Conversion of Isobutene and Formaldehyde to Diol using Praseodymium-Doped CeO2 Catalyst

Article abstract of DOI:10.1021/acscatal.6b02134

Conversion of low-carbon olefins to higher alcohols or olefins via the formation of C-C bonds is an increasingly important topic. We herein report an example of converting isobutene and formaldehyde (38 wt % aqueous solution) to 3-methyl-1,3-butanediol (MBD), a precursor for isoprene. The reaction occurs through a Prins condensation-hydrolysis reaction over a praseodymium (Pr)-doped CeO₂ catalyst. The best MBD yield (70%) is achieved over the Pr-doped CeO₂ catalyst. Catalyst characterizations with high-angle annular dark field transmission electron microscopy (HAADF-TEM), pyridine adsorption infrared (IR) and Raman spectroscopy, and density functional theory (DFT) calculations show that the doped Pr is uniformly and highly dispersed in the CeO₂ crystalline phase. In addition, the Pr doping creates more oxygen vacancy sites on CeO₂ and thus enhances the Lewis acidity of the catalyst, which is responsible for the catalytic performance of the Pr-CeO₂ catalyst. (Chemical Equation Presented).

Full text of DOI:10.1021/acscatal.6b02134

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Article
Conversion of Isobutene and Formaldehyde to
Diol Using Praseodymium-doped CeO2 Catalyst
Zhixin Zhang, Yehong Wang, Jianmin Lu, Chaofeng
Zhang, Min Wang, Mingrun Li, Xuebin Liu, and Feng Wang
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02134 • Publication Date (Web): 01 Nov 2016
Downloaded from http://pubs.acs.org on November 4, 2016
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Conversion of Isobutene and Formaldehyde to Diol Using
Praseodymium-Doped CeO Catalyst
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a,b
a
a
a
a
Zhixin Zhang, Yehong Wang, Jianmin Lu, Chaofeng Zhang, Min Wang,
a
c
a
Mingrun Li, Xuebin Liu, and Feng Wang*
a
State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy,
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P. R.
China.
b
State Key Laboratory of Fine Chemicals, Faculty of Chemical Environmental and
Biological Science and Technology, Dalian University of Technology, Dalian 116024, P.
R. China
c
Energy Innovation Laboratory, BP Office (Dalian Institute of Chemical Physics), Dalian
116023, P.R. China
*
Corresponding author. Tel: +86-411-84379762; Fax: +86-411-84379798; E-mail:
wangfeng@dicp.ac.cn
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ABSTRACT: Conversion of low-carbon olefins to higher ones via the formation of C–C
bond is an increasingly concerned topic. We herein report an example of converting
isobutene and formaldehyde (aqueous solution of 38 wt%) to 3-methyl-1,3-butanediol
(MBD), a precursor for isoprene. The reaction occurs through a Prins
condensation-hydrolysis reaction over a praseodymium (Pr)-doped CeO catalyst. The
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best 70% MBD yield is achieved over the Pr-doped CeO catalyst. Catalyst
2
characterizations with high-angle annular dark field-transmission electron microscopy
(HAADF-TEM), pyridine adsorption infrared (IR), Raman spectroscopy, and density
functional theory (DFT) calculations show that the doped Pr is uniformly and highly
dispersed in the CeO crystalline phase. Besides the Pr-doping creates more oxygen
2
vacancy sites on CeO and thus enhances the Lewis acidity of the catalyst, which is
2
responsible for the catalytic performance of the Pr-CeO catalyst.
2
Keywords: CeO · metal-doped · Prins condensation · hydrolysis · Lewis acid · oxygen
2
vacancy
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Introduction
Alkanediols are important chemical intermediates for polymers and surfactants.
Dehydration of alkanediols, such as 3-methyl-1,3-butanediol (MBD), produces isoprene
1
,2
3,4
5
-7
for synthesizing rubber. Schneider et al. ever studied the selective hydroxylation of
isoamyl alcohol using -nitroperbenzoic acid as catalyst and obtained 5% yield of MBD
p
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as a minor product. MBD can also be synthesized by Prins condensation of isobutene
and formaldehyde, but it was only obtained as byproduct with the yield of < 10% using an
9
FeMCM-41 catalyst. Up to now, no study targeting at MBD as the major product has
ever been successfully achieved from isobutene and formaldehyde. This require a
catalyst which contains water-tolerant acidic sites and the acidity should be suitably
strong enough to activate isobutene and formaldehyde and also avoid catalyze their
polymerization.
In aspiring for sustainable route for petroleum-based chemicals, it is interesting to
extend the biomass product chain from C –C alcohols to more functionalized alcohols,
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2
1
0-13
such as C + alcohols. For example, isobutene can be obtained from bioethanol
via
19,20
4
14-18
sugar platforms.
Formaldehyde is industrially produced by oxidation of methanol.
Therefore, the success of converting isobutene and formaldehyde to MBD can fill the
gap between sustainable biomass and valuable chemicals (Scheme 1).
Gas
Dehydration
This work
Biomass
Coal
CH
3
OH
HCHO
Prins
Condensation
+
Chemical intermediates for
polymers and surfactants
Oil
Scheme 1. MBD production based on the biomass.
The Prins condensation of olefins and their derivatives with aldehydes can produce
alkyl- -dioxanes and alkanediols, and hydrolysis of dioxanes over acid catalysts can
also give alkanediols.
m
6,21,22
Thus, the Prins condensation forming a new C–C bond is a
subject of study. Prins condensation reactions can be catalyzed by homogeneous
21
catalysts, such as mineral acids (e.g. H SO ) or Lewis acids (e.g. SnCl ). In recent
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4
4
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years, heterogeneous acid catalysts (such as zeolites,
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sulfates,
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phosphates,
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heteropolyacids , and oxides ) have been pursued because of the
reusability and operational simplicity. Recent studies by ours and others further show
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that the pristine CeO can efficiently catalyze various organic reactions.
We find
2
that the pristine CeO possesses weak and water-tolerant Lewis acidity (54 ꢀmol/g).
2
This nature makes it catalytically active in one-pot condensation-hydrolysis reaction
even under hydrothermal reaction conditions, such as the synthesis of 1,3-butanediol
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from propene and formaldehyde. But the pristine CeO was inefficient in synthesizing
2
MBD from isobutene and formaldehyde (merely 21% yield). Hegde, Nolan, Luo, and
Fornasiero et al. report that the addition of metal dopants into the CeO lattice
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4
4-56
generates oxygen vacancies in CeO2.
Recently, the group led by Metiu has reported
a series of works on theoretical calculations of CeO . Their results reveal that the
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metal-doping can increase the acidity or basicity of CeO2.
But it is experimentally
unknown how the metal-doping modifies the acid-base property of CeO and which
2
metal is suitable for doping.
Multivalent cation-doped CeO materials have been considered as outstanding
2
5
9
60-63
catalysts for CO oxidation or soot oxidation.
Among the various multivalent
dopants, Pr is expected to be the most promising for dissolution into the host CeO to
2
form a solid solution, due to its similar ionic radius to the Ce ions and the same fluorite
3
+/4+
structure of its oxide PrO as CeO . Doping with multivalent Pr (Pr ) in CeO -based
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6
4
materials is known to enhance the formation and migration of oxygen vacancies. And
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the oxygen vacancies can be generated and increased gradually by the addition of
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multivalent Pr ions up to 50 mol%. In the multivalent Pr-doped CeO material, Pr ions
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play an important role to create oxygen vacancies and induce a local distortion, which
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4
+
will improve the degree of unsaturated coordination of Ce ions, and Pr can contribute
to diminishing reduction energy via the formation of an additional redox couple
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+/4+ 65
(
Pr ).
In this study we focus on this topic and employ the condensation-hydrolysis of
isobutene and formaldehyde to MBD as a test reaction. We discovered that the
Pr-doped CeO shows excellent activity and selectivity toward MBD synthesis.
2
Combined catalyst characterizations suggest that the metal-doping created more
oxygen vacancies on CeO , and its concentration is proportional to the catalytically
2
active acidic sites required for the isobutene-formaldehyde condensation reaction.
Particularly the ionic radius of the metal dopants has distinctive influence on the density
of observed oxygen vacancies and furthermore on catalytic activity. Catalytic reaction
over the Pr-doped CeO offers 55% yield of MBD, which is 2.6 times higher than that of
2
the undoped CeO , and to the best of our knowledge, is the best result reported up to
2
now.
Results and Discussion
XRD and TEM Characterization of the Metal-doped CeO2
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Al-CeO2
Zr-CeO2
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Eu-CeO2
Pr-CeO2
La-CeO2
JCPDS 04-0593
CeO2
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60
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2θ (degree)
Figure 1. XRD patterns of the doped M-CeO (M = Al, Zr, Eu, Pr, or La)
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The CeO and doped CeO catalysts were obtained with a precipitation and
2
2
co-precipitation approach, respectively. The molar ratio of Ce to the doped metal was
set to 4:1. For comparison, we selected some other doped metal, including Al, Zr, Eu,
3
+
and La, based on their radii, valences, and properties. The ion radii of Al (50 pm) and
4
+
3+/4+
Zr (80 pm) are smaller than that of Ce , and they have different valences. The ion
3+
4+
3+
radius of Eu (95 pm) is larger than that of Ce (92 pm) and smaller than that of Ce
3
+
3+/4+
(
103.4 pm). And the ion radius of La (106.1 pm) is larger than that of Ce . The
4
+
3+
3+
3+
substitution of Ce by the low-valence dopant (such as Al , Eu , and La ) generates
58
oxygen vacancies via the charge neutrality criterion. Although the fact that
4
+
4+
substitution of Ce by the isovalent Zr is limited in terms of self-generation of oxygen
vacancies for “charge compensation”, the oxygen vacancies can also be generated by
large local relaxation in the crystalline lattice. And Zr-doped CeO is well known for
2
5
4,66
various applications due to its high oxygen storage capacity.
Doping of CeO with
2
rare-earth elements (RE), such as Eu and La, has been widely applied in the field of
60,67
catalysis, due to its unusual properties induced by oxygen vacancy defects.
For
brevity, metal-doped CeO catalysts were denoted as M-CeO (M = Al, Zr, Eu, Pr, or
2
2
La), respectively. XRD shows all samples were indexed to be (111), (200), (220), (311),
222), (400), (331), and (420) crystal facets of the fluorite structure of CeO (JCPDS
(
2
0
4-0593) (Figure 1). No doped metal oxide phase was found, indicating the doped
species were highly dispersed in CeO . A slight peak shift of 2θ toward higher angles
2
4+
was observed on Al- and Zr-CeO . The shift was probably due to the occupancy of Ce
2
3
+
4+
(
92 pm) sites by the smaller Al (50 pm) and Zr (80 pm) ions, causing lattice
68
contraction. In contrast, the 2θ shift to smaller angles when CeO was doped with the
2
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larger metal ions Eu (95 pm), Pr (101.3 pm) and La (106.1 pm) indicates lattice
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expansion.
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Figure 2. HAADF-TEM EDS elemental mapping images of the Pr-CeO catalyst. The HAADF-TEM
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image (a), EDS elemental mapping images representing the Pr (b) and Ce (c).
Figure S1 shows the SEM, TEM and EDS elemental mapping results of the doped
CeO . Pr-CeO has a rod-like morphology as shown in TEM images (Figure S1f).
2
2
HAADF-TEM EDS elemental mappings (Figure 2) show that Pr is uniformly and highly
dispersed in CeO . EDS analysis by randomly chosen eight zones indicated the Ce/Pr
2
atomic ratios were 4.33 ± 0.04 (Figure S1g). The slight higher Ce/Pr ratio than the
added value of 4 may indicate the little enriching of Ce and Pr infusion into CeO lattice.
2
The HRTEM images shows the Pr-CeO is well crystallized and consists of crystallites
2
of CeO or PrO (Figure S2b). The selected area electron diffraction (SAED) pattern
2
2
also indicates the as-prepared Pr-CeO is polycrystalline and the diffraction rings were
2
attributed to (111), (200), (311), (222), (400), and (331), respectively, which is in
agreement with the XRD results (Figure S2c). Thus, no amorphous PrO could be found
x
in the Pr-CeO . Furthermore, similar results were observed on Al, Zr, Eu, and
2
La-doped CeO (Figure S1 and S5).
2
Prins Condensation-hydrolysis Reactivity of the M-CeO Catalysts
2
Table 1: Prins condensation-hydrolysis of isobutene with formalin over CeO and M-CeO catalysts
2
2
(
M = Al, Zr, Eu, Pr, or La)
Formaldehyde
Conv. (%)
Sel. (%)
GC yield (%)
Entry
Catalyst
MBD + DMD
MBD
DMD
1
2
CeO2
Al-CeO2
27
35
> 99
> 99
21
30
6
5
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La-CeO2
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Reaction conditions: 50 mg catalyst, 1.5 mL H O, 0.21 mL formalin (38 wt% HCHO), 3.0 g
isobutene, 120 °C, 2 h. At 135 °C, 3 h.
2
a
Catalytic results showed that the reaction of isobutene and formaldehyde gave
3
4
-methyl-1,3-butanediol (MBD) as the major product, together with minor
,4-dimethyl- -dioxane (DMD) (Table 1). These two products may be obtained via a
m
7
0
parallel mechanism reported by Dumitriu et al. The mechanism starts from the
interaction of isobutene and HCHO to create a C5 hydroxyl-carbocation, which can
react with water or a second HCHO molecule to form MBD or DMD, respectively. The
43
reaction was conducted in water-phase. Thus, hydrolysis of DMD can also give MBD.
71
Furthermore, the hydrolysis reaction is reversible. The overall selectivity of (MBD +
DMD) was >99%. No isoprene was observed among products, which can be obtained by
o
70
the dehydration of MBD in vapor-phase at high temperatures (usually >250 C).
Pristine CeO gave a relatively low formaldehyde (FA) conversion (27%) (Table 1, entry
2
1
). All dopants increased the FA conversion in the order of Al-CeO < Zr-CeO <
2 2
Eu-CeO < La-CeO < Pr-CeO . The Pr-CeO was the best catalyst with a 67%
2
2
2
2
conversion and 66% yield of (MBD + DMD) (Table1, entry 5). Further experiments at
1
35 °C using Pr-CeO catalyst offered 93% yield of (MBD + DMD) in 3 h (Table1, entry
2
6
). After reaction, Pr-CeO was recovered by centrifugation, dried, and then used for a
2
subsequent cycle. The catalyst retained its activity during its fourth run (Figure S6).
These results indicated Pr-CeO was an effective and stable heterogeneous catalyst in
2
this reaction.
Correlation between Catalytic Activity Variation and Metal Dopants
2
1
Prins condensation-hydrolysis is an acid-catalyzed reaction. The acidity of M-CeO
2
catalysts is a critical factor for the reaction. Pyridine-adsorption IR (Figure S7) shows
no Brønsted acidic sites were present on the doped CeO . The presence of the strong
2
–1
band at 1440 cm , which was attributed to the coordinatively bound pyridine on Lewis
7
2-74
acidic sites,
indicated the M-CeO oxides contain Lewis acidic sites. Pyridine-IR
2
showed the metal doping remarkably increased the Lewis acidity (Table 2). Particularly
the Lewis acidity of Pr-CeO increased by 3.5 times compared with the pristine CeO .
2
2
7
5
In literatures, Sn-doping increased CeO acidity by 0.4–0.6; Zr-doping increased
2
76
CeO acidity by 0.21–0.57; and Al-doping increased Ce-Zr mixed oxides acidity by
2
7
7
0.15–0.85. All these metals increase both Brønsted and Lewis acidity of CeO2.
However, it is rarely known that the Pr-doping only increases Lewis acidity.
Table 2: The concentration of Lewis acidic sites of CeO and M-CeO (M = Al, Zr, Eu, Pr, or La)
2
2
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a
–1
Entry
Catalyst
CL (µmol g )
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2
3
4
5
6
CeO2
Al-CeO2
Zr-CeO2
Eu-CeO2
Pr-CeO2
La-CeO2
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82
74
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a
CL means the concentration of Lewis acidic sites and is calculated according to the Formula 1 (See
Experimental Section).
4
3
The oxygen vacancies of CeO were related to its Lewis acidity and were
2
7
8
quantified by Raman spectroscopy. Two major Raman peaks at about 460 and 530–600
–1
cm are ascribed to the F symmetrical stretching vibration mode of metal-doped CeO
2
g
2
5
1,67,79,80
in a fluorite structure and oxygen vacancies, respectively（Figure 3）.
Different
metal dopant results in different peak position. The assignments of Raman peaks are
–1
summarized in Table 3. The bands of F mode around 463 cm in the pristine CeO ,
2
g
2
Al-CeO , and Zr-CeO samples are almost same. However, for the Eu-CeO , La-CeO ,
2
2
2
2
–1
and Pr-CeO , this F mode band is around 457 cm , which shifts to the lower frequency
2
2g
3+
3+
3+
because of the larger ionic radius of metal ions (Eu 95 pm, La 106.1 pm, and Pr
4
+
1
01.3 pm) than that of Ce (92 pm). This observation agrees with the XRD results. The
–
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bands ascribed to the oxygen vacancies are located at 600 cm for the pristine CeO ,
2
82,83
Al-CeO , and Zr-CeO samples.
And the oxygen vacancies induced band are
2
2
–
1
84
67
65
situated at 537, 550, and 570 cm for the Eu-CeO , La-CeO , and Pr-CeO ,
2
2
2
–
1
respectively. Furthermore, two bands at about 640 and 694 cm appearing on the
spectra of Eu-CeO sample is closely correlated with crystalline Eu O , because of the
2
2
3
8
5
europium (Eu) segregation on the Eu-CeO surface.
2
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Al-CeO2
Zr-CeO2
Eu-CeO2
La-CeO2
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CeO2
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00
400
500
600
700
800
900
1000
-1
Raman shift (cm )
Figure 3. Raman spectra of CeO and M-CeO (M = Al, Zr, Eu, La, or Pr)
2
2
Table 3: Raman peaks assignment and the observed oxygen vacancy concentration of CeO and
M-CeO (M = Al, Zr, Eu, La, or Pr).
2
2
F mode frequency
Oxygen vacancy
Cov
2
g
Entry
Catalyst
–
1
–
1
a
(
cm )
463
462
466
459
453
457
frequency (cm )
A(V
Ӧ
)/A(F )
2
g
1
2
3
4
5
6
CeO2
600
600
600
537
550
570
0.009
0.015
0.021
0.166
0.215
2.601
Al-CeO2
Zr-CeO2
Eu-CeO2
La-CeO2
Pr-CeO2
a
Ӧ
2g 2g
A(V )/A(F ) means the peak area of F mode be divided by the peak area of oxygen vacancy,
representing the observed oxygen vacancy concentration.
The observed oxygen vacancy concentration (Cov) was calculated based on the
Ӧ
integrated peak area (A) of the bands of oxygen vacancy (V ) and F mode. The ratio of
2g
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A(V
Ӧ
)/A(F ) indicates the Cov (Table 3).
The Al and Zr do not increase the Cov value
2
g
obviously, while the Eu-, La-, and Pr-doping remarkably increase the Cov value. Among
the five samples, Pr-CeO has the highest Cov value, 289 times than that of pristine
2
CeO2.
3
+
4+
When Pr is doped into the CeO , Pr and Pr ions are incorporated as majority and
2
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3+
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minority ions, respectively. And doping with Pr generates the oxygen vacancies for
charge compensation”. And the oxygen vacancies could survive under the reaction
“
4
3
environments, as proved by our previous work , in which the Lewis acidic sites induced
by oxygen vacancy over CeO surface is water-tolerate and can catalyze the Prins
2
condensation-hydrolysis of propene with formalin. But the other dopants seem to be not
good at introducing the oxygen vacancies. Under the same doping level and synthesis
conditions, for the Al and Zr-CeO , the dopant may be incorporated as interstitial ions
2
rather than incorporated into the lattice of CeO due to their small radii compared to
2
87
their host cations. Thus, the Cov values increase un-obviously. The concentration of
oxygen vacancies over Eu-CeO sample increases but cannot compare with that of
2
Pr-CeO , due to the aggregation of Eu observed by Raman spectra. For the La-CeO2
2
sample, the Cov value also increase but less than the value of Pr-CeO , the possible
2
3
+/4+
reason is the larger radius of La than that of Ce
.
Figure 4. Catalytic activity for Prins condensation-hydrolysis of isobutene and formalin over
M-CeO and its relation to the ionic radius of the dopant and the concentration of Lewis acidic site
2
(
unit: mmol/g) and oxygen vacancy.
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A plot of the conversion of FA, the CL, and the Cov against the dopant ionic radius
is also shown in Figure 4. The concentration of Lewis acidic sites (CL) and the
conversion of FA against the doped metal ionic radius had a similar relationship,
indicating that CL is vital for the condensation-hydrolysis of isobutene with formalin and
the value can be modulated by different metal doping. The Cov has a similar trend as CL
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The consistency apparently reveal that the observed oxygen vacancy of the doped CeO
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32,33
is related to its Lewis acidity. Based on our previous studies
and others,
we
propose that the coordinatively unsaturated Ce cations next to the oxygen vacancies
function as Lewis acidic sites for the Prins condensation-hydrolysis reaction.
DFT Calculations
For the doped oxides, it comes with a question that what is the nature of catalytic
active sites: the doped phase or the parent oxide phase? It is rather difficult to
experimentally tackle the issue. Density functional theory (DFT) calculations may shed
light on this issue. Formaldehyde adsorption and activation is a key step for the
88,89
condensation.
We constructed several adsorption models of HCHO on CeO (111)
2
and Pr-CeO (111) with one oxygen vacancy, respectively and calculated the adsorption
2
9
0,91
energy of HCHO. In some studies,
solvation could improve the stability of transition
states and affect the calculated adsorption energy of substrates. In the present study,
when we compared the calculated adsorption energy of HCHO on CeO and Pr-CeO ,
2
2
the effect should exhibit the similar trend for these two catalysts and the net difference
value between these two adsorption models should be constant. As shown in Figure 5,
HCHO is adsorbed on the (111) crystal face of CeO with one oxygen vacancy, which is
2
optimized on the Lewis acid-base pair of Ce–O (Figure 5a) next to the oxygen vacancy
with an adsorption energy of -1.39 eV. When the Pr is doped into the lattice of CeO2,
the calculated adsorption energy of HCHO on Pr-CeO on the same adsorption site is
2
-1.60 eV, which is 0.21 eV lower than that on the CeO surface and is found to be the
2
lowest state. In comparison, the adsorption energy of HCHO on Pr–O site of Pr-CeO is
2
-
1.33 eV, which is close to that on the CeO surface. Next, the bond length of carbonyl
2
group in HCHO adsorbed on Pr-CeO is also slightly larger (1.391 Å) than the value of
2
the HCHO (1.370 Å) adsorbed on CeO due to the different interaction between the
2
carbonyl group in HCHO and the Lewis acid-base pair (Ce–O). These two values are
larger than that of the isolated HCHO (1.215 Å), also indicating the CeO -based
2
material can be a good catalyst of the activation of HCHO. These DFT calculation
results demonstrate an obvious promoting effect of the highly dispersed Pr dopant on
CeO surface for the adsorption and activation of HCHO.
2
It is known that the presence of oxygen vacancies can promote the interaction of
9
2
aldehydes with oxide surfaces via the C=O group. We have also considered the
adsorption model of HCHO at oxygen vacancy sites of CeO and Pr-doped CeO2
2
catalyst and calculated the adsorption energies of HCHO. The adsorption energies are
-
1.13 eV and -0.85 eV, respectively, which are weaker than that of HCHO on the Ce–
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O sites next to the oxygen vacancies. Thus, the adsorption model of HCHO on the Ce–
O sites is optimized. Therefore, the increased quantity of oxygen vacancies enhances
the interaction (activation) of HCHO with oxide surface via the effect on the Ce–O sites
next to the oxygen vacancies, and thus the reactivity with isobutene.
Prins condensation of olefin with HCHO is a bimolecular reaction. In heterogeneous
catalysis, two main reaction mechanisms proposed are the Langmuir-Hinshelwood (LH)
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mechanism and the Rideal-Eley (RE) mechanism.
For the vast majority of surface
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catalytic reactions, the LH mechanism is preferred. In this study, the adsorption of
isobutene was also considered. As shown in Figure 6, isobutene is adsorbed on the
oxygen vacancy over CeO and Pr-CeO with an adsorption energy with -0.72 and
2
2
-0.28 eV, respectively. The adsorption of isobutene is 0.67 eV weaker than that of
HCHO on CeO , and 1.32 eV weaker on the Pr-CeO surface. This reveals that the
2
2
Prins condensation may proceed via the RE mechanism in the catalytic system. Similar
calculation results were obtained by Fu et al., who found the reaction between HCHO
8
9
and propylene over MgY zeolite proceeded via the R-E mechanism. Thus, the
adsorption and activation of HCHO is the vital step for the Prins reaction.
Figure 5. Summary of adsorption models (top views) of HCHO on CeO (111) and Pr-CeO (111).
2
2
Adsorption energies (eV) and bond length (Å) are labeled in the figure. Atom colors: H = white; C =
black; O = red; Ce = pale yellow; Pr = green. (a) HCHO adsorbed on O-Ce site over CeO . (b)
2
HCHO adsorbed on O-Ce site over Pr-CeO . (c) HCHO adsorbed on O-Pr site over Pr-CeO .
2
2
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Figure 6. Summary of adsorption models of isobutene on CeO (111) (top view) and Pr-CeO (111)
2
2
(
top and sided views). Adsorption energies (eV) are labeled in the figure. Atom colors: H = white; C
= black; O = red; Ce = pale yellow; Pr = green. (a) Isobutene adsorbed on oxygen vacancy over CeO2.
b) HCHO adsorbed on oxygen vacancy over Pr-CeO2.
(
Conclusions
In summary, we have demonstrated that Pr-doped CeO acts as a reusable and efficient
2
heterogeneous catalyst for the 3-methyl-1,3-butanediol synthesis from isobutene and
formalin via the Prins condensation-hydrolysis reaction. This route is both scientifically
and industrially meaningful for synthesizing higher and complex alcohols from
sustainable biomass. HAADF-TEM, pyridine adsorption IR, and Raman spectroscopy
indicate that the superior catalytic activity can be ascribed to the enhanced Lewis
acidity of catalyst induced by the more oxygen vacancy sites via the Pr-doping. DFT
calculations show that the adsorption and activation of HCHO is the vital step for the
Prins reaction and the Pr dopant on the CeO surface promotes the adsorption and
2
activation of HCHO.
Experimental Section
Chemicals and Reagents
All chemicals were of analytical grade, purchased from Aladdin Chemicals, and used
without further purification.
Preparation of CeO and the Metal-doped CeO Catalysts
2
2
4
3
The CeO sample was prepared by a conventional precipitation method. Briefly, 5.0 g
2
of Ce(NO ) ·6H O was dissolved in 100 mL of Millipore-purified water (18 mΩ·cm), and
3
3
2
the solution was adjusted to pH = 11.0 by the addition of NH OH (3.4 M) under
4
magnetic stirring at room temperature. The resulting gel mixture was washed with pure
–
water, dried in an oven at 120 °C for 12 h, and calcined at 550 °C in air (50 mL min
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1
)
for 4 h.
The metal-doped CeO catalysts (Al, Zr, Eu, La, or Pr-doped CeO ) were prepared
2
2
by a co-precipitation method. Ce(NO ) ·6H O and the corresponding metal nitrates
3
3
2
(
Al(NO ) ·9H O, ZrO(NO ) ·6H O, Eu(NO ) ·6H O, La(NO ) ·6H O, or Pr(NO ) ·6H O)
3 3 2 3 2 2 3 3 2 3 3 2 3 3 2
were used as precursors. Ce(NO ) ·6H O and doped-metal nitrate were dissolved by
3
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2
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deionized H O. The ammonia aqueous was added drop-wise to the above solutions
2
under stirring conditions. Consequently, the mixture was stirred for 2 h. Then the
mixture was filtered and dried at 120
°C overnight, and then calcinated at 550 °C in
–1
air (50 mL min ) for 4 h to obtain the final materials. The molar ratio of Ce to added
metal was about 4:1. For brevity, metal-doped CeO catalysts were denoted as M-CeO2
2
(M = Al, Zr, Eu, La, or Pr), respectively.
Catalyst Phase Analysis by Powder X-ray Diffraction
Powder X-ray diffraction patterns are conducted on a PANalytical X-Pert PRO
diffractometer, using Cu-K radiation at 40 kV and 20 mA. Continuous scans are
α
o
collected in the 2θ range of 5 ~ 80 .
Prins Condensation-hydrolysis Reaction
Formaldehyde (38% formalin, 0.21 mL, 3.0 mmol of formaldehyde), catalyst (50 mg),
water (1.5 mL), and a magnetic stir bar were loaded into a 15 mL Teflon-lined autoclave
reactor. The reactor was then sealed and placed in a preheated red copper mantle at the
desired temperature. Isobutene (99.99%) was supplied from a cylinder and charged into
the reactor and its amount was based on the difference of weight. Products were
analyzed by gas chromatography−mass spectrometry (GC−MS) using an Agilent
7890A/5975C instrument equipped with an HP-5MS column. The formaldehyde solution
was analyzed by a GC instrument equipped with a packed column and a TCD detector.
An external standard method was used to quantify the formaldehyde conversion.
Acidity Measurement by Pyridine-IR Spectra
Quantification of acidity by the pyridine-adsorption IR method was conducted on a
Bruker 70 IR spectrometer. The sample was pressed into a self-supporting disk (13 mm
diameter, 21.4‒38.3 mg) and placed into a homemade IR cell attached to a closed
glass-circulation system. Prior to pyridine-adsorption, the sample disk was pretreated
‒
2
by heating at 350
mbar) and then cooled to 30
exposed to pyridine vapor. IR spectra of the chemisorbed pyridine were recorded after
evacuation at 150 C for 30 min to eliminate physically adsorbed pyridine. Spectra
were collected after cooling to 30 C. The calculation of the number of Lewis acidic
sites was based on the following formula:
ꢁ = 1.42 × IA × ꢂ ∕ ꢄ (Formula 1)
°
C for 30 min under Ar and for 30 min in vacuum (pressure < 10
°
C. After a spectrum was collected, the sample disk was
°
°
96
ꢃ
ꢀ
‒
1
Where
the L band (cm ),
mg).
Raman Characterization
C is the concentration [mmol (g of catalyst) ], IA is the integrated absorbance of
‒1
R
is the radius of the catalyst disk (cm), and W is the mass of the disk
(
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Raman spectra were recorded on a micro-Raman spectrometer (Renishaw) equipped with
a CCD detector using a He/Ne laser with a wavelength of 532 nm.
DFT Calculation Settings
The periodic plane wave based density functional theory program VASP (Vienna ab initio
9
7,98
simulation package)
electron-ion interactions are described by the projector-augmented wave method
PAW), which is a frozen core all-electron method using the exact shape of the valence
has been employed to perform all the DFT calculations. The
0
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wave functions instead of pseudo-wave functions. The exchange correlation energy has
been calculated within the generalized gradient approximation by the
1
00,101
Perdew-Burke-Ernzerhof formulation (GGA-PBE).
plane-wave basis sets was fixed to 500 eV in all calculations. 2s 2p electrons of O,
The kinetic energy cutoff of
2
4
2
6
2
6
3
2
5
s 5p 4f5d6s of Ce and 5p 4f 6s of Pr are explicitly taken into consideration. We used
102
103
DFT+U corrections to describe the CeO with the value of U-J = 4.5 eV for Ce 4f
2
104
orbitals and U-J = 6.0 eV for Pr 4f orbitals.
This level of theory computes the lattice constant of a = 5.495 Å for bulk CeO , in
2
1
05
reasonable agreement with the experimental value (a = 5.411 Å). This lattice constant
has been used to construct a periodic CeO (111) (3 × 3) slab with 9 atomic layers
which are equal to 3 stoichiometric layers) separated by a vacuum layer of 15 Å to
p
2
(
eliminate interactions between the slab and its periodic images. In total, there are 27 Ce
atoms and 54 O atoms in the slab. The bottom 4 atomic layers are fixed to their
optimized bulk configuration during all computations, and the top 5 atomic layers and
surface intermediates are fully relaxed. All atomic coordinates of the adsorbates and the
atoms in the relaxed layers are optimized to a force of < 0.03 eV/Å on each atom. All
−5
self-consistent field calculations are converged to 1 × 10 kJ/mol. Brillouin zone
integration is performed using a 3 × 4 × 1 Monkhorst−Pack grid and a Gaussian
smearing of 0.05 eV.
ASSOCIATED CONTENT
Supporting Information Available: Description of TEM-EDX and HAADF-TEM
characterization, Catalyst recycling tests, and pyridine adsorption IR spectra of
M-CeO . This material is available free of charge via the Internet at
2
http://pubs.acs.org.
Acknowledgements
This work was supported by the National Natural Science Foundation of China
(21403216, 21233008 and 21303189) and by Strategic Priority Research Program of
Chinese Academy of Sciences (XDB17020300). Computing resources from National
Supercomputing Center in Shenzhen, China and National Supercomputing Center in
Tianjin, China are gratefully acknowledged.
References
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