from first principles PP-I-1

PP-III-85MgAlY Layered Double Hydroxides as Catalysts for Organic SynthesesCatalysed by Base SitesPavel O.D, Zavoianu R., Angelescu E., Neatu F.University of Bucharest, Faculty of Chemistry, Department of Organic Chemistry,Biochemistry and Catalysis, Bd. Regina Elisabeta, No. 4-12, S3, Bucharest 030018, Romaniarodica.zavoianu@g.unibuc.roThis contribution aims to bring new highlights concerning the effect induced by theincorporation of Y in the brucite-type layer of MgAl layered double hydroxides (LDH)structures on the basicity of these structures, the possibility of their reconstruction via the“memory effect” and their catalytic activities in organic syntheses mediated by base sites. Aseries of Mg x Al (1-y) Y y -LDH compounds (where x = Mg/(Al+Y) = 1-10, y = 0.04-0.66) havebeen prepared by co-precipitation at constant pH = 10 under low supersaturation conditionsusing aqueous solutions of metal nitrates, NaOH and Na 2 CO 3 according to a previouslydescribed method [1]. The obtained solids were calcined at 460 o C for 18 h, yielding thecorresponding mixed oxides Mg x Al (1-y) Y y -MO. The reconstruction of the LDH phases via“memory effect” was performed by hydration of the mixed oxides at different temperatures inthe range 25-100 o C, yielding Mg x Al (1-y) Y y -LDHR T (T = temperature). All the solids havebeen characterized by XRD, DRIFTS, DR-UVVis-NIR, and BET. The surface base sites ofthe catalysts were determined using a method suitable for both dried and calcined samplesconsisting in the irreversible adsorption of organic acids with different pK a [2]. The catalyticactivity of the samples was tested in two types of base-mediated organic syntheses:i) cyanoethylation of ethanol and ii) selective oxidation of cyclohexene by molecular oxygenin the presence of isobutyraldehyde. The characterization results showed that both surfaceareas and crystallinities of Mg x Al (1-y) Y y materials decreased with the increase of Y content inthe range y = 0.01-0.66 for all x values in the range 1-10. Meanwhile, the basicity followedthe opposite trend. The reconstruction of the LDH structure was possible only for the sampleswith lower content of Y (e.g. y = 0.04-0.33) and the heating during the rehydration processhad beneficial effects on the crystallinity, the texture and the basicity of the materials. Thecatalytic activity of Mg x Al (1-y) Y y materials for both catalytic processes investigated washigher than that of the corresponding Mg x Al materials, indicating the importance of thebasicity increase following the incorporation of Y in the brucite-type layers.References:[1] E. Angelescu, O.D. Pavel, M. Che, R. Bîrjega, G. Costentin, Catal. Commun. 5 (2004) 647.[2] R. Ionescu, R. Birjega, E. Angelescu, O.D. Pavel, R. Zavoianu, Catal. Lett. 34 (2010) 309.242