Molecular engineering approach in the selection of catalytic strategies for upgrading of biofuels

Abstract

F uel is one of the archetypical commodities, which one rarely associates with precise molecular design. However, the ever increasing severity of environmental regulations on fuels have made refiners during the last decade embrace the concepts of molecular management and molecular engineering, which still keep elements of process design, but also aim at product design. The advent of different biofuels in the energy scene, particularly those from secondand third-generation technologies, produced from nonfood biomass resources, poses new challenges and research opportunities for fuel development and catalytic upgrading. The concept of molecular management has been implemented in refining operations for some time. In simple terms, molecular management implies having the right molecule in the right place, at the right time and at the right price. By applying these concepts, refiners have developed separation and conversion processes that allow them to more accurately select the mix of crudes with properties that maximize the performance of products with higher demand at a given time (gasoline, kerosene, or diesel). The closely related concept of molecular engineering as applied to fuels implies a higher level of molecular manipulation, indicating a purposeful design of molecules with precise structures and well-defined properties. To achieve this high level of chemical specificity, the continuous improvement of catalytic materials is essential. A number of properties determine the quality of a given fuel. We can mention octane number, cetane number, sooting tendency, water solubility, freezing point, viscosity, flash point, cloud point, autoignition temperature, flammability limits, sulfur content, aromatic content, density, boiling temperature, vapor pressure, heat of vaporization, heating value, thermal and chemical stability, and storability. Many of these properties can be modified by catalytic upgrading. In designing a catalytic upgrading strategy, a refiner must know how each of these properties is affected by the structure of the molecule and how a given catalytic conversion of that structure in turn affects the properties. For example, catalytic cracking on an acidic zeolite, converting long alkanes into shorter and branched hydrocarbons would increase octane number and vapor pressure, while decreasing viscosity and density. Of course, fuels have a large number of components and for many fuel properties the overall value for the mixture depends nonlinearly on the individual properties of the components. However, it is certainly of great value to understand how the structure of a given molecule in the mixture affects each of the properties of interest. This knowledge can serve as a guide to determine what reaction paths would be the best candidates to optimize a specific fuel property of a complex mixture. There are many examples in the literature in which the molecular engineering approach has been applied for the upgrading of fossil fuels. By contrast, the same rational approach has been used more sporadically in the upgrading of biofuels. The purpose of this contribution is to point out this opportunity to the chemical engineering research community.