Methylation of 2-methylnaphthalene over metal-impregnated mesoporous MCM-41 for the synthesis of 2,6-triad dimethylnaphthalene isomers

Article abstract of DOI:10.1007/s11164-020-04098-8

2,6-Dimethylnaphthalene (2,6-DMN) is one of the key intermediates for the production of polyethylene naphthalate (PEN), which demonstrates superior properties compared with the polyethylene terephthalate. However, the complex synthesis procedure of 2,6-DMN increases the production cost and decreases the commercialisation of PEN. In this study, selective synthesis of 2,6-triad DMN isomers (1,5-DMN, 1,6-DMN and 2,6-DMN) has been investigated by the methylation of 2-methylnaphthalene (2-MN) over mesoporous Cu/MCM-41 and Zr/MCM-41 zeolite catalysts. On the contrary of other DMN isomers, 2.6-triad isomers can effectively be converted to be profitable 2,6-DMN with an additional isomerisation reaction, which is a new approach to reach higher 2,6-DMN yield. The methylation reactions of 2-MN were investigated in a fixed-bed reactor at 400?°C and weight hourly space velocity of 1–3?h^?1. The results showed that the activity of MCM-41 on the methylation of 2-MN has been enhanced with the impregnation of Cu. The conversion increased from about 17% to 35 wt% with the impregnation of Cu. Similarly, the 2,6-triad DMN selectivity and 2,6-/2,7-DMN ratio reached the maximum level (48 wt% and 1.95, respectively) over Cu-impregnated MCM-41 zeolite catalyst.

Full text of DOI:10.1007/s11164-020-04098-8

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-020-04098-8
Methylation of 2‑methylnaphthalene
over metal‑impregnated mesoporous MCM‑41
for the synthesis of 2,6‑triad dimethylnaphthalene isomers
Aysel Niftaliyeva1 · Fatih Güleç2,3 · Ali Karaduman4
Received: 27 August 2019 / Accepted: 29 January 2020
© Springer Nature B.V. 2020
Abstract
2,6-Dimethylnaphthalene (2,6-DMN) is one of the key intermediates for the pro-
duction of polyethylene naphthalate (PEN), which demonstrates superior properties
compared with the polyethylene terephthalate. However, the complex synthesis pro-
cedure of 2,6-DMN increases the production cost and decreases the commercialisa-
tion of PEN. In this study, selective synthesis of 2,6-triad DMN isomers (1,5-DMN,
1,6-DMN and 2,6-DMN) has been investigated by the methylation of 2-methylnaph-
thalene (2-MN) over mesoporous Cu/MCM-41 and Zr/MCM-41 zeolite catalysts.
On the contrary of other DMN isomers, 2.6-triad isomers can efectively be con-
verted to be proftable 2,6-DMN with an additional isomerisation reaction, which is
a new approach to reach higher 2,6-DMN yield. The methylation reactions of 2-MN
were investigated in a fxed-bed reactor at 400 °C and weight hourly space velocity
of 1–3 h−1. The results showed that the activity of MCM-41 on the methylation of
2-MN has been enhanced with the impregnation of Cu. The conversion increased
from about 17% to 35 wt% with the impregnation of Cu. Similarly, the 2,6-triad
DMN selectivity and 2,6-/2,7-DMN ratio reached the maximum level (48 wt% and
1.95, respectively) over Cu-impregnated MCM-41 zeolite catalyst.
Keywords Methylation · 2-Methylnaphthalene · 2,6-Dimethylnaphthalene · 2,6-
Triad DMN · MCM-41 · PEN
*
Aysel Niftaliyeva
Aysel.Niftaliyeva@gmail.com
1
2
Chemical Engineering, Faculty of Engineering, Selçuk University, 42030 Konya, Turkey
Chemical Engineering, Faculty of Engineering, University of Nottingham,
Nottingham NG7 2RD, UK
3
4
Energy and Power Theme, School of Water, Energy and Environment, Cranfeld University,
Cranfeld MK43 0AL, UK
Chemical Engineering, Faculty of Engineering, Ankara University, 06100 Ankara, Turkey
Vol.:(0123456789)
1 3

A. Niftaliyeva et al.
Introduction
2,6-Dimethylnaphthalene (2,6-DMN) is one of the most convenient chemicals
used in the synthesis of 2,6-naphthalene-dicarboxylic acid, which is the monomer
being used in the production of polyethylene naphthalate (PEN). Superior proper-
ties of PEN, such as thermal, mechanical and chemical stability, high gas barrier,
high tensile strength, heat resistance, high stability against UV and X-ray, enable
to be a better polymer in industrial applications compared to PET [1–6]. However,
the commercialisation of PEN has been limited due to the excessive production
cost of 2,6-DMN. Therefore, the studies have been recently focused on an alterna-
tive method for the selective synthesis of 2,6-DMN by alkylation (methylation) of
naphthalenes/methylnaphthalenes [7], disproportionation of methylnaphthalenes
[8, 9] and isomerisation of dimethylnaphthalenes [10]. Thanks to these synthesis
processes, ten diferent DMN isomers can be obtained, which consists of 2,6-triad
(1,5-DMN; 1,6-DMN; 2,6-DMN), 2,7-triad (1,8-DMN; 1,7-DMN; 2,7-DMN),
2,3-triad (1,4-DMN; 1,3-DMN; 2,3-DMN) and 1,2-DMN [5, 10–12].
Studies on DMN synthesis mainly focus on heterogeneous shape-selective zeo-
lites catalysts because of their controllable porous structures and surface acidic
characteristics, strong thermal and water stability, large surface area and broad
pore [13]. The methylation of naphthalene was investigated over HZSM-5, Hβ,
HUSY and SAPO-11 zeolites modifed by 0.1 wt% PdO under atmospheric pres-
sure. The stability of reaction, the highest 2,6-DMN selectivity, 2,6-DMN/2,7-
DMN ratio and long catalyst lifetime were enhanced after modifed by PdO2
[2]. NH4F and Pt-modifed HZSM-5 zeolites can be used for 2-MN methylation
with methanol. The catalytic activity and products distribution depend strongly
on zeolite acidity. The conversion of 2-MN and selectivity of 2,6-DMN change
with the addition of Pt and the increase in the content of NH4F [5]. Güleç et al.
[14] also investigated Fe-modifed ZSM-5 zeolite on the methylation of 2-MN.
The selectivity of 2,6-DMN in the methylation of 2-MN increased while the con-
version decreased by the steam and TEOS modifcation of HZSM-5 zeolite [15].
Under high-pressure reaction conditions, compared with macro-sized MTW-type
zeolites, nano-sized MTW-type zeolites exhibited higher 2-MN conversion and
dimethylnaphthalene (DMN) selectivity [16]. Niftaliyeva et al. [17] studied meth-
ylation of 2-MN over La- and Cu-modifed Y zeolite. The results show that Cu-
modifed catalysts show higher 2-MN conversion compared with parent Y zeolite.
Recently, various medium and large-pore zeolite catalysts, mainly ZSM-5, ZSM-
11, ZSM-12, MCM-22, Mordenite, beta, Y zeolite, have been utilised as catalysts
in the methylation of 2-MN and naphthalene [5, 17–21].
However, without a selective catalyst, in addition to 2,6-DMN, the other DMN
isomers are also produced by methylation of naphthalenes. The similarity of the
chemical and physical properties of DMN isomers complicates the separation of
these substances [22]. Wang et al. [23] and Fang et al. [24] reported that the reac-
tivity of the C-6 position is higher than the C-7 position since the electron density
at C-6 is remarkably higher than that at C-7 on the methylation of 2-MN inside
of catalyst channel. Therefore, 1,5-DMN and 1,6-DMN in the 2,6-triad group are
1 3

Methylation of 2-methylnaphthalene over metal-impregnated…
also considered as valuable products which can be easily isomerised to 2,6-DMN
[25–27]. It is reasonable to evaluate the overall selectivity for 2,6-triad DMN
with the sum of 2,6-, 1,6- and 1,5-DMNs synthesised from the methylation of
2-MN as mentioned by Zhang et al. [2] and Güleç et al. [8].
MCM-41 zeolite has attracted considerable catalytic performance for selective
methylation due to its periodic framework of regular mesopores, large surface area
(>1000 m2/g), high porosity (50–500 Å), highly uniform hexagonal structure of
tuneable size, which helps the mass transfer of relatively large molecules of reac-
tants to active sites in pores, excellent thermal stability (1173 K) [28–34]. The com-
bination of large pores and mild acidity in Al-containing MCM-41 has enabled to
use in a wide range of application such as alkylation, cracking and hydrocracking
[29, 34–38]. Although pure MCM-41 has very weak acidity, it exhibits low acid-
ity in the presence of Al in small quantities and in consequence of Al-atoms does
not exist in the structure of MCM-41 compared to other zeolites. The addition of
Al-atoms into the MCM-41 structure can produce active sites with Brønsted acid
sites for adsorption, ion exchange and catalysis. For instance, García-Sancho et al.
[39], Ding et al. [40], Güleç et al. [41], Juarez et al. [42] have investigated Zr- and/or
Cu-modifed MCM-41 samples on diferent reactions. They found that the acidity of
MCM-41 can be enhanced by adding metals such as Zr and Cu.
However, the catalytic performance of Cu- and/or Zr-modifed MCM-41 zeolite
catalysts has not been investigated for the methylation of 2-MN, which is the nov-
elty of the present work. Therefore, in this study, the methylation of 2-MN has been
investigated in a fxed-bed reactor over MCM-41 and Cu-, Zr-impregnated MCM-41
zeolite catalysts. The conversion of 2-MN, the ratio of 2,6-DMN/2,7-DMN and the
selectivity of 2,6-triad DMN are investigated to demonstrate the catalytic perfor-
mance of the MCM-41 type zeolite catalysts.
Experimental
Impregnation of Cu‑ and Zr‑ on mesoporous MCM‑41
Mesoporous MCM-41 (surface area 900 m2/g) zeolite was obtained from Zeolyst
(USA) in the form of powder. Before the impregnation of Cu- and Zr-metals, MCM-
41 were mechanically mixed with γ-Al2O3 with a ratio of 5:1 and then cylindrical
pellets were prepared from the mixture of MCM-41/γ-Al2O3 (after this point, the
MCM-41 represents a mixture of MCM-41/γ-Al2O3 having a ratio of 5:1). The pel-
lets were then dried at 393 K for 4 h and calcined 823 K for 6 h, respectively. The
calcined pellets were used for the preparation of metal, Cu- and Zr- and bimetal, Cu/
Zr, modifed mesoporous MCM-41 zeolite catalysts. The wet impregnation method
was carried out for the modifcation of mesoporous MCM-41, and the process was
reported in our previous publications [8, 9, 14, 17, 41]. Briefy, for the prepara-
tion of Cu/MCM-41 (10 wt% of Cu) and Zr/MCM-41 (10 wt% of Zr), 3.31 gm of
Cu(NO3)2.3H2O and 2.23 gm of ZrO(NO3)2.xH2O (ACROS, 99.5%) were dissolved
in 20 ml of deionized water, respectively. Then, 8 gm of MCM-41 was added into
the aqueous solutions. Cu/Zr/MCM-41 (5 wt% of Cu-Zr) catalyst was also prepared
1 3

A. Niftaliyeva et al.
using 1.60 gm of Cu(NO3)2.3H2O and 1.067 gm of ZrO(NO3)2.xH2O. The impreg-
nation was held at room temperature for 24 h. The mixture was then dried at 393 K
for 4 h and calcined at 823 K for 6 h. After the calcination, parent MCM-41, Cu/
MCM-41, Zr/MCM-41, and Cu/Zr/MCM-41 catalysts were prepared.
Characterisation of the prepared catalysts
The pore structure of the catalysts was measured by nitrogen adsorption–desorp-
tion isotherms at liquid nitrogen temperature (77 K) using the Quantachrome NOVA
2200 series volumetric gas adsorption instrument. The surface area and pore size
distribution of the prepared catalysts were determined using the BET (Bulrunauer-
Emmetr-Teller) and BJH (Barrett-Joyner-Halenda) methods. The X-ray difraction
patterns of the catalysts were obtained using a Bruker D8 Advance with DaVinci
X-ray difractometer (XRD) with Cu-Kα radiation at 40 kV. The scanning range
was set from 5° to 90° (2θ). Additionally, the IR spectroscopy of adsorbed pyridine
(Py-IR) was carried out on a Mattson 1000 Fourier transform infrared spectrometer
(FTIR) to identify the Brønsted and Lewis acid sites present in each material. About
1 g of the sample was weighed and treated with pyridine before prepared pellets.
After a background spectrum was recorded, the pyridine absorbed samples were
scanned. The scanning electron microscopy (SEM) images and elemental analysis of
the samples were performed using an energy-dispersive X-ray spectrometer (EDX)
analysis were performed using a ZEISS EVO 40 microscope with an accelerating
voltage of 20 kV. The samples were carbon-coated prior to the SEM investigation.
The quantitative chemical analysis of chemical elements of prepared catalysts based
on the measurements of the wavelengths and intensities of their X-ray spectral lines
emitted by secondary (emissions) excitation were identifed using a Spectra XLAB-
2000 PEDX-ray fuorescence (XRF).
Catalytic activity of the prepared catalysts
The methylation of 2-methylnaphthalene (as demonstrated in Fig. 1) was investi-
gated over the prepared zeolite catalysts, MCM-41, Cu/MCM-41, Zr/MCM-41, and
Cu/Zr/MCM-41, using a fxed-bed experimental setup, which has already presented
2,7-triad
Methylation
2,6-triad
Isomerisation
2,3-triad
Methylation
Fig. 1 Possible reaction pathways for the methylation of 2-MN
1 3

Methylation of 2-methylnaphthalene over metal-impregnated…
in our previous publications [8, 9, 14, 17, 41]. As demonstrated in our previous stud-
ies, a fxed-bed reactor having a length of 30 cm and diameter of 1 cm is located
in a tubular furnace where the temperature is controlled by both a thermocouple
connected by the catalyst bed and a PID temperature controller connected by fur-
nace itself. The volumetric fow rate of the methylation feed consisting of 2-MN,
methanol and 1,3,5-trimethyl benzene (mesitylene) having a molar ratio of 1:5:5 is
adjusted using ISCO Model 2350 HPLC Pump.
Approximately 1.0 g of zeolite catalyst which was pelletised in the shape of a
cylindrical was frstly placed in the middle zone of the reactor which was located in
a high-temperature tubular furnace. Before the methylation tests, the catalysts were
activated at 500 °C under a nitrogen fow of 5 mL/min for about 30 min to purify the
pores from the water. After the catalyst activation, the temperature was then down to
the reaction temperature. The methylation experiments were carried out at 400 °C
and three diferent weight hourly space velocities, WHSV: 1 h−1, 2 h−1, 3 h−1. The
reaction products were condensed at −10 °C and separated from gases using a phase
separator. The products were then analysed by gas chromatography (GC; Thermo
Finnigan DSQ250) equipped with mass spectroscopy (MS; Zebra brand capillary
column having a length of 60 m internal diameter of 0.25 mm and the flm thickness
of 0.25 μm working −60 to 370 °C). The conversion of 2-MN, the selectivity of
2,6-triad dimethylnaphthalene and the ratio of 2,6-DMN/2,7-DMN were determined
using the following equations:
[
]
W2-MN,0 − W2-MN,t
Conversion of 2-MN =
∗ 100
(1)
(2)
(3)
W2-MN,0
[
]
W2,6-triad DMN
2,6-triad DMN selectivity =
2,6−∕2,7-DMN ratio =
∗ 100
WDMNs
[
]
W2,6-DMN
W2,7-DMN
W2-MN,0 and W2-MN,t are the weight of 2-MN in the feed and product mixtures,
respectively. The 2,6-triad DMN selectivity is the mass percentage of 2,6-DMN,
1,6-DMN and 1,5-DMN in total DMN isomers. WDMNs, W2,6-DMN and W2,7-DMN
are pointing the weight percentage of total DMN isomers, 2,6-DMN, 2,7-DMN,
respectively.
Results and discussion
Characterisation of prepared catalysts
The N2 adsorption–desorption isotherms and mesopore size distributions of
the prepared catalysts are demonstrated in Fig. 2 and Table 1. Figure 2 shows
that the isotherms of the prepared MCM-41 samples demonstrate adsorption of
1 3

A. Niftaliyeva et al.
Fig. 2 N2 adsorption–desorp-
tion isotherms of (a) parent
MCM-41, (b) Cu/MCM-41, (c)
Zr/MCM-41, and (d) Cu/Zr/
MCM-41
d)
c)
b)
a)
Adsorption
Desorption
0
0.2
0.4
0.6
0.8
1
P/P0
Table 1 BET surface area, BJH
Pore volume and pore diameter
of the prepared catalysts
Catalysta
MCM-41
Cu/MCM-41
Zr/MCM-41
Surface area
Pore volume
Pore diam-
(BET) (m2/g)
(BJH) (cm3/g) eter (BJH)
(Å)
833.1
742.2
773.9
631.0
0.717
0.808
0.683
0.678
37.21
31.03
30.94
30.98
Cu/Zr/MCM-41
aMCM-41 represents the mixture of MCM-41/γ-Al2O3 having a ratio
of 5:1
monolayer-multilayer and capillary condensation trends on the outer particle sur-
faces, which named hysteresis with a sharp infexion value of P/P0 is higher than 0.4.
Apparently, MCM-41 samples show capillary condensation step, indicating a uni-
formity of the mesopore size distribution having a diameter of 2–50 nm with type IV
isotherm according to International Union of Pure and Applied Chemistry (IUPAC)
classifcation [34, 40, 42]. The capillary condensation step of metal-impregnated
MCM-41 samples starts earlier than parent MCM-41 due to a contraction in pore
diameter (as seen in Table 1) [43].
Table 1 demonstrates that the BET surface area decreased from 833 m2/g
to about 742, 773, and 631 m2/g with the impregnation of Cu, Zr, and Cu/Zr,
respectively. Similarly, the BJH pore diameter has also demonstrated a decrease
with the impregnation of metal and bimetal into MCM-41 zeolite pores. How-
ever, pore volume demonstrates an increase from about 0.72 cm3/g to 0.81 cm3/g
1 3

Methylation of 2-methylnaphthalene over metal-impregnated…
with the impregnation of Cu, although it has been insignifcantly afected by the
impregnation of Zr and Cu/Zr. The decrease in the surface area by the impregna-
tion of metals maybe therefore attributed to pore blocking by the impregnation.
The micropores in the zeolite frames tend to block by the impregnation of metals,
which makes disable these surfaces in the micropores [8, 44–46].
The low- and wide-angle XRD patterns (Fig. 3) show the refection peaks
of the MCM-41 zeolite at 2θ = 1.8°, 3.2°, and 5° before and after modifcation,
corresponding to (100), (110), and (200) characteristic peaks of the amorphous
mesoporous silica suggesting that metal impregnation has insignifcant efect on
the structure of MCM-41 zeolite integrity [44, 46–50]. The wide-angle difrac-
tion peaks at 2θ = 35.5°, 38.6°, 48.8°, 53.3°, 58.1°, 61.5°, 65.6°, and 67.7° were
attributed to Cu2+ form on Cu/MCM-41 and Cu/Zr/MCM-41 catalysts [34, 46,
51]. However, no specifc peak representing Zr2+ (or any other forms of Zr) was
observed over Zr/MCM-41, which may be attributed to the impregnation of Zr on
the internal surface of MCM-41 zeolite.
The FTIR spectra of the prepared catalysts, parent MCM-41, Cu/MCM-41,
Zr/MCM-41 and Cu/Zr/MCM-41, are presented in the range of 400–4000 cm−1
in Fig. 4. The results indicate that six bending vibration bands are observed for
the zeolite catalysts prepared. There are hydroxyl groups on the surface as indi-
cated with the broadband around 3440 cm−1 and the band around 1610 cm−1 are
defned as the Si–OH deformational vibrations of adsorbed molecules [33, 34].
The internal and external asymmetric stretching of Si–O–Si groups is observed
at around 805 cm−1 and 1090 cm−1 with a corresponding shoulder at 1239 cm−1
and the band at 465 cm−1 attributed to symmetric stretching modes of tetrahe-
dral Si–O–Si and M–O–Si (M; metal) bending modes [31, 34, 35, 43, 49, 50].
As the adding metals on the zeolite, the absorption bands became stronger at
450–960 cm−1, indicating the metals in the MCM-41 framework. Although par-
ent MCM-41 material also exhibits these bands at the same range, the bands for
Cu/Zr/MCM-41
Cu/MCM-41
Zr/MCM-41
MCM-41
1
3
5
10
20
30
40
2 theta (°)
50
60
70
Fig. 3 Low- and wide-angle XRD patterns of parent MCM-41, Cu/MCM-41, Zr/MCM-41, and Cu/Zr/
MCM-41
1 3

A. Niftaliyeva et al.
d)
c)
b)
a)
4000
3500
3000
2500
2000
1500
1000
500
Wavenumbers (cm-1)
Fig. 4 Py-IR spectrums of (a) parent MCM-41, (b) Zr/MCM-41, (c) Cu/MCM-41, and (d) Cu/Zr/MCM-
41
parent MCM-41 are weak and the intensity of this band increases with metal
incorporation in the framework [30, 37].
After the pyridine was impregnated on the catalyst, the acid sites of the catalysts
presented between 1450 and 1550 cm−1 bands. The peak at around 1540 cm−1 was
assigned to pyridine adsorbed on Brønsted (B) acid site. The interaction of pyridine
molecule with Lewis (L) acid site gives rise to a bending vibration at 1450 cm−1,
and the band at 1500 cm−1 was attributed to both Brønsted and Lewis acid sites [37,
44]. Generally, Lewis acid sites defne as weak acid sites [37]. The Lewis acid and
Brønsted acid sites were not detected on the pure MCM-41. The amount of Brøn-
sted and Lewis acid sites was obviously increased by the metals loading. After the
Cu metal is impregnated, Lewis acidity peaks increase (as seen in Fig. 4c) and give
the Cu/MCM-41 catalyst a weak acidity property. Zr-impregnated MCM-41 shows
strong Bronsted+Lewis peak due to strong Brønsted acidity.
Table 2 shows the XRF measurements of the prepared zeolite catalysts. The Al
in the framework of MCM-41 was replaced by impregnated metals, which increased
Table 2 XRF results of parent
Catalysta
Al (wt%) Si (wt%) Cu (wt%) Zr (wt%) Si/Al
and metal-impregnated MCM-
41 zeolites
MCM-41
Cu/MCM-41
Zr/MCM-41
5.01
1.50
1.65
40.27
32.46
32.57
35.64
–
9.8
–
–
–
9.0
5.9
20.7
18.9
20.8
Cu/Zr/MCM-41 1.64
4.9
4.6
aMCM-41 represents the mixture of MCM-41/γ-Al2O3 having a ratio
of 5:1
1 3

Methylation of 2-methylnaphthalene over metal-impregnated…
the Si/Al molar ratio. This increase may be attributed to the dealumination, which
occurs simultaneously with desilication, so Cu and Zr have mostly substituted Al.
The mass ratio of impregnated metals was found about 10% for Cu and Zr and
5–5% for Cu/Zr impregnation. Figure 5 illustrates the EDX mapping of MCM-41,
Cu/MCM-41, Zr/MCM-41, and Cu/Zr/MCM-41 where both Cu and Zr were suc-
cessfully and homogenously impregnated over MCM-41 zeolite. Additionally, the
impregnation of metal and bimetals has an insignifcant efect on the morphology of
MCM-41.
Methylation of 2‑methylnaphthalene
The methylation activity of mesoporous MCM-41 was enhanced by the impregna-
tion of Cu, Zr, and Cu/Zr at lower WHSV conditions as shown in Fig. 6. When
WHSV was kept at 1 h−1, the conversion of 2-MN increased from about 17 wt% to
approximately 35 wt%, 30 wt% and 22 wt% with the impregnation of Cu, Zr, and
Cu/Zr, respectively.
However, the conversion of 2-MN decreased with an increase in the WHSV as
the residence time of reactants in the catalyst bed decreased with increasing WHSV.
Additionally, at the highest WHSV (3 h−1), the conversion decreased with the
impregnation of Zr and Cu/Zr, which may be attributed to the narrowing of pore
size, where the reactants cannot reach the active sites efciently. On the other hand,
the pore dimension has demonstrated an expansion after the Cu impregnation which
may explain the better conversion on the Cu/MCM-41 mesoporous. Consequently,
the improvement in the catalytic performance on methylation reaction of 2-MN was
mainly attributed to both the weakening of acid strength and the expansion of pore
dimension that occurred after Cu incorporation into mesopore framework instead
of Al. Similarly, Jin et al. [52] proved that the increase in conversion is convenient
with the weakening of acid strength. Additionally, Güleç et al. [8, 53] presented that
Al
Si
Al
Si
(b)
(a)
Cu
O
O
Al
Si
Al
Si
(c)
(d)
Zr
O
Zr
Cu
Fig. 5 EDX mapping of a MCM-41, b Cu/MCM-41, c Zr/MCM-41, and d Cu/Zr/MCM-41
1 3

A. Niftaliyeva et al.
40
35
30
25
20
15
10
5
0
MCM-41
17.1
8.8
Cu/MCM-41
35.2
Zr/MCM-41
29.9
Cu/Zr/MCM-41
WHSV1
WHSV2
WHSV3
22.6
17.8
4.9
22.2
20.2
25.7
5.3
7.5
Fig. 6 Conversion of 2-MN over MCM-41, Cu/MCM-41, Zr/MCM-41, Cu/Zr/MCM-41 at 400 °C for
diferent WHSVs
the impregnation of Cu and Zr over MCM-41 zeolite was considerably enhanced the
conversion in the disproportionation of 2-MN. Furthermore, Morin et al. [54] sug-
gested that the particular selectivity of MCM41 samples was most likely due to the
presence of regular non-interconnected long channels, where the molecules undergo
successive reactions of disproportionation and transalkylation before desorption.
The selectivity of 2,6-triad DMN over MCM-41, Cu/MCM-41, Zr/MCM-41, Cu/
Zr/MCM-41 is presented in Fig. 7. The selectivity was found approximately 40 wt%
for MCM-41, Zr/MCM-41, and Cu/Zr/MCM-41. It was slightly enhanced to about
48 wt% over Cu/MCM-41, which was attributed to the weakening of acid strength
and enlargement of pore dimensions, similarly [4, 52]. As the weak acid density
may lead to slow deactivation of the catalyst, increased mesoporosity which is sus-
ceptible to methylation reaction. The enlargement of the pore size may result in a
reduction of unit cell dimensions, which can lead to the contraction of pore dimen-
sions. The increase in mesopore volume was found to be parallel to the increase in
selectivity to 2.6-triad DMN by decreasing the volume of unit cells [3, 4]. More
reactants can therefore easily difuse in and out of pore channels, which may help to
increase 2,6-triad DMN selectivity [3].
As it is known that the higher ratio of 2,6-/2,7-DMN can enable an easy subse-
quent separation because a eutectic mixture would be formed at a ratio of 0.7 [11].
However, the dimension of 2,6-DMN is somewhat larger than that of 2,7-DMN,
with their molecular dimensions in length, thickness and cylindrical diameter being
10.06 Å, 2.76 Å, 6.44 Å for 2,6-DMN and 9.73 Å, 2.76 Å, 6.03 Å for 2,7-DMN
[52]. Therefore, 2,7-DMN has less difusion problem than 2,6-DMN once the prod-
ucts difusing out from the pore channel of catalyst. The suitable pore structure is,
therefore, highly important for higher 2,6-DMN/2,7-DMN ratio. Figure 8 shows that
1 3

Methylation of 2-methylnaphthalene over metal-impregnated…
50
45
40
35
30
25
20
15
10
5
0
MCM-41
40.6
38.7
Cu/MCM-41
44.8
Zr/MCM-41
40.3
Cu/Zr/MCM-41
WHSV1
WHSV2
WHSV3
37.2
33.7
30.9
40.8
47.9
40.4
39.3
36.1
Fig. 7 Selectivity of 2,6-triad DMN over MCM-41, Cu/MCM-41, Zr/MCM-41, Cu/Zr/MCM-41 at
400 °C for diferent WHSVs
2.5
2
1.5
1
0.5
0
MCM-41
1.69
1.74
Cu/MCM-41
1.95
Zr/MCM-41
1.68
Cu/Zr/MCM-41
WHSV1
WHSV2
WHSV3
1.83
1.81
1.63
1.8
1.92
1.86
1.73
1.76
Fig. 8 Ratio of 2,6-/2,7-DMN over MCM-41, Cu/MCM-41, Zr/MCM-41, Cu/Zr/MCM-41 at 400 °C for
diferent WHSVs
1 3

A. Niftaliyeva et al.
the 2,6-/2,7-DMN ratio was higher than 1.7 for all the catalysts tested in this study.
The highest ratio was found as 1.95 for Cu/MCM-41, and Zr/MCM-41 and Cu/Zr/
MCM-41 follows as 1.86 and 1.83, respectively. Jin et al. [52] mentioned that after
the impregnation of Zr on ZSM-5, the unit cell volume has an expansion of 33.39
Å, which can reduce the negative “product selectivity” between 2,6-DMN and 2,7-
DMN. Similarly, in this study, after Cu impregnation on MCM-41, the pore diam-
eter of the catalyst decreases from 37.21 Å to about 31.03 Å while the pore volume
increases from 0.71 to 0.80 cm3/g. Therefore, the increase in the 2,6-/2,7-DMN ratio
may directly be attributed to the pore structure of MCM-41 after Cu impregnation.
Product distributions
Naphthalene-based product distributions obtained from the methylation of 2-MN
over parent MCM-41, Cu/MCM-41, Zr/MCM-41 and Cu/Zr/MCM-41 are presented
in Table 3. The DMN distribution was found as 14.4 wt% as the highest value once
the methylation of 2-MN carried out over Cu/MCM-41, whereas it is only about 3
wt% for MCM-41, 7.7 wt% for Zr/MCM-41 and 1.5% for Cu/Zr/MCM-41. Further-
more, nearly half of the DMN isomers are consisting of 2,6-triad DMN (2.6-DMN,
1,5-DMN and 1,6-DMN) for Cu/MCM-41.
Another critical issue is the isomerisation of 2-MN over the catalysts. However,
the isomerisation level was kept minimal thanks to the large pores in the catalyst
where the 2-MN molecules difuse in the pore structure and give a methylation reac-
tion over the active surface and both DMN isomers and unreacted 2-MN can easily
Table 3 Product distributions of methylation of 2-MN over parent and metal-impregnated MCM-41 zeo-
lite catalysts for WHSV1
Catalystsa
MCM-41
Cu/MCM-41
Zr/MCM-41
Cu/Zr/MCM-41
Naphthalene
Methylnaphthalenes
1-MN
2-MN
Dimethylnaphthalenes
2.6-DMN
2.7-DMN
1.3+1.7-DMN
1.4-DMN
1.6-DMN
1.5-DMN
5.1 1.5
85.6 2.3
1.9 0.1
98.1 0.1
3.0 0.9
15.7 0.2
8.5 0.8
40.2 0.6
8.5 0.1
21.6 0.9
3.3 1.0
1.0 0.3
0.2 0.1
0.1 0.1
0.4 0.1
16.4 0.9
55.6 1.5
2.8 0.1
97.2 0.1
14.4 0.5
18.6 0.3
9.6 0.9
35.4 0.7
8.1 0.1
15.6 0.3
10.6 1.2
0.5 0.0
0.9 0.2
0.4 0.0
0.4 0.3
11.2 0.3
73.0 0.7
2.2 0.1
97.8 0.1
7.7 0.1
16.9 0.7
10.5 0.5
38.4 0.4
9.1 0.1
19.7 0.3
3.4 0.2
0.9 0.3
0.4 0.1
0.5 0.2
1.1 0.1
1.6 0.1
90.5 0.6
1.9 0.1
98.1 0.1
1.5 0.5
16.3 0.2
8.7 0.6
41.6 1.5
7.7 0.2
16.1 0.3
4.7 0.4
2.1 0.8
0.4 0.1
0.3 0.1
1.9 0.1
1.2-DMN
2.3-DMN
1.8-DMN
Tri-methylnaphthalene
aMCM-41 represents the mixture of MCM-41/γ-Al2O3 having a ratio of 5:1
1 3

Methylation of 2-methylnaphthalene over metal-impregnated…
be difuse outside without further isomerisation of 2-MN to 1-MN. Therefore, the
formation of 1-MN was kept minimal over MCM-41-type zeolite catalysts. Simi-
larly, due to the pore structure, DMN isomers difuse outside without further meth-
ylation reaction to TMN isomers. Thus, the formation of TMN isomers was also
kept minimal.
Conclusions
The selective synthesis of 2,6-triad DMN from methylation of 2-MN was investi-
gated over pure MCM-41, Cu/MCM-41, Zr/MCM-41, Zu/Zr/MCM-41, which has
attracted considerable attention among M41S members due to its regular mesopores,
large surface area, low acidity and good thermal stability. The conversion of 2-MN
increased with the impregnation of Cu, Zr, Cu/Zr over MCM-41. The highest con-
version of 2-MN was found as 35 wt% over Cu/MCM-41 at WHSV1. Similarly, the
highest 2,6-triad DMN selectivity (48 wt%) and the highest 2,6-/2,7-DMN ratio
(1.95) were also found over Cu/MCM-41. Furthermore, nearly half of the DMN iso-
mers are consisting of 2,6-triad DMN (2.6-DMN, 1,5-DMN and 1,6-DMN) for Cu/
MCM-41. The characterisation results demonstrated that the Cu is well connected
on the MCM-41 framework to the enhanced pore structure of MCM-41 for selective
synthesis of 2,6-triad DMN isomers. It is proposed as future work that the synthesis
of Cu/MCM-41/Al2O3 by co-precipitation method would increase the catalyst per-
formance in the methylation reaction. In order to reach higher conversion and selec-
tivity values, optimum Cu ratio over MCM-41 zeolite catalyst would be tested.
Acknowledgements This work was supported by The Scientifc and Technological Research Council of
Turkey [TÜBİTAK, Project No: 112M297].
References
1. X. Wang, F. Guo, X. Wei, Z. Liu, Y. Wang, S. Guo, Russ. J. Phys. Chem. A 93(3), 431 (2019)
2. Y. Zhang, J. Feng, Z. Lyu, X. Li, Mod. Res. Catal. 3(02), 19 (2014)
3. J.-N. Park, J. Wang, S.-I. Hong, C.W. Lee, Appl. Catal. A 292, 68 (2005)
4. L. Zhao, H. Wang, M. Liu, X. Guo, X. Wang, C. Song, H. Liu, Chem. Eng. Sci. 63(21), 5298 (2008)
5. Z. Liang, G. Xinwen, L. Min, W. Xiangsheng, S. Chunshan, Chin. J. Chem. Eng. 18(5), 742 (2010)
6. L. Jin, X. Zhou, H. Hu, B. Ma, Catal. Commun. 10(3), 336 (2008)
7. B. Viswanathan, B. Jacob, Catal Rev. 47(1), 1 (2005)
8. F. Güleç, A. Niftaliyeva, A. Karaduman, Res. Chem. Intermed. 44(12), 7205 (2018)
9. A. Niftaliyeva, A. Karaduman, Anadolu Univ. J. Sci. Technol. A Appl. Sci. Eng. 16(2), 275 (2015)
10. T. Chen, N. Kang, C. Lee, H. Kim, S. Hong, H. Roh, Catal. Today 93, 371 (2004)
11. R. Millini, F. Frigerio, G. Bellussi, G. Pazzuconi, C. Perego, P. Pollesel, U. Romano, J. Catal.
217(2), 298 (2003)
12. S.-B. Pu, T. Inui, Appl. Catal. A 146(2), 305 (1996)
13. M. Liu, W. Wu, O. Kikhtyanin, L. Xiao, A. Toktarev, G. Wang, Microporous Mesoporous Mater.
181, 132 (2013)
14. F. Güleç, A. Özen, A. Niftaliyeva, A. Aydın, E.H. Şimşek, A. Karaduman, Res. Chem. Intermed.
44(1), 55 (2018)
15. C. Zhang, X. Guo, C. Song, S. Zhao, X. Wang, Catal. Today 149(1), 196 (2010)
1 3

A. Niftaliyeva et al.
16. G. Watanabe, Y. Nakasaka, T. Taniguchi, T. Yoshikawa, T. Tago, T. Masuda, Chem. Eng. J. 312,
288 (2017)
17. A. Niftaliyeva, F. Güleç, E.H. Şimşek, M. Güllü, A. Karaduman, Anadolu Univ. J. Sci. Technol. A
Appl. Sci. Eng. 16(2), 167 (2015)
18. K. Bobuatong, M. Probst, J. Limtrakul, J. Phys. Chem. C 114(49), 21611 (2010)
19. C. Li, L. Li, W. Wu, D. Wang, A. Toktarev, O. Kikhtyanin, Procedia Eng. 18, 200 (2011)
20. T. Tsutsui, K. Ijichi, T. Inomata, T. Kojima, K. Sato, Chem. Eng. Sci. 59(19), 3993 (2004)
21. C. Zhang, X.W. Guo, Y.N. Wang, X.S. Wang, C.S. Song, Chin. Chem. Lett. 18(10), 1281 (2007)
22. A. Chobsa-Ard, N. Kraikul, P. Rangsunvigit, S. Kulprathipanja, Chem. Eng. J. 139(1), 78 (2008)
23. X. Wang, W. Zhang, L. Zhao, Iran. J. Chem. Chem. Eng. 34(3), 19 (2005)
24. Y. Fang, H. Hu, Catal. Commun. 7(5), 264 (2006)
25. N. Kraikul, P. Rangsunvigit, S. Kulprathipanja, Chem. Eng. J. 114(1–3), 73 (2005)
26. N. Kraikul, P. Rangsunvigit, S. Kulprathipanja, Adsorption 12(5–6), 317 (2006)
27. N. Kraikul, P. Rangsunvigit, S. Kulprathipanja, Chem. Eng. J. 131(1–3), 145 (2007)
28. K. Lin, P.P. Pescarmona, K. Houthoofd, D. Liang, G. Van Tendeloo, P.A. Jacobs, J. Catal. 263(1),
75 (2009)
29. R. Luque, J.M. Campelo, D. Luna, J.M. Marinas, A.A. Romero, J. Mol. Catal. A. Chem. 269(1–2),
190 (2007)
30. S.K. Roy, D. Dutta, A.K. Talukdar, Mater. Res. Bull. 103, 38 (2018)
31. X. Zhang, J. Dong, Z. Hao, W. Cai, F. Wang, Trans. Tianjin Univ. 24(4), 361 (2018)
32. S. Haemi-myun, Int. J. BioSci. BioTechnol. 8(3), 171 (2016)
33. K. Guo, F. Han, Z. Arslan, J. McComb, X. Mao, R. Zhang, Water Air Soil Pollut. 226(9), 288
(2015)
34. C. Huo, J. Ouyang, H. Yang, Sci. Rep. 4, 3682 (2014)
35. K. Murthy, S. Kulkarni, M. Chandrakala, K.K. Mohan, P. Pal, T.P. Rao, J. Porous Mater. 17(2), 185
(2010)
36. S. Udayakumar, A. Pandurangan, P. Sinha, Appl. Catal. A 272(1–2), 267 (2004)
37. B. Xue, J. Xu, P. Liu, L. Lv, C. Xu, Y. Li, J. Mol. Catal. A Chem. 357, 50 (2012)
38. K.G. Bhattacharyya, A.K. Talukdar, P. Das, S. Sivasanker, J. Mol. Catal. A Chem. 197(1–2), 255
(2003)
39. C. García-Sancho, R. Moreno-Tost, J. Mérida-Robles, J. Santamaría-González, A. Jiménez-López,
P. Maireles-Torres, Appl. Catal. A 433, 179 (2012)
40. J. Ding, T. Ma, M. Cui, R. Shao, R. Guan, P. Wang, Mol. Catal. 461, 1 (2018)
41. F. Güleç, F. Sher, A. Karaduman, Pet. Sci. 16(1), 161 (2019)
42. R. Juarez, A. Padilla, A. Corma, H. Garcia, Catal. Commun. 10(5), 472 (2009)
43. M.S.A. Salam, M.A. Betiha, S.A. Shaban, A.M. Elsabagh, R.M.A. El-Aal, Egypt. J. Pet. 24(1), 49
(2015)
44. D.P. Gamliel, B.P. Baillie, E. Augustine, J. Hall, G.M. Bollas, J.A. Valla, Microporous Mesoporous
Mater. 261, 18 (2018)
45. L.P. Ozorio, R. Pianzolli, L. da Cruz Machado, J.L. Miranda, C.C. Turci, A.C. Guerra, Appl. Catal.
A. 504, 187 (2015)
46. H. Kim, E. Jang, Y. Jeong, J. Kim, C.Y. Kang, C.H. Kim, Catal. Today 314, 78 (2018)
47. I. Graça, M. Bacariza, A. Fernandes, D. Chadwick, App. Catal. B Environ. 224, 660 (2018)
48. C. López-Aguado, M. Paniagua, J. Iglesias, G. Morales, J.L. García-Fierro, J.A. Melero, Catal.
Today 304, 80 (2018)
49. X. Dai, F. Qiu, X. Zhou, Y. Long, W. Li, Y. Tu, Electrochim. Acta 144, 161 (2014)
50. R. Tayebee, M. Amini, M. Akbari, A. Aliakbari, Dalton Trans. 44(20), 9596 (2015)
51. X. Wang, Y. Zhang, P. Ning, S. Yan, L. Wang, Q. Ma, RSC Adv. 7(89), 56638 (2017)
52. L. Jin, Y. Fang, H. Hu, Catal. Commun. 7(5), 255 (2006)
53. F. Güleç, E.H. Simsek, A. Karaduman, J. Fac. Eng. Archit. Gazi Univ. 31(3), 610 (2016)
54. S. Morin, P. Ayrault, S. El-Mouahid, N. Gnep, M. Guisnet, Appl. Catal. A 159(1–2), 317 (1997)
Publisher’s Note Springer Nature remains neutral with regard to jurisdictional claims in published
maps and institutional afliations.
1 3