Enhancement of concentration and functionality of active sites
in zeolite based catalysts for acid-catalysed reactions
Petr Sazama1*, Radim Pilar1, Lukasz Mokrzycki1, Vasile I. Parvulescu2, Edyta Tabor1, Alena Vondrova1,
Dalibor Kaucky1, Petr Klein1, Jiri Dedecek1, Stepan Sklenak1, Jaroslava Moravkova1
1J.
Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejskova 3, 182 23 Prague, Czech
Republic
2University of Bucharest, Department of Organic Chemistry and Catalysis, Romania
*Corresponding author: [email protected]
Keywords: Acid-catalysed reactions; zeolites; isomerization; alkylation; cracking; hydroamination.
1. Introduction
The discovery of new synthesis procedures of the
beta zeolite employing seeding of Si-rich beta
crystals in the complete absence of an organic
structure-directing agent [1-3] led to the synthesis of
Al-rich beta zeolites with a high concentration of
tetrahedrally coordinated Al in the framework Si/Al
≥ 4. The high concentration of Al in the framework
of the template-free synthesized beta zeolite
provides charge balance for very high concentration
of protons of comparable strength with that of the
protons of Si-rich zeolites [4-7].
In pursuing the present work, 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 concentration of Al in the
zeolite framework. We wanted to determine the
effect of variations in the corresponding high
concentration protons located in the close vicinity on
the reaction rate and selectivity in relevant acidcatalysed reactions. The obtained results illustrate
the unparalleled potential of Al-rich beta zeolites for
obtaining significantly increased concentrations of
active centres with enhanced specific activities for
variety of acid-catalysed reactions.
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
[8].
Cracking
of
n-decane,
hydroisomerization of n-hexane, alkylation of
benzene and hydroamination of styrene over H*
BEA were investigated regarding the concentration
and nature of acid sites.
3. Results and Discussion
We exploited recent progress in the synthesis of the
beta zeolite and manipulated the framework
aluminum content in a very broad range (Si/Al 4 20) with highly predominant tetrahedrally
coordinated Al in the framework [4-7]. The acid
strength of protons of the bridging OH groups in Alrich H-*BEA (Si/Al 4) is not significantly decreased,
as supported by the very small shift in the structural
OH vibrations and less-than-significant differences
in the deprotonation energies for OH groups related
to the individual Si-Al sequences [4, 6].
Si
Al
Si
H
Si
Al
Al
H
2. Experimental
Al-rich beta zeolites were hydrothermally
synthesized by a procedure based on the reports in
Refs. [1-6] and characterised by XRD, N2 adsorption
at -196 °C, SEM, 27Al and 29Si MAS NMR, XPS and
FTIR spectroscopy. The high-silica zeolites used as
standards for comparing the catalytic properties
were kindly supplied by the Tricat 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). A hierarchical
mordenite zeolite with optimal micro-mesoporous
H
Al
Si
Si
H
H
H
Al
Al
Si
Figure 1. Schematic representations of the main Al-O-SiO-Al sequences in Al-rich H-*BEA. The Al-Si-Al
sequences cross the zeolite wall and the corresponding H +
are located in two different channels in Al-rich H-*BEA.
Oxygens in red.
Al-Si-Al sequences in Al-rich *BEA zeolites are
mostly located in the zeolite wall separating two
channels and the Al atoms of the sequence thus face
two channels (Figure 1). The negative charge of the
framework originating from these sequences is
balanced 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
groups but the Al-Si-Al sequences forming the
zeolite beta wall provide H+ sites like in a Si-rich
zeolite but in significantly increased concentrations.
We examined the extent to which the cracking
of n-decane, hydroisomerization of n-hexane,
alkylation of benzene and hydroamination of styrene
reactions are affected by the 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. The dramatic
increase in the yield of branched isomers in the
hydroisomerization reaction over Al-rich Pt/H-*BEA
resulted in the shift of the operation window to low
temperatures (Table 1). A high increase in the
conversions in alkylation of benzene with benzyl
alcohol to diphenyl methane, and hydroamination of
styrene with aniline to phenyl-[2-phenylethyl]amine
and phenyl-[1-phenylethyl]amine over Al-rich H*
BEA are given by a threefold increase in the
concentration of active sites and synergetic effects
increasing the specific activity of the protonic sites
(Table 1).
Table 1. Activity of Al-rich H-BEA* in acid-catalysed reactions
compared to representative Si-rich H-BEA* zeolite.
Catalytic process
Cracking of n-decane
r (molC10H22.kgcat-1.h-1)
at 500 °C
Hydroisomerization of
n-hexane
riso (mol.gcat-1.s-1 *108)
at 175 °C
Alkylation of benzene
with benzyl alcohol
Xbenzyl alcohol (%)
Hydroamination of
styrene with aniline
Xaniline (%)
Si-rich
HBEA Si/Al~12
Al-rich H-BEA
Si/Al~4.5
650
1200
0.51
4.35
15.9
49.0
14.6
86.8
4. Conclusions
The critical function of the density of the acidic
protons for acid-catalysed reactions relevant for the
processing of oil, in petrochemistry, and for organic
syntheses was elucidated using the H-forms of the
beta zeolites with the 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 their
catalytic properties 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 Alrich H-*BEA zeolite (Si/Al 4) lowers the activation
barrier and results in multiplying the reaction rates.
In conclusion, the achievement of highly
increased concentrations of non-interacting acidic
protons in the zeolite catalyst allowed a significant
increase in the activity in relevant acid catalysed
reactions. This finding enables a rational design of
zeolite catalysts with reaction rates exceeding the
established state-of-the-art zeolite catalysts. It opens
a new potential for a development of new functional
zeolite catalysts as well as for the fundamental
understanding of acid-catalysed reactions.
Acknowledgments
This work was supported by the Grant Agency of the Czech
Republic (project # 15-12113S). The authors acknowledge the
assistance provided by the Research Infrastructures NanoEnviCz
and Pro-NanoEnviCz, supported by the Ministry of Education,
Youth and Sports of the Czech Republic under Project No.
LM2015073.
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