H. Miao et al. / Catalysis Communications 78 (2016) 68–70
69
Table 1
Acidity of MMZ-5 zeolite catalysts .
located on the external surface [9,10]. What's more, the MMZ-5 zeolites
expose more Brönsted acid sites on the external surface owing to the
presence of secondary mesopores. For MMZ-5-2 zeolite, a percentage
of 89.0% acid sites on the external surface is obtained. Meanwhile, the
external Brönsted and Lewis acid sites are quite strong as indicated by
the high concentration of them observed after desorption of Py and
DTBPy at 623 K.
a
Brönsted acid sites
%)
FTIR-Py
FTIR-DTBPy
(
Samples
Temperature
L
Py
B
Py
B
DTBPy
Internal External
(
μmol/g) (μmol/g) (μmol/g)
ZSM-5-0 423
4
2
1
85
83
75
76
69
63
73
62
44
72
71
65
77
66
57
104
95
80
1
–
–
98.8
52.6
11.0
30.6
13.0
61.5
1.2
47.4
89.0
69.4
87.0
38.5
5
6
23
23
3.2. Catalytic activity
MMZ-5-1 423
20
14
10
25
20
15
21
15
12
30
22
13
27
16
12
36
18
10
65
58
52
50
41
32
67
57
36
40
30
17
5
6
23
23
The benzylation of aromatics with benzyl chloride (BC) is totally in
accordance with the classical mechanism of Friedel–Crafts reaction
and presented in Scheme 1. The catalytic results of the samples are
summarized in Table 2. The apparent rate data for the benzylation of
aromatics in excess of aromatics over the zeolite catalysts could be fitted
well to a pseudo-first order rate law:
MMZ-5-2 423
5
6
23
23
MMZ-5-3 423
5
6
23
23
MMZ-5-4 423
ln (1 / (1 − x)) = k
where k is an apparent rate constant, x is the fractional conversion of
benzyl chloride, t is reaction time, and t is the induction period corre-
sponding to the time required to reach the reaction temperature.
A plot of ln (1 / (1 − x)) versus (t − t ) gives a linear graph over the
range of BC conversion. The apparent rate constant k of the samples can
a 0
(t − t ),
5
6
23
23
a
0
MMZ-5-5 423
5
6
23
23
0
a
a
Py-IR spectra and DTBPy-IR spectra of the zeolite samples were supported as Fig. S3
be easily obtained. All MMZ-5-n catalysts have much higher BC conver-
sion than ZSM-5-0. In benzylation of benzene and toluene, MMZ-5-2
zeolite has the highest catalytic activity, over which BC conversions
reach 13.8% and 47.2% for benzene and toluene, respectively. The appar-
ent rate constant for MMZ-5-2 is 31.0 times and 43.3 times that of ZSM-
5-0. Interestingly, BC conversion can reach above 90% for all the MMZ-5
zeolites in benzylation of mesitylene and anisole. The apparent rate con-
and Fig. S4.
The concentration of total Brönsted and Lewis acid sites of the
samples was calculated from peaks intensities at 1545 cm
and at 1455 cm (PyL) in the Py-IR spectra. The concentration of exter-
nal Brönsted acid sites was determined by peak intensity at 1616 cm
−
1
+
(PyH )
−
1
−
1
+
4
−1
(
DTBPyH ) in DTBPy-IR spectra.
The liquid phase benzylation of different aromatics with benzyl chlo-
stants for zeolite MMZ-5-3 are 252.1 × 10 min for mesitylene and
4
−1
282.7 × 10 min for anisole, which are 1.5 times and 1.1 times that
for MMZ-5-2, respectively.
ride (BC) was conducted in a 50 ml three-necked flask equipped with a
reflux condenser. The aromatics and BC (26 mmol) in a required molar
ratio of 13:1 were added to the system including the catalyst (0.15 g),
while the excess of aromatics was applied as a solvent. The reaction
mixture was continuously stirred under the reaction temperature. The
reactants and products were withdrawn periodically to be analyzed
on a gas chromatograph which was equipped with a FID detector and
a 60 m HP-5 capillary column.
In general, Lewis and Brönsted acid sites in the catalysts play a major
+
role in the production of the electrophile (C
6
H
5
CH
2
), which transition
stage is very critical in the reactions [2,11,12]. After the acid sites
+
6 5 2
interact with BC to produce the electrophile (C H CH ), the generated
electrophilic species attack the benzene rings, resulting in the formation
of the benzylated aromatics. Obviously, with the increase of the
electron-donating ability of the substituents in benzen ring, the
conversion of BC increases on the all ZSM-5 catalysts. The reactivities
of these aromatics with BC are in the following order:
anisole N mesitylene N toluene N benzene. The MMZ-5-n catalysts
possess significantly more external acid sites (LPy and BDBTPy), which
are responsible for a much higher catalytic activity of these zeolite cat-
alysts in benzylation of different aromatics without diffusion and/or ste-
ric limitation. As a result, MMZ-5-2 and MMZ-5-4 catalysts have higher
activity in benzylation of benzene or toluene because of their signifi-
cantly more strong acid sites (65 μmol/g and 67 μmol/g), although
they have the different Si/Al ratio. By comparing the activities of
MMZ-5-3 and MMZ-5-5, it is further confirmed that the number of
external Brönsted acid sites, especially strong Brönsted acid sites, can
influence the activity of the zeolite catalysts. This conclusion is also
approved in benzylations of anisole and mesitylene. However, MMZ-
5-3 shows the best catalytic activity among all the MMZ-5-n zeolite cat-
alysts in these reactions although the amount of external acid sites is
lower than that of MMZ-5-2 or MMZ-5-4. This could be assigned to
3
. Results and discussion
3
.1. Porosity and acidity of the samples
All the as-synthesized samples have well-resolved XRD characteris-
tic peaks of MFI structure (Fig. S1) and contain very small crystalline
grains of 20–30 nm (Table S1). The BET surface area and the external
surface area for MMZ-5 zeolites obviously increase owing to their nano-
crystalline feature and the presence of mesopores centered at 3–7 nm
(
Fig. S2b). The BET surface area and the external surface area of MMZ-
2
2
5
1
-2 zeolite reached 544 m /g and 327 m /g, respectively, which are
.3 and 8.8 times the value of ZSM-5-0 zeolite.
The acidic properties of the samples are investigated by FTIR spec-
troscopy of adsorbed molecules (Py and DTBPy). A higher concentration
of Lewis acid sites is observed for all MMZ-5 zeolites compared to ZEM-
5
-0 zeolite (Table 1), and these Lewis acid sites are predominantly
Scheme 1. Benzylation reaction scheme of aromatics with benzyl chloride.