Volumetric-calorimetric measurements

Abstract The physical principles and experimental techniques of pure gas- and multi-component gas adsorption measurements hy the volumetric (or manometric) method are outlined. Examples are given. Thermovolumetric and combined volumetric-calorimetric measurements are presented. Pros and cons of the method are discussed. References. List of Symbols. 

As we here are mainly interested in adsorption measurement techniques for industrial purposes, i. e. at elevated pressures (and temperatures), we restrict this chapter to volumetric instruments which on principle can do this for pure sorptive gases (N = 1), Sect. 2. Thermovolumetric measurements, i. e. volumetric/manometric measurements at high temperatures (300 K - 700 K) are considered in Sect. 3. In Section 4 volumetric-chromatographic measurements for multi-component gases (N>1), are considered as mixture gas adsorption is becoming more and more important for a growing number of industrial gas separation processes. In Section 5 we discuss combined volumetric-calorimetric measurements performed in a gas sensor calorimeter (GSC). Finally pros and cons of volumetry/manometry will be discussed in Sect 6, and a hst of symbols. Sect. 7, and references will be given at the end of the chapter. 

Measurement of the thermokinetic parameter can be used to provide a more detailed characterization of the acid properties of solid acid catalysts, for example, differentiate reversible and irreversible adsorption processes. For example, Auroux et al. [162] used volumetric, calorimetric, and thermokinetic data of ammonia adsorption to obtain a better definition of the acidity of decationated and boron-modified ZSM5 zeolites (Figure 13.7). 

Heats of adsorption of ammonia were measured with a twin-conduction-type microcalorimeter equipped with a volumetric vacuum line. The details and procedures have been described previously [6-8], Prior to calorimetric measurements, samples were activated by calcination under 1 mPa pressure on increasing the temperature at a rate of 3 K min and at the final temperature, in general 723 K, for 10 h. Adsorption of ammonia was carried out at 473, 573 and 623 K. The Si-MAS-NMR spectra were taken using a JEOL GX-270. 

The heats of adsorption of the probe molecules were measured in a heat-flow microcalorimeter of the Tian-Calvet type from Setaram, linked to a glass volumetric line to permit the introduction of successive small doses of gases [6]. The equilibrium pressure relative to each adsorbed amount was measured by means of a differential pressure gauge (Datametrics). Successive doses were sent onto the sample until a final equilibrium pressure of 133 Pa was obtained. The adsorption temperature was maintained at 353 K in order to limit physisorption interactions between the probe molecules and the zeolites. All the samples were pretreated at 773 K under vacuum overnight prior to any calorimetric measurement. 

The adsorption calorimetric measurements were carried out at 423 K on a SETARAM microcalorimeter of calvet-type connected with a standard volumetric adsorption apparatus. The pressure measurements were made using a MKS Baratron membrane manometer. Prior to the ammonia adsorption, the samples (900 mg) were carefully calcined in high vacuum at 673 K for 15 h. 

CO adsorption volumetric/calorimetric data. Heats of CO adsorption and relevant adsorbed amounts were measured for the systems A1273IO23, ACE3i273l023, and ACE2012731023. 

The ideal-gas and ideal-solution approaches also differ because they are based on different kinds of experimental data. The residual properties and fugadty coeffidents depend on volumetric data measurements of P, v, T, and x. But the excess properties and activity coeffidents depend on density measurements for calorimetric measurements for h, and phase-equilibrium data for and y,-. Modem modeling tends to rely on volumetric data (equations of state), and a prindpal feature of this chapter has been to establish how excess properties can be computed from residual properties and how activity coefficients can be computed from fugadty coeffidents. But note that such calculations can be performed in either direction that is, at least in principle. 

Calorimetric measurements are less common than the measurements mentioned above and yield a different physical quantity. To be effective, the calorimetric method needs to be combined with either the volumetric technique, which is normal, or with the gravimetric technique which is a little more difficult for high-quality work. Both methods are used. Calorimetry measures the temperature change as the adsorption occurs. This along with a heat capacity measurements of the resultant adsorbate-adsorbent combination yields the heat of adsorption as a function of pressure. Less precise calorimetric measurements measure only the heat evolved which gives... 

Figures 5.24 and 5.25 present as examples the results of a volumetric and a calorimetric measurement on poly (vinylacetate). The glass transition has a characteristic signature which shows up in the curves. As we can see, the transition is associated with steps in the expansion coefficient dp /dT and the heat capacity d H/dT, i.e. changes in the slope of the functions p T) and H T). The transition extends over a finite temperature range with typical widths in the order of 10 degrees. The calorimetric experiment also exhibits another characteristic feature. One can see that the location of the step depends on the heating rate T, showing a shift to higher temperatures on increasing the rate.

Hence, a calorimetric measurement and also a volumetric experiment produce the step at a temperature where the relaxation time of the a-modes is in the order of minutes. 

The measurement of the heat of adsorption by a suitable calorimeter is the most reliable method for evaluating the strength of adsorption (either physical or chemical). Tian-Calvet heat-flow microcalorimeters are an example of high sensitivity apparatus which are suitably adapted to the study of gas-solid interactions when connected to sensitive volumetric systems [10-14, 50-55]. Volumetric-calorimetric data reported in the following were measured by means of either a C-80 or MS standard heat-flow microcalorimeter (both by Setaram, F), connected to ahigh vacuum (residual pressure... 

Dunne et al. [54] employed the Clapeyron equation for determining the isosteric heat of a variety of molecules adsorbed on Silicalite. The obtained data were compared by the same authors to the heat of adsorption measured calorimetrically by means of a home-made volumetric-calorimetric apparatus. The agreement between the two methods was fairly good. 

A few examples of adsorption processes accompanied by an endothermic step due to the deformation/reconstruction of the surface in interaction with molecules were illustrated. In the reported cases, the heat measured within the calorimetric cell was the combination of an exothermic (adsorption) and an endothermic (surface reconstruction) effect, which caused the calorimetrically measured heat to be lower than what expected on the basis of a plain adsorption. An extra-care in interpreting (at molecular level) the experimental calorimetric results should be addressed in several cases, and in this respect it is quite fhiitful to complement the molar volumetric-calorimetric data with results from other approaches, typically the various spectroscopic methods and/or the ab initio molecular modeling. 

Experiments Sorption equihbria are measured using apparatuses and methods classified as volumetric, gravimetric, flow-through (frontal analysis), and chromatographic. Apparatuses are discussed by Yang (gen. refs.). Heats of adsorption can be determined from isotherms measured at different temperatures or measured independently by calorimetric methods. 

The idea of calorimetry is based on the chemical reaction characteristic of molecules. The calorimetry method does not allow absolute measurements, as is the case, for example, with volumetric methods. The results given by unknown compounds must be compared with the calibration curve prepared from known amounts of pure standard compounds under the same conditions. In practical laboratory work there are very different applications of this method, because there is no general rule for reporting results of calorimetric determinations. A conventional spectrophotometry is used with a calorimeter. The limitations of many calometric procedures lie in the chemical reactions upon which these procedures are based rather than upon the instruments available . This method was first adapted for quinolizidine alkaloid analysis in 1940 by Prudhomme, and subsequently used and developed by many authors. In particular, a calorimetric microdetermination of lupine and sparteine was developed in 1957. The micromethod depends upon the reaction between the alkaloid bases and methyl range in chloroform. 

FIGURE 13.5 Calorimetric and volumetric data obtained from adsorption calorimetry measurements Raw pressure and heat flow data obtained for each dose of probe molecule and Thermokinetic parameter (a), Volumetric isotherms (b), Calorimetric isotherms (c), Integral heats (d), Differential heats (e), Site Energy Distribution Spectrum (f). (From Damjanovic, Lj. and Auroux, A., Handbook of Thermal Analysis and Calorimetry, Further Advances, Techniques and Applications, Elsevier, Amsterdam, 387-438, 2007. With permission.)... 

Qv and Qg should be evaluated at the same temperature and pressure, usually the relief pressure. QG, the volumetric rate of gas evolution, can be obtained from measurements in a calorimetric test by the use of equations (A2.3) or (A2.4) (see Annex 2). Qv is the volumetric rate of vapour generation and can be calculated, as follows, from the rate of temperature rise in a closed calorimetric test or in an open test with a high superimposed containment pressure (see Annex 2). 

Cobalt, copper and nickel metal ions were deposited by two different methods, ionic exchange and impregnation, on an amorphous silica-alumina and a ZSM-5 zeolite. The adsorption properties towards NH3 and NO were determined at 353 and 313 K, respectively, by coupled calorimetric-volumetric measurements. The average acid strength of the catalysts supported on silica-alumina was stronger than that of the parent support, while the zeolite-based catalysts had (with the exception of the nickel sample) weaker acid sites than the parent ZSM-5. The oxide materials used as supports adsorbed NO in very small amounts only, and the presence of metal cations improved the NO adsorption [70]. 

Four types of sorption measuring instruments ire on the market volumetric/manometric, gravimetric, carrier gas and calorimetric. 

Infrared spectroscopy measurements were performed using a Perkin Elmer 580 apparatus. The acid strength distribution of the samples was measured using both calorimetric and volumetric gas-solid titration. Ammonia, pyridine, and branched pyridines (2,6-lutidine and 3,5-lutidine) were the selected probes. They were further dried over activated 3A molecular sieve extrudates and were purified by freeze-thaw techniques. 

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