One-Step synthesis of acetals from phenol/alcohols mixtures using a Pd/Aluminoborate catalyst

Article abstract of DOI:10.1007/s10562-012-0891-4

A one-step synthesis of cyclohexanone acetals from phenol/alcohol mixtures has been explored using bifunctional aluminum borates supported Pd catalysts. Three aluminum borates have been prepared via either a high temperature calcination (9Al₂O₃·2B₂O₃) or a selfpressured thermal synthesis (PKU-1 and ABO-X) using various aluminum precursors and boric acid. Well dispersed Pd nanoparticles loaded on these materials show excellent phenol hydrogenation activity in both water and ethanol. The reactivity follows the trend of Pd/ABOX[ Pd/9Al₂O ₃·2B₂O₃[Pd/PKU-1, which is similar to the sequence of BO₄/BO₃ ratio presented in the aluminoborates as shown by NMR spectra. When ethanol is used as a solvent, 13.5 % yield of cyclohexanone diethyl acetal has been obtained under the conditions investigated on Pd/ ABO-X, which is attributed to its high activity in both hydrogenation and acetalization for the one-step synthesis of cyclohexanone acetals. Springer Science+Business Media New York 2012.

Full text of DOI:10.1007/s10562-012-0891-4

Catal Lett (2012) 142:1225–1233
DOI 10.1007/s10562-012-0891-4
One-Step Synthesis of Acetals from Phenol/Alcohols Mixtures
Using a Pd/Aluminoborate Catalyst
•
Yejun Guan Damin Zhang Yimeng Wang
•
Received: 7 June 2012 / Accepted: 9 August 2012 / Published online: 21 August 2012
Ó Springer Science+Business Media, LLC 2012
Abstract A one-step synthesis of cyclohexanone acetals
from phenol/alcohol mixtures has been explored using
bifunctional aluminum borates supported Pd catalysts.
Three aluminum borates have been prepared via either a
high temperature calcination (9Al2O3ꢀ2B2O3) or a self-
pressured thermal synthesis (PKU-1 and ABO-X) using
various aluminum precursors and boric acid. Well dis-
persed Pd nanoparticles loaded on these materials show
excellent phenol hydrogenation activity in both water and
ethanol. The reactivity follows the trend of Pd/ABO-
X [ Pd/9Al2O3ꢀ2B2O3 [ Pd/PKU-1, which is similar to
the sequence of BO4/BO3 ratio presented in the alumin-
oborates as shown by NMR spectra. When ethanol is used
as a solvent, 13.5 % yield of cyclohexanone diethyl acetal
has been obtained under the conditions investigated on Pd/
ABO-X, which is attributed to its high activity in both
hydrogenation and acetalization for the one-step synthesis
of cyclohexanone acetals.
chemical industry such as perfumes, flavours, pharmaceu-
ticals, solvents, and polymer chemistry [1]. Traditionally,
acetalization of aldehydes and ketones is performed using
trimethyl orthophosphate in the presence of acid catalysts
such as HCl, H2SO4, p-toluene sulphonic acid and ferric
chloride [2]. Cyclohexanone is one of the mostly used
ketones and its acetalization has been widely investigated
[3, 4], preferably on heterogeneous catalysts [5, 6].
Cyclohexanone itself is industrially produced by partial
oxidation of hexane or phenol hydrogenation [7]. It would
be beneficial to synthesize cyclohexanone acetal as a one-
step synthesis. In this case, the process may start from
phenol instead of cyclohexanone. The operation of highly
flammable and toxic cyclohexanone is then avoided. To
this end, the desired catalyst should be able to hydrogenate
phenol first and subsequently catalyze the acetalization
reaction. To our knowledge, such a process has not been
reported before. Clearly there are several challenges. First,
the catalyst should be highly active and selective for phenol
hydrogenation to cyclohexanone, otherwise the acetaliza-
tion will not take place. In fact, supported Pd catalysts are
promising for low temperature liquid phase selective
hydrogenation of phenol [8–13]. Liu et al. [9] reported a
dual supported Pd–Lewis acid catalyst, on which the AlCl3
acts as a Lewis acid and strongly coordinates with cyclo-
hexanone. This suppresses the hydrogenation of cyclo-
hexanone. Some supports, e.g., C3N4, MIL-101 and hybrid
TiO2/carbon, are also found to hydrogenate phenol, with
99.9 % selectivity to cyclohexanone at high conversion
[10–13]. Second, the catalyst should be acidic enough to
catalyze the subsequent acetalization. In this regard, some
solid acids like zeolites, Al or Zr incorporated mesoporous
materials would be possible candidates [5]. However,
palladium supported on these materials seems to show very
low activity in hydrogenation under mild conditions
Keywords Phenol hydrogenation ꢀ Palladium ꢀ
Aluminum borate ꢀ Acetalization ꢀ Cyclohexanone
1 Introduction
Acetals are an important class of compounds that have
found their direct applications in diverse areas in the
Y. Guan (&) ꢀ D. Zhang ꢀ Y. Wang (&)
Shanghai Key Laboratory of Green Chemistry and Chemical
Processes, East China Normal University, North Zhongshan
Road 3663, Shanghai 200062, China
e-mail: yjguan@chem.ecnu.edu.cn
Y. Wang
e-mail: ymwang@chem.ecnu.edu.cn
123

1226
Y. Guan et al.
[9, 14]. The incorporation of boron oxide to Al2O3 leads to
an alumina–boria catalyst with an increase in the acid
character which owes to its Bronsted acid sites [15, 16]. In
another study, a new zeolitic aluminoborates containing B
and Al centers that serve as Lewis acid sites have been also
reported [17, 18]. These materials can catalyze alkene
isomerization [19], alcohol dehydration [20, 21], and vapor
phase Beckmann rearrangement of cyclohexanone oxime
[22]. Moreover, doping boron onto alumina surface has
been found to be an effective way to increase the hydro-
genation activity of supported CoMo or Pt catalysts in oil
refinery [23, 24]. Inspired by these studies, we use alum-
inoborate as a support of palladium to carry out the one-
step synthesis of cyclohexanone acetal in this study.
This bifunctional catalyst efficiently converted phenol into
cyclohexanonediethylacetalwithethanolusedasasolvent. In
contrast, a conventional Al2O3 or HZSM-5 supported Pd did
not catalyze this reaction. Reaction and characterization
results suggest that the crystallized phase and surface struc-
ture, most likely the boria groups of aluminum borate play a
vital role in determining the one-step synthesis activity.
(Pd: 18.4 mg/mL) was diluted into 10 mL. It should be
noted that the H2PdCl4 solution is very acidic, which may
partially dissolve the alumina based supports [25]. There-
fore, prior to deposition, the pH value of the H2PdCl4
solution was adjusted to 6 by dropwise adding 1 M NaOH
solution. 0.95 g of support was added into the resulting
solution and thoroughly stirred. The final pH value of the
mixture was further adjusted to 9 by adding more NaOH
solution (1 M). Afterwards, NaBH4 solution was added to
the solution in order to reduce the Pd(OH)2 colloid while
cooling using ice bath [26]. The solid was filtered and dried
at 70 °C under vacuum overnight.
2.3 Characterization
Nitrogen adsorption–desorption isotherms at 77 K were
obtained using a BELSORP-max instrument. Prior to
measurement, the samples were outgassed at 150 °C under
vacuum for 6 h. Specific surface areas were calculated
according to the BET-method using five relative pressure
points in the interval of 0.05–0.3. The powder X-ray dif-
fraction (XRD) patterns were collected on a Rigaku Ultima
IV X-ray diffractometer using Cu Ka radiation
˚
(k = 1.5405 A) operated at 35 kV and 25 mA. Scanning
2 Experimental
electron microscopy (SEM) was performed on a Hitachi
S-4800 microscope. Transmission electron microscopy
(TEM) images were taken on a JEOL-JEM-2100 micro-
scope at an accelerating voltage of 200 kV. The average Pd
particle size was calculated by dTEM ¼ ðRnidi3Þ=ðRnidi2Þ by
measuring at least 100 particles. The Pd loading was
determined by a Thermo Elemental IRIS Intrepid II XSP
inductively coupled plasma emission spectrometer (ICP-
AES). The desired amount of sample was dissolved in
10 mL of aqua regia. This mixture was heated to remove
the extra acid and diluted into a 50 mL volumetric flask.
11B and 27Al solid-state magic angle spinning nuclear
magnetic resonance (MAS-NMR) spectra were recorded at
20 °C on a Varian 400 MHz spectrometer working at
104.18 MHz (27Al) and 128.27 MHz (11B), with a rotation
frequency of 10 kHz and recycle delay of 4 s. The refer-
ences of 27Al and 11B were taken from KAl(SO4)2ꢀ12H2O
(-0.21 ppm) and NaBH4 (-8.16 ppm), respectively.
2.1 Preparation of Aluminum Borate Supports
a. 9Al2O3ꢀ2B2O3: A conventional calcination synthesis
procedure was employed [15, 16]. Typically, 2.4 g of
AlCl3ꢀ6H2O and 12.2 g of boric acid were mixed and
ground thoroughly. The resulting mixture was heated
to 1,200 °C at 10 °C/min and held in static air for 8 h.
b. PKU-1: PKU-1 is a new material synthesized by Ju
et al. [17, 18]. In a typical synthesis, 0.76 g of
Al(NO3)ꢀ9H2O and 12.2 g of boric acid were mixed
thoroughly. The resulting mixture was transferred into
a Teflon-lined autoclave (50 mL), sealed tightly, and
heated in an oven at 240 °C for 4 days. After cooling
the autoclave to room temperature, the mixture was
dispersed in large amount of water (70 °C) with severe
stirring. The hot mixture was subsequently filtered to
remove the soluble boric acid, after which the PKU-1
sample was recovered and dried.
c. ABO-X. The synthesis procedure is similar to the one
employed for PKU-1. AlCl3ꢀ6H2O was used as aluminum
precursor. An unidentified structure was found for the
resulting material, which isdenoted as ABO-X in this study.
2.4 Catalytic Tests
2.4.1 Hydrogenation of Phenol
A teflon-lined (100 mL) steel batch reactor was used to
carry out the liquid phase hydrogenation. Prior to reaction,
the catalyst (100 mg) was reduced under a H2 flow at
200 °C for 30 min in a three-neck glass reactor. After
cooling down to room temperature, 10 mL of phenol
2.2 Preparation of the Supported Pd Catalysts
The supported Pd catalysts were prepared by a deposition-
reduction method. Typically, 2.4 mL of H2PdCl4 solution
123

One-Step Synthesis of Cylcohexanone Acetals
1227
solution (0.25 M, either of water or of alcohols) was
introduced into the glass reactor. The mixture of catalyst
and solution was then transferred into the teflon-lined
(100 mL) steel batch reactor. The reactor was purged three
times with H2 and then pressurized with 5 bar H2. The
reaction mixture was heated up to 80 °C and held for 4 h.
Once the reactor was cooled down, the products were
analyzed on a Shimadzu GC 2014 equipped with a RTX-
Wax capillary column (30 m length and 0.25 mm internal
diameter).
c
#
2.4.2 Acetalization of Cyclohexanone
Prior to reaction, 10 mg of each sample was loaded in a
50 mL round-bottom flask and dried at 75 °C under vac-
uum overnight to remove the physisorbed water. After
cooling down to room temperature, 5 mL of solvent
(methanol or ethanol) was added and 100 lL of cyclo-
hexanone was introduced. The mixture was placed in an oil-
bath and refluxed for 1 h at 80 °C for ethanol and 60 °C for
methanol, respectively. Then the liquid was filtered, diluted
with ethanol, and analyzed on a Shimadzu GC 2014.
#
#
#
#
#
b
a
*
*
*
*
*
*
*
*
*
*
*
*
*
*
10
20
30
40
50
60
70
80
2 Theta (degree)
3 Results and Discussion
Fig. 1 X-ray
diffraction
patterns
of
three
aluminum
borates:a 9Al2O3ꢀ2B2O3 (*),b PKU-1 (#), c ABO-X
3.1 XRD, BET, SEM and NMR Characterization
of Aluminum Borates
length and width (Fig. 2c). Compared with PKU-1, the
diameter of ABO-X was substantially larger. This might be
ascribed to the properties of aluminum sources used.
The incorporation of boron to Al2O3 leads to an alu-
mina–boria catalyst with an increase in acidity compared to
pure Al2O3. Acid behavior of the alumina–boria catalyst is
reported to be due to bronsted acid sites, which are asso-
ciated to the boria groups [27–29]. The local environments
of B and Al are essential to elucidate the distribution of
surface hydroxyl groups. Figure 3a, b display the 11B and
27Al NMR spectra of three aluminum borates, respectively.
Distinct differences are clearly observed in these spectra.
The crystal structure of the aluminum borate 9Al2O3ꢀ
2B2O3 contains four distinct aluminum sites: one tetrahe-
dral, two pentahedral and one octahedral [27]. These res-
onances can be clearly observed in Fig. 3B-a. Boron is
dominated by BO3 species in 9Al2O3ꢀ2B2O3, as indicated
by the characteristic quadrupolar doublet pattern in the
range 2–20 ppm. The chemical shift at 0 ppm indicates the
coexistence of BO4 species. BO4 species are considered to
be created by a reversible hydration–dehydration reaction
of BO3 species [19]. Only those BO3 species situated near
the surface are able to transform into BO4 species, which
may be accessible to molecules in catalysis [28, 29]. The
NMR spectroscopy of PKU-1 is consistent with previous
Figure 1 shows the XRD patterns of the synthesized alu-
minum borate supports. When AlCl3 and H3BO3 was
mixed and calcined at 1,200 °C, the product was ortho-
rhombic 9Al2O3ꢀ2B2O3 (Fig. 1a). This phase is the only
one thermodynamically stable upon calcination above
1,000 °C. The surface area of 9Al2O3ꢀ2B2O3 was 111 m2/
g. Using Al(NO3)ꢀ9H2O as the precursor, an aluminum
borate known as PKU-1 (Fig. 1b) was obtained by self-
pressured thermal treatment (240 °C for 4 days). This
material had a surface area of 112 m2/g. With AlCl3ꢀ6H2O
being the aluminum precursor, a new aluminum borate
with unidentified structure (ABO-X, Fig. 1c) was synthe-
sized using the same thermal treatment. This material had a
surface area of 116 m2/g. No narrow pore distribution is
observed for all samples.
Figure 2 shows the SEM images of these materials.
Upon high temperature calcination, a complex morphology
was observed for 9Al2O3ꢀ2B2O3 (Fig. 2a). Both irregular
aggregated particles and long rods were present. Uniformly
distributed rods with 1 lm in length and 100 nm in
diameter were found for PKU-1 (Fig. 2b), which is con-
sistent with a previous study reported by Ju et al. [17].
ABO-X displayed the same morphology as PKU-1
regardless of crystalline phases, but with non-uniform
123

1228
Y. Guan et al.
18-ring windows and 10-ring rectangular windows,
respectively. The 11B NMR of ABO-X was clearly dif-
ferent from both of PKU-1 and 9Al2O3ꢀ2B2O3. A sharp 11
B
chemical shift was observed centered at 0 ppm. This result
means that the boron in ABO-X is mainly present as BO4
species, with a small fraction of BO3. The aluminum
coordination is similar to that of PKU-1 according to 27Al
NMR chemical shift, indicating that the framework also
consists of AlO6 octahedra, which attach to each other by
both BO3 and BO4 groups. The exact structure still needs to
be resolved.
3.2 XRD and TEM Characterization of Pd/Aluminum
Borates
Figure 4 displays the XRD patterns of the supported Pd
catalysts. The diffraction peak at 40° assigned to the (111)
reflection of the Pd nanoparticles was unfortunately over-
lapped by the strong diffractions of aluminum borates.
Accordingly it is not possible to calculate the particle size
by using the Scherrer equation. Figure 5 shows the typical
TEM images of aluminum borates supported Pd catalysts
and the particle size distributions. Well dispersed Pd
nanoparticles were found on all catalysts. The average
particle sizes of Pd on Al2O3 (Fig. 5a), 9Al2O3ꢀ2B2O3
(Fig. 5b), PKU-1 (Fig. 5c), and ABO-X (Fig. 5d) were 8.5,
15, 5, and 6.5 nm, respectively, when NaBH4 was used as a
reductant. Clearly boron modified alumina are a group of
excellent support materials for the preparation of Pd cata-
lysts. The Pd particle size on 9Al2O3ꢀ2B2O3 was the largest
among these catalysts, which may be due to its specific
surface property upon high temperature calcination. The
effect of reducing reagent was also compared using ABO-
X support as an example. Even smaller Pd particles in the
range of 1–4 nm (Fig. 5e) were observed when a diluted
hydrazine solution was employed as the reducing reagent.
It can be concluded that the Pd particle size is dependent on
support surface properties and the reducing reagents used.
3.3 Phenol Hydrogenation in Aqueous Solution
The catalysts were tested for the hydrogenation of phenol
in aqueous media under mild conditions, i.e. 5 bar of
hydrogen and 80 °C for 4 h. Entries 1–4 in Table 1 show
the activities of Pd samples obtained using NaBH4 as the
reducing reagent. These results show that the catalysts
exhibited high selectivity to cyclohexanone under the
reaction conditions investigated. Pd/9Al2O3ꢀ2B2O3 showed
21.2 % of phenol conversion and cyclohexanone selectiv-
ity of 81.6 % (Table 1, Entry 1). The phenol conversion on
Pd/PKU-1 was higher than that found on Pd/9Al2O3ꢀ
2B2O3, with phenol conversion of 48.5 % and cyclohexa-
none selectivity of 93.3 % (Table 1, Entry 2). Pd/ABO-X
Fig. 2 SEM images of three aluminum borates: a 9Al2O3ꢀ2B2O3,
b PKU-1, c ABO-X
reports [17, 18], which is characterized by the quadrupolar
doublet pattern. The authors proposed that the Al and B
atoms are exclusively in octahedral and triangular coordi-
nation, respectively. The structure can be considered as an
octahedral framework, which is enveloped by triangular
borate groups [i.e., BO2(OH) and B2O4(OH)]. These
BO2(OH) and B2O4(OH) groups are located within the
123

One-Step Synthesis of Cylcohexanone Acetals
1229
Fig. 3 11B (A) and 27Al NMR
(B) spectra of three aluminum
borates: a 9Al2O3ꢀ2B2O3,
b PKU-1, c ABO-X
0.4
A
B
11.7
4.0
c
c
b
b
a
a
40
20
0
-20
-40
100 80
60
40
20
0
-20 -40 -60
(ppm)
(ppm)
slightly higher selectivity to cyclohexanone (99 vs.
94.2 %). When the Pd loading was lowered further to
0.88 wt%, this led to a slight decrease of phenol conversion
to 94 % (Table 1, Entry 4). In this case, the selectivity to
cyclohexanone was as high as 99.6 %. Clearly, ABO-X
showed a superior performance as a support of palladium
catalysts. Chen et al. [23] proposed that the interaction of
boron with Pt metal affected the electronic structure of the
metal and the activity for hydrogenation. Another report
speculated that the acidic property of B-OH benefited to the
interaction between Co–Mo–O groups and support, thus
leading to a higher HDS activity [24]. The boria located in
the framework as in the case of PKU-1 and 9Al2O3ꢀ2B2O3
does not likely contribute to the hydrogenation activity of
Pd. When the diluted hydrazine solution was used as the
reducing reagent, much smaller particles with average size
of 3 nm were obtained on ABO-X. This catalyst showed
comparable activity (Table 1, Entry 5) to that prepared
using NaBH4 as the reducing reagent.
e
d
c
b
a
3.4 Phenol Hydrogenation and Acetalization Coupling
30
35
40
45
50
2 Theta (degree)
Given the excellent hydrogenation activity of these alu-
minum borates supported Pd catalysts in aqueous solution
of phenol, we tested their activity for the production of
cyclohexanone acetal in a one-step process by combining
the phenol hydrogenation and acetalization in an alcohol
solution. The results are shown in Table 2. Pd/PKU-1
catalyst did not produce any acetal (Table 2, Entry 1).
Under the identical conditions, the other two Pd catalysts
were both active for acetal production. Taking Pd/ABO-X
as an example, the obtained products contained 50 % of
Fig. 4 X-ray diffraction spectra of c-Al2O3 and aluminum borates
supported Pd catalysts: a Pd/c-Al2O3 (NaBH4), b Pd/9Al2O3ꢀ2B2O3
(NaBH4), c Pd/PKU-1 (NaBH4), d Pd/ABO-X (NaBH4), e Pd/ABO-X
(N2H4)
showed the best catalytic property among these catalysts,
with phenol conversion of 99 % and cyclohexanone
selectivity of 99 % (Table 1, Entry 3). This reactivity was
comparable to that of Pd/Al2O3 (Table 1, Entry 6), but with
123

1230
Y. Guan et al.
Fig. 5 TEM images and
particle distribution of c-Al2O3
and aluminum borates
supported Pd catalysts:
a Pd/c-Al2O3 (NaBH4), b Pd/
9Al2O3ꢀ2B2O3 (NaBH4), c Pd/
PKU-1 (NaBH4), d Pd/ABO-X
(NaBH4), e Pd/ABO-X (N2H4)
123

One-Step Synthesis of Cylcohexanone Acetals
1231
Table 1 Loading, particle size,
Entry
Catalyst
Loading (wt%)
dPd (nm) TEM
Conv. (%)
Sel. (%)
C=O
and aqueous phase phenol
hydrogenation activities of
C–OH
various palladium catalysts
1
Pd/9Al2O3ꢀ2B2O3
Pd/PKU-1
4.6
4.7
5.1
0.88
5.3
5.7
15
21.2
48.5
99.0
94.0
99.4
99.9
81.6
93.3
99.0
99.6
98.6
94.2
18.4
6.7
1.0
0.4
1.4
5.8
2
5.0
6.5
n.d.
3.0
8.5
Reaction conditions 10 mL of
3
4
5
6
Pd/ABO-X
Pd/ABO-X
Pd/ABO-Xa
Pd/Al2O3
0.25 M phenol aqueous
solution, 100 mg of catalyst,
T = 80 °C, PH2 = 0.5 MPa,
t = 4 h
a
Reduced by N2H4
Table 2 Results of phenol
hydrogenation in alcohols on
various Pd-based catalysts
Entry
Catalyst
Solvent
Conv. (%)
Sel. (%)
Acetal
C=O
C–OH
1
2
3
4
5
6
7
Pd/PKU-1
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Methanol
9.1
12.3
45.0
30.0
62.9
4.0
0.0
19.0
30.0
10.0
1.1
92.0
69.0
50.0
70.0
94.0
48.0
65.7
8.0
12.0
20.0
20.0
4.9
Pd/9Al2O3ꢀ2B2O3
Pd/ABO-X
Reaction conditions 10 mL of
0.25 M phenol/ethanol
(methanol) solution, 100 mg of
catalyst, T = 80 °C,
PH2 = 0.5 MPa, t = 4 h
Pd/ABO-X (N2H4)
Pd/Al2O3
Pd/ABO-X
Pd/Al2O3 ? HZSM-5a
19.5
27.3
32.5
7.0
a
100 mg of Pd/Al2O3 catalyst
and 100 mg of HZSM-5
Ethanol
49.7
cyclohexanone, 20 % of cyclohexanol, and 30 % of cyclo-
hexanone diethyl acetal, with an acetal yield of 13.5 %
(Table 2, Entry 3). The yield of acetal for Pd/9Al2
O3ꢀ2B2O3 catalyst was 2.3 % (Table 2, Entry 2). The
activity for cyclohexanone dimethyl acetal with methanol
as solvent was also investigated using Pd/ABO-X as an
example. 19.5 % selectivity to acetal was found but with
low phenol conversion (Table 2, Entry 6). It should be
noted that Pd/Al2O3 only gave trace amount of acetal under
the identical reaction conditions (Table 2, Entry 5). The
introduction of HZSM-5 (Table 2, Entry 7) to Pd/Al2O3
remarkably increased the acetal yield. Apparently, pres-
ence of acidic sites is favorable to acetalization. We con-
sider that both the hydrogenation ability of Pd and acidity
of support are essential for the production of acetal, as
depicted in Scheme 1. Indeed Pd/HZSM-5 barely gave any
acetal products in phenol hydrogenation due to its very low
hydrogenation activity (not shown), meaning that the
interaction between Pd and the acidic site is also relevant.
It seems that the metal/support interface may be involved
or some specific structures are required for this process. We
have investigated the reusability and leaching of Pd cata-
lyst. However, a severe activity loss was found for Pd/
ABO-X, for both hydrogenation and acetalization. No
leaching of Pd was observed in the hot solution, meaning
that the surface property of Pd/ABO-X changed during the
hydrogenation process.
Scheme 1 A schematic illustration of a Pd/aluminum borate catalyst
and its bifunctional role in cyclohexanone acetals production from
phenol in alcohol
substantially from 99 % (Table 1, Entry 3) to 45 %
(Table 2, Entry 3). On the other hand, the selectivity to
cyclohexanol increased from 1 % (Table 1, Entry 3) to
20 % (Table 2, Entry 3). This effect is even more pro-
nounced when methanol was used, in which case 4 % of
phenol conversion (Table 3, Entry 6) was observed under
identical conditions as for Pd/ABO-X. The hydrogenation
activity of Pd on ABO-X decreased 10 times when the
solvent changed from ethanol to methanol. It should be
stressed that Pd/ABO-X was the most active catalyst for
phenol hydrogenation in ethanol, whereas Pd/PKU-1 was
the least active one. This result indicates that the boron
Another finding should be noted is that the phenol
conversion decreased for all catalysts when ethanol was
used as a solvent compared to that observed in water. As an
example, the phenol conversion on Pd/ABO-X decreased
123

1232
Y. Guan et al.
explain the acetalization activity [2–4]. Generally speak-
ing, the hydroxyl groups in aluminum borates are associ-
ated to boria species, namely BO3 and BO4 [19, 28]. Their
concentration is more likely ascribed to the number of
tetrahedrally coordinated BO4 sites [15, 19, 29]. ABO-X
tentatively presents the most amounts of BO4 sites among
three aluminum borates based on the 11B NMR results.
However, we could not determine the exact concentration
of surface hydroxyl groups due to their low thermal sta-
bility [17]. According to in situ FTIR results, the amount of
hydroxyl group decreased fast upon calcination and even-
tually disappears at 400 °C. No clear trend was found for
the activities of 9Al2O3ꢀ2B2O3 and ABO-X in acetaliza-
tion, as ABO-X was more active than 9Al2O3ꢀ2B2O3 in
methanol and vice versa in ethanol. In this study, trace
amounts of water in the solvent might adsorb on the surface
bronsted acid although all aluminum borates were dried at
75 °C under vacuum overnight prior to the reaction, and
thus affected the activity. Indeed, when some aluminum
borates (PKU-1 and PKU-2) were used for acid-catalyzed
reactions, the samples needed to be dried thoroughly and
the solvent should be degassed prior to reactions [18]. In
summary, these results suggest that aluminum borates are
indeed active in acetalization and their activities are
dependent on the surface properties.
Table 3 Cyclohexanone acetalization activities on aluminoborates
Sample
Conv. (%)a
Methanolb
Ethanolc
PKU-1
0
1.3
27.2
17.5
0
9Al2O3ꢀ2B2O3
ABO-X
2.9
35.5
0
Al2O3
HZSM-5
87.4
55.4
Reaction conditions 10 mg of catalyst, 100 lL of cyclohexanone in
5 mL of alcohol, t = 1 h
a
The conversion was based on cyclohexanone
b
T = 60 °C
T = 80 °C
c
species are likely involved in the phenol adsorption and
hydrogenation process. Velu et al. [30] also reported a
three-fold drop in phenol conversion using ethanol as a
solvent while no product selectivity change was observed.
The authors claimed that the drop in phenol conversion
could be due to a competitive adsorption of reactants
(phenol and/or H2) and solvents (ethanol) molecules on the
same active site [30]. Indeed, the solvent effect is com-
monly observed in liquid phase hydrogenation, which is
usually interpreted by the competitive adsorption of sub-
strates and solvent or solvent dependent hydrogen solu-
bility [31, 32].
4 Conclusion
3.5 Acetalization of Cyclohexanone with Various
Alcohols
Crystallized aluminum borate materials with distinct vari-
ous structures and morphologies were synthesized by dif-
ferent methods. These materials have different ratios of
BO4/BO3 species, following the trend of ABO-X [
9Al2O3ꢀ2B2O3 [ PKU-1. The presence of BO4 species
contributes to both the acidity of the obtained aluminum
borates and the phenol hydrogenation activity of supported
Pd nanoparticles. Pd/ABO-X catalyst shows the highest
hydrogenation activity for phenol in water and ethanol.
Moreover, a one-step synthesis of cyclohexanone diethyl
acetal on Pd/ABO-X catalyst is achieved by simple
hydrogenation of phenol in an ethanol solution.
To further determine the catalytic role of aluminoborates in
acetalization, we have conducted the acetalization of
cyclohexanone on these materials, in comparison to that on
Al2O3 and HZSM-5. The results in various alcohols are
listed in Table 3. Alumina showed no activity under the
reaction conditions regardless of the type of the alcohols
used. In contrast, HZSM-5 showed the highest cyclohex-
anone conversion, with 87.4 and 55.4 % in methanol and
ethanol, respectively. This result emphasizes the impor-
tance of bronsted acidity in acetalization. Among alumi-
num borates, PKU-1 turned out to be the least active one,
with 1.3 % cyclohexanone conversion in ethanol and even
no conversion in methanol. On 9Al2O3ꢀ2B2O3, the con-
version of cyclohexanone was 2.9 and 27.2 % in methanol
and ethanol, respectively. Under the identical conditions,
the conversion of cyclohexanone on ABO-X was 35.5 and
17.5 % in methanol and ethanol, respectively. The activity
of acetalization is in good agreement with that observed in
one-step phenol hydrogenation conditions. Properties of
the catalyst, such as the number of acid sites, acid strength
and pore size have been brought forward previously to
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