Shape-selective catalysts of Mg²⁺ ion exchange modified SAPO-11 molecular sieves for alkylation of 2-methylnaphthalene

The conversion of 2-methylnaphthalene (2-MN) and methanol by molecular zeolites to 2,6-dimethylnaphthalene (2,6-DMN) is significant for the preparation of polyethylene naphthalate. However, there is still a challenge to develop catalysts with efficient selectivity and activity. In this work, the hydrothermally synthesized SAPO-11 samples were modified by ion-exchange of Mg²⁺ for the catalytic alkylation of 2-MN, and thus the selectivity of 2,6-DMN was greatly improved. The results show that the Mg²⁺ ion-exchanged SAPO-11 molecular sieves could maintain the original main structure and morphology. However, the introduction of Mg²⁺ regulated the pore structure, acid content, acid strength and the distribution ratio of Brönsted acid and Lewis acid sites in the molecular sieves, and thus the Mg/SAPO-11 catalyst presented better catalytic performance than the pure SAPO-11. After 6 h of reaction, the selectivity of 2,6-DMN could still maintain 41.3% and the ratio of 2,6-/2,7-DMN was 1.87 over 0.06M-Mg/SAPO-11 sample. The deep isomerization of methylnaphthalene was reduced due to the Mg²⁺ ion exchange resulting in a shape-selective effect from passivation of the outer surface of the catalyst and adjustment of the open pore size. Therefore, this work presents a shape-selective catalysis in zeolites using Mg²⁺ ion exchange strategy for the alkylation of 2-MN.