MONTE-CARLO SIMULATIONS AND SPECTROSCOPIC STUDIES OF THE SORPTION ANDCOSORPTION OF GROUP-VI METAL HEXACARBONYLS IN FAUJASITIC ZEOLITES

Molecular simulations of the sorption and cosorption energetics and si ting locations of M(CO)(6) (M = Cr, Mo, W) in faujasitic M'(n)FAU zeol ites (n = 0-96, Si/Al = 100-1, extraframework cations M' = Li+, Na+, K +, Rb+, Cs+, Mg2+, and Ca2+) have been presented in combination with D iffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS), A ttenuated Total Reflectance (ATR-FTIR), and Raman scattering experimen tal study at low temperature. From the present Monte Carlo calculation s and the earlier and present experimental works a coherent picture of the sorption and the cosorption of M(CO)(6) has been drawn as a funct ion of the Si/Al ratio of the zeolite framework, the cation size, and the cationic site occupancy as well as the coverage of the sorbates. T he most favorable siting regions of the completely siliceous FAU zeoli te pore space are the windows of the supercages, with intercage and in tracage rapid motions of the M(CO)(6) molecules. In aluminated M'(n)FA U zeolites the calculated average energy provides evidence to the cont ribution of significant site II cation/sorbate interactions whereas th e site I,I' cation/ sorbate and sorbate/sorbate interactions remain we ak even at high loading. In M'(56)FAU zeolites with bulky cations (M' = K+, Rb+, Cs+), the M(CO)(6) molecules are rigidily held within the w indows of the supercages to the six octahedral disposed site II cation s. The sorption sites are randomly filled from low coverage to saturat ion. In M'(56)FAU zeolites with small cations (M' = Li+, Na+), the dis tribution of positions occupied by M(CO)(6) is located in the vicinity of the windows. It is suggested that M(CO)(6) is predominently entrap ped as an isolated molecule at low coverage whereas at high loading th e filling is primarily pairwise. In M'(28)FAU zeolites with site II pa rtial occupancy (M' = Mg2+, Ca2+), the distribution of positions occup ied by M(CO)(6) is located in the vicinity of the windows in two sligh tly different sorption sites.