Phosphate Functionalization of CeO2–ZrO2 Solid Solutions for the Catalytic Formation of Dimethyl Carbonate from Methanol and Carbon Dioxide

Article abstract of DOI:10.1002/cctc.201701105

Phosphate surface groups on CeO₂–ZrO₂ solid solutions were generated by treating Ce–Zr–hydroxide precursors with phosphoric acid. In the catalytic formation of dimethyl carbonate (DMC) from methanol and CO₂, the performance of the P-modified samples was markedly affected relative to that of the unmodified ones: Phosphate treatment caused remarkable changes in the phase composition, acid–base properties, and the ability to form monodentate methoxy intermediates. The DMC yield (1.6 %) was successfully improved from 0.24 to 1.6 % by phosphate modification of CeO₂–ZrO₂ (Ce/Zr=4.7, P/Zr=0.13) and by performing the reaction at 170 °C and 6.5 MPa for 1 h.

Full text of DOI:10.1002/cctc.201701105

Accepted Article
Title: Phosphate functionalization of CeO2-ZrO2 solid solutions for
the catalytic formation of dimethyl carbonate from methanol and
carbon dioxide
Authors: Iuliia Prymak, Oleg Prymak, Junhui Wang, Venkata Narayana
Kalevaru, Andreas Martin, Ursula Bentrup, and Wohlrab
Sebastian
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ChemCatChem
10.1002/cctc.201701105
COMMUNICATION
Phosphate functionalization of CeO -ZrO solid solutions for the
2
2
catalytic formation of dimethyl carbonate from methanol and
carbon dioxide
[
a]
[b]
[a,c]
[a]
[a]
Iuliia Prymak, Oleg Prymak, Junhui Wang, Venkata Narayana Kalevaru, Andreas Martin,
*
[a]
*[a]
Ursula Bentrup and Sebastian Wohlrab
Abstract: Phosphate surface groups on CeO
2
-ZrO
2
solid solutions
dialkyl carbonates from CO
2
, a balance of basic and acidic
[
9]
were generated by treating Ce-Zr-hydroxide precursors with
phosphoric acid. In the catalytic formation of dimethyl carbonate
centers on the catalyst surface is needed. Moreover, Ikeda et
al. have performed surface modifications of ZrO with
catalysts exhibited
remarkably improved catalytic activities for DMC formation, in
particular at lower temperatures. It was found that both, ZrO
2
[10]
from methanol and CO
2
the performance of P-modified samples is
phosphoric acid.
These modified ZrO
2
markedly affected compared to unmodified ones: The phosphate
treatment has caused remarkable changes in phase composition,
acid-base properties and the ability to form monodentate methoxy
intermediates. The DMC yield (1.6%) was successfully improved
2
4+
and ZrO
2
/H
3
PO
with P/Zr = 0.05 has additional weak Brønsted acid
The authors claimed that Brønsted acid sites can
4
, have Lewis acid sites (Zr ), whereas only
ZrO /H PO
2
3
4
[
11]
from 0.24% to 1.6% by phosphate modification of CeO
2
-ZrO
2
(Ce/Zr
sites.
=
4.7, P/Zr = 0.13) running the reaction at 170 °C and 65 bar for 1 h.
contribute to MeOH activation and enhances the catalytic activity.
Inspired by these results, we extended this approach to
CeO
conversion of carbon dioxide with methanol over several CeO
ZrO catalysts (Table SI1, maximal DMC yields ∼ 0.24%), which
were in good agreement with previous findings,
ZrO solid solution catalyst with a theoretical Ce/Zr ratio of 4
C80Z) was chosen and used in subsequent runs especially for
modification with H PO to boost the yield of DMC beyond
.24%. The promoting effect of H PO was then investigated by
varying the P-contents in C80Z with the aim to identify efficient
catalyst compositions for achieving enhanced DMC yields.
Furthermore, the key catalyst properties should be explored
which play a pivotal role for an improved catalytic performance.
In Table 1 the elemental ratios, surface areas, and DMC
yields of the unmodified C80Z and three P-modified C80Z
2 2
-ZrO catalysts. Based on initial tests of the catalytic
Alternative routes towards “green” dimethyl carbonate (DMC)
are intensively under study due to the unique application
2
-
2
[
1]
potential of this compound: DMC can be used as a solvent, as
[5a, 12]
2
the CeO -
[
2]
[3]
raw material for polycarbonate resins, electrolyte solvent, or
2
[
4]
in pharmacy. Among different possibilities for DMC synthesis,
the most desirable, environmentally attractive and direct route is
(
3
4
from CO
2
and methanol. However, the reaction is
0
3
4
thermodynamically difficult due to equilibrium limitations, and
[5]
hence, the yield of DMC reported so far is relatively low.
Attempts to use dehydrating agents for removing the formed
water from the product stream to shift the equilibrium towards
[
6]
product side still remain challenging. Nevertheless, concerted
efforts of various research groups are underway to develop
efficient catalyst compositions to produce DMC in an attractive
3 4
samples obtained from H PO pretreatment of a Ce-Zr hydroxide
precursor (C80ZP1-3, P/Zr = 0.03-0.3) are compared. In addition,
the data of another catalyst are included (C80ZP*) which was
way. Among others, ZrO
widely used as catalysts for this reaction.
properties of these CeO -ZrO solid solutions can be adjusted by
2
, CeO2, and their solid solutions are
[
5, 7]
The catalytic
2
2
3 4
prepared by post-treatment of C80Z with H PO .
tuning the Ce/Zr ratios. Due to the different charges and radii of
the cations, the acid strength of the resulting oxides may be
Table 1 Effect of the H
molar Ce/Zr and P/Zr ratios, the surface areas as well as the DMC yields
Reaction conditions: catalyst weight 50 mg, CH OH = 247 mmol, T = 170 °C,
3 4 2 2
PO modification of CeO -ZrO catalysts on the overall
4
+
altered. Hence, Zr with the smaller ionic radius exhibits a more
(
3
[
8]
acidic nature in solid solution. But, for an efficient synthesis of
p = 65 bar, t = 1 h).
Sample
Ce/Zr
ICP)
P/Zr
(ICP)
BET-SA
[m g ]
Y
DMC
[mol%]
2
-1
(
[a]
[b]
[c]
I. Prymak, Dr. J. Wang, Dr. V. N. Kalevaru, Dr. habil. A. Martin, Dr.
U. Bentrup, Dr. S. Wohlrab
Leibniz-Institut für Katalyse e.V. an der Universität Rostock (LIKAT)
Albert-Einstein Straße 29a, 18059 Rostock, Germany
E-mail: sebastian.wohlrab@catalysis.de
Dr. O. Prymak
Institute of Inorganic Chemistry and Center for Nanointegration
Duisburg-Essen (CeNIDE)
University of Duisburg-Essen
C80Z
5.5
4.6
4.7
4.6
1.5
-
46
0.24
C80ZP-1
C80ZP-2
C80ZP-3
0.04
0.13
0.47
0.13
73
0.24
63
1.62
74
0.11
[
a]
Universitaetsstr. 5-7, 45117 Essen, Germany
Dr. J. Wang
C80ZP*
22
0.004
Research Center for Eco-Environmental Sciences
Chinese Academy of Sciences
3 4
[a] post-treated sample (H PO -addition to already calcined solid).
18 Shuangqing Road, Haidian District, Beijing, 100085, China
Supporting information for this article is given via a link at the end of
the document.
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ChemCatChem
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COMMUNICATION
Although no correlation was found between catalytic
performance and surface area, the yield of DMC significantly
increases with increasing P/Zr ratio and reaches a maximum at
composition of P/Zr=0.13 (C80ZP-2; YDMC = 1.62%). A greater
P/Zr of 0.47 resulted in a significant decrease of the DMC yield.
displayed in (Fig. 2a). Depending on desorption temperature
range the presence of acid sites can be classified as weak,
moderate and strong. . Both, unmodified Ce-Zr mixed oxides
as well as P-modified solids, exhibit mainly weak (<180 °C) and
[
14]
2
moderate (180-410 °C) acid sites (Table SI5). When CO is used
Similar findings were also reported for the ZrO
2
/H
3
PO
4
system
as acidic TPD probe molecule basicity of surface hydroxyl
(forms hydrogen carbonate species) or oxygen ions (forms
by Ikeda et al. but to a much lower efficiency: catalysts with low
P/Zr ratio (0.025-0.1) were more effective for DMC formation
[
15]
carbonate species) on metal oxides can be studied (Fig. 2b,
Table SI5). C80Z exhibits a broad range of CO desorption
[
11]
compared to un-phosphated ZrO
C80ZP* a considerably lower yield of DMC was obtained. This
suggests, that i) the method of H PO treatment plays a crucial
2
.
For the post-treated
2
temperature indicating the presence of three different types of
basic sites such as weak (<230 °C), moderate (230-500 °C) and
3
4
[
16]
role on the catalyst properties and ii) a certain amount of
phosphorus in the C80Z catalyst (P/Zr = 0.13) is essential for
proper catalyst formation.
strong (above 500 °C).
HAADF-STEM images of C80Z and C80ZP-2, shown in Fig.
SI1, revealed nano-sized irregular particles densely
agglomerated in all cases in the range of 7-15 nm. This was in a
good agreement to the crystallite sizes calculated by XRD (see
Table SI3). The sizes of pores measured by nitrogen adsorption
(
3.7 and 4.5 nm, Table SI1) are in good agreement with those
obtained from TEM images (~ nm). However, energy
5
dispersive X-Ray microanalysis (EDX) showed that the
distribution of cerium and zirconium concentrations differ on the
nanometer scale within the samples (Table SI2). The X-ray
diffraction patterns of samples with a Ce content ≥ 60 mol%
generally reveal the coexistence of a tetragonal and a cubic
[13]
phase, which is in accordance to results of Alifanti et al. (Fig.
SI2 and Table SI3). Thus, the unphosphated C80Z sample
contains a tetragonal Ce-rich Ce0.62Zr0.38
a cubic Ce0.93Zr0.07 phase (76%). In contrast, the formation of
a Zr-rich tetragonal Ce0.22Zr0.78 phase (13%) besides the cubic
phase (87%) is observed in the phosphated
2
O phase (24%) besides
3 2
Figure 2. Normalized NH -TPD (a) and CO -TPD profiles (b) of C80Z and all
C80ZP samples.
O
2
O
2
Ce0.93Zr0.07
O
2
C80ZP-2 sample (Figs. 1, SI3 and SI4). Crystalline P-containing
phases were not detected in the phosphated samples.
The C80ZP1-3 samples exhibit CO
2
desorption peak at
around 150 °C that indicated the presence of only weak basicity,
whereas this feature is distinctly reduced for C80ZP-3. This
suggests that P-modification from hydroxide derived catalyst
suppresses the concentration of both moderate and strong basic
sites which is not the case for C80ZP*. From previous
investigations, we have observed that CeO
with balanced concentration of acid-base sites performed the
best in the continuous esterification of CO to dialkyl
carbonates. Obviously, it is possible to reduce moderate and
2 2
-ZrO solid solutions
2
[
9]
3 4
strong basic sites to a higher extent by H PO treatment which in
turn causes enhanced activity.
For further inspecting the surface properties, hydroxyl
groups and possible carbonate surface species FTIR spectra of
the pretreated samples (synthetic air, 400 °C) at 170°C were
recorded (Fig. 3). Compared to C80Z, the C80ZP-2 catalyst
-
1
[17]
-1
shows additional bands at 3671 cm (νZr−OH) and 3510 cm ,
-1
[17]
besides of a band at 3649 cm (νCe−OH) . The latter can be
assigned to a νOH band from a cerium hydroxide microphase.
The (νC−H) bands at 2935, 2844, and 2723 cm in C80Z stem
from formate species , the respective ν /ν COO bands of
which appear at 1546 and 1372/1358 cm . These formate
bands are missing at C80ZP-2, instead, bands at 1542/1523 cm
and 1253/1220 cm can be seen pointing to surface bidentate
carbonate species.
[
18]
-
1
Figure 1. Representative X-ray powder diffractograms with Rietveld
refinement of the samples C80Z and C80ZP-2 with additionally highlighted
[
19]
−
as
s
tetragonal Ce
Zr1-xO
x 2
phases.
-1
-
1
-1
Temperature-programmed desorption (TPD) of NH
suitable characterization technique to estimate the total acidity of
the catalysts (Table SI4). The respective NH -TPD profiles are
3
is a
3
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For studying the influence of H
3
PO
4
modification on the
bands changes dramatically by H
3
PO
4
modification in favour of
4
+
surface properties, in situ FTIR experiments were performed in
which MeOH/CO gas mixtures were adsorbed exemplarily on
3
Zr −OCH (Table 2). This goes along with an increasing Lewis
4
+
2
acidity caused by higher percentage of unsaturated surface Zr
C80Z and C80ZP-2 solids at 170°C (Fig. 4).
cations in the modified C80ZP-2 sample. In parallel, the total
basicity drops by modification while the overall acidity does not
change significantly (Table 2). Considering the findings of
[
22]
Tomishige et al. intermediate methoxy carbonate species over
ZrO catalysts are easily formed by reaction of adsorbed
monodentate methoxy species with CO . Based on these results,
it can be concluded that the favoured formation of Zr −OCH
2
2
4
+
3
species is responsible for the enhanced yield of DMC over
C80ZP-2 catalyst.
Table 2. Surface properties of the unmodified and modified C80Z samples
as examined by MeOH/CO and pyridine adsorption (FTIR) as well as NH -
2 3
2
TPD and CO -TPD.
Sample
Methoxy
Lewis acid.
(py. ads.)
Total acidity
NH uptake
(µmol m )
Total basicity
CO uptake
(µmol m )
[
b]
formation
3
2
4+
4+[a]
-2
-2
Figure 3. In situ FTIR spectra of C80Z and C80ZP-2 recorded at 170°C after
pretreatment in synthetic air at 400°C.
Ce /Zr
C80Z
40/1
2/1
15.9
30.5
2.0
1.7
-1
2.2
0.5
-1
C80ZP-2
Under these conditions both catalysts are also active in the
2
conversion of CO with methanol (see ESI). The bands in the
[a] Ratio of νC−O band intensities (~1100 cm and ~1150 cm ), normalized
-1
νC−H region around 2940, 2915, and 2810 cm can be
assigned to C−H stretching vibrations of methoxy species, while
-3 -1
to BET-SA. [b] Band intensity ×10 (1440 cm ), normalized to BET-SA.
-1
those in the frequency range of 1200-1000 cm are due to νC−O
[
20]
vibrations.
Depending upon the νC−O band positions the
nature of surface methoxides can be estimated. Hence, bands
In summary, the phase composition of a Ce rich CeO -ZrO2
2
solid solution changed considerably after phosphoric acid
pretreatment leading to Ce rich cubic and Zr rich tetragonal
Ce Zr O nanophases with intimate contact on the nanoscale.
-1
around 1150 and 1100 cm are related to monodentate methoxy
-
1
species while the bands around 1050/1030 cm
are
4
+
characteristic for bidentate methoxy species adsorbed on Zr
x
1-x
2
4
+ [17, 21]
or/and Ce .
From the band positions of the monodentate
Crucial for the increased activity are the specific surface
properties such as surface composition (Ce/Zr and P/Zr ratios)
methoxy species, it is possible to distinguish between
4
+
-1 [21]
4+
-1 [17]
Ce −OCH
3
(1103 cm )
and Zr −OCH
3
(1148 cm ).
The
and acid-base properties. The H PO4 modification caused a
3
spectrum of C80Z is characterized by a strong band of
decrease of moderate and strong basic sites and an increase of
Lewis acidic sites from most likely unsaturated surface Zr
4
+
4+
4+
Ce −OCH
3
and only a weak band of Zr −OCH
3
species, while
the latter is essentially more intensive over C80ZP-2 (Fig. 4).
cations which are responsible for an enhanced ability to form
monodentate methoxy species as an intermediate of the
catalytic cycle.
Experimental Section
Experimental details are provided in the accompanying Supporting
Information.
Acknowledgements
The authors like to thank the Leibniz Association for funding
(
Project SAW-2011-LIKAT-2). Thanks are also due to Mr. R.
Eckelt for BET-SA; Ms. A. Simmula for ICP-OES; Dr. M.-M. Pohl
for TEM results, Dr. H. Atia for NH - and CO -TPD.
Figure 4. In situ FTIR of C80Z and C80ZP-2 recorded after 30 min exposure
to MeOH/CO at 170 °C and subsequent flushing with He.
2
3
2
Keywords: Ce-Zr mixed oxides • in situ FTIR • crystal phases •
Although the overall intensities of the methoxy bands is
4
+
4+
acid-base properties • dimethyl carbonate
decreased for C80ZP-2, the intensity ratio of the Ce /Zr −OCH
3
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
COMMUNICATION
[
a]
Esterification of CO
with methanol over
phosphate
2
Iuliia Prymak, Oleg
[
b]
[a,c]
*[a]
Prymak, Junhui Wang,
Venkata Narayana
Kalevaru, Andreas
Martin, Ursula Bentrup
and Sebastian Wohlrab
[
a]
functionalized CeO
ZrO solid solutions
2
-
[
a]
2
*
[a]
results in more than 6
times higher yields of
dimethyl carbonate
Page No.1 – Page No.4
(
DMC) compared to the
Phosphate
functionalization of
CeO -ZrO solid
2 2
solutions for the
catalytic formation of
dimethyl carbonate from
methanol and carbon
dioxide
untreated mixed
oxides. Changed
surface properties of
the phosphated
catalysts lead to a
decrease of moderate
and strong basic sites
and an increase of
Lewis acidic sites which
are responsible for an
enhanced ability to
form Zr-OCH
an intermediate of the
aimed product DMC.
3
species
–
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