Mechanistic insights into the hydroconversion of cinnamaldehyde using mechanochemically-synthesized Pd/Al-SBA-15 catalysts

Article abstract of DOI:10.1039/c4gc01354a

The hydroconversion of cinnamaldehyde as an α,β-unsaturated compound was studied using a simple and efficient hydrogen-donating protocol catalyzed by mechanochemically synthesized bifunctional Pd/Al-SBA-15 catalysts. Materials were characterized using TGA-TDA, nitrogen physisorption, TEM and EDX analyses. Catalytic results pointed to the presence of competitive pathways that are able to provide a selectivity switch from the expected fully hydrogenated aromatic ring (e.g. cyclohexane) and hydrogenated products (e.g. hydrocinnamaldehyde, cinnamyl alcohol and 3-phenylpropan-1-ol) to the unexpected ethylbenzene and oxalic acid products from hydrodeformylation and hydrocarboxylation reactions of cinnamaldehyde and formic acid, respectively.

Full text of DOI:10.1039/c4gc01354a

View Article Online
View Journal
Green
Chemistry
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: A. Yepez, J. M.
Hidalgo Herrador, A. Pineda, R. Cerny, P. Jisa, A. Garcia, A. A. ROMERO REYES and R. Luque, Green
Chem., 2014, DOI: 10.1039/C4GC01354A.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/greenchem

Page 1 of 21
Green Chemistry
View Article Online
DOI: 10.1039/C4GC01354A
Mechanistic insights on the hydroconversion of cinnamaldehyde using
mechanochemicallyꢀsynthesized Pd/AlꢀSBAꢀ15 catalysts
Alfonso Yepeza, José M. Hidalgo*a,b, Antonio Pinedaa, Radek Černýb, Petr Jíšab, Angel
Garciaa, Antonio A. Romeroa, Rafael Luquea
aDepartamento de Química Orgánica, Universidad de Córdoba, Campus de Rabanales,
Edificio Marie Curie (C-3), Ctra Nnal IV-A, Km 396, E-14014, Córdoba (Spain), Tel: +34
957211050, Fax: +34 957212066, e-mail: q62alsor@uco.es
bResearch Institute of Inorganic Chemistry (VUAnCh), Department of Efficient Refining
Technologies (UniCRE-EFFRET), Chempark Litvínov, 43670 Litvínov, Czech Republic,
email: jose.hidalgo@vuanch.cz
Abstract
The hydroconversion of cinnamaldehyde as α,βꢀunsaturated compound was studied using a
simple and efficient hydrogenꢀdonating protocol catalyzed by mechanochemically
synthesized bifunctional Pd/AlꢀSBAꢀ15 catalysts. Materials were characterized using
TGAꢀTDA, nitrogen physisorption, TEM and EDX analyses. Catalytic results pointed to the
presence of competitive pathways able to provide a selectivity switch from expected fully
hydrogenated aromatic ring (e.g. cyclohexane) and hydrogenated products (e.g.
hydrocinnamaldehyde, cinnamyl alcohol and 3ꢀphenylpropanꢀ1ꢀol) to unexpected
ethylbenzene and oxalic acid products from hydrodeformylation and hydrocarboxylation
reactions of cinnamaldehyde and formic acid, respectively.
Keywords: Pd/AlSBA-15, cinnamaldehyde conversion, microwave irradiation

Green Chemistry
Page 2 of 21
View Article Online
DOI: 10.1039/C4GC01354A
Introduction
The selective hydrogenation of α,βꢀunsaturated aldehydes has attracted significant
interest in the production of fine chemicals especially for the fragrances and flavoring
industries.1 In particular, the hydroconversion of cinnamaldehyde to various important
derivatives including cinnamyl alcohol and hydrocinnamaldehyde has been the subject of
mechanistic studies with different types of catalytic systems to investigated C=C vs C=O
hydrogenation selectivities and the potential existence of structure sensitive processes.2ꢀ4 In
any case, the process has also relevance from the industrial point of view as typically obtained
products (e.g. cinnamyl alcohol, hydrocinnamaldehyde) can find applications as additives in
the food and pharmaceutical industries.5
Reaction pathways and mechanism in cinnamaldehyde hydroconversion have been
proposed under a rather general consensus that hydrogenations, hydrogenolysis and
dehydrations are key processes leading to most compounds illustrated in Scheme 1.2ꢀ7 A wide
range of conditions and several heterogeneous catalysts have been utilized in the reaction,
mostly supported noble metal catalysts (e.g. Pd, Pt, Ru) on porous materials including
zeolites, silicates (SBA and MCMꢀtype materials), alumina, etc. at moderate temperatures and
high pressures of hydrogen.5, 8ꢀ12 Interestingly, no literature reports could be found on the use
of hydrogenꢀdonating solvents for cinnamaldehyde hydroconversion.
Formic acid is a biomassꢀderived chemical that readily decomposes under certain
metalꢀcatalysed and temperature conditions (e.g. microwaveꢀirradiation) into H2 and a
mixture of CO + CO2, providing a potentially useful alternative to conventional hydrogenꢀ
donors (e.g. isopropanol) in hydrogenꢀtransfer reactions13ꢀ15 as well as in nanomaterials
synthesis.16, 17 Due to its acid properties, formic acid has additionally been proven to be highly
effective in promoting acidolytic cleavage of CꢀC and CꢀO bonds in starting materials as
recalcitrant as lignin.18

Page 3 of 21
Green Chemistry
View Article Online
DOI: 10.1039/C4GC01354A
Scheme 1. Reaction scheme of the cinnamaldehyde hydrogenation. Adapted from reference 3.
In light of the excelling features of formic acid as hydrogenꢀdonor and (coꢀ)catalyst as
well as current possibilities to be derived from biomass (concomitantly produced with
levulinic acid from cellulose hydrolysis/dehydration19, 20), we were prompted to explore a
novel cinnamaldehyde hydroconversion strategy using formic acid as both hydrogenꢀdonating
solvent and coꢀcatalyst under both conventional heating and microwave irradiation.
Microwave protocols have been extensively reported to accelerate rates of reaction as
compared to conventionally heated protocols, often changing selectivities to products, due to
the rapid a homogeneous heating achieved in the bulk solution as compared to conventional
heating.21 Previous work from the group also pointed out a synergetic effect between the use
of microwave irradiation and formic acid decomposition catalyzed by supported metal
nanoparticles.17, 18

Green Chemistry
Page 4 of 21
View Article Online
DOI: 10.1039/C4GC01354A
Herein, we report the mechanochemical preparation of various Pdꢀcontaining
AlꢀSBAꢀ15 with different Pd contents as bifunctional catalysts for the hydroconversion of
cinnamaldehyde using formic acid. Results of the activity of supported Pd aluminosilicates
were compared to that of a commercial 5%Pd/C.
Experimental
All chemicals and reagents used in this study (at least 99% purity) were purchased
from Sigma Aldrich (unless otherwise stated) and were not further purified.
Synthesis of supports
AlꢀSBAꢀ15 materials (Si/Al=20 ratio) were synthesized according to a previously
reported protocol. A solution (300 mL) of hydrochloric acid at pH = 1.5 was prepared, then
8.07 g of surfactant (Poly (ethyleneꢀglycol)ꢀblock (P123 Aldrich) was added to this solution.
The mixture was stirred until having one final transparent solution. 0.82 g of Aluminum
isopropoxide 98 wt. % (Aldrich) was then added to the mix followed by 18 mL TEOS
(tetraethylortosilicate) dropwise (time 5ꢀ10 min). The final gel was stirred during 24 h and
introduced in a furnace at 100 °C, t = 24 h. The products obtained were filtered and calcined
in air at 600 °C during 8 h (2 h in nitrogen atmosphere and 6 h in air).
Mechanochemical synthesis of Pd/Al-SBA-15
Pd/AlꢀSBAꢀ15 materials were synthesized following a previously reported
mechanochemical protocol.17, 22 Different quantities of solid Pd precursor [palladium (II)

Page 5 of 21
Green Chemistry
View Article Online
DOI: 10.1039/C4GC01354A
acetate] to reach theoretical Pd contents of 0.5, 1, 2 and 4 wt.% Pd in the materials were
milled together with the preꢀformed AlꢀSBAꢀ15 in the solid phase in a Retsch PM 100
bioMETA planetary ball mill (optimum conditions: 350 rpm, 10 minutes milling17). Upon
milling, the final mechanochemical material obtained was calcined at 450°C in air
(10 °C/min) for 2 hours. For simplicity, supported Pd nanoparticle AlꢀSBAꢀ15 materials have
been denoted as PdXꢀAl where X stands for the Pd content in the materials and Al refers to
AlꢀSBAꢀ15. A commercial 5%Pd/C catalyst (SigmaꢀAldrich) has been also employed as
reference catalyst for comparative purposes.
Catalyst characterisation
Thermal analysis was performed by simultaneous TGꢀDTA measurement using a
System Setaram Setsys 12 TGA instrument. Samples were heated at a heating rate of
10ºC minꢀ1, in air (40 mL minꢀ1) at the temperature range 50ꢀ1000 ºC. The compound αꢀAl2O3
was used as a reference material.
Nitrogen adsorption measurements were carried out at 77 K using an ASAP 2000
volumetric adsorption analyzer from Micromeritics. The samples were outgassed for 2 h at
373 K under vacuum (p < 10ꢀ2 Pa) and subsequently analyzed. The linear part of the BET
equation (relative pressure between 0.05 and 0.30) was used for the determination of the
specific surface area. The pore size distribution was calculated from the adsorption branch of
the N2 physisorption isotherms and the BarretꢀJoynerꢀHalenda (BJH) formula. The cumulative
mesopore volume VBJH was obtained from the PSD curve.
Energyꢀdispersive Xꢀray spectroscopy (EDX) measurements were performed using a
Scanning Electron Microscope (SEM) JEOL JSM 6300 provided with a microanalysis system

Green Chemistry
Page 6 of 21
View Article Online
DOI: 10.1039/C4GC01354A
Inca Energy 250, with a SiLi detector. Detection interval: from boron to uranium, resolution:
137 eV to 5.9 keV.
Catalytic experiments
Conventional heating reactions
Reactions under conventional heating were conducted in a Carousel Radleys
multiꢀreaction system (12 reaction vials) with controllable temperature using a thermocouple.
In a typical experiments, 0.5 mL cinamaldehyde (4 mmol), 1.5 mL formic acid (40 mmol),
10 mL acetonitrile, 1 g catalyst were stirred together at 82 ºC (reaction temperature) for the
desired time of reaction. The reaction was conducted at atmospheric pressure. Samples were
then withdrawn from the reaction mixture and analyzed by GC using an Agilent 6890N fitted
with a capillary column HPꢀ5 (30 m × 0.32 mm × 0.25 m) and a flame ionisation detector
(FID). The identity of the products was confirmed by GC–MS.
Microwave-assisted reactions
Microwave experiments were carried out in a CEMꢀDISCOVER model with PC
control. Experiments were performed on a closed vessel mode (pressure controlled) under
continuous stirring. The microwave method was generally powerꢀcontrolled where reaction
mixtures were irradiated with a power output of 200 W (180ºC maximum temperature
reached, average temperature = 150ºC) under autogenously generated pressures for 15 min
(maximum 250 PSI, 17.25 bar). In a typical reaction, 0.1 mL cinnamaldehyde (0.8 mmol) and
0.3 mL formic acid (8 mmol) in 2 mL acetonitrile were microwaved in the presence of the
catalyst (0.5 g) for 15 min at 200 W. Products were identified and quantified in a similar way
as reported above.

Page 7 of 21
Green Chemistry
View Article Online
DOI: 10.1039/C4GC01354A
Results and discussion
TGAꢀDTA analyses were carried out to study the desorptionꢀcalcination of the parent
materials (filtered material during the synthesis, previous to calcination at 600 °C to obtain
the SBAꢀ15 solids without added Pd). The first weight loss at 100 °C (endothermic – TDA
signal) of 2ꢀ5 wt.% was due to the presence of water over the catalyst. The next weight loss
(exothermic signals; T = 200 – 500 °C; 35 wt.% mass loss, Figure 1) can be assigned to the
combustionꢀdesorption of carbonaceous species used for the catalyst synthesis (mainly
surfactants). From 500 °C only a small amount of weight loss was detected (dehydroxylation
of –OH species over the surface of the support). DTA experiments showed an endothermic
signal at ca. 100 °C and then two clear exothermic signals (230 and 270 °C) from the
desorption ꢀ combustion of the surfactant species (Figure 2).
Temperature (°C)
50
250
450
650
850
0
ꢀ10
ꢀ20
ꢀ30
ꢀ40
ꢀ50
Figure 1. TGA of AlꢀSBAꢀ15 prior to calcination.

Green Chemistry
Page 8 of 21
View Article Online
DOI: 10.1039/C4GC01354A
Temperature (°C)
50
250
450
650
850
0
ꢀ4
ꢀ8
ꢀ12
ꢀ16
ꢀ20
Figure 2. DTA of parent AlꢀSBAꢀ15 before calcination.
Textural properties of mechanochemically synthesized supported nanomaterials have
been included in Table 1. As expected, a significant decrease on surface area of all catalysts
could be observed upon metal incorporation. Pore sizes were also observed to decrease
particularly for samples with higher metal loading (e.g. Pd4ꢀAl = 4%Pd/AlꢀSBAꢀ15).
Interestingly, no significant pore blocking could be observed at low incorporated metal
loadings (nanoparticles are distributed within the external surface and the pores of the
material as previously demonstrated17, 22). Pd4ꢀAl experienced a remarkable reduction in pore
size as compared to parent AlꢀSBAꢀ15 (from 8.9 to 6.8) which may be due to a partial pore
blocking upon Pd incorporation within the pores of the material. Pore volumes also followed a
similar trend, being significantly reduced upon metal incorporation in AlꢀSBAꢀ15.
Representative TEM images of the metalꢀcontaining SBAꢀ15 materials as compared to the
parent AlꢀSBAꢀ15 have been included in Figure 3. A well structurally ordered AlꢀSBAꢀ15
material can be observed in Figure 3A, with clearly visible mesopore sizes in good agreement
with porosity data (< 10 nm, Figure 3A).

Page 9 of 21
Green Chemistry
View Article Online
DOI: 10.1039/C4GC01354A
Table 1. Nitrogen physisorption and EDX elemental analysis.
Si/M
Metal
SBET DBJH
(m2gꢀ1) (Å)
VBJH
Catalyst
Molar ratioa Loadinga
(M= Zr, Al) (wt. %)
(mL gꢀ1)
AlꢀSBAꢀ15
Pd0.5ꢀAl
Pdꢀ1ꢀAl
ꢀ
ꢀ
756
625
533
363
527
1220
8.9
8.6
8.5
8.1
6.8
ꢀb
0.72
0.50
0.32
0.24
0.51
1.5
29.5
30.2
28.2
33.5
ꢀ
0.4
1.2
1.9
2.9
5
Pdꢀ2ꢀAl
Pdꢀ4ꢀAl
5%Pd/C
aMeasured by EDX; bPd/C is a microporous material
Upon Pd incorporation via ball milling, TEM images clearly depict a fairly good
dispersion of tiny Pd nanoparticles in the materials, with <10 nm as average NP size (ca. 7ꢀ9
nm). No observable sintering could be found for low loaded materials (e.g. 0.5 to 2%Pd,
Figure 3B and C) although the presence of significantly larger aggregates could be observed
at higher loadings (Figure 3Cꢀbottom of imageꢀand 3D). The structural order in SBAꢀ15 was
largely preserved upon milling although the presence of some amorphous silica domains was
also evidenced in TEM images (Figures 3B and C).17
The catalytic activity of mechanochemically synthesized Pd/AlꢀSBAꢀ15 was
subsequently investigated in the hydroconversion of cinnamaldehyde with formic acid under
conventional heating using acetonitrile as solvent under reflux conditions. Formic acid has
been previously reported to readily decompose into CO + CO2, hydrogen and water under

Green Chemistry
Page 10 of 21
View Article Online
DOI: 10.1039/C4GC01354A
heating conditions in the presence of noble metals, providing an in-situ source of hydrogen for
hydrogenꢀtransfer reactions.12
Figure 3. TEM images of A) AlꢀSBAꢀ15 support; B) Pd1ꢀAl; C) Pd2ꢀAl; D) Pd4ꢀAl
Results of catalytic activity were compared to those of a commercial 5%Pd/C. Most
Pd/AlꢀSBAꢀ15 catalysts exhibited an almost negligible conversion to products (< 10 %) even
after 24 h reaction with the exception of Pdꢀ4ꢀAl for which a 50% conversion was obtained
after 24 h (Table 2, Figure 4). The commercial 5%Pd/C system provided almost quantitative
conversion after 24 h, with a remarkably different selectivity to that of Pd/AlꢀSBAꢀ15
catalysts (Figures 4 and 5). Taking into account the Pd loadings in the catalysts (2.9 wt.% in
Pd4ꢀAl and 5 wt.% in Pd/C) and the reaction profiles depicted in Figure 4, it can be clearly

Page 11 of 21
Green Chemistry
View Article Online
DOI: 10.1039/C4GC01354A
visualized that Pd/C possessed almost comparable rates of reaction with respect to Pd4ꢀAl
with very slow initial reaction rates, after which Pd/C provided improved reaction rates after
prolonged reaction times (>4 h). These results may indicate some diffusional/mass transfer
limitations in Pd/C as compared to the lower active Pd/AlꢀSBAꢀ15 materials (with
significantly reduced Pd content) as expected from the inherent microporous nature of Pd/C.
Table 2. Comparison of catalytic activity between Pd/AlꢀSBAꢀ15 materials and commercial
5%Pd/C in the hydroconversion of cinnamaldehyde under conventional heating.
Catalyst
Time of reaction Conversion
(h)
24
1
(mol%)
<1
ꢀ
Blank
Pd0.5ꢀAl
4
<5
<5
ꢀ
24
1
Pd1ꢀAl
Pd2ꢀAl
Pd4ꢀAl
5%Pd/C
4
<5
<5
ꢀ
24
1
4
5
24
1
10
10
17
50
12
36
96
4
24
1
4
24
Reaction conditions: 0.5 mL cinamaldehyde, 1.5 mL formic acid, 10 mL acetonitrile, 1 g cat,
83ºC reaction temperature.

Green Chemistry
Page 12 of 21
View Article Online
DOI: 10.1039/C4GC01354A
Selectivities were significantly different between both systems as illustrated in
Figure 5. The three main competitive and most plausible pathways identified for the products
obtained in the hydroconversion of cinnamaldehyde with formic acid have been summarized
in Scheme 2. In pathway I, cinammaldehyde undergoes C=O hydrogenation to cinnamyl
alcohol (Pd sites) followed by dehydration to βꢀmethylstyrene (promoted by acid sites,
Scheme 2ꢀI). C=C reduction to hydrocinnamaldehyde can further lead to hydrocinnamyl
alcohol (Scheme 2ꢀII) which can be followed by subsequent dehydration to βꢀmethylstyrene..
Ethylbenzene can also be produced through pathway III via hydrogenation of styrene)
produced via cinnamaldehyde hydrodeformylation (Scheme 2ꢀIII) or by direct
hydrodeformylation of hydrocinnamaldehyde generared in pathway II upon C=C bond
reduction of cinnamaldehyde.
Figure 4. Reaction profiles for Pd4ꢀAl (dotted line) and Pd/C (solid line) in the
hydroconversion of cinnamaldehyde with formic acid under conventional heating. Reaction
conditions: 0.5 mL cinamaldehyde, 1.5 mL formic acid, 10 mL acetonitrile, 1 g cat, 83ºC.
Results indicated that Pd/C favored typical hydrogenation reactions for the production
of major quantities of hydrocinnamaldehyde (pathway II). Since no cinnamyl alcohol was
detected as product for Pd/C and only low selectivities to βꢀmethylstyrene were obtained at

Page 13 of 21
Green Chemistry
View Article Online
DOI: 10.1039/C4GC01354A
short times of reaction (<10 mol%, times of reaction under 4ꢀ6 h), it is clear that the C=C
reduction to hydrocinnamaldehyde is the rate determining step in the Pd/C catalyzed
hydroconversion of cinnamaldehyde. These results were in good agreement with previous
4, 23
literature reports.
Interestingly, as the reaction progresses, C=O reduction leads to the
formation of hydrocinnamyl alcohol which undergoes dehydration to βꢀmethylstyrene as
major product (ca. 60 mol% selectivity) observed after 24 h (Figure 4, Mestyrene for Pd/C).
Due to the low acidity of Pd/C, dehydration may be promoted by formic acid present in
excess in the reaction media.
Scheme 2. Possible competitive pathways (I to III) for the production of ethylbenzene,
hydrocinnamaldehyde and βꢀmethylstyrene, main products different from cyclohexane
obtained in the hydroconversion of cinnamaldehyde with formic acid.
Cyclohexane from hydrogenation of the aromatic ring as well as ethylbenzene and
methylstyrene as minor products together with traces of propylbenzene and short amides (e.g.
formamide) were also observed in Pd/C catalyzed reactions under conventional heating
(Figure 5). Findings seemed to point out a favored competitive reaction for the production of

Green Chemistry
Page 14 of 21
View Article Online
DOI: 10.1039/C4GC01354A
ethylbenzene to the typical C=C and C=O reductions steps observed in most reports as
proposed in Scheme 2. Ethylbenzene can be most plausibly obtained via hydrodeformylation
of hydrocinnamaldehyde in a similar way to previous reports using Ni/Al2O3.23 The catalytic
decarbonylation of cinnamaldehyde was also reported many years back using Pd and Pt/C,24
27
Ni raney catalysts25 and Ru and Rh homogeneous complexes.26,
Alternatively,
hydrodeformylation of cinnamaldehyde could generate styrene and ethylbenzene upon
subsequent hydrogenation on the metal sites. This route, however, is not likely to generare
ethylbenzene as no traces of styrene could be detected in the reaction under the investigated
conditions.
Pd4ꢀAl comparatively exhibited
a
major hydrogenation selectivity to
hydrocinnamaldehyde (C=C bond reduction, pathway II) and cyclohexane (aromatic ring
hydrogenation). Interestingly, a high selectivity to oxalic acid (15ꢀ20 mol%) was observed
even after 24 h reaction (Figure 5). Minor quantities of βꢀmethylstyrene and traces of
ethylbenzene were also detected in the catalytic system under conventional heating.
The formation of ethers or acetals from cinnamaldehyde and/or hydrocinnamaldehyde
via acidꢀcatalysed reactions (Scheme 1) was not observed in any Pdꢀcontaining system even
in the presence of AlꢀSBAꢀ15 as solid acid support and formic acid under the investigated
reaction conditions in conventional heating.
Based on these findings, particularly related to oxalic acid production for
Pd/AlꢀSBAꢀ15 materials, we were prompted to investigate the proposed hydroconversion
reaction at shorter times of reaction under microwaveꢀirradiation pushing the reaction
conditions to maximize product yields.

Page 15 of 21
Green Chemistry
View Article Online
DOI: 10.1039/C4GC01354A
5%Pd/C
BzProp
Oxacid
MeStyrene
Chxane
EthylBz
Pd4-Al
0
10
20
30
40
50
60
Figure 5. Selectivity comparison (X axis, mol%) between Pd4ꢀAl containing ca. 2.9 wt.% Pd
and a commercial 5%Pd/C in the hydroconversion of cinnamaldehyde in formic acid.
Reaction conditions: 0.5 mL cinamaldehyde, 1.5 mL formic acid, 10 mL acetonitrile, 1 g cat,
83ºC reaction temperature, 24 h reaction. Products legend: BzProp= hydrocinnamyl alcohol;
Oxacid= oxalic acid; MeStyrene= βꢀmethylstyrene; Chxane= cyclohexane; EthylBz=
ethylbenzene
Main findings for the microwaveꢀassisted hydroconversion of cinnamaldehyde have
been summarized in Table 3. Blank runs provided almost negligible activity in the systems
(<10%). Interesting catalytic trends were observed in the proposed microwaveꢀassisted
system for Pdꢀcontaining catalysts (Table 3). Quantitative conversion was achieved for all
Pd/AlꢀSBAꢀ15 materials as compared to a 71% conversion comparatively obtained for Pd/C.
The activities of Pdꢀcontaining systems were remarkably superior to Pd/C as compared to
reactions conducted under conventional heating. These findings are in good agreement with
previously reported results from the group28 and illustrate the advantages of
microwaveꢀactivation in mechanochemically synthesized Pd systems as compared to
commercial Pd catalysts (e.g. Pd/C). Most importantly, inherent mass transfer and diffusional

Green Chemistry
Page 16 of 21
View Article Online
DOI: 10.1039/C4GC01354A
limitations have already been demonstrated in the Pd/C system under conventional heating
present at short times of reaction due to its microporous nature (Figure 4 reaction profiles)
are. Hence, short times of reaction under microwave irradiation can justify the reduced
activity of Pd/C under the investigated reaction conditions. Previously reported
Pdꢀaluminosilicate interactions as well as the most important preferential external deposition
of highly accessible, active and easier to activate nanoparticles in the proposed bifunctional
Pd/AlꢀSBAꢀ15 materials may also partially contribute to the improved activities under
microwave irradiation.28
Table 3. Total conversion (mol %) and selectivities to products (mol %) of different supported
Pd nanoparticles on AlꢀSBAꢀ15 as compared to a commercial 5%Pd/C material in the
microwaveꢀassisted hydroconversion of cinnamaldehyde in formic acid.
Selectivity (mol %)
Conversion
Catalyst
Blank
(mol %)
<5
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
<5
ꢀ
ꢀ
ꢀ
ꢀ
Pd0.5ꢀAl
Pd1ꢀAl
97
<5
<5
52
64
26
11
16
18
>99
Pd2ꢀAl
Pd4ꢀAl
5%Pd/C
98
85
71
<5
13
21
66
44
48
18
41
27
13
<5
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
<5
Reaction conditions: 0.1 mL cinnamaldehyde (0.8 mmol), 0.3 mL formic acid (8 mmol),
2 mL acetonitrile, 0.5 g catalyst, microwave irradiation, 200 W (180ºC maximum temperature
reached, averaged temperature 150ºC, 250 PSI maximum pressure), 15 min reaction.

Page 17 of 21
Green Chemistry
View Article Online
DOI: 10.1039/C4GC01354A
Selectivities in the systems followed the similar trend observed under conventional
heating, with expected high activities towards complete aromatic ring hydrogenation as well
as hydrogenolysis of the side chain to generate cyclohexane as final product. The formation of
βꢀmethylstyrene, ethylbenzene and oxalic acid accounted for most remaining product
selectivity. An increasing in Pd loading generally favored the formation of βꢀmethylstyrene as
major byꢀproduct, with the exception of Pd0.5ꢀAl (Table 3).
The formation of relevant quantities of oxalic acid was repeatedly observed in
microwaveꢀassisted reactions for catalysts containing low Pd content (0.5ꢀ2 wt.%), generally
decreasing with Pd content in the materials. Oxalic acid was not detected in blank runs, using
similar Ptꢀbased catalysts or in the Pd/C catalyzed hydroconversion of cinnamaldehyde
(Table 3). With formic acid decomposing into CO + CO2, hydrogen and water under the
investigated microwaveꢀassisted conditions12, the most plausible mechanism for oxalic acid
production is a Pd catalysed hydrocarboxylation. This is a type of reaction which has been
previously reported to be catalyzed by various type of Pd catalysts29 including analogous
heterogeneous Pd complexes on MCMꢀ41 and MCMꢀ48 materials.30
A plausible explanation for the observed decrease in concentration of oxalic acid at
increasing Pd loadings may relate to the presence of large Pd clusters in materials over 2 wt.%
Pd which favor hydrogenation reactions as compared to hydrocarboxylations promoted in
smaller size Pd nanoparticles. The proposed change in selectivity in sizeꢀsensitive reactions
31
has been previously reported.4,
Calculated TON values of the catalysts for
microwaveꢀassisted reactions were in the 5ꢀ10 range while a maximum catalyst TOF for
cinnamaldehyde conversion to products of 42 hꢀ1 (Pd1ꢀAl) and minimum of 15 hꢀ1 (Pd4ꢀAl)
were obtained.

Green Chemistry
Page 18 of 21
View Article Online
DOI: 10.1039/C4GC01354A
Conclusions
Mechanochemically synthesized Pd/AlꢀSBAꢀ15 materials with different Pd loadings
(0.5, 1, 2 and 4 wt.%) have been synthesized and characterized using a range of analytical
techniques including TG/DTA, N2 physisorption and TEM and subsequently employed as
heterogeneous catalysts in the hydroconversion of cinnamaldehyde with formic acid both
under conventional heating and microwave irradiation. In contrast to potentially expected
products, an interesting selectivity to products including ethylbenzene (via Pdꢀcatalysed
hydrodeformylation reactions) and oxalic acid (via Pdꢀcatalysed hydrocarboxylation) were
observed. Formic Acid was proved to be an effective hydrogen donor in the hydroconversion
of cinnamaldehyde under heating (both conventional and microwave). The proposed Pd
systems are envisaged to be of potential interest to the extension of the proposed chemistries
to related hydro(de)formylation reactions which will be reported in due course.
Acknowledgments
Rafael Luque acknowledges support from the Spanish MICINN via the concession of a
Ramon y Cajal contract (ref. RYC 2009ꢀ04199) and funding under project CTQ2011ꢀ28954ꢀ
C02ꢀ02. Part of this publication was created using the infrastructure supported by UniCRE
project (reg. no. CZ.1.05/2.1.00/03.0071) funded by the EU Structural Funds and the state
budget of the Czech Republic.
References
[1] W. Kooꢀamornpattana, J.M. Winterbottom, Catal. Today 2001, 66, 277–287.
[2] A.Y. Hammoudeh, S.M. Sa’ada, S.S. Mahmoud, Jordan J. Chem. 2007, 2, 53ꢀ67.

Page 19 of 21
Green Chemistry
View Article Online
DOI: 10.1039/C4GC01354A
[3] J. Hájek, N. Kumara, P. MäkiꢀArvela, T. Salmi, D. Yu Murzin, I. Paseka, T. Heikkilä, E.
Laine, P. Laukkanen, J. Väyrynen, Appl. Catal. A 2003, 251, 385–396.
[4] Y. Zhu, F. Zaera, Catal. Sci. Technol. 2014, 4, 955ꢀ962.
[5] S. Handjani, E. Marceau, J. Blanchard, J.ꢀM. Krafft, M. Che, P. MäkiꢀArvela, N. Kumar,
J. Wärnå, D. Yu. Murzin, J. Catal. 2011, 282, 228–236.
[6] http://lib.hut.fi/Diss/
[7] K.ꢀY. Jao, K.ꢀW. Liu, Y.ꢀH. Yang, A.ꢀN. Ko, J. Chinese Chem. Soc. 2009, 56, 885ꢀ890.
[8] E. Erasmus, S. Afr. J. Chem. 2013, 66, 216–220.
[9] M. Zou, X. Mu, N. Yan, Y. Kou, Chin. J. Catal. 2007, 28, 389–391.
[10] C. Ge, Y. Li, J. Zhao, R. Zhou, Indian J. Chem. Sec. A, 2010, 49A, 281ꢀ287.
[11] M. Jahjah, B. Caron, S. Menuel, E. Monflier, L. Djakovitch, C. Pinel, ARKIVOC 2011, 7,
406ꢀ415.
[12] S. Narayanan, Bull. Catal. Soc. India 2003, 2, 107ꢀ121.
[13] D.A. Bulushev, S. Beloshapkin, J.R.H. Ross, Catal. Today 2010, 154, 7–12.
[14] D.A. Bulushev, J.R.H. Ross, Catal. Today 2011, 163, 42–46.
[15] X. Wang, R. Rinaldi, Energy Environ. Sci. 2012, 5, 8244ꢀ8260.
[16] M. Grasemann, G. Laurenczy, Energy Environ. Sci. 2012, 5, 8171ꢀ8181.
[17] A. Pineda, A.M. Balu, J.M. Campelo, A.A. Romero, D. Carmona, F. Balas, J.
Santamaria, R. Luque, ChemSusChem 2011, 4, 1561ꢀ1565.

Green Chemistry
Page 20 of 21
View Article Online
DOI: 10.1039/C4GC01354A
[18] a) A. Toledano, L. Serrano, A. Pineda, A.M. Balu, R. Luque, J.
Labidi, ChemSusChem 2013, 6, 529ꢀ536; b) A. Toledano, L. Serrano, J. Labidi, A.M. Balu,
A. Pineda, R. Luque, ChemCatChem 2013, 5, 977ꢀ985; c) A. Toledano, L. Serrano, A.
Pineda, A.A. Romero, J. Labidi, R. Luque, Appl. Catal. B 2014, 145, 43ꢀ55.
[19] J.C. Serrano, R. Luque, A. SepulvedaꢀEscribano, Chem. Soc. Rev. 2011, 40, 5266ꢀ5281.
[20] M. Watanabe, T.M. Aida, R. L. Smith, H. Inomata, J. Japan Petroleum Institute 2012,
55, 219ꢀ228.
[21] a) C. Oliver Kappe, Angew. Chem. Int. Ed. 2004, 43, 6250 –6284; b) C.O. Kappe, B.
Pieber, D. Dallinger, Angew. Chem. Int. Ed. 2013, 52, 1088ꢀ1094.
[22] M. Ojeda, A.M. Balu, V. Barrón, A. Pineda, A. Garcia, A.A. Romero, R. Luque, J.
Mater. Chem. A 2014, 2, 387ꢀ393.
[23] S.ꢀY. Chin, F.ꢀJ. Lin, A.ꢀN. Ko, Catal. Lett. 2009, 132, 389ꢀ394.
[24] N.E. Hoffmann, A.T. Kanakkanatt, R.F. Schneider, J. Org. Chem. 1962, 27, 2687ꢀ2689.
[26] A. Mailhe, Bull. Soc. Chim. Fr. 1926, 39, 22.
[27] J.G. de Vries, G. Roelfes, R. Green, Tetrahedron Lett. 1998, 39, 8329ꢀ8332.
[28] a) M. Ojeda, A. Pineda, A.A. Romero, V. Barron, R. Luque, ChemSusChem. 2014, DOI:
10.1002/cssc.201400055; b) M. Ojeda, A.M. Balu, V. Barron, A. Pineda, A. Garcia, A.A.
Romero, R. Luque, J. Mater. Chem A 2014, 2, 387ꢀ393.
[29] a) D. Zargarian, H. Alper, Organometallics 1993, 12, 712ꢀ724; b) J. Takaya, N.
Iwasawa, J. Am. Chem. Soc. 2008, 130, 15254ꢀ15255; c) M. AlꢀMasum, Y. Yamamoto, J.
Am. Chem. Soc. 1998, 120, 3809ꢀ3810.

Page 21 of 21
Green Chemistry
View Article Online
DOI: 10.1039/C4GC01354A
[30] K. Mukhopadhyay, B.R. Sarkar, R.V. Chaudhari, J. Am. Chem. Soc. 2002, 124,
9692ꢀ9693.
[31] M.S. Ide, B. Hao, M. Neurock, R.J. Davis, ACS Catal. 2012, 2, 671ꢀ678.