Quantum chemical study of Li-, Na- and K-faujasites

I. Phys Chm

Printed in Great

Solids Vol. 43. No. 3, pp. 221-225 Brihin.

1982

002236971831030221J35SO3.0010 0 1962 Pergamon Press Ltd.

QUANTUM CHEMICAL STUDY OF Li-, Na- AND K-FAUJASITES S. BERAN J. Heyrovsky Institute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, Machova 7, 12138 Prague 2, Czechoslovakia

(Received 27 April 1981;accepted in revised form 28 July 1981)

Abstract-The physico-chemical properties of Li-, Na- and K-zeolites modelled by T606(OH)r2clusters were studied by the CNDO/Z method. It was shown that the physical characteristics of the zeolite skeleton (charge density, bond strength, electron structure) are practically independent of the type of cation coordinated in its Sn and Si’cation posicons. The studied zeolites differ in the cation charge, which has values of about 0.0,0.3 and 0.6 for Li, Na and K, respectively, and in the character of the cation-skeleton bonds in the zeolite, whose ionicity decreases in the order, K > Na > Li. The calculated characteristics of zeolites are employed in the discussion of their interactions with HzO.

Zeolitic alumino-silicatesare among the most important catalysts used, especially in the petrochemicalindustry. Attempts to clarify the factors determiningtheir physical, chemicaland catalytic properties (especiallyin connection with the possibilityof modifyingthese properties for effective use in the conversion and synthesis of hydrocarbons) have led in recent years to very intense study of these substances by various experimental[l-31 and theoretical[4-211methods. One of the most important factors with a markedinfluenceon the mentioned zeolite characteristicsis the type of cation compensating the negative charge on the zeolite skeleton. Changesin the zeolite properties as a result of the different electron donor-acceptorproperties of the cations can be studied, for example,on the series of univalentcations, Li’, Na’ and K’. This work was carried out in order to theoretically study the physico-chemicalproperties of faujasite zeolites containingLi’, Na’ and K’ cations and to compare these properties. MODELANDMETHOD

-2, respectively[l7], were studied. It remains unclear whether it is more suitable to use the geometry of these clusters obtained from X-ray data, correspondingto the averaged bond lengths and angles of windowswith and without cations (i.e. to a certain extent including the effect of the cations on the skeleton, but not distinguishing between the Si-0 and Al-0 bond lengths), or to employ an idealized geometry adapted to the different Si-0 and Al-0 bond lengths[7,8]. A previous study in this series[17],dealingwith Na-faujasites,employed the geometric parameters obtained from X-ray data[22]. Description of the physico-chemicalcharacteristics of zeolites obtained in this way is not very different from that for idealized zeolite skeleton geometry[7,8]. Consequently, the geometric characteristics obtained from X-ray data for Li-faujasites (for oxygen atoms of the 01, 02,0) and 0, type the T-0, bond length has the value of 1.62, 1.69, 1.71 and 1.64~ lo-“m; the T-Oi-T angle takes on the value of 149.5,128.7,126.9and 143.7 degrees, respectively[23])as well as for K-faujasites[24] were also used in the calculations. The standard version of the CNDO/ZmethodI and an s,p base for the Si and Al atoms[14,17]were used for the calculationof the electronic structure of the zeolitic clusters. The orbital electronegativity l/2(1, + A,) for the 4s and 4p atomic orbitals of the K atom was obtained in the manner described earlier[26,27] and attains values of 1.83 and 1.07eV, respectively[28]. Bonding parameter &“, obtained from the fiti value using the relationship

In the study of solid phases using quantum chemical methods, it is necessary to work with models containing a finite number of atoms-clusters.Especiallyin ionic (or partially ionic) crystals, this method represents rather considerable simplificationas it does not take sufficient account of the electrostatic field (long-rangeinteractions) in the solid phase. It has, however, been demonstratedin a number of works[4-211 that this procedure yields quantitatively correct information for characteristics which are determined primarily by short-range inter&.4sU0 + U4,.4,(K) actions (charge density, bond orders, orbital energies, pKo = Uh,b (Li) + U2,.&i) etc.). Consequently, the reliable models of six-sided windows leading into large cavities (cluster A, Fig. 1) /340[29] has a value of 5.13eV[28]. with the cation in the Sn positionand a six-sidedwindow RESULTS AND DWUSSION leadinginto a prism (cluster B, Fig. 1) with the cation in the Sr’ position, terminated by hydrogen atoms[17,20] The electronic structure of the studied zeolites has were used to model the studied system. Clusters with a band character. The occupied molecular orbitals (MO) Si: Al ratio of m,5 and 1 with overall chargesof 1,Oand are divided into two bands separated by a gap with a 221

S. BERAN

222

I

x%

0.3

/gT\ H

%\,/O’

/

H

I I

4

/T\‘“,H

9,

d ‘0 / \

”

ClusterA

Clusrer B

Fig. 1. Depictionof clusters A and B with indication of the individual types of atoms. (T stands for Ai or Si atoms). width of about 13 eV (see Fig. 2), in dependence on the Si: Al ratio. The electron donor and acceptor properties (Lewis basicity and acidity) of the studied zeolites are reflected in the values of the energies of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) or of the non-charged clusters given in Table 1. In all cases, the HOMO is located on the skeletal oxygen atoms and the LUMO primarily on the cations. The calculated HOMO and

Lln.

MO

*o

-1.5

-1.0

-0.5 I

0.0

OS

‘.O

a.”

Fii. 2. Representationof the band structure of clusters B-Li, B-Na and B-K for a Si: AI ratio of 5, by a histogramof the number of MO’svs the energy at 0.1 a.u. intervals. The shaded areas representoccupiedMO’S

LUMO energy values for the individual types of zeolite differ too little (maximal difference of 0.5 eV, see Table 1) for them to be a source of the different behaviour of these zeolites during their interactions with molecules. Similarly, their absolute values, compared with those for the Al-zeolites or dehydroxylated H-zeolites [20,21] do not suggest marked Lewis activity. The charge densities on the Si and Al atoms of the studied zeolites, given in Tables 1 and 2, are practically not affected by the type of cation. Similarly, the charge on the individual types of skeletal oxygen atoms is not very different for the studied zeolites. The small differences are apparently connected with differences in the geometric parameters used for the individual types of zeolites. Only the negative charge on the type 0, (cluster A) and type 0, (cluster B) oxygen atoms of the Lizeolites is somewhat lower compared with the corresponding oxygen atoms in the Na and K zeolites. These oxygen atoms play the main role in coordinating the Li cation and differences in the charge value apparently depend on the character (ionicity) of the cationzeolite skeletal bond. Substantially larger differences were observed in the charge densities on the cations. Depending on the Si:AI ratio and the cationic position, the charge on the Li atom attains values close to zero; i.e. the Li-zeolite skeleton bond has practically covalent character (see Tables l-3). Calculations of the solvation of the Li’ ion by water and other oxygen-containing solvents (H,CO), also carried out by the CNDO/Z method, indicated that the charge on the Li atom depends on the number and kind of solvating molecules[30]. The charge on the Li atom for 0, 1, 2 and 4 water (H2CO) molecules solvating the Li’ atom attains values of 1.00 (l.OO), 0.83 (0.70). 0.67 (0.40) and 0.44 (0.39), respectively. Thus, for coordination number 4, which roughly corresponds to the coordination number of the Li cation coordinated in the cation position of the zeolite (the cation is mostly coordinated by three oxygen atoms of the 0, or 0, type, see the Wiherg bond orders in Tables 2 and 3) the CNDO/2 method yields considerably higher charge values on the Li atoms. Although it is probable that the covalency of the bond is

Quantum chemical study of Li-, Na- and K-faujasites

223

Table 1. CNDO/Z charge densities on cations, q,-,and energies of the highest occupied molecular orbital, EM, and of the Lowest Unoccupied Molecular Orbital, E Lu~o, for ClustersA-Li, A-Na, A-K, B-Li, B-Na and B-K (the Si : Al ration is 5 : 1) A-M

A-Na

A-K

5Li

B-K

BNe

EHOYO

-12.18 -12.18 -11.81

-11.94 -11.70 -11.80

ELUnO

- 1.71 - 1.86 - 1.59

- 1.33 - 1.77 - 1.87

SC

0.04

0.26

0.60

0.05

0.34

0.63

Table 2. CNDO/Zchargedensitieson atoms, q, and Wibergbond orders,p, for clusters A-Li, A-Na and A-K Si:Al ratio 00

Si:kl ratio 1:l

A-Li

A-Na

A-K

1.65

1.61

1.61

QO4 s, QSi-O2

-0.60 -0.69' 0.05 0.71

-0.75 -0.70 0.29 0.76

-0.73 -0.69 0.62 O.&l

QA1-02 QSi-Oq

0.82

0.79

0.82

0.36 0.06

0.15 0.08

0.04 0.04

qSi %l QO2

QA1-04

PC-O2 PC-O4

A-Li

1.47 1.39 -0.65 -0.70 0.03 0.66 0.53 0.98 0.58 0.3e 0.06

A-Na

1.50 1.37 -0.72 -0.70 I. O.&j 0.93 0.56 0.93 0.59 0.17 0.09

A-K

1.38 1.36 G.71 -0.69 0.56 0.96 0.60 c.95 0.60 c.05 0.05

Table 3. CNDO/Zchargedensitieson atoms, q, and Wibergbond orders,p, for clusters B-Li, B-Na and B-K Si:Al ratio -8

qSi qA1

PC3 z2 QSi-O2 QAl-O2

Si:Al ratio 1:l

B-Li

B-Na

B-K

1.63

1.64

1.63

-0.67 -0.64 0.08 0.81

-0.73 -0.71 0.36 0.80

-0.72 -0.74 0.65 0.82

0.70

0.75

0.82

0.05 0.39

0.08 0.15

0.04 0.04

*Al-O3 QSi-O3

PC-O2 PC-O3

overestimated using the CNDO/Z methodl91, the given charge densities on the Li atoms coordinated in the zeolite skeleton indicate that the electron donor ability of the lone electron pairs on the skeletal oxygen atoms is much higher than the ability of the lone electron pair of water or other oxygen-containing solvent. Similarly, the

B-Li

E-Na

B-K

1.44 1.38 -0.66 -C.64 0.01 0.95 0.57 0.84 0.52 0.05 0.42

1.43 1.37 -0.73 -0.65 0.29 0.95 0.59 0.90 0.55 0.09: 0.17

1.42 1.3& -0.71 -0.71 0.59 0.98 0.60 0.97 0.63 0.05 0.05

rather low positive charge on the Na atom (about 0.3) was attributed to strong solvation of the Na’ ions by lone electron pairs on the skeletal oxygen atoms[17]. In agreement with the electron donor-acceptor abilities of the studied cations, the highest positive charge value was exhibited by the K’ cation (about 0.6). It can thus be stated

224

S. BERAN

that the charge of the cations coordinated in the cation positions of zeolite is correlated with their electron donor-acceptor abilities, even though their absolute values are rather low as a result of marked electron density transfer from the lone electron pairs of the oxygen atoms to the cation. The Wiberg bond orders pco given in Tables 2 and 3 reflect the character and strength of the bonds between the cation and the zeolite skeleton. The K’ cation in the S,, cation position (cluster A) is bonded to the same degree by skeletal oxygen atoms of the O2 and O4 types; coordination of the Na’ cation by O2 type oxygen atoms is roughly twice as strong as coordination with type 0, oxygen atoms; the Li’ cation is bonded primarily by oxygen atoms of the OZtype (see Table 2). Conditions are similar in the Sr’ cationic position; the K’ cation is coordinated to the same degree by type O2 and 0, oxygen atoms; the coordination of the Na’ cation by type 0, oxygen atoms is roughly twice as strong as coordination by type 0, oxygen atoms. The Li’ cation is then coordinated primarily by type 0, oxygen atoms (see Table 3). The strength of the electron donor-acceptor cation-zeolite skeleton bonds for a given cation depends on the Si: Al ratio; with increasing Si: Al ratio it decreases. Depending on the type of cation, the strength of the cation-zeolite skeleton bond exhibits the following trend: Li > Na > K. While the Li-zeolite skeleton bond was found to be practically covalent, the K-zeolite skeleton bond have marked ionic character. The Na-zeolite skeleton bond has only partial ionic character. The effect of the cation on the zeolite skeleton is apparent from the Wiberg bond orders psi_o and p&O. The Wiberg bond orders for S-0 in clusters A and without cations and with Si:Al ratios equal to 21and 1 attain values of over 0.80 and about 1.00, respectively; the Wiberg bond order for the Al-O bond is over 0.60. It is thus apparent that introduction of a cation into the cationic position of the zeolite results in a slight weakening of the T-O bonds of those skeletal oxygens which participate in its coordination. The degree of weakening is then proportional to the strength of the cation-zeolite skeleton bond. Thus the weakening of the T-O bonds of the Li-zeolite is greatest (see Tables 2 and 3). The results of the above calculations may be employed for qualitative interpretation of the interactions of molecules with the studied zeolites. In the literature there exist a lot of experimental data concerning the interaction of water molecules with the alkali metal zeolitites[31-341. These measurements indicate that the strength of the water-zeolite bond (the heats of water adsorption) decreases from Li to CS [32, 331and that the stretching vibrational frequencies of the interacting water molecule decrease in the series Cs
similarly to the cation-skeletal oxygen atom bonds, an electron donor-acceptor character as the calculation of water interactions with Ca-faujasites indicates[35]. Therefore, it is very probable that the strength of the cation-water oxygen atom bond for individual cations correlates with the strength of the cation-skeletal oxygen atom bonds decreasing from Li to K (see Tables 2 and 3). The strength of water interaction with the alkali metal zeolites (e.g. measured by the adsorption heat of water[32]) decreases from Li to K, in agreement with experimental data[32,33]. As a consequence of the formation of the cation-water bond a weak donation of electron density from the water molecule to the cation takes place, resulting in a small positive charge localized on the water molecule [35]. This effect may cause a slight strengthening of the O-H bond in the water molecule. On the other hand, another stronger effect operates at the same time. Due to the electrostatic field of the zeolite, a polarization of the O-H bond occurs leading to O--H’ dipole moment as indicated by the electrostatic potential maps (for the studied cations these maps are similar to those presented by Mortier et al. for Mg-faujasites[7]) as well as by the calculations of water interactions with Ca-faujasites [35]. The degree of O-H bond polarization correlates with the value of the electrostatic field formed by a cation (i.e. it correlates with the value of qJr,, where qc is the calculated charge on the cation and r, is the ionic radius of the cation) which decreases from K to Li. The O-H bond polarization then results in the corresponding weakening of the O-H bond. (The model calculation of the water molecule polarization by homogeneous electrostatic field showed that the strength of the O-H bond decreases with the increasing field[36]. Moreover, in the case of water interaction with Ca-faujasites, the O-H bond is polarized and the Wiberg order of the O-H bond for the water molecule interacting with the Ca cation has a value of 0.96 compared to the value of 0.98 for the isolated water molecule of the same geometry[35].) Simultaneously it is well known that a weakening of a bond brings about the corresponding decrease in its stretching vibrational frequency. It can be, therefore, concluded that the stretching vibrational frequencies of water interacting with the alkali metal cations localized in the zeolites should decrease from Li to K. This conclusion is in accordance with the observed vibrational frequencies [31,33,34].

CONCLUSIONS

The given results of the calculation of the physical characteristics of Li-, Na- and K-zeolites indicate that the properties of the zeolite skeleton (charge density on the atoms, bond orders, Lewis acidity and basicity characterized by the MOM0 and LUMO energies) depend very little on the type of cation. Especially the differences between Ha- and L-zeolites are negligible; small differences can be observed only for Li-zeolites. Marked differences between the studied zeolites were observed only in the charge densities on the individual types of cations and the character of their coordination

Quantum chemical study of Li-, na- and K-faujasites

in the zeolite skeleton. The charge density values localized on the cations, as well as the ionicity of the cationzeolite skeleton bonds, which decreases in the order K > Na > Li, indicate that there is marked electron density transfer from the skeletal oxygen atoms to the cation, reflectingthe strongelectron donor abilitiesof the oxygen atoms in the zeolite rings. Differencesin the effect of the individualstudied zeolites on the interactingmoleculescan be observed only in the different electrostatic fields of these zeolites, which have the following trend for the studied cations, K > Na > Li. On the basis of these results conclusionswere drawn about the characteristics of the water molecule interacting with the cations of the studied zeolites which agree well with the experimentalfindings.

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