Poly-(styrene sulphonic acid): An acid catalyst from polystyrene waste for reactions of interest in biomass valorization
Graphical abstract
Introduction
Important achievements have been already accomplished in the catalytic processing of biomass to biofuels and biobased products. Many breakthroughs are still to come but there is a consensus that many of the processes are being or will be conducted in liquid phase at relatively high reaction temperature and by utilizing polar solvents (like water and/or oxygenated organic solvents). This implies that the catalysts must withstand phenomena like leaching of active species, thermal and chemical deterioration and fouling by deposition of heavy products. Another relevant property of the catalyst must be its low price.
We have demonstrated that commercial poly-(styrene sulphonic acid) (PSSA) can be used as an effective catalyst for biodiesel synthesis and xylose to furfural dehydration [1]. PSSA is essentially not crosslinked (it is a linear macromolecule) and it is soluble in water and in other polar organic solvents. Therefore the transport restrictions of the reactants to (or products from) all the active sites are diminished with respect to other solid porous catalysts. Leaching, thermal degradation and fouling phenomena were not detected during utilization in the reaction mixture and PSSA could be reused for a number of runs in both reactions.
However reutilization of PSSA requires the separation from the reaction medium by ultrafiltration which is not a conventional procedure [1]. To circumvent the ultrafiltration, we have proposed to retain the PSSA polymer by anchoring it on an inorganic solid: SiO2. Thus the so formed solid organic–inorganic nanocomposite can be separated from the reaction medium by more conventional procedures like filtration or centrifugation. Part of the PSSA polymer chains are still exposed to the liquid phase, solvated by the solvent molecules and in good contact with reactants and products. A sol–gel methodology to hydrolyse and condensate SiO2 organosilane precursors (TEOS) was used. Organosilane with aminopropyl functionalities (APTES) is also involved (see Scheme 1). Acid PSSA itself catalyzes the hydrolysis and condensation reactions implicated in the sol–gel process. Electrostatic interactions between the amine groups and part of the sulphonic functionalities are responsible of retaining the polymer molecules on SiO2. The solid catalyst displayed satisfactory hydrothermal stability and could be reutilized in the xylose to furfural reaction [2].
The objective of this article is to explore if PS waste can be effectively sulphonated to form PSSA catalysts and therefore be reutilized as an acid catalyst (either as soluble catalyst or as a nanocomposite). This reclamation approach represents a cheaper and more environmental friendly route to synthesize acid sulphonic catalysts than preparing catalyst by polymerization of fresh monomers. PS waste is widely accessible as it is present for example in CD covers, yogurt packaging and expanded polystyrene. Reclamation of PS waste by sulphonation has been demonstrated for many different applications [3], [4], [5], [6], [7], [8], [9], [10]. To our knowledge PS waste has been never recycled as a catalyst (Waste To Catalyst strategy, WTC). We have to take into account that specific additives (antioxidants, UV stabilizers, fillers, pigments, lubes for processing, rubbers, antistats and flame retardants) are incorporated during the manufacture to render a given end PS product. So we have to verify if these additives may interfere in the sulphonation route or in the usage as catalyst.
In this article, it will be shown that PSSA from PS waste, either as soluble WTC-PSSA or as WTC-SiO2-nanocomposite, presents promising properties in biodiesel reaction, xylose dehydration to furfural and in the oxidation of furfural with H2O2 to obtain succinic (SAc) and maleic acids (MAc). These three reactions cover a wide range of reaction conditions: from mild to relatively high reaction temperatures and both polar solvents or aqueous solutions.
This work is a preliminary exploration and it is not intended to find the best PSSA waste based catalyst but to demonstrate that the concept of preparing acid catalysts from polystyrene waste is possible. Further research must be conducted to find the best synthesis conditions to prepare the optimum catalyst for any of the biomass valorization reaction here tested.
Section snippets
Conditioning of PSSA
Commercial soluble PSSA was supplied by Sigma-Aldrich as aqueous solutions. Raw PSSA (as supplied) was conditioned by using ultrafiltration membranes prior utilization as catalyst. The fraction of PSSA molecules with sizes larger than 5kDa were retained by the ultrafiltration membranes and used for catalytic studies. Further details are given elsewhere [1]. The same ultrafiltration conditioning was applied to the PSSA derived from waste.
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WTC catalysts by sulphonation of waste PS
With respect to sulphonation of waste polystyrene to
WTC-PSSA prepared by sulphonation of polystyrene waste
Two different liquid phase routes to sulphonate PS waste were selected as they are frequently used procedures in sulphonation of commercial PS [11], [12]. Gas phase sulphonation is not very effective in extensive sulphonation. Moreover liquid phase routes involve the dissolution and re-precipitation of the polymer molecules that can help in removing additives present in waste PS that may interfere in the final product. Three different types waste PS were selected: CD covers (CD) and yoghurt (Y)
Conclusions
The investigation here reported demonstrates that acid catalysts derived from PS waste by sulphonation can be used in reactions of interest in biomass valorization, namely, biodiesel synthesis, xylose dehydration to furfural and furfural oxidation to maleic and succinic acids. Two types of waste derived catalysts (WTC, Waste To Catalysts) were studied: soluble PSSA (WTC-PSSA) and solid SiO2-PSSA nanocomposite (WTC-SiO2-PSSA). The soluble WTC-PSSA shows reasonably good activity for these three
Acknowledgments
This work was funded by the Spanish Ministry of Economy and Competitiveness (Project CTQ2012-38204-C03-01, CARBIOCAT) and the Autonomous Government of Madrid (S2009/ENE-1660, CARDENER-CM partly funded by FSE funds).
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Current address: Department of Chemical and Biological Engineering, University of Wisconsin-Madison, 1415 Engineering Drive, Madison, WI 53706, USA.