Narrow Slit-like Pores Research Articles

Microscopic pore structure characteristics and controlling factors of marine shale: a case study of Lower Cambrian shales in the Southeastern Guizhou, Upper Yangtze Platform, South China

A systematic study of the pore structure characteristics of Lower Cambrian shales in the southeastern Upper Yangtze Platform, was conducted using organic geochemistry, mineralogy, nitrogen adsorption, physical property analysis, and scanning electron microscopy. The results indicate that: 1) The Total organic carbon (TOC) content shows a strong correlation with quartz and clay minerals. Shales with low TOC content and rich in clay minerals primarily exhibit slit-shaped and narrow slit-like inter-clay particle pores with pore size distribution is dominated by mesopores and macropores. Shales with high TOC content predominantly feature narrow slit-like and ink bottle-shaped pores with pore size distribution dominated by micropores and mesopores. 2) Shale pore structures vary significantly under different gas content and preservation conditions. Shales under favorable preservation conditions exhibit a relatively “high porosity, low permeability, and high gas content” pattern, with well-developed organic pores and a strong pore-permeability correlation. In contrast, shales under unfavorable preservation conditions appear dense, with excessively developed fractures increasing both average pore size and local permeability. The pore-permeability correlation is weak, presenting a relatively “low porosity, high permeability, and low gas content” pattern. 3) TOC content plays a crucial role in controlling pore structure, showing overall positive correlations with pore volume, specific surface area, and porosity, and negative correlations with pore size. High TOC content enhances shale plasticity, resulting in lower pore diameters. Factors such as compaction and unfavorable preservation conditions lead to the shrinkage, collapse, and closure of some narrow pore throats, negatively impacting pore volume, specific surface area, brittleness, and fractal dimension, exhibiting a negative correlation with TOC content. 4) The pore structure of Lower Cambrian shales is complex, with fractal dimensions D1 and D2 exhibiting negative correlations with average pore size and positive correlations with TOC, specific surface area, and total pore volume. A high D1 value indicates well-preserved nanoscale pore surface structures with low complexity, suggesting minimal alteration by external fluids and better shale gas preservation. D1 serves as an indicator for shale gas content and preservation conditions. D2 shows better correlations with various pore structure parameters, making it suitable for characterizing pore structures.

MOLECULAR SIMULATION OF WATER STRUCTURE IN NARROW SLITLIKE PORES

The structure of water in narrow slitlike pores has been studied by the methods of molecular dynamics simulation. Pores with interwall distances of 6.2–15.5 Å have been considered. Water structures resulting from spontaneous crystallization upon cooling to T = 300 K have been clarified on the basis of twoand three-dimensional order parameters. It has been shown that the observed structures can be described as sections of FCC or HCP crystals.

From n-layer planar ordering to the monolayer homeotropic structure of confined hard rods: The effect of shape anisotropy and wall-to-wall separation.

Using the Parsons-Lee theory, we examined the effect of shape anisotropy and the wall-to-wall separation (H) on the phase behavior of the hard parallelepiped rods with dimensions L, D, and D (L>D) in such narrow slitlike pores which only one homeotropic layer can form. The phase structures, including biaxiality, planar nematic layering transition as well as planar to homeotropic, were studied for some separations in the range 2.5D≤H≤10.0D for H-D≤L<H.

Comparisons of activated carbons produced from sycamore balls, ripe black locust seed pods, and Nerium oleander fruits and also their H2 storage studies

Starting materials are very significant to produce activated carbons because every starting material has a different chemical structure; hence they affect the surface functional groups and surface morphologies of obtained activated carbons. In this study, sycamore balls, ripe black locust seed pods, and Nerium oleander fruits have been used as starting materials by ZnCl2 chemical activations for the first time. Firstly, activated carbons were obtained from these starting materials with ZnCl2 chemical activation by changing production conditions (carbonization time, carbonization temperature, and impregnation ratio) also affecting the structural and textural properties of the resultant activated carbons. Then, the starting materials and resultant activated carbons were characterized by utilizing diverse analysis techniques, such as TGA, elemental analysis, proximate analysis, BET surface areas, pore volumes, pore size distributions, N2 adsorption–desorption isotherms, SEM, FTIR spectra, and H2 adsorption isotherms. The highest surface areas were determined to be 1492.89, 1564.84, and 1375.47 m2/g for the activated carbons obtained from sycamore balls, ripe black locust seed pods, and N. oleander fruits, respectively. The yields of these activated carbons with the highest surface areas were calculated to be around 40%. As the carbonization temperature increased with sufficient ZnCl2 amount, N2 adsorption–desorption isotherms began to turn into Type IV isotherms given by mesoporous adsorbents with its hysteresis loops. Also, their hysteresis loops resembled Type H4 loop generally associated with narrow slit-like pores. Moreover, hydrogen uptakes under 750 mmHg at 77 K were determined to be 1.31, 1.48, and 1.24 wt% for the activated carbons with the maximum surface areas produced from sycamore balls, ripe black locust seed pods, and N. oleander fruits, respectively. As a result, the highest surface areas of the activated carbons with different structural properties produced in this study were obtained with different production conditions.

Orientational ordering and layering of hard plates in narrow slitlike pores

We examine the ordering behavior of hard platelike particles in a very narrow, slitlike pore using the Parsons-Lee density functional theory and the restricted orientation approximation. We observe that the plates are orientationally ordered and align perpendicularly (face-on) to the walls at low densities, a first-order layering transition occurs between uniaxial nematic structures having n and n+1 layers at intermediate densities, and even a phase transition between a monolayer with parallel (edge-on) orientational order and n layers with a perpendicular one can be detected at high densities. In addition to this, the edge-on monolayer is usually biaxial nematic, and a uniaxial-biaxial nematic phase transition can be also seen at very high densities.

Effect of lithofacies on pore structure and new insights into pore-preserving mechanisms of the over-mature Qiongzhusi marine shales in Lower Cambrian of the southern Sichuan Basin, China

Research on shale pore structures has furthered the understanding of shale oil and gas accumulation. However, understanding of mechanisms of shale lithofacies that affect pore structures remains poor, restricting evaluations of marine shale and accurate estimations of gas content. This study focuses on the over-mature Qiongzhusi marine shales in Lower Cambrian of the Southern Sichuan Basin. A series of laboratory experiments were conducted on core samples, including X-ray diffraction (XRD), scanning electron microscopy (SEM), low-pressure gas adsorption (N2 and CO2) and high-pressure mercury intrusion porosimetry (MIP). XRD results show that four main types of shale lithofacies are found in the Qiongzhusi Formation according to the content of siliceous, carbonate and clay minerals, including silica-rich argillaceous shale lithofacies (SAL), argillaceous-rich siliceous shale lithofacies (ASL), carbonate/argillaceous containing siliceous shale lithofacies (C/ASL) and argillaceous/siliceous mixed shale lithofacies (A/SML). SEM images reveal that three kinds of pores within the shale samples (organic matter, intergranular and dissolved pores) with the majority being slit-like and narrow slit-like pores. Silica-rich argillaceous shale lithofacies exhibit the highest pore surface areas and volumes, while other shale lithofacies and carbonate/clay containing siliceous shale lithofacies exhibit relatively lower pore surface areas and volumes. It can be inferred that silica-rich argillaceous shale lithofacies facilitate shale gas storage and flow. Both TOC and clay mineral content are more positively correlated with the microscopic pore structure parameters of micropores than that of mesopores, and thus further interpreted as two dominant factors controlling the development and distribution of micropores. A relatively high quartz content in Qiongzhusi shale of Lower Cambrian are conductive to contribute a large macropore volume, but most importantly, the development of rigid frameworks established by authigenic quartz/pyrite and clay minerals are favorable for the preservation of primary organic matter micropores by providing effective grain supporters. Clay-rich siliceous shale could be deemed as one of the most typical lithofacies types displaying this unique pore-preserving mechanism. Shale lithofacies with different TOC and mineral compositions exhibit distinct differences in pore structures.

Biaxial layering transition of hard rodlike particles in narrow slitlike pores

The phase behavior of hard rectangular rods with length L and diameter D is studied in a narrow slit-like pore using the Parsons-Lee density functional theory. Using the restricted orientation approximation, we find strong adsorption at the walls with planar ordering, second order uniaxial-biaxial ordering transitions and first order layering transitions. The layering transition takes place between two fluids having n and n+1 layers, where the layer spacing is in the order of D. In the case of weak shape anisotropy (L/D=3), the coexisting fluids can be either uniaxial or biaxial, while both phases are found to be biaxial for L/D=6 and L/D=9. Interestingly, even two or more layering transitions can be observed with increasing density at a given shape anisotropy and pore width.

A novel hydrophobic adsorbent of electrospun SiO2@MUF/PAN nanofibrous membrane and its adsorption behaviour for oil and organic solvents

With increasing oil spill accidents, the development of effective and low-cost adsorbents with good hydrophobicity is highly desirable. To cope with the clean-up of oil spill, a hydrophobic adsorbent was synthesized by electrospinning using inexpensive raw materials. By ingeniously combining melamine with polyacrylonitrile (PAN) as well as SiO2 nanoparticles, a novel composite nanoadsorbent named SiO2@MUF/PAN nanofibrous membrane was prepared and characterized. The adsorbents were conducted based on uniform nanofibre networks and were abundant with narrow slit-like pores, which are significant for the retention of oil and organic solvents. The hydrophobicity of the as-prepared membranes was enhanced with an increasing amount of SiO2, and the highest water contact angle was 128.3°. Furthermore, the combination of SiO2 and melamine increased the thermal stability of the membranes. With the unique pore structures and hydrophobicity, the membranes were able to selectively remove not only oil but also organic solvents from water surface. The adsorption capacities of the membranes with SiO2 nanoparticles (0.9 wt%) were the highest and that for peanut oil, diesel, pump oil and engine oil were 19.09, 13.12, 18.48 and 22.67 g g−1, respectively, while that for organic solvents ranged from 12.92 to 22.16 g g−1. After 10 adsorption–regeneration cycles, the adsorption capacity was still around 35% of the initial value. Due to its high oil adsorption capacity, excellent reusability and the cost-effective hydrophobic, SiO2@MUF/PAN have a great potential for oil spill clean-up.

Ordering transitions of weakly anisotropic hard rods in narrow slitlike pores.

The effect of strong confinement on the positional and orientational ordering is examined in a system of hard rectangular rods with length L and diameter D (L>D) using the Parsons-Lee modification of the second virial density-functional theory. The rods are nonmesogenic (L/D<3) and confined between two parallel hard walls, where the width of the pore (H) is chosen in such a way that both planar (particle's long axis parallel to the walls) and homeotropic (particle's long axis perpendicular to the walls) orderings are possible and a maximum of two layers is allowed to form in the pore. In the extreme confinement limit of H≤2D, where only one-layer structures appear, we observe a structural transition from a planar to a homeotropic fluid layer with increasing density, which becomes sharper as L→H. In wider pores (2D<H<3D) planar order with two layers, homeotropic order, and even combined bilayer structures (one layer is homeotropic, while the other is planar) can be stabilized at high densities. Moreover, first-order phase transitions can be seen between different structures. One of them emerges between a monolayer and a bilayer with planar orders at relatively low packing fractions.

Effect of supplementation different hydrophobic silicas on the stability of foams

In this paper, we investigated two types of silicas obtained by the sol-gel synthesis (silica 1) and solgel synthesis of endotemplates (fructose) (silica 2). In the first phase of work, the surface characteristics of the synthesized silicas were evaluated. Area measurement and pore volume of silica samples showed that the surface of silica 2 had a narrow slitlike pores, and the pore size was 3.15–3.17 nm. Thermochemical studies of silica showed that the content of hydroxyl groups per 1 g of silica was greater in the case of silica synthesized by endotemplate technology. The obtained data on the texture parameters and chemical activity of the surface were used in the study of systems containing surfactant and silica in aqueous media. Common for particles of synthesized silica is that the form of particles in the solutions remains spherical and the zeta potential decreases. Moreover, the neutralization of the surface charge of silica 2 takes place to a greater extent than the neutralization of the charge on the surface of particles of silica 1. The greatest practical interest may be the pre-foaming solution with the addition of silica 2, because the maximum lifetime was recorded in this solution.

Experimental investigations on the geometry and connectivity of pore space in organic-rich Wufeng and Longmaxi shales

The nano-scale pore systems of organic-rich shale reservoirs were investigated from Upper Ordovician Wufeng and Lower Silurian Longmaxi Formations in southeast Sichuan Basin. These two formations are the most important target plays of shale gas development in China. The purpose of this article is to assess the geometry and connectivity of multi-scale pore systems, and to reveal the nature and complexity of pore structure for these over-mature gas shales. To achieve these objective, total organic carbon, mineralogy, image analyses by focused ion beam-SEM, low pressure nitrogen adsorption, mercury injection capillary pressure (MICP) and spontaneous fluid [deionized (DI) water and n-decane] imbibition were performed.Most of the visible pores from SEM work in Wufeng and Longmaxi shales are within nm- and μm-size regimes and belong to organic matter (OM) pores. The shapes of OM pore in Longmaxi samples are elliptical, bubble-like, irregular or rounded. Wufeng pores are mainly irregular, linear and faveolated, even though two shales have small depth difference, as well as similar thermal maturity, kerogen type and TOC content. Nano-scale pores in Longmaxi are mainly associated with narrow platelike or slitlike pores with pore size of 3–50 nm; while inkbottle pores are dominant in Wufeng samples and over 88% of the pore volume is contributed by pores with diameter <20 nm. Overall, porosity, pore volume and surface area values from Wufeng samples are much higher than those in Longmaxi, which is mainly correlated with the different TOC contents and mineral compositions. MICP tests show that a total of 5 inflection points (indicative of different connected pore networks) are identified in all pressure regions for Longmaxi, while only 2 for Wufeng in high pressure region with the associated permeability at nano-darcy range. Imbibition curves of n-decane are divided into three stages: the initial stage (Stage Ⅰ), linear imbibition stage (Stage Ⅱ) and late imbibition stage (Stage Ⅲ), and the slopes of linear imbibition stage are around 0.5, suggesting well-connected pore spaces for n-decane. In contrast, imbibition curves for DI water are divided in two stages with linear slopes of between 0.25 and 0.5, indicating moderately-connected pore networks for the movement of DI water. This is consistent with the mixed-wet nature of these shales, with observed weak wettability for hydrophilic, while complete wetting for hydrophobic fluids.

A facile method to fabricate N-doped graphene-like carbon as efficient electrocatalyst using spent montmorillonite

Montmorillonite (Mt), as an efficient adsorbent, has been widely applied in the treatment of wastewater. However, it is important to develop appropriate disposal methods, which are currently lacking for spent Mt after adsorption. We utilized Mt after adsorption of tetracyclines (TC) as carbon source to obtain a by-product, N-doped graphene-like carbon-based electrocatalysts (TC-C), by a facile and low cost pyrolysis strategy. Physicochemical and electrochemical characterization of TC-C indicated that the resultant TC-C was made up of many ultrathin nanosheets, which resembled the morphology of graphene, and it had a large Brunauer-Emmett-Teller surface area with narrow slit-like pores. Nitrogen doping was evaluated through X-ray photoelectro-spectra measurements and four nitrogen-bonding configurations were identified and quantified, including pyridinic N, pyrrolic N, graphitic N and pyridinic N+–O−. Electrochemical studies showed that TC-C exhibited high catalytic activity for the oxygen reduction reaction (ORR) in alkaline medium with outstanding methanol tolerance compared to the commercial Pt/C catalysts. Furthermore, the effect of the adsorbed amount of TC on the ORR activity of TC-C was slight and the pyrolysis temperature of 700°C was demonstrated to be the optimal temperature for the pyrolysis process in the recycling of spent Mt.

Preparation of Ru/graphene-meso-macroporous SiO2 composite and their application to the preferential oxidation of CO in H2-rich gases

Preparation for graphene (GE)-meso-macroporous SiO2 composite was studied, the composite was used as the support for Ru and the resulted catalyst was applied to the preferential oxidation of CO (CO-PROX) in H2-rich gases. The composite was characterized by using techniques of SEM, TEM, XRD, TPR, N2 adsorption/desorption and XPS. In the preparation, polystyrene foam and a tri-block copolymer of P123 were used as the template for the macropores and the mesopores, respectively. In addition to the macro and the meso pores, there are narrow slit-like pores with width of several nanometers between GE and SiO2. The highly dispersed Ru nanoparticle can be loaded on the surface of GE in the composite by impregnating. Thus prepared Ru/GE-SiO2 showed very good catalytic performance for CO-PROX, especially exhibited high activity at low temperature. The GE-SiO2 composite should possess the properties of both meso-macroporous oxides and graphene, and is a kind of new promising material, in which SiO2 can be replaced by other oxides.

Motion of a vapor—liquid meniscus in narrow slitlike pores

In this work, a microhydrodynamic approach is used to investigate the motion of the meniscus on a vapor—liquid interface in narrow slitlike pores. Calculations are performed on the basis of Navier—Stokes—type equations, in which the transport coefficients and equation of state for a compound are calculated within the simplest molecular model of a gas lattice that takes into account the inherent volume of molecules and their interaction with each other and the pore wall. The dynamic modes of the flow of a liquid monatomic gas (argon) bearing a vapor bubble through nanosized pores at a given pressure difference across the pore are studied. The differences in the average rates of the liquid and vapor-bubble motion in a quasi-stationary regime caused by the intensive nonequilibrium processes of molecule exchange on the liquid—vapor and vapor—liquid boundaries, i.e., by the phase transitions on both boundaries of a bubble, are revealed.

Phase diagrams for model heteronuclear dimers confined in narrow slitlike pores

Monte Carlo simulation is used to study the phase behavior of a model of heteronuclear dimers on a lattice. The calculations are carried out for two- and three-dimensional bulk systems and for dimers confined in slitlike pores of different widths and for different segment-wall interactions. The calculations recover a rich variety of phase behaviors that includes transitions between gaslike and liquidlike phases, the latter being ordered or disordered. The topology of the obtained diagrams depends on the energy of interactions between segments forming a dimer, on the segment-wall interaction, and on the pore width.

The special features of molecular transport in nanosized channels

The molecular theory of the transport of pure substances and mixtures of molecules of different shapes in narrow slit-like pores, in which the potential of surface forces creates a strongly anisotropic distribution of molecules across pores and thereby makes the hydrodynamics equation inapplicable, is considered. The new microhydrodynamic approach is based on the lattice gas model, which takes into account the intrinsic volume of molecules and intermolecular interactions in the quasi-chemical approximation. Self-consistent calculations of dissipative coefficients taking nonlocal fluid properties into account were performed on the basis of the transition state model including information about equilibrium adsorbate distribution. Changes in fluid concentrations from the gaseous to liquid state and a broad temperature range, including the critical region, are analyzed. This allows vapor, liquid, and vapor-liquid fluid flows to be considered in the presence of capillary condensation. An increase in the size of pores transforms the equations of the theory into hydrodynamic transfer equations for gas or liquid flows, while preserving the relation of transfer coefficients to intermolecular potentials. The use of microhydrodynamic approach equations in numerical calculations and the possibility of applying this approach are discussed.

Synthesizing nanocrystal-assembled mesoporous magnesium oxide using cotton fibres as exotemplate

A novel biomorphic nanocrystal-assembled mesoporous (NAM) magnesium oxides have been synthesized via an exotemplating pathway using cotton fibres as template and magnesium acetate as MgO precursor. These mesoporous MgO materials replicate the microtubular or zonal structure of the cotton morphology, and their frameworks are constructed by plenty of uniform MgO nanocrystals, which are different from those of the cotton template and reference MgO prepared by directly thermal-decomposition of MgO precursor. The mesoporous structure with narrow slit-like pores can be formed, if suitably chosen amounts of precursor magnesium acetate are used. These biomorphic NAM MgO materials have high surface areas, exhibit high selectivity in the catalytic decomposition of isopropanol, and display their potential for an effective basic catalyst or support.

Approximation of contacts for calculating the velocity of the thermal motion of rodlike molecules in narrow slitlike pores

Expressions for calculating the thermal velocities of the motion of rodlike molecules in slitlike pores were obtained within the framework of a lattice gas model valid over a wide range of fluid densities (from rarefied gases to liquids) and temperatures, including the critical region. The translational and rotational motion of molecules was described using the transition-state theory for nonideal reaction systems, which takes into account the effect of the neighboring molecules on the activation barrier height. The local distributions of the components of the mixture under equilibrium conditions were calculated by describing lateral interactions in the approximation of isolated contacts. The equations of the model reflect the fact that the distributions of the components in the direction perpendicular to the pore walls (due to the effect of adsorption forces) and along the pore axis (if capillary condensation occurs) exhibit a strong anisotropy.

Storage of Hydrogen at 303 K in Graphite Slitlike Pores from Grand Canonical Monte Carlo Simulation

Grand canonical Monte Carlo (GCMC) simulations were used for the modeling of the hydrogen adsorption in idealized graphite slitlike pores. In all simulations, quantum effects were included through the Feynman and Hibbs second-order effective potential. The simulated surface excess isotherms of hydrogen were used for the determination of the total hydrogen storage, density of hydrogen in graphite slitlike pores, distribution of pore sizes and volumes, enthalpy of adsorption per mole, total surface area, total pore volume, and average pore size of pitch-based activated carbon fibers. Combining experimental results with simulations reveals that the density of hydrogen in graphite slitlike pores at 303 K does not exceed 0.014 g/cm(3), that is, 21% of the liquid-hydrogen density at the triple point. The optimal pore size for the storage of hydrogen at 303 K in the considered pore geometry depends on the pressure of storage. For lower storage pressures, p < 30MPa, the optimal pore width is equal to a 2.2 collision diameter of hydrogen (i.e., 0.65 nm), whereas, for p congruent with 50MPa, the pore width is equal to an approximately 7.2 collision diameter of hydrogen (i.e., 2.13 nm). For the wider pores, that is, the pore width exceeds a 7.2 collision diameter of hydrogen, the surface excess of hydrogen adsorption is constant. The importance of quantum effects is recognized in narrow graphite slitlike pores in the whole range of the hydrogen pressure as well as in wider ones at high pressures of bulk hydrogen. The enthalpies of adsorption per mole for the considered carbonaceous materials are practically constant with hydrogen loading and vary within the narrow range q(st) congruent with 7.28-7.85 kJ/mol. Our systematic study of hydrogen adsorption at 303 K in graphite slitlike pores gives deep insight into the timely problem of hydrogen storage as the most promising source of clean energy. The calculated maximum storage of hydrogen is equal to approximately 1.4 wt %, which is far from the United States Department of Energy (DOE) target (i.e., 6.5 wt %), thus concluding that the total storage amount of hydrogen obtained at 303 K in graphite slitlike pores of carbon fibers is not sufficient yet.

Self-diffusion, mass transfer, and viscosity coefficients for a binary mixture in narrow slit-like pores

The concentration dependences of the dynamic characteristics of a binary mixture in narrow slit-shaped pores of different widths are considered. The local and mean partial self-diffusion, label transfer, mass transfer (mutual diffusion), and shear viscosity coefficients for binary mixtures of various compositions were calculated. The calculation was based on the lattice-gas model in the quasichemical approximation for spherical components with approximately the same size. The calculation of dynamic characteristics took into account collisions between the molecules that determine the direction of their motion. All the kinetic coefficients depend substantially on the mixture density, the direction of motion, and the distance to the pore wall. The effect of the pore width on the calculated dynamic characteristics is considered.