Elsevier

Applied Catalysis A: General

Volume 501, 25 July 2015, Pages 48-55
Applied Catalysis A: General

Recoverable and reusable aluminium solvated species used as a homogeneous catalyst for biodiesel production from brown grease

https://doi.org/10.1016/j.apcata.2015.04.031Get rights and content

Highlights

  • Brown grease was efficiently converted into biodiesel.

  • The reaction factors for catalysed direct esterification were optimised.

  • Recyclability and reusability of solvated aluminium species were tested.

  • The effect of water into direct esterification of FFAs was estimated.

  • The behaviour of AlCl3·6H2O into methanol was studied in ESI-MS.

Abstract

Homogeneous aluminium species, obtained by dissolving AlCl3·6H2O into methanol, were characterised and tested as catalysts into the direct esterification of free fatty acids with methanol. The nature and the role of this catalyst was further investigated through ESI-MS and FTIR spectroscopy, by revealing an immediate exchange reaction between methanol and the water molecules originally bounded to the aluminium, producing a final mixed methanol-aquo-complex whose reactivity was found to be comparable to that of a methanolic solution of hydrogen chloride. Reaction conditions were optimised using the desirability function applied on the response surface methodology analysis of a Box–Behnken factorial design of experiments. By carrying out the reaction at 72 °C for 120 min and using a catalyst amount of 1.5% (mol of Al respect to fatty acids), almost 94% of the starting acids were converted. At the end of the reaction, a biphasic system was obtained in which the upper methanolic phase, which contained most of the starting catalyst, was separated from the heaviest phase, mainly composed of fatty acid methyl esters. Such a distribution not only allowed the biodiesel to be easily separated, but also the catalysts were efficiently recovered and reused for at least four times, determining a total TON greater than 200, without revealing any loss of its activity.

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.RCOOH+MeOHRCOOMe+H2O

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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