(465a) Mechanism for Selective Fructose Etherification on Hierarchical Sn-SPP Zeolite

Isomerization of glucose to
fructose is a key reaction for enabling the conversion of sugars into a wide
range of biorenewable chemicals through furanic intermediates. The highest glucose to fructose
yields on Sn-Beta are 33%¹, but a new catalyst, Sn-SPP can achieve a
record 65% yield², even in excess of the glucose/fructose
equilibrium (50%). Sn-SPP catalyzes a new reaction: fructose etherification
(formally a ketalization) to ethyl fructoside, which reverts to fructose after water is added post-reaction (see Scheme 1). After these
initial experimental results, many fundamental questions were unanswered,
including how the structure of the active site enables this chemistry, why
Sn-SPP performs ketalization while Sn-Beta does not, and why fructose
ketalization is catalyzed but not glucose acetalization.

Scheme 1 -
Reaction scheme for improving fructose yields from glucose using tandem
reactions with fructose etherification.

We answered these questions using
periodic DFT calculations and Monte Carlo simulations. A screening algorithm
identified the most thermodynamically stable active site geometries; Sn-SPP has
a “closed” Sn site with three nearby silanol groups, while Sn-Beta has a
hydrolyzed “open” Sn site with a single, more distant silanol group.
Identification of the most favorable ketalization mechanism revealed that the
silanols in Sn-SPP facilitate ketalization through key H-bonding interactions
at the transition state, particularly via a Sn-O-Si-OH moiety. However, the
silanol in the Sn-Beta active site is not positioned to stabilize the TS,
giving a larger barrier and indicating why Sn-Beta is not active for this
chemistry. Analyzing glucose acetalization revealed differences in stability of
the key oxonium intermediate at the respective
transition states, indicating the reason for the remarkable selectivity of this
process for fructose ketalization over glucose acetalization.

1.   Moliner, M.; Roman-Leshkov, Y.;
Davis, M. E. E.; Román-Leshkov, Y.; Davis, M. E. E. Proc Natl Acad Sci U S A
2010, 107 (14), 6164–6168.

2.   Ren, L.; Guo,
Q.; Kumar, P.; Orazov, M.; Xu, D.; Alhassan, S. M.; Mkhoyan, K. A.;
Davis, M. E.; Tsapatsis, M. Angew.
Chemie - Int. Ed. 2015, 54 (37),
10848–10851.