K. Liu et al. / Journal of Molecular Catalysis A: Chemical 380 (2013) 84–89
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2. Experimental
amount of water adsorbed via thermogravimetric analysis (TG) on
a Rigaku Thermoflex System. The samples were hydrated in a des-
iccator over a saturated NaCl solution for 24 h at room temperature
prior to the measurements.
2.1. Catalyst preparation
Alkyl/aryl functionalized porous pillared-zirconium phosphates
were prepared following the procedures in the literature [6,7].
Typically, 14.4 g cetyltrimethylammonium bromide (CTAB) was
dissolved in 100 ml n-propanol. 3.04 g 85 wt% orthophosphoric
acid and zirconium tetra-n-propoxide (70 wt% n-propanol solu-
tion) were then added dropwise under vigorous stirring. The
reaction mixture was stirred for 30 min. The resulting gel was
separated by centrifugation, washed first with the mixture of n-
propanol/water and then with distilled water, dried at 60 ◦C. The
solid obtained is denoted as CTAB-ZrP. 1 g CTAB-ZrP was then
suspended in 100 ml water and a certain amount of hexadecy-
lamine in n-propanol (35 g/l) solution was added as co-surfactant
with hexadecylamine/P molar ratio of 0.4. After being stirred for
one day at room temperature, a solution (50%, v/v) of tetraethy-
lorthosilicate and organo-triethoxysilane mixture (molar ratio 9:1)
in n-propanol was added with Si/P molar ratio of 2. This sus-
pension was stirred for three days at room temperature. Then
the solid obtained was centrifuged, washed several times with
ethanol. The surfactants were removed by refluxing in 200 ml
ethanol containing 1.5 ml concentrated hydrochloric acid at 70 ◦C
overnight and then the solid obtained was centrifuged, washed
three times with ethanol and finally dried at 60 ◦C. The obtained
catalysts by using methyltriethoxysilane, ethyltriethoxysilane, n-
propyltriethoxysilane and phenyl-triethoxysilane were denoted as
ZrP-Me, ZrP-Et, ZrP-Pro and ZrP-Ph, respectively. Porous pillared-
zirconium phosphate synthesized without organo-triethoxysilane
was denoted as ZrP-E.
2.3. Catalytic reactions
2.3.1. Alkylation of hydroquinone
Alkylation of hydroquinone with tert-butanol was carried out
in an autoclave equipped with a magnetic stirrer. Typically, 0.5 g
hydroquinone, 1.0 g tert-butanol, and 0.20 g catalyst were added
to the autoclave, along with 2 g of xylene as a solvent. The reac-
tion lasted 4 h under 150 ◦C. The products were analyzed with
a gas chromatograph equipped with a SE-30 capillary column
(30 m × 0.25 mm × 0.3 m) and a flame ionization detector. The
temperature of the capillary column was set at 180 ◦C. The main
products were 2-tert-butyl hydroquinone (2-TBHQ), 2,5-di-tert-
butyl hydroquinone (2,5-TBHQ), 2,5-di-tert-butyl benzoquinone
(2,5-DTBBQ) and tert-butyl phenol ether (TBPE). The conversion
and the yield of 2-TBHQ are calculated in mol% by the following
equations:
ꢀ
ꢁ
ꢂꢃ
Remaining hydroquinone
Total hydroquinone
Conversion (%) = 1 −
× 100%
2 − TBHQ (mol)
Yield (%) =
× 100%
Total hydroquinone(mol)
2.3.2. Esterification of lauric acid
The esterification of lauric acid with methanol was carried out in
a 25 ml three-necked flask equipped with a condenser under 60 ◦C.
Typically, 0.2 g catalyst, 4.167 g lauric acid and 10 g methanol were
added into the flask with magnetic stirring. The products were ana-
lyzed by timing sampling with a gas chromatograph equipped with
a SE-30 capillary column (30 m × 0.25 mm × 0.3 m) and a flame
ionization detector. The temperature of the capillary column was
set at 200 ◦C.
2.2. Catalyst characterization
The N2 adsorption/desorption isotherms were measured on a
Micromeritics ASAP2010 instrument at liquid N2 temperature. Spe-
cific surface areas (A) of the samples were calculated from the
adsorption isotherms by BET method, and the pore volume (V) was
calculated from the adsorption isotherm by single-point method.
Average pore diameter was obtained by 2V/A.
3. Results
29Si MAS NMR spectra were recorded using Bruker MSL-300
NMR spectrometer. A resonance frequency of 59.627 MHz, a recy-
cle delay of 3 ms, 3.5 s pulses, a spectral width of 16.667 kHz and
a spin rate of 4 kHz were applied. External tetramethylsilane was
used as reference. Thermal Gravity Analysis (TGA) was taken on
a Rigaku Thermoflex thermal analysis system under a flowing N2
out with a Philips XL30 using an accelerating voltage of 15 kV. Phos-
phorus and silicon content in the samples was determined using a
Bruker-AXS S4 Explorer XRF elemental analysis spectrometer.
The acidity was measured by means of potentiometric titration
[13–15]. The solid (0.05 g) was suspended in 10 ml acetonitrile,
agitated for 3 h, and then titrated with 0.1 mol/l n-butylamine
in acetonitrile. The electrode potential variation was recorded
with a Mettler Toledo FE20 potentiometer. The acid strength
can be classified according to the following scale [13,16,17]:
Ei > 100 mV (very strong sites), 0 < Ei < 100 mV (strong sites) and
−100 < Ei<0 mV (weak sites). The amount of absorbed n-butylamine
with Ei > −100 mV was the total number of the acid sites.
3.1. Catalyst characterization
methyl, ethyl, propyl and phenyl groups were prepared by co-
condensation method. In our previous work, the best experimental
respectively) were obtained for getting a solid with large specific
surface area and high accessible acid sites [11]. These conditions
were used in preparation of the above alkyl/aryl functionalized
porous phosphates. Their N2 adsorption/desorption isotherms are
shown in Fig. 1, together with that of unmodified one. Type IV
the silica or organo-silica inserting. These similar isotherms also
indicate that the mesoporous structure changes little after the
introduction of alkyl/aryl groups.
Table 1 summarizes the textural properties of these sam-
ples. The obtained samples exhibit high BET surface area and
large pore volume, which are in the range of 607–698 m2/g and
0.46–0.96 cm3/g, respectively. This again proves the presence of
porous structure between the interlayer spaces of the phosphate,
since the BET surface area of unpillared sample (CTAB-ZrP) is only
24 m2/g.
DTBPy adsorption in liquid phase was tested by adding 20 mg
catalyst into DTBPy/xylene or DTBPy/methanol solution. The mix-
ture was stirred for 5 h at room temperature. The remaining DTBPy
was analyzed with a gas chromatograph equipped with a SE-30
capillary column (30 m × 0.25 mm × 0.3 m) and a flame ionization
detector while trace amount of tetradecane was utilized as internal
standard.
The morphologies of alkyl/aryl functionalized porous phos-
phates were studied by SEM. The SEM images for all these samples