Quasi-equilibrated Temperature-programmed Desorption And Adsorption Research Articles

Co-Adsorption of Alcohols and Water in JUK-8 Studied Using Quasi-Equilibrated Thermodesorption.

JUK-8 ([Zn(oba)(pip)]n, oba2- = 4,4'-oxybis(benzenedicarboxylate), pip = 4-pyridyl-functionalized benzene-1,3-dicarbohydrazide) is a hydrolytically stable flexible metal-organic framework. Owing to its unusual adsorptive properties, JUK-8 can be considered as a promising sensing material for construction of detectors of volatile organic compounds (VOCs) in air. Quasi-equilibrated temperature-programmed desorption and adsorption (QE-TPDA) is a versatile method dedicated to characterization of porous materials. In this work, QE-TPDA was employed to study co-adsorption of water and selected alcohols in JUK-8. For the first time an infrared detector sensitive to organic compounds was used in the QE-TPDA measurements, allowing the study of the influence of water vapor on sorption of VOCs. The QE-TPDA profiles of the studied alcohols, exhibiting two desorption maxima and two adsorption minima, are consistent with the standard sorption isotherms, revealing a two-step adsorption-desorption mechanism. The profiles recorded in the presence of water are noticeably changed in different ways for different alcohols. While at low relative humidity (RH) (ca. 20%) the low temperature adsorption states of ethanol and 1-propanol were only slightly destabilized, for 2-propanol almost complete suppression of adsorption was observed. The results found for moderate RH levels (ca. 50%) indicated that the opening of the JUK-8 structure, responsible for its breathing behavior, was followed by the filling of the just generated pores with a water-alcohol mixture.

The Influence of UiO-66 Metal-Organic Framework Structural Defects on Adsorption and Separation of Hexane Isomers.

In this work, adsorption properties of the UiO-66 metal-organic framework were investigated, with particular emphasis on the influence of structural defects. A series of UiO-66 samples were synthesized and characterized using a wide range of experimental techniques. Type I adsorption isotherms for low-temperature adsorption of N2 and Ar showed that micropore volume and specific surface area significantly increase with the number of defects. Adsorption of hexane isomers in UiO-66 was studied by means of quasi-equilibrated temperature-programmed desorption and adsorption (QE-TPDA) experimental and Monte Carlo simulation techniques. QE-TPDA profiles revealed that only defect-free UiO-66 exhibits distinct two adsorption states. This technique also yielded high-quality adsorption isobars that were successfully recreated using Grand-Canonical Monte Carlo molecular simulations, which, however, required refinement of the existing force fields. The calculations demonstrated the detailed mechanism of adsorption and separation of hexane isomers in the UiO-66 structure. The preferred tetrahedral cages provide suitable voids for bulky molecules, which is the reason for unusual "reverse" selectivity of UiO-66 towards di-branched alkanes. Interconnection of the tetrahedral cavities due to missing organic linkers greatly reduces the selectivity of the defected material.

Adsorption of n-alkanes in ZIF-8: Influence of crystal size and framework dynamics

Due to its exceptional chemical and thermal stability, ZIF-8 is one of the most promising representatives of nanoporous metal-organic frameworks. In this work, we investigate adsorption properties of this material both experimentally and theoretically. The experiments were carried out on 8 preparations differing in morphology of the crystals. Adsorption was studied in isothermal approach exploiting standard adsorbates, such as N 2 or CO 2 , as well as in isobaric regime with C5–C9 linear alkanes. The latter were performed with the novel quasi-equilibrated temperature-programmed desorption and adsorption (QE-TPDA) technique showing that a complexity of n-alkanes adsorption mechanism in ZIF-8 depends on the nature of adsorbate. Unexpectedly, for adsorption of C7–C9 n-alkanes a two-step process was found. QE-TPDA yielded high quality adsorption isobars which were successfully reproduced by Grand-Canonical Monte Carlo molecular simulations. The calculations showed that the specific adsorption behaviour of ZIF-8 is due to the fact that its structure undergoes conformational changes in order to adapt to the guest molecules. QE-TPDA measurements with n-nonane were performed at conditions close to saturation of the adsorbate. This allowed to observe surface-related adsorption on the ZIF-8 crystals, which was correlated with their size. • Combined theoretical and experimental studies on adsorption of C5–C9 n-alkanes in ZIF-8. • Investigations on the influence of crystal size on sorption properties. • Two-step adsorption of specific n-alkanes influenced by framework flexibility. • Packing of n-alkanes similar as for LTA-type zeolites. • Hyperbolic trend between surface adsorption amount and the size of the crystals.

Gate-Opening Mechanism of Hydrophilic–Hydrophobic Metal–Organic Frameworks: Molecular Simulations and Quasi-Equilibrated Desorption

Adsorption of polar and nonpolar molecules in hydrophilic–hydrophobic STAM-1 metal–organic framework was studied by means of quasi-equilibrated temperature-programmed desorption and adsorption (QE-TPDA) experimental technique and molecular simulation. The QE-TPDA measurements revealed that thermal stability of the studied material in the presence of water may be lower than determined from thermogravimetric analysis. Molecular dynamics showed evident impact of diffusion on the adsorption mechanism in STAM-1. The QE-TPDA profiles recorded for adsorption of n-alkanes, water, and alcohols indicate the gate-opening effect occurring only upon adsorption of polar molecules, which was confirmed by in situ IR spectroscopy. Monte Carlo molecular simulations agree with experimental data revealing preferable adsorption sites for the molecules of alcohols in STAM-1. Simulations also showed that the molecular mechanism of the gate-opening is dependent on the size of polar guest molecules.

Water-Stable Metal-Organic Framework with Three Hydrogen-Bond Acceptors: Versatile Theoretical and Experimental Insights into Adsorption Ability and Thermo-Hydrolytic Stability.

A new microporous cadmium metal-organic framework was synthesized both mechanochemically and in solution by using a sulfonyl-functionalized dicarboxylate linker and an acylhydrazone colinker. The three-dimensional framework is highly stable upon heating to 300 °C as well as in aqueous solutions at elevated temperatures or acidic conditions. The thermally activated material exhibits steep water vapor uptake at low relative pressures at 298 K and excellent recyclability up to 260 °C as confirmed by both quasi-equilibrated temperature-programmed desorption and adsorption (QE-TPDA) method as well as adsorption isotherm measurements. Reversible isotherms and hysteretic isobars recorded for the desorption-adsorption cycles indicate the maximum uptake of 0.19 g/g (at 298 K, up to p/p0 = 1) or 0.18 g/g (at 1 bar, within 295-375 K range), respectively. The experimental isosteric heat of adsorption (48.9 kJ/mol) indicates noncoordinative interactions of water molecules with the framework. Exchange of the solvent molecules in the as-made material with water, performed in the single-crystal to single-crystal manner, allows direct comparison of both X-ray crystal structures. The single-crystal X-ray diffraction for the water-loaded framework demonstrates the orientation of water clusters in the framework cavities and reveals their strong hydrogen bonding with sulfonyl, acyl, and carboxylate groups of the two linkers. The grand canonical Monte Carlo (GCMC) simulations of H2O adsorption corroborate the experimental findings and reveal preferable locations of guest molecules in the framework voids at various pressures. Additionally, both experimental and GCMC simulation insights into the adsorption of CO2 (at 195 K) on the activated framework are presented.

Ordering of n-Alkanes Adsorbed in the Micropores of AlPO4-5: A Combined Molecular Simulations and Quasi-Equilibrated Thermodesorption Study

Adsorption of n-alkanes from methane to decane (C1–C10) on AlPO4-5 aluminophosphate was studied by means of Monte Carlo simulations. The additional experimental study with use of quasi-equilibrated temperature-programmed desorption and adsorption (QE-TPDA) yielded high quality adsorption isobars of C5–C10 n-alkanes. The QE-TPDA based isotherms were accurately reproduced in the simulation, thus validating the applied force field. The agreement between experimental and simulated data indicated that the QE-TPDA is a reliable method for studying porosity-related adsorptive properties. The simulations revealed differences in the adsorption mechanism of the n-alkane series, with unmistakable site-based adsorption for short molecules (C1–C5) at saturation. Kinetic properties of the studied systems determined with the use of molecular dynamics exposed chain length dependency of the self-diffusion coefficient, indicating probably a resonant diffusion mechanism.

Adsorption of n-Alkanes in MFI and MEL: Quasi-Equilibrated Thermodesorption Combined with Molecular Simulations

Adsorption of n-alkanes on high silica MFI and MEL zeolites was studied by means of experimental quasi-equilibrated temperature-programmed desorption and adsorption (QE-TPDA) and Monte Carlo simulations. An unusual, isobaric approach to adsorption measurements and simulations was applied. Good agreement between the experimental and calculated data observed for MFI indicates that the QE-TPDA is a reliable method for studying porosity-related adsorptive properties of molecular sieves. The calculated average occupation profiles confirmed limited mobility of hexane and heptane molecules adsorbed in the sinusoidal channels on the MFI, thus proving the concept of order–disorder phase transition postulated in explanation of the two-step desorption profiles of these alkanes observed for MFI zeolites. Partial agreement of the calculated isobars with the experimental data found for zeolite MEL indicated that adsorption of n-alkanes in this structure is more complex than assumed in the simulation model. However, two...

Thermoporosimetry of n-alkanes for characterization of mesoporous SBA-15 silicas – Refinement of methodology

This report focused on setting up general experimental protocols for n-alkanes thermoporosimetry (TPM) in application for the study of porosity in a series of mesoporous SBA-15 silicas. The results were referred to the porosity characteristics obtained from nitrogen adsorption/desorption at −196 °C and quasi-equilibrated temperature programmed desorption and adsorption (QE-TPDA) of nonane. n-Hexane, n-heptane and n-octane were used as liquid probes in TPM experiments. For the first time the role of heating and cooling rate and liquid to solid mass ratio in n-alkane-TPM measurements was evaluated. n-Heptane was found to be the most appropriate n-alkane probe, n-octane admissible one, and n-hexane unsuitable due to the strong dependence of the DSC effects on choice of experimental conditions. Empirical calibration equations for n-heptane and n-octane thermoporosimetry were established.

Unusual adsorption behavior of volatile hydrocarbons on MOF-5 studied using thermodesorption methods

Adsorption of n-alkanes (C5–C10), isooctane, benzene, toluene and p-xylene on MOF-5 was studied by means of quasi-equilibrated temperature programmed desorption and adsorption (QE-TPDA) and temperature programmed desorption (TPD). Unusual thermodesorption profiles showing increasing complexity with the size of a probe molecule have been observed. Number of peaks in the thermodesorption profile increased from one for pentane or benzene to five for decane. Considerable adsorption–desorption hysteresis was found for isooctane. These unusual thermodesorption profiles were discussed in comparison to those obtained for zeolites and ordered mesoporous silicas.

Application of quasi-equilibrated thermodesorption of linear and di-branched paraffin molecules for detailed porosity characterization of the mono-layered zeolite MCM-56, in comparison with MCM-22 and ZSM-5.

The pore characteristics of zeolite samples including two kinds of ZSM-5 crystals as a base case and the unique mono-layered MCM-56 in different structural forms have been studied by the new method QE-TPDA (quasi-equilibrated temperature-programmed desorption and adsorption) in comparison with the standard nitrogen adsorption. Both approaches produce consistent results in terms of micro- and meso-porous features as well as quantitative pore volume values. The benefits of QE-TPDA include fast data acquisition (hours) and small sample size (milligrams). It is very flexible in using various hydrocarbons as probe molecules, which may reveal additional details associated with pores, their internal environment and dimensions/shape of the sorbate molecules. Hence, QE-TPDA is a valuable complementary tool for porosity characterization of the ever increasing diversity of porous materials and their pore structures. This was demonstrated by the results for the desorption of nonane and 2,2-dimethyloctane (DMO). The latter showed an additional maximum in the intermediate temperature range (between 'micro-' and 'mesopore' regions) which could be attributed to adsorption in the subsurface micropores (i.e. the pore mouths) where DMO could be partially adsorbed with t-butyl groups remaining on the outside. This was also reflected in the discrepancy between apparent volumes of micro- and mesopores calculated from the nonane and DMO experiments. Pillared MCM-56 revealed visibly enhanced subsurface micropore adsorption compared to the parent (mono-layer MWW) and MCM-22 (multi-layered MWW) consistent with the expected increase in the content of external 12 ring surface cups.

Porosity and accessibility of acid sites in desilicated ZSM-5 zeolites studied using adsorption of probe molecules

Porosity of zeolites ZSM-5 (Si/Al=32 or 164) desilicated with NaOH or NaOH/TBAOH (tetrabutylammonium hydroxide) solutions was studied by means of quasi-equilibrated temperature programmed desorption and adsorption (QE-TPDA) of selected hydrocarbons. QE-TPDA of n-nonane confirmed substantial formation of mesopores upon desilication, observed also in N2 adsorption/desorption isotherms. Very narrow mesopore size distributions with maxima at about 4nm, typical for partially constricted mesopore networks, were obtained for all desilicated zeolites. It was also observed that desilication with NaOH/TBAOH results in formation of additional slightly larger mesopores (ca. 4–6nm) while even larger mesopores (6–10nm) were found after leaching with NaOH only. Thermodesorption of n-hexane revealed that the micropores in the desilicated zeolites remained unchanged. Acidity of the zeolites was characterized by quantitative IR measurements of pyridine and collidine (2,4,6-trimethylpyridine) adsorbed on acid sites. Increased numbers of Brønsted acid sites accessible for large collidine molecules found for the modified zeolites indicate that during desilication redistribution of the framework Al atoms to the surface of mesopores occurred. The influence of TBAOH presence in desilicating mixture on porosity and accessibility of the acid sites was also discussed.

Application of quasi-equilibrated thermodesorption of hexane and cyclohexane for characterization of porosity of zeolites and ordered mesoporous silicas

Quasi equilibrated temperature programmed desorption and adsorption (QE-TPDA) of hexane and cyclohexane was applied for characterization of zeolites 5A, ZSM-5, 13X, Y, NaMOR and ordered mesoporous silicas MCM-41, MCM-41/TMB, SBA-15 and HMS. Similar QE-TPDA profiles of hexane and cyclohexane with a single desorption maximum were observed for the wide pore zeolites. No adsorption of cyclohexane for zeolite 5A and a single desorption maximum for ZSM-5 were found, while two-step desorption profiles of hexane were observed for these zeolites. Similar values of the adsorption enthalpy and entropy of hexane and cyclohexane were obtained by fitting the Langmuir model functions for the zeolites X and Y. For NaMOR and ZSM-5 larger differences in these parameters were found. A single desorption peak found at low temperatures in the QE-TPDA profiles of hexane and cyclohexane for the studied silicas was attributed to the multilayered adsorption on their mesopore surface. The adsorption isobars calculated from the thermodesorption profiles were fitted with the BET function. This way values of the specific surface area and the adsorption heat were calculated. Additionally values of the initial heat of adsorption were found by fitting the Henry’s law to the high-temperature sections of the linearized isobars. The largest deviations from the BET and Henry functions and the largest values of the adsorption heats found for SBA-15 indicated the greatest heterogeneity of the adsorption sites on its surface.

Porosity of micro–mesoporous zeolites prepared via pseudomorphic transformation of zeolite Y crystals: A combined isothermal sorption and thermodesorption investigation

The porosity of micro–mesoporous zeolite crystals was studied by sorption measurements with different probe molecules. Recrystallization of a commercial zeolite Y with addition of a cationic surfactant as the mesostructure directing agent was used as a ‘constructive’ synthesis route to generate mesoporosity inside the zeolite crystals. The obtained material was characterized by X-ray powder diffraction, scanning and transmission electron microscopy and thermogravimetric analysis. The adsorption–desorption isotherms of N2 and Ar at 77K and quasi-equilibrated temperature programmed desorption and adsorption (QE-TPDA) of n-hexane and n-nonane were performed in order to characterize the hierarchical porous network. The QE-TPDA of n-hexane on the recrystallized zeolite confirmed the faujasite type structure and showed some distortion of the micropores. The pore size distributions calculated from the QE-TPD of n-nonane are more precise than those determined from nitrogen desorption isotherms, since determined from numerous experimental data points recorded in one desorption cycle.

Characterization of the porosity and surface chemistry of mesoporous silicas by quasi-equilibrated thermodesorption of 1-butanol and n-nonane

Mesoporous silicas MCM-41 and SBA-15 were studied using the quasi-equilibrated temperature-programmed desorption and adsorption (QE-TPDA) of n-nonane and 1-butanol. The QE-TPDA profiles of n-nonane containing one desorption maximum were typical for the mesoporous materials. The pore size distributions from the thermodesorption data were similar to those calculated from N 2 adsorption isotherms. The QE-TPDA profiles of 1-butanol exhibited low temperature maxima (below 70 °C) resulting from desorption of the adsorbate filling the mesopores and high temperature maxima (at ca. 100 °C) attributed to desorption of the molecules specifically adsorbed on the hydrogen-bonded silanols. These findings were confirmed by IR measurements. Hydrothermal treatment of MCM-41 resulted in increase of the H-bonded silanol concentration, while for SBA-15 an inverse effect was observed.

Studies on the equilibrated thermodesorption of n-hexane from ZSM-5 zeolite

Equilibrated thermodesorption (TPED) and quasi-equilibrated temperature programmed desorption and adsorption (QE-TPDA) were employed as methods for studying the influence of different extraframework cations (Na+, K+, Li+, Cu2+, Zn2+, or Mg2+) on adsorption of n-hexane on ZSM-5 zeolite with high Al content (Si/Al = 15). Considerable influence of the cations on both initial adsorption in the micropores and ordering of the adsorbed molecules, occurring at high coverages, has been observed. This influence is reflected by the values of the adsorption enthalpy and entropy, determined by fitting the dual site Langmuir (DSL) adsorption function to the equilibrated thermodesorption profiles. However, no clear correlation between the determined parameters and properties of the extraframework cations could be found.

Characterization of Acidity and Porosity of Zeolite Catalysts by the Equilibrated Thermodesorption of n-Hexane and n-Nonane

Thermodesorption of n-hexane and n-nonane from the acidic and non-acidic zeolites ZSM-5 and Y was studied by means of the thermogravimetric temperature programmed equilibrated desorption (TPED) and quasi-equilibrated temperature programmed desorption and adsorption (QE-TPDA). Micropore volumes determined from the adsorption capacity of n-hexane were close to those determined by N2 adsorption. Content and strength of acid sites in the acidic zeolites estimated by fitting the Arrhenius equation to the high temperature parts of QE-TPDA profiles of n-hexane attributed to its cracking were in agreement with their acidity characteristics obtained by IR spectroscopy of chemisorbed pyridine. The mesopore volume was determined from the QE-TPDA profiles of n-nonane. Coking observed only for QE-TPDA of n-nonane on H-USY zeolite resulted in blocking the micropores without affecting the mesopores.

Determination of the adsorption heat of n-hexane and n-heptane on zeolites beta, L, 5A, 13X, Y and ZSM-5 by means of quasi-equilibrated temperature-programmed desorption and adsorption (QE-TPDA)

Measurements of temperature-programmed desorption and adsorption of n-hexane and n-heptane under quasi-equilibrium conditions (QE-TPDA) were performed for several zeolites in TPD apparatus equipped with a TCD detector. One-step adsorption profiles for zeolites X and Y and two-step adsorption profiles for beta, L, 5A and ZSM-5 were observed. Approximate adsorption isobars calculated from the QE-TPDA profiles did not depend on the heating/cooling rate, proving equilibrium control of the adsorption degree. Values of the adsorption enthalpy and entropy, computed by fitting the Langmuir or dual site Langmuir model functions to the temperature derivatives of the experimental adsorption isotherms, were in agreement with the literature data. New method for determination of the initial (zero coverage) adsorption heat by fitting the linearized Henry function to the transformed high temperature experimental data was proposed. Values of the initial adsorption heat were close to those obtained using the inverse gas chromatography infinite dilution method.