Ultrasonic-assisted Oil-conjugated Reaction Using Solid Catalyst

The heterogeneous catalytic process reduces the cost and minimizes the environmentalimpacts by reason of its milder reaction conditions, simpler test steps and easier catalystrecycling. Consequently, solid catalysts have attracted widespread attention and are expectedto be the latest generation of environmentally benign catalytic materials. Nevertheless, thecritical issue associated with the heterogeneous catalysis lies in its lower reaction speed owingto the weak interaction between oils and solid catalysts, originating from their mutualimmiscibility. The introduction of continuous and vigorous mixing is thus necessary forincreasing the interfacial region between these two phases. An alternative means of enhancingthe mixing action is the application of low frequency ultrasound, which is theoretically treatedas an efficient, energy saving and economically viable way to accelerate the conjugatedreaction of oil molecules. Ultrasonic irradiation contributes to an enormous advancement ofthe contact surface through the acoustic energy propagation. The most crucial phenomenon ofultrasonic transmission in liquid is considered to be the cavitation effect. The collapse ofinnumerable cavitation bubbles is responsible for the formation of intense shock waves andplentiful extreme micro areas with high temperature and high pressure, which is in favor ofimproving reaction conditions, quickening reaction rate and boosting resultant conversion. Onthe basis of the reasons above mentioned, a practical method, i.e. the combination ofultrasound and solid catalyst, has been conducted to increase the product concentration in thepresent study. Besides, the effect of ultrasonic irradiation on oil-conjugated reaction usingsolid catalyst has been investigated.Through single factor tests, batch processes assisted with20kHz ultrasound wereperformed to study the impacts of ultrasonic power, temperature, catalyst loading and reactiontime on the content of conjugated linoleic acid (CLA). As the system temperature rose, theconversion of CLA experienced a constant increase. When the reaction time varied within thescope of20-60min, the greatest CLA production could be achieved at the duration of60min.However, in order to avoid the occurrence of possible side reactions, the reaction time shouldbe limited to60min. The content of CLA rocketed as ultrasonic irradiation mounted from80W to160W. When ultrasonic power ranged from160W to400W, there was a continuousdecrease in the product conversion with the increase in output power, which resulted from thatan excess of ultrasonic power weakened the cavitation effect. The formation of CLA wasaccelerated with the weight%of catalyst to oil climbing from4wt.%to10wt.%., and the highest CLA production of1.791mg/mL was attained at10wt.%, i.e.3.410g. At catalystloadings above10wt.%, however, there was a rapid decline in the concentration of CLA.Besides, the resultant content under ultrasonic cavitation realized an adequate raise, comparedwith that under the conventional magnetic stirring reactor.The results of response surface methodology (RSM) indicated that the optimum reactionconditions were ultrasonic power of240W, temperature of180℃, catalyst loading of8wt.%and reaction time of60min, at which the highest product yield of4.644mg/mL was achieved.Reaction time was found to exert the most significant weight on the variability of the CLAconcentration, followed by system temperature.In the mechanism experiment of oil-conjugated reaction, Ni catalyst powder reacted with Hradical to generate Ni[H]. The ultrasound enhanced the dissociation reaction of linoleic acidmolecules, produced large numbers of H radical and boosted the effective collision amongNi[H] and the intermediate product with conjugated double bonds. Nevertheless, the directreaction of H radical and intermediate product to form CLA could not occur in the absence ofNi catalyst due to the higher activation energy of these two radicals. Ni catalyst generated astable and longer-lived substance Ni[H] and lowered the activation energy needed in thereaction of H radical and intermediate product, accordingly promoting the oil-conjugatedreaction. In addition, the reaction of Ni[H] and intermediate product occurred in thegas-liquid interface of cavitation bubbles.