Selective synthesis of 2,6-triad dimethylnaphthalene isomers by disproportionation of 2-methylnaphthalene over mesoporous MCM-41

Article abstract of DOI:10.1007/s11164-018-3551-5

2,6-Dimethylnaphthalene (2,6-DMN) is one of the crucial intermediates for the synthesis of polybutylenenaphthalate and polyethylene naphthalate (PEN). The complex synthesis procedure and the high cost of 2,6-DMN production significantly reduce the commercialisation of PEN even though PEN demonstrates superior properties compared with polyethylene terephthalate. 2,6-DMN can be produced by methylation of 2-methylnaphthalene (2-MN) and/or naphthalene, disproportionation of 2-MN, and/or isomerisation of dimethylnaphthalenes (DMNs). In this study, synthesis of 2,6-triad DMN isomers consisting of 2,6-DMN, 1,6-DMN, and 1,5-DMN have been investigated with the disproportionation of 2-MN over unmodified and Zr-modified mesoporous MCM-41 zeolite catalysts. In contrast to other DMN isomers, both 1,5-DMN and 1,6-DMN can be effectively isomerised to be profitable 2,6-DMN. The disproportionation of 2-MN experiments were carried out in a catalytic fixed-bed reactor in the presence of 1?g of catalyst at a temperature range of 350–500?°C and weight hourly space velocity between 1 to 3?h^?1. The results demonstrated that mesoporous MCM-41 zeolite catalyst has a selective pore shape for 2,6-triad DMN isomers, which may allow a decrease in the production cost of 2,6-DMN. Additionally, 2,6-DMN was successfully synthesised by the disproportionation of 2-MN over MCM-41 zeolite catalyst. Furthermore, both the conversion of 2-MN and the selectivity of 2,6-DMN were considerably enhanced by the Zr impregnation on MCM-41.

Full text of DOI:10.1007/s11164-018-3551-5

Research on Chemical Intermediates
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Selective synthesis of 2,6-triad dimethylnaphthalene
isomers by disproportionation of 2-methylnaphthalene
over mesoporous MCM-41
1
Aysel Niftaliyeva2 • Ali Karaduman3
Fatih G u¨ leç
•
Received: 14 May 2018 / Accepted: 4 August 2018
Ó Springer Nature B.V. 2018
Abstract
2
,6-Dimethylnaphthalene (2,6-DMN) is one of the crucial intermediates for the
synthesis of polybutylenenaphthalate and polyethylene naphthalate (PEN). The
complex synthesis procedure and the high cost of 2,6-DMN production significantly
reduce the commercialisation of PEN even though PEN demonstrates superior
properties compared with polyethylene terephthalate. 2,6-DMN can be produced by
methylation of 2-methylnaphthalene (2-MN) and/or naphthalene, disproportionation
of 2-MN, and/or isomerisation of dimethylnaphthalenes (DMNs). In this study,
synthesis of 2,6-triad DMN isomers consisting of 2,6-DMN, 1,6-DMN, and 1,5-
DMN have been investigated with the disproportionation of 2-MN over unmodified
and Zr-modified mesoporous MCM-41 zeolite catalysts. In contrast to other DMN
isomers, both 1,5-DMN and 1,6-DMN can be effectively isomerised to be prof-
itable 2,6-DMN. The disproportionation of 2-MN experiments were carried out in a
catalytic fixed-bed reactor in the presence of 1 g of catalyst at a temperature range
-
1
of 350–500 °C and weight hourly space velocity between 1 to 3 h . The results
demonstrated that mesoporous MCM-41 zeolite catalyst has a selective pore shape
for 2,6-triad DMN isomers, which may allow a decrease in the production cost of
2
,6-DMN. Additionally, 2,6-DMN was successfully synthesised by the dispropor-
tionation of 2-MN over MCM-41 zeolite catalyst. Furthermore, both the conversion
of 2-MN and the selectivity of 2,6-DMN were considerably enhanced by the Zr
impregnation on MCM-41.
Keywords 2-Methylnaphthalene Á 2,6-Dimethylnaphthalene Á Disproportionation Á
MCM-41 Á PEN
&
Fatih G u¨ le c¸
Fatih.Gulec@nottingham.ac.uk
Extended author information available on the last page of the article
123

F. Güleç et al.
Introduction
Polyethylene naphthalate (PEN) is a thermoplastic polyester which demonstrates
superior properties such as thermal and mechanical stability, oligomer extraction,
gas barrier, heat resistance, tensile strength, and stability against UV light compared
with polyethylene terephthalate (PET) [1–6]. These properties clearly reveal that
PEN is expected as an alternative material for innovative applications [1, 3, 5] such
as high-performance containers for carbonated beverages, advanced photosystems,
films, textile and industrial fibres, storage of water and other liquids, etc. [3–5, 7]. It
also has a wide application possibility for the modern electronic devices in view of
its thermal and mechanical stability. The PEN synthesis process involves three main
parts; (1) the oxidation of 2,6-dialkylnaphthalenes (2,6-DKN) to synthase
dicarboxylic acid (2,6-NDA), (2) the esterification of 2,6-NDA to produce dimethyl
2
,6-naphthalene dicarboxylate (2,6-NDC), and (3) copolymerization of 2,6-NDC
with ethylene glycol to PEN [3, 8]. 2,6-Dimethylnaphthalene (2,6-DMN) is
preferred over dialkyl naphthalene thanks to the fact that no carbon atoms are lost in
the oxidation step [8]. However, the complex synthesis procedure and the high cost
of 2,6-DMN production significantly reduce the industrial applications of PEN
[
1, 3, 4, 7–9]. To produce 2,6-DMN, a few synthesis pathways have been
investigated, such as disproportionation of methylnaphthalenes [4, 10], methylation
of naphthalenes [6, 11–19], and isomerisation of dimethylnaphthalenes (DMNs)
[
7, 20].
In the methylation of 2-methylnaphthalene (2-MN), a higher ratio of 2,6-DMN-
to-2,7-DMN and higher selectivity of b,b-DMN were observed over Fe- and Zn-
modified ZSM-5 silicates while ZSM-5 demonstrated higher MN conversion and
less deactivation than metal silicates [6]. However, both lifetime and catalytic
activity of ZSM-5 zeolite catalyst were enhanced by NaOH treatment [16].
Furthermore, a higher ratio of 2,6-DMN-to-2,7-DMN and selectivity and yield of
2
,6-DMN were observed over ZSM-5 modified by Pt and Zr [16, 19]. Although
-MN is one of the widely used starting materials in methylation reaction,
2
naphthalene is also another alternative starting material for the production of 2,6-
DMN [5, 14, 21]. The methylation of naphthalene was investigated over Al, Ga, and
Al/Ga impregnated ZSM-12 by Wu et al. [21]. According to their results, the
highest naphthalene conversion was obtained over ZSM-12 modified by Al-Ga,
whereas the highest ratio of 2,6-DMN-to-2,7-DMN was observed over Ga-modified
ZSM-12. Song et al. [14] also revealed that Fe-impregnated ZSM-5 demonstrated
higher naphthalene conversion and better catalytic activity as to compare with Ti-
and Co-modified ZSM-5. In the isomerisation of DMNs, Motoyuki et al. [7] state
that an effective synthesis process of 2,6-DMN is possible by utilising a non-
valuable feed stream that mainly includes naphthalene and DMN isomers in which
2,6-DMN can be effectively enriched with isomerisation reaction over MCM-22
zeolite catalyst [7].
As for the disproportionation of 2-MN, G u¨ le c¸ et al. [4] revealed that Zr
modification efficiently enhanced the disproportionation activity of ZSM-5.
Moreover, it was found by Matsuda et al. [22] that while ZSM-5 was
1
23

Selective synthesis of 2,6-triad dimethylnaphthalene…
stable against deactivation for the disproportionation of 2-MN, the catalyst was
deactivated in the isomerisation of MNs and disproportionation of 1-MN. However,
although HZSM-5 was defined as one of the most selective catalysts for the
synthesis of 2,6-DMN and 2,7-DMN, these two isomers were easily isomerised to
other DMN isomers [23]. On the other hand, isomerisation of 2,6-DMN and 2,7-
DMN was suppressed by removal of external acid sites by dealumination using
(
NH )SiF with the disproportionation activity being unchanged [23]. However,
4 6
catalysts, such as H-ZSM-5, H-ZSM-11, H-ZSM-48, and H-mordenite, having
medium pores, demonstrated lower disproportionation activity compare with H-Y
and H-beta zeolite catalysts [23]. Niftaliyeva and Karaduman [10] also investigated
the disproportionation of 2-MN over mesoporous Y zeolite catalyst. Although
mesoporous MCM-41 aluminosilicate zeolite catalyst has never been tested in the
disproportionation of 2-MN reaction, it is a promising catalyst in the dispropor-
tionation reactions. The particular selectivity of MCM-41 was attributed by Morin
et al. to its regular non-interconnected long channels where the xylene molecules
undergo, before desorption, successive reactions of disproportionation and
transalkylation [24].
Although disproportionation of 2-MN is an alternative pathway for the synthesis
of DMN isomers, especially 2,6-triad DMN isomers, there is a relatively small
number of studies which have been concerned with the shape-selective conversion
of 2-MN to 2,6-triad DMN isomers. Investigations into different zeolite catalysts
with different pore shapes are therefore of utmost importance to find a selective
catalyst for the disproportionation of 2-MN. In previous studies, neither MCM-41
nor Zr-impregnated MCM-41 zeolite catalysts have been used for the dispropor-
tionation of 2-MN which is the novelty of the present work. In this study, the
disproportionation of 2-MN was carried out over Zr-modified MCM-41 (Zr/MCM-
4
1) and un-modified MCM-41 zeolite catalysts. The conversion of 2-MN, the ratio
of 2,6-DMN-to-2,7-DMN, and the selectivity of 2,6-DMN were investigated to
demonstrate the disproportionation activity of non-modified and Zr-modified MCM-
4
1zeolite catalysts.
Materials and method
Preparation of Zr/MCM-41 zeolite catalyst
2
MCM-41 zeolite catalysts, supplied from Zeolyst (surface area of 900 m /g), were
modified with about 10 wt% of Zr using ZrO(NO ) Á2H O (Acros Organics, purity
3
2
2
of 99.5%) by the wet impregnation method. Before the impregnation, the powder
MCM-41 zeolite catalysts were firstly pelletized using the following process; 15 g
of MCM-41 zeolite catalyst was mechanically mixed with 3 g of c-Al O powder
2
3
with a mass ratio of 5:1. The mixture was then mixed with deionized water before it
was extruded to cylindrical pellets. The cylindrical pellets were then dried at room
temperature for 24 h and then at 120 °C for 4 h in an oven. To investigate the
baseline experiments of MCM-41, the pelletized MCM-41 zeolites were calcined at
5
50 °C for 6 h. The impregnation method of Zr, which has also been presented in a
123

F. Güleç et al.
previous study [5], was used as follows: approximately 9 g of the pelletized MCM-
4
1 zeolite catalyst was placed in a beaker and then an aqueous solution of 2.53 g of
ZrO(NO ) Á2H O was added onto the MCM-41 catalysts. The impregnation was
3
2
2
carried out at 25 °C for 20 h. The samples were then dried at 110 °C for 4 h before
the calcination at 550 °C for 6 h. The zeolite catalyst having a cylindrical shape (2-
mm diameter, 10-mm length) was, finally, designated as Zr/MCM-41. The activity
of the MCM-41 zeolite catalyst would, therefore, be expected to be enhanced by the
impregnation of Zr as active sites of the catalyst.
Characterisation of Zr/MCM-41 zeolite catalyst
To characterise Zr-impregnated MCM-41 and un-modified MCM-41 zeolite
catalysts, the textural properties of zeolite catalysts were derived from nitrogen
sorption measurements at liquid nitrogen temperature, -196 °C (77 K), using a
Quantachrome NOVA 2200 N sorption instrument. The specific surface area and
2
the pore volume of the zeolite catalysts were calculated according to Brunauer-
Emmett-Teller (BET) [25, 26] and Barrett-Joyner-Halenda (BJH) [25] methods,
respectively. The wide-angle X-ray diffraction (XRD) analysis of the prepared
catalysts was investigated using an Inel Equinox 1000 instrument with Cu-Ka
radiation at 40 kV and 100 mA. The patterns were recorded under the 2h range of
˚
5
°–60° with a step size of 0.02 A. The energy-dispersive X-ray (EDX) and scanning
electron microscopy (SEM) were performed using a ZEISS EVO 40 microscope
operated at 20 kV. Additionally, the amount of Zr impregnated in the pore surfaces
of MCM-41 zeolite catalyst was measured using a Spectra XLAB-2000 PED X-ray
fluorescence (XRF) spectrometer. Furthermore, the infrared analyses of Zr/MCM-
4
1
1 and un-modified MCM-41 were investigated using a spectrometer (Mattson
000). The MCM-41 and Zr/MCM-41 zeolite pellets were prepared by pressing the
sample powders at 10 kPa, and each pellet was then scanned 30 times per analysis at
-
1
-1
a 16-cm resolution within the 400–4000-cm infrared region. The nature of acid
sites of the prepared catalysts was characterised by the same FTIR spectroscopy
(Py-IR). In the Py-IR experiments, pyridine was used as a probe molecule and it was
dosed onto the samples till saturation and kept in a desiccator for 24 h.
Disproportionation of 2-MN
The disproportionation tests of 2-MN were investigated in a fixed-bed experimental
set-up which consisted of a high-pressure liquid pump, a flow meter controlling the
carrier gas flow rate, a specially designed steel fixed-bed flow reactor with a 30-cm
length and inner diameter of 1 cm, surrounded by a tubular furnace with a PID
temperature controller, a condenser (working at - 5 °C), and a phase separator. A
schematic diagram of the experimental set-up was supplied in the previous
publications [5, 10]. A mixture of 2-MN and benzene having a mole ratio of 1:1 was
prepared as the disproportionation feed stream. Approximately 2 ml (nearly 1 g) of
zeolite catalyst (MCM-41 or Zr/MCM-41) was located in the centre of the fixed-bed
reactor, and the catalysts were then activated at 500 °C for 30 min with a nitrogen
flow rate of 5 ml/min. The disproportionation feed was then injected into the fixed-
1
23

Selective synthesis of 2,6-triad dimethylnaphthalene…
bed reactor by the liquid pump just after the catalyst activation. The dispropor-
tionation of 2-MN was carried out at different temperatures of 350–500 °C and
-
1
various weight hourly space velocities (WHSVs) of 1–3 h . The reaction products
were then condensed and collected in glass vials. The disproportionation feed
stream and liquid products were analysed using a gas chromatograph (GC; Thermo
Finnigan DSQ250) with a Zebra brand mass spectroscopy (60 m long, 0.25-mm
internal diameter, 0.25 lm film thickness, from - 60 to 370 °C). DMN isomers in
the product stream were easily separated using a temperature programme; from
9
5 °C to 125 °C with a heating rate of 1 °C/min, then from 125 to 200 °C with a
heating rate of 5 °C/min, and then finally from 200 to 230 °C with a heating rate of
0 °C/min. The conversion of 2-MN [4, 5, 13], the ratio of 2,6-DMN-to-2,7-DMN,
and the selectivity of 2,6-DMN [4] were calculated using the following equations;
2
W2
ÀMN;0
À W2ÀMN;t
ÀMN;0
Conversion of 2-MN ¼
à 100
ð1Þ
ð2Þ
ð3Þ
W2
W2;6ÀDMN
WDMNs
S2;6ÀDMN ¼
à 100
W2;6ÀDMN
W2;7ÀDMN
R2
¼
;6À=2;7ÀDMN
where W2-MN,0, and W2-MN,t are the weight of 2-MN in the feed and product mix-
tures, respectively. S2,6-DMN shows the selectivity of 2,6-DMN. W2,6-DMN, W2,7-DMN
and WDMNs represent the weight percentage of 2,6-DMN, 2,7-DMN, and total DMN
,
isomers, respectively.
Results and discussion
Proposed reaction mechanism
It is known that the zeolite catalysts are widely used for methylation [9, 11, 15, 27],
isomerisation [1–3, 7, 28], and disproportionation [4, 10] processes. Because of the
pore shape of zeolite catalysts and the structure of the model feed, both
isomerisation and disproportionation reactions could be observed in this study as
demonstrated in Fig. 1. The isomerisation reaction may occur among MN isomers
and DMN isomers. DMN isomers may be produced by either disproportionation of
2
-MN or 1-MN which is produced by isomerisation of 2-MN as seen in Fig. 1. The
experimental results supported the proposed disproportionation reaction from 2-MN
or 1-MN to DMNs (more information is presented in the ‘‘Disproportionation of
2
-methylnaphthalene’’ section).
123

F. Güleç et al.
Dimethylnaphthalenes
2
,6-triad
Disproportionation
Disproportionation
1,5-
1,6-
1,7-
2,6-
1
-MN
2
2
,7-triad
Isomerisation
+
1
,8-
2,7-
Naphthalene
,3-triad
1
,4-
1,3-
2,3-
2
-MN
1
,2-
Fig. 1 Possible disproportionation and isomerisation reaction pathways of 2-MN
Characterisation of prepared catalysts
N2 adsorption–desorption isotherms and XRD analysis
Nitrogen sorption isotherms of MCM-41 and Zr/MCM-41 zeolite catalysts are
presented in Fig. 2. The isotherms consist of three well-distinguished regions; (1)
adsorption of monolayer-multilayer, (2) capillary condensation, and (3) multilayer
adsorption on the outer particle surfaces [29]. According to IUPAC classification,
the isotherms may be defined as ‘‘type IV’’, typical isotherm of mesoporous
˚
materials, as porous materials having a diameter of 20–500 A are classified as
mesoporous [30]. Additionally, the isotherms contain a hysteresis loop at high P/P0
values (higher than 0.4) which is typical for mesoporous materials [31]. The BET
surface area, BJH pore volume, and pore diameter of MCM-41 were found as
Fig. 2 Nitrogen adsorption–
desorption isotherms obtained at
-196 °C; a MCM-41 and b Zr/
MCM-41
600
a)
b)
500
400
300
200
100
0
a Ad sd. sorption
d De s e. sorption
0
0.2
0.4
0.6
0.8
1
P/Po
1
23

Selective synthesis of 2,6-triad dimethylnaphthalene…
2
3
˚
33 m /g, 0.8833 cm /g, and 33.96 A, respectively. For Zr/MCM-41, the BET
8
surface area and BJH pore volume, however, decreased to 759.80 m /g and
2
3
.7822 cm /g, respectively, whereas the BJH pore diameter was insignificantly
0
˚
affected (33.89 A). The decrease on the surface area and pore volume may be
attributed to the blocking of some pores by Zr impregnation into the catalyst pores.
Figure 3 illustrates the wide-angle XRD patterns of the catalysts; MCM-41 and
Zr/MCM-41. The wide-angle XRD patterns of both MCM-41 and Zr/MCM-41 are
quite similar with the wide-angle XRD patterns of MCM-41 zeolite catalysts
presented in the literature [29, 32, 33]. There is no clear diffraction peak
corresponding to the Zr compound in Fig. 3b which may be explained by high
dispersion of Zr compounds in the catalyst [29]. The hump at 27.13° may be
attributed to the amorphous nature of MCM-41 zeolite catalyst.
SEM, EDX mapping, and XRF analysis
SEM images of the zeolite catalysts are presented in Fig. 4. The SEM images reveal
no morphological decomposition on the MCM-41 zeolite with the modification of
Zr. Additionally, the MCM-41 zeolite surface looks much smoother after Zr
modification compared with unmodified MCM-41.
EDX mappings of Zr/MCM-41 and MCM-41 zeolite catalysts are presented in
Fig. 5. Figure 5b proves the successful impregnation of Zr on the MCM-41 zeolite
surface. Additionally, the dispersion of Zr metal on the surface seems quite
homogenous. Both SEM and EDX mapping results demonstrate that there was no
agglomeration on the zeolite surface after Zr impregnation. The amount of
impregnated Zr was measured by XRF analysis before and after disproportionation
of 2-MN. According to the XRF analysis, fresh Zr/MCM-41 zeolite catalyst
consisted of approximately 9.0 wt% of Zr, 40 wt% of Si, and 1.5 wt% of Al.
However, the XRF analysis of used Zr/MCM-41 demonstrated a slightly lower Zr
Fig. 3 Wide-angle X-ray
diffraction (XRD) patterns of
a MCM-41 and b Zr/MCM-41
b)
a)
1
0
20
30
40
50
60
2
Theta (°)
123

F. Güleç et al.
Fig. 4 SEM images of a MCM-41 and b Zr/MCM-41 zeolite catalysts before testing on the
disproportionation reaction of 2-MN
(
a)
Al
O
Si
(
b)
Al
O
Si
Zr
Fig. 5 EDX mapping of a MCM-41 and b Zr/MCM-41 zeolite catalysts before testing on the
disproportionation reaction of 2-MN
content of 8.7 wt%. Although the chemical composition of MCM-41 consists of Si
and O, the other metal, Al, comes from c-Al O used in the pelletizing process.
2
3
FTIR analysis
Figure 6 shows that the FTIR spectrums contain a group of absorption bands where
-
the peak at around 805 cm indicates the symmetric stretching modes of Si–O–Si
1
1
23

Selective synthesis of 2,6-triad dimethylnaphthalene…
Fig. 6 FTIR spectrum of the
MCM-41, Zr/MCM-41, and
used MCM-41
3
100-3700 cm-1
Used MCM-41
Zr/MCM-41
MCM-41
1
090 cm-1
1500
Wavenumbers (cm )
500
2500
3500
-
1
-
1
groups [29, 33]. Additionally, the shoulder band at 968 cm may be attributed to
stretching vibration of either Si–O–Si or M–O–Si [29]. The other band around
-
1
1
090 cm
29, 33, 34] and the shoulder on the right side of the peak may be attributed to the
external asymmetric stretching of Si–O–Si [29]. The other wide band at about
indicates the internal asymmetric stretching of the Si–O–Si bond
[
-
1
-1
3
100–3700 cm may be linked with H-bonded hydroxyl groups at 3530 cm
35–38]. As neither the bridging hydroxyl groups, SiOHAl, which are usually
[
characterised by a sharp band at 3550–3650 cm [39], nor isolated SiOH groups
-
1
-
1
characterised by a band at 3745 cm [39] were observed. These peaks are the same
for both MCM-41 and Zr/MCM-41 zeolite catalyst. Additionally, the structure of
used MCM-41 zeolite catalyst remains the same after the disproportionation
reaction, as demonstrated in Fig. 6 where the external and internal symmetric
stretching modes of Si–O–Si groups still remain. The Py-IR results of MCM-41 and
Zr/MCM-41, which has not been presented in this paper, demonstrate that Zr
impregnation has clearly decreased the absorbance values for Lewis (about
-
1
-1
1
450 cm ), Lewis ? Br o¨ nsted (about 1495 cm ), and Br o¨ nsted (about
-
1
550 cm ) acidities. Similarly, Jin et al. [13] have mentioned that the total acidic
1
amount and proportion of strong acid sites decrease with increase in the ratio of Zr-
to-Al.
Disproportionation of 2-MN
Catalytic activity of Zr/MCM-41
The activity of MCM-41-type zeolite catalysts is identified by the conversion of
2
-MN, the selectivity of 2,6-DMN, and the ratio of 2,6-DMN-to-2,7-DMN with
using Eqs. 1–3. Figure 7 shows that an increase in temperature leads to an increase
in the conversion, although it decreases with an increase in the WHSV. When the
MCM-41 was used in the disproportionation of 2-MN, the conversion values were
found lower than 4 wt% as shown in Fig. 7a. However, the conversion was clearly
enhanced by the modification of Zr. For instance, the conversion of 2-MN clearly
123

F. Güleç et al.
3
3
5
0
5
4
3
2
1
0
WHSV1
WHSV2
WHSV3
WHSV1
WHSV2
WHSV3
(b)
(
a)
25
2
1
1
0
5
0
5
0
3
50
400
450
400
450
500
Temperature (°C)
Temperature (°C)
Fig. 7 Conversion of 2-MN at different WHSVs and temperatures for a MCM-41 and b Zr/MCM-41
increased to about 20 wt% at 400 °C and nearly 30 wt% at 500 °C for WHSV1, as
observed in Fig. 7b, when Zr/MCM-41 was used as a catalyst. A similar increase
was also observed for WHSV2 and WHSV3 in Fig. 7. It is known that the reaction
rate constant (k) depends on the temperature because of the exponential term of the
Arrhenius equation (k = k * exp(- E /RT)). According to the equation, the
0
A
increase in the temperature can increase the value of the exponential term that may
result in an increase in the reaction rate. Additionally, the activity increase may be
attributed to the reaction mechanism of disproportionation on the catalyst surface.
The acid sites may be attached to the weak C–C bound on the 2-MN molecules
adsorbed on the active site of the catalyst and result in a bond cleavage. As it is
known that the weakest C–C bond on the 2-MN is the methyl bond which is
connected to the 2nd carbon of the naphthalene molecule. Thus, the conversion of
2
-MN may be enhanced thanks to the transfer of methyl group to another 2-MN
molecule. Furthermore, the increase in the conversion may also be attributed to both
the weakening of Lewis and Br o¨ nsted acidities and the constriction of the pore
dimension of MCM-41 after Zr modification. These findings correlate with those of
Jin et al. [13], where the improvement in the catalytic performance on methylation
reaction of 2-MN was also mainly attributed to both the weakening of acid strength
and the expansion of pore dimension that occurred after Zr impregnation into ZSM-
5
4
. Additionally, Morin et al. [24] suggested that the particular selectivity of MCM-
1 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-DMN demonstrated an increasing trend with an increase in
temperature, but a decreasing trend with an increase in WHSV for MCM-41 and Zr/
MCM-41 zeolite catalysts, as seen in Fig. 8. For instance, the selectivity of 2,6-
DMN increased from about 7–18 wt% with temperature increasing from 350 to
450 °C for WHSV1 for MCM-41. A similar increase was observed for both
WHSV2 and WHSV3, as seen in Fig. 8a. However, the selectivity of 2,6-DMN
decreased from about 7 wt% to approximately 1 wt% with WHSV increasing from
-
1
1
to 3 h at 350 °C. Additionally, a similar decrease in the selectivity was clearly
observed at 400 and 450 °C. These decreases may be attributed to the decreasing
1
23

Selective synthesis of 2,6-triad dimethylnaphthalene…
3
2
2
1
1
0
5
0
5
0
5
0
30
25
20
15
10
5
WHSV1
WHSV2
WHSV3
WHSV1
WHSV2
WHSV3
(b)
(
a)
0
3
50
400
450
400
450
500
Temperature (°C)
Temperature (°C)
Fig. 8 The selectivity of 2,6-DMN at different WHSVs and temperatures for a MCM-41 and b Zr/MCM-
41
residence time in the catalyst bed with increasing WHSV. The decrease in the
interaction between catalyst and reactant may also decrease the selectivity of 2,6-
DMN. As for Zr/MCM-41, the selectivity of 2,6-DMN was found to be higher than
that for MCM-41. It is clear that Zr modification enhanced both the conversion of
2
-MN shown in Fig. 7b, and the selectivity of 2,6-DMN illustrated in Fig. 8b. The
selectivity of 2,6-DMN for Zr/MCM-41 was found mostly higher than 20 wt% at
00, 450, and 500 °C for WHSV1, WHSV2, and WHSV3. On the contrary of
4
MCM-41 and Zr/MCM-41 can almost keep the selectivity of 2,6-DMN values at the
same level regardless of different temperatures and WHSV used in these
experiments.
Figure 9 shows the ratio of 2,6-DMN-to-2,7-DMN for MCM-41 and Zr/MCM-41
at different temperatures and WHSVs. The ratio of 2,6-DMN-to-2,7-DMN is
generally greater than 1.5 for MCM-41 and Zr/MCM-41 zeolite catalysts. The
higher ratio of 2,6-DMN-to-2,7-DMN can enable an easy subsequent separation
because an eutectic mixture would be formed at a ratio of 0.7 [8]. Similarly, the
(
a)3
(b)
3
WHSV1
WHSV2
WHSV3
WHSV1
WHSV2
WHSV3
2
1
0
.5
2
2.5
2
.5
1
1.5
1
.5
0
0.5
0
3
50
400
450
400
450
500
Temperature (°C)
Temperature (°C)
Fig. 9 The ratio of 2,6-DMN-to-2,7-DMN at different WHSVs and temperatures for a MCM-41 and
b Zr/MCM-41
123

F. Güleç et al.
ratio of 2,6-DMN-to-2,7-DMN was found to be higher than 1.0 in the dispropor-
tionation reaction over Zr-modified ZSM-5 zeolite catalyst in our previous study [4].
However, the great increase in the ratio by about 2.5–3.0 of 2,6-DMN-to-2,7-DMN
after Zr impregnation on ZSM-5 was investigated on methylation of 2-MN by Jin
et al. [13]. The ratio of 2,6-DMN-to-2,7-DMN is among 1.4–2.4 for MCM-41 and
1
.3–1.6 for Zr/MCM-41 as shown in Fig. 9.
Dimethylnaphthalene distribution
The weight % distributions of DMN isomers in the products stream is another
crucial issue in this study for the synthesis of 2,6-DMN. 2,6-DMN has nine other
DMN isomers that can be produced as by-products in the disproportionation
reaction of 2-MN, as demonstrated in Fig. 1. These DMN isomers can be divided
into three main triads: (1) 2,6-triad; 1,5-DMN, 1,6-DMN, and 2,6-DMN, (2) 2,3-
triad; 1,4-DMN, 1,3-DMN, and 2,3-DMN, and (3) 2,7-triad; 1,8-DMN, 1,7-DMN,
and 2,7-DMN [1]. Among these three triads, the 2,6-triad has the utmost importance
as both 1,5-DMN and 1,6-DMN can be effectively isomerised to be profitable 2,6-
DMN [1, 2]. 1,6-DMN can be supplied by a single methyl shift of 1,5-DMN and
then 2,6-DMN can be produced by one more methyl shift of 1,6-DMN [1, 3]. The
DMN distributions are presented in Table 1 for WHSV1 space velocity in which the
highest conversion of 2-MN and selectivity of 2,6-DMN were achieved. The 2,6-
DMN and other DMN isomers such as 2,7-DMN, 1,3-DMN, 1,4-DMN, 1,7-DMN,
1
,5-DMN, 1,6-DMN, 1,2-DMN, 2,3-DMN, and 1,8-DMN were synthesised with
disproportionation of 2-MN over MCM-41 and Zr/MCM-41. It is clear that Zr
modification enhanced the selectivity of 2,6-DMN at low temperature (400 °C).
Additionally, the disproportionation of 2-MN over the Zr/MCM-41 catalyst mainly
leads to 2,6-DMN, 2,7-DMN, 1,5-DMN, and 1,6-DMN in very high proportions
compared with the thermodynamically attainable level. A similar contribution was
Table 1 DMN isomer distributions over MCM-41and Zr/MCM-41 at different temperatures for WHSV1
Catalyst type
MCM41
350
ZrMCM41
Thermodynamic
equilibrium [23]
Temperature (°C)
400
450
400
450
500
Composition of DMNs (wt%)
2
2
1
1
1
2
1
,6-DMN
6.8
3.6
5.9
74.8
1.9
1.5
5.5
–
12.4
8.2
18.3
12.7
19.1
40.9
5.6
3.4
0
19.3
13.5
20.7
29.2
7.2
19.8
15.8
13.6
31.9
9.8
23.1
16.5
10.2
35.2
9.3
9.0
9.0
45.6
27.3
5.7
2.9
–
,7-DMN
,3-, 1,4- and 1,7-DMN
,5- and 1,6-DMN
,2-DMN
12.7
56.2
3.4
,3-DMN
2.3
6.4
8.1
5.4
,8-DMN
4.5
0.5
0.9
0.1
Unknown
Sum
0.3
–
2.2
–
–
–
100
99.7
100
97.8
100
100
99.5
1
23

Selective synthesis of 2,6-triad dimethylnaphthalene…
also investigated with disproportionation of 2-MN over H-ZSM-11 and H-Mor by
Kikuchi et al. [23]. The disproportionation of 2-MN demonstrates that MCM-41
zeolite catalyst has a selective pore shape for 2,6-triad isomers; 1,5-DMN, 1,6-
DMN, and 2,6-DMN. These three isomers have the highest percentage among the
other DMN isomers for both MCM-41 and Zr/MCM-41 zeolite catalysts.
Conclusions
The selective synthesis of 2,6-triad DMN isomers has been investigated with the
disproportionation of 2-MN over both MCM-41 and Zr-modified MCM-41 zeolite
catalysts. This is the first study in which the MCM-41 zeolite catalyst has been
investigated in the disproportionation of 2-MN reaction to synthesize 2,6-triad
DMN isomers. The conversion of 2-MN is considerably enhanced by the Zr
modification on MCM-41 zeolite catalyst, and reaches a peak level of 30 wt% at
5
00 °C and WHSV1. In addition to the improvement in the conversion, the
selectivity of 2,6-DMN is also increased substantially by the Zr modification. It was
increased to nearly 20–25 wt% after Zr modification at 400, 450, and 500 °C.
Furthermore, the ratio of 2,6-DMN-to-2,7-DMN is about 1.5 for both MCM-41 and
Zr/MCM-41 zeolite catalysts. Additionally, in terms of the synthesis of 2,6-tirad
isomers, MCM-41 zeolite catalyst shows a selective pore shape for 2,6-triad
isomers; 1,5-DMN, 1,6-DMN, and 2,6-DMN, which can allow for a decrease in the
production costs of 2,6-DMN. Thanks to its selective pore shape for 2,6-triad
isomers, the other transition metals can also be impregnated onto MCM-41 zeolite
catalyst and investigated in the disproportionation reaction as a future study.
Furthermore, a cheap naphthalene source, such as coal tar naphthalene oil fractions,
which is a sub-product of the iron-steel industry, could be used as a raw material
which could also reduce the production cost of 2,6-DMN.
Acknowledgements This work was supported by The Scientific and Technological Research Council of
¨
Turkey (TUBITAK, Project No: 112M297).
˙
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Affiliations
1
Aysel Niftaliyeva2 • Ali Karaduman3
Fatih G u¨ leç
•
1
Department of Chemical Engineering, Faculty of Engineering, University of Nottingham,
Nottingham NG7 2RD, UK
2
3
Department of Chemical Engineering, Faculty of Engineering, Sel c¸ uk University,
42030 Konya, Turkey
Department of Chemical Engineering, Faculty of Engineering, Ankara University,
06100 Ankara, Turkey
1
23