Correlation between Brønsted Acid Strength and Local Structure in

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Correlation between Brønsted Acid Strength and Local Structure in Zeolites Naonobu Katada,*,† Katsuki Suzuki,†,‡ Takayuki Noda,† German Sastre,§ and Miki Niwa† Department of Chemistry and Biotechnology, Graduate School of Engineering, Tottori UniVersity, 4-101 Koyama-cho Minami, Tottori 680-8552, Japan, Japan Society for the Promotion of Science, Chiyoda-ku, Tokyo 102-8471, Japan, and Instituto de Tecnologia Quimica UPV-CSIC, UniVersidad Politecnica de Valencia, AVenida Los Naranjos s/n, 46022 Valencia, Spain ReceiVed: April 24, 2009; ReVised Manuscript ReceiVed: September 19, 2009

As an index of acid strength, ammonia adsorption energies (Eads) were calculated with density functional theory on cluster models of Brønsted acid sites belonging to FAU, BEA, MFI, FER, MWW, and MOR structures, which were selected because of the availability of experimental data and industrial importance. The calculated Eads were reasonably consistent with experimental results from the ammonia IRMS-TPD (infrared mass spectroscopy-temperature-programmed desorption) method. The calculated value was slightly (10-20 kJ mol-1) lower than the observed value, and its change with varying structure was approximately in agreement with the experiments. A thorough study was carried out to find the geometric parameters of the zeolite clusters (in the H and NH4 forms) relevant to Eads and to discuss parameters controlling the acidic property. Hydrogen bonding interactions between ammonium cations and neighboring zeolitic oxygens were found to affect Eads observed in small cavities. When NH4+ was stabilized in relatively open spaces (large cavities), acid strength was controlled by the local geometry of the Brønsted acid site, indicating a contribution of strain around Si(OH)Al to acid strength. In these cases, a shorter Al-O distance (a) gave a higher Eads. This is consistent with the explanation that Lewis acidic Al withdraws the electron charge of the SiOH contributing to Brønsted acid strength. A relationship was found between a and the distance (b) and planar angle (ω) between two triangles consisting of three oxygens each, which surrounded the Si(OH)Al unit, and finally, a relationship was found in which a smaller b and ω brought a higher Eads. The strain (compression) on atoms surrounding the Si(OH)Al unit is reflected in the extent of b and ω, and this contributes to vary Brønsted acid strength. Introduction Zeolites, crystalline and microporous aluminosilicates, are one of the most important groups of functional materials. Their astounding structural richness, with a growing family currently with nearly 200 types, is only one of the reasons that make zeolites widely used as solid acid catalysts in petroleum refinery and petrochemical industries. Newly designed zeolites will be applied in the future production of alternative fuels from heavier hydrocarbon feedstock to environmentally benign organic synthesis in place of conventional liquid acid catalysts. The origin of the catalytic activity of zeolites is in most cases the Brønsted acidic Si(OH)Al group, which is generated by isomorphous substitution of Si4+ by Al3+. Acid strength of the Si(OH)Al group is believed to depend on the composition of the framework (i.e., SiO2/Al2O3 ratio),1 presence of extraframework cation(s),2,3 and zeolite type (crystal structure).4 Among these factors, the zeolite type has been accepted to significantly affect acid strength even at a fixed composition of zeolite, provided the influence of extraframework species is not taken into account.4 The distribution of acid strength of a highly crystalline zeolite is narrow5 compared to that of an amorphous silica-alumina or a zeolite containing defects.6 These observations indicate a marked contribution of geometric parameters (i.e., bond angles and lengths around the acid site) to the acid strength. Compared to amorphous (or nonstoichiometric) mixed * Corresponding author. Telephone: +81-857-31-5684. Fax: +81-85731-5684. E-mail: [email protected].. † Tottori University ‡ Japan Society for the Promotion of Science § Universidad Politecnica de Valencia

oxide catalysts, where the structure cannot be fully determined, the crystallinity of zeolites allows a clear determination of their structure. Catalyst design will be possible for a specific purpose on the basis of advances in zeolite chemistry; for example, a catalyst strongly acidic and accessible by relatively large molecules can be designed, if a reliable relationship between the microstructure and catalytic function is known. In order to study the dependency of acid strength with local geometry, quantum chemistry calculations have focused on small clusters involving the Si(OH)Al unit.7-12 Geometric parameters affect acid strength, as shown by the sensitivity of acid strength on the geometric parameters.12,13 According to principles based on bond conservation order,14 a balance is expected between the strengths of Al-O and O-H bonds in the Si(OH)Al unit, resulting in a correlation in which the shorter (stronger) Al-O bond yields the weaker O-H bond. A weak O-H bond corresponds to a high ability of the Si(OH)Al center to release a proton to other molecules, hence a strong Brønsted acidity.8 Another way to explain this is as follows: tetracoordinated Al is a Lewis acid center that can withdraw electrons from the neighboring OH, thus contributing to a positive charge at the acidic H atom; the shorter (stronger) the Al-O bond is, the higher the Brønsted acid strength of Si(OH)Al.7 Si-O-Al angles have often been claimed to influence Brønsted acid strength. The p-character of the O-H bond should increase (s-character should decrease) with increasing the Si-O-Al angle, and an increase in the p-character is in turn related to the weakness of the O-H bond.8,15 Beran and co-researchers systematically studied relationships among the acid strength and structural parameters through

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Correlation between Acidity and Structure of Zeolites quantum chemistry studies [nonempirical SCF (self-consistent field) and semiempirical CNDO/2 (complete neglect of differential overlap)] on a small cluster and showed that large Si-O-Al angles yielded low O-H dissociation energies (higher acid strength) under conditions where the other factors were fixed. Also, the shorter the Al-O distance is, the lower the dissociation energy, with all other factors fixed.8 The significance of the effects of these structural parameters was compared with the influence of the composition. Influence of the amount and kind of extraframework cation was small, whereas increasing Al in the assumed cluster gave a considerable effect;16 this effect was smaller than that of the structural parameters.17 However, the real zeolite framework is flexible; and therefore, the Al-O bond length and Si-O-Al bond angle should vary simultaneously due to the complex force field present in the real zeolite framework. This should bring complexity to the relationships between acid strength and structural parameters. In addition, Senchenya et al.8 and Datka et al.9 pointed out that the influence of factors other than the geometry, e.g., number of Al atoms at adjacent positions, should be considered in the real zeolite system. In order to study the effects of crystal structure through the local geometry in real zeolites, quantum chemical calculations on cluster models cut from the structure or periodic models were consequently carried out. Relationships between the Si-O-Al angle and acid strength were observed in limited cases,18-20 and in some cases, the relationship was weak21 or unclear.22 Finally, a significant influence on acid strength was suggested to be due to long-range factors, such as electric field gradient,23 other than the local geometry. The conclusions of the above studies are significant but nevertheless different conclusions appear, some times contradictory, suggesting that multiple factors influence acid strength in the real zeolite. Then, discussion on the validity of models based upon experimental confirmation should be important. Recently, a method of ammonia IRMS-TPD (infrared mass spectroscopy-temperature-programmed desorption) has been developed as follows.24 The conventional ammonia TPD can determine the number and strength of acid sites, but identification of Brønsted and Lewis acid sites is impossible. On the other hand, IR spectroscopy can detect NH4+ and NH3 adsorbed on Brønsted and Lewis acid sites, respectively. However, the molar absorption coefficients of the adsorbed species must be varied with the coordination environments. The coordination environment varies on different zeolites; and thereby, quantitative analysis of the acidic property only from the IR spectrum should be incorrect. In order to discuss the precise acid amount, we quantified the number of ammonium cations and number of OH groups by combining the information from IR and MS. The temperature dependence of the NH4 and OH band intensities were compared with the temperature dependence of the gaseous concentration of ammonia (so-called conventional TPD spectrum), and the desorption rate of ammonia from each of the Brønsted or Lewis acid site is quantified without assumption of the molar absorption coefficient. The desorption rate of ammonia was controlled by an equilibrium, and readsorption of ammonia freely occurs.25 Thereby the analysis of the desorption rate5 gives us the adsorption energy or heat of ammonia from each of the Brønsted and Lewis acid site. This method provides us with not only the total number and averaged strength of acid sites but also the number and strength of Brønsted and Lewis acid sites. In addition, the number and acid strength of each OH group distinguishable in an IR (infrared) spectrum on a zeolite can be measured. The measured ammonia

J. Phys. Chem. C, Vol. 113, No. 44, 2009 19209 adsorption heat has been correlated with OH band frequency in the IR spectrum and catalytic activity for the alkane cracking.26 Ammonia adsorption energy or enthalpy,5 which is thus considered to be a representative parameter showing acid strength, can be determined using the ammonia IRMS-TPD method. This makes it possible to combine such experimental studies with quantum chemical calculations because the nature of the OH group with a known geometry can be analyzed by the IRMS-TPD method. As a preliminary study,27 we have reported that the ammonia adsorption energies of Brønsted acid sites due to the framework Al on zeolites of different types (FAU,28 *BEA,29 FER, MFI, MWW,26 and MOR24) were in good agreement with the values calculated according to a density functional theory (DFT) based on cluster models, where not only the trend but the absolute value were in agreement. In this study, the effect of the neighboring Al was discarded through the use of clusters with only one Brønsted site surrounded by a large neighborhood (about 40 Si atoms) resembling an isolated Brønsted acid site. The influence of the assumed cluster size was discussed elsewhere,30 and ab initio calculations showed that acid strength reached a plateau at cluster sizes larger than about 30 T.13 We use a GGA (generalized gradient approximation) DFT functional approach, which has been shown to improve significantly the energy calculations compared to LDA (local density approximation).31 In previous studies within our group, a considerable agreement between calculations and experiments27 was found through the introduction of GGA rather than LDA. It is convenient to study the effect of the zeolite type and geometrical aspects on the ammonia adsorption energy with this level of calculation. We here attempted to find and analyze a correlation between the ammonia adsorption energy and local structure, i.e., bond lengths and angles around the acid site in the present study. Calculations under periodic boundary conditions provide better results than the calculations based on cluster models, and we have also carried out calculations under periodic boundary conditions for structures such as FAU,32 CHA,33 MOR, and BEA,34 where the size of unit cell is relatively small. However, the time for DFT calculation is approximately dependent on the third power of number of electrons in the system, and therefore, the calculation time is very long in the periodic conditions for a unit cell with a large number of atoms. In spite of the advantage of periodic boundary conditions, we mainly chose the cluster models to study a wide variety of industrially important zeolites by taking into account the calculation time and cost. For only some selected structures, calculations were also performed in periodic boundary conditions. As stated above, the ammonia adsorption energies calculated on the basis of both methods were generally in reasonable agreement with the experimental results, supporting the validity of our approach. Such a parameter as deprotonation energy has been used as a measure of intrinsic acidity,10,11,21,22,35 but there are not experiments that can measure such a property. In contrast, ammonia adsorption energy is widely utilized to be a measurable index of solid acid strength. However, the adsorption of ammonia is affected by structural fitting of ammonia to pore wall.22 Different basic probe molecules, e.g., pyridine, CO, pivalonitrile, and acetonitrile, show a different order of zeolites in adsorption strength.36,37 In addition, the catalytic activity is directly affected by the micropore structure through confinement effect and diffusion control.38 In order to extract the correlation between the Brønsted acid strength and geometric feature from these complex adsorption properties, an extensive study using

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multiple probe molecules is required, as has been recently carried out.36-38 The present study adopted a slightly different approach. We concentrate upon the analysis of ammonia adsorption energy. In order to eliminate the influences of confinement effect and steric hindrance, coordination environment of the adsorbed NH4+ cation is carefully discussed. Methods and Models 1. Structural Optimization and Energy Calculations. DFT calculations were carried out with the Dmol3 software developed by Accelrys, Inc., as reported.27 The coordinates of the initial structures were obtained from the Material Studio 4.0 library. The calculations were performed in cluster and periodic methods. In both methods, geometry optimization and energy calculations were performed using the double numerical with polarization (DNP) basis set. The convergence criteria (energy, force, and displacement) were set as 2 × 10-5 Ha (1 Ha ) 4.36 × 10-18 J), 4 × 10-3 Ha Å-1, and 0.005 Å (1 Å ) 0.1 nm), respectively. After structural optimization, the zero-point vibrational energy was calculated for a partial Hessian consisting of atoms in the Si(OH)Al unit. The energy convergence criteria was set as 1 × 10-4 Ha, and the double numerical (DN) basis set was used for the vibration calculations as preliminary calculations showed little influence on the difference in calculation accuracy within this region for the zero-point energy. The adsorption energy (more exactly, internal energy increase upon desorption) was calculated from the following equation: Eads ) EH-Z + ENH3 - ENH4-Z, where EH-Z, ENH3, and ENH4-Z refer to the total energy of each structure, which had been corrected with zero-point vibrational energy. Both structures of H and NH4 forms were optimized to have minimum energies (CIF files are included in the Supporting Information). Cluster Method. For these calculations, clusters consisting of 38 to 48 T atoms (and connected oxygen atoms) were cut off from a zeolite structure with a Si(OH)Al unit in the central part of the cluster. The edge of the cluster was terminated by adding H atoms to form SiH3 groups, which is a common practice.13 The Si/Al molar ratio of cluster models was hence 37-47, which is in all cases sufficiently high so as to consider that acid strength is not affected by neighboring centers. Clusters modeled contain an inner part (Figure 1) where the interactions between the NH4+ cation (formed upon adsorption) and O atoms neighboring the acid site will be particularly important. Such “special” O atoms will be responsible for some repulsive or attractive interaction, e.g., hydrogen bonding, (discussed later) with the NH4+ cation and are specifically labeled (Figure 1). The structures of NH3, H-Zeolite (H-Z), and NH4-Z were optimized at the GGA level using BLYP (Becke-Lee-YangParr) exchange and correlation functional. The positions of atoms involving the central Si(OH)Al unit plus six Si atoms and corresponding O atoms (Figure 1) were optimized to search for the lowest energy, while the outer part (involving 30-40 Si atoms) was fixed according to the coordinates of the initial structure. Periodic Method. For some selected structures (FAU Al1O1-Si1, MOR Al4-O10-Si4, and BEA Al1-O4-Si8), calculations were made in the periodic boundary conditions at the GGA level using the HCTH (Hamprecht-Cohen-Tozer-Handy) exchange and correlation functional. One Al atom and one acidic proton were introduced in a unit cell assumed as below. 2. Centers Considered. In the cluster method, a Si(OH)Al unit was located in the central part of the cluster, around which the total cluster grows to a size of 38-48 T atoms. There was

Figure 1. Schematic of the cluster assumed in the cluster method. H is the Brønsted acid site. The oxygen directly connecting to the H is termed Oa. Among the three oxygen atoms attached to the Al on the right-hand side of this graphic, the oxygen atom closest to the H is termed OAl,1 in this paper. Positions of the atoms (eight T atoms) shown in the white region were optimized to minimize the total energy. Atoms in the outer part (gray region, 30-40 T atoms) were fixed at the initial positions.

one Al atom and the other T atoms were Si; the Si/Al ratio was hence 37-47. The positions of the proton and Al were selected by taking into account investigations on quantum chemical calculations and spectroscopic measurements as follows. Here, we have to note that distributions of the proton and Al are generally under controversy, and the distributions are affected by the synthesis conditions.39 We assumed the positions with current knowledge as shown below. For assuming the initial coordination of the NH4 form zeolite, the proton was replaced with NH4+, and the initial position of NH4+ was set on the Oa atom (Figure 1). Hopping of the adsorbate was ignored. (a) MFI. Distributions of the aluminum and proton in the MFI are still under discussion.40 Preferential occupation of the T12 site by Al in a quantum chemical study41 and preferential locations of the Al(OH)Si unit at Al7-O7-Si8 (QM-pot, quantum mechanical treatment to a small part combined with interatomic functional treatment to a periodic structure),42 Al7-O17-Si4 (ab initio22,43 and DFT44), Al9-O18-Si6 (ab initio),20 and Al11-O11-Si12 (DFT)44 were estimated. In addition, Al12-O24-Si12 was often assumed as a proton location.13,22,45 These five positions shown in Table 1 were considered. (b) FER. Preferential occupation of T4 and T3 by Al was observed by neutron diffraction46 and hence, positions bonded to Al4 and Al3, i.e., Al4-O6-Si4 and Al3-O4-Si1, were studied. On the other hand, preferential occupation of T2 and T4 by Al was estimated based on an ab initio study,47 and this is in agreement with an IR study showing the presence of Al2-O1-Si2.48 Moreover, a DFT study showed the stability of Al2-O7-Si4.44 Therefore, four positions shown in Table 1 were studied. Among them, only Al3-O4-Si1 is located in the ferrierite cagem while the others are in the 10-ring (10 oxygen-membered ring). (c) FAU. There are four crystallographically different positions for the acid site, of which Al1-O4-Si1 has never been observed in the FAU structure. Among the other three, an acid site Al1-O1-Si1 located in the supercage was analyzed in this study, while Al-O2-Si1 and Al-O3-Si1 in the sodalite cage and D6R (double six-membered rings), respectively, were not considered in order to eliminate the effect due to small cavities already observed by the IRMS-TPD.28 In addition, the periodic method was applied to a rhombohedral unit cell of FAU (48

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TABLE 1: Calculated Clusters and Coordination Environment of NH4+ crystal phase and pore system

position of acid site

method

pair of O-H atoms (H in NH4 and O in wall) with distance FAU,4 8-ring >12-ring in MOR24 or 6-ring >12-ring in FAU.28 Our finding also allows us to explain that it has been difficult to detect strong acid sites on so-called mesoporous zeolites, although many attempts have been carried out. On a flat surface or a wall with a relatively wide pore with a small curvature, b and ω should be large, resulting in low acid strength, according to the present proposal. This study deals with only the acid site generated by framework Al, and extraframework Al or other cations can vary the acid strength by different rules.2,3 This study precisely indicates that only two geometric parameters can be related to the acid strength (Eads). One of the parameters, a (AlOa bond length), relates to the local structure of the Si(OH)Al unit. The other is a combination of two parameters (b and ω), both related to the distance and relative orientations of the tetrahedra located at both ends of the Si(OH)Al Brønsted site, and in turn, this is related to the framework structure. Our results relate both findings (Eads is related to a, and a is related to b and ω), and this indicates the intricate and intertwined relationships between short-range and long-range factors influencing Brønsted acid strength in zeolites. Many studies have pointed out how both short- and long-range factors influence acid strength, and our present study has attempted to unveil in a mathematically and physically meaningful way to what extent this is true. Although this analysis requires some tedious study of what correlates and what does not, we hope that the conclusion shed some new light onto the fundamental issue of Brønsted acid site of zeolites. Conclusion Cluster models of Brønsted acid sites of various zeolites were considered in order to calculate ammonia adsorption energies (Eads) by DFT. The calculated Eads was consistent with the experimental results of the ammonia IRMS-TPD method. On the models where NH4+ is coordinated to form a bidentate in a relatively open space, a correlation was found between the Al-O distance and ammonia adsorption energy, with the shorter Al-O distance giving the higher ammonia adsorption energy. Also, the OH Mulliken charge was correlated with the Al-O distance, in agreement with a fundamental explanation that Lewis acidic Al withdraws electron charge and contributes to an increase in the acid strength of the Si(OH)Al unit. Additionally, we were able to relate local and framework geometry parameters, and this allowed us to correlate ammonia adsorption energies with parameters related to the distance (b) and planar angle (ω) between the two tetrahedra at both ends of the Si(OH)Al acid center. As a result, the following relationship between the ammonia adsorption energy and structural parameters is proposed.

Figure 10. Examples of local geometry of strong and weak acid sites: (I) Al1-O1-Si1 model of FAU with 1.967 Å and 103.8 kJ mol-1 of AlOa distance (a) and Eads, respectively; (II) Al9-O18-Si6 model of MFI with 1.824 Å and 137.3 kJ mol-1 of AlOa distance (a) and Eads, respectively. The fixed part is shown with the wire frame, and the optimized part is shown with the ball and stick model.

Eads ≈ 419 - (67b + 0.84ω)

(3)

This means that a high acid strength results when the atoms surrounding the acid site are compressed from both sides of the Si(OH)Al unit. Zeolite structures providing such a strain should generate strong acid sites. Acknowledgment. This study was partly supported by Grantin-Aids for Scientific Research from Japan Society for the Promotion of Science (C, 20560721; A, 08636016). Supporting Information Available: Analysis of relationship between Eads and local geometric parameters θ and φ, numerical analysis of relationship between the Al-O(a) distance (a) and structural parameters (b and ω), supplementary figures. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Jacobs, P. A. Catal. ReV. Sci. Eng. 1982, 24, 415. (2) Niwa, M.; Suzuki, K.; Isamoto, K.; Katada, N. J. Phys. Chem., B 2006, 110, 264. (3) Noda, T.; Suzuki, K.; Katada, N.; Niwa, M. J. Catal. 2008, 259, 203. (4) Katada, N.; Niwa, M. Catal. SurV. Asia 2004, 8, 161. (5) Katada, N.; Igi, H.; Kim, J.-H.; Niwa, M. J. Phys. Chem., B 1997, 101, 5969. (6) Suzuki, K.; Aoyagi, Y.; Katada, N.; Choi, M.; Ryoo, R.; Niwa, M. Catal. Today 2008, 132, 38. (7) Kawakami, H.; Yoshida, S.; Yonezawa, T. J. Chem. Soc., Faraday Trans. 2 1984, 80, 205.

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