Design of active sites in zeolite catalysts using modern semiempirical methods: The case of mordenite
Graphical abstract
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.
References (54)
- et al.
Dimethyl ether carbonylation over pyridine-modified MOR: enhanced stability influenced by acidity
Catal. Today
(2018) - et al.
Effect of Cu and Zn ion-exchange locations on mordenite performance in dimethyl ether carbonylation
Microporous Mesoporous Mater.
(2018) - et al.
Relationship between acid amount and framework aluminum content in mordenite
Zeolites
(1990) - et al.
Comparison of the ion exchange uptake of ammonium ion onto New Zealand clinoptilolite and mordenite
Water Res.
(2004) - et al.
On the enhancing effect of Ce in Pd-MOR catalysts for NOxCH4-SCR: a structure-reactivity study
Appl. Catal. B Environ.
(2016) - et al.
Control of released volatile organic compounds from industrial facilities using natural and acid-treated mordenites: the role of acidic surface sites on the adsorption mechanism
Chem. Eng. J.
(2014) - et al.
Rietveld refinement of several structural models for mordenite that account for differences in the X-ray powder pattern
Zeolites
(1994) Location of Brønsted sites in mordenite
Zeolites
(1997)- et al.
Acid properties of H-type mordenite studied by solid-state NMR
Microporous Mesoporous Mater.
(2011) - et al.
A periodic DFT study of the isomerization of thiophenic derivatives catalyzed by acidic mordenite
J. Catal.
(2002)
Siting of B, Al, Ga or Zn and bridging hydroxyl groups in mordenite: an ab initio study
J. Mol. Catal. Chem.
Periodic density functional investigation of Lewis acid sites in zeolites: relative strength order as revealed from NH3 adsorption
Appl. Surf. Sci.
H-MOR: Density functional investigation for the relative strength of Brønsted acid sites and dynamics simulation of NH3 protonation–deprotonation
J. Mol. Catal. Chem.
Trivalent ions modification for high-silica mordenite: a first principles study
Appl. Surf. Sci.
Structural and physico-chemical properties of high-silica mordenite
Microporous Mesoporous Mater.
Synthesis and characteristic properties of Rb-mordenite
Microporous Mesoporous Mater.
Distribution of Al and adsorption of NH3 in mordenite: a computational study
J. Fuel Chem. Technol.
A first-principles evaluation of the stability, accessibility, and strength of Brønsted acid sites in zeolites
J. Catal.
Violations of Löwenstein’s rule in zeolites
Chem. Sci.
Recent experimental and theoretical studies on Al siting/acid site distribution in zeolite framework
Curr. Opin. Chem. Eng.
Selective isomerization of n-butane over mordenite nanoparticles fabricated by a sequential ball milling–recrystallization–dealumination route
Energy Fuels
Investigations into the mechanisms of zeolite-catalyzed transalkylation of iso-propylbenzene with toluene
J. Phys. Chem. C
Activation of hydrogen and hexane over Pt, H-mordenite hydroisomerization catalysts
J. Phys. Chem. C
Reaction mechanism of dimethyl ether carbonylation to methyl acetate over mordenite – a combined DFT/experimental study
Catal. Sci. Technol.
Monocopper active site for partial methane oxidation in Cu-exchanged 8MR zeolites
ACS Catal.
Optimization of lead adsorption of mordenite by response surface methodology: characterization and modification
J. Environ. Health Sci. Eng.
Effects of exchangeable cations on the adsorption character of mordenite
Kolloid-Z. Z. Für Polym.
Cited by (9)
DFT study on zeolites’ intrinsic Brønsted acidity: The case of BEA
2024, Computational Materials ScienceInfluence of platinum on mordenite properties and catalytic activity towards cyclohexene epoxidation
2023, International Journal of Hydrogen EnergyCitation Excerpt :Enhancement of the active catalytic site is due to the higher surface area of the catalyst. Acid leaching treatment has increased the surface area of the sample [38–40]. The elimination of aluminium from the mordenite framework was validated by X-ray fluorescence investigation.
Aluminum distribution in mordenite-zeolite framework: A new outlook based on density functional theory calculations
2022, Journal of Solid State ChemistryInfluence of Al location on formation of silver clusters in mordenite
2021, Microporous and Mesoporous MaterialsCitation Excerpt :In this study, investigating the T3+T4 substitution provides the opportunity to evaluate the role of a consistent Al position in the framework on Ag0 nanocluster formation. Additionally, recent DFT studies have identified that under anhydrous conditions the Löwenstein rule can be violated and still result in thermodynamically stable zeolites [21,60]. An Al-MOR structure with simultaneous T3+T4 substitution would be challenging to synthesize, though synthesis methods have been reported to increase Al proximity in other zeolite frameworks (ZSM-5, CHA, MFI) [14].
Catalytic improvement of biomass conversion: Effect of adding mesoporosity on MOR zeolite for esterification with oleic acid
2021, Renewable EnergyCitation Excerpt :Hydrogen atoms were added to dangling bonds to stabilize the structure. There are fourteen different possibilities for introducing the Al atom into the four different T sites [42], we chose the T4 site to replace a Si atom by an Al atom according to Boronat and Corma [43]. The resulting cluster model has a total of 586 atoms with a general composition of H130O324Si129Al(OH), as shown in Fig. 1.
DFT Study of Pt Particle Growth inside β-Zeolite Cages
2023, Journal of Physical Chemistry C