Microcalorimetric measurements

Alongside tliese teclmiques, microbalance measurements of adsorjDtion capacities and kinetics, microcalorimetric measurements of adsorjDtion processes and temperature-programmed desorjDtion of base molecules have provided useful infonnation about tire tliennochemistry of adsorjDtion processes and tire acidity characteristics of zeolites [46]. 

The microcalorimetric measurements of Della Gatta and his co-workers in their investigation of the interaction of water vapour with highly dehydroxylated y-alumina confirm that in this system also, the nondissocia-tive chemisorption of water is nonactivated, whilst the dissociative chemisorption is always activated. Thus the pseudo-equilibrium between the two chemisorbed states is displaced towards dissociative chemisorption as the temperature is increased above 150 C. 

Microcalorimetric measurements also confirm the occurrence of the phase transition [440]. A small peak (AH 1500 J-mole 1) was observed at about 480K for RbsNbsOFig. A second peak (AH 4850 J-mole 1) occurs at 590K. The second peak also appears upon cooling, but with a significantly smaller thermal effect (AH 1800 J-mole 1). This implies that even if the transition takes place at 590K, it probably occurs along with at least partial decomposition. 

With Ga-Beta it was found that, when the Si/Ga ratio increased from 10 to 40, the number of strong sites decreased drastically for Si/Ga between 10 and 25 and then reached a plateau above Si/Ga = 25 [53], The strength and density of acid sites in H(Ga, La)-Y were also found to be lower than those in HY crystals of the type used in FCC preparation (LZY-82) [250], Similar catalytic selectivities were obtained for both Ga-ZSM5 and A1-ZSM5 in Prins condensation of isobutylene with formaldehyde. Catalytic tests coupled with microcalorimetric measurements have shown that medium to weak acid strength sites favor the selectivity to isoprene [254],... 

The catalytically active sites of isomorphous substituted MFI structures (Al-Sil and In-Sil) have been characterized by infrared spectroscopy and microcalorimetric measurements using ammonia and acetonitrile as probes [261]. The hrst derivative of the heat of adsorption curves, dg/dfl, as function of the loading, a, gave maxima at about 140 and 100 kJ/mol for Al-Sil and In-Sil, respectively. Janchen and colleagues... 

For adiabatic type calorimeters, the initial and the final temperatures are by definition different. To which temperature will a derived A//-value then refer In connection with microcalorimetric measurements, the problem may merely be academic as the temperature change may be very small. However, it is at this point instructive to analyze in some detail what we do when we calibrate an adiabatic calorimeter. Let the initial and the final state of the experimental process be represented by A and B and the corresponding temperatures by TA and TB, respectively. The experimental process is thus ... 

Wadso, I. (1993). On the accuracy of results from microcalorimetric measurements on cellular systems. Thermochim. Acta 219,1-15. 

Clark, D.G., Brinkman, M., Neville, SD. (1986). Microcalorimetric measurements of heat production in brown adipocytes from control and cafeteria-fed rats. Biochem. J. 235,337-342. 

The Boltzmann law computes to a configurational AS governed by Eq. (3.22). A configurational AS represents dissolution of a perfectly ordered, pure solid polymer in pure solvent (Allcock and Lampe, 1981). van Oss (1991) cautions against designating physical processes as AH- or AS-driven unless careful microcalorimetric measurements have been made, because many thermodynamic suppositions (imputed to modeling or intuition) have not been substantiated by experimentation. Although descriptive analyses of... 

Buck, L.T., P.W. Hochachka, A. Schon, and E. Gnaiger (1993). Microcalorimetric measurement of reversible metabolic suppression induced by anoxia in isolated hcpatocytcs. Am. J. Physiol. 265 R1014-R1019. 

Perkin Elmer MPF-3 spectrofluorometer. X- and Q-band measurements of EPR spectra were carried out at liquid nitrogen and liquid helium temperatures. Microcalorimetric measurements were performed on a LKB 10700 batch microcalorimeter. Temperature-jump relaxation kinetics were measured using a double beam instrument (18) with a cell adapted for anaerobic work. The relaxation signals were fed into an H.P. 2100 computer and analyzed as described in Ref. 7. The pulse radiolysis exepriments were carried out on the 5-MeV linear accelerator at the Hebrew University. Details of the system have been published previously (19). 

Of the four types of wetting phenomena examined in the previous section, only immersional wetting lends itself to direct microcalorimetric measurement spreading and adhesion experiments would involve too small interfacial areas (say, no more than c. 100 cm2), whereas condensational wetting would require measurements up to p/p° = 1. As we saw in Chapter 3, these are the conditions where accurate measurements of the amounts of gas adsorbed are difficult to achieve. For this reason we confine the following recommendations to immersion microcalorimetry.. ... 

The enhancement of the potential energy in micropores can be directly assessed by microcalorimetric measurements of Aads li. Moreover, a comparison between N2 and Ar adsorption allows one to distinguish between the enhancement due to the confinement in micropores and that due to specific adsorbent-adsorbate interactions. Both effects are manifested in the low-pressure range of the nitrogen isotherm, but the specific interactions are virtually absent with argon. 

The acidity of pillared clays has been characterized by both microcalorimetric measurements of the adsorphon of aromatic molecules and pyridine and the catalytic ethylbenzene test reaction [111]. The aromatic probe molecules used were a reactant and a product of the catalytic reaction ethylbenzene and m-diethylben-zene, respectively. In this way, only the strongest of the accessible acid sites were htrated. The heats of adsorphon of these molecules indicate that a zirconium oxide pillared clay had stronger acidity than an aluminum oxide pillared clay, whereas the pyridine results were equal for both samples. 

We have previously shown that a most interesting information provided by immersion microcalorimetry into organic liquids was, in the case of carbons, a direct assessment of the internal surface area of the micropores [1]. This conclusion was based on calculations of adsorption potentials in micropores, on geometrical considerations and on microcalorimetric measurements on a number of activated carbons. It was only validated for carbons. 

Addition of flourine to H-mordenite enhanced considerably the acid strength of this catalyst but decreased the ratio of Brpnsted to Lewis acidity (167). Using IR spectroscopy of adsorbed pyridine, X-Ray diffraction, catalytic activity tests for cumene cracking, and microcalorimetric measurements of ammonia adsorption, it was shown that some of the acidic hydroxyl groups were substituted with fluorine and that the inductive effect of fluorine increased the acid strength of the remaining hydroxyl groups. 

When these bases are compared in terms of their respective proton affinities, the order of basic strength is ammonia < n-butylamine < pyridine < trimethylamine < piperidine < triethylamine, which is the same order observed with microcalorimetric measurements. In fact, plots of the initial differential heat of adsorption of ammonia, pyridine, trimethylamine, and triethylamine on silica-alumina and on silica as a function of the proton affinity give linear correlations, as can be seen in Fig. 7 (18, 105). 

As discussed above, there have been few systematic studies in which the acid or basic strength of materials relevant to catalysis has been correlated on a quantitative scale. The utility of microcalorimetric measurements of the heats of adsorption of various molecules is evident. These measurements can be used to determine the acid or basic strength of surfaces and establish their effect on the catalytic behavior of the materials. If we desire to control these acid-base properties to tailor and improve catalysts for existing processes and to design improved catalysts for new catalytic processes, a quantitative scale of the acid-base interactions is required. Appropriate correlations, perhaps involving electronegativity scales, would allow the prediction of the acid-base strength of the surface sites which can then be related to the catalytic activity of those sites. Additional research in this area is required. 

Fig. 6 Results of microcalorimetric measurements of spray-congealed carnauba wax. Untreated sample and samples annealed at different temperatures and stored for 5 days at ambient conditions all measurements taken at 45°C. (From Ref> l)...

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