Interaction of methanol with OH groups of zeolites:
Comparison of theory and experiment (*)
D. NACHTIGALLOVA´ and L. KUBELKOVA´
J. Heyrovsky´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic Dolejsˇkova 3, 182 23 Prague 8, Czech Republic
(ricevuto il 28 Febbraio 1997; approvato l’8 Maggio 1997)
Summary. — The nature of methanol interaction complexes with bridging
hydro-xyls of zeolites is studied experimentally and theoretically. Results of calculations using HF, DFT, and MP2 methods on a large H2OH-Si-O-SiH2-OH-AlH2-O-SiH2OH are
compared with experimental studies, particularly1H broad line and FTIR fragment
spectroscopies on H-mordenite and H-ZSM-5. Both experimental and theoretical studies predict the hydrogen-bonded complex of methanol with zeolites. Two local minima of the hydrogen-bonded complex with a different orientation of methanol are found on the potential energy surface. In both structures, two active sites of zeolite are separated by one skeletal oxygen. The calculated adsorption enthalpies of both structures fall into the interval of measured adsorption heats. The calculated vibrational OH frequencies are in reasonable agreement with the experimentally measured frequencies. The calculations are also performed on a larger fragment which includes 4-oxygen ring and a part of the zeolite channel. The results obtained with this fragment are similar to those obtained with smaller fragments.
PACS 71.15.Fv – Atomic- and molecular-orbital methods (including tight binding approximation, valence-band method, etc.).
PACS 01.30.Cc – Conference proceedings.
1. – Introduction
Despite a large number of experimental and theoretical studies, the nature of the surface species of methanol formed upon adsorption on zeolitic sites is still subject of controversy.
Formation of either neutral hydrogen-bonded or ion-pair complex is predicted. Keeping with the notion of zeolites as strong acid, spectral data were originally interpreted in terms of chemisorption, i.e. transfer of the acidic proton from zeolite to
(*) Paper presented at the “First International Workshop on Reactivity of Oxide Materials. Theory and Experiment”, Como, 8, 9 November 1996.
HO O OH O OH
0
±
0
±
0
±
0
±
SiH2 SiH2 AlH2 SiH2
Fragment I
methanol and formation of a methoxonium cation [1]. Some authors assume that both forms exist in an equilibrium [2]. However, in the recent quantum-chemical calculations [3-6] the formation of neutral hydrogen-bonded rather than ionic complex as a minimum on the potential energy surface is now predicted. Based on the calculations at both DFT and MP2 levels, the protonated intermediate is predicted to be a transition state for hydrogen exchange between the different zeolite lattice oxygens adjacent to aluminium. The calculated value of the proton transfer energy is very low [3, 7]. A new application of Fermi resonance theory for the interpretation of FTIR spectra supports the concept of the neutral stable hydrogen-bonded complex [3, 8]. In a very recent study Shah et al. [9] calculated a methanol-zeolite complex using density functional theory with generalized gradient approximation and plane-wave basis set. They found the protonated form of methanol as a stable minimum in open-ring structures, as found in chabazite. On the other hand, methanol was found to be physisorbed in sodalite structure. Despite the effort in the interpretation of experimental data of methanol-zeolite complex, large differences between experimental and calculated vibrational frequencies make the characterization of the methanol-zeolite complex still unclear [3, 7]. A major uncertaintity of theoretical studies is due to the modelling of a zeolite fragment. In most of the studies, only few atoms of the zeolite structure are considered.
In this contribution, two models of zeolite were used to study the methanol-zeolite adsorption complex. Calculations with a smaller model (fragment I) were performed at
the Hartree-Fock (HF) and MP2 levels. Calculations on a larger model (fragment II) which imitates the four-fold ring of the zeolite structure were performed at the HF and DFT levels. Results of calculations are compared with the results of FTIR and 1H
broad-line NMR spectra of the complex of H-mordenite and H-ZSM-5 with methanol [10, 11].
2. – Methods
The geometries of the stationary points of the complex of methanol with the zeolite modelled by fragment I were optimized at the HF/6-31G* and MP2/6-31G* [12] levels. Zero-point vibrational energies (ZPE) as well as the fundamental vibrational frequencies were obtained at the HF/6-31G* level. The optimization of methanol interacting with fragment II was performed at the HF/6-31G* and B3LYP/6-31G* [13, 14] levels. ZPE and vibrational frequencies were calculated at the HF/6-31G* level. The interaction energies were corrected for the basis set superposition error (BSSE) by the counterpoise correction method. Vibrational frequencies calculated at the HF level were scaled by the factor 0.9. All the calculations were performed using the GAUSSIAN94 program [15].
3. – Results
Fragment I mimics part of the 4-oxygen ring and part of the zeolite pore or window. Two local minima corresponding to two possible orientations were found on the potential-energy surface (PES) of the zeolite-methanol complex. In these two orientations the methanol crosses either the 4-oxygen ring or the window (structure 1 or structure 2, respectively, fig. 1). The fragment II was chosen to model the positioning of the methanol on the 4-oxygen ring and partially include interactions with atoms coordinated on the Si(Al) atoms of this ring. Only one orientation where methanol crosses the 4-oxygen ring was considered (structure 3, fig. 2). The ring was optimized on a smaller system and was fixed during further optimization. In all structures, methanol is bonded to two sites of zeolite: via hydrogen bond between acidic hydrogen of zeolite (HZ) and methanol oxygen (OM) and hydrogen bond between
zeolite oxygen ( OZ) and methanol hydrogen (HM). In all structures, two adsorption
sites are separated by one skeletal oxygen.
The selected geometrical parameters for structure 1 and 2 calculated at the HF/6-31G* and MP2/6-31G* levels and for structure 3 calculated at the HF/6-31G* and B3LYP/6-31G* levels are given in table I. In all cases, the HZ-OMbond length is shorter
than the HM-OZ bond length. In fragment I, the intermolecular distances (OM-HZ and
OZ-HM) are shorter and intramolecular (OM-HM and OZ-HZ) longer in structure 2
than in structure 1. Inter- and intramolecular distances calculated at the HF level for structure 3 are similar to those for structure 1 calculated at the same level of theory. Compared to the results of calculations at the HF level, the intermolecular OH bonds become shorter and intramolecular OH bonds longer by 10–20 pm and 3-10 pm, respectively, when the correlation energy is included (MP2 and DFT methods). These results are in agreement with those calculated by Haase and Sauer [3].
The distance between HZ and HM
(
r (HM-HZ))
is important for interpretation of1Hbroad-line NMR spectra. The difference between this distance calculated for neutral and ion-pair complex of structure 1 was calculated at the HF level. Due to the
Fig. 1. – Neutral hydrogen-bonded complex of methanol with fragment I (structure 1 and structure 2) optimized at the MP2/6-31G* level.
instability of the ion-pair complex, the complex was optimized with the fixed distance HZ-OM 111 pm. The resulting
(
r (HM-HZ))
distance is 160 pm, which is about 50 pmshorter than in the neutral structure.
The adsorption energies (DEad) calculated with the HF and MP2 (or DFT) methods
corrected for the basis set superposition error (BSSE) are given in table II. Because the optimization procedure of fragment I resulted in the formation of unrealistic intramolecular hydrogen bonds, the calculation of this monomer had to be performed
Fig. 2. – Neutral hydrogen-bonded complex of methanol with fragment II optimized at the HF/6-31G* level.
with the geometry of the zeolite fragment obtained for the zeolite-methanol system. The same structure was used for the calculations of ZPE. Due to the size of the system, we were not able to calculate fundamental vibrational frequencies of structures of fragment I at the MP2 level. Therefore, we used values of zero-point energy (ZPE) calculated at the HF level for the estimation of adsorption enthalpies at 0 K (DH0
ad) at
both the HF and MP2 levels. The same approximation was used for calculations of DHad0
of structure 3 at the DFT level.
Results of adsorption enthalpies for fragment I indicate that structure 2 is more stable than structure 1, by 12.6 kJ/mol at the HF level and 17.7 kJ/mol at the MP2 level, respectively. Compared to calculations at the HF level on structure 1 where the orientation of methanol is the same as in structure 3, the interaction energies and enthalpies of these two complexes are almost the same. The interaction energies and enthalpies calculated with DFT method are about 10 kJ/mol higher compared to structure 1 calculated with MP2.
The results of calculations of fundamental vibrational frequencies of all structures are given in table III. As mentioned above, these calculations were performed at the TABLEI. – Selected optimized bond lengths (pm) for the structure 1, structure 2, and structure 3 of the neutral hydrogen-bonded complex of methanol with the zeolite fragment.
Structure 1 Structure 2 Structure 3
HF MP2 HF MP2 HF DFT r ( HZ-HM) r ( OM-HZ) r ( OZ-HM) r ( OZ-HZ) r ( OM-HM) 218.5 172.1 207.2 97.7 95.2 198.5 154.1 190.0 103.7 98.0 214.2 165.9 188.8 98.4 95.7 194.8 147.2 178.4 106.1 98.8 224.0 177.6 222.6 97.4 95.2 207.7 161.6 203.6 101.6 97.8
TABLEII. – Adsorption energies DE and adsorption enthalpies DHad0 (kJ/mol) of the Structure-1
and -2, and Structure-3 of the neutral hydrogen-bonded complex of methanol with the zeolite fragment.
Structure 1 Structure 2 Structure 3 DE (a) HF/6-31G*//HF/6-31G* MP2/6-31G*//MP2/6-31G* B3LYP/6-31G*//B3LYP/6-31G* 262.3 286.4 — 274.9 2104.1 — 260.6 — 276.3 DHad(b) HF/6-31G*//HF/6-31*+ZPE(HF) MP2/6-31G*//MP2/6-31G*+ZPE(HF) B3LYP/6-31G*//B3LYP/6-31G*+ZPE(HF) 253.6 277.8 — 266 .3 295.2 — 251.4 — 267.1 (a) Interaction energy calculated as a difference between the total energy of the complex and the sum of the energies of its monomers corrected for BSSE: DEad4 Etot2 Emet2 Ezeol2BSSE.
(b) The BSSE corrected interaction enthalpy at 0 K: DHad4 DEad2 DZPE, where zero-point energies were calculated at
the HF/6-31G* level.
TABLE III. – Calculated (a) and observed (b) vibrational frequencies ( cm21) for the complex of methanol with zeolite fragment I and II.
Mode Methanol (c) Methanol on zeolite
Structure 1 Structure 2 Structure 3 Observed nOH(M) nOH(Z) nCD dOH(M) dOH(Z) nCO gOH(Z) 3706 — 2206 — 2059 1289 — 1167 — 3628 3216 2238 — 2081 1334 1252 1148 815 3550 3066 2262 — 2077 1389 1292 1149 841 3652 3282 — — — 1366 1244 1046 807 3560 1790 2268 — 2083 1430 1375 962 866 (a) Calculated at the HF/6-31G* level.
(b) Experimental and calculated using Fermi resonance theory, from ref. [10, 11]. (c) Calculated at the HF/6-31G* level.
HF level only. The stretching and bending vibrational frequencies of the free methanol are not significantly different compared to those calculated for methanol adsorbed on the zeolite (table III). As expected from the geometrical parameters, the OH stretching frequencies of bridging hydroxyls of zeolite
(
nOH( Z ))
and methanol(
nOH( M ))
are lower for structure 2 than for structure 1. The frequencies calculated for structure 3 at the HF/6-31G* level are similar to those calculated at the structure 1 with the same level of theory.4. – Discussion
The ab initio calculations of the hydrogen-bonded complex of methanol with the zeolite fragments used in the previous studies do not provide a good agreement with the experimental data obtained with HM and HZSM-5 (table III, ref. [10, 11]). We performed the calculations on larger fragments of zeolites (fragment I and II) in order to better describe the curvature of the pores which can be important for the orientation and interaction of methanol with the active site of zeolite. The adsorption enthalpies of both structures of fragment I calculated at the MP2 level fall into the interval of measured adsorption heats (75–120 kJ/mol) measured for HZSM-5 and HM [16, 17]. The adsorption enthalpies calculated for Structure-3 with the DFT method are also in a reasonably good agreement with experiment. Comparison of adsorption enthalpies calculated for structures used by us and those found in the literature suggests that several arrangements of the methanol in the zeolite pores close in the energy can be found on the PES.
A reasonably good agreement between experimental and calculated (at the HF level) vibrational frequencies has been found for all structures for stretching vibrations of CD3(nCD) and OH groups of methanol nOH(M), in-plane bending vibrations of OH
groups of methanol dOH(M) and zeolite dOH(Z) and out-of-plane bending vibrations of zeolite OH group gOH(Z) [10, 11]. However, a large error was found in the calculations of OH stretching vibrational frequency of zeolite for all structures considered in this study. The difference between calculated and observed frequencies is 1420 and 1280 cm21for structure 1 and structure 2, respectively, and 1490 cm21for structure 3.
As already mentioned in the previous section, the inclusion of correlation effects elongates the intramolecular OH bonds of both methanol and zeolite. Therefore, the relevant vibrational frequencies calculated with the MP2 method are lower compared to those calculated with the HF method. It has been calculated by Haase and Sauer [3] using the H3Si-OH-Al(OH)2-O-SiH3 fragment that these differences are 360 and
750 cm21 for nOH(M) and nOH(Z), respectively. If we assume the same shifts for our
structures we can estimate the nOH(M) in the range of 3200–3300 cm21and nOH(Z) in
the range of 2300–2500 cm21, respectively. After these corrections, the nOH(M)
becomes low and the nOH(Z), although significantly improved, still too large compared to the experiment. Comparison of the results of structure 1 and 3 with a similar orientation of methanol in the zeolite cavity suggests that inclusion of other part of zeolite channels or windows does not have a significant effect on the orientation and interaction of methanol with the fragment.
The r (HM-HZ) distance calculated for all structures (in their neutral form) is in a
good agreement with the distance 200 pm estimated from 1H broad-line NMR spectra [10, 11] (measured at 32% coverage). For the ion-pair complex this distance was significantly shorter.
5. – Conclusion
The ab initio calculations at the HF and MP2 levels on fragment I reveal two stable structures of neutral hydrogen-bonded complex between methanol and zeolite. Orientations of methanol in the zeolite cavity with two active sites separated by one skeletal oxygen have been found. The calculations on larger fragment II do not show any significant changes compared to those on fragment I. Despite larger errors in the
calculations of vibrational spectra, our calculations support the statement that methanol is physisorbed hydrogen-bonded to the zeolite surface.
* * *
This study has been carried out in the framework of the COST project D5/0002/94 partly under the PECO grant 246042. Authors acknowledge the support of the Ministry of Education of the Czech Republic (OCD5.10).
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