Structure and base properties of calcined hydrotalcites

Abstract

JOURNAL OF CHEMETRY 1999 Structure and base properties of calcined hydrotalcites Abetrrd Five hydmtalcites with MglAl molar d o range of3-15 were pepared. llle ehucture and basicity of Mg-Al mixed oxides (Mg(AI)O) transformed fmm hydrotalcites were investigated by IPD, XPS, XRD, FT-IR and NMRtechniques. llle r e s u l t s o f e l~~ anal* and m indicate that Al is enriched in the SUTface +om of Mg(M)O, and its ammt increases with the MglAI molar d o and the Calcination temperatwe. nAl-MASNMR results show that AI exists in two chenrid environments: t e i m t d d aluminium (Al(t)) and octahedlal aluminium (Al(0)) in Mg(Al)O. l l l e amopLnt of Al(t) increases with the higlAl molar ratio and the calcination tempmhm. It is assumed that A(t) may be mainly h the surface Al. Tenpnmm-pmpmmed de-soption (IPD) of Cq shows that the number of basic sites of Mg ( Al) 0 samples increases with the Mg/AI molar ratio, and the maximum number of basic sites is obtained for hydmtalcite calcined at 773 K. Mrad spectra of adembed C@ and B(OCH3)s ~v e d that there are two kinds of basic sites: weak basic OHsites and s h n g basic bsites on the hfg( Al) 0 samples, the base strength depends on the M d A l molar d o and calcination temperature.it is proposed that the s d a c e Al( t) probably results from the Mg-Al inverse spinel, MgA1204, formed by migrat;on of M3+ to intelstitial site with the tetrahedral symmetry in the rock-salt lattice of MgO. The spinel structure exists in small and local domains. It is dinerent from the bulk MgAl2O4 and is not ob-~erved in the XRD patterns. According to this proposal, increasing the calcination temperature and Mg/ Al molar ratios may reduce the stability of the Mg( Al) 0 and promote the migration of Ai3' to form MgAlZO4. 'Ihis is further substantiated by nAl-MASNMR results. From the chemical analysis and XPS results, Al is enriched in the surface regions of Mg( Al) 0 rather than unifomdy distributed in the bulk, and the enrichment becomes greater with the increase of Mg/Al molar ratios and calcination temperatm. It is suggested that Al(t) may be derived from surface Al rather than bulk Al, and the enrichment of Al in surface and the appearance of Al( t) is related to the formation of the inverse spinel, MgA1204, located in small and local domains on the Mg( M ) O surface. ?he basic properties of MgO have been extensively studied. There are two kinds of basic sites: strong basic dand weak basic O H . C Q adsorption produces the bicarbonate and monodentate carbonate on the surface. lltm bands at 3623, 1665 and 1224 cm-' due to the bi&nate species are observed for all Mg( Al)O samples. Lercher et ul . l7 suggested that the wavenumbers of U,(CO) of HCG-species de-cre8se6 with the increase of electron-pair donor ( EF'D) strength of the OHsites, i . e . , the base strength of the O K sites. ?he wavenumbers of us(G0) of HCQspecies decrease in the order of 1444, 1411, 1409, 1405 ard 1392 an-' respectively for M203, three Mg( Al) 0 samples and MgO . It is apparent that the base strength of the O H sites incr...

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