Zinc-Based Nanocrystalline Aluminum Oxide Adsorbents for Desulfurization Application
Xiangxin Yang1, Chundi Cao1, Larry E. Erickson1, Keith L. Hohn1, and Kenneth J. Klabunde2. (1) Department of Chemical Engineering, Kansas State University, Manhattan, KS 66502, (2) Chemistry, Kansas State University, Willard Hall, Chemistry Dept., Manhattan, KS 66506
Deep removal of sulfur from fuels has received more and more attention worldwide because of environmental consideration and the need to produce ultra-low sulfur fuels for fuel cell applications. The state of the art in desulfurization technology is hydrodesulfurization (HDS). In this process, under high temperature (300-340°C) and high pressure (20-100 atm of hydrogen), sulfur-containing compounds are converted to hydrogen sulfide and corresponding hydrocarbon on CoMo/ Al2O3 or NiMo/ Al2O3 catalyst. The reactivity of organosulfur compounds varies widely depending on their structure and local sulfur atom environment. HDS is very efficient for aliphatic organosulfur compounds (mercaptans, sulfides, and disulfides) because of high electron density on S atom and weaker C-S bonds. Thiophenic compounds are less reactive in HDS due to aromatic stabilization of the thiophene and benzothiophene rings, especially for dimethylbenzothiophene (DMDBT) alkkylated at the 4 and 6 positions. Ultra-low sulfur specifications for fuels can only be produced under severe reaction conditions with respect to pressure, temperature and residence time, which significantly increase the cost of HDS. Consequently, development of new and affordable deep desulfurization processes for removing the refractory sulfur compounds is one of the major challenges for refineries and fuel cell research. Adsorptive desulfurization is considered to be the most promising alterative to catalytic hydrodesulfurization (HDS). Adsorbents based on nanocrystalline oxides have several advantages: high surface area, open pore structure, and high pore volume. In the present study, zinc based nanocrystalline aluminum oxide adsorbents are synthesized by the xerogel method. Thiophene adsorption from hydrocarbon liquid is conducted by a batch method. Brunauer-Emmett-Teller specific surface area (BET), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) are used to characterize the adsorbents. The adsorptive experiment and characterization results show that theomovacuum treatment is critical for the occurrence of thiophene adsorption. An “aluminate-type” phase (surface spinel) is formed during thermal treatment. Compared with the treatment conducted in air, the interaction between zinc ions and alumina support is weaker and that between thiophene molecules and zinc ions is stronger when the treatment is carried out under vacuum. In addition, the surface areas of the adsorbent are larger when adsorbents are treated under vacuum.