Methylation of naphthalene to prepare 2,6-dimethylnaphthalene over acid-dealuminated HZSM-12 zeolites

The dealumination of HZSM-12 zeolite (Si/Al = 65) was carried out with citric acid and hydrochloric acid. The structure and acidity were characterized by XRD, XRF, SEM, N2-adsorption, NH3-TPD, Py-IR and 29Si MAS NMR. The zeolites’ catalytic properties were studied using the methylation of naphthalene to prepare 2,6-dimethylnaphthalene as a test reaction. The results show that the two kinds of acid can remove the framework Al of and decrease the amount of Brønsted acid sites, while not destroying the framework structure of the zeolite under the proper conditions. Hydrochloric acid removed both the strong and weak acid sites simultaneously, resulting in improved catalytic stability in the methylation of naphthalene (NAPH). In contrast, the citric acid preferred to remove the strongly acid sites and also formed secondary mesopores in the sample, resulting in improved reaction performance. Under the proper reaction conditions including: a reaction temperature of 350 °C, a reaction pressure of 4 MPa, a nitrogen flow rate of 10 mL/min, NAPH:CH3OH:1,2,4-TMB 1:2:8 (mol), and WSHV = 3 h−1 with a reaction time of 4 h, the conversion of naphthalene was 30.5% and the ratio of 2,6-/2,7-DMN was 1.83 over dealuminated HZSM-12 by 10 mol/L hydrochloric acid for 6 h.

The low concentration of hydrochloric acid removed the aluminum oxide species, which block the micropores, therefore, more acid sites are exposed. The high concentration of hydrochloric acid eliminates the same number of weak and strong acid sites, simultaneously. In contrast, citric acid preferentially eliminates the strong acid sites. By prolonging the dealumination time, the weak acid sites also begin to decrease.Figure optionsDownload as PowerPoint slide