M. M. Dubinin

Adsorption energy of water vapor on high-silica and pure-silica zeolites

The high heats of water adsorption on silicalite with the adsorption of 0.4 mmole/g result from an additional interaction with the silicalite OH groups, while the high heats of water adsorption on NaZSM-5 with the adsorption of 0.5 mmole/g result from an additional strong interaction with 0.5 mmole/g sodium cations. Tetraaqua complexes are formed about these sodium cations at the channel intersection sites in NaZSM-5. The first water molecule adds with an energy of about 100 kJ/mole, while the other three molecules add with the same energy of 56 ± 2 kJ/mole. The water adsorption isotherms on silicalite and NaZSM-5 are completely and satisfactorily described by two- and three-term VMOT equations.

Differential adsorption heats of water on powdered synthetic zeolite NaX

1. Employing a Tyan-Calvet calorimeter, a four-step curve was obtained for the differential adsorption heat of water on zeolite NaX. 2. Four mechanisms for the adsorption of H2O molecules on various energetically homogeneous sections of the large cavities in zeolite NaX were examined.

Heats of adsorption of methanol and ethanol on high-silicon ZSM-5 zeolite

The heats of adsorption of lower alcohols on NaZSM-5 have a stepwise appearance and each step corresponds to the stoichiometric formation of adsorption complexes of Na+ ions with from one to four alcohol molecules. All the adsorption complexes are located at zeolite channel intersections, while the alkyl groups enter these channels. The heats of adsorption of alcohols on NaZSM in the region of the formation of adsorption complexes with cations markedly exceed the heats of adsorption on silicalite, while on the noncationic part of the NaZSM-5 structure, they are identical to the heats of adsorption on silicalite. The mean molar integral adsorption entropies of alcohols are significantly less than the entropy of the liquid. The adsorbed molecules are in a solidlike state. The isotherms for the adsorption of alcohols on NaZSM-5 are completely described by VMOT equations.

Heats of adsorption of CO2 on high-silicon zeolites ZSM-5 and silicalite

The heat of adsorption of C02on NaZSM-5 at zero occupancy is 50.0 kJ/mole. The differential heats have two linearly descending segments, corresponding to the formation of two types of adsorption complexes with one or two C02 molecules, on the average. The heat of adsorption on silicalite coincides with the heat of adsorption of CO2 on the noncationic segment of the NaZSM-5 zeolite structure (28–29 kJ/mole). The adsorbate-adsorbate interaction forces are not evident on the zeolites up to 1.5 mmole/g occupancy. The isotherms for the adsorption of C02 on zeolite NaZSM-5 and silicalite at 303 K in the occupancy region of 0–1.5 and 0–0.5 mmole/g are completely described by VMOT equations.

Adsorption energetics of hydrocarbons on silicalite

The differential adsorption heat curves for hydrocarbons on silicalite feature Langmuir invariance without apparent interaction for heptane, a linear increase with increasing occupancy for pentane, and great complexity with both minima and maxima for benzene. The adsorption heat increment per CH2group is 10.0 kJ/mole from the adsorption heat data for ethane, butane, pentane, and heptane, while the free regression term corresponding to the adsorption of 2H or H2is 11.5 kJ/mole. The mean molar entropies of pentane and heptane is less than the entropies of the liquids by -60 J/mole · K, while the state of normal alkanes in silicalite channels is solidlike. The isotherms for the adsorption of hydrocarbons on silicalite are described completely by the volumetricmicropore occupancy theory equations.

Heat of adsorption of methanol on the ultrahigh-silica zeolite silicalite

Conclusions1.The DAK-1-1 calorimeter is similar to the standard Tian-Calvet calorimeter, but it also acquires the features of a double calorimeter in the compensation mode. The operating characteristics of the instrument and its advantages and disadvantages were analyzed; ways of improving the device were indicated.2.The different heats of adsorption of methanol vapors on silicalite were measured. The zeolite contains a small amount of hydroxyl groups which chemically react with methanol with the formation of surface methoxyl groups. The methanol fills the sorption space of the silicalite by only 80%. The adsorbing channels of silicalite are a homogeneous microporous system in which collective adsorbate-adsorbate interactions are clearly manifested.3.Calculation of the heat of adsorption of water vapors on silicalite according to an additive scheme as the difference between the heat of adsorption of methanol and the increment of the heat of adsorption on the CH2 group produced a value of ∼33 kJ/mole, which is significantly lower than the heat of condensation of water vapors. This the cause of the hydrophobicity of silicalite.

Energy of adsorption of gases and vapors on microporous adsorbents

1. The differential heats of adsorption of water vapors on crystalline NaA zeolite were measured with a Tian-Calvet calorimeter, and a reconstructed adsorption apparatus with mercury seals in place of the stopcocks with grease. The results obtained are in satisfactory agreement with analogous measurements performed earlier for molded NaA zeolite. 2. The time of establishment of adsorption equilibrium, obtained from the thermokinetic curve, varies depending on the degree of filling of the zeolite with water. At 2.3 mmoles/g it passes through a maximum and corresponds to blockage of all of the eight-membered oxygen apertures of the zeolite, entering the large cavities, by adsorbed water molecules. 3. A minimum and a second maximum were detected on the thermokinetic curves. Their appearance is associated with a process of nonequilibrium adsorption and the specifics of the energy processes that occur in this case.