NANOSCALE METAL-OXIDE PARTICLES AS CHEMICAL REAGENTS - INTRINSIC EFFECTS OF PARTICLE-SIZE ON HYDROXYL CONTENT AND ON REACTIVITY AND ACID-BASE PROPERTIES OF ULTRAFINE MAGNESIUM-OXIDE

Two sizes of ultrafine magnesium oxide particles have been carefully compared for intrinsic surface chemistry differences. Ultrahigh surface area MgO (400-650 m2/gm) prepared by an autoclave hypercritical drying (aerogel) procedure has been compared with conventionally prepared MgO (100-300 m2/gm) for hydroxyl surface concentration (also residual OCH3), Pyridine, benzene, xylene, and nitrobenzene adsorption. The aerogel-prepared material exhibited the highest surface area (650 m2/g) after a 300-degrees-C heat treatment, while the conventional sample's surface area went up to 300 m2/g after a 400-degrees-C treatment. Both FT-IR/photoacoustic and conventional transmission IR spectroscopies were employed to monitor the adsorbed species. Amounts of material adsorbed were also quantitatively determined. Careful comparisons between the smaller and larger particle MgO were made. It was shown that surface -OH concentrations were similar but that the larger particles allowed more geminal pair reactions of -OH with AlEt3 to yield EtAl-(O2)(ad) rather than Et2Al-O(ad). It was also found that the MgO particles adsorbed pyridine on the Mg2+ sites and not the Bronsted sites. However, benzene did adsorb on Bronsted sites, and IR shifts showed that the smaller particles exhibited weaker Bronsted acidity. Nitrobenzene adsorption suggested weaker Lewis basicity as well. It is proposed that basic/reducing character depends more on domains and is encouraged by larger particle size and Bronsted acidity is also encouraged by -OH surface island domains. Overall, the results show that particle size can have intrinsic effects on surface chemistry.