Biolubricant synthesis using immobilised lipase: Process optimisation of trimethylolpropane oleate production

Abstract

Synthetic esters based on polyols and fatty acids possess suitable technical and ecological properties for applications as biolubricants, and can replace the mineral oil based lubricants in several applications. In this work, the synthesis of trimethylolpropane (TMP) esters with oleic acid using immobilised lipase B from Candida antarctica (Novozym^®435) has been studied. TMP-trioleate has suitable properties for use as hydraulic fluids, especially at extreme temperatures. The effect of different reaction parameters on the reaction efficiency has been evaluated. The study showed that the formation of the triester product was facilitated at high temperature and biocatalyst concentration, as well as stoichiometric amounts of oleic acid and TMP. The product with the highest triester content exhibited the lowest pour point (−42 °C). The stability of the biocatalyst was however limited at high temperature and polyol concentration. Loss of activity during recycling of the biocatalyst at 70 °C was reduced to some extent by washing it with 2-propanol prior to subsequent run.

Introduction

Synthetic oleochemical esters are regarded as environmentally benign alternatives for mineral oil based lubricants, exhibiting a combination of excellent technical performance with favourable ecological properties [1], [2]. They are easily biodegradable and possess low aquatic toxicity. Such esters are thus highly suited as high performance lubricants for different industrial and automotive applications such as hydraulic fluids, metal working fluids, drilling oils, gear oils and lubricants for power saw chains [1], [3].

Oleochemical esters are characterised by the presence of one or more ester bonds with alkyl chains that are normally of plant or animal origin. Polyol esters are made of multifunctional synthetic alcohols like trimethylolpropane (TMP), neopentyl glycol or pentaerythritol. They contain at least one quaternary carbon atom that imparts higher chemical stability to the molecules as compared to e.g. the esters of glycerol, in spite of which they are relatively easily biodegradable. Polyol esters are so far the most common group of biolubricants with TMP esters of oleic acid being the most widely applied for hydraulic fluids [4].

Production of trimethylolpropane trioleate on industrial scale is achieved by reaction of TMP with free fatty acids or esters catalysed by a homogeneous or heterogeneous chemical catalyst such as acidic resins, acid oxides, and organic ion exchange resins [5]. Synthesis of polyol esters by transesterification with triglycerides using zinc (II) oxalate as catalyst has also been reported [6]. Yet another catalyst used for esterification and transesterification reactions has been the enzyme lipase [7], [8], [9], [10]. Earlier studies with immobilised lipases from Rhizomucor miehei and Candida rugosa have shown that the concentration of biocatalyst needed for efficient production was as high as 20–50% (w/v). Recently, the synthesis of TMP-oleate using different heterogeneous catalysts has been compared [11]. The reaction performance using immobilised C. antarctica lipase B, Novozym^®435 (N435), and silica sulphuric acid were comparable, however, the product quality was better with the enzymatically catalysed process. High yields (95%) of the triester were achieved in the process run at 70 °C and the product was found to have suitable properties as biolubricant.

In this paper, the effect of different reaction parameters on the efficiency of the synthesis of TMP-oleate using N435 and on enzymatic stability was studied, with an aim to find conditions for an economical and environmentally benign process.