The Effect of Synthesis Conditions and Nature of Heteroelement on Acidic Properties of Isomorphously Substituted UTL Zeolites
Abstract
The possibility to modify the acidic properties of extra-large pore UTL zeolite, possessing unique two-dimensional 14- and 12-ring pore system, was evidenced. Acid strength increases in order B-UTL<Fe-UTL<Ga-UTL<Al-UTL and the concentration of Bronsted acid centers in Al-, Ga-, and Fe-UTL frameworks could be adjusted by changing the pH value of the initial gel.
The acidic properties of isomorphously substituted UTL zeolites with different heteroelements (B, Al, Ga, Fe) in the framework, were investigated using FTIR of adsorbed pyridine, ammonia temperature programmed desorption (TPD of NH3 and Hammett indicator method. Pure germanosilicate UTL zeolites contain two types of acid centers: (i) weak silanol and germanol groups with strength 48<H0 < 68 (characterized by the band at 1446 cm-1 in IR spectra of adsorbed pyridine and by desorption peak at 180 C in TPD of NH3 and (ii) Lewis acid centers attributed to 4-coordinated Ge with strength -82 < H0 < -56 (characterized by the band at 1452 cm-1 in pyridine IR and by the desorption peak at 350 C in NH3 TPD).
Introduction of B, Al, Ga and Fe into UTL framework leads to the formation of Bronsted acid sites (band at 1542 cm-1 in the IR spectra of adsorbed pyridine and additional desorption peaks in the range of 250-490 C in NH3 TPD).