1072
Chemistry Letters Vol.32, No.11 (2003)
Liquid Phase Hydrogenation of Phenol to Cyclohexenone Over A Pd–La–B Amorphous Catalyst
Li Zhuang, Hexing Li,ꢀ Weilin Dai,y and Minghua Qiaoy
Department of Chemistry, Shanghai Normal University, Shanghai 200234, P. R. China
yDepartment of Chemistry, Fudan University, Shanghai 200433, P. R. China
(Received August 5, 2003; CL-030728)
A La-doped Pd–B amorphous catalyst (Pd–La–B) was pre-
pared by chemical reduction of mixed PdCl2 and LaCl3 with
KBH4. The La-dopant played a key role in stabilizing the amor-
phous structure since only the crystalline Pd–B was obtained with-
out the La-dopant. During liquid phase phenol hydrogenation, the
as-prepared Pd–La–B exhibited higher activity and better selectiv-
ity to cyclohexanone than the undoped Pd–B.
The selected area electron diffraction (SAED) picture of the
fresh Pd–La–B sample displayed diffractional cycles indicative
of the amorphous structure which was further confirmed by the
XRD pattern since only one broad peak around 2ꢁ ¼ 45ꢁ was ob-
served. After being treated at 873 K for 2 h in N2 flow, a series of
diffractional peaks appeared on the XRD pattern, indicating the
crystallization of the Pd–La–B amorphous alloy. The undoped
Pd–B alloy displayed a similar XRD pattern to that of the Pd–
La–B after being treated at 873 K, implying that the Pd–B alloy
was originally present in the crystalline structure. The DSCanaly-
sis revealed that the Pd–La–B amorphous alloy began to crystallize
at 573 K corresponding to an exothermic peak. However, no exo-
thermic peak was observed from the DSCcurve of the undoped
Pd–B sample, which again confirmed that only the crystalline
Pd–B alloy was obtained without the La-dopant. Thus, one could
conclude that the La-dopant played a key role in stabilizing the
Pd–B amorphous alloy structure.
According to ICP analysis, the compositions of the Pd–La–B
(ꢀLa ¼ 1%) and the undoped Pd–B were determined as Pd92B8 and
Pd96B4, respectively, indicating the B-enrichment induced by the
La-dopant. Meanwhile, the BET surface area of the Pd–La–B cat-
alyst was determined as 27 m2/g, much higher than that of the un-
doped Pd–B catalyst (18 m2/g). The TEM morphologies demon-
strated that the La-dopant resulted in a remarkable increase in
the dispersion degree of the as-prepared catalyst, corresponding
to much smaller particle size and more homogenous distribution
in the Pd–La–B catalyst than those in the undoped Pd–B catalyst,
which could account for the higher surface area of the Pd–La–B
catalyst. The XPS spectra of the undoped Pd–B sample revealed
that almost all the Pd species were present in the metallic state cor-
Cyclohexanone is widely used in industry (>400 million ton/
year) for producing caprolactam, a monomer used in the synthesis
of Nylon-6. Traditionally, cyclohexanone is produced via com-
plete hydrogenation of phenol to cyclohexanol, followed by the de-
hydrogenation of cyclohexanol. Obviously, the selective hydroge-
nation of phenol to cyclohexanone is superior in saving energy,
simplifying reaction steps and reducing waste disposal. However,
this is a challenging task since the cyclohexanone is an active in-
termediate which could be further hydrogenated to cyclohexanol.
Though very few catalysts have been reported to be suitable for
the title reaction, among them, the Pd-Based catalysts are most fre-
quently used. In order to achieve as high as possible selectivity to
cyclohexanone, great attempts have been made to develop power-
ful catalysts, such as the modification of Pd-Based catalysts by
metal or nonmetal additives, the change of the support etc.1–3 Some
catalysts exhibit pretty good activity and selectivity during gas
phase phenol hydrogenation. However, our preliminary studies re-
vealed that these catalysts were not suitable for the liquid phase
phenol hydrogenation due to the low activity and selectivity and
even the rapid deactivation. As well known, the amorphous alloy
catalysts usually exhibit higher activity and better selectivity as
well as stronger resistance against poisoning during various hydro-
genation reactions.4 Although a great number of metal boride
amorphous alloys have been prepared through chemical reduction
with BH4À, the Pd–B amorphous alloy has never been obtained
due to its extremely poor thermal stability. In this paper, we report-
ed a novel La-doped Pd–B amorphous alloy (Pd–La–B) which was
more active and selective to cyclohexanone during liquid phase
phenol hydrogenation. The stabilizing effect of the La-dopant on
the amorphous structure and its promoting effect on the catalytic
performance were discussed briefly.
The Pd–La–B sample was prepared by reducing mixed PdCl2
and LaCl3 with excess KBH4 in aqueous solution at room temper-
ature. Briefly, 2.0 M KBH4 solution containing 0.2 M NaOH was
added dropwise into 10 mL solution containing mixed PdCl2 and
LaCl3 under magnetic stirring. The reaction was lasted for about
1.5 h to ensure the complete reduction of metallic ions in the solu-
tion. Then, the resulting Pd–La–B black solid was washed thor-
oughly with H2O, then with ethanol (EtOH), and finally, kept in
EtOH until the time of use. The content of the La-dopant in the
Pd–La–B sample, expressed in the La/(Pd + La + B) molar ratio
(ꢀLa), was adjusted by changing the amount of LaCl3 in the solu-
tion. The undoped Pd–B sample (ꢀLa ¼ 0) was also prepared in the
similar way but using the solution containing PdCl2 alone.
responding to the binding energies (BE) of 335 eV in Pd3d level.
5=2
However, the B species were present in the states of both the B al-
loying with Pd and the oxidized B, corresponding to BE of 188.0
and 191.7 eV in B1S level. The BE value of the alloying B shifted
positively by 0.9 eV in comparison with the standard BE value of
the pure B (187.1 eV), showing that the alloying B donated partial
electrons to the metallic Pd which could be understood by consid-
ering the assumption that the bonding electrons of the B occupied
the vacant d-orbitals of metallic Pd.5 The failure in observing the
BE shift of the metallic Pd could be mainly attributed to the rela-
tively bigger size of the Pd atom and the lower B content in the Pd–
B alloy. Concerning the Pd–La–B sample at ꢀLa ¼ 1%, two peaks
around BE of 835.1 and 838.0 eV were observed in La3d level,
5=2
corresponding to LaH2 and La2O3, respectively. As no metallic
La was present, it could be concluded that only the Pd–B amor-
phous alloy was formed during the chemical reduction of mixed
Pd2þ and La3þ by KBH4. The presence of the La2O3 could act
as a support for the Pd–B alloy particles, resulting in higher BET
surface area. Comparing the XPS spectra between the Pd–La–B
and undoped Pd–B, it was found that, in the presence of the La-
dopant, the BE values of both the metallic Pd and the alloying B
shifted negatively by 0.3 and 0.5 eV, respectively. While, the BE
of the La(III) in the La2O3 shifted positively by 0.4 eV in compar-
Copyright Ó 2003 The Chemical Society of Japan