Improving the Catalytic Performance of Keggin [PW12O40]3- for Oxidative Desulfurization: Ionic Liquids versus SBA-15 Composite

Abstract

Different methodologies were used to increase the oxidative desulfurization efficiency of the Keggin phosphotungstate [PW₁₂O₄₀]^3- (PW₁₂). One possibility was to replace the acid proton by three different ionic liquid cations, forming the novel hybrid polyoxometalates: [BMIM]₃PW₁₂ (BMIM as 1-butyl-3-methylimidazolium), [BPy]₃PW₁₂ (BPy as 1-butylpyridinium) and [HDPy]₃PW₁₂ (HDPy as hexadecylpyridinium. These hybrid Keggin compounds showed high oxidative desulfurization efficiency in the presence of [BMIM]PF₆ solvent, achieving complete desulfurization of multicomponent model diesel (2000 ppm of S) after only 1 h, using a low excess of oxidant (H₂O₂/S = 8) at 70 °C. However, their stability and activity showed some weakness in continuous reused oxidative desulfurization cycles. An improvement of stability in continuous reused cycles was reached by the immobilization of the Keggin polyanion in a strategic positively-charged functionalized-SBA-15 support. The PW₁₂@TM⁻SBA-15 composite (TM is the trimethylammonium functional group) presented similar oxidative desulfurization efficiency to the homogeneous IL⁻PW₁₂ compounds, having the advantage of a high recycling capability in continuous cycles, increasing its activity from the first to the consecutive cycles. Therefore, the oxidative desulfurization system catalyzed by the Keggin-type composite has high performance under sustainable operational conditions, avoids waste production during recycling and allows catalyst recovery.

Keywords: benzothiophene derivatives; functionalized-SBA-15; hydrogen peroxide; ionic liquids polyoxometalates; sustainable oxidative desulfurization.

Figure 1

FT-Raman spectra of (A) the PW₁₂-hybrids and (B) the amine-functionalized TM–SBA-15 and the corresponding PW₁₂@TM–SBA-15 composite before and after catalysis. Asterisks (*) denote bands associated with organosulfur compounds that remained adsorbed to catalysts.

Figure 2

Powder XRD patterns of amine-functionalized SBA-15 (TM–SBA-15) and the corresponding PW₁₂@TM–SBA-15 composite before and after catalysis (abbreviated as ac).

Figure 3

SEM images of the PW₁₂@TM–SBA-15 composite material at different magnifications: (A) ×5000; (B) ×25,000; (C) ×60,000 and (D) energy dispersive X-ray spectroscopy (EDS) spectrum.

Figure 4

Kinetic desulfurization profiles of the extractive and catalytic oxidative desulfurization (ECODS) process catalyzed by PW₁₂, IL–PW₁₂ compounds, composite material and blank experiments (without catalyst) using (A) [BMIM]PF₆ and (B) MeCN as extraction solvents at 70 °C.

Figure 5

Kinetic desulfurization profiles catalyzed by [BPy]₃PW₁₂ for three consecutive ECODS cycles using ionic liquid ([BMIM]PF₆) as extraction solvent at 70 °C.

Figure 6

Desulfurization data of multicomponent model diesel obtained after 1 h in the presence of the support (TM–SBA), [BPy]₃PW₁₂, PW₁₂ and PW₁₂@TM–SBA-15 with MeCN or IL ([BMIM]PF₆) as extraction solvent.

Figure 7

Kinetic desulfurization profiles of multicomponent model diesel catalyzed by PW₁₂@TM–SBA-15 for three continuous recycling cycles using ionic liquid ([BMIM]PF₆) as an extraction solvent at 70 °C.