| Abstract
| - The diffuse reflectance UV−visible absorption and Raman scattering spectrometry provide evidence of sorptionof 2,2‘-bipyridine (bpy, C10H8N2) as intact molecules into nonacidic MFI zeolites, silicalite-1, and Al-ZSM-5. The effect of the aluminum content n, the nature of the charge-balancing cation Mn+ as well as bpy loadingin Mm/n(AlO2)m(SiO2)96-m (Mm/nZSM-5; m = 0, 3, 6; n =1, 2; M = Na+, Zn2+) were examined. The Ramanscattering as well as the X-ray powder diffraction experiments provide evidence of weak structural deformationsof hosts upon bpy sorption. Clear evidence has emerged from Raman and UV−visible spectroscopicexperiments to support the contention that sorption of bpy in dehydrated MFI type zeolites results in threedifferent bpy conformations according to the aluminum content of the framework and the nature ofextraframework cations. The modeling investigations using Monte Carlo simulations, molecular mechanics,and molecular dynamics calculations provide coherent structural support to the interpretation of spectroscopicresults. It appears that the size and shape of straight channel itself of MFI type zeolites does not generateenergetic and steric constraints to change the trans planar structure of free bpy molecule. Thus, the sorptionof 2,2‘-bipyridine into purely siliceous silicalite-1 retains the trans-planar conformation. The nonbondinginteractions between the extraframework Na+ cation and occluded bpy in NanZSM-5 are the main drivingforces that stabilize the cisoid nonplanar conformer, whereas the coordination bonding between Zn2+ and Natoms in Zn3ZSM-5 requires the cis-planar conformation. The spectroscopic and modeling results were foundto be in agreement with previous quantum calculations concerning the conformational behavior of free 2,2‘-bipyridine relative to the inter-ring rotation.
|
