Solvent effects on heterogeneous catalysis in the selective hydrogenation of cinnamaldehyde over a conventional Pd/C catalyst

Article abstract of DOI:10.1039/c8cy00943k

Solvent effects in the selective hydrogenation of an α,β-unsaturated aldehyde of cinnamaldehyde (CAL) were investigated under gas-liquid-solid reaction conditions. A conventional 5 wt% Pd/C catalyst and twelve organic solvents were used, including apolar solvents (three) and protic (two) and aprotic (seven) polar ones. The total rate of CAL hydrogenation strongly depended on the solvents used. The relationship between the rate of CAL hydrogenation observed and a few different solvent parameters was examined. The main product was hydrocinnamaldehyde (HCAL) with a selectivity of 80% or higher in most of the solvents examined except for pyridine and 4-methylpyridine. In these two solvents, the CAL hydrogenation was slow and the main product was changed to cinnamyl alcohol (COL) with a selectivity of about 60%. The addition of a small volume of pyridine to other solvents such as 2-propanol and tetrahydrofuran could change the rate of hydrogenation and switch the main product from HCAL to COL. Possible interactions among the solvents, CAL substrate, and Pd/C catalyst were examined by FTIR (attenuated total reflection (ATR-) and diffuse reflectance modes (DR-)) and TPD to discuss the solvent effects observed in the Pd-catalyzed heterogeneous CAL hydrogenation.

Full text of DOI:10.1039/c8cy00943k

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Catalysis Science & Technology
ARTICLE
Solvent effects on heterogeneous catalysis in selective
hydrogenation of cinnamaldehyde over a conventional Pd/C
catalyst
Received 00th January 20xx,
Accepted 00th January 20xx
Yan Li,a,b,c Haiyang Cheng,*a,c Weiwei Lin,a,c Chao Zhang,a,c Qifan Wu,a,c Fengyu Zhao,a,c and
Masahiko Arai*a,c
DOI: 10.1039/x0xx00000x
www.rsc.org/
Solvent effects in selective hydrogenation of an α, β-unsaturated aldehyde of cinnanaldehyde (CAL) were investigated
under gas – liquid – solid reaction conditions. A conventional 5 wt.-% Pd/C catalyst and twelve organic solvents were used,
including apolar solvents (three) and protic (two) and aprotic (seven) polar ones. The total rate of CAL hydrogenation
strongly depended on the solvents used. The relationship between the rate of CAL hydrogenation observed and a few
different solvent parameters was examined. The main product was hydrocinnamaldehyde (HCAL) in a selectivity of 80% or
larger in most of the solvents examined except for pyridine and 4-methylpyridine. In these two solvents, the CAL
hydrogenation was slow and the main product was changed to cinnamyl alcohol (COL) in a selectivity of about 60%. The
addition of a small volume of pyridine to other solvents such as 2-propanol and tetrahydrofuran could change the rate of
hydrogenation and switch the main product from HCAL to COL. Possible interactions among solvents, CAL substrate, and
Pd/C catalyst were examined by FTIR (attenuated total reflection (ATR-) and diffuse reflectance modes (DR-)) and TPD to
discuss the solvent effects observed in the Pd-catalyzed heterogeneous CAL hydrogenation.
physical factors should be considered to deal with solvent
effects in heterogeneous reactions as compared with
homogeneous ones. In the literature, one can find several
Introduction
Solvents and solvent effects are an important issue for
designing and processing effective organic synthetic reactions
as reviewed comprehensively by Reichardt and Welton.1 The
physicochemical properties of solvents such as polarity,
dielectric constant, acid/base properties, and so on would
cause interactions, in hydrogenation and related reactions,
with aromatic ring, organic substrate molecules, and catalysts,
affecting the reaction rate and the product selectivity of
synthetic reactions. Recently, the solvent effects in
homogeneous catalytic reactions have been reviewed by
Dyson and Jessop.2 Up to now, however, little attention has
been paid to the solvent effects in heterogeneous catalytic
reactions, in which interfaces exist between liquid and solid
(catalyst) phases and between liquid and gas phases (if
gaseous reactants are included); mass transfer across these
interfaces and interactions among reacting species at these
interfaces should be considered. Additional chemical and/or
works on the influence of solvents in heterogeneous
carbon-carbon double/triple bond,3,4 nitro,5,6 nitrile,7,8 and
carbonyl compounds9 using supported metal catalysts.
Recently, a few studies reported the solvent effects in selective
hydrogenation of carbonyl compounds over supported metal
catalysts.10-16 Yoshida et al. investigated the hydrogenation of
acetophenone over Rh/C and Rh/Al2O3 catalysts in 12
conventional organic solvents.10 The solvent effects were
observed to significantly depend on the catalyst supports used;
interactions of solvent with the substrate (forming hydrogen
bond) was influential for Rh/C, while those of solvent with the
catalyst surface for Rh/Al2O3. McManus et al. examined
aromatic, alcohol, and alkane solvents for Pt-catalyzed
hydrogenation of 4-phenyl-2-butanone and found different
solvent effects for Pt/TiO2 and Pt/SiO2 catalysts.11 For the
former, alkane solvents favored the ring hydrogenation while
in aromatic and alcohol solvents the carbonyl hydrogenation
also occurred. Trasarti et al. reported metal-dependent solvent
effects in hydrogenation of acetophenone using
SiO2-supported metal catalysts.12 The rate of hydrogenation
was slow in toluene and benzene for Ni/SiO2, due to the
blockage of Ni active sites by these solvent molecules, but not
for Co/SiO2 and Cu/SiO2 in the same solvents.
The selective hydrogenation of α, β-unsaturated aldehydes
such as citral,17,18 acrolein,19,20 cinnamaldehyde (CAL),21-24 and
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crotonaldehyde25,26 is a practically important reaction as it benzene from Beijing Chemical Works. Ultra-pure H2, N2, and
produces useful intermediates for pharmaceuticals and Ar gases were purchased from Dalian Special Gases.
fragrances. More attention has been paid to control the
product selectivity in these reactions over supported metal
CAL hydrogenation
catalysts in various ways through modifying the nature of
Liquid-phase hydrogenation of CAL was carried out in a 50 mL
alloyed metals, and so on.27 However, the solvent effects in
stainless steel autoclave. The reactor was loaded with 5 mg
supports, controlling the size of metal nanoparticles, using
Pd/C catalyst, 5 mmol CAL, and 5 mL solvent. It was flushed
with H2 three times to remove the air, heated up to 50 oC, and
studied well although the product selectivity was known to
these selective hydrogenation reactions have little been
depend on the solvent used.28-32 For example, it was found for
then 4 MPa H2 was introduced. The reaction runs were
conducted for 30 min under strong stirring with a magnetic
Pd/C that apolar toluene was more selective to
stirrer. At the end of reaction, the autoclave was cooled to
hydrocinnimaldheyde (HCAL), while diacetal was produced as
room temperature and slowly depressurized. The composition
by-product in such polar solvents as n-propanol and
n-butanol.28 The formation of such undesired byproducts was
of reaction mixture was analyzed by a gas chromatograph
also found in CAL hydrogenation with Pt/SiO2 in methanol.33
(Shimadzu, 2010) equipped with a capillary column (Rtx-50 30
m × 0.25 mm × 0.25 μm) using a flame ionization detector. The
Although there are some works on the solvent effects in
conversion was calculated by moles of CAL consumed ÷ initial
heterogeneous catalytic reactions in the literature as
moles of CAL used; the selectivity to a product was calculated
above-mentioned, studies are limited up to now. This
by moles of the product produced ÷ moles of CAL converted.
motivated the authors to further investigate the important
The formation of undesired acetal byproducts was seldom
issue in organic synthetic reactions. In the present work,
found even in 2-propanol and in 1-butanol under the
selective hydrogenation of CAL was used as a target reaction
conditions used, and the carbon balance was in the range of
and it was examined with a conventional 5 wt.-% Pd/C catalyst
97-102% according to external standard method.
in 12 solvents including protic (2-propanol, 1-butanol), aprotic
polar (N, N’-dimethylformamide, diethyl ether, triethylamine,
tetrahydrofuran, 4-methylpyridine, pyridine), and aprotic Fourier-transform infrared (FTIR) experiments
apolar (toluene, n-hexane, benzene) solvents. Some
Interactions between CAL substrate and solvents in the liquid
solvatochromic parameters and solubility of H2 of those
phases were examined by ATR-FTIR on a Thermo Scientific
solvents are given in Table S1†. The solvent effects on the
Nicolet iS50 spectrometer with
a MCT detector at a
wavenumber resolution of 2 cm−1. A liquid mixture of CAL and
a solvent in a similar volume ratio as used in the reaction was
mixed with a magnetic stirrer and then loaded into an ATR cell
(22 mL) attached with mechanical stirrer. ATR-FTIR spectra
were collected at a reaction temperature of 50 oC.
reaction rate and the product selectivity were first examined,
showing significant differences in the rate and the selectivity
among the solvents examined. Secondly, interactions of the
substrate and the solvent in the liquid phase were examined
by attenuated total reflection (ATR)-FTIR method and
interactions between the substrate and the solvent on the
surface of catalyst were also examined by diffuse reflectance
(DR)-FTIR and temperature programed desorption (TPD)
methods under gas-solid conditions. Then, the solvent effects
observed in the heterogeneous Pd-catalyzed CAL
hydrogenation are discussed at molecular level, with particular
attention to pyridine that is significantly different in the
solvent effects compared to the other solvents. The main
product is HCAL in the latter but it can be switched to cinnmaly
alcohol (COL) by the addition of a small amount of pyridine.
The new insights obtained in this work will benefit the
selection of a suitable solvent in the selective hydrogenation of
unsaturated aldehydes.
The adsorption of CAL on Pd/C was also examined by FTIR
under gas-solid conditions by diffuse reflectance
Fourier-transform infrared spectroscopy (DR-FTIR). The vapor
pressure of CAL is very low at room temperature and the
amount of CAL adsorbed is insufficient to measure FTIR
spectra. The CAL adsorption was made as follows: 5 wt.% Pd/C
and KBr (with a weight ratio of 1:100) were grinded and
compressed into a tablet, and the sample tablet was placed in
a hydrothermal reactor containing CAL in a small glass tube.
o
The reactor was heated to 80 C and CAL was adsorbed at this
temperature for 20 h. Then, the sample tablet was cooled to
room temperature, transferred to the IR cell, and exposed to
pure He stream at 10 mL/min for 60 min at room temperature.
Afterwards, the FTIR spectrum was measured.
Experimental
Temperature-programmed desorption (TPD) experiments
Materials
Interactions of CAL with solvent molecules on the surface of
Pd/C catalyst was examined under gas-solid conditions using
TPD. The experimental procedures used are as follows: 20 mg
Pd/C sample was loaded in U-quartz tube and pretreated in a
stream of He (10 mL/min) at 50 oC for 30 min. The catalyst
sample was exposed to a stream of solvent in He (10 mL/min)
at 50 oC for 20 min, which was prepared by passing He through
A commercial 5 wt.% Pd/C supplied by Tangshan Chemical was
used. The following commercially available chemicals were
used as received: CAL from J&K Chemical, 4-methylpyridine
and pyridine from Tokyo Chemical Industry, 2-propanol,
1-butanol, N, N’-dimethylformamide, diethyl ether,
triethylamine, tetrahydrofuran, toluene, n-hexane, and
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o
a solvent vapor generator/saturator at 70 C. Then, physically
adsorbed species were removed by passing He at 10 mL/min
for 60 min and the catalyst sample was heated in a He stream
Results and discussion
The reaction pathways for CAL hydrogenation are shown in
Scheme 1, in which COL and HCAL are formed by parallel
hydrogenation reactions of C=O and C=C bonds, respectively,
followed by their further hydrogenation to produce HCOL as
the final product. In the present work, twelve organic solvents
were used for heterogeneous CAL hydrogenation over a
conventional 5 wt.-% Pd/C catalyst. The rate of hydrogenation
and the product selectivity were observed to be significantly
influenced by organic solvents used. Unique solvent effects
o
o
(10 mL/min) at a heating rate of 10 C/min from 50 to 800 C.
The amount of desorbing species was measured by a thermal
conductivity detector. Similar TPD was made for an organic
solvent, CAL, and their mixture. As its boiling point was high (>
120 oC), the adsorption was made by mixing of the catalyst and
o
the solvent at 50 C for 30 min. The catalyst was filtered from
the liquid (CAL, CAL + solvent) and vacuum dried overnight at
room temperature. Then, TPD was made in the same way as
described above.
appeared
with
pyridine
compounds
(pyridine,
4-methylpyridine).
Hydrogenation of CAL in different organic solvents
The hydrogenation of CAL was examined in a series of organic
solvents at 50 oC under the same conditions. The results
obtained are shown in Fig. 1. Fig. 1a indicates significant
differences in the CAL conversion among the solvents used and
it was larger in the order of aprotic > protic >> apolar except
for a few solvents. The conversion values obtained in pyridine
and 4-methylpyridine were even smaller compared with those
in the other aprotic solvents. As shown in Fig. 1b the main
product was HCAL, independent of the conversion, with a
selectivity of 80% or larger in most of the solvents examined,
in which the hydrogenation of C=C bond mainly occurred. In
contrast, COL was produced in a selectivity of about 60% in
pyridine and 4-methylpyridine, which were thus better
solvents for the hydrogenation of C=O bond rather than C=C
bond. The solvent effects of pyridine will be discussed in detail
later. Jiang et al. reported solvent effects in CAL hydrogenation
Scheme 1 Hydrogenation of Cinnamaldehyde (CAL).
o
(at 80 C) with a 0.5 wt.-% Pd/Al2O3 catalyst in seven organic
solvents including pyridine, THF, and others.29 The authors
showed that the conversion was larger in the order of
p
-xylene,
N,
’-diethylaniline > N, N’-dimethylaniline >> pyridine. A very
n
-hexane, cyclohexane
>
tetrahydrofuran (THF)
>
N
small CAL conversion observed in pyridine is the same as in our
results but the conversion obtained in an apolar solvent of
n
-hexane was large in contrast to our result (Fig. 1). Note also
that HCAL was produced in any solvent used in a selectivity of >
80% with a minor product of HCOL but no product could be
obtained in pyridine. The differences in the support materials
(Al2O3, C), the metal loadings (0.5, 5.0 wt.-%), and/or the
temperatures (80, 50 oC) would be responsible for the
different solvent effects between their and our heterogeneous
CAL hydrogenation reactions. Zhang et al. also reported that
HCAL was the main product in CAL hydrogenation over a 5 wt.-%
Pd/C in apolar solvent of toluene and undesired diacetal was
produced in protic polar solvents of
-butanol.28 Similar results were reported with Ru catalysts on
MCM-4130 and Y zeolite supports31.
n-propanol and
n
Fig.
1 Conversion (a) and selectivity (b) obtained in CAL
hydrogenation in various solvents over Pd/C. Reaction conditions:
The above-mentioned solvent effects in the heterogeneous
CAL hydrogenation observed will also be compared with those
in similar heterogeneous hydrogenation reactions of
acetophenone. Yoshida et al. reported for Rh/C catalyst that
the order of acetophenone conversion was protic ≥ aprotic >
apolar solvents,10 similar to the order in the CAL
o
CAL 5 mmol, 5 wt.-% Pd/C 5 mg, solvent 5 mL, H2 4 MPa, 50 C, 30
min. Solvents: 1-butanol (n-BuOH), 2-propanol (2-PrOH), diethyl
ether (DEE), tetrahydrofuran (THF), N, N’-dimethylformamide
(DMF), triethylamine (Et3N), 4-methylpyridine (4-Mepy), pyridine
(Py), n-hexane (HEX), toluene (TOL), benzene (BZ).
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hydrogenation with Pd/C (Fig. 1a). Carbon-supported noble
metal catalysts were used in common for these hydrogenation
reactions of acetophenone and CAL. For Rh/Al2O3 catalyst,
however, a different order of apolar > protic ≥ aprotic solvent
was observed. Trasarti et al. also examined the hydrogenation
of acetophenone using SiO2-supported Ni, Co, and Cu catalysts
in different solvents of 2-propanol (2-PrOH), cyclohexane,
toluene, and benzene.12 The rate of hydrogenation was slow in
such apolar solvents as toluene and benzene for Ni/SiO2,
similar to the present solvent effects (Fig. 1a), due to the
blockage of Ni active sites by these solvent molecules, but not
for Co/SiO2 and Cu/SiO2 in the same solvents.
Those results obtained so far indicate that the solvent
effects in heterogeneous catalytic hydrogenation would
include interactions between substrate and solvent, those
between metal and solvent, and/or those between support
and solvent; for the metal-solvent interactions, the size of
metal nanoparticles and the oxidation state of surface metal
species should be important. Jiang et al. made DFT calculations
for the adsorption of a CAL molecule on a minute Pd cluster
and on a Pd(111) surface.29 Although they give useful
information on the adsorption of CAL on Pd, we believe that
molecular interactions between CAL substrate and organic
solvent on the surface of supported metal nanoparticles
should be taken into consideration for the actual adsorption
and activation of CAL molecules, which will be discussed later.
Then, the relationship between the CAL conversion observed
(Fig. 1a) and the different properties of solvents used (Table
S1†) was examined. The conversion is plotted against the
solubility of a gaseous reactant of H2 in solvents but no good
correlation is seen (Fig. S1†). Other factors should be more
important in determining the rate of CAL hydrogenation. But
the conversion was also not correlated well with such solvent
parameters as (hydrogen-bond-donation ability) and
(hydrogen-bond-acceptance ability) (Fig. S1†). Fig. 2 gives the
Fig. 3 Conversion and yield (a) and selectivity (b) obtained in
CAL hydrogenation in the absence and presence of a small
amount of pyridine. Reaction conditions: CAL 5 mmol, 5 wt.-%
Pd/C 5 mg, solvent 4.8 mL (without pyridine, the volume of
solvent is 5 mL), pyridine 0.2 mL, H2 4 MPa, 50 oC, 30 min. For
solvent abbreviations, see Fig. 1’s caption.
the CAL conversion observed is even smaller than expected
from the trend. Interestingly, THF with a medium π* value
gives the highest conversion (Fig. 2) and furan with a similar
oxo-structure of THF also gives a high conversion (Fig. 1), in
which the CAL conversion observed is even larger than
expected from the trend. For apolar solvents, the conversion is
very small and does not depend on π* value.
plot of the conversion ag ainst
a par ameter of
polarity/polarizability index (π*), indicating a trend that the
conversion increases with π* value for protic and aprotic polar
solvents except for pyridine and 4-methylpyridine, in which
Hydrogenation of CAL in the pyridine-containing mixed solvent
As above-mentioned, HCAL was mainly produced in the
present CAL hydrogenation over Pd/C in most of the organic
solvents examined, in agreement with the literature that
supported Pd catalysts are more selective to the
hydrogenation of C=C bond compared to C=O one in CAL
hydrogenation.23,28,29 It should be noted that the main product
can be switched from HCAL to COL on the same Pd/C catalyst
when the reaction is conducted in pyridine (and
4-methylpyridine); that is, pyridine is suitable for the
hydrogenation of –C=O bond. The CAL hydrogenation was
further examined in selected organic solvents to which a small
amount of pyridine was added (Fig. 3). It was observed that
the addition of pyridine increased the CAL conversion in
Fig. 2 The total conversion in CAL hydrogenation (Fig. 1(a)) vs. the
π* value of the solvents used (Table S1†). For solvent abbreviaꢀons,
see Fig. 1’s caption.
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2-PrOH, diethyl ether, and toluene while it lowered the
conversion in -butanol, N, N’-dimethylformamide, and THF. In
n
addition to the CAL conversion, the product selectivity was
also influenced by the addition of pyridine. The main product
was HCAL in a selectivity of 80% or larger in the pure solvents
examined, with a minor product of HCOL and a very marginal
product of COL. It is interesting that in the mixed solvents with
a small amount of pyridine, COL was produced in a comparable
selectivity to that for HCAL and HCOL was also produced in a
larger amount. With the addition of pyridine, the increased
yield of COL and HCOL is larger than the decreased yield of
HCAL in 2-PrOH and diethyl ether, and in contrary, the
increased yield of COL and HCOL is lower than the decreased
Fig. 4 Influence of the volume of pyridine added on onversion and
selectivity in CAL hydrogenation in 2-PrOH. Reaction conditions: CAL
5 mmol, 5 wt.-% Pd/C 5 mg, 2-PrOH + pyridine 5 mL, H2 4 MPa, 50
oC, 30 min.
yield of HCAL in n-butanol, N, N’-dimethylformamide, and THF.
It is likely that pyridine influences both the first step of CAL
hydrogenation to either HCAL or COL and their subsequent
hydrogenation to HCOL. The competitive adsorption of
solvents and CAL on Pd surface is likely to play an important
role determining the product selectivity in the mixed solvents.
Furthermore, the influence of pyridine addition was
examined by changing the amount of pyridine added to a
selected solvent of 2-PrOH. Fig. 4 shows the results obtained
for CAL hydrogenation in a fixed volume (5 mL) of 2-PrOH and
pyridine. The conversion was increased by the addition of
pyridine when the volume of pyridine was increased to 0.2 mL
from 29.5% to 48.6% but it was then slightly decreased with
the further increasing volume of pyridine added. HCAL was the
main product with a selectivity of about 85% in 2-PrOH with no
pyridine added. The product selectivity was observed to
change significantly by the addition of a small amount of
pyridine (< 0.4 mL); the HCAL selectivity decreased but the
addition of pyridine increased the selectivity to either COL or
HCOL. With the further increasing pyridine volume from 1 to 5
mL, the selectivity to HCAL changed slightly, but the selectivity
to COL increased to 62.6% with a decrease in the selectivity to
HCOL. The time profiles of conversion and selectivity of CAL
hydrogenation in a mixed solvent of 2-PrOH and pyridine are
presented in Fig. 5. In the mixed solvent, in the addition of a
small amount of pyridine (0.4 mL), the conversion went up
smoothly with time, but in the addition of a large amount of
pyridine (2.5 mL), the conversion went up to 66.5% and
changed slightly with further extending time. In these cases,
the product selectivity remained almost unchanged during the
reaction. In pure pyridine, the rate of hydrogenation was very
small even at a higher temperature of 80 oC (Fig. S2†),
suggesting strong effects of pyridine on the reactivity of CAL
substrate and/or the activity of supported Pd nanoparticles.
Possible actions of pyridine causing these effects will be
discussed later. Thiols and amines modified supported metal
catalysts were reported in CAL hydrogenation in the literature.
It is reported that for the amine-capped Pt3Co, oleylamine can
prevent CAL from lying flat on the metal surface, resulting in
high selectivity to COL. However, the selectivity to COL
gradually decreases from 90% to 23% with shortening the
chain length of the amine modifiers from oleylamine to C4NH2;
both C=O and C=C bonds can competitively access to the metal
surface.34 Recently, thiols modified supported Pt catalysts have
Fig. 5 Conversion and selectivity obtained in solvent of 2-PrOH and
pyridine over Pd/C. Reaction conditions: CAL 5 mmol, 5 wt.-% Pd/C
5 mg, H2 4 MPa, 50 oC, (a) 2-PrOH 4.6 mL, pyridine 0.4 mL, (b)
2-PrOH 2.5 mL, pyridine 2.5 mL.
been shown to improve the hydrogenation selectivity of CAL to
COL, and 3-phenyl-1-propanethiol modified Pt/Al2O3 give a
high selectivity to COL (90%) at complete conversion of CAL.35
The modification of Pd/Al2O3 with 3-phenyl-1-propanethiol can
also improve the selectivity to COL in the hydrogenation of CAL,
but a low selectivity to COL (<10%) could be obtained at a
conversion about 70%.36 In the present work, the authors used
a commercial Pd/C in the mixture of pyridine and 2-PrOH and
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obtained better results, in which a high COL selectivity of 60%
was achieved at a conversion of 71%.
As described above, the hydrogenation of the partially
hydrogenated products of COL and HCAL is likely to be
influenced by the presence of pyridine. Table 1 shows the
results of COL and HCAL hydrogenation reactions using
selected solvents of 2-PrOH and THF. When COL was a starting
substrate, the hydrogenation to HCOL was fast in the two neat
solvents while it became very slow in the coexistence of
pyridine; namely, the hydrogenation of C=C bond of COL
became difficult. On the other hand, the C=O bond of HCAL
was not hydrogenated in any solvent used and, therefore, the
final product of HCOL was produced from COL but not from
HCAL. In the co-existence of HCAL, the conversion of CAL
decreased obviously.
Fig. 6 ATR-FTIR spectra of CAL in different solvents measured at 50
oC. CAL 5 mmol, solvent 4.8 mL, pyridine 0.2 mL.
The switching in the product selectivity from HCAL to COL by
the addition of pyridine should result from interactions of
pyridine with substrate, solvent, and/or catalyst, which were
then examined by different techniques.
Molecular interactions among solvent, substrate, and catalyst
At first, the interaction between CAL and solvent was checked
by using FTIR (Fig. S3†). When CAL was dissolved in organic
solvents, the absorption band of ν(C=O) at 1670 cm-1 was
blue-shifted to different extents depending on the solvents
used but the absorption band of ν(C=C) remained at 1624 cm-1.
These results indicate that the solvents may have interactions
with the carbonyl group of CAL substrate. Fig. S4 shows the
plot of CAL conversion against the extent of the blue-shift in
different solvents, giving no good correlation. In addition, FTIR
measurements were made for CAL in the mixed solvents of
either 2-PrOH or THF containing a small amount of pyridine
Fig. 7 FTIR spectra of CAL adsorbed on 5 wt.-% Pd/C. (a) CAL on
blank KBr tablet, (b) CAL on KBr tablet mixed with Pd/C, and (c) CAL
on KBr tablet mixed with Pd/C after pre-adsorption of pyridine.
Table 1 Hydrogenation of either COL or HCAL in the absence
(Fig. 6). The absorption band of ν(C=C) and ν(C=O) were
observed to appear at the same wavenumber in the presence
and absence of pyridine. Those results indicate that the
interactions of organic solvents with CAL molecules exist in the
liquid phase but are unlikely to activate or inactiviate C=C or
C=O bond without Pd/C.
and presence of a small amount of pyridine
Pyridine
Time
(min)
5
Conversion
Substrate
Solvent
(mL)
--
(%)
72.0
54.6
99.9
15.0
1.0
0
COL
COL
2-PrOH
2-PrOH
THF
0.2
-
5
COL
5
Secondly, the interactions of supported Pd particles with
CAL and pyridine molecules were examined by FTIR for
gas-solid systems. Fig. 7 gives FTIR spectra for CAL adsorbed on
Pd/C catalyst in the range of (C=O) and (C=C) absorption of CAL
molecules. Spectra (b) and (c) are those for CAL in the absence
and presence of pyridine pre-adsorption, showing that the
pre-adsorption of pyridine (1) decreases the amount of CAL
adsorbed and (2) red-shifts the wavenumber of (C=O)
absorption band but changes slightly that of (C=C) absorption
band. The former suggests that the pre-adsorption of pyridine
would cover the active sites and prevent the adsorption of CAL.
The latter means that the reactivity of C=O bond may be
enhanced but the reactivity of C=C bond may little change.
Such a correspondence was previously observed for
hydrogenation reactions, in which the increased (decreased)
COL
THF
0.2
5
5
COL
--
5
HCAL
HCAL
HCAL
HCAL
HCAL
CALa
2-PrOH
2-PrOH
THF
--
30
30
30
30
30
60
60
0.2
-
0
0
THF
0.2
5
0
--
0
2-PrOH
2-PrOH
--
45.2 b
15.7 b
CAL + HCAL a,c
--
Reaction conditions: Substrate (COL or HCAL) 5 mmol, 5 wt.-%
Pd/C 2 mg for COL, 5 mg for HCAL, volume of solvent +
O
pyridine 5 mL, H2 4 MPa, 50 C. For both COL and HCAL, the
selectivity to HCOL was 100%.
a CAL 5 mmol, 5 wt.-% Pd/C 5 mg,
2-PrOH 5 mL, H2 4 MPa, 50 oC. b Determined from the consumption
amount of CAL/the initial amount of CAL added. c HCAL 3 mmol.
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correlation that the conversion increases with the π* value for
protic and aprotic polar solvents (Fig. 2). HCAL was mainly
produced in these solvents in a selectivity of 80% or larger and
the high selective hydrogenation of CAL to HCAL is a feature of
supported Pd catalysts.23,28,29 Probably, a solvent of a larger π*
value is more likely to assist the hydrogenation steps, the
adsorption of CAL on the surface of Pd nanoparticles, the
addition of hydrogen to C=C bond, and/or the desorption of
the hydrogenated product, HCAL, bearing a polar C=O bond.
Among the solvents examined, THF seems to show a larger CAL
conversion than expected from the correlation, which will be
discussed later. For apolar aromatic solvents of toluene and
benzene with electron-rich ring, the CAL conversion was very
small (Fig. 1); active Pd sites may be blocked by these solvent
molecules, preventing the adsorption and the subsequent
hydrogenation of CAL. Furthermore, surface functional groups
of active carbon can interact with aromatic rings of toluene
and benzene through interactions, inhibiting the adsorption of
CAL on Pd nanoparticles nearby the support, making the
reaction hard to occur.42 As shown in Fig. 1, however, the CAL
hydrogenation reactions in those solvents give similar product
distribution patterns except for pyridine and 4-methylpyridine.
Therefore, the functional group on the surface of support is
less important for determining the product selectivity.
In contrast to those solvents, the main product was COL in a
selectivity of about 60% in pyridine and 4-methylpyridine and
the rate of reaction was very small (Fig. 1). In pyridine, the
hydrogenation of CAL was very slow (Fig. S2†), COL was
hydrogenated to HCOL at a very small rate, and no HCOL was
formed from HCAL (Table 1). Scheme 2 displays the difference
in the rate of hydrogenation steps between pyridine (and
4-methylpyridine) and the other organic solvents examined.
Pyridine molecules are likely to be adsorbed on Pd/C catalyst
and cover the surface of supported Pd nanoparticles, on which
the CAL substrate molecules cannot be adsorbed. For pyridine
(and 4-methylpyridine), due to lone electron pair on its
nitrogen atom, it may coordinate with the unoccupied d
orbital of Pd atoms.37 Such N, O, and S-containing molecules as
pyridine and THF (discussed below) are known to act as
coordinating agents.43 In pyridine, therefore, the number of
active Pd sites on Pd/C is decreased, resulting in a significant
decrease in the rate of CAL conversion. In addition to the rate
of CAL hydrogenation, interestingly, the product selectivity
was switched to COL from HCAL. There are two possible
Fig. 8 TPD curves for 5 wt.-% Pd/C catalyst adsorbing CAL, solvent,
and pyridine. Solvent: 2-PrOH (a), THF (b).
reactivity of C-O and N-O bonds corresponded with the
red-shift (blue-shift) of their frequencies.37-41 The authors
believe that these effects of pyridine on the adsorption of CAL
molecules should also appear in the present liquid phase
hydrogenation with Pd/C catalyst.
Furthermore, TPD was used to examine the behavior of CAL
on the surface of Pd/C catalyst in the coexistence of organic
solvent molecules (Fig. 8). The adsorption of 2-PrOH, THF,
pyridine was quite low compared to CAL, thus the desorption
peak of the mixture of solvent and CAL was mainly contributed
by CAL. Fig. 8a shows that CAL desorbs at around 150 oC in the
coexistence of 2-PrOH and 2-PrOH + pyridine, suggesting no
significant interactions of CAL with these organic solvent
molecules on the catalyst surface. However, the desorption
occurs at lower and higher temperatures on the co-adsorption
of THF and THF + pyridine, respectively (Fig. 8b). The results
suggest that there are some interactions among CAL, THF, and
pyridine molecules adsorbed on the catalyst, for which their
carbonyl and oxygen and nitrogen atoms should be involved.
Discussion
Scheme 2 Positive and negative effects of pyridine on the reaction
The relationship between the total CAL conversion and the π*
parameter will be discussed. There is a relatively good
steps of CAL hydrogenation.
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the addition of pyridine (Fig. 3), considering an action of HCAL.
In 2-PrOH alone, CAL was mainly hydrogenated to HCAL and
HCAL was not further hydrogenated to HCOL. There is a C=O
group in HCAL similar to CAL and HCAL (and other similar
aldehydes) can also be adsorbed on the surface of Pd;23 some
HCAL molecules should remain adsorbed after these have
been produced from CAL, occupy the active sites, and retard
the desired hydrogenation of CAL as shown by the results in
Table1 that the conversion of CAL decreased obviously in the
presence of HCAL. However, on the addition of pyridine into
2-PrOH, the selectivity to HCAL is decreased and the amount of
the Pd sites occupied by HCAL should be decreased, exposing
more Pd sites to hydrogenate the C=O bond of CAL to COL.
For THF, the addition of pyridine was also found to change
the product selectivity and the total CAL conversion but in
different manners (Fig. 3). The yield of HCAL was remarkably
decreased and the total yield of COL and HCOL was increased
Scheme 3 Adsorption CAL substrate molecule on the surface of Pd by the addition of pyridine. The surface of Pd nanoparticles
in the absence (upper) and presence (lower) of pyridine.
may be covered by THF as well as pyridine in the mixed solvent.
Therefore, the number of THF and pyridine free Pd active sites
is reduced to a large extent, resulting in the decreased total
rate of CAL conversion. As seen in Fig. 8, the desorption of CAL
occurs at lower temperatures in the coexistence of THF alone
while the presence of THF and pyridine shifts the desorption of
CAL to higher temperatures. Assuming that the higher
desorption temperature of CAL is a measure of its lower
reactivity, the reactivity should be lowered in the presence of
both THF and pyridine due to interactions among the substrate
and solvent molecules on the surface of Pd/C catalyst (Scheme
4). Then, these interactions among THF, pyridine, and CAL also
contributed to the retardation of the CAL hydrogenation, as
observed. The change of the product selectivity may also be
ascribed to the cooperation between THF and pyridine. The
co-adsorption of THF and pyridine molecules is more likely to
promote the adsorption of CAL molecule with its C=O group
compared to the case of THF alone (Scheme 4).
reasons for the high COL selectivity in pyridine. One is the
presence of interactions between CAL and pyridine on the
surface of Pd/C catalyst as indicated by FTIR, which enhance
the reactivity of C=O but not C=C bond (Fig. 7). The other is the
change in the geometrical structure of CAL adsorption on the
catalyst. When pyridine molecules are co-adsorbed and so the
surface of Pd nanoparticles is crowded, a CAL molecule is likely
to be adsorbed sterically with its C=O bond rather than C=C
one (Scheme 3). Thus, pyridine may assist the selective
hydrogenation of CAL to COL.
The influence of the addition of pyridine to such other
solvents as 2-PrOH and THF on CAL hydrogenation will then be
discussed. For the former solvent, HCAL was the main product
with a smaller amount of HCOL (Fig. 3). HCOL was not
produced from HCAL but from COL since the rate of
hydrogenation of COL to HCOL was much larger than that of
HCAL to HCOL, but the hydrogenation of COL to HCOL became
difficult in the presence of pyridine (Table1 and Fig. 3). When a
small amount of pyridine was added, the conversion was
increased by a factor of 1.6 and the product selectivity was
also changed (Fig. 3). On the addition of pyridine, the yield of
HCAL was a little decreased while the total yield of COL and
HCOL was significantly enhanced. As above-mentioned,
pyridine may (a) increase the reactivity of C=O bond, and (b)
facilitate the adsorption of CAL with its C=O bond (Scheme 3).
These would promote the hydrogenation of the C=O bond of
CAL substrate, resulting in the increases in the CAL conversion
and in the yield of COL and HCOL by the addition of pyridine as
observed (Fig. 3). However, these promotional effects should
be set off by a negative effect that the number of active Pd
sites decreases by the co-adsorption of pyridine; the negative
effect should become superior with an increase in the amount
of pyridine added. This may explain the change in the CAL
conversion with the volume of pyridine added that it is
maximized at a certain volume (Fig. 4). The authors suggest a
different factor for the increased CAL conversion in 2-PrOH by
It is known that pyridine and THF molecules may act as
coordinating agents and be important in the synthesis of
stable metal nanoparticles (stable colloidal metal dispersion).43
A similar molecule of
N-methylpyrrolidone would interact
Scheme 4 Possible interactions among CAL, pyridine, and THF
molecules on the surface of Pd.
Scheme 5 Possible interactions between CAL and 2-PrOH molecules
on the surface of Pd.
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with supported Pd nanoparticles and cause the leaching of Pd selectivity. The surface of supported Pd nanoparticles is
species into the liquid phase under Heck coupling conditions covered in part by pyridine molecules, in which pyridine-free
but in the absence of such a ligand as phosphines.44 McManus Pd sites should be separated in small fragments, facilitating the
et al. examined the influence of solvents in hydrogenation of hydrogenation of C=O bond rather than C=C bond. There are
4-phenyl-2-butanone over Pt/TiO2 and indicated that alkanes interactions of C=O bond of CAL substrate with solvent
favored ring hydrogenation while aromatics and alcohols molecules in the liquid phases but these do not affect the
facilitated carbonyl hydrogenation. They considered the results of CAL hydrogenation. The reactivity of C=O bond of
competitive adsorption on the surface of Pt/TiO2 catalyst CAL on the surface of Pd/C catalyst is increased by the
between the substrate and solvent molecules; in their case, coexistence of pyridine molecules; interactions between CAL
oxygen vacancies of TiO2 close to Pt nanoparticles are and 2-propanol and those among CAL, THF, and pyridine
important for determining the adsorption of –C=O of the contribute to the promotion of C=O bond hydrogenation. The
substrate.11
difference in those interactions between 2-propanol and THF
Finally, the reactions in 2-PrOH and in THF in the absence of should be responsible for the difference in the influence of
pyridine will be compared (Fig. 3). The main product was HCAL pyridine addition on the rate of CAL hydrogenation. The results
in the two solvents but the CAL conversion was larger in THF obtained would give useful pieces of information about
by a factor of about 2 than in PrOH. The reactivity of CAL was solvents and solvent effects in heterogeneous hydrogenation
increased in the presence of THF alone via interactions and other reactions.
between CAL and THF molecules on the surface of Pd/C
catalyst (Fig. 8b). This may be a reason for the higher CAL
Conflicts of interest
conversion obtained in THF than expected from the
conversion-π* relationship (Fig. 2). Furthermore, the authors
There are no conflicts to declare.
believe that the adsorption of aldehyde molecules of CAL and
HCAL in 2-PrOH is stronger via interactions with the solvent
molecules (Scheme 5) than that in THF, for which the
Acknowledgements
desorption of CAL occurs at a lower temperature in the
This work was supported by the National Key Research and
presence of THF (Fig. 8b). The catalytic steps are more
Development Program of China (2016YFA0602900), the
effectively cycled on Pd/C in THF than in 2-PrOH, resulting in
National Natural Science Foundation of China (21603212,
the larger CAL conversion in the former than in the latter. In
21672204), Youth Innovation Promotion Association CAS
the presence of pyridine, however, the adsorption of CAL on
(2016206), Jilin Provincial Science and Technology Program of
the surface of Pd in 2-propanol (+ pyridine) is stronger than in
China (20160203007GX), and Chinese Academy of Sciences
THF (+ pyridine), as described above, and so the addition of
President’s International Fellowship Initiative (2018VCA0012).
pyridine has different effects on CAL hydrogenation in THF and
2-propanol (Fig. 3).
Notes and references
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Table of contents
The surface of Pd particles is modified by pyridine and this promotes the
hydrogenation of C=O in CAL over Pd/C.