Ferrierite-Catalysed Branching of Unsaturated Fatty Acids

Abstract

The light alkyl branching in isostearic acid (ISAC) imparts it with unique physical properties and functionalities among fatty acids, making it a valuable raw material in the production of lubricants, cosmetics and surface-active agents. Being derived from renewable vegetable oils, ISAC has the potential to become a key oleochemical building block in the growing bio-based industrial sector. However, it can currently be manufactured only as a co-product in the polymerisation of unsaturated acids, utilising swelling clays as catalyst. This leads to product balance issues, and a costly purification process. A more selective route to alkyl isomerisation (branching) of unsaturated fatty acids would lower production costs and open up new markets and applications. The prospect of a direct route has recently moved closer, when ferrierite, a medium-pore zeolite, was found to be an effective catalyst in its acid form (H-FER) for the isomerisation of oleic acid (OA), resulting in a step change in the yield of branched unsaturated fatty acids. The goal of this study is to understand the relationship between ferrierite structure and performance, in order to further improve the selectivity, activity and robustness of the zeolite, and thus enable optimisation of the catalytic process for the industrial production of ISAC. To better understand catalyst deactivation, the formation and nature of “coke” species have been studied during the H-FER-catalysed isomerisation of OA. UV-Vis and FT-IR spectroscopic analyses of the spent catalyst materials, complemented by NMR and mass spectrometry analyses of the carbonaceous deposits extract, provide new insights into the deactivation mechanisms. The initial, high catalyst activity is quickly lost, despite conservation of the framework integrity. Dodecyl benzene is the major “coke” constituent, while polyenylic carbocations poison the active sites. Pore blockage is detected very early in the reaction, and only the pore mouth is actively employed. Large zeolite crystals were studied to get more detailed spatial insight into deactivation of ferrierite in the OA alkyl isomerisation, using a combination of visible micro-spectroscopy and fluorescence microscopy. The limited activity observed is in line with the pore mouth catalysis hypothesis; the visible micro-spectroscopy data furthermore shows that the accumulation of aromatic species associated with deactivation (and therefore the reaction) is limited to the crystal edges. Fluorescence microscopy data strongly suggest the presence of polyenylic carbocations, and light polarisation reveals that these carbonaceous species are aligned only in the larger 10-membered ring (10-MR) channels of ferrierite at all crystal edges; the reaction is thus further limited to these specific pore mouths. Finally, a study of the relationship between bulk acidity and performance is reported for five commercial ferrierite catalysts; significant differences were found, independently of their bulk Si/Al ratios. Initial pore conversion correlates with Brønsted acidity in the 10-MR channels, as determined by pyridine adsorption/desorption and FT-IR. A low density of external acid sites reduces OA oligomerisation, while a high ratio of Brønsted to Lewis sites explains the high branching yield. FT-IR combined with CO adsorption and temperature-programmed NH3 desorption, confirm that having a few, strong Brønsted acid sites increases selectivity to mono-branching.