On the impact of co-feeding aromatics and olefins for the methanol-to-olefins reaction on HZSM-5

Article abstract of DOI:10.1016/j.jcat.2014.03.013

The impact of adding various aromatic molecules (benzene, toluene, and xylenes) or olefins (ethene, propene, 1-butene, 1-pentene, and 1-hexene) to methanol over a HZSM-5 catalyst on activity and selectivity was systematically studied. Addition of a low concentration of aromatic molecules (16-32 C%), which are free of diffusion constraints, significantly enhanced the aromatics-based catalytic cycle and greatly suppressed the olefin-based cycle. This led to enhanced methane and ethene formation and methylation of aromatic rings at the expense of propene and C₄₊ higher olefins. The ratio of propene to ethene is controlled by the concentration of the aromatic molecules added. Co-feeding the same molar concentration of benzene, toluene and p-xylene influenced the methanol conversion to a nearly identical extent, as none of them experience transport constraints and the methylation rapidly equilibrates the aromatic molecules retained in the pores. In stark contrast, addition of small concentrations (10-40 C%) of C_3-6 olefins with 100 C% methanol does not selectively suppress the catalytic cycle based on aromatic molecules. This led to unchanged selectivities to ethene and higher olefins (C₃₊). Within the C₃₊ fraction, the selectivity to propene decreased and the selectivity to butenes were enhanced with increasing concentration of the co-fed olefin. Because of the relatively fast rates in methylation and cracking of C_3-6 olefins in the olefin-based cycle, the product distributions at high methanol conversion were identical when co-feeding C_3-6 olefins with the same carbon concentrations. This work provides further insights into the two distinct catalytic cycles operating for the methanol conversion to produce ethene and propene over HZSM-5 catalysts.

Full text of DOI:10.1016/j.jcat.2014.03.013

Journal of Catalysis 314 (2014) 21–31
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
On the impact of co-feeding aromatics and olefins for the
methanol-to-olefins reaction on HZSM-5
Xianyong Sun, Sebastian Mueller, Hui Shi, Gary L. Haller, Maricruz Sanchez-Sanchez, Andre C. van Veen,
⇑
Johannes A. Lercher
Department of Chemistry and Catalysis Research Center, Technische Universität München, Lichtenbergstrasse 4, D-85747 Garching, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
The impact of adding various aromatic molecules (benzene, toluene, and xylenes) or olefins (ethene, pro-
pene, 1-butene, 1-pentene, and 1-hexene) to methanol over a HZSM-5 catalyst on activity and selectivity
was systematically studied. Addition of a low concentration of aromatic molecules (16–32 C%), which are
free of diffusion constraints, significantly enhanced the aromatics-based catalytic cycle and greatly
suppressed the olefin-based cycle. This led to enhanced methane and ethene formation and methylation
of aromatic rings at the expense of propene and C4+ higher olefins. The ratio of propene to ethene is con-
trolled by the concentration of the aromatic molecules added. Co-feeding the same molar concentration
of benzene, toluene and p-xylene influenced the methanol conversion to a nearly identical extent, as none
of them experience transport constraints and the methylation rapidly equilibrates the aromatic mole-
cules retained in the pores. In stark contrast, addition of small concentrations (10–40 C%) of C3–6 olefins
with 100 C% methanol does not selectively suppress the catalytic cycle based on aromatic molecules. This
led to unchanged selectivities to ethene and higher olefins (C3+). Within the C3+ fraction, the selectivity to
propene decreased and the selectivity to butenes were enhanced with increasing concentration of the
co-fed olefin. Because of the relatively fast rates in methylation and cracking of C3–6 olefins in the
olefin-based cycle, the product distributions at high methanol conversion were identical when co-feeding
C3–6 olefins with the same carbon concentrations. This work provides further insights into the two
distinct catalytic cycles operating for the methanol conversion to produce ethene and propene over
HZSM-5 catalysts.
Received 20 December 2013
Revised 19 March 2014
Accepted 26 March 2014
Available online 19 April 2014
Keywords:
Methanol-to-olefins
Hydrocarbon pool
Selectivity
Co-feeding
Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction
for many of these commercialized processes has remained limited
since their inception, and substantial recycling is required.
Methanol-to-hydrocarbon processes using microporous zeolites
or zeo-type catalysts are regarded as a vital family of conversion
technologies to bypass petroleum-based routes for the production
of specific fuels and platform petrochemicals [1–6]. Methanol can
be readily produced by proven technologies via synthesis gas,
which in turn is generated by reforming carbon resources including
coal, natural gas, and biomass. On the other hand, special
significance of the methanol chemistry originates from its versatil-
ity of enabling selective transformations toward various products
by proper choice of catalysts and reaction conditions [1–6]. Suc-
cessfully implemented processes include Methanol-To-Gasoline
[7,8], Methanol-To-Propene (both based on HZSM-5) [9–12], and
Methanol-To-Olefins (based on SAPO-34, producing both ethene
and propene) [13–15]. However, the typical single-pass selectivity
Fundamental insights into the reaction mechanism play a vital
role in achieving selectivity control. Ever since the first report by
Chang and Silvestri [16], three decades of considerable experimen-
tal and computational research efforts have been dedicated to
unraveling the complex reaction mechanism. Instead of the direct
mechanisms, which involve the initial C–C bond formation directly
from C1 entities, the indirect ‘‘hydrocarbon pool’’ mechanism
[17–19] is generally accepted for explaining the formation of light
(C2ÀC4) olefins from methanol during steady-state operation. In
the original ‘‘hydrocarbon pool’’ concept, the active site has been
proposed to be located in the pores or cages of a microporous solid,
comprised of an hydrocarbon species (organic part) and a proxi-
mate Brønsted acid site (inorganic part) [17–19]. It is described to
act as a virtual scaffold for the assembly of light olefins and avoids
unstable and high-energy intermediate species required for the
direct CÀC coupling mechanisms [17–23]. In a proposed cycle,
methanol successively reacts with the hydrocarbon species via
⇑
Corresponding author. Fax: +49 89 28913544.
E-mail address: johannes.lercher@ch.tum.de (J.A. Lercher).
http://dx.doi.org/10.1016/j.jcat.2014.03.013
0021-9517/Ó 2014 Elsevier Inc. All rights reserved.

22
X. Sun et al. / Journal of Catalysis 314 (2014) 21–31
methylation, and subsequently elimination of light olefin products
such as ethene and propene regenerates the initial hydrocarbon
species [17–23].
distributions by co-processing low concentrations of toluene and/
or propene with dimethylether, but the experiments were mainly
performed at reaction temperatures as low as 548 K and a dimethyl
ether pressure of 70 kPa [40]. To simulate industrial process
conditions, experiments were performed with methanol pressure
fixed at 10 kPa on a highly siliceous HZSM5 catalyst at 723 K.
Various aromatic co-feeds including benzene, toluene and xylenes,
and olefins including ethene, propene, 1-butene, 1-pentene, and
1-hexene were evaluated.
Recent experimental and theoretical work demonstrated that
focusing on polymethylbenzenes as the sole active species would
cause a biased understanding on the MTO mechanism, and olefins
may act as another kind of active ‘‘hydrocarbon pool’’ species in
zeolites such as the medium-pore ZSM-5 zeolite with 3-D 10-ring
channels, while aromatic intermediates seem to be kinetically-rel-
evant for catalysts with large pores or voids [24,25]. This leads to
the proposal and establishment of the ‘‘dual-cycle’’ mechanism
[24,25], as shown in Scheme 1. Thus, considering that both aromat-
ics and olefins exist in the zeolite pores, the corresponding olefin-
and methylbenzene-mediated routes operate on a competing
basis. Taking advantage of the different activities and selectivities
of olefin- and aromatics-populated cycles toward ethene and pro-
pene formation, it has been hypothesized that one could optimize
the product distribution through selectively propagating or sup-
pressing one of the two (aromatics- and olefin-based) catalytic
cycles.
Three potential strategies can be conceived for achieving selec-
tivity control. Given that turnover of the aromatics-based cycle
demands generally a larger space for the transition states than
the olefin-based cycle, one approach is to adjust the pores by vary-
ing zeolite topologies [26,27]. Indeed, very recent experiments on
methanol conversion over the one-dimensional 10-MR H-ZSM-22
zeolite without intersections showed that the sterically restricted
topology suppressed selectively the reactions via the aromatics-
based cycle and secondary aromatization via hydrogen transfer
which would require larger transition states and reaction interme-
diates [28–31]. Thus, methanol conversion at 673 K proceeded
exclusively via the olefin-based cycle, leading to a product mixture
rich in C3+ branched alkenes, very low in ethene and almost negli-
gible in aromatics [28–31]. The second strategy is to tune the inor-
ganic part, i.e., the Brønsted acidity, through zeolite synthesis or
post-synthetic anion or cation modifications, which have been doc-
umented in a large body of literature [2]. The third approach for
selective propagation of a catalytic cycle is to influence the organic
part, i.e., the concentration of olefin or aromatic species, by adding
specific hydrocarbons together with methanol.
2. Experimental
2.1. Catalyst and reagents
The specific synthesis method of the HZSM-5 (Si/Al = 90) was
reported previously [42]. The as-synthesized material has a crystal
size of 500 nm. The zeolite powder was pressed into a wafer,
crushed, and sieved to a fraction of particle size in the range of
200–280 lm. Methanol (99.93%), 1-hexene, 1-heptene, benzene,
toluene, para- and meta-xylenes (99.0%) were supplied by
Sigma–Aldrich. Gases of C2–5 olefins (5% or 10% in volume diluted
in N2) were supplied by Westfalen GmbH.
2.2. Catalytic testing
All catalytic tests were performed on a bench-scale plug flow
reaction unit. The catalyst pellets were homogeneously diluted
with silicon carbide (ESK-SIC, 1:15 wt:wt) with a comparable par-
ticle size to ensure temperature uniformity. Catalysts were placed
in a quartz tube (26 cm in length, 6.0 mm i.d.) and supported
between two quartz wool plugs. The samples were activated at
753 K with the temperature control at the external surface of the
quartz tube with 50 ml minÀ1 N2 for 2 h prior to switching to feed.
The reaction temperature was held at 723 K, and the total pressure
was 108 kPa. The methanol partial pressure was maintained at
10 kPa. The total flow rate was held at 55 ml minÀ1. Methanol
vapor was fed by passing dry N2 flow (29 ml minÀ1) through the
methanol-containing saturator which was thermo-stated at
298 K. Flow rates of gaseous olefin co-feeds (C2–5) were controlled
by mass flow controllers (Bronkhorst). For aromatics, 1-hexene or
1-heptene, the co-fed vapor was introduced by passing dry N2 flow
through a saturator containing the liquid reactant. Catalyst loading
(2–100 mg) and reactant flow velocity were varied to achieve a
wide range of contact time and methanol conversion. Here the con-
tact time is defined as the ratio of catalyst mass to the molar flow
rate of methanol. The reactor effluent was kept at 393 K and trans-
ferred via a heated line into a gas chromatograph (HP 5890)
In this contribution, we explore this third approach by varying
the nature and concentration of the co-processed hydrocarbons,
to adjust the product selectivity under industrially relevant
reaction conditions. Several reports on co-reacting hydrocarbons
with methanol including various olefins and aromatics have
appeared, but their main intentions were to elucidate the mecha-
nistic features via isotopic labeling under conditions far away from
realistic MTO(P) operations, and the impacts of co-feeding on
product distributions is largely lacking [32–41]. Most recently, Ilias
and Bhan reported in an elegant paper the impact on the product
equipped with a HP PLOT-Q column (30 m  0.32 mm  0.5
lm)
connected to a flame ionization detector for on-line analysis. Prod-
uct analysis was performed at steady-state conversions.
Both methanol and dimethyl ether were treated as reactants.
The concentration of a co-feed is given as a molar ratio of its partial
pressure to methanol partial pressure (10 kPa). The product distri-
butions (concentration and yield) were given on a carbon basis,
and the carbon in the methanol feed with a partial pressure of
10 kPa was defined as 100%. For instance, a feed of 0.4 kPa toluene
and 10 kPa methanol is depicted as co-feeding 4 mol.% toluene. As
one toluene molecule has seven carbon atoms, 28% toluene with
100% methanol in carbon, the feed was referred to as containing
a total carbon concentration of 128% (28 C% from toluene with
100 C% from methanol) in the feed.
In the experiments of methanol conversion with aromatic co-
feeds, the final aromatics increment after reaction is defined as
the aromatics concentration (in C%) from which the initial concen-
tration of the aromatics co-feed (in C%) is subtracted.
Scheme 1. Proposed ‘‘dual-cycle’’ mechanism in methanol-to-olefins conversion
over HZSM-5 [24,25].

X. Sun et al. / Journal of Catalysis 314 (2014) 21–31
23
The concentrations of propene and C5 hydrocarbons reached a
maximum approximately at the contact time when methanol con-
version attained 100%, followed by a slight decline with increasing
contact times (Fig. 1a). Ethene, light paraffins (C1–4), and aromatics
were formed with low concentrations (ca. 8 C% in total), all
increasing with contact time. This is generally in line with the sim-
plified pathway shown on Scheme 2.
3. Results and discussion
3.1. Reaction pathway of methanol-to-olefins conversion over HZSM-5
catalysts
A detailed view on MTO conversion and product yields as a
function of contact time over HZSM-5 at 723 K and methanol
pressure of 10 kPa, is depicted in Fig. 1a. Methanol conversion
leads to a wide variety of products including methane, ethene,
propene, butenes, C5 hydrocarbons, C6+ aliphatics, other light par-
affins (C2–4), and aromatics. Hereby, the C5 fraction designates all
hydrocarbons with five carbon atoms, and the C6+ aliphatics
includes all other heavier hydrocarbons other than aromatics.
Light paraffins include methane, ethane, propane, n-butane and
isobutane, while aromatics include benzene, toluene, xylenes,
trimethylbenzenes, and tetramethylbenzenes. Hydrogen transfer
(HT) products include aromatics and C2–4 light paraffins. In the
absence of an added reactant, the effect of contact time on meth-
anol conversion is characterized by an initiation phase preceding
the on-set of methanol conversion. At the initiation phase, meth-
anol is dehydrated to dimethyl ether (DME) until thermodynamic
equilibrium is established. Only when
hydrocarbon species had been built up, the methanol conversion
started up at a critical contact time (0.03 min kgcat mol
methanol conversion showed
complete within a narrow range of contact times. The typical
S-shaped curve, characteristic of an autocatalytic phenomenon,
indicates that some of the products provide new reaction routes
of methanol conversion [43].
However, two product groups, C6+ aliphatics and ethene,
deserve further attention. The concentrations of C6+ aliphatics
peaked when the methanol conversion was ca. 80%, then decreased
with further increasing methanol conversion and finally leveled off
at even longer contact times (Fig. 1a). Considering that methanol
inhibits olefin adsorption, even in low concentrations (e.g., until
only 20% remaining) [41], we attribute the formation of C6+, partic-
ularly at lower contact times, mainly to the methylation of C3–5
olefins, and not to olefin oligomerization. Noted that there was
only a slight increase in the yields of hydrogen transfer products
with longer contact time, it is concluded that further conversion
of these higher olefins is mainly due to the formation of C3–5 light
olefins by cracking rather than ring closure and aromatization. This
behavior indicates that olefin methylation to higher olefins (mainly
C=6 and C=7) and cracking of these higher olefins are two critically
important steps in the reaction network, and C6+ olefins act as reac-
tion intermediates [44,45]. Very different from C3+ olefins, ethene
formation followed a similar trend to that of aromatics with
increasing contact time (Fig. 1a), suggesting that ethene formation
has a mechanistic relationship with the formation of aromatic
molecules [25]. These observations are generally in line with the
current dual-cycle mechanism over HZSM-5.
a certain amount of
À1
). The
MeOH
a
self-acceleration and was
Fig. 1. Conversion/concentration profiles as a function of contact time for feeds of pure methanol (a) and methanol with 4 mol.% p-xylene (b) over HZSM-5 catalyst at 723 K.
Product concentrations were calculated on the carbon-number basis through defining the carbon concentration in methanol as 100 C%. Hereby the aromatics increment was
calculated by subtracting the product concentration (in C%) by 32 C% (the initial carbon in 4 mol.% p-xylene which contained eight carbon atoms).

24
X. Sun et al. / Journal of Catalysis 314 (2014) 21–31
Scheme 2. A simplified reaction pathway of methanol conversion over HZSM-5 catalysts.
3.2. Impact of aromatics co-feeding on the methanol conversion
To investigate the influence of specific aromatic co-feeds on the
methanol conversion, experiments were performed with several
aromatic molecules at 723 K and a methanol partial pressure of
10 kPa. The aim was to explore whether co-feeding a low concen-
tration of aromatic molecules would selectively propagate the aro-
matics-based cycle and if so, to study the effect of co-feeding
aromatics on modulating the product distribution. First, the impact
of co-feeding different concentrations (molar ratios of aromatics to
methanol being 2 or 4 to 100% methanol) of p-xylene is shown, fol-
lowed by comparison with that of co-feeding m-xylene. Then, the
effect of co-feeding lower substituted aromatics, toluene and ben-
zene, was examined.
3.2.1. Impact of co-feeding p- or m-xylene
Co-processing of methanol with xylenes was carried out at reac-
tion conditions similar to the experiments in the absence of co-
feed. The feed was composed of methanol and p-xylene in a molar
ratio of 50 or 25 (2 or 4 mol.% of p-xylene). In contrast to most pre-
vious reports [41,38,46], low molar ratios of co-feeds to methanol
were used in this study for several reasons, i.e., (i) to mimic the
realistic on-site conditions of a MTP operation; (ii) to avoid meth-
ylation of aromatics rather than methanol conversion as the main
catalytic reaction; and (iii) to keep the initial catalyst surface cov-
ered mainly by methanol or intermediates derived from it rather
than by the co-fed aromatics.
The initiation phase for methanol conversion was dramatically
shortened when p-xylene (2 mol.%) was co-fed. However, the S-
shaped curve still remained. Further increase in p-xylene concen-
tration from 2 to 4 mol.% led to insignificant enhancement in
methanol conversion at any contact time (Fig. 2). If aromatic spe-
cies would act as classic components in autocatalysis kinetics,
higher concentrations of an intermediate product should result in
progressively higher conversion rates. Thus, we conclude that p-
xylene cannot be the main product responsible for autocatalysis
under the conditions of these experiments.
Fig. 2. Impact of p-xylene co-feeding on methanol conversion as a function of
contact time. Methanol pressure was 10 kPa and reaction temperature was 723 K.
The partial pressure of p-xylene was 0, 0.2, 0.4 kPa, leading to a ratio of p-xylene to
methanol being 0, 2 or 4 to 100 (0%, 2% or 4%).
The yields of ethene, aromatics and methane increased, while
those of propene and higher olefins decreased, with increasing
concentration of p-xylene in the feed (Fig. 3). In particular, the
yields of ethene and aromatics increased by ca. 4 times, from 3.5
to 14 C% and from 0.8 to 3.6 C%, respectively, when the feed chan-
ged from pure methanol to 4 mol.% p-xylene-containing methanol.
The effect of contact time on the product concentrations, with a
mixed feed of methanol and p-xylene in a molar ratio of 25, is
shown in Fig. 1b. At methanol conversion of 8.4%, the main prod-
ucts were ethene and propene, the selectivity being 39% and 36%,
respectively, and the other products were mainly butene isomers
(10%) and C5–7 olefins (4% and 6% each). At higher methanol con-
versions, the propene concentration drastically increased and sur-
passed that of ethene by a factor of ca. 3 at the highest contact
time. As shown in Fig. 1b, while ethene concentration nearly lev-
eled off at a methanol conversion of 70%, the propene concentra-
tion continued to increase. Concurrently, butenes and C5–7 olefins
evolved in a manner similar to that observed with pure methanol.
During the increase in methanol conversion from 70% to 100%, 30%
carbon in methanol, together with 10% carbon in C6–7 aliphatics,
was converted to C3–5 olefins (Fig. 1b). This indicates that although
the aromatics-based cycle contributes significantly to methanol
conversion and ethene formation, the olefin-based cycle, which
contributes to the formation of propene and higher olefins, is not
completely suppressed by co-feeding p-xylene.
Fig. 3. Influence of the p-xylene addition on the product distribution in terms of
methane (a), ethene and propene (b), C=4, C5, and C6+ aliphatics (c), C2–À41paraffins and
aromatics increment (d) at a contact time of 0.23 min kgcat molMeOH. Reaction
temperature was 723 K, methanol partial pressure 10 kPa and partial pressures for
p-xylenes 0, 0.2 or 0.4 kPa.

X. Sun et al. / Journal of Catalysis 314 (2014) 21–31
25
These observations, along with the high fractions of trimethylbenz-
enes and tetramethylbenzenes in aromatics (Fig. S1), demon-
strated that co-feeding p-xylene propagates the aromatics-based
cycle via aromatics methylation and elimination of methane and
light olefins (predominantly ethene and propene). On the other
hand, the olefin-based cycle was suppressed when p-xylene was
co-fed, as shown by the lower yields of propene and C4–7 olefins.
The total yield of ethene and propene slightly increased with
increasing p-xylene concentration in the feed (Fig. 3b). Therefore,
by varying the concentration of co-fed p-xylene, the ratio of pro-
pene to ethene can be varied without loss of selectivity to total
light olefins, and both propene-targeted and light-olefins-targeted
production can be realized without changing the reaction temper-
ature and methanol pressure. As depicted in Fig. 4a, the yields of
C2–4 paraffins, formed via hydrogen transfer reactions of olefins,
were comparable between pure methanol and feeds containing
p-xylene.
In a typical hydrogen transfer reaction, the paraffin and aromat-
ics molecules are produced with a stoichiometric ratio of three to
one during hydrogen transfer between olefins (Scheme 3, route
1). As a result, aromatics produced from such routes should have
been observed with at most one third of the yield of C2–4 paraffins.
As the actual fraction of aromatics formed was significantly higher
than C2–4 paraffin yield when co-feeding p-xylene (Fig. 4b), the aro-
matics surplus must come from the incorporation of carbon in
methylating C1 species derived from methanol (Scheme 3, route
2). This interpretation is further supported by the detailed analysis
on the carbon distribution in aromatics increment, shown in
Fig. S2. While the fraction of carbon in the aromatic ring
(Fig. S2a) is comparable for the feeds with p-xylene co-fed or with
pure methanol, co-feeding p-xylene enhances the aromatics meth-
ylation, leading to the significantly increased carbon concentration
in the side chains.
methanol conversion became detectable was shortened by about
50% (Fig. 5a). Compared to the values observed for pure methanol
feed, ethene yield was promoted from 3.5 to 5.1 C%, while the pro-
pene yield decreases slightly from 49.9 to 49.0 C% when 4 mol.% m-
xylene was co-fed (Fig. 5c). These effects were much less signifi-
cant compared to those imposed by co-feeding of p-xylene.
The main reason is attributed to steric constraints. The dimen-
sion of the pore size of MFI zeolite is about 0.6 nm, while the
molecular size for p- and m-xylene is 0.58 and 0.68 nm, respec-
tively. The diffusion rate for m-xylene to access the zeolite
Brønsted acid sites is lower than that of p-xylene. Therefore, the
enhancement by m-xylene on the aromatics-based cycle is less
prominent than that of p-xylene. This is also in line with the incre-
ment of aromatics concentration (Fig. 5d). The aromatics incre-
ment (2 C%) in the feed containing m-xylene was significantly
lower than the increase (3.1 C%) with the feed containing p-xylene,
indicating that m-xylene faced diffusional constraints and con-
straints of being methylated, a pre-requisite for the aromatics-
based cycle to initiate and turnover.
3.2.2. Impact of toluene and benzene co-feeding on methanol
conversion
Having shown that the aromatics-based cycle operates over
HZSM-5 and can be selectively propagated through co-feeding
low concentrations of xylenes and that the olefin-based cycle
can be suppressed, the question arises how other aromatic
molecules, such as toluene and benzene, influence activity and
selectivity.
At a molar methanol-to-aromatics of 25, methanol conversion
changed with increasing contact time in a manner independent of
the nature of co-fed aromatics (Fig. 6). After full methanol
conversion was reached, ethene and propene yields from
a
toluene-containing feed were slightly lower than those from a
reactant stream containing p-xylene. Co-feeding with benzene led
to even lower yields to ethene and propene, i.e., 12.8% and 40.4%,
When m-xylene was used as the co-feed with a methanol-to-
xylene molar ratio of 25, the critical contact time after which
Fig. 4. Impact of co-feeding different concentrations (2 or 4 mol.%) of p-xylene on the yield of C2–4 paraffins (a), aromatics increment (b), ethene (c) and propene (d). Reaction
temperature was 723 K, methanol partial pressure 10 kPa and partial pressures for xylenes 0, 0.2 or 0.4 kPa.

26
X. Sun et al. / Journal of Catalysis 314 (2014) 21–31
Scheme 3. Two representative routes for aromatics increment.
Fig. 5. Impact of m-xylene co-feeding on methanol conversion (a), ethene yield (b), propene yield (c) and aromatics increment (d). Pure methanol feed and p-xylene co-
feeding were included as references. Reaction temperature was 723 K, methanol partial pressure 10 kPa and partial pressures for xylenes 0.4 kPa.
respectively. Compared to the feed containing p-xylene, the
toluene-containing feed gave rise to significantly larger increase
in the aromatic products (6.0 C% vs. 3.1 C%), while yielding a compa-
rable amount of C2–4 light paraffins. This is attributed to a higher
degree of methylation of toluene than of p-xylene. In turn, the aro-
matics increment when co-feeding benzene was ca. 4% higher than
that from toluene, indicating that all the co-fed 4% benzene was
methylated. As more methanol was consumed in methylation of
the aromatic ring with benzene and toluene than with p-xylene, a
slightly lower concentration of ethene and propene was observed
than with p-xylene. A similar trend was also observed from compar-
ison of the impact of co-feeding 2 mol.% benzene, toluene, and p-
xylene, as shown in Fig. S3.
3.3. Impact of co-feeding olefins on methanol conversion
Analogous to the experiments carried out with various aromat-
ics, the impact of co-feeding a specific olefin on methanol conver-
sion has been investigated. Experiments were performed with
various olefins at 723 K and methanol partial pressure of 10 kPa.
The aim was to study whether co-feeding a certain concentration
of olefins could selectively propagate the olefin-based cycle and
could impact the product distribution. First, the impact of co-feed-
ing different concentrations of 1-pentene (molar ratios of metha-
nol-to-olefin were 50, 25 and 12.5) was shown, followed by
comparison of the impact of the size of the alkene (ethene, pro-
pene, 1-butene, 1-hexene).
In consequence, we conclude that the relative selectivity to eth-
ene and propene can be modulated by recycling a suitable fraction
of aromatics products. As co-feeding of benzene, toluene and
xylenes essentially influences the methanol conversion in an iden-
tical way, only the ratio between the aromatic molecules and
methanol controls this selectivity.
3.3.1. Impact of 1-pentene co-feeding on the methanol conversion
Co-feeding olefins is another conceived potential strategy for
tuning product selectivities. 1-pentene was co-fed with methanol
in a molar methanol-to-1-pentene ratio of 50, 25 and 12.5. In anal-
ogy with the addition of aromatics, co-feeding 1-pentene led to a

X. Sun et al. / Journal of Catalysis 314 (2014) 21–31
27
Fig. 6. Impact of co-feeding 4 mol.% toluene or benzene on methanol conversion (a), propene yield (b), ethene yield (c), the yield of C2–4 paraffins (d, open symbols), and
concentration increment of aromatics (d, closed symbols). Pure methanol and 4% p-xylene were shown as references.
shorter initiation phase (Fig. 7). In stark contrast to the case of aro-
matics co-feeding, however, further increase in 1-pentene concen-
tration from 10 to 40 C% in the feed led to higher rates. Therefore,
we conclude that olefins, rather than aromatic molecules act as the
crucial species enhancing the conversion rate. The S-shaped curve
characteristic of autocatalysis was observed with 8% co-fed 1-pen-
tene. It is interesting to note that at very high concentrations of
olefin addition (a ratio of one on a molar basis), methanol conver-
sion followed a pseudo first-order kinetics instead of a sigmoid
curve [41]. Under the conditions of this latter study, methanol
tends to be converted via methylation of the co-fed pentene, when
the molar concentration of olefin is at least comparable to the
methanol concentration. In this case, the conversion of methanol
followed a first-order rate law, because methylation of 1-pentene
had higher rate constant than the methylation of any other lower
olefins [41]. When only a low concentration of pentene was co-
fed with methanol, most methanol molecules would undergo auto-
catalytic MTO reaction, rather than direct methylation of the co-fed
pentene [4].
Fig. 8 depicts the evolution of product concentrations with
increasing contact time for the feed containing 1-pentene
(20 C%). At the shortest contact time, 1-pentene (and likely its iso-
mers) underwent a rapid methylation, evidenced by the rapid
increase in the concentration of C6+. The product distribution at
the shortest contact time studied is listed in Table 1. From these
Fig. 8. Concentration of various hydrocarbons as a function of contact time for the
feed of methanol with 20 C% from 1-pentene. Reaction temperature was 723 K,
methanol partial pressure 10 kPa, 1-pentene co-feed partial pressure 0.4 kPa. The
horizontal dashed line indicated the concentration of initial 20 C% from 1-pentene.
The vertical dashed line indicated the contact time at which the 100% conversion of
methanol was reached.
Fig. 7. Impact of the concentration of co-fed 1-pentene on the methanol conver-
sion. Reaction temperature was 723 K, methanol partial pressure 10 kPa, 1-pentene
partial pressure 0.2, 0.4, 0.8 kPa, respectively.

28
X. Sun et al. / Journal of Catalysis 314 (2014) 21–31
Table 1
Conversions and product distributions at low contact times, i.e., <0.01 (min kgcat molMeOH). Reaction temperature was 723 K, methanol partial pressure 10 kPa, co-feeding 20 C%
from propene, 1-butene, 1-pentene or 1-hexene. The flow rate was kept identical at 55 ml minÀ1
.
Co-feed (20 C%)
Catalyst amount (mg)
Methanol conversiona
Hydrocarbons (C%)
HT products
C1
C2=
C3=
C4=
2.52
14.4
0.32
6.69
C5
0.82
4.16
17.8
4.56
C6
C7+
C3=
2.5
2.5
1
1.9
4.1
1.5
0.04
0.06
0.04
0.09
0.01
0.04
0.02
0.21
17.4
1.38
0.37
8.80
0.63
2.89
2.22
8.29
0.45
1.09
0.43
0.93
1-C=4
1-C=5
1-C=6
0.05
0.17
5
13.9
a
The lowest conversion for each co-feeding experiment.
experiments, we derive that at differential conditions, the conver-
sion rate of methanol was estimated to be twice the rate of pentene
disappearance. This suggests that pentenes are not all methylated
by methanol in a 1:1 ratio, and that multiple methylation events of
pentenes to heptenes likely occur. Alternatively, this can be
ascribed to the fact that a part of the methanol is not consumed
in methylating pentenes; this fraction converts to olefins, as evi-
denced by the formation of C2–4 olefins at such a short contact time
(Table 1).
After a certain contact time, the pentene concentration started
to increase, and the following trend for each group was similar to
that observed for the conversion of methanol alone (Fig. 1a), indi-
cating that co-feeding 1-pentene does not alter the dominant reac-
tion pathway. At a certain contact time, the yield of a specific
product or group of products, e.g., propene, from methanol with
co-fed pentene was higher than that from pure methanol (Fig. 8
compared with Fig. 1a), as 1-pentene was fully converted via meth-
ylation and cracking.
Fig. 9 shows the comparison of the product distribution for the
feeds without or with various concentrations of 1-penteneÀc1o-feed
at the same methanol contact time of 0.23 min kgcat molMeOH. In
stark contrast to the addition of aromatic molecules, co-feeding
10–40 C% 1-pentene significantly reduced the formation of meth-
ane, which is concluded to have formed mainly via demethylation
of polymethylaromatics. For pure methanol or methanol with aro-
matics, the initial phase of the methanol reaction is dominated by
the aromatics-based cycle, leading to methane formation. In the
presence of pentene, the olefin-based cycle dominates the
conversion pathway, in which b-scission necessarily leads to insig-
nificant rates of methane formation. On the other hand, co-feeding
Fig. 9. Influence of the concentration of co-fed 1-pentene on the total product distriÀb1ution. Reaction temperature was 723 K, methanol partial pressure 10 kPa, 1-pentene co-
feed partial pressure 0.2, 0.4, 0.8 kPa, respectively, contact time 0.23 min kgcat molMeOH
.

X. Sun et al. / Journal of Catalysis 314 (2014) 21–31
29
1-pentene showed little impact on the selectivities to ethene and
2–4 paraffins resulting from hydrogen transfer. In marked contrast
are in turn more active than butenes and propene in methylation
and, thus, have a high tendency to be methylated further to hex-
enes. At the prevalent reaction conditions, hexenes are much more
reactive than butenes and pentenes in cracking. In consequence
only some hexene desorbed as product, some molecules were
cracked to propene, and some further methylated to heptene,
which either desorbs (minor) as product or undergoes cracking
(major) to form propene and butenes. Cracking of hexenes and
heptenes was relatively fast. Therefore, the carbon chain growth
practically terminated at heptenes.
The fast methylation and cracking of C3+ higher olefins results in
extensive scrambling of the co-fed carbon and, thus, the final prod-
uct distribution is dominated by the concentration of added car-
bon, irrespective of the nature of the co-fed olefin. When C3+
olefins were co-fed with 40 C% concentration with respective to
methanol (100 C%), the product distribution also remained
unchanged at methanol conversions >60% for any C3–6 olefin frac-
tion (Fig. S5). Accordingly, there is practically no need to co-feed a
specific olefin fraction, except for ethene (vide infra). As shown in
Figs. 8, S5 and 10, the yield of propene was improved, e.g., from
50% to 60%, by co-feeding C3–6 olefin with methanol, but the total
C-based selectivity to propene was either indifferent or slightly
lower by olefin co-feeding (Fig. 9).
Compared to the co-feeding of C3–6 olefins, co-feeding ethene
led to a different impact on methanol conversion with increasing
contact time (Fig. 11). The reactivity of ethene in methylation
has been reported to be at least one order of magnitude lower than
the C3+ olefins. The low activity of ethene has led, therefore, to a
slower conversion of methanol compared to the addition of other
higher olefins. It also indicates a slow incorporation and scram-
bling of the carbon of ethene into the other higher products.
The fact that the product distribution was independent from the
nature of olefin added is a further evidence that the olefin-based
cycle is the dominant reaction pathway over HZSM-5 under the
studied reaction conditions (at higher conversion of methanol).
However, addition of olefins favors hydrogen transfer and the
C
to the case of co-feeding aromatic molecules, the addition of low
concentrations of 1-pentene neither suppressed the aromatics-
based cycle nor promoted significantly the olefin-based cycle. The
main consequence of co-feeding 1-pentene was the decrease in
the selectivity to propene and the increase in the selectivity to
butenes (Fig. 9).
This impact can also be demonstrated with Fig. S4. Compared to
the pure methanol feed, the feed with 10 C% from 1-pentene pro-
duced 0.3 C% more ethene, 5.1 C% more propene and 3.4 C% more
butenes (in total 8.8 C% to C2–C4 olefins). On the other hand, the
yield of C5 fraction was 1.2 C% higher for the feed with 1-pentene.
Considering that 10 C% was co-fed, the net impact is that 8.8% of
the co-fed C5 was transformed into C2–4 olefins, while the selectiv-
ities to other products from methanol conversion remained almost
unchanged.
3.3.2. Impact of the nature of co-fed olefins on the methanol
conversion
Fig. 10 shows the methanol conversion and product yields as a
function of contact time for a methanol-olefin mixture, which con-
tained 100% C from methanol and an equal amount of 20% C from
one of the C3–6 1-olefins, i.e., propene, 1-butene, 1-pentene, or 1-
hexene. At methanol conversions below 50%, the nature of co-fed
olefin impacted methanol conversion and product distribution.
As methylation of the co-fed Cn olefins was the main reaction
(Table 1), the Cn+1 olefins were the main products except for the
case of 1-hexene.
At methanol conversion higher than 70%, the product distribu-
tion no longer depended on the nature of the olefin co-feeds
(Fig. 10). The methylation rate coefficients for C3+ olefins increase
in order of carbon chain length [6]. Taking the propene co-feeding
as an example, the propene molecules are methylated to form
butenes, which have higher methylation reactivities than propene.
This results in further methylation of butenes to pentenes, which
Fig. 10. Reaction path for the conversion of 100% methanol with 20% carbon from propene (blue), 1-butene (red), 1-pentene (green), 1-hexene (purple), in terms of
conversion and propene (a), butenes and ethene (b), C5 and HT products (c), and C6+ (d). Reaction temperature was 723 K, methanol partial pressure 10 kPa. Partial pressures
for propene, 1-butene, 1-pentene or 1-hexene were 0.67, 0.5, 0.4, 0.33 kPa, respectively, leading to an equivalent 20% co-fed carbon in each feed mixture. (For interpretation of
the references to color in this figure legend, the reader is referred to the web version of this article.)

30
X. Sun et al. / Journal of Catalysis 314 (2014) 21–31
Fig. 11. Comparison of reaction path for 100% methanol with 20% carbon from ethene (green) and 1-hexene (red), in terms of methanol conversion and propene (a), butenes
and C5 (b), ethene and HT products (c), and C6+ (d). Reaction temperature was 723 K. Methanol partial pressure was 10 kPa, and partial pressures for ethene and 1-hexene
were 1 and 0.33 kPa, respectively, leading to the equivalent 20 C% in each feed mixture. (For interpretation of the references to color in this figure legend, the reader is referred
to the web version of this article.)
formation of aromatics. In turn, the aromatic molecules based
route remains active, as shown by the methanol conversion rates
in Section 3.3.1. The overall consequence is, therefore, that co-feed-
ing olefins does not selectively propagate the olefin-based cycle. In
other words, the ratio of activities of aromatics and olefin-based
cycles is not significantly changed by co-feeding small amount of
olefins compared to the case of a pure methanol feed.
methane, ethene formation and aromatics methylation at the
expense of propene and C4+ higher olefins. The ratio of propene
to ethene can be easily controlled by modulating the concentration
of the aromatics co-feeds. Co-feeding the same molar concentra-
tion of benzene toluene and p-xylene influences the methanol con-
version. The aromatic molecules with
a lower degree of
substitution become rapidly methylated to form the same active
carbon species than xylenes.
We note, however, that the results of impact of co-feeding
olefins demonstrated in Fig. 9 is in marked contrast with the
recent work by Ilias and Bhan [40], who report that co-process-
ing propene significantly propagates the olefin-based cycle. In
this case, results were reported at a reaction temperature of as
low as 548 K and the comparison was made at a DME conver-
sion of around 20% [40]. These conditions are highly different
from the realistic industrial MTO(P) operations (reaction temper-
ature higher than 723 K and methanol/DME conversion nearly
100%) [9–12] and the reaction conditions adopted in the present
work. Under such lower reaction temperature and low methanol/
DME conversion, methylation was much more favored than
cracking. This led these authors to observe a remarkable increase
in the selectivity of C4–C7 aliphatics when co-processing propene
[40]. However, as showed in the present work, in a complete
reaction pathway, these methylated higher olefins would
undergo rapid cracking at higher methanol conversion facilitated
by the usually higher reaction temperature. This essentially leads
to the scrambling and re-distribution of the co-fed carbon,
instead of selectively promoting the olefin-based cycle, as shown
in Figs. 9 and S5.
In stark contrast, co-feeding small concentrations (10–40 C%) of
C3–6 olefins with 100 C% methanol does not selectively suppress
the aromatics-based cycle, resulting in unchanged selectivities to
ethene and higher olefins (C3+). Within the C3+ fraction, propene
selectivity decreases and the selectivity to butenes is enhanced
with increasing concentration of the co-fed olefin. Due to the rela-
tively fast methylation rate of C3–6 olefins, the same product distri-
bution at 100% methanol conversion and an identical impact were
observed when co-feeding C3–6 olefins.
The present work provides insights into the practical MTO(P)
applications, such as Lurgi’s MTP process[6–9,38]. In this process,
Methanol/DME is introduced to the HZSM-5 catalyst bed loaded
in an adiabatic fix-bed reactor [9–12]. After fractionalization of
the reactor effluent, the alkene products other than propene, such
as C=2 and C4–C6 aliphatics, are recycled for further conversion to
maximize propene production [9–12,41]. Globally, this process
converts about 65% carbon to propene, balanced by LNG and gaso-
line (mainly aromatics).
Based on the results shown here, the MTP process has a great
potential to be tuned from maximum propene production into pro-
ducing both ethene and propene to meet the fluctuating market
demands. Conceptually, this could be achieved through selective
recycling of the on-site produced aromatic by-products without
changing the reactor configuration or the catalyst. One should note,
however, that this approach also promotes to some degree the for-
mation of undesired methane.
4. Conclusions
Under reaction conditions close to those of the industrial MTP
process operation, co-feeding a small concentration of aromatic
molecules (16–32 C%), which are free of diffusion constraints,
significantly propagates the aromatics-based catalytic cycle and
greatly suppresses the olefin-based cycle, leading to higher
On the other hand, adding C4–C6 olefins leads to the product dis-
tribution independent of the nature of olefinic co-feeds. This indi-
cates that a more fractionated recycling of olefins would not

X. Sun et al. / Journal of Catalysis 314 (2014) 21–31
31
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Recycling of the higher olefin fraction with methanol yields,
however, more propene than the single pass methanol conversion,
because the majority of the carbon in the recycle reacts and is
re-distributed into propene through fast olefin methylation and
cracking.
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Acknowledgments
The financial support from Clariant Produkte (Deutschland)
GmbH and fruitful discussion within the framework of MuniCat
is gratefully acknowledged.
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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.2014.03.013.
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