Effective Pore Width Research Articles

Adsorption of Diffusing Tracers, Apparent Tortuosity, and Application to Mesoporous Silica.

The apparent tortuosity due to adsorption of diffusing tracers in a porous material is determined by a scaling approach and is used to analyze recent data on LiCl and alkane diffusion in mesoporous silica. The slope of the adsorption isotherm at small loadings is written as β = qA/qG, where qA is the adsorption-desorption ratio and qG = ϵ/(as) - 1 is a geometrical factor depending on the range a of the tracer-wall interaction, the porosity ϵ, and the specific surface area s. The adsorption leads to a decrease of effective diffusion coefficient, which is quantified by multiplying the geometrical tortuosity factor τgeom by an apparent tortuosity factor τapp. In wide pores or when the adsorption barrier is high, τapp = β + 1, as obtained in previous works, but in narrow pores there is an additional contribution from frequent adsorption-desorption transitions. These results are obtained in media with parallel pores of constant cross sections, where the ratio between the effective pore width ϵ/s and the actual width is ≈0.25. Applications to mesoporous silica samples are justified by the small deviations from this ideal ratio. In the analysis of alkane self-diffusion data, the fractions of adsorbed molecules predicted in a recent theoretical work are used to estimate τgeom of the silica samples, which is ≫1 only in the sample with the narrowest pores (nominal 3 nm). The application of the model to Li+ ion diffusion leads to similar values of τgeom and to a difference of energy barriers of desorption and adsorption for those ions of ∼0.06 eV. Comparatively, alkane self-diffusion provides the correct order of magnitude of τgeom, with adsorption playing a less important role, whereas adsorption effects on Li+ diffusion are much more important.

Modeling transport and filtration of nanoparticle suspensions in porous media.

Recently membrane filters have gained in significance due to the need to provide protection against airborne pollution. A question of importance, and some controversy, is the efficiency of filters for small nanoparticles with diameters below 100nm as these are considered particularly dangerous due to possible penetration into the lungs. The efficiency is measured by the number of particles blocked by the pore structure after passing though the filter. To study the penetration into pores by nanoparticles suspended in a fluid, a stochastic transport theory based on an atomistic model is used to calculate particle density and flow within the pores, resulting pressure gradient, and filter efficiency. The importance of pore size relative to particle diameter and of the parameters of the pore wall interactions are investigated. The theory is applied to aerosols in fibrous filters and found to reproduce common trends in measurements. As particles enter the initially empty pores on relaxation to the steady state the small penetration measured at the onset of filtration increases faster in time the smaller the nanoparticle diameter. Control of pollution by filtration is achieved by strong repulsion of pore walls for particle diameters greater than twice the effective pore width. For smaller nanoparticles the steady-state efficiency decreases as the pore wall interactions weaken. Effective efficiency is increased when the suspended nanoparticles inside the pores combine into clusters of sizes greater than the filter channel width.

The role of volatile organic compounds in the oxidation-driven fragmentation of soot particles

In this work, the role of volatile organic compounds (VOCs) in soot oxidation and fragmentation was investigated in a lean, premixed CH4 flame. An aerosol generator was used to homogeneously disperse soot with and without VOCs (termed Soot-W and Soot-WO, respectively) into the flame. Because minimal soot was formed in the flame itself, information on the fragmentation of the Soot-W and Soot-WO during the oxidation process was readily captured to determine the role of VOCs in soot fragmentation. The integrated number concentration of fine mode particles and the degree of particle aggregation show that the presence of VOCs suppresses soot aggregate fragmentation, primarily by the mitigating consumption of the neck sites between primary particles due to the decreased specific surface area. The actual oxygen diffusion route and effective pore width between the crystallite structures were determined using the image analysis technique. To determine the characteristic regime of oxygen diffusion, the distribution of effective pore width was studied using high-resolution transmission electron microscopy and image analysis. The estimated effectiveness factor indicates that the presence of VOCs lowers the propensity of internal burning via intra-particle oxygen diffusion, and reduces the extent of primary particle fragmentation, as evidenced by a relatively smaller peak of ultrafine particles. The analysis of the nanostructure of neck sites vs. the bulk discovers significantly more disordered and thus reactive structure at neck sites, explaining the prevalence of aggregate fragmentation over primary particle fragmentation. The finding of the role of VOCs in oxidation-driven fragmentation is beneficial to the optimization of existing models for predicting the number concentration of fine and ultrafine particles. Furthermore, a better understanding of this role may aid in the control of the most harmful fine and ultrafine particle emissions.

From excess to absolute adsorption isotherm: The effect of the adsorbed density

Estimating an accurate quantity of adsorbed density to calculate the absolute adsorption in shale reservoirs is critical for production, sequestration, and enhanced oil recovery purposes. Recent studies have shown that assuming a constant adsorbed density can generate erroneous results. Despite efforts to estimate the adsorbed density via methane density profile, there isn’t a consensus to select the appropriate range. In this study, hybrid molecular dynamics/grand canonical Monte Carlo (MD/GCMC) simulations for investigating the methane adsorbed density in graphene layers with different pore widths (0.7–6 nm), temperatures (303.15–348.15 K), and pressures (0–40 MPa) under 13 different systems based on the methane density profile were used. Also, helium expansion and helium probe were utilized to calculate the accessible pore volume. The results showed that various ranges that are chosen from the methane density profile for calculating the adsorbed density have a significant effect on the trend and estimation of the absolute adsorption isotherm. It was found that adsorbed density calculated with a starting point of the effective pore width obtained from the helium probe would be the most appropriate one when the helium probe is used to calculate the accessible pore volume in the excess adsorption isotherm.

Activated carbon-based CO2 uptake evaluation at different temperatures: The correlation analysis and coupling effects of the preparation conditions

As potential CO2 adsorbents, carbon materials have recently gained attention because of their low energy penalty, abundant sources and good stability. In this study, the response surface methodology (RSM) was used to guide the pitch-based activated carbon preparation and CO2 adsorption at different temperatures. Based on this experimental design, the correlations of CO2 uptakes at different temperatures, as well as the coupling effects of the preparation conditions on CO2 uptake, were analyzed by mathematical modelling. The obtained optimal carbon sample AC650-1.25-2.5 showed a high atmospheric CO2 uptake of 3.15(±0.14), 2.05(±0.02), and 1.37(±0.11) mmol/g at 25 °C, 50 °C, and 75 °C, respectively. Furthermore, the analysis of variance confirmed that the key factor to determine CO2 uptake at 25 °C was the activator ratio, contributing to 77.8 % of the total variance; however, it gradually shifted to the activation temperature and its coupling effect with other factors for CO2 uptake at 75 °C. The linear regression and significance test of CO2 uptakes showed that using CO2 uptake at 25 °C to predict the uptake at 50 °C was feasible; however, it failed to work when predicting the CO2 uptake at 75 °C. The good linear relationship of CO2 uptakes at similar adsorption temperatures benefited from the existence of sharable effective pore width. It was also concluded that the strict requirement for extremely narrow ultra-micropores led to the non-coincidence of suitable preparation conditions, corresponding to a good CO2 uptake at different adsorption temperatures; this further led to the bad predictability when the adsorption temperature varied greatly.

Propane simulated in silica pores: Adsorption isotherms, molecular structure, and mobility

Molecular dynamics simulations were conducted for bulk propane in contact with fully protonated slit-shaped silica pores. The effective pore width was of either 0.8 or 2.7nm. The temperature was set at 343, 368, and 373K. The TraPPE-UA and CLAYFF force fields were implemented to model propane and silica, respectively. Each individual simulation yields the density of confined fluid as a function of the bulk pressure. For a given temperature, adsorption isotherms were estimated by repeating the simulations at various bulk pressures. The results qualitatively agree with available experimental data; namely, at fixed temperature the excess sorption is found to show a maximum near the pressure at which the pores fill; at fixed pressure the excess adsorption is found to decrease as the temperature increases and as the pore width expands. At equilibrium, pronounced layering was observed for propane near the pore surface, especially in the narrower pore and at the highest densities considered. The propane molecules at contact with silica tend to lay with their CH3-CH3 vector parallel to the pore surface. The mean square displacement as a function of time was used to quantify the self-diffusion coefficient of confined propane as a function of temperature, pressure and pore width. These results will be useful for enhancing the interpretation of experimental data.

ChemInform Abstract: Catalytic Hydrocracking—Mechanisms and Versatility of the Process

AbstractReview: 114 refs.

Catalytic Hydrocracking—Mechanisms and Versatility of the Process

AbstractHydrocracking of saturated hydrocarbons can proceed by means of four distinctly different mechanisms. On bifunctional catalysts comprising hydrogenation/dehydrogenation and Brønsted acid sites alkenes and carbocations occur as intermediates. The current mechanistic views of bifunctional hydrocracking of long‐chain n‐alkanes are discussed in detail with emphasis on the now widely accepted concept of ideal hydrocracking. Other mechanisms are hydrogenolysis and Haag–Dessau hydrocracking which proceed, respectively, on monofunctional metallic and acidic catalysts. Even without a catalyst, thermal hydrocracking occurs in chain reactions via radicals. The chemistry of hydrocracking naphthenes on bifunctional catalysts resembles that of alkanes. A peculiarity, however, is the pronounced reluctance of cyclic carbenium ions to undergo endocyclic β‐scissions. The effect manifests itself in the so‐called paring reaction, which, in turn, forms the basis for measuring the Spaciousness Index for characterizing the effective pore width of zeolitic catalysts. Hydrocracking on bifunctional catalysts is among the very important processes in modern petroleum refining. It is primarily used for converting heavy oils into diesel and jet fuel. Besides, hydrocracking is appreciated for its pronounced versatility: numerous process variants exist which help to meet specific requirements in refineries or petrochemical plants. Two recent developments are briefly discussed in this review, viz. the conversion of surplus aromatics, e.g., in pyrolysis gasoline, into a synthetic feedstock for steam crackers, and quality enhancement of diesel fuel by selective ring opening of polynuclear aromatics.

State of Hydrogen in Idealized Carbon Slitlike Nanopores at 77 K

The purpose of this letter is to clarify recent findings and answer to the question: "What is the state of hydrogen in carbon slitlike pores at 77 K?" For this purpose, we determined the volumetric density of hydrogen in idealized carbon pores of molecular dimension at 77 K and pressure up to 1 MPa. We used quantum corrected grand canonical Monte Carlo simulation. We recognized the highest volumetric density of confined hydrogen (around 71% of hydrogen liquid at boiling point) for effective pore width 5.6 angstroms (H* = 3.04) in the considered pressure range. Our computational results are in agreement with the calculations performed by Wang and Johnson and Rzepka et al. In contrast, we did not observe the high volumetric density of hydrogen in slitlike carbon pores exceeding the density of hydrogen liquid at the boiling point as was reported by Jagiello and Thommes. Moreover, we obtained qualitative agreement between the simulation results and some experimental findings reported by Nijkamp.

Chiral open-framework uranyl molybdates. 1. Topological diversity: synthesis and crystal structure of [(C 2H 5) 2NH 2] 2[(UO 2) 4(MoO 4) 5(H 2O)](H 2O)

Crystals of a new framework uranyl molybdate, [(C 2H 5) 2NH 2] 2[(UO 2) 4(MoO 4) 5(H 2O)](H 2O), were synthesized from a solution of UO 2(CH 3COO) 2 · 2H 2O, MoO 3, diethylamine and HCl in H 2O at 220 °C. The structure was studied at −127 °C. The compound is hexagonal, space group P6 522, a = 11.3612(13), c = 52.698(8) Å, V = 5890.7(13) Å 3, Z = 3. The structure has been refined to R 1 = 0.050 ( wR 2 = 0.100; S = 0.985) on the basis of 3103 unique observed reflections. The structure consists of a three-dimensional framework of composition [(UO 2) 4(MoO 4) 5(H 2O)] 2− that consists of UO 7 pentagonal bipyramids that share equatorial corners with MoO 4 tetrahedra. The framework contains a three-dimensional system of channels. The first system of channels is parallel to [0 0 1] and has the dimensions 7.0 × 7.0 Å, which result in a crystallographic free diameter (effective pore width) of 4.3 × 4.3 Å (based on an oxygen radius of 1.35 Å). Other channels run parallel to [1 0 0], [1 1 0] and [0 1 0] and have dimensions 5.2 × 7.1 Å (giving an effective pore width of 2.5 × 4.4 Å). One symmetrically unique [(C 2H 5) 2NH 2] + cation and H 2O molecule reside in the framework cavities. The channels parallel to [0 0 1] are chiral and are centered about a 6 5 screw axis. The uranyl framework with U:Mo = 4:5 in the structure of [(C 2H 5) 2NH 2] 2[(UO 2) 4(MoO 4) 5(H 2O)](H 2O) represents the third known type of chiral uranyl molybdate framework. Its relationships to the frameworks with U:Mo = 5:7 and 6:7 can be understood in terms of fundamental chains that have related but yet different topological structure.

Improving the models used for calculating the size distribution of micropore volume of activated carbons from adsorption data

A widely used model for analyzing micropore distribution in activated carbons assumes an array of semi-infinite, rigid slits of distributed width whose walls are modeled as energetically uniform graphite. Adsorption isotherms can be simulated for this system using GCMC or DFT. Micropore size distributions determined from experimental isotherms using such models usually show minima near two and three molecular diameters effective pore width, regardless of the simulation method used. This may prove to be a model-induced artifact, rather than the real situation.

5491273 Catalytic conversion of methanol to linear olefins

Zeolite ZSM-57 can be synthesized from sodium-containing alumosilicate gels using hexaethyl-C5-diquat as organic template and silica sol as silica source. ZSM-57 can be readily distinguished from the structurally closely related zeolites with the FER framework topology by its X-ray powder pattern and mid-infrared spectrum. From thermogravimetric analysis of as-synthesized ZSM-57 it follows that applying an air calcination at 550°C for several hours is sufficient to remove the organic template from the pores of the zeolite. The results of two selected test reactions for probing the effective pore width of zeolites, viz. ethylbenzene disproportionation and n-decane isomerization suggest that the pore width is larger for ZSM-57 as compared to ZSM-35 (ferrierite) and ZSM-5. This is in agreement with their crystallographic structures and offers new opportunities in shape selective catalysis and molecular sieve separation.

Grand canonical Monte Carlo simulation for nitrogen adsorption in graphitic slit micropores: effect of interlayer distance

Adsorption isotherms of N 2 by slit-shaped graphitic micropores at 77 K were simulated using grand canonical Monte Carlo (GCMC) simulation and compared the simulated isotherms with experimental adsorption isotherms of N 2 on pitch-based activated carbon fibers. The pore was modeled as the slit space between parallel smooth graphite surfaces. The concept of the effective pore width which can be determined experimentally was used for the simulation. The effect of the layer distance ( d 002) of the graphitic structure on the simulated N 2 adsorption isotherm was examined. The observed d 002 of activated carbon fiber (ACF) well simulated the experimental result.

ChemInform Abstract: Shape Selective Hydroconversion of Hydrocarbons: A Powerful Tool for Characterizing the Effective Pore Width of Zeolites and Related Materials

Abstract Up to now, a variety of catalytic test reactions have been proposed for probing the effective pore widths of zeolites and related molecular sieve materials. Almost all these reactions rely on shape selective conversion of hydrocarbons, either in an acidic or in a afunctional form of the zeolite. The advantages offered by bifunctional catalysis are underlined. The most powerful test reactions available are (i) isomerization and hydrocracking of long chain n-alkanes in the range from n-nonane to n-hexadecane and (ii) hydrocracking of butylcyclohexane. A number of quantitative criteria are based on the first reaction, among them is the Modified Constraint Index CI*. The second test reaction enables the determination of the Spaciousness Index SI. While CI* is particularly useful for molecular sieves with 10-membered ring (10-MR) pores, SI is the method of the choice for materials with 12-MR pores. The mechanistic background of both methods is discussed. A novel method is proposed for a safe discrimination between 10-MR and 12-MR zeolites. It is based on the findings that a steep jump exists in the relative rates of hydrocracking of homologous n-alkanes and that the position of this jump depends on the pore width of the zeolite catalyst. Recommendations for the routine application of the preferred test reactions are given.

ChemInform Abstract: Synthesis and Catalytic Properties of Zeolite ZSM‐23.

Abstract Zeolite ZSM-23 was synthesized in the presence of pyrrolidine. An increase of the synthesis time and the crystallite size with decreasing alkalinity (NaOH content) suggests that not only hydroxyl ions but also sodium ions play an important role in the nucleation of ZSM-23. One of the synthesized samples was characterized by means of selected catalytic test reactions, viz. hydrocracking of butylcyclohexane (determination of the Spaciousness Index, SI) and the isomerization and hydrocracking of n-decane. The observed selectivities suggest that the effective pore width of ZSM-23 is slightly smaller than that of ZSM-5.

Synthesis and catalytic properties of zeolite ZSM-23

Zeolite ZSM-23 was synthesized in the presence of pyrrolidine. An increase of the synthesis time and the crystallite size with decreasing alkalinity (NaOH content) suggests that not only hydroxyl ions but also sodium ions play an important role in the nucleation of ZSM-23. One of the synthesized samples was characterized by means of selected catalytic test reactions, viz. hydrocracking of butylcyclohexane (determination of the Spaciousness Index, SI) and the isomerization and hydrocracking of n-decane. The observed selectivities suggest that the effective pore width of ZSM-23 is slightly smaller than that of ZSM-5.

The spaciousness index: A novel test reaction for characterizing the effective pore width of bifunctional zeolite catalysts

The spaciousness index: A novel test reaction for characterizing the effective pore width of bifunctional zeolite catalysts

Theory of adsorption of macromolecules in cylindrical pores and at surfaces of cylindrical shape

A rigorous theory of adsorption of a Gaussian chain of infinite length into cylindrically shaped pores and onto cylindrical surfaces of arbitrary diameter, D, has been developed for any energy of interaction −ϵ. The change in the conformational free energy of a chain arriving inside the pores from an unrestricted volume is proportional to (D∗) −2 ( D∗ is the effective pore width) over the entire molecular-sieve range (− ϵ < − ϵ c ). In the exclusion range (− ϵ < 0), when the interaction of chain units with the adsorbent exhibits repulsive type forces, D∗ is approximately equal to D. As the adsorption forces of attraction increase (0 < − ϵ < − ϵ c ) the value of D∗ increases and becomes infinite at the critical point. This makes it possible to use a single adsorbent for effective chromatographic separation of polymers over a wide range of molecular weights. In the adsorption range, where − ϵ > − ϵ c , the conformational free energy of the chain is virtually independent of the width and shape of the pores and is determined mainly by the value of −ϵ. At the critical point, − ϵ = − ϵ c , the probability of the arrival of a polymer chain in the pore is independent of both the width and shape of the pore and is determined only by the ratio of geometrical volumes of the free volume to the pore volume. For cylindrical pores, a weak dependence of − ϵ c on D is observed only for narrow pores. This dependence virtually disappears on passing to adsorbents with wide pores. Consideration of the adsorption of the macromolecule on the outer surface of the cylinder showed that at any finite value of the diameter of the cylinder, D, adsorption occurs as an ‘infinite-order’ phase transition. The maximum value of the specific heat (C p max k = 25 3 =ϵ 2 c does not depend on D. The degree of bonding of the macromolecule to the adsorbent increases with decreasing curvature of the surface, rapidly attaining values characteristic of chain adsorption on a plane.

Adsorption of the polypeptides on a solid surface. III. Behavior of stiff chains in a pore

AbstractA strict analytical theory has been developed describing the behavior of a model lattice polymer chain of arbitrary stiffness in a slitlike pore at polymer–adsorbent interaction energies –ε. The thermodynamic characteristics of the system were calculated. It was shown that the transition of the macromolecule from the solution volume inside a pore occurs by the first‐order phase transition with evolution of latent heat of adsorption. The transition point –ε = –εc is determined by the chain stiffness and is independent of the pore width D. It is shown that in the precritical range, –ε &lt; –εc, the free energy ΔF of the macromolecules in the pores is adequately described by the universal dependence ΔF = ΔF(D*/A), where D* is some effective pore width depending on the value of –ε, and A is the length of the Kuhn segment. At high attraction energies, –ε ≫ –εc, the macromolecules are bonded to the pore walls by a great number of units and their free energy depends only on –ε and the chain stiffness, ΔF = ΔF(A, ε). Close to the critical energy –ε ≃ –εc (transition range), ΔF is determined by both the stiffness of the macromolecule and the pore width D: ΔF ∼ A2D−1 for fairly high values of A and D. The possibilities of using porous media as protein stabilizers are discussed, and the value of the stabilizing effect depending on the chain stiffness is estimated.