M.I.M. Alzeer et al. / Applied Catalysis A: General 524 (2016) 173–181
175
The 29Si spectra were acquired with a 5 mm Doty MAS probe and
a zirconia rotor spun at ∼6 kHz. The excitation pulse for 29Si was
7 s with a recycle time of 30 s and the spectra were referenced
with respect to tetramethylsilane (TMS).
Table 2
Molar ratios of the geopolymersa.
Geopolymer
H2O/Al2O3
Na2O/Al2O3
K2O/Al2O3
K-N
K-hiSi
Na-N
3.59
5.19
3.54
6.40
7.62
13.26
14.81
13.59
22.88
22.88
0.02
0.02
1.26
2.27
1.11
1.30
0.01
0.0006
0.0006
The N2 adsorption-desorption isotherms were measured at
−196 ◦C using a Micromeritics ASAP 2010. All the samples were
degassed at 110 ◦C to 3 mTorr pressure using the instrument’s
degassing system. The specific surface area was measured by the
Brunauer–Emmett–Teller (BET) method over a p/p◦ range of 0.05-
0.3. The total pore volume was measured by single point adsorption
at p/p◦ = 0.995. The mesopore volumes and the average adsorption
pore widths were determined by the Barrett-Joyner-Halenda (BJH)
method. Transmission electron microscopy (TEM) was carried out
using a JEOL 2010 transmission electron microscope operated at
200 kV.
The ion-exchange process was monitored by FTIR in which the
powder samples were suspended in a KBr disk and the spectra
acquired using a Perkin Elmer Spectrum One FTIR spectrometer
in the range 4000–450 cm−1. A quantitative analysis of the amount
of pyridine adsorbed was obtained by thermogravimetric analysis
(TGA) using a Shimadzu TGA-50 thermal analyser at a heating rate
of 10 ◦C min−1 up to 800 ◦C in flowing air (50 ml min−1). The dif-
ference in the weight loss between the sample with and without
pyridine indicated the total content of acid sites in the sample (see
supporting information (SI), Fig. SI3). The adsorption-desorption of
pyridine on the catalyst surface was carried out as follows: 0.1 g of
the catalyst was heated to 450 ◦C for 10 min then degassed at 250 ◦C
for 12hr. at 200 mTorr vacuum using a Micromeritics VacPrep061
sample degassing system. The powder then was left to cool to room
temperature and exposed to pyridine at 150 ◦C for 1 hr. to allow
the surface to become saturated. The physisorbed pyridine was
then desorbed at 100 ◦C for 1 hr. under the same vacuum condi-
tions. The nature of the active sites was further investigated by
FTIR spectroscopy of the adsorbed pyridine (see Fig. SI2).
Na-hiSi
Na-hiSi-Seqb
0.0002
a
b
double this amount of silica, designated Na-hiSi and K-hiSi were
also prepared. The molar compositions of all these samples are
shown in Table 2. The compositions of the high-silica geopolymers
were adjusted by the addition of fine silica fume (Elkem 971-U,
Elkem, Norway) simultaneously with the clay. After thorough mix-
ing for 10 min. the geopolymer resins were cured in covered plastic
molds at 80 ◦C for 6 hr., then uncovered and oven-dried at 40 ◦C
overnight. The hardened blocks were then broken into pieces and
ground in a vibratory mill (Bleuler, Switzerland) fitted with a tung-
sten carbide pot and milling rings and sieved to pass a 105 m
mesh.
2.2. Catalyst preparation
The alkali ions of the geopolymers prepared as above were
exchanged with NH4 by the method of O’Connor et al. [35]. One
+
gram of geopolymer powder was treated with 100 ml of 0.1 M
NH4Cl solution (Panreac) with vigorous stirring at room tempera-
ture for 12 hr., and then washed thoroughly with a freshly prepared
solution of 0.1 M NH4Cl, filtered, then washed thoroughly with dis-
tilled water to remove any remaining alkali ions and dried at 40 ◦C
overnight. The zeolites Y and ZSM-5 used for comparison purposes
in the catalytic reactions were ion-exchanged in the same manner.
2.5. Catalytic activity
In some cases, the geopolymers were sequentially dealuminated
and desilicated after ion exchange, by the procedure of Verboekend
et al. [33] in which a weighed amount of the geopolymer catalyst
was first dealuminated by treatment with 20 ml/g 0.11 M Na2H2
EDTA (Merck) for 5 hr. at 85 ◦C. This was followed by desilication by
treatment with 30 ml/g 0.1 M NaOH for 30 min at 65 ◦C in a plastic
container placed in a thermostatic bath. The third step of the treat-
ment was an acid wash, performed as in the first dealumination but
for only 2 hr. Between each of these steps, the solid was filtered,
washed with distilled water and dried at 50 ◦C overnight. Finally
The rearrangement of cyclohexanone oxime to -caprolactam
was carried out in a magnetically stirred 50-ml two-necked round
bottom flask equipped with a reflux condenser and placed in a
thermostatic bath. In a typical run 0.1 g of cyclohexanone oxime
was dissolved in 20 ml solvent (benzonitrile) and heated to 100 ◦C
followed by the addition of 0.1 g of the catalyst and the reaction
temperature was set at 130 ◦C under atmospheric pressure for
5 hr. (wt.% 1:200:1 respectively). The catalysts were the NH4+-ion-
exchanged form of the geopolymers, heated at 450 ◦C for 10 min
prior to the reaction which was monitored by periodic sampling in
which 0.1 ml samples were taken and analysed using a Shimadzu
QP20-Plus GC–MS with a 30 meter Rxi-5sil MS capillary column
(Full details of the GC–MS method are reported in the SI). Cali-
bration curves covering the range of 0.044–0.009 mmol/ml of each
reactant and product were used for quantitative analysis. Each reac-
tion was repeated at least three times and the reproducibility is
expressed in the form of standard errors that were measured at the
conclusion of each reaction. The conversion and selectivity of the
reaction were determined according to the IUPAC recommenda-
tions [44] as follows:
+
these samples were exchanged with NH4 as described above, but
in three 6-hr. treatments with NH4Cl solution. The geopolymer
samples treated in this way are designated Seq (e.g. Na-hiSi-Seq). It
should be noted that only the results for the Na-hiSi samples sub-
jected to this post-synthetic treatment are presented here since
these produced superior catalytic performance.
2.4. Catalyst characterization
The formation of the geopolymer was confirmed by X-ray
powder diffraction (Bruker D8 Avance X-ray diffractometer with
Ni-filtered Cu K␣ radiation operated at 45 kV and 40 mA.). 27Al and
29Si solid-state magic angle spinning nuclear magnetic resonance
(MAS NMR) spectra were acquired at a magnetic field of 11.7 T using
a Bruker Avance III 500 spectrometer operating at a 27Al frequency
of 130.24 MHz and a 29Si frequency of 99.29 MHz. The 11.7 T 27Al
solid-state spectra were acquired using a 4 mm Doty MAS probe
with a silicon nitride rotor spun at 10–12 kHz, a 1 s pulse and a
amount of oxime converted
Conversion% =
x100%
amount of oxime fed
Where the amount of oxime converted = the amount fed − the
amount left over.
amount of product formed
Selectivity% =
amount of oxime converted
x100%
1 s recycle time, the spectra referenced with respect to Al(H2O)3+
.