Influence of Intracrystalline Ionic Strength in MFI Zeolites on Aqueous Phase Dehydration of Methylcyclohexanols

Article abstract of DOI:10.1002/anie.202107947

The impact of the concentration of hydrated hydronium ions and in turn of the local ionic strength in MFI zeolites has been investigated for the aqueous phase dehydration of 4-methylcyclohexanol (E1 mechanism) and cis-2-methylcyclohexanol (E2 mechanism). The E2 pathway with the latter alcohol led to a 2.5-fold higher activity. The catalytic activity normalized to the hydronium ions (turnover frequency, TOF) passed through a pronounced maximum, which is attributed to the increasing excess chemical potential of the alcohols in the pores, increasing in parallel with the ionic strength and the additional work caused by repulsive interactions and charge separation induced by the bulky alcohols. While the maximum in rate observed is invariant with the mechanism or substitution, the reaction pathway is influencing the activation parameters differently.

Full text of DOI:10.1002/anie.202107947

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Accepted Article
Title: Influence of intracrystalline ionic strength in MFI zeolites on
aqueous phase dehydration of methylcyclohexanols
Authors: Lara Milaković, Peter Hintermeier, Yue Liu, Eszter Barath,
and Johannes Lercher
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Influence of intracrystalline ionic strength in MFI zeolites on
aqueous phase dehydration of methylcyclohexanols
Lara Milaković,+[a] Peter H. Hintermeier,+[a],[b] Yue Liu,[a] Eszter Baráth,*[a] and Johannes A. Lercher*[a],[b]
[a]
L. Milaković,[+] Dr. P. H. Hintermeier,[+] Dr. Y. Liu, Dr. E. Baráth, Prof. Dr. J. A. Lercher
Department of Chemistry and Catalysis Research Center
Technische Universität München
Lichtenbergstraβe 4, 85748 Garching (Germany)
E-mail: eszter.barath@tum.de
johannes.lercher@ch.tum.de
[b]
Dr. P. H. Hintermeier, Prof. Dr. J. A. Lercher
Institute for Integrated Catalysis
Pacific Northwest National Laboratory
902 Battelle Boulevard, Richland, WA 99352 (USA)
These authors contributed equally to this work.
+
[ ]
Supporting information for this article is given via a link at the end of the document.
Abstract: The impact of the concentration of hydrated hydronium ions
and in turn of the local ionic strength in MFI zeolites has been
investigated for the aqueous phase dehydration of 4-
methylcyclohexanol (E1 mechanism) and cis-2-methylcyclohexanol
(E2 mechanism). The E2 pathway with the latter alcohol led to a 2.5-
fold higher activity. The catalytic activity normalized to the hydronium
ions (turnover frequency, TOF) passed through a pronounced
maximum, which is attributed to the increasing excess chemical
potential of the alcohols in the pores, increasing in parallel with the
ionic strength and the additional work caused by repulsive interactions
and charge separation induced by the bulky alcohols. While the
maximum in rate observed is invariant with the mechanism or
substitution, the reaction pathway is influencing the activation
parameters differently.
the zeolite. We have shown its composition to be H+(H2O)8 in MFI,
creating an empty void between these clusters.[10] Sorbed organic
substrates, e.g., cyclohexanol or phenol, may be sorbed in these
voids between neighboring hydrated hydronium ions.[10,11]
This transforms the zeolite pores in water into a strongly ionic
environment, with the concentration of hydronium ions in the
volume of the zeolite pores, approximating liquids with high ionic
strength. We have shown that a high intracrystalline ionic strength
induces a strong non-ideality and destabilizes a sorbed organic
substrate by increasing its excess chemical potential compared to
a zeolite pore without acid sites, similarly to the increase in the
excess chemical potential as the ionic strength of an electrolyte
solution is increased. The intracrystalline ionic strength increases
proportionally to the H3O+ concentration, leading to a monotonic
increase of the standard free energy of adsorption of nonpolar or
less polar substrates in zeolite pores.[12]
Zeolitic Brønsted acid sites (BAS) are generated by substitution
of metal cations with a 3+ charge to the framework Si4+-oxygen
tetrahedra. Although often denoted as a “proton”, i.e., a H+, the
acid site is intrinsically a charge-neutral hydroxyl group in which
the hydrogen atom is covalently bonded to the oxygen bridging
between Si and Al based tetrahedra. In gas phase, BAS of a
specific type of zeolite tend to have a similar acid strength and,
therefore, act catalytically similarly on reactive substrates.[1]
Stronger deviations occur, if the zeolite has pronounced
differences in the Al locations, a substantial concentration of
extra-framework Al, and Al concentrations that lead to substantial
concentration of aluminum as next nearest neighbors.[2-5]
Zeolites tend to be unstable in aqueous phase at elevated
temperatures.[6] Recent studies showed, however, that the zeolite
frameworks tend to be stable for a prolonged time in aqueous
phase, if temperatures do not exceed 180°C.[7] The suitability of
the solid acids as catalyst for converting oxo-functionalized
molecules derived from renewable resources, has led to a quite
intense exploration of molecular sieves in such aqueous
environment.[8]
Using cyclohexanol dehydration to cyclohexene catalyzed by MFI
and BEA in water it was shown recently that the ionic environment
in zeolite crystallites stabilizes the cationic transition state, thus,
decreasing the reaction barrier and enhancing the reaction
rate.[12] This provides a new path to influence the catalytic activity
of Brønsted acidic zeolites.
In this work, we investigate, how the intracrystalline ionic strength
influences the reaction rate in the dehydration of substituted cyclic
alcohols, independent of the reaction mechanism (E1 vs. E2
elimination) respond to the ionic strength and whether variations
in the steric hindrance changes the impact of ionic strength.
Two isomers of methylcyclohexanol were chosen for the study,
i.e., 4-methylcyclohexanol (4-McyOH) and cis-2-methylcyclo-
hexanol (cis-2-McyOH). The former is shown to dehydrate via an
E1 mechanism and can access to all micropore space in MFI
channels, while the latter dehydrates via an E2 mechanism[13] and
can only access part of the MFI micropore space due to its
bulkiness. The comparison of the dehydration rates of the two
substrates catalyzed by a series of MFI zeolites with varying BAS
concentrations allows to address the role of steric challenges and
of the nature of the transition state.
It has been shown that in presence of water the covalently bound
OH group balancing the charge of aluminum-oxygen tetrahedron
is deprotonated, forming a hydrated hydronium ion (abbreviated
as H3O+hydr.).[9] The hydration shell forms a fluxional, positively
charged cluster that remains close to the anion in the zeolite
lattice. The size of the cluster depends on the micropore size of
The most important physicochemical properties of the zeolites are
compiled in Table 1 (additional in SI, Table S1). With increasing
Si/Al ratio, the BAS (and hence H3O+hydr.) decreased from 1.14 to
0.09 mmol g-1. The micropore volumes (Vmirco) were found to be
low for MFI with high Si/Al ratio and tended to increase with
1
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decreasing Si/Al ratio. The calculated unit cell volumes in Table 1
show that the unit cell does not change significantly across all
samples studied. Thus, the differences in Vmirco are tentatively
attributed to (presumably silicia) debris in the pores. By
normalizing the BAS concentration (forming quantitively hydrated
hydronium ions) to the micropore volumes the intracrystalline
ionic strength was calculated to range from 0.89 to 6.44 mol L-1
(Ionic strength = c(BAS) Vmicro-1).
shows a 30 kJ mol-1 lower activation barrier than the trans isomer
and, therefore, is converted preferentially in a racemic mixture of
both isomers. The reaction order in 2- and 4-McyOH was zero for
MFI zeolites; consequently, it is assumed that the measured
activation parameters are representing intrinsic values, i.e., the
energy difference between transition state and sorbed substrate
(SI, Figures S1 – S2 and Tables S2 – S3).[13]
The turnover frequencies (TOF) of 4-McyOH dehydration at
150°C on all MFI zeolites are compiled in Table 1. The highest
TOFs appeared on zeolites (MFI-40 and MFI-60) with BAS
concentrations of 0.23 – 0.31 mmol g-1 and ionic strengths of 1.51
– 2.07 mol L-1, respectively. Figure 1 A shows a volcano-shaped
dependence of the TOF on the ionic strength. A similar trend is
also seen when correlating the TOF with the concentration of the
hydrated hydronium ions (H3O+hydr.). (SI, Figure S3). While for
lower ionic strength a sharp increase of the TOF was observed, a
decreasing trend is present for catalysts with high ionic strength.
The volcano-shaped dependence of the TOF on the ionic strength
was also consistently found at other reaction temperatures (160 –
190°C, SI, Figure S4).
As we demonstrated previously,[12] the increasing local ionic
strength in the zeolite pores causes the increase in TOFs. This
conclusion was drawn unequivocally from a series of Na+ partly
exchanged H-MFI, in which the ionic strength was kept constant
Scheme 1. Dehydration mechanism of 4-McyOH (E1) and cis-2-McyOH (E2).[13]
while at the same time the H3O+
concentration was
hydr.
Previous investigations have established that trans-2-McyOH and
4-McyOH (cis/trans) dehydrate via carbenium ion intermediates
following an E1 mechanism.[13] 4-McyOH is thereby
predominately dehydrated to 4-methylcyclohexene (4-MCH),
which represents the Hofmann-product (Scheme 1).[13,14]
In contrast, the concerted E2 mechanism was concluded to
dominate in the dehydration of cis-2-McyOH, which almost
exclusively resulted in the formation of the Saytzeff-product 1-
methylcyclohexene (1-MCH) (Scheme 1).[13,15] The cis isomer
decreased.[12] Consequently, we conclude that also in the present
study, the high ionic strength is responsible for the increasing
TOFs by inducing non-ideality to the system. More precisely, the
induced ionic environment destabilizes the uncharged sorbed
reactant and simultaneously stabilizes the positively charged
transition state intermedium (carbenium ion), which in turn results
in an overall lowering of the free energy barrier and, therefore, in
higher TOFs (Table 1).
Table 1. Characterization of the investigated MFI zeolites, measured kinetic (150°C) and activation parameters of 4-McyOH dehydration over MFI zeolites.
Entry
Zeolite
Vmicro (cm3 g-1)
Unit cell
volume (Å3)[a]
c (BAS)
(mmol gMFI
Ionic strength
(mol L-1)
TOF
(s-1)
ΔG°‡
(kJ mol-1)
ΔH°‡
(kJ mol-1)
ΔS°‡
(J mol-1 K-1)
-1
)
1
2
3
4
5
6
7
MFI-193
MFI-90[b]
MFI-60
MFI-45
MFI-40
MFI-15
MFI-12
0.10
0.13
0.15
0.12
0.15
0.18
0.18
5374.4
5376.8
5376.8
5376.7
5372.8
5380.9
5377.3
0.09
0.15
0.23
0.36
0.31
0.86
1.14
0.89
1.12
1.51
3.00
2.07
4.92
6.44
0.009
0.024
0.030
0.015
0.031
0.005
0.004
121 (±2)
-
149 (±1)
-
66 (±3)
-
117 (±6)
120 (±2)
117 (±9)
124 (±7)
124 (±6)
143 (±3)
145 (±1)
141 (±5)
157 (±4)
153 (±3)
60 (±6)
59 (±3)
56 (±10)
77 (±8)
68 (±7)
[a] Derived from XRD (lattice parameters a,b,c in SI, Table S1). [b] Experiment conducted only at 150°C.
Figure 1. (A) TOF as a function of ionic strength in the dehydration of 4-McyOH at 150°C. (B) ΔG°‡ and (C) ΔH°‡ (black) and ΔS°‡ (blue) as a function of the distance
between hydronium ions (db-b).
2
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H+(H2O)8[10] from the distance between two centers of the clusters
(dh-h
ratios and increasing H3O+
)
[16] (Figure 2 A), which is decreasing with decreasing Si/Al
concentrations, respectively
hydr.
(Figure 2 B). Figure 1 B illustrates that for the dehydration of 4-
McyOH, ΔG°‡ reaches a minimum at a db-b between 0.4 and 0.6
nm. For zeolites with smaller db-b, the free energy is increasing
although featuring a higher ionic strength.
Once the void space between the hydronium ions is smaller than
the volume of one substrate molecule, the repulsion induced by
the sorption of molecules and the partial separation of charge via
a
rearrangement of the hydronium ions (combination of
electrostatic, hydrogen bonding, and dispersion interactions) sets
in in constrained systems forcing reorganization in the highly ionic
strength environment.[12] The additional work resulting from the
partial separation of the negative charge at the zeolite lattice and
the positive charge (particularly for the transition state) that has to
be overcome, causes an increase of the free energy and,
therefore, a decrease of the TOF. In open systems[12] the TOF
increased monotonically with increasing ionic strength without
passing through a maximum. The sorption in the environment of
higher ionic strength is compensated by volume expansion, which
causes minimal additional work.
Alcohol adsorption measurements (SI, Figure S17, Table S16)
confirm that the uptake of 4-McyOH starts to decrease on zeolites
with ionic strength higher than 2 mol L-1. However, it should be
noted that the reaction is not transport limited despite steric
constraints as also on these zeolites the reaction order in the
alcohol was zero. In parallel with the variations of the free energy,
the activation enthalpy and entropy obtained with the investigated
MFI zeolites show an equal dependency on db-b. As soon as db-b
falls below 0.4 nm, a sharp increase in ΔH°‡ and ΔS°‡ is observed.
Despite the beneficial gain in entropy, the high enthalpic barrier
more than offsets this contribution, resulting in the
aforementioned increase in ΔG°‡. Longer db-b again favor ΔS°‡,
but suffer from a lower stabilization of ΔH°‡ than for zeolites with
higher ionic strength.
Figure 2. (A) Illustration of the distance between the boundaries of neighboring
hydronium ions (db-b) in the MFI pores adapted from reference 10. The distance
between the centers of hydrated hydronium ions (dh-h) is estimated by the cubic
root of the average zeolite volume normalized to the number of hydronium
ions.[10,16] The H+(H2O)8 cluster is assumed to be cylindric with the diameter of
the H-MFI zeolite micropore channel.[17] (B) Dh-h and db-b as function of the BAS
concentration.
The marked decrease of the TOF at very high ionic strengths,
however, is hypothesized to result from reorganization of the ion
pair by the spatial constraints brought by the neighboring H3O+
hydr.
to the organic substrate residing in between them. In order to
explore this hypothesis, the distance between the boundaries of
neighboring H3O+
(db-b) in the investigated MFIs is plotted
hydr.
against the corresponding activation parameters (Figure 1 B and
C). Db-b is calculated by subtracting the length of a hydrated
hydronium ion cluster consisting of eight water molecules
Table 3. Characterization of the investigated MFI zeolites, measured kinetic (150°C) and activation parameters of cis-2-McyOH dehydration over MFI zeolites.
Entry
Zeolite
Ionic strength
(mol L-1)
TOF
(s-1)
ΔG°‡
(kJ mol-1)
ΔH°‡
(kJ mol-1)
ΔS°‡
(J mol-1 K-1)
1
2
3
4
5
6
MFI-193
MFI-60
MFI-45
MFI-40
MFI-15
MFI-12
0.89
1.51
3.00
2.07
4.92
6.44
0.019
0.076
0.056
0.081
0.045
0.021
118 (±6)
114 (±4)
115 (±7)
114 (±6)
116 (±4)
119 (±4)
113 (±3)
115 (±2)
111 (±4)
114 (±3)
103 (±2)
103 (±2)
-13 (±7)
2 (±4)
-10 (±8)
0 (±7)
-30 (±4)
-37 (±4)
Figure 3. (A) TOF as a function of ionic strength in dehydration of cis-2-McyOH at 150°C. (B) ΔG°‡ and (C) ΔH°‡ (black) and ΔS°‡ (blue) as a function of the distance
between hydronium ions (db-b).
3
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The dehydration of cis-2-McyOH shows a similar volcano-shaped
dependency of the TOF on the ionic strength and the BAS
concentration, respectively (Table 3, Figure 3 A, SI, Figure S3).
The highest TOFs are again obtained at ionic strengths between
1.51 – 2.07 mol L-1. As also in a concerted E2 dehydration
pathway, the β-H abstraction (and simultaneous C-O bond
cleavage) is the kinetically relevant step, the rate enhancement
shows that also transition states in a concerted elimination benefit
from a high ionic strength.
Figure 4 displays that all tested zeolites follow a linear correlation
between the entropy and enthalpy. Remarkably, the correlation
even falls on the same line as all other secondary alcohols
converted over various catalysts as reported recently.[18] This
reflects the significant influence of the position of the OH-group
on the overall catalytic activity (SI, Figure S18).
Furthermore, Figure 4 highlights again that an increasing ionic
strength has a different influence on the enthalpy and entropy
when following an E1 or E2 mechanism. While a pronounced
stabilization of ΔH°‡ is characteristic for the E2 mechanism
(absence of a carbenium ion), this pathway suffers from low or
even negative ΔS°‡ due to a highly ordered and multicomponent
transition state. An increasing ionic strength has, in this case, a
stronger beneficial impact on the entropy and shifts the
parameters towards a more E1-like character (Figure 4, blue
arrow). In contrast, an increasing ionic strength seems to shift an
E1 mechanism more towards E2-like parameters by reducing the
characteristic high enthalpic barrier at the expense of a lowering
in entropy (Figure 4, black arrow).
Interestingly, the drop of the reaction rates occurs at the
symmetric situation as for 4-McyOH and prior for the non-
substituted CyOH,[12] irrespective of the higher steric hindrance
through the position and orientation of the substituted group or the
mechanism pathway. ΔG°‡ is increasing for all three substrates at
the same boundary (Figure 1 B and 3 B), suggesting that the
cyclohexyl ring determines the critical size (distance) after which
the contribution of the repulsions exceeds the gain from the high
local ionic strength. The reorganization penalty seems to have a
less serve impact on the associated-complex than on the
carbenium ion intermediate. This is demonstrated by the not as
sharply decreasing volcano-plot for the E2 compared to the E1
mechanism (TOF decrease from MFI-40 to MFI-45 31 % for E2
vs. 52 % for E1).
Moreover, for the dehydration of cis-2-McyOH, the dependency of
the activation enthalpy and entropy on db-b shows an opposite
trend (Figure 3 C). The enthalpic stabilization is now increasing
for distances below 0.4 nm and above 0.6 nm, while a significant
decrease of the entropy to negative values is observed. This loss
in entropy is caused by the associated complex formed in the
transition state of the concerted elimination consisting of a proton,
the alcohol and water acting as the proton-abstracting base
(Scheme 1).
The adsorption uptake of cis-2-McyOH is reduced, in line with the
TOF decrease, after an ionic strength of 2.07 mol L-1 (SI, Figure
S17, Table S16). This isomer showed an overall lower uptake
than 4-McyOH due to its higher steric hindrance (on MFI-40: 0.34
mmol g-1 for cis-2-McyOH vs. 1.08 mmol g-1 for 4-McyOH).
Nevertheless, the dehydration of cis-2-McyOH results in more
than 2.5-fold higher TOFs (Table 1 and 3). As it was concluded
previously, the antiperiplanar arrangement of the protonated
hydroxyl group and the adjacent -H allows the cis-2-McyOH to
proceed via a concerted E2 mechanism, thereby resulting in an
increased selectivity towards the energetically more favored
Saytzeff-product (1-MCH) and simultaneously avoiding the
energetically demanding formation of a carbenium ion (SI, Table
S17).[13]
In conclusion, we investigated the impact of the concentration of
H3O+
and the intracrystalline ionic strength on the aqueous
hydr.
phase dehydration of 2- and 4-methylcyclohexanol. The increase
of the turnover frequency in the demonstrated volcano-plot is
caused by increasing local ionic strength in the zeolite pores. The
highest dehydration rates were obtained by zeolites of moderate
Si/Al ratios, i.e., on MFI-40 and MFI-60. The decrease, on the
other hand, is arising from the additional work to overcome the
strong repulsions once the void space between neighboring
hydronium ions falls below the critical distance of 0.4 nm and a
reorganization of the ion pairs is required. The position of the
maximum is consistently found regardless of the substitution or
whether the dehydration proceeds via an E1 or E2 mechanism.
The reaction pathway strongly affects the activation entropy and
enthalpy and the mode by which they are influenced by the ionic
strength. While the formation of the carbenium ion primarily
resulted in an enthalpic stabilization at high ionic strength, the
formation of the associated complex was mainly entropically
supported. The significantly higher rates for the cis-2-McyOH over
the 4-McyOH dehydration, despite the higher steric bulkiness, are
a consequence of the E2 pathway and the selective conversion
to the Saytzeff-product.
Acknowledgements
This work was supported by the U.S. Department of Energy
(DOE), Office of Science, Office of Basic Energy Sciences (BES),
Division of Chemical Sciences, Geosciences and Biosciences
(Transdisciplinary Approaches to Realize Novel Catalytic
Pathways to Energy Carriers, FWP 47319). Special gratitude is
expressed to Prof. Gary L. Haller for fruitful discussions and P.
Waligorski and P. Pfauser for their help in performing experiments.
Keywords: alcohol dehydration • cyclic alcohols • ionic strength
• volcano • zeolites
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5
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Entry for the Table of Contents
The volcano shape and position of the maximum in the dehydration of methylcyclohexanols in aqueous phase over various MFI zeolite
catalysts is a consequence of the intracrystalline ionic strength and a spatial constraint. The individual TOFs depend on the substrate
related mechanism (E1 vs. E2).
6
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