Rational design of hierarchically structured catalysts: multiscale modeling, optimization and experiments

Abstract

Industrial nanoporous catalysts suffer from diffusion limitations and catalyst deactivation caused by active site poisoning and pore blockage. While significant attention has been paid to designing new and improved catalysts at the nanoscale, there has been no concerted effort at linking the nanoscale to the macroscale within the framework of rational catalyst design. The questions that need to be answered are: (i) which pore network is optimal to best resist catalyst deactivation, and (ii) how to establish a framework for rational catalyst design at all length and time scales?

In order to demonstrate the idea of a framework for rational catalyst design by combining theory and experiments, the total meso- plus macroporosity of a hierarchically structured zeolite is numerically optimized to maximize the production of ethylbenzene in the alkylation of benzene with ethylene. The optimum structure which maximizes the catalytic yield has a total meso- plus macroporosity of 32% with 300 nm ZSM-5 crystals occupying 70% of the volume in a single composite grain. ZSM-5 composites with varying textural properties are then synthesized using a versatile two-step synthesis route. Fixed bed reactor experiments with the composites show qualitative agreement with reactor simulations, indicating that numerical optimizations of the broad pore network can be used to guide the synthesis of more active hierarchically structured catalysts.

To answer the first question, model driven numerical optimizations of the broad pore network in hierarchically structured catalysts are carried out with the goal of minimizing diffusion limitations and increasing stability to catalyst deactivation. It is shown that a broad pore network with an optimized uniform macroporosity and an optimized uniform broad pore size is nearly as optimal as a broad pore network with an optimized distribution in macroporosities and broad pore sizes. For catalyst deactivation in the hydrodemetalation of crude oil containing a single asphaltene species, a doubling in useful lifetime at the pellet scale and a nearly 40% increase in lifetime at the reactor scale is demonstrated, as compared to a non-hierarchical structure.