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Sn-beta/HCl and homogeneous CrII catalytic systems, the pres-
ent heterogeneous catalyst, which contains both isomerization
and dehydration catalytic sites and a moderate pore size (4–
5 nm), gives comparable furfural yields in water as the reactive
phase. The recyclability experiments show that the catalyst re-
tained full activity after four consecutive cycles, and the loss in
activity, in terms of HMF yield, was only 3%.
The microwave-assisted conversions of all substrates were per-
formed by using a CEM Corporation Discover TM Microwave reac-
tor at the standard operating frequency and 100 W power. HMF
and furfural yields were measured by both UV/Vis spectrophotom-
etry by using a Shimadzu UV-2501PC spectrophotometer and HPLC
by using a Waters HPLC instrument equipped with a Waters 2487
photodiode array (PDA) and 2414 refractive index detectors.
1H NMR spectra of HMF were recorded by using a Bruker ARX
400 MHz instrument, and NMR data were processed with
XWinNMR software.
Experimental Section
Materials
Conversion of carbohydrates to HMF and Ff
Fructose, glucose, xylose, sucrose, cellobiose, xylulose, MeTHF,
DMSO, lithium chloride, DMA, titanium isopropoxide, pure HMF,
and furfural were purchased from Sigma–Aldrich and used as re-
ceived. Unless otherwise mentioned, Millipore water was used as
the aqueous phase for all reactions.
The dehydration reactions of carbohydrates were performed by
charging a 10 mL microwave tube with the substrates, solvent, and
catalyst. The loaded microwave tube was then inserted into the
microwave reactor preset to the desired temperature and reaction
time. Upon completion of the allotted reaction time, the reactor
was opened. The reaction mass was cooled to RT, and the solution
was filtered through a 0.22 mm cut-off syringe filter (25 mm diame-
ter) for analysis. In the case of the biphasic-solvent-mediated reac-
tions, both the organic and aqueous phase were analyzed sepa-
rately for the quantification of furfurals yield and carbohydrate
conversion. The conversion of the starting carbohydrate substrate
was calculated from HPLC analysis by determining the unconvert-
Catalyst synthesis
The biorenewable, solid, TiO2-containing carbonaceous acid cata-
lyst, Glu-TsOH-Ti, was prepared by the thermal treatment of TsOH,
glucose, and titanium(IV) isopropoxide at 1808C. In a specific prep-
aration, glucose (2 g), TsOH (2 g), and titanium isopropoxide (0.5 g)
were mixed well, transferred to a 25 mL Teflon-sealed autoclave,
and maintained at 1808C for 24 h. The obtained black material was
ground to powder by using a mortar and pestle, washed with
water and ethanol, and oven-dried at 808C. Glu-TsOH catalyst was
prepared by following a similar synthesis method but without
using titanium isopropoxide.
1
ed substrate in the aqueous phase. For H NMR spectroscopic anal-
ysis, MeTHF was removed from the organic phase by rotary evapo-
ration, and the oily product was dissolved in [D6]acetone. DMF was
used as an internal standard.
Recyclability of Glu-TsOH-Ti
Instrumentation
The recycling efficiency of the catalyst was determined for the de-
hydration of fructose as a representative reaction. In this study,
a 10 mL microwave tube was charged with fructose (0.2 mmol),
catalyst (22 mg), MeTHF (2 mL), and water (1 mL). The tube was
placed in the microwave reactor, and the mixture was heated at
1808C using 100 W microwave power for 60 min. After the reac-
tion, the tube was cooled to RT, and an aliquot was collected for
analysis. The solid catalyst left in the tube was collected and
reused for three consecutive cycles by adding fresh substrate and
solvent. Fresh catalyst was not added to compensate any loss of
the catalyst during recovery. The yield of HMF was determined
from each run.
Powder X-ray diffraction (PXRD) patterns were recorded by using
a Bruker D-8 Advance diffractometer operated at 40 kV and 40 mA
and calibrated with a standard silicon sample using nickel-filtered
CuKa (l=0.15406 nm) radiation. A JEOL JEM 6700F field-emission
scanning electron microscope was used to determine the morphol-
ogy of powder samples. The pore structure was investigated by
using a JEOL JEM 2010 transmission electron microscope operated
at an accelerating voltage of 200 kV. The FTIR spectrum of the ma-
terial as well as pyridine desorption studies at variable tempera-
tures were recorded by using a PerkinElmer Spectrum 100 spectro-
photometer. For pyridine-IR studies, the sample was saturated with
pyridine vapor in a closed vessel at 758C for 2 h, and desorption
spectra were recorded at elevated temperatures. N2 adsorption–de-
sorption isotherms were obtained by using a Beckman Coulter SA
3100 Surface Area Analyzer at 77 K. The sample was degassed at
1358C for 4 h under high vacuum prior to the N2 sorption analysis.
The NH3-TPD was conducted by using a Micrometrics AutoChem II
2920 V4.04 in the temperature range of 120–3008C, which em-
ployed a thermal conductivity detector. This experiment was per-
formed at the Micrometritics analytical facility, USA. For the NH3-
TPD measurement, the sample was activated at 3008C inside the
reactor of the TPD furnace under a He flow. After cooling to
1208C, 10.02% ammonia balanced with He was injected, and the
system was allowed to equilibrate. The temperature was then
raised at a linear heating rate of 108CminÀ1 up to 3008C. AAS anal-
ysis of the sample was performed by using a Shimadzu AA-6300
atomic absorption spectrometer. The percentage of sulfur in the
catalyst sample was measured by ICP-AES by using an ICP-OES
optima instrument of model 3300DV at Galbraith Laboratory, USA.
Determination of HMF yield
UV/Vis spectrophotometric method
The UV/Vis spectra of pure HMF (Figure S1) and Ff have distinct
peaks at lmax =284 and 268 nm with corresponding extinction co-
efficient (e) values of 1.66ꢁ104 and 1.53ꢁ104 LmolÀ1 cmÀ1, respec-
tively. The mole percentage of furfurals in each of the reaction
product was calculated from the measured absorbance values at
respective lmax values for HMF and Ff and the corresponding ex-
tinction coefficient values. First, standard HMF and Ff solutions of
98–99% purity were analyzed separately to correlate the actual
and calculated amounts of HMF and Ff. Once good correlations
were established, HMF product samples were measured, and the
percentage HMF and Ff yields were calculated. Repeated measure-
ment of the same solution showed that the percentage of error as-
sociated with this measurement was in the range of Æ5%.
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