Bonny W. M. Kuipers

Glycerol Etherification over Highly Active CaO‐Based Materials: New Mechanistic Aspects and Related Colloidal Particle Formation

Glycerol is an attractive renewable building block for the synthesis of di- and triglycerols, which have numerous applications in the cosmetic and pharmaceutical industries. In this work, the selective etherification of glycerol to di- and triglycerol was studied in the presence of alkaline earth metal oxides and the data are compared with those obtained with Na2CO3 as a homogeneous catalyst. It was found that glycerol conversion increased with increasing catalyst basicity; that is, the conversion increases in the order: MgO90 % at 60 % conversion) are obtained over CaO, SrO, and BaO. For these catalysts no substantial acrolein formation was observed. Furthermore, at the start of the reaction mainly linear diglycerol was produced, whereas at higher conversion degrees branched diglycerol started to form. In another series of experiments different types of CaO materials were prepared. It was found that these CaO-based materials not only differed in their surface area and number of basic sites, but also in their Lewis acid strength. Within this series the CaO material possessing the strongest Lewis acid sites had the highest catalytic activity, comparable to that of BaO, pointing towards the important role of Lewis acidity for this etherification reaction. Based on these observations a plausible alternative reaction scheme for glycerol etherification is presented, which considers the facilitation of the hydroxyl leaving process. Finally, the stability of the catalytic solids under study was investigated and it was found that colloidal CaO particles of about 50-100 nm can be spontaneously generated during reaction. Catalytic testing of these CaO colloids, after isolation from the reaction medium, revealed a very high etherification activity. Understanding the nature of these Ca-based colloids opens new opportunities for investigating supported colloidal particle catalysts to take advantage of both their hetero- and homogeneous nature.

Fluorous derivatives of [Rh(COD)(dppe)]BX4 (X=F, Ph): synthesis, physical studies and application in catalytic hydrogenation of 1-alkenes and 4-alkynes

Abstract Fluorous derivatives of 1,2-bis(diphenylphosphino)ethane (dppe) ( 1 ), containing a para-(1H,1H,2H,2H-perfluoroalkyl)silyl function, were used in the synthesis of fluorous derivatives of [Rh(COD)(dppe)]BF4. The single crystal X-ray crystallographic structure of [Rh(COD)( 1a )]BPh4 ( 7 ) was determined ( 1a =[CH2P(C6H4–SiMe2CH2CH2C6F13-p)2]2). Large differences in catalytic activity and selectivity (higher hydrogenation activity and lower isomerization activity) relative to [Rh(COD)(dppe)]BF4 were observed in the hydrogenation of 1-octene using non-fluorous, silyl-substituted [Rh(COD)( 2 )]BF4 ( 5 ; 2 =[CH2P(C6H4–SiMe3-p)2]2 or [Rh(COD)( 3 )]BF4 ( 6 ; 3 =[CH2P(C6H4–SiMe2C8H17-p)2]2) and even more so with fluorous [Rh(COD)( 1a )]BF4 ( 4a ). For 4a and 6 , the presence of aggregates was demonstrated by dynamic light scattering (DLS) and cryogenic transmission electron microscopy (cryoTEM), which is most probably responsible for these differences. Similar differences between (fluorous) silylated catalysts and non-substituted [Rh(COD)(dppe)]BF4 were observed in the semi-hydrogenation of 4-octyne. Recycling of highly fluorous catalyst [Rh(COD)( 1c )]BF4 ( 4c ; 1c =[CH2P(C6H4-Si(CH2CH2C6F13)3-p)2]2) was investigated in two different fluorous biphasic solvent systems. The catalyst could be recycled with 97.5% (for PFMCH/acetone, 1:1 v/v) and >99.92% (for FC-75/hexanes, 1:3 v/v) retention of rhodium, respectively. The leaching of phosphorus was comparable to the leaching of rhodium (in PFMCH/acetone), showing that dissociation and leaching of free ligand does not take place for these rhodium diphosphine catalysts.

Directed synthesis of stable large polyoxomolybdate spheres.

Polyoxometalates or POMs, a class of inorganic transition metal-oxide based clusters, have gained significant interest owing to their catalytic, magnetic, and material science applications. All such applications require high surface area POM based materials. However, chemically synthesized POMs are still at most in the range of a few nanometers, with their size and morphology being difficult to control. Hence, there is an immediate need to develop design principles that allow easy control of POM morphology and size on mesoscopic (50-500 nm) length scales. Here, we report a design strategy to meet this need. Our method reported here avoids a complex chemical labyrinth by using a prefabricated cationic 1,2-dioleol-3-trimethylammonium-propane (DOTAP) vesicle as a scaffold/structure directing agent and gluing simple anionic heptamolybdates by electrostatic interaction and hydrogen bonds to form large POM spheres. By this method, complexity in the resulting structure can be deliberately induced either via the scaffold or via the oxometalate. The high degree of control in the matter of the size and morphology of the resulting POM superstructures renders this method attractive from a synthetic standpoint.

Model Independent Determination of Colloidal Silica Size Distributions via Analytical Ultracentrifugation

We report a method to determine the particle size distribution of small colloidal silica spheres via analytical ultracentrifugation and show that the average particle size, variance, standard deviation, and relative polydispersity can be obtained from a single sedimentation velocity (SV) analytical ultracentrifugation (AUC) experiment. The particle size distribution (psd) from the enhanced van Holde-Weischet (vHW) analysis accounts for the dynamic light scattering results quite well. In addition, the vHW psd equals the psd from a continuous distribution of sedimentation coefficients analysis where whole sedimentation velocity boundaries are fitted. The SV AUC interference optical data also yield the specific particle volume such that distributions of sedimentation coefficients for colloidal spheres can be converted directly to particle size distributions. Our results show that SV AUC experiments may yield a quantitative particle size distribution without a priori knowledge of the particle size and the shape of the size distribution.

Complex magnetic susceptibility setup for spectroscopy in the extremely low-frequency range

A sensitive balanced differential transformer was built to measure complex initial parallel magnetic susceptibility spectra in the 0.01-1000 Hz range. The alternating magnetic field can be chosen sufficiently weak that the magnetic structure of the samples is only slightly perturbed and the low frequencies make it possible to study the rotational dynamics of large magnetic colloidal particles or aggregates dispersed in a liquid. The distinguishing features of the setup are the novel multilayered cylindrical coils with a large sample volume and a large number of secondary turns (55 000) to measure induced voltages with a good signal-to-noise ratio, the use of a dual channel function generator to provide an ac current to the primary coils and an amplitude- and phase-adjusted compensation voltage to the dual phase differential lock-in amplifier, and the measurement of several vector quantities at each frequency. We present the electrical impedance characteristics of the coils, and we demonstrate the performance of the setup by measurement on magnetic colloidal dispersions covering a wide range of characteristic relaxation frequencies and magnetic susceptibilities, from chi approximately -10(-5) for pure water to chi>1 for concentrated ferrofluids.

Rotational dynamics of colloidal spheres probed with fluorescence recovery after photobleaching

We report a polarized fluorescence recovery after photobleaching (pFRAP) method to measure the rotational dynamics of fluorescent colloids over a wide dynamic range. The method is based on the polarization anisotropy in the fluorescence intensity, generated by bleaching of fluorescently labeled particles with an intense pulse of linearly polarized laser light. The rotational mobilities of the fluorescent particles can be extracted from the relaxation kinetics of the postbleach fluorescence polarization anisotropy. Our pFRAP setup has access to correlation times over a range of time scales from tens of microseconds to tens of seconds, and is highly sensitive, so very low concentrations of labeled particles can be probed. We present a detailed description of the theoretical background of pFRAP. The performance of the equipment is demonstrated for fluorescent colloidal silica spheres, dispersed in pure solvents as well as in fd-virus suspensions.

Second virial coefficients of dipolar hard spheres

An asymptotic formula is reported for the second virial coefficient B(2) of a dipolar hard-sphere (DHS) fluid, in zero external field, for strongly coupled dipolar interactions. This simple formula, together with the one for the weak-coupling B(2), provides an accurate prediction of the second virial coefficient for a wide range of dipole moments, including those that are experimentally accessible in magnetite ferrofluids. The weak-coupling B(2) also yields an estimate of the magnetic moment minimally needed for isotropic gas-liquid phase-separation, if any, in the DHS fluid.

Measurement of the zero-field magnetic dipole moment of magnetizable colloidal silica spheres

The magnetic properties of dispersions of magnetic silica microspheres have been investigated by measuring the magnetization curves and the complex magnetic susceptibility as a function of frequency and field amplitude. The silica spheres appear to have a net permanent magnetic dipole moment, even in zero field, which is increased significantly after a temporary exposure of the silica colloids to a saturating magnetic field. The magnetic properties of the microparticles in zero field are discussed in terms of the number and the orientations of the embedded nanoparticle dipoles along an easy axis of magnetization in the absence of an external field.

Algebraic Repulsions between Charged Planes with Strongly Overlapping Electrical Double Layers

Langmuirs disjoining pressure between two flat, charged planes was calculated analytically for strongly overlapping double layers in the limit of zero electric field between the planes. The resulting repulsion has a long-range algebraic decay that stems from the thermodynamic equilibrium between homogeneously distributed interplate ions and ions in the surrounding electrolyte reservoir. Together with the van der Waals attraction, the repulsion forms the zero-field pendant of the exponentially screened DLVO potential, a pendant that is always repulsive at large plate-plate distances. The experimental occurrence of algebraic repulsions can be simply predicted from surface charge density and ionic strength.

CaO as Drop‐In Colloidal Catalysts for the Synthesis of Higher Polyglycerols

Glycerol is an attractive renewable building block for the synthesis of polyglycerols, which find application in the cosmetic and pharmaceutical industries. The selective etherification of glycerol to higher oligomers was studied in the presence of CaO colloids and the data are compared with those obtained from NaOH and CaO. The materials were prepared by dispersing CaO, CaCO3, or Ca(OH)2 onto a carbon nanofiber (CNF) support. Colloidal nanoparticles were subsequently dispensed from the CNF into the reaction mixture to give CaO colloids that have a higher activity than equimolar amounts of bulk CaO and NaOH. Optimization of the reaction conditions allowed us to obtain a product with Gardner color number <2, containing no acrolein and minimal cyclic byproducts. The differences in the CaO colloids originating from CNF and bulk CaO were probed using light scattering and conductivity measurements. The results confirmed that the higher activity of the colloids originating from CaO/CNF was due to their more rapid formation and smaller size compared with colloids from bulk CaO. We thus have developed a practical method for the synthesis of polyglycerols containing low amounts of Ca.