Water sorption on coal:effects of oxygen-containing function groups and pore structure

Coal-water interactions have profound influences on gas extraction from coal and coal utilization.Experimental measurements on three coals using X-ray photoelectron spectroscopy(XPS),low-temperature nitrogen adsorption and dynamic water vapor sorption(DVS)were conducted.A mechanism-based isotherm model was proposed to estimate the water vapor uptake at various relative humidities,which is well validated with the DVS data.The validated isotherm model of sorption was further used to derive the isosteric heat of water vapor sorption.The specific surface area of coal pores is not the determining parameter that controls water vapor sorption at least during the primary adsorption stage.Oxidation degree dominates the primary adsorption,and which togethering with the cumulative pore volume determine the secondary adsorption.Higher temperature has limited effects on primary adsorption process.The isosteric heat of water adsorption decreases as water vapor uptake increases,which is found to be close to the latent heat of bulk water condensation at higher relative humidity.The results confirmed that the primary adsorption is controlled by the stronger bonding energy while the interaction energy between water molecules during secondary adsorption stage is relatively weak.However,the thermodynamics of coal-water interactions are complicated since the internal bonding interactions within the coal are disrupted at the same time as new bonding interactions take place within water molecules.Coal has a shrinkage/swelling colloidal structure with moisture loss/gain and it may exhibit collapse behavior with some collapses irreversible as a function of relative humidity,which further plays a significant role in determining moisture retention.

Molecular simulation of the CH_4/CO_2/ H_2O adsorption onto the molecular structure of coal

Clarification of the molecular mechanism underlying the interaction of coal with CH4, CO2, and H2 O molecules is the basis for an in-depth understanding of the states of fluid in coal and fluid-induced coal swelling/contraction. In terms of instrumental analysis, molecular simulation technology based on molecular mechanics/dynamics and quantum chemistry is a powerful tool for revealing the relationship between the structure and properties of a substance and understanding the interaction mechanisms of physical-chemical systems. In this study, the giant canonical ensemble Monte Carlo（GCMC） and molecular dynamics（MD） methods were applied to investigate the adsorption behavior of a Yanzhou coal model（C222H185N3O17S5）. We explored the adsorption amounts of CH4, CO2, and H2 O onto Yanzhou coal, the adsorption conformation, and the impact of oxygen-containing functional groups. Furthermore, we revealed the different adsorption mechanisms of the three substances using isosteric heat of adsorption and energy change data.（1） The adsorption isotherms of the mono-component CH4, CO2, and H2 O were consistent with the Langmuir model, and their adsorption amounts showed an order of CH4CO2〉CH4. In addition, at higher temperatures, the isosteric heat of adsorption decreased; pressure had no significant effect on the heat of adsorption.（3） CH4 molecules displayed an aggregated distribution in the pores, whereas CO2 molecules were cross arranged in pairs. Regarding H2 O molecules, under the influence of hydrogen bonds, the O atom pointed to surrounding H2 O molecules or the H atoms of coal molecules in a regular pattern. The intermolecular distances of the three substances were 0.421, 0.553, and 0.290 nm, respectively. The radial distribution function（RDF） analysis showed that H2 O molecules were arranged in the most compact fashion, forming a tight molecular layer.（4） H2 O molecules showed a significantly stratified distribution around oxygen-containing functional groups on the coal surface, and the bonding strength showed a descending order of hydroxyl〉 carboxyl〉carbonyl. In contrast, CO2 and CH4 showed only slightly stratified distributions.（5） After the adsorption of CH4, CO2, and H2 O, the total energy, the energy of valence electrons, and the non-bonding interaction of the system in the Yanzhou coal model all decreased. The results regarding the decrease in the total energy of the system indicated an order of H2O〉CO2〉CH4 in terms of the adsorption priority of the Yanzhou coal model. The results regarding the decrease in the energy of valence electrons showed that under certain geological conditions, a pressure-induced “coal strain” could lead to a structural rearrangement during the interaction of coal with fluid to form a more stable conformation, which might be the molecular mechanism of coal swelling resulting from the interaction between fluid and coal. An analysis of the contribution of Van der Waals forces, electrostatic interactions and hydrogen bonds to the decrease in non-bonding interactions revealed the mechanism underlying the interactions between coal molecules and the three substances. The interaction between coal molecules and CH4 consisted of typical physical adsorption, whereas that between coal molecules and CO2 consisted mainly of physical adsorption combined with weak chemical adsorption. The interaction between coal molecules and H2 O is physical and chemical.

Adsorption of benzene, cyclohexane and hexane on ordered mesoporous carbon

Ordered mesoporous carbon（OMC） with high specific surface area and large pore volume was synthesized and tested for use as an adsorbent for volatile organic compound（VOC）disposal. Benzene, cyclohexane and hexane were selected as typical adsorbates due to their different molecular sizes and extensive utilization in industrial processes. In spite of their structural differences, high adsorption amounts were achieved for all three adsorbates, as the pore size of OMC is large enough for the access of these VOCs. In addition, the unusual bimodal-like pore size distribution gives the adsorbates a higher diffusion rate compared with conventional adsorbents such as activated carbon and carbon molecular sieve. Kinetic analysis suggests that the adsorption barriers mainly originated from the difficulty of VOC vapor molecules entering the pore channels of adsorbents. Therefore, its superior adsorption ability toward VOCs, together with a high diffusion rate, makes the ordered mesoporous carbon a promising potential adsorbent for VOC disposal.