Design of active sites in zeolite catalysts using modern semiempirical methods: The case of mordenite

https://doi.org/10.1016/j.comptc.2019.112572Get rights and content

Highlights

  • Modern semiempirical methods dftb2 and pm7 were used for modeling of mordenite.

  • Stability of mordenite with single and double Al substitution was studied.

  • Acid strength for Brønsted sites was determined.

  • Several mordenite samples were identified with very high acid strength and catalytic activity.

Abstract

Zeolites are widely used for numerous processes for production of a vast number of chemicals, fuels and commercial goods. Preparation of zeolite catalysts that have improved selectivity for the desired products, operate at lower temperature and possess increased stability is therefore of great interest. The key to such improved zeolite catalysts is in the design of active sites and facilitation of mass transfer via optimization of the porous structure. At the same time, undesirable sites that inhibit desirable properties of the active sites need to be removed or blocked. The strength and structure of either the Brønsted or Lewis acid sites, directly determines their catalytic activity and selectivity for each reaction. In the present study, the structure and acidity of active sites in zeolites are investigated for the example of mordenite using modern semiempirical methods pm7 and scc-dftb (dftb2). Models AlHSi95O192 and Al2H2Si94O192 are used for Brønsted acid sites and Al2Si94O191 for Lewis acid sites. In agreement with previous studies, the stability of T1, T2, T3 and T4 sites is similar. Many different configurations of pair-wise located Al atoms were studied. In the present work it was found that some of the pair-wise located Al atoms possess Brønsted acid sites with strength much higher than that for single Brønsted acid sites. However, since their stability is not the highest among other double sites, special preparation methods need to be developed for selectively obtaining these very active sites. The stability of different Lewis acid sites is also considered.

Introduction

Mordenite is industrially important zeolite that is used for a wide variety of applications, such as catalytic hydrocarbons processing [1], [2], [3], organic synthesis [4], [5], carbonylation of dimethyl ether [6], conversion of methanol to gasoline [7] and natural gas to methanol [8], removal of water [9] and air pollutants via adsorption [10], [11], and selective catalytic reduction [12]. Mordenite is one of the zeolites most widely found in volcanic sediments on the Earth. Such kind of natural zeolites can be used in pristine or activated forms for the removal of pollutants from water [11] and air [13].

Mordenite can have a Si/Al ratio in the range from 5.0 to 50 [7]. The structure of mordenite has been subject of many studies since the first report of its XRD characterization interpretation had appeared in 1961 [14]. It was deduced that the mordenite space group is Cmc21. However, it was not until 1994 that major differences between the experimental and theoretical models were attributed to stacking faults along the c-axis, and its space group was assigned to Cmmm [15]. Mordenite has four symmetry different tetrahedral sites occupied by Si or Al and designated T1, T2, T3 and T4. There are ten different positions occupied by oxygen atoms that are designated from O1 to O10 [16].

Mordenite possesses three types of channels: large 12-membered ring (12-MR) channels that extend along c-axis, narrow 8-membered ring (8-MR) channels along c-axis, and so called side pocket 8-membered ring (SP8-MR) short channels along b-axis that are perpendicular to c-axis and interconnect 12-MR channels [17]. There are two varieties of mordenite, small-port and large port mordenite that exhibit very different behavior in the adsorption of molecules. While large-port mordenite can adsorb relatively large molecules like benzene even in its Na-form, small-port mordenite cannot adsorb molecule of size above 4.9 Å in Na-form but can in NH4- and H-form [10]. Naturally found mordenite demonstrates small-port character while synthetically obtained mordenite is a large-port zeolite [10]. The cause for this different adsorption behavior is not well understood. It was suggested that amorphous material present in mordenite partially blocks channels and prevents adsorption of large molecules in small-port mordenite. Indeed, extraframework Al in various amounts is detected using MAS 27Al NMR spectroscopy in initial and dealuminated mordenite samples as octahedrally coordinated Al while framework Al is known to be tetrahedrally coordinated [7].

The exact localization of framework Al and associated Brønsted and Lewis catalytically active acid sites has been subject of intensive research by diverse experimental and computational methods. Loewenstein suggested in 1954 a rule called by his name that two Al atoms cannot be in the adjacent sites but have to be separated by at least one site occupied by Si [18]. Alberti [19] suggested framework O2 and O7 as the Brønsted sites in the 12MR channel and O9 in the 8MR channel on the basis of analysis of experimental structural data. However, this assignment could not be considered conclusive. One and two-dimensional NMR techniques were used to determine the Si atom neighborhood and Brønsted sites structure which produce different signals for different number of Al neighbors but cannot determine the sites [16], [20]. It is often believed that the ratio of T1:T2:T3:T4 sites is 18:10:43:29 [21]. However, direct evidences for this are absent. The next-nearest-neighbor (NNN) model was suggested to explain the distribution of acid sites according to which Brønsted acid strength depends on the number of Al atoms located around each single SiO4 site. An increase in the number of neighbor Al sites decreases Brønsted acid strength. Mordenite’s ideal structure contains 36 nonequivalent NNN locations [22].

Due to the numerous applications of mordenite in multi-million ton industrial catalytic processes [23], [24], determination of preferred positions for catalytically active sites has remained a very important task for constructing more active, selective and stable catalysts that function at a lower temperature. The knowledge of the mordenite structure is also important for the adequate modeling of chemical reactions [25] and adsorption processes [26] proceeding over its surface. A large number of theoretical computational studies were devoted to mordenite structure. Table 1 summarizes the major results of such previous studies with an accent to location of acid sites.

Table 1 shows that there is no agreement on the most probable location of framework aluminum atoms and their acidity was not studied. This can be explained at least partially by the strong sensitivity of computational results to the size of computational models for mordenite [39]. Larger models favor T3 and T4 locations for framework Al atoms while other models suggest different sorts of distributions.

During the last decade, advanced semiempirical methods dftb and pm7 have been developed on the basis of expanded experimental and higher level computational methods, and these methods often demonstrated very good precision and promising results [40], [41]. In the present computational investigation, we employ these modern semiempirical methods to determine the most stable Brønsted and Lewis acid sites and acid strength for Brønsted sites that serve as catalytic sites for numerous industrially important reactions. We found that some Brønsted sites configurations have much higher acidity compared to other sites and these very strong acid sites can serve as very active sites for a variety of acid-catalyzed reactions.

Section snippets

Methods

The present investigation employs two types of models for zeolite mordenite. The first type of the model was a cluster model consisting of 1 × 1 × 2 unit cells with all terminal oxygen atoms saturated with hydrogen atoms. The chemical formula of this cluster model is Si96O224H64. The second type of the model was a supercell slab model periodically repeated in 3D space. The slab consisted also of 1 × 1 × 2 unit cells and its chemical formula is Si96O192. The structure of the starting zeolite

Results and discussion

It was pointed out in the Introduction and showed in Table 1 that slightly different computational models and methods generated very different orders of stability for Al sites in mordenite. Therefore, it was considered very important to utilize (1) large set of models of equivalent Al substitution sites; (2) cluster and periodic boundary conditions models; (3) two independent methods pm7 and scc-dftb; (4) statistically averaged results in order to obtain the values hopefully close to the truth.

Conclusions

The present computational study considers single and double Brønsted and Lewis acid sites in mordenite using moderately sized models containing 96 Si atoms before substitutions with Al employing modern semiempirical methods pm7 and scc-dftb. Major results of the study are as follows.

  • 1.

    The anionic PBC models computed with pm7 method predicts the following order of stability for singly Al substituted slabs: T4 > T2 > T3 > T1.

  • 2.

    The cluster model computed with scc-dftb predicts the following order of

Declaration of Competing Interest

There are no conflicts involved in the presented research.

Acknowledgements

The work was supported by a grant from CONICYT FONDECYT/Regular 1170694.

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