Rational design of shape selective separation and catalysis—I: Concepts and analysis

A molecule can enter a zeolite channel when it fits the size and shape of the opening. Zeolites have been used commercially for their ability to admit selectively molecule A but not molecule B. The search and design of zeolites for separation has been based on finding a channel with a diameter that is larger than molecule A, but smaller than molecule B. This method is not sufficiently accurate when the molecule is not a sphere or the channel is not a circle. We present here first a more accurate screening method based on comparing both the major and minor diameters of the molecule projection, called the “footprint”, with the diameters of the channel. Then we present an even better method that is based on the activation energy required to strain and distort a molecule to fit a given channel, thus leading to a lowering of its Boltzmann concentration in this channel. This paper compiles the activation energies encountered among thirty-eight (38) molecules and two hundred and seventeen (217) zeolite channels into a database, and shows how one can use this database to identify the most promising zeolites for the separation of a set of molecules of interest. Such a screening method would help to identify a zeolite that has much lower activation energy for molecule A than for molecule B. The appropriate temperature range for separation would be centered around the optimal temperature T^*, where the degree of selectivity is at a maximum. The separation of the three pentane molecules was selected as an illustration.