10.1002/cssc.202001323
ChemSusChem
that of 1-butanol solvent (i.e., ∆∆EB.E.‘s of 5.36, 7.93, and 9.47 for
water, ethanol and butanol conditions, respectively), indicating the
reduced selectivity on mannitol formation at water and ethanol as
shown in the experimental observation.
10
ISad,m
ISad,s
40
DDEB.E.
8
6
4
2
0
35
30
5
0
water
ethanol
1-butanol
Figure 8. Magnified view of the optimized configurations of fructose at the
(A) ISad,m, and (B) ISad,s states. (C) Reaction energy profiles of two fructose
hydrogenation reactions. In the (A) and (B), closely interacting atom pairs
and their interaction types are indicated by dashed lines with red for
Cu‧‧‧HO-C, black for Cu‧‧‧H-C, and blue for Cu-H‧‧‧O=C interactions,
respectively. For the clear view, top layer of Cu surface and adsorbed
hydrogen are shown by space-filling model, while the fructose molecule is
shown by ball-and-stick model. Copper and adsorbed hydrogen are colored
in orange and sky blue, respectively. For fructose molecule, carbon,
hydrogen, and oxygen atoms are colored in gray, white, red, respectively.
Figure 9. Binding energies of mannitol and sorbitol (∆EB.E.) and differences
of binding energies (∆∆EB.E.) in each solvent.
4. CONCLUSION
We developed a promising protocol for the production of
mannitol crystal from fructose using Cu(80)-SiO2 nanocomposite
and 1-butanol solvent for the first time. More importantly, this
catalyst did not show any Cu leaching and its physico-chemical
properties were maintained after the liquid-phase hydrogenation. In
addition, the highly-pure mannitol crystal could be recovered from
the sorbitol-containing 1-butanol solution by simple filtration.
Therefore, the present protocol is a novel and effective method to
produce a pure mannitol from fructose in both an environmental
and an industrial context.
We further investigated the selective formation by tracking
hydrogenation mechanisms for the formations of mannitol and
sorbitol on Cu surface, respectively (Fig. 8C and Fig. S7). By
considering the previous mechanistic study on the hydrogenation
of fructose34, a hydrogen atom adsorbed on the copper surface was
transferred to the carbon of the C=O bond in fructose at the first
step (i.e., from IS to IM). The activation energy (Ea) and heat of
reaction (∆E) for the first hydrogen transfer in mannitol formation
were estimated to be 7.89 and 6.36 kcal/mol, respectively, while in
the sorbitol formation reaction, the energies of 10.39 and 7.20
kcal/mol were required for Ea and ∆E, respectively. After the
hydrogen transfer step, at the IM to FS step, another hydrogen atom
was attached to the oxygen in C(H)-O- of intermediate structure. In
this reaction step, each reaction route for mannitol and sorbitol
formation required marginal amount of Ea and exhibited large
exothermic heat of ∆E (i.e., Ea and ∆E for mannitol pathway were
5.47 and -30.29 kcal/mol, respectively, and 3.24 and -25.97
kcal/mol for sorbitol pathway, respectively). Thus, comparing the
∆EB.E., Ea and ∆E obtained from theoretical calculations, in addition
to the stable initial structure of adsorbed fructose at ISad,m, mannitol
pathway was more thermodynamically favorable than sorbitol
pathway. Theoretically, mannitol was expected to be formed
selectively on the Cu-SiO2 catalyst, which was consistent with
experimental observation.
We conducted further calculations under the other solvent
conditions. In this calculation, we considered the implicit solvent
condition (i.e., COSMO) for estimating the difference between
binding energies (∆∆EB.E.) of ISad,m and ISad,s under each solvent
(Fig. R1). Here, ∆∆EB.E represents the selective formation of
mannitol over sorbitol. The binding energies of ISad,m under water
and ethanol solvent conditions were slightly smaller than that
estimated in 1-butanol (i.e., ∆EB.E.‘s of -37.87, -38.56, and -38.85
for water, ethanol and 1-butanol conditions, respectively). On the
contrary, the binding energies of ISad,s under water and ethanol
were larger than that estimated in 1-butanol (i.e., ∆EB.E.‘s of -32.51,
-30.63, and -29.38 for water, ethanol and 1-butanol conditions,
respectively). Thus, the differences of binding energies between
ISad,m and ISad,s under water and ethanol solvents were smaller than
ACKNOWLEDGEMENTS
This work was supported by the R&D Program of the
Institutional Research Program of the KRICT (SI2011-10) and
Korea Research Fellowship Program funded by the National
Research Foundation of Korea [2017H1D3A1A02053077]. S.K.K.
acknowledges
the
financial
support
from
NRF-
2014R1A5A1009799.
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