Catalytic arene alkylation over H-Beta zeolite: Influence of zeolite shape selectivity and reactant nucleophilicity

Article abstract of DOI:10.1016/j.jcat.2019.09.035

Renewable arenes and aromatic alcohols can be derived from lignocellulose by biorefineries, which has been considered as a sustainable alternative to replace petrochemical feedstocks in the synthesis of monobenzylation products, key industrial intermediates, via benzylation reactions. Zeolites with micropores are the most widely used catalysts in the benzylation of arenes, however, their performance suffers from diffusion limitations in converting large arenes. In this work, mesoporous and microporous H–Beta zeolites were prepared and applied in the systematic study of benzylation of arenes (benzene, toluene, p-xylene and mesitylene) with benzyl alcohol (BA). The porous structure of these zeolites has been confirmed by XRD, BET and TEM techniques. The catalytically active Br?nsted acid sites (BAS) were determined by quantitative ¹H magic-angle spinning (MAS) nuclear magnetic resonance (NMR) experiments. The benzylation studies have shown that introducing mesopores into H–Beta zeolites can significantly increase the diffusion/access of arenes to surface sites, particularly for bulky arenes (e.g. mesitylene), while micropores are mainly selective for the conversion of small arenes (e.g. benzene). Increasing the nucleophilicity of arenes with more alkyl groups can enhance their catalytic performance in mesopores, however, the increase hinders their conversion in micropores because of the shape selectivity due to their increasing molecular size. Compared to mesopores, micropores promote the conversion of small arenes (e.g. benzene), which can be additionally enhanced by a high Br?nsted acidity. Therefore, introducing a suitable porosity balanced with acidity are keys in the tailoring of the catalytic performance of H-Beta zeolites for target benzylation reactions.

Full text of DOI:10.1016/j.jcat.2019.09.035

Journal of Catalysis 380 (2019) 9–20
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Catalytic arene alkylation over H-Beta zeolite: Influence of zeolite shape
selectivity and reactant nucleophilicity
a
a
b
c
d
Xin Zeng a, Zichun Wang a,b, , Jia Ding , Leizhi Wang , Yijiao Jiang , Catherine Stampfl , Michael Hunger ,
⇑
Jun Huang a,
⇑
a Laboratory for Catalysis Engineering, School of Chemical and Biomolecular Engineering & Sydney Nano Institute, The University of Sydney, NSW 2006, Australia
b Department of Engineering, Macquarie University, Sydney, New South Wales 2109, Australia
c School of Physics & Sydney Nano Institute, The University of Sydney, Sydney, New South Wales 2006, Australia
d Institute of Chemical Technology, University of Stuttgart, 70550 Stuttgart, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Renewable arenes and aromatic alcohols can be derived from lignocellulose by biorefineries, which has
been considered as a sustainable alternative to replace petrochemical feedstocks in the synthesis of
monobenzylation products, key industrial intermediates, via benzylation reactions. Zeolites with micro-
pores are the most widely used catalysts in the benzylation of arenes, however, their performance suffers
from diffusion limitations in converting large arenes. In this work, mesoporous and microporous H–Beta
zeolites were prepared and applied in the systematic study of benzylation of arenes (benzene, toluene,
p-xylene and mesitylene) with benzyl alcohol (BA). The porous structure of these zeolites has been
confirmed by XRD, BET and TEM techniques. The catalytically active Brønsted acid sites (BAS) were deter-
mined by quantitative 1H magic-angle spinning (MAS) nuclear magnetic resonance (NMR) experiments.
The benzylation studies have shown that introducing mesopores into H–Beta zeolites can significantly
increase the diffusion/access of arenes to surface sites, particularly for bulky arenes (e.g. mesitylene),
while micropores are mainly selective for the conversion of small arenes (e.g. benzene). Increasing the
nucleophilicity of arenes with more alkyl groups can enhance their catalytic performance in mesopores,
however, the increase hinders their conversion in micropores because of the shape selectivity due to their
increasing molecular size. Compared to mesopores, micropores promote the conversion of small arenes
(e.g. benzene), which can be additionally enhanced by a high Brønsted acidity. Therefore, introducing a
suitable porosity balanced with acidity are keys in the tailoring of the catalytic performance of H-Beta
zeolites for target benzylation reactions.
Received 6 May 2019
Revised 21 September 2019
Accepted 23 September 2019
Keywords:
Beta zeolite
Benzylation reaction
Shape selectivity
Reactant nucleophilicity
Solid-state NMR spectroscopy
Ó 2019 Elsevier Inc. All rights reserved.
1. Introduction
aromatics. Recently, various arenes and aromatic alcohols have
been derived from lignocellulose biomass [3–5]. The conversion
Non-edible lignocellulose biomass is regarded as a renewable
and environmentally friendly alternative to fossil fuels in the pro-
duction of energy and valuable chemicals [1,2]. Lignocellulose
consists of cellulose, hemicellulose, and lignin. Cellulose and hemi-
cellulose are polymers of glucose and monosaccharides, respec-
tively, and have been widely utilized in biorefineries around the
world. Lignin is composed of disordered polymerized phenyl-
propane monomers via hydroxyl or methoxy substituents, which
of these bio-derived aromatic compounds has high potential for
the sustainable production of aromatic-containing chemicals, and
can fit into current chemical processes.
Benzylated arenes are key industrial intermediates and building
blocks in the production of petrochemicals, pharmaceuticals, fine
chemicals, and polymers [6]. These compounds can be derived
from the benzylation of arenes with benzyl alcohol (BA) or benzyl
chloride, which are typical acid-catalysed Friedel–Crafts alkylation
reactions [7,8]. Lewis acidic halides (e.g. AlCl3, FeCl3, InCl3, BF3) and
protic acids (e.g. H2SO4, HCl, HNO3) are most frequently used in the
synthesis of benzylated arenes [8], however, they cause sever envi-
ronmental issues and problems with catalyst separation and
regeneration [9]. Environmentally friendly heterogeneous catalysts
have been developed to replace homogeneous acids, including
is interesting as
a renewable raw feedstock for bio-derived
⇑
Corresponding authors at: Laboratory for Catalysis Engineering, School of
Chemical and Biomolecular Engineering & Sydney Nano Institute, The University of
Sydney, NSW 2006, Australia (J. Huang, Z. Wang).
E-mail addresses: zichun.wang@mq.edu.au (Z. Wang), jun.huang@sydney.edu.au
(J. Huang).
https://doi.org/10.1016/j.jcat.2019.09.035
0021-9517/Ó 2019 Elsevier Inc. All rights reserved.

10
X. Zeng et al. / Journal of Catalysis 380 (2019) 9–20
zeolites and mesoporous silica-alumina [7,10–22], acidic oxides
and sulphides [23–30], and heteropoly acids (HPAs) [15,31].
Zeolites, having crystalline framework, high surface area, regu-
lar porous structure, and strong acidity, are the most popular cat-
alysts used in current chemical processes [10]. Brønsted acid sites
(BAS) on zeolites, which are protons compensating the negatively
charged, tetrahedrally coordinated Al atoms substituting Si atoms
in the zeolite framework, are active sites for benzylation reactions
[7,11–13]. The suitable micropores (<2 nm) and strong acidity of
zeolites can remarkably promote the formation of diphenyl-
methane with a high selectivity in comparison to other catalysts
[7,11,14,15]. However, microporous zeolites often suffer from dif-
fusion limitations, poor selectivity, and a strong shortening of the
catalyst lifetime for bulky substrates [16].
Introducing mesopores (2–50 nm in diameter) or hierarchical
pore architectures offer promising solutions to facilitate molecule
transportation and improve the catalytic performance of zeolites
in the benzylation [16–20]. The application of hierarchical archi-
tectures of H-ZSM-5 zeolites in the benzylation of mesitylene with
benzyl chloride has been reported to increase the reaction rate k
for over 65 times compared to microporous H-ZSM-5 zeolites
[19]. Improving the catalytic performance of zeolites by increasing
the mesoporosity is also observed on mesoporous mordenite in
alkylation reactions [32,33]. In the catalytic benzylation of naph-
thalene with benzyl chloride, mesoporous H-Beta (Hb) zeolites
afford a benzyl chloride conversion of 90–100%, much higher than
the conversion of ca. 25% obtained with microporous Hb zeolites
[20].
(50–56% Al2O3, 37–45% Na2O, Sigma-Aldrich), sodium hydroxide
(NaOH, reagent grade, ꢀ98%, pellets (anhydrous), Sigma-Aldrich),
aluminum sulfate (98% Al2(SO4)3∙18 H2O, Sigma-Aldrich), fumed
silica (Sigma-Aldrich, 395 m2/g).
As described earlier [34], microporous Na-Beta zeolites (Nab)
were prepared by dissolving sodium aluminate in an aqueous
solution of TEAOH. Then, NaOH and silica sol were added to the
mixture under vigorous stirring for half hour. Afterwards, the gels
with molar composition of Na2O: (TEA)2O:H2O:SiO2:Al2O3:
= 5.1:12.5:1300:62:x, where x is adjusted to the desired Si/Al ratio,
were transferred to a Teflon lined stainless autoclave. The mixture
was kept at 413 K for 4 days for crystal growth under static condi-
tions. The solid products were collected by centrifugation, filtra-
tion, washing and drying at 353 K for 12 h. Finally, the solid
products were transferred to quartz crucibles and calcined in air
at 823 K for 2 h.
Mesoporous Nab zeolites were prepared as described earlier
[36]. In brief, a mixture of NaOH (0.066 g), Al2(SO4)3∙18 H2O (x g),
and aqueous TEAOH (6 g) was stirred until clear, where x is
adjusted to the desired Si/Al ratio. Then, fumed silica (2.0 g) was
added, and the mixture was further stirred for another 1 h. The
resulting gel was dried by heating at 333 K. The dry precursor
lumps were ground and transferred into a 4 mL Teflon cup, which
was placed into a 20 mL Teflon liner. Then, 0.3 mL of water was
added into the liner without contacting the dry gel. The Teflon
set was transferred into a 20 mL autoclave, which was subse-
quently heated at 453 K for 3 days. The resulting gel was recovered
by washing, filtrating, and drying, followed by calcination in air at
823 K for 6 h.
In general, benzylation of arenes is proceeded by replacing a
hydrogen atom of the aromatic ring with a benzyl group from a
benzylating agent in the presence of a catalyst [8]. Zeolites having
strong Brønsted acidity can facilitate the formation of benzyl
cations (C6H5CH+2), which then attack the aromatic ring for an elec-
trophilic substitution [34]. Alkyl groups as nucleophilic reagents
can contribute via electron donation effects when bounded to aro-
matic rings. Therefore, more substituted methyl groups on the aro-
matic ring is often considered to promote benzylation reactions
with a higher catalytic performance [6]. However, the shape selec-
tivity of zeolites limits the diffusion and access of aromatics with
more alkyl groups, i.e. reducing their catalytic performance [11,21].
In this work, we provide a systematic study on the influence of
acidity and pore size of microporous zeolites and the reactant
nucleophilicity effects on the catalytic benzylation over Hb zeo-
lites, since Hb zeolites are highly active and selective to monoben-
zylation products compared to other zeolites (e.g. H-Y, H-ZSM-5
and mordenite) [7,11–13,21,22]. Hb zeolites with different Si/Al
ratios and micro- and mesopores have been successfully synthe-
sized for the benzylation of arenes (benzene, toluene, p-xylene
and mesitylene) with benzyl alcohol. Benzyl alcohol is a kind of
environmentally friendly alkylating agent to replace benzyl chlo-
ride for green chemical processes, producing water rather than
stoichiometric hydrochloride. Compared to benzyl chloride, the
application of benzyl alcohol often suffers from fast self-
condensation to generate dibenzyl ether (DBE), but is limited by
the further conversion of DBE with arenes [35], leading to a low
selectivity to monobenzylation products.
Ammonia exchange of the obtained Nab zeolites for obtaining
Hb zeolites was employed as described in our earlier work [37].
Briefly, the Nab zeolites were mixed with a certain amount of
0.1 M NH4NO3 aqueous solution and stirred at 353 K for 3 h. After
filtration and washing with deionized water till no nitrate ions
could be detected, the obtained products were dried at room tem-
perature overnight. The above procedure was repeated for four
times to ensure an ion-exchange degree of >99% in the final Hb
zeolites. The nomenclature of Hb zeolites is defined as Micro-Hb-
x and Meso-Hb-x, for microporos and mesoporous Hb zeolites,
respectively, where x is assigned to 1 and 2 for samples with differ-
ent Si/Al ratios listed in Table 1.
2.2. Catalyst characterization
Nitrogen adsorption-desorption measurements were performed
at 77 K by an Autosorb IQ-C system to determine the specific sur-
face areas. Before measurements, the zeolite samples were
degassed at 423 K under vacuum to remove adsorbates from the
surface. The specific surface areas were determined by the
Brunauer–Emmett–Teller (BET) method and are summarized in
Table 1.
X-ray diffraction characterization (XRD) studies was performed
on a Rigaku D/Max-RB diffractometer instrument with Cu-Ka (tube
voltage 40 kV, tube current 100 mA, k = 0.154 nm) radiation. The
scanning angle is from 5° to 50° with a scanning rate of 2°/min
were used.
Transmission electron microscopy (TEM) images were obtained
by using a Philips CM120 BioFilter with samples that were
mounted on a carbon-coated copper grid by drying a droplet of a
suspension of the ground sample in ethanol.
2. Experimental Section
2.1. Catalyst preparation
The local structure and acidity of Beta zeolites were character-
ized by solid-state nuclear magnetic resonance (NMR) spec-
troscopy. For the 27Al and 29Si magic-angle (MAS) NMR
investigations, all samples were fully hydrated at ambient temper-
ature overnight in a desiccator containing a saturated Ca(NO3)2
solution. Before the 1H and 13C MAS NMR experiments, the
All chemicals were purchased in analytical standard without
further purification, including silica sol (30 wt% SiO2 suspension
in H2O, Sigma-Aldrich), tetraethyl ammonium hydroxide aqueous
solution (35 wt% TEAOH in water, Sigma-Aldric), sodium aluminate

X. Zeng et al. / Journal of Catalysis 380 (2019) 9–20
11
Table 1
The Si/Al ratios, surface areas, pore volumes, and average pore sizes of the Hb zeolites under study.a
Samples
Micro-Hb-1
Micro-Hb-2
Meso-Hb-1
Meso-Hb-2
Si/Al ratio
12.5
390
0.34
0.26
0.08
1.1
20
15
22
Specific surface areas (m2/g)
Total pore volume (cm3/g)
Micropore volume (cm3/g)
Mesopore volume (cm3/g)
Pore size (NLDFT, nm)
404
0.33
0.27
0.06
1.2
564
1.02
0.20
0.82
13.5
597
1.11
0.19
0.92
15.7
a
Determined by Ar adsorption-desorption isotherms at 87 K. Surface areas and pore size were obtained by BET and NLDFT methods, respectively.
samples filled in glass tubes were dehydrated at 723 K for 12 h and
at a pressure of less than 10ꢁ2 bar. These dehydrated samples were
sealed in the glass tubes or directly loaded with acetonitrile-d3 or
acetone-2-13C (99.5% 13C-enriched, Sigma-Aldrich) using a vacuum
line. Subsequently, the adsorbate-loaded samples were evacuated
at room temperature for 10 min (for acetonitrile-d3) or 2 h (for ace-
tone) to remove weakly physisorbed molecules. Then, the samples
were transferred into the MAS NMR rotors under dry nitrogen gas
inside a glove box.
time is regard as the initial reaction time. The samples of the reac-
tion mixture (30 L) were collected periodically, and subsequently
filtrated and dissolved in ethanol (1 mL) for gas chromatography
(GC) analysis. The reaction products were analysed by a Shimadzu
GCMS-QP2010 Ultra instrument equipped with a Rtx-5 capillary
l
column (30 m ꢂ 0.32 mm ꢂ 3
lm) connected with a GC-FID detec-
tor. The benzyl alcohol conversion and product selectivity were
calculated by
Ap1 þ Ap2
1H, 27Al, and 13C MAS NMR investigations were carried out on
Bruker Avance III 400 WB spectrometer at resonance frequencies
of 400.1, 104.3, and 100.6 MHz, respectively, with a sample spin-
ning rate of 8 kHz using 4 mm MAS rotors. Spectra were recorded
Conversion ½%ꢃ ¼
Selectivity ½%ꢃ ¼
ꢂ 100;
Ap1 þ Ap2 þ Ar
A
p1orAp2
ꢂ 100
Ap1 þ Ap2
after single-pulse
p/2 and p/6 excitation with repetition times of
20 s and 0.5 s for studying 1H and 27Al nuclei, respectively. Quanti-
tative 1H MAS NMR measurements were performed using a dehy-
drated H,Na-Y zeolite (35% ion-exchanged) as an external intensity
standard, which contains 58.5 mg zeolite H,Na-Y with 1.776 mmol
OH/g. 13C cross-polarization (CP) MAS NMR spectra were recorded
with the contact time of 4 ms and the repetition time of 4 s. 29Si
MAS NMR experiments were performed on the same spectrometer
at the resonance frequency of 79.5 MHz and with the sample spin-
ning rate of 4 kHz using 7 mm MAS rotors. For 29Si MAS NMR spec-
where Ap1 and Ap2 are the peak areas of the benzylation products
obtained by GC analysis, and Ar is the peak area of benzyl alcohol.
3. Results and discussion
3.1. Physical characterization of Hb Zeolites
The crystal structure of the obtained Hb zeolites samples were
confirmed by powder X-ray diffraction (XRD). No obvious diffrac-
tion signal of crystalline impurities could be detected. Three
diffraction peaks at 2 Theta = 8.7°, 23.4° with shoulder peaks at
22.4°, and 44.5° corresponding to (1 1 0), (3 0 2), and (2 2 0) planes,
respectively, were clearly observed in the obtained XRD patterns
(Fig. 1), which is typical for Hb zeolites with a BEA topological
structure type BEA [38].
The N2 adsorption-desorption isotherms of the Hb zeolites are
shown in Fig. 2. Micro-Hb zeolites showed a type I isotherm typical
for microporous materials according to IUPAC classification, while
the Meso-Hb zeolites exhibited a type VI isotherm, typically for
mesoporous solids [39]. The specific surface areas and the pore
tra, single-pulse p/2 excitation and high-power proton decoupling,
and a recycle delay of 20 s were applied. To separate the different
signals and for the quantitative evaluation of the spectra, the data
were processed using the Dmfit software.
The acidity of the zeolite catalysts was also characterized by
NH3-TPD. The NH3-TPD measurements were performed on a Chem
BET TPR/TPD Chemisorption Analyzer, CBT-1, QuantaChrome
instruments. Prior to the NH3-TPD measurements, the catalyst
(100 mg) was loaded and degassed at 823 K for 1 h under N2 flow
(50 mL/min) to completely remove molecules adsorbed on the
samples. Then the sample was cooled to 323 K and saturated in
the gas mixture of NH3 gas (10 vol% in N2, 25 mL/min) for
30 min, and flushed with N2 flow (50 mL/min) for 1 h at 373 K.
Ammonia desorption was carried out with increasing temperature
from 373 to 873 K at a constant heating rate of 5 K /min, and the
concentration of desorbed ammonia was obtained by a thermal
conductivity detector connected to the TPD system.
Micro-Hβ-1
Micro-Hβ-2
Meso-Hβ-1
2.3. Benzylation of arenes with benzyl alcohol
Prior to reaction, the catalyst was activated in a quartz tube
under a N2 flow of 30 mL/min at 723 K overnight, followed by cool-
ing down to room temperature under flowing N2 gas. A three-neck
round-bottom flask (10 mL) equipped with a reflux condenser,
nitrogen gas tube, and needle for the reactant injection and sample
collecting were employed for the catalytic studies. The flask was
purged with dry nitrogen, and subsequently, 3.5 mL of arenes (ben-
zene, toluene, p-xylene and mesitylene) and 0.25 mL of benzyl
alcohol were added and mixed in the flask. After keeping the mix-
ture at the desired reaction temperature for 10 min under stirring,
the activated catalyst (25 mg) was added into the flask, and this
Meso-Hβ-2
10
20
30
40
50
2 Theta / degree
Fig. 1. XRD patterns of the Hb zeolites under study.

12
X. Zeng et al. / Journal of Catalysis 380 (2019) 9–20
Fig. 2. Nitrogen adsorption/desorption isotherms of Hb zeolites (a) and TEM images of Meso-Hb-2 (b-d).
sizes are summarised in Table 1. Both the surface areas and pore
sizes of the Micro-Hb zeolites are similar to those reported earlier
[36]. The surface areas of the Meso-Hb zeolites are much larger
than those of the Micro-Hb zeolites, which is indicated by the
strong increase of isotherms at higher relative pressures (P/
P0 > 0.8). The average pore diameters of the Meso-Hb zeolites are
in the range of 13.5–15.7 nm, i.e. 13–15 times higher than the aver-
age pore size (1.1–1.2 nm) of the Micro-Hb zeolites. TEM images
revealed that the Meso-Hb zeolites were formed from the assembly
of nanoparticles of 20–70 nm (Fig. 2b). These nanoparticles are
nanosized crystals, as evidenced by lattice fringes as shown in
Figs. 2c and d. The effective intergrowth of the nanosized crystals
from the bulk gave rise to the substantial mesoporosity and high
surface area. The obtained surface areas, pore diameters, and total
pore volumes of the Meso-Hb zeolites are in good agreement with
a previous work [36], which indicates that both the Micro-Hb and
Meso-Hb zeolites were successfully synthesized.
Si(OSi)3OH species. The signal of which is often overlapped with
the signal of Si(1Al) species at ꢁ104 ppm [41]. Since the formation
of catalytically active BAS is accompanied by tetrahedrally coordi-
nated Al atoms incorporated into the silica framework, the nature
of Al species was further studied by 27Al MAS NMR spectroscopy.
The 27Al MAS NMR spectra of all Hb zeolites are shown in Fig. 4.
Nearly no octahedrally coordinated Al (AlVI) atoms at 0 ppm,
related to the extra-framework Al species (EFAl) were detected.
EFAl are generally considered as the surface Lewis acid site (LAS)
[42]. This indicates nearly no LAS exist in the Hb zeolites under
study. The dominant signal at 54 ppm was assigned to tetrahe-
drally coordinated Al (AlIV) species. In zeolites, AlIV species are
introduced by the substitution of Si atoms by Al atoms in the zeo-
lite framework [37,43], resulting in a negative charge. Hence, the
27Al MAS NMR spectra shows the majority of Al atoms were incor-
porated into the silica framework of Hb zeolites.
After ammonia exchange and calcination, the negatively
charged aluminium atom can be balanced by protons, forming
bridging OH groups (SiOHAl), acting as BAS in acid-catalysed reac-
tions, such as in benzylation reactions [35,44,45]. Unlike the
Brønsted acidic OH groups in amorphous silica-alumina causing
1H MAS NMR signals, which overlap with the signals of terminal
SiOH groups at 1.2–2.2 ppm [42,46,47], bridging groups in zeolites
are characterized by low-field signals at chemical shifts of 3.6–
5.2 ppm [42]. Therefore, the signals of BAS in zeolites can be clearly
distinguished from those of non-acidic SiOH groups, and the BAS
densities can be quantified via simulating the 1H MAS NMR spectra
of the dehydrated Hb zeolites (Fig. 5). As shown in the simulation
results of 1H MAS NMR spectra, the signals at 1.8 ppm, and 0 and
2.4 ppm, used for the simulation, are assigned to terminal SiOH
groups and AlOH groups, respectively [48]. The strong low-field
signal at ca. 4.1 ppm, with a broad hump at 5.2 ppm are widely
accepted as signals of bridging SiOHAl groups and disturbed bridg-
3.2. Local structure and acid sites of Hb zeolites studied by 27Al, 29Si,
1H, and 13C NMR spectroscopy
Solid-state NMR spectroscopy was applied for the investigation
of the local structure and acidity of Hb zeolites, since these proper-
ties strongly influence their catalytic performance. The 29Si MAS
NMR spectra of Hb zeolites are dominated by two strong signals
at ꢁ104 ppm and ꢁ111 ppm, as shown in Fig. 3. The simulation
of the 29Si MAS NMR spectra required three signals at ꢁ104,
ꢁ111 and ꢁ115 ppm, which were assigned to Q3 + Si(1Al), Si
(0Al), Si(0Al). The Si(1Al) and Si(0Al) species are ascribed to 0 or
1 Al atom located in the first coordination sphere of T-positions,
for example, Si(OSi)3OAl. The split of the Si(0Al) signal is caused
by two groups of different crystallographic sites [40]. Q3 species
are assigned to terminal hydroxyl groups bonded to Si atoms as

X. Zeng et al. / Journal of Catalysis 380 (2019) 9–20
13
-111
a) Micro-Hβ-1
b) Micro-Hβ-2
Q3+Si(1Al)
Experiment
Si(0Al)
Experiment
Simulation
Simulation
-104
-115
Components
Components
-70
-90
-110
-130
-70
-90
-110
-130
δ29Si / ppm
δ29Si / ppm
c) Meso-Hβ-1
Experiment
d) Meso-Hβ-2
Experiment
Simulation
Components
Simulation
Components
-70
-90
-110
-130
-90
-130
-70
-110
δ29Si / ppm
δ29Si / ppm
Fig. 3. 29Si MAS NMR spectra of the Hb zeolites under study.
with the acid strength of surface OH groups. A larger
a higher strength of surface protons [42,43]. Therefore, the acid
strength of surface protons can be evaluated by the adsorbate-
D
d1H means
54
a) Micro-Hβ-1
0
induced low-field shift
[46]. As shown in Fig. 6, the signal of terminal SiOH groups
(d1H = 1.8 ppm) was shifted to low field ( d1H = 3.2–3.3 ppm),
which is typical for weak-acidic silanol protons. A much stronger
low-field shift ( d1H = 7.2–8 ppm) was observed for the bridging
Dd1H, e.g. occurring upon CD3CN adsorption
D
b) Micro-Hβ-2
c) Meso-Hβ-1
D
hydroxyl protons having a much stronger acid strength than sila-
nol protons. Notably, two kinds of bridging OH groups were probed
in Fig. 5. They seem to provide a similar acid strength as reported
in literature [50,51]. A similar mobility (or line shape of 1H MAS
NMR) was observed with all Hb zeolites upon adsorption of CD3CN.
d) Meso-Hβ-2
Therefore, the small increase of
to 8 ppm indicates a slight increase of the acid strength with
increasing the Si/Al ratio.
Dd1H of the signal centers from 7.2
100
50
0
-50
δ27Al / ppm
Fig. 4. 27Al MAS NMR spectra of the Hb zeolites under study.
The acidity of Hb zeolites was also studied using NH3-TPD
experiments. The NH3-TPD profile is shown in Fig. 7, and the num-
ber of BAS was summarized in Table 2. In general, a higher desorp-
tion temperature in NH3-TPD indicates a higher binding energies
between the adsorbed ammonia molecules and the surface acid
sites depends on the higher strength of acid sites [52]. Two signals
at high and low temperature can be distinguished in Fig. 7, repre-
senting for two types of BAS with strong and weak strength,
respectively, in Hb zeolites. The total number of BAS decreased in
the order of Micro-Hb-1 > Micro-Hb-2 > Meso-Hb-1 > Meso-Hb-2,
well in line with the results obtained from 1H MAS NMR experi-
ments. Moreover, the number of both strong and weak BAS are
decreased in the same order, and thus, Micro-Hb zeolites possess
higher Brønsted acidity compared Meso-Hb zeolites.
ing SiOHAl groups, respectively [41,48]. Their signal intensities
(d1H = 4.1–5.2 ppm) were utilized to calculate the densities of
BAS as summarised in Table 2. In both Micro-Hb and Meso-Hb zeo-
lites, the BAS density decreases with increasing the Si/Al ratio. At a
similar Si/Al ratio, Micro-Hb zeolites yielded a higher BAS density
than the Meso-Hb zeolites counterparts, which indicates that some
of the tetrahedrally coordinated Al atoms in Meso-Hb zeolites
incorporated into defect structures similar to those of amorphous
silica-alumina.
Because of the higher electronegativity of silicon atoms in
comparison with aluminium atoms, a higher Si/Al ratio affords a
higher electronegativity of the framework. This higher framework
electronegativity is accompanied by an increased charge with-
drawal from the hydroxyl protons of the bridging OH groups, and
therefore, an increase of their acid strength [49]. Upon adsorption
of a weak base, e.g. acetonitrile-d3 (CD3CN), hydrogen bonds
(O–Hꢄ ꢄ ꢄNCD3C) are generated between surface OH groups and
probe molecules. These hydrogen bonds can induce characteristic
3.3. Benzylation of arenes with benzyl alcohol
The obtained Hb zeolites were utilised in the benzylation of
arenes, such as benzene, toluene, p-xylene and mesitylene with
benzyl alcohol at 353 K for 4 h. The role of the acidity and shape
selectivity of the Hb zeolites, and the reactant nucleophilicity
and molecular size of arenes on the benzylation of BA were
low-field shifts (D
d1H) of 1H MAS NMR signals, which correlate

14
X. Zeng et al. / Journal of Catalysis 380 (2019) 9–20
Fig. 5. 1H MAS NMR spectra of the dehydrated Hb zeolites and their simulations (bottom).
Table 2
Brønsted acid sites (BAS) of the Hb zeolites under study.
Samples
Micro-Hb-1
Micro-Hb-2
Meso-Hb-1
Meso-Hb-2
BAS density (mmol/g)a
0.52
7.2
0.66
0.58
0.17
0.41
0.45
7.5
0.50
0.42
0.15
0.27
0.35
7.6
0.46
0.21
0.12
0.09
0.27
8
0.27
0.14
0.08
0.06
BAS strength (D
d1H/ppm)b
Number of BAS (per nm2)c
BAS density (mmol/g)d
Strong BAS (mmol/g)d
Weak BAS (mmol/g)d
a
b
c
Determined by the corresponding 1H MAS NMR spectra.
Determined by 1H MAS NMR spectra of dehydrated Hb zeolites loaded with CD3CN as shown in Fig. 6.
Calculated by (BAS density ꢂ Avogadro constant)/(specific surface area).
d
Determined by NH3-TPD experiments, and high-temperature peak at 673–723 K and low-temperature peak at 473–573 K were utilized to quantify the strong and weak
BAS, respectively.
investigated. In all reactions, only monobenzylation products and
DBE could be detected by GC analysis. The conversion of benzyl
alcohol at 353 K as a function of time is shown in Fig. 8, and the
reaction results are summarized in Table 3.
toluene increased when using Micro-Hb-2, but less effect when
using p-xylene and mesitylene occurred in comparison with
Micro-Hb-1 as shown in Table 3. These observations have been
attributed to the higher acid strength of Micro-Hb-2, which can
promote the formation of benzyl cations and the activation of less
active arenes (benzene and toluene) for benzylation reaction. Sim-
ilar trends were also observed with Meso-Hb zeolites. However,
Micro-Hb zeolites cause higher conversions when using small are-
nes (benzene, toluene) than Meso-Hb zeolites, but lower conver-
sions when using arenes having large size (p-xylene, mesitylene).
This is due to the limitation of the mass transfer of large arenes
inside the micropores and will be discussed later.
3.3.1. Effect of the acidity of the Hb zeolites
In the benzylation of BA with benzene or substituted benzene,
the direct benzylation of aromatics with benzyl alcohol to yield
monobenzylation products (1) competes with a two-step reaction
mechanism that the condensation between benzyl alcohols to gen-
erate DBE (2), which can be further converted into the monobenzy-
lation product with aromatic addition (3) (see Scheme 1). The
direct benzylation of aromatics of pathway (1) is initiated by polar-
izing BA to generate benzyl cations as an electrophile, followed by
an electrophilic addition to the aromatic ring [11]. A higher acid
strength facilitates the formation of benzyl cations as electrophile,
which promotes the reaction [34]. With similar textual properties,
Micro-Hb-2, having a higher acid strength of the BAS, causes a
higher BA conversions with all arenes compared with Micro-Hb-1
having a higher BAS density, as shown in Fig. 8. Moreover, the
selectivities to the monobenzylation products of benzene and
3.3.2. Effect of nucleophilicity/proton affinity (PA)
Alkyl groups as nucleophilic reagents are electron donating sub-
stituents when bonded to aromatic rings. As shown in Table 4, the
nucleophilicity and PA of aromatic rings increase with more substi-
tuted methyl groups on the aromatic ring. In the Friedel–Crafts
reactions, more alkyl substituents on aromatic rings can donate
more electrons to enhance the stability of electron-deficient carbo-
cations. Therefore, increasing the nucleophilicity (more alkyl

X. Zeng et al. / Journal of Catalysis 380 (2019) 9–20
15
Δδ1H = 3.2
Δδ1H = 3.3
1.8
a) Micro-Hβ-1
b) Micro-Hβ-2
1.8
5
5.1
Δδ1H = 7.5
4
11.5
Δδ1H = 7.2
4
11.2
Dehydrated
Dehydrated
+CD3CN
15.0
+CD3CN
15.0
10.0
5.0
0.0
-5.0
10.0
5.0
0.0
-5.0
δ1H / ppm
δ1H / ppm
Δδ1H = 3.2
Δδ1H = 3.3
5.1
Δδ1H = 7.6
c) Meso-Hβ-1
d) Meso-Hβ-2
1.8
5
1.8
Δδ1H = 8
3.9
4
11.9
11.6
Dehydrated
Dehydrated
+CD3CN
15.0
+CD3CN
15.0
10.0
5.0
δ1H / ppm
0.0
-5.0
10.0
5.0
δ1H / ppm
0.0
-5.0
Fig. 6. 1H MAS NMR spectra of the dehydrated Hb zeolites recorded before (top) and after (bottom) adsorption of CD3CN at room temperature and purged under a N2 flow of
50 mL for 10 min.
diffusion/access of large molecules to surface active sites by shape
selectivity, such as mesitylene.
3.3.3. Effect of porosity of zeolites
As shown in Table 4, the molecular size of arenes (0.5–0.8), BA
(0.7 nm), all monobenzylation products (0.9–1 nm) and DBE
(1.2 nm), is comparable to or even larger than the pore sizes of
Micro-Hb zeolites (1.1–1.2 nm), however, far smaller than the
mesopores in Meso-Hb zeolites (13.5–15.7 nm). Therefore, ben-
zene and toluene reacted with BA in both micropores and meso-
pores; however, the conversion of p-xylene and mesitylene is
favoured in mesopores due to the diffusion limitation of microp-
ores. This results in the significant difference in catalytic activity
as shown in Table 5. Since the porous structure has less effect on
the diffusion/access of small arenes (benzene and toluene),
Micro-Hb zeolites with higher BAS density can promote the benzy-
lation with BA via pathway (1) in Scheme 1, leading to 1.5 times
Fig. 7. NH3-TPD profiles of Micro-Hb-1, Micro-Hb-2, Meso-Hb-1 and Meso-Hb-2.
higher BA conversion than over Meso-Hb zeolites. However, when
the arenes having large molecular size (p-xylene and mesitylene)
were applied, Meso-Hb zeolites exhibited a much higher activity
substituents) can facilitate the attack of benzylic cations on aro-
matic rings for electrophilic aromatic substitution reactions [6].
As expected, the conversion of BA (Table 5) and the yield of
monobenzylation products increased in the order of mesitylene >
p-xylene > toluene over Meso-Hb zeolites, except for benzene.
However, the conversion of the benzylating agent (BA) was
significantly decreased in the order of benzene > toluene > p-
xylene > mesitylene, for both Micro-Hb zeolites. This dependence
of the catalytic activity on the nucleophilicity and PA of arenes
has also been reported for mesoporous [Al]SBA-15 (pore size of
9.7–11.2 nm) in the benzylation of benzyl chloride with arenes,
which is attributed to a stronger adsorption of arenes on active
sites with a higher acid strength [53]. It should be noted that
increasing the nucleophilicity of arenes by more substituted
methyl groups can significantly increase the size of reactant/prod-
ucts as shown in Table 4. Since Meso-Hb zeolites, which have sim-
ilar or higher acid strength than Micro-Hb zeolites, the low activity
with using large arenes over Micro-Hb zeolites is attributed to the
constraint of the micropore size in this work, which hinders the
than Micro-Hb zeolites, providing an up to 2 times higher BA
conversion.
The above-mentioned catalytic results have been further
revealed by investigating the turnover frequency (TOF) of the ben-
zylation reaction over Hb zeolites. TOF is often employed to evalu-
ate the performance of per active site on the catalyst surface during
a reaction under study. The TOFs were determined at the initial
reaction time (10 min). As shown in Fig. 9, these measurements
revealed that the surface sites on Meso-Hb zeolites are more active
than those on Micro-Hb zeolites for all arenes, not only for large
reactants. The high BA conversion obtained with small arenes over
Micro-Hb zeolites is attributed to their high BAS density, compared
to Meso-Hb zeolites. It is obvious that the TOFs obtained for Micro-
Hb zeolites significantly decrease with increasing the molecular
size of arenes, which was not observed for Meso-Hb zeolites. This
finding demonstrates the accessibility of BAS for reactants and
products was dramatically improved inside mesopores, while dif-
fusion/access limitations occur for large molecules in micropores.
In theory, the TOF should increase with increasing the

16
X. Zeng et al. / Journal of Catalysis 380 (2019) 9–20
12
25
20
15
10
5
(b)
(a)
Micro-Hβ-1
Micro-Hβ-2
Meso-Hβ-1
Meso-Hβ-2
10
8
6
4
2
0
0
0
50
100
150
200
250
0
50
100
150
200
250
Time(min)
Time(min)
20
18
16
14
12
10
8
20
18
16
14
12
10
8
(c)
(d)
6
6
4
4
2
2
0
0
0
50
100
150
200
250
0
50
100
150
200
250
Time(min)
Time(min)
Fig. 8. Benzyl alcohol conversion in the alkylation of arenes with different catalysts: (a) benzene, (b) toluene, (c) p-xylene, and (d) mesitylene. Conditions: 25 mg catalysts,
3.5 mL arenes, 0.25 mL benzyl alcohol, reaction temperature of 353 K.
Table 3
Yield and selectivity in the benzylation of benzyl alcohol with arenes over Hb zeolites.a
Arenes
Products
Yield % (Selectivity %)
Micro-Hb-1
Micro-Hb-2
Meso-Hb-1
Meso-Hb-2
15.6 (89.7)
19.1 (98.2)
9.4 (74.0)
12.7 (88.6)
7.7 (85.5)
0.8 (8.8)
8.9 (87.6)
0.9 (9.1)
3.8 (63.7)
0.4 (6.1)
6.1 (80.6)
0.7 (8.7)
5.2 (76.0)
4.6 (90.7)
7.8 (75.5)
7.5 (93.5)
13.5 (89.8)
15.2 (95.4)
14.9 (96.5)
15.9 (97.1)
a
Conditions: 25 mg catalysts, 3.5 mL arenes, 0.25 mL benzyl alcohol, reacted at 353 K for 4 h. DPM, BMB, BDB and BTB are for diphenylmethane, benzyl-methylbenzene, 2-
benzyl-1,4-dimethylbenzene and 2-benzyl-1,3,5-trimethylbenzene.
Pathway (1)
Pathway (2)
Pathway (3)
Scheme 1. Reaction pathway of arenes with benzyl alcohol.

X. Zeng et al. / Journal of Catalysis 380 (2019) 9–20
17
Table 4
Nucleophilicity and PA values of arenes, and molecular diameters of arenes and products.
Arenes
Nucleophilicitya
PA Value (kJ/mol)
ꢁ4.38
750.2
0.5
ꢁ3.94
784.1
0.54
ꢁ3.39
794.5
0.67
ꢁ2.82
836.4
0.8
Molecular diameter of arenes (nm)b
Molecular diameter of products (nm)b
0.7
1.2
0.9
0.94/1.0
0.94
1.0
a
Nucleophilicity parameter as acquired from Mayr’s equation, log(k) = S(N + E), in which E (Electrophilicity parameter) of An2CH+ was defined as 0 and S (nucleophile-
specific slope parameter) of 2-methyl-1-pentene was defined as 1.
b
The molecular diameters of arenes are derived from refs. [54–56], and the size of BA and products are obtained from Chem 3D.
mesopores to facilitate the formation of DBE, but is hindered in
Table 5
BA conversion over Hb catalysts in the benzylation of arenes.
the micropores via shape selectivity. The conversion of DBE with
arenes is inhibited by the rate limiting step in pathway (3) in
Scheme 1. This is in agreement with previous studies [35,57], that
BA is more favourable to proceed by self-condensation, rather than
the direct benzylation with arenes having low nucleophilicity. As
observed by Chiu et al. [35] for the benzylation of toluene with
BA at 383 K, the rate constant k in the conversion of BA to DBE
(pathway (2) in Scheme 1) are 3 ꢂ 10ꢁ3 sꢁ1 and 3.5 ꢂ 10ꢁ3 sꢁ1
for microporous H-Beta (Si/Al = 75, pore size of 0.56 nm) and
mesoporous SBA-15 (Si/Al = 70, pore size of 5.2 nm), respectively.
Micro-Hb-1
Micro-Hb-2
Meso-Hb-1
Meso-Hb-2
17.4
19.5
12.7
14.4
9
10.3
10.2
6.0
7.5
6.9
15.1
15.5
5.0
8.0
15.9
16.4
The rate constant
k in the conversion of DBE to benzyl-
methylbenzene (BMB) (pathway (3) in Scheme 1) are 1.5 ꢂ 10ꢁ5
sꢁ1 and 9.0 ꢂ 10ꢁ4 sꢁ1 for microporous H-Beta and mesoporous
SBA-15, respectively. Obviously, increasing the pore size can pro-
mote the formation of DBE with BA even with much lower
Brønsted acidity on SBA-15, however, the conversion of DBE to
BMB is limited. Therefore, Micro-Hb zeolites provided a higher
selectivity to DPM and BMB than Meso-Hb zeolites. With increas-
ing the molecular size of arenes, the nucleophilicity of arenes is
increased, which can promote their reaction with BA over the for-
mation of DBE, and thus, enhances the selectivity (>95%) to BDB
and BTB over Meso-Hb zeolites. Over Micro-Hb zeolites, even a
small DBE selectivity (<10%) was still achieved with high selectiv-
ities to BDB and BTB. The yields of BDB and BTB are only a quarter
to half of those obtained with Meso-Hb zeolites, due to the limita-
tion of mass transfer. The yields of monobenzylation products
increased in the order of BMB < BDB < BTB over Meso-Hb zeolites,
which is in line with the nucleophilicity of arenes, similar to those
reported for homogeneous catalysts [6]. This indicates that the
presence of mesopores can remarkably enhance the catalytic per-
formance of Hb zeolites in the benzylation of large arenes.
a
Conditions: 25 mg catalysts, 0.25 mL benzyl alcohol in 3.5 mL arenes, reacted at
353 K for 4 h.
In the literature [29,58], benzylation of benzene is often carried
out at its boiling point of 353 K. In the benzylation of toluene, p-
xylene, and mesitylene, 373–383 K is the frequently applied reac-
tion temperature, which is the boiling point of toluene [16,34,35].
Therefore, benzylation with arenes, except benzene, was further
carried out at 383 K to confirm above observations. The conver-
sion of BA is significantly enhanced as shown in Fig. 10. All cata-
lysts retained their high selectivity to the monobenzylation
products (Table 6) as obtained at 353 K. Micro-Hb zeolites provide
higher BA conversion with using toluene, while Meso-Hb zeolites
are more active with using p-xylene and mesitylene. For example,
in the benzylation of mesitylene, the conversion of BA was signif-
icantly enhanced from ca. 16% at 353 K to ca. 95% at 383 K over
Meso-Hb zeolites, while only increased from 5 to 8% to 40% over
Micro-Hb zeolites. Moreover, the yield obtained with Meso-Hb
zeolites is almost 2–4 times higher than that obtained with
Micro-Hb zeolites. This is similar as those observed above that
the small pore size of Micro-Hb zeolites constrains the mass
transfer of large arenes/products by shape selectivity. The TOF val-
ues obtained at 383 K are remarkably enhanced with temperature
Fig. 9. Turnover frequencies (TOF) of BA in the benzylation with different arenes
over various Hb zeolites. Conditions: 25 mg catalysts, 3.5 mL arenes, 0.25 mL benzyl
alcohol, reacted at 353 K for 10 min.
nucleophilicity of arenes under the same conditions without the
shape selectivity of zeolite pore size. However, a similar or slight
decrease of TOF values was observed for Meso-Hb zeolites. This
behaviour could be caused by a molecular size similar to the dis-
tance between two active sites, indicated by the number of BAS
per nm2 in Table 2. As an example, the average distance between
two sites on the Meso-Hb-1 zeolite is close to the molecular size
of p-xylene, mesitylene and BA, and thus, the reaction at one site
and the benzylation product with a larger size could inhibit the
reaction at neighbouring sites.
In the following, the shape-selectivity effect of zeolites on the
selectivity/yield of desired products has been considered. Com-
pared to Micro-Hb zeolites, a high DBE selectivity was obtained
in the conversion of benzene and toluene over Meso-Hb zeolites.
This indicates that the mass transfer can be strongly enhanced in

18
X. Zeng et al. / Journal of Catalysis 380 (2019) 9–20
100
80
60
40
20
0
100
(a)
(b)
80
60
40
Micro-Hβ-1
Micro-Hβ-2
Meso-Hβ-1
Meso-Hβ-2
20
0
0
50
100
150
200
250
0
50
100
150
200
250
Time(min)
Time(min)
1400
1200
1000
800
600
400
200
0
100
80
60
40
20
0
Toluene
P-xylene
Mesitylene
(c)
(d)
0
50
100
150
200
250
Micro-Hß-1 Micro-Hß-2 Meso-Hß-1 Meso-Hß-2
Time(min)
Fig. 10. Benzyl alcohol conversion in the benzylation of toluene (a), p-xylene (b), and mesitylene (c) and the TOF of BA in the benzylation with arenes for different catalysts
(d). Conditions: 25 mg catalysts, 3.5 mL mesitylene, 0.25 mL benzyl alcohol, reaction temperature of 383 K.
Table 6
Yield and selectivity in the benzylation of benzyl alcohol with arenes over Hb zeolites.a
Arenes
Products
Yield % (Selectivity %)
Micro-Hb-1
Micro-Hb-2
Meso-Hb-1
Meso-Hb-2
80.4 (80.4)
79.9 (79.9)
62.7 (62.7)
81.6 (81.6)
8.4 (8.4)
9.5 (9.5)
6.0 (6.0)
7.7 (7.7)
31.3 (67.6)
46.8 (76.0)
66.3 (69.7)
79.4 (79.4)
15.2 (59)
26.5 (64.5)
62.3 (69.4)
69.2 (73.1)
a
Conditions: 25 mg catalysts, 3.5 mL arenes, 0.25 mL benzyl alcohol, reacted at 383 K for 4 h.
compared to those obtained at 353 K. The TOF values obtained
with the Meso-Hb-2 zeolite significantly increased for 140 times
using toluene, while increased from 70 times using mesitylene.
This indicates that the temperature would have a significant effect
on the mass transfer of low reactive arenes at their boiling point,
such as for benzene and toluene. As observed in Fig. 9, the TOF
values obtained with Meso-Hb zeolites are slightly change with
using different arenes, while those obtained Micro-Hb zeolites
are significantly reduced with increasing the molecular size of
arenes. These observations clearly demonstrate that the shape
selectivity by porosity of zeolites has strong effect on their
activity.
Obviously, mesopores in Hb zeolites can significantly enhanced
the activity of these catalysts due to the improved mass transfer
and the large pore size facilitates the formation of BTB, compared
to micropores. This is in an agreement with the observations of
Jin et al. [57] and Liu et al. [16] in the comparison of Micro-H-
ZSM-5 and Meso-H-ZSM-5 zeolites. In the benzylation of benzene
with BA, Wang et al. [58] have shown that Meso-ZSM-5 is more
active in the conversion of BA than Micro-ZSM-5 (71% for Meso-
ZSM-5 vs. 11% for ZSM-5) at 353 K for 6 h. Chiu et al. [35] reported
that the application of mesoporous SBA-15 for the benzylation of
toluene to BMB provides a two times higher of reaction rate k than
using microporous Hb zeolites (1.5 ꢂ 10ꢁ3 sꢁ1 vs. 8.0 ꢂ 10ꢁ4 sꢁ1),

X. Zeng et al. / Journal of Catalysis 380 (2019) 9–20
19
even the latter has much higher Brønsted acidity. Recently,
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This work was supported by the Australian Research Council
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