Research Highlight 6 – Benzoic Acid and other High value Chemicals from Waste Polystyrene Recycling

Next time make sure to put your takeaway coffee/cold drink cups, food boxes, yogurt containers, and so on, in the bin. Do you know why? The research team of Prof. Jianliang Xiao from the University of Liverpool, UK, Prof. Xiaotian Qi from the Wuhan University, China and Prof. Eric J. L. McInnes from the University of Manchester, UK, have recently found a lucrative way to chemically recycle these our daily-use plastic wastes – which are made out of polystyrene (PS). Their innovative strategy involves acid catalysed aerobic oxidation of PS waste in the presence of visible light at room temperature, which converts PS into high value bulk chemicals such as benzoic acid (50%), formic acid (67%) and acetophenone (2%) – from which benzoic acid can be selectively isolated in high purity. The work is published in ‘Journal of the American Chemical Society’ (JACS) [1].

The conventional chemical recycling of polystyrene into styrene monomer involves thermal cracking or pyrolysis (via β-scission of C-C bonds in the main chain) at a high temperature of 300-600 °C, either in the presence (acid or base) or absence of a catalyst [2,3,4]. It produces an oil that consists of styrene (~ 50-70%) and many other aromatic compounds such as dimer/trimer of styrene, ethyl benzene, toluene, α-methyl styrene, benzene, indan derivatives (the latter two were only observed when using acid catalysts – formed via proton attack on phenyl group) and 3 to 4-ring polycyclic aromatic hydrocarbons (PAHs : phenanthrene, pyrene, chrysene). This process is energy-intensive and requires high cost for specially designed reactors. Not only that, the isolation of styrene monomer from this oil is an arduous task because of the small difference between the boiling points of these compounds. It requires vacuum distillation technique, and the chances of styrene monomer to undergo polymerization during such isolation process is very high and need inhibitors to prevent it. There was an attempt to ‘close-the-loop’ by direct use of this oil containing styrene to (re)produce polystyrene. However, other aromatic compounds in the oil were found to act as chain transfer agents during free-radical polymerization (for example toluene, which has abstractable hydrogens), which led to inferior properties (low molecular weight and low Tg) as compared to polystyrene produced from pure styrene monomer. More importantly, carcinogenic PAHs in the styrene oil persisted even after polymerization in the final PS produced – thus it is not suitable for food packaging applications [4]. The authors chemical recycling strategy is not only beneficial over conventional chemical recycling, but also a remedy for current industrial practice of mechanical recycling of PS waste. Because, it allows recycling of a wide range of products made from polystyrene – both solid and expanded form. By using authors strategy, it is possible to recycle them together without any special sorting technique or pre-treatment. This is not the case with the mechanical recycling of PS, in which prior to reprocessing, the expanded polystyrene (EPS) was employed in a de-foaming process (for example by using solvents) to reduce its volume; otherwise EPS waste is recycled via dissolution-precipitation technique. During mechanical recycling of PS, the polymer chains are prone to scission, which results in poor mechanical properties. For these reasons, current PS recycling rate is very low (< 1%) [2,5,6]. Therefore, the conversion of PS waste into a high value chemical such as benzoic acid is a powerful strategy to improve PS waste recycling and thereby circular economy.

Benzoic acid is used in the production of food preservatives, plasticizers, medicines, dyes, cosmetics, and also as a chemical intermediate in organic synthesis. It had a market value of ~US$ 1028 million in 2021 [7,8]. Current industrial production of benzoic acid, that is aerobic oxidation of toluene in the presence of cobalt(II) acetate catalyst, produces a large amount of hazardous and flammable waste – “it is called benzoic acid industrial residue” which mainly consists of 20% of benzoic acid, 10% of benzyl benzoate, 4% of 9-fluorenone, and the rest is accounted for many different types of aromatic compounds. This industrial residue is currently either landfilled or incinerated because no cost-effective waste treatment method is available – causing environmental pollution [7]. Thus, the authors work is not only a solution to mitigate plastic pollution, but also overcome the hurdles associated with the benzoic acid industrial production.

Previously, benzoic acid production via PS recycling has been reported. However, it needs very harsh conditions for oxidative degradation of PS – that is, at a high temperature of 170 °C and a mixture of nitrogen oxides (NO) and dioxygen (O2) as oxidants [9]. Moreover, this strategy resulted in poor yield of benzoic acid, and other compounds such as 3-nitro benzoic acid, 4-nitro benzoic acid and 3,5-dinitro benzoic acid were also co-produced. There is also a very recent report via iron(II) chloride (FeCl2) catalyzed aerobic oxidation of PS using light emitting diode (LED) [10]. This strategy was found highly efficient on model substrate (1,3-diphenyl propane), however when it was implemented on real world plastic waste such as ‘plastic cup’, a longer reaction time of 66 hours was needed to achieve full conversion and isolable yield of benzoic acid. The highlighted work stands out from these reports in terms of low cost and mild reaction conditions, at the same time selective isolation of benzoic acid is possible in a quantitative manner. Their strategy only requires catalytic amount of cheap and industrially accepted acid catalyst such as para-toluene sulfonic acid (p-TSOH. H2O) and a shorter degradation time of 15 hours. Other acid catalysts such as sulfuric acid, methane sulfonic acid and triflic acid were also found equally effective for this chemical recycling of PS.

Before extending this strategy to real life PS waste, the authors have conducted screening experiments by using commercial PS of 192,000 molecular weight. They have found that both acid catalyst and irradiation under visible light (365-420 nm) are essential for this chemical recycling process – without these the degradation of PS was not observed. The only by-product of this strategy is PS oligomer (that is PS of smaller average molecular weight) which can be re-used in recycling process. Benzoic acid is isolated from the crude mixture by using simple aqueous extraction and recrystallization techniques. According to the authors, singlet oxygen (1O2) is the reactive oxygen species responsible for oxidative degradation of PS. The singlet oxygen abstracts hydrogen from the tertiary C-H bonds of PS chain, which led to hydroperoxidation and subsequent C-C bond cleavage via radical pathways. The authors were able to confirm this by performing control experiments, for example using singlet oxygen trap or scavenger such as sodium azide (NaN3) or 9,10-diphenyl anthracene (DPA) and radical trapping agent such as 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) in the recycling process – in all cases, no PS degradation or desired products were observed. In the authors strategy, no external photosensitizers were used and the PS---acid catalyst adduct is thought to act as a photosensitizer, which initiates singlet oxygen formation under violet blue light irradiation.

They have also demonstrated the feasibility of this methodology using other PS substrates such as poly(4-tert-butyl styrene) of 50,000-100,000 of molecular weight – in this scenario, 4-tert-butyl benzoic acid (46%) is produced along with formic acid (49%) and 4-tert-butyl acetophenone (3%). The successful demonstration using real world plastic waste such as cup lids, yogurt containers, loose-fill PS chips, expanded polystyrene (EPS) foam (thermocol box), food boxes and laboratory weighing boats also reported in the article – in all cases the authors were able to isolate 38-48% of benzoic acid in pure white crystalline powder form, with 58-64% of formic acid and 2-3% of acetophenone were detected in the crude mixture structural analysis.

The authors have also found a perfect solution to isolate formic acid from these waste PS recycling process – as ‘formanilide’ derivative. For this, after oxidation of PS, the authors have conducted reaction further for 3 days at room temperature in the presence of an amine compound such as aniline or p-toluidine. The authors were able to isolate both benzoic acid (44%) and formanilide (55% in case of aniline) in good yields and high purity via flash column chromatography of the depolymerized mixture (using hexane and ethyl acetate mixtures as eluent). They have further shown the scale up of this process in a continuous-flow micro reactor by using gram scale (18.72 grams) of waste PS food box, which could yield 6.15 grams of benzoic acid. Overall, the authors work provides a sustainable route for the benzoic acid production – by utilizing plastic waste as the resource, and it has the potential to be implemented industrially by using current infrastructure.

References:

[1]. Huang, Z.; Shanmugam, M.; Liu, Z.; Brookfield, A.; Bennett, E. L.; Guan, R.; Vega Herrera, D. E.; Lopez-Sanchez, J. A.; Slater, A. G.; McInnes, E. J. L.; Qi, X.; Xiao, J. Chemical recycling of polystyrene to valuable chemicals via selective acid-catalyzed aerobic oxidation under visible light. J. Am. Chem. Soc. 2022, 144, 6532-6542. (Featured article!)

[2]. Maharana, T.; Negi, Y. S.; Mohanty, B. Review article: Recycling of polystyrene. Polym.-Plast. Technol. Eng. 2007, 46, 729.

[3]. Ukei, H.; Hirose, T.; Horikawa, S.; Y. Takai, Y.; Taka, M.; Azuma, N.; Ueno, A. Catalytic degradation of polystyrene into styrene and a design of recyclable polystyrene with dispersed catalysts. Catal. Today 2000, 62, 67-75.

[4]. Achilias, D. S.; Kanellopoulou, I.; Megalokonomos, P.; Antonakou, E.; Lappas, A. A. Chemical recycling of polystyrene by pyrolysis: Potential use of the liquid product for the reproduction of polymer. Macromol. Mater. Eng. 2007, 292, 923-934.

[5]. Schyns, Z. O. G.; Shaver, M. P. Mechanical recycling of packaging plastics: A review. Macromol. Rapid Commun. 2021, 42, 2000415.

[6]. Rahimi, A.; García, J. M. Chemical recycling of waste plastics for new materials production. Nat. Rev. Chem. 2017, 1, 0046.

[7]. Zhang, X.; Qiao, P.; Xuan Ji, X.; Han, J.; Liu, L.; Weeks, B. L.; Yao, Q.; Zhang, Z. Sustainable recycling of benzoic acid production waste: Green and highly efficient methods to separate and recover high value-added conjugated aromatic compounds from industrial residues. ACS Sustainable Chem. Eng. 2013, 1, 974-981.

[8]. https://www.globenewswire.com/en/news-release/2022/05/02/2433395/0/en/Benzoic-Acid-Market-Size-Worth-US-1-496-54Mn-Globally-by-2028-at-5-5-CAGR-Exclusive-Report-by-The-Insight-Partners.html

[9]. Pifer, A.; Sen, A. Chemical recycling of plastics to useful organic compounds by oxidative degradation. Angew. Chem. Int. Ed. 1998, 37, 23.

[10]. Wang, M.; Wen, J.; Huang, Y.; Hu, P. Selective degradation of styrene related plastics catalyzed by iron under visible light. ChemSusChem 2021, 14, 5049-5056.

Created by: Sini Nalakathu Kolanadiyil