Superior activity of non-interacting close acidic protons in Al-rich Pt/H-*BEA zeolite in isomerization of n-hexane

Article abstract of DOI:10.1016/j.apcata.2016.12.016

Skeletal isomerization of linear alkanes, an essential reaction for the production of gasoline, relies on environmentally questionable chlorinated catalysts, whose activity exceeds that of alternative zeolite catalysts. This work describes an attempt to understand relations between the local arrangement of active sites and skeletal isomerization of n-hexane in order to adapt the structure of zeolite catalysts to increase the reaction rates. For this purpose, we used a combination of synthesis of zeolites of^*BEA structural topology with unique density and distribution of strongly acid sites, analysis of the nature of the acid sites by¹H MAS NMR spectroscopy and FTIR spectroscopy of the OH groups and adsorbed d₃-acetonitrile, UV–vis-NIR spectroscopy of carbocations formed by protonization, and kinetic analysis. We demonstrate that the high density of non-interacting but close and strongly acidic structural hydroxyl groups significantly lower the activation barrier in the isomerization reaction compared to far-distant acid sites. The organotemplate-free synthesized Al-rich Pt/H-^*BEA zeolite (Si/Al 4.2) with an unparalleled high concentration of the non-interacting close H⁺ions balancing the charge of the Al-Si-Al sequences forming a wall between the two channels yields 6 times higher reaction rates compared to state-of-the-art Si-rich Pt/H-zeolite catalysts.

Full text of DOI:10.1016/j.apcata.2016.12.016

Applied Catalysis A: General 533 (2017) 28–37
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Superior activity of non-interacting close acidic protons in Al-rich
Pt/H-*BEA zeolite in isomerization of n-hexane
Petr Sazamaa,∗, Dalibor Kauckya, Jaroslava Moravkovaa, Radim Pilara, Petr Kleina,
Jana Pastvovaa,b, Edyta Tabora, Stepan Sklenaka, Ivo Jakubecc, Lukasz Mokrzyckia
a J. Heyrovsky´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejskova 2155/3, 182 23 Prague 8, Czech Republic
b University of Pardubice, Studentská 95, Pardubice, 532 10, Czech Republic
c Institute of Inorganic Chemistry, Academy of Sciences of the Czech Republic, Husinec-Rez, 25068 Rez, Czech Republic
a r t i c l e i n f o
a b s t r a c t
Article history:
Skeletal isomerization of linear alkanes, an essential reaction for the production of gasoline, relies on
environmentally questionable chlorinated catalysts, whose activity exceeds that of alternative zeolite
catalysts. This work describes an attempt to understand relations between the local arrangement of
active sites and skeletal isomerization of n-hexane in order to adapt the structure of zeolite catalysts
to increase the reaction rates. For this purpose, we used a combination of synthesis of zeolites of *BEA
structural topology with unique density and distribution of strongly acid sites, analysis of the nature of
the acid sites by 1H MAS NMR spectroscopy and FTIR spectroscopy of the OH groups and adsorbed d3-
acetonitrile, UV–vis-NIR spectroscopy of carbocations formed by protonization, and kinetic analysis. We
demonstrate that the high density of non-interacting but close and strongly acidic structural hydroxyl
groups significantly lower the activation barrier in the isomerization reaction compared to far-distant
acid sites. The organotemplate-free synthesized Al-rich Pt/H-*BEA zeolite (Si/Al 4.2) with an unparalleled
high concentration of the non-interacting close H+ ions balancing the charge of the Al-Si-Al sequences
forming a wall between the two channels yields 6 times higher reaction rates compared to state-of-the-art
Si-rich Pt/H-zeolite catalysts.
Received 12 October 2016
Received in revised form
15 December 2016
Accepted 17 December 2016
Available online 3 January 2017
Keywords:
Isomerization
Alkanes
Hexane
Zeolites
Al-rich beta (*BEA)
Mordenite
© 2017 Elsevier B.V. All rights reserved.
1. Introduction
Since the chlorination occurs by reversible chemical exchange of
the surface hydroxyl groups of the alumina support, the use of
Hydroisomerization of linear alkanes into branched alkanes rep-
resents a key reaction for the production of automotive fuels.
This complex catalytic reaction consists of dehydrogenation of a
linear alkane to an alkene on a noble metal, skeletal isomeriza-
tion of the formed alkene via protonization and a cyclopropyl
intermediate on a strongly acidic site and re-hydrogenation of
the branched alkene to the alkane (Fig. 1) [1–6]. The dehydro-
genation/hydrogenation reactions on a bifunctional catalyst are
in thermodynamic equilibrium and practically do not affect the
reaction rate of the hydroisomerization [4]. In contrast, the acid-
catalysed reaction steps determine the rate of isomerization and
this catalyst is associated with constant supply and formation of
organic chlorines [11], which leads to serious questions about
the environmental impact. Zeolite-based acid catalysts, such as
Pt-mordenite zeolite with improved texture [12–14] or sulfated
Pt-zirconia [15,16] were developed; however, the acid centers of
these catalysts facilitate a sufficient rate of isomerization at much
higher temperatures. Conversely, low-temperature activity is cru-
cial for achieving sufficient conversion of linear alkanes to branched
ones due to the thermodynamic equilibrium of the isomerization
reaction, wherein the lower temperatures shift the equilibrium
towards the desired di-branched products, whereas higher temper-
atures hinder their formation. Development of an acidic catalyst for
hydroisomerization of linear alkanes, particularly n-pentane and n-
hexane, functional at low temperatures without streams containing
low concentrations of chlorine additives, remains a great challenge
for heterogeneous catalysis.
require strongly acidic centers [7]. The labile protons of the O
H
groups with acid strength strongly enhanced by the presence of
chlorine on the traditional chlorinated Pt-alumina catalysts [8] pro-
vide sufficient conversion at temperatures as low as 150 ◦C [9,10].
Knaeble et al. [7] and Macht et al. [5] documented well that the
rate of hexane skeletal isomerization at acid sites with an opened
coordination sphere and located in an unconfined space in tung-
∗
Corresponding author.
E-mail address: petr.sazama@jh-inst.cas.cz (P. Sazama).
http://dx.doi.org/10.1016/j.apcata.2016.12.016
0926-860X/© 2017 Elsevier B.V. All rights reserved.

P. Sazama et al. / Applied Catalysis A: General 533 (2017) 28–37
29
Fig. 1. Reaction scheme for n-hexane hydroisomerization on bifunctional Pt/H-zeolite catalysts. The fast dehydrogenation–hydrogenation reactions at Pt sites equilibrate
alkanes and all alkene isomers of a given carbon chain structure providing a very low and constant concentration of alkenes. In the acid catalysed reaction steps, the alkene
molecules undergo skeletal isomerization and, at high temperatures, also produces lower molecular weight alkanes [5,6].
sten Keggin polyoxometalates supported on silica is proportional
to the concentration of acid sites and decreases exponentially with
decreasing acid strength. However, in the case of zeolite catalysts,
the location of protons in the individual pores of high-silica zeo-
lites and, in addition, the distances between the protons might be
also expected to affect their function and reaction pathway in the
isomerization reaction. The rate of skeletal isomerization per pro-
ton in the 8-member ring (8-MR) side channels of the mordenite
structure was reported to be five times higher in comparison with
that in the large straight 12-member ring (12-MR) channels [17].
Since the pioneering work of Haag et al. [18], suggesting a linear
relationship between the concentration of tetrahedral aluminum
in the framework, hence the acid centers, and the reaction rate in
the cracking of hexane, it has been well established that, not only
the concentration, but also the local spatial arrangement of acidic
centers attached to crystallographically different sites in the chan-
nels [19,20] and the distance between them [21] fundamentally
influence both the reaction rate and the mechanism.
In pursuing the present work on the isomerization of linear alka-
nes over zeolites, we wanted to specify the role of an increase in
the density of the strongly acidic protons countering the negative
charge of the framework in the H-forms of zeolites in relation to the
occurrence of Al-Si-Al sequences, inevitably formed at high con-
centration of Al in the zeolite framework. We wanted to determine
the effect of variations in the distribution of aluminum providing
charge balance for the corresponding high concentration protons
located in the close vicinity on the reaction rate and selectivity.
We therefore employed zeolite *BEA topology, which can be pre-
pared in a broad range of Al concentrations and which offers fast
intra-crystalline diffusion of reactants and products through chan-
nels with three-dimensional architecture and 12-MR openings. We
exploited recent progress in the synthesis of the beta zeolite that
opened a new potential to manipulate the framework aluminum
content in a very broad range and employed Al-rich beta zeo-
lites with very high concentration of aluminum (Si/Al ≥ 4) with
highly predominant tetrahedrally coordinated Al in the framework
[22–34]. This approach enabled us to examine the extent to which
the isomerization reaction is affected by close proximity of strongly
acidic centers. We found that the high density of non-interacting
strongly acidic sites facilitates extraordinarily high reaction rates
due to a synergetic effect significantly decreasing the activation
barrier of the reaction. This enabled more rational design of iso-
merization zeolite catalysts providing superior activity.
2. Experimental
2.1. *BEA and MOR zeolites and preparation of Pt/H-catalysts
Al-rich beta zeolite (molar Si/Al 4.2), denoted as *BEA/4.2, was
hydrothermally synthesized from aluminosilicate synthesis gel
prepared from NaAlO2 and fumed silica (Cabosil) in the absence
of an organic structure-directing agent and using seeding of cal-
cined beta crystals (TZB-212, Tricat). Details of the procedure were
reported previously [33]. The high-silica zeolites used as standards
for comparing the catalytic properties kindly supplied by the Tri-
cat Company (now part of Clariant), (*BEA, Si/Al 11.5, TZB-212) and
Zeolyst International (*BEA, CP814B-25, Si/Al 12.5 and MOR, CBV
20A, Si/Al 12.1) were denoted as *BEA/11, *BEA/12, and MOR/12,
respectively. A hierarchical mordenite zeolite with optimal micro-
mesoporous structure was prepared and used for comparing the
catalytic properties of Al-rich beta zeolite with a state-of-the-art
hydroisomerization zeolite-based catalyst [12]. The hierarchical
mordenite was prepared by treatment of MOR/12 in alkaline solu-
tions (30 ml 0.2 M NaOH per 1 g mordenite stirred in a beaker at
85 ◦C for 2 h) and subsequently in acid solution (10 ml 0.1 M oxalic
acid per 1 g alkaline treated zeolite stirred in a beaker at 85 ◦C
for 20 h). All the zeolites were ion-exchanged with 0.5 mol dm−3
NH4NO3 at RT (1 g of a zeolite per 100 cm3 of solution, three times
over 12 h).
Pt was introduced into the zeolites by incipient wetness impreg-
nation of pre-dried (105 ◦C/2 h) powder zeolites with a H2PtCl6
solution to yield 1.5 wt.% of Pt. The Pt-impregnated granulated zeo-
lite was activated before the catalytic test in a stream of O2 at 450 ◦C
for 3 h, then purged by a N2 stream at 450 ◦C, then cooled down to
250 ◦C, and finally activated in a mixture of 80 mol% H2 and 20 mol%
N2 at 250 ◦C for 1 h.
2.2. Structural analysis
X-ray powder diffraction (XRD) patterns were obtained with
a graphite monochromator and a position sensitive detector
(Våntec-1) using a Bruker AXSD8 Advance diffractometer with
CuK␣ radiation in Bragg–Brentano geometry. The porosity of
the zeolites was determined by the analysis of the adsorption
isotherms of nitrogen at 77 K carried out using an ASAP2010
apparatus (Micromeritics). Before the adsorption experiment the
samples were outgassed at 240 ◦C for at least 24 h. The crys-

30
P. Sazama et al. / Applied Catalysis A: General 533 (2017) 28–37
WSHV 0.25 h−1 and GHSV 4000 h−1. The temperature of the reac-
tor was controlled with an internal thermocouple and kept at
the desired temperature in the range 125–250 ◦C. The concentra-
tions of n-hexane, branched hexanes and lower molecular weight
products (methane, ethane, propane, butane, and iso-butane) were
analyzed by an on-line connected Finnigan 9001 gas chromato-
graph equipped with a 50m × 0.32mm × 5 ␮m Al2O3/KCl capillary
column and an FID detector. Steady-state conditions were achieved
within 0.5–3 h of reaction time-on-stream. The reaction rates of the
n-hexane to iso-hexane were calculated at low conversion values
and close to 100% selectivity for iso-hexanes. Only the conversion
values <6% are considered for our calculation of the reaction rates
(moliso-hexanes gcat−1 s−1) and the TOF values for the reaction per Al
(moliso-hexanes molAl−1 s−1).
The conversions and yields of branched hexane isomers and
lower molecular weight by-products were also analyzed under
process-like conditions. These catalytic tests were performed using
a stainless-steel gas flow tubular PID Eng&Tech Microactivity − Ref-
erence reactor at a pressure of 10 bar with a H2 to hexane molar
ratio of 6 and amount of catalyst equal to 2.5 g (5 ml) with a flow
rate corresponding to WHSV 0.7 h−1 and GHSV 638 h−1. The tem-
perature was controlled by an internal thermocouple and kept at
the desired temperature in the range 200–215 ◦C. Liquid n-hexane
was continuously loaded by a Gilson 307 pressure pump and evap-
orated in the hot flow of hydrogen gas. The reaction products
were analyzed using an on-line connected Perkin Elmer Clarus
580 gas chromatograph equipped with CP-Sil Pona CB column
(100 m × 0.25 mm × 0.5 ␮m) and a FID detector.
tal morphology and the Pt dispersion were analyzed using a
scanning electron microscope (SEM, JEOL JSM-5500LV) and a high-
resolution transmission electron microscope (HR-TEM, JEOL JEM
3010), respectively. The chemical compositions of the parent and
prepared Pt-zeolites were determined by X-ray fluorescence spec-
troscopy using a PW 1404 (Philips). A Nicolet Nexus 670 FTIR
spectrometer was used to collect the FTIR spectra of the H-forms of
zeolites. The samples in the form of a self-supporting wafers were
activated at 450, 500 and 550 ◦C in a dynamic vacuum for 3 h in-situ
in the IR cell. CD3CN was adsorbed at a partial pressure of 13 mbar
at RT for 20 min and then evacuated for 15 min at RT. The spec-
trometer with a MCT-B detector was operated at 1 cm−1 resolution
and 256 scans were averaged for one spectrum. The intensity of the
absorption bands was normalized using the bands of the overtone
vibrations of the zeolite lattice as an internal standard. The spectra
in the stretching vibration of C N were processed by deconvolution
and curve fitting by the procedure according Wichterlova et al. [35]
to obtain the integral area of the band characteristic of the Brønsted
and Lewis sites. The extinction coefficients εB = 2.05 cm ␮mol−1 and
εL = 3.60 cm ␮mol−1 were used for calculations of the correspond-
ing concentrations of acid sites. Solid state 1H and 29Si MAS NMR
experiments were carried out on a Bruker Avance 500 MHz Wide-
Bore spectrometer (11.7 T) equipped with 4 mm double-resonance
MAS NMR probe-head. 1H MAS NMR single pulse spectra were col-
lected after 128 scans with a ␲/2 (4 ␮s) excitation pulse and 2 s
repetition delay at a rotation speed of 11 kHz. To obtain the pro-
tonic form (H-*BEA), “parent” NH4-*BEA samples were dehydrated
in-situ in 4 mm ZrO2 MAS NMR rotors at 450 ◦C (ramp 1 ◦C.min−1
)
under dynamic vacuum of 5.10−1 Pa for 6 h. 29Si MAS NMR sin-
gle pulse spectra of hydrated zeolites were measured at a rotation
speed of 7 kHz, with a ␲/6 (1.7 ␮s) excitation pulse and relaxation
delay of 30 s for single pulse spectra. The framework aluminum
content (Si/AlFR) was estimated from the intensity of the 29Si NMR
spectra according to Refs. [36,37]. The UV–vis spectra of Al-rich and
Si-rich H-*BEA zeolites dehydrated at 500 ◦C in a vacuum after inter-
action with hexamethylbenzene (HMB) at 200 ◦C for 1, 2 and 3 h
and after subsequent interaction with NH3 were measured in the
range from 20 000 to 45 000 cm−1 using a Perkin-Elmer Lambda 950
spectrometer equipped with a Spectralon integration sphere. HMB
was introduced into the dehydrated zeolite by mixing the pow-
dered zeolite and HMB in a glovebox and the measurements were
performed using a cell enabling collection of the spectra without
exposure of the mixture to the air.
3. Results and discussions
An optimized organotemplate-free hydrothermal synthesis was
employed for the preparation of the Al-rich *BEA zeolite (molar
Si/Al 4.2). Its structural as well as acid and catalytic properties were
examined in detail to compare with the high-silica *BEA zeolite
(molar Si/Al 11.3) to obtain an insight into the nature and role of
the high density of acid sites in hydroisomerization of n-hexane.
Commercial high silica zeolites of *BEA and MOR structure with
the molar Si/Al 12.5 and 12.1, respectively, and a hierarchical mor-
denite zeolite (molar Si/Al 9.5) with optimal micro-mesoporous
were further used to demonstrate the functionality of the Al-rich
*BEA zeolite in the hydroisomerization reaction in comparison with
a state-of-the-art hydroisomerization zeolite-based catalyst. The
main characteristics of the *BEA and MOR zeolites are listed in
Table 1.
2.3. Kinetic analysis
The kinetic analysis of hydroisomerization of n-hexane to the
corresponding iso-hexanes was performed in a regime implying
equilibrium between hydrocarbons in the micropores and gas
phase reached by a high H2/hydrocarbon molar ratio and corre-
spondingly low concentration of hexanes in the reaction stream
[38]. The kinetic regime under the reaction conditions was con-
firmed by variation in the total gas flow and the weight of the
catalyst. Small crystallites of the zeolites (∼0.05–0.4 ␮m) guaran-
teed the absence of intra-crystalline diffusion constraints to the
overall reaction rates as the internal diffusion limitations have
been shown to be irrelevant for crystal sizes at least up to 12 ␮m
for both the n-hexane hydroisomerization [38] and cracking [39].
The concentration of platinum of 1.5 wt.% in the prepared PtH-
zeolite catalysts provides a sufficient rate of (de)hydrogenation
reactions yielding hexene/hexane in equilibrium and not limit-
ing the overall alkane hydroisomerization [38]. The catalytic tests
were carried out in a glass flow-through tubular U-shaped reactor
under atmospheric pressure. 0.50 g of a catalyst and the reaction
stream consisting of 79 mol.% H2, 20 mol.% N2 and 1 mol.% of n-
hexane kept at a total flow rate 66 cm3 min−1 corresponded to
3.1. Physicochemical characterization
The patterns of the X-ray diffraction lines are characteristic for
the beta and mordenite zeolites without impurity phases (Fig. 2A).
The high intensities of the X-ray diffraction lines and the absence of
a diffuse halo peak indicate a well-developed crystalline structure
without the presence of an amorphous phase. The representative
SEM image shows that the Al-rich *BEA/4.2 sample consists of
well-developed intergrown crystals with a uniform morphology of
partially truncated octahedrons with an average size of ∼0.4 ␮m
(Fig. 2D). The N2 sorption isotherm of the *BEA/4.2 sample is char-
acteristic of purely microporous material (Type I, isotherm profile),
with a high microporous volume and the apparent surface area of
510 m2 g−1 (Table 1). The *BEA/11 and *BEA/12 samples consist of
very small crystallites with an average size of ∼0.05 and ∼0.1 ␮m,
respectively. Analysis of the adsorption isotherms (Fig. 2B) showed
high apparent surface areas of 617 and 605 m2 g−1 for *BEA/11 and
*BEA/12, respectively, and a high adsorption on the external surface
of the small crystallites at higher relative pressures. The SEM image

P. Sazama et al. / Applied Catalysis A: General 533 (2017) 28–37
31
Table 1
Characteristics of the beta (*BEA) and mordenite zeolites.
b
a
c
c
Sample
Zeolite
Si/Ala
Si/AlFR
cAl
cB
cL
Crystal size
␮m
S
mmol g−1
mmol g−1
mmol g−1
m2 g−1
H-*BEA/4.2
H-*BEA/11
H-*BEA/12
H-MOR/12
H-MOR/10
Al-rich *BEA
4.2
4.7
3.0
1.4
1.2
1.3
1.6
1.80
0.63
0.48
1.06
0.77
0.22
0.32
0.30
0.11
0.35
∼0.4
510
617
605
454
405d
Si-rich *BEA
11.3
12.5
12.1
9.5
11.5
15.0
12.5
11.2
∼0.05
∼0.1
Si-rich *BEA
Si-rich MOR
Micro-mesoporous Si-rich MOR
∼0.15
∼0.15
a
From chemical analysis of the Na+ form of zeolites.
From 29Si MAS NMR spectra of the Na+ form of zeolites.
b
c
Concentration of Brønsted and Al-Lewis sites from FTIR spectra of adsorbed d3-acetonitrile on the H+ form of zeolites.
d
External surface area 110 m2 g−1
.
Fig. 2. A) X-ray powder diffractograms, B) adsorption isotherms of N2 at 77 K, C) 27Al MAS NMR spectra, and D) SEM images of the *BEA and MOR zeolites.
of the microporous MOR/12 (Fig. S1) shows aggregated crystals of
an average size of ∼0.15 ␮m and the HR-TEM image of its micro-
mesoporous analogue MOR/10 exhibits the presence of a secondary
mesoporous structure characterized by numerous cavities about
5–20 nm in size (Fig. S1). The XRD patterns are slightly broadened
for the micro-mesoporous mordenite compared to the microporous
analogue indicating a smaller effective crystallite size of the zeolite.
The adsorption isotherm of MOR/12 is typical for a purely micro-
porous zeolite while the MOR/10 zeolite exhibits an adsorption in
the zeolite micropores and a hysteresis loop typical for mesopores
with the mesoporous surface area of 110 mg−1 (Table 1). 27Al MAS
NMR spectra of all the investigated zeolites show the absence of a
signal at 0 ppm and the presence of a strong resonance envelope
with the chemical shift of about 55 ppm reflecting the tetrahedral
coordination of Al atoms (Fig. 2C). The tetrahedral coordination of
Al atoms is consistent with comparable values of the framework
Si/AlFR ratios obtained using analysis of the 29Si MAS NMR spectra
and those obtained from the chemical analysis (Table 1).
teristic of terminal and internal SiOH, extra-framework AlOH and
bridging Si(OH)Al groups, respectively [40,41]. The main difference
in the spectra of H-*BEA/4.2 and H-*BEA/11 is in the intensity of
the signal characteristic of bridging Si(OH)Al groups. The 3 times
higher intensity of the signal of the Brønsted sites for H-*BEA/4.2
corresponds to the increase in the concentration of Al in the zeolite
with Al atoms incorporated predominantly in the regular Td coor-
dination in the framework. The close positions of the band maxima
*
at 3.6 and 3.8 ppm for Si- and Al-rich BEA, respectively, can indi-
cate that the acid strength does not greatly differ. The maxima of
the signals of the OH groups in the zeolites are not unambiguously
connected with their acid strengths; however, the shift matches the
changes in properties for a single zeolitic structure [42,43]. The sig-
nal of silanol groups forming a surface termination of the crystalline
structure is weak for H-*BEA/4.2 due to the low external surface
of well-developed crystallites compared to high intensity of the
band for *BEA-11 characteristic of small crystallites. It is obvious
that the signal of the internal silanol groups reflecting structural
defects associated with removal of the T-atoms or intergrowth
structure also exhibits barely distinguishable intensity for the Al-
rich *BEA sample compared to the Si-rich *BEA. As the intensities
3.2. Analysis of acid sites
l
of the H MAS NMR bands do not depend on the band positions,
The pair of samples H-BEA/4.2 and H-BEA/11 zeolites, exhibiting
different concentration of the framework Al atoms corresponding
to molar Si/AlFR 4.7 and 11.5, respectively (Table 1), was ana-
lyzed for the concentration and strength of the Brønsted sites,
the concentration of Lewis acid sites and the presence of struc-
tural defects using 1H MAS NMR spectroscopy, FTIR spectroscopy of
the OH groups and adsorbed d3-acetonitrile, and UV-VIS-NIR spec-
troscopy of carbocations formed by protonization upon adsorption
of hexamethylbenzene (HMB). The 1H MAS NMR spectra of the
dehydrated H-*BEA/4.2 and H-*BEA/11 zeolites (Fig. 3A) exhibit
bands with maxima at 1.6, 2.0, 2.4–2.5, and 3.6–3.8 ppm charac-
they accurately reflect the ratio between the concentrations of the
individual hydroxyls. Thus the low intensity signal of the internal
silanol groups and the low signal of the OH groups bound to extra-
framework and/or perturbed framework Al atoms indicate that the
*
Al-rich BEA/4.2 sample has a well-developed low-defective crys-
talline structure.
Fig. 3B and C show the FTIR spectra in the region of hydroxyl
group vibrations of H-*BEA/4.2 and H-*BEA/11 evacuated at
temperatures from 450 up to 550 ◦C and after adsorption of d3-
acetonitrile, respectively. The FTIR spectra in the region of the

32
P. Sazama et al. / Applied Catalysis A: General 533 (2017) 28–37
Fig. 3. Analysis of acid sites in H-*BEA zeolites. A) 1H MAS NMR spectra of H-*BEA zeolites dehydrated at 500 ◦C, and FTIR spectra for zeolites dehydrated at increasing
temperatures in B) the region of the ␯0→1OH vibration and C) in the region of the ␯0→1C N vibration after adsorption of d3-acetonitrile.
hydroxyl group exhibit approximately 3 times higher intensity of
the bands with maxima at about 3610–3613 cm−1 of the stretching
vibration of the bridging hydroxyls, negligible intensity of the bands
at 3650–3660 cm−1 of perturbed framework or extra-framework
AlOH [44–46] and low intensity of the band at 3727–3745 cm−1
of silanols for the H-*BEA/4.2 sample compared to H-*BEA/11 in
good agreement with the 1H MAS NMR results. Only evacuation at
550 ◦C produced a considerable loss of the intensity for the bridg-
ing hydroxyls. This indicates sufficient structural stability of Al-rich
H-*BEA zeolite at temperatures relevant for the isomerization reac-
tions. Adsorption of d3-acetonitrile resulted in the appearance of
the bands at 2325 and 2297 cm−1 corresponding to the stretch-
ing mode of ␯(C N) of d3-acetonitrile adsorbed on Lewis and
Brønsted sites, respectively [35]. Quantitative analysis of the acid
sites using the integral intensities of the IR bands and the extinc-
tion coefficients for the C N group interacting with the Brønsted
and Lewis sites [35] indicated a slightly predominant concentration
of Brønsted sites compared to concentration of Lewis sites in the
H-*BEA/4.2 sample (Table 1). The formation of a significant concen-
tration of Lewis sites is connected with a reversible change in the
coordination of the framework Al atoms characteristic of the struc-
ture of the *BEA zeolite [34,47–52] and also observed for Al-rich
*BEA [53]. Thus the 1H MAS NMR and FTIR spectra and OH groups
and adsorbed d3-acetonitrile together show that the increase in
the concentration of the aluminum content does not led to signifi-
cantly increased dehydroxylation and formation of Lewis acid sites
and the concentration of Brønsted hydroxyls in Al-rich beta zeo-
lite is proportional to the increase in the concentration of Al in the
zeolite framework.
The ability of the Brønsted hydroxyls to protonate hexene,
the essential step in the isomerization reaction, is controlled by
interplay of the concentration and strength of the acidic centers
and confinement of the hydrocarbon molecule in a constrained
environment of the zeolite channels [17]. However, the proto-
nation of hexene cannot be experimentally followed because of
the unmeasurable hexene concentration under realistic reaction
conditions and at higher concentrations due to the rapid oligomer-
ization of the akenes and the formation of complex hydrocarbon
molecules [54]. Therefore, the ability of Al-rich H-*BEA zeolite to
form carbenium ions was analyzed using the hexamethylbenzene
(HMB) molecule, whose protonation yields the relatively stable
hexamethylbenzenium cation providing electronic transitions with
characteristic spectral components in the range of UV–vis light [55].
The UV–vis spectra of HMB and carbocations formed by protoniza-
tion of adsorbed hexamethylbenzene (HMB) in Al-rich and Si-rich
H-*BEA zeolites are shown in Fig. 4. The UV–vis spectrum of neutral
HMB exhibited a typical absorption band at 37 000 cm−1 character-
istic of the ꢀ-electron system of the aromatic ring [56]. Adsorption
of HMB in both Si- and Al-rich *BEA zeolites resulted in the appear-
ance of an intense band with maximum at 25 500 cm−1 with a
shoulder around 29 500 cm−1 corresponding to the formation of
the hexamethylbenzenium cation [55,57]. The spectra obtained for
the two zeolites do not show significant differences in the positions
and shapes of the high intensity bands of the carbenium ions and
are very similar to those of protonated benzene in a superacidic
solution [56]. The assignment of the bands to the hexamethylben-
zenium cation was demonstrated experimentally by co-adsorption
of ammonia, resulting in disappearance of the band at 25 500 cm−1
consistent with proton transfer between ammonia as a stronger
base and the hexamethylbenzenium cation forming the ammo-
nium ion and neutral HMB.
3.3. Nature and local arrangement of acid sites in Al-rich H-*BEA
The negligible intensity of the internal silanol groups, the low
signal of the OH groups bound to extra-framework and/or per-
turbed framework Al atoms and very high intensity of the structural
bridging OH groups observed in the 1H MAS NMR and FTIR spectra
of Al-rich H-*BEA are characteristic of a non-defective crystalline
structure with a high concentration of non-interacting (isolated)
acid sites associated with tetrahedrally coordinated Al atoms
in the zeolite framework. Consistently, the distinctively ordered
nature and essentially defect free structure for Al-rich H-*BEA were
reported by Yilmaz et al. [58] and Sasaki et al. [59]. In Al-rich *BEA
zeolites (Si/Al 4–5), the Al atoms are predominantly present in the
Al-Si-Al sequences [29,33,53,60,61] with the concentrations of Al-
Si-Al ranging from 40% to 100% of the total Al depending on their
arrangement in rings or as long sequences [53]. It follows that
the majority of the H+ ions in the H-*BEA/4.2 sample compensate
the negative charge resulting from substitution of the framework
Si(IV) by Al(III) in the Al-Si-Al sequences; however, the respective
OH groups are reflected in the 1H MAS NMR and FTIR spectra as
non-interacting OH groups. Our previous 27Al and 29Si (CP) MAS
NMR studies supplemented by FTIR of adsorbed d3-acetonitrile
and UV–vis spectroscopy of Co(II) ions as probes of close Al atoms
and further supported by DFT molecular dynamic calculations of
the Co(II) sites in the *BEA zeolites revealed that these Al-Si-Al
sequences in Al-rich *BEA zeolites are mostly located in the zeo-
lite wall separating two channels and the Al atoms of the sequence
thus face two channels (see Fig. 5 and Ref. [53]). The negative
charge of the framework originating from these sequences is bal-
anced by two H+ ions located in different channels. Therefore the
high concentration of Al atoms in the framework of Al-rich beta
zeolites does not result in increased formation of interacting OH

P. Sazama et al. / Applied Catalysis A: General 533 (2017) 28–37
33
A)
B)
0.5
0.5
25 500 cm-1
25 500 cm-1
Carbocationic
H-*BEA/11+HMB+NH3
H-*BEA/4.2+HMB+NH3
f)
Carbocationic
HMB
f)
3 h
2 h
1 h
HMB
3 h
2 h
HMB
HMB
1 h
H-*BEA/4.2+HMB
H-*BEA/4.2
c-e)
c-e
)
H-*BEA/11+HMB
H-*BEA/11
b)
a)
b)
a)
40000
35000
30000
25000
20000
40000
35000
30000
25000
20000
Wavenumber (cm-1)
Fig. 4. UV-vis spectra of carbocations formed by protonization of hexamethylbenzene (HMB) in A) Al-rich and B) Si-rich H-*BEA zeolites. Spectra of a) zeolites dehydrated
at 500 ◦C, b) solid HMB, c–e) zeolite after interaction with HMB at 150 ◦C for 1, 2 and 3 h, respectively, and f) after subsequent interaction with NH3.
Fig. 5. Schematic representations of the main Al-O-(SiO)n-Al sequences in A) Al-rich and B) Si-rich H-*BEA zeolites. H+ compensates the charge mainly from Al-O-Si-O-Al
sequences in Al-rich H-*BEA whereas the closest Al atoms form an Al-O-(SiO)2-Al sequence in Si-rich H-*BEA. The Al-O-Si-O-Al sequences cross the zeolite wall and the
corresponding H+ are located in two different channels in Al-rich H-*BEA. Oxygens in red.
NH3 desorbs from Al-rich H-*BEA. The pair of Al- and Si-rich H-*BEA
zeolites thus represents unique material that allows elucidation of
the effect of the close vicinity of non-interacting acidic protons on
activation of a hydrocarbon molecule in acid-catalysed reactions
over zeolite catalysts, moreover, in the three-dimensional chan-
nel system with 12-MR openings providing easy intra-crystalline
diffusion of reactants and products.
groups but the Al-Si-Al sequences forming the zeolite beta wall pro-
vide H+ sites like in a Si-rich zeolite but in significantly increased
concentrations. The calculated deprotonation energies used as a
measure of the acid strengths of the Brønsted sites corresponding
to AlSiAl and AlSiSiAl sequences in Al- and Si-rich *BEA, respec-
tively, reported in the previous study [33], were in the range of
differences in the deprotonation energy values among the protons
associated with AlOHSi groups at individual T sites in the frame-
work of *BEA zeolites. This is consistent with the small observed
shift in the maxima of the band of bridging hydroxyl groups in the
1H MAS NMR and FTIR spectra, and the similar protonating ability
of HMB molecules. Suganuma et al. [62] assessed the Brønsted acid
strength and related acidic property of Al-rich H-*BEA zeolites using
analysis of the enthalpy of ammonia desorption combined with
FTIR measurements. The enthalpy distributed mainly in the range
between 115 and 145 kJ mol−1 demonstrated that the Al-rich H-
*BEA is a strongly acidic zeolite, essentially having the acid strength
in the region generated by isomorphous substitution of Si by Al
in the common *BEA framework [62]. Similarly, De Baerdemaeker
et al. [29] deduced the presence of a large amount of remarkably
strong acid sites from the amount and the temperature at which the
3.4. The effect of the density of acid sites on n-hexane
hydroisomerization
Fig. 6 depicts the yields of branched hexanes and low molec-
ular products as a function of temperature and Arrhenius plots
for hydroisomerization of n-hexane over Al- and Si-rich Pt/H-*BEA
zeolites. The high concentration of close strongly acidic sites in
Al-rich *BEA led to a significantly improved yield of branched iso-
mers with very low yield of undesired C1–C5 products compared
Si-rich *BEA zeolites over the entire temperature range from 125
to 225 ◦C. The high density of Brønsted sites facilitated increased
conversion of n-hexane to iso-hexane from 5.9 to 35.5% practically
without formation of side-products (0.2% yield of C1–C5) at 175 ◦C

34
P. Sazama et al. / Applied Catalysis A: General 533 (2017) 28–37
Fig. 6. Hydroisomerization of n-hexane over Al-rich *BEA (PtH-*BEA/4.2) compared with Si-rich *BEA zeolite (PtH-*BEA/11). A) Effect of temperature on the yields of branched
C6 isomers and C1–C5 by-products and B) Arrhenius plot. Reaction conditions: WHSV 0.25 h−1, molar ratio H2/n-C6 79, atmospheric pressure.
Table 2
surements. HR-TEM images of all the prepared samples (Fig. S2)
showed the presence of well-dispersed platinum clusters located
on the external surface of zeolite crystals with a wide distribu-
Hydroisomerization of n-hexane to 2-methyl- and 3-methylpentanes, reaction rates
(rISO) and turnover frequencies (TOF) for 145 ◦C.
Sample
ayiso-C6
%
riso
bTOF
s−1
tion of particle sizes ∼1–20 nm. However, we cannot exclude the
presence of platinum also in the zeolite channels. The compari-
son of the absorption bands in the FTIR spectra in the region of
hydroxyl group before and after Pt introduction into Si- and Al-rich
H-*BEA zeolites indicated only very small changes in the inten-
sity of the bridging OH groups (not shown). This indicates that
the Pt species are not coordinated into cationic sites in a signifi-
cant extent after the reduction of Pt in a hydrogen atmosphere. The
oxidation state and nature of the platinum species located inside
the zeolite channels and the outer surface was analysed using the
FTIR spectra of CO adsorbed on the Al-rich Pt/H-*BEA compared
with the Si-rich Pt/H-MOR and the Pt/H-*BEA zeolites (Fig. 7). The
positions of the FTIR absorption bands of adsorbed CO on Pt2+ coun-
molng−1 s−1
PtH-BEA/4.2
PtH-BEA/12
5.68
0.99
5.7 10−8
9.8 10−9
1.8 10−5
8.1 10−6
Reaction conditions: WHSV 0.25 h−1, molar ratio H2/n-C6 79, atmospheric pressure.
a
Yield of 2-methyl- and 3-methylpentanes while the formation of di-branched
hexane isomers and lower molecular weight by-products was negligible.
b
TOF calculated for total concentration of Al.
(Table S1). The reaction rate per gram of Al-rich *BEA zeolite, calcu-
lated at low conversions of n-hexane, high selectivity for 2-methyl-
and 3-methylpentanes (>95%), low selectivity for di-branched hex-
ane isomers (<5%), and negligible selectivity for lower molecular
weight by-products (<0.1%), exceeds six fold that of Si-rich *BEA
(Table 2). More than twice higher TOF calculated per total concen-
tration of Al for Al-rich *BEA clearly shows the substantially higher
specific activity of the active sites. The remarkable improvement in
the catalytic performance is reflected in the lower apparent activa-
tion energy of 104.2 vs. 118.9 kJ mol−1 obtained from the Arrhenius
plots for Al- and Si-rich *BEA, respectively, in accordance with the
higher specific activity of the active sites. The six-fold higher reac-
tion rate, higher TOF and the lower apparent activation energy by
␦+
terions charge balanced by the zeolite framework, Ptn clusters,
and reduced Pt metallic clusters were reported at 2170–2150 cm−1
,
2130–2140 cm−1, and 2050–2100 cm−1, respectively [64,65]. The
analysis of the spectrum of Al-rich Pt/H-*BEA reveals the presence
of a major absorption band at 2078 cm−1 characteristic for reduced
Pt metallic clusters of 1–20 nm in size [64,65] located on the exter-
nal surface of zeolite crystal without the presence of any unusual
absorption bands. The absence of a significant absorption inten-
sity at about 2170–2130 cm−1 indicates a reduction of Pt into the
zero-valent oxidation state of metallic Pt in the Al-rich Pt/H-*BEA
zeolite. The broad intensity of the band reflects the presence of
platinum species of non-uniform sizes which is typical for Pt intro-
duced by an impregnation method. The representative HR-TEM
image of Pt/H-*BEA/4.2 shows well-dispersed cubic and tetrahedral
Pt-nanoparticles located on the external interior of zeolite crystals
with a wide distribution of particle sizes ∼1–20 nm (Fig. 7B). The
spectrum of adsorbed CO on Si-rich Pt/H-*BEA exhibits compared to
Al-rich Pt/H-*BEA higher intensity of the band at about 2080 cm−1
indicating a better dispersion of Pt on the large external surface of
small crystals of Si-rich Pt/H-*BEA. These results are consistent with
the observations that the reactions occurring on Pt for high metal
loadings (∼1% Pt) are much faster than on acid sites and the rate of
hydroisomerization is independent of the metal area of dispersed
Pt clusters [63,66]. Thus Pt dispersed in the form of metallic clus-
ters can provide sufficient functionality regardless of the specifics
of their structure for all the Pt/H-zeolite catalysts.
ca. 15 kJ mol−1 for Al-rich BEA zeolite clearly indicate a synergis-
tic effect of the high concentration and close proximity of the OH
groups.
*
3.5. Effect of platinum on n-hexane hydroisomerization
A sufficient concentration of functional clusters of metallic plat-
inum is essential for ensuring the dehydrogenation/hydrogenation
reactions not limiting the overall alkane hydroisomerization on the
bifunctional Pt-zeolite catalysts [38,63]. Ribeiro et al. [63] showed
that the hydrogenation-dehydrogenation reaction was much faster
than the skeletal isomerization on acid centers and the isomeriza-
tion did not depend on the surface of the metal for the metal loading
from 0.5 to 6 wt.% of dispersed Pt on the faujasite zeolite. Van de
Runstraat demonstrated that the metal function was not rate deter-
mining for Pt-*BEA with the metal loading ∼1.5 wt.% Pt. For our
study, all the Pt-zeolites were prepared with a concentration of 1.5%
Pt. The platinum dispersity in the prepared zeolites was checked
by HR-TEM and a CO sorption followed by FTIR spectroscopy mea-
To further analyse whether the activity of the Al-rich Pt/H-
*BEA zeolite could also be associated with the specificity of Pt

P. Sazama et al. / Applied Catalysis A: General 533 (2017) 28–37
35
Fig. 7. Analysis of Pt in Pt/H-zeolites. A) FTIR spectra of CO adsorbed on the Pt/H-zeolites reduced by hydrogen at 250 ◦C and B) the representative HR-TEM image of
Pt/H-*BEA/4.2.
from the acid sites is important for the high activity of the Al-rich
Pt/H-*BEA catalyst.
3.6. Implication of the enhanced activity of Al-rich Pt/H-*BEA on
isomerization of n-hexane
It is well established that mordenite zeolites with strongly
acidic bridging Si-OH-Al sites are the most active and selective
among all the types of investigated zeolitic hydroisomerization
catalysts [3,13,14,17,67,68]. However, considerably lower activity
of mordenite zeolites compared to chlorinated alumina necessi-
tates isomerization at higher temperatures leading to unfavorable
thermodynamic equilibrium for branched isomers. An essential
solution of the problem lies in significant enhancement of the iso-
merization activity that enables reaching high conversions at lower
temperatures in a favorable area of the thermodynamic equilib-
rium. The remarkable increase in the yield of branched isomers and
an increase in the selectivity for the desired products limiting crack-
ing reactions over Al-rich Pt/H-*BEA enables a shift of the operation
window to lower temperatures. To analyze the potential of the Al-
rich *BEA zeolite as an environmentally sustainable low/medium
temperature hydroisomerization catalyst, the activity, selectivity
and durability of Al-rich Pt/H-*BEA were compared with state-
of-the-art microporous and micro-mesoporous mordenite zeolites
under model reaction conditions and also the relevant conditions
of the hydroisomerization process, i.e. at elevated pressure and
at high concentrations of n-hexane in the reaction stream. The
yields of branched hexane isomers and C1–C5 by-products as a
function of temperature at low pressures of n-hexane compared
for Al-rich Pt/H-*BEA/4.2 and microporous and micro-mesoporous
Pt/H-mordenite zeolites are shown in Fig. 9. The distribution
of the formed mono-branched isomers (2-methylpentane and
3-methylpentane) and di-branched isomers (2,2-dimethylbutane
and 2,3-dimethyl butane) is listed in Table S1. The yield of branched
isomers was approximately 7 times higher (at 175 ◦C) over Al-rich
BEA than over MOR, while the yield of C1–C5 by-products was com-
parable or lower to that over mordenite catalysts. The performance
of the mordenite catalysts was comparable to Si-rich *BEA catalysts.
The performance characteristics of Al-rich Pt/H-*BEA compared
to Si-rich Pt/H-*BEA and MOR zeolites under elevated pressure and
at high concentration of hexane in the hydrogen stream relevant to
the catalytic processes are listed in Table 3. A threefold increase in
the concentration of the active sites and the synergetic effect clearly
resulted in the desired increase in activity and selectivity. The Al-
rich Pt/H-*BEA/4.2 gives an isomer yield of 51.1% at a temperature of
Fig. 8. Comparison of the conversions of n-hexane and the yields of branched C6
isomers and C1–C5 by-products in the hydroisomerization reaction over *BEA and
MOR catalysts with Pt introduced by a conventional incipient wetness impregnation
(Pt/H-*BEA/4.2 and Pt/H-MOR/12) and the mechanical mixtures of the protonic form
of zeolite and Pt/Al2O3 (H-*BEA/4.2-Pt/Al2O3 and H-MOR/12-Pt/Al2O3) at 225 ◦C. The
reaction conditions same as in Fig. 6.
species, additional samples of Pt/H-MOR and Al-rich Pt/H-*BEA
were prepared with the same concentration of platinum (1.5 wt.%)
but differing in the form of Pt species and their distance from
the acidic sites. The samples H-*BEA/5-Pt/Al2O3 and H-MOR/12-
Pt/Al2O3 were prepared as mechanical mixtures of the protonic
form of the irrespective zeolite and Pt/Al2O3. The Pt/Al2O3 was
obtained using conventional incipient wetness impregnation of
Al2O3 and subsequent calcination in air and reduction in hydrogen.
The mixture was mixed to give a final Pt concentration of 1.5 wt.%
and the resulting material was not calcined to preclude migra-
tion of Pt. Note that the hydroisomerization activity of resulting
Pt/Al2O3 was negligible at a temperature up to 225 ◦C. Fig. 8 com-
pares the yields of branched C6 isomers and C1–C5 by-products over
the catalysts obtained by a conventional incipient wetness impreg-
nation and the mechanical mixtures of the H-zeolites and Pt/Al2O3
at 225 ◦C. It is obvious that the yields were practically identical for
both the MOR samples, however, slightly lower for the H-*BEA/5-
Pt/Al2O3 compared the Pt/H-*BEA/5. It indicates a minor role of the
distance of Pt sites from the active acid sites on the activity of the
Pt/H-*BEA/5. It indicates that the activity of the Al-rich Pt/H-*BEA
is not connected with a specific Pt dispersion but the distance of Pt

36
P. Sazama et al. / Applied Catalysis A: General 533 (2017) 28–37
Table 3
The yields of branched hexane isomers and lower molecular weight by-products for hydroisomerization of n-hexane analyzed under process-like conditions.
PtH-BEA/4.2 PtH-BEA/12 PtH-MOR/12 PtH-MOR/10
200 ◦
C
215 ◦
C
200 ◦
C
215 ◦
C
200 ◦
C
215 ◦
C
200 ◦
C
215 ◦
C
yꢁiso-C6 (%)
y2MP+3MP (%)
y2,2DMB (%)
y2,3DMB (%)
yby-productsC1–C4 (%)
51.1
44.8
1.5
4.8
0.11
76.8
62.1
6.7
8
12.1
11.9
0.08
0.2
31.8
29.1
1.5
1.2
26.7
10.1
8.8
0.32
0.93
0.58
25.4
21.8
1
2.6
1.3
10.5
9.5
0.95
0.86
0.65
28.1
24.5
0.95
2.67
1.7
2.0
10.7
Reaction conditions: WHSV 0.7 h−1, molar ratio H2/n-C6 6, pressure 10 bar.
acid-catalysed reactions requiring strongly acidic centers like alky-
lation and hydroamination of aromatics [33,58], and cracking and
hydrocracking of aromatics and alkanes [27,29,33] are substantially
improved over Al-rich H-*BEA exceeding significantly the Si-rich
H-*BEA zeolites. Thus, the Al-rich H-*BEA provides the unique con-
centration of strongly acidic sites in the three-dimensional channel
system with 12-MR openings. Such density of the acid sites is
not available in zeolites with the three-dimensional channel sys-
tem with 10-MR openings, e.g. MFI structural topology, due to the
absence of a synthesis procedure for the preparation of zeolite with
molar Si/Al ratio <10. Al-rich H-*BEA is also advantageous to Y
zeolites, wherein the protonic sites are mutually affected due to
the presence of Al-Si-Al sequences in the framework and exhibit
much lower acid strengths [69]. The three-dimensional channel
system with 12-MR openings of Al-rich H-*BEA is also a significant
advantage over traditional mordenite based isomerization cata-
lysts where the pseudo-monodimensional channel structure and
the restricted accessibility of acid sites located in 8-MR channels
limit the efficiency of the catalytic process by mass transfer effects
[70].
Fig. 9. Comparison of Al-rich *BEA (PtH-*BEA/4.2) with microporous PtH-MOR/12
and partially dealuminated micro-mesoporous PtH-MOR/10 zeolites in hydroiso-
merization of n-hexane. Effect of temperature on the yields of branched C6 isomers
and C1–C5 by-products. The reaction conditions same as in Fig. 6.
The reduced apparent activation energy and increased TOF for
Al-rich H-*BEA zeolite indicate only a partial contribution of the
increased concentration of protonic sites to the overall enhance-
ment of the catalytic properties and therefore another factor
associated with the close proximity of the active sites plays an
important role. Conversely, there are no spectroscopic signatures
of significantly different acidic characteristics in the Al-rich beta
zeolite. Because the enhancement of the strength of the acid groups
has not been observed, the increase in specific activity of close non-
interacting OH groups must be accompanied with another specific
synergistic effect. Effects of the proximity of Brønsted and Lewis
sites [71], the arrangement of acid centers influencing the entropy
of the system or an interaction of one molecule with two close cen-
ters could possibly affect the overall catalytic process. However,
we can not rule out that the overall increase in specific activity is
given by contributions from others synergistic effects. The analy-
sis of effects of the proximity of acid sites on reaction mechanism
of skeletal isomerization of alkanes needs to be accessed in fur-
ther studies to obtain full comprehending the consequences of the
specific structure.
Fig. 10. Dependence of the yield of branched hexane isomers and C1–C4 by-products
on time-on-stream in hydroisomerization of n-hexane over Al-rich *BEA (PtH-
*BEA/4.2). Reaction conditions: Temperature 215 ◦C, WHSV 0.7 h−1, molar ratio
H2/n-C6 6, pressure 10 bar.
4. Conclusions
200 ◦C, whereas the Si-rich Pt/H-zeolites lead to isomers yields from
10.1 to 12.1% at the same temperature. Stable values of the yields
of isomers ∼74–77% and by-products ∼2% as a function of time-on-
stream were obtained at 215 ◦C for 76 h that indicates stability of
the Al-rich Pt/H-*BEA (Fig. 10).
It is clear that the unique density and distribution of the strongly
acidic sites are ultimately connected with catalytic performance
highly exceeding that of state-of-the-art Si-rich zeolite catalysts.
There is no evidence in the scientific literature that the strength
of the acidity of the Al-rich H-*BEA zeolite is significantly weak-
ened due to its high aluminum content, at least with respect to
acid-catalysed transformations of hydrocarbons. On the contrary,
The critical function of the density of the acidic protons for
hydroisomeroization of n-hexane was elucidated using the H-forms
of the beta zeolites with a very high concentration of aluminum
(Si/Al ≥ 4) with highly predominant tetrahedrally coordinated Al
atoms in the framework. Analysis of the relationships between the
density and distribution of strongly acidic sites and n-hexane iso-
merization identified a specific arrangement of Brønsted acid sites
directing the reaction toward higher reaction rates. A high density
of strongly acidic non-interacting close OH groups in the Al-rich
H-*BEA zeolite (Si/Al ∼ 4) lowers the activation barrier in the iso-
merization reaction and results in multiplying the reaction rates by

P. Sazama et al. / Applied Catalysis A: General 533 (2017) 28–37
37
a factor of six compared to the hitherto most active Si-rich zeolite
catalysts. The arrangement of Brønsted acid sites is unique for the
Al-rich H-*BEA zeolite (Si/Al ∼ 4) and is given by the arrangement of
Al atoms in the Al-Si-Al sequences charge-balanced by two H+ ions
located in different zeolite channels. In conclusion, the achieve-
ment of the high concentration of non-interacting acidic protons
in the zeolite catalyst allowed a significant increase in the isomer-
ization activity and a shift of the operation temperature window
into the thermodynamically more favourable region for desired
di-branched isomers.
[30] H. Zhang, B. Xie, X. Meng, U. Mueller, B. Yilmaz, M. Feyen, S. Maurer, H. Gies, T.
Tatsumi, X. Bao, W. Zhang, D. De Vos, F.-S. Xiao, Microporous Mesoporous
Mater. 180 (2013) 123–129.
[31] Y. Kubota, K. Itabashi, S. Inagaki, Y. Nishita, R. Komatsu, Y. Tsuboi, S. Shinoda,
T. Okubo, Chem. Mater. 26 (2014) 1250–1259.
[32] B. Zheng, Y. Wan, W. Yang, F. Ling, H. Xie, X. Fang, H. Guo, Chin. J. Catal. 35
(2014) 1800–1810.
[33] P. Sazama, B. Wichterlova, S. Sklenak, V.I. Parvulescu, N. Candu, G. Sadovska, J.
Dedecek, P. Klein, V. Pashkova, P. Stastny, J. Catal. 318 (2014) 22–33.
[34] R.B. Borade, A. Clearfield, Chem. Commun. (Cambridge, U. K.) (1996) 625–626.
[35] B. Wichterlova, Z. Tvaruzkova, Z. Sobalik, P. Sarv, Microporous Mesoporous
Mater. 24 (1998) 223–233.
[36] C.A. Fyfe, Y. Feng, H. Grondey, G.T. Kokotailo, H. Gies, Chem. Rev. (Washington
DC, U. S.) 91 (1991) 1525–1543.
[37] G. Engelhardt, U. Lohse, E. Lippmaa, M. Tarmak, M. Magi, Z. Anorg, Allg. Chem.
482 (1981) 49–64.
Acknowledgements
[38] A. Van De Runstraat, J.A. Kamp, P.J. Stobbelaar, J. Van Grondelle, S. Krijnen,
R.A. Van Santen, J. Catal. 171 (1997) 77–84.
[39] W.O. Haag, R.M. Lago, P.B. Weisz, Faraday Discuss. Chem. Soc. 72 (1981)
317–330.
[40] M. Muller, G. Harvey, R. Prins, Microporous Mesoporous Mater. 34 (2000)
135–147.
[41] H. Huo, L. Peng, Z. Gan, C.P. Grey, J. Am. Chem. Soc. 134 (2012) 9708–9720.
[42] P. Strodel, K.M. Neyman, H. Knozinger, N. Rosch, Chem. Phys. Lett. 240 (1995)
547–552.
This work was supported by the Czech Science Foundation
under project # 15-12113S and RVO: 61388955. The authors
acknowledge the assistance provided by the Research Infrastruc-
tures NanoEnviCz and pro-NanoEnviCz, supported by the Ministry
of Education, Youth and Sports of the Czech Republic under Project
No. LM2015073.
[43] M. Sigl, S. Ernst, J. Weitkamp, H. Knozinger, Catal. Lett. 45 (1997) 27–33.
[44] I. Kiricsi, C. Flego, G. Pazzuconi, W.O. Parker, R. Millini, C. Perego, G. Bellussi, J.
Phys. Chem. 98 (1994) 4627–4634.
Appendix A. Supplementary data
[45] J. Brus, L. Kobera, W. Schoefberger, M. Urbanová, P. Klein, P. Sazama, E. Tabor,
S. Sklenak, A.V. Fishchuk, J. Dedecek, Angew. Chem. Int. Ed. 54 (2015)
541–545.
[46] P. Sazama, B. Wichterlova, J. Dedecek, Z. Tvaruzkova, Z. Musilova, L. Palumbo,
S. Sklenak, O. Gonsiorova, Microporous Mesoporous Mater. 143 (2011) 87–96.
[47] E. Bourgeat-Lami, P. Massiani, F. Di Renzo, P. Espiau, F. Fajula, T. Des Courieres,
Appl. Catal. 72 (1991) 139–152.
[48] L.W. Beck, J.F. Haw, J. Phys. Chem. 99 (1995) 1076–1079.
[49] L.C. De Menorval, W. Buckermann, F. Figueras, F. Fajula, J. Phys. Chem. 100
(1996) 465–467.
[50] P.J. Kunkeler, B.J. Zuurdeeg, J.C. Van Der Waal, J.A. Van Bokhoven, D.C.
Koningsberger, H. Van Bekkum, J. Catal. 180 (1998) 234–244.
[51] G.H. Kuehl, H.K.C. Timken, Microporous Mesoporous Mater. 35-36 (2000)
521–532.
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.apcata.2016.12.
016.
References
[1] E. Iglesia, S.L. Soled, G.M. Kramer, J. Catal. 144 (1993) 238–253.
[2] E. Iglesia, D.G. Barton, S.L. Soled, S. Miseo, J.E. Baumgartner, W.E. Gates, G.A.
Fuentes, G.D. Meitzner, Stud. Surf. Sci. Catal. (1996) 533–542.
[3] A. Chica, A. Corma, J. Catal. 187 (1999) 167–176.
[4] Y. Ono, Catal. Today 81 (2003) 3–16.
[5] J. Macht, R.T. Carr, E. Iglesia, J. Am. Chem. Soc. 131 (2009) 6554–6565.
[6] R. Roldán, F.J. Romero, C. Jiménez-Sanchidrián, J.M. Marinas, J.P. Gómez, Appl.
Catal. A 288 (2005) 104–115.
[7] W. Knaeble, R.T. Carr, E. Iglesia, J. Catal. 319 (2014) 283–296.
[8] M. Digne, P. Raybaud, P. Sautet, D. Guillaume, H. Toulhoat, J. Am. Chem. Soc.
130 (2008) 11030–11039.
[52] J. Penzien, A. Abraham, J.A. Van Bokhoven, A. Jentys, T.E. Muller, C. Sievers, J.A.
Lercher, J. Phys. Chem. B 108 (2004) 4116–4126.
[53] P. Sazama, E. Tabor, P. Klein, B. Wichterlova, S. Sklenak, L. Mokrzycki, V.
Pashkkova, M. Ogura, J. Dedecek, J. Catal. 333 (2016) 102–114.
[54] J.R. Anderson, Y.F. Chang, R.J. Western, J. Catal. 118 (1989) 466–482.
[55] M. Bjørgen, F. Bonino, S. Kolboe, K.P. Lillerud, A. Zecchina, S. Bordiga, J. Am.
Chem. Soc. 125 (2003) 15863–15868.
[56] M. Ma, K.E. Johnson, J. Am. Chem. Soc. 117 (1995) 1508–1513.
[57] K. Hemelsoet, Q. Qian, T. De Meyer, K. De Wispelaere, B. De Sterck, B.M.
Weckhuysen, M. Waroquier, V. Van Speybroeck, Chem. Eur. J. 19 (2013)
16595–16606.
[58] B. Yilmaz, U. Muller, M. Feyen, S. Maurer, H. Zhang, X. Meng, F.S. Xiao, X. Bao,
W. Zhang, H. Imai, T. Yokoi, T. Tatsumi, H. Gies, T. De Baerdemaeker, D. De
Vos, Catal. Sci. Technol. 3 (2013) 2580–2586.
[59] Y. Sasaki, Y. Yoshida, C.A.J. Fisher, T. Ikeda, K. Itabashi, T. Okubo, Microporous
Mesoporous Mater. 225 (2016) 210–215.
[60] P. Sazama, L. Mokrzycki, B. Wichterlova, A. Vondrova, R. Pilar, J. Dedecek, S.
Sklenak, E. Tabor, J. Catal. 332 (2015) 201–211.
[61] P. Sazama, R. Pilar, L. Mokrzycki, A. Vondrova, D. Kaucky, J. Plsek, S. Sklenak, P.
Stastny, P. Klein, Appl. Catal. B 189 (2016) 65–74.
[62] S. Suganuma, H. Zhang, C. Yang, F.S. Xiao, N. Katada, J. Porous Mater. 23 (2016)
415–421.
[63] F. Ribeiro, C. Marcilly, M. Guisnet, J. Catal. 78 (1982) 267–274.
[64] A.Y. Stakheev, E.S. Shpiro, O.P. Tkachenko, N.I. Jaeger, G. Schulz-Ekloff, J. Catal.
169 (1997) 382–388.
[65] P. Kubanek, H.W. Schmidt, B. Spliethoff, F. Schüth, Microporous Mesoporous
Mater. 77 (2005) 89–96.
[66] R. Monteiro, C.O. Ania, J. Rocha, A.P. Carvalho, A. Martins, Appl. Catal. A 476
(2014) 148–157.
[67] K.J. Chao, H.C. Wu, L.J. Leu, Appl. Catal. A 143 (1996) 223–243.
[68] N. Viswanadham, L. Dixit, J.K. Gupta, M.O. Garg, J. Mol. Catal. A Chem. 258
(2006) 15–21.
[69] N. Wang, M. Zhang, Y. Yu, Microporous Mesoporous Mater. 169 (2013) 47–53.
[70] M. Tromp, J.A. van Bokhoven, M.T. Garriga Oostenbrink, J.H. Bitter, K.P. de
Jong, D.C. Koningsberger, J. Catal. 190 (2000) 209–214.
[71] J.A. Van Bokhoven, B.A. Williams, W. Ji, D.C. Koningsberger, H.H. Kung, J.T.
Miller, J. Catal. 224 (2004) 50–59.
[9] L. Gora, J.C. Jansen, J. Catal. 230 (2005) 269–281.
[10] H. Weyda, E. Köhler, Catal. Today 81 (2003) 51–55.
[11] A.N. Shakun, M.L. Fedorova, Catal. Ind. 6 (2014) 298–306.
[12] P. Sazama, Z. Sobalik, J. Dedecek, I. Jakubec, V. Parvulescu, Z. Bastl, J.
Rathousky, H. Jirglova, Angew. Chem. Int. Ed. 52 (2013) 2038–2041.
[13] M. Tromp, J.A. van Bokhoven, M.T.G. Oostenbrink, J.H. Bitter, K.P. de Jong, D.C.
Koningsberger, J. Catal. 190 (2000) 209–214.
[14] S.V. Konnov, I.I. Ivanova, O.A. Ponomareva, V.I. Zaikovskii, Microporous
Mesoporous Mater. 164 (2012) 222–231.
[15] D.G. Barton, S.L. Soled, G.D. Meitzner, G.A. Fuentes, E. Iglesia, J. Catal. 181
(1999) 57–72.
[16] G.D. Yadav, J.J. Nair, Microporous Mesoporous Mater. 33 (1999) 1–48.
[17] H. Chiang, A. Bhan, J. Catal. 283 (2011) 98–107.
[18] W.O. Haag, R.M. Lago, P.B. Weisz, Nature 309 (1984) 589–591.
[19] A. Janda, A.T. Bell, J. Am. Chem. Soc. 135 (2013) 19193–19207.
[20] A.J. Jones, R.T. Carr, S.I. Zones, E. Iglesia, J. Catal. 312 (2014) 58–68.
[21] P. Sazama, J. Dedecek, V. Gabova, B. Wichterlova, G. Spoto, S. Bordiga, J. Catal.
254 (2008) 180–189.
[22] B. Xie, J. Song, L. Ren, Y. Ji, J. Li, F.S. Xiao, Chem. Mater. 20 (2008) 4533–4535.
[23] K. Shanjiao, D. Tao, L. Qiang, D. Aijun, Z. Yanying, P. Huifang, J. Porous Mater.
15 (2008) 159–162.
[24] G. Majano, L. Delmotte, V. Valtchev, S. Mintova, Chem. Mater. 21 (2009)
4184–4191.
[25] Y. Kamimura, W. Chaikittisilp, K. Itabashi, A. Shimojima, T. Okubo,
Chem.—Asian J. 5 (2010) 2182–2191.
[26] Y. Kamimura, S. Tanahashi, K. Itabashi, A. Sugawara, T. Wakihara, A.
Shimojima, T. Okubo, J. Phys. Chem. C 115 (2011) 744–750.
[27] B. Xie, H. Zhang, C. Yang, S. Liu, L. Ren, L. Zhang, X. Meng, B. Yilmaz, U. Muller,
F.-S. Xiao, Chem. Commun. (Cambridge U. K.) 47 (2011) 3945–3947.
[28] K. Itabashi, Y. Kamimura, K. Iyoki, A. Shimojima, T. Okubo, J. Am. Chem. Soc.
134 (2012) 11542–11549.
[29] T. De Baerdemaeker, B. Yilmaz, U. Muller, M. Feyen, F.S. Xiao, W. Zhang, T.
Tatsumi, H. Gies, X. Bao, D. De Vos, J. Catal. 308 (2013) 73–81.