Adsorptive Desulfurization of Diesel Fuel over TiO2-CeO2 mixed Oxides-based Adsorbents

Open Access
Author:
Xiao, Jing
Graduate Program:
Energy and Mineral Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
November 05, 2012
Committee Members:
  • Chunshan Song, Dissertation Advisor
  • Chunshan Song, Committee Chair
  • Yongsheng Chen, Committee Member
  • Caroline Elaine Clifford, Committee Member
  • Michael John Janik, Committee Member
  • Andre Louis Boehman, Special Member
Keywords:
  • adsorptive desulfurization
  • TiO2-CeO2
  • adsorbents
  • diesel fuel
Abstract:
The ultimate goal of this dissertation is to develop and fundamentally understand the adsorptive desulfurization (ADS) of diesel fuel over TiO2-CeO2 mixed oxides-based adsorbents in the proposed new approaches, including air-promoted ADS, selective ADS from light-irradiated fuel, and activated carbon (AC) guard-bed assisted ADS. Adsorptive desulfurization test was carried out in a flow adsorption system. Adsorption selectivity was studied in a model fuel containing the same concentration of different compounds.The sulfur chemistry during adsorption was investigated by ANTEK Total Sulfur Analyzer, Gas Chromatography-Pulsed Flame Photometric Detector (GC-PFPD), and Sulfur K-edge X-ray Absorption Near Edge Structure (XANES). N2 adsorption test was used to characterize the textural properties of the adsorbents. The surface chemical properties of the adsorbents were characterized by Ti L2,3-edge and Ce M4,5-edge XANES,Fourier Transfer Infrared (FT-IR) Spectroscopy, Zeta potential, and boehm titration. Air has been introduced into ADS for the first time as a promoter for ultra-deep desulfurization of diesel fuel under ambient conditions. Air significantly promotes ADS by facilitating the formation of highly polar sulfoxide species from original sulfur compounds. Moreover, by incorporating Ce into TiO2, ADS capacity increased 20-fold due to a significant synergistic effect between TiO2 and CeO2, which can be mainly attributed to the increased amount of reduced centers of Ti3+ and Ce3+ by doping Ce into TiO2. The reduced centers possibly resulted in active O-vacancy sites for O2 activation to form superoxide species when feeding air at room temperature, which further chemically transform organosulfurs to sulfoxides. In the presence of air, light-irradiation has been reported in petroleum chemistry in the last century to degrade or partially oxidize hydrocarbons to form oxygenates, i.e.peroxides. Photooxidation is also widely known for pollutant cleanup in water and air. Taking advantage of the peroxides generation by light-irradiation of fuels, lightirradiation of fuel has been studied for the first time for ADS over TiO2-CeO2-based adsorbents. The light-irradiation of diesel fuel dramatically enhanced the selective removal of refractory sulfur compounds over the TiO2-CeO2/MCM-48 adsorbent,resulting in a superior adsorption capacity of 95 ml-F/g-A, which is over 30-fold higher than that for the original diesel fuel(~ 3 ml-F/g-A). Light-irradiation of fuel promoted the in-situ generation of peroxides, which further served as active oxygen species to catalytically transform sulfur species to highly polar sulfones over TiO2-CeO2/MCM-48. Fuel composition, i.e. nitrogen compounds, affects ADS over TiO2-CeO2/MCM-48.Activated carbon, a promising material for the removal of nitrogen compounds, has been further studied as the guard bed material for ADS over TiO2-CeO2/MCM-48 adsorbent bed from light-irradiated diesel fuel. Using AC-1 as the guard bed further doubled ADS capacity over TiO2-CeO2/MCM-48 to 195 ml-F/g-sorb. AC guard bed further enhanced ADS by pre-selectively adsorb nitrogen compounds, which could block TiO2-CeO2 active sites for sulfur oxidation, but not peroxides, which served as active oxygen species for sulfur transformation from light-irradiated fuel.