- Email: [email protected]
Effects of structural characteristics of zeolites on the properties of their bridging and terminal hydroxyl groups
Applied Surface Science 130-132 Ž1998. 555–560
Effects of structural characteristics of zeolites on the properties of their bridging and terminal hydroxyl groups A. Chatterjee a , T. Iwasaki b, T. Ebina b, H. Tsuruya a , T. Kanougi a , Y. Oumi a , M. Kubo a , A. Miyamoto a,) a
Department of Materials Chemistry, Graduate School of Engineering, Tohoku UniÕersity, Sendai 980-77, Japan b Tohoku National Industrial Research Institute, 4-2-1 Nigatake, Miyagino-ku, Sendai-983, Japan Received 1 September 1997; accepted 24 October 1997
Abstract Hydrogen forms of zeolites are active catalysts in numerous acid-catalyzed reactions. Hydroxyl groups are responsible for the Brønsted acidity of amorphous silica–aluminas and of zeolites. Generally, the acid strength of OH groups in H forms of zeolite depends on the SirAl ratio, the degree of cation exchange and the degree of dehydroxylation. The present aim of our investigation is to rationalize an understanding of the role of hydroxyl groups both bridging and terminal on the acidity of zeolites with the help of local density functional ŽLDF. vibrational frequency calculation. The effects of geometry ŽSi–O and Al–O bond lengths and Si–O–H as well as Si–O–Al angles. of zeolites on the vibrational frequencies of their OH groups were modeled by H 3 SiOHAlH 3; whereas to study the influence of chemical composition models with formula H 3 SiOSiŽOH. 3 and H 2 AlOSiŽOH. 3 have been studied. It is observed that structural characteristic influences the vibrational frequency more in comparison to chemical composition. A case study with H–Y zeolite has been performed with the models having formula ŽHO. 3 SiOHAlŽOH. 3 and wŽHO. 3 SiOAlŽOH. 3 xy. It is observed that between O 1 and O4 type oxygens present in H–Y zeolite, the O1 –H group lies at a higher frequency than the O4 –H group. All the results were compared with experimental IR frequencies. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Zeolites; Hydroxyl group; Vibrational frequency
1. Introduction For a number of years, various types of aluminosilicates termed as zeolites have been extensively used for many acid catalyzed reactions like cracking reaction, hydro-isomerization and also finds its way in synthesis of fine chemicals w1,2x. Proton donor sites or electron-acceptor sites are at solid surfaces, i.e. Brønsted or Lewis sites, respectively are at the )
Corresponding author. Fax: q81-22-217-7235; e-mail: [email protected].
origin of heterogeneous catalyst. Hydroxyl groups are responsible for the Brønsted acidity of amorphous silica alumina and of zeolites. The isomorphous substitution of the Si 4q species by Al 3q induces a negative charge in the framework. This charge can be balanced by addition of a proton in the bridging oxygen between silica and aluminum resulting in the formation of Si–OH–Al. Generally, the acid strength of OH groups in hydrogen forms of zeolite depends on the SirAl ratio, the degree of cation exchange and the degree of dehydroxylation w3x. Since the increased presence of aluminum in the
0169-4332r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 1 1 4 - 7
556
A. Chatterjee et al.r Applied Surface Science 130-132 (1998) 555–560
zeolite tends to weaken it structurally, the ratio of Si:Al measures both acidity and stability. Zeolites that are essentially devoid of aluminum still show presence of acidity, which is due to the terminal hydroxyls on the zeolite exterior. Many studies based on ab initio w4,5x and Hartree Fock method w6x has been devoted for the understanding of the intrinsic phenomenon in zeolite acidity. Very few density functional ŽDF. studies w7x are aimed at this direction, although DF method shows its viability over ab initio methods w8x. With zeolites, the variation of vibrational frequencies of the bridging hydroxyl group was accounted for by the different composition of the third coordination sphere w9x. The shifts in the frequency of the OH groups of zeolites with changes in the chemical composition are often explained by average electronegativity approach w10x. This approach, however, neglects the effect of the coordination sphere. So the previous studies underestimated the influence of structural factor as well as composition of the framework on the vibrational frequency of the OH. The present investigation aims to rationalize the understanding of zeolite acidity resulting from both structural and compositional variation by vibrational frequency calculations using
local density functional ŽLDF. study on cluster models of zeolite framework. The effects of geometry ŽSi–O and Al–O bond lengths and Si–O–H as well as Si–O–Al angles. of zeolites on the vibrational frequencies of their OH groups were modeled by H 3 SiOHAlH 3 ; whereas to study the influence of chemical composition models with formula H 3 SiOSiŽOH. 3 and H 2 AlOSiŽOH. 3 have been studied. It is observed that structural characteristic influences the vibrational frequency more in comparison to chemical composition. A case study with H–Y zeolite have been performed with the models having ŽH O . 3 S iO H A lŽO H . 3 fo rm u la and wŽHO. 3 SiOAlŽOH. 3 xy. It is observed that between O 1 and O4 type oxygens present in H–Y zeolite, the O 1 –H group lies at a higher frequency than the O4 –H group. All the results were compared with experimental IR frequencies.
2. Method and models Local density functional ŽLDF. calculations have been performed by running the DMOL software w11,12x on a Silicon Graphics IndigoII workstation.
Fig. 1. The cluster model Ž1. H 3 SiOHAlH 3 Ž1.1. to study the structural influence and to study the effect of chemical composition the cluster models Ž2. H 3 SiOSiŽOH. 3 Ž1.2. and Ž3. H 2 AlOSiŽOH. 3 Ž1.3. are chosen. All unlabeled atoms stand for hydrogens.
A. Chatterjee et al.r Applied Surface Science 130-132 (1998) 555–560
557
Fig. 2. The cluster models Ž1. ŽHO. 3 SiOHAlŽOH. 3 neutral Ž2.1. and Ž2. wŽHO. 3 AlOSiŽOH. 3 xy anionic Ž2.2. are chosen for H–Y zeolite to model different oxygens present in the system.
The LDF formalism is based on the work of von Barth and Hedin w13x. A double numerical with polarization ŽDNP. basis with spin restricted energy calculations was performed with fine mesh grid and frozen core electrons. The fine grid gives a reasonable compromise between accuracy and computational efficiency and was used for all calculations presented here. The self consistent field ŽSCF. tolerance was set to 10y4 and the gradient convergence to 10y3 harBohr. We determined the harmonic frequencies and the corresponding normal modes by diagonalizing the Cartesian–Hessian matrix constructed by numerical differentiation of the analytical gradients obtained at the equilibrium geometry. We used localized cluster model approach to build the cluster models. The effects of geometry ŽSi–O and Al–O bond lengths and Si–O–H as well as Si–O–Al angles. of zeolites on the vibrational frequencies of their OH groups were modeled by H 3 SiOHAlH 3 Ž1.1.; whereas to study the influence of chemical composition models with formula H 3 SiOSiŽOH. 3 Ž1.2. and H 2 AlOSiŽOH. 3 Ž1.3. have been studied. The models are shown in Fig. 1 as Ž1., Ž2. and Ž3. respectively. Hydrogens are not labeled for better visibility, all the other atoms unlabeled are hydrogen. The geometries were fully optimized for all the above models. A case study with H–Y zeolite have been performed. H–Y zeolite structure has been modeled from the X–ray crystal data w14x. In case of H–Y zeolite it is observed w15x that there are two kinds of oxygen present alternatively in the 12-membered ring. The pore opening has a C 6v
symmetry. Considering the typical nature cluster m o d e ls a re g e n e ra te d h a v in g fo rm u la ŽHO. 3 SiOHAlŽOH. 3 Ž2.1. and wŽHO. 3 SiOAlŽOH. 3 xy Ž2.2. for both the oxygens namely O 1 and O4 . These models are shown in Fig. 2 for O 1 oxygen as Ž1. s 2.1 and Ž2. s 2.2, the orientation of the two models shown in Fig. 2 are different for better visibility.
3. Results and discussion LDF calculations have been performed on three different clusters first to analyze the structural effect on vibrational frequency of the bridging hydroxyl, followed by the analysis of chemical composition on the same. Both the effects were compared to rationalize the dominant contributor for the vibrational frequency of hydroxyl both bridging and terminal in zeolites. This was followed by a case study on H–Y zeolite cluster models to justify the above proposition of dominance of the structural contribution in the properties of the bridging hydroxyls. In studying the effects of the structural factor on the bridging OH group we used the cluster 1.1 and performed LDF calculations over it. Now starting from the equilibrium geometry we varied the Si–O, Al–O distance and Si–O–H and Si–O–Al angles at a regular interval. The difference between the initial and final bond distance variation was noted as D R and the variation for the corresponding angles as mentioned above is termed as D f . The variation of
558
A. Chatterjee et al.r Applied Surface Science 130-132 (1998) 555–560
˚ and the variabond distances performed were 0.02 A tion in angles were 108. After varying the bond distances and lengths together in the said cluster model, it was optimized fully. The optimized geometries were given in Table 1. The total energy as well as the relative energy are also given in Table 1. It is observed that the total energy values listed in Table 1 have a maximum relative instability of around 1.2 kcalrmol. The interesting phenomena which is to be noted here is that the O–H stretching vibrational frequency Ž mOH . changes by 40 cmy1 . This can be explained by the molecular orbital contribution in agreement with the proposition of Newton w16x. In his analysis, an increase in the valence bond angle on the O atom Žhere, the Si–O–H angle. results in an increase in the percentage s character of the hybrid atomic orbitals forming the bonds Žhere, Si–O and O–H bonds., we observed the same trend in the contribution of s orbitals from density of states ŽDOS. results. The contribution of s orbitals increases for Si and O in the abovementioned bond formation process. As a consequence the O–H distance will decrease resulting to an increase in the vibrational frequency. Now, silica hydrogel consists of incompletely polymerized silicic acid, SiŽOH.4 , and contains a considerable amount of free hydroxyls terminating the polymer network which are called terminal hydroxyls w3x. Now, with the current model the terminal hydroxyls do not show any change with the change in the structural parameters, justifying the fact that the existence of these types of hydroxyl at the defect sites of the silicalite will be studied in future. The vibrational frequency shows the same
order as reported by other calculation methods reported earlier w17x. Comparison of the results of calculations on cluster models 1.2 and 1.3 were performed to study the effect of chemical composition which reveals that the substitution of silica by aluminum in the third coordination sphere has little influence on the O–H stretching vibrational frequency. The observed vibrational frequency of 4159 and 4162 cmy1 for the respective clusters justifies the fact that the structural factor plays a decisive role in the determination of vibrational frequencies of the bridging hydroxyl group in comparison to that due to variation in chemical composition. The terminal hydroxyls are not affected by the structural or compositional variation. This makes us interested to study the structural effect in a real situation, like that of H–Y zeolite. The electronic properties of the H–Y zeolite cluster models 2.1 and 2.2 were calculated using LDF. The calculations were performed on both type of oxygens to compare the shift in vibrational frequency for the respective oxygens. It is observed that the two oxygens have different geometry in terms of bond distances, angles and the reaction enthalpies w18x D H ŽkJrmol. of H-migration in between various O–H groups present in zeolite H–Y are as follows: Si– O 1 –Al s 135.388 and Si–O4 –Al s 136.278; Si–O 1 ˚ and Si–O4 s 1.69 A; ˚ Al–O1 s 1.65 A˚ and s 1.68 A ˚ D H for O1 –H s 7 kJrmol and Al–O4 s 1.62 A; D H for O4 –H s 58 kJrmol. The LDF calculation results are given in Table 2. It shows that the net charge on oxygens are somewhat the same, but the total energy for the individual
Table 1 Geometric parameters Žas a function of bond distance D R and bond angle D f ., total energy, relative energy and vibrational frequency of bridging OH Ž mOH . calculated for 1.1 cluster Geometric parameters
Equilibrium geometry
Deformations introduced into the cluster
˚ D R s q0.02 A D f s q108 ˚. R SiO ŽA ˚. R AlO ŽA f Si–O–H Ž8. f Al–O–Si Ž8. Total energy Žkcalrmol. Relative energy Žkcalrmol. mOH Žcmy1 .
˚ D R s q0.02 A D f s y108
˚ D R s y0.02 A D f s q108
˚ D R s y0.02 A D f s y108
1.687
1.696
1.702
1.678
1.683
1.829 114.2 128.9 y663014.5 0.0 4186
1.844 112.6 135.5 y663013.22 1.28 4218
1.851 111.9 138.3 y663013.81 0.69 4202
1.816 117.3 120.2 y663013.86 0.64 4222
1.827 116.8 119.9 y663014.05 0.45 4214
A. Chatterjee et al.r Applied Surface Science 130-132 (1998) 555–560
559
Table 2 Total energy, deprotonation energy, net charge on oxygen and O–H stretching frequency for both the oxygens O1 and O4 in cluster model 2.1 of H–Y zeolite Type of oxygen in H–Y zeolite
Total energy Žkcalrmol.
Deprotonation energy Žkcalrmol.
Net charge on oxygen
O–H stretching frequency Žcmy1 .
O1 O4
y663059.19 y663055.5
y339.56 y335.87
y0.58 y0.56
3712 3619
oxygens show that O 1 cluster is energetically more stabilized in comparison to that of O4 cluster. The relative acidity for the different oxygen present in H–Y can be estimated from the calculated proton affinity ŽPA. of zeolite models. The cluster which will show a higher PA will act as a poor proton donor and hence will show low Brønsted acidity. In the reverse, clusters having less PA will behave as better proton donor and hence show higher Brønsted acidity. This PA can be approximated by the deprotonation energy w19x. Now, the deprotonation energies for the individual clusters were calculated by using the following relation: ZeolOH™ ZeolOyq Hq The deprotonation energies were calculated from the difference of energy between neutral and anionic cluster models for both the oxygens. The energy values along with OH stretching vibrational frequency are also shown in Table 2. It is observed that the deprotonation energy is less in case of O 1 oxygen. The difference in deprotonation energy is 3.69 kcalrmol, which shows that O 1 oxygen will be a better proton donor than the O4 oxygen. Now the vibrational frequency results show that in case of O 1 oxygen the O–H stretching frequency comes at a higher value of wave number, in comparison to O4 . This trend is in agreement with the experimental observation which shows that the OH frequency at O 1 is at 3650 cmy1 whereas that of O4 falls at around 3550 cmy1 . The reason may be ascribed as follows, in case of O4 oxygen the proton can form a weak hydrogen bond to another oxygen atom of the tetrahedral six-ring in which it is present. This additional H-bond will weaken the OH bond which explains its lower reactivity. These results justify the fact that structural parameter influences the vibrational frequency. The optimistic results enable us to think in direction to formulate a priori rule for
correlating acidity with vibrational frequency, hence future studies are warranted. 4. Conclusion LDF studies have been performed on localized cluster models. The full geometry optimization with smaller models have been performed to get more accurate results. In the first place the comparison of influence on vibrational frequency of OH group both in terms of structural parameters and chemical composition have been performed. The results show the remarkable influence of structural parameters on vibrational frequency. The influence is much pronounced for the bridging hydroxyls in comparison to the terminal one in this model. The role of structural parameters have been further tested for H–Y zeolite with a different model to verify the reactivity of the two different oxygens present in H–Y zeolite. The calculation successfully reproduce the experimental trend. References w1x H. Karge, J. Weitkemp, in: Zeolites as Catalyst, Sorbents and Detergent Builders, Elsevier, Amsterdam, 1989, p. 645. w2x J. Weitkemp, S. Ernst, H. Daums, E. Gallie, Chem. Eng. Tech. 58 Ž1986. 623. w3x W.J. Mortier, J. Sauer, J.A. Lercher, H. Noller, J. Phys. Chem. 88 Ž1984. 905. w4x G. Coudurier, J.C. Vedrine, Pure Appl. Chem. 58 Ž1986. 1389. w5x V.A. Durrant, D.A. Walker, S.N. Gussou, J.E. Lyons, US Patent 4918249 Ž1972.. w6x N.S. Gnep, J.D. Diyemet, M.D. Guisnet, J. Mol. Catal. 45 Ž1988. 281. w7x M.S. Stave, J.B. Nicolas, J. Phys. Chem. 97 Ž1993. 9630. w8x J.C. White, A.C. Hess, J. Phys. Chem. 97 Ž1993. 8703. w9x R. von Balmoos, N.H. Meier, Nature 289 Ž1987. 782. w10x W.J. Mortier, K.V. Genechen, J. Gasteiger, J. Am. Chem. Soc. 107 Ž1985. 829.
560
A. Chatterjee et al.r Applied Surface Science 130-132 (1998) 555–560
w11x W. Kohn, L.J. Sham, Phys. Rev. A 140 Ž1965. 1133. w12x W. Kohn, L.J. Sham, InsightII user guide, version 3.00, Biosym Tech., San Diego, 1995. w13x U. von Barth, L. Hedin, J. Phys. C 5 Ž1972. 1629. w14x D.H. Olson, J. Phys. Chem. 74 Ž1970. 2758. w15x A. Chatterjee, R. Vetrivel, R. Sreekumar, Y.V.S.N. Murthy, C.N. Pillai, J. Mol. Catal. Žin press..
w16x M.D. Newton, in: M. Okeefe, A. Navtrosky ŽEds.., Structure and Bonding in Crystals, Vol. 1, Academic Press, New York, p. 981. w17x J. Sauer, P. Ugliengo, E. Garrone, V.R. Saunders, Chem. Rev. 94 Ž1994. 2095. w18x K.B. Wibeg, J. Am. Chem. Soc. 90 Ž1968. 59. w19x R.A. van Santen, G.J. Kramer, Chem. Rev. 95 Ž1995. 637.
Effects of structural characteristics of zeolites on the properties of their bridging and terminal hydroxyl groups A. Chatterjee a , T. Iwasaki b, T. Ebina b, H. Tsuruya a , T. Kanougi a , Y. Oumi a , M. Kubo a , A. Miyamoto a,) a
Department of Materials Chemistry, Graduate School of Engineering, Tohoku UniÕersity, Sendai 980-77, Japan b Tohoku National Industrial Research Institute, 4-2-1 Nigatake, Miyagino-ku, Sendai-983, Japan Received 1 September 1997; accepted 24 October 1997
Abstract Hydrogen forms of zeolites are active catalysts in numerous acid-catalyzed reactions. Hydroxyl groups are responsible for the Brønsted acidity of amorphous silica–aluminas and of zeolites. Generally, the acid strength of OH groups in H forms of zeolite depends on the SirAl ratio, the degree of cation exchange and the degree of dehydroxylation. The present aim of our investigation is to rationalize an understanding of the role of hydroxyl groups both bridging and terminal on the acidity of zeolites with the help of local density functional ŽLDF. vibrational frequency calculation. The effects of geometry ŽSi–O and Al–O bond lengths and Si–O–H as well as Si–O–Al angles. of zeolites on the vibrational frequencies of their OH groups were modeled by H 3 SiOHAlH 3; whereas to study the influence of chemical composition models with formula H 3 SiOSiŽOH. 3 and H 2 AlOSiŽOH. 3 have been studied. It is observed that structural characteristic influences the vibrational frequency more in comparison to chemical composition. A case study with H–Y zeolite has been performed with the models having formula ŽHO. 3 SiOHAlŽOH. 3 and wŽHO. 3 SiOAlŽOH. 3 xy. It is observed that between O 1 and O4 type oxygens present in H–Y zeolite, the O1 –H group lies at a higher frequency than the O4 –H group. All the results were compared with experimental IR frequencies. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Zeolites; Hydroxyl group; Vibrational frequency
1. Introduction For a number of years, various types of aluminosilicates termed as zeolites have been extensively used for many acid catalyzed reactions like cracking reaction, hydro-isomerization and also finds its way in synthesis of fine chemicals w1,2x. Proton donor sites or electron-acceptor sites are at solid surfaces, i.e. Brønsted or Lewis sites, respectively are at the )
Corresponding author. Fax: q81-22-217-7235; e-mail: [email protected].
origin of heterogeneous catalyst. Hydroxyl groups are responsible for the Brønsted acidity of amorphous silica alumina and of zeolites. The isomorphous substitution of the Si 4q species by Al 3q induces a negative charge in the framework. This charge can be balanced by addition of a proton in the bridging oxygen between silica and aluminum resulting in the formation of Si–OH–Al. Generally, the acid strength of OH groups in hydrogen forms of zeolite depends on the SirAl ratio, the degree of cation exchange and the degree of dehydroxylation w3x. Since the increased presence of aluminum in the
0169-4332r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 1 1 4 - 7
556
A. Chatterjee et al.r Applied Surface Science 130-132 (1998) 555–560
zeolite tends to weaken it structurally, the ratio of Si:Al measures both acidity and stability. Zeolites that are essentially devoid of aluminum still show presence of acidity, which is due to the terminal hydroxyls on the zeolite exterior. Many studies based on ab initio w4,5x and Hartree Fock method w6x has been devoted for the understanding of the intrinsic phenomenon in zeolite acidity. Very few density functional ŽDF. studies w7x are aimed at this direction, although DF method shows its viability over ab initio methods w8x. With zeolites, the variation of vibrational frequencies of the bridging hydroxyl group was accounted for by the different composition of the third coordination sphere w9x. The shifts in the frequency of the OH groups of zeolites with changes in the chemical composition are often explained by average electronegativity approach w10x. This approach, however, neglects the effect of the coordination sphere. So the previous studies underestimated the influence of structural factor as well as composition of the framework on the vibrational frequency of the OH. The present investigation aims to rationalize the understanding of zeolite acidity resulting from both structural and compositional variation by vibrational frequency calculations using
local density functional ŽLDF. study on cluster models of zeolite framework. The effects of geometry ŽSi–O and Al–O bond lengths and Si–O–H as well as Si–O–Al angles. of zeolites on the vibrational frequencies of their OH groups were modeled by H 3 SiOHAlH 3 ; whereas to study the influence of chemical composition models with formula H 3 SiOSiŽOH. 3 and H 2 AlOSiŽOH. 3 have been studied. It is observed that structural characteristic influences the vibrational frequency more in comparison to chemical composition. A case study with H–Y zeolite have been performed with the models having ŽH O . 3 S iO H A lŽO H . 3 fo rm u la and wŽHO. 3 SiOAlŽOH. 3 xy. It is observed that between O 1 and O4 type oxygens present in H–Y zeolite, the O 1 –H group lies at a higher frequency than the O4 –H group. All the results were compared with experimental IR frequencies.
2. Method and models Local density functional ŽLDF. calculations have been performed by running the DMOL software w11,12x on a Silicon Graphics IndigoII workstation.
Fig. 1. The cluster model Ž1. H 3 SiOHAlH 3 Ž1.1. to study the structural influence and to study the effect of chemical composition the cluster models Ž2. H 3 SiOSiŽOH. 3 Ž1.2. and Ž3. H 2 AlOSiŽOH. 3 Ž1.3. are chosen. All unlabeled atoms stand for hydrogens.
A. Chatterjee et al.r Applied Surface Science 130-132 (1998) 555–560
557
Fig. 2. The cluster models Ž1. ŽHO. 3 SiOHAlŽOH. 3 neutral Ž2.1. and Ž2. wŽHO. 3 AlOSiŽOH. 3 xy anionic Ž2.2. are chosen for H–Y zeolite to model different oxygens present in the system.
The LDF formalism is based on the work of von Barth and Hedin w13x. A double numerical with polarization ŽDNP. basis with spin restricted energy calculations was performed with fine mesh grid and frozen core electrons. The fine grid gives a reasonable compromise between accuracy and computational efficiency and was used for all calculations presented here. The self consistent field ŽSCF. tolerance was set to 10y4 and the gradient convergence to 10y3 harBohr. We determined the harmonic frequencies and the corresponding normal modes by diagonalizing the Cartesian–Hessian matrix constructed by numerical differentiation of the analytical gradients obtained at the equilibrium geometry. We used localized cluster model approach to build the cluster models. The effects of geometry ŽSi–O and Al–O bond lengths and Si–O–H as well as Si–O–Al angles. of zeolites on the vibrational frequencies of their OH groups were modeled by H 3 SiOHAlH 3 Ž1.1.; whereas to study the influence of chemical composition models with formula H 3 SiOSiŽOH. 3 Ž1.2. and H 2 AlOSiŽOH. 3 Ž1.3. have been studied. The models are shown in Fig. 1 as Ž1., Ž2. and Ž3. respectively. Hydrogens are not labeled for better visibility, all the other atoms unlabeled are hydrogen. The geometries were fully optimized for all the above models. A case study with H–Y zeolite have been performed. H–Y zeolite structure has been modeled from the X–ray crystal data w14x. In case of H–Y zeolite it is observed w15x that there are two kinds of oxygen present alternatively in the 12-membered ring. The pore opening has a C 6v
symmetry. Considering the typical nature cluster m o d e ls a re g e n e ra te d h a v in g fo rm u la ŽHO. 3 SiOHAlŽOH. 3 Ž2.1. and wŽHO. 3 SiOAlŽOH. 3 xy Ž2.2. for both the oxygens namely O 1 and O4 . These models are shown in Fig. 2 for O 1 oxygen as Ž1. s 2.1 and Ž2. s 2.2, the orientation of the two models shown in Fig. 2 are different for better visibility.
3. Results and discussion LDF calculations have been performed on three different clusters first to analyze the structural effect on vibrational frequency of the bridging hydroxyl, followed by the analysis of chemical composition on the same. Both the effects were compared to rationalize the dominant contributor for the vibrational frequency of hydroxyl both bridging and terminal in zeolites. This was followed by a case study on H–Y zeolite cluster models to justify the above proposition of dominance of the structural contribution in the properties of the bridging hydroxyls. In studying the effects of the structural factor on the bridging OH group we used the cluster 1.1 and performed LDF calculations over it. Now starting from the equilibrium geometry we varied the Si–O, Al–O distance and Si–O–H and Si–O–Al angles at a regular interval. The difference between the initial and final bond distance variation was noted as D R and the variation for the corresponding angles as mentioned above is termed as D f . The variation of
558
A. Chatterjee et al.r Applied Surface Science 130-132 (1998) 555–560
˚ and the variabond distances performed were 0.02 A tion in angles were 108. After varying the bond distances and lengths together in the said cluster model, it was optimized fully. The optimized geometries were given in Table 1. The total energy as well as the relative energy are also given in Table 1. It is observed that the total energy values listed in Table 1 have a maximum relative instability of around 1.2 kcalrmol. The interesting phenomena which is to be noted here is that the O–H stretching vibrational frequency Ž mOH . changes by 40 cmy1 . This can be explained by the molecular orbital contribution in agreement with the proposition of Newton w16x. In his analysis, an increase in the valence bond angle on the O atom Žhere, the Si–O–H angle. results in an increase in the percentage s character of the hybrid atomic orbitals forming the bonds Žhere, Si–O and O–H bonds., we observed the same trend in the contribution of s orbitals from density of states ŽDOS. results. The contribution of s orbitals increases for Si and O in the abovementioned bond formation process. As a consequence the O–H distance will decrease resulting to an increase in the vibrational frequency. Now, silica hydrogel consists of incompletely polymerized silicic acid, SiŽOH.4 , and contains a considerable amount of free hydroxyls terminating the polymer network which are called terminal hydroxyls w3x. Now, with the current model the terminal hydroxyls do not show any change with the change in the structural parameters, justifying the fact that the existence of these types of hydroxyl at the defect sites of the silicalite will be studied in future. The vibrational frequency shows the same
order as reported by other calculation methods reported earlier w17x. Comparison of the results of calculations on cluster models 1.2 and 1.3 were performed to study the effect of chemical composition which reveals that the substitution of silica by aluminum in the third coordination sphere has little influence on the O–H stretching vibrational frequency. The observed vibrational frequency of 4159 and 4162 cmy1 for the respective clusters justifies the fact that the structural factor plays a decisive role in the determination of vibrational frequencies of the bridging hydroxyl group in comparison to that due to variation in chemical composition. The terminal hydroxyls are not affected by the structural or compositional variation. This makes us interested to study the structural effect in a real situation, like that of H–Y zeolite. The electronic properties of the H–Y zeolite cluster models 2.1 and 2.2 were calculated using LDF. The calculations were performed on both type of oxygens to compare the shift in vibrational frequency for the respective oxygens. It is observed that the two oxygens have different geometry in terms of bond distances, angles and the reaction enthalpies w18x D H ŽkJrmol. of H-migration in between various O–H groups present in zeolite H–Y are as follows: Si– O 1 –Al s 135.388 and Si–O4 –Al s 136.278; Si–O 1 ˚ and Si–O4 s 1.69 A; ˚ Al–O1 s 1.65 A˚ and s 1.68 A ˚ D H for O1 –H s 7 kJrmol and Al–O4 s 1.62 A; D H for O4 –H s 58 kJrmol. The LDF calculation results are given in Table 2. It shows that the net charge on oxygens are somewhat the same, but the total energy for the individual
Table 1 Geometric parameters Žas a function of bond distance D R and bond angle D f ., total energy, relative energy and vibrational frequency of bridging OH Ž mOH . calculated for 1.1 cluster Geometric parameters
Equilibrium geometry
Deformations introduced into the cluster
˚ D R s q0.02 A D f s q108 ˚. R SiO ŽA ˚. R AlO ŽA f Si–O–H Ž8. f Al–O–Si Ž8. Total energy Žkcalrmol. Relative energy Žkcalrmol. mOH Žcmy1 .
˚ D R s q0.02 A D f s y108
˚ D R s y0.02 A D f s q108
˚ D R s y0.02 A D f s y108
1.687
1.696
1.702
1.678
1.683
1.829 114.2 128.9 y663014.5 0.0 4186
1.844 112.6 135.5 y663013.22 1.28 4218
1.851 111.9 138.3 y663013.81 0.69 4202
1.816 117.3 120.2 y663013.86 0.64 4222
1.827 116.8 119.9 y663014.05 0.45 4214
A. Chatterjee et al.r Applied Surface Science 130-132 (1998) 555–560
559
Table 2 Total energy, deprotonation energy, net charge on oxygen and O–H stretching frequency for both the oxygens O1 and O4 in cluster model 2.1 of H–Y zeolite Type of oxygen in H–Y zeolite
Total energy Žkcalrmol.
Deprotonation energy Žkcalrmol.
Net charge on oxygen
O–H stretching frequency Žcmy1 .
O1 O4
y663059.19 y663055.5
y339.56 y335.87
y0.58 y0.56
3712 3619
oxygens show that O 1 cluster is energetically more stabilized in comparison to that of O4 cluster. The relative acidity for the different oxygen present in H–Y can be estimated from the calculated proton affinity ŽPA. of zeolite models. The cluster which will show a higher PA will act as a poor proton donor and hence will show low Brønsted acidity. In the reverse, clusters having less PA will behave as better proton donor and hence show higher Brønsted acidity. This PA can be approximated by the deprotonation energy w19x. Now, the deprotonation energies for the individual clusters were calculated by using the following relation: ZeolOH™ ZeolOyq Hq The deprotonation energies were calculated from the difference of energy between neutral and anionic cluster models for both the oxygens. The energy values along with OH stretching vibrational frequency are also shown in Table 2. It is observed that the deprotonation energy is less in case of O 1 oxygen. The difference in deprotonation energy is 3.69 kcalrmol, which shows that O 1 oxygen will be a better proton donor than the O4 oxygen. Now the vibrational frequency results show that in case of O 1 oxygen the O–H stretching frequency comes at a higher value of wave number, in comparison to O4 . This trend is in agreement with the experimental observation which shows that the OH frequency at O 1 is at 3650 cmy1 whereas that of O4 falls at around 3550 cmy1 . The reason may be ascribed as follows, in case of O4 oxygen the proton can form a weak hydrogen bond to another oxygen atom of the tetrahedral six-ring in which it is present. This additional H-bond will weaken the OH bond which explains its lower reactivity. These results justify the fact that structural parameter influences the vibrational frequency. The optimistic results enable us to think in direction to formulate a priori rule for
correlating acidity with vibrational frequency, hence future studies are warranted. 4. Conclusion LDF studies have been performed on localized cluster models. The full geometry optimization with smaller models have been performed to get more accurate results. In the first place the comparison of influence on vibrational frequency of OH group both in terms of structural parameters and chemical composition have been performed. The results show the remarkable influence of structural parameters on vibrational frequency. The influence is much pronounced for the bridging hydroxyls in comparison to the terminal one in this model. The role of structural parameters have been further tested for H–Y zeolite with a different model to verify the reactivity of the two different oxygens present in H–Y zeolite. The calculation successfully reproduce the experimental trend. References w1x H. Karge, J. Weitkemp, in: Zeolites as Catalyst, Sorbents and Detergent Builders, Elsevier, Amsterdam, 1989, p. 645. w2x J. Weitkemp, S. Ernst, H. Daums, E. Gallie, Chem. Eng. Tech. 58 Ž1986. 623. w3x W.J. Mortier, J. Sauer, J.A. Lercher, H. Noller, J. Phys. Chem. 88 Ž1984. 905. w4x G. Coudurier, J.C. Vedrine, Pure Appl. Chem. 58 Ž1986. 1389. w5x V.A. Durrant, D.A. Walker, S.N. Gussou, J.E. Lyons, US Patent 4918249 Ž1972.. w6x N.S. Gnep, J.D. Diyemet, M.D. Guisnet, J. Mol. Catal. 45 Ž1988. 281. w7x M.S. Stave, J.B. Nicolas, J. Phys. Chem. 97 Ž1993. 9630. w8x J.C. White, A.C. Hess, J. Phys. Chem. 97 Ž1993. 8703. w9x R. von Balmoos, N.H. Meier, Nature 289 Ž1987. 782. w10x W.J. Mortier, K.V. Genechen, J. Gasteiger, J. Am. Chem. Soc. 107 Ž1985. 829.
560
A. Chatterjee et al.r Applied Surface Science 130-132 (1998) 555–560
w11x W. Kohn, L.J. Sham, Phys. Rev. A 140 Ž1965. 1133. w12x W. Kohn, L.J. Sham, InsightII user guide, version 3.00, Biosym Tech., San Diego, 1995. w13x U. von Barth, L. Hedin, J. Phys. C 5 Ž1972. 1629. w14x D.H. Olson, J. Phys. Chem. 74 Ž1970. 2758. w15x A. Chatterjee, R. Vetrivel, R. Sreekumar, Y.V.S.N. Murthy, C.N. Pillai, J. Mol. Catal. Žin press..
w16x M.D. Newton, in: M. Okeefe, A. Navtrosky ŽEds.., Structure and Bonding in Crystals, Vol. 1, Academic Press, New York, p. 981. w17x J. Sauer, P. Ugliengo, E. Garrone, V.R. Saunders, Chem. Rev. 94 Ž1994. 2095. w18x K.B. Wibeg, J. Am. Chem. Soc. 90 Ž1968. 59. w19x R.A. van Santen, G.J. Kramer, Chem. Rev. 95 Ž1995. 637.