A descriptor for the relative propagation of the aromatic- and olefin-based cycles in methanol-to-hydrocarbons conversion on H-ZSM-5

Article abstract of DOI:10.1016/j.jcat.2013.03.021

The observed product distribution in methanol-to-hydrocarbons (MTH) catalysis can be rationalized based on the relative rates of propagation of the aromatic- and olefin-based cycles that operate on the zeolite catalyst. We report that the ratio of ethene to 2-methylbutane + 2-methyl-2-butene (ethene/2MB) yield can be used to describe the propagation of aromatic and olefin methylation/cracking cycles. The co-reaction of ¹²C-ethene with ¹³C-dimethyl ether (DME) shows that the rate of DME conversion (1.62 mol C (mol Al s)^-1) is ～20 times faster than ethene conversion (0.08 mol C (mol Al s)^-1), suggesting that ethene can be considered as terminal product for MTH at 623 K. At iso-conversion conditions at 548 K, propene is co-fed with DME to increase propagation of the olefin-based cycle and correspondingly a 1.7-fold decrease in the ethene/2MB yield is observed. Similarly, the co-reaction of toluene with DME increases propagation of the aromatic-based cycle and a 2.1-fold increase in the ethene/2MB yield is observed. The ethene/2MB yield also increased by a factor of 2 as DME conversion increased from 5% to 62%, which is consistent with the observed concurrent increase in selectivity to ethene and methylbenzenes. For the reaction of DME alone, increasing the temperature from 548 K to 723 K increases the propagation of the olefin-based cycle and a corresponding decrease in the ethene/2MB yield from 4.7 to 1.3 is also observed. The ethene/2MB yield varies systematically with feed composition, conversion, and temperature, showing that this ratio describes the relative propagation of the aromatic to olefin methylation/cracking cycles in MTH conversion on H-ZSM-5.

Full text of DOI:10.1016/j.jcat.2013.03.021

Journal of Catalysis 303 (2013) 135–140
Contents lists available at SciVerse ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
A descriptor for the relative propagation of the aromatic- and olefin-based
cycles in methanol-to-hydrocarbons conversion on H-ZSM-5
Samia Ilias a, Rachit Khare a, Andre Malek b, Aditya Bhan a,
⇑
a Department of Chemical Engineering and Materials Science, University of Minnesota – Twin Cities, 421 Washington Avenue SE, Minneapolis, MN 55455, USA
b Feedstocks and Energy, The Dow Chemical Company, 1776 Building, Midland, MI 48674, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The observed product distribution in methanol-to-hydrocarbons (MTH) catalysis can be rationalized
based on the relative rates of propagation of the aromatic- and olefin-based cycles that operate on the
zeolite catalyst. We report that the ratio of ethene to 2-methylbutane + 2-methyl-2-butene (ethene/
2MB) yield can be used to describe the propagation of aromatic and olefin methylation/cracking cycles.
The co-reaction of 12C-ethene with 13C-dimethyl ether (DME) shows that the rate of DME conversion
(1.62 mol C (mol Al s)À1) is ꢀ20 times faster than ethene conversion (0.08 mol C (mol Al s)À1), suggesting
that ethene can be considered as terminal product for MTH at 623 K. At iso-conversion conditions at
548 K, propene is co-fed with DME to increase propagation of the olefin-based cycle and correspondingly
a 1.7-fold decrease in the ethene/2MB yield is observed. Similarly, the co-reaction of toluene with DME
increases propagation of the aromatic-based cycle and a 2.1-fold increase in the ethene/2MB yield is
observed. The ethene/2MB yield also increased by a factor of 2 as DME conversion increased from 5%
to 62%, which is consistent with the observed concurrent increase in selectivity to ethene and methyl-
benzenes. For the reaction of DME alone, increasing the temperature from 548 K to 723 K increases the
propagation of the olefin-based cycle and a corresponding decrease in the ethene/2MB yield from 4.7
to 1.3 is also observed. The ethene/2MB yield varies systematically with feed composition, conversion,
and temperature, showing that this ratio describes the relative propagation of the aromatic to olefin
methylation/cracking cycles in MTH conversion on H-ZSM-5.
Received 21 November 2012
Revised 15 February 2013
Accepted 26 March 2013
Keywords:
Methanol-to-hydrocarbons
Methanol-to-olefins
Zeolite
Hydrocarbon pool
Olefin and aromatic methylation
HZSM-5
Selectivity
Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction
sional 10-MR pores of H-ZSM-22, which are too small for aromatics
to be reactive for olefin elimination [8–11]. In these two zeolites,
Methanol-to-hydrocarbons conversion on acid zeolites pro-
ceeds through an indirect mechanism known as the ‘‘hydrocarbon
pool’’ mechanism in which olefins and aromatics act as scaffolds
for carbon–carbon bond formation [1–4]. As a result, two catalytic
cycles are at work: one in which olefins are repeatedly methylated
to form branched species which are susceptible to cracking and an-
other in which aromatics are repeatedly methylated and dealkylat-
ed to form light olefins (Scheme 1) [5,6]. The observed product
distribution of MTH can be rationalized as an effect of the relatve
propagation of these two cycles. For some zeolites, such as SAPO-
34 and ZSM-22, the product distribution can easily be understood
in this manner. The observed product selectivity of SAPO-34 is rich
in light olefins because small 8-MR windows hinder C5+ hydrocar-
bons from escaping the larger 12-MR cavities, resulting in the aro-
matic-based cycle dominating over the olefin-based cycle [1–3,7].
In contrast, the olefin-based cycle dominates in the one-dimen-
one cycle predominantly propagates over another; however, in
other zeolites, such as H-ZSM-5, both cycles contribute to the ob-
served product distribution and the product distribution is tunable,
showing that the aromatic- and olefin-based cycles can be propa-
gated to different extents depending on the reaction conditions
[12].
Kinetic studies for the different reactions depicted in Scheme 1
are sparse, thus far available only for C2–C4 olefin methylation and
benzene methylation [13–19]. Global kinetic models have also
been used to determine activation energies for aromatic
methylation, but are limited in that they do not always account
for secondary reactions and diffusion [20–26]. Other than compu-
tational studies, there have been few experimental studies on the
kinetics of aromatic dealkylation, olefin cracking, cyclization, and
hydrogen transfer. Understanding how these steps contribute to
the overall product distribution and which cycle is dominant on
the zeolite is critical for developing structure–function relation-
ships for MTH catalysis [27]. In the absence of experimental studies
detailing kinetic parameters for the various elementary steps in
MTH, we propose that the ratio of ethene to 2-methylbutane +
⇑
Corresponding author.
E-mail addresses: ilias003@umn.edu (S. Ilias), khare014@umn.edu (R. Khare),
AMalek@dow.com (A. Malek), abhan@umn.edu (A. Bhan).
0021-9517/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jcat.2013.03.021

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S. Ilias et al. / Journal of Catalysis 303 (2013) 135–140
+
CH3
H+
+
H+
CH3
H+
Higher
Olefins
+
CH3
alkanes
H+
+
CH3
+
CH3
+
CH3
H+
Olefin-based cycle
H+
Aromatic-based cycle
Scheme 1. Dual olefin and aromatic methylation/cracking cycles for methanol-to-hydrocarbons on H-ZSM-5.
2-methylbutene (ethene/2MB) yield can be used as a descriptor to
assess the relative propagation of the aromatic- and olefin-based
cycles and we show that this ratio varies systematically and pre-
dictably with olefin and aromatic co-feeds at iso-conversion. Previ-
ously, we reported that the selectivity of MTH could be
systematically tuned at iso-conversion using aromatic and olefin
co-feeds [12]. Co-processing propene with DME resulted in in-
creased propagation of the olefin-based cycle relative to the aro-
matic-based cycle, whereas toluene co-fed with DME resulted in
increased propagation of the aromatic-based cycle. In this work,
we show that the ethene/2MB yield is a valid descriptor for the rel-
ative propagation of these two cycles as a function of co-feed com-
position, reaction temperature, and DME conversion.
4–100 mg catalyst with DME at 70 kPa (Matheson Tri-Gas, 99.5%
purity; 0.033–0.448 cm3 sÀ1) with a mixture of CH4 and argon as
an internal standard (10% methane, 90% argon, Airgas; 0.031–
0.175 cm3 sÀ1) and a balance of helium (0–0.195 cm3 sÀ1) was used
if necessary to keep the concentration of internal standard compa-
rable to the concentration of the effluent products. In some reac-
tions, propene (50% propene, 50% argon, Praxair) and toluene
(99.9% purity, Sigma–Aldrich) co-reactants were fed so that the
olefin or aromatic co-feed partial pressure was 4 kPa (0.013–
0.026 cm3 sÀ1). Toluene was fed as a liquid using a Cole Parmer
EW-74900-00 syringe pump. The resulting DME space velocity var-
ied from 2.7 to 833 g DME (g catalyst h)À1. The total pressure of the
reactor was 130 kPa. Heat-traced lines (423 K) were used to trans-
fer toluene to the reactor and the reactor effluent to a gas chro-
matograph (Agilent 7890) equipped with
a
methyl siloxane
m) connected
2. Materials and methods
capillary column (HP-1, 50.0 m  320 m  0.52
l
l
to a flame ionization detector. Product distributions shown in Sec-
tion 3 include C8+ hydrocarbons that were not identified and are
grouped as ‘‘others.’’ The H/C stoichiometry listed for these species
in figure captions are determined based on the weighted difference
of the stoichiometry of converted feed and stoichiometry of known
C2–C7 hydrocarbon products.
2.1. Catalyst preparation
The catalyst, H-ZSM-5 (CBV8014), Si/Al = 42.6, was obtained in
the ammonium form from Zeolyst International. Structural and
chemical characterization of the commercial H-ZSM-5 sample used
in this study is reported in Section S.1 of the Supplemental Infor-
mation section. H-ZSM-5 with 17 lm crystals (Si/Al = 28) was also
2.3. Reactions using 13C-labeled DME with ethene over H-ZSM-5
used in this work. This material was synthesized according to the
procedure in Ref. [28], and the characterization of the material is
shown in the Supplementary Material section of Ref. [29]. The sil-
icon to aluminum ratio of the commercial H-ZSM-5 sample was
determined by ICP-OES elemental analysis (performed by Galbra-
ith Laboratories). The ammonium-form zeolite was sieved to
For the experiment using 13C-DME with 12C-ethene, the reac-
tion was run using 10 mg of catalyst at 623 K with 0.153 cm3 sÀ1
of DME (99% 13C-atom purity) and 0.017 cm3 sÀ1 of ethene (Math-
eson Tri-Gas, chemical purity grade) with a balance of CH4/Ar to
achieve a total flow rate of 0.278 cm3 sÀ1. Isotopologue distribu-
tions were determined from mass fragmentation patterns using
the method outlined by Price and Iglesia [30].
obtain aggregate particle sizes between 180 and 425 lm (40–
80 mesh) and treated in 1.67 cm3 sÀ1 of dry air (20–21% O2,
<10 ppm H2O, Minneapolis Oxygen) at 773 K for 4 h (heating rate
of 0.0167 K sÀ1) to convert it to the proton-form zeolite. The cata-
lyst was pre-treated in situ in 1.67 cm3 sÀ1 of helium flow (99.995%
purity, Minneapolis Oxygen) at 773 K overnight using a heating
rate of 0.0167 K sÀ1 prior to reaction.
3. Results and discussion
3.1. Reactivity of ethene, 2-methylbutane, and 2-methyl-2-butene in
MTH catalysis
2.2. Catalytic reactions of DME with and without co-feeds over H-
ZSM-5
Isotopic switching experiments at 598–623 K on H-ZSM-5, H-
BEA, and H-SAPO-34 show that the 13C-incorporation of ethene
closely matches that of methylbenzenes, indicating that ethene is
a product of the aromatic-based carbon pool [5–7,31–33]. Addi-
tionally, kinetic studies show that the rate of ethene methylation
is at least an order of magnitude slower than propene or butene
methylation, suggesting that ethene can be considered to be a ter-
mination product of the aromatic-based cycle [13,14,17–19]. To
confirm that ethene can be considered a termination product of
DME conversion to hydrocarbons on H-ZSM-5, 13C-DME
A stainless steel packed-bed reactor (0.25 in o.d.; 0.215 in i.d.)
equipped with a concentric thermal well (0.0625 in o.d.; 0.0485
in i.d.) aligned along the tube center was used for the conversion
of DME. The catalyst bed was supported between quartz wool
plugs and operated at isothermal conditions using an ARI heating
coil regulated by a Watlow Temperature Controller (96 Series).
Reactions were run at 548 K, 623 K, and 723 K. To achieve the de-
sired chemical conversions, reactions were performed using

S. Ilias et al. / Journal of Catalysis 303 (2013) 135–140
137
(70.2 kPa) was co-processed with 7.7 kPa of 12C-ethene. These re-
sults, summarized in Table 1, show that although some ethene is
converted when co-processed with DME, ethene reacts ꢀ20 times
slower than DME; hence, it can be considered termination product
of the aromatic-based cycle of MTH on H-ZSM-5.
Alkanes may be considered as termination products for the ole-
fin-based cycles. Olefins can form alkanes through hydrogen trans-
fer reactions, and in general, alkanes are less reactive than either
olefins or aromatics. The selectivity to both alkanes and olefins in
MTH is, however, dependent on the extent of hydrogen transfer.
Fig. 1 shows the extent of hydrogen transfer, measured by the ratio
of alkanes to alkenes, with a higher ratio indicating a greater extent
of hydrogen transfer, occurring for different MTH reaction condi-
tions used in this work. The extent of hydrogen transfer in MTH
is dependent on the conversion of DME, the reaction temperature,
and the identity of the aromatic or olefin co-feed. Therefore, an
individual alkane or olefin cannot be used as a representative of
the olefin-based cycle because the relative amount of olefins and
alkanes is dependent on hydrogen transfer, which is necessary
for the formation of aromatics. To eliminate the effect of hydrogen
transfer on a descriptor for the relative propagation of the aro-
matic-based cycle compared to the olefin-based cycle, we use the
sum of the yields of 2-methylbutane and 2-methyl-2-butene to
represent the propagation of the olefin-based cycle.
The alkane-to-alkene ratio increases with increasing chemical
conversion for the reaction of DME at 623 K showing that hydrogen
transfer increases with increasing conversion (Fig. 1b). As a result,
the selectivity to 2-methylbutane increases by a factor of 2.6 while
the selectivity to 2-methyl-2-butene decreases a factor of 2.6;
however, the sum of the two remains relatively constant as conver-
sion increases from 5% to 62% (Supplementary Information
Fig. S.2a). Similar trends are also seen with the effect of tempera-
ture on the alkane-to-alkene ratio for the reaction of DME on H-
ZSM-5 between 548 K and 723 K. As temperature increases, the al-
kane-to-alkene ratio decreases. The selectivity to 2-methyl butane
decreases by a factor of 3.1 while the selectivity to 2-methyl-2-bu-
tene increases a factor of 8.9 as temperature increases and the al-
kane-to-alkane ratio decreases (Supplementary Information
Fig. S.2b). The carbon selectivity to total 2MB, however, remains
relatively constant with the alkane-to-alkene ratio. The differing
trends observed for 2-methylbutane and 2-methyl-2-butene and
the relatively invariant selectivity to 2MB with varying tempera-
ture and chemical conversion further support our claim that both
an alkane and an olefin must be used to represent the propagation
of the olefin-based cycle.
The ratio of the yield of ethene to other C3–C5 hydrocarbons and
a detailed description of the composition of the C5 fraction for the
various reaction conditions used in this work is shown in Sec-
tion 2.3 of the Supplementary Material. Although other ratios, such
as ethene/(i-C4 + 1-butene) and ethene/propene, also follow the
same trends as ethene/2MB, branched C5 hydrocarbons were cho-
sen specifically because isotopic experiments in which methanol/
DME is co-processed with aromatics on H-ZSM-5 have shown that
propene in the effluent contains a significant fraction (10–42%) of
carbons originating from aromatics showing that the aromatic-
based cycle is one route for propene formation [12,34]. Addition-
ally, similar isotopic experiments on H-BEA show that isobutane
also contains aromatic carbons, suggesting that the aromatic-based
Fig. 1. The ratio of alkane to alkene C4 (j), C5 (s), and C6
(
) products on H-ZSM-5
(Si/Al = 42.5) for the reaction of 70 kPa DME (a) with and without 4 kPa of co-feed at
548 K and 17.9–18.6% DME conversion, (b) at 623 K and 5–62% conversion, and (c)
at 548–723 K and 59.4–60.7% DME conversion.
cycle is a route to i-C4 formation on H-BEA [35]. Although there is
no direct experimental evidence linking isobutene to the aromatic-
based cycle on H-ZSM-5, branched C5 hydrocarbons are used in
this work to represent the propagation of the olefin-based cycle be-
cause methanol conversion on H-ZSM-22, a catalyst in which the
Table 1
Feed rate, fractional conversion, and conversion rate for co-reaction of 70 kPa DME with 7.7 kPa of ethene 623 K.
Co-feed
Ethene
Feed rate (mol C (mol Al s)À1
)
Conversion (%)
DME
Conversion rate (mol C (mol Al s)À1
)
DME
4.09
Co-feed
0.44
Co-feed
19.1
DME
1.62
Co-feed
0.08
39.6

138
S. Ilias et al. / Journal of Catalysis 303 (2013) 135–140
Fig. 2. The effect of feed composition on (a) the product selectivity and (b) the ethene/2MB yield on a carbon basis for the reaction of 70 kPa DME (+4 kPa co-feed) at 548 K
and 17.9–18.6% DME conversion over H-ZSM-5 (Si/Al = 42.5); for (a) j C2, C3, C4–C7, methylbenzenes, h others. The H/C stoichiometry for the fraction labeled ‘‘others’’
is the following: 2.03 for DME + propene, 1.85 for DME, and 1.29 for DME + toluene.
olefin-based cycle dominates over the aromatic-based cycle, has a
much higher selectivity to branched C5 hydrocarbons compared to
branched C4 hydrocarbons [10].
system by ꢀ580%. The co-reaction of DME with toluene increases
the ethene/2MB yield by a factor of 3.2 compared to the reaction
of DME alone as a result of increased propagation of the aro-
matic-based cycle relative to the olefin-based cycle, which is sim-
ilar to the observation at 548 K shown in Fig. 2b. Similar trends in
selectivity and ethene/2MB yield for propene, toluene, and ethyl-
benzene co-feeds with DME are also observed at 723 K (Section S.4
of the Supplemental Information).
Co-feeding p-xylene with DME has similar effects on both the
product distribution (shown in Fig. S.7 of the Supplemental Infor-
mation) and the ethene/2MB yield as co-feeding toluene with
DME, with the ethene/2MB yield increasing by a factor of 3.2 com-
pared to the reaction of DME alone (Fig. 3). The similarity in eth-
ene/2MB yield and product distribution for the co-reaction of p-
xylene and toluene with DME shows that both aromatic co-feeds
influence the propagation of the aromatic-based cycle in a similar
manner – increasing the overall partial pressure of aromatics in the
system increases the propagation of the aromatic-based cycle rel-
ative to the olefin-based cycle.
3.2. Effect of olefin and aromatic co-feeds on ethene/2MB yield
Previously, we reported that at iso-conversion conditions at
548 K on H-ZSM-5, co-feeding 4 kPa propene with 70 kPa DME
propagates the olefin-based cycle evidenced by selectivity to C4–
C7 increasing 2.5-fold compared to when 4 kPa toluene is co-fed
with 70 kPa DME [12]. Co-feeding toluene increased the relative
propagation of the aromatic-based cycle and a corresponding in-
crease in ethene and methylbenzene selectivity was observed
[12]. Fig. 2 shows the effect of feed composition on the product
selectivity and the ethene/2MB yield (on a carbon basis) at iso-con-
version conditions at 548 K (17.9–18.6% DME). For these experi-
ments, propene and toluene feeds were completely labeled with
13C-atoms and any 13C3-propene or 13C7-toluene in the reaction
effluent is not included in the assessment of product selectivity.
Additionally, the isotopic distribution of propene and toluene in
the reaction effluent was used to determine chemical conversions
of the 13C3-propene and 13C7-toluene co-feed, which were 52.3%
and 40.3%, respectively [12]. The ethene/2MB yield decreases by
a factor of 1.9 when propene is co-processed with DME compared
to the conversion of DME alone as a result of increased propagation
of the olefin-based cycle (Fig. 2b). In contrast, the ratio increases by
a factor of 2.1 when toluene is co-processed with DME due to in-
creased propagation of the aromatic-based cycle (Fig. 2b). These
results showing systematic trends in the ethene/2MB yield support
our hypothesis that ethene/2MB yield can be used a descriptor for
the propagation of the aromatic-based cycle relative to the propa-
gation of the olefin-based cycle.
3.3. Effect of conversion on ethene/2MB yield
At 623 K, the product selectivity for reaction of DME alone over
H-ZSM-5 varied slightly for DME chemical conversions between 5%
Fig. 3 shows the effect of co-processing propene, toluene, and p-
xylene with DME at 623 K on selectivity and the ethene/2MB yield
at iso-conversion conditions (33.3–36.7% DME). The product distri-
butions for the results shown in Fig. 3 are shown in Fig. S.6 of the
Supplemental Information. The effect of co-processing propene
with DME on the ethene/2MB yield compared to DME alone was
relatively small, only a 26% decrease in the ratio was observed.
We postulate that this small change in ethene/2MB yield is a result
of the large fraction of olefins already produced when DME alone is
reacted at 623 K, with olefin pressure increasing by less than 50%
with the addition of a propene co-feed. In contrast, the addition
of toluene as a co-feed increases the amount of aromatics in the
Fig. 3. The effect of feed composition on the ethene/2MB yield on a carbon basis for
the reaction of 70 kPa DME (+4 kPa co-feed) at 623 K over H-ZSM-5 (Si/Al = 42.5).

S. Ilias et al. / Journal of Catalysis 303 (2013) 135–140
139
and 62% (Fig. 4a). Correspondingly, the ethene/2MB yield on a car-
bon basis varied slightly, between 0.9 and 2.1 (Fig. 4b). The small
increase in the ethene/2MB yield at 623 K as DME conversion in-
creases from 5% to 62% suggests that the rate of propagation for
the aromatic-based cycle increases slightly compared to the prop-
agation of the olefin-based cycle as conversion increases. The effect
C3–C7 hydrocarbons is also seen. These trends in selectivity show
that as temperature increases, the rate of propagation of the ole-
fin-based cycle increases more than the rate of propagation of
the aromatic-based cycle. The selectivity of MTH systematically
varies with reaction temperatures at iso-conversion conditions
which results in a monotonic decrease in the ethene/2MB yield
as shown in Fig. 5b.
of conversion was also examined for the reaction of DME on 17 lm
crystals of H-ZSM-5 at 623 K between 8.7% and 32.3% conversion.
Product selectivity for the large crystal H-ZSM-5 sample was nearly
invariant for the range of conversions studied (Fig. S.8a of the Sup-
plementary Information), and correspondingly, the ethene/2MB
yield (ꢀ4.1) was also relatively invariant (Fig. S.8b of the Supple-
mentary Information).
At 723 K, the selectivity to ethene and methylbenzenes is low
compared to that at lower temperatures (Fig. 5b). To further under-
stand if ethene selectivity is lower at higher temperatures at least
in part because of the lower concentration of methylbenzenes, tol-
uene was co-fed with DME at 723 K. At 72.6% DME conversion and
73.9% net toluene conversion, ethene selectivity increased to 14.7
C% and correspondingly, the ethene/2MB yield increased to 3.2,
compared to the reaction of DME alone at 723 K and 60% conver-
sion which results in 3.0% selectivity to ethene and an ethene/
2MB yield of 0.4. This increase in ethene selectivity shows that
one reason that selectivity of DME conversion to ethene is low at
high temperatures (ꢀ723 K) is due to the low concentration of
methylbenzenes, not necessarily because the rate of aromatic deal-
kylation is slower relative to olefin methylation and cracking rates.
The selectivity to methylbenzenes is low for the reaction of DME
3.4. Effect of temperature on ethene/2MB yield
Fig. 5a shows the selectivity of the reaction of DME only over H-
ZSM-5 at 548 K, 623 K, and 723 K at iso-conversion conditions
(59.4–60.7% DME conversion). At these conditions, the selectivity
to C2 hydrocarbons (>99% of which is ethene), methylbenzenes,
and ‘‘others’’ systematically decreases with increasing tempera-
ture. With increasing temperature, an increase in selectivity to
Fig. 4. The effect of fractional conversion of DME on (a) product selectivity and (b) ethene/2MB yield on a carbon basis for the reaction of 70 kPa DME at 623 K over H-ZSM-5
(Si/Al = 42.5); for (a) j C2, C3, C4–C7, methylbenzenes, e others (H/C stoichiometry of 1.87–1.96).
Fig. 5. The effect of temperature on (a) product selectivity and (b) ethene/2MB yield on a carbon basis for the reaction of 70 kPa DME over H-ZSM-5 (Si/Al = 42.5); for (a) j C2,
C3, C4–C7, methylbenzenes, h others. The H/C stoichiometry for the fraction labeled ‘‘others’’ is the following: 1.85 at 548 K, 1.88 at 623 K, and 1.77 at 723 K.

140
S. Ilias et al. / Journal of Catalysis 303 (2013) 135–140
alone at 723 K because methylbenzene formation, which requires
olefins to undergo both cyclization and dehydrogenation reactions,
occurs to a lesser extent at higher temperatures, suggesting that at
higher temperatures, olefins prefer other pathways, such as meth-
ylation and cracking.
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The ethene/2MB yield is used to describe the relative propaga-
tion of the aromatic- and olefin-based cycles in MTH catalysis on
H-ZSM-5, where a high ethene/2MB yield indicates increased prop-
agation of the aromatic-based cycle and low ethene/2MB yield
indicates increased propagation of the olefin-based cycle. Isotopic
experiments of 13C-DME with 12C-ethene show that the reaction
rate of DME conversion is ꢀ20 times faster than ethene. At iso-con-
version conditions at 548 K, propene and toluene are co-fed with
DME to increase propagation of the olefin- and aromatic-based cy-
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factor of 2.1 with the addition of toluene. The ethene/2MB yield in-
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62%) at 623 K on commercial H-ZSM-5, showing that aromatic-
based cycle propagates more relative to the olefin-based at higher
conversions. For the reaction of DME alone, a monotonic increase
in selectivity to C3–C7 hydrocarbons and monotonic decrease in
selectivity to methylbenzenes and ethene with increasing reaction
temperature from 548 K to 723 K is observed, showing that selec-
tivity can be tuned at iso-conversion conditions and that at higher
temperatures, the olefin-based cycle dominates over the aromatic-
based cycle resulting in lower ethene/2MB yield. The ethene/2MB
yield varies systematically with varying feed composition and tem-
perature, showing that the relative rates of propagation of the aro-
matic- and olefin-based cycles in MTH catalysis on H-ZSM-5 can be
described by this ratio.
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Acknowledgments
The authors acknowledge financial support from Dow Chemical
Company and the National Science Foundation (CBET 1055846).
The authors also acknowledge Professor Dongxia Liu, University
of Maryland, for synthesis and characterization of H-ZSM-5 with
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Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jcat.2013.03.021.