Hydrogenolysis of C?O Chemical Bonds of Broad Scope Mediated by a New Spherical Sol–Gel Catalyst

Article abstract of DOI:10.1002/open.201700185

The new spherical sol–gel hybrid material SiliaCat Pd⁰ selectively mediates the hydrogenolysis of aromatic alcohols, aldehydes, and ketones by using an ultralow catalytic amount (0.1 mol % Pd) under mild reaction conditions. The broad reaction scope as well as the catalyst's superior activity and pronounced stability open the route to green and convenient reductive deoxygenation processes of primary synthetic relevance in chemical research as well as in the fine chemical and petrochemical industries.

Full text of DOI:10.1002/open.201700185

DOI: 10.1002/open.201700185
Hydrogenolysis of CÀO Chemical Bonds of Broad Scope
Mediated by a New Spherical Sol–Gel Catalyst
[a]
[b]
[a]
[a]
Valerica Pandarus, Rosaria Ciriminna, Genevi›ve Gingras, FranÅois BØland,*
[b]
[c]
Mario Pagliaro,* and Serge Kaliaguine*
This study is dedicated to Professor Franck Dumeignil, Lille University and CNRS (France), for all he has done to advance the second
generation biorefinery from a conceptual to a practical reality
0
The new spherical sol–gel hybrid material SiliaCat Pd selective-
ly mediates the hydrogenolysis of aromatic alcohols, alde-
hydes, and ketones by using an ultralow catalytic amount
nounced stability open the route to green and convenient re-
ductive deoxygenation processes of primary synthetic rele-
vance in chemical research as well as in the fine chemical and
petrochemical industries.
(
0.1 mol% Pd) under mild reaction conditions. The broad reac-
tion scope as well as the catalyst’s superior activity and pro-
1. Introduction
[
8]
The hydrogenolysis of CÀO and C=O chemical bonds in alco-
performance, for example, through encapsulation of Cu,
[
9–11]
[12]
hol, ester, and carbonyl compounds over supported metal
Ni,
and Pd NPs in the inner porosity of porous oxides.
nanoparticles (NPs) with H is an important reaction used in
However, as put by Samec and co-workers in 2013, when re-
porting on the Pd/C-catalyzed transfer hydrogenolysis of pri-
mary, secondary, and tertiary benzylic alcohols by formic
2
synthetic organic chemistry and in manufacturing active phar-
[1]
maceutical ingredients, as well as in biomass conversion in
[
13]
the emerging biorefinery to cleave CÀOH bonds with hydro-
acid, the hydrogenolysis of benzylic alcohols has been stud-
ied to a lesser extent, with the noticeable exception of Yang
[2]
gen to access valuable platform chemicals. Numerous solid
[
14]
catalysts are available to selectively mediate the scission of CÀ and co-workers who reported a clean and simple method
[
3]
[4]
0
O bonds with H including supported Ni, PtO (Adams cata-
(and proposed a reaction mechanism based on Pd formation)
2
[
5]
[6]
[7]
lyst), Rh and Ir, Cu (copper chromite), and Pd catalysts.
for the reductive deoxygenation of aromatic ketones and ben-
The reaction is usually carried out under hydrogen pressure at
relatively high temperature (usually above 1508C and often
above 2008C), promoting NP aggregation. Hence, intense re-
search efforts are being devoted to finding catalysts of stable
zylic alcohols using palladium chloride (PdCl ) as the catalyst,
2
and polymethylhydrosiloxane as the hydride source. Starting
from conventional palladium on activated charcoal (Pd/C), nu-
merous solid palladium catalysts are available to selectively
mediate hydrogenolysis reactions under mild or very mild con-
ditions. However, even though there is broader utilization of
said catalysts using valued palladium, it requires that high
product yields are achieved by using low catalyst amounts
[
a] V. Pandarus, G. Gingras, Dr. F. BØland
SiliCycle
2
500, Parc-Technologique Boulevard
1
Quebec City, Quebec G1P 4S6 (Canada)
E-mail: francoisbeland@silicycle.com
while minimizing leaching of the valued palladium. This is par-
ticularly important in light of forthcoming wide utilization of
reactions under flow conditions for active pharmaceutical in-
gredient (API) manufacturing, in which long-term stability of
[
b] R. Ciriminna, Dr. M. Pagliaro
Istituto per lo Studio dei Materiali Nanostrutturati, CNR
via U. La Malfa 153, 90146 Palermo (Italy)
E-mail: mario.pagliaro@cnr.it
[
15]
the catalyst is an essential requirement.
0
SiliaCat Pd , namely a hybrid organically modified silica (OR-
[
c] Prof. S. Kaliaguine
Department of Chemical Engineering, UniversitØ Laval
MOSIL) glass encapsulating palladium NPs, which combines
[
16,17]
2
325 Rue de l’UniversitØ
high porosity with organic functionality,
is a hydrogena-
Quebec City, Quebec G1V 0A6 (Canada)
E-mail: serge.kaliaguine@gch.ulaval.ca
tion catalyst that selectively mediates a number of important
reactions including the highly selective debenzylation of aryl
Supporting Information and the ORCID identification number(s) for the
author(s) of this article can be found under https://doi.org/10.1002/
open.201700185.
[
18]
benzyl ethers, benzyl esters, and benzyl amines, and the hy-
[
19]
[20]
drogenation of functionalized nitroarenes, olefins, vegeta-
[
21]
[22]
ꢀ
2017 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
ble oils, and squalene. More recently, we reported that the
This is an open access article under the terms of the Creative Commons
Attribution-NonCommercial-NoDerivs License, which permits use and
distribution in any medium, provided the original work is properly cited,
the use is non-commercial and no modifications or adaptations are
made.
1
À1
Leaching values are given in mgkg of API. Limit of detection:
À1
À1
LODPd,Si =0.05 ppm in solution (100 mgmL concentration) or 0.50 mgkg
in the crude product.
ChemistryOpen 2018, 7, 80 – 91
80
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solvent-free hydrogenation of the latter oil requires a catalyst
General Procedure for the Catalytic Hydrogenolysis of
Aromatic Alcohols, Aldehyde and Ketones over SiliaCat Pd
[23]
0
with nanostructured spherical morphology. In this work, we
0
report that the glassy hybrid spherical SiliaCat Pd material is a
Multigram hydrogenolysis reactions of aromatic alcohols, aldehyde,
and ketones were carried out in an Ace Glass 6437 system
equipped with a batch glass reactor, under a maximum pressure of
highly selective mediator for the hydrogenolysis of aromatic al-
cohols, aldehydes, and ketones under very mild conditions,
with ultralow amounts of Pd leaching in the final product. We
compared the performance of this new catalyst with that of
several commercial catalysts including Pd/polyethylenimine/
3 bar H at 70–1008C. The 500 mL glass reactor was charged with
2
250 mL reagent solution (375 mmol reagent) and 1.56 g SiliaCat
0
Pd for 0.1 mol% Pd. Mechanical stirring was then set at 800 rpm
0
and the reaction mixture degassed five times, replacing three
SiO , Pd/SiO , and Pd/C. SiliaCat Pd is a recyclable catalyst and
2
2
times the vacuum by Ar, and twice Ar by H , after which the de-
2
retains its ideal spherical morphology, thereby providing re-
search laboratories and industry with a suitable economic
method to make substances in high demand at low cost by
using an entirely green process.
sired hydrogen pressure was set and kept at 3 bar. The reaction
temperature was then raised from 228C to the desired reaction
temperature and kept at this temperature for several hours until
maximum conversion, after which the reaction mixture was cooled
to 208C. The solid catalyst recovered by filtration was washed,
dried, and stored prior to reuse. The isolated yield was assessed by
simple isolation of product by rotary evaporator distillation at
208C. Thus-obtained products are generally very pure and no fur-
ther chromatographic purification required.
Experimental Section
Materials
GC-MS analyses were performed by using a 7890B GC System (Agi-
lent Technologies) equipped with a HP-5MS 30 m capillary column
All reactions were performed on a multigram scale, using anhy-
drous ethanol or solvents of HPLC grade. Unless otherwise noted,
reagents were commercially available and used without purifica-
[
0
(5%-phenyl)-methylpolysiloxane, 0.25 mm inner diameter and
.25 mm film thickness] and with a mass spectrometer 5977B mass-
tion. The commercial palladium catalyst 5% Pd/C (from Sigma Al-
selective detector operated in electron impact ionization mode
(70 eV). GC-MS analyses were carried out in split mode, using
helium as the carrier gas (1 mLmin flow rate). The injection tem-
0
drich), ROYER catalyst (3 wt% Pd /polyethylenimine/SiO , from
2
0
À1
Strem Chemicals) and Pd Escat 1351 (5 wt% Pd /SiO , from Strem
2
Chemicals) were used as received from the suppliers.
perature was 2508C, the interface was set at 3258C, and the ion
source was adjusted to 2308C. The column was maintained at an
initial temperature of 508C for 4.5 min, and then ramped to 3258C
À1
at 1008Cmin , where it was maintained for 5 min. Mass spectra
À
were recorded at 5.5 scanss (m/z 50–550). The identification of
Sample Characterization
the compounds was based on comparison of their retention times
with those of authentic samples, and on comparison of their EI-
mass spectra with the NIST/NBS, Wiley library spectra, and the
literature.
0
Physical properties of the SiliaCat Pd catalyst were determined by
using X-ray diffraction (XRD), transmission electron microscopy
TEM), N isotherms, Si MAS NMR spectroscopy, and inductively
2
coupled plasma–optical emission spectroscopy (ICP-OES). The XRD
analyses were performed on a Siemens D-5000 X-ray diffractome-
ter equipped with a monochromatic CuKa radiation source (l=
2
9
(
2
. Results and Discussion
1
.5418). The spectra were recorded in the 2q=0–308 range for the
0
undoped ORMOSIL support and 2q=10–908 for the SiliaCat Pd
catalyst, in both cases at a scan speed of 18m and a step scan of
0
0
SiliaCat Pd is a heterogeneous catalyst obtained through the
À
alcohol-free sol–gel polycondensation of alkoxysilanes such as
.028. The Powder Diffraction File of The International Centre for
[
24]
methyltrimethoxysilane (MTES). The catalyst, now available
in spherical morphology (Figure 1), is comprised of catalytically
active Pd NPs encapsulated within the spherical mesoporous
ORMOSIL matrix.
Diffraction Data was used to identify the diffraction peaks charac-
teristic of crystalline Pd with a face centered cubic (fcc) lattice. A
tube voltage of 40 kV and a current of 100 mA were used for scan-
ning. The TEM images were taken by using a JEOL-2010 micro-
scope equipped with a LaB6 electron gun source operated at
0
The presence of highly dispersed palladium NPs (ca. 2–4 nm)
entrapped within the inner porosity of the lipophilic organosili-
2
00 kV. Nitrogen adsorption and desorption isotherms were mea-
sured at 77 K by using a Micrometrics TriStar II 3020 system. The
resulting data were analyzed with the TriStar II 3020 version 3.02
software. Barrett–Joyner–Halenda (BJH) desorption branches were
[
25]
ca matrix was clearly revealed by TEM analysis before and
after scale-up catalyst synthesis (Table 1, Figures 2a and 2b).
The crystalline nature of the active nanophase is evident from
the XRD pattern of the catalyst powder (Figure 3), characteris-
tic of the fcc structure of metallic Pd.
29
used to calculate the pore-size distribution. Solid-state Si NMR
spectra were recorded on a Bruker Avance spectrometer (Milton,
ON, Canada) at a silicon frequency of 79.5 MHz. The sample was
spun at 8 kHz at magic angle at room temperature in a 4 mm ZrO
rotor. A Hahn echo sequence synchronized with the spinning
speed was used while applying a TPPM15 composite pulse decou-
pling during acquisition. A total of 2400 acquisitions were recorded
with a recycling delay of 30 s. The leaching of Pd and Si was as-
sessed by ICP-OES analysis of the crude product in DMF (concen-
The peaks at 2q=408 correspond to the most intense dif-
fraction line of the (111) plane of metallic palladium. Further-
more, the peaks corresponding to diffraction lines relative to
0
the (200) and (220) planes of Pd were detected only after
scale-up at 1.6 kg, when a weak agglomeration of palladium
NPs from 2.1 to 3.5 nm was observed (Figure 3B–C). The amor-
phous nature of 100% MeSiO1.5 used as an encapsulating
matrix was confirmed by the characteristic wide XRD diffrac-
À1
tration 100 mgmL ), using a PerkinElmer Optima 2100 DV system.
Values measured are reported as milligrams of Pd and as milli-
grams of Si per kilogram of crude product.
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29
Figure 5 shows the Si MAS NMR spectra of the support and
of the catalyst. The spectra were analyzed according to the
chemical shifts of organotrialkoxysilanes, for which the silicon
n
atoms appear as T band type, where n is the number of silox-
[
27]
ane bonds of the Si atom.
The absence of any signal at
À40 ppm related to ethoxy groups derived from non-hydro-
2
lyzed MTES, and the presence of signals only at À56 ppm [T ,
3
MeSi(OSi) OH] and À66 ppm [T , MeSi(OSi) ], corresponds to
2
3
almost complete hydrolysis of the MTES organosiloxane
[
28]
precursor.
For this study, benzyl alcohol (BA) was examined as a model
compound carrying out the hydrogenolysis reaction at 30–
7
08C and 3 bar hydrogen pressure. Other BA derivatives such
as a-methylbenzyl alcohol (MBA), an intermediate in the pro-
duction of propylene oxide by the ethylbenzene hydroperox-
ide-oxidation process, were tested as well. In this process, eth-
ylbenzene is peroxidized to ethylbenzene hydroperoxide
[
29]
(
EBHP) in the first step,
and then propylene oxide is ob-
tained by the epoxidation of propylene with EBHP. During the
epoxidation step, EBHP turns into a-MBA, which can be further
hydrogenolysed to ethylbenzene. The hydrogenolysis of MBA
is the key step to achieve the recycling process.
0
Figure 1. SEM images at 100 and 250” magnification of SiliaCat Pd ,
50 =100 mm. Lab-scale synthesis (top) and 1.6 kg scaled-up synthesis
bottom).
d
(
0
Table 1. TEM and XRD NP size in the SiliaCat Pd catalyst used in present
study.
2.1. Effect of Solvent
Material
Pd loading
wt%]
Scale-up level
[kg]
NP size
[nm]
A series of hydrogenolysis reactions were performed to identify
the optimum molar concentration of BA solution in MeOH. The
molar concentration of BA in the reaction volume (250 mL)
was thus increased from 0.4 to 4m by charging the 500 mL re-
actor with 250 mL BA solution from 0.4m (10.81 g, 100 mmol)
to 4.0m (108.14 g, 1000 mmol) and 0.1 mol% Pd of SiliaCat
[
0
0
SiliaCat Pd (Pd /MeSiO1.5
)
2.5
0.1
ca. 2.1
ca. 3.5
1.6
0
tion pattern of the spherical MeSiO1.5 xerogel devoid of metal
NPs (Figure 3A).
Pd . Table 3 shows the outcomes of the hydrogenolysis under
The Brunauer–Emmett–Teller (BET) and BJH values of specific
surface area, pore size, and pore volume of the undoped orga-
Table 3. Effect of BA concentration in methanol solution in the hydroge-
nolysis over SiliaCat Pd .
0
[a]
0
nosilica support and of the SiliaCat Pd catalyst are given in
Table 2.
[b]
Entry
Benzyl alcohol
m]
Time
[h]
Toluene yield
[%]
[
1
2
3
4
5
0.4
0.7
1.0
1.5
2.0
1.0
2.0
1.0
95
100
98
100
97
100
98
100
73
0
Table 2. Textural properties of the SiliaCat Pd catalyst and blank support
MeSiO1.5
.
2.0
Material
Pd loading
wt%]
BET surface
area [m /g]
Pore volume
[cm /g]
Pore size
[nm]
0.5
1.0
0.5
2
3
[
MeSiO1.5
SiliaCat Pd
–
2.5
865
391
1.15
0.82
5.3
8.4
1.0
0
0.5
1
2
.0
.0
97
100
62
6
7
8
2.5
3.0
4.0
0.5
.0
1
84
The type IV N adsorption–desorption isotherms of both ma-
2
[26]
2.0
1.0
100
65
87
100
77
98
terials are typical of mesoporous materials. The organosilica
support (Figure 4, left) presents a large BET surface area
2.0
2
À1
(
>800 m g ) and narrow pore-size distribution (6.0 nm) of
3.0
3.0
5.0
mesopores capable of adsorbing a large volume of cryogenic
3
À1
0
nitrogen (>1.1 cm g ). On the other hand, the SiliaCat Pd
catalyst (Figure 4, right) has a lower BET surface area (average
6.0
100
0
2
À1
[a] Experimental conditions: 0.1 mol% Pd of SiliaCat Pd under 3 bar H
2
,
4
8
00 m g ) and a broader pore-size distribution centered at
.4 nm.
708C. [b] Toluene yield evaluated by GC-MS analysis.
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0
Figure 2. a) SEM image (A), TEM images (B–D), and palladium particle-size distribution histogram of SiliaCat Pd catalyst (0.1 kg scale-up). b) TEM images and
0
palladium particle-size distribution histogram of SiliaCat Pd catalyst (1.6 kg scale-up).
0
0
Figure 3. Powder X-ray diffraction (XRD) patterns of MeSiO1.5 blank material (A) and Pd NP-loaded MeSiO1.5 organosilica catalyst SiliaCat Pd : 0.1 kg scale-up
B); 1.6 kg scale-up (C).
(
0
3
bar H pressure at 708C, over SiliaCat Pd at 0.1 mol% Pd
almost complete conversion (95–98%) after 1 h only. By in-
creasing the concentration of the substrate solution to 1.0m
and then to 1.5m, a total conversion to toluene was achieved
after 1 h (entries 3 and 4). Further increase of the concentra-
2
loading (molar ratio of palladium/BA).
Entries 1–2 show that, for both 0.4 and for 0.7m BA solu-
tions, full conversion to toluene was obtained after 2 h, with
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Figure 4. N
NP organosilica catalyst SiliaCat Pd (right).
2
-adsorption and desorption isotherms and BJH desorption pore-size distribution of the amorphous MeSiO1.5 support (left) and of the Pd/MeSiO1.5
0
29
0
Figure 5. Si MAS NMR spectra of the blank support MeSiO1.5 (left) and of SiliaCat Pd (right).
tion from 3 to 4m required to extend the reaction time to 3
and 6 h, respectively (entries 5–8).
The GC-MS analysis showed that, in cases in which conver-
sion of BA to toluene was incomplete, a large number of inter-
mediary products of BA such as benzyl methyl ether, benzalde-
hyde, benzaldehyde dimethyl acetal, and benzyl ether were
present in the reaction mixture (Figure 6). However, these in-
termediary products were completely converted in the final
product and no more additional purification is required.
BA solution with a concentration of 1.5m was used to test
other solvents including HPLC grade ethanol, tetrahydrofuran,
heptanes, and ethyl acetate (Table 4). The reactions were run
0
again over 0.1 mol% Pd SiliaCat Pd at 708C under 3 bar H2
pressure and performed in a 375 mmol BA scale. The 500 mL
Figure 6. Relative concentration of different intermediates formed during
the hydrogenolysis reaction of BA in methanol (3m) over 0.1 mol% Pd of Sil-
0
reactor was charged with 250 mL BA solution (1.5m; 38.84 mL
iaCat Pd versus time.
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2.3. Effect of Reaction Catalyst Loading
[
a]
Table 4. Hydrogenolysis of BA to toluene in different solvents.
[
b]
Entry
Solvent
MeOH
Benzyl alcohol
m]
Time
[h]
Toluene yield
[%]
By using the conditions developed in entry 1 of Table 4
375 mmol BA in 250 mL MeOH solution, 1.5m), the amount of
SiliaCat Pd spherical catalyst was decreased from 0.2 to
[
(
0
1
2
3
4
1.5
1.5
1.5
1.5
1.5
0.5
97
100
99
100
99
100
97
100
42
1
.0
0.5
.0
0.5
.0
0.5
.0
0.5
0
5
.025 mol% Pd to assess its catalytic activity. Again, the
c
EtOH
00 mL reactor was charged with 250 mL of 1.5m BA methanol
1
0
solution (375 mmol BA) and solid SiliaCat Pd catalyst
THF
À1
1
(0.24 mmolg Pd loading), varying the amount from 0.2 mol%
heptane
AcOEt
Pd to 0.025 mol% Pd.
1
Entry 1 in Table 5 shows that the hydrogenolysis of BA over
[d]
5
0
.2 mol% Pd proceeds with full conversion to toluene after
1
1
2
.0
.5
.0
69
97
98
15 min only. At lower palladium loading (0.1 mol% Pd), the
conversion of BA to toluene was complete after 1 h (entry 2).
Further, decreasing the palladium reaction loading to 0.05
and to 0.025 mol%, the substrate conversion to toluene was
complete after 2 and after 6 h, respectively.
0
[
[
[
2
a] Experimental conditions: 0.1 mol% Pd of SiliaCat Pd , 3 bar H , 708C.
b] Toluene yield evaluated by GC-MS. [c] Anhydrous ethanol was used.
d] Ethylcyclohexane (2%).
of BA in 211.16 mL solvent) and 0.1 mol% Pd (1.56 g SiliaCat
Table 5. Effect of catalyst concentration in hydrogenolysis of BA over Sil-
iaCat Pd .
0
[a]
0
Pd ). The variety of solvents ranging in polarity from heptane
to EtOH worked well in the Pd-catalyzed hydrogenolysis of BA
with quantitative conversion to toluene within 0.5–1 h, except
for reaction in EtOAc with complete conversion in only 2 h
0
[d]
Entry
SiliaCat Pd loading
Time
[h]
Toluene yield
[%]
[b]
[c]
[mol% Pd]
[wt% catalyst]
1
2
0.2
0.1
8
4
0.25
0.5
100
97
(
entry 5).
1
0.5
1.0
.0
100
56
72
3
0.05
2
2.2. Effect of Reaction Temperature
1.5
84
The effects of reaction temperature on the catalytic activity of
2.0
2.0
100
51
72
0
4
0.025
1
SiliaCat Pd are shown in Figure 7, in which toluene yield is
4
6
.0
.0
plotted against the reaction temperature. Reactions were per-
97
0
formed over 0.1 mol% Pd of SiliaCat Pd using a 1.5m BA solu-
[
2
a] Experimental conditions: 3 bar H , T=708C. [b] Molar ratio of palladi-
um/BA. [c] Mass ratio of catalyst/BA. [d] Toluene yield evaluated by GC-
MS analysis.
tion in methanol. As expected, the reaction rate increases with
increasing reaction temperature. After 1 h reaction, the conver-
sion of BA to toluene increases from 36% at 308C to 100% at
7
08C. The reaction temperature should, therefore, be higher
than 508C in order to obtain a high substrate conversion in
short reaction time.
2.4. Reaction Scope
A variety of benzylic alcohols and different primary benzylic al-
cohols containing different functionalities were investigated in
0
the hydrogenolysis reaction (Table 6). SiliaCat Pd was thus
tested in multigram reactions under 3 bar H pressure in meth-
2
anol (or ethanol) over 0.1 mol% Pd. The yields were evaluated
by product isolation or by GC-MS.
The optimized reaction conditions in MeOH at 708C devel-
oped in Table 4, entry 1, were first applied to hydrogenolysis of
a variety of primary (entries 1–8 in Table 5), secondary (en-
tries 6–14), and tertiary benzylic alcohols (entries 15–19) to
their corresponding hydrocarbons. Once again, the 500 mL re-
actor was charged with 250 mL of 1.5m substrate solution in
methanol or in anhydrous ethanol (375 mmol substrate) using
0
À1
the SiliaCat Pd spherical catalyst of 0.24 mmolg of palladium
loading. In these conditions, BA (1) was quantitatively reduced
to 2 within 1 h (entry 1). Substitution of the phenyl group in
the para position by electron-donating groups under the same
reaction conditions gave the corresponding hydrocarbons in
lower yields (entries 2–6 in Table 5). To increase the yield in 4-
Figure 7. Effect of reaction temperature on the hydrogenolysis of BA to tolu-
ene in methanol over 0.1 mol% Pd of SiliaCatPd(0).
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0
[a]
Table 6. Hydrogenolysis of different alcohol substrates over SiliaCat Pd .
Entry
Benzylic alcohol
Solvent
T
[
Product
t
[h]
Alcohol
conversion [%]
Hydrocarbon
yield [%]
Isolated
yield [%]
o
[b]
[b]
C]
0
1
.5
100
100
97
100
–
95
1
MeOH
70
1
3
5
1
3
5
1
3
32
55
74
42
64
79
83
100
25
43
51
40
63
77
79
98
[
[
c]
c]
2
MeOH
EtOH
70
80
–
–
3
[
d]
4
EtOH
80
–
96
3
5
3
5
100
100
100
100
45
61
69
97
[
[
e]
d]
5
6
–
EtOH
EtOH
80
80
–
95
3
5
3
5
36
44
53
75
33
40
47
71
7
8
–
–
[
d]
1
3
5
1
2
70
91
98
90
100
67
85
91
88
97
9
1
MeOH
EtOH
70
80
–
[f]
0
–
94
1
3
5
1
2
3
65
85
93
84
97
100
62
80
85
82
92
96
1
1
MeOH
EtOH
70
80
–
[g]
12
–
–
94
1
3
5
1
3
5
31
48
59
40
67
91
31
47
58
40
66
84
1
3
4
MeOH
EtOH
70
80
–
–
[h]
1
1
2
1
2
76
100
98
73
96
95
98
–
94
–
[
i]
i]
1
5
6
MeOH
EtOH
70
80
[
1
100
96
1
3
5
3
5
5
75
78
82
100
100
10
31
37
40
55
87
10
[
j]
k
1
1
1
7
8
9
EtOH
80
80
80
–
–
–
[
d,j]
[k]
EtOH
[
k]
THF
[
2
a] Experimental conditions: 3 bar H , 70–808C, catalyst loading=0.1 mol% Pd. [b] Evaluated by GC-MS. [c] Intermediates observed: 4-methoxybenzyl
methyl ether. [d] 0.2 mol% Pd. [e] 4-Aminobenzyl alcohol in 39% yield. [f] Ethylcyclohexane (3%). [g] Propylcyclohexane (4%). [h] Trace of (cyclohexylme-
thyl)benzene. [i] Presence of isopropylcyclohexane. [j] Intermediates observed: 1,1-diphenylethyl ethyl ether. [k] Molar concentration of 1,1-diphenylethanol
solution in anhydrous ethanol or in THF=1.0 m.
methoxy toluene (4), the reactions were carried out in ethanol
at 808C using 0.1 mol% Pd (entry 3) or 0.2 mol% Pd with
quantitative conversion within 3 h (entry 4). Dissolved in etha-
5 h (entry 5). However, the conversion to 4-aminotoluene hy-
0
drocarbon over 0.2 mol% Pd of SiliaCat Pd was quantitative
after 3 h (entry 6). Entries 7 and 8 show that substitution of
the phenyl group in the para position by an electron-with-
drawing group gave the corresponding hydrocarbon in very
poor yield.
0
nol at 808C over 0.1 mol% Pd of SiliaCat Pd substrate, 4-nitro-
BA (5) was quantitatively reduced to 4-aminotoluene (6) in
6
1% yield and to 4-aminobenzyl alcohol in 39% yield within
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Different secondary benzylic alcohols were then tested. a-
MBA (9) in methanol was reduced to the ethylbenzene (10) in
glass reactor was charged with 250 mL of 1.5m carbonyl com-
pound (375 mmol) solution in anhydrous solvent and
À1
9
1% yield within 5 h at 708C (entry 9) and quantitatively
0.1 mol% Pd of SiliaCat Pd catalyst (0.24 mmolg Pd loading).
within 2 h in ethanol at 808C (entry 10). Substitution of the
methyl group in the a-MBA by an ethyl group in 1-phenyl-1-
propanol (11) or by a phenyl group in diphenylmethanol (13)
gave the corresponding hydrocarbons in lower yield.
In general, the obtained results show that the aromatic car-
bonyl compounds can readily be converted to the correspond-
ing hydrocarbons in excellent yields over only 0.1 mol% Pd
under 3 bar H pressure and 70–808C. In addition, the reac-
2
Dissolved in methanol, 1-phenyl-1-propanol (11) was re-
duced to propylbenzene (12) in 85% yield within 5 h (entry 11)
and quantitatively within 3 h when dissolved in ethanol
tions were very clean and no chromatographic separation was
required to obtain higher purity product.
The hydrogenolysis reaction of benzaldehyde (19) and ace-
tophenone (27) were first tested in different solvents (en-
tries 1–5 for benzaldehyde and entries 12–15 for acetophe-
none, in Table 7) such as methanol, ethanol, tetrahydrofuran,
and heptane by using the conditions developed for BA in
Table 4.
(
entry 12). Diphenylmethanol (13) dissolved in methanol at
08C was reduced to diphenylmethane (14) in 58% yield
8
within 5 h (entry 13) and quantitatively within the same reac-
tion time by carrying out the reaction in ethanol (entry 14).
Overall, as expected, these results indicate that the reactivity
strongly depends on the aromatic alcohol structure, with the
In all solvents tested, the quantitative conversions of benzal-
dehyde and acetophenone were observed after 1–2 h. The
presence of BAs or a-MBA intermediates detected by GC-MS
confirmed that the catalytic hydrogenolysis of aromatic car-
bonyls to hydrocarbon compounds proceeds via the formation
of benzylic alcohol intermediates.
0
activity of the sol–gel SiliaCat Pd catalyst changing in the
order: BA (1) > a-methylbnenzyl alcohol (9) >1-phenyl-1-
propanol (11) > diphenylmethanol (13).
The steric influence observed on the activity of Pd/MeSiO1.5
catalyst can be explained by a mechanism in which the aro-
matic ring of alcohols is first adsorbed on the Pd nanocrystal
surface with the CÀO bond lying in close proximity to catalyti-
cally active Pd atoms subject to the steric hindrance of the al-
Compared to the hydrogenolysis reaction of BA, the catalytic
hydrogenolysis of benzaldehyde in different solvents requires
longer reaction times. In methanol at 708C, the conversion to
toluene was quantitative within 5 h while in THF, heptanes,
and ethanol the conversion was quantitative in almost 3 h (en-
tries 2–4). A shorter reaction time of 2 h was obtained in etha-
nol at 808C (entry 5).
[
30]
cohol substrates. Similarly to what happens for the aromatic
alcohol hydrogenolysis reaction over Pd/SiO and Pd/TiO cata-
2
2
[
31]
lysts, the easier reduction of primary (1) versus secondary al-
cohol substrates (9 or 11) suggests that, for the unhindered BA
(
1), associative and dissociative hydrogenation mechanisms
Substitution of the phenyl group in the para position by
electron-withdrawing group gave the corresponding hydrocar-
bons in excellent yields when carrying out the reaction in etha-
nol at 808C over 0.1 mol% Pd catalyst. Methyl 4-formylben-
zoate (20) was quantitatively reduced (99% yield) within 2 h
(entry 6). However, under the same conditions, 4-carboxyben-
zaldehyde (22) was completely converted within 2 h in a mix-
ture of p-toluic acid (24) and 4-(hydroxymethyl)benzoic acid
(23) [Eq. (1)], and in benzyl ethyl ether intermediate generated
by the presence of EtOH in 24, 65, and 11 % yields, respectively
(entry 7). When the reaction temperature was increased to
908C (entry 8) and further to 1008C (entry 9), the yield in p-
toluic acid after 3 h increased to 81% and to 100%, respective-
ly [Eq. (1)]:
occurs, whereas the reduction mechanism for secondary alco-
hols could take place through a radical substitution of the
carbon-bonded OH group by a hydrogen atom from the metal
surface.
Similar results were obtained when tertiary benzylic alcohols
were tested in the hydrogenolysis reaction. Substrate 2-
phenyl-2 propanol (15) dissolved in methanol at 708C was re-
duced quantitatively to isopropylbenzene (16) within 2 h, and
also in ethanol at 808C (entries 15, 16), whereas the reduction
of the more hindered 1,1-diphenylethanol (17) in ethanol at
8
08C gave the corresponding hydrocarbon 1,1-diphenylethane
0
(
(
18) in 40% yield within 5 h over 0.1 mol% Pd of SiliaCat Pd
entry 17), and in 87% yield when doubling the catalytic
amount to 0.2 mol% (entry 18). To avoid the generation of the
,1-diphenyl-1-ethoxyethane intermediate identified by GC-MS,
the reaction was run in tetrahydrofuran at 808C. In this solvent
,1-diphenylethane (18) was generated only in 10% yield
1
1
within 5 h (entry 19). Results for bulky tertiary alcohol substrate
indicate that the reactivity once again strongly depends on the
aromatic alcohol structure.
2.5. Hydrogenolysis of Carbonyl Compounds
We next studied the heterogeneously catalyzed C=O cleavage
4-Carboxybenzaldehyde (22) is abundantly obtained in in-
reaction of both aryl-substituted aldehydes and ketones over
dustry as a by-product in the terephthalic acid synthesis,
0
[33]
SiliaCat Pd (Table 7). The catalytic hydrogenolysis of aromatic
mainly as a precursor to polyethylene terephthalate
pro-
carbonyls to methylene compounds occurs smoothly via the
duced in very large amounts in the ninth largest industrial
[32]
[34]
formation of the intermediate BA. Once again, the 500 mL
chemical process. Until now, the main upgrading step in re-
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0
[a]
Table 7. Catalytic hydrogenolysis of aldehyde and ketones over SiliaCat Pd .
Entry
Carbonyl
Solvent
T
[
Hydrocarbon
product
t
[h]
C=O
conversion [%]
Hydrocarbon
yield [%]
Alcohol
yield [%]
Isolated
yield [%]
o
[b]
[b]
C]
1
3
5
1
2
3
1
2
3
1
2
3
1
2
97
100
100
85
100
100
100
100
100
97
27
66
96
55
76
97
86
88
91
45
79
92
92
98
36
28
0
45
24
0
8
2
0
27
4
[
[
c]
1
MeOH
THF
70
70
–
d]
2
–
–
95
–
[
[
[
d]
c]
c]
3
4
5
heptanes
EtOH
70
70
80
–
100
100
100
100
0
0
0
EtOH
–
96
1
2
100
100
85
99
0
0
–
95
6
EtOH
80
1
2
2
3
2
3
96
100
100
100
100
100
17
24
60
81
95
100
67
65
30
11
5
[
[
f]
f]
7
8
80
90
–
–
[
g]
EtOH
EtOH
[f]
9
100
–
99
0
2
4
2
4
91
100
99
30
34
48
62
25
28
11
3
[
f]
1
0
80
80
–
–
[
f]
11
100
1
3
5
1
3
5
1
3
5
1
2
3
100
100
100
100
100
100
72
100
100
100
100
100
47
82
96
37
90
92
34
76
91
69
86
98
41
10
0
63
10
5
33
24
9
31
14
0
[
g]
1
2
MeOH
THF
70
70
–
–
1
3
1
1
4
5
heptanes
EtOH
70
80
–
[f]
–
–
96
1
3
98
100
84
98
0
0
–
96
16
17
18
EtOH
EtOH
EtOH
80
80
80
0
1
.5
.0
99
100
70
75
0
0
–
1
2
97
100
97
99
0
0
–
95
[
2
a] Experimental conditions: 3 bar H , T=70–1008C. [b] Evaluated by GC-MS analysis. [c] Intermediates observed: benzyl methyl ether, benzaldehyde di-
methyl acetal, and benzyl ether. [d] Presence of ethylcyclohexane. [g] 4-Carboxybenzaldehyde ethanol solution (1.0m). [f] Intermediate observed: benzyl
ethyl ether. [g] Intermediate observed: benzyl methyl ether.
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fining terephthalic acid has been the catalytic hydrogenolysis
of 4-carboxybenzaldehyde to p-toluic acid over Pd/C or Pd-Ru/
[
35]
C under 14 bar H at 250–2708C, or over Pd@SiO core–shell-
2
2
[36]
particles under 14 bar H at 160–1758C.
Pd has a much better catalytic performance, being capable of
Spherical SiliaCat
2
0
catalyzing 4-carboxybenzaldehyde to p-toluic acid effectively
at 1008C under 3 bar H in five consecutive cycles without a
2
decrease in catalytic activity, which translates into a significant
decrease in the production cost, as well as in the industrial
hazard reduction (Figure S1 in the Supporting Information).
Finally, entries 10 and 11 in Table 7 show that substitution of
the phenyl group in the para position by an electron-donating
group gave the corresponding hydrocarbon in lower yield.
Also, the catalytic hydrogenolysis of acetophenone (27) in
different solvents (entries 12–15) requires longer reaction times
when compared to the hydrogenolysis of BA (Table 4), of a-
MBA (Table 6, entries 10 and 11), and of benzaldehyde (Table 7,
entries 1–6). In methanol, THF, or heptanes at 708C under
0
Figure 8. Reusability of SiliaCat Pd in the hydrogenolysis of BA.
3
9
bar H2 pressure, the conversion to ethylbenzene (10) was
0–96% within 5 h (entries 12–14), whereas in EtOH at 808C
the conversion was quantitative within 3 h (entry 15). Substitu-
tion of the phenyl group in the para position by electron-do-
nating groups gave the corresponding hydrocarbon in higher
yield within 2 h, except for 4’-hydroxyacetophenone (30) when
only 75% yield was obtained (entries 16–18). When MeOH or
EtOH were used as solvents, the GC-MS analysis invariably
showed the presence of benzyl alkyl ether, and benzaldehyde
dialkyl acetal in the reaction mixture eventually converted in
the hydrocarbon final product.
0
Figure 9. TEM images of the SiliaCat Pd NP organosilica before (left) and
after the eighth reaction run (right).
2.6. Catalyst Stability
The reusability of the catalyst was evaluated in several consec-
utive multigram hydrogenolysis reactions of 375 mmol of BA
0
The amounts of Pd and Si leached from the SiliaCat Pd
(
40.55 g) dissolved in 250 mL MeOH solution (1.5m) over
.1 mol% Pd SiliaCat Pd under 3 bar hydrogen pressure at
08C. In each consecutive reaction, once the maximum conver-
during catalysis in the isolated crude product are presented in
Table S1 in the Supporting Information, which were obtained
under different conditions after filtration of the catalyst
0
0
7
[
37]
sion of the BA to toluene was achieved, the catalyst was col-
lected by filtration, extensively washed with THF (”3 during 10
minutes), dried at room temperature, and reused in a subse-
quent cycle.
through ICP-OES,
and are extremely low. In detail, the
amount of leached Pd never exceeds 3 ppm, and neither does
that of Si with the exception (for the latter metal only) of 1,1-
diphenylethanol (entry 7), and for ketones 4’-methoxyaceto-
phenone (entry 12) and 4’-methylacetophenone (entry 14),
which are somehow able to extract the metal from the organo-
silica surface.
0
Figure 8 shows that the SiliaCat Pd heterogeneous catalyst
was reused in eight consecutive cycles. In the first four consec-
utive tests, complete conversion was obtained in 2 h, after
which the selectivity to toluene after 2 h decreased, now re-
quiring 3 h to reach very high selectivity values originally ob-
tained in 2 h from the 5th through the 8th reaction run. We as-
cribe this slight decrease in activity to weak agglomeration of
palladium NPs from 2.1 nm to average 6 nm after eight con-
secutive experiments, as determined by the TEM image
0
2
.7. SilliaCat Pd versus Different Heterogeneous Pd
Catalysts
The optimal reaction conditions developed for the hydrogenol-
0
ysis reaction of BA over SiliaCat Pd (0.1 mol% Pd, 708C, 3 bar
(
Figure 9).
The spherical SiliaCat Pd catalyst, indeed, is stable and
H on a 375 mmol scale in the 500 mL reactor) were used to
2
0
compare the catalytic activity of the latter material with that of
different commercial solid palladium catalysts including Pd/C,
ROYER catalyst, Pd Escat 1351, as well as with an organically
robust, as shown by the SEM images (Figure S2 in the Support-
ing Information) and the N adsorption isotherms (Figure S3 in
2
0
the Supporting Information) before the 1st and after the 8th
reaction run.
modified aluminosilicate (2.1 wt% Pd /MeSiO -Al O ,) made by
1.5
2
3
our group (unpublished results). The textural properties of the
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Table 8. Textural properties of different palladium heterogeneous catalysts.
Entry
Catalyst
Pd loading
wt%]
Particle
size [mm]
BET surface
area [m g
Total pore
volume [mLg ]
Average pore
size [nm]
2
À1
À1
[
]
0
1
2
3
4
5
Pd /MeSiO1.5
2.5
2.1
3.0
5.0
5.0
d
d
d
d
d
d
d
d
d
d
50 =100
90/10 =2
50 =100
90/10 =2
50 =111
90/10 =18
50 =43
402
613
234
144
596
0.80
0.70
0.2
8.0
4.6
4.7
27
0
SiliaCat Pd
0
Pd /MeSiO1.5-Al
2
O
3
0
Pd /polyethylenimine/SiO
2
ROYER Pd catalyst
Pd/SiO
Pd Escat 1351
Pd/C
2
0.97
0.61
TM
90/10 =6
50 =18
4.1
90/10 =34
heterogeneous catalyst used in this study are presented in
Table 8.
0
Invariably (Figure 10), the SiliaCat Pd showed much higher
catalytic activity with complete conversion of BA to toluene
after 1 h only over 0.1 mol% Pd and after 2 h over 0.05 mol%
Pd (Table 9, entries 1 and 2).
0
The organically modified aluminosilicate 2.1 wt% Pd /
MeSiO -Al O was the second best catalyst, with complete BA
1.5
2
3
conversion after 3 h (entry 3 in Table 9).
Under the conditions specified above, the only commercial
catalyst showing reasonable activity was the 5 wt% Pd/C cata-
lyst, affording full conversion of BA to toluene after 5 h. Both
0
Pd/SiO2 (entry 4) and Pd /polyethylenimine/SiO (entry 5)
2
showed modest activity with 11 and 20% yield after 3 h,
respectively.
We ascribe the high catalytic activity of the SiliaCat Pd cata-
lyst in the BA hydrogenolysis reaction to its spherical morphol-
ogy, as well as to the hydrophobic nature of the spherical or-
ganosilica material that prevents the accumulation of water
formed during the reaction. Excellent robustness and mechani-
cal stability add to the well-known chemical stability of methyl-
silica, with the spherical catalyst contributing excellent stability
to the catalytic activity.
Figure 10. Benzyl alcohol hydrogenolysis to toluene mediated by different
palladium solid catalysts.
3. Conclusions
0
The hybrid sol–gel SiliaCat Pd material with a spherical mor-
phology is a highly selective mediator for the hydrogenolysis
[a]
Table 9. Hydrogenolysis reaction of BA over different heterogeneous catalysts.
[
b]
[c]
[d]
Entry
Catalyst
Pd reaction loading
Pd [mol%]
t
[h]
Yield
[%]
Isolated
yield [%]
Leaching (mg/kg)
Catalyst [wt%]
Pd
Si
0
1
2
2.5 wt% Pd /MeSiO1.5
SiliaCat Pd
0.1
4
0.5
1
0.5
1
2
2
3
1
3
1
95
100
60
72
100
79
99
4
11
7
20
55
92
98
–
95
–
1.07
1.11
0.88
0
0.05
2
1.39
96
3
4
5
6
2.1 wt%
0.1
0.1
0.1
0.1
5
–
96
–
1.75
1.49
1.63
1.47
0.14
1.85
1.07
0
0
Pd /MeSiO1.5-Al
5 wt% Pd/SiO
Pd Escat 1351
2
O
3
2
2
TM
3 wt%
0
3.3
2
–
Pd /polyethylenimine/SiO
5 wt% Pd/C
2
ROYER Pd catalyst
3
1
3
5
–
–
92
[
a] Experimental conditions: 0.1 mol% Pd, 708C, 3 bar H
2
. [b] Pd/BA mass and molar ratio. [c] Yield evaluated by GC-MS. [d] Leaching of Pd and Si by
ICP-OES.
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Conflict of Interest
2
The authors declare no conflict of interest.
Keywords: hydrogenolysis
palladium · sol–gel · sphericity
·
organically modified silica
·
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Received: November 17, 2017
Version of record online December 11, 2017
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