Characterization immersion calorimetry

Since the porosity of carbons is responsible for their adsorption properties, the analysis of the different types of pores (size and shape), as well as the PSD, is very important to foresee the behavior of these porous solids in final applications. We can state that the complete characterization of the porous carbons is complex and needs a combination of techniques, due to the heterogeneity in the chemistry and structure of these materials. There exist several techniques for the analysis of the porous texture, from which we can underline the physical adsorption of gases, mercury porosimetry, small angle scattering (SAS) (either neutrons—SANS or x-rays—SAXS), transmission and scanning electron microscopy (TEM and SEM), scanning tunnel microscopy, immersion calorimetry, etc. 

If properly used, immersion calorimetry is a versatile, sensitive and accurate technique which has many advantages for the characterization of porous solids and powders. An indication of these possibilities is given in Figure 5.7. The major areas of application are outlined in this section and reference made to specific examples which are discussed in more detail in other chapters. 

Immersion calorimetry was in use over 70 years ago for the characterization of activated charcoals and silica gels. The measurement of heat of wetting appeared to provide a relatively simple way of determining the surface area of a porous adsorbent (Brunauer, 1945). 

Immersion calorimetry has much to offer for the characterization of powders and porous solids or for the study of adsorption phenomena. The technique can provide both fundamental and technologically useful information, but for both purposes it is essential to undertake carefully designed experiments. Thus, it is no longer acceptable to make ill-defined heat of immersion measurements. To obtain thermodynamically valid energy, or enthalpy, or immersion data, it is necessary to employ a sensitive microcalorimeter (preferably of the heat-flow isothermal type) and adopt a technique which involves the use of sealed glass sample bulbs and allows ample time (usually one day) for outgassing and the subsequent temperature equilibration. 

Immersion calorimetry is a very useful technique for the surface characterization of solids. It has been widely used with for the characterization of microporous solids, mainly microporous carbons [6]. The heat evolved when a given liquid wets a solid can be used to estimate the surface area available for the liquid molecules. Furthermore, specific interactions between the solid surface and the immersion liquid can also be analyzed. The appropriate selection of the immersion liquid can be used to characterize both the textural and the surface chemical properties of porous solids. Additionally, in the case zeolites, the enthalpy of immersion can also be related to the nature of the zeolite framework structure, the type, valence, chemistry and accessibility of the cation, and the extent of ion exchange. This information can be used, together with that provided by other techniques, to have a more complete knowledge of the textural and chemical properties of these materials. 

Exchanged zeolites were characterized by N2 adsorption at 77K, X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), immersion calorimetry and NHs-temperature programmed desorption (NH3-TPD). X-ray diffraction patterns (XRD) were obtained with a JSO Debye-Flex 2002 system, from Seifert, fitted with a Cu cathode and a Ni filter, using CuXa radiation (A,=1.5419) and 2°min of scanning rate. X-ray photoelectron spectroscopy (XPS) spectra were acquired with a VG-Microtech Multilab 3000 spectrometer equipped with a hemispherical electron analyzer and Mg Ka (1253.6 eV) 300W X-ray source. 

As described above, immersion calorimetry constitutes a powerful technique for the textural and chemical characterization of porous solids. In the absence of specific adsorbate-adsorbent interactions, heats of immersion can be related to the surface area available for the molecules of the liquid. However, the use of polar molecules or molecules with functional groups produces specific adsorbent-adsorbate interactions related to the surface chemical properties of the solid. An adequate selection of the immersion liquid can be used to study hydrophilicity, acid-base character, etc. Table 2 reports the enthalpies of immersion (J/g) into different lineal and branched hydrocarbons (n-hexane, 2-methyl-pentane and 2,2-dimethyl-butane) for Zn exchanged NaX zeolites. 

Water has been widely used as a probe molecule for the characterization of zeolites, especially of those with a high aluminium content [9]. Water adsorption on hydrophilic zeolites has been used to measure their pore volume, and it has been shown that the amount of water adsorbed is a linear function of the aluminium content [10]. Additionally, water adsorption is also highly sensitive to the nature, valence and accessibility of extra-framework cations [11]. Immersion calorimetry allows for the measurement of the degree of interaction between the zeolite and water, and this can be compared with the interaction between the zeolite and other molecules with different polarity. In this way, the polar character of the zeolite surface can be assessed. 

Zn(II) content would be responsible for the decrease in the acid character observed by immersion calorimetry into n-butylamine. This trend in the acid character has also been confirmed by the NH3-TPD experiments. The amount of NH3 desorbed increases from the parent NaX zeolite up to a maximum for the zeolite ZnNaX (60), and decreases thereafter for higher Zn loaxlings. This correlation between both techniques verifies the applicability of immersion calorimetry as a useful and fast technique for acid strength characterization, mainly if liquids with different basicity are used. 

A considerable number of different techniques has been employed in the past to characterize the porosity and surface chemistry of porous carbon materials. These include gas adsorption (mostly N2 and CO2) [9-14], immersion calorimetry [9], small-angle X-ray [11,15] and neutron [14] scattering, inverse gas chromatography [12,13], differential thermal analysis [12], Fourier transform infrared [12], Raman [16] and X-ray photoelectron [17] spectroscopies and electron spin resonance [16]. It is worth mentioning that the information about the porous structure of the material provided by this array of techniques is only indirect... 

The porous structure of active carbons can be characterized by various techniques adsorption of gases (Ni, Ar, Kr, CO ) [5.39] or vapors (benzene, water) [5,39] by static (volumetric or gravimetric) or dynamic methods [39] adsorption from liquid solutions of solutes with a limited solubility and of solutes that are completely miscible with the solvent in all proportions [39] gas chromatography [40] immersion calorimetry [3,41J flow microcalorimetry [42] temperature-programmed desorption [43] mercury porosimetry [36,41] transmission electron microscopy (TEM) [44] and scanning electron microscopy (SEM) [44] small-angle x-ray scattering (SAXS) [44] x-ray diffraction (XRD) [44]. 

The porous structure of chars from a high volatile bituminous coal from mine Pumarabule in Spain, initial and preoxidized, then steam activated, was characterized by carbon dioxide and benzene adsorption measurements, as well as by immersion calorimetry molecular probes with increasing critical dimensions were used. The influence of preoxidation of the coal on the values of parameters describing the pore size distribution, with particular attention to micropores, evaluated according to each of the applied methods, is discussed. 

Characterization of the pore structure of amorphous adsorbents and disordered porous catalysts remains an important chemical engineering research problem. Pore structure characterization requires both an effective experimental probe of the porous solid and an appropriate theoretical or numerical model to interpret the experimental measurement. Gas adsorption porosimetry [1] is the principal experimental technique used to probe the structure of the porous material, although various experimental alternatives have been proposed including immersion calorimetry [2-4], positron... 

Adsorption calorimetry, based on the use of different adsorbates, is now employed to probe the effects of various types of surface modification treatments on the surface chemistry of sohds. The technique is employed in particular to investigate and characterize activated carbons. Recent studies show good agreement between the results from this technique and immersion calorimetry. 

Porous Texture and Surface Characterization from Liquid-Solid Interactions Immersion Calorimetry AND Adsorption from Solution... 

Silvestre-Albero, J., Gomez de Salazar, C., Sepulveda-Escribano, A., and Rodriguez-Reinoso, F. (2001). Characterization of microporous solids by immersion calorimetry. Colloids Suif. A Physicochem. Eng. Aspects, 187-188, 151-65. 

The heat of immersion is a parameter that is measured directly in a calorimeter, while the surface energy of a solid is not easily measured. Indeed, the heat of immersion provides an indirect measure of the surface energy it also provides information on the surface heterogeneity of carbonaceous solids. F.arly work on this subject has been reviewed by Zettlemoyer and Narayan [39]. The use of immersion calorimetry to characterize the porous texture and also the surface chemistry of activated carbons has been reviewed by Rodriguez-Reinoso and coworkers [40,41]. 

Stoeckli and Kraehenbuehl [42] discussed the derivation of an exact expression for the enthalpy of immersion of activated carbons using Dubinin s theory as a starting point They tested this expression with experimental data for 10 different carbons immersed in benzene and -heptane. In a subsequent paper, Kraehenbuehl et al. [43] reported the use of immersion calorimetry to determine the micropore size distribution of carbons in the course of then-activation. Later on, Stoeckli and Centeno [44] pointed out that immersion calorimetiy is a useful tool for characterizing solid surfaces in general, but in the case of microporous solids it usually requires complementary information obtained from the adsorption isotherms. They also discussed the limitations and possibilities of the technique and recommended that at least one adsorption isotherm from the vapor phase (e.g., CH2CI2 or CsH ) be determined to remove all the uncertainties. 

Denoyel et al. [45] derived the pore size distributions of two sets of activated carbons (one activated in water vapor and the other activated with phosphoric acid) using immersion calorimetric data. They concluded that immersion calorimetry is a convenient technique to assess the total surface area available for a given molecule and the micropore size distribution. More recently, Villar-Rodil et al. [46] have followed this approach to characterize the porous texture of a series of NomexO-derived carbon fibers activated to various bum-offs using liquids with different molecular dimensions as well as N2 and CO2 adsorption Isotherms. Table 3 includes the immersion enthalpies and corresponding surface areas. Relative changes in surface area accessible to the different adsorbates were ascribed to... 

When coupled to gas adsorption data, calorimetric data can be very useful for the textural characterization of carbons. The use of chemical probes with different molecular sizes allow determining the pore size distribution [288-295]. On the other hand, relevant information concerning chemical properties of the carbon surfaces and their influence on the sorption properties of carbons can be obtained when using the appropriate calorimetric technique. Immersion, flow adsorption and gas-adsorption calorimetry have been employed for the study of surface chemistry of carbons. For instance, immersion calorimetry provides a direct measurement of the energy involved in the interaction of vapor molecules of the immersion liquid with the surface of the solid. This energy depends on the chemical nature of the solid surfajoe and the probe molecules, i.e. the specific interaction between the solid and the liquid. Comparison between enthalpies of immersion into liquids with different polarities provides a picture of the surface chemistry of the solid. Although calorimetric techniques are not able to completely characterize the complex surface chemistry of carbons, they represent a valuable complement to other techniques. 

However, there is uncertainty about this method because of networking effects of some adsorbents including activated carbons and carbon nanostructures. Other experimental techniques that usually implement for characterizing the pore stmcture of porous materials are mercury porosim-etry. X-ray diffiaction (XRD) or small angle X-iay scattering (SAXS), and immersion calorimetry. 

Enrichment in S/L interfaces is of great importance in numerous industrial purification processes (solvent purification, separation, water treatment, decoloriza-tion, flotation, oil recovery, detergency, and so on). The surface area of industrial adsorbents is also often derived from S/L adsorption isotherms. Adsorption at S/L interfaces can be divided into two types, namely adsorption from pure liquids and adsorption firom solutions. Interaction with pure liquids is often characterized by immersion calorimetry. 

In addition to the aforementioned methods, the characterization of acidic and basic sites on carbon surfaces has been carried out by using immersion calorimetry [72,78]. As will be analyzed further in Section IV, calorimetry has not been employed widely for carbon characterization and particularly not for the study of acid/base site distribution [78], Nevertheless, in recent papers, Stoeckli and... 

Porosity, with the dimensions of nanometers or less, cannot be precisely imaged even in the most recent of transmission and scanning electron microscopes and recourse has to be made to the powerful experimental techniques of physical adsorption of gases, of immersion calorimetry and of small-angle scattering of X-rays (SAXS) and neutrons (SANS) to characterize porosity. Microporosity has the dimensions of molecules and such molecules, as adsorbates, become the experimental probes providing significant information about the adsorption site. Hence, the phenomena of porosity and adsorption are inseparable. 

Characterization of microporous adsorbents by immersion calorimetry is not as straightforward as for non-porous adsorbents. Atkinson et al. (1982) measured the heats of immersion of a microporous carbon cloth and a microporous activated carbon in a series of organic liquids and, for a given solid, obtained a significant dependence of the heat of immersion with the liquid used. They concluded that the heat of immersion is a measure of the volume of pores accessible to the molecule of the immersing liquid, thus opening the possibility of using immersion calorimetry as a tool to obtain PSDs in microporous carbons. 

However, it is obvious that the accessibility of the larger molecules to the total microporosity of the two samples, activated to a lesser extent (8 and 19% bum-off), is very limited as compared with nitrogen. This is clear evidence of the potential of immersion calorimetry to characterize, easily, the micropore size distribution in molecular sieves, based on the use of wetting liquids of different molecular sizes and with similar interactions with the solid surface. 

In a further study, a series of CMS was prepared from coconut shells by carbonization and activation with carbon dioxide (De Salazar et al., 2000). This series was characterized by carbon dioxide adsorption at 273 K and by immersion calorimetry using liquids with different molecular sizes, dichloromethane (0.33 nm), benzene (0.37 nm), cyclohexane (0.48 nm), 2,2-dimethylbutane (0.56 nm) and a-pinene (0.7 nm). Immersion data were analyzed following the two methods described above. A graphitized carbon black, V3G, with a BET surface area of 62 m g (N2,77 K), was used as a non-porous reference to obtain the area enthalpy of immersion of a carbonaceous surface into the different liquids. With these values, and the enthalpies of immersion of the CMS into the dilferent liquids, the surface areas accessible to the liquids were obtained. These are plotted in Figure 4.50 as a function of the molecular dimension samples are identified by a number that indicates their activation time (De Salazar et al., 2000). 

Immersion calorimetry using polar liquids gives another insight to the characterization of the solid surfaces. Here, specific interactions between the liquid molecules and active centers at the solid surface play a major role. The comparison between enthalpies of immersion of liquids with different polarity provides a refined picture of the surface properties of the solid. This point can be illustrated by the following example. Stoeckli et al. (1983, 1998) measured the enthalpies of immersion of two non-porous illites, with different BET surface areas (N2 77 K) into benzene and water. Whereas the area enthalpy of immersion into benzene was similar for both samples, about 73mJm , the area enthalpies of immersion into water were quite different, 371 and 782 mJ m respectively, Stoeckli et al. (1998). Clearly, the compositions of these illite surfaces differ. 

Stoeckli F, Centano TA. On the characterization of microporous carbons by immersion calorimetry alone. Carbon 1997 35(8) 1097-1100. 

The use of adsorption methods to characterize porosity in carbons, and the (then considered) undisputable position of the N2 BET isotherm appeared to be permanent. However, the advent of immersion calorimetry and its application to carbon chemistry, together with the availability of commercial calorimeters, presented a significant challenge to the supremacy of the N2 (77 K) adsorption isotherm. The School of Adsorption, University of Alicante, made use of immersion calorimetry and reviewed their work over several years (Rodrfguez-Reinoso et al., 1997 Silvestre-Albero et al., 2001). Immersion calorimetry, as a method of characterization, is discussed at length in Section 4.7. 

Silvestre-Albero J, Sepulveda-Escribano A, Rodrfguez-Reinoso F. Characterization of microporous solids by immersion calorimetry. Colloid Surf A Physico chem Eng Aspect 2001 187-I88 151-165. 

Porosity and pore-size distributions were determined by gas adsorption and immersion calorimetry, with the measurement of helium and bulk densities. Volumes of micropores were calculated using the Dubinin-Radushkevich (DR) equation (Section 4.2.3) to interpret the adsorption isotherms of N2 (77 K), CO2 (273 K) and n-C4H o (273 K). Volumes of mesopores were evaluated by subtracting the total volume of micropores from the amount of nitrogen adsorbed at p/p° = 0.95. The two density values for each carbon were used to calculate the volume of the carbon skeleton and the total volume of pores (including the inter-particle space in monolithic disks). Immersion calorimetry of the carbon into liquids with different molecular dimensions (dichloromethane 0.33 run benzene 0.37 nm and 2,2-dimethylbutane 0.56 nm) permits the calculation of the surface area accessible to such liquids and subsequent micropore size distributions. The adsorption of methane has been carried out at 298 K in a VTI high-pressure volumetric adsorption system. Additional techniques such as mercury porosimetry and scanning electron microscopy (SEM) have also been used for the characterization of the carbons. 

This paper deals with the characterization of activated carbons obtained from Polyethylene Terphtalate (PET). This has been carried out by using several techniques. Among them immersion calorimetry of several organic vapours (n-hexane, benzene, cyclohexane and 2,2-DMB) and adsorption of the same vapours. Nice agreement is found between the textural characteristics determined by both techniques. 

Immersion calorimetry is a useful technique for the characterization of porous materials. The heat evolved in the immersion process is directly related to the integral enthalpy of adsorption if the experiment is carried out at constant pressure and temperature [1]. The experimental data, which are obtained by immersion measurements, are normally used to determine the textural characteristics of the adsorbent, i.e. the micropore volinne or the surface areas accessible to the wetting liquid. In relation to the former Stoeekli et al [2] consider that a thermodynamic consequence of the Dubinin theory is the equation 1. 

The micropore structure can be determined by several methods such as immersion calorimetry, small-angle X-ray scattering (SAXS) high resolution transmission electron microscopy (HRTEM) and s- and liquid-phase adsorption, among which the most widely us is gas adsorption[7]. The pore structure of activated carbon is usually characterised in terms of the pore size distribution (PSD), perhaps die most imporlant aspect of characterization of die structural heterogeneity of porous solids used in industrial applications. This PSD could be obtained as an arbitrarily chosen form such as, for instance, mma or C ssian distribution[8]. For a local isodierm one may choose traditional mmlels, statistical mechanical methods such as DFT, or, most accurate for micropores, methods based on Monte Carlo simulation. 

Adsorption from solution measurements have been employed for many years to characterize industrial adsorbents, but the data obtained are often difficult to interpret. Rouquerol and his co-workers have now made a systematic study of a series of activated charcoals in which the results of adsorption from solution are compared with data obtained by gas adsorption and immersion calorimetry. By adapting the a -method, they have shown that the adsorption of benzene from ethanol solutio is comparable with that of nitrogen from the gas phase and that the adsorption from solution data obtained with probe molecules of different shape provide a useful means of studying the enlargement of mciropore entrances. 

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