Diffusion-Selective Tandem Catalysis for Alkane Hydroisomerization.

Area of Science:

• Catalysis
• Chemical Engineering
• Materials Science

Background:

• Molecular diffusion significantly impacts selectivity in catalytic systems, similar to zeolite shape selectivity.
• This 'diffusion-selectivity' phenomenon is often overlooked and not fully understood.
• Understanding diffusion effects is crucial for optimizing catalytic processes.

Purpose of the Study:

• To demonstrate and elucidate intermediate diffusion-selective effects in catalytic hydroisomerization.
• To precisely modulate a tandem diffusion system for n-alkene and i-alkene intermediates.
• To investigate how manipulating diffusion pathways influences reaction outcomes and product selectivity.

Main Methods:

• Fabrication of Pt/zeolite composites with controlled Pt nanoparticle deposition.
• Systematic variation of zeolite structure to alter diffusion distances for intermediates.
• Analysis of surface permeability and intracrystal diffusion rates.
• Quantification of isomer yield and side-reaction mitigation.

Main Results:

• Prolonging n-alkene intermediate diffusion distance by external Pt deposition reduced surface permeability by 20%, enhancing dispersion and minimizing side reactions.
• Decreasing i-alkene intermediate diffusion length via reduced zeolite channel length increased intracrystal diffusion by two orders of magnitude.
• This modulation prevented secondary cracking, leading to a high isomer yield of 62%.

Conclusions:

• Precisely controlling intermediate diffusion pathways offers a powerful strategy for enhancing catalytic performance.
• The demonstrated diffusion-selective approach significantly outperforms conventional methods.
• This concept is generalizable to reactant and product diffusion-selectivity in various catalytic applications.