Structure Features and Thermochemistry of the Intercalation of Pyridine and α-, β-, and γ-Picolines into Crystalline α-Titanium Hydrogen Phosphate

Abstract

The insertion of polar organic molecules inside the gallery space of the synthetic crystalline lamellar R-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 R-, -, and γ-picolines (R-, -, and γ-pic), respectively, with expansion of the original interlamellar distance of 0.76 nm, to accommodate these molecules. The interlamellar distances for R-and -pic of 1.15 and 1.12 nm indicate a parallel orientation to the inorganic sheets, while γ-pic with 1.32 nm 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 Brønsted 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 R-, -, and γ-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 (∆G and ∆S) are in agreement with a spontaneous and favorable intercalation process.

The Intercalation of Some Heterocyclic Amines into α-Titanium Hydrogenphosphate—Structural and Calorimetric Data

“…This kind of disturbance caused by the methyl groups was previously observed with picoline amines, where the position of the attached group on the ring causes a drastic change in the amount of the amine intercalated (21). The respective structures of the sequence of cyclic amines used in this investigation are illustrated below.…”

“…However, for piperidine and pyrazine the lamellar structure of the solids changed to poor crystalline materials as the time of intercalation increased (27). The decrease in crystallinity was also detected for the set pyridine and -, -, and -picolines (21). On the other hand, for pyridine the interlamellar distance of 1800 pm seemed to indicate an apparent contrast to other reports (6,14,28).…”

“…Piperidine and pyrazine molecules are very similar in structure and both caused di!erent increases in the observed interlamellar distance (21), suggesting that di!erent orientations can be expected for these inserted species. Thus, the interlamellar expansion can re#ect about the possible arrangement of these amines into the free cavity of the matrix, if we consider the relationship between the interlamellar distance and the length of the molecules involved, as was previously observed for the couple pyridine and 4-aminopyridine (14,21).…”

“…This interlamellar distance for this phase contained 3.75 mmol of pyridine, changing to 1800 pm as the "nal phase is formed. From 3.75 to 15.0 mmol of pyridine only this peak is detected due to the establishment of the "nal phase and the di!ractogram showed the appearance of some amorphous material (21).…”

“…Each thermal e!ect caused by the reaction was recorded after addition of the titrant. The same procedure was used to follow the thermal e!ect of dilution of the host and the guest solution in water (21,22).…”