Effect of pretreatment conditions on acidity and dehydration activity of CeO2-MeOx catalysts

Article abstract of DOI:10.1016/j.apcata.2020.117722

A series of MeO_x-modified CeO₂ (CeO₂-MnO_x, CeO₂-ZnO, CeO₂-MgO, CeO₂-CaO, and CeO₂-Na₂O) catalysts were prepared by the impregnation of CeO₂ with corresponding metal nitrates. Acidity and oxidation state of cerium were investigated on both oxidized and reduced catalysts by employing Fourier Transform Infrared spectroscopy (FTIR) on adsorbed pyridine and in situ H₂-Temperature Programmed Reduction/X-ray Absorption Spectroscopy (H₂-TPR/XAS) techniques, respectively. Metal oxide addition tended to alter both type and number of acid sites on ceria. EXAFS data showed a significant difference in N_Ce-O between unmodified and CeO₂-MeO_x, suggesting that added MeO_x interferes with vacancy formation on ceria during reduction. In comparison with air-pretreated samples, H₂-pretreated ones under similar conversion of 1,5 pentanediol exhibited a higher selectivity towards linear alcohols. Alcohol conversion found to correlate with total acidity (i.e., Br?nsted and Lewis). CeO₂ benefited from the addition of alkali (Na) or alkaline earth metals (Mg, Ca) by producing unsaturated alcohols.

Full text of DOI:10.1016/j.apcata.2020.117722

Journal Pre-proof
Effect of pretreatment conditions on acidity and dehydration activity of
CeO2-MeOx catalysts
Muthu Kumaran Gnanamani (Conceptualization) (Methodology)
(Investigation), Richard Garcia (Formal analysis) (Investigation),
Gary Jacobs (Formal analysis) (Data curation) (Validation) (Writing -
´
review and editing), Kinga Gora-Marek (Formal analysis)
(Investigation) (Data curation), Donald C. Cronauer
(Investigation)<ce:contributor-role>Reources), A. Jeremy Kropf
(Formal analysis)<ce:contributor-role>Data Curation) (Investigation)
(Resources), Christopher L. Marshall (Investigation) (Resources)
PII:
S0926-860X(20)30315-X
DOI:
https://doi.org/10.1016/j.apcata.2020.117722
APCATA 117722
Reference:
To appear in:
Applied Catalysis A, General
Received Date:
Revised Date:
Accepted Date:
24 February 2020
15 June 2020
24 June 2020
´
Please cite this article as: Gnanamani MK, Garcia R, Jacobs G, Gora-Marek K, Cronauer DC,
Kropf AJ, Marshall CL, Effect of pretreatment conditions on acidity and dehydration activity of
CeO2-MeOx catalysts, Applied Catalysis A, General (2020),
doi: https://doi.org/10.1016/j.apcata.2020.117722

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Effect of pretreatment conditions on acidity and dehydration activity of CeO2-MeOx
catalysts
Muthu Kumaran Gnanamania*, Richard Garciab, Gary Jacobsb,c, Kinga Góra-Marekd, Donald C.
Cronauere, A. Jeremy Kropfe, Christopher L. Marshalle
a
Center for Applied Energy Research, University of Kentucky, 2540 Research Park Drive,
Lexington, KY 40511, USA
b Department of Chemical Engineering and Biomedical Engineering, UTSA, One UTSA Circle,
San Antonio, TX 78249, USA
c Department of Mechanical Engineering, UTSA, One UTSA Circle, San Antonio, TX 78249, USA
d
Department of Chemistry, Jagiellonian University in Krakow, 2 Gronostajowa St., 30-387
Krakow, Poland
e Argonne National Laboratory, Argonne, IL 60439, USA
*Corresponding author: Tel: +1 859-257-0240
Email address: muthu.gnanamani@uky.edu (MK Gnanamani)
1

Graphical abstract
Highlights
 Metal oxide was added to modify the acid-base characteristics of ceria
 Pretreatment conditions (H2, air) has a profound effect on both redox and acid-base
functions of ceria
 B and L acidity were correlating with 1,5-pentanediol or cyclohexanol conversion
 Oxygen vacancies in conjunction with acid-base sites control the dehydration selectivity
 H2-treatment seems more favorable than air-pretreatment to achieve higher conversion
and selectivity
Abstract
2

A series of MeOx-modified CeO2 (CeO2-MnOx, CeO2-ZnO, CeO2-MgO, CeO2-CaO, and
CeO2-Na2O) catalysts were prepared by the impregnation of CeO2 with corresponding metal
nitrates. Acidity and oxidation state of cerium were investigated on both oxidized and reduced
catalysts by employing Fourier Transform Infrared spectroscopy (FTIR) on adsorbed pyridine and
in situ H2-Temperature Programmed Reduction/X-ray Absorption Spectroscopy (H2-TPR/XAS)
techniques, respectively. Metal oxide addition tended to alter both type and number of acid sites
on ceria. EXAFS data showed a significant difference in NCe-O between unmodified and MeOx-
CeO2, suggesting that added MeOx interferes with vacancy formation on ceria during reduction. In
comparison with air-pretreated samples, H2-pretreated ones under similar conversion of 1,5
pentanediol exhibited a higher selectivity towards linear alcohols. Alcohol conversion found to
correlate with total acidity (i.e., Brønsted and Lewis). CeO2 benefited from the addition of alkali
(Na) or alkaline earth metals (Mg, Ca) by producing unsaturated alcohols.
Keywords: Dehydration; CeO2-MeOx; 1,5-pentanediol; X-ray Absorption Spectroscopy; Acidity
1. Introduction
Metal oxides are known to catalyze dehydration reactions [1-5]. The selectivity of
dehydration is often hampered by many side products originating from reactions such as
dehydrogenation, dehydrocyclization, and dimerization. In general, preparation and pretreatment
conditions of metal oxides have played a significant role in modifying the activity and selectivity
of dehydration and other acid-catalyzed reactions [6]. Alumina and zirconia are considered to be
highly selective dehydration catalysts, but they can still produce a wide range of products if
subjected to different pretreatment conditions [7-9]. Several mixed metal oxides, such as SiO2-
Al2O3 [10], CeO2-ZrO2 [11], and ZrO2/SiO2 [12], were tested as dehydration catalysts with the aim
3

of modifying their acid-base function, which in turn controls catalyst activity and product
selectivity. Viter and Nagornyi [13] studied SnO2/B2O3 binary oxides as solid catalysts for
dehydration of 2-methyl-3-butyn-2-ol (MBOH) and the authors found that dehydration activity of
SnO2/B2O3 was close to that of ZrO2/SiO2.
CeO2 is a well-known oxygen storage material used in three-way catalytic converters [14].
Apart from redox chemistry, CeO2 also possesses mild acidity and significant basicity, found to
be effective for carrying out many organic transformations, including dehydration [14-18]. When
CeO2 is pretreated at elevated temperatures and/or exposed to reducing conditions, oxygen
vacancies and other reduced defect sites (e.g., bridging OH groups) are formed, accompanied by
a change in the oxidation state of ceria from Ce4+ to Ce3+ [19]. On the other hand, under oxidizing
conditions, CeO2 surfaces can be terminated by O or OH groups [20]. This degree of coordinative
unsaturation at the surface influences the nature of intermediates formed upon adsorption of
alcohols. For instance, primary alcohols favor dehydrogenation over dehydration on oxidized
CeO2 (111) while reduced CeOx (111) surfaces yield more olefins (i.e., dehydration) compared to
aldehyde (i.e., dehydrogenation) [21]. Mudiyanselage et al. conducted ethanol conversion
reactions on CeO2 and Ru/CeO2, and the authors [22] found that dehydration to ethylene occurs
mainly at about 700 K, with intermediates being ethoxides adsorbed on surface oxygen defect sites
of CeO2. Jacobs and co-workers [23] pointed out the significance of defect sites of CeO2 on
stabilizing the ethoxide intermediate, which in the presence of steam undergoes oxidative
dehydrogenation to produce acetate (analogous to formate in methanol steam reforming), with
subsequent demethanation resulting in the production of CO2 during steam reforming of ethanol
over metal/CeO2. In a study by Ohtake et al., hydrothermally developed ceria was shown to have
both higher ethanol conversion and ethylene selectivity in comparison with a reference catalyst.
4

The authors pointed out that higher surface area and Lewis basicity were influential properties of
ceria [24]. Zaki and Sheppard [25] evaluated the effects of calcination on the dehydration of 2-
propanol using IR spectroscopy, and the authors concluded that higher calcination temperature
leads to a higher degree of surface shell reduction of ceria, which decreases electron mobility. This
may explain the observed drop in the dehydration activity of ceria.
The Sato group has investigated CeO2 and other rare-earth oxides (REO’s) for the selective
dehydration of terminal diols to unsaturated alcohols [26-31]. The latter compounds find many
applications within the fine chemicals industry. The authors have shown that CeO2 produces
unsaturated alcohol selectively from 1,3-butanediol, while a mixture of products forms using 1,5-
pentanediol (i.e., a terminal diol). The authors argued that differences in the carbon chain length
between these two alcohols could cause the respective intermediate to be stabilized on the catalytic
sites of CeO2. In our previous work [32], we reported that Na acts as an acid-base moderator for
CeO2, allowing control of the product selectivity from 1,5-pentanediol. This leads us to believe
that surface modification of CeO2 by adding another oxide [33] may avail an entirely different
chemistry that might have an impact on both conversion and product selectivity for dehydration of
1,5-pentanediol.
Thus, in this contribution, we investigate the influence of pretreatment conditions (air or
H2) of CeO2 and MeOx-modified CeO2 catalysts on the dehydration of 1,5-pentanediol reaction to
better understand the role of oxygen vacancies in addition to acid sites present on the surface of
activated CeO2, with the aim of controlling both conversion and selectivity. The acidity was
measured using FTIR of adsorbed pyridine. In situ H2-Temperature Programmed Reduction X-
ray Absorption Near-Edge Spectroscopy and Extended X-ray Absorption Fine Structure (H2-TPR
XANES/EXAFS) were utilized for the analysis of partially reduced cerium in CeO2. The reaction
5

of cyclohexanol - which is considered a tool for measuring acidity and basicity of metal oxides –
was carried out on all catalysts after subjecting them to reducing or oxidizing conditions. Results
from the dehydration of 1,5-pentanediol on CeO2 and MeOx-modified CeO2 catalysts suggests
that, irrespective of the pretreatment conditions, total acidity correlates with catalytic activity but
that selectivity to linear alcohols varies depending on the nature of the MeOx added, as well as the
catalyst pretreatment conditions.
2. Experimental
2.1. Sample preparation
In this work, CeO2-MeOx (90Ce:10Me) catalysts (CeO2-MnOx, CeO2-ZnO, CeO2-MgO,
CeO2-CaO, and CeO2-Na2O) were prepared by incipient-wetness impregnation using respective
metal nitrates and commercially available CeO2. More details can be found in our earlier
publication [33]. CeO2 was purchased from Rhodia, and had a high surface area of 114 cm2/g. All
metal nitrates [manganese (II) nitrate hydrate 98%, zinc (II) nitrate hexahydrate 98%, magnesium
(II) nitrate hexahydrate 98%, calcium (II) nitrate tetrahydrate 99.9%, and sodium (I) nitrate 99.9%]
were purchased from Sigma Aldrich and used as is, without further purification. Briefly, the
required metal nitrates were dissolved in deionized water and added to previously weighed CeO2
to produce a Ce to Me ratio of 90:10. The samples were then dried in an air oven held at 120 °C
overnight and finally calcined in a muffle furnace at 500 ℃for 4 h. For the purpose of comparison,
pure CeO2 was subjected to similar treatments as described above, excluding the metal nitrate
addition.
2.2. Characterization
The catalyst acidity was determined by FTIR pyridine adsorption using a Bruker Vertex
70 spectrometer equipped with an MCT detector with a spectral resolution of 2 cm-1. The catalyst
6

sample was pressed into a self-supporting disc (3.5 cm2; ca.15 mg/cm2) and pretreated in situ in
an IR cell by flowing either air at 350 ℃or H2 at 500 ℃for one hour. Both temperature and nature
of pretreating environment are major factors that influence the surface characteristics of ceria. One
hour of pretreatment time is enough for characterization to attain the reduced or oxidized state of
CeO2. After outgassing at the same temperature, the sample was cooled to 130 ℃ prior to
contacting with pyridine (>99.8%, Sigma Aldrich) vapor. The physisorbed pyridine was
subsequently removed by evacuation at the same temperature. The Brønsted and Lewis acidity
was inferred from the intensities of the 1545 cm-1 band for pyridinium ions (PyH+) and the 1444,
1450 cm-1 bands for Py coordinatively bonded to Lewis sites (PyL). The absorption coefficients
0.07 and 0.1 cm2/mol were used to characterize the Brønsted and Lewis acid sites, respectively
[34,35].
H2-TPR XAFS experiments were performed at the Materials Research Collaborative
Access Team (MR-CAT) beamline at the Advanced Photon Source, Argonne National Laboratory.
A cryogenically cooled Si (111) monochromator was used to select the incident energy, and a
rhodium-coated mirror was used to reject higher-order harmonics of the fundamental beam energy.
The experimental setup was as outlined by Jacoby [36]. A stainless-steel multi-sample holder (4.0
mm i.d. channels) allowed for testing the in-situ reduction of six samples during temperature-
programmed heating. The samples were diluted with silica gel at a weight ratio (silica/sample) of
approximately 5. Approximately 6 mg of each sample was loaded as a self-supporting wafer in
each channel. The holder was placed in the center of a quartz tube, equipped with gas and
thermocouple ports and Kapton windows. The amount of sample used was optimized for the Ce
L3 edge (5723 eV). The quartz tube was placed in a clamshell furnace mounted on a positioning
table. Each sample cell was positioned relative to the beam by finely adjusting the position of the
7

table to an accuracy of 20 μm for repeated scans. Once the sample positions were fine-tuned, the
reactor was purged with helium for more than 5 min at 30 ml/min. Then, the reactant gas (3.5%
H2/He) was flowed through the samples (95 ml/min), and a temperature ramp of ∼1.8 °C/min was
initiated for the furnace. The final temperature was ∼975 °C, and it was held for ~45 min. The
Ce L3-edge spectra were recorded in transmission mode, and a Cr metallic foil spectrum was
measured simultaneously with each sample spectrum for energy calibration. Data reduction of the
XAFS spectra was carried out using the WinXAS program [37], and raw data were processed to
give the normalized XANES spectra. Linear combination fitting of spectra with appropriate
reference compounds was carried out. The initial spectrum of each catalyst was used as a
fingerprint for the Ce4+ oxidation state, while the final spectrum of a 0.5%Pt/Ca0.50Ce0.50OX
catalyst following TPR (30 ml/min of 4%H2/He, 820 °C, 1.5 h hold time) from [38] was used as a
fingerprint of the Ce3+ oxidation state for all samples, since its lineshape resembled Ce3+
compounds, such as Ce(NO3)3 or Ce(Cl3)3. EXAFS data reduction and fitting were carried out
using the WinXAS program [see above], Atoms [39], FEFF [40], and FEFFIT [40] programs. The
k- and r-ranges were chosen to be approximately Δk = 2.5 – 10 Å-1 and ΔR = 1.0 – 2.5 Å,
respectively.
2.3. Catalytic reaction
The dehydration of 1,5-pentanediol or cyclohexanol was carried out in a fixed-bed flow
reactor under an atmosphere of N2 (air pretreated) or H2 (H2 pretreated) at a flow rate of 50 cm3
min-1. The reactor was a stainless-steel tube having a length of 35.6 cm with an internal diameter
of 0.9 cm. The total catalyst bed length would be approximately 2.5 cm for 1.0 g of catalyst sample
(40-100 microns) mixed with an equal amount of the powdered glass beads of similar sizes and
fixed between the quartz wool, which acts as a catalyst bed. Before each reaction, the sample was
8

preheated in the reactor either in airflow (50 cm3 min-1) at 350 ℃for two hours or H2 flow (50 cm3
min-1) at 500 ℃for two hours. After the catalyst bed temperature reached to 350 ℃, the alcohol of
choice [1,5-pentanediol (0.6 ml/h) or cyclohexanol (3.0 ml/h)] was fed into the reactor along with
N2 or H2 gas flow (50 cm3 min-1). The selection of a reaction temperature of 350 °C was based on
the literature [27] and the LHSV was chosen to be 0.6 ml/h for 1,5-PDO and 3.0 ml/h for
cyclohexanol to achieve a reasonable level of conversion. The liquid products collected
periodically in the trap maintained at 5 ℃were analyzed by gas chromatography using an Agilent
7890 instrument equipped with a DB-5 capillary column and an FID detector. The corresponding
FID response factor was applied for each oxygenated material found in the reactor mixture [41].
The products are identified as propanol, tetrahydropyran, tetrahydropyran-2-one, 4-penten-1-ol, 1-
pentanol, cyclopentanol, and cyclopentanone, ethanol, 3,4-dihydropyran, 2-methyl
cyclopentanone, cyclohexane methanol, 4-penten-1-ol propanoate, valeric acid pentenyl ester,
cyclohexane carboxylic acid cyclohexyl ester, and 2,2-dimethyl 3-octanol. As you can see, there
are esters with high molecular weights whose formation mechanism is uncertain. This precludes
a complete material balance. The effluent gases were collected in a Tedlar gas bag during various
time intervals and analyzed using a micro GC (Inficon Fusion) to identify the formation, if any, of
lower hydrocarbons.
Alcohol conversion and selectivity of each product were calculated by referring the outlet
molar flow to the inlet molar flow of the alcohol, as follows:
9

3. Results and discussion
3.1. Acidity characterization by FT-IR pyridine adsorption
Polycrystalline CeO2 possesses different surface Ce to O ratios depending primarily on the
type of crystalline planes exposed at the surface. To reduce the surface free energy of Ce,
reconstruction occurs by reaction between the ceria surface and molecules from the environment
(e.g., water). This leads to the formation of new surface species, such as -OH groups (usually
referred to as Brønsted acid sites). The oxygen vacancy is created on CeO2 by removal of adsorbed
water by thermal treatment in the presence of either air or H2. The unsaturated metal site acts as a
Lewis acid center.
IR spectroscopy determines both the nature and strength of catalyst acidity by adsorbing
pyridine as a probe molecule. Typically, pyridine adsorbs on Brønsted (Py-B) and Lewis (Py-L)
acid sites of CeO2-MeOx as shown in Scheme 1. The Ce and other metals exposed to surfaces act
as Lewis acid (Py-L) sites, and those hydroxyl groups associated with Ce or metals are Brønsted
acid (Py-B) sites. Apart from this, pyridine can also adsorb onto oxygen vacancies referred to as
redox sites. There is uncertainty in distinguishing the structural differences between the Lewis
acid site and oxygen vacancy [42]. In this study, additional complications arise due to the presence
of metal other than Ce that might contribute to Brønsted and Lewis acidity as well. Hence, no
attempt has been made to differentiate among Lewis acid sites. Fig. 1 shows the FT-IR spectra of
pyridine adsorption on CeO2 and CeO2-MeOx catalysts after pretreatment in H2 at 500℃. The IR
bands at 1550-1540 cm-1 correspond to Brønsted acidity, and 1460-1440 cm-1 bands are attributed
to Lewis acid sites, as shown in Table 1.
10

Scheme 1. Adsorption of pyridine on different sites of H2-treated CeO2-MeOx
The H2-pretreated CeO2 shows bands at 1545, 1490, and 1445 cm-1. The amount of
Brønsted (B) acidity in CeO2 was 35 mol/g, and Lewis (L) acidity was 240 mol/g, as shown in
Scheme 2a. The oxygen vacant redox sites were generated upon reducing CeO2 to high
temperatures. The addition of 10 wt.% MnOx increases Brønsted acidity of CeO2 from 35 to 50
mol/g, and on the other hand, Lewis acidity dropped from 240 to 143 mol/g. As MnOx interacts,
it probably consumed a small number of hydroxyl groups on CeO2, but the acidity data shows the
opposite, which indicates that additional B sites must come from MnOx. The decreasing L sites
with MnOx addition can be explained based on the formation of fewer coordinatively unsaturated
Ce metal sites on CeO2 during reduction. Both B and L sites increase with the addition of ZnO on
CeO2. However, this increase becomes very pronounced for B rather than L with CeO2-ZnO. The
spectra show two types of Lewis acid sites with bands at 1445 and 1455 cm-1. The higher wave
number at 1455 cm-1 corresponds to pyridine adsorbed on L sites of Zn2+ [43]. This suggests that
11

the addition of ZnO generates new B and L sites in addition to sites that belong to CeO2. These
new B and L sites should be associated with ZnO.
On the other hand, alkaline earth metals (Mg, Ca) and alkali metal (Na) additions tend to
decrease both B and L sites. This could be due to the exchange of the acidic proton of CeO2 with
metal cations such as Na+, Ca++, and Mg++, or due to their oxide phases being well-dispersed on
the surface of ceria. However, the former explains only a decrease in B sites, but the latter could
explain the depletion of L sites as alkali metal ions show significantly lower acidity than ceria.
Additionally, alkali and alkali earth metals could reduce L sites by assisting CO2 to interact more
strongly with CeO2, preventing oxygen vacancy formation (Scheme 2a). Zaki et al. [44] found
that Na poisoned the Lewis acid sites on CeO2 but did not offer any explanation. It is believed that
active sites for acid/base catalysis on CeO2 were generated upon removal of CO2 and/or H2O with
high-temperature treatments [45]. If this is the case, Na could hamper CO2 dissociation, as
explained earlier in this discussion, leading to fewer oxygen vacancies accompanied by diminished
acidity compared to undoped CeO2. As with CeO2-MnOx and CeO2-ZnO, alkali earth metals (Mg
and Ca) also generated much stronger L sites, which displayed a band at 1450-1455 cm-1
attributable to the associated metal cations [46,47].
Unlike H2-pretreatment, air pretreated CeO2 showed no Brønsted acidity (Fig. 2). The air
pretreated samples exhibited half of the Lewis sites (117 mol/g) compared to H2-pretreated ceria
(240 mol/g). On oxidized CeO2, Lewis acidity arises due to exposed Ce4+ sites with different
degrees of coordinative unsaturation as shown in Scheme 2b. Among the air-pretreated samples,
CeO2-MnOx has the maximum number of B sites (250 mol/g), followed by CeO2-ZnO (105
mol/g). The CeO2 modified by Mg and Ca show traces of B sites while CeO2-Na2O displayed
12

no B sites. The band positions corresponding to B and L sites did not vary appreciably between
H2-pretreated and air-pretreated samples.
In comparison to acid-base properties, the redox function of ceria has been well studied.
In the present work, an analysis of FT-IR pyridine adsorption on CeO2 and CeO2-MeOx catalysts
after different pretreatment conditions infers that new B and L sites created by the addition of
MnOx or ZnO are due to a charge imbalance between the added metal cation and ceria. Remember
ZnO or MnOx alone can contribute to total acidity. In contrast, both B and L acidity of CeO2 were
found to decrease with the addition of Mg, Ca or Na.
13

Scheme 2. Proposed structures of acidic and basic sites of (a) H2-pretreated and (b) oxidized
surfaces of CeO2 and CeO2-MeOx catalysts.
3.2 X-ray Absorption Fine Structure Analysis: In-situ H2-Temperature Programmed Reduction
The XANES technique was employed to monitor changes in the oxidation state of cerium
oxide during H2 treatment at different temperatures. The experimental details of H2-TPR XANES
are provided in Section 2.2. XANES spectra of reference compounds for Ce4+ (CeO2) and Ce3+
(cerium carbonate) were published by our group earlier, and we used those spectra for qualitative
and quantitative purposes [48]. In general, Ce4+ compounds show a white line having two peaks
corresponding to 2p3/2 → (4f1L)5d* and 2p3/2 → (4f0)5d* transitions. However, Ce3+ compounds
have a strong absorption edge due to a 2p3/2 → (4f1)5d* electronic transition (d* denotes an excited
electron in the d orbital). Fig. 3 shows a typical XANES spectrum of CeO2 and CeO2-MeOx
catalysts. Before reduction, CeO2 is in a 4+ oxidation state, and the reduction takes place in two
stages. The first dip in XANES between 300-500 ℃ is for surface shell reduction of CeO2 from
Ce4+ to Ce3+ and may be associated with O-vacancy generation and Type-II bridging OH group
formation. The second dip at high temperature (>700 ℃) was attributed to the bulk reduction of
CeO2 to Ce2O3. Differences in reduction patterns are noticed between samples, especially in the
higher temperature region, indicating that the addition of metal oxides influences ceria reduction.
EXAFS analysis reveal the structural characteristics such as the cerium coordination
number and the bond length of Ce – O. Fig. 4 shows the EXAFS spectrum of CeO2 and CeO2-
MeOx catalysts. The quantitative EXAFS data are shown in Table 2 for various samples before
and after H2 treatment. The Ce – O bond length in CeO2 is 2.341 Å ; this distance decreased to
2.338 Å for CeO2 after high-temperature H2 treatment, indicating a contraction when ceria loses
O, such that the remaining O atoms are held more tightly. This explanation is supported by the
14

literature [49]. The Ce – O bond length in CeO2-ZnO and CeO2-MgO show similar trends as CeO2
but CeO2-MnO, CeO2-CaO, and CeO2-Na2O all show either the same Ce – O bond length or even
an increase after H2 treatment. The reason for such an observation is not apparent. As expected,
the Ce coordination number (N) for CeO2 decreased from 4.74 to 2.94 after the high-temperature
H2 treatment due to the loss of surface O. The percentage decrease of NCe-O from oxidized to
reduced for different catalysts follows in the order, CeO2-Na2O < CeO2-MgO < CeO2-CaO < CeO2
< CeO2-ZnO < CeO2-MnOx. The results of the fittings suggest that Mn and Zn containing CeO2
lose relatively more oxygen than CeO2 or Ca-, Mg-, and Na-modified CeO2 during hydrogen
reduction.
The degree of ceria reduction, as shown in Fig. 5, was quantified using the XANES spectra
of different treated catalysts. The initial Ce4+ concentration is assumed to be 100%; however, the
Ce4+ content falls below 90% at 500 ℃ and further diminishes to 50% at 700 ℃. In contrast, the
Ce3+ concentration increases with temperature. CeO2 and CeO2-Na2O catalysts show a very
similar reduction profile, while metal oxide addition inhibited ceria reduction for CeO2-MnOx,
CeO2-ZnO, CeO2-MgO, and CeO2-CaO. Surface shell reduction of ceria began at 200 ℃, and this
point shifted slightly to higher temperature for all metal oxide modified CeO2 catalysts as directed
by the arrow shown in Fig. 5. It indicates that the added metal oxides on ceria somehow stabilize
O on the surface by forming solid solutions [50]. However, reports on Mn or Zn additions have
found enhanced ceria reduction due to the formation of more labile oxygen with mixed solid
solutions compared to undoped CeO2. Given the fact that the magnitude of the difference in the
oxidation state of Ce is trivial between samples up to a pretreatment temperature of 500 ℃, it is
difficult to make any definitive conclusion. Nevertheless, H2-TPR XANES/EXAFS provided
15

quantitative details on the cerium oxidation state, as well as oxygen vacancy formation, before and
after the treatment in H2.
3.3 Dehydration of 1,5-pentanediol and cyclohexanol
Table 3 shows conversion and selectivity for the dehydration of 1,5-pentanediol on CeO2
and metal-oxide modified-CeO2 catalysts after different pretreatment conditions. The products are
1-propanol, tetrahydropyran, tetrahydropyran-2-one, 4-penten-1-ol, 1-pentanol, cyclopentanol,
cyclopentanone, 3,4-dihydropyran, 2-methylcyclopentanone, cyclohexylmethanol, 4-pentenyl
propionate, 4-pentenyl pentanoate, cyclohexyl cyclohexanecarboxylate, 2,2-dimethyl 3-octanol.
There are also esters of high molecular weight formed whose mechanism of formation remains
uncertain. Due to this reason, a complete material balance cannot be achieved. The conversion of
1,5-PDO with H2-treated CeO2-MnOx and CeO2-ZnO samples is relatively higher than CeO2. On
the other hand, basic metals such as Mg, Ca, and Na tended to decrease the diol conversion.
However, catalytic activity of air-pretreated catalysts followed the order: CeO2 > CeO2-MnOx >
CeO2-MgO > CeO2-ZnO > CeO2-CaO > CeO2-Na2O. Irrespective of catalyst and treatment
conditions, five-membered cyclic products such as cyclopentanol and cyclopentanone were
formed to a significant extent (20-35%) other than THP and THP-2-one. The selectivity to linear
alcohols (4-penten-1-ol+1-pentanol) on H2-treated catalysts were 34.4%, 30.6%, 28.1%, 41.5%,
30.8% and 64.9% for CeO2, CeO2-MnOx, CeO2-ZnO, CeO2-MgO, CeO2-CaO and CeO2-Na2O,
respectively. Similarly, linear alcohol selectivity for air-pretreated catalysts varied between 25-
46%. The product labeled as “Others” contributed about 5-22% to total products formed. The
data showed that pretreatment conditions do affect the dehydration capability of CeO2 and
modified-CeO2 catalysts. Fig. 6 shows a linear relationship between the selectivity of different
products and the conversion of 1,5-PDO. H2-pretreated samples show relatively higher selectivity
16

to linear alcohols (4-penten-1-ol and 1-pentanol) than air-pretreated catalysts at a similar
conversion level. The results in Fig. 6 indicated that at higher conversions, cyclization is more
preferred than linear products. The air-pretreated samples show a steeper slope (negative for linear
alcohols and positive in the case of cyclized products) compared to H2-pretreated samples,
indicating H2-treatment is an added advantage over air pretreatment to achieve a higher selectivity
to linear alcohols.
The dehydration capability of the catalysts was investigated using cyclohexanol
dehydration as a probe reaction. Cyclohexanol can undergo dehydration to cyclohexene over
Brønsted acid sites; on the other hand, basic sites - in this case, lattice oxygen ions - catalyze
dehydrogenation that lead to cyclohexanone [51]. Hence, the ratio of the products sheds light on
the acidic and basic sites present on these catalysts. The reaction data shown in Table 4 were
obtained using H2 and air-pretreated samples. In general, CeO2-based catalysts are not as selective
as other rare-earth oxides for the dehydration reaction [27]. In the current study, the ratio of
cyclohexene/cyclohexanone was below 0.5, which indicates that dehydrogenation is a preferred
reaction either on air-pretreated or H2-pretreated samples. The percentage cyclohexanol
conversion on CeO2-MnOx and CeO2-ZnO was higher than CeO2 alone under both pretreatment
conditions, while CeO2-MgO, CeO2-CaO, and CeO2-Na2O exhibited lower activity. These results
coincide with those from the dehydration of 1,5-pentanediol reaction.
On the other hand, CeO2 and CeO2-MgO catalysts produce dehydration products
equivalently (~30%), and far better than any samples tested for this study. The H2-pretreated
catalysts all show much less dehydration selectivity (<13%) compared to air-pretreated samples.
Overall, MnOx and ZnO accelerate cyclohexanol conversion, and this is probably linked to the
production of additional acid sites. The results also show that basic sites associated with lattice
17

=
oxygen are responsible for higher dehydrogenation selectivity (C6 /C6-one) on H2-pretreated
samples.
3.4 Relationship between acidity and dehydration activity and selectivity
It is generally accepted that the dehydration on heterogeneous catalysts requires both acidic
and basic sites [4]. In this respect, 1,5-pentanediol conversion, as shown in Fig. 7, increases with
increasing Brønsted acidity, irrespective of pretreatment conditions and the nature of the catalyst.
Similarly, H2-pretreated catalysts display a linear relationship between conversion of 1,5-PDO and
total acidity, indicating that both Brønsted and Lewis acidity contribute to the transformation of
1,5-PDO. However, air-pretreated samples show scattered points for the plot of Lewis acidity
versus dehydration activity. The added metal oxide might have interfered with the acidic function,
as well as the adsorption and desorption characteristics of 1,5-PDO on CeO2, that ultimately
determine catalyst activity. The air-pretreated catalysts show twice the amount of acid sites as
compared to H2-pretreated catalysts; indeed, acidity did not translate entirely to catalyst activity.
For example, CeO2-MnOx and CeO2-ZnO after air-pretreatment displayed Lewis acidity > 300
μmol/g, which is nearly twice that of H2-treated catalysts. Despite its lower acidity, H2-pretreated
CeO2-MnOx and CeO2-ZnO samples showed higher diol conversion compared to corresponding
air-pretreated samples. One possible reason is that the structure and location of Lewis acid sites
on the oxidized catalyst may be different from the H2-treated sample. Another reason could be the
lack of associated basic sites near Lewis acid sites on the oxidized surface, which will limit H
abstraction from the β-carbon required to form dehydration products. In both cases, acidity alone
cannot contribute to the conversion of 1,5-PDO. The cyclohexanol conversion on both air- and
H2-pretreated CeO2 and CeO2-MeOx catalysts show a linear correlation with Brønsted acid sites.
18

A similar relationship can be seen between Lewis acidity and cyclohexanol conversion (Fig. 8)
which implies that both B and L sites contribute to cyclohexanol conversion.
The ratio of linear-to-cyclized products obtained on both H2- and air-pretreated samples
for dehydration of 1,5-PDO decreases with increasing the number of B sites. However, the ratio
achieved a maximum Lewis acidity of 200 μmol/g and then dropped with further increases in the
number of L sites (Fig. 9). At the same time, air-pretreated samples showed a decreasing trend
over the entire range of Lewis acidity. Unlike 1,5-PDO, which involves a variety of reactions,
cyclohexanol undergoes either dehydration or dehydrogenation on ceria. Fig. 10 shows the
relationship between acidity and dehydration/dehydrogenation products ratio. Apart from CeO2
and CeO2-MgO, the dehydration/dehydrogenation ratio remains below 0.15 for all other samples,
indicating that they are highly selective towards dehydrogenation.
dehydration/dehydrogenation ratio decreases with increasing B and L sites.
The
H2-reduced ceria exhibits oxygen vacancies (i.e., redox sites) along with acid-base sites.
Several groups reported that dehydration that leads to the production of olefins from alcohol is
considered an acid-catalyzed reaction, while dehydrogenation is considered basic catalysis. Rare-
earth oxides like CeO2 contain oxygen vacancies or defect sites, providing an additional active
center for dehydration. The vacancy site on ceria is viewed as the primary reaction site for diol
[27]. Terminal diols like 1,5-pentenediol readily undergo cyclization over ceria producing
tetrahydropyran and cyclopentanol. In this study, we propose mechanisms for the formation of
different products from 1,5-pentanediol dehydration. Several factors, including geometry, the
combination of acid-base and redox sites, and their location, determine the mode of adsorption of
1,5-PDO. The sites available next to those oxygen vacancies (i.e., Lewis base or Brønsted acid)
probably guiding the reaction pathway (i.e., via β-H abstraction if Lewis base sites are involved or
19

via unimolecular H+ addition by Brønsted acid sites, as shown in Scheme 3a, 3b). Alternatively,
the adsorbed diol might undergo cyclization to tetrahydropyran as well (Scheme 3c).
The air-pretreated samples, particularly CeO2-MnOx and CeO2-ZnO, contain relatively
more acid sites compared to H2-pretreated samples, but alcohol conversion on air-pretreated
catalysts was rather low. It is believed to be due to differences in the oxygen vacancy densities
between air pretreated and H2-pretreated samples, because the former is less active than the latter.
Thus, we conclude that oxygen vacancies (redox sites) coupled with basic or acidic sites seem to
be responsible for the activity of dehydration of 1,5-PDO on CeO2-based catalysts. The product
selectivity to unsaturated or saturated linear alcohols decreases with increasing B and L sites,
independent of the pretreatment conditions. CeO2-Na2O and CeO2-MgO show higher unsaturated
alcohol selectivity than any other catalysts tested, probably due to complementary basic sites
provided by those metal oxides (i.e., without adding significantly more acidity to the catalysts).
20

Scheme 3. A plausible reaction mechanism, (a) Lewis base-assisted; (b) Brønsted acid-assisted;
and (c) Brønsted acid-assisted ring closure, for the formation of different products from
dehydration of 1,5-PDO on reduced CeO2 catalysts.
4. Conclusions
The dehydration of 1,5-pentanediol was investigated on CeO2 and MeOx-modified CeO2
catalysts after air and H2-pretreatments. The acidity measured by pyridine adsorption on oxidized
and reduced catalysts reveals that MnOx and ZnO increase the Brønsted and Lewis acidity of CeO2.
From a H2-TPR XANES/EXAFS study, the degree of ceria reduction was observed to be
influenced by the presence of the added metal oxide. The conversion of 1,5-PDO or cyclohexanol
increases with the addition of MnOx and ZnO on ceria while MgO, CaO, and Na2O suppressed the
overall alcohol conversion, but improved the selectivity to linear alcohols. The catalyst activity
increases linearly with increasing Brønsted and Lewis acidity, indicating that both sites take part
in the reaction. In contrast, the linear-to-cyclized product ratio in the case of 1,5-PDO, or
dehydration-to-dehydrogenation product ratio in the case of cyclohexanol, decreased with
increasing Brønsted acidity on H2 and air-pretreated catalysts. There must be an optimum density
of Lewis acid sites (160-200 mmol/g) required for CeO2-based catalysts to achieve higher
dehydration selectivity. The oxygen vacancies are primary adsorption sites on CeO2 for 1,5-PDO,
and further reaction of dehydration, dehydrocylization or dehydrogenation depends mainly on the
mode of interaction of alcohol and acid-base sites located within close vicinity to oxygen
vacancies.
CRediT author statement
21

M.K. Gnanamani: Conceptualization, Methodology, Investigation, Writing R. Garcia: Formal
analysis, Investigation G. Jacobs: Formal analysis, Data Curation, Validation, Review & Editing
K.Góra-Marek: Formal analysis, Investigation, Data curation D.C. Cronauer: Investigation,
Resources A.J. Kropf: Formal analysis, Data Curation, Investigation, Resources C.L. Marshall:
Investigation, Resources
Acknowledgment
The work carried out at the CAER was supported in part by funding from the Commonwealth of
Kentucky. Research conducted at UTSA was supported by UTSA, the State of Texas, and the
STARs program. Richard Garcia would like to acknowledge funding from a UTSA College of
Engineering Scholarship. His work was also supported by the USDA National Institute of Food
and Agriculture, Interdisciplinary Hands-on Research Traineeship and Extension Experiential
Learning in Bioenergy/Natural Resources/Economics/Rural project, U-GREAT (Undergraduate
Research, Education and Training) program (2016-67032-24984). Argonne’s research was
supported in part by the U.S. Department of Energy (DOE), Office of Fossil Energy, National
Energy Technology Laboratory (NETL). Advanced Photon Source was supported by the U.S.
Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No.
DE-AC02-06CH11357. MRCAT operations are supported by the Department of Energy and the
MRCAT member institutions.
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