A novel Cu-Mn/Ca-Zr catalyst for the synthesis of methyl formate from syngas

Article abstract of DOI:10.1039/c5ra13555a

A novel catalyst comprised of Cu-Mn mixed oxides and CaO-ZrO₂ solid base has contributed to a high-performance methyl formate (MF) synthesis from syngas in a slurry reactor. Cu-Mn mixed oxides and mesoporous CaO-ZrO₂ solid base were prepared by complexing method and alcohothermal route, respectively, and they were characterized by N₂ isotherm adsorption-desorption, XRD, SEM, TEM, XPS and CO₂-TPD techniques. Under the optimum reaction conditions of 160 °C, 3 MPa, 3 : 7 for the ratio of methanol to N,N-dimethylformamide, 40 g L^-1 Cu-Mn sample, and 30 g L^-1 CaO-ZrO₂ sample, a low CO conversion of 22.4% was obtained over Cu-Mn/Ca-Zr, whereas the MF selectivity of 82.3% was higher than that of the traditional catalyst (e.g. Cu-catalyst and NaOCH₃), which was due to the synergism between Cu-Mn and CaO-ZrO₂ samples.

Full text of DOI:10.1039/c5ra13555a

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A novel Cu-Mn/Ca-Zr catalyst for the synthesis of methyl
formate from syngas
Haijun Zhaoa, b, Minggui Lina, *, Kegong Fanga, Juan Zhoua, Ziyu Liuc, Gaofeng Zengc,
Yuhan Sunc, *
a State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of
Sciences, Taiyuan 030001, Shanxi, P.R. China
b University of Chinese Academy of Sciences, Beijing, 100049, P.R. China
c Key Laboratory of Lowꢀcarbon Conversion Science and Engineering, Shanghai Advanced
Research Institute, Chinese Academy of Sciences, Shanghai, 201203, P.R. China
Abstract
CuꢀMn mixed oxides and mesoporous CaOꢀZrO2 solid base were prepared by
complexing method and alcohothermal route, respectively, and they were mixed
together as combined CuꢀMn/CaꢀZr catalyst which was evaluated for the direct
synthesis of methyl formate (MF) from syngas in a slurry reactor. CuꢀMn and
CaOꢀZrO2 samples were characterized by N2 isotherm adsorptionꢀdesorption, XRD,
SEM, TEM, XPS and CO2ꢀTPD techniques. Under the optimum reaction conditions
of 160 oC, 3 MPa, 3:7 for the ratio of methanol to N, N dimethylformamide, 40 g Lꢀ1
for CuꢀMn sample, and 30 g Lꢀ1 for CaOꢀZrO2 sample, a lower CO conversion of
22.4 % was obtained over CuꢀMn/CaꢀZr, whereas the MF selectivity of 82.3% was
higher than that of traditional catalyst (e.g. Cuꢀcatalyst and NaOCH3), which was due
to the synergism between CuꢀMn and CaOꢀZrO2 samples.
* Corresponding authors.
Eꢀmail address: linmg@sxicc.ac.cn (Minggui, Lin), yhsun@sxicc.ac.cn (Yuhan, Sun)

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1. Introduction
Methyl formate (MF) is one of the most important chemical intermediates, which
can be used to synthesize a large number of chemicals such as formic acid, acetic acid,
ethylene glycol, methyl propionate and methyl glycolate.1ꢀ5 Besides, MF is considered
to be a promising substitute for methyl tertiary butyl ether (MTBE) and ecological
fuel for vehicles.6,7 In agriculture, MF is regarded as pesticide, bactericide and fruit
desiccant. Thus, the catalytic synthesis of MF has attracted much attention of
researchers.
There are many effective processes developed for MF synthesis so far. The
commercial method to synthesize MF is the carbonylation of methanol over alkali
o
methoxide catalysts at 80 C in liquid phase8ꢀ10. However, the catalyst deactivation
caused by CO2 and H2O in system exist in above process. Recently, the selective
oxidation of methanol to MF on noble metal catalysts such as Au, Ru, Ag and Pd has
been the research hotspot due to its high conversion and selectivity.11ꢀ15 Wojcieszak et
al.16 studied the oxidation of methanol to methyl formate over Pd nanoparticles
supported on γꢀFe2O3 at 80 oC with 76% methanol conversion and 81% MF selectivity.
Wang et al.17 reported methanol selective oxidation, with a methanol conversion of
o
90.2% and selectivity of 100% to methyl formate, at 70 C on grapheneꢀsupported
AuꢀPd nanoparticles, owing to the synergism of Au and Pd particles as well as the
strong interaction between graphene and Au–Pd nanoparticles. The major
disadvantage of this process is the cost of catalyst due to the presence of noble metal.
Besides, the photocatalytic reduction of CO2 to MF in methanol also aroused the
interests of researchers.18,19 Chen et al.20,21 employed the photocatalytic reduction of
CO2 to MF on B2S3 and Niꢀdoped ZnS catalyst with the rate of MF production 175

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ꢁmol gꢀ1 hꢀ1 and 121 ꢁmol gꢀ1 hꢀ1, respectively, while the low yield and poor stability
limit the application of this process. This has stimulated a desire for the development
of an alternative process.
Direct synthesis of MF from syngas in liquidꢀphase system is significantly
concerned by researchers due to its atomic economy and short reaction process. 22ꢀ26
R
The direct process follows the stoichiometry: 2CO+2H2 → HCOOCH3, △H0
=
ꢀ157.2 kJ molꢀ1 (1).27 In the direct synthesis process, MF formed as an intermediate of
methanol synthesis from syngas on mixed catalyst comprised of alkali or alkali earth
compounds and Cuꢀbased catalysts,22 which contains homogeneous carbonylation of
methanol on alkali or alkali earth compounds and heterogeneous hydrogenation of
MF on Cuꢀbased catalysts. Palekar et al.23 controlled the selectivity of methanol and
MF by changing reaction temperature and the ratio of CO to H2. They found that high
reaction temperature and low CO/H2 were beneficial to methanol; on the contrary, low
reaction temperature and high CO/H2 condition favored MF formation, and the similar
results were also reported by Chen et al.28 However, a critical problem in this
process is that homogeneous alkali or alkali earth compounds is susceptible to CO2 or
H2O in reaction system, which accelerate the fall of the catalytic activity.29ꢀ31
Moreover, alkali or alkali earth compounds distributed on the surface of Cuꢀbased
catalyst caused the blockage of active sites.32
In this work, CaOꢀZrO2 solid base and CuꢀMn mixed oxides were prepared and
characterized by N2 isotherm adsorptionꢀdesorption, XRD, TEM, SEM, XPS and
CO2ꢀTPD measurements. More importantly, the combined CuꢀMn/CaꢀZr catalyst
prepared by mechanical mixing above two asꢀprepared samples was applied to the
direct synthesis of MF from syngas in a batch slurry reactor for the first time. In
addition, the influences of reaction parameters such as temperature, pressure, the ratio

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of methanol to DMF, the concentration of CaOꢀZrO2 and CuꢀMn smaples on catalytic
performance were thoroughly investigated.
2. Experimental
2.1 Preparation of catalysts
2.1.1 Preparation of Cu-Mn mixed oxides
The preparation of CuꢀMn mixed oxides is described as follows: 1 M Mn(NO3)2
aqueous solution with pH = 3ꢀ4 was dropped into 1 M copper ammonia solution of
pH = 9ꢀ10 with vigorous stirring at 35 °C at a dropping rate of 1mL minꢀ1. After aging
for 5 h, the precipitate was washed with deionized water for several times, then
filtered, dried, and calcined at 550 °C for 4 h. The normal molar ratio of Cu to Mn
was 1.0.
2.1.2 Preparation of CaO-ZrO2 solid base
CaOꢀZrO2 solid base was prepared by solꢀgel method as the reported procedure
previously.33 Typically, 1 g of amphiphilic poly block copolymers (PEO20PPO70PEO20,
Pluronic P123, Aldrich) and 1.5 g Ca(NO3)2ꢂ4H2O were dissolved in 24.2 mL absolute
ethanol, which noted as solution A. Meanwhile, 4 g of zirconium nꢀpropoxide (70
wt. % in nꢀpropanol, Alfa Aesar) and 0.43 g acetylacetone were dissolved in 16.1 mL
absolute ethanol to produce solution B. Then solution B and 1.6 mL H2O were
dropped slowly into solution A sequentially. The resulting solution was stirred for 2 h
before it was aged at 60 °C for 24 h. After aging, the acquired white gel was refluxed
in 0.5 M NaOH solution for 24 h. Subsequently, the product was washed by deionized
water to remove residual Na+ and filtered. The white hybrid was then dried at 100 °C
overnight and calcined at 600 °C for 6 h to produce solid base, and the normal mole

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ratio of Ca to Zr is 0.5.
2.1.3 Preparation of Cu-Mn/Ca-Zr catalyst
CuꢀMn/CaOꢀZrO2 combined catalyst, noted as CuꢀMn/CaꢀZr, was prepared by
mechanical mixing CuꢀMn mixed oxides and CaOꢀZrO2 solid base. The mass ratio of
CuꢀMn to CaOꢀZrO2 is determined by the concentration of two samples in slurry
phase. The concentration of CuꢀMn catalyst is 0ꢀ65 g Lꢀ1, and that of CaOꢀZrO2
catalyst is 0ꢀ50 g Lꢀ1.
2.2 Catalyst characterization
The specific surface area of the sample was measured with ASAP 2020 analyzer
using the multipoint Brunauer, Emmett and Teller (BET) adsorption. Xꢀray diffraction
(XRD) characterization including smallꢀangle (SXRD) was conducted using a Bruker
diffractometer employing Cu Kα radiation (40 kV and 40 mA). Scanning electron
microscopy (SEM) images were taken with a JSMꢀ7001F electron microscope
operating at 10 or 5 kV. Transmission electron microscopy (TEM) images were
recorded using a JEMꢀ2100F microscope operated at 200 kV. Xꢀray photoelectron
spectroscopy (XPS) experiments were carried out with an AXIS ULTRA DLD X
photoelectron spectrometry employing an AlꢀKα Xꢀray source. The binding energy of
the samples was corrected with the energy of C1s (284.6 eV) as the reference.
Temperatureꢀprogrammed desorption of CO2 (CO2ꢀTPD), 0.2 g samples were
pretreated in a flow of He (99.99%) at a rate of 10 oC minꢀ1 from room temperature to
o
o
700 C and kept for 2 h. Prior to adsorption of CO2 (99.999%) at 30 C, blank
o
o
experiment was carried out from 30 C to 700 C to confirm no CO2 desorption
occurred. When the temperature elevated, the CO2 desorbed was detected by TCD.

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2.3 Catalyst performance testing
A batch stainless steel autoclave reactor with an inner volume of 100 mL was
employed. The designed amount of CuꢀMn/CaꢀZr catalyst and solvent including
methanol (99.95%) and N, N dimethyl formamide (99.5%, DMF) were poured into
the reactor. After replacing the air in reactor with N2 for 2ꢀ3 times, feed gas was
permitted into the reaction system at room temperature until the pressure of the
system increased to the required level. Then the temperature was increased in 30 min.
The stirring speed was 1000 rpm to resist mass transfer effect. The gas phase was
analyzed using Carbosieveꢀpacked column equipped with thermal conductivity
detector (TCD) and PorapackꢀQ column equipped with flame ionization detector
(FID), and liquid samples were analyzed on PorapackꢀT column equipped with
thermal conductivity detector (TCD). The composition of the feed gas is H2/CO/N2 =
64/32/4, where nitrogen was employed as an internal standard.
3. Results and discussion
3.1 Catalyst characterization
3.1.1 Physicochemical properties
Fig. 1 shows the N2 isotherm adsorptionꢀdesorption and pore size distribution
(inserted) profiles of CaOꢀZrO2. As can be seen, the CaOꢀZrO2 solid base shows the
type IV isotherm and H2 hysteresis loop. The presence of type IV isotherm plot
indicates the mesoporous structure of CaOꢀZrO2, and H2 hysteresis loop is the
characteristic of “inkꢀbottle” or “network” type of pores formed from the voids
between nanoꢀparticles in this sample.34 Meanwhile, the CaOꢀZrO2 sample exhibits an
narrow pore size distribution ranging from 1.8 nm to 10 nm with a center at 3.7 nm in
the inserted of Fig. 1, suggesting a rather uniform pore structure of CaOꢀZrO2. The

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SXRD plot of CaOꢀZrO2 shows a broad peak with maximum at 1.28o (Fig. 2), which
indicates the uniform pore structure of CaOꢀZrO2 in line with the result of Fig.1.
Besides, the CaOꢀZrO2 possesses a specific surface area of 53 cm2 gꢀ1 determined by
BET surface area. However, the CuꢀMn catalyst with a very small specific surface
area of 14 cm2 gꢀ1 is obtained.
3.1.2 XRD measurements
The crystalline phases of CaOꢀZrO2 and CuꢀMn oxide samples are investigated by
XRD analysis (Fig. 3). For the CaOꢀZrO2 solid base, only tetragonal ZrO2 (PDF
88ꢀ1007) diffraction peaks at 30.3o, 35.2o, 50.6o and 60.3o are detected, while no
peaks belonging to CaO are found, which indicates that CaO is highly dispersed in
solid base with amorphous structure, or Ca2+ incorporates in ZrO2 lattice and
homogeneous CaOꢀZrO2 solid solution is formed.35,36 The diffraction peaks of
CaOꢀZrO2 are broad and weak, which implies that the CaOꢀZrO2 solid base possesses
very small crystalline size or low crystallinity. For the CuꢀMn mixed oxides, CuO
(Tenorite, PDF 72ꢀ0629) and incomplete spinel Cu1.5Mn1.5O4 (PDF 70ꢀ0262) with
monoclinic and cubic structures are observed, and no diffraction peaks corresponding
to Mn oxides are detected, which is due to partial Mn species existing in amorphous
or being in highly dispersed.
3.1.3 SEM and TEM
The morphology information of CuꢀMn and CaOꢀZrO2 catalysts is characterized by
SEM in Fig. 4a and b. The CuꢀMn is mainly comprised of uniform particles with the
size of 50ꢀ100 nm, which aggregate together in random (Fig. 4a). For the CaOꢀZrO2
catalyst, both large lumps (≈10 ꢁm) with flat surfaces and smaller particles on top are

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observed, which might be formed from the clusters of primary particles (See Fig. 4d).
TEM is used to investigate the particle size and textural structure of CuꢀMn and
CaOꢀZrO2 samples (Fig. 4cꢀf). As shown in Fig. 4c, the CuꢀMn exhibits loose
agglomerates with the particle size about 20ꢀ100 nm. The lattice spacings of 0.27 nm
and 0.21 nm could be assigned to the (110) planes of CuO and (400) planes of
Cu1.5Mn1.5O4 as given in Fig. 4e, which indicates the coexistence of CuO and
Cu1.5Mn1.5O4 phases in CuꢀMn mixed oxides, in line with the result of XRD (Fig. 3b).
The CaOꢀZrO2 is highly porous in nature and consists of the narrowly distributed
nanoparticles with the size less than 10 nm (Fig. 4d). It can be seen in Fig. 4f that
nanoꢀcrystals and amorphous nanoꢀparticles are coexistence in CaOꢀZrO2 sample,
indicating that CaOꢀZrO2 is semiꢀcrystalline structure, in accordance with the weak
and broad diffraction peak in XRD pattern (Fig. 3a).
3.1.4 XPS measurements
The oxidation states of Cu and Mn component in CuꢀMn catalyst are determined
via XPS surface analysis (Fig. 5). Five peaks are distinguished by deconvoluting Mn
2p peak. The peak at 641.1 eV and 652.9 eV are attributed to Mn2+ and the peak at
642.7 eV and 654.4 eV are assigned to Mn3+ or Mn4+ cations.37,38 Moreover, a satellite
peak at 648.2 eV could be detected, which suggests the presence of Mn2+.39ꢀ41
However, there are no Mn2+ oxide peaks in XRD patterns (Fig. 3), indicating that the
Mn2+ oxides coexist in amorphous structure with Cu1.5Mn1.5O4 and CuO in CuꢀMn
sample.
From the Cu 2p XPS plot of CuꢀMn catalyst (Fig. 5b), it can be seen that Cu 2p3/2
main peaks located at 933.7 eV and 934.9 eV which are assigned to Cu2+ species. The
shakeꢀup satellite peaks at 941.4 eV and 943.8 eV corresponding to Cu 2p3/2 also

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confirm the presence of Cu2+, and these peaks do not appear in Cu+ or Cu0 species,
since shakeꢀup transitions do not occur in filled 3d shells and metallic states.41,42 The
Cu 2p XPS profile also shows a peak at the binding energy of 930.9 eV, which could
be attributed to cuprous or metallic copper.40,42 Unfortunately, Cu0 cannot be clearly
distinguished from Cu+ by the XPS profile of Cu 2p due to their similar position of
binding energy. It is reported that the following redox equilibrium is established in
Cu1.5Mn1.5O4,40ꢀ43 Cu2+ + Mn3+ = Cu+ + Mn4+. Therefore, it can be assumed that the
peak at 930.9 eV is characteristic of Cu+ cation. Auger electron spectroscopy is taken
to further distinguish the chemical state between Cu+ and Cu0 (Fig. 6). According to
the literatures,40,44ꢀ45 the peak of Auger spectra with kinetic energy of 918.5 eV is
assigned to Cu0, the peak at 917.6 eV, which is attributed to Cu2+, for the peak at
916.5 eV, which is related to Cu+. In this case, the corresponding kinetic energy
spectra of Auger electron includes two peaks at 917.8 eV and 916.8 eV (Fig. 6), which
could be assigned to Cu2+ and Cu+. Besides, a low kinetic energy broad peak at 913.1
eV can be observed, which belongs to Cu2+ or Cu+ according to the literature,46
indicating the absence of Cu0. In addition, there is no possibility to form Cu0 during
o
the preparation process of CuꢀMn oxide, for the samples were calcined at 550 C in
air, in line with the results of Mn 2p XPS and XRD.
3.1.5 CO2-TPD
The CO2 desorption temperature can be used to evaluate the base strength, and the
basicity of each type of basic site is determined by the desorption amount of CO2. As
shown in Fig. 7, the basic property of CaOꢀZrO2 is characterized by CO2ꢀTPD. The
CO2 desorption profile can be roughly divided into three broad peaks: low
temperature peak below 235 oC, medium temperature peak from 240 oC to 515 oC and

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high temperature peak around 603 oC, which are named α, β and γ, respectively. The α
o
peak centered at 163 C with about 62 ꢁmol gꢀ1 CO2 uptake could be assigned to the
CO2 desorption on the weak basic site of ZrO2 surface. The β peak centered at 362 oC
with about 219 ꢁmol gꢀ1 CO2 uptake is related to the moderately strong basic site from
the surface of a solid solution, in which the presence of neighboring Ca2+ and Zr4+
affect the basicity of lattice oxygen on the surface,47 and the γ peak at 603 oC with 42
ꢁmol gꢀ1 CO2 uptake is attributed to the strong basic site on the surface of CaO in
CaOꢀZrO2.33 It could be concluded from the results above that there are three basic
sites with different basic strength on the CaOꢀZrO2 solid base surface, and the basicity
of three basic sites follows β > α > γ.
3.2 Influence of reaction variables on MF synthesis
Slurry phase is a complex reaction system, and many factors can influence the
catalytic activity and the selectivity of products. In the present work, reaction
variables such as temperature, pressure, the ratio of methanol to DMF, the
concentration of CaOꢀZrO2 and CuꢀMn samples have been investigated. To simplify
the effect of other parameters, only one parameter is changed while others kept
unchanged.
3.2.1 Reaction temperature
The effect of reaction temperature on the catalytic performance of MF synthesis is
given in Fig. 8. It can be seen that CO conversion monotonically increases with the
rise of temperature from 140 °C to 200 °C. The spaceꢀtime yield (STY) of MF is
relatively low at 140 °C, while it sharply reaches a maximum value of 4.18 g Lꢀ1 hꢀ1 at
160 °C and then decreases. The MF selectivity firstly increases from 29.3% to 83.5%

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as the temperature rises from 140 °C to 160 °C, and then begins to decrease after
further increasing the temperature. It can be inferred from equation (1) that the
synthesis of MF from syngas is a strongly exothermal reaction. The reaction proceeds
slowly at low temperature, which leads to a low CO conversion. However, the
selectivity of MF reaches maximum at the low temperature of 160 °C and begins to
decrease at higher temperature. Therefore, the reaction temperature of 160 °C is
beneficial to the formation of MF. The selectivity of formaldehyde and ethanol shows
the opposite trend to that of MF.
3.2.2 Reaction pressure
Equation (1) indicates that the reaction of MF synthesis from syngas is a gas
volume reducing process before and after reaction. As a consequence, the process
would be favorable to the conversion of CO at higher reaction pressure in a
thermodynamics point view, and vice versa. The relationship between catalytic
performance and initial pressure is shown in Fig. 9. With increasing the pressure, CO
conversion firstly increases from 11.0% at 2 MPa to 20.2% at 3 MPa and then
decreases slowly as the pressure increases above 3 MPa. This result seems to be
opposite to the thermodynamic rule that higher pressure would make the balance
move towards the side where the gas volume decreases, vice versa, which could be
explained by the evaluation method of catalyst. In the present work, the amount of
feed gas purged into the batch reactor at low pressure is less than that at high pressure,
which leads to the phenomenon that the CO conversion at high pressure is smaller
than that at low pressure. The monotonic rise of STY (MF) from 2.85 g Lꢀ1 hꢀ1 at 2
MPa to 4.03 g Lꢀ1 hꢀ1 at 6 MPa could also demonstrate that the process of MF
synthesis follows the thermodynamic rule above. The selectivity of MF consistently

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keeps about 80% with increasing pressure from 2 MPa to 6 MPa, indicating the little
influence of reaction pressure on product selectivity.
3.2.3 The ratio of methanol to DMF
The polarity of solvent in this work is controlled by changing the volume ratio of
methanol to DMF, and its influence on catalytic performance is listed in Table 1. As
can be seen, CO conversion rises firstly and then declines with decreasing the ratio of
methanol to DMF. The STY of MF reaches the maximum value of 4.18 g Lꢀ1 hꢀ1 at the
ratio of 3:7 and begins to decline with further decreasing the ratio. For selectivity, it
can be seen that the MF selectivity firstly enhances with decreasing the ratio of
methanol to DMF from 9:1 to 3:7 and then reaches a plateau level about 83.5%,
whereas the selectivity of formaldehyde and ethanol substantially descend until the
ratio of methanol to DMF decreases to 3:7. It is wellꢀknown that the polarity of
methanol is higher than that of DMF, and the polarity of the liquid phase decreases
with decreasing the ratio of methanol to DMF. Therefore, the appropriate polarity of
solvent is indispensable for the high conversion of CO and STY of MF. The lower
polarity of liquid phase is beneficial to the selectivity of MF, while the higher polarity
of liquid phase supports the conversion of CO.48 Moreover, methanol or DMF
molecular in solvent could interact with products, which may also have a significant
influence on CO conversion and MF selectivity.
3.2.4 The concentration of CaO-ZrO2 or Cu-Mn catalysts
The influence of CaOꢀZrO2 concentration on catalytic performance is shown in Fig.
10. CO conversion drastically varies from 3.3% to 12.0% as the CaOꢀZrO2
concentration increases from 0 to 30 g Lꢀ1. In this concentration range, the STY and

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the selectivity of MF also enhance significantly from 0.28 g Lꢀ1 hꢀ1 and 14.4% to 2.08
g Lꢀ1 hꢀ1 and 72.4%, respectively, while the selectivity of formaldehyde and ethanol
decreases. Further increasing the CaOꢀZrO2 concentration, its influence on CO
conversion and selectivity could be ignored. The influence of CuꢀMn concentration on
catalytic performance is shown in Fig. 11. CO conversion and MF selectivity firstly
rise up to 12.2% and 74.2% as CuꢀMn concentration increases from 0 to 40 g Lꢀ1, and
subsequently reduce with further increasing CuꢀMn concentration. As for the STY of
MF, it monotonically increases until the concentration is more than 55 g Lꢀ1.
More active sites would be exposed with increasing the catalyst concentration,
when the concentration of CaOꢀZrO2 or CuꢀMn catalysts increases below 30 g Lꢀ1 and
40 g Lꢀ1, leading to the increase of CO conversation and MF selectivity. However, as
the concentration of CaOꢀZrO2 and CuꢀMn catalysts is higher than 30 g Lꢀ1 and 40 g
Lꢀ1, the mass transfer resistance of system increases with further increasing the
concentration of samples.49 More specifically, the slurry dispersion of catalyst is
defined and becomes worse; the apparent viscosity of system increases resulting in the
decay of interface mobility, and the solubility of gas phase in slurry decreases. As a
consequence of this, CO conversation and MF selectivity increase slowly or begin to
decrease, and the reaction process might transfer to thermodynamics control from
dynamics control.
3.3 Catalytic performance for MF synthesis
According to the above results, 160 °C, 3 MPa, 3:7 for the ratio of methanol to
DMF, 40 g Lꢀ1 for CuꢀMn concentration and 30 g Lꢀ1 for CaOꢀZrO2 concentration are
the optimum reaction variables to directly synthesize MF from syngas in liquid
phase. Under this reaction conditions, the highest catalytic activity and MF

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selectivity can be obtained over CuꢀMn/CaꢀZr catalyst. The catalytic performance of
CuꢀMn/CaꢀZr catalyst is listed and compared to those of previous literatures in Table
2. As can be seen in Table 2, the CO conversion of 22.4% and the MF selectivity of
82.3% could be obtained on CuꢀMn/CaꢀZr catalyst under the optimum reaction
conditions in this work. Although a relatively low CO conversion is presented on
CuꢀMn/CaꢀZr compared to NaOCH3/Cuꢀcatalyst, HCOOK/Cuꢀcatalyst and
KOCH3/Ni(CO)4 in previous literatures,49ꢀ51 CuꢀMn/CaꢀZr has higher MF selectivity
than traditional catalysts above, which is due to the presence of synergistic effect
between CuꢀMn and CaOꢀZrO2 samples.
3.4 Roles of CaO-ZrO2 and Cu-Mn in MF synthesis
It is worth noticing that CO conversion and MF selectivity are extremely low
when CaOꢀZrO2 or CuꢀMn catalyst is absence from liquid phase, as shown in Fig. 10,
11. However, they sharply increase when the CaOꢀZrO2 or CuꢀMn catalyst is
introduced in system, indicating that both CaOꢀZrO2 and CuꢀMn catalysts play
important roles in MF direct synthesis from syngas. Higher catalytic activity could be
obtained when the concentration of CaOꢀZrO2 and CuꢀMn catalysts is 30 g Lꢀ1 and 40
g Lꢀ1 respectively, which might be due to the existence of synergistic effect between
CaOꢀZrO2 and CuꢀMn samples. Two steps of reaction route might present in the direct
synthesis of MF on CuꢀMn/CaꢀZr catalytic system, and different elementary reactions
proceed on CaOꢀZrO2 and CuꢀMn catalysts, respectively (see 3.5). An optimum
catalytic activity and MF selectivity can be obtained while two elementary reactions
on CaOꢀZrO2 and CuꢀMn catalysts proceed synergistically.
CaOꢀZrO2 is a strong basic catalyst and can be applied to amounts of
baseꢀcatalyzed reactions, such as Knoevenagel reaction, Michael additions, Tischenko

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reaction, ClaisenꢀSchmidt condensation and acetonylacetone cyclization. With the
presence of mesoporous CaOꢀZrO2 solid base, CuꢀMn/CaꢀZr catalyst has an optimum
catalytic activity in the synthesis of MF from syngas. The mesoporous structure and
high specific surface of CaOꢀZrO2 make the basic sites expose on the surface of
sample, and its basic property is beneficial to the synthesis of MF according to the
previous literature.52 The specifically proofs will be provided in another article of
authors. On the other side, incomplete spinel of Cu1.5Mn1.5O4 is formed in CuꢀMn
oxide, in which two JahnꢀTeller ions of Cu2+ and Mn3+ are presented and confirmed
by XPS, leading to a large number of surface oxygen on CuꢀMn catalyst.53 In addition,
a strong interaction exists between Cu and Mn cations due to the formation of
Cu1.5Mn1.5O4 phase. Therefore, two factors of CuꢀMn mixed oxides mentioned above
provide well catalytic activity and MF selectivity for direct MF synthesis from syngas
over CuꢀMn/CaꢀZr catalyst.
3.5 Understanding of MF synthesis
In the traditional direct synthesis of MF, carbonylation and hydrogenation reaction
mechanism has been generally accepted.22ꢀ26 However, in this work, a new catalyst
system of CuꢀMn/CaꢀZr is used, and different products are formed besides MF; thus
the reaction mechanism might differ from carbonylation and hydrogenation. Basing
on the results of Fig.10 and Fig. 11, it can be known that CaOꢀZrO2 or CuꢀMn
catalysts are indispensable in the direct synthesis of MF to obtain high CO conversion
and MF selectivity, and two elementary reactions on CaOꢀZrO2 and CuꢀMn catalysts
proceed synergistically. According to the previous literatures,30,54 the following
reaction mechanism might exist in direct synthesis of MF on CuꢀMn/CaꢀZr catalyst,
as shown in Fig. 12. At first adsorption formate species are formed on CuꢀMn catalyst,

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and then the reaction between adsorption fromate species on CuꢀMn catalyst and
methoxide adsorbed on CaOꢀZrO2 catalyst proceeds to produce methyl formate, at last
partial methyl formate may hydrogenate with hydrogen atom on CuꢀMn catalyst to get
methanol, which also illustrates the significant of synergism between CuꢀMn and
CaOꢀZrO2. However, this reaction mechanism badly needs to be further confirmed in
later work. We can conclude from above reaction route that the methanol in this
system is not only a solvent but an intermediate to synthesize MF also.
4. Conclusions
CuꢀMn/CaꢀZr catalyst prepared by mechanical mixing CuꢀMn and CaOꢀZrO2 are
highly active to the formation of methyl formate from syngas. Its methyl formate
selectivity is higher than that of traditional catalyst (e.g. Cuꢀcatalyst and NaOCH3 ).
Incomplete spinel of Cu1.5Mn1.5O4 phase is formed in CuꢀMn mixed oxides, and
CaOꢀZrO2 solid base exhibits mesoporous structure with three basic sites on the
surface. The optimum reaction conditions for methyl formate synthesis are 160 °C, 3
MPa, 3:7 for the ratio of methanol to DMF, 40 g Lꢀ1 for CuꢀMn concentration and 30
g Lꢀ1 for CaOꢀZrO2 concentration. Under this optimum reaction conditions, CO
conversion of 22.4 % and MF selectivity of 82.3% are obtained, which can be
attributed to the presence synergistic effect between CuꢀMn and CaOꢀZrO2 samples.
The present work prepared a novel CuꢀMn/CaꢀZr combined catalyst, which seems
great potential in the green and efficient route to synthesize methyl formate from
syngas.
Acknowledgements

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The authors acknowledge the financial support from the National Natural Science
Foundation of China (No. 21103217), the Strategic Priority Research Program of the
Chinese Academy of Sciences (No. XDA01020304).
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Table 1 Influence of methanol/DMF ratios on catalytic performance
STY (g Lꢀ1 hꢀ1)
MF
Selectivity (%)
HCHO MF C2H5OH CO2
V (CH3OH) : V (DMF) Conv.CO
9:1
7:3
5:5
3:7
1:9
10.9
13.8
12.4
11.9
7.5
0.67
17.4
15.7
4.0
42.1 30.0 10.5
50.9 22.8 10.6
0.75
4.09
80.6 11.3
83.5 11.0
83.8 11.8
4.1
3.8
2.5
4.18
1.7
2.44
2.0
Catalyst: C (CaOꢀZrO2) = 30 g Lꢀ1, C (CuꢀMn) = 15 g Lꢀ1, p = 4 MPa, T = 160 °C, t = 8 h, solvent:
V = 50 mL.
Table 2 Catalytic performance of different catalysts
Catalyst
Conv. CO (%) Sel. MF (%) Reference
CuꢀMn/CaꢀZr
22.4
57.7
ꢀ
82.3
66.9
73.2
40.3
5.6
This worka
Ni(CO)4, KOCH3
CuꢀMn, NaOCH3
CuCl, NaOCH3
CuꢀCr, KOCH3
HCOOK, Cu catalyst
26
49
49
50
51
ꢀ
88.6
75.6
2.6
a C (CaOꢀZrO2) = 30 g Lꢀ1, C (CuꢀMn) = 40 g Lꢀ1, p = 3 MPa, T = 160 °C, t = 8 h, solvent: V
(CH3OH) = 15 mL, V (DMF) = 35 mL.

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Fig. 1 N2 isotherm adsorptionꢀdesorption and pore size distribution (inserted) profiles
of CaOꢀZrO2
Fig. 2 SXRD plot of CaOꢀZrO2 catalyst

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Fig. 3 XRD patterns of CaOꢀZrO2 (a) and CuꢀMn (b) catalysts

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b
a
c
d
f
e
Fig. 4 SEM (a, b) and TEM (cꢀf) images of the CuꢀMn (a, c, e) and CaOꢀZrO2 (b, d, f)
catalysts

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Fig. 5 Mn 2p (a) and Cu 2p (b) XPS of the CuꢀMn catalyst

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Fig. 6 XPS of Cu L3MM Auger electron of CuꢀMn catalyst
Fig. 7 CO2ꢀTPD plot of CaOꢀZrO2 catalyst

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Fig. 8 The influence of reaction temperature on catalytic performance
Catalyst: C (CaOꢀZrO2) = 30 g Lꢀ1, C (CuꢀMn) = 15 g Lꢀ1, p = 4 MPa, t = 8 h, solvent: V (CH3OH)
= 15 mL, V (DMF) = 35 mL.
Fig. 9 The influence of reaction pressure on catalytic performance
Catalyst: C (CaOꢀZrO2) = 30 g Lꢀ1, C (CuꢀMn) = 15 g Lꢀ1, T = 160 °C, t =8 h, solvent: V (CH3OH)
= 15 mL, V (DMF) = 35 mL.

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Fig. 10 Influence of CaOꢀZrO2 concentration on catalytic performance
Catalyst: C (CuꢀMn) = 15 g Lꢀ1, p = 4 MPa, T = 160 °C, t= 8 h, solvent: V (CH3OH) = 15 mL, V
(DMF) = 35 mL.
Fig. 11 Influence of CuꢀMn concentration on catalytic performance
Catalyst: C (CaOꢀZrO2) = 20 g Lꢀ1, p = 4 MPa, T = 160 °C, t = 8 h, solvent: V (CH3OH) = 15 mL,
V (DMF) = 35 mL.

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Fig. 12 Schematic for MF direct synthesis over CuꢀMn/CaꢀZr catalyst