A hybrid
of high level and low level quantum mechanics (QM) methods
has been employed to predict intrinsic and apparent energy barriers
for the direct proton exchange mechanism of methane, ethane, propane, n-butane, and i-butane on Brønsted
sites of H-MFI. The specific hybrid MP2:PBE+D2 + ΔCC implementation used is known to yield the so-called “chemical
accuracy” (±4 kJ/mol). Whereas the apparent enthalpy barriers
decrease with increasing C number from 104 to 63 kJ/mol, in line with
the decreasing heat of adsorption, the intrinsic enthalpy barriers
are constant within 124–127 kJ/mol at 500 K. For methane, ethane,
propane, and n-butane, we find the expected agreement
of apparent barriers with activation energies from batch recirculation
reactor experiments. The activation energies derived from NMR experiments
(103–113 kJ/mol) are similarly constant as the predicted intrinsic
barriers but systematically lower. For i-butane the
predicted intrinsic and apparent barriers for the direct proton exchange
step are the same as for n-butane with deviations
of 2–5 kJ/mol, while the experiments yield values that are
50–60 kJ/mol lower, far outside the estimated range of combined
experimental and computational uncertainty (±14 kJ/mol). A change
to the indirect proton exchange mechanism, in which a hydride ion
is transferred between the alkane and a tert-butyl
carbenium ion can be excluded, because we confirm previous findings
that the barrier for dehydrogenation that would create a tert-butyl cation from i-butane is much too high, 188
and 132 kJ/mol for the intrinsic and apparent enthalpy barriers, respectively,
at 500 K. The possible role of extraframework- and framework-bound
alumina species is discussed.