Eduardo Humeres

The desulfurization of carbons modified with SO₂ was studied as a dispersion in boiling cyclohexane (81°C) using activated carbon (mAC) and graphene oxide (mGO), modified by SO₂. The steady‐state species in the carbon matrix after the catalytic reduction of SO₂ was considered a trisulfane. For mAC, there was a burst of a sulfur species identified as S₂ by UV spectrum with a maximum at 217 nm (ε_M at 217 nm = 2.56 × 10³) that showed a second‐order decay of absorbance with = 47.29 M^‐1·sec^‐1 ( = 18.1 kcal·mol^‐1). The product was postulated to be S₄. No other consecutive reaction was observed because of the possible adsorption of S₄ in the carbon matrix. The desulfurization of mGO was shown by XPS and the kinetics were a second‐order decay up to 16 min ( 18.41 M^‐1·sec^‐1; = 18.8 kcal·mol^‐1) followed by a second‐order increase of absorbance with = 3.84 M^‐1·sec^‐1 ( = 19.9), where the product showed a double maximum at 260‐285 nm typical of S₈. These results are consistent with a mechanism of consecutive thermodynamically favorable dimerizations of S₂ and S₄ and with the desulfurization mechanism that has been previously postulated.

The kinetics of the ozonation of graphite with different particle sizes (106 μm, G106; 6.20 μm, G6.2) was studied at several temperatures under a flow of O3 diluted in O2. The reaction was first-order with respect to graphite and to the consumption of ozone. XPS spectrum showed that the reactions occurring in the solid under steady-state conditions maintain the original stoichiometry, as predicted by the postulated mechanism for SO2. The deoxygenation reaction occurred along with the ozonation reaction at 100 oC. The rate of oxygen elimination in the flow system has the same rate determining kinetic barrier as ozone insertion. Ozonation and deoxygenation reactions are sequentially related. The ozonation occurs with the insertion of O3 forming a 1,2,3-trioxolane followed by an oxygen transfer that produces a peroxide valence tautomer in equilibrium with a 1,3-dicarbonyl, [peroxide↔dicarbonyl], and an oxirene that eliminates atomic oxygen. The decarboxylation reaction was studied at 600 oC from the ozonated G106 (ΔG≠ = 83.60±0.08 kcal·mol-1). Total decarboxylation at 600 oC, matched the number of moles of CO2 removed and the oxygen content after ozonation, showing that the reduction of ozone on graphite was essentially a clean reduction with no secondary oxidations. When ozonized graphite was heated to 600 oC, only [peroxide↔dicarbonyl] species remained in the matrix. The peroxide tautomer isomerized to dioxirane and eliminated CO2 as a dioxicarbene. Total deoxygenation of decarboxylated graphite G106 was obtained by pyrolysis. There was a residual oxygen that arise from the atomic oxygen eliminated from the oxirene, intercalated in graphite layers and forming basal epoxy groups. Also incoming O atoms reacted with the intercalated O atoms to produce O2 molecules. Thermal annealing de-intercalated molecular oxygen (600-900 oC).

Photolyses at 254 nm of aqueous solutions of phenylalanine (Phe) were carried out at 25°C in the absence and presence of multiwalled carbon nanotubes (CNT). The photolyses were performed in distilled, deoxygenated water and inert Ar atmosphere or in distilled water and air atmosphere. Kinetics of the photolyses were followed by UV spectrophotometry at 220 nm. Initial rates were obtained from the plot of ΔA versus Δt, both in the presence of CNT (ΔA/Δt = p*) and in their absence (ΔA/Δt = ). The ratio p*/ gave the ratio k^*/ of the rate constants of the photolysis in the presence and in the absence of nanotubes, respectively. The mass of nanotubes produced an exponential attenuation of the photolytic decomposition of Phe with minor formation of tyrosine and an attenuation constant κ of the shielding effect of the nanotubes against the photolytic degradation of the amino acid. The results show the effects of CNT on the photolysis of Phe and support the previously observed attenuation effect of carbons on the denaturalizatoion of amino acids and proteins. It is concluded that carbons produce a shielding effect on the photolysis, absorbing the photons and decreasing substantially their denaturalization while at the same time excites the active sites of the carbon for the insertion reaction of the organic moiety.

Graphene oxide, MPGO, was obtained by oxidation of graphite microparticles by a H₂SO₄/KMnO₄/H₂O₂ mixture followed by thermal exfoliation. Non‐thermal plasma treatment of MPGO under SO₂ atmosphere resulted in the insertion of the oxidized intermediates only. Refluxing this material in CS₂ (46 °C) showed sulfur elimination and interconversion of the oxidized into non‐oxidized intermediates with an energetic barrier of ΔG^‡ = 25.81 kcal mol⁻¹ without decarboxylation. The episulfide content in modified MPGO increases with respect to the oxidized intermediates when increasing temperature. Modification of MPGO with SO₂ at 630 °C presented exclusively the non‐oxidized intermediate. These results support the hypothesis that there are two major reactions with different energetic demand in the SO₂ reduction on carbons: desulfurization and decarboxylation. The selectivity of thiolysis and aminolysis towards the intermediates inserted in the MPGO matrix allowed the amino‐thiolysis of a sample containing both intermediates with the insertion of the amino group on the episulfide site followed by an intramolecular thiolysis with double functionalization of the matrix. Copyright © 2016 John Wiley & Sons, Ltd.

Photolyses at 254 nm of phenylalanine (Phe) in aqueous solutions, were carried out in the presence of oxidized carbon nanotubes modified by the reaction with SO₂ (mNTO). Kinetics of the photolyses were followed by UV spectrophotometry at 220 nm, and the products were characterized by HPLC, XPS, and ¹³C-SSNMR. The ratio of the initial rates of photolysis in the presence and absence of mNTO, k*/k_o*, showed a systematic decrease. The photolytic decay of Phe occurs with minor formation of tyrosine. The mass of nanotubes produced an exponential attenuation of the photolytic decomposition of Phe. Total carbon analyses (TCA) showed no inorganic carbon formation after the photolyses. The first-order rate constant of photofunctionalization of mNTO by the insertion of phenylalanine onto the nanotube matrix was calculated from TCA to be k_in = 30.1 min^–1. Comparison of the XPS spectra of the mNTO before and after the photolysis, using the atom inventory technique, suggests the insertion of Phe along with the extrusion of a sulfide radical anion (^•S^–) which undergo subsequent oxidation to SO₄^2–. The obtained results show the effects of mNTO on the photolysis of Phe and provide a new method of photofunctionalization of carbon materials, modified by the intermediates of the reduction of SO₂, with an organic moiety.