G.K. Chuah

Supported zirconium propoxide—a versatile heterogeneous catalyst for the Meerwein–Ponndorf–Verley reduction

Grafting of zirconium 1-propoxide on SBA-15 resulted in highly active catalysts for the MPV reduction. The activity increased with zirconium loading up to a monolayer coverage. In most cases, there were no side products other than the desired alcohol. Electron-donating groups adjacent to the carbonyl group in the substrate facilitate the reaction. The grafted zirconium catalysts did not lose their activity in the presence of moisture or on exposure to ambient atmosphere, making them easy to handle and reuse. No leaching of the grafted zirconium 1-propoxide into the reaction mixture was observed. The addition of pyridine and water to the reaction medium had only a small effect on its activity while benzoic acid led to severe deactivation. The deactivation is attributed to strong adsorption of benzoic acid at the Zr metal centres which could be reversed on removal of the poison. Aluminum 2-propoxide grafted on SBA-15 resulted in a less active catalyst than the zirconium catalysts. The good resistance to hydrolysis of the zirconium catalysts makes them superior to the aluminum 2-propoxide catalysts.

High surface area zirconia by digestion of zirconium propoxide at different pH

The microstructure of hydrous zirconia (ZrO2) prepared by the hydrolysis of zirconium propoxide followed by digestion at pH 1, 3 and 9 and the ZrO2 derived from this precursor have been studied. The undigested hydrous oxides formed plate-like aggregates with low porosity. After digestion at pH 9, the hydrous oxides became highly porous with surface areas up to 550 m2/g. Calcination at 500°C for 12 h results in ZrO2 with surface areas of more than 380 m2/g. ZrO2 formed after digestion in acidic medium had lower surface areas of less than 100 m2/g and were mixed in tetragonal and monoclinic phases. The water/propoxide ratio during gel formation influences the surface area of the resulting ZrO2. The thermal stability of the material is remarkable: ZrO2 resulting from long digestion retained up to 100 m2/g after successive heating to 900°C. The observed microstructural properties are attributed to the nature of the alkoxide precursor formed under different water/alkoxide ratios and to the effects of digestion under varying pH conditions.

Copper oxidation and surface copper oxide stability investigated by pulsed field desorption mass spectrometry

Abstract Pulsed field desorption mass spectrometry (PFDMS) has been used to examine several types of oxide layers formed by the in situ oxidation of clean copper surfaces. Films of Cu 2 O, CuO/Cu 2 O and CuO were produced by direct oxidation of Cu field emitters and examined for their behavior at different temperatures and for the effects of high fields. The high positive field pulses applied to the Cu 2 O overlayers result in field-enhanced ion injection at the copper/oxide interface, field-enhanced Cu + ion desorption or cation vacancy injection at the oxide/vacuum interface and produce only Cu + ions in the mass spectrum. CuO produces only Cu + , CuO + and CuO 2 + ions and CuO/Cu 2 O bilayers produce these ions until the CuO overlayer is removed from the Cu 2 O layer by field desorption. Removal of the CuO overlayer re-establishes the Cu 2 O field ionization process, namely Cu + ions which occur at approximately one half the field strength necessary for CuO field desorption at the same temperature. The low field strength required to produce the Cu + ions from a Cu 2 O overlayer reflects its ion conduction character and the ionic binding state of the Cu + . Heating the tip covered with Cu 2 O overlayers to higher than 470 K causes the overlayer to decompose and results in loss of its characteristic mass spectrum. It is concluded that PFDMS can be used to study the nature of the oxide overlayers formed during the early stages of oxidation and is projected to be a useful tool to examine electrostatic field effects during oxide growth.

Liquid-phase regioselective benzylation of bromobenzene and other aromatics over microporous zeolites

Various solid acids were investigated for the liquid-phase benzylation of aromatics to industrially important substituted diphenylmethanes. Large-pore zeolites, particularly H-beta (BEA), were active and regioselective for the para-substituted product in the benzylation of arenes. With deactivated aromatics such as bromobenzene, the selectivity towards the para-substituted product decreased with reaction time. The decline in selectivity is attributed to the partial blockage of the zeolite channels by high molecular weight deposits, leaving only non-selective catalytic sites at the outside of the zeolite crystallites. These deposits result from self-condensation of benzyl chloride, and their formation can be greatly suppressed when the concentration of benzyl chloride in the reaction mixture is kept low. By adding benzyl chloride continuously to the reaction mixture, a selectivity towards the para-substituted product of more than 80% was achieved. Mesoporous materials such as montmorillonite K10 and H-Al-MCM-41 were active but not regioselective for the reaction. The pores of these materials are too large to impose restraints on the geometry of the products or the transition state. H-ZSM-5 with smaller pores was less active and showed no regioselectivity, because the active sites inside the channel system of this medium-pore zeolite are less accessible to the reactants, and most of the reaction is catalyzed at the outer surface.

Al-free Zr-zeolite beta as a regioselective catalyst in the Meerwein?Ponndorf?Verley reactionElectronic supplementary information (ESI) available: 29Si MAS NMR spectra. See http://www.rsc.org/suppdata/cc/b3/b309191k/

Al-free Zr-zeolite beta catalyzes the transfer reduction of ketones to the corresponding alcohols in high yield and with exceptional stereocontrol. Notably, the catalyst is very robust and gives good results even with 10% water content in the reaction mixture.

Surface properties of mesoporous catalytic supports

The influence of the processing parameters on the properties of zirconia, alumina and silica MCM-41 has been investigated. Digestion of the precursors leads to zirconia, alumina and silica MCM-41 with higher surface area and better thermal stability. The effects of digestion are attributed to increased dehydroxylation, strengthening of the network between primary particles, and to a decrease in the number of surface defects. In the case of zirconia and alumina, digestion also increased the surface acidity. This may be due to the smaller crystallites which expose low-coordinated sites at the surface. Al-MCM-41 prepared by post-synthesis grafting of aluminum is compared with MCM-41 prepared by direct incorporation of aluminum during synthesis. The surface Si/Al ratio was determined using XPS and correlated to the catalytic activity of the Al-MCM-41 for the synthesis of jasminaldehyde.