A nanostructured CeO2 promoted Pd/α-alumina diethyl oxalate catalyst with high activity and stability

Article abstract of DOI:10.1039/c4ra08170f

A Pd/α-Al₂O₃ nanocatalyst was synthesized and investigated as a catalyst for CO oxidative coupling to diethyl oxalate and CeO₂ was used as a promoter. With the highest activity and stability found so far, great CO conversion and diethyl oxalate selectivity were achieved due to the addition of CeO_2. This journal is

Full text of DOI:10.1039/c4ra08170f

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A nanostructured CeO promoted Pd/a-alumina
2
diethyl oxalate catalyst with high activity and
Cite this: RSC Adv., 2014, 4, 48901
stability†
Received 5th August 2014
Accepted 23rd September 2014
a
a
b
a
ac
Erlei Jin, Leilei He, Yulong zhang, Anthony R. Richard and Maohong Fan*
DOI: 10.1039/c4ra08170f
www.rsc.org/advances
A Pd/a-Al
catalyst for CO oxidative coupling to diethyl oxalate and CeO
used as a promoter. With the highest activity and stability found so far, production plant in 2009.
2
O
3
nanocatalyst was synthesized and investigated as a leaded the word in this area and has successfully built the
2
was world's rst annual 200 thousand ton coal to ethylene glycol
13
great CO conversion and diethyl oxalate selectivity were achieved due
Syngas to ethylene glycol process contains several steps. The
step of CO oxidative coupling to di-alkyl oxalate is the critical
one since di-alkyl oxalate is needed for production of EG with
hydrogenation. Two major chemical reactions are involved with
the overall CO oxidative coupling step, CO coupling and RONO
regeneration, as shown in R1 and R2 respectively. R in both R1
and R2 could be methyl, ethyl or butyl groups. R1 needs catalyst
while R2 does not. Esterication between oxalic acid and
alcohol has been employed as a traditional way of synthesizing
oxalic ester. However, this method has several problems, such
as severe pollution, high energy consumption and high capital
costs. Therefore, oxidative coupling CO with alkyl nitrite for
to the addition of CeO2.
Ethylene glycol (EG) is a crucial raw chemical with a global
demand of around 25 million tons each year, which is mostly
1,2
produced through traditional petrochemical technology.
However, the cost of this production is relatively high due to the
continuously increasing price of natural gas and crude oil, and
dwindling sources of petroleum. Furthermore, strong acids or
alkalis such as sulphuric acid or sodium hydroxide have to be
used during the traditional method, which causes severe
3
corrosion to the equipment and environmental problems.
3,14–20
formation of oxalic ester is gaining increasing interest.
Therefore, a green route which is independent of petroleum
while achieves a high yield of EG is in demand and of great
signicance.
2CO + 2RONO / (COOR) + 2NO
(R1)
(R2)
2
1
Coal is the most abundant energy reserve in the world that
some people like because of their needs while others hate due to
2ROH þ 2NO þ
O
2
/2RONO þ H
2
O
2
4
the various emissions resulting from its combustion. To reduce
Various supported palladium catalysts for gas-phase
synthesis of dimethyl oxalate (DMO) or diethyl oxalate (DEO)
have been investigated, and the results have demonstrated that
CO emission and produce high-value fuels and chemicals from
2
coal, coal gasication and liquefaction technologies have
5
–8
attracted increasing interest during the past few decades.
2 3
higher conversion and selectivity are realized on Pd/a-Al O
Coal to ethylene glycol, as a potentially green and economic coal
liquefaction technology, has been attracting extensive attention
21,22
compared to Pd on active carbon or g-Al
2
O
3
.
However, the
9–12
relatively high Pd loading (around 2 wt%) is always an issue for
industrial application of CO oxidative coupling to DMO which
greatly increases the cost of production. Therefore, the design of
low Pd loaded catalysts with high performance is important to
industry. A Pd/a-Al O nanocatalyst with ultra-low Pd loading
in both academic and business circles in the past decades.
Although it is challenging to achieve high industrial production
levels, due primarily to achieving good performance of the
catalysts, this technology has been scaled-up to industrial levels
of production in China and Europe. Until now, China has
2
3
that exhibits high activity and stability for CO oxidative coupling
23
to DMO was developed recently. This catalyst was prepared by
a
Department of Chemical and Petroleum Engineering, University of Wyoming, Laramie,
2+
a Cu assisted in situ reduction method at room temperature,
WY 82071, USA. E-mail: mfan@uwyo.edu
which signicantly increased the dispersion and the specic
surface area of active component Pd, and also decreased the
ensemble size of Pd nanoparticles dispersed over the
Pd/a-Al O To further enhance the activity and stability of
b
Western Research Institute, Laramie, WY 82070, USA
c
School of Energy Resources, University of Wyoming, Laramie, WY 82071, USA
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c4ra08170f
1
2 3.
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24,25
Pd/a-Al
2
O
3
, several metals such as Fe,
2
Ni, and Ce were the red circles in Fig. S5a† indicate the dispersion of CeO on
reported as promoters to enhance the dispersion of Pd on the the a-Al O support, which is conrmed by energy dispersive
2
3
2
4–27
support or decrease the Pd particles size.
CeO was reported X-ray (EDX) spectra. The dispersion of CeO particles was not as
2 2
28
as a promoter for Pd/a-Al O catalyst. However, the activity of good as Pd particles. Small portion of the added CeO2
2
3
2
the CeO promoted catalyst was only evaluated up to 100 min. aggregated into the large particles while most of it which played
Although methyl nitrite has been maturely used, especially the promoter role existed in the form of small nanoparticles
in China, for the industrial synthesis of DMO, it is controlled in that hardly detectable with SEM or TEM. In summary of the
the US due to its highly ammable, explosive, and toxic prop- results from TEM and SEM, it can be concluded that the
erties. Ethyl nitrite, however, is a safe and non-explosive alkyl promoter CeO
2
not only promotes the dispersion of Pd on the
nitrite that also can be used for CO oxidative coupling reac- support, but also decreases the nanoparticle size of Pd.
18,20,29–32
tion.
Therefore, to nd a good catalyst with low Pd
The two catalysts, Pd/a-Al O and Pd–CeO /a-Al O , were
2 3 2 2 3
loading and high catalytic activity for CO oxidative coupling to analyzed with X-ray photoelectron spectroscopy (XPS) (Pd 3d)
ꢀ
DEO is of great signicance. Herein, we report a Pd–CeO
a-Al
CeO
the preparation and characterization of two catalysts with and values for both Pd(0) and Pd(II) were consistent with the
2
before and aer the reaction with CO and EN at 140
C
2
O
3
nanocatalyst with 0.8% Pd (wt%) loading and 0.2 wt% (Fig. S3†). Although there were small differences between Pd/
2
as a catalyst for CO oxidative coupling to DEO. We present a-Al and Pd–CeO /a-Al , the obtained Pd 3d3/2 and Pd 3d5/2
2
O
3
2
2 3
O
33–35
without CeO
2
as a promoter. The comparison of catalytic published literatures.
In Fig. 2a and c, both the Pd 3d5/2 and
activities between the two catalysts is discussed and the inter- Pd 3d3/2 of catalysts Pd/a-Al O and Pd–CeO /a-Al O are
2
3
2
2 3
action among Pd, ceria and the support leading to the activity around 335 and 340 eV, respectively, which indicates that the
differences is also presented.
oxidation state of Pd in the catalysts is Pd(0). However, aer
The textural characteristics of Pd–CeO
investigated by transmission electronic microscopy (TEM) (Fig. S3c and d†), which are assigned to Pd(II), indicating that
2
/a-Al
2
O
3
catalyst were reaction, two new peaks appeared in both of the catalysts
35
(
Fig. 1), scanning transmission electronic microscopy (STEM) some Pd(0) in the two catalysts was oxidized to Pd(II) by ethyl
13
3 2 2 3
and scanning electronic microscopy (SEM) (Fig. S5†). TEM nitrite to form an intermediate, CH CH O–Pd(II)–OCH CH .
images presented in Fig. 1a and c clearly show that the Pd The peaks area of the Pd(II) in Fig. S3d† is much bigger than the
nanoparticles of Pd–CeO /a-Al O are dispersed on the a-Al O peaks area in Fig. S3b,† which indicates that more intermediate
relatively uniformly while the dispersion of Pd nanoparticles of was generated on the surface of Pd–CeO /a-Al O catalyst, and
2
2
3
2 3
2
2 3
Pd/a-Al O is poor. Moreover, the results in Fig. 1b and d show therefore Pd–CeO /a-Al O may have higher catalytic activity
2
3
2
2
3
that the average Pd nanoparticles size of the Pd–CeO
2
/a-Al
2
O
O
3
with the addition of CeO
2
. Furthermore, the percentage of the
catalyst is 13.2 nm which is smaller than that of the Pd/a-Al
2
3
Pd on both catalysts was calculated using the peaks area of the
/a-Al catalyst showed higher Pd
catalyst is narrower than that of the concentration (0.92%) than that of the Pd/a-Al catalyst
catalyst as well. CeO was difficult to detect by TEM (0.81%), which suggests that the promoter CeO can also
catalyst (17.3 nm). The Pd nanoparticles size distribution of XPS and the Pd–CeO
the Pd–CeO /a-Al
Pd/a-Al
2
2 3
O
2
2
O
3
2 3
O
2
O
3
2
2
21
which may be due to its low loading concentration. However, enhance the Pd loading concentration on the support. X-ray
diffraction (XRD) (Fig. S4†) was also attempted to further
conrm the XPS results. However, no detectable CeO or Pd
2
peak was found which may be due to their low concentrations
28
and the high dispersion of Pd.
The catalytic performances of the two catalysts were evalu-
ated under the same conditions. With the addition of CeO , the
conversion of CO and EN was increased from 39% to 65% and
4–92%, respectively (Fig. 2a), which is 50% more conversion
for both of the reactants. The STY of DEO with Pd–CeO /a-Al O
2
6
2
2 3
Fig. 2 Conversion of CO (blue lines) and EN (red lines) of CO oxidative
coupling to DEO with different catalysts within 72 h (a) and DEO
; TEM (c) selectivity of CO oxidative coupling to DEO with different catalysts
within 72 h (b).
Fig. 1 TEM (a) and size distribution (b) of catalyst Pd/a-Al
2 3
O
and size distribution (d) of catalyst Pd–CeO /a-Al
2
2 3
O .
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a
Table 1 CO oxidative coupling to DEO with different catalysts
b
ꢁ1 ꢁ1
Catalysts
Pd content (wt%)
Ce content (wt%)
Conversion (%)
Selectivity (%)
STY (g L
h )
Pd/a-Al
Pd–CeO
2
O
3
0.8
0.8
—
—
0.15
0.2
39
65
—
95
93
—
195
318
—
2
/a-Al
/a-Al
2
O
3
CeO
2
2 3
O
a
ꢁ1
ꢀ
Reaction conditions: 3.5 g of catalyst, 1200 h of gas hourly space velocity (GHSV), reactants volume ratio CO–EN is 1.2. 0.1 MPa, 140 C.
Conversion of CO.
b
was also greatly increased, which is 60% higher than that of Pd/ (Fig. S6†), the peak area of Pd–CeO
2
/a-Al
at 140 C (Table 1). Meanwhile, the selectivity of DEO bigger than that of the catalyst without CeO
with these two catalysts was almost the same (92–95%). Since strates the superior catalytic activity of Pd–CeO
there was no catalytic activity found for the catalyst CeO /a- to Pd/a-Al , consistent with the results of catalytic activity
Al O , the CeO must play an important role as a promoter and evaluation, TEM, and XPS results.
2
O
3
catalyst is 20%
, which demon-
/a-Al O relative
2 3
ꢀ
a-Al
2
O
3
2
2
2
2 3
O
2
3
2
the interaction of CeO with Pd was responsible for the high
The effect of temperature on both EN and CO conversion
2
activity and selectivity in CO oxidative coupling to DEO. Most of where Pd–CeO /Al O was used as the catalyst for CO oxidative
2
2 3
all, the catalytic activity of catalyst Pd–CeO
2 2 3
/a-Al O can be coupling to DEO reaction was also evaluated (Fig. S7†). The
maintained for at least 72 h (Fig. 2b), which lays a good foun- conversions of EN and CO increased by 20% when the
ꢀ ꢀ
temperature increased from 120 C to 140 C, while the selec-
dation for a further long-term stability test.
Fig. 3 illustrates the in situ DR-FTIR spectra for the reaction tivity of DEO had little change until the temperature reached to
ꢁ
1
ꢀ
ꢀ
of CO with ethyl nitrite to DEO. The band at 1768 cm is 180 C. Both the CO conversion and DEO selectivity decreased at
attributed to the C]O stretching vibrations of the DEO product. 180 C due to the decomposition of the EN. Therefore, the
It is obvious to note that the intensity of the band at 1768 cm
ꢁ1
future plan for this project is optimizing the Pd–CeO /Al O
2 2 3
in the spectrum of Fig. 3b is stronger than that in the spectrum catalyst to achieve a lower reaction temperature while main-
of Fig. 3a and from the integration results of the two peaks taining high DEO selectivity.
In summary, a low Pd loading Pd/Al
.8% Pd (wt%) loading and an average Pd size of 13.2 nm was
synthesized for CO oxidative coupling to DEO. Aer the intro-
duction of 0.2 wt% CeO , Pd–CeO /Al catalyst showed
2 3
O nanocatalyst with
0
2
2
2 3
O
remarkably higher catalytic activity compared with the catalyst
without CeO . The CO conversion was increased by 50% (from
2
3
9% to 62%) with the DEO selectivity higher than 90% when the
CeO was used as a promoter and, importantly, the high activity
and selectivity could be maintained up to 72 h without visible
decrease. TEM results showed clearly that CeO not only
2
2
improved the dispersion of palladium on the surface of the
support but also decreased the palladium size as well, thus
resulting in the excellent catalytic activity. In consideration of
the facile synthesis and low Pd loading of this catalyst as well as
the risky factors of methyl nitrite, this highly efficient and stable
nanocatalyst may have a promising industrial application,
especially in the US, of coal to ethylene glycol.
Acknowledgements
This research was supported by the Department of Energy and
Wyoming Clean Coal Program.
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Fig. 3 In situ FTIR spectra for the CO oxidative coupling to DEO
reaction with Pd/a-Al (a) and Pd–CeO /a-Al (b).
O
2 3
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O
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