Insight into the active site nature of zeolite H-BEA for liquid phase etherification of isobutylene with ethanol

Article abstract of DOI:10.1039/c9ra07721a

The nature of active acid sites of zeolite H-BEA with different Si/Al ratios (15-407) in liquid phase etherification of isobutylene with ethanol in a continuous flow reactor in the temperature range 80-180 °C has been explored. We describe and discuss data concerning the strength and concentration of acid sites of H-BEA obtained by techniques of stepwise (quasi-equilibrium) thermal desorption of ammonia, X-ray diffraction, low-Temperature adsorption of nitrogen, FTIR spectroscopy of adsorbed pyridine and solid-state ²⁷Al MAS NMR. The average values of the adsorption energy of NH₃ on H-BEA were experimentally determined as 63.7; 91.3 and 121.9 mmol g^-1 (weak, medium, and strong, respectively). In agreement with this, a correlation between the rate of ethyl-Tert-butyl ether synthesis and the concentration of weak acid sites (E_NH3 = 61.6-68.9 kJ mol^-1) has been observed. It was concluded that the active sites of H-BEA for this reaction are Br?nsted hydroxyls representing internal silanol groups associated with octahedrally coordinated aluminum in the second coordination sphere.

Full text of DOI:10.1039/c9ra07721a

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Insight into the active site nature of zeolite H-BEA
for liquid phase etherification of isobutylene with
ethanol
Cite this: RSC Adv., 2019, 9, 35957
*
Nina V. Vlasenko,
and Peter E. Strizhak
Yuri N. Kochkin, German M. Telbiz, Oleksiy V. Shvets
The nature of active acid sites of zeolite H-BEA with different Si/Al ratios (15–407) in liquid phase
etherification of isobutylene with ethanol in a continuous flow reactor in the temperature range 80–
180 ꢀC has been explored. We describe and discuss data concerning the strength and concentration of
acid sites of H-BEA obtained by techniques of stepwise (quasi-equilibrium) thermal desorption of
ammonia, X-ray diffraction, low-temperature adsorption of nitrogen, FTIR spectroscopy of adsorbed
pyridine and solid-state 27Al MAS NMR. The average values of the adsorption energy of NH3 on H-BEA
were experimentally determined as 63.7; 91.3 and 121.9 mmol gꢁ1 (weak, medium, and strong,
respectively). In agreement with this, a correlation between the rate of ethyl-tert-butyl ether synthesis
Received 23rd September 2019
Accepted 31st October 2019
and the concentration of weak acid sites (ENH ¼ 61.6–68.9 kJ molꢁ1) has been observed. It was
3
DOI: 10.1039/c9ra07721a
concluded that the active sites of H-BEA for this reaction are Brønsted hydroxyls representing internal
silanol groups associated with octahedrally coordinated aluminum in the second coordination sphere.
rsc.li/rsc-advances
acid sites in zeolites constitutes a main factor determining the
catalytic activity and the selectivity of zeolites, and hence,
1. Introduction
a reliable method to quantify acidity is an importance. Several
instrumental techniques can be used for this purpose; among
them the ammonia thermal desorption (NH3 TPD) are typically
used.20–23 High basicity and low size of ammonia molecule allow
characterizing all acid sites independently on their nature,
localization, and strength. Other experimental techniques are
informative for evaluating the acid properties of the hydroxyl
groups in zeolites, e.g., calorimetric measurements of simple
probe molecules,24 titration of the surface with Hammett indi-
Zeolites are the most important solid acids as highly active and
selective catalysts in a number of commercial chemical
processes.1–4 The acidity allows protonic zeolites to have wide-
spread application as solid catalysts in a large range of chemical
processes, e.g., catalytic cracking,5–7 isomerization,8–10 and
alkylation.11–13 The catalytic activity of zeolites is mainly asso-
ciated with the Brønsted acid and (or) Lewis acid sites, and
a result of the replacement of Al for Si in the tetrahedral units,
which generates a net negative charge that has to be balanced by
extra framework cations or protons. The concentration of acid
sites depends on the Si/Al ratio and their acid strength is
believed to be dependent on the structure of the three-
dimensional network as well as on the local atomic environ-
ment of the site.14 The active sites of zeolites are situated in
pores and channels of different dimensions as well as on the
external surface of the microcrystals in the catalytic bed.
Depending on their location, acid sites in zeolites might be
hidden and unavailable to participate in catalytic reactants. The
recent developments and trends in tailoring the nature and
local properties of active sites in zeolite-based catalysts were
reviewed in ref. 15 and 16.
27
cators,25,26 Al MAS NMR,27–29 13C NMR chemical shis of
adsorbed acetone,30 FTIR spectroscopic studies,31–37 ammonia
infrared/mass spectroscopy (IRMS) temperature-programmed
desorption (TPD).38,39
Recently, large pore HBEA zeolites became a subject of
numerous studies in catalysis. Zeolite b (BEA) has a three-
dimensional channel system with 12-membered ring aper-
tures of 5.6–7.3 A and high external surface area.40,41 Zeolite BEA
˚
is more active than other zeolites especially if the processing of
bulk molecules is needed. The small crystal size of BEA zeolites
(20–50 nm) causes a short diffusion path of reactants and
products, and therefore transport limitations do not play
perceptible role in the catalytic reactions.42 Due to the
successful combination of structural and acid characteristics,
namely, open pore structure and high acidity,43,44 the protonic
form of BEA is an active solid acid catalyst for many kinds of
The catalytic performance of zeolites depends in a different
way on the strength, concentration, and structure of their
Brønsted acid sites.17–19 The concentration and strength of the
industrially
important
reactions,
like
acylation,45,46
L. V. Pisarzhevskii Institute of Physical Chemistry, National Academy of Sciences of
Ukraine, Prospect Nauki 31, 03039 Kiev, Ukraine. E-mail: nvvlasenko@gmail.com
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alkylation,47–49 esterication,45,50,51 etherication,52,53 isomeriza- commercial ion-exchange resin Amberlyst-15 that has been
tion,54,55 dehydration56,57 etc.
explained as a result of the adsorption of methanol on the
In last decades the etherication of alcohols, especially of silanols of the zeolite BEA.80 Contrary, zeolites HY and HZSM-5
bioethanol which is considered as a renewable crude, becomes showed substantially lower activity in the MTBE synthesis due
important commercial processes for the production of a variety to the methanol is mainly adsorbed on the bridging OH groups.
of oxygenates. Oxygenated compounds are known as octane
Another example of studying role of weak acid sites is pre-
improvers for liquid fuels.58,59 Their use as additives to fuel sented by the effect of zeolites acidity on their catalytic activity
reduces the exhaust of carbon monoxide and unburned in etherication of 2-naphthol with ethanol over H-beta, H-
hydrocarbons, minimizing the emission of volatile organic MOR and H-ZSM-5.81 t was expected that the etherication
compounds.60,61 In industry, alkyl tert-butyl ethers are produced activity of zeolites may be affected by the weak acid sites. In
using various sulfonic cation-exchange resins as catalysts.62 cyclohexanone oxime Beckmann rearrangement reaction
They are characterized by high activity and selectivity, however, MgSiAlPO-5 molecular sieve having weak acidity demonstrated
they have certain drawbacks: low mass exchange characteris- considerably superior performance than the relatively stronger
tics, poor heat resistance, and tendency to swell under the acidic MgAlPO-5 catalyst.82
action of polar reagents, which causes technological disadvan-
tages of their practical use.
It has been found that H-BEA zeolites with Si/Al ratios of 12.2
and 36 were more active in the MTBE synthesis than industrial
The worldwide production and consumption of the most catalyst Amberlyst-15 at temperatures between 40 and 100 ꢀC.80
popular fuel additive MTBE have sharply decreased for envi- The most active H-BEA sample with Si/Al ¼ 36 contains a higher
ronmental reasons, whereas production of oxygenates from bio- concentration of silanols which contribute to the adsorption of
resources, particularly, ethyl tert-butyl ether (ETBE), is growing. the alcohol. However, no correlation between acid and the
ETBE exhibits higher octane rating, higher boiling point, lower catalytic characteristics was found.
ash point, lower blending Reid vapor pressure, and reasonably
In our previous work, we have established a crucial role of
high oxygen contents.63,64 Moreover, the attraction of the ETBE the weak acid sites of zeolites for their catalytic behavior in the
industrial application is dened by the possibility to use bio- ETBE synthesis.83 However, the nature of these sites was not
ethanol, and also to obtain gasoline with reduced volatility.
clear. In this study we present the results to highlight the nature
Various acid catalysts have been studied in ETBE synthesis of the weak acid sites of H-BEA zeolites which are active in the
from ethanol and isobutylene, particularly, zeolites,65,66 macro- etherication of isobutylene with ethanol. For this purpose, H-
porous sulfonic acid resins,67,68 and heteropolyacids.69 ETBE BEA zeolites with a wide range of Si/Al ratios were studied. The
synthesis is commercially operated in the liquid phase over ammonia Quasi-Equilibrium ThermoDesorption (QETD) has
various types of acidic ion-exchange resins.70 Typically, these been chosen to characterize both concentration and strength of
catalysts are presented as sulfonated resins with styrene– acid sites. Comparison of these data with results of the catalytic
divinylbenzene matrix. The catalytic efficiency of sulfonic acid tests allows us to establish a relationship between acidic and
groups located both on the polymer surface and inside the grain catalytic characteristics for H-BEA zeolites. Moreover, these
is governed by their accessibility to molecules of reagents. results highlight an effect of the Si/Al ratio on the catalytic
Therefore, both the mass-transfer process and the adsorption– performance of zeolite H-BEA. These data supported by FTIR
desorption dynamics should be important factors inuencing spectroscopy of adsorbed pyridine and solid-state 27Al MAS
the catalytic behavior in etherication processes. These prop- NMR gives an understanding of the nature and structure of the
erties are determined by morphology, porous structures and weak acid sites which are active in the etherication of isobu-
size of catalyst grain.71 Decreasing the optimum temperature of tylene with ethanol.
the ETBE synthesis with simultaneous increase of efficiency has
appeared possible under the condition of maintaining the
accessibility of zeolite acid sites.72–75
2. Experimental
2.1. Materials
Usually, the activity of zeolites catalyst in acid-catalyzed
reactions binds to the total amount of acid sites. In ETBE
synthesis over zeolite beta with different Si/Al, it was found that
the yield at maximum isobutylene conversion is proportional to
the external surface area.66 However, the role of strong and weak
acid sites in a catalytical transformation of the reactants on BEA
surface has not been claried.
Commercially available ethanol (azeotropic mixture with
4.43 wt% H2O), isobutylene, and helium were used as
purchased without additional purication. For investigation of
catalysts acidity commercially available ammonia was dried by
passing through the trap with NaA zeolite.
Impact of weak acid sites of zeolites on the catalytic behav-
iors has gathered a lot of attention, mainly as it was considered
2.2. Catalysts preparation
in the aspect of their non-acidity. However, recently it has been The synthesis of the BEA zeolite using hydrothermal template
shown that external silanols in zeolites have weak acidic synthesis was performed as follows. In typical synthesis, solu-
nature.76,77 The signicance of weak acid surface groups in some tion containing 0.044 g of LiOH and 0.149 g of Al(NO3)3 were
catalytic reactions occurring in so conditions was reported.78,79 dissolved in 6.7 ml of deionized water and were mixed with
The good performance of zeolite BEA in MTBE synthesis from 6.51 ml of 35% tetraethylammonium hydroxide. The mixture
methanol and isobutylene showed activity comparable to the was maintained at room temperature for 30 min and then
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dropwise added 2.83 ml of colloidal solution SiO2 (Ludox HS- was achieved (typically 10–15 min). As a result, experimental
40). The reaction mixture was transferred into a Teon-lined conditions corresponded to desorption at constant pressure, i.e.
ꢀ
stainless steel autoclave and hydrothermally treated at 150 C desorption isobar. The concentration of each type of acid sites
for 4 days. Resulting product was ltered, washed with distilled was calculated from the amount of ammonia sorption (mmol
water, dried at 80 ꢀC for 10 h and calcined at 550 ꢀC for 5 hours. gꢁ1) in a certain temperature range determined by the form of
H-BEA zeolites were obtained by ion exchange with 1 M NH4NO3 the QETD curve. The total concentration of the acid sites (acid
aqueous solution at 80 ꢀC under stirring. The resulting solid capacity) was determined as a quantity of ammonia adsorbed
was ltered, washed with deionized water, dried in air at 100 ꢀC on a sample surface at 50 ꢀC.78,83 The obtained QETD data allows
for 10 h, and calcined at 450 ꢀC for 5 h. The different Si/Al ratio one to estimate not only the total acid capacity as an amount of
in samples was regulated by changing of SiO2/Al2O3 molar ratio ammonia adsorbed in all temperature range but also amounts
of in reaction mixture. Marking of samples corresponds to the of weak and strong sites of various strength.
Si/Al ratio in zeolites.
The acidity of all zeolites was investigated by adsorption of
pyridine used as a probe molecule followed by FTIR spectros-
copy. Generally studied zeolites were activated in a vacuum (5 ꢂ
10ꢁ3 torr) at 450 ꢀC during 2 h. The adsorption and desorption
of pyridine was carried out in hand-made IR cell with NaCl
2.3. Sample characterization
The silicon and aluminum content was determined by standard
methods with an error less than 2%.84,85 Crystallinity and phase
purity of solids were determined by XRD. XRD patterns weꢁre1
recorded in the 2q range of 5–50ꢀ with a scan speed of 2ꢀ min
on a Bruker AXS D8 ADVANCE diffractometer in the Bragg–
Brentano geometry using CuKa radiation (l ¼ 0.1542 nm, 40 kV,
20 mA) with a NaI dynamic scintillation detector. Nitrogen
adsorption isotherms were performed at ꢁ196 ꢀC in the 0.001–1
relative pressure range using Sorptomatic 1990 apparatus
(Thermo Electron Corporation). Prior to the adsorption
measurements, all samples were degassed in owing N2 for 5 h
at 300 ꢀC. The surface area (SBET) was calculated by the standard
BET method using nitrogen adsorption data in the relative
pressure range from 0.01 to 0.10 because deviations from line-
arity of the BET plot were observed at relative pressures below
0.01 and above 0.10. The micropore volume (Vmi) was calculated
from the cumulative pore size distribution as the volume of
pores with sizes less than 2 nm.
ꢀ
windows at temperature 150 C (for selected samples – in the
ꢀ
ꢀ
temperature range of 150–500 C with step 50 C) and investi-
gated on a Spectrum One (PerkinElmer) spectrometer with
a resolution of 1 cmꢁ1. The concentration of Brønsted and
Lewis type acid sites on Py-IR data was determined directly from
the integral intensity of the corresponding bands in the IR-
spectra (in adsorption mode) aer adsorption of pyridine
using the equations derived by Emeis.86
2.5. Catalytic measurement
The reaction of the ETBE synthesis was investigated in a ow
reactor with a xed bed. Ethanol was measured out from vessel
standing under pressure 1.0 MPa with the help of dropwise
sampler and “Whitey” needles. The space velocity of carrier gas
(helium) was regulated by a needle and was xed by a differen-
tial manometer. The temperature in the reactor was established
and controlled by the controller/programmer type 812 “EURO-
THERM” (Great Britain). The pressure in the reactor and in the
vessel with ethanol was measured by model manometers.
Catalyst loading (volume of catalyst) was 1.0 cm3, grain size
0.25–0.50 mm. Molar ratio ethanol/isobutylene was 1.5, LHSV 1
hꢁ1, carrier gas helium. The pressure was always 1.0 MPa.
Studies were performed in the temperature range of 80–180 ꢀC.
Reaction products were amassed in a trap and analyzed by gas
chromatograph Agat (“Mashpriborkomplekt”, Russia) equipped
with Chromaton N-AW column with 10% Carbowax 600 (3 mm
i.d., 2 m length) and a thermal conductivity detector.
27Al Magic Angle Spinning Nuclear Magnetic Resonance
(MAS-NMR) spectroscopy measurements were carried out using
an “Avance 400” (“Bruker”) spectrometer equipped with
a multinuclear probe, with 4 mm zirconia rotors spinning at 10
kHz. A single pulse sequence was used, with a pulse length of 15
ms, an observation frequency for 27Al of 52.12 MHz. The
chemical shis (d, ppm) are reported in ppm from 0.1 M AlCl3
solution.
2.4. Acidity measurement
Acid properties of samples were investigated by Quasi-
Equilibrium Thermodesorption (QETD) of ammonia.78,83
Experiments were carried out at 50–500 ꢀC under vacuum
(constant pressure 0.133 Pa was maintained by vacuum pump)
using thermogravimetric apparatus with quartz spring balance
of McBain type. Prior to the experiment, the sample was out-
gassed at the temperature of its calcination during a time
sufficient to reach a constant weight of the sample (typically
1.5–2 h). Then, the temperature was lowered to ambient
The catalytic activity of zeolites for ETBE formation was
characterized by the apparent ETBE formation rate, measured
as the amount of ETBE formed per unit time per unit mass of
catalyst (mol sꢁ1 gꢁ1).
3. Results and discussion
3.1. Structure and composition of zeolites H-BEA
conditions and ammonia was admitted into sample stepwise The XRD patterns of H-BEA zeolites is shown in Fig. 1. The XRD
(10–20 torr) until no uptake was observed. Finally, an excess of patterns of all H-BEA samples are identical, indicating no
ꢀ
ammonia was evacuꢁa1ted at 50 C. The temperature was raised structural changes under the Si/Al ratio variation within the
stepwise (5 ꢀC min between steps, step size 50 ꢀC). Aer studied range. All the solids are highly crystalline and no
raising, the temperature was kept xed until constant weight reections other than those from BEA zeolite are observed. The
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Table
1
The composition and texture characteristics of H-BEA
zeolites
Al content, CAl
Sample
mmol gꢁ1 % wt Si/Al SBET, m2 gꢁ1 Vmicro, cm3 gꢁ1
H-BEA 15
H-BEA 32
H-BEA 72
H-BEA 124 0.13
H-BEA 407 0.04
1.06
0.50
0.23
2.87
1.35
0.62
0.36
0.11
14.8 558
32.4 612
71.5 632
123.9 646
407.4 670
0.21
0.21
0.20
0.20
0.19
Fig. 1 XRD patterns for H-BEA zeolites: (1) H-BEA 15, (2) H-BEA 32, (3) areas of intensive weight loss due to the ammonia desorption
H-BEA 72, (4) H-BEA 124; (5) H-BEA 407.
and the horizontal sections where ammonia desorption is not
observed. Using CONTIN method allows to calculate the
ammonia adsorption energy for acid sites of studied
zeolites.78,83 The corresponding ammonia adsorption energy
distribution proles for H-BEA samples are shown in Fig. 4,
highlighting the energetic heterogeneity of acid sites. These
proles clearly show that generally the acidity spectrum of BEA
zeolites includes three types of sites of different strengths:
weak, medium, and strong. The data on the concentration and
strength (ammonia adsorption energy) for each type of sites are
presented in Table 2.
Increasing the Si/Al ratio for H-BEA results in decreasing the
total acid capacity, which is in a good agreement with a general
tendency that acid capacity for zeolites, is caused by the pres-
ence of aluminum in their structure. However, it is worth noting
that the total acid capacity of H-BEA samples determined by
ammonia QETD signicantly exceeds the corresponding value
that may be calculated from the aluminum content as it follows
from a comparison of the data presented in Tables 1 and 2.
Therefore, a noticeable portion of acid sites in H-BEA is not
related directly to the presence of aluminum in the zeolite
framework. Probably, these weak acid sites correspond to sila-
nol groups on the surface of the zeolite.
XRD patterns of all zeolites match perfectly those reported in
the literature for zeolite BEA.87,88
Nitrogen adsorption isotherms for all studied H-BEA zeolites
are characteristic of a microporous material lacking secondary
micropores and mesopores. The nitrogen adsorption isotherms
of H-BEA zeolites is shown in Fig. 2.
The most part of nitrogen adsorption isotherms (up to p/ps
about 0.9) belongs to type I of IUPAC classication, which is
characteristic of microporous adsorbents.89 Small hysteresis of
type H3 together with increasing adsorption of nitrogen at high
relative pressures with no limiting uptake is characteristic to
aggregates of particles. The uptake of nitrogen at high relative
pressures contains about one-third of total nitrogen adsorption
for H-BEA samples. The composition of H-BEA samples and
their textural properties are listed in Table 1.
The BET surface area for H-BEA samples is found in the
range of 558–670 m2 gꢁ1 and the microporous volume of crys-
talline zeolites samples is in the range of 0.19–0.21 cm3 gꢁ1. The
BET surface area shows a tendency to increase with decreasing
the Al content, whereas the micropore volume slightly decrease.
Our studies give the ammonia adsorption energy in the
range of 59–69 kJ molꢁ1 for the weak acid sites which is close to
the ammonia adsorption energy of 58.4 kJ molꢁ1 calculated for
3.2. Acid capacity and strength
QETD curves of ammonia desorption for H-BEA samples are
shown in Fig. 3. They have a complicated character with several
Fig. 2 N2 adsorption isotherms for H-BEA zeolites: (1) H-BEA 15, (2) H- Fig. 3 QETD-curves of ammonia thermal desorption from a surface of
BEA 32, (3) H-BEA 72, (4) H-BEA 124; (5) H-BEA 407.
H-BEA zeolites.
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the heat of the ammonia adsorption decreases slightly with
increasing the Si/Al ratio. These sites are absent in zeolite H-
BEA-407.
3.3. FTIR study
The IR spectra of adsorbed pyridine for all samples in the 1400–
1650 cmꢁ1 range are shown in Fig. 5. They present the bands
corresponding to formation of protonated, PyH+ (bands at 1637,
1490 and 1547 cmꢁ1), coordinated, PyL (bands at 1621, 1490,
1445 and 1455 cmꢁ1) and H-bonded (bands at 1600 and
1575 cmꢁ1) pyridine species.32,92,93 According to94 the band at
1445 cmꢁ1 can be assigned to pyridine bonded to a relatively
stronger Lewis site formed on three-coordination or extra-
framework Al species. The band centered at 1490 cmꢁ1
displays pyridine adsorbed on both the Lewis and Brønsted acid
sites.
The concentrations of Brønsted and Lewis acid sites were
calculated based on the published extinction coefficients86 for
Fig. 4 Ammonia adsorption energy distribution for H-BEA zeolites.
the silanol groups in silica.90 The higher ammonia adsorption
heat for silanol groups of zeolites, compared with silanols in
silica, may be associated with the induced action of
aluminum.21,91 An increase of the aluminum content for H-BEA
zeolite is accompanied by an increase in ENH for weak acid sites
3
as it follows from the data presented in Table 2.
According to the data presented in Table 2, the heat of
ammonia adsorption at zero coverage is in the range of 120–
150 kJ molꢁ1 which is in a good agreement with data obtained
for zeolite H-BEA by a microcalorimetric method.24 The heat of
ammonia adsorption at zero coverage depends nonmonotoni-
cally on the Si/Al ratio. It reaches the maximum value for H-BEA-
32.
The same tendency also holds for the heat of ammonia
adsorption at strong acid sites. This result is in agreement with
previously reported data for the heat of ammonia adsorption
dependence on Si/Al ratio for H-ZSM-5 zeolite that showed
a maximum strength for Si/Al in the range between 18 and
35.21,24 The strength of medium acid sites changes non-
monotonically too. The strength of weak acid sites presented by
Fig. 5 FTIR spectra of pyridine adsorption for H-BEA zeolites with
various Si/Al ratio: (1) H-BEA 15, (2) H-BEA 32, (3) H-BEA 72, (4) H-BEA
124, (5) H-BEA 407.
Table 2 Acid characteristics of H-BEA samples
Si/Al
Parameter
Acid sites
15
32
72
124
407
Aluminum content, mmol gꢁ1
Acid capacity, mmol gꢁ1
1.06
1.64
0.59
0.47
2.70
68.9
99.1
115.9
130
0.5
0.23
0.96
0.97
0.14
2.07
61.6
94.3
129.8
142
0.13
0.27
0.54
0.67
1.48
59.6
89.7
121.4
133
0.04
0.00
0.45
0.45
0.90
—
86.9
112.4
120
w
m
s
Total
w
m
s
0.83
0.36
0.84
2.03
64.8
86.2
130.0
150
Average NH3 adsorption heat, kJ molꢁ1
Zero coverage
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the bands at 1545 cmꢁ1 and, 1455 cmꢁ1 correspondingly. Fig. 6
gives these concentrations as a function of the aluminum
relative content in zeolites H-BEA. Increasing aluminum
content increases the concentrations of both Brønsted acid sites
and Lewis acid sites.
The FTIR spectra in the hydroxyl range for different samples
are show in Fig. 7. They present a clearly distinguishable bands
at 3745 and 3735 cmꢁ1 and a broad shoulder about 3680 cmꢁ1
.
The most intensive bands observed at 3745 and 3735 cmꢁ1
may be attributed to external and internal silanol groups
respectively. The band at 3735 cmꢁ1 corresponds to silanol
groups localized inside the zeolite cavities, and the band
3745 cmꢁ1 corresponds to terminal silanol groups located on
the exterior surface.33,95 The band centered at 3680 cmꢁ1
assigned to amorphous extra-framework A1–O–OH species.95
The IR spectra for the most of studied zeolites contain clearly
resolved band at 3735 cmꢁ1. The intensity of this band
decreases with an increase in the Si/Al ratio up to 124. Contrary
to all other samples, the band at 3735 cmꢁ1 disappears in the
FTIR spectra for H-BEA-407, whereas band at 3745 cmꢁ1 is
clearly resolved. Therefore, exterior surface and it does not
contain internal silanol groups localized inside zeolite cavities.
The band at 3745 cmꢁ1 is also presented as a shoulder in the
spectrum of H-BEA-124. The small broad shoulder in FTIR
spectra in the region between 3600 and 3700 cmꢁ1 is probably
associated with OH groups bonded to extra-framework
aluminium.95–97 These groups are presented in the samples
with high Si/Al ratio (H-BEA-124 and H-BEA-407).
Fig. 7 FTIR spectra of the hydroxyl region of the zeolite H-BEA
samples with various Si/Al ratio: (1) H-BEA 15, (2) H-BEA 32, (3) H-BEA
72, (4) H-BEA 124; (5) H-BEA 407.
distorted tetra-coordinated framework aluminum as a result of
strong quadrupole interactions.101–103 NMR spectra also contain
a small signal at around 0 ppm, associated with the octahedrally
coordinated extra-framework aluminum.99,104 Fig. 8 shows that
the band of 0 ppm, corresponding to octahedrally coordinated
aluminum, present in all samples except H-BEA 407.
3.4. 27Al MAS NMR study
3.5. ETBE synthesis over H-BEA zeolites
27Al MAS spectra of all hydrated solids are presented in Fig. 8
where intensity is given in arbitrary units.
All samples show catalytic activity in the ETBE synthesis except
H-BEA-407. The products of the catalytic transformations of
ethanol and isobutylene over H-BEA zeolites are ETBE and tert-
butyl alcohol (t-BuOH). Appearance of t-BuOH in reaction
products is a consequence of the water present in the reaction
mixture as a part of the azeotropic ethanol-water mixture. At
For almost all samples, 27Al MAS NMR spectra are charac-
terized by two main signals at ꢃ54–58 and 0 ppm which may be
assigned to tetrahedral and octahedral Al atoms, respec-
tively.98–100 The broad peak at 55 ppm may be attributed to the
Fig. 8 The 27Al MAS spectra demonstrate a signal of octahedrally
Fig. 6 Dependence of concentrations of Brønsted and Lewis acid coordinated aluminum detected in the H-beta-zeolites: 1–H-BEA 15;
sites of H-BEA zeolites on aluminum relative content.
2–H-BEA 72; 3–H-BEA 407.
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ꢀ
temperatures above 120 C, over H-BEA 32 and H-BEA 72, the
diethyl ether (DEE) and di-isobutylene formation is observed.
Fig. 9 and 10 shows the temperature dependences of iso-
butylene conversion and ETBE selectivity for studied H-BEA
samples. Zeolite H-BEA-15 shows the highest catalytic activity,
whereas zeolite H-BEA-124 exhibits very low activity. It is worth
noting that zeolite H-BEA-407 does not show any catalytic
activity for all temperature range.
Catalytic behavior of zeolites H-BEA signicantly depends on
the Si/Al ratio. Increasing the Si/Al ratio results in decreasing
the iso-butylene conversion that is accompanied by an increase
in the optimal temperature of the process. Increasing temper-
ature above 100–120 ꢀC decreases the i-C4H8 conversion due to
the thermodynamics limitations resulting from the process
exothermicity.101 For samples of H-BEA 15, H-BEA 32 and H-BEA
72 the ETBE selectivity coincides closely. Wherein, ETBE selec-
tivity sharply decreases above 120 ꢀC due to the contribution of
the side reactions of DEE and di-isobutylene formation. The
low-active H-BEA-124 catalyst is characterized by the 100%
ETBE selectivity; however, the conversion values for this sample
do not exceed 1%.
The porous structure of the zeolite catalysts is almost the
same as it follows from the XRD studies and nitrogen adsorp-
tion data. The catalytic experiments were carried out under the
same conditions for each sample. Therefore, the apparent ETBE
formation rate (mol sꢁ1 gꢁ1) gives a comparative characteriza-
tion of the catalytic activity of the studied samples. Fig. 11
shows a temperature dependence of the apparent ETBE
formation rate for the studied samples.
Fig. 10 Temperature dependence of ETBE selectivity for H-BEA
zeolites.
temperatures, the reaction of the ETBE formation follows the
thermodynamic equilibrium.105,106
A linear correlation is observed for zeolites H-BEA-15, H-BEA-
32, and H-BEA-72, i.e. for zeolites with low Si/Al ratio. Increasing
the Si/Al ratio results in a sharp falling down the activity for
zeolites H-BEA-124 and H-BEA-407.
Fig. 12 illustrates that catalytic activity increases with
increasing the concentration of the weak acid sites. The data
presented in Table 2 show that increasing the concentration of
the weak acid sites is accompanied by an increase in the total
concentration of acid sites. As a result, the apparent ETBE
formation rate of zeolites H-BEA in the ETBE synthesis
increases with increasing the total concentration of acid sites.
However, this is just a coincidence resulted from similar
dependencies of both, apparent ETBE formation rate and the
total concentration of acid sites, on the concentration of the
weak acid sites. Our results show the absence of the catalytic
3.6. A relation between acid and catalytic properties for
zeolites H-BEA
Fig. 12 gives the dependence of the apparent ETBE formation
rate on the concentration of w-sites for zeolites H-BEA. The
catalytic activity and acidity of H-BEA zeolites were compared at
the temperature of 100 ꢀC because at this temperature only one
side reaction, the tert-butanol formation, takes place. At higher
Fig. 9 Temperature dependence of i-C4H8 conversion for H-BEA Fig. 11 Temperature dependence of apparent ETBE formation rate for
zeolites.
zeolites H-BEA.
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second coordination environment.24 The proton mobility in
silanol groups may be enhanced by the presence of aluminum
in zeolite structure. The presence of aluminum in the rst
coordination sphere of silanol group forms strong bridging
Brønsted site. If it presents in the second coordination sphere,
it also affects the proton mobility in silanol group. As a result,
the corresponding silanol group exhibits properties of a weak
Brønsted acid site.
The performed studies allow us to suggest the structure of
the active site which is responsible for the catalytic activity of
zeolites H-BEA in the isobutylene etherication with ethanol.
This structure is shown in Scheme 1. This weak Brønsted acid
site represents a silanol group with tetrahedral aluminum in the
second coordination sphere. Its occurrence is accompanied by
the formation of octahedral aluminum species. These struc-
tures can form by the partial hydrolysis of Si–O–Al bonds during
the calcination, when the strong Brønsted acid site (bridging
OH group) breaks up into the Lewis acid site (tri-coordinated
aluminum) and the weak Brønsted acid site (Si–OH
group).100,110,111 This process is illustrated in Scheme 2.
Fig. 12 Correlation of apparent ETBE formation rate on the concen-
tration of weak acid sites of zeolites H-BEA.
activity for the sample H-BEA-407 which does not contain the
weak acid sites.
We note the corresponding samples contain octahedral
aluminum what is in a good agreement with our 27Al MAS NMR
studies that showed the presence of octahedral aluminum in all
3.7. The nature of active sites of zeolite H-BEA
A correlation between the catalytic activity and the concentra- samples except catalytically inactive H-BEA-407.
tion of weak acid sites supports that the weak acid sites are
The proposed structure of the active site is also in a good
mainly responsible for the catalytic performance of zeolites H- agreement with the IR study. The band near 3735 cmꢁ1 corre-
BEA. Catalytic studies illustrate that zeolite H-BEA-407 does sponds to OH groups bonded to Si atoms linked to three-
not show catalytic activity. According to the QETD studies, coordinated Al atoms.97 Its integral intensity correlates with
zeolite H-BEA-407 does not contain weak acid sites as it follows the aluminum content (see Fig. 13). The linear correlation
from the data presented in Table 1. Therefore, the catalytic between the band intensity at 3735 cmꢁ1 and the concentration
activity of zeolites H-BEA in the isobutylene etherication with of aluminum is observed for the all zeolites with Si/Al ratio up to
ethanol is associated with the weak acid sites. The ammonia 124. This band corresponding to internal silanol groups totally
QETD studies show that zeolites H-BEA may contain three types disappear for zeolites with low aluminium content, Si/Al > 124.
of acid sites, which are typical for zeolites.21,83 The strong acid
A disappearance of the band at 3735 cmꢁ1 in IR spectra at Si/
sites, apparently, can be assigned to the Lewis sites representing Al ratio less than 124 can be explained from the NMR data.100 It
three-coordinated aluminum atoms. The sites of medium has been shown that the amount of octahedral aluminium
strength can be assigned to the “classic” Brønsted acid sites species decreases with increasing Si/Al ratio and octahedral
represented by the bridging hydroxyl groups, Si(OH)Al.21,107 The aluminium was absent for the high-silica H-BEA samples.
nature of weak acid sites is less clear. For the weak acid sites, According to the Loewenstein's rule, two aluminium sites,
our data give the ammonia adsorption energy in the range of which are adjacent or close to each other, are required for the
59–69 kJ molꢁ1. The range of the ammonia adsorption energy is hydrolysis of a Si–O–Al bond for the formation of octahedrally
in agreement with the reported value of 62 kJ molꢁ1 for the coordinated aluminium. Therefore, observed dependence
Brønsted acid sites of zeolite H-BEA.108 Therefore, we may (Fig. 13) can conrm the identication of the absorption band
assume that the weak acid sites are associated with hydroxyl of 3735 cmꢁ1 with the acid sites associated with octahedral
groups.
Comparison of the QETD and FTIR studies for zeolites H-
aluminum.
A presence of the aluminum also promotes the proton
BEA suggests that the band at 3735 cmꢁ1 corresponds to weak mobility which shows the correlation between the strength of
acid sites which are presented in all samples except H-BEA-407
as it follows from the data presented in Table 2. According to
data presented in the literature,33,95 the band at 3735 cmꢁ1
corresponds to silanol groups localized inside the zeolite cavi-
ties, i.e. to internal silanol groups. It has been reported that the
internal silanol groups display a signicant Brønsted acidity,
which is higher compared to the acidity of silanol groups
located at the external crystal surface.33,97,109 Moreover, acidic
properties of the surface functional groups are dependent on
Scheme 1 Schematic drawings of the structure of weak acid sites of
the nature of the neighboring atoms, and even atoms of the H-BEA zeolites.
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Scheme 2 Schematic drawings of the formation of weak acid sites of H-BEA zeolites.
A formation of the weak acid sites with the discussed
structure is accompanied by a formation of three-coordinated
aluminium species. According to a number of works103,111,113
a signal at 0 ppm in NMR spectra associated with the octahedral
aluminum in zeolites structure represents an extra-framework
aluminum. Our suggestions about the formation of active
sites of H-BEA zeolites allows us to speculate on the role of the
extra-framework aluminum in catalytic behavior of zeolites
which has been discussed in several publications and still did
not nd a logical explanation. Studies have shown a positive
effect of the extra-framework aluminium on the catalytic activity
of zeolites. However, a pronounced correlation was not found
between the catalytic activity and the concentration of the extra-
framework aluminium.114–116 We suggest that the extra-
framework aluminum is a “by-product” of the formation of
proposed weak acid sites. As a result, the apparent catalytic
activity may be associated with a presence not of the extra-
framework aluminum but with the silanol groups in a specic
environment. These silanol groups are responsible for the weak
Brønsted acidity and for the catalytic activity of zeolites in the
etherication of isobutylene with ethanol.
Fig. 13 Correlation between the 3735 cmꢁ1 IR band intensity and the
aluminum content.
weak acid sites and the aluminum content in H-BEA zeolites
(Fig. 14). A similar effect was recently reported for ZSM-5
zeolites with varying Si/Al ratio.112 The observed correlation
also supports the proposed structure of weak acid sites of
zeolites H-BEA illustrating the aluminium effect on a sharp
decrease in strength of the weak acid sites with decreasing the
aluminium content in the zeolite.
4. Conclusions
Decreasing the aluminum content results in lowering
a probability to nd two adjacent or closely located aluminum
atoms in the zeolite structure. Consequently, the proposed
structure of the active site cannot be realized if the aluminum
content is too small.100
In the present work, we determined the nature of the acid sites
of the H-BEA zeolites responsible for the heterogeneous cata-
lytic synthesis of ethyl tert-butyl ether from ethanol and isobu-
tylene. QETD and catalytic studies emphasize that etherication
of isobutylene with ethanol over zeolites occurs with the
participation of weak Brønsted acid sites, which are character-
ized by an ammonia adsorption energy in the range of 59–
69 kJ molꢁ1. The adsorbed pyridine FTIR studies and NMR data
suggest that these weak acid sites are identied as the Brønsted
hydroxyls representing internal silanol groups. We concluded
that the structure of the active site forms by the partial hydro-
lysis of the Si–O–Al bonds during the zeolite calcination and
accompanied by the formation of particles of three-coordinated
aluminum. This is proved by the correlation of absorption band
3735 cmꢁ1 intensity in FTIR spectrum with the aluminum
content. The aluminum content also has an impact on the
strength of weak acid sites. The disappearance of the weak acid
sites for zeolites H-BEA with low aluminum content is associ-
ated with the absence of two nearby aluminum atoms in zeolite
framework. Hence, the disruption of linear correlation of cata-
lytic activity and weak acidity for high-silica H-BEA zeolites
results from the difficulty of formation of internal silanol
groups at low aluminum content. Our correlations show an
Fig. 14 The dependence of the strength of weak acid sites on
aluminum content in H-BEA zeolite.
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importance of the Brønsted weak acid sites in the catalytic
performance of zeolites. The weak acid silanol groups may play
an important role not only in the etherication reactions but
also in the alkylation and acylation processes, which occur
under so conditions and do not require a break of chemical
bonds in molecules.
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Conflicts of interest
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There are no conicts to declare.
Acknowledgements
The work is partially supported by the Target Integrated
Programs of Fundamental Researches of the National Academy
of Sciences of Ukraine “Fundamental Problems of New Nano-
materials and Nanotechnologies” (grant 29/18-H). Authors are
grateful to Prof. A. M. Puziy (Institute for Sorption and Problems
of Endoecology, National Academy of Sciences of Ukraine) for
the CONTIN calculations, Dr V. V. Trachevskii (Technical
Center, National Academy of Sciences of Ukraine) for the MAS-
NMR spectroscopy measurements, and P. S. Yaremov (L. V.
Pisarzhevskii Institute of Physical Chemistry, National Academy
of Sciences of Ukraine) for the adsorption measurements.
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