Low-temperature, highly selective, gas-phase oxidation of benzyl alcohol over mesoporous K-Cu-TiO2 with stable copper(I) oxidation state

Article abstract of DOI:10.1021/ja9032499

(Graph Presented) A newly developed mesoporous mixed metal oxide (K-Cu-TiO₂) catalyst is capable of highly selective, gas-phase benzyl alcoholbenzaldehyde transformation at excellent yields (>99%) under surprisingly low temperatures (203°C, bp

Full text of DOI:10.1021/ja9032499

Published on Web 10/12/2009
Low-Temperature, Highly Selective, Gas-Phase Oxidation of Benzyl Alcohol
over Mesoporous K-Cu-TiO2 with Stable Copper(I) Oxidation State
Jie Fan,*,† Yihu Dai,† Yunlong Li,† Nanfeng Zheng,‡ Junfang Guo,† Xiaoqing Yan,† and
Galen D. Stucky*,‡
Key Lab of Applied Chemistry of Zhejiang ProVince, Department of Chemistry, Zhejiang UniVersity,
Hangzhou, Zhejiang ProVince 310027, China, and Department of Chemistry & Biochemistry, UniVersity of
California at Santa Barbara, Santa Barbara, California 93106
Received May 2, 2009; E-mail: jfan@zju.edu.cn; stucky@chem.ucsb.edu
Selective oxidation of alcohols to aldehydes, in particular benzyl
alcohol-to-benzaldehyde, is one of the most common organic
transformations and is of fundamental importance for laboratory
and commercial processes.1 Currently, benzaldehyde is produced
through stoichiometric oxidation by manganese and chromium salts
in the laboratory or by liquid-phase chlorination and oxidation of
toluene in industrial processes.1 In the wake of increasing concern
about the environment, the benzyl alcoholbenzaldehyde transforma-
Figure 1. (a) TEM image of mesoporous 3K-Cu-50TiO2 and (b) element
mapping of a single mesoporous 3K-Cu-50TiO2 particle.
tion should preferably be accomplished by using a highly selective
and recyclable “green” catalyst that is able to use molecular oxygen
as the oxidant.2
nm shows uniform X-ray intensities of K, Cu, Ti, and O signals
In practice, the oxidation of alcohols with molecular oxygen can
throughout the mesoporous particles, revealing the existence of a
be performed in the liquid or gas phase, depending mainly on the
homogeneous distribution of multiple components (Figure 1b).
thermal stability and volatility of the reagents and products. The
Wide-angle X-ray diffraction (WXRD) analysis reveals only the
convenience of catalyst separation and solvent-free conditions make
anatase crystalline phase of 3K-Cu-50TiO2 despite the copresence
a gas-phase process more attractive for industrial applications. While
of Cu and K.
many recent reports deal with the liquid phase aerobic oxidation
The oxidation state of copper ions is a key factor that determines
with/without solvents,3 less attention has been paid to the gas-phase
their catalytic property in selective oxidation reactions. X-ray
reaction, and attempts to develop a successful gas-phase industrial
photoelectron spectroscopy (XPS) was used to characterize the Cu
application have failed owing to selectivity and stability problems.4
oxidation state. The coexistence of Cu(I) and Cu(II) in fresh 3K-
The biggest disadvantage of gas-phase oxidation is the high reaction
Cu-20TiO2 catalyst is evidenced by a shoulder observed on the
temperature, which tends to favor energy consumption, low
main peak at 931.6 eV (Figure 2a), which is assigned to Cu(I)
selectivity, and deactivation of active sites. The reaction temper-
species. The higher binding energy (BE) Cu 2p3/2 peak at 933.7
atures currently reported for gas-phase benzyl alcohol oxidation
eV and its shakeup satellites are due to Cu(II). The Cu(I) percentage
in fresh mesoporous 3K-Cu-20TiO2 is estimated to be 44%.6 For
mesoporous 3K-Cu-50TiO2, only Cu(I) species are confirmed by
XPS. These Cu(I) species can be completely oxidized to Cu(II) by
oxygen at 250 °C as evidenced by temperature programmed
oxidation (TPO) and XPS data. The electron paramagnetic reso-
nance (EPR) spectrum of oxidized 3K-Cu-50TiO2 exhibits a
pronounced peak at g⊥ ) 2.132 and a poorly resolved quadruplet
at g| ) 2.383, corresponding to isolated ions located in octahedral
sites. It implies that the original Cu(I) also exists as isolated ions
incorporated within the TiO2 matrix.
Mesoporous K-Cu-TiO2 is highly active in the selective oxidation
of benzyl alcohol at a reaction temperature as low as 203 °C, the
bp of benzyl alcohol. In our example, a high selectivity (>98%) to
benzaldehyde with a conversion of ∼72% is obtained between 203
and 223 °C over mesoporous 3K-Cu-20TiO2 (Figure 2c), and the
mesoporous 3K-Cu-50TiO2 displays a stable yield of >99% for the
benzyl alcohol-to-benzaldehyde transformation in this low temper-
ature range (Figure 2d). Such a stable, excellent catalytic perfor-
mance at this low temperature in gas-phase benzyl alcohol oxidation
is very unusual.
are generally above 300 °C, 100 °C higher than the boiling point
(bp) of benzyl alcohol (203 °C), over various catalysts (e.g., Au/
SiO2,4a K/Ag/SiO2,4b Cu/Na/ZSM-5,4c and Au-Cu/SiO24d), and no
appreciable benzyl alcoholbenzaldehyde oxidation below 250 °C
has been reported.
In this communication, we report a newly developed mesoporous
ternary metal oxide (K-Cu-TiO2) that is capable of activating the
selective gas-phase oxidation of benzyl alcohol at a surprisingly
low temperature, the bp of benzyl alcohol (203 °C). The low-
temperature reaction conditions and the homogeneous integration
of K and Cu(I) components into the TiO2 matrix enable the
stabilization of the active Cu(I) oxidation state and the resulting
stable, excellent catalytic performance.
Mesoporous K-Cu-TiO2 catalysts (3K-Cu-50TiO2 and 3K-Cu-
20TiO2) were synthesized according to our previously reported
AcHE method (Supporting Information (SI)).5 The catalysts possess
a two-dimensional hexagonal mesophase structure as confirmed by
small-angle X-ray scattering and transmission electron microscopy
(TEM) analysis (Figure 1). Mesoporous 3K-Cu-50TiO2 and 3K-
Cu-20TiO2 have high surface areas, 150 and 144 m2 ·g-1, respectively.
A detailed energy dispersive X-ray (EDX) element mapping
measurement of mesoporous 3K-Cu-50TiO2 at a resolution of ∼10
The high surface area, integrated K, Cu(I), and TiO2 matrix is
responsible for the high activity of the K-Cu-TiO2 catalyst at low
temperatures, for the gas-phase benzyl alcohol oxidation. Pure TiO2
shows very poor activity for this reaction (only 6.9% conversion).
† Zhejiang University.
‡ University of California at Santa Barbara.
9
15568 J. AM. CHEM. SOC. 2009, 131, 15568–15569
10.1021/ja9032499 CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
K ions to Cu and Ti species so that the electron transfer from Cu(I)
is less favorable.7
Clarification of the reaction mechanism is important for the future
design of catalysts with enhanced performance. That the reaction
occurs stoichiometrically without added oxygen supports the so-
called classic dehydrogenation mechanism where the alcohol
dehydrogenation and oxidant reduction steps are sequential, and
not coupled (SI).2 The lower yield in the absence of O2 and
formation of the surface hydride species as determined by EPR
and spin trapping are both consistent with this mechanism (SI).8
In addition, benzaldehyde is readily oxidized by a free radical
mechanism to benzoic acid, so that the very high benzaldehyde
yield argues against a free radical mechanism dominating the
chemistry here. For the catalyst performed with added O2, no surface
hydride species are observed, suggesting that O2 acts as an efficient
hydrogen acceptor and accelerates the reaction by creating the free
active Cu(I) sites.
In summary, using a high surface area catalyst and the homo-
geneous distribution of multiple components integrated into a
matrix, we have shown that the gas-phase oxidation of benzyl
alcohol to benzaldehyde can be accomplished at low temperatures,
with a high TOF (up to 108 h-1). We believe that this strategy will
be of general importance to other multicomponent heterogeneous
catalytic reactions and has promise as an alternative to current
alcohol oxidation processes.3
Figure 2. XPS spectra of mesoporous (a) 3K-Cu-20TiO2 and (b) Cu-20TiO2
before and after the reaction; (c) benzaldehyde selectivity and benzyl alcohol
conversion as a function of the setup temperature (Text) over 3K-Cu-20TiO2;
and (d) benzaldehyde yields vs time on stream using Cu-TiO2 (200 °C),
3K-Cu-20TiO2 (210 °C), and 3K-Cu-50TiO2 (210 °C) as the catalysts.
The incorporation of 0.2% Cu (Ti/Cu ) 500) can significantly boost
the benzyl alcohol-to-benzaldehyde conversion of TiO2, from 6.9
to 46.1%. For comparison, mesoporous Cu-ZrO2, Cu-SiO2, and Cu-
Al2O3 were also synthesized and investigated as catalysts for gas-
phase benzyl alcohol oxidation. They all show very poor activity
(1%-7% conversion), implying that there is a synergistic effect
between Cu and TiO2.
Acknowledgment. We are grateful for financial support from
the National Science Foundations of China (20873122 and J0830413)
and the National Science Foundation (DMR 02-33728). We thank
Dr. Peter Ford for helpful discussions.
One of the biggest challenges in selective oxidation is the
deactivation of catalysts during the course of a long-term reaction.
Continually exposing copper catalysts to oxidizing and reducing
environments generally leads to a change of their oxidation state
and a concurrent major modification of their catalytic properties.
In our reaction system, two factors play very important roles in
the stabilization of the Cu(I) oxidation state in mesoporous Cu-K-
TiO2 catalysts and their stable, excellent catalytic performance: (i)
a low reaction temperature (203-223 °C). TPO studies reveal that
the Cu(I)-to-Cu(II) conversion in mesoporous 3K-Cu-20TiO2 starts
Supporting Information Available: The synthesis of mesoporous
materials, material characterization, and catalytic result analysis. This
material is available free of charge via the Internet at http://pubs.acs.org.
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