Ti-SBA-15 supported Cu-MgO catalyst for synthesis of isobutyraldehyde from methanol and ethanol

Article abstract of DOI:10.1039/c6ra09379e

Ti-SBA-15 supported Cu and MgO catalysts were prepared and used for the first time in the one-step conversion of methanol and ethanol to isobutyraldehyde (IBA). The results show that the loadings of Cu and MgO, and catalyst calcination temperature have strong effects on the catalyst activity. A high yield of IBA, 32.7%, and high ethanol conversion, 96.6%, were achieved at 360 °C with WHSV of 3.0 mL (g^-1 h^-1) on the catalyst calcined at 400 °C when the loadings of Cu and Mg were 20.0 wt% and 6.5 wt%, respectively. The physicochemical properties of the catalysts were analyzed by various techniques including XRD, N₂ adsorption and desorption, FT-IR, H₂-TPR, CO₂-TPD and XPS. The ordered mesoporous structure of the catalysts was retained with the introduction of CuO and MgO. The size of CuO particles on the catalysts was retained though they suffered from a varied calcination temperature. H₂-TPR measurements revealed that the increase of calcination temperature from 400 °C to 700 °C resulted in the decrease of basicity of the catalysts, and enhanced the interaction between the Cu and Mg species and the support. The results from XPS analysis indicated that the binding energy of Cu 2p was increased with the introduction of MgO, while the increased calcination temperature easily resulted in the decrease of Cu content on the catalyst surface due to probable migration of Cu species into internal pores or their incorporation into the framework of the Ti-SBA-15 support.

Full text of DOI:10.1039/c6ra09379e

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Ti-SBA-15 supported Cu–MgO catalyst for
synthesis of isobutyraldehyde from methanol and
Cite this: RSC Adv., 2016, 6, 85940
ethanol†
a
a
b
a
a
a
Junfeng Zhang, Yingquan Wu, Li Li, Xiaoxing Wang, Qingde Zhang, Tao Zhang,
a
a
Yisheng Tan* and Yizhuo Han*
Ti-SBA-15 supported Cu and MgO catalysts were prepared and used for the first time in the one-step
conversion of methanol and ethanol to isobutyraldehyde (IBA). The results show that the loadings of Cu
and MgO, and catalyst calcination temperature have strong effects on the catalyst activity. A high yield of
ꢀ
ꢁ1 ꢁ1
IBA, 32.7%, and high ethanol conversion, 96.6%, were achieved at 360 C with WHSV of 3.0 mL (g
h )
ꢀ
on the catalyst calcined at 400 C when the loadings of Cu and Mg were 20.0 wt% and 6.5 wt%,
respectively. The physicochemical properties of the catalysts were analyzed by various techniques
2 2 2
including XRD, N adsorption and desorption, FT-IR, H -TPR, CO -TPD and XPS. The ordered
mesoporous structure of the catalysts was retained with the introduction of CuO and MgO. The size of
CuO particles on the catalysts was retained though they suffered from a varied calcination temperature.
ꢀ
ꢀ
H
2
-TPR measurements revealed that the increase of calcination temperature from 400 C to 700
C
resulted in the decrease of basicity of the catalysts, and enhanced the interaction between the Cu and
Mg species and the support. The results from XPS analysis indicated that the binding energy of Cu 2p
was increased with the introduction of MgO, while the increased calcination temperature easily resulted
in the decrease of Cu content on the catalyst surface due to probable migration of Cu species into
internal pores or their incorporation into the framework of the Ti-SBA-15 support.
Received 12th April 2016
Accepted 15th August 2016
DOI: 10.1039/c6ra09379e
www.rsc.org/advances
The catalysts applied for IBA synthesis from methanol and
ethanol mainly include two types: copper-based catalyst (Cu-
1
. Introduction
Isobutyraldehyde (IBA) as a basic organic compound is widely catalyst) and vanadium-based catalyst (V-catalyst). Although V-
5
used for the synthesis of isobutanol, methyl isopropyl ketone, catalysts have higher catalytic activity than Cu-catalysts, most
neopentyl glycol, methyl acrylic acid, etc. which are important of them are hypertoxic, expensive and insoluble in common
raw materials and intermediates in the manufacture of addi- solvents. Cu is, thereby regarded as a promising catalytic
tives, adhesives, resin, and medicine agents. At present, IBA is component. In 1991, Wang et al. developed a novel method for
obtained as a by-product from the production of butyraldehyde the synthesis of IBA from methanol and ethanol via an one-step
(
BA), butanol and octanol, which is a petroleum route (also process by using titanium oxide supported vanadium oxide
6
known as the carbonylation process of hydroformylation of (V
2
O
5
/TiO
2
) as catalyst. The IBA yield of 50.0% was achieved
1–3
ꢀ
propylene with CO and H2). However, the productivity of IBA over 2.5% V
2
O
5
/TiO
2
catalyst at 350 C, wherein, some by-
does not meet the rising demand from industries due to the products such as acetaldehyde (AA), propaldehyde (PA), etc.
expansion of IBA application and accelerated petroleum were also observed. Later, Wang et al. envisaged a plausible
exhaustion. The motivation of the present work is to nd an mechanism of IBA synthesis from methanol and ethanol with
7
environment-friendly and promising approach for the produc- well-designed experiments. It was deduced that ethanol was
tion of IBA from the mixture of methanol and ethanol, which rstly dehydrogenated to acetaldehyde which subsequently
4
can be obtained from renewable biomass resources.
reacted with methanol via two-steps condensation, nally
formed the target product IBA. that is, the AA and PA are the
8
medium products for synthesis if IBA. Here, it should be noted
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese that the two-step condensation is only an overall reaction (the
a
Academy of Sciences, Taiyuan 030001, China. E-mail: hanyz@sxicc.ac.cn; tan@
reaction is mentioned as overall condensation reaction in below
context), which is probably composed of several elementary
reactions. The more detailed mechanism needs to been inves-
tigated in-depth.
sxicc.ac.cn; Tel: +86-351-4049747; +86-351-4044388
b
Integrated Chemical Reaction Facility, Department of Chemistry, Durham University,
South Road, Durham, DH1 3LE, UK
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c6ra09379e
1
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9
Reddy et al. reported that a high selectivity of IBA, up to of Cu and Mg species, and changing catalyst calcination
0.0% was achieved over Cu–ZnO–Al O catalyst. However, space temperature. Based on the results of the characterization, we
9
2
3
time yield of IBA (STY) was remarkably low due to a high water attempted to discuss in detail the origin of this catalyst
content, 50.0 wt% in the reaction system, which resulted in supremacy as well as the reasons for the difference in charac-
10
catalyst deactivation. Qiu et al. suggested that Cu and ZnO teristics and catalytic performance caused by different Cu and
species on Cu–ZnO–Al catalyst favored the conversion of Mg loading and calcination temperature.
2 3
O
methanol and ethanol, and IBA selectivity, respectively. Wu and
Hu et al. compared the activity of Cu-catalyst and V-catalyst for
the conversion of methanol and ethanol to IBA. Their results
2
. Experimental section
showed that the former one was less active than the latter 2.1 Material
1
1–13
one.
increased reaction time, though the initial value was as high as
0.0%. Less than 20.0% of IBA yield was obtained during
IBA yield decreased dramatically over Cu-catalyst with
Methanol, ethanol, muriatic acid, Cu(NO
3
)
2
$3H
2
O
and
Mg(NO $6H O (analytic grade) were all obtained from Sino-
3
)
2
2
5
pharm Chemical Reagent Co. Tetrabutyl titanate (TBTA), tet-
raethyl orthosilicate (TEOS) and acetylacetone (AAT) were
purchased from Tianjing Guangfu Fine Chemical Research
Institute. The template agent triblock copolymer (P123) was
purchased from Aldrich.
14
a reaction period of 4 h. In order to increase the catalyst
activity, some alkaline metal oxides, e.g. MgO, ZnO and CaO,
were used as promoters and doped into Cu and V based cata-
2 2
lysts, and transition metal oxides, such as TiO and ZrO , were
9,15
used as catalyst supports. Notably, the addition of promoters
improved the dispersion of active components (Cu and V O )
2
5
2
.2 Catalyst preparation
signicantly, leading to dehydrogenation of alcohol to aldehyde.
Factually, Cu–MgO catalyst is very suitable and effective for The Ti-SBA-15 support was synthesized according to the method
25–28
majority of hydrogenation/dehydrogenation of alcohol and reported by B ´e rub ´e and Visuvamithiran et al.
In a typical
16–20
further condensation.
tion for the condensation reaction is attributed to its moderate water and 180 mL of hydrochloric acid (2.0 mol L ). The
generally, it was thought that promo- synthesis, 6.0 g of P123 was dissolved in 45.0 g of deionized
ꢁ1
21,22
ꢀ
basicity and prohibition to the formation of light alkane.
As resulting mixture was stirred rigorously for 4 h at 40 C. Then,
a result, the IBA yield was increased. It is plausible that Cu is 12.6 g of TEOS and the mixture of 1.0 g of TBTA and 0.25 g of
active for dehydrogenation of ethanol, and yet MgO can promote AAT were added into the initial homogeneous P123 solution in
subsequent two-step overall condensation reaction.
sequence (the molar ratio of Si : Ti ¼ 15 : 1). The resulting
ꢀ
The catalyst support is another important factor affecting the mixture was stirred for 24 h at 40 C continuously, followed by
catalytic performance in IBA synthesis. Reddy et al. prepared the hydrothermal treatment in an autoclave for an additional 24
ꢀ
V
Al
2
O
5
-based catalysts by using metal oxide composites (TiO
2
–
h at 100 C. The obtained suspension was ltrated and washed
15,23
2
O
3
, TiO
2
–SiO
2
and TiO
2
–ZrO
2
) as supports.
The results with deionized water and ethanol thoroughly. The solid fraction
showed that about 98.0% conversion of ethanol and a high IBA was dried at 60 C in a vacuum drying oven overnight. Finally,
selectivity of 85.0% were achieved over the TiO –SiO supported the products were calcined at 550 C for 4.5 h for removing the
ꢀ
ꢀ
2
2
catalyst with high surface area. It was suggested that the low template. Thus, the Ti-SBA-15 support was obtained. For
catalytic activity of TiO –Al O and TiO –ZrO supported cata- comparison, mesoporous SBA-15 support was also synthesized
2
2
3
2
2
lysts was mainly attributed to their strong acidity and low by following the same procedures detailed above in the absence
surface area, compared with SiO supported ones. It was also of TBTA and AAT.
reported that a catalyst support with high surface area was The wet-impregnation approach was adopted for the
2
favourable for the dispersion of active components, and small synthesis of Cu–MgO/Ti-SBA-15 catalysts. Typically, 2.22 g of Ti-
active particles showed good performance in IBA synthesis from SBA-15 or SBA-15 was added into 15.0 mL of a aqueous solution
13,14
methanol and ethanol.
TiO–SiO composites are regarded as containing 2.42 g of Cu(NO ) $3H O and 1.28 g of Mg(NO ) -
2 3 2 2 3 2
ideal catalyst supports for IBA synthesis to date due to their high $6H O (corresponding to 20.0 wt% of Cu and 6.5 wt% of Mg
2
surface area and moderate surface acid–base properties.
SBA-15 as an orderly mesoporous material has been widely treated with ultrasonic at 70 C until the water was evaporated
used for various catalytic processes due to its high surface area, completely. Then, the obtained solid gel was dried at 100 C
loadings on Ti-SBA-15 or SBA-15). The resulting suspension was
ꢀ
ꢀ
ꢀ
developed pore structure. Moreover, introduction of specic overnight and calcined at 400, 500, 600, 700 C for 2 h,
metals or metal oxides (such as Al, Fe, Mn, Ti, etc.) into SBA-15 respectively. Thus, the desired Ti-SBA-15 supported catalysts
can obtain surprising effects in improving the performance of were obtained and denoted as 20C6.5MTS-400, 20C6.5MTS-500,
SBA-15-supported catalysts due to the creation of special acid sites 20C6.5MTS-600 and 20C6.5MTS-700, respectively. For compar-
24
and high synergy between active components and supports.
ison, the same procedures were followed for preparing SBA-15
In the present study, with a view to developing a low cost Cu- supported catalysts without Ti modication, denoted as
based catalyst for effective IBA synthesis from methanol and 20C6.5MS. Other catalysts with varied loadings of Cu and Mg
ethanol, Ti-modied SBA-15 (Ti-SBA-15) with high surface area were also prepared and denoted as xCyMTS, where x (equal to 5
was used as the catalyst support and MgO as the promoter for and 10) and y (equal to 1.6 and 3.25) are the expected loadings of
adjusting the catalyst basicity. Effects of catalyst structure on its Cu and Mg, respectively. The obtained catalyst was then pressed
catalytic performance were investigated by tuning the loadings and crushed into particles of 20–40 mesh.
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2
.3 Catalyst evaluation
The evaluation of catalyst activity was carried out in an upright
xed-bed quartz tube reactor (ID 12 mm and length 500 mm).
temperature for 0.5 h. Then the sample was cooled down to 50
C under Ar atmosphere, followed by switching the ow gas
ꢀ
from Ar to H /Ar (10% vol H ). Finally, H -TPR measurement
2
2
2

ꢀ
was carried out with the sample being heated to 400 C from 50
The reaction was operated at atmospheric pressure. Simply,
catalyst (1.50 g) was placed in the middle of the reactor, and the
upper part of the reactor was lled with inert ceramic rings (ꢂ20
mesh) for preheating and vaporizing the in-coming mixture of
methanol and ethanol (mole ratio of methanol/ethanol ¼ 3 : 1).
Before the introduction of the alcohol mixture, the catalyst was
heated to 360 C at a rate of 5 C min in a ow of pure H (20
2
mL min ) and kept at 360 C for 4 h. Aer that, the H was shut
off and a ow of the alcohol mixture (4.5 mL h ) was intro-
duced into the reactor by a syringe pump, but without any inert
ꢀ
ꢀ
ꢁ1
C at a ramp rate of 5 C min . The consumption of H
measured by TCD.
2
CO temperature-programmed desorption (CO -TPD) was
2
was
2
also conducted to measure the dispersion of CuO on catalyst
surface. Practically, the catalyst (0.05 g) was rst heated up to
ꢀ
ꢁ1
ꢀ
ꢀ
ꢁ1
360 C in a H
2
/Ar ow (20 mL min ) and reduced for 4 h at this
2
ꢀ
ꢁ
1
ꢀ
temperature. Then the sample was cooled down to 50 C in
ꢁ
1
owing H
/Ar, followed by switching the ow gas to Ar. Aer
2
ꢁ1
that, CO
2
(ow rate 20 mL min ) was introduced into the ow
ꢀ
system for the adsorption process at 50 C for 30 min. Finally,
gas (such as N
2
Ar, etc.) as carrier, wherein the reaction
ꢀ
the CO
2
desorption process was carried out with the sample
temperature was also kept at 360 C. The products were
collected through a cold trap. The volume of outlet gas was
measured with a wet ow meter. The collected liquid fractions
were analyzed by gas chromatograph (GC) equipped with
a ame ionization detector (FID) and a capillary column (HP-
FFAP, length 60 m, ID 0.32 mm), and tetrahydrofuran was
used as the internal standard. The gas-phase products were
analyzed by GCs equipped with FID and thermal conductivity
detector (TCD). Based on the amount of consumed ethanol, the
ꢀ
ꢀ
ꢁ1
being heated to 500 C at a rate of 5 C min
.
X-ray photoelectron spectroscopy (XPS) measurement was
performed on a AXIS ULTRA DLD instrument with Al Ka radi-
ation. The binding energy (BE) was calibrated against the C 1s
signal (284.8 eV) of contaminant carbon. Elemental surface
composition was estimated on the basis of peak areas which
were normalized using atomic sensitivity factors.
conversion (X ) of ethanol, and product yields (Y ) and selectivity 3. Results and discussion
e
i
(Si) were calculated using the following eqn (1)–(3).
3.1 Catalytic performance
ðne;in ꢁ ne;out
e;in
Þ
The loadings of Cu and Mg components was rstly optimized
for the synthesis of IBA, wherein the mass ratio of Cu/Mg was
kept constant based on our previous pre-research results. Fig. 1
X
e
¼
ꢃ 100%
(1)
N
n
i
Y
i
¼ n ꢃ 100%
(2) shows, in liquid products, the conversion and selectivities
Conv. and Sel.) as a function of time on stream (TOS) over three
e;in
(
catalysts, i.e. 5C1.6MTS, 10C3.25MTS and 20C6.5MTS, which
were all calcined at 400 C.
Y
i
S
i
¼ X ꢃ 100%
(3)
ꢀ
e
It can be observed that the ethanol conversions of three
catalysts are all close to 100% in the running period of 12 h
though the product distributions differ from one another, and
nevertheless the methanol conversions decrease with TOS
gradually. It also shows that the conversions of methanol and
ethanol were remarkably complicated over the different cata-
lysts. Clearly, various products were produced such as acetal-
dehyde (AA), propaldehyde (PA), methyl acetate (MA), ethyl
where, ne,in is the molar quantity of ethanol fed into the reactor,
e,out is the molar quantity of ethanol in the effluent, and n is
the molar quantity of ethanol equated from the i component of
n
i
products.
2.4 Catalyst characterization
Powder X-ray diffraction (XRD) measurements were conducted acetate (EA), propanol (POL), isobutanol (IBOL), etc. apart from
on a Rigaku MiniFlex II X-ray diffractometer with Cu Ka radi- the target product IBA. By the way, a small number of several
1
˚
ꢀ
ꢀ
ation (l ¼ 1.5406 A) over a 2q range of 10 –80 . The anode was products have been identied yet in our collected liquid prod-
6
operated at 40 kV and 40 mA. The Fourier transform infrared ucts. Wang et al. ever reported that products such as AA, PA,
(
FT-IR) spectra were recorded on a Bruker Tensor 27 instrument acrolein, etc. were observed, but MA, EA, POL and IBOL were
ꢁ
1
with a MCT detector (64 scans, resolution 4 cm ). Each sample also detected in this work. The reason is not clear by now. The
was ground with KBr and put in an in situ infrared cell. The IBA selectivity increased with TOS over the 5C1.6MTS catalyst
specic surface areas and pore volumes of catalysts were during a period of initial 8 h, then declined obviously with
2
measured by N adsorption and desorption at 77 K on a Micro- further prolonged reaction time, wherein, a highest IBA selec-
metrics Tristar 3000 instrument. The specic surface areas were tivity of 25.9% was achieved at 8 h reaction. When the loadings
estimated using the BET method, while the pore volumes were of Cu and Mg were increased, the product distribution changed
calculated with the t-plot method.
accordingly and the IBA selectivity showed an increasing trend
H temperature-programmed reduction (H -TPR) measure- in general. On the 20C6.5MTS catalyst, a highest IBA selectivity,
2
2
ments were conducted to evaluate the dispersion of CuO on ꢂ32.7%, appears at 6 h reaction time, and then the selectivity
catalyst surface. Typically, the catalyst sample (0.05 g) was rst decreases slightly, which apparently indicates that increase of
ꢀ
ꢁ1
heated up to 200 C in an Ar ow (20 mL min ) and kept at this loading of Cu and Mg not only improves catalytic activity but
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intermediates. In terms of side-products, it can be observed
from Fig. 1 that, the AA and PA selectivities increase with TOS
on all of the prepared catalysts. According to Reddy's proposed
mechanism, it was deduced that the rst step, ethanol dehy-
drogenation, and subsequent overall condensation reaction
(conversion of acetaldehyde and methanol to propaldehyde)
were mainly proceeded when reaction time was prolonged, but
the second step overall condensation (conversion of prop-
aldehyde and methanol to IBA) was weakened, thereby leading
to a decreasing trend of IBA selectivity. Additionally, the selec-
tivities of other side-products such as MA, EA, POL and IBOL
were also affected by loadings of Cu and Mg components, which
are detailed in the following section. The results show that Cu–
MgO/Ti-SBA-15 catalyst has a high catalytic activity for IBA
synthesis from methanol and ethanol. The loadings of Cu and
Mg components have signicant impacts on both catalytic
performance and product distributions. Gas composition in the
outlet obtained from GC or GC-MS revealed that gas products is
mainly composed of low-carbon hydrocarbons (C1–C4+),
methanol and small amount of DME, e.g. gas composition in
the outlet aer 6 h reaction over the 20C6.5MTS catalyst was
quantied and listed in Table S2.† Wherein, the carbon balance
was also calculated and is at 90–95% regardless of unknown
liquid products. Moreover, we found that the carbon balance
was increased slightly with time on stream. As is well-known,
catalyst stability is an important specication for developing
a promising industrial catalyst. The present catalysts all showed
deactivation with the increased reaction time. However, as
above mentioned, ethanol conversion still reached about 100%
aer 12 h reaction, unlike the decreased methanol conversion
with the increased reaction time. It is plausible that dehydro-
genation activity of the catalysts was not affected and the
decreased activity probably is because the subsequent overall
condensation reaction is retarded, which can be conrmed
from the decreased methanol conversion. In addition, from the
observed colour change of catalyst aer reactions, we deduce
that catalyst deactivation maybe attributed to carbon deposition
rather than the aggregation of Cu particles. In fact, our recent
results indicated that the regenerated catalyst, which was ever
Fig. 1 Catalytic performance of the catalysts with different loadings of
Cu and Mg components.
ꢀ
calcined at 500 C under air atmosphere, still showed good
catalytic performance comparable to that of fresh catalyst. Here,
it is probably the effect of MgO got affected due to carbon
coverage. Catalyst deactivation keeps further investigation in
the further.
also retards catalyst deactivation. In our experiment, higher
loading catalyst (30Cu9.75MTS-500) was also used for IBA
synthesis, and the evaluation results indicated the catalyst
showed much poorer performance (Table S1†), probably due to
the deteriorated Cu and MgO dispersion. Just observing rela-
tively high activity of the Cu–Zn–Al catalysts reported by Reddy
et al., we attempted to prepare Cu–Zn–Al catalyst with Red-
dy's method and used it for IBA synthesis. However, the results
revealed that the catalyst showed a poor catalytic activity under
a high WHSV. In this paper, the effect of mesoporous structure
of Ti-SBA-15 on catalyst activity was not studied specically, but
we are sure that the specic pore properties of this zeolite are
favourable for IBA synthesis because of its promotion for
contact time and collision probability of the reactants and
20C6.5MTS catalyst with the optimized loadings of Cu and
Mg was chosen for the further investigation on the effects of
calcination temperature on catalytic performance. As shown in
Fig. 2, the ethanol conversion over 20C6.5MTS-400 was close to
1
00% during the initial 4 h and retained at this level with the
increased reaction time. Compared with 20C6.5MTS-400,
0C6.5MTS-700 showed lower ethanol conversion (only 88.0%)
9,10
2
at the initial 4 h reaction. Moreover, the ethanol conversion
decreased with the increased TOS on the catalysts calcined at
high temperature. Especially on 20C6.5MTS-700, the ethanol
conversion decreased dramatically to 60.0% aer 10 h reaction,
corresponding to a low methanol conversion of 27.3%. The low
methanol conversion indicated that the subsequent two steps
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Fig. 2 Catalytic activities and stabilities of catalysts calcined at different temperatures.
overall condensations were inhibited somehow. It can be leading to its absence in outlet at present high reaction
observed from Fig. 2 that the IBA selectivity decreased gradually temperature. Our recent results showed that acrolein could be
ꢀ
with increased calcination temperature and TOS. An extremely produced at 320 C when formaldehyde and acetaldehyde were
low IBA selectivity was obtained over 20C6.5MTS-700, and less used as reactants. Here, the reaction between formaldehyde and
than 1% of IBA selectivity was determined aer 6 h. It can also acetaldehyde is ascribed to aldol condensation. It is plausible
be noticed from Fig. S1 in the ESI.† In the case of the side- that the aldol condensation probably proceeded during above-
products (AA, PA, MA, EA, POL and IBOL), the selectivity AA mentioned overall condensation reaction given that formalde-
substantially showed an increasing trend with TOS. Also, it was hyde can be also produced via dehydrogenation of methanol.
favored by increasing calcination temperature from 400 to 700 Based on above illustration, the possible main reaction path-
ꢀ
C. As observed from the results of the 20C6.5MTS-700 catalyst, ways of conversion of methanol and ethanol to chemicals in IBA
AA selectivity was up to 60.0% aer 10 h reaction from the initial synthesis system was proposed by us and shown in Fig. 3, in
4
0.0%, wherein, there was no signicant inuence on the which gas products and unidentied products were not
selectivities of the other side-products. This indicates that designed.
catalyst calcined at a relatively high temperature is still active It is worthy to mention that although the mechanism of IBA
8
for dehydrogenation of ethanol. However, the conversion of AA formation had been proposed by Wang et al. the tandem
with methanol was seriously affected, thereby leading to a low process is too complicated to explain the whole reaction.
IBA selectivity, which can be also veried from the gradually Especially, overall condensation process and further conversion
decreased PA selectivity over 20C6.5MTS-700 with the increased
TOS.
It was reported that in the system with the presence of
29–32
aldehyde and alcohol, Tishchenko reaction proceeds easily,
that is, the aldehyde can react with the alcohols to form corre-
sponding esters. In the present system, since a remarkable
amount of MA and EA was detected, it indicated that Tish-
chenko reaction proceeded over the catalysts. It is plausible that
there is a competition between the overall condensation reac-
tion and Tishchenko reaction. POL and IBOL were produced
through the hydrogenation of PA and IBA with the presence of
2
H during the reactions. No acrolein was observed at present
ꢀ
33
reaction temperature of 360 C. we deduced that the C]C
Fig. 3 Proposed pathways of the main reactions in methanol and
ethanol conversion to IBA over CuMg based catalyst.
double bond for acrolein was easily dehydrogenated, thereby
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of the intermediates (AA, hemiacetal, etc.) as well as catalyst
deactivation are still unclear and need to be further
investigated.
3.2 Catalyst characterization
3
.2.1 XRD. The small-angel XRD patterns of the catalysts
ꢀ
calcined at 400, 500, 600 and 700 C are shown in Fig. 4. As
observed, an ordered mesoporous structure of the catalysts is
basically restrained even though suffering from high calcina-
34
tion temperatures. With the elevation of calcination temper-
ature, there is a gradual increase of peak intensity and a shi of
ꢀ
peak position within the small-angle range (2q ¼ 0.7–5 ). As
detailed in the experimental section, the Ti-SBA-15 support was
obtained via calcination at 550 C. Calcination at a relatively
ꢀ
Fig.
5
Wide-angel XRD patterns of the representative catalysts
calcined at different temperatures.
high temperature can result in pore collapse, leading to
a decrease of pore wall thickness and relatively high crystalli-
zation. Therefore, the small-angel XRD patterns show a shi of
the diffraction peaks and increased peak intensity with the
elevation of catalyst calcination temperature. Also, a high
calcination temperature is favorable for dispersing the fused
reported in literatures, Ti species are difficult to be determined
directly with XRD technology due to their high dispersion and
27
incorporation into the framework of SBA-15. It is shown in the
previous section that calcination temperature had signicant
effects on catalytic performance. However, the CuO particle size
of each catalyst is similar. Therefore, it is plausible that, in
present reaction, the different performance of the catalysts is
caused by other factors rather than Cu particle size.
3 2 3 2
Cu(NO ) and Mg(NO ) from the external surface into the inner
pores of Ti-SBA-15 support or incorporating partial Cu species
into the framework. However, it needs to be pointed out that
calcination at a relatively high temperature can also induce an
irreversible growth of the metal oxide particles and enhance the
interaction between the components and the support.
The wide-angle XRD patterns of as-prepared catalysts
calcined at different temperatures are shown in Fig. 5. The
diffraction peaks of CuO were observed on the catalysts
3.2.2 FT-IR. Fig. 6 shows the FT-IR spectra of as-prepared
Ti-SBA-15 support and Cu–MgO/Ti-SBA-15 samples calcined at
different temperatures. Generally, the IR absorption peaks at
ꢁ1
1
230 and 1070 cm are attributed to nas (Si–O–Si) vibrations of
the silica framework of the SBA-15 support, and the peak at 802
(JCPDF48-1541), and the peak intensity does not increase with
ꢁ1
ꢁ1
cm is attributed to n
s
(Si–O–Si) vibration. And the peak at 960
the elevation of calcination temperature, which implies that the
particle size of CuO is hardly affected by the calcination
temperature. Based on the results calculated from the Scher-
rer's equation, it was found that the CuO particle size is about
26,36
cm
belongs to nas (Si–OH) or nas (Si–O–Ti),
while the
ꢁ1
absorption peaks at 3700 cm are attributed to s-OH, of which
amount is related to Ti deposition on the surface of SBA-15. As
36
observed in Fig. 5, the unloaded Ti-SBA-15 shows a vibration
10–20 nm, but the effect of calcination temperature on their size
ꢁ1
peak at 960 cm , indicating that Ti was successfully incorpo-
was insignicant seemingly. Additionally, though 6.5 wt% Mg
was loaded on the catalysts, the characteristic diffraction peaks
of MgO are not observed in Fig. 5. It is deduced that MgO might
present on the catalyst surface with an amorphous structure,
rated into the framework of SBA-15 during the in situ hydro-
thermal synthesis. For the catalysts calcined at different
ꢁ1
temperatures, the peak at 960 cm apparently disappear and
ꢁ1
the peak at 3700 cm is weakened dramatically, indicating that
35
that is, the MgO is highly dispersed on the support. As
Fig. 4 Small-angel XRD patterns of representative catalysts calcined Fig. 6 FT-IR spectra of Ti-SBA-15 support and the representative
at different temperatures.
catalysts calcined at different temperatures.
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the introduction of Cu and MgO probably induces the surface
groups such as –OH and Ti–O–Si to be overlaid. On the other
hand, it can be observed that, in spite of suffering from different
calcination temperatures, there is no obvious difference in the
IR spectra of the catalysts.
3.2.3
2
N adsorption and desorption. The nitrogen adsorp-
tion and desorption on the samples was conducted and their
textural properties are shown in Table 1. It is observed that the
2
Ti-SBA-15 has a remarkably high surface area (Sbet ꢂ 847 m
ꢁ1
g
), indicating its good crystallization. With the introduction
of Cu and Mg species, the surface areas, the pore volume of the
samples decreased dramatically, but the remained surface areas
2
ꢁ1
were still as high as 300 m g . A slight increase of the surface
area can be observed over the catalysts calcined between 400–
ꢀ
6
00 C. Further increased calcination temperature would lead to
a certain decrease of surface area, which is in agreement with
the results of pore volume in Table 1. In comparison of BJH pore
diameters of the catalysts in Table 1, one can observe that
introduction of Cu and MgO components also results in an
Fig. 7 The isotherms liner plots of the catalyst calcined at different
temperatures.
obvious decrease due to their coverage to catalyst surface. In temperature. In spite of presence of the texture parameter
addition, 20C6.5MTS-400 has 5.81 nm of pore size, which is change with the varied calcination temperature, it is difficult to
much smaller than that of 20C6.5MTS-500. It is thought that the correlate the catalytic performance with catalyst texture prop-
impregnated Cu and Mg nitrates did not well spread into the erties directly. Only, high specic surface area of these catalysts
inner pores of Ti-SBA-15 completely at calcination temperature is a positive factor for IBA synthesis due to well dispersion of Cu
ꢀ
of 400 C, leading to the blockage of pores, which is also and MgO species.
affirmed from the distorted IV-type isotherms of 20C6.5MTS-
00 (as shown in Fig. 7). And nevertheless, with the increase loadings of Cu and Mg on Ti-SBA-15 support had a great effect
2
3.2.4 H -TPR. The catalyst test above has revealed that the
4
of calcination temperature, the fused Cu and Mg nitrates driven on catalytic performance. In this section, the catalysts prepared
by the thermal energy would migrate into inner pores, thereby with different Cu and Mg loadings were characterized with H2-
inducing the slight increase of BET surface area, BJH pore TPR and XRD, and the results are shown in Fig. 8. As observed,
diameter and pore volume. Due to the limitation of pores, the the samples show an overlapped H -consumption outgrowth
2
ꢀ
size of the formed CuO particles increased slightly with the amongst 220–287 C, wherein, the main reduction peaks at
ꢀ
increased calcination temperature, which is veried by the XRD around 251 C are attributed to the reduction of nely dispersed
ꢀ
results discussed above. For 20C6.5MTS-700, it is deduced that CuO species while the shoulder reduction peaks at 273
further high-temperature calcination caused lattice shrinking represent the reduction of CuO species that strongly interact
C
37,39
(a0 in Table 1) and pore distortion of Ti-SBA-15 support, thereby with MgO.
With the increase of loadings of Cu and Mg, the
resulting in the loss of surface area. However, it is worthy to H -consumption peak increases, but the shoulder peak at 273
2
ꢀ
mention that high-temperature calcination can induce strong
C is merged, and form a single peak. Different from other two
interaction between Cu and Mg components, and the support, samples, only 20C6.5MTS shows a reduction peak shi to a high
ꢀ
and incorporation of Cu or Mg into the framework of Ti-SBA-15 temperature, 260 C, which implies the CuO particles are
as well, which might result in disordered framework somewhat. aggregated, probably caused by higher CuO loadings. Also, XRD
From right-hand raw in Table 1, it can be observed that the results shown in the inset of Fig. 6 have proved that particle size
introduction of Cu and MgO induced an obvious increase of of CuO increased with their increased loading. Generally,
wall thickness, but as for supported catalysts, wall thickness a small particle size of CuO aer reduction is signicantly
becomes thin gradually with increase of calcination favorable for IBA synthesis. However, our test results showed
Table 1 Textural properties of Ti-SBA-15 support and as-prepared catalysts calcined at different temperatures
2
ꢁ1
3
ꢁ1
b
a
Catalyst
S
bet/m g
Volume of pore/m g
BJH pore diameter /nm
d
100 /nm
a
0
/nm
Wall thickness/nm
Ti-SBA-15
847
281
322
342
320
1.25
8.00
5.81
6.15
5.92
5.42
9.38
9.28
9.23
8.95
8.38
10.8
10.7
10.7
10.3
9.7
1.72
4.89
4.55
4.38
4.28
2
2
2
2
0C6.5MTS-400
0C6.5MTS-500
0C6.5MTS-600
0C6.5MTS-700
0.485
0.559
0.575
0.539
a
b
Calculated by XRD data. From pore distribution proles in Fig. S2 in ESI.
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aldol condensation, which are promoted by catalyst basicity. In
this section, the basicity of the fresh catalysts and the spent
catalysts were measured by CO -TPD and the results are shown
2
in Fig. 10. As observed in Fig. 10a, the features of the proles
differ with each other for the fresh catalysts. The catalysts
ꢀ
calcined at 400 and 500 C need a relatively high temperature
for CO
2
desorption compared with the other two calcined at 600
ꢀ
and 700 C, respectively. Also, the peak areas of 20C6.5MTS-400
and 20C6.5MTS-500 are bigger than those of the other two
catalysts. It suggests that high-temperature calcination can
cause a decrease of basic sites, i.e. low basicity. Since the cata-
lyst basicity is closely related with subsequent aldol condensa-
tion, the decrease of basicity certainly induces a poor catalytic
activity. This is in agreement with our previous results of cata-
lyst evaluations. Therefore, it is apparently concluded that high-
temperature calcination induces the change of catalyst struc-
ture such as the thickness of pore wall and pore size, strongly
Fig. 8
2
H -TPR profiles and XRD patterns (in the inset) of as-prepared
catalysts with different Cu and Mg loadings.
that the catalytic performance of the catalyst 5Cu1.6MTS with affects the dispersion of active Cu species and MgO within the
the lowest Cu loading (1.6%) was much poorer than 20C6.5MTS. pores of Ti-SBA-15 and their interaction, and nally leads to
This indicates that the improvement of catalytic activity a decreased catalytic performance.
induced by small CuO particle size does not compensate the
Compared CO -TPD proles of fresh catalysts, those of the
2
decline of activity caused by the decreased amount of active spent catalysts showed an obvious shi to low-temperature with
sites.
Fig. 9 shows the proles of H
calcined at different temperatures. It can be seen that the H
increase of calcination temperature (in Fig. 10b). CO desorp-
2
ꢀ
2
-TPR of 20C6.5MTS catalysts tion peak of the spent catalyst calcined at 700 C shied to
ꢀ
2
-
about 73 C, indicating that the basicity on catalyst surface can
consumption peaks of 20C6.5MTS-400 and 20C6.5MTS-500 are be weakened during reaction. For the spent catalysts calcined at
ꢀ
ꢀ
located between 240–280 C. With the elevation of calcination 400 and 500 C, it was observed that their CO desorption peak
2
temperature, the reduction peaks of 20C6.5MTS-600 and
20C6.5MTS-700 shi to high temperature and shoulder peaks
appear on the right of the main peaks. According to the XRD
results shown in the previous section, the CuO particle size did
not increase in spite of elevating calcination temperature. Based
on this, it is deduced that the shi is probably caused by the
enhanced interaction between Cu species and other compo-
37,39
nents, including MgO, and the catalyst support.
The
appearance of the shoulder peaks is probably relative with Cu
incorporation into the framework of Ti-SBA-15.
3
.2.5 CO -TPD. As our previous illustration, a good catalyst
2
for IBA synthesis from methanol and ethanol should possess
simultaneously two functions such as dehydrogenation and
Fig. 9
H
2
2
-TPR profiles of 20C6.5MTS catalysts calcined at different Fig. 10 CO -TPD profiles of fresh 20C6.5MTS catalysts calcined at
temperatures.
different temperatures and the spent catalysts.
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areas aer reaction were much smaller than that of the fresh
catalysts. According to previous catalyst evaluation, the catalysts
ꢀ
calcined at 400 and 500 C had better performance. This indi-
cated that the better performance the catalyst showed, the
severer the basicity loss was. In addition, we found that the
colour of spent catalyst is slightly black, even aer H re-
2
reduction. Based on that, we deduced that catalyst surface
might be covered by carbon residue, thereby resulting in
basicity loss.
3.2.6 XPS. XPS measurements were preformed to further
investigate the states of each component on the catalyst
surface. Fig. 11 shows the Cu 2p binding energy spectra of
2
0C6.5MTS catalysts calcined at different temperatures. As
observed, the peaks at 933.14 and 935.54 eV are ascribed to Cu
Fig. 12 Cu 2p XPS spectra of 20Cu/SBA-15, 20C6.5MS and
2
+
2p3/2 of Cu species, and that at 955.52 eV is otherwise
20C6.5MTS.
3
8
ascribed to Cu 2p1/2
. The binding energy corresponding to Cu
2
p
1/2 was due to a spin–orbit coupling, with an energy gap of
3
8,40
about 20 eV of Cu 2p3/2. In some literatures,
there is
The surface elemental compositions of 20C6.5MTS calcined
generally a single peak of Cu 2p3/2 at ꢂ934 eV, but the present
spectra shows two peaks at 933.14 eV and 935.54 eV, respec-
tively. It suggests that two types of interactions present on
these catalysts.
at different temperatures were calculated from the deconvolu-
tion of XPS spectra (as shown in Table 2). The results show that
the molar ratios of Cu/Si and Cu/Mg on the catalyst surface
decrease with the increase of calcination temperature. Notably,
the ratio of Mg/Si keeps constant. It implies that the impreg-
nated Cu(NO ) immigrated into the internal pores of Ti-SBA-15
3 2
support or incorporated into the framework during the calci-
nation, which resulted in the shi of reduction peak to high
Auger electron spectra (AES) of Cu 2p indicate that the
+
present catalysts do not contain Cu species (Fig. S3 in ESI†). In
order to clear this, some catalysts such as 20Cu/SBA-15,
20C6.5MS and 20C6.5MTS were prepared and characterized
with XPS technology (results shown in Fig. 12). It is revealed that
only Cu-containing catalyst, 20Cu/SBA-15, shows a weak peak at
temperature, shown in the H -TPR proles. It can be also
2
observed that the ratios of Cu/Mg are much higher than bulk
composition (in second right-hand row). It is thought that the
935.54 eV. However, the peak intensity was strongly enhanced
when MgO was introduced, which indicates that the interaction
between MgO and Cu existed. It is concluded that the peak at
impregnated Cu(NO
is as low as 170 C, was rstly fused and decomposed, while
3
)
2
, whose decomposition temperature (DT)
ꢀ
933.14 eV is ascribed to Cu 2p3/2 BEs of the isolated CuO, while
ꢀ
Mg(NO
3
)
2
with DT of 330 C was relatively stable. Thus, owing to
the peak at 935.54 eV is ascribed to the Cu 2p3/2 BEs caused by
the strong interaction between Cu and other components such
distinct spread process, obtained CuO were more stranded at
catalyst surface than MgO during calcination, leading to higher
ratio of Cu/Mg on catalyst surface. Besides, the ratio of Cu/Mg
obtained by XRF is much lower than the added theoretical
amount (the best right-hand row in Table 2). We deduced that
part of Cu might remain at the surface of the porcelain crucible
during calcination, which was factually observed to be black
aer calcination. The binding energies of other components
such as Mg and Si of the catalysts are found to be unchanged in
spite of varied calcination temperature.
37,39
as MgO and the support.
In addition, it can be observed
from Fig. 11 that there is no obvious difference in the Cu 2p
binding energies in spite of being subjected to the high-
temperature calcination.
For the catalysts with different calcination temperature, the
XPS spectra (Fig. 11) are similar with each other. It was difficult
Table 2 Surface elemental compositions of 20C6.5MTS catalysts
calcined at different temperatures
Calcination
temperature ( C)
ꢀ
a
a
a
b
c
Cu/Mg
Cu/Si
Mg/Si
Cu/Mg
Cu/Mg
4
5
6
7
00
00
00
00
10.1
8.73
8.53
7.55
0.12
0.010
0.010
0.0096
0.010
1.16
1.05
1.08
1.10
2
2
2
2
0.091
0.084
0.077
a
b
c
Fig. 11 Cu 2p XPS spectra of 20C6.5MTS catalysts calcined at different
By XPS technique. By XRF technique. Theoretical amount.
temperatures.
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to correlate specic Cu species with catalytic activities. The
analysis of elemental composition indicated that the difference
in performance was mainly caused by the decrease of Cu with
the increase of calcination temperature. In fact, the doping of
Mg is a key factor which promotes the catalytic performance. As
observed in Fig. 12, the binding energies of Cu 2p were strongly
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4
. Conclusions
from methanol and ethanol, M.D., East China University of
Ti-SBA-15 supported Cu and MgO catalyst was used for the rst
Science and Technology, Shanghai, China, 2008.
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40, 550.
ꢀ
ꢁ1 ꢁ1
lyst at 360 C under WHSV of 3.0 mL (g
h
). This result was 15 (a) B. M. Reddy, E. P. Reddy and B. Manohar, Appl. Catal., A,
superior to the one on the Cu-based catalyst ever reported.
The introduction of MgO affected the binding energy of Cu
species. The high-temperature calcination enhanced the inter-
1993, 96, L1; (b) H. Hu, X. Meng and L. Shi, Nat. Gas Chem.
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Oil Gas, 2007, 36, 356.
action between Cu, Mg species and the support, and neverthe- 16 (a) G. Y. Bai, H. L. Wang, H. S. Ning, F. He and G. F. Chen,
less decreased the basicity of the catalysts. A portion of Cu
species migrated into the internal pores of Ti-SBA-15 or prob-
ably incorporated into the framework while Mg still remained
on the support surface during the high-temperature calcina-
tion. The ratios of Cu/Mg are much higher than bulk compo-
React. Kinet. Catal. Lett., 2008, 94, 375; (b) R. K. Marella,
C. K. P. Neeli, S. R. R. Kamaraju and D. R. Burri, Catal. Sci.
Technol., 2012, 2, 1833; (c) B. M. Nagaraja, A. H. Padmasri,
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sition due to the tremendous difference in decomposition 17 (a) N. F. Balsamo, C. M. Chanquia, E. R. Herrero,
temperature of Cu(NO
3
)
2
and Mg(NO
3
)
2
. The catalyst activity
S. G. Casuscelli, M. E. Crivello and G. A. Eimer, Ind. Eng.
Chem. Res., 2010, 49, 12365; (b) B. M. Nagaraja,
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was mainly affected by Cu content on the catalyst surface, the
interaction of Cu and Mg species as well as Cu–Mg and support,
and the basicity of the catalysts.
The research work is still carried on for further investigation
on catalyst deactivation and the reaction mechanism.
1
9 (a) R. Tesser, V. Maradei, M. Di Serio and E. Santacesaria,
Ind. Eng. Chem. Res., 2004, 43, 1623; (b) R. K. Marella,
K. S. Koppadi, Y. Jyothi, K. S. R. Rao and D. R. Burri, New
J. Chem., 2013, 37, 3229.
Acknowledgements
The authors gratefully acknowledge the nancial support from
the National Natural Science Foundation of China (21303242)
and Natural Science Foundation of Shanxi Province 20 (a) V. Chikan, A. Molnar and K. Balazsik, J. Catal., 1999, 184,
(
2015011021), China.
134–143; (b) C. Angelici, F. Meirer, A. M. J. van der Eerden,
H. L. Schaink, A. Goryachev, J. P. Hofmann, E. J. M. Hensen,
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