Lewis acid sites in MOFs supports promoting the catalytic activity and selectivity for CO esterification to dimethyl carbonate

Article abstract of DOI:10.1039/c9cy02330e

CO esterification to dimethyl carbonate (DMC) is an attractive route, although the catalyst is the main problem restricting the industrialization of this route. Supported palladium catalysts have been often used for this route. However, the effect of the support is not always clear. In this work, we firstly introduced three metal-organic frameworks (MOFs) materials as catalyst supports, namely UiO-66, MIL-101 and MOF-5, and studied the effect of Lewis acid sites in the MOFs supports on the catalytic activity and selectivity for CO esterification to DMC. Interestingly, the Pd(ii)/UiO-66 catalyst exhibits superior catalytic performance to the other catalysts, with 87.9% CO conversion and 98.5% DMC selectivity due to the abundant Lewis acid sites in UiO-66. It was proven that UiO-66 with more defects would have a large number of Lewis acid sites, and the catalytic performance was positively correlated with the number of Lewis acid sites in MOFs supports, which can be demonstrated by characterization by NH₃-TPD, pyridine-IR, XPS and in situ DRIRS. The current work not only revealed the relationship between the Lewis acidity of the support and catalytic performance, but also promotes the development of practical catalysts for CO esterification to DMC.

Full text of DOI:10.1039/c9cy02330e

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ISSN 2044-4761
PAPER
Chaoqiu Chen, Yong Qin et al.
Highly dispersed Pt nanoparticles supported on carbon
nanotubes produced by atomic layer deposition for hydrogen
generation from hydrolysis of ammonia borane
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DOI: 10.1039/C9CY02330E
ARTICLE
Lewis Acid Sites in MOFs Supports Promoting the Catalytic Activity
and Selectivity for CO Esterification to Dimethyl Carbonate
Yu-Ping Xu,a,b Zhi-Qiao Wang,b Hong-Zi Tan,b Kai-Qiang Jing,b Zhong-Ning Xu,b* Guo-Cong Guob*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
CO esterification to dimethyl carbonate (DMC) is an attractive route, while the catalyst is the main problem to restrict the
industrialization of this route. The supported palladium catalysts have been often used for this route. However, the effect
of support is always unclear. In this work, we have firstly introduced three metal-organic frameworks (MOFs) materials as
catalyst supports, such as UiO-66, MIL-101 and MOF-5, and studied the effect of Lewis acid sites in MOFs supports on the
catalytic activity and selectivity for CO esterification to DMC. Interestingly, the Pd(II)/UiO-66 catalyst exhibits superior
catalytic performance with 87.9% CO conversion and 98.5% DMC selectivity than the other catalysts due to the abundant
Lewis acid sites in UiO-66. It was proved that UiO-66 with more defects would have a large amount of Lewis acid sites, and
the catalytic performance was positively correlated with the amount of Lewis acid sites in MOFs supports, which can be
demonstrated by the characterizations of NH3-TPD, Pyridine-IR, XPS and in situ DRIRS. Current work not only revealed the
relationship between supports Lewis acidity and catalytic performance, but also promoted to develop practical catalysts
for CO esterification to DMC.
efficiency of the existent technologies, it was desired to explore a
new route.
1. Introduction
Dimethyl carbonate (DMC, (CH3O)2CO) is a significant and green raw
material for the synthesis of polycarbonate.1, With two methyl
2
CO esterification to DMC (CO + 2CH3ONO = (CH3O)2CO + 2NO)
developed by UBE company has attracted widespread attention
because of its unique technical advantage. No water is produced
during the DMC synthesis process, which can enhance the stability
of catalyst. NO can be cycled to synthesize CH3ONO by reaction
with CH3OH and O2 in another reactor. Moreover, the reaction
conditions are mild at atmospheric pressure and low temperature
(363–403 K).18 The catalyst is the main problem to restrict the
industrialization of this route. Chlorine-containing and chlorine-free
Pd-based catalysts have been reported.19 Although the chlorine-
containing catalyst exhibits superior initial catalytic activity and
selectivity, it would rapidly deactivate due to the loss of chloride
ions in the form of methyl chloroformate byproduct20 It is necessary
to supplement HCl gas to keep the stability of chlorine-containing
catalyst, which would raise corrosion to the production equipment.
By comparison, chlorine-free catalysts can get rid of the
deactivation restriction resulting from the loss of chloride ions.19, 21,
groups and one carbonyl group, DMC is used as non-toxic
methylation and carbonylation reagents in place of the toxic
dimethyl sulfate and phosgene.3 Due to high ionic conductivity,
DMC is also widely used as electrolyte solvent in lithium battery
industry.4 Additionally, it could be used as a potential fuel additive
instead of methyl tertiary butyl ether (MTBE) to improve the
gasoline octane number.3, 5, 6 The technologies of DMC production
involves phosgenation,7, transesterification9-12 and liquid-phase
methanol oxidative carbonylation routes13-16
8
.
Liquid-phase
methanol oxidative carbonylation to synthesis DMC developed by
Enichem (Italy) had been industrialized.17 The production process
uses CuCl as a catalyst, but there are some problems to be solved.
Such as H2O and by-product will lead to catalyst deactivation;
equipment corrosion will result in additional costs. Owing to the
problems of production security, raw cost, and conversion
22
Our group has developed a mononuclear-isolated Pd(II)/NaY
chlorine-free catalyst, which demonstrates
a high selectivity
(>99.5%) to target DMC over 100 hours.23 The reported supports in
chlorine-free catalysts mainly include traditional metal oxides24 and
zeolites25. However, the effect of support on catalytic performance
is always unclear.
a. College of Chemistry and Materials Science, Fujian Normal University, Fuzhou,
Fujian 350007, P. R. China.
b. State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the
Structural of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R.
China.
† Dedicated to Professor Jin-Shun Huang on the occasion of his 80th birthday.
† E-mail: gcguo@fjirsm.ac.cn, znxu@fjirsm.ac.cn;
† Electronic Supplementary Information (ESI) available: BET, Py-IR, in situ IR
As an emerging porous material, metal-organic frameworks
(MOFs) have attracted wide attention as supports in heterogeneous
catalysis, owing to its designable metal nodes and coordination
unsaturated as Lewis acid sites.26-31 Compared with the traditional
See DOI: 10.1039/x0xx00000x
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supports, MOFs exhibits many superior characters: i) defined and refluxed with ethanol solution for 12 h at 353 K. Finally, the green
View Article Online
designable crystal structures; ii) the uniform pores; iii) high surface product was dried under vacuum at 343 K foDrO12I: 1h0..1039/C9CY02330E
area. With these advantages, the MOFs materials are expected to
MOF-5 (Zn): MOF-5 (Zn) was prepared following previous
as heterogeneous catalysts or catalyst supports. From the catalysis
reported literatures.37 Generally, 580 mg Zn(NO3)2·6H2O and 160
perspective, the advantages of MOFs such as porosity and
mg H2BDC were added to 40 mL DMF at room temperature. The
extraordinary surface area can offer a host number of accessible
mixture solution was stirred constantly until completely dissolved.
active sites and allow efficient diffusion of reactant, thus enhancing
Then the mixture was sealed in a Teflon-lined stainless steel
the catalytic activities. In recent years, the defects of MOFs had
autoclave and then placed in a preheated oven at 393 K for 21 h.
attracted great attention. Defects in the MOFs framework will
After cooling to room temperature, the microsized crystals was
expose a large number of Lewis acid sites, which will have a huge
filtered and washed with DMF for several times. Finally, the white
impact on acid catalysed reactions. As Lewis acids catalysts, MOFs
crystallite product was dried under vacuum at room temperature
materials have great potential in biomass conversion catalysis.32-34
for two days.
In this work, we have firstly introduced three MOFs as chlorine- Preparation of Pd(II)/MOFs catalysts:
free catalyst supports, such as UiO-66 (Zr), MIL-101 (Cr) and MOF-5
The Pd(II)/MOFs catalysts were prepared by the traditional wet
(Zn). The effect of Lewis acid sites in MOFs supports on the catalytic
impregnation method. Typically, 300 mg MOFs supports [UiO-66
activity and selectivity for CO esterification to DMC has been
(Zr), MIL-101 (Cr), MOF-5 (Zn)] were added into the 0.0126 g
investigated. The Pd(II)/UiO-66 catalyst exhibits superior catalytic
Pd(OAc)2 and 10 mL acetone solution, respectively. And the
performance with 87.9% CO conversion and 98.5% DMC selectivity
compounds were ultrasonic dispersed for 30 minutes. Then mixture
than the other catalysts due to the abundant Lewis acid sites in
solution was stirred for overnight at room temperature. Then, the
UiO-66. This work will promote to develop practical catalysts for CO
suspension was heated at 333 K to evaporate the solvent. These
esterification to DMC.
three catalysts were denoted as Pd(II)/UIO-66 (Zr), Pd(II)/MIL-101
(Cr) and Pd(II)/MOF-5 (Zn), respectively.
2. Experimental Section
2.2 Catalysts Characterizations
2.1 Catalysts Preparation
PXRD: Powder X-ray diffraction (PXRD) patterns were recorded
using a Rigaku MiniFlex II diffractometer with a Cu-Kα X-ray source
(λ=1.5406 Å) at a scanning rate of 3° min-1 from 5° to 50°. The X-ray
tube was operated at 40 kV and 30 mA.
Materials:
Pd(OAc)2 was purchased from Kunming Institute of Precious Metals.
ZrCl4 and 1,4-benzodicarboxylic (H2BDC) were purchased from
Aladdin Chemcial Reagent Co., Ltd. DMF, methanol, Cr(NO3)·9H2O,
Zn(NO3)2·6H2O, Acetic acid (HOAc) and HF were purchased from
Sinopharm Chemcial Reagent Co., Ltd, China. Deionized water (18
MΩ·cm) were used throughout the experiments.
ICP-AES: Inductively coupled plasma (ICP) spectroscopy was
performed with an Ultima 2 plasma emission spectrometer from
Jobin Yvon.
TG: Thermogravimetric (TG) analysis was performed using a
METTLER TOLEDO thermogravimetric analyzer with a heating rate
of 5 K·min-1 from 298 to 1073 K in nitrogen (N2).
Preparation of MOFs supports:
UiO-66 (Zr): UiO-66 (Zr) was prepared following previous
reported literatures.35 Typically, 820 mg ZrCl4, 580 mg H2BDC, 0.25
mL H2O and 6 mL HOAc were dissolved in 40 mL DMF at room
temperature with stirring, and then the obtained mixture was
stirred for 3 h at room temperature. Subsequently, the substrate
mixture was sealed in a Teflon-lined stainless steel autoclave and
then placed in a preheated oven at 393 K for 24 h, followed by
cooling to room temperature. After the solvothermal reaction, the
formed solid product was filtered off, then filtered and washed with
methanol and DMF (1 : 4) for several times, and finally dried under
vacuum at 353 K for 10 h. The obtained UiO-66 as catalyst support
use for the catalytic reaction.
BET: Brunauer-Emmett-Teller (BET) surface areas were calculated
from the N2 adsorption–desorption isotherms obtained using the
BET method, which was performed on an ASAP 2020 instrument
(Micromeritics) at the temperature (77 K) of liquid N2. All samples
were outgassed at 423 K for 12 h. The pore volume was calculated
at a relative pressure of 0.99.
TEM: The high angle annular dark field scanning transmission
electron microscopy (HAADF-STEM) and high angle annular dark
field scanning transmission electron microscopy-energy dispersive
X-ray (HAADF-STEM-EDX) mapping were performed on a G2 F20
field-emission transmission electron microscope (FEI Tecnai)
operating at an acceleration voltage of 200 kV. All the catalyst
samples were dispersed in ethanol ultrasonically, and then dropped
onto carbon coated copper grids in preparation for the
measurements.
MIL-101 (Cr): MIL-101 (Cr) was prepared following previous
reported literatures.36 Firstly, 400 mg Cr(NO3)·9H2O and 166 mg
H2BDC were dissolved in 4.8 mL H2O. Then 44 µL HF was slowly
added to the above solution and stirred until completely dissolved
at room temperature for 3 h. Transferring the mixture solution to a
100 mL of hydrothermal reaction vessel with Teflon liner, and kept
493 K for 8 h under static conditions. After cooling to room
temperature, the solid product was filtered and washed one time
with H2O, DMF and ethanol, respectively. Next, the product was
XPS: X-ray photoelectron spectroscopy (XPS) was performed
using an Escalab 250 spectrometer (VG Systems) equipped with an
Al-Kα (1486.7 eV) anode as X-ray source. The binding energies were
2 | J. Name., 2012, 00, 1-3
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corrected by C 1s peak at 284.6 eV, providing experimental error is
within ± 0.1 eV.
SDMC and SDMO are the peak areas of DMC and dimethyl oxalate
(DMO), respectively and R-FDMC and R-DFDOMI:O10a.1r0e39t/Che9CYr0e2la3t3i0vEe
correction factors of DMC and DMO, respectively.
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NH3-TPD: NH3 temperature programmed desorption (NH3-TPD)
were carried out to detect the acid properties of supports. The
experiment process is as follows: 100 mg of the sample was treated
with helium at 423 K for 1 h to remove the adsorbed impurities.
After cooling to 323 K under a He flow, the sample was exposed to
a 10% NH3–He mixture (50 mL·min−1) for 1 h, followed by purging
with He for 30 min, and then heated to 673 K by ramping at 10
K·min-1 under flowing He. The signals were recorded by monitoring
the desorbed ammonia with a TCD detector.
3. Results and Discussions
3.1 Catalytic activity
Pd(II)/UiO-66, Pd(II)/MIL-101 and Pd(II)/MOF-5 chlorine-free
catalysts were all synthesized by traditional wet impregnation
method and evaluated under the same conditions for CO
esterification to DMC. Fig. 1 summarizes the catalytic performance
over these three Pd(II)/MOFs catalysts in which Pd(II)/UiO-66
catalyst exhibits the highest catalytic performance (87.9% CO
conversion and 98.5% DMC selectivity), far exceeding those of
Pd(II)/MIL-101 and Pd(II)/MOF-5 catalysts. The order of WTY over
these three Pd(II)/MOFs catalysts (Table S1) were Pd(II)/UiO-66
(1653 g·kgcat.-1·h-1) > Pd(II)/MIL-101 (647 g·kgcat.-1·h-1) > Pd(II)/MOF-5
(316 g·kgcat.-1·h-1). The three pure MOF supports showed poor CO
conversion with the absence of Pd species (Fig. S1). When a
physical mixture of MOF and Pd(OAc)2 as catalyst, the catalytic
performance were significantly improved due to Pd species as
active species. However, these catalysts were quickly deactivated.
When Pd species were loaded by wet impregnation method, and
the catalytic stability also was enhanced, which is attributed to the
interaction between the supports and Pd species.
Py-IR: Pyridine infrared (Py-IR) spectra were used to discriminate
acid type of samples on a Thermo fisher Nicolet IS10. Wafers of
compressed catalysts samples were mounted in the FTIR cell and
degassed in vacuum of 10-7 Pa at 573 K for 2 h. Samples were
saturated with pyridine vapor at 303
K for 10 min. After
equilibration, the samples were evacuated at 373 K, 473 K and 573
K to remove the excess of probe molecules, respectively. And the IR
spectra were recorded in the range of 400–4000 cm-1.
In situ DRIRS: In situ diffuse reflectance infrared spectroscopy
(DRIRS) measurements were performed on
a 6700 Fourier
transform-infrared (FT-IR) spectrometer (Nicolet) equipped with a
stainless steel in situ IR flow cell and set up for diffuse reflectance
sampling. The powder samples were placed into the cell, and pre-
treated in the N2 flow for 30 min at 423 K. After cooling down to the
293 K, a reference spectrum was recorded. Then, the reactants
were introduced in a constant flow, and the spectra were recorded
at a resolution of 4 cm-1.
As shown in Table 1, the Pd loading of Pd(II)/UiO-66 is 1.32%,
which is close to that of Pd(II)/MIL-101 (1.42%) and Pd(II)/MOF-5
(1.36%). This result excludes the influence of Pd loading on catalytic
performance, indicating that the catalytic performance may be
strongly dependent on the nature of the catalyst support materials.
2.3 Catalysts Evaluation
Catalytic performances of catalysts for CO esterification to DMC
were measured using a fixed-bed quartz tubular reactor. The center
of the quartz tubular reactor was loaded with 200 mg of catalyst.
This reaction was performed under atmospheric pressure using
reactant gases composed of 19% CO, 45% methyl nitrite, 3% Ar (as
an internal standard), and 33% N2 (as a balance gas) at a weight
hour space velocity (WHSV) of 2500 L·kgcat.-1·h-1 and the
temperature of 393 K. The composition of the reactant gases and
reaction products were monitored using an on-line GC-2014 gas
chromatograph (Shimadzu) equipped with a thermal conductivity
detector and a flame ionization detector.
The conversion of CO, the selectivity to DMC and the weight time
yields (WTY) of DMC were calculated using the following formulae:
Conversion of CO (%) = ([CO]in/[Ar]in – [CO]out/[Ar]out)/([CO]in/[Ar]in)
× 100%
Fig. 1 The conversion of CO and selectivity to DMC over Pd(II)/MOFs catalysts
(Pd(II)/UiO-66, Pd(II)/MIL-101 and Pd(II)/MOF-5) for CO esterification to DMC.
a
Reaction conditions: 200 mg catalyst, weight hour space velocity (WHSV) =
Selectivity to DMC (%) = (SDMC × R-FDMC)/(SDMC × R-FDMC + SDMO × R-
FDMO) × 100%
2500 L·kgcat.-1·h-1, CO : CH3ONO : Ar : N2 = 19 : 45: 3 : 33 (volume), 393 K, 0.1
b
c
MPa. Selectivity to DMC based on CO. WTY represents the weight-time yield,
grams of DMC per kilograms of the catalyst per hour (g·kgcat.-1·h-1).
WTY of DMC (g·kgcat.-1·h-1) = conversion of CO (%) × selectivity to
DMC (%) × WHSV of CO (L·kgcat.-1·h-1) ×
90.08 g·mol-1/(22.4 L·mol-1)
Where [Ar]in and [Ar]out are the concentrations of Ar at the inlet
and outlet, respectively, and [CO]in and [CO]out are the
concentrations of CO at the inlet and outlet, respectively.
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DOI: 10.1039/C9CY02330E
Fig. 2 XRD patterns of various catalysts (a) Pd(II)/UiO-66 (b) Pd(II)/MIL-101 and (c) Pd(II)/MOF-5 support without Pd, before and used of reaction, respectively.
3.2 Catalyst characterization
defect sites and these can influence the Lewis acidity of the
framework. Typically, there could be two types of defects in the
framework, a missing-linker defect and a missing-cluster defect.38
The first detailed investigation into missing linker defects was
performed by the Zhou and coworkers.39 Moreover, many MOFs
benefit from additional Lewis acid sites on their clusters, which
were recently attributed to the presence of structural defects in the
MOFs framework. Thus, the thermogravimetric analysis to
quantitate the number of coordinated BDC as well as the missing-
linker defects per metal cluster of three MOFs supports. For specific
calculation methods, we refer to the reported literature.40
The TGA curves (Fig. S3) are normalized in order to calculate the
number of missing linkers, and the minimum weight attained by the
sample, WLfinal is taken to be 100%. WLPth represents the
theoretical weight loss plateau, Tlink is the temperature after which
the weight loss is attributed to the linker combustion, and WLPexp
represents the experimental weight loss plateau. The estimates
number of BDC missing-linker defects per Zr6 cluster, Cr3 cluster and
Zn4 cluster is around 3.4, 1.13 and 0.91, respectively. The defects
Fig. 2 presents XRD patterns of various Pd(II)/UiO-66, Pd(II)/MIL-
101 and Pd(II)/MOF-5 catalysts. It was found that the XRD patterns
of Pd(II)/UiO-66, Pd(II)/MIL-101 and Pd(II)/MOF-5 catalysts matched
well with the patterns of UiO-66, MIL-101 and MOF-5 supports,
indicating that structure of supports were well maintained during
preparation of catalysts and corresponding catalysts were
successfully synthesized, respectively. Across all catalysts samples
of the XRD patterns have a very low background signal and sharp
reflections, indicating their superior crystalline properties. As
shown in Fig. 3, the TEM, HAADF-STEM and select-area element
mapping analyses were carried out to investigate the distribution of
the Pd species. There is no obviously aggregation of Pd species. The
select-area element mapping of Pd, Zr, N and O for the fresh
Pd(II)/UiO-66 catalysts means that Pd species were uniformly
distributed on the support of the UiO-66.
The hydrothermal stability of three supports using
thermogravimetric analysis was performed. As shows in the Fig. 4,
UiO-66 with an obvious weight loss peak between 747 and 853 K,
which correspond to the complete decomposition of the UiO-66
framework. In addition, MIL-101 and MOF-5 have significant weight
loss peaks at 650 K and 668 K, respectively.(Fig. S3) Obviously, the
first weight loss was observed in the range of 308–373 K and it was
attributed to the volatilization of the adsorbed H2O. The second
weight loss was in the range of 373–623 K and it was attributed to
the dehydroxylation of the Zr6 cluster and to the removal of the
monocarboxylate ligands and it was accompanied with a peak of
the DTG curve. The third weight loss was the destruction of the
framework by the combustion of the linker in the structure in the
range of 623–823 K. These supports with different numbers of
Fig. 4 TGA/DTG profiles of UiO-66.
Black solid line, left axis TGA curve, Red solid line, right axis DTG curve. WLfinal
(orange dashed line) represents the end of the theoretical TGA weight-loss
plateau; WLPth (blue dashed line) represents the start of the theoretical TGA
weight-loss plateau; WLPexp (green dashed line) represents the start of the
experimental TGA weight-loss plateau; Tlink(pink dashed line) represents the
temperature of the combustion of the linker.
Fig. 3 TEM image (a), HAADF-STEM image (b), and select-area element mapping
analyses of O, N, Pd, Zr of fresh Pd(II)/UiO-66 catalysts.
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number of UiO-66 is higher than the others, which means that the
Zr6 cluster could product more Lewis acid sites to enhance catalytic
performance.
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DOI: 10.1039/C9CY02330E
As shown in Table 1, the prepared sample Pd(II)/UiO-66 (Zr)
exhibits a high surface area of 1386.9 m2·g-1 and a pore volume of
0.6 cm3·g-1, which are higher than those of MIL-101 (734.6 m2·g-1)
and MOF-5 (922.6 m2·g-1). And three catalysts' pore volumes have
the similar tendency. Fig. S2 illustrates the N2 adsorption-
desorption isotherms and BJH pore size distribution plots
determined from the desorption branch of the N2 isotherms for the
three MOFs supports. The pore size distribution, derived from
desorption data and calculated from the isotherm using the BJH
model, shows that most of the pores fall into the average size range
of 2.3 nm. Interestingly, the order of the BET surface area
(Pd(II)/UiO-66 > Pd(II)/MIL-101 > Pd(II)/MOF-5) is consistent with
that of the catalytic activity. Obviously, the enlargement of pore
size and pore volume in catalysts may also contribute to the
catalytic performance, which is favorable for increasing the catalytic
efficiency of the active sites located in the channel of supports. The
large BET surface area can expose more active sites to promote the
adsorption of CO on the surface. The result indicates that the large
BET surface area and pore volume could benefit the catalytic
activity.
XPS was performed to investigate the composition and the
surface chemical state of catalyst support. The related work
reported that the electronic state of the active species plays an
essential role in catalytic process. Therefore, three catalysts exhibit
different electron density of Pd, due to the mutual interactions
between supports and Pd species. As show in Fig. 5, the binding
energy (BE) of the Pd 3d5/2 peaks of fresh Pd(II)/UiO-66, Pd(II)/MIL-
101 and Pd(II)/MOF-5 catalysts were 337.74 eV, 337.65 eV and
337.28 eV, respectively, decreasing from Pd(II)/UiO-66 to
Pd(II)/MOF-5. As shown in Fig. S5, the peaks at 332.5 eV and 347 eV
were attributed to Zr 3d3 and Zr 3d1 in Pd window, respectively. It is
clearly that the catalytic activity consistent with the trend of BE of
the Pd species in different catalysts, indicating that the Pd species
with higher binding energy exhibited lower electron density and
higher catalytic performance. It means that the UiO-66 support
Fig. 5 Pd 3d XPS spectra of fresh (a) Pd(II)/UiO-66, (b) Pd(II)/MIL101 and (c) Pd(II)/MOF5
catalysts.
possess stronger electron-withdrawing property with the Pd
species. By analyzing the atomic orbitals of the metal nodes of
MOFs, the empty orbits of Zr4+ are more than Cr3+ and Zn2+,
respectively. The Zr based MOFs has a stronger ability to accept
electrons.
The different electron acceptability of supports was exhibited
different Lewis acidity, which also led to the difference in catalytic
performance. It is well known that the supports surface acidity of
the catalyst plays a dominant role in the Lewis acid catalyzed
reaction, and a large amount of acidic sites on the support provide a
rich adsorption sites for CO.41
Fig. 6 In situ DRIR spectra of CO adsorption on catalysts at 293 K range from 2500
to 1800 cm-1 Pd(II)/UiO-66.
Table 1 The physical parameters of Pd(II)/MOFs catalysts with different supports.
Entry
Catalysts
a Pd loading
1.32%
b SBET(m2·g-1)
1386.9
c DPore (nm)
2.37
d Vt(cm3·g-1)
0.62
1
2
3
Pd(II)/UiO-66 (Zr)
Pd(II)/MIL-101 (Cr)
Pd(II)/MOF-5 (Zn)
1.42%
734.8
2.36
0.37
1.36%
922.6
3.21
0.38
a measured by ICP; b Brunauer-Emmett-Teller surface area; c The BJH desorption average pore width; d Single point adsorption total pore volume at P/P0=0.99.
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The in situ DRIRS of CO adsorption on catalysts can further proves produce a large number of by-products to reduceVietwheArtcicaletaOlnyltiniec
that the electron acceptability of supports can affect the adsorption activity.23 As shown in Fig.7, it can be oDbvOiIo:u10sl.1y03s9e/eCn9CtYh0a2t33th0Ee
of the reactants. As shown in Fig. 6, the peaks of 1998 cm-1 and amount of Lewis acids sites both weak and medium acid sites of
1987 cm-1 were attributed to the bridge adsorption of CO on the Pd(II)/UiO-66. Pd(II)/UiO-66 is larger than those of other catalysts
catalyst.42, 43 The peaks of 2032 cm-1 and 2024 cm-1 were assigned to with MIL-101 and MOF-5 as a support, suggesting that UiO-66 is
linear absorbed CO on the surface of the Pd species.44
beneficial to CO esterification to DMC.
The stronger linear adsorption peaks of CO on the Pd(II)/UiO-66
According to the results of NH3-TPD, we investigated the Lewis
catalyst, suggesting that catalysts with more Lewis acid sites can acid sites over the catalysts supports and found that the amount of
provide more CO adsorption sites. Obviously, compared to the acid sites is an important factor affecting the catalytic performance.
Pd(II)/UiO-66 catalyst, the linear adsorbed CO on the Pd(II)/MIL-101 Therefore, three supports were further carried out to distinguish
and Pd(II)/MOF-5catalyst is slightly shifted toward low the Lewis acid sites and Brönsted acid sites by FT-IR using pyridine
wavenumbers (Fig. S7). This slight red shift in the frequency of CO as a probe molecule. As is well-known, MOFs materials were
adsorption is due to the electron transfer from the support to the generated most of Lewis acid sites through the thermal treatment
Pd species. The linear adsorption of UiO-66 at high wavenumbers to remove hydroxyl groups and partial framework linkers.35, 47, 48 We
means that the support has a stronger electron withdrawing ability know Zr(IV) as a hard Lewis acid, which the multi-functionality and
and promotes the adsorption of CO, which is consistent with the strong acidity of the Zr-based node make it become a potential
result of XPS. This result is consistent with the catalytic catalyst support. Generally, the band around 1450 cm-1 described to
performance, indicating that catalysts with higher Lewis acid sites the vibration of pyridine, which was used to assign to the Lewis acid
can provide more CO adsorption sites and enhance the catalytic sites. The band around 1540 cm-1, corresponding to the vibration of
performance.
pyridinium cations, was used to determine Brönsted acid sites.49
To clarify the nature of catalyst active sites for CO esterification However, those bands appear in the 1400–1600 cm-1 range which is
to DMC. The NH3-TPD was used to investigate the acid not visible in the case of MOFs materials with stronger bands due to
characteristics of Pd(II)/MOFs catalysts with different supports. carboxylate and ring vibrations.50 Therefore, we mainly concerned
There are three NH3 desorption peaks in Fig. 7 which can be on the signal peak between the wave numbers of 1000-1100 cm-1 in
ascribed to three types of acid sites with different intensities. First, this work. Fig. S6 shows the IR spectra of UiO-66, MIL-101 and
NH3 desorption between 232 K and 473 K was assigned to weak acid MOF-5 after adsorption of pyridine at 303 K followed by vacuum
sites, which correspond to the physical adsorption of NH3 and NH3 treatment at different temperatures 373 K, 473 K, 573 K. We only
adsorbed on weak Lewis acid sites. Second, NH3 desorption focus on the infrared spectrum of pyridine at 473 K, which
between 473 K and 573 K was assigned to medium acid sites, which represents the total amount of acid.
means NH3 adsorbed on strong Lewis acid sites. And the last one, As shown in Fig. 8, the intensities of Lewis acid strength over the
NH3 desorption between 573 K and 673 K was assigned to strong three catalysts can be determined by three major signal peaks at
acid sites, which were probably associated with NH3 adsorbed on 1014, 1043 and 1070 cm-1. According to the reported works, MIL-
Brönsted probably associated with NH3 adsorbed on Brönsted acid 101 is attributed to the ʋ1, ʋ12 and ʋ18 ring modes of pyridine
sites.20, 45, 46 UiO-66 shows a broad desorption signal corresponding adsorption, respectively.51 Those peaks were assigned to pyridine
to the medium intensity, confirming that there exists more medium coordinated to Cr CUSs, which plays important roles as Lewis acid
acid sites than the MIL-101 (Cr) and MOF-5 (Zn). We found that NH3 sites interacting with pyridine. The peaks area of pyridine
desorption at high temperatures could exposed more adsorption in UiO-66, MIL-101 and MOF-5 were 13.636, 5.388 and
coordinatively unsaturated Zr sites (Zr-CUSs) to form Lewis acid 3.398, respectively. Therefore, the amount of Lewis acid sites in
sites. In addition, it is clear that the UiO-66 sample could expose three MOFs supports decreased from UiO-66 to MOF-5, which was
more Lewis acid sites that may originate from the Zr-CUSs. This consistent with the order of their catalytic performance.
result was consistent with the thermogravimetric analysis. Ma et al
reported have shown that excessive Brönsted acids could led to
CH3ONO decomposition and
Fig. 8 FT-IR spectra observed after adsorption of pyridine of UiO-66, MIL-101 and
MOF-5 supports followed by vacuum treatment at 473 K.
Fig. 7 NH3-TPD profiles of Pd(II)/MOFs catalysts with different supports.
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Catalysis Science & Technology
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Journal Name
The UiO-66 support composed of Zr6(µ3-O)4(µ3-OH)4 inorganic
nodes and Lewis acid sites stem from µ3-OH, Zr-OH, Zr-OH2 sites, but also confirms the electron transfer between the Pd active
and exposed Zr4+, respectively. The μ3-OH groups of Zr6 nodes species and the UiO-66 support. Stronger electronic receptivity
and Cr3 nodes have great potential to anchor Pd active species means strong Lewis acidity, which is consistent with the previous
to balance the charge.52-55 The Pd species may be dispersed in results of NH3-TPD and Py-IR. The defects on the support greatly
both inner channels and outer surfaces of UiO-66 support and increase the amount of Lewis acid sites, which promotes the
formed Zr-O-Pd bonds. As shown in Fig. 9, three supports performance for CO esterification to DMC.
structure that exposed a large amount of Lewis acid sites due
ARTICLE
It is not only provides evidence for the generation of Lewis acid
View Article Online
DOI: 10.1039/C9CY02330E
to the deletion of the organic ligand can serve as a strong
4. Conclusion
adsorption site. The UiO-66 contains one Zr6(µ3-O)4(µ3-OH)4
unit and nine coordinated 1, 4-benzodicarboxylic acid ligands.
In summary, we obtained a high-performance Pd(II)/UiO-66
And the zirconium atom is coordinated by six oxygen atoms
catalyst with 87.9% CO conversion, 98.5% DMC selectivity. Notably,
and zirconium atom is connected to zirconium atom via Zr-O-
-
the WTY of DMC over Pd(II)/UiO-66 catalyst reaches to 1653 g·kgcat.
Zr links. The MIL-101 contains one Cr3 cluster and six
coordinated 1, 4-benzodicarboxylic acid ligands. And the
1·h-1. By compared with Pd(II)/MIL-101 and Pd(II)/MOF-5 catalysts,
the abundant Lewis acid sites in UiO-66 may be responsibility for
chromium atom is coordinated by six oxygen atoms and
the superior catalytic performance of Pd(II)/UiO-66. Meanwhile, the
chromium atom is connected to chromium atom via Cr-O-Cr
structures of the three MOFs supports have been analyzed in detail
links. The MOF-5 contains one Zn4O nodes coordinated with
to illustrate the origin of the Lewis acid sites. The number of MOFs
six 1, 4-benzodicarboxylic acid ligands. And four zinc atoms are
missing-linkers was quantified by thermogravimetric analysis. It was
coordinated by six oxygen atoms and zinc atom is connected
proved that UiO-66 with more defects would have a large amount
to zinc atom via Zn-O-Zn links. Both UiO-66 and MIL-101 are
of Lewis acid sites, and the catalytic performance was positively
trimeric structures of nodal metals, and the Zr6 cluster has
correlated with the amount of Lewis acid sites in MOFs supports,
much more μ3-OH groups than Cr3 cluster. It was found that
the MIL-101 and MOF-5 structural units exposed much less
Lewis acid sites than UiO-66 support, which is also a key factor
for the strong acidity of UiO-66. The previous
which can be demonstrated by the characterizations of NH3-TPD,
Pyridine-IR, XPS and in situ DRIRS. In addition, catalysts with more
Lewis acid sites are beneficial for CO adsorption and greatly
improve the reaction performance. This work investigated the
thermogravimetric analysis strongly proves that UiO-66 have a
application of MOFs as
a solid acid catalyst support in
large number of defects, and also indicates that defects can
influence the Lewis acidity of the framework The amount of
heterogeneous catalysis and provided guidance for further
potential applications of MOFs materials in heterogeneous
catalysis.
.
Lewis acid sites were consistent with the order of catalytic
performance, which means that a large amount of Lewis acid
sites were favorable for producing DMC. It was found that the
support have strong ability to attract electrons due to a large
number of Lewis acid sites. Thus, the active Pd species is more
likely to be stable on the support and exhibits excellent
catalytic performance. Stronger electronic receptivity means
strong Lewis acidity, which was consistent with the results of
XPS and in situ DRIRS. Therefore, Lewis acid sites have been
Conflicts of interest
There are no conflicts to declare.
considered to play crucial roles in the CO esterification to DMC, Acknowledgements
which was consistent with the result of NH3-TPD. Obviously,
This work was supported by the National Key R&D Program of
Pd(II)/UiO-66 catalysts exhibited much better performance than the
others, which suggests that the exposed Lewis acids sites could as
active centers to enhance catalytic performance, which also
indicates Pd species has a strong interaction with the UiO-66
support.
China (2017YFA0206802, 2017YFA0700103), the National
Natural Science Foundation of China (91545201, 91645116),
the Programs of the Chinese Academy of Sciences (ZDRW-CN-
2016-1, QYZDJ-SSW-SLH028), the China Postdoctoral Science
Foundation (2016LH0018) and the Natural Science Foundation
of Fujian Province (2018J06005).
Notes and references
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