Characterization of the active sites on the surface of alumina ethanol dehydration catalysts by EPR using spin probes, страница 2

Commercial alumina sample supplied by Condea (S.A. = 210 m²/g)was used in the experiments on the development of the method used for testing weak acceptor sites. According to the chemical analysis data provided by the manufacturer, the sample contained about 20 ppm Na₂O, 100 ppm Fe₂O₃ and 120 ppm SiO₂ as the main impurities.

Commercial aluminum oxide (AOK-63-22) was used as the starting material for synthesis of chlorinated and sulfated alumina catalysts. Chlorinated aluminum oxides were prepared by incipient wetness impregnation of the commercial Al₂O₃ sample at 20°C. About 3 g of the 0.5-1.0 mm granules of the Al₂O₃ samples were impregnated with 3.4-3.6 ml aqueous solutions with HCl concentrations 0.24, 0.48 and 0.96 M, followed by drying at 120°C for 6 h and calcination at 550°C for 4 h. The concentrations of the hydrochloric acid were calculated to yield catalysts containing 1, 2 and 4 wt.% Cl^- ions.

Sulfated alumina samples were prepared by similar procedure. About 3 g of the 0.5-1.0 mm granules of the Al₂O₃ samples (AOK-63-22) were impregnated with 3.4-3.6 ml a aqueous solutions of ammonium sulfate with concentrations 0.18, 0.36 and 0.72 M were used for incipient wetness impregnation. The concentrations were calculated to yield sample containing 2, 4 and 8 wt.% SO₄^2- ions. Modification of the AOK-63-22 alumina with the sulfates and chlorides did not affect much its surface area that remained about 240 m²/g for all studied acid-doped catalysts.

The alumina samples containing 0, 0.3, 0.6 and 0.9 wt.% sodium concentrations were prepared using ultrapure aluminum hydroxide containing 0.001 wt.% Na₂O and 0.01 wt.% Fe₂O₃ as the main impurities. It was subjected to the incipient wetness impregnation with NaOH solutions of calculated concentrations followed with drying and calcination in air at 550°C. The synthesized catalysts had the surface areas between 210 and 220 m²/g and pore volumes 0.42-0.45 cm³/g. 

The surface areas of the samples were determined by the BET method using Micromeritics ASAP 2400 instrument. (методом низкотемпературной адсорбции азота при 77К на сорботометре Asap 2400 (Micrometrics, США) - BET surface areas of the catalysts were calculated from nitrogen adsorption isotherms at 77 K in an Micromeritics ASAP 2400 instrument(equipment).

BET surface area (S_BET in m² g⁻¹) was derived from nitrogen adsorption isotherms measured on test samples at liquid nitrogen temperature, using an automatic ASAP 2400 Sorptometer of Micromeritics (USA).

The EPR spectra were recorded at room temperature using an X-band ERS-221 spectrometer. The EPR spectra were recorded at 20 dB attenuation with typical microwave power 3 mWt. The spectrometer operation and the analysis of the obtained results were performed using a PC and a software package EPR-CAD developed in our laboratory. The concentrations of the paramagnetic species were determined by numerical double integration with baseline compensation. A DPPH standard was used for calibration.

The experimental procedure was generally similar to the one used earlier for characterization of the donor sites and strong acceptor sites [17]. Before adsorption the samples were activated in air at 400°C. Electron donor sites were characterized by adsorption of 1,3,5-trinitrobenzene from 2 x 10^-2 M solution in toluene. Strong acceptor sites were characterized using toluene as the donor molecule. Weak acceptor sites were characterized using anthracene adsorption from 4 x 10^-2 M solution in toluene. After an activated sample was filled with a desired solution, the ampoule was sealed. The concentrations of the corresponding sites were assumed to equal the maximum concentrations of the radical species obtained after heating the samples with the adsorbed probe solution at 80°C for 12 h.