Transfer hydrogenation of cinnamaldehyde with 2-propanol on Al2O3 and SiO2-Al2O3 catalysts: Role of Lewis and Br?nsted acidic sites

Article abstract of DOI:10.1039/c7cy01569k

Transfer hydrogenation of α,β-unsaturated aldehyde is an important reaction (MPV reaction) for the synthesis of allyl alcohol because of its high selectivity for the hydrogenation of the carbonyl group. This reaction can proceed over many oxide catalysts

Full text of DOI:10.1039/c7cy01569k

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Revised manuscript submission: CY-ART-08-2017-001569
Transfer Hydrogenation of Cinnamaldehyde with
2ꢀPropanol on Al2O3 and SiO2ꢀAl2O3 Catalysts: Role of
Lewis and Brϕnsted Acidic Sites
ZhanꢀKun Gao, YongꢀChun Hong, Zhun Hu, BoꢀQing Xu*
Innovative Catalysis Program, Key Lab of Organic Optoelectronics & Molecular Engineering,
Department of Chemistry, Tsinghua University, Beijing 100084, China
* Corresponding author. Eꢀmail address: bqxu@mail.tsinghua.edu.cn
Abstract: Transfer hydrogenation of α,βꢀunsaturated aldehyde is an important reaction (MPV
reaction) for the synthesis of allyl alcohol because of its high selectivity for the hydrogenation
at the carbonyl group. The reaction can proceed over many oxide catalysts but the role of
acidic and basic sites on the oxide catalyst surface has remained an issue of debate. We report
herein the catalytic data of serial Al2O3 and SiO2ꢀAl2O3 catalysts for the MPV of
cinnamaldehyde at 150 °C using 2ꢀpropanol as the Hꢀdonor. The surface acidity of the
catalysts is adjusted by changing of the calcination temperature between 400 and 1000 °C,
and quantified to correlate with the catalyst performance. Surface Lewis acidic sites that are
poisoned by pyridine but not by hindered pyridine (lutidine) under the catalyst working
conditions are identified as the catalytic sites for the MPV reaction. The lutidineꢀsensitive
surface Brϕnsted acidic sites, which are among the spectator sites during the MPV reaction,
are shown to serve as the catalytic sites for the cascade crossꢀetherification reaction between
cinnamyl alcohol and 2ꢀpropanol. The MPV reaction is also conducted under higher pressure
CO2, the results of which exclude a possible involvement of surface basic sites during the
reaction.
Keywords: Solid acids, Acidꢀbase catalysis, Transfer hydrogenation, Selective poisoning,
α,βꢀunsaturated aldehyde, Active sites
1

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Introduction
The selective reduction of unsaturated carbonyl compounds to their corresponding
unsaturated alcohols is an important reaction in fine chemical synthesis.1ꢀ4 Compared to
traditional methods of carbonyl reduction by molecular hydrogen (H2) employing precious
metal (such as Au, Pt and Pd) catalyst,5ꢀ7 carbonyl reduction via transferꢀhydrogenation using
alcohols as Hꢀdonor under the presence of acidꢀbase oxide catalyst, that is
MeerweinꢀPonndorfꢀVerley (MPV) reaction,8 would be more promising because of its
distinctly high selectivity for producing unsaturated alcohols,9 better economic feasibility in
terms of catalyst cost,10 and environmental friendness.11
MPV reaction between aldehyde (Hꢀacceptor) and alcohol (Hꢀdonor) was known earlier
to be catalyzed by homogenous Lewis acids (such as aluminum alkoxides) via a sixꢀmember
ring structured transition state, as shown in Scheme 1.8 Heterogeneous acidꢀbase catalysts,
including acidic (zeolites, Al2O3, etc.),12ꢀ14 basic (MgO, etc.)15ꢀ26 and acidꢀbase bifunctional
ones (ZrO2, etc.),27ꢀ31 were also reported to be efficient for the reaction. However, the nature
of the catalytically active sites on these solid oxide catalysts has long remained an issue of
much debate.32ꢀ37
Lewis acid
( R: C6H5-CH=CH )
Transfer hydrogenation
Scheme 1. Homogeneous Lewis acid catalyzed transfer hydrogenation of cinnamaldehyde
with 2ꢀpropanol.8
As an analog to homogenous catalyst, heterogenous Lewis acids are also found active for
MPV reaction, probably proceeding via a surface transition state with a structure similar to the
sixꢀmembered ring in Scheme 1. In their study of a series of Zrꢀcontaining catalysts, including
bulk ZrO2, supported ZrO2 (ZrO2/SiO2, ZrO2/TiO2, ZrO2/CeO2), and zirconosilicates (ZrꢀBEA
and ZrꢀMCMꢀ41), for the MPV reaction between crotonaldehyde and ethanol in the
temperature range of 473–573 K, Ivanova et al.12 showed that the catalyst activity positively
correlates to the number of tetrahedral Zr4+ (Lewis acid) sites in the catalyst samples. Similar
2

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correlation was also reported by Urbano et al35, who worked on liquidꢀ as well as gasꢀphase
reactions between crotonaldehyde and 2ꢀpropanol on ZrO2 catalysts modified with Al2O3,
Ga2O3 and In2O3. In the kinetic study of the reaction between methyl levulinate and 2ꢀbutanol
on Lewis acidic ZrꢀBEA zeolites, Leshkov et al.13 revealed that Hꢀtransfer to the carbon atom
in the carbonyl group from the CꢀH (βꢀH) of the secondary alcohol could be the rateꢀlimiting
step.
Basic solids, like MgO, were also found active for MPV reactions.15ꢀ26 Shen et al.18
reported that the activity of MgO for the MPV reaction between benzaldehyde and ethanol
increased with increasing the basic site density according to CO2ꢀTPD. Similar observations
were reported for other MPV reactions on high surface area MgO,19 CaOꢀAl2O3 mixed
oxides22 and La2O3/Al2O3 catalysts.23 The contribution of surface basic sites (O2ꢀ anions),
however, would be complicated by the possible contribution of their conjugated Lewis acidic
sites (surface cations such as Mg2+ and Ca2+).27ꢀ31 The basic sites were proposed to assist the
formation of sixꢀmembered ring transition state (Scheme 1) at their conjugated Lewis acid
sites during the reaction between cyclohexanone and 2ꢀpropanol over ZrO2,27 which seems to
be supported by the observation that mixed oxide SnO2ꢀMgOꢀAl2O3 was more efficient than
MgO and MgOꢀAl2O3 in catalyzing the MPV reactions of benzaldehyde and cyclohexanone
with 2ꢀpropanol.29
Surface Brϕnsted acidic sites, usually in the form of surface hydroxyl groups, were also
regarded as the possible active sites for MPV reactions.32,33 On ZrO2 doped with boron and
alkaline earth metal cations, for instance, Brϕnsted acidic sites of medium to strong acidity, as
determined by Raman spectra of adsorbed pyridine, were considered to be active for the
reaction between cinnamaldehyde and 2ꢀpropanol.33 However, transformation of surface
hydroxyl (Mn+ꢀOH) based Brϕnsted acidic sites to Lewis acidic sites (Mn+) by ‘ligandꢀ
exchange’, e.g., with alkoxy group,34 could also occur during reaction, making it difficult to
identify the function of Brϕnsted acidic sites under the reaction conditions. In addition, the
presence of Brϕnsted sites at the catalyst surface would lead to possible cascade etherification
reaction during MPV reaction, as the intermolecular dehydration between the alcohol
products was considered to happen easily over the Brϕnsted sites.38ꢀ40 However, little effort
has been made to date to quantitatively correlate the surface Brϕnsted acidity with the
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reactivity and product selectivity of the carbonyl reduction during MPV reactions.
Reported herein are the catalytic data of serial Al2O3 (Lewis acidic) and SiO2ꢀAl2O3 (both
Brϕnsted and Lewis acidic) catalysts for the reduction of cinnamaldehyde (CAL) to cinnamyl
alcohol (COL) at 150 °C by transfer hydrogenation using 2ꢀpropanol (2POL) as the Hꢀdonor.
The surface acidity of the catalysts was fineꢀtuned by systematical change of their calcination
temperature between 400 and 1000 °C and quantified to correlate with the catalyst
performance. We show that surface Lewis acidic sites are solely responsible for the transfer
hydrogenation catalysis. Surface Brϕnsted acidic sites are confirmed to be responsible for
catalyzing the cascade crossꢀetherification reaction, leading to formation of 1ꢀcinnamyl
2ꢀpropyl ether (CPE) from COL.
1. Experimental
1.1 Sample preparation
The Al2O3 samples were prepared by calcination of an Al(OH)3 hydrogel at different
o
temperatures T (T = 450−1000 C) in flowing air (50 mL⋅minꢀ1, STP) for 5 h, as previously
reported.41 The hydrogel was obtained by hydrolysis of Al(NO3)3·9H2O (AR, Dongfang
Chemical Reagent Company, Beijing) with an aqueous solution of ammonia with the final pH
controlled at pH = 10. The calcined samples were denoted as A−T according to the calcination
temperature T. SiO2ꢀAl2O3 was obtained from Sasol (SIRAL30, SiO2:Al2O3 = 30:70 wt/wt),
which was calcined at 400−800 oC in flowing air (50 mL⋅minꢀ1, STP) for 5 h prior to use. The
calcined SiO2ꢀAl2O3 samples were denoted as SA−T (T = 400–800 oC).
1.2 Characterizations
Surface areas of the samples were determined using N2 adsorption–desorption isotherms
o
measured at ꢀ196 C on a Micromeritics ASAP 2010C instrument using BET model. Prior to
o
the measurements, the samples were degassed under vacuum at 200 C for 5 h. Xꢀray
diffraction (XRD) patterns were recorded on a Bruker D8 Advance Xꢀray diffractometer with
a Niꢀfiltered Cu Kα (λ = 0.15406 nm) radiation at 40 kV and 40 mA. Data analysis was
performed using JADE (Materials Data, Inc., Livermore, CA).
The surface acidity of A−T was measured by temperature programmed desorption of NH3
(NH3ꢀTPD)41ꢀ46 and FTIR spectroscopy of adsorbed pyridine.41ꢀ43 The NH3ꢀTPD measurement
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was conducted on a catalyst analyzer (BELꢀA, Japan) equipped with a mass spectrometer
(Inprocess Instruments, GAM200) as the detector. The sample (ca. 50 mg) was loaded in a
quartz reactor (I.D.= 4 mm) and pretreated in a synthetic air (20 vol% O2 in Ar) flow (50
mL⋅minꢀ1, STP) at the sample calcination temperature (T) for 0.5 h, and then cooled to 100 oC
in flowing dry Ar (50 mL⋅minꢀ1, STP) for adsorption of NH3, which was conducted by
switching the reactor to a flow of 1.9 vol% NH3 in Ar for 0.5 h. The reactor was then purged
with flowing dry Ar (40 mL⋅minꢀ1) until a stabilized baseline was obtained. NH3ꢀTPD profiles
was measured by recording the signal of m/z = 15 with a temperature ramp of 10 °C⋅minꢀ1 up
o
to 800 C
.
FTIR measurement of adsorbed pyridine was performed on a PerkinꢀElmer FTIR
System 2000 spectrophotometer in the transmission mode with a quartzꢀlined stainless steel in
situ IR cell with optical path length less than 5 mm.41ꢀ43 Selfꢀsupporting sample wafers (10
o
mg⋅cmꢀ2) were pretreated in flowing 20 vol% O2/Ar (40 mL⋅minꢀ1) at 400 C for 2 h and
o
cooled to 100 C in flowing Ar before switching to a pyridine/Ar mixture (1 vol% pyridine,
40 mL⋅minꢀ1) for pyridine adsorption (0.5 h). The spectra were measured by averaging 20
scans at a resolution of 4 cmꢀ1 after purging the sample wafers with flowing Ar (40 mL⋅minꢀ1)
o
at 150 and 200 C for 1 h. Relative population of Lewis and Brønsted acidic sites on the
catalyst surfaces was calculated using the integrated area of peaks at 1450 cmꢀ1 (for
coordinated pyridine on Lewis acidic sites) and 1545 cmꢀ1 (for protonated pyridine on
Brønsted acidic sites), assuming that the integrated molar extinction coefficients were 2.22
and 1.67 cm⋅ꢁmolꢀ1, respectively, for pyridine adsorbed on Lewis and Brønsted acidic sites, as
measured by Emeris 47
.
1.3 Catalytic activity measurement
o
The MPV reaction of CAL with 2POL was performed at 150 C and a molar ratio
2POL/CAL = 33 in a 25 mL stainlessꢀsteel autoclave batch reactor. Unless otherwise specified,
the autoclave was loaded with 10 mL 2POL (AR, Dongfang Chemical Reagent Company,
Beijing), 0.5 mL CAL (AR, SigmaꢀAldrich, CAL), and 200 mg catalyst. After purging with
dry N2 (1.0 MPa) for six times, the autoclave was heated to the reaction temperature (150 oC)
under 1 MPa dry N2. Initiation of the reaction was done by switching on the stirring (900 rpm)
as soon as the reaction temperature was reached. The stirring speed (900 rpm) for the reaction
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was determined in preliminary experiments using varying stirring speeds (500ꢀ1200 rpm),
which would guarantee absence of diffusion limitation during the catalytic reaction
measurements. Termination of the reaction was done by dropping the autoclave in a waterꢀice
bath to quench the reaction. Before workup, 0.5 mL dimethylformamide (DMF) was added as
an internal standard into the reaction mixture. The liquid mixture was analyzed, after
separation from the solid catalyst by filtration, on an HP7890 gas chromatography (GC)
equipped with a Shimadzu HiCap CBP20ꢀS25ꢀ050 capillary column (0.32 mm × 25 m) and
an flame ionization detector (FID). As only cinnamyl alcohol (COL) and 1ꢀcinnamyl 2ꢀpropyl
ether (CPE) were detected as the reaction products in the reactor, the CAL conversion and
product selectivity data were obtained according the calculations:
moles of CAL before rxn − mole of CAL after rxn
ꢀ ꢁ
CAL conversion % =
×100%
mole of CAL before rxn
moles of a defined product
ꢀ
ꢁ
Product selectivity mol% =
×100%
mole of CAL consumed
The carbon balance of the reaction was estimated to be 98−108%.
Selective poisoning experiments using hindered pyridine (2,6ꢀdimethyl pyridine or
lutidine) or pyridine were carried out by injection prior to the reaction of different amounts of
lutidine (99%, J&K Scientific Co., Ltd) or pyridine (99%, J&K Scientific Co., Ltd) into the
reactor loaded with the reactants.
2. Results and discussion
2.1 Physicochemical properties of AꢀT and SAꢀT samples
Table 1 shows the BET surface area (SBET), pore volume (VP) and average pore diameter
(DP) data for the A−T and SA−T samples. The BET surface area declined from 413 to 50 m2
⋅
gꢀ1 on increasing the calcination temperature T from 400 to 1000 oC for the A−T samples,
accompanied by shrinking pore volume (0.38 to 0.20 cm3 gꢀ1), pore diameter enlargement (3.5
⋅
to 12.9 nm), as well as phase transformation from γꢀAl2O3 to θꢀAl2O3 when the calcination
o
temperature was higher than 850 C.49,50 For the SAꢀT samples, the calcination temperature
effects on the BET surface area (382−270 m2 gꢀ1), pore volume (0.73−0.67 cm3 gꢀ1) and
⋅ ⋅
diameter (5.8−7.7 nm) were less prominent. Moreover, all the calcined SA−T (T = 400−800
oC) samples remained amorphous, as shown in Fig. 1, except that residual ammonium salts
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were detected by diffractions at 2θ = 26, 37 and 49 °. Thus, the existence of SiO2 in the SA
samples apparently improved the resistance of the coꢀexisting Al2O3 towards sintering and
crystallization.51
Table 1. Physical properties and surface acidity of A−T and SA−T samples
Physical properties
Acidic properties
BASd
LASe
a
a
a
Crystal Total acidityc
Sample
SBET
VP
DP
density
(ꢁmol⋅mꢀ2) (ꢁmol⋅mꢀ2)
density
B/Lf
(nm)
phaseb
(m2⋅gꢀ1)
(cm3⋅gꢀ1)
(ꢁmol⋅mꢀ2)
413
316
196
160
112
68
0.38
0.36
0.36
0.37
0.33
0.24
0.20
3.5
4.0
γ
γ
γ
γ
γ
θ
θ
1.28
1.36
1.94
2.13
2.86
2.35
3.20
0
0
0
0
0
0
0
1.28
1.36
1.94
2.13
2.86
2.35
3.20
0
0
0
0
0
0
0
A−450
A−600
A−700
A−800
A−850
A−900
A−1000
5.4
6.5
7.1
12.1
12.9
50
396
395
402
382
270
0.73
0.74
0.75
0.74
0.67
5.8
5.8
5.9
6.0
7.7
ah
a
0.96 g
1.10
1.04
1.10
1.56
0.17
0.21
0.27
0.25
0.28
0.79
0.83
0.77
0.85
1.29
0.21
0.25
0.35
0.30
0.21
SA−400
SA−500
SA−600
SA−700
SA−800
a
a
a
a BET surface area (SBET), pore volume (VP) and pore diameter (DP) estimated using the wellꢀknown BJH
e
model.48 b Crystal phase determined by XRD. c Determined by NH3ꢀTPD. d Brϕnsted acidic site (BAS)
f
g
Lewis acidic site (LAS). Molar BASꢀtoꢀLAS ratio. This number was obtained by subtraction of the
residual NH4+ salts from the desorbed NH3 in the NH3ꢀTPD measurement. h Amorphous.
NH3ꢀTPD and IR spectroscopy of adsorbed pyridine were conducted to measure the
surface acidity of the A−T and SA−T samples. The data for the A−T samples were reported in
ref 41, which are also listed in Table 1 for comparison with those measured here in this study
for the SA−T samples. No surface Brφnsted acidic sites (BAS) was detected for the A−T
samples, and the Lewis acidic site density for these samples increased with the calcination
temperature.
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γꢀAl2O3
θꢀAl2O3
Aꢀ1000
Aꢀ900
Aꢀ850
Aꢀ800
Aꢀ700
Aꢀ600
Aꢀ450
SAꢀ800
SAꢀ700
SAꢀ600
SAꢀ500
SAꢀ400
45
30
40
20
25
35
50
2θ (o)
Figure 1. XRD profiles of A−T and SA−T samples. The asterisks ( ) mark the diffractions
from residual ammonium nitrate.
The NH3ꢀTPD profiles for the SAꢀT samples except SAꢀ400 (Fig. 2A) feature a broad
o
peak with maxima at ca. 200 C. Increasing the calcination temperature T generally lowered
the intensity of the peak but had little effect on the shape of the NH3ꢀTPD peak. The SAꢀ400
sample seems unique, which gave a shoulder peak on the high temperature side (ca. 470 oC).
This high temperature shoulder peak can be arisen from decomposition of residual
+
ammonium ions (NH4 ) in the sample. This assignment was further corroborated in Fig. 2B,
which shows a thermogravimetry−MS analysis (TGA/MS) of a fresh SAꢀ400 sample and
clearly detected a weight loss in the temperature range of 400−800 oC; the MS signals reveal
+
evolutions of NH3 and H2O. The presence of NH4 ꢀinvolving entities in the fresh SAꢀ400
sample were also detected as ammonium salts by XRD (Fig.1). Therefore, this SAꢀ400
+
sample contained residual NH4 ions before NH3 adsorption, which was deducted from the
NH3ꢀTPD acidity for measuring the surface acidity of SAꢀ400.
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100
B
A
2Eꢀ6
2Eꢀ6
TGA
98
96
94
92
90
m/z = 18 (×10ꢀ4)
T = 400
m/z = 15
500
600
700
800
100 200 300 400 500 600 700 800
100 200 300 400 500 600 700 800
Temperature (oC)
Temperature (oC)
Figure 2. (A) NH3ꢀTPD profiles of SA−T samples, and (B) TGA/MS curves of SA−400.
Figure 3 shows the FTIR spectra of chemisorbed pyridine on the SA−T samples. The
absorption band centered at 1450 cmꢀ1 features coordinately adsorbed pyridine on surface
Lewis acidic sites (LAS) or coordinately unsaturated surface Al3+ sites on the surface of the
SA samples.41 The absorption band centered at 1545 cmꢀ1 is characteristic of surface
pyridinium ion or protonated pyridine due to pyridine adsorption on BAS. Both of these
absorption bands (1450 cmꢀ1 and 1545 cmꢀ1) were significantly weakened with increasing the
o
purging temperature from 150 to 200 C (Fig. 3 A and B). Figure 3C shows the
corresponding BASꢀtoꢀLAS ratios (B/L) according to the purging temperature for every
o
SA−T sample. Regardless of purging temperature (150 or 200 C), the sample calcined at
o
600 C (i.e., SAꢀ600) showed the maximum B/L ratios; the increase with T of B/L at the
o
lower temperature side (T ≤ 600 C) would be accounted for by proton (BAS) generation as
+
a result of NH4 decomposition due to the use of a higher temperature for the sample
o
calcination, while the decrease with T of B/L at T ≥ 600 C can be explained by generation
of LAS due to surface dehydroxylation, as depicted in Scheme 2.
SA−T (T > 600 oC)
Fresh SA−T (T ≤ 400 oC)
SA−T (T = 400ꢀ600 oC)
Scheme 2. Evolution of Brϕnsted and Lewis acidic sites on SiO2ꢀAl2O3 samples.
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1450 cmꢀ1
1450 cmꢀ1
A
B
0.05
0.05
1545 cmꢀ1
1545 cmꢀ1
T = 400
T = 400
500
600
500
600
700
800
700
800
1700
1650
1600
1550
1500
1450
1400
1700
1650
1600
1550
1500
1450
1400
Wavenumber (cmꢀ1)
Wavenumber (cmꢀ1)
0.7
C
0.6
0.5
0.4
0.3
0.2
0.1
0.0
300 400 500 600 700 800 900
Calcined temperature (oC)
Figure 3. FTIR spectra of adsorbed pyridine on SA−T samples after purging in flowing Ar at
o
(A) 150 oC and (B) 200 C, respectively. (C) Effect of calcination temperature on the molar
BASꢀtoꢀLAS (B/L) ratio after purging at 150 oC ( ) and 200 oC ( ).
The density of BAS and LAS on the surface of each SAꢀT sample can be therefore
determined according to the total surface acidity obtained in the NH3ꢀTPD measurements
and the B/L ratio obtained in the FTIR measurements of adsorbed pyridine. It should be
noted that we employed the B/L ratios obtained from the FTIR spectra collected after
o
purging at 150 C, as the temperature of the transfer hydrogenation reaction of CAL and
o
2POL (presented in the next section) was set at 150 C. The obtained results are listed in
Table 1. It is seen that Lewis acidity (0.77−1.29 ꢁmol⋅
mꢀ2) was always higher than Brϕnsted
acidity (0.17ꢀ0.28 ꢁmol
⋅
mꢀ2) on the SA catalysts, agreeing with earlier documentaion.52ꢀ54
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2.2 Catalytic performance of AꢀT and SAꢀT for Transfer hydrogenation of CAL and
2POL: insight into the nature of surface active sites
The MPV reaction or transfer hydrogenation of CAL and 2POL was conducted at 150
oC under massꢀtransfer free conditions using 200 mg catalyst, unless otherwise specified.
Table 2 presents the catalytic performance data of every A−T and SA−T sample, together
with the surface Lewis acidity data for the catalysts. The reaction was also conducted using a
smaller amount of catalyst (20 mg) to limit the CAL conversion to less than ca. 6% for
reliable measurement of the specific activity of the catalysts, and the data in the parentheses
of Table 2 give the CAL conversion and COL selectivity in these reaction tests. Note that
COL was always the only product over the different A−T catalysts, except a coꢀproduction of
acetone (not shown) from the Hꢀdonor (2POL). However, CPE, a product of cascade
crossꢀetherification between COL and 2POL,49.50 was always detected together with COL
over the SA−T catalysts, and the selectivity for CPE clearly increased with the CAL
conversion.
The areal activity data in Table 2 are obtained by normalizing the catalytic reaction
rates measured at CAL conversion levels of lower than ca. 6% to the catalyst surface areas.
The areal activity for A−T samples of T = 450−850 oC increased with increasing T from 0.61
µmol⋅minꢀ1⋅mꢀ2 for A−450 to 0.65 µmol⋅mꢀ2⋅minꢀ1 for A−600, and then to 1.55
µmol⋅mꢀ2⋅minꢀ1 for A−850. Despite of their high surface Lewis acidity (2.35−3.2 µmol mꢀ2),
⋅
the areal activity for the two θꢀAl2O3ꢀbased samples (i.e., A−900 and A−1000) were much
lower (0.24−0.35 µmol⋅mꢀ2⋅minꢀ1) than that for the other A−T. The lower catalytic activity of
these θꢀAl2O3ꢀbased samples would be related to the fact that the Lewis acidity on the
surface of θꢀAl2O3 was significantly weaker than that of γꢀAl2O3,41 as stronger Lewis acidic
sites would be more efficient in catalyzing the MPV reaction.55,56
Compared with A−T, the SA−T samples showed lower areal activity and lower
selectivity for COL in the MPV reaction of this study. The Lewis acid site density increased
with increasing the calcination temperature T of the SAꢀT samples, from 0.79 µmol⋅mꢀ2 for T
o
= 400 oC to 1.29 µmol⋅mꢀ2 for T = 800 C. Unlike in the case of A−T catalysts, the areal
activity of these SA−T samples showed an insignificant dependence on T and LAS density.
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However, the selectivity for COL over these SA−T samples declined to less than 35 mol%
and the major product became CPE (> 65 mol%) when the CAL conversion was made higher
than 20%. BAS on the surface of the SA−T samples should be responsible for the formation
of CPE as it was at all not observed over all of the A−T catalysts having only LAS.
o
Table 2. Lewis acid site density and catalytic performance at 150 C of A−T and SA−T
catalysts
LAS density CAL conv.a
COL sel.a
(mol%)
Areal activityb
(µmol⋅mꢀ2⋅minꢀ1)
TOFb
(minꢀ1)
0.52
Catalyst
(µmol⋅mꢀ2)
(%)
1.28
41.7 (8.3)
40.7c (6.2)
47.8 (6.1)
38.5 (5.2)
44.1 (4.8)
4.8
>99 (>99)
>99 (>99)
>99 (>99)
>99 (>99)
>99 (>99)
>99 (>99)
>99 (>99)
0.61
0.65
1.04
1.08
1.43
0.24
0.35
A−450
A−600
A−700
A−800
A−850
A−900
A−1000
1.36
0.48
1.94
0.53
2.13
0.51
2.86
0.50
2.35
0.10
3.20
5.3
0.11
0.79
0.83
0.77
0.85
1.29
19.2d (3.0)
21.6d (2.9)
23.1 (3.1)
21.1 (3.0)
23.1 (3.5)
35.1e (91.6)e
32.7e (92.3) e
28.4e (90.6)e
31.5e (92.1) e
33.2e (93.2) e
0.25
0.24
0.26
0.26
0.43
0.31
0.30
0.33
0.31
0.34
SA−400
SA−500
SA−600
SA−700
SA−800
Rxn conditions: 0.5 mL CAL, 10 mL 2POL, 1.0 MPa N2 , 900 rpm, 1 h. a Number outside the parenthesis
shows the conversion over 200 mg catalyst while those in the parenthesis over 20 mg catalyst; b Obtained
at CAL conversion of no higher than ca. 8%; c 35 min; d 4 h; e The balance (i.e., 100%− COL selectivity)
gives the selectivity for CPE.
In order to further understand the nature of the active sites on the catalyst surface for the
MPV reaction, hindered pyridine (2,6ꢀdimethyl pyridine or lutidine) and pyridine were
added, respectively, to the reaction mixture in varying amounts for investigating possible
effects of their poisoning to the acidic sites on the catalyst surface. The hindered pyridine has
known as a specific poison to BAS while pyridine a nonꢀspecific poison to both BAS and
LAS.42,57,58 Using SA−600 as a representative SA catalyst, Fig. 4 shows effects of lutidine
and pyridine addition on CAL conversion and product selectivity of the MPV reaction over
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SiO2ꢀAl2O3 catalysts. The addition of even small amounts of lutidine (e.g., molar
lutidine/BAS < 0.1) produced a remarkable effect on the product selectivity: the selectivity
for COL was significantly enhanced at the expense of CPE (Fig. 4A). The selectivity for
COL was kept on increasing with the amount of added lutidine until lutidine/BAS ≥ 1, when
the formation of CPE (from the cascade crossꢀetherification reaction) was completely
inhibited and COL became the only product from CAL. However, the addition of lutidine
had no effect on the reactivity of CAL, whose conversion level was kept constant regardless
of the amount of added lutidine (Fig. 4A). These results clearly confirm that BASs have
nothing to do with the COLꢀproducing MPV reaction but are responsible for catalyzing the
cascade crossꢀetherification reaction.
Figure 4. Poisoning effects of lutidine (A) and pyridine (B and C) during the MPV reaction
of CAL and 2POL on CAL conversion (■), and selectivity for COL ( ) and CPE ( ) over
SA−600. Other conditions: 200 mg catalyst, 0.5 mL CAL, 10 mL 2POL, 1.0 MPa N2, 900 rpm, 1 h.
Similar effects were observed when small amounts of pyridine [pyridine/BAS < ca. 0.5
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or pyridine/(BAS + LAS) < ca. 0.1] were added; the selectivity for COL increased while that
for CPE decreased, with the reactivity of CAL being little affected (Fig. 4 B and C). Both the
reactivity of CAL and product selectivity became sensitively affected, however, when higher
amounts of pyridine were added to reaction mixture. The sensitiveness of the product
selectivity and relative insensitiveness of the CAL reactivity to the addition of small amounts
of pyridine would indicate that the surface BAS is more sensitive than the LAS to the
poisonous pyridine. Therefore, adsorption of pyridine on surface BAS was stronger than on
LAS. These results further confirm that the LASs were responsible for catalyzing the
COLꢀproducing MPV reaction12ꢀ14 while the BASs for catalyzing the cascade
crossꢀetherification, 38ꢀ40 as highlighted in Scheme 3.
Brønsted acid
Lewis acid
( R: C6H5-CH=CH )
Crossꢀetherification
Transfer hydrogenation
Scheme 3. Heterogeneous Lewis acid catalyzed MPV reaction of CAL with 2POL, followed
by cascade crossꢀetherification via Brϕnsted acid catalysis.
The selective poisoning experiments were also conducted with the γ−Al2O3 catalysts.
Table 3 shows the results over Aꢀ600. Regardless of the amounts of added lutidine, the
reactivity of CAL and selectivity for COL were not at all affected by the added lutidine. This
completely inertness of the MPV reaction to the addition of this BASꢀspecific poison well
demonstrates that the surface of γ−Al2O3 does not have BAS59 and thus excludes the
possibility of BAS participation in catalyzing the MPV reaction. In contrast, the reactivity
(conversion) of CAL was lowered significantly when the nonꢀspecific pyridine was added
instead of lutidine, due pyridine poisoning of the catalytic LASs on the catalyst (γ−Al2O3)
surface. The MPV reaction was also carried out under highꢀpressure CO2 on the Aꢀ600
catalyst, attempting to gain an insight into a possible effect of CO2 poisoning of the surface
basic sites. As indicated by the data shown in the bottom of Table 4, the MPV reaction was
also completely insensitive to the presence of even a large amount of CO2 (2 MPa). This
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insensitiveness to CO2ꢀpoisoning suggests no participation of surface basic sites in
catalyzing the MPV reaction.
Table 3. Poisoning effects of lutidine, pyridine and CO2 on the MPV reaction at 150 oC over
A−600
Npoison
(mmol)
0
npoison/ntotal acidity
(mol/mol)
0.0
CAL conv.
(%)
COL sel.
(%)
Poison
40.7
40.2
40.5
38.5
＞99
＞99
＞99
＞99
4.3
50.2
93.0
502.3
lutidine
8.0
43 a
0
0.0
2.3
11.6
23.3
93.0
726.7
40.7
35.1
33.4
32.8
24.7
8.1
＞99
＞99
＞99
＞99
＞99
＞99
0.2
1.0
2.0
8.0
62 b
pyridine
CO2
14.4 c
ꢀ
40.1
＞99
Other conditions：200 mg catalyst, 0.5 mL CAL, 10 mL 2POL, 1.0 MPa N2, 900 rpm, 35 min. a 5 mL
lutidine (hindered pyridine), 5 mL 2POL. b 5 mL pyridine, 5 mL 2POL. c 2 MPa CO2.
Figure 5. Correlation between the catalytic activity and surface Lewis acidity of the A−T and
SA−T catalysts.
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We therefore have reason to correlate the catalyst activity for the MPV reaction with the
number of LAS on the catalyst surface. The correlation is presented in Fig. 5 by plotting the
catalyst areal activity (µmol⋅mꢀ2 minꢀ1) against the LAS density at the catalyst (A−T and
⋅
SA−T) surfaces. Again, the relationships between the areal activity and the LAS density (or
surface Lewis acidity) for the serial γ−Al2O3 (A−T of T ≤ 850 °C) and SA catalysts
demonstrate that the catalyst surface Lewis acidity is responsible for the hydrogenꢀtransfer
catalysis.12ꢀ14 Note that the dependence slope for the A−T samples was significantly higher
than for the SA−T ones, which would be related with the effect of Lewis acidity strength on
the catalyst activity.55,56 Indeed, the average acidity strength of LASs on the surface of A−T
was stronger than that on the surface of SA−T as the highest temperature of the NH3ꢀTPD
profiles extended to ca. 600 °C for A−T of T ≤ 850 °C41but no higher than 500 °C for SA−T
(Fig. 2). The even much bigger activity difference between θ−Al2O3 (A−T of T ≥ 900 °C)
and γ−Al2O3 (A−T of T ≤ 850 °C) catalysts further confirms the effect of Lewis acidity
strength on the catalysis by LAS. As we showed earlier,41 the surface Lewis acidity of
γ−Al2O3, characterized by the highest temperature of the NH3ꢀTPD profile, would be much
stronger than that of θ−Al2O3. The last column of Table 2 shows the catalytic turnover
frequency (TOF) per LAS, which was obtained by normalizing the catalytic reaction rates
measured at CAL conversion of lower than ca. 6% to the density of LAS on the catalyst
surface. Again, the respectively similar TOF numbers for γ−Al2O3 (A−T of T ≤ 850 °C, 0.5
minꢀ1), SiO2ꢀAl2O3 (SA−T, 0.3 minꢀ1) and θ−Al2O3 (A−T of T ≥ 900 °C, 0.1 minꢀ1) catalysts
are consistent in supporting the conclusion that the specific activity by TOF of CAL
consumption is determined mainly by the Lewis acidity strength.
3. Conclusions
This work compares the catalytic performance of serial Al2O3 and SiO2−Al2O3 catalysts
in the MPV reaction of CAL with 2POL and correlates the performance with the catalyst
surface acidity and nature (type) of the acidic sites. The catalytically active sites are
identified as the surface LASs that are insensitive to the poisoning of hindered pyridine
(lutidine). The surface BASs appear to be among the spectator sites during the MPV reaction
but they serve as the active sites for catalyzing the cascade crossꢀetherification reaction
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between COL and 2POL, reducing the selectivity for COL. Also, LAS of stronger acidity is
found to show higher activity for the MPV reaction. A possible involvement of surface basic
sites during the reaction is reasonably excluded by comparing the reactivity of CAL under
the presence and absence of higher pressure CO2.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (grants:
21533004 and U1463208) and the National Basic Research Program of China (grant:
2013CB933103).
References
1. C. F. Degraauw, J. A. Peters, H. Vanbekkum and J. Huskens, Synthesis (Stuttgart), 1994,
10, 1007ꢀ1017.
2. J. R. Ruiz and C. J. Sanchidrian, Curr. Org. Chem., 2007, 11, 1113ꢀ1125.
3. G. K. Chuah, S. Jaenicke, Y. Z. Zhu and S. H. Liu, Curr. Org. Chem., 2006, 10
,
1639ꢀ1654.
4. J. S. Cha, Org. Process Res. Dev., 2006, 10, 1032ꢀ1053.
5. Y. C. Hong, K. Q. Sun, G. R. Zhang, R. Y. Zhong and B. Q. Xu, Chem. Commun., 2011,
47, 1300ꢀ1302.
6. K. Q. Sun, Y. C. Hong, G. R. Zhang and B. Q. Xu, ACS Catal., 2011,
7. P. Claus, Top. Catal., 1998, , 51ꢀ62.
8. L. M. Jackman and J. A. Mills, Nature, 1949, 164, 789ꢀ790.
1, 1336ꢀ1346.
5
9. E. D. Williams, K. A. Krieger and A. R. Day, J. Am. Chem. Soc., 1953, 75, 2404ꢀ2407.
10. V. A. Ivanov, J. Bachelier, F. Audry and J. C. Lavalley, J. Mol. Catal., 1994, 91, 45ꢀ59.
11. J. L. Song, B. W. Zhou, H. C. Zhou, L. Q. Wu, Q. L. Meng, Z. M. Liu and B. X. Han,
Angew. Chem., Int. Ed., 2015, 54, 9399ꢀ9403.
12. V. L. Sushkevich, I. I. Ivanova, S. Tolborg and E. Taarning, J. Catal., 2014, 316
,
121ꢀ129.
13. H. Y. Luo, D. F. Consoli, W. R. Gunther and Y. R. Leshkov, J. Catal., 2014, 320
,
198ꢀ207.
17

Catalysis Science & Technology
Page 18 of 21
View Article Online
DOI: 10.1039/C7CY01569K
14. P. P. Samuel, S. Shylesh and A. P. Singh, J. Mol. Catal. A: Chem., 2007, 266, 11ꢀ20.
15. M. Glinski, A. Czajka and U. Ulkowska, React. Kinet., Mech. Catal., 2015, 114
,
279ꢀ294.
16. M. Glinski, Appl. Catal., A, 2008, 349, 133ꢀ139.
17. M. Glinski and U. Ulkowska, Pol. J. Chem., 2008, 82, 1117ꢀ1119.
18. F. Wang, N. Ta and W. Shen, Appl. Catal., A, 2014, 475, 76ꢀ81.
19. J. K. Bartley, C. Xu, R. Lloyd, D. I. Enache, D. W. Knight and G. J. Hutchings, Appl.
Catal., B, 2012, 128, 31ꢀ38.
20. J. J. Ramos, V. K. Diez, C. A. Ferretti, P. A. Torresi, C. R. Apesteguia and J. I. Di
Cosimo, Catal. Today, 2011, 172, 41ꢀ47.
21. V. K. Diez, C. A. Ferretti, P. A. Torresi, C. R. Apesteguia and J. I. Di Cosimo, Catal.
Today, 2011, 173, 21ꢀ27.
22. M. Mora, M. I. Lopez, C. J. Sanchidrian and J. R. Ruiz, Catal. Lett., 2010, 136
,
192ꢀ198.
23. Z. Boukha, L. Fitian, M. L. Haro, M. Mora, J. R. Ruiz, C. J. Sanchidrian, G. Blanco, J. J.
Calvino, G. A. Cifredo, S. Trasobares and S. Bernal, J. Catal., 2010, 272, 121ꢀ130.
24. M. Glinski and U. Ulkowska, React. Kinet. Catal. Lett., 2008, 95, 107ꢀ112.
25. F. O. Figueras, Top. Catal., 2004, 29, 189ꢀ196.
26. J. I. Di Cosimo, A. Acosta and C. R. Apesteguia, J. Mol. Catal. A: Chem., 2004, 222
,
87ꢀ96.
27. T. Komanoya, K. Nakajima, M. Kitano and M. Hara, J. Phys. Chem. C, 2015, 119
,
26540ꢀ26546.
28. J. Wang, S. Jaenicke and G. K. Chuah, RSC Adv., 2014, 4, 13481ꢀ13489.
29. C. J. Sanchidrian and J. R. Ruiz, Appl. Catal., A, 2014, 469, 367ꢀ372.
30. J. M. Hidalgo, C. J. Sanchidrian and J. R. Ruiz, Appl. Catal., A, 2014, 470, 311ꢀ317.
31. V. Borzatta, E. Capparella, R. Chiappino, D. Impala, E. Poluzzi and A. Vaccari, Catal.
Today, 2009, 140, 112ꢀ116.
32. S. Axpuac, M. A. Aramendia, J. H. Carrillo, A. Marinas, J. M. Marinas, V. M. Jimenez, F.
J. Urbano and V. Borau, Catal. Today, 2012, 187, 183ꢀ190.
33. F. J. Urbano, M. A. Aramendia, A. Marinas and J. M. Marinas, J. Catal., 2009, 268
,
18

Page 19 of 21
Catalysis Science & Technology
View Article Online
DOI: 10.1039/C7CY01569K
79ꢀ88.
34. Y. Z. Zhu, S. H. Liu, S. Jaenicke and G. K. Chuah, Catal. Today, 2004, 97, 249ꢀ255.
35. J. F. Minambres, M. A. Aramendia, A. Marinas, J. M. Marinas and F. J. Urbano, J. Mol.
Catal. A: Chem., 2011, 338, 121ꢀ129.
36. M. Boronat, A. Corma and M. Renz, J. Phys. Chem. B, 2006, 110, 21168ꢀ21174.
37. R. Cohen, C. R. Graves, S. B. T. Nguyen, J. M. L. Martin and M. A. Ratner, J. Am.
Chem. Soc., 2004, 126, 14796ꢀ14803.
38. J. Iglesias, J. A. Melero, G. Morales, J. Moreno, Y. Segura, M. Paniagua, A. Cambra and
B. Hernandez, Catal. 2015, 5, 1911ꢀ1927.
39. S. H. Liu, S. Jaenicke and G. K. Chuah, J. Catal., 2002, 206, 321ꢀ330.
40. Y. Zhu, G. K. Chuah and S. Jaenicke, J. Catal., 2006, 241, 25ꢀ33.
41. Z. Hu, W. Z. Li, K. Q. Sun and B. Q. Xu, Catal. Sci. Technol., 2013,
42. Y. R. Xing, B. Yan, Z. F. Yuan and K. Q. Sun, RSC Adv., 2016, , 59081ꢀ59090.
43. L. Z. Tao, B. Yan, Y. Liang and B. Q. Xu, Green Chem., 2013, 15, 696ꢀ705.
44. B. Yan, L. Z. Tao, Y. Liang and B. Q. Xu, ChemSuschem, 2014, , 1568ꢀ1578.
45. B. Yan, A. Mahmood, Y. Liang and B. Q. Xu, Catal. Today, 2016, 269, 65ꢀ73.
3, 2062ꢀ2071.
6
7
46. B. Yan, L. Z. Tao, A. Mahmood, Y. Liang and B. Q. Xu, ACS Catal., 2017,
47. C. A. Emeis, J. Catal., 1993, 141, 347ꢀ354.
7, 538ꢀ550.
48. E. P. Barrett, L. G. Joyner and P. P. Halenda, J. Am. Chem. Soc., 1951, 73, 373ꢀ380.
49. C. Pecharroman, I. Sobrados, J. E. Iglesias, T. G. Carreno and J. Sanz, J. Phys. Chem. B,
1999, 103, 6160ꢀ6170.
50. M. Trueba and S. P. Trasatti, Eur. J. Inorg. Chem., 2005, 3393ꢀ3403.
51. S. F. Yin and B. Q. Xu, ChemPhysChem, 2003, 4, 277ꢀ279.
52. K. Tanabe, Stud. Surf. Sci. Catal., 1989, 51, 117ꢀ119.
53. S. Nassreddine, S. Casu, J. L. Zotin, C. Geantet and L. Piccolo, Catal. Sci. Technol.,
2011, 1, 408ꢀ412.
54. M. Stekrova, H. Minarikova, E. Vyskocilova, J. Kolena and L. Cerveny, J. Porous
Mater., 2014, 21, 757ꢀ767.
55. J. Wang, K. Okumura, S. Jaenicke and G. K. Chuah, Appl. Catal., A, 2015, 493
,
19

Catalysis Science & Technology
Page 20 of 21
View Article Online
DOI: 10.1039/C7CY01569K
112ꢀ120.
56. J. Wang, S. Jaenicke and G. K. Chuah, RSC Adv., 2014,
57. P. A. Jacobs and C. F. Heylen, J. Catal., 1974, 34, 267ꢀ274.
4, 13481ꢀ13489.
58. J. G. Santiesteban, J. C. Vartuli, S. Han, R. D. Bastian and C. D. Chang, J. Catal., 1997,
168, 431ꢀ441.
59. Q. L. Dai, B. Yan, Y. Liang, B. Q. Xu et al., Catal. Today, 2017, 295, 110ꢀ118.
20

Page 21 of 21
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TOC graphic
:
Surface Brϕnsted acidic and basic sites are neither involved in the catalytic transfer
hydrogenation reaction of Cinnamaldehyde with 2ꢀPropanol.
21