Surfactant-directed mesoporous zeolites with enhanced catalytic activity in tetrahydropyranylation of alcohols: Effect of framework type and morphology

Article abstract of DOI:10.1016/j.apcata.2017.02.019

Nanosponge zeolite beta was hydrothermally synthesized using multi-quarternary ammonium surfactant as a meso-micro hierarchical structure directing agent. The nanosponge morphology of the beta zeolite consisted of randomly interconnected nanocrystals with thickness of 10–20?nm. The beta nanosponges with highly mesoporous structure exhibited enhanced catalytic activity in tetrahydropyranylation of alcohols (methanol, 1-octanol, cyclohexanol and 2-adamantanol) when compared with the conventional beta zeolites and nanosponge zeolites of MTW and MFI framework types. The enhanced catalytic performance (～60% conversion with 90–100% selectivity towards tetrahydropyranyl ether and over two times higher initial rate compared to other zeolites) of hierarchical beta nanosponges can be attributed to the high accessibility of acid sites and facile diffusion of reactants and products through the mesopores.

Full text of DOI:10.1016/j.apcata.2017.02.019

Accepted Manuscript
Title: Surfactant-directed mesoporous zeolites with enhanced
catalytic activity in tetrahydropyranylation of alcohols: effect
of framework type and morphology
Authors: Hye Sun Shin, Maksym Opanasenko, Carlos
´
Palomino Cabello, Ryong Ryoo, Jirnullı Cnullejka
PII:
S0926-860X(17)30086-8
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2017.02.019
APCATA 16155
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
3-10-2016
21-2-2017
24-2-2017
Please cite this article as: Hye Sun Shin, Maksym Opanasenko, Carlos Palomino
Cabello, Ryong Ryoo, Jirx30c;iacute; Cx30c;ejka, Surfactant-directed mesoporous
zeolites with enhanced catalytic activity in tetrahydropyranylation of alcohols:
effect of framework type and morphology, Applied Catalysis A, General
http://dx.doi.org/10.1016/j.apcata.2017.02.019
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Surfactant-directed mesoporous zeolites with enhanced
catalytic activity in tetrahydropyranylation of alcohols: effect
of framework type and morphology
Hye Sun Shin1,2, Maksym Opanasenko3,4,*, Carlos Palomino Cabello3, Ryong Ryoo1,2
and Jiꢀí ꢁejka3
1Center for Nanomaterials and Chemical Reactions, Institute for Basic Science(IBS),
Daejeon 34141, Republic of Korea
2Department of Chemistry, Korea Advanced Institute of Science and
Technology(KAIST), Daejeon 34141, Republic of Korea
3J. Heyrovskꢂ Institute of Physical Chemistry, Academy of Sciences of Czech
Republic, v.v.i., Dolejꢃkova 3, 182 23 Prague 8, Czech Republic
4Department of Physical and Macromolecular Chemistry, Faculty of Science,
Charles University, Hlavova 8, 128 43 Prague 2, Czech Republic
*Corresponding author
Dr. Maksym Opanasenko
Department of Physical and Macromolecular Chemistry, Faculty of Science,
Charles University, Hlavova 8, 128 43 Prague 2, Czech Republic; J. Heyrovskꢂ
Institute of Physical Chemistry, Academy of Sciences of Czech Republic, v.v.i.,
Dolejꢃkova 3, 182 23 Prague 8, Czech Republic. E-mail: maksym.opanasenko@jh-
inst.cas.cz; Tel: (+420) 26605 3055
1

Graphical abstract
Highlights
 The influence of zeolite topology and morphology on the catalytic
behaviour in tetrahydropyranylation of alcohols was investigated.
 Variables include nanosponge vs. bulk morphology, zeolite type (BEA,
MTW, MFI), Si/Al and the size of reactant molecules (methanol, 1-
octanol, cyclohexanol and 2-adamantanol).
 Due to the high accessibility of active sites and facile diffusion of
reactant and product molecules BEA nanosponges showed enhanced
catalytic performance (TON = 529 – 827) compared with conventional
analogue (TON = 198 – 375).
2

Abstract
Nanosponge zeolite beta was hydrothermally synthesized using multi-quarternary
ammonium surfactant as a meso-micro hierarchical structure directing agent. The
nanosponge morphology of the beta zeolite consisted of randomly interconnected
nanocrystals with thickness of 10–20 nm. The beta nanosponges with highly
mesoporous structure exhibited enhanced catalytic activity in tetrahydropyranylation of
alcohols (methanol, 1-octanol, cyclohexanol and 2-adamantanol) when compared with
the conventional beta zeolites and nanosponge zeolites of MTW and MFI framework
types. The enhanced catalytic performance (~60% conversion with 90–100% selectivity
towards tetrahydropyranyl ether and over two times higher initial rate compared to other
zeolites) of hierarchical beta nanosponges can be attributed to the high accessibility of
acid sites and facile diffusion of reactants and products through the mesopores.
Keywords:
Mesoporous beta, nanosponge
zeolite,
hierarchical zeolite,
tetrahydropyranylation, external acidity
1. Introduction
Tetrahydropyranyl ethers have been extensively applied in organic synthesis to obtain a
variety of polyfunctional compounds due to their low prices and good stability under
harsh reaction conditions [1]. The tetrahydropyranyl ethers can be easily synthesized
through tetrahydropyranylation reaction protecting hydroxyl groups in organic
compounds with 3,4-dihydro-2H-pyran (DHP) over acid catalysts (Scheme 1).[2,3]
Usually, both Brønsted acids (e. g. HCl [4], p-toluenesulfonic acid [5], etc) and Lewis
acids (e. g. boron tri-fluoride etherate [6], niobium chloride [7], tantalum chloride [8],
etc) were used as catalysts for tetrahydropyranylation of alcohols under organic solvent
mediated reaction conditions. However, a mild and efficient method for
tetrahydropyranylation of alcohols is required to limit the effect of moisture sensitivity
of catalysts, long reaction time under reflux conditions, handling problem and
environmental hazard [9,10]. In order to improve such drawbacks caused by use of the
strong acids and chemical reagents, heterogeneous catalysts such as ion-exchange resins,
silica-based sulfonic acid catalysts [11], natural clays [12,13], and zeolites [10,14-16]
have been investigated for tetrahydropyranylation of alcohols under relatively mild
reaction conditions. Particularly, zeolites have attracted much attention because of their
tunable acidity, shape-selectivity, high structural stability and easy regeneration [17].
Conventional H-FAU [15] and H-beta [18] zeolites and zeolite analogues [19] (e. g.
AlPO4) were utilized in tetrahydropyranylation reaction of alcohols, phenols, naphthols
including bulky substrates such as cholesterol. The H-beta zeolite exhibited high yield
of target ether and good recyclability under mild reaction conditions. However, the
reaction of bulky alcohols in the microporous materials proceeded slowly due to
diffusion limitations requiring longer reaction times. Hence, mesoporous materials were
used to resolve the diffusion restriction of bulky substrates. Nedumaran et al. reported
that mesoporous silica synthesized by incorporation of zirconium and subsequent
sulfation (i. e., sulfated Zr-Si-MCM-41) exhibited an enhanced catalytic activity in
3

tetrahydropyranylation of phenol [20]. In addition, Corma et al. employed the zeolitic
material (ITQ-2) with thin layered structure to increase the accessibility of active sites
[21]. ITQ-2 showed higher conversions compared to H-beta zeolites in
tetrahydropyranylation of bulky substrates. Most recently, Al incorporated
germanosilicate ITH zeolites exhibited improved catalytic behavior in
tetrahydropyranylation reaction, resulting from simultaneous mesoporosity and
presence of strong acid centers generated by alumination treatment [22]. The additional
post-treatment such as incorporation of heteroatom or delamination was necessary to
introduce the strong acid sites and/or mesoporosity into the materials.
Additional strategy for the enhancement of substrate diffusion in zeolites is to
decrease the diffusion path length inside the zeolite micropores by reducing the crystal
or framework thickness [23-25]. The zeolites with reduced crystal thickness can be
obtained through the zeolite crystallization for nanocrystal morphology or hierarchical
structure possessing the secondary mesoporosity within the microporous zeolite crystals.
Such zeolites with hierarchical structures have been produced in a number of synthetic
routes including post-steaming [26], selective framework etching through desilication or
dealumination [27,28], and addition of mesopore-generating agents (e. g. carbon
nanoparticles [29], silylated organic polymers [30], organosilane surfactants [31-33] and
multi-quarternary ammonium surfactants [34,35]) into the synthesis compositions.
Particularly, the direct synthesis route for hierarchical zeolite using the organosilane or
multi-ammonium surfactants as mesopore-directing agents has been well-established.
The resultant zeolites exhibited remarkably enhanced catalytic activity in some catalytic
reactions involving bulky molecules that fail to enter the micropores. Recently, Ryoo
and coworkers reported the direct synthesis of mesoporous zeolites (e. g., MFI, MTW,
MRE and beta) with nanosponge-like morphology using multi-quarternary ammonium
surfactants as meso-micro hierarchical structure directing agents (SDAs) [34-39]. The
ammonium head groups and long alkyl chains of the multi-ammonium surfactants led to
the formation of microporous zeolite frameworks and disordered mesostructures.
Typically, nanosponge beta zeolites were synthesized using [C22H45-N+(CH3)2-C6H12-
N+(CH3)2-CH2-(C6H4)-CH2-N+(CH3)2-C6H12-N+(CH3)2-CH2-(C6H4)-CH2-N+(CH3)2-
C6H12-N+(CH3)2-C22H45](Br-)2(Cl-)4 surfactants. Moreover, Jo et al. proposed the
synthesis method of mesoporous beta zeolite using a piperidinium-functionalized multi-
ammonium surfactant as the SDA in near-neutral pH and/or high pH conditions quite
recently [40]. The beta nanosponges composed of randomly assembled tiny
nanocrystals possessed uniform mesopores (~4.5 nm of diameter). This nanosponge
beta zeolite exhibited high catalytic performance in the Friedel-Craft alkylation of
benzene [38,41], acylation of aromatic compounds [35], and Diels-Alder cycloaddition
of biomass derived 2,5-dimethylfuran and ethylene [42]. The high catalytic activity of
the nanosponge beta zeolite was attributed to a high concentration of strong acid sites
on the external surface inducing facile diffusion of reactants and products through the
mesopores. In this respect, mesoporous beta zeolite obtained via the direct synthesis can
be an efficient catalyst outperforming the conventional H-beta zeolite in liquid-phase
tetrahydropyranylation reaction.
The aim of this contribution was to carry out the comparative investigation of zeolite
materials with different topology (beta, MTW and MFI) and morphology (nanosponge
vs. bulk) as catalysts in tetrahydropyranylation of alcohols different in size (methanol,
4

1-octanol, cyclohexanol and 2-adamantanol). Particular attention was focused on the
understanding of key reaction parameters as zeolite structure, porosity and chemical
composition on the acidic properties and catalytic activity (yield, turnover frequency
and turnover number) of respective materials. Since the Si/Al ratio of mesoporous beta
zeolites can vary widely, materials with the Si/Al ratio of 15, 50 and 100 have been used
in present study.
2. Experimental
2.1. Materials
Hexane (≥97.0%, Sigma-Aldrich), 3,4-dihydro-2H-pyran (97%, Sigma-Aldrich),
methanol (Lachner), 1-octanol (Honeywell Riedel-de Haën), mesitylene (≥99.8%,
Sigma-Aldrich), cyclohexanol (99%, Sigma-Aldrich) and 2-adamantanol (TCI) were
used as received without further purification. Tetraethylorthosilicate (TEOS, 95%,
Junsei), water glass (29 wt% SiO2, Si/Na = 1.75, Shinheung Chemical), aluminum
isopropoxide (≥98%, Sigma-Aldrich), sodium aluminate (50 - 56 wt% Al2O3, 40 - 45 wt%
Na2O, Sigma-Aldrich), aluminum sulfate octadecahydrate (98%, Sigma-Aldrich) and
sulfuric acid (47%, Wako) were used for the synthesis of zeolites. Three kinds of zeolite
SDA were prepared in the present work, according to the previous reports [37-40]. The
zeolite SDAs have the molecular formulas of 4,4´-trimethylene-bis-[1-(CH3)-1-{CH2-
C6H4-CH2-N+(CH3)2-C6H12-N+(CH3)2-C22H45}piperidinium](Br‒)2(Cl‒)4,
N+(CH3)2-C6H12-N+(CH3)2-CH2-(C6H4)-CH2-N+(CH3)2-C6H12-N+(CH3)2-CH2-(C6H4)-
[C22H45-
CH2-N+(CH3)2-C6H12-N+(CH3)2-C22H45](Br-)2(Cl-)4
and
[C18H45-N+(CH3)2-C6H12-
N+(CH3)2-C6H13](Br-)2 (See Table S1 for their molecular structure). For brevity, the
three SDAs are denoted by [C22-6Npipe-C22](Br-)2(Cl-)4, [C22-6Nph-C22](Br-)2(Cl-)4 and
[C18-2N-C6](Br-)2, respectively, hereafter. The purity of the zeolite SDAs was checked
with 1H NMR spectroscopy. Commercial zeolites (beta: CP814C* and CP811C-300, all
from Zeolyst) were used for comparison in catalysis.
2.2. Preparation of zeolites
The zeolite beta nanosponges were hydrothermally synthesized using a hydroxide
form of [C22-6Npipe-C22](Br-)2(Cl-)4 surfactant as a zeolite SDA [40]. The [C22-6Npipe-
C22](Br-)2(Cl-)4 was ion-exchanged with OH‒-exchange resin (MTO-Dowex SBR
LCNG OH form, Supelco) to convert the Cl‒ and Br‒ ions with OH‒ ions. The
concentration of the hydroxide form of the surfactants in distilled water ranged from 7
to 10 wt%. For the zeolite synthesis, aluminum isopropoxide was added to an aqueous
solution of [C22-6Npipe-C22](OH-)6 and then stirred at room temperature for 1 h. TEOS
was added dropwise to the mixture solution and vigorously stirred at room temperature
for 4 h. The mixture solution was placed in an oven at 333 K for 12 h. The final gel
composition was 30 SiO2: x Al2O3: 1.8 SDA: 5000 H2O (x = 0.15, 0.30 and 1). After
cooling down to the room temperature, the resultant mixture was transferred to a Teflon-
lined autoclave and then heated at 413 K for 9 d (x = 0.15 and 0.30) or 12 d (x = 1). The
solid products were collected from the aqueous solution by centrifugation, washed twice
with distilled water and dried at 373 K overnight.
The MTW nanosponge was hydrothermally synthesized using [C22-6Nph-C22](Br-
)2(Cl-)4 surfactant as the zeolite SDA [38]. For a typical synthesis, TEOS was dropwise
5

added to an aqueous solution of NaOH containing [C22-6Nph-C22](Br-)2(Cl-)4 surfactant.
Sodium aluminate was dissolved in the distilled water and then the clear solution was
dropwise added to the solution with continuous stirring. The resultant gel mixture was
aged at 333 K for 12 h. The final synthesis gel of a composition of 100 SiO2: 0.5 Al2O3:
3.3 SDA: 13 Na2O: 4500 H2O was transferred to a Teflon-lined stainless autoclave and
then heated at 423 K for 5 d under tumbling conditions. The solid product was filtered,
washed with distilled water and dried in an oven at 373 K for 12 h.
The MFI nanosponge was synthesized with a seed-assisted hydrothermal synthesis
method using [C18-2N-C6](Br-)2 as the zeolite SDA, following the previous procedure
with a slight modification [37]. The aluminum sulfate octadecahydrate was dissolved in
distilled water. The solution was added to the mixture of aqueous NaOH solution, SDA
and H2SO4 (47%). The mixture was stirred for 30 min at room temperature. Then, water
glass was added dropwise with vigorous stirring. The gel mixture was stirred at 333 K
for 2 h and subsequently, a calcined commercial MFI zeolite was added to the gel
mixture as a seed, amounting to 5 wt% of total silica source. The final gel composition
was 100 SiO2: 1 Al2O3: 7.5 SDA: 30 Na2O: 17 H2SO4: 5000 H2O in molar ratio. The
final mixture was further stirred in an oven at 333 K for 12 h and then heated at 423 K
for 3 d in a Teflon-lined autoclave under agitation conditions. The solid product was
filtered, washed with distilled water and dried in an oven at 373 K for 12 h.
All the synthesized zeolites were calcined at 853 K for 4 h in air and then ion-
+
exchanged to the NH4 form with 1 M NH4NO3 aqueous solution corresponding to the
+
five-fold access of NH4 for the Al content in zeolites. The exchange treatment was
+
repeated three times. The NH4 -exchanged zeolites were calcined at 823 K in air for
conversion to the proton form.
Commercial beta zeolites were calcined at 823 K in a stream of air.
All samples under study are denoted as X-ABC-n, where X = C (for conventional
zeolites) or N (nanosponge), ABC is the zeolite framework types (BEA*, MTW or
MFI) and n = Si/Al ratio.
2.3. Characterization
X-ray powder diffraction (XRD) patterns were obtained with a Rigaku Multiflex
diffractometer using Cu Kα radiation (λ = 0.1541 nm) at 30 kV and 40 mA (1.2 kW).
Scanning electron micrograph (SEM) images were taken with a Verios 460 SEM
microscope under operating condition at 1 kV (decelerating voltage: 3.0 kV) without a
metal coating. High-resolution TEM (HR-TEM) images were taken with a FEI Tecnai
G2 F30 microscope at an acceleration voltage of 300 kV. For TEM imaging, the powder
samples were suspended in acetone by sonication and the solution was dropped on a
holey carbon grid.
The nitrogen adsorption-desorption isotherms were obtained at 77 K with a
Micromeritics Tristar II instrument. For adsorption measurements, the samples were
degassed in vacuum at 573 K for 6 h. The specific surface area was calculated from the
adsorption branch in the P/P0 range between 0.01 and 0.30 using the Brunauer-Emmett-
Teller (BET) equation. Approximate pore size distributions were derived from the
adsorption branch according to the Barrett-Joyner-Halenda (BJH) algorithm. The argon
sorption measurement was performed using a Micromeritics ASAP 2020 instrument at
87 K. For the Ar adsorption measurements, the samples were degassed in vacuum at
6

573 K for 6 h. The micropore volume and average micropore diameter of the catalysts
were determined according to the nonlocal density functional theory (NLDFT).
The Si/Al ratio was determined by inductively coupled plasma-optical emission
spectrometry (ICP-OES) using an iCAP 6300 Duo instrument (Thermo Scientific Co.,
UK). For the ICP analysis, the zeolite samples were completely dissolved in a dilute
acid solution containing hydrochloric acid, nitric acid and hydrofluoric acid. To this
clear solution, boric acid was added and dissolved completely to prevent the damage of
ICP sample transport system by the complexation of the fluoride.
The concentration of Lewis and Brønsted acid sites was determined by Fourier
transform infrared (FTIR) spectroscopy using a Nicolet Protꢄgꢄ 460 Magna equipped
with a transmission MCT/A detector after adsorption of pyridine. For FTIR
measurement, zeolite samples were pressed into self-supporting wafers with a density of
8.0 - 12 mg cm-2 and then activated in situ at 723 K and high vacuum (10-5 Torr) during
4 h. Pyridine adsorption was carried out at 423 K for 20 min at a partial pressure of 3.3
Torr. Then, the samples were degassed at 423 K for 20 min. The stepwise thermal
desorption of the probe molecule was monitored by evacuating the sample at 423, 523,
623 and 723 K and cooling sample to room temperature between each step. Pyridine
was purified through Freeze-pump-thaw cycles. All the spectra were recorded with a
resolution of 4 cm-1 by collecting 128 scans for a single spectrum at room temperature.
The spectra were normalized to a wafer density of 10 mg cm-2. The concentration of
Lewis and Brønsted acid sites was evaluated from the integral intensities of bands at
1454 cm-1 (Lewis acid sites) and at 1545 cm-1 (Brønsted acid sites) using extinction
coefficients, ε(L) = 2.22 cm μmol-1 and ε(B) = 1.67 cm μmol-1 [43].
Thermogravimetric analysis (TGA) was carried out using a TGA Q50 (Thermal
Analysis Instruments Inc.). The temperature was increased to 1073 K at a constant
ramping rate of 20 K min-1 under air flow (60 ml min-1).
2.4. Catalytic measurements
The tetrahydropyranylation reaction of alcohols with DHP was performed in the
liquid phase at 300 or 333 K under atmospheric pressure in a multi-experiment
workstation StarFish (Radleys Discovery Technologies). Before use, the catalyst was
activated at 723 K for 90 min at a rate of 5 K min-1. The alcohol, mesitylene (internal
standard), hexane (solvent) and DHP were placed in a three-necked vessel equipped
with a thermometer. After the solution reached the reaction temperature, the activated
zeolite catalyst was added into the vessel. The details of reaction conditions for different
kinds of alcohol are shown in the Table 1. Particular reaction conditions for each alcohol
were established, where the reaction temperature or relative amount of the alcohol and
DHP were systematically changed to show a comparative activity between hierarchical
and commercial beta zeolites (Table 1). Samples of the reaction mixture were taken
periodically and centrifuged. The liquid part was analyzed using an Agilent 6850 GC
equipped with a nonpolar DB-5 column (length 20 m, diameter 0.180 mm and film
thickness 0.18 mm) and a flame ionization detector. The GC analysis proceeded using
10 μl of injection volumes, 20 K min-1 of oven ramp rates from 333 K to 573 K and 4
min of hold times. The reaction products were identified by using a Thermo Finnigan
Focus DSQ II single quadrupole GC/MS. The conversion and selectivity were
calculated on the basis of the GC analysis. The conversion of alcohols was calculated
7

based on the amount of converted alcohol reactants using internal standard. The
selectivity was determined using the response factors of the products. The deviation of
the carbon balance calculated from the alcohol consumption and the sum of all products
was smaller than ±5% in all experiments.
2.5. Recyclability test
The recyclability of the zeolite beta nanosponge with Si/Al = 110 was tested in the
tetrahydropyranylation of 1-octanol with DHP at 333 K. After 1 h of reaction, the
catalyst was separated from the reaction mixture by centrifugation and washed with
hexane and acetone several times. The collected sample was dried at 423 K for 6 h. In
the 2nd catalytic run, the sample was used without further activation at high temperature.
Before the 3rd and 4th cycles, the used catalyst was calcined at 853 K for 4 h in air flow
to remove the organic residues and subsequently activated at 723 K for 90 min.
3. Results and discussion
3.1. Structural and textural properties of the zeolites
Figs. 1A and 1B show representative SEM and TEM images of the nanosponge beta
zeolite with Si/Al = 110 (N-BEA*-110). The images display no visible additional
phases. The SEM image of N-BEA*-110 exhibits that the sample was composed of very
thin nanocrystals of approximately 10 – 20 nm. The nanocrystals were randomly
interconnected forming a nanosponge-like assembly. On the other hand, the SEM image
of C-BEA*-103 indicates that the sample consisted of aggregates (0.5 – 1 µm) of the
particles with a diameter of 30 – 50 nm (Fig. 1C). The powder XRD patterns of N-
BEA*-110 and C-BEA*-103 are shown in Fig. 1D. The wide-angle XRD pattern of C-
BEA*-103 sample exhibited several representative Bragg reflections, which are in a
good agreement with those of the beta zeolites in the literature [44]. Compared to the C-
BEA*-103, the N-BEA*-110 showed a broad small-angle peak at about 2θ = 1.4°,
indicating that the nanosponge had modest mesostructural order. In addition, the wide-
angle XRD pattern of the N-BEA*-110 sample exhibited that characteristic Bragg
diffraction lines (2θ = 7.5° and 22°) of the beta nanosponge sample were attributed to
the small crystal domains of nanomorphic beta zeolite. However, other reflections
corresponding to the beta zeolite structure could not be identified due to the broadening
and low intensity of the peaks [35,40].
Textural properties of the N-BEA*-110 and C-BEA*-103 samples were evaluated
using N2 adsorption and desorption isotherm measured at 77 K (Fig. 1E). The isotherm
exhibited a sharp increase in the adsorbed amount in the low pressure region of P/P0 <
0.1 and a gradual rise in the range of 0.4 < P/P0 < 0.6, followed by the hysteresis loop.
The increase in the adsorbed amount in the low and high relative pressure region can be
attributed to the filling of micropores and capillary condensation in the mesopores of the
zeolite, respectively. The mesopore size distribution of nanosponge obtained by the BJH
algorithm showed the peak centered at 4.5 nm. The N-BEA*-110 sample possessed
high BET area (770 m2 g-1) and external surface area (540 m2 g-1) determined using the
t-plot method. The micropore volume (Vmic) and average micropore diameter (Dmic)
were accurately assessed from Ar adsorption measurement at 87 K in the range of P/P0
8

< 0.01. The values (Vmic = 0.18 cm3 g-1, Dmic = 0.64 nm) determined according to a
NLDFT method corresponded to a typical microporous feature of beta zeolite (See
Table 2 and Fig. S1) [33,45]. The results revealed that the nanosponge beta zeolite had
highly hierarchical meso-microporous structure.
The nanosponge beta zeolites with different Si/Al ratios of 15 and 53 (N-BEA*-15
and N-BEA*-53, respectively) exhibited very similar morphologies in SEM and TEM
images and reflections in the XRD patterns (Figs. S2 and S3). Both N-BEA*-15 and N-
BEA*-53 samples also showed mesoporous characteristics similar to that of the N-
BEA*-110, possessing high BET areas (770 and 850 m2 g-1), external surface areas (580
and 590 m2 g-1) and large total pore volumes (1.3 and 1.4 cm3 g-1), respectively (Fig. S3
and Table 2).
In comparison with N-BEA* zeolites, the N2 adsorption-desorption isotherm of C-
BEA*-103 exhibited type I isotherm (Fig. 1E), indicating a typical feature of
microporous material although the sample did not show the smooth surface in the SEM
image (Fig. 1C). C-BEA*-103 also displayed considerably smaller external surface area
(77 m2 g-1), compared with the nanosponge beta samples (Table 2). For the conventional
beta zeolite with Si/Al = 20 (C-BEA*-20), the morphology was similar with the C-
BEA*-103, representing the aggregation of zeolite crystals with a diameter ranging
from 30 – 80 nm (Fig. S4). The C-BEA*-20 possessed typically microporous structures
and thereby had low external surface area (58 m2 g-1) and small pore volume (0.29 cm3
g-1) (Fig. S4 and Table 2).
Nanosponge zeolites with MTW and MFI structure were similarly characterized by
SEM, TEM, XRD and N2 adsorption analysis. The N-MTW-110 and N-MFI-80
materials exhibited a nanosponge-like morphology in which the zeolite nanocrystals
were randomly assembled (Figs. S5A and S5D). The N-MTW-110 consisted of the
nanocrystals with thicknesses of < 10 nm, which were assembled in a disordered
manner. The N-MFI-80 was composed of the randomly self-connected zeolite
nanolayers with 2.5 nm thickness exhibiting intercrystalline mesopores. The XRD
patterns of the N-MTW-110 and N-MFI-80 samples exhibited the characteristic Bragg
reflections corresponding to the MTW and MFI zeolite structures [37,38,41]. In
addition, the both diffraction patterns showed the peak (2θ = 1.1° and 1.3°, respectively)
in small-angle region of the XRD pattern, indicating that the nanosponge zeolites
possessed a mesoporous structure (Figs. S5B and S5E) [36-38]. In a good agreement
with the XRD and TEM analysis, N-MTW-110 and N-MFI-80 samples exhibited both
inherent microporosity of the zeolites and mesoporous characteristics, corresponding to
the hysteresis loops in the adsorption isotherms and narrow distributions of mesopore
diameters centered at 4 nm (Table 2, Figs. S5C and S5F) [37,38]. Accordingly, the
mesoporous MTW and MFI zeolites had high BET surface area and external surface
area (Table 2).
3.2. Acidic properties of the zeolites
The concentration and type of acid sites in the conventional beta and nanosponge
zeolites (beta, MTW and MFI) were determined by FTIR spectroscopy using pyridine
9

as a probe molecule. The pyridine with a kinetic diameter of about 0.5 nm can enter not
only the 12-ring channels of the BEA (0.66 x 0.67 nm; 0.56 x 0.56 nm) and MTW
(0.57 x 0.61 nm) zeolite but also 10-ring channels of the MFI (0.53 x 0.56 nm; 0.51 x
0.55 nm) zeolite [46,47]. Fig. 2 shows the concentrations of Lewis and Brønsted acid
sites in the N-BEA* and C-BEA* samples, which were calculated from the integral
intensities of the bands at 1454 and 1545 cm-1, respectively, by using extinction
coefficients reported in the literature [43]. For both nanosponge and conventional beta
zeolites, with decrease of the Si/Al ratio of the zeolites, the total amounts of acid sites
increased (N-BEA*-15 > N-BEA*-53 > N-BEA*-110 and C-BEA*-20 > C-BEA*-103).
This result showed a good agreement with the Al contents of the nanosponge and
conventional beta samples, determined by ICP-OES (Table 2). Notably, all the
nanosponge beta zeolites possessed significantly higher Lewis/Brønsted acid centers
ratio in comparison to that in the conventional analogues. This feature can be attributed
to the increased concentration of the defect sites in the hierarchical beta nanosponge
zeolites [48]. The amounts of the Lewis and Brønsted acid centers in N-MTW-110 and
N-MFI-80 samples were comparable with that in the N-BEA*-110 possessing similar
Si/Al ratio (Fig. S6).
The acid properties related to the relative strength of acid sites in the conventional
and nanosponge zeolites were further investigated by thermal desorption of the
adsorbed pyridine with the increase of desorption temperature from 423 K to 723 K.
The IR spectra and acid concentrations calculated from the evaluation of the IR spectra
in the range of 1400 cm-1 to 1600 cm-1 are shown in the Fig. S7 and Table S2. As shown
in Table S2, the amount of acid sties was decreased systematically as the desorption
temperature increased for both Lewis and Brønsted acid sites in the zeolites. For all the
zeolites, the concentration of Lewis acid sites was relatively high in comparison with
the Brønsted acid sites after pyridine desorption at 723 K. This can be explained by the
general mechanism based on the increasing the number of Lewis acid sites with
decrement of the bridged Si-OH-Al sites, associated with Brønsted acid sites [49].
Furthermore, all the zeolite samples possessed the reasonable amount of medium to
strong acid sites, which was indicated by maintenance of the ratio of acid sites detected
at 623 and 423 K (i. e., C623/C423) in the range of 0.5 – 0.9 for both Lewis and Brønsted
acid (Table S2). In the case of beta zeolites with different Si/Al ratios, the ratios of
C623/C423 tend to be increased with the increase of Si/Al ratio of the zeolite. Such
tendency can be due to either lower stability of the zeolite framework or lower acid
strength of the surface acid sites, regarding the close proximity of Al sites for the zeolite
with low Si/Al ratios [50].
3.3. Catalytic performance of nanosponge beta zeolites in tetrahydropyranylation
reaction
3.3.1. Comparison of catalytic performance between nanosponge and conventional beta
zeolites
Catalytic properties of nanosponge and conventional beta zeolites were investigated
in the liquid-phase tetrahydropyranylation of 1-octanol with DHP. Both Lewis and
Brønsted acid sites can act as active centers in this reaction (Scheme 2) [17,51]. Fig. 3A
10

shows the catalytic results obtained over N-BEA*-110 and C-BEA*-103 by means of
conversion of 1-octanol plotted as a function of reaction time. The plot displays that the
1-octanol conversion over N-BEA*-110 and C-BEA*-103 catalysts increased in the 30
min of the initial reaction period. After initial reaction period, the catalytic conversion
increased gradually with the reaction time and converged to a maximum value. The N-
BEA*-110 exhibited a significantly higher catalytic conversion than the C-BEA*-103
(56% vs. 16%, respectively) at 24 h of reaction time. This result is consistent with
previous report showing that the nanosponge beta zeolite exhibits higher catalytic
conversion in Friedel-Crafts alkylation reaction of aromatics than conventional beta
zeolite catalyst [41]. The N-BEA*-110 and C-BEA*-103 showed a comparable
selectivity (90 – 100%) to the desired tetrahydropyranyl ether during the whole reaction
time. C-BEA*-103 was deactivated rapidly, indicating the maximum value of the
conversion of reactant within 30 min. The result well corresponded to the data obtained
from TGA analysis where the spent catalyst possessed substantial amounts of
carbonaceous materials within 30 min of the reaction time (Fig. S8). Thus, the rapid
deactivation of C-BEA*-103 can be explained by limitation of the accessibility of
reactants to the active sites by formation of polymeric compounds as cokes in the
micropores. This was the main reason of the rapid deactivation for microporous zeolites
in various catalytic reactions [52-54]. We calculated ‘total number of reactant alcohol
molecules converted per acid site’ during 24 h (i. e. turnover number, ‘TON’) and ‘the
number of reactant molecules per total acid sites per unit time (h) (i. e. turnover
frequency, ‘TOF’) based on the total concentration of both Lewis and Brønsted acid
sites accessible for pyridine molecules (Fig. 2). The reaction for determination of the
TOF was carried out in a mild condition by reducing the amount of catalyst and reaction
temperature owing to significantly high reactivity (above 30% conversion within 15 min)
in the reaction condition described in Table 1. The TOF was calculated using the linear
part of the conversion-time dependence (in the range of < 10% conversion) to exclude
the influence of deactivation (inset in Fig. 3A). Fig. 3B shows the TON and TOF of N-
BEA*-110 and C-BEA*-103 in the tetrahydropyranylation reaction of 1-octanol. The
result was obtained taking into consideration that a deviation within 5% was calculated
from a set of three different experiments. As the conversion of N-BEA*-110 constantly
increased in contrast to C-BEA*-103 (Fig. 3A), the TON value of N-BEA*-110 (584)
was much higher than that of C-BEA*-103 (171). This result may be related to the well-
developed mesopore structure and large external surface of the hierarchical zeolite
facilitating easier diffusion of reactants and products, and therefore a slow rate of
deactivation. In addition, the N-BEA*-110 showed about 2-fold higher TOF (111 x10-2,
h-1) in comparison with C-BEA*-103 (64 x10-2, h-1). The higher catalytic activity of N-
BEA*-110 can be attributed to the high concentration and accessibility of the active
sites located on the external surface of nanosponge zeolite.
In order to investigate the influence of the reactant molecule size on the catalytic
behavior of bulk and nanomorphic materials, different alcohols (methanol, cyclohexanol
and 2-adamantanol) have been used in the tetrahydropyranylation reaction (Table 1).
The TOF and TON values of N-BEA*-110 and C-BEA*-103 were evaluated for each
reaction (Table 3). For both conventional and nanosponge samples, catalytic activity
gradually decreased with increase of alcohol size (TOFmethanol > TOFcyclohexanol ≈ TOF1-
> TOF2-adamantanol). Similarly to the results obtained in the tetrahydropyranylation
octanol
11

of 1-octanol, N-BEA*-110 exhibited higher values of the TOF and TON compared to
C-BEA*-103 for all the alcohols applied. This difference became especially pronounced
when the most bulky 2-adamantanol was used: TOFN-BEA*-110/TOFC-BEA*-103 increased to
4.1 from 2.3 – 2.7 characteristic for transformation of methanol and cyclohexanol.
These results confirmed the preferences of nanosponge materials over conventional
zeolites in terms of diffusional limitations and accessibility of acid sites mentioned
above.
3.3.2. Effect of Si/Al ratio in nanosponge beta zeolites
The catalytic behavior of nanosponge beta zeolites with different Si/Al ratio (N-
BEA*-15,
N-BEA*-53
and
N-BEA*-110)
was
investigated
in
the
tetrahydropyranylation of different alcohols (methanol, 1-octanol, cyclohexanol and 2-
adamantanol). In all reactions, with increasing the Al content in the N-BEA* samples,
the yield of tetrahydropyranyl ether increased, which was reasonable in accordance with
increase in the total acid site concentrations (Figs. 2 and S9). For all of used alcohols,
both TOF and TON values increased with decreasing Al contents (N-BEA*-15 < N-
BEA*-53 < N-BEA*-110) (Fig. 4). This tendency of decreased catalytic activities with
increasing Al content is commonly known in acid-catalyzed reaction over zeolites,
which can be due to the closely located Al sites and/or reduced hydrophobicity in
zeolite frameworks [55,56]. Similarly, Kim et al. reported such tendencies of the
catalytic activity in the Pechmann condensation reaction over the nanosponge MFI
zeolites with different Si/Al ratios [57].
3.3.3. Effect of the framework type of nanosponge zeolite
We performed the tetrahydropyranylation reaction of 1-octanol and 2-adamantanol
over the beta, MTW and MFI nanosponge zeolites in order to investigate the effect of
framework topology of the zeolite. In the case of 1-octanol, N-BEA*-110 and N-MTW-
110 exhibited higher yields of tetrahydropyranyl ether in comparison with N-MFI-80
during whole reaction time (Fig. S10A). The TOF and TON values of N-BEA*-110
(TOF = 111 x10-2, h-1, TON = 584) and N-MTW-110 (TOF = 89 x10-2, h-1, TON = 613)
were also higher than those of N-MFI-80 (TOF = 40 x10-2, h-1, TON = 428) (Fig. 5A).
On the other hand, the catalytic performance of the nanosponge zeolites showed a
notable difference in the tetrahydropyranylation of 2-adamantanol (Fig. S10B). N-
BEA*-110 (TOF = 74 x10-2, h-1, TON = 529) exhibited the highest catalytic activity,
presenting the higher TOF and TON values than N-MTW-110 (TOF = 27 x10-2, h-1,
TON = 370) and N-MFI-80 (TOF = 7.6 x10-2, h-1, TON = 224) (Fig. 5B). The results
indicate that the nanosponge MTW and especially beta zeolites showed a better
catalytic performance rather than the nanosponge MFI zeolite possessing the larger
amount of total acid sites (Figs 3 and S6). This observation can be ascribed to the
differences in the pore structure of zeolites used. While the beta zeolite possessed 3-
dimensionally (3D) intersecting pore structure composed of 12-ring channels, pore
structure of the MTW (1D, 12-ring) and MFI (3D, 10-ring) zeolites resulted in the
12

diffusional obstructions for reactant and product molecules. As a result, the catalytic
activities of the nanosponge beta, MTW and MFI zeolites can be said to be dependent
on their intrinsic pore structures, considering that the three nanosponge zeolites
possessed enough high external surface area and large mesopores for the
tetrahydropyranylation reaction of linear alkyl and bulky cyclic alcohols.
3.4. Recycling test
The recyclability of N-BEA*-110 was studied in the tetrahydropyranylation reaction
of 1-octanol with DHP. After the first kinetic run, the used sample was washed with
organic solvent (hexane and acetone) and then dried only at 423 K without further
activation at high temperature. The catalytic conversion in 2nd cycle decreased to 15.2 %
from that obtained in the 1st cycle (41.7 %), while the selectivity was maintained during
the recycling runs. The deactivation can be attributed to pore blocking by organic
compounds since the catalyst collected after the first reaction showed weight loss of 20 %
by thermal decomposition as confirmed by TGA analysis (Fig. S11). The collected
sample therefore was calcined at 853 K under air atmosphere to remove the organic
compounds before 3rd cycling test. The catalytic activity of the calcined catalyst could
be recovered to 96 % of original activity in the 3rd cycle. In addition, the recyclability
was preserved until the 4th cycle, exhibiting a negligible difference in conversion of 1-
octanol from the 3rd cycle. In accordance with the recyclability of N-BEA*-100, the
catalyst collected after 3rd or 4th cycle and subsequent calcination exhibited the
maintenance of the mesostructure and crystallinity as well as acid properties,
comparable to the fresh catalyst (Figs. S12 and S13). Therefore, the nanosponge beta
zeolite could be regenerated by the calcination, showing the recovered catalytic activity
in recycle test (Table 4).
4. Conclusions
The nanosponge beta zeolites with a meso-micro hierarchical structure were synthesized
using piperidinium-functionalized multi-ammonium surfactants as SDAs. The resulting
beta nanosponges possessed large surface areas and uniform mesopores. In the liquid-
phase tetrahydropyranylation of alcohols (methanol, 1-octanol, cyclohexanol and 2-
adamantanol) with 3,4-dihydro-2H-pyran, the beta nanosponges exhibited enhanced
catalytic performance with the higher conversion and initial reaction rate compared to
the conventional beta catalyst.
Remarkable enhancement of the catalytic activity of the nanosponge was found in the
tetrahydropyranylation reaction of bulky alcohol (e.g. 2-adamanthanol), which can be
attributed to high accessibility of the active sites and facile diffusion of reactant and
product molecules in the mesoporous beta zeolites. Furthermore, the catalytic behavior
of nanosponge beta zeolites with different Si/Al ratios showed a measurable relationship
with increase in the Al contents, evidencing the decrease of TOF and TON or increase in
the yield toward target ethers. Notably, the beta nanosponge exhibited superior catalytic
activity in the tetrahydropyranylation reaction of 1-octanol and 2-adamantanol in
13

comparison with mesoporous MTW and MFI zeolites, exhibiting higher initial reaction
rate. We postulate that the enhanced catalytic activity of beta nanosponge can be the
result of the intrinsic pore structure of the nanosponge beta zeolite (3D, 12-ring)
different from nanosponge MTW (1D, 12-ring) and MFI (3D, 10-ring) zeolites,
considering their high external surface areas, large mesopores and comparable acidic
properties.
Acknowledgements
Jꢁ thanks the Czech Science Foundation for the support of this research (P106/12/0189).
R.R. acknowledges the Institute for Basic Science (IBS-R004-D1).
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16

Fig. 1. SEM (A) and HR-TEM (B) images of N-BEA*-110, SEM image (C) of C-
BEA*-103, small and wide angle XRD patterns (D) of N-BEA*-110 (a) and C-BEA*-
103 (b), N2 adsorption-desorption isotherm (E) of N-BEA*-110 (a) and C-BEA*-103 (b)
and pore size distributions corresponding to the adsorption branch of N-BEA*-110 (a).
17

Fig. 2. Concentration of Brønsted and Lewis acid sites in nanosponge and commercial
beta zeolites determined by FTIR spectroscopy using pyridine as a probe molecule.
18

Fig. 3. Conversion of 1-octanol versus reaction time** (A) and TOF* and TON** (B)
obtained in the tetrahydropyranylation reaction of 1-octanol with DHP over N-BEA*-
110 and C-BEA*-103, respectively. The inset of (A) is conversion of 1-octanol over N-
BEA*-110 as a function of reaction time for determination of initial reaction rate (i. e.,
TOF*). Error bars (red lines) in (B) represent the deviation within 5% calculated from a
set of three different measurements. Reaction conditions: *5 mmol of 1-octanol, 15
mmol of DHP, 0.5 g of mesitylene (internal standard), 3 ml of hexane (solvent), 10 mg
of catalyst, 300 K. **5 mmol of 1-octanol, 78 mmol of DHP, 0.5 g of mesitylene
(internal standard), 50 mg of catalyst, 333 K.
19

Fig. 4. TOF* (A) and TON** (B) obtained in the tetrahydropyranylation reaction of
methanol, 1-octanol, cyclohexanol and 2-adamantanol with DHP over N-BEA*-15, N-
BEA*-53 and N-BEA*-110, respectively. Reaction conditions: *5 mmol of alcohol, 15
mmol of DHP, 0.5 g of mesitylene (internal standard), 3 ml of hexane (solvent), 10 mg
a
of catalyst, 300 K. **9 mmol of methanol, 15 mmol of DHP, 0.05 g of mesitylene
(internal standard), 10 ml of hexane (solvent), 25 mg of catalyst, 300 K, 24 h. 5 mmol
of b1-octanol, ccyclohexanol and d2-adamantanol, b78, c150 and d220 mmol of DHP, 0.5
g of mesitylene (internal standard), 50 mg of catalyst, 333 K, 24 h.
20

Fig. 5. TOF* and TON** obtained in the tetrahydropyranylation reaction of 1-octanol
(A) and 2-adamantanol (B) with DHP over N-BEA*-110, N-MTW-110 and N-MFI-80,
respectively. Reaction conditions: *5 mmol of alcohol, 15 mmol of DHP, 0.5 g of
mesitylene (internal standard), 3 ml of hexane (solvent), 10 mg of catalyst, 300 K. **5
a
b
a
b
mmol of 1-octanol and 2-adamantanol, 78 and 220 mmol of DHP, 0.5 g of
mesitylene (internal standard), 50 mg of catalyst, 333 K, 24 h.
21

Scheme 1. Tetrahydropyranylation of alcohols with DHP over acid catalysts.
22

Scheme 2. Possible reaction mechanism of the tetrahydropyranylation of alcohols over
zeolite catalysts.
23

Table 1. The reaction conditions for tetrahydropyranylation of the different kinds of
alcohol.
Alcohol
(mmol)
Mesitylene
(g)
Hexane
(ml)
DHP
(mmol)
Catalyst
(mg)
T
(K)a
methanol
(9)
0.05
0.5
0.5
0.5
10
15
25
50
50
50
300
333
333
333
1-octanol
(5)
not used 78
not used 150
not used 220
cyclohexanol
(5)
2-adamantanol
(5)
a Reaction temperature.
24

Table 2. Physicochemical properties of nanosponge zeolites (BEA*, MTW and MFI)
and conventional beta zeolite catalysts.
b
c
d
e
SBET
Sext
Vtot
Vmic
Catalyst
Si/Ala
(m2 g-1)
(m2 g-1)
(cm3 g-1) (cm3 g-1)
C-BEA*-20
C-BEA*-103
N-BEA*-15
N-BEA*-53
N-BEA*-110
20
580
550
770
850
790
430
610
58
0.29
0.29
1.30
1.40
1.30
0.42
0.69
0.28
0.25
0.14
0.18
0.18
0.12
0.13
103
15
77
580
590
540
290
540
53
110
N-MTW-110 110
N-MFI-80 80
b
aThe Si/Al molar ratio was determined by ICP-OES analysis. SBET is the BET surface
area obtained from N2 adsorption in the relative pressure range (P/P0) of 0.1-0.3. cSext is
d
the external surface area evaluated from the t-plot method. Vtot is total pore volume
e
obtained at P/P0 = 0.95. Vmic is the micropore volume, obtained from Ar adsorption at
P/P0 < 0.01. The Vmic was determined according to the NLDFT.
25

Table 3. TOF and TON obtained in the tetrahydropyranylation reaction of methanol,
cyclohexanol and 2-adamantanol with DHP over N-BEA*-110 and C-BEA*-103,
respectively.
Alcohol
Catalyst
TOF (x10-2, h-1)a
TON
827b
319b
571c
375c
529d
198d
N-BEA*-110
C-BEA*-103
N-BEA*-110
C-BEA*-103
N-BEA*-110
C-BEA*-103
259
97
Methanol
120
53
Cyclohexanol
74
2-Adamantanol
18
Reaction conditions: a5 mmol of alcohol, 15 mmol of DHP, 0.5 g of mesitylene (internal
b
standard), 3 ml of hexane (solvent), 10 mg of catalyst, 300 K. 9 mmol of methanol, 15
mmol of DHP, 0.05 g of mesitylene (internal standard), 10 ml of hexane (solvent), 25
c
d
c
mg of catalyst, 300 K, 24 h. 5 mmol of cyclohexanol and 2-adamantanol, 150 and
d220 mmol of DHP, 0.5 g of mesitylene (internal standard), 50 mg of catalyst, 333 K, 24
h.
26

Table 4. Recyclability test of N-BEA*-110 in the tetrahydropyranylation reaction* of 1-
octanol with DHP.
Cycle
1
Conversion (%)
41.7
15.2
39.9
38.7
2a
3b
4b
*Reaction conditions: 5 mmol of 1-octanol, 78 mmol of DHP, 0.5 g of mesitylene
nd
a
(internal standard), 50 mg of catalyst, 333 K, 1 h. For the 2 cycle, the used catalyst
rd
th
b
was washed with hexane and acetone and then dried at 423 K for 6 h. The 3 and 4
cycles were carried out with the catalyst calcined at 853 K for 4 h in air flow after using
in the previous reaction.
27