The catalytic conversion of sustainable biomass-based feedstocks into commercial fuels and chemicals is critical for reducing dependence on fossil fuel resources. A variety of zeolite-based materials, particularly with Lewis rather than Brønsted acidic character, have been identified as catalysts for such processes. Here, we demonstrate a gas-solid ion exchange process between alkali-metal form zeolites and organic acids or esters wherein Brønsted acid sites are generated in situ, turning a purely Lewis acidic surface into a multifunctional one. Transmission FTIR of pyridine adsorption and probe reactions including isopropanol dehydration were employed to characterize the density and acid strength of the in situ generated sites, respectively, on NaY (Si/Al = 1.6). The acidity of the dopant molecule, defined by the gas-phase deprotonation enthalpy (GPDE), plays a significant role in both the extent of ion exchange and the strength of the resulting Brønsted site. Up to 20% of available sites (400 µmol/gcat) are exchanged by acidic species such as formic acid (GPDE = 1445 kJ/mol), and the site density decreases with increasing GPDE until a threshold is reached at GPDE ~1475 kJ/mol above which ion exchange does not proceed, e.g. trifluoroethanol (1513 kJ/mol) and water (1633 kJ/mol). Formic acid also generates sites with acid strength equal to “intrinsic” sites (such as those in HY), as measured by the activation energy for isoproanol dehydration. By contrast, weaker acids (e.g. acetic acid) generate weaker sites, leading to Ea increases of up to 25 kJ/mol. The Brønsted acid site generated by a methyl ester species is always weaker than the corresponding acid, with the Ea difference varying from 20 kJ/mol to 60 kJ/mol, possibly due to incomplete hydrolysis leading to a surface complex with -CHx. These findings could have significant consequences for the catalysis of acidic biomass-derived molecules by zeolites, as the fundamental nature of the catalyst is altered by the reactant and/or product.

Characterization of Brønsted acid sites generated in situ on alkali-metal form zeolites via gas-solid ion exchange

254th National Meeting (2017)

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