Catalysis of Organic..

358Synthesis of PseudoiononesMgO PericlaseLi 2 CO 32Intensity (kcps)Li/MgO-5Li/MgO-4Li/MgO-3Li/MgO-2Li/MgO-1MgO10 20 30 40 50 60 70 802θ (°)Figure 1. X-ray diffraction patterns of MgO and Li/MgO-x catalystsdesorption temperature for MgO and Li/MgO-x catalysts. The total amount ofdesorbed CO 2 (n b ) was measured by integration of TPD curves and the resultingvalues are reported in Table 2; n b shows a maximum value of 11.3 µmol/m 2 for a Liloading of 0.5 wt. % and clearly decreases for higher Li loadings. The complex TPDprofiles of Fig. 2A suggest that the surface of the Li/MgO-x oxides are nonuniformand contain several species formed by CO 2 adsorption; in other words, the solidsurface contains oxygen atoms of different chemical nature that bind CO 2 withdifferent coordination and binding energy. In previous work (10), we investigated thechemical nature of CO 2 adsorbed species on similar alkaline- promoted MgOcatalysts by using FTIR and identified at least three different species: unidentatecarbonate, bidentate carbonate, and bicarbonate. These three adsorption species aredepicted in Fig. 2B. Unidentate carbonate formation requires isolated surface O 2-ions, i.e., low-coordination anions, such as those present in corners or edges andexhibits a symmetric O-C-O stretching at 1360-1400 cm -1 and an asymmetric O-C-Ostretching at 1510-1560 cm -1 . Bidentate carbonate forms on Lewis acid-Brönstedbase pairs (M n+ -O 2- pair site, where M n+ is the metal cation Mg 2+ or Li + ), and shows asymmetric O-C-O stretching at 1320-1340 cm -1 and an asymmetric O-C-O stretchingat 1610-1630 cm -1 . Bicarbonate species formation involves surface hydroxyl groupsand shows a C-OH bending mode at 1220 cm -1 as well as symmetric and asymmetricO-C-O stretching bands at 1480 cm -1 and 1650 cm -1 , respectively. We alsodetermined (10) the following base strength order: low coordination O 2- anions >oxygen in M n+ -O 2- pairs > OH groups. Based on these FTIR results, the TPD profilesof Fig. 2A were deconvoluted in three desorption bands in order to quantify thedensity of weak, medium and strong base sites. The