The hydroisomerization of n-hexane over highly selective Pd/ZSM-22 bifunctional catalysts: The improvements of metal-acid balance by room temperature electron reduction method

Abstract

The preparation of bifunctional catalysts with high catalytic selectivity and stability for the n-alkanes hydroisomerization still remains challenging for the production of clean fuel. Herein, two series of Pd/ZSM-22 bifunctional catalysts are prepared by the room temperature electron reduction (Pd/Z22(x)-E) and conventional hydrogen reduction methods (Pd/Z22(x)-C), respectively. The physico-chemical property and catalytic performance for the n-hexane hydroisomerization over all catalysts are studied. Owing to the avoidance of high temperature measurement in the preparation process, the deposited Pd nanoparticles of Pd/Z22(x)-E catalysts reveal higher dispersion, smaller size and larger {1 1 1} facets fraction compared with Pd/Z22(x)-C catalysts. The results of n-hexane hydroisomerization demonstrate that Pd/Z22(x)-E catalysts show promoted catalytic performance because of the stronger metal function and more favourable metal–acid balance. Especially for the Pd/Z22(90)-E catalyst, the maximum i-hexane yield is 76.3% at the n-hexane conversion of 83.9%, which is 8.3% higher than that of the Pd/Z22(90)-C catalyst. In addition, the products distribution and calculated thermodynamic equilibrium constants suggest that the modified characteristics of Pd/Z22(90)-E catalysts are beneficial for both the generation of mono-branched i-hexane products and the limitation of their further isomerization and cracking. Furthermore, the results of long-term tests of 72 h indicate that the Pd/Z22(90)-E catalyst shows enhanced stability due to the weaker Pd aggregation and less carbon deposition. Therefore, the room temperature electron reduction method is an effective way to prepare bifunctional catalysts with improved catalytic performance for n-alkane hydroisomerization.