Effective regulation of acid sites is essential to strengthen the reaction performance and to modulate product distribution of an isomerization reaction. In this study, ZSM-50 (Al-EUO) and Fe-substituted ZSM-50 zeolites, namely FeAl-EUO-n (n = 0.25, 0.50 and 0.75) and Fe-EUO, are synthesized by a seed-assisted hydrothermal method. The study has shown that Fe has effectively entered the Al-EUO framework as a tetrahedrally-coordinated species. More importantly, the Fe-substitution has resulted in a significant increase in the amount of surface weak- and medium-strength Brönsted acids and a reduction in the strength of strong acids of FeAl-EUO-n. This study also evaluates the catalytic performance of Pt/FeAl-EUO-n catalysts in n-heptane hydroisomerization. It shows that the introduction of Fe in ZSM-50 greatly enhances the isomer yield, achieving a maximum yield of 66.1 % at 320 °C over Pt/FeAl-EUO-0.50 compared to 16.4 % at 300°C over Pt/Al-EUO. At the same time, the fraction of multi-branched iso -heptane also increases noticeably in the reaction product.
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.
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