J. Phys. Chem. C 2008, 112, 581-589
581
Reactivity of the Thermally Stable Intermediates of the Reduction of SO2 on Carbons and Mechanisms of Insertion of Organic Moieties in the Carbon Matrix Eduardo Humeres,*,†,⊥ Karen Mendes de Castro,† Regina de F.P.M. Moreira,‡ Maria da Gloria B. Peruch,‡ Wido H. Schreiner,§ Abil E. Aliev,| Moise´ s Canle,⊥ J. Arturo Santaballa,⊥ and Isabel Ferna´ ndez⊥ Departamento de Quı´mica and Departamento de Engenharia Quı´mica e Engenharia de Alimentos, UniVersidade Federal de Santa Catarina, 88040-670 Floriano´ polis, SC, Brazil; Departamento de Fı´sica, UniVersidade Federal do Parana´ , 81531-970 Curitiba, PR, Brazil; Department of Chemistry, UniVersity College London, 20 Gordon Street, London, WC1H OAJ United Kingdom; and Department of Physical Chemistry and Chemical Engineering I, UniVersity of A Corun˜ a, Ru´ a Alejandro de la Sota, 1, E 15008 A Corun˜ a, Spain ReceiVed: August 29, 2007
The reduction of SO2 on carbons proceeds through reactive intermediates bound to the carbon matrix, which were postulated to be 1,2-oxathiene 2-oxide (or sultine), and 1,3,2-dioxathiolane that decomposes to produce an episulfide and CO2. The reactivity of these intermediates was studied in this work through several reactions, using XPS and NMR spectra to postulate their mechanisms. When modified activated carbon obtained after reaction with SO2 at 630 °C was heated at 900 °C, it was observed that the changes of the XPS spectrum resulted from the forward reaction of decomposition of the oxidized intermediate with S-transfer to produce the episulfide and CO2 and the reverse reaction with expulsion of SO2. Strong bases hydrolyzed the dioxathiolane intermediate and the episulfide. The thiolysis, aminolysis, and reaction of alkyl halides with modified activated carbon occurred with the insertion of the organic moiety in the carbon matrix. Laser photolysis at 266 nm in t-butanol showed insertion of t-butoxide on the matrix. Consistent mechanisms for these reactions were postulated. These results provide additional evidence on the mechanism of reduction of SO2 on carbons and the chemical nature of the intermediates, offering a new method to modify the physical and chemical properties of a carbon matrix by functionalization with an organic moiety.
1. Introduction When the reduction of SO2 on different carbons (graphite, charcoal, activated carbon, and cokes) was studied using strict kinetic and differential reactor conditions, under chemical control, it was shown that it proceeds through the stoichiometric reaction (1).1
SO2 + C f CO2 + 1/2 S2
(1)
All carbons produced CO2 and sulfur in the ratio 2:1 (considering the sulfur as S2) as the main products that are formed through the same path. Analysis of product distribution strongly suggested that the byproducts (CO, COS, and CS2) were produced from consecutive reactions of the primary products. The reactivity of the different carbons increased with decreasing cristallinity and increasing oxygen content. Activated carbon was ca. 105 times more reactive than graphite.2 The reduction of SO2 on activated carbon at 630 °C increased the sulfur content on the carbon until the reaction reached the steady-state condition. The sulfur content remained constant * Corresponding author. E-mail:
[email protected]. † Departamento de Quı´mica, Universidade Federal de Santa Catarina. ‡ Departamento de Engenharia Quı´mica e Engenharia de Alimentos, Universidade Federal de Santa Catarina. § Universidade Federal do Parana ´. | University College London. ⊥ University of A Corun ˜ a.
during this period, and it was chemically bound to the carbon matrix. The superficial complexes behaved as reactive intermediates because, besides the constant concentration during the steady state, the residual carbon reacted with SO2 at the same rate as the pure activated carbon and also reacted with CO2 to produce SO2 alone by the reverse reaction.2 The XPS spectrum of the residual carbon C(S) was interpreted as having two forms of sulfur bound to carbon: nonoxidized and oxidized sulfur. It was proposed that the reduction of SO2 on activated carbon occurred through a primary mechanism (shown in Scheme 1) where, after the adsorption of SO2 on the carbon, a 1,3,2-dioxathiolane 1 and/or 1,2-oxathietane 2-oxide 2 were formed and decomposed to produce CO2 and an episulfide 3. Consecutive insertions of atomic sulfur form a trisulfide that extrudes S2, regenerating the episulfide and establishing a transport mechanism where an equilibrium sulfide-disulfide-trisulfide works as a capture-release cycle of sulfur.3 Theoretical calculations supported that the reduction of SO2 on a graphite surface at 900 °C proceeds through 1 and 2 (possibly in equilibrium) inserted mainly on the zigzag edge.4 In this work, we studied some properties of these intermediates and several reactions in order to gather more evidence on their postulated structure and to explore future uses of these reactions on carbon materials including graphitizable and turbostratic carbons as well as nanocarbon particles.
10.1021/jp076941g CCC: $40.75 © 2008 American Chemical Society Published on Web 12/22/2007
582 J. Phys. Chem. C, Vol. 112, No. 2, 2008 SCHEME 1: Mechanism of the Primary Reaction
Humeres et al. Quantification of XPS Spectrum Components after the Reaction. The change of atomic composition of the surface after a reaction was calculated considering the reactions involved in the possible mechanisms. If +ni is the number of atoms of the element i inserted (or excluded, -ni) from the matrix after the reaction, the total balance of atoms of the elements involved in the reaction is ∑ni. The extent of the reaction, ∆, of the element i is given by eq 2
C ii - C fi
∆)
C fi
2. Experimental Section Reagents and Methods. All reagents were of analytical grade and were used without further purification. Sulfur dioxide, from White & Martins, was 99.9% pure. The graphite, 99.98%, was from Nacional de Grafite Ltda. The carbon samples were characterized by proximate analysis. The specific surface area was determined by the static method, using CO2 at room temperature as adsorbate and the DubininPolanyi isotherm equation to fit the experimental data.5 The sulfur content was determined in a LECO SC132 analyzer. The activated carbon, from Carbomafra S.A., Santa Catarina, Brazil, was steam activated at 700 °C. It was demineralized in the laboratory by HCl and HF treatment.6 It had a particle size of 1.68 mm; 0.29% ash content; surface area, 384 m2.g-1; no sulfur was detected. X-ray diffraction analysis showed that graphite was highly crystalline and that activated carbon was mainly amorphous. Solid-state NMR experiments were carried out on an MSL300 (Bruker) with a 7.05 T wide-bore magnet and a standard 7 mm double-resonance MAS probe (Bruker). High-resolution solidstate 13C NMR spectra at 75.5 MHz were recorded using crosspolarization (CP). The RIDE pulse sequence was also used for direct detection of 13C with suppression of acoustic ringing effects.7 Both CP and RIDE spectra were recorded using MAS and high-power 1H decoupling. Samples were spun at MAS frequencies in the range 4.5-5.0 kHz with stability better than (3 Hz. Typical acquisition conditions for 13C CPMAS experiments were as follows: 1H 90° pulse duration ) 5 µs; contact time ) 2 ms; recycle delay ) 2 s. Typical operating conditions for the 13C RIDE experiment were as follows: 13C 90° pulse duration ) 4.5 µs; recycle delay ) 10 s. The 13C chemical shifts are given relative to tetramethylsilane. The XPS spectra were obtained using a VG Microtech ESCA 3000 spectrometer operating with a Mg KR source. The base pressure of the system was in the low 10-10 mbar range, and the operating pressure was maintained below 10-8 mbar during the measurements. The calibration was carried out with respect to the main C1s peak at 284.5 eV. The concentration of the elements was calculated using the system database. The deconvolution of the various peaks was done using the SDP software from XPS International.8 The samples of the residual carbon from the reaction with SO2 were allowed to cool under N2 atmosphere, after shutting off the stream of SO2. The samples were extracted in a Soxhlet with CS2 to eliminate physically bound elemental sulfur. The solvent was eliminated by heating under vacuum. No difference in sulfur content was observed before or after the extraction. The sulfur content was determined in a LECO SC132 analyzer.
∑n - n 100
(2) i
where C ii and C fi are the initial and final concentration of the element i in atom %. The correction divisor f to transform the new surface composition after the reaction in atom % is eq 3 and the final concentration C fi for each element is obtained from eq 4
f)
100 + (
C fi )
∑n)∆
100 C ii + ni∆ f
(3)
(4)
For a one step mechanism, ∆ and f should be the same for all of the elements of the spectum, and therefore, calculation of ∆ and f for one element by eqs 2 and 3 allowed the calculation of the final concentration of the rest of elements through eq 4, if the reaction was correct. If the mechanism consists of several steps, the final concentrations calculated for one step were used as initial concentrations of the following. The standard deviation of the atom % of the elements calculated for the spectra according to the acceptable mechanisms was less than (0.7 atom %. Modification Reactions. ActiVated Carbon. The sample of demineralized activated carbon was dried at 110 °C for 12 h, and after cooling, it was placed in a tubular 316 stainless steel reactor (30 cm height, 2.5 cm diameter) fitted with a temperature controller and heated by an electric oven in a system that has been described in detail previously.1 The sample was pretreated at 700 °C for 3 h under a gas flow of nitrogen controlled at 80 mL/min by a mass flow controller. The temperature was then adjusted to the experimental temperature, and the total gas flow of SO2 (20% in N2) was 80 mL/min. The sample was then allowed to react for 3 h. It was observed previously that, during this time, the sulfur concentration increased and the intermediates reached a steadystate concentration.2 This residual carbon will be referred as modified carbon. All of the reactions studied with the modified carbon were run also with samples of nonmodified activated carbon, and the XPS spectra were obtained for comparison. The observed changes were within the standard deviation of the spectra. Graphite. The sample was dried at 110 °C for 48 h and allowed to react at 630 °C with SO2 under the same conditions as the activated carbon. Reactions of Modified Activated Carbon. Alkaline Hydrolysis. The alkaline hydrolysis of the modified carbon was carried out by refluxing for 24 h a dispersion of a sample in 1 M NaOH aqueous solution. The solid was washed with water and ethanol, and finally dried under vacuum.
Intermediates of the Reduction of SO2 Thiolysis. Sodium dodecane-1-thiolate was prepared by addition to 60 mL of dried benzene, 10.0 g of dodecane-1-thiol, and 2.0 g of sodium and refluxing the mixture for 3 h in a dried atmosphere. The excess of sodium was eliminated and the benzene was evaporated in a rotatory evaporator. The residue was dried under vacuum. A dispersion of 1.03 g of modified carbon in 33 mL of dried DMSO was treated with 0.89 g of sodium dodecane-1-thiolate refluxing for 48 h in dried atmosphere. The solid was filtered, washed with water and ethanol, refluxed for 6 h with CS2, washed with ethanol, and dried under vacuum. 13C NMR, CPMAS: Two very broad isotropic peaks with half-height linewidths of ca. 2 kHz; ppm: 25 (alkyl); 121 (carbon) with spinning sidebands at (65 ppm. Aminolysis. A dispersion of 0.34 g of modified carbon in 17.8 mL of dried DMSO was treated with 0.45 g of dodecylamine and refluxed for 48 h in dried atmosphere. The solid was filtered, washed with water and ethanol, and dried under vacuum. 13C NMR, CPMAS: Two isotropic peaks; ppm: 27 (alkyl); 123 (carbon) with spinning sidebands at (50 ppm. Reaction with Alkyl Halide. A dispersion of 0.37 g of modified carbon in 31 mL of dried DMSO was treated with 0.65 g of hexadecyl bromide and refluxed for 48 h in dried atmosphere. The solid was filtered, washed with water and ethanol, and dried under vacuum. 13C NMR, CPMAS: ppm: 28 (alkyl); 127, broad isotropic carbon peaks with spinning sidebands at (57 ppm. Photolysis. A Brilliant B Nd:YAG laser from Quantel was used to irradiate the samples with a discrete number of pulses of known energy (ca. 70 pulses of 15 mJ). The same laser was coupled to a LKS-60 laser flash photolysis system from Applied Photophysics in order to monitor the initial stages of the process. The samples were prepared as suspensions of the modified carbon in prismatic quartz cells containing 3 mL of the solvent (H2O, EtOH, and t-buOH), previously saturated with Ar or O2. All experiments were carried out at the natural pH of the samples and at 25 °C. The solvent was separated by filtration and the solid was washed and dried under vacuum. Solid from the photolysis in t-buOH saturated with Ar at 25 °C; 13C NMR, CPMAS; ppm: 23 (alkyl), the aliphatic carbon peak was relatively narrow with a half-height line width of ca. 200 Hz; 121 (carbon), very broad with a half-height line width of ca. 2.9 kHz and spinning sidebands at (56 ppm. Anions, and particularly SO42- analysis of the solvent was carried out by electrophoresis, in a Waters system bearing an interchangeable positive-negative power supply, with a UV detector, able to apply potentials between 0 and 30 kV. The detection limit for SO42- was ca. 0.1 ppm. The solvent samples were analyzed by GC/MS performed in a Thermo Finningan Trace GC 2000/Polaris Q system, using electron ionization. Three different colums were used (J&W, DB-XLB 60 m × 0.25 mm × 0.25 µm; J&W, DB-5MS, 30 m × 0.25 mm × 0.25 µm; and Supelco SP 2330 30 m × 0.25 mm × 0.25 µm), in order to reinforce the obtained results. The injection and detection conditions were the same in all cases: injection in PTV mode, initial temperature 50 °C, split flow 50 mL‚min-1, splitless time 5 min, injection time 0.5 min, transfer rate 4 °C‚min-1, final temperature 290 °C, injection volume 9 µL, constant flow at 1 mL‚min-1; full scan detection between 45 and 450 uma. The programmed elutions were: 50 to 150 °C at 10 °C‚min-1, 3 °C‚min-1 to 300 °C (5 min), transfer line at 290 °C, for the DB-XLB column; 80 °C (1 min), 30 °C‚min-1 to 180 °C (3 min), and 3 °C‚min-1 to 300 °C (7 min), transfer line at 290 °C, for the DB-5MS column; 100 °C
J. Phys. Chem. C, Vol. 112, No. 2, 2008 583
Figure 1. Free energy profile of the reduction of SO2 on graphite.
at 1.5 °C‚min-1 to 162 °C (13 min), 1.5 °C‚min-1 to 200 °C, transfer line at 230 °C, for the SP 2330 column. 3. Results and Discussion Theoretical study of the chemisorption process of SO2 on the graphite surface4 showed two possible adsorption sites: the benzyne structure of the armchair edge and the triplet birradical species on the zigzag edge. The adsorption on the armchair edge had energies in the range of -5 to -51 kcal mol-1, whereas the adsorption on the zigzag edge was the most favorable, with energies of -61 to -100 kcal mol-1. At 900 °C, corresponding to the experimental conditions of graphite, the armchair edge was estimated to be covered by only 5%, whereas the zigzag edge was fully covered with SO2 molecules bound to the surface through two C-O bonds forming a six-membered ring 1,3,2dioxathiolane I, or one C-O and one S-O bond through a fivemembered ring 1,2-oxathiane 2-oxide or γ-sultine II. β-Sultines have only a limited thermal stability and they readily lose SO2 producing olefins, while thermolysis of γ-sultines occurs at much higher temperatures.9-13
Decomposition by pyrolysis of a three-membered episulfone ring was rationalized on the basis of expansion to a fourmembered ring β-sultine followed by further expansion to a five-membered dioxathiolane ring with relief of strain. The dioxathiolane can lose SO2 concertedly to yield olefin stereospecifically. A similar mechanism is possible from the β-sultine, although in this case the loss of sulfur dioxide seems to be a high-energy process.14 The decomposition of the dioxathiolane (or γ-sultine) extruding SO2 is the reverse reaction of the reduction of SO2 on carbons, and the energetics of the mechanism suggests that during the steady state the two intermediates are in equilibrium and coexist with the episulfide product that is more stable by only 1-3 kcal mol-1 (Figure 1).2,4 Temperature Effects on Modified Carbons. Kinetics of the reduction of SO2 on graphite has been observed at 900 °C, because of the low reactivity due to the high cristallinity. In this work, when graphite was modified at 630 °C, the XPS spectrum in the S2p region showed two bands, at 164.1 and 168.8 eV, of the nonoxidized and oxidized sulfur, respectively,
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TABLE 1: Bond Energies and Composition of XPS Spectrum of Modified Activated Carbon Obtained at 630 °C and Heated at 900 °Ca sample
after heating at 900 °C c
Initial b
element
eV (weight%)
atom %
eV (weight%)
S2p non-oxi
163.9 (58.1)
4.1
oxi total C1s
167.7 (41.9)
3.0 7.1 62.2 19.7 81.9 8.0 3.0 11.0
163.5 (40.1) 164.8 (59.9) ND
total O1s total
284.5 (76.0) 285.9 (24.0) 532.1 (72.3) 534.7 (27.7)
284.4 (77.3) 285.8 (22.7) 531.9 (59.7) 534.3 (40.3)
atom % 2.2 3.3 5.5 68.9 20.2 89.1 3.2 2.2 5.4 %at SD
TABLE 2: Bond Energies and Composition from XPS Spectrum of Modified Activated Carbon after Basic Hydrolysis at 100 °Ca sample
calcd atom % 2.3 3.4 < 0.1 5.7 68.7 20.2 88.9 3.2 2.2 5.4 (0.17e
a Spectrum calibrated by reference to C1s (284.5 eV). b Modified activated carbon obtained at 630 °C. c Heated for 2 h at 900 °C under nitrogen. d Calculated from the following reactions: (1) C(SO2) f C(S) + CO2 ∆Snon-oxi ) 1.183; fSnon-oxi ) 0.965. (2) C(SO2) f C() + SO2 ∆ ) (∆Soxi + ∆C + ∆O)/3 ) 1.934; f ) (fSoxi + fC + fO)/3 ) 0.942. e Standard deviation per element.
as has been found for the activated carbon intermediates.2 Since the reduction of SO2 on graphite produced the same intermediates, it occurs through the same mechanism as has been proposed before.2 The quantification of XPS spectrum components of the modified graphite was obtained assuming that the chemical reaction of SO2 with graphite produces oxidized and nonoxidized intermediates along with the desorption of O2. When modified activated carbon obtained at 630 °C was heated at 900 °C, the XPS spectrum showed that the oxidized sulfur (168 eV) was eliminated and the nonoxidized sulfur (164 eV) was partially decomposed to form another species at 165 eV. According to the method outlined in the Experimental Section, the XPS spectrum resulted from two reactions: the forward reaction of decomposition of the oxidized intermediate with S-transfer, producing the episulfide and CO2, and the reverse reaction with extrusion of SO2 (Table 1). These results are consistent with, and support, the primary mechanism of Figure 1. The extra band observed at 165 eV suggests that at 900 °C some of the species involved in the transport mechanism (disulfide-trisulfide-S2) were formed.3,15,16 Basic Hydrolysis. The XPS spectrum of the product of the alkaline hydrolysis at 100 °C of modified activated carbon (Table 2) showed that the reaction decreased the total sulfur content, mainly the oxidized sulfur, and negative centers neutralized by sodium ions were formed. The description of the spectrum and the sequence of reactions that had to be considered in order to obtain the final spectrum with a standard deviation per element of (0.40 can be observed in Table 2. The implicit mechanism is shown in Scheme 2. According to the XPS spectrum deconvolution, the basic hydrolysis produced the decrease of oxidized sulfur as a consequence of the expected hydrolysis of the dioxathiolane intermediate.17-18 It is also known that basic hydrolysis of sultines can easily occur with nucleophilic attack on the sulfinyl sulfur that would form a sodium salt.19-25 The nucleophilic attack of hydroxide ion on the carbon atom of the episulfide ring would form a sulfide anion with Na+ as the counterion in the first step,26-27 eliminating S2- in the consecutive step. However, with the present information, we cannot decide whether the sulfinylate or the sulfide sodium salt (or both) are formed.
element
eV (weight%)
S2p non-oxi 163.9 (55.2) oxi 168.2 (44.8) total C1s 284.5
total O1s
after basic hydrolysisb
initial
531.8
total Na1s
atom %
eV (weight%)
atom %
calcc atom %
4.0 3.2 7.2
163.7 (60.4) 2.5 2.5 167.7 (39.6) 1.7 1.6 4.2 4.1 284.5 (75.5) 59.3 285.9 (13.0) 10.2 286.9 (7.0) 5.5 288.5 (4.5) 3.5 82.6 78.5 79.6 531.7 (36.1) 5.6 533.4 (63.9) 9.8 10.3 15.4 14.6 1072 1.9 1.9 atom % SDd (0.40
a Spectrum calibrated by reference to C1s (284.5 eV). b Refluxed in 1 M NaOH for 24 h. c Calculated from the following reactions:
NaOH
H2O
(1) C(SO2) 98 98 HO-C()-OH + Na2SO3; ∆Soxi ) 1.566, fSoxi ) 0.984. (2) C(S) + 2NaOH f HO-C()-OH + Na2S; ∆Snon-oxi ) 1.437, fSnon-oxi ) 1.014. (3a) C(S) + NaOH f HO-C(S-) Na+; ∆Na+ ) 1.921, f Na+ ) 1.038. (3b) C(SO2) + NaOH f HO-C(SO2-) Na+. d Standard deviation per element.
SCHEME 2: Basic Hydrolysis of Modified Activated Carbon at 100 °C
A similar procedure was used to investigate, from the XPS spectra, the mechanisms of the following reactions of the intermediates. The results were summarized in Table 3, and the mechanisms are described in the corresponding Schemes (see Supporting Information for details). Thiolysis. The thiolysis of the modified activated carbon with sodium 1-dodecathiolate kept the sulfur content, observed in the XPS spectrum, practically constant (Table 3), but the ratio of oxidized to nonoxidized sulfur decreased 10-fold, suggesting that the reaction occurred with elimination of oxidized sulfur. The solid state 13C NMR spectrum after the thiolysis showed, besides the carbon signal at 121 ppm, the peak of dodecathiolate at 25 ppm, indicating that the thiolate moiety was inserted in the carbon matrix. The reactions that were considered to calculate the atom % distribution shown in Table 3 are detailed in Scheme 3. Thermal reactions of 2,5-disubstituted thienosultines with nucleophiles support this mechanism.28 Sealed tube reaction at 180 °C in benzene of 2,5-dimethyl thienosultine 4 gave the corresponding sulfolene 6 (Scheme 4). The sultine may undergo C-O bond cleavage to form an alkyl sulfinyl biradical 5, producing 6 by intramolecular rearrangement. In our case, this rearrangement is unlikely because it requires the formation of a strained four-membered ring sulfolene. In the presence of
Intermediates of the Reduction of SO2
J. Phys. Chem. C, Vol. 112, No. 2, 2008 585
TABLE 3: Bond Energies and Composition of XPS Spectrum of Modified Activated Carbon after Several Reactions sample
thiolysis,b atom %
initiala
aminolysis,c atom %
alkyl halide,d atom %
photolysis, e atom %
element
ev (weight%)
atom %
exp
calcf
exp
calcg
exp
calch
exp
calci
S2p non-oxi oxi total C1s
163.9 (58.1) 167.7 (41.9)
4.1 3.0 7.1 62.2 19.7 81.9 8.0 3.0 11.0
5.7 0.4 6.1 63.0 24.8 87.8 3.9 2.2 6.1
5.7 0.4 6.0 62.9 24.7 87.6 4.1 2.3 6.4
6.6 ND 6.6 50.4 34.4 84.8 4.9 1.5 6.4 2.2
5.2 0.0 5.2 51.2 35.0 86.2 4.9 1.5 6.4 2.2
5.0 2.1 7.1 59.6 23.0 82.6 5.8 2.6 8.4
3.9 2.0 6.0 60.1 23.1 83.2 6.2 2.7 8.9
3.5 1.1 4.6 61.9 24.1 86.0 6.3 3.0 9.4
4.0 1.1 5.1 61.9 24.1 86.0 6.0 2.8 8.8
1.9
1.9 (0.46
total O1s total N1s Br3d atom %, SDj a
284.5 (76.0) 285.9 (24.0) 532.1 (72.3) 534.7 (27.7)
(0.20 b
(0.70
(0.28
c
Spectrum calibrated by reference to C1s (284.5 eV). Sodium dodecane-1-thiolate in dried DMSO, refluxed 48 h. Dodecylamine in DMSO, refluxed for 48 h. d 1-Bromohexadecane in DMSO, refluxed for 48 h. e Photolysis in t-buOH under Ar, λirrad ) 266 nm, 70 pulses of 2 ns and 15 mJ, 25 °C. f Calculated values from Figure 4. g Calculated values from Figure 6. h Calculated values from Figure 7. i Calculated values from Figure 9. j Standard deviation per element.
SCHEME 3: Reactions of the Oxidized Intermediates with Sodium 1-Dodecathiolate (C12SNa)
radical trapping reagents RXH, sultine 4 underwent SO2 extrusion and gave the trapping product 8 as well as sulfolene 6. Although the formation of adduct 8 may be explained by a radical trapping mechanism through 5, it may also be formed through the ionic intermediate 7.29-31 The trapping by 1,4-cyclohexadiene was not found to be efficient, although it has been frequently used in the trapping of biradicals.30 This result favors the alternative mechanism of nucleophilic displacement of RXH on the O-carbon atom to yield a sulfinic acid which loses SO2, producing 8. Some of the few reported reactions of nucleophilic ring opening of sultines with Grignard reagents as well as organocopper-lithium reagents were observed mainly with aliphatic sultines.32 The nucleophilic attack of 2,5-dichloro-thienosultine by nBuLi produced sulfinyl alcohol at the comparable yield reported for aliphatic sultines.28 Therefore, it seems reasonable in our case to consider that the reaction occurs with nucleophilic displacement by thiolate anion on the O-carbon atom to yield a sulfinate, which, after losing SO2, gives the observed product. Aminolysis. The aminolysis of modified activated carbon was carried out refluxing the dispersion in a DMSO solution of dodecylamine (198 °C). The XPS spectrum after the reaction showed that the total sulfur remained practically constant; however, oxidized sulfur was not detected (Table 3). The presence of nitrogen at 399 eV in the XPS spectrum and the alkyl peak at 27 ppm observed in the NMR spectrum showed that dodecylamine was inserted in the matrix. The deconvolution
of the XPS spectrum of modified activated carbon after aminolysis shown in Table 3 was obtained considering the reactions of Scheme 5. The nonoxidized intermediate reacted with the amine, forming the insertion product. Thiiranes react with amines with SN2 ring opening, which is very sensitive to steric effect.33-37 There is also an S-transfer from the oxidized intermediate, forming the episulfide and CO2 that remains adsorbed on the carbon matrix and is further partially released as free CO2. This step is similar to the rate determining step of the reaction of reduction of SO2 (Figure 1), although in this case it occurs unexpectedly at near 200 °C as a consequence of a possible catalytic effect of the amine. We note that the nucleophilic attack of the amine on the sultine with extrusion of SO2, as described by the mechanism for 2,5-dimethylthienosultine (Scheme 4), does not operate in this case. It may be due to the fast decomposition of the oxidized intermediate to produce the episulfide by the catalyzed path because the step of amine insertion with SO2 elimination would not produce a XPS spectrum consistent with the experiment. Reaction with Alkyl Halides. After refluxing in a DMSO solution of 1-bromohexadecane, the XPS spectrum of the modified activated carbon showed no change of sulfur content (Table 3), but it did show incorporation of bromine at 69.1 eV and insertion of the alkyl moeity according to the peak shown by the NMR spectrum at 28 ppm. The lowest standard deviation of the quantification of XPS spectrum components was obtained with the mechanism of Scheme 6. The nucleophilic attack of the sulfur of 1,3,2dioxathiolane and episulfide intermediates on the alkyl halide produced an alkyl sulfinyl ester and an alkyl sulfide, respectively, releasing a Br- ion that was inserted in the matrix. The sulfinyl ester would react with the alkyl halide in a second step forming a dibromide and liberating a substituted alkyl sulfinyl ester. The sulfur of the dioxathiolane intermediate may act as an effective nucleophile for the displacement of the halide of an alkyl halide. One important feature of thiolate complexes is their ability to undergo S-alkylation by SN2 mechanism.38 This reactivity is also found in episulfides that behave as nucleophilic agents.26 There are no reports on the reactivity of sulfinyl esters as SN2 nucleophiles, but the bridge-O could act as a nucleophile. It has been proposed that the hydrolysis of substituted anisoles, at 300 °C, under near critical water conditions, occurs predominantly by SN2 nucleophilic attack by water.39 Irradiation Experiments. Suspensions of the modified carbon were irradiated with a variable number of 266 nm laser
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SCHEME 4: Mechanisms of the Reaction of 2,5-Dimethylthienosultine in Benzene at 180 °C with Radical Trapping Reagents RXH
SCHEME 5: Reactions of the Intermediates with Dodecylamine (C12NH2)
SCHEME 6: Reactions of the Intermediates with Hexadecylbromide (C16Br)
pulses at 25 °C in different hydroxylic solvents (t-buOH, EtOH, and H2O) saturated with Ar or O2. Photolysis in t-buOH saturated with Ar led to insertion of the t-buO group in the
carbon matrix, as shown by the relatively narrow peak at 23 ppm of the t-butoxide in the NMR spectrum. The XPS spectrum components after the irradiation could be quantified assuming the insertion of t-buOH with extrusion of SO2 (Table 3). This reaction must be analyzed in connection with the pyrolysis of 2,5-dimethylthienosultine in the presence of methanol, described previously (Scheme 4).28 In that case, the reaction in the absence of radical trapping reagents goes via formation of the alkyl sulfinyl biradical 5 that then undergoes intramolecular rearrangement. Similar trapping of the reactive intermediates by methanol-d4 gave 8 (RX ) CD3O; H ) D) with insertion of the methoxy group and abstraction of a hydrogen. As it was mentioned above, although the formation of the adducts may be explained by a biradical trapping mechanism, it may also be alternatively explained via ionic intermediates. Photolysis of the modified carbon in different hydroxylic solvents (t-buOH, EtOH, and H2O) showed the formation of sulfate anions. No organic sulfur compounds or other organic derivatives were detected by GC/MS analysis after exhaustive examination, as described in the experimental section. The yield of SO42- was higher, by a factor of 2, when the dispersion was saturated with oxygen instead of argon (Table 4). Laser flash photolysis experiments showed the formation of a broad undefined band between 300 and 500 nm that decayed within ca. 1 µs, as shown in Figure 2. Most probably, this broadening effect was due to light scattering by the suspension of modified carbon. More detailed observation at different wavelengths within this broad band concluded that the rate of decay was uniform at different wavelengths (Figure 2, parts b and c, Table 5), which points to a single process being observed. There is no effect of O2 on the decay of the transient species, which allows a triplet state nature to be discarded for the excitedstate generated upon initial excitation and suggests that the oxidation to sulfate occurs after the rate determining step of the decay. The observed effect of ground-state oxygen on the
Intermediates of the Reduction of SO2
J. Phys. Chem. C, Vol. 112, No. 2, 2008 587
Figure 2. (a) Time-resolved spectra observed at 25 °C after 266 nm laser flash photolysis of an Ar-saturated suspension of modified carbon in H2O; (b) decay at 320 nm; (c) decay at 400 nm.
TABLE 4: Effect of Oxygen on the Formation of Sulfatea 2-
[SO4 ]/mg‚L Ar O2 a
-1
H2O
EtOH
4.50 8.10
1.65 2.43
Irradiation with 60 laser pulses of 266 nm light (E ) 15 mJ).
TABLE 5: Rate Constants for the Decay of the Observed Transient Species in Different Solventsa solvent H2O EtOH t-buOH
λ, nm
10-6 k, s-1
300 400 300 300 300 550
8.1 ( 0.3 8.5 ( 0.3 1.3 ( 0.4 1.9 ( 0.3b 3.4 ( 0.6 3.1 ( 0.2
a At 25 °C after 266 nm laser flash photolysis of a suspension of 10 mg of modified carbon in 3 mL of solvent saturated with Ar unless indicated. b Saturated with O2.
yield of sulfate points to a radical nature of the sulfur species that eventually leads to sulfate anion. No transient was observed in experiments carried out in the presence of ascorbic acid at pH 5.10, which is consistent with a radical nature of the transient species, since ascorbic acid is a well-known radical scavenger.40 The differences observed in the rate of decay in different solvents (Table 5) must be attributed to an effect of the polarity of the solvent and are indicative of a relevant change of charge of the species involved in solution. It is convenient at this stage to note that the initial transient species are bound to the solid phase. In a mechanism involving biradical intermediates, the rate will be dependent only on the sultine or dioxathiolane concen-
tration if the trapping reagent concentration is kept high and a steady-state approximation of the biradical concentration is satisfied. In our case, the rate constants are pseudo-first-order because of the huge solvent’s excess and may be corrected for its concentration. However, for reactions at the solid-liquid interphase, the reaction would be independent of the bulk concentration of the solvent, and the corresponding correction for the concentration of the solvent in the vicinity of the reaction site on the surface is not known, making it necessary to use the pseudo-first-order rate constant for the sake of comparison. From these experimental pieces of evidence, it is reasonable to assume that the excited-state of the intermediates generated upon light absorption may decay in energy by C-O (dioxathiolane) and/or C-S (sultine) bond homolysis to yield a biradical species bound to the carbon surface. The C-S bond homolysis is expected to be preferred, since it demands ca. 86 kJ‚mol-1 less than a hypothetical C-O bond homolysis [D(C-O) ) 358 kJ.mol-1; D(C-S) ) 272 kJ.mol-1].41 The so-formed solidphase bound transient biradical species would then react with the solvent with expulsion of a sulfur dioxide radical anion (SO2•-) and loss of a proton, yielding the carbon matrix with a t-butoxide moiety of the solvent inserted and an empty radical position. The so-formed SO2•- subsequently leads to SO42through either dimerization and/or mild oxidation (SO3•-) releasing one solvated electron that is captured by the proton, generating a hydrogen atom that covers the empty radical position (Scheme 7).23 As there is no spectral evidence of the solvated electron, its eventual formation and subsequent disappearance could only take place after the rate determining step. Since the intermediates are in equilibrium, a similar mechanistic proposal could be imagined for both species, the sultine and the dioxathiolane.
588 J. Phys. Chem. C, Vol. 112, No. 2, 2008 SCHEME 7: Mechanism of Phototransformation of the Oxidized Intermediates upon Irradiation at 266 nm in t-Butanol
Humeres et al. of carbon-oxygen and carbon-sulfur biradicals would justify the broad, undefined spectrum observed.47 4. Conclusions The reactions of graphite with SO2 at 630 °C and modified activated carbon heated at 900 °C showed the formation of the same intermediates and products proposed for the primary mechanism (Scheme 1). The change of atomic composition of the surface after a reaction, observed by the XPS spectrum, can be used to postulate the mechanisms involved. Basic hydrolysis of modified activated carbon produced the hydrolysis of the sultine intermediate and the attack of hydroxide ion on the episulfide. The thiolysis, aminolysis, and reaction of alkyl halide with modified activated carbon occurred with insertion of the organic moiety. The photolysis in t-buOH saturated with Ar led to insertion of t-butoxide in the carbon matrix with expulsion of SO2•- which oxidized to sulfate anion. The reactions and mechanisms postulated in this work are consistent with the simultaneous presence of the dioxathiolane, sultine, and episulfide on the surface of the modified activated carbon. The reduction of SO2 is an important process to decrease air pollution, and the search for catalysts that improve the method is an active field.48-51 However, such research requires understanding the mechanism of reduction in order to rationalize the design of potential catalysts. There is an intense interest in carbon allotropes, especially in the form of nanoparticles, with respect to their reactivity, properties, and construction of nanoscale devices. Modification of their properties can be readily obtained by functionalization of the carbon matrix with organic moieties using the reactions studied in this work.
It has been proposed that the thermal decomposition of episulfones with extrusion of SO2 does not occur through homolysis of the S-C bond but through expansion of the threemembered ring to a less strained four-membered ring sultine followed by further expansion to a five-membered 1,3,2dioxathiolane ring. The loss of SO2 from this last species becomes preferred over the sultine because the loss can be concerted and stereospecific.42-46 However, the reaction pathways after thermal or photochemical activation are not necessarily the same, since the excited-state from which the reaction starts after light absortion may have geometric, energetic, and thermodynamic properties that are very different from those of its corresponding ground state. Thus, the proposal in Scheme 7 is the one that fulfills better the experimental observations. The fact that the same decay rate was observed for the transient species regardless of the wavelength supports the proposed mechanism. Similar rates of displacement of SO2•by t-buOH from the sultine and the dioxathiolane biradicals are expected, leading to the same radical species shown in Scheme 7. Therefore, the simultaneous decay of the sultine and the dioxathiolane derived radicals would appear like a single process. Also, the simultaneous presence of different proportions
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