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AB INITIO AND DENSITY FUNCTIONAL STUDIES HYDROCARBON ADSORPTION IN ZEOLITES*

OF

Larry A. Curtiss, 1 Stanislaus A. Zygmunt, l~zand Lennox E. Itonl lMaterials Science and Chemistry Divisons, Argonne National Laboratory, Argonne, IL 60439 2Department of Physics and Astronomy, Valparaiso University, Valparaiso, IN 46383

submitted for publication

in the

Proceedings of the 12th International Zeolite Conference Baltimore, Maryland July 5-10, 1998

The submitted manuscript has been created by the University of Chicago as Operator of Argonne National Laboratory(“Argonne”) under Contract No. W-31- 109-ENG-38 with the U.S. Department of Energy. The U.S. Government retains for itself. and others acting on its behalf. a paid-u~, nonexclusive. imevoc=ble worldwide license m said article to reproduce, prepare derivative works, $istnbute c?pies to M. public, and perform publtcl and drsplay pubhcl y, by or on behalf of the J ovemment.

*Work supported by the U.S. Department of Energy, —. BES-Materials Sciences, under Contract . w-3 1-109-ENG-38.

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DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, make any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or manufacturer, or service by trade name, trademark, otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

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AB INITIO AND DENSITY FUNCTIONAL ADSORPTION IN ZEOLITES

STUDIES

OF HYDROCARBON

LARRY A. CURTISS,* STANISLAUS A. ZYGMUNT,+ AND LENNOX E. ITON*

*Argonne National Laboratory, Argome, IL 60439 USA; [email protected]. anl.gov + Valparaiso University, Valparaiso, IN 46383 USA; szygmunt(ljlexodus.valpo.edu

ABSTRACT

The adsorption energies of methane and ethane in zeolites are investigated with ab initio molecular orbital theory and density functional theory. In this work we have used zeolite cluster models containing two, three, and five tetr~edral (Si, Al) atoms and have found equilibrium structures for complexes of methane, ethane, and propane with an acid site. If a large enough cluster is used and correlation effects are included via perturbation theory, the calculated adsorption energy for ethane is about 5 kcal/mol compared with the experimental value of 7.5 kcal/mol. The B3LYP density fictional method gives a much smaller binding of -1 kca~mol for ethane. The reason for the failure of density fictional theory is unclear.

INTRODUCTION

The transfer of a proton ilom the Bransted acid site to an adsorbed molecule is an important step in acid catalysis by zeolites. However, this process is not yet fi.dly understood at an atomic level. We are studying this proton transfer process using quantum chemical techniques.’ The adsorption process is also important and it is necessary to know the adsorption energy to investigate the barriers for proton transfer and cracking. While there has been a great deal of computational work done on the adsorption of polar molecules such as water, ammonia, and methanol in zeolites,2 little has been reported on hydrocarbon adsorption. Experimental studies3-Ghave reported adsorption energies for hydrocarbons that range horn -7 kcal/mol for ethane to -15 kcal/mol for n-hexane. A density fictional

(DF) study’ has found ethane to be

unbound at an acid site in a zeolitic cluster. In this study we have carried out high level ab initio molecular orbital calculations, as well DF calculations, of the adsorption energy of methane and ethane on zeolitic clusters.

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THEORETICAL

METHODS

The ab initio calculations

were done using G2(MP2,SVP) theory.9 This method uses

MP2/6-31 G* geometries and scaled HF/6-3 lG* zero-point energies. Single point energies at the MPY6-311 +G(3dfi2p) and QCISD(T)/6-3 IG* levels are combined to obtain a final total energy. The 6-31 l+G(3df,2p) basis set should be large energy enough to handle the interaction between the hydrocarbon and the acid site. The density fictional

method used is the gradient corrected

B3LYP method.8 The geometries were calculated at the B3LYP/6-31 G* level and then a single point energy was calculated at the B3LYP/6-31 l+G(3df,2p) level. After obtaining each of the optimized geometries, we also calculated the harmonic zero-point energy (ZPE) of each system at the B3LYP/6-31 G* level. The adsorption energies are calculated for the reaction ZH-C~H2~ + ZH + CnH2~. The cluster models for the zeolite Iia.mework used in this study have two, three, and five tetrahedral (Si, Al) atoms and are denoted 2T (SiAIOHG),3T (SizAKhH*), ~d 5 T (S~O&lG), respectively. The clusters having an acid site are denoted ZH and the all silicon clusters are denoted Z. All of the structures were filly optimized and have all positive frequencies.

RESULTS

AND DISCUSSION

The MP2/6-31 G(d) structures of the ethane-ZH and methane-ZH complex using the 2T cluster are shown in Fig. 1. Selected geometrical parameters are shown in the figure. In both complexes the two hydrogens of the hydrocarbon are bonded to the acidic proton with H-H distances of about 2 & The optimized structures of the ethane-ZH complex using the 3T and 5T clusters are shown in Fig. 2. Selected geometrical parameters are shown in the figure. Ethane is bonded to the acidic proton with H-H distances of about 2 ~. Also, only one end of the ethane is bonded to the ZH cluster even though the clusters are large enough to allow for interaction of both ends of ethane. The adsorption energies of the methane and ethane complexes with the 2T and 3T clusters from the G2(MP2,SVP) calculation (based on the MP2/6-31 G(d)) optimized clusters are summarized in Table 1. The results indicate that the ethane is bound by 2.8 kcal/mol (including zero-point effects) to the acid site. This increases slightly to 3.3 kcal/mol for the larger 3T

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cluster. Methane is bound by 2.1 kcal/mol to the acid site, The larger binding for ethane than methane agrees with the trend of larger adsorption energies as the hydrocarbon size increases. The adsorption energies from the B3LYP/6-31 l+G(3df,2p)

calculations (B3LYP/6-

31G(d) geometries) are listed in Table 2. This method, which is generally considered the most reliable density fi.mctional method for bond energies in molecules,’0 indicates that ethane is bound by only 0.1 to 0.6 kcal/mol to the acid site. This is considerably smaller than the G2(MP2,SVP) results which should be more reliable. The adsorption energy for propane, 1.0 kcal/mol, is slightly more than that of ethane (see Table 2). It is known that for weak van der Wads interactions large basis sets and a good account of correlation effects are required for obtaining accurate binding energies. Apparently the treatment of correlation in density functional theory is not adequate for calculating these weak interaction energies.

Figure 1. MP2/6-31 G(d) optimized structures for ethane-ZH and methane-ZH complexes using the 2T cluster model for the zeolitic acid site. There have been numerous experimental measurements of the hydrocarbon adsorption energies in zeolites and generally they are significantly larger than what is given by our calculations. The adsorption energy of methane in ZSM-5 has been measured to be about 6 kcal/mo13 compared to our value of 2.8 kcal/mol. The adsorption energy of ethane in ZSM-5 has been measured to be about 7.5 kcal/mo14 compared to our value of 3.3 kcal/mol. The adsorption energies of other hydrocarbons increase by about 2 kcal/mol per CH2 group.

c % 2.046 i“, 2.1

‘ 1 , ,“;.01 ,,

4 Figure 2. Optimized structures for ethane-ZH complexes using the 3T cluster (MP2/6-31G(d)) and 5T cluster (133LYW6-3lG(d)).

Cluster Size 2T

Cluster Type ZH

Adsorbed Molecule

CH4 CZH6

3T

z

C&

ZH

CzH(j CZH6

AE.,kcal/mol 2.7 3.3 1.1 2.2 3.8

AEO,kcal/mol 2.1 2.8 0.8 2.0 3.3

Table 1. Adsorption energies for methane and ethane complexes with ZH and Z clusters from G2(MP2,SVP) calculations.

The reason for the discrepancy for between experiment and theory is not clear. The theoretical calculations indicate that ethane and methane form a weak van der Waals bond with the acid site, which is what is to be expected when a nonpolar molecule interacts with a strong dipole. Results for the all silicon clusters (Z) in Table 1 indicate that the bonding to a non-acidic site is also weak (2 kcal/mol or less).

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Cluster Size 2T 3T 5T 5T

Cluster Type ZH ZH ZH ZH

Adsorbed Molecule CH4 CZH6 CZH6 c3Hg

AEO,kcal/mol 0.1 0.2 0.6 1.0

Table 2. Adsorption energies for methane, ethane, and propane complexes with ZH clusters fromB3LYP/6-31 l+G(3df,2p) //B3LYP/6-3 lG(d) calculations.

Cluster Size 2T 3T 5T

Adsorbed Molecule HF/6-3 lG(d) CZHfj “ 0.9 1.0 CZHG 1.0 CzH(j

MP2/6-31 G(d) 2.8 3.2 4.3

Table 3. Adsorption energies (A&, kcal/mol) for the ethane complex with different size ZH clusters flom MP2/6-3 lG(d) andHF/6-31 G(d) calculations. In order to fhrther investigate the cluster size effect, we have compiled MP2/6-31G(d) and HF/6-31 G(d) adsorption energies of ethane-ZH in 2T, 3T and 5T clusters in Table 3. At the HF/6-31 G(d) level there is little dependence of the binding on the cluster size. In contrast, at the MP2/6-3 lG(d) level the results indicate that the binding of the ethane to the acid site increases significantly with cluster size. Since the 3T result of 3.2 kcal/mol (MP2/6-31 G(d)) is close to the more accurate G2(MP2,SVP) result of

3.3 kcal/mol in Table 1, there

appears to be a

cancellation of higher level effects (correlation, basis set, and zero-point energies). Thus, the MP2/6-3 lG(d) values for the binding may be reasonably reliable. For the 5T cluster the adsorption energy is 4.3 kcah’mol at the MP2/6-31 G(d) level. We have also investigated the effect of using a much larger cluster, 38T, on the adsorption energy. At the HF/6-31 G(d) level the binding increases by 0.8 kcal/mol tlom 5T to 38T. If the long-range electrostatic effects are described reasonably well at the Hartree-Fock level, this gives an estimate for the ethane adsorption energy of 5.1 kcal/mol (4.3 kcal/mol from 5T and 0.8 kcal/mol flom 5T ~ 38T). The experimental value is 7.5 kcal/mol. It may be necessary to do a more complete study with larger clusters to account for the remaining difference between experiment and theory.

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CONCLUSIONS

The interaction of methane and ethane with an zeolitic acid site using cluster models containing two and three tetrahedral (Si, Al) atoms is found to be very weak with binding of 2-3 kcal/mol. This is significantly less than experimental measurements of hydrocarbon adsorption, which give an adsorption energy of-6-7

kcal/mol for these molecules. Using larger clusters the

adsorption energy of CzHfj increases to -5 kcal/mol compared to the experimental value of 7.5 kcal/mol. The B3LYP density fictional

method appears to greatly underestimate the binding

energies compared to the ab initio molecular orbital methods. In work in progress we are using larger basis sets and larger zeolitic clusters to fin-ther investigate theoretical adsorption energies of hydrocarbons in zeolites.

Acknowledgement

This work was supported by the Division of Materials Sciences, Office of Basic Energy Sciences, Department of Energy, under Contract No. W-31-109-ENG-38. REFERENCES

1. S. A. Zygrnunt, L. A. Curtiss, L. E. Iton, to be published. 2.

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(1996).

3. H. Papp, W. Hinsen, N. T. Do, and M. Baems, Therrnochim. Act., 82137 (1984). 4.

R. E. Richards and L. V.S.Rees,Langmuir,3335

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5. F. Eder and J. A. Lercher, J Phys. Chem., B101 1273 (1997). 6. J. A. Dunne, R. Maiwala, M. Rae, S. Sircar, R. J. Gorte, and A. L. Myers, Langmuir, 125888

(1996). 7. S. Blaszkowski, M. Nascimento, R. van Santen, 1 Phy.s. Chem., 1003463

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8. L, A. Curtiss, P. Redfem, B. J. Smith, and L.Radom, J Chem. Phy.s., 1045148 (1996). 9. M. J. Frisch et al., Gaussian 94, Gaussian, Inc. Pittsburgh, PA.

10. L. A. Curtiss, K. Raghavachari, P. C. Redfem, J. A. Pople, J Chem. Phy.s., 1061063 (1997).