Catalytic and physicochemical properties of modified natural clinoptilolite

Article abstract of DOI:10.1016/j.cattod.2015.04.039

A natural specimen from the deposit at Ku?in (Slovakia), rich in clinoptilolite type zeolite, was dealuminated using HCl solutions of increasing concentration (0.05-11.5 M). The samples were characterized by XRD, sorption of nitrogen, TPD of ammonia, FT IR and NMR spectroscopies. The preparations modified under mild conditions (acid concentration, temperature of dealumination) retained largely their crystallinity and acidity, and were active in the liquid phase isomerization of α-pinene. Upon more severe treatments, the samples became partially amorphous and lost their catalytic activity. The kinetics of α-pinene isomerization was studied over the most active catalysts. The reaction rate constants and apparent energies of activation were obtained. Initial reaction rates over the clinoptilolite type catalysts were compared with other acidic catalysts, including ferrierite-type zeolites.

Full text of DOI:10.1016/j.cattod.2015.04.039

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Catalysis Today
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Catalytic and physicochemical properties of modified natural
clinoptilolite
Anna Dziedzicka, Bogdan Sulikowski∗, Małgorzata Ruggiero-Mikołajczyk
Jerzy Haber Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Niezapominajek St. 8, 30-239 Kraków, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
A natural specimen from the deposit at Kucˇin (Slovakia), rich in clinoptilolite type zeolite, was dealumi-
nated using HCl solutions of increasing concentration (0.05–11.5 M). The samples were characterized by
XRD, sorption of nitrogen, TPD of ammonia, FT IR and NMR spectroscopies. The preparations modified
under mild conditions (acid concentration, temperature of dealumination) retained largely their crys-
tallinity and acidity, and were active in the liquid phase isomerization of ␣-pinene. Upon more severe
treatments, the samples became partially amorphous and lost their catalytic activity. The kinetics of
␣-pinene isomerization was studied over the most active catalysts. The reaction rate constants and appar-
ent energies of activation were obtained. Initial reaction rates over the clinoptilolite type catalysts were
compared with other acidic catalysts, including ferrierite-type zeolites.
Received 26 January 2015
Received in revised form 1 April 2015
Accepted 16 April 2015
Available online xxx
Keywords:
Natural clinoptilolite
Dealumination
␣-Pinene isomerization
Kinetics
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
clinoptilolite-Na, -K and -Ca [1]. It was also postulated that each
clinoptilolite or heulandite may be structurally different, even
Clinoptilolite, a mineral discovered already in 1890 in USA, is
the most common zeolite occurring in nature. With morphology
very similar to heulandite, both zeolites reveal the same tetrahe-
dral, aluminosilicate framework denoted as HEU, while differ in the
Si/Al ratio and the alkali-dominant composition. The clinoptilolite-
type framework contains three systems of mutually intersecting
channels. Secondary building units forming 2-dimensional chan-
nel system in clinoptilolite/heulandite are 4–4–1. There are two
channels running parallel to the c-axis: (i) 10-membered rings (10-
when exhibiting the same Si/Al ratio, the latter parameter known to
affect significantly diffusion and ion-exchange of cations, sorption
of gases and finally catalytic properties. The natural samples with
the same Si/Al ratios might differ in silicon–aluminum ordering [4].
One of the most popular and effective methods of zeolite
modification is dealumination. Dealumination of zeolites can be
accomplished using either ion exchange with ammonium ions
combined with (hydro)thermal treatment, or by extracting alu-
minum with a number of chemical agents, like organic/inorganic
acids salts and other agents [5]. In this contribution we have focused
on the use of a mineral acid. It should be emphasized, however, that
during the acid treatment different reactions may proceed in zeolite
crystals, depending on the particular conditions used (tempera-
ture, concentration of acid and contact time). At low temperatures,
acid concentrations and/or short contact times replacement of the
native cations by H+/H3O+ ions takes place. At more severe condi-
tions aluminum from the zeolite framework is depleted, without
visible changes occurring in the zeolite structure. Finally, at still
higher temperatures and acid concentrations coupled with the
longer contact time advanced dealumination might be observed,
leading to a partial decomposition of the zeolite crystals and for-
mation of an amorphous phase [5,6]. First account on clinoptilolite
dealumination was published by Barrer and Makki [7]. The clinop-
tilolite specimen from Mount Hector in California of 85–95% purity
was dealuminated using hydrochloric acid; its thermal stability was
exceptional, up to 750 ◦C. The dealuminated preparations showed
increased sorption properties for water and low alcohols. However,
˚
˚
MR), type A channels with the dimension of 3.0 A × 7.6 A; and (ii)
˚
˚
the smaller, 8 MR channels (3.3 A × 4.6 A). The third type C chan-
nels are also 8 MR and run parallel to the a-axis, with dimension
˚
˚
(2.6 A × 4.7 A) comparable to the B type. Crystals of clinoptilolite
are monoclinic, with the space group C2/m, however sometimes
the reported symmetry is lower [1,2].
It was suggested to retain two mineral names, clinoptilolite
and heulandite, despite that the two zeolites are characterized
by the same framework type HEU [3]. The distinction between
them is made taking into account the Si/Al ratio. The more
siliceous structures with the Si/Al ratios ≥4.0 are attributed to
the name clinoptilolite, while the more aluminous isomorphs are
denoted heulandite. Additionally, depending on the cations con-
tent, the three variety of clinoptilolite were discerned, that is
∗
Corresponding author. Tel.: +48 12 6395 159; fax: +48 12 4251923.
E-mail address: ncsuliko@cyf-kr.edu.pl (B. Sulikowski).
http://dx.doi.org/10.1016/j.cattod.2015.04.039
0920-5861/© 2015 Elsevier B.V. All rights reserved.
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sorption of benzene was the most spectacular, as it increased five
times in comparison with the parent sample [7].
specimen, as chemical dealumination with hydrochloric acid of
different concentrations, or carrying out a multi-fold ammonium
ion-exchange combined with calcination at high temperatures.
The kinetics of ␣-pinene transformation towards camphene and
limonene was studied, and the results were compared to the
performance of other zeolite and non-zeolite catalysts.
Isomerization of ␣-pinene in the liquid phase, a hydrocarbon
of the monoterpene class, was chosen as a test reaction. There
were several reasons for this choice. First of all, terpene feedstocks
as renewable resources are becoming increasingly important for
producing a variety of fine and bulk chemicals. Of these, isomers
of pinene are used to manufacture a number of products for the
pharmaceutical and chemical industries [8]. In particular, the iso-
merization of ␣-pinene, a reaction catalyzed by acid centers, gives
bicyclic products, exemplified by camphene, and monocyclic prod-
ucts such as limonene and p-cymene (Scheme 1) [9,10]. Camphene
is used for producing polymers, lacquers, explosives and medicines.
Camphene is also an intermediate compound for manufacturing
isoborneol, isobornyl acetate and camphor. Limonene is used in
cosmetic and food products, and also as a special solvent in cars’
industry. Nowadays, camphene and limonene are produced in the
transformations of ␣-pinene, ␤-pinene or pinene-containing mate-
rials (e.g., wood turpentine, gum turpentine or sulphate turpentine
obtained by delignification of wood) over acidic catalysts [11]. Cur-
rently, isomerization of ␣-pinene is carried out on an industrial
scale over a weakly acidic, hydrated TiO2 catalyst [12,13], leading to
main components limonene, camphene and tricyclene, and accom-
panied by small amounts of fenchenes and bornylene. To prepare
an active catalyst, TiO2 is treated with sulphuric acid. In this way a
layer of titanic acid supported on the oxide surface is formed. The
initial reaction rate increases with the amount of H2SO4 used, up to
a certain value, but then decreases leading to amorphous, inactive
material. Consumption of a catalyst prepared in this way is about
0.6 kg per 1 t of ␣-pinene, however activity and selectivity of ␣-
pinene transformations over the titania catalysts are not very high.
As the reaction rate is rather low, it is important to search for a
new catalytic system in order to attain better activity coupled with
enhanced selectivity to desirable products [13].
2. Experimental
2.1. Preparation of the samples
Clinoptilolite sample used in this work was mined from the
deposit at Kucˇin, Slovakia. Zeolite content in this deposit is about
83%, and the other phases present in the pristine material were
low-cristobalite, feldspar and mica. The parent sample was crushed
in a ball mill and sieved to obtain size fraction <250 ␮m, washed
with distilled water (twice) and dried at 80 ◦C. A purified sample
was dealuminated with hydrochloric acid solutions of different
concentrations (0.05 ÷ 11.5 M) for 4 h at 95–97 ◦C. The solid:liquid
ratio was always kept at 1 g of zeolite per 15 cm3 of HCl solution.
The resultant samples were washed with distilled water until no
Cl− ions could be detected in the solution and dried at 80 ◦C. Before
NMR measurements, the samples were hydrated in a desiccator
over saturated magnesium nitrate solution. A natural, unmodi-
fied zeolite specimen was labeled HEU. The names of the other
samples obtained were coined from the acid concentration used
(e.g., clinoptilolite dealuminated with 0.05 M HCl was denoted as
HEU-0.05HCl).
A hydrogen form of pristine clinoptilolite was prepared for com-
parison purposes. First, clinoptilolite purified with distilled water
was transformed into the ammonium form by triple ion exchange
with 10% solution of NH4NO3 for 4 h at 80 ◦C. Next, the NH4-HEU
sample was transformed into the hydrogen forms by calcination
in air flow (50 cm3/min) at 300 ◦C (6 h), 400 ◦C (4 h) and 500 ◦C
(4 h). The resultant samples were labeled H-HEU-300, H-HEU-400,
and H-HEU-500, respectively. During high-temperature calcination
some dealumination of the samples took place (Table 1) and new
A large choice of acidic catalysts, such as oxides treated with
acids [14,15], modified zeolites [10,13,16–20], activated clays [21]
and heteropoly acids [22,23] have been suggested in the literature
for the isomerization of ␣-pinene. On the other hand, basic cata-
lysts (e.g., alkaline earth-metal oxides) have been reported for the
isomerization of ␣- to ␤-pinene [24].
siloxane bonds ( Si
O Si ) are formed, thereby stabilizing the
zeolite framework after aluminum removal [25,26].
Transformations of pinene proceed along the two parallel
routes. One yields monocyclic products: limonene, terpinenes, ter-
pinolene and p-cymene, while the other leads to bicyclic and
tricyclic hydrocarbons, of which camphene is the most abun-
dant and important. The reaction commences by protonation of
the ␣-pinene double bond and forming the pinanyl cation. Then
Wagner–Meerwein type rearrangement takes place leading to
isobornyl and p-menthenyl cations. Transformation of isobornyl
cations yields camphene and other bi- and tricyclic products
[18,20].
2.2. Characterization
Powder X-ray diffraction (XRD) patterns of the hydrated sam-
ples in the range of 5–70◦ 2ꢀ were recorded on a X’Pert PRO MPD
˚
diffractometer (Panalitycal) with Cu K␣ radiation (ꢁ = 1.5406 A).
Silicon powder was used as the internal standard (ca. 5 wt.%) for
calibrating the diffraction angle and estimating the crystallinity
of the pristine and modified samples. Crystallinity of the samples
was calculated as follows: degree of crystallinity (%) = sum total of
sample’s intensities/sum total of parent’s zeolite intensities ×100%.
Only well-separated signals were taken for estimating crystallinity
of the samples. The (h k l) reflexes affected mostly during the
dealumination were: (0 2 0), (2 0 0), (−2 0 1), (−3 1 1), (1 1 1), (4 0 0),
(−4 2 2) and (4 4 0). Rietveld refinement was used for estimation
the content of different phases in the pristine material. The amount
of zeolite in the natural specimen was also estimated by quan-
titative 27Al MAS NMR spectroscopy (Supplementary Data). The
aluminum signal revealed the presence of two components, the
largest one was due to clinoptilolite phase, and the second smaller
at −59.6 ppm was assigned to feldspar. Finally, taking into account
the chemical composition of the sample, Rietveld refinement and
the NMR data, the following phase content in the pristine sample
was obtained: 83% of clinoptilolite, 9% SiO2 (low-cristobalite), 6%
of feldspar and 2% of mica.
Despite commercial importance, isomerization of ␣-pinene is a
useful test reaction, as it proceeds in the liquid phase under mild
conditions (60–155 ◦C, atmospheric pressure). ␣-Pinene is a rela-
tively small molecule, comparable with benzene, with the kinetic
˚
˚
diameter of 6.8 A × 6.9 A. In general, therefore, it cannot access the
internal channel system of small pore zeolites, exemplified here by
clinoptilolite. Whenever the mesopores are formed upon modifica-
tion of clinoptilolite, then the additional pores would accommodate
the ␣-pinene molecules thus allowing their further transforma-
tions. Taking this into account, transformations of ␣-pinene is a
choice reaction for monitoring how a secondary pore system in
zeolite is developing upon different treatments.
The objective of this work was to use a natural clinoptilolite
specimen of Slovakia origin and to modify it in such a way as
to obtain viable catalysts for the liquid-phase transformations
of ␣-pinene. We have applied various treatments of natural
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Table 1
Characterization of the pristine specimen and modified clinoptilolite samples: the Si/Al framework ratios and the loss of crystallinity after modification.
Sample
Modification
(Si/Al)fr.a
Crystallinity loss (%)
HEU
Pristine material
0.05 M
0.1 M
0.25 M
0.5 M
4.11
4.50
4.62
0
2
HEU-0.05HCl
HEU-0.1HCl
HEU-0.25HCl
HEU-0.5HCl
HEU-1.0HCl
HEU-2.0HCl
H-HEU-300
H-HEU-400
H-HEU-500
14
21
39
49
54
0
6.64
10.06
10.97
11.33
4.72
4.85
6.56
1.0 M
2.0 M
+
NH4 exchange, calcination: 300 ◦C, 6 h
+
NH4 exchange, calcination: 400 ◦C, 4 h
2
12
+
NH4 exchange, calcination: 500 ◦C, 4 h
a
Calculated from 27Al MAS NMR spectra.
The chemical composition of the samples was obtained by ICP-
AES. The overall Si/Al ratio in the pristine sample, obtained from the
chemical analysis, was equal to 3.80. The Si/Al framework ratios in
the modified clinoptilolite samples were derived from 27Al MAS
NMR spectra and are listed in Tables 1, 5 and 6, respectively.
Adsorption of nitrogen was studied using the Quantachrome
Autosorb gas sorption system. Before the measurements, the sam-
ples were outgassed at 200 ◦C for 24 h.
FT IR spectra were recorded on a Nicolet 6700 spectrometer at
ambient temperature. Typically, 64 scans at 0.482 cm−1 resolution
in the region 400 ÷ 4000 cm−1 were collected using zeolite samples
dispersed in the standard KBr pellets.
if dealumination is very advanced it would eventually led to the
collapse of the zeolite framework yielding an amorphous alumi-
nosilicate.
Natural clinoptilolite was treated with HCl solutions of increas-
ing concentration and crystallinity of the resultant samples was
estimated by XRD. The results are listed in Table 1. As it is
seen, crystallinity of the samples continuously decreases with
the increase of acid concentration. Even mild treatment with the
diluted 0.05–0.1 M HCl affects crystallinity of the preparations.
After treatment with 1 M HCl crystallinity decreases to ca. 50%.
When using the more concentrated solutions of the acid, the sam-
ples became largely amorphous.
Acidity of the samples was assessed using a home-made
temperature-programmed desorption (TPD) apparatus, equipped
with a quadrupole MS detector RGA 200 (Stanford Research
Systems). The samples were outgassed at 300 ◦C for 1 h before
adsorption of ammonia at 120 ◦C.
Earlier observations on clinoptilolite specimens from different
deposits showed various resistance of a zeolite against acids. For
example, dealumination of clinoptilolite from Mount Hector in Cal-
ifornia at temperature slightly exceeding 100 ◦C was described by
Barrer and Makki [7]. While the XRD crystallinity was not reported,
the nitrogen sorption data showed a continuous decrease of the
equivalent monolayer areas from about 390 to 250 m2/g, after
treatment with 2–15 N HCl. Later works documented not so good
resistance and more or less advanced amorphization always took
place during the acid treatment [31,32].
2.3. Catalytic testing
The conversion of ␣-pinene was carried out at atmospheric pres-
sure in a two-necked flask immersed in bath oil. The reactor was
equipped with a reflux condenser, a stirrer and a temperature con-
troller. Before each catalytic test a catalyst was activated at 80 ◦C.
Typically, a 100 mg portion of catalyst and 5 ml of ␣-pinene were
charged into the flask, and the catalytic tests were carried out at
75–150 ◦C for 3 h under stirring. After selected times reaction prod-
ucts were sampled, filtered off and analyzed with a HP 6890 gas
chromatograph equipped with a thermal conductivity detector and
a home-made column (8% bentonite-34, 6% didecyl phthalate and
1% silicon oil A on Chromosorb W) [27].
Thermal stability of zeolites is important for catalytic appli-
cations. In general, the stability of zeolites depends on the Si/Al
framework ratio, and increases for the more siliceous samples. For
example, in FAU type zeolites the highly siliceous isomorphs with
the Si/Al ratios 51–105.5 were prepared [33]. Such preparations
were stable upon heating to 1150 ◦C, moreover, could be boiled for
several hours in concentrated HCl without impairing their crys-
tallinity. Crystallinity depends also on the zeolite structure type
and the presence/absence of different cations neutralizing the neg-
ative charge of the framework. The presence of K+ and probably Na+
ions in clinoptilolite was even more important for thermal stability
+
3. Results and discussion
than the Si/Al ratio itself [32]. Ion exchange for NH4 cations can
affect structural stability of clinoptilolite and result in decrease of
crystallinity after samples calcination. Lower conversion would be
invariably observed for the calcined clinoptilolite samples exhibit-
ing lower crystallinity [6]. Finally, a clinoptilolite sample from
Tokaj-Hills in Hungary, transferred into the ammonium form, was
stable until 460 ◦C [34].
3.1. XRD
Despite the chemical agents used for extraction of aluminum,
one of the issues of paramount importance is the stability of the
clinoptilolite framework under treatment of a mineral acid of dif-
ferent concentration. Attack of H+/H3O+ ions on the zeolite results
in hydrolysis of some of the
O
Al
O
bonds, the tetrahedrally
coordinated Al is depleted from the framework T sites, while
hydroxyl nests are formed around the aluminum vacancies [28].
Obviously, such a process will be affected by the kinetics of Al
removal, and its fast extraction would lead to a partial/total loss
of zeolite crystallinity [29,30]. Moreover, inhomogeneous removal
of Al might lead to formation of a non-crystalline, silicon enriched
layers/zones in the zeolite crystals. On the other hand, treatment
of zeolites with diluted solutions of acids (0.001–0.l M) results
essentially in ion exchange of cations located in the pore sys-
tem, while the zeolite framework is hardly affected. Ultimately,
3.2. IR studies
FT IR spectra of clinoptilolites from different deposits may dif-
fer from each other due to the changes in chemical composition,
the Si/Al ratio and the presence of additional phases. Infrared spec-
tra of the pristine and modified clinoptilolite samples are shown in
Fig. S2 (Supplementary Data). The assignments of individual bands
are summarized in Table 2 [35,36]. The band around 1066 cm−1
is associated with the asymmetric internal T–O stretching vibra-
tions of the tetrahedra. The position of this band is sensitive to
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Table 2
Assignment of the IR bands for the natural clinoptilolite specimen from the deposit
at Kucˇin (Slovakia).
Frequency (cm−1
471
)
Vibration mode
Internal tetrahedral T
bending
O
606
798
External tetrahedral
double ring
External tetrahedral
linkage symmetric
stretching
1065
External tetrahedral
linkage asymmetric
stretching
1645
3618
3438
Bending frequency of
water molecules
Bridging hydroxyls
Si O(H) Al
Hydrogen bonded Si OH
groups and water adsorbed
on free silica
Table 3
Position of the main asymmetric internal Si
and modified samples.
O
Al stretching vibration in the parent
Sample
Main asymmetric stretching vibration (cm−1
)
HEU (pristine material)
HEU-0.05HCl
HEU-0.1HCl
HEU-0.25HCl
HEU-0.5HCl
HEU-1.0HCl
HEU-2.0HCl
HEU-5.0HCl
HEU-8.0HCl
HEU-11.5HCl
H-HEU-300
1066.0
1070.3
1073.7
1080.9
1084.8
1085.3
1088.6
1089.1
1092.5
1092.5
1066.0
1069.4
1077.1
H-HEU-400
H-HEU-500
the composition of the zeolite framework. It shifts considerably
towards higher wavenumbers when the Si/Al framework ratio in a
sample is increased. Therefore the position of this band can be used
for quantitative estimation of the Si/Al framework ration without
performing NMR or XRD studies.
The positions of the main asymmetric vibration for the parent
and modified samples are shown in Table 3. The band observed
for HEU at 1066 cm−1 is shifting continuously to 1093 cm−1 for the
sample treated with 11.5 M HCl.
Similarly, upon calcination of the ammonium form of clinop-
tilolite at 300–500 ◦C, the shifts of the main IR band to 1069 and
1077 cm−1 were observed, respectively (Table 3). Clearly, some
dealumination of the sample during calcination of the ammonium
form took place, and some of the tetrahedral framework aluminum
was expelled to extraframework positions. This was observed quan-
titatively by 27Al MAS NMR spectroscopy (spectra not shown).
XRD crystallinity loss of was negligible after calcination at 400 ◦C,
and 12% at 500 ◦C (Table 1). The Si/Al framework ratios for all
samples studied were estimated from the 27Al MAS NMR spec-
tra, and the corresponding framework compositions are shown in
Tables 1, 5 and 6.
Fig. 1. Adsorption/desorption isotherms of nitrogen on: (A) natural clinoptilolite
from Slovakia; (B) HEU-1.0HCl (modified with 1 M HCl); and (C) H-HEU-300 (cal-
cined at 300 ◦C).
As it is seen, type IV adsorption isotherms with the H4 hys-
teresis loops according to the IUPAC classification were obtained
both for the parent and modified materials. The H4 hysteresis loops
have characteristic step down at relative pressure p/po ≈ 0.4. Such
isotherms are typical for zeolitic adsorbents containing both micro-
pores due to structural features of the framework and mesopores
introduced to the crystals by various post-synthesis treatments,
e.g. ZSM-5 [37]. The micropores volume of the pristine material
was low, 0.003 cm3/g (the same as reported for a sample from Moa
(Cuba) [36]), and the overall specific surface area was ca. 34 m2/g
3.3. Adsorption of nitrogen
Evolution of porosity in clinoptilolite upon acid dealumination
was monitored by sorption studies of nitrogen. The adsorp-
tion/desorption isotherms of natural clinoptilolite are visualized
in Fig. 1A.
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Table 4
Adsorption of nitrogen on the parent and modified clinoptilolite samples.
Sample
SSA (m2/g)a
External surface area (m2/g)b
Micropores surface area (m2/g)b
Total pore volume (cm3/g)
Micropores volume (cm3/g)b
HEU
33.5
62.6
130.6
143.4
44.1
59.1
98.1
143.3
164.7
140.2
80.9
27.9
32.0
56.0
56.0
31.1
36.0
40.0
52.9
63.1
52.4
62.2
45.2
54.9
5.6
30.6
74.6
87.4
12.9
23.1
58.2
90.4
101.6
87.8
18.7
33.9
35.2
0.135
0.150
0.167
0.174
0.140
0.133
0.192
0.193
0.232
0.230
0.137
0.176
0.213
0.003
0.017
0.040
0.047
0.007
0.013
0.033
0.049
0.053
0.050
0.011
0.019
0.018
H-HEU-300
H-HEU-400
H-HEU-500
HEU-0.05HCl
HEU-0.1HCl
HEU-0.25HCl
HEU-0.5HCl
HEU-1.0HCl
HEU-2.0HCl
HEU-5.0HCl
HEU-8.0HCl
HEU-11.5HCl
79.1
90.0
a
Specific sample area, multipoint BET.
t-Method.
b
(Table 4). These values are comparable with other studies on nat-
ural clinoptilolite specimens of different origin, like 30 m2/g for
a sample from California [7], 20 m2/g from Moa [36], 13.4 m2/g
for a sample from Ukraine [38], and 14 m2/g for a specimen from
Bigardic¸ (Turkey) [39]. Such differences can be attributed to vari-
ous chemical compositions, amount of the major zeolite phase, the
presence of other phases and morphology of natural materials stud-
ied. The natural clinoptilolites accommodate various cations (Na+,
Ca2+, Mg2+) which might block the access to the structural micro-
pores [38]. Higher SSA value found for the specimen of Kucˇin may
point to higher clinoptilolite content (83%). The specific surface area
increases by 1/3 even after treatment with 0.05 M HCl.
found under these conditions were camphene and limonene,
and small amounts of other isomers were obtained only after
30 min of reaction. Moreover, high-retention-time products (HRTP)
were sometimes observed over zeolite catalysts [17,41]. These
oligomeric products consist probably of dimers, trimers and higher
oligomers, and were formed especially at higher temperatures
over the zeolite catalysts containing strong Brønsted acid cen-
ters [41], for example on the H-ferrierite material from Zeolyst
Intern. Company [20]. No HRTP were found over the clinoptilolite-
type zeolites at 90 ◦C. The conversion of ␣-pinene and selectivity
to the main products, camphene and limonene, are listed in
Table 5.
Upon using more concentrated HCl solutions, the SSA values
were increasing continuously until 165 m2/g was reached for the
HEU-1.0HCl sample. This was accompanied by parallel increase of
the external surface area and micropores volume. If more concen-
trated acid solutions were used, the opposite trend was observed
and SSA were diminishing until 79–90 m2/g. Amorphization of the
samples took place and hence structural microporosity due to the
zeolite phase was decreasing considerably (Table 4). In order to
obtain the highest SSA of the sample, 1 M HCl solution should be
used. A sample after such a treatment is then characterized by
the most developed specific surface area and reveals the highest
sorption capacity. Interestingly, the sample from Mount Hector
exhibited the similar trend, and sorption of water, ethanol, ben-
zene and isopentane reached maximum upon 1 M HCl treatment
[7].
Apart from the samples prepared by hydrochloric acid, the
regular hydrogen form of parent clinoptilolite was prepared for
comparison purposes by three-fold ammonium cations exchange
followed by calcination at 300–500 ◦C. The results of nitrogen
sorption data for the H-forms and dealuminated samples are sum-
marized in Table 4. It can be concluded that specific and external
surface areas as well as micropores volume for the sample after ion
exchange and calcination at 300 ◦C are comparable with clinoptilo-
lite treated with diluted, 0.1 M HCl solution. Higher temperatures
of calcination lead to the increase of specific surface area, total pore
volume as well as the micropore volume.
As it is seen, the pristine sample is non-active with the conver-
sion 0.5%. However, already upon transformation to the hydrogen
form H-HEU at 300 ◦C, the conversion level of ␣-pinene increases
sharply to 36%, in accord with the high acidity exhibited by this
sample (Table 5). However, when the temperature of calcination
was increased further to 400 and 500 ◦C, a systematic decrease of
activity to 24% and 9% was found (Table 5). As the sample’s crys-
tallinity loss at 400 ◦C is negligible, and at 500 ◦C is also insignificant
(12%, Table 1), therefore this factor cannot be responsible for lower
activity. On the other hand, NMR studies revealed that the Si/Al
framework ratio of clinoptilolite is increasing systematically from
4.1 to 4.7, 4.9 and 6.6, pointing to dealumination of the samples
occurring during calcination at 300–500 ◦C. Dealumination leads to
expelling part of aluminum from framework tetrahedral sites, thus
resulting in decrease of the Brønsted acid sites and hence lower-
ing the catalytic activity. Lower acidity of HEU-0.25 HCl illustrates
well this trend (ca. 300 ␮mol/g, Table 5). Moreover, acid sites in
this material are of lower strength (Fig. S4). For the more siliceous
+
clinoptilolite specimen, not subjected to ion-exchange with NH4
,
Brønsted acidity also decreased with increasing temperature of
activation [42].
The most active catalyst was obtained by treatment of a nat-
ural zeolite with 0.05 M HCl solution. Fig. 2 depicts conversion
of ␣-pinene and selectivity to camphene and limonene over the
two most active catalysts: (i) the hydrogen form of parent mate-
rial obtained via the ammonium ions exchange and calcination
at 300 ◦C (H-HEU-300), and (ii) clinoptilolite treated with 0.05 M
HCl. The conversion level of ␣-pinene over the slightly dealu-
minated HEU-0.05HCl sample was about 10% higher, while its
acidity (507 ␮mol/g) was inferior in comparison with H-HEU-300
(738 ␮mol/g), and the acid strength was similar (Fig. S4). Thus, eas-
ier access to acid centers in the dealuminated sample has to play
its role. Selectivity to camphene over the two samples at 90 ◦C
was comparable (55–62%), a few percent higher for H-HEU-300
(Fig. 2). We note that selectivity to limonene was either constant
(for the dealuminated sample) or decreased insignificantly with the
3.4. Catalytic studies of ˛-pinene isomerization
Transformation of ␣-pinene is a typical reaction catalyzed by
acid sites [40]. For catalytic purposes, the key element is a prepa-
ration of a catalyst in such a way that the access of reactants to the
zeolite structural pores is maintained while the crystallinity of a
zeolite sample is maintained at high level.
Isomerization of ␣-pinene over the pristine and modified
samples was compared at 90 ◦C (Table 5). The main products
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A. Dziedzicka et al. / Catalysis Today xxx (2015) xxx–xxx
Table 5
Catalytic performance of the parent and modified natural clinoptilolite at 90 ◦C after 180 min (reaction conditions: 100 mg catalyst/5 cm3 ␣-pinene).
Sample
(Si/Al)fr.
Acidity (␮mol/g)a
Conversion (mol%)
Selectivity (%) to
Camphene
Limonene
Other
HEU
4.11
4.72
4.85
6.56
4.50
4.62
6.64
474
738
–
0.5
35.7
23.9
9.0
41.0
12.0
1.8
100
64
51
55
57
49
63
–
23
34.4
29
32
–
13
14.5
16
10.6
15
H-HEU-300
H-HEU-400
H-HEU-500
HEU-0.05HCl
HEU-0.1HCl
HEU-0.25HCl
–
507
500
300
36
37
–
a
Amount of acid sites as obtained from TPD of ammonia.
Fig. 3. Conversion of ␣-pinene vs. the reaction time over (A) unmodified natural
clinoptilolite and (B) clinoptilolite modified with 0.05 M at a temperature range of
75–150 ◦C (100 mg catalyst/5 cm3 ␣-pinene).
Fig. 2. Conversion of ␣-pinene and selectivity vs. the reaction time over: (A) HEU-
0.05HCl; and (B) H-HEU-300 at 90 ◦C (100 mg catalyst/5 cm3 ␣-pinene).
the boiling point of ␣-pinene is 155–156 ◦C.) The performance
of a pristine clinoptilolite was compared with the dealuminated
catalyst exhibiting the highest conversion (HEU-0.05HCl), and the
results are visualized, showing the conversion of ␣-pinene vs. time
(Fig. 3). The pristine sample is inactive below 120 ◦C, and continu-
ous increase of activity is seen until reaching the conversion level
of ca. 90% at 150 ◦C. For the dealuminated sample, similar trend of
conversion vs. time was generally observed, except the more steep
increase at the beginning of the reaction. The HEU-0.05HCl sample
was active already at 75 ◦C, and 95% conversion was obtained after
90 min at 120 ◦C (Fig. 3B). Apart from camphene and limonene,
no other products were observed during first several minutes, and
only minor amounts appeared later or at higher temperatures.
It was of interest to consider the kinetics of ␣-pinene isomer-
ization. Therefore, the first- and second-order kinetics were taken
into account, and the appropriate plots ln(c0/c) and (1/c) vs. time
are shown in Fig. 4. At the beginning of the reaction one can
assume the first-order kinetics since a plot is linear (Fig. 4B). Some
reaction time when using the H-form. The amount of other by-
products was 11–16%, depending on the catalyst (Table 5). Under
the conditions studied, the major products were camphene and
limonene. The selectivity to limonene was around 30% (Fig. 2). In
general, limonene is more reactive than camphene, and after for-
mation as a primary product it can be transformed further to yield
terpinenes, terpinolene and p-cymene. It can be therefore con-
cluded that the consecutive transformations of limonene into the
secondary products are suppressed over the clinoptilolite studied,
in contrast to the ferrierite (Zeolyst) type catalysts [20].
Dealumination of natural clinoptilolite from the Kucˇin deposit
under more severe conditions (>0.25 M HCl) led to significant crys-
tallinity loss of the resultant materials combined inevitably with
poor or negligible catalytic activity.
After identifying the most active catalysts a series of experi-
ments at temperatures 75–150 ◦C was carried out. (The highest
attainable in practice temperature of reaction was 150 ◦C, since
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7
Fig. 4. Plots of the reaction rate assignment over clinoptilolite modified with 0.05 M HCl at 110 ◦C: (A) beginning of the reaction when first order kinetics can be considered,
(B) ln(c0/c) vs. reaction time for overall tested reaction – first order kinetics cannot be considered, (C) 1/c vs. reaction time – second order kinetics can be considered.
It was also of interest to calculate the apparent activation
energy of reaction over the representative catalysts. Assuming
the first-order kinetics, apparent activation energy of ␣-pinene
isomerization over the representative clinoptilolite catalysts was
calculated. The corresponding equation can be written in the form:
c0
ln
= kt
(1)
c
Scheme 1. Transformations of alpha-pinene towards camphene and limonene by
two parallel, irreversible reactions.
where:
k – reaction rate constant (1/s)
c0 – initial concentration of ␣-pinene (mol/dm3)
c – concentration of ␣-pinene after time t(mol/dm3).
published data indeed suggest that ␣-pinene isomerization over
zeolites proceeds according to the first-order kinetics [11,42]. Such
an assumption would be true if camphene and limonene were
formed only. In fact, within first 15 min essentially no other prod-
ucts were found. Camphene is less reactive than limonene, and only
under more severe conditions it gives tricyclene in the reversible
reaction camphene ⇔ tricyclene. As it was shown earlier, limonene
is a more reactive hydrocarbon than camphene does [20]. In Fig. 4A
the same plot is shown, now obeying whole period (180 min) of the
process studied. Obviously, after the prolonged time of reaction the
first-order kinetics cannot be applied. Instead, second-order kinet-
ics might be considered and the corresponding plot is depicted in
Fig. 4C.
The overall reaction rate constants k were determined from
experimental data for the first 30 min of the process using the Eq.
(1). The overall reaction rate constants for different catalysts and
temperatures are listed in Table 6.
In addition to the rate constant k, it was also possible to calculate
the reaction rate constants kC for camphene and kL for limonene
[11]. The ratio kC/kL is equal to the value determined from the
slope of the line shown in Fig. S3 (Supplementary Data). Taking
into account the previously calculated reaction rate constants k,
the kC/kL experimental ratios, and Eq. (2):
Taking into account the first-order kinetics at the beginning of
reaction, transformations of ␣-pinene to camphene and limonene
can be described in the form of two parallel, irreversible reactions
as shown in Scheme 1, where kC and kL denote the reaction rate
constants towards camphene and limonene, respectively.
c0
ln
= (kC + kC )t
(2)
c
the individual reaction rates for camphene kC and limonene kL
were obtained. Finally, the initial reaction rates ro for the ␣-pinene
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Table 6
The initial reaction rates constants of ␣-pinene transformations to camphene and limonene over pristine and modified clinoptilolite samples (reaction conditions as in Table
5).
Catalyst
(Si/Al)fr.
Reaction temperature (◦C)
Initial reaction rate
Initial reaction rate r0
kC/kL
kC (1/s)a
kL (1/s)a
constant k (1/s)a
(mmol␣-pinene/gcat min)a
120
130
140
150
1.3 × 10−5
2.4 × 10−5
1.7 × 10−4
5.1 × 10−4
0.25
0.45
2.16
5.96
2.4
2.1
1.9
1.8
9.4 × 10−6
1.6 × 10−5
1.1 × 10−4
3.3 × 10−4
3.9 × 10−6
7.7 × 10−6
5.9 × 10−5
1.8 × 10−4
HEU (pristine)
4.11
H-HEU-300
H-HEU-400
H-HEU-500
4.72
4.85
6.56
90
90
90
75
90
100
110
120
8.2 × 10−5
6.0 × 10−5
2.1 × 10−5
3.7 × 10−5
9.6 × 10−5
2.2 × 10−4
4.3 × 10−4
1.0 × 10−3
1.37
1.01
0.35
0.67
1.61
3.27
5.35
8.68
2.0
1.5
2.1
1.5
1.8
1.7
1.4
1.7
5.4 × 10−5
3.6 × 10−5
1.4 × 10−5
2.2 × 10−5
6.1 × 10−5
1.4 × 10−4
2.5 × 10−4
6.4 × 10−4
2.7 × 10−5
2.4 × 10−5
6.8 × 10−6
1.5 × 10−5
3.5 × 10−5
8.2 × 10−5
1.8 × 10−4
3.7 × 10−4
HEU-0.05HCl
4.50
HEU-0.1HCl
4.62
90
2.8 × 10−5
0.51
1.4
1.6 × 10−5
1.2 × 10−5
a
Determined after first 30 min of the reaction.
Analysis of the initial reaction rates reveals a broad range, from
0.25 to 8.68 mmol ␣-pinene/gcat min (Table 6). The rate close to 6
was observed for the pristine sample at 150 ◦C. Values between 2.3
and 5 were characteristic for parent and dealuminated ferrierites,
but at the temperature of 90 ◦C [18]. The kC/kL ratios were higher
on parent HEU, while for the modified samples the opposite trend
was observed, especially at higher temperatures. The best catalytic
performance was obtained for the HEU-0.05HCl clinoptilolite. For
this sample, the initial reaction rate ro was equal to 8.7 mmol ␣-
pinene/gcat min at 120 ◦C. It can be therefore concluded that in
order to obtain a viable catalyst the sample should be treated with a
very diluted acid. Such a treatment affects the clinoptilolite frame-
work slightly, the level of dealumination observed is not advanced
(the Si/Al increased from 4.11 to 4.50), and acidity is kept at the
high level (Table 5). Simultaneously, removal of impurities, proba-
bly blocking the entrance to the channel system, takes place, what
is clearly seen from the adsorption studies (Table 4). The perfor-
mance of the HEU-0.1HCl and HEU-0.25 HCl samples was inferior
in comparison with HEU-0.05HCl (Table 6).
4. Conclusion
A natural specimen rich in clinoptilolite type zeolite was used as
a starting material for chemical modification. The pristine sample
contained clinoptilolite (83%), low-cristobalite (9%), feldspar (6%)
and mica (2%). The pristine sample was systematically modified
by HCl solutions of increasing concentration (0.05–11.5 M). Upon
modification, dealumination of the parent material took place, and
the Si/Al molar ratios of the clinoptilolite framework were assessed
using 27Al MAS NMR spectroscopy. Simultaneously, evolution of
the pore system was monitored by nitrogen adsorption. The SSA
values were increasing continuously until reaching 165 m2/g. The
1 M HCl solution should be used to obtain the most developed SSA
with the highest sorption capacity. Amorphization of the samples
took place and hence structural microporosity due to the clinop-
tilolite phase was decreased considerably if more concentrated acid
solutions were used.
The liquid phase transformation of ␣-pinene over represen-
tative samples was studied. First-order kinetics of ␣-pinene
consumption was found at the beginning of the process (up to
30 min), while the second-order kinetics described the process
during 3 h studied. The overall reaction rate constants k, as well as
kC and kL constants for camphene and limonene, were calculated.
The kC/kL ratios were higher on parent HEU, while for the modified
samples the opposite trend was observed, especially at higher
temperatures. It was shown that apparent activation energy on
the pristine HEU sample 179 kJ/mol decreased significantly to
Fig. 5. The Arrhenius plots for: (A) pristine clinoptilolite, and (B) HEU-0.05HCl.
conversion were calculated for comparison purposes with other
acid-type catalysts studied previously (Table 6).
The apparent activation energies were determined from the cor-
responding Arrhenius plots visualized in Fig. 5. Apparent activation
energies for the pristine clinoptilolite and the HEU-0.05HCl sam-
ple were 178.5 and 91.7 kJ/mol, respectively. The pristine sample is
characterized by high activation energy, and upon modification it
drops down considerably. The process is therefore not limited by
diffusion constraints. On natural tuff rich in clinoptilolite similar
activation energy 80.9 kJ/mol was reported [13], while on a hybrid
catalysts H3PW12O40 supported on clinoptilolite 65.4 kJ/mol [39].
In one case low activation energy was found (28 kJ/mol) pointing
to diffusion limitation of the reaction [43].
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9
92 kJ/mol for the most active HEU-0.05 HCl material. For the
latter sample, the initial reaction rate ro was equal to 8.7 mmol
␣-pinene/gcat min at 120 ◦C. Values ro between 2.3 and 5 were
characteristic for parent and dealuminated ferrierite-type zeolites
described earlier, but at the temperature of 90 ◦C [18]. In order to
obtain a viable catalyst, the pristine material from Kucˇin should
be treated with a very diluted acid solution. Finally, it can be
concluded that the catalytic activity of natural clinoptilolite is an
entangled function of chemical composition, crystallinity, overall
acidity and access of a substrate to the active sites.
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Please cite this article in press as: A. Dziedzicka, et al., Catalytic and physicochemical properties of modified natural clinoptilolite, Catal.
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