Rapid Synthesis of ZSM-50 Zeolites for Effective n-Heptane Hydroisomerization

In this study, a rapid synthesis of ZSM-50 zeolites is achieved by a two-step aging–crystallization hydrothermal method. By optimizing the temperature/time for aging (393 K/48 h) and crystallization (423 K/72 h), the synthesis time for obtaining pure-phase ZSM-50 zeolites has been significantly shortened to 4 days from typically weeks. The gel compositions to form pure-phase ZSM-50 have also been mapped out in the ternary composition diagram of 5Al2O3/SiO2–OSDA/SiO2–OH–/SiO2 under the optimized aging–crystallization conditions. Furthermore, three ZSM-50 samples are synthesized using different gel compositions and/or crystallization conditions. The samples have different acidity characteristics and average crystal sizes from 5 to 17 μm. As catalysts for n-heptane hydroisomerization, the three Pt-loaded ZSM-50 catalysts deliver the maximum iso-heptane yield directly correlated to their surface weak Brönsted acid sites on the external surface. The ZSM-50 (5 μm) catalyst has a superior catalytic activity, achieving a maximum iso-heptane yield of 71.4% at 543 K, along with a high fraction of multibranched isomers of 29.1%. This study highlights the pivotal role of particle size in modulating acid properties and pore accessibility in ZSM-50 and offers a facile strategy for designing effective bifunctional catalysts for n-alkane isomerization.