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X. Chan et al. / Catalysis Communications 72 (2015) 11–15
washed with methanol and water 3 times. The separated WO3 catalysts
were initially dried overnight at ambient temperature, and then cal-
cined in air (Praxair, Extra dry) at 400 °C for 4 h with a ramping rate
of 4 °C/min. The calcined WO3 catalysts were investigated by visible
Raman spectroscopy and X-ray diffraction and oligomerization reaction
was also performed with regenerated WO3 catalysts under the same
experiment condition (100 °C and 6 h). Compositions of OFA were iden-
tified by a PerkinElmer Clarus 680 GC equipped with a PerkinElmer
SQ8T mass detector. Quantitative analyses were carried out using a
PerkinElmer Clarus 680 GC equipped with a flame ionization detector
and a split/splitless injector. The split ratio was maintained at 30:1.
PerkinElmer Elite-5MS (30 m × 0.25 mm × 1.0 μm) capillary column
was used in both GC and GC/MS. Response factor for lower molecular
weight oligomers (dimer and trimer) were estimated using a computa-
tional approach where FA was used as the internal standard [14]. Due to
the limitation of column temperatures, we mainly investigated and an-
alyzed FA dimer and trimers in detail. Undetected and smaller GC peaks
were added together and reported as heavy oligomers, Ntrimer.
and reach 60% in 24 h (Fig. 1). Fig. S1 (GC/MS, Supporting information)
and Table 1 provide that five dimers (C9–C10) and two trimers (C14–C15
)
were produced over WO3 catalysts, which are similar to previously
reported FA oligomers catalyzed by homogeneous catalysts [9,13,
15–17]. Although conjugated diene and diketone structure were pro-
posed in the previous paper [7,9,10], we could not identify these oligo-
mers within our experimental and equipment conditions. Based on the
carbon balance calculation (0.5–6 h: 100–91%, 24 h: 75%), we confirm
the possible presence of long chain-length oligomer (e.g. tetramer,
pentamer, or even hexamer) and hypothesize that undetected conju-
gated diene and diketone structure might exist in these large oligomers
or polymers. Another possibility is unidentified peaks at longer reten-
tion times (up to 78 min) in GC/MS might contain these two proposed
structure oligomers. In the 5 different dimers, we were able to identify
both 4-furfuryl-2-pentenoic acid γ-lactone (PAL, D4) and 5-furfuryl-
furfuryl alcohol (HFF, D5) although E.M. Wewerka et al. claimed that
both HFF and PAL can't be coexisted at the same catalyzed FA oligomers
[13].
Fig. 2(a) shows that both low (C9–C15) and heavy (NC15) oligomers'
formations increase as a function of time. Up to 6 h, C9–C15 wt.% in prod-
ucts is higher than NC15. After 6 h, however, NC15 oligomers gradually
exceed C9–C15 oligomers. At 24 h, the concentration of NC15 oligomers
and C9–C15 oligomers are ~29 wt.% and ~26 wt.%, respectively. As ex-
pected, heavy oligomer (Ntrimer) selectivity increased as a function of
time, while the selectivity of dimer + trimer decreased with reaction
time, as shown in Fig. 2(b). Especially dimer + trimer selectivity de-
creased sharply b1.5 h and then slightly decreased up to 24 h. In order
to acquire a higher selectivity of short chain-length oligomers, short re-
action times, typically b1.5 h, are required. Fig. 2(c) and (d) show de-
tailed dimers' and trimers' wt.% for 24 h reaction. Ether bridged (D3),
terminal alcohol (D5), and linear-shape trimer (T1) are dominant di-
mers and trimer products. It was, however, observed that D5 dimer's
wt.% wasn't changed after 5 h, while D1, D2, and D4 formation were in-
creased up to 24 h. Possibly, D5 dimer converts into T1 via dehydration
and condensation reaction. The calculated enthalpy of dimers' forma-
tion from FA monomer with one proton showed that D5 formation is
thermodynamically more favorable than D3 by ~12 kcal/mol [15]. D5
(terminal OH dimer) was the dominant dimer with sulfuric acid [15]
which is similar to the current experimental results with WO3 up to 5 h.
In Fig. S2, we compared the catalytic performance of WO3 with other
two acid catalysts (sulfuric acid and γ-Al2O3). In general, WO3 showed
highest conversion, as well as the highest C9–C15 selectivity compare
to H2SO4 and γ-Al2O3. These results provide that WO3 is a strong candi-
date as a heterogeneous catalyst to replace H2SO4, which has been fre-
quently used in FA oligomerization.
2.2. Characterization of WO3 catalyzed FA samples
UV Raman (325 nm) spectra were recorded on a Horiba-Jobin Yvon
LabRam HR Raman spectrometer equipped with a confocal microscope.
The spectral acquisition time was 30 s/scan for a total of ~9 min/spec-
trum (20 scans). The visible excitation was generated by a diode-
pumped solid state continuous wave laser (532 nm, BaySpec Nomadic™
Raman Microscope, 50 mW). For the visible Raman spectra, the spectral
acquisition time was 1 scan for a total of 3 s/spectrum (3 scans). Infrared
spectra were obtained with a PerkinElmer Frontier Fourier transform
infrared spectroscopy (FTIR), which is equipped with an attenuated
total reflectance (ATR) accessory, with a 0.4 cm−1 resolution and 4
scans. The X-ray diffraction (XRD) spectra were collected using a Rigaku
MiniFlex600, where a Cu target Kα ray was used as the X-ray source.
The 2θ scans were measured at room temperature in the angle range
of 10–60°.
3. Results and discussion
3.1. Composition analysis
Catalytic activity and product distributions in the FA oligomerization
reaction over WO3 catalysts were analyzed by GC and GC/MS. Quantita-
tive data was plotted in a range of 0.5–24 h while the trend between 6
and 24 h was deduced from 6 h and 24 h data. It was observed that
WO3 catalyzed FA conversion gradually increased with time on stream
3.2. Vibrational spectroscopy analysis
3.2.1. FT-IR spectroscopy studies of the FA monomer molecular structure
evolution
In order to obtain more information of the FA oligomer molecular
structure changing over WO3, IR spectroscopy was applied during the
reaction at different reaction times, as shown in Fig. 3. With increasing
the reaction time, peak intensity of both 1561 cm−1 and 1715 cm−1
are increased. According to previous studies, 1561 cm−1 peak is
assigned to C_C stretching vibration inside furan ring structure [18].
R. Zavaglia et al. reported that observed 1565 cm−1 peak is C_C bond
of a furan-methylene-furan group [16]. Several papers reported that
the band at 1715 cm−1 is attributed to C_O stretching mode in the
diketone structure which was not obtained in our GC/MS results [6,19,
20–22]. It has been claimed that open-ring structure (γ-diketone)
forms during the acid catalyzed FA polymerization reaction via protonic
species react with oxygen in furan ring [17,20–22]. However, experi-
ment evidence of diketone structure was not reported. In addition to
diketone structure, we can't avoid the possibility of PAL. However,
because concentration of PAL was very low (see Fig. 2(d)), while
Fig. 1. FA conversion in the presence of WO3 catalysts. Reaction conditions: Temp =
100 °C, pressure = atmosphere, time = 0–24 h.