Raw natural gas contains various concentrations of hydrogen sulfide (H2S) gas, which needs to be reduced to a concentration below 4 parts per million in the US. Conventionally, natural gas desulfurization is accomplished by contacting the gas stream with a liquid stream that absorbs the H2S. Although the liquid absorption process is technologically mature and has been used for nearly 100 years, it is highly energy intensive and is also plagued by equipment corrosion and solvent loss problems. Adsorption is a technology that is ideally suited for gas purification, provided an efficient sorbent with high adsorption capacity and H2S-selectivity is found. In this work, new approaches for synthesizing amine-grafted silicas with superior sulfur capacity will be developed. Results of this project may guide the design of a new, more efficient process for natural gas desulfurization. This project will involve active participation of a diverse group of graduate as well as undergraduate students, particularly minority and female students.
Amine-grafted mesoporous silicas are currently the best desulfurization sorbents because they have good sulfur capacity and selectivity, high moisture resistance/stability, fast uptake rates, and regenerability; however, the sulfur capacity needs to be further improved to make adsorption competitive to the liquid scrubbing process. Amine grafting is accomplished by the reaction between the surface hydroxyl groups of silica (i.e., silanols) and an aminosilane dopant. The approach used in this work is to increase the silanol density on the mesoporous silica, thereby directly increasing the grafted amines and consequently the H2S capacity. The work includes three basic strategies to increase the silanol density. The mesoporous silica MCM-48 will be the main silica to be used because of its high BET surface area (~1400 m2/g). Other silicas such as SBA-15 (with large pores, 7-20 nm) and MCM-x with expanded pores will also be used. The three strategies are all scalable and environmentally sound, outlined in three tasks: 1) new and improved organic-template removal processes; 2) post-treatment (i.e., after template removal) of the mesoporous silica in a hydroxyl abundant environment; 3) in-situ synthesis of mesoporous silicas with maximized silanol densities. Task 1 is the main task, which includes five sub-tasks aimed at new template removal processes to increase silanol density: microwave digestion; special gaseous oxidants; supercritical extraction; extraction using organic solvents and ionic liquids, and nonthermal ("cold") plasma. The resulting mesoporous silicas with the highest silanol densities will be subjected to aminosilane grafting and H2S adsorption/desorption measurements including both equilibrium adsorption isotherms and rates. Because the results of the three strategies may be synergistic, the PI will study combined treatments during the later phase of the proposed work. This work will lead to the development of superior H2S adsorbents that may enable adsorption technology to replace the wet amine absorption technology that is currently used for natural gas desulfurization. The increased use of clean natural gas--compared to coal and oil--will help mitigate global warming.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
