Y. Cao et al.
sealed. This procedure was performed for five cycles.[26] After
separation of the solid that remained at the end of the reac-
tion, the aqueous solution contained 12 wt% LA and 5 wt%
FA (the overall yields of LA and FA from cellulose are ca. 50%).
Notably, without isolation of the insoluble hydrolysis by-prod-
ucts, the subsequent BL reduction was very slow under the re-
action conditions described, which could owe to the strong
adsorption of humins by the Au/ZrO2 catalyst. The hydrolyzed
solution was transferred into a 50 mL round-bottomed flask
that contained n-butanol (0.2 mol) and subjected to a reaction
at 908C for 2 h. Analysis of the reaction mixture after the
esterification step showed that the yields of BL and BF were 60
and 50%, respectively. Under these conditions, the organic
stream also contains 5.8 mmol of unconverted LA and
6.8 mmol of unconverted FA. Thus, the organic phase contains
83% LA and 77% FA that were present in the aqueous feed
solution initially. To this hydrophobic, organic stream were
added the Au/ZrO2 catalyst (0.1 mol%) and 4 equiv. of water,
and the catalytic reduction of BL and LA using BF and FA as
the in situ sources of hydrogen at 1708C produced GVL in
94% yield. The overall yield of GVL from cellulose was ca. 36%.
The yield of GVL was improved by concentrating the cellu-
lose-derived LA and FA solutions. As shown in Scheme S1, the
cellulose-derived LA (12 wt%) and FA (5 wt%) solutions were
concentrated under reduced pressure using a rotary evapora-
tor. Some of the water and FA were then removed (the boiling
point of FA is close to water) by evaporation to obtain a more
concentrated solution of LA (ca. 6 m), FA (ca. 3 m), and sulfuric
acid. Subjection to reactive extraction with n-butanol at 908C
after 2 h led to the almost complete conversion of the mixture
of LA and FA into a hydrophobic layer of BL and BF. The BL
was converted into GVL using BF and FA (obtained from the
evaporation step), as an in situ source of hydrogen, in the pres-
ence of Au/ZrO2 (0.1 mol%). In this case, the overall yield of
GVL from cellulose was 47%.
Experimental Section
Preparation of Au/ZrO2 catalyst
Zirconia, with a Brunauer–Emmet–Teller (BET) surface area of
110 m2 gÀ1
was prepared by conventional precipitation
,
a
method.[18] ZrOCl2·8H2O (12.9 g) was dissolved quickly in deionized
water (200 mL) at room temperature and the pH adjusted to 9.0
by dropwise addition of NH4OH (2.5m). The resultant hydrogel was
washed with deionized water until free of chloride ions. The pre-
cipitate was then dried at 1108C overnight and calcined at 4008C
for 2 h in air. The Au/ZrO2 catalyst was prepared by a modified
deposition–precipitation method. ZrO2 powder (2 g) was mixed
with appropriate amounts of aqueous solutions of HAuCl4 at room
temperature. The pH was adjusted to 9.0 by the dropwise addition
of 0.25m NH4OH. After 6 h of stirring at room temperature, the cat-
alyst was washed with deionized water (5ꢁ100 mL) and separated
by filtration. The samples were dried at 1108C in a forced air oven
for 1 h and reduced with a stream of 5 vol% H2/Ar at 3508C for
2 h. The BET surface area of the resultant Au/ZrO2 catalyst (final
mass ca. 1.9 g) was 113 m2 gÀ1. The concentration of gold was
0.8 wt%, which was measured by inductively coupled plasma
atomic emission spectroscopy. A large fraction of the Au particles
in the catalyst was within 1.2–2.5 nm in diameter. XPS measure-
ments showed that all the gold in the catalyst was in its metallic
state.
Esterification of biomass-derived LA and FA
A mixture of LA (10 mmol), FA (10 mmol), n-butanol (80 mmol),
H2SO4 (2.5 mmol), and water were charged to a 50 mL round-bot-
tomed flask and stirred at a rate of 800 rpm under a 0.5 MPa N2 at-
mosphere for the given reaction time (see Table 1). The mixture
was heated to the desired temperature in less than 10 min. The or-
ganic liquid products BL and BF were analyzed by using a Shimad-
zu GC-17A gas chromatograph equipped with a capillary column
HP-FFAP (30 mꢁ0.25 mm) and FID detector. Aqueous samples
were analyzed by using a HPLC (HP 1100, Agilent, USA) system
with a Platisil ODS C18 column and a refractive index detector.
H2SO4 (0.5mm) was used as the mobile phase at a flow rate of
1 mLminÀ1. Both the column temperature and the detector tem-
perature were 408C.
Conclusions
Decomposition of BF
We have described a simple and efficient alcohol-mediated re-
active extraction protocol for the production of g-valerolactone
(GVL) via butyl ester intermediates starting from cellulose, in
which supported gold nanoparticles were facilitated the reduc-
tion of levulinic acid and its ester without using external hy-
drogen gas. This protocol simplifies the recovery and recycling
of sulfuric acid for cellulose deconstruction, which allows for
improved control of the overall process economics. The mix-
ture of levulinic and formic esters, along with residual levulinic
and formic acids, can be directly converted to an aqueous so-
lution of GVL and n-butanol over a single Au/ZrO2 catalyst, in
which H2, generated in situ from formic acid and its ester, is
used for the reduction of levulinic acid and its ester to GVL.
The operational simplicity and the improved efficiency of our
new catalyst system are expected to contribute to its utiliza-
tion for the cost-effective production of GVL or its derivatives
from renewable lignocellulosic resources.
A mixture of BF (20 mmol), supported metal catalyst (metal
0.1 mol%), and water (60 mmol) were charged to a 25 mL Hastel-
loy-C high pressure Parr reactor and stirred at a rate of 800 rpm
under a 0.1 MPa He atmosphere for the given reaction time (see
Table 2). The mixture was heated to 1708C in less than 15 min.
After the reaction, the concentration of residual BF was analyzed
by using a Shimadzu GC-17A gas chromatograph equipped with a
capillary column HP-FFAP (30 mꢁ0.25 mm) and FID detector (ex-
ternal standard: 2-methoxyethyl ether). The gaseous products were
analyzed by using a GC analyzer equipped with a TDX-01 column
and a thermal conductivity detector.
Conversion of BL and BF into GVL
A mixture of BF (20 mmol), BL (20 mmol), supported metal catalyst
(metal 0.1 mol%), and water (80 mmol) were charged to a 25 mL
Hastelloy-C high pressure Parr reactor and stirred at a rate of
800 rpm under 0.1 MPa N2 for the given reaction time. The mixture
1842
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ChemSusChem 2011, 4, 1838 – 1843