Latin American Applied Research 33:79-86 (2003) EFFECT OF THE ACID-BASE PROPERTIES OF Mg-Al MIXED OXIDES ON THE CATALYST DEACTIVATION DURING ALDOL CONDENSATION REACTIONS V.K. DÍEZ, C.R. APESTEGUÍA and J.I. DI COSIMO* Instituto de Investigaciones en Catálisis y Petroquímica -INCAPE- (UNL-CONICET). Santiago del Estero 2654, (3000) Santa Fe, Argentina, e-mail: [email protected] Abstract−− The effect of chemical composition of have reported that Mg-Al mixed oxides efficiently Mg-Al mixed oxides on both the acid-base properties catalyze the gas-phase self-condensation of acetone to and the deactivation process during the gas phase α,β-unsaturated ketones such as mesityl oxides and self-condensation of acetone was studied. The isophorone (Di Cosimo et al., 1998a). Unfortunately, in activity and selectivity for acetone oligomerization coupling reactions like aldol condensations, basic depended on the catalyst acid-base properties. Mg- catalysts are often deactivated either by the presence of rich catalysts selectively yielded mesityl oxides byproducts such as water in the gas phase or by coke whereas Al-rich MgyAlOx oxides produced mainly build up through secondary side reactions. Deactivation isophorone. The initial deactivation rate, increased has traditionally limited the potential of solid basic linearly with the density of surface basic sites, catalysts to replace environmentally problematic and thereby suggesting that although MgyAlOx oxides corrosive liquid bases. However, few works in the promote the self-condensation of acetone by both literature deal with the deactivation of solid bases under acid- and base-catalyzed mechanisms, the reaction conditions. Studies relating the concerted and deactivation rate would be closely related to the sequential pathways required in the deactivation surface basic properties. The MgyAlOx activity mechanism with the acid-base properties of the catalyst declines in the acetone oligomerization reaction due surface are specially lacking. In this work, we studied to a blockage of both base and acid active sites by a the deactivation of Mg-Al mixed oxides in the gas phase carbonaceous residue formed by secondary oligomerization of acetone. We prepared and reactions. The amount and the nature of the carbon characterized calcined Mg-Al hydrotalcites with Mg/Al deposits were characterized by temperature- atomic ratios of 1-9. The effect of composition on both programmed oxidation technique. MgyAlOx and the surface and catalytic properties as well as the Al2O3 formed more and heavier coke than pure MgO catalyst deactivation were investigated by combining but the latter deactivates faster. The deactivation several characterization methods with catalytic data. rate and coke composition are defined by the nature of the active site involved in the coke-forming II. METHODS reactions at different catalyst compositions rather than by the carbon amount or polymerization A. Synthesis Procedures degree. Mg/Al hydrotalcite with Mg/Al atomic ratios (y) of 1, 3, Keywords−− Catalyst Deactivation, Acetone 5 and 9 were prepared by coprecipitation of Mg(NO3)2.6 Oligomerization, Acid-Base Catalysis H2O and Al(NO3)3.9 H2O at 333 K and pH = 10. After drying overnight at 348 K, precursors were decomposed in N2 at 673 K in order to obtain the MgyAlOx mixed I. INTRODUCTION oxides. More experimental details are given elsewhere Mg-Al mixed oxides obtained by thermal (Di Cosimo et al., 2000). Reference samples of Al2O3 decomposition of anionic clays of hydrotalcite structure, and MgO were prepared following the same procedure. present acidic or basic surface properties depending on their chemical composition (Di Cosimo et al., 2000). B. Methods for the Sample Characterization These materials contain the metal components in close The crystalline structure of the samples was determined interaction thereby promoting bifunctional reactions that by powder X-ray diffraction (XRD) methods. Acid site are catalyzed by Brönsted base-Lewis acid pairs. densities were measured by temperature-programmed Among others, hydrotalcite-derived mixed oxides desorption (TPD) of NH3 preadsorbed at room promote aldol condensations (Reichle, 1985), temperature. The structure of CO2 chemisorbed on alkylations (Velu and Swamy, 1996) and alcohol hydrotalcite-derived samples was determined by eliminations (Di Cosimo et al., 2000). In particular, we infrared spectroscopy (IR). CO2 adsorption site * To whom correspondence should be addressed 79 Latin American Applied Research 33:79-86 (2003) densities were obtained from TPD of CO2 preadsorbed gaseous evolution develops a significant porous at 298 K. Total surface areas (Sg) were measured by N2 structure and gives rise to relatively high surface area physisorption at 77 K. mixed oxides as shown in Table 1. In the compositional range of this study, the analysis by XRD of the C. Acetone Self-Condensation Procedures MgyAlOx oxides showed only one crystalline phase, Self-condensation of acetone was carried out at 473 K MgO periclase (ASTM 4-0829). No traces of and 100 kPa in a flow system with a differential fixed- hydrotalcite phases were detected after the thermal bed reactor. Acetone was vaporized in H2 (H2/acetone = decomposition at 673 K. 12) before entering the reaction zone. The standard Table 1 presents the acid and base site densities 0 contact time (W/F Acetone) was 49 g catalyst h/mole measured for all the samples by TPD of preadsorbed acetone. Main reaction products were mesityl oxides NH3 and CO2, respectively. The base site density (nb) (MO's), isophorone (IP) and mesitylene (MES). Traces of the MgyAlOx oxides was always between the values of phorone and light hydrocarbons were also identified. measured for MgO and Al2O3. Basic sites of different Coke formed on the catalysts was characterized after chemical nature were observed by FTIR of CO2 and reaction, ex-situ, in a temperature-programmed deconvolution of the CO2 TPD traces; the strength of oxidation (TPO) unit. The TPO experiments were the basic sites follows the order: isolated low- 2- 2- carried out in a microreactor loaded with 50 mg of coordination O ions > O in metal-oxygen pairs > OH catalyst and using a 3 % O2/N2 carrier gas. Sample groups (Di Cosimo et al., 1998b). The density of temperature was increased linearly from room metal–oxygen pairs measured by CO2 TPD decreased temperature to 973 K at 10 K/min. The reactor exit with increasing the Al content at expenses of the surface gases were fed into a methanator operating at 673 K to hydroxylation. convert COx in methane and then analyzed by flame The acid site density (na), on the other hand, ionization detector. increases with increasing Al content. By deconvolution of the NH3 TPD traces it was found that the MgyAlOx III. RESULTS AND DISCUSSION oxides contain both Brönsted OH groups (low temperature peak) and Lewis acid sites provided by A. Catalyst Characterization metal cations (high temperature peak) (Prinetto et al., 2000; Berteau et al., 1991). Since NH adsorption was The hydroxycarbonate precursors showed diffraction 3 performed at room temperature, the measured Brönsted patterns consistent with the layered double hydroxide acidity may be attributed on all the samples to the structure proposed for hydrotalcite-like compounds, i.e., reversible H-bonded NH adsorption on surface brucite-like sheets formed by OH groups and cations 3 hydroxyls. Lewis acidity of Mg AlO oxides (Mg2+ and Al3+) in octahedral coordination. Water y x corresponds to the irreversible coordinated NH molecules and charge-compensating CO 2- anions are 3 3 adsorption on surface defects with accessible Al3+ located in the interlayers.The hydrotalcite-like precursor + − cations whereas on alumina, the Lewis acidity was more r 2 3+ composition is [Mg1−rAlr (OH)2 ] (CO3 )r/2 .mH 2O likely assigned to Al cations in the abundant surface 3+ 2- with r = Al/(Al+Mg); the stoichiometric hydrotalcite Al -O pairs. On MgO, on the other hand, the high compound, Mg6Al2(OH)16CO3.4H2O, is obtained for r = temperature peak may result from the irreversible 2+ 0.25. Upon thermal decomposition at 673 K, the coordinated NH3 adsorption on Mg cations combined hydroxycarbonate precursors lose water and CO2. This with heterolytic dissociative adsorption leading to Table 1. Chemical Composition, Crystalline Structure, Surface Area and Acid-Base Properties of MgyAlOx, MgO and Al2O3. 10 Acetone Catalyst r = Al/(Al+Mg) Crystalline Sg Site Density b 2 ) µ 2 (molar)a Phase (m2/g) ( mol/m ) c d MO's nb na MgO 0.00 MgO 143 3.13 0.51 mol/hm 1 Mg AlO 0.11 MgO 114 1.17 0.81 µ 9 x ( j r IP Mg5AlOx 0.18 MgO 184 0.46 0.85 Mg3AlOx 0.24 MgO 238 0.59 1.02 Mg AlO 0.47 MgO 231 0.83 1.57 0.1 1 x 0246810 Al2O3 1.00 Amorphous 230 0.34 1.34 a b c d Time (h) by AAS; by XRD; by TPD of CO2; by TPD of NH3 Figure 1: Time dependence of the acetone conversion and product formation rates on Mg9AlOx (T = 473 K, PAcetone = 7.7 kPa) 80 V. K. DÍEZ, C. R. APESTEGUÍA, J. I. DI COSIMO 45 3.6 80 IP 40 r 0 ) 3.2 Acetone 2 35 2.8 60 ( µ mol/m 30 2.4 mol/h m µ ( 40 a 2.0 25 1.6 20 + n + MO's 2 b 20 ) (carbon atom %) (carbon atom n j 1.2 15 0 S MES 0.8 10 0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 r = Al /(Al + Mg) r = Al/(Al+Mg) Figure 2: Initial acetone conversion rate and total Figure 3: Initial product selectivity as a function of number of active sites as a function of composition of composition on MgyAlOx catalysts (T = 473 K, PAcetone = 7.7 kPa, X0 = 12-14 %) MgyAlOx oxides (T = 473 K, PAcetone = 7.7 kPa, Acetone 0 W/F Acetone = 49 g h/mol) - + 2+ 2- NH2 - H species on Mg - O pairs (Garrone and Mg9AlOx sample which lost about 60 % of its initial 0 Stone, 1985).