Herman van Bekkum

Synthesis and characterization of zeolite (MFI) membranes on porous ceramic supports

Continuous polycrystalline films of ZSM-5 (MFI-type) crystals have been grown on porous ceramic (clay) supports. The films are thermomechanically stable upon calcination at 400 °C in air to remove template ions (tetrapropylammonium), during which process the porous support seems to have a stabilizing effect. When the hydrothermal conditions induce substantial aluminium leaching from the support, large analcime crystals are grown on the support. Gas permeation experiments with both pure gases (permanent gases, alkanes and difluorodichloromethane) and mixtures thereof have been performed on the MFI composite to investigate the separation potential of this new type of membrane. Expected selectivities as a result of large differences in diffusivity are found to be strongly reduced by differences in sorption of the same order of magnitude, and the reduced mobility of weakly adsorbing (fast moving) molecules caused by the slower moving species. However, from the observed low permeation rates it is expected that, owing to the presence of a porous support with relatively low porosity, the molecular sieving effect is reduced as well.

High-temperature stainless steel supported zeolite (MFI) membranes: Preparation, module construction, and permeation experiments

Abstract Continuous layers of MFI (silicalite-1; Si-rich ZSM-5) have been prepared on porous, sintered stainless steel supports. Similar metal supported MFI membranes of ∼ 50 μm thickness have been grown within stainless steel membrane modules in order to perform (high-temperature) permeation experiments. As-synthesized layers are found to be gas-tight even for small molecules such as neon. The supported MFI layers remain thermomechanically stable upon calcination at 400°C in air to remove template ions (tetrapropylammonium). Gas permeation experiments have been performed using neon, methane, n-butane, and isobutane according to the Wicke—Kallenbach principle with helium as a purge gas. The sequence of the pure gas permeabilities at room temperature and 0.3 bar partial pressure difference is methane >n-butane > neon a isobutane, demonstrating that the permeation is based on both adsorption and diffusion. The deviating permeation behaviour between the butane isomers is attributed to the bulkiness of isobutane, which is also reflected in the substantially lower adsorption capacity as compared with n-butane. In experiments using binary mixtures of strongly (butane isomers) and weakly (methane) adsorbing species, the permeation rate of the former is hardly affected, whereas for the latter a drop in permeability of some two orders of magnitude is observed. At higher temperatures (up to 350°C) with a constant feed composition, the methane permeation rate increases as a result of the decreased adsorption of n-butane. The MFI layer retains its separation potential after several heating and cooling cycles.

Selective oxidation of primary alcohols mediated by nitroxyl radical in aqueous solution. Kinetics and mechanism

The kinetics of the TEMPO-mediated oxidation of methyl α-D- glucopyranoside to sodium methyl α-D-glucopyranosiduronate were studied. An intermediate was found which was identified as the hydrated aldehyde. This was oxidised in the same manner as the alcohol, with pseudo first order rate constants ratio k(obs,ald)/k(obs,alc) = 7. The reaction mechanism is discussed with emphasis on steric factors and compared to literature data. Two different reaction pathways are postulated; under basic reaction conditions via a cyclic transition state 3 and under acid reaction conditions through an acyclic transition state 4.

On the remarkable behaviour of zeolite Beta in acid catalysis

Abstract Zeolite Beta is developing into a major catalyst in organic chemicals conversion, contributing to low waste technology. In comparison with other zeolites, zeolite Beta possesses unique acid properties which are related to local defects. These defects are generated when a tertiary building unit (TBU) is rotated 90° around the c -direction with respect to the neighbouring TBUs in the same layer. The rotated TBU cannot connect properly with the adjacent layers. This results in T -atoms that are not fully coordinated to the framework, thereby creating potential Lewis-acid sites. The high stereoselectivity observed in the Meerwein-Ponndorf-Verley (MPV) reduction of 4- tert -butylcyclohexanone to the corresponding cis -alcohol and the absence of MPV reduction of the bulky probe molecule 1-(3,5-di- tert -butylphenoxy)propan-2-one on the external surface, indicate that Lewis-acid sites are mainly present in the micropore walls. By contrast, Bronsted-acid sites are present on the internal as well as the external surface.

The structure and (local) stability constants of borate esters of mono- and di-saccharides as studied by 11B and 13C NMR spectroscopy

The formation of borate esters of various mono- and di-saccharides in aqueous solution was studied by 11B and 13C NMR spectroscopy. Association constants K(B-L) at a carbohydrate-borate molar ratio of 1:1, pH 7, and 25 degrees C were determined and compared with literature values obtained from potentiometry. The association constants K(B-L) were converted into local association constants Kloc(B-L) by using the distribution of the various anomeric forms in D2O. In this way, values of Kloc(B-L) were obtained, which appear to be characteristic of the configuration concerned. They explain the favourable effect of borate in the alkaline isomerisation of lactose into lactulose. At a low molar ratio (1:3) of carbohydrate-borate, predominantly diborate esters (B-)2L were formed.

Bromide-free TEMPO-mediated oxidation of primary alcohol groups in starch and methyl α-d-glucopyranoside

TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl)-mediated oxidation of potato starch and methyl alpha-D-glucopyranoside (MGP) was performed in the absence of sodium bromide (NaBr) as co-catalyst, solely using sodium hypochlorite (NaOCl) as the primary oxidant. The low reaction rate associated with a bromide-free process was increased by performing the oxidation at increased temperatures. The reaction proceeded stoichiometrically and with high selectivity and with only minor depolymerisation, provided that temperature and pH were kept < or = 20 degrees C and < 9.0, respectively. At 20 degrees C and pH 8.5, the reaction rate was comparable to that of a corresponding oxidation catalysed by NaBr at 2 degrees C. Consequently, this is a simple approach to raise the TEMPO/NaOCl reaction rate under bromide-free conditions while still maintaining good product properties. At higher oxidation temperatures (> or = 25 degrees C) and under more alkaline conditions (pH > or = 9.0) degradation of the starch skeleton occurred. Simultaneously, side-reactions of the nitrosonium ion lowered the yield of the oxidation. Despite the absence of the NaBr catalyst, the reaction rate-controlling step was found to be the oxidation of the primary hydroxyl groups with the nitrosonium ion. The reaction was first-order in MGP and in TEMPO.

Hydrothermal formation of 1,2,4-benzenetriol from 5-hydroxymethyl-2-furaldehyde and D-fructose

Thermolysis of 0.05 M aqueous 5-hydroxymethyl-2-furaldehyde (HMF) at 27.5 MPa and 290 to 400°C led to the formation of 1,2,4-benzenetriol in yields of up to 46% at 50% HMF conversion. The reaction temperature and water density have a significant effect on the product composition. Pseudo-first-order reaction rate constants for HMF conversion under these conditions range from 0.107 to 0.308 min−1. For the region 290 to 350°C, the activation energy for HMF conversion was found to be 47.7 kJ.mol−1. When subjecting d-fructose to hydrothermolysis, the main products are HMF, 1,2,4-benzenetriol, and furfural.

Application of zeolite titanium Beta in the rearrangement of α-pinene oxide to campholenic aldehyde

Zeolite titanium Beta is found to be an effective catalyst for the industrially relevant rearrangement of α-pinene oxide to campholenic aldehyde, giving selectivities of up to 89% in the liquid phase. In the gas phase, up to 94% selectivity is obtained at conversions above 95%, which exceeds the best values obtained with homogeneous Lewis acid catalysts. The selectivity obtained is profoundly influenced by the solvent or co‐adsorbate applied.

MCM-41 type molecular sieves as catalysts for the Friedel-Crafts acylation of 2-methoxynaphthalene

Abstract Molecular sieves of the MCM-41 type were studied as catalysts for the Friedel-Crafts acylation of 2-methoxynaphthalene in the liquid phase. When MCM-41 in the H + form is used as Bronsted-acid catalyst, acylation with acetic anhydride gives substitution predominantly at the 1-position. Increased reaction temperatures lead to a slight decrease of selectivity to acylation at the 1-position. The regenerability of H-MCM-41 is found to be excellent. For the acylation of 2-methoxynaphthalene with acetyl chloride Zn-MCM-41 was studied as a Lewis-acid catalyst, but both selectivity and regenerability are less favourable than in the case of the H-MCM-41 catalysed acylation of with acid anhydride.

Zeolite catalysed synthesis of coumarin derivatives

The direct synthesis of coumarin derivatives from m-substituted phenols and α,β-unsaturated carboxylic acids catalysed by solid-acid catalysts, such as zeolite H-Beta or Amberlyst-15, in toluene as solvent was studied. The conversion involves esterification followed by alkylation (ring closure). Ring closure of the ester is promoted both by an appropriate substituent on the aromatic ring and by Michael activation of the β-carbon of the ester. These influences were studied by variation of the reactants. 7-Hydroxy-3,4-dihydrocoumarin is formed in high yield when resorcinol and propenoic acid are used as reactants.