2.67 to 6.06 mmol CO2/g adsorbent. Moreover, CO2 adsorption on the TEPA-Mg/DOBDC adsorbent was promoted by water vapor, and the adsorption capacity was enhanced to 8.31 mmol CO2/g absorbent. The adsorption capacity of the TEPA-Mg/DOBDC adsorbent dropped only 3% after 5 consecutive adsorption]desorption cycles. Therefore, this kind of adsorbent can be considered as a promising material for the capture of CO2 from flue gas.

Numerical simulations and comparative analysis of two- and three-dimensional circulating fluidized bed reactors for CO2 capture

Carbon dioxide(CO2),the main gas emitted from fossil burning,is the primary contributor to global warming.Circulating fluidized bed reactor(CFBR)is proved as an energy-efficient method for post-combustion CO2 capture.The numerical simulation by computational fluid dynamics(CFD)is believed as a promising tool to study CO2 adsorption process in CFBR.Although three-dimensional(3D)simulations were proved to have better predicting performance with the experimental results,two-dimensional(2D)simulations have been widely reported for qualitative and quantitative studies on gas-solid behavior in CFBR for its higher computational efficiency recently.However,the discrepancies between 2D and 3D simulations have rarely been evaluated by detailed study.Considering that the differences between the 2D and 3D simulations will vary substantially with the changes of independent operating conditions,it is beneficial to lower computational costs to clarify the effects of dimensionality on the numerical CO2 adsorption runs under various operating conditions.In this work,the comparative analysis for CO2 adsorption in 2D and 3D simulations was conducted to enlighten the effects of dimensionality on the hydrodynamics and reaction behaviors,in which the separation rate,species distribution and hydrodynamic characteristics were comparatively studied for both model frames.With both accuracy and computational costs considered,the viable suggestions were provided in selecting appropriate model frame for the studies on optimization of operating conditions,which directly affect the capture and energy efficiencies of cyclic CO2 capture process in CFBR.

Mean Residence Time of CO2 Molecules in Flexible ZIF-8 Cages Explored by Molecular Dynamics Simulations

The adsorption sites and diffusion mechanism of CO2 molecules in the flexible Zn（MeIM）2 （MeIM=2-methylimidazole） （ZIF-8） have been investigated by grand canonical Monte Carlo and molecular dynamics simulations. A reasonable time correlation function is for the first time constructed to explore the mean residence time of CO2 molecules in the ZIF-8 cages, suggesting that C02 molecules can remain in the same cage for up to several tens of picoseconds. Furthermore, we find that the mean residence time almost linearly increases with the increasing pressure （or loading） at 273 and 298 K.

Recent progresses on improving CO_(2)adsorption and proton production for enhancing efficiency of photocatalytic CO_(2)reduction by H_(2)O

Photocatalytically reducing CO_(2)by H_(2)O into valuable carbon-containing chemicals is one of the greatest concerns for both scientific and industrial communities,due to its great potential in solving energy and environmental problems.However,the photocatalytic reduction efficiency is still much lower than the need of large-scale applications.Abilities of photocatalysts in adsorbing CO_(2)and splitting H_(2)O to produce protons are crucial factors determining the photocatalytic reduction efficiency.Significant research efforts have been devoted to addressing the issues on CO_(2)adsorption and proton production.The present review discusses the recent progresses in improving CO_(2)adsorption and proton production on photocatalysts for increasing the photocatalytic reduction efficiency.Future research opportunities and challenges are also discussed.It is hoped that the present review can stimulate more deep insights on adsorption of CO_(2)and production of proton for increasing the photocatalytic reduction efficiency。

Microwave-assisted Ionothermal Synthesis and Characteriza- tion of Zeolitic Imidazolate Framework-8

The zeolitic imidazolate framework-8 （ZIF-8） was successfully synthesized using ionic liquids as struc- ture-directing agent under microwave irradiation. Ionic liquids are green solvents with low vapour pressure and good thermal stability. They are appropriate templates for microporous materials and ideal microwave absorbers. The microwave-assisted ionothermal synthesis applied in this paper was expected to be a promising method for the preparation of microporous materials. Results showed that the as-synthesized samples （300---500 nm in diameter） could be synthesized in a short time （60 min） and possessed regular morphology, stable structure and high thermal stability （up to 720 ~C in argon atmosphere）. Nitrogen adsorption-desorption test illustrated that samples produced by microwave heating had a higher surface area. Carbon dioxide adsorption test indicated that the samples synthe- sized by microwave heating had better carbon dioxide adsorption ability than those by conventional heating.