Methanol Adsorption in Isomorphously Substituted AlPO-34 Clusters and Periodic Density Functional Theory Calculations

Methanol adsorption in isomorphously substituted MAPO-34 (M = Mn, Zn, Mg, Si, Ti, or Zr) zeolite clusters was investigated, and periodic density functional theory (DFT) calculations were carried out. All structures are optimized and charactered at B3LYP/LANL2DZ-6-31G** (the LANL2DZ basis set for Mn, Zn, Mg, Ti, and Zr atoms and 6-31G** basis set sequentially for Si, Al, O, C, and H atoms) and generalized gradient approximation and Perder−Burke−Ernzerhof theoretical levels. Both methods demonstrate that the type of metal dopant used plays an active role in methanol protonation. In Mn, Zn, and Mg-AlPO-34, the stable form of methanol is protonated. However, in Si, Ti, and Zr-AlPO-34, methanol is unprotonated and is simply physisorbed. In the protonated mode, the methoxonium cation forms two very strong hydrogen bonds (1.019−1.073 Å) with the negatively charged zeolite. On the other hand, in the physisorbed mode, methanol interacts with the zeolite framework to form an eight-member ring through two hydrogen bonds, one that is short and rather strong (1.392−1.676 Å) and one that is much weaker (1.941−3.036 Å).