Fischer–Tropsch diesel emulsions stabilised by microfibrillated cellulose and nonionic surfactants

Abstract

Water-in-diesel emulsion fuels have been prepared with a combination of sorbitan monolaurate and glycerol monooleate as emulsifier and with microfibrillated cellulose (MFC) of different hydrophilic/hydrophobic character as stabilizer. The MFC was treated with either octadecylamine or poly(styrene-co-maleic anhydride), resulting in very hydrophobic fibrils. The most stable emulsion was achieved with a combination of hydrophilic (untreated) and hydrophobic MFC and only minute amounts of the stabilizer gave a pronounced effect. Even with the optimized formulation the lifetime of the emulsion was shorter than previously reported when a conventional polymeric stabilizer was used, however. The water drop sizes in the emulsions were determined by three methods: optical images, light scattering, and NMR diffusometry. All three methods gave water drops sizes of ca 2 μm. The NMR diffusometry indicated that besides the micrometer-sized emulsion drops a significant fraction of the water is present in small droplets of micelle size. The chemical exchange of water between these two populations of pools is believed to be the reason for the relatively low stability of the system.

Introduction

Water-in-diesel emulsion fuels are known to give an improved emission profile compared to regular diesel fuel in combustion in internal combustion engines. We have previously shown that the positive effect, in particular with respect to particulate matter, is obtained for Fischer–Tropsch diesel (FT-diesel), as well as for regular diesel of fossil origin [1]. Since FT-diesel can be produced from biomass, water-in-FT-diesel emulsions constitute a hydrocarbon fuel with an exceptionally low environmental impact.

Emulsion fuels have been prepared before with different types of surfactants as emulsifiers and polymers as stabilizers [2]. Various kinds of nonionic surfactants can be used, such as alcohol ethoxylates, sorbitan monooleate and ethoxylated sorbitan monooleate [1], [2], [3], [4], [5], [6] but cationic gemini surfactants have also been found useful as emulsifier [7]. For the long term stability of the emulsion a stabilizer needs to be added. The stabilizer prevents the water drops to coalesce. Relatively low molecular weight nonionic surface-active polymers, such as poly(isobutylene)-succinic anhydride ester of monomethyl-capped poly(ethylene glycol) have been found useful for the purpose [1], [2]. Particles may also be efficient emulsion stabilizers. Particle stabilised emulsions were described by Ramsden [8] and Pickering [9] already in the early 20th century and they are commonly referred to as Pickering emulsions. The particles that have been described for the purpose vary from solid materials like silica [10], [11] and clay minerals [12] to paraffin wax crystals [13].

During the last 10 years there has been a significant and continuous interest in research on microfibrillated (delaminated) cellulose (MFC). This relatively new material originates from the disintegration of cellulose fibres into sub-micron and nano-sized fibrils and was introduced in the beginning of the 1980s [14], [15]. The wood fibres are built from microfibrils with diameters of approximately 3.5 nm [16], [17], [18]. Mechanical fibrillation of the cellulose fibres give a material consisting of fibrils in several size classes, with diameters typically ranging from 20 to 100 nm. Fig. 1 shows an example of a cellulose fibre and a network of fibrils produced by fibrillation of such fibres [19], [20]. Having excellent mechanical properties and high water adsorption capacity and being based on a renewable feedstock, MFC constitutes an attractive new material. Several applications have been suggested for this material, particularly for paper-making but also in food and cosmetics. A relatively unexplored application of MFC, although suggested already in 1983 [14] is as stabilizer of oil-in-water emulsions.

Cellulose microfibrils are hydrophilic, with ability to form interfibrillar hydrogen bonds and, hence, strong networks. The fibrils may be modified so that their surface becomes hydrophobic. Every glucose monomer in the cellulose chain has three hydroxyl groups that are possible sites for modifications. Hydrophobic microfibrils can be obtained by silylation [21], or by grafting of polymer brushes and layers [22]. This opened for the use of hydrophobic MFC as stabilizer for water-in-oil emulsions [23], [24]. Very stable water-in-toluene emulsions could be formed and stabilized by silylated MFC and an interesting feature of such systems was that the emulsion remained stable also when the drops became very large. Evidently, the hydrophobized MFC was an effective stabilizer of water-in-toluene emulsions.

For fuel emulsions hydrophobization of the MFC by silylation is not suitable. The silicon atom may poison the exhaust catalyst. For that reason two other hydrophobization agents were explored in this work, octadecylamine and poly(styrene-co-maleic anhydride). Periodate oxidation was used to introduce aldehyde groups on the cellulose fibres [25], [26]. The oxidation leads to a dialdehyde formed by oxidative cleavage at the C2–C3 diol of the glucose ring. Subsequent reductive amination with octadecylamine gave a hydrophobized MFC. The other hydrophobization procedure involved direct reaction of the MFC with poly(styrene-co-maleic anhydride). In this reaction glucose hydroxyl groups react with anhydride rings of the copolymer, attaching the hydrophobic polymer to the MFC by ester bonds. The two types of hydrophobized MFC were evaluated as stabilizer for water-in-diesel emulsions in combination with different types of low molecular weight emulsifiers. The emulsions obtained were characterized by light microscopy, light scattering and NMR diffusometry.