Materials Chemistry and Physics, 18 (1988) 553-575
BASICITY AND BASIC CATALYTIC PROPERTIES OF ZEOLITES
D. BARTHOMEUF Laboratoire de rCactivit& de surface et structures, Universite Paris VI, F 75252 Paris CQdex 05 (France) G. COUDURIER and J.C. VEDRINE* Institut de Recherches sur la Catalyse, CNRS, 2 avenue Albert Einstein, F 69626 Villeurbanne Cedex (France) Received
April 13, 1987; accepted
ABSTRACT In the present review article one describes basicity for zeolites from theoretical and experimental points of view. Zeolites appear to be rather soft bases,their basicity increasing within the series Li < Na < K < Rb < Cs. The physical methqts used_for characterizing basic sites either themselves (anionic) species : 0 OH-) or through adsorption of acidic probes, as r A104,, C02, S02, organic acids, pyrrole, etc., are described.A review of basic type reactions catalyzed by zeolites is also given. The influence of Lewis acidity of exchangeable cations and of basicity of framework oxide ions on adsorption processes
Acidity and basicity of catalysts are features which are often mentioned to explain their catalytic
constant by the following relation AH 2 H+ + ABH+=
K = IH+IIA-I/I~I a = I?31 lH+l/@i+l Ka
b + H+
for acidity for basicity
with pKa = - log 10 Ka In the 1920's Brijnsted and Pedersen [l] and further Hammett and Deyrup [Z] quantified the relationship between the acid strength of an acid in solution and the rate of the catalytic reaction. For solids Hammett has defined the H
log(aH+. Yi/YiH+) the
the of also
* Author to whom correspondence should be sent. 0254-0584/88/$3.50
0 Elsevier Sequoia/Printed in The Netherlands
554 H = PUBH' - log( IBH+I/ ISI) (2) 0 The ]BH+/]B] ratio is directly observable Prom the Hammett indicator in its two different colored forms. Acid catalysts are characterized by the value of Ho which is negative, the highest negative value corresponding to the strongest acid strength. Often an acid strength distribution is observed. The Ho acidity function evaluates the trend to give a proton to an indicator. Indicators are chosen such as yi/yBH+ is identical for all of them and therefore measures the acidic property of the medium whatever the indicator. After analogy with the acid strength of solids, their basic strength can be defined from the Brijnstedconcept [l], as the proton-accepting solid surface. It is quantitatively
ability of the
expressed by the Hammett and Deyrup H_
function, . Considering an acid indicator BH reacting with a solid base g BH+B
The basic strength H
of % is given by an equation similar to the well
known Ho equation 121 for acid strength H _ = eKBH + log(]B-]/]BH]) where
(4) ]BH] are
indicators. Equation (4) means that the basic strength of a solid surface increases with an increase in H .
Basic character of zeolites Zeolite materials are now well known in the field of catalysis for their acidic properties. This acidity feature was assigned to the tetracoordinationof Al in a tetrahedral Si02 framework which results in a negatively charged AlO; anion : this
charge is compensated
by cations and particularly by
protons resulting in Briinstedacidity. A question then arises : could we prepare a positively charged zeolitic framework? The idea was to incorporate a pentavalent element rather than the trivalent Al into the Si02 framework and to neutralise the charge with anions of basic character. Phosphorus as a pentavalent element was largely investigated without success to our knowledge. New molecular sieves discovered by Union Carbide
 and designated ALP04 (AlO;, PO;), SAP0 ]AlO;, (Si02)x, (PO:),_x],
MePO et ElAPO where Me and El are metals or elements incorporated in the ALP0 framework might have some basic properties. The second idea was to exchange protons by cations in order to neutralize the acidity and hopefully develop basicity. As cations are soft acids one should then expect that the basic character of the zeolite framework should be higher than the cationic Lewis acidity, if one would have any chance to obtain basic 2zeolite. The basicity of the framework should correspond to anions such as 0 , A104- or OH-.
555 Because much
has been paid up to now to acidic
and much less to basic
for many reactions
[3, 41. In this review the
article we have tried to summarize
The present baslcity
in the field of catalysis
As described from
by the oxide
Such calculations method
have been carried
for faujasite over
the basic properties
in the xeolite
out in order
adsorption (iv) basic
as the zeolite
in the compound
[17,18] in an
and the cation
size goes up in the
was then found to be the most basic
are only based on the chemical
are based on quantum
i.e. do not reflect
to the intermediate
Other calculations taking
[15,16] and extended
. The negative
from 1 to 6 [lo]. The
The first data in connection (sulfates,
and X-ray emission
IlO,ll]. It was shown that the charges
 or more recent
X-ray diffraction One
(T = Si or Al) were found not to be fully ionic as one could and
a SiO2 matrix
by selective oxide
and data may be useful
ions and the charge
has been applied
in the introduction
: (i) theoretical
role of exchangeable
on the role of basicity
of the structure. chemical
ab initio calculations
as model where M is the Al or B. The
cations as H , Li
on the charge distribution and the Si-O-Al bridge
stability has been calculated. It was found that the M-O bond becomes more ionic + when H is replaced by more electropositive cations. The negative charge, originallylargely delocalized in the cluster, gets more localized on the oxygen + of the Si-O-T bond as charge compensating M . It follows that bonds are primarily covalent in character, i.e. the anionic framework has to be considered as a soft base .
CHARACTERIZATIOR OF BASICITY The usual way is to use acidic adsorbates of different strengths in order to probe
of solid materials.
appropriate choice of acidic molecules. For acidity measurement there exists a large number of basic probes useful for zeolites although the use of large organic molecules as Hammett indicators may be limited for small pore zeolites because of steric hindrance. For basicity measurement  the number of acidic probes able to cover a wide range of pKa is rather small. Moreover a difficulty stems from the fact that the acidic probe molecules may interact with cations. For instance CO2 may either be adsorbed on the cations or physisorbed or may react with
to give a carbonated species
[22-251. The adsorption of molecules on a catalyst involves several interactions as depicted by Barrer and Gibbons . The energy of adsorption may be written as the summation
p, being the dispersion; aR the short range repulsion, $6Fthe polarization, !i$, the field dipole, $?J;+the field gradient quadrupole and $
the sorbate-sorbate sP The first two terms depend on interaction (sp denotes self potential) energies.
the sorbent while the three following terms depend on both the heteropolarityOf the sorbent and the nature of the sorbate. The fourth and fifth terms are depending on the orientation of the sorbate molecule with respect to the sorption site. The $KD term equals -A/r6 where r is the distance between the centers of the interacting adsorbent-adsorbateand A is a constant calculated 12 from London and Kirkwood-Muller formulae. p, is depending on B/r function (Lennard Jones potential), lap= field strength) and !K)i_Q=
polarixability of the adsorbate, F
- QF(3cos*@-1)/4r with Q being the quadrupole
moment. In order to calculate some of these terms, fields and field gradients have been evaluated by means of various approximations for the charge distributions on cations and on oxide ions [26-311. Some values as calculated following the principles described above are given in Table I.
Table I. Experimental initial values of sorption energies E and calculateq values H for co 2 adsorbed on X-type zeolite with various cations in kJ.mol from ref. 26b.
51.4 55.2 15.9 0.4 9.6 30.9 1.7
Cationic form K Na
45.2 38.9 13.0 0.8 5.0 21.3 1.3
43.9 28.8 7.1 3.3 2.1 17.6 1.3
42.2 23.4 4.6 4.6 0.8 14.6 1.3
36.8 22.2 4.6 9.2 0 9.6 1.3
go: estimated zero point energy It is worthwhile
noting that the energy of CO2 sorption in X zeolite
decreases from Li to Cs, which is mainly due to the decrease of the quadrupole energy term while the dispersion energy term 9, value increases for cations and decreases for oxide ions. Probe molecules The ideal
interaction with oxide ion and hydroxyls. It should also be stable with time and with temperature. CO2 has widely been chosen to characterize basicity of solids [22-251 but it interacts with several types of basic sites and may give rise to chemical reaction (carbonation).
Carboxylic acids or bensoic acid have
used [7,21] and phenol as well . However for oxides and mixed oxides phenol may decompose, which precludes its practical use. Pyrrole has also be used particularly
at room temperature
for short contact
decomposition [19,33]. Benzene has not be used to titrate basicity but has been shown to interact presumably with oxides ions in the 12-R window of faujasite as a function of their basic strength [34,35]. Colored indicators Few results are available. Attemps for basicity measurements using benzoic acid solution in dry benzene shows the presence of strong basicity in highly exchanged CsX samples  Hammett indicators have also been used to determine the change in weak acidity of basic seolite by the addition of acidic cation. For instance Li+ ion was shown to exhibit stronger acidic nature than the other = + 3.3 for LiX and KbLiX against + 4.0 for RbX and RbKX .
1261 and to as
a maximum was
may be suggested
heat , corresponding As the
Eoor CO2 studies
of the Na form to the sites
and entropy (421
shows that acidification
characterization such basic
may reasonably etc.)
when the bands
to the T-O found
bond could be modified.
to be different
the nature of the Al-O bond was
Al or to extra
in the future.
the study of adsorbates. characterize
pressure Co-A of
have been employed.
be drawn: decreases (ii)
in the strongest
is due to the interaction
ion increased trend
of this shift with respect
to the charge
equalization was observed
the high asymmetry
of basic sites thevNH
and may be compared
the ions -1 cm
in Ca-Y and Mg-Y zeolites
low wavenumbers , which
while the calculated
and in the presence
VNH shift for the same oxide (i) except
the basic strength.
when the negative
as a probe
ions as calculated
liquid pyrrole Fig. 1 for
and acidic hydroxyl
has also be used
law. If the total one may
[22-251. CO2 may also interact with basic hydroxyl
formed by water decomposition carbonated
and the corresponding
using the Clausius-Clapeyron for instance
. The nature of the cations
be determined by IR with increasing CO2 partial -1 band intensity for Na-A, NaCa-A and = 2350 cm
heat may be measured,
rise in 2+ infrared to a shift due to the adsorption on cations, for instance for Ca and -1 2+ an IR band near 2350 cm . The in NaCaMg-Y zeolites 1231 with Mg adsorption
the most currently
(X and an may
of the oxygen
0.30 0.35 0.40 0.45 Negative oxygen charge
. Variations of VNR values against the negative lattice oxide ion
charge calculated using the Sanderson electronegativityequalization principle  for various zeolites
: 2 mordenite, _b L, _c Y and _d X in different
cationic forms (A) Li, (0) Na, (A) K, (0) Rb and (0) Cs .
as the oxide ion charges do by themselves. It follows
that pyrrole turns out to be a reliable probe for basicity characterization except
(pyridine) and IR spectroscopy
and Datka have shown
amphiprotic properties of NaHY zeolite. An IR study of the methanol decomposition on alkali-metal X zeolites (47) correlates the formation of surface formates to the basicity strength of the material. Benzene is also a rather valuable probe molecule. It has been shown (34,35) that for faujasites the ITelectrons of the benzene ring interact with cations in S
and S sites giving rise to v and v,6 + v,, combination bands of CH II III 5 + "17 out-of-plane vibrations slightly shifted towards higher wavenumbers with respect -1 to liquid benzene absorbing in the 1800-2100 cm region. The interaction of CR with the 0, and O4 oxide ions in the 12-R window, when benzene is located in the plane of this aperture, gives the highest VNH shift for the V5 + V,7 and V 10
+V 17 bands. It was shown that the wavenumber values could be related to
the charge on the oxide ions calculated from the Sanderson
561 equalization principle
. In addition the amount of benzene molecules
interacting at high loading with the 12-R window increased when one went from Li to Cs in the alkaline series and as the Si/Al ratio decreased (Table II). Table II. Determination by IR studies of the number of benzene molecules adsorbed on various zeolites from ref. L34.351.
C6H6 per S.C.(a)
27 6 6
oxygen charge (b)
- 0.223 - 0.275 - 0.303
0.6 + 0.3 0.6 + 0.1
- 0.350 - 0.380
1.2 1.2 1.2
0.45 + 0.25 1.2 + 0.2 1.2 + 0.1
- 0.410 - 0.449 - 0.463
(a) number of benzene molecules per supercage interacting with 0 ions of the 12-R window (b) Calculated from Sanderson electronegativity (c) D means samples dealuminated with ammonium hexafluorosilicate This means that in the more basic zeolites, benzene molecules prefer to be located in the 12-R window where they can interact with basic oxide ions. This suggestion shows that, at least for faujasite where benzene molecules just fit the 12-R window, the amount and energy of adsorption of benzene may be used to characterize the 0 1 and O4 oxide ion basicity.
Raman Spectroscopy Adsorption of benzene on alkali metal-X and -Y zeolites was also studied by Raman laser spectroscopy  and W
reflectance spectroscopy . It has been
shown that Raman shifts of the ring breathing mode of adsorbed benzene depend on the electrostatic fields within the supercage and that the excess cations in X versus Y results in higher fields at the aromatic nucleus of initially adsorbed benzene. Further increases in field are caused by crowding in the supercage as the size of cation increases. UV reflectance studies show the n electron interaction with the cation in the supercage : the position of the vibronic maxima with both the degree of surface coverage and the nature of metal ion exchanged.
NMRand neutron diffraction techniques They have been used with benefit for the study of benzene adsorption on Na-X and Na-Y zeolites [50,51].They show that benzene molecules are adsorbed in the 12-R window and on cations in SII sites, which supports the infrared data.
562 ESR technique When a surface exhibits electron donating properties with one electron transfer, radical anions may be formed. Such ions are paramagnetic and may be studied by ESR, this idea gave rise 20 years ago to many works concerning charge transfer complexes on oxides and zeolites [52-541. Molecules anthracene, the strength
to this aspect
low electron donating
on the ionisation devoted
etc. were ionised
as tri-, di-, mono-nitro
of the adsorbates. radical
In contrast, zeolites
of the works
anions were detected
instance anion radicals were detected for trinitrobenzene adsorbed on H-Y zeolite dehydroxylated tri-nitrobenzene
, for tetracyanoethylene
adsorbed on dehydroxylated H-Y zeolite , for SO2 adsorbed
on dehydroxylated H-Y, H-M  and H-L
 and for triphenylamine
adsorbed on dehydroxylated H-Y zeolite . The structure of the anion sites formed upon dehydroxylation was postulated to correspond to AlO; anion : H+
Such a conclusion was supported by the observation by Naccache and Ben Taarit [SO]
anthracene cation, were decreasing upon dehydroxylation.Note also that it was shown that oxides as alumina, silica-alumina, zeolite exhibit simultaneously both electron donating and electron accepting sites [61,62]. Some studies on alkaline zeolite (Nay -fCsY) showed low electron donating properties as evidenced by ESR, particularly
non dehydroxylated. This indicates that the ESR technique
sites as 0
or OH- presumably
is not reliable of their too low
XPS techniaue This technique may be used with profit to characterize basicity. It is well established that factors, value
one being the charge
in a crude binding
the negative of
energy value of the Ols peak
by the element. charge
potential is expected
by the element
it is difficult
0 ls binding
their 71 electrons
CO 2 from
for directing of the
or is adsorbed adsorbed contents
due to the existence
it was possible
form as discussed
zeolite . The
: Rb-X > K-Y > Na-Y
. The direction
with the less basic aromatics of conjugate
and Na-Y as the zeolite basicity both with the cations
as for instance
Let us consider
: ethyl benzene, para-, ortho- and
from Rb-X to
sites flat on Na
are based on non geometric
as Lewis acid sites
or the separations
and the zeolite
in the hexagonal
in the plane of the 12-R window
to any diffusion
on the the
ions, the interaction In the
 and meta-xylene
are in the following
and non acidic
the more basic zeolites
of one specific
It has been
it may be
with the metal
are known to be preferentially
A large number of industrial favoring
in a mixture
of one adsorbate
Most of the work done on adsorption materials.
in that field
the amount of benzene
shown that the
face of the supercage
In a study
of benzene different
564 site and to show that the energy of the corresponding interaction depends on the cation Lewis acidity, the oxide ion basicity and the cation loading
The interaction cation-benzene TT ring is smaller for instance for Rb than for Na and the interaction CH-framework oxide ions increases simultaneously with Al content, i.e. with its basicity. Hence the more basic Rb-X interacts more strongly with benzene
interaction with the cations
than with benzene
. These results strongly suggest that the
interaction of any aromatic and more generally any adsorbate with zeolite framework depends not only on the molecule geometry and on the zeolite structure itself but also on the chemical properties of the zeolite. Since the only accessible atoms of the framework are cations and oxide ions, the charge on both i.e. the cation Lewis acidity and the oxygen basicity are factors to consider in adsorption processes.
BASIC REACTIONS IN CATALYSIS BY ZEOLITES The application of zeolites as acidic catalysts in reactions proceeding through
hydrocarbons, etc.) has received widespread attention in contrast with their use as basic catalysts. homogeneous bases
for many industrial reactions catalyzed by
1721, the replacement of liquid bases by heterogeneous
catalysts may be an appreciable amelioration such as limitation of reactor corrosion, easy separation of used catalyst and its possible regeneration, etc. etc. Generally,
intermediates are characterized by higher activity and selectivity than the solid-acid or metal catalyzed reactions. The mechanism of base catalyzed reactions and the active sites on solid bases have been discussed in some review articles
[5,6] [72-741. More particularly the reactions catalyzed by
basic zeolites have been reviewed by Ono [61. The most important ones are the side-chain alkylation of substituted aromatics, the dehydrogenation of alcohols, the ring transformation ofy
butyrolactone or tetrahydrofuran with hydrogen
sulfide or primary amines. Side-chain alkylation of substituted aromatics The acid-catalyzed alkylation of alkylaromatics with olefins or methanol results in selective alkylation of the benzene ring . H+, di and trivalent cation exchanged zeolites are efficient catalysts for this ring alkylation while alkali exchanged zeolites are inactive in the same temperature range (< 25O'C) [75,76], However, Sidorenko et al. 
observed that at higher temperature
(425-475.C) the alkylation oE toluene with methanol over alkali cation exchanged zeolites produces a mixture of xylenes, ethylbenzene and styrene showing that the alkylation may occur both on the side-chain as over catalysts with basic
over alkali exchanged depends
X or Y type.
of the alkali
than for Y-type
on the basicity
(Na K Rb Cs) and
was more under
(styrene and ethylbenzene)
(of the order of 10 8). Appreciable
with the addition zeolites.
of a boron or a phosphorus
Exxon Co patented
boron and phosphorus
nature of the cation, reaction nature
(77) was developped chemistry.
side-chain toluene implied
for the alkylation
= CH2 + Hz0
in reaction (1). This
of toluene with
(2) is hydrogenated
site is necessary
acidic and basic
by Itoh et al.  and was
Fig. 2. Configurational
-t CSH5 CH2CH3
2 CO + Hz
agent. Styrene by
H 2 produced
X or Y-zeolite
+ HCHO+CSH5-CH = CR2 + Hz
HCHO + H
gas , etc, were studied
the use of alkali exchanged
 or methanol proceeds
the role of the alkali metal cation,
of the carrier
of these yields was claimed
et al. quantum to the
sites on the zeolite. This model and
The basic site determines the selectivity to the side-chain alkylation whereas the
: the importance of the basic sites is generally demonstrated
[77,80,81,85] and it has been recently shown that addition of an acidic cation to a basic zeolite accelerates side-chain alkylation . For the same reaction Garces et al.  found that LiNa-X zeolite gives xylenes
for the metal
of the alkyl
for the activation
tOluene with metal
of alkylaromatics. appears
such as boron
to be formaldehyde
has to be favored which
[BO, 811, silver
[85,87] the reaction is presumably
, cobalt, iron, manganese
Similarly, with methanol converted
the alkylating agent
the basic sites needed
Na, K, Rb and Cs-X zeolites
by the reduction
side chain alkylation
on the side chain
and vinyl naphtalenes
zeolite were alkylated
over KX and RbX zeolites
over RbX zeolite gave cumene
WI. Dehydrogenation of alcohols Transformation
of alcohols is a catalytic probe to determine the relative
importance of catalysis by acidic or basic sites. Dehydration products (olefins
dehydrogenation products The decomposition
(ketones or aldehydes), on basic catalysts [5,73].
of isopropanol on acid zeolites proceeds via ionic
intermediates to propene and diisopropyl ether [74,89,90] the selectivity of the reaction over alkali cation.
on the content
high selectivity -Y zeolites
and the decrease show
K-, Rb-, Cs-X
of a basic compound
Na-X or -Y zeolites
and of an acidic
of ion and increases
with the increase of ionic
of the alkali metal cation.
K, Rb, Cs zeolites exhibit both acidic and basic sites.
567 The acidic centers are suggested to be decationated sites and possibly the alkali cation and the basic sites are the lattice A10 - paired with alkali 4 cation [911. Similarly, isopropanol is converted to propylene on acidic sites 1931 and to acetone on basic sites [941. On K- and Cs- exchanged ZSM-5 and mordenite
the selectivity of this reaction depends on the reaction temperature and on the nature of the exchanged cation. At ZOO'C, formation of propylene is essentially observed whatever the zeolite or the exchanged cation. At 3OO'C, formation of acetone is detected in each case and is higher on Cs-ZSM-5 zeolite. These results suggest that acetone formation is favored on the more basic zeolite and at
dehydrogenation reaction. The dehydrogenation of methanol over sodium modified silicalite was reported to give formaldehyde with high selectivity thermodynamically
[961 though this reaction is
unfavorable. Since the catalytic activity does not depend on
the aluminium content (present as impurities) but only on the sodium excess, it was assumed that the active sites are the sodium ions and not the aluminium ion and alkali-metal cation pairs as proposed by Yashima et al. [761 for the decomposition of isopropanol.
Oxygen-sulfur or nitrogen interchange reactions The interchange reactions may be resumed by the following scheme where furan is converted to thiophene or pyrrole by reacting respectively with H2S or NH3 [741.
For these reactions, the optimum activities are obtained with seolites of low OK
negligible acidity such as NaX or NaY zeolites. The
zeolites and acidic zeolites [6,97].
studied on alkali metal cation exchanged X or Y
It was shown that the alkali metal cation exchanged zeolites are much more active than the acidic zeolites, that the catalytic activity depends on the alkali metal cation in the order LiY
and that NaX is more
active than Nay. Kinetic study has allowed to calculate the rate constant k which increases in the order Li c Na < Cs and to determine the activation energy -1 for k to be 163, 130 and 109 kJ.mol for LiY, Na and CsY, respectively.The effect of addition of hydrogen chloride which inhibits the ring conversion and of pyridine which enhances the catalytic activity indicates that the active centers are associated with basic sites. As already proposed
, the basic sites are described as oxygen anions
bound to aluminium cations AlO;.
The negative charge of the site AlO; is
neutralized by the alkali metal cation which is more weakly bound to the basic site as its ionic radius increases. Hence the order of catalytic activity LiY < NaY < KY < RbY ( CsY may be correlated to an increasing basic strength of the AlO; sites, which is confirmed by the decrease of the activation energy. The higher activity of NaX as compared to NaY can be explained by the higher number and basic strength of AlO; sites due to its higher Al/(Al + Si) ratio . For the ring transformation of tetrahydrofuran [6,9Sl, the alkali metal
cation exchanged zeolites also exhibited the higher
activity and the addition of hydrogen chloride inhibited the reaction. However, addition of pyridine greatly decreased the activity. Therefore,
reaction, acidic and basic sites appeared essential. Since the ring opening of tetrahydrofuran
[99,100] necessitates Brijnsted acidity, it was assumed that
interaction of basic sites with hydrogen sulfide produces acidic OH groups which react with tetrahydrofuran according to the following scheme
H2S + Na+ + OZ-
C-J -( 0
A (II (1)
(I) + NaSH
first step was confirmed by infrared studies of hydrogen sulfide
adsorption on NaX which revealed that hydrogen sulfide dissociated on NaX  Among the other interchange reactions catalyzed by alkali metal cation exchanged zeolites, one may mention :
- the conversion of acetic anhydride to thioacetic acid  - the ring transformation of Y butyrolactone into I-alkyl pyrrolidinone  - the reduction of nitro compounds with hydrogen sulfide into amines  Other basic reactions involving anionic intermediates are mentioned such as the aldol condensation of n-butyraldehyde to 2 ethyl 2 hexanol over Na-, K- or H- Y zgolites , the dimerization of cyclopropenes over KA or NaA
the dialkylation of o-ethylphenol over NaX and the dehydrocyclisation
o-ethylphenol with COS over NaX . Hydrocarbon aromatization on platinum alkaline zeolite The
It was shown that a monofunctional
scheme is operating on non acidic zeolites fully exchanged with alkaline cations and that L type zeolite is the most selective for aromatic formation . Table
% % % % %
nC6 conversion (wt%)
of CO adsorption
2.9 8.3 2.0 0.5 0.8
80 66 58 49
41.2 54.0 23.1 15.1 0.9
It was further
from Li to Cs in
[110 1 and
and on zeolite
of the L type
by the chemical
state of platinum
This may be r-elated to the increase
3.5 5.3 12.4 7.4 5.3 9.8 0.4 15 4 11.2
and to the usual acidic
59 83 41 31 17
Pt/KL Pt/NaX Pt/NaY Pt/Na Pt/Na mordenite
0.6 0.6 0.6 0.6 0.6
induced by the alkaline
of the basic
the framework oxygen from Li to Cs zeolites . The importance of the absence of any acidity is also emphasized in more recent results obtained with Pt/BaKL zeolite prepared in such a way to locate the which precludes the formation of acidity
Ba ions in small cages,
The catalysts are very active
for both paraffin and alkyl cycloalkane aromatizations.
570 CONCLUSION Basic type reactions in heterogeneous catalysis by zeolites remain of low practical interest at the present time by comparison to acidic reactions, presumably because there have been too few studies on that subject. Basic type zeolites or molecular sieve type materials with a cationic framework have not yet been synthesized but basicity may be obtained in anionic lattices by alkalineion exchange, the resulting basic strength depending
size of cations. Acidic and basic sites are shown to coexist simultaneously. Basicity is a feature rather difficult to characterize. Physical techniques and the use of acidic probes are useful to try to quantify basic sites with regards to their strength, nature and amount. Lattice oxide ions which bear a more or less large negative charge, AlO; species and hydroxyl groups are the three main basic sites to consider. Their characterizationby physical technique is actually not
useful for the understanding of basic
type reactions. More emphasis on basic type reactions zeolite should be done with promising applications in the future for new catalytic selectivities.
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