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.