Marc Pera-Titus

Pickering Interfacial Catalysis for Biphasic Systems: From Emulsion Design to Green Reactions

Pickering emulsions are surfactant-free dispersions of two immiscible fluids that are kinetically stabilized by colloidal particles. For ecological reasons, these systems have undergone a resurgence of interest to mitigate the use of synthetic surfactants and solvents. Moreover, the use of colloidal particles as stabilizers provides emulsions with original properties compared to surfactant-stabilized emulsions, microemulsions, and micellar systems. Despite these specific advantages, the application of Pickering emulsions to catalysis has been rarely explored. This Minireview describes very recent examples of hybrid and composite amphiphilic materials for the design of interfacial catalysts in Pickering emulsions with special emphasis on their assets and challenges for industrially relevant biphasic reactions in fine chemistry, biofuel upgrading, and depollution.

Tunable Catalysts for Solvent-Free Biphasic Systems: Pickering Interfacial Catalysts over Amphiphilic Silica Nanoparticles

Stabilization of oil/oil Pickering emulsions using robust and recyclable catalytic amphiphilic silica nanoparticles bearing alkyl and propylsulfonic acid groups allows fast and efficient solvent-free acetalization of immiscible long-chain fatty aldehydes with ethylene glycol.

Homogeneity of flexible metal–organic frameworks containing mixed linkers

Very sophisticated porous materials known as multivariate functional MOFs (also known as MixMOFs) can be designed using a synthesis method that starts from solutions composed of two or more different linkers. For this procedure to be successful, one must have access to techniques that characterize the homogeneity of MOF crystallites containing two different linkers. This is of particular relevance for MOFs made of 2-aminobenzene-1,4-dicarboxylate (abdc), which are excellent platforms for the introduction of additional functions by post-modification. In this paper, we show that adsorption/desorption isotherms and thermodiffraction studies on flexible structures can indirectly characterize the homogeneity of MOFs made from a mixture of linkers. Breathing pressures and temperatures for a series of MIL-53(Al) functionalized with amino tags, i.e. Al(OH)(bdc)1−n(abdc)n, were measured as a function of the amino content. The linear relationship between the CO2 breathing pressure and the amine content in the MIL-53(Al) structure clearly illustrates the homogeneity of the crystallite composition; in other words, the crystallites have the same abdc : bdc ratio. On the other hand, the functionalization of MIL-53(Al) with low amine content (10% abdc) results in a profound modification of the breathing properties triggered by the temperature. Much higher temperatures are required for full conversion of the np (narrow pore) to the lp (large pore) phase. We also suggest an interplay between coexisting np and lp microcrystalline domains that may “smooth” the breathing properties at the macroscopic level.

Catalytic amination of biomass-based alcohols.

Amines are key intermediates in the chemical industry with extensive applications in the manufacture of agrochemicals, pharmaceuticals, detergents, fabric softeners, lubricants, polymers, and food additives. The basis of synthetic amine chemistry and amine industry relies on ammonia, and all the organic amines are produced directly or indirectly from this commodity. Excluding the classical process for amine synthesis by direct reaction of ammonia with alkyl halides and the reduction of nitro or nitrile compounds, 2] various processes such as hydroamination, hydroaminomethylation, and reductive amination have been developed and elegant results were obtained. Besides the reactions mentioned above, direct alcohol amination constitutes an attractive process since water is generated as the main byproduct. Indeed, this reaction constitutes a common method for the production of low alkyl chain amines such as N-methyl-, N,N-dimethyl-, and N,N,N-trimethyl-amines in the presence of Brønsted and Lewis acid catalysts (Scheme 1 a). However, the reaction temperature is usu-

Polyethylenimine covalently grafted on mesostructured porous silica for CO2 capture

Polyethylenimine (PEI) has been grafted on a 2D hexagonal mesostructured porous silica of MCM-41 type (LUS silica) using 3-glycidoxypropyltrimethoxysilane (GTMS) as a grafting agent to develop sorbents for CO2 capture. The advantage of this tether is to create ethanolamine units upon reaction of the epoxy group with the amine functions of PEI. Two synthetic routes have been explored: (1) reaction of GTMS and PEI and then grafting on a calcined MCM-41 silica (M-1), and (2) grafting of GTMS on the silica and then reaction with PEI (M-2). In both cases, the grafted solids are well structured according to the XRD patterns. The amounts of glycidoxypropylsilane (GS) and PEI are 14 and 9 wt%, and 21 and 16 wt%, respectively, for samples M-1 and M-2. The CO2 adsorption capacity of both materials has been tested at 303 K and 101 kPa and compared to a bare LUS silica sample impregnated with 25 wt% PEI (M-3-25). Samples M-1 and M-2 containing ethanolamine groups show higher CO2 adsorption capacities, with loading of about 150 and 134 mgCO2 / gPEI (36 and 43 mgCO2 / g-sorbent), respectively, while the CO2 adsorption capacity was about 55 mgCO2 / gPEI (14 mgCO2 / g-sorbent) for the impregnated solid M-3-25.

Selective oxidation of 5-hydroxymethylfurfural to 2,5-diformylfuran over intercalated vanadium phosphate oxides

The selective oxidation of 5-hydroxymethylfurfural (5-HMF) to 2,5-diformylfuran (DFF) was studied over vanadium phosphate oxide (VPO)-based heterogeneous catalysts in the liquid phase. The selectivity to DFF was highly increased when using intercalated vanadium phosphate oxides under mild conditions (1 atm of oxygen, 110 °C) in an aromatic solvent. We found that the length of the intercalated ammonium alkyl chain had no clear influence on the catalytic performances, and a maximum yield of 83% could be achieved over C14VOPO4 and C14VOHPO4 after 6 h of reaction. Recycling of the catalyst was successfully performed, and we further obtained some insights in the reaction pathway: while the desired oxidation reaction indeed proceeded over the catalyst, the formation of by-products was linked to the presence of free radicals in solution.

Catalytic etherification of glycerol with short chain alkyl alcohols in the presence of Lewis acids

Here we report the homogeneously-catalyzed etherification of glycerol with short chain alkyl alcohols. Among the large variety of Bronsted and Lewis acids tested, we show here that metal triflates are not only the most active but are also capable of catalyzing this reaction with an unprecedented selectivity. In particular, in the presence of Bi(OTf)3, the targeted monoalkylglyceryl ethers were obtained with up to 70% yield. Although tested Bronsted acids were also capable of catalyzing the etherification of glycerol with alkyl alcohols, they were found however less active and less selective than Bi(OTf)3. By means of counter experiments, we highlighted that the high activity and selectivity of Bi(OTf)3 may rely on a synergistic effect between Bi(OTf)3 and triflic acid, a Bronsted acid that can be released by in situ glycerolysis of Bi(OTf)3. The scope of this methodology was also extended to other polyols and, in all cases, the monoalkylpolyol ethers were conveniently obtained with fair to good yields.

Enhanced H2 Uptake in Solvents Confined in Mesoporous Metal–Organic Framework

Hydrogen uptake at 298 K and 30 bar in hybrid sorbents consisting of n-hexane confined in MIL-101 is found to be 22 times larger than in sole n-hexane. The enhanced solubility in MIL-101, found to be 3 times larger than in mesoporous silica of similar pore size, highlights the key roles played by surface chemistry and accessible surface area.

Xylene Vapor Mixture Separation in Nanocomposite MFI-Alumina Tubular Membranes: Influence of Operating Variables

In this study, we present the results of a preliminary investigation on the influence of operating variables (temperature, sweep gas flow rate, and total feed vapor pressure) on xylene vapor mixture separation using tubular nanocomposite MFI-alumina zeolite membrane prepared by the pore-plugging synthesis technique. Within the detection limit of our analytical system, neither m- nor o-xylene was detected in the permeate stream, the membranes displaying therefore “infinite” p-xylene selectivity. The mixtures p-xylene flux displayed a maximum value of ca. 3.5 µmol·m−2·s−1, corresponding to a mixture permeance of 11 nmol·m−2·s−1·Pa−1, at 473 K and for a feed composition 0.63 kPa p-xylene/0.27 kPa m-xylene/0.32 kPa o-xylene, being almost unchanged for sweep gas flow rates (N2) higher than 20 mL(STP)/min and increasing with the total xylene vapor pressure at 1 : 1 : 1–3 p/m/o-xylene composition. The experimental p-xylene fluxes can be well predicted by a Maxwell-Stefan model, as expected for a mass transfer process driven by competitive adsorption / surface diffusion. Unlike film-like MFI membranes, the membranes presented here preserved their selectivity to p-xylene for total xylene pressures as high as 150 kPa. This behavior is attributed to the intimate contact between the alumina confining pores and MFI nanoparticles, reducing long-term stresses and thus preventing distortion of the MFI framework during p-xylene adsorption. These results open up potential applications of nanocomposite MFI-alumina for selective p-xylene separations at high loadings, for instance in pervaporation, where the use of film-like MFI membranes is discouraged.

Pickering Interfacial Catalysts for solvent-free biomass transformation: Physicochemical behavior of non-aqueous emulsions

A key challenge in biomass conversion is how to achieve valuable molecules with optimal reactivity in the presence of immiscible reactants. This issue is usually tackled using either organic solvents or surfactants to promote emulsification, making industrial processes expensive and not environmentally friendly. As an alternative, Pickering emulsions using solid particles with tailored designed surface properties can promote phase contact within intrinsically biphasic systems. Here we show that amphiphilic silica nanoparticles bearing a proper combination of alkyl and strong acidic surface groups can generate stable Pickering emulsions of the glycerol/dodecanol system in the temperature range of 35-130°C. We also show that such particles can perform as Pickering Interfacial Catalysts for the acid-catalyzed etherification of glycerol with dodecanol at 150°C. Our findings shed light on some key parameters governing emulsion stability and catalytic activity of Pickering interfacial catalytic systems. This understanding is critical to pave the way toward technological solutions for biomass upgrading able to promote eco-efficient reactions between immiscible organic reagents with neither use of solvents nor surfactants.