Catalysis of Organic..

Kiss, Rothenberg and Dimian 413Preparation of Cs2.5 catalyst [Cs 2.5 H 0.5 PW 12 O 40 ].Cs 2 CO 3 (1.54 g, 10 ml, 0.47 M) aqueous solution were added dropwise toH 3 PW 12 O 40 (5 ml, 10.8 g, 0.75 M aq. sol.). Reaction was performed at roomtemperature and normal pressure while stirring. The white precipitate was filteredand aged in water for 60 hours. After aging, the water was evaporated in an oven at120 o C. White solid glass-like particles of Cs 2.5 H 0.5 PW 12 O 40 (9.0375 g, 2.82 mmol)were obtained.Table 3. Catalyst characterization.Catalyst sampleSurfaceareaPore volumePore diametermax./mean/calc.SulfurcontentCs 2.5 H 0.5 PW 12 O 40 163 m 2 /g 0.135 cm 3 /g 2 / 5.5 / 3 nm N/AZrO 2 /SO 2- 4 / 650°C 118 m 2 /g 0.098 cm 3 /g 4.8 / 7.8 / 7.5 nm 2.3 %TiO 2 /SO 2- 4 / 550°C 129 m 2 /g 0.134 cm 3 /g 4.1 / 4.3 / 4.2 nm 2.1 %SnO 2 /SO 2- 4 / 650°C 100 m 2 /g 0.102 cm 3 /g 3.8 / 4.1 / 4.1 nm 2.6 %Catalyst characterization.Characterization of mixed metal oxides was performed by atomic emissionspectroscopy with inductively coupled plasma atomisation (ICP-AES) on a CEInstruments Sorptomatic 1990. NH 3 -TPD was used for the characterization of acidsite distribution. SZ (0.3 g) was heated up to 600 °C using He (30 ml min –1 ) toremove adsorbed components. Then, the sample was cooled at room temperature andsaturated for 2 h with 100 ml min –1 of 8200 ppm NH 3 in He as carrier gas.Subsequently, the system was flushed with He at a flowrate of 30 ml min –1 for 2 h.The temperature was ramped up to 600 °C at a rate of 10 °C min –1 . A TCD was usedto measure the NH 3 desorption profile. Textural properties were established from theN 2 adsorption isotherm. Surface area was calculated using the BET equation and thepore size was calculated using the BJH method. The results given in Table 3 are ingood agreement with various literature data. Indeed, stronger acid sites lead to highercatalytic activity for esterification.Catalyst leaching. The mixture may segregate leading to possible leaching of sulfategroups. The leaching of catalyst was studied in organic and in aqueous phase. First, asample of fresh SZ catalyst (0.33 g) was stirred with water (50 ml) while measuringthe pH development in time. After 24 h, the acidity was measured by titration withKOH. The suspension was then filtered and treated with a BaCl 2 solution to test forSO 2– 4 ions. In a second experiment, the catalyst was added to an equimolar mixtureof reactants. After 3 h at 140 °C, the catalyst was recovered from the mixture, driedat 120 °C and finally stirred in 50 ml water. The pH was measured and thesuspension titrated with a solution of KOH. SO 2–4 ions in the suspension weredetermined qualitatively with BaCl 2 . In a third experiment, the procedure wasrepeated at 100 °C when the mixture segregates and a separate aqueous phase isformed. From the leaching tests it can be concluded that SZ is not deactivated byleaching of sulfate groups when little water is present in the organic phase but it iseasily deactivated in water or aqueous phase. There are several methods to preventaqueous phase formation and leaching of acid sites: 1) use an excess of one reactant,