Table 1 Catalytic activities of copper-silica catalysts in vapor phase
dehydrogenation of 1,4-BDO to GBL
After 5 h
After 100 h
Catalyst
H2 Conv. (%) Sel. (%) Conv. (%) Se.l (%)
O
X
O
X
X
96.0
99.7
95.2
99.7
98.0
99.5
99.6
99.0
94.7
95.8
86.7
77.9
97.0
94.6
98.6
95.8
91.8
93.6
92.1
99.0
Cu(12)/SiO2
Na(1)–Ca(7)–Cu(12)/SiO2
Cu(80)–SiO2(20)
Reaction conditions: 250 ◦C; 1 atm; catalyst weight, 1.0 g; 1,4-BDO feed
rate, 2.0 cm3 h-1; N2 carrier gas flow rate, 45 cm3 min-1; H2/BDO = 2
(mole ratio); O: with H2; X: without H2.
without H2 (from 77.9% to 86.7%), which was summarized in
Table 1. It is thought that the inactive copper species (CuI or
CuII) generated from the active Cu0 during the dehydrogenation
reaction could be regenerated by reduction with H2. For alkali-
doped Cu/SiO2, the conversion after 100 h in the presence of
H2 was almost identical with the initial activity without H2.
However, the selectivity was still lower due to the presence of
the strong basic sites. Although the introduction of H2 was a
little effective in regeneration of the active Cu0 species during
the reaction thereby an increase of the catalyst lifetime,15 this
approach is not desirable from economic and environmental
point of views since the use of H2 in the alkali-promoted catalyst
makes the process more expensive and hazardous.
Therefore, we paid attention to a new type of copper catalyst
for the dehydrogenation of BDO which overcomes the sintering
of metallic copper nanoparticles as well as the partial oxidation
of active copper species. For this purpose, Cu(80)–SiO2(20)
with unusually high Cu loading on silica was prepared by
precipitation of copper onto silica nanoparticles.
TEM image of fresh Cu(80)–SiO2(20) catalyst after reduction
shows that Cu nanoparticles with 15 nm were mixed in the nano-
sized silica matrix leading to the formation of nanocomposite.
Notably, the particle size of Cu was almost constant even after
reaction of 100 h, which was also confirmed by XRD analysis in
Fig. 4. The surface area of the fresh Cu(80)–SiO2(20) was about
105 m2 g-1, which was decreased a little to 77 m2 g-1 after reaction
of 100 h. This decrease of the surface area seems to be due to
the sintering of silica matrix.
Fig. 4 XRD patterns of Cu(80)–SiO2(20) catalyst; (a) after calcination,
(b) after reduction, and (c) after reaction of 100 h.
SiO2(20) at 0.15 g loading than Cu(12)/SiO2 at 1 g loading
was ascribed to the insufficient number of Cu active site due
to lower Cu surface area (5.9 m2 g-1 or 0.9 m2/0.15g) than
Cu(12)/SiO2 (3.9 m2 g-1) (Table S1, ESI†). However, it is notable
that the turnover number based on Cu surface area of Cu(80)–
SiO2(20) was 2 times higher than that of Cu(12)/SiO2. After
100 h of reaction, the Cu surface area of Cu(12)/SiO2 was
decreased rapidly to 0.9 m2 g-1, while that of Cu(80)–SiO2(20)
was decreased gradually to 3.1 m2 g-1 (or 0.5 m2/0.15g). This
indicates clearly that Cu(80)–SiO2(20) catalyst is much more
stable than Cu(12)/SiO2 even at the same Cu content.
Therefore, it is evident that Cu(80)–SiO2(20) has higher GBL
productivity (GBL yield/catalyst weight) as well as turnover
number based on Cu surface area (GBL yield/Cu surface area)
than Cu(12)/SiO2, which would increase the economics of GBL
production from BDO in the vapor-phase dehydrocyclization
definitely.
In this nanocomposite catalyst, the further growth of copper
crystallites seems to be prevented during the reaction due to the
inclusion of silica nanoparticles between Cu nanoparticles. It
is thought to be a main driving force of the superior catalyst
life time. Moreover, compared with Cu(12)/SiO2 catalyst, the
Cu(80)–SiO2(20) nanocomposite catalyst is shown to be effective
in the aspect of regeneration of the active Cu metallic species. It
is likely that hydrogen species adsorbed on adjacent Cu metallic
species produced during the reaction suppress the generation of
the oxidized Cu species.
Conclusions
The copper-silica nanocomposite catalyst with unusually high
loading of Cu showed an unprecedented high activity in
the vapor phase dehydrocyclization of 1,4-butanediol to g-
butyrolactone without deactivation until 400 h (99% conversion
and 99% selectivity). The present novel catalyst is expected to
be a key technology to convert biomass-derived 1,4-butanediol
into g-butyrolactone selectively.
Accordingly, the catalytic activity of this catalyst with high Cu
content was maintained up to 400 h as illustrated in Fig. 3. When
BDO was dehydrogenated over 0.15 g of Cu(80)–SiO2(20), which
contains the same amount of Cu with 1.0 g of Cu(12)/SiO2, the
initial conversion was about 50% with GBL selectivity of 88%
and this activity maintained without a significant deactivation
until 250 h (Fig. S1, ESI†). The lower conversion of Cu(80)–
Acknowledgements
This work was supported by Institutional Research Program of
KRICT and by a grant (B551179-10-03-00) from the cooperative
R&D Program funded by the Korea Research Council Industrial
Science and Technology, Republic of Korea.
1674 | Green Chem., 2011, 13, 1672–1675
This journal is
The Royal Society of Chemistry 2011
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