Narrow Pore System Research Articles

Application of Thermal Response Measurements to Investigate Enhanced Water Adsorption Kinetics in Ball-Milled C2 N-Type Materials.

Sorption-based water capture is an attractive solution to provide potable water in arid regions. Heteroatom-decorated microporous carbons with hydrophilic character are promising candidates for water adsorption at low humidity, but the strong affinity between the polar carbon pore walls and water molecules can hinder the water transport within the narrow pore system. To reduce the limitations of mass transfer, C2 N-type carbon materials obtained from the thermal condensation of a molecular hexaazatriphenylene-hexacarbonitrile (HAT-CN) precursor were treated mechanochemically via ball milling. Scanning electron microscopy as well as static light scattering reveal that large pristine C2 N-type particles were split up to a smaller size after ball milling, thus increasing the pore accessibility which consequently leads to faster occupation of the water vapor adsorption sites. The major aim of this work is to demonstrate the applicability of thermal response measurements to track these enhanced kinetics of water adsorption. The adsorption rate constant of a C2 N material condensed at 700 °C remarkably increased from 0.026 s-1 to 0.036 s-1 upon ball milling, while maintaining remarkably high water vapor capacity. This work confirms the advantages of small particle sizes in ultramicroporous materials on their vapor adsorption kinetics. It is demonstrated that thermal response measurements are a valuable and time-saving method to investigate water adsorption kinetics, capacities, and cycling stability.

Efficient adsorptive separation of propene over propane through a pillar‐layer cobalt‐based metal–organic framework

AbstractThe purification of propene from the propene/propane binary mixture is one of the most important and challenging separation processes in the chemical industry. In this study, a cobalt‐based pillar‐layer metal–organic framework, Co(AIP)(BPY)0.5, was synthesized. It exhibited excellent water and moisture stability and efficient C3H6/C3H8 separation performance. At 298 K and 100 kPa, the adsorption capacity of C3H6 was 1.99 mmol/g and the IAST adsorption selectivity was 21, which has exceeded most investigated MOFs adsorbents. The cyclic breakthrough experiments of C3H6/C3H8 binary mixture confirmed its efficient dynamic separation property and excellent recyclability. The molecular simulation showed that the C3H8 molecules with multiple binding sites (C─H) were restricted at the middle of the one‐dimensional channel. Compared with C3H6, the large steric hindrance of C3H8 caused lower diffusion rate (with a C3H6/C3H8 kinetic selectivity of 29.7, at 303 K) in the narrow pore system of 1, which has been confirmed by the kinetic adsorption experiments.

Thermal dislocation and physical changes during preparation of active silicate of group 2 Periods 3, 4 and 5 elements from spent fines of finishing crystal glass

This study reports on the thermal dislocation and crystal growth changes during preparation of active metasilicate of group 2 periods 3.4 and 5 elements from waste fines of finishing process of crystal glass (CG) industry. Sodium carbonate fused the fines at ≈1273 K for 120-180 minutes followed by water leaching. Alkali silicate goes into solution. Primary chloride and sulphate salts of elements of group 2 periods 2, 4 and 5 added to the alkali silicate solution at temperatures 300 K to 353 K for 60 minutes to give metasilicate as a white precipitate followed by heating at 473 K to 1073 K for 30 minutes. The study discussed the physical changes taking place after heating. Results revealed that surface area and adsorption capacity of Mn ions from underground water increased with the surface area of the silicate associating the decrease in atomic radius of the element. The effect of pH, concentration of the reactants and temperature on the synthesis and physical changes of the silicate salts has been investigated. Formation of the silicate salt proceeded in a reaction sequence involving atom rearrangement to form a honeycomb structure with very narrow pore diameter amounting to 1.538 Ằ. Heating silicate at 773 K dislocate the oxygen atoms around metal atoms to form unit crystals with mild pore diameter of 15.29 Ằ. With further increase in temperature, narrow pore system collapses to form sponge structure with wider pore diameter and lower surface area. The manuscript tabulated the physical changes in digital presentation derived mathematically.

Polymer translocation dynamics in the quasi-static limit

Monte Carlo (MC) simulations are used to study the dynamics of polymer translocation through a nanopore in the limit where the translocation rate is sufficiently slow that the polymer maintains a state of conformational quasi-equilibrium. The system is modeled as a flexible hard-sphere chain that translocates through a cylindrical hole in a hard flat wall. In some calculations, the nanopore is connected at one end to a spherical cavity. Translocation times are measured directly using MC dynamics simulations. For sufficiently narrow pores, translocation is sufficiently slow that the mean translocation time scales with polymer length N according to <τ> ∝ (N - N(p))(2), where N(p) is the average number of monomers in the nanopore; this scaling is an indication of a quasi-static regime in which polymer-nanopore friction dominates. We use a multiple-histogram method to calculate the variation of the free energy with Q, a coordinate used to quantify the degree of translocation. The free energy functions are used with the Fokker-Planck formalism to calculate translocation time distributions in the quasi-static regime. These calculations also require a friction coefficient, characterized by a quantity N(eff), the effective number of monomers whose dynamics are affected by the confinement of the nanopore. This was determined by fixing the mean of the theoretical distribution to that of the distribution obtained from MC dynamics simulations. The theoretical distributions are in excellent quantitative agreement with the distributions obtained directly by the MC dynamics simulations for physically meaningful values of N(eff). The free energy functions for narrow-pore systems exhibit oscillations with an amplitude that is sensitive to the nanopore length. Generally, larger oscillation amplitudes correspond to longer translocation times.

Hypercrosslinked Aromatic Heterocyclic Microporous Polymers: A New Class of Highly Selective CO2 Capturing Materials

Aromatic heterocyclic microporous polymers with high surface areas are obtained by directly crosslinking of the heterocyclic monomers under mild conditions. Owing to the narrow pore system and the heteroatom-rich pore surface, these networks exhibit high CO(2) adsorption capacity and selectivity. At 273 K, the CO(2)/N(2) selectivity of Py-1 is about 117, which is among the highest the reported microporous materials.

Monitoring adsorption-induced switching by 129Xe NMR spectroscopy in a new metal–organic framework Ni2(2,6-ndc)2(dabco)

The synthesis and structure of a new flexible metal-organic framework Ni(2)(2,6-ndc)(2)(dabco) (DUT-8(Ni), DUT = Dresden University of Technology, 2,6-ndc = 2,6-naphthalenedicarboxylate, dabco = 1,4-diazabicyclo[2.2.2]octane) as well as its characterization by gas adsorption and (129)Xe NMR spectroscopy is described. The compound shows reversible structural transformation without loss of crystallinity upon solvent removal and physisorption of several gases. Xenon adsorption studies combined with (129)Xe NMR spectroscopy turn out to be favorable methods for the detection and characterization of the so-called "gate-pressure" effect in this novel MOF material. The linewidth and chemical shift of the (129)Xe NMR signal are shown to be very sensitive parameters for the detection of this structural transition from a narrow pore system with low porosity to a wide pore state. The transition and threshold temperature is clearly detected.

Calculation of the adsorption-desorption hysteresis curves in narrow-pore systems

The gas and liquid spinodal branches for an adsorbate located in narrow slit-shaped, cylindrical, and spherocylindrical pores were calculated. The adsorbate is modeled by Lennard-Jones spherical particles. The calculation was based on the lattice gas model taking into account the intermolecular interactions of nearest neighbors in the quasichemical approximation. The density-temperature diagrams for the gas and liquid spinodal branches in the pores are similar to the equilibrium vapor-liquid phase diagrams: they have a common critical point; the dense-phase branches are shifted to lower pore fillings, while the rarefied-phase branches are shifted toward higher pore fillings. The width of adsorption-desorption hysteresis loop in the adsorption isotherms for Lennard-Jones particles was analyzed as a function of the pore size and the interaction potential of the adsorbate with the pore walls. The effect of pore wall roughness and the accuracy of isotherm calculation on the width of the adsorption-desorption hysteresis loop in cylindrical pores is discussed

Calculation of the equilibrium and transport characteristics of gaseous and liquid mixtures of noble gases in complex narrow-pore systems

The molecular-kinetic theory of transfer of the components of the gaseous and liquid mixtures of inert gases is generalized to the case of narrow pores of complex structure. The theory is based on the latticegas model, which provides a self-consistent method for calculating the equilibrium and transport characteristics of noble gases and liquids. This model is used to describe the supramolecular structure of finely dispersed bodies and to calculate the distributions of the mixture components within the pore space. The supramolecular structure is composed of slitlike, cylindrical, spherical, and globular model elements and their joints. An analysis of the effect of the molecular properties of the mixture components, physicochemical properties of the pore walls and the width of the cylindrical and complex sections of variable diameter of spherocylindrical pores on the concentration dependences of the local label transfer coefficients and viscosity coefficients for mixtures of spherical molecules at supercritical temperatures range is performed. Correlations between the partial degrees of filling of pores with mixture components and the local dynamic characteristics of the mixture in the spherocylindrical systems are considered. It is shown that an increase in the binding energy increases the local flow viscosity and reduces the diffusion coefficients of the labeled mixture components.

Carbon molecular sieve membranes: Sorption, kinetic and structural characterization

Carbon molecular sieve membranes (CMSM) from Carbon Membranes Ltd. (Israel) were characterized. These membranes possess a very narrow pore system, in the range of 0.3–0.5 nm, and a sharp pore size distribution, allowing for high selectivities and permeabilities in gas separations. Penetrants O 2, N 2, CO 2, SF 6, Xe and He were used in monocomponent adsorption, permeability and uptake experiments for obtaining adsorption and diffusion data, leading to new insights on the internal pore structure of CMSM. A number of complementary morphological characterizations were also performed. The permeability and adsorption data were obtained over a temperature range from 273 to 323 K and over a pressure up to 5 bar. A relation between the adsorption and permeability data was used, allowing for the estimation of the diffusivity coefficients, which were further compared with those directly obtained from the uptake experiments. Permeation and uptake experiments showed a similar strong dependence of CO 2 diffusivity on concentration.

Molecular grounds of the calculation of equilibrium and transport characteristics of inert gases and liquids in complex narrow-pore systems

A self-consistent approach to the calculation of equilibrium and transport characteristics of inert gases and liquids in complex narrow-pore systems based on the lattice-gas model is proposed. A supramolecular structure for fine-grained solids was constructed and the adsorbate distribution within the pore volume is described. The supramolecular structure is simulated using slit-shaped, cylindrical, spherical, and globular segments. Additionally, junctions of pore systems with different structures are included, and the heterogeneity of their walls and the presence of structural defects in the pore segments are taken into account. The distributions of molecules are described in the quasi-chemical approximation to take into account intermolecular interactions using calibration functions to correct this approximation in the near-critical area. Expressions for local and integrated flow transfer coefficients are constructed, in particular, self-diffusion, shear viscosity and heat conductivity. The contributions of the near-wall areas and the core parts of pores to the general form of phase diagrams, the effect of the pore size on the conditions of capillary condensation, and the role of surface mobility of molecules are discussed.

Statistics of an ideal polymer chain near the bifurcation region of a narrow tube

Conformational statistics of an ideal polymer chain in the vicinity of the bifurcation region of a narrow pore system is considered. It is shown that the bifurcation region plays the role of an effective entropic trap. Due to this fact the chain adopts globular conformation with the average dimensions of the order of the dimensions of the bifurcation region. It is concluded that the statistical properties of an ideal polymer chain and of random walks near the bifurcation point are essentially different.