culations of the effect of poisons (e.g., sulfur) and promoters (e.g., K+) have yielded information on the manner in which the components affect the adsorption of molecules. Computed potential energy surfaces can be used to calculate the rate coefficients for elementary processes such as adsorption, diffusion, desorption, and reaction. By use of dynamic simulation techniques and dynamically corrected transition state theory, estimates of rate parameters have been obtained that are in reasonable agreement with experimental observation.
The application of theoretical methods to zeolites has proved fruitful. Advances in theory have contributed to understanding the thermodynamics of adsorption in zeolites and the dynamics of diffusion. Monte Carlo calculations can now provide accurate predictions of the isotherms and heats of adsorption of hydrocarbons in a number of zeolites. Molecular dynamics calculations can be used to describe the motion of molecules through the void space of zeolites. Such calculations have been used to determine the molecular diffusion coefficients, various properties of motion, and the spatial distribution of sorbates within the zeolite void space. Promising progress has also been made recently in the use of ab initio quantum chemical techniques to characterize the interaction of reactant molecules with acid sites within a zeolite. Such calculations can provide a description of the reactant-zeolite potential surface, from which it will then be possible to define the reaction intermediates and to determine the rate coefficients for chemical transformations.
Yet another area in which progress is being made is the modeling of catalyst particle and reactor performance. Such models combine information about intrinsic catalyst activity and selectivity, intraparticle mass and heat transfer, and heat, mass, and fluid transport within the reactor to predict product conversion and yield, and catalyst performance with time on-stream. Models of this type can be used to determine the extent to which transport phenomena affect catalyst or reactor performance and the mode by which catalyst deactivation occurs (e.g., sintering, poisoning, fouling).
It has also become possible to use catalyst and reactor models to determine the optimal design of catalyst particles and structures. In such cases, calculations are made to maximize or minimize a desired objective function for a given set of design and operating parameters. Typical catalyst design parameters include descriptors of the structure and composition of the catalyst surface, the pore structure, the activity distribution in the catalyst particle, and the size and shape of the catalyst particle. Typical operating parameters are reactor type (fixed bed, ebullated bed, fluid bed, and so on), inlet conditions (temperature, pressure, composition, flow rate), and parameters related to heat or solid flow management. The objective function (or functions) to be maximized (or minimized) provides a quantitative measure by which the performance of the catalyst, catalytic reactor, or catalytic process is to be judged