PdNP@Titanate Nanotubes as Effective Catalyst for Continuous-Flow Partial Hydrogenation Reactions

Article abstract of DOI:10.1002/cctc.201501126

Pd nanoparticles were easily immobilized onto titanate nanotubes by a straightforward procedure. The material (0.50 wt % Pd) was used as catalyst in the continuous-flow, liquid-phase hydrogenation reaction of unsaturated C-C bonds and it showed excellent performance and durability under very mild conditions (room temperature, 1-2 bar H₂, residence time 13-36 s). In particular, very high productivity was obtained in the synthesis of the perfumery component cis-3-hexen-1-ol (40.6 mol g_Pd^-1 h^-1) without additives or metal contamination, with clear benefits in terms of process economy and environmental impact compared with conventional catalysts. The catalyst performance is discussed in the light of comparable systems.

Full text of DOI:10.1002/cctc.201501126

DOI: 10.1002/cctc.201501126
Full Papers
PdNP@Titanate Nanotubes as Effective Catalyst for
Continuous-Flow Partial Hydrogenation Reactions
Noemi Linares, Carmen Moreno-Marrodan, and Pierluigi Barbaro*[a]
Pd nanoparticles were easily immobilized onto titanate nano-
tubes by a straightforward procedure. The material (0.50wt%
Pd) was used as catalyst in the continuous-flow, liquid-phase
hydrogenation reaction of unsaturated CÀC bonds and it
showed excellent performance and durability under very mild
conditions (room temperature, 1–2 bar H2, residence time 13–
36 s). In particular, very high productivity was obtained in the
synthesis of the perfumery component cis-3-hexen-1-ol
(40.6 molgPdÀ1 hÀ1) without additives or metal contamination,
with clear benefits in terms of process economy and environ-
mental impact compared with conventional catalysts. The cata-
lyst performance is discussed in the light of comparable sys-
tems.
Introduction
Catalytic hydrogenation of carbon–carbon multiple bonds is
a main strategy for the synthesis of fine and commodity chem-
icals.[1] In particular, the stereo- and chemoselective hydrogena-
tion reaction of functionalized alkenes and alkynes is of funda-
mental importance in the manufacture of several food addi-
tives, flavors, and fragrances.[2] Partial hydrogenation reactions
are also crucial in polymerization processes to achieve com-
plete elimination of alkynes and dienes from alkene feed-
stocks.[3] For these purposes, solid catalysts are preferred by in-
dustry owing to their easier integration in existing reactor
equipment, cleaner downstream processing, and facile reuse
of precious catalysts compared with homogeneous systems.[4]
Among the heterogeneous catalysts developed over the years,
sophisticated Pd nanoparticle-based materials containing varia-
ble amounts of additives or modifiers have shown unsurpassed
activity and selectivity for the production of a variety of popu-
lar market products.[5,6]
generated/kg product),[11] by optimizing the selectivity of
a given catalyst while reducing the number of synthetic steps,
waste emission, and purification operations of the overall pro-
cesses.[12] Indeed, as detailed by Newman and Jensen in
a recent review,[13] the development of continuous-flow manu-
facture methods has been identified as a pivotal field of re-
search in the engineering and green chemistry practice for the
pharmaceutical industry.
Continuous-flow catalysis is most usually performed by con-
tacting a stream of gas or liquid with a bed of catalytic materi-
al packed into an open tubular reactor, wherein suitable cata-
lysts include solid-supported metal nanoparticles (MNPs). While
on one hand, problems with packed-bed reactors relate to in-
homogeneous flow patterns, formation of hot spots, pressure
drop, catalyst abrasion, and mass transfer,[14] on the other hand
a number of issues are often associated with MNPs, for exam-
ple, i) synthetic procedures requiring toxic reagents, harsh con-
ditions, or sophisticated equipment, ii) lack of control over
MNP size and distribution, iii) lack of reproducibility, iv) lack of
stability under reaction conditions, v) applicability only to spe-
cific reaction–support combinations.[15,16]
However, on the industrial scale most processes are still per-
formed by using conventional batch setups and multiple reac-
tor units,[7] with significant drawbacks in terms of volume and
time consumption, energy input, safety, costs, and scale-up.[8,9]
A more intensive, productive, and environmentally friendly
option is provided by continuous-flow catalysis.[10] In the fine-
chemicals sector, catalysis under flow may significantly contrib-
ute to the abatement of the high E-factor (kg waste
Nanotubular materials with 1D straight pores and large sur-
face areas have been deeply investigated because of their po-
tential applications in a variety of areas of heterogeneous cat-
alysis, including photocatalysis, organo-, and organometallic
catalysis.[17] If used as a support for catalytically active species,
the morphology of these materials may allow for a better site-
accessibility than other structures, that is, 2D silica mesoporous
frameworks, SBA-15 or MCM-41, or 3D frameworks, such as ac-
tivated carbon. In addition, compared with conventional
powder and granular beds, 1D fibrous catalysts have proven to
be beneficial in continuous-flow operations, mainly owing to
their lower resistance to the flow of gas or liquid.[18] Although
the most recent studies on nanotubes focused on carbon[19,20]
and titanium dioxide,[21] titanate nanotubes (TiNT) obtained by
[a] Dr. N. Linares,+ Dr. C. Moreno-Marrodan, 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
[+] Current address:
Molecular Nanotechnology Lab
Department of Inorganic Chemistry
University of Alicante
Carretera San Vicente s/n, E-03690 Alicante (Spain)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201501126.
[22]
alkali hydrothermal treatment of TiO2 have received increas-
ChemCatChem 2016, 8, 1001 – 1011
1001
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers
ing attention either as solid acid catalysts[23] or as support ma-
terials.[24] Indeed, high loadings, even distributions, and high
metal dispersions may be obtained thanks to their high cation
exchange capacity,[25] which makes them excellent candidates
for nanocatalyst supports. Examples of applications under flow
conditions can be found in the literature, mostly for oxidation
reactions.[26]
X-ray spectroscopy) sodium data were consistent with this con-
clusion. The XRD spectra showed that the structure of the
nanotubes was not significantly affected by ion exchange with
PdII species and their subsequent reduction, nor was evidence
of Pd nanoparticles detected, in agreement with both the low
content of homogeneously distributed Pd in the final material
and the small Pd particles size (see TEM below).[31]
Herein, we describe the synthesis of palladium nanoparticles
on titanate nanotube catalysts (Pd@TiNT) and their use in se-
lective hydrogenation reactions under continuous-flow condi-
tions. Substrates of industrial interest for the production of
fine chemical intermediates were examined under very mild
conditions (258C, 1–2 bar H2), with special attention to the
preparation of cis-alkenes through stereoselective partial hy-
drogenation reactions of substituted alkynes. The performance
of the Pd@TiNT catalyst was compared with those reported for
similar systems and with that of 1.2wt% Pd@SiO2 and com-
mercial 5wt% Pd@C.
The porosity of the titanate materials was examined by ni-
trogen sorption isotherms at 77 K. The resulting textural pa-
rameters are summarized in Table 1. The isotherms of TiNT
before and after immobilization of PdNP are shown in Figure 2.
Both materials displayed type IV isotherms typical of mesopo-
rous solids, with two distinctive nitrogen uptakes at P/P0 =0.5
and 0.8–0.9, which indicate the presence of a bimodal porosity.
The inset in the Figure 2 shows the corresponding pore size
distribution. The first peak is due to capillary nitrogen conden-
sation in the intraparticle mesopores related to the inner cavi-
ties of the nanotubes (ca. 3 nm diameter). The secondary po-
rosity (uptake at P/P0 >0.8) is characteristic of textural interpar-
ticle mesopores (ca. 50 nm diameter).[32] After immobilization
of the PdNP, the diameter, surface area and pore volume of
the mesopores slightly decreased (Table 1), showing that at
least a portion of the PdNPs are located within the meso-
pores.[33]
Results and Discussion
Synthesis and characterization of the catalyst
Titanate nanotubes were prepared by a slight modification of
the reported hydrothermal procedure.[27] Pd nanoparticles
were then smoothly immobilized therein through an ion-ex-
change/in-situ reduction sequence, by using an appropriate
water-soluble palladium(II) precursor and 1 bar H2 at room
temperature as the metal reducing agent.[28] The Pd-free and
Pd@TiNT materials were characterized by XRD, physisorption,
TEM, and X-ray photoelectron spectroscopy (XPS) measure-
ments. Inductively coupled plasma optical emission spectros-
copy (ICP-OES) indicated a loading of 0.50wt% Pd in the final
material.
The morphology of the mesoporous material and the loca-
tion and size of the Pd nanoparticles were investigated by
TEM. Figure 3 shows representative TEM images of the titanate
nanotubes before (left) and after (center) immobilization of
PdNP.
A more detailed picture is reported in Figure S1 in the Sup-
porting Information. The corresponding PdNP size distribution
Table 1. Textural parameters of mesoporous titanate nanotubes before
and after immobilization of PdNPs.[a]
Figure 1 shows the XRD patterns of the nanotubes before
and after immobilization of PdNPs. The XRD peaks of the Pd-
free material observed at 2qꢀ108, 24.48, and 48.58 are charac-
Material
SBET
[m2 gÀ1
Dmesopore
[nm]
Vmesopore
Vinterparticle
Vtotal
[cmgÀ1
]
]
[cmgÀ1
]
[cmgÀ1
]
teristic of a NaxH2ÀxTi3O7 trititanate layered 1D structure.[29]
A
TiNTs
Pd@TiNT
215
190
2.9
2.5
46
37
0.13
0.11
0.79
0.60
0.92
0.71
broad peak is also observed at 2qꢀ288, which, as previously il-
lustrated, should be of extremely poor intensity if a portion of
sodium ions is exchanged by protons during the synthetic pro-
cedure;[30] in our case this did not occur, thus suggesting a low
degree of replacement of Na+ by H+. EDS (energy-dispersive
[a] The surface areas, pore sizes, and pore volumes were calculated as de-
tailed in the Experimental Section.
Figure 2. N2 adsorption/desorption isotherms of titanate nanotubes at 77 K
before (filled circles) and after (empty circles) immobilization of PdNPs. The
inset shows the corresponding pore size distributions calculated from the
adsorption branch by using the Barrett–Joyner–Halenda (BJH) method.
Figure 1. XRD patterns for titanate nanotubes, before (solid line) and after
(dashed line) the incorporation/reduction of palladium.
ChemCatChem 2016, 8, 1001 – 1011
www.chemcatchem.org
1002
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers
Hydrogenations under continuous flow: activity and
selectivity control
The Pd@TiNT nanotubes were tested as catalyst precursors in
the hydrogenation of unsaturated CÀC bonds under continu-
ous-flow conditions. A home-made mesoreactor based on con-
current flows of substrate solution and gas reactant was used
for this purpose, wherein the catalytic material was packed
into commercial glass tubes. Several probe substrates were
chosen to test the efficiency of the catalyst in terms of produc-
tivity and selectivity. These include the terminal alkynes phe-
nylacetylene (1), 1,1-diphenyl-2-propyn-1-ol (2), 2-methyl-3-
butyn-2-ol (3), the internal alkyne 3-hexyn-1-ol (4), the bis-
olefin 1,5-cyclooctadiene (5), and the a,b-unsaturated ketone
trans-4-phenyl-3-buten-2-one (6; Scheme 1).
All reactions were tested at room temperature under differ-
ent combinations of solution and H2 gas flow rates. Several sol-
vents were examined, of which methanol resulted in the best
catalyst performance. As expected from similar systems, the
conversion and selectivities could be tuned by adjusting the
flow rates. Typical findings are illustrated below for the hydro-
genation reaction of 1, as a representative example. At a fixed
solution flow rate (i.e., for the same residence time, t), an in-
crease in the H2 flow resulted in significantly higher substrate
conversion. An enhancement of conversion from 57% to 83%
was observed on going from 3.0 to 12.0 mLminÀ1 H2 flow at
a residence time of 36 s (Figure 5, left). This effect is attributed
to the increase in the H2/substrate molar ratio upon varying
the hydrogen flow rate, as the overall H2 pressure at the reac-
tor inlet did not significantly change. Notably, the selectivity
for the partial hydrogenation product, styrene (1a), was affect-
ed to a lesser extent; a selectivity drop from 93% to 82% was
observed under these conditions.
Figure 3. Representative TEM images of titanate nanotubes without (a) and
with (b) immobilized Pd nanoparticles (arrows). c) PdNP size histogram.
in Pd@TiNT is reported in Figure 3, right. The titanate material
was shown to consist of nanotube bundles with no isolated ti-
tania particles, confirming the success of the hydrothermal
treatment. Hollow tubes of 3–5 nm internal diameter, 10–
15 nm outer diameter, and 150–250 nm length were typically
measured. The overall morphology was not altered by ion ex-
change or reduction of the PdII ions. A narrow dispersion of
PdNPs (3.1Æ0.7 nm diameter) was found, which were homo-
geneously distributed on the external and the inner nanotube
surfaces, with no preferential anchoring sites. A surface/bulk
palladium atom ratio of 36% was estimated from the TEM
data.[34]
XPS measurements were carried out to analyze the oxidation
state of the supported Pd particles. Figure 4 shows the XPS
spectrum in the Pd3d region where the usual palladium dou-
blet is observed together with a lower intensity, large peak at-
tributed to an Auger transition of sodium (Na KLL), in agree-
ment with the XRD data. The Pd3d peaks were deconvoluted
into two oxidation states, namely metallic Pd0 at lower binding
energies (Pd3d5/2 334.8Æ0.1 eV) and PdII at higher binding en-
ergies (Pd3d5/2 336.5Æ0.1 eV) with the latter likely a result of
PdO.[35] No other palladium species were detected. The calcu-
lated abundance of the two species was 81.8% for Pd0 and
18.2% for PdII. As was previously reported,[36] the presence of
PdO can be safely attributed to a layer covering the core of
the metallic palladium particles as a result of sample manipula-
tion. This was confirmed by the observation that the percent-
age of Pd0 increased to 86.1% after sputtering with Ar, a proce-
dure that removes the most superficial atoms of the nanoparti-
cle.[37] This effect is also highlighted by the shrinkage of the
XPS peaks shown in Figure S2 (in the Supporting Information).
On the other hand, an increase in the solution flow rate (i.e.,
a decrease in the residence time while keeping a constant H2/
substrate molar ratio) invariably led to a conversion drop and
to a selectivity enhancement. Figure 5, center, shows the con-
Figure 5. Continuous-flow hydrogenation of 1 to 1a over Pd@TiNT catalyst
(50 mg 0.50wt% Pd, RT, methanol 0.1m, reactor volume 240 mL). a) Conver-
*
*
sion ( ) and selectivity ( ) as a function of H2 flow rate and H2/1 molar ratio
under fixed residence time, t=36 s (solution flow rate=0.4 mLminÀ1).
b) Conversion (filled symbols) and selectivity (open symbols) as a function of
solution flow rate and residence time under fixed H2/1 molar ratios of 10.7
* *
(
, ) and 19.8 (~,~). c) Selectivity/conversion diagram at fixed H2/
*
1 ratio=19.8 and residence time 21–48 s ( ), and a fixed residence time
t=36 s and H2/1 ratio range 7.0–26.0 ( ; H2 flow rate=3.0–12.0 mLminÀ1).
*
Figure 4. XPS spectrum of Pd@TiNT in the Pd3d region.
ChemCatChem 2016, 8, 1001 – 1011
www.chemcatchem.org
1003
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers
Scheme 1. Range of hydrogenation reactions studied with our Pd@TiNT catalyst with detected products and labeling scheme. Percentage values refer to rep-
resentative selectivity data.
version and selectivity trend for the hydrogenation of 1 upon
changing the solution flow. At a fixed H2/substrate ratio 19.8,
the conversion decreased from 92% to 49% and the selec-
tivity increased from 70% to 97%, on going from 0.3 to
0.7 mLminÀ1 solution flow rate.
amount of Pd leached into the solution was below the ICP-
OES detection limit over the entire experiment duration, and
no significant changes in Pd particle size (3.5Æ0.9 nm, TEM)
nor significant Pd depletion of the recovered catalyst
(<0.01%, ICP-OES) was detected after use in catalysis.
A reproducible selectivity/conversion diagram could be ob-
tained on the above basis, as reported in Figure 5, right. Thus,
appropriate selection of the experimental parameters allowed
for the optimization of the partial hydrogenation product yield
that, in the case of 1a, was approximately 70%.
As above described, the hydrogenation of 1 resulted in
a steady 83% conversion and in 82% selectivity to 1a in 36 s
residence time and with 1.2 bar H2 pressure drop, correspond-
ing to 0.83 kgLÀ1 hÀ1 space-time-yield productivity (STY=
kgproduct/Lreactorvolume ”h),[38] 850 hÀ1 turnover frequency (TOF=
molproduct/molPd ”h),[39] and 6.5 mol1a gPdÀ1 hÀ1 mass productivity.
The catalytic hydrogenation of 1 under continuous flow has
been scarcely investigated in the past. The best results were
obtained over the polymeric monolithic catalysts Pd@MonoBor
(95.5% selectivity at 98.1% conversion)[40] or the amorphous
Pd81Si19 alloy catalyst in supercritical CO2 (80% selectivity
at 90% conversion).[41] However, compared with the reported
systems, the Pd@TiNT catalyst was able to process higher
phenylacetylene flows, thus resulting in overall better produc-
tivity.
Hydrogenations under continuous flow: versatility and
stability
The experimental conditions resulting in the best compromise
between conversion and selectivity for all the substrates exam-
ined are summarized in Table 2. High conversions (70–97%)
and very good selectivities (82–94%) for the partial hydrogena-
tion products were typically achieved under very mild condi-
tions, that is, residence times in the range 13–36 s, at 2.2–
2.8 bar H2 pressure, and room temperature. All reactions were
monitored for conversion, selectivity, and metal leaching over
at least 5 h of time-on-stream. Irrespective of the reaction, the
Under similar conditions, both alkynols 2 and 3 were hydro-
genated by Pd@TiNT with high ene selectivity at high substrate
conversion, namely 89% selectivity to 2a at 78% conversion
ChemCatChem 2016, 8, 1001 – 1011
www.chemcatchem.org
1004
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers
Table 2. Representative data for continuous-flow hydrogenation reactions using Pd@TiNT catalyst.[a]
Substrate
1
2
3
4
5
6
Solution flow rate [mLminÀ1
Residence time [s]
]
0.4
36
0.1
12.0
2.2
26
83
82 (1a)[d]
0.6
24
0.2
11
2.6
9.8
1.1
13
0.5
22.0
2.8
4.6
75
83 (3a)
1.1
13
0.2
13.5
2.4
6.1
88
94 (4a+4b)[e]
93 (4a)[f]
0.85
17
0.05
5
2.5
12.2
97
0.4
36
0.1
5
2.7
14
Concentration [m]
H2 flow rate [mlminÀ1
H2 pressure [bar]
]
H2/substrate molar ratio[b]
Conversion [%][c]
78
89 (2a)
70
90 (6a)
Selectivity [%]
94 (5a)
[g]
TOF [hÀ1
]
850
0.83
6.5
2300
2.35
19.2
11 706
10.35
91.3
4945
4.86
40.6 (4a)
1032
715
0.70
6.0
[h]
STY [kgLÀ1 hÀ1
Productivity [molprod gPdÀ1 hÀ1
]
1.03
9.1
]
[a] Reaction conditions: RT, methanol, 50 mg Pd@TiNT, 0.50wt% Pd loading, reactor volume 240 mL. Conversion and selectivity data (optimized values)
from GC analysis. H2 pressure measured at the reactor inlet. [b] Calculated at the reactor inlet. [c] Average value over 5 h time-on-stream. [d] 1a/(1a+1b).
[e] (4a+4b)/(4a+4b+4c). [f] 4a/(4a+4b). [g] Turnover frequency: (mol substrate converted)/(molPd”h). Calculated at the conversion indicated with
bulk Pd content. [h] Space-time-yield calculated with the overall conversion indicated.
and 83% selectivity to 3a at 75% conversion. A remarkably
high catalyst productivity was again observed in these cases.
For instance, a TOF of 2300 hÀ1 and a STY of 2.35 kgLÀ1 hÀ1 was
obtained in the hydrogenation of 2, corresponding to
19.2 mol2a gPdÀ1 hÀ1 hourly production of 2a. Although no cata-
lytic activity decay was detected in the hydrogenation reaction
of 2, a minor efficiency loss was observed for 3, as shown in
Figure 6. As Pd leaching was not detected, the activity drop
can be attributed to the adsorption of oligomers and other by-
products onto the catalyst surface, as previously reported.[42] 2-
Methyl-3-butyn-2-ol (3) is among the most studied alkyne sub-
strates in semi-hydrogenation reactions, mainly owing to the
importance of 3a as an intermediate in the synthesis of vita-
mins (A, E) and several perfumes.[43] It is currently manufac-
tured in high yields (95–97%) under batch conditions by using
the Lindlar[44] or other heterogeneous catalysts,[45] which, how-
ever, often suffer from fast deactivation as a result of degrada-
tion of the support or sintering of the PdNPs.[46] Few reports
exist on the partial hydrogenation of 3 under continuous-flow
conditions. The best performance in terms of selectivity (97%
at 99.9% conversion, 608C) was observed by using a capillary
microreactor coated with Pd25–Zn75/TiO2 catalyst in the pres-
ence of pyridine, however, this had very low mass productivi-
ty.[47]
The continuous-flow hydrogenation reaction of 3-hexyn-1-ol
(4) was investigated in detail (see below), as the cis-partial hy-
drogenation product, leaf alcohol 4a, is an important ingredi-
ent in the fragrance industry,[52] which is currently produced on
a scale of 400 tyÀ1 with approximately 96% selectivity at 99%
conversion by a batch process using the Lindlar catalyst.[53]
Pd@TiNT afforded the mono hydrogenation products 4a+4b
in 94% selectivity (93% of which was the cis isomer) at 88%
conversion under 2.4 bar hydrogen and only 13 s residence
time. No significant catalytic efficiency decay was observed
over 5 h time-on-stream (Figure 6), which corresponds to an
excellent values of 4945 hÀ1 TOF, 4.86 kgLÀ1 hÀ1 STY, and
40.6 mol4a gPdÀ1 hÀ1 productivity. The catalyst could be reused
several times by switching off the reactor system and restart-
ing it, with no need for regeneration provided that it was
stored under nitrogen or hydrogen. No traces of overhydroge-
nation to n-hexane, nor the 4- and 2-hexen-1-ol isomerization
products were detected.
The continuous-flow partial hydrogenation reaction of 5 to
cyclooctene 5a has been previously described using supported
PdNP catalysts, either on polymeric or inorganic materials. Rep-
resentative literature data and typical 5a yields are reported in
Table 3. The best results in terms of 5a purity were reported
by using a pore-through-flow Al2O3 membrane support to give
95% selectivity at full conversion under 508C and 10 bar H2.[54]
The catalyst had to be regenerated at 2508C under H2 for 2 h.
Pd@TiNT showed comparable performance (94% selectivity at
97% conversion) under milder reaction conditions (room tem-
perature, 2.5 bar H2), however, with practical advantages in
terms of volume consumption (1.0 vs. 0.1 kgLÀ1 hÀ1 STY). The
only byproduct was cyclooctane 5b, with none of the isomeri-
zation products 1,3- or 1,4-cyclooctadiene detected.
Catalyst selectivity toward competitive C=O bond hydroge-
nation was tested by using 6 as a substrate, which resulted in
the formation of 4-phenyl-butan-2-one (6a) with 90% selectivi-
ty at 70% conversion. No traces of 1-phenyl-1-buten-3-ol were
observed. This behavior can be justified by the greater affinity
of Pd for C=C rather than for C=O bonds in aliphatic com-
pounds.[55]
Figure 6. Continuous-flow data (solid symbols=conversion, open
symbols=selectivity) for the hydrogenation of 3 and 4 by Pd@TiNT catalyst
(0.50 wt% Pd, RT, methanol). Start time: attainment of steady-state condi-
tions (ca. 30 min).
ChemCatChem 2016, 8, 1001 – 1011
www.chemcatchem.org
1005
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers
Table 3. Continuous-flow hydrogenation reaction of 5 to 5a by heterogeneous PdNP catalysts.[a]
Catalyst
Conversion [%]
Selectivity [%]
Yield [%]
TOF[b] [hÀ1
]
STY[b] [kgLÀ1hÀ1
]
Ref.
Pd@Dowex-Li
87
95
75
99
91
97
97
90
91
95
88
94
84
86
68
93
80
91
395
38
153
3192
282
1032
1.3
0.2
0.4
0.1
0.1
1.0
[48]
[49]
[50]
[54]
[51]
This work
Pd@SiO2 monolith
Pd@TiO2 monolith
[c]
Pd@Al2O3
Pd@NKZPDB5
Pd@TiNT
[a] Room temperature, H2 pressure 1–3 bar. [b] Calculated from overall conversion. [c] At 508C and 10 bar H2.
The best selectivity so far obtained for the catalytic hydroge-
nation reaction of 6 under continuous flow was recently re-
ported by using Pd on the organic monolith MonoBor (96%
selectivity at 99.5% conversion).[56] However, as described
above for the other substrates, the productivity of Pd@TiNT is
significantly higher (6.0 mol6a gPdÀ1 hÀ1 vs. 1.6 mol6a gPdÀ1 hÀ1).
a similar effect also holds for Pd on titanate nanotubes.
Indeed, compared with Pd on conventional mesoporous tita-
nia, Pd@TiNT show approximately 50% higher activity under
batch conditions.[50] Other factors may affect the Pd hydroge-
nation activity, including size, shape, and distribution of the
embedded PdNPs,[59] as well as the metal–support interac-
tions.[60] However, details of the catalytic active sites for most
catalysts, including Lindlar,[61] are still unknown, hence a direct
comparison is not possible on this basis.
Hydrogenations under continuous flow: comparison be-
tween different catalytic systems
When used under continuous-flow conditions, Pd@TiNT pro-
vided 4a in lower purity, although with higher productivity,
compared with the corresponding batch setup under similar
reaction conditions (1715 hÀ1 vs. 4945 hÀ1, Table 4 entry 1 vs.
3). In addition, miniaturization of the reactor flow system al-
lowed for a much higher production per unit volume, resulting
The hydrogenation reaction of alkyne 4 was used to compare
the efficiency of the Pd@TiNT catalyst with that of other batch
(stirred tank) and continuous-flow reactor systems. Representa-
tive results are reported in Table 4 in terms of conversion, se-
lectivity, and productivity.
in
a 2.5 order magnitude greater STY value (0.01 vs.
Compared with the industrial batch catalysts (Lindlar, 5% Pd
on CaCO3 doped with 2–3% Pb), Pd@TiNT showed much
higher efficiency (ca. 6 times) at similar conversions and selec-
tivity levels (Table 4, entry 1 vs. 2). However, the justification
for this finding is not straightforward. Indeed, the Pd@TiNT cat-
alyst avoids Pd poisoning from a variety of contaminants (quin-
oline, phosphines, CO, sulfides, dimethyl sulfoxide, Pb) that are
employed in other systems to decrease the Pd hydrogenation
activity and enhance its selectivity.[57] On the other hand,
a “proton-acceleration” effect on the metal hydrogenation ac-
tivity by Brønsted acid sites from the support has been in-
voked in some instances for PdNP hydrogenation catalysts on
solid acids, including Amberlyst-15.[58] We cannot rule out that
4.86 kgLÀ1 hÀ1). Superior activity of the continuous-flow versus
batch design has been outlined in the past for catalytic entities
immobilized on inorganic supports,[50,62] which was explained
in terms of faster molecular flow to and from the active sites
owing to the large surface area of the solid in contact with the
flow (convective mass transfer). Minimization of active site in-
hibition by non-accumulation of strongly adsorbed co-prod-
ucts may be also of great relevance under flow conditions.[63]
With respect to other packed-bed catalysts under analogous
flow conditions, Pd@TiNT invariably showed better per-
formance (Table 4, entry 3 vs. 4–7). Particularly, compared with
conventional 3D supports, that is, the microporous Dowex-Li
Table 4. Catalytic hydrogenation reactions of 3-hexyn-1-ol (4).[a]
Entry
Reactor
Catalyst
Conversion
[%]
Selectivity
Yield 4a
TOF[d]
[hÀ1
STY[e]
Mass product.
[mol4a gPdÀ1 hÀ1
]
Ref.
ene
Z/E
[%][c]
[%][b]
[%]
]
[kgLÀ1 hÀ1
]
1
2
3
4
5
6
7
8
Batch
Flow
0.50% Pd@TiNT[f]
5% Pd(Pb)@CaCO3
0.50% Pd@TiNT
5% Pd(Pb)@CaCO3
1.2% Pd@Dowex-Li
1.2% Pd@SiO2
5% Pd@C
0.73% Pd@TiO2
0.2% Pd@TiO2 monolith
1.3% Pd@SiO2 monolith
0.7% Pd@MonoBor
93
99
88
99
75
40
94
84
61
85
99
98
98
94
64
80
87
22
80
63
80
94
97
96
93
62
89
93
81
85
87
80
93
88
93
77
39
53
32
17
57
33
54
86
1715
555
4945
44
352
929
530
296
350
80
0.01
n.d.[h]
4.86
0.25
1.02
1.77
0.90
0.29
0.93
0.27
0.85
15.3
4.9
40.6
0.2
2.3
7.1
0.9
19.2
1.8
0.5
6.8
This work
[53]
This work
[40]
[g]
[48]
This work
This work
This work
[50]
[49]
[40]
9
10
11
829
[a] Reaction conditions: room temperature, methanol. [b] (4a+4b)/(4a+4b+4c). [c] 4a/(4a+4b). [d] Turnover frequency: (mol substrate converted)/
(molPd”h). Calculated at the conversion indicated with bulk Pd content. [e] Space-time-yield calculated with the overall conversion indicated. [f] Reactor
volume 25 mL, 1 bar H2. [g] Reaction conditions: À108C, ethanol. [h] Not determined.
ChemCatChem 2016, 8, 1001 – 1011
www.chemcatchem.org
1006
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers
catalytic contribution by the well-exposed Pd sites on the
outer surface of the nanotubes.
Compared with the catalytic flow systems examined, the
higher productivity of Pd@TiNT may be attributed both to the
accessibility of Pd sites and to the low flow resistance of the
tubular material, which allows for a high flow rate of the sub-
strate solution. Indeed, a weight hourly space velocity of
5180 hÀ1 (WHSV=greactant gPdÀ1 hÀ1)[39] could be attained in the
case of alkyne 4, whereas in a check experiment under analo-
gous flow conditions using a packed bed of Pd immobilized
on the pores of conventional mesoporous powder titania,
a much lower efficiency was observed (Table 4, entry 8). Fila-
mentous materials have been previously reported to provide
significant benefits in both liquid- and gas-phase operations in
terms of low pressure drops, homogeneous flow patterns, and
minimization of mass transfer limitations. Nanosized fibrous
materials, either zeolites on sintered metal fibers[72] or metal
nanoparticles on activated carbon,[73] have been successfully
shaped into macro-structured catalytic beds by the group of
Kiwi-Minsker.
Figure 7. Continuous-flow hydrogenation reaction of 4 over packed-bed cat-
alysts. Left: conversion versus time-on-stream. Right: typical composition (%)
of the reaction mixture. Data for Pd@Dowex-Li taken from ref. [48].
resin, the mesoporous Davisil silica, and commercial carbon,
Pd@TiNT showed approximately 1 order magnitude higher TOF
values and a better partial cis-hydrogenation selectivity that,
coupled with a steady conversion over 5 h (Figure 7, left), re-
sulted in an overall better yield of 4a (Figure 7, right). Produc-
tivity for 4a was in the order Pd@TiNT@Pd@SiO2 >Pd@Do-
wex-Li>Pd@C. The latter catalyst resulted in a higher rate of
over hydrogenation to 1-hexanol 4c (yield>70%).
Conclusions
The performance of Pd@TiNT was also compared with that
of unconventional monolithic reactors.[64] The best results so
far reported in terms of conversion and selectivity were recent-
ly obtained by using Pd on the macroporous polymeric mono-
lith MonoBor (99% conversion, 94% ene selectivity, 93% cis,
Table 4, entry 11).[40] Somewhat lower catalytic efficiency was
reported for Pd on inorganic titania[50] and silica[49] monoliths
featuring hierarchically ordered meso- and macroporosity
(Table 4, entries 9 and 10). The Pd@TiNT system ranks just
below Pd@MonoBor. However, to the best of our knowledge,
once again Pd@TiNT shows the highest STY and 4a productivi-
ty among the reported systems.
The catalytic selective hydrogenation of unsaturated CÀC
bonds is a reaction of high current interest because of its cen-
tral relevance in the production of several fine-chemical inter-
mediates. In the case of alkynes and their partial hydrogena-
tion to alkenes, conventional Pd-based heterogeneous cata-
lysts are often problematic because of selectivity issues, over
hydrogenation to alkanes, isomerization, and limited resistance
of other functional groups. Strategies have been developed to
enhance the selectivity of these systems by addition of varia-
ble, often large, amounts of (toxic) contaminants. On the in-
dustrial scale, the reaction is accomplished in the liquid phase
by using Lindlar-type batch catalysts, with serious drawbacks
in terms of environmental impact, process economy, and cata-
lyst deactivation.
The above results illustrate the advantages of the 1D
Pd@TiNT catalyst, both in terms of selectivity and productivity,
in large-scale continuous-flow partial hydrogenation reactions.
If one skips the peculiar cases of heterogenized homogenous
catalysts[65] and the addition of modifiers to classic heterogene-
ous catalysts,[66] selectivity enhancement in porous heterogene-
ous catalysts has been previously addressed by the confine-
ment effect of mesoporous materials (particularly in enantiose-
lective catalysis),[67] by the size and shape control of the em-
bedded metal nanoparticles[68] and by the electronic effects
generated by the support,[69] particularly by the SMSI (strong
metal–support interaction) enhancement for titanium oxide-
based materials.[70] In the case of the partial hydrogenation of
alkynes, a short contact time for the intermediate alkene,
which quickly leaves the active metal after formation with no
possibility of further reaction, has been invoked for 3D egg-
shell Pd@TiS catalysts[71] and for Pd@MonoBor, whose scarce
swelling of the support in the reaction solvent limits the acces-
sibility of reactants only to the sites on the surface of the solid
catalyst.[40] We believe that this effect may also take place for
Pd@TiNT. Indeed, the high turnover frequency observed at
very short residence times (13 s, see below) indicates a major
Herein, we described a novel type of heterogeneous contin-
uous-flow system based on PdNPs on titanate nanotubes,
which is usable in the hydrogenation of various alkyne sub-
strates under mild reaction conditions, without additives or re-
generation steps, and that shows excellent performance in
terms of mass and space-time-yield productivity, selectivity,
and resistance. In particular, the leaf alcohol fragrance cis-3-
hexen-1-ol was continuously obtained in good yields and in an
unprecedented hourly productivity with no evidence of metal
contamination. The results obtained point to a reaction mecha-
nism involving adsorption of the alkyne substrate onto the
easily accessible Pd sites on the outer surface of the solid cata-
lyst. At high substrate flow rates, the short residence time re-
sults in a very fast interaction with the active sites and in the
continuous removal of the intermediate alkene formed by hy-
drogenation, with no possibility of further reduction, thus it is
extremely beneficial in terms of both selectivity and productivi-
ty of the semi-hydrogenation product. As previously reported
for comparable heterogeneous porous catalysts,[49,50] if the sub-
strate undergoes a significant adsorption on the metal sites
ChemCatChem 2016, 8, 1001 – 1011
www.chemcatchem.org
1007
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers
ground. GC analyses were performed on a Shimadzu GC-2010 Plus
gas chromatograph equipped with a flame ionization detector and
a 30 m (0.25 mm ID, 0.25 mm FT) Varian VF-WAXms capillary
column. GC-Ms analyses were performed on a Shimadzu QP2010S
spectrometer equipped with an identical capillary column. The pal-
ladium content in the supported catalysts was determined by in-
ductively coupled plasma optical emission spectroscopy (ICP-OES)
with a Varian 720ES instrument at a sensitivity of 500 ppb. Each
sample (20–50 mg) was treated in a microwave-heated digestion
bomb (Milestone, MLS-200) with concentrated HNO3 (1.5 mL), 98%
H2SO4 (2 mL), 37% HCl (0.5 mL), and a pellet of a digestion aid re-
agent (0.5 g, 0.1% Se in K2SO4). The content of palladium leached
into the catalytic solutions was determined by ICP-OES analysis by
using the above instrument. The solutions were analyzed directly
after 1:5 dilution in 0.1m hydrochloric acid. The Pd detection limit
was 0.006 ppm.
inside the pores of the mesoporous material, the formed
alkene cannot diffuse away fast enough and it is hydrogenated
before leaving the catalyst, resulting in a lower selectivity at
the same conversion level.
As for catalyst productivity, it is clear that microstructure of
the support is associated with the catalytic efficiency in rela-
tion to the minimization of mass transfer limitations typical of
tubular materials. Additionally, this feature combines favorably
with both the chemical and electronic structure of titanate,
which results in a strong anchoring and in a negligible sinter-
ing of the PdNPs, and with their location on the outer surface
of the support, which results in optimal site-accessibility.
Finally, it is worth mentioning that the clean method of cata-
lyst preparation fulfils the fundamental principles of greener
nanosynthesis.[74] Overall, the system reported may represent
a significant contribution to the development of more sustain-
able industrial processes.[75]
In Table 1, the Brunauer–Emmett–Teller (BET) surface area was cal-
culated by using the multipoint BET method using the adsorption
data in the relative pressure (P/P0) range of 0.05–0.30. The average
size of mesopores and larger mesopores were calculated from the
adsorption branch of the nitrogen isotherm by using the BJH
method. The mesopore volume was calculated from the adsorp-
tion branch of the nitrogen isotherm by using the BJH method,
the volume was measured at the plateau of the cumulative adsorp-
tion pore volume plot. The total pore volume was measured at
P/P0 =0.99. The pore volume of the larger mesopores, interparticle
volume, was calculated by subtracting both values.
Experimental Section
General information
All reagents were used as received from commercial suppliers
without further purification, except for benzylideneacetone, which
was recrystallized from n-pentane prior to use. 5% Pd@C was ob-
tained from Aldrich, Davisil silica was obtained from Grace. Quartz
double-distilled water was used throughout. TEM (transmission
electron microscopy) measurements were carried out on a CM12
PHILIPS instrument at 120 keV accelerating voltage. Samples were
prepared by grinding the samples in an agate mortar, followed by
sonication in ethanol and deposition of the supernatant on
a carbon-coated Cu TEM grid. Statistical nanoparticle size distribu-
tion analysis was typically carried out on 300–400 particles. ESEM
(environmental scanning electron microscopy) measurements were
performed on a FEI Quanta 200 microscope operating at 25 keV ac-
celerating voltage in the low-vacuum mode (1 Torr) and equipped
with an EDAX energy dispersive X-ray spectrometer (EDS). XRD (X-
ray diffraction) spectra were recorded on a PANanalytical XPERT
PRO powder diffractometer, employing CuKa radiation (l=
1.54187 Š), a parabolic MPD-mirror, and a solid-state detector
(PIXcel). The samples were prepared on a silicon wafer (zero back-
ground) that was rotating (0.5 rotations per second) during spectra
acquisition. All XRD spectra were acquired at room temperature in
a 2q range from 5 to 708, applying a step size of 0.02638 and
a counting time of 77.5 s. Nitrogen sorption measurements at 77 K
were carried out on a Micromeritics ASAP 2020 instrument after
outgassing the samples at 1508C. X-ray photoelectron spectrosco-
py (XPS) experiments were carried out in an ultrahigh vacuum
(UHV, 10À9 mbar) system equipped with a VSW HAC 500 hemi-
spherical electron-energy analyzer using a non-monochromatic
MgKa X-ray source operating at 120 W (10 kV”10 mA). The sam-
ples were introduced in the UHV system through a loadlock under
an inert gas (N2) flux to minimize the exposure to air contaminants,
and kept in the introduction chamber for at least 12 h before the
measurements. Ion sputtering was performed by using an argon
beam (chamber pressure 10À7 mbar) at 2 kV and 20 mA current for
3 min. Survey and high-resolution spectra (C1s, O1s, Pd3d, Ti 2p)
were acquired in the constant analyzer energy mode at a pass
energy Epass =22 eV with a step size of 1.0 and 0.1 eV, respectively.
The peaks were fitted by using CasaXPS software employing
Gauss–Lorentz curves after subtraction of a Shirley-type back-
Synthesis of titanate nanotubes
Titanates nanotubes (TiNT) were obtained by hydrothermal treat-
ment of commercial anatase powder (Alfa Aesar). In a typical pro-
cedure, TiO2 (0.8 g) were dispersed in 10.0m NaOH (30 mL) for
30 min under vigorous stirring (600 rpm). Then, the suspension
was placed in a Teflon bottle and heated at 110 8C for 90 h. The re-
sultant solid was recovered by centrifugation and washed with
0.1m HCl and water until a neutral pH of the eluate was obtained.
The final material was air dried at 408C for 2 days.
Synthesis of the Pd@TiNT catalyst
PdII species were incorporated into TiNT by an ion-exchange proce-
dure adapted from that reported.[31a] Thus, TiNT (0.5 g) was orbitally
stirred (50 rpm) in a water solution of Pd(NO3)2·2H2O (25 mL,
1.5 mm) for 3 h at room temperature. After that time, the yellow
color of the solution was transferred to the solid, whereas the re-
maining solution was colorless. The yellow solid was recovered by
filtration, washed with water (4”25 mL), and air dried at 408C for
2 days. Prior of use in catalysis, the TiNT-supported palladium(II)
species were reduced to Pd0 under a hydrogen atmosphere and at
room temperature by using the in-situ procedure described below
for the batch and continuous-flow setups. In both cases, the Pd
loading in the final material was 0.50wt% (ICP-OES).
Catalytic hydrogenation reactions
Batch setup: Catalytic batch reactions under a controlled pressure
of hydrogen were performed by using a non-metallic Büchi Mini-
clave (100 mL internal volume) equipped with a pressure controller.
In a typical reaction, methanol (6 mL) was loaded under nitrogen
into the autoclave containing the TiNT-supported PdII catalyst pre-
cursor (100 mg, 0.50 wt% Pd) and nitrogen was replaced by hydro-
ChemCatChem 2016, 8, 1001 – 1011
www.chemcatchem.org
1008
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers
gen with three cycles of pressurization (1 bar)/depressurization.
The autoclave was finally charged with H2 (1 bar) and stirred at
room temperature (200 rpm) for 1 h to allow for the reduction of
the metal. After that time, the color of the solid changed from
yellow to dark brown, indicating the formation of Pd0 nanoparti-
cles.
Centro Microscopie Elettroniche—Area di Ricerca CNR, Firenze,
for technical support.
Keywords: heterogeneous catalysis
·
hydrogenation
·
nanoparticles · nanotubes · supported catalysts
A deaerated substrate solution in methanol was then added by
using a gas-tight syringe (6 mL, 0.2m, substrate/Pd=255:1 mol).
The reaction progress was monitored by periodic sampling of the
reaction solution by using a gas-tight syringe equipped with a fil-
tering needle, followed by GC analysis. After the desired time, the
reactor was depressurized and the solution analyzed by ICP-OES
for the determination of the amount of palladium leached into the
solution. A sample of the solid catalyst recovered at the end of the
reaction was analyzed by ICP-OES (Pd loading) and TEM (particle
size). Parallel hydrogenation experiments showed no catalytic ac-
tivity of the solutions recovered after catalysis (Maitlis test[76]).
[1] The Handbook of Homogeneous Hydrogenations, Vol. 1 (Eds.: J. G.
de Vries, C. J. Elsevier), Wiley-VCH, Weinheim, 2007.
[2] a) G. J. Hutchings, J. Mater. Chem. 2009, 19, 1222–1235; b) B. Chen, U.
Dingerdissen, J. G. E. Krauter, H. G. J. Lansink Rotgerink, K. Mçbus, D. J.
Ostgard, P. Panster, T. H. Riermeier, S. Seebald, T. Tacke, H. Trauthwein,
Appl. Catal. A 2005, 280, 17–46.
[3] A. Molnµr, A. Sarkany, M. Varga, J. Mol. Catal. A 2001, 173, 185–221.
[4] a) D. J. Cole-Hamilton, R. P. Tooze, Catalyst Separation, Recovery and Re-
cycling; Chemistry and Process Design, Springer, Dordrecht, 2006; b) Fine
Chemicals Through Heterogeneous Catalysis (Eds.: R. A. Sheldon, H. van
Bekkum), Wiley-VCH, Weinheim 2004.
[5] a) S. Nishimura, Handbook of Heterogeneous Catalytic Hydrogenation for
Organic Synthesis, Wiley, New York, 2001, p. 720; b) A. O. King, R. D.
Larsen, E. Negishi, in Handbook of Organopalladium Chemistry for Or-
ganic Synthesis (Ed.: E. Negishi), Wiley, New York, 2002, p. 2719; c) H. U.
Blaser, C. Malan, B. Pugin, F. Spindler, H. Steiner, M. Studer, Adv. Synth.
Catal. 2003, 345, 103–151.
Continuous-flow setup: Catalytic flow hydrogenations were car-
ried out by using an home-made continuous-flow system con-
structed at ICCOM-CNR, Firenze, and allowing for concurrent flows
of substrate solution and hydrogen gas.[77] A commercial 6.6 mm
i.d. and adjustable bed length Omnifitꢁ glass column, equipped
with 2 mm polyethylene (PE) frits, was packed with 50 mg of the
TiNT-supported PdII catalyst precursor, then connected to the
system. The solid sample was treated with hydrogen (1.8 bar,
5 mLminÀ1 H2) at room temperature for 1 h under solvent-free con-
ditions, showing a fast color change from yellow to dark brown
and indicating the formation of the Pd0 catalyst.
[6] J. A. Dumesic, G. W. Huber, M. Boudart, in Handbook of Heterogeneous
Catalysis, 2nd ed. (Eds.: G. Ertl, H. Knçzinger, F. Schüth, J. Weitkamp),
Wiley-VCH, Weinheim, 2008, pp. 1–56.
[7] X. Y. Mak, P. Laurino, P. H. Seeberger, Beilstein J. Org. Chem. 2009, 5, 19.
[8] G. Jas, A. Kirschning, Chem. Eur. J. 2003, 9, 5708–5723.
[9] P. Styring, A. I. R. Parracho, Beilstein J. Org. Chem. 2009, 5, 29.
[10] For
a review, see: a) U. Hintermair, G. Franciò, W. Leitner, Chem.
Commun. 2011, 47, 3691–3701; b) J. Wegner, S. Ceylan, A Kirschning,
Adv. Synth. Catal. 2012, 354, 17–57; c) M. Irfan, T. N. Glasnov, C. O.
Kappe, ChemSusChem 2011, 4, 300–316; d) D. Webb, T. F. Jamison,
Chem. Sci. 2010, 1, 675–680; e) A. Kirschning, W. Sodolenko, K. Men-
necke, Chem. Eur. J. 2006, 12, 5972–5990.
After that time, a deaerated solution of substrate in methanol
(0.2m) was flowed through the column reactor at a constant rate
(1.1 mLminÀ1) and at room temperature, together with a constant
H2 flow (13.5 mLminÀ1). These flows resulted in a H2 pressure at
the reactor inlet of ca. 2.4 bar (corresponding to a H2/substrate
molar ratio of ca. 6.0), while the hydrogen gas was released at at-
mospheric pressure at the outlet of the reactor. Therefore, the
pressure drop generated by the packed-bed reactor was ca.
1.4 bar. The attainment of steady-state conditions (ca. 1 h) was
taken as the catalytic reaction start time. The reaction solution was
periodically analyzed for conversion by GC, and 6.0 mL aliquots
were sampled every 1 h for subsequent Pd leaching analysis by
ICP-OES. At the end of the reaction, a sample of the solid catalyst
was recovered and analyzed by ICP-OES (Pd loading).
[11] R. A. Sheldon, I. W. C. E. Arends, U. Hanefeld, Green Chemistry and Catal-
ysis, Wiley-VCH, Weinheim, 2007.
[12] a) C. Lucarelli, A. Vaccari, Green Chem. 2011, 13, 1941–1949; b) I. An-
drews, P. Dunn, J. Hayler, B. Hinkley, D. Hughes, B. Kaptein, K. Lorenz, S.
Mathew, T. Rammeloo, L. Wang, A. Wells, T. D. White, Org. Process Res.
Dev. 2011, 15, 22–30; c) C. JimØnez-Gonzµlez, P. Poechlauer, Q. B. Brox-
terman, B.-S. Yang, D. am Ende, J. Baird, C. Bertsch, R. E. Hannah, P. Del-
l’Orco, H. Noorman, S. Yee, R. Reintjens, A. Wells, V. Massonneau, J.
Manley, Org. Process Res. Dev. 2011, 15, 900–911.
[13] S. G. Newmana, K. F. Jensen, Green Chem. 2013, 15, 1456–1472.
[14] a) A. Pashkova, L. Greiner, Chem. Ing. Tech. 2011, 83, 1337–1342; b) V.
Hessel, presented at the 1st RSC/SCI Symposium on Continuous Process-
ing and Flow Chemistry, Stevenage, 3rd–4th November 2010.
[15] a) C. A. Witham, W. Huang, C. K. Tsung, J. N. Kuhn, G. A. Somorjai, F. D.
Toste, Nat. Chem. 2010, 2, 36–41; b) A. Fukuoka, P. L. Dhepe, Chem. Rec.
2009, 9, 224–235; c) N. Semagina, L. Kiwi-Minsker, Catal. Rev. Sci. Eng.
2009, 51, 147–217.
Synthesis of the Pd@SiO2 catalyst
The Pd@SiO2 catalyst was prepared by wet impregnation. The pal-
ladium precursor [Pd(NH3)4](NO3)2 (0.042 g) in distilled H2O (10 mL)
was added to a flask containing Davisil SiO2 (1 g). The pH of the
mixture was adjusted to 10 by addition of NH3. The solution was
kept under magnetic stirring at 100 rpm for 4 h. After that time,
the solid was recovered by filtration, dried in the oven at 408C for
72 h, and calcined at 5508C for 8 h. The reduction of palladium
was carried out by using a H2 flow at 2808C for 2 h. The Pd silica
material was obtained as a black powder with a 1.2wt% Pd load-
ing (ICP-OES).
[16] J. M. Campelo, D. Luna, R. Luque, J. M. Marinas, A. A. Romero, Chem-
SusChem 2009, 2, 18–45.
[17] Y. Li, X. Y. Yang, Y. Feng, Z. Y. Yuan, B. L. Su, Crit. Rev. Solid State 2012, 37,
1–74.
[18] Y. Matatov-Meytal, M. Sheintuch, Appl. Catal. A 2002, 231, 1–16.
[19] D. S. Su, S. Perathoner, G. Centi, Chem. Rev. 2013, 113, 5782–5816.
[20] a) Y. Liang, J. Li, S. Z. Wang, X. Z. Fu, Q. C. Xu, J. D. Lin, D. W. Liao, Catal.
Lett. 2008, 120, 236–242; b) S. Dominguez-Dominguez, A. Berenguer-
Murcia, B. K. Pradhan, A. Linares-Solano, D. Cazorla-Amoros, J. Phys.
Chem. C 2008, 112, 3827–3834; c) J. Garcia, H. T. Gomes, P. Serp, P.
Kalph., J. L. Figueiredo, J. L. Faria, Carbon 2006, 44, 2384–2391; d) A.
Chambers, T. Nemes, N. M. Rodriguez, R. T. K. Baker, J. Phys. Chem. B
1998, 102, 2251–2258.
[21] K. Lee, A. Mazare, P. Schmuki, Chem. Rev. 2014, 114, 9385–9454.
[22] a) D. Yang, H. Liu, Z. Zheng, S. Sarina, H. Zhu, Nanoscale 2013, 5, 2232–
2242; b) D. V. Bavykin, F. C. Walsh, Eur. J. Inorg. Chem. 2009, 977–997;
c) H. H. Ou, S. L. Lo, Sep. Purif. Technol. 2007, 58, 179–191; d) D. V. Bavy-
Acknowledgements
Thanks are due to the EC Marie-Curie Initial Training Network
nano-HOST for funding (Grant Agreement PITN-215193) and to
ChemCatChem 2016, 8, 1001 – 1011
www.chemcatchem.org
1009
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers
kin, J. M. Friedrich, F. C. Walsh, Adv. Mater. 2006, 18, 2807–2824; e) D. V.
Bavykin, V. N. Parmon, A. A. Lapkin, F. C. Walsh, J. Mater. Chem. 2004, 14,
3370–3377.
mann, N. Xanthopoulos, A. Renken, L. Kiwi-Minsker, J. Catal. 2007, 251,
213–222.
[46] M. Crespo-Quesada, F. Cµrdenas-Lizana, A.-L. Dessimoz, L. Kiwi-Minsker,
ACS Catal. 2012, 2, 1773–1786.
[47] E. V. Rebrov, E. A. Klinger, A. Berenguer-Murcia, E. M. Sulman, J. C.
Schouten, Org. Process Res. Dev. 2009, 13, 991–998.
[48] C. Moreno-Marrodan, P. Barbaro, M. Catalano, A. Taurino, Dalton Trans.
2012, 41, 12666–12669.
[49] A. Sachse, N. Linares, P. Barbaro, F. Fajula, A. Galarneau, Dalton Trans.
2013, 42, 1378–1384.
[50] N. Linares, S. Hartmann, A. Galarneau, P. Barbaro, ACS Catal. 2012, 2,
2194–2198.
[51] F. Liguori, P. Barbaro, H. Sawa, Appl. Catal. A 2014, 488, 58–65.
[52] a) J. Warr, S. Fraser, O. Gouault (Takasago Perfumery Co., Ltd.) Pat. EP
1964542, 2008; b) J. Y. Choi, B. B. Jeon, H. J. Seo (Amorepacific Corp.)
Pat. WO 2007055493, 2007.
[53] a) H. Bçnnemann, W. Brijoux, K. Siepen, J. Hormes, R. Franke, J. Poll-
mann, J. Rothe, Appl. Organomet. Chem. 1997, 11, 783–796; b) H. Bçn-
nemann, W. Brijoux, A. Schultze Tilling, K. Siepen, Top. Catal. 1997, 4,
217–227.
[23] M. Kitano, K. Nakajima, J. N. Kondo, S. Hayashi, M. Hara, J. Am. Chem.
Soc. 2010, 132, 6622–6623.
[24] a) X. Yang, L. Wu, L. Ma, X. Li, T. Wang, S. Liao, Catal. Commun. 2015, 59,
184–188; b) V. Bem, M. C. Neves, M. R. Nunes, A. J. Silvestre, O. C. Mon-
teiro, J. Photochem. Photobiol. A 2012, 232, 50–56; c) C. Y. Hsu, T. C.
Chiu, M. H. Shih, W. J. Tsai, W. Y. Chen, C. H. Lin, J. Phys. Chem. C 2010,
114, 4502–4510; d) K. Jabou, H. Kochkar, G. Berhault, A. Ghorbel, J.
Mater. Sci. 2009, 44, 6677–6682; e) L. Torrente-Murciano, A. A. Lapkin,
D. V. Bavykin, F. C. Walsh, K. Wilson, J. Catal. 2007, 245, 272–278; f) L. M.
Sikhwivhilu, N. J. Coville, B. M. Pulimaddi, J. Venkatreddy, V. Vishwana-
than, Catal. Commun. 2007, 8, 1999–2010.
[25] a) S. Jiao, Y. Chen, M. Xu, Y. Zhang, D. Wang, G. Pang, S. Feng, Mater.
Lett. 2010, 64, 1704–1706; b) J. A. Toledo-Antonio, M. A. Cortes-Jµcome,
C. Angeles-Chavez, E. López-Salinas, P. Quintana, Langmuir 2009, 25,
10195–10201; c) J. Jiang, Q. Gao, Z. Chen, J. Hu, C. Wu, Mater. Lett.
2006, 60, 3803–3808.
[26] a) M. MØndez-Cruz, J. Ramírez-Solís, R. Zanell, Catal. Today 2011, 166,
172–179; b) A. V. Grigorieva, E. A. Goodilin, L. E. Derlyukova, T. A. Anu-
frieva, A. B. Tarasov, Y. A. Dobrovolskii, Y. D. Tretyakov, Appl. Catal. A
2009, 362, 20–25; c) J. Y. Tsai, J. H. Chao, C. H. Lin, J. Mol. Catal. A 2009,
298, 115–124; d) J. Jiang, Q. Gao, Z. Chen, J. Mol. Catal. A 2008, 280,
233–251; e) L. M. Sikhwivhilu, N. J. Coville, D. Naresh, K. V. R. Chary, V.
Vishwanathan, Appl. Catal. A 2007, 324, 52–61; f) D. V. Bavykin, A. A.
Lapkin, P. K. Plucinski, J. M. Friedrich, F. C. Walsh, J. Catal. 2005, 235, 10–
17.
[54] a) A. Schmidt, A. Wolf, R. Warsitz, R. Dittmeyer, D. Urbanczyk, I. Voigt, G.
Fischer, R. Schomäcker, AIChE J. 2008, 54, 258–268; b) J. Caro, K. J.
Caspary, C. Hamel, B. Hoting, P. Kollsch, B. Langanke, K. Nassauer, T.
Schiestel, A. Schmidt, R. Schomäcker, A. Seidel-Morgenstern, E. Tsotsas,
I. Voigt, H. Wang, R. Warsitz, S. Werth, A. Wolf, Ind. Eng. Chem. Res.
2007, 46, 2286–2294.
[55] E. Sulman, V. Matveeva, V. Doluda, L. Nicoshvili, L. Bronstein, P. Valetsky,
I. Tsvetkova, Top. Catal. 2006, 39, 187–190.
[56] F. Liguori, P. Barbaro, Catal. Sci. Technol. 2014, 4, 3835–3839.
[57] a) T. Mitsudome, Y. Takahashi, S. Ichikawa, T. Mizugaki, K. Jitsukawa, K.
Kaneda, Angew. Chem. Int. Ed. 2013, 52, 1481–1485; Angew. Chem.
2013, 125, 1521–1525; b) Y. Takahashi, N. Hashimoto, T. Hara, S. Shima-
zu, T. Mitsudome, T. Mizugaki, K. Jitsukawa, K. Kaneda, Chem. Lett. 2011,
40, 405–407; c) M. P. R. Spee, J. Boersma, M. D. Meijer, M. Q. Slagt, G.
van Koten, J. W. Geus, J. Org. Chem. 2001, 66, 1647–1656; d) T. A. Nij-
huis, G. van Koten, F. Kapteijn, J. A. Moulijn, Catal. Today 2003, 79–80,
315–321.
[58] a) A. M. R. Galletti, C. Antonetti, V. De Luise, M. Martinelli, Green Chem.
2012, 14, 688–694; b) T. Seki, J. D. Grundwaldt, N. van Vegten, A. Baiker,
Adv. Synth. Catal. 2008, 350, 691–705; c) T. Miller, B. L. Mojet, D. E. Ram-
aker, D. C. Koningsberger, Catal. Today 2000, 62, 101–114; d) W. Laufer,
J. P. M. Niederer, W. F. Hoelderich, Adv. Synth. Catal. 2002, 344, 1084–
1089.
[27] T. Kasuga, M. Hiramatsu, A. Hoson, T. Sekino, K. Niihara, Adv. Mater.
1999, 11, 1307–1311.
[28] C. Moreno Marrodan, D. Berti, F. Liguori, P. Barbaro, Catal. Sci. Technol.
2012, 2, 2279–2290.
[29] E. Morgado, Jr., M. A. S. de Abreu, G. T. Moure, B. A. Marinkovic, P. M.
Jardim, A. S. Araujo, Chem. Mater. 2007, 19, 665–676.
[30] L. Q. Weng, S. H. Song, S. Hodgson, A. Baker, J. Yu, J. Eur. Ceram. Soc.
2006, 26, 1405–1409.
[31] a) H. Kochkar, A. Turki, L. Bergaoui, G. Berhault, A. Ghorbel, J. Colloid In-
terface Sci. 2009, 331, 27–31; b) X. Sun, Y. Li, Chem. Eur. J. 2003, 9,
2229–2238.
[32] G. Abellµn, A. I. Carrillo, N. Linares, E. Serrano, J. Garcia-Martinez, J. Solid
State Chem. 2009, 182, 2141–2148.
[33] D. V. Bavykin, A. A. Lapkin, P. K. Plucinski, L. Torrente-Murciano, J. M. Frie-
drich, F. C. Walsh, Top. Catal. 2006, 39, 151–160.
[34] R. L. Johnston, Atomic and Molecular Clusters, Taylor & Francis, London,
2002.
[35] a) N. G. Semaltianos, P. Petkov, S. Scholz, L. Guetaz, J. Colloid Interface
Sci. 2013, 402, 307–311; b) K. McEleney, C. M. Crudden, J. H. Horton, J.
Phys. Chem. C 2009, 113, 1901–1907.
[36] E. Coronado, A. Ribera, J. Garcia-Martinez, N. Linares, L. M. Liz-Marzan, J.
Mater. Chem. 2008, 18, 5682–5688.
[37] M. Muniz-Miranda, S. Caporali, P. Marsili, E. Giorgetti, Mater. Chem. Phys.
2015, 167, 188–193.
[38] The accepted definition of STY is: “the number of molecules of a speci-
fied product made per unit volume of reactor per unit time”. See, e.g.:
D. Pletcher, F. Walsh, Industrial Electrochemistry, Springer, London, 1990,
p. 83.
[39] a) M. Boudart, Chem. Rev. 1995, 95, 661–666; b) IUPAC, Manual of Sym-
bols and Terminology for Physicochemical Quantities and Units—Ap-
pendix II, R. L. Burwell, Jr., Pure Appl. Chem. 1976, 46, 71–90.
[40] F. Liguori, P. Barbaro, J. Catal. 2014, 311, 212–220.
[41] R. Tschan, R. Wandeler, M. S. Schneider, M. M. Schubert, A. Baiker, J.
Catal. 2001, 204, 219–229.
[59] a) Y. Yuan, N. Yan, P. J. Dyson, ACS Catal. 2012, 2, 1057–1069; b) B.
Corain, M. Kralik, J. Mol. Catal. A 2000, 159, 153–162.
[60] W. P. Zhou, A. Lewera, R. Larsen, R. I. Masel, P. S. Bagus, A. Wieckowski, J.
Phys. Chem. B 2006, 110, 13393–13398.
[61] Yields of 4a by using Pd@TiNT under batch conditions also compare
positively with those reported for Pd on charcoal and hydrotalcite, see:
P. T. Witte (BASF Catalysts LLC) Pat. WO 2009096783, 2009 and
J. C. A. A. Roelofs, P. H. Berben, Chem. Commun. 2004, 970–971.
[62] A. El Kadib, R. Chimenton, A. Sachse, F. Fajula, A. Galarneau, B. Coq,
Angew. Chem. Int. Ed. 2009, 48, 4969–4972; Angew. Chem. 2009, 121,
5069–5072.
[63] N. G. Anderson, Org. Process Res. Dev. 2012, 16, 852–869.
[64] The IUPAC definition of monoliths is “a shaped, fabricated intractable
article with a homogeneous microstructure that does not exhibit any
structural components distinguishable by optical microscopy” Defini-
tions of terms relating to the structural and processing of sols, gels, net-
works and inorganic–organic hybrid materials, IUPAC Recommendations,
2007, p. 1812.
[65] Heterogenized Homogeneous Catalysts for Fine Chemicals Production
(Eds.: P. Barbaro, F. Liguori), Springer, London, 2010.
[42] M. Besson, P. Gallezot, Catal. Today 2003, 81, 547–559.
[43] 3-Buten-2-ol, 2-methyl, CAS No.: 115-18-4, UNEP Publications, Interna-
tional Programme on Chemical Safety (IPCS).
[66] T. Mallat, A. Baiker, Appl. Catal. A 2000, 200, 3–11.
[67] a) C. Yu, J. He, Chem. Commun. 2012, 48, 4933–4940; b) X. Pan, X. Bao,
Acc. Chem. Res. 2011, 44, 553–562; c) F. Goettmann, C. Sanchez, J.
Mater. Chem. 2007, 17, 24–30.
[44] H. Lindlar, Helv. Chim. Acta 1952, 35, 446–450.
[45] a) L. N. Protasova, E. V. Rebrov, K. L. Choy, S. Y. Pung, V. Engels, M. Cabaj,
A. E. H. Wheatley, J. C. Schouten, Catal. Sci. Technol. 2011, 1, 768–777;
b) M. Crespo-Quesada, M. Grasemann, N. Semagina, A. Renken, L. Kiwi-
Minsker, Catal. Today 2009, 147, 247–254; c) N. Semagina, M. Grase-
[68] a) E. Gross, J. H. C. Liu, F. D. Toste, G. A. Somorjai, Nat. Chem. 2012, 4,
947–952; b) P. Christopher, S. Linic, J. Am. Chem. Soc. 2008, 130, 11264–
11265.
[69] P. Gallezot, D. Richard, Catal. Rev. Sci. Eng. 1998, 40, 81–126.
ChemCatChem 2016, 8, 1001 – 1011
www.chemcatchem.org
1010
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers
[70] For strong metal–support interactions, see: A. Lewera, L. Timperman,
A. Roguska, N. Alonso-Vante, J. Phys. Chem. C 2011, 115, 20153–20159.
[71] P. T. Witte, S. Boland, F. Kirby, R. van Maanen, B. F. Bleeker, D. A. Matthijs
de Winter, J. A. Post, J. W. Geus, P. H. Berben, ChemCatChem 2013, 5,
582–587.
[75] R. Dach, J. J. Song, F. Roschangar, W. Samstag, C. H. Senanayake, Org.
Process Res. Dev. 2012, 16, 1697–1706.
[76] J. P. Collman, K. M. Kosydar, M. Bressan, W. Lamanna, T. Garrett, J. Am.
Chem. Soc. 1984, 106, 2569–2579.
[77] C. Moreno-Marrodan, P. Barbaro, Green Chem. 2014, 16, 3434–3438.
[72] I. Yuranov, A. Renken, L. Kiwi-Minsker, Appl. Catal. A 2005, 281, 55–60.
[73] T. Yuranova, L. Kiwi-Minsker, C. Franch, A. E. Palomares, S. Armenise, E.
García-Bordeje, Ind. Eng. Chem. Res. 2013, 52, 13930–13937.
[74] J. A. Dahl, B. L. S. Maddux, J. E. Hutchison, Chem. Rev. 2007, 107, 2228–
2269.
Received: October 14, 2015
Revised: December 29, 2015
Published online on February 4, 2016
ChemCatChem 2016, 8, 1001 – 1011
www.chemcatchem.org
1011
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim