Structure features and thermochemistry of the intercalation of pyridine and alpha-, beta-, and gamma-picolines into crystalline alpha-titanium hydrogen phosphate
Lm. Nunes et C. Airoldi, Structure features and thermochemistry of the intercalation of pyridine and alpha-, beta-, and gamma-picolines into crystalline alpha-titanium hydrogen phosphate, CHEM MATER, 11(8), 1999, pp. 2069-2075
The insertion of polar organic molecules inside the gallery space of the sy
nthetic crystalline lamellar alpha-titanium hydrogen phosphate was followed
by a batch method at 298 +/- 1 K. The maximum amounts intercalated were 3.
34, 1.54, 1.85, and 2.21 mmol g(-1) for pyridine (py) and alpha-, beta-, an
d gamma-picolines (alpha-, beta-, and gamma-pic), respectively, with expans
ion of the original interlamellar distance of 0.76 nm, to accommodate these
molecules. The interlamellar distances for alpha- and beta-pic of 1.15 and
1.12 nm indicate a parallel orientation to the inorganic sheets, while gam
ma-pic with 1.32 nn forms a perpendicular monolayer of the matrix. The valu
e of 1.80 nm for py suggests that the guest molecules are interacting simul
taneous and perpendicularly to adjacent layers, in a typical Bronsted base
interaction. The intercalated compounds lost water of hydration, organic ma
tter, and water of condensation of the phosphate to give pyrophosphate on h
eating. The intercalation processes were also calorimetrically followed at
298.15 +/- 0.02 K. From thermal effect data, the variation in enthalpy for
each system was calculated. Either the intercalation based on batch method
or the thermal effects were fitted to a modified Langmuir equation. The exo
thermic enthalpic results for these intercalation gave the following sequen
ce of values: -44.62 +/- 0.15, -1.24 +/- 0.03, -1.95 +/- 0.08, and -2.90 +/
- 0.24 kJ mol(-1) for py and alpha-, beta-, and gamma-pic, respectively. Th
e variation in enthalpic values for pyridine derivatives can be interpreted
to reduction of the steric hindrance associated with the methyl group atta
ched to the heterocyclic ring. Thermodynamic data (Delta G and Delta S) are
in agreement with a spontaneous and favorable intercalation process.