AquivionÂ®-Carbon Composites with Tunable Amphiphilicity for

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Aquivion -Carbon Composites with Tunable Amphiphilicity for Pickering Interfacial Catalysis Shi Zhang, Bing Hong, Zhaoyu Fan, Jingya Lu, Yisheng Xu, and Marc Pera-Titus ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08649 • Publication Date (Web): 12 Jul 2018 Downloaded from http://pubs.acs.org on July 16, 2018

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ACS Applied Materials & Interfaces

Aquivion-Carbon Composites with Tunable Amphiphilicity for Pickering Interfacial Catalysis Shi Zhang,a,b Bing Hong,b Zhaoyu Fan,b Jingya Lu,c Yisheng Xua* and Marc PeraTitusb* a

State Key Laboratory of Chemical Engineering and Department of Chemistry, East China

University of Science and Technology, Shanghai, 200237, China. b

Eco-Efficient Products and Processes Laboratory (E2P2L), UMI 3464 CNRS-Solvay, 3966 Jin

Du Road, Xin Zhuang Ind. Zone, 201108 Shanghai, China. c

Solvay (China), Ltd, 3966 Jin Du Road, Xin Zhuang Ind. Zone, 201108 Shanghai, China.

KEYWORDS: Aquivion®, Carbon, Composite, Pickering Interfacial Catalysis, Emulsion

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ABSTRACT

A key demand in biomass conversion is how to achieve a high reactivity with immiscible reagents with the use of neither co-solvent nor additive. Pickering Interfacial Catalysis encompassing the design of amphiphilic catalysts behaving concomitantly as emulsifiers offers an elegant solution. In this study, we prepared a systematic series of amphiphilic Aquivion®carbon composites by hydrothermal carbonization of guar gum with Aquivion® perfluorosulfonic superacid.

By

tuning

the

Aquivion®-carbon

composition,

materials

with

tunable

hydrophilic/hydrophobic properties could be achieved, showing high versatility for conducting biphasic reactions without stirring. In particular, an optimal formulation based on 5:1 Aquivion : carbon could be developed, showing high activity in the transesterification reaction of glyceryl trioleate with methanol at 100 oC with good reusability due to the genesis of stable Pickering emulsions


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1. INTRODUCTION The production of biomass chemicals and fuels is challenge to achieve a real transition towards sustainability for chemical industry towards sustainability.1,2 Typical organic reactions using vegetable oils usually encompass immiscible reagents.3 Solvents are often used for enhancing the miscibility of reactants, also favoring in some cases product separation and catalyst recovery.4 Nonetheless, solvents need extraction/purification for recovery and reuse, adversely influencing the green footmark of the processes. Indeed, 80% and 50% of the mass and energy utilization is involved in chemical conversions, respectively. It is anticipated that growth of solvent consumption has an annual rate of 4.0% until 2021.5 In this view, the conception of novel solvent-free reactions has become a priority, being consistent with a 100% atom economy horizon. Surfactants and phase-transfer catalysts such as quaternary ammonium and phosphonium salts, crown ethers are often employed for distributing the catalyst between the phases.6,7 Also, surfactant-combined catalysts have been developed, increasing the interfacial surface by the origin of supramolecular assemblies (i.e. micelles, µ-emulsions, emulsions). Such catalytic systems include Brφnsted and Lewis acid catalysts for etherification, esterification and transesterification reactions (e.g, 4-dodecylbenzene sulfonic acid, metal dodecyl sulfates),8,9 polyoxometallates self-assembled with cationic surfactants for oxidation/epoxidation reactions with H2O2 in W/O and O/W emulsions,10,11 and organometallic complexes for the hydroformylation of fatty aldehydes in water.12,13 Despite their potential benefits, all these methods are discouraged for industrial use, since they require additives that can be hardly recyclable.



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Amphiphilic nanoparticles (NPs) can also form stable Pickering emulsions enabling per se potential particle separation and recycling after the reaction.14,15 In this view, two concepts can be proposed depending on the position of the catalyst in the system: (1) Pickering-Assisted Catalysis (PAC), where the liquid phases are emulsified using NPs, colloidosomes, polymersomes or microgels and a homogeneous catalyst/enzyme is located in the bulk of one of the phases,16-18 and (2) Pickering Interfacial Catalysis (PIC) in which the NPs function concomitantly as solid emulsifiers and interfacial catalysts, allowing the reaction to occur at the L/L interface.19-22 Recently, we among other authors have shown the potentials of Aquivion® for conducting acid-catalyzed biphasic reactions under the PIC principle.23-25 Aquivion® constitutes a new type of perfluorosulfonic superacid

(PFSA)

resin

(H0∼12)

consists

of a backbone of

tetrafluoroethylene and sides chains of sulfonyl fluoride vinyl ether as shown in Figure S1. The hydrophilic/hydrophobic properties of Aquivion® can be tuned by its immobilization in a carbon matrix, generating Aquivion®-carbon composites. Noteworthy, amphiphilic carbon materials based on single and multi-walled nanotubes,19,26,27 hollow spheres,28 and ‘onion-like’ NPs29 are known to stabilize Pickering emulsions and catalyze interfacial reactions. Aquivion®-carbon composites present the additional advantage of a potential cost reduction, as well as an improvement of their recycling and reuse compared to the parent Aquivion®. In this study, we prepared a library of Aquivion®-carbon composites with tunable hydrophilic/hydrophobic balance. The catalytic properties of the different composites were tested and compared in two acid-catalyzed biphasic reactions: (1) acetalization of dodecyl aldehyde (C12-aldehyde) with ethylene glycol (EG), used here as a model reaction; and (2) water-free transesterification of pure glyceryl trioleate (GTO) with methanol (MeOH) for the production of 4 ACS Paragon Plus Environment

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methyl oleate (biodiesel). Acid-catalyzed transesterification reactions are particularly challenging owing not only to the lower intrinsic activity of solid acids in contrast to alkaline catalysts, but also to resilient external and internal mass transfer resistances, hampering the reaction rate.30,31 In practice, high temperatures (130-250 oC), long reaction times (>10 h), high alcohol-to-oil ratios, a high concentration of free fatty acids (FFAs) in the starting oil and vigorous stirring are often required to enhance the catalytic activity.32 These stringent conditions act as a deterrent for the industrial implementation of solid acid catalysts in transesterification reactions. To date, most of the reported studies have focused mainly on the engineering of acid catalysts with accessible pore architectures,33-35 and hydrophobic properties (mainly oleophilic polymers and carbons) for boosting the diffusion of bulky oil molecules to/from the acid centers.36-40 In parallel, few examples of catalysts with balanced hydrophilic/hydrophobic properties have been reported, showing promising credentials for designing performing catalysts for transesterification reactions under the PIC principle.41-43

2. EXPERIMENTAL SECTION 2.1. Chemicals Aquivion®PW98-S (1.0 mmolH+/g) was procured from Solvay Specialty Polymers (Italy). Guar gum (natural-type, food purity) was provided by Solvay China. Ethylene glycol (EG, >99%), dodecyl aldehyde (C12-aldehyde, 92%), glyceryl trioleate (>99%), methanol (99%), isopropanol (>99%), all purchased from Sigma-Aldrich, were used for the acetalization and transesterification reactions. Sulfuric acid (H2SO4, 95%-98%, Sinopharm), p-toluenesulfonic acid monohydrate (PTSA, 99%, Dow Chemical) were used as reference and benchmark catalysts, respectively. 5 ACS Paragon Plus Environment


2.2. Synthesis of Aq-C composites A suspension of Aquivion®PW98-S (Aq-PW98) was first prepared by adding a given amount of the resin to 60 mL of deionized water at 50 oC under stirring for 2 h. Subsequently, guar gum was added slowly to the suspension at 50 oC under vigorous stirring keeping a total mass of AqPW98 and guar gum at 6.0 g, the solution evolving into a gel within 5 minutes. The gel was stirred at 50 oC until the resin was dissolved. The mixture was then transferred into a 100-mL Teflon®-lined autoclave for hydrothermal carbonization (HTC) at 180 oC for 24 h. Subsequently, the temperature was decreased to RT and the final solid was filtered, rinsed with DI water until neutral pH, and vacuum-dried overnight at 80 oC. The as-synthesized composites were labeled as Aq-C (x-y), where x-y refers to the Aquivion® / guar gel weight ratio in the composites. 2.3. Catalyst characterization The catalysts were inspected by powder X-ray diffraction (PXRD) with a Rigaku D/max 2200 diffractometer using Cu Kα radiation (λ=1.5418 Å). The PXRD patterns were recorded in the range 5-50° with a scanning rate of 4°/min and a scan step of 0.01° and were indexed using the Joint Committee on Powder Diffraction (JCPDS) database. The structural features of the Aq-C composites were assessed by N2 adsorption/desorption at 196 oC using a Micromeritics ASAP 2010 Surface Area Analyzer. The Brunauer-Emmett-Teller (BET) method was used for measuring the specific surface areas (pressure range 0.05 < P/P0 < 0.25), whereas the total pore volumes were recorded at P/P0 = 0.99. The interparticle pore size distributions were measured using the Barrer-Joyner-Halenda (BJH) method. All catalysts were degassed at 100 oC for 3 h before the analyses. Thermogravimetric analysis (TGA) was used to assess the stability of the Aq-C composites. The thermal profiles were measured on a TA SDT Q600 instrument with a flow gas system. The 6 ACS Paragon Plus Environment

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catalysts (~10 mg in an open alumina crucible) were treated from room temperature to 700 °C with a heating rate of 10 °C.min-1 under a air flow rate of 100 mL(STP).min-1. Acid-base titration was performed with NaOH (10 mM) using a Metrohm 794 apparatus. Before titration, the Aq-C composites (100 mg) were ion exchanged with 10 mL of a NaCl solution (1 M) at 25 °C during 24 h. The morphology and elementary particle size of the Aq-C composites was examined by scanning electron microscopy (SEM) using a ZEISS EVO 18 microscope operating at HV mode after coating with Pt (30 mA x 30 s). The particle size distributions (PSD) were measured on a Malvern Mastersizer 3000. In a typical measurement, 0.50 ± 0.01 g sample were dispersed into 50 mL of deionized water. The resulting suspension was dispersed by ultrasonication at 25 oC for 10 min and then added to the sampler under stirring (2,400 rpm) at an obscurantation

AquivionÂ®-Carbon Composites with Tunable Amphiphilicity for

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