Adsorption microcalorimetry isotherms

Mesoporous materials (SBA-15 and Al-SBA-15 with various Si/Al ratios) were synthesized and investigated in relation to their capacity to be used as adsorbents for depollution of the contaminated air or wastewater. The compositional and the structural properties were determined by XRD, N2 isotherms, NMR, chemical analysis and XPS. The acidity and adsorption properties of the solids were checked by adsorption microcalorimetry using various basic or polluting molecules in gas phase. 

FIGURE 13.6 Types of generalized thermograms obtained in isothermal adsorption microcalorimetry. (From Solinas, V. and Ferino, 1., Catal. Today, 41, 179-89, 1998 and Andersen, P. J. and Kung, H. H., Catalysis, 11, 441-66, 1994. With permission.)... 

Heat-flow adsorption microcalorimetry. The most important type of isothermal calorimeter in current use is that based on the principle of the heat flowmeter, which was first applied by Tian (1923) and improved by Calvet (Calvet and Prat, 1958,... 

In a standard set-up for heat-flow gas adsorption microcalorimetry, the adsorbent bulb, (which is located in the thermopile (Figure 3.15)), is connected to a device to allow the simultaneous determination of the adsorption isotherm by one of the techniques listed in the previous sections. The simultaneous determination of the amount adsorbed and corresponding heat evolution allows one to assess the energy of adsorption (for the exact procedure, see Section 3.3.3) provided the following points are... 

Adsorption microcalorimetry, finally, allows to check the differential enthalpies of adsorption derived from various adsorption isotherms through the isosteric method. What is more, it is much safer and meaningful in the first, raising, part of the isotherm, specially when it is close to the ordinate, such as for instance for carbon dioxide adsorption. This part is probably the most interesting from the viewpoint of specific interactions and gas separation or storage. 

Rouquerol and co-workers have recently described the experimental determination of entropies of adsorption using isothermal adsorption microcalorimetry by a slow and constant introduction of adsorbate under quasiequilibrium conditions (77) or by discontinuous introduction of the adsorbate in an open system (72). 

Rouquerol et al. (11, 12) have recently described the experimental determination of entropies of adsorption by applying thermodynamic principles to reversible gas-solid interactions. Theoretically, the entropy change associated with the adsorption process can only be measured in the case of reversible heat exchange. The authors showed how isothermal adsorption microcalorimetry can be used to obtain directly and continuously the integral entropy of adsorption by a slow and constant introduction of adsorbate under quasi-equilibrium conditions (11) or by discontinuous introduction of the adsorbate in an open system (12). 

A prior study of the adsorption of nitrogen at 77 K on 5A and 13X zeolites using quasiequilibrium, isothermal, adsorption microcalorimetry experiments at 77K [16] has detected a step in the differential enthalpies of adsorption, towards the end of micropore filling. At the time, this was interpreted as a consequence either of the adsorbate-adsorbate interactions, or... 

An adsorption isotherm is a necessary but not sufficient way of describing the thermodynamics of ionic surfactant adsorption because a full description of the phenomenon requires knowledge of mutual interactions between all the components of the system. Such opportunity is offered by flow and batch liquid adsorption microcalorimetry [25-30]. 

The determination of adsorption thermodynamic quantities such as adsorption heats can now be performed through direct or indirect methods with a great degree of accuracy. The foundations of gas—solid interface calorimetry have been well established by combining adsorption microcalorimetry with adsorption in quasi-equilibrium. The experimental results reported so far, obtained from different calorimetries, concur with the values calculated from adsorption isotherms. 

The adsorption of three argon/nitrogen binary mixtures at 310 K and up to 0.6 bar are presented. A continuous, quasi-equilibrium flow technique of adsorptive introduction was used to allow high-resolution isotherms to be obtained. These are compared to differential enthalpies of adsorption determined using adsorption microcalorimetry. 

Llewellyn and Maurin (2005) demonstrated that gas (nitrogen and argon) adsorption microcalorimetry can be used a powerful technique for depth examination of the surface state of adsorbents and a minute following of adsorption mechanisms such as phase changes and transitions. The use of this technique in parallel to the measurements (and appropriate analysis) of the adsorption isotherms of the same gases, and DSC and/or NMR cryoporometry measurements can provide deeper insight into the interfacial phenomena over a broad temperature range. 

In Fig. 1.21a, the differential heats of adsorption of CO on H—BEA zeolite and on MFI-Silicalite are reported as a function of the adsorbed amounts. Volumetric isotherms are illustrated in the figure inset. In both cases the adsorption was fully reversible upon evacuation of the CO pressure, as typical of both physical and weak, associative chemical adsorption. For H-BEA a constant heat plateau at 60kJ mol was measured. This value is typical of a specific interaction of CO with coordinative unsaturated Al(III) atoms, as it was confirmed by combining adsorption microcalorimetry and molecular modeling [73, 74, 78, 89] Note that the heat value was close to the heat of adsorption of CO at cus Al(III) sites on transition catalytic alumina, a typical Lewis acidic oxide [55, 73], Once saturated the Al(III) defects, the heat of adsorption started decreasing down to values typical of the H-bonding interaction of CO with the Br0nsted acidic sites (- 30 kJ mol , as reported by Savitz et al. [93]) and with polar defects, either confined in the zeolite nanopores or at the external surface. 

In this preliminary Chapter it has been illustrated that adsorption microcalorimetry is very fruitfully employed in describing quantitatively the processes occurring at the gas-solid interface. The population of the surface sites active towards suitably chosen probe molecules is evaluated through the volumetric adsorption isotherms the... 

The results of CO adsorption microcalorimetry described in this overview were collected with a differential and isothermal microcalorimeter (Tian-Calvet Microcalorimeter) linked to a static volumetric system. The equipment permits the introduction of successive small doses of CO onto the catalyst. Both the calorimetric and the volumetric data were stored and analyzed by microcomputer processing. The obtained data are presented as differential heats versus the amount of CO adsorbed... 

Adsorption microcalorimetry of N2 and Ar at 77K was carried out with an equipment described by Rouquerol (ref. 8) and which associates quasi equilibrium adsorption volumetry with isothermal low temperature microcalorimetry (using Tian Calvet heat flow-meters) so that two curves are continuously recorded (heat flow and quasi equilibrium pressure) as a function of the amount of gas introduced into the systems. Continuous plots of the adsorption isotherm and of the derivative enthalpy of adsorption h vs surface coverage may easily be doived (refs. 4,7). 

Chemical composition was determined by elemental analysis, by means of a Varian Liberty 200 ICP spectrometer. X-ray powder diffraction (XRD) patterns were collected on a Philips PW 1820 powder diffractometer, using the Ni-filtered C Ka radiation (A, = 1.5406 A). BET surface area and pore size distribution were determined from N2 adsorption isotherms at 77 K (Thermofinnigan Sorptomatic 1990 apparatus, sample out gassing at 573 K for 24 h). Surface acidity was analysed by microcalorimetry at 353 K, using NH3 as probe molecule. Calorimetric runs were performed in a Tian-Calvet heat flow calorimeter (Setaram). Main physico-chemical properties and the total acidity of the catalysts are reported in Table 1. 

This is why we thought it worthwhile to switch to immersion microcalorimetry into either liquid nitrogen or - even better - liquid argon, by making use of an isothermal, heat-flux, microcalorimeter, initially designed and built in our laboratory for the sake of gas adsorption experiments at 77 or 87 K. 

The adsorption up to 50 bars was carried out by means of a Tian-Calvet type isothermal microcalorimeter built in the former CNRS Centre for Thermodynamics and Microcalorimetry. For these experiments, around 2 g of sample was used which were outgassed by Controlled Rate Thermal Analysis (CRTA) [7]. The experiments were carried out at 30°C (303 K). Approximately 6 hours is required after introduction of the sample cell into the thermopile for the system to be within 1/100 of a degree Celsius. At this point the baseline recording is taken for 20 minutes. After this thermal equilibrium was attained, a point by point adsorptive dosing procedure was used. Equilibrium was considered attained when the thermal flow measured on adsorption by the calorimeter returned to the base line. For each point the thermal flow and the equilibrium pressure (by means of a 0-70 bar MKS pressure transdueer providing a sensitivity of 0.5% of the measured value) were recorded. The area under the peak in the thermal flow, Q eas, is measured to determine the pseudo-differential... 

The continuous thermodynamic analysis of the evolution of the differential enthalpies of adsorption at 77 K directly measured by isothermal microcalorimetry can quite easily highlight such phenomena that have thus far been overlooked. 

Results for water sorption obtained by isothermal microcalorimetry at various temperatures and RH values are shown in Figure 51.2, which shows that the metastable form. A, exhibits greater heat of adsorption than form B under all conditions of temperature and RH investigated. At the three lower temperatures (20, 25, and 35°C), form A shows a sharp increase in heat of adsorption as function of RH at RH values above the 70 -80% range. Form B also shows a similar sharp increase, but in this case the sharp raise is only observed only at 25°C. Figure 51.3 also shows that for each of the two polymorphs, the heat of adsorption reaches the highest values at 25°C in comparison with the other temperatures. 

Surfactant adsorption close to the cmc may appear Langmuirian, but this does not automatically imply a simple orientation. For example, rearrangement from a horizontal to a vertical orientation or electrostatic interaction and counterion binding may be masked by simple adsorption isotherms. It is essential, therefore, to combine the adsorption isotherms with other techniques such as microcalorimetry and various spectroscopic methods in order to obtain a full picture of surfactant adsorption. 

Pudipeddi, M., T. D. Sokoloski, S. P. Duddu, and J. T. Carstensen. 1996. Quantitative characterization of adsorption isotherms using isothermal microcalorimetry. / Pharm. Sci. 85 381-386. 

Microcalorimetry is an extremely sensitive technique that determines the heat emitted or adsorbed by a sample in a variety of processes. Microcalorimetry can be used to characterize pharmaceutical solids to obtain heats of solution, heats of crystallization, heats of reaction, heats of dilution, and heats of adsorption. Isothermal microcalorimetry has been used to investigate drug-excipient compatibility [82]. Pikal and co-workers have used isothermal microcalorimetry to investigate the enthalpy of relaxation in amorphous material [83]. Isothermal microcalorimetry is useful in determining even small amounts of amorphous content in a sample [84]. Solution calorimetry has also been used to quantitate the crystallinity of a sample [85]. Other aspects of isothermal microcalorimetry may be obtained from a review by Buckton [86]. 

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