Using Pore Window Size to Control Selectivity for Acetylene Hydrogenation on Pd@LTA Zeolite Catalysts

Selective hydrogenation of acetylene in ethylene-rich streams is important for industrial applications of ethylene and is commonly performed on Pd-based catalysts. To improve the ethylene selectivity, we incorporated highly dispersed Pd into 4A, a Linde type A zeolite with a pore size of ∼4 Å, with the assistance of mercaptosilanes. Reaction studies indicated that incorporation of Pd within the zeolite (Pd@4A) resulted in higher ethylene selectivity compared to deposition on the external surface of the zeolite (Pd/4A), even at high acetylene conversions near 99%. Pressure decay adsorption measurements showed that diffusion was markedly faster for acetylene compared to ethylene within the 4A. To test the hypothesis that pore (window) size determined selectivity, the Pd@4A zeolite pore size was further modified by variable loadings of NaOH. Breakthrough curve measurements and kinetic adsorption studies indicated that the ethylene diffusion rate decreased with increasing deposition of NaOH, suggesting that accessibility of ethylene to the encapsulated Pd within the LTA zeolite was hindered. The NaOH-modified zeolites exhibited a higher ethylene selectivity (∼80% under 93% acetylene conversion). However, excessive NaOH loading led to a low activity. This strategy provides an avenue for enhancing the selectivity by tuning the accessibility of active sites inside of the porous material.