Structure features and thermochemistry of the intercalation of pyridine and alpha-, beta-, and gamma-picolines into crystalline alpha-titanium hydrogen phosphate

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.