Theoretical study of the adsorption of C1-C4 primary alcohols in H-ZSM-5.

The adsorption of C1-C4 primary alcohols at the Al12-O24(H)-Si12 intersection site in H-ZSM-5 is investigated using periodic density functional theory (DFT) calculations and adding a damped interatomic potential to the DFT results to account for dispersive van der Waals interactions (DFT-D). A good agreement between predicted and experimentally available adsorption enthalpies for C1-C3 alcohols is obtained. The effect of the H-ZSM-5 framework is sampled for adsorption of the C1-C4 alcohols in the straight and the zigzag channel. A variety of possible geometries, including hydrogen-bonded (physisorbed) and protonated (chemisorbed) complexes, are located as stable minima indicating that the potential energy surface connecting them is broad and very shallow. Experimental infrared (IR) spectra of the C1-C4 alcohols are interpreted based on harmonic frequency calculations for the most stable physisorbed and chemisorbed complexes. The stability of the adsorbed alcohols is governed by an interplay between their intrinsic basicity, van der Waals dispersive interactions and steric constraints exerted by the zeolite framework. In essence, steric constraints destabilize local hydrogen bonding and/or Coulomb alcohol-Brønsted acid site interactions while dispersive van der Waals interactions enhance the stability of physisorbed and chemisorbed complexes. Due to the prevalence of van der Waals interactions over steric constraints, C1-C4 alcohols adsorb preferentially in the more compact zigzag channel than in the straight channel. Both the physisorption and chemisorption energies increase linearly with 13 to 15 kJ mol(-1) per carbon atom in the straight and the zigzag channel, respectively.