Metal-Organic Framework For Separating Paraffin Isomers


Metal-organic frameworks (MOFs) constitute a large family of microporous solids exhibiting high surface areas tunable pore dimensions and adjustable surface functionality. With these attributes MOF performance is beginning to rival traditional solid adsorbents such as zeolites and activated carbons for some key gas-storage and molecular-separation applications. With regard to the latter the possibility of creating pore characteristics that cannot readily be achieved in zeolites or carbons expands the opportunities for molecular recognition. In the presence of the right surface environment separations currently carried out inefficiently could potentially be performed with a substantially reduced energy cost. The efficient separation of alkane isomers by adsorption is especially challenging because the molecules are chemically inert and have similar polarizabilities leaving shape as the main handle available for their differentiation. This separation is critical to the production of gasoline which is composed of ~10% pentanes and hexanes. Hexanes are generated at enormous scale through a catalytic isomerization reaction that results in a thermodynamically controlled product stream composed of 10 to 30% of each of the five different isomers. The value of a particular isomer as a component in the gasoline pool is related to its research octane number and is highest for the dibranched hexanes 23-dimethylbutane and 22-dimethylbutane. Currently about two million barrels of pentanes and hexanes are processed daily.To address this situation researchers at UC Berkeley have developed a MOF featuring sharply angled pore walls of a type not encountered in zeolites and capable of fractionating alkane isomers according to the degree of branching. Using this Berkeley MOF as an improved hexane-separation process would selectively isolate the most valuable products 23-dimethylbutane and 22-dimethylbutane while returning the less valuable monobranched isomers to the isomerization reactor along with n-hexane. Further it would potentially benefit public health because it could reduce the usage of toxic aromatics which are currently added to boost the octane number of gasoline. Performing this separation at or near the isomerization temperature would save a great deal of energy in the production of high-quality gasoline. Numerous attempts have been made to identify solid adsorbents capable of effecting an efficient separation of hexane isomers according to octane number including zeolites silicas and very recently metal-organic frameworks. Although zeolite BETA facilitates the separation of 22-dimethylbutane from the monobranched isomers and some 23-dimethylbutane (the highest value isomer) elutes with it these results are only seen at 0.01 bar and are therefore not useful for a separation.

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