2.2 Catalyst preparation
Pure silica MEL and Pure silica Fe containing MEL zeolites were synthesized by hydrothermal method, using TEOS as a silicon source, iron citrate as the iron source and TBAOH as the organic template, respectively. Typically, the TEOS was slowly added to TBAOH solution under vigorous stirring, and after mixing at room temperature for 3 hours, iron citrate and deionized water were added. The final molar composition of the gel was 1 TEOS : 0.25 TBAOH : 25 H2O : x Fe2O3 (x = 0 and 0.007, respectively). After aging for 6 hours at 30 oC, the prepared mixture was placed into stainless steel autoclave and crystallized at 170 oC for 72 hours under static conditions. After crystallization, the product was filtered, washed several times with deionized water, and dried overnight at 100oC. Finally, the organic templates were removed by calcination at 550 oC for 6 hours in an air atmosphere. Those samples were denoted as P-MEL (pure silica MEL zeolite) and P-MEL@Fe (Pure silica Fe containing MEL zeolite), respectively.
The above synthesized P-MEL and P-MEL@Fe samples were further treated. Typically, a certain amount of aluminum isopropoxide was dissolved in 0.1 M/L of TBAOH solution, and then the concentration of aluminum isopropoxide solution was tuned to 0.015 M/L, 0.024 M/L and 0.03 M/L, respectively. The amount of solution was 30 ml/g as-prepared P-MEL or P-MEL@Fe. Then, it was placed into a stainless-steel autoclave and crystallized at 170 oC for 72 hours under static conditions. In the same way, the product was filtered, washed several times with deionized water, and dried overnight at 100oC. Finally, the organic template was removed by calcination at 550 oC for 6 hours in an air atmosphere. Those hierarchical zeolites were denoted as H-MEL-31 (it was obtained from P-MEL and the Si/Al = 31) or H-MEL@Fe-34, H-MEL@Fe-23 and H-MEL@Fe-20 (they were prepared from P-MEL@Fe and the Si/Al = 34, 23 and 20), respectively. The corresponding Si/Al ratios of zeolites were determined by ICP-OES.