Catalytic conversion of glycerol to polymers in the presence of ammonia
Graphical abstract
Introduction
In the context of glycerol valorisation, oligomers and polymers of glycerol have been investigated at both industrial and laboratory scales [1], [2], [3]. Their commercial applications seem to depend strictly on the molecular weight of the synthesised product. Whilst low molecular weight fractions obtained directly from glycerol as monomer are used in food, cosmetic and polymer industries, high molecular weight products from glycidol are of significant interest in the biomedical field due to their resemblance to the polyethylene glycol (PEG) structure [2]. When synthesising di-glycerol (simplest oligomer of glycerol) most of the routes are multi-step processes with very specific and costly starting substances and generation of significant varieties of by-products [1]. Glycerol can also undergo oligomerisation by thermal degradation in an inert atmosphere, however, controlling product selectivity, meeting product specifications and conducting downstream separation procedures remain challenging aspects for commercial application of these reactions [1]. Caesium or magnesium impregnated mesoporous materials have been examined in the selective production of polyglycerol aiming at lowering the rate of formation of acrolein and cyclic polyglycerols [4]. Over iron catalysts, glycerol is converted to acrolein, acetaldehyde, allyl alcohol, hydroxyacetone, carboxylic acids, amongst others [5], [6], [7]. Moreover, these and several other glycerol based chemicals contain functional moieties such as acrylates, vinyl and allyl groups which can also polymerise [8].
With different approaches (i.e. amination or ammoxidation), ammonia has been introduced in processes where glycerol is transformed [9], [10], [11]. Amino alcohols, amines, diamines and polyalkylenepolyamines are typical products [9]. These species together with those produced from glycerol conversion could lead to the formation of polymers bearing nitrogen atoms. For instance, polyamides have been previously obtained from amines and carboxylic acids using 3,4,5-trifluorophenylboronic acid as catalyst [12]. Oxazolines have been reported to spontaneously polymerise with α,β-unsaturated acids through a ring opening mechanism via a zwitterion [13]. Likewise, the spontaneous polymerisation of acrylamide has been claimed to occur in glycerol solution [14]. However, to the best of our knowledge, the formation of polymeric products has not been reported directly from glycerol in the presence of ammonia.
In the current study, primary laboratory data has been produced. This data includes a detailed characterisation of the product formed, which is of particular relevance due to the novel character of the work. Insights into the formation of the building blocks present in the polymeric mixture have been developed. From a chemical engineering point of view, information such as this is essential for understanding and optimising the process, which are necessary for commercial exploitation of the studied reaction. It is believed that this communication contributes towards shaping a glycerol value chain by reporting the one-pot synthesis of high value polymeric products at mild conditions over an alumina-supported iron catalyst.
Section snippets
Materials and methods
A 35 wt.% glycerol aqueous solution and 3% gas phase ammonia (99.5% from Coregas) were reacted over an alumina-supported iron catalyst. Details on catalyst preparation and composition are provided in previous publications [7], [15]. The reaction was carried out with nitrogen as carrier gas and with a GHSV of approximately 1190 h−1 operating for 180 min at 340 °C and atmospheric pressure, in a reactor system described elsewhere [15]. Once the catalytic experiments were terminated, the experimental
Polymer characterisation
Results from CHN analysis of the polymeric mixture, including both cross-linked and non cross-linked fractions, confirmed the presence of nitrogen in the polymeric product, as listed in Table 1. The polymer was not formed when the aqueous glycerol solution was reacted over the iron catalyst in the absence of ammonia with nitrogen as gas carrier. This suggests that the source of nitrogen in the polymer is from feed ammonia. Considering our reactants, the other possible element is oxygen which
Conclusions
Potentially valuable polymers are synthesised in a single step gas phase reaction under an inert atmosphere from glycerol and ammonia over an alumina-supported iron catalyst. NMR and FTIR spectroscopy techniques allowed characterisation of the products as a mixture containing both ester and amide functional groups. Specifically, CH2OCO, CHOCO, CH2NHCH, NHCOCH2, CHCONH and CH2CH type structures were confirmed to be present in the polymeric mixture. A polymerisation mechanism different to
Acknowledgements
Gizelle Sánchez and Vaibhav Gaikwad would like to thank the University of Newcastle for their post graduate research scholarships. Funding by African Explosives Ltd. is gratefully acknowledged. Authors thank Dr. Sazal Kundu and Dr. Monica Rossignoli for their help with preliminary NMR analyses.
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