Density Functional Studies of Weak Base Interactions with Hydroxyl

J. Phys. Chem. B 1998, 102, 4507-4515

4507

Density Functional Studies of Weak Base Interactions with Hydroxyl Groups: Models for Adsorption Complexes of Weak Bases in Microporous Materials Patrick J. O’Malley* and Kevin J. Farnworth Department of Chemistry, UMIST, Manchester, M60 1QD, England ReceiVed: January 14, 1998; In Final Form: March 11, 1998

Density functional calculations, utilizing the BLYP and Becke3LYP functionals, have been used to calculate the vibrational frequencies and interaction energies for a variety of intermolecular complexes modeling the interaction of weak bases with acidic hydroxyl groups in microporous materials. The interaction complexes of nitrogen, acetylene, ethene, and benzene have been studied. Comparison is made between calculated vibrational frequencies, vibrational frequency shifts, and interaction energies with experimental determinations. Good agreement is observed between calculated vibrational properties and experimental determinations for nitrogen, acetylene, and ethene interactions. Interaction energies are significantly underestimated in most cases.

Introduction The active sites for many catalytic reactions occurring over microporous materials are Bronsted acid in behavior and correspond to hydroxyl groups generated in the framework usually by ion exchange. The electronic structure of such hydroxyl groups is a key aspect of the investigation of catalytic behavior of zeolites and related microporous materials. Experimentally, the intrinsic acid site strength can be probed by the interaction of small weak base molecules with the hydroxyl groups.1-6 The observed shift in the O-H bond vibrational stretching frequency, after interaction of the probe molecule with the free or bridged hydroxyl group, can be reliably used as a measure of the acid site strength.6 Many experimental studies have been published concerning the use of carbon monoxide,4 nitrogen,5,8 acetylene,9,10 ethene,1,11 and benzene.12,13 Theoretical attempts to predict the experimental frequency shifts using Hartree-Fock theory have been very limited in their success.14,15 More recent studies at the MP216 and density functional level17 have shown improvement over the HartreeFock level of theory when compared to the experimental data. The hydroxyl groups in microporous materials were investigated previously by us with density functional theory.17 It was found that both the free and bridged hydroxyl groups present in zeolites could be described extremely well by the BLYP and Becke3LYP density functionals. Bathochromic frequency shifts calculated for complexes with carbon monoxide were also in good agreement with experimental findings. The current report develops and extends on this study. We examine the interaction of nitrogen, acetylene, ethene, and benzene with hydroxyl models to assuage the generality of our previous findings for carbon monoxide interaction.17 Previous theoretical reports15,17,18 have highlighted the need to account for the anharmonicity of the O-H bond, and similar to our previous report,17 anharmonicity is allowed for and shown to be of significance in this study. Nitrogen and acetylene interactions with zeolite active sites have previously been investigated both experimentally5,8 and theoretically.17 The interaction of ethene and benzene with zeolite acid sites is a very important part of the cracking process.

Theoretical investigation of the interaction of ethene has also been made,19 but this study is the first to concentrate on the vibrational frequency shifts. Methods All calculations were performed, using GAUSSIAN,20 CERIUS,21 and SPARTAN,22 were used to facilitate data and graphical analysis. The molecules and molecular complexes studied are shown in Figures 1-5. As for the carbon monoxide study of ref 17, intermolecular complexes with free hydroxyl group models(silanol) and bridged hydroxyl groups are studied. Density functional calculations were all self-consistent KohnSham calculations using the BLYP and Becke3LYP density functionals. BLYP combines the exchange functional of Becke23 with the correlation functional proposed by Lee, Yang, and Parr.24 Becke3LYP differs in its exchange functional, where the Becke325 hybrid functional is employed. These particular functionals have been shown to give geometries and energies comparable to MP2 theory for a wide range of molecules.26 They have also been shown to predict vibrational frequencies of comparable accuracy to experimental data.27-29 The basis set used was 6-31G**.30 Geometry optimizations were performed using the Berny gradient optimization method.31 Normal mode vibrational frequencies were calculated from the analytical harmonic force constants. Zero-point energy corrections were made to the interaction energies and basis set superposition errors (BSSE) were accounted for by employing the Boys-Bernardi counterpoise full correction method.32 Anharmonic corrections were performed as described in ref 17. Results and Discussion 1. Interaction Of Nitrogen. Nitrogen is a very good probe molecule for measuring the intrinsic acidity of hydroxyl groups in zeolites. Its small size eliminates contributions for peripheral framework atoms and its weak base nature makes it a very suitable probe for investigating strong acid sites. 1.1. Free Nitrogen. The results of BLYP and Becke3LYP calculations on nitrogen are given in Figure 1 and Table 1. The

S1089-5647(98)00792-5 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/19/1998

4508 J. Phys. Chem. B, Vol. 102, No. 23, 1998

Figure 1. Geometric data. for nitrogen and the silanol:nitrogen complexes calculated with BLYP/6-31G** and in brackets Becke3LYP/ 6-31G**. All values given in angstroms.

Figure 2. Geometric data for the bridged hydroxyl group complexes with nitrogen, BLYP/6-31G** and in brackets Becke3LYP/6-31G**. All values given in angstroms.

BLYP bond length of 1.118 Å is slightly longer than the experimental26 value of 1.098 Å, whereas Becke3LYP predicts an excellent bond length of 1.106 Å. Both BLYP and Becke3LYP N-N vibrational stretching frequencies compare well with the experimental26 harmonic value of 2360 cm-1.

O’Malley and Farnworth

Figure 3. Geometric data for acetylene and the complexes of acetylene with the silanol and bridged hydroxyl groups. BLYP/6-31G** with Becke3LYP/6-31G** in brackets. All values given in angstroms.

1.2. Interaction Of Nitrogen With The Free Hydroxyl Group. Three different configurations were investigated for the interaction of nitrogen with the free hydroxyl group. The complexes are shown in Figure 1 along with the relevant geometric data predicted by the BLYP and Becke3LYP density functionals. It was found that configuration IIA did not represent a stationary point and on optimization converts to structure IIC. It was also found that configuration model IIB is lower in total energy than model IIC, (i.e., it corresponds to a more stable minimum). By comparison with the silanol monomer,17 it can be seen that the interaction of nitrogen only causes very small geometric changes. These changes are much less pronounced than those observed with CO interaction.17 The Becke3LYP predicted distance of the nitrogen monomer to the acidic hydrogen of the silanol monomer is 2.303 Å compared to 2.238 Å for the CO interaction.17 These factors imply a much weaker interaction between the free hydroxyl group and the nitrogen than was found with the CO probe molecule. Further evidence of the low level of interaction for the nitrogen system is given by the vibrational frequency shifts shown in Table 1. At the harmonic level of theory BLYP predicts values of ∆ωeOH ) -7 cm-1 and ∆ωeNN ) +8 cm-1 for model IIB. Similar values of ∆ωeOH ) -10 cm-1 and ∆ωeNN ) +7 cm-1 were obtained with Becke3LYP. The predicted shifts can be compared to the anharmonic experimental O-H bond vibrational shift of -37 cm-1 obtained by Geobaldo et al.44 for external hydroxyls in hydrogen exchanged mordenite. The small shifts are indicative of a weakly interacting system and this corroborates the earlier geometric evidence. The O-H vibrational frequency shifts predicted by the density functionals

Hydroxyl Groups in Zeolites

Figure 4. Geometric data for ethene and its complexes with silanol and the bridged hydroxyl group. BLYP/6-31G** with Becke3LYP/ 6-31G** values in brackets. All values given in angstroms.

approach the experimental value but are still quite low in comparison perhaps reflecting the inability of present functionals to describe purely dispersive forces. In the case of model IIC, the BLYP density functional predicts an anomalous ∆ωeOH value of 5 cm-1 along with a value of ∆ωeCO ) +5 cm-1. The Becke3LYP functional predicts more realistic values of ∆ωeOH ) -12 cm-1 and ∆ωeNN ) +7 cm-1. Harmonically corrected O-H vibrational frequency shifts are given in Table 2. In all cases the shifts are slightly improved. The BLYP predicted shift for model IIC is still anomalous, with a value of -0.42 cm-1. There is no obvious reason this has occurred. Even with the slight improvements obtained with the inclusion of anharmonicity the O-H vibrational shifts are still found to be low. With the current results in mind it is advisable that care should be taken when analyzing results obtained with the current method if the vibrational shifts under investigation are very low. Calculated interaction energies are given in Table 3 along with zero-point energies and BSSE values. The BLYP and Becke3LYP interaction energies are calculated to be 0.40 and 0.51 kcal mol,-1 respectively. For model IIC, BLYP predicts the system to be unbound, while Becke3LYP predicts an interaction energy of 0.29 kcal mol-1. On the basis of the total energy of the interacted systems and the adsorption energies model IIB is shown to be the most favorable configuration. If we compare the Becke3LYP predicted adsorption energy of 0.51 kcal mol-1 for model IIB with the value of 1.07 kcal mol-1 for the corresponding CO interaction,17 the weakly bound nature of the nitrogen complex is clearly illustrated.

J. Phys. Chem. B, Vol. 102, No. 23, 1998 4509

Figure 5. Geometric data for benzene and its complexes with silanol and the bridged hydroxyl group. BLYP/6-31G** with Becke3LYP/ 6-31G** values in brackets. All values given in angstroms.

TABLE 1: Calculated Harmonic (ωeOH, ωeNN) Vibrational Stretching Frequencies (cm-1) for the Free Hydroxyl Group, the Bridged Hydroxyl group, and Nitrogena model H3SiOH-N2 model IIB BLYP Becke3LYP model IIC BLYP Becke3LYP exptl H3Al(OH)SiH3-N2 BLYP Becke3LYP exptl

ωeOH

3729 3890

3919 3727 3868 3770-3820

ωeNN ∆ωeOH ∆ωeNN

ref

2340 2460

-7 -10

+8 +7

this work this work

+5 +7

2360

+5 -12 -24

this work this work 26, 36, 45

-92 -83 -106

+12 +11 +5

this work this work 3-5, 36, 5

a The calculated harmonic (∆ω -1 eOH, ∆ωeNN) vibrational shifts (cm ) for the complexes of nitrogen with the free hydroxyl and bridged hydroxyl groups are also included. The experimental vibrational shifts presented for comparison are anharmonic, as no experimental harmonic shifts are currently available.

1.3. Interaction of Nitrogen with the Bridged Hydroxyl Group. The two configurations used to represent nitrogen interaction with the bridged hydroxyl group are given in Figure 2. Model A represents interaction of the nitrogen π clouds with the acidic hydroxyl group. This was not found to correspond to a stable minimum and on optimization converts to model B. Model B corresponds to interaction with the bridged hydroxyl group via the lone pair on one of the nitrogen atoms. The



TABLE 2: Calculated Anharmonic (ωOH) Vibrational Stretching Frequencies (cm-1) for the Free Hydroxyl and Bridged Hydroxyl Groupsa ωOH

model H3SiOH-N2 model IIB BLYP Becke3LYP model IIC BLYP Becke3LYP exptl H3Al(OH)SiH3-N2 BLYP Becke3LYP exptl

∆ωOH

ref

3571 3730

-12 -11

this work this work

3730-3748

-0.42 -13 -24

this work this work 14, 45

3562 3708 3610-3660

-114 -119 -106

this work this work 3-5 -1

a

Calculated anharmonic (∆ωOH) vibrational shifts (cm ) for the complexes of nitrogen with the free hydroxyl and bridged hydroxyl groups are also included.

TABLE 3: Calculated Interaction Energies (kcal mol-1) for Complexes of Nitrogen with the Free Hydroxyl and Bridged Hydroxyl Groups

model H3SiOH-N2 model IIB BLYP Becke3LYP model IIC BLYP Becke3LYP H3Al(OH)SiH3-N2 BLYP Becke3LYP exptla a

TABLE 4: Calculated Harmonic (ωeOH, ωeCC) Vibrational Stretching Frequencies (cm-1) for the Free Hydroxyl Group, the Bridged Hydroxyl Group, and Acetylenea

corrected adsorption energy

De

zero-point energy

BSSE

1.77 1.77

0.54 0.58

0.83 0.68

0.40 0.51

1.33 1.60

0.67 0.70

0.98 0.61

-0.32 0.29

3.20 3.15

0.78 0.86

1.04 0.84

1.38 1.45 2.81

Reference 5.

geometric data predicted by BLYP and Becke3LYP for this endon complex is given in Figure 2. The geometry of the bridged hydroxyl group is shown to have changed, with an increase in O-H bond length from 0.9652 to 0.9694 Å when using Becke3LYP and a corresponding decrease of 0.002 Å in the N-N bond length. The perturbation of the bridged hydroxyl group is noticeable, but not as pronounced as was the case for the CO interaction.17 This is consistent with the results of the free hydroxyl study. Nitrogen is shown to be a much weaker base than CO. This may well be advantageous for studies which aim to concentrate attention on the more acidic sites present in zeolites. Vibrational shifts at the harmonic level of theory are given in Table 1. BLYP predicts shifts of ∆ωeOH ) -92 cm-1 and ∆ωeNN ) +12 cm-1. These values can be compared with the experimental values of -106 and +5 cm-1 obtained by Wakabayashi et al.,5 for nitrogen sorption on H-Mordenite. It is clearly evident that the O-H frequency shift can be well reproduced with the density functional methods. The N-N vibrational frequency shift is rather too large, which once again highlights the difficulties that can be encountered when calculating very small vibrational shifts. The Becke3LYP results are slightly lower than at the BLYP level with values of ∆ωeOH ) -83 cm-1 and ∆ωeNN ) +11 cm-1. The only other theoretical work done on this model was by Neyman et al.18 They used a density functional method, which did not include nonlocal gradient corrections until after the optimization had completed. They obtained a value of ∆ωeOH ) -107 cm-1, which is exceptionally close to the experimental value. However, the

model H3SiOH-C2H2 BLYP Becke3LYP HF/DZP MP2/DZP exptl H3Al(OH)SiH3-C2H2 BLYP Becke3LYP HF/DZP MP2/DZP exptl

ωeOH 3729 3890 4231 3959 3919 3727 3868 3770-3820

ωeCC ∆ωeOH ∆ωeCC 2019 -111 -8 2086 -101 -9 -39 -4 -73 -4 2011 -120 -12

ref this work this work 16 16 26, 36, 16

-285 -14 this work -286 -15 this work -127 -9 16 -234 -9 16 -350 -24 3-5, 36, 10

Calculated harmonic (∆ωeOH, ∆ωeCC) vibrational shifts (cm-1) for the complexes of acetylene with the free hydroxyl and bridged hydroxyl groups are also included. The experimental vibrational shifts presented for comparison are anharmonic, as no experimental harmonic shifts are currently available. a

models involved were heavily constrained and the harmonic vibrational frequency of the bridged hydroxyl monomer was exceptionally low at 3625 cm-1 compared to the experimental range of 3770-3820 cm-1. Anharmonically corrected O-H stretching frequencies for the bridged hydroxyl group are given in Table 2. The BLYP and Becke3LYP values of -114 and -119 cm-1 are in good agreement with the experimental value of -106 cm-1. The calculated interaction energies are given in Table 3. The BLYP and Becke3LYP adsorption energies are 1.38 and 1.45 kcal mol-1, respectively. These calculated values are again lower than the experimental value of 2.81 kcal mol-1 obtained by Wakabayashi et al.5 2. Interaction of Acetylene. The polymerization of acetylene in zeolites has been reported in the literature.9,10 This is an important process, as polyacetylene exhibits a high level of electrical conductivity when it has been doped with oxidizing or reducing agents.45 It has been shown that H-ZSM5 can protonate acetylene and initiate a carbocationic polymer chain. The reaction has been followed using temperature-programmed desorption, thermogravimetric analysis, 13C NMR, IR, and UVvis spectroscopies.9,10,46 The first stage in this important process is the interaction of acetylene with the bridged hydroxyl group. An adsorption complex in which the acetylene molecule interacts via its π electrons with the hydrogen of the bridged hydroxyl group has been proposed on the basis of IR and 1H NMR studies.9,10,46 Theoretical methods could be used to help confirm this proposed complex and its configuration. If the hydrogenbonded complex can be well described theoretically, further investigation of the acetylene polymerization process could be undertaken. Experimental work has also been published10 concerning the interaction of acetylene with the free hydroxyl groups present in amorphous silica. The experimental work proposed a T-shaped conformer, similar to the configuration proposed for the bridged hydroxyl group. The current study aims to use BLYP and Becke3LYP to confirm the structure of the complexes proposed above on the basis of experimental data. It also aims to reproduce the observed spectroscopic data. Positive results in these areas will show that the density functional methods could be applied to the polymerization reaction of acetylene over zeolite catalysts. 2.1. Free Acetylene. The results of BLYP and Becke3LYP calculations on acetylene are given in Figure 3 and Table 4.

Hydroxyl Groups in Zeolites

J. Phys. Chem. B, Vol. 102, No. 23, 1998 4511

TABLE 5: Calculated Anharmonic (ωO-H) Vibrational Stretching Frequencies (cm-1) for the Free Hydroxyl and Bridged Hydroxyl Groupsa model H3SiOH-C2H2 BLYP Becke3LYP exptl H3Al(OH)SiH3-C2H2 BLYP Becke3LYP exptl

ωOH

∆ωOH

TABLE 6: Calculated Interaction Energies (kcal mol-1) for Complexes of Acetylene with the Free Hydroxyl and Bridged Hydroxyl Groups

ref

3571 3730 3730-3748

-139 -123 -120


3562 3708 3610-3660

-338 -341 -350

this work this work 3-5, 10

a Calculated anharmonic (∆ωOH) vibrational shifts (cm-1) for the complexes of acetylene with the free hydroxyl and bridged hydroxyl groups are also included.

The experimental geometric data is well reproduced by the density functionals, especially Becke3LYP, which calculates a C-C bond length of 1.205 and a C-H bond length of 1.066 Å. This is in excellent agreement with the experimental values26 of 1.203 and 1.061 Å. The BLYP stretching frequency for the C-C bond of acetylene was calculated to be 2019 cm-1, which is almost identical to the experimental harmonic value of 2011 cm-1. Becke3LYP calculates a slightly higher value of ωeCC ) 2086 cm-1. 2.2. Interaction of Acetylene with the Free Hydroxyl Group. The only configuration of acetylene with the free hydroxyl group found to correspond to a stable minimum is given in Figure 3. The geometric data for the T-shaped conformer is included at both the BLYP and Becke3LYP levels of theory. The T-shaped conformer proposed on the basis of experimental work with amorphous silica is backed up theoretically by the density functional study. This is a positive result, and it emphasizes how theoretical and experimental studies can be used together. Vibrational shifts at the harmonic level of theory are given in Table 4. BLYP predicts values of ∆ωeOH ) -111 cm-1 and ∆ωeCC ) -8 cm-1. The Becke3LYP results are ∆ωeOH ) -101 cm-1 and ∆ωeCC ) -9 cm-1. Both sets of results are in excellent agreement with the experimental values of -120 and -12 cm-1, which were obtained from the study of amorphous silica.16 Comparison can also be made with the results of Ugliengo et al.16,19 who used Hartree-Fock and MP2 theory with a DZP basis set. The Hartree-Fock results of ∆ωeOH ) -39 cm-1 and ∆ωeCC ) -4 cm-1 greatly underestimate the experimental values. Even at the correlated MP2 level of theory the results are only ∆ωeOH ) -73 cm-1 and ∆ωeCC ) -4 cm-1. These results, while improving on the Hartree-Fock level, are inferior to the results of the current density functional study. Anharmonic O-H vibrational frequency shifts are given in Table 5. The BLYP functional very slightly overestimates the anharmonic O-H shift with a value of ∆ωeOH ) -139 cm-1. However, the Becke3LYP functional predicts an ∆ωeOH value of -123 cm-1, which is almost exactly equal to the experimental value of -120 cm-1. Calculated interaction energies are given in Table 6. Unfortunately, there is no experimental value available from literature for this type of interaction. Both BLYP and Becke3LYP predict very similar interaction energies of 1.52 and 1.58 kcal mol-1, respectively. This implies a stronger interaction than is found with either the nitrogen or carbon monoxide probe molecules. Such a conclusion is consistent with the higher vibrational shift observed for acetylene, both theoretically in the present study and experimentally as well. 2.3. Interaction of Acetylene with the Bridged Hydroxyl Group. The T-shaped interaction geometry proposed on the

model H3SiOH-C2H2 BLYP Becke3LYP H3Al(OH)SiH3-C2H2 BLYP Becke3LYP

De

zero-point energy

BSSE

corrected adsorption energy

3.62 3.68

0.74 0.75

1.36 1.35

1.52 1.58

6.49 6.63

1.48 1.29

0.79 0.85

4.22 4.49

basis of IR and 1H NMR spectroscopies9,10,46 is backed up by the current density functional study. The only stable minimum on the potential energy surface of the complex is found to correspond to the T-shaped geometry shown in Figure 3. By comparison with the free H3Al(OH)SiH3 monomer geometry,17 acetylene is shown to interact with the bridged hydroxyl group very strongly. The strong interaction causes a large perturbation in the structure of the bridged hydroxyl group. The O-H bond is significantly longer for the acetylene complex, while the Si-O and Al-O bonds are shorter in length. With the Becke3LYP functional the O-H bond increases from 0.9652 to 0.9788 Å, while the Si-O and Al-O bond lengths of 1.721 and 2.017 Å, respectively, decrease to values of 1.710 and 2.00 Å. The vibrational shifts at the harmonic level are given in Table 4. BLYP predicts shifts of ∆ωeOH ) -285 cm-1 and ∆ωeCC ) -14 cm-1, while the Becke3LYP values are -286 and -15 cm-1, respectively. The results are a good approximation to the experimental values of ∆ωeOH ) -350 cm-1 and ∆ωeCC ) -24 cm-1 obtained by Bordiga et al.10 for H-ZSM5. Comparison of the results can be made with the Hartree-Fock and MP2 calculations of Ugliengo et al.16,19 Using a DZP basis set, the Hartree-Fock results were ∆ωeOH ) -127 cm-1 and ∆ωeCC ) -9 cm-1, while the MP2 results were ∆ωeOH ) -234 cm-1 and ∆ωeCC ) -9 cm-1. These results reflect similar trends that were seen for the interaction of acetylene with the free hydroxyl group and also the previous interacted complexes with nitrogen and carbon monoxide probe molecules. The HartreeFock vibrational shifts are greatly underestimated. MP2 theory predicts improved O-H vibrational shifts, but the results are not as accurate as those of the density functional methods obtained in the current study. It can be seen that although the density functional methods improve on the Hartree-Fock and MP2 levels of theory the vibrational shifts are still lower than the corresponding experimental values. As previously shown this is most likely to be a result of anharmonicity.17 Once again, accounting for anharmonic behavior is shown to be very important. The anharmonic O-H frequency shifts, predicted by BLYP and Becke3LYP, are -338 and -341 cm-1, respectively. These results are very close to the experimental value of ∆ωOH ) -350 cm-1. Calculated interaction energies are given in Table 6. Again, no experimental data is available for comparison. The very high BLYP and Becke3LYP adsorption energies of 4.22 and 4.49 kcal mol-1 are indicative of the strong interaction between acetylene and the bridged hydroxyl group. This work is extremely encouraging as it illustrates the ability of the density functionals to describe the bridged hydroxyl/ acetylene complex. This leaves the way open for the investigation of the polymerization reaction of acetylene using either BLYP or Becke3LYP density functionals. 3. Interaction of Ethene. The oligomerization reaction of ethene on H-ZSM5 has been studied by fast FTIR spectros-



TABLE 7: Calculated Harmonic (ωeOH, ωeCC) Vibrational Stretching Frequencies (cm-1) for the Free Hydroxyl Group, the Bridged Hydroxyl Group, and Ethenea model H3SiOH-C2H4 BLYP Becke3LYP exptl H3Al(OH)SiH3-C2H4 BLYP Becke3LYP exptl

ωeOH 3729 3890 3919 3727 3868 3770-3820

ωeCC

∆ωeOH ∆ωeCC

ref

1661 -124 1715 -117 1655 -430

-11 -11

this work this work 36, 26, 1

-328 -307 -389

-19 -18 -11

this work this work 3-5, 11, 36

a Calculated harmonic (∆ω -1 eOH, ∆ωeCC) vibrational shifts (cm ) for the complexes of ethene with the free hydroxyl and bridged hydroxyl groups are also included. The experimental vibrational shifts presented for comparison are anharmonic, as no experimental harmonic shifts are currently available.

copy.11 The work showed that a protonation and resultant polymerization process occurred, which was similar to that previously discussed for acetylene above. It was discovered that the reaction proceeded via a hydrogen-bonded complex of ethene with the bridged hydroxyl group. It was proposed that the plane of symmetry of the ethene π bond was perpendicular to the axis of the bridged hydroxyl. This structure was also proposed by White et al.46 on the basis of a 1H NMR study. The current density functional study aims to confirm the ethene π complex configuration. It also aims to give a good theoretical description of the spectroscopic properties of the complex. If the density functionals can adequately describe the interacted species, it enables further work to be embarked upon. Theoretical investigation of the carbenium ion species would be possible and also a theoretical study of the ethene oligomerization reaction in zeolites. Experimental studies on the interaction of ethene with free hydroxyl can be found in and Busca et al.1 A bathochromic shift of -115 cm-1 was reported. 3.1. Free Ethene. The results of BLYP and Becke3LYP calculations on ethene are given in Figure 4 and Table 7. The C-C and C-H bond lengths can be compared to the experimentally determined values26 of 1.339 and 1.085 Å. BLYP is shown to calculate a very good C-C bond length, but to slightly overestimate the C-H bond length. Becke3LYP, on the other hand, predicts an excellent C-H bond length of 1.087 Å, but slightly underestimates the C-C bond length. Both BLYP and Becke3LYP predicted C-C vibrational stretching frequencies compare well with the experimental harmonic value of 1655 cm-1. The BLYP predicted value of 1661 cm-1 can be seen to be extremely close to the experimental frequencies.26 3.2. Interaction of Ethene with the Free Hydroxyl Group. The model used to represent the interaction of ethene with the free hydroxyl group is given in Figure 4. The observed changes in the silanol structure are almost identical to those seen for the interaction of the acetylene probe molecule. The O-H bond length in the silanol:ethene complex is calculated with Becke3LYP to be 0.9686 Å, while the Si-O bond length is 1.658 Å. For the silanol:acetylene complex, the bond lengths were 0.9682 and 1.658 Å, respectively. The isolated silanol monomer has bond lengths of 0.9631 and 1.665 Å.17 Both sets of density functional results show an elongation of the O-H bond length and a reduction in the Si-O bond length. The similarity of the geometric data for the ethene and acetylene complexes is an indication that both probe molecules exhibit a similar level of interaction. This would lead us to expect similar interaction energies and a similar value for the

TABLE 8: Calculated Anharmonic (ωOH) Vibrational Stretching Frequencies (cm-1) for the Free Hydroxyl and Bridged Hydroxyl Groupsa model H3SiOH-C2H4 BLYP Becke3LYP exptl H3Al(OH)SiH3-C2H4 BLYP Becke3LYP exptl

ωOH

∆ωOH

ref

3571 3730 3730-3748

-154 -142 -430


3562 3708 3610-3660

-394 -389 -389


a Calculated anharmonic (∆ωOH) vibrational shifts (cm-1) for the complexes of ethene with the free hydroxyl and bridged hydroxyl groups are also included.

TABLE 9: Calculated Interaction Energies (kcal mol-1) for Complexes of Ethene with the Free Hydroxyl and Bridged Hydroxyl Groups

model H3SiOH-C2H4 BLYP Becke3LYP H3Al(OH)SiH3-C2H4 BLYP Becke3LYP exptl

De

corrected adsorption zero-point energy BSSE energy

ref

3.69 3.81

1.01 1.04

1.27 1.15

1.41 1.62

this work this work

6.40 6.61 3.90

1.54 1.44 11

1.70 1.48

3.16 3.69

this work this work

O-H vibrational frequency shift. The value of the harmonic O-H vibrational shift for the acetylene complex with the Becke3LYP functional was -101 cm-1. The actual vibrational shifts calculated within the harmonic approximation are given in Table 7. BLYP predicts values of ∆ωeOH ) -124 cm-1 and ∆ωeCC ) -11 cm-1. The Becke3LYP results are ∆ωeOH ) -117 cm-1 and ∆ωeCC ) -11 cm-1. The calculated values for the O-H vibrational shift are close to the values obtained for acetylene interaction. This is what was expected from the points discussed earlier. Harmonic shifts calculated with Hartree-Fock and MP2 methods using double-ζ basis sets19 predicted much smaller shifts. The density functional values reported here are in good agreement however with the experimental determination of Busca et al.1 who reported an experimental value of -115 cm-1. Anharmonically corrected O-H vibrational frequency shifts are given in Table 8. The BLYP functional predicts an O-H vibrational shift of -154 cm-1, while the value with Becke3LYP is slightly lower at -142 cm-1. This suggests that the interaction of ethene with the free hydroxyl group is quite a strong interaction. It is slightly greater in strength than the interaction of acetylene with the free hydroxyl group. This point is backed up when the calculated interaction energies given in Table 9 are considered. BLYP predicts an interaction energy of 1.41 kcal mol-1, while Becke3LYP predicts a slightly higher value of 1.62 kcal mol-1. These results are very similar to the values found for the interaction of acetylene with the free hydroxyl group. 3.3. Interaction of Ethene with the Bridged Hydroxyl Group. As previously discussed, experimental IR and 1H NMR studies have suggested that the plane of symmetry of the ethene π bond lies perpendicular to the axis of the bridged hydroxyl. This configuration, shown in Figure 4, was found to correspond to a stable minimum. The geometry of the bridged hydroxyl site changes in a similar way to that found for the acetylene complex. With the Becke3LYP functional the O-H bond increases from 0.9652 to 0.9798 Å. The Si-O and Al-O bond lengths both

Hydroxyl Groups in Zeolites decrease from 1.721 and 2.017 Å to 1.710 and 1.995 Å, respectively. The calculated harmonic vibrational shifts for the O-H bond and the C-C double bond in ethene are given in Table 7. BLYP predicts vibrational shifts of ∆ωeOH ) -328 cm-1 and ∆ωeCC ) -19 cm-1, while the Becke3LYP values are -307 and -18 cm-1. The results compare well with the experimental values of ∆ωeOH ) -389 cm-1 and ∆ωeCC ) -11 cm-1 obtained by Spoto et al.11 for H-ZSM5. As for the acetylene complex, the O-H vibrational shift is underestimated and the vibrational shift for the C-C double bond of ethene is overestimated. As with the previous interacted complexes the errors are most likely due to a combination of anharmonicity, simplicity of the bridged hydroxyl model and limitations of the theoretical method. The problem of anharmonicity is the most serious area for error. Anharmonically corrected stretching frequencies for the bridged hydroxyl group are given in Table 8. The shifts for the anharmonic frequency of the O-H bond are -394 and -389 cm-1 for the BLYP and Becke3LYP density functionals, respectively. These values can be compared to the experimental value of -389 cm-1 obtained by Spoto et al.11 for zeolite H-ZSM5. The density functional methods are shown to be extremely accurate in their prediction of the O-H vibrational shift. The Becke3LYP predicted value of -389 cm-1 is exactly equal to the experimental value. This shows that the density functional methods can describe the spectroscopic nature of the ethene complex extremely well. Hartree-Fock and MP2 studies predict much lower shifts.19 The calculated interaction energies are given in Table 9. BLYP and Becke3LYP predict values of 3.16 and 3.69 kcal mol-1. 4. Interaction of Benzene. One of the most widely used probe molecules for the investigation of zeolite acid site strength is benzene. It is a very useful probe molecule as it’s size precludes it from acid sites which are only accessible to small molecules. For the cracking process it is very important to investigate the acid sites present in easily accessible parts of the framework. It is essential that large bulky hydrocarbons can gain access to the acid sites present in the framework, in order for the zeolite to be an efficient catalyst for the cracking process. Benzene is the ideal probe for these acid sites. On the basis of FTIR and 1H NMR studies,12,13 the benzene ring is believed to interact with the free and bridged hydroxyl groups via the π electrons of the benzene ring. The ring lies perpendicular to the hydroxyl group, as shown in Figure 5. Investigation of benzene interaction with the free and bridged hydroxyls is very difficult theoretically. Experimentally, it has been shown that the benzene:hydroxyl complex is stabilized by dispersion interactions between benzene and the zeolite framework.47 To account for the extra lattice framework theoretically, we would not be able to use density functional methods as the calculations would be computationally unfeasible. Therefore, we would not expect the results of the current density functional calculations to follow the experimental data as accurately as they have for the previous probe molecules. However, because benzene is such an important probe molecule, it is worthwhile finding out if it is possible to model these interactions to any appreciable accuracy. It will also give us an insight into the effect the zeolite framework actually exerts on the benzene interaction. 4.1. Free Benzene. The results of BLYP and Becke3LYP calculations on benzene are given in Figure 5 and Table 10. The C-C and C-H bond lengths were found experimentally48 to be 1.397 and 1.084 Å, respectively. The BLYP predicted

J. Phys. Chem. B, Vol. 102, No. 23, 1998 4513 TABLE 10: Calculated Harmonic (ωeOH, ωeCC) Vibrational Stretching Frequencies (cm-1) for the Free Hydroxyl Group, the Bridged Hydroxyl Group, and Benzenea ωeOH

model H3SiOH-C6H6 BLYP Becke3LYP exptl H3Al(OH)SiH3-C6H6 BLYP Becke3LYP exptl

ωeCC

3729 3890 3919

1480 1590

3727 3868 3770-3820

∆ωeOH ∆ωeCC -68 -66

ref

-1 this work -2 this work -110 12, 36 -246 -2 -214 -3 -318 3-5, 36, 50

a Calculated harmonic (∆ω -1 eOH, ∆ωeCC) vibrational shifts (cm ) for the complexes of benzene with the free hydroxyl and bridged hydroxyl groups are also included. The experimental vibrational shifts presented for comparison are anharmonic, as no experimental harmonic shifts are currently available.

TABLE 11: Calculated Anharmonic (ωOH) Vibrational Stretching Frequencies (cm-1) for the Free Hydroxyl and Bridged Hydroxyl Groupsa model H3SiOH-C6H6 BLYP Becke3LYP exptl H3Al(OH)SiH3-C6H6 BLYP Becke3LYP exptl

ωOH

∆ωOH

ref

3571 3730 3730-3748

-73 -73 -110


3562 3708 3610-3660

-296 -257 -318


a Calculated anharmonic (∆ω ) vibrational shifts (cm-1) for the OH complexes of benzene with the free hydroxyl and bridged hydroxyl groups are also included.

bond lengths of 1.406 and 1.093 Å are slightly too long, whereas the Becke3LYP values of 1.396 and 1.086 Å are exceptionally close to the experimental value. 4.2. Interaction of Benzene with the Free Hydroxyl Group. The model used to represent the interaction of benzene with the free hydroxyl group is given in Figure 5. This corresponds to the structure proposed on the basis of FTIR and 1H NMR, whereby the benzene ring sits on top of the hydroxyl group.12,13 As with the other probe molecules we note an increase in the O-H bond length along with a corresponding decrease in the Si-O bond length. The calculated harmonic vibrational shifts are given in Table 10, along with the experimental anharmonic O-H vibrational shift for comparison purposes.12 No theoretical work of this nature could be found in the literature. Experimentally it was found that the O-H vibrational frequency shifted by -110 cm-1, while the C-C vibrational stretch of benzene is only slightly shifted on interaction with the free hydroxyl groups. The small shift in the C-C vibrational frequency is highlighted in the density functional calculations. BLYP predicts practically no change with a shift of ωeCC ) -1 cm-1, while Becke3LYP predicts an equally low shift of -2 cm-1. The calculated harmonic vibrational shifts for the O-H bond were -68 and -66 cm-1 for BLYP and Becke3LYP, respectively. These are lower than the experimental value of 110 cm-1 by quite a large degree. Anharmonically corrected O-H vibrational frequency shifts are given in Table 11. Both the BLYP and Becke3LYP density functionals predict a value of -73 cm-1. Unlike the interactions previously discussed, this is still quite a lot lower than the experimental value of 110 cm-1. This is a strong indication that the zeolite or silica framework, which surrounds the free hydroxyl group, has a large effect on the interaction of benzene



TABLE 12: Calculated Interaction Energies (kcal mol-1) for Complexes of Benzene with the Free Hydroxyl and Bridged Hydroxyl Groups

model H3SiOH-C6H6 BLYP Becke3LYP exptla H3Al(OH)SiH3-C6H6 BLYP Becke3LYP exptlb a

De

zero-point energy

BSSE

2.31 2.63

0.47 0.37

1.01 0.84

5.78 6.25

0.96 0.90

1.80 1.69

corrected adsorption energy 0.83 1.42 10.4 3.02 3.66 14.0

Reference 12. b Reference 50.

with the acid site. Computationally, the inclusion of the extraframework is expensive for any adequate level of theory. Hence, calculations based on the interaction of benzene with the free hydroxyl will be limited in their representation of the experimental data. The calculated interaction energies are given in Table 12. The BLYP value of 0.83 kcal mol-1 is very low compared to the experimental value of 10.40 kcal mol-1.12 The Becke3LYP adsorption energy of 1.42 kcal mol-1 is an improvement on the BLYP level of theory, but it is still low compared to the experimental value. The experimental interaction energy of 10.4 kcal mol-1 is very high when compared to the equivalent interaction energy of 2.62 kcal mol-1 for the CO molecule.17 As shown for the other bases above, the predicted interaction energy is significantly below the experimental value. 4.2. Interaction of Benzene with the Bridged Hydroxyl Group. As previously mentioned, IR and 1H NMR studies have suggested that the center of the benzene ring is situated directly above the bridged hydroxyl group and perpendicular to it. This configuration is shown in Figure 5 and was found to correspond to a stable minimum. As in the case of acetylene and ethene, the O-H bond is found to increase, along with a corresponding decrease in the Si-O and Al-O bond lengths. The calculated harmonic vibrational shifts for the O-H bond and C-C bond in benzene are given in Table 10. As in the case of the free hydroxyl group, the C-C vibrational stretching frequency is practically unchanged on interaction with the bridged hydroxyl group. However, the O-H vibrational stretching frequency is altered quite significantly. BLYP predicts a shift of ∆ωeOH ) -246 cm-1 and Becke3LYP predicts a shift of ∆ωeOH ) -214 cm-1. These values are lower than the value of ∆ωeOH ) -318 cm-1 obtained by Su et al.49 for zeolite HY. This again emphasizes the effect of the surrounding zeolite lattice on the interaction of the benzene molecule. Anharmonically corrected stretching frequencies for the bridged hydroxyl group are given in Table 11. The shifts in the anharmonic frequency of the O-H bond are -296 and -257 cm-1 for the BLYP and Becke3LYP density functionals, respectively. Although there is obvious improvement over the harmonic frequency shifts, the values are still lower than the experimental value of 318 cm-1. This is strong evidence that the zeolite lattice does have an effect on the interaction of benzene with the bridged hydroxyl group. This is consistent with the experimental studies of Bull et al.47 The calculated interaction energies are given in Table 12. BLYP and Becke3LYP predict values of 3.02 and 3.66 kcal mol-1. The experimental value for benzene sorption on zeolite HY was found to be 14.0 kcal mol-1.50 As in the case for the silanol:benzene complex, the predicted interaction energy is much lower than the experimental value.

Benzene is often used as a measure of intrinsic acid site strength. This study strongly suggests that the interaction of benzene with zeolitic acid sites may be highly dependent on the surrounding lattice structure. Hence, any deductions as to intrinsic acid site strength based on the experimental benzene shifts should be viewed with caution. Conclusions The BLYP and Becke3LYP density functionals are shown to describe the vibrational properties of the interaction complexes extremely well. For the nitrogen, acetylene, and ethene probe molecules, the density functional results show excellent agreement with experimental data. In the case of acetylene and ethene the proposed experimental structures were backed up by the theoretical work. Both BLYP and Becke3LYP predicted T-shaped configurations. Excellent agreement between experimental frequencies and frequency shifts were also observed. Such agreement bodes well for future theoretical work on similar systems. In the case of acetylene, its polymerization reaction over zeolites is a potential area for study. The equivalent oligomerization reaction of ethene over H-ZSM5 is also of interest. More significantly, the cracking process can be investigated (i.e., the occurrence of carbenium ions adsorbed on the bridged hydroxyl group). This study has also highlighted some areas in which the calculations are limited. For nitrogen sorption on the free hydroxyl group, the vibrational frequency shifts are low, due to the weak interactions. The current functionals approach the values found experimentally, but were still quite low. The other area of error was found for the sorption of benzene. This interaction is known47 to be higher because of dispersion forces between the benzene molecule and the zeolite lattice which surrounds the acid site. The small cluster model could not account for this effect and the theoretical vibrational frequency shifts were found to be low as a consequence. This highlighted the effect of the surrounding zeolite structure on the benzene interaction. The results show this effect to be quite strong. This means that experimentalists should excercise caution when comparing the intrinsic acidity of acid sites in different zeolites on the basis of benzene shifts. In general, interaction energies for all complexes are significantly underestimated compared with experimental determinations. The theoretical calculations predicted the nitrogen probe to exhibit the weakest interaction. The strongest interactions were observed for the acetylene and ethylene probe molecules, which were very similar to each other. These findings are consistent with those of Makarova et al.,8 who investigated a range of probe molecules interacting with zeolite HY. References and Notes (1) Busca, G.; Ramis, G.; Lorenzelli, V.; Janin, A.; Lavalley, J. C. Spectrochim. Acta 1987, 43A, 489. (2) Zecchina, A.; Bordiga, S.; Spoto, G.; Scarano, D.; Petrini, G.; Leofanti, G.; Padovan, M.; Arean, C. O. J. Chem. Soc., Faraday Trans. 1992, 88, 2959. (3) Kubelkova, L.; Beran, S.; Lercher,. J. A. Zeolites 1989, 9, 539. (4) Hedge, S. G.; Ratnasamy, P.; Kustov, L. M.; Kazansky, V. B. Zeolites 1988, 8, 137. (5) Wakabayashi, F.; Kondo, J.; Wada, A.; Domen, K.; Hirose, C. J. Phys. Chem. 1993, 97, 10761. (6) Datka, J. J. Chem. Soc., Faraday Trans. 1981, 77, 511. (7) Echoufi, N.; Gelin, P. J. Chem. Soc. Trans. 1992, 88, 1067. (8) Makarova, M. A.; Ojo, A. F.; Karim, K.; Hunger, M.; Dwyer, J. J. Phys. Chem. 1994, 98, 3619. (9) Pereira, C.; Kokotailo, G. T.; Gorte, R. J. J. Phys. Chem. 1991, 95, 705. (10) Bordiga, S.; Ricchiardi, G.; Spoto, G.; Scarano, D.; Carnelli, L.; Zecchina, A.; Area´n, C. O. J. Chem. Soc., Faraday Trans. 1993, 89, 1843.

Hydroxyl Groups in Zeolites (11) Spoto, G.; Bordiga, S.; Ricchiardi, G.; Scarano, D.; Zecchina, A.; Borello, E. J. Chem. Soc., Faraday Trans. 1994, 90, 2827. (12) Knozinger, H. In The Hydrogen Bond; Schuster, P., Zundel, G., Sandorfu, C., Eds.; North-Holland: Amsterdam, 1976; Vol. III, p 1263. (13) Geerlings, P.; Tariel, N.; Botrel, A.; Lissillour, R.; Mortier, W. J. J. Phys. Chem. 1984, 88, 5752. (14) Ugliengo, P.; Saunders V. R.; Garrone, E. J. Phys. Chem. 1989, 93, 5210. (15) Bates, S.; Dwyer, J. J. Phys. Chem. 1993, 97, 5897. (16) Sauer, J.; Ugliengo, P.; Garrone, E.; Saunders, V. R. Chem. ReV. 1994, 94, 2095. (17) Farnworth, K. J.; O’Malley, P. J J. Phys. Chem. 1996, 100, 1815. (18) Neyman, K. M.; Strodel, P.; Ruzankin, S. P.; Schlensog, N.; Kno¨zinger, H.; Ro¨sch, N. Catal. Lett. 1995, 31, 273. (19) Ugliengo, P.; Ferrari; A. M.; Zecchina, A.; Garrone, E J. Phys. Chem. 1996, 100, 3632. (20) Frisch, M. J.; Trucks, G. W.; Head-Gordon, M.; Gill, P. W. M.; Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M. A.; Replogle, E. S.; Gomperts, R.; Andres, J. L.; Raghavachari, K.; Binkley, J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.; DeFrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A.; GAUSSIAN92/DFT; Gaussian, Inc.: Pittsburgh, PA, 1993. (21) Cerius2: Molecular Modelling Software For Materials Research; Molecular Simulations, Inc.: Burlington, MA, Cambridge, U.K. (22) SPARTAN, Version 3.1; Wavefunction, Inc.: 18401 Von Karman Ave., 370 Irvine, CA 92715. (23) Becke, A. D. Phys. ReV. B 1988, 38, 3098. (24) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (25) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (26) Johnson, B. G. 1988 Gill, P. M. W. 1988 Pople, J. A. J. Chem. Phys. 1993, 98, 5612. (27) Sosa, C. 1988 Andzelm, J. 1988 Elkin, B. C.; Winner, E.; Dobbs, K. D.; Dixon, D. A. J. Phys. Chem. 1992, 96, 6630. (28) Murray, C. W.; Laming, G. J.; Handy, N. C.; Amos, R. D. J. Phys. Chem. 1993, 97, 1868. (29) Hutter, J.; Luthi, H. P.; Diederich, F. J. Am. Chem. Soc. 1994, 116, 750.

J. Phys. Chem. B, Vol. 102, No. 23, 1998 4515 (30) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56, 2257. (31) Schlegel, H. B. J. Comput. Chem. 1982, 3, 214. (32) Boys, S. F.; Bernardi, F. Mol. Phys. 1970, 19, 553. (33) Ugliengo, P.; Garrone, E. J. Mol. Catal. 1989, 54, 439. (34) Hehre, W. J.; Randon, L.; Schleyer, P. V. R.; Pople, J. A. In Ab Initio Molecular Orbital Theory; John Wiley & Sons: New York, 1986. (35) Garrone, E.; Kazansky, V. B.; Kustov, L. M.; Sauer, J.; Senchenya, I. N.; Ugliengo, P. J. Phys. Chem 1992, 96, 1041. (36) Beebe, T. P.; Gelin, P.; Yates, J. T. Surf. Sci. 1984, 148, 526. (37) Stave, M. S.; Nicholas, J. B. J. Phys. Chem. 1993, 97, 9630. (38) Gupta, N. M.; Kamble, V. S.; Rao, K. A.; Iyer, R. M. J. Catal. 1989, 120, 432. (39) Nicholas, J. B.; Winans, R. E.; Harrison, R. J.; Iton, L. E.; Curtiss, L. A.; Hopfinger, A. J. J. Phys. Chem. 1992, 96, 10247. (40) Kubelkova´, L.; Beran, S.; Lercher, J. A. Zeolites 1989, 9, 539. (41) Brand, H. V.; Curtiss, L. A.; Iton, L. E. J. Phys. Chem. 1992, 96, 7726. (42) Brand, H. V.; Curtiss, L. A.; Iton, L. E. J. Phys. Chem. 1993, 97, 12773. (43) Sauer, J.; Hill, J. R. Chem. Phys. Lett. 1994, 218, 333. (44) Geobaldo, F.; Lamberti, C.; Ricchardi, G.; Bordiga, S.; Zecchina, A.; Palomino, T. G.; Arean, C. O. J. Phys. Chem. 1995, 99, 11167. (45) Chiang, C. K.; Fisher, C. R.; Park, Y. W.; Heeger, A. J.; Shirakawa, H.; Louis, E. J.; Gau, S. C.; MacDiarmid, A. G. Phys. ReV. Lett. 1977, 39, 1098. (46) White, J. L.; Beck, L. W.; Haw, J. F. J. Am. Chem. Soc. 1992, 114, 6182. (47) Bull, L. M.; Cheetham, A. K.; Powell, B. M.; Ripmeester, J. A.; Ratcliffe, C. I. J. Am. Chem. Soc. 1995, 117, 4328. (48) In Nuffield AdVanced Science Book of Data; Ellis, H., Ed.; Longman Group Limited: New York, 1984. (49) Su, B. L.; Barthomeuf, D. Zeolites 1993, 13, 627. (50) Barthomeuf, D.; Ha, B. H. J. Chem. Soc., Faraday Trans. 1 1973, 12, 2158.

Density Functional Studies of Weak Base Interactions with Hydroxyl

Recommend Documents