Impact of pore topology and crystal thickness of nanosponge zeolites on the hydroconversion of ethylbenzene

Article abstract of DOI:10.1039/c5cy02029h

The gas-phase hydroconversion of ethylbenzene was investigated in the presence of intimate mixtures of ?MRE, MFI and MTW-type zeolite nanosponges and a hydrogenating component (Pt/Al₂O₃). The nanomorphic zeolites were prepared using multiammonium surfactants acting as dual-porogenic agents directing the formation of micro- and mesopores simultaneously. The effects of the zeolite topology (pore size and dimensionality) and crystal thickness on the product selectivity of ultra-thin zeolite frameworks (<10 nm) were investigated. The enhanced catalytic activity confirmed the importance of improved molecular diffusion. These nanosponges were unique in producing more xylenes, suggesting lower confinement effects. The selectivity for p-xylene and the selectivity towards ethylbenzene hydroisomerization, dealkylation, disproportionation, transalkylation and hydrocracking were evaluated. Despite the similar <10 nm crystal thickness of all the nanosponge zeolites, the presence of spacious channel interconnections in MFI was concluded to remarkably impact the product selectivity compared to straight channels as in ?MRE and MTW. Our findings clarify the relatively unexplored transformation of alkyl-aromatics over ultra-thin zeolite crystals, through five typical catalytic reactions of major industrial interest.

Full text of DOI:10.1039/c5cy02029h

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ARTICLE
Impact of pore topology and crystal thickness of nanosponge
zeolites on the hydroconversion of ethylbenzene
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
a,b
F. Marques Mota, P. Eliášová, J. Jung, and R. Ryoo*
DOI: 10.1039/x0xx00000x
The gas-phase hydroconversion of ethylbenzene was investigated in the presence of intimate mixtures of *MRE, MFI and
MTW-type zeolite nanosponges and a hydrogenating component (Pt/Al ). The nanomorphic zeolites were prepared
2
O
3
www.rsc.org/
using multiammonium surfactants acting as dual-porogenic agents directing formation of micro- and mesopores
simultaneously. The effects of the zeolite topology (pore size and dimensionality) and crystal thickness on the product
selectivity of ultra-thin zeolite frameworks (<10 nm) were investigated. Enhanced catalytic activity confirmed the
importance of improved molecular diffusion. These nanosponges were unique in producing more xylenes, suggesting
lower confinement effects. The selectivity to p-xylene and the selectivity towards ethylbenzene hydroisomerization,
dealkylation, disproportionation, transalkylation and hydrocracking was evaluated. Despite the similar <10 nm crystal
thickness of all nanosponge zeolites, the presence of spacious channel interconnections in MFI improved the product
selectivity compared to straight channels as of *MRE and MTW. Our findings clarify the relatively unexplored
transformation of alkyl-aromatics over ultra-thin zeolite crystals, through five typical catalytic reactions of major industrial
interest.
xylenes and ethylbenzene is preferred, ethylbenzene is
hydrogenated into ethylcyclohexene isomers in the presence
1
. Introduction
2 3
of a strong hydrogenating component (Pt/Al O ), followed by
Innovative use of existing benzene-toluene-xylene (BTX)
resources has been greatly extended following the increased
isomerization into dimethylcyclohexenes and further
9
dehydrogenation to xylenes. This process is nonetheless
1
-3
global demand for para-xylene. Separation of p-xylene by
accompanied by numerous reactions, e.g., naphtha ring
opening, followed or otherwise by hydrocracking,
aromatization, and polymerization of aliphatic hydrocarbon
chains, as well as dealkylation, disproportionation, and
adsorption and the conversion of the remaining C aromatics
8
have become important branches in the integrated aromatics
complex. The C8 aromatic cut from catalytically reformed
naphthas and pyrolysis distillates contains not only xylenes but
also ethylbenzene in a wide composition range. Although the
latter has large scale application for the production of styrene
monomer, it is rarely isolated as such from C8 aromatic
1
0
transalkylation reactions of alkyl-aromatics.
With a focus on the acid component of the bifunctional
catalyst, continuous research has described the impact of the
11,12
4
zeolite topology on the catalytic performance.
In
mixtures due to the difficulty and cost of the separation.
particular, recent studies highlighted the dependence of the
selectivity on the size, dimensionality, presence of large
cavities, and even the shape and tortuosity of zeolitic pore
Xylene hydroisomerization preferentially occurs over Brønsted
acid sites traditionally provided by zeolites. Based on the
balance between the supply of benzene and xylenes and their
market demand, ethylbenzene may be dealkylated or
alternatively hydroisomerized depending on the selected
8
,13
systems.
Whereas a 3-dimensional (3-D) 10-ring (10-R) MFI
zeolite has found general application for the dealkylation of
ethylbenzene, the zeolite of choice in the selective
hydroisomerization of ethylbenzene is still under discussion.
Optimized formulations of 1-D large-pore mordenite catalysts
3
-7
catalyst. Catalysts for the conversion of C aromatics have
8
ordinarily been classified by the manner of processing
6
ethylbenzene associated with the xylene isomers. The
are generally used for ethylbenzene transformation towards
11,14-17
ethylbenzene content in the C aromatic fraction can range up
8
the formation of xylenes.
Yet, zeolites with narrow non-
to 50%, which further highlights the choice of the zeolite
8
applied here. When the simultaneous hydroisomerization of
intersected 10-R channels have recently emerged as a new
8
-10,12,13,18,19
generation of more selective catalysts.
Sensitivity
to coke retention in the core of the zeolite crystals has been a
crucial factor for choosing the catalyst when ethylbenzene is
preferentially hydroisomerized. For the simultaneous
hydroisomerization of xylenes and ethylbenzene, the zeolite-
based catalyst is required to balance the desired shape-
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selectivity, the tendency toward fast deactivation by coke transalkylation, and hydrocracking – in the transformation of
formation, and the limited molecular transport. alkyl-aromatic hydrocarbons, with ethylbenzene as a probe
In a wide range of catalytic applications, hierarchical molecule.
zeolites are of primary interest as they may overcome main
zeolite drawbacks associated with limited molecular diffusion,
2
. Experimental
resulting in low catalyst effectiveness. Over the years,
numerous approaches have been designed to tailor the
synthesis of hierarchical zeolites exhibiting mesoporosity in
addition to zeolitic-micropores, using both top-down and
2
.1. Synthesis of zeolites
Nanosponge zeolites were prepared in accordance with
previously established protocols optimized for each zeolite
2
0-32
bottom-up approaches.
In particular, Ryoo and
3
3,46
structure.
In order to differentiate between the acidity
collaborators have synthesized MFI, *MRE, BEA, and MTW
zeolites, and AEL, AFI and ATO-type aluminophosphates, in the
form of nanosponges with improved molecular diffusion.
These investigators designed special multiammonium
impact and the influence of the pore topology, each zeolite
was synthesized with a similar framework Si/Al molar ratio
close to 100. MTW and *MRE-type zeolites were synthesized
+
+
using the same multiammonium surfactant SDA, [C22
H
45
-
surfactants, e.g.,
C
22
H
45N (CH
3
)
2
C
6
H
12N (CH
3
)
2
C
6
H
13
,
for
+
N (CH
+
N (CH
+
+
-N (CH
3
3-36
3
)
)
2
-C
6
H
12-N (CH
3
)
2
-CH
2
-(C
6
H
4
)-CH
2
3
)
)
2
-C
6
H
12
-
application as dual-porogenic agents.
The microporous
+
-N (CH
+
-
3
2
-CH
46
2
-(C )-CH
6
H
4
2
3
)
2
-C
6
H
12-N (CH
3
2
-C22H45](Br
framework is directed by the ammonium groups while the long
hydrocarbon chains can direct the formation of the
mesostructure due to their self-assembly ability similar to
mesoporous materials. The resulting nanosponge materials
have nano-sized crystals and a narrow mesopore size
distribution. Recent works demonstrated the major impact of
the crystal thickness of these ultra-thin materials on the
catalyst lifetime and catalytic activity in several catalytic
-
(Cl )
)
2
4
.
Tetraethylorthosilicate (TEOS, 95%, Junsei) and
sodium aluminate (53 wt%, Sigma-Aldrich) were used as silica
and alumina sources, respectively. For MFI-type zeolite, the
+
+
-
33
SDA was [C16
H
33-N (CH
3
)
2
-C
6
H12-N (CH
3
)
2
-C
6
H13](Br )
2
. Sodium
silicate solution (Si/Na = 1.75, 15 wt% SiO
2
) and sodium
aluminate (53 wt%, Sigma-Aldrich) were in this case used as
silica and alumina sources. The molar composition of the gel
3
3,37-44
was 100 SiO
2
: 0.5 Al
2 3 2 2
O : x SDA: y Na O: z H O with (x, y, z) as
reactions.
Yet, the impact of the crystal thickness on the
(
1.67, 13, 3000) for *MRE, (7.5, 27, 5000) for MFI, and (3.33,
attained catalytic selectivity has been relatively unexplored so
far. Our research aims to shed some light into this area.
1
3, 4500) for MTW. The nano-zeolites were obtained after 3, 3
and 10 days at 423 K. The nanosponge samples were denoted
as nano-*MRE, nano-MTW, and nano-MFI. The corresponding
bulk counterparts (bulk-*MRE, bulk-MTW, and bulk-MFI) were
also synthesized for comparison purposes. Detailed synthesis
In the present work, three zeolite nanosponge materials,
namely *MRE, MFI, and MTW, and their corresponding bulk
counterparts were evaluated in the hydroconversion of
ethylbenzene to inspect the effect of acid site accessibility on
the resulting activity and, in particular, on the catalytic
selectivity. In each case, the nanosponge crystals exhibited
dimensions below 10 nm. Whereas in previous reports the
procedures are given in the ESI
†.
2
.2. Preparation of bifunctional catalysts
impact of the zeolite topology determines the reaction The preparation of bifunctional catalysts involved the
selectivity, the question of whether the unique molecular mechanical mixing of each solid acid with alumina
shape-selectivity of each zeolite is preserved with such thin impregnated with platinum (Pt/Al O ). The indicated catalyst
2 3
materials remains unanswered. In this regard, we assessed the formulation was based on long-term literature results and
multitude of side reactions occurring during ethylbenzene well-established patent data available for the industrial
1
0-13,15
transformation to evaluate the impact of crystal size on the process.
product distribution.
We intentionally varied the exchange of
dimensionality and pore opening sizes of the chosen zeolite the presence of 0.2 M HCl following literature reports.
The hydrogenating phase was prepared by ion
γ
-Al
2
O
3
with a hexachloroplatinic acid solution in
1
1,12
The
-1
3
-1
topologies, *MRE, MFI, MTW, to examine the roles these hydrogenating phase was then calcined under 2 dm g min
factors play in the studied catalytic reaction. Corresponding of dry air flow at 773 K for 4 h (1 K min ). The Pt content was
-1
4
5
illustrations of their channel systems are presented in Fig. 1.
MRE possesses symmetrical, straight, 1-D 10-R channels with behavior. The Pt content, based upon literature results, was
dimensions of 5.3×5.6 Å. MFI zeolite has a 3-D pore believed sufficient to balance the comparatively weak acid
equal to 1.2 wt.% to assure an ideal bifunctional catalytic
*
1
3
architecture with 10-R straight channels (5.1×5.5 Å) phase of the tested zeolites with high Si/Al ratios. High Pt
interconnected through 10-R sinusoidal channels (5.3×5.6 Å), dispersion was corroborated by collected scanning
thus generating 8.6 Å intersections. The MTW zeolite exhibits transmission electron microscopy (STEM) images. Metal
straight, 1-D, 12-R channels with openings 5.6×6.0 Å. The particle domains ranged from 0.8 to 1.2 nm-diameter (Fig. S1,
comparison of the catalytic performance of their nanosponge ESI
forms should extend our understanding of the impact of the zeolite samples with the prepared Pt/Al
†
). The bifunctional catalysts were prepared by milling the
solid in 10:90 wt.
2 3
O
framework topology on the obtained product spectrum in the proportion. The samples were pelletized, crushed, and sieved
case of zeolites as ultra-thin crystals. This effect is reflected in to consistently obtain a particle size of 0.200 to 0.355 mm.
five typical catalytic reactions of major industrial interest –
hydroisomerization,
dealkylation,
disproportionation, 2.3. Characterization techniques
2
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Powder X-ray diffraction patterns of all zeolites were recorded SEM microscopy. Bulk-MFI crystals (20-μm in length) were
with a Rigaku Multiflex diffractometer using Cu K radiation strikingly voluminous compared to the approximately 2-μm
40 kV, 30 mA). Scanning electron microscopy (SEM) images long and narrow MTW and *MRE bulk crystals. The individual
α
(
were obtained with a FEI VERIOS operating at 1 kV. Crystal nanocrystals of all nano-zeolites were arranged in an irregular
thickness was evaluated through TEM photographs taken with manner resulting in the nanosponge-type morphology.
a Philips F30 Tecnai at an accelerating voltage of 300 kV Representative TEM images of the nanosponge samples
(Cs=0.6 mm, point resolution 0.17 nm). STEM micrographs of evidenced the presence of ultra-thin nanocrystals in each case
the hydrogenating phase were obtained using a spherical (Fig. 2g-i). The thickness of the nano-MFI crystals
aberration-corrected Titan ETEM G2 60-300 system operated corresponded to a single-unit-cell-size, 2.5-nm along the b axis,
at 300 kV and equipped with a high angle annular dark field with the zig-zag channels extended along the ac plane. Both
(HAADF) detector. Nitrogen adsorption-desorption isotherms nano-MTW and nano-*MRE are characterized by about 10 nm
were measured using an Tristar volumetric adsorption analyzer sized crystals with the pore openings on the bc and ac planes,
at 77 K, with the samples being outgassed for 4 h at 573 K respectively. The uniform thickness of the nanocrystals agreed
3
3,46
under vacuum prior to measurement. The Brunauer-Emmett- with previous reports,
Teller (BET) equation was used to estimate the surface area the synthesis procedures applied. For the bulk samples, the
from the adsorption branch of isotherm in the 0.05-0.2 range measured N sorption at 77 K revealed type I isotherms
of relative pressures. The total pore volume was equated to characteristic of microporous zeolites (Fig. S3, ESI ). The nano-
the amount of adsorbed N at P/P = 0.98. The content of Si, Al zeolites additionally exhibit a prominent hysteresis loop in the
and Pt in the samples was determined by inductively coupled range 0.5<p/p <0.9 region. This result points to the presence
plasma-atomic emission spectroscopy (ICP-AES) carried out on of an intercrystalline mesoporous network in addition to
evidencing a good reproducibility of
2
†
2
0
0
2
7
47
an OPTIMA 4300 DV instrument (Perkin Elmer). Al NMR framework micropores. The nanosponge zeolites have higher
spectra were acquired in a solid state with magic angle total BET surface areas in comparison to the bulk ones (Table
spinning (MAS) using a Bruker Avance 400WB spectrometer at 1). Larger surface areas were attributed to the presence of
room temperature.
mesopores. Relatively narrow mesopore size distributions in
the range 2.5-6.0 nm correspond to the typical features of
3
5
2.4. Ethylbenzene hydroconversion
nanosponge zeolites.
Elemental analysis using ICP-AES
confirmed that the silicon to aluminum atomic ratios were
The gas-phase hydroconversion of ethylbenzene (Sigma-
Aldrich, 99%) was conducted in a fixed bed stainless steel
2
close to the nominal value (Table 1). According to Al NMR the
7
quasi-totality of the aluminum in all nanosponge zeolites had
2
reactor at 10 bar and a H : ethylbenzene molar ratio of 4. The
the tetrahedral coordination (Fig. S4, ESI
†). This outcome
bifunctional catalyst (1.0 g) was loaded into the reactor and
1
-
highlights the noteworthy stability of these solid acids.
2
activated in situ at 753 K for 4 hours under H flow (5 K min ,
at 10 bar). The reaction was carried out at decreasing weight
hourly space velocity (WHSV) values at a fixed reaction
3
.2. Catalytic testing
temperature (683 K). WHSV values were varied over the range 3.2.1. Catalytic activity Nanosponge *MRE, MFI, and MTW
-1
-1
of 2 to 45 g of ethylbenzene g h , by adjusting the zeolites were evaluated in the gas-phase hydroconversion of
ethylbenzene flow. Reaction products were analyzed online by ethylbenzene and compared with their corresponding bulk
flame ionization detector (FID) gas-chromatography (GC, counterparts. The conversion increased stepwise with
Agilent 7890A) using a CP-Chirasil-Dex CB capillary column increasing contact time, taken as the reverse of the
(0.25 mm × 25 m).
corresponding WHSV (Fig. 3). The catalytic activity was
assessed at the end of the catalytic test at a lower WHSV value
-1
(
15 h ). All bifunctional catalysts remained relatively
3
. Results and Discussion
insensitive to deactivation in the evaluated range. The catalyst
stability was attributed to their low acidity (see Table 1),
whereas reduced crystal thickness was believed to further play
a crucial role in the prolongation of their lifetimes as reported
3.1. Zeolites characterization
Zeolites under study were synthesized as traditional bulk
crystals and as nanosponge materials. For this purpose we
applied the method reported by Ryoo and coworkers using
multivalent cation surfactants acting as a dual-porogenic
33,36,38,46
previously.
To eclipse the potential impact of slightly varied Al content
in each zeolite on its performance, the turn-over frequencies
3
3,34
agent.
The prepared nanosponge zeolites with *MRE, MFI,
(TOF), defined as the number of moles of ethylbenzene
and MTW topologies exhibited powder X-ray diffraction (XRD)
patterns similar to those of the corresponding bulk
converted per mole of Al per second, were determined at a
-1
WHSV value of 15 h . At a fixed contact time, hierarchical
counterparts (Fig. S2, ESI†). The overall decrease in the
nano-zeolites reveal higher TOF values compared to their
purely microporous counterparts (see Table 1). Assuming
relatively similar acid strength, these profiles affirmed the
expected enhanced molecular diffusion in ultra-thin crystals (<
intensities and peak-broadening observed for the nanosponge
samples corroborated the changes in their crystal size towards
smaller dimensions and was in a good agreement with
3
3,46
previous reports.
The zeolite morphologies of both bulk
1
0 nm) compared to conventional micrometer-sized ones.
(Fig. 2a-c) and nanosponge solids (Fig. 2d-f) were studied by
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Catalytic results confirmed that we generated nanosponge R zeolite, the slightly divergent diameters, as compared to
zeolites with easier access to the acid sites. Over the bulk both MFI and *MRE, appear to be sufficient to generate a
-1
zeolites, the TOF (at 15 h WHSV) followed the order MFI notable difference in the xylene distribution over both solids.
0.24) < *MRE (0.35) < MTW (0.50). In principle, a 3-D channel
(
system enables a faster diffusion of organic molecules. 3.2.3. Catalytic selectivity During the simultaneous
Nevertheless, the involved molecules are believed to hydroisomerization of xylene and ethylbenzene, the latter can
experience higher diffusion hindrance inside the 20-μm long undergo other reactions along with the desired formation of
bulk-MFI crystals compared to the only 2-μm long MTW and xylenes. All reactions products were carefully identified by gas
*MRE crystals (Fig. 2). The ethylbenzene hydroconversion may chromatography to gain advanced insight into the impact of
be significantly limited to the outer part of the MFI zeolite decreased crystal thickness and pore topology on the shape-
crystals. As only a smaller part of the protonic sites may selective ability of the studied zeolites. Fig. 5 illustrates a
participate in the reaction, it seems reasonable that bulk-MFI simplified scheme of the ethylbenzene intramolecular
shows the lowest TOF value. On the other hand, the sequence hydroisomerization mechanism and considers the main side
in the nanosponge counterparts is quite different, *MRE (0.65) reactions based on the obtained catalytic results as follows: (1)
<
MTW (0.76) < MFI (1.10). The MFI pair exhibited the most the disproportionation of ethylbenzene into benzene and
striking activity discrepancy. Furthermore, these results diethylbenzene; (2) transalkylation of ethylbenzene and
suggest that our nanosponge zeolites provide easier access to xylenes, leading to ethyltoluene/toluene and to
the acid sites.
benzene/dimethylethylbenzene; (3) dealkylation followed by
hydrogenation of ethylene; and (4) bifunctional hydrocracking
3 8
3.2.2. Xylene isomers distribution Shape-selectivity of the into light C -C hydrocarbons following an initial naphthene
micropore channels often plays a critical role in the selectivity ring opening step. With every tested sample, at low contact
toward p-xylene. The change in the para-selectivity in the times, only hydrogenation products could be observed. This
xylene mixture was plotted against conversion (Fig. 4). For result reflects the activity of the metallic phase, and
each catalyst, the p-selectivity shows the tendency to converge corresponds to the catalytic behavior of an individually tested
2 3
to the thermodynamic equilibrium value at higher conversion zeolite-free catalyst containing only Pt/Al O . Methane could
degrees. In the case of bulk-*MRE, the mixture of xylenes is not be observed, confirming the absence of monofunctional
remarkably richer in the p-isomer, attaining ca. 90%, at an hydrogenolysis over platinum sites. The result further
early stage of the reaction. This isomer distribution highlights highlights the achieved balance between the acidic and
the shape-selective properties of the *MRE-type narrow metallic functions of these ideal bifunctional catalysts. The
channel system, in which the less bulky p-isomer (6.7 Å) is consecutive disproportionation of xylenes, the formation of C
preferred over m- and o-xylene dimensions of 7.1 and 7.4 Å, naphthenes and heavy aromatic products, or the
The critical size of p-xylene further transalkylation of ethylbenzene with other alkylbenzene
5
-
C
7
4
8,49
respectively.
corroborates the occurrence of the catalytic reaction within species were confirmed to be minor in each case.
the *MRE microporous network. Interestingly, this selectivity Aromatization and oligomerization reactions were not taken
2
pattern was not observed with our nano-*MRE. The thickness into account assuming that all C resulted from dealkylation of
of the nano-zeolite (about 10 nm) is likely unable to generate ethylbenzene. The performance of the individual zeolitic
transition-state and diffusion shape-selectivity responsible for topology is discussed below at the same conversion levels for
the hampered formation of both m- and o-isomers. Diffusion each zeolite topology (Fig. 6).
pathways of a certain length are needed to increase the para-
selectivity, which may not be fulfilled in micro-mesoporous *MRE topology This zeolite showed similar selectivity trends for
3
,50
zeolites. The steric hindrance was also evidenced over bulk- both bulk and nano crystals in the scoped conversion range. In
MFI as conspicuously reported for the transformation of alkyl- the hydroisomerization, higher selectivity towards the
aromatics, e.g., toluene disproportionation and alkylation, and formation of xylenes (up to ca. 95%) was attained over nano-
5
1,52,53
corroborated by molecular dynamics simulations.
*MRE (Fig. 6). At an early stage of the reaction, the nano-
Despite similar a 10-R pore size, bulk-*MRE and bulk-MFI zeolite proved highly advantageous compared to the bulk-
exhibited distinct para-selectivity at low conversion degrees, *MRE, enhancing the initial hydroisomerization reaction rate
9
0% and 70%, respectively. The lower propensity over bulk- 2.6-fold (Fig. S5, ESI
†). A higher number of exposed protonic
MFI may be tentatively explained based on the findings of sites, which was deduced from a 5-fold increase of the external
5
Llopis et al. The presence of cavities or interconnections is surface area of nano-*MRE compared to bulk-*MRE (from 76
2
believed to allow consecutive reactions that lead to the up to 354 m /g), is believed to be favorable for higher xylenes
1
thermodynamic equilibrium, and consequently lower p- selectivity. Indeed, similar observations were reported for
1
2,13
selectivity. Conversely, direct channels (*MRE) allow diffusion industrially applied EUO and other 10-R zeolites.
In each
without trapping if their free diameter is large enough. case, the improved selectivity towards hydroisomerization was
Surprisingly, no noticeable changes in the xylene distribution associated with the presence of highly accessible acid sites,
were observed over bulk- and nano-MTW with 12-R channels. which are most likely located close to the external surface, in
With both catalysts the para-selectivity is only ca. 30% at the the pore mouths and in the exposed side pockets. The quasi-
initial reaction stages. Whereas MTW is a relatively narrow 12- isolation of acid sites, the easy access by hydrogenated
4
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cycloalkene intermediates, and short residence time in the sufficient content of the metallic function and mutual distance
micropore channels have been identified as potential key between the two catalytic functions, which indicates that the
9
factors in this matter.
Weisz’s intimacy criterion is respected. The almost entire
While the concentration and accessibility of the acid sites absence of aliphatic compounds with ≥ 6 carbons further
define the conversion rates, the proximity of protonic acid supports the lack of oligomerization reactions.
sites often presents a determining effect on the reaction
1
1
product distribution.
In particular, low diethylbenzene MFI topology The MFI-type zeolites exhibited an obviously
selectivity has been ascribed to the high Si/Al molar ratio. An distinct catalytic behavior based on different crystal thickness
increase in the distance between the protonic sites at lower Al (Fig. 6). Over the purely microporous bulk-MFI, a notable
content favors the selectivity towards the formation of maximal selectivity (65%) towards dealkylation was observed
isomers at the expense of acid-catalyzed reactions which at 42% conversion. The stoichiometry between benzene and
demand more than one protonic acid site (e.g., ethane seemed, however, not to be respected as lower
2 6
disproportionation and transalkylation). The density of acid amounts of the C H were consistently determined. This
sites has been underlined as the crucial parameter directing mismatch may indicate that ethylene undergoes rapid
the disproportionation rate, while the acidic strength has secondary transformations, e.g., aromatization, before the
5
4
apparently no effect. In a context of high acid sites hydrogenation of the C
2
species over the platinum sites
When high dealkylation rates are witnessed,
primary interest to reduce xylene losses by diethylbenzene transalkylation or disproportionation of alkylaromatics can
formation. further proceed not only via a bimolecular mechanism
3
,60
accessibility, the choice of low Al content should hence be of occurs.
A lower propensity to dealkylation was also confirmed in involving diphenylalkane intermediates but also through
the presence of nano-*MRE (<1%). Based on the current data, dealkylation-realkylation pathways, which include free
5
6,61
the 5-fold increase of the external surface area in nano-*MRE olefins.
This appears to be the case for bulk-MFI as
may serve as a tentative explanation. We may expect that indicated by the significant maintenance in the diethylbenzene
Brønsted acid sites located on the exposed framework can be selectivity, ca. 72% at 5% conversion. This particular selectivity
less strained from the perfect tetrahedral geometry and towards dealkylation and disproportionation has been
therefore can form weaker acid centers. In that case, the attributed to the presence of strong acid sites and
nano-*MRE would possess fewer strong acid sites compared to confinement effects in the MFI-type architecture. Conversely,
its bulk counterpart. Nevertheless, the dealkylation reaction is the presence of spacious intersections of the 3-D pore system
supported by strong acids centers that are necessary for the (8.6 Å) connected to narrow 10-R channels is known to be
+
55
13
formation of a very unstable ethyl carbenium ion CH
Multi-step reactions notably depend on the reaction
temperature and contact time of the molecular species with hydroisomerization selectivity was merely 17%. When
3
-CH
2
.
disadvantageous for the hydroisomerization of ethylbenzene.
At ca. 50% conversion, the maximal attained
5
6
the acid surface.
Herein, ethylbenzene underwent compared with the bulk-*MRE with 82% selectivity at a similar
transalkylation and hydrocracking at a higher extent at higher conversion degree, the bulk-MFI was confirmed to be a poor
conversion degrees. Multi-step reactions occurring in the 10-R hydroisomerization catalyst. This behavior reflects the outlined
channels of bulk-*MRE crystals suffered from higher diffusive industrial acceptance of MFI-type zeolite when the
restrictions in comparison with reactions proceeding in the 10- isomerization of xylenes is preferentially accompanied by the
nm-thick crystals of nano-*MRE. Among the side reactions transformation of ethylbenzene into valuable products by
studied here, the reduced crystal thickness had the most disproportionation (acid process) or dealkylation (bifunctional
6
2
striking impact on hydrocracking. At ca. 50% conversion, the process). Herein, the selected operating conditions (high
selectivity of light hydrocarbon by-products was decreased temperature) are believed to favor the dealkylation
5
6
from 11% to 2% for bulk and nano-*MRE, respectively. Overall, mechanism.
In agreement, single-event microkinetic
the result was quite notable since over bulk-*MRE, the methodology applied towards the transformation of
involved molecules are believed to experience diffusion ethylbenzene on a Pt/H-MFI zeolite catalyst confirmed that
-
1
hindrance inside the mere 2-μm long channels. The dealkylation was energetically most demanding (194 kJ mol )
hydrocracking products were rich in paraffins with 3, 4 and 5 compared to alkyl shift and transalkylation reactions (134 and
-
1
63
carbon atoms even at high degrees of conversion (Fig. S6, 125 kJ mol , respectively).
ESI ). The obtained distribution was symmetrically centered The ultra-thin crystals of nano-MFI led to an impressive
around half of the original molecule (8 carbon atoms), boost in selectivity towards hydroisomerization with 71%
although it is visually disturbed by a higher ethane formation selectivity at ca. 50% conversion (Fig. 6, Fig. S5, ESI ). Based on
caused by ethylbenzene dealkylation. The result shows a the previous studies, ca. 30% of the Brønsted acid sites are
†
†
3
9,44,52
similarity between the theoretical molar carbon number located at the external surface of the MFI nanosponge.
distribution of cracked products and the ideal hydroconversion The desired accommodation of the aromatic species at the
8
of a considered n-paraffin. An ideal hydroconversion catalyst channel intersections, where the strong protonic sites are
achieves the balance between a number and an activity or located in bulk-MFI, was somehow lost. In addition, rapid
strength of a metal and acid sites in the absence of striking molecular diffusion through the MFI nanostructure is believed
5
7-59
steric hindrances.
The results thus corroborate the to favor the reaction towards the formation of xylenes. In this
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ARTICLE
Journal Name
context, our MFI nanosponge represents
a
valuable conversion bulk-MTW showed higher selectivity to
hydroisomerization
catalyst
in
the
simultaneous transalkylation (5% vs. 3%) and hydrocracking (8% vs. 4%)
transformation of ethylbenzene and xylenes with improved compared to nano-MTW. The bulk-MTW catalyst displayed a
molecular transport compared with the purely microporous product spectrum with substantial amounts of hydrocracking
bulk-zeolite.
Fig. 6 confirms a continuous increase in the hydrocracking for ideal hydroconversion catalysts (Fig. S6, ESI
selectivity with conversion with both MFI-type zeolites. As The bulk-MTW zeolite exhibits a conspicuous selectivity
products reflecting a theoretical carbon number distribution
†
).
molecular diffusion was highly affected in the case of bulk-MFI, toward disproportionation with diethylbenzene formation.
it can only be expected that long contact times would lead to This tendency (selectivity 25%) was better illustrated
higher formation of light hydrocarbon by-products. Following compared with the bulk-*MRE sample (with selectivity 8%) at
the first naphthene ring opening step, the resultant long-chain the initial stages of the reaction. The pore size of both zeolites
alkyl species are assumed to rapidly undergo hydrocracking is believed to play a key role. The assumption is corroborated
inside the MFI zeolite. In both cases the hydrocracked product by the striking divergence of the attained para-selectivity of
spectrum clearly differs from the theoretical molar carbon xylenes with 90% over bulk-*MRE and 30% over bulk-MTW at
number distribution for ideal hydroconversion catalysts (Fig. an early stage of the reaction (Fig. 4). During the
S6, ESI†). The impact of pore dimensionality on the product transformation of the involved alkyl-aromatic species, the
selectivity in the hydroconversion of paraffins with MFI-based larger MTW channel system facilitated the formation and
catalysts has been comprehensively discussed in the diffusion of the involved species in the monomolecular ethyl-
6
4-66
literature.
structure would allow the formation of bulky alkyl-branched accommodate the demanded bulky transition-state needed for
species that are unable to diffuse out of narrow 10-R pores. the bimolecular disproportionation of ethylbenzene. The
The large interconnections within the zeolite transfer reaction pathway. These 12-R pores may further
Under the selected conditions, nano-MFI and nano-*MRE assumption has been widely explored in the presence of MOR-
showed strikingly different hydroisomerization selectivity over type 1-D 12-R channels applied in the simultaneous xylene and
1
1,15,16,54
the evaluated conversion range. Conversely, nano-MFI ethylbenzene industrial hydroisomerization.
Whereas
revealed higher ethylbenzene disproportionation (12% vs. 1%), transalkylation or disproportionation of alkyl-aromatics can
dealkylation (4% vs. 0.5%), and hydrocracking selectivity (14% proceed via a dealkylation/alkylation mechanism, as suggested
vs. 2%) at the same conversion degree (ca. 50%) (Fig. 6). Hence with our bulk-MFI, bimolecular reactions are likely to occur
6
8
intrinsic preferential reaction mechanisms in ultra-thin inside larger channels. The reaction was shown to proceed
nanocrystals appear to still depend on the zeolite pore via a carbocation chain mechanism involving the benzylic
5
6
dimensionality even when very similar pore-sizes are carbocation and diarylmethane intermediates. Lower Si/Al
considered (5.1×5.5 Å in MFI vs. 5.3×5.6 Å in *MRE). We ratio suggesting higher acid density is, however, further
believe that this divergence reflects the molecular believed to play a role in the increasing diethylbenzene
confinement provided through a single layer of narrow 3-D formation. Conversely, our nano-MTW was able to restrain the
interconnections in the MFI nanosponge (Fig. 2). The formation of diethylbenzene with selectivity ca. 3% below 10%
framework termination in the b crystallographic direction with conversion. Nonetheless, disproportionation occured to a
narrow 10-membered rings has been thought essential for the higher extent than over the 10-R nano-*MRE. In spite of the
shape-selectivity of ultra-thin MFI crystals applied in the similar channel length provided by these <10 nm-thick
6
7
hydroconversion of n-decane.
Our results seem to nanocrystals, the catalytic behavior of ultra-thin
corroborate this assumption. The nano-MFI appears able to aluminosilicates with similar acidity was still believed to be
retain its shape-selectivity up to a certain extent in the dependent on the varying zeolite pore aperture size. In this
hydroconversion of ethylbenzene. This result is of particular context, larger pore size though appears to have a minor
interest if the decrease of the crystal thickness is optimized to impact in the attained hydrocracking and dealkylation
achieve higher catalytic stability and activity while selectivity when both 1-D channel nanosponge zeolites (MTW,
simultaneously affording the desired selectivity in a wide *MRE) are compared. Furthermore, the similar profiles
multitude of reactions involving the hydroconversion of alkyl- obtained with both zeolites suggest an impact of the pore size
aromatics.
at a significantly lower extent compared to the effect of pore
dimensionality observed with our MFI nanosponge. High
MTW topology The trend in xylenes formation over both MTW- selectivity towards hydroisomerization has been reported to
type zeolites was comparable (Fig. 6). Higher maximal be a particular feature of exposed protonic sites located in the
hydroisomerization selectivity was attained over the pore-mouths and exposed side-pockets of tubular 10-R
1
2,13
nanosponge (91% over nano vs. 83% over bulk) enhancing the zeolites.
initial reaction rate 1.8-fold (Fig. S5, ESI†). Similarly to the within highly accessible 1-D 12-R zeolites, a remarkable xylene
Nonetheless, the present results proved that
aforementioned cases, the catalytic benefit of reducing the selectivity (92%) can also be achieved as demonstrated with
zeolite crystal thickness was evident also with the MTW our nanosponge MTW.
topology. Experimental results demonstrated that a decrease
of the crystal thickness in nano-MTW could suppress the
Conclusions
occurrence of multi-step reactions. In particular, at 80%
6
| J. Name., 2012, 00, 1-3
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Cat Pa ll ey as si es dS oc ni eo nt ca ed j &u s Tt me ca hr gn i on sl ogy
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ARTICLE
Herein, we synthesized a series of nanosponge zeolites with
topologies *MRE, MFI, and MTW, and their bulk counterparts. The
nanosponge-zeolites possess, in addition to their intrinsic
microporous architecture, a significant volume of intercrystalline
8
E. Guillon, S. Lacombe, T. Sozinho, P. Magnoux, S. Gnep, P.
Moreau and M. Guisnet, Oil Gas Sci. Technol., 2009, 64, 731.
M. Guisnet, Catal. Today, 2013, 218, 123.
9
1
0 E. Guillon and L. Brandhorst, U.S. Patent 2014/0135550 A1,
2
014.
mesoporosity with a narrow size distribution. For the first time they 11 L.D. Fernandes, J.L.F. Monteiro, E.F. Sousa-Aguiar, A.
were investigated in the gas-phase hydroconversion of
ethylbenzene (EB) in the presence of a hydrogenating phase
Martinez and A. Corma, J. Catal., 1998, 177, 363.
2 F. Moreau, P. Moreau, N.S. Gnep, P. Magnoux, S. Lacombe
1
1
and M. Guisnet, Microporous Mesoporous Mater., 2006, 90
27.
3 P. Moreau, N.S. Gnep, P. Magnoux, E. Guillon, S. Lacombe
,
2 3
(Pt/Al O ). Improved catalytic activity compared to bulk zeolites
3
suggested an enhanced molecular transport within zeolitic crystals,
which is a challenging industrial sidestep for the hydroconversion of
and M. Guisnet, Stud. Surf. Sci. Catal., 2008, 174, 1179.
alkyl-aromatic species. A lower confinement effect inside these 14 Y.S. Hsu, T.Y. Lee and H.C. Hu, Ind. Eng. Chem. Res., 1988, 27
,
9
42.
nanosponge zeolites proved to be unique to increase the xylene
formation. This effect was further reflected on the selectivity to p-
xylene and on the selectivity towards ethylbenzene dealkylation,
disproportionation, transalkylation, and hydrocracking (see Fig. 7).
1
1
5 C. Travers, F. Raatz, C. Marcilly, F. Ramôa Ribeiro and M.F.
Gomes Ribeiro, European Patent 0,363,253, 1989.
6 F. Moreau, N.S. Gnep, S. Lacombe, E. Merlen and M. Guisnet,
Ind. Eng. Chem. Res., 2002, 41, 1469.
In particular, when the MFI crystal thickness is decreased from 17 H.H. John, H.D. Neubauer and P. Birke, Catal. Today, 1999,
, 211.
8 W. Souverijns, L. Rombouts, J.A. Martens and P.A. Jacobs,
Microporous Mater., 1995, , 123.
4
9
2
0 μm to 2.5 nm, a remarkable performance is observed when the
1
1
simultaneous hydroisomerization of xylenes and ethylbenzene is
desired.
4
9 J.L. Casci, B.M. Lowe and T.V. Whittam, U.S. Patent
4,537,754, 1985.
Assuming relatively similar acidity, we attributed the
divergence in the product selectivity to the pore size and 20 R. Ryoo, K. Cho and F. Marques Mota, “Mesostructured
zeolites”,
in
Sustainable
Chemistry:
Synthesis,
channel dimensionality of ultra-thin nanosponge zeolites. We
observed a dramatic impact of the presence of spacious
interconnections within narrow 10-R channels (MFI zeolite) on
the resulting product selectivity compared to straight channels
Characterization and Catalytic Applications, ed. F.-S. Xiao and
X. Meng, Springer, 2015, ch. 4, p. 101.
1 C.J.H. Jacobsen, C. Madsen, J. Houzvicka, I. Schmidt and A.
Carlsson, J. Am. Chem. Soc., 2000, 122, 7116.
2
in *MRE and MTW. Conversely, this work emphasizes ultra- 22 M. Choi, H.S. Cho, R. Srivastava, C. Venkatesan, D.-H. Choi
and R. Ryoo, Nat. Mater., 2006,
3 D. Verboekend and J. Pérez-Ramírez, Catal. Sci. Technol.,
011, , 879.
4 M. Milina, S. Mitchell, P. Crivelli, D. Cooke and J. Pérez-
Ramírez, Nat. Commun., 2014, 5:3922, DOI:
10.1038/ncomms4922.
5, 718.
thin zeolites with tubular-like micropores, exhibiting
outstanding maximal isomer product yields, as catalysts of
choice when ethylbenzene is preferentially hydroisomerized.
Whereas EB has a crucial impact in the choice of the zeolite
2
2
2
1
8
applied in the hydroconversion of the C aromatic, this work
2
2
5 M.S. Holm, E. Taarning, K. Egeblad and C.H. Christensen,
Catal. Today, 2011, 168, 3.
6 A. Corma, V. Fornes, S.B. Pergher, T.L.M. Maesen and J.G.
additionally sheds light on the study of five catalytic reactions
of major industrial interest in the transformation of alkyl-
aromatic species over ultra-thin nanosponge zeolites.
Buglass, Nature, 1998, 396, 353.
Accordingly, it serves as a new effort in understanding the 27 I. Ogino, M.M. Nigra, S. Hwang, J. Ha, T. Rea, S.I. Zones and
shape-selectivity provided by <10 nm zeolite crystals, to
further define and improve their potential utility in a wide
number of applications.
A. Katz, J. Am. Chem. Soc., 2011, 133, 3288.
2
2
8 P. Eliášová, M. Opanasenko, P.S. Wheatley, M. Shamzhy, M.
Mazur, P. Nachtigall, W.J. Roth, R.E. Morris and J. Čejka,
Chem. Soc. Rev., 2015, 44 (20), 7177.
9 W.J. Roth, P. Nachtigall, R.E. Morris, P.S. Wheatley, V.R.
Seymour, S.E. Ashbrook, P. Chlubná, L. Grajciar, M. Položij, A.
Acknowledgements
Zukal, O. Shvets and J. Čejka, Nat. Chem., 2013,
0 W.J. Roth, P. Nachtigall, R.E. Morris and J. Čejka, Chem. Rev.,
014, 114, 4807.
1 M. V. Opanasenko, E. Montanari, M. V. Shamzhy,
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5, 628.
3
3
This work was supported by IBS-R004-D1.
2
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Tables
Table 1 Physicochemical properties of bulk and nanosponge zeolites and corresponding TOF for the hydroisomerization of ethylbenzene
2
-1
Surface area (m g )
V
Turn-over frequency,
total
-1
Sample
3
(Si/Al)*
-1
-1
BET
173
468
307
614
224
441
External
76
(cm g )
0.129
0.785
0.141
0.630
0.116
0.420
TOF (s ) at WHSV value 15 h
bulk-*MRE
nano-*MRE
bulk-MFI
99
100
82
0.35
0.65
0.24
1.10
0.50
0.76
354
21
nano-MFI
bulk-MTW
nano-MTW
383
48
90
75
276
103
*determined by ICP

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Figures
Fig. 1. Representative illustrations of the porous channel system of *MRE, MFI, and MTW zeolites.
Fig. 2. SEM images of (a) *MRE, (b) MFI, and (c) MTW conventional zeolite samples and of corresponding nanosponge counterparts (d, e
and f). TEM images of (g) nano-*MRE, (h) nano-MFI and (i) nano-MTW.

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Fig. 3. Ethylbenzene conversion against contact time taken as the reverse of WHSV over the bulk (ꢀ) and nanosponge samples (ꢁ) with
-
1
*
MRE, MFI, and MTW-type zeolites. (ꢂ,) Corresponding back-points at the end of the catalytic test at a WHSV=15 h .
Fig. 4. p-Xylene content (mol %) against conversion over the bulk (ꢀ) and nanosponge samples (ꢁ) with *MRE, MFI, and MTW-type
zeolites.
Fig. 5. Illustration of a simplified reaction scheme of ethylbenzene hydroisomerization and the main side reactions considered in this
contribution: ꢃ ethylbenzene disproportionation, ꢄ ethylbenzene transalkylation with xylene, ꢅ ethylbenzene dealkylation and ꢆ
hydrocracking following naphthene ring opening.

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Fig. 6. Comparison of the selectivity towards hydroisomerization, dealkylation, disproportionation, transalkylation and hydrocracking
against conversion over the bulk (ꢀ) and nanosponge samples (ꢁ) with (a) *MRE, (b) MFI and (c) MTW-type zeolites.
Fig. 7. Comparison of the selectivity towards hydroisomerization, dealkylation, disproportionation, transalkylation and hydrocracking at
ca. 50% conversion over *MRE, MFI and MTW-type bulk and nanosponge zeolites.

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5
5x30mm (300 x 300 DPI)