Synthesis of Al-MTW with low Si/Al ratios by combining organic and inorganic structure directing agents

Article abstract of DOI:10.1039/c5nj01203a

A rationalized combination of alkali cations and bulky dicationic organic structure directing agents (OSDAs) has allowed the synthesis of the Al-rich MTW zeolites with Si/Al ratios of ～12 and large pore accessibility. ²⁷Al MAS NMR spectroscopy indicates that most of the aluminum atoms are in tetrahedral coordination in framework positions, and in situ infrared pyridine adsorption/desorption spectroscopy reveals strong Br?nsted acidity after cationic exchange for the Al-rich MTW. In addition, another MTW material with a Si/Al ratio of 30 has been synthesized under alkali-free conditions using a bulky dicationic molecule such as OSDA, the lowest Si/Al ratio being achieved for a MTW zeolite synthesized in the absence of alkali-cations in the synthesis media. The catalytic activity of these MTW materials has been tested for the n-decane cracking reaction, achieving higher catalytic activities and olefin yields than other related large pore zeolites.

Full text of DOI:10.1039/c5nj01203a

NJC
View Article Online
View Journal
PAPER
Synthesis of Al-MTW with low Si/Al ratios by
combining organic and inorganic structure
directing agents
Cite this:DOI: 10.1039/c5nj01203a
Cecilia Paris,a Nuria Martın,a Joaquın Martınez-Triguero,a Manuel Moliner*a and
´
´
´
Avelino Corma*ab
A rationalized combination of alkali cations and bulky dicationic organic structure directing agents (OSDAs)
has allowed the synthesis of the Al-rich MTW zeolites with Si/Al ratios of B12 and large pore accessibility.
27Al MAS NMR spectroscopy indicates that most of the aluminum atoms are in tetrahedral coordination
in framework positions, and in situ infrared pyridine adsorption/desorption spectroscopy reveals strong
Received (in Montpellier, France)
12th May 2015,
Bronsted acidity after cationic exchange for the Al-rich MTW. In addition, another MTW material with a Si/Al
¨
Accepted 30th July 2015
ratio of 30 has been synthesized under alkali-free conditions using a bulky dicationic molecule such as
OSDA, the lowest Si/Al ratio being achieved for a MTW zeolite synthesized in the absence of alkali-cations in
the synthesis media. The catalytic activity of these MTW materials has been tested for the n-decane cracking
reaction, achieving higher catalytic activities and olefin yields than other related large pore zeolites.
DOI: 10.1039/c5nj01203a
www.rsc.org/njc
following a seed-assisted methodology.12 This seeding metho-
dology has allowed the synthesis of Al-MTW zeolite with Si/Al
1. Introduction
MTW is a one-dimensional large pore zeolite with pore openings ratios in the final product of almost 12 when the Si/Al ratio in
of 5.6 ꢀ 6.0 Å in its structure.1 The first synthesized MTW structure the synthesis gel was fixed at 30.12 This large difference between the
was the high-silica ZSM-12 zeolite, by researchers of Mobil Oil Co theoretical and the final Si/Al ratios indicates that the zeolite yields
with Si/Al ratios above 40.2 In addition to ZSM-12, other zeolites achieved in the synthesis were very low. Nevertheless, the main
presenting the MTW structure have been reported in the literature, problem of the Al-rich MTW zeolites synthesized following the
such as TPZ-12,3 Nu-13,4 Theta-3,5 CZH-5,6 and VS-12,7 where the OSDA-free methodology is their very low pore accessibility, since a
main differences between them are the chemical composition micropore volume of 0.006–0.035 cm3 gꢁ1 compared to regular
(introduction of different heteroatoms, such as V, Fe, Ti, among MTW acid materials with a micropore volume of 0.12 cm3 gꢁ1 was
others) and/or the organic structure directing agents (OSDAs) used measured.12 The possible reason for the low micropore volume of
for their synthesis. Nevertheless, the most important form of the the MTW materials presented with the OSDA-free method could be
MTW structure is aluminosilicate (Al-MTW), since the incorpora- the partial blocking of the one-dimensional channels due to the
tion of aluminum atoms in the MTW framework creates strong presence of structural defects.13 Indeed, the defects would be formed
¨
Bronsted acid sites, allowing the application of Al-MTW as a by the excessive positive charges introduced by the alkali-cations
catalyst for several chemical processes, such as isomerization,8 when used as single structure directing agents. In that case, the
alkylation and disproportionation of aromatic hydrocarbons,9 alkaline content is larger than the Al framework and the excess of
or cracking of hydrocarbons.10
positive charge is compensated by Si–Oꢁ framework groups, which
Since the discovery of the high-silica ZSM-12 zeolite, the become internal sylanols when the alkaline content is removed.
synthesis of the Al-rich MTW material has become a clear On the other hand, during the synthesis of MTW assisted
objective to improve its acidic properties.11 Okubo et al. have by organic structure directing agents, samples with higher
recently described the synthesis of the Al-rich MTW zeolite framework Si/Al ratios are achieved, and the filling of the pores
under OSDA-free conditions using sodium aluminosilicate gels requires a relatively small number of organic cations within
the pores of the MTW zeolite. Then, their positive charge can
be compensated by the negative charge associated with the
presence of the Al framework, reducing therefore the formation
of structural defects (internal sylanols).
Tetraethylammonium (TEA), methyltriethylammonium (MTEA),
and benzyltrimethylammonium (BTMA)14 have been described as
a
´
´
´
Instituto de Tecnologıa Quımica (UPV-CSIC), Universidad Politecnica de Valencia,
´
Consejo Superior de Investigaciones Cientıficas, Valencia, 46022, Spain.
E-mail: acorma@itq.upv.es, mmoliner@itq.upv.es
b King Fahd University of Petroleum and Minerals, P.O. Box 989, Dhahran 31261,
Saudi Arabia
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2015
New J. Chem.

View Article Online
Paper
NJC
prepared by reacting piperidine (0.1 mol) and 1,2-dichloro-
ethane (0.025 mol) at 100 1C (80% yield). In the second step,
the quaternary salt was achieved by direct alkylation of 1,1-bis-
piperidinoethane (0.02 mol) in acetonitrile with iodomethane
(0.16 mol) at 40 1C. OSDA-C2 was obtained as a white solid.
Elemental analysis. N = 5.84; C = 35.17; H = 6.64 (found, %).
N = 5.83; C = 35.02; H = 6.30 (calculated, %). Formula
(C14H30N2I2).
OSDA-C3. Piperidine (0.1 mol) was introduced into a two-
neck round flask and, an aqueous NaOH solution (0.11 mol)
was added to the flask and left to react for 15 min under
vigorous stirring. Finally, 1,3-dibromopropane (0.05 mol) was
added drop-wise and the resulting biphasic solution was heated
at 80 1C for 12 h. Once the reaction was completed, the organic
layer was separated from the crude, and washed with water
(3 ꢀ 15 ml) and Brine solution (1 ꢀ 20 ml). Then, the organic
layer was dried with anhydrous MgSO4 and filtered. Finally,
1,3-bispiperidinopropane was obtained as a pale yellow oil,
with 78% yield.
In a second step, 1,3-bispiperidinopropane (0.039 mol) was
subjected to alkylation with iodomethane (0.195 mol) in a
methanolic solution at 40 1C. The quaternary ammonium salt
OSDA-C3 was obtained as white crystals after recrystallization
(70% yield).
Fig. 1 Diquaternary ammonium compounds proposed as OSDA for the
synthesis of the MTW structure.
the most common OSDAs for the synthesis of Al-MTW. Never-
theless, the lowest Si/Al ratios reported for the synthesis of pure
Al-MTW phases using the above described monocationic
OSDAs in combination with small inorganic cations are still
limited to 25–30.14
Very recently, Li et al. have reported the synthesis of the
Al-rich MTW structure using a bulky and complex dicationic
OSDA, such as N,N,N0,N0-tetraethylbicyclo[2.2.2]oct-7-ene-2,3:5,6-
dypyrrolidinium, combined with the presence of potassium
cations in the synthesis media.15
In addition, Zones et al. have systematically studied the
synthesis of zeolites with OSDAs formed by diquaternary
ammonium compounds of C4–C6 chain lengths with different
heterocyclic end-groups.16 They observed that when the hetero-
cyclic end-group was N-methylpiperidine (see OSDA-C4 in Fig. 1),
the synthesis of aluminosilicates in alkaline media was very
selective towards MTW, but the Si/Al ratios were larger than 40.16
The high selectivity of this linear OSDA towards MTW under
alkaline synthesis conditions together with its dicationic nature,
provide a unique opportunity to modulate the presence of positive
charges within the MTW pores either in combination or in the
absence of small inorganic cations and, therefore, to control the
amount of aluminum atoms in the framework positions of MTW.
We will show here that the use of related linear dicationic
OSDAs of different sizes (see Fig. 1), either in combination or in
the absence of alkali metal cations (Na+ or K+), can direct the
synthesis of Al-MTW zeolites toward materials with low Si/Al
ratios in the final solids, while preserving the pore accessibility
when the zeolite is activated. If this was achieved, the catalytic
activity of MTW for acid catalyzed reactions should increase,
opening further possibilities for its use, as will be shown here.
Elemental analysis. N = 5.60; C = 36.35; H = 6.64 (found, %).
N = 5.67; C = 36.45; H = 6.53 (calculated, %). Formula
(C15H32N2I2).
OSDA-C4. 1-Methylpiperidine (0.15 mol) was dissolved in
an argon atmosphere, in anhydrous N,N-dimethylformamide
(50 ml). Then, under continuous stirring, 1,4-dibromobutane
(0.075 mol) was slowly added and the reaction mixture was
allowed to react at room temperature for 24 h. OSDA-C4 is
obtained as a white solid (61% yield).
Elemental analysis. N = 6.78; C = 45.70; H = 9.21 (found, %).
N = 6.76; C = 46.39; H = 8.27 (calculated, %). Formula (C16H34N2Br2).
2.1.2. Zeolite synthesis. In a typical synthesis, alumina
(74.6%, Condea) was dissolved in an aqueous solution formed
by the hydroxide salts of the OSDA and the alkali cation (Na+ or
K+). Colloidal silica (Ludox AS-40, Aldrich) was then added, and
the gel was allowed to reach the desired silica to water ratio by
evaporation. Finally, the gel was transferred to Teflon lined
stainless autoclaves and heated at 175 1C for 14 days. The solids
were recovered by filtration, extensively washed with distilled
water, and dried at 90 1C overnight.
2.2. Characterization
Powder X-ray diffraction (PXRD) measurements were per-
formed using a multisample Philips X’Pert diffractometer
equipped with a graphite monochromator, operating at 45 kV
and 40 mA, and using Cu Ka radiation (l = 0,1542 nm).
The chemical analyses were carried out in a Varian 715-ES
2. Experimental
2.1. Synthesis
2.1.1. OSDA preparation
OSDA-C2. The preparation was performed following a two- ICP-Optical Emission spectrometer, after solid dissolution in
step procedure. In the first step, 1,2-bispiperidinoethane was HNO3/HCl/HF aqueous solution. The organic content of as-made
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2015

View Article Online
NJC
Paper
Table 1 Experimental design for the synthesis of zeolites using different
diquaternary ammonium compounds
materials was determined by elemental analysis performed with
a SCHN FISONS elemental analyser.
The morphology of the samples was studied by scanning
electron microscopy (SEM) using a JEOL JSM-6300 microscope.
Textural properties were obtained from the N2 adsorption
Variable
Values
OSDA type OSDA-C2, OSDA-C3, OSDA-C4
Si/Al
INF, 30, 15
isotherms measured at 77 K with a Micromeritics ASAP 2020 H2O/Si
10, 30
OSDA/Si
0.25 (for alkali-free), 0.2 (for synthesis with alkali cations)
0, Na/Si = 0.1, K/Si = 0.1
apparatus.
Alkali/Si
Solid NMR spectra were recorded at room temperature with
a Bruker AV 400 MAS spectrometer. 27Al MAS NMR spectra were
recorded at 104.2 MHz at a spinning rate of 10 kHz and a 91
pulse length of 0.5 ms with 1 s repetition time. 27Al chemical
shift was referred to as Al3+(H2O)6.
Infrared spectra were measured with a Nicolet 710 FT IR
spectrometer. Pyridine adsorption–desorption experiments
were made on self-supported wafers (10 mg cmꢁ1) of original
samples previously activated at 673 K and 10ꢁ2 Pa for 2 hours.
After wafer activation, the base spectrum was recorded and
pyridine vapour (6.5 ꢀ 102 Pa) was admitted in the vacuum IR
cell and adsorbed onto the zeolite. Desorption of pyridine was
performed in vacuum over three consecutive one-hour periods
of heating at 423, 523 and 623 K, each of them followed by the
IR measurement at room temperature. The spectra were scaled
according to the sample weight.
Fig. 2 Phase diagram obtained for the synthesis of zeolites using different
diquaternary ammonium compounds.
2.3. Catalytic test
NON is a cage-like chlatrasil type structure with pore open-
ings lower than 3 Å.18 This dense phase is preferentially
obtained when using the smaller dicationic OSDAs (OSDA-C2
and OSDA-C3, see Fig. 1), always in the presence of alkali metal
cations (both Na+ and K+) in the synthesis gels (see Fig. 2).
Similar structure directing effects towards chlatrasil frame-
works have been described in the literature by using small
OSDAs combined with alkali metal cations in zeolite synthesis
under alkaline conditions.19 MTW is the only competing crystal-
line phase when using small OSDAs (OSDA-C2 and OSDA-3) and
aluminum in the synthesis gels (see Fig. 2). These results would
suggest that bulkier OSDAs could help in directing the crystal-
lization towards more open crystalline phases than the dense
NON phase.
As expected, the use of the bulkier OSDA-C4 molecule directs
the crystallization of the open crystalline MTW structure, pre-
cluding the formation of the undesired dense NON phase
(see Fig. 2). MTW is preferentially achieved as pure crystalline
phase when alkali metal cations, either sodium or potassium,
are present in the synthesis gels under high silica or even pure
silica conditions (see Fig. 2). Nevertheless, by properly combin-
n-Decane cracking experiments were performed at 500 1C, 60 s
time on stream, and different zeolite to n-decane ratios in a
microactivity test unit as described previously.17 Zeolites were
pelletized, crushed, and sieved, and the 0.59 to 0.84 mm
fraction was taken and diluted in 2.5 g of inert silica. For
each catalyst, five experiments were performed, maintaining
the amount of catalyst (cat) constant and equal to 0.5 g, and
varying the n-decane amount fed (oil) between 0.77 and 1.54 g.
Apparent kinetic rate constants were calculated by fitting the
conversions (X) to a first-order kinetic equation for a plug flow
reactor (1), assuming that the deactivation is enclosed in the
kinetic constant and taking into account the volumetric expansion
factor (2).
K = ꢁ(cat oilꢁ1 TOS)ꢁ1 [eX + (1 + e) ln(1 ꢁ X)]
(1)
(2)
P
e = ( molar selectivities of products) ꢁ 1
3. Results
3.1. Syntheses of Al-rich MTW zeolites
Based on the previous results,16 three related linear dicationic ing potassium cations and OSDA-C4, it was possible to synthe-
OSDAs with different sizes were synthesized and used as OSDAs size a highly crystalline MTW zeolite with a Si/Al ratio in the
for the synthesis of Al-rich MTW zeolites (see Fig. 1). The synthesis gel of 15 (see MTW-1 in Fig. 2). In this case, MTW
experimental design containing the synthesis conditions is with the same Si/Al ratio as the starting gel could be obtained,
summarized in Table 1. 54 synthesis experiments have been as it will be shown later. Note that the only synthesis we have
performed where the OSDAs, Si/Al, H2O/Si, and alkali/Si ratios found in the literature giving a similar low Si/Al ratio is the
where varied. The phase diagram, achieved after the crystal- OSDA-free procedure described by Okubo et al. and, as said
lization process at 175 1C for 14 days, shows the presence of above, the resultant MTW crystalline material had low pore
amorphous materials and the formation of two different crystal- accessibility, due to the presence of a large number of struc-
line phases, such as NON and MTW (see Fig. 2).
tural defects.12 We were also able to prepare a MTW as a pure
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2015
New J. Chem.

View Article Online
Paper
NJC
Table 2 Chemical analyses and acid properties of the crystalline MTW
materials
crystalline phase in the absence of alkali-cations with OSDA-C4
and with a Si/Al ratio of 30 (see MTW-2 in Fig. 2). It has to be
remarked that the synthesis of Al-MTW under alkali metal cation-
free synthesis conditions is very unusual. Indeed, zeolites synthe-
sized with alkali metal cations require post-synthetic ion-exchange
treatments before their use as acid catalyst. Very recently,
researchers at BASF have reported the alkali-free synthesis of
the Al-MTW zeolite using 4-cyclohexyl-1,1-dimethylpiperazinium
as OSDA, and a Si/Al ratio of almost 50 was required for crystal-
lizing this material.20 Thus, MTW-2 could be synthesized in our
work under alkali-free conditions with lower Si/Al ratios than
those reported by researchers at BASF.
Aciditya (mmol Py grꢁ1
)
Sample
(Si/Al)gel (Si/Al)ICP K/Al
B150
B250
B350
MTW-1
MTW-1-Exc
MTW-2
15
—
30
12.5
12.6
30.3
0.3
0.035
0.025
0.120
0.082
0.007
0.082
0.063
0.005 0.138
0.095
—
a
Calculated by in situ infrared pyridine adsorption/desorption at
different temperatures.
3.2. Physico-chemical characterization of the Al-rich MTW
zeolite samples
PXRD patterns of the as-prepared MTW-1 and MTW-2 materials
reveal the presence of the pure MTW crystalline phase (see Fig. 3),
and scanning electron microscopy (SEM) images show a similar
average crystal size (B1–2 mm) for both MTW materials (see Fig. 4).
Chemical analyses of the crystalline MTW materials indicate
that the Si/Al ratios in the final solids are similar to the Si/Al
ratios introduced in the synthesis gels (see Table 2). Interestingly,
the MTW-1 material presents a Si/Al ratio in the solid of 12.5,
revealing that a relatively Al-rich MTW material can be obtained
by properly combining potassium cations and OSDA-C4 as struc-
ture directing agents.
Fig. 5 Solid 27Al MAS NMR spectra of the MTW materials.
MTW-2 show a single band at B55 ppm that has been assigned to
tetrahedrally coordinated Al at framework positions (see Fig. 5). If
this is so, then it can be said that the synthesis methodology
presented here allows the efficient introduction of aluminum
atoms at framework positions of the MTW materials.
The coordination of aluminium atoms in the MTW materials
has been studied by solid 27Al MAS NMR spectroscopy. MTW-1 and
N2 adsorption measurements have been performed to evaluate
the pore accessibility of the two MTW samples after calcination in
air at 550 1C. As described above, pore blocking could be a critical
issue for one-dimensional zeolites.12 In the case of the calcined
MTW-1 zeolite, since this material has been synthesized using
potassium cations (see potassium content of MTW-1 in Table 2), a
previous cationic exchange with ammonia followed by calcination
at 500 1C is required to remove the extra-framework inorganic
cations and properly evaluate the pore accessibility. Practically
+
all potassium cations were removed by NH4 exchange (see
potassium content in MTW-1-Exc sample in Table 2), and the
N2 isotherm of the MTW-1-Exc material indicates high micro-
porosity (see Fig. 6), with a micropore surface area and micro-
Fig. 3 PXRD patterns of the as-prepared crystalline MTW-1 and MTW-2
materials.
pore volume of 261 m2 gꢁ1 and 0.12 cm3
g
ꢁ1, respectively
(see Table 3). It is important to note that this micropore volume
is similar to the values reported for highly crystalline high Si/Al
MTW materials in the literature.11 The N2 adsorption isotherm
of MTW-2 can be directly obtained after calcination of the
synthesized sample since this sample was obtained in the
absence of alkali ions and a post-synthesis cation exchange is
not required to produce the acid sample. As seen in Fig. 6 and
Table 3, MTW-2 also shows high microporosity, with a micro-
pore area of 274 m2 gꢁ1 and a micropore volume of 0.13 cm3 gꢁ1
.
The acid properties of the Al-rich MTW-1 material have
been characterized by IR spectroscopy of adsorbed pyridine.
Fig. 4 SEM images of the MTW materials.
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2015

View Article Online
NJC
Paper
adsorption (see MTW-1-Exc in Fig. 7). The intensity of this band
remains important after increasing the pyridine desorption tem-
perature, even at 350 1C (see MTW-1-Exc in Fig. 7 and Table 2),
¨
revealing a strong Bronsted acid behavior of the MTW-1-Exc
zeolite, which is useful for carbocation type reactions.
¨
When the Bronsted acidity of the MTW-2 sample was mea-
sured, pyridine adsorption results show a lower intensity of the
1545 cmꢁ1 band for MTW-2 than for MTW-1-Exc. The amount of
¨
Bronsted acid sites per gram of zeolite was determined from the
area of that IR band using the extinction coefficient reported by
Emeis,21 and the values are given in Table 2.
3.3. Catalytic results
The catalytic activity of the Al-rich MTW samples has been
studied here for the cracking of n-decane at 500 1C and 60 s of
time on stream. n-Decane is a linear alkane able to diffuse
through the one-dimensional 12-ring channels of MTW. The
catalytic activity of these MTW samples has been compared
with two mordenite (MOR) samples of similar Si/Al ratios. The
framework of MOR contains large pores (6.5 ꢀ 7.0 Å), which are
slightly higher than those of the MTW zeolite (6.0 ꢀ 5.6 Å),
interconnected by 8-ring pores (3.4 ꢀ 4.8 Å). In general, the
results presented in Table 4 show that MTW samples present
higher catalytic activity in terms of the kinetic rate constant
than mordenites. This higher catalytic activity compared to
Fig. 6 N2 adsorption isotherms of the MTW materials.
Table 3 Textural properties calculated from N2 adsorption isotherms
BET surface
area (m2 gꢁ1
Micropore area
Micropore volume
Sample
)
(m2 gꢁ1
)
(cm3 gꢁ1
)
MTW-1-Exc
MTW-2
295
325
261
274
0.12
0.13
The results in Fig. 7 show that the characteristic IR band of the
pyridinium ion at 1545 cmꢁ1, which is associated with the
+
¨
presence of Bronsted acid sites, is very small for the K -
containing MTW-1 material. This is due to the fact that K+ is
compensating the negative charges associated with tetra-
hedrally coordinated aluminum atoms, and also because the
relatively large size of these extra-framework potassium cations
hinder the diffusion of pyridine molecules through the one-
dimensional pores of MTW. After cationic exchange of MTW-1
with ammonia followed by calcination at 500 1C, the 1545 cmꢁ1
IR band of pyridinium ions is clearly observed after pyridine
Table 4 Catalytic activity of MTW samples in the cracking of n-decane at
500 1C and 60 s time on stream compared with Mordenite samples
Catalyst
MOR-1
MOR-2
MTW-1-Exc
MTW-2
Si/Al
10
4.09
20
3.26
12.5
7.04
30.3
4.30
Kinetic conꢁst1ant
(gdecane gcat
s
ꢁ1) ꢀ 100
Yields (wt%) at 50% conversion of decane
C5–C9
Gases H2 + C1–C4
Coke
Hydrogen
Methane
Ethane
16.05
38.39
5.55
0.13
0.36
0.45
1.87
11.25
6.37
9.55
5.04
1.30
0.98
2.35
0.96
13.31
42.34
4.34
0.13
0.39
0.59
2.25
12.36
7.51
7.36
5.50
1.48
1.10
2.58
1.08
15.03
42.14
2.83
0.13
0.33
0.73
2.75
10.46
8.96
5.08
5.61
1.86
1.43
3.44
1.36
17.48
39.85
2.67
0.06
0.19
0.46
1.69
7.48
9.08
6.02
5.12
2.26
1.66
4.15
1.65
Ethylene
Propane
Propylene
Isobutane
n-Butane
trans-2-Butene
1-Butene
Isobutene
cis-2-Butene
Ratios of interest
Butene/butane
Propylene/propane
Isobutane/n-butane
C1 + C2
Isobutene/n-butenes
Isobutene/isobutane
C1 + C2/isobutane
C3/C4
IB = +IB/TOTC4
Iso/normal-C4
C1 + C2/IB = +IB
Ethylene/ethane
0.38
0.57
1.89
2.68
0.72
0.25
0.28
0.87
0.59
0.38
0.23
4.13
0.49
0.61
1.34
3.23
0.70
0.35
0.44
1.04
0.52
0.53
0.33
3.83
0.76
0.86
0.91
3.81
0.74
0.68
0.75
1.03
0.45
0.82
0.45
3.74
0.87
1.21
1.18
2.34
0.75
0.69
0.39
0.79
0.49
0.63
0.23
3.65
Fig. 7 Transmission FTIR spectra in the stretching C–C region after
adsorbing pyridine followed by desorption at 150, 250, and 350 1C: (a)
calcined MTW-1, (b) MTW-1-Exc, and (c) calcined MTW-2.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2015
New J. Chem.

View Article Online
Paper
NJC
MOR zeolites could be attributed to the lower dimensions of References
the channels of MTW that are close to those of medium pore
1 (a) R. B. LaPierre, A. C. Rohrman, J. L. Schlenker,
J. D. Wood, M. K. Rubin and W. J. Rohrbaugh, Zeolites,
1985, 5, 346; (b) H. Gies and B. Marler, Zeolites, 1992, 12, 42;
(c) C. A. Fyfe, H. Gies, G. T. Kokotailo, B. Marler and
D. E. Cox, J. Phys. Chem., 1990, 94, 3718.
2 E. J. Rosinski, M. K. Rubin and U.S. Pat., 3832449, 1974.
3 K. Sumitani, T. Sakai, Y. Yamasaki and T. Onodera, U.S. Pat.,
4557919, 1985.
4 T. V. Whittam, Eur. Pat. Appl. EPA, 0059059, 1982.
5 T. M. Barlow, Eur. Pat. Appl. EPA, 162719, 1985.
6 D. A. Hickson, UK Pat. Appl. GB, 2079735A, 1981.
7 K. M. Reddy, I. Moudrakovski and A. Sayari, J. Chem. Soc.,
Chem. Commun., 1994, 1491.
8 L. T. Nemeth and G. F. Maher, U.S. Pat., 6872866, 2005.
9 (a) G. Pazzucconi, C. Perego, R. Millini, F. Frigeiro, R. Mansani
and D. Rancati, U.S. Pat., 6147270, 2000; (b) R. Millini,
F. Frigerio, G. Bellussi, G. Pazzucconi, C. Perego, P. Pollesel
and U. Romano, J. Catal., 2003, 217, 298; (c) C. Perego,
S. Amarilli, R. Millini, G. Bellussi, G. Girotti and G. Terzoni,
Microporous Mater., 1996, 6, 395; (d) C. W. Jones, S. I. Zones and
M. E. Davis, Appl. Catal., A, 1999, 181, 283.
10 W. Zhang and P. G. Smirniotis, Catal. Lett., 1999, 60, 223.
11 A. Katovic, B. H. Chiche, F. Di Renzo, G. Giordano and
F. Fajula, Stud. Surf. Sci. Catal., 2000, 130, 857.
12 (a) Y. Kamimura, K. Itabashi and T. Okubo, Microporous
Mesoporous Mater., 2012, 147, 149; (b) Y. Kamimura, K. Iyoki,
P. Elangovan, K. Itabashi, A. Shimojima and T. Okubo,
Microporous Mesoporous Mater., 2012, 163, 282.
13 J. P. Coulomb and N. Floquet, Stud. Surf. Sci. Catal., 2008,
174B, 913.
14 (a) S. Gopal, K. Yoo and P. G. Smirniotis, Microporous Mesoporous
Mater., 2001, 49, 149; (b) A. S. Araujo, A. Silva, M. Souza,
A. Coutinho, J. Aquino, J. A. Moura and A. G. Pedrosa, Adsorption,
2005, 11, 159.
structures. In this sense, the lower dimensions of the channel
of the MTW should add a stronger confinement effect boosting
the activity for the activation of linear alkanes.
In terms of selectivity, MTW materials show a higher yield of
C3–C4 olefins compared to MOR zeolites, with a remarkable higher
propylene yield (see Table 4). The reason for this could be explained
by the presence of less hydrogen transfer reactions towards satu-
rated paraffins, such as propane or butanes. This lower extension
of hydrogen transfer reactions results in higher olefinicity ratios for
MTW compared to MOR zeolites (see propylene/propane, butenes/
butanes and isobutene/isobutane in Table 4). The lower contribu-
tion of hydrogen transfer reactions for MTW materials should also
be attributed to the lower dimensions of the large pore channels
that preclude bimolecular reactions, which are more favoured in
the larger pores of MOR.
4. Conclusions
The synthesis of the Al-rich MTW zeolite with low Si/Al ratios
(B12) and large pore accessibility has been described by using
the proper combination of alkali metal cations, such as potas-
sium, and bulky dicationic OSDAs, such as OSDA-C4. This zeolite
presents all the aluminum atoms in framework positions, result-
¨
ing in a material with strong Bronsted acidity after cationic
exchange, as revealed by in situ infrared pyridine adsorption/
desorption at different temperatures. In addition, another MTW
material with a Si/Al ratio of 30 has been synthesized under
alkali-free conditions using OSDA-C4 as the only structure
directing agent. This Si/Al ratio is the lowest described in the
literature for the synthesis of MTW in the absence of alkali-
cations, which is an important issue since post-synthetic cationic
exchange procedures are not required to create its acid-form.
These MTW samples are very active for n-decane cracking and
show lower extension of hydrogen transfer reactions leading to a
higher yield of olefins compared to other large pore zeolites, as
mordenites.
15 J. Li, L. Lou, C. Xu and S. Liu, Catal. Commun., 2014, 50, 97.
16 A. Jackowski, S. I. Zones, S. J. Hwang and A. W. Burton,
J. Am. Chem. Soc., 2009, 131, 1092.
17 A. Corma, J. Martinez-Triguero, S. Valencia, E. Benazzi and
S. Lacombe, J. Catal., 2002, 206, 125.
18 B. Marler, N. Dehnbostel, H. H. Eulert, H. Gies and
F. Liebau, J. Inclusion Phenom., 1986, 4, 339.
19 A. B. Pinar, R. Garcia, L. Gomez-Hortiguela and J. Perez-
Pariente, Top. Catal., 2010, 53, 1297.
Acknowledgements
Financial support from the Spanish Government-MINECO 20 T. De Baerdemaeker, U. Muller and B. Yilmaz, Microporous
through ‘‘Severo Ochoa’’ (SEV 2012-0267), Consolider Ingenio
2010-Multicat and, MAT2012-37160 is acknowledged.
Mesoporous Mater., 2011, 143, 477.
21 C. A. Emeis, J. Catal., 1993, 141, 347.
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2015