Oxidative Desulfurization Process Assisted Simultaneously by Phase
3.2.4 Analysis
The residual concentration of DBT in the aliquots of reaction mixture was analyzed using Shimadzu Ultra High Performance Liquid Chromatography (UHPLC, Model: SPD–20A) equipped with a reverse phase C–18 column (5 µm, 4.6 mm × 250 mm) and UV detector at 287 nm. The mobile phase was a mixture of acetonitrile and water (80:20 v/v) with a flow rate of 1.8 mL/min. The formation of sulfone in oxidative desulfurization was confirmed using FTIR analysis of aliquots of reaction mixture in category B.2 (as a representative of all experimental protocols listed in Table 3.1 after completion of sonication. To identify the intermediates during DBT oxidation, GC–MS analysis of the same reaction sample was performed using Varian 240–GC equipped with VF–5ms column (30 m × 0.25 m ID DF = 0.25). 1 µL of sample was injected with a split ratio of 100:1. The temperature program was as follows: injection temperature – 250oC, column temperature – 100oC for zero min and increased to 300oC at a ramping rate of 7oC/min. The scanned mass range was from 50 – 1000 m/z and helium gas was used as a carrier gas.
3.3 R
ESULTS ANDD
ISCUSSIONBefore presentation of the experimental results and their analysis, we give herewith a brief discussion about the individual mechanisms of effect of ultrasound and PTA on oxidative desulfurization reaction system. The mechanism of action of PTA is explained in Fig. 3.2 (Wan et al., 2007; Mei et al., 2003). Either formic acid or acetic acid reacts with H2O2 to form peracids, which then dissociate to form an anion species (either HCOOO– or CH3COOO–) and a proton. PTA forms a complex with the anion oxidant species and transports it to the organic phase, where it can induce the
oxidation reaction of sulfur compound to sulfone.
Table 3.1: Summary of DBT reduction in different experimental categories
Experimental Category % DBT
oxidation
k (min–1) R2
A.1 STR + AA (4 mL) + H2O2 + TOAB
18.89 ± 0.33 2.82 × 10–3 0.70 A.2 US + AA (4 mL) + H2O2 (2 mL)
+ TOAB
28.25 ± 0.26 4.03 × 10–3 0.98 A.3 STR + AA (4 mL) + H2O2 (4 mL)
+ TOAB
12.41 ± 0.14 1.82 × 10–3 0.86 A.4 US + AA (4 mL) + H2O2 (4 mL)
+ TOAB
24.26 ± 0.17 3.89 × 10–3 0.87 B.1 STR + FA (4 mL) + H2O2 (2 mL)
+ TOAB
26.94 ± 0.55 3.86 × 10–3 0.93 B.2 US + FA (4 mL) + H2O2 (2 mL)
+ TOAB
42.00 ± 0.96 7.50 × 10–3 0.93 B.3 STR + FA (4 mL) + H2O2(4 mL)
+ TOAB
20.30 ± 0.67 3.23 × 10–3 0.82 B.4 US + FA (4 mL) + H2O2 (4 mL)
+ TOAB
28.48 ± 0.63 4.79 × 10–3 0.86 C.1 US + AA (4 mL) + H2O2 (2 mL)
+ TOAB + ESP (1.8 bar)
32.31 ± 0.47 1.39 × 10–2 0.82 C.2 US + FA (4 mL) + H2O2 (2 mL)
+ TOAB + ESP (1.8 bar)
47.05 ± 0.59 2.11 × 10–2 0.89
Notations: US – ultrasound, STR – stirring, TOAB – tetraoctyl ammonium bromide, AA –acetic acid, FA – formic acid, ESP – elevated static pressure (1.8 bar), η – DBT reduction in (%), k – pseudo 1st order kinetic constant in (s–1), R2 – regression coefficient, Reaction solution: All experiments were conducted using [TOAB] = 0.05 g (0.091 mmol)
This transport is, of course, dependent on the interfacial area between the organic and aqueous phase, which in turn depends on the convection present the system. After oxidation reaction both species, i.e. the anion of acid (HCOO– or CH3COO–) and the cation of PTA (Q+) return to aqueous phase, and the cycle continues. PTA thus assists
faster transport of oxidant species across interface and helps overcome the mass transfer limitations of the process that result in enhancement of kinetics of oxidation.
The selectivity of sulfur hydrocarbon compounds towards oxidation in comparison of other hydrocarbons in the reaction mixture is an important issue. Chen et al. (2010) have stated that organic sulfur compounds have higher oxidative reactivity than their analogue pure hydrocarbons in fossil fuels. In another publication, Chen et al.(2012) have assessed the relative reactivities of different sulfur compounds, viz. thiophene, benzothiophene and dibenzothiophene towards oxidation to corresponding sulfones. Essentially, the electron density on the sulfur atom of these compounds and their oxidative rate constants were examined. This chapter revealed that oxidative reactivity of sulfur compounds increased with electron density on sulfur atoms. The least oxidative reactivity of thiophene among the compounds mentioned above is attributed to low electron density of sulfur atom and low boiling point. On the other hand, the highest oxidative reactivity was observed for dibenzothiophene, which is also the model sulfur hydrocarbon species in the present study.
HO O formic acid
O O HO
performic acid
HO OH
hydrogen peroxide H
O H
water HCOOO-Q+
+ Q+ - H+ HCOO- + Q+
+ H+ sulfone
S
Organic phase
Aqueous phase sulfoxide
O S O
S O
Figure 3.1: The cyclic mechanism of phase transfer agent (PTA) during oxidative desulfurization with perorganic acid (e.g. performic acid).
and emulsification that enhance the mass transfer in the system through different mechanisms, viz. micro–streaming, micro–convection, shock (or acoustic) waves and micro jet. Greater description of these mechanisms is given elsewhere (Kuppa and Moholkar, 2010; Shah et al., 1999). Moreover, the organic solvent can get evaporated into the cavitation bubbles during the expansion phase of the bubble. Some of this vapor gets entrapped into the bubble at the instance of transient collapse. The temperature and pressure inside the bubble reach extreme at which the solvent vapor undergoes thermal cleavage that results in formation of reducing species such as CO, H2 and CH4. These species can competitively consume the oxidant species that are transferred from aqueous to organic medium, and hamper their utilization for oxidation of sulfur species. Thus, the physical effect of ultrasound and cavitation is beneficial towards enhancement of oxidation kinetics, while the sonochemical effect has an adverse effect. In our earlier studies (Chakma and Moholkar, 2013; Morya et al., 2008; Sivasankar et al., 2008), we have reported simulations of cavitation bubble dynamics that give quantitative estimates of the physical and chemical effects of cavitation bubbles in different reaction systems.