High temperature calcination for a highly efficient and regenerable B2O3/ZrO2
catalyst for the synthesis of e-caprolactam
Bo-Qing Xu,*a Shi-Biao Cheng,b Xin Zhang,a Shuang-Feng Yinga and Qi-Ming Zhua
a State Key Lab of C1 Chemical Technology and Department of Chemistry, Tsinghua University, Beijing 100084,
China. E-mail: bqxu@mailtsinghua.ed.cn
b Research Institute of Petroleum Processing, SINOPEC, Beijing 100083 China
Received (in Cambridge, UK) 20th April 2000, Accepted 16th May 2000
Published on the Web 8th June 2000
High temperature calcination of boria-supported zirconia
leads to highly selective and regenerable B2O3/ZrO2 catalysts
for the synthesis of e-caprolactam by Beckmann rearrange-
ment of cyclohexanone oxime.
and the gas-phase Beckmann reaction have been described
elsewhere.10,11 Regeneration of the deactivated catalyst was
performed by calcination in air at 600 °C for 8 h.
The precalcination temperature (PCT) of the support pre-
cursor [ZrO(OH)2·xH2O] before loading with boria, the loading
of boria, and the catalyst activation (calcination) temperature
(CAT) after the loading are important parameters for catalyst
preparation.12,13 When the CAT is fixed for the preparation, the
catalyst activity and selectivity at a fixed loading of boria can be
significantly modified by changing the PCT. When the
temperature for the catalyst activation is kept at or below 500 °C
(CAT @ 500 °C), support precalcination at 300 °C (i.e. PCT =
300 °C) leads to the highest oxime conversion activities among
10% B2O3/ZrO2 samples with various PCT histories.13 Cata-
lysts with PCT !300 °C also effectively reduce the formation of
undesirable by-products when compared with samples prepared
using lower PCTs.13 In this work, the PCT is fixed at 300 °C for
the catalyst samples to examine the effect of varying the CAT
on the Beckmann reaction. A catalyst loaded with 9.5% B2O3
has been used for this prupose. This loading of boria is close to
the optimum load with the ‘standard’ preparation (CAT = 350
°C) on a zirconia support precalcined at 500 °C (PCT = 500
°C).11 Table 1 shows that oxime conversion and the selectivity
and yield of the lactam are little affected over the entire period
on an 8 h reaction time on stream (TOS) when the CAT is
increased from the ‘standard’ activation temperature (350 °C) to
500 °C over this 9.5% B2O3/ZrO2 catalyst. However, a further
increase in the CAT to 600 °C results in dramatically improved
selectivity and yield of the desired lactam; the selectivity
increases from 70–80% to values greater than 95%, and the
average lactam yield increases from ca. 70% to 92%. The last
group of data at the bottom of the Table 1 gives the results of the
reaction over a ‘pure’ zirconia support that was activated at 600
°C. Except at the very beginning of the reaction, the desired
lactam product is basically not produced. This observation
agrees with our conclusions from earlier work that the active
catalytic sites for the Beckmann reaction are connected with
boria in the sample.13 ICP-MS analysis showed that the B2O3
loading in the three samples with CAT = 350, 500 and 600 °C
are in the range 9.0–9.7%, which indicates that no significant
loss of boron occurs during catalyst activation (calcination) with
CAT > 400 °C. It is, therefore, clear that high temperature
activation (calcination) is essential for the preparation of highly
efficient B2O3/ZrO2 catalysts. The ability of B2O3 to withstand
the high temperature calcinations suggests a strong interaction
between B2O3 and the support surface.
The Beckmann rearrangement of cyclohexanone oxime is an
important industrial reaction for the production of e-capro-
lactam. The conventional technology makes use of fuming
sulfuric acid as the catalyst in the liquid phase.1 This technique
is not environmentally friendly and is considered to be one of
the most inefficient chemical processes, since it produces 2–5
equivalents of valueless by-product (ammonium sulfate) for
every unit of the desired lactam product. It has long been hoped
that this process could be replaced with one based on a solid
acid catalyst.2 However, the development of such a specific
solid catalyst has proven to be a big challenge in heterogeneous
catalysis. Borias supported on oxide supports are reported to be
efficient catalysts for the Beckmann reaction.3–7 The main
obstacle to the practical use of these catalysts comes from the
rapid deactivation and poor regeneration properties of these
catalysts.8,9 Since supported boria was believed to be volatile
above 400 °C, the preparation of supported boria catalysts in the
literature was deliberately developed in order to avoid using
high calcination temperatures to activate the impregnated boria
catalysts. Thus, calcination at 350 °C has become a ‘standard’
activation procedure for preparing supported boria catalysts.3–7
Following the ‘standard’ preparation, we reported recently that
boria supported on zirconia, B2O3/ZrO2, was highly active and
selective for lactam synthesis; the average lactam yield during a
6 h reaction over properly prepared B2O3/ZrO2 catalyst was
greater than 90%.10,11 ‘Pure’ zirconia containing no boria
showed very poor selectivity for the lactam synthesis.11 Here,
we show that activation of boria-loaded zirconia with high
temperature (600–700 °C) calcinations produces highly se-
lective (96–98%) catalysts for the Beckmann reaction. Al-
though the loading of boria affects the catalyst activity and
selectivity, deactivated catalysts can be regenerated, regardless
of the boria loading, to their initial activity and selectivity by
calcination at 600 °C.
Samples of B2O3/ZrO2 were prepared by impregnation of the
zirconia support with an aqueous solution of boric acid.10,11 The
boria loading in the catalyst was determined by ICP-MS
analysis, and is expressed as a weight percentage of the catalyst
sample. Zirconyl hydroxide, which was obtained by hydrolysis
of ZrOCl2·8H2O with an aqueous solution of ammonia (25–28
wt% NH3), was used as the precursor for the support. Before
introduction of the boria, this support precursor was precalcined
in air at various temperatures. After the support had been loaded
with boria, the catalyst sample was activated by a further
calcination at elevated temperatures in air. Both the calcination
of the support and of the catalyst was carried out for 10 h. The
Beckmann rearrangement reaction was performed on a down-
flow fixed bed reactor at 300 °C with 10wt% cyclohexanone
oxime in benzene (solvent), and with N2 as the carrier gas. The
weight hourly space velocity (WHSV) of the oxime reactant
was 0.32 h21. Experimental details for the catalyst preparation
With CAT = 600 °C for the catalyst preparation, samples of
B2O3/ZrO2 catalysts were prepared with various loadings of
boria. Fig. 1 presents the time course of the oxime conversion
over two B2O3/ZrO2 samples with 5 and 13.5% B2O3 by weight,
respectively. While the oxime conversion decreases more or
less with reaction time, no significant change ( < 2%) in the
lactam selectivity is observed over both catalysts; the lactam
selectivity is 81–83% with the 5% B2O3/ZrO2 sample and
96–97% with the 13.5% B2O3/ZrO2 sample (not shown in the
figure). Experiments were undertaken in order to ascertain the
DOI: 10.1039/b003217o
Chem. Commun., 2000, 1121–1122
This journal is © The Royal Society of Chemistry 2000
1121