Direct synthesis of isobutyraldehyde from methanol and ethanol on Cu-Mg/Ti-SBA-15 catalysts: The role of Ti

Article abstract of DOI:10.1039/c7nj01513e

Herein, Cu-Mg/Ti-SBA-15 catalysts were prepared through the modification of Cu and Mg to mesoporous Ti-SBA-15 zeolites with different Ti/Si ratios and used for the synthesis of isobutyraldehyde (IBA) from methanol and ethanol. The catalysts were characterized via various techniques including XRF, XRD, TEM, N₂ sorption, CO₂-TPD, FT-IR, and XPS. With an increase in Ti content, CuO was well dispersed accordingly, and the amounts and strength of the basic sites were reduced. However, an excess introduction of Ti led to the accumulation of single TiO₂ crystals, inducing a decrease in the surface area and a deviation from the regular pattern such that the binding energies of Cu 2p, Mg 2p, and Si 2p shifted to lower values. This precisely affected the catalytic behaviors of the prepared catalysts synergistically. The catalyst stability was improved with the increasing Ti content accordingly, and over the catalyst with a Ti/Si ratio = 4/15, the IBA selectivity, after 24 h reaction, could still reach 25%, which was the best durability ever reported for IBA synthesis from methanol and ethanol. The catalytic performance test conducted using a regenerated catalyst and IR measurement of the spent catalyst indicated that carbon deposition on the catalyst surface could be depressed to some extent with the increasing Ti content.

Full text of DOI:10.1039/c7nj01513e

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PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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ARTICLE TYPE
Direct synthesis of isobutyraldehyde from methanol and ethanol on Cu-
Mg/Ti-SBA-15 catalyst : Role of Ti
a
a,b
a
a
a
a,
Junfeng Zhang, Meng Zhang, Xiaoxing Wang, Qingde Zhang, Faen Song, Yisheng Tan, * and
Yizhuo Han *
a,
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Cu-Mg/Ti-SBA-15 catalysts were prepared through the modification of Cu and Mg to mesoporous Ti-
SBA-15 zeolites with different Ti/Si ratios, and used for isobutyraldehyde (IBA) synthesis from methanol
and ethanol. The catalysts were characterized by various techniques including XRF, XRD, TEM, N2
1
0
sorption, CO -TPD, FTIR, and XPS. With increase of Ti content, CuO is well dispersed accordingly, and
2
the amounts and the strength of the basic sites reduce. However, excess introduction of Ti leads to the
accumulation of single TiO crystals, inducing the decrease of surface area and the deviation from the
2
regular pattern that the binding energies of Cu2p, Mg2p and Si2p shift to low BEs. The above precisely
affect the catalytic behaviors of the prepared catalysts synergistically. The catalyst stability is improved
with increase of Ti content accordingly, and over the catalyst with Ti/Si ratio = 4/15 the IBA selectivity,
after 24 h reaction, still highly reached to 25%, which is the best durability ever reported for IBA
synthesis from methanol and ethanol. The catalytic performance test using regenerated catalyst and IR
measurement of the spent catalyst indicate that carbon deposition on catalyst surface can be depressed to
the extent with increase of Ti content.
1
5
2
1-25
dispersion
.
2
6
2
0
1. Introduction
In our previous paper , we have ever reported a novel
mesoporous Ti-SBA-15 supported Cu-Mg catalysts for IBA
synthesis, and the effect of interaction between Cu and MgO
addition on the synthesis of IBA was also discussed. However,
the role of Ti in previous work was not well investigated due to
Isobutyraldehyde (IBA) is an important chemical and widely used
for synthesis of isobutanol, methyl isopropyl ketone, neopentyl
glycol, methyl acrylic acid, etc. . Currently, IBA is derived from
the carbonylation process of hydroformylation of propylene with
CO and H2 (known as OXO process) . However, as an
alternative route, IBA can be synthesized via direct condensation
of methanol and ethanol , which can be obtained from coal or
natural gas via gasification or reforming reaction , petroleum
route and biomass fermentation . Therefore, it is remarkably
significant and promising to develop a non-petroleum and
environmentally friendly route for IBA production.
5
5
6
6
7
0
5
0
5
0
1
26
quite low addition amount of Ti in those catalysts . Factually, Ti
as promoter or support, even as catalyst
important effects on bettering catalyst activity. Luo et al. studied
the aldol condensation of acetaldehyde to form high molecular
weight compounds on TiO2 and found that the higher
concentration of acid sites may be responsible for higher rate of
chain growth on Degussa TiO , in agreement with Rekoske’s
. Fischer et al. prepared high surface area TiN catalyst
via nitriding of TiO for the C-C coupling of alcohol and ketone,
and found that the resulting nitride is an effective and versatile
catalyst for the alkylation of ketones with alcohols. Other
literatures also reported the application of TiO in condensation
reaction of organic molecules, mostly based on the consideration
of the abundant acid-basic properties present on TiO2
2
-4
2
3
3
4
4
5
0
5
0
5
27-29
, has significantly
5, 6
7
, 8
9
30
2
31,32
33
results
IBA synthesis from methanol and ethanol is a cascade
reaction consisted of a series of dehydrogenation, condensation,
dehydration and hydrogenation steps , wherein, the
2
6
dehydrogenation and condensation are regarded to be two key
steps. Thus, the catalyst employed must possess multiple
functions in order to obtain IBA with high yield by doping
2
34, 35
.
1
0-13
adequate promoters.
. For Cu catalyst for IBA synthesis, the
Besides, TiO doped with small amount is used to promote the
dispersion of metals such as Ni, Cu, Co, etc., because TiO , as a
n-type semiconductor, strongly interacts with the metals and
suppresses the migration of metal particles and formation of new
2
addition of moderate alkaline promoters (MgO, ZnO, CaO, CeO2
etc.) is approved to be very essential because they were able to
promote not only the dehydration of the alcohols
subsequent aldol condensation of the intermediates
addition, other transition metal oxides such as TiO , ZrO and
2
1
1, 14-17
, but also
1
8-20
. In
36
crystals
.
2
2
Based on the above, the motivation of the present work was to
investigate the role of Ti in Ti-SBA-15 supported Cu-Mg catalyst
for IBA synthesis from mixed methanol and ethanol. We
Al O were also used as supports and promoters since they are
2
3
thought to be favourable for improvement of Cu or V
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Page 2 of 10
synthesized the Ti-SBA-15 zeolites with different Ti doping
amount and adopted them to disperse Cu and MgO. The effects of
Ti doping on the physicochemical properties and performance of
catalyst were investigated. Based on characterization using XRD,
mm), and tetrahydrofuran was used as the internal standard. The
gaseous products were analyzed by another two GCs equipped
with FID and thermal conductivity detector (TCD). The main
components in the liquid phase products were found to be
5
BET, H -TPR, CO -TPD, XPS, etc., the structure-activity relation 60 acetaldehyde(AA), propionaldehyde(PA), IBA, methyl acetate
2 2
was correlated and discussed in-depth.
(MA), ethyl acetate (EA), propanol (POL), isobutanol (IBOL),
methanol and ethanol. Minor products including methyl
propionate (MP), and some unidentified compounds were
denoted as ‘‘others’’. The main gaseous products were CH , CO ,
2
. Experimental
4
2
2
.1 Catalyst Preparation
65
hydrocarbons (C -C ), dimethyl ether, etc.. The ethanol
2 5+
The Ti-SBA-15 supports were synthesized using one-step
hydrothermal method, similar to that described by Bérubé et al.
conversion, product yields and product selectivity were calculated
2
6
1
1
2
2
3
3
0
5
0
5
0
5
based on the amount of consumed ethanol.
.4 Catalyst Characterization
The elemental composition of the catalyst bulk was obtained
2
6-29
and us
. The amount of Ti doping was controlled by
2
regulating the amount of tetrabutyl titanate (TBTA) as Ti source
in synthesized parent liquor precursor. Typically, 6.0 g of P123
was added in 45.0 g of deionized water and 180 mL of 70 using a ZSX100e X-ray fluorescence spectrophotometer. Powder
-
1
hydrochloric acid (2.0 mol·L ) and dissolved completely. The
resulting solution was stirred continuously for 4 h at 40 °C. After
that, 12.6 g of tetraethylorthosilicate (TEOS) and the mixture of
TBTA and acetylacetone (AAT) were added into the P123
X-ray diffraction of the as-synthesized TS-15 and CMTS-15 was
conducted on Rigaku MiniFlex II X-ray diffractometer with Cu
K
radiation (λ = 1.5406 Å ) over a 2θ range of 10°-80°. The
specific surface areas and pore volumes of the catalysts were
adsorption and desorption at 77 K on a
α
solution in turn. Wherein, the ratio of TBTA/AAT was kept at 75 measured by N
2
about 4:1, and the normal molar ratio of Ti/Si was designedly
adjusted as 1/15, 1.5/15, 2/15, 3/15 and 4/15, based on the added
amount of Ti source. After stirred for 24 h at 40 °C continuously,
the precursor gel was hydrothermally treated at 100 °C for an
Micrometrics Tristar 3000 instrument. The Fourier transform
infrared (FT-IR) spectra were recorded on a Bruker Tensor 27
instrument.
H
2
temperature-programmed
reduction
(H -TPR)
2
additional 24 h. The obtained suspension was filtrated and 80 measurements were conducted to evaluate the reduction of CuO
washed with the deionized water and ethanol thoroughly. The
collected solid was dried at 60 °C in vacuum, followed by
calcination in air at 550 °C for 4.5 h. Finally, the Ti-SBA-15
support was obtained and simplified as TS-15.
on catalyst surface. Typically, 0.05g of the catalyst was firstly
heated to 200 °C in Ar (20 mL/min) and kept at this temperature
for 0.5 h. Then the sample was cooled down to 50 °C, followed
by switching the flow gas from Ar to H
-TPR measurement was carried out with the sample
being heated up to 400 °C from 50 °C at a ramp rate of 5 °C/min.
The consumption of H was recorded by TCD.
CO temperature-programmed desorption (CO
conducted to measure the basicity of the catalyst surface.
/Ar (10% vol. H ).
2
2
Cu–MgO/Ti-SBA-15 catalysts were prepared using the wet- 85 Finally, H
2
2
6
impregnation method, similar to our previous report , wherein,
all catalysts were calcined at 400 °C for 2 h. Based on our
optimized results, loading amount of CuO and MgO is 0.0035
mmol of CuO and 0.004 mmol of MgO per 2.2g Ti-SBA-15,
2
-TPD) was
2
2
respectively. Final catalyst was defined as CMTS-15(m/n), 90 Typically, 0.05 g of catalyst was firstly heated to 400 °C to degas,
wherein m and n represents the molar ratio of Ti and Si (eg. 1/15,
and then cooled down to 360 °C, accompanied by being switched
to a H /Ar flow (20 mL/min). After the reduction for 4 h, the
1
.5/15, 2/15, 3/15, 4/15, etc.)
2
sample was cooled down to 50 °C in H /Ar flow. Afterward, the
2
2
.2 Catalyst Activity
catalyst was exposed to a flow of CO (20 mL/min) for 30 min.
2
All reactions were carried out at atmospheric pressure in a 95 Finally, CO2 desorption was carried out at a heating rate of
vertical fixed-bed quartz reactor with 12 mm in internal diameter
and 500 mm in length. Simply, 1.5 g portion of catalyst was
placed in the middle of the reactor, and the space above catalyst
bed was filled with inert ceramic rings (~20 mesh) to ensure
preheating and vaporizing the in-coming mixture of methanol and 100 (284.8 eV) of contaminant carbon.
ethanol (molar ratio of methanol/ethanol = 3:1), which was
5 °C/min within a range of 50-500 °C.
4
4
5
0
5
0
X-ray photoelectron spectroscopy (XPS) measurement was
performed on an Axis Ultra Dld instrument with Al Kα radiation.
The binding energy (BE) was calibrated against the C1s signal
introduced by a syringe pump. Before the start of the reaction, the
catalyst was heated up to 360 °C at a rate ramp of 5 °C/min in a
3. Results and Discussion
3
.1 Catalyst characterization
flow of pure H (20 mL/min) and kept for 4 h at this temperature.
2
Then, H was shut off, followed by introduction of the alcohol
3.1.1 XRF.
According to the literatures
2
2
6-29
, the final Ti content on catalyst is
mixture (4.5 mL/h) into the reactor. The liquid products were
collected through a cold trap, while the uncondensed outlet gas 105 not in proportion to its added amount during preparation.
was measured with a wet flow meter.
.3 Product Analysis
The collected liquid fractions were analyzed by gas
Therefore, it is necessary to measure the actual Ti content in
order to conveniently investigate its effect on the
physicochemical properties of the catalyst as well as the catalytic
performance. Thus, the bulk composition of the catalysts was
2
chromatograph (GC) equipped with a flame ionization detector 110 measured using XRF technique, and the results are tabulated in
5
5
(FID) and a capillary column (HP-FFAP, length 60 m, ID 0.32
Table 1. As observed, the Ti content increases with the increase
2
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Table 1 XRF results of as-prepared Ti-SBA-15 with different Ti/Si
ratios.
Ratio of Ti/Si
Sample
Theoretical(added amount) by XRF technique
CMTS-15(1/15)
CMTS-15(1.5/15)
CMTS-15(2/15)
CMTS-15(3/15)
CMTS-15(4/15)
0.0667
0.1000
0.1333
0.2000
0.2667
0.0016
0.0024
0.0030
0.0088
0.0764
of addition of Ti in preparation, but the ratios of Ti/Si by XRF are
far below the theoretical values. This indicates that only a small
section of Ti component, as modifier, was resided on catalyst and
most of the added Ti was removed off during filtration due to
water and ethanol washing. In addition, it is also noted that,
below the ratio of Ti/Si=3/15, the increased values of Ti are
relatively small with the same order of magnitude. Further
addition of Ti into the mother liquor precursor such as the one
with Ti/Si=4/15 leads to a noteworthy increase of Ti content in
obtained catalyst (about 9 times more than that on CMTS(3/15)).
It was plausibly speculated that, due to large amount of Ti source
added(TBTA), the hydrolyzed product probably aggregated to
larger particles, which could not be washed off during the
filtration, leading to an obvious increase of Ti content. To verify
the above speculation, it was attempted to synthesize TS-15 with
higher content of Ti using the parent liquor with ratio of
Ti/Si=5:15. Sure enough, XRF result showed that the ratio of
Ti/Si on the synthesized sample was as high as 0.145.
5
1
0
Fig. 2 TEM images of the representative CMTS catalysts with different
Ti/Si ratios. (a) Ti/Si=1/15; (b) Ti/Si=1/15; (c) Ti/Si=1/15;
(d) Ti/Si=1/15;(e) Ti/Si=1/15
1
5
35
incorporated into the framework of SBA-15, but, as single
crystals, is present on catalyst surface. In addition, the
characteristic diffraction peaks ascribed to MgO are not observed
from XRD patterns in Fig. 1, indicating that MgO species, in
amorphous form, are highly dispersed on catalyst surface.
2
0
3.1.2 XRD and TEM.
XRD patterns of the prepared catalysts were conducted, and the
results were shown in Fig. 1. It is observed that the intensity of
the diffraction peaks ascribed to CuO(JCPDF48-1541) declines
with the increase of introduced amount of Ti, indicating that the
dispersion of CuO is more and more bettered. On CMTS-15(4/15)
40
TEM images of the prepared catalysts are shown in Fig. 2. It
is observed that the mesoporous structures of all samples are well
kept in spite of the addition of Cu and MgO. Whereas, with the
increase of Ti content, the dull black aggregations are noted (such
as the catalyst with the Ti/Si ratio of 3/15). Even for the catalysts
2
5
catalyst, the best dispersion of CuO is observed. However, it is 45 with Ti/Si ratio of 4/15, the small irregular bulks were produced
also observed that the diffraction peaks ascribed to TiO appear
in the edge as shown in Fig. 2(e), probably ascribed to the
formation of TiO2.
2
3
7
on CMTS-15(4/15). The results reported by Zhu et al. revealed
that, when the ratio of Ti/Si is higher than 1/12.5 in synthesis
parent liquor, titanium dioxide crystals are easily formed. This is
3
0
3.1.3 N sorption
2
slight inferior to our results, in which more Ti can be seemingly 50 Table 2 shows the textural properties of the as-prepared TS-15
added. Previous XRF results have revealed that a relatively more
Ti (0.0764) could be introduced into CMTS-15(4/15). Therefore,
it is deduced that large amount of Ti on this catalyst is not well
supports and CMTS-15 catalysts. For TS-15 samples, the BET
surface area increases with the increase of Ti content when the
ratio of Ti/Si is below 3/15. This indicates that introducing
moderate amount of Ti is favourable for increase of surface area .
However, further increase of Ti content in parent precursor such
as Ti/Si=4/15 leads to an obvious decline of BET surface area
3
7
5
6
6
5
0
5
(
ABET). In combination of XRD and XRF results, it is easily
understandable that the aggregated TiO crystals on TS-15(4/15)
2
blocked somewhat meso- and micropores, leading to the decrease
of surface area. Other textural parameters of the TS-15 samples,
such as Amicro, Vtotal and Vmicro do not give regular understandings
in spite of the varied Ti content. However, one can obviously
observe that all TS-15 samples have relatively large microporous
surface area as well as microporous volume, indicating their well-
developed channel structure. From the pore distribution plot
shown in Fig. 3, it is observed that a wide pore diameter
distribution from about 6.3 to 15.3 nm presents on TS-15(1/15).
However, when Ti content is increased, the most probable
distribution of pore shifts to large pore diameter. This indicates
Fig. 1 XRD patterns of the representative catalysts with different Ti/Si
ratios.
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Fig. 3 Pore diameter distribution of as-prepare TS-15s
that Ti introduction is favorable for the expansion of pore, which
certainly affects molecular or intermediate spread during
chemical reaction. Unexpectedly, TS-15(4/15), in spite of
presence of TiO2 on it, still retains the pore structure with
relatively larger diameter. This implies that the mesoporous
property is not seemingly affected by the aggregated TiO2
crystals, which is also affirmed from the isotherm line of TS-
5
Fig. 4 Isotherms liner plots of the catalysts with different Ti/Si ratios.
1
5(4/15) as shown in Fig. 4.
Compared with TS-15s, Cu and Mg loaded catalysts show
-
1
attributed to σ-OH. It is seen that the peaks at 1230 and 1070 cm
3
3
4
4
0
5
0
5
are rarely affected by varied Ti introduction amount, but the peak
at 802 cm seems to show slightly decrease in intensity with
increase of Ti amount. In addition, the peak at 960 cm is
enhanced with the increased Ti content, and nevertheless the one
at 3700 cm , the intensity of which is related to Ti deposition on
1
0
remarkably obvious decline in ABET, Amicro, Vtatol and Vmicro,
-1
which is attributed to that the Cu and MgO components covered
catalyst surface and also blocked one part of pores. Among the
loading samples, the CMTS(3/15) shows the largest ABET(432
m /g), but CMTS(4/15), rather, gives the smallest one. It was
thought that, in spite of the best dispersion of Cu on CMTS(4/15),
the formed TiO on it further blocked some part of the pores and
covered the catalyst surface during Cu and Mg species
introduction via impregnation.
-
1
-1
2
3
8
the surface of SBA-15 , shows gradual decline in intensity. Here,
1
2
2
5
0
5
-
1
the enhancement of the peak at 960 cm indicates that more Ti
might be incorporated into the framework of SBA-15. However,
it should be bear in mind that the skeletal Ti amount introduced is
2
3
7
limited when TS-15 was synthesized using one-step method ,
and excess Ti addition results in the aggregation easily and the
3.1.4 FT-IR
2
6
formation of single TiO crystals. As our previous reported , the
Cu and MgO introduction would result in the disappearance of
the peak at 960 cm and also intensity decrease of peak at 3700
2
FT-IR was used to measure the vibration of the chemical bond on
as-prepared TS-15 supports with different ratios of Ti/Si, and the
recorded spectra are shown in Fig. 5. In reference to the
-
1
-
1
2
9, 38
-1
cm .
literatures
, the peaks at 1230 and 1070 cm are attributed to
νas (Si–O–Si) vibrations of the silica framework of the SBA-15,
and the peak at 802 cm is attributed to ν (Si–O–Si) vibration.
Likewise, the peak at 960 cm belongs to νas (Si–OH) or νas (Si–
O–Ti) vibration, and the absorption peak at 3700 cm is
-
1
3.1.5 H -TPR
2
s
Shown in Fig. 6 are the H -TPR profiles of the as-prepared
CMTS-15 catalysts with different Ti contents. As observed, the
-
1
2
-
1
Table 2 Textural properties of the as-prepared samples with
different Ti/Si ratios
A
BET
2
A
m /g)
micro
2
(
V
total(cm³/
g)
Catalyst
Vmicro(cm³/g)
(m /g)
TS-15(1/15)
TS-15(1.5/15)
TS-15(2/15)
TS-15(3/15)
TS-15(4/15)
CMTS-15(1/15)
CMTS-
847
882
919
916
807
398
106
147
130
120
90
1.25
1.20
1.20
1.29
1.15
0.728
0.0359
0.0555
0.0491
0.0433
0.0310
0.0143
28
3
56
36
0.709
0.0175
1
5(1.5/15)
CMTS-15(2/15)
CMTS-15(3/15)
CMTS-15(4/15)
383
432
342
31
29
29
0.658
0.705
0.559
0.0147
0.0129
0.0140
Fig. 5 FT-IR spectra of TS-15 supports with different Ti/Si ratios.
4
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maximum of H -consumption peak on CMTS-15(1/15) is
2
positioned at about 280 °C. With the increase of Ti content,
nevertheless, the peaks of H -consumption shift to low
2
temperature regularly. Since XRD results have indicated that
CuO dispersion was improved efficiently with the increase of Ti
content, that is, the particle size of CuO on TS-15 decreased with
the increase of Ti content, it is easily understandable that the
decrease of particle size certainly leads to the shift of H2-
consumption peak to low temperature. In addition, the peak area
of the catalyst with 4/15 of Ti/Si ratio, on which remarkably high
Ti content (Ti/Si=0.0764) was introduced according to XRF
results, is much smaller than others and two reduction peaks can
be observed, implying that Cu component as two types of
interactions probably exist on the catalysts.
5
1
0
Fig. 6 H
2
-TPR profiles of the as-prepared CMTS-15 catalysts with
different Ti/Si ratios.
1
5
3
.1.6 CO -TPD
2
For the IBA synthesis from methanol and ethanol, the presence of
basicity on catalyst surface, which is originated from the basic
metal oxides species such as MgO, CaO, etc. existing on catalyst,
is significantly necessary to promote the aldol condensation
during the reaction. Therefore，in this section，the basicity of
species can be completely removed. However, it can be
concluded that increase of Ti content can reduce the numbers of
basic sites.
2
0
4
5
3
.1.7 XPS
the prepared CMTS-15 catalysts was measured by CO -TPD
2
XPS spectra of the as-prepared CMTS catalysts were recorded
and the results are shown in Fig. 8(ad). From Fig. 8a, there are
two peaks of Cu2p BEs at 933.1 and 935.7 eV presented on the
CMTS-15(1/15). Based on Auger electron spectra (AES) of Cu
technique. As observed in Fig. 7, below the Ti/Si=2/15, the CO2
desorption peaks are located at about 117 °C, which is ascribed to
3
9
2
3
3
4
5
0
5
0
the weak basic sites , and nevertheless the peak would shift to
low temperature with further increase of Ti amount, indicating
the decline of basic strength, wherein, the medium strong and
strong basic sites are not observed on the prepared catalysts
50
2
p, we have ever affirmed that these two peaks were probably
attributed to two types of interaction, but not to different valence
26
40, 41
of Cu species , which is consistent with literatures
. Here,
3
9
unlike the literiture . Moreover, CO -desorption peak area also
2
the peak at 933.1 eV is ascribed to the isolated CuO and the one
at 935.7 eV is ascribed to the strong interaction between Cu and
decreases with increase of amount of Ti, which is relative with
the decrease of numbers of basic sites. As evidenced in some
5
6
6
5
0
5
26
other components such as MgO and the support . With the
3
2-35
literatures
, remarkably high acidic density exists on TiO2.
increase of Ti content, the peak at 935.7 eV is seemingly
enhanced and shifts to low BEs when the Ti/Si ratio is below 3/15,
but further increase of Ti content (4/15) leads to the decline of
intensity of the peak at 935.7 eV and the enhancement of the one
at 933.1 eV. This indicates that the presence of lots of Ti could
weaken the interaction between Cu and other components (MgO
and the support). Likewise, the introduction of Ti also affects the
BEs of Mg2p and Si2p. As shown in Fig. 8b and 7c, below the
Ti/Si ratio of 3/15, the peaks of Mg2p and Si2p slightly shift to
Therefore, it is easily understandable that Ti doping is prone to
induce the decrease of basic strength as well as numbers of basic
sites. For CMTS-15(4/15) catalyst, the surface basic sites are
majorly covered due to the formation of the aggregated TiO2,
leading to the smallest amounts of basic sites and also the
weakest basic strength. Though it is known that one part of Ti
species has been incorporated into the framework of SBA-15, the
contribution of the framework Ti species to the adjustment of
acidity and basicity is still unclear unless the non-framework Ti
Fig. 7 CO
2
-TPD profiles of as-prepared CMTS-15 catalysts with
Fig. 8 XPS spectra (Cu2p, Mg2p, Si2p and Ti2p) of the representative
different Ti/Si ratios.
CMTS-15s with different Ti/Si ratios.
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New Journal of Chemistry
Page 6 of 10
low BEs with the increase of Ti content, and nevertheless further
increase of Ti content does not induce the sequential shift. Below 25 The prepared CMTS-15 catalysts with different Ti/Si ratios were
3.2 Catalytic performance
3
/15 of Ti/Si ratio, it is thought that most of Ti as the bonding
applied to the IBA synthesis from methanol and ethanol, and the
results are shown in Fig. 9(ae). It can be observed that the
conversions of ethanol are all close to 100% on all representative
catalysts in their respective running period, and nevertheless
atom (such as Si-O-Ti) are present on the catalyst by being
incorporated into the framework of SBA-15, and the amount of
Si-O-Ti groups increase with the increase of Ti content. It can be
5
0
5
0
understandable that the presence of more and more Si-O-Ti 30 methanol conversions decrease with time on stream(defined as
2
6
groups regularly affects the BEs of Cu2p and Mg2p as well as
Si2p. However, for CMTS-15(4/15) catalyst, the existing single
TiO crystals cover the surface of the support easily (here, the
support refers to the bonding Ti-SBA-15(Ti-O-Si)), and the
subsequent introduced Cu and Mg components are separated with
the support by the formed TiO2 crystal layer. Thus, the
interactions between the support SiO and the loaded Cu and Mg,
are partially weakened due to the interaction between Cu and Ti,
even Mg and Ti. This may explain two H -consumption peaks of
CMTS-15(4/15) in Fig. 6. Therefore, the BEs of Cu2p and Mg2p
on CMTS-15(4/15) catalyst are deviated with the regular pattern.
According to Fig. 8d, Ti could be not detected in our present
catalysts below the Ti/Si ratio of 3/15 unless Ti content is further
increased (eg. CMTS-15(4/15)), which is probably attributed to
the coverage of Cu and Mg to the support surface.
TOS), which was noted in our previous work . With further
notion, it is found that, the methanol conversion decreases to 56%
on the CMTS-15(1/15) after 12 h reaction time from initial
1
1
2
2
8
2%(4 h) and the average decreasing rate is about 3.25% per hour.
3
4
4
5
0
5
With the increase of Ti/Si ratio, the rate can be further reduced
and finally decreased to 1.52% per hour on the CMTS-15(4/15),
indicating the increase of Ti amount doped is favourable for the
retention of catalytic performance.
Fig. 9 also gives the selectivity of main products containing
AA, PA, IBA, MA, EA, POL, IBOL. As observed, the selectivity
to IBA on CMTS-15(1/15) in 4 h reaction is about 20% and
higher than that on other catalysts with higher Ti content. On the
CMTS-15(4/15), the IBA selectivity in 4 h reaction decreases to
around 10%. Moreover, it is also observed that the IBA
selectivity on CMTS-15(1/15) adds to the highest in 8 h reaction,
and then rapidly decreases with time on stream. However, for
2
2
Fig. 9 Performances of the as-prepared catalysts with different the ratios of Ti/Si(ae) and regenerated catalyst(f) (calcined for 4 h at 500 °C in air).
6
| Journal Name, [year], [vol], 00–00
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New Journal of Chemistry
other three catalysts, the appearance of the highest selectivities
are delayed, especially on the CMTS-15(4/15), the highest value
and IBA, respectively. In the meantime, the catalyst with further
increase of Ti content (Ti/Si=5/15) was also used to synthesize
appears at 16 h reaction, and subsequent decreasing rate becomes 45 IBA in our experiments. However, the test results showed that the
lower and lower with increase of Ti content. Wherein, it is clearly
noted that the IBA selectivity, after 24 h reaction, is as high as
performance is quite inferior to our present catalyst, probably due
to the severe blockage of mesopores caused by the presence of
excess Ti.
5
0
5
0
5
0
5
0
2
5%, which is the best durability ever reported for IBA synthesis
from methanol and ethanol. The IBA selectivity as a function of
time on stream is plotted in Fig. 10. It must be confessed that the 50 other several liquid by-products with small amounts, which have
increase of Ti content does not induce the improvement of IBA
selectivity accordingly, instead of the decrease of the selectivity
in early stage. As a comparison, the catalytic performance of the
catalyst without Ti doping was investigated in our experiment.
Necessary to say, except the above-mentioned resultants,
not been identified yet, and gaseous products, such as H , light
2
1
1
2
2
3
3
4
hydrocarbons(C -C ), dimethyl ether and CO , could be also
1
4+
2
produced in the reaction system. Through calculation of carbon-
containing quantities of the identified products, the carbon
The test results(as shown in Table S1) showed that, after 8 h 55 balance of the present reaction is above 93%, wherein, another 7%
reaction, IBA selectivity reached to the maximum value, about
35%, and then quickly decreased to 16% after 14 h reaction. This
indicates that, in initial stage, Ti doping is unhelpful for the
formation of IBA, but the catalyst is able to run in longer period
with the increase of Ti content. That is, Ti addition is preferable 60 amount of liquid products was very small in the initial stage of 2
to improve catalyst durability compared to the catalyst activity.
The further analysis will be done in discussion section.
In Table 3, we list the results of IBA synthesis from methanol
and ethanol over the representative catalyst systems. It is
observed that V-catalyst has a relatively higher IBA productivity
of carbon might be attributed to the unidentified products and
deposited carbon. In addition, we found that the gaseous products
were largely produced in early stage of the reaction due to the
decomposition of reactant alcohols on Cu-based catalyst. The
h reaction.
3
.3 Catalyst regeneration
2
6, 45
Literature and our previous results
indicated that the V-based
than Cu-catalyst except Cu-ZnO-Al O catalyst, which was used 65 and Cu-based catalysts for IBA synthesis deactivated with
2
3
to synthesize IBA from the mixture of methanol and ethanol (2: 1
molar ratio) with 50% water (V/V). Owing to shortage of
important operation parameters (especially reaction time), it is
hard to directly compare the inherent supremacy of the
representative catalyst systems. Factually, in our early study, the 70 to the presence of H
Cu-Zn-Al catalyst prepared using Reddy’s method was used for
IBA synthesis on present operation condition. The results
revealed that IBA production on the prepared Cu-Zn-Al catalyst
was obviously inferior to that on our present CMTS catalysts.
reaction time. However, the mechanism of deactivation on the
two types of catalysts is quite different. The deactivation on V-
based catalyst is resulted from carbon deposition on catalyst
5
+
4+
surface and reduction of V to V in present reaction system due
4
5
, which is produced through the alcohol
dehydrogenation, while that of Cu-based catalyst is probably
2
2
6
related to the sintering of Cu particles and carbon deposition . In
this section, the deactivation mechanism was further studied
through the catalytic performance test using regenerated catalyst
In the case of other side-products on the prepared catalysts, 75 and IR characterization of the spent catalysts.
AA and PA selectivities increase with time on stream and
nevertheless the selectivities on the catalyst with low Ti/Si ratio
are moderately higher than that on the catalyst with high Ti/Si
ratio. MA selectivities are below 5% along with decreasing trend
From the test result in Fig. 9f, it is observed that, on the
catalyst regenerated through calcination for 4 h at 500 °C in air,
the activity can be recovered to some extent. Compared to the
fresh catalyst (CMTS-15(4/15)), the regenerated catalyst shows
in whole test time, while EA selectivities increase with time on 80 relatively lower methanol conversion and IBA selectivity and but
stream, probably because of the enhancement of Tishchenko
higher AA and PA selectivities. In our experiment, the fresh
4
0, 42-44
reaction
. The POL and IBOL selectivities are both below 5% catalyst was obtained after calcination at 400 °C, but the spent
and their variation with time on stream is similar to that of PA
catalyst was regenerated via calcination at 500 °C. Thus, at the
Table 3 IBA synthesis from methanol and ethanol over the representative catalyst systems
Ethanol conv.
%)
IBA Sel.
(%)
≈59
≈82
85
≈55
≈60
87
55.3
43.4
90
Catalyst
V/TiO
Methanol/ Ethanol
Temp. (°C)
RT(h)
LHSV
Refs
(
2
10
2
2
2
2
2
3
3c
2
\
350
350
350
350
350
350
375
280
350
300
300
320
\
\
\
\
\
\
\ a
86
≈55
98
≈35
≈40
92
96.4
84.2
100
95. 9
93.9
71.6
6
22
22
22
22
21
12
13
14
V
V
V
2
O
2
5
/TiO -Al
/TiO -SiO
-ZrO
2
2
O
2
3
2-4
2-4
2-4
2-4
a
O
5
2
a
2
O
5
/TiO
2
2
a
V
2
O
5
/TiO
2
-SiO
2
-ZrO
2
2
a
V
2
O
Nb
FeO
5
/TiO
-V
-V
CuO-ZnO-Al
CuO-MnO -A l
CuO-MnO -SiO
CuO-CaO/SiO -TiO
2
-SiO
2-4
a
2
O
x
3
2
O
5
2
2
O
5
\
\
\
\
\
\ a
2
O
3
4-6
2
2
O
3
\
\
\
11
71
50.9
10
10
11
2
2
\
2
2
2
b
26，
This study
CuO-MgO/TS(15/1)
3
360
6
3
≈100
32.0
a
b
c
:
h-1; : mL·g-1·h-1, :(with 50% water(V/V)), RT: reaction time.
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DOI: 10.1039/C7NJ01513E
New Journal of Chemistry
Page 8 of 10
Fig. 12 Proposed mechanism of IBA formation from methanol and
ethanol.
content. As is well known, the activity in dehydrogenation of
alcohols is closely related to the size of Cu particles, and small
size of Cu particles is significantly favorable for the
4
7
3
3
4
4
5
5
6
0
5
0
5
0
5
0
dehydrogenation of alcohols , which facilitates the subsequent
aldol condensation. However, our results shown in Fig. 9,
factually, revealed that the catalyst with Ti/Si=4/15 showed
lowest selectivity to IBA in early stage of reaction. By metering
outlet gas, it is affirmed that the majority of alcohols were
Fig. 10 IBA selectivity as a function of time on stream over the
representative catalysts with different Ti/Si ratios.
converted to gaseous products, such as H , alkanes, etc. in early
stage. This indicates that the decomposition of alcohols
methanol and ethanol) is much more preferential than their
dehydrogenation on the catalyst with small-size Cu particles
caused by relatively more Ti content. With time on stream, the
decomposition activity is decreased, and thus dehydrogenation
activity is selectively enhanced due to probable Cu sintering and
carbon deposition. Thus the AD, PD and IBA selectivities are
improved. But, appearance of the highest value is delayed after
2
time of removal of the deposited carbon, the growth of Cu
particles may lead to the loss of activity. In order to further affirm
that carbon deposition is one of the reasons for catalyst
deactivation, the spent catalysts were measured on IR instrument.
(
5
0
5
From Fig. 11, it is obvious that the vibration bands of CH - and
3
-
1
CH - groups are observable at 2850-3000 cm and intensity of
2
the peaks decreases with the increase of Ti/Si ratio. This indicates
that catalyst deactivation is closely related to the carbon
deposition. Otherwise, for the majority of Cu-based catalysts, Cu
particles on the catalyst surface are easily sintered during reaction
1
6 h reaction. The further Cu sintering and carbon deposition on
1
catalyst is prone to lead to the decrease of IBA selectivity due to
the decreased catalytic activity. It is plausible that Ti addition is
preferable to promote catalyst durability rather than the catalyst
activity.
Generally, the strong basicity of MgO can accelerates the
carbon deposition on catalyst surface . CO -TPD result above
revealed that the amounts of basic sites decreased with the
increase of Ti content. This implies that carbon deposition is also
weakened with increase of Ti content as shown in Fig. 11, and the
catalyst can be run well in longer period. Besides, the interactions
between the components, such as Cu, Mg, Si, etc., as well as
textural properties, are affected with the variation of Ti content.
Therefore, it is thought that the catalytic behaviours of the
prepared catalysts are overdetermined by the multi-factor above
due to various Ti introduced amount.
4
6
when the reaction temperature is higher than 300 °C . Given that
the present reaction was carried out at 360 °C, Cu particle size
certainly grew up. Therefore, it can be apparently concluded that
catalyst deactivation in present reaction is mainly resulted from
Cu sintering and carbon deposition on catalyst surface.
1
48
2
3
.4 Discussion
IBA synthesis from methanol and ethanol belongs to a tandem
2
6
reaction. According to our previous proposed mechanism , the
dehydrogenation of alcohols and subsequent aldol condensation
are the two most important steps in whole reaction process(as
shown in Fig. 12). For our present CMTS-15 catalysts, Cu is
usually considered as a component for dehydrogenation, while
MgO is responsible for the aldol condensation. The
2
0
It needs to be pointed out that the promotion of TiO itself for
2
2
5
characterizations such as XRD and H -TPR have revealed that Cu
dispersion could be greatly improved with the increase of Ti
2
IBA synthesis is unobvious, though the literatures indicated that
TiO is helpful for the aldol condensation of aldehydes. Further
2
research is needed.
65
4. Concluding remarks
In the present study, mesoporous Ti-SBA-15 with different Ti/Si
ratio was synthesized and used as support to prepared Cu-Mg/Ti-
SBA-15 catalysts for IBA synthesis from methanol and ethanol.
The Ti addition can somehow modify the morphology of
SBA-15 and improve the dispersion of Cu. The increase of Ti
doping amount is significantly favourable for the improvement of
BET surface area and the incorporation of more Ti into the
framework of SBA-15, but the excess doping of Ti easily results
70
in the accumulation of single TiO crystals, inducing the decrease
2
Fig. 11 IR spectra of the spent catalysts with different Ti/Si ratios.
75 of surface area.
8
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New Journal of Chemistry
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Acknowledgements
The authors gratefully acknowledge the financial support from
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and Natural Science Foundation of Shanxi Province
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University of Chinese Academy of Sciences, Beijing 100049, China.
Electronic Supplementary Information (ESI) available: [details of any
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Table of content entry
Direct synthesis of isobutyraldehyde from methanol and ethanol on
Cu-Mg/Ti-SBA-15 catalyst: Role of Ti
Junfeng Zhang, Meng Zhang, Xiaoxing Wang, Qingde Zhang, Faen Song, Yisheng Tan, Yizhuo Hana,∗
a
a,b
a
a
a
a,*,
a
:
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan
0
30001, China;
b:
.
University of Chinese Academy of Sciences, Beijing 100049, China
Increase of Ti content significantly betters catalyst durability due to its effect on Cu dispersion and the basicity of
catalyst
∗
:
Corresponding authors.
Institute of Coal Chemistry, Chinese Academy of Sciences, Shanxi, 030001, China. Tel: +86-351-4049747, E-mail:
hanyz@sxicc.ac.cn (Y.-Z. Han); Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001,
China. Tel: +86-351-4044287, E-mail: tan@sxicc.ac.cn (Y.S. Tan).