High catalytic performance of surfactant-directed nanocrystalline zeolites for liquid-phase Friedel-Crafts alkylation of benzene due to external surfaces

Article abstract of DOI:10.1016/j.apcata.2013.11.019

Beta zeolite is known as an efficient catalyst for Friedel-Crafts alkylation. In liquid phase reactions, however, beta zeolite catalyst is often deactivated rapidly. We discovered that the maximum possible catalytic turnovers in benzene alkylation with benzyl alcohol could be increased by six times by using a beta zeolite with a nanosponge-like morphology, in comparison to bulk beta zeolites. The nanomorphic zeolite was obtained using a hydrothermal synthesis method which uses multiammonium surfactants as a meso-micro hierarchical structure-directing agent. The origins of the high catalytic performance were investigated by measuring the catalytic conversions after selectively poisoning acid sites located on external surfaces and in internal micropores selectively. The result indicated that the high catalytic performance was due to the alkylation reactions occurring on external surfaces. External active sites were able to perform the catalytic function even after active sites inside the zeolite micropores were deactivated. Similar results were obtained with other nanomorphic zeolites such as MFI nanosheets, MTW nanosponge and MRE nanosponge.

Full text of DOI:10.1016/j.apcata.2013.11.019

Applied Catalysis A: General 470 (2014) 420–426
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
High catalytic performance of surfactant-directed nanocrystalline
zeolites for liquid-phase Friedel–Crafts alkylation of benzene due to
external surfaces
Jeong-Chul Kima,b, Kanghee Cho , Ryong Ryoob,c,∗
b
a
Graduate School of Nanoscience and Technology, KAIST, Daejeon 305-701, Republic of Korea
Center for Nanomaterials and Chemical Reactions, Institute for Basic Science (IBS), Daejeon 305-701, Republic of Korea
Department of Chemistry, KAIST, Daejeon 305-701, Republic of Korea
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Beta zeolite is known as an efficient catalyst for Friedel–Crafts alkylation. In liquid phase reactions,
however, beta zeolite catalyst is often deactivated rapidly. We discovered that the maximum possible
catalytic turnovers in benzene alkylation with benzyl alcohol could be increased by six times by using a
beta zeolite with a nanosponge-like morphology, in comparison to bulk beta zeolites. The nanomorphic
zeolite was obtained using a hydrothermal synthesis method which uses multiammonium surfactants
as a meso–micro hierarchical structure-directing agent. The origins of the high catalytic performance
were investigated by measuring the catalytic conversions after selectively poisoning acid sites located on
external surfaces and in internal micropores selectively. The result indicated that the high catalytic per-
formance was due to the alkylation reactions occurring on external surfaces. External active sites were
able to perform the catalytic function even after active sites inside the zeolite micropores were deac-
tivated. Similar results were obtained with other nanomorphic zeolites such as MFI nanosheets, MTW
nanosponge and MRE nanosponge.
Received 27 September 2013
Received in revised form 8 November 2013
Accepted 13 November 2013
Available online 22 November 2013
Keywords:
Nanomorphic zeolite
Mesoporous zeolite
Zeolite external acidity
Friedel–Crafts alkylation
Catalytic lifetime
©
2013 Published by Elsevier B.V.
1
. Introduction
catalyst [11–15]. The advantage of this mode is high producibility
and long catalytic lifetime. However, for low volatile organic prod-
ucts, FC alkylation is difficult to operate in gas phase. Instead, the
alkylation can be operated in liquid phase using a batch reactor
[16,17]. In many previous works on liquid phase FC alkylation, the
conversion of reactants was often reported to reach almost 100%
using beta zeolite as a catalyst [18]. The benzylation of benzene with
benzyl alcohol is a typical example. This reaction is often reported
to go to almost 100% conversion with zeolites or mesoporous alumi-
nosilicate catalysts [19–24]. If a limited amount of catalyst is used,
however, high conversion is difficult to achieve in liquid phase. The
conversion does not increase beyond a certain value even if a very
long reaction time is given. To show this, we measured the con-
version of benzyl alcohol (limiting agent) with various amounts of
reactants per catalyst using a conventional aluminosilicates beta
zeolite. The result indicated that the maximum possible benzyl
alcohol conversion (%) decreased against the reactant-to-Al ratio.
We then converted the maximum conversion to the number of ben-
zyl alcohol molecules per Al atom contained in the zeolite, and
plotted the conversion per Al (CPA) as a function of the reactant
amount. This plot showed that the CPA stopped increasing after
reaching a maximum value about 40. That is, the zeolite catalyst
was completely deactivated after CPA reaching 40.
Friedel–Crafts (FC) alkylation is one of the most important
chemical processes to produce value-added aromatic compounds
through the formation of new C–C bonds between aromatic
molecules and alkylating agents such as olefins, alkyl halides and
alcohols [1–3]. Many FC alkylated aromatic products are used as
key intermediates for the production of petrochemicals, cosmet-
ics, dyes and pharmaceuticals [2–5]. FC alkylation can be catalyzed
by both Lewis and Brönsted acids. The catalysts can be used
as homogeneously dissolved components in reaction media (i.e.,
homogeneous catalysts), or as solid acids (heterogeneous cata-
lysts) [1–3,6–9]. Among the heterogeneous FC alkylation catalysts,
zeolites are widely used in industrial processes, due to many advan-
tages such as strong acidity, shape selectivity, high stability and
easy regenerability [10].
In petrochemical processes, FC alkylation for easily volatile
products such as cumene synthesis and toluene alkylation are oper-
ated in gas phase, continuous plug-flow mode using beta zeolite
∗ Corresponding author. Tel.: +82 42 350 2830; fax: +82 42 350 8130.
E-mail address: rryoo@kaist.ac.kr (R. Ryoo).
0
926-860X/$ – see front matter © 2013 Published by Elsevier B.V.
http://dx.doi.org/10.1016/j.apcata.2013.11.019

J.-C. Kim et al. / Applied Catalysis A: General 470 (2014) 420–426
421
The purpose of the present work was to investigate why
the liquid-phase FC alkylation zeolite catalyst was so rapidly
deactivated, and therefore, to seek a solution to the deactiva-
tion problem. We approached the problem, focusing particularly
on the effect of zeolite particle size. We noted that zeolites for
catalytic applications are normally composed of crystallites on a
micrometer scale (∼1 ␮m). Despite so small particle sizes, even
such micro-crystallites are still much larger than the diameter of
the internal micropores (<1 nm) that constitute the crystal struc-
ture. Such zeolites are called bulk zeolites, and their external crystal
surface area is very small as compared to internal surfaces cor-
responding to a large number of micropores. When a bulk beta
zeolite is used as a catalyst, the benzyl alcohol–benzene reac-
tion can occur mostly inside the micropores. Reactions on the
external surfaces can be disregarded, due to the relatively small
surface area. The phenomenon is similar in various reactions where
bulk zeolites are used as catalysts [25,26]. On the other hand, in
nanomorphic zeolites (i.e., zeolites with nanoscale morphologies),
the catalytic function of the external surfaces can be quite signifi-
cant, as recently reported by Kim et al. [27]. Kim et al. synthesized
MTW, MRE and beta zeolites with nanosponge-like morphologies
via a recently-developed synthesis route which used multiammo-
nium surfactants as meso–micro dual structure-directing agents
effective means of achieving high catalytic turnovers in the liquid-
phase benzylation reaction.
2. Experimental
2.1. Material preparation
Nanomorphic zeolite samples were synthesized with multi-
ammonium surfactant SDAs as described elsewhere [27,28]. Beta,
MTW and MRE zeolites were synthesized using the same SDA,
+
+
+
[
(
(
C22H45–N (CH ) –C H –N (CH ) –CH –(C H )–CH –N
3 2
+
6
12
3 2
2
6
4
2
CH ) –C H –N (CH ) –CH –(C H )–CH –N (CH ) –C H –N+
CH ) –C22H45](Br ) (Cl ) . Tetraethylorthosilicate (TEOS, 95%,
+
3
2
6
12
3 2
2
6
4
2
3 2
6
12
−
−
3
2
2
4
Junsei) was used as the silica source for the nanomor-
phic zeolite samples. Sodium aluminate (53 wt%, Sigma-Aldrich)
was the alumina source. The details of the synthesis con-
ditions were differently optimized for each zeolite struc-
ture. The optimized synthesis conditions are the same as
described by Kim et al. [27]. For MFI zeolite, the SDA was
+
+
−
[
C16H33–N (CH ) –C H –N (CH ) –C H (Br ) ]. Sodium sili-
3 2 6 12 3 2 6 13
2
cate solution (Si/Na = 1.75, 15 wt% SiO ) was used as a silica source
2
for the MFI zeolite. Sodium aluminate (53wt%, Sigma-Aldrich) was
used as an alumina source. The synthesis conditions were the
same as those reported by Kim et al. [28]. These nanomorphic
zeolite samples are denoted as nanozeolites. Their correspond-
ing bulk samples were also synthesized following the procedures
(
SDAs). These nanomorphic zeolites exhibited superior catalytic
performance to their bulk counterparts in a gas-phase cumene
synthesis study. It was particularly notable that the nanomor-
phic zeolites maintained high catalytic activities for a long time,
whereas bulk zeolites lost activity almost completely. Kim et al.
were able to separate the catalytic deactivation curve into two
exponential functions. Based on this analysis, they attributed the
high catalytic performance of the nanomorphic zeolites to ben-
zene alkylation reactions occurring mainly at catalytic sites that
were located at external surfaces. If this was correct, the catalytic
reactions in these zeolites should have occurred dominantly at the
external active sites rather than the internal acid sites. In addition,
the external active sites exhibited much longer catalytic lifetimes
than the internal sites. Nevertheless, such effects of external acid
sites to the catalytic performances were not yet generally con-
firmed in other zeolite catalysis, particularly in liquid-phase FC
alkylation.
For the purposes of the present research, four structure types
of zeolite (i.e., beta, MTW, MRE, and MFI) with nanosponge mor-
phologies were synthesized via the multi-ammonium surfactant
route [27,28]. These nanomorphic zeolites were characterized in
terms of their catalytic conversion rates and the maximum pos-
sible conversions in liquid-phase FC alkylation of benzene with
benzyl alcohol. Efforts were made to quantify the catalytic con-
versions taking place on the external surfaces in comparison to
reactions occurring inside micropores, using a nanomorphic beta
zeolite sample. One strategy to this end was to treat the zeolite
sample with triphenylphosphine before benzylation reaction mea-
surement. Triphenylphosphine was a strong base molecule so that
it could poison acid sites in a zeolite by strong chemisorption. The
poisoning was limited to the external surfaces as the molecule was
too large to enter a micropore aperture in a beta zeolite. Another
strategy was to expose the zeolite sample to a high-temperature gas
flow containing benzene and i-propene, prior to benzylation. This
treatment could deactivate the internal catalytic sites more rapidly
than the external sites due to the preferential deposition of coke in
micropores [29,30]. We analyzed the result of the catalytic reaction
measurements after such selective deactivation treatments. This
investigation indicated that the cause for the rapid catalytic deacti-
vation of bulk beta zeolites in liquid-phase FC alkylations was due
to internal pore blockage by the bulky side products. Compared
to the internal sites, the external sites were very slowly deacti-
vated. Thus, zeolite synthesis in nanomorphic form turned out as an
[
28,31–33]. An Al-MCM-41 sample was prepared via the postsyn-
thetic incorporation of Al, following a procedure reported in the
literature [23]. All zeolite and Al-MCM-41 samples were calcined
in air at 853 K after hydrothermal synthesis.
2.2. Characterization
Powder X-ray diffraction (XRD) patterns were measured with
a Rigaku Multiflex diffractometer equipped with Cu K␣ radiation
30 kV, 40 mA). The Ar adsorption isotherms were measured at the
(
liquid argon temperature with an ASAP 2020 volumetric adsorp-
tion analyzer [34]. Si/Al ratios were determined by inductively
coupled plasma-atomic emission spectroscopy (ICP-AES) using an
OPTIMA 4300 DV instrument (Perkin Elmer). Scanning electron
micrographs (SEM) images were taken with a Hitachi S-4800 micro-
scope operating at 2 kV without a metal coating. Transmission
electron micrographs (TEM) images were obtained using a Tec-
31
naiG2 F30 at an operating voltage of 300 kV. P NMR spectra were
acquired in a solid state with magic angle spinning (MAS) using a
Bruker AVANCE400WB spectrometer at room temperature, follow-
ing the method reported in the literature [35].
2.3. Catalytic measurements
Calcined zeolites in powder form were slurried in a 1-M NH NO
4
3
+
solution three times in total for the ion exchange into NH . The
4
+
zeolites were calcined again in air at 823 K to convert to a H -
ion-exchanged form. The liquid-phase FC alkylation reaction was
+
performed using a Pyrex batch reactor. Typically, 50 mg of H -form
zeolite was degassed at 573 K. This sample was added to a Pyrex
glass reactor that contained 190 mmol of benzene and (7.2, 14.4,
or 28.8) mmol of benzyl alcohol. The mixing was carried out in
a glove box to prevent moisture contamination. The reactor tem-
perature was quickly (<3 min) increased to 353 K under magnetic
stirring. Small aliquots (0.1 ml) of samples were taken at various
times afterward. At each sampling, the solid catalysts were fil-
tered out after immediate cooling to room temperature. The liquid
phase was analyzed on a gas chromatograph equipped with a flame

4
22
J.-C. Kim et al. / Applied Catalysis A: General 470 (2014) 420–426
Fig. 1. EM micrographs of nano beta (left column) and bulk beta (right column): SEM images with low-magnification (a and b) or high-magnification (c and d), and
high-magnification TEM images (e and f).
ionization detector and a HP–1 column (Agilent, 30 m long,
adsorption isotherms measured at the liquid argon temperature
(87 K). The XRD pattern of the bulk beta zeolite exhibited sev-
eral representative Bragg reflections corresponding to the typical
beta zeolite structure with polymorphs. The XRD peaks appearing
0.32 mm i.d., and 1 ␮m thick coating).
3
. Results and discussion
◦
around 2ꢀ = 7.5 were somewhat broad, which was due to the pres-
ence of more than two polymorphs. Except this peak, the XRD
pattern of the bulk zeolite exhibited quite sharp peaks. Further-
more, there was no detectable background increase in the 2ꢀ region
3
.1. Large external surface area of beta zeolite nanosponge
Fig. 1 shows representative SEM and TEM images taken from
◦
◦
between 15 and 25 that could indicate presence of any amor-
phous phases. Compared to the bulk zeolite, the nano beta sample
beta zeolite samples. The EM images of the nano zeolite indicate
that the sample was composed of very thin (approximately 20 nm)
beta zeolite crystals. The nanocrystals were randomly and loosely
interconnected to form a nanosponge-like mesoporous assembly.
On the other hand, the SEM images of the bulk zeolite indicate that
the zeolite was composed of particles of approximately 2 ␮m in
diameter. The external surface of the particles showed many crys-
tal steps and terraces, but overall, the surfaces did not indicate the
presence of mesopores. Fig. 2 shows the XRD patterns and argon
◦
◦
showed only two detectable peaks (2ꢀ = 7.5 and 22 ). These peaks
were very broad, but nevertheless, they were consistent with (1 0 1)
and (3 0 2) reflections of a beta zeolite. The broad peak widths
should be attributed to the small crystal domains in the nanocrys-
talline morphologies. Other reflections were difficult to identify
due to their low intensities in addition to the peak broadening
[
37].

J.-C. Kim et al. / Applied Catalysis A: General 470 (2014) 420–426
423
Fig. 2. Powder XRD patterns (a) and argon adsorption isotherms (b) of bulk and nano
beta zeolites. Inset of (b) is pore size distributions corresponding to the adsorption
branch of nano beta and bulk beta.
Fig. 3. Conversion per Al (a) and Diphenylmethane (desired product, in short, ‘DPM’)
selectivity (b) plotted as a function of reaction time: nano beta (᭹), bulk beta (ꢀ)
and Al-MCM-41 (ꢁ). Reaction condition: 7.2 mmol of benzyl alcohol, 190 mmol of
benzene, 50 mg of catalyst, 353 K.
Argon adsorption isotherm of bulk zeolite exhibited a very
were analyzed by solid-state 31P NMR spectroscopy using phos-
phine oxides as a probe molecule, following previous works
[38,39]. This technique was known as an effective method to
quantify the concentration of the acid sites at different locations
(e.g., at internal micropores or external surfaces) and to classify the
acid groups according to their strengths [27,35]. Trimethylphosphi-
neoxide, which is much smaller than the micropore size of zeolite
beta, was used as probe molecule to evaluate the total amount of
Brönsted acid over the entire region of the zeolite. Table 1 includes
the results of 31P NMR measurement, showing that that bulk and
nano beta had a similar amount of Brönsted acid, which is in good
agreement with elemental analysis and Al NMR measurement. On
the other hand, the number of external acid sites was evaluated
by P NMR spectroscopy, adsorbing tributylphosphine oxide into
the zeolites. Table 1 shows that nano beta contained 34% of the
total Brönsted acids at the external surface, whereas only 5% of the
total Brönsted acids were located at the external surfaces of bulk
beta. This result shows similar results with those of MFI zeolite
nanosheets synthesized via the same synthesis approach, as shown
in a previous report [35].
sharp increase in the low-pressure region of P/P < 0.1, which is
0
interpreted as due to the argon filling in the micropores. Such an
isotherm shape is a typical feature of microporous material. The
micropore diameters were determined from the isotherm in the
region of P/P < 0.1 using nonlocal density functional theory. A very
0
sharp distribution peak appeared at 0.70 nm. On the other hand,
the isotherm of nano zeolite showed another jump in the medium-
pressure region of 0.4 < P/P < 0.5, as well as a sharp increase in the
0
low-pressure region. The former is due to capillary condensation
in the mesopores. The latter is due to argon filling in quite uni-
form micropore with an average diameter of 0.70 nm as in bulk beta
zeolite. The Barrett–Joyner–Halenda algorithm was used to deter-
mine the size of the mesopores. The mesopore-size distribution
obtained in this manner showed a narrow peak center at 4.5 nm.
The specific surface area of the two zeolite samples was determined
by the Brunauer–Emmett–Teller (BET) method (see Table 1). BET-
specific areas of bulk and nano beta showed a significant difference:
31
2
−1 −1
2
4
60 m g for bulk and 930 m g for nano zeolites. Such a differ-
ence is due to a significant increase of the external surface area
that was exposed to mesopores in the nano beta zeolite. The fol-
lowing external surface areas were determined according to the
2
−1
2
−1
t-plot method: 50 m g for bulk and 700 m g for nano beta.
3.2. High catalytic turnover of zeolite beta nanosponge
The Si/Al ratios of nano and bulk beta zeolites were 16 and
1
5, respectively, according to elemental analysis using inductively-
The catalytic properties of bulk and nano beta zeolites were
investigated in the liquid-phase benzylation of benzene with ben-
zyl alcohol (BA) where Brönsted acid sites are used as active sites.
Al-MCM-41 with a Si/Al ratio of 15 was also tested in the ben-
zylation as a catalyst containing very weak Brönsted acidity for
comparison with the zeolite catalysts. Fig. 3a shows the catalytic
results of two zeolite catalysts and Al-MCM-41 by the means of CPA
plotted as a function of reaction time. Nano and bulk beta showed
coupled plasma atomic emission spectroscopy (Table 1). Solid
Al NMR measurement was performed for zeolite beta samples,
showing that almost all aluminum atoms in two zeolites were
incorporated in tetrahedral sites of two zeolites. The elemental
analysis and Al NMR investigation indicated that two zeolites con-
tained a similar number of Brönsted acid sites in a unit weight.
The strength and distribution of the Brönsted acid in two zeolites
Table 1
Physicochemical properties of the nano and bulk beta catalysts.
Si/Ala
SBETb (m2
g
−1
)
Sextc (m2
g
−1
)
Vtotd (cm3
g
−1
)
BAinie (␮mol g−1
)
BAextf (␮mol g−1
)
BAex/BAtotg (%)
Catalysts
Nano beta
Bulk beta
16
15
930
460
700
50
1.40
0.24
644
1090
326
55
34
4.8
a
Si/Al mole ratio obtained from ICP/AES analysis.
b
c
d
e
f
SBET is the BET surface area obtained from Ar adsorption in relative pressure range (P/P0) of 0.05–0.20.
Sext is the external surface area determined according to the t-plot method.
Vtot is the total pore volume obtained at P/P0 = 0.95.
BAint is the concentration of internal Brönsted acid sites, which was calculated by subtracting the concentration of external acid sites from that of total acid sites.
BAext is the concentration of external Brönsted acid sites.
g
BAex/BAtot is the molar ratio of the external acids to total acids.

4
24
J.-C. Kim et al. / Applied Catalysis A: General 470 (2014) 420–426
The bulk beta contains a negligible amount of the external acid
sites so that the catalytic performance of the bulk zeolite could be
explained by only the behavior of the internal acid site. In nano zeo-
lite, however, a portion of external acid sites is not negligible, so the
catalytic reaction occurring at the external surface should be sig-
nificantly concerned in the catalytic performance (catalytic activity
and lifetime) of the zeolite catalyst. The catalytic performance of the
external surfaces might be different from that of the internal micro-
pores. To understand the effect and contribution of each internal
or external active site in affecting the catalytic results, the catalytic
tests were performed under same conditions after selectively deac-
tivating one of two different active sites in the nano beta [27,36].
The external acid sites of nano beta could be selectively deacti-
vated by the chemisorption of triphenylphosphine (TPP) following
a procedure reported in the literature [36]. 31P NMR investigation
was performed to count the number of TPP at the external acid
sites of the treated sample, confirming that almost all external acid
sites adsorbed strongly the TPP molecule (i.e., poisoning). However,
the internal sites remained strongly acidic. After the TPP poisoning
treatment, the nano zeolite was immediately tested in the liquid-
phase benzylation reaction. During the reaction, the TPP adsorbed
at the external surface was not dissolved into the reaction solu-
tion so that the external acid sites remained deactivated. Fig. 4a
shows that the treated nano beta exhibited significant but some-
what smaller catalytic conversion than pristine nano beta, early in
the reaction time (<1 h). The slight decrease of the catalytic activity
is attributed to the deactivation of external acid sites that can act as
catalytic sites in this reaction. However, the treated nano beta was
very rapidly deactivated in comparison with pristine nano beta. The
treated nano beta was deactivated as fast as bulk beta zeolite was,
meaning that the internal acid sites of the nano beta exhibited very
short catalytic lifetime like that of the bulk beta.
Fig. 4. Conversion of benzyl alcohol over bulk beta (᭹), nano beta (ꢀ), nano beta
with external-acid poisoning by TPP (ꢁ) and nano beta with micropore-blocking
by coke (ꢂ), plotted as a function of reaction times: 0–1 h (a) and 0–120 h (b).
Reaction condition: 14.4 mmol of benzyl alcohol, 190 mmol of benzene, 10 mg of
triphenylphosphine, 50 mg of fresh and coked catalyst, 353 K.
significant CPA with similar product selectivity (approximately 70%
of the product mixture was diphenylmethane and the others were
mostly dibenzyl ether, which was generated by self-condensation
of BAs). In contrast to the zeolite catalysts, Al-MCM-41 exhibited
insignificant value of CPA, and the value did not increase for long
time of reaction. This result indicates that the liquid-phase benzy-
lation reaction requires strong Brönsted acid sites of the crystalline
zeolite framework, which is stronger than the acid sites in an amor-
phous framework [27,37].
As mentioned above, both zeolite betas are quite active catalysts
in the liquid-phase benzylation reaction, but they showed signifi-
cant difference in the maximum value of CPA (Fig. 3a). In the case
of bulk beta, CPA increased gradually with the reaction time and
converged to a maximum value of 35 (conversion of BA: 25%) in
The internal acid sites of nano beta could be preferably deacti-
vated through micropore-blocking by coke which was generated
during benzene isopropylation reaction [27]. Benzene isopropy-
lation was carried out in a fixed bed Pyrex reactor (10 mm
inside diameter) with gas-phase continuous plug-flow mode
−
1
−1
). The reaction was con-
2
h. This value did not further increase, although the reaction was
(WHSVbenzene = 8.8 gbenzene gcatalyst
tinued until the micropores were completely filled with carbon
species, which was confirmed by argon sorption analysis. 31
h
continued for long time. On the other hand, the CPA of nano beta
gradually increased up to 135. Considering the reaction condition,
this value of CPA corresponds to almost 100% BA conversion. 100%
conversion means that nano zeolite could be still active after con-
verting all BA molecules. If more reactant was added, nano beta
might exhibit much higher CPA value.
The catalytic investigation for the zeolite beta catalysts was also
carried out with an increase in the amount of the reactant. Fig. 4
shows the catalytic result of the catalytic test with high dosing of
the reactant (14.4 mmol). Fig. 4a shows plots of BA conversion early
in the reaction time, and Fig. 4b shows conversion plots of the over-
all range of reaction time. Until 1 h of the benzylation reaction, the
conversionrate of BAusing nano and bulk beta increasedlinearly. At
P
NMR measurements were performed after adsorbing phosphonium
probe molecules such as trimethylphosphine and tributylphos-
phine, which confirmed that treated nano beta lost almost all
internal acid sites. The treated sample was also tested in the liquid-
phase benzylation reaction without further treatment, and the
catalytic conversion was plotted as a function of reaction time in
Fig. 4. Fig. 4a shows that the treated nano beta exhibited somewhat
less catalytic activity than pristine nano beta, early in the reaction
time (<1 h). However, the catalytic conversion of treated nano beta
increased gradually for a long time (80 h) and achieved as high of
a maximum conversion as that of pristine nano beta. This result
indicated that the external acids can act as active sites in liquid-
phase catalytic benzylation and exhibit significantly long catalytic
lifetime. This aspect of external acid sites resembles the previous
results in a gas-phase cumene reaction [27]. The longer catalytic
lifetime of nano beta than bulk beta in a liquid-phase benzyla-
tion reaction, as well as gas-phase cumene synthesis [27], could
be attributed to the nano beta possessing a significant amount of
such an external acid sites which exhibit long catalytic lifetime.
1
h of the reaction, the two zeolites exhibited less than 15% conver-
sion. In this situation, the catalytic activities of the catalysts could
be directly compared by the conversion rates. The result indicates
that nano beta shows 1.3 times the activity of bulk beta in the ben-
zylation reaction. In the long period until 100 h (see Fig. 4b), the
BA conversion rate of nano beta increased gradually for a long time
(
∼50 h) and converged to 80%. On the other hand, bulk beta became
deactivated rapidly to exhibit a maximum BA conversion of approx-
imately 13%. Considering the initial amount of BA and the number
of acid sites in the zeolites, the maximum CPAs of nano and bulk
beta could be evaluated to 230 and 35, respectively. That is, CPA for
nano beta was almost six times as much as CPA for bulk beta.
The difference in CPA of the two zeolites could be attributed
to the different crystal size and, accordingly, the concentration
of the Brönsted acid sites at the external surface of the zeolite.
3.3. Catalytic deactivation by organic deposition
The conventional zeolite catalysts were known to deactivate
rapidly in the liquid-phase FC alkylation reactions due to the depo-
sition of organic species inside the micropores or at the external
surfaces [26,40]. The organic species thereby block the access of

J.-C. Kim et al. / Applied Catalysis A: General 470 (2014) 420–426
425
by carbon residue and/or water. Nevertheless, as shown above,
the nano zeolite exhibited much lower deactivation rate than the
bulk zeolite in liquid-phase FC benzylation reaction. The difference
might be due to different rate and location (e.g., in micropores or
at the external surfaces) of the carbon species generated during
the reaction. The generation of the organic residue in bulk and
nano zeolites was observed in detail using argon sorption analysis,
thermogravimetric analysis and elemental analysis. Experimental
details of the coke analysis was described in Appendix A. The bulk
and nano beta zeolites were sampled at various times of the benzy-
lation reaction, and the nature and amounts of the organic residues
generated in micropores and external surfaces were analyzed. As
shown in Fig. 5, bulk beta generated the organic species almost
in its micropores during the reaction. The organic species gener-
ated at the external surfaces was almost negligible. The content of
the carbon deposit increased with the reaction time to achieve the
−
1
maximum content of 0.1 g g of the bulk beta. At this time, the
catalyst was completely deactivated. This amount of the organic
species could fill or completely block all of the whole micropores,
which was confirmed by argon sorption analysis (see Fig. 5). In the
case of nano beta, the amount of organic deposit also increased
according to the reaction time, while the zeolite became increas-
ingly deactivated. The differing features from bulk beta are that
the organic component was more preferentially deposited in the
mesopores (i.e., at the external surfaces) than inside the micro-
pores. The amount of carbon deposits in the micropores of nano
beta was smaller than that of bulk beta. After 20 h of the benzyla-
tion reaction, the total carbon contents and even the amount of only
the external carbon species in the nano beta sample is larger than
the maximum contents in total carbon deposits in bulk beta. Never-
theless, the nano beta catalyst was still active in the reaction. Since
the mesopore size and volume is much larger than the micropores
Fig. 5. Content and location of organic deposits on nano beta (a) and bulk beta (b)
samples which were collected at various times of FC benzylation of benzene with
benzyl alcohol. Gray line with hollow circle is conversion per Al over two zeolite
beta catalysts plotted as a function of reaction time. Reaction condition: 14.4 mmol
of benzyl alcohol, 190 mmol of benzene, 50 mg of catalyst, 353 K.
reactants into the active sites. Water, which was generated from the
benzyl alcohol during the reaction, can also fill micropores quickly
to hinder the access of reactants to active sites inside micropores
[
24,41,42]. In contrast to the carbon deposits, the water molecules
can be reversibly removed from the active sites. The deactivation
of nano zeolite might also be due to the covering the active sites
Fig. 6. Conversion per Al over beta (a), MTW (b), MRE (c), and MFI (d) zeolites plotted as a function of reaction time. Reaction condition: 28.8 mmol of benzyl alcohol,
90 mmol of benzene, 50 mg of catalyst, 353 K.
1

4
26
J.-C. Kim et al. / Applied Catalysis A: General 470 (2014) 420–426
in the beta zeolite, the mesopore retained enough volume of empty
space to make the reactants approach the active sites even after
significant formation of carbon species. Argon sorption analysis
confirmed that mesopore size and volume somewhat decreased
but were still measurable after 20 h of the benzylation reaction.
The organic deposit in both nano beta and bulk beta was con-
firmed as various poly-alkylated aromatic compounds by LC–MS
analysis. The poly-alkylated aromatics are likely to be generated
via side reaction routes, such as the self-condensation of benzyl
alcohol or the benzylation of benzene, and their repetition. Bulk
beta contained relatively small-sized organic species consisting of
two or three aromatic groups, whereas nano beta contained larger-
sized organic species consisting of more than four aromatic groups,
as well as the smaller poly-aromatic components. The large-sized
organic molecules are somewhat larger than the micropore size.
These species are probably located in the mesopores of nano beta.
As compared to the small nanometer size of the mesopore, such a
bulky molecule remains quite small in order to provide sufficient
space for access of the reactant molecules.
Acknowledgments
This work was supported by the Institute for Basic Science in
Korea (IBS) [CA1301].
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.apcata.
2013.11.019.
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