Experimental Heats Of Adsorption Research Articles

Improving gas adsorption modeling for MOFs by local calibration of Hubbard U parameters.

While computational screening with density functional theory (DFT) is frequently employed for the screening of metal-organic frameworks (MOFs) for gas separation and storage, commonly applied generalized gradient approximations (GGAs) exhibit self-interaction errors, which hinder the predictions of adsorption energies. We investigate the Hubbard U parameter to augment DFT calculations for full periodic MOFs, targeting a more precise modeling of gas molecule-MOF interactions, specifically for N2, CO2, and O2. We introduce a calibration scheme for the U parameter, which is tailored for each MOF, by leveraging higher-level calculations on the secondary building unit (SBU) of the MOF. When applied to the full periodic MOF, the U parameter calibrated against hybrid HSE06 calculations of SBUs successfully reproduces hybrid-quality calculations of the adsorption energy of the periodic MOF. The mean absolute deviation of adsorption energies reduces from 0.13eV for a standard GGA treatment to 0.06eV with the calibrated U, demonstrating the utility of the calibration procedure when applied to the full MOF structure. Furthermore, attempting to use coupled cluster singles and doubles with perturbative triples calculations of isolated SBUs for this calibration procedure shows varying degrees of success in predicting the experimental heat of adsorption. It improves accuracy for N2 adsorption for cases of overbinding, whereas its impact on CO2 is minimal, and ambiguities in spin state assignment hinder consistent improvements of O2 adsorption. Our findings emphasize the limitations of cluster models and advocate the use of full periodic MOF systems with a calibrated U parameter, providing a more comprehensive understanding of gas adsorption in MOFs.

Linking Experimental and Ab Initio Thermochemistry of Adsorbates with a Generalized Thermochemical Hierarchy.

Enthalpies of formation of adsorbates are crucial parameters in the microkinetic modeling of heterogeneously catalyzed reactions since they quantify the stability of intermediates on the catalyst surface. This quantity is often computed using density functional theory (DFT), as more accurate methods are computationally still too expensive, which means that the derived enthalpies have a large uncertainty. In this study, we propose a new error cancellation method to compute the enthalpies of formation of adsorbates from DFT more accurately through a generalized connectivity-based hierarchy. The enthalpy of formation is determined through a hypothetical reaction that preserves atomistic and bonding environments. The method is applied to a data set of 60 adsorbates on Pt(111) with up to 4 heavy (non-hydrogen) atoms. Enthalpies of formation of the fragments required for the bond balancing reactions are based on experimental heats of adsorption for Pt(111). The comparison of enthalpies of formation derived from different DFT functionals using the isodesmic reactions shows that the effect of the functional is significantly reduced due to the error cancellation. Thus, the proposed methodology creates an interconnected thermochemical network of adsorbates that combines experimental with ab initio thermochemistry in a single and more accurate thermophysical database.

A new empirical potential for zeolite with variable Si/Al ratio: Simulations vs. experiments

A new empirical potential for zeolite framework is proposed as a function of Si/Al ratio. The potential is able to reproduce the variation in the lattice parameter with Si/Al ratio for faujasite obtained from literature and experiments reported in this work. We also report results of calculation with the proposed potential on zeolite A. Maximum deviation is 2.11% which is for Si/Al = 1.18. The atomic positions are within 10−4 of the experimental crystallographic positions. It is also able to reproduce the pressure-volume relationship and isothermal compressibility. Experimental heat of adsorption of methane in faujasite as well as the adsorption isotherms are also reproduced well. Infra-red spectra for siliceous faujasite shows good agreement with experimental spectra. It is shown that the distribution of aluminum atoms in the lattice has only a marginal role in the value of the lattice parameter.

Highly rapid mechanochemical synthesis of a pillar-layer metal-organic framework for efficient CH4/N2 separation

Efficient capture of methane (CH4) from nitrogen (N2) by adsorption separation remains a challenge. In this work, we demonstrate the first solvent-free mechanochemical synthesis of a zinc-based pillar-layer MOF Zn2(5-aip)2(bpy) (1) within minutes, making 1 accessible via a rapid and green route. By rational control of grinding time, the resulting product of 1–4 (which stands for grinding 4 min) has the optimal porosity confirmed by using CO2 as the probe gas. For the recovery of CH4 from N2 at 5.0 bar and 298 K, the adsorption uptakes of CH4 and N2 for 1–4 are 1.10 and 0.27 mmol/g, exhibiting preferential adsorption CH4 over N2 with high CH4/N2 selectivity of 7.0. The high selectivity of CH4/N2 in 1–4 is mainly ascribed to the difference in affinity interactions between the pore walls and gas molecules, confirmed by experimental adsorption heat and molecular simulation. Two CH4 molecules can enter in the same pores, but most of the pores can only adsorb one N2 molecule. Furthermore, the dynamic separation performance for CH4/N2 mixture in 1–4 is verified by simulated breakthrough curves. The unprecedentedly rapid solvent-free mechanochemical synthesis within minutes makes 1 easily accessible at the gram scale via an environmentally friendly method for accelerating potential application of CH4/N2 separation.

A New Approach in Predicting Gas Adsorption Isotherms and Isosteric Heats Based on Two-Dimensional Equations of State

The present study provides a modified approach in determining gas adsorption equilibria based on two-dimensional equations of state (2-D EOS). The proposed model utilizes temperature-dependent parameters in the general form of the 2-D EOS. These parameters were considered similar to other well-known isotherm models, such as Langmuir as specifics function of temperature. The proposed model was examined against various experimental single- and multi-component adsorption isotherm data. In most of the investigated cases, the proposed model reduces the error of predictions compared with temperature-independent two-dimensional equations of state. Moreover, utilizing temperature-dependent two-dimensional equations of state, isosteric heat of adsorption was theoretically obtained and compared with experimental heats of adsorption for different homogeneous and heterogeneous adsorption systems. Applying temperature-dependent parameters within 2-D EOS enables us to describe the heterogeneity of considered adsorption systems quite well. Predicted isosteric heats are in good accordance with the experimental data.

Temperature Dependence of Carbon Monoxide Adsorption on a High-Silica H-FER Zeolite

The temperature dependence of the isosteric heat of the CO adsorption on a high-silica protonic ferrierite (H-FER) zeolite was investigated using microcalorimetry and density functional theory/coupled-cluster (DFT/CC) atomistic simulations. A large change in the experimental heat of adsorption was observed at the zero-coverage limit for the CO/H-FER system (from 32.2 kJ/mol at 200 K to 25.4 kJ/mol at 300 K). This can be explained by a dramatic change in the CO dynamics in the 200–300 K temperature range. During our ab initio molecular dynamics simulation at 200 K, the CO molecule is localized; at 300 K, the molecule jumps between adjacent Brønsted sites. The only exception has been found for the T4″ site, where the fast desorption of the CO molecule is prevented by a curved ferrierite wall enclosing this site. The previously reported variable temperature IR heat of adsorption of the CO/H-FER with Si/Al 27.5 (28.4(±2) kJ/mol) is consistent with the Perdew–Burke–Ernzerhof/CC predictions for individual sites when the statistical distribution of adsorption sites and temperature effects are taken into account.

Prediction of the monocomponent adsorption of H2S and mixtures with CO2 and CH4 on activated carbons

Monocomponent adsorption of hydrogen sulfide and multicomponent adsorption with CO2 and CH4 mixtures were predicted in two samples of commercial activated carbons. We perform Monte Carlo calculations in the grand canonical ensemble in representative slit-pores of each carbon material and proposed a new energy parameter for H2S-carbon interaction from experimental adsorption heat in selected activated carbons. Further analysis of adsorption in representative pores demonstrated the importance of ultramicropores in H2S retention and a cooperative effect of CO2 molecules favoring H2S adsorption. To the best of our knowledge this is the first time that a method based on molecular simulation is proposed to predict H2S adsorption on carbon. This study contributes to a better understanding of the role of pore size distribution and adsorption regimes in carbon materials.

Part II: Quantum mechanical prediction of heats of adsorption for C2-C4 hydrocarbons in MOF-74-Mg/Zn periodic structures

Periodic boundary condition models are used to investigate the interaction of small hydrocarbons (C2-C4) with MOF-74-Mg/Zn. In contrast to other studies where molecular cluster approaches are used, the complete chemical environment of the framework is included. This allows correcting the binding energies for basis-set superposition error, molecular lateral interaction, zero-point energy, and thermal contributions. As such, the presented results are directly comparable to experimental calorimetric values. This work discusses, for the first time, the origin of the fictitious agreement between binding energies obtained with molecular clusters and experimental heats of adsorption, identifying its origin as due to compensation of errors.

A DFT-D study of the interaction of methane, carbon monoxide, and nitrogen with cation-exchanged SAPO-34

Abstract Density-functional theory calculations including a semi-empirical dispersion correction (DFT-D) are employed to study the interaction of small guest molecules (CH4, CO, N2) with the cation sites in the silicoaluminophosphate SAPO-34. Eight different cations from three different groups (alkali cations, alkaline earth cations, transition metals) are included in the study. For each case, the total interaction energy as well as the non-dispersive contribution to the interaction are analysed. Electron density difference plots are used to investigate the nature of this non-dispersive contribution in more detail. Despite a non-negligible contribution of polarisation interactions, the total interaction remains moderate in systems containing main group cations. In SAPOs exchanged with transition metals, orbital interactions between the cations and CO and N2 lead to a very strong interaction, which makes these systems attractive as adsorbents for the selective adsorption of these species. A critical comparison with experimental heats of adsorption shows reasonable quantitative agreement for CO and N2, but a pronounced overestimation of the interaction strength for methane. While this does not affect the conclusions regarding the suitability of TM-exchanged SAPO-34 materials for gas separations, more elaborate computational approaches may be needed to improve the quantitative accuracy for this guest molecule.

Interaction of hydrogen and carbon dioxide with sod-type zeolitic imidazolate frameworks: a periodic DFT-D study

Dispersion-corrected density-functional theory (DFT-D) calculations are used to study the interaction of hydrogen and carbon dioxide with ZIF-8, a prototypical zeolitic imidazolate framework (ZIF) with sodalite topology. Four distinct adsorption sites are identified for each of the two guest species. Two of the sites are associated with the six-ring windows, a third site is located close to one imidazolate moiety, and the fourth site is situated in the close proximity of two methyl substituents of the methylimidazolate linkers. For the case of hydrogen, where experimental data are available, the positions and the energetic ordering obtained in the DFT-D calculations agree well with these data. The investigation is then extended to two groups of isostructural systems. The first group consists of two boron imidazolate frameworks (BIFs), in which the tetrahedrally coordinated atoms (T atoms) differ from those in ZIF-8, while the methylimidazolate linker remains the same. The calculations show that the nature of the T atoms has only a very limited effect on the interaction with the guest molecules. The second group of derivatives comprises four systems that incorporate the same T atom as ZIF-8 (zinc), but linkers with different substituents X, with X = –H, –NO2, –NH2, –CHO. In these cases, the interaction with CO2 and, to a much lesser extent, hydrogen is increased at the adsorption site that is associated with the substituents, most prominently in the nitro- and aldehyde-functionalised systems. A detailed analysis of the adsorption geometries is used to explain the favourable effect of the substituents. Furthermore, it is shown how a reasonable estimate of the average interaction energy can be obtained from a weighted average over the different adsorption sites, accounting for their possible occupancy. In the case of ZIF-8, this averaged value is compared to experimental heats of adsorption, and the deviations are discussed. Finally, possible applications in hydrogen storage and CO2/H2 separation are discussed. All materials show similar affinities for hydrogen, indicating that their performance in H2 storage applications is largely independent of the structural modifications considered. Because the CO2/H2 selectivity is related to the difference in affinity towards the two species, it can be expected from the DFT-D results that the nitro- and aldehyde-functionalised systems will perform considerably better than ZIF-8, especially for the removal of relatively small amounts of carbon dioxide from a hydrogen feed. This finding is particularly encouraging as both systems are synthetically accessible.

Controlling the Adsorption Enthalpy of CO2 in Zeolites by Framework Topology and Composition

Zeolites are often investigated as potential adsorbents for CO(2) adsorption and separation. Depending on the zeolite topology and composition (Si/Al ratio and extra-framework cations), the CO(2) adsorption heats at low coverages vary from -20 to -60 kJ mol(-1), and with increasing surface coverage adsorption heats either stay approximately constant or they quickly drop down. Experimental adsorption heats obtained for purely siliceous porous solids and for ion-exchanged zeolites of the structural type MFI, FER, FAU, LTA, TUN, IMF, and -SVR are discussed in light of results of periodic density functional theory calculations corrected for the description of dispersion interactions. Key factors influencing the stability of CO(2) adsorption complexes are identified and discussed at the molecular level. A general model for CO(2) adsorption in zeolites and related materials is proposed and data reported in literature are evaluated with regard to the proposed model.

Heats of Adsorption of CO and CO2 in Metal–Organic Frameworks: Quantum Mechanical Study of CPO-27-M (M = Mg, Ni, Zn)

Density functional theory is applied with a hybrid functional to which a parametrized damped 1/r6 term has been added to account for dispersion (B3LYP+D*). This method is used with periodic boundary conditions to get the structures of the adsorption complexes. Dispersion has a substantial share on the calculated adsorption energies (46–77%). For these structures, adsorption energies are also calculated with a hybrid high-level (MP2 with complete basis set extrapolation):low level (B3LYP+D*) method. The MP2 calculations are performed on cluster models. Comparison is made with experimental heats of adsorption. B3LYP+D* underestimates heats of adsorption by about 5 kJ/mol, whereas hybrid MP2:B3LYP+D* slightly overestimates them by about 2 kJ/mol. With MP2:B3LYP+D*, also the mean absolute error is somewhat smaller, 3.8 kJ/mol compared to 5.6 kJ/mol for B3LYP+D*. Both the B3LYP+D* and the hybrid MP2/CBS:B3LYP+D* method predict the same sequence of binding energies for carbon monoxide (Ni > Mg > Zn) and carbon ...

Enhanced Hydrogen Spillover on Carbon Surfaces Modified by Oxygen Plasma

Hydrogen storage capacity by hydrogen spillover on metal-doped carbons can be significantly enhanced by introducing surface oxygen functional groups to the carbon. It is shown that the enhancement is greater by oxygen plasma treatment compared to air oxidation because different surface groups were formed. Oxygen plasma treatment generated mainly semiquinone (C═O) groups while air oxidation formed mainly hydroxyl (C−OH) groups. Experimental heats of adsorption, X-ray photoelectron spectroscopy (XPS) analyses, and ab initio molecular orbital calculations showed that the binding energies between the spiltover hydrogen and the different groups followed the following order: lactone > semiquinone > carboxyl > basal plane. Thus, the H2 storage capacity at 298 K and 10 MPa was increased from 1.17 wt % (without O2 treatment) to 1.74 wt % on Pt-doped on a templated carbon that was pretreated with O2 plasma. Similar enhancements were seen on Pt doped on a superactivated carbon, AX-21. However, there was a decrease in storage capacity during the first three adsorption−desorption cycles (at 298 K and 10 MPa). The capacity decreased from 1.74 wt % to 1.30 wt % and remained unchanged after three cycles. XPS results showed that this decrease was caused by the very strong (and irreversible) binding of the spiltover hydrogen with the lactone groups (HO−C═O). Nonetheless, the main groups of semiquinone remain functional as receptor sites upon cycling, and the 1.3 wt % storage capacity is among the highest reversible capacities reported in the literature.

Desulfurization of high-sulfur jet fuel by mesoporous π-complexation adsorbents

Desulfurization of JP-5 jet fuel (1172 ppmw S) was investigated by π-complexation adsorption with AgNO 3 supported on mesoporous silica SBA-15 and MCM-41. The average pore sizes of AgNO 3/SBA-15 and AgNO 3/MCM-41 were 48.8 and 19.1 Å, respectively. The results of JP-5 desulfurization showed that significant sulfur breakthrough occurred at ∼10.0 and ∼15.0 mL/g by AgNO 3/SBA-15 and AgNO 3/MCM-41, respectively, at a space velocity of 1.25 h −1. The spent AgNO 3/MCM-41 was regenerated by a simple process (heating in air at 200 °C) and ∼50% of the sulfur capacity was recovered after the first cycle. Molecular orbital calculations show that Cu + (as that in CuY zeolite) formed stronger π-complexation bonding with the thiophenic compounds than Ag + (in AgNO 3), as evidenced by experimental heats of adsorption. However, pore diffusion limitation of the large sulfur molecules (alkylated benzothiophenes) became an important factor for desulfurization of high sulfur jet fuels such that the AgNO 3-supported mesoporous sorbents yielded substantially better results than Cu(I)Y, although Cu(I)Y was better for a model fuel that contained only small sulfur molecules. Among all sorbents that have been investigated, the AgNO 3/MCM-41 sorbent showed the best desulfurization performance for high sulfur jet fuels.

Nature and Tunability of Enhanced Hydrogen Binding in Metal−Organic Frameworks with Exposed Transition Metal Sites

Metal−organic framework (MOF) compounds with exposed transition-metal (TM) sites were recently found to exhibit significantly larger experimental heats of adsorption of H2 than classical MOFs, thus attracting greater attention. Here we show that the hydrogen binding in Mn4Cl−MOF is not of the expected Kubas type because there is (a) no significant charge transfer from TM to H2, (b) no evidence of any H2-σ* Mn-d orbital hybridization, (c) no significant H−H bond elongation, and (d) no significant shift in H−H stretching mode frequency. We make predictions for the magnetic superexchange interactions in Mn4Cl−MOF and determined low- and high-spin states of the Mn ion as local minima with very different hydrogen binding energies. We show that, by replacing Cl with F or Br, one can tune the H2 binding energy. We further reveal that the major contribution to the overall binding comes from the classical Coulomb interaction which is not screened due to the open-metal site and explains the relatively high binding energies and short H2−TM distances observed in MOFs with exposed metal sites compared to traditional ones. Finally, we show that the orientation of H2 has a surprisingly large effect on the binding potential, reducing the classical binding energy by almost 30%.

An alternative approach for ab initio fitted potentials: The n-pentane/silicalite-1 system

Quantum chemical calculations at the second-order Møller–Plesset perturbation (MP2) levels were performed to evaluate n-pentane/ n-pentane and n-pentane/silicalite-1 interactions where several hundred configurations of the pair were generated. The silicalite-1 crystal structure was represented by a 10-T ring, in which the chemical composition is O[10]Si[10]H[20]. The energies of these configurations were fitted to analytical functions. Our goal was to reproduce both the experimental heat of adsorption ( Q st ) as well as the self-diffusion coefficient ( D s ). The MP2/6-31+G(d,p) was applied and the collision constants were introduced into the ab initio fitted potential function. The extrapolated Q st from the newly developed function of 52.28 kJ/mol is 9.39% lower than that of the experiment and the dependence of the D s as a function of temperatures is in good agreement with that observed experimentally. The obtained function was applied for a series of molecular dynamics simulations by varying temperatures and the concentration of n-pentane. The obtained structural data were interpreted in terms of residence distributions. Their changes were, then, described in terms of a competition between potential fields exerted by the zeolite and molecular movement. The attractive potential fields dominate the molecular movement at low temperature and this dominance decreases when the temperature increases.

Microkinetic modeling of ethylene oxidation over silver

In the present work a detailed microkinetic model based on the surface science of ethylene oxidation over silver has been developed. The active phase of the catalyst is suggested to be a surface oxide on which atomic oxygen and ethylene can adsorb and react to form an oxametallacycle. The oxide oxygen creates Ag δ+ sites which promotes the adsorption of ethylene and is often termed nucleophilic or ionic oxygen in the literature. A less stable atomic oxygen often referred to as electrophilic or covalent in the literature is suggested to compete with ethylene for the Ag δ+ sites. The oxametallacycle is a common intermediate in epoxidation, ethylene combustion, and ethylene oxide combustion. The microkinetic model reproduces experimental heats of adsorption, sticking, TPD, and TPR measurements. The kinetics of epoxidation, ethylene combustion, and ethylene oxide combustion has been simulated by the model. The model has been validated successfully by comparing model output to measured initial rates for both oxygen- and ethylene-rich mixtures at different temperatures. Selectivity, apparent activation energies, isotope effects, and reaction orders are reproduced by the model. The rate and selectivity controlling steps in the reaction mechanism and the critical parameters of the model have been identified.

Kinetic interpretation of catalytic activity patterns based on theoretical chemical descriptors

We introduce descriptors of bond energies, calculated at the DFT level for bulk crystalline structures. We show the relevance of these descriptors for surface bonds, insofar as they are well correlated with experimental heats of adsorption. Based on these correlations, and combining adapted Brønsted–Evans–Polanyi free energy relationships and simple Langmuir–Hinshelwood rate expressions, we show that the trends in experimental activity patterns across the periodic table can be explained by a single variable, the relevant bond energy descriptor. We obtain “volcano” curves with strong maxima, at bond energies that indicate optimal chemical properties. Activities of metallic formulations involving binary alloys in hydrogenation and hydrogenolysis reactions, as well as of ternary sulfides in hydrodesulfurization, are correctly predicted by the metal–carbon or metal–sulfur bond energy descriptors calculated for simple structural models. Our approach therefore provides useful exploratory guidelines.

A Molecular Dynamics Simulation Study Towards Understanding the Effects of Diameter and Chirality on Hydrogen Adsorption in Singlewalled Carbon Nanotubes

ABSTRACTA force field methodology has been developed for the description of carbon-carbon and carbon-molecular hydrogen interactions that is ideally suited to modeling hydrogen adsorption on single-walled carbon nanotubes (SWNT). The method makes use of existing parameters of potential functions developed for sp2 and sp3 hybridized carbon atoms and allows accurate representation of molecular forces on curved carbon surfaces. This approach has been used in molecular dynamics (MD) simulations for hydrogen adsorption in SWNT. The results reveal significant nanotube deformations, consistent with ab initio MD simulations, and the calculated energies of adsorption at room temperature are comparable to the reported experimental heats of adsorption for H2 in SWNT. The efficiency of this new method has permitted the MD simulation of hydrogen adsorption on a wide range of SWNT types, varying such parameters as nanotube diameter and chirality. The results show that these SWNT physical parameters have a substantial effect on the energies of adsorption and hydrogen capacities.

Influence of carbon curvature on molecular adsorptions in carbon-based materials: a force field approach.

A general force field methodology is developed for description of molecular interactions in carbon-based materials. The method makes use of existing parameters of potential functions developed for sp(2) and sp(3) carbons and allows accurate representation of molecular forces in curved carbon environment. The potential parameters are explicitly curvature and site dependent. The proposed force field approach was used in molecular dynamics (MD) simulations for hydrogen adsorption in single-walled carbon nanotubes (SWNTs). The results reveal significant nanotube deformations and the calculated energies of adsorption are comparable to the reported experimental heat of adsorption for H2 in SWNTs.