Differences in Mechanism and Rate of Zeolite-Catalyzed Cyclohexanol Dehydration in Apolar and Aqueous Phase

Cyclohexanol Dehydration in Apolar and Aqueous Phase

Research Article

Diﬀerences in Mechanism and Rate of Zeolite-Catalyzed

Feng Chen, Manish Shetty, Meng Wang, Hui Shi,* Yuanshuai Liu, Donald M. Camaioni,

Oliver Y. Guti é rrez, and Johannes A. Lercher *

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sı Supporting Information

ABSTRACT: The rate of acid−base-catalyzed dehydration of alcohols strongly

depends on the solvent and the environment of the acid sites. We ﬁnd that Brønsted

acidic sites in large-pore zeolites, but not in medium-pore zeolites, catalyze cyclohexanol

dehydration in decalin at signiﬁcantly higher rates than hydrated hydronium ions in

aqueous phase. Speciﬁcally, the diﬀerence in turnover rates between the two solvents

amounts to 2−3 orders of magnitude on H-BEA and H-FAU, while being very modest

(

within a factor of 2) for H-MFI. Combining kinetic, isotopic tracer, and H NMR

measurements, it is established that cyclohexanol dehydration generally follows an E1-

elimination pathway in decalin. A notable exception is the monomer dehydration route

on H-MFI, which exhibits a much lower activation energy and a substantially negative

activation entropy that appear to be associated with an E2-type mechanism. The C−O

bond cleavage displays a dominant degree of rate control in decalin, which stands in

contrast to deprotonation (C−H cleavage) being rate-limiting in aqueous-phase

dehydration.

KEYWORDS: alcohol dehydration, solid−liquid interface, conﬁnement eﬀect, zeolites, elimination mechanism

. INTRODUCTION

compounds. This often leads to substantial rate enhancements

compared to pure water for both homogeneous and solid acids

in the presence of polar aprotic solvents and their mixtures

Acid-catalyzed reactions in the liquid phase are central to many

synthetic strategies and have become central in the context of

upgrading biomass-derived oxygenates into liquid fuels and

21−23

with water.

−11

In order to understand these eﬀects, this work aims to

chemicals.

Considering the wide application of these

provide a benchmark for liquid-phase alcohol dehydration in

reactions, it seems surprising that the mechanisms of acid−

base catalysis at solid−liquid interfaces are not well understood

at an elementary level. This is related to challenges in

elucidating the fundamental structure−function relations at

solid−liquid interfaces,

reacting substrates, their interactions with the catalyst, the

dynamic nature of the active sites, and subtle variations in the

27−36

the absence of strong solvent interactions.

Having

reported how nanoscopic conﬁnement induced by zeolite

pores aﬀects the dehydration rates in water, we investigate in

2−20

this study how dehydration catalysis is impacted by such pore

constraints in the apolar solvent decalin, which was found to

dissolve the alcohol more readily than the linear hydrocarbons

and therefore allowed for a wider range of alcohol

including the solvation of the

reaction pathways.

concentrations to be accessed for kinetic studies. Furthering

Solvents bring about another layer of complexity because of

intra- and intermolecular interactions (e.g., dipolar, electro-

static, H-bonding) that inﬂuence not only the nature of active

sites but also the stability of intermediates and transition states

along a reaction pathway.

chemical potentials of water, the acidic proton of the zeolite

framework is transformed to a hydrated hydronium ion. We

recently discovered that these hydronium ions conﬁned largely

inside zeolite pores lead to up to 2 orders of magnitude higher

dehydration rates of cyclohexanol compared with rates

37,38

our previous work

focused on a single zeolite (H-BEA with

Si/Al = 75), we report herein thermochemical and kinetic

measurements in conjunction with isotope labeling, to

elucidate the reaction mechanism of cyclohexanol dehydration

in decalin, to quantitatively assess the inﬂuence of steric

environment on the reactivity of Brønsted acid sites without

21−23

In the presence of even modest

Received: December 25, 2020

Revised: February 3, 2021

Published: February 17, 2021

25,26

measured in acidic aqueous solutions.

Dumesic and co-

workers showed that the stabilization of the acidic proton

relative to the positively charged transition states determines

the rates of acid-catalyzed reactions of hydroxyl-containing

2021 American Chemical Society

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Research Article

complete water solvation, and to contrast with alcohol

25,26

dehydration previously reported for aqueous media.

The

results show a signiﬁcantly diﬀerent manifestation of conﬁne-

ment eﬀects than that observed for aqueous-phase dehydra-

tion, which primarily results from the stabilities and, thus, the

relative surface concentrations of alcohol-derived intermediates

and kinetically relevant transition states that have divergent

sizes and polarities.

. RESULTS AND DISCUSSION

.1. Kinetics of Zeolite-Catalyzed Cyclohexanol

Dehydration in Decalin. The present work compares three

framework types with distinct pore topologies, namely, FAU,

BEA, and MFI (implicitly in their H-forms), which have been

previously investigated for the aqueous-phase dehydration of

5,26

cyclohexanol.

leads to cyclohexene as the dominant product (selectivity: 95−

00%) on all zeolites. A minor product, dicyclohexyl ether, was

In decalin, the dehydration of cyclohexanol

detected only on the large-pore zeolites, BEA and FAU (Figure

S1; 0.33 M alcohol, 413 K). The initial rates on zeolites,

assessed at conversions of 5−20%, were normalized to the

concentrations of Brønsted acid sites (BAS).

Figure 1. Measured turnover frequencies (TOFs) of cyclohexene

formation from dehydration of cyclohexanol (0.33 M in decalin) with

diﬀerent zeolites: FAU (Si/Al = 30), BEA (Si/Al = 75), and MFI (Si/

Al = 40). Rates were determined at conversions <20%. Solid lines are

ﬁts to the Arrhenius equation. Turnover numbers (TONs) were all

much larger than 10, demonstrating the catalytic nature of the

reaction.

Before discussing rate data, we ﬁrst brieﬂy address the

chemical state of the acid sites. In the case of MFI, according

to the estimated maximum vapor pressure of water in the

reactor and the water adsorption isotherms reported earlier,

the initial state of the BAS is a framework-bound proton at a

bridging Si−OH−Al group. At increased alcohol conversions,

the amount of water formed potentially changes the localized

BAS to a hydronium ion. It can be also estimated, however,

that the equilibrium size of hydronium ions during the

0.53 × 0.56 nm, BEA: 0.66 × 0.67 nm and 0.56 × 0.58 nm,

FAU: 0.74 × 0.74 nm), in an apparent reversed order

compared with the trend for aqueous-phase dehydration of the

25,26

same molecule.

For BEA and FAU, the reaction proceeded

dehydration of cyclohexanol in decalin remains substantially

at rates ∼50 to ∼800 times faster in decalin than in water,

smaller than in the aqueous phase, where H(H O)₇₋₈is the

whereas for MFI, the TOF in decalin was comparable (within a

25,26

most probable form of intrapore hydronium ions in MFI.

Detailed discussions regarding the state of BAS in the presence

of intraporous decalin, alcohol, and water are presented in

Section 5 of the Supporting Information (SI).

factor of 2) to that in aqueous phase.

2.2. On the Mechanism of Cyclohexanol Dehydration

in Decalin over Zeolites. For all zeolites, cyclohexanol

dehydration rates exhibited zero-order dependence with

Rate data on BEA, MFI, and FAU zeolites with varying Si/Al

ratios are summarized in Supplementary Figure S1 (0.33 M

alcohol in decalin, 413 K). For each framework type, varying

the Si/Al ratio resulted in up to a factor of 3 variations in the

turnover frequency (TOF, based on BAS concentrations) of

cyclohexanol dehydration. Without the formation of hydro-

nium ions in the pore, this rate dependence on the Si/Al ratio

may be attributed to diﬀerent crystallographic and topological

locations of framework Al and the associated protons, as well

as to constraints in the zeolite pores induced by extraframe-

work Al debris. If decalin, cyclohexanol and hydronium ions

coexist in the zeolite pores (SI, Section 5), it is plausible that

such rate changes are induced by the varied polar environ-

ments that would aﬀect the stability of charged intermediates

and transition states involved in alcohol dehydration. In the

spirit of focusing on the elucidation of reaction mechanism and

the impact of steric conﬁnes on kinetic parameters, catalysts

with the highest TOF for each framework type, that is, FAU

respect to cyclohexanol (>0.1 M) in aqueous phase. In

contrast, dehydration rates in decalin decreased with increasing

concentration of cyclohexanol (0.025−1.0 M) for these

zeolites (Figure 2). This trend resembles the gas-phase alcohol

0,41

dehydration catalyzed by zeolites

clusters.

and polyoxometalate

In the gas phase, this has been attributed to the

coverage of active sites by unreactive or much less reactive

42−45

alcohol−alcohol dimers with increasing alcohol pressure. As

decalin is an apolar and only weakly interacting solvent, we

hypothesize that the intramolecular dehydration of cyclo-

hexanol to cyclohexene occurs via monomer- and dimer-

mediated routes on acidic zeolites in decalin, analogous to gas-

phase alcohol dehydration on solid Brønsted acids. The

dimer route is much slower than the monomer route, as

37,38

previously shown.

Note, however, that the magnitude of

the decrease in rates at 413 K with increasing alcohol

concentration is the smallest on MFI and the greatest on

FAU, reﬂecting the combined eﬀects of coverages and

reactivities of monomer and dimer species in zeolites of

diﬀerent conﬁning environments. At higher temperatures (e.g.,

443 K), the rates on MFI depended only weakly on the alcohol

concentration, the cause of which is analyzed later on the basis

of kinetic ﬁtting (Section 2.3).

(

Si/Al = 30), BEA (Si/Al = 75), and MFI (Si/Al = 40), were

chosen for further studies; the physicochemical properties of

these materials are shown in Supplementary Table S1. It is

important to mention that the concentration of Lewis acid sites

was relatively low (<50 μmol g ) in all three samples.

The TOFs for cyclohexene formation, at an alcohol

concentration of 0.33 M in decalin (Figure 1), increased

with increasing pore dimensions (MFI: 0.51 × 0.55 nm and

−

To investigate the reaction mechanism in decalin, H/D

isotope tracer studies were used in conjunction with H NMR

experiments. The dehydration of 1-D-cyclohexanol was carried

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cyclohexanol catalyzed by FAU, BEA, and MFI (Figure 3 and

Research Article

would form through 1,2-hydride shifts of carbenium ions

following the cleavage of the C−O bond in 1-D-cyclohexanol

(

Scheme 1). Along the E2 pathway, 3-D- and 4-D-isotopomers

would not form directly from the dehydration of 1-D-

cyclohexanol; instead, they would form via secondary reactions

(

readsorption, protonation, and label shifts) of 1-D-cyclo-

hexene.

Deuterium label shifts were observed in the reactions of 1-D-

Figure 2. TOFs for cyclohexanol dehydration in decalin on diﬀerent

zeolites as a function of cyclohexanol concentration. The inset shows

an enlarged view of the rate data on MFI. Reaction conditions:

.025−0.90 M (r.t.) cyclohexanol in decalin, 413 K. Rates were

determined at conversions of <20%.

Figure 3. Ratio of 1-D-cyclohexene to 3-D-cyclohexene from 1-D-

cyclohexanol dehydration with FAU, BEA, and MFI in decalin up to

alcohol conversions of 20%.

out on zeolites in decalin, and the products were analyzed with

H NMR at varying conversions and concentrations of

cyclohexanol. 1-D-Cyclohexene and 3-D-cyclohexene were

both detected, while the signal of 4-D-cyclohexene overlapped

with that of decalin (nonlabeled) and, thus, was not quantiﬁed

reliably. 1-D-Cyclohexene may be generated from 1-D-

cyclohexanol dehydration via both E1 and E2 elimination

pathways. Along the E1 pathway, 3-D- and 4-D-cyclohexenes

Supplementary Figures S6 −S11). For example, the ratio of 1-

D-cyclohexene to 3-D-cyclohexene remained relatively con-

stant (6.7 on average) as the reaction proceeded on BEA until

secondary reactions involving oleﬁn readsorption and proto-

Scheme 1. Possible Reaction Pathways (E1 and E2 Elimination) for 1-D-Cyclohexanol Dehydration

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nation set in at high conversions (Supplementary Figure S6).

The presence of these label-shifted oleﬁn isotopomers even at

low conversions suggests that a major portion of cyclohexanol

dehydration passes through a carbenium ion-type intermediate

following an E1 elimination pathway, though it remains

diﬃcult to fully exclude that label shifts occur while the oleﬁn

diﬀuses out of the pores and interacts with protons along the

diﬀusion path.

stepwise. Speciﬁcally, cyclohexanol ﬁrst adsorbs on a BAS to

form cyclohexanol monomer (Step 1). The cleavage of the C−

O bond of the monomer produces a surface-bound carbenium

ion-type intermediate, likely a surface alkoxy group in the

absence of signiﬁcant concentrations of water (Step 2). Finally,

deprotonation (i.e., C−H bond cleavage) of the intermediate

leads to the formation of cyclohexene, which desorbs to

regenerate the BAS (Step 3). A protonated dimer is

hypothesized to be generated through the interaction of the

formed monomer with another cyclohexanol (Step 4). This

dimer species potentially also converts to the alkene (e.g., via

Step 5 that is not necessarily an elementary step), which is

energetically more demanding and, hence, slower than the

monomer route according to recent density functional theory

With BEA, the ratio of 1-D-cyclohexene to 3-D-cyclohexene

(measured at alcohol conversions up to 20% in most cases) did

not change with the alcohol concentration from 0.02 to 2.0 M

and was only slightly smaller than that observed for the

dehydration of neat cyclohexanol (ca. 9.6 M; Supplementary

Figure S6). Interestingly, however, the ratio of 1-D-cyclo-

hexene to 3-D-cyclohexene was 1.9−2.6 for MFI and 10−14

for FAU (Figure 3). While we attribute this to a better

stabilization of the transition state for 1,2-hydride shifts in the

medium-pore zeolite (MFI) than in larger pores, we cannot

fully exclude that oleﬁn readsorption and protonation at the

BAS and the ensuing hydride shifts occurs with higher rates

along the pore-exiting path of the oleﬁn product in MFI. Taken

together, we conclude that the elimination of cyclohexanol

displays a dominant E1 character across the studied range of

alcohol concentration (0.02−2.0 M) in decalin, while we

caution that the experiments do not provide a stringent

mechanistic proof for the complete absence of an E2-

elimination path.

6−48

(DFT) calculations of 1-butanol

and 1-propanol

dehydration in zeolites and our earlier measurements in gas-

phase alcohol conversions. Some prior literature even treated

14,43,41

dimers as unreactive species.

Along the E1 elimination path, either C−O or C−H bond

cleavage is the rate-determining step (RDS), or in other words,

45−47

one of the two steps has a greater degree of rate control.

This question was probed by measuring kinetic H/D isotope

eﬀects (KIEs) for cyclohexanol dehydration on the three

zeolites. In one set of experiments (only performed on BEA),

the TOFs of oleﬁn formation from cyclohexanol-d 12 and 1-D-

cyclohexanol were measured at 403 K (Supplementary Table

S6 ). The KIE for the pair of cyclohexanol-d

and cyclohexanol-

The proposed sequence for cyclohexanol dehydration

catalyzed by zeolites in decalin (Scheme 2) illustrates that

for the monomer route, the elimination generally occurs

d (i.e., TOF _O_H)/TOF_C₆_D₁₁_O_D) was 1.58 and 1.42, while

6 11

C H

for the pair of cyclohexanol-d and 1-D-cyclohexanol, the KIE

was 1.42 and 1.19, at alcohol concentrations of 0.025 and 0.1

M, respectively. In another set of experiments, a mixture of

cyclohexanol-d and cyclohexanol-d was reacted on the three

Scheme 2. Proposed Sequence of Key Reaction Steps in

Cyclohexanol Dehydration (in Gas Phase or in Decalin)

with the Relevant Kinetic and Thermodynamic Coeﬃcients

Denoted

zeolites (50 mol % of each substrate with a total concentration

of 0.025 M solution at 413 K, that is, each at 0.0125 M). The

initial molar ratio of cyclohexene-d to cyclohexene-d , also

representing a KIE, varied from 1.79:1 to 2.15:1 on the three

zeolites (Supplementary Table S7 ). Because the measured

KIEs are based on rate ratios, rather than ratios of rate

constants, the increase in the measured KIE (which contains

contributions from KIEs for both monomer and dimer routes,

weighted by the individual coverages of the monomer and

dimer species) with decreasing alcohol concentration points to

higher intrinsic KIEs (∼2) associated with the monomer-

mediated elimination path.

At these temperatures, rehybridization of α-C (in a

protonated alcohol monomer species) to α-C_s_p(resembling

the TS) would lead to secondary KIE values (1.2−1.4) for C−

O bond cleavage as the RDS. Thus, the measured KIEs

indicate that the C−O bond cleavage exerts a higher degree of

rate control than the C−H cleavage for cyclohexanol

dehydration in decalin. This is in contrast to the same reaction

occurring in aqueous phase where the C−H bond cleavage

shows a dominant degree of rate control, displaying primary

KIEs of 2.9−3.5 that are close to the theoretical maximum for

a late TS of C−H cleavage.

The somewhat larger KIEs than the values expected solely

from rehybridization of the α-carbon in the C−O cleavage step

in the formation of carbenium ion might indicate a small

degree of rate control by the C−H bond cleavage step. By

the same token, the decrease in KIE with increasing alcohol

concentration suggests a greater degree of rate control by the

C−O cleavage step along the dimer dehydration route

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compared with the monomer route, as expected from the larger

coverage of cyclohexanol dimers at higher concentrations

in Section 3 of the SI). Note also that the derivations of eqs 1

and 2 do not require that E1 elimination be the sole

dehydration mechanism; assumptions of a series of steps

along a concerted E2 path would lead to the same rate

expressions, albeit with somewhat diﬀerent meanings assigned

to the rate constants.

(section 2.3). Thus, we deduce that cyclohexanol dehydration

in decalin (an apolar medium) proceeds primarily via E1

elimination on the zeolites, with the highest degree of rate

control exerted by the C−O cleavage step. This (tentative)

conclusion is further analyzed using activation enthalpies and

entropies.

The rate constants, obtained by regression, for both

monomer- and dimer-mediated dehydration routes increase

with the increase in pore dimensions from MFI to FAU,

following the same trends observed for TOFs at any given

concentration. The contribution from the monomer pathway

for dehydration decreases with increasing cyclohexanol

concentration. The rate constant for the monomer pathway

is 30−44 times greater than that for the dimer pathway on the

two large-pore zeolites, whereas the diﬀerence in monomer-

path rate constant is much smaller (∼9 times) on MFI. As a

result, at the lowest concentration studied here (∼0.02 M), the

ratio of the rates along the monomer and dimer routes is 80−

90 on FAU and BEA and ∼150 on MFI. This ratio becomes

unity, that is, equal contributions by the monomer and dimer

routes, at ∼1.5−2.0 M of cyclohexanol. For the dehydration of

neat alcohol (9.6 M), this ratio would drop to 0.1−0.2,

showing the preponderance of the dimer route.

.3. Impact of Pore Constraints on the Rate

Constants and Activation Parameters. On the basis of

the proposed sequence of steps in Scheme 2, the following

expression for the TOFs (eq 1), is derived, which captures the

contribution from monomer- and dimer-mediated pathways

for oleﬁn formation (detailed derivations in the SI):

k K [C H OH] + k K K [C H OH]

olefin

6 11

TOF =

[

H ]

1 + K [C H OH] + K K [C H OH]

6 11

(1)

in which the kinetic and thermodynamic constants carry the

meanings as indicated in Scheme 2: speciﬁcally, k and k are

the intrinsic rate constants for the monomer- and dimer-

elimination steps, respectively; K is the equilibrium constant

for alcohol monomer formation from the BAS and an alcohol

in the liquid phase; K is the equilibrium constant for alcohol

Using the same protocol, k and K were derived (k being

dimer formation from a H-bonded monomer at the BAS and

ﬁxed to give the TOF of neat alcohol dehydration) from rate-

concentration data acquired at temperatures in the ranges

403−433 K for BEA and FAU and 413−443 K for MFI. Table

an alcohol in the liquid phase; [H ] is the total concentration

of BAS.

When the latter two terms in the denominator of eq 1

dominate, the dependence of the TOF on cyclohexanol

concentration is approximated by eq 2:

compiles enthalpies and entropies of activation obtained

from the Eyring plots of rate constants, k and k , as well as

free energies of activation calculated for the regressed intrinsic

rate constants on the three zeolites at 423 K. Interestingly,

activation enthalpies and entropies for monomer and dimer

dehydrations were comparable on the two large-pore zeolites,

FAU and BEA. MFI appears to be quite diﬀerent, showing a

much lower activation enthalpy and a negative activation

entropy for the monomer route but a much higher activation

enthalpy and a positive activation entropy for the dimer route.

Conceptually, the negative entropy of activation for the

monomer route on MFI is an indication for an E2-elimination

path, which appears, at ﬁrst sight, to contradict our foregoing

mechanistic analysis. We surmise that the observed label shifts

k + k K [C H OH]

6 11

TOF =

+ K [C H OH]

(2)

6 11

Consequently, the kinetic parameters for cyclohexanol

dehydration on the three zeolites in decalin were obtained

by ﬁtting the kinetic data obtained at 413 K (Figure 2) to eq 2

(Table 1). To reduce the degrees of freedom in ﬁtting the data

Table 1. Intrinsic Rate Constants (Normalized to the BAS

Concentration) for the Formation of Cyclohexene from

Cyclohexanol by Monomer- and Dimer-Mediated

Dehydration Routes and the Monomer-Dimer Equilibrium

on MFI ( H NMR; measured in an alcohol concentration

Constant over Zeolites FAU, BEA, and MFI in Decalin at

range of 0.25−2.0 M) may not reﬂect the intrinsic character-

istics of the monomer route and could be understood in terms

of readsorption, protonation, and hydride shifts before exiting

the pore. Adsorption of decalin (cross section: 0.70 × 0.52

nm) is signiﬁcantly impeded in MFI, with its vapor-phase

saturation uptake accounting for only 10% of the micropore

volume, suggesting low diﬀusion rates of decalin and similarly

sized molecules in MFI pores (sinusoidal channels: 0.51 × 0.55

nm and straight channels: 0.53 × 0.56 nm). As a consequence,

van der Waals stabilizations provided by decalin are lacking for

the monomer route in MFI, unlike the other two large-pore

zeolites that allow decalin to coadsorb in the pore. In this case,

a stepwise pathway is hypothesized to be unfavorable on MFI

because of the lack of coadsorbed species that assist in the

stabilization of charged TS. For the aqueous-phase dehy-

dration, the pore is fully occupied by water and alcohol

molecules that are more eﬀective in stabilizing charged

intermediates and transition states. Hence, an E1-type

mechanism was concluded to be dominating for the

13 K

zeolite

k_M(s⁻¹)

k_D(s⁻¹

)

k /k

K₃(M⁻¹)

FAU

BEA

MFI

1.744 ± 0.107

1.010 ± 0.041

0.052 ± 0.004

0.058

0.023

0.006

30.0

43.9

8.7

18.6 ± 2.5

23.5 ± 1.9

2.9 ± 0.7

^kM ^aⁿ^d^kD are the intrinsic rate constants for the monomer- and

dimer-mediated dehydration pathways, respectively. In practice, k_D

was always set to give the measured TOF of cyclohexene formation in

the neat phase (100% cyclohexanol, 9.6 M). K is the equilibrium

constant for alcohol dimer formation from a monomer and an alcohol

in the liquid phase.

as well as to increase the precision in ﬁtted values for k and

K , k was constrained to values that reproduce the measured

TOF for cyclohexene formation in neat cyclohexanol

assuming a complete coverage by cyclohexanol dimers at

(

concentration of ∼9.6 M). The term [C H OH] refers to the

thermodynamic activity of alcohol rather than its concentration

in decalin (an extended discussion of this matter is presented

25,26

aqueous-phase dehydration of cyclohexanol on all zeolites.

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Table 2. Intrinsic Activation Parameters for Dehydration of Cyclohexanol in Decalin with the Chosen Zeolites

ΔH^o^‡(kJ mol⁻¹

)

ΔS^o^‡(J K⁻¹mol⁻¹

monomer

)

ΔG^o^‡

(kJ mol⁻¹

)

423 K

zeolite

monomer

dimer

monomer

dimer

FAU

BEA

MFI

127 ± 4

130 ± 7

76 ± 6

119 ± 1

120 ± 11

194 ± 15

60 ± 9

68 ± 16

−88 ± 14

16 ± 1

11 ± 1

176 ± 14

102 ± 1

101 ± 1

113 ± 1

111 ± 1

114 ± 1

119 ± 1

^aThe error bars for ΔH^o^‡and ΔS^o^‡represent the 1-σ standard deviations.

Note that using eq 1 (that accounts for low BAS coverage

regime) to ﬁt the rate data, a diﬀerent set of regressed rate and

equilibrium constants would be found. In this approach, to

and activation parameters for dehydration in water and in

decalin. In the cases of BEA and FAU, the rate constants were

more than 2 orders of magnitude smaller for aqueous-phase

dehydration than the rate constants for monomer elimination

route in decalin. Our earlier study demonstrated that the C−O

cleavage/recombination TS lies at a Gibbs free energy close to

reduce the degrees of freedom in the ﬁtting procedure, k was

again set to reproduce the TOF of neat alcohol dehydration

and additionally, K (i.e., adsorption of an alcohol molecule

−

from the decalin solution to form a H-bonded monomer on

BAS) was obtained from independent calorimetric and

adsorption measurements at room temperature and extrap-

olation to reaction conditions, while k and K were regressed

or slightly lower than (by ∼5 kJ mol at most) that of the C−

H cleavage TS for aqueous-phase cyclohexanol dehydration in

zeolites. As a result, the rate constant for aqueous-phase

dehydration corresponds to the free energy barrier of C−H

cleavage and almost also reﬂects that of the C−O bond

cleavage. In turn, we consider the measured free energies of

activation to primarily reﬂect the diﬀerence in the free energy

discussed in Section 3 of the SI). As exempliﬁed for MFI (cf.

Table S3 and Table S5 ), using the complete rate eq (eq 1), the

regressed rate constant for the monomer route (k ) would

increase by a factor of 2−3 compared to that obtained from the

previous ﬁtting protocol. Regardless, the conclusion does not

change that rate constants (both k and k ) on MFI are much

smaller than those on FAU and BEA. For the activation

enthalpies and entropies, the diﬀerence between the two ﬁtting

approaches is not substantial (cf. Table 2 and Table S4); in

both cases, relatively low activation enthalpy and negative

activation entropythe hallmark of E2-eliminationwere

determined for the monomer route on MFI, as opposed to the

other two zeolites.

barrier for C−O cleavage between aqueous-phase and decalin-

‡

phase dehydration of cyclohexanol (ΔΔG

C−O, water‑decalin

speciﬁcally, the diﬀerences in the free energies of activation

in water and in decalin are estimated from the respective rate

−

constants to be ∼21−27 kJ mol for FAU and BEA at 423 K

‡

−1

‡

(i.e., ΔG

= 101−102 kJ mol ; ΔG

C−O, decalin

C−O, water

−

∼122−129 kJ mol at 423 K). A closer inspection indicates

that such diﬀerences for both zeolites stem primarily from

diﬀerent activation enthalpies (∼159 and 166 kJ mol in

water vs 130 and 127 kJ mol in decalin for BEA and FAU,

respectively, i.e., ΔΔH

BEA and FAU). Diﬀerences in the activation entropy make a

−

‡

−1

.4. Cyclohexanol Dehydration: A Tale of Two

= ∼29−39 kJ mol for

C−O, water‑decalin

Solvents. In both water and decalin, we have demonstrated

that cyclohexanol dehydration follows an E1 elimination path

at the prevalent reaction conditions, with the only exception

being the monomer route on MFI that seems to exhibit a

pronounced E2-character. However, the rate constants in water

and in decalin are associated with diﬀerent rate-determining

‡

relatively small contribution (ΔΔS

= ∼20−27 J

C−O, water‑decalin

−

−1

‡

−1

mol K , or TΔΔS

403−433 K).

= ∼8−12 kJ mol at

C−O, water‑decalin

This lowering of the C−O scission TS relative to the

monomer precursor state in decalin can be understood on the

basis of the Hammond postulate, in view of a substantially

more stable intermediate (i.e., surface alkoxide) that connects

the C−O cleavage TS and the C−H cleavage TS (Scheme 3).

In the case of MFI, the rate constant of the aqueous-phase

steps. In water, the C −O recombination is considerably faster

than deprotonation (C −H cleavage) such that the latter step

5,26

is rate-determining

has the highest degree of rate control

(or, according to Campbell, this step

9−51

). In the presence of

low thermodynamic activities of intraporous water, as is the

case for gas-phase dehydration and in decalin, the deprotona-

tion of the alkoxy intermediate by framework oxygen (a base)

occurs at a rate suﬃciently faster than that of C−O

recombination. Under these conditions, C−O recombination

occurs infrequently, at least because there are few water

molecules present to reform the C−O bond. In this case

dehydration was similar to that of the monomer route

−

(k_a_q_u_e_o_u_s= 0.03−0.18 vs k

= 0.05−0.15 s at 413−433

M, decalin

K) in decalin. Surprisingly, however, a marked diﬀerence in the

activation enthalpy was deduced for the two solvents (for the

‡

−1

monomer-path: ΔΔH

∼60 kJ mol ), which

C−O, water‑decalin

was almost totally oﬀset by entropic factors

‡

−1 −1

(ΔΔS

∼150 J mol K ) at the studied range

C−O, water‑decalin

≪ r

), C−O bond cleavage becomes

of temperature. In view of these diﬀerent enthalpic and

entropic components in the two solvents, the similar rate

constants for MFI in decalin and in water are concluded to be

coincidental.

C−O recombination

deprotonation

the rate-determining step (see more detailed discussions in

Section 4 of SI). Thus, the measured rate constant of aqueous-

phase dehydration reﬂects the Gibbs free energy of the

deprotonation TS relative to that of the associated complex of

alcohol and hydrated hydronium ions, whereas the rate

constants (for monomer- and dimer routes) of dehydration

in decalin reﬂect the Gibbs free energy diﬀerence between the

Transition states for both C−O and C−H scissions typically

occur late (i.e., resembling the products) along the reaction

26,47,48,52

coordinate of alcohol dehydration

in aqueous- and

gas-phase dehydration of alcohols catalyzed by protonic

zeolites (e.g., much longer distances between the leaving

H O-elimination TS (i.e., C−O cleavage) and the respective

adsorbed state (i.e., H-bonded monomer or protonated

dimer).

water and the C atom than in the original alcohol, signiﬁcant

positive charges on the organic moiety). The zeolite framework

or the pore conﬁnement, however, does not seem to have a

pronounced or systematic impact on the “lateness” of the

The above understanding of the mechanism facilitates the

interpretation of the observed diﬀerences in the rate constants

884

ACS Catal. 2021, 11, 2879−2888

ACS Catalysis

Research Article

Scheme 3. An Illustration of the Energy Landscapes for

Aqueous- and Decalin-Phase Dehydration of Cyclohexanol

That Seeks to Explain, on the Basis of the Hammond

Postulate That Links the Stability of Transition State (TS)

to That of the Product State (in This Case, the Intermediate

State between C−O and C−H Bond Cleavages), the Lower

Measured Activation Barrier for C−O Cleavage in the

Decalin-Phase Dehydration than in Aqueous-Phase

size substantially smaller than that in aqueous-phase catalysis

may form in zeolite pores. These intraporous active sites lead

to disparate kinetics of alkanol dehydration in decalin and in

water: for large pore zeolites FAU and BEA, dehydration rates

are 2−3 orders of magnitude higher in decalin than in aqueous

solutions, while for MFI, the rates are similar in the two

solvents.

In contrast to the aqueous-phase dehydration of cyclo-

hexanol that exhibits a dominant degree of rate control by C−

H bond cleavage (deprotonation of the cyclohexyl cation), C−

O bond cleavage is more rate-controlling for the dehydration

in decalin. The higher catalytic activity in apolar media is

primarily driven by lower activation enthalpies than in aqueous

solutions because of the greater stability of the alkoxy

intermediate (compared with a less stable hydrated carbenium

ion-like intermediate in the case of aqueous-phase dehydra-

tion) that forms upon C−O bond cleavage prior to subsequent

deprotonation to oleﬁn. The favorable enthalpic stabilization

from the intraporous decalin and coadsorbed cyclohexanol

molecules inside the conﬁnes of BEA and FAU lead to higher

rate constants in decalin (than those in water), while this

stabilization assumes lesser signiﬁcance inside the narrower

MFI pores, making Brønsted acid sites in FAU and BEA much

more active than in MFI for the apolar-phase dehydration of

cyclohexanol, an opposite trend to that observed in aqueous

phase.

Dehydration

Note that C−O TS and C−H TS lie at similar energies in the

aqueous case . The most populated initial states for C−O cleavage,

shown as ROH. . .H , are diﬀerent, being an adsorbed alcohol

4. MATERIALS AND METHODS

monomer at the zeolite proton in the decalin case, while being an

ROH associated with hydronium ions in the water case; these two

states are referenced to the same energy level in this scheme. Dashed

lines are used to indicate that some steps (e.g., protonation)

connecting two given states are omitted.

.1. Chemicals and Catalysts. Commercial zeolites were

−

obtained in the H-form, activated (1 K min to 723 K for 6 h)

and stored in closed containers to prevent from contamination

by volatile organics. Speciﬁcally, BEA (Si/Al = 75) was

provided by Clariant AG, while MFI (CBV8014, Si/Al = 40)

elimination or deprotonation TS according to theoretical

and FAU (CBV760;Si/Al_t_o_t_a_l= 30, Si/Al = 60) were obtained

calculations. The high activation entropies observed for BEA

and FAU in both water and decalin further suggest similar size

of the TS relative to the ground state. Moreover, a very recent

study from this group suggested that intraporous stabilization

assumes greater signiﬁcance over van der Waals stabilization

provided by the larger pores of BEA for aqueous-phase alkanol

dehydration. Therefore, we hypothesize that the favorable

enthalpic stabilization from the intraporous decalin and

coadsorbed cyclohexanol lead to higher rate constants in

decalin than water inside the conﬁnes of BEA and FAU.

Presumably, this stabilization assumes lesser signiﬁcance inside

the narrower MFI pores.

from Zeolyst International. Cyclohexanol (ReagentPlus, 99%),

cyclohexene (99%), 1,3-dimethoxybenzene (99%), sodium

sulfate (99%, anhydrous), dichloromethane (HPLC grade),

cyclohexanol-d₁₂(98−99 atom % D; containing a small but

undetermined fraction of C D HOD) and sodium borodeu-

teride (98 atom % D, 90% (CP)) were purchased from

Sigma−Aldrich and used as received without further

puriﬁcation. Water (H O, resistivity 18.2 MΩ·cm) was taken

from an ultrapure water dispenser system.

4.2. Preparation of 1-D-Cyclohexanol. Cyclohexanone

(3.8 g, 38.5 mmol) was mixed with 100 mL of water in a 250

. CONCLUSIONS

On zeolites, the dehydration of cyclohexanol in decalin

generally proceeds via E1-elimination routes mediated by

alcohol monomer and dimer species. The only exception found

in this study is the monomer route catalyzed by MFI, for which

the low activation barrier and negative activation entropy point

to a concerted elimination. The formation of cyclohexanol-

cyclohexanol dimers at the Brønsted acid sites leads to a

decrease in turnover frequency with increasing concentrations

of cyclohexanol in decalin, most markedly in zeolites with large

pores. However, at concentrations less than ∼1 M, the reaction

rates are dominated by cyclohexanol monomers that exhibit 1

to 2 orders of magnitude higher rate constant than dimers.

Most likely, the framework-bound proton remains a

predominant presence at relatively low conversions, though

there is a ﬁnite possibility that hydronium ion clusters with a

mL round-bottom ﬂask. Then 0.775 g (19.4 mmol) of NaOH

pellets was added to the solution under continuous stirring at

room temperature. After NaOH dissolved, 0.82 g (17.6 mmol)

of NaBD was slowly added to the ﬂask. After 5 min, the

solution was heated and reﬂuxed for 2 h with stirring. After it

was cooled to room temperature, the solution was extracted

twice with diethyl ether (100 mL × 2). The combined ether

solution was dried over anhydrous Na SO . Finally, the

product 1-D-cyclohexanol was obtained after distillation in

an oil bath, with its purity checked by NMR.

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ACS Catal. 2021, 11, 2879−2888

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https://pubs.acs.org/doi/10.1021/acscatal.0c05674.

Research Article

.3. Kinetic Measurements. The dehydration reactions

was loaded with identical compositions, without zeolite.

Uptake (q) was determined using GC by measuring the

diﬀerence in alcohol concentration between the starting

solution and the solution at equilibrium (alcohol concentration

were performed in a 300 mL Hastelloy Parr reactor, a

gradientless batch reactor. Preliminary tests showed that the

zeolite samples (protected from contaminants) could be used

oﬀ-the-shelf without reactivation procedures. For reactions

catalyzed by solid acids, the external diﬀusion limitation (mass

transport of the dissolved reactants from the liquid bulk to the

outer surface of the catalyst particles) was ruled out in

preliminary experiments by varying the stirring speed (400−

−

c ): q = V(c −c )m . Adsorption isotherms were obtained by

immersing 100 mg of zeolite in a cyclohexanol-decalin solution

of a deﬁned alcohol concentration for at least 24 h.

4.6. Infrared (IR) Spectroscopy of Adsorbed Pyridine.

IR spectra of adsorbed pyridine were recorded with a Perkin−

−

00 rpm) and catalyst loading (10−100 mg). In a typical

experiment, 2.4 g of cyclohexanol in 78 mL of decalin solution

0.3 M) with 100 mg of zeolite were loaded in the reactor. The

Elmer 2000 spectrometer at a resolution of 4 cm . The

catalyst samples were prepared as self−supporting wafers and

−

(

activated in vacuum (p = 10 mbar) at 723 K for 1 h at a

−

sealed reactor was then pressurized with 40 bar of H at room

heating rate of 10 K min . After it was cooled to 423 K, the

sample was equilibrated with 0.1 mbar pyridine for 0.5 h

followed by outgassing for 1 h and the acquisition of the

spectrum. Finally, desorption program (up to 723 K with 10 K

temperature and heated to the set temperature. During

heating, the stirring rate was typically less than 100 rpm.

The stirring rate was adjusted to 700 rpm once the set

temperature was reached. The reaction time was based on the

point when the set temperature was reached (typically 30

min). H and its pressure had no eﬀect on the dehydration

^r^e^a^c^tⁱ^oⁿ^aⁿ^d^w^a^s^f^o^uⁿ^d^t^o^b^e^t^o^t^a^l^l^y^r^e^p^l^a^c^e^a^b^l^e^wⁱ^t^h^N2^,

except for an observation of a faster heating proﬁle under high

pressure H while under low stirring (<100 rpm). At the end of

a reaction, the reactor was cooled with an ice/water mixture to

at least 278 K. The reaction mixture was isolated by

centrifugation, and an aliquot of the solution was analyzed

on an Agilent 7890A GC equipped with a HP-5 ms 25m ×

.25 mm (i.d.) column and a ﬂame ionization detector. 1,3-

Dimethoxybenzene was used as the standard for quantiﬁcation

added to the liquid after opening the reactor and extracted

−

min and 0.5 h at 723 K) was initiated and the spectra were

recorded until equilibrium was achieved. The concentrations of

BAS and LAS are quantiﬁed using the integrated areas of peaks

−

at 1540 and 1450 cm , respectively. The number of pyridine

molecules retained after evacuation at 423 and 723 K were

used to determine the concentrations of total and strong acid

sites, respectively. For quantiﬁcation, molar integral extinction

−

coeﬃcients of 0.73 and 0.96 cm μmol were used for BAS and

54,55

LAS, respectively.

ASSOCIATED CONTENT

■

sı Supporting Information

(

together).

The Supporting Information is available free of charge at

By measuring the composition of the reaction mixture at

diﬀerent time points and dividing the moles of oleﬁn

cyclohexene) formed by the time elapsed and the mass of

Physicochemical properties of parent zeolites, additional

(

rate data, results of H NMR experiments and isotopic

catalyst (or the BAS concentration), the rate of oleﬁn

formation was obtained (on a mass basis or site basis). As

discussed in Section 2.2, the rate of dehydration decreases with

increasing concentration of cyclohexanol, and thus, this rate

assessment protocol provides an average rate value within the

studied conversion range (albeit relatively low) and somewhat

underestimates the true initial rates at the starting alcohol

concentration.

tracer experiments, derivations of rate equations, and

additional discussion on ﬁtting results, the rate-

determining step and chemical state of acid sites

(PDF )

AUTHOR INFORMATION

■

.4. H/D KIE Measurements. For H/D KIE measure-

Corresponding Authors

ments, 0.025 or 0.1 M 1-D-cyclohexanol or cyclohexanol-d in

decalin solution with 10−100 mg of zeolite were loaded in the

reactor. The sealed reactor was then pressurized with 40 bar of

Hui Shi − Department of Chemistry and Catalysis Research

Center, Technische Universität München, 85748 Garching,

Germany; orcid.org/0000-0003-1180-7443 ;

Email: shihui@yzu.edu.cn

Johannes A. Lercher − Institute for Integrated Catalysis,

Paciﬁc Northwest National Laboratory, Richland,

Washington 99352, United States; Department of Chemistry

and Catalysis Research Center, Technische Universität

München, 85748 Garching, Germany; orcid.org/0000-

0002-2495-1404 ; Email: Johannes.lercher@pnnl.gov

H at room temperature and heated to the set temperature

while being stirred vigorously (700 rpm). The reaction time

was based on the point when the set temperature was reached

ca. 30 min after heating was started). At the end of a reaction,

the reactor was cooled with an ice/water mixture to at least

78 K. The reaction mixture was isolated by centrifugation,

and an aliquot of the solution was analyzed by GC, using 1,3-

dimethoxybenzene as the standard for quantiﬁcation (added to

the liquid after opening the reactor).

(

Authors

.5. Liquid-Phase Adsorption and Calorimetry. Heat

Feng Chen − Institute for Integrated Catalysis, Paciﬁc

Northwest National Laboratory, Richland, Washington

99352, United States; orcid.org/0000-0002-5227-6119

Manish Shetty − Institute for Integrated Catalysis, Paciﬁc

Northwest National Laboratory, Richland, Washington

99352, United States; orcid.org/0000-0002-8611-7415

Meng Wang − Institute for Integrated Catalysis, Paciﬁc

Northwest National Laboratory, Richland, Washington

99352, United States; orcid.org/0000-0002-3380-3534

of adsorption of cyclohexanol from decalin solutions onto

zeolites was determined by liquid calorimetry using a Setaram

Calvet C80 calorimeter. Reversal mixing cells were used to

separate the adsorptive from the adsorbent. The lower

compartment was loaded with 0.03 g of zeolite (m) immersed

in 1.0 mL of decalin. The upper compartment was loaded with

.0 mL of the desired cyclohexanol solution resulting in a total

volume (V) of 2 mL with a concentration c . The reference cell

886

ACS Catal. 2021, 11, 2879−2888

ACS Catalysis

Research Article

Yuanshuai Liu − Department of Chemistry and Catalysis

Research Center, Technische Universität München, 85748

Garching, Germany

Donald M. Camaioni − Institute for Integrated Catalysis,

Paciﬁc Northwest National Laboratory, Richland,

Washington 99352, United States; orcid.org/0000-0002-

added-value chemicals to fuelsvia aqueous-phase processing. Chem.

Soc. Rev. 2011, 40, 5266−5281.

12) Deshlahra, P.; Carr, R. T.; Chai, S.-H.; Iglesia, E. Mechanistic

Details and Reactivity Descriptors in Oxidation and Acid Catalysis of

Methanol. ACS Catal. 2015, 5, 666−682.

(13) Macht, J.; Carr, R. T.; Iglesia, E. Consequences of Acid

Strength for Isomerization and Elimination Catalysis on Solid Acids. J.

Am. Chem. Soc. 2009, 131, 6554−6565.

213-0960

Oliver Y. Gutiérrez − Institute for Integrated Catalysis, Paciﬁc

Northwest National Laboratory, Richland, Washington

(14) Jones, A. J.; Zones, S. I.; Iglesia, E. Implications of Transition

State Confinement within Small Voids for Acid Catalysis. J. Phys.

Chem. C 2014, 118, 17787−17800.

9352, United States; orcid.org/0000-0001-9163-4786

(15) Jones, A. J.; Carr, R. T.; Zones, S. I.; Iglesia, E. Acid strength

Complete contact information is available at:

https://pubs.acs.org/10.1021/acscatal.0c05674

and solvation in catalysis by MFI zeolites and effects of the identity,

concentration and location of framework heteroatoms. J. Catal. 2014,

16) Deshlahra, P.; Carr, R. T.; Iglesia, E. Ionic and Covalent

12, 58−68.

Notes

The authors declare no competing ﬁnancial interest.

Stabilization of Intermediates and Transition States in Catalysis by

Solid Acids. J. Am. Chem. Soc. 2014, 136, 15229−15247.

17) Simonetti, D. A.; Carr, R. T.; Iglesia, E. Acid strength and

ACKNOWLEDGMENTS

■

solvation effects on methylation, hydride transfer, and isomerization

rates during catalytic homologation of C1 species. J. Catal. 2012, 285,

This work is supported by the U.S. Department of Energy

DOE), Oﬃce of Science, Oﬃce of Basic Energy Sciences

BES), Division of Chemical Sciences, Geosciences and

(

9−30.

(

(18) Bregante, D. T.; Johnson, A. M.; Patel, A. Y.; Ayla, E. Z.;

Cordon, M. J.; Bukowski, B. C.; Greeley, J.; Gounder, R.; Flaherty, D.

W. Cooperative Effects between Hydrophilic Pores and Solvents:

Catalytic Consequences of Hydrogen Bonding on Alkene Epoxidation

in Zeolites. J. Am. Chem. Soc. 2019, 141, 7302−7319.

Biosciences (Transdisciplinary Approaches to Realize Novel

Catalytic Pathways to Energy Carriers, FWP 47319). Portions

of the experiments were performed at the William R.

Environmental Molecular Science Laboratory (EMSL), a

national scientiﬁc user facility sponsored by the DOE Oﬃce

of Biological and Environmental Research located at Paciﬁc

Northwest National Laboratory (PNNL). PNNL is a multi-

program national laboratory operated for DOE by Battelle.

19) Di Iorio, J. R.; Johnson, B. A.; Roman-Leshkov, Y. Ordered

Hydrogen-Bonded Alcohol Networks Confined in Lewis Acid

Zeolites Accelerate Transfer Hydrogenation Turnover Rates. J. Am.

Chem. Soc. 2020, 142, 19379−19392.

(20) Cordon, M. J.; Harris, J. W.; Vega-Vila, J. C.; Bates, J. S.; Kaur,

S.; Gupta, M.; Witzke, M. E.; Wegener, E. C.; Miller, J. T.; Flaherty,

D. W.; Hibbitts, D. D.; Gounder, R. Dominant Role of Entropy in

Stabilizing Sugar Isomerization Transition States within Hydrophobic

Zeolite Pores. J. Am. Chem. Soc. 2018, 140, 14244−14266.

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