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Reactive Site Model of the Reduction of SO on Graphite Eduardo Humeres, Nito Angelo Debacher, Regina de Fátima Peralta Muniz Moreira, J. Arturo Santaballa, and Moises L. Canle J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b03787 • Publication Date (Web): 21 Jun 2017 Downloaded from http://pubs.acs.org on June 24, 2017

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The Journal of Physical Chemistry

Reactive Site Model of the Reduction of SO2 on Graphite

Eduardo Humeres [http://orcid.org/0000-0002-9740-2756],*, a Nito Angelo Debacher [http://orcid.org/0000-0002-8809-8457],a Regina de F. P. M. Moreira [http://orcid.org/0000-0002-2863-7260],b J. Arturo Santaballa [http://orcid.org/0000-0003-0593-8009],*, c Moisés Canle.c [http://orcid.org/0000-0002-4814-7795]

a

Departamento de Química and bDepartamento de Engenharia Química e Engenharia de

Alimentos, Universidade Federal de Santa Catarina, 88040-670 Florianópolis, SC, Brazil c

Universidade da Coruña, Facultade de Ciencias & CICA, Grupo Reactividad Química e Fotorreactividade, E-15071 A Coruña, Spain.

* Corresponding authors Eduardo Humeres. Tel: +55 48 3282 1078 (home). Email: [email protected] J. Arturo Santaballa. Tel: +34 881012195. Email: [email protected] [email protected]

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Abstract Computational quantum chemistry calculations were carried out for the reduction of SO2 on graphite to produce elemental sulfur and CO2. Two models of the reactive site of graphite were used and a viable mechanism was proposed for the reaction pathways based on experimental results and known reactions. The SO2 OO approach to the zigzag edge of the model cluster yielded a sulfur oxidized intermediate 1,3,2-dioxathiolane 1. Sulfur transfer step takes place from 1 to a neighbor benzyne site forming a reduced sulfur intermediate thiirene 2 along with a 1,3-dicarbonyl in equilibrium with the peroxide valence tautomer. The calculated barrier from 1 to thiirene 2 at 900 oC was 39.4, kcal·mol-1. The peroxide tautomer isomerizes to a dioxirane intermediate that is eliminated as dioxicarbene to produce CO2. The total free energy of activation for the decarboxylation reaction at 900 oC was in the range 110.8 - 122.7 kcal·mol-1 depending on the model (∆G‡experimental, 114.3 kcal·mol-1). The reduced intermediate thiirene 2 decomposes through a transport mechanism where polysulfane species with increasing number of sulfur atoms eleminate elemental Sx.


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INTRODUCTION As a consequence of the fast development of surface characterization techniques a remarkable growth of solid surface chemistry has occurred and the discipline has become one of the frontiers area of chemistry.1 Carbon surface has been particularly studied because the importance of carbon-based nanomaterials. Surface structure and chemistry of polyaromatics as graphite, graphene, fullerenes and nanotubes needs appropriate models to understand the origin the surface reactivity. The model of graphite is extracted from an infinite solid where the nature of the limits raises the issue on boundaries saturation. Hydrogen has been used to terminate the boundaries although is not enough to achieve theoretical calculations close to experimental data. Accordingly, non-hydrogen saturated carbons were proposed as the most liable atoms to react (active sites).2 These sp2-hybridized carbons crucially define the nature and concentration of these edge sites.3 Consequently, individual graphene layers are used as graphite models, where the main surface functionalities are the free dehydrogenated sites at the zigzag (carbene type) and armchair edges (carbyne type).2,3 Chemisorption of NO and N2O were analyzed considering both, the zigzag and armchair edges,4 as well as the reliability to describe the multi radicalary nature of the zigzag edge.5 X-ray diffraction (XRD) and transmission electron microscopy of several anthracites produced consistent experimental results showing that graphene edge sites are not completely saturated with hydrogen.6,7 Depending on the model structure, carbenes may have a triplet or a singlet electronic ground state.8 Zigzag and armchair sites present different reactivity 9,10 while the stability of the carbene and carbyne structures increases with increasing size of the graphene sheet, due to higher resonance energy contributions.11 3 ACS Paragon Plus Environment


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Some studies of organic chemistry of solid surfaces allowed to postulate mechanisms such as the dissociation of saturated C–H bonds12 and the reactivity of alkyl halides13-18 on solids. Surface chemistry of semiconductors, as Si, Ge, and C, has been recently studied because of the important applications of these elements in microelectronics ,19-25 and the need of understanding the surface chemistry involved.26 By the other hand, important solid-gas reaction mechanisms are unknown. For instance, it is surprising that despite the technological relevance the carbon-oxygen reactions, it is still unknown how CO2 is formed from the oxidation of carbon materials.27,28 In order to postulate the reaction mechanisms in which these materials are involved, it is critical to have a realistic models of edge reactivity.29,30 The reduction of SO2 on carbons, producing sulfur and CO2, is an important solidgas reaction that allows the elimination of SO2 from flue gases.31,32 The reaction can also be used to activate carbon particles for further functionalization.33,34 The X-ray photoelectron spectrum (XPS) of the residual carbon, after the reaction, showed two forms of sulfur bound to the matrix: non-oxidized sulfur (at 164 eV) and oxidized sulfur (at 168 eV).35-41 These forms of sulfur represent the reactive intermediates of the reduction of SO2

42

and

have been characterized in detail with respect to their specific reactivity to thiols, amines, alkyl halides and alcohols.33,34,43 The reduction of SO2 on different carbons (graphite, charcoal, and cokes) presents the same stoichiometry (eq 1), similar entropies of activation of the decarboxylation reaction32,42 and the same selective reactivity of the intermediates for the thiolysis and aminolysis reaction,33,34,43 leading to the conclusion that the mechanisms involved are essentially similar, except for some differences due to the properties of the carbon matrix such as the oxidation state.


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SO2 + C → CO2 + 1/2 S2

(1)

Theoretical calculations using pyrene and dehydrogenated derivatives as models of active sites of graphite,44 showed that the exothermic chemisorption of SO2 is most favorable to occur on the triplet biradical zigzag edge. Carbene-like zigzag sites have been characterized as intermediates in organic reactions45 and are considered as active sites of carbon materials.3 The carbyne-type armchair site of the pyrene model showed an enthalpy of adsorption in the range of -5 to -51 kcal mol-1, while the adsorption on the zigzag edge was the most favorable, with energies of –61 to –100 kcal mol-1, consequently at 900 oC the zigzag edge was estimated to be fully covered with SO2 molecules.44 It is known that the covalent functionalization of graphite and exfoliated graphene sheets occurs preferentially at the edges rather than in the domains of the basal plane46-48 because the chemisorption of SO2 on basal fused aromatic rings occurs with high positive energy.44 The SO2 molecules are inserted in the biradical zigzag edge of the surface by a SO2 OO approach that produces a [3+3]cycloaddition forming a six-membered ring 1,3,2dioxathiolane 1, or a OS approach producing a [2+3]cycloaddition generating a fivemembered ring 1,2-oxathietene 2-oxide or γ-sultine 2; both species are in equilibrium (at 900 oC, ∆Go = 1-2 kcal·mol-1).44,49 The S approach is not considered because is endothermic and therefore, it is not a viable process.49 These species constitute the oxidized sulfur intermediates observed at 168 eV in the XPS spectrum. Similar viable species of SO2 chemisorption on graphite were reached when models of graphite with unsaturated zigzag edge sites and four, six, and seven fused benzene rings were used in the calculations.49 The



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interactions of the dehydrogenated edge sites and SO2 play a significant role in the reaction process. Selected models of graphite are two-dimensional graphene clusters with some dehydrogenated edge sites because they yield parameters in excellent agreement with experimental data.49 Adjacent layers would produce second-order effects and larger clusters with periodic boundary conditions do not improve the general results. The postulated primary mechanism of the SO2 reduction on carbons considers the above results shown in Figure 1.50,51 The by-products of the reaction observed for some carbons (CO, COS, CS2) were shown to be due to consecutive reactions of these primary products. 50

Sulfur transport route Sx

O

S

+ C(CO2)

O S 3

1 SO2 O O

S

CO2 Decarboxylation route

2 Figure 1. Primary mechanism of the reduction of SO2 on carbons.

In Figure 1 the zigzag diradical fragment represents the active site of carbon matrix where SO2 is inserted. Sulfur transfer from the oxidized intermediate 1 produces the reduced sulfur intermediate episulfide 3, observed at 164 eV, by insertion on an unsaturated 6 ACS Paragon Plus Environment

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center along with a corresponding oxidized moiety defined as C(CO2), that will produce CO2 in a later step. Episulfide 3 initiates the transport of sulfur out the carbon matrix.52 The mechanism of CO2 formation has not been studied yet and will be discussed in this work. For this reason, the fragment of the residual carbon matrix in Figure 1 is not described. The reactive site model. The primary mechanism shown in Figure 1 indicates that there are two active sites involved in the reaction. One active site forms the oxidized intermediates 1 and 2 inserted on a zigzag edge, and the second allows the formation of the reduced sulfur intermediate episulfide 3 from the insertion of sulfur on a carbyne armchair center as will be discussed below. The relationship between both sulfurized sites depends on their properties. When the reaction with SO2 reached the steady state, the residual carbon reacted with CO2 to produce only SO2 through the reverse reaction42 that requests the transformation of the episulfide into the oxidized intermediates. This condition is supported by the observed easy interconversion of oxidized and reduced sulfur intermediates43 that also suggests that both sites are physically close each other. This geometrical proximity was shown by the selectivity of thiolysis and aminolysis towards the intermediates. Thiolysis occurs only on the oxidized intermediate while aminolysis occurs only on the episulfide.33,34,43 A sample containing both intermediates reacted with an alkyl aminothiol inserting the amino group on the episulfide followed by the intramolecular thiolysis with double functionalization of the carbon matrix.43 These considerations lead to a reduction reaction site where the oxidized sulfur site is close to the reduced sulfur site. The simplest geometry is a structure such as benzoanthracene where the reactive site model, based on the primary mechanism, is a 1,2,11,12-tetradehydrogenated-benzo[α]anthracene (TBA) with two dehydrogenated edge 7 ACS Paragon Plus Environment


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reactive sites: a 11,12-diradical zigzag edge, where SO2 is inserted, and an armchair 1,2benzyne where the episulfide would be inserted. As will be shown below, this geometry requires a six-membered intermediate for the sulfur transfer while the transfer from anthracene, the simplest model of a polycyclic aromatic hydrocarbon, would need a strain five-membered ring.

2 1 11

3

12

4

10 9

5 8

7

6

TBA model

In this work ab initio molecular orbital calculations on the possible pathways of the reduction of SO2 on carbons were performed, that were consistent with the experimental results that we have obtained so far. The mechanism includes the transfer, reduction and elimination of sulfur, and the generation of the site that eliminates CO2 in the last step. This was accomplished using computational quantum chemistry on a polycyclic aromatic hydrocarbon molecule as reactive site model of graphite.

THEORETICAL METHODS The study of the minimum of the structures and transition states with 1,2,11,12tetradehydrogenated-benzo[α]anthracene (TBA model) was carried out with the model chemistry B3LYP/6-311G(d) as implemented in the G09/b1 Gaussian suite of programs.53,54 In spite of its deficiencies B3LYP was used as it is probably the one of the most successful functionals, and the 6-311G(d) basis set due to its reasonable 8 ACS Paragon Plus Environment

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computational cost. Following previous work44 graphite structure was chosen to keep the model as simple as possible.

We have also used an embedded two-layer QM/MM ONIOM model for computational studies of a molecular system involving TBA model plus five crown aromatic rings (TBA+5 model), where carbons and hydrogens not participating in the reaction region were treated by molecular mechanics, while those atoms directly involved in the “reactive site” were calculated quantum mechanically. This model was also used to better simulate graphite structure.

TBA+5 model

High-level-theory layer was treated at the B3LYP/6-311G(d) level, and the outer, low-level-theory layer with the UFF force field. ONIOM optimization was carried out with a quadratic coupled algorithm, option QuadMacro, to better take into account the coupling between atoms in the QM layer and those in the MM layer. Electronic embedding was used to get a better description of the electrostatic interaction between the QM and MM regions.



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UB3LYP/6-311G(d) was the model chemistry for triplet state calculations. All Gibbs free energy values are at 900ºC. All stationary points were characterized as either minima or transition state structures (TS) by computing the vibrational harmonic frequencies for each level of theory. TS were properly characterized by one imaginary frequency, whereas minima have only real frequencies.

RESULTS AND DISCUSSION The selective insertion of the sulfur intermediates using nonthermal plasma and thermal reaction51 and their easy interconversion,43 without formation of CO2, showed that the SO2 reduction on carbons occurs through two main reaction paths, decarboxylation and desulfurization, with different energy demand. The decarboxylation route. The addition of SO2 on the diradical carbene is similar to the first step of the mechanism of ozonolysis of alkenes to form a molozonide or 1,2,3trioxolane,55,56 that by a retro-1,3-cycloaddition produces a carbonyl + carbonyl oxide, but in this case the flip of the carbonyl to produce a 1,2,4-trioxolane cannot occur in the ozonization of the rigid carbon matrix.57 The dioxathiolane ring should open heterolytically with a positive charge on the sulfur atom because of the difference in electronegativity between sulfur and oxygen (Figure 2). Subsequently, the sulfur cation would undergo an electrophilic attack on a properly situated benzyne-type, the 1,2-dehydrogenated-benzene site of TBA, which is very reactive toward electrophiles. Quantum chemical calculations showed a large change of reactivity between cyclic alkynes and arynes and their stable, linear counterparts. Strained cycloalkynes and arynes typically react as electrophiles and they behave as diradicals or dicarbenes because bending of triple bonds reduces orbital 10 ACS Paragon Plus Environment

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overlap and increases reactivity.58 This mechanism is described in Figure 2 through steps 23-4. The sulfur transfer from 1 to the benzyne site produces a 1,2-thiirene along with a peroxide 4a in equilibrium with the valence tautomer 11,12-dicarbonyl 4b. The O-S ring opening of the 1,2-oxathiolane intermediate 3 would produce the aryl episulfide intermediate or 11,12-dehydrogenated-benzo[α]anthracene-1,2-thiirene. When ozone substitutes SO2 in the reaction described in Figure 2, it would produce in step 4 an aryl epoxide, or oxirene, instead of the thiirene, but the same peroxide as with SO2.59



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S+

1 O-

O

S S

Ib

O

O

S O-

O

O

O

TS1 2

3

3

S O

Ia

O

TS2

S O

3

O

4a

O

O

S

O

S

O

S O

O

TS3 S

5a

6

O

O

4b

CO2

S

+ 7

Figure 2. Decarboxylation mechanism of the reduction of SO2 on carbons. TBA model as reactive site model.


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According to this reactive model the oxidized moiety defined as C(CO2) in Figure 1 is

a

peroxide

that

corresponds

to

the

tetradehydrogenated-benzo[α]anthracene-

cyclopentaperoxide 4a in equilibrium with its valence tautomer tetradehydrogenatedbenzo[α]anthracenequinone 4b, an aryl 1,3-dicarbonyl. Only 1,3-dicarbonyls from βketoesters or β-diketones have been studied and applied to the synthesis of polycyclic heterocycles60 and geminal diazides.61 The bicyclic valence-isomeric congener 4a is stabilized by aromaticity and conjugation, but is destabilized by the steric strain of the fivemembered ring, and because the more electronegative O-heteroatoms, the more stable form is the quinone 4b (eq 2).62 Tautomer 4b is expected to be more stable than orthobenzoquinone because of its insertion in a highly resonance stabilized polycyclic aromatic cluster.3,63 S O

S

O

O

O

4a

4b

(2)

The functionalization of graphite with episulfide and peroxide groups creates the sites from where the desulfurization and decarboxylation routes proceed independently. In Figure 2 species 4 to 6 are represented including the 1,2-thiirene sulfur because under steady-state conditions (carbon + SO2 flow), the oxidized and non-oxidized sulfur intermediates stay in equilibrium.42,43 Step 4 to 5 define a plausible pathway of isomerization of the valence tautomers 4a-4b to form a dioxirane 5a (Figure 3).



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S O

O

5

5a

Figure 3. Peroxy radical intermediate 5 could close as a dioxirane 5a (TBA+5 model)

The C-O ring breaking of 4a produces a peroxy radical intermediate 5 that could close as a dioxirane 5a. It is known that the ring of the phenyl dioxetanyl radical can open to form a phenylperoxy radical that close to form a dioxiranyl ring; there are 21.8 kcal·mol-1 of free energy preference for closure to form the dioxiranyl rather than the dioxetanyl ring, with loss of aromaticity in the ring, even when the unpaired electron is delocalized over the π system of the sp2 carbons in the ring.64 The isomerization of carbonyl oxides to dioxiranes is also thermodynamically favorable, although it involves a substantial activation barrier.65-67 Because of the endothermicity of its formation (118.9 kcal·mol-1, Figure 4), dioxirane 5a should be very short-lived. The C-O breaking is the only productive path of 4a because the O-O bond fission would lead back to tautomer 4b. However, a concerted isomerization is also possible because the high energy involved in breaking the C-O bond.


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G, kcal.mol-1

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+ TS3

T = 900 oC

14.8 (14.5)

5a

1

110.8 (122.7)

1b

+ TS1+

21.7 3

69.2

6

39.4

34.8

2 0.0 47.3

3 3.3 (5.4)

158.1 (132.8)

118.9 (135.0)

1a

19.6 (21.4)

+ TS2 74.1

3.7 (5.8)

4a

4b 7

Figure 4. Free energy profile of the decarboxylation reaction. 2, dioxathiolane intermediate; 3, oxathiolane intermediate; 4, peroxide intermediate; 5a, dioxirane; 6, dioxicarbene; 7, CO2. TBA model: tetradehydrogenated-benzo[α]anthracene. Level of theory Ab initio DFT-B3LYP/6-311++G(d,p). Decarboxylation reaction, ∆G‡experim 114.3 kcal·mol-1. In parenthesis values calculated from hybrid model with aromatic crown TBA+5 model, ONIOM [QM/MM], b3lyp/6-311++g(d,p)/uff).

The free energy profile calculated from the decarboxylation mechanism described in ±

Figure 2 is shown in Figure 4. The calculated barrier from 2 to thiirene 4 was ∆G = 39.4, kcal·mol-1 for graphite at 900 oC, in reasonable agreement with the experimental values observed for oxidized carbons (activated carbon 42.39 kcal.mol-1, at 630 oC ;42 graphene oxide 25.81 kcal.mol-1, at 46 oC).51,43



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The free energy profile of the mechanism using a more rigid model with five additional aromatic rings, was also calculated (TBA+5 model). The calculated energy barriers were slightly higher than those of the simpler TBA model, and the total free energy of activation for the decarboxylation reaction (step 2 to ࢀࡿ‡૜ ) was 122.7 kcal·mol-1 as compared to 110.8 kcal·mol-1 for the simpler model; comparison between both models does not intend to draw quantitative conclusions, the sole purpose is to show that similar trend is obtained. In considering differences between calculated and experimental values one has to take into account that the theoretical values are expressed per mole of model molecule, while the experimental values were obtained considering the units of weight or specific surface, which varies according with the gas used in the determination. In Table 1 it can be observed that ∆H‡ is independent of the units used, but ∆S‡ is inversely dependent on the specific surface, and consequently, ∆G‡ increases with the value of specific surface within a range of ±7 kcal·mol-1. Therefore, the calculated values are within the range of the experimental values.

Table 1. Initial reactivity (Ro) at different temperatures and activation parameters of the decarboxylation reaction of SO2 and graphite.32,68 Specific surface, m2·g-1 Temperature, oC

a

800

mol·seg-1·atm-1·g-1 5.00

1.30 (N2) 108Ro mol·seg-1·atm-1·m-2 3.85

mol·seg-1·atm-1·m-2 0.23

850

12.5

9.62

0.57

900

26.7

20.5

1.21

∆H‡, kcal·mol-1

39.72

39.64

39.57

∆S‡, cal·mol-1K-1

-57.5

-58.0

-63.7

∆G‡, kcal·mol-1 a

107.12

107.74

114.33

At 900 oC 16 ACS Paragon Plus Environment

21.94 (CO2)

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Photochemical and thermal activation of dioxiranes are well established reactions and these induced decompositions occur with formation of a diradical that produces CO2.6971

Decarboxylation of dioxirane 5a produces a dioxicarbene that rapidly decomposes to

CO2.28 The calculated ∆G‡ for the dioxirane decarboxylation reaction was 14.8 kcal·mol-1 (Figure 4). There is another possible route of decarboxylation of intermediates 4 through a lactone 5b that is more stable than the oxirane 5a, but thermochemical values of the desorption of CO2 from C19H10O2 prototypical graphene clusters, favor the dioxirane (28.4 kcal·mol-1) (5a) against the lactone (90.2 kcal·mol-1) (5b).28

Lactone 5b, TBA+5 model

The desulfurization route. When graphite is heated under a constant flow of SO2, the steady-state is reached when the oxidized sulfur intermediates observed at 168 eV of the XPS spectrum are in equilibrium with the non-oxidized intermediate (164 eV). As mentioned above (Figure 1), this intermediate was postulated to be an episulfide,33,42 but the detailed chemical nature of this group was not defined. Episulfides are characterized by 18 ACS Paragon Plus Environment

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the three-membered C2S ring and can be generated by the reaction of elemental sulfur with three different unsaturated chemical groups: a) olefin (thiirane); b) arene (thiepin); or c) obenzino (thiirene). They all tend to decompose to the corresponding unsaturated compound and elemental sulfur and the reaction must involve several steps. Arrhenius’ activation energy (40 kcal mol-1) for the thermolysis of thiirane C2H4S is much lower than the enthalpy of the hypothetical sulfur extrusion reaction (58 kcal mol-1)72 and, therefore, the reaction does not proceed through a monomolecular mechanism (eq 3), but through several bimolecular reactions as was shown by theoretical calculations.73 However, such mechanism is not acceptable for reactions taking place on rigids carbon matrixes.

S H H C C H H

Polycyclic

arene

∆

episulfides

+ S (3P)

C C

(3)

(thiiranes)

are

highly

unstable

both

thermodynamically74 and kinetically,75 and consequently, only few polycyclic aromatic episulfides have been synthezised so far. They undergo easy thermal desulfurization because the driving force towards aromatization, therefore, their thermodynamic stability increases the higher is the aromatic character of the ring system. In general, arene episulfides are less thermodynamically stable than the arene oxides towards the hypothetical extrusion of sulfur as S(3P). For benzo[α]anthracene-1,2-episulfide this reaction was calculated to be exothermic (∆Ho = -30 to -33 kcal mol-1).76 Because this instability arene episulfide is not a likely candidate as episulfide intermediate. 19 ACS Paragon Plus Environment


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S

H

H

Benzo[α]anthracene-1,2-episulfide

The mechanism proposed for the TBA model assumes that the S-transfer from the dioxathiolane intermediate to the benzyne site produces a benzothiirene. Benzyne is highly reactive and strongly electrophilic, and it is a reasonable site of the non-oxidized sulfur intermediate from where could start the sulfur transport out of the carbon matrix. We proposed the mechanism of sulfur transport52 shown in Figure 5 (adapted to the TBA model) where, after the extrusion of atomic sulfur from the benzothiirene site 3, the sulfur reacts with another benzothiirene site through a thiosulfoxide intermediate77 to generate a disulfane which eliminates diatomic sulfur and the original unsaturated bond, or consecutively, accepts a third atom and form a trisulfane able to extrude S2 regenerating the benzothiirene site. Formation of a trisulfane from the disulfane or from the benzothiirene site allows the extrusion of a more stable form of sulfur, a singlet diatomic sulfur 1S2,78 that would regenerate the benzothiirene site, establishing an equilibrium sulfane-disulfanetrisulfane that would function as a cycle of capture-release of sulfur. Trisulfides can also result from the insertion of S2 into a disulfide, followed by the extrusion of atomic sulfur.78


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S S+ S

S

+ S2

(-32.7) S

S S

S

S S2 (-49.3)

3

S

S

S

+ S2

(+62.0)

3

+S Figure 5. Mechanism of sulfur transport. Values in parenthesis calculated from TBA model, in kcal·mol-1.

It has been observed that benzyne, generated by thermolysis of diphenyliodonium2-carboxylate, reacted with sulfur to give benzopentathiepin and thianthrene, suggesting an initial formation of benzothiirene (o-C6H4S) (Figure 6),79,80 that exhibits enhanced selectivity in electrophilic substitution reactions,81 and has also been postulated as intermediate in the thermolysis of sodium o-bromobenzenethiolates.82 Figure 5 also shows the relative stabilities calculated from the TBA model, with respect to the benzothiirene site, of some of the species considered in the transport mechanism. The stability increases with the number of sulfur atoms in the polysulfane ring from 32.7 kcal mol-1 for the disulfane, to 49.3 kcal mol-1 for the trisulfane.



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+ IPh CO2 ∆ Sx

S

S

S

S S S S S

Figure 6. Reaction of benzyne with elemental sulfur.

Recently, a study of the reaction of o-benzyne with elemental cyclo-octasulfur (S8) was carried out83 providing evidence of the initial presence of o-C6H4S8 intermediate that decomposed to give, among other products, penta- and trithiepin. Theoretical calculations showed that the stability of the ortho-fused heterocycles o-C6H4Sx (where x = 1-8) increases rapidly from the strained ring x = 1 to x = 3. Thus, benzodithiete (o-C6H4S2) is -39 kcal·mol-1 and trithiole (o-C6H4S3) is -48 kcal·mol-1 more stable than benzothiirene (oC6H4S). These values are very close to the calculated values for the sulfur transport given in Figure 5. The mechanism involving consecutive reactions from the primordial extrusion of monoatomic sulfur of the episulfide, considers reactions between reactive sites. In this context, the reactive site has to be visualized as part of an ensemble of sites where the model represents the statistically predominant structure present when the steady-state is reached.84 The sulfur species extruded from a reactive site must diffuse to other sites to produce the sequence of consecutive insertions. Surface diffusion of oxygen on graphite has been study experimentally and theoretically,85-87 as well as the surface diffusion of individual sulfur atoms on several metals.88-93


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These results also show that the desulfurization route has a much lower energetic demand than the decarboxylation route (Figure 1), raising the important possibility of using the reaction of reduction of SO2 on carbons to reduce the acid rain, producing elemental sulfur as the main product, without increasing the greenhouse effect due to the formation of CO2.

CONCLUSIONS The reactive site model 1,2,11,12-tetradehydrogenated-benzo[α]anthracene (TBA), or this molecule plus five crown aromatic rings (TBA + 5), can be considered a viable model for the reduction of SO2 on graphite. The calculated mechanism considers that the experimentally observed sulfur intermediates are a 1,3,2-dioxathiolane in equilibrium with 1,2-oxathietene 2-oxide (γsultine) at the position 11,12 of the biradical zigzag edge, and the episulfide (benzothiirene) at 1,2-armchair of the benzyne site. The sulfur reduction step occurs with sulfur transfer from the dioxathiolane to the benzyno site with formation of a peroxide valence tautomer in equilibrium with a 1,3dicarbonyl. The decarboxylation reaction from the dicarbonyl valence tautomer occurs by isomerization to a dioxirane which produces a dioxicarbene that generates CO2. The total free energy of activation calculated for the decarboxylation reaction between the dioxathiolane and the transition state of the thermolysis of the dioxirane was within the range of the experimental values.



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The relative stabilities of the species considered in the transport mechanism of sulfur, with respect to the benzothiirene site, increase with the number of sulfur atoms in the polysulfane ring and their values are consistent with those reported in the literature.

ACKNOWLEDGEMENTS Thanks are due to the Centro de Supercomputacion de Galicia (CESGA) for computing facilities and CPU time. We thank the Brazilian Government Agency Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Projeto CAPES/DGU 219/2010, and the Spanish Ministerio de Educación, Cultura y Deporte, DGU, Project PHB20090057-PC, for financial support.

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