A Systematic Study of Isomorphically Substituted H-MAlPO-5 Materials for the Methanol-to-Hydrocarbons Reaction

Article abstract of DOI:10.1002/cphc.201701024

Substituting metals for either aluminum or phosphorus in crystalline, microporous aluminophosphates creates Br?nsted acid sites, which are well known to catalyze several key reactions, including the methanol to hydrocarbons (MTH) reaction. In this work, we synthesized a series of metal-substituted aluminophosphates with AFI topology that differed primarily in their acid strength and that spanned a predicted range from high Br?nsted acidity (H-MgAlPO-5, H-CoAlPO-5, and H-ZnAlPO-5) to medium acidity (H-SAPO-5) and low acidity (H-TiAlPO-5 and H-ZrAlPO-5). The synthesis was aimed to produce materials with homogenous properties (e.g. morphology, crystallite size, acid-site density, and surface area) to isolate the influence of metal substitution. This was verified by extensive characterization. The materials were tested in the MTH reaction at 450 °C by using dimethyl ether (DME) as feed. A clear activity difference was found, for which the predicted stronger acids converted DME significantly faster than the medium and weak Br?nsted acidic materials. Furthermore, the stronger Br?nsted acids (Mg, Co and Zn) produced more light alkenes than the weaker acids. The weaker acids, especially H-SAPO-5, produced more aromatics and alkanes, which indicates that the relative rates of competing reactions change upon decreasing the acid strength.

Full text of DOI:10.1002/cphc.201701024

DOI: 10.1002/cphc.201701024
Articles
A Systematic Study of Isomorphically Substituted
H-MAlPO-5 Materials for the Methanol-to-Hydrocarbons
Reaction
Magnus Mortꢀn,[a] Łukasz Mentel,[a] Andrea Lazzarini,[a] Ilia A. Pankin,[b, c] Carlo Lamberti,[b, c]
Silvia Bordiga,[b] Valentina Crocellꢁ,[b] Stian Svelle,[a] Karl Petter Lillerud,[a] and Unni Olsbye*[a]
Substituting metals for either aluminum or phosphorus in crys-
talline, microporous aluminophosphates creates Brønsted acid
sites, which are well known to catalyze several key reactions,
including the methanol to hydrocarbons (MTH) reaction. In this
work, we synthesized a series of metal-substituted alumino-
phosphates with AFI topology that differed primarily in their
acid strength and that spanned a predicted range from high
Brønsted acidity (H-MgAlPO-5, H-CoAlPO-5, and H-ZnAlPO-5)
to medium acidity (H-SAPO-5) and low acidity (H-TiAlPO-5 and
H-ZrAlPO-5). The synthesis was aimed to produce materials
with homogenous properties (e.g. morphology, crystallite size,
acid-site density, and surface area) to isolate the influence of
metal substitution. This was verified by extensive characteriza-
tion. The materials were tested in the MTH reaction at 4508C
by using dimethyl ether (DME) as feed. A clear activity differ-
ence was found, for which the predicted stronger acids con-
verted DME significantly faster than the medium and weak
Brønsted acidic materials. Furthermore, the stronger Brønsted
acids (Mg, Co and Zn) produced more light alkenes than the
weaker acids. The weaker acids, especially H-SAPO-5, produced
more aromatics and alkanes, which indicates that the relative
rates of competing reactions change upon decreasing the acid
strength.
1. Introduction
Controlled synthesis of advanced, functional materials at the
nanoscale is now becoming achievable. However, decoupling
individual material properties and assessing their influence on
material functionality remains a challenge.
by novel applications emerging to exploit the advanced mate-
rial properties.[8,9]
The quest for linking structure and material properties to ac-
tivity, “the materials genome”, is ongoing, with the main chal-
lenge of identifying the key properties of the materials.[10] By
understanding and quantifying structure–activity relationships,
a more rational approach to catalyst design is possible, im-
proving existing and developing new processes. In surface sci-
ence, considerable effort has been made to establish a descrip-
tor-based approach to transition-metal catalysis, linking meas-
urable properties of the catalyst structure with activity and se-
lectivity in catalyzed reactions.[11,12] Control of material proper-
ties is thus of key importance in this quest, as it makes it
possible to investigate the influence of the individual factors
on the performance of the materials. The understanding
gained from those investigations could subsequently be used
to optimize the performance of the materials for selected
applications.
For zeolites and zeotypes, a class of microporous aluminosili-
cates and aluminophosphates, control over material properties
is already quite advanced. Extended research efforts devoted
to finding procedures driving the synthesis towards materials
with desired properties have resulted in methodologies that
allow control over particle size and morphology,[1–4] incorpora-
tion and distribution of active sites,[5,6] and the construction of
new topologies.[7] Moreover, the efforts are further stimulated
[a] M. Mortꢀn, Ł. Mentel, A. Lazzarini, S. Svelle, K. P. Lillerud, Prof. U. Olsbye
Centre for Materials Science and Nanotechnology
Department of Chemistry, University of Oslo
P.O. Box 1033, Blindern, 0315 Oslo (Norway)
E-mail: unni.olsbye@kjemi.uio.no
[b] I. A. Pankin, C. Lamberti, S. Bordiga, V. Crocellꢁ
Department of Chemistry, CrisDi Interdepartmental Centre
and INSRM reference, University of Turin
A particularly challenging process is the methanol-to-hydro-
carbons reaction (MTH), in which methanol or dimethyl ether
(DME) is converted into larger hydrocarbons over a Brønsted
acidic zeotype catalyst. Numerous studies have aimed to
reveal how each of the material properties (e.g. catalyst com-
position, topology, morphology, crystal size, and density of
acid sites) can be optimized to improve conversion capacity,
activity, and product selectivity.[13,14]
via Pietro Giuria 7, 10125 Turin (Italy)
[c] I. A. Pankin, C. Lamberti
International Research Center “Smart Materials”
Southern Federal University, Zorge Street 5
344090 Rostov-on-Don (Russia)
Supporting Information and the ORCID identification number(s) for the
author(s) of this article can be found under:
The mechanism of MTH has been debated extensively with
more than 20 unique mechanisms proposed to explain the ini-
tial carbon–carbon bond-formation process.[13] However, under
https://doi.org/10.1002/cphc.201701024.
An invited contribution to a Special Issue on Reactions in Confined
Spaces.
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steady-state conditions Dahl and Kolboe have proposed that
reactions occur through a hydrocarbon pool mechanism, by
which sequential methylation and elimination reactions occur
on adsorbed carbon species at the active site of the catalyst,
co-catalyzing product formation.[15–17] Alkenes and aromatics
constitute important adsorbed pool species promoting the
two distinct product-forming cycles (Scheme 1).
cycle being promoted over the more acidic material, whereas
the alkene cycle was favored for the weaker acid.[30] More re-
cently, a study in which methanol and DME were compared as
MTH feedstock provided additional insight into the matter. Im-
portantly, it was found that methanol promoted hydrogen-
transfer reactions to a much higher extent than DME. Hence,
the DME/methanol ratio in the reactor had substantial influ-
ence on product selectivity and catalyst conversion capacity.
Furthermore, the study showed that the reversible methanol
condensation reaction to DME and water was equilibrated
over H-SAPO-5 but not over H-SSZ-24 under the MTH reaction
conditions.[31] This study showed that the difference in product
selectivity observed for H-SSZ-24 and H-SAPO-5 could at least
partly be ascribed to their different activities for the methanol
condensation reaction relative to subsequent methylation reac-
tions.
The effect of BAS strength on a key MTH reaction step,
olefin methylation, was explored computationally by Brogaard
and Wang et al.[28,29,32] Ammonia adsorption enthalpy was iden-
tified as a descriptor for BAS strength over a series of metal-
substituted aluminophosphates, ranking the materials on the
basis of the BAS strength of the site created by isomorphic
substitution. A scaling relationship for methylation rates of
ethene and propene and BAS strength was established for sev-
eral topologies and material compositions, yielding a predic-
tive relationship for turnover frequencies of propene methyla-
tion on the basis of the ammonia adsorption enthalpy on the
BAS.
Scheme 1. General reaction scheme for the dual-cycle mechanism. Figure
from ref. [18].
In the alkene cycle, alkenes undergo methylation and crack-
ing reactions typically to give C3 to C5 alkenes as major prod-
ucts.[19] In the arene cycle, aromatics are methylated to substi-
tuted aromatics and undergo elimination reactions to form
light alkenes and aromatics with fewer substituents as pro-
ducts.[19b,20] This latter reaction links the arene and alkene
cycles. Another main reaction linking the two cycles is hydro-
gen transfer from either methanol or DME to an adjacent
alkene, followed by addition of the dehydrogenated species to
another alkene molecule, which leads to diene formation. Sub-
sequent cyclization and hydrogen-transfer reactions finally lead
to the formation of aromatic products.[21–23]
One of the investigated topologies, AFI, with unidimensional
12-ring channels (7.3 ꢃꢄ7.3 ꢃ), is ideal for experimental verifi-
cation, as it allows for bulky molecules to diffuse out of the
structure, thereby limiting secondary reactions. In addition,
since the discovery of AlPO-5 in 1982 by Wilson et al.,[33] nu-
merous examples of compositional variants substituting heter-
oatoms in the structure have been reported, including the ele-
ments Mg, Zn, Co, Ti, Zr, and Si.[5,34] It has been debated
whether isomorphic substitution of metals for either Al or P in
the AFI structure, thereby creating a Brønsted acid site, is even
possible.[35] However, several elements have been subject to
rigorous study by employing advanced spectroscopic tech-
niques, which have confirmed framework incorporation.[36]
Metal-substituted AlPO-5 materials have been tested as cata-
lysts in several reactions, including methanol to hydrocar-
bons,[37] ethylation and propylation of aromatics,[38,39] cracking
of alkanes,[40] and aromatic isomerization.[41]
The active site of the MTH reaction is a Brønsted acid site
(BAS). Although the fundamental aspects of solid Brønsted
acidity are still under active investigation, there is extensive lit-
erature covering experimental and computational approaches
for the construction of relative acid strength scales.[24,25] Nu-
merous approaches to quantify acidity have been suggested
by using probe molecules, usually basic, in combination with
an experimental probe detection technique. Examples include
NH3, CO, alcohols, nitriles, and aromatic molecules and
common probe detection methods such as infrared spectros-
copy, NMR spectroscopy, and temperature-programmed de-
sorption with mass spectrometry.[24,25] A computational alterna-
tive is the deprotonation energy (DPE)[26,27] and NH3 adsorption
enthalpy,[28,29] which allow quantifiable ranking of acidity.
A few recent studies have addressed the influence of BAS
strength on the MTH reaction. By using infrared spectroscopy
and CO as a probe, the acidity difference of two isostructural
zeolite/zeotype materials, H-SSZ-24 and H-SAPO-5, was estab-
lished.[18,30] The more acidic zeolite H-SSZ-24 produced more
aromatics and C2–C3 hydrocarbons upon feeding MeOH,
whereas more weakly acidic H-SAPO-5 showed increased selec-
tivity towards C4–C6+ alkenes. It was concluded that the differ-
ences in product selectivity could be ascribed to the arene
In this work, we aim to study the effect of metal incorpora-
tion into the AlPO-5 structure and the effect of the created
BAS on the MTH reaction. This will be a single-parameter study
attempted by synthesizing a series of metal-substituted AlPO-5
materials with the same topology, similar crystal size, same
morphology, and same BAS density. The materials were exten-
sively characterized to ensure that the catalyst lattices, includ-
ing the immediate surroundings of the heteroatoms, were
intact during testing. Furthermore, to avoid the influence of
feed composition, DME was used as MTH feedstock instead of
MeOH, keeping the DME/MeOH/H2O ratio in the reactor similar
across the materials. To the best of our knowledge, this is the
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first detailed, systematic study of metal-substituted ALPO-5
materials used in the MTH reaction.
2. Results and Discussion
2.1. Synthesis and Catalyst Characterization
A summary of the obtained characterization data, including
the size of the particles [from scanning electron microscopy
(SEM) images], specific surface area (from BET), acid-site densi-
ty estimated from n-propylamine temperature-programmed
desorption (TPD), and the heteroatom content obtained on
the basis of energy-dispersive X-ray spectroscopy (EDS) meas-
urements, for all of the synthesized materials can be found in
Table 1.
The X-ray diffractograms were measured for the as-synthe-
sized (Supporting Information, Section S1) and calcined materi-
als (Figure 1). All materials were highly crystalline and showed
Figure 1. XRD powder patterns of the calcined H-MAlPO-5 materials and
AlPO-5 used in this work (see Section S1 for the XRD powder patterns for
the as-synthesized samples).
Table 1. Key material characteristics for the synthesized H-MAlPO-5 mate-
rials.
sired density of the Brønsted acid sites was 0.1 mmolgꢀ1, a
value that was previously reported for most of the materials
synthesized in this work.[37,43] Table 1 shows that the density of
[a]
Material
Crystal size
[mm]
SABET
Acid-site
density[b]
[mmolgꢀ1
Density
of M[c]
[mmolgꢀ1
]
[m2 gꢀ1
]
]
H-MgAlPO-5
H-ZnAlPO-5
H-CoAlPO-5
H-SAPO-5[d]
H-ZrAlPO-5
H-TiAlPO-5
AlPO-5
1ꢄ2.5
1ꢄ2.5
1.5ꢄ2
1ꢄ2
2ꢄ4.5
3.5ꢄ3
1ꢄ1
360
362
347
340
352
329
370
0.102
0.094
0.081
0.068
0.065
0.060
–
0.1
0.2
0.2
0.3
[e]
–
0.2
[f]
–
[a] BET surface area. [b] Determined by n-propylamine TPD. [c] Estimated
from EDS on the basis of the ratio of M and either Al or P. [d] See
ref. [42]. [e] Overlap between Zr/P peaks; no estimate obtained. [f] No
heteroatom detected by EDS.
the AFI structure as the only phase present. The materials were
stable up to 5508C during calcination and showed no sign of
degradation after heating, as is evident from the X-ray diffrac-
tion (XRD) patterns of the calcined samples (Figure 1).
The BET areas of the samples were determined to fall be-
tween 329 and 362 m2 gꢀ1, with H-TiAlPO-5 and H-ZnAlPO-5
being the two extremes of the metal-substituted series. Non-
substituted AlPO-5 showed a slightly higher surface area of
370 m2 gꢀ1 (Table 1). These results indicate that little pore
blocking due to extra framework metal species occurs.
Representative SEM images (Figure 2) show the hexagonal
barrel-shaped morphology of the samples. The crystallites have
a homogenous size distribution with typical averages between
1 and 2 mm in diameter and 2 and 3 mm along the c axis. H-
ZrAlPO-5 and H-TiAlPO-5 show slightly larger crystallites, with
averages from 2 to 4 mm in diameter and 3 to 5 mm along the
c axis. The morphology of both samples resembles hexagonal
barrels, but with edges that are less well defined than those of
the rest of the synthesized materials.
n-Propylamine TPD was used to probe the Brønsted acidity
and to quantify the density of the Brønsted acid sites. The de-
Figure 2. SEM images of the H-MAlPO-5 materials and AlPO-5.
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acid sites varies somewhat between the +2 and +4 metals.
The density of acid sites is lower for the +4 metals than for
the +2 metals, ranging from 0.060 to 0.065 and
0.068 mmolgꢀ1 for H-TiAlPO-5, H-ZrAlPO-5, and H-SAPO-5, re-
spectively. This is in agreement with the +4 metals being less
likely to substitute into the framework, as PꢀO bonds are more
covalent in character than AlꢀO bonds in the ALPO-5 frame-
work.[44] For the +2 metals, the density of acid sites varies
from 0.081 to 0.094 and 0.102 mmolgꢀ1 for H-CoAlPO-5, H-
ZnAlPO-5, and H-MgAlPO-5, respectively. To investigate the
effect of varying the acid-site density on the MTH reaction, ex-
periments with a higher density sample (Mg) and lower density
sample (Zn) were performed (Section S2). The product selectiv-
ities do not vary significantly between the high- and low-densi-
ty samples under these conditions, so it is concluded that the
variation in the acid-site density between the samples is satis-
factory for the present study.
The estimates of metal content from EDS provide comple-
mentary information to the density of Brønsted acid sites ob-
tained from n-propylamine TPD, from which we can infer the
amount of metal present in the sample that is not incorporat-
ed into the lattice as BAS. As can be seen in Table 1, the total
density of metals varies somewhat between the samples, rang-
ing from 0.1 to 0.2 mmolgꢀ1 for the materials synthesized in
this work. Rough agreement between the samples is expected,
as the same synthetic procedure was followed, adding the
same amount of metal salts to the synthesis gel. The excess
amount of metal can exist as a variety of species that are hard
to identify precisely; however, except for the H-SAPO-5
sample, the amounts of those species are small.
Figure 3. DRIFT spectra of AlPO-5 and H-MAlPO-5 after activation at 4508C
under a He atmosphere. The spectra were collected at 1208C to avoid mois-
ture contamination and water adsorption.
To investigate the nature of the acid sites in more detail, se-
lected samples (i.e. Mg, Co, Si, and pure AlPO-5) were studied
by using CD3CN as a probe molecule (see Section S3). All of
the samples analyzed exhibit the following spectral features:
1) a broad band centered at n˜ =2310–2315 cmꢀ1, ascribed to
the probe adsorbed on Al3+ Lewis acid sites; 2) a sharp band
at n˜ =2263 cmꢀ1, due to physisorbed CD3CN; 3) a band be-
tween n˜ =2285 and 2275 cmꢀ1, assigned to the probe ad-
sorbed on terminal (phosphanols) and bridged hydroxy groups
(Brønsted sites); 4) samples including Co2+ and Mg2+ also
show a feature (well visible in the last desorption step; gray
spectra in the insets) centered at n˜ =2301–2304 cmꢀ1, assigned
to acetonitrile adsorbed on Lewis sites created by the incorpo-
rated M2+ cations.[46,47] These observations are in line with
older literature, which showed that the AlPO-5 framework was
so flexible that Al could interact with strong basic probes as a
Lewis acid without losing its tetrahedral lattice coordination.[48]
The complexity of the IR spectra further suggests that ranking
of Brønsted acid strength for these materials must rely on the-
oretical predictions.
Direct evidence of metal incorporation into the AlPO-5
framework and the consequent formation of strong Brønsted
acid sites were assessed through IR spectroscopy. Due to the
prohibitive particle dimensions in transmission measurements,
the samples were studied in reflectance mode (see the Experi-
mental Section). Figure 3 shows the spectra collected after
sample activation at 4508C under helium flow for the full set
of samples.
As already described, the IR spectra of the H-MAlPO-5 mate-
rials (AFI topology) are characterized by strong bands in the
region below n˜ =2500 cmꢀ1, which are assigned to overtone
and combination vibrations of the lattice. Above n˜ =
2500 cmꢀ1, absorption bands attributed to the stretching vibra-
tions of the hydroxy groups are observed.[45] In all spectra, a
characteristic band at n˜ =3674 cmꢀ1 ascribed to the terminal
PꢀOH groups is present. Moreover, in H-SAPO-5 and AlPO-5
the respective bands for the SiꢀOH and AlꢀOH terminal
groups are present at n˜ =3790 and 3735 cmꢀ1. The only
sample that shows well-defined maxima ascribable to bridging
Brønsted sites is H-SAPO-5 (n˜ =3625 cmꢀ1), whereas other H-
MAlPO-5 materials are characterized by a continuum in the
spectra profile, which suggests that the OH group is engaged
in hydrogen bonding.[37] All of these OH species in the spectral
region below n˜ =3650 cmꢀ1 are usually ascribed to OH bonds
presenting Brønsted acidity, as reported in the literature for
these materials.[46]
To sum up, the characterization data presented in this sec-
tion show that the materials form a series with similar key
properties such as particle size, morphology, surface area, and
acid-site density. The materials differ in one respect, namely,
the nature of the isomorphically substituted heteroatom. This
makes the series of materials well suited for a one-parameter
variation study, in which the catalytic properties of the created
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Brønsted acid sites can be ascribed to differences in acid
strength.
Table 2. H-MAlPO-5 theoretical acid strength (DENH ; given in kJmolꢀ1),
3
turnover frequency (TOF; values in molprod mol ꢀ1 hꢀ1), and rate of DME
½Hþꢁ
conversion (rconv; values in 10ꢀ2 molprod gcatꢀ1 hꢀ1).[a]
[b]
Material
DENH
TOF
rconv
3
2.2. Catalytic Testing
H-MgAlPO-5
H-CoAlPO-5
H-ZnAlPO-5
H-SAPO-5
H-ZrAlPO-5
H-TiAlPO-5
AlPO-5
ꢀ128
ꢀ121
ꢀ119
ꢀ83
ꢀ75
ꢀ75
–
243
279
217
118
30
2.48
2.27
2.04
0.80
0.20
0.24
0.13
2.2.1. Catalytic Activity of H-MAlPO-5 Materials at 4508C in
the MTH Reaction
Although metal-substituted aluminophosphates have been
studied extensively for over 30 years, there are only a few sys-
tematic studies in which a consistent series of materials was
examined to uncover the dependence of catalytic properties
on the substituted element forming the BAS. In pioneering
studies on H-MAlPO-5 materials with M=Mg, Co, Zn, Si, and Zr
were tested for MeOH conversion at 3708C.[37] Lischke et al.
found that H-SAPO-5 was the most-active catalyst, followed by
H-CoAlPO-5. H-MgAlPO-5 and H-ZnAlPO-5 were found to be
significantly less active, and H-ZrAlPO-5 showed negligible con-
version. In an analogous study of H-SAPO-5 and H-CoAlPO-5,
Popova et al. reported that the initial conversion of MeOH was
similar over H-CoAlPO-5 and H-SAPO-5 at 4508C.[49]
In the present contribution, the Brønsted acidic H-MAlPO-5
materials were designed to span a spectrum of acid strengths
covering the high, medium, and low values[32] while keeping
properties such as particle size and morphology and density of
acid sites maximally similar to a degree allowed by the syn-
thetic procedure. The choice of the substituting elements was
strongly influenced by the work of Brogaard and Wang et al.,
who used ammonia adsorption enthalpy as an acidity descrip-
tor; they established a ranking of the materials that correlated
with rates of propene methylation.[28,29]
39
–
[a] Conditions: 4508C, pDME =13 mbar, WHSV=1.9 hꢀ1. [b] See Section S6.
values of 243, 279, and 217 molprod mol ꢀ1 hꢀ1, respectively. H-
SAPO-5 gives an initial TOF of 118 molprod mol ꢀ1 hꢀ1, a value
½Hþꢁ
½Hþꢁ
that is twofold smaller than the TOFs of the three strongest
acids, whereas H-ZrAlPO-5 and H-TiAlPO-5 give TOF values of
30 and 39 molprod mol ꢀ1 hꢀ1, respectively. The activity of the
½Hþꢁ
catalysts corresponds well with the theoretical acidity ranking
based on ammonia adsorption energy. This trend is in line
with the scaling relation developed by Brogaard and Wang
et al. between NH3 adsorption enthalpy and alkene methyla-
tion rate over H-MAlPO-5 materials.[32] Furthermore, previous
mechanistic studies point to a reaction between sorbed spe-
cies as the rate-determining step of both hydrogen-transfer re-
actions (leading to the first CꢀC bond formation; i.e. to initia-
tion of the MTH reaction) and alkene/arene methylation reac-
tions (the propagation reactions of the dual-cycle MTH mecha-
nism).[31,50,51] The rate of such reactions generally increases
upon increasing the coverage of the active site, which is, in
this case, induced by a higher acid strength. The catalysts can
be grouped on the basis of both their performance, measured
by the theoretical acidity, and initial TOF/rconv, as high acidity/
activity (Mg, Co, Zn), medium acidity/activity (Si), and low acidi-
ty/activity (Zr/Ti). This grouping coincides well with a grouping
that can be made on the basis of the chemical nature of the
doping element. Mg, Co, and Zn are formally +2 and are in-
corporated in place of Al atoms; Zr and Ti both have +4 oxi-
dation state and substitute P lattice position, the same as Si.
However, Si is the only metalloid in the +4 series, and it is the
most similar to P in size, which suggests that it will exert the
smallest perturbation on the framework compared to Zr and
Ti, which are transition metals and have larger ionic radii. The
different behavior of the +2 and +4 sites can be further ra-
tionalized in view of the findings presented by Corꢁ and
Catlow, who showed that AlPOs exhibited molecular-ionic
character that was a consequence of being composed of Al3+
All synthesized H-MAlPO-5 materials were tested as catalysts
in the MTH reaction to evaluate catalytic activity. The MTH re-
action is catalyzed by Brønsted acid sites and thus serves both
as an indirect means of confirming framework incorporation of
metals by demonstrating activity[35] and as a probe for the in-
fluence of the strength of the created BAS through the activi-
ties and selectivities across the materials. The H-MAlPO-5 mate-
rials were tested at 4508C and low feed rates (weight hourly
space velocity, WHSV=1.9 hꢀ1) and a low DME partial pressure
(13 mbar, 1 mbar=0.1 kPa) to ensure catalytic activity across
the series. Table 2 summarizes the activity of the catalysts ex-
pressed as initial turnover frequency (TOF) and initial conver-
sion rate per gram of catalyst (rconv). The theoretically calculat-
ed value for ammonia adsorption energy is included in the
table as an acid-strength descriptor and serves as a theoretical
ranking of the materials from strong to weak Brønsted acids.
This ranking will be used throughout the discussion upon ad-
dressing acid strength.
3ꢀ
and PO4 ions.[44]
Before proceeding to the test results, it should be noted
that the DME-to-methanol ratio versus conversion graphs in
the reactor effluent overlap for all tested materials (Section S4),
which demonstrates that the activity and selectivity differences
observed between the materials are not related to the relative
abundances of DME, methanol, and water in the reactor.
As can be seen in Table 2, the three most-active catalysts are
H-MgAlPO-5, H-CoAlPO-5, and H-ZnAlPO-5 with initial TOF
Unsubstituted AlPO-5 is also included in the series. Given
that it does not contain any Mꢀ(OH)ꢀP/Al bridge sites, it can
serve as a benchmark for the low-activity samples. Under the
studied conditions, AlPO-5 showed conversion of DME into hy-
drocarbons, however, at a lower rate than H-ZrAlPO-5 and H-
TiAlPO-5 (0.13 vs. 0.20 and 0.24ꢄ10ꢀ2 molprod gcatꢀ1 hꢀ1). The
conversion of DME over AlPO-5 may be ascribed to weakly
acidic PꢀOH-groups. The PꢀOH groups can catalyze the dehy-
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dration of methanol to DME and at higher temperatures con-
vert MeOH and DME into hydrocarbons.[52] To investigate the
difference in the activities of H-ZrAlPO-5 and AlPO-5, a series
of experiments were performed on the two materials and a
physical mixture of ZrO and AlPO-5 (Section S7). The activity
and product selectivity of H-ZrAlPO-5 is substantially different
from those of AlPO-5 and the ZrO/AlPO-5 physical mixture.
The significant difference in activities between AlPO-5 and
either H-ZrAlPO-5 or H-TiAlPO-5 indicates that isomorphic sub-
stitution of Zr or Ti into the AlPO-5 framework creates a
Brønsted acid bridge site that alters the materials’ catalytic
properties. Overall, the MTH results show that the materials in
this series have BAS with significant activity differences, and
this corresponds well with theoretical predictions of Brønsted
acid strength.
terials (Mg, Co and Zn) give significantly higher alkene selectiv-
ity than medium acidic H-SAPO-5. Weakly acidic H-ZrAlPO-5
gives the lowest alkene selectivity of the tested materials.
Moving on to selectivity to alkanes and aromatics (Fig-
ure 4b,c), their selectivity trends are generally opposite to
those of the alkenes. Their selectivity decreases from the weak-
est Brønsted acid, H-ZrAlPO-5, to the strongest acid, H-
MgAlPO-5, and H-CoAlPO-5 lies slightly outside the general
trend for alkanes (see below). In the MTH reaction scheme, al-
kanes and aromatics are typical products originating from hy-
drogen-transfer reactions.[13,20] The results presented in
Figure 4 suggest that isomorphic substitution of a metal into
the aluminophosphate structure significantly alters the chemis-
try associated with the created Brønsted acid sites, shifting the
product spectrum towards alkenes for the more acidic materi-
als.
Turning next to individual alkenes, ethene, propene, and C5+
selectivity versus conversion for each material is shown in
Figure 5. A striking observation in Figure 5a is the systematic
increase in propene selectivity with increasing catalyst acid
strength. At 75% conversion, the selectivity to propene spans
a gap from 25 C% for H-SAPO-5 to almost 50 C% for H-
MgAlPO-5. The difference in propene selectivity is also signifi-
cant at lower conversion levels, ranging from 30 C% over H-
MgAlPO-5, via 15 C% over H-SAPO-5, to 10 C% over H-ZrAlPO-
5, all at 20% conversion. Another evident feature of the pro-
pene selectivity plot is the rapid increase in propene selectivity
with increasing conversion. The slope of the curve is similar to
that for the Mg-, Co-, and Zn-containing materials and is some-
what lower than that for the Si- and Zr-containing materials.
An opposite trend is observed for the C5+ alkenes (Figure 5b):
their selectivity decreases with increasing acid strength and
with increasing conversion; the slope is steep and negative for
the Mg-, Co-, and Zn-containing materials; slightly negative for
H-SAPO-5; and positive for H-ZrAlPO-5.
2.2.2. Product Selectivity
To investigate the influence of the different isomorphic substi-
tutions into the AlPO-5 framework on product selectivity
during the MTH reaction, the synthesized H-MAlPO-5 materials
were tested at 4508C with variable feed rates (WHSV=0.23–
1.9 hꢀ1) to obtain comparable initial conversion levels. By re-
ducing the feed rate, an initial conversion level of about 80%
is obtained for H-MgAlPO-5, H-CoAlPO-5, H-ZnAlPO-5, and H-
SAPO-5. For H-ZrAlPO-5 and H-TiAlPO-5, the highest conver-
sion levels obtained are 30 and 12%, respectively, by employ-
ing very-low feed rates (see the Experimental Section). Previous
studies of the MTH reaction showed that conversion–selectivity
trends were often linear between 15 and 85% conversion but
differed widely for higher and lower conversion.[53] This is
indeed what we observe for all materials except for H-TiAlPO-
5, for which the conversion does not rise above 12%, and
therefore, the H-TiAlPO-5 sample will not be included in the re-
mainder of the discussion.
The conversion–selectivity results reported in Figures 4–8
were obtained during deactivation of the samples. This proce-
dure was warranted by previous studies of H-SAPO-5 as a MTH
catalyst. It was shown that the conversion–selectivity graphs
overlapped for fresh versus deactivated samples.[42] Additional
tests performed in the present study over H-MgAlPO-5 at two
different space velocities led to the same conclusion (Sec-
tion S8).
Alkenes are intermediate, autocatalytic species in the MTH
reaction. Reactions leading to their formation and conversion
include the methylation, oligomerization, and cracking of
short- and long-chain alkenes, as well as dealkylation of aro-
matic compounds, in particular polymethylated benzene mole-
cules.[13,20,30,54,55] The results presented in Figure 5 suggest that
a high acid strength favors cracking of higher alkenes to form
propene. This conclusion is in line with literature reports,
which showed that only strongly acidic zeolites could crack
C5+ alkenes to light C2 and C3 alkenes, whereas less-acidic zeo-
types favored oligomerization followed by cracking to higher
alkenes.[56,57] Interestingly, Miyaji et al. showed that 2-methyl-2-
butene and 1-pentene were not cracked in a monomolecular
mechanism over H-SAPO-5,[75] in line with the results presented
in Figure 5. Furthermore, Meusinger et al. studied cracking of
n-hexane and n-butane over H-MgAlPO-5, H-CoAlPO-5, and H-
SAPO-5.[58] These authors observed that comparable samples
of the materials (acid-site density, crystal size) gave a ranking
based on the TOF as H-MgAlPO-5>H-CoAlPO-5>H-SAPO-5. A
comparison between the ethene and propene selectivity plots
(Figure 5a,c) shows that the two products have opposite
trends with respect to selectivity versus predicted catalyst acid
The selectivity to three main categories of products, namely,
alkenes, alkanes, and aromatics, was chosen to illustrate the
principal trends and to highlight selectivity variations with het-
eroatom substitution (see Section S5 for the full set of selectiv-
ities).
Light alkenes, and propene in particular, are currently the
target products of the majority of industrial MTH plants.[53] Fig-
ure 4a shows that the selectivity to alkenes for Mg, Co, Zn,
and Si is approximately constant, at a very high level (70–
90%), between 15 and 85% conversion, whereas for Zr it
steadily increases towards 40% with increasing conversion.
The most striking feature of alkene selectivity is the distinct be-
havior exhibited by material groups categorized according to
the predicted acid strength. The stronger Brønsted acidic ma-
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Figure 4. Product selectivity for DME conversion into three major product categories at 4508C over the H-MAlPO-5 materials. Alkenes include ethene,
propene, butenes, pentenes, and identified C6+ alkenes.
Figure 5. Product selectivity for DME conversion into C3, C2, and C5+ at 4508C over the H-MAlPO-5 materials.
strength. This result suggests that the two products are
formed by mechanistically distinct routes. Ethene formation
was previously assigned to polymethyl benzene dealkylatio-
n,[13,19a,20,59] although at higher temperatures, a parallel route to
ethene from higher alkene cracking was shown to increase in
abundance over H-ZSM-5 zeolite.[21] Together, the results pre-
sented in Figures 4 and 5 may suggest that propene is mainly
formed from the alkene cycle, whereas ethene is mainly
formed from the arene cycle under the conditions used in this
study (see Scheme 1). Furthermore, an increase in the predict-
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ed catalyst acid strength favors the alkene cycle over the arene
cycle.
ed acid strength than other competing reactions in the MTH
reaction scheme.
The end products of hydrogen-transfer reactions, that is, the
alkanes, can be analyzed in view of the hydrogen-transfer
index (HTI). The HTI for C2–C5 products versus conversion over
H-MgAlPO-5 and H-SAPO-5 is plotted in Figure 6. The two
plots show that the HTI value observed for each individual
alkane/alkene pair is higher for H-SAPO-5 than for H-MgAlPO-
5. This result links the higher total alkene selectivity observed
for the more acidic material of the two, H-MgAPO-5 (see Fig-
ure 4a), directly to the relative hydrogen-transfer activity of
the two materials. We further note that the HTI is zero for C3,
very low for C2 (but increasing at decreasing conversion), and
substantially higher for C4 and C5 over both materials. The dif-
ference between C2, C4, and C5 is in line with that reported by
Iglesia et al. for H-beta zeolite[60] and reflects the relative stabili-
ty of primary versus secondary and tertiary carbocation inter-
mediates. The zero HTI value observed for propene, if com-
bined with the non-zero value observed for ethene, is surpris-
ing and cannot be explained at this point. However, we note
that the same observation is made for both materials.
Methane formation from two DME molecules via a methoxy
methylene species was recently directly linked to the formation
of the first CꢀC bond in the MTH reaction.[50,61] Whereas such a
direct hydrogen transfer between two oxygenate molecules is
feasible, the activation energy of this reaction is far higher
than that for hydrogen transfer from a DME or methanol mole-
cule to an adjacent alkene molecule.[23,51] Methane formation
associated with the MTH initiation reaction is, therefore, as-
sumed to take place mainly in the first part of the reactor, in
which the concentration of hydrocarbons is negligible.[62] An-
other pathway to methane formation is associated with hydro-
gen transfer from coke precursors to oxygenates, thereby
forming polyaromatic molecules. This pathway becomes more
abundant with increasing deactivation.[63]
Figure 7 shows the selectivity to methane over the H-
MAlPO-5 materials. In general, the selectivity to methane is low
over the stronger Brønsted acidic materials, slightly higher
over H-SAPO-5, and much higher over H-ZrAPO-5. At low con-
version (<15%), the selectivity curves for the medium and
strong acid materials converge to a common value. Deeper
analysis of the methane formation rates is complicated by the
different alkene selectivities over the studied materials. Howev-
er, we note that the methane selectivity trends are in line with
those obtained for the C2+ HTI, that is, that materials with low
predicted acid strength favor hydrogen-transfer reactions.
Finally, we turn to reactions leading to coke formation and
deactivation of the catalysts. Conversion versus time on stream
curves for all catalysts are shown in Figure 8, as are the C4 HTI
and C5 HTI versus aromatics yield plots. Considering first the
conversion versus time on stream curves, it is interesting to
note that among the materials with high acid strength (i.e.
Mg-, Co-, and Zn-substituted AlPO-5), which have very similar
effluent product selectivities, the Co- and Zn-substituted mate-
rials deactivate much more rapidly than the Mg-containing ma-
terial. Indeed, upon considering the different WHSVs used in
these tests, their DME conversion capacity is comparable to, or
lower than, that of H-SAPO-5, which has substantially higher
activity to hydrogen-transfer products in the effluent than the
more acidic materials (Figures 4, 6, and 7). A possible explana-
tion for this result could be leaking of Co and Zn from the H-
MAlPO-5 structure during testing. For this reason, in operando
The mechanism of the hydrogen-transfer reactions in MTH
has been scarcely studied, mainly due to the complexity of the
product spectrum obtained even at low conversion (e.g. see
ref. [30]). Recently, however, studies in which methanol and
DME were compared as methylating agents of benzene and
isobutene, mainly over H-ZSM-5, yielded new insight into
these reactions. Briefly, the studies revealed a key role of meth-
anol and DME as hydrogen-transfer agents between the alkene
and arene cycles, as well as between monocyclic arenes and
heavier analogues.[31,51] Furthermore, hydrogen-transfer reac-
tions were found to take place at the Brønsted acid sites,
whereas isolated Lewis acid sites had negligible activity for
both the methylation and hydrogen-transfer reactions.[23] The
role of Lewis acid sites in MTH is still not fully revealed. Anoth-
er recent study of MTH, performed with a H-ZSM-5 catalyst, re-
ported that the rate of methanol conversion correlated with
the number of Brønsted acid sites, whereas the aromatics yield
correlated with the number of Lewis acid sites in the materi-
al.[21] Although the underlying reason for this result is not fully
understood, the HTI results obtained suggest that the hydro-
gen-transfer activity of H-MAPO-5 is less influenced by Brønst-
Figure 6. C2, C4, and C5 HTI values for H-MgAlPO-5 and H-SAPO-5 at 4508C. The HTI for C3 was zero at all conversion levels and is not included in the figure.
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demonstrating that these products are formed by (oxygenate-
promoted) hydrogen-transfer reactions. In the Co- and Zn-con-
taining materials, on the other hand, the HTI values are too
low to account for the aromatics yield, which suggests another
parallel pathway to aromatics formation. Co- and Zn-ex-
changed zeolites were previously shown to have alkane dehy-
drogenation activity,[64,65] and we speculate that this is also the
case for the isomorphically substituted H-CoAlPO-5 and H-
ZnAlPO-5 materials used in this study. Such activity could con-
tribute to more rapid deactivation of the Co- and Zn-contain-
ing catalysts.
Overall, the product selectivity data obtained over the iso-
morphically substituted H-MAPO-5 materials strongly suggest
that under the conditions used in this study, an increasing acid
strength favors methylation and cracking reactions over hydro-
gen-transfer reactions. Whereas the detailed interaction be-
tween the reactant molecules and the active catalyst is not
fully revealed, this study clearly shows that an increasing acid
strength enhances the alkene versus arene reaction cycle for
total alkene selectivity and also enhances the propene-to-
ethene ratio of the H-MAlPO-5 catalysts.
Figure 7. Methane selectivity over the H-MAlPO-5 materials at 4508C.
X-ray absorption spectroscopy (XAS) measurements were per-
formed for these two materials (Section S9). Briefly, both Co
and Zn are in the expected +2 oxidation state after calcination
and the +2 oxidation state is retained during activation in air
at 5508C and, more importantly, during catalytic testing mim-
icking high and low conversion of DME/methanol (see Sec-
tion S9 for a detailed analysis). With the metal-leaking hypoth-
esis excluded, an alternative explanation was sought. In
Figure 8, a clear, positive correlation between HTI and aromat-
ics yield is observed for the Mg- and Si-containing materials,
As a final note, the MTH activity trend observed for the ma-
terials presented in this study are in line with those previously
published by our group for two isostructural zeolite/SAPO
pairs, that is, H-SSZ-13 versus H-SAPO-34 (CHA structure) and
H-SSZ-24 versus H-SAPO-5 (AFI structure). In those contribu-
tions, the acid strength was assessed experimentally by the rel-
ative shift in the OꢀH stretch frequency of the Brønsted acid
site upon CO adsorption, as measured by IR spectroscopy. In
both contributions, the material with the highest acid strength,
Figure 8. Deactivation plots and C4/C5 HTI values versus aromatics yield for DME conversion at 4508C, pDME =13 mbar, WHSV=0.23–1.9 hꢀ1
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80% of the total added water for 30 min, followed by adding the
metal acetate dissolved in the remaining 20% of the water. Ortho-
phosphoric acid was then added dropwise with stirring, followed
by the dropwise addition of Et3N. The solution was stirred for
30 min after the addition of Et3N and was then crystallized for 4 h
at 2008C in a rotating Teflon-lined steel autoclave. The total weight
of the synthesis gel was typically 30 g.
the zeolite, showed substantially higher activity for the MTH
reaction than the corresponding SAPO material.[18,66] The prod-
uct selectivity trends could be reasonably assessed only for the
H-SSZ-24 versus H-SAPO-5 pair due to the diffusional restric-
tions of the H-SSZ-13 versus H-SAPO-34 pair. For H-SSZ-24
versus H-SAPO-5, the selectivity towards ethene and aromatic
products was higher for H-SSZ-24 than for H-SAPO-5 upon
using methanol as the feed,[18,31] whereas it was similar for the
two materials upon using DME as the feed.[31] These results
may seem disparate from those presented in the current con-
tribution. However, taking into account the different properties
of the material surrounding the active site in the SiꢀOꢀSi zeo-
lite lattice, which is more covalently bonded than the AlꢀOꢀP
MAlPO lattice, these results rather point to a more complex
material–performance correlation than previously assessed.
H-ZrAlPO-5 and H-TiAlPO-5 were made by following a similar pro-
cedure, with the exception of adding zirconium(IV) acetylacetonate
and titanium(IV) isopropoxide directly to the stirring solution of
Catapal B and all of the added H2O without dissolving the metal
prior to the addition. AlPO-5 was made following the procedure
without adding any metal to the Catapal B and H2O solution.
The recovered material was washed and centrifuged three times
with deionized water and dried at 808C for 18 h. Removal of Et3N
was performed by heating the sample to 5508C at a rate of
18Cminꢀ1 under a 20% O2 in N2 atmosphere, followed by calcina-
tion at this temperature for 10 h.
3. Conclusions
Careful design and rigorous execution of the synthesis of a
consistent series of isomorphically substituted H-MAlPO-5 ma-
terials differing only by the substituted metal and homoge-
nous with respect to the remaining properties allowed us to
attribute unambiguously the observed differences in catalytic
performance to acid strength associated with modified Brønst-
ed acid sites (BAS). Substantial characterization evidence was
provided, firstly showing the homogeneity of the samples with
respect to particle size, morphology, surface area, and acid-site
density, and secondly strongly indicating the incorporation of
the metals as BAS and their stability under the reaction condi-
tions. All materials were shown to be active in the methanol to
hydrocarbons (MTH) reaction at 4508C with clear activity differ-
ences, agreeing well with the predicted acid strength.
Catalyst Characterization
Temperature-programmed desorption of n-propylamine was per-
formed at atmospheric pressure in a fixed-bed glass reactor (i.d.
11 mm), similar to the procedure described by Gorte et al.[67] The
catalyst powder was pressed and sieved to 250–420 mm and was
pretreated in a flow of air at 5508C. The catalyst was then cooled
to 1508C. N2 bubbled (80 mLminꢀ1) through a saturator containing
n-propylamine at room temperature was then fed over the catalyst
for 20 min. The excess amount of n-propylamine was removed by
flowing N2 at a rate of 80 mLminꢀ1 for 4 h at 1508C. The tempera-
ture was then ramped at 208Cminꢀ1 up to 5508C. The amount of
propene desorbed was quantified by using an on-line Pfeiffer Om-
nistar quadrupole mass spectrometer.
The powder X-ray diffraction (XRD) patterns of the as-synthesized
and calcined catalysts were recorded by using a Siemens Bruker
D8 Discover instrument with Bragg–Brentano geometry by using
CuKa radiation (l=1.5406 ꢃ).
The materials exhibited clear selectivity differences that
could be categorized according to acid strength. Strong acids
(Mg, Co, Zn) showed the highest selectivity towards the de-
sired alkene products, with low selectivity towards alkanes and
aromatic products. The medium-strength acid (Si) showed in-
creased selectivity towards aromatics and alkanes and a dimin-
ished proportion of alkenes. The weakly acidic Zr-substituted
material followed the trend of decreasing alkene selectivity
with decreasing acid strength.
The size and morphology of the zeolite particles were analyzed by
scanning electron microscopy (SEM), recorded with a Hitachi SU
8230 FE-SERM. Utilizing energy-dispersive X-ray spectroscopy (EDS)
with the same instrument, the elemental composition of the sam-
ples was determined.
The BET surface areas and pore volumes were determined by using
N2 physisorption at 77 K by using a Belsorp-mini II instrument.
Fresh catalysts were outgassed under vacuum for 4 h, 1 h at 808C,
followed by a period of 3 h at 3008C. The BET surface areas were
determined on the basis of a linear fit of the data in the relative
pressure (p/p0) range of 0.01 to 0.1.
These encouraging results will be pursued in future studies
of individual reactions over the same series of catalysts.
Experimental Section
Catalyst Synthesis
FTIR spectra were collected by using a Bruker Vertex70 instrument
equipped with a liquid-nitrogen-cooled mercury cadmium telluride
detector and a Harrick Praying Mantis accessory able to collect
data in diffuse reflectance mode (diffuse reflectance infrared Fouri-
er transform, DRIFT). A total of 128 spectra were averaged for each
acquisition with a spectral resolution of 2 cmꢀ1. Data are reported
in log (R) versus wavenumber. The samples were activated in a
15 mLminꢀ1 helium flow from RT to 4508C (58Cminꢀ1) to remove
water and were then cooled down to 708C to dose vapors of deu-
terated acetonitrile (CD3CN, supplied by Sigma–Aldrich) as probe
molecule, through a 15 mLminꢀ1 helium flow. The desorption of
the probe was performed by flowing pure helium (under the same
H-MAlPO-5 (M=Mg, Co, Zn) was synthesized hydrothermally from
water, triethylamine (Sigma–Aldrich, 99%), orthophosphoric acid
(VWR, 85%), Catapal B (Sasol), and Mg(OAc)2·4H2O or Co(OA-
c)2·4H2O or Zn(OAc)2·4H2O (Sigma–Aldrich, 99.99%). H-ZrAlPO-5 or
H-TiAlPO-5 was synthesized from water, triethylamine, orthophos-
phoric acid (VWR, 85%), Catapal B (Sasol), and titanium(IV) isoprop-
oxide (Sigma–Aldrich, 97%) or zirconium(IV) acetylacetonate
(Sigma–Aldrich, 97%), respectively.
For Mg, Co, and Zn, a representative gel composition of 1.0Al2O3/
1.0P2O5/0.1MO/1.0Et3N/50H2O was made by stirring Catapal B in
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conditions) for approximately 15 h. CD3CN was used instead of
CH3CN to avoid the well-known spectroscopic complication due to
Fermi resonance between the n(CN) vibration and the combination
mode d(CH3)+n(CC).
space velocity of 1.9 hꢀ1. In the second experimental regime, to
obtain comparable selectivity data at 4508C, catalyst mass was
varied under
a fixed total flow reactant and inert gas of
50 mLminꢀ1 and pDME =13 mbar. This gave space velocities of 1.9
(H-MgAlPO-5), 1.1 (H-CoAlPO-5), 0.8 (H-ZnAlPO-5), 0.47 (H-SAPO-5),
0.23 (H-ZrAlPO-5), 0.70 (H-TiAlPO-5), and 0.25 gDME gcatꢀ1 hꢀ1 (AlPO-
5).
The X-ray absorption spectra at the Co and Zn K-edges were col-
lected in transmission mode at the Swiss–Norwegian Beamline
BM31[68,69] of the European Synchrotron Radiation Facility (ESRF). A
double-crystal Si(111) monochromator in the continuous scanning
mode was used to obtain a monochromatic X-ray beam. Mono-
chromator crystals were detuned to minimize the higher harmonic
contribution. A third ionization chamber was used to measure the
adsorption of the corresponding metal foil for internal energy cali-
bration.[70] For H-ZnAlPO-5 samples, extended X-ray absorption fine
structure (EXAFS) spectra under static conditions were collected up
to k=15.5 ꢃꢀ1 in a 15 min acquisition scan, whereas sample activa-
tion was monitored by collecting X-ray absorption near-edge struc-
ture (XANES) every 3 min. Due to the lower Co content, for H-
CoAlPO-5 samples the EXAFS spectra were collected only up to k=
14.0 ꢃꢀ1 in a 30 min acquisition scan; XANES spectra were collect-
ed in 5 min. Totals of 3 and 5 equivalent EXAFS scans were collect-
ed for H-ZnAlPO-5 and H-CoAlPO-5, respectively, and were aver-
aged in k space before the EXAFS data analysis, as described else-
where.[71]
The effluent from the reactor was analyzed after 5 min of reaction,
and subsequently, every 75 min, by an on-line GC–MS instrument
(Agilent 7890 with flame ionization detector and 5975C MS detec-
tor) by using two Restek Rtx-DHA-150 columns. Hydrogen (Praxair,
purity 6.0) was used as the carrier gas.
Acknowledgements
The authors would like to thank Maria Mykland for synthesis of
the AlPO-5 sample and Mustafa Kømurcu, Reynald Henry, and
Michael Dyballa for assistance during XAS measurements at the
Swiss–Norwegian beamline at ESRF. Kirill A. Lomachenko and
Aram L. Bugaev are acknowledged for fruitful discussion on the
XAS data. The ESRF is acknowledged for granting beamtime at
BM01. M.M., L.M., A.L., and U.O. acknowledge the Norwegian Re-
search Council for financial support through contract 239193.
I.A.P. and C.L. acknowledge the Mega-grant of the Russian Feder-
ation Government to support scientific research at the Southern
Federal University, No. 14.Y26.31.0001.
The measurements were performed in operando mode by using a
microtomo reaction cell (hosting the sample in form of self-sup-
ported pellet) that allowed sample temperature and gas flows to
be remotely controlled and that allowed the gas outlet to be ana-
lyzed by a Pfeiffer Omnistar mass spectrometer.[72–74] For H-CoAlPO-
5 and H-ZnAlPO-5, the following protocol was followed: 1) sample
activation by heating from RT to 5508C in air (58Cminꢀ1), moni-
tored by XANES; 2) subsequent cooling in He flow to 4508C
(EXAFS collection); 3) the reaction gas feed (2 mLminꢀ1 helium
sent through a saturator containing MeOH at room temperature)
was sent directly through the microtomo cell (hereafter named 1st
cycle) (EXAFS collection after stabilization, monitored by online
MS); 4) He was flushed at 4508C for 2 h; 5) the reaction gas feed
(CH3OH+He) was then sent to a reactor loaded with a commercial
zeolite H-ZSM-5 sample (ZeoChem, Si/Al=59) to convert methanol
into hydrocarbons, which subsequently was sent to the sample cell
to mimic high conversion over the samples (2nd cycle). Also, for
step 5 the EXAFS collection started after stabilization, monitored
by online MS.
Keywords: acidity · aluminophosphates · Brønsted acids ·
hydrocarbons · zeotypes
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Catalytic tests were performed at atmospheric pressure in fixed-
bed quartz reactors with the catalyst powder pressed and sieved
to 250–420 mm. The inner diameter of the reactor was 8 mm. Reac-
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a 3 mm wide quartz sleeve inserted into the middle of the catalyst
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Manuscript received: September 20, 2017
Revised manuscript received: October 24, 2017
Version of record online: && &&, 2017
&
ChemPhysChem 2017, 18, 1 – 13
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ꢂ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

ARTICLES
Acid test: Metal-substituted alumino-
phosphates are tested for the metha-
nol-to-hydrocarbons reaction at 4508C
by using dimethyl ether (DME) as feed.
Stronger acids convert DME significantly
faster than medium and weak Brønsted
acidic materials, and the stronger
Brønsted acids produce more light
alkenes than the weaker acids, which
indicates that the relative rates of
competing reactions change upon
decreasing the acid strength.
M. Mortꢀn, Ł. Mentel, A. Lazzarini,
I. A. Pankin, C. Lamberti, S. Bordiga,
V. Crocellꢁ, S. Svelle, K. P. Lillerud,
U. Olsbye*
&& – &&
A Systematic Study of Isomorphically
SubstitutedH-MAlPO-5 Materials for
the Methanol-to-Hydrocarbons
Reaction
ChemPhysChem 2017, 18, 1 – 13
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&
These are not the final page numbers! ÞÞ