Catalysis of Organic..

Diez, Di Cosimo and Apesteguia 357The cross aldol condensation of citral (Millennium Chemicals, 40 % cis-isomer+ 55 % trans-isomer) with acetone (Merck, PA) was carried out at 353 K in N 2atmosphere under autogenous pressure (≈ 250 kPa) in a batch PARR reactor, usingan acetone/citral = 49 (molar ratio) and a catalyst/(citral+acetone) = 1 wt.% ratio.Catalysts were pre-treated ex-situ in flowing N 2 at 773 K for 2 h to remove adsorbedwater and carbon dioxide and then quickly transferred to the reactor withoutexposing them to air. Reaction products were analyzed by gas chromatography.Selectivities (S j , mol of product j/mol of citral reacted) were calculated as S j (%) = C jx 100/ ΣC j where C j is the concentration of product j. Product yields (η j , mol ofproduct j/mol of citral fed) were calculated as η j = S j X Cit . Thirteen samples of thereaction mixture were extracted and analyzed during the 6-hour reaction. The mainreaction product of citral conversion was pseudoionone, PS (cis- and trans- isomers).Results and DiscussionThe chemical composition, BET surface area, pore volume, and XRD analysis resultsof MgO and Li-promoted MgO samples are shown in Table 1.Table 1. Physicochemical properties of the catalysts used in this workCatalyst Li loading(wt.%)Li/Mg(molar ratio)Sg(m 2 /g)Vg(cm 3 /g)Crystallite size(Å)MgO 0 -- 149 0.27 83Li/MgO-1 0.13 0.008 138 -- 107Li/MgO-2 0.30 0.017 100 0.23 109Li/MgO-3 0.50 0.029 87 0.25 114Li/MgO-4 1.20 0.070 66 0.18 125Li/MgO-5 2.60 0.151 56 0.13 139Promotion of MgO with Li decreased the MgO surface area and pore volume.XRD analysis of the Li/MgO-x catalysts revealed that a single phase of MgOpericlase is present at low Li loadings, but on sample Li/MgO-5 containing 2.6 wt. %Li an incipient Li 2 CO 3 phase was also detected (Figure 1). MgO crystallite sizeincreased with the Li loading which is in line with the observed concomitant drop ofthe sample surface area. These results are explained by particle agglomeration in thepresence of the Li salt during MgO aqueous impregnation and further thermaldecomposition of the resulting Li(OH)/Mg(OH) 2 which involves Li(OH) melting (9).From XRD diffractograms of Fig.1, we also calculated for Li/MgO-x samples theunit cell parameter a corresponding to the MgO structure with face-centered cubicsymmetry. The obtained a values were almost constant indicating no detectablestructural modification of the MgO lattice by Li doping. This result suggested that Liis located on the surface of Li/MgO-x samples rather than inside of the MgO matrix,probably forming small domains of amorphous Li 2 O or crystalline Li 2 CO 3 . Thesurface base properties of the samples were investigated by TPD of CO 2 preadsorbedat room temperature. Figure 2A shows the CO 2 desorption rate as a function of