Research Article pubs.acs.org/acscatalysis
High Catalytic Performance of Aquivion PFSA, a Reusable Solid Perfluorosulfonic Acid Polymer, in the Biphasic Glycosylation of Glucose with Fatty Alcohols Ayman Karam,† Karine De Oliveira Vigier,† Sinisa Marinkovic,‡ Boris Estrine,‡ Claudio Oldani,§ and François Jérôme*,† †
Institut de Chimie des Milieux et Matériaux de Poitiers, CNRS/Université de Poitiers, 1 rue Marcel Doré, ENSIP, TSA 41105 86073 Poitiers cedex 9, France ‡ ARD-Agro-industrie Recherches et Développements, Green Chemistry Department, Route de Bazancourt, F-51110 Pomacle, France § Solvay Speciality Polymers, Viale Lombardia 20, 20021 Bollate, Milan, Italy S Supporting Information *
ABSTRACT: The catalytic performance of Aquivion PFSA, a solid superacid, was investigated in the catalytic glycosylation of glucose with fatty alcohols. Four main criteria were considered to evaluate the catalytic performance of Aquivion PFSA: (1) the selectivity of the reaction, (2) the turnover frequency, (3) the reactor productivity, and (4) the catalyst stability/recycling. To shed light on the catalytic performance of Aquivion PFSA, it was systematically compared to H2SO4, which is industrially employed in such reactions. We discovered that Aquivion PFSA surpassed the catalytic performance of H2SO4 in terms of activity, selectivity, and reactor productivity. In particular, Aquivion PFSA selectively converted glucose to alkyl polyglucosides (DP = 1.2) with 85% yield, which corresponded to a reactor productivity as high as 477 kg/(m3 h). Conversely to H2SO4, Aquivion PFSA was also capable of selectively producing alkyl polyglucosides directly from glucose syrup, a cheap source of glucose. This result was ascribed to the amphiphilic nature of Aquivion PFSA, which facilitated the Pickering-like emulsification of the biphasic reaction medium. Finally, owing to its high chemical and mechanical properties, Aquivion PFSA was highly stable under our working conditions and could be recycled at least 10 times without obvious loss of its catalytic performance. KEYWORDS: biphasic catalysis, acid catalysis, carbohydrates, glycosides, Aquivion PFSA, alkyl polyglycosides
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(AAGs), which are valuable nonionic bio-based surfactants.3a Thanks to their low toxicity, biodegradability, and stability (even under alkaline conditions), these nonionic surfactants have attracted great interest in the food, detergent, cosmetic, and pharmaceutical industries. For instance, AAGs are the main constituents of Glucopon 600 and Milcoside 200, produced by BASF and LG, respectively. The production of AAG is industrially achieved by glycosylation of glucose with an excess of fatty alcohols (10 equiv) in the presence of 1 wt % of H2SO4.4 This reaction releases a stoichiometric amount of water. The reaction is conducted under vacuum (100 mbar) to distill water out of the reactor and to shift the process toward the formation of AAG. At the end of the reaction, H2SO4 is neutralized by NaOH, leading to the production of Na2SO4. As this salt is nontoxic and is present in a low quantity, it is not separated at the end of the reaction and
INTRODUCTION
Acids are widely employed in industry to catalyze diverse reactions from carbohydrates such as dehydration, esterification, glycosylation, and polymerization or depolymerization, among many other examples.1 In most cases, these reactions imply water as a solvent, reactant, or coproduct. Industrially, H2SO4 is often the preferred catalyst for carbohydrate processing because it is cheap (0.15 $/kg), is highly active, and also leads to high reactor productivities. From a sustainable point of view, solid acid catalysts are obviously more desirable owing to their possible recycling, the absence of a neutralization step at the end of the reaction, and also their low corrosiveness.1g,h Solid acid catalysts, however, often suffer from low productivity and/ or deactivation caused by the presence of water or thermal instability.2 All of these aspects currently hamper the industrial deployment of solid acid catalysts for carbohydrate processing. The catalytic glycosylation of glucose with alkyl alcohols is a typical example.3 This reaction is of prime importance in industry. In particular, the glycosylation of glucose with longchain alkyl alcohols affords amphiphilic alkyl monoglycosides © 2017 American Chemical Society
Received: December 16, 2016 Revised: February 27, 2017 Published: March 15, 2017 2990
DOI: 10.1021/acscatal.6b03561 ACS Catal. 2017, 7, 2990−2997
Research Article
ACS Catalysis Scheme 1. Synthesis of Amphiphilic Alkyl Glycosides
glycosylation of glucose in the presence of a heterogeneous catalyst, examples of heterogeneously catalyzed glycosylation of glucose with fatty alcohols (C8−C24) are very scarce due to the biphasic nature of the reaction medium, inducing mass transfer problems and, thus, poor selectivity. Chaubal et al. reported the heterogeneously catalyzed glycosylation of glucose with myristyl alcohol in the presence of spinels of the type ZnFe2O4 supported on ZrO2.8 The reaction was carried out in toluene, and myristyl glycosides were obtained with 87% yield. Wu et al. reported the synthesis of octyl glycosides over H2SO4/SiO2, affording APGs with an average degree of polymerization of 1.37.9 The same catalyst was employed by Amin et al. for the glycosylation of dextrose with n-octanol.10 In another example, Kim et al. investigated the glycosylation of glucose with n-octanol in the presence of polyvinyl-bound trisulfonate ethylamine chloride and polyvinyl bound disulfonate ethylamine as Brønsted solid acid catalysts.11 Under optimized conditions, APGs were obtained with excellent yields. Although excellent selectivity to APGs was claimed in all of these examples, the deactivation/leaching of solid catalysts and the low reactor productivity remain important obstacles for implementation on a larger scale. To date, there is no commercialized heterogeneously catalyzed process for the glycosylation of carbohydrates and, in this field, H2SO4 remains the reference in terms of activity, selectivity, productivity, and cost efficiency. Aquivion PFSA PW98 is a perfluorosulfonic acid ionomer which is synthesized by the free radical copolymerization of tetrafluoroethylene and sulfonyl fluoride vinyl ether, CF2 CFO(CF2)2SO2F. The −SO2F moieties are then converted to −SO3H groups by treatment with a mineral base (NaOH or KOH) followed by a cation exchange under acid conditions.12 Thanks to the presence of perfluorinated chains, Aquivion PFSA is considered to be a solid superacid with a Hammett acidity function of −12, which is similar to that of H2SO4 and higher than that of classical sulfonated polystyrenes such as Amberlyst-15 for instance (H0 = −2).13 The structure of Aquivion PFSA PW98 is provided in Figure S1 in the Supporting Information. The proton exchange capacity of Aquivion PFSA PW98 used in this study is 1.0 mmol/g. In contrast to Nafion NR-50, the mechanical and chemical integrity of Aquivion PFSA PW98 is preserved at high temperature (up to 150− 160 °C).14 This large operating temperature window is an important aspect with respect to the long-term recycling. In this work, we report that Aquivion PFSA PW98 is not only a highly robust and recyclable heterogeneous catalyst
thus remains with AAG. Industrial processes generally afford AAG with 70% yield. Other products are mostly alkyl polyglycosides (APG). On average, the final product contains 1.1− 1.5 glucose units per fatty chain. According to the experimental conditions, glucose may also partly polymerize, leading to the formation of unwanted polydextroses. According to the grade of glucose, there are two different pathways to produce AAGs on a large scale (Scheme 1). From refined glucose (anhydrous or monohydrate crystalline powder, 0.45 $/kg), AAGs are produced by the direct glycosylation of glucose with fatty alcohols in the presence of H2SO4. From glucose syrup (0.30 $/kg), direct glycosylation with a fatty alcohol is not efficient due to the very low solubility of the fatty alcohol in the syrup, leading to uncontrolled and dominant polymerization of glucose to polydextroses. In this case, transglycosylation reactions are thus generally preferred. Typically, the syrup is first glycosylated with n-butanol followed by a transglycosylation with a fatty alcohol, both reactions taking place subsequently in the same reactor generally with H2SO4 as a homogeneous catalyst. Nowadays, the price of AAG is a critical issue (about 2.0 $/kg) and it is too high in comparison to the price for fossilderived surfactants (
