Unveiling the thermodynamic-kinetic trade-off effect on acid sites in zeolite-catalyzed alcohol dehydration

Domain: chemistry
Published: 2026-03-09
Editorial outcome: major_revision

Abstract
Understanding the distinct roles of Brønsted and Lewis acid sites remains a great challenge in designing
zeolite catalysts, as their coexistence often obscures mechanistic understanding. Here, we combine solid-
state NMR spectroscopy with density functional theory to elucidate the site-specific pathways of ethanol
dehydration to ethylene over ZSM-5 zeolite. Two key intermediates are identified: chemisorbed ethanol
on Lewis acid sites (LAS) and surface ethoxy species on Brønsted acid sites (BAS), both formed via -OH
activation followed by β-H elimination to yield ethylene. Comparative analysis reveals a thermodynamic–
kinetic trade-off between the two sites. LAS facilitates low-temperature -OH activation but exhibits high
barriers for β-H abstraction, limiting ethylene formation. In contrast, BAS requires higher activation
energy for -OH activation but enables more facile β-H elimination, promoting ethylene production. This
intrinsic trade-off, governed by the thermodynamics of -OH activation, provides a mechanistic basis for
understanding and tuning alcohol dehydration on zeolite acid sites.

Introduction
Alcohol molecules, including methanol and higher-chain alcohols, are important platform chemicals that
can be produced in abundance from various feedstocks such as coal, natural gas, and biomass.1-5 The
catalytic dehydration of these alcohols has attracted considerable interest due to its environmental benefits
and potential economic advantages over traditional crude oil processes.6-11 Zeolites play a crucial role as
catalysts in this transformation, characterized by their tunable active sites, well-defined molecular-sized
channels, and excellent thermal stability (Fig. 1).12-15 However, their high catalytic activity in alcohol
conversion often poses challenges in achieving precise product selectivity and managing coking issues.
To develop high-performance catalysts, it is essential to thoroughly understand the mechanisms governing
alcohol conversion. However, this understanding is complicated by the inherent complexities of the active
sites and the surface species involved in the reaction.16-19

Both Brønsted acid sites (BAS) and Lewis acid sites (LAS) serve as critical active centers in the
dehydration of alcohols over zeolites.20,21 BAS arise from the isomorphous substitution of aluminum for
silicon within the zeolite lattice, enabling alcohol dehydration via either concerted or stepwise mechanistic
pathways.3 In contrast to the well-defined nature of BAS, LAS exhibit greater structural and functional
complexity.22 The (hydro)thermal treatment of zeolites induces the disintegration of framework Al (tetra-
coordinated) into coordinatively unsaturated tri-coordinated framework Al and extra-framework Al
species, both of which act as LAS.23,24 Moreover, the spatial proximity of BAS and LAS within the zeolite
channel can give rise to synergistic active sites that integrate the properties of both acid types.25-29
Consequently, ethanol dehydration is governed by the interplay of cooperative and competitive
interactions between these distinct acidic sites.

The dehydration of alcohols over zeolite catalysts is a complex process comprising multiple sequential
steps, including adsorption, activation, and subsequent reaction pathways, which collectively give rise to
a diverse array of surface-bound species. In the context of alcohol dehydration catalyzed by BAS, the
process is initiated by the activation of the alcohol’s hydroxyl (-OH) group, facilitating the formation of
active intermediates such as alkyloxonium ions and surface alkoxy species.30-32 Specifically, in ethanol
dehydration, the activation of the -OH group at BAS leads to the generation of surface ethoxy species
(SES), which subsequently undergo β-H elimination to yield ethylene.33 While BAS are recognized as
primary active sites for ethanol dehydration on zeolites, LAS can also contribute to the overall catalytic
activity. Extra-framework LAS have been demonstrated to enhance the formation of SES on BAS at
reduced temperatures, thereby promoting ethylene production.34 The electron-deficient character of LAS
facilitates strong interactions with electron-rich molecules like water and alcohols.22,35-38 However, the
detailed mechanisms underlying ethanol activation and conversion on LAS within zeolite remain
insufficiently understood, hindering a comprehensive elucidation of the distinct roles played by different
active sites in the ethanol dehydration process.

In this work, we elucidate the roles of distinct active sites in ethanol dehydration to ethylene over ZSM-
5 zeolite. Three critical surface intermediates are involved in ethanol dehydration over zeolites: surface
ethoxy species on Brønsted acid sites (SES-BAS), chemisorbed ethanol on Lewis acid sites (CSE-LAS),
and triethyloxonium ions (TEO). Mechanistic analysis reveals that BAS and LAS catalyze ethanol
dehydration through parallel two-stage pathways, with each site exhibiting a kinetic trade-off that governs
ethylene formation (Fig. 1). Specifically, LAS preferentially promotes the initial -OH activation at lower
temperatures but imposes kinetic limitations on the subsequent β-H elimination step, whereas BAS favors
β-H elimination at elevated temperatures. This mechanistic compensation dictates overall catalytic
efficiency. Comparable kinetic behaviors are also observed during isopropanol dehydration on zeolites,
supporting the generality of this mechanism.

Results
Effect of Acid Sites on Catalytic Activity
ZSM-5 zeolite samples (Si/Al = 11.5, Zeolyst) were calcined in static air at 650 °C, resulting in samples
designated as ZSM-5-650 (refer to the methods section for detailed procedures). Structural and acidic
property analyses reveal the presence of BAS and LAS at concentrations of 347 µmol/g and 85 µmol/g,
respectively (as detailed in Figs. S1-S4 and Table S1). To investigate the origin of the LAS, 27Al magic-
angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy was performed on NH4+
exchanged ZSM-5-650 samples (Fig. S5) under hydration condition. On ZSM‑5‑650, the 0 ppm signal
assigned to octahedrally coordinated Al originates from water coordination to Lewis acidic Al sites. Upon
NH₄⁺ exchange, these octahedral species reversibly transform into tetrahedrally coordinated Al, giving
rise to a signal at ~55 ppm. This reversible coordination change demonstrates that the observed
octahedrally coordinated Al species result from water adsorption on framework-associated Al rather than
from extra-framework Al species.22 The acidic characteristics of the zeolites were further probed using
31P NMR spectroscopy, which involved introducing trimethylphosphine oxide (TMPO) as a probe
molecule (Fig. 2a). TMPO, a Lewis base, interacts with both BAS and LAS, and the resulting changes in
its 31P NMR signal provide information about the nature and type of acidity present on the dehydrated
zeolite. The observed signals at 76 ppm and 88 ppm were assigned to TMPOH+ ions formed on BAS and
synergistic Brønsted/Lewis (B/L) acid sites,39 respectively. Additionally, signals within the 63-69 ppm
chemical shift range correspond to interactions between TMPO and framework-associated Al species
possessing zero (FTA), one (FTA-OH), or two (FTA-2OH) hydroxyl groups.40 To further investigate the
interactions between hydrogen and Al species within TMPO-adsorbed ZSM-5-650 zeolites, a two-
dimensional (2D) 1H-27Al dipolar-mediated heteronuclear multiple quantum coherence (D-HMQC) NMR
experiment was conducted. (Fig. 2b). This spectroscopic technique leverages dipolar coupling, a through-
space interaction between nuclear magnetic moments, to reveal spatial proximities and correlations
between 1H and 27Al nuclei. A prominent signal at (2.5, 51) ppm is attributed to Al-OH groups from tri-
coordinated Al sites bound to TMPO, which consequently form tetrahedral Al configurations.
Furthermore, a signal at (1.8, 51) ppm reflects the interaction between the methyl group of TMPO and
these tetrahedral Al sites. Notably, the absence of correlation signals linking Al-OH/TMPO to higher-
coordinated Al species (i.e., penta- or hexa-coordinated) confirms the lack of extra-framework Al in the
zeolites. Additionally, the absence of correlation signals between BAS and framework Al is attributed to
the formation of delocalized TMPOH+ species. Therefore, these data demonstrate that the predominant
acid sites in the ZSM-5-650 samples are BAS and framework-associated Al LAS. It should be noted that
upon zeolite hydration, tri-coordinated Al sites exposed under dehydrated conditions would transform into
higher-coordination states (tetra-, penta-, or hexa-coordinated Al), leading to a reduction in the number of
LAS.40

To further delineate the individual roles of BAS and LAS in ethanol dehydration, two additional ZSM-5
zeolite samples were synthesized: one predominantly containing BAS and another primarily comprising
LAS (Figs. S1-S5). The BAS-rich sample, designated ZSM-5-650-AHFS, was prepared by selectively
removing framework Al LAS from ZSM-5-650 using ammonium hexafluorosilicate (AHFS). Conversely,
the LAS-rich sample, denoted ZSM-5-650-Na, was obtained by ion-exchanging BAS in ZSM-5-650 with
sodium ions (Na+) using sodium nitrate (NaNO3). Ethanol dehydration activity was then evaluated over
these zeolites in the temperature range of 200-275 °C (Fig. 2c). At 200 °C, ethanol dehydration over all
tested samples primarily yields diethyl ether (DEE) and ethylene. For the BAS-rich ZSM-5-650-AHFS,
ethanol conversion reaches 77.8%, with DEE and ethylene yields of 58.0% and 19.8%, respectively. In
contrast, the LAS-rich ZSM-5-650-Na exhibits 47.4% ethanol conversion, yielding 43.9% DEE and 3.5%
ethylene. The high DEE yields on both zeolites suggest that DEE formation proceeds with a relatively
low activation barrier over both BAS and LAS. It is noteworthy that DEE serves as a reactive intermediate
in the ethanol dehydration network over zeolites and can subsequently convert to ethylene (Fig. S6).
However, the markedly lower ethylene yield over LAS-rich zeolite indicates the limited reactivity toward
alkene formation (Fig. S7). Notably, ZSM-5-650, which contains both BAS and LAS sites, exhibits a
significantly higher ethylene yield of ~67.3% within the first 2 h. This can be attributed to the cooperation
of BAS and LAS in promoting the β-H elimination step, which is further supported by turnover frequency
(TOF) analysis (Tables S2 and S3).

As the reaction temperature was incrementally raised from 200 °C to 275 °C, all three zeolite samples—
ZSM-5-650, ZSM-5-650-AHFS (BAS-rich), and ZSM-5-650-Na (LAS-rich)—show a marked increase
in catalytic activity. However, a notable trend emerges with ZSM-5-650: the selectivity toward ethylene,
the primary dehydration product, decreases as the temperature increases. This decline in ethylene
selectivity is likely attributable to the initiation of secondary reactions, such as oligomerization of ethylene.
To further explore how the BAS-to-LAS ratio influences ethanol dehydration, a series of dealuminated
ZSM-5 catalysts were synthesized by calcining the parent ZSM-5 zeolite at temperatures ranging from
500 °C to 700 °C, yielding samples with varying BAS-to-LAS ratios (Table S1). Catalytic testing of these
samples reveals that ZSM-5-600 and ZSM-5-650, characterized by moderate BAS-to-LAS ratios,
outperform their counterparts in terms of catalytic efficiency. These results indicate that an optimal
equilibrium between BAS and LAS is essential for maximizing the efficiency of ethanol dehydration to
ethylene, as illustrated by the performance trends (Fig. S8), highlighting the critical role of acid site
distribution in achieving high selectivity and conversion under these reaction conditions.

Characterization of Surface Species during Ethanol Dehydration over ZSM-5
Investigating the formation and evolution of surface species on zeolites provides crucial insights into the
dynamic mechanisms governing ethanol transformation. To characterize these surface species during
ethanol dehydration, 13C MAS NMR spectroscopy was employed. Specifically, CH313CH2OH was reacted
over ZSM-5-650, ZSM-5-650-AHFS, and ZSM-5-650-Na at 200 °C, with the resulting surface species
analyzed via 13C MAS NMR (Fig. 3a). The spectroscopic data reveals a diverse range of adsorbed species
on the zeolite surfaces, as evidenced by distinct 13C NMR signals spanning the chemical shift range of
60-90 ppm. Signals at 61 and 63 ppm are assigned to the methylene groups of physically adsorbed ethanol,
reflecting its intact presence on the surface.9,41 A signal at 69 ppm is attributed to the methylene groups of
DEE, a common intermediate to ethylene or byproduct in ethanol dehydration. Notably, signals at 72 ppm
and 85 ppm are exclusively observed on ZSM-5-650 and the BAS-rich ZSM-5-650-AHFS. These signals
were identified as surface ethoxy species bonded to BAS (SES-BAS) and triethyloxonium ions (TEO),
respectively.30-32,42 Our previous experimental and theoretical studies have shown that the reaction
pathway involving the TEO intermediate is more favorable than the conventional ethanol dehydration
route on ZSM-5, offering a more plausible mechanism for ethylene formation at lower temperatures.32,41
In this pathway, TEO serves as the intermediate leading to the formation of SES-BAS, which then
decomposes to ethylene.

Furthermore, a distinct surface species, characterized by a prominent 13C NMR signal at 66 ppm, is
predominantly observed on the LAS-rich ZSM-5-650-Na. While this species is also detected on ZSM-5-
650, it is only marginally observable on the BAS-rich ZSM-5-650-AHFS, indicating a strong correlation
with the presence of LAS. Based on these findings, it is deduced that the formation of this specific surface
species is primarily driven by framework LAS on zeolite. Remarkably, this species forms readily even at
room temperature (Fig. 4), indicating that LAS can effectively activate ethanol. It is proposed to serve as
a key intermediate in ethanol dehydration to ethylene over zeolites.

To gain deeper insights into the bonding characteristics of surface species on zeolites, the interactions
between these species and framework active sites were further investigated. To this end, 13C-{27Al}
symmetry-based rotational-echo saturation-pulse double-resonance (S-RESPDOR) NMR experiments
were conducted (Fig. 3b), enabling the assessment of spatial proximity and interactions between 13C
nuclei in surface species and 27Al nuclei in framework active sites.43,44 In this technique, when 27Al nuclei
are in close proximity to 13C nuclei, irradiation of the 27Al nuclei induces significant dipolar dephasing
effects on the 13C signals—quantified as the fractional signal reduction (ΔS/S₀)—leading to a pronounced
decrease in signal intensity. Conversely, if the 13C and 27Al nuclei are spatially distant, irradiation of 27Al
results in minimal changes to the 13C signals. As depicted in Fig. 3b, the 13C signals corresponding to
physically adsorbed ethanol (at 61 and 63 ppm) and DEE (at 69 ppm) exhibit negligible alteration upon
27Al irradiation, suggesting that these species are positioned away from the zeolite's active sites. Similarly,
the 13C signal of TEO at 85 ppm shows no discernible change with 27Al irradiation, indicating a lack of
close interaction between the cationic center of TEO and the anionic framework sites of the zeolite. In
contrast, the 13C signal at 72 ppm, attributed to SES-BAS, displays a clear dipolar dephasing effect (ΔS/S
0 = 0.52), reflecting a strong spatial coupling with framework Al. Similarly, the 13C signal at 66 ppm,
observed exclusively in ZSM-5-650 and LAS-rich ZSM-5-650-Na, exhibit substantial dipolar dephasing
(ΔS/S0 = 0.51), signifying a robust interaction with framework Al. Based on this evidence, the 66 ppm
signal is assigned to surface ethoxy-like species linked to framework LAS, herein termed as chemisorbed
ethanol on LAS (CSE-LAS). To aid in the interpretation of the 66 ppm 13C NMR signal, we performed
DFT calculations on two types of surface species—chemisorbed ethanol (–OHCH2CH3) and directly
bonded ethoxy groups (–OCH2CH3)—on three representative framework-associated LAS: FTA, FTA-
OH, and FTA-2OH (Fig. S9). The predicted 13C chemical shifts for the chemisorbed ethanol species (CSE-
LAS) are 66 ppm (FTA), 68 ppm (FTA-OH), and 64 ppm (FTA-2OH), closely matching the experimental
observation. In contrast, the calculated chemical shifts for the ethoxy species on the same LAS types are
lower: 58 ppm (FTA), 61 ppm (FTA-OH), and 59 ppm (FTA-2OH). These results confirm that the 66 ppm
NMR signal originates from CSE-LAS. The corresponding structure is illustrated in the lower panels of
Fig. 3. In situ FTIR spectroscopy was further employed to monitor the formation of surface intermediates
during ethanol dehydration over the zeolite catalysts (Fig. S10). A distinct absorption band at 2912 cm-1,
attributed to chemisorbed ethanol, is observed exclusively in ZSM-5-650 and the LAS-rich ZSM-5-650-
Na samples, but is absent in the BAS-rich ZSM-5-650-AHFS. This selective appearance of the 2912 cm-
1 band reinforces the conclusion that CSE-LAS forms specifically on LAS, consistent with our NMR
results and DFT predictions. Structurally, in CSE-LAS, the oxygen atom adopts a tri-coordinated
configuration, resembling the configuration in SES-BAS; however, unlike SES-BAS, where the oxygen
is bonded to a silicon atom, in CSE-LAS it is coordinated to a hydrogen atom, implying distinct reactivity
in the ethanol dehydration process.

To further probe the configuration of CSE-LAS species, we experimentally determined the interatomic
distance between the carbon atom of CSE-LAS (methylene group) and the Al atom associated with
framework LAS. This was achieved by measuring the dipolar dephasing effect (ΔS/S₀) as a function of
recoupling time in 13C-{27Al} S-RESPDOR NMR experiments (Fig. 3c). By fitting the resulting S-
RESPDOR build-up curve, a distance of 2.83 Å is derived, which closely matches the theoretically
calculated distance of 2.80 Å, providing structural insight into the surface-bound CSE-LAS on zeolite.
Furthermore, to elucidate the coordination environment of Al species during ethanol dehydration, 2D 13C-
{27Al} population transfer dipolar-mediated heteronuclear multiple quantum coherence (PT-D-HMQC)
NMR experiments were performed (Fig. 3d). The PT-D-HMQC results indicate that SES-BAS (72 ppm)
interacts with tetra-coordinated Al at 48 ppm. This Al is part of the framework and is responsible for
Brønsted acidity in the zeolite. In contrast, the CSE-LAS (66 ppm) correlates with a distinct tetra-
coordinated Al species at 53 ppm. This observation indicates that the tri-coordinated framework Al LAS
transitions to a tetra-coordinated state upon ethanol adsorption, forming a coordination bond during
dehydration. This process mirrors the adsorption behavior of TMPO on LAS (Fig. 2a), where the oxygen
atom’s lone pair electrons coordinate with the electron-deficient Al atom of LAS, exemplifying a classic
Lewis acid-base interaction.

Understanding the Reactivity of Surface Species in Ethanol Dehydration over ZSM-5
To elucidate the reactivity of surface species during ethanol dehydration over ZSM-5-650, their
temperature-dependent (25 °C ~350 °C) behavior was monitored using variable-temperature 13C MAS
NMR spectroscopy (Fig. 4a). Ethanol predominantly binds to the acid sites of zeolites, activating the -OH
group to generate surface species like CSE-LAS, SES-BAS, TEO, and DEE, which then convert into
ethylene and potentially secondary products at elevated temperatures. The normalized 13C NMR signals,
plotted against reaction temperature (Fig. 4b), demonstrate the formation and subsequent transformation
of CSE-LAS, SES-BAS, TEO, and adsorbed DEE on the zeolite surface, providing insights into the
mechanistic steps of ethanol dehydration. This process unfolds in two primary steps: (1) the initial
formation of surface intermediates through the activation of ethanol’s -OH group, and (2) the
transformation stage, involving the β-H abstraction, which leads ethylene. Energetically, the activation of
the -OH group is more facile than the subsequent β-H abstraction, resulting in a temperature-driven
accumulation of surface intermediates that peaks as the reaction progresses. Notably, CSE-LAS forms at
temperatures as low as below 25 °C (Fig. 4a and Fig. S11), in stark contrast to SES-BAS and TEO, which
require temperatures exceeding 100 °C to emerge. This low-temperature formation of CSE-LAS
underscores the exceptional catalytic activity of framework LAS in facilitating ethanol activation and
generating chemisorbed ethanol species, compared to the higher thermal threshold demanded by BAS-
driven intermediates (SES-BAS, TEO and DEE). DEE appears at a lower temperature than SES-BAS and
TEO, suggesting that it serves as a precursor for the subsequent formation of these intermediates, in
agreement with our previous theoretical results.32,41 Further 13C MAS NMR analysis of the surface species
formed after reaction with DEE over ZSM-5-650 shows that DEE is converted to SES-BAS more readily
than ethanol, confirming its higher reactivity and its role as a key intermediate in ethanol dehydration (Fig.
S12). Following their maximum accumulation (Fig. 4b), these surface species undergo decay at elevated
temperatures due to their transformation into ethylene. The rate of this decay serves as an indicator of the
intermediates’ reactivity in the β-H abstraction step. Analysis of the decay rate reveals a reactivity order
of TEO > SES-BAS > CSE-LAS > DEE (Fig. 4b), directly reflecting a corresponding order of intrinsic
reactivity for these intermediates in the ethanol dehydration reaction.

These observations indicate a mechanistic trade-off in the two-stage ethanol dehydration process, which
is governed by the distinct characteristics of the active sites. Specifically, LAS facilitates the activation of
ethanol’s -OH group with remarkable efficiency, initiating the formation of CSE-LAS at room
temperature. However, the subsequent transformation of CSE-LAS into ethylene, requiring β-H
abstraction, demands significantly higher temperatures, resulting in a broad evolution curve that reflects
a slower and more thermally intensive decay process. Conversely, BAS requires elevated temperatures to
activate ethanol’s -OH group and form SES-BAS, yet the transformation of SES-BAS proceeds more
readily once formed, exhibiting a narrower evolution curve indicative of a faster and energetically more
favorable β-H abstraction step. This stark contrast illustrates two parallel reaction pathways on BAS and
LAS, each involving distinct energetics in the two-stage mechanism (Fig. 4c): LAS exhibits superior
activity in low-temperature ethanol activation but are less efficient in subsequent transformation steps,
whereas BAS, while displaying lower initial activity, promotes a more rapid product formation. This
dichotomy underscores the distinct catalytic strengths and inherent limitations of each acid site type within
the ethanol dehydration pathway.

To further validate the applicability of the proposed mechanistic framework, we extended our
investigation to isopropanol dehydration over zeolitic catalysts. As shown in Fig. S13, isopropanol is more
effectively converted to propene on ZSM-5-650 and BAS-rich ZSM-5-650-AHFS, whereas significantly
lower activity is observed on LAS-rich ZSM-5-650-Na, mirroring the reactivity trends observed for
ethanol. To probe the underlying surface chemistry, temperature-dependent solid-state 13C NMR
experiments were conducted over ZSM-5-650 in the range of 25–250 °C (Fig. S14). At room temperature,
chemisorbed isopropanol species are clearly observed on both LAS (76 ppm, denoted as CSI-LAS) and
BAS (68 ppm), and their assignments are further supported by 13C–27Al S-RESPDOR NMR analysis (Fig.
S15). The presence of CSI-LAS at low temperature indicates that –OH activation readily occurs on LAS,
forming stable chemisorbed species. Upon increasing the reaction temperature above 100 °C, the CSI-
LAS signal diminishes and eventually disappears, suggesting its conversion via β-H elimination to
propene. Concurrently, significant amounts of adsorbed hydrocarbon species, including alkenes and
aromatics, are detected.

Despite this, the enhanced propene formation observed on BAS-rich catalysts (Fig. S13) indicates that
CSI-LAS is unlikely to be the dominant pathway for propene production. While isopropoxy species on
BAS (typically at ca. 80 ppm) are widely considered as the key intermediates for propene formation, they
are not detected at low temperatures and remained undetectable throughout the reaction process. This
absence is likely due to their high reactivity and rapid turnover at elevated temperatures once formed.
Collectively, these findings demonstrate that isopropanol dehydration exhibits behavior analogous to
ethanol dehydration, reinforcing the existence of a trade-off between –OH activation and β-H elimination
on both BAS and LAS. These results support the general validity of the proposed mechanism for alcohol
dehydration over zeolites.

Reaction mechanism of ethanol dehydration reaction
To gain mechanistic insights into the ethanol dehydration reaction and the reactivity of different active
sites, density functional theory (DFT) calculations were performed (Fig. 5 and Figs. S16-18). The
dehydration process was computationally modeled over framework LAS, including FTA, and its
hydroxylated forms (FTA-OH, FTA-2OH). For comparative analysis, ethanol dehydration over BAS was
also computationally evaluated. We focused on the TEO-mediated pathway, which has been identified as
the most energetically favorable route compared to both unimolecular and bimolecular dehydration
mechanisms.32,41

Ethanol dehydration over both BAS and LAS involves two-stage process: (1) activation of the ethanol -
OH group, and (2) β-H abstraction leading to ethylene formation. For framework LAS (Fig. 5), the initial
step involves direct chemisorption of ethanol, forming CSE-LAS, accompanied by substantial exothermic
energy release: −53.5 kcal/mol for FTA, −35.4 kcal/mol for FTA-OH, and −33.4 kcal/mol for FTA-2OH.
This indicates a strong stabilization of CSE-LAS, with adsorption strength decreasing as the number of
hydroxyl groups on the LAS increases. However, the subsequent β-H abstraction from CSE-LAS is
endothermic, revealing a critical energetic barrier. A clear inverse relationship emerges between the
stability of CSE-LAS (reflected by the exothermic heat of adsorption) and the facility of ethylene
formation: greater adsorption strength correlates with a higher energy barrier for decomposition.
Specifically, the activation energy for β-H abstraction rises in the sequence FTA-2OH (38.8 kcal/mol) ≈
FTA-OH (40.5 kcal/mol) < FTA (51.5 kcal/mol), illustrating a trade-off wherein stronger initial binding
to LAS hinders the kinetics of the transformation step.

Unlike the CSE-LAS pathway, ethanol activation on BAS proceeds via SES-BAS formation, with TEO
identified as a key intermediate, as supported by DFT calculations (Fig. 5 and Fig. S17). This process
entails an activation energy of 25.6 kcal/mol (denoted TS3 in Fig. S17) for SES-BAS formation, rendering
it energetically more demanding than the formation of CSE-LAS. The SES-BAS formation pathway is
endothermic, largely due to the transformation of TEO to SES-BAS, and thus becomes more favorable at
elevated temperatures, aligning with temperature-dependent experimental observations (Fig. 4). For the
subsequent decomposition of SES-BAS into ethylene, the activation energy is calculated at 30.5 kcal/mol
(TS in Fig. 5), a value notably lower than those for all framework LAS systems (FTA: 51.5 kcal/mol,
FTA-OH: 40.5 kcal/mol, FTA-2OH: 38.8 kcal/mol). To clarify DEE’s role in ethanol dehydration, DFT
calculations were also conducted for its formation and conversion over BAS and LAS. On BAS, ethanol
and DEE readily interconvert with similar barriers (24.8 vs. 29.2 kcal/mol), and DEE can form TEO (23.5
kcal/mol), which decomposes to SES-BAS and ultimately ethylene (Fig. S17). On LAS (FTA, FTA-OH,
FTA-2OH; Fig. S19), ethanol–DEE equilibrium is also favorable, with low forward/reverse barriers
(26.5–28.2 vs. 16.4–18.2 kcal/mol). However, direct DEE-to-ethylene conversion is kinetically hindered
(40.8–67.1 kcal/mol). These results indicate that while DEE can reversibly form from ethanol over both
BAS and LAS, its further conversion to ethylene is kinetically feasible only via the BAS-mediated TEO
route. On LAS, DEE preferentially reverts to ethanol, which then proceeds through the CSE-LAS pathway
toward ethylene formation, as illustrated in Fig. 5.

We further conducted kinetic studies of ethanol dehydration using BAS- and LAS-enriched samples, as
shown in Fig. S20. To ensure intrinsic kinetic behavior, apparent activation energies were determined
from Arrhenius plots obtained under low-conversion conditions (below 15%), thereby minimizing the
influence of heat and mass transfer limitations. The experimentally determined activation energies are
31.7 kcal/mol for the BAS-rich ZSM-5-650-AHFS sample and 38.5 kcal/mol for the LAS-rich ZSM-5-
650-Na sample. These values are in close agreement with the DFT-predicted energy barriers: 30.5
kcal/mol for the decomposition of SES-BAS and 38.8 kcal/mol for the conversion of CSE-LAS (Fig. 5).
This strong correlation between experimental kinetics and theoretical calculations provides compelling
evidence that the SES-BAS-mediated pathway offers a more favorable route for ethylene formation,
highlighting a kinetic advantage over the CSE-LAS mechanism.

These differences in activation energies reflect how BAS and LAS distinctly influence each reaction step.
Consequently, the site-specific trade-off observed in the two-stage ethanol dehydration process stems
from the intrinsic acid characteristics of BAS and LAS, which control the thermodynamics and kinetics
of intermediate formation and transformation. On LAS, the covalent bond strength between SES and the
zeolite framework scales directly with LAS acidity, following the order FTA > FTA-OH ≈ FTA-2OH, as
evidenced by their respective adsorption heats (−53.5, −35.4, and −33.4 kcal/mol). This stronger bonding,
while stabilizing CSE-LAS, elevates the activation energy for β-hydrogen abstraction due to the increased
stability of the carbenium ion-like transition state (Fig. 5 and Fig. S18), impeding ethylene formation.
Conversely, SES-BAS formation on BAS requires an endothermic -OH abstraction step, which is less
favorable at low temperatures but becomes viable at higher temperatures due to the decomposition of
TEO. Despite this higher energy requirement for formation, SES-BAS exhibits a pronounced carbenium
ion character that enhances the ease of β-hydrogen abstraction, lowering the energy barrier for
decomposition relative to CSE-LAS. This disparity in carbenium ion character elucidates why SES-BAS,
though more challenging to form, undergoes kinetically more favorable decomposition. Thus, the
mechanistic trade-off observed on both BAS and LAS during two-stage ethanol dehydration stems from
the balance between adsorption strength, carbenium ion stabilization, and the activation energy required
for β-hydrogen abstraction at each site. In light of the entropy changes associated with adsorption and
desorption during ethanol dehydration, Gibbs free energy calculations were performed for the reaction
pathways (Figs. S21 and S22). At 200 °C, SES-BAS exhibits higher Gibbs free energy than CSE–LAS,
while the activation Gibbs free energy for the subsequent β-H elimination remains lower for SES-BAS
than for CSE-LAS. These findings confirm that the thermodynamic–kinetic trade-off persists under
reaction conditions. Given that water is a byproduct of ethanol dehydration, we further assessed its
influence on the reaction mechanism. As shown in Figs. S23-S25, the inclusion of a water molecule
markedly stabilizes all key adsorbed species, intermediates, and transition states compared with the
anhydrous models (Figs. 5 and S17). Despite this enhanced stabilization, the thermodynamic-kinetic
compensation persists, with the stability order (CSE-FTA-2OH > CSE-FTA > CSE-FTA-OH > SES-BAS)
remaining inversely correlated with the β-H elimination barriers (60.4, 51.4, 44.5, and 26.8 kcal/mol,
respectively). These results confirm that the mechanistic insights derived under anhydrous conditions
remain robust upon inclusion of water.

Methods
Sample preparation and characterization.
The preparation of calcined ZSM-5 zeolites was carried out as follows: ZSM-5 zeolites (with a Si/Al ratio
of 11.5, obtained from Zeolyst) were arranged in a shallow bed configuration on a quartz crucible and
subsequently calcined at 500 - 700 °C for 6 h in air. The heating temperature gradually elevated at a rate
of 1 °C/min raised from room temperature to a target temperature. The calcined samples are denoted as
ZSM-5-500, ZSM-5-600, ZSM-5-650 and ZSM-5-700, respectively. The ZSM-5-650 sample was washed
with ammonium hexafluorosilicate (AHFS) by the following procedure (denoted as ZSM-5-650-AHFS):
Catalysts were suspended in AHFS solution at a concentration of 1 g/10 ml with an AHFS: Al ratio of 2.
The resultant mixture was heated to 90 °C under intense stirring. After 6 h stirring, the final mixture was
filtered and subjected to three successive washings with deionized water. Subsequently, the samples were
dried in air at 100 °C overnight and then calcined at 550 °C for 6 h in air with a rate of 2 °C/min raised
from room temperature to a target temperature to remove NH4+. Furthermore, the calcined catalyst ZSM-
5-650 was also converted to the sodium form (denoted as ZSM-5-650-Na) and ammonium form (denoted
as ZSM-5-650-NH4) through the following procedure: The catalysts were suspended in 1.0 M solution of
sodium nitrate or ammonium nitrate respectively, with a concentration of 1 g/10 ml. The mixture was
heated to 90 °C under intense stirring for 2 h. The final mixture was filtered and then subjected to three
successive washings with deionized water, then, the samples were dried in air at 100 °C for 2 h. This
procedure was iterated three times to obtain zeolite in sodium form and ammonium form.

TMPO adsorption was performed on pre-dehydrated samples. Prior to adsorption of TMPO, the sample
was dehydrated on a vacuum line. The temperature gradually increased at a rate of 2 °C min-1 and the
sample was kept at a final temperature of 673 K at a pressure below 10-3 Pa overnight. Typically, solid
TMPO was dissolved in CD2Cl2 solution and a certain amount of TMPO solution was added into the
sample in the glass sample tube in a N2 glovebox followed by removal of the CD2Cl2 solvent by evacuation
on a vacuum line at 80 °C for 1 h. To ensure uniform adsorption of TMPO molecules in the channels of
the zeolite, the sealed sample was further heated at 180 °C for 2 h. Finally, the sample was transferred to
the ZrO2 MAS rotor in the N2 glovebox prior to the solid-state NMR experiments.

The structure and crystalline nature of H-ZSM-5 zeolites were investigated by X-ray diffractometer
(X’Pert3 Powder) using a CuKα radiation with a step of 0.02° at a respective voltage of 40 kV and a
current of 40 mA. The scanning range was from 5° to 40°. The acid concentration of zeolites was
determined by FT-IR of pyridine adsorption experiments. The measurements were conducted on a Bruker
Tensor 27 spectrometer. Prior to the measurements, catalysts were pre-activated at 350 °C under high
vacuum (<10−5 Pa) for 6 h. After cooling down to 25 °C, a background spectrum was collected.
Subsequently, an excessive amount of pyridine was then introduced to the infrared cell and held for 10
min to allow equilibrium. The residual pyridine was removed by vacuum at 200 °C for 2 h. The FT-IR
spectra of pyridine-adsorbed samples were measured after cooling down to 25 °C.

All the catalytic reactions were conducted in a fixed bed reactor with inner diameter of 6 mm.45 The ZSM-
5 powder was compacted into pellets between 40-60 mesh. The pellets (0.1 g) were activated at 400 °C
in flowing helium for 1 h prior to the reaction. Then both ethanol and isopropanol were reacted upon the
zeolite for 1 min at preset temperature, in which the total gas flow through the reactor was 100 sccm. Then
the reaction was thermally quenched by pulsing liquid nitrogen onto the catalyst bed, and the temperature
was cooled down to ambient temperature within a very short period (0.2 s). After the reaction was
quenched, the reactor containing the catalyst was sealed by means of valves. The sealed reactor was then
transferred to a glove box filled with pure N2 and the catalyst was packed into an NMR rotor for NMR
measurements. For the kinetic measurements, a feed containing 12 mbar ethanol was passed over 25 mg
of catalyst at a total gas flow rate of 100 mL/min. The reaction temperature was varied between 160 and
225 °C, with the conversion rate kept below 15%. All the effluent was analyzed by online GC-FID
chromatograph (Shimadzu GCMS-QP2010) equipped with a Supelco Supel-QTM PLOT capillary
column (30 m× 0.32 mm× 15 μm). The temperature programming started at 40 ℃ (maintained for 1 min),
followed by a rate of 5 ℃ min-1 to a temperature of 140 ℃ (maintained for 2 min) and a rate of 10 ℃
min-1 to a final temperature of 250 ℃ (maintained for 10 min).

Solid-State NMR experiments
All the 1D 1H, 13C, 27Al and 31P solid-state NMR experiments were performed at 11.7 T on a Bruker
Avance III-500 spectrometer, equipped with a 4 mm probe, with resonance frequencies of 500.57, 125.87,
130.44 and 202.65 MHz for 1H, 13C, 27Al and 31P, respectively. For the entirety of the experiments, a magic
angle spinning rate of 10 kHz was employed. For the 1H MAS NMR experiments, single-pulse excitation
was performed using a 1H π/2 pulse length of 4.0 μs and a repetition time of 2 s. The 1H chemical shifts
were referenced to adamantane (1.78 ppm). For the 1H→13C CP/MAS NMR experiments, the Hartmann-
Hahn condition was achieved using hexamethylbenzene (HMB), with a contact time of 3.5 ms and a
repetition time of 2 s. The 13C chemical shifts were referenced to HMB (a second reference to TMS). The
27Al MAS NMR spectra were acquired using small-flip angle technique with a pulse length of 0.3 μs (π/18
flip-angle (calculated from the Al(NO3)3 solution)) and a recycle delay of 1 s. The 27Al chemical shifts
were referenced to 1 M Al(NO3)3 aqueous solution (0 ppm). The 31P MAS NMR spectra were recorded
with a π/2 pulse length of 3.7 μs, a recycle delay of 30 s and a 1H decoupling RF strength of 60 kHz. The
31P chemical shifts were referenced to ammonium dihydrogen phosphate (0.81 ppm). The 29Si solid-state
NMR experiments were carried out at 9.4 T on a Bruker Avance III-400 spectrometer equipped with a 4
mm probe. The 29Si MAS NMR spectra were acquired with a 1H decoupling RF strength of 54 kHz, a
recycle delay of 100 s and a spinning rate of 10 kHz. The 29Si chemical shifts were referenced to kaolinite
(-91.5 ppm).

The 2D 1H-27Al D-HMQC experiment was performed at 18.8 T on a Bruker-Avance III-800 spectrometer,
for which 1H and 27Al Larmor frequencies are equal to 800.36 and 208.56 MHz, respectively, using a 3.2
mm probe under 20 kHz MAS speed. The recoupling pulses were applied on the proton channel with the
SR4 sequence,46 with the recoupling time of 3.0 ms. And the pulses applied on Al channel were central-
transition-selective pulses with low RF power lasting 12.8 µs for π/2 pulses. The spectra were acquired
by averaging 32 scans for each of the 12.5 μs t1 increments with a recovery delay of 1 s. Presaturation
has been applied with a train of 10 π/2 1H pulses 10 ms apart to spoil residual longitude magnetizations
at the beginning of each scan.

The 13C-{27Al} symmetry-based rotational-echo saturation-pulse double-resonance (S-RESPDOR)
experiments were also performed at 11.7 T on a Bruker-Avance III-500 spectrometer with a 4 mm probe
under 10 kHz MAS speed. 1H-13C cross polarization (CP) with a contact time of 5.0 ms was employed to
prepare the initial 13C signal. The recycle delay was set to 2 s. SR4 dipolar recoupling was used on the
13C channel with νnut,13C =20 kHz. Continuous-wave 1H decoupling with an amplitude of 71.4 kHz was
used during the dipolar recoupling, while a SPINAL-64 (small-phase incremental alternation with 64 steps)
1H decoupling with an amplitude of 75.8 kHz was used during acquisition. A π pulse length of 8 μs was
used on the 13C channel. The saturation pulse on the 27Al channel with an amplitude of 53.8 kHz and a
length of 100 μs = TR was irradiated at the 54 ppm 27Al signal.

The simulations for measuring the distance of 13C and 27Al spins were performed using the SIMPSON
program.47 An S-RESPDOR pulse sequence with an SR4 recoupling strategy was simulated to examine
the evolution of RESPDOR curves versus 13C-27Al distances. To saturate 27Al spins, a pulse with a power
level of 53.8 kHz was employed on the 27Al channel. Other parameters such as magnetic field (11.7 T),
spinning speed (10 kHz), and saturation pulse length (100 μs) were kept the same as those in 13C-{27Al}
S-RESPDOR experiments. The RESPDOR curves for different 13C-27Al dipolar interactions (distances)
were obtained by varying the duration of the recoupling modules. In addition, a uniform powder sample
(320 α, β crystallites selected using the REPULSION method48 and 9 γ angles) was considered for the
simulation.

The 2D 13C-{27Al} PT-D-HMQC experiments were also performed at 18.8 T on a Bruker-Avance III-800
spectrometer, for which 1H, 13C and 27Al Larmor frequencies are equal to 800.36, 201.26 and 208.56 MHz,
respectively, using a 3.2 mm probe under 10 kHz MAS speed. For cross polarization from 1H to 13C,
spectra were acquired using a 1H pulse width of 5.6 μs, and a contact time of 3000 μs. Repetitive sideband-
selective (SS) WURST-80 adiabatic pulses with a length 98 µs were applied on Al channel during SR4
recoupling to accelerate coherence transfers between 13C and 27Al. The frequency offset of the WURST
pulses was alternated between 148 kHz. The spectra were acquired by averaging 3600 scans for each of
the 33.3 μs rotor-synchronized t1 increments with a recovery delay of 1 s.

Computational method
The framework tri-coordinated Al Lewis acid sites, without a hydroxyl group (FTA) (Fig. S16a), with one
hydroxyl group (FTA-OH) (Fig. S16b), and with two hydroxyl groups (FTA-2OH) (Fig. S16c), are
represented using 72T models. The Bronsted acid site (Fig. S16d) of ZSM-5 zeolites is represented by
88T models. Both sets of models were derived from crystallographic structural data obtained from the
IZA Structure Database (http://www.iza-structure.org/databases). All the zeolite models incorporate the
full double 10-MR intersection pores characteristic of ZSM-5 zeolite. Both the BAS and LAS were
modeled on symmetrically equivalent T12 crystallographic sites within the MFI framework. While tri-
coordinated framework Al species can, in principle, be formed via direct cleavage of Al–O–Si bonds
during dealumination, such processes typically generate adjacent silanol groups. These silanol groups can
introduce local structural distortions, thereby compromising the stability and accessibility of the resulting
Lewis acid sites. To avoid these limitations and in line with previous reports,49,50 we constructed well-
defined tri-coordinated framework Al LAS through controlled Si defect engineering. Specifically, the FTA
site (Al(OSi)3) was modeled by introducing a single Si vacancy adjacent to a tetra-coordinated Al center.
The FTA-OH site (HO–Al(OSi)2) was generated by removing two neighboring Si atoms, and the FTA-
2OH site ((HO)2–Al(OSi)) was formed by introducing three Si vacancies. This approach enables the
simulation of stable, isolated LAS with varying degrees of hydroxylation while preserving a realistic
framework environment. The BAS was modeled using the Si12–O24(H)–Al12 unit at the T12 position,
located at the channel intersection. This configuration was chosen based on its high accessibility to
reactants and its demonstrated structural and catalytic relevance in previous reports.51-53 The terminal Si-
H bond length was set at 1.47 Å, with the bond oriented in the same direction as the corresponding Si-O
bond.

In this work, the active site atoms represented as ball and stick view and the adsorbed hydrocarbon
complex were treated as the high-level layer (Fig. S9), while the rest of the frameworks were treated as
the low-level layer. To maintain the structural integrity of the modeled ZSM-5 zeolite, partial structure
optimizations were conducted by relaxing the atoms within the high-level layer, while the remaining
atoms were kept fixed at their crystallographic positions. All the transition state (TS) structures were
identified using the QST3 method with the Gaussian program. Then the IRC (Intrinsic Reaction
Coordinate) method was used to determine the structures of the corresponding reactant and product. Based
on the imaginary vibrational frequencies of the optimized TS, we slightly adjusted the positions of the
vibrating atoms along the calculated reaction coordinate, in both directions towards the reactant and the
product. Finally, we optimized the resulting structures to achieve their minimum energy configurations.

A combined theoretical approach, specifically ONIOM (ωB97XD/6-31G(d,p):AM1), was utilized for the
geometry optimization of adsorption states and transition states (TS). The ωB97XD method, which
accounts for both short- and long-range dispersion effects, has been proven effective and has been
extensively applied to investigate the reaction mechanisms occurring within zeolites.54,55 Since the AM1
method is known to underestimate the low-level interaction energies, all the energies reported in this work
were calculated at the ωB97XD/6-31G(d,p) level of theory, using the structures optimized previously.
Harmonic frequency calculations employing partial Hessian vibrational analysis (PHVA)，including the
high layer active acid site and organic species, were performed to check whether the stationary points
found exhibit the proper number of imaginary frequencies. The intrinsic energy barriers, defined as the
zero-point corrected electronic energy difference between the transition state and the reactant complex for
each elementary step. The corresponding Gibbs free energies at 473 K were then computed within the
rigid-rotor harmonic oscillator approximation, incorporating vibrational and thermal corrections obtained
from harmonic frequency analysis.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The manuscript entitled “Unveiling the Dynamic Interplay of Lewis and Brønsted Acid Sites in Zeolite-Catalyzed Alcohol Dehydration” describes Brønsted and Lewis acid-catalyzed ethanol dehydration sites in ZSM-5 materials. The authors use several solid-state NMR methods and density functional theory to evaluate the contribution of these different sites to ethanol dehydration reactions. The NMR characterization approach, including with isotopically labeled ethanol, is an interesting approach to quantify surface intermediates. I believe that the theoretical methods are mostly appropriate, although I have some additional comments about how the theory was employed below.\nThe results are somewhat interesting but appear preliminary and are not connected well enough to draw any strong conclusions. Additional results are needed to support conclusions strong enough to merit publication. Importantly, some details are missing from the methods that are important for reproducibility. I do not believe the manuscript is appropriate for Nat. Comm. without significantly more data and significant restructuring of the manuscript. I have some more specific comments the authors must address before this work is published in any journal:\n1. The authors use multiple methods (TMPO titration, NMR, and pyridine adsorption and IR) to quantify the number of Brønsted and Lewis acid sites in their materials. They use these results to attempt to determine the different behavior of Brønsted and Lewis acid sites. They provide an equation on line 155 that contains a γ parameter that represents the increase in the ethanol dehydration rate on Brønsted acid sites that are near Lewis acid sites. This effect is reminiscent of superacids like fluoroantimonic acid, whose acidity is enhanced by tandem Lewis-Brønsted acids (as the authors noted on their Chem. Sci. paper discussing 31P NMR to characterize Lewis acid Al sites, Ref. 39). The authors do not determine the value of γ, though it is important for this work. I believe the authors should calculate turnover rates per Brønsted and Lewis acid site for ethanol dehydration on each of their samples (those with Na-exchange, AHFS etching, and no modification) to determine the value of γ on their ZSM-5-650 sample rather than simply saying that γ > 1. I believe they have done all of the necessary characterization to do this calculation.\n2. The authors identify a peak at 66 ppm in their NMR spectra that appears to be unique to an ethanol-derived intermediate on LAS. Could the authors provide additional evidence to support the structure that they assign to this peak? The purple image at the bottom of Figure 3 shows a chemisorbed ethanol molecule that has not decomposed on a three-coordinate Al site. The authors could confirm this structure by calculating NMR shifts for different intermediates on Lewis acid sites of their DFT models. These tools are available in Gaussian. Alternatively, IR spectroscopy might be able to resolve the structure of intermediates on LAS; in situ FTIR would provide much needed corroboration of the NMR results.\n3. The authors appear to observe significant concentrations of adsorbed diethyl ether on Brønsted acid sites in Figure 3, but their catalytic tests in Figure 2 show extremely high selectivity to ethylene (near 100%). How is this possible? This finding contrasts with work from the Bhan group (DOI:10.1016/j.jcat.2010.01.021), for example. Although these results are from different zeolites, they suggest that ethylene formation rates are much lower than those of diethyl ether. The authors are operating at a contact time and conversion that favors very high conversions of their reactants, where side reactions and high partial pressures of products may distort any kinetic observations that would be useful from studies at lower conversions. I suspect that the diethyl ether is decomposing into ethylene, but the authors do not address this issue in their work at all.\n4. At what crystallographic position in the MFI material was the Al placed in the DFT calculations? This information is important for reproducibility. Similarly, the authors must provide more information about how they constructed their Lewis acid Al models. Figure S9 in the supporting information appears to show this Al in a different crystallographic position than the Brønsted acid site. Additionally, it appears that the sites are formed by hydrolyzing Al-O bonds of the framework, but it is not obvious. The authors must be extremely clear about how these models were constructed and provide much more detail than they have.\n5. Did the authors perform DFT calculations only at a single O site around the Al in the Brønsted acid model? If so, they must specify which one. Even so, it would be more rigorous to study the reaction at each possible O site where appropriate because there can be significant differences in energy even for a single Al position (see, for example work from the Grabow group, DOI:10.1016/j.catcom.2014.04.005).\n6. The authors provide a decent amount of evidence throughout the paper that I believe supports some of their conclusions, but it is not presented in a way that each piece of evidence is connected. In particular, the DFT data are presented and not tied to the experimental results at all—although such a comparison for a reaction mechanism would be very difficult without estimates of reaction barriers either from Arrhenius plots or partial pressure kinetic studies at low conversion. I am left with many questions that the authors could, but do not, address: do the DFT results corroborate the surface intermediates that the authors observe with NMR? Can the chemical shifts observed experimentally be validated with DFT and identify the precise structure of the intermediates the authors observe? Can DFT help identify where the diethyl ether is going?\n7. The authors have performed reaction studies of isopropanol but don’t discuss it anywhere in the main text. Were these experiments not informative? Do they support or contradict the findings from their ethanol dehydration studies? The presence of these results but the absence of this discussion is extremely confusing.", "reviewer_2": "The manuscript entitled “Unveiling the Dynamic Interplay of Lewis and Brønsted Acid Sites in Zeolite-Catalyzed Alcohol Dehydration” describes a study on the transformation of ethanol over a zeolite catalyst involving the two different types of acid sites. Under the reaction conditions, ethanol may suffer an intramolecular dehydration reaction to produce one molecule of water and one molecule of ethylene. By an alternative pathway, two molecules of ethanol may react to form diethyl ether and one molecule of water, which is an intermolecular dehydration reaction.\nThe incorporation of aluminium into crystallographic sites instead of silicon creates Brönsted acid sites due to charge compensation. These are strong acid sites that can activate molecules for certain reaction which include the dehydrations, intramolecular and intermolecular version. The activation occurs by protonation of the oxygen atom of the hydroxy group, converting it into a good leaving group which ends up as neutral water molecule.\nIn my opinion, it is a severe drawback of the present manuscript that the direct protonation of ethanol is not considered for the mechanistic discussion. From Organic Chemistry point of view, even inside zeolites, this is the most probable reaction and the most common species. Even more, an Al-containing zeolite should also be able to protonate the diethyl ether which is another species that is not considered in the present work. Instead, a triethyloxonium ion (TEO) is taken into account. This is a very unstable species, much less stable than the protonated diethyl ether. Transition state stabilization by gas-phase water or ethanol is not included.\nAs several relevant scenarios are not taken into account, the results of the study are not meaningful. The present manuscript should not be published in any journal, no matter whether high impact or not.", "reviewer_3": "In the manuscript entitled “Unveiling the Dynamic Interplay of Lewis and Bronsted Acid Sites in Zeolites-Catalyzed Alcohol Dehydration”, Hu et al. present a mechanistic study of ethanol dehydration into ethylene catalyzed by H-ZSM-5 zeolitic catalysts, supported by experimental and molecular modeling (DFT) data. More specifically, the authors address the catalytic activity of materials containing various proportions of Bronsted, framework sites and Lewis framework sites (EFAl species are excluded based on several observations). They observe a synergistic effect between LAS and BAS which, as far as the reviewer can understand, they assign to different roles played by both types of sites (LAS activate easily the -OH group of ethylene, while BAS activate more efficiently the β-H abstraction).\nThe experimental part seems rather sound to the reviewer, and interesting NMR data are provided. However, in view of the data provided in the manuscript, in the opinion of the reviewer, the main caveat of the document is that the synergistic effect (or “trade-off” effect) claimed by the authors, while observed, is not clearly explicit. The authors seem to imply (but it is not very explicit) that the synergy arise from a cooperation between LAS and BAS for activating two steps of the mechanisms: (i) “activation” of the OH group, and (ii) β-H abstraction. This is however not convincing, because :\na- The ‘activation’ of the OH group promoted by LAS and BAS is very different in nature. Activation by framework BAS involves the formation of surface ethoxide (SES-BAS in the manuscript), hence the breaking of the C-O bond of the alcohol to remove water (and formation a C-Oframework bond), that have a strong carbocation character according to modeling studies on the matter. By contrast, activation by framework LAS (PSES-LAS in the manuscript) involves the formation of an ethanol-zeolite adduct, with formation O-Al bond without breaking of the C-O bond of the alcohol (and according to the DFT structures provided on Figure S7, the H atom from the hydroxyl group remains on the alcohol without being transferred). Hence, the naming of ‘pseudo surface ethoxide species-LAS’ is misleading in many respects and should be changed. Consequently, the β-H abstraction step is also very different, as they originate from fundamentally different structures.\nb- No interconversion between both types of SES-BAS and PBES-LAS can be envisaged, since, as mentioned, they are different species. Not even mentioning diffusion from one site to the other, converting a PSES-LAS into SES-BAS necessitates the rupture of the C-O bond in PSES-LAS, which would require an additional activation barrier ,not considered by the authors (and actually it would just boil down to the energetics of the SES-BAS pathway).\nHence, the two steps on both sites cannot be considered as interchangeable parts of a same mechanism, which renders dubious the explanation of the authors that step (i) is catalyzed by LAS and step (ii) by BAS.\nAdditional comments include :\n1- The site concentration data for sample ZSM-5-650-Na are not provided in TableS1. This is however an important point of discussion since the authors employ this sample as a “LAS-rich” sample.\n2- No reference is provided for the assignment of some of the 13C MAS-NMR signals of ethanol. Please complete. Besides, regarding the assignment of PSES-LAS, the authors rely on DFT calculations (Figure S7). The authors should add similar simulations for the other types of species to better substantiate their assignments of the different species.\n3- The intermediacy of TEO in the formation of SES-BAS, as claimed by the authors (in fact implicitly referring to their earlier work https://doi.org/10.1038/s41467-019-09956-7, which by the way should be better emphasized as it is not the conventional finding), is not obvious to the reviewer given the data shown in the manuscript. Would it be the case, shouldn’t its concentration on Figure 4b rise before that of SES-BAS ? Here they seem to rise simultaneously instead, which rather points to parallel processes. Besides, the mechanisms suggested by the authors for this sequence involves the intermediacy of DEE. The reviewer suggests adding the evolution of the corresponding signal to Figure 4b, and why is no DEE observed in the gas phase in that case, regardless of the temperature ?\n4- Regarding DFT calculations, it is unclear how the molecular models for the FTA, FTA-OH etc was generated. Please explain carefully in the method section."}, "author_response": "Response to comments\nReviewer #4:\nComment 1. The authors have attempted to address my previous comments. The reply\nto comment 2 is fine. However, the turnover frequency reported (comment 1) is at high\nand different conversions on the different materials, and this makes the comparison\ndoubtful. TOFs need to be estimated at differential conditions for comparison of\nmaterials to be meaningful.\nReply: We thank the reviewer for this important comment. To ensure a rigorous and\nmeaningful comparison of turnover frequencies (TOFs), we have performed additional\nkinetic measurements under strictly differential conditions (ethanol conversion <10%)\nat 200 °C. The resulting overall TOF values are summarized in Table R1 (Table S2 in\nthe SI). Furthermore, the individual TOFs for ethylene and diethyl ether (DEE)\nformation were re-evaluated and are reported in Table R2 (Table S3 in the SI).\nThe TOF for DEE formation follows the trend: ZSM-5-650-Na (LAS-rich, 805.3\nmol .mol -1 h-1) > ZSM-5-650 (BAS+LAS, 762.4 mol .mol -1 h-1) > ZSM-5-650-\nethanol site ethanol site\nAHFS (BAS-rich, 455.5 mol .mol -1 h-1). In contrast, the TOF for ethylene formation\nethanol site\nexhibits the opposite sequence: ZSM-5-650 (68.4 mol .mol -1 h-1) > ZSM-5-650-\nethanol site\nAHFS (21.4 mol .mol -1 h-1) > ZSM-5-650-Na (11.4 mol .mol -1 h-1).\nethanol site ethanol site\nThese results unambiguously demonstrate that LAS-rich zeolites preferentially promote\nDEE formation, whereas the coexistence of Brønsted and Lewis acid sites, as realized\nin ZSM-5-650, leads to the highest intrinsic activity toward ethylene production.\nAccordingly, the calculated enhancement factor (𝛾 = 3.86), which quantifies the\nactivity difference in ethylene formation, specifically reflects the promotional effect\narising from BAS-LAS cooperation. This interpretation is further corroborated by the\nproduct selectivity data (Table R1), where ZSM-5-650 consistently exhibits the highest\nethylene selectivity among the investigated catalysts.\nTable R1. Turnover frequencies (TOFs) for ethylene formation on BAS and LAS\nin zeolites. Ethanol dehydration kinetics is measured under differential conditions\n(ethanol conversion <10%) at 200 °C. Ethanol conversion is defined by the extent of\nethanol transformation, and TOFs are calculated from ethylene formation rates\nnormalized to the corresponding BAS or LAS site densities.\nDEE Ethylene TOF (mol .mol -1 h-1)\nEthylene ethanol site\nConversion\nZeolite selectivity selectivity\n(%) BAS LAS\n(%) (%)\nZSM-5-650 7.6 91.8 8.2 82.3 11.4\nZSM-5-650-AHFS 3.3 95.5 4.5 21.4 -\nZSM-5-650-Na 1.4 98.6 1.4 - 11.4\nTable R2. Turnover frequencies (TOFs) for DEE and ethylene formation over\ndifferent zeolites. Ethanol dehydration kinetics is measured under differential\nconditions (ethanol conversion <10%) at 200 °C, with TOFs calculated from DEE or\nethylene formation rates normalized to the total acid-site density.\nZeolite TOF DEE (mol ethanol .mol site -1 h-1) TOF Ethylene (mol ethanol .mol site -1 h-1)\nZSM-5-650 762.4 68.4\nZSM-5-650-AHFS 455.5 21.4\nZSM-5-650-Na 805.3 11.4\nReviewer #5:\nComments\nI read the revised manuscript and the comments and rebuttal. The manuscript contains\nbeautiful data that deserve to be published. However, there remain deficiencies in the\ntext that require adaptations. I made suggestions in that regard.\nComment 1. The role of water is insufficiently addressed. What about direct\nstabilization of transition states? are they (not) affected by H O? Fig S22-S24 are not\n2\nobvious to interpret: how is H O affecting the adsorbed states? There seems to no direct\n2\ninteraction, but simply a water adsorbed nearby(?). It must be shown that there is or is\nno stabilization from direct H O interaction with adsorbed and transition states.\ninteraction with transition state, or like in S22 on a distance? So the figure must be\nmade clearer and any direct interaction described.\nReply: We sincerely thank the reviewer for raising these important points concerning\nthe role of water. In the revised manuscript, we now explicitly demonstrate that water\nmolecules directly interact with and stabilize both adsorbed intermediates and transition\nstates, as supported by detailed energetic comparisons and structural analyses.\nSpecifically, by comparing anhydrous conditions (Fig. 5 and Fig. S16) with hydrated\nmodels for LAS and BAS, respectively (Fig. R1 and Fig. R2; Figs. S22 and S23 in the\nSI), the introduction of a single water molecule is found to substantially lower the\nenergies of all key adsorbed species and transition states, confirming its direct\nstabilizing role. For example, the surface ethoxy species (SES) on BAS is stabilized\nfrom −5.4 to −12.9 kcal mol-1 (Fig. S16 vs. Fig. R2). Similarly, chemisorbed ethanol on\nLAS shows pronounced stabilization upon hydration (FTA: −53.5 → −70.1 kcal mol-1;\nFTA–OH: −35.4 → −54.9 kcal mol-1; FTA–2OH: −33.4 → −72.5 kcal mol-1; Fig. 5 vs.\nFig. R1).\nWater also directly stabilizes the transition states for the subsequent β-H elimination\nstep. On BAS, the transition-state energy decreases from 25.1 to −13.9 kcal mol-1 (Fig.\nS16 vs. Fig. R2). On LAS, the corresponding transition-state energies are reduced from\n−2.0, 5.1, and 5.4 kcal mol-1 to −18.7, −10.4, and −12.1 kcal mol-1 for the FTA, FTA–\nOH, and FTA–2OH configurations, respectively (Fig. 5 vs. Fig. R1).\nTo visually emphasize these direct interactions, we have revised the structural\nschematics in Figs. R1 and R2 (Figs. S22 and S23 in the SI) to explicitly highlight the\ninteractions between water and the adsorbed species or transition states. In the detailed\ntransition-state models (Fig. R3 as Fig. S24 in the SI), water molecules are highlighted\nwith blue dashed circles, and the associated hydrogen-bond distances—each within 3.2\nÅ—are clearly annotated. These revisions provide unambiguous evidence for close-\nrange, stabilizing interactions between water and the reactive intermediates and\ntransition states.\nThe following discussion has been added to the revised manuscript:\n“As shown in Figs. S22-S24, the inclusion of a water molecule markedly stabilizes all\nkey adsorbed species, intermediates, and transition states compared with the anhydrous\nmodels (Figs. 5 and S16).”\nFigure R1. Calculated reaction energy profiles for ethanol dehydration on ZSM-5\nin the presence of water. Ethanol dehydration pathways over BAS, FTA, FTA-OH and\nFTA-2OH. Upper panels: key reaction intermediates and transition states (TS) for SES-\nBAS’/CSE-LAS’ decomposition to ethylene. Dashed line: simplified SES-BAS’\nformation (see Figure S23 in the revised manuscript for details). Energies in kcal/mol.\nFigure R2. Theoretical analysis of elementary reaction steps for ethanol\ndehydration on BAS in the presence of water. Calculated reaction energy profile for\nethanol dehydration to ethylene on BAS in the presence of water, with relative energies\n(kcal mol⁻¹) indicated for each elementary step. DEE, TEO, and TS denote diethyl ether,\ntriethyloxonium ion, and transition state, respectively; asterisks (*) indicate adsorbed\nspecies.\nFigure R3. Optimized transition state (TS) structures for ethanol dehydration in\nthe presence of water. Optimized local TS structures for ethylene formation via ethanol\ndehydration on a, FTA, FTA-OH, and FTA-2OH; and b, BAS. Water molecules are\nhighlighted with blue dashed circles. Key geometric parameters are shown in Å with\nthe shortest O(H O)‑TS distances highlighted in red.\noff and synergy effects. To me it looks like there is a different understanding of such\nterms, which means it is likely confusing to others as well. One way out is to rewrite\nand avoid the use of such terms. I agree with the reviewer that the synergy effect is\ninsufficiently proven and explained and the authors should not make any claims in this\nrespect. Identifying different reaction paths over LAS and BAS and showing their\ndifferent reactivity is what the experimental data show. When phrased carefully, this is\nworthy of publishing in Nature Commun.\nReply: We thank the reviewer for this constructive comment and suggestion. To avoid\nconceptual ambiguity and enhance the rigor of the manuscript, we have removed all\nreferences to a “synergy effect” in the revised version. The discussion now focuses\nexclusively on delineating the distinct ethanol dehydration pathways on Lewis and\nBrønsted acid sites and on comparing their intrinsic catalytic activities. We believe that\nthese revisions lead to a more precise and rigorous presentation, while fully preserving\nthe core mechanistic insights into the site-dependent kinetics of ethanol dehydration.\nComment 3. I have some minor additional comments: The D-HMQC NMR experiment\nis done on dehydrated zeolite, such dehydration may affect Al coordination; Oh may\ntransform into Td or trigonal for example.... Al MAS NMR is done under hydrated\nconditions favoring the high coordination. This requires more explanation in the text.\nReply: We agree with the reviewer that the hydration–dehydration state plays a critical\nrole in determining Al coordination in zeolites. To specifically probe the intrinsic acid\nsites relevant to catalysis, the 31P MAS NMR and 1H–27Al D-HMQC NMR experiments\nin this study were conducted on fully dehydrated zeolites. Under these conditions, both\nBAS and framework-associated LAS, assigned to tri-coordinated Al species, are clearly\nobserved. As reported in our previous work (Angew. Chem. Int. Ed. 2022, e202207400),\nthese tri-coordinated Al species are highly sensitive to moisture and readily transform\ninto tetra-, penta-, or hexa-coordinated Al upon hydration, resulting in a concomitant\ndecrease in the population of LAS. To aid interpretation of the experiments presented\nhere, we have explicitly clarified this point in the revised manuscript.\nThe following statement has been added to the text:\n“It should be noted that upon zeolite hydration, tri-coordinated Al sites exposed under\ndehydrated conditions would transform into higher-coordination states (tetra-, penta-,\nor hexa-coordinated Al), leading to a reduction in the number of LAS [ref 40].”\nComment 4. Selectivities must be compared at identical conversion levels.\nReply: We fully agree with the reviewer that product selectivities are most\nappropriately compared at identical conversion levels. In Fig. 2c in the main text, we\nemployed product yields (%) measured as a function of reaction temperature to\ncompare the activity of different acid sites during ethanol dehydration. Because the\nintrinsic activities of BAS and LAS differ substantially, achieving the same conversion\nunder identical reaction conditions-particularly at low temperatures-is experimentally\nchallenging. The use of product yield therefore enables a more direct comparison of\nsite-dominated reaction pathways while minimizing bias arising from differing\nconversion levels.\nTo further assess product selectivity under identical conversion, we conducted\nadditional measurements at 275 °C, where complete ethanol conversion (100%) was\nachieved on all samples (Table R3 as Table S4 in the SI). Under these conditions, the\ncatalyst containing both BAS and LAS (ZSM-5-650) exhibited the lowest ethylene\nselectivity (83.5%), accompanied by the formation of 16.5% C products. In contrast,\n3+\nthe BAS-rich ZSM-5-650-AHFS and the LAS-rich ZSM-5-650-Na catalysts both\ndisplayed ethylene selectivities exceeding 99%. These results demonstrate that,\nalthough the coexistence of BAS and LAS enhances overall catalytic activity, it also\nfacilitates secondary reactions of ethylene at full conversion, leading to a reduction in\ntarget product selectivity.\nTable R3. Conversion and product selectivity over different zeolites. Product\nselectivity is measured during ethanol dehydration at 275 oC under full-conversion\nconditions (100% ethanol conversion).\nConversion Selectivity (%)\nZeolite\n(%) ethylene DEE C\n3+\nZSM-5-650 100 83.5 0 16.5\nZSM-5-650-AHFS 100 99.5 0 0.5\nZSM-5-650-Na 100 99.1 0 0.9\nComment 5. Do the samples change color, and is this different with temperature, which\nmay be indicative of coke formation?\nReply: We thank the reviewer for this comment. We confirm that none of the spent\ncatalysts exhibited any visible color change, even after extended reaction times of up\nto 20 h. This observation is consistent across the entire temperature range investigated\n(200–275 °C). The absence of color change agrees with the high catalytic stability\nobserved under these relatively mild reaction conditions and indicates that coke\nformation is negligible within the studied temperature window.\nComment 6. The role of DEE in remains vague. An experiment with DEE as reactant\nmay provide insight into ethoxy formation.\nReply: We thank the reviewer for this valuable suggestion. To further elucidate the role\nof DEE, in addition to the DEE dehydration control reaction experiments (Fig. S6 in\nthe SI), we performed 13C MAS NMR analysis of surface species formed on ZSM-5-\n650 after reaction (Fig. R4 as Fig. S11 in the SI). The spectrum reveals the presence of\nphysically adsorbed ethanol and DEE, surface ethoxy species on BAS (SES-BAS),\nchemisorbed ethanol on LAS (CSE-LAS), and triethyloxonium ions (TEO). The overall\ndistribution of surface species closely resembles that observed during ethanol\ndehydration. Notably, the SES-BAS signal is substantially more intense than that\nobserved after ethanol dehydration (cf. Fig. R4 and Fig. 3a), indicating that DEE is\nmore readily activated to form SES-BAS than ethanol. This conclusion is consistent\nwith effluent analysis, which shows that DEE converts to ethylene more readily than\nethanol under comparable conditions (cf. Fig. S6 and Fig. 2c). Together, these results\ndemonstrate a rapid equilibrium between ethanol and DEE under reaction conditions\nand establish DEE as a highly reactive intermediate during ethanol conversion,\ncapable of efficiently generating SES-BAS via TEO formation and subsequently promoting\nethylene production.\nThe relevant discussion has been added to the revised manuscript:\n“Further 13C MAS NMR analysis of the surface species formed after reaction with DEE\nover ZSM-5-650 shows that DEE is converted to SES-BAS more readily than ethanol,\nconfirming its higher reactivity and its role as a key intermediate in ethanol dehydration\n(Fig. S11).”\nFigure R4. Identification of surface species formed after DEE reaction on zeolite.\n13C MAS NMR spectrum of trapped species obtained from the reaction of\nCH 13CH O13CH CH over ZSM-5-650 at 200 °C for 1 min. The upper panels indicate\n3 2 2 3\nthe assignments of the observed signals to DEE, SES-BAS, CSE-LAS, and TEO.\nComment 7. Figure S5 needs to be described in the text; how is the conclusion about\nframework Al made?\nReply: We thank the reviewer for this comment. To clarify the identification of\nframework-associated Al species, we have added a description of Fig. S5 in the main\ntext. The presence of framework Al is confirmed by the reversible coordination change\nbetween octahedral (0 ppm) and tetrahedral (55 ppm) Al observed upon back-exchange\nof the proton form sample to its ammonium form (ZSM-5-650-NH ; Fig. S5). This\n4\nreversible transformation is a well-established criterion for distinguishing framework\nAl from extra-framework Al (EFAL) species (Nat. Mater. 2020, 19, 1047). The\nfollowing discussion has been added to the revised manicurist:\n“On ZSM‑5‑650, the 0 ppm signal assigned to octahedrally coordinated Al originates\nfrom water coordination to Lewis acidic Al sites. Upon NH + exchange, these\n4\noctahedral species reversibly transform into tetrahedrally coordinated Al, giving rise to\na signal at 55 ppm. This reversible coordination change demonstrates that the observed\noctahedrally coordinated Al species result from water adsorption on framework-\nassociated Al rather than from extra-framework Al species [ref. 22]”"}, {"round": 2, "reviewer_comments": {"reviewer_2": "In their response to the comments the Authors did not provide new arguments.\nIn the theoretical studies the concept is of outmost importance. Resasco et al. have shown that the presence of water, a by-product of the ethanol dehydration, may change energy profiles of a chemical reaction: please cf.\nhttps://doi.org/10.1021/acscatal.9b04637\nThe same effect has been applied to other reactions like epoxidation: https://doi.org/10.1038/s41929-021-00672-4\nIn light of these examples, the study described in the present manuscript are too limited and a wider view may change the results, even only incorporating one water molecule. There is no hard proof provided, only interpretation. Therefore, the article should not be published in Nature Communication since this means a approval of the results.", "reviewer_3": "Please see attached document.", "reviewer_4": "The comments have been sufficiently reviewed. Some clarifications are necessary.\nComment 1. In Table R1, the authors should report the conversion used in the calculation of TOF. Can the authors also estimate individual TOFs for alkene and ether formation and discuss a little more whether the enhancement of 3.08 is toward the alkene or the ether? i.e., Is there a change in selectivity?\nComment 6. If I understood, the authors computed the barrier on the BAS and LAS. Are these values zero-point energy corrected? Are these apparent or intrinsic barriers? If the former, how do the authors estimate these? The authors should elaborate on these points. Also, what is the barrier at the defected sites? Can the authors capture the synergy between sites in the barriers? Discussion on these points is worthy."}, "author_response": "Response to comments\nReviewer #4:\nComment 1. The authors have attempted to address my previous comments. The reply\nto comment 2 is fine. However, the turnover frequency reported (comment 1) is at high\nand different conversions on the different materials, and this makes the comparison\ndoubtful. TOFs need to be estimated at differential conditions for comparison of\nmaterials to be meaningful.\nReply: We thank the reviewer for this important comment. To ensure a rigorous and\nmeaningful comparison of turnover frequencies (TOFs), we have performed additional\nkinetic measurements under strictly differential conditions (ethanol conversion <10%)\nat 200 °C. The resulting overall TOF values are summarized in Table R1 (Table S2 in\nthe SI). Furthermore, the individual TOFs for ethylene and diethyl ether (DEE)\nformation were re-evaluated and are reported in Table R2 (Table S3 in the SI).\nThe TOF for DEE formation follows the trend: ZSM-5-650-Na (LAS-rich, 805.3\nmol .mol -1 h-1) > ZSM-5-650 (BAS+LAS, 762.4 mol .mol -1 h-1) > ZSM-5-650-\nethanol site ethanol site\nAHFS (BAS-rich, 455.5 mol .mol -1 h-1). In contrast, the TOF for ethylene formation\nethanol site\nexhibits the opposite sequence: ZSM-5-650 (68.4 mol .mol -1 h-1) > ZSM-5-650-\nethanol site\nAHFS (21.4 mol .mol -1 h-1) > ZSM-5-650-Na (11.4 mol .mol -1 h-1).\nethanol site ethanol site\nThese results unambiguously demonstrate that LAS-rich zeolites preferentially promote\nDEE formation, whereas the coexistence of Brønsted and Lewis acid sites, as realized\nin ZSM-5-650, leads to the highest intrinsic activity toward ethylene production.\nAccordingly, the calculated enhancement factor (𝛾 = 3.86), which quantifies the\nactivity difference in ethylene formation, specifically reflects the promotional effect\narising from BAS-LAS cooperation. This interpretation is further corroborated by the\nproduct selectivity data (Table R1), where ZSM-5-650 consistently exhibits the highest\nethylene selectivity among the investigated catalysts.\nTable R1. Turnover frequencies (TOFs) for ethylene formation on BAS and LAS\nin zeolites. Ethanol dehydration kinetics is measured under differential conditions\n(ethanol conversion <10%) at 200 °C. Ethanol conversion is defined by the extent of\nethanol transformation, and TOFs are calculated from ethylene formation rates\nnormalized to the corresponding BAS or LAS site densities.\nDEE Ethylene TOF (mol .mol -1 h-1)\nEthylene ethanol site\nConversion\nZeolite selectivity selectivity\n(%) BAS LAS\n(%) (%)\nZSM-5-650 7.6 91.8 8.2 82.3 11.4\nZSM-5-650-AHFS 3.3 95.5 4.5 21.4 -\nZSM-5-650-Na 1.4 98.6 1.4 - 11.4\nTable R2. Turnover frequencies (TOFs) for DEE and ethylene formation over\ndifferent zeolites. Ethanol dehydration kinetics is measured under differential\nconditions (ethanol conversion <10%) at 200 °C, with TOFs calculated from DEE or\nethylene formation rates normalized to the total acid-site density.\nZeolite TOF DEE (mol ethanol .mol site -1 h-1) TOF Ethylene (mol ethanol .mol site -1 h-1)\nZeolite TOF DEE (mol ethanol .mol site -1 h-1) TOF Ethylene (mol ethanol .mol site -1 h-1)\nZSM-5-650 762.4 68.4\nZSM-5-650-AHFS 455.5 21.4\nZSM-5-650-Na 805.3 11.4\nReviewer #5:\nComments\nI read the revised manuscript and the comments and rebuttal. The manuscript contains\nbeautiful data that deserve to be published. However, there remain deficiencies in the\ntext that require adaptations. I made suggestions in that regard.\nComment 1. The role of water is insufficiently addressed. What about direct\nstabilization of transition states? are they (not) affected by H O? Fig S22-S24 are not\n2\nobvious to interpret: how is H O affecting the adsorbed states? There seems to no direct\n2\ninteraction, but simply a water adsorbed nearby(?). It must be shown that there is or is\nno stabilization from direct H O interaction with adsorbed and transition states.\ninteraction with transition state, or like in S22 on a distance? So the figure must be\nmade clearer and any direct interaction described.\nReply: We sincerely thank the reviewer for raising these important points concerning\nthe role of water. In the revised manuscript, we now explicitly demonstrate that water\nmolecules directly interact with and stabilize both adsorbed intermediates and transition\nstates, as supported by detailed energetic comparisons and structural analyses.\nSpecifically, by comparing anhydrous conditions (Fig. 5 and Fig. S16) with hydrated\nmodels for LAS and BAS, respectively (Fig. R1 and Fig. R2; Figs. S22 and S23 in the\nSI), the introduction of a single water molecule is found to substantially lower the\nenergies of all key adsorbed species and transition states, confirming its direct\nstabilizing role. For example, the surface ethoxy species (SES) on BAS is stabilized\nfrom −5.4 to −12.9 kcal mol-1 (Fig. S16 vs. Fig. R2). Similarly, chemisorbed ethanol on\nLAS shows pronounced stabilization upon hydration (FTA: −53.5 → −70.1 kcal mol-1;\nFTA–OH: −35.4 → −54.9 kcal mol-1; FTA–2OH: −33.4 → −72.5 kcal mol-1; Fig. 5 vs.\nFig. R1).\nWater also directly stabilizes the transition states for the subsequent β-H elimination\nstep. On BAS, the transition-state energy decreases from 25.1 to −13.9 kcal mol-1 (Fig.\nS16 vs. Fig. R2). On LAS, the corresponding transition-state energies are reduced from\n−2.0, 5.1, and 5.4 kcal mol-1 to −18.7, −10.4, and −12.1 kcal mol-1 for the FTA, FTA–\nOH, and FTA–2OH configurations, respectively (Fig. 5 vs. Fig. R1).\nTo visually emphasize these direct interactions, we have revised the structural\nschematics in Figs. R1 and R2 (Figs. S22 and S23 in the SI) to explicitly highlight the\ninteractions between water and the adsorbed species or transition states. In the detailed\ntransition-state models (Fig. R3 as Fig. S24 in the SI), water molecules are highlighted\nwith blue dashed circles, and the associated hydrogen-bond distances—each within 3.2\nÅ—are clearly annotated. These revisions provide unambiguous evidence for close-\nrange, stabilizing interactions between water and the reactive intermediates and\ntransition states.\nThe following discussion has been added to the revised manuscript:\n“As shown in Figs. S22-S24, the inclusion of a water molecule markedly stabilizes all\nkey adsorbed species, intermediates, and transition states compared with the anhydrous\nmodels (Figs. 5 and S16).”\nFigure R1. Calculated reaction energy profiles for ethanol dehydration on ZSM-5\nin the presence of water. Ethanol dehydration pathways over BAS, FTA, FTA-OH and\nFTA-2OH. Upper panels: key reaction intermediates and transition states (TS) for SES-\nBAS’/CSE-LAS’ decomposition to ethylene. Dashed line: simplified SES-BAS’\nformation (see Figure S23 in the revised manuscript for details). Energies in kcal/mol.\nFigure R2. Theoretical analysis of elementary reaction steps for ethanol\ndehydration on BAS in the presence of water. Calculated reaction energy profile for\nethanol dehydration to ethylene on BAS in the presence of water, with relative energies\n(kcal mol⁻¹) indicated for each elementary step. DEE, TEO, and TS denote diethyl ether,\ntriethyloxonium ion, and transition state, respectively; asterisks (*) indicate adsorbed\nspecies.\nFigure R3. Optimized transition state (TS) structures for ethanol dehydration in\nthe presence of water. Optimized local TS structures for ethylene formation via ethanol\ndehydration on a, FTA, FTA-OH, and FTA-2OH; and b, BAS. Water molecules are\nhighlighted with blue dashed circles. Key geometric parameters are shown in Å with\nthe shortest O(H O)‑TS distances highlighted in red.\noff and synergy effects. To me it looks like there is a different understanding of such\nterms, which means it is likely confusing to others as well. One way out is to rewrite\nand avoid the use of such terms. I agree with the reviewer that the synergy effect is\ninsufficiently proven and explained and the authors should not make any claims in this\nrespect. Identifying different reaction paths over LAS and BAS and showing their\ndifferent reactivity is what the experimental data show. When phrased carefully, this is\nworthy of publishing in Nature Commun.\nReply: We thank the reviewer for this constructive comment and suggestion. To avoid\nconceptual ambiguity and enhance the rigor of the manuscript, we have removed all\nreferences to a “synergy effect” in the revised version. The discussion now focuses\nexclusively on delineating the distinct ethanol dehydration pathways on Lewis and\nBrønsted acid sites and on comparing their intrinsic catalytic activities. We believe that\nthese revisions lead to a more precise and rigorous presentation, while fully preserving\nthe core mechanistic insights into the site-dependent kinetics of ethanol dehydration.\nComment 3. I have some minor additional comments: The D-HMQC NMR experiment\nis done on dehydrated zeolite, such dehydration may affect Al coordination; Oh may\ntransform into Td or trigonal for example.... Al MAS NMR is done under hydrated\nconditions favoring the high coordination. This requires more explanation in the text.\nReply: We agree with the reviewer that the hydration–dehydration state plays a critical\nrole in determining Al coordination in zeolites. To specifically probe the intrinsic acid\nsites relevant to catalysis, the 31P MAS NMR and 1H–27Al D-HMQC NMR experiments\nin this study were conducted on fully dehydrated zeolites. Under these conditions, both\nBAS and framework-associated LAS, assigned to tri-coordinated Al species, are clearly\nobserved. As reported in our previous work (Angew. Chem. Int. Ed. 2022, e202207400),\nthese tri-coordinated Al species are highly sensitive to moisture and readily transform\ninto tetra-, penta-, or hexa-coordinated Al upon hydration, resulting in a concomitant\ndecrease in the population of LAS. To aid interpretation of the experiments presented\nhere, we have explicitly clarified this point in the revised manuscript.\nThe following statement has been added to the text:\n“It should be noted that upon zeolite hydration, tri-coordinated Al sites exposed under\ndehydrated conditions would transform into higher-coordination states (tetra-, penta-,\nor hexa-coordinated Al), leading to a reduction in the number of LAS [ref 40].”\nComment 4. Selectivities must be compared at identical conversion levels.\nReply: We fully agree with the reviewer that product selectivities are most\nappropriately compared at identical conversion levels. In Fig. 2c in the main text, we\nemployed product yields (%) measured as a function of reaction temperature to\ncompare the activity of different acid sites during ethanol dehydration. Because the\nintrinsic activities of BAS and LAS differ substantially, achieving the same conversion\nunder identical reaction conditions-particularly at low temperatures-is experimentally\nchallenging. The use of product yield therefore enables a more direct comparison of\nsite-dominated reaction pathways while minimizing bias arising from differing\nconversion levels.\nTo further assess product selectivity under identical conversion, we conducted\nadditional measurements at 275 °C, where complete ethanol conversion (100%) was\nachieved on all samples (Table R3 as Table S4 in the SI). Under these conditions, the\ncatalyst containing both BAS and LAS (ZSM-5-650) exhibited the lowest ethylene\nselectivity (83.5%), accompanied by the formation of 16.5% C products. In contrast,\n3+\nthe BAS-rich ZSM-5-650-AHFS and the LAS-rich ZSM-5-650-Na catalysts both\ndisplayed ethylene selectivities exceeding 99%. These results demonstrate that,\nalthough the coexistence of BAS and LAS enhances overall catalytic activity, it also\nfacilitates secondary reactions of ethylene at full conversion, leading to a reduction in\ntarget product selectivity.\nTable R3. Conversion and product selectivity over different zeolites. Product\nselectivity is measured during ethanol dehydration at 275 oC under full-conversion\nconditions (100% ethanol conversion).\nConversion Selectivity (%)\nZeolite\n(%) ethylene DEE C\n3+\nZSM-5-650 100 83.5 0 16.5\nZSM-5-650-AHFS 100 99.5 0 0.5\nZSM-5-650-Na 100 99.1 0 0.9\nComment 5. Do the samples change color, and is this different with temperature, which\nmay be indicative of coke formation?\nReply: We thank the reviewer for this comment. We confirm that none of the spent\ncatalysts exhibited any visible color change, even after extended reaction times of up\nto 20 h. This observation is consistent across the entire temperature range investigated\n(200–275 °C). The absence of color change agrees with the high catalytic stability\nobserved under these relatively mild reaction conditions and indicates that coke\nformation is negligible within the studied temperature window.\nComment 6. The role of DEE in remains vague. An experiment with DEE as reactant\nmay provide insight into ethoxy formation.\nReply: We thank the reviewer for this valuable suggestion. To further elucidate the role\nof DEE, in addition to the DEE dehydration control reaction experiments (Fig. S6 in\nthe SI), we performed 13C MAS NMR analysis of surface species formed on ZSM-5-\n650 after reaction (Fig. R4 as Fig. S11 in the SI). The spectrum reveals the presence of\nphysically adsorbed ethanol and DEE, surface ethoxy species on BAS (SES-BAS),\nchemisorbed ethanol on LAS (CSE-LAS), and triethyloxonium ions (TEO). The overall\ndistribution of surface species closely resembles that observed during ethanol\ndehydration. Notably, the SES-BAS signal is substantially more intense than that\nobserved after ethanol dehydration (cf. Fig. R4 and Fig. 3a), indicating that DEE is\nmore readily activated to form SES-BAS than ethanol. This conclusion is consistent\nwith effluent analysis, which shows that DEE converts to ethylene more readily than\nethanol under comparable conditions (cf. Fig. S6 and Fig. 2c). Together, these results\ndemonstrate a rapid equilibrium between ethanol and DEE under reaction conditions\nand establish DEE as a highly reactive intermediate during ethanol conversion,\ncapable of efficiently generating SES-BAS via TEO formation and subsequently promoting\nethylene production.\nThe relevant discussion has been added to the revised manuscript:\n“Further 13C MAS NMR analysis of the surface species formed after reaction with DEE\nover ZSM-5-650 shows that DEE is converted to SES-BAS more readily than ethanol,\nconfirming its higher reactivity and its role as a key intermediate in ethanol dehydration\n(Fig. S11).”\nFigure R4. Identification of surface species formed after DEE reaction on zeolite.\n13C MAS NMR spectrum of trapped species obtained from the reaction of\nCH 13CH O13CH CH over ZSM-5-650 at 200 °C for 1 min. The upper panels indicate\n3 2 2 3\nthe assignments of the observed signals to DEE, SES-BAS, CSE-LAS, and TEO.\nComment 7. Figure S5 needs to be described in the text; how is the conclusion about\nframework Al made?\nReply: We thank the reviewer for this comment. To clarify the identification of\nframework-associated Al species, we have added a description of Fig. S5 in the main\ntext. The presence of framework Al is confirmed by the reversible coordination change\nbetween octahedral (0 ppm) and tetrahedral (55 ppm) Al observed upon back-exchange\nof the proton form sample to its ammonium form (ZSM-5-650-NH ; Fig. S5). This\n4\nreversible transformation is a well-established criterion for distinguishing framework\nAl from extra-framework Al (EFAL) species (Nat. Mater. 2020, 19, 1047). The\nfollowing discussion has been added to the revised manicurist:\n“On ZSM‑5‑650, the 0 ppm signal assigned to octahedrally coordinated Al originates\nfrom water coordination to Lewis acidic Al sites. Upon NH + exchange, these\n4\noctahedral species reversibly transform into tetrahedrally coordinated Al, giving rise to\na signal at 55 ppm. This reversible coordination change demonstrates that the observed\noctahedrally coordinated Al species result from water adsorption on framework-\nassociated Al rather than from extra-framework Al species [ref. 22]”"}, {"round": 3, "reviewer_comments": {"reviewer_4": "The paper has been adequately reviewed and can now be published.", "reviewer_5": "The authors gave serious efforts to modify the manuscript and reply to each of the comments.\nExcept for the comment about comparing selectivity at equal conversion level, they have been responded to satisfactorily. In the remark about comparing selectivity at equal con version level, the authors compared selectivities at 100% conversion; this is the least relevant case, as intrinsic selectivities in such case are maximally affected by secondary reactions, thus not providing insight into cataylst properties. This must be removed from the text. If equal conversion levels cannot be achieved, the authors could do a set of experiments and extrapolate selectivities to zero % conversion. If such experiment is not possible (which I doubt), this must be explained clearly in the main text."}, "author_response": "Response to comments\nReviewer #4:\nComment 1. The authors have attempted to address my previous comments. The reply\nto comment 2 is fine. However, the turnover frequency reported (comment 1) is at high\nand different conversions on the different materials, and this makes the comparison\ndoubtful. TOFs need to be estimated at differential conditions for comparison of\nmaterials to be meaningful.\nReply: We thank the reviewer for this important comment. To ensure a rigorous and\nmeaningful comparison of turnover frequencies (TOFs), we have performed additional\nkinetic measurements under strictly differential conditions (ethanol conversion <10%)\nat 200 °C. The resulting overall TOF values are summarized in Table R1 (Table S2 in\nthe SI). Furthermore, the individual TOFs for ethylene and diethyl ether (DEE)\nformation were re-evaluated and are reported in Table R2 (Table S3 in the SI).\nThe TOF for DEE formation follows the trend: ZSM-5-650-Na (LAS-rich, 805.3\nmol .mol -1 h-1) > ZSM-5-650 (BAS+LAS, 762.4 mol .mol -1 h-1) > ZSM-5-650-\nethanol site ethanol site\nAHFS (BAS-rich, 455.5 mol .mol -1 h-1). In contrast, the TOF for ethylene formation\nethanol site\nexhibits the opposite sequence: ZSM-5-650 (68.4 mol .mol -1 h-1) > ZSM-5-650-\nethanol site\nAHFS (21.4 mol .mol -1 h-1) > ZSM-5-650-Na (11.4 mol .mol -1 h-1).\nethanol site ethanol site\nThese results unambiguously demonstrate that LAS-rich zeolites preferentially promote\nDEE formation, whereas the coexistence of Brønsted and Lewis acid sites, as realized\nin ZSM-5-650, leads to the highest intrinsic activity toward ethylene production.\nAccordingly, the calculated enhancement factor (𝛾 = 3.86), which quantifies the\nactivity difference in ethylene formation, specifically reflects the promotional effect\narising from BAS-LAS cooperation. This interpretation is further corroborated by the\nproduct selectivity data (Table R1), where ZSM-5-650 consistently exhibits the highest\nethylene selectivity among the investigated catalysts.\nTable R1. Turnover frequencies (TOFs) for ethylene formation on BAS and LAS\nin zeolites. Ethanol dehydration kinetics is measured under differential conditions\n(ethanol conversion <10%) at 200 °C. Ethanol conversion is defined by the extent of\nethanol transformation, and TOFs are calculated from ethylene formation rates\nnormalized to the corresponding BAS or LAS site densities.\nDEE Ethylene TOF (mol .mol -1 h-1)\nEthylene ethanol site\nConversion\nZeolite selectivity selectivity\n(%) BAS LAS\n(%) (%)\nZSM-5-650 7.6 91.8 8.2 82.3 11.4\nZSM-5-650-AHFS 3.3 95.5 4.5 21.4 -\nZSM-5-650-Na 1.4 98.6 1.4 - 11.4\nTable R2. Turnover frequencies (TOFs) for DEE and ethylene formation over\ndifferent zeolites. Ethanol dehydration kinetics is measured under differential\nconditions (ethanol conversion <10%) at 200 °C, with TOFs calculated from DEE or\nethylene formation rates normalized to the total acid-site density.\nZeolite TOF DEE (mol ethanol .mol site -1 h-1) TOF Ethylene (mol ethanol .mol site -1 h-1)\nZeolite TOF DEE (mol ethanol .mol site -1 h-1) TOF Ethylene (mol ethanol .mol site -1 h-1)\nZSM-5-650 762.4 68.4\nZSM-5-650-AHFS 455.5 21.4\nZSM-5-650-Na 805.3 11.4\nReviewer #5:\nComments\nI read the revised manuscript and the comments and rebuttal. The manuscript contains\nbeautiful data that deserve to be published. However, there remain deficiencies in the\ntext that require adaptations. I made suggestions in that regard.\nComment 1. The role of water is insufficiently addressed. What about direct\nstabilization of transition states? are they (not) affected by H O? Fig S22-S24 are not\n2\nobvious to interpret: how is H O affecting the adsorbed states? There seems to no direct\n2\ninteraction, but simply a water adsorbed nearby(?). It must be shown that there is or is\nno stabilization from direct H O interaction with adsorbed and transition states.\ninteraction with transition state, or like in S22 on a distance? So the figure must be\nmade clearer and any direct interaction described.\nReply: We sincerely thank the reviewer for raising these important points concerning\nthe role of water. In the revised manuscript, we now explicitly demonstrate that water\nmolecules directly interact with and stabilize both adsorbed intermediates and transition\nstates, as supported by detailed energetic comparisons and structural analyses.\nSpecifically, by comparing anhydrous conditions (Fig. 5 and Fig. S16) with hydrated\nmodels for LAS and BAS, respectively (Fig. R1 and Fig. R2; Figs. S22 and S23 in the\nSI), the introduction of a single water molecule is found to substantially lower the\nenergies of all key adsorbed species and transition states, confirming its direct\nstabilizing role. For example, the surface ethoxy species (SES) on BAS is stabilized\nfrom −5.4 to −12.9 kcal mol-1 (Fig. S16 vs. Fig. R2). Similarly, chemisorbed ethanol on\nLAS shows pronounced stabilization upon hydration (FTA: −53.5 → −70.1 kcal mol-1;\nFTA–OH: −35.4 → −54.9 kcal mol-1; FTA–2OH: −33.4 → −72.5 kcal mol-1; Fig. 5 vs.\nFig. R1).\nWater also directly stabilizes the transition states for the subsequent β-H elimination\nstep. On BAS, the transition-state energy decreases from 25.1 to −13.9 kcal mol-1 (Fig.\nS16 vs. Fig. R2). On LAS, the corresponding transition-state energies are reduced from\n−2.0, 5.1, and 5.4 kcal mol-1 to −18.7, −10.4, and −12.1 kcal mol-1 for the FTA, FTA–\nOH, and FTA–2OH configurations, respectively (Fig. 5 vs. Fig. R1).\nTo visually emphasize these direct interactions, we have revised the structural\nschematics in Figs. R1 and R2 (Figs. S22 and S23 in the SI) to explicitly highlight the\ninteractions between water and the adsorbed species or transition states. In the detailed\ntransition-state models (Fig. R3 as Fig. S24 in the SI), water molecules are highlighted\nwith blue dashed circles, and the associated hydrogen-bond distances—each within 3.2\nÅ—are clearly annotated. These revisions provide unambiguous evidence for close-\nrange, stabilizing interactions between water and the reactive intermediates and\ntransition states.\nThe following discussion has been added to the revised manuscript:\n“As shown in Figs. S22-S24, the inclusion of a water molecule markedly stabilizes all\nkey adsorbed species, intermediates, and transition states compared with the anhydrous\nmodels (Figs. 5 and S16).”\nFigure R1. Calculated reaction energy profiles for ethanol dehydration on ZSM-5\nin the presence of water. Ethanol dehydration pathways over BAS, FTA, FTA-OH and\nFTA-2OH. Upper panels: key reaction intermediates and transition states (TS) for SES-\nBAS’/CSE-LAS’ decomposition to ethylene. Dashed line: simplified SES-BAS’\nformation (see Figure S23 in the revised manuscript for details). Energies in kcal/mol.\nFigure R2. Theoretical analysis of elementary reaction steps for ethanol\ndehydration on BAS in the presence of water. Calculated reaction energy profile for\nethanol dehydration to ethylene on BAS in the presence of water, with relative energies\n(kcal mol⁻¹) indicated for each elementary step. DEE, TEO, and TS denote diethyl ether,\ntriethyloxonium ion, and transition state, respectively; asterisks (*) indicate adsorbed\nspecies.\nFigure R3. Optimized transition state (TS) structures for ethanol dehydration in\nthe presence of water. Optimized local TS structures for ethylene formation via ethanol\ndehydration on a, FTA, FTA-OH, and FTA-2OH; and b, BAS. Water molecules are\nhighlighted with blue dashed circles. Key geometric parameters are shown in Å with\nthe shortest O(H O)‑TS distances highlighted in red.\noff and synergy effects. To me it looks like there is a different understanding of such\nterms, which means it is likely confusing to others as well. One way out is to rewrite\nand avoid the use of such terms. I agree with the reviewer that the synergy effect is\ninsufficiently proven and explained and the authors should not make any claims in this\nrespect. Identifying different reaction paths over LAS and BAS and showing their\ndifferent reactivity is what the experimental data show. When phrased carefully, this is\nworthy of publishing in Nature Commun.\nReply: We thank the reviewer for this constructive comment and suggestion. To avoid\nconceptual ambiguity and enhance the rigor of the manuscript, we have removed all\nreferences to a “synergy effect” in the revised version. The discussion now focuses\nexclusively on delineating the distinct ethanol dehydration pathways on Lewis and\nBrønsted acid sites and on comparing their intrinsic catalytic activities. We believe that\nthese revisions lead to a more precise and rigorous presentation, while fully preserving\nthe core mechanistic insights into the site-dependent kinetics of ethanol dehydration.\nComment 3. I have some minor additional comments: The D-HMQC NMR experiment\nis done on dehydrated zeolite, such dehydration may affect Al coordination; Oh may\ntransform into Td or trigonal for example.... Al MAS NMR is done under hydrated\nconditions favoring the high coordination. This requires more explanation in the text.\nReply: We agree with the reviewer that the hydration–dehydration state plays a critical\nrole in determining Al coordination in zeolites. To specifically probe the intrinsic acid\nsites relevant to catalysis, the 31P MAS NMR and 1H–27Al D-HMQC NMR experiments\nin this study were conducted on fully dehydrated zeolites. Under these conditions, both\nBAS and framework-associated LAS, assigned to tri-coordinated Al species, are clearly\nobserved. As reported in our previous work (Angew. Chem. Int. Ed. 2022, e202207400),\nthese tri-coordinated Al species are highly sensitive to moisture and readily transform\ninto tetra-, penta-, or hexa-coordinated Al upon hydration, resulting in a concomitant\ndecrease in the population of LAS. To aid interpretation of the experiments presented\nhere, we have explicitly clarified this point in the revised manuscript.\nThe following statement has been added to the text:\n“It should be noted that upon zeolite hydration, tri-coordinated Al sites exposed under\ndehydrated conditions would transform into higher-coordination states (tetra-, penta-,\nor hexa-coordinated Al), leading to a reduction in the number of LAS [ref 40].”\nComment 4. Selectivities must be compared at identical conversion levels.\nReply: We fully agree with the reviewer that product selectivities are most\nappropriately compared at identical conversion levels. In Fig. 2c in the main text, we\nemployed product yields (%) measured as a function of reaction temperature to\ncompare the activity of different acid sites during ethanol dehydration. Because the\nintrinsic activities of BAS and LAS differ substantially, achieving the same conversion\nunder identical reaction conditions-particularly at low temperatures-is experimentally\nchallenging. The use of product yield therefore enables a more direct comparison of\nsite-dominated reaction pathways while minimizing bias arising from differing\nconversion levels.\nTo further assess product selectivity under identical conversion, we conducted\nadditional measurements at 275 °C, where complete ethanol conversion (100%) was\nachieved on all samples (Table R3 as Table S4 in the SI). Under these conditions, the\ncatalyst containing both BAS and LAS (ZSM-5-650) exhibited the lowest ethylene\nselectivity (83.5%), accompanied by the formation of 16.5% C products. In contrast,\n3+\nthe BAS-rich ZSM-5-650-AHFS and the LAS-rich ZSM-5-650-Na catalysts both\ndisplayed ethylene selectivities exceeding 99%. These results demonstrate that,\nalthough the coexistence of BAS and LAS enhances overall catalytic activity, it also\nfacilitates secondary reactions of ethylene at full conversion, leading to a reduction in\ntarget product selectivity.\nTable R3. Conversion and product selectivity over different zeolites. Product\nselectivity is measured during ethanol dehydration at 275 oC under full-conversion\nconditions (100% ethanol conversion).\nConversion Selectivity (%)\nZeolite\n(%) ethylene DEE C\n3+\nZSM-5-650 100 83.5 0 16.5\nZSM-5-650-AHFS 100 99.5 0 0.5\nZSM-5-650-Na 100 99.1 0 0.9\nComment 5. Do the samples change color, and is this different with temperature, which\nmay be indicative of coke formation?\nReply: We thank the reviewer for this comment. We confirm that none of the spent\ncatalysts exhibited any visible color change, even after extended reaction times of up\nto 20 h. This observation is consistent across the entire temperature range investigated\n(200–275 °C). The absence of color change agrees with the high catalytic stability\nobserved under these relatively mild reaction conditions and indicates that coke\nformation is negligible within the studied temperature window.\nComment 6. The role of DEE in remains vague. An experiment with DEE as reactant\nmay provide insight into ethoxy formation.\nReply: We thank the reviewer for this valuable suggestion. To further elucidate the role\nof DEE, in addition to the DEE dehydration control reaction experiments (Fig. S6 in\nthe SI), we performed 13C MAS NMR analysis of surface species formed on ZSM-5-\n650 after reaction (Fig. R4 as Fig. S11 in the SI). The spectrum reveals the presence of\nphysically adsorbed ethanol and DEE, surface ethoxy species on BAS (SES-BAS),\nchemisorbed ethanol on LAS (CSE-LAS), and triethyloxonium ions (TEO). The overall\ndistribution of surface species closely resembles that observed during ethanol\ndehydration. Notably, the SES-BAS signal is substantially more intense than that\nobserved after ethanol dehydration (cf. Fig. R4 and Fig. 3a), indicating that DEE is\nmore readily activated to form SES-BAS than ethanol. This conclusion is consistent\nwith effluent analysis, which shows that DEE converts to ethylene more readily than\nethanol under comparable conditions (cf. Fig. S6 and Fig. 2c). Together, these results\ndemonstrate a rapid equilibrium between ethanol and DEE under reaction conditions\nand establish DEE as a highly reactive intermediate during ethanol conversion,\ncapable of efficiently generating SES-BAS via TEO formation and subsequently promoting\nethylene production.\nThe relevant discussion has been added to the revised manuscript:\n“Further 13C MAS NMR analysis of the surface species formed after reaction with DEE\nover ZSM-5-650 shows that DEE is converted to SES-BAS more readily than ethanol,\nconfirming its higher reactivity and its role as a key intermediate in ethanol dehydration\n(Fig. S11).”\nFigure R4. Identification of surface species formed after DEE reaction on zeolite.\n13C MAS NMR spectrum of trapped species obtained from the reaction of\nCH 13CH O13CH CH over ZSM-5-650 at 200 °C for 1 min. The upper panels indicate\n3 2 2 3\nthe assignments of the observed signals to DEE, SES-BAS, CSE-LAS, and TEO.\nComment 7. Figure S5 needs to be described in the text; how is the conclusion about\nframework Al made?\nReply: We thank the reviewer for this comment. To clarify the identification of\nframework-associated Al species, we have added a description of Fig. S5 in the main\ntext. The presence of framework Al is confirmed by the reversible coordination change\nbetween octahedral (0 ppm) and tetrahedral (55 ppm) Al observed upon back-exchange\nof the proton form sample to its ammonium form (ZSM-5-650-NH ; Fig. S5). This\n4\nreversible transformation is a well-established criterion for distinguishing framework\nAl from extra-framework Al (EFAL) species (Nat. Mater. 2020, 19, 1047). The\nfollowing discussion has been added to the revised manicurist:\n“On ZSM‑5‑650, the 0 ppm signal assigned to octahedrally coordinated Al originates\nfrom water coordination to Lewis acidic Al sites. Upon NH + exchange, these\n4\noctahedral species reversibly transform into tetrahedrally coordinated Al, giving rise to\na signal at 55 ppm. This reversible coordination change demonstrates that the observed\noctahedrally coordinated Al species result from water adsorption on framework-\nassociated Al rather than from extra-framework Al species [ref. 22]”"}]

Decision letter