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Properties of iron-containing ZSM-5 zeolite: A theoretical study based on quantum chemical calculations
JournaJ of Molecular Catalysis, 66 (1991) 385-397
385
Properties of iron-containing ZSM-5 zeolite: a theoretical study based on quantum chemical calculations* R. Vetrivel**, S. Pal and S. Krishnan Na.ti.un.& ChemicaL L&oratory, Pune-411008 @u&j (Received August 22, 1990; revised January 2, 1991)
Abstract We report here the results of cluster calculations on ferrisilicate zeolite models. The different possible sites in the ZSM-5 zeolite are modelled by the H,T,07 cluster (where T=Si). Further, one of the ‘T’ sites is substituted with Fe, and the electronic properties of the resulting clusters are studied by performing EHMO (Extended Huckel Molecular Orbital) calculations. The properties studied include the preference energy for substitution of Fe at various ‘T’ sites, the pore size modifications and variations in the Briinsted as well as Lewis acidity due to Fe substitution. The acidity determination carried out by TFD of NHa on ZSM-5 type zeolites containing ‘Fe’ at framework positions and their shape selectivity for ethylbenzene conversion reactions are compared with the results of the calculations. The observed catalytic and acidic behaviour of iron-containing ZSM-5 zeolites is explained on the basis of their electronic structure.
Introduction Zeolitic structures containing cations other than Si4+ and A13+ at the framework sites are gaining importance due to their interesting catalytic and structural properties, It has been proved that it is possible to substitute iron in the zeolitic framework [ 11. The catalytic interest of these materials arises because the heteroatoms help in fine-tuning the strength of acidic sites and in introducing a bifunctional nature to the catalysts [ 11. For example, incorporation of iron includes hydrogenation centres in the catalyst and reduces the number of superacid sites. The structural interest arises from the following observations: (i) the isomorphous substitution modilles the pore size and hence the shapeselectivity, and (ii) higher ahuninium content in the ZSM-5 framework is known to decrease the transition temperature for monoclinic to orthorhombic phase change. There seems to be a limit on the Al incorporation in the framework. The lowest SW ratio known in the ZSM-5 lattice is 10 [ 21. However, the limit of isomorphous substitution in ZSM-5 framework is found to vary with different elements, e.g. the lowest ratios SiRi=67.5 [3a], Si/Fe= 36 [3b], lNCL Communication No. 4972. **Author to whom correspondence should be addressed.
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386
Si/s=l6.1[3c],Si/Ga=10.7[3d]andSi/Ge=2[3e]arereported.Furthermore, the substitution of tetravalent Ge ion in the framework is reported to increase the transition temperature for the monoclinic to orthorhombic phase change, in contrast to the effect of Al substitution. Therefore, in cases where the values of preference energy for substitution are large, even new zeolitic phases may me possible. Quantum chemical calculations are being widely used [4, 51 to study the structure and acidity of zeolitic systems. In the present study Extended Huckel Molecular Orbital (EHMO) calculations are undertaken to study the preferential site for the substitution of Si4+ by Fe3+ in the ZSM-5 framework. This is important information necessary to determine if these heteroatoms are going to be the part of lo-member channels or part of smaller channels, which in turn has many catalytic implications in shape-selective reactions. with X-ray diffraction studies, it is difficult to locate the position of Fe atoms since they are in low concentrations, and the lack of order in their distribution also makes it impossible to predict their site occupancy. Furthermore, the results obtained from these calculations answer the questions regarding the effects of incorporation of Fe3+ III . the ZSM-5 framework, on the pore geometry and on the acidic properties.
Methodology
and model
Method
We have used the standardExtended Huckel method [ 61for the electronic structure calculations. Although more accurate ab in&So calculations are desirable, semi-empirical EHMO calculations are adopted in the present study considering the following facts: (i) EHMO calculations are computationally efficient and are feasible for performing a multitude of calculations on related clusters. (ii) The absolute values of the energy calculated are not used; their relative difference is compared for different geometries of sim&,r clusters. (iii) The method has been used extensively in the past, for the successful study of transition metal systems [ 7, 81, and (iv) EHMO calculations have been found to be a valid method for the comparison of energies of chemical reactions in which the number of atoms and each kind of formal chemical bond is conserved, as is the case in the present study [9]. The parameters used in the present calculation are given in Table 1. Model
The cluster model chosen for the present study is (OH),-T-0-T-(OH), (where T = Si4+ or Fe3+). The T atoms and the oxygen atoms were located in the framework positions of ZSM-5 structure as determined by Olson et. al. [lo]. The cluster model represents two adjacent TO4 groups which share a corner. Considering that the electronic effects are short range forces and
387 TABLE 1 Parameters usedin the EHMO calculations Orbital
Hu
3,
(W
H
IS
- 13.600
1.300
0
2s 2P
- 32.300 - 14.800
2.275 2.275
Si
3s 3P
- 17.300 - 9.200
1.383 1.383
Fe
4s 4P 3d
-9.100 - 5.320 - 12.600
1.556 1.257 5.656
2.331
0.4850
0.6600
*Contraction coefficients used in the double-S exponent of the 3d orbital.
systemfollowed.T sites (O), ozqgen sites (0).
anice, showing the numbering
substitution of silicon by iron will be a localized phenomenon, this cluster model is adequate for studying the substitution process. Further, in earlier reports [ 111, ab initti calculations on dimeric cluster models as in the present study, as well as on larger pentsmeric cluster models, predicted the same preference for Al substitution at various T sites. The numbering system followed for the different sites is shown in F’ig. 1 for the repeating secondary building unit of ZSM-5. The terminal oxygen atoms were bonded to hydrogen atoms to ma&ain neutrality of the cluster. The positions of hydrogen atoms
388
were located at the nearest neighbour T-sites. The cluster charge was 0, when the T atoms are silicon and the charge was - 1 when one of the Si4’ was substituted by Fe3+. Again the cluster charge was 0 when hydrogen was added to the bridging oxygen between Si4+ and Fe3+ to represent the Briinsted acid site. Results and discussion
Among the 12 unique locations for T atoms in the ZSM-5 lattice, shown in Fig. 1, heteroatoms such as iron can substitute silicon in any one or more sites. There are 4 possible oxygens around each T atom where the chargecompensating proton could be attached. Hence, if one considers the substitution of a Fe3+ ion for a Si4+ ion, then theoretically 48 possible Bronsted sites could be generated. It is important to know the site at which Si4+ is replaced by Fe 3+. This information is still not obtainable from any viable experimental spectroscopic techniques. Cluster models representing all the possible Bronsted acid sites were therefore generated and EHMO calculations carried out. The calculations were initially carried out for the cluster models where all T sites were occupied by silicon atoms. The electronic energy calculated for all possible H&&O, clusters are listed in Table 2. Their energies relative to the most stable dimer, namely (HO)3-Tr-023-T7-(OH)3 are also given. The relative energy values for each of the 12 sites are given in Table 3. The energy for a given silicon site is an average energy of 4 clusters, where the particular Si is bonded through oxygen to one of the 4 possible neighbours. For example, the energy for site 1 is calculated as an average energy of SiISiz, SilSi4, SirSi5 and SiISilo clusters. These values are compared with similar results of ab initio calculations [ 111 available in the literature. The most stable Si-0-Si linkage is 7,7. The relative energies for the 12 sites are plotted in Fig. 2. The results of ab imitio calculations by Fripiat et al. [ 111 are also included in Fig. 2 for the purpose of comparison. Indeed, the absolute values of energies will be different for the obvious reason that the calculation methodology and the rigourousness involved in these techniques are different. However, it is encouraging to see the excellent congruity in the relative energy predicted by both methods. This correlation provides support for the validity of EHMO calculations and for further extension of this calculation procedure to study iron substitution in the ZSM5 framework. The finding of Fripiat et al. [ 111 that T site energies of the ZSM-5 framework fall within a wide range is confirmed by the EHMO calculations. at the framework sites Fe3+ was substituted in place of Si4+ at all the 12 possible framework sites. Dimer clusters were generated in which the Fe3+ ion at each of these
Fe
substitution
389
TABLE 2 The electronic energy and the relative energy values of various possible (OH),SiOSi(OH), clusters in the ZSM-5 lattice Serial No.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 me
Cluster
SilSi2 SilSi4 SilSi5 SilSilO Si2Si3 Si2Si6 Si2Si8 Si3Si4 Si3Si6 Si3Si12 Si4Si5 Si4Si7 Si5Si6 Si5Sill Si6Si9 Si7Si7 Si7Si8 SiPSi 1 SiSSi9 SiSSi12 SiSSilO SilOSilO SilOSill Sil lSi12 Si12Si12
Sum of one-electron energy (ev)
Relative energy
-1111.09 -1110.97 - 1110.40 - 1110.21 - 1111.59 -1111.11 - 1110.95 -1111.09 - 1110.76 -1111.58 -1110.93 - 1110.62 -1110.90 -1111.13 - 1110.58 -1111.94 - 1110.97 - 1111.23 -1110.68 -1110.77 - 1110.21 - 1110.72 -1111.24 -1110.99 -1111.12 -1111.07
19.60 22.37 35.51 39.89 8.07 19.14 22.83 19.60 27.21 8.30 23.29 30.44 23.98 18.68 31.36 0.00 22.37 16.37 29.06 26.98 39.89 28.13 16.14 21.91 18.91 20.06
mol - ‘)
lowest energy cluster, namely Si7Si7, is taken as the reference.
sites is connected to one of the 4 neighbouring Si4+ ions. In these cluster models it is assumed that: (i) there are no Fe -O-Fe linkages in the framework, (ii) the long-range effects due to next to nearest neighbour are negligible. These assumptions are valid considering the fact that the SiPe ratio is usually high in ferrisihcates. The process of substitution of Si”+ by Fe3+ is considered as follows: [(OH),-Si-0-Si-(OH),]
+Fe3’
-
[(OH)3-Si-O-Fe-(OH)s]-’
+Si4’
These calculations were carried out with the aim of deriving certain structural, acidic and shape-selective catalytic properties of ferrisillcate systems, which are discussed in the following sections.
390 TABLE
3
The relative energy values for the 12 sites in the ZSM-5 lattice Site
Adjacent
1 2 3 4 5 6 7 8 9 10 11 12
T-sites
Average AE (kcal mo1-‘)
Ab initio AE ([ll]) (kcal mol-‘)
29.34 17.41 15.80 23.93 25.37 25.42 17.30 25.31 32.11 26.52 18.97 18.56
65.7 34.4 35.3 68.5 67.7 54.2 50.1 65.9 72.8 66.4 51.5 28.3
2,
4,
5, 10
1, 2,
3, 4,
6, 8 6, 12
3, 4,
1, 1,
5, 7 6, 11
5, 2, 3, 9 7, 4, 8, 11 7, 2, 9, 12 8, 9, 10, 6 9, 10, 11, 1 10, 5, 12, 7 11, 3, 12, 8
I I
2
3
4
5
6
i’
9
9
IO
II
Site !n ZSM- 5 2. Plot showing the relative energies for the 12 sites in ZSM-5. (a) calculations, (b) from ub initio calculations reported in [ 111.
Fig.
Present
EHMO
Structural properties The values of the sums of one-electron energy for various iron-substituted clusters and their relative energies with respect to the Si,Fe7 cluster are shown in Table 4. The iron substitution energies in the 12 T sites of ZSM5 framework are listed in Table 5. The iron substitution energy at site 1 is calculated as an average of energies of FelSi,, Fe,,%,, Fe,& ‘5 and Fe,Sl,, clusters, and so on for all the sites.
391 TABLJZ 4 The electronic energy and the relative energy values of various possible (OH),FeOSi(OH), clusters and their proton binding energies
Serial No.
Cluster
sum of one-electron energy
Relative energy” AE (kcal mol-‘)
Proton
1.84 0.46 22.37 13.61 16.14 6.23 17.30 12.91 5.07 3.92 7.84 8.30 11.76 - 1.84 2.54 11.30 6.69 7.38 14.30 4.84 3.92 14.07 10.84 15.45 14.99 0.00 10.84 1.15 0.69 1.15 10.15 18.22 15.22 9.69 18.22 8.07 10.15 5.77 - 6.69 8.53 0.92 5.53 0.69 11.76 5.53 17.06 9.92 13.37
3.77 3.93 3.49 3.52 3.73 3.63 3.41 3.29 3.62 3.23 3.20 3.63 3.82 3.19 3.42 3.56 3.52 3.39 3.59 3.41 3.40 3.23 3.61 3.68 3.50 3.57 3.45 3.59 3.29 3.41 3.50 3.55 3.70 3.54 3.47 3.55 3.34 3.56 3.58 3.02 3.37 3.56 3.09 3.56 3.67 3.58 3.51 3.66
a=w (ev)
(eV
11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 f: 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48
’
FelSi2 FelSi4 FelSi5 FelSilO Fe2Sil FeZSi3 Fe2Si6 Fe2Si8 Fe3Si2 Fe3Si4 Fe3Si6 Fe3Si12 Fe4Sil Fe4Si3 Fe4Si5 Fe4Si7 FeSSi Fe5Si4 Fe5Si6 Fe5Sill Fe6Si2 Fe6Si3 Fe6Si5 Fe6Si9 F&7Si4 Fe7Si7 Fe7Si8 Fe7Sill Fe8Si2 Fe8Si7 Fe8Si9 Fe8Si12 Fe9Si6 Fe9Si8 Fe9Si9 FeSSi FelOSil FelOSi9 FelOSilO FelOSill FellSi FellSi FellSilO FellSi Fe12Si3 Fe12SiS Fel2Sill Fe12Si12
- 1169.37 -
1169.43 1168.48 1168.86 1168.75 1169.18 1168.70 1168.89 1169.23 1169.28 1169.11 1169.09 1168.94 1169.53 1169.34 1168.96 1169.16 1169.13 1168.83 1169.24 1169.28 1168.84 1168.98 1168.78 1168.80 1169.45 1168.98 1169.40 1169.42 1169.40 1169.01 1168.66 1168.79 1169.03 1168.66 1169.10 1169.01 1169.20 1169.74 1169.08 1169.41 1169.21 1169.42 1168.94 1169.21 1168.71 1169.02 1168.87
Y%e Fe7Si7 cluster is taken as the refereWe.
TABLE 5 The relative energy values for the substitution of Fe 3+ in each of the 12 sites in the ZSM-5 lattice and the average proton afEnity values T-Site 1 2 3 4 5 6 7 8 9 10 11 12
Average AE (kcal mol-‘) 9.57
13.15 6.28 5.94 8.30 11.07 6.75 7.55 12.80 4.44 4.73 11.47
Proton affinity (ev> 3.68 3.52 3.42 3.50 3.48 3.48 3.53 3.44 3.57 3.38 3.40 3.61
From the results reported for a series of clusters, it could be inferred that unlike Si-Si clusters, the energies of Fe-Si clusters fall within a short energy range, indicating that the iron substitutionenergies at different possible sites are quite close. However, site 10 seems to be energetically the most favourable for the substitution of iron. As stated earlier, there is no precise experimental technique to obtain this information on iron occupation in ZSM5. The topography of site 10 in ZSM-5 is such that it lies on the lo-member ring of the sinusoidal channel and it does not occur in the lo-member ring of the straight channel. Hence the geometric changes occurring due to iron substitution are expected to modify the diameter of the sinusoidal channel only. Shape-selective catalytic pruperties In general, iron-oxygen distances are known to be larger than sili-
con-oxygen distances. Hence, when silicon is substituted by iron, the pore geometry will be altered. We have varied the Fe-O distance systematically from 1.2 A to 2.0 A. Similarly the Fe-O -Si bond angle was varied from 100” to 180”. These variations were carried out for the cluster with the lowest energy when Fe3+ is substituted at site 10, namely the Silo-Fe10 cluster. There are two ways in which the T-O distance can elongate when iron is incorporated in the framework, as shown in Fig. 3. In Scheme I of Fig. 3, the T-O-T angle remains constant and the Fe3+ projects into the channels of zeolite, thus decreasing the pore diameter. In Scheme II of Fig. 3, the T-O-T angle increases and hence the circumference and thus the pore diameter increases. The energetics for these geometric variations have been analysed by EHMO calculations for both Fe-O stretching at equilibrium angle and Si-O-Fe bending at equilibrium distance. Figure 4 shows the plot of energy
393
0
0
0
R YY
Y e
e
Si
0
0
0
0
0
Si
Si
0
0
0
0
SCHEME
\ Fe
0
I
< 0
0
0
e
Si
0
S
0
0
SUBSTITU~ Si By Fe
0
00 SCHEME
0
0
II
F’lg. 3. Two possible schemes through which the T-O distances can elongate substitution: Scheme I with constant Si-O-Fe angle, and Scheme II with varying
during Fe .%--O-Fe
angle.
“4170.75I. 1.41
I.61
tBi
2.01
Fe - 0 distance ( k)
F&f. 4. Plot shokg
the variation of electronic
energy of Si-Fe
cluster with respect to Fe-O
distance.
vs. the Fe-O distance for a constant T-O-T ahgle which is the equilibrium value taken from crystal structure. This plot shows a shallow curve, where dE is a small value for the changes in Fe-O distances. This plot *indicates the possibility of stretching the Fe - 0 distance without much destabilisation of the energy of the system. Figure 5 shows the plot of energy IUS. the Si-O-Fe angle, which is the value obtained from Fig. 4. This plotgives a steep curve, indicating that even for a small deviation in the T-O-T angle
394
-1163.15
\ I
p m E -1166.15 e s 2 m 2 -1164.15 -: “s -1168.15 s
I’ / \ \
/ x
E z -I 170.15 -1171.15
x/ I 135.8
I
I 140.8
1
I 145.8
I
1 150.8
I
I 155.8
Si - 0 - Fe Angle(dag.)
5. Plot showing the variation of electronic energy of Si-Fe with respect to O-Fe-O angle. Fig.
TABLE 6 Shape selectivity of Fe-ZSM-5 and Al-ZSM-5 in the ethylbenzene reaction (from 1121); (conditions: 713 K, atm. pt., WHSV= 15 h, H/oil=O, EB conversion * 45wt.%) Product
Fe-ZSM-5
Al-ZSM-5
diphlltk?
2.7 9.0 4.0 66.8 13.7 3.8
2.1 9.9 5.1 62.8 14.6 5.6
benzene toluene xylenes ethylbenzene cg aromatics
there is a large destabilisation. The exact geometric parameters obtained at the minimum of curves in Figs. 4 and 5 are not accurate values, since the core-core interactions are neglected in EHMO calculations. However, the trend in the variation of energetics clearly shows that the elongation of Fe-O bond occurs according to Scheme I of Fig. 3. Hence, the iron-containing zeolites are expectedto be better shape-selective catalysts.Ferrisilicate pentasil zeolite was synthesised and characterised by Ratnasamy et al. [ 121. In the conversion of C8 aromatics, it has been observed that Fe-ZSM-5 is more selective for xylene production than Al-ZSM-5, as shown in Table 6 [ 121. Another example of enhanced product selectivity, achieved in the catalytic dewaxing of petroleum oils to gasoline by the isomorphous substitution of Si4’ by Fe3’ in ZSM-5 zeolite, has been reported by Sivasanker et al. [ 13). It has also been shown that in the conversion of methanol to olefins, the selectivity for the formation of C2-4 olefins over H-ferrisilicate is as much as 5 times that of Al-HZSM-5 [ 141.Thus the experimentally observed higher
395
shape selectivity of iron-containing ZSM-5 corresponds with the predictions of the present calculations.
In order to study the electronic structure of Bronsted acid sites, a proton is attached to the bridging oxygen connecting the Si4’ and Fe3+ ions, thus neutrahsing extra negative charge due to Fe3+ substitution. This proton attached to the bridging oxygen represents the Brijnsted acid sites in ZSM5 catalysts. The geometry of the proton was chosen based on our previous calculations [4]; the O-H distance was 1.08 A and the hydrogen was in the plane formed by Fe3.‘, Si4+ and bridging 02-, forming an angle exactly half the Fe-0-Si angle on the obtuse side. The proton affinities calculated for various clusters are also listed in Table 4. The proton afiinily values are a measure of the Bronsted acidity. However, due to inherent approximations in the EHMO calculations, the values of proton affmity are overemphasised and hence only the relative differences in the values are considered to assess the strength of acidity at various sites. The proton afhnity is calculated as the difference in energy between (OH)sSi-OH-Fe(OH)3 and [(OH)3SiOFe(OH)3]‘-. The Bronsted acidity due to substitution of iron at a specific site is calculated in the same way as the substitution energy. The proton affinities are found to be dependent on the net charge on the bridging oxygen. In general, the values of proton afhnity calculated for ahnninium-substitutedclusters of ZSM-5 lattice [ 15 ] were smaller than the proton a&&y values calculated for iron-substituted clusters. It has been shown in earlier experiments [ 161that the iron-containing zeolites are less acidic than the ahnninium-containing zeolites since the electronegativity of Al is less than that of Fe. This is understandable, based on their ionic radii (A13+=0.51 & Fe3+ =0.64 A), despite having same net charge. Among the 12 sites, sites 10 and 11 are found to have the lowest proton affinity values and hence expected to be strong acid sites. Since Fe substitution is favoured at site 10, the strong acid sites will be found around site 10. From the results of these calculations, we propose that the 6 10 K peak reported in the TPD of NH3 [ 121 is due to the presence of NH4+ ion bonded to either one of the following oxygens 09, 01,,,015 and Ozs which are bonded to Tlo. The Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital (LIMO) energies were analysed in order to estimate the Lewis acidity of these systems. In Si-Si clusters, the HOMO were contributed mainly by the 2p atomic orbitals of oxygen, wbile’the 3s of silicon and 1s of hydrogen made the major contribution to the LUMO. The HOMO and LUMO formed two distinct bands with a band gap of - 3.0 eV. Since the electronic configuration of A13+ is close to Si4+, the band picture is also found to be the same for Al-containing zeolite clusters. However, for Fe-containing zeolite clusters, the energy of the 3d orbit+ of Fe3+ falls in the band gap region. Hence it fornk .a contimnnk, as shown
396 Si -Si
Si -Fe
CLUSTER
LUMO
.
(3s ot Si ad 1s ot HI I
30 ev
I
CLUSTER
_-___-_______------contlnwm ~_=_-_~~---_ 36 o+ Fe ------__-_-___
1 HOMO (2p
ot allygem)
F’ig. 6. Frontier energy level diagram of silicalite and ferrisilicate clusters.
in Fig. 6, and the LUMO levels are higher in energy compared to the Si-Si system. Thus the ability to accept electrons, namely the Lewis acidity, is low in Fe-containing zeolites.
Conclusions Quantumchemical calculations on a series of cluster models representing Fe-containing ZSM-5 zeolite is reported. Realistic models based on the crystal structure were used and the favourable sites for the substitution of Fe3+ instead of Si4+ was determined based on the relative energies of these chrsters. The correlation between the results of experimental study and the calculations on the shape selectivity as well as acidic properties are emphasised. The main outcome of the present study can be summarised as follows: (i) The preference energy for the substitution of Fe in ZSM-5 lattice predicts the presence of iron in the lo-member ring of the sinusoidal channel. (ii) The pore diameter of the eliptical, sinusoidal channel is reduced due to the incorporation of Fe3+ in the lattice. (iii) Fe-containing ZSM-5 is predicted to be less acidic than the corresponding Al-containing ZSM-5.
Acknowledgement The EHMO calculations were carried out at our computer system using the 1989-HP version of the EHMO program developed by Prof. R. Hoffmann and his research group at the Department of Chemistry, University of Cornell, Ithaca, N.Y. We thank Dr. P. Ratnasamy for the encouragement to pursue this work.
References 1 Eur. Pat. AppL EP.260 235 (1985) to P. Raksamy, H&e, I. BaU&hmn and B. S. Rao.
R. B. Borade, S. B. Kulkami, S. G.
397
2 D. H. Olson, W. 0. Haag and R. M. Lago, J. Catal., 61 (1980) 390. 3 (a) Q. ShiIu, P. Wenqin and Y. Shangoing, Stud Su@ce Sci. Cu.&&, 49A (1989) 133; (b) L. M. Kustov, V. B. Kezaneky and P. Ratnasazny, Zeolites, 7 (1987) 79; (c) J. C. Jansen, E. Biron and H. Van Bekkum, Stud. Su&ce Sci. CataL, 37 (1988) 133; (d) A. Endoh, K. Nishimiya, K. Tsutsumi and T. Take&hi, Stud. Su@zce Sci. Catal., 46 (1989) 779; (e) Z. Gabeka end J. L. Gluth, Stud. Surface Sci. Catal., 49A, (1989) 42 1. 4 R. Vetrivel, C. R. A. Catlow and E. A. Colbourn, Proc. R. Sot. Landon, A417 (1988) 81. 5 J. Sauer, C%em. Rev., 89 (1989) 199. 6 R. HofFmann, J. Chem. P&s., 39 (1963) 1397. 7 R. Hoffmann, Science, 211 (1988) 995. 8 B. Viiathan and R. Vetrivel, J. Mol. Catal., 37 (1986) 157. 9 W. J. Hehre, Act. Chem. Res., 3 (1976) 399. IfI D. H. Olson, G. T. Kokotailo, S. L. Lawton and W. M. Meier, J. Phys. Chem., 85 (1981) 2238. 11 (a) J. G. Fripiat, F. B. Andre, J. M. Andre and E. G. Derouane, Zeolites, 3 (1983) 306; (b) E. G. Derouane and J. G. Fripiat, Zeolites, 5 (1985) 165. 12 P. Ratnasamy, R. B. Borade, S. Sivasanker, V. P. Shiralkar and S. G. Hedge, Acta Phys. Chem., 31 (1985) 137. 13 S. Sivansanker, K. J. Waghmare, M. Reddy and P. Ratnasemy, in M. J. Phillips and M. Temam (eds.), Proc. Chemical Inst. Canada, Ottawa, p. 120. 14 S. B. Kulkarni, V. P. Shirakar, A. N. Kothasthene, R. B. Borade and P. Ratnasamy, Zeolites, 2 (1982) 313. 15 R. Vetrivel (unpublished results). 16 P. Ratnssamy, Reuct. Kim& Catd L&t., 35 (1987) 219.
385
Properties of iron-containing ZSM-5 zeolite: a theoretical study based on quantum chemical calculations* R. Vetrivel**, S. Pal and S. Krishnan Na.ti.un.& ChemicaL L&oratory, Pune-411008 @u&j (Received August 22, 1990; revised January 2, 1991)
Abstract We report here the results of cluster calculations on ferrisilicate zeolite models. The different possible sites in the ZSM-5 zeolite are modelled by the H,T,07 cluster (where T=Si). Further, one of the ‘T’ sites is substituted with Fe, and the electronic properties of the resulting clusters are studied by performing EHMO (Extended Huckel Molecular Orbital) calculations. The properties studied include the preference energy for substitution of Fe at various ‘T’ sites, the pore size modifications and variations in the Briinsted as well as Lewis acidity due to Fe substitution. The acidity determination carried out by TFD of NHa on ZSM-5 type zeolites containing ‘Fe’ at framework positions and their shape selectivity for ethylbenzene conversion reactions are compared with the results of the calculations. The observed catalytic and acidic behaviour of iron-containing ZSM-5 zeolites is explained on the basis of their electronic structure.
Introduction Zeolitic structures containing cations other than Si4+ and A13+ at the framework sites are gaining importance due to their interesting catalytic and structural properties, It has been proved that it is possible to substitute iron in the zeolitic framework [ 11. The catalytic interest of these materials arises because the heteroatoms help in fine-tuning the strength of acidic sites and in introducing a bifunctional nature to the catalysts [ 11. For example, incorporation of iron includes hydrogenation centres in the catalyst and reduces the number of superacid sites. The structural interest arises from the following observations: (i) the isomorphous substitution modilles the pore size and hence the shapeselectivity, and (ii) higher ahuninium content in the ZSM-5 framework is known to decrease the transition temperature for monoclinic to orthorhombic phase change. There seems to be a limit on the Al incorporation in the framework. The lowest SW ratio known in the ZSM-5 lattice is 10 [ 21. However, the limit of isomorphous substitution in ZSM-5 framework is found to vary with different elements, e.g. the lowest ratios SiRi=67.5 [3a], Si/Fe= 36 [3b], lNCL Communication No. 4972. **Author to whom correspondence should be addressed.
0304-5102/91/$3.50
(B Elsevier Sequoia/FYLuted in The Netherlands
386
Si/s=l6.1[3c],Si/Ga=10.7[3d]andSi/Ge=2[3e]arereported.Furthermore, the substitution of tetravalent Ge ion in the framework is reported to increase the transition temperature for the monoclinic to orthorhombic phase change, in contrast to the effect of Al substitution. Therefore, in cases where the values of preference energy for substitution are large, even new zeolitic phases may me possible. Quantum chemical calculations are being widely used [4, 51 to study the structure and acidity of zeolitic systems. In the present study Extended Huckel Molecular Orbital (EHMO) calculations are undertaken to study the preferential site for the substitution of Si4+ by Fe3+ in the ZSM-5 framework. This is important information necessary to determine if these heteroatoms are going to be the part of lo-member channels or part of smaller channels, which in turn has many catalytic implications in shape-selective reactions. with X-ray diffraction studies, it is difficult to locate the position of Fe atoms since they are in low concentrations, and the lack of order in their distribution also makes it impossible to predict their site occupancy. Furthermore, the results obtained from these calculations answer the questions regarding the effects of incorporation of Fe3+ III . the ZSM-5 framework, on the pore geometry and on the acidic properties.
Methodology
and model
Method
We have used the standardExtended Huckel method [ 61for the electronic structure calculations. Although more accurate ab in&So calculations are desirable, semi-empirical EHMO calculations are adopted in the present study considering the following facts: (i) EHMO calculations are computationally efficient and are feasible for performing a multitude of calculations on related clusters. (ii) The absolute values of the energy calculated are not used; their relative difference is compared for different geometries of sim&,r clusters. (iii) The method has been used extensively in the past, for the successful study of transition metal systems [ 7, 81, and (iv) EHMO calculations have been found to be a valid method for the comparison of energies of chemical reactions in which the number of atoms and each kind of formal chemical bond is conserved, as is the case in the present study [9]. The parameters used in the present calculation are given in Table 1. Model
The cluster model chosen for the present study is (OH),-T-0-T-(OH), (where T = Si4+ or Fe3+). The T atoms and the oxygen atoms were located in the framework positions of ZSM-5 structure as determined by Olson et. al. [lo]. The cluster model represents two adjacent TO4 groups which share a corner. Considering that the electronic effects are short range forces and
387 TABLE 1 Parameters usedin the EHMO calculations Orbital
Hu
3,
(W
H
IS
- 13.600
1.300
0
2s 2P
- 32.300 - 14.800
2.275 2.275
Si
3s 3P
- 17.300 - 9.200
1.383 1.383
Fe
4s 4P 3d
-9.100 - 5.320 - 12.600
1.556 1.257 5.656
2.331
0.4850
0.6600
*Contraction coefficients used in the double-S exponent of the 3d orbital.
systemfollowed.T sites (O), ozqgen sites (0).
anice, showing the numbering
substitution of silicon by iron will be a localized phenomenon, this cluster model is adequate for studying the substitution process. Further, in earlier reports [ 111, ab initti calculations on dimeric cluster models as in the present study, as well as on larger pentsmeric cluster models, predicted the same preference for Al substitution at various T sites. The numbering system followed for the different sites is shown in F’ig. 1 for the repeating secondary building unit of ZSM-5. The terminal oxygen atoms were bonded to hydrogen atoms to ma&ain neutrality of the cluster. The positions of hydrogen atoms
388
were located at the nearest neighbour T-sites. The cluster charge was 0, when the T atoms are silicon and the charge was - 1 when one of the Si4’ was substituted by Fe3+. Again the cluster charge was 0 when hydrogen was added to the bridging oxygen between Si4+ and Fe3+ to represent the Briinsted acid site. Results and discussion
Among the 12 unique locations for T atoms in the ZSM-5 lattice, shown in Fig. 1, heteroatoms such as iron can substitute silicon in any one or more sites. There are 4 possible oxygens around each T atom where the chargecompensating proton could be attached. Hence, if one considers the substitution of a Fe3+ ion for a Si4+ ion, then theoretically 48 possible Bronsted sites could be generated. It is important to know the site at which Si4+ is replaced by Fe 3+. This information is still not obtainable from any viable experimental spectroscopic techniques. Cluster models representing all the possible Bronsted acid sites were therefore generated and EHMO calculations carried out. The calculations were initially carried out for the cluster models where all T sites were occupied by silicon atoms. The electronic energy calculated for all possible H&&O, clusters are listed in Table 2. Their energies relative to the most stable dimer, namely (HO)3-Tr-023-T7-(OH)3 are also given. The relative energy values for each of the 12 sites are given in Table 3. The energy for a given silicon site is an average energy of 4 clusters, where the particular Si is bonded through oxygen to one of the 4 possible neighbours. For example, the energy for site 1 is calculated as an average energy of SiISiz, SilSi4, SirSi5 and SiISilo clusters. These values are compared with similar results of ab initio calculations [ 111 available in the literature. The most stable Si-0-Si linkage is 7,7. The relative energies for the 12 sites are plotted in Fig. 2. The results of ab imitio calculations by Fripiat et al. [ 111 are also included in Fig. 2 for the purpose of comparison. Indeed, the absolute values of energies will be different for the obvious reason that the calculation methodology and the rigourousness involved in these techniques are different. However, it is encouraging to see the excellent congruity in the relative energy predicted by both methods. This correlation provides support for the validity of EHMO calculations and for further extension of this calculation procedure to study iron substitution in the ZSM5 framework. The finding of Fripiat et al. [ 111 that T site energies of the ZSM-5 framework fall within a wide range is confirmed by the EHMO calculations. at the framework sites Fe3+ was substituted in place of Si4+ at all the 12 possible framework sites. Dimer clusters were generated in which the Fe3+ ion at each of these
Fe
substitution
389
TABLE 2 The electronic energy and the relative energy values of various possible (OH),SiOSi(OH), clusters in the ZSM-5 lattice Serial No.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 me
Cluster
SilSi2 SilSi4 SilSi5 SilSilO Si2Si3 Si2Si6 Si2Si8 Si3Si4 Si3Si6 Si3Si12 Si4Si5 Si4Si7 Si5Si6 Si5Sill Si6Si9 Si7Si7 Si7Si8 SiPSi 1 SiSSi9 SiSSi12 SiSSilO SilOSilO SilOSill Sil lSi12 Si12Si12
Sum of one-electron energy (ev)
Relative energy
-1111.09 -1110.97 - 1110.40 - 1110.21 - 1111.59 -1111.11 - 1110.95 -1111.09 - 1110.76 -1111.58 -1110.93 - 1110.62 -1110.90 -1111.13 - 1110.58 -1111.94 - 1110.97 - 1111.23 -1110.68 -1110.77 - 1110.21 - 1110.72 -1111.24 -1110.99 -1111.12 -1111.07
19.60 22.37 35.51 39.89 8.07 19.14 22.83 19.60 27.21 8.30 23.29 30.44 23.98 18.68 31.36 0.00 22.37 16.37 29.06 26.98 39.89 28.13 16.14 21.91 18.91 20.06
mol - ‘)
lowest energy cluster, namely Si7Si7, is taken as the reference.
sites is connected to one of the 4 neighbouring Si4+ ions. In these cluster models it is assumed that: (i) there are no Fe -O-Fe linkages in the framework, (ii) the long-range effects due to next to nearest neighbour are negligible. These assumptions are valid considering the fact that the SiPe ratio is usually high in ferrisihcates. The process of substitution of Si”+ by Fe3+ is considered as follows: [(OH),-Si-0-Si-(OH),]
+Fe3’
-
[(OH)3-Si-O-Fe-(OH)s]-’
+Si4’
These calculations were carried out with the aim of deriving certain structural, acidic and shape-selective catalytic properties of ferrisillcate systems, which are discussed in the following sections.
390 TABLE
3
The relative energy values for the 12 sites in the ZSM-5 lattice Site
Adjacent
1 2 3 4 5 6 7 8 9 10 11 12
T-sites
Average AE (kcal mo1-‘)
Ab initio AE ([ll]) (kcal mol-‘)
29.34 17.41 15.80 23.93 25.37 25.42 17.30 25.31 32.11 26.52 18.97 18.56
65.7 34.4 35.3 68.5 67.7 54.2 50.1 65.9 72.8 66.4 51.5 28.3
2,
4,
5, 10
1, 2,
3, 4,
6, 8 6, 12
3, 4,
1, 1,
5, 7 6, 11
5, 2, 3, 9 7, 4, 8, 11 7, 2, 9, 12 8, 9, 10, 6 9, 10, 11, 1 10, 5, 12, 7 11, 3, 12, 8
I I
2
3
4
5
6
i’
9
9
IO
II
Site !n ZSM- 5 2. Plot showing the relative energies for the 12 sites in ZSM-5. (a) calculations, (b) from ub initio calculations reported in [ 111.
Fig.
Present
EHMO
Structural properties The values of the sums of one-electron energy for various iron-substituted clusters and their relative energies with respect to the Si,Fe7 cluster are shown in Table 4. The iron substitution energies in the 12 T sites of ZSM5 framework are listed in Table 5. The iron substitution energy at site 1 is calculated as an average of energies of FelSi,, Fe,,%,, Fe,& ‘5 and Fe,Sl,, clusters, and so on for all the sites.
391 TABLJZ 4 The electronic energy and the relative energy values of various possible (OH),FeOSi(OH), clusters and their proton binding energies
Serial No.
Cluster
sum of one-electron energy
Relative energy” AE (kcal mol-‘)
Proton
1.84 0.46 22.37 13.61 16.14 6.23 17.30 12.91 5.07 3.92 7.84 8.30 11.76 - 1.84 2.54 11.30 6.69 7.38 14.30 4.84 3.92 14.07 10.84 15.45 14.99 0.00 10.84 1.15 0.69 1.15 10.15 18.22 15.22 9.69 18.22 8.07 10.15 5.77 - 6.69 8.53 0.92 5.53 0.69 11.76 5.53 17.06 9.92 13.37
3.77 3.93 3.49 3.52 3.73 3.63 3.41 3.29 3.62 3.23 3.20 3.63 3.82 3.19 3.42 3.56 3.52 3.39 3.59 3.41 3.40 3.23 3.61 3.68 3.50 3.57 3.45 3.59 3.29 3.41 3.50 3.55 3.70 3.54 3.47 3.55 3.34 3.56 3.58 3.02 3.37 3.56 3.09 3.56 3.67 3.58 3.51 3.66
a=w (ev)
(eV
11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 f: 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48
’
FelSi2 FelSi4 FelSi5 FelSilO Fe2Sil FeZSi3 Fe2Si6 Fe2Si8 Fe3Si2 Fe3Si4 Fe3Si6 Fe3Si12 Fe4Sil Fe4Si3 Fe4Si5 Fe4Si7 FeSSi Fe5Si4 Fe5Si6 Fe5Sill Fe6Si2 Fe6Si3 Fe6Si5 Fe6Si9 F&7Si4 Fe7Si7 Fe7Si8 Fe7Sill Fe8Si2 Fe8Si7 Fe8Si9 Fe8Si12 Fe9Si6 Fe9Si8 Fe9Si9 FeSSi FelOSil FelOSi9 FelOSilO FelOSill FellSi FellSi FellSilO FellSi Fe12Si3 Fe12SiS Fel2Sill Fe12Si12
- 1169.37 -
1169.43 1168.48 1168.86 1168.75 1169.18 1168.70 1168.89 1169.23 1169.28 1169.11 1169.09 1168.94 1169.53 1169.34 1168.96 1169.16 1169.13 1168.83 1169.24 1169.28 1168.84 1168.98 1168.78 1168.80 1169.45 1168.98 1169.40 1169.42 1169.40 1169.01 1168.66 1168.79 1169.03 1168.66 1169.10 1169.01 1169.20 1169.74 1169.08 1169.41 1169.21 1169.42 1168.94 1169.21 1168.71 1169.02 1168.87
Y%e Fe7Si7 cluster is taken as the refereWe.
TABLE 5 The relative energy values for the substitution of Fe 3+ in each of the 12 sites in the ZSM-5 lattice and the average proton afEnity values T-Site 1 2 3 4 5 6 7 8 9 10 11 12
Average AE (kcal mol-‘) 9.57
13.15 6.28 5.94 8.30 11.07 6.75 7.55 12.80 4.44 4.73 11.47
Proton affinity (ev> 3.68 3.52 3.42 3.50 3.48 3.48 3.53 3.44 3.57 3.38 3.40 3.61
From the results reported for a series of clusters, it could be inferred that unlike Si-Si clusters, the energies of Fe-Si clusters fall within a short energy range, indicating that the iron substitutionenergies at different possible sites are quite close. However, site 10 seems to be energetically the most favourable for the substitution of iron. As stated earlier, there is no precise experimental technique to obtain this information on iron occupation in ZSM5. The topography of site 10 in ZSM-5 is such that it lies on the lo-member ring of the sinusoidal channel and it does not occur in the lo-member ring of the straight channel. Hence the geometric changes occurring due to iron substitution are expected to modify the diameter of the sinusoidal channel only. Shape-selective catalytic pruperties In general, iron-oxygen distances are known to be larger than sili-
con-oxygen distances. Hence, when silicon is substituted by iron, the pore geometry will be altered. We have varied the Fe-O distance systematically from 1.2 A to 2.0 A. Similarly the Fe-O -Si bond angle was varied from 100” to 180”. These variations were carried out for the cluster with the lowest energy when Fe3+ is substituted at site 10, namely the Silo-Fe10 cluster. There are two ways in which the T-O distance can elongate when iron is incorporated in the framework, as shown in Fig. 3. In Scheme I of Fig. 3, the T-O-T angle remains constant and the Fe3+ projects into the channels of zeolite, thus decreasing the pore diameter. In Scheme II of Fig. 3, the T-O-T angle increases and hence the circumference and thus the pore diameter increases. The energetics for these geometric variations have been analysed by EHMO calculations for both Fe-O stretching at equilibrium angle and Si-O-Fe bending at equilibrium distance. Figure 4 shows the plot of energy
393
0
0
0
R YY
Y e
e
Si
0
0
0
0
0
Si
Si
0
0
0
0
SCHEME
\ Fe
0
I
< 0
0
0
e
Si
0
S
0
0
SUBSTITU~ Si By Fe
0
00 SCHEME
0
0
II
F’lg. 3. Two possible schemes through which the T-O distances can elongate substitution: Scheme I with constant Si-O-Fe angle, and Scheme II with varying
during Fe .%--O-Fe
angle.
“4170.75I. 1.41
I.61
tBi
2.01
Fe - 0 distance ( k)
F&f. 4. Plot shokg
the variation of electronic
energy of Si-Fe
cluster with respect to Fe-O
distance.
vs. the Fe-O distance for a constant T-O-T ahgle which is the equilibrium value taken from crystal structure. This plot shows a shallow curve, where dE is a small value for the changes in Fe-O distances. This plot *indicates the possibility of stretching the Fe - 0 distance without much destabilisation of the energy of the system. Figure 5 shows the plot of energy IUS. the Si-O-Fe angle, which is the value obtained from Fig. 4. This plotgives a steep curve, indicating that even for a small deviation in the T-O-T angle
394
-1163.15
\ I
p m E -1166.15 e s 2 m 2 -1164.15 -: “s -1168.15 s
I’ / \ \
/ x
E z -I 170.15 -1171.15
x/ I 135.8
I
I 140.8
1
I 145.8
I
1 150.8
I
I 155.8
Si - 0 - Fe Angle(dag.)
5. Plot showing the variation of electronic energy of Si-Fe with respect to O-Fe-O angle. Fig.
TABLE 6 Shape selectivity of Fe-ZSM-5 and Al-ZSM-5 in the ethylbenzene reaction (from 1121); (conditions: 713 K, atm. pt., WHSV= 15 h, H/oil=O, EB conversion * 45wt.%) Product
Fe-ZSM-5
Al-ZSM-5
diphlltk?
2.7 9.0 4.0 66.8 13.7 3.8
2.1 9.9 5.1 62.8 14.6 5.6
benzene toluene xylenes ethylbenzene cg aromatics
there is a large destabilisation. The exact geometric parameters obtained at the minimum of curves in Figs. 4 and 5 are not accurate values, since the core-core interactions are neglected in EHMO calculations. However, the trend in the variation of energetics clearly shows that the elongation of Fe-O bond occurs according to Scheme I of Fig. 3. Hence, the iron-containing zeolites are expectedto be better shape-selective catalysts.Ferrisilicate pentasil zeolite was synthesised and characterised by Ratnasamy et al. [ 121. In the conversion of C8 aromatics, it has been observed that Fe-ZSM-5 is more selective for xylene production than Al-ZSM-5, as shown in Table 6 [ 121. Another example of enhanced product selectivity, achieved in the catalytic dewaxing of petroleum oils to gasoline by the isomorphous substitution of Si4’ by Fe3’ in ZSM-5 zeolite, has been reported by Sivasanker et al. [ 13). It has also been shown that in the conversion of methanol to olefins, the selectivity for the formation of C2-4 olefins over H-ferrisilicate is as much as 5 times that of Al-HZSM-5 [ 141.Thus the experimentally observed higher
395
shape selectivity of iron-containing ZSM-5 corresponds with the predictions of the present calculations.
In order to study the electronic structure of Bronsted acid sites, a proton is attached to the bridging oxygen connecting the Si4’ and Fe3+ ions, thus neutrahsing extra negative charge due to Fe3+ substitution. This proton attached to the bridging oxygen represents the Brijnsted acid sites in ZSM5 catalysts. The geometry of the proton was chosen based on our previous calculations [4]; the O-H distance was 1.08 A and the hydrogen was in the plane formed by Fe3.‘, Si4+ and bridging 02-, forming an angle exactly half the Fe-0-Si angle on the obtuse side. The proton affinities calculated for various clusters are also listed in Table 4. The proton afiinily values are a measure of the Bronsted acidity. However, due to inherent approximations in the EHMO calculations, the values of proton affmity are overemphasised and hence only the relative differences in the values are considered to assess the strength of acidity at various sites. The proton afhnity is calculated as the difference in energy between (OH)sSi-OH-Fe(OH)3 and [(OH)3SiOFe(OH)3]‘-. The Bronsted acidity due to substitution of iron at a specific site is calculated in the same way as the substitution energy. The proton affinities are found to be dependent on the net charge on the bridging oxygen. In general, the values of proton afhnity calculated for ahnninium-substitutedclusters of ZSM-5 lattice [ 15 ] were smaller than the proton a&&y values calculated for iron-substituted clusters. It has been shown in earlier experiments [ 161that the iron-containing zeolites are less acidic than the ahnninium-containing zeolites since the electronegativity of Al is less than that of Fe. This is understandable, based on their ionic radii (A13+=0.51 & Fe3+ =0.64 A), despite having same net charge. Among the 12 sites, sites 10 and 11 are found to have the lowest proton affinity values and hence expected to be strong acid sites. Since Fe substitution is favoured at site 10, the strong acid sites will be found around site 10. From the results of these calculations, we propose that the 6 10 K peak reported in the TPD of NH3 [ 121 is due to the presence of NH4+ ion bonded to either one of the following oxygens 09, 01,,,015 and Ozs which are bonded to Tlo. The Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital (LIMO) energies were analysed in order to estimate the Lewis acidity of these systems. In Si-Si clusters, the HOMO were contributed mainly by the 2p atomic orbitals of oxygen, wbile’the 3s of silicon and 1s of hydrogen made the major contribution to the LUMO. The HOMO and LUMO formed two distinct bands with a band gap of - 3.0 eV. Since the electronic configuration of A13+ is close to Si4+, the band picture is also found to be the same for Al-containing zeolite clusters. However, for Fe-containing zeolite clusters, the energy of the 3d orbit+ of Fe3+ falls in the band gap region. Hence it fornk .a contimnnk, as shown
396 Si -Si
Si -Fe
CLUSTER
LUMO
.
(3s ot Si ad 1s ot HI I
30 ev
I
CLUSTER
_-___-_______------contlnwm ~_=_-_~~---_ 36 o+ Fe ------__-_-___
1 HOMO (2p
ot allygem)
F’ig. 6. Frontier energy level diagram of silicalite and ferrisilicate clusters.
in Fig. 6, and the LUMO levels are higher in energy compared to the Si-Si system. Thus the ability to accept electrons, namely the Lewis acidity, is low in Fe-containing zeolites.
Conclusions Quantumchemical calculations on a series of cluster models representing Fe-containing ZSM-5 zeolite is reported. Realistic models based on the crystal structure were used and the favourable sites for the substitution of Fe3+ instead of Si4+ was determined based on the relative energies of these chrsters. The correlation between the results of experimental study and the calculations on the shape selectivity as well as acidic properties are emphasised. The main outcome of the present study can be summarised as follows: (i) The preference energy for the substitution of Fe in ZSM-5 lattice predicts the presence of iron in the lo-member ring of the sinusoidal channel. (ii) The pore diameter of the eliptical, sinusoidal channel is reduced due to the incorporation of Fe3+ in the lattice. (iii) Fe-containing ZSM-5 is predicted to be less acidic than the corresponding Al-containing ZSM-5.
Acknowledgement The EHMO calculations were carried out at our computer system using the 1989-HP version of the EHMO program developed by Prof. R. Hoffmann and his research group at the Department of Chemistry, University of Cornell, Ithaca, N.Y. We thank Dr. P. Ratnasamy for the encouragement to pursue this work.
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R. B. Borade, S. B. Kulkami, S. G.
397
2 D. H. Olson, W. 0. Haag and R. M. Lago, J. Catal., 61 (1980) 390. 3 (a) Q. ShiIu, P. Wenqin and Y. Shangoing, Stud Su@ce Sci. Cu.&&, 49A (1989) 133; (b) L. M. Kustov, V. B. Kezaneky and P. Ratnasazny, Zeolites, 7 (1987) 79; (c) J. C. Jansen, E. Biron and H. Van Bekkum, Stud. Su&ce Sci. CataL, 37 (1988) 133; (d) A. Endoh, K. Nishimiya, K. Tsutsumi and T. Take&hi, Stud. Su@zce Sci. Catal., 46 (1989) 779; (e) Z. Gabeka end J. L. Gluth, Stud. Surface Sci. Catal., 49A, (1989) 42 1. 4 R. Vetrivel, C. R. A. Catlow and E. A. Colbourn, Proc. R. Sot. Landon, A417 (1988) 81. 5 J. Sauer, C%em. Rev., 89 (1989) 199. 6 R. HofFmann, J. Chem. P&s., 39 (1963) 1397. 7 R. Hoffmann, Science, 211 (1988) 995. 8 B. Viiathan and R. Vetrivel, J. Mol. Catal., 37 (1986) 157. 9 W. J. Hehre, Act. Chem. Res., 3 (1976) 399. IfI D. H. Olson, G. T. Kokotailo, S. L. Lawton and W. M. Meier, J. Phys. Chem., 85 (1981) 2238. 11 (a) J. G. Fripiat, F. B. Andre, J. M. Andre and E. G. Derouane, Zeolites, 3 (1983) 306; (b) E. G. Derouane and J. G. Fripiat, Zeolites, 5 (1985) 165. 12 P. Ratnasamy, R. B. Borade, S. Sivasanker, V. P. Shiralkar and S. G. Hedge, Acta Phys. Chem., 31 (1985) 137. 13 S. Sivansanker, K. J. Waghmare, M. Reddy and P. Ratnasemy, in M. J. Phillips and M. Temam (eds.), Proc. Chemical Inst. Canada, Ottawa, p. 120. 14 S. B. Kulkarni, V. P. Shirakar, A. N. Kothasthene, R. B. Borade and P. Ratnasamy, Zeolites, 2 (1982) 313. 15 R. Vetrivel (unpublished results). 16 P. Ratnssamy, Reuct. Kim& Catd L&t., 35 (1987) 219.