Conversion of methanol into light olefins over ZSM-5 zeolite: Strategy to enhance propene selectivity

Article abstract of DOI:10.1016/j.apcata.2012.09.025

Four ZSM-5 zeolite materials with varying crystal size, acid site density and morphology were prepared, characterized by BET, XRD, SEM, FT-IR, H/D exchange and n-hexane cracking experiments and tested as catalysts for the Methanol to Olefins (MTO) reaction at 350 °C, WHSV = 1.8 g/gh under atmospheric pressure. Optimal propene to ethene ratios (>5) were obtained over a material prepared via the fluoride route. Its superior selectivity was tentatively ascribed to its low density of strong acid sites in combination with a long diffusion pathway and few crystal defects.

Full text of DOI:10.1016/j.apcata.2012.09.025

Applied Catalysis A: General 447–448 (2012) 178–185
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Conversion of methanol into light olefins over ZSM-5 zeolite: Strategy
to enhance propene selectivity
Francesca Lønstad Blekena, Sachin Chavana, Unni Olsbyea,∗, Marilyne Boltzb,
Fabien Ocampob, Benoit Louisb,∗∗
a inGAP Center of Research Based Innovation, Department of Chemistry, University of Oslo, P.O. Box 1033, Blindern, 0315 Oslo, Norway
b Laboratoire de Synthèse, Réactivité Organiques et Catalyse, Institut de Chimie, Université de Strasbourg, UMR 7177 CNRS, 4 rue Blaise Pascal, 67000
Strasbourg Cedex, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Four ZSM-5 zeolite materials with varying crystal size, acid site density and morphology were prepared,
characterized by BET, XRD, SEM, FT-IR, H/D exchange and n-hexane cracking experiments and tested
as catalysts for the Methanol to Olefins (MTO) reaction at 350 ◦C, WHSV = 1.8 g/gh under atmospheric
pressure.
Optimal propene to ethene ratios (>5) were obtained over a material prepared via the fluoride route.
Its superior selectivity was tentatively ascribed to its low density of strong acid sites in combination with
a long diffusion pathway and few crystal defects.
Received 10 August 2012
Received in revised form
11 September 2012
Accepted 14 September 2012
Available online 8 October 2012
Keywords:
Propene
© 2012 Elsevier B.V. All rights reserved.
MTO
ZSM-5 zeolite
Light olefins
Fluoride route
1. Introduction
undertaken [16–23]. In addition, the increasing market worldwide
demand toward ethylene and propylene is gradually raising and
The rational design of heterogeneous catalysts to optimize activ-
ity and selectivity in a targeted reaction often remains a key
challenge in state-of-the-art catalysis research [1–3]. In this con-
text, zeolites occupy a prominent place since they are used in
numerous acid-catalyzed reactions [4,5].
Discovered in Mobil laboratories, the transformation of
methanol-to-hydrocarbons (MTH) to make high-octane gasoline
[6,7], represents a timely and valuable process involving zeolite
catalysts. Indeed, the MTH technology has received considerable
industrial interest [8–10]. For instance, the Mobil Oil methanol to
gasoline (MTG) process [11], the Topsøe integrated gasoline syn-
thesis (TIGAS) process [12], the Lurgi methanol to propene (MTP)
process [13] and the Norsk Hydro/UOP’s methanol to olefins (MTO)
process [14], have either been commercialized or are ready for
commercialization [15].
projected growth rates in coming decades are expected to remain
high [24,25]. SAPO-34 silico-aluminophosphate, having the CHA
structure, is recognized as a valuable catalyst to generate a high
selectivity to light olefins due to its moderate acid strength and
small pore opening [22]. However, a high rate of deactivation is
usually observed for this kind of material due to the rapid coke
deposition [26,27]. ZSM-5 zeolite catalysts are therefore often used
despite usually lower olefin yield [28–31].
Several strategies targeted to achieve an increasing selec-
tivity toward light olefins over these MFI catalysts have been
tempted: modification of the reaction conditions, raising the steric
constraints to increase shape selectivity, change in the reactor
configuration [32], or proper tailoring of the Brønsted acidity (den-
sity and strength) [29,33]. Moreover, in order to limit/inhibit the
consecutive formation of higher alkenes, alkanes and aromatics,
the development of new catalyst design at the mesoscopic and
macro-scale have also been explored [28–30]. Nevertheless, the
preparation of such catalysts remains quite complex and costly.
During the past decade, the exploration of ethane-rich shale
gas in the US has made ethane crackers economically favorable
over naphta crackers, and led to the construction of ethane crack-
ers as well as retrofitting of naphta crackers [34]. Naphta cracker
co-products such as propene are therefore in increasingly short
supply, and tuning of MTH selectivity toward propene production
The MTO reaction is considered to be a valuable option for the
valorization of stranded gas reserves, and therefore several studies
devoted either to the reaction mechanism or to the technology were
∗
Corresponding author. Tel.: +47 22855457.
Corresponding author. Tel.: +33 368851344.
E-mail addresses: unni.olsbye@kjemi.uio.no (U. Olsbye),
∗∗
blouis@unistra.fr (B. Louis).
0926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2012.09.025

F.L. Bleken et al. / Applied Catalysis A: General 447–448 (2012) 178–185
179
becomes increasingly attractive. Only few studies have yet been
devoted to the optimization of propene yields in the MTH reac-
tion over ZSM-5 catalysts. Chang et al. explored the interaction
between catalyst Si/Al ratio, temperature and pressure in MTH reac-
tion over ZSM-5 zeolite [35]. Two SiO2/Al2O3 ratios were tested, i.e.,
35 and 1670, while intermediate Brønsted acid site densities were
obtained by partial H+–Na+ cationic exchange. At 370 ◦C and full
conversion, the C2–C4 olefins selectivity increased from 30% to 46%
for SiO2/Al2O3 = 35 and 1670, respectively. Increasing the temper-
ature to 500 ◦C led to higher olefins selectivities; with a maximum
of 38% propene selectivity and a propene to ethene ratio of 4.9 at
full methanol conversion for the SiO2/Al2O3 = 1670 catalyst. Lower
Si/Al ratios led to higher ethene selectivities and lower propene
selectivities, e.g. 10.5 and 22.6%, respectively, for SiO2/Al2O3 = 35
[35].
classical alkaline route was prepared as a reference material and
named ZSM-5R. In addition, the former procedure was slightly
modified to introduce mesoporosity by allowing the growth of MFI
crystals on a structured glass monolith [40,41]. This material was
named ZSM-5MS (mesoporous and structured). To further inves-
tigate the incidence of the length of diffusion within the crystal,
we have prepared MFI nanocrystals and thus called this sample
ZSM-5N (for nano). Finally, very large prismatic ZSM-5 crystals
were prepared in fluoride medium, named ZSM-5F (fluoride), for
comparison.
The influence of these modifications on the catalytic perfor-
mance in the MTH reaction was investigated in terms of degree of
methanol conversion and selectivity toward propene and ethene.
2.2. Preparation of the different MFI catalysts
In line with those results, Prinz and Riekert tested ZSM-5 cata-
lysts with varying Si/Al ratios (34–660) or crystal sizes (0.5–8 ␮m)
for the MTO reaction at 290–360 ◦C in a batch reactor, and reported
that increasing Si/Al ratios led to increasing light olefins selectivi-
ties and propene-to-ethene ratios. A similar effect was reported for
decreasing crystal sizes (0.5 versus 2 × 2 × 6 micron-sized crystals);
however, it is worth noting that for conversions exceeding 80%, the
selectivity differences due to crystal size vanished [36]. Recently,
Hua et al. reported that mesopore formation by desilication of ZSM-
5 zeolite with Si/Al ratio 72 led to enhanced propene selectivities;
up to 42% with a propene to ethene ratio of 10 at full methanol
conversion at 470 ◦C. The authors ascribed the selectivity increase
to a shorter diffusion pathway [37].
Zhao et al. studied the effect of adding ZrO2 and H3PO4 to an
H-ZSM-5 catalyst with Si/Al ratio of 84, and obtained a propene
to ethene ratio of 10 (45% propene selectivity) at 450 ◦C and full
DME conversion. After 30 h on stream, the propene to ethene
ratio increased to 16, due to lower ethene selectivity, exhibiting
still full conversion and 45% propene selectivity [38]. The authors
ascribed the propene yield improvements to the modification of
acid strength toward lower values while the number of acid sites
was increased. A slight reduction of pore width was also observed.
Later, Liu et al. tested ZSM-5 samples with Si/Al ratios ranging
from 12 to 360 at 460 ◦C, and observed a gradual increase in propene
selectivity from 16% to 52%, respectively, at full methanol con-
version [39]. Concurrently, the propene to ethene ratio increased
from 2.0 to 6.5. Screening tests of the Si/Al = 220 sample after
modification with a large variety of promoter elements showed
that phosphorus gave the highest propene selectivity, 56%, at full
methanol conversion, with a propene to ethene ratio of 7. Aromat-
ics production was almost fully suppressed (<1% selectivity) after P
modification. The P/ZSM-5 sample was thoroughly characterized. It
was found that the main change induced by P doping was a reduc-
tion of the number and strength of acid sites in the catalyst, while
the pore size was maintained or slightly increased [39].
2.2.1. Synthesis of ZSM-5 under alkaline conditions, ZSM-5R
(reference)
ZSM-5 zeolite was prepared following the procedure previously
reported [42]. This method consisted of adding at room tempera-
ture sodium aluminate (52.5 wt.% NaAlO2, Riedel-de Haën), sodium
chloride (99.5 wt.% NaCl, Fluka), tetrapropylammonium hydroxide
(20 wt.% in water, TPA-OH, Fluka) in demineralized water. After
20 min, tetraethylorthosilicate (TEOS, 98%, Sigma–Aldrich) was
introduced under vigorous stirring. The molar ratio was set as fol-
lows: TPA-OH:TEOS:NaCl:NaAlO2:H2O = 2.16:5.62:3.43:0.13:1000.
Aging and homogenization of the mixture were performed during
2 h. Subsequently, the gel was introduced in a 70 mL Teflon-lined
autoclave and the hydrothermal treatment was performed at 170 ◦C
during 48 h. After filtration, washing with demineralized water, and
drying at 110 ◦C, a white powder was obtained. These synthesis
steps were followed by calcination over night at 500 ◦C in air to
remove the structure directing agent.
2.2.2. Synthesis of large ZSM-5 crystals in fluoride medium,
ZSM-5F
The synthesis gel was prepared by adding, at room temperature,
2.07 g of aerosil 200 and 70 mg of glass monolith (used as a co-
silica source) [43] in distilled water (30 mL). The gel was left 15 min
under vigorous stirring to homogenize the system. Then, 0.65 g
of tetrapropylammonium bromide (TPA-Br), 0.028 g of NaAlO2, an
additional 20 mL of distilled water and 1.44 g of ammonium fluoride
(NH4F) were introduced into the mixture. Few drops of hydroflu-
oric acid were then added to reach a pH value of about 6. Once the
gel had formed, aging and homogenization of the mixture were per-
formed during 2 h. The gel was then placed in a 75 mL Teflon-lined
autoclave. The temperature was increased within 1 h to 170 ◦C. The
synthesis time was set to 22 h. The catalyst was recovered by fil-
tration, washed with distilled water and dried at 100 ◦C for 2 h. The
NH4-zeolite form was obtained and calcined at 450 ◦C for 5 h in air
to obtain the zeolite H-form.
In the present study, we investigate the influence of Brønsted
acid site density and also the crystal size in the performance in the
MTH reaction. The principle aim is to raise and then maintain a high
selectivity toward propene, and therefore develop a simple strategy
to design a new generation of MTP (methanol-to-propene) catalyst
exhibiting the MFI structure.
2.2.3. Synthesis of ZSM-5 nanocrystals, ZSM-5N
The synthesis mixture was prepared by first mixing deionized
water with cationic template solution (TPAOH, 1 M, Sigma–Aldrich)
under stirring for 10 min. Sodium hydroxide (NaOH, 98%, Fluka)
and sodium aluminate (NaAlO2, Riedel de Häen) were then
added and the solution was again stirred for 15 min. The silica
source, TEOS (98%, Fluka), was then introduced dropwise under
vigorous stirring. The mole ratio was as follows: NaAlO2:TPA-
OH:TEOS:NaOH:H2O = 1:56:23:14:7714, respectively. Afterwards,
300 mg of sacrificial sugar cane bagasse [44] was added to the
gel and the mixture was aged for another 2 h under vigorous stir-
ring. It was then transferred into an autoclave and the synthesis
was performed during 48 h at 167 ◦C under autogeneous pressure.
After the hydrothermal synthesis, the solid was filtered off, rinsed
2. Experimental
2.1. Methodology for the catalysts preparation: molecular and
microscopic design
Four different MFI zeolites were synthesized via different routes
in order to vary several factors: Brønsted acidity, Si/Al ratio, crys-
tal size and internal porosity. One ZSM-5 zeolite prepared via the

180
F.L. Bleken et al. / Applied Catalysis A: General 447–448 (2012) 178–185
with distilled water and dried in an oven at 100 ◦C for 15 h. The
amine-derived organic template and the lignocellulosic biomass
were removed by treating the material under air atmosphere at
450 ◦C for 5 h.
by Aga. In order to compare the IR band intensities, spectra were
normalized to the overtone mode at 2005 cm−1
.
2.4. n-Hexane cracking procedure
2.2.4. Synthesis of structured ZSM-5, ZSM-5MS
The number of strong Brønsted acid sites of as-prepared zeo-
lites was evaluated in a conventional n-hexane cracking reaction
at 500 ◦C. The catalytic cracking of n-hexane (99.89% pure, water
0.005 wt.%) was performed in a high throughput unit as already
detailed elsewhere [47]. A flow of 60 ml/min of 11% (v/v) n-
hexane in nitrogen was used. A typical run was carried out with
0.1–0.03 g of catalyst (the amounts of catalyst were adjusted in
order to provide iso-conversion). Reaction products were analyzed
on-line after three different times on stream (3, 17 and 32 min)
by gas chromatography using a Shimadzu GC-2010 equipped with
a Chrompack KCl/Al2O3 column operated under isothermal con-
ditional (303 K) and nitrogen flow (2 ml/min). Catalytic activities
were estimated at degrees of conversion between 9 and 11%, as the
average of results obtained after 17 and 32 min time-on-stream.
These values differed by less than 10% (relative) in activity (each
catalyst were analyzed three times). The catalysts showed a mod-
erate to low deactivation with an activity at 3 min on time on
stream differing less than 15% (relative), than the one measured
after 32 min.
The zeolite was prepared according to our earlier proce-
dure [42]. The same recipe as described in Section 2.2.1 was
followed, thus adding at room temperature sodium aluminate,
sodium chloride, tetrapropylammonium hydroxide in distilled
water. Afterwards, TEOS as a silica source was introduced under
vigorous stirring. Likewise, once the gel has formed, aging and
homogenization of the mixture were performed during 2 h. Then,
the stirring was stopped and 0.180 g of mesoporous glass monoliths
(approximately 30–35 mg per piece) were placed into the synthesis
mixture for 30 min. The gel was then poured in a 75 mL Teflon-lined
autoclave containing the support packing. The temperature was
increased within 1 h to 170 ◦C. The synthesis duration was 24 h. The
packing was recovered upon filtration, washed with distilled water
and dried at 100 ◦C for 2 h. Then, the composite was kept in an ultra-
sonic bath (45 kHz) for 20 min to remove weakly attached crystals
and dried at 100 ◦C for 2 h. Finally, the monolith was calcined at
450 ◦C in air over night to remove the template.
2.2.5. Ion-exchange of ZSM-5 samples
All ZSM-5 samples obtained in the Na-form were ion-exchanged
and calcined. Ion exchange was repeated three times, using 20 mL
1.0 M NH4NO3 solution per gram of catalyst. The solution was kept
at 75 ◦C for 2 h before separation of catalyst and liquid by cen-
trifugation. The catalyst was dried overnight in an oven at 100 ◦C
before calcination in static air during 6 h at 550 ◦C, subsequently to
a heating ramp of 5 ◦C/min.
2.5. Catalysts evaluation in the methanol to hydrocarbons (MTH)
reaction
The zeolites were pressed into wafers and subsequently crushed
and sieved to obtain particles in the range 250–420 ␮m. Each cat-
alytic test was performed with 60 mg of catalyst in a fixed bed
quartz reactor configuration, with 3.0 mm inner diameter. 30 mL He
was passed through a saturation evaporator containing methanol
(BDH Laboratory Supplies, >99.8% chemical purity) kept at 0 ◦C,
before being fed to the reactor, thus giving a WHSV (weight hourly
space velocity) of 1.8 g methanol/gzeolite/h. The product stream was
analyzed with an Agilent 6890 A gas chromatograph with FID detec-
tor and automatic sampling (Supelco SPB-5 capillary column, 60 m,
0.530 mm internal diameter, stationary phase thickness 3 ␮m). The
pressure of the carrier gas was 42 kPa and the temperature was
programmed to 5 min at 45 ◦C before heating (25 ◦C/min to 260 ◦C).
The catalyst temperature was adjusted to 350 ◦C before onset of
methanol for all tests, and once the reaction had started the oven
temperature was not further modified. The test was stopped when
the catalyst had deactivated or after 48 h on stream.
Additional catalytic tests for sample ZSM-5F were performed by
varying catalyst mass (5–60 mg) while all other parameters were
kept constant as described above. WHSV was varied between 1.8
and 21 g methanol/gzeolite/h.
The activity of the catalysts is expressed in terms of methanol
conversion, calculated from the difference between inlet and outlet
concentrations of methanol (and dimethyl ether, DME, which was
considered as a reactant). The Q factor for reaction (1):
2.3. Characterization of the catalysts
BET surface areas (SBET) of the ZSM-5 samples were determined
by N2 adsorption–desorption mesurements performed at −196 ◦C
employing the BET-method (Micromeritics sorptometer Tri Star
3000). Prior to nitrogen adsorption, the samples were outgassed at
350 ◦C for 4 h in order to remove moisture adsorbed at the surface
and inside the porous network.
X-ray diffraction (XRD) was used to assess the fingerprint of as-
prepared zeolite structure. The different patterns were compared
with those from the JCPDS tables. Patterns were recorded on a
Bruker D8 Advance diffractometer, with a Ni detec◦tor side filtered
˚
Cu K␣ radiation (1.5406 A) over a 2ꢀ range of 5–60 and a position
sensitive detector using a step size of 0.02◦ and a step time of 2 s.
Scanning Electron Microscopy (SEM) micrographs were
recorded on a JEOL FEG 6700F microscope working at 9 kV accel-
erating voltage. The Si/Al ratios of the materials were determined
by energy dispersive X-ray analysis (EDX) coupled with the SEM
chamber.
An evaluation of the Brønsted acid site density of the samples
was performed following an H/D isotope exchange method devel-
oped by Louis et al. [45,46].
2CH3OH ↔ (CH3)2O + H2O
(1)
Infrared spectroscopy (FTIR) spectra were recorded in con-
trolled atmosphere at 2 cm−1 resolution on a Bruker Vertex 80
FTIR spectrophotometer, equipped with a MCT detector and using
a homemade IR cell. FTIR spectra were collected in transmission
mode on a thin film prepared by deposition of zeolite water suspen-
sion on a silicon wafer. The difference in the Brønsted acid strength
of these samples was derived from CO adsorption. Prior to the CO
adsorption, sampleswere pretreatedfor 1 h at 500 ◦C in vacuumand
CO adsorption–desorption was followed at liquid N2 temperature
(i.e. −196 ◦C). Carbon monoxide (99.97% purity) used was supplied
Q = P[(CH3)2O]·P(H2O)/P[(CH3OH)]2
where P(i) represents the partial pressure of compound i, was cal-
culated and found to be constant under all conditions (except at
close to full methanol conversion where the measurement uncer-
tainty is high), thus indicating that reaction (1) was at equilibrium
and that there was no bypass of reactant gas in any experiment.
The Q value curves are shown in Supporting information, Figs. S1
and S2.

F.L. Bleken et al. / Applied Catalysis A: General 447–448 (2012) 178–185
181
Fig. 1. SEM micrographs of as-prepared zeolites ZSM-5R (a); ZSM-5F (b); ZSM-5N (c) and ZSM-5MS (d).
Product selectivity was defined as the mole ratio of each prod-
uct (on C1 basis) referred to the moles of converted methanol.
The following products were detected during reaction: light olefins
(ethene, propene), higher alkenes, alkanes (C5–C8) and aromatics.
nanometer-sized mesopores [43]. EDX analysis allowed to estimate
the Si/Al ratio of the different zeolites (Table 1).
H/D isotope exchange was performed to evaluate the number of
Brønsted acid sites present in the different materials. It appears that
the number of Brønsted acid sites diminished among the zeolites
in the order: ZSM-5R > ZSM-5N > ZSM-5F > ZSM-5MS. It is notewor-
thy that the fluoride-mediated route allowed a decrease in the
total number of Brønsted acid sites when compared to MFI cat-
alyst prepared under alkaline conditions. Such influence of the
preparation method on the density of Brønsted acid sites for ZSM-
5 and [F]ZSM-5 zeolites has already been reported in an earlier
study [48]. In parallel to this loss in the number of Brønsted acid
sites, the Si/Al ratio was enhanced as expected. The ZSM-5MS sam-
ple exhibited a low number of acid sites, 0.26 mmol/g; however
it is important to remember that this structured composite con-
tained only 34 wt.% of zeolite crystals coated on a glass monolith.
The catalytic activity of these zeolites was evaluated in the n-
hexane cracking reaction and nearly the same rate was achieved
over ZSM-5R and ZSM-5F zeolites, 120 and 107 mmoln-hexane con-
verted/g min, respectively, in spite of the much higher Al content
in the ZSM-5R material. This result is further discussed below.
Zeolite nanocrystals (ZSM-5N) led to an improved reactivity for
n-hexane cracking, thus confirming the higher effectiveness of
nanocrystals (183 mmoln-C6 converted/g min) compared to their
micrometer counterparts [49–51]. Interestingly, ZSM-5MS zeolite
3. Results
3.1. Characterization of the zeolites
The XRD patterns of each zeolite sample confirmed the MFI
structure [42–44]. Fig. 1 shows the SEM images of the four cat-
alysts. ZSM-5R and ZSM-5MS exhibited the classical coffin-shape
morphology (Fig. 1a and d) along with crystal sizes of 5 and 2 ␮m,
respectively. The material prepared via the fluoride-mediated route
led to the formation of large prismatic crystals having ∼15–20 ␮m
in size (Fig. 1b). In contrast, ZSM-5N catalyst consisted of rectan-
gular nanocrystals, 0.05–0.10 ␮m in size, which assembled into a
spherical superstructure of approximately 1–2 ␮m (Fig. 1c).
Table 1 presents the BET surface areas (SBET), the total number
of Brønsted acid sites, Si/Al ratios, crystal sizes and the rate of n-
hexane cracking for all catalysts. SBET values are in line with those
usually found for ZSM-5 zeolites. However, ZSM-5MS exhibited a
higher SBET value of 453 m2/g. This finding is in line with previ-
ous HRTEM measurements which showed that it contains 10–20
Table 1
BET surface areas, Si/Al ratio, crystal sizes, number of Brønsted acid sites and rate of n-hexane cracking.
Catalyst
Si/Al
Crystal size (␮m)
SBET (m2/g)
Number of Brönsted acid sites (mmol/g)
Rate of n-hexane cracking (mmol/g min)
ZSM-5R
ZSM-5F
ZSM-5N
ZSM-5MS
25
53
19
43
5
320
305
345
453
1.00
0.31
0.78
0.26
120
107
183
322
15–20
0.05–0.10
2

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F.L. Bleken et al. / Applied Catalysis A: General 447–448 (2012) 178–185
35
C1
30
C2
C3
C4=
C4-
25
C5
C6+ar
C6+al
20
15
10
5
0
0
20
40
60
80
100
Conversion (%)
Fig. 3. Methanol conversion–product selectivity plot for ZSM-5F at 350 ◦C.
n-hexane cracking rate observed for sample ZSM-5R compared to
sample ZSM-5N in Table 1 and aforementioned, in spite of the sim-
ilar Si/Al ratios determined for those samples by EDX analysis and
H/D exchange experiments, may be explained by the higher frac-
tion of EFAL sites present in the ZSM-5R zeolite, when assuming
that a high acid strength is required for alkane activation.
Fig. 2. FTIR spectra (ꢁ(O H) region) of ZSM-5X samples collected at room temper-
ature after vacuum degassing at 500 ◦C for 1 h. These spectra are normalized to the
overtone modes in the 1944–2092 cm−1 region. For clarity spectra have been shifted
along the ordinate.
3.2. Evaluation of catalytic properties in the methanol to
hydrocarbons (MTH) reaction
MTH test data for the four samples are shown in Table 3. Con-
version versus time on stream curves for the various samples are
shown in Supporting information (Fig. S4). All samples gave an ini-
tial conversion close to 100%. The initial C3/C2 ratio in the effluent
increased from 1.6 over the ZSM-5N material, via 1.8 over the ref-
erence sample ZSM-5R, to 2.5 over ZSM-5MS and finally to 5.5 over
ZSM-5F, thus:
led to a higher rate of n-hexane cracking, i.e.; 322 mmoln-hexane con-
verted/g min, thus suggesting a facilitated diffusion of the alkane
through the mesoporous zeolite.
Fig. 2 shows the IR spectra of ZSM-5N, ZSM-5F, ZSM-5R, and
ZSM-5MS zeolites in the range 3800–3000 cm−1 (ꢁ(O H) region).
The wavenumbers and assignment of the observed IR compo-
nents are given in Table 2. In general, the IR spectra supported the
observations made by other methods: The isolated silanol peak at
3750 cm−1 increased in abundance with a decrease in zeolite crystal
size, as shown by the SEM images in Fig. 1 (ZSM-5F > ZSM-5R > ZSM-
5MS > ZSM-5 N). All samples contained Si OH Al bridge sites, as
well as extra-framework Al (EFAL) sites. The mesoporous sample,
ZSM-5MS, further contained a significant amount of silanol nests.
Spectra measured during interaction with CO at 77 K are shown in
Supporting Information, Fig. S3. The peak of the Brønsted acid sites
(3610 cm−1) was red shifted to 3305 cm−1 for all samples upon
interaction with CO, indicating similar acid strength for all sam-
ples. When considering the ꢁ(C O) region (2050–2250 cm−1) the
main peak was observed at 2174 cm−1 for all samples, correspond-
ing to interaction with the Si OH Al bridge sites. Furthermore,
a significant component centered at 2169 cm−1, associated to CO
interaction with EFAL sites, was observed, in particular for ZSM-
5R sample, followed by ZSM-5F sample. The significantly lower
ZSM-5F > ZSM-5MS > ZSM-5R > ZSM-N
An inverse correlation was found between the C3/C2 ratio and
the hydrogen transfer index (HTI), which was calculated for the
C4 fraction [i.e. ꢂC4−/(ꢂC4− + ꢂC4 )]. As one would expect, a pos-
itive correlation was found between the HTI and the selectivity to
aromatic products (Table 3).
From the Conversion versus Time on stream plots shown in
Supporting Information (Fig. S4), no direct correlation was found
between product selectivity and deactivation rate. The deactivation
rate decreased in the order:
ZSM-5R > ZSM-5MS > ZSM-5N ∼ ZSM-F
With increasing times on stream, an increase in the C3 /C2
ratio and a decrease in HTI were observed for all catalysts. The ref-
erence catalyst, ZSM-5R, was the only catalyst which deactivated
below 50% conversion. At such low conversions, the C2 selectiv-
ity increased faster than the C3 selectivity, leading to a gradual
decrease in the propene-to-ethene ratio with decreasing conver-
sion.
Since the aim of this study was to optimize a catalyst for high
propene yields, more tests were carried out with the most promis-
ing material, ZSM-5F, using different contact times. Product yield
versus methanol conversion data for ZSM-5F are shown in Fig. 3.
The yield versus conversion curves were linear for all products in
the 2 to 98% methanol conversion range, and the C3/C2 ratio was in
the range 5–7 for all contact times. The yield to aromatic products
never exceeded 5%.
Table 2
Assignments of infrared spectra.
Wavenumber (cm−1
)
Assignment
3745 (intense)
3663 (small band)
3612 (intense)
3510 (broad)
2174
Isolated external silanol (SiO H)
Extra-framework aluminum species (AlO H)
Bridging hydroxyls (Si OH Al)
Silanol nests
CO interacting with bridging OH
CO interacting with EFAL
2169
2157
2138
CO interaction with silanol
Liquid like CO

F.L. Bleken et al. / Applied Catalysis A: General 447–448 (2012) 178–185
183
Table 3
Test data for the MTH reaction over ZSM-5 samples at 350 ◦C.
Sample
Conv. (%)
TOS (min)
Selectivities [%]
C3 /C2
C4/C4
HTI (C4)
C2
C3
C4
C4−
C5
C6+ ar
C6+ al
ZSM-5R
ZSM-5N
ZSM-5MS
ZSM-5F
ZSM-5N
99
100
99
98
99
5
5
5
15
14
12
7
27
23
30
36
29
14
11
18
23
19
13
17
9
4
8
11
10
11
13
12
9
17
9
5
10
10
7
11
12
11
1.8
1.6
2.5
5.5
2.9
0.9
1.5
0.5
0.2
0.4
0.5
0.6
0.3
0.2
0.3
5
2490
10
(1) Hydrogen Transfer Index (HTI) was calculated for the C4 fraction as ꢂC4−/(ꢂC4− + ꢂC4
)
4. Discussion
4.1. Product selectivity versus crystal size
The crystal size of the materials decreases in the order:
In order to rationalize the observed data, a brief recapitulation
of the mechanistic aspects of the Methanol to Hydrocarbons (MTH)
reaction is required: Chang and Silvestri published the first study of
the MTH reaction using a ZSM-5 catalyst in 1977. They showed how
product selectivity was shifted from mainly light olefins toward a
mixture of aromatics and alkanes with increasing contact times [6].
Two years later, Chen and Reagan re-plotted the rate versus con-
tact time data and suggested that the reaction was autocatalytic,
with olefins acting as the autocatalytic species [52]. During the
past three decades, huge efforts have been made to reveal further
mechanistic details of the MTH reaction. Today, the general view
is that both C3+ alkenes and various arenes, polymethylated ben-
zenes in particular, may act as autocatalytic species in ZSM-5 (see
ref. [15] and references therein). Methylation of C3+ alkenes have
been found to produce mainly more C3+ alkenes by cracking reac-
tions, while methylation of polymethylated benzenes have been
shown to produce mainly ethene and propene, in ZSM-5 zeolite
[31,53].
Taking the mechanistic aspects into account, a high C3 /C2
ratio, and a low selectivity to aromatics, may be ascribed either to
low acid site density and/or low diffusion hindrance, thus facilitat-
ing transport of alkene products out of the catalyst before they are
eventually converted to aromatic products. Such a hypothesis has
already been validated by previous studies referred in the Introduc-
tion, which indicated that high Si/Al ratios, as well as small crystal
sizes, favor light olefins selectivity and elevated propene-to-ethene
ratios [35–37]. An alternative hypothesis could be an enhanced
selectivity for one or the other catalytic cycle, i.e., alkene-mediated
versus arene-mediated, beyond the two parameters already men-
tioned. Such an explanation would be in line with the studies
showing that addition of phosphorous to the ZSM-5 material led to
lower acid strengths and concurrently to higher propene-to-ethene
ratios and high light olefins selectivities in the materials studied
[38,39].
ZSM-5F (15–20 ␮m) > ZSM-5R (5 ␮m) > ZSM-5MS (2 ␮m)
> ZSM-5N (0.05–0.10 ␮m)
A larger crystal size represents longer diffusion pathways,
but also a lower fraction of external sites. A positive correlation
between crystal size and light olefins selectivity was indicated
when considering the two ends of the row. However, an inversion
was observed for the two materials in the middle, thus suggest-
ing that there is no direct correlation between crystal size and
selectivities for the samples studied here.
4.2. Product selectivity versus accessible Brønsted acid sites
The number of accessible Brønsted acid sites in the materials
decreases in the order:
ZSM-5R (1.00) > ZSM-5N (0.78) > ZSM-5F (0.31)
> ZSM-5MS (0.26) (H/D titration)
ZSM-5MS (322) > ZSM-5N (183) > ZSM-5R (120)
> ZSM-5F (107) (n-hexane cracking)
The two methods gave quite different results, possibly because
H/D exchange may take place at Si OH Al bridge sites as well
as EFAL sites, while n-hexane cracking may only proceed at the
Si OH Al bridge sites with higher acid strength.
The FTIR spectra indicated that the acid strength of the
Si OH Al bridge sites is similar for all materials. However, they
further suggested that the fraction of EFAL sites is highest for ZSM-
5R, followed by ZSM-5F, while ZSM-5MS and ZSM-5N contain a low
fraction of EFAL sites. In either case, it is not obvious that there is
a direct correlation between the number of Si OH Al bridge sites
and selectivity in the materials tested.
A key challenge when comparing several catalysts for a tar-
get reaction is that the materials normally differ by more than
one parameter, and that several parameters may influence prod-
uct selectivity. This is the case even for the four samples studied
here. Let us therefore compare product selectivities versus each of
the parameters suggested above:
4.3. Tentative reaction path occurring for raising propylene
formation
The aforementioned catalytic results remain quite difficult to
explain in terms of reaction pathway occurring especially in the
ZSM-5F giant crystals. Indeed, the nearly five-times higher pro-
pylene formation versus ethylene may involve a rather complex
mechanism. It is important to remind that Froment and Park
have already presented a detailed MTO mechanism involving 726
elementary steps including protonation, deprotonation, hydride
transfer, methyl-shift, methylation, oligomerization, cracking [54].
Nevertheless, one may expect that under specific reaction condi-
tions or catalyst formulation, it can be conceived that one path
becomes dominant with respect to the others. Wu et al. [55] have
demonstrated, while co-feeding methanol and C2–C6 alkenes in
Overall, the selectivity to C2–C4 olefins diminishes in the order:
ZSM-5F (66%) > ZSM-5MS (60%) > ZSM-5R (56%)
> ZSM-5N (48%)
Furthermore, the propene-to-ethene ratio decreases in the
order:
ZSM-5F (5.1) > ZSM-5MS (2.5) > ZSM-5R (1.8) > ZSM-5N (1.6)

184
F.L. Bleken et al. / Applied Catalysis A: General 447–448 (2012) 178–185
Fig. 4. Schematic representation of the methanol reaction over Brønsted acid sites in the ZSM-5R and ZSM-5F crystals.
a multi-stage adiabatic fixed-bed reactor, that methylation and
cracking of C4–C7 olefins became predominant.
The question arises what would induce this impressive pro-
pylene selectivity at complete methanol conversion over ZSM-5F
synthesized via the fluoride-mediated route?
BA sites in the ZSM-5F regular prismatic crystals, whilst ZSM-5R
presents a high degree of twinning, one can understand the higher
selectivity toward light olefins over the former catalysts. In spite
of their huge diffusion length, giant ZSM-5F crystals which con-
tain few defects, being rather hydrophobic, exhibits few BA sites
and warrant therefore reduced aromatization reactions (and thus
lower extend of aromatics hydrocarbon pool), allowing the cata-
lyst to maintain its steady-state behavior during the whole course
of the reaction (Fig. S4). Fig. 4 tends to rationalize the main reaction
path occurring over this highly MTP-selective zeolite. The reduced
acidity of ZSM-5F catalyst with respect to ZSM-5R plays in favor
of higher propylene selectivity as already observed by Liu et al.
[39] while reducing the acidity of ZSM-5 zeolites via doping with
phosphorous.
In line with these tentative interpretations, we have recently
performed an analysis of coke deposits (via TPO measurements)
after long-term experiments by flowing methanol at 400 ◦C over
the two zeolites prepared in alkaline and fluoride-media, respec-
tively. It appeared that the amount of carbon present on the former
ZSM-5R zeolite was about nine times higher than for the ZSM-5F
zeolite [60]. One can therefore conclude that the zeolite prepared
via the fluoride route is much more resistant to coke formation
when compared to the other zeolite catalysts. Thanks to its reduced
number of Brønsted acid sites and an important diffusion length,
thus to a drastically reduced density of acid sites, ZSM-5F appears
to be at the same time an active and stable MTP-catalyst. Whereas
the peculiar behavior of this catalyst remains partly unrevealed, the
high selectivity to propene may be related to (some extend at least)
Following the Mulhouse’s group key researches [56–58], it is
now well admitted that larger zeolite crystals with fewer defects
are produced via the non-alkaline procedure. In addition, the F−
anion has been reported to be occluded in [415262] cage, penta-
coordinated and interacting with a silicon framework atom, thus
forming [SiO4/2F]− entities [59]. Guth and Kessler have reported
a possible modification of the catalytic properties in calcined
materials even after complete fluorine removal [56]. Indeed, the
co-templating role of fluoride anion contributes to the stabiliza-
tion of small 4MR cavities [58,59] and may also induce distortion in
the geometry of tetrapropylammonium (TPA) cations located at the
channel intersections. Besides those materials exhibit very few con-
nectivity defects and thus allow the synthesis of rather hydrophobic
zeolites.
The main difference between ZSM-5R and ZSM-5F zeolites relies
in different BA sites densities. Whereas the later catalyst exhibits a
three- to four-time larger crystal size, it possesses at the same time
nearly the third of BA sites. One can therefore roughly estimate a
ten-time higher density of BA sites per crystal of ZSM-5R zeolite.
The higher deactivation rate observed is directly connected to this
high acidity. A higher density of BA sites should also induce more
proximity among these sites, thus to ease consecutive reactions up
to extensive coke formation. If we consider a high dispersion of

F.L. Bleken et al. / Applied Catalysis A: General 447–448 (2012) 178–185
185
aforementioned peculiarities: lower BA sites density, huge diffu-
sion length, absence of crystal twinning, presence of EFAL. Fig. 4
tempts to summarize an eventually favored alkene methylation-
cracking cycle over large ZSM-5F crystals. Further studies are under
progress to evaluate the exact nature of the coke formed over these
zeolites. In addition, we aim to deeper investigate via a multi-
nuclear NMR study the structure of ZSM-5 zeolites prepared in
fluoride medium.
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A high propylene to ethylene ratios (∼5) was achieved over
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MB and BL would like to thank the Agence Nationale de la
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project (SelfAsZeo). FLB acknowledges the Research Council of
Norway for financial support through contract no. 171158/V30.
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.apcata.2012.09.
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