Ind. Eng. Chem. Res. 2009, 48, 1081–1089
1081
Low Trans and Saturated Vegetable Oil Hydrogenation over Nanostructured Pd/Silica Catalysts: Process Parameters and Mass-Transfer Features Effects Khaled Belkacemi* and Safia Hamoudi Department of Soil Sciences and Agri-Food Engineering, UniVersite´ LaVal, Que´bec, Canada, G1K 7P4
Sunflower and canola oils were hydrogenated over a novel Pd catalyst and a commercial N catalyst. The Pd catalyst consisted of palladium nanoparticles highly dispersed on mesostructured silica material. The effect of temperature (80-130 °C) and H2 pressure (3.6-9.3 atm) on the activity, selectivity, and trans (TFA) and saturated (SFA) fatty acids formation were studied for both oils and types of catalysts. It was found that the reaction temperature and the hydrogen pressure increased the activity of both catalysts and controlled the solubility of hydrogen and the cis/trans isomerization. The Pd catalyst exhibited a greater selectivity toward the formation of monoene in comparison to the commercial Ni catalyst. Partial hydrogenation of both oils from initial iodine (IVo) value of 120-130 to a final IV of 90 with the nanostructured Pd catalyst using a conventional stirred reactor equipped with a surface aeration turbine-type impeller yielded modified oils with low TFA level (7-10%) and also controlled the formation SFA (∼7%) under mild process conditions. Further reduction of TFA level was achieved when the hydrogenation of vegetable oils over Pd catalyst was carried out with a reactor equipped with a gas-inducing sparger-type impeller having enhanced mass-transfer features. Very low TFA ( 1. Again, the hydrogentransfer limitations still occur but with at least 2-fold lower impact in comparison to the case of a turbine-type impeller. The fractional mass-transfer resistance ζ for sparger impeller would be up to 62% (See Table 5) if we can admit at a first approximation that the volumetric-transfer coefficient kLa remains equal to that obtained for a turbine-type impeller (kLa ) 0.107 s-1). Actualy, kLa could be greater that 0.107 s-1, and the fractional mass-transfer resistance ζ for a spargertype impeller could be lower than 62% (see Figure 12). In other words, the probability that hydrogen could be transferred using a sparger-type impeller to the catalyst’s sites is at least twice the probability when using a turbine-type impeller. This might be correlated, with no doubt, to the possibility of producing much less trans-fatty acids, thus confirming our results. Consequently, it is possible to control the duality between the two competing reactions, hydrogenation and cis-trans isomerization, by modulating the H2-Weiz-Prater number. When there are not limitations for the hydrogen molecule to reach the metal, an excess of H2 on the active site favors the triglycerides to saturate before their isomerization. On the other hand, when the restrictions on the hydrogen are significant (at high values of Weisz-Prater module), cis/trans isomerization occurs favorably to the detriment of the double bonds saturation.18 This effect of H2-Weiz-Prater module was reported on the trans-fatty acids content and it was observed that at ΦH2 > 15 the trans-isomer content increases significantly and then levels off at higher ΦH2 values.
The novel Pd catalyst supported on nanostructured silica material was more active and selective for hydrogenation of sunflower and canola oils under mild process conditions than what can be obtained with the selective commercial Ni catalyst at a similar activity, and this was achieved at a lower metal loading. It produced less saturated acid and reached a better selectivity toward the formation of monoenes. Partial hydrogenation of both oils from an initial IV value of 120-130 to a final IV value of 90 with the nanostructured Pd catalyst using a stirred reactor equipped with a turbine-type impeller, yielded modified oils with low TFA level (7-10%), and also controlled the formation of SFA (∼ 7%) under mild process conditions. Furthermore, the novel Pd catalyst was active even at low process conditions of temperature and H2 pressure, 80 °C and 3.6 atm, respectively. Under these conditions, the Ni catalyst does not exhibit any activity. Further reduction of TFA level was achieved when the hydrogenation of vegetable oils (initial IV value of 130) with Pd catalyst was carried out with a reactor equipped with a sparger-type impeller enhancing the mass-transfer features. Unprecedented very low TFA (
