Relating Zeolites Activity to their Structure and Acidity
J. Čejka
J. Heyrovský Institute of Physical Chemistry
Academy of Science of the Czech Republic, Prague, CZ-182 23, Czech Republic
Fax: +420 286 582 307, jiri.cejka@jh-inst.cas.cz
Zeolites are the most important group of industrial catalysts with a very broad range of
applications starting from cracking, petrochemistry, and environmental catalysis up to the
synthesis of fine chemicals. At present, almost 200 different structural types of zeolites and
zeotypes were recognized by the International Zeolite Association. The critical factors –
textural and chemical ones – control the behavior of zeolites in catalytic reactions. The
individual types of zeolites differ each other in the size of the channels (8-, 10-, 12-, 14- or
even larger rings), their connectivity and the presence/absence of cages in channel
intersections or channels themselves [1]. On the other side, chemical composition of zeolites
controls the number, type and strength of acid sites, which can be modified during the
synthesis or by post-synthesis methods.
There is a critical imperative for a better design of highly active and selective zeolite
catalysts, which is the understanding of the relationship among their structure – location and
properties of active sites – activity and selectivity in different reactions [2].
Several decades zeolite researchers believed that zeolite activities increase mainly
with increasing channel dimensions and the connectivity of their channels. Constraint Index,
a test presented by Frillette, Haag and Lago [3], tries to relate the activity and selectivity of a
zeolite to the ratio of the reaction rates of cracking of n-hexane to 3-methylpentane. Csicsery
used 1-methyl-2-ethylbenzene isomerization/disproportionation for exploring the structure of
intracrystalline reaction volume of zeolites [4]. Further attempts to rationalize the relationship
between the zeolite structure and its catalytic behavior were discussed e.g. by Zones and
Harris [5] clearly evidencing that there is no direct relationship between the size and
dimensionality of zeolite channel systems and their catalytic behavior. It has been observed
that some novel zeolites with previously unknown structural features frequently show
unexpected catalytic behavior. Correspondingly, much research effort has been devoted to the
application of various “standard” reactions for recognizing the catalytic properties of these
zeolites [6].
To understand in a detail the relationship between textural/chemical parameters and
catalytic performance of zeolites the analysis of reaction variables is needed. Two types of
reaction parameters have to be considered: i) those associated with variables of catalytic
reaction (reaction temperature, space velocity, concentration and molar ratio of reactants, and
thermodynamic equilibrium), and ii) those related to the properties of the zeolite catalysts
under study (crystal size, channel size and dimensionality of the channel system, type and
strength of acid sites, their location and density, as well as the rate of coking).
As for the factors ad i) the situation is rather straightforward, these factors can be
maintained constant during all catalytic experiments. Much more complex is the situation
with variables ad ii). Chemical parameters control the inherent properties of zeolites and
some of them are still not completely understood and even less controlled in a reasonable
manner. These parameters include Si/Al ratio, concentration of Brønsted and Lewis sites and
their density as well as crystal size. These are the factors that we cannot fully adjust for all
zeolites under study. The crystal size of zeolites is expected to substantially influence the
conversion of aromatic hydrocarbons as well as the resulting selectivity and in particular
para-selectivity [7]. Unfortunately, most of the literature data show the selectivities for
different conversion levels, which make it impossible to provide clear conclusions.
In the case of acid catalyzed reactions over zeolites, type of acid sites, their
concentrations, distribution in inner/outer surface and acid strength are critical to achieve the
optimum performance of zeolites in catalytic reactions. With respect to that, detailed analysis
of acidic properties of zeolites provides a nice opportunity to correlate the catalytic behavior
of these zeolites and their structural features. FTIR spectroscopy employing probe molecules
differing in size and basicity is the mostly used to technique to get relatively precisely this
types of information [8]. While the determination of acidity is relatively straightforward,
catalytic activity is mostly related to the acidic properties of zeolites determined at ambient
temperature while the reaction proceeds usually in the range from 150 to at least 500 oC.
Temperature-programmed desorption of different probe molecules allows to determine the
concentration of both Lewis and Broensted acid sites decreases. However, to test the acidic
properties of zeolites we use probe molecules, which differ from real reactants, which makes
our correlation much more complex and not unambiguous.
This contribution focuses on comparison of acidic and catalytic properties of novel
zeolites with mostly 3-D channel systems of 10-ring zeolites (TNU-9, SSZ-74, and IM-5).
Structures of TNU-9, SSZ-74 and IM-5 exhibit rather peculiar properties as for their acidity
and catalytic ability. This makes the comparison of their catalytic behavior rather complex.
The performance of these zeolites is compared with conventional ZSM-5 and ZSM-11 and
Absorbance, a. u.
also with SSZ-57, the structure of which is not yet solved.
A
0.2 B
e
e
0.1 C
0.2
e
d
d
d
c
c
c
b
b
b
a
a
a
4000
3800
3600
3400 4000
3800
3600
3400 4000 3800 3600 3400 3200
Wavenumber, cm
-1
Figure 1
Infrared spectra of hydroxyl region during pyridine adsorption/desorption
cycles of TNU-9 (A), IM-5 (B), SSZ-74 (C) - (a) before adsorption, (b) desorption at 150 °C,
(c) 250 °C, (d) 350 °C, (e) 450 °C.
Figure 1 presents the typical features of hydroxyl region of zeolites with absorption
bands of hydroxyl groups (around 3740-5 cm-1), OH groups attached to Al species (3660-70
cm-1) and bridging acidic Si-OH-Al groups (3610-20 cm-1). While the spectra of TNU-9 and
IM-5 are rather similar, in the case of SSZ-74 we can see additional broad band of disturbed
silanol groups, which arises from the structural defect – one missing Si atom per unit cell.
After pyridine adsorption the acidic bands disappear while with increasing temperature of
pyridine desorption they appear again as soon as pyridine is desorbed from sites of lower acid
strength. Based on the knowledge of extinction coefficients, the absolute concentrations of
acid sites can be determined [8].
To characterize the structural and acidic properties of these zeolites toluene
disproportionation and its alkylation with isopropyl alcohol were chosen. Toluene alkylation
with isopropyl alcohol is a reaction of a particular use as one can follow toluene conversion,
selectivity to isopropyltoluenes, p-isopropyltoluene as well as to n-propyltoluenes. While the
ratio between isopropyltoluenes and n-propyltoluenes provides important information about
the special arrangement in channel intersections or presence of cavities, the value of paraselectivity is related to the size of 10-ring channels of the individual zeolites [9].
In this contribution it will be shown that depending on the reaction and structural
features of the zeolite, the acidic properties are of major or minor importance for the behavior
of zeolites in catalytic reactions. Proper understanding of both textural and chemical features
of zeolites is of a critical importance for optimizing their catalytic performance in different
reactions. The objective of this contribution is to stress the importance of appropriate
approaches to the determination of the relevant properties of zeolites related to their catalytic
behavior.
Acknowledgement
The author thanks the Academy of Sciences of the Czech Republic for financial support of
this project and also is indebted to Dr. Stacey I. Zones (Chevron) for fruitful discussions and
providing some zeolites for this study.
References
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