Combined DFT Theoretical Calculation and Solid-State NMR Studies

Dec 3, 2005 - and Mathematics, the Chinese Academy of Sciences, Wuhan 430071, ... functional theory (DFT) calculations as well as magic angle spinning...
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J. Phys. Chem. B 2005, 109, 24273-24279

24273

Combined DFT Theoretical Calculation and Solid-State NMR Studies of Al Substitution and Acid Sites in Zeolite MCM-22 Anmin Zheng, Lei Chen, Jun Yang, Mingjin Zhang, Yongchao Su, Yong Yue, Chaohui Ye, and Feng Deng* State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, the Chinese Academy of Sciences, Wuhan 430071, China ReceiVed: May 24, 2005; In Final Form: October 27, 2005

The distributions of Bro¨nsted acidic protons and their acid strengths in zeolite H-MCM-22 have been characterized by density functional theory (DFT) calculations as well as magic angle spinning (MAS) NMR experiments. The embedded scheme (ONIOM) that combines the quantum mechanical (QM) description of active sites and semiempirical AM1 treatment of the neighboring environment was applied to predict the aluminum substitution mechanism and proton affinity (PA), as well as adsorption behaviors of acetone and trimethylphosphine oxide (TMPO) onto the zeolite. Our theoretical results indicate that the Al substitution takes place in the order of Al1-OH-Si2 > Al8-OH-Si8 > Al5-OH-Si7. The DFT theoretical and NMR experimental results suggest that the acid strength of the three Bro¨nsted acid sites in H-MCM-22 zeolite is slightly lower than that of H-ZSM-5 zeolite and the accessible Bro¨nsted acidic protons most likely reside in both the supercages (at the Al8-OH-Si8 and Al1-OH-Si2 sites) and external surface pocket (at the Al8OH-Si8 site) rather than in the sinusoidal channels (Al5-OH-Si7), with the Al1-OH-Si2 site having the strongest acid strength (as probed by TMPO). This may partially explain the special selectivity of acidcatalyzed reactions occurring inside the channels of H-MCM-22 zeolite.

Introduction Zeolites are crystalline microporous molecular sieve materials consisting of SiO4 and AlO4 tetrahedra that link together in three-dimensional networks of pores and cages of various sizes. Generally, incorporation of heteroatoms, such as aluminum atom, into the siliceous porous materials will introduce a charge imbalance in the framework, which is balanced by protons. Thus Bro¨nsted acid sites are generated in these materials that have shown a promising potential to act as a catalyst in chemical and petrochemical processes, such as the transformation of methanol into hydrocarbons, isomerization of aromatic compounds, and cracking of n-alkanes.1-4 Since the pore structure and acidic property of zeolite catalysts are two main factors that affect the catalytic activity, comprehensive understanding of these two properties would be important for the application of the solid acid catalysts. The pore structure of zeolites can be well characterized by the N2 adsorption and 129Xe NMR measurements.5-7 Several spectroscopic methods such as IR, temperature programmed desorption, microcalorimetry, and solid-state NMR are usually employed to characterize the acidity of solid acid catalysts including zeolites. Among those techniques, the solid-state NMR method is a powerful tool to characterize the nature of different acid sites on zeolites. 1H MAS NMR can provide structural information about various hydroxyl groups including the bridging OH groups (Bro¨nsted acid site). Sorption of the probe molecules on acidic catalysts, in combination of magic angle spinning (MAS), is one of the widely used methods to characterize the solid acidity as well as the interactions between the probe molecules and the Bro¨nsted and Lewis acid sites.8-13 13C MAS NMR of adsorbed 2-13C* Author to whom correspondence should be addressed. E-mail: dengf@ wipm.ac.cn. Fax: 86-27-87199291.

acetone was used to determine the acid strength of various solid catalysts, while 31P NMR of adsorbed trimethylphosphine (TMP) or trimethylphosphine oxide (TMPO) was employed to discriminate Bro¨nsted and Lewis acid sites, with the latter probe molecule more sensitive to Bro¨nsted acid strength. Pyridine and other probe molecules were also widely used for this purpose. However, all the above spectroscopic methods are unable to give rise to information about the detailed locations of both aluminum and acidic proton in zeolite. Using the X-ray diffraction (XRD) technique it is difficult to discriminate silicon and aluminum atoms (both of them having almost the same atomic weight and diameter size) as well as the precise location of the acidic proton (having much less atomic weight) in zeolites. As is well-known, the distribution of aluminum and acidic proton will considerably affect the acidity and catalytic performance of molecular sieves, so it is necessary to obtain such information with the help of other less direct methods. Theoretical calculation can reveal the substitution mechanism of aluminum, the position of the acidic proton, and the interaction between the probe molecule and the acidic centers, as well as the reaction transition state formed in zeolites.14-22 The ab initial molecular orbital method has been applied to calculate the substitution of aluminum in the various zeolites through comparing the relative substitution energies.16-18 Haw et al. calculated the PA and 13C chemical shift of adsorbed 2-13Cacetone to measure the acid strength of various solid acids.19-22 Zeolite MCM-22, first synthesized by Mobil researchers in 1990, has a unique and unusual pore structure.23,24 Its internal structure is composed of two different independent pore systems (Figure 1): the 10-membered ring (MR) two-dimensional sinusoidal channels and the 12-MR large supercages (0.71 × 0.71 × 1.82 nm3) interconnected by the 10-MR window systems. The free diameters of these apertures are 0.4 × 0.59

10.1021/jp0527249 CCC: $30.25 © 2005 American Chemical Society Published on Web 12/03/2005

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Zheng et al. Al substitution. In addition, the interaction of acidic protons with probe molecules, such as acetone and TMPO, was thoroughly studied by using DFT calculation as well as 13C and 31P MAS NMR experiments. The theoretical calculation results were very consistent with NMR experimental observations. Experimental Section

Figure 1. (a) Projection of the MCM-22 topology indicating the eight crystallographically unequivalent tetrahedral sites;24 (b) schematic illustration of two layers of MCM-22 showing the supercage (A) with its 10-ring windows and the surface pockets (B) joined together through double six-rings (C). When two layers join together in this manner, two pockets form the supercage.29 For clarity, only T atoms are shown.

nm for the sinusoidal channel and 0.4 × 0.54 nm for the entrance to the supercage, respectively. Such a combination in one zeolite structure could enable large pores to convert large molecules and medium pores to gain shape selectivity. Many investigations have been carried out to clarify the acid property of MCM-22 by both experiments and theoretical calculations.9,25-29 Previous works used relatively simply methods or models to predict the order of Al substitution in MCM22 zeolite and the corresponding results did not agree with one another. Corma et al.29 predicted by force field method that Al substitution preferentially occurred at the T1 site of MCM-22. As pointed out by the authors, a more precise theoretical calculation was required to obtain a quantitative analysis of the strength and location of acid centers. Using DFT calculation with a 5T cluster model, Bao et al.26 concluded that Si atoms at T2-Si3 and T3-Si2 sites were preferentially substituted by Al atoms. However, the 5T cluster is only a simplified model that ignores all structural and long-range influence of the zeolite lattice. Considering the important effect of zeolite framework structure on the acid properties, using a large cluster to represent the zeolite structure would, of course, be necessary. However, accurate quantum calculation with a large cluster model is very expensive. Fortunately, a new theoretical method ONIOM (Our own N-layered Integrated molecular Orbital + molecular Mechanics) provides a better description of the electrostatic interaction between the high-layer and low-layer regions as well as the effect of environment on the properties of interested atoms.30 Our previous studies demonstrated that the combined ONIOM method was an efficient method to calculate the large systems, such as the supramolecular assembly of amino acids31 and the adsorption of organic species inside zeolites.32 Burk et al. have also investigated the vibrational frequencies of the bridging hydroxyl groups in zeolite ZSM-5 by the ONIOM method.33 In this paper, to reveal the locations of acid sites and their acid strengths that are responsible for the catalytic activity of H-MCM-22 zeolite, we calculated the substitution energies of aluminum at eight distinct Si sites of the zeolite by the newly introduced ONIOM method to find the preferential Si sites for

Sample Preparation. MCM-22 zeolite was synthesized according to the procedures described in the literature,23,34 using hexamethyleneimine (HMI) as a structure-directing agent. The synthesis mixtures were prepared by using silica sol (26.5 wt % SiO2), sodium aluminate, HMI, sodium hydroxide, and deionized water. Sodium hydroxide (0.8 g) and sodium aluminate (1.7 g) were dissolved in 135 g of deionized water, and then 10.3 g of HMI and 46.8 g of silica sol were added with stirring. The molar compositions of the resulting mixtures were 13.5 Na2O, 5 Al2O3, 100 SiO2, 4500 H2O, and 50 HMI. After aging and stirring for 60 min at room temperature, the mixture was transferred into a Teflon-lined autoclave and heated to 423 K for 96 h. The crystalline product was filtered, washed with deionized water until it reached a pH Al8-O13-Si8 > Al5-O-Si7 > Al4-O-Si2. Although Corma et al.29 also predicted that Al preferentially substituted the T1 site, no detailed substitution order was given. As is shown in Table 1, the most difficult

24276 J. Phys. Chem. B, Vol. 109, No. 51, 2005 substitution site is T7, having a substitution energy of ca. 38 kcal/mol higher than that of the T1 site, while the substitution energy differences for T1, T8, T5, and T4 sites are smaller than 7 kcal/mol. If the substitution in MCM-22 is thermodynamically controlled, all the T1, T8, T5, and T4 sites are Al-rich sites. In the high-field (17.6 T) 27Al MAS NMR spectra of zeolite MCM-22, Kennedy et al.41 could resolve and assign three tetrahedral Al signals at ca. 50 (T6 and T7), 56 (T1, T3, T4, T5, and T8), and 61 ppm (T2) with relative areas of 15:62:23. The signal assignment was made based on a well-accepted empirical correlation between the isotropic chemical shifts of tetrahedral Al and average Al-O-Si angles in the zeolite framework.42 By comparing the predicted (assuming a random Al distribution) and experimental relative areas of the three signals, the authors concluded that one or more of the sites T1, T3, T4, T5, and/or T8 are aluminum rich.41a On the basis of our theoretical calculation, we determine aluminum substitution taking place in the order of Al1-O-Si2 > Al8-O13-Si8 > Al5-O-Si7 > Al4-O-Si2. From a thermodynamic point of view, all these sites are Al rich. In the following calculations, we only calculate the acid properties of Al1-OH-Si2, Al8O13H-Si8, and Al5-OH-Si7 sites since these sites are Al rich and the corresponding protons are accessible to reaction molecules, whereas the unaccessible proton site (Al4-OHSi2) and the other Al-deficient sites are neglected. Proton Affinity of H-MCM-22 Zeolite. Proton affinity (PA) is a criterion to evaluate the intrinsic acidity strength of zeolites and other solid acids. The PA value can be obtained by comparing the energy difference between protonated and deprotonated zeolite models.21 The smaller the PA value, the more easily the Bro¨nsted acidic proton can be deprotonated, and thus the much stronger the acid strength would be. It is well-known that the small cluster is unable to represent the structure and reproduce acid properties of zeolites. Bao et al.26 calculated PA values in the range from 323 to 330 kcal/mol on a 5T cluster model for H-MCM-22 zeolite, almost equal to that of the silanol group (326 kcal/mol).43 Taking framework environment around the bridging hydroxyl proton into account for each site by the ONIOM method, our models give rise to PA values of 308.9, 304.2, and 307.3 kcal/mol for the Al1OH-Si2, Al5-OH-Si7, and Al8-OH-Si8 sites, respectively, corresponding to acid strengths slightly lower than that of H-ZSM-5 zeolite (having a PA value of 294 kcal/mol predicted at the same level). Since the PA difference of the three acid sites is very small with the maximum being 4.7 kcal/mol, their acid strengths are similar with the Al5-OH-Si7 site being most acidic. Acetone Adsorbed on H-MCM-22 Zeolite. Since the acidity of zeolites is considerably affected by its pore topology, it is necessary to examine their acidity in the presence of a proton acceptor. We used two probe molecules (acetone and TMPO) to characterize the acid strength as well as acidic proton distribution in H-MCM-22 zeolite. When the acetone molecule adsorbs onto a Bro¨nsted acid site, an acetone-zeolite complex is generated through a hydrogen bond between the bridging hydroxyl group and the oxygen atom of the carbonyl group (Figure 3). Both the adsorption energy and the 13C isotropic chemical shift of the carbonyl carbon of 2-13C-acetone can also be used as a mark to evaluate the acid strength of solid acids.11,12 The zeolite-OH bond length, about 0.97 Å in the bare H-MCM22, is elongated to 1.025-1.041 Å in the adsorption complex, and the CdO bond length, about 1.211 Å in the free gas acetone, is elongated to 1.225, 1.227, and 1.226 Å for Al1-OH-Si2, Al5-OH-Si7, and Al8-OH-Si8 sites, respectively. Generally,

Zheng et al.

Figure 3. The B3LYP/TZVP-optimized geometry for acetone adsorbed on the Al5-OH-Si7 acid site of H-MCM-22 zeolite. Adsorption energies (∆E, kJ mol-1) and selected interatomic distances (Å) are also included.

TABLE 2: Proton Affinity, Adsorption Energy, and 13C Chemical Shifts of Acetone Adsorbed on the Three Acid Sites of H-MCM-22 Zeolite Al1-OH-Si2 Al5-OH-Si7 Al8-OH-Si8 proton affinity (kcal/mol) adsorption energy(kJ/mol) σ adsolute (ppm) chemical shift (ppm)a experimental (ppm)

308.9 49.32 -37.0 217.1 218

304.2 65.21 -41.0 220.0 218

307.3 57.68 -40.0 219.1 218

a The second-order Moller-Plesset (MP2) chemical shift predicted by restricted Hartree-Fock (RHF) calculation, using the linear correlation σMP2 ) 1.13σRHF - 43.1 in ref 22.

the stronger the Bro¨nsted acidity, the stronger the hydrogen bond between the carbonyl oxygen and the acidic proton, resulting in a longer CdO bond length. The variation of the CdO bond lengths suggests that the acid strength of the different Bro¨nsted sites follows an order of Al5-OH-Si7 > Al8-OH-Si8 > Al1-OH-Si2. As shown in the Table 2, the adsorption energies of the different acid sites also indicate a similar order of acid strength. The longer CdO bond length leads to a more deshielding effect on the carbonyl carbon atom, and a much more 13C downfield shift would be observed. The calculated 13C isotropic chemical shifts for the carbonyl carbon of acetone adsorbed on the three acid sites are also listed in Table 2, indicating that the acid strength follows the order of Al5-OHSi7 (220.0 ppm) > Al8-OH-Si8 (219.1 ppm) > Al1-OHSi2 (217.1 ppm), consistent with the order predicted by both the CdO bond length and the acetone adsorption energy. Figure 5a shows the 13C CP/MAS spectrum of 0.2 mmol/g of 2-13C-acetone adsorbed on H-MCM-22 zeolite. Only one signal at 218 ppm, due to unreacted acetone adsorbed on Bro¨nsted acid sites, can be observed (the peak at 31 ppm can be assigned to the methyl group). Since acid strengths of the three acid sites are very close, 13C NMR of adsorbed acetone is unable to distinguish the difference and an average chemical shift was observed for the carbonyl carbon. The experiment value (218 ppm) is in good agreement with the DFT computational values (217-220 ppm), indicating that our computational model and method are reasonable to predict the acid strength. The 13C chemical shifts suggest that the acid strength of H-MCM-22 zeolite is slightly lower than that of H-ZSM-5 zeolite, where adsorbed 2-13C-acetone gives rise to a 13C NMR resonance at 223 ppm, but similar to that of H-Y zeolite (having an isotropic 13C chemical shift of ca. 219 ppm).22 TMPO Adsorbed on H-MCM-22 Zeolite. TMPO is another widely used probe molecule that is much more sensitive to

Al Substitution and Acid Sites in Zeolite MCM-22

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Figure 4. The B3LYP/TZVP-optimized geometries for TMPO adsorbed on the different acid sites of H-MCM-22 zeolite: (a) Al1-OH-Si2, (b) Al5-OH-Si7, and (c) Al8-OH-Si8. Adsorption energies (∆E, kJ mol-1) and selected interatomic distances (Å) are also included.

Figure 5. 13C CP/MAS NMR spectrum (a) of 2-13C-acetone and 31P MAS NMR spectrum (b) of TMPO adsorbed on H-MCM-22 zeolite. Asterisks denote spinning sidebands.

TABLE 3: Geometry Parameters and 31P Chemical Shifts for TMPO Adsorbed on the Three Acid Sites of H-MCM-22 Zeolite Al1-OH-Si2 Al5-OH-Si7 Al8-OH-Si8 adsorption energy (kJ/mol) PdOTMPO (Å) OTMPO‚‚‚H (Å) H‚‚‚OMCM-22 (Å) σ adsolute (ppm)a chemical shift (ppm)a experimental (ppm)

116.80 1.579 1.047 1.470 274 80 80

111.73 1.577 1.055 1.446 278 76

98.35 1.572 1.065 1.411 286 68 66

a Calculated by the HF/DZVP2 approach on the optimized structure at the B3LPY/TZVP level.

Bro¨nsted acid sites with similar acid strength.34 The optimized structures of TMPO adsorbed on H-MCM-22 zeolite are shown in Figure 4, and the corresponding calculated results are given in Table 3. The most obvious change after the adsorption of TMPO is that the zeolite-OH bond length has been elongated from 0.97 Å in the bare H-MCM-2 to 1.411-1.470 Å in the adsorption complexes, indicative of the occurrence of proton transfer from the zeolite to TMPO. The zeolite-OH bond lengths at the three acid sites suggest the following acid strength order: Al1-OH-Si2 > Al5-OH-Si7 > Al8-OH-Si8. In addition, both the elongation extent of the PdO bond lengths in the TMPO-zeolite adsorption complexes and the calculated adsorption energies (see Table 3) also predict the same order

of acid strength: Al1-OH-Si2 > Al5-OH-Si7 > Al8-OHSi8. It is noteworthy that the acid strength order probed by TMPO is quite different from that either predicted by PA values or probed by acetone. For example, both the PA values and the acetone adsorption results indicate the same acid strength order: Al5-OH-Si7 > Al8-OH-Si8 > Al1-OH-Si2. Generally, the PA value of an isolated Bro¨nsted acid site only gives an indication of the relative strengths of the O-H bond, whereas when the probe molecule is adsorbed on the acid sites, its acidity depends not only on the proton affinity but also on the accessibility of the acidic OH group that will be controlled by both pore size and local steric constraint. Trout et al.44 have also found that four basic probe molecules (acetonitrile, methanol, ammonia, and pyridine) reveal different acid strength orders for the different acid sites inside chabazite zeolite by the theoretical calculation. The authors pointed out that the smaller base molecules, ammonia and methanol, were more stable when situated in smaller channels, whereas the larger base molecules, acetonitrile and pyridine, were preferentially situated in the 8T ring of chabazite zeolite. It can be seen from Figure 4 that the Al8-OH-Si8 site has the largest steric hindrance for the TMPO adsorption since the two neighboring OSiH3 groups around the acid site point toward the adsorbed TMPO molecule, whereas for the Al1-OH-Si2 and Al5-OH-Si7 sites, only the left OSiH3 group points toward adsorbed TMPO but the two right OSiH3 groups point away from adsorbed TMPO. It is apparent that the larger the steric hindrance, the smaller the adsorption energy, as well as the 31P chemical shift of TMPO, corresponding to a weaker acid strength. Since the molecular size of acetone is relatively small compared with TMPO, the steric constraint imposed by the zeolite wall for the adsorption of acetone is not so pronounced as in the case of TMPO, and the acid strength order of the three acid sites probed by acetone is identical with that predicted by PA values. We also calculated the 31P isotropic chemical shifts of TMPO adsorbed on the three acid sites. At the HF/DZVP2 level, the computational results are 80, 76, and 68 ppm for the Al1-OHSi2, Al5-OH-Si7, and Al8-OH-Si8 sites, respectively. The experimental 31P MAS spectrum of TMPO adsorbed on HMCM-22 zeolite was shown in Figure 5b, where three peaks at 53, 66, and 80 ppm were observed. It is well accepted that the formation of TMPOH+ ion due to the interaction of TMPO with Bro¨nsted acid sites will give rise to a 31P resonance at about 55 to 85 ppm, while TMPO molecules bound to Lewis acid sites of zeolite will result in resonances at the shift range from 50 to 55 ppm, and physisorbed TMPO molecules will give rise to

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NMR signal at about 39 ppm.45 Therefore, the two signals at 66 and 80 ppm in the 31P NMR spectrum can be assigned to TMPO bound to two Bro¨nsted acid sites with different acid strengths, while the 53 ppm peak is likely due to TMPO adsorbed on the Lewis acid site that results from nonframework aluminum species.46 In addition, on the basis of the above 31P chemical shifts calculation, the two downfield signals can be attributed to TMPO adsorbed on the Al8-OH-Si8 (66 ppm) and Al1-OH-Si2 (80 ppm) sites, respectively. Although MCM-22 contains both 10-MR sinusoidal channels and 12-MR supercage, many studies have shown that, for certain acid catalytic processes, it behaves more like a 12-MR zeolite than a 10-MR zeolite.3,4 This behavior was explained by the fact that the large 12-MR supercage might accommodate larger intermediates, whereas the sinusoidal channels would restrict the diffusion of reactants or products. Since the size of TMPO (kinetic diameter ca. 0.55 nm) is much smaller than the inner free diameter of the supercage (ca. 0.71 nm) of MCM-22, but larger than the free diameters of the aperture (0.40 × 0.59 nm2) for the sinusoidal channels, it can be expected that the TMPO molecule can easily enter into the supercages, but is unable to diffuse into the sinusoidal channels not only due to its smaller channel size but also due to its high degree of tortuosity. Our previous 129Xe NMR results47 also suggested that xenon atoms (with a kinetic diameter of ca. 0.44 nm) were preferentially adsorbed in the supercages of MCM-22 zeolite at low xenon adsorption pressure. Therefore, we are unable to observe the 31P NMR signal from TMPO adsorbed on the Al5-OH-Si7 site (residing in the 10-MR sinusoidal channel). According to the topological structure of MCM-22, the concentration ratio of T8:T1 is 3:1,36 and the T8 site can reside inside both the supercage and the external surface pocket (consisting of a half supercage). This may be the reason that the intensity of the signal at 66 ppm (Al8-OH-Si8) is much stronger than that at 80 ppm (Al1-OH-Si2). As revealed by Lawton et al.,48 the T8 Bro¨nsted acid sites inside the pockets would play an important role in certain catalytic processes. On the basis of our 31P TMPO NMR experimental results, it can be concluded that the accessible acidic protons mainly reside on the Al1-O-Si2 site in the 10-MR crossing window or the Al8-O-Si8 site in either the supercage or the external surface pockets, while the acidic protons at the Al5-O-Si7 site inside the 10-MR sinusoidal channel are inaccessible to large probe molecules, such as TMPO. Conclusion The Al substitution, the acidic protons distribution, and their acid strengths in H-MCM-22 zeolite have been studied by using DFT theoretical calculation combined with solid-state NMR methods. The aluminum substitution order has been determined on a large cluster containing all eight different tetrahedral sites in the zeolite framework by the ONIOM theoretical approach, indicative of an aluminum substitution order of Al1-OH-Si2 > Al8-OH-Si8 > Al5-OH-Si7. The PA values and 13C chemical shifts of adsorbed acetone on H-MCM-22 zeolite suggest that the acid strength of H-MCM-22 is slightly lower than that of the H-ZSM-5, but similar to that of the HY zeolite. The DFT theoretical calculation and 31P NMR of TMPO adsorbed on H-MCM-22 zeolite reveal that the accessible acidic protons mainly locate in the supercage (at the Al8-OH-Si8 and Al1-OH-Si2 sites) and external surface pocket (at the Al8-OH-Si8 site) of H-MCM-22, whereas because of the local steric constraint, the acidic protons located in the 10-MR sinusoidal channel are inaccessible to large probe molecules.

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