Compensation between Enthalpy and Entropy

Micropore mass transfer resistance of zeoUte crystals is quantified in units of time by r /Dc, where is the crystal radius and Dc is the intracrystalline diffusivity. In addition to micropore resistance, zeolitic catalysts may offer another type of resistance to mass transfer, that is resistance related to transport through the surface barrier at the outer layer of the zeoHte crystal. Finally, there is at least one additional resistance due to mass transfer, this time in mesopores and macropores Rp/Dp. Here Rp is the radius of the catalyst pellet and Dp is the effective mesopore and macropore diffusivity in the catalyst pellet [18]. 

While microscopic techniques like PFG NMR and QENS measure diffusion paths that are no longer than dimensions of individual crystallites, macroscopic measurements like zero length column (ZLC) and Fourrier Transform infrared (FTIR) cover beds of zeolite crystals [18, 23]. In the case of the popular ZLC technique, desorption rate is measured from a small sample (thin layer, placed between two porous sinter discs) of previously equilibrated adsorbent subjected to a step change in the partial pressure of the sorbate. The slope of the semi-log plot of sorbate concentration versus time under an inert carrier stream then gives D/R. Provided micropore resistance dominates all other mass transfer resistances, D becomes equal to intracrystalline diffusivity while R is the crystal radius. It has been reported that the presence of other mass transfer resistances have been the most common cause of the discrepancies among intracrystaUine diffusivities measured by various techniques [18]. 

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The term compensation temperature arises from the compensation between enthalpy and entropy at the temperature Tc This temperature turns out to be virtually independent of the solute in an RPLC system [339]. The magnitude of Tc (200 - 300 °C) usually implies that it is a hypothetical rather than a practical temperature. 

Often if one compares the effect of chai ng structure on the thermodynamic properties of a series of related compounds in an acid-base reaction, he will find that both the enthalpy and the entropy undeigo apparently meaningless fluctuations, but that the free energy changes in a systematic manner. This is the result of a remarkable compensation between enthalpy and entropy which is often observed, and which has been the subject of several valuable discussions (29,117,220,276). Although the fundamental theory of the compensation phenomenon is not simple, one can see intuitively that any interaction between two particles, be they acid and base or ion and solvent, must involve energy and entropy terms in complemen-... 

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