Els De Canck

Periodic mesoporous organosilicas functionalized with a wide variety of amines for CO2 adsorption

The adsorption of CO2 on amine modified periodic mesoporous organosilicas (PMOs) has been investigated. An ethenylene-bridged PMO is modified with a wide range of diamines and polyamines. A variety of dangling N-containing functionalities, i.e., diaminobutane, diaminohexane, diaminododecane, diethylenetriamine and tetraethylenepentamine, as nucleophiles in a substitution reaction is used. The CO2 adsorption capacity of these materials is probed and compared with amine functionalized SBA-15 material, in an effort to reach the maximal CO2/N ratio of 0.5 when using dry conditions in a chemisorption process. The materials showed good CO2 adsorption behaviour and this maximum amine efficiency value has been approximated by the PMO material modified with diaminododecane.

New ultrastable mesoporous adsorbent for the removal of mercury ions

To find a more stable adsorbent for the selective removal of mercury ions, a new mesoporous adsorbent is developed and compared with a number of carefully selected mesoporous silica adsorbents described in literature. This new adsorbent is based on a pure trans-ethene bridged periodic mesoporous organosilica (PMO) which is subsequently modified to obtain a suitable adsorbent. The outcome is a new thiol-containing ethene bridged PMO which combines the adsorption efficiency of the thiol group toward mercury ions with the stability of ethene bridged PMOs. During the adsorption process, this material not only maintains its mesoporous structure and ordering, it also completely preserves the amount of organic functionalities, allowing recycling and reuse of the adsorbent. Additionally, this PMO is able to reduce the Hg(2+) amount in aqueous solutions below 0.5 microg/L, and the adsorbent has a maximal adsorption capacity of 64 mg/g which means an apparent 1:1 ratio mercury(II) ion to thiol.

Formation and functionalization of surface Diels–Alder adducts on ethenylene-bridged periodic mesoporous organosilica

The Diels–Alder cycloaddition of two common dienes (cyclopentadiene and anthracene) to the double carbon–carbon bonds of an ethenylene-bridged periodic mesoporous organosilica was studied and compared to that of benzocyclobutene. The resulting materials were characterized by several techniques such as X-ray and thermal analyses, DRIFT, 13C and 29Si MAS NMR and porosimetry. They showed that the mesopores were decorated with the Diels–Alder adducts with negligible structural degradation and with the concomitant reduction of the surface area and pore size. The formation of surface adducts was even successful with a relatively hindered diene such as anthracene. The surface Diels–Alder adducts were stable and susceptible to further functionalization. Thus, after being sulfonated, the resulting solids were used as acid catalysts in the esterification of acetic acid with ethanol, a process in which they proved to be very active. In fact, each acid site of these hybrid materials was up to three times more active than a similar site in Amberlyst-15, a commercial acid resin.

Silanol‐Assisted Aldol Condensation on Aminated Silica: Understanding the Arrangement of Functional Groups

Free silanol groups are known to promote the activity of aminated silica. In this work the effect of the silanol‐to‐amine ratio on the aldol condensation of 4‐nitrobenzaldehyde and acetone is investigated in a range from 0 to 2.4. Irrespective of the amine density, identical, moderate turnover frequencies are obtained if the silica exclusively has amines on its surface. The turnover frequency increases with increasing silanol‐to‐amine ratio until an upper limit is reached at a silanol‐to‐amine ratio of 1.7. At this upper limit the turnover frequency is a factor 5 higher than the turnover frequencies obtained with the monofunctional amine‐based catalysts. This increase is ascribed to hydrogen‐bridge interactions between the silanols and the carbonyl moiety of the reactants that provoke a more easy interaction between the carbonyl moiety and the amine as required for the aldol condensation. The observation that higher values than one for the silanol‐to‐amine ratio are required is rationalized by computer simulations. It was found that amine groups were grafted on the silica surface in a clustered manner, originating from positive deviations from ideality in the synthesis mixture, that is, from clustering of the amine precursor in the liquid phase.

Synthesized mercaptopropyl nanoporous resins in DGT probes for determining dissolved mercury concentrations

3-Mercaptopropyl functionalized SBA-15 (SH-SBA) and 3-mercaptopropyl functionalized ethenylene bridged periodic mesoporous organosilica (SH-PMO) were included in a Diffusive Gradients in Thin film (DGT) probe and compared to similar commercially available resins also containing thiol functional groups, such as Sumichelate Q10R (SQR) and 3-mercaptopropyl functionalized silica gel (SH-KG), and also to the Chelex-100 resin for the determination of labile Hg concentrations. An agarose gel was used as the diffusive gel because the classic polyacrylamide gel shows more than 20% of Hg adsorption. According to our results, the Chelex-100 resin presents a much lower affinity for Hg than the thiol based resins. The non-linear accumulation profile of mercury with time for the Chelex-100 resin makes it in fact impossible to use Ficks law for estimating the diffusion coefficient of Hg. The 4 other resins all show a linear accumulation profile of Hg with time. Although the highest accumulation rate is observed for SH-PMO followed by SQR, SH-SBA and SH-KG, these values do not differ very much.

Developing a new and versatile ordered mesoporous organosilica as a pH and temperature stable chromatographic packing material

Periodic Mesoporous Organosilicas (PMOs) have been successfully spray dried for the first time using a novel cyclic multifunctional organosilane molecule. By reducing the siloxane bridge formation both in the polymerisation and in the anchoring of the side groups an organic/inorganic hybrid material was designed, depicting inherently improved chemical stability and much increased organic content. The spherical silica particles were applied as a hydrolytically stable stationary phase in reversed phase HPLC. They proved to be both highly retentive and stable in a pH range from 1.75 to 12 and up to 150 °C in 60 v% water. Both the backbone and the functionalities of the material exhibit this high stability.

Periodic mesoporous organosilica functionalized with sulfonic acid groups as acid catalyst for glycerol acetylation

A Periodic Mesoporous Organosilica (PMO) functionalized with sulfonic acid groups has been successfully synthesized via a sequence of post-synthetic modification steps of a trans-ethenylene bridged PMO material. The double bond is functionalized via a bromination and subsequent substitution obtaining a thiol functionality. This is followed by an oxidation towards a sulfonic acid group. After full characterization, the solid acid catalyst is used in the acetylation of glycerol. The catalytic reactivity and reusability of the sulfonic acid modified PMO material is investigated. The catalyst showed a catalytic activity and kinetics that are comparable with the commercially available resin, Amberlyst-15, and furthermore our catalyst can be recycled for several subsequent catalytic runs and retains its catalytic activity.

A new sulphonic acid functionalized periodic mesoporous organosilica as a suitable catalyst

Abstract A new solid acid catalyst is developed by the direct sulphonation of the ethene bond of a pure trans ethene bridged Periodic Mesoporous Organosilica. The catalytic activity of this mesoporous material is evaluated in an esterification reaction and compared with p-toluenesulphonic acid. The sulphonated ethene PMO can compete with a homogeneous catalyst and maintains its porosity.

Periodic Mesoporous Organosilicas as Catalysts for Organic Reactions

Periodic Mesoporous Organosilicas (PMOs) emerged on the scene of Materials Chemistry in 1999. Since then, the literature concerning their synthesis, characterization and applications has rapidly grown. From the very beginning, one of their main applications has been for heterogeneous catalysis. In order to impart them a catalytic behaviour, they have been functionalized to create catalytic sites on their pore walls, sometimes following well-established strategies reminiscent from mesoporous silicas but other times carrying out new methods based on the existence of organic bridges in their framework. Herein, we will give an overview on their use as catalysts, but being focused on the properties that distinguish them from mesoporous silicas and on those processes where their application involves a real advantage over other materials.

Heterogeneous Ru(III) oxidation catalysts via ‘click’ bidentate ligands on a periodic mesoporous organosilica support

A 100% monoallyl ring-type Periodic Mesoporous Organosilica (PMO) is prepared as a novel, versatile and exceptionally stable catalytic support with a high internal surface area and 5.0 nm pores. Thiol–ene ‘click’ chemistry allows straightforward attachment of bifunctional thiols (–NH2, –OH, –SH) which, exploiting the thioether functionality formed, give rise to ‘solid’ bidentate ligands. [Ru(acac)2(CH3CN)2]PF6 is attached and complex formation on the solid is studied via density functional theory. All resulting solid catalysts show high activity and selectivity in alcohol oxidation reactions performed in green conditions (25 °C/water). The PMO catalysts do not leach Ru during reaction and are thus easily recuperated and re-used for several runs. Furthermore, oxidation of poorly water-soluble (±)-menthol illustrates the benefits of using hydrophobic PMOs as catalytic supports.