Isosteric Heat Of Adsorption Qst Research Articles

Methane adsorption on the Zr-BDC metal-organic framework structure at supercritical temperatures and pressures

Methane adsorption by the sample of the metal-organic framework structure (MOF) Zr-BDC (BDC is benzene-1,4-dicarboxylate) containing zirconium ions was studied in a pressure range of 0.1–40 MPa at 303, 313, 323, and 333 K. The excess adsorption isotherms Γ(P,T) and adsorption isotherms of total amount a(P, T) were calculated, and the adsorption volume W was determined. The adsorption isosteres were constructed on the basis of these isotherms, and the isosteric qst(a,T) and initial q(0,T) heats of adsorption were calculated. The average isosteric heats of adsorption q st * (a, T) were determined taking into account the dependence of qst(a,T) on the amount adsorbed. These values were compared with the average heats of adsorption q(E) calculated with allowance for the characteristic energy of adsorption E(T), which were determined, in turn, from the adsorption isotherms a(P,T). Two calculation methods were used: via the characteristic curve and via the adsorption isotherm presented in the linear coordinates of the Dubinin—Astakhov equation. In both cases, new parameters were introduced in the adsorption equation to determine E(T): “Ps” that replaces the vapor pressure at saturation and critical temperature Tc. The Zr-BDC MOF sample is a microporous adsorbent, which can be used for methane storage.

Solid Sorbents as a Retrofit Technology for CO2 Removal from Natural Gas Under High Pressure and Temperature Conditions

The capture of CO2 under high pressure and temperature is challenging and is required in a number for industrial applications including natural gas processing. In this work, we examine the use of benchmark hybrid ultraporous materials HUMs for their potential use in CO2 adsorption processes under high-pressure conditions, with three varying temperatures (283, 298 and 318 K). NbOFFOVE-1-Ni and SIFSIX-3-Ni were the selected HUMs given their established superior CO2 capacity under low pressure (0–1 bar). Both are microporous with highly ordered crystalline structures as compared to the mesoporous hexagonal silica (Santa Barbara Anhydrous-15 (SBA-15)). SBA-15 was previously tested for both low and high-pressure applications and can serve as a benchmark in this study. Sorbent characterization using XRD, SEM, FTIR and N2 adsorption were conducted to assure the purity and structure of the sorbents. TGA analysis were conducted to establish the thermal stability of the sorbents under various temperatures. High-pressure CO2 adsorption was conducted from 0–35 bar using magnetic suspension balance (Rubotherm). Although the SBA-15 had the highest surface (527 m3/g) are of the three adsorbents, the CO2 adsorption capacity (0.42 mmol/g) was an order of magnitude less than the studies HUMs with SIFSIX-3-Ni having 2.6 mmol/g, NbOFFIVE-1-Ni achieving 2.5 mmol/g at 298 K. Multistage adsorption isotherms were obtained at different pressures. In addition, results indicate that electrostatics in HUMs are most effective at improving isosteric heat of adsorption Qst and CO2 uptake. Higher temperatures had negative effect on adsorption capacity for the HUMs and SBA-15 at pressures between 7–9 bar. In SAB-15 the effect of temperature is reversed in what is known as a cross over phenomena.

Enhanced CO2 adsorption capacity of bi-amine co-tethered flue gas desulfurization gypsum with water of hydration

An alcoholamine, 2-[2-(dimethylamino)ethoxy]ethanol (DMAEE), was co-tethered with pentaethylenehexamine (PEHA) on the surfaces of the flue gas desulfurized gypsum (FGDG) carriers to synthesize a solid bi-amine adsorbent. The effects of temperature, loading of the blended amines, mass ratio of DMAEE to PEHA, CO2 partial pressure on the CO2 adsorption performance were discussed. The water of hydration contained in the FGDG carriers promoted the CO2 adsorption capacity of the bi-amine adsorbent. Both the breakthrough time and the saturation adsorption capacity of the bi-amine adsorbent were distinctly higher than the DMAEE or PEHA singly tethered adsorbents. The adsorption capacity of the bi-amine adsorbent decreased by only 1.86% over twelve cycles. The isosteric heat of adsorption Qst and the mean free energy of adsorption Efe was determined and both the values indicated that the adsorption was primarily chemisorption. In the kinetics analysis, Avrami model fitted best with the measured adsorption capacity. With the aid of the 13C–NMR quantitative analysis results, the CO2 adsorption mechanism of the bi-amine adsorbent in the presence of the water of hydration was revealed. For the bi-amine adsorbents supported on some porous carriers, FGDG particles were testified to replace up to 7 wt.% of montmorillonite clay or 7 wt.% of MCM–41 zeolite or 10.5 wt.% of ZSM–5 zeolite without any decrease in the CO2 adsorption capacity.

Porous 10- and 12-vertex (bi)-p-dicarba-closo-boranedicarboxylates of cobalt and their gas adsorptive properties

Two new porous coordination polymers, [Co3(μ3-OH)(Bcb10DC)2.5(DEF)3(EtOH)1.5(H2O)0.5] · DEF · EtOH (3b) and [Co2(Bcb12DC)2(DMF)4(EtOH)] · 2EtOH (4) based on 10- and 12-vertex p-bicarboranedicarboxylic acids, 1,1′-bi-(1,10-dicarba-closo-decaborane)-10,10′-dicarboxylic acid (H2Bcb10DC) and 1,1′-bi-(1,12-dicarba-closo-decaborane)-12,12′-dicarboxylic acid (H2Bcb12DC) were synthesized. Their properties were compared with the two known analogues based on shorter ligand-homologues, [Co(cb10DC)(DMF)] (1) and [Co4(OH)2(cb12DC)3(DMF)2(H2O)4] · 3DMF · EtOH (2). The structure of 3b is based on a trinuclear cluster and has a 3D structure with a 5-c bnn underlying net, while 4 is based on a binuclear cluster and composed of stacked 2D sql layers. Activated 3′ with altered structure has SBET(N2) of 1012 m2 g−1 and 4′ with mostly retained structure 920 m2 g−1. Gas adsorption by 3′ and 4′ at 1 bar is: 80.1 cm3 g-1 (0.72%wt) and 81.3 cm3 g-1 (0.73%wt) H2 at 77 K; 16.7 and 21.9 cm3 g-1 CO2 at 273 K; 6.2 and 10.0 cm3 g-1 of CH4 at 273 K. The isosteric heat of adsorption Qst at zero coverage are respectively: 5.0 and 4.5 kJ mol−1 for H2, 16.7 and 21.9 for CO2; 6.2 and 10.0 for CH4. 2′ with ∼645 m2 g−1 (calc. 883 m2g−1) has the highest mass-specific H2 adsorption, >30% than other compounds, while Qst is close to 1′ and slightly higher than for 3′ and 4´. Adsorption of CO2 and CH4 and Qst are ∼1.5–2 times lower for 3′ and 4′ compared to 1′ and 2´. The surface area specific adsorption of H2, CO2 and CH4 is the highest for 1´. The CO2/CH4 IAST selectivities (0.15/0.85; 1 bar, 298 K) are in 2–4.5 range. 3′ and 4′ retain a significant part of porosity after soaking in water unlike 1′ and 2´.

Green Synthesis of a Microporous, Partially Fluorinated ZnII Paddlewheel Metal–Organic Framework: H2/CO2 Adsorption Behavior and Solid‐State Conversion to a ZnO–C Nanocomposite

AbstractA microporous, partially fluorinated metal–organic framework consisting of ZnII dimeric paddlewheel units, formulated as [Zn(hfipbba)(4bpdb)0.5]·H2O (1) [hfipbba = 4,4′‐(hexaflouroisopropylene)bis(benzoic acid); 4bpdb = 1,4‐bis(4‐pyridyl)‐2,3‐diaza‐1,3‐butadiene], was synthesized by room‐temperature self‐assembly and characterized structurally by single‐crystal XRD. Compound 1 adopts a 3D microporous framework structure constituted by six‐connected ZnII dimeric paddlewheel nodes with {44.610.8}‐net topology. Further, rapid synthesis of phase‐pure 1 by green synthetic approaches such as mechanochemical and sonochemical routes was achieved. The 3D framework of 1 houses 1D helical channels with narrow pore dimensions of about 3.11 × 4.17 Å and exhibits interesting gas‐uptake (H2/CO2) properties. The isosteric heats of adsorption Qst for H2 and CO2 were estimated to be 8.8 and 36.4 kJ mol–1, respectively. Interestingly, solid‐state conversion of 1 to phase‐pure hexagonal ZnO nanocrystals of 6.1 ± 0.63 nm in size, embedded in carbonaceous layers to form a ZnO–C nanocomposite (NC), occurred on heating of 1 to 600 °C under vacuum for 2 h. The ZnO–C NC was characterized by XRD, UV/Vis, energy‐dispersive X‐ray, and TEM analyses. The as‐synthesized ZnO–C NC exhibits good photocatalytic activity for the degradation of methylene blue.

The adsorption of H2O and D2O on porous polystyrene adsorbents

The adsorption of H2O and D2O on porous polymers, Chromosorb-102 (styrene-divinylbenzene copolymer) and MN-200 (supercross-linked polystyrene), was studied by gas chromatography. Test adsorbates used to study the properties of the surface of these polymers were n-alkanes (C6-C9), C6H6, and the polar compounds CHCl3, CH3NO2, CH3CN, (CH3)2CO, C2H5COOCH3, and (C2H5)2O. The experimental data on the retention of the sorbates were used to determine the contributions of dispersion and specific intermolecular interactions to the total energy of adsorption for the systems studied. The electron donor KD and electron acceptor KA characteristics of the surfaces of Chromosorb-102 and MN-200 were determined. The KD and KA values obtained allow these polymers to be classified as weakly specific adsorbents with the predominance of electron acceptor properties. The adsorption isotherms of H2O and D2O were measured at 55, 67, and 80°C. The dependences of the isosteric heats of adsorption Qst on adsorption values were determined. The conclusion was drawn that H2O interacted with the surface of the polymers by the adsorption mechanism, whereas absorption likely made a noticeable contribution to the retention of D2O.

General HETP Equation for the Study of Mass-Transfer Mechanisms in RPLC

Classical HETP equations including the Van Deemter and the Knox equations, are semiempirical, approximate equations that provide apparent mass-transfer coefficients with little sound physical justifications. The conventional A and B coefficients are revisited, the former through the use of the fundamental theory of eddy diffusion due to Giddings, the latter by taking into account the intraparticle diffusion (pore and surface diffusion). Our work confirms that eddy diffusion originated from three different sources in RPLC: trans-channel, short-range interchannel, and long-range interchannel velocity biases. Accordingly, the eddy diffusion term is given by the ratio of two third-degree polynomials. Finally, the C term is the sum of two terms corresponding to the resistance to mass transfer due to diffusion through the external stationary film of liquid phase surrounding the silica particles and to the classical resistances to mass transfer due to diffusion through the silica particles. It is easily related to the physical characteristics of the phenomena involved. Experimental HETP data were derived from moment analysis for phenol on a C(18)-Sunfire column, with a mixture of acetonitrile and water as the mobile phase (15/85, v/v). The linear interstitial velocity ranged between 0.027 cm/s and 4.7 mL/min, and six temperature (21, 36, 45, 55, 67, and 77 degrees C) were applied successively. The HETP equation obtained was tested to study the mass-transfer mechanism. An excellent agreement was found between the experimental and theoretical HETP. The model allows the precise calculation of the activation energy for surface diffusion (E(S) = 31.3 kJ/mol) and the coefficient beta that relates the restriction energy for molecular diffusion on the C(18)-bonded surface to the isosteric heat of adsorption Q(st) (beta = 0.80).

Comparison of the adsorption properties of krypton on multi-walled carbon nanotubes and on graphite.

AbstractKrypton adsorption has been investigated on multi-walled carbon nanotubes prepared by catalytic decomposition of acetylene. Isotherms measured between 77.5 and 83.7K from the first stages of adsorption to the adsorbate saturation vapor pressure are compared to those obtained in the same conditions on graphite. The results are discussed in the light of the nanotube morphology, as determined by transmission electron microscopy.Krypton adsorption proceeds, as on graphite, by successive monomolecular layer condensations on a same surface. From the morphology of the tubes, adsorption probably occurs only on their external surfaces. The first adsorbed layer is commensurate with the substrate. At its completion, it undergoes a transition into an incommensurate solid of higher density. The curvature of the graphene sheets, with respect to those of graphite, produces a stabilization of the commensurate film. The isosteric heat of adsorption Qst was measured to be 11.6 kJ/mol (ΔQst/Qst = 6%) and is lower than that of krypton on graphite.Moreover, the monolayer condensation pressures are higher than those observed with graphite, which probably leads to an incomplete wetting of the surface by limiting the number of krypton monomolecular layers adsorbed before its bulk condensation.

Energetics and kinetics of ethylbenzene adsorption on epitaxial FeO(111) and Fe3O4(111) films studied by thermal desorption and photoelectron spectroscopy

The adsorption of ethylbenzene (EB) has been studied on thin films of FeO(111) and Fe3O4(111) grown epitaxially on Pt(111) using thermal desorption spectroscopy (TDS), ultraviolet photoelectron spectroscopy (UPS) and low energy electron diffraction (LEED). Applying a threshold analysis of the TDS data, desorption energies Edes and the corresponding frequency factors are deduced. The UPS measurements are performed under adsorption–desorption equilibrium conditions: The spectra are taken at varying sample temperature at constant EB gas phase pressures. From the spectra, the EB-coverages ΘEB are deduced. From the adsorption isobars obtained in this way, isosteric heats of adsorption qst(ΘEB) are obtained which are compared to the desorption energies Edes deduced from TDS. On the oxygen-terminated FeO(111) surface, two adsorption states are observed, a physisorbed first layer (β-EB) followed by condensation (α-EB). Their UP spectra are almost identical and very similar to the spectrum of gas phase EB. On Fe3O4(111), a more strongly chemisorbed species (γ1-EB) is adsorbed first, followed by physisorbed β- and condensed α-EB. The chemisorbed phase exhibits a strong shift and split of the highest occupied π orbitals of the phenyl group. This indicates a strong interaction between the substrate and the adsorbed molecules that are adsorbed with the phenyl ring lying flat on the surface. The desorption energies Edes and the isosteric heats of adsorption qst, respectively, are 91 (85) kJ/mol for γ1-, 55 (58) kJ/mol for β- and 50 (52) kJ/mol for α-EB and agree generally well. The differences are discussed in terms of different coverage ranges accessible for both methods, the nonequilibrium character of the TDS method and to the threshold analysis which yields only data for the most loosely bound molecules desorbing first in each desorption track.

Surface Diffusion of Alkylbenzenes on Octadecylsilyl−Silica Gel

Surface diffusion phenomena of alkylbenzenes in reversed-phase liquid chromatography (RP-LC) using an octadecylsilyl−silica gel and methanol/water (70/30, v/v) were analyzed on the basis of a restricted molecular diffusion model, in which surface diffusion coefficient, Ds, was correlated with molecular diffusivity, Dm, by introducing a restriction energy, Er. The average ratio of Er to isosteric heat of adsorption Qst, γ, in RP-LC was about 0.3. It was also indicated that γ at different temperatures could be represented as a linear function of the reciprocal of −Qst when a linear free-energy relation held and an enthalpy−entropy compensation for adsorption equilibrium was established. An estimation procedure of Ds was proposed on the basis of the results. It was demonstrated that Ds at a given temperature could be calculated with an error of less than about 10% from only one datum of the adsorption equilibrium constant.

Acetylene C2H2 Adsorbed onto MgO(001): Structure and Thermodynamics

Structural and thermodynamical LEED experiments were performed on MgO(001) single crystal cleaved in situ under ultrahigh vacuum conditions within the temperature range of 40–94 K. We observe a commensurate (2 × 2) herringbone structure containing two molecules and having two glide planes. The isosteric heat of adsorption Qst = 6.9 ± 0.4 kcal · mole -1 and lateral interoction energy Q// = 3.1 ± 0.4 kcal · mole -1 have been measured and compared with semiempirical potential calculations. The influence of the molecular motions on the calculated isosteric heat of adsorption is evaluated to take into account temperature effects.

Sorption studies of microporous sol-gel modified ceramic membranes

Sorption experiments with H2, CO2, CH4 and iso-C4H10 were performed on microporous SiO2 and SiO2/TiO2 (30 mol% TiO2) non-supported membrane top-layers using volumetric and gravimetric techniques. For silica, the sorption capacity decreases in the order CO2>iso-C4H10>CH4>H2 at temperatures <373 K. The isosteric heat of adsorption q st is 23, 24, 10 and 6 kJ·mol−1 for respectively CO2, iso-C4H10, CH4 and H2. The sub-atmospheric adsorption isotherms are of Henry-type for temperatures equal and higher than 348 K for CO2, temperatures higher than 373 K for iso-C4H10, temperatures equal and higher than 194 K for H2 and for temperatures equal and higher than 273 K for CH4. The sorption capacity for the SiO 2/TiO 2 sample was only slightly lower than for silica, as may be expected due to the lower porosity.

Solvent effect on adsorption phenomena in reversed‐phase liquid chromatography

AbstractThe effect of solvent and mobile‐phase composition on adsorption characteristics in liquid‐phase absorption is studied. Comparing experimental adsorption data for several organics in both gas‐ and liquid‐phase systems confirms that adsorption equilibrium constant K and isosteric heat of adsorption Qst are smaller than those in corresponding gaseous systems. The logarithm of K almost linearly increases as methanol composition decreases. Solvophobic theory applied quantitatively analyzes these solvent effects. Estimation methods for three parameters influence calculation results: solvent effects on K can be quantitatively analyzed; the adsorbability of adsorbates can be estimated from the value of ΔGsolv for each homologue in a reversed‐phase liquid chromatographic system; and the solvent effect on Qst cannot be satisfactorily interpreted. It is, however, confirmed that Qst is influenced by a solvent and an apparent small value is observed in liquid‐phase adsorption.

Adsorption isotherms and heats of adsorption of neon and hydrogen on zeolite and charcoal between 20 and 90 K

Measurements have been made of the adsorption isotherms of neon and hydrogen on Linde synthetic zeolite type 13X and on Fisher activated coconut charcoal in the temperature ranges 20–37 and 77–90 K. In the lower temperature range, below the critical points of each adsorbate, it was found that the adsorption isotherms, when plotted giving the volume V, of gas adsorbed in cm3 (STP) per gram of adsorbent as a function of the parameter (p/p0)T/100, yielded a unique isotherm curve independent of temperature for each adsorbate-adsorbent system (p0 is the saturated vapor pressure at each temperature of measurement). This result is what would be expected from potential theory, as emphasized by Chesteret al. in 1974. It was found, moreover, for neon on both charcoal and zeolite 13X that the unique temperature-independent curve for each system was the same for adsorption data taken below the triple point Ttr of bulk neon as for data taken above Ttr. From the data, the isosteric heats of adsorption Qst were calculated at various temperatures and coverages and these showed a marked dependence on coverage, Qst decreasing rapidly with increasing coverage.