Multi-scale pore fractal characteristics of differently ranked coal and its impact on gas adsorption

Well-developed pores and cracks in coal reservoirs are the main venues for gas storage and migration.To investigate the multi-scale pore fractal characteristics,six coal samples of different rankings were studied using high-pressure mercury injection(HPMI),low-pressure nitrogen adsorption(LPGA-N2),and scanning electron microscopy(SEM)test methods.Based on the Frankel,Halsey and Hill(FHH)fractal theory,the Menger sponge model,Pores and Cracks Analysis System(PCAS),pore volume complexity(D_(v)),coal surface irregularity(Ds)and pore distribution heterogeneity(D_(p))were studied and evaluated,respectively.The effect of three fractal dimensions on the gas adsorption ability was also analyzed with high-pressure isothermal gas adsorption experiments.Results show that pore structures within these coal samples have obvious fractal characteristics.A noticeable segmentation effect appears in the Dv1and Dv2fitting process,with the boundary size ranging from 36.00 to 182.95 nm,which helps differentiate diffusion pores and seepage fractures.The D values show an asymmetric U-shaped trend as the coal metamorphism increases,demonstrating that coalification greatly affects the pore fractal dimensions.The three fractal dimensions can characterize the difference in coal microstructure and reflect their influence on gas adsorption ability.Langmuir volume(V_(L))has an evident and positive correlation with Dsvalues,whereas Langmuir pressure(P_(L))is mainly affected by the combined action of Dvand Dp.This study will provide valuable knowledge for the appraisal of coal seam gas reservoirs of differently ranked coals.

Synthesis, Crystal Structure and Gas Adsorption of a Two-fold Interpenetrated Three-dimensional Framework {[Cd(2-NO_2-BDC)(4,4？-DPA)]·(DMF)}_n

The reaction of Cd（NO_3）_2·4H_2O with 4,4？-dipyridylacetylene（4,4？-DPA） and 2-nitroterephthalic acid（2-NO_2-H_2BDC） in DMF/H_2O mixed solvent has afforded a compound {[Cd（2-NO_2-BDC）（4,4？-DPA）]·（DMF）}_n（1）. Compound 1 has been characterized by single-crystal X-ray diffraction, powder X-ray diffraction, thermogravimetry analysis, and IR spectrum. Compound 1 crystallizes in the monoclinic system, space group P21/n, with a = 12.1488（3）, b = 14.6689（3）, c = 13.1615（3） ？, β = 111.809（3）o, V = 2177.63（9） ？~3, Z = 4, C_（23）H_（18）N_4O_7 Cd, M_r = 574.81, D_c = 1.753 g/cm^3, μ = 8.523 mm^（-1）, F（000） = 1152, the final R = 0.0411 and wR = 0.1064 for 3589 observed reflections with I 〉 2s（I）. In compound 1, the Cd（Ⅱ） ions are linked by the carboxylate groups of 2-NO_2-BDC ligands to give a two-dimensional layered structure based on the centrosymmetric dinuclear Cd_2（COO）_2 units, which are further connected by the 4,4？-DPA ligands to produce a three-dimensional framework with pcu topology. Careful examination revealed that compound 1 is a 2-fold interpenetrating framework. Furthermore, the gas adsorption properties of 1 for N_2 and CO_2 have also been investigated.

Real-time pore structure evolution difference between deep and shallow coal during gas adsorption:A study based on synchrotron radiation small-angle X-ray scattering

Coal seam CO_(2) sequestration is an important option to address global warming.A better knowledge on coal pore structure evolution during gas adsorption can provide guidance for coal seams CO_(2) seques-tration.However,few investigations on the pore structure evolution differences between the deep and shallow coal were conducted during gas adsorption.In this study,based on the real-time synchrotron radiation small-angle X-ray scattering(SAXS)observation,the average pore diameter and pore surface fractal dimension evolution differences between deep and shallow coal were investigated from the as-pects of coal compositions and stress history.Two types of coal deformation(inner-swelling and outer-swelling)coexist during gas adsorption.Coal compositions have significant impact on the dominance of deformation type.The dominance of inner-swelling in deep coal is induced by the higher ash contents,and there is the decrease of average pore diameter during gas adsorption.The impact of stress-history(burial depth)on adsorption-induced deformation is more prominent than that of gas adsorption ca-pacity.In deep coal,the surface fractal dimension evolution presents a negative correlation with the evolution of pore diameters.In shallow coal,the surface fractal dimension evolution presents a Langmuir-type correlation with the adsorption time.

Massive Preparation of Coumarone-indene Resin-based Hyper-crosslinked Polymers for Gas Adsorption

Hyper-crosslinked polymers （HCPs） are promising materials for gas capture and storage because of their low cost and easy preparation. In this work, we report the massive preparation of coumarone-indene resin-based hyper-crosslinked polymers via one-step Friedel-Crafts alkylation. Low-cost coumarone-indene resin serves as the new building block and chloroform is employed as both solvent and external crosslinker. A maximum surface area of 966 m2·g-1 is achieved, which is comparable with that of previously-reported coal tar-based porous organic polymers. Most importantly, a large number of heteroatoms including inherent oxygen atoms and introduced chlorine atoms in obtianed HCPs further enhance the interaction between specific sorbate molecule and adsorbent. Therefore, optimal structural and chemical property endow the new coumarone-indene resin-based HCPs with decent gas storage capacity （14.60 wt% at 273 K and 0.1 MPa for CO2; 1.18 wt% at 77.3 K and 0.1 MPa for H2）. These results demonstrate that new HCPs are potential candidates for applications in CO2 and H2 capture.

A Novel Co(Ⅱ)-organic Framework with Multiple Active Sites for Selective Gas Adsorption

A novel metal-organic framework[Co(BTTA)(H_(2)O)_(2)]_(n)(FJI-H_(2)4)has been prepared from H_(2)BTTA ligand and CoCl_(2),and its structure was determined by single-crystal X-ray diffraction,thermogravimetric analysis,and Fourier transform infrared spectroscopy.It has relatively narrow pores and high density of open metal ions and free Lewis base sites.Gas adsorption tests demonstrate that FJI-H_(2)4 has moderate CO_(2)(34.0 cm^(3)·g^(–1))and C_(2)H_(2)(53.0 cm^(3)·g^(–1))adsorption capacity,but displays high selectivity of CO_(2)/N_(2)(87)and C_(2)H_(2)/CH_(4)(66)under ambient conditions(298 K,1 atm),which may be attributed to its relatively narrow pores and polar environment.This work will provide a potential strategy for preparing practical porous metal-organic frameworks for gas adsorption and purification.

Tetra-armed conjugated microporous polymers for gas adsorption and photocatalytic hydrogen evolution

Two novel tetra-armed conjugated microporous polymers with different geometries have been designed and synthesized via Suzuki-Miyaura cross coupling polycondensation. Both polymers are stable in various organic solvents tested and are thermally stable. The pyrene-containing polymer of PrPy with the rigid pyrene unit shows a higher Brunauer-Emmet-Teller specific surface area of 1219 m^2 g^(-1) than the tetraphenylethylene-containing polymer of PrTPE(770 m^2 g^(-1)), which leads to a high CO_2 uptake ability of 3.89 mmol g^(-1) at 1.13 bar/273 K and a H_2 uptake ability of 1.69 wt% at 1.13 bar/77 K. The photocatalytic hydrogen production experiments revealed that PrPy also shows a better photocatalytic performance than PrTPE due to the higher conjugation degree and planar structure, the broader UV-visible(UV-Vis) absorption, the lower photoluminescence lifetime, and the higher specific surface area.

Component fractionation of temporal evolution in adsorption-desorption for binary gas mixtures on coals from Haishiwan Coal Mine

Adsorption-desorption experiments on CO2-CH4 gas mixtures with varying compositions have been conducted to study the fractionation characteristics of CO2-CH4 on Haishiwan coal samples. These were carried out at constant temperature but different equilibrium pressure conditions. Based on these experimental results, the temporal evolution of component fractionation in the field was investigated. The results show that the CO2 concentration in the adsorbed phase is always greater than that in the original gas mixture during the desorption process, while CH4 shows the opposite characteristics. This has confirmed that CO2 , with a greater adsorption ability has a predominant position in the competition with CH4 under different pressures. Where gas drainage is employed, the ratio of CO2 to CH4 varies with time and space in floor roadways used for gas drainage, and in the ventilation air in Nos.1 and 2 coal seams, which is consistent with laboratory results.

Achieving a stable COF with the combination of“flat”and“twist”large-size rigid synthons for selective gas adsorption and separation

A new type of covalent organic framework(COF)was achieved using combination of structrally rigid and conformationally othorganal building blocks.The N-2-aryl-substituted triazole derivative(NAT-CHO)was prepared with co-planar conformation among the three aromatic rings as the“flat”building block.The 4,4,4,4-(ethene-1,1,2,2-tetrayl)tetraaniline)(ETTA)was applied as the“twist”building block.A 2 D sheet of network was obtained through imine formation.The resulting NAT-COF gave excellent thermal and chemical stability,survived aqueous solutions from p H 5 to 13.With large-size building blocks,the porous framework NAT-COF gave efficient gas adsorption with excellent selectivity of C3 propane over C1 me-thane,suggesting its potential application for selective gas capture and separation.

Determination of gas adsorption capacity in organic-rich marine shale: a case study of Wufeng-Lower Longmaxi Shale in the southeast Sichuan Basin

Determination of gas adsorption capacity under geological conditions is essential in evaluating shale gas resource potential.A quantitative determination of gas adsorption capacity was proposed through 1)investigating controlling geological factors(including both internal ones and external ones)of gas adsorption capacity in organic-rich marine shale with geochemical analysis,XRD diffraction,field-emission scanning electron microscopy,and methane sorption isotherms;2)defining the relationship between gas adsorption capacity and single controlling factor;3)establishing a comprehensive determination model with the consideration of all these controlling factors.The primary controlling factors of the sorption capacity for the studied O3wLower S1l shale are TOC,illite and quartz,temperature,pressure,Ro,and moisture(water saturation).Specifically,TOC,thermal maturity,illite,and pressure are positively correlated with sorption capacity,whereas,quartz and temperature contribute negatively to the sorption capacity.We present the quantitative model along with application examples from the Wufeng-Lower Longmaxi Shale in the southeast Sichuan Basin,west China,to demonstrate the approach in shale gas evaluation.The result shows that the comprehensive determination model provides a good and unbiased estimate of gas adsorption capacities with a high correlation coefficient(0.96)and bell-shaped residues centered at zero.

Porous Organic Frameworks-derived Porous Carbons with Outstanding Gas Adsorption Performance

A series of porous carbon materials was synthesized via high temperature pyrolysis from well-defined and thermally stable precursors, namely porous organic frameworks（POFs）, in inert atmosphere. The porous carbon materials showed enhanced gas adsorption capacities together with increased heat of adsorption and stronger affinity between the frameworks and the gases as compared to the precursor materials. To exemplify, sample C-POF-TBBP-1000 with a high BET surface area of 1290 m^2/g can adsorb 2.8 mmol/g CH4（273 K, 101.325 kPa）, 5.4 mmol/g CO2（273 K, 101.325 kPa） and 2.2% H2（mass fraction, 77 K, 101.325 kPa）, thereby surpassing most other porous adsorbent materials reported till date. The study highlights the potential of porous carbons derived from novel porous organic framework structures for gas adsorption applications.

Effect of Fluorination on the Crystal Structure,Stability and Gas Adsorption Property in Zinc (Ⅱ) Metal-organic Frameworks

Three zinc(Ⅱ) metal-organic frameworks (x F-MAC-3) have been synthesized by using Zn(Ⅱ) salts,3,5-dimethyl-1H-1,2,4-triazole (Hdmtrz) and different fluorination degree carboxylate ligands,which are analogic structures and can be described as (6,6)-connected pcu-b net.We find that the fluorine atoms have structural regulation effect on x F-MAC-3,which can not only enlarge the torsion angle φ of carboxylate ligands but also elevate the space group of structures.Besides,the CO_(2)-273 K uptake increased from 23.21 cm^(3)·g^(-1) (MAC-3) to36.13 cm^(3)·g^(-1) (4F-MAC-3) and H_(2)-77 K uptake increased from 24.33 cm^(3)·g^(-1) (MAC-3) to 59.79 cm^(3)·g^(-1)(4F-MAC-3),which means fluorination can enhance the gas adsorption uptake of x F-MAC-3 analogues.Furthermore,the results of fluorination in x F-MAC-3 analogues offer a potential way to study the ligand pre-functionalization effect on the structures and properties of MOFs analogues.

Effects of O_(2)adsorption on secondary electron emission properties

The surface adsorption of gas molecules is a key factor limiting the secondary electron yield(SEY)of a material in many areas of applied physics.The influence of O_(2)adsorption on the SEY of metallic Ag is investigated in this work.To account for the particle distribution,we propose a BET theory based on multilayer O_(2)physisorption model.Furthermore,based on the phenomenological model of secondary electron(SE)emission and by taking into account the different scattering processes between electrons and particles in the adsorbed layer,we develop a numerical model of SEY in the adsorbed state using Monte Carlo simulations.The relationships among O_(2)adsorption,adsorption layer thickness,and SEY variation characteristics are then examined through a series of experiments.After 12-h exposure to O_(2),the clean samples increases12%-19%of the maximum value of SEY and 2.3 nm in thickness of the adsorbed layer.Experimental results are also compared with the results from the MC model to determine whether the model is accurate.

Adsorption Regularity and Characteristics of sp^3-Hybridized Gas Molecules on Anatase TiO_2(101) Surface

We report the anatase titanium dioxide （101） surface adsorption of sp3-hybridized gas molecules, including NH3, 1-12 0 and CH4, using first-principles plane-wave ultrasoft pseudopotential based on the density functional theory. The results show that it is much easier for a surface with oxygen vacancies to adsorb gas molecules than it is for a surface without oxygen vacancies. The main factor affecting adsorption stability and energy is the polarizability of molecules, and adsorption is induced by surface oxygen vacancies of the negatively charged center. The analyses of state densities and charge population show that charge transfer occurs at the molecule surface upon adsorption and that the number of transferred charge reduces in the order of N, 0 and C. Moreover, the adsorption method is chemical adsorption, and adsorption stability decreases in the order of NH3, tt2 0 and CH4. Analyses of absorption and reflectance spectra reveal that after absorbed CH4 and H2 O, compared with the surface with oxygen vacancy, the optical properties of materials surface, including its absorption coefficients and reflectivity index, have slight changes, however, absorption coefficient and reflectivity would greatly increase after NH3 adsorption. These findings illustrate that anatase titanium dioxide （101） surface is extremely sensitive to NH3.

Synthesis,Crystal Structure and Water Vapor Adsorption Property of a New Supramolecular Architecture

A new complex, [Cu（HL）（phen）（H2O）]·4H2O（1, H3L·HCl = 5-（（4-carboxypiperidin-1-yl）methyl）isophthalic acid hydrochloride, phen = 1,10-phenanthroline）, has been hydrothermally prepared and characterized by single-crystal X-ray diffraction, IR spectroscopy, elemental analysis and PXRD. Complex 1 crystallizes in monoclinic, space group P21/c with a = 14.5520（14）, b = 12.6659（12）, c = 15.5006（14） A, β = 97.224（2）o, V = 2834.3（5） A3, Z = 4, C27H33N3O11 Cu, Mr = 639.10, Dc = 1.498 g/cm3, μ = 0.837 mm-1, S = 1.047, F（000） = 1332, the final R = 0.0423 and w R = 0.1118 for 18772 observed reflections（I 〉 2σ（I））. The compound is a Cu（Ⅱ） centre mononuclear molecule in the asymmetric unit. The independent binuclear [Cu2（HL）2（phen）2] units are bridged to form a three-dimensional（3D） supramolecular polymer by extensive hydrogen bonds and π-π non-covalent bonding interactions. Moreover, thermogravimetric（TG） analysis and gas adsorption property of 1 were also discussed.

Improvement in Sensitivity and Recovery of Single-Walled Carbon Nanotube-Based Gas Sensors

In this paper,an improvement in sensitivity and recovery of the single-walled carbon nanotube (SWNT)-based gas sensors was reported.The sensors were fabricated from SWNT powder by a screen-printing method,followed by annealing for 2 h in open-air at various temperatures.The Raman spectra of the SWNT bundles and the response to ammonia (NH_3) exposure of the sensors indicated that the annealing at a relevant temperature improved the sensor sensitivity.The sensor annealed at 200℃exhibited a high sensitivity in NH_3 detection.After 20 min of exposure to NH_3 at room temperature,the resistance of the sensor increased up to 12% in comparison with its initial value.By evacuating combined with maintaining the flux of carrier gas at 300 sccm,the sensor recovery was significantly speeded.The sensor resistance retrieved to its initial value after only 25 min degassing. These results would be considered in the development of the SWNT-based gas sensors.

Temperature influence on macro-mechanics parameter of intact coal sample containing original gas from Baijiao Coal Mine in China

Investigation of temperature effect on mechanical parameters of coal is very important for understanding the mechanical response of coal bed at high temperature.It is especially benefcial for mitigating the thermal-induced disasters occurred in those coal mines suffering from heat hazard.In this work,coal samples,obtained from the No.2442 working face of Baijiao Coal Mine,were subjected to uniaxial compression ranging from 20 to 40℃ with an interval of 5℃.The apparatus used was designed to obtain deformation of a stressed sample,as well as the emission of gases desorbing from coal matrix.The adsorbed gas desorption caused by heating is measured during the entire testing.It is evident that the concentrations of releasing gas(containing methane,carbon dioxide and ethane)slightly rise with increasing temperature.Gas movement observed is closely related to the deformation of coal sample.Both uniaxial compressive strength and elastic modulus of coal samples tend to reduce with temperature.It reveals that increasing temperature can not only result in thermal expansion of coal,but also lead to desorption of preexisting gas in coal which can in turns harden coal due to shrinks of the coal matrix.Even though desorption of adsorbed gas can contribute to the hardening effect for the heated coal,by comparison to the results,it could be inferred that the softening of coal resulted from thermal expansion still predominates changes in mechanical characters of coal sample with temperature at the range from20 to 40℃.

Apparent activation energy of mineral in open pit mine based upon the evolution of active functional groups

This study aimed to investigate the mechanism of mineral spontaneous combustion in an open pit. On the study of coal and mineral mixture in open pit mines, as well as through the specifc surface area and Search Engine Marketing (SEM) experiments, the specifc surface area and aperture characteristics of distribution of open pit coal sample and pit mineral mixture samples were analyzed. Thermal analysis experiments were used to divide the oxidation process was divided into three stages, and the thermal behavior characteristics of experimental samples were characterized. On the basis of the stage division, we explored the transfer law of the key active functional groups of the experimental samples. The apparent activation energy calculation of the key active groups, performed by combining the Achar diferential method with the Coats–Redfern integral method, microstructural and oxidation kinetic properties were revealed. The resulted showed that the mixed sample had high ash, the fxed carbon content was reduced, the specifc surface area was far lower than the raw coal, the large aperture distribution was slightly higher than the medium hole, the micropore was exceptionally low, the gas adsorption capacity was weaker than the raw coal, the pit coal sample had the exceedingly more active functional groups, easy to react with oxygen, more likely to occur naturally, and its harm was relatively large. The mixed sample contained the highest C–O–C functional group absorbance. The functional groups were mainly infuenced by the self-OH content, alkyl side chain, and fatty hydrocarbon in the sample. The main functional groups of the four-like mixture had the highest apparent activation energy, and the two reactions were higher in the low-temperature oxidation phase.

Controlling effects of differential swelling index on evolution of coal permeability

Coal permeability measurements are normally conducted under the assumption that gas pressure in the matrix is equalized with that in fracture and that gas sorption-induced swelling/shrinking strain is uniformly distributed within the coal.However,the validity of this assumption has long been questioned and differential strain between the fracture strain and the bulk strain has long been considered as the primary reason for the inconsistency between experimental data and poroelasticity solutions.Although efforts have been made to incorporate the impact into coal permeability models,the fundamental nature of those efforts to split the matrix strain between fracture and coal bulk remains questionable.In this study,a new concept of differential swelling index(DSI)was derived to theoretically define the relation among sorption-induced strains of the coal bulk,fracture,and coal matrix at the equilibrium state.DSI was a function of the equilibrium pressure and its magnitudes were regulated by the Langmuir constants of both the matrix and the coal bulk.Furthermore,a spectrum of DSI-based coal permeability models was developed to explicitly consider the effect of differential strains.These models were verified with the experimental data under the conditions of uniaxial strain,constant confining pressure,and constant effective stress.

Hydrothermal synthesis and chromic properties of hexagonal WO_3 nanowires

This paper reports that highly purified hexagonal WO3 nanowires are synthesized by a simple hydrothermal method. The as-synthesized WO3 nanowires are investigated in detail by ultraviolet-visible-near infrared spectroscopy and electrical transport measurements under different conditions. It finds that the optical band gap and the diffuse reflection coefficient in the wavelength region above 450 nm of WO3 nanowires decrease observably upon exposure to ultraviolet light or NH3 gas. It is also found that there are electrons being trapped or released in individual WO3 nanowires when scanning bias voltage in different directions upon exposure to ultraviolet and NH3 gas. The experimental results suggest that the chromic properties might be attributed to the injection/extraction of hydrogen ions induced by ultraviolet light irradiation in air or creation/annihilation of oxygen vacancies induced by NH3 gas exposure, which serve as colour centres and trap electrons as polarons. The experimental results also suggest that the hexagonal WO3 nanowires will be a good candidate for sensing reduced gas such as NH3.

Two open metal sites on the same metal: Dynamics of CO_(2) in MOF UTSA-74--Dedicated to Professor Xiuwen Han on the occasion of her 80th birthday

Metal-organic frameworks(MOFs)are an emerging dass of porous materials with many unique properties that make them promising candidates for carbon dioxide(CO_(2))capture and storage.A better understanding of the behavior of CO_(2) adsorbed inside MOF will enable researchers to develop the ability of designing new MOF based materials with high CO_(2) adsorption capabilities.It is well known that presence of open metal sites(OMSs)can greatly enhance the gas adsorption capability of MOFs.MOF UTSA-74,a framework isomer of well-known MOF-74 has a unique feature that upon activation,a single metal ion has two OMSs.Therefore,it presents a unique opportunity to probe the dynamics of CO_(2) molecules adsorbed on these OMSs.In this work,the CO_(2) adsorptive properties of MOF UTSA-74 are examined with particular attention being paid to the dynamics of the CO_(2) adsorbed on these OMSs.Specifically,variable temperature 13C static solid-state nuclear magnetic resonance(SSNMR)experiments were conducted to directly monitor the behavior of BCO_(2) in UTSA-74 at different loadings.All CO_(2) molecules undergo localized wobbling.At low loadings,some CO_(2) molecules jump among three OMSs from three different Zn atoms in the cOsS section of the channel.Others hop back and forth between the two neighboring OMSs.At high loading,the three site jump has ceased,but two-site hopping persists.The dynamical behavior of CO_(2) in UTSA-74 results from the unique Zn coord ination environment It was discovered that CO_(2) is less mobile in UTSA-74 than in its.framework isomer,MOF-74-Zn.