Amorphous SiO2 catalyst for vapor-phase aldol condensation of butanal

Article abstract of DOI:10.1016/j.apcata.2018.11.012

Vapor-phase aldol condensation of butanal to form 2-ethyl-2-hexenal was carried out over several oxide catalysts such as SiO₂-Al₂O₃, Al₂O₃, ZrO₂, and SiO₂. Catalysts with moderate and strong acid sites such as Al₂O₃ and SiO₂-Al₂O₃ were active for the reaction in the initial period, whereas they deactivated rapidly. In contrast, SiO₂ with weak acidity showed a low but a stable catalytic activity for the formation of 2-ethyl-2-hexenal. Thermogravimetric analyses of the samples used after the reactions indicate that SiO₂ has the smallest amount of carbonaceous species that contributed to its stable activity among the tested catalysts. SiO₂ catalysts with different pore sizes and specific surface areas were examined: SiO₂ with a mean pore diameter of 10 nm and a surface area of 295 m² g^?1 showed the best catalytic performance and gave a 2-ethyl-2-hexenal selectivity of 90% at a conversion of 48% at 240 °C. In the catalytic test using deuterated SiO₂, which was prepared by contacting SiO₂ with deuterated water before the reaction, it was confirmed by a mass spectrometer that the deuterium atom of SiOD was transferred to a 2-ethyl-2-hexenal molecule during the reaction. It is indicated that silanol groups on the SiO₂ surface played a role as an active site.

Full text of DOI:10.1016/j.apcata.2018.11.012

AppliedCatalysisA,General570(2019)113–119
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Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
Amorphous SiO2 catalyst for vapor-phase aldol condensation of butanal
Daolai Suna,b, Yasuhiro Yamadab, Satoshi Satob,⁎
a School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin, 300130, China
b Graduate School of Engineering, Chiba University, Chiba, 263-8522, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords:
SiO2
Aldol condensation
Butanal
Catalytic deactivation
Silanol groups
Vapor-phase aldol condensation of butanal to form 2-ethyl-2-hexenal was carried out over several oxide catalysts
such as SiO2-Al2O3, Al2O3, ZrO2, and SiO2. Catalysts with moderate and strong acid sites such as Al2O3 and SiO2-
Al2O3 were active for the reaction in the initial period, whereas they deactivated rapidly. In contrast, SiO2 with
weak acidity showed
a low but a stable catalytic activity for the formation of 2-ethyl-2-hexenal.
Thermogravimetric analyses of the samples used after the reactions indicate that SiO2 has the smallest amount of
carbonaceous species that contributed to its stable activity among the tested catalysts. SiO2 catalysts with dif-
ferent pore sizes and specific surface areas were examined: SiO2 with a mean pore diameter of 10 nm and a
surface area of 295 m2 g−1 showed the best catalytic performance and gave a 2-ethyl-2-hexenal selectivity of
90% at a conversion of 48% at 240 °C. In the catalytic test using deuterated SiO2, which was prepared by
contacting SiO2 with deuterated water before the reaction, it was confirmed by a mass spectrometer that the
deuterium atom of SiOD was transferred to a 2-ethyl-2-hexenal molecule during the reaction. It is indicated that
silanol groups on the SiO2 surface played a role as an active site.
1. Introduction
viewpoint of industrial application. However, serious deactivation of a
catalyst is reported in the previous studies performed in a vapor phase
In the chemical industry, aldol condensation is an important reac-
tion for the synthesis of long-chain chemicals [1,2], and it has wide
applications in the field of biomass conversion to useful chemicals and
fuels [3–5] as well as in the field of petrochemistry. Aldol condensation
proceeds between aldehydes and/or ketones, and either an acid or a
base can catalyze the reaction [1,2]. Industrially, sulfuric acid or so-
dium hydroxide is generally used as a catalyst in aldol condensation
[1,2,6–9], while it is obvious that the utilization of inorganic acid and
base is not friendly to the environmental. The development of solid
catalysts for aldol condensation has attracted much attention in recent
years. Solid acids such as zeolites and heteropolyacids [7,10–14] as well
as solid bases such as Na/SiO2 and MgO [15–18] exhibit catalytic ac-
tivity for aldol condensation, whereas the catalysts are not stable and
they deactivated during the reaction due to a poisoning of the active
sites by the deposition of carbonaceous species in most cases.
[20–22,25]. For example, the conversion of butanal over Pd/Na/SiO2
decreased from 81.8 to 49.1% with time on stream from 4 to 8 h at
350 °C [21]. In our recent report, SiO2-Al2O3, Nb2O5, and TiO2 were
tested at 200 °C in H2 flow: all the catalysts gradually deactivated and
the conversion of butanal decreased with a range of 10–40% during the
initial 5 h of time on stream [25]. In contrast, SiO2-Al2O3 with the
strong acidity showed the most serious catalytic deactivation. On the
other hand, we found that Ag-modified TiO2 with hydrogen flow was
effective for inhibiting the formation of coke and improving the cata-
lytic stability.
Amorphous SiO2 is a porous material with weak acidity [26–28],
and it is widely used as a catalyst support [29–31]. Recently, we found
that SiO2 can be used as a catalyst for vapor-phase dehydration such as
the dehydration of aldoximes to nitriles [26] and the cyclization of
levulinic acid to angelica lactones [27]. In the dehydration, the cata-
lysts with strong acidity such as SiO2-Al2O3 exhibited high initial ac-
tivities, whereas strong acid sites took part in the side reactions to form
oligomers, which poisoned the active sites and decreased the stability of
catalytic activity. In contrast to the strong acid catalysts, the catalytic
activity of SiO2 was usually low, while it was stable [26,27]. Over the
weak acidic SiO2, also, high conversion levels can be achieved by
Self-aldol condensation of butanal to form 2-ethyl-2-hexenal (2E2H)
is investigated as a model reaction to study aldol condensation reac-
tions [14,19–25]. In addition, the reaction is attracting great im-
portance because 2E2H has been used as an intermediate for producing
perfumes, cosmetics, and plasticizers [19,20,23,24]. A vapor-phase
continuous process for 2E2H production is attractive from the
⁎ Corresponding author.
E-mail address: satoshi@faculty.chiba-u.jp (S. Sato).
https://doi.org/10.1016/j.apcata.2018.11.012
Received 4 September 2018; Received in revised form 7 November 2018; Accepted 12 November 2018
Availableonline14November2018
0926-860X/©2018ElsevierB.V.Allrightsreserved.

D. Sun et al.
AppliedCatalysisA,General570(2019)113–119
Table 1
a
Aldol condensation of butanal over various solid acid catalysts.
Catalyst
SBET
Conversionb
Selectivity (mol%)b
2E2Hc
(m2 g−1
)
(mol%)
Butanoic acid
1-Butanol
Others
Al2O3
SiO2-Al2O3
ZrO2
10-Li2O/ZrO2
SiO2 (Q-10)
198
397
39
–
295
13.7
20.7
2.5
2.3
9.4
87.2
77.2
26.6
10.4
74.2
5.6
4.0
62.7
63.4
21.1
2.1
2.0
5.6
3.5
1.5
5.1
16.8
5.1
22.7
3.2
a
Reaction conditions: reaction temperature, 200 °C; catalyst weight, 0.5 g; H2 flow rate, 5 cm3 min−1
Conversion and selectivity were averaged between 1–5 h of time on stream, TOS.
2E2H, 2-ethyl-2-hexenal.
.
b
c
increasing the contact time. It should be emphasized that the selectivity
to the dehydration products over SiO2 can be maintained since no side
reactions proceed even at high conversions. For example, in the cycli-
zation of levulinic acid to angelica lactones performed at 250 °C [27],
the selectivity to angelica lactones over SiO2 was ca. 90% at a con-
version of ca. 50%, whereas that was only ca. 80% over SiO2-Al2O3 at
the same conversion level due to the further oligomerization of angelica
lactones.
In the present study, several solid catalysts including different kinds
of SiO2 were investigated in the vapor-phase aldol condensation of
butanal. This study aims to achieve a high selectivity to 2E2H with
stable and high conversions using appropriate solid catalysts.
Furthermore, the active sites on the SiO2 surface were investigated and
discussed.
2.3. Characterization of catalysts
The specific surface area (SBET) of catalysts was calculated by the
Brunauer-Emmett-Teller (BET) method in the N2 adsorption-desorption
isotherm at −196 °C. The thermogravimetry (TG) analysis was per-
formed using Thermoplus 8120E2 (Rigaku). The TG analysis conditions
were as follows: sample weight, ca. 10 mg; the rate of temperature in-
crement, 5 °C min−1; the range of heating temperature, between room
temperature and 900 °C. The temperature-programmed desorption of
adsorbed NH3 (NH3-TPD) measured by neutralization titration was the
method introduced elsewhere [33,34], and the details were described
in the supplementary information. The diffuse reflectance infrared
Fourier transform (DRIFT) spectra of catalysts were recorded on spec-
troscopy using FT/IR-4200 (JASCO Corp., Japan) under vacuum con-
ditions. Before the DRIFT measurement, a sample was pretreated in
vacuum at 150 °C for 1 h to remove the H2O adsorbed on the catalyst
surface.
2. Experimental
2.1. Samples
3. Results and discussion
Four kinds of SiO2 (CARiACT Q-3, Q-6, Q-10, and Q-15 with mean
pore diameters of 3, 6, 10 and 15 nm, respectively) samples were sup-
plied by Fuji Silycia, Ltd., Japan. γ-Al2O3 (DC-2282) was supplied by
Dia Catalyst & Chemicals Ltd., Japan. SiO2-Al2O3 (N631HN) was pur-
chased from Nikki Chemical Co. Ltd., Japan. Monoclinic ZrO2 (RSC-HP)
was supplied by Daiichi Kigenso Kagaku Kogyo Co. Ltd., Japan. 10 mol
% Li2O-modified monoclinic ZrO2, denoted as 10-Li2O/ZrO2, was pre-
pared by an impregnation followed by calcination at 600 °C for 3 h, and
the details are described in our previous report [32]. Butanal, which
was purchased from Wako Pure Chemical Industries, Japan, was uti-
lized for the catalytic reaction without further purification.
3.1. Aldol condensation of butanal over various solid catalysts
Table 1 shows the catalytic reaction results of the aldol condensa-
tion of butanal over solid catalysts such as SiO2-Al2O3, Al2O3, ZrO2, 10-
LiO/ZrO2, and SiO2 (Q-10) where the reaction was performed at 200 °C
and a catalyst loading of 0.5 g. 2E2H, butanoic acid, and 1-butanol were
detected as the major products in all the reactions. Values of the con-
version and the selectivity were averaged between 1–5 h. Scheme 1
shows the proposed formation route of each detected product. Aldol
condensation of butanal firstly forms 2-ethyl-3-hydroxy-hexanal, which
further dehydrates to form 2E2H. Tsuji et al. reported that Tishchenko
esterification of butanal occurred over both acid and base catalysts
[15], which means that the reactions, Tishchenko esterification and
aldol condensation, require similar active sites. Therefore, Tishchenko
esterification usually proceeds as the major side reaction in aldol con-
densation [15,22,25]. In the present study, the major by-products,
butanoic acid and 1-butanol, were probably formed by the hydrolysis of
butyl butyrate, which was formed by the Tishchenko esterification of
butanal. However, the selectivity to butanoic acid was higher than that
to 1-butanol in all the cases. The yield of butanoic acid, which was
calculated by the butanal conversion multiplied by the selectivity to
butanoic acid, was less than 2.5% and it was almost constant at ca. 2%
in the cases at 200 °C. Some undetectable products, summarized as
others in Table 1, were formed in the reactions, and they are supposed
to be mainly attributed to the oligomers of butanal.
2.2. Catalytic reaction
Vapor-phase aldol condensation of butanal to 2E2H was performed
in a fixed-bed glass tube reactor in a similar way to the previous report
[25]. The details were described in the supplementary information, and
the typical reaction conditions were as follows: butanal was fed into the
reactor at 200 °C and a feed rate of 1.3 g h−1 together with an H2 flow
at a flow rate of 5 cm3 min-1 and an ambient pressure.
A pretreatment of SiO2 (Q-10) with D2O was performed in the fixed-
bed down-flow reactor. Before the pretreatment of D2O, 4.0 g of silica
placed on the catalyst bed was heated at 250 °C for 1 h. D2O was fed at
250 °C and a feed rate of 1.8 g h−1 for 1 h, and then the silica sample
was heated at 250 °C for 1 h to remove the residual D2O. After the
pretreatment, the temperature was decreased to 200 °C, and the reac-
tion was started by feeding butanal under the same conditions as
mentioned above. A mass spectrometer in a gas chromatograph system
(GC–MS, QP5050, Shimadzu, Japan) was used for the mass analysis of
2E2H in the reaction effluent collected during an appropriate period of
the reaction.
In our recent report, ZrO2 showed a catalytic activity in the vapor-
phase intramolecular aldol condensation of 2,5-hexanedione to form 3-
methylcyclopent-2-enone, and 10-Li2O/ZrO2 exhibited an enhanced
activity, providing 97.9% selectivity to 3-methylcyclopent-2-enone
with a conversion of 18.7% at a W/F of 0.29 h and a reaction tem-
perature of 250 °C [32]. In the present study, however, ZrO2 and 10-
114

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AppliedCatalysisA,General570(2019)113–119
Scheme 1. Proposed formation routes of the detected products. (2Ethyl-3-hydroxy-hexanal and butyl butyrate were not detected in the products).
Fig. 1. Changes in catalytic conversion of butanal over SiO2-Al2O3, Al2O3, and
SiO2 (Q-10) with time on stream.
Reaction conditions: temperature, 200 °C; catalyst weight, 0.5 g; H2 flow rate,
5 cm3 min−1
.
Fig. 2. NH3-TPD profiles of SiO2-Al2O3, Al2O3, and SiO2 (Q-10).
Li2O/ZrO2 showed poor selectivity for the formation of 2E2H while
they had a high selectivity to butanoic acid. This indicates that the
reactions, the intramolecular aldol condensation of 2,5-hexanedione
and the self-aldol condensation of butanal, would require different ac-
tive sites with different reaction mechanisms. In contrast to the ZrO2-
based catalysts, SiO2-Al2O3, Al2O3, and SiO2 (Q-10) exhibited high ac-
tivities, and the selectivity to 2E2H was higher than 70%. Among the
tested catalysts, SiO2-Al2O3 showed the highest conversion of 20.7%
with a 2E2H selectivity of 77.2%, while Al2O3 showed the highest 2E2H
selectivity of 87.2% with a conversion of 13.7%. Fig. 1 shows the
changes in the activity of SiO2-Al2O3, Al2O3, and SiO2 (Q-10) with time
on stream. It is obvious that the conversion of butanal gradually de-
creased with time of stream in the case of SiO2-Al2O3 and Al2O3, in-
dicating the deactivation of catalysts occurs during the reaction.
However, SiO2 (Q-10) was stable during the reaction, and no decrease
in conversion was observed irrespective of a low conversion. These
results show that SiO2 (Q-10) is the most stable catalyst although the
intrinsic activity is low.
Fig. 2 shows the NH3-TPD profiles of SiO2-Al2O3, Al2O3, and SiO2
(Q-10). SiO2-Al2O3 shows a broad desorption peak located from ca. 100
to 700 °C with a peak temperature at 293 °C. This indicates that SiO2-
Al2O3 includes weak, medium, and strong acid sites, which were de-
fined by NH3 desorption peaks at 100–300, 300–500, and 500–700 °C,
respectively, and the amount of medium acid sites is the largest. Al2O3
shows a desorption peak appeared at temperatures from ca. 100 to
600 °C with a peak temperature at 296 °C, which is similar to that of
SiO2-Al2O3. However, the total acid amount of Al2O3 is much smaller
than that of SiO2-Al2O3. SiO2 shows a desorption peak appeared from
ca. 100 to 300 °C with a peak temperature of 201 °C, meaning that SiO2
has the weakest acidity.
Meanwhile, SiO2 has the smallest amount of acid sites. The order of
the number of acid sites, SiO2-Al2O3 > Al2O3 > SiO2, agrees with the
order of their catalytic activity, indicating that the aldol condensation
of butanal is catalyzed by the acid sites on the catalysts. In general, the
catalyst with strong acidity is easy to be deactivated, which has been
demonstrated in our previous studies dealing with dehydration pro-
cesses such as the cyclodehydration of diethylene glycol to 1,4-dioxane
[35] and the dehydration of 1,2-propanediol to propanal [29,30]. In the
present study, the deactivation of SiO2-Al2O3 and Al2O3 is supposed to
be attributed to their relatively strong acidity because polymerizable
compounds such as the oligomers of butanal would be easily formed
over strong acid sites. In another word, SiO2 is stable without side re-
actions because of its weak acidity.
Fig. 3 depicts the TG profiles of SiO2-Al2O3, Al2O3, and SiO2 (Q-10)
samples used after the reactions shown in Fig. 1. All the used samples
show a weight loss between ca. 200 and 500 °C, which is attributed to
the coke decomposition [25,35]. The order of the amount of deposited
carbon is SiO2-Al2O3 > Al2O3 > SiO2, which agrees with the order of
the amount of their medium and strong acid sites, demonstrating that
carbon is readily formed and deposited on the surface of a strong acid
catalyst. The used SiO2 sample shows the smallest amount of accumu-
lated carbonaceous species, which agrees with the stable activity of
SiO2, demonstrating that the accumulation of carbonaceous species
primarily causes the catalytic deactivation.
115

D. Sun et al.
AppliedCatalysisA,General570(2019)113–119
Table 3
Aldol condensation of butanal over SiO2 (Q-10) at different reaction tempera-
tures.a
Temperature
(oC)
Conversionb
(mol%)
Selectivity (mol%)b
2E2Hc
Butanoic acid
1-Butanol
Others
180
200
220
240
260
27.3
38.6
43.9
48.3
51.6
92.2
88.6
89.1
90.0
87.9
4.3
4.4
2.7
4.0
4.0
1.6
2.1
2.8
2.8
4.1
1.9
4.9
5.4
4.2
4.0
a
Reaction conditions: catalyst weight, 4.0 g; H2 flow rate, 5 cm3 min−1
Conversion and selectivity were averaged between 1–5 h of TOS.
2E2H, 2-ethyl-2-hexenal.
.
b
c
large contact time was effective for achieving a high yield of 2E2H from
butanal. The formation of butanoic acid was inhibited at large contact
time. A high 2E2H selectivity of 88.6% with a conversion of 38.6% was
achieved over Q-10 at 200 °C and a catalyst loading of 4.0 g.
Fig. 3. TG profiles of SiO2-Al2O3, Al2O3, and SiO2 (Q-10) used after the reac-
tion.
Reaction conditions: temperature, 200 °C; catalyst weight, 0.5 g; H2 flow rate,
5 cm3 min−1; time on stream, 5 h.
The effect of reaction temperature on the catalytic activity of Q-10
was investigated at temperatures from 180 to 260 °C with a catalyst
loading of 4.0 g (Table 3). The conversion of butanal increased with
increasing the reaction temperature, while the selectivity to 2E2H was
kept at 88–92% in the reactions irrespective of reaction temperature.
This indicates that the reaction temperatures did not significantly affect
the selectivity to 2E2H over Q-10. In the previous study, however, high
temperatures such as 220 and 240 °C decreased the selectivity to 2E2H
over TiO2 with a relatively strong acidity [25]. In addition, high reac-
tion temperatures decreased the selectivity to 2E2H in the case of over
basic Na/SiO2 [21] and MgO/SiO2 with acid/base properties [22]. In
the present study, the high selectivity to 2E2H over Q-10 can be
maintained at such a high temperature of 260 °C at high conversion
levels. This could be attributed to the weak acidity of Q-10 which would
be less active for catalyzing the side reactions such as the oligomer-
ization of butanal.
Fig. 4 shows the changes in the activity of Q-10 with time on stream
at different reaction temperatures. In the reactions especially at high
temperatures such as 240 and 260 °C, the catalytic activity was stable,
and a significant decrease in either the conversion or the selectivity to
2E2H was not observed. In contrast, both the conversion and the se-
lectivity to 2E2H decreased with time on stream in the case of Pd/Na/
SiO2 at 350 °C [21], MgO/SiO2 at 300 °C and 350 °C [22], Pd/KX zeolite
at 150 °C [20], and TiO2 at 240 °C [25]. Fig. 5 shows the TG profiles of
Q-10 used after the reactions at 200 and 260 °C. Compared with the TG
profile of fresh Q-10, the weight loss of the samples used after the
3.2. Aldol condensation of butanal over SiO2 catalysts
From the viewpoint of industrial production, catalytic stability is
highly required. Although SiO2 (Q-10) showed the lowest activity for
2E2H formation among the three acid catalysts (Table 1), it showed the
highest stability among the tested catalysts. Therefore, various SiO2
catalysts with different properties were tested and investigated in the
following studies. Table 2 shows the catalytic activity of various SiO2
for aldol condensation of butanal at 200 °C. Four types of SiO2, Q-3, Q-
6, Q-10, and Q-15, with different pore sizes and surface areas, exhibited
different catalytic activities. Among the tested SiO2 samples, Q-10
showed both the highest conversion of butanal and the highest se-
lectivity to 2E2H. Q-3 and Q-6 had larger surface areas and larger acid
amount than Q-10 [27], whereas they showed lower catalytic activities.
It is supposed that the small mesopores of Q-3 and Q-6 would disturb
the diffusion of the dimers and oligomers of butanal in the pores, which
leads to their poor activity. On the other hand, Q-15 with the largest
pore size but the smallest surface area showed the lowest activity, in-
dicating that the surface area is an important factor for the aldol con-
densation over SiO2. These results demonstrate that both large pore
sizes and sufficient surface areas of SiO2 are required for the efficient
aldol condensation of butanal to form 2E2H.
Because of the high catalytic activity of Q-10 among SiO2, it was
selected for investigating the effect of the catalyst loading, i.e., contact
time. As shown in Table 2, both the conversion and the selectivity to
2E2H increased with increasing the loading of Q-10, indicating that
Table 2
Aldol condensation of butanal over various SiO2 catalysts.a
Catalyst SBET
(m2 g−1
Conversionb Selectivity (mol%)b
)
(mol%)
2E2Hc Butanoic acid 1-Butanol Others
Q-3
Q-6
Q-10
Q-10d
Q-10e
Q-15
705
4.0
8.1
9.4
20.5
38.6
1.9
42.7
65.2
74.2
86.0
88.6
40.7
45.0
28.9
21.1
10.0
4.4
0.0
1.0
1.5
1.9
2.1
0.0
12.3
4.9
3.2
2.1
4.9
0.0
401
295
295
295
226
59.3
a
Reaction conditions: reaction temperature, 200 °C; catalyst weight, 0.5 g;
H2 flow rate, 5 cm3 min−1
.
b
Conversion and selectivity were averaged between 1–5 h of TOS.
2E2H, 2-ethyl-2-hexenal.
Catalyst weight, 2.0 g.
c
d
e
Fig. 4. Changes in the catalytic activity over SiO2 (Q-10) with time on stream at
different reaction temperatures.
Reaction conditions: catalyst weight, 4.0 g; H2 flow rate, 5 cm3 min−1
.
Catalyst weight, 4.0 g.
116

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AppliedCatalysisA,General570(2019)113–119
Therefore, H2 was mainly used as the carrier gas in the present study.
However, we found no significant difference between H2 and N2, as
shown in Fig. 6. It is evident that the conversion of butanal decreased
with increasing the flow rate of gas, either N2 or H2. Because the aldol
condensation of butanal is a dimerization process, it can be expected
that the high concentration of butanal would promote the contact of
butanal molecules with each other and accelerate the reaction. In this
study, the flow rate of carrier gas significantly affects the partial pres-
sure of butanal and thus changes the conversion of butanal. Although
the selectivity to 2E2H was not influenced by the flow rate of N2, the
selectivity to 2E2H significantly decreased with increasing the flow rate
of H2. It is an important finding that carrier gases are not necessary for
the vapor-phase aldol condensation. Since the aldol addition is an
equilibrium reaction, the high partial pressure of butanal without car-
rier gas would be the best reaction conditions to shift the equilibrium to
the right. In other words, usage of carrier gas at high partial pressure
would shift the equilibrium to the left.
Fig. 5. TG profiles of SiO2 (Q-10) used at different reaction temperatures.
Reaction conditions: catalyst weight, 4.0 g; H2 flow rate, 5 cm3 min−1; time on
stream, 5 h.
3.3. Study of the active sites on SiO2
In the cyclization of levulinic acid to angelica lactones [27], we
have confirmed that the silanol groups on the surface of SiO2 are the
active sites. Also, the cyclization of 5-amino-1-pentanol to piperidine
was performed over D2O-treated SiO2 [28]: SiOH groups were ex-
changed to SiOD groups, and it was confirmed that the deuterium
atoms in silanol groups shifted to the product piperidine in the initial
periods when SiOD remained.
In the present study, to confirm the active sites on Q-10, the aldol
condensation of butanal was performed over Q-10 deuterated by D2O.
The pretreatment of D2O was performed in the fixed-bed down flow
reactor under D2O flow conditions at 250 °C for 1 h. Fig. 7 compares the
DRIFT spectra of fresh Q-10 and deuterated Q-10. An absorption peak
at 3737 cm−1, which is assigned to the stretching of SiO-H in isolated
silanol groups [36], can be observed in the spectra of both fresh Q-10
and deuterated one. In the DRIFT spectra of the deuterated Q-10, an
absorption peak attributed to the stretching of SiO-D in isolated silanol
groups was observed at 2711 cm−1 [36], indicating the partial in-
troduction of SiOD groups on the surface of Q-10.
reaction was large, indicating that carbonaceous species was accumu-
lated on the surface of Q-10 during the reaction. On the other hand, the
weight loss of the sample used at 260 °C was smaller than that of the
sample used at 200 °C. This indicates that the carbonaceous species
deposited on the samples are not a heavy coke because of the ready
desorption at low temperatures. We have studied the dehydration of
1,2-alkanediols to aldehydes over WO3/SiO2 with a moderate acidity:
high temperatures promoted the formation of coke in that case [30]. In
contrast to the present SiO2, it is probable that a high temperature
promotes the formation of coke through dehydrogenation over WO3/
SiO2 with stronger acidity. This difference in temperature dependence
on carbon formation between the two studies is considered to be at-
tributed to the different acid strength of the catalysts as well as the
different type of reactions. In the present study, a low temperature of
200 °C could not promote the desorption of residual product on the
catalyst surface that increases the amount of carbonaceous species,
which is a precursor of coke.
Fig. 8 shows the mass profiles of 2E2H and butanal collected in the
effluents over deuterated Q-10 at 200 °C in different periods of time on
stream. In the catalytic reaction of butanal over undeuterated Q-10,
2E2H showed a major molecular peak at m/z of 126, which corre-
sponded to the molecular weight of C8H14O, as well as a small isotope
peak at m/z of 127 (Fig. 8a). In the reaction of butanal over deuterated
Q-10, the 2E2H collected in the initial 30 min of time on stream had
peaks at m/z of 127 and 128 with stronger intensity than that at 126
(Fig. 8b). Therefore, most of the produced 2E2H molecules include
Fig. 6 shows the effects of carrier gas on the catalytic activity of SiO2
(Q-10) at 250 °C. The reaction was performed in either N2 or H2 at-
mosphere with different flow rate. In our previous report dealing with
the dehydration of 1,2-propanediol to propanal over WO3/SiO2, H2
showed a better performance than N2 for keeping the catalytic activity
[29]. Also, H2 carrier gas effectively works as an inhibitor of deacti-
vation over Ag/TiO2 for the aldol condensation of butanal [25].
Fig. 6. Effects of carrier gas on the catalytic activity of SiO2 (Q-10) catalyst at
250 °C.
Reaction conditions: catalyst weight, 4.0 g. The carrier gas was either N2 or H2.
Fig. 7. DRIFT spectra of SiO2 (Q-10) and deuterated SiO2 (Q-10).
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AppliedCatalysisA,General570(2019)113–119
– > C8H12D2O + D2O. These results demonstrate that hydrogen of si-
lanol groups transfers to the product of 2E2H and the reactant butanal
during the reaction, and suggest that the silanol groups on the SiO2
surface are the active sites for the reaction. However, there remains a
question about whether SiOH catalyzes the aldol condensation or not.
Aldol condensation of butanal has been recently studied over Ti species
supported on silica at 80 °C and proposed a reaction mechanism using
isolated titanol, TiOH [37]. However, we could not propose a probable
mechanism using isolated silanol in this paper. Because the evidence to
considering the reaction mechanism is not enough in this experiment
(Fig. 7), we need further work on the isotope experiment in detail to
clarify the mechanism of the aldol condensation and the role of SiOH
groups.
4. Conclusions
In this study, aldol condensation of butanal to form 2E2H was
performed over several solid catalysts. Although strong acidic catalysts
such as SiO2-Al2O3 and Al2O3 were active, they were deactivated ra-
pidly due to the significant accumulation of carbonaceous species on
the catalyst surface. In contrast, SiO2 with a weak acidity showed a low
activity, but the activity was the most stable because carbonaceous
species was relatively difficult to be accumulated on the surface of SiO2.
Both the pore size and the surface area affected the activity of SiO2:
SiO2(Q-10) with a mean pore diameter of 10 nm and a surface area of
295 m2 g−1 showed the best catalytic performance. High conversion
levels could be obtained by increasing the loading of SiO2, and the
2E2H selectivity of ca. 90% could be maintained at high conversion
levels. To study the active sites on SiO2, the surface SiOH groups on
SiO2 were exchanged to SiOD groups by contacting SiO2 with deuter-
ated water at 250 °C. It was confirmed by mass analysis that most of the
produced 2E2H molecules contained deuterium atoms, indicating that
the deuterium atom transferred from Q-10 to a 2E2H molecule during
the reaction. It is demonstrated that the silanol groups on the surface of
SiO2 could be the active sites for the vapor-phase aldol condensation of
butanal.
Acknowledgments
We thank Mr. Shota Tamaki, Ms. Mami Tsumura, Ms. Asuka Yabe,
and Ms. Minami Iwakiri for experimenting with the catalytic reactions
and TG analysis at Chiba University.
Fig. 8. Mass profiles of 2-ethyl-2-hexenal (a,b,c) and butanal (d,e,f) in the re-
action over undeuterated and deuterated SiO2 (Q-10) with different periods of
time on stream.
(a,d), a sample collected between 0–60 min of time on stream in the usual re-
action over undeuterated SiO2 (Q-10); (b,e) collected between 0–30 min of time
on stream in the reaction over deuterated SiO2 (Q-10); (c,f) collected between
70–90 min of time on stream in the reaction over deuterated SiO2 (Q-10).
Reaction conditions: temperature, 200 °C; catalyst weight, 4.0 g; H2 flow rate,
5 cm3 min−1. Mass profiles in a full range are presented in the supplementary
information (Fig. S1).
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.apcata.2018.11.012.
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