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\begin{document}

\title{Achieving acid sites accessibility and pore compatibility in MWW
nanosheets for alkylation of benzene with 1-dodecene}



\author[1]{Baoyu Liu}%
\author[1]{Zhantu Liao}%
\author[1]{Jianwen Zhang}%
\author[2]{Yeqing Huang}%
\affil[1]{Affiliation not available}%
\affil[2]{Guangdong University of Technology - University Town Campus}%


\vspace{-1em}



  
  \date{June 16, 2024}


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\selectlanguage{english}
\begin{abstract}
Regularly spaced MWW nanosheets combined with single-unit-cell thickness
and optimized Al species distribution are highly effective for the
alkylation between benzene with 1-dodecene, high conversion of
1-dodecene (ca. 85 \%) and selectivity of 2-LAB (ca. 80 \%) can be
obtained. Detailed studies of this MWW nanaosheets system reveal that
ultra-thin MWW nanosheets with ordered arrangement can expose more
accessible Br\selectlanguage{ngerman}ønsted acid sites (ca. \textasciitilde{}89 \%), and
specially place framework Al species on the T2 sites (ca.
\textasciitilde{}13 \%), in which the alkylation mainly occurred under
the catalysis of accessible Brønsted acid sites. Besides, the desired
2-LAB is produced on the unique 12-MR hemicavities with solid binding
energy and perfect steric configuration. As a proof-of-concept study, we
integrate the highly exposed Brønsted acid sites and unrivalled
shape-selectivity in the regularly spaced MWW nanosheets system with
well characterized acid sites accessibility and pore adaptability,
leading to the excellent catalytic performance.%
\end{abstract}\selectlanguage{ngerman}%



\sloppy


\textbf{Introduction}

Long chain linear alkylbenzene (LAB) is one of critical chemicals since
its sulphonated products are the raw materials for industrial and
domestical detergents.\textsuperscript{1}Commercially, the LAB is
produced through the alkylation between benzene with \selectlanguage{greek}α-\selectlanguage{english}olefins
(C\textsubscript{8}-C\textsubscript{16}), which is catalyzed under the
HF and AlCl\textsubscript{3}system.\textsuperscript{2} Notability, the
position of phenyl group in long chain \selectlanguage{greek}α-\selectlanguage{english}olefin is crucial to
biodegradability and solubility, among various LAB isomers, the 2-LAB is
the most biodegradable and possesses the best
solubility.\textsuperscript{3} Therefore, it's highly desired to develop
an efficiency catalysis technology to improve the selectivity of 2-LAB
isomer. However, the traditional HF and AlCl\textsubscript{3} catalysis
technology for the production of LAB brings some severe issues: 1) these
homogeneous catalysts can generate a large number of waste acids during
the producing process, they also cause serious corrosion towards
reactor, and the separation with the reactants and products is a tough
energy-consuming process.\textsuperscript{4} In addition, the HF
requires special handling and equipment, which further increase cost and
risk; 2) HF and AlCl\textsubscript{3} catalysts exhibit low 2-LAB
selectivity with \textasciitilde{}15\% and 30\%,\textsuperscript{5}
respectively. This is far from the target that develops an
environmental-friendly, noncorrosive, reusable and green alkylation
technology with desired 2-LAB selectivity.

To overcome these problems, a large number of clean technology has been
investigated based on the solid acid catalysts such as
zeolites,\textsuperscript{6,7}ion exchange resin,
\textsuperscript{8}metal
oxides,\textsuperscript{2}clay\textsuperscript{9} and heteropoly
acid.\textsuperscript{10} Among them, the UOP company firstly realized
the production of LAB in the industrial scale based on the Detal
technology by employing HF or NH\textsubscript{4}F modified
SiO\textsubscript{2}-Al\textsubscript{2}O\textsubscript{3} as the solid
catalyst,\textsuperscript{11} which open a window for the development of
green alkylation technology between benzene with long chain \selectlanguage{greek}α-\selectlanguage{english}olefins.
Currently, the production of LAB mainly depends on the HF,
AlCl\textsubscript{3} and Detal technology, and the corresponding market
share is \textasciitilde{}75 \%, 10 \% and 15 \%, respectively.
Unsatisfactorily, the selectivity of 2-LAB is only increased to 25 \%
through Detal technology.\textsuperscript{12} Besides, the acid strength
of HF or NH\textsubscript{4}F modified
SiO\textsubscript{2}-Al\textsubscript{2}O\textsubscript{3} is not enough
to catalyze the alkylation of benzene with long chain \selectlanguage{greek}α-\selectlanguage{english}olefins, and the
reaction must be operated under the high temperature and pressure,
resulting in high cost compared with conventional HF technology.
Therefore, it's still highly expected to obtain an efficient green
technology for the production of 2-LAB.

Zeolites is a class of widely used solid catalysts with tunable acidity
and pore size, which is an ideal candidate for alkylation between
benzene with long chain linear \selectlanguage{greek}α-\selectlanguage{english}olefins. In the past decades, great
efforts have been made to search suitable zeolites for 2-LAB
synthesis.\textsuperscript{13} The major issues that prevent the
successful industrialization of zeolite catalysts for LAB process are
the non-ideal distribution of LAB isomers and deactivation of
zeolites.\textsuperscript{6} On the one hand, the activity of zeolites
is from acid sites that can catalyze alkylation between benzene and
\selectlanguage{greek}α-\selectlanguage{english}olefins. Thus, zeolites must expose more accessible acid sites in
order to improve activity, the effective strategy for improving
accessible acid sites in zeolite framework is fabrication of mesopore
zeolites with open hierarchical channels. The mesopores not only improve
the accessibility of acid sites, but also enhance the diffusion of coke
precursor and increase the life time of catalysts. On the other hand,
the shape-selective catalysis for zeolites is mainly based on the size
or shape in sterically confined environments.\textsuperscript{14} If the
mesopores are introduced into the framework or special fabrication of
layer zeolites with ultra-thin thickness along one crystal axis, the
shape-selective effect of zeolites is lineally decreased. Therefore,
it's still a huge challenge to fabricate suitable zeolite catalysts for
the production of 2-LAB.

In present research, the structure of MWW zeolite are optimized to solve
the above-mentioned issues. Dual template strategy with conventional
hexamethyleneimine (HMI) structure directing agent and long chain
quaternary ammonium surfactant is employed to delaminate the MWW
zeolites, which expect to expose the external acid sites and improve the
accessibility of acid sites. Our finds confirm that external surface
Br\selectlanguage{ngerman}ønsted acid sites can be systematically tailored by tuning the
hydrophobic chain length of surfactant. Moreover, the mono-layer
delaminated MWW zeolite is obtained by increasing the ratio between
quaternary ammonium surfactant and HMI. Computational results reveals
that the external isolated 12-ring hemicavities located on the upper and
lower surface of the MWW lamella can stabilize the 2-LAB due to the most
appropriate sterical configuration and highest binding energy with
12-ring hemicavities, which is benefit to the formation of 2-LAB. Our
study shows that a regularly spaced MWW zeolite nanosheets system can
greatly improve the activity of catalysts in the alkylation between
benzene with 1-dodecene without sacrificing the selectivity of 2-LAB. In
addition, the catalytic performance of MWW zeolite nanosheets are
further maximized by altering the distribution of Al species on the
exterior surfaces and inhibiting the stacking of MWW zeolite layers, the
optimized MWW zeolite layers also show a superior stability with robust
selectivity of 2-LAB, which is highly desired and expected as an ideal
solid catalyst for the industrial production of 2-LAB.

\textbf{Experimental section}

Catalyst preparation

The MWW zeolite nanosheets were prepared by employing the dual template
strategy following the previous report,\textsuperscript{15} and
regularly spaced MWW zeolite nanosheets were fabricated according to the
method developed by Tsapatsis et al.\textsuperscript{16} The detailed
synthesis produces and characteristic technology can be found in the
experimental procedures section in supporting information.

Computational Results

The calculations in this work were performed using Forcite module in
Materials Studio.\textsuperscript{17} To calculated the interaction
energy between LAB isomers and zeolite, we first build a cleaved model
with the bowl shape structure exposed on the surface to provide
effective adsorption space. This model was optimized with energy, force,
displacement convergence thresholds set to 1×10\textsuperscript{-4}
kcal/mol, 5×10\textsuperscript{-3} kcal/mol/Å and
5×10\textsuperscript{-3} Å, respectively. The van der Waals interaction
between the atoms was modeled using universal force field, while the
electrostatic interaction was described with atomic partial charges
calculated using QEq method and summed with Ewald method. The van der
Waals interaction was truncated within a cutoff distance of 15.5 Å, and
the Ewald accuracy for computing electrostatic interaction was set to
1×10\textsuperscript{-4} kcal/mol. After the optimization, one molecule
of the LAB isomers was respectively adsorbed in the bowl shape structure
of the model as an adsorbed configuration. These configurations were
sequentially optimized with the same parameters as optimizing the bare
model. Finally, using the same parameters to describe van der Waals
interaction and electrostatic interaction, we calculated the single
point energy of these adsorbed configurations, the bare adsorbent model
and the LAB isomers, and obtained the binding energy using Eq.1,

E\textsubscript{binding} = E\textsubscript{configuration} -
E\textsubscript{bare model} - E\textsubscript{LAB} (1)

Catalytic property

The liquid alkylation between benzene and 1-dodecene was performed in a
three-necked round-bottom flask equipped with condensation under
stirring condition. Prior to measurement, the resultant zeolite samples
were ion exchanged three time with 1 M
NH\textsubscript{4}NO\textsubscript{3} solution at
80\textsuperscript{o}C for 12 h, and benzene, 1-dodecene with ethyl
cyclohexane was dried over 4A zeolite for 12 h. In a typical experiment,
0.1 g zeolite catalyst, 112 mmol benzene and 7.2 mmol ethyl cyclohexane
was added into the flask, then the flask was stirred at
80\textsuperscript{o}C for 0.5 h, subsequently 12.8 mmol 1-dodecene was
added. This moment was marked as the initial reaction time. Liquid
samples were withdrawn and analyzed by a gas chromatograph (Agilent 7890
A) equipped with a polyorganosiloxane capillary column (HP-5, 30.0 m ×
0.32 mm, 0.25 \selectlanguage{greek}μ\selectlanguage{english}m) connected to a flame ionization detector (FID). For
the recycling experiment, zeolites after cycle were filtered and dried
at 120  for 12 h, then they were reused to evaluate the catalytic
performance in next cycle without further treatment.

Results and discussion

Synthesis of MWW zeolite nanosheets

The dual template synthesis strategy with HMI and long chain quaternary
ammonium surfactant was used to delaminate the MWW zeolites, which can
expect to generate MWW zeolite nanosheets and expose more exterior
Br\selectlanguage{ngerman}ønsted acid sites for alkylation. It was reported that
cetyltrimethylammonium bromide (CTAB) could act as a positive counterion
role to balance negative charge of framework Al atoms at surface sites
on the MWW zeolite lamellas in the zeolite synthesis
gel,\textsuperscript{15} the accumulation of long chain quaternary
ammonium surfactants on exterior surfaces can impede layer\selectlanguage{english}-layer
interactions owing to the tangling effect of long hydrophobic tail of
surfactant, resulting in the separation between MWW layers. Herein, the
long chain quaternary ammonium surfactants with different tails such as
dodecyl trimethylammonium bromide (DTAB), myristyltrimethylammonium
bromide (MTAB) and cetyltrimethylammonium bromide (CTAB) were employed
to tune the degree of delaminated MWW nanosheets based on discrepant
long tails that can generate the various tangling effect due to the
steric hindrance.\textsuperscript{18} In addition, the excess surfactant
was added into synthesis gel to ensure that the sufficient coverage of
incorporated surfactant on the exterior surface of MWW layers was
achieved in order to effectively impede layer alignment.

Fig. 1 gave the result of powder x-ray diffraction (PXRD) patterns of
multiple-layer and mono-layer MWW zeolite nanosheets, which was commonly
used to evaluate the arrangement of MWW layers in the 2\selectlanguage{greek}θ \selectlanguage{english}range of
6-10@.\selectlanguage{english}\textsuperscript{19} As shown in Fig. 1, MCM-22 presented three
peaks at \textasciitilde{}7.2@, \selectlanguage{english}8.0@ \selectlanguage{english}and 10.0@, \selectlanguage{english}corresponding (100),
(101) and (102) reflections, indicating ordered staking of MWW
layers.\textsuperscript{20} When the long chain quaternary ammonium
surfactant was added into the gel, these samples such as
MZN\textsubscript{4}-12, MZN\textsubscript{4}-14 and
MZN\textsubscript{4}-16 also exhibited the peaks corresponding (101) and
(102) reflections, but the bands assigned to the (101) and (102) peaks
became broad, suggesting that the degree of long-rang order of MWW layer
along the \emph{c} axis was decreased and the multilayered structure was
formed.\textsuperscript{21} Besides, when the addition of MTAB was
increased to the 8.0 wt \%, the intensity of (101) and (102) reflections
for MZN\textsubscript{8}-14 was obviously decreased, and these
reflections were transformed into a broad peak owing to disordered
arrangement of MWW nanosheets along the \emph{c}direction and the
mono-layer MWW zeolites may be produced.\textsuperscript{15} In
addition, for all the multiple-layer and mono-layer samples, the (100)
reflection can be observed, indicating that crystalline structure of MWW
framework was maintained.\textsuperscript{22} Moreover, the (002)
reflection can be observed in the as-synthesized
MZN\textsubscript{4}-12, MZN\textsubscript{4}-14 and
MZN\textsubscript{4}-16 (Fig. S1) owing to the presence of interlayer
structure.\textsuperscript{15} As the concentration of surfactant was
increased to 8.0 wt \% in synthesis gel, the (002) reflection of
MZN\textsubscript{8}-14 dramatically decreased, indicating the loss of
long-range order along the \emph{c} -axis (Fig. S1).

To further investigate the delaminated degree of MWW zeolite nanosheets,
the N\textsubscript{2} adsorption/desorption experiments were carried
out. It was observed from Fig. S2 that MCM-22 and
MZN\textsubscript{4}-12, MZN\textsubscript{4}-14 and
MZN\textsubscript{4}-16 presented a characteristic of micropore
materials at \emph{P/P\textsubscript{0}}\textless{} 0.9, while the sharp
increase in the relative pressure ranges above 0.9 was attributed to the
meso/macropores formed by MWW layers stacking.\textsuperscript{23} For
MZN\textsubscript{8}-14, it exhibited a typical type-III isotherm with an
obvious hydrolysis loop in the range of
\emph{P/P\textsubscript{0}}\textgreater{} 0.6, indicating the presence
of mesopores arised from intergrown and stacking of MWW
nanosheets.\textsuperscript{21} Table S1 summarized the textural
parameters of resultant MWW zeolites, it was noted that the micropore
volume of MWW zeolite nanosheets was lower than MCM-22, which was
reasonably attributed to the fact that the MWW zeolite nanosheets
destroy the conventional stacking of MWW layer as like 3D order in
MCM-22, resulting in the loss of 10-membered ring (MR) channels and
12-MR supercages along the \emph{c}axis.\textsuperscript{15} However,
the MWW zeolite nanosheets showed an enhanced fraction of mesoporous
surface or volume compared with MCM-22 owing to the delaminated effect,
which was expected to expose more accessible Br\selectlanguage{ngerman}ønsted acid sites. In
addition, the concept of disorder index as ratio between the external
and total surface areas was employed to identify the delaminated degree
of MWW layers following Rimer et al's report,\textsuperscript{15} it was
found that the degree of disorder for MWW zeolite nanosheets increased
in the order of MZN\textsubscript{4}-12 \textless{}
MZN\textsubscript{4}-14 \textless{} MZN\textsubscript{4}-16 \textless{}
MZN\textsubscript{8}-14, indicating that the delaminated degree of MWW
zeolite nanosheets can be tailored by tuning the length of long tail in
surfactants due to distinct steric hindrance. Specially, when the amount
of MTAB was increased to 8.0 wt \%, it greatly improved the separation
of MWW layers since the sufficient surfactant can graft on every single
MWW layer, which can inhibit the inductive effects between HMI with
surface silanols,\textsuperscript{24} leading to the formation of
mono-layer and presence of the highest disorder index.

The morphology of MCM-22 and MWW zeolite nanosheets was investigated by
scanning electron microscope (SEM) images, as shown in Fig. 2 and Fig.
S3. The MCM-22 showed bird's nest-like shape (Fig. 2a), this was typical
morphology of non-agitated MCM-22 sample.\textsuperscript{25} While the
MZN\textsubscript{4}-12, MZN\textsubscript{4}-14 and
MZN\textsubscript{4}-16 exhibited a thick plate-like morphology (Fig.
S3). Particularly, MZN\textsubscript{8}-14 presented an interpenetrating
network of nanosheets with ultrathin flake-like morphology (Fig. 2b),
demonstrating that the additional surfactant can impede MWW layer
alignment act as an exfoliating agent, and increasing the amount of
surfactant was a more effective method to achieve the delaminated MWW
zeolite with ultrathin nanosheets compared with tailoring the tail
length of surfactants.

The delaminated nature of resultant MWW zeolite nanosheets was further
investigated by transmission electron microscope (TEM) images, as
depicted in Fig. 3 and Fig. S4. It was observed from Fig. S3 that the
MZN\textsubscript{4}-12, MZN\textsubscript{4}-14 and
MZN\textsubscript{4}-16 exhibited the discrete lamellar feature, but the
thickness of these MWW layers was still thick, indicating that the
additional surfactant can delaminate the 3D MWW zeolite to generate the
lamellar structure on a certain degree. Specially, the
MZN\textsubscript{8}-14 present an ultrathin nanosheets morphology (Fig.
3a-c) compared with conventional 3D MCM-22 with a thickness of
\textasciitilde{}23 nm (Fig. 3e-f), and the thickness of the MWW zeolite
nanosheets was measured by atomic force microscope (AFM), which showed a
uniform nanosheet thickness of 2.5 ± 0.4 nm (Fig. 3d and Fig. S5), the
average content of ultrathin nanosheets estimated from AFM images was
about 73 ± 5\%, suggesting that the mono-layer MWW nanosheets with
single-unit-cell thickness was produced.\textsuperscript{22}

Comparison of the catalytic properties of MWW zeolite nanosheets

The catalytic properties of MWW zeolite nanosheets were evaluated in the
alkylation between benzene and 1-dodecene. It was noted from Fig. 4a
that the MWW zeolite nanosheets exhibited a higher conversion than
MCM-22 owing to the enhanced external Brønsted acid sites (Table S2 and
Fig. S6), which originated from the delaminated process that can
generate the MWW nanosheets and expose more accessible Brønsted acid
sites. To verify the external Brønsted acid sites was main active sites
for alkylation between benzene with 1-dodecene since it involves the
bulk molecules, the pyridine and 2,6-di-tert-butylpyridine (DTBP)
poisoning experiment of typical MZN\textsubscript{4}-14 were performed,
as shown in Fig. S7. It was found that the pyridine poisoning
MZN\textsubscript{4}-14 totally lost the activity, and the DTBP
poisoning MZN\textsubscript{4}-14 maintained \textasciitilde{}8 \%
conversion, the conversion dropped by 82 \% compared with fresh
MZN\textsubscript{4}-14, indicating that the external Brønsted acid
sites of MWW zeolites were main active sites in alkylation since they
can be easily accessed compared with Brønsted acid sites located in the
limited micropore channels. In addition, the conversion of MWW zeolite
nanosheets increased in the order of MZN\textsubscript{4}-12 \textless{}
MZN\textsubscript{4}-14 \textless{} MZN\textsubscript{4}-16 \textless{}
MZN\textsubscript{8}-14, which was in agreement with concentration of
external Brønsted acid sites as shown in Table S2.

Moreover, the MWW zeolite nanosheets presented a comparable selectivity
of 2-LAB (\textgreater{} 70 \%) compared with MCM-22, as listed in Fig.
4b. As is known to everyone, the shape-selective catalysis for zeolites
mainly depends on zeolite confined environments. Here, the alkylation
mostly occurred on the external surface of MWW zeolites based on the
aforementioned analysis, and MWW zeolite is a class of layer zeolite
containing external isolated 12-MR hemicavities with free internal
diameter of \textasciitilde{}0.71 nm, in which the alkylation takes
place under exposed acid sites to produce the LAB.\textsuperscript{11}
Therefore, the interaction between LAB isomers with 12-MR hemicavities
was calculated by using Forcite module in Materials Studio in order to
reveal why the MWW zeolite possessed comparable selectivity of 2-LAB
regardless of MWW layers arrangement, as shown in Fig. 5. It was
observed that 2-LAB had the highest binding energy and the best matched
steric configuration with 12-MR cage located on MWW lamellar surface
than other isomers, indicating that 2-LAB can be steadily formed in the
external confined 12-MR hemicavities of MWW zeolites, resulting in high
selectivity of 2-LAB. With the reaction time increasing, the
thermodynamic equilibrium mixture of LAB isomers was formed by reverse
alkylation and isomerization steps,\textsuperscript{26} leading to the
selectivity of 2-LAB decreasing with reaction time increasing.

Fabrication of regularly spaced MWW zeolite nanosheets

Comprehensive analysis of the above data, it was found that tuning the
tail length of long chain quaternary ammonium surfactant was not an
appropriate strategy to effectively delaminate the MWW zeolites to
produce the ultrathin MWW zeolite nanosheets. When excessive surfactant
was used, it can produce the mono-layer MWW zeolites, but the nucleation
of MWW zeolite was disrupted on a certain degree, resulting in low
crystallinity (Fig. 1 and Table S1). For alkylation of benzene and
1-dodecene, the external Brønsted acid sites were main active sites.
Thus, it's necessary to develop a strategy to fabricate the ultrathin
MWW zeolite nanosheets without destroying their crystallinity in order
to maximize the accessible Brønsted acid sites. The desired ultrathin
MWW zeolite nanosheets not only have the single MWW unit cell thickness
but also possess enough stability that can efficiently inhibit the
layers stacking. Naturally, the pillaring ultrathin MWW zeolite
nanosheets was an ideal approach to produce ordered MWW lamellas that
can satisfy the above requirements.

The structure of regularly spaced MWW zeolite nanosheets was identified
by PXRD technology, as presented in Fig. 6. It was noted that both of
PMWW(31) and PMWW(74) exhibited the (001) reflections in the low-angle
region of 1-5\selectlanguage{english}@, and the corresponding (002) and (003) can be found in
PMWW(74), indicating that the resultant PMWW(74) possessed the
characteristic of long-rang ordered structure even after multi-pillaring
steps.\textsuperscript{16} For PMWW(31), the second-order reflection was
missing due to mismatch between the thickness of a single MWW nanosheet
and one-half of the adjacent-layer space,\textsuperscript{27} which
demonstrated that the PMWW(31) maintained the long range order, but the
degree of long range order was inferior compared with PMWW(74). By
comparation, the MCM-22 didn't show any reflection in the low angle
region of 1-5@ \selectlanguage{english}owing to its typical 3D structure. In addition, the (101)
and (102) reflections were obviously visible in PMWW(74) and PMWW(31),
which was different from the previous MCM-36 material that the (101) and
(102) reflections were transformed into a broad peak due to the partial
loss of the vertical alignment order of MWW layers along the\emph{c}
-axis.\textsuperscript{19} The presence of these reflections indicated
that the ordered lamellar structure of MWW layers with the vertically
aligned layers being ordered perpendicular to the \emph{c} axis in
PMWW(74) and PMWW(31) was well maintained even after
pillaring,\textsuperscript{28-30} which was also the typical
characteristic of long-range order of MWW nanosheets in the\emph{c}
-direction.\textsuperscript{31}Therefore, the regularly spaced MWW
zeolite nanosheets with long range order were fabricated based on the
analysis of low-angle and high-angle PXRD data. Moreover, the only
difference between PMWW(31) and PMWW(74) was discrepant Si/Al ratios,
PMWW(31) had the same Si/Al ratio with aforesaid MWW zeolite nanaosheets
in the synthesis of MCM-22(P) that was the precursor for fabrication of
pillared MWW zeolites, and PMWW(74) increased the Si/Al ratio.

For silica pillaring process, it can be performed through formation the
Si-O-Si bonds between silanol groups from the MWW layers with silica
pillar species,\textsuperscript{16} and the high Si/Al ratio of MWW
layers can definitely influence the concentration of silanol groups
located on the external surface of MWW layers.\textsuperscript{32} It's
postulated that the MWW layers with high Si/Al ratio have sufficient
opposing silanol groups in the neighboring layers, which is benefit to
formation of ordered silica pillars. While the Si/Al ratio of MWW layers
decreases, there is an increased probability of Al atoms becoming
incorporated at external surface sites on the MWW layers, it reduces the
concentration of terminal silanol groups in the adjacent layers, and
further disrupts the pillaring process on a certain degree due to the
slight deviations or mismatch between the position of silanol groups on
each MWW layers,\textsuperscript{15} resulting in the low degree of long
range order for PMWW(31), and the proposed influence of Si/Al ratio on
the pillaring process was illustrated in Scheme 1.

The pillared MWW zeolites were further investigated by
N\textsubscript{2} adsorption experiments, it was observed from Fig. S8a
that PMWW(31) and PMWW(74) exhibited an increased adsorption
at\emph{P/P\textsubscript{0}} \textgreater{} 0.4, indicating the
presence of mesoporosity caused by pillaring process, which was in
accordance with previous report.\textsuperscript{16}By comparison,
MCM-22 achieved a saturation at a relative pressure of 0.1 owing to the
sole micropores. The corresponding NLDFT (nonlocal density functional
theory) pore size distribution of PMWW(31) and PMWW(74) (Fig. S8b) also
presented a substantial increase in the range of
(1.2\textasciitilde{}3.3 nm) as a result of pillaring. Moreover, the
textural data (Table S1) further corroborated that PMWW(31) and PMWW(74)
had increased \emph{S\textsubscript{ext/}S\textsubscript{BET}}
and\emph{V\textsubscript{meso}/V\textsubscript{tot}} owing to the
effective pillaring procedure. In addition, the PMWW(31) and PMWW(74)
showed a thin flake-like morphology with layers stacked in a lamellar
arrangement (Fig. 7), and the framework structure of PMWW(31) and
PMWW(74) were further investigated by TEM images, as shown in Fig. 8 and
Fig. S9. It was noted from Fig. 8 that both PMWW(31) and PMWW(74)
exhibited an ordered multilamellar architecture with a vertical distance
of \textasciitilde{}1.8 nm, the interlayer distance
(\textasciitilde{}1.8 nm) between the MWW nanosheets was almost equal to
the theoretical length of MTAB surfactant (\textasciitilde{}1.75
nm),\textsuperscript{18} which was also in agreement with previous XRD
data (\textasciitilde{}1.6 nm), and the thickness of regularly spaced
MWW nanosheets was \textasciitilde{}2.5 nm corresponding to one unit
cell of MCM-22 in the \emph{c} -axis. However, the multilamellar
arrangement of PMWW(31) presented a litter deviation compared with
PMWW(74) due to the low Si/Al ratio, which was in accordance with above
analysis.

Initial optimization of the catalytic performance

Fig. 9 exhibited the catalytic performance of PMWW(31) and PMWW(74) in
the liquid alkylation between benzene with 1-dodecene. As shown in Fig.
9a, PMWW(31) presented the comparable conversion of 1-dodecene compared
with MZN\textsubscript{4}-14 since they had the similar concentration of
external Br\selectlanguage{ngerman}ønsted acid sites (Table S2). While PMWW(74) showed the best
activity in all of investigated MWW zeolites due to its largest
concentration of external Brønsted acid sites (Table S2, Fig. S10), and
the fraction of external Brønsted acid sites for PMWW(74) achieved 88.5
\%, indicating that the optimized pillaring process was an effective
method to expose the accessible Brønsted acid sites. Moreover, the
\textsuperscript{27}Al multiquantum magic angle spinning (MQMAS) NMR
spectroscopy was further conducted to reveal the potential differences
in the spatial distribution of Al sites between PMWW(31) and PMWW(74)
samples, as shown in Fig. 10.

The framework of MWW zeolite was composed of eight different T sites and
three types of pores: 12-MR surface pockets, 10-MR windows accessing
internal supercages and 10-MR sinusoidal channels, which was depicted in
Fig. S11. It was noted from Fig. 10 that both PMWW(31) and PMWW(74)
exhibited two peaks at around \textasciitilde{}56 ppm and 0 ppm, which
was assigned to the 4-coordinated framework (Al\textsubscript{F}) and
6-coordinated extra-framework aluminum (Al\textsubscript{EF}) species,
respectively.\textsuperscript{33}According to previous
study,\textsuperscript{34} the broad peak around \textasciitilde{}56 ppm
can be resolved into three overlapped signals at 50, 56, and 61 ppm,
which was attributed to T\textsubscript{6}+ T\textsubscript{7} (\selectlanguage{greek}δ \selectlanguage{english}= 50
ppm), T\textsubscript{1}+ T\textsubscript{3} + T\textsubscript{4} +
T\textsubscript{5} + T\textsubscript{8} (\selectlanguage{greek}δ \selectlanguage{english}= 56 ppm) and
T\textsubscript{2} (\selectlanguage{greek}δ \selectlanguage{english}= 61 ppm), respectively. Following this way, the
broad peak around \textasciitilde{}56 ppm was deconvoluted, as presented
in Table S3 and Fig. 10. It was observed that the proportion of
Al\textsubscript{F(50)}species for PMWW(31) was about 49 \% (the
combination of \selectlanguage{greek}δ \selectlanguage{english}= 50 and \selectlanguage{greek}δ \selectlanguage{english}= 51), indicating the Al\textsubscript{F}
species mainly located in the supercages and surface
pockets,\textsuperscript{35} in which the alkylation occurred as a
result of accessible Br\selectlanguage{ngerman}ønsted acid sites arised from the
Al\textsubscript{F(50)} species. While the Al\textsubscript{F(61)}
species for PMWW(31) was missing, which demonstrated that proportion of
Al\textsubscript{F(61)} species in PMWW(31) was negligible. However,
Al\textsubscript{F(61)} species were related to T\textsubscript{2} sites
that located on the exterior surface of MWW nanosheets close to the
outer edge of the 12-MR pockets, and the T\textsubscript{2} sites was
the most accessible to bulky adsorbates.\textsuperscript{15} In this
point of view, it was reasonable that PMWW(74) had an advantage over
PMWW(31) in the alkylation due to the presence of considerable
Al\textsubscript{F(61)} species (\textasciitilde{}13 \%). In addition,
PMWW(31) and PMWW(74) possessed the similar quantity of
Al\textsubscript{EF(0)} species. Fig. 9b showed that PMWW(31) and
PMWW(74) had comparable selectivity of 2-LAB regardless of the
multilamellar arrangement of MWW zeolite nanosheets, because PMWW(31)
and PMWW(74) shared the same 12-MR hemicavities structure on the upper
and lower surface of MWW layers,\textsuperscript{36} which was in line
with above computer calculation data (Fig. 5). These analyses
corroborated that the fabrication of order multilamellar arrangement of
MWW nanosheets and precise control of Al species distribution can
greatly expose the accessible Brønsted acid sites.

Stability, acidity and catalytic performance of regularly spaced MWW
zeolite nanosheets

The stability, acidity and catalytic performance of optimized PMWW(74)
was further investigated in the alkylation between benzene with
1-dodecene. As shown in Fig. 11a, the conversion of 1-dodecene for
PMWW(74) and MCM-22 was 85 \% and 40 \% in the first cycle,
respectively. With the number of cycles increasing, the activity of
PMWW(74) and MCM-22 all displayed the tendency declining. For MCM-22,
the conversion dropped to 19 \% after five cycles, while PMWW(74) still
maintained 68 \% conversion even after five recycling experiments,
indicating that PMWW(74) possessed strong stability compared with MCM-22
due to the enhanced diffusion of PMWW(74) with open single-layered
structure.\textsuperscript{21} As shown in Fig. S12 and Table S4, which
clearly verified that the effective diffusional time constants (\emph{D}
\textsubscript{eff}/\emph{r\textsuperscript{2}} ) of PMWW(74) was higher
than MCM-22. Interestingly, the selectivity of 2-LAB for MWW zeolite
catalysts was almost not influenced after cycle experiments (Fig. 11b),
because MWW zeolite possessed special 12-MR pockets on the exterior
surfaces that had the best matched steric configuration and charge force
with 2-LAB isomer.

To examine the structure of zeolite catalysts before and after cycles,
the corresponding XRD, N\textsubscript{2} adsorption-desorption isotherm
and acidity of fresh and spent catalysts after five cycles were
performed, as given in Fig. 12, Fig. S13 and Fig. S14, respectively. It
was observed from Fig. 12 that both PMWW(74) and MCM-22 exhibited the
robust stability with similar reflection patterns before and after
cycles. However, the coalescence of the (101) and (102) diffraction
peaks into a broadband reflection for spent PMWW(74) was observed,
indicating partial loss of vertical alignment order in the
stacking\emph{c} direction in the recycle
experiments,\textsuperscript{37} but (001) and (002) reflections were
still preserved, suggesting that multilamellar structure of PMWW(74) was
also totally preserved.\textsuperscript{16} In addition, the fresh
PMWW(74) and spent PMWW(74) showed the comparable N\textsubscript{2}
adsorption-desorption isotherms (Fig. S13) and textural parameters
(Table S1), suggesting that multilamellar arrangement of MWW nanosheets
over PMWW(74) was not destroyed, but the spent MCM-22 exhibited a
decreased BET surface area (557
m\textsuperscript{2}g\textsuperscript{-1} to 451
m\textsuperscript{2}g\textsuperscript{-1}, Table S1), indicating that
the channels of MCM-22 was blocked on a certain degree due to diffusion
limitation under the sole micropores. Moreover, the total Brønsted acid
sites and external Brønsted acid sites of PMWW(74) and MCM-22 after five
cycles were examined, as presented in Fig. S14 and Table S2. It was
observed from Table S2 that the total Brønsted acid sites and external
Brønsted acid sites of spent MCM-22 dropped by 83.2\% and 34.6 \%,
respectively. While the total Brønsted acid sites and external Brønsted
acid sites of spent PMWW(74) decreased by 32.3 \% and 30.6 \%,
respectively. These data confirmed that multilamellar arrangement of MWW
nanosheets for PMWW(74) exhibited the evident advantage of architecture
compared with conventional 3D MCM-22, in which the sole micropores of
MCM-22 suffered from serious diffusion limitation during the alkylation
and their narrow micropore entrance was easily blocked by bulk
alkylbenzene products,\textsuperscript{21} resulting in the
concentration of total Brønsted acid sites drastically decreasing after
cycles.

\textbf{Conclusions}

In summary, multiple-layer and mono-layer MWW zeolite nanosheets were
fabricated through the dual template synthesis strategy with
conventional HMI and long chain surfactants, in terms of multi-layered
MWW thickness or single-unit-cell thickness along the \emph{c} -axis
with a large number of exposed Brønsted acid sites. This strategy works
well on a layered MWW zeolite system that simultaneously integrates
exposed Brønsted acid sites and shape-selectivity, which exhibits
excellent catalytic activity and selectivity towards 2-LAB in the
alkylation between benzene with 1-dodecene. Mechanistic insights are
provided that the superior catalytic performance originates from the
raised accessible Brønsted acid sites, enhanced molecular diffusion and
solid interactions between adsorbate and 12-MR pockets on the exterior
surface. From our investigation, it's clear that in order to achieve
higher 1-dodecene conversion with MWW zeolite nanosheets system, a
modification of the MWW nanosheets to allow i) delamination of layered
MWW zeolites into the single-unit-cell nanosheet, ii) assembling the
single-unit-cell nanosheets into ordered multilamellar arrangement and
iii) precise control of Al species distribution in discrepant T sites
must be found. Therefore, our present work lays the foundation for
catalysts design in the future to fulfill this task, but our study
already produces regularly spaced MWW zeolite nanosheets with harmonic
Al distribution, representing an important finding.

\textbf{Associated content}

\textbf{Supporting Information}

Experimental contents for synthesis and characterization of MWW
zeolites; the study of catalytic and diffusion performance of MWW
zeolites; additional XRD, AFM, SEM, \textsuperscript{27}Al MAS NMR,
FTIR, TEM, N\textsubscript{2} isotherm data and Acidity analysis.

\textbf{Acknowledgment}

This work was supported by National Natural Science Foundation of China
(No. 21978055 and 21808040), and ``High-level Talents Program'' of Pearl
River (2017GC010080).

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\textbf{Figure Captions}

\textbf{Figure. 1} XRD patterns of various calcined MWW zeolite samples.

\textbf{Figure. 2} SEM images of (a) MCM-22 and (b)
MZN\textsubscript{8}-14.

\textbf{Figure. 3} (a-c) TEM images of MZN\textsubscript{8}-14 with
different magnitude, (d) AFM topographic image of
MZN\textsubscript{8}-14 on a mica disc (inset, the profile analysis of
the topographic height along the line scan of the MWW nanosheet shown in
(d)), (e-f) TEM images of MCM-22 with different magnitude.

\textbf{Figure. 4} (a) Conversion of 1-dodecene and (b) selectivity of
2-LAB with reaction time over various MWW zeolites.

\textbf{Figure. 5} The binding energy of LAB isomers with 12- membered
ring pockets on the exterior surface of MWW layer and the corresponding
adsorption configuration.

\textbf{Figure. 6} XRD patterns of calcined PMWW(31), PMWW(74) and
MCM-22 samples.

\textbf{Figure. 7} SEM images of (a) PMWW(31) and (b) PMWW(74).

\textbf{Figure. 8} TEM images of (a) PMWW(31) and (b) PMWW(74).

\textbf{Figure. 9} (a) Conversion of 1-dodecene and (b) selectivity of
2-LAB with reaction time over PMWW(31) and PMWW(74), respectively.

\textbf{Figure. 10} Al\textsuperscript{27} MAS NMR spectra of (a)
PMWW(31) and (b) PMWW(74) with the deconvolution and curve fitting.

\textbf{Figure. 11} (a) Conversion of 1-dodecene and (b) selectivity of
2-LAB over MCM-22 and PMWW(74) with different cycles.

\textbf{Figure. 12} XRD patterns of MCM-22 and PMWW(74) before and after
5 cycles.

\textbf{Scheme 1} The proposed arrangements of pillared MWW lamellar at
(a) high and (b) low Si/Al ratios.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

\textbf{Figure. 1} XRD patterns of various calcined MWW zeolite samples.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

\textbf{Figure. 2} SEM images of (a) MCM-22 and (b)
MZN\textsubscript{8}-14.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

\textbf{Figure. 3} (a-c) TEM images of MZN\textsubscript{8}-14 with
different magnitude, (d) AFM topographic image of
MZN\textsubscript{8}-14 on a mica disc (inset, the profile analysis of
the topographic height along the line scan of the MWW nanosheet shown in
(d)), (e-f) TEM images of MCM-22 with different magnitude.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image5/image5}

\end{center}
\end{figure}

\textbf{Figure. 4} (a) Conversion of 1-dodecene and (b) selectivity of
2-LAB with reaction time over various MWW zeolites.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

\end{center}
\end{figure}

\textbf{Figure. 5} The binding energy of LAB isomers with 12- membered
ring pockets on the exterior surface of MWW layer and the corresponding
adsorption configuration.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image7/image7}

\end{center}
\end{figure}

\textbf{Figure. 6} XRD patterns of calcined PMWW(31), PMWW(74) and
MCM-22 samples.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image8/image8}

\end{center}
\end{figure}

\textbf{Figure. 7} SEM images of (a) PMWW(31) and (b) PMWW(74).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image9/image9}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image10/image10}

\end{center}
\end{figure}

\textbf{Figure. 8} TEM images of (a) PMWW(31) and (b) PMWW(74).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image11/image11}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image12/image12}

\end{center}
\end{figure}

\textbf{Figure. 9} (a) Conversion of 1-dodecene and (b) selectivity of
2-LAB with reaction time over PMWW(31) and PMWW(74), respectively.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image13/image13}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image14/image14}

\end{center}
\end{figure}

\textbf{Figure. 10} Al\textsuperscript{27} MAS NMR spectra of (a)
PMWW(31) and (b) PMWW(74) with the deconvolution and curve fitting.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image15/image15}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image16/image16}

\end{center}
\end{figure}

\textbf{Figure. 11} (a) Conversion of 1-dodecene and (b) selectivity of
2-LAB over MCM-22 and PMWW(74) with different cycles.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image17/image17}

\end{center}
\end{figure}

\textbf{Figure. 12} XRD patterns of MCM-22 and PMWW(74) before and after
5 cycles.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image18/image18}

\end{center}
\end{figure}

\textbf{Scheme 1} The proposed arrangements of pillared MWW lamellar at
(a) high and (b) low Si/Al ratios.

\selectlanguage{english}
\FloatBarrier
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