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structures for the interaction between benzene, AlCl3, and SiO2
were studied. It was found that the formation enthalpy (DH2)
of the complex benzene–AlCl3–H7Si8O12(OH) shown in Fig-
ure 2c is À9.1 kcalmolÀ1. In the complex, there are hydrogen
bonds between a Cl atom (Cl1) of AlCl3 and a H atom (H7) of
an OH group of SiO2 (Cl···HÀO), and between a H atom (H1) of
benzene and a O atom (O1) of an OH group of SiO2 (H···OÀH).
Importantly, the formation of the complex also results in an
uneven distribution of charges in the benzene ring, as shown
in Figure 2d. This is favorable for the hydrogenation of ben-
zene because the stable structure of the benzene is destroyed.
On the basis of the above discussion, we propose a possible
mechanism for the excellent cooperative effect of AlCl3 and Pd
nanocatalysts on supports with abundant surface hydroxyl
groups for the hydrogenation of aromatic compounds. This
proposed mechanism is discussed by using benzene as an ex-
ample. Firstly, the Lewis acid interacts with the benzene ring,
leading to an uneven charge distribution over the benzene
ring and making the aromatic ring more active. The Pd species
is responsible for the activation of molecular hydrogen by dis-
sociative adsorption,[23] the H+/HÀ pair resulting from the het-
erolytic cleavage of H2 transfers to the polarized aromatic ring
and the hydrogenation reaction occurs. These two effects work
together to make the hydrogenation of benzene proceed
smoothly. The surface hydroxyl groups play an important role
in enhancing the selectivity of the reaction and preventing un-
desired reaction pathways.[24] The hydroxyl groups prevent the
side reactions initiated by Lewis acids in two ways. Firstly, for
the formation of the structure shown in Figure 2c the amount
of hydroxyl-group functionality needs to be much larger than
that of AlCl3. Hence, all of the AlCl3 in the reaction system is
bound on the support. Thus, the AlCl3-activated benzene mole-
cules are also bound on the solid particles, preventing efficient
collision between AlCl3-activated benzene molecules for the
Scholl reaction. Secondly, once a benzene molecule is activated
it can be hydrogenated quickly because it is close to the Pd
nanocatalysts on the surface of the support. Therefore, the
Scholl reaction, or other side reactions, between benzene mol-
ecules is prohibited. To provide further evidence to support
this argument, we carried out the reaction with an excess
amount of AlCl3 (Table 2, entry 14), so that free AlCl3 existed in
the reaction system. The results indicated that the byproduct
was formed under these conditions.
benzene, AlCl3, and the hydroxyl groups on the catalyst sup-
port form a complex, through hydrogen bonding, which pre-
vents the side reactions. We believe that the excellent cooper-
ative effect between AlCl3 and the supports with surface hy-
droxyl groups provides new opportunities for the highly effi-
cient transformation of nonpolar or weakly polar aromatic
compounds under mild and solvent-free conditions.
Experimental Section
The chemicals used, the procedures for catalyst preparation, and
the characterization details are described in the Supporting
Information.
The reactions were performed in a 15 mL Teflon-lined stainless-
steel autoclave equipped with a magnetic stirrer. In a typical ex-
periment, reactant, catalyst, Lewis acid, and solvent (if used) were
loaded into the reactor. The autoclave was sealed and purged with
hydrogen several times to remove the air at 08C. The reactor was
then placed into a water bath at the desired temperature. Hydro-
gen was introduced into the reactor after the desired temperature
was reached and the stirrer was started with a rate of 400 rpm.
After the reaction, the autoclave was cooled in ice water and the
H2 in the reactor was released. Then an internal standard (n-octane
or n-decane, depending on the reactant) was added into the reac-
tor. The product was analyzed by gas chromatography (Agilent
4890, equipped with HP-INNOWax capillary column and an FID).
Identification of the products and reactant was achieved by GC-MS
(SHIMADZU-QP2010, equipped with DB-5ms capillary column) anal-
ysis as well as by comparison of the retention times of the stand-
ards in the GC traces. The conversion and selectivity were calculat-
ed from the GC data.
Acknowledgements
The authors thank the National Natural Science Foundation of
China (21373230, 21273253), and the Chinese Academy of Scien-
ces (KJCX2.YW.H30).
Keywords: arenes · hydrogenation · Lewis acids · palladium ·
surface hydroxyl groups
don, I. Arends, U. Hanefeld, Green Chemistry and Catalysis, Wiley-VCH,
Weinheim, 2007; c) D. Bianchi, R. Bortolo, R. Tassinari, M. Ricci, R. Vigno-
4491–4493; d) G. Zhou, X. Tan, Y. Pei, K. Fan, M. Qiao, B. Sun, B. Zong,
[2] R. L. Augustine, Heterogeneous Catalysis for the Synthetic Chemistry,
Marcel Dekker, New York, 1996.
[4] a) G. A. Olah, Friedel–Crafts Chemistry, Wiley, New York, 1973; b) M.
Rueping, B. J. Nachtsheim, Beilstein J. Org. Chem. 2010, 6, 6; c) M. Bandi-
5078; b) G. A. Olah, A. Molnar, Hydrocarbon Chemistry, Wiley, Hoboken,
2003.
In conclusion, it has been found that Al2O3 or SiO2 solid
oxide particles that have abundant surface hydroxyl groups
can prevent undesirable reactions in benzene–AlCl3, toluene–
AlCl3, and naphthalene–AlCl3 mixtures, whereas the aromatic
compounds condensed quickly to form oligomers in the ab-
sence of the solid oxide particles. Further studies showed that
AlCl3 can promote hydrogenation of the aromatic compounds,
catalyzed by Pd/SiO2 or Pd/Al2O3, very efficiently under mild
and solvent-free conditions without forming any byproduct.
The hydrogenation reactions catalyzed by Pd/C produce
a large number of byproducts from side reactions owing to
the lack of surface hydroxyl groups on the carbon support.
DFT studies indicated that AlCl3 interacts with benzene,
making the benzene ring much more active. At the same time,
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