(590c) Development of Sulfur Tolerant Reforming Catalyst for Diesel and Jet Fuel: Understanding the Reaction Pathways and Catalyst Characterization
AIChE Annual Meeting
Thursday, November 16, 2006 - 4:15pm to 4:45pm
Sulfur poisoning and coke formation during reforming of logistic fuels (i.e. diesel, jet fuel, and gasoline) to produce hydrogen for fuel cell systems are two major factors leading to catalyst deactivation. Desulfurization of the fuel prior to reforming through the addition of selective sulfur sorbent is one option to address this concern. However, the weight and volume of the sorbent may be substantial, and such desulfurization systems may not be practical if large fuel volumes will be used. Therefore, development of more stable catalyst formulations is essential for reforming of heavy fuels. Several studies have shown that the rate of deactivation is increased by the presence of sulfur in the fuel, due to interactions between the catalyst surface and the sulfur component in the fuel, leading to an increase in the surface acidity and thus greater rates of coke formation.
We have evaluated a number of catalyst formulations that combine high reforming activity with the ability to react with the sulfur to produce a stable solid catalyst. In our present study, for studying diesel fuel steam reforming, catalysts were prepared through wet impregnation of commercially available catalyst supports. These catalysts were tested with 25-100 ppm sulfur content in the fuel. Thiophene doped n-hexadecane was used as surrogate fuel for diesel. Steam reforming was carried out at 800°C at steam to carbon ratio of 5. In a proposed commercial application, we envision a reforming system in which the catalyst is sequentially deactivated and regenerated. In order to test the capacity of the proposed catalysts for use in this system, we regenerated the deactivated catalyst with air. Three sequential reaction and regeneration cycles were carried out to evaluate catalytic activity and stability of the reforming system throughout the proposed process.
Several catalysts were stable for greater than 100 hr during reforming with 25 ppm sulfur. At 100 ppm sulfur loading, the catalysts showed substantial deactivation within 24 hr, but could be regenerated through combustion with air. These regenerated catalysts then provided high activity and good stability when used in the presence of 25 ppm sulfur. The addition of selected catalyst promoters enhanced the performance of the catalytic system, giving both increased hydrogen productivity and improved catalyst stability.
For steam reforming of jet fuel, metal oxide modified noble metal catalysts were prepared on stabilized catalyst supports. Steam reforming experiments were performed on toluene (used as a jet fuel surrogate) with and without sulfur (50 ppm equivalent of thiophene). The reforming conditions for these experiments remained constant, and were: steam to carbon (S/C) ratio = 3, operating temperature = 825°C at ambient pressure (1 atm.), and GHSV = 11,000 hr-1. The addition of a base metal oxide promoter increased the activity and the H2 yield and, depending on the choice of promoter, also increased the stability of the catalyst.
In order to understand the catalytic system and prepare more highly active and stable catalysts, more information is needed describing the specific processes that lead to catalyst deactivation during steam reforming. Steam reforming of logistics fuels (or fuel simulants) is a complex mix of reaction steps, interacting through the catalytic metals to produce desired and undesired products. Modeling through lumped reaction kinetics has often been used to understand the complex reaction networks associated with cracking of large molecular weight compounds. A similar methodology is being used in the current work, in order to better understand the kinetics of steam reforming and the specific reaction steps leading to catalyst deactivation. In situ steam reforming using FTIR-DRIFTS should elucidate specific reaction steps associated reforming, and indicate changes in the catalyst surface leading to deactivation.
Surface analysis of fresh and used catalyst provide details regarding the structural changes of the surface throughout the deactivation/regeneration process, providing critical information into understanding the role of the promoters in enhancing the sulfur tolerance of the catalyst. Surface characterization was done primarily by BET, TPR, TPO and chemisorption. Preliminary results show that coking and sulfur adsorption is accompanied by active metal sintering in reducing atmosphere. Further analysis using XPS and XPD will reveal changes in metal-metal and metal-support interaction as the deactivation occurs.