Structure features and thermochemistry of the intercalation of pyridine and α-, β-, and γ-picolines into crystalline α-titanium hydrogen phosphate

The insertion of polar organic molecules inside the gallery space of the synthetic 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-, and gamma-picolines (alpha-, beta-, and gamma-pic), respectively, with expansion 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 gamma-pic with 1.32 nn forms a perpendicular monolayer of the matrix. The value of 1.80 nm for py suggests that the guest molecules are interacting simultaneous and perpendicularly to adjacent layers, in a typical Bronsted base interaction. The intercalated compounds lost water of hydration, organic matter, and water of condensation of the phosphate to give pyrophosphate on heating. 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 exothermic enthalpic results for these intercalation gave the following sequence 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. The variation in enthalpic values for pyridine derivatives can be interpreted to reduction of the steric hindrance associated with the methyl group attached to the heterocyclic ring. Thermodynamic data (Delta G and Delta S) are in agreement with a spontaneous and favorable intercalation process.