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H.-J. Eom et al. / Applied Catalysis A: General 493 (2015) 149–157
production of phenol. It was postulated that phenol was produced
via multiple routes that could be divided largely into two pathways:
heterolytic ˇ-ether bond cleavage and ˛-hydrogen abstraction of
Correlating the reaction results with the properties of catalyst
was attempted, and the reaction mechanisms were proposed based
of those correlations.
+
the Na -PPE intermediate [16]. In principle, heterogeneous catal-
ysis is preferred over homogeneous catalysis because the latter
requires additional costs due to catalyst recovery and waste treat-
ment, which account for approximately 30% of the entire processing
cost on average [17]. Moreover, the use of water-soluble catalyst
bears a risk that the metallic salt may precipitate in supercriti-
cal water as solid particles, depending on the salt concentration
and temperature [18]. Thus, our next step was verifying the fea-
sibility of heterogeneous (solid) catalysis to dissociate lignin’s
ˇ-ether bonds (PPE) under high-temperature water. Under high-
temperature water conditions, PPE becomes miscible with water,
forming a single-phase reactant, which is advantageous to hetero-
geneous catalysis as much as to homogeneous one in that it relieves
mass transfer resistance at the catalyst-reactant interface.
2. Experimental
2.1. Materials
Phenethyl phenyl ether (99%) was purchased from Frinton
Laboratories Inc. Poly(ethylene oxide)-poly(propylene oxide)-
poly(ethylene oxide) (Aldrich, PEO20PPO70PEO20, Pluronic P123),
zirconium (IV) n-propoxide (Aldrich, 70 wt.% solution in 1-
propanol), acetyl acetone (Aldrich, >99%), sodium hydroxide
(Aldrich, 98%), and ethanol (J. T. Baker, HPLC grade) were used
to prepare the catalysts. Water was purified to a resistivity of
≥18.2 Mꢀ/cm using an AquaMax-Ultra water purification system
(Younglin). The gas chromatography mass spectrometry (GC–MS)
spectra of phenol (Aldrich, >99%), toluene (Aldrich, >99.9%), ethyl-
benzene (Aldrich, >99%), and styrene (Aldrich, >99%) authentic
standards were used to calibrate the GC signals of the products.
Tetrahydrofuran (Aldrich, HPLC grade) was used as the recovery
solvent without further purification.
Zirconia (ZrO ) is an amphoteric solid widely used to cat-
2
alyze various types of acid- and base-catalyzed reactions [19,20].
Zirconia-based materials perform well as catalysts when applied
to the reactions involving H O and CO specifically, because the
2
2
acid and base sites of zirconia are not poisoned by H O and CO
2
2
[
19,21,22]. The basic properties of ZrO2 could be enhanced easily
+
by incorporating an external component such as Na [17], CaO [23],
TiO2 [24], etc. Additionally, ZrO2 shows good chemical, mechanical
and thermal stability, which makes it useful for a wide range of
industrial catalytic applications.
2.2. Preparation of the zirconia catalysts
The synthesis of ZrO2 catalysts was based on the method of Liu
et al. [17]. First, 7.0 g of Pluronic P123 was dissolved in 140 mL
of ethanol upon stirring. Then, 14 g of zirconium (IV) n-propoxide
(70 wt.% solution in 1-propanol) was separately mixed with 1.5 g
of acetylacetone while stirring. The two solutions were mixed
together under vigorous stirring for 1 h at room temperature. Then,
5.4 g of deionized water was dropped (20 ml/h) into the solution,
and the stirring was continued for 1 h. The mixture was gelated in a
ZrO2 exists in three types of polymorphs: monoclinic, tetrago-
nal, and cubic phases [25,26]. The tetragonal ZrO2 is known to
catalyze C C bond cracking/formation reactions such as isomeriza-
tion of n-butane [27] and benzylation of toluene [28]. The tetragonal
◦
ZrO2 is thermodynamically stable at 1170–2370 C. At lower tem-
peratures, it is possible for ZrO2 to exist in a metastable tetragonal
phase; however, it is transformed into a monoclinic phase when
◦
closed vessel at 60 C for 48 h and then evaporated in a rotary evapo-
◦
◦
temperatures reach higher than 400 C. The transformation can be
rator (100 C) under reduced pressure. The resulting transparent gel
prevented with an alkaline solution post-treatment method, which
is also known to improve the thermal stability [29,30], specific
surface area [29,30] and base catalytic activity [17] of tetragonal
zirconia. Liu et al. reported that when a zirconia gel is refluxed
was refluxed under 0.5 mol/L NaOH aqueous solution for 48 h. Then,
the slurry was washed thoroughly with deionized water until NaOH
was completely removed. NaOH removal was confirmed with a pH
◦
meter. Finally, the resulting solid products were dried at 100 C for
+
◦
◦
under aqueous NaOH solution for a sufficient time, Na ions are
24 h and calcined at 500 C for 5 h (ramping rate: 1 C/min).
The prepared catalysts were named Na-ZrO2 or ZrO2 in accor-
dance with whether the catalyst was refluxed under an aqueous
solution of NaOH.
incorporated into the zirconia framework, slowing down the rate
of crystallization of zirconia during calcination and inhibiting the
transition from the tetragonal to the monoclinic phase [17]. The net
result is the stabilization of the specific surface area [17,31]. Such
◦
effects were valid at temperatures up to 800 C [17]. Additionally, it
2.3. Characterization of catalysts
was reported that the NaOH treatment generates strong basic sites
by dispersing Na O nanocrystals over the zirconia framework.
FT-IR analysis was performed with a Spectrum GX FTIR spec-
trometer (PerkinElmer Inc.) to determine if the residual surfactant
(Pluronic P123) remained in the final form of the catalyst. The sam-
ples were diluted to 1 wt.% in KBr powder and then pelletized. The
spectrum was recorded by co-adding 64 spectra in the range of
4000–370 cm (resolution of 4 cm ) on a deuterated triglycine
sulfate (DTGS) detector.
The Na content of the Na-ZrO2 catalyst was determined by ICP-
AES on an ULTIMA 2C HR instrument (Jobin Yvon Horiba). The
measurement was conducted at the Korea Basic Science Institute
(Seoul Center).
2
In this study, mesoporous ZrO2 catalysts post-treated with
NaOH solution were applied in the hydrolysis of PPE under
hydrogen-pressurized supercritical water. There are several pub-
lished papers which studied the use of solid catalysts for the
degradation (gasification) of lignin under near or super-critical
water conditions [32,33]. However, to the best of our knowledge,
no study has been reported yet, which deals with ˇ-ether bond
cleavage of PPE under high-temperature water using solid ZrO2-
based catalysts, with a purpose to obtain insights for heterogeneous
catalytic dissociation of lignin.
−
1
−1
To elucidate the compositional, crystallographic, textural and
acid/base properties of the prepared catalysts, various character-
ization techniques were adopted, including inductively coupled
plasma-atomic emission spectrometry (ICP-AES), low and wide
angle X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) anal-
ysis, transmission electron microscopy (TEM), Fourier transform-
infrared spectroscopy (FT-IR), carbon dioxide-temperature pro-
The specific surface areas of the catalysts were measured by the
BET method on an ASAP 2010 instrument (Micromeritics). Before
measurement, the sample (200 mg) was degassed at 300 C and
◦
4 mmHg for 12 h. The nitrogen adsorption–desorption isotherms
◦
were measured at −196 C. The pore size distributions of cata-
lysts were obtained by applying the Barrett–Joyner–Halenda (BJH)
method to the adsorption branch of the isotherms.
grammed desorption (CO -TPD) and ammonia-temperature pro-
Wide-angle XRD analysis was performed at room temperature
on a D/MAX-2500 V/PC instrument (Rigaku). The X-ray source was
2
grammed desorption (NH -TPD).
3