Unconventional Pd@Sulfonated Silica Monoliths Catalysts for Selective Partial Hydrogenation Reactions under Continuous Flow

Article abstract of DOI:10.1002/cctc.201700381

Doubly functionalized, hierarchical-porosity silica monoliths were synthesized by postgrafting of sulfonic groups and in situ growth of Pd nanoparticles in that order. PdNP of 3.1 nm size located in the mesopores of the material showed to be evenly distributed within 4.6 % wt Pd monoliths. The system was explored in the continuous-flow, catalytic partial hydrogenation reaction of 3-halogeno-nitrobenzenes and 3-hexyn-1-ol in the liquid phase, showing remarkable conversion, selectivity, and resistance under very mild conditions.

Full text of DOI:10.1002/cctc.201700381

Accepted Article
Title: Unconventional Pd@sulfonated silica monoliths catalysts for
selective partial hydrogenation reactions under continuous flow
Authors: Francesca Liguori, Pierluigi Barbaro, Bilel Said, Anne
Galarneau, Vladimiro Dal Santo, Elisa Passaglia, and
Alessandro Feis
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10.1002/cctc.201700381
ChemCatChem
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Unconventional Pd@sulfonated silica monoliths catalysts for
selective partial hydrogenation reactions under continuous flow
Francesca Liguori,[a] Pierluigi Barbaro,*[a] Bilel Said,[b] Anne Galarneau,[b] Vladimiro Dal Santo,[c] Elisa
Passaglia,[d] and Alessandro Feis[e]
Abstract: Doubly functionalized, hierarchical porosity silica monoliths were synthesized by post-grafting of sulfonic groups and in-situ
growth of Pd nanoparticles in the order. PdNP of 3.1 nm size located in the mesopores of the material showed to be evenly distributed
within 4.6% wt Pd monoliths. The system was explored in the continuous flow, catalytic partial hydrogenation reaction of 3-halogeno-
nitrobenzenes and 3-hexyn-1-ol in the liquid phase, showing remarkable conversion, selectivity and resistance under very mild conditions.
Introduction
volume and porosity changes with swelling, which may result in
non-uniform permeability, variation of residence times, back
pressure evolution at high flow rates.[8] In order to avoid these
problems, different types of inorganic monoliths have been
synthesized featuring better resistance and rigidity.[9] However,
despite the above favorable features, use of hierarchically
porous monoliths in catalysis has been scarcely investigated so
far. One reason for this is that specific, sometimes troublesome,
functionalization of the monolithic support is required for use in
catalysis.[ 10 ] Indeed, few inorganic monoliths with grafted
functionalities have been described, either based on immobilized
noble metal nanoparticles (MNP) for metal-type catalysis
(Pd@TiO2,[ 11 ] Pd@SiO2,[ 12 ] Ag@SiO2[ 13 ]), or acidic/basic
Continuous flow fine chemical synthesis by mean of supported
heterogeneous catalysts is receiving increasing interest in large
scale applications, due to the considerable safety, environmental
and economic benefits compared to conventional batch
operations.[ 1 ] However, the need to overcome the common
drawbacks associated with the mesoporous catalysts usually
employed to this purpose (including mass transfer limitations,
pore clogging, support degradation, active sites accessibility and
deactivation, lack of reproducibility) requires a constant effort to
develop innovative materials with improved properties.[2]
Unconventional monoliths[
featuring an isotropic,
3
]
functionalities (-SO3H, -NH2@SiO2,[14] zirconium phosphate[15]
for organic-type catalysis.
)
hierarchically porous structure of interconnected flow-through
macropores (1 - 30 m) and diffusive mesopores (6 - 20 nm)[4]
have shown a unique hydrodynamic behavior in the liquid-phase,
which is able to address the need of both efficient processing
(within small pores) and effective mass transport (by
macropores).[5] Monolithic reactors may also circumvent most
problems typical of packed-bed systems, such as pressure drop,
low contacting efficiency, broad distribution of residence times,
formation of hot-spots, which result in uncontrolled fluid
dynamics, hence in low catalyst activity and selectivity.[ 6 ]
Polymeric materials were the first to demonstrate the benefits of
monolithic supports in the continuous flow, catalytic production
of fine-chemicals.[ 7 ] However, polymeric monoliths may be
affected by a number of limitations in relation to thermal,
mechanical and chemical stability, shrinking phenomena,
Silica and titania monoliths featured by a hierarchical macro-
and mesoporosity have proven their highly efficient mass
transport properties due to the very good permeability, [5] which
have led to successful catalytic application under continuous
[11,12]
flow,[14,15] particularly for selective hydrogenations.
The
hydrogenation of cyclohexene, cyclooctadiene and 3-hexyn-1-ol
have been performed on such monoliths after immobilization of
PdNP through classical impregnation methods.[11,12a] The
catalytic performance of these monoliths have been compared
as a single piece or packed-bed (ground monolith 60 - 120 m)
in continuous flow and in batch arrangements. The better access
to the PdNP active sites was clearly demonstrated for the
monolith in one piece in comparison to packed-bed and batch
reactors, for instance in the hydrogenation of cyclohexene,
resulting in TOF values of 1673, 1131 and 932 h-1, and space-
time-yields of 4.02, 0.95 and 0.01 kg L-1 h-1, respectively.[11]
Much lower activity values were observed under continuous flow
using PdNP onto conventional mesoporous supports. In the
partial hydrogenation reaction of 3-hexyn-1-ol, selectivity using
the monolithic SiO2 reactor was higher both than that of
conventional mesoporous silica under continuous flow
conditions and the corresponding batch catalyst.[12a] Irrespective
of the support material, the monolithic catalysts could be used
for more than 70 hours continuous stream with no significant
deactivation and very low pressure drops. Similar favorable
comparisons with respect to the corresponding batch and
mesoporous catalysts were observed for metal-free
transesterification reaction of triacetine using a sulfonated silica
monolith.[14b] All the above demonstrate the advantages in terms
of mass transport and reactivity of these kinds of monoliths
prepared by spinodal decomposition with respect of conventional
[a]
Dr. F. Liguori, Dr. P. Barbaro
Consiglio Nazionale delle Ricerche, Istituto di Chimica dei Composti
Organo Metallici,
Via Madonna del Piano 10, 50019 Sesto Fiorentino, Firenze (Italy)
E-mail: pierluigi.barbaro@iccom.cnr.it
[b]
Dr. B. Said, Dr. A. Galarneau
Institut Charles Gerhardt Montpellier, UMR 5253 CNRS-Université
de Montpellier-ENSCM, ENSCM
8 rue de l’Ecole Normale, 34296, Montpellier Cedex 05 (France)
Dr. V. Dal Santo
Consiglio Nazionale delle Ricerche, Istituto di Scienze e Tecnologie
Molecolari, Via Golgi 19, 20133 Milano (Italy)
Dr. E. Passaglia
Consiglio Nazionale delle Ricerche, Istituto di Chimica dei Composti
Organo Metallici, Via Moruzzi 1, 56124 Pisa (Italy)
Dr. A. Feis
[c]
[d]
[e]
Department of Chemistry, University of Florence, Via della
Lastruccia 3-13, 50019, Sesto Fiorentino, Firenze (Italy)
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/cctc.xxxx
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10.1002/cctc.201700381
ChemCatChem
FULL PAPER
2 Pd2+
SO3H
- 1
SO2Cl
SO3H
SO3
/
Pd
Si
Si
O
Si
O
Si
O
H2
H2O
Pd(NO3)2
CSPTMS
OH OH OH
O
O
O
O
O
O
O
O
O
toluene,
SiO2
SiO2
SiO2
MonoSil-ArSO3
SiO2
SiO2
reflux 48h
Pd@MonoSil-ArSO3
MonoSil
Scheme 1. Synthetic procedure for Pd-functionalized, sulfonated silica monoliths.
5
Si
4
3
2
1
Results and Discussion
0
1.0
S
Synthesis and characterization of the catalysts
0.5
0.0
4
Hierarchical macro- / mesoporous silica monoliths were first
synthesized using the combination of sol-gel process and
spinodal decomposition accordingly to our previous works.[5,12a]
The resulting monoliths (MonoSil) were then sulfonated by
grafting of 2-(4-phenylsulfonic)ethyl silane (MonoSil-ArSO3) and
1,3-propylsulfonic (MonoSil-PrSO3) groups. A sketch of the
synthetic procedure is reported in Scheme 1 for MonoSil-ArSO3.
Due to the harmful properties of the pure silylation reagent 2-(4-
chlorosulfonylphenyl) ethyltrimethoxy silane (CSPTMS),
MonoSil-ArSO3 was obtained using an alternative method to the
literature reported flow protocol.[14b]
Pd
3
2
1
0
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
depth (mm)
Figure 1. ESEM image (left) and line (red) EDS maps of an equatorial section
of Pd@MonoSil-ArSO3. From the top: Si (K1), S (K1) Pd (L1), (20 keV,
100 points, 43 magnifications).
systems. Since sulfonic resins are known to enable the easy
incorporation of metal ions by ion exchange and to stabilize
metal particles of small size for use in catalysis through
electrostatic stabilization,[ 16 ] we decided to explore the same
approach to the immobilization of Pd particles onto hierarchically
porous silica monolith, and to examine the influence of these
functions on selective hydrogenation reactions.
Thus, the one-pot, batch treatment of MonoSil with a
CSPTMS solution in refluxing toluene, followed by hydrolysis
of the chlorosulfonyl group, afforded MonoSil-ArSO3 monolith
showing features analogous to those of the previously reported
material. ICP-OES analysis indicated the bulk sulfur content to
be 3.0% wt (corresponding to 0.93 mmol g-1, 1.07 molecule
nm-2, 0.28 mmol mL-1), in line with the literature value (0.95
mmol g-1).[14b] FT-IR, FT-Raman (reported in the Supporting
Information, Figure S8) and TGA (Figure S9) experiments
showed all grafted sulfur was in the -SO3H form. The FT-
Raman spectrum of MonoSil-ArSO3 closely resembles that of
the model compound 4-toluenesulfonic acid.[21] In particular,
the bands at 1126 and 1176 cm-1 can be attributed to modes
involving the -SO3 group.[22] TGA analysis showed a weight
loss (15.9 %) between 410 and 800°C, due to the thermal
decomposition of the sulfonic acid group -(CH2)2-Ph-SO3H
(desorption peak at 574°C),[23] whose sulfur loading estimate
(0.86 mmol g-1) is consistent with the value obtained from ICP-
OES. ESEM analysis (Figure 1) showed the macroscopic
structure of the silica monolith was not altered by sulfonation,
while EDS line maps, both transversal and longitudinal,
demonstrated the sulfonic groups to be evenly distributed
within the material. EDS transversal line maps of an equatorial
section of Pd@MonoSil-ArSO3 monolith (vide infra), in which
silica and sulfur emissions are reported for comparison, are
shown in Figure 1. Point-to-point EDS analyses recorded at
different monolith depth confirmed this result, while providing
sulfur content values comparable to that of ICP-OES within the
experimental errors. The mesoporosity of MonoSil-ArSO3 was
Herein, we report for the first time the dual functionalization of
silica monoliths synthesized by a combination of sol-gel process
and spinodal decomposition and featuring bimodal hierarchical
porosity (MonoSil, macropore size 5 m, skeleton thickness of 3
m, mesopore size 6.6 nm) with both sulfonic groups and
supported PdNP, and the engineering of the resulting material
into continuous flow, catalytic hydrogenation mesoreactors.[17]
The performance of the catalyst was evaluated in highly
significant reactions for the process industry, i.e. the selective,
partial hydrogenation of halogeno-nitroarenes and alkynes,
particularly 1-chloro-3-nitrobenzene and 3-hexyn-1-ol. The
stereo- and chemo-selective hydrogenation of alkynes in the
presence of other functional groups is of fundamental
importance in the manufacture of several food additives, flavors
and fragrances.[ 18 ] Partial hydrogenation reactions are also
crucial in the polymerization industry to achieve the complete
elimination of alkynes and dienes from alkene feedstocks.[19] The
catalytic hydrogenation of halogeno-substituted nitroarenes to
the corresponding amines is a reaction of much practical interest
in the polymer and pharmaceutical sectors.[20]
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10.1002/cctc.201700381
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analyzed by nitrogen sorption / desorption isotherms at 77 K
(shown in Figure S7). Representative data are reported in
Table 1. Compared to the native MonoSil, the phenylsulfonic
derivative showed a decrease of surface area (322 vs. 521 m2
g-1), pore volume (0.64 vs. 1.07 cm3 g-1) and pore diameter
(6.0 vs. 6.6 nm), in accordance with what previously reported
and expected for the grafting of molecules inside silica
mesopores.[23b, 24 ] The covalent functionalization of MonoSil
was ascertained by 13C CP and 29Si MAS NMR spectroscopy,
after grinding of the sample. The 13C CP-MAS NMR spectrum
of MonoSil-ArSO3 (Supporting Information, Figure S10)
showed signals at 13.4 and 29.2 ppm corresponding to
methylene CH2Si and CH2Ph carbons, respectively. Signals at
128.3, 141.5 and 151.9 ppm are attributed to aromatic CH, C-
CH2 and C-SO3 nuclei, respectively.[23b,25] The 29Si MAS NMR
spectrum was dominated by a broad resonance centered
around -108 ppm, attributable to the overlap of peaks Q4
[(SiO)4Si] and Q3 [(SiO)3SiOH] condensed silica network
(expected at -110 and -100 ppm, respectively), plus a weak
signal at ca. -60 ppm due to functionalized silica species T2
[(SiO)2Si(OH)-C][14b,26] The concentration of Lewis (LAS) and
Brønsted (BAS) solid acid sites onto MonoSil-ArSO3 was
further investigated by pyridine desorption FT-IR spectroscopy
at various temperatures (i.e. rt, 50, 100, 150, 200 and 250°C)
(full set of spectra in Figure S11).[27] Integration of peaks at
1546 and 1446 cm-1,[ 28 ] revealed a high concentration of
Brønsted acid sites (0.149 mmol g-1 @ 150 °C)[ 29 ] and a
Figure 2. Optical images of cladded a) MonoSil-ArSO3 and b) Pd@MonoSil-
ArSO3 monoliths.
The propylsulfonic monolithic derivative MonoSil-PrSO3 was
prepared using a two-step, grafting-oxidation procedure using
mercaptopropyltrimethoxy silane as anchoring agent and H2O2
as oxidant. However, besides a lower sulfur content compared
to MonoSil-ArSO3, ICP-OES, TGA (TGA and DTG in Supporting
Information, Figure S3), FT-IR and NMR analyses indicated the
partial conversion of -SH groups to -SO3H and a significant loss
of sulfur upon oxidation,[30] in line with some literature findings.[31]
Therefore, we decided not to investigate further the use of
MonoSil-PrSO3 in catalysis.
After sulfonation of the silica monolith, Pd nanoparticles were
grown in-situ onto MonoSil-ArSO3 by a simple two-steps, one-
pot flow procedure (Scheme 1). Thus, after allowing a water
solution of Pd(NO3)2 to flow cyclically through a Teflon-cladded
monolith, the Pd(II)-supported material was reduced under
concurrent flows of 0.5 mL min-1 methanol and 2 mL min-1 H2, 1
bar pressure and room temperature. Experimental details are
provided in the Supporting Information. The reduction of the
metal was evidenced by a color change of the monolith from
brownish to black (Figure 2). Selection of palladium nitrate was
motivated by the combination of several key factors: i) the need
for a cationic palladium precursor, owing to the ion-exchange
strategy chosen for the first step of synthesis; ii) high solubility
and dissociation in environmentally friendly water solvent; iii)
higher reduction potential of "naked" Pd2+ ions,[32] which allows
for the quantitative metal reduction by H2 under milder conditions
compared to other precursors, e.g. [Pd(NH3)4]2+ or Pd(CH3CO2)2
which requires high temperatures or NaBH4 treatment;[33,34] iv)
lower cost and better stability compared to [Pd(CH3CN)4]2+.[35] An
analogous ion-exchange / H2 reduction flow procedure, devoid of
harsh conditions (1 bar, rt), toxic reagents or elaborate
apparatuses, was previously adopted by us for the
immobilization of PdNP onto organic-type monolithic resins,
taking advantage of the palladium nitrate precursor.[36]
negligible quantity of Lewis acid sites (0.002 mmol g-1
@
150 °C), respectively (Table 2). The bare silica monolith
MonoSil showed the almost complete absence of Brønsted
and Lewis acidity.
Table 1. Mesoporosity features of monolithic materials.[a]
S
V
D
Material
[m2 g-1]
[cm3 g-1]
[nm]
MonoSil
521
322
322
1.07
0.64
0.62
6.6
6.0
6.1
[b]
MonoSil-ArSO3
[c]
Pd@MonoSil-ArSO3
[a] BET surface area (S), porous volume (V), pore diameter (D). [b] 0.93
meq g-1 SO3 sites. [c] 4.6 wt % Pd.
Table 2. Lewis (LAS) and Brønsted (BAS) acid sites concentration on silica
monoliths.[a]
[b]
LAS
BAS
Tdes
Material
MonoSil
The as-prepared Pd@MonoSil-ArSO3 monolith could be
directly used in continuous flow catalytic hydrogenation
reactions (vide infra) without disconnecting from the flow
equipment nor any further treatment, or dried under a stream of
N2 before being characterized in the solid state. ICP-OES
showed the bulk Pd loading to be 4.6% wt (0.43 mmol g-1), while
EDS linear maps, recorded both on radial and longitudinal
sections of the monolith, proved the metal to be evenly
distributed within the support (Figure 1). These results were
possible thanks to the ion-exchange properties of the sulfonated
monolith (Scheme 1). Indeed, ICP-OES analyses indicated that
all sulfonic sites were replaced with (easily reducible)[37] Pd2+
[μmol g-1]
[μmol g-1]
[°C]
0
0
100
150
100
150
0
0
MonoSil-ArSO3
80
2
229
149
[a] LAS 1446 cm-1, BAS 1546 cm-1. [b] Desorption temperature.
ions by ion-exchange (molar ratio
S / Pd ca. 2). The
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10.1002/cctc.201700381
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incorporation of Pd did not lead to any significant modification of
the macroporous silica skeleton as shown by ESEM analysis
(Figure 3, left). Compared to the metal-free monolith (Table 1),
nitrogen sorption isotherms showed a slight decrease of the
mesoporous volume (0.62 vs. 0.64 cm3 g-1) and a slight increase
of mesopore diameter (6.1 vs. 6.0 nm) (Figure 3, right), thus
indicating that the PdNP are located within the silica mesopores.
Analogous findings were previously observed for Pd onto
hierarchically porous titania and silica monoliths[11,12a] and
line with those of TEM, within the experimental errors. A
surface/bulk palladium atoms ratio of 36% could be estimated
from TEM data.[ 39 ] XPS measurements were carried out to
analyze the oxidation state of the supported Pd particles. Figure
5 shows the XPS spectrum in the Pd 3d region where the usual
palladium doublet is observed. The Pd 3d peaks were
deconvoluted into two oxidation states, namely metallic Pd(0) at
lower binding energy (Pd 3d5/2 335.0 ± 0.1 eV) and Pd(II) at
higher BE (Pd 3d5/2 337.1 ± 0.1 eV) with the latest likely due to
PdO.[ 40 ] No other palladium species were detected. The
calculated abundance of the two species was 86.7% for Pd(0)
and 13.3% for Pd(II). As it was previously reported,[ 41 ] the
presence of PdO can be safely attributed to a layer covering the
core of metallic palladium particles due to sample manipulation
in air prior of measurements, since the percentage of Pd(0) was
increased up to 91.1% after sputtering with Ar, a procedure that
removes the most superficial atoms of the nanoparticle.[ 42 ]
Pyridine desorption experiments showed Pd@MonoSil-ArSO3 to
38
]
titanate nanotubes.[
Indeed, TEM analysis showed
Pd@MonoSil-ArSO3 to contain spheroidal PdNP of average
diameter 3.1 ± 1.0 nm, with no particles larger than 6.3 nm
detected. A typical TEM image and the corresponding PdNP
size distribution are reported in Figure 4. The XRD data were in
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
have a higher density of Lewis acid sites (0.017 mmol g-1
150 °C) and a lower density of Brønsted sites (0.037 mmol g-1
@
@
150 °C), compared to the metal free MonoSil-ArSO3 monolith.
These findings are in line with those previously reported for
other solid-supported Pd0 materials, and attributed to the
residual Lewis acid contribution of PdNP[43] and to the partial
recovery of sulfonic acid sites after H2 reduction of palladium
under mild conditions,[44] respectively. Quantitative restoration of
acidic sites may require rinsing with strong mineral acid solution,
in that case.[45]
0
10 20 30 40 50
pore size (nm)
Figure 3. Pd@MonoSil-ArSO3 monolith. Left: ESEM image (secondary
electrons, 27 keV, 1600 magnifications). Right: BJH mesopore size distribution.
The immobilization of PdNP onto inorganic oxide monoliths
(SiO2, TiO2) has been previously described in the literature
through impregnation / (calcination) reduction procedures, either
using Pd(NH3)4(NO3)2, Pd(NO3)2 or Na2PdCl4 precursors.
Particularly, Pd particles of 6.5 ± 1.0 nm size were obtained by
40
30
20
10
0
impregnation
of
unsulfonated
silica
monoliths
using
Pd(NH3)4(NO3)2, followed by treatment under harsh, batch
conditions (280 °C).[12a] The amount of Pd incorporated showed to
be strongly pH-dependent in that case, with a 1.3% wt maximum
loading achieved at pH 10. Reaction of the silica surface with a
strong base was required to increase the negative charge of the
silica support, that can be justified by the point of zero charge of
silica below pH 7.5.[46] PdNP onto unfunctionalized SiO2 monoliths
were also obtained by incipient wetness impregnation using
Na2PdCl4 and 340 °C H2 reduction, resulting in Pd particles of 2 -
10 nm size.[12b] To further ascertain the efficacy of the synthetic
0
1
2
3
4 5 6 7
diameter (nm)
Figure 4. Left: TEM image of Pd@MonoSil-ArSO3. Right: PdNP size
histogram. Statistical nanoparticle size distribution analysis was carried out on
300-400 particles at 120 keV.
protocol herein described, we performed
a countercheck
1200
3d5/2
immobilization experiment using the unsulfonated MonoSil
monolith and the same experimental conditions adopted for the
preparation of Pd@MonoSil-ArSO3, i.e. Pd nitrate precursor, room
temperature, H2 1 bar flow reduction. This protocol resulted in ca.
0.3% wt Pd loading and 4.7 ± 2.1 nm PdNP (see Supporting
Information). This clearly indicates that the synthetic strategy
devised in the present work for the immobilization of PdNP onto
silica monolith, involving ion-exchange of sulfonated materials,
provides multiple benefits over previously reported methods: i)
milder reactions conditions and more friendly procedure, ii) easier
access to high Pd loading, iii) better control and higher dispersion
of PdNP since, irrespective of the precursor, size was twice
smaller than in silica monoliths without sulfonation of the support,
whereas the monoliths feature analogous textural properties
(mesopore size, surface area, mesoporous volume).
raw data
Pd (0)
Pd (0)
Pd (II)
Pd (II)
envelope
1000
3d3/2
800
600
400
200
0
346
344
342
340
338
336
334
332
binding energy (eV)
Figure 5. XPS spectrum of Pd@MonoSil-ArSO3 in the Pd 3d region.
The above findings can be attributed to the atomic level
dispersion of the Pd ions within MonoSil-ArSO3, as a consequence
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10.1002/cctc.201700381
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Scheme 2. Common reaction pathways for the hydrogenation reactions of 1-chloro-3-nitrobenzene (1) and 3-hexyn-1-ol (2).
atmospheric pressure at the outlet of the reactor. Reactions were
typically monitored for conversion, selectivity and metal leaching
over 8 h continuous time-on-stream, followed by overnight switch-
offs and restart the day after under the same conditions.
Benchmark reaction conditions were room temperature and
methanol solvent.
Both processes were investigated under a wide range of H2
and substrate solution flows and the combinations thereof. As
expected for similar systems, conversion and selectivity showed
to be dependent from the reactants flow rates. As a common
trend, under a fixed solution flow rate (i.e. for the same
residence time ), an increase of the H2 flow (i.e. of the
H2:substrate molar ratio) resulted in higher conversions (up to
100 %) and in lower selectivities, Under a fixed H2:substrate
molar ratio, a decrease of the solution flow rate (i.e. an increase
of residence time ) resulted in higher conversions and in a
selectivity decrease.
solution flow (mL min-1
)
H2 flow (mL min-1
)
0.4 0.3
0.2
0.2 0.3
0.5
l
l
l
l
l
l
90
80
70
60
50
40
100
90
80
70
60
50
40
100
90
80
70
60
50
40
sel. 1a
conv.
sel. 1a
conv.
ratio 4.3

28 s
15 25 35 45
2
4
6
8
70 80 90 100
conversion (%)
 (s)
H2 : 1 molar ratio
Figure 6. Continuous flow hydrogenation of 1 to 1a over Pd@MonoSil-ArSO3
catalyst (4.6% wt Pd, rt, methanol 0.01 M, reactor volume 141 L). Left:
conversion and selectivity as a function of residence time and solution flow
rate under fixed H2:1 molar ratio 4.3. Centre: conversion and selectivity as a
function of H2:1 molar ratio and H2 flow rate under fixed residence time  28 s
(solution flow rate 0.30 mL min-1). Right: selectivity / conversion diagram at: 
fixed H2:1 ratio = 4.3 and residence time 21 - 42 s,  fixed residence time 28 s
and H2:1 ratio range 2.8 - 7.3 (H2 flow rate 0.2 - 0.5 mL min-1).
Conversion of 1-chloro-3-nitrobenzene (1) to 3-
chloroaniline (1a): The hydrogenation of 1 was examined in the
H2 flow range 0.2 - 0.5 mL min-1 (H2:1 molar ratio = 2.8 - 7.3)
and in the solution flow range 0.4 - 0.2 mL min-1 ( = 21 - 42 s).
Under these conditions, the hydrogen pressure measured at the
reactor inlet was 1.1 - 1.2 bar, hence the pressure drop
generated by monolithic reactor was lower than 0.2 bar in any
case. The H2 pressure at the reactor inlet was not significantly
affected by moderate changes in the H2 flow rate, that can be
attributed to the scarce flow resistance of the monolithic support
owing the open-cell macroporous structure. Figure 6 left shows
the conversion and selectivity change upon variation of the
residence time under fixed H2:substrate ratio 4.3. The
conversion increased from 79.5% to 95.9% and the selectivity to
1a decreased from 82.6% to 70.1%, on passing from 0.4 to 0.2
mL min-1 solution flow. A conversion enhancement from 88.3%
to 98.0% and a selectivity drop from 78.4% to 64.1% was
observed on passing from 0.2 to 0.5 mL min-1 H2 flow at fixed 28
s residence time (Figure 6 center). A reproducible selectivity /
conversion diagram could be obtained on these bases, as
reported in Figure 6 right. A more complete picture of 1a yield as
a function of and H2 : substrate ratio is shown in the diagram of
Figure 7. The reaction conditions (28 s, H2:1 ratio = 4.3)
resulting in the best compromise results between conversion
(95%) and selectivity (76%), corresponding to a 1a yield of 72%,
are reported in Table 3, in which productivities are express both
of the ion-exchange procedure,[47] and to the smooth reduction
conditions available, thanks to the use of Pd(NO3)2 precursor.
The present synthetic approach has been widely used for the
immobilization of metals onto organic polymers,[16] and it was
successfully applied here for the first time to the incorporation of
MNP onto inorganic monoliths.
Continuous flow catalytic reactions
Owing to the peculiar hydrodynamic properties of the
hierarchically porous monolithic support and to the high Pd loading,
Pd@MonoSil-ArSO3 was specifically designed for application to
continuous flow processes requiring an efficient interaction, but a
short contact time, with a dispersed metal active phase to achieve
the best selectivity at the highest conversions.[ 48 ] Suitable
examples to this regard are partial hydrogenation reactions. [49] We
thus investigated the catalytic performance of Pd@MonoSil-ArSO3
in the selective hydrogenation of 1-chloro-3-nitrobenzene and 3-
hexyn-1-ol under continuous flow conditions, as test reactions
(Scheme 2). Catalytic experiments were carried out by connecting
the cladded monolith to a home-made reactor system allowing for
the controlled, concurrent flows of substrate solution and H2 gas
(sketch of reactor equipment in Supporting Information, Scheme
S1). The reaction start time was taken as the attainment of steady
state conditions, ca. 1h. Hydrogen gas was released at
as molprod gPd h-1 and space-time-yield (STY, kgprod Lreactor h-
1).[50] Irrespective of the reaction conditions, the only by-product
-1
-1
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Table 3. Selected data for continuous flow hydrogenation reactions by Pd@MonoSil-ArSO3 monolithic catalyst [a]
Reaction conditions
Conv.[e]
[%]
Selectivity[f]
[%]
Productivity[g,h]
[mol gPd-1 h-1]
STY[h]
[kg L-1 h-1]
Solution
H2
Substrate
H2 / sub.
Ratio[d]
Flow rate
[mL min-1]
[b]
Flow rate
[mL min-1]
Pressure[c]
[bar]
[s]
1-chloro-3-nitrobenzene (1)[i]
3-hexyn-1-ol (2)[l]
0.30
0.48
28
28
0.30
1.25
1.07
1.15
4.3
1.2
95 ± 2
94 ± 1
76 ± 1 (1a)
0.07 (1a)
0.80 (2a)
0.12 (1a)
91 ± 1 (2a+2b)
94 ± 1 (2a)
1.09 (2a+2b)
[a] Best compromise results in terms of conversion and selectivity to the partial hydrogenation product indicated. Reaction conditions: methanol 0.1 M, room
temperature, 4.6% wt Pd. Start time attainment of steady state conditions 1h. [b] Residence time (). [c] Pressure at the reactor inlet. [d] Hydrogen to substrate
molar ratio at the reactor inlet. [e] Conversion average value over 8 h time-on-stream. [f] Selectivity to the product indicated in brackets. Average value over 8 h
time-on-stream. Selectivity 2a+2b = (2a+2b)/(2a+2b+2c+2d). Selectivity to cis isomer 2a = 2a/(2a+2b). [g] Calculated on bulk Pd content. [h] Calculated on the
products indicated. [i] Solution 0.01 M, reactor volume 141 L. [l] Reactor volume 226 L.
42
reused with no significant efficiency decay, provided it was
68
69
yield of 1a (%)
stored under nitrogen or hydrogen overnight. Conversions and
selectivities values were within the specified ranges after 3 days
catalyst reuse. In no case the amount of palladium leached in
solution was above the detection limit of ICP-OES, nor catalytic
activity was shown by the recovered reaction solution, thus
ruling out the loss of active species during catalysis.[52] At the
same time, the sulfur content in the recovered catalyst was the
same as in the starting material (ICP-OES), thus indicating the
stability of the grafted phenylsulfonic groups under the
conditions of catalysis.[ 53 ] The dimension of PdNP did not
change significantly after use of Pd@MonoSil-ArSO3 in catalytic
experiments (for the statistical distribution and data, see Figure
S13), which confirms the effective stabilization of metal particles
by the grafted sulfonic groups.[37]
59
60
61
62
63
64
65
66
67
68
69
70
71
70
71
28
66
64
68
66
21
2.8
4.3
7.3
H2 : 1 molar ratio
Figure 7. Contour plot of 1a yield in the hydrogenation of 1 by Pd@MonoSil-
ArSO3 catalyst under continuous flow conditions (4.6% wt Pd, rt, methanol
0.01 M, reactor volume 141 L). Residence time () range 21 - 42 s, H2 : 1
molar ratio range 2.8 - 7.3
3-Chloroaniline is an intermediate for the manufacture of
agricultural chemicals, pigments, pharmaceuticals and
polymers.[ 54 ] Current production methods base on the batch
hydrogenation of
1
by noble metal catalysts. Several
monometallic heterogeneous catalysts have been described for
the lab-scale batch hydrogenation of 1 to 1a with selectivity >
78% and conversion > 95%, however with catalyst deactivation
as common drawback. Representative data are reported in
Supporting Information, Table S2. Activity decay of chloro-
nitrobenzene hydrogenation catalysts was previously attributed
to catalyst leaching,[ 55 ] poisoning[ 56 ] or MNP aggregation.[ 57 ]
Deposition of carbonaceous materials was also observed at
medium temperatures and low H2:substrate ratios.[ 58 ] Few
examples have been reported in the liquid phase for the
continuous flow hydrogenation reaction of 1 by supported metal
catalysts. A reason for this may reside in the fact that, among
the chloro-nitrobenzene isomers, the meta one is the most
recalcitrant to (selective) hydrogenation.[ 59 ] The continuous
hydrogenation of 1 to 1a, in 99% conversion and 98% selectivity,
was described using 1.9% Au@-Al2O3 packed catalysts in
toluene at 80 °C, 10 bar H2 pressure and very high H2:1 ratio
(107).[60] However, the catalyst deactivated rapidly, since ca.
20% of its starting activity was lost after 2 h continuous stream.
We performed a series of studies aimed at comparing the
performance of: i) the bimodal macro/mesoporous monolithic
catalyst versus conventional mesoporous catalysts, and ii)
continuous flow versus batch setups. To this purpose, the
continuous flow hydrogenation of 1 was carried out using
commercial 5% wt Pd onto mesoporous silica under reaction
100
90
80
70
60
50
40
conv.
sel. 1a
1
2
3
4
5
6
7
8
time-on-stream (h)
Figure 8. Conversion and selectivity vs. time on stream in the continuous flow
hydrogenation of
1 over Pd@MonoSil-ArSO3 catalyst (4.6% wt Pd, rt,
methanol 0.01 M, reactor volume 141 L, solution 0.30 mL min-1, H2 0.30 mL
min-1). Start time: attainment of steady state conditions, ca. 1 h. Filled symbols,
conversion; empty symbols, selectivity to 1a.
observed was aniline (1c). No traces of nitrobenzene (1b),
benzene (1d) or condensation products (e.g. 3,3’-dichloro-
hydrazobenzene) were detected by GC-Ms analysis. The
catalyst showed a minor activity decay over 7 h time-on-stream
(ca. 4%), whereas conversion stabilized with ± 2% fluctuation for
longer reaction periods (Figure 8). Selectivity was pretty
constant with a mean 76 ± 1% value over 8 h time-on-stream.
Productivities of 0.07 mol1a gPd h-1 and 0.12 kg1a L-1 h-1 STY
could be calculated on these bases.[51] The catalyst could be
-1
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10.1002/cctc.201700381
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close packing of the material. The above findings can be
attributed to the specific texture of the hierarchically porous
monolith, which ensures efficient mass transport and fast
desorption of the intermediate hydrogenation product from the
metal site, thus enhancing selectivity, in contrast to purely
mesoporous materials.[11,12a] Similar results were obtained using
commercial 5% Pd onto carbon (Table 4, entry 3). In line with
previous findings,[10a,11,14b] the monolithic Pd@MonoSil-ArSO3
catalyst showed to be more efficient under flow rather than
under batch setup (crushed 60−120 μm fraction), the latter
providing similar conversion (98%) but complete over-
hydrogenation to 1c under analogous conditions (Table 4, entry
4). High stirring rates were adopted to ensure minimization of
mass transfer limitations.[14b]
Table 4. Continuous flow hydrogenation of 1-chloro-3-nitrobenzene (1) by
supported Pd catalysts.[a]
Conv.
[%]
Sel. 1a
Productivity 1a [b]
[molgPd-1 h-1]
Entry Catalyst
[%]
1
2
3
4
4.6% Pd@MonoSil-ArSO3
95[ c]
93[d]
99[e, f]
98
76
8
0.07
0
5% Pd@SiO2
5% Pd@C
0
0
4.6% Pd@MonoSil-ArSO3[g]
0
0
[a] Reaction conditions: solution 0.01 M, room temperature, 28 s, H2:1
ratio = 4.3, weight amount of catalyst corresponding to 0.022 mmol Pd.
Solution flow rate 0.3 mL min-1, H2 flow rate 0.3 mL min-1. [b] Calculated on
bulk Pd content. [c] Pressure drop 0.07 bar. [d] Pressure drop 0.57 bar. [e]
H2:1 ratio = 7.4 [f] Pressure drop 0.84 bar. [g] Batch conditions, 1 bar H2,
60-120 m sieved fraction, 500 rpm.
Conversion of 3-hexyn-1-ol (2) to cis-3-hexen-1-ol (2a):
The hydrogenation of 2 by Pd@MonoSil-ArSO3 was carried out
analogously to that of 1 using similar ranges of H2 and substrate
solution flows. The changes in conversion and selectivity
observed at different residence time, under a fixed H2:substrate
ratio of 1.2, are graphically reported in Figure 10 left, while the
corresponding selectivity / conversion diagram is shown in
Figure 10 right. The best compromise between conversion and
selectivity, resulting in the highest yield of the semi-
hydrogenation alkene product 2a, was obtained for 0.48 ( 28 s)
and 1.25 mL min-1 (H2 : 2 ratio = 1.2) solution and H2 flows,
respectively. Under these conditions, 3-hexen-1-ol (2a+2b) was
obtained with a 91 ± 1% selectivity, whose 94 ± 1% was the cis
isomer 2a, at 94 ± 1% conversion over 8h time-on-stream (Table
3). Neither significant catalyst efficiency decay nor Pd leaching
in solution was observed over that time, as well as upon 3 days
catalyst reuse. For comparative purposes, the continuous flow
hydrogenation of 2 was also carried out using commercial 5%
Pd@SiO2 mesoporous silica catalyst under analogous reaction
conditions, showing much lower optimal 2a yield (54% selectivity,
84% conversion, Table 5, entry 2) and much higher H2 pressure
drop (2.06 versus 0.15 bar). Compared to Pd@MonoSil-ArSO3,
yields of 2a were lower and pressure drops were higher in the
entire range of flow rates examined. A complete selectivity /
conversion diagram for 5% Pd@SiO2 is reported in Supporting
Information, Figure S19.
1.2

= 2.8 s, H2 : 1 = 2.1 - 7.3
80
60
40
20
0
1.0
0.8
0.6
0.4
0.2
0.0
2.8
4.3
7.3
Pd@MonoSil-ArSO3
Pd@SiO2
2.1
4.3
40
60
80
100
0
1 2 3 4
H2 flow (cm min-1)
conversion (%)
Figure 9. Continuous flow hydrogenation of 1 to 1a over 4.6% Pd@MonoSil-
ArSO3 () and 5% Pd@SiO2 () catalyst (0.022 mmol Pd, rt, methanol 0.01
M). Left: selectivity / conversion diagram at fixed residence time 28 s and H2:1
ratio range 2.1 - 7.3 (H2 flow rate 0.1 - 0.5 mL min-1). Right: pressure drops
observed as a function of the H2 linear flow rate ( 28 s).
conditions analogous to those adopted for Pd@MonoSil-
ArSO3.[30] Using the conditions of optimized yield found for
Pd@MonoSil-ArSO3 (28 s, H2:1 ratio = 4.3), the packed
Pd@SiO2 catalyst showed a lower substrate conversion (93%)
and only 8% selectivity to 1a, since the major product observed
was the over-hydrogenated aniline 1c, while the pressure drop
generated was one order magnitude higher compared to the
monolithic catalysts (0.57 vs. 0.07 bar) (Table 4, entry 2). The
reaction using Pd@SiO2 was investigated under a range of
reactants flow rates and selectivity / conversion diagrams were
obtained, as e.g. the one reported in Figure 9, left for fixed
residence time 28 s and H2:1 ratio 2.1 - 7.3 (H2 flow 0.1 - 0.5 mL
min-1). The superior performance of the monolithic catalyst was
observed in any case. The back pressure generated to the
mesoporous catalyst was higher than that of Pd@MonoSil-
ArSO3 irrespective of the flow rates. Figure 9 right shows the
pressure drop divided by the length of the reactor as a function
of the H2 linear flow rate, from which the Darcy permeability
coefficient of the porous materials can be calculated.[5b] It is
worth also noticing that, compared to the conventional
mesoporous support, the monolithic reactor shows much shorter
equilibration times: slight increases of back pressure with time-
on-stream were observed in the former case due to increasingly
solution flow (mL min-1)
ratio 1.2
0.52 0.48 0.45
l
l
l
100
90
100
90
80
70
60
50
26 s
28 s
80
sel. 2a+2b
conv.
30 s
70
24
28
32
80
90
100
conversion (%)
 (s)
Figure 10. Continuous flow hydrogenation of 2 over Pd@MonoSil-ArSO3
catalyst (4.6% wt Pd, rt, methanol 0.1 M, reactor volume 226 L). Left:
conversion and selectivity to 2a+2b as a function of residence time  and
solution flow rate under fixed H2:2 molar ratio 1.2. Right: selectivity/conversion
diagram at fixed H2:2 ratio = 1.2 and residence time 26 - 30 s.
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Table 5. Continuous flow catalytic hydrogenations of 3-hexyn-1-ol (2).[a]
Yield 2a [d]
Productivity 2a
[mol gPd-1 h-1]
STY [e]
[kg L-1 h-1]
Selectivity [%]
Conversion
[%]
Entry
Catalyst
Ref.
[b]
ene
Z/E [c]
[%]
1
4.6% Pd@MonoSil-ArSO3
5% Pd@SiO2
94
85
94
99
75
40
84
88
85
61
99
91
54
22
64
80
87
80
94
80
63
94
94
70
81
62
89
93
85
93
80
87
93
80
32
17
39
53
32
57
77
54
33
86
0.8
0.3
0.9
0.2
2.3
7.1
19.2
40.6
0.5
1.8
6.8
1.09
0.52
0.20
0.16
0.82
1.54
0.23
4.57
0.22
0.59
0.80
This work
2
This work
64
3
5% Pd@C
4
5% Pd(Pb)@CaCO3
1.2% Pd@Dowex-Li
1.2% Pd@SiO2
65
5
66
6
64
7
0.73% Pd@TiO2
64
8
0.50% Pd@TiNT
1.3% Pd@MonoSil
0.2% Pd@TiO2 monolith
0.7% Pd@MonoBor
64
9
12a
11
10
11
65
[a] Best compromise results in terms of 2a yield, i.e. between conversion and selectivity. Reaction conditions: methanol solution, room temperature. [b] Selectivity to the
alkene product 2a+2b = (2a+2b)/(2a+2b+2c+2d). [c] Selectivity to the Z-alkene product 2a = 2a/(2a+2b). [d] Yield of 2a is calculated as Conv. ·Sel.ene ·Sel.Z/E
2a/(2+2a+2b+2c+2d ), where 2 is the concentration at the reactor outlet. Data from GC analysis. [e] Calculated on the ene product 2a+2b.
=
Cis-3-hexen-1-ol (2a) is an important ingredient in the
fragrance industry (leaf alcohol).[61] It is currently manufactured
from 2 in 400 t/y and ca. 96 % selectivity at 99 % conversion by
a batch process using the Lindlar catalyst (5 % Pd on CaCO3
doped with 2-3 % Pb)[ 62 ] which, however, shows serious
drawbacks in terms of recovery, deactivation, presence of toxic
lead, need of excess of amine modifier.[63]
groups on the metal sites in Pd@MonoSil, as previously
suggested.[12a] The Pd@MonoSil-ArSO3 catalyst performed
better than Pd@MonoSil even in term of mass productivity (0.8
vs. 0.5 mol gPd h-1, entries 1, 9), that can be attributed to the
-1
smaller PdNP size. Noteworthy, Pd@MonoSil-ArSO3 was more
productive than the analogous hierarchically porous silica and
titania monolith catalysts in term of STY (1.09 vs. 0.22 and 0.59
kg L-1 h-1, entries 1, 9 and 10) that can be safely ascribed to
higher Pd loading per unit reactor volume. The productivity of
Pd@MonoSil-ArSO3 was somewhat lower than the best ever
reported for the hydrogenation of 2, namely Pd@TiNT (entry 8),
and that was attributed to the scarce flow resistance of the
tubular titanate material, which allows for high substrate flow
In the recent years, a number of catalysts have been
reported for the synthesis of 2a under continuous flow conditions
(Table 5). These include packed-bed systems based onto
commercial (Pd@C,[64] Lindlar,[65] entries 3, 4) and laboratory
66
]
catalysts (Pd@ion-exchange resins,[
Pd@SiO2,TiO2
mesoporous xerogels,[64] Pd@titanate nanotubes,[64] entries 5-8),
as well as monolithic systems (Pd@MonoSil,[12a] Pd@TiO2
monolith,[11] Pd@MonoBor,[65] entries 9-11). In terms of product
yield, the Pd@MonoSil-ArSO3 system ranks just below the best
catalyst so far reported for this reaction (Pd@MonoBor organic
monolith, entry 11) and whose selectivity is the only one
comparable to that of the industrial batch process. It must be
underlined that Pd@MonoSil-ArSO3 was more efficient than the
parent, unsulfonated Pd@MonoSil catalyst (entry 1 vs. 8).
Particularly, while the selectivity was 91% at 94% conversion
using Pd@MonoSil-ArSO3 (80% 2a yield), selectivity was 80% at
85% conversion (yield 2a 54%) and 40% at 94% conversion
(yield 2a 30%) using Pd@MonoSil.[12a] Given the same textural
properties the two catalysts, this finding can be ascribed to the
combination of multiple factors: a) the smaller size of PdNP in
the sulfonated monolith, since a positive effect of an higher
percentage of surface atoms on the selectivity of 3-hexyn-1-ol
hydrogenation by solid-supported Pd catalysts was highlighted
in the past,[37,67,68] b) the “proton-acceleration” effect on the metal
hydrogenation activity due to the Brønsted acid sites from the
support, as invoked for other solid acid-supported MNP
catalysts,[69] c) a negative influence of the excess surface silanol
rates.[65] Overall, Pd@MonoSil-ArSO3 provides
a
good
compromise between selectivity and productivity for the catalytic
hydrogenation of 2 under continuous flow, with clear operational
advantages over packed-bed reactors.
The above results point out to hydrogenation pathways
involving a fast interaction of the substrate with easily available
Pd sites onto the solid catalyst. As previously reported for
comparable heterogeneous systems,[64,65] the high substrate flow
rates and the short residence times allowed by the monolithic
catalyst may result in a fast interaction of the active sites with
the intermediate semi-hydrogenation products, that quickly leave
the metal after formation with no possibility to react further, thus
to be beneficial in terms of both selectivity and productivity of
partial hydrogenation. If the substrate would have a prolonged
adsorption with the metal sites inside the pores, the intermediate
formed cannot diffuse away fast enough and it is hydrogenated
before leaving the catalyst, resulting in a lower selectivity at the
same conversion level. In the case of the batch partial
hydrogenation reaction of alkynes, a short contact time of the
intermediate alkene has been invoked for the high selectivity
observed by 3-D egg-shell Pd@TiS catalysts.[67]
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HO
HO
H2
experimental data and selectivity/conversion diagrams for all
substrates investigated are reported in the Supporting
Information (Table S3, Figures S20-S25).
Pd@MonoSil-ArsO3
3a
3
2-Methyl-3-buten-2-ol (3a) is an important intermediate for
the synthesis of vitamins (A, E) and perfumes, that is currently
obtained via conventional Lindlar hydrogenation of 3, however
with fast catalyst deactivation.[70,71] Cis-2-buten-1,4-diol (4a) is
used in the manufacture of antibiotics, vitamins A and B6,
insecticides and pharmaceuticals. It is industrially produced from
4 in 5000 t/y using 0.5% Pd@Al2O3 batch catalysts, doped with
Cd, Zn, Bi or Te, under elevated pressures / temperatures.[72,73 ]
Use of Pd@MonoSil-ArSO3 catalyst provided 3a in good yields
with no conversion decay over 10 h continuous reaction (56%
selectivity, 78% conversion), Table 6, entry 1). Better
performance was observed in the reduction of 4, wherein the
partial hydrogenation product 4a+4b was obtained in 91%
selectivity, whose 99% the cis isomer 4a, at 72% conversion,
under very mild conditions (Table 6, entry 2). Despite higher
product purity has been reported on the lab scale for the
continuous hydrogenation of 3 and 4 in the liquid phase using
H2
OH
HO
OH
+
HO
HO
OH
Pd@MonoSil-ArsO3
4a
4b
4
NO2
NH2
NO2
H2
+
Pd@MonoSil-ArsO3
F
F
5
5a
1b
NO2
NH2
NO2
Cl
Cl
H2
+
Pd@MonoSil-ArsO3
6
6a
1b
H2
+
Pd@MonoSil-ArsO3
1% Pd25Zn75@TiO2 (capillary reactor)[74] and 0.5% Pd@Al2O3
(honeycomb reactor)[
catalysts, respectively, it is worth
75 ]
7b
7
7a
7c
noticing that STY for the pure 3a and 4a partial hydrogenation
products was ca. one order magnitude higher using
Pd@MonoSil-ArSO3. This finding can be safely attributed to both
the textural properties of the monolithic support, which allows for
very fast reactants' flow rates at low back pressures, and to the
high Pd content per unit reactor volume. Indeed, the
hydrogenation of alkynes substrates over Pd@MonoSil-ArSO3
was carried out at remarkable weight hourly space velocities,
Scheme 3. Continuous flow hydrogenations by Pd@MonoSil-ArsO3 catalyst.
Substrates investigated and main reaction products with labelling.
Table 6. Continuous flow hydrogenations by Pd@MonoSil-ArSO3 catalyst [a]
Conv.[b]
[%]
Selectivity[c]
[%]
STY [d]
[kg L-1 h-1]
Entry Substrate
ranging from 1.8 to 4.3 h-1 (WHSV = gsubstrate /gcatalyst h-1). The
-1
partial hydrogenation reaction of short chain internal alkynes
over unmodified Pd0 catalysts is usually achieved, at
comparable conversion level, with higher selectivity compared to
terminal alkynes,[76] as indeed observed for 2 and 4 versus 3.
This substrate effect was tentatively justified on the basis of the
so-called thermodynamic selectivity concept, i.e. the higher
stability of the Pd-adsorbed intermediate terminal alkene, which
favors further reaction with H-species, thereby reducing
selectivity.[77]
The liquid phase hydrogenation of 1-fluoro-3-nitrobenzene (5)
over Pd@MonoSil-ArSO3 was accomplished in excellent yields (>
99% selectivity to 5a at 99% conversion, STY 0.08 kg L-1 h-1)
under very undemanding reaction conditions, i.e. room
temperature, residence time 85 s, H2 pressure 1.15 bar (Table 6,
entry 3).To the best of our knowledge, no catalyst has been
previously reported for the partial hydrogenation of 5 under
continuous flow. Under experimental conditions comparable to
that of Table 6, the selective hydrogenation of 5 to 5a was
described using batch reactors and silica gel supported
palladium[78] or platinum[79] catalysts, with an estimated STY of
0.01 and 0.02 kg L-1 h-1 respectively. Compared to the chlorine
1
2
2-methyl-3-butyn-2-ol (3)
2-butyn-1,4-diol (4)[e]
78 ± 2
72 ± 2
56 ± 1 (3a)
0.70 (3a)
91 ± 1 (4a+4b) 0.43 (4a)
> 99 (4a)
3
4
5
1-fluoro-3-nitrobenzene (5)[f] 99 ± 1
> 99 (5a)
0.08 (5a)
0.29 (6a)
0.50 (7a)
1-chloro-2-nitrobenze (6)[f]
1,5-cyclooctadiene (7)
90 ± 1
94 ± 1
73 ± 1 (6a)
87 ± 1 (7a)[g]
85 ± 1 (7a)[h]
[a] Best compromise results in terms of yield, i.e. between conversion and
selectivity, to the partial hydrogenation product indicated. Reaction conditions:
methanol 0.1 M, room temperature. H2 pressure 1.09 - 1.18 bar. Start time
attainment of steady state conditions 1h. [b] Conversion average value over
8 h time-on-stream. [c] Selectivity to the product indicated in brackets.
Average value over
8 h time-on-stream. Selectivity 4a+4b = mol
(4a+4b)/(mol substrate converted). Selectivity to cis isomer 4a = 4a/(4a+4b).
[d] Calculated on the product indicated. [e] Solution 0.025 M. [f] Solution
0.01 M. [g] Hydrogenation selectivity = 7a / (7a + 7c). [h] Overall selectivity =
7a / (7a + 7b + 7c).
Catalyst and substrate scope. Having investigated in detail
the catalytic performance of Pd@MonoSil-ArSO3 in the liquid
phase continuous flow partial hydrogenation reaction of probe
substrates, including comparison with conventional systems, we
examined the hydrogenation of other alkynes and nitro arene
substrates of industrial interest, under a broad range of solution
and H2 flow rates and benchmark reaction conditions (Scheme
3). Best compromise results between conversion and selectivity
are reported in Table 6, together with the corresponding STY
values of the desired partial hydrogenation products. Complete
analogue 1, the nitrobenzene
5 was more resistant to
hydrogenation using Pd@MonoSil-ArSO3 catalyst. In fact, under
the same flow conditions, a conversion of 45% was observed for 5,
against 95% of 1, resulting in a productivity of 0.04 mol gPd-1 h-1 for
5a and 0.07 mol gPd h-1 for 1a, respectively (Supporting
-1
Information, Table S3).
Similarly, Pd@MonoSil-ArSO3 catalyst was less active in the
hydrogenation of 1-chloro-2-nitrobenze (6) than in the
hydrogenation of the meta isomer 1. Under the same reactants'
flow rates, 6a was obtained in ca. 30% lower conversion (62%)
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10.1002/cctc.201700381
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FULL PAPER
and productivity (0.04 mol gPd h-1) compared to 1 (Table S3).
The catalyst activity showed to decrease in the series 1 > 6 > 5,
that can be attributed to deactivating effect of the substituents on
the benzene ring. Nonetheless, the reaction conditions could be
optimized ( 17 s, H2:6 ratio 9) so as to achieve a 73% 6a
selectivity at 90% 6 conversion, with a remarkable 6a STY of
0.29 kg L-1 h-1 (Table 6, entry 4). As above described for 1, the
liquid phase continuous flow hydrogenation of 6 was previously
reported in 99% conversion and 99% selectivity using 1.9%
Au@-Al2O3 packed catalysts, however under harsher conditions
compared to Pd@MonoSil-ArSO3 catalyst (80 °C, 10 bar H2,
H2:6 ratio 107, catalyst regeneration at 400 °C required).[60] The
batch hydrogenation of 6 by Pd catalysts was previously
reported using Pd@SiliaCat[79] and 5% Pd@C in scCO2,[ 80 ]
however with complete and significant (16%) de-chlorination,
respectively.
-1
ratio 1.6
100%
80%
60%
40%
20%
0%
7
100
90
7c
7b
7a
23
45
68
85
80
70
50 60 70 80 90 100
conversion (%)
23
45
68
85
residence time (s)
Figure 11. Continuous flow hydrogenation of 7 over Pd@MonoSil-ArSO3
catalyst (rt, methanol 0.1 M). Left: selectivity/conversion diagram at fixed H2:7
ratio = 1.6 and residence time in the range 23 - 85 s. Right: reaction mixture
composition (mol %) under these conditions.
Finally, the catalytic hydrogenation of the unfunctionalized
olefin 1,5-cyclooctadiene (7) was investigated, owing the
challenging selectivity issues and the available literature data
using comparable monolithic flow reactors. The reaction
produces cyclooctene (7a), an important monomer for the
manufacture of a variety of polymeric materials.[81] During the
process, besides over hydrogenation to cyclooctane (7c),
competitive hydro-isomerization to 1,4-cyclooctadiene (7b) is
known to occur over supported PdNP systems, particularly at
low H2 pressures (Scheme 3).[82] Under the optimized condition
for 7a yield ( 85 s, H2:7 ratio 1.6, H2 pressure 1.08 bar) use of
Pd@MonoSil-ArSO3 provided the desired partial hydrogenation
product 7a with 85% overall selectivity [7a / (7a + 7b + 7c)] and
87% selectivity in hydrogenation reaction [7a / (7a + 7c)] at 94%
conversion, corresponding to a 80% 7a yield (Table 6, entry 5).
Analogously to the other substrates examined, a reproducible
selectivity / conversion diagram could be obtained by varying
The stability of the Pd@MonoSil-ArSO3 catalyst can be
ascribed to both the stabilization of embedded PdNP, thanks to
the combined effect of mesoporosity (steric stabilization)[84] and
sulfonated groups (electrostatic stabilization),[85] and to the mild
operating conditions attained, thanks to the efficiency of the
monolithic catalyst. As a matter of fact, neither leaching of metal
in solution (ICP-OES) nor sintering of PdNP (TEM) has been
observed upon use in catalysis. Minimization of active site
inhibition by non-accumulation of adsorbed by-products, e.g.
dimers in the case of alkyne hydrogenation,[86] may be also of
relevance under the conditions of continuous flow.[87]
Conclusion
A facile method was described for the synthesis of PdNP- doped
hierarchical porosity silica monoliths, based onto an ion-
exchange / reduction sequence of post-functionalized sulfonated
materials. The procedure afforded PdNP of small size evenly
distributed within the monolith and resulting in a 4.6% wt bulk
metal loading. Compared to known synthetic methods, the
actual strategy provides significant advantages in terms of
higher metal content and smoother, environmentally friendlier,
procedure.[ 88 ] The as-prepared material was scrutinized as
catalysts for the continuous flow, partial hydrogenation reaction
of various substrates, showing remarkable versatility and
efficiency under very mild conditions. Very good activity and
selectivity was achieved in the hydrogenation of halogeno-
nitrobenzenes. Particularly, no significant de-halogenation was
observed at full conversion in the hydrogenation of 1-fluoro-3-
nitrobenzene. The hydrogenation of alkynols showed to be
significantly substrate-dependent in terms of chemoselectivity to
the alkene product. Better results were obtained for internal
alkynes, and particularly for 3-hexyn-1-ol, whose corresponding
leaf alcohol product was obtained with yield comparable to the
best ever reported for a continuous flow catalytic system.
Irrespective of the substrate, compared to similar hierarchically
porous materials (and also most of the conventional systems),
the monolithic catalyst showed better performance in terms of
STY of desired product at comparable conversion levels. This
can be attributed to the high Pd loading per unit volume catalyst
and to the high permeability of the support material. Highly
reproducible flow processes with very short equilibration times
and low pressure drops were achieved in any case. The catalyst
either the H2:substrate ratio or the residence time.
A
representative example is shown in Figure 11, left for fixed H2:7
ratio 1.6. A typical composition of the reaction mixture under
different residence times (23 - 85 s) and the same H2:7 ratio is
reported in Figure 11, right, showing the decrease of 7 and 7b,
and the increase of 7a and 7c, upon increasing . As expected,
the amount of isomerization product 7b obtained is lower for
higher H2 pressures (Supporting Information, Figure S26).
Compared to the previously reported Pd@MonoSil[12a] and
Pd@TiO2 monolith[11] catalysts, under analogous flow conditions
Pd@MonoSil-ArSO3 provided 7a with similar or better yields (85,
68 and 80%, respectively) and higher STY (0.07, 0.40 and 0.50
kg L-1 h-1, respectively) at remarkably lower H2:7 ratio (4.0, 4.8
and 1.6) (Table S4). The continuous flow partial hydrogenation
of 7 was previously accomplished in ca. 95% 7a yield, but in
much lower STY (ca. 0.1 kg L-1 h-1), using a pore-through-flow
Pd@Al2O3 catalytic membrane reactor under 50 °C and 10 bar
H2, however with periodic regeneration at 250°C under H2
required.[83]
Importantly, irrespective of the substrate, all above processes
could be achieved with full conversion by selection of
appropriate, very mild flow conditions ( 21 - 100 s, H2 1.07 -
1.16 bar, room temperature), showing no significant change of
activity and selectivity over one week continuous reaction time.
Palladium leaching in solution was below the ICP-OES detection
limit, in any case.
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10.1002/cctc.201700381
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FULL PAPER
toluene, ethanol, methanol : H2O = 50 : 50, acetone and dried under high
vacuum at rt for 1h, then at 75°C for 3 days.
showed pretty nice constant efficiency over prolonged time on
stream, with no regeneration treatments required.
Selectivity enhancement in catalytic, partial hydrogenation
reactions is a critical goal for industrial applications, which is
often problematic because of over hydrogenation reactions,
isomerization, limited resistance of other functional groups.
Strategies have been developed in order to enhance the
selectivity of these systems, e.g. in the case of alkynes
hydrogenation, by addition of variable, often large, amounts of
(toxic) modifiers.[89] Compared to the known catalysts for the flow
processes herein examined, if any, the reported catalysts
provide a good compromise solution between product purity and
productivity, with no need of additives to achieve satisfactory
selectivity.
Synthesis of the Pd catalysts Pd@MonoSil-ArSO3
In a typical procedure, the cladded MonoSil-ArSO3 monolith (6 mm
diameter, 1 cm length, 81.8 mg, 0.08 meq sulfonic sites) was connected
through PFA Swagelok fittings and Teflon tubing to
a diaphragm
metering pump. The system was conditioned by flowing deionized-water
0.3 mL min-1 for 30 min. A solution of Pd(NO3)22H2O in H2O (0.025 M,
15.0 mL, 0.375 mmol) was allowed to cyclically flow through the cladded
monolith at 0.5 mL min-1 for 3h. The brownish monolith obtained was
then thoroughly washed with H2O (0.5 mL min-1 for 1h) and methanol (0.5
mL min-1 for 30 min).
After connection to the flow reactor system described above, H2 (2
mL min-1, pressure at the reactor inlet 2 bar) and methanol (0.5 mL min-1)
were flowed simultaneously through the monolith for 3h at rt. The as-
prepared black-monolith was directly used in catalytic hydrogenation
reactions without disconnecting from the system or dried under a stream
of N2 (2 mL min-1 for 12 h at rt) before being characterized.
Experimental Section
General information
Unfunctionalized, dual porosity silica monoliths (MonoSil) of 6 mm
Catalytic reactions in continuous flow mode
diameter, featuring a skeleton thickness of 3 m, a macropore size of 5
m and
a
macroporous volume of 1.8 cm3 g-1 (SEM, mercury
In a typical experiment, a deaereated solution of substrate in methanol
(0.1 M) was allowed to flow through the cladded Pd@MonoSil-ArSO3
monolith (6 mm diameter, 1 cm length, 81.8 mg, 0.283 cm3, 4.6% wt Pd)
at 0.48 mL min-1 rate, together with a H2 flow of 1.25 mL min-1 under rt.
This resulted in a H2 pressure at the reactor inlet of ca. 1.15 bar
(corresponding to a H2 : substrate molar ratio of ca. 1.2), while the
hydrogen gas was released at atmospheric pressure at the outlet of the
reactor. Therefore, the pressure drop generated by monolithic reactor
was ca. 0.2 bar. The attainment of the steady state conditions (ca. 1 h)
was taken as the reaction start time. The reaction was monitored
periodically by analyzing the product solution for conversion and
selectivity by GC. Aliquots were sampled at 1 h intervals for Pd leaching
determination by ICP-OES.
porosimetry) , were prepared as previously described.[5a] The BET
surface area was 521 ± 6 m² g-1, the BJH desorption cumulative pore
volume and desorption average pore width was 1.07 cm3 g-1 and 6.6 nm,
respectively. 5% Pd@SiO2 (Escat™ 1351, 40 m grain size, surface area
400 m2 g-1) and 5% Pd@C were obtained from Strem Chemicals and
Aldrich, respectively. Reactions under continuous flow were carried out
using
a home-made reactor system constructed at ICCOM-CNR
(Scheme S1). The system was designed to allow for a concurrent flow of
substrate solution and hydrogen gas through the monolithic catalyst. The
reactor was completely inert since all wet parts were made of PEEK, PFA
or PFTE. The flow of the substrate solution was regulated by an Alltech®
model 426 HPLC pump in PEEK. A constant flow of hydrogen gas was
adjusted by a flow controller while the hydrogen pressure in the reactor
was monitored by a pressure meter. The concurrent flows of gas and
liquid were driven through a T-shaped PEEK mixer equipped with a 2 m
PE filter to ensure efficient gas dispersion. The monolithic catalyst was
cladded into a heat-shrinkable 1/4” Deray-KY175 PTFE tube together
with two glass tubes (4 mm inner diameter) at each end, and connected
in a top-down arrangement to the system by PFA Swagelok fittings,
Chemraz® O-rings and 1/16” PEEK tubing. At the outlet of the reactor,
the reaction solution was collected for GC, GC-Ms and ICP-OES analysis
and the excess amount of the hydrogen gas released to the atmospheric
pressure. Commercially available H2 (99.995%) was used as received.
The reaction products were unequivocally identified by the GC retention
times and mass spectra of those of authentic specimens. Quantitative
analysis of the reaction products was carried out via GC based on
calibration curves of the pure compounds
Catalytic reactions in batch mode
Reaction conditions analogous with those used for the continuous flow
experiments were adopted. Particularly, in order to establish a proper
comparison, the catalytic reactions were performed by contacting the
same amount of substrate per unit catalyst and time as in the flow mode.
Thus, in a typical procedure, the Pd@MonoSil-ArSO3 monolith was
crushed in an agate mortar and sieved to collect a 60-120 m fraction
using a glove-box. The ground catalyst (53 mg, 4.6% wt Pd) was placed
under nitrogen into a glass non-metallic Büchi Miniclave® and a solution
of 1-chloro-3-nitrobenzene (1) in methanol (18 mL, 0.01 M,) was added
via a Teflon capillary under nitrogen. Nitrogen was then replaced by
hydrogen with three cycles pressurization (1 bar) / depressurization. The
autoclave was finally charged with a constant H2 pressure of 1 bar. The
mixture was stirred under room temperature at 500 rpm, in order to avoid
mass transfer limitations.[14b] After 1 h, the reactor was depressurized and
the solution analyzed via GC to yield a 98 %, conversion and a 100%
selectivity to aniline 1c.
Synthesis of sulfonated silica monolith MonoSil-ArSO3
Prior of functionalization, the bare silica monolith (0.424 g, 7.07 mmol)
was activated under vacuum at 100 °C for 18h then at 150°C for further
18h. After cooling to rt under nitrogen, the monolith was transferred
under nitrogen into a two-necked, round-bottomed flask equipped with a
refrigerator. Dry toluene (13 mL) was added under nitrogen by a gas-tight
syringe and the mixture was degassed by three cycles vacuum/nitrogen.
Acknowledgements
A
50% wt solution of CSPTMS in dichloromethane (0.442 mL
Thanks are due to Centro Microscopie Elettroniche (CNR,
Firenze) for technical assistance in TEM analysis and to Dr.
Stefano Caporali (Università di Firenze) for XPS experiments.
corresponding to 0.297 g, 0.91 mmol of CSPTMS) was added dropwise
under nitrogen. The mixture was refluxed for 48h without stirring then
cooled to rt. The monolith was decanted, washed sequentially with
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10.1002/cctc.201700381
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FULL PAPER
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