Thermal decomposition of ammonia-borane under pressures up to 600 bar

The thermal decomposition of ammonia-borane BH₃NH₃ in the temperature range up to 450 K has been studied by differential scanning calorimetry (DSC) and volumetric analysis of the released volatile decomposition products. Measurements were performed in a transitiometer ST6-VI under pressures up to 600 bar and in a DSC C-80 in the pressure range 1-100 bar hydrogen. Above 360 K ammonia-borane undergoes an exothermic decomposion, which proceeds in two steps with rising temperature. The decomposition is accompanied by hydrogen release. Formation of further volatile products, beside hydrogen, seems to be negligible. The heat evolution and hydrogen release terminates near 430 K. The final amount of released hydrogen is approximately equal to 2 mol H₂/mol ammonia-borane. Variation of pressure does not influence significantly the reaction enthalpy and hydrogen release. The transitiometer ST6-VI is well-suitable for the monitoring of solid-gas reaction under high-pressure conditions. This instrument enables a reliable determination of the reaction heat and the amount of gas release/gas uptake.     

Determination of the micropore volume distribution function of activated carbons by gas adsorption

A new method for the determination of the micropore volume distribution function of activated carbons is presented. It is based on the treatment of pure gas adsorption isotherms by a theoretical model derived from the Hill-de Boer theory. Adsorption data (isotherms and heat curves) for carbon dioxide, ethane and ethylene on activated carbon (F30/470 CHEMVIRON CARBON) have been provided by a thermobalance coupled to a calorimeter (TG-DSC 111 SETARAM) at different temperatures (233, 273, 303 and 323 K) for pressures up to 100 kPa. Adsorption isotherms of carbon dioxide and ethane at 303 and 323 K have been used for the determination of the micropore volume distribution function of the activated carbon of interest. The knowledge of its structure has then allowed the simulation of adsorption isotherms and heats for the same adsorbates at the same temperatures as those experimentally studied. Similar calculations have been conducted for ethylene. Whatever the adsorbate (carbon dioxide and ethane used for the determination of the micropore volume distribution function or ethylene), the mean deviation between experimental and calculated isotherms does not exceed 4% at quasicritical and supercritical temperatures (303 and 323 K). In the same temperature conditions, discrepancies between calculation and experiment reach about 10% for adsorption heats. For both isotherms and heats, large discrepancies appear at low temperature (233 and 273 K). This method allows the determination of the micropore volume distribution function of activated carbons. The validity of the results is insured using several isotherms of several adsorbates and taking into account the calorimetric effect of the phenomenon. That is the reason why this method can also be seen as a new possible model for pure gas adsorption data prediction. This paper also presents a brief summary of the state of the art in this field.     

Gas sorption properties of microporous metal organic frameworks

A low-temperature gas sorption study has been carried out on four three-dimensional microporous metal organic framework (MMOF) structures and two two-dimensional layered structures. The pore characteristics are analyzed based on the argon adsorption-desorption isotherms at 87 K. The results from hydrogen sorption experiments conducted at 77 and 87 K show that all MMOFs have a relatively high hydrogen uptake, with adsorbed hydrogen densities falling in the range of liquid hydrogen. Isosteric heats of hydrogen adsorption data calculated based on the Clausius-Clapeyron equation are consistent with these observations, indicating strong sorbent-sorbate interactions.     

Adsorption of chlorobenzene and benzene on γ-Al2O3/atl>

Adsorption of chlorobenzene and benzene on -Al₂O₃ was investigated in the 413--572 K temperature region at an adsorbate partial pressure ranging from 2 to 1000 Pa. The adsorption isotherms were measured and the isosteric heats and the entropy characteristics of adsorption were determined. The experimentally found and theoretically calculated entropy changes upon adsorption were compared. The mobility of the molecules of both adsorbates in the adsorption layer was limited with respect to that predicted by the ideal two-dimensional gas model. The mechanism of adsorption of benzene and chlorobenzene is discussed.     

Heat of Adsorption of Pure Sulfur Hexafluoride on Micro-Mesoporous Adsorbents

The isotherms and the isosteric heats of adsorption of pure SF₆ were measured on two microporous zeolites (NaX and Silicalite), one mesoporous alumina, and two activated carbons (BPL and PCB) at 305 K. The adsorption isotherms were Type I by Brunauer classification. The PCB carbon adsorbed SF₆ most strongly and the alumina adsorbed SF₆ most weakly. The adsorption of SF₆ on the other three materials were comparable in the low pressure region despite their drastic differences in the physicochemical properties. The heat of adsorption of SF₆ on the silicalite and the alumina remained practically constant over a large range of coverage. The heat of adsorption of SF₆ increased with increasing adsorbate loading on the NaX zeolite in the high coverage region. The heat of adsorption of SF₆ on the activated carbons decreased with increasing adsorbate loading before leveling off in the high coverage region.     

Adsorption phenomena at high pressures and temperatures 1. Method of investigation; adsorption of nitrogen on NaA zeolite

A simple precision volumetric-gravimetric setup was elaborated and created for determining the equilibrium and kinetic parameters of adsorption in the 1–160 MPa pressure range and the 300–600 K temperature range. The isotherms of adsorption of nitrogen on crystalline NaA zeolite in the indicated temperature and pressure range were measured. A method was proposed for determining the pore volume of zeolites based on conducting adsorption measurements at high pressures of the gas phase. The isosteric heats of adsorption of nitrogen on NaA zeolite at 305, 334, and 373 K were estimated.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1233–1238, June, 1990.     

Influence of CO2 activation on hydrogen storage behaviors of platinum-loaded activated carbon nanotubes

In this work, platinum (Pt) metal loaded activated multi-walled carbon nanotubes (MWNTs) were prepared with different structural characteristics for hydrogen storage applications. The process was conducted by a gas phase CO₂ activation method at 1200 °C as a function of the CO₂ flow time. Pt-loaded activated MWNTs were also formulated to investigate the hydrogen storage characteristics. The microstructures of the Pt-loaded activated MWNTs were characterized by XRD and TEM measurements. The textural properties of the samples were analyzed using N₂ adsorption isotherms at 77 K. The BET, D-R, and BJH equations were used to observe the specific surface areas and the micropore and mesopore structures. The hydrogen storage capacity of the Pt-loaded activated MWNTs was measured at 298 K at a pressure of 100 bar. The hydrogen storage capacity was increased with CO₂ flow time. It was found that the micropore volume of the activated MWNTs plays a key role in the hydrogen storage capacity.     

Heats of adsorption of CO2 on high-silicon zeolites ZSM-5 and silicalite

The heat of adsorption of C0₂on NaZSM-5 at zero occupancy is 50.0 kJ/mole. The differential heats have two linearly descending segments, corresponding to the formation of two types of adsorption complexes with one or two C0₂ molecules, on the average. The heat of adsorption on silicalite coincides with the heat of adsorption of CO₂ on the noncationic segment of the NaZSM-5 zeolite structure (28–29 kJ/mole). The adsorbate-adsorbate interaction forces are not evident on the zeolites up to 1.5 mmole/g occupancy. The isotherms for the adsorption of C0₂ on zeolite NaZSM-5 and silicalite at 303 K in the occupancy region of 0–1.5 and 0–0.5 mmole/g are completely described by VMOT equations.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2636–2638, November, 1989.     

Estimation of the average heat of gas adsorption from the adsorption isotherm measured at supercritical temperatures and pressures

Using a high-precision volumetric—gravimetric experimental unit, the adsorption iso-therms of methane, argon, and nitrogen were measured at pressures of 0.1—40 MPa and at temperatures of 303—373 K on four carbon adsorbents with different porosities. To calculate the characteristic energy and average heat of adsorption of gases in the supercritical region, two parameters, which characterize an adsorptive were introduced: critical temperature T _c of the gas and the analog of the saturated vapor pressure “P _s”, which is defined assuming that densities of an adsorbate and an adsorptive are equal. The average heats of gas adsorption well agree with the average isosteric heats for the adsorption systems over the entire pressure and temperature intervals under study.     

Adsorption micro calorimeter

In this work, it is described an innovative heat flux micro calorimeter Tian-Calvet type designed to measure adsorption heats and reactions as well as adsorption isotherms. It consists in an adsorption instrument for volumetric gases, which is coupled to the micro calorimeter. The changes in the pressure are monitored by means of high sensitivity and high precision pressure transducers. The micro calorimeter has thermo elements that work by a Seebeck effect, in a twin cells system. The cells are inside a box in which the temperature can be adjusted from 77 to 300 K. The sensitiveness of the calorimeter is established by applying a perfectly known electric work. The results corresponding to the electric calibration, the base line stability determination and the time constant in the equipment are shown.     

Adsorption isotherms of CO2, CO,N2, CH4, Ar and H2 on activated carbon and zeolite LiX up to 1.0 MPa

The adsorption isotherms of CO₂, CO, N₂, CH₄, Ar, and H₂ on activated carbon and zeolite LiX were measured using a volumetric method. Equilibrium experiments were conducted at 293, 308, and 323 K and pressures up to 1.0 MPa. The adsorption isotherm and heat of adsorption were analyzed for two pressure regions of experimental data: pressures up to 0.1 MPa and up to 1.0 MPa. Each experimental isotherm was correlated by the Langmuir, Sips, Toth and temperature dependent Sips isotherm models, and the deviation of each model was evaluated. The Sips and Toth models showed smaller deviation from the experimental data of adsorbents than the Langmuir model. Isosteric heats of adsorption were calculated by the temperature dependent Sips model and are presented along with surface loading. From deviation analysis, it is recommended that the isotherm in the proper pressure range be used to appropriately design adsorptive processes.     

Accurate correlation,structural interpretation,and thermochemistry of equilibrium adsorption isotherms of carbon dioxide in zeolite NaX by means of the GSTA model

Zeolite NaX (commonly known as zeolite 13X) has found wide use in industry for the separation of carbon dioxide from air, methane-containing landfill gas, and flue gases. Capture and sequestration of carbon dioxide has become of utmost importance to mitigate severe environmental problems associated to burning of fossil fuels, such as the greenhouse effect and the consequential warming of global climate. Due to its low energy consumption and ease of operation, the zeolite-13X molecular-sieve pressure-swing adsorption process has become the method of choice for the recovery and capture of carbon dioxide from air and flue gas. Accurate correlation of the equilibrium adsorption isotherms of carbon dioxide in zeolite NaX is required for the reliable modeling and simulation of that process. In this paper, we firstly show that none of the traditional adsorption isotherm models (such as those of Langmuir, Sips, Toth, UNILAN, and Dubinin–Astakhov) is entirely capable of correlating a published set of equilibrium adsorption isotherms of carbon dioxide in zeolite NaX that were measured over a range of eight decades of pressure. A generalized statistical thermodynamic adsorption (GSTA) model, which we had already derived and successfully applied to the adsorption of water vapor in zeolite 3A, is employed in this work to obtain a very accurate correlation of that set of adsorption isotherms of carbon dioxide in zeolite NaX, for the pressure range from 0.2 Pa to 6.4 MPa and in the temperature range from −78 to 150 °C. We also provide thermochemical and structural interpretations of the isotherms fit and make predictions for the isosteric heat of adsorption that are in excellent agreement with the available experimental data.     

Distribution coefficients in the water-dodecane system and the heats of adsorption of C<Subscript>1</Subscript>-C<Subscript>8</Subscript> alcohols on silica

Distribution coefficients in the water-dodecane system and the heats of adsorption of C₁-C₈ alcohols on silica (Silochrom S-80) were determined by gas chromatography. At low temperatures, C₁-C₄ alcohols were largely distributed in the aqueous phase, and C₆-C₉ alcohols were predominantly dissolved in the organic phase; C₅ alcohols had intermediate properties. Permittivity was found to correlate with log K _d, which allowed us to predict the character of the distribution of alcohols in the water-oil system. The heats and isotherms of adsorption were determined for the alcohols studied. The heats of adsorption of alcohols on silica monotonically increased in the series under consideration. For C₅-C₈ alcohols, they exceeded the heat of adsorption of water. The isotherms of adsorption were described by the Freundlich equation.     

Adsorption of carbon dioxide on microporous carbon adsorbents

Adsorption isotherms of carbon dioxide on microporous carbon adsorbents prepared by activation with potassium sulfide in water vapor were measured. The measurements were carried out in the pressure interval from 1 Pa to 0.1 MPa at temperatures from 216.2 to 293.15 K. Based on the theory of volumetric filling of micropores, the main structural and energetic parameters of the microporous carbon adsorbents were calculated. The adsorption isosters of carbon dioxide were calculated from the adsorption isotherms in the same pressure and temperature ranges and approximated by linear dependences. The plots of the differential mole isosteric heats of adsorption vs amount adsorbed were constructed by using the adsorption isosters.     

Adsorption isotherms and ideal selectivities of hydrogen sulfide and carbon dioxide over methane for the Si-CHA zeolite: comparison of carbon dioxide and methane adsorption with the all-silica DD3R zeolite

Adsorption isotherms of H₂S, CO₂, and CH₄ on the Si-CHA zeolite were measured over pressure range of 0–190 kPa and temperatures of 298, 323, and 348 K. Acid gases adsorption isotherms on this type of zeolite are reported for the first time. The isotherms follow a typical Type-I shape according to the Brunauer classification. Both Langmuir and Toth isotherms describe well the adsorption isotherms of methane and acid gases over the experimental conditions tested. At room temperature and pressure of 100 kPa, the amount of CO₂ adsorption for Si-CHA zeolite is 29 % greater than that reported elsewhere (van den Bergh et al. J Mem Sci 316:35–45 (2008); Surf Sci Catal 170:1021–1027 (2007)) for the pure silica DD3R zeolite while the amounts of CH₄ adsorption are reasonably the same. Si-CHA zeolite showed high ideal selectivities for acid gases over methane at 100 kPa (6.15 for H₂S and 4.06 for CO₂ at 298 K). Furthermore, H₂S adsorption mechanism was found to be physical, and hence, Si-CHA can be utilized in pressure swing adsorption processes. Due to higher amount of carbon dioxide adsorbed and lower heats of adsorption as well as three dimensional channels of Si-CHA pore structure, this zeolite can remove acid gases from methane in a kinetic based process such as zeolite membrane.     

Adsorption of CO2 and N2 on synthesized NaY zeolite at high temperatures

NaY zeolite particles with a high surface area of 723 m²/g were synthesized by a hydrothermal method. Adsorption isotherms of pure gases CO₂ and N₂ on the synthesized NaY particles were measured at temperatures 303, 323, 348, 373, 398, 423, 448 and 473 K and pressures up to 100 kPa. It was found that the adsorption isotherm of CO₂ on the synthesized zeolite is higher than that on other porous media reported in the literature. All measured adsorption isotherms of CO₂ and N₂ were fitted to adsorption models Sips, Toth, and UNILAN in the measured temperature/pressure range and Henry’s law adsorption equilibrium constants were obtained for all three adsorption models. The adsorption isotherms measured in this work suggest that the NaY zeolite may be capable of capturing CO₂ from flue gas at high temperatures. In addition, isosteric heats of adsorption were calculated from these adsorption isotherms. It was found that temperature has little effect on N₂ adsorption, while it presents marked decrease for CO₂ with an increase of adsorbate loading, which suggests heterogeneous interactions between CO₂ and the zeolite cavity.     

Adsorption of benzene on the V2O5/γ-Al2O3 catalyst

Adsorption of benzene on the V₂O₅/-Al₂O₃ catalysts was studied in the temperature interval from 443 to 493 K and at partial pressures of the adsorbate ranging from 1 to 400 Pa. The adsorption isotherms were plotted. The isosteric heats and various entropy characteristics of adsorption were determined. Mobility of benzene in the adsorption layer is restricted compared to the model of ideal dimeric gas. The adsorbed amounts of benzene and chlorobenzene are compared.     

Modeling of water vapor adsorption isotherms onto polyacrylic polymer

The adsorbed amounts of water vapor onto polyacrylic polymer (polymer ×10) were measured using a thermogravimetry method as a function of pressure at 298 and 313 K. The adsorption isotherms are categorized to type II isotherms by IUPAC classification leading to a hysteresis loop between adsorption and desorption branches. The current study was completed by the measurement of the adsorption heats at 298 K using a differential scanning calorimetry. The calorimetric curves showed two adsorption heats domains. These domains have been attributed to the adsorption of “equivalent monolayer” and the condensation of water between polymeric chains. The correlation of experimental data to some chosen theoretical models shows that the GAB model is the most adequate to describe water vapor sorption isotherms.     

Adsorption equilibria of CO/H2 with a magnetic suspension balance: Purely gravimetric measurement

Adsorption equilibria of pure gases and binary gas mixtures can be measured with a new magnetic suspension balance. For this measurement no additional concentration measurement is required, neither for the gas phase nor for the adsorbed phase. The new instrument allows gravimetric adsorption measurements in a wide range of pressure (UHV...50 MPa) and temperature (210 K...570 K) to be performed. The experimental procedure and the instrument set up is fairly easy and can be compared to pure gas adsorption experiments. The new instrument and experimental procedure have been tested by performing coadsorption measurements with CO/H₂ mixtures on a commercial 5A zeolite.This revised version was published online in November 2005 with corrections to the Cover Date.