Nanosized MCM-22 zeolite using simple non-surfactant organic growth modifiers: Synthesis and catalytic applications

Article abstract of DOI:10.1039/c8cc05356a

Non surfactant cyclic alkylammonium can selectively decrease the rate of crystal growth along the x-y crystal axes during the synthesis of MWW zeolite. This results in an ～60 nm nanocrystalline zeolite that shows remarkable catalytic properties for the production of cumene.

Full text of DOI:10.1039/c8cc05356a

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Nanosized MCM-22 zeolite using simple non-surfactant organic
growth modifiers: synthesis and catalytic application
Received 00th January 20xx,
Accepted 00th January 20xx
Eva M. Gallego, Cecilia Paris, Cristina Martínez, Manuel Moliner,* Avelino Corma*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Non surfactant cyclic alkylammonium can selectivelly decrease the two independent pore systems: one is composed by 12-ring
rate of crystal growth along the x-y crystal axes during the cages connected by 10-ring apertures and the other one is
8
synthesis of MWW zeolite. This results in a ∼60 nm nanocrystalline defined by a 10-ring circular channel. The external surface of
zeolite that shows remarkable catalytic properties for the the MCM-22 crystals exposes half of these 12-ring cages,
8
production of cumene.
leading to external “cups” with accessible acid sites, which
have been described as active sites for selective aromatic
alkylation reactions, such as the benzene alkylation with
The search for effective methodologies to synthesize
nanosized zeolitic materials with crystal sizes below 100 nm
9
propylene to obtain cumene.
1
has supposed an important challenge in the last twenty years.
Along these two decades, many post-synthetic or one-pot
1
methods have been described in the literature showing their
Two different post-synthetic treatments on the as-
prepared layered MCM-22 precursor were first described for
increasing the exposed fraction of the external cups,
1
0
effective application in diverse areas, such as medicine,
2
electronics or catalysis, among others.
pillarization of the MCM-22 precursor to MCM-36, or its
delamination with surfactants to obtain the single MWW
1
1
Regarding catalysis, the synthesis of zeolites in the
nanometer scale remarkably decreases diffusion path lengths,
not only favouring the accessibility of the reactants to the
active sites, but also preventing undesired side reactions of the
layers, known as zeolite ITQ-2. Very recently, new one-pot
strategies based on the use of bifunctional surfactant-type
cations presenting long alkyl-aliphatic chains, alone or in
combination with the traditional MCM-22 OSDA, have been
described as efficient methods to obtain highly-delaminated
3
primary products. Among the different techniques that can be
1
2
found in the literature describing the crystallization of
1
MWW materials (MIT-1 and DS-ITQ-2, respectively).
-4
nanosized zeolites,
the use of bulky organic structure-
Nevertheless, these bulky OSDA molecules are costly and, as it
has been stated above, the use of simpler organic molecules
would be required to make the direct synthesis of the MWW
with controlled particle sizes industrially attractive.
directing agents (OSDAs) with long aliphatic chains is one of
the preferred methodologies since, in general, it allows
obtaining high solid yields and well-controlled chemical
4
compositions. However, the cost associated to the
nanozeolite manufacture is probably the most critical factor
for its industrial implementation and, consequently, current
research efforts must focus on the design of direct and
inexpensive procedures that permit an efficient control of the
5
kinetic crystal growth during the crystallization process.
Among the 235 zeolites recognized by the International
6
Zeolite Association (IZA), the MCM-22 zeolite (MWW
Fig. 1 Organic structure directing agents employed for the
synthesis of MCM-22 materials
structure) is one of the very few zeolites that currently are in
7
commercial application. The MWW structure is formed by
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In this sense, we have recently described a new TEM microscopy confirms that the average particle diameter
methodology employing simple cycloalkyl-ammonium OSDA for the MCM-22(n) is remarkably lower (below 60 nm, see Fig.
molecules to prepare Beta and ZSM-5 zeolites in their
1
3
nanosized form. These simple OSDAs combine a cyclic part
and a short alkyl-chain group (preferably C4, see for instance
BMH in Fig. 1).
Thus, in order to control the particle size of the MCM-22, we
1
have modified the conventional MCM-22 synthesis, replacing
4
2
0% of the former OSDA, hexamethyleneimine (HMI, see Fig.
), by a related cyclic organic molecule but also containing a
1
short alkyl-chain, N-butyl-N-methylhexamethyleneiminium
BMH, see Fig. 1). The overall synthesis conditions for the
conventional MCM-22 zeolites were: SiO /0.042 Al O /0.3
(
2
2 3
NaOH/0.5 HMI/40 H O, whereas those for the modified MCM-
2
22 were: SiO /0.042 Al O /0.3 NaOH/0.1 BMH/0.4 HMI/40
2 2 3
H2O. The gels were introduced under tumbling conditions at
23 K for 10 days. After that period, the resultant solids were
4
4
b) than for the conventional MCM-22 zeolite (∼200-250 nm,
filtered and washed with abundant deionized water, and dried
at 363 K overnight [more details can be found in the Electronic
Supplementary Information (ESI†)].
see Fig. 4a), but deeper inspections on the platelets thickness
by TEM do not reveal appreciable differences between both
MWW zeolites [see Fig. S1 and Fig.S2 in (ESI†)].
Fig. 3 FE-SEM images of the conventional MCM-22 zeolite
(a), and modified MCM-22(n) zeolite (b).
Fig. 4 TEM images of the conventional MCM-22 zeolite
(a), and modified MCM-22(n) zeolite (b).
These results clearly suggest that the BMH molecule added
to the synthesis media is acting as a Zeolite Growth Modifier
ZGM). We have found that the use of BMH cation as co-OSDA
(
does not preclude the crystal growth along the z-axis, forcing
the crystallization of very thin MWW zeolites with one or very
few layers but, in contrast, its presence prevents the crystal
growth along the x and y-axes, resulting in small hexagonal
Fig. 2 PXRD patterns of the as-prepared (a) and calcined
1
2
(
b) MCM-22 samples
platelets of 0.5-1 µm. This exposes more pore mouths giving
access to the circular 10-ring channels and decreases the mean
path length through the layer formed by 12-ring cavities,
improving in this way the diffusion of the molecules trough the
two layers, i.e. the one formed by 12-ring cavities connected
by 10-ring windows, and the one formed by circular 10-ring
channels with many openings along the x- and y-axes.
The PXRD patterns of both as-prepared solids reveal the
crystallization of the MWW structure (see Fig. 2a). However,
when the intensity of the diffraction peaks are compared, it
can be observed that the one synthesized in the presence of
the zeolite growth modifier BMH shows lower intensities and
broader diffraction peaks (see MCM-22(n) in Fig.2a), which
could imply smaller crystal sizes. FE-SEM microscopy images
show that both MWW zeolites exhibit similar platelet crystal
morphology (see Fig. 3), although a noteworthy reduction of
the crystal size for the MCM-22(n) is evidenced (see Fig.3b).
The role of the BMH cations on the control of crystal
growth along the x and y-axes cannot be precisely described at
the moment but, according to the previous works by Rimer et
1
5
al., this type of cationic molecules could present particular
interactions with the pre-zeolite nuclei, influencing the aspect
2
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1
2b,13
ratio (diameter/width) of the resultant zeolite crystals. In this lifetime.
This is not surprising if one takes into account
sense, the introduction of BMH molecules in the synthesis that larger diffusion limitations for the reaction products, i.e.
media does modify the aspect ratio of the MWW crystals, as it cumene, will drive into a higher extension of consecutive
can be clearly stated when comparing the conventional MWW reactions, as for instance the formation of dialkylated products
1
2
zeolite and the surfactant-modified MWW nanosheets.
The textural properties of the MWW samples have been micropores and, especially, within the 12-ring cavities may
measured by N adsorption after the calcination treatments. lead to a faster deactivation of the catalyst with time on
and oligomerization products that, if build within the
2
As it could be expected from its smaller crystal size, the stream. On the other hand, if the dialkylbenzenes are formed
nanosized MCM-22(n) sample shows higher BET and external at the external surface of the zeolite they can still be desorbed.
surface area than conventional MCM-22 (482.7-124.4 and Following the above, we can expect that a nanosized MWW
2
4
57.3-83.8 m /g respectively, see Table 1).
with much external surface area and lower diffusional
should give higher conversion, more
11, see diisopropylbenzene, and lower rate of catalyst deactivation.
Table 1). Interestingly, the amount of aluminum species in Indeed, the nanosized MCM-22(n) zeolite shows
tetrahedral coordination is also comparable for both MWW remarkably higher propylene conversion compared to the
zeolites, with 77-80% of the Al species remaining in conventional MCM-22 zeolite, presenting negligible
framework positions after the calcination treatments [see Al deactivation with time-on-stream (see Fig. 5), even working at
Regarding the chemical composition, ICP analyses reveal problems
similar Si/Al molar ratios in the final MWW solids (
∼
a
∼
a
2
7
MAS NMR spectra in Fig. S3 in (ESI†)].
very low contact times. In addition, the product yield towards
cumene is substantially higher for the nanosized MCM-22(n)
Table. 1 Physico-chemical properties of the synthesized catalyst, achieving continuous values of
∼80% with TOS (see
MWW materials
Fig. S5a in (ESI†)]. These catalytic improvements observed for
the MCM-22(n) zeolite compared to the conventional MCM-22
zeolite could be ascribed to the nanocrystalline nature of the
former.
Crystal Area BET Ext. Surf.
Microp.
Sample Si/Al)ICP
2
size (nm) (m /g) area (m /g) area (m /g)
2
2
MCM-22
10.6
∼
200
457.3
482.7
83.8
373.5
358.3
MCM-22(n) 11.1
∼
50-60
124.4
MWW-related materials are often described as efficient
catalysts for industrially-relevant reactions that mainly require
3
a,9,11,16
external surface acid sites.
Thus, to evaluate the
accessibility to the acid sites of the synthesized MWW samples
and their acid strength, we have adsorbed two probe
molecules with different sizes, pyridine and ditertbutyl
pyridine (DTBP), and measured their desorption at different
1
7
temperatures by FT-IR spectroscopy. As it can be seen in Fig.
-1
S4a in (ESI†), the intensity of the IR bands at 1545 cm ,
assigned to the pyridinium ions retained on the Brønsted acid
sites, are analogous for both MWW materials after the
desorption treatment at 150°C. This suggests that the two
Fig. 5 Propylene conversion with TOS for the liquid phase
synthesized MWW-materials present similar amount of acid
sites accessible to pyridine [see Table S1 in (ESI†)]. However,
when a larger probe molecule, such as DTBP, is adsorbed, the
alkylation of benzene with propylene using the synthesized
MWW catalysts (reaction conditions: T=125°C, P=3.5 MPa,
-2
−
1
benzene/propene=3:1 mol/mol, τ=10 h).
intensity of the 3360 cm band associated with protonated
DTBP is remarkably higher for the nanosized MCM-22(n) as
compared to the conventional MCM-22 [see Fig. S4b and Table
S1 in (ESI†)]. These results indicate that the nanosized MWW
zeolite presents a higher amount of acid sites accessible to the
larger molecule DTBP, owing to its larger external surface area,
which exposes a larger number of channel pore mouths (see
Table 1). This should have an important impact on reactions
involving diffusion limitations either for reactants or products.
The synthesis of cumene, a chemical precursor to phenol
In conclusion, we have prepared a nanosized MCM-22
zeolite with average particle sizes of 60 nm, remarkably
∼
smaller than the particle sizes of conventional MCM-22 zeolite.
The crystal size control has been achieved by selectively
decreasing the rate of crystal growth along the x- and y-axes.
This has been accomplished by including a simple non-
surfactant cyclic alkyl-ammonium molecule, which acts as a
zeolite growth modifier. The resultant nanosized MCM-22
zeolite shows improved catalytic activity and stability as
compared to conventional MCM-22 zeolite for the industrially-
relevant liquid-phase benzene alkylation with propylene to
synthesize cumene. These results allow envisioning that the
controlled addition of simple non-surfactant alkyl-substituted
1
8
and acetone, is a key petrochemical reaction . Although
MWW and Beta zeolites are well-known industrial catalysts for
1
8
liquid phase alkylation of benzene with propylene, the ability
to design these catalysts in their nanocrystalline form has been
shown to result in a remarkable increase of the catalyst
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cyclic cations to the synthesis media as zeolite growth
modifiers, could be extended to the synthesis of other relevant
zeolites in order to improve their physico-chemical properties
and to enhance their catalytic performance for target
reactions.
and A. Corma, Angew. Chem. Int. Ed., 2015, 54, 13724-
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3 E. M. Gallego, C. París, M. R. Díaz-Rey, M. E. Martínez-
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This work has been supported by the European Union
through ERC-AdG-2014-671093 (SynCatMatch), by the Spanish
Government-MINECO through ‘‘Severo Ochoa” (SEV 2016-
0
683), MAT2015-71261-R and CTQ2015-70126-R, and by the
Fundación Ramón Areces through a research contract of the
Life and Materials Science” program. The Electron Microscopy
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Marek, K. Tarach and M. Choi, J. Phys. Chem. C, 2014, 118
“
,
Service of the Universitat Politècnica de València (UPV) is
acknowledged for their help in sample characterization. E.M.G.
acknowledges “La Caixa - Severo Ochoa” International PhD
Fellowships (call 2015). We thank V. J. Margarit for helpful
discussions and B. Esparcia for technical assistance.
12266-12274.
8 W. Vermeiren and J. P. Gilson, Top. Catal., 2009, 52, 1131-
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Conflicts of interest
There are no conflicts to declare.
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