Description of Si and Al Release from Aluminosilicate in the Acidic Condition Using Density Functional Theory: Protonated Terminal Oxygen

Abstract

The molecular clusters ((HO)₃Si-O-Si(OH)₃ and (HO)₃Al-O-Si(OH)₃) representative of aluminosilicate mineral surface were employed to study the dissolution of aluminosilicate in acidic condition via density functional theory (DFT) with the M06-2X+G(d,p) methodology. The surface termination sites (Si and Al) were both tetra-coordinated and the terminal oxygen was protonated in an acidic condition. In the dissolution reaction, the calculated barrier height of the six-membered ring transition state complex containing two water molecules was predicted to be 76.13 kJ/mol, lower than that of the four-membered ring transition state complex containing one water molecule. The barrier height of the reaction decreased to 6.17 kJ/mol and was 91.90% lower than that for the Si_ter-O-Si without protonation. In addition, the calculated barrier heights for Al-terminated sites were predicted to be 22.23 kJ/mol, lower than those for the Si-terminated sites, suggesting that breaking the Al-O bond is easier than the Si-O bond in the aluminosilicate mineral surface. With the fracture of Si-O and Al-O bonds, the Si and Al release from the aluminosilicate. The results indicate that the acidic condition facilitates the release of Si and Al from the aluminosilicate, and the concentration of Al leaching from the aluminosilicate is higher than the Si.

1. Introduction

The dissolution of aluminosilicate minerals in an aqueous solution occurs nearly everywhere on Earth and has an effect on a number of processes such as soil chemistry, water contaminants, and the global CO₂ cycle. Furthermore, investigating the dissolution mechanisms in an acidic condition will provide some implication for the acid activation of aluminosilicate minerals and energy saving. High sintering temperature is required in the porcelain industry as the destruction of Si-O and Al-O bonds of aluminosilicate minerals in the raw material needs high activation energy [1], which results in high energy consumption. In order to decrease the energy consumption, methods for reducing sintering temperature are investigated. Activating the raw material can destroy the Si-O and Al-O bonds in the mineral and then reduce the sintering temperature [2]. Acid activation is a common activation method [3,4,5] since the dissolution rate of the mineral increases in an acid aqueous solution. Aluminosilicate minerals (such as kaolin and feldspar) account for a large proportion in the raw ceramic material. It is therefore worth investigating the dissolution mechanism of aluminosilicate minerals. A. Steudel [6] has reported that under acidic conditions, the dissolution rate of Si and Al from kaolin and feldspar increased significantly and a large number of Si-O and Al-O bonds in mineral are destructed.

The dissolution of minerals in aqueous solutions has been studied for nearly a century. Most authors have investigated the dissolution rate law of minerals based on the concentration of Si and Al released from minerals [7,8,9]. However, the hydrolysis of minerals generally occurs in a number of steps rather than as written stoichiometrically. For example, orthoclase dissolution can be described thermodynamically by

KAlSi₃O₈ + 4H₃O⁺ → K⁺ + Al³⁺ + 3H₄SiO₄

However, the probability of an orthoclase formula unit reacting with 4H₃O⁺ molecules in one step is vanishingly small. Instead, the reaction occurs in a number of elementary steps, such as ion exchange of K⁺, the hydrolysis of individual Al-O-Si and Si-O-Si linkages, the formation of K⁺ and Al³⁺ species. Among these elementary steps, close attention has been paid to the breakage mechanisms of Al-O and Si-O bonds in the mineral surfaces. To investigate the reaction pathways, some scholars have simulated the Si-O and Al-O bond cleavage at the molecular level using quantum mechanical calculations in recent decades [10,11].

Theoretical studies have been carried out on the effect of different molecules on the clusters of (SiO)_n, (AlO₄)_n or (Al/Si-O-Si)_n [12,13,14]. C. Zhang [15] explored the stability of the Al-O/water system and Si-O/water system, using the Si-O layer (Si₆O₁₈H₁₂) or Al-O layer (Al₆O₂₄H₃₀) cluster model of kaolinite at the B3LYP. A. Rimola [16] studied the ring opening reactions of (SiO)_n ring motifs at silica surfaces by their interaction with H₂O and HCOOH by means of quantum chemical methods, adopting the B3LYP-D2/6-311++G(d,p) level of calculations for computing the energetics of the reaction; the calculation shows that the energy barriers of the ring opening reactions with H₂O is significantly lowered by the presence of an additional H₂O molecule acting through the proton relay mechanism. However, the basic structure unit of aluminosilicate minerals was not presented and the effect of the proton was also not discussed in detail.

Two typical molecular clusters (Figure 1) on aluminosilicate mineral surface were chosen for calculation. The dotted line in Figure 1 represents the surface of the mineral, and each tetra-coordinated Si or Al is terminated with hydroxyl groups. Both the Si and Al terminal sites (Si_ter and Al_ter) are connected to the bulk of mineral by a bridge oxygen atom (O_bri). Si sites on the surface is depicted by the Si_ter-O-Si to simulate the breakage of Si_ter-O bond and the release of Si. Similarly, the Al terminated sites is depicted with Al_ter-O-Si to indicate the breakage of Al_ter-O bond and the release of Al. The oxygen in the hydroxyl group attached to the Si_ter and Alte on the surface is called terminal oxygen.

In the presence of an acidic aqueous solution, the H⁺ is easily captured by the terminal oxygen on the hydroxylated aluminosilicate mineral surface for the weak space steric effect. This study focuses on the reaction pathway for hydrolyzing Si_ter-O-Si and Al_ter-O-Si, and the effect of the proton on the breakage of Si_ter-O and Alter-O bonds. The reaction rate constants were calculated by the Arrhenius equation via the barrier height computed by data in the output file. Before the investigation of the barrier height for the Si_ter-O-Si and Al_ter-O-Si surface sites, the effect of the configuration of transition state complexes containing four- and six-membered rings (Figure 2) on the activation energy is discussed.

2. Materials and Methods

2.1. Reaction Models

The most important aspect of a reaction pathway is determining the transition state complex, which has no stability in time and has no real finite concentration, by definition. Recent descriptions of quartz and aluminosilicate mineral dissolution included different transition state complexes during the hydrolysis of Al-O-Si and Si-O-Si linkage on the mineral surface [17,18]. However, the configurations of the transition state complexes were not discussed. In this paper, the four- and six-membered ring transition state complexes for the hydrolysis reaction of aluminosilicate are presented. The four-membered ring configuration contained an Si and O atom in the reaction center of Si_ter-O, and H and O from a water molecule (Figure 2a), whereas the addition of H and O from in the six-membered ring configuration (Figure 2b) came from another water molecule.

In the beginning of the hydrolysis reaction, the water molecule approached the reaction center and the coordination of Si and Al was changed from four to five. Then the Si_ter-O and Al_ter-O bonds lengthened and the coordination of Si_ter and Al_ter converted into four. Finally, the Si and Al species left away from the mineral surface and migrated to the liquid phase.

To model dissolution process for the surface reaction sites above, the following six reactions were studied. Equations (1) and (2) represent the Si_ter-O-Si (Figure 3a) and Al_ter-O-Si (Figure 3b) sites without protonated terminal oxygen, respectively. With the breakage of the Si_ter-O or Al_ter-O bond, the Si and Al species are extracted from the reaction sites as H₄SiO₄ and [Al(OH)₄]⁻, respectively. Equations (3) and (4) represent the Si_ter-O-Si (Figure 4a), and Al_ter-O-Si (Figure 4b) sites with protonated terminal oxygen, respectively (Figure 3). After the hydrolysis reaction, the protonated monosilicic acid molecule releases from the Si_ter-O-Si, and the [Al(OH)₃·H₂O] forms from the Al_ter-O-Si site. The bridge oxygen in the surface sites becomes the terminal oxygen after reaction. The charge of the molecular cluster in the Equations (1)–(4) is retained during calculation.

[(HO)₃Si_ter-O-Si(OH)₃] + H₂O → 2Si(OH)₄

(1)

[(HO)₃Al_ter-O-Si(OH)₃]⁻ + H₂O → [Al(OH)₄]⁻ + Si(OH)₄

(2)

[(H₂O)(HO)₂Si_ter-O-Si(OH)₃]⁺ + H₂O → [Si(OH)₃H₂O]⁺ + Si(OH)₄

(3)

[(H₂O)(HO)₂Al_ter-O-Si(OH)₃] + H₂O → [Al(OH)₃H₂O] + Si(OH)₄

(4)

[(H₂O)(HO)₂Si_ter-O-Si(OH)₃]⁺ + 2 H₂O → [Si(OH)₃H₂O]⁺ + Si(OH)₄ + H₂O

(5)

[(H₂O)(HO)₂Al_ter-O-Si(OH)₃] + 2 H₂O → [Al(OH)₃H₂O] + Si(OH)₄ + H₂O

(6)

2.2. Reaction Rate

Once the reactant and transition state has been optimized in the gas-phase (1 atm, 298 K) for each reaction, the frequency in output files is used to calculate the rate constant via the Arrhenius equation:

$$k=A{e}^{\frac{-\Delta {\rm E}}{RT}}$$

where $A$ is referred to as the pre-exponential factor, $\Delta E$ is the activation energy, $R$ is the gas constant and $T$ is the temperature in degrees Kelvin.

3. Computational Methods

Some articles have successfully applied the DFT method to explain the hydrolysis reaction of the silicate mineral surface [10,19,20,21]. Thus, the application of the M06-2X functional method [22] here to model the hydrolysis of aluminosilicate is reasonable. In the dissolution process, the coordination number of the Si atom and the Al atom with d orbitals and p orbitals is altered; the H atom is important and calculated. Thus, the 6-311+G(d,p) basis set is employed in this study. All calculations were performed on the reactants, products and transition state complexes given in Equations (1)–(6) with the Gaussian 09 package [23] without any constrains. The transition state was obtained by requiring that there was one and only one imaginary frequency for the configuration of the constructed cluster; otherwise, we readjusted or even redesigned the configuration to find the correct transition state configuration.

4. Results

The configurations of transition state complexes were firstly discussed. Then the hydrolysis mechanisms of Si_ter-O-Si and Al_ter-O-Si sites in an acidic condition were investigated.

4.1. The Configuration of Transition State

The hydrolysis process of Si_ter-O-Si with H₂O is shown in Figure 5 and Figure 6, distinguished by the configuration of transition state complexes (

Table 1. Selected geometry parameters of the transition state complex containing a four-membered ring (TS4) and a six-membered ring (TS6). Bond Lengths in Å, Bond Angles in Degrees.

Model	Si-Obri	Siter-Obri	Obri-H	1Si-10O	1Si-16O	Siter-Obri-Si
TS4	1.68016	1.80964	1.11736	1.84944	--	141.62679
TS6	1.62984	1.84868	1.49800	--	1.72330	127.61374

). When the Si_ter-O-Si surface site reacted with a H₂O molecule (Figure 5), the transition state complex contained a four-membered ring. Along the reaction path, the H in the water molecule was absorbed by the bridge O and the rest of water molecule (OH) was captured by the Si_ter, while simultaneously, the Si_ter-O bond fractured. The barrier height of this reaction (labelled as R_f) was 121.11 kJ/mol. When the Si_ter-O-Si surface site reacted with two H₂O molecules (Figure 6), six-membered ring complexes of transition state formed. Along this reaction path, the H in the H₂O beside the bridge O side bonded with the bridge O and the OH in the other H₂O near the Si_ter was captured by the Si_ter. In addition, a new water molecule formed with the rest of H and OH from different H₂O molecules. The barrier height of this reaction (labelled as R_s) was 76.13 kJ/mol.

According the Arrhenius equation, the rate constant of R_s was 7.60 × 10⁷ time of the R_f. The addition of the H₂O insignificantly affects the activation energies. The lower activation energy and larger rate constant of R_f is possibly attributed to the weaker bond strength of Si_ter-O in the reaction of the six-membered ring. Compared with the experimental data for the hydrolysis activation energy of quartz, only containing Si-O-Si site, which was 67~89 kJ/mol [24,25,26], the barrier height of six-membered ring transition state was reasonable. Thus, the six-membered ring configuration of the transition state was appropriate and is used in the following text.

4.2. Hydrolysis Reaction at Siter-O-Si Site

The energy profiles of Si_ter-O-Si with protonated terminal oxygen reacting with two H₂O molecules are shown in Figure 7 and

Table 2. Selected geometry parameters of the transition state of Si_ter-O-Si with protonated terminal oxygen reacting with two H₂O molecules (TS6-1). Bond Lengths in Å, Bond Angles in Degrees.

Model	Si-Obri	Siter-Obri	Obri-H	21Si-16O	Siter-Obri-Si
TS6-1	1.65078	1.75820	1.52743	1.71474	129.06299

. After the breakage of the Si_ter-O bond on the mineral surface, the Si species was left as [Si(OH)₃(H₂O)]. The barrier height of the reaction decreased to 6.17 kJ/mol and was 91.90% lower than that for the Si_ter-O-Si without protonation. The rate constant of this hydrolysis reaction was 1.81 × 10¹² times of that of the Si_ter-O-Si without protonated terminal oxygen. The proton significantly affected the electron cloud of the terminal oxygen and the bond strength of Si-O, resulting in the lower barrier height. This indicated that the proton facilitates the fracture of the Si_ter-O bond on the mineral surface, corresponding to the experimental data that the dissolution rate of Si release from quartz and feldspar increased in the acidic condition [27,28].

4.3. Hydrolysis Reaction at Al_ter-O-Si Site

The energy profiles of hydrolysis reaction at Al_ter-O-Si site are shown in Figure 8 and

Table 3. Selected geometry parameters of the transition state of Al_ter-O-Si site (TS6-2) and Al_ter-O-Si site with protonated terminal oxygen reacting (TS6-3) with two H₂O molecules. Bond Lengths in Å, Bond Angles in Degrees.

Model	Si-Obri	Alter-Obri	Obri-H	Alter-16O	Alter-15O	Alter-Obri-Si
TS6-2	1.64804	1.97997	1.16485	1.89625	--	120.14675
TS6-3	1.63210	1.94006	1.25021	--	1.83037	123.71870

. The barrier height for the Al_ter-O-Si without protonation was 22.23 kJ/mol. The results suggest that the Al_ter-O-Si site was easier to hydrolyze than the Si_ter-O-Si site. This may be attributed to the higher bond energy and symmetry of Si_ter-O-Si, which needed higher energy to break.

Figure 9 and Table 3 show the energy profile and hydrolysis mechanism of Al_ter-O-Si with protonated terminal oxygen. The barrier height was −0.86 kJ/mol. The negative activation energy suggested that the intermediate existed with a lower energy than the transition state [29]. The two reactants formed an intermediate with lower energy, due to the strong hydrogen bond before the transition state formed. These results led to the same conclusion, that the protonated site was easier to hydrolyze than the unprotonated site.

Compared to barrier height for the hydrolysis of Si_ter-O-Si clusters, the barrier height for the hydrolysis of Al_ter-O-Si clusters was lower, due to the longer chemical bond of Al_ter-O_bri, and the chemical bond strength was weakened. This implies that the Al_ter-O-Si site reacted more easily with H₂O, which indicates the hydrolysis reaction may take place preferentially at the Al_ter-O-Si site on the aluminosilicate mineral.

5. Conclusions

The Si_ter-O-Si and Al_ter-O-Si clusters representative of aluminosilicate mineral surface were employed to study hydrolysis reactions in an acidic condition via density functional theory with the M06-2X+G(d,p) methodology. The transition state complex containing a six-membered ring for Si_ter-O-Si linkage yielded barrier heights of 76.13 kJ/mol, which was lower than 121.11 kJ/mol for the complex containing a four-membered ring and was closer to the experimental data of 67~89 kJ/mol for the hydrolysis activation energy of quartz.

In the hydrolysis reaction, the calculated barrier heights of Si_ter-O-Si and Al_ter-O-Si clusters without protonated terminal oxygen molecule were 76.13 and 22.23 kJ/mol, respectively; the calculated barrier heights of Si_ter-O-Si, and Al_ter-O-Si clusters with protonated terminal oxygen molecule were 6.71 and −0.86 kJ/mol, respectively. It was obvious that the barrier height decreased as the terminal oxygen was protonated and the hydrolysis activation energy of Al_ter-O was lower than that of Si_ter-O before and after protonation. The barrier height of the three surface sites was ranged as: Al_ter-O-Si < Si_ter-O-Si. With the fracture of the Si_ter-O and Al_ter-O bonds, the Si and Al were released from the aluminosilicate. These results confirm that the proton facilitates the release of Si and Al, and the dissolution of Al was easier than Si in the aluminosilicate mineral surface.