Facile Preparation of Methyl Phenols from Ethanol over Lamellar Ce(OH)SO4&#183; xH2O

Jinqiu Guo, Zongjing Feng, Jun Xu, Jie Zhu, Guanghui Zhang, Yaping Du,* Hongbo Zhang,*

Research Article

Facile Preparation of Methyl Phenols from Ethanol over Lamellar

Ce(OH)SO ·x H O

and Chunhua Yan

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sı Supporting Information

ABSTRACT: Ethanol transformation with high product selectiv-

ity is a great challenge, especially for high weight molecules. Here,

we show a combination study of kinetic, thermodynamic, and in

situ spectroscopy measurements to probe the selective upgrading

of ethanol over lamellar Ce(OH)SO ·xH O catalysts, with 60−

(

0% Ce preserved during the catalysis. High methyl phenols

MPs) selectivity at ∼80% within condensation products was

achieved at ∼50% condensation yield (3.0 kPa C H OH, 15 kPa

H , Ar balanced, 693 K, 1 atm, gas hourly space velocity (GHSV)

−

∼

5.4 min ), with acetaldehyde, acetone, 4-heptanone, and 2-

pentanone as the key reaction intermediates. Kinetic measure-

ments with the assistance of isotope labeling proved that MPs generated from the kinetically relevant step (KRS) of C−C bond

coupling of enolate nucleophilically attacks surface C H O following a Langmuir−Hinshelwood model. Low ethanol and water

pressures and high acetaldehyde and hydrogen pressures were proved to be favored for MPs generation rather than dehydration, in

which hydrogen could reduce the amount of lattice oxygen and facilitate the preparation of MPs while water and ethanol both

compete with acetaldehyde for active sites during catalysis. On the basis of in situ X-ray diﬀraction (XRD), quasi-in situ X-ray

photoelectron spectroscopy (XPS), and Raman characterizations, the Ce(OH)SO crystal structure was proved to be maintained

along with ethanol activation, and the Ce −OH Lewis acid−base pair was proved to be the active species for the selective C−C

bond coupling. The KRS assumption was also supported by the apparent activation energy assessment within the reaction network

on dehydration, dehydrogenation, aldol condensation, and cyclization and a series of negligible kinetic isotope eﬀects (KIEs). This

system can be easily extended to some other systems related to C−C bond coupling and is attracting attention on converting

oxygenate platform molecules over lanthanide species.

KEYWORDS: lamellar rare earth material, ethanol transformation, methyl phenols, kinetics, rate-determining step

. INTRODUCTION

also has low MPs yield with mesitylene as the main byproduct.

Therefore, it is urgent to ﬁnd a new way to produce MPs

Phenols and substituted phenols, such as methyl phenols (MPs),

are important chemical materials and crucial precursors for

resin, pesticide, antioxidants, and vitamin E formation. The

global demand for phenols reached 1200 tpa (tons per annual)

in 2018 and has increased with an annual growth rate of ∼5%,

thus attracting great attention. Phenols are usually generated

from coal tar distillation, which requires complex processes and

is not an environmentally friendly process, which is why

alternative strategies for phenolic compound preparation have

eﬃciently.

MPs might be obtained from C5+ ketone cyclization, which

would be eﬀectively generated from ethanol following a series of

18−22

C−C bond coupling steps.

That is clearly environmentally

benign and promising. For example, Ramasamy et al. directly

prepared C5+ ketone (with 2-pentanone (2-PNO) and 4-

heptanone (4-HPO) as the main components) from ethanol

over 0.1% Pd-ZnO-ZrO at 643 K, 2 MPa, with acetone as the

key reaction intermediate. Small amounts of MPs were

been investigated for decades. For example, substituted phenols

−9

determined from their system, which indicates that the selective

can be produced by phenol alkylation, which suﬀers from low

selectivity controlled by the positioning group of the benzene

ring, thus leading to low yield of desired products. They could

also be prepared by p-isopropyltoluene or benzene/xylene

Received: March 9, 2021

Revised: April 16, 2021

Published: May 6, 2021

0−16

hydroxylation.

However, some strong acids, such as

H SO , may be involved in these methods, thus leading to

high cost and causing damage to the environment. In addition,

MPs could be produced by isophorone transformation, which

2021 American Chemical Society

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generation of MPs from ethanol is possible. Preparation of value-

added chemicals from ethanol not only saves fossil energy

resources but also could eﬀectively solve the problem of

environmental pollution. However, there are some severe

diﬃculties in ethanol transformation. One of the problems is

that in most cases of Lewis acid/base catalytic systems, due to

the weakness of the catalysts in hydrogenation/dehydrogen-

ation steps, the initial ketone group is scarcely generated from

ethanol, thus causing low eﬃciencies in directly converting

ethanol to any high-weight products, even MPs, etc. In order to

improve the dehydrogenation capacity, some silver (Ag) or

copper (Cu) nanoparticles are incorporated into the catalytic

system and are unfortunately likely to facilitate the formation of

traditional ethanol transformation process. The impact on MPs

formation of C H OH, C H O, H O, and H as well as the

active oxygen from the surface has been systematically studied.

Dehydration to ethylene was determined to be the main side

reaction along with MPs formation through aldol condensation

reaction. A Langmuir−Hinshelwood model has been estab-

lished to describe the formation of MPs from ethanol with

nucleophilic attack from enolate to the molecularly adsorbed

acetaldehyde as the rate-determining step (RDS). The physical

properties of the LRHs variation as well as the active species

along with ethanol upgrading were investigated by in situ XRD,

quasi-in situ and ex-situ XPS, and Raman measurements. The

isotopic substitution experiments were introduced to conﬁrm

the RDS suggested. Factors on thermodynamic and kinetic

inﬂuencing the reaction activity and selectivity have been

studied systematically.

23,24

the C−O bond (esteriﬁcation)

rather than the C−C bond.

Another diﬃculty in MPs formation is that the complex reaction

network occurs between the oxygenates with multicarbonyl

groups and β-C−H bonds by aldol condensation, thus leading

to great diﬃculty in selective generation of value-added

2. EXPERIMENTAL SECTION

chemicals with high yield. Only a few studies found that

methylbenzyl alcohols and methylbenzaldehydes could be

.1. Catalyst Synthesis. Synthesis of Ce(OH)SO ·xH O:

4 2

In a typical preparation process, 6 mmol of CeCl ·7H O (99.9%,

Aladdin) and 6 mmol of (NH ) SO (99.9%, Damao Chemical

24,26

produced with relatively high yield (<25%) from ethanol.

Many studies indicated that Lewis acid−base strength is a vital

Reagent Factory) were dissolved in 60 mL of deionized water,

and then the solution was transferred into a 100 mL Teﬂon-lined

stainless steel autoclave under continuous stirring until all of the

precursors were completely dissolved. After that, 1 mL of NH₃·

factor in controlling the product distribution of this consecutive

25,27−33

condensation reaction network.

In recent years, lamellar rare-earth metal hydroxides (LRHs)

have been widely investigated, which have unique lamellar

H O (25%, Meryer) was added dropwise. Then the sample was

crystal structure, alternating lanthanide cations (Ln ) and the

heated to 423 K for 24 h. The as received solid material was

washed with deionized water and ethanol. Finally, the as

prepared Ce(OH)SO ·xH O was obtained after drying at 60 °C

−

2− 34−36

interlayer anions (OH , NO , Cl, Br, and SO ),

leading

to ﬂexible interactions, which could continuously adjust the

distribution and strength of Lewis acid sites, and might be the

most attractive properties for LRHs in catalysis. For example,

for 2 h. The synthesis methods for Y (OH) SO and SiO −SO

are available in Supporting Information Section 1.

.2. Catalyst Characterization. 2.2.1. X-ray Diﬀraction

Gandara et al. found that YbAQDS can be used as Lewis acid

to catalyze the hydrodesulfurization of thiophene; after a 26 h

(XRD). XRD measurements were used to determine the identity

and crystal structure of all the catalysts (Cu Kα radiation, λ=

run, 50% thiophene was removed by reducing with H under 7

−

bar at 70 °C. Zhang et al. prepared fatty acid methyl ester

FAME) from canola oil and methanol over potassium-doped

lamellar Ln O CO . The FAME yield can reach ∼100% at 95

0.15418 nm, 40 kV, 40 mA, scanning rate 5°·min ; Rigaku

(

Smart Lab 3 kW diﬀractometer).

2.2.2. In Situ XRD. In situ XRD measurements were applied to

observe the change of the Ce(OH)SO ·xH O crystal structure

C. They found that there were two active sites in this lamellar

material; one is the potassium doping site and the other is the

carbonate group between the layers, both of which are related to

oxygen vacancies. Additionally, cerium (Ce) cations were found

19,39,40

during methyl phenols formation from ethanol (Cu Kα

radiation, λ= 0.15418 nm, 45 kV, 200 mA, scanning rate 6°·

−

min ). The ﬁrst XRD pattern of Ce(OH)SO ·xH O was

to be active for aldol condensation, especially the Ce .

obtained at room temperature after the catalyst was placed into

the sample holder in the atmosphere, and then the sample holder

For example, Wang et al. prepared 4-HPO from the biomass

fermented ABE (acetone + butanol + ethanol) aqueous solution

was heated to 573 K in N and kept for 60 min to remove the

over tin-doped ceria (Sn-CeO ), the additive of Sn could

preadsorbed species. After the pretreatment, ethanol and H₂

were introduced into the in situ chamber at 693 K for 6 h

(reaction conditions: 6.5 mL/min H , 32 mL/min N , 5 kPa

improve the Ce concentration from 23.3% to 32.8% and the

oxygen vacancy concentration increases in the meantime. In this

transformation, the oxygen vacancies of CeO could be used as

C H OH). The XRD patterns were recorded every 20 min along

2 5

dehydrogenation, Guerbet reaction, and aldol condensation

sites, which were favorable for 4-HPO formation. In another

example, Wang et al. prepared 1,3-butanediol from form-

this procedure.

2.2.3. Transmission Electron Microscopy (TEM). TEM

measurements were measured on JEOL JEM-2800 microscope,

which was operated at an accelerating voltage of 200 kV with

0.21 nm resolution. The samples were suspended in ethanol by

ultrasonication. Then the suspension was dropped onto the

copper grid for TEM measurements.

aldehyde and propylene over the Lewis acid sites of CeO .

Similarly, Li et al. synthesized 2-butanal from acetaldehyde

AA) through aldol condensation over CeO catalysts. They all

found that the high proportion of Ce in CeO is beneﬁcial for

(

preparing target product at high yield. However, compared with

2.2.4. X-ray Photoelectron Spectroscopy (XPS). XPS analysis

was conducted on a Thermo Scientiﬁc ESCALAB 250Xi

photoelectron spectrometer equipped with a monochromatic

microfocused Al Kα X-ray source (1486.8 eV). The C 1s peak at

284.8 eV was taken as an internal standard.

2.2.5. Quasi-In Situ XPS. Quasi-in situ XPS measurements

were performed on a Thermo Scientiﬁc ESCALAB Xi

photoelectron spectrometer. The ﬁrst spectroscopy was

Ce , Ce is relatively unstable and could be easily oxidized to

Ce within an oxidative atmosphere.

Here, we design a lamellar Ce(OH)SO ·xH O catalyst with a

large amount of Ce³(∼70% of all the Ce ) exposed to

selectively convert ethanol to MPs, which eﬀectively solved the

problems of insuﬃcient dehydrogenation capacity of traditional

Lewis acids and simpliﬁed the complex reaction system in

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Figure 1. Elemental analysis of (a) Ce, (b) O, (c) S using energy-dispersive spectrometer (EDS) spectroscopy. (d) HAADF-STEM image of the same

region of Ce(OH)SO ·xH O as in (a)−(c). (e) Quasi-in situ XPS spectra of Ce(OH)SO ·xH O during ethanol coupling. (Reaction conditions: 15

kPa H , 3.0 kPa C H OH, Ar balanced, 1 atm, from 298 to 693 K.)

obtained after the catalyst was placed in the sample stage at room

estimated based on the peak area from GC and the response

factor of each component in the reaction system, as per the

following equations

temperature, and then it was heated in situ to 693 K at a heating

−

ramp of 10 K·min in ﬂowing Ar. The second spectroscopy was

gained before ethanol feeding-in (reaction conditions: 3 kPa

C H OH, 15 kPa H , 83.325 kPa Ar, 6 h) at 693 K, and the last

conversion (%) = ⁿ⁽^e^t^h^aⁿ^o^l

converted⁾

× 100

quasi-in situ XPS spectroscopy was then collected after ethanol

was fed-in for 6 h. All of the spectra were obtained after the

catalyst pretreatment under the reaction conditions and transfer

to the XPS analysis chamber by a special reaction chamber, in

order to avoid the sample changing after exposure to air.

n(ethanolfed‐in)

(1)

(2)

(3)

ⁿ⁽^p^r^o^d^u^c^t⁾^×^Ncarbon‐atom

selectivity (%) =

turnover rate =

× 100

2n(ethanol_c_o_n_v_e_r_t_e_d)

.3. Catalysis Measurements. Methyl phenols formation

∑

n(ethanol_c_o_n_v_e_r_t_e_d

N_a_c_t_i_v_e_s_i_t_e_s× t

)

catalyzed by a series of catalysts was performed in a ﬁxed bed

reactor (tubular glass with 6 mm inner diameter). The

temperature of the catalytic bed was controlled by a PID

controller equipped with a K-type thermocouple, which was

placed in direct contact with the wall of the reactor adjacent to

the catalytic bed. The catalysts (1000−1500 mg, mechanically

mixed with silica (99%, Sigma-Aldrich, CAS no. 112926-00-8))

were treated in situ by heating to 573 K and held for 2 h under

where n(ethanol_c_o_n_v_e_r_t_e_d) and n(ethanol_f_e_d_‑_i_n) represent the molar

weight of ethanol molecules converted and the ethanol

molecules fed in, respectively; n(product) are the molar weight

of the products; N_c_a_r_b_o_n_‑_a_t_o_mis the carbon number of each

product, and N_a_c_t_i_v_e_s_i_t_e_sis the number of active sites.

All of the kinetic measurements were performed in the

di ﬀerential conditions within a linear conversion range (Figure

S1 ), indicating that the diﬀusion limitations were eliminated. As

for the pressure dependence studies, ethanol reaction order was

measured by changing the C H OH pressure from 1.5 to 5 kPa

−

ﬂowing Ar (100 mL·min ) to remove the preadsorbed species

before any catalytic test. Flow rates of all the gases were

controlled by mass ﬂow controllers (Brooks, 5850E). Liquid-

phase reactants (such as ethanol (C H OH, 99.9%, Saifurui)

and acetaldehyde (CH CHO, 99.5%, Sigma-Aldrich), etc.) were

while holding other conditions constant (reaction conditions:

fed into the reactor through syringe pumps (Longer Pump, TJP-

1.5−5 kPa C H OH, 15 kPa H , Ar balanced, 1 atm, 693 or 673

A) and were transferred through a.1/8 in. stainless steel tube

K). The reaction orders of other reactants (C H O, H , H O,

2 4 2 2

preheated to 423 K with electrical heating tape to prevent

potential condensations of the chemicals. The total pressure of

the reaction system was maintained at 101.325 kPa with Ar as

the equilibrium gas. The system was allowed to reach steady

state under similar conditions (15 kPa H , 3.0 kPa C H OH,

balanced Ar, 693 K, 1 atm). The qualitative and quantitative

analyses of products were employed based on gas chromatog-

raphy (SCION, 436-GC) and gas chromatography−mass

spectrometer (GC−MS, ESI, Waters APGC/UPLC Xevo TQ-

S) characterization equipped with a capillary column (DB-WAX,

Agilent, 30 m × 0.25 mm × 0.25 μm) and a ﬂame ionization

detector (FID), respectively. After 8 h steady-state measure-

ment, conversion, product selectivity, and reaction rate were

C H O, CO, and CO ) were also obtained by changing the

3 6 2

pressure of a single material only and holding others constant.

The activation energies of methyl phenols formation over

diﬀerent catalysts were calculated by obtaining the turnover

rates of products at diﬀerent temperatures within the diﬀerential

conditions.

2.4. Estimations for Active Site Counts by Butyric Acid

Titration during Methyl Phenols Formation. The number

of Lewis acid sites over Ce(OH)SO ·xH O were estimated by in

situ butyric acid (BA, >99.0%, TCI chemicals, CAS no. 107-92-

6) titration (Figure S2), in which each reaction was carried out

for at least 9 h before the injection of BA (3.0 kPa C H OH, 15

kPa H , balanced with Ar, 1 atm, 643 K). BA was introduced in

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controlled by GHSVs. As suggested in Figure 2 , for ethanol

Research Article

situ to the reaction system while holding other specie pressures

connection with the loss of Ce³⁺content after ethanol

transformation.

as constants (3.0 kPa C H OH, 0.34 kPa C H O , 15 kPa H ,

balanced with Ar, 1 atm, 643 K); the cumulative BA uptake and

the methyl phenols formation rate changes were measured as a

function of time. The result suggested that there were 0.985

mmol _a _c _t _i _v _e _s _i _t _e _s·mmol_C_e⁻over Ce(OH)SO ·xH O as described

u + v + u′ + v′

x_C_e3+

^∑i (u + v )

(4)

.2. Preparation of Methyl Phenols from Ethanol.

.2.1. Eﬀects of Gas Hourly Space Velocity (GHSV) on Aldol

Condensation Product Yield and Selectivities. As has been

seen, reaction selectivity correlates closely with conversions and

ultimately determined by the residence time of reactants

in Figure S2 .

. RESULTS AND DISCUSSION

.1. Catalyst Characterization. The lamellar Ce(OH)-

SO ·xH O compound was successfully synthesized by hydro-

thermal strategy. According to the literature report, the

structure of the lamellar Ce(OH)SO ·xH O is shown in Figure

S3 . Our research shows that the compounds can obtain two

crystallographic states: Ce(OH)SO and Ce (OH) SO ·2H O

(

2θ = 10.8° and 17.8°) (Figure S4 ). The crystallization water of

the above compounds will be removed when the reaction

temperature reaches 693 K as conﬁ rmed by in situ XRD and

thermogravimetry measurements (Figures S5 and S6), which

means that Ce(OH)SO is the active phase during the catalysis

at 693 K. The size of the lamellar structure is about 200 nm

(

Figure 1a−d). Ce, O, and S are uniformly distributed within the

lamellar structure. Combined with Raman, in situ XRD, and

quasi-in situ XPS, the crystal structure of Ce(OH)SO can be

maintained during the reaction. For example, Raman’s signal

only changes in intensity before and after the reaction, without

any shift of the spectral peak (Figure S7). The intensity and

position of the peaks were not changed within in situ XRD, and

Figure 2. Relationship between gas hourly space velocity (GHSV) and

catalytic performance (aldol condensation product selectivities and

aldol condensation product yield (red

○

)) over Ce(OH)SO ·xH O

the diﬀraction signal of Ce(OH)SO was always displayed,

4 2

(

Reaction condition: 5−1700 mg of cat., 3.0 kPa C H OH, 15 kPa H ,

indicating that the structure of Ce(OH)SO was well preserved.

balanced Ar, 693 K, 1 atm).

According to the molecular composition, the Ce ratio of the

material should be close to 100%. Through quasi-in situ and ex-

situ XPS characterization (Figure 1e, Figures S8 −S10 , and Table

), it was found that ∼70% of Ce could be obtained, which was

catalytic conversion over Ce(OH)SO ·xH O, the aldol

4 2

condensation product yield increased as GHSV decreased and

reached a steady state at around 50%, and aldol condensation

product selectivities changed from mainly (over 70% selectivity)

Table 1. Quasi-In Situ and Ex Situ XPS Measurements on

Ce Contents over Various Catalysts

−

acetaldehyde (GHSV > 600 min ) at ∼1.2% condensation

catalyst

Ce³⁺content (%)

−

yield to mainly acetone (GHSV ∼300 min ) at 4.9%

Ce(OH)SO ·xH O, 298 K

70.8

70.3

60.5

67.7

61.4

18.7

condensation yield and then to mainly (over 50% selecticvity)

Ce(OH)SO ·xH O, 693 K

−1

MPs (GHSV ∼30 min ) at ∼50% condensation yield. Further

Ce(OH)SO ·xH O, 693 K, 6 h

decreasing the GHSV will not change the condensation yield

fresh Ce(OH)SO ·xH O

(

∼50%), but the selectivity of MPs in condensation products

spent Ce(OH)SO ·xH O

will be promoted to ∼80%. As the selectivity of MPs increases,

the amount of acetone decreases, which suggests that acetone is

one of the intermediates for MPs formation. This is consistent

with previous results reported by Wang et al., which suggested

that 2-PNO and 4-HPO are stable reaction intermediates along

the multiple steps of C−C bond coupling. The overall changes of

ethanol conversion and aldol condensation product selectivities

are shown in Figure S15. A signiﬁcant amount of acetaldehyde

CeO₂

calculated by eq 4. In particular, the quasi-in situ XPS

characterization showed that the Ce³ratio was still more than

0% after 6 h of continuous reaction. Although 10% Ce was

lost after 6 h ethanol transformation, this is still a very high

−1

content of Ce active species reported, which lays a foundation

for the subsequent related catalytic research. At the same time,

compared with the electron microscope photos before and after

the reaction (Figures S11 and S12), it was found that the

lamellar structure of the material was slightly damaged, which

may be due to the fact that the overall structure of the compound

was diﬃcult to maintain completely under such a high

hydrothermal environment (693 K with water), but the catalyst

was identiﬁed at high GHSV (>600 min ), which suggests that

the acetaldehyde formation experiences low activation barrier

and would be easily achieved under small contact time between

ethanol and the active sites. The existence of acetaldehyde and

acetone increases the complexity of the reaction network since

they are both active species for aldol condensation. The

products were conﬁrmed by gas chromatography−mass

spectrometry (GC−MS) (Figure S16 ). The eﬀects of reaction

temperature to the aldol condensation product selectivity

(Figure S17 ) were displayed in the Supporting Information,

which showed that 693 K is the optimal temperature for methyl

phenols formation through C−C bond coupling.

showed relatively superior stability in terms of Ce content,

Ce(OH)SO phase, and catalytic reaction stability (Figure S13),

and only a small deactivation was observed at ∼70% ethanol

conversion over Ce(OH)SO ·xH O (Figure S14 ), which was in

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atically compared below. Similarly, we found that (Figure 3 ) the

Research Article

As suggested in the literature, ethanol upgrading undergoes

dehydration, dehydrogenation, aldol condensation, decarbon-

ylation, and possibly cyclization, which could selectively

generate ethylene, ethyl ether (not shown in Scheme 1 due to

Figure 3 is included in Figure S19), which suggests that

acetaldehyde is primary products formed by ethanol dehydro-

genation, while C ketones (mainly 2-butenal) are the secondary

products formed by acetaldehyde condensation, and acetone is

the tertiary product generated from decarbonylation of C₄

ketones with the assistant of water, while methyl phenols are

the ﬁnal products generated by aldol condensation, hydro-

genation, dehydrogenation, dehydration, and cyclization. The

decarbonylation rather than decarboxylation was conﬁrmed by

the considerable activity of water gas shift (WGS) reaction over

Ce(OH)SO ·xH O as shown in Figure S20. Along the reaction

Scheme 1. Reaction Pathways to Form Methyl Phenol

Products

pathway of MPs formation, adding 2-PNO and 4-HPO

separately could also promote the formation of MPs (Figures

S21 and S22 ). Thus, reaction pathway of MPs formation was

proposed as shown in Scheme 1. The stability test (Figure S13)

showed that after about 86 h, the selectivity of methyl phenols in

aldol condensation could still be maintained at around 70% at

∼

99% ethanol conversion.

.2.2. Eﬀects of Ln , OH , SO₄²⁻, and Lamellar Structure

−

on Methyl Phenols Selectivity. Table 2 and Figure 4 show the

aldol condensation product selectivities of ethanol trans-

formation over various catalysts and the overall comparison

with some other typical Lewis acid−base catalysts were shown in

Figure S23, the conversions of all these catalysts are at around

00%, in which Ce(OH)SO ·xH O had the best reaction

4 2

performance of 76.9% methyl phenols selectivity within the

aldol condensation products. The complete conversions and

product selectivities (including aldol condensation products and

trace amount determined), acetaldehyde, C alcohol/aldehyde,

acetone, 2-PNO, 4-HPO, and methyl phenols (Scheme 1).

(

CH ) ) are displayed in the Supporting Information (Table

x n

Ethanol dehydration to hydrocarbon (CH ) (ethylene is the

x n

S2). Considering about the inﬂuence of Ln , Y (OH) SO was

2 4 4

main component in (CH ) as shown in Figure S18) is the main

x n

selected as a comparison; however, there were a small amount of

methyl phenols (16.6%) generated and most of the aldol

condensation product was acetone (53.8%), which suggests that

side reaction along with the formation of methyl phenols

through aldol condensation reaction, which would be system-

Ce cation was required for directly generating MPs from

ethanol. Additional comparison with La(OH)SO was included

in the Supporting Information (Figure S23), which shows

mainly acetaldehyde (∼77.7%) in aldol condensation products

at ∼34.6% ethanol conversion (due to the space limitation of our

reactor, 34.6% ethanol conversion is the maximum we can reach

over La(OH)SO ). In order to conﬁrm the eﬀect of Ce ,

Ce (C O ) ·xH O, and Ce(OH) were introduced as compar-

4 3

ison, they all show high MPs selectivity (∼68%). H temper-

ature-programmed reduction (TPR) of Ce(OH) in Figure S24

suggested that Ce could be partially reduced to Ce during

ethanol conversion at 693 K, while Ce could not be reduced to

Ce over Ce(SO ) ·4H O and CeO at reaction temperature

693 K), which is in line with the catalytic performance in

Figure 3. Aldol condensation product selectivities of methyl phenols

(

red ●), acetone (olive □), C ketone (dark yellow ◇), and

methyl phenols formation, indicating that Ce , especially Ce ,

acetaldehyde (black △) as a function of ethanol conversion over

Ce(OH)SO ·xH O. (Reaction condition: 5−1700 mg of cat., 3.0 kPa

is critical for methyl phenols formation. To investigate the eﬀect

2−

of SO₄, pure CeO (commercial sample from Alpha-99.5%,

C H OH, 15 kPa H , balanced Ar, 693 K, 1 atm).

CAS no. 1306-38-3) was introduced, which shows a high

ethanol conversion at 99.9%; however, the selectivity of methyl

phenols is only as low as 35.0%, while acetone (∼46.4%)

dominated within its aldol condensation product distributions.

aldol condensation product selectivities of ethanol trans-

formation change (methyl phenols, acetone, C₅₊ketone and

acetaldehyde) as a function of ethanol conversion over

Ce(OH)SO ·xH O (Typical reaction condition: 3.0 kPa

−

We also prepared SiO −SO (SO -functionalized SiO₂),

which showed high dehydration activity at 98.9% conversion

(shown in Table S2 ) and suggests a strong correlation between

(CH ) formation and the SO functional group. The crystal

C H OH, 15 kPa H , balanced Ar, 693 K, 1 atm) by varying

2−

the GHSV (Figure 2). With the conversion increased from 1.9%

to 99.9%, the selectivity of methyl phenols in aldol condensation

increased from ∼0 to 76.9% and the AA selectivity decreased

from 73.7% to ∼0 (bellow our GC detection limitation), and

acetone and C ketone experienced a ﬁrst increasing and

x n 4

phase (Figures S4 and S5 ), thermogravimetric property (Figure

S6 ), and morphology (Figures S11 and S12 ) information on

fresh and spent Ce(OH)SO ·xH O are shown in the Supporting

Information , indicating that the Ce(OH)SO ·xH O lamellar

decreasing selectivity afterward trend (an enlarged ﬁgure of

structure has been cracked (decreasing of the Raman peak

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Table 2. Conversions and Aldol Condensation Product Selectivities from Ethanol over Various Catalysts

aldol condensation product selectivities (%)

catalyst

conv (%)

MPs

C₅₊ketone

acetone

C₄

others

Ce(OH)SO ·xH O

99.9

98.6

99.5

99.9

99.3

99.9

84.9

59.7

98.9

76.9

68.8

67.2

35.0

33.3

16.6

6.0

6.8

10.3

10.1

15.9

4.9

4.8

0.4

9.9

6.8

15.1

20.5

20.7

46.4

13.6

53.8

0.8

0.1

1.2

0.3

1.5

1.9

4.5

19.8

29.4

30.1

3.3

Ce (C O ) ·xH O

Ce(OH)₄

CeO₂

0.5

0.8

6.1

8.5

39.3

15.9

Ce(SO ) ·4H O

37.6

5.0

54.8

13.4

66.7

Y (OH) SO

TiO₂

MgO

6.0

0.6

1.2

6.7

SiO −SO₄

Reaction conditions: 3.0 kPa C H OH, 15 kPa H , balanced with Ar, 1 atm, 693 K.

unsaturated ﬁllings of the 4f orbitals surrounding Ce . That

causes connections of the Ln cations on the boundaries

between transition metal and Lewis acidic catalyst. Previously,

people tried to increase the ability of hydrogenation/

dehydrogenation of the Lewis acidic/basic sites via Ag, Cu,

23,43−48

etc. transition-metal doping.

Well, with the existence of

Cu or Ag, the esteriﬁcation of acetaldehyde/ethanol occurs

readily. Clearly, Ce(OH)SO ·xH O catalyst shows advantages

at the presence of Ce in hydrogenation/dehydrogenation

compared to TiO , MgO, etc. Additionally, we also compared

the catalytic performance and stability among Ce(OH)SO₄·

xH O and the catalysts reported in the literature (Table S7),

which showed that preparation of methyl phenols from ethanol

over Ce(OH)SO ·xH O had the highest yield reported under

the optimized reaction conditions (3.0 kPa CH CH OH, 15 kPa

−

Figure 4. Aldol condensation product selectivities of ethanol

transformation over various catalysts (Reaction condition: 3 kPa

CH CH OH, 15 kPa H , Ar balanced, 1 atm, 693 K).

H , Ar balanced, GHSV = 5.4 min , 693 K, 1 atm).

3.3. Mechanism for Preparation of Methyl Phenols

from Ethanol over Lamellar Ce(OH)SO ·xH O. 3.3.1. Ele-

mentary Steps for Preparation of Methyl Phenols over

intensity from spent Ce(OH)SO ·x H O catalyst while the peak

Lamellar Ce(OH)SO

phenols formation from ethanol over Ce(OH)SO

included in Scheme 2, which shows ethanol (step 1), acetone

(step 11) and CH (step 18) adsorption,

product desorption (steps 8, 10, 12, 17, 24, and 25),

dehydrogenation (step 2), hydrogenation (steps 5, 6, 7, 15,

19, and 23), dehydration (steps 16 and 22), acetaldehyde (step

3), acetone (step 13) and 2-PNO (step 20) deprotonation,

decarbonylation (step 9), C−C bond coupling (steps 4, 14, and

21) and 4-HPO cyclization (step 24) elementary steps.

·xH

O. A schematic illustration of methyl

position does not change, Figure S7 and Table S1) after ethanol

transformation; however, the Ce³content (con ﬁ rmed by ex-

situ/quasi-in situ XPS analysis, Figure 1e and Figure S8) was

maintained. The thermogravimetry analysis (TGA) also

indicated that Ce(OH)SO ·xH O and Ce(OH) both have

·xH O was

COCHCHCH

−

OH and Ce species below 700 K. Combining with the results

of measurements and catalytic performance of various catalysts,

it could be concluded that Ce −OH Lewis acid−base pair

probably is the active species for MPs formation. We also

compared the Brunauer−Emmett−Teller (BET) surface area

and Lewis acid intensities of the catalysts shown in Figure 4, the

BET surface area and the turnover rate (per surface area) were

included in Tables S3 and S4, respectively. The results of Lewis

The molecular adsorption/desorption steps were assumed to

be quasi-equilibrated (QE). The possible kinetically relevant

events for the methyl phenols formation include: (i)

deprotonation of C H O by a neighboring base site to create

2 4

acid intensities of various catalysts studied by acetonitrile-d and

enolate (step 3 in Scheme 2), (ii) enolate nucleophilically

pyridine adsorption are displayed in Figures S25 and S26 and

Tables S5 and S6 . These results suggested that there is no clear

and strong correlation between the BET surface area, Lewis acid

intensities, and methyl phenols formation under steady-state

conditions. Some typical Lewis acid (or base) catalysts were also

introduced to elucidate the eﬀect of the Lewis acidity on the

whole reaction rate and MPs selectivities. However, most Lewis

acidic/basic catalysts are lacking dehydrogenation capacity, such

attacking adjacent surface adsorbed C H O to generate the C−

δ−

C bond (step 4 in Scheme 2), (iii) CH CH(O) CH CHO

protonation to form aldol molecule (step 5 in Scheme 2), (iv)

the molecule of aldol hydrogenation to create

δ−

CH CHOHCH CH (O)

(step 6 in Scheme 2), (v)

δ −

C H C H O H C H C H ( O )

h y d r o g e n a t i o n t o

CH CHOHCH CH OH (step 7 in Scheme 2), (vi) 3-ketobutyl

aldehyde (CH CHOHCH CHO) reacting with water to make

as TiO and MgO, etc. Then AA and n-butanol were found to be

acetone, carbon dioxide, and hydrogen (step 9 in Scheme 2),

δ−

the main aldol condensation product instead of MPs (<6.0%

selectivity in aldol condensation products). That suggests that

our Ce(OH)SO ·xH O catalyst has unique properties on

(vii) deprotonation of acetone to CH CO(CH ) (step 13 in

δ−

Scheme 2), (viii) CH CO(CH ) nuclephilically attacking

C H O to form CH COCH CH(O )CH (step 14 in Scheme

δ−

catalyzing ethanol dehydrogenation and promoting C−C

bond coupling, which would be tentatively attributed to the

2). In order to determine which of these steps is kinetically

relevant for the selective methyl phenols and ethylene

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Scheme 2. Reaction Pathways of Ethanol Upgrading over Ce(OH)SO ·xH O to Form Methyl Phenols

A and *B indicate unoccupied Lewis acid and base sites, respectively; the marked arrow (step 4) represents the elementary step that was assumed

to be kinetically relevant and irreversible.

formation, respectively, pressure dependence studies were

carried out and recorded in Figure 5, Langmuir−Hinshelwood

model was solely considered based on the results determined

subsequent sections were measured after the diﬀusion

limitations were eliminated and will be represented by brackets

([X]) for clarity.

49,50

and people discussed in the literature.

Methyl phenols formation rates show a ﬁrst-order (gradient =

In addition to the elementary steps mentioned above, some

other steps of molecule adsorption and water formation were

included in Scheme 3, which includes acetaldehyde adsorption

.54) and second-order (gradient = 1.66) dependence on

[

(

C H O] at 693 K without and with 1 kPa H O, respectively

measured at a constant 15 kPa H , 3 kPa C H OH, 0 or 1 kPa

(

step 26 in Scheme 3), carbon dioxide adsorption (step 27 in

H O). We cofeed water to make a cleaner surface during ethanol

Scheme 3), water adsorption (step 28 in Scheme 3), hydrogen

adsorption (step 29 in Scheme 3), reactive surface oxide species

hydrogenation to water (step 30 in Scheme 3), and hydroxide

hydrogenation to water (step 31 in Scheme 3). All these steps

were assumed to be QE.

transformation. Combing the reaction order at 693 and 673 K,

we could conﬁrm that methyl phenols formation rates show a

negative second-order (gradient = −1.14 at 693 K, gradient =

−

and [H O] at 693 and 673 K, respectively. Additionally, MPs

formation rates increase with a nearly one and half-order

dependence on [H ] (gradient = 1.1) at 693 K.

1.2 at 673 K) and an inverse second-order (gradient = −1.12 at

93 K, gradient = −1.67 at 673 K) dependence on [C H OH]

.3.2. Pressure Dependency Studies for Methyl Phenols

Formation. To develop a deeper understanding for the interplay

of the methyl phenols formation and the most abundant reactive

intermediates (MARIs) from the surface, we have carried out a

systematic study (Figure 5 and Figure S27) to investigate the

methyl phenols formation rate as a function of partial pressures

of the reactants (C H OH) and the coproducts (C H O, H O,

Overall, the manner in which the methyl phenols formation

rates over the Ce(OH)SO

pressures of C O, C OH, H

Figure S28 ) can be summarized as

·xH

O catalyst depend on the

O, and H (Figure 5 and

and H ) on Ce(OH)SO ·xH O. All partial pressures in this and

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δ−

r_M_P_s

k [(CH ) CHO*A][CH CHO*A]

[

[LA]

(6)

Considering about the quasi-equilibrium (QE) assumption

on CH CHO adsorption (step 26 in Scheme 3), CH CHO

deprotonation (step 3 in Scheme 2), and hydrogenation

adsorption (step 29 in Scheme 3), the reaction rate could be

expressed as

r_M_P_s

k K K [CH CHO] [*A]

[

^K²9 [H ] [LA]

(7)

where the surface Lewis acid active sites could be occupied as

follows:

[

LA] = [*A] + K [C H OH][*A] + K [C H O][*A]

K [MPs] + K [C H O] + K [H O][*A]

[C H ] + [HO*A] + [O*A]

C H

(8)

Figure 5. Methyl phenols formation rates over Ce(OH)SO ·xH O

determined at 693 K as a function of C H O (dark yellow ◇) (0.1−0.7

Considering about the QE assumption on water moleculely

adsorption (step 28 in Scheme 3), hydrogen dissociation (step

29 in Scheme 3), and water formation by hydroxide hydro-

genation (step 31 in Scheme 3), the [HO*A] can be expressed

as follows:

kPa C H O, 15 kPa H , 3.0 kPa C H OH, Ar balanced, 1 atm, 693 K),

C H OH (blue ○) (1.5−5 kPa C H OH, 15 kPa H , Ar balanced, 1

atm, 693 K), H O (black

▽

) (0.2−3 kPa H O, 15 kPa H , 3.0 kPa

C H OH, Ar balanced, 1 atm, 693 K), H (red △) (14.5−50 kPa H ,

.0 kPa C H OH, Ar balanced, 1 atm, 693 K), and C H O (pink □)

pressures (0.1−0.75 kPa C H O, 15 kPa H , 3.0 kPa C H OH, 1 kPa

K [H O][*A]

H O, Ar balanced, 1 atm, 693 K).

[

HO*A] =

K K

^[^H2^]

(9)

Scheme 3. Elementary Steps of Molecule Adsorption and

Reactive Surface Oxide Species Hydrogenation to Water and

Hydroxide Hydrogenation to Water during Methyl Phenols

After considering about the QE assumption on water

moleculely adsorption (step 28 in Scheme 3), hydrogen

dissociation (step 29 in Scheme 3), and water formation by

reactive oxide species hydrogenation (step 30 in Scheme 3), the

Formation over Ce(OH)SO ·xH O

[O*A] can be expressed as follows:

K [H O][*A]

[

O*A] =

K₂₉²K [H ]

(10)

Then the surface Lewis acid active sites could be simpliﬁed

into:

[

LA] = [*A]j1 + K [C H OH] + K [C H O]

A and *B indicate unoccupied Lewis acid and base sites,

respectively; the circled arrow represents the elementary step that

was assumed to be quasi-equilibrated.

K [C H O] + K [MPs] + K [H O]

K [H O]

K [H O] z

.5−(−0.5)

2−0

+ K

[C H ] +

C H

k_a_p_p[H₂]

[C H O]

K K [H ]

K [H ]

29 30

r_M_P_s

∼

{

0−2

[

C H OH] [H O]

(5)

(11)

The reaction is under diﬀerential reaction conditions, which

suggests that the coverage of all the products are much lower

than the surface intermediates derived from the reactants. That

is why [MPs*], [CH COCH *], and [C H *] were omitted in

where k_a_p_prepresents the apparent reaction constant for methyl

phenols formation and [X] represents the pressure of species X

in the reactor. The form of eq 5 suggests that two C H O-

derived intermediates participate in the kinetically relevant step

on active sites that are mostly covered by C H OH-derived

the surface intermediates listed. Then

species. The additive of H O can reduce the coverage of C H O

and show the intrinsic reaction order. Based on our knowledge,

the dependencies on hydrogen are mainly coming from the

inﬂuence of reducible surface oxygen.

[

LA] = [*A]j1 + K [C H OH] + K [C H O]

K [H O]

K [H O] z

We assume here that enolate nucleophilically attacks C H O

K [H O] +

to form C−C bond (step 4 in Scheme 2) is the rate-determining

step (RDS). With this assumption, the reaction rate for methyl

phenols formation is expressed as

K K [H ]

K [H ]z

29 30

(12)

{

Finally, the reaction rate could be simpliﬁed into:

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conﬁrms our assumption that there is strong correlation

between surface hydrogen and MPs formation as described in

Figure 6. The reduced active oxygen shown in quasi-in situ O 1s

XPS spectra also indicated the participation of active oxygen in

methyl phenols formation process.

r_M_P_s

k K K [CH CHO]

mK [H₂]

[

j1 + K [C H OH] + K [C H O] + K [H O]

3.2.3. Eﬀects of H/D-Atom Substitution on Rates of Methyl

Phenols and (CH ) Formation. To conﬁrm the kinetically

x n

relevant step(s) assessment on the formation of methyl phenols,

a steady-state isotopic replacement experiment was introduced,

y o

K [H O]

K [H O] z o

z }

z o

where C H OH was replaced by C H OD or C D OD with all

K K [H ]

K [H ] o

2 5 2 5 2 5

29 30 2 {

(13)

the other gas pressures held as constants (2.6 kPa C H OH/

By assuming diﬀerent species are MARIs, the rate expression

simpliﬁes to either zero or negative second order in ethanol,

second or zero order in acetaldehyde, zero or negative second

C H OD/C D OD, 15 kPa H , Ar balanced, 1 atm, 693 K).

2 5 2 5 2

Table 3 and Figure S29 show that steady-state methyl phenols

order in water, one and half or negative half order in H , which

Table 3. Methyl Phenols and (CH ) Formation Rate

x n

match the trends observed in our study very well. So enolate

Changes (i.e., k /k ) during Isotopic Substitution at

nucleophilically attacks C H O to form C−C bond (step 4 in

Scheme 2) should be the RDS. This assessment is quite similar

to our previous studies of acetaldehyde/ethanol transformation

Constant C H OH/C H OD/C D OD (2.6 kPa), H /D

2 5 2 5 2 5 2 2

(4.99 kPa) and C H O/C D O (0.3 kPa) Pressures

isotopic substitution

experiments

methyl phenols formation rate changes

on TiO following aldol condensation process with the only

(k /k )

exception of hydrogen.

As reported in early literature,

2−56

OH → C

OD → C D

2 5

1.1

active oxygen could be

_g_e_n_e_r_a_t_e_d_f_r_o_m_t_h_e_r_e_d_o_x_c_o_u_p_l_i_n_g_o_f_C_e4 and Ce and then

C H O → C D O; C H OH

participate in CO oxidation through Mars−van Krevelen (MvK)

OH → C

OD; C

D O

1.06

0.96

mechanism. This provides a hypothesis that active oxygen

might participate in methyl phenols formation through the O*A

and HO*A group, which could well explain the positive

H → D ; C D OD

dependence of MPs formation against H . A correlation of the

^M^P^s^f^o^r^m^a^tⁱ^oⁿ^r^a^t^e^s⁽^m^u^l^tⁱ^p^lⁱ^e^d^b^y^[^H2^]

were plotted as a function of the [O*A] and [HO*A] from the

surface in Figure 6 . As discussed above, the O *A and HO*A

−5

formation rate over Ce(OH)SO ·xH O is 1.8 × 10 mmol_M_P_s·

4 2

1/2

based on eq 13)

−1

(mmol ·min) when C H OH was fed in (2.6 kPa C H OH,

5 kPa H , Ar balanced, 1 atm, 693 K), which decreases by a

factor of ∼8% when C H OH was replaced by partially

deuterated ethanol (C H OD) in the reactant stream (i.e., k /

k ∼ 1.1) and subsequently shows a negligible variation when

C H OD was further replaced by perdeuterated ethanol

(

C D OD) (i.e., k /k ∼ 1.1) while holding all other pressures

as constant. These results suggest that dehydrogenation of

C H OH to C H O (step 2 in Scheme 2) could not be the RDS;

otherwise, a typical normal KIE would be determined. Previous

51,58,59

studies

also considered that deprotonation of acetalde-

hyde or nucleophilic attack of enolates was the RDS during the

aldol condensation reaction, then acetaldehyde was fed in with

ethanol. Negligible KIE was determined either on acetaldehyde

isotopic replacement (Figure S30 , i.e., k /k ∼1.0−1.1). In

order to explore the relationship between hydrogenation and

methyl phenols formation, H was replaced by D while holding

all other parameters constant (Figure S31). Clearly, the

hydrogenation (steps 5−7 in Scheme 2) or dehydrogenation

(

steps 8, 12, and 24 in Scheme 2) will not be involved in the KRS

based on the negligible KIE determined (k /k ∼ 0.96).

.2.4. Pressure Dependency Studies for MPs/(CH ) Ratio.

x n

Figure 6. MPs formation rate multiplied by [H₂]¹^/²over Ce(OH)SO₄·

xH O determined at 693 K as a function of: active oxygen ([H O]/

Dehydration reaction is the only main side reaction along with

MPs formation determined that is why the reaction mechanism

of (CH ) formation was systematically investigated here.

[

H ]) and OH group ([H O]/[H ] ) (black □) pressure (1 kPa H O,

x n

According to previous studies, the C −H bond breakage is the

5−55 kPa H , 3 kPa C H OH, Ar balanced, 1 atm, 693 K).

0−65

only KRS during alcohols dehydration,

which was

supported by the normal KIE (k /k ∼ 2.04) obtained once

group concentrations could be expressed by [H O] and [H ]

normal ethanol and acetaldehyde was replaced by perdeuterated

pressures (eq 9 and 10), then we use “α[H O]/[H ] + β[H O]/

ethanol and acetaldehyde during (CH ) formation (Figure

x n

[

H₂] ” to express the concentration of O*A and HO*A, while

α” and “β” are apparent constants for these two species

formation. Figure 6 shows that the MPs reaction rates

S32 ).

“

To compare the two competing reactions, the ratio of MPs to

(CH ) formation rates was calculated (eq 14), which suggests

x n

(

multiplied by [H₂]¹) has a reversed second order (gradient

that the relative selectivity of MPs and (CH ) formation is

x n

1/2

−2.03) on “α[H O]/[H ]+β[H O]/[H ] ”, which further

determined by acetaldehyde, ethanol, H O, and H pressures.

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https://pubs.acs.org/doi/10.1021/acscatal.1c01096.

Research Article

The MPs/(CH ) ratios decreased with an negative second-

acetone aldol condensation, C decarbonylation, and 4-HPO

x n

order and negative ﬁrst-order dependence on [C H OH] and

cyclization at diﬀerent temperatures (623−693 K) over

Ce(OH)SO ·xH O catalyst, the values are included in Table 4

[

H O], respectively, in a stark contrast, it increased with a ﬁrst

order in [H ] and second order in [C H O] (Figure 7 ). These

Table 4. Activation Energies for Dehydration,

Dehydrogenation, Aldol Condensation, Decarbonylation,

and Cyclization during Ethanol Transformation over

Ce(OH)SO ·xH O

step

activation energy of each step (kJ·mol⁻¹)

ethanol dehydration

ethanol dehydrogenation

aldol condensation

decarbonylation

66.2

−31.3

80.7−93.0

49.5

cyclization

59.8

Reaction conditions: 15 kPa H , balanced with Ar, 1 atm, 623−693

and Figure S33 , and all of the apparent activation energies and

reaction orders were obtained at diﬀerential conversion

conditions (Figure S1 ). The highest apparent activation energy

during ethanol transformation is aldol condensation at 80.7−

−

3.0 kJ·mol , which is in good agreement with previous results

proposed that this step is the kinetically relevant for ethanol

upgrading. Then enolate nucleophilically attacks C H O to form

C−C bond (step 4 in Scheme 2) should be the only possible

rate-determining step for MPs formation.

Figure 7. MPs/(CH_x)_nselectivity ratios over Ce(OH)SO ·xH O

determined at 693 K as a function of C H OH (blue ○) (1.5−5 kPa

C H OH, 15 kPa H , Ar balanced, 1 atm, 693 K), H O (black ▽) (0.2−

kPa H O, 15 kPa H , 3.0 kPa C H OH, Ar balanced, 1 atm, 693 K),

2 2 2 5

H (red △) (14.5−50 kPa H , 3.0 kPa C H OH, Ar balanced, 1 atm,

4. CONCLUSION

93 K) and C H O (pink □) pressures (0.1−0.75 kPa C H O, 15 kPa

Ultrahigh selectivity of methyl phenols has been selectively

H , 3.0 kPa C H OH, 1 kPa H O, Ar balanced, 1 atm, 693 K).

achieved at ∼80% in aldol condensation products at around 50%

condensation yield over lamellar Ce(OH)SO ·xH O from

dependencies suggest that at a relatively high reaction

conversion, due to the consumption of ethanol and generation

of C H O in situ, the MPs selectivity would be promoted. That is

ethanol in this work. The signiﬁcant amount of Ce (∼70%)

was determined to be the active species based on quasi-in situ/

ex-situ XPS and Raman characterizations and the amount of

why ∼80% MPs selectivity in condensation products was

determined at high (∼100%) ethanol conversion. At the same

time, the existence of surface active oxygen (O*A) would

facilitate dehydration; that is why more hydrogen would

suppress the formation of ethylene via reacting with surface

oxygen. Finally, the existence of water would challenge the

possibilities of acetaldehyde dimer generation from the surface

and reduce the possibility of C−C bond formation. Thus, we

propose that the MPs formation yield might be further improved

by subtracting the moisture from the system.

Ce has been maintained during ethanol upgrading. Dehy-

dration to ethylene was found to be the main side reaction

controlled by β-C−H bond rupture. The MPs/(CH )

selectivity ratio was proved to be highly sensitive to

acetaldehyde, ethanol, water, and hydrogen pressure, in which

ethanol and water were believed to challenge the acetaldehyde

adsorption while surface H reacts with surface oxygen and

would facilitate MPs formation and suppress hydrocarbon

generation. With a combination of kinetic study, thermody-

namic measurement and in situ/ex situ characterizations, the

methyl phenols formation was found to be controlled by enolate

nucleophilically attacks surface C H O to form the C−C bond,

.76

0.55

k_a_p_p[C H O] [H₂]

^rMPs/(CHx)n

∼

1.54

0.74

[

C H OH] [H O]

(14)

which was supported by a prominent apparent activation energy

assessment over C−C bond coupling, dehydration, dehydrogen-

ation, and cyclization within the reaction network identiﬁed and

conﬁrmed by negligible KIEs determined. All of the information

was summarized into a well-deﬁned Langmuir−Hinshelwood

model to explain the reaction mechanism of MPs formation,

which would clarify the confusion on ethanol transformation

over Lewis acidic/basic catalysts, help researchers optimize the

reaction system on other oxygenate platform molecules

upgrading, and attract interest in exploring the role of reaction

intermediates during C−C bond coupling.

.2.5. Activation Energy Assessment for Methyl Phenols

Formation. As described in Table S8, the reaction barrier for

−

dehydration is much smaller at 66.2 kJ·mol than the value for

−

C−C bond coupling at 159.6 kJ·mol over Ce(OH)SO ·xH O.

The activation energy of ethanol dehydrogenation indicates that

C H O, one of the key reactive intermediates in methyl phenols

formation, could be easily generated from ethanol at 693 K over

Ce(OH)SO ·xH O, suggesting that ethanol dehydrogenation is

a faster step, which is the main diﬀerence between Ce(OH)SO₄·

xH O and any Lewis acidic/basic catalyst compared (Table S8).

Probably, the MPs formation shown in Figure 2 is within a

kinetic controlled regime rather than thermodynamic regime.

Additionally, apparent activation energy of each step during

ethanol transformation were estimated by measuring the rates of

ethanol dehydrogenation, ethanol dehydration, acetaldehyde, or

ASSOCIATED CONTENT

sı Supporting Information

The Supporting Information is available free of charge at

■

171

ACS Catal. 2021, 11, 6162−6174

ACS Catalysis

Nankai University, Tianjin 300350, China; orcid.org/

Research Article

Experimental procedures, catalytic characterizations, de-

R&D Program of China (2017YFA0208000); National Natural

Science Foundation of China (21971117, 21771156); State Key

Laboratory of Rare Earth Resource Utilization

(RERU2019001); and Beijing-Tianjin-Hebei Collaborative

Innovation Project (63201058).

tailed derivations of kinetic assessment on determining

the rate-determining step, and some other supported

results (PDF )

Notes

AUTHOR INFORMATION

Corresponding Authors

Hongbo Zhang − School of Materials Science and Engineering

and National Institute for Advanced Materials, Tianjin Key

Laboratory for Rare Earth Materials and Applications,

■

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

The authors thank Dr. Yue Lin from the University of Science

and Technology of China for help wih HAADF-STEM. The

authors also thank Prof. Tiehong Chen from Nankai University

000-0002-4065-6499 ; Email: hbzhang@nankai.edu.cn

for help with H -TPR experiments.

Yaping Du − School of Materials Science and Engineering and

National Institute for Advanced Materials, Tianjin Key

Laboratory for Rare Earth Materials and Applications,

Nankai University, Tianjin 300350, China; orcid.org/

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000-0002-9937-2087 ; Email: ypdu@nankai.edu.cn

Authors

Jinqiu Guo − School of Materials Science and Engineering and

National Institute for Advanced Materials, Tianjin Key

Laboratory for Rare Earth Materials and Applications,

Nankai University, Tianjin 300350, China

(5) Liu, Y. S.; Baráth, E.; Shi, H.; Hu, J. Z.; Camaioni, D. M.; Lercher, J.

Zongjing Feng − School of Materials Science and Engineering

and National Institute for Advanced Materials, Tianjin Key

Laboratory for Rare Earth Materials and Applications,

Nankai University, Tianjin 300350, China

(

catalysts. Appl. Catal., A 2000, 193, 123−128.

Jun Xu − School of Materials Science and Engineering and

National Institute for Advanced Materials, Tianjin Key

Laboratory for Rare Earth Materials and Applications,

Nankai University, Tianjin 300350, China; orcid.org/

A. Solvent-Determined Mechanistic Pathways in Zeolite-H-BEA-

Catalysed Phenol Alkylation. Nat. Catal. 2018 , 1 , 141 −147.

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Alkenylation of Phenols with Alcohols: Expedient Synthesis for

Substituted Phenols and Benzofurans. J. Am. Chem. Soc. 2012, 134,

000-0003-3507-0159

7) Zhao, C.; Camaioni, D. M.; Lercher, J. A. Selective Catalytic

325−7328.

Jie Zhu − School of Chemical Engineering, Dalian University of

Technology, Dalian, Liaoning 116024, China; orcid.org/

Hydroalkylation and Deoxygenation of Substituted Phenols to

000-0003-0651-8022

Bicycloalkanes. J. Catal. 2012, 288, 92−103.

Guanghui Zhang − School of Chemical Engineering, Dalian

University of Technology, Dalian, Liaoning 116024, China;

orcid.org/0000-0002-5854-6909

Chunhua Yan − Beijing National Laboratory for Molecular

Sciences, State Key Laboratory of Rare Earth Materials

Chemistry and Applications, PKU-HKU Joint Laboratory in

Rare Earth Materials and Bioinorganic Chemistry, College of

Chemistry and Molecular Engineering, Peking University,

Beijing 100871, China; orcid.org/0000-0002-0581-2951

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to Phenol by Cytochrome P450BM3 Triggered by Amino Acid

Derivatives. Angew. Chem., Int. Ed. 2017, 56, 10324−10329.

Complete contact information is available at:

https://pubs.acs.org/10.1021/acscatal.1c01096

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0675.

Author Contributions

For the work details, H.B.Z. conceived the research ideas and

designed the experiments. J.Q.G. did the experimental work.

Z.F. and J.X. did the material preparation, and J.Z. and G.Z.

helped with in situ XRD characterization. All of the authors

discussed the results and commented on the diﬀerent versions of

the manuscript.

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Quinolinium and Polyoxovanadate-Based Supramolecular Catalysts.

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Chem., Int. Ed. 2017, 56, 12260−12263.

Funding

(14) Al-Sabagh, A. M.; Yehia, F. Z.; Eshaq, G.; ElMetwally, A. E.

This work was supported by the “111 Project” of China

B18030); the Fundamental Research Funds for the Central

Universities, Nankai University (63201071 and ZB19500202);

open foundation of Key Laboratory of Industrial Ecology and

Environmental Engineering of Ministry of Education, Dalian

University of Technology (KLIEEE-19-07); Natural Science

Foundation of Tianjin (19JCYBJC20500); the National Key

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