Role of Bronsted Surface Acid Sites of B2O3-Al2O3 Catalyst Supports in Perylene Radical Cation Formation Studied by Probe EPR with TEMPO Radicals

For the series of borate-modified alumina samples with a wide range of B2O3 content (0-30 wt %), the regularities of the change in acid properties were studied by probe EPR spectroscopy of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and solid-state H-1 NMR. An increase in the concentration of Bronsted acid sites (BAS) in the series with an increase in the boron oxide content to 30 wt % was found. The obtained dependences of the BAS concentration for the samples determined from TEMPO adsorption and H-1 MAS NMR data are in good agreement. Also, the results of determining the BAS concentration by a direct method (via the concentration of TEMPO complexes with BAS) are close in values to the concentration of perylene radical cations determined on the surface of the corresponding B2O3-Al2O3 samples. The obtained results show that the proposed EPR approaches are promising for the quantitative analysis of surface BAS in the study of various porous acid solids.