Catalysis of Organic..

Kiss, Rothenberg and Dimian 409Table 1. Advantages and disadvantages of tested catalysts.Catalyst Advatanges DisadvantagesH 2 SO 4 Highest activity Liquid catalystIon-exchange resins Very high activity Low thermal stabilityH 3 PW 12 O 40 Very high activity Soluble in waterCs 2.5 H 0.5 PW 12 O 40 Super acid Low activity per weightZeolitesControlable acidity and Small pore size(H-ZSM-5, Y and Beta)Sulfated metal oxides(zirconia, titania, tin oxide)hydrophobicityHigh activityThermally stableLow activityDeactivates in water(but not in organic phase)In a separate set of experiments, we tested the catalyst reusability androbustness. In five consecutive runs, the activity dropped to ~90% of the originalvalue, and remained constant thereafter. Re-calcination of the used catalyst restoredthe original activity (Figure 3). Considering the promising results with 2-ethylhexanol, we tested the applicability of SZ also for esterification with methanol(Figure 4). High conversions can be reached even at 140 °C providing that anincreased amount of catalyst is used. Note that the esterification with methanol takesplace at higher rates compared to the one with 2-ethylhexanol. This can be explainedby the alcohols’ relative sizes.For a convenient comparison, Table 1 summarizes the pros and cons for eachcatalyst tested. Clearly, the sulfated metal oxides are the best choice.Although the reaction mechanism for the heterogeneous acid-catalysedesterification was reported to be similar to the homogeneously catalysed one (17),there is a major difference concerning the relationship between the surfacehydrophobicity and the catalyst’s activity (Figure 5, left). Reaction pockets arecreated inside a hydrophobic environment, where the fatty acid molecules can beabsorbed and react further. Water molecules are unlikely to be absorbed on sitesenclosed in hydrophobic areas. SZ has large pores, thus not limiting the diffusion ofthe fatty acid molecules. It does not leach under the reaction conditions nor give riseto side reactions. This makes SZ a good candidate for catalytic biodiesel production.REACTANTSFatty AcidAlcoholesterificationCOCH +O O HH +O HH 2 OH 2 OH H + 2 OH +H + H + H +H H ++H 2 OetherificationdehydrationcatalystcatalystFatty EsterEtherWaterAlkeneMain productSecondary productsFigure 5. Surface influence on activity (left). Reaction pathways (right).