Recoverable and reusable aluminium solvated species used as a homogeneous catalyst for biodiesel production from brown grease
Graphical abstract
Introduction
The dwindling of fossil reserves, coupled with the continuous increase in liquid fuel demand have emphasised the utilisation of new resources never previously considered. First generation biofuels, namely fatty acid methyl esters (FAMEs), were initially produced from refined oil via a basic transesterification process using methanol. Since these oils are too costly as a starting source [1], attention was lately focused on the use of cooking oil [2], non-edible oils [3], or animal fat [4], which are defined as yellow grease, with a free fatty acid (FFA) content between 2 and 15% [5].
Attention has more recently focused on grease with an FFA content bigger than 15%, named brown grease (BG), which represents the cheapest source of biodiesel. BG is typically the fatty matter recoverable from sewers. However, only the fraction that is trapped in grease-interceptors for fat, oils and grease (GI-FOG), or grease-traps (GTs) has been largely studied in terms of composition [6], mechanism of formation [7], [8] and successive conversion into biofuels [9]. More specifically, in these cases, having GI-FOG a reasonable content of glycerides (more than 50%), a two-step approach has been adopted to obtain biofuel [10], as carried out for yellow grease [5].
Actually, GI-FOG represents only a fraction of the whole BG produced, because it is recovered from devices strictly used only by restaurants or larger producers of cooking-oil waste. Most of domestic users do not have installed any GIs or GTs, directly disposing of their oily-wastes in sewers. In addition, GIs and GTs are not able to completely capture and separate the lipid fraction [11] from wastewater. These points make wastewater treatment plants (WTTPs) new and profitable sources of BG (WWTP-BG), because they are able to intercept, in preliminary floatation processes, either the domestic oily-wastes as well as the residual oily-phase not trapped by GIs and GTs. Presently, this WWTP-BG, defined also sewage scum, are collected as special waste and disposed of in landfill, or in the best of cases, in incinerators after proper dewatering. Concerning this waste, very limited characterisation data and alternative uses are effectively available [12] and only recently, promising results have been reported concerning its efficient conversion into biodiesel [13].
For converting FFAs into FAMEs, the acid-catalysed direct esterification is used.
Homogeneous mineral acids efficiently promote such a reaction under relatively mild conditions, but they are difficult to be manipulated and recovered. Interestingly, very recently, recoverability and reuse of conventional homogeneous mineral acids [14] were positively studied only for hydrochloric acid. On the other hand, heterogeneous catalysts, which are easier to be recovered, needed harsher operative conditions [15] and often required a tedious regeneration. When some heterogeneous systems worked at milder conditions, as well as several iron (III) salts [16], it was demonstrated that the real active species was its soluble part [2].
In this work, optimisation of direct esterification of FFAs contained into sewage scum into FAMEs by using cheap homogeneous aluminium species, obtained by dissolving AlCl3·6H2O into methanol, was determined through response surface methodology (RSM). Then, the best conditions were successively tested for larger systems, even isolating the final biodiesel by distillation. The role and the nature of the catalyst in its active form was deeply investigated through ESI-MS technique and using FTIR. Finally, the recoverability and reusability of this catalyst was positively proven: it maintained its good activity for at least four cycles, even working on such a so raw material.
Section snippets
Reagents and instruments
Methanol (99.9%), hexane (>99%), undecane (99%) and methyl-heptadecanoate (>99.0%) were all obtained as pure-grade reagents (Sigma–Aldrich). AlCl3·6H2O (>99.0%) was purchased by Baker, and formic acid (99%), KOH (>85%), diethyl-ether (>99.8%), ethanol (96%), nitric acid (>69%) and hydrochloric acid (>36.5%) were Carlo Erba pure reagents.
Gas chromatography–mass spectroscopy (GC–MS) for qualitative analysis was carried out using a Perkin Elmer Clarus 500 gas chromatograph interfaced with a Clarus
Results and discussion
After a preliminary optimisation study, a first scale-up of the process was performed adopting the best operative conditions and very pure FAMEs were distilled under vacuum. Then, the recovery and reusability of the catalyst were tested, while the ESI-MS and FTIR completed the study of characterisation of the active species.
Conclusions
The efficient direct esterification with methanol of FFAs contained into sewage scum was obtained using mild and sustainable conditions. The reaction conditions were optimised through RSM: 94% of the starting acids were converted into the respective FAMEs, even positively testing a first scalability of the proposed technology. The formation of a biphasic system at the end of the reactive cycle allowed the biodiesel from the unreacted FFAs and the catalyst to be efficiently separated.
Acknowledgements
This study was financially supported by CNR through “Bioraffineria di terza generazione integrate con il territorio e biocombustibili” Progetto Premiale. The authors would like to acknowledge Vito Locaputo for its effort into the use of ESI-MS.
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