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Chemisorption of aluminum chloride on the surface of amorphous silica
Materials Today: Proceedings xxx (xxxx) xxx
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Chemisorption of aluminum chloride on the surface of amorphous silica Alsu Yusupova a, Alexey Khatsrinov a, Lenar Shafigullin b,⇑ a b
Kazan National Research Technological University, K. Marksa Street 68, Kazan, Republic Tatarstan 420015, Russian Federation Kazan Federal University, Sjujumbike Street 10a, 1 Naberezhnye Chelny, Republic Tatarstan 420008, Russian Federation
a r t i c l e
i n f o
Article history: Received 8 May 2019 Accepted 2 July 2019 Available online xxxx Keywords: Aluminum chloride Polysulfide aluminum silicate
a b s t r a c t The paper shows that modification of amorphous silica with aluminum chloride leads to an increase in number of active sites on the surface of amorphous silica. The physical chemical and quantum chemical studies showed the chemical interaction between sulfur and aluminum (attached to the surface of amorphous silicate), as well as oxygen and silicon (a part of amorphous filler) using the donor-acceptor mechanism. It was shown that it was more efficient from the thermodynamic point of view when sulfur was added using the donor-acceptor mechanism due to vacant d-orbitals in the amorphous silica-aluminum chloride system. Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Modern Trends in Manufacturing Technologies and Equipment 2019.
1. Introduction There is a known method for changing the properties of surfaces, specifically for silica and silicates, by treating them with chlorides and organochlorine derivatives of some elements. In the case of silica derivatives, this method is based on the ability of surface hydroxyl groups to react with the corresponding chlorine substances. Thus, when AlCl3 reacts with silica gel at a high concentration of OH-groups, together with polysilicic acid, it forms supramolecular oxychloride (SiO2)xO(AlCl2)2, and at a low concentration – (SiO2)xOAlCl2 [1,2]. Therewith, the active sites with vacant d-orbitals form on the surface of silica gel. Sulfur’s electron configuration is 3s23p43d0. The presence of lone sulfur pairs defines its activating ability under the influence of electrophiles, such as AlCl3 [3]. After opening of sulfur rings, the polysulfide radicals are formed, and we assume that they can interact with the active surface of the modified silica gel by the donor-acceptor mechanism due to vacant d-orbitals on the surface and lone sulfur pairs. Molecular layering in silica gel was successful for a wide range of substances [2]. This method for activating the surface of amorphous silica with aluminum chloride was used in the paper. It assessed the possibility of interaction between hydroxyl groups of amorphous silica and molecules of aluminum chloride, their ⇑ Corresponding author. E-mail address: [email protected] (L. Shafigullin).
attachment to the surface, and the formation of an active layer with vacant d-orbitals. It studied the physicochemistry and contribution of active sites on the surface of amorphous silica to an increase in sulfur reactivity and formation of aluminum silicate polysulfides.
2. Experimental For modification of amorphous silica and further production of inorganic aluminum silicate polysulfides, the following components were used: sulfur (S), which was the waste from the Nizhnekamsk oil refinery with 99.98% wt. of base material (GOST 127-93); aluminum chloride (GOST 3759-75); silica-containing rock from the Dobrinskoe site, Saratov oblast (mineral composition, % wt.: 78 ± 7 of opal-cristobalite, 7 ± 2 of zeolite, 5 of montmorillonite). The composite materials were studied using the methods of physical chemical analysis. The IR spectral studies were performed with a Vector 22 FTIR spectrometer (Bruker, Germany) (4000– 300 cm 1) and SPECORD 75 IR spectrometer. The samples were prepared by depositing on KBr pellets. The X-ray studies were carried out with a DRON 3 diffractometer using monochromatic Cu K-alpha radiation. The electron paramagnetic resonance studies were performed with a RE-1306 radio spectrometer (at frequencies of 9100 and 9370 MHz) at 77 and 300 K, respectively. Single-crystal ruby with Cr3+ was used as an
https://doi.org/10.1016/j.matpr.2019.07.044 2214-7853/Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Modern Trends in Manufacturing Technologies and Equipment 2019.
Please cite this article as: A. Yusupova, A. Khatsrinov and L. Shafigullin, Chemisorption of aluminum chloride on the surface of amorphous silica, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.07.044
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A. Yusupova et al. / Materials Today: Proceedings xxx (xxxx) xxx
internal standard. The spectra were recorded in three scanning ranges of the external magnetic field. The quantum chemical calculations mentioned in the paper were made with Gaussian 98 (hybrid B3LYP method and semiempirical PM3 method) and Priroda 6 (density functional GGA PBE method) application software [4]. The unrestricted HartreeFock method was used to estimate radical decomposition reactions and study the structure of biradical transit state. It involved mixing of the highest occupied and lowest unoccupied molecular orbitals for singlet biradicals at the start of calculations. Transition structures and reaction barriers were evaluated to find the most efficient mechanism of interaction between sulfur and modified amorphous silica. The Gaussian software algorithms of relaxed scan (Scan) or quadratic synchronous transit (QST2) were used to define initial geometry of the transit states. The existence of the transit state in the studied process was verified by steps down the reaction path from the transit states to the reactants and products.
3. Results and discussion There is a well-known technique - molecular layering which is used to produce some catalysts for organic synthesis and absorbents [3,5]. It involves chemisorption of chlorides of p- and d-block elements on the surface of amorphous silica in the inert carrier gas. In this case, the modifying agent (metal chlorides) interacts primarily with hydroxyl groups of the silica surface. The chemisorption mechanism depends on the concentration of hydroxyl groups on the surface. According to the molecular layering technique [2,3], the temperature of silica heat treatment should not exceed 100–200 °C for surface dehydroxylation because higher temperatures change the structure of a material. Due to a high activity of amorphous silica, its usage allows us to initiate chemical interaction in the sulfur-silica system. For this purpose, we ensured activation of the silicate component with aluminum chloride. A series of advanced physico-chemical analysis and theoretical studies was used to analyze the activation of the surface of amorphous silica with aluminum chloride and chemical interaction between the activator and amorphous silica. The results of IR spectroscopic studies are shown in Fig. 1. After the amorphous silica was modified with aluminum chloride the triplet was observed in the area 2850–2950 cm 1. It indicates that new chemical bonds in the system and active sites were formed when a temperature increased up to 400–500 °C. The X-ray diffraction analysis showed that crystallinity of the sulfur samples with aluminum chloride was 61 per cent, and without it – 69 per cent. A lower crystallinity of the sample with the modified amorphous silica indicates that a portion of crystalline sulfur was used to form a covalent bond with aluminum, silicon, and oxygen of the filler, and create X-ray amorphous compounds. The samples of amorphous silica were studied using the method of electron paramagnetic resonance (EPR). The electron-hole centers detected with EPR method represent the defects of the crystalline structure of the studied objects. The chemical modification with aluminum chloride results in an intended change in the chemical properties of the amorphous silica surface. The vacant d-orbitals in the resulting amorphous silica-aluminum chloride system are electron-hole centers of the surface. Modification with aluminum chloride (AlCl3 5%) leads to a higher number of electron-hole centers (up to 96 arb. un.). Therefore, we get the material with catalytic properties. The results of physical chemical studies allow us to presume that the resulting samples have high physical mechanical proper-
Fig. 1. IR spectra of aluminum chloride (1), amorphous silica (2), samples of amorphous silica modified with 5% of AlCl3 at different temperatures of heat treatment: 200 °C (3); 500 °C (4); and sulfur composite material based on amorphous silica with 5% of AlCl3 (500 °C) (5).
ties due to the chemical interaction between sulfur and aluminum (attached to the surface of amorphous silica), as well as oxygen and silicon (a part of amorphous filler) by the donor-acceptor mechanism [6,7]. In order to get a deeper insight into the mechanisms of the system processes and confirm the formation of new chemical bonds S–S, Si–O–S, Si–O–Al, Si–O–Al–S, the quantum chemical studies were performed. The important prerequisite for successful quantum chemical studies is a correct choice of calculation method, primarily, a method of electron correlation measurement and used basis set. First, various quantum chemical methods were tested for proper transfer of energy behavior with broken bonds in different molecules. The second test was to define the transfer accuracy for a geometrical structure. Analysis of the determined thermodynamic properties of the compounds shows that values closest to experimental evaluations are provided with Priroda (3z.bas) [3] and B3LYP 6-31G(d). But it should be noted that the evaluation with eB3LYP 6-31G(d) method gave the closest values. However, in most cases there is no significant difference between them and Priroda data. So, the Priroda and B3LYP 6-31G(d) data can be used for qualitative evaluation of molecule geometry and energies. For studies of polyatomic systems, the Priroda software is more preferable as the calculations do not take a lot of computing time. The literature describes various types of heterolytic transformations involving the sites on the surface of primary silica [4]. We studied the two most likely ways of interaction between sulfur and amorphous silica: substitution of hydroxyl groups on silicon and insertion to oxygen of hydroxyl group. These two mechanisms of interaction between sulfur and silica were analyzed with Priroda software for quantum chemical calculations. The dissociation energy and lengths of Si-S, O-S bonds were calculated, the results are shown in Table 1.
Please cite this article as: A. Yusupova, A. Khatsrinov and L. Shafigullin, Chemisorption of aluminum chloride on the surface of amorphous silica, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.07.044
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A. Yusupova et al. / Materials Today: Proceedings xxx (xxxx) xxx Table 1 Changes in dissociation energies and bond lengths due to sulfur addition to silica. Number of sulfur atoms in the chain
S1 S2 S3 S4 S5 S8
Addition of sulfur to silicon
Addition of sulfur to oxygen
Energy of Si-S bond, kJ/mol
Length of Si-S bond, pm
Energy of O-S bond, kJ/mol
Length of O-S bond, pm
383.09 294.57 291.21 292.29 290.33 292.46
217 211 219 219 218 219
290.20 253.84 247.82 238.24 236.10 236.56
171 172 172 172 171 171
According to the calculations, the strongest bonds are formed by one or two sulfur atoms. With a higher number of sulfur atoms in the chain, the bond energy (Si-S, O-S) lowers and becomes stable. Stronger bonds are formed when sulfur is added to silicon atom (substitution of hydroxyl group). Nucleophilic substitution and addition can be distinguished among the reactions involving the functional groups of the amorphous silica surface. It is the reaction which involves attack by a nucleophile (sulfur radical) and nucleophilic substitution of OH– group, or nucleophilic addition (insertion to oxygen). Transition structures and reaction barriers were evaluated to analyze the most efficient mechanism of interaction between sulfur and surface of amorphous silica. The search for a transit state started from the structure suggested by chemical intuition and analysis of enthalpy differences in formation of the reactants and products, i.e. reaction heat (in case of exothermic reaction, the transit state geometry should be closer to reactants, in different case, – to products [8,9]). In the forward search of a transition state, the Baker’s eigenvectorfollowing algorithm was used. Also, the quadratic synchronous transit algorithm (QST2) was used to generate automatically a starting structure for a transition state based upon the reactants and products [10]. All the extreme points on the potential energy surface are identified as minimums or transit states, with the calculation of vibration frequencies. The existence of the transition state in the studied process is verified by steps down the reaction path to the reactants and products [11]. It was assessed if it was possible to insert sulfur to oxygen atom, to add sulfur to silicon atom with the substitution of O–H group, and to add to amorphous silica modified with aluminum chloride. According to the findings of our studies, cross-linking of amorphous silica by disulfide fragments shows greater stability. That’s why, it is more pertinent to assess the activation energy of similar reactions for diatomic sulfur. The search for a transition state for triplet and singlet sulfur was performed. The reaction of diatomic sulfur (singlet) insertion to an oxygen atom is endothermic (8.36 kJ/mol). In the transition state, O–H bond lengthens to 121.4 pm, and O–S and S–H bonds shorten to 185.7 and 173.0 pm respectively. The activation energy is 133.76 kJ/mol. The insertion of triplet sulfur proceeds endothermically (149.06 kJ/mol). The activation energy is 166.91 kJ/mol. The addition of diatomic sulfur (triplet) to a silicon atom with substitution of OH-group proceeds endothermically (267.02 kJ/mol). In the transition state, Si–O bond lengthens to 223.8 pm, and Si–S bond shortens to 221.0 pm. The activation energy of the nucleophilic substitution is 261.33 kJ/mol. The reaction of singlet sulfur insertion to a silicon atom is endothermic (24.46 kJ/mol). In the transition state, Si–O bond lengthens to 189.4 pm, and Si–S, S–O bonds shorten to 251.5 and 189.6 pm. The activation energy is 147.55 kJ/mol. The more stable and thermodynamically efficient (with regard to activation energy) compounds are formed as a result of the insertion of singlet sulfur to oxygen (Eact = 133.76 kJ/mol), and
insertion of singlet sulfur to a silicon atom (Eact = 147.55 kJ/mol). The products of these reactions create tight valence bonds between sulfur and atoms of oxygen (280.3 kJ/mol) and silicon (310.4 kJ/mol), which are responsible for synthesis of strong sulfur composite materials. From the point of view of energy, the insertion of diatomic sulfur (M = 1) to oxygen and silicon is more efficient. The potential barrier is 134–148 kJ/mol. Thus, the strength of a composite material is achieved by insertion reactions with oxygen and silicon. The addition of diatomic sulfur to the surface of amorphous silica modified with aluminum chloride proceeds exothermically and without activation. The addition reaction of triplet sulfur is thermodynamically stable. Sulfur is added to aluminum using the donor-acceptor mechanism, lengths and energies of the bonds that form the compounds are shown in Fig. 2.
Fig. 2. Diagram of sulfur addition to the surface modified with aluminum chloride.
Please cite this article as: A. Yusupova, A. Khatsrinov and L. Shafigullin, Chemisorption of aluminum chloride on the surface of amorphous silica, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.07.044
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A. Yusupova et al. / Materials Today: Proceedings xxx (xxxx) xxx
Fig. 3. Lengths and energies of the bonds in the biradical sulfur chain (1), cross-linking of the sulfur chain by a silicon atom (2), by an oxygen atom (3), and by aluminum (4).
The strongest bond is Al–S bond, which is formed with one sulfur atom. With a higher number of sulfur atoms in the chain, it weakens and becomes stable (from 70.30 to 43 kJ/mol), and its length increases to 250.9 pm. No bond weakening is observed in the sulfur chain when sulfur interacts with aluminum chloride (Fig. 3). The weak bonds between sulfur and aluminum are responsible for electron density redistribution and stronger bonds in the sulfur chain. This is illustrated by the diagram in Fig. 3, which shows the energy and geometry of the sulfur chain for different crosslinking mechanisms. In sulfur biradical S4, as well as after cross-linking with molecule S4 by atoms of silicon and oxygen, S2–S3 bond weakens dramatically in comparison with S1-S2 bond by 76, 87, 112 kJ/mol respectively, and upon interaction with aluminum it remains sufficiently strong (the bond weakens by 29 kJ/mol).
It was assessed how aluminum chloride contributed to the weakening of the bonds in rings S4, S8. S–S bonds, that are closest to aluminum, lengthen from 216.0 to 216.8 pm, and from 209.7 to 231.4 pm in rings S4, S8. The energy of S–S bond (213.4 pm) in ring S8, which is attached to aluminum, decreases by 27.59 kJ/mol. Aluminum chloride contributes to a decrease in opening energy for sulfur ring S4 by 10 kJ/mol. We can therefore say that aluminum chloride encourages the destabilization of the rings and triggers their opening. 4. Conclusions Therefore, modification of amorphous silica with aluminum chloride leads to an increase in the number of active sites on the surface of amorphous silica and opening of sulfur rings, as well as production of a compact and solid structure. By starting a rearrangement, we encourage the removal of inner molecules S2, S4, S6
Please cite this article as: A. Yusupova, A. Khatsrinov and L. Shafigullin, Chemisorption of aluminum chloride on the surface of amorphous silica, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.07.044
A. Yusupova et al. / Materials Today: Proceedings xxx (xxxx) xxx
and prevent the reverse processes of formation of long sulfur chains. The chemical interaction between sulfur and atoms of oxygen and silicon of the amorphous silica surface results in the formation of solid material with high density, strength, and resistance to aggressive environments. Aluminum chloride which modifies the amorphous silicate surface initiates opening of sulfur rings. References [1] G.V. Lisichkin, Modified Silica in Sorption, Catalysis and Chromatography, Nauka Publ, Moscow, 1986 (In Russian). [2] V.B. Aleskovsky, Supramolecular Chemistry, SPbGU Publ., Saint Petersburg, 1996 (In Russian). [3] G.V. Lisichkin, A.Ya. Yuffa, Heterogeneous Metal-complex Catalysts, Khimiya Publ, Moscow, 1981 (In Russian). [4] D.N. Laykov, YuA. Ustynyuk, Russ. Chem. Bull. 3 (2005) 804–810 (In Russian).
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[5] [5] V.I. Roldugin, Surface physicochemistry, Moscow, 2008, (In Russian). [6] A.A. Yusupova, A.I. Khatsrinov, R.T. Akhmetova, Activating effect of aluminum chloride in the preparation of sulfur concrete from sulfur and silica, Inorg. Mater. 54 (8) (2018) 809–814. [7] A.A. Yusupova, A.A. Bobryshev, A.A. Treschev, Development of sulfur and silicon dioxide activation method in the sulfur concrete technology, materials engineering and technologies for production and processing IV, Solid State Phenomena 284 (2018) 1114–1118, https://doi.org/10.4028/www.scientific. net/SSP.284.1114. [8] I.V. Aristov, A.G. Shamov, Herald of Kazan Technological University, vol. 18, No. 21 (2015) pp. 5-9 (In Russian). [9] L.A. Gribov, S.P. Mushtakova, Qunatum Chemistry, Gardariki Publ, Moscow, 1999 (In Russian). [10] A. Frisch, J.B. Foresman, Exploring Chemistry with Electronic Structure Methods, second ed., Gaussian Inc, Pittsburgh PA, 1996. [11] R.M. Minyaev, Gradient lines on multidimensional potential energy surfaces and chemical reaction mechanisms, Uspekhi Khimii 63 (11) (1994) 939–961 (In Russian).
Please cite this article as: A. Yusupova, A. Khatsrinov and L. Shafigullin, Chemisorption of aluminum chloride on the surface of amorphous silica, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.07.044
Contents lists available at ScienceDirect
Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr
Chemisorption of aluminum chloride on the surface of amorphous silica Alsu Yusupova a, Alexey Khatsrinov a, Lenar Shafigullin b,⇑ a b
Kazan National Research Technological University, K. Marksa Street 68, Kazan, Republic Tatarstan 420015, Russian Federation Kazan Federal University, Sjujumbike Street 10a, 1 Naberezhnye Chelny, Republic Tatarstan 420008, Russian Federation
a r t i c l e
i n f o
Article history: Received 8 May 2019 Accepted 2 July 2019 Available online xxxx Keywords: Aluminum chloride Polysulfide aluminum silicate
a b s t r a c t The paper shows that modification of amorphous silica with aluminum chloride leads to an increase in number of active sites on the surface of amorphous silica. The physical chemical and quantum chemical studies showed the chemical interaction between sulfur and aluminum (attached to the surface of amorphous silicate), as well as oxygen and silicon (a part of amorphous filler) using the donor-acceptor mechanism. It was shown that it was more efficient from the thermodynamic point of view when sulfur was added using the donor-acceptor mechanism due to vacant d-orbitals in the amorphous silica-aluminum chloride system. Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Modern Trends in Manufacturing Technologies and Equipment 2019.
1. Introduction There is a known method for changing the properties of surfaces, specifically for silica and silicates, by treating them with chlorides and organochlorine derivatives of some elements. In the case of silica derivatives, this method is based on the ability of surface hydroxyl groups to react with the corresponding chlorine substances. Thus, when AlCl3 reacts with silica gel at a high concentration of OH-groups, together with polysilicic acid, it forms supramolecular oxychloride (SiO2)xO(AlCl2)2, and at a low concentration – (SiO2)xOAlCl2 [1,2]. Therewith, the active sites with vacant d-orbitals form on the surface of silica gel. Sulfur’s electron configuration is 3s23p43d0. The presence of lone sulfur pairs defines its activating ability under the influence of electrophiles, such as AlCl3 [3]. After opening of sulfur rings, the polysulfide radicals are formed, and we assume that they can interact with the active surface of the modified silica gel by the donor-acceptor mechanism due to vacant d-orbitals on the surface and lone sulfur pairs. Molecular layering in silica gel was successful for a wide range of substances [2]. This method for activating the surface of amorphous silica with aluminum chloride was used in the paper. It assessed the possibility of interaction between hydroxyl groups of amorphous silica and molecules of aluminum chloride, their ⇑ Corresponding author. E-mail address: [email protected] (L. Shafigullin).
attachment to the surface, and the formation of an active layer with vacant d-orbitals. It studied the physicochemistry and contribution of active sites on the surface of amorphous silica to an increase in sulfur reactivity and formation of aluminum silicate polysulfides.
2. Experimental For modification of amorphous silica and further production of inorganic aluminum silicate polysulfides, the following components were used: sulfur (S), which was the waste from the Nizhnekamsk oil refinery with 99.98% wt. of base material (GOST 127-93); aluminum chloride (GOST 3759-75); silica-containing rock from the Dobrinskoe site, Saratov oblast (mineral composition, % wt.: 78 ± 7 of opal-cristobalite, 7 ± 2 of zeolite, 5 of montmorillonite). The composite materials were studied using the methods of physical chemical analysis. The IR spectral studies were performed with a Vector 22 FTIR spectrometer (Bruker, Germany) (4000– 300 cm 1) and SPECORD 75 IR spectrometer. The samples were prepared by depositing on KBr pellets. The X-ray studies were carried out with a DRON 3 diffractometer using monochromatic Cu K-alpha radiation. The electron paramagnetic resonance studies were performed with a RE-1306 radio spectrometer (at frequencies of 9100 and 9370 MHz) at 77 and 300 K, respectively. Single-crystal ruby with Cr3+ was used as an
https://doi.org/10.1016/j.matpr.2019.07.044 2214-7853/Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Modern Trends in Manufacturing Technologies and Equipment 2019.
Please cite this article as: A. Yusupova, A. Khatsrinov and L. Shafigullin, Chemisorption of aluminum chloride on the surface of amorphous silica, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.07.044
2
A. Yusupova et al. / Materials Today: Proceedings xxx (xxxx) xxx
internal standard. The spectra were recorded in three scanning ranges of the external magnetic field. The quantum chemical calculations mentioned in the paper were made with Gaussian 98 (hybrid B3LYP method and semiempirical PM3 method) and Priroda 6 (density functional GGA PBE method) application software [4]. The unrestricted HartreeFock method was used to estimate radical decomposition reactions and study the structure of biradical transit state. It involved mixing of the highest occupied and lowest unoccupied molecular orbitals for singlet biradicals at the start of calculations. Transition structures and reaction barriers were evaluated to find the most efficient mechanism of interaction between sulfur and modified amorphous silica. The Gaussian software algorithms of relaxed scan (Scan) or quadratic synchronous transit (QST2) were used to define initial geometry of the transit states. The existence of the transit state in the studied process was verified by steps down the reaction path from the transit states to the reactants and products.
3. Results and discussion There is a well-known technique - molecular layering which is used to produce some catalysts for organic synthesis and absorbents [3,5]. It involves chemisorption of chlorides of p- and d-block elements on the surface of amorphous silica in the inert carrier gas. In this case, the modifying agent (metal chlorides) interacts primarily with hydroxyl groups of the silica surface. The chemisorption mechanism depends on the concentration of hydroxyl groups on the surface. According to the molecular layering technique [2,3], the temperature of silica heat treatment should not exceed 100–200 °C for surface dehydroxylation because higher temperatures change the structure of a material. Due to a high activity of amorphous silica, its usage allows us to initiate chemical interaction in the sulfur-silica system. For this purpose, we ensured activation of the silicate component with aluminum chloride. A series of advanced physico-chemical analysis and theoretical studies was used to analyze the activation of the surface of amorphous silica with aluminum chloride and chemical interaction between the activator and amorphous silica. The results of IR spectroscopic studies are shown in Fig. 1. After the amorphous silica was modified with aluminum chloride the triplet was observed in the area 2850–2950 cm 1. It indicates that new chemical bonds in the system and active sites were formed when a temperature increased up to 400–500 °C. The X-ray diffraction analysis showed that crystallinity of the sulfur samples with aluminum chloride was 61 per cent, and without it – 69 per cent. A lower crystallinity of the sample with the modified amorphous silica indicates that a portion of crystalline sulfur was used to form a covalent bond with aluminum, silicon, and oxygen of the filler, and create X-ray amorphous compounds. The samples of amorphous silica were studied using the method of electron paramagnetic resonance (EPR). The electron-hole centers detected with EPR method represent the defects of the crystalline structure of the studied objects. The chemical modification with aluminum chloride results in an intended change in the chemical properties of the amorphous silica surface. The vacant d-orbitals in the resulting amorphous silica-aluminum chloride system are electron-hole centers of the surface. Modification with aluminum chloride (AlCl3 5%) leads to a higher number of electron-hole centers (up to 96 arb. un.). Therefore, we get the material with catalytic properties. The results of physical chemical studies allow us to presume that the resulting samples have high physical mechanical proper-
Fig. 1. IR spectra of aluminum chloride (1), amorphous silica (2), samples of amorphous silica modified with 5% of AlCl3 at different temperatures of heat treatment: 200 °C (3); 500 °C (4); and sulfur composite material based on amorphous silica with 5% of AlCl3 (500 °C) (5).
ties due to the chemical interaction between sulfur and aluminum (attached to the surface of amorphous silica), as well as oxygen and silicon (a part of amorphous filler) by the donor-acceptor mechanism [6,7]. In order to get a deeper insight into the mechanisms of the system processes and confirm the formation of new chemical bonds S–S, Si–O–S, Si–O–Al, Si–O–Al–S, the quantum chemical studies were performed. The important prerequisite for successful quantum chemical studies is a correct choice of calculation method, primarily, a method of electron correlation measurement and used basis set. First, various quantum chemical methods were tested for proper transfer of energy behavior with broken bonds in different molecules. The second test was to define the transfer accuracy for a geometrical structure. Analysis of the determined thermodynamic properties of the compounds shows that values closest to experimental evaluations are provided with Priroda (3z.bas) [3] and B3LYP 6-31G(d). But it should be noted that the evaluation with eB3LYP 6-31G(d) method gave the closest values. However, in most cases there is no significant difference between them and Priroda data. So, the Priroda and B3LYP 6-31G(d) data can be used for qualitative evaluation of molecule geometry and energies. For studies of polyatomic systems, the Priroda software is more preferable as the calculations do not take a lot of computing time. The literature describes various types of heterolytic transformations involving the sites on the surface of primary silica [4]. We studied the two most likely ways of interaction between sulfur and amorphous silica: substitution of hydroxyl groups on silicon and insertion to oxygen of hydroxyl group. These two mechanisms of interaction between sulfur and silica were analyzed with Priroda software for quantum chemical calculations. The dissociation energy and lengths of Si-S, O-S bonds were calculated, the results are shown in Table 1.
Please cite this article as: A. Yusupova, A. Khatsrinov and L. Shafigullin, Chemisorption of aluminum chloride on the surface of amorphous silica, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.07.044
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A. Yusupova et al. / Materials Today: Proceedings xxx (xxxx) xxx Table 1 Changes in dissociation energies and bond lengths due to sulfur addition to silica. Number of sulfur atoms in the chain
S1 S2 S3 S4 S5 S8
Addition of sulfur to silicon
Addition of sulfur to oxygen
Energy of Si-S bond, kJ/mol
Length of Si-S bond, pm
Energy of O-S bond, kJ/mol
Length of O-S bond, pm
383.09 294.57 291.21 292.29 290.33 292.46
217 211 219 219 218 219
290.20 253.84 247.82 238.24 236.10 236.56
171 172 172 172 171 171
According to the calculations, the strongest bonds are formed by one or two sulfur atoms. With a higher number of sulfur atoms in the chain, the bond energy (Si-S, O-S) lowers and becomes stable. Stronger bonds are formed when sulfur is added to silicon atom (substitution of hydroxyl group). Nucleophilic substitution and addition can be distinguished among the reactions involving the functional groups of the amorphous silica surface. It is the reaction which involves attack by a nucleophile (sulfur radical) and nucleophilic substitution of OH– group, or nucleophilic addition (insertion to oxygen). Transition structures and reaction barriers were evaluated to analyze the most efficient mechanism of interaction between sulfur and surface of amorphous silica. The search for a transit state started from the structure suggested by chemical intuition and analysis of enthalpy differences in formation of the reactants and products, i.e. reaction heat (in case of exothermic reaction, the transit state geometry should be closer to reactants, in different case, – to products [8,9]). In the forward search of a transition state, the Baker’s eigenvectorfollowing algorithm was used. Also, the quadratic synchronous transit algorithm (QST2) was used to generate automatically a starting structure for a transition state based upon the reactants and products [10]. All the extreme points on the potential energy surface are identified as minimums or transit states, with the calculation of vibration frequencies. The existence of the transition state in the studied process is verified by steps down the reaction path to the reactants and products [11]. It was assessed if it was possible to insert sulfur to oxygen atom, to add sulfur to silicon atom with the substitution of O–H group, and to add to amorphous silica modified with aluminum chloride. According to the findings of our studies, cross-linking of amorphous silica by disulfide fragments shows greater stability. That’s why, it is more pertinent to assess the activation energy of similar reactions for diatomic sulfur. The search for a transition state for triplet and singlet sulfur was performed. The reaction of diatomic sulfur (singlet) insertion to an oxygen atom is endothermic (8.36 kJ/mol). In the transition state, O–H bond lengthens to 121.4 pm, and O–S and S–H bonds shorten to 185.7 and 173.0 pm respectively. The activation energy is 133.76 kJ/mol. The insertion of triplet sulfur proceeds endothermically (149.06 kJ/mol). The activation energy is 166.91 kJ/mol. The addition of diatomic sulfur (triplet) to a silicon atom with substitution of OH-group proceeds endothermically (267.02 kJ/mol). In the transition state, Si–O bond lengthens to 223.8 pm, and Si–S bond shortens to 221.0 pm. The activation energy of the nucleophilic substitution is 261.33 kJ/mol. The reaction of singlet sulfur insertion to a silicon atom is endothermic (24.46 kJ/mol). In the transition state, Si–O bond lengthens to 189.4 pm, and Si–S, S–O bonds shorten to 251.5 and 189.6 pm. The activation energy is 147.55 kJ/mol. The more stable and thermodynamically efficient (with regard to activation energy) compounds are formed as a result of the insertion of singlet sulfur to oxygen (Eact = 133.76 kJ/mol), and
insertion of singlet sulfur to a silicon atom (Eact = 147.55 kJ/mol). The products of these reactions create tight valence bonds between sulfur and atoms of oxygen (280.3 kJ/mol) and silicon (310.4 kJ/mol), which are responsible for synthesis of strong sulfur composite materials. From the point of view of energy, the insertion of diatomic sulfur (M = 1) to oxygen and silicon is more efficient. The potential barrier is 134–148 kJ/mol. Thus, the strength of a composite material is achieved by insertion reactions with oxygen and silicon. The addition of diatomic sulfur to the surface of amorphous silica modified with aluminum chloride proceeds exothermically and without activation. The addition reaction of triplet sulfur is thermodynamically stable. Sulfur is added to aluminum using the donor-acceptor mechanism, lengths and energies of the bonds that form the compounds are shown in Fig. 2.
Fig. 2. Diagram of sulfur addition to the surface modified with aluminum chloride.
Please cite this article as: A. Yusupova, A. Khatsrinov and L. Shafigullin, Chemisorption of aluminum chloride on the surface of amorphous silica, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.07.044
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Fig. 3. Lengths and energies of the bonds in the biradical sulfur chain (1), cross-linking of the sulfur chain by a silicon atom (2), by an oxygen atom (3), and by aluminum (4).
The strongest bond is Al–S bond, which is formed with one sulfur atom. With a higher number of sulfur atoms in the chain, it weakens and becomes stable (from 70.30 to 43 kJ/mol), and its length increases to 250.9 pm. No bond weakening is observed in the sulfur chain when sulfur interacts with aluminum chloride (Fig. 3). The weak bonds between sulfur and aluminum are responsible for electron density redistribution and stronger bonds in the sulfur chain. This is illustrated by the diagram in Fig. 3, which shows the energy and geometry of the sulfur chain for different crosslinking mechanisms. In sulfur biradical S4, as well as after cross-linking with molecule S4 by atoms of silicon and oxygen, S2–S3 bond weakens dramatically in comparison with S1-S2 bond by 76, 87, 112 kJ/mol respectively, and upon interaction with aluminum it remains sufficiently strong (the bond weakens by 29 kJ/mol).
It was assessed how aluminum chloride contributed to the weakening of the bonds in rings S4, S8. S–S bonds, that are closest to aluminum, lengthen from 216.0 to 216.8 pm, and from 209.7 to 231.4 pm in rings S4, S8. The energy of S–S bond (213.4 pm) in ring S8, which is attached to aluminum, decreases by 27.59 kJ/mol. Aluminum chloride contributes to a decrease in opening energy for sulfur ring S4 by 10 kJ/mol. We can therefore say that aluminum chloride encourages the destabilization of the rings and triggers their opening. 4. Conclusions Therefore, modification of amorphous silica with aluminum chloride leads to an increase in the number of active sites on the surface of amorphous silica and opening of sulfur rings, as well as production of a compact and solid structure. By starting a rearrangement, we encourage the removal of inner molecules S2, S4, S6
Please cite this article as: A. Yusupova, A. Khatsrinov and L. Shafigullin, Chemisorption of aluminum chloride on the surface of amorphous silica, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.07.044
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and prevent the reverse processes of formation of long sulfur chains. The chemical interaction between sulfur and atoms of oxygen and silicon of the amorphous silica surface results in the formation of solid material with high density, strength, and resistance to aggressive environments. Aluminum chloride which modifies the amorphous silicate surface initiates opening of sulfur rings. References [1] G.V. Lisichkin, Modified Silica in Sorption, Catalysis and Chromatography, Nauka Publ, Moscow, 1986 (In Russian). [2] V.B. Aleskovsky, Supramolecular Chemistry, SPbGU Publ., Saint Petersburg, 1996 (In Russian). [3] G.V. Lisichkin, A.Ya. Yuffa, Heterogeneous Metal-complex Catalysts, Khimiya Publ, Moscow, 1981 (In Russian). [4] D.N. Laykov, YuA. Ustynyuk, Russ. Chem. Bull. 3 (2005) 804–810 (In Russian).
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[5] [5] V.I. Roldugin, Surface physicochemistry, Moscow, 2008, (In Russian). [6] A.A. Yusupova, A.I. Khatsrinov, R.T. Akhmetova, Activating effect of aluminum chloride in the preparation of sulfur concrete from sulfur and silica, Inorg. Mater. 54 (8) (2018) 809–814. [7] A.A. Yusupova, A.A. Bobryshev, A.A. Treschev, Development of sulfur and silicon dioxide activation method in the sulfur concrete technology, materials engineering and technologies for production and processing IV, Solid State Phenomena 284 (2018) 1114–1118, https://doi.org/10.4028/www.scientific. net/SSP.284.1114. [8] I.V. Aristov, A.G. Shamov, Herald of Kazan Technological University, vol. 18, No. 21 (2015) pp. 5-9 (In Russian). [9] L.A. Gribov, S.P. Mushtakova, Qunatum Chemistry, Gardariki Publ, Moscow, 1999 (In Russian). [10] A. Frisch, J.B. Foresman, Exploring Chemistry with Electronic Structure Methods, second ed., Gaussian Inc, Pittsburgh PA, 1996. [11] R.M. Minyaev, Gradient lines on multidimensional potential energy surfaces and chemical reaction mechanisms, Uspekhi Khimii 63 (11) (1994) 939–961 (In Russian).
Please cite this article as: A. Yusupova, A. Khatsrinov and L. Shafigullin, Chemisorption of aluminum chloride on the surface of amorphous silica, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.07.044