Isothermal measurement of heats of hydration in zeolites by simultaneous thermogravimetry and differential scanning calorimetry

Abstract

A calorimetric method for determining isothermal partial and integral heats of hydration reactions (ΔH¯R,T,P${\rm{\Delta }}{\bar H_{{\rm{R,}}T,\,P}}$ and ΔH∼R,T,P${\rm{\Delta }}{\tilde H_{{\rm{R,}}T,\,P}}$, respectively) in zeolites and other mineral hydrates is presented. The method involves immersing a dehydrated sample in a humid gas stream under isothermal conditions within a thermal analysis device that records simultaneous differential scanning calorimetric (DSC) and thermogravimetric analysis (TGA) signals. Monitoring changes in sample mass (corresponding to extent of reaction progress) coincident with a quantitative measurement of heat flow allows for direct detection of ΔH¯R,T,P${\rm{\Delta }}{\bar H_{{\rm{R,}}T,\,P}}$ as a function of the extent of hydration, which can be integrated to determine ΔH∼R,T,P${\rm{\Delta }}{\tilde H_{{\rm{R,}}T,\,P}}$. In addition, it eliminates uncertainties associated with imprecise knowledge of the starting and final states of a sample during hydration. Measurement under isothermal conditions removes uncertainties associated with heat capacity effects that complicate interpretations of DSC measurements of dehydration heats conducted under traditional scanning temperature conditions. Example experiments on the zeolites natrolite, analcime and chabazite are used to illustrate strategies for quantifying ΔH¯R,T,P${\rm{\Delta }}{\bar H_{{\rm{R,}}T,\,P}}$ and ΔH∼R,T,P${\rm{\Delta }}{\tilde H_{{\rm{R,}}T,\,P}}$ and minimizing errors associated with baseline uncertainties. Results from this method agree well with previously published values determined by other calorimetric techniques and regression of phase equilibrium data. In the case of chabazite, the results allowed detailed measurements of the variation in ΔH¯R,T,P${\rm{\Delta }}{\bar H_{{\rm{R,}}T,\,P}}$ for energetically different water types encountered progressively as the sample absorbed water. This technique complements and in many cases improves the quality of thermodynamic data obtained through phase equilibrium observations and other calorimetric techniques.

Anomalous Temperature Dependence of the Heat of Hydration of Natrolite

“…It is shown below that these reaction behaviors change progressively between these temperatures. The dependence of reaction rate on degree of reaction in natrolite is markedly different from that found in other zeolite hydration and dehydration reactions which typically exhibit an exponential decrease in rate as the reaction progresses [14,23,25]. This behavior is only apparent in the last half of the experiment shown in Fig.…”

“…One potential explanation is that this disparity is related to the decrease in hydration capacity with temperature noted above, with differences in the energetics of the unoccupied water sites from those filled during the experiments. If this were true, however, H hyd for the sites filled last would most likely be less exothermic than the first ones to fill, as observed in other zeolites [16,23]. Furthermore, there is no indication of changes in H hyd as a function of hydration state in any of the results from the present study.…”

“…(2 and 3) are probably artifacts of the induction and cessation of reaction, respectively. The curious aspect of these experiments is the nearzero order reaction kinetics that dominates reaction behavior at 382 K and occurs over about the first half of the reaction at 482 K. This behavior has been noted previously [23,26] both during hydration and dehydration reactions in natrolite although its temperature dependence has not been previously studied in detail. Fig.…”

“…The hydration behavior and heats of hydration of natrolite were assessed under isothermal conditions through isothermal DSC measurements [23]. In each experiment, 20 to 30 mg of hydrated natrolite was placed into an unsealed PtRh crucible with perforated lid and dehydrated by scanning heating from 298 to 733 K at the rate of 15 K/min.…”

Microcalorimetric Study of the Catalytic Properties of SBA-15 Modified with Cu or Fe for Adsorption/oxidation of Methyl mercaptane

“…Thus, it is often difficult to prepare clean metal surfaces and maintain them in a zero-valence state for microcalorimetry. New advances in adsorption microcalorimetry have made it possible to measure differential heats of adsorption on single metal crystals 22,23 , polycrystalline metal films 24,25 , and metal powders and metal based catalysts that are reactive towards oxygenates 26 . Microcalorimetry has been used effectively to probe sites on supported metal catalysts (e.g.…”