Catalytic transfer hydrogenolysis of 2-phenyl-2-propanol over palladium supported on activated carbon

Article abstract of DOI:10.1016/j.molcata.2006.02.055

Transfer hydrogenolysis of 2-phenyl-2-propanol to isopropylene using formic acid as hydrogen donor was performed over the supported palladium, platinum and ruthenium catalysts. The hydrogen-donating abilities of various hydrogen donors and the effect of the pH value in a reaction medium were studied. It was found that Pd/C is an effective catalyst and formic acid is much better than formate salts and other agents of donating-hydrogen for the catalytic transfer hydrogenolysis of 2-phenyl-2-propanol. The protons in a reaction medium have a promotive effect on the catalytic hydrogenolysis of 2-phenyl-2-propanol. At the reaction condition of using formic acid as hydrogen donor and the Pd/C catalyst at 80 °C for 4 h, the conversion of 2-phenyl-2-propanol and selectivity to isopropylbenzene reaches 95.2 and 98.4%, respectively.

Full text of DOI:10.1016/j.molcata.2006.02.055

Journal of Molecular Catalysis A: Chemical 252 (2006) 176–180
Catalytic transfer hydrogenolysis of 2-phenyl-2-propanol over palladium
supported on activated carbon
∗
Xiaohui Liu, Guanzhong Lu , Yanglong Guo, Yun Guo, Yunsong Wang, Xinhong Wang
Lab for Advanced Materials, Research Institute of Industrial Catalysis, East China University of Science and Technology, Shanghai 200237, PR China
Received 7 September 2005; accepted 21 February 2006
Available online 29 March 2006
Abstract
Transfer hydrogenolysis of 2-phenyl-2-propanol to isopropylene using formic acid as hydrogen donor was performed over the supported
palladium, platinum and ruthenium catalysts. The hydrogen-donating abilities of various hydrogen donors and the effect of the pH value in a
reaction medium were studied. It was found that Pd/C is an effective catalyst and formic acid is much better than formate salts and other agents
of donating-hydrogen for the catalytic transfer hydrogenolysis of 2-phenyl-2-propanol. The protons in a reaction medium have a promotive effect
on the catalytic hydrogenolysis of 2-phenyl-2-propanol. At the reaction condition of using formic acid as hydrogen donor and the Pd/C catalyst at
◦
8
0 C for 4 h, the conversion of 2-phenyl-2-propanol and selectivity to isopropylbenzene reaches 95.2 and 98.4%, respectively.
©
2006 Elsevier B.V. All rights reserved.
Keywords: Transfer hydrogenolysis; Pd/C catalyst; 2-Phenyl-2-propanol; Formate salts; Formic acid
1
. Introduction
most useful protecting groups of O-benzyl group, but also is a
very important intermedium to manufacture the propylene oxide
(PO) by isopropylbenzene hydroperoxide oxidizing propylene
[12,13]. When propylene is epoxidized to PO by isopropyl-
benzene hydroperoxide as an oxidant, 2-phenyl-2-propanol was
obtained as a side-product, and should be hydrogenolyzed to
isopropylbenzene that is the raw material to produce isopropy-
lbenzene hydroperoxide, which forms a cycle of oxidizing iso-
propylbenzene to isopropylbenzene hydroperoxide, and then
being reduced to isopropylbenzene.
For the transfer hydrogenolysis or hydrogenation, it is nec-
essary to select an efficient catalyst and suitable hydrogen
donors. In this paper, CTH of 2-phenyl-2-propanol was used
as a model reaction, the preparation of the Pd/C catalyst, the
effects of various hydrogen donors and the pH value in reaction
medium on the hydrogen-donating ability were studied. Fur-
thermore, the mechanism for the catalytic transfer hydrogenol-
ysis of 2-phenyl-2-propanol over Pd/C by formic acid was
discussed.
Catalytic transfer hydrogenolysis (CTH) is an important syn-
thetic technique in organic chemistry [1–3]. As neither hydrogen
containment nor a pressure vessel is required, the mild reaction
conditions offer considerable advantages over the conventional
method of catalytic hydrogenolysis. CTH is available for a wide
variety of functional groups including halo, nitro and benzyl
groups [4–7], in which formic acid or formate salts was used
often as hydrogen donors. Generally, metal (VIII group ele-
ments) such as palladium, ruthenium and Raney nickel [8–10]
were employed as the catalysts for the transfer hydrogenolysis.
Oxygen-containing benzyl (O-benzyl) groups are among the
most useful protecting groups in synthetic organic chemistry,
because of their ease of formation, their relative stability, and an
efficiency of cleavage under mild conditions [11], in which 2-
phenyl-2-propanol (ArPrOH) is often used as one of the model
compounds including oxygen-containing benzyl groups, and is
widely used in the perfumeries. As a model reaction, the transfer
hydrogenolysis of 2-phenyl-2-propanol has been studied by our
group, because that 2-phenyl-2-propanol includes not only the
2. Experimental
H2PdCl4, H2PtCl and RuCl3·nH2O (AR) aqueous solution
6
∗
were used as the precursors of the catalyst active components.
Corresponding author. Fax: +86 21 64253703.
E-mail address: gzhlu@ecust.edu.cn (G. Lu).
Commercial ␥-Al2O3, activated carbon and MCM-41 molecu-
1
381-1169/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2006.02.055

X. Liu et al. / Journal of Molecular Catalysis A: Chemical 252 (2006) 176–180
Table 1
177
lar sieve (TEOS as silica precursor) that was prepared according
PerformanceofcatalystforCTHof2-phenyl-2-propanolbyHCOOHasH-donor
at 80 C for 4 h
to the reference [14] were used as the carriers of catalysts.
Their BET surface areas are 215, 1080 and 1023 m /g, respecti-
◦
a
2
vely.
Catalyst
Conversion of
ArPrOH (%)
Selectivity to
i-PrAr (%)
Yield of
i-PrAr (%)
The supported catalysts were prepared as follows. The acti-
vated carbon was added into 0.01 M aqueous solution of the
Pd/C
Pd/C
Pt/C
Ru/C
Pd/␥-Al2O3
Pd/MCM-41
95.2
92.6
53.4
37.1
35.8
33.1
98.4
97.1
1.2
–
11.1
3.9
93.7
89.9
0.6
–
4.0
1.3
◦
b
active metal precursors under stirring at 60 C, and the pH value
of the slurry solution was adjusted to 6–7 by the aqueous solution
of NaHCO3. After 2 h, the solid sample was filtered and dried
at room temperature. Pd/Al2O3 and Pd/MCM-41 were prepared
◦
by an impregnation method and calcined at 550 C for 5 h in air.
ArPrOH: 2-phenyl-2-propanol; i-PrAr: isopropylbenzene.
◦
The catalysts should be treated with H2 at 120 C for 1 h before
a
1.5 mmol ArPrOH, 3 mmol HCOOH, 0.2 g catalyst, 10 ml EtOH and 2 ml
they were used.
H2O.
b
The active metal loading on the catalysts was about 3.8%
Untreated.
(
wt.) that was determined by inductively coupled plasma atomic
emission spectroscopy (ICP-AES), which was done on an IRIS
3.2. Effect of the support
1
000 (Thermo Elemental, USA) instrument.
The XRD patterns of the catalysts were determined on a
Riguku D-MAX RB X-ray diffractometer at 40 kV and 40 mA,
and the radiation source was Cu K␣.
The catalytic transfer hydrogenolysis of 2-phenyl-2-propanol
was operated as follows. 2-Phenyl-2-propanol (1.5 mmol),
hydrogen donor (3 mmol), catalyst (0.2 g) and ethanol solvent
Effects of the support on the catalytic performance of the
supported palladium catalysts are shown in Table 1. The results
show that Pd/C reduced or unreduced behaves a high activity for
the transfer hydrogenolysis of ArPrOH. When MCM-41 or ␥-
Al2O3 is used as the support, the catalytic performance of the Pd
catalyst is remarkably lower than that of Pd/C. For the ␥-Al2O3
support, its comparable stronger surface acidity would promote
the dehydration of 2-phenyl-2-propanol to ␣-methyl styrene to
reduce the selectivity to isopropylbenzene, and its low surface
area would reduce its activity. Pd/MCM-41 has a larger surface
area like Pd/C, but its performance is very low and is similar to
that of Pd/␥-Al2O3. This implies that the performance of the Pd
catalyst is independent of the support’s surface area, anddepends
on the properties of the Pd sites on the support.
(
10 ml EtOH and 2 ml H2O) were put into a flask equipped
with a water-cooled condenser and a magnetic stirrer. The flask
◦
was placed in an oil bath and maintained at 80 C and atmo-
sphere. After the reaction with reflux was carried out for 4 h,
the hydrogenolysis products were withdrawn and analyzed by
the gas chromatograph of Perkin-Elmer Autosystem XL with
the FID detector and fused silica capillary column (methyl 5%
phenyl silicone, 25 m × Ø0.32 mm).
The hydrogenolysis products obtained with HCOOH as
hydrogen donor over the Pd/C catalyst were analyzed qualita-
tively by GC-Mass (HP 5890 II gas chromatograph and Micro-
mass GCT CA 055) and GC-IR (HP 5890 II gas chromatograph
and a BIO-RAD FTS-40 IR spectrometer).
Fig. 1 shows the XRD patterns of the palladium catalysts
untreated. For Pd/␥-Al2O3 and Pd/MCM-41, their XRD patterns
display the diffraction peaks of PdO that is formed during cal-
cination in air. It is interesting there are the diffraction peaks of
0
metallic Pd in the XRD spectrum of the Pd/C catalyst untreated,
The initial pH value of the reaction mixtures was measured
by the pH meter (Leici PHS-3C, Shanghai).
which is the reason of the Pd/C catalyst untreated having a great
catalytic activity (Table 1, entry 2). In the preparation of Pd/C,
after H2PdCl4 in aqueous solution adsorbs on the surface of
3
. Results and discussion
3
.1. Performance of the catalyst
Using HCOOH as hydrogen donor, the catalytic transfer
hydrogenolysis of 2-phenyl-2-propanol over the Pd/C, Pt/C and
Ru/C catalyst was studied. As shown in Table 1, the catalytic
activity of Pd/C is far superior to the Pt/C and Ru/C catalysts.
Over the Pd/C catalyst reduced, the conversion of ArPrOH and
the selectivity to cumene reaches 95.2 and 98.4%, respectively.
Palladium is widely used for the cleavage of the benzylic–O
bonds, because it has a high activity for hydrogenolysis with a
low activity for the saturating aromatic rings. However, plat-
inum and ruthenium catalysts are usually effective catalyst
to saturate aromatic rings with minimal hydrogenolysis [11].
Therefore, for the catalytic transfer hydrogenolysis of 2-phenyl-
2
low.
-propanol, the performance of Pt/C or Ru/C catalyst is very
Fig. 1. XRD patterns of the catalysts untreated of Pd/Al2O3 (a), Pd/MCM-41
(b) and Pd/C (c). ꢀ, Pd ; ᭹, PdO.
0

1
78
X. Liu et al. / Journal of Molecular Catalysis A: Chemical 252 (2006) 176–180
carbon, two competitive reactions may occur, that is, H2PdCl4
PdCl2 + 2HCl) is reduced to form metallic Pd particles (process
) and reacts with the C C fragments of carbon matrix to form
the ␲-complexes of PdCl2 (process 1) [15,16]:
ing ester. For formate salts as hydrogen donor, its hydrogen-
donating ability is in the inverse proportion of an initial pH
value of the reaction solution, that is, the higher the pH value
of the reaction solution, the poorer hydrogen-donating ability
of formate salt is. The fact of HCOOK and HCOONH4 (not
CH3COOH) having the hydrogen-donating ability indicates that
(
2
PdCl2
C
Pd0
−
2Cl
+
2HClꢀH2PdCl4 + Cꢀ
+
2C
+ 2HCl
⊕
−
(1)
(2) C
H of HCOO plays the role of hydrogen resource for the transfer
hydrogenolysis.
2
+
Therefore, the Pd cations can be reduced during an adsorp-
tion of H2PdCl4 in aqueous solutions on the surface of activated
carbon. It is due to the high reducing potential and the conduc-
tivity of activated carbon [17]. Furthermore, Simonov et al. [15]
have reported an adsorption of H2PdCl4 on carbon from a dilute
3.4. Effect of water content
In order to investigate an influence of the water content in sol-
0
vent on the hydrogenolysis of 2-phenyl-2-propanol with formic
acid, the mixture solvent of water/ethanol was used. The results
in Fig. 2 show that the conversion of 2-phenyl-2-propanol and
selectivity to isopropylbenzene increase with an increase of the
amount of water, and they reach the maximums as 2 ml water is
used. This is because that increasing the water amount in solvent
makes the HCOOH concentration in the medium decrease and
the ionizability of HCOOH increase. However, water is one of
products in the hydrogenolysis reaction, and more water added
in a reaction system would restrain the hydrogenolysis of 2-
phenyl-2-propanol.
solution can produce greater amount of deposited Pd than from
a concentrated solution. As the H2PdCl4 aqueous solution used
in this paper is dilute (0.01 M), there will be a relatively smaller
quantity of Pd2 and a mass of Pd adsorbed on the carbon sur-
+
0
face. However, there are no diffraction peaks of PdCl2 in the
XRD patterns of the Pd/C catalyst. It is probably because that
the adsorption of PdCl2 is present either as particles of less than
3
nm size, or in molecularly dispersed form [15]. At one time,
the complex features of the activated carbon surface are very
important for the Pd/C catalyst, such as, the residual acid prop-
erties on the surface of carbon can help to increase the reaction
rates [11].
3.5. Effect of the pH value in a reaction medium
3
.3. Effect of hydrogen donor
The studies above mentioned show that, formic acid is more
efficient hydrogen donor than formate salts for the transfer
hydrogenolysis of 2-phenyl-2-propanol. This situation is differ-
ent from the transfer hydrogenolysis of aryl chlorides or benzyl
acetate catalyzed by palladium, in which formate salts as hydro-
gen donors are superior to formic acid [18–20].
It has been reported that the hydrogenolysis of benzyl ester is
mucheasierthanthatofbenzylalcoholinthetraditionalcatalytic
hydrogenolysis by molecular hydrogen [21]. When HCOOH is
used as a hydrogen donor, whether the hydrogenolysis of 2-
phenyl-2-propanol can be promoted by forming an intermediate
of ester of 2-phenyl-2-propanol and HCOOH? The analysis for
the products obtained with HCOOH as hydrogen donor (Table 2,
entry 4) by GC-Mass and GC-IR shows there a few formate of 2-
phenyl-2-propanol in the products. If sodium formate was added
Using the Pd/C catalyst, the effect of hydrogen donor on
the transfer hydrogenolysis of 2-phenyl-2-propanol is shown in
Table 2. The results show that the kind of hydrogen donor has
a great influence on the transfer hydrogenolysis of 2-phenyl-2-
propanol. Among HCOOH, 2-propanol, HCOOK, HCOONH4,
N2H4·H2O, HCOONa and CH3COOH, formic acid is most
efficient hydrogen donor, in which the conversion of 2-phenyl-
-propanol and selectivity to isopropylbenzene reaches 95.2 and
8.4%, respectively.
When CH3COOH is used as hydrogen donor, the conver-
2
9
sion of 2-phenyl-2-propanol is 27.8% and selectivity to iso-
propylbenzene was ∼0%. It is attributed to the reaction of
acetic acid with 2-phenyl-2-propanol to form the correspond-
Table 2
Effect of hydrogen donor on the transfer hydrogenolysis of 2-phenyl-2-propanol
◦
a
over the 3.8 wt.% Pd/C catalyst at 80 C for 4 h
Hydrogen donor
pHb
Conversion of
ArPrOH (%)
Selectivity to
i-PrAr (%)
2
-Propanol
6.4
0.4
27.8
95.2
1.9
1.4
–
65.8
∼100
0
98.4
51.8
100
–
N2H4·H2O
CH3COOH
HCOOH
HCOOK
HCOONH4
HCOONa
2.93
5.83
6.97
8.02
ArPrOH: 2-phenyl-2-propanol; i-PrAr: isopropylbenzene.
a
ArPrOH (1.5 mmol), hydrogen donor (3 mmol), Pd/C (0.2 g), EtOH (10 ml),
H2O (2 ml).
Fig. 2. Effect of water content in ethanol solvent on the transfer hydrogenolysis
with formic acid over Pd/C catalyst at 80 C for 4 h.
b
◦
The initial pH value of the reaction mixture.

X. Liu et al. / Journal of Molecular Catalysis A: Chemical 252 (2006) 176–180
179
Table 3
Effect of pH value on the transfer hydrogenolysis of 2-phenyl-2-propanol with
◦
a
HCOONa over Pd/C catalyst at 80 C for 4 h
Additive
PHb
Conversion of
ArPrOH (%)
Selectivity to
i-PrAr (%)
None
8.02
7.52
5.90
4.81
3.78
7.21
4.71
–
1.4
2.0
17.9
43.8
1.7
–
CH3COOHc
27.6
55.5
95.0
95.7
33.3
98.9
0
0
1
.15 mmol CH3COOH
.3 mmol CH3COOH
.8 mmol CH3COOH
c
HBO3
HCOOH
d
20.6
a
ArPrOH (1.5 mmol), hydrogen donor (3 mmol), Pd/C (0.2 g), EtOH (10 ml),
H2O (2 ml).
b
c
d
The initial pH value of the reaction mixture.
The pH value of HAc solution equals to that of HBO3 solution.
The amount of HCOOH equals to that in Table 2.
in the hydrogenolysis system using formic acid as hydrogen
donor (Table 2, entry 4), the rate of hydrogenolysis fell sharply
(Table 3, entry 7). Differing from formic acid, boric acid is an
inorganicacidanditsacidityisweakerthanformicacid, anddoes
not react with 2-phenyl-2-propanol to form ester. Adding boric
acid in the hydrogenolysis system using HCOONa as hydrogen
donor reduces the pH value of the reaction solution from 8.02
to 7.21, and increases the rate of hydrogenolysis of 2-phenyl-
Scheme 1. Mechanism of transfer hydrogenolysis of 2-phenyl-2-propanol over
Pd/C with formic acid.
3.6. Mechanism of the catalytic transfer hydrogenolysis of
2-phenyl-2-propanol
2
-propanol (Table 3, entry 6). This implies it is not necessary
to promote the hydrogenolysis of 2-phenyl-2-propanol by a for-
mation of an intermediate of ester, and the initial pH value of
the reaction mixture is an important factor to affect the rate of
hydrogenolysis.
Based on the results and discussion above, the mechanism
for the catalytic transfer hydrogenolysis of 2-phenyl-2-propanol
over Pd/C by formic acid has been proposed as shown in
−
+
Scheme 1. HCOOH is ionized to HCOO and H in a reaction
−
To further demonstrate the role of the pH value of reaction
medium, the effects of the pH value of reaction medium mod-
ified by acetic acid on the hydrogenolysis were investigated,
and the results are shown in Table 3. With an increase of the
amount of acetic acid, the pH value of medium decreases, and
the rate of hydrogenolysis enhances. When 1.8 mmol HAc is
added, the conversion of 2-phenyl-2-propanol and selectivity to
isopropylbenzene reaches 43.8 and 95.7%, respectively. But the
conversion of alcohol is much lower than that using formic acid
as H-donor, even the amount of acetic acid added is much more
than the stoichiometric molar weight to react with 2-phenyl-
medium. HCOO adsorbed on the palladium sites dissociates
−
+
H adsorbed and CO2, and H attacks the carbon–oxygen bond
to form a carbonium ion. Then the carbonium ion reacts with H
−
adsorbed on the sites of palladium to form isopropylbenzene.
From the mechanism proposed in Scheme 1, it can be seen
that the amount of protons and formyloxy ions is important for
the transfer hydrogenolysis of 2-phenyl-2-propanol with formic
acid over Pd/C. Formate salts would dissociate more formyloxy
ions than formic acid, but the performance of Pd/C with formate
salts as H-donor is poorer than that with formic acid. Low pH
value in a reaction medium is in favor of increasing the perfor-
mance of catalyst. Based on the facts above, it can be concluded
that the step of forming carbonium ion is more difficult than the
2
-propanol (Table 3, entry 5), which attributes to its higher pH
value (3.78) than that of medium including formic acid as hydro-
gen donor (pH 2.93).
−
−
step of HCOO adsorbed dissociating H adsorbed and CO2,
that is to say, the former is the controlling step for the catalytic
transfer hydrogenolysis of 2-phenyl-2-propanol over Pd/C.
The results mentioned above show that, the acidic medium is
in favor of the transfer hydrogenolysis of 2-phenyl-2-propanol,
which is perhaps induced by protons in a medium. The less
the pH value of reaction medium, the higher the concentra-
tion of proton is. The protons are available to a formation of
4. Conclusions
+
the leaving group, i.e. O H2 instead of OH [22], and a cleav-
For the transfer hydrogenolysis of 2-phenyl-2-propanol by
formic acid, Pd/C is an effective catalyst. As a hydrogen donor,
formic acid is much better than formate salts and other agents of
donating-hydrogen. With a decrease of the pH value of reaction
medium, the catalytic performance of Pd/C increases for the
transfer hydrogenolysis of 2-phenyl-2-propanol. The reason is
there is a promotive effect of protons in a reaction medium on
the catalytic hydrogenolysis of 2-phenyl-2-propanol.
age of the carbon–oxygen bond to give a carbonium ion. Then
the carbonium ion reacts with hydrogen on the surface of cat-
alyst to form isopropylbenzene (Scheme 1). On the contrary, if
the reactants include basic groups, more acid molecules need
to be added to protonize the basic groups. That is consis-
tent with the catalytic hydrogenolysis by molecular hydrogen
[
23].

1
80
X. Liu et al. / Journal of Molecular Catalysis A: Chemical 252 (2006) 176–180
Acknowledgment
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[
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We are grateful to the Committee of Science and Technol-
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