10.1002/anie.202009835
Angewandte Chemie International Edition
RESEARCH ARTICLE
interactions of the TS with the pore walls, and 9) intraporous
intermolecular interactions considering there exists more than
one alkanols near TS in the zeolite pores. The thermochemical
cycle enables representation of the activation enthalpy of late Cβ–
H bond cleavage TS with respect to the steps (1) through (9).
significance. Bregante et al. have reported enthalpically
unfavorable interactions between long aliphatic chain of olefin
epoxidation TS with water clusters.[7] Therefore, the higher
entrainment of alkanol molecules in BEA pores increases the
relative importance of TS-alkanol interactions than TS-water
interactions. The increased stabilization of TS of 2-methyl-2-
hexanol as compared to secondary alcohols inside BEA pores
relative to MFI may be attributed to this larger enthalpic
stabilization driven by favorable TS-intraporous alkanol
interactions. While the role of alkanol structure on the enthalpic
stabilization of Cβ-H TS is clear from our data, the defect sites and
pore hydrophilicity can also contribute to the differences in alkanol
uptakes inside zeolite pores and the stabilization of Cβ-H TS.[7, 16]
We first consider that step 2, 4, 5, 6, and 7 are expected to be
similar for MFI and BEA. The deprotonation energy of dry proton
(step 4), which is independent of the confinement as interaction
of zeolite pores and protons are not appreciably affected by the
confinement.[15] The electrostatic component (step 7) would
depend on the charge distribution in the TS and their interaction
with the anion. Such charge distributions can be considered to be
similar across all zeolite frameworks because of similar acid
strengths and similar stabilities of conjugate anions at all
framework locations.[15]
This can be further explored with zeolites with different Si:Al ratio
16]
and different synthesis methods.[7,
Despite the difficulty in
deconvolution of the number of such complex interactions that
affect the reactivity on a solid-liquid interface, our study provides
an important step in furthering the understanding of the catalysis
at solid-liquid interfaces.
In contrast, the deprotonation (step 2) and protonation energies
(step 5) may be considered to be slightly different across zeolites
due to varying hydronium ion cluster size. However, proton
affinities of hydronium-ion size clusters start approaching a
constant value as the cluster size starts approaching a size of n =
5, as compared to a size larger than 8 ± 1 inside MFI and BEA
pores.[5c] Therefore, the enthalpic values of steps 2 and 5 are not
expected to vary appreciably. Step 6, hydronium-ion catalyzed
dehydration in water without zeolite confinement, leading up to
the TS is also independent of the zeolites.
Conclusion
We show here how the steric constraints of zeolite pores influence
the catalytic activity of hydronium ions and how the environment
influences the local organization of solvents and substrate
molecules. The higher dehydration rates of secondary alkanols,
3-heptanol and 2-methyl-3-hexanol, in MFI zeolite with pores
smaller than those of zeolite BEA, is caused by a lower activation
enthalpy in the tighter confines of MFI. It offsets a less positive
activation entropy. With the increasing ease in the formation of C-
H TS for 2-methyl-2-hexanol, the stabilization provided by the
confinement assumes lesser significance and an additional
enthalpic stabilization of the TS due to dispersive interactions with
other alcohol molecules become important. This makes the
larger-pore BEA zeolite more reactive than the smaller-pore MFI
zeolite for dehydration of 2-methyl-2-hexanol. Our results
demonstrate additional avenues for tuning the microenvironment
inside the zeolites to enhance rate kinetics inside nanoscopic
confinements.
Therefore, the activation enthalpy difference between MFI and
BEA for a given alkanol, ∆∆H o‡MFI-BEA is given by:
ꢃ‡
(2)
∆∆ꢀ
= ∆∆ꢀꢇꢃꢂꢈ ꢙꢄꢊꢃℎꢃꢄ,ꢚꢛꢘ−ꢜꢑꢙ
+
ꢚꢛꢘ−ꢜꢑꢙ
∆ꢀꢇꢃꢂꢈ ꢎ2ꢏ ꢍ,ꢚꢛꢘ−ꢜꢑꢙ − (∆∆ꢀꢕꢃꢖꢗ,ꢚꢛꢘ−ꢜꢑꢙ
+
(
)
∆∆ꢀꢘꢃꢍꢆꢂꢌꢒꢃꢄꢂꢊꢔꢄꢉꢌ ꢙꢄꢊꢃℎꢃꢄ,ꢚꢛꢘ−ꢜꢑꢙ
)
The contribution from desorption of alcohol (step 1) can be
estimated by adsorption enthalpies calculated from the isotherms
(Table S21). Together with the measured activation enthalpy, the
combined vdW (step 8) and intermolecular interactions (step 9)
along with the desorption of water cluster (step 3) contributing
towards the difference in stabilization of TS in BEA pores
compared to MFI is estimated to be 25, 20, and 5 kJ mol-1 for 3-
Acknowledgments
This work was part of the Chemical Transformations Initiative at
the Pacific Northwest National Laboratory (PNNL), conducted
under the Laboratory Directed Research and Development
Program at PNNL. D.M.C. and J.A.L. were supported by the U.S.
Department of Energy (DOE), Office of Science, Office of Basic
Energy Sciences, Division of Chemical Sciences, Geosciences
and Biosciences (Transdisciplinary Approaches to Realize Novel
Catalytic Pathways to Energy Carriers, FWP 47319) for
contributing to the discussion of the results, planning, and writing
of the manuscript. Some of the experiments were performed at
the William R. Environmental Molecular Science Laboratory, a
national scientific user facility sponsored by the DOE Office of
Biological and Environmental Research located at Pacific
Northwest National Laboratory.
heptanol,
2-methyl-3-hexanol,
and
2-methyl-2-hexanol,
respectively. The desorption enthalpies of water cluster are not
expected to be alkanol-structure dependent, but rather dependent
on the zeolite pore. We infer, therefore, the pore environment
inside the BEA pore, which enables the combined vdW and
intermolecular interactions accounts for the higher enthalpic
penalties in the formation of the TS going from 2-methyl-2-
hexanol to 3-heptanol.
The question arises now as to how structural differences of the
alkanols, especially between the tertiary 2-methyl-2-hexanol and
the secondary 2-methyl-3 hexanol and 3-heptanol, impact the
vdW interaction of TS with zeolite pore and the intraporous
intermolecular interactions differently in MFI and BEA. On one
hand, with the decreasingly enthalpically demanding TS formation
from 3-heptanol to 2-methyl-2-hexanol (Figure 2), the vdW
stabilization provided by the narrower MFI pore over BEA
assumes reduced significance from the secondary to tertiary
alkanols. On the other hand, with the increasing ease of TS
formation, the intraporous stabilization provided by the co-
adsorbed alkanol molecules inside BEA assumes greater
significance than the weaker vdW stabilization, making BEA more
reactive. As BEA is entrained with more alkanol molecules than
MFI, these intermolecular interactions assume greater
Keywords: Hydronium ion • confinement effect • dehydration •
aliphatic alcohols • enthalpy-entropy compensation
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