Catalytic conversion of glycerol to polymers in the presence of ammonia

Highlights

• First synthesis of polymers from glycerol and ammonia over an iron catalyst.

• Characterisation revealed unsaturated, amido and ester functionalities.

• Unsaturated, branched and cross-linked structures were present in the polymer.

• Oxidation of glycerol dehydration products is required for the polymerisation.

Abstract

In this contribution, the development of a process for the synthesis of potentially highly valuable polymeric products from the reaction of waste glycerol with ammonia is reported for the first time. The polymers were the result of a single step, continuous gas phase process, catalysed by an alumina-supported iron catalyst, operating under relatively mild reaction conditions. The solid product was characterised using 1D and 2D NMR spectroscopy, FTIR spectroscopy, qualitative chemical tests and elemental analysis. Characterisation revealed building blocks with unsaturated, amido and ester functionalities shaping a mixture of polymers. Nitrogen atoms were present in the main chain of the resultant polymers. NMR analyses of the polymer denotes the formation of structural defects such as unsaturation and branching; whilst the partial solubility of the polymer in solvents such as CDCl₃ and THF is indicative of the formation of cross-linked structures. Insights into the mechanism of formation of these functional groups were based on the liquid and gas phase product distribution. Polymers with chain structures similar to those synthesised in this work are currently manufactured from fossil fuels and are widely used in biomedical applications not only because of their architecture but also due to their response to changes in pH and temperature.

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.