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2
D. Cao et al. / Applied Catalysis A: General 528 (2016) 59–66
and catalyst was added into the autoclave and purged with N2 for
several times to remove air. Then, the reactor was heated under
magnetically stirring at 800 rpm. After the reaction, the reactor was
quickly quenched in an ice-water bath and a small amount of DI
water was added. Subsequently, the mixture was filtrated and the
obtained liquid product was analyzed by HPLC (Wates e2695) with
an Shodex SUGAR SC1011 column (8 × 300 mm). The operating
◦
temperature of column and detector was selected to be 70 C and
3
◦
5 C, respectively. Distilled water was selected as mobile phase
(
0.6 mL/min). The same experimental procedures were repeated at
least two times to verify the reproducibility. The deviation of the
results was observed below 5% ruling out the effect of the experi-
mental error.
Sorbitol conversion and isosorbide selectivity was calculated
according to the following equation:
Csorbitol% = (moles of reacted sorbitol/mol
of initial sorbitol) ∗ 100%
(2)
Fig. 5. FT-IR spectra of the ZrP catalyst.
The microscopy of the ZrP catalyst was firstly analyzed using
SEM and the results were shown in Fig. 3. Apparently, both the
spherical particles with sizes in the range of 10–20 nm and the
worm like porous structure were observed which were highly in
line with the previous XRD result. The TEM micrograph, depicted
in Fig. 4 also revealed that the ZrP sample consisted of conglomer-
ated particles which connected each other and a clear pore channel
was observed confirming the formation of porous structure. It was
noted that an average pore size of ca.3–5 nm was in line with the
value achieved from BET results. And it also demonstrated a short
range order of porous structure. Combining with the BET analysis,
the wormlike structure might have good interconnection to result
into the high surface area and pore volume observed.
Sisosorbide% = (moles of carbon in the produced isosorbide/mol of
carbon in the reacted sorbitol) ∗ 100%
(3)
2.4. Catalyst recyclability
The reusability of the ZrP catalyst was determined in order to
assess its stability for the possible industrial application. The tests
were conducted under the identical experimental conditions as
mentioned above. After each reaction, the used catalyst was fil-
Fig. 5 shows the FT-IR spectra of the ZrP catalyst. An intense
−1
band in the range of 1000–1100 cm was attributed to the P-O
◦
tered and thoroughly washed with hot DI water and dried at 90 C
symmetrical vibration of −PO group [47] and the spectral band
4
◦
over night. The recovered catalyst was finally calcined at 450 C for
−1
at ca.524 cm was assigned to the Zr O stretching vibration [48].
2
h in static air for the next run.
−1
A slight peak at ca.748 cm
corresponded to the P
O P vibra-
tion suggesting the formation of P
O
P band. Moreover, the bands
−
1
3
. Results and discussion
centered at ca.3439 and 1631 cm were associated to the −OH
−
1
vibration and the peak located at ca.1402 cm was due to bending
vibration mode of OH groups in good consistent with the previous
reports [49]. These OH groups might be the origin of the Brön-
ICP result indicated that the resulting molar ratio of P to Zr in the
as-prepared ZrP catalyst was ca.1.93, which was very close to the
stoichiometry indicating no loss of the active component occurred
during the synthesis process of the catalyst. Fig. 1 presents the XRD
patterns of the ZrP sample. Only two broad peaks in the 2 ranges
−
1
sted acidy of ZrP catalyst. The clear peak at ca.3439 cm also could
not rule out the partial contribution of the stretching vibration of
adsorbed lattice water molecules.
◦
◦
of 10–40 and 40–70 respectively were observed suggesting its
amorphous structure in good agreement with the previous litera-
ture [43]. No characteristic diffraction peaks assigned to zirconium
phosphate and/or zirconia were observed.
Briefly, the surface acidity is one of the important factors for
solid acid catalysts to catalyze dehydration reaction. It is recognized
that FT-IR spectra of pyridine adsorption is an effective technique
to determine the types of acid sites on the catalyst surface. Typ-
ically, pyridine molecules formed pyridinium ions with Brönsted
acid centers. In contrast, they were coordinated with Lewis acid
sites because of their electron-pair deficient property [50]. The
pyridine adsorption spectra of the ZrP catalyst is shown in Fig. 6
revealing the co-existence of Brönsted and Lewis acid sites. An
The textual properties of the ZrP catalysts were further stud-
ied using nitrogen adsorption-desorption isotherms as presented
in Fig. 2. The specific surface area and average pore diameter was
around 148 m /g and 3.57 nm, respectively. Interestingly, three dif-
ferent stages were observed from the isotherm curve. The first
stage was observed at p/p < 0.4, the second one was observed in
the region of 0.4 < p/p < 0.9 and the third one was observed at
p/p > 0.9. Notably, the N2 adsorption-desorption isotherms cor-
responded to a type IV isotherm with a typically hysteresis loop
implying the porous structure of the synthesized ZrP catalyst. It was
2
−
1
intense band at around 1540 cm , typical of pyridinium ions, con-
firmed the presence of Brönsted acid sites. Another strong peak at
0
0
−
1
ca.1446 cm was attributed to the pyridine adsorption on Lewis
0
−
1
sites and the peak at ca.1489 cm revealed the presence of a mix-
ture of Brönsted and Lewis acid sites [51]. It was noted that the
intensity of the adsorption peaks corresponded to the quantity of
acid sites. Additionally, the ratio of Brönsted and Lewis sites was
calculated according to the method described by Emeis [52]. The
result indicated that most of the acid sites (ca.62%) were Brönsted
generally accepted that the observed hysteresis loop from p/p = 0.4
0
was assigned to the capillary condensation occurring in the meso-
pores [44–46]. Additionally, the distribution of pore size calculated
by the Cranston and Inkley method (inserted in Fig. 2) also eluci-
dated a narrow pore size range centered around 3.5 nm supporting
the mesoporous nature of the ZrP sample.
◦
acid sites. Meanwhile, upon further heating to 400 C, only a slight
decline of the intensity of the peak at ca.1540 cm was observed
−
1