The Activation of H2 by Zeolitic Zn(II) Cations

The adsorption and dissocn. of mol. hydrogen on the Zn(II) cations located in the channels of several zeolites has been studied theor. by periodical d. functional theory (DFT). The influence of the zeolite structure on the reactivity of this cation, placed in the eight-membered ring, was investigated by analyzing three different frameworks; chabazite, mordenite, and ferrierite. The relative position of two Al framework cations in the eight-membered ring has been also explored to evaluate the stability of the divalent cationic center as a function of the Al distribution. The Zn(II) cation is most stable when surrounded by two nearby Al-O-Si centers, regardless of the zeolite type. Ferrierite shows the strongest interaction. Hydrogen adsorbs either molecularly or in a dissociative way in chabazite and mordenite, depending on the relative position of the two Al centers in the eight-membered ring. The opposite trend is found for ferrierite, in which the mol. adsorption is always preferred. The large band shifts found in the exptl. IR spectra of hydrogen adsorption are reproduced in the current calcns. The largest Dn's are calcd. for CHA (AlSiAl) and FER (Al-far-Al). The first seems to be a result of the combined interaction of the hydrogen mol. with the Zn(II) and the oxygen atoms of the ring, whereas the second is a consequence of the strong interaction of the Zn(II) cation and the probe mol. [on SciFinder (R)]