(444e) Synthesis of Hierarchically Structured Zeolite Monoliths as Desulfurization Adsorbents for Logistics Fuel
Logistics fuel reformation to hydrogen for fuel cell based electrical generation for military use is highly desirable and under development. However, the high sulfur content in logistics fuels such as JP-8 (<3000ppmw-S) irreversibly poisons reformer catalysts. Fuel cell catalysts are also poisoned by sulfur species in the hydrogen stream from the reformer. Organosulfur component removal from the fuel prior to reformation eliminates negative effects from sulfur in both reformers and fuel cells. Adsorption of organosulfur diesel fuel components at ambient conditions by nickel-exchanged zeolite Y in previous studies by Hernandez-Maldonado et al. (2004) has produced 19mL of deeply desulfurized (<1ppmw-S) diesel per gram of sorbent. Highly accessible, high surface area monolithic macroporous mesoporous zeolite (3MZ) materials based on nickel exchanged zeolite Y nanocrystals are proposed as high capacity adsorbents for organosulfur compounds from the logistics fuel JP-8.
The distance organosulfur molecules must diffuse to reach the Ni(II) sites in the zeolite crystals can be minimized by reducing the zeolite crystal size. Instead of diffusing to the center of large, commercial 2ìm zeolite crystals, organosulfur molecules would only have to find their way 50nm into the most buried Ni(II) sites in the proposed 100nm zeolite nanocrystals. Reducing the crystal size from 2000nm to 100nm (assuming a spherical geometry) also results in a 20-fold increase in external surface area per unit volume of zeolite material. The many external adsorption sites could easily accommodate the larger organosulfur molecules with plenty of pore access remaining, and the guard bed previously required in other work could also be eliminated.
Small zeolite nanocrystal size can also introduce complications along with their promising advantages, however. Loose nanocrystals can be hard to confine in one location, therefore a hierarchical zeolite monolith is proposed. With the zeolite nanocrystals fused together in a mesoporous and macroporous monolith, liquid fuel will easily pass through while achieving a high degree of contact with the zeolite adsorbent. A monolithic structure also has many other benefits, including no bed settling, orientational freedom, and ease of handling.