Y.-Y. Zhang et al. / Catalysis Communications 54 (2014) 6–10
7
2.2. Catalyst characterization
hand, ZnO/HZSM-5 shows the highest catalytic activity, and benzene
conversion reaches 75.9% (Table 1). We hence modified HZSM-5 with
both phosphoric acid and zinc nitrate for better para-selectivity
and p-xylene yield.
The morphology of HZSM-5 and modified HZSM-5 was examined
using a field emission scanning electron microscope (FE-SEM) (Hitachi
S-4800). The BET surface areas and pore volumes of HZSM-5 and modi-
fied HZSM-5 were measured using a Micromeritics 2010C instrument.
Powder X-ray diffraction (XRD) analysis was conducted on a Rigaku
Automatic Diffractometer (Rigaku D-MAX) using monochromatized Cu
Kα radiation (λ = 0.15406 nm) at a setting of 40 kV and 80 mA. Exper-
iments of temperature-programmed desorption of ammonia (NH3-TPD)
were conducted using a set of Micromeritics 2920 apparatus equipped
with a thermal conductivity detector (TCD).
3.2. Effect of P2O5 loading
It was reported by Ghiaci et al. that the modification with P2O5 is an
effective way to enhance para-selectivity in the methylation of toluene
with methanol [32]. We modified HZSM-5 with phosphoric acid and
investigated the effect of P2O5 loading (Fig. 1(a)). It is observed that
with increasing P2O5 loading from 0 to 8 wt.%, benzene conversion
decreases from 60.7% to 8.1%, whereas para-selectivity rises from 27.0%
to 100%. Ghiaci et al. reported that P2O5 stops p-xylene isomerization
by covering most of the B acid sites on the external surface of catalyst
[32]. However, P2O5 can enter into the channels of catalyst and covers
the strong acid sites of inner surface, leading to poor benzene conversion.
It is apparent that P2O5 modification is an effective method to enhance
para-selectivity over HZSM-5. Since para-selectivity is over 90% at a
P2O5 loading of 6 wt.%, we chose the HZSM-5 modified with 6 wt.%
P2O5 for the optimization of ZnO loading.
2.3. Catalyst evaluation
The alkylation of benzene with CH3Br was carried out over a continu-
ous fixed-bed quartz reactor, and the experimental setup is similar to
that described elsewhere [10,29]. The CH3Br was prepared through the
catalytic bromination of methane using H2O, HBr, and O2 as mediators
over an Rh/SiO2 catalyst [11,30,31] at 660 °C. As reported by Wang
et al., benzene can be prepared through the dehydrogenation and aroma-
tization of methane in the absence of oxidant over a Mo/HZSM-5 catalyst
at 700 °C [12].
3.3. Effect of ZnO loading
In order to simplify the experimental procedures (Fig. S2, SI), CH3Br
was prepared from methanol and hydrobromic acid over HZSM-5
(SiO2/Al2O3 = 380) at 210 °C in this study. We observed complete
conversion of methanol and 100% selectivity to CH3Br. The synthesized
CH3Br gas was carried by N2 into the methylation reactor, and benzene
was introduced directly (by syringe pumping) rather than being synthe-
sized on-site. The liquid products generated in the second hour were
collected using an ice-water condenser and qualitatively analyzed on
a GC/MS (6890N/5973N) equipped with an Agilent HP-5MS capillary
column (30 m × 0.45 mm × 0.8 μm). For quantitative determination,
the collected liquid products were analyzed on an Agilent 7820A GC
equipped with FID and Agilent AB-FFAP capillary column (30 m ×
0.25 mm × 0.25 μm). The liquid products were quantitatively analyzed
using CCl4 as internal standard substance.
It was reported by Gao et al. that the acid strength and acid sites
of HZMS-5 molecular sieves can be adjusted by the introduction of
a zinc salt [33]. Herein we investigated the effect of ZnO loading
on 6 wt.%P2O5/HZSM-5 in the methylation of benzene with CH3Br
(Fig. 1(b)). With an increase of ZnO loading from 0 to 6%, benzene
conversion increases from 8.4% to 41.5% while p-xylene selectivity
decreases from 94.3% to 87%. At a ZnO loading of 4 wt.%, p-xylene
selectivity is 90%. It is clear that the modification with ZnO results in
significant rise of activity but only slight decrease in p-xylene selectivity.
The 4 wt.%ZnO–6 wt.%P2O5/HZSM-5 catalyst is denoted hereinafter as
P-Zn/HZSM-5. We attribute the variation of CH3Br conversion and aro-
matic yield to the changes of surface acidity and pore channels upon
the loading of P2O5 and ZnO2 on HZSM-5. At this optimized stage (P2O5
loading = 6 wt.% and ZnO loading = 4 wt.%), benzene conversion is
38.2%, p-xylene selectivity is 90.0%, and p-xylene ratio in liquid product
is 26.3%.
3. Results and discussion
3.1. Catalyst screening
3.4. Effect of reaction conditions
In this study, modified HZSM-5 was used to catalyze the alkylation of
benzene with CH3Br. As shown in Table 1, the Bi2O3-, ZnO-, Al2O3-,
La2O3-, ZrO2-, and SiO2-modified HZSM-5 are superior to HZSM-5 in
terms of activity. As for p-xylene selectivity, MgO, P2O5, Fe2O3 and B2O3
are good modifiers, showing promotion effects in the order of P2O5
(94.3%) N B2O3 (82.5%) N MgO (67.1%) N Fe2O3 (57.0%). It is clear that
P2O5/HZSM-5 shows the highest p-xylene selectivity. On the other
3.4.1. Benzene weight hourly space velocity
The effect of benzene weight hourly space velocity (WHSVBenzene
)
was studied over P-Zn/HZSM-5 at 450 °C with CH3Br/benzene molar
ratio fixed at 2. One can see that with WHSVBenzene varied from 1.0 to
2.5, benzene conversion declines from 49.0% to 35.5% and p-xylene
selectivity increases from 87.3% to 90.3% (Fig. S3, SI). With rise of
Table 1
Performance of HZSM-5 and MO-modified HZSM-5 in the alkylation of benzene with CH3Br.
MO/HZSM-5
X (benzene) (%)
Y (p-xylene) (%)
Ratio (p-xylene) (%)
Xylene fraction (%)
p-Xylene
m-Xylene
o-Xylene
HZSM-5
60.7
4.7
8.4
6.2
0.3
0.6
0.4
6.4
5.9
6.7
0.6
5.4
5.5
9.5
10.2
0.2
6.6
6.4
8.8
27.0
67.1
94.3
57.0
23.4
23.5
29.2
82.5
30.8
23.5
35.6
52.4
17.5
5.7
20.6
15.4
0
19.3
22.5
21.5
18.4
6.0
MgO/HZSM-5
P2O5/HZSM-5
Fe2O3/HZSM-5
Bi2O3/HZSM-5
ZnO/HZSM-5
Al2O3/HZSM-5
B2O3/HZSM-5
La2O3/HZSM-5
ZrO2/HZSM-5
SiO2/HZSM-5
6.0
23.7
54.1
55.0
52.4
11.5
51.4
53.9
47.9
72.3
75.9
65.6
5.6
74.9
65.8
67.7
7.7
10.3
11.4
10.0
8.4
17.8
22.6
16.5
14.0
Reaction conditions: T = 450 °C, WHSVBenzene = 1.5 h−1, nCH3Br: nBenzene = 2, N2 = 10 ml/min.