Theoretical Investigation on the Spontaneous Transformation of Framework Octahedral to Tetrahedral Aluminum in Zeolites via Proton-Cation Exchange

Abstract

First-principles calculations are employed to systematically investigate the dynamic evolution from Al(O_h) to Al(T_d) in zeolites induced by proton–cation exchange (Cu⁺, Li⁺, Na⁺, NH₄⁺). The protons directly bonded to Al(O_h) are found to be essential for structural stability. Single cation exchange preserves the six-coordinated Al(O_h), while double exchange triggers spontaneous conversion to four-coordinated Al(T_d), accompanied by stepwise detachment of two water molecules. Different cations exhibit variations in spatial occupation patterns and water-binding strength. The coordination effect of metal cations and the hydrogen bonding effect of NH₄⁺ dominate the transformation of the aluminum coordination configurations. Protons directly bonded to Al(O_h) serve as strong Brønsted acid sites. Single exchange indirectly reduces the activity of adjacent protons, whereas double exchange eliminates Al–O–H bonds to stabilize Al(T_d). This work reveals a cooperative mechanism among cation species, exchange number, water binding, and electronic coupling that controls the Al(O_h) to Al(T_d) transformation, providing a theoretical basis for activating Al species and for designing high-performance catalysts with controlled acid site distributions via ion exchange.

1. Introduction

Zeolites are an important class of microporous aluminosilicate materials, finding widespread applications in catalysis, adsorption, and ion exchange due to their unique pore structure, tunable acidity, and excellent thermal stability [1,2,3,4,5]. The performance of zeolites is largely determined by the coordination state and distribution of aluminum within their framework [6,7,8]. By precisely regulating the coordination environment of aluminum, the acidity, pore accessibility, and stability of zeolites can be optimized, thereby enhancing their catalytic performance in specific reactions [9,10,11,12]. In recent years, the development of advanced characterization techniques has led to a more profound understanding of the dynamic changes in the aluminum coordination state within zeolites [13,14]. Considerable interest has arisen in the conversion of framework-associated octahedral aluminum (Al(O_h)) to tetrahedral aluminum (Al(T_d)). This dynamic transition not only reveals the tunable nature of acid sites in zeolites but also provides new insights for the design of high-performance zeolite catalysts.

Al(O_h) is a special type of aluminum species. Although it exhibits octahedral coordination, it remains connected to the zeolite framework through chemical bonds [15,16,17,18,19,20,21,22,23,24]. Bourgeat-Lami et al. [15] utilized NMR and other techniques to reveal that a portion of the aluminum in zeolite beta can exist in an octahedral coordination state under specific conditions, such as following the introduction of protons. They proposed that this octahedral aluminum is an integral part of the zeolite framework. More importantly, the authors observed that this octahedral aluminum could reversibly revert to tetrahedral coordination after ion exchange with sodium or potassium cations. The reversible transformation of framework-associated octahedral aluminum to tetrahedral aluminum in zeolites was clearly reported in zeolite Y by B. H. Wouters et al. [16]. Through the application of NMR spectroscopy, the research revealed that in mildly calcined zeolite Y, interaction with water leads to the conversion of a fraction of framework aluminum from tetrahedral to octahedral coordination, with the aluminum remaining connected to the framework. The study further demonstrated that this octahedral aluminum could be reversibly converted back to the tetrahedral form upon subsequent adsorption of ammonia. Our research group, utilizing ²⁷Al NMR experimental characterization and first-principles calculations, has revealed for the first time the detailed structure and reversible transformation mechanism of Al(O_h) [17]. The study found that the critical prerequisite for the existence of this octahedral Lewis acid site is the presence of multiple Brønsted acid sites (BAS) in its vicinity, specifically requiring three protons. When the proton concentration decreases (e.g., by increasing the Si/Al ratio or through ion exchange to convert it into a non-acidic form), the tetrahedral BAS becomes thermodynamically more stable. The above studies not only confirm the reversible transformation of the coordination state of framework aluminum in zeolites but also elucidate that its core regulatory mechanism lies in variations in the local proton concentration. We further investigated the specific manifestation of this dynamic process during typical chemical modification procedures. It was found that the proton–cation exchange process can effectively disrupt the original thermodynamic equilibrium, thereby spontaneously inducing the transformation of Al(O_h) to Al(T_d).

The transformation of Al(O_h) to Al(T_d) elicited by the proton–cation exchange has garnered considerable attention [10,13,25,26,27,28,29,30,31,32], as it offers a fundamental insight into the intrinsic tunability of acid sites in zeolites. Drake et al. [30] found that in hydrated H⁺-USY and H⁺-ZSM-5, aluminum exists in both tetrahedral and octahedral coordination. Upon dehydration, the octahedral aluminum reversibly converts back to the tetrahedral form. When H⁺ was replaced with NH₄⁺ or Cu⁺, a significant reduction in octahedral aluminum was observed, attributed to fewer water molecules associated with these cations. This directly demonstrates that H⁺ (and the water molecules it attracts) is a key factor leading to the increase in aluminum coordination number. In contrast, upon exchange with Na⁺, NH₄⁺, or Cu⁺, this proton-induced effect is weakened or reversed, thereby favoring the stabilization of aluminum in its tetrahedral form. Sartbaeva et al. [28], using high-resolution NMR and pair distribution function analysis, found that the NH₄⁺ exchange process itself can alter the local environment of some Al atoms in Na-Y and Na-A zeolites. Notably, they observed a partial transformation of aluminum from tetrahedral to octahedral coordination, which demonstrates that NH₄⁺, as an exchange cation, can directly influence the coordination state of aluminum, although the specific outcome may depend on the initial form and the experimental conditions. The research results of Lisboa et al. [26] indicate that aluminum in non-tetrahedral coordination, such as octahedral coordination, may be connected to the framework and can be reversibly restored to tetrahedral coordination through ion exchange with cations like Na⁺ and K⁺ or by adsorbing NH₃. Liu et al. [13] investigated mordenite zeolite as a model system and modulated the states of aluminum species through various methods, revealing the dynamic evolution of aluminum coordination environments in zeolites. Their study found that when NH₄-type mordenite (NH₄-MOR) is converted to proton-type mordenite (HMOR) via calcination, a portion of the aluminum species, originally present entirely in tetrahedral coordination within the zeolite framework, transforms into octahedral and distorted tetrahedral states. Accumulated experimental evidence has demonstrated that the coordination environment of Al in zeolites is strongly modulated by cation species, proton distribution and hydration conditions. Hydrated protons tend to stabilize octahedral Al, whereas the exchange of protons by cations such as NH₄⁺, Cu⁺, and Na⁺ reduce content of bound water and weakens proton-induced structural effects, thereby facilitating the stabilization and recovery of tetrahedral Al. In addition, cation exchange directly alters the local microenvironment of framework Al and triggers reversible interconversion between octahedral and tetrahedral Al, which is highly dependent on the zeolite topology and experimental conditions. Although such a reversible Al coordination transition has been widely validated across various zeolite frameworks, the underlying mechanistic details remain poorly understood. Current knowledge is primarily established on ex situ post-characterization, which only identifies the initial and final structural states rather than capturing the real-time atomic-scale dynamic evolution during coordination conversion. Specifically, how the zeolite framework accommodates geometric distortion upon ion exchange, the functional role of intermediate hydration states, and the atomic-level structural evolution accompanying the Al(O_h) to Al(T_d) transformation still lack systematic theoretical interpretation, highlighting the critical requirement for further investigations to fill this critical research gap.

In this work, the mechanism by which proton-cation exchange influences the coordination structure of Al species in zeolites will be systematically investigated. This study aims to unravel the intricate dynamic interactions among cation species, the number of exchanged protons, water binding strength, and electronic coupling effects during the Al(O_h) to Al(T_d) transformation; thereby, it provides new insights into the evolution of the Al coordination environment. By employing proton–cation exchange as a tool to regulate aluminum coordination, researchers are expected to simultaneously optimize multiple catalytic parameters.

2. Results and Discussion

2.1. Structure and Stability

The structure of the Al(O_h) in the primitive cell is shown in Figure 1a, which corresponds to one of the aluminum atoms within the Al–O–Si–O–Al pair, in which the positions of all three protons (see Figure 1a, ①, ②, and ③) are displayed and labeled as H⁺-1, H⁺-2, and H⁺-3. To more clearly illustrate the structural features, we also present the structure of Al(O_h) in the double-6-ring (D6R), as shown in Figure 1b, where one of the Al–O–Si bonds in the 6-ring is hydrolyzed, thus forming a silanol. None of the three Brønsted protons originally present in the CHA cavity remain solvated. Instead, two of these protons (H⁺-1 and H⁺-2) become firmly bonded to the framework oxygen atoms associated with the Al(O_h) site, while the third is consumed during the formation of a silanol group (H⁺-3). Consequently, the transformation from Al(T_d) to Al(O_h) results in the complete loss of the three original Brønsted acid sites. The Al in the Al(O_h) is coordinated to three framework oxygen atoms (O_f) and three water oxygen atoms (O_w) in the fac-Al(O_f)₃(H₂O)₃ arrangement (as shown in Figure 1c).

The AIMD results show that Al(O_h) exhibits superior stability, regardless of whether the temperature is 300 K or 450 K, as shown in Figure 1d,e. In the same manner as in previous reports [33], a running average was employed to obtain accurate averages of the internal energies for configurations along the AIMD trajectories. The breadth of the sampling bins was 2000 configurations per data point, and the running average traces showed good convergence between running average traces for both simulations. Reported average internal energies are given by the final 2000 configurations from the AIMD simulations (see Figure 1d,e), which ensures that no spurious contributions are collected from unequilibrated configurations at the start of the simulation. Compared with the ambient temperature of 300 K, the total internal energy of the system at 450 K increases by 92 kJ·mol⁻¹. From a kinetic perspective, this substantial energy enhancement provides a direct driving force for the water detachment process. Elevated temperature intensifies atomic thermal motion and weakens the interfacial interaction between the substrate and confined water molecules, thereby affecting the migration and detachment rates of adsorbed water. Although high temperature may kinetically promote water detachment, the overall framework of the Al(O_h) system remains intact and stable. Considering the lower intrinsic energy of the system at 300 K and the highly consistent structural evolution observed across different temperatures, subsequent discussions focus exclusively on the results obtained under 300 K conditions.

2.2. One Cation Exchange One H⁺

The effect of proton-cation exchange on the structural stability of the Al(O_h) was systematically investigated. The examined cations included Cu⁺, Li⁺, Na⁺, and NH₄⁺, all of which have been experimentally observed to readily exchange with protons and promote the transformation of the Al(O_h) to Al(T_d) [10,13,25,26,27,28,29,30,31,32]. A case of exchanging one proton with one cation was first examined, using Cu⁺ as a representative cation, and focused on the exchange of Cu⁺ with H⁺-1 or H⁺-2. The results are presented in Figure 2. The average energy calculations reveal that the system energy for Cu⁺ exchanging with the proton H⁺-2 is 20 kJ·mol⁻¹ lower than H⁺-1. The other cations exhibit similar trends: the system energies at H⁺-2 are 15 kJ·mol⁻¹, 13 kJ·mol⁻¹, and 19 kJ·mol⁻¹ lower than those at H⁺-1 for Li⁺, Na⁺, and NH₄⁺ exchanging with one H⁺, respectively (see Figure S1 for details). Upon summarizing the exchange behavior of all four cations, a key conclusion is that single proton-cation exchange exerts no influence on the coordination configuration of Al(O_h). Following 20 ps of AIMD simulations, the Al(O_h) maintains high structural stability, remaining coordinated with three O_f and three O_w.

Single proton-cation exchange at H⁺-2 yields a lower system energy; thus, only the structures at H⁺-2 were analyzed in detail (see Figure 3). The Cu⁺ is mainly adsorbed near the center of the bottom 6-ring. In addition to bonding with the oxygen atom at the original H⁺ site, the Cu⁺ also interacts with other adjacent O_f, forming Cu-O bonds of approximately 1.86–2.59 Å. Similarly, Li⁺ and Na⁺ exhibit comparable adsorption sites, forming Li-O bonds of 1.86–2.59 Å and Na-O bonds of 2.32–2.51 Å, respectively. In contrast, the adsorption site of NH₄⁺ differs from those of the other three cations. NH₄⁺ forms a hydrogen bond of 1.78 Å with the oxygen atom at the original H⁺ site. These findings demonstrate that Cu⁺, Li⁺, and Na⁺ share similar coordination environments and interact with the zeolite framework mainly via metal-oxygen coordination bonds. In contrast, NH₄⁺adopts a distinct adsorption configuration stabilized by hydrogen bonding rather than ionic coordination, reflecting a fundamentally different stabilization mechanism. Such differences arise from the ionic radii, charge densities, and chemical properties of these cations, which govern their adsorption preferences and stability after proton exchange.

2.3. Two Cations Exchange Two H⁺

The ion exchange behavior between two cations and two protons was systematically investigated. Taking Li⁺ as a representative cation, two Li⁺ ions were simultaneously exchanged with the H⁺-1 and H⁺-2 (see Figure 4). During the AIMD simulations, Al(O_h) spontaneously transformed into Al(T_d), and the reduced Al(T_d) structure exhibited higher stability, with a system internal energy reduction of 41 kJ·mol⁻¹. Specifically, the intact six-coordinated Al structure could be clearly observed at 7.5 ps. By 15 ps, the water molecules bound to the central Al interacted with the cations, where each Li⁺ coordinates to one water molecule. Consequently, only one of the three water molecules originally coordinated to the central Al remained, leading to the structural transformation from six-coordinated to four-coordinated Al. Other cations, including Cu⁺, Na⁺, and NH₄⁺, yielded analogous results, forming Al(T_d) structures with system internal energy decreases of 66 kJ·mol⁻¹, 42 kJ·mol⁻¹, and 64 kJ·mol⁻¹, respectively (see Figure S2). Notably, for Na⁺ and NH₄⁺, the spontaneous structural transformation occurred rapidly within the initial 3 ps of AIMD simulations. These results demonstrate that the cation-exchange induced Al(O_h) to Al(T_d) transformation is general, with the transformation rate depending on the cation species. The significant decrease in internal energy after the transformation indicates that the four-coordinate Al(T_d) structure is thermodynamically more favorable. These findings also provide a new route for tuning the coordination environment of Al species in zeolites.

The optimal configuration for the transformation of Al(O_h) to Al(T_d) after the exchange of two cations with two H⁺ is displayed in Figure 5. For metal cations, the cation exchanged at H⁺-2 occupies the center of the bottom 6-ring, while the cation exchanged at H⁺-1 is located in the side 4-ring (see Figure 5a–c). Among the three water molecules originally adsorbed on Al(O_h), two interact with metal cations, forming Cu–O_w, Li–O_w, and Na–O_w bonds with lengths of approximately 1.88 Å, 2.00 Å, and 2.26 Å, respectively; the resulting Al(T_d) structure is connected to three O_f and one O_w. For NH₄⁺, it does not occupy the structural center but forms hydrogen bonds of 1.77 Å and 1.86 Å with the O_f at the original H⁺ positions. Meanwhile, the two water molecules detached from Al(O_h) are linked to the residual water molecule adsorbed on Al via hydrogen bonds with lengths of 1.42 Å and 1.79 Å (see Figure 5d). Notably, during the structural transformation of Al species induced by cation exchange, inorganic metal cations (Cu⁺, Li⁺, Na⁺) and NH₄⁺ exhibit remarkably different spatial occupation modes and interfacial interaction characteristics. Inorganic metal cations share identical spatial occupation configurations, whereas their interaction strengths with the zeolite framework and adsorbed water molecules display evident quantitative discrepancies. The metal–water (M–O_w) bond lengths follow the sequence: Cu⁺ (1.88 Å) < Li⁺ (2.00 Å) < Na⁺ (2.26 Å), which directly indicates a gradual decrease in interaction intensity. This result suggests that cations with higher charge density tend to form shorter coordination bonds and possess stronger binding ability with water molecules. In stark contrast, NH₄⁺ adopts a completely different spatial distribution and interaction pattern. Rather than occupying the central sites of six-membered or four-membered rings, NH₄⁺ forms two stable hydrogen bonds with O_f at the original H⁺ exchange sites, implying strong interaction between NH₄⁺ and the zeolite framework. Moreover, the unique hydrogen bond network constructed by NH₄⁺ and water molecules provides an extra driving force for the transformation from Al(O_h) to Al(T_d). Overall, the coordination effect of metal cations and the hydrogen-bonding effect of NH₄⁺ synergistically dominate the topological reconstruction of local microstructures in zeolites. The quantitative differences in bond length and interaction strength are the fundamental reason for the distinct structural transformation behaviors induced by different cations.

The change in coordination number of the central Al atom during the transformation from Al(O_h) to Al(T_d) arises from the detachment of two water molecules. Our previous research has demonstrated that individual water molecules are firmly bound to Al(O_h) species, with an interaction energy of at least 118 kJ·mol⁻¹ [17]. In this work, we systematically investigated the binding of water molecules within the Al(T_d)_2M⁺ structure. For the four systems with different cation-exchanged forms, water molecules were sequentially removed from the zeolite cages, and the results are summarized in

Table 1. Detachment energies of individual water molecules from the Al(T_d)_2M⁺ structures. (see Equation (1)).

(kJ·mol−1) Removal of	2H+ → 2Cu+	2H+ → 2Li+	2H+ → 2Na+	2H+ →2 NH4+
water no. 1	108	113	108	107
water no. 2	124	89	85	104
water no. 3	217	140	133	121

. The detachment energies of water molecule no. 2 indicate that the binding strength of a single water molecule with the three metal cations follows the order Cu⁺ > Li⁺ > Na⁺. Cu⁺ exhibits the strongest binding due to its high polarizing ability arising from the d¹⁰ electronic configuration and partial covalent contribution. Although Li⁺ possesses the highest charge density and strong ion-dipole interactions, its lack of covalent character results in only moderate binding. In contrast, Na⁺ shows the weakest binding owing to its largest ionic radius and lowest charge density. This sequence reflects that the binding energy decreases linearly with increasing ionic radius and increases with increasing charge density. Upon exchanging 2NH₄⁺ with 2H⁺, the detachment energies of water molecules no. 1 and no. 2 indicate that approximately 104 kJ·mol⁻¹ is required to detach a single water molecule from the hydrogen-bonding network. This value is lower than the metal–water coordination detachment energies observed in the Cu⁺ and Li⁺ systems but exceeds some values in the Na⁺ system, reflecting the moderate binding capability of the NH₄⁺-induced hydrogen-bonding network. Furthermore, the detachment energy of water molecule no. 3 is significantly higher than those of water molecules no. 1 and no. 2, indicating that the four-coordinate hydrated Lewis acidic site (LAS) [34] interacts most strongly with water molecules, requiring the highest energy for water removal.

The detachment energy of water molecules from aluminum species in zeolites is closely related to their local coordination environment and is significantly influenced by the intrinsic properties of extra-framework cations. For metal cations, the binding strength with water molecules is jointly determined by the covalent contribution and charge density, whereas for NH₄⁺, the strength of the hydrogen-bonding network is the dominant factor. Moreover, the strong interaction between the four-coordinated hydrated Lewis acidic site and water molecules highlights the critical role of such active sites during the hydration and dehydration processes of zeolites. While experimental characterization can capture various intermediate Al coordination states [14,34], it remains challenging to track in real time, at atomic resolution, the dissociative adsorption of water on Al sites, the subsequent coordination transformation, and the evolution of the hydrogen-bonding network. Our theoretical calculations elucidate this dynamic pathway and provide geometric configurations and energy profiles for Al species with varying numbers of adsorbed water molecules, thereby offering a molecular level mechanistic basis for the water-induced evolution of acidic sites.

We further investigated the case where all three protons in Al(O_h) were exchanged by cations and obtained conclusions similar to those for the exchange with two cations. The calculation results represented by Cu⁺ are shown in Figure S3. Water molecules initially adsorbed on Al(O_h) interact with metal cations, thereby converting hydrated LAS from six-coordinated to four-coordinated, presenting a distinct structural evolution pathway.

2.4. Electronic Coupling

To elucidate the evolution of the electronic structure of Al(O_h) during the dynamic changes in its coordination environment induced by the progressive exchange of framework protons with cations, total and partial density of states (DOS/PDOS) analyses were performed, using NH₄⁺ as a representative. The results are presented in Figure 6. The case of metal ion exchange is presented in the Supporting Information (see Figure S4). In the fully protonated structure (top panel), the protons H⁺-1 and H⁺-2 directly coordinated to Al(O_h) exhibit intense and sharp characteristic peaks in the conduction band region (~6–8 eV). This observation confirms that H⁺-1 and H⁺-2 serve as strong Brønsted acid sites with highly localized electron-accepting orbitals, which are favorable for proton transfer reactions. In contrast, the silanol group (H⁺-3), which is not directly bonded to Al(O_h), shows negligible contribution in this energy region, consistent with its intrinsic weak acidity. After the single exchange of H⁺-2 with NH₄⁺ (middle panel), the Al center retains its octahedral coordination geometry. Meanwhile, the intense conduction band peak associated with H⁺-1 is significantly attenuated, demonstrating a pronounced electronic coupling effect between adjacent acid sites. Specifically, the exchange of H⁺-2 with NH₄⁺-2 indirectly suppresses the electronic activity of the neighboring H⁺-1, leading to a moderate reduction in the overall acidity of the system. Upon the double exchange of both H⁺-1 and H⁺-2 with NH₄⁺ (bottom panel), the direct Al-O-H bonding interactions are lost, which dismantles the octahedral coordination shell and reconstructs the Al center into a tetrahedral configuration. Consequently, the primary strong Brønsted acid sites in the system are completely eliminated, forming a stable Al(T_d) framework supported by adsorbed NH₄⁺ cations. Notably, the PDOS of the silanol proton (H⁺-3) remains almost unchanged across all three structural models. This confirms that H⁺-3 is electronically isolated from the Al(O_h) center and its local environment, and thus is not affected by the proton substitution process or the change in the Al coordination environment.

2.5. Generality

To assess the generality of our findings, we also considered the stability of framework-associated Al(O_h) species in a large-pore FAU model with Si/Al = 15 (containing one Al–O–Si–O–Al pair). The results are shown in Figure S5. Similar to the CHA case, our AIMD trajectories show that the FAU model is kinetically stable and that at least one proton bound to Al(O_h) is necessary for stabilizing the Al(O_h) species. When one of the H⁺ ions in the system was exchanged for one Na⁺, the system remained stable, with the Al(O_h) species persisting throughout the simulation and exhibiting almost no fluctuation in internal energy. When both H⁺ ions in the system were exchanged by Na⁺, the Al(O_h) spontaneously transformed to Al(T_d), and this process was accompanied by a decrease in internal energy of 47 kJ·mol⁻¹. These FAU results confirm the generality of our findings from the CHA system. The persistence of Al(O_h) upon single Na⁺ exchange demonstrates that one proton is sufficient to kinetically stabilize the octahedral moiety, regardless of zeolite topology. Conversely, double Na⁺ exchange triggers an irreversible transformation to Al(Td) indicating that the instability of fully deprotonated Al(O_h) is an intrinsic property, not a confinement effect specific to small-pore zeolites. This understanding provides a transferable framework for controlling Al coordination via ion exchange across different zeolite topologies.

3. Computational Details

3.1. Model

The model is a primitive cell of CHA zeolite, which contains 12 T sites, and the selected structure is sourced from our previously published paper [17], which is in excellent agreement with experimental results. The re-optimized unit cell parameters are: a = b = c = 9.286 Å and α = β = γ = 96.015°. The Si/Al ratio used in the calculation is 3, and the primitive cell contains 3 water molecules. We tested all possible proton sites, and the lowest energy configuration was chosen.

3.2. Methods

All theoretical calculations in this work were carried out on the basis of plane-wave density functional theory (DFT), as realized via the Vienna Ab initio Simulation Package (VASP 5.4.4) [35,36]. The generalized gradient approximation formulated by Perdew, Burke, and Ernzerhof (PBE) [37] was adopted to handle electron exchange-correlation interactions, while the D3 van der Waals dispersion correction coupled with the Becke-Johnson damping function was employed to describe weak intermolecular forces [38,39]. A plane-wave cutoff energy of 400 eV was set for the expansion of electronic wave functions. Only the gamma k-point was selected for sampling in electronic structure calculations. Structural optimization was completed through the conjugate gradient algorithm, with rigorous convergence criteria set at an energy threshold of 10⁻⁶ eV and a residual force limit of 0.01 eV/Å for all relaxed atoms. The stabilities of systems were determined based on ab initio molecular dynamics (AIMD) simulations of at least 20 ps. We extended the simulation time to 80 ps and found that once Al(O_h) spontaneously transforms into Al(T_d), the structure of the system no longer changes, and the average internal energy exhibits only minor fluctuations, as shown in Figure S6. The extended simulation yields the same conclusion as the 20 ps simulation. Therefore, for the structural transition focused on in this study, a simulation time of 20 ps is sufficient. The timestep of all AIMD simulations was set to 0.75 fs with the atomic mass of hydrogen increased to three. All AIMD simulations were performed in the NVT ensemble; the target temperatures are 300 K (ambient temperature) and 450 K (experimental hydrolysis temperature of zeolites) [17]. The temperature value is controlled by the Nosé-Hoover thermostat [40].

The energy cost of removing one water molecule from the Al(T_d)_2M⁺ species is calculated as follows:

$$E_{rem}=E_{Al\left(T_d\right)\_2M^{+}\_(n-1)H_{2}O}+E_{H_{2}O}-E_{Al\left(T_d\right)\_2M^{+}}$$

(1)

where $E_{Al\left(T_d\right)\_2M^{+}}$ are the total energies of the Al(Td) species after the exchange of two cations with two protons and $E_{H_{2}O}$ is the energy for a single isolated water molecule in the gas phase.

4. Conclusions

The dynamic evolution process of the transformation from Al(O_h) to Al(T_d) induced by proton–cation (Cu⁺, Li⁺, Na⁺, NH₄⁺) exchange in zeolites has been systematically investigated at the atomic level using first-principles calculations. It is found that in the six-coordinated Al(O_h) structure, the protons directly attached to Al(O_h) play a crucial role in maintaining structural stability. When one proton is exchanged with a cation, the Al(O_h) structure remains stable and retains its six-coordinate configuration. However, when two protons directly bonded to Al(O_h) are exchanged with cations, Al(O_h) spontaneously transforms into Al(T_d), a schematic diagram of this process is presented in Figure 7. The cation–proton exchange results indicate that the number of exchanged protons is the core determining factor, and retaining at least one proton on the Al(O_h)-O-Si oxygen is essential for maintaining the structural stability of the six-coordinated aluminum species. The transformation from Al(O_h) to Al(T_d) is accompanied by the stepwise detachment of two water molecules. Different cations exhibit distinct spatial occupation modes and water interaction characteristics. For inorganic metal cations, they share the same spatial occupation configuration, but the binding strength of water molecules with different metal cations follows the order Cu⁺ > Li⁺ > Na⁺, which is governed by the combined effects of ionic polarization, covalent contribution, and electrostatic interactions. In stark contrast, the spatial occupation mode of NH₄⁺ is completely different from that of metal cations: NH₄⁺ forms stable hydrogen bonds with the O_f at the original H⁺ exchange sites. The coordination effect of metal cations and the hydrogen-bonding effect of NH₄⁺ collectively dominate the topological reconstruction of the local zeolite framework. The electronic coupling modulates the acidity of the structure. Single exchange remotely suppresses the electronic activity of the adjacent proton, thereby reducing the acidity, whereas double exchange completely eliminates the Al-O-H bonds and converts Al to a stable Al(T_d) framework. This work establishes a complete physical picture in which the cation species, the number of exchanged protons, the water binding strength, and electronic coupling effects cooperatively control the Al(O_h) to Al(T_d) transformation. These findings not only provide a theoretical basis for the activation and reutilization of Al species in zeolites but also open up new routes for the rational design of Al active sites with tailored coordination environments via ion exchange.