Quantum chemical study of the effect of the structural characteristics of zeolites on the roperties of their bridging oh groups

Journal of Molecular Catalysis, 26 (1984) 31 - 36

31

QUANTUM CHEMICAL STUDY OF THE EFFECT OF THE STRUCTURAL CHARACTERISTICS OF ZEOLITES ON THE PROPERTIES OF THEIR BRIDGING OH GROUPS

S. BERAN The J. Heyrovskg Institute of Physical Chemistry and Electrochemistry, Academy of Sciences, es-121 38 Prague 2 (Czechoslovakia)

Czechoslovak

(Received October 11,1983)

Summary It has been demonstrated using the CNDO/B method and (OH),SiOAl(OH)3 and (OH),SiOHAl(OH), clusters, with various Si-0 and Al-O bond lengths and SiOAl angles, that the dissociation energy of the bridging hydroxyl groups decreases with increasing SiOAl angle and with decreasing lengths of the Si-0 and Al-O bonds. This indicates that the probability of the existence of the individual types of skeletal OH groups in the zeolite should increase with increasing SiOAl angle and with decreasing lengths of the Si-0 and Al-O bonds. The acidity of these OH groups in particular zeolites should also depend on their structural parameters. The dependencies found were employed in a discussion of the proton localization in faujasites and in a comparison of the acidity of OH groups in faujasites and ZSM-5 zeolites.

Introduction In recent years various types of aluminosilicates have found broad application as catalysts in hydrocracking, hydroisomerization, hydration, etc. One of the most important factors determining the catalytic activity and selectivity of zeolites is the character and properties of their hydroxyl groups. The accessibility of these groups (the types of oxygen atoms to which they are attached) for the reacting molecules, and their acidity are especially important. For this reason the properties of hydroxyl groups have been extensively studied both experimentally [ 1, 21 and theoretically [3 - 51. It has been demonstrated that various quantum chemical methods used in calculations of cluster models of the solid phase can provide a number of very useful and otherwise unobtainable pieces of information on the properties of crystalline materials [ 4 - 61. For aluminosilicates, these are primarily calculations of various structural characteristics (equilibrium bond lengths 0304-5102/84/$3.00

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and valence angles), the strength and character of the individual bonds, the charge density distribution and various energy characteristics (overall energy, molecular orbital energies) and their dependence on a number of factors (structural type, Si:Al ratio, cation type, etc.). In connection with theoretical work concerned with the dependences between the structure and physicochemical properties of various crystalline substances (cf. ref. 6 and references cited therein), we have attempted in this work to employ model qu~tum chemical calculations to establish relationships between some structural characteristics (TOT angles and T-43 bond lengths; T = Si or Al) and the properties of the skeletal hydroxyl groups of aluminosilicates.

Model and method The properties of the bridging oxygen atoms of aluminosilicates and the hydroxyl groups formed by these 0 atoms were calculated using the already successfully applied [4 - 8] cluster models of the (O~)~SiOAl(OH)~- type, with the following geometric parameters: (i) The 0 ligands were arranged tetrahedrally around the Si or Al atom with Si-OH and Al-OH bond lengths of 1.60 and 1.70 X lo-” m, respectively . (ii) The terminal H atoms were always located on a straight line determined by the particular Si-0 or Al-O bond, and the O-H bond length was 1.08 X lo-lo m (iii) The SF-0 bridging bonds had lengths of 1.55, 1.60, 1.65 and 1.70 X lo-i0 m and the Al-0 bond lengths were 1.65,1.70,1.75 and 1.80 X lOBi m, respectively. (iv) The SiOAl angles had values of 120 to 170”. Protons in protonated clusters of the (OH),SiOHAl(OH)s type were located on the axis of the SiOAl angle at a distance of 1.5 X lO_” m from the bridging oxygen atom, which is the equ~ib~um position. Calculations were carried out using the standard version of the CNDO/Z method with an s, p basis set for the Si and Al atoms [9, lo]. It is known that this method provides good quality information on the valence angles or vibration frequencies as well as on charge densities [lo]. Information obtained on the bond lengths for elements of the first period are also sufficiently accurate, but for elements of the second period the CNDO/Z method overestimates the bond lengths [ll]. Similarly, the bond dissociation energies and stretching vibration frequencies are markedly overestimated by the CNDOf2 method [lo]. Consequently, the dissociation energies of the O-H bonds listed below must be considered as qualitative (relative) data. The clusters were then characterized using the atomic charge densities, the Wiberg bond orders, the energies of the highest occupied molecular orbital, E HOMO > or lowest unoccupied molecular orbital, ELuMO,and the dissociation energies of the bridging O-H bonds, ED, obtained as the difference between the overall cluster energy with and without the proton.

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Results and dicussion Some physical characteristics of protonated and unprotonated (OH),SiOAl(OH)s- clusters calculated for various values of the SiOAl angle between 120 and 170” are listed for illustration in Table 1. These characteristics exhibit several trends found earlier using ab irzitio calculations on (OH),SiOSi(OH), clusters [6]. For both types of clusters, the strength of the Si-0 bonds, characterized by the Wiberg bond orders, increases with increasing SiOAl angle, as does the negative charge on the bridging oxygen atom or the charge on the Si and Al atoms. The Al-O bond strength exhibits a maximum in the region of the equilibrium SiOAl angle. Similarly, the energy of the highest occupied molecular orbital, located primarily on the bridging 0 atom in the (OH),SiOAl(OH)s- cluster (and representing a lone electron pair interacting with a proton), has the lowest value for the equilibrium SiOAl angle. Practically the same trends as those listed in Table 1 for Si-0 and Al-O bond lengths of 1.60 and 1.75 X 10-r’ m, respectively, were found for clusters with the other Si--0 and Al-O bond lengths studied. TABLE

1

CNDO/Z energies of frontier orbitals, EHoMo and ELUMO (eV), total energies, E (kJ mol-‘), charges on atoms, q, and bond orders, p, calculated for the clusters (OH)&OAl(OH), and (OH)sSiOHAl(OH)s with various SiOAl angles (degree)a SiOAl

angles (degree)

120

125

130

135

-0.291 0.188 358 143 -0.677 1.534 1.373 1.078 0,510

-0.294

-0.295 0.185 0.186 358 149358147 -0.680 -0.684 1.536 0.539 1.379 1.383 1.098 1.089 0.512 0.513

-0.466 -0.066 360 022 -0.515 1.724 1.364 0.179 0.650 0.302

-0.465 -0.465 -0.067 -0.068 360023360023 -0.512 -0.509 1.728 1.726 1.366 1.368 0.176 0.173 0.653 0.655 0.302 0.301 0.935 0.933

140

150

160

170

-0.294 0.184 358 146 -0.688 0.540 1.388 1.106 0.514

-0.293 0.182 358 142 -0.696 1.543 1.396 1.119 0.514

-0.290 0.181 358 137 -0.703 1.545 1.402 1.127 0.512

-0.288 0.180 358 133 -0.707 1.547 1.406 1.131 0.510

-0.464 -0.069 360 017 -0.506 1.729 1.370 0.170 0.656 0.301 0.931

-0.464 -0.070 360 003 -0.501 1.723 1.373 0.165 0.656 0.299 0.924

-0.463 -0.070 359 983 -0.496 1.734 1.376 0.159 0.653 0.296 0.916

-0.463 -0.071 359 956 -0.491 1.736 1.378 0.153 0.646 0.293 0.905

(OH)sSiOAl(OH)sEHOMO ELUMO

E 40 QSi

QAl Psi-0 PAl-0

-0.277 0.191 358 127 -0.673 1.529 1.365 1.067 0.506

(OH)sSiOHAl(OH)3 EHOMO ELUMO

E 40 4Si qA1 qH

Psi-o PAl-0

-0.459 -0.064 360 010 -0.518 1.720 1.360 0.182 0.649 0.301

PO-H

adsi-

0.938 = 1.60

X lo-”

0.936

m and dAl-o

= 1.75

x lo-r0

m.

34

ED kJ/md) _ l&IO-

1800;

--‘\ 120

140

160 4SiOAl

Fig. 1. Plots of the total CNDO/P energy of (OH)#iOAl(OH), and (OH)$iOHAl(OH)3 clusters against the SiOAl angle, and the dependence of the dissociation energy of the bridging OH group on the SiOAl angle (d,_0 = 1.60 X 1O-1o and d~l-0 = 1.70 X 1O-1o m).

It is apparent from the energy dependence of the protonated and unprotonated clusters on the SiOAl angle (cf. Fig. 1) that the equilibrium angle is roughly identical for both forms and, depending on the lengths of the Si-3 and AI-O bonds assumed, attains values from 125 to 140”. This equilibrium angle decreases with the increasing Si-0 and Al-O bond lengths in agreement. with ab irzitio calculations [ 61. For angles less than the equilibrium value, the energy of the protonated cluster increases with increasing SiOAl angle more steeply than for the unprotonated cluster. Consequently, the dissociation energy of the protons of the bridging OH groups decreases with increasing SiOAl angle (cf. Table 2). Thus CNDO/B calculations indicate that the acidity of the bridging OH groups (or the affinity of the bridging oxygen atom for a proton) depends strongly on the SiOAl angle. In addition, it has been found that the dependence of the dissociation energy of the O-H bonds is correlated with the strength of the particular bond expressed in terms of the Wiberg bond orders. On the other hand, it. is rather surprising

TABLE2 CND0/2 dissociation energies of bridging OH groups(kJ mol-')calculated forvarious Si-0 and Al-O bondlengths,da_o and dam (X lo-'Om),as wellas TOT angles(degree)for(OH)3SiOHAl(OH)aclusters 10rOd*,-_O 10'0dS,_O (m) (m) 1.65 1.65 1.65 1.65 1.70 1.70 1.70 1.70 1.75 1.75 1.75 1.75 1.80 1.80 1.80 1.80

1.55 1.60 1.65 1.70 1.55 1.60 1.65 1.70 1.55 1.60 1.65 1.70 1.55 1.60 1.65 1.70

TOTangles(degree) 120

125

1843 1840 1858 1855 1871 1869 1882 1880 1855 1852 1870 1867 1882 1881 1893 1892 1867 1864 1881 1879 1893 1892 1904 1903 1878 1876 1891 1890 1903 1903 1914 1914

130

135

140

150

160

170

1835 1851 1866 1878 1848 1866 1878 1890 1861 1877 1891 1903 1873 1889 1903 1914

1829 1847 1862 1875 1844 1861 1872 1889 1858 1874 1889 1902 1870 1887 1902 1914

1823 1841 1858 1872 1838 1856 1862 1886 1852 1871 1887 1900 1866 1884 1900 1912

1807 1828 1846 1862 1825 1845 1850 1879 1841 1861 1879 1896 1856 1877 1894 1909

1786 1808 1820 1847 1804 1828 1844 1866 1823 1846 1865 1884 1841 1861 1883 1900

1751 1780 1803 1823 1777 1802 1835 1846 1797 1823 1845 1864 1817 1841 1864 1883

that the charge on the H atoms, that was also used to describe the acidity of the OH groups, decreases with increasing SiOAl angle. It would also be interesting to determine the manner in which the lengths of the SF-0 and Al-O bonds affect the properties of the bridging OH group. However, the CNDO/B method strongly overestimates the lengths of the Si-0 and Al-O bonds and thus it is not possible to employ bond lengths optimized using this method. Calculations carried out with the Si-0 and Al-O bond lengths optimized using the ab initio method [6] by varying bond lengths indicate that the dissociation energies of the O-H bonds (affinity of the oxygen for a proton) increases with the length of the Si-0 or Al-O bond for all the studied SiOAl angles (cfi Table 2). This trend can be explained by the fact that the equilibrium Si-0 and Al-0 bond lengths in the protonated clusters are greater than for the unprotonated ones [6]. Thus the longer Si-0 and Al-O bonds in the protonated cluster are closer to the equilibrium bond lengths and the overall energy is lower. This then leads to an increase in the dissociation energy of the O-H bonds for longer Si-0 and Al-O bond lengths. Thus the probability of the existence of the given OH group (the affinity of the bridging 0 atom for a proton) correlates with the lengths of the Si-0 and Al-O bonds formed by the particular bridging 0 atom. It follows from these calculations that, in addition to other factors important for actual aluminosilicates (particularly the Si:Al ratio), the properties of the bridging hydroxyl groups depend on the value of the particular

36

SiOAl angle as well as on the lengths of the neighbouring SF-0 and Al-O bonds in this group. The affinity of the bridging 0 atom for a proton decreases with increasing SiOAl angle and with decreasing length of the Si-0 and Al-O bonds. The acidity of the bridging OH group then exhibits the opposite behaviour. The obtained relations between the properties of the OH groups and their structural parameters can be employed to explain the properties of these groups in real zeolite structures. Values of the TOT angles for skeletal bridging 0 atoms of the Oi, 02, O3 and O4 types in faujasites attain values of 138.6,147.4,139.7 and 145.3”, respectively, and the corresponding T-O bond lengths are 1.65, 1.63, 1.66 and 1.62 X lOWi m, respectively [12]. The results obtained then support the conclusions of ref. 12 that the oxygen atoms of the O1 and 0s types should exhibit greater affinity for a proton than the remaining two types, i.e. in faujasites the protons should preferably be localized on these Oi and 0s atoms. Experimental results actually indicate the existence of OH groups of the Oi-H and 0,-H types (cf. ref. 13 and references cited therein). The TOT angles in ZSM zeolites attain values from 140 to 177” and the T-O bond lengths are clearly shorter (average T-O bond length = 1.59 X lo-” m [14]) than in faujasites. The skeletal oxygen atoms in faujasites should exhibit greater affinity for a proton than these atoms in ZSM-5 zeolites. It is thus apparent that the higher acidity of the OH groups in the ZSM-5 zeolite compared with faujasites could be explained on the basis of the present results, even though it is highly probable that the differing Si:Al ratio, electrostatic fields and other factors in ZSM zeolites and faujasites also play important roles.

References

6 7 8 9 10 11 12 13 14

P. A. Jacobs, Carboniogenic Activity of Zeolites, Elsevier, New York, 1977. H. W. Haynes, Jr., Catal. Rev. Sci. Eng., 17 (1978) 273. S. Beran,J. Mol. Catal., 10 (1981) 177. G. M. Zhidomirov, Kinet. Katal., 18 (1977) 1192. H. H. Dunken and V. I. Lygin, Quantenchemie der Adsorption an Festktirperoberfliichen, Verlag Chemie, Weinheim, New York, 1978. G. V. Gibbs, E. P. Meagher, M. D. Newton and D. K. Swanson, Struct. Bonding Cryst., 1 (1981) 195. P. Hobza, J. Sauer, Ch. Morgeneyer, J. Hurych and R. Zahradnik, J. Phys. Chem., 85 (1981) 1061. J. Dubsky, S. Beran and V. Bos&Eek, J. Mol. Catal., 6 (1979) 321. S. Beran and J. Dubskjr, J. Phys. Chem., 83 (1979) 2538. J. A. Pople and D. LBeveridge, Ap&o&ate ~oleculor Orbital Theory, McGrawHill, New York. 1970. F. J, Marsh and’M. S. Gordon, J. Mol. Struct., 31 (1976) 345. D. H. Olson and E. Dempsey, J. Catal., 13 (1969) 221. V. Bo&ek, S. Beran and Z. Jirak, J. Phys. Chem., 85 (1981) 3856. D. H. Olson, G. T. Kokotailo, S. L. Lawton and W. M. Meier, J. Phys. Chem., 85 (1981) 2238.