RETRACTED ARTICLE: Study on Methylation of Naphthalene with Methanol over ZSM-5 (core)/SAPO-11(shell) Composite Molecular Sieve

Article abstract of DOI:10.1134/S0036024419030269

Abstract: ZSM-5 (core)/SAPO-11 (shell) composite molecular sieve was synthesized by the hydrothermal method in order to combine the advantages of ZSM-5 and SAPO-11 materials for the methylation of naphthalene with methanol. For comparison, the catalytic performance of mechanical mixture of ZSM-5 and SAPO-11 was also evaluated. The characterization results proved that ZSM-5/SAPO-11 composite has a core-shell structure. The pore size of the composite was between that of ZSM-5 and SAPO-11. The composite had fewer acid sites than ZSM-5 and mechanical mixture, but more acid sites than SAPO-11. The experimental results indicated that the composite exhibited good catalytic performance in the methylation of naphthalene with methanol.

Full text of DOI:10.1134/S0036024419030269

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2019, Vol. 93, No. 3, pp. 431–436. © Pleiades Publishing, Ltd., 2019.
CHEMICAL KINETICS
AND CATALYSIS
Study on Methylation of Naphthalene with Methanol over ZSM-5
(core)/SAPO-11(shell) Composite Molecular Sieve1
Xiaoxiao Wanga, Fang Guob, Xianxian Weic, Zhenmin Liua, Yingchun Wanga, Shaoqing Guoc,*,
Yuanyang Wanga,**, and Yue Yud
aTaiyuan University of Science and Technology, School of Chemical and Biological Engineering, Taiyuan, 030024 PR China
bJinzhong University, College of Chemistry and Chemical Engineering, Yuci, 030619 PR China
cTaiyuan University of Science and Technology, College of Environment and Safety, Taiyuan, 030024 PR China
dLinyi Academy of Technology Cooperation and Application, Linyi, 276000 PR China
*e-mail: guosq@sxicc.ac.cn
**e-mail: yywangs@163.com
Received March 31, 2018
Abstract—ZSM-5 (core)/SAPO-11 (shell) composite molecular sieve was synthesized by the hydrothermal
method in order to combine the advantages of ZSM-5 and SAPO-11 materials for the methylation of naph-
thalene with methanol. For comparison, the catalytic performance of mechanical mixture of ZSM-5 and
SAPO-11 was also evaluated. The characterization results proved that ZSM-5/SAPO-11 composite has a
core-shell structure. The pore size of the composite was between that of ZSM-5 and SAPO-11. The compos-
ite had fewer acid sites than ZSM-5 and mechanical mixture, but more acid sites than SAPO-11. The exper-
imental results indicated that the composite exhibited good catalytic performance in the methylation of naph-
thalene with methanol.
Keywords: ZSM-5/SAPO-11, core/shell, synthesis, methylation of naphthalene with methanol
DOI: 10.1134/S0036024419030269
INTRODUCTION
DMN at acceptable naphthalene conversion. Gener-
ally, the product mixture in this process has very com-
plex composition. Dimethylnaphthalene (DMN) has
10 different isomers with similar boiling points. Partic-
ularly, it is very difficult to separate 2,6-DMN from
2,7-DMN due to the very low difference in boiling
point of only 0.3°C.
2,6-Dimethylnaphthalene (2,6-DMN) is a crucial
chemical for the production of 2,6-naphthalenedicar-
boxylic acid, which is used in synthesis of high-perfor-
mance polyester resins such as poly(ethylene naph-
thalate) (PEN) and poly(butylene naphthalate)
(PBN). Compared with poly(ethylene terephthalate)
(PET), PEN shows superior properties such as lower
gas permittivity, tensile strength, heat resistance and
stability against UV light and x-rays. leading to its wide
application in electronics, production of refillable
bottles, advanced photo systems, food containers, avi-
ation, etc. [1, 2]. PBN is mainly used in electronics,
insulators, and automobile industry [3]. However,
nowadays PEN and PBN are not widely used due to
high cost and low availability of 2,6-naphthalenedi-
carboxylic acid and its precursor 2,6-DMN. There are
few studies on production of 2,6-DMN by different
methods [4–13]. Among them, the synthesis of 2,6-
DMN by methylation of naphthalene with methanol
catalyzed by molecular sieves gets more attention due
to its simplicity and low production cost. The main
problem in development of this process is to find a
molecular sieve providing high selectivity towards 2,6-
Recently, the methylation of naphthalene has been
studied over molecular sieves such as ZSM-5, HY,
mordenite, SAPO-11, and Hβ [14–17]. Among them,
SAPO-11 and ZSM-5 are the most known catalysts.
Thus, many studies have been reported regarding their
catalytic properties [18–22]. SAPO-11, which is a
member of the silico-aluminophosphate (SAPO-n)
family, was synthesized by the first time at Union Car-
bide Corporation. It is a one-dimensional pore molec-
ular sieve with pore size of 0.39 × 0.63 nm [23, 24] and
has a milder acidity. Its pore size lies between the pore
sizes of large-pore and medium-pore zeolites, making
it an interesting material for catalytic cracking,
reforming and alkylation processes. The pore size of
SAPO-11 is larger than 0.60 nm, so it can effectively
sieve the products of naphthalene methylation. In
addition, SAPO-11 shows excellent resistance to deac-
tivation by carbonaceous deposits due to its one-
dimensional pore structure [17]. It has been widely
1
The article is published in the original.
431

432
XIAOXIAO WANG et al.
recognized that ZSM-5 molecular sieve shows excel- mixing the ZSM-5 and SAPO-11 samples prepared as
lent activity in the methylation of naphthalene with described above.
methanol [18, 19]. Therefore, a combination of ZSM-
5 and SAPO-11 might be an optimal catalyst for this
purpose. In the present work, the ZSM-5
Catalyst Characterization
X-ray powder diffraction (XRD) analysis was per-
formed on Rigaku D/MAX-RB diffractometer. Dif-
fraction patterns were recorded using CuKα radiation
at 40 kV and 100 mA in the 2θ range from 5° to 50°.
SEM images were acquired on a LEO-435VP scan-
ning electron microscope operated at 20 kV and 50 pA
beam current.
The textural properties of the samples were derived
from N2 adsorption-desorption measurements per-
formed on a Micromeritics Tristar 3000 unit. In each
case, the sample was outgassed under vacuum at
300°C for 3 h before N2 adsorption. The specific sur-
face area was calculated by BET method, and the pore
volume was obtained by t-plot analysis of the adsorp-
tion isotherm.
(core)/SAPO-11 (shell) composite was synthesized
through hydrothermal method, and its catalytic per-
formance in naphthalene methylation was evaluated.
For comparison, the mechanical mixture prepared
through mixing of ZSM-5 with SAPO-11 was also
studied. The samples of ZSM-5, SAPO-11, the com-
posite and the mechanical mixture were characterized
by XRD, SEM, FT-IR, N2 adsorption–desorption,
and NH3-TPD.
EXPERIMENTAL
Catalyst Preparation
ZSM-5 was synthesized by a hydrothermal method
from a gel with molar composition of template : SiO2 :
Al2O3 : Na2O : H2O = 0.2 : 1 : 0.02 : 0.09 : 35. Tetra-
ethyl orthosilicate (TEOS) (40 wt % SiO2) and sodium
aluminate (80 wt % Al2O3) were used Si and Al
sources, tetrapropylammonium hydroxide (TPAOH)
Infrared spectra were recorded on a Nicolet 380
FT-IR spectrometer.
The acidity of the catalysts was examined by tem-
perature programmed desorption of ammonia (NH3-
was used as the template. The gel was transferred into TPD). The measurements were carried out in a flow
a stainless steel autoclave and heated in an oven at system with a thermal conductivity detector. All sam-
200°C for 24 h. The product thus obtained was ples were preheated from room temperature to 500°C
washed, dried at 120°C for 12 h, and calcined in air at in argon flow and kept at 500°C for 1 h, which was fol-
550°C for 6 h to remove the template completely.
lowed by NH3 saturation in NH3/Ar flow at 40°C for
5 min. Then the samples were evacuated at 40°C for
40 min to remove physically adsorbed NH3. Finally,
the catalyst was heated to 600°C at a linear heating rate
of 10°C/min, and the detector signal of NH3 desorp-
tion was recorded.
SAPO-11 was synthesized from gel with the com-
position Al2O3 : P2O5 : SiO2 : template : H2O = 1 : 1 :
0.6 : 1.2 : 49. Pseudoboehmite (72 wt % Al2O3), ortho-
phosphoric acid (85% H3PO4) and silica sol (30 wt %
SiO2) were used as starting materials. Di-n-propyl-
amine (99 wt %) was used as the template. The gel was
kept for crystallization at 180°C for 24 h. The as-syn-
thesized material was washed with distilled water, then
dried at 120°C for 12 h and calcined at 600°C for 4 h.
ZSM-5/SAPO-11 composite molecular sieve was
synthesized via hydrothermal method. ZSM-5 powder
(5.0 g) was added to 30 mL of distilled water and
stirred for 30 min to form gel A. Pseudoboehmite
(4.6 g, 72 wt % Al2O3) was added in small portions to
a solution of orthophosphoric acid (7.8 g, 85%
H3PO4) diluted with 70 g of distilled water, and the
mixture was stirred for 30 min. Afterwards, silica sol
(30 wt % SiO2) was added dropwise with further stir-
ring for 120 min to get a homogeneous gel B. Finally,
di-n-propylamine (6.2 g) was added to the mixture of
equal amounts of gels A and B. The mixture was stirred
for 150 min and transferred to a stainless steel auto-
clave. The autoclave was then heated at 180°C for 24 h.
The solid product was washed with distilled water,
dried at 120°C overnight, and calcined at 600°C for 6 h
to remove the template.
Catalyst Evaluation
The experiments were performed in a continuous
flow fixed-bed reactor (20 × 600 mm stainless steel
tube). Zeolite catalysts (2.5 g of 20–40 mesh) were
loaded in the reactor tube. The reaction mixture was
fed into the reactor by a quantity measuring pump and
the pressure was kept by N2. The weight hourly space
velocity (WHSV) of naphthalene was 0.19 h–1 in all
experiments. The reaction temperature was 400oC and
the liquid reactant including naphthalene, methanol
and mesitylene (solvent) in a molar ratio of 1 : 5 : 3.5
was preheated before feeding to the reactor. The reac-
tion products were analyzed by gas chromatography
(GC9560) with a Beta-Dex120 capillary column. The
conversion of naphthalene was calculated as follows:
Naphthalene conv. (%)
nN,0 − nN 
=
× 100%,



nN,0

For comparison, the mechanical mixture of ZSM- where nN,0 and nN are the molar percentage of naph-
5 and SAPO-11 (ZSM-5&SAPO-11) was prepared by thalene before and after the reaction. The product dis-
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STUDY ON METHYLATION OF NAPHTHALENE
433
tribution includes the molar percentages of ethylnaph-
thalene (EN), methylnaphthalene (MN), dimethyl-
naphthalene (DMN), and trimethylnaphthalenes
(TMN) in the total product mixture.
7.83°
8.66°
9.47°
The selectivity towards 2,6-DMN is the corre-
sponding molar percentage in the sum of all DMN
isomers; 2,6-/2,7-DMN stands for the molar ratio of
2,6-DMN to 2,7-DMN: 2,6-DMN yield = (naphtha-
lene conversion×2,6-DMN distribution) /100%.
d
c
b
RESULTS AND DISCUSSION
Characterization of Catalysts
a
XRD. Figure 1 shows the XRD patterns of ZSM-5,
SAPO-11, ZSM-5&SAPO-11 and ZSM-5/SAPO-11
molecular sieves. The characteristic peaks of ZSM-5
and SAPO-11 are identified both for ZSM-5&SAPO-
11 and ZSM-5/SAPO-11 samples, and there are some
overlapping forms between the typical characteristic
peaks of ZSM-5 and SAPO-11. The characteristic
peaks at 2θ = 7.83° and 8.66° are attributed to ZSM-5,
and the peak at 2θ = 9.47° is attributed to SAPO-11
[18–20]. The crystallinity of ZSM-5/SAPO-11 is
higher than that of ZSM-5&SAPO-11. Meanwhile,
the XRD peak positions slightly shifts to a lower 2θ for
ZSM-5/SAPO-11, which is due to the formation of
defects and/or lattice contraction in the framework. It
implies that the interaction between SAPO-11 and
ZSM-5 can be present in ZSM-5/SAPO-11 composite.
SEM. SEM results of ZSM-5, SAPO-11, ZSM-
5&SAPO-11, and ZSM-5/SAPO-11 molecular sieves
are presented in Figs. 2a–2d. It is clear that ZSM-
5&SAPO-11 appears as a disordered material, but
ZSM-5/SAPO-11 shows unique spherical crystal
morphology indicating that ZSM-5 is embedded in
SAPO-11.
10
20
30
40
50
2θ, deg
Fig. 1. (Color online) XRD patterns of ZSM-5&SAPO-11
composite molecular sieve and the relative molecular sieves
samples. a, SAPO-11; b, ZSM-5; c, ZSM-5&SAPO-11;
d, ZSM-5/SAPO-11.
ZSM-5, and ZSM-5&SAPO-11 samples, the bands at
1094, 796, 542, and 457 cm–1 of ZSM-5/SAPO-11 shift
to 1092, 792, 544, and 454 cm–1, it suggests that there
are strong interactions between SAPO-11 and ZSM-5
in the ZSM-5/SAPO-11 composite molecular sieve.
NH3-TPD. The acidic properties of the investi-
gated samples were characterized by NH3-TPD. The
acidity was determined by the ammonia desorption
peak temperature, while the amount of acid sites was
estimated by the areas under the TPD curves. The
NH3-TPD profiles of four samples (Fig. 4) show three
ammonia desorption peaks at ca. 220, 300, and 450°C,
corresponding to weak, medium and strong acid sites.
The TPD results show that ZSM-5 has the strongest
acidity, while SAPO-11 has the weakest one. The
amount of acid sites of ZSM-5&SAPO-11 is
decreased (compared to pure ZSM-5), because ZSM-
5 is mixed with SAPO-11, but the peak temperatures
have not changed. Compared with ZSM-5&SAPO-
11, ZSM-5/SAPO-11 exhibits lower acidity, which
further indicates the occurrence of strong interaction
between ZSM-5 and SAPO-11 in the composite. As
shown in Fig. 4, the acid strength and the total acid
sites amount decreases in the order ZSM-5 > ZSM-
5&SAPO-11 > ZSM-5/SAPO-11 > SAPO-11.
FT-IR. The FT-IR spectra of the four samples are
shown in Fig. 3. For the SAPO-11 sample (Fig. 3a), all
peaks are assigned as follows according to [25]:
1119 cm–1 corresponds to asymmetric stretching vibra-
tion of inner tetrahedral, 1228 cm–1 is assigned to
asymmetric stretching vibration of outer tetrahedra;
708 cm–1 is ascribed to the symmetric stretching vibra-
tion of inner tetrahedra and 754 cm–1 to the symmetric
stretching vibration of outer tetrahedra; the bands at
630 and 663 cm–1 are due to the deformation vibration
of four-membered and of six-membered rings, respec-
tively. The 473 cm–1 band corresponds to the flexural
vibration of T–O bond. In Fig. 3b (ZSM-5), the band
at 454 cm–1 corresponds to the flexural vibration of T-
O‒T bond of inner tetrahedra; 546 cm–1 is assigned to
the absorption peak of double bow; 1094 cm–1 and
796 cm–1 is due to the asymmetric stretching vibration
of outer tetrahedra and the symmetric stretching
vibration of outer tetrahedra, respectively. 1221 cm–1
corresponds to the asymmetric stretching vibration of
N2 adsorption–desorption. The textural properties
of the samples were characterized by N2 adsorption-
desorption (Table 1). As shown in Table 1, ZSM-5 has
the maximum SBET, while SAPO-11 has the lowest
one. Compared with ZSM-5&SAPO-11, pore size of
inner tetrahedra [26]. Compared with SAPO-11, ZSM-5/SAPO-11 is enlarged.
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XIAOXIAO WANG et al.
1 ꢀm
1 ꢀm
(а)
(b)
10 ꢀm
10 ꢀm
(c)
(d)
Fig. 2. SEM images of ZSM-5/SAPO-11composite molecular sieve and the relative molecular sieves samples. (a) SAPO-11;
(b) ZSM-5; (c) ZSM-5&SAPO-11; (d) ZSM-5/SAPO-11.
SAPO-11
d
c
SAPO-11/ZSM-5
SAPO-11&ZSM-5
ZSM-5
b
a
400 600 800 1000 1200 1400 1600 1800 2000 2200
Wavenumber, cm−1
100
200
300
400
500
600
Temperature, ꢁC
Fig. 3. (Color online) FTIR spectra of ZSM-5/SAPO-
11composite molecular sieve and the relative molecular sieves
samples: a, SAPO-11; b, ZSM-5; c, ZSM-5&SAPO-11;
d, ZSM-5/SAPO-11.
Fig. 4. (Color online) Results of NH -TPD of ZSM-
3
5/SAPO-11 composite molecular sieve and the relative
molecular sieves samples.
Formation Mechanism of ZSM-5 (core)/SAPO-11
(shell) Composite Molecular Sieve
Then, orthophosphoric acid reacts with pseudoboeh-
mite and silica sol to form the colloid (containing Al,
P and Si) around ZSM-5 core. At last, di-n-propyl-
amine (template) is added to the colloid and reacts
with it to form SAPO-11 crystals around ZSM-5.
Generally, ZSM-5 serves as a seed for the crystalliza-
We can propose the following possible formation
mechanism of ZSM-5 (core)/SAPO-11 (shell) com-
posite. At the first stage, as one of orthophosphoric
acid (which is a SAPO-11 precursor) reacts with the
surface of ZSM-5, and the ZSM-5 is fixed in this way. tion of the composite during the synthesis. The pro-
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STUDY ON METHYLATION OF NAPHTHALENE
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Table 1. Textural properties of ZSM-5/SAPO-11 compos-
the contrary, ZSM-5/SAPO-11 shows lower initial
naphthalene conversion and its stability is the highest
among all the samples, although its acidity is not the
weakest. These indicate that there is no coincidence
between the naphthalene conversion and the acidity of
the samples.
ite molecular sieve and the relative molecular sieves samples
Pore
Pore sizec,
Surface
Sample
volumeb,
cm3 g–1
areaa, m2 g–1
nm
Table 2 shows that the selectivity towards 2,6-
DMN decreases in the order ZSM-5/SAPO-11 >
SAPO-11 > ZSM-5&SAPO-11 > ZSM-5, which is
not arranged by the acidity of the catalysts. This indi-
cates that the selectivity and the acidity of the catalysts
are not explicitly linked. The pore size of ZSM-5 is
near to the kinetic diameter of the naphthalene mole-
cule (0.62 nm), which inhibits naphthalene molecule
from entering into the pore channels, thus the selectiv-
ity towards 2,6-DMN attained over ZSM-5 is the low-
est. Compared with ZSM-5 and ZSM-5&SAPO-11,
SAPO-11 and ZSM-5/SAPO-11 possess suitable pore
size, which make it more favorable to the diffusion of
reactants and products, thus finally leading to higher
2,6-DMN selectivity. The 2,6-DMN yield decreases
in the order of ZSM-5/SAPO-11 > SAPO-11 > ZSM-
5&SAPO-11 > ZSM-5. So, the catalytic performance
of ZSM-5/SAPO-11 is the highest among the studied
samples.
SAPO-11
ZSM-5
SAPO-11&ZSM-5
SAPO-11/ZSM-5
149
256
223
0.145
0.187
0.163
0.172
0.55
0.63
0.62
0.58
182
a
b
c
BET method; t-plot method; volume adsorbed at p/p = 0.99.
0
posed mechanism explains the interactions and chem-
ical bond formation between the components of the
composite.
Catalytic Performance in the Methylation
of Naphthalene
The data on catalytic performance of ZSM-5,
SAPO-11, ZSM-5&SAPO-11, and ZSM-5/SAPO-11
samples are compared in Table 2. The initial naphtha-
lene conversion on the studied samples decreases in
the order of ZSM-5 > ZSM-5&SAPO-11 > ZSM-
5/SAPO-11 > SAPO-11, which is consistent with the
results of NH3-TPD. The stronger acidity of molecu-
lar sieve provides higher initial naphthalene conver-
sion. From Table 2 we can see that over ZSM-5 cata-
lyst the highest conversion of naphthalene (65.2%)
was achieved at the reaction time of 1 h, and it
decreases to 10.6% after 6 h. The conversion of naph-
thalene on SAPO-11 is 55.2 and 32.5% at the reaction
times of 1 and 6 h, respectively. The naphthalene con-
version over ZSM-5&SAPO-11 is 59.4% at the reac-
tion time of 1 h, but it decreases only to 25.5% after
6 h. The conversion of naphthalene over ZSM-
5/SAPO-11 is 58.2% after 1 h and 28.3% at the reac-
tion time of 6 h. From the results above, it can be con-
cluded that the initial catalytic activity of ZSM-5 is
higher than the activity of the other samples because it
has more active sites than other studied catalysts owing
to its strong acidity. However, the conversion of naph-
CONCLUSIONS
ZSM-5 (core)/SAPO-11 (shell) composite molec-
ular sieve was synthesized by the hydrothermal
method and was applied in methylation of naphtha-
lene with methanol. The characterization results
showed the interaction between SAPO-11 and ZSM-5
components of the ZSM-5/SAPO-11 composite
molecular sieve. The pore diameter of ZSM-5/SAPO-
11 is suitable for the diffusion of 2,6-DMN, leading to
higher 2,6-DMN selectivity and yield. Meanwhile, the
composite showed relatively higher stability. Thus, the
ZSM-5/SAPO-11 composite molecular sieve showed
higher catalytic performance in the methylation of
naphthalene with methanol.
ACKNOWLEDGMENTS
The work was supported by the National Science
thalene does not decrease too quickly, although its Foundation
of
Shanxi
Province,
China
strong acidity could accelerate the coke formation. On (201601D102011), the National Science Foundation
Table 2. Effect of different molecular sieves catalysts on catalytic performances
TOS
ZSM-5
SAPO-11
ZSM-5/SAPO-11 ZSM-5&SAPO-11
Naphthalene conversion, %
2,6-DMN selectivity
2,6-DMN yield, %
1h
6h
1h
6h
1h
6h
65.2
10.6
24.6
15.4
4.94
1.09
55.2
32.5
24.9
36.5
6.96
3.50
58.2
28.3
27.2
38.3
7.85
4.62
59.4
25.5
24.5
15.7
5.46
2.32
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