Pieter C. M. M. Magusin

Nucleation and Growth of Monodisperse Silica Nanoparticles

Although monodisperse amorphous silica nanoparticles have been widely investigated, their formation mechanism is still a topic of debate. Here, we demonstrate the formation of monodisperse nanoparticles from colloidally stabilized primary particles, which at a critical concentration undergo a concerted association process, concomitant with a morphological and structural collapse. The formed assemblies grow further by addition of primary particles onto their surface. The presented mechanism, consistent with previously reported observations, reconciles the different theories proposed to date.

Dual template synthesis of a highly mesoporous SSZ-13 zeolite with improved stability in the methanol-to-olefins reaction

The dual template synthesis of zeolite SSZ-13 by use of trimethyl-adamantanammonium hydroxide and a diquaternary-ammonium mesoporogen induces considerable mesoporosity without impeding zeolite microporosity. The strongly improved accessibility of Brønsted sites in mesoporous SSZ-13 increases its stability during application as an acid catalyst in the methanol-to-olefins reaction.

Click chemistry as a means to functionalize macroporous PolyHIPE

The surface functionalization of macroporous polyHIPE (pHIPE) was achieved by Huisgen-type ‘click’ chemistry. In the first step a 600–800 nm thick layer of poly(glycidyl methacrylate) (pGMA) was grafted from the pHIPE surface by atom transfer radical polymerization (ATRP). Near quantitative azidation of the pGMA layer was achieved by the ring-opening reaction of the epoxide groups with sodium azide. The influence of the reaction conditions on the uniformity of the ‘click’ reaction on the three-dimensional macroporous materials was shown in model reactions with propargyl alcohol. Under optimized conditions, azide conversions of around 80% were estimated from IR-spectra. Visualization of the homogeneous functionalization was achieved by the attachment of a fluorescent molecule. Moreover, the first proof of the versatility for biofunctionalization of pHIPE by this method was provided by the attachment of several protected amino acids. The hydrolytic stability of the triazole ring allows for the successful deprotection of the amino acids on the pHIPE.

Influence of Extraframework Aluminum on the Brønsted Acidity and Catalytic Reactivity of Faujasite Zeolite

A series of faujasite zeolites was modified by extraframework Al (AlEF) with the goal to investigate the influence of such species on the intrinsic Brønsted acidity and catalytic activity towards paraffin cracking. The chemical state of AlEF and zeolite acidity were investigated by 27Al MAS NMR and COads IR spectroscopy, H/D exchange reaction, and propane cracking. Strongly acidic defect‐free Y zeolites were obtained by substitution of framework Al by Si with (NH4)2SiF6. In accordance with the next‐nearest‐neighbor model, the intrinsic acidity of the protons increased with decreasing framework Al density. This increased acidity was evidenced by an increased shift of the OH stretching vibration upon CO adsorption in COads IR spectroscopy and by an increased H/D exchange rate in H/D exchange reactions with perdeuterobenzene. All of the acid sites in these zeolites were of equal strength beyond a certain Si/Al ratio. The increased acidity resulted in an enhanced propane cracking activity. Modification of a model dealuminated Y zeolite by AlEF only resulted in a small fraction of cationic AlEF species, because it was difficult to control the ion exchange process. In comparison, commercial ultrastabilized Y zeolites contained less AlEF and these species were predominantly present in cationic form. The rate of propane cracking strongly correlated to the concentration of Brønsted acid sites perturbed by cationic AlEF species. The results of MQMAS 27Al NMR spectroscopy confirmed the presence of sites perturbed by AlEF and unaffected framework Al sites. Zeolites with higher intrinsic cracking activities contained a higher proportion of perturbed sites. Although COads IR and H/D exchange methods proved to be suitable methods to probe the acidity of Y zeolites free from AlEF, they were less suitable to predict the reactivity if the Brønsted acid sites were affected by cationic AlEF species.

Combined in situ 29Si NMR and small-angle X-ray scattering study of precursors in MFI zeolite formation from silicic acid in TPAOH solutions

Silicic acid powder was dissolved and polymerized in a concentrated aqueous tetrapropylammonium (TPA) hydroxide solution at room temperature. Two complementary techniques were employed to follow this process leading to silicalite-1 zeolite upon heating. The formation of small silicates and specific oligomers involved in the assembly of silicalite-1 nanoprecursors was investigated using 29Si NMR. Small-angle X-ray scattering (SAXS) was used to follow processes at a colloidal level. Dissolution and polymerization of silicic acid could then be related to events occurring at both molecular and colloidal scales. The appearance of very well-defined colloidal particles was linked to a specific intermediate already observed in systems using an organic and monomeric silica source. In situ time-resolved ultra-small-angle X-ray scattering (USAXS) using synchrotron radiation showed a linear growth of the average crystal diameter, which was slower than of that encountered in Na+ containing synthesis mixtures. Using the results presented here, we propose a mechanism describing the TPA-mediated self-assembly of silicalite-1 from silicic acid powder as silica source. This model is in agreement with rising evidence of a common mechanism involving nanoblock aggregation for organic mediated crystallization of high-silica zeolites.

Characterization of Ga/HZSM-5 and Ga/HMOR synthesized by chemical vapor deposition of trimethylgallium

Chemical vapor deposition (CVD) of trimethylgallium (TMG) has been studied as a method to disperse extraframework Ga in acidic ZSM-5 and mordenite zeolite. Various samples were extensively characterized by ICP, XPS, NMR, and FTIR. Silylation with tetramethyldisilazane is explored as a method for deactivating the external zeolite surface. The deposition of TMG in silylated ZSM-5 results in a gallium-to-aluminum ratio close to unity, which indicates a homogeneous metal distribution in the micropore space. However, pore blockage in the one-dimensional channels of mordenite results in a inhomogeneous distribution and a low Ga loading. Upon exposure to moistened air, the adsorbed methylgallium species decompose and alkoxy groups are formed. Subsequent oxidation or reduction leads to the complete removal of methyl groups. The reductive route is the preferred one resulting in a better dispersion of Ga, since oxidation of the methyl groups leads to water formation and hydrolysis of cationic Ga species.

Super-microporous organosilicas synthesized from well-defined nanobuilding units

Super-microporous organosilica with bridging ethylene and pendant vinyl groups has been synthesized by assembling predefined nanobuilding block polyhedral oligomeric silsesquioxanes (POSS) with nonionic surfactant Brij-76 as the template. The material shows wormhole-like super-micropores with uniform size of 1.9 nm, high BET surface area of 872 m2 g–1 and pore volume of 0.52 cm3 g–1. IR and NMR results show that the bridging ethylene, the pendant vinyl groups and the double-4-membered ring structure were successfully transferred from the building blocks to the super-microporous organosilica material. The material shows high hydrothermal stability and can further react with Br2. The advantage of the present approach lies in that the relative contents and proximity of the different organic functionalities in the final material can be well controlled through the starting nanobuilding blocks.

Molecular Promoting of Aluminum Metal–Organic Framework Topology MIL-101 by N,N-Dimethylformamide

In situ NMR and DFT modeling demonstrate that N,N-dimethylformamide (DMF) promotes the formation of metal-organic framework NH2-MIL-101(Al). In situ NMR studies show that upon dissociation of an aluminum-coordinated aqua ligand in NH2-MOF-235(Al), DMF forms a H-Cl-DMF complex during synthesis. This reaction induces a transformation from the MOF-235 topology into the MIL-101 topology. Electronic structure density functional theory (DFT) calculations show that the use of DMF instead of water as the synthesis solvent decreases the energy gap between the kinetically favored MIL-101 and thermodynamically favored MIL-53 products. DMF therefore promotes MIL-101 topology both kinetically and thermodynamically.

Facile synthesis of the DD3R zeolite: performance in the adsorptive separation of buta-1,3-diene and but-2-ene isomers

Small pore size and hydrophobic nature of DD3R make this material a unique zeolite with high potential in industrial separation applications. However, the reproducible rapid synthesis of this zeolite is still a problem. In this work, a thorough assessment of different synthetic methods revealed that synthesis reproducibility relies on two main pillars: the use of properly cleaned autoclave liners and the synthesis composition. High quality DD3R crystals are obtained when KOH is used as a cleaning agent, eliminating memory effects, and when KF is used in the synthesis as a mineralizing agent. The effect of fluoride addition is investigated by use of several characterization techniques (13C, 19F and 29Si MAS-NMR and (2D) 29Si–1H correlation spectra), while monitoring the temporal crystallization of DDR. 29Si–1H NMR reveals that template molecules accommodated within the cages are sticking to these 8-ring windows through their amine group. High quality DD3R crystals are applied in the adsorptive separation of buta-1,3-diene and but-2-ene isomers, one of the most energy intensive separations in chemical industry. Mixture separation experiments revealed that the 8-ring apertures of the DD3R cages are only accessible to trans-but-2-ene and buta-1,3-diene, while excluding but-1-ene and cis-but-2-ene molecules, resulting in shape-selective separation in the presence of C4 mixtures.

The role of the amorphous phase in melting of linear UHMW-PE; implications for chain dynamics

In ultra-high molecular weight polyethylene (UHMW-PE), it is possible to obtain single chain forming single crystals, where chains are adjacently re-entrant. Depending on the heating rate, it is feasible to melt these crystals either by simple consecutive detachment of chain stems from the crystalline substrate or by cluster melting, where several chain stems are involved. The consecutive detachment of chain stems occurs at the melting point predicted from the Gibbs–Thomson equation, whereas the cluster melting at much higher temperatures. Melting by the consecutive detachment of chain stems from the crystal substrate and their diffusion in the melt ultimately result in a new melt state having a heterogeneous distribution of physical entanglements, which invokes differences in local mobility. With combined DSC, rheology and solid-state NMR studies, it is concluded that the disentangled domains present within the entangled matrix possess higher local mobility than the entangled domains, ultimately causing lower elastic modulus. The fraction of the entangled and disentangled domains is maintained at higher temperatures, leading to a thermodynamically non-equilibrium melt state. In contrast, in cluster melting, where several chain stems (initially disentangled) can simultaneously adopt the random coil state, entanglements that are formed are homogeneously distributed in the melt. The paper invokes the influence of the topological differences present in the amorphous phase of the semi-crystalline polymer on the melting kinetics of crystals. The reported findings have implications for the melting behaviour and the resulting melt state of polymers in general.