Kristof Houthoofd

Synthesis Modulation as a Tool To Increase the Catalytic Activity of Metal–Organic Frameworks: The Unique Case of UiO-66(Zr)

The catalytic activity of the zirconium terephthalate UiO-66(Zr) can be drastically increased by using a modulation approach. The combined use of trifluoroacetic acid and HCl during the synthesis results in a highly crystalline material, with partial substitution of terephthalates by trifluoroacetate. Thermal activation of the material leads not only to dehydroxylation of the hexanuclear Zr cluster but also to post-synthetic removal of the trifluoroacetate groups, resulting in a more open framework with a large number of open sites. Consequently, the material is a highly active catalyst for several Lewis acid catalyzed reactions.

Zeolite-catalysed conversion of C3 sugars to alkyl lactates

The direct conversion of C3 sugars (or trioses) to alkyl lactates was achieved using zeolite catalysts. This reaction represents a key step towards the efficient conversion of bio-glycerol or formaldehyde to added-value chemicals such as lactate derivatives. The highest yields and selectivities towards the desired lactate product were obtained with Ultrastable zeolite Y materials having a low Si/Al ratio and a high content of extra-framework aluminium. Correlating the types and amounts of acid sites present in the different zeolites reveals that two acid functions are required to achieve excellent catalysis. Bronsted acid sites catalyse the conversion of trioses to the reaction intermediate pyruvic aldehyde, while Lewis acid sites further assist in the intramolecular rearrangement of the aldehyde into the desired lactate ester product. The presence of strong zeolitic Bronsted acid sites should be avoided as much as possible, since they convert the intermediate pyruvic aldehyde into alkyl acetals instead of lactate esters. A tentative mechanism for the acid catalysis is proposed based on reference reactions and isotopically labelled experiments. Reusability of the USY catalyst is demonstrated for the title reaction.

Short-ranged structural rearrangement and enhancement of mechanical properties of organosilicate glasses induced by ultraviolet radiation

The short-ranged bonding structure of organosilicate glasses can vary to a great extent and is directly linked to the mechanical properties of the thin film material. The combined action of ultraviolet (UV) radiation and thermal activation is shown to generate a pronounced rearrangement in the bonding structure of thin organosilicate glass films involving no significant compositional change or film densification. Nuclear magnetic resonance spectroscopy indicates loss of –OH groups and an increase of the degree of cross-linking of the organosilicate matrix for UV-treated films. Fourier transform infrared spectroscopy shows a pronounced enhancement of the Si–O–Si network bond structure, indicating the formation of more energetically stable silica bonds. Investigation with x-ray reflectivity and ellipsometric porosimetry indicated only minor film densification. As a consequence, the mechanical properties of microporous organosilicate dielectric films are substantially enhanced while preserving the organosili...

Productive sugar isomerization with highly active Sn in dealuminated β zeolites

A water-tolerant Lewis acid catalyst was synthesized by grafting SnIV in isopropanol under reflux onto dealuminated zeolites with the BEA (β) topology. This synthesis method allows the production of highly active Snβ-type catalysts without the need for long hydrothermal syntheses or hydrogen fluoride, while using cheap Sn-precursors, industrially available β zeolites and standard catalyst synthesis unit operations. Extensive characterization of the best catalyst shows highly dispersed Sn in the zeolite matrix (XRD, 29Si MAS NMR and 1H MAS NMR) without the formation of SnO2 (XRD and UV-Vis). The catalyst was tested for the model isomerization of sugars such as glucose to fructose. The catalytic activity proved to be purely heterogeneous and the catalyst was recycled and reused without significant loss in activity. Isomerization productivities above 4 kg product per kg of catalyst per hour are reported with appreciably low Sn loadings, corresponding to exceptionally high turnover frequencies, viz. 500 cycles per Sn per hour at 110 °C, which surpass the activity per Sn of the original hydrothermally synthesized Snβ.

Design of zeolite by inverse sigma transformation

Although the search for new zeolites has traditionally been based on trial and error, more rational methods are now available. The theoretical concept of inverse σ transformation of a zeolite framework to generate a new structure by removal of a layer of framework atoms and contraction has for the first time been achieved experimentally. The reactivity of framework germanium atoms in strong mineral acid was exploited to selectively remove germanium-containing four-ring units from an UTL type germanosilicate zeolite. Annealing of the leached framework through calcination led to the new all-silica COK-14 zeolite with intersecting 12- and 10-membered ring channel systems. An intermediate stage of this inverse σ transformation with dislodged germanate four-rings still residing in the pores could be demonstrated. Inverse σ transformation involving elimination of germanium-containing structural units opens perspectives for the synthesis of many more zeolites.

NH2-MIL-53(Al): A High-Contrast Reversible Solid-State Nonlinear Optical Switch

The metal-organic framework NH(2)-MIL-53(Al) is the first solid-state material displaying nonlinear optical switching due to a conformational change upon breathing. A switching contrast of at least 38 was observed. This transition originates in the restrained linker mobility in the very narrow pore configuration.

Copper Benzene Tricarboxylate Metal–Organic Framework with Wide Permanent Mesopores Stabilized by Keggin Polyoxometallate Ions

Porous solids with organized multiple porosity are of scientific and technological importance for broadening the application range from traditional areas of catalysis and adsorption/separation to drug release and biomedical imaging. Synthesis of crystalline porous materials offering a network of uniform micro- and mesopores remains a major scientific challenge. One strategy is based on variation of synthesis parameters of microporous networks, such as, for example, zeolites or metal-organic frameworks (MOFs). Here, we show the rational development of an hierarchical variant of the microporous cubic Cu(3)(BTC)(2) (BTC = 1,3,5-benzenetricarboxylate) HKUST-1 MOF having strictly repetitive 5 nm wide mesopores separated by uniform microporous walls in a single crystal structure. This new material coined COK-15 (COK = Centrum voor Oppervlaktechemie en Katalyse) was synthesized via a dual-templating approach. Stability was enhanced by Keggin type phosphotungstate (HPW) systematically occluded in the cavities constituting the walls between the mesopores.

Ternary Ag/MgO-SiO2 catalysts for the conversion of ethanol into butadiene.

Ternary Ag/Magnesia-silica catalysts were tested in the direct synthesis of 1,3-butadiene from ethanol. The influence of the silver content and the type of silica source on catalytic performance has been studied. Prepared catalysts were characterized by (29) Si NMR, N2 sorption, small-angle X-ray scattering measurements, XRD, environmental scanning electron microscopy with energy dispersive X-ray analysis (ESEM/EDX), FTIR spectroscopy of adsorbed pyridine and CO2 , temperature-programmed desorption of CO2 and UV/Vis diffuse reflectance spectroscopy. Based on these characterization results, the catalytic performance of the catalysts in the 1,3-butadiene formation process was interpreted and a tentative model explaining the role of the different catalytically active sites was elaborated. The balance of the active sites is crucial to obtain an active and selective catalyst to form 1,3-butadiene from ethanol. The optimal silver loading is 1-2 wt% on a MgO-silica support with a molar Mg/Si ratio of 2. The silver species and basic sites (Mg−O pairs and basic OH groups) are of prime importance in the 1,3-butadiene production, catalyzing mainly the ethanol dehydrogenation and the aldol condensation, respectively.

Itraconazole/TPGS/Aerosil ® 200 solid dispersions Characterization, physical stability and in vivo performance

Solid dispersions were successfully prepared by co-spray-drying of TPGS-stabilized itraconazole nanosuspensions with Aerosil200, followed by heat treatment of the powders. The itraconazole/Aerosil200 weight ratios amounted to 50/50, 30/70, 40/60 and 20/80. The itraconazole content of the powders was close to the expected value, with relative errors between 0.3% and 7.8%. X-ray powder diffraction (XRPD), solid state NMR (SSNMR) and differential scanning calorimetry (DSC) evaluation on the powders revealed the formation of amorphous itraconazole and the absence of glassy itraconazole. Dissolution of the powders was enhanced compared to crystalline and glassy itraconazole (a 2-dimensional structured form of itraconazole). However, no clear trend could be observed between drug loading and dissolution performance of the solid dispersions. Upon storage, conversion to crystalline itraconazole was observed for the 50/50 powder based on XRPD, SSNMR and DSC measurements. Although the 40/60 powder remained X-ray amorphous upon storage, DSC did reveal that a small fraction (7.5+/-1.6% after 10 months of storage) of itraconazole crystallized upon storage. For the 30/70 and 20/80 dispersions, no crystallization could be seen. After 10 months of storage, important changes in the dissolution behavior of the powders were observed. A decrease in dissolution performance was seen for the 50/50 dispersion, which could be attributed to the crystallization of itraconazole. On the other hand, the 40/60, 30/70 and 20/80 dispersions showed an increase in dissolution rate (more than 60% after 10 min). Although not completely clear at this stage, adsorption of itraconazole onto the Aerosil200 surface during storage might be responsible for this behavior. Finally, in vivo experiments were performed in the rat. Oral bioavailability of the 30/70 dispersion was, although lower compared to the marketed Sporanox formulation, significantly enhanced compared to the crystalline drug.

Dissolution enhancement of the anti-HIV drug UC 781 by formulation in a ternary solid dispersion with TPGS 1000 and Eudragit E100

The present research deals with the improvement of the dissolution properties of the anti-HIV drug UC 781. A ternary solid dispersion consisting of a high amount of TPGS 1000 and exhibiting good powder properties with respect to flowability was developed. Eudragit E100 was selected as a polymer based on supersaturation studies. DSC analysis of solid dispersions containing drug doses from 0 to 80% w/w revealed eutectic phase behaviour of the ternary TPGS 100-Eudragit E100-UC 781 mixture. The release of UC 781 in a medium simulating the gastrointestinal lumen was markedly enhanced, reaching a release of 70% w/w after 4h. XRD results pointed to the presence of crystalline drug in the solid dispersion. The presence of UC 781 in the dispersion had an influence on the TPGS 1000-Eudragit E100 carrier, favoring folding of the polyethylene glycol chains in TPGS 1000. Moreover, the addition of UC 781 to the binary polymer-surfactant mixture was physically expressed by an increase in fluidity of the samples up to a drug load of 50% w/w. NMR was used to investigate this phenomenon, revealing a shielding and/or deshielding effect of the carrier on aromatic C atoms and methyl groups in UC 781. Polyethylene glycol chains present in TPGS 1000 seemed to play a role in this process. In addition, combining UC 781 with the TPGS 1000-Eudragit E100 mixture led to the appearance of TPGS 1000 clusters with a glass transition temperature well below the T(g)s of the pure compounds.