(312d) Lipase Immobilization on Mesoporous Carbon and Evaluation As Catalysts in the Hydrolysis of Waste Oleochemical Streams

The increasing global generation of used cooking oils (UCOs) is resulting in a cascade of economic, environmental, and public health issues as mismanagement is a common practice worldwide. Hence industrial exploitation and valorization is required towards a circular economy within the oleochemical industry. However main challenges for UCOs harnessing are their heterogeneity and the high content of impurities, acidity, and water. Alternatively, waste UCOs could be subjected to complete hydrolysis to recover free fatty acids (FFAs), which are widely used oleochemical feedstocks. Different catalysts have been explored for this process; however, lipases are of particular interest because they are able to work on low-quality feedstocks under milder operating conditions and with high selectivity, thus reducing capital and energy costs and the associated environmental impacts. On the other hand, the high pressure and temperature hydrolysis of refined vegetable oils and fats is typically used in the manufacture of fatty acids, this process is highly energy-intensive, and the severe conditions can drive a variety of undesirable side reactions. Despite the high conversion, selectivity, and benign conditions, the industrial implementation of the enzymatic processes is still challenging owing to the high cost of the enzymes, the lower reaction rates compared with homogeneous catalysts, and the deactivation that can be exacerbated by impurities contained in the feedstock.

In order to improve the enzymatic processes, immobilization techniques have been used to enhance enzymes stability and selectivity, simultaneously facilitating recovery and reutilization. As the immobilized enzymes are more resilient to reactive media, they are then effectively used as catalysts in traditional and ultrasound-assisted reactors [1] [2]. At the same time, the use of immobilized catalysts can help to overcome the stability issues when processing waste streams as the feedstock of the hydrolysis process. This is particularly important because reclaiming and exploiting waste oleochemical streams (e.g. used cooking oil, trap greases, yellow-brown-black fat, crop mill effluents) is paramount to reducing major environmental impacts on the food/feed production chain [3]. Currently, there are commercial immobilized enzymes, but they are characterized by low yields and slow kinetics in the hydrolysis of refined oils and fats [4]. This occurs partly because the enzymes have been immobilized under microporous and highly hydrophobic materials (e.g. silica, alumina, zeolites).

In this regard, the immobilization of enzymes on mesoporous and hydrophilic supports seems a feasible pathway to enhance their effectiveness in the hydrolysis of waste oleochemical streams. First, three different immobilization method were tested, showing that amino-functionalized activated carbons have an enormous potential for either further lipase immobilization or activation with glutaraldehyde. The biocatalyst obtained were tested in hydrolysis reactions, for the experiments ultrasound-assisted conditions at 37 and 80 kHz at 37 ºC were used, since reaction media is a critical point different molar ratios of water to oil ( 9:1, 5:1, 1:1 and 1:3) and lipase concentration were studied. On the other hand, it is well known that long exposure time might induce denaturation for this reason experiments with and without ultrasound and mechanical stirring were carried out. For comparison purposes, the reaction was carried out under the best hydrolysis conditions previously reported for a commercial lipase (Novozyme® 435). It was found that the immobilized lipases on the highly hydrophobic mesoporous material had better performance on the hydrolysis of waste oleochemicals than the commercial one, in terms of yield and reusability, in the same way high contents of water may allow to obtain hydrolysis degrees up to 90% but with low reusability, this indicates that hydrophilic medium induces enzyme leaching, it was also identified that ultrasound has no influence in the structural changes in the lipase during the reaction.