Adsorption of acetic acid and methanol on H-Beta zeolite: An experimental and theoretical study

Abstract

The adsorption of acetic acid and methanol on H-Beta zeolite as a model of reaction of first step of the esterification reaction has been investigated with TGA-IR coupled, ATR-FTIR spectroscopy together with Density Functional Theory (DFT) calculations at M06–2X/6-31G(D) level. From the theoretical viewpoint, different models of adsorption of acetic acid and methanol on the surface of H-Beta zeolite are studied. TGA-IR experiments show that both reactants are molecularly adsorbed on H-Beta, beyond 200 °C for methanol and 250 °C for acetic acid other species are also formed due to the surface reactivity being strongly adsorbed on the catalyst. Results from ATR-FTIR spectroscopy and theoretical calculations reveal that the predominant adsorption mode of acetic acid involves the ads_AA(C[dbnd]O) complex where the acetic acid is molecularly adsorbed by the carbonyl group on the Brønsted acid site of catalyst, and the OH group is oriented to the Al-O-Si bridge. The mechanism of adsorption of both acetic acid and methanol is also discussed at molecular level. The complexes where the acetic acid is adsorbed by the carbonyl group are clearly the most stable one.