Kai C. Szeto

Heteronuclear NMR spectroscopy as a surface-selective technique: a unique look at the hydroxyl groups of γ-alumina.

The surface hydroxyl groups of γ-alumina dehydroxylated at 500 °C were studied by a combination of one- and two-dimensional homo- and heteronuclear (1)H and (27)Al NMR spectroscopy at high magnetic field. In particular, by harnessing (1)H-(27) Al dipolar interactions, a high selectivity was achieved in unveiling the topology of the alumina surface. The terminal versus bridging character of the hydroxyl groups observed in the (1)H magic-angle spinning (MAS) NMR spectrum was demonstrated thanks to (1)H-(27) Al RESPDOR (resonance-echo saturation-pulse double-resonance). In a further step the hydroxyl groups were assigned to their aluminium neighbours thanks to a {(1)H}-(27) Al dipolar heteronuclear multiple quantum correlation (D-HMQC), which was used to establish a first coordination map. Then, in combination with (1)H-(1) H double quantum (DQ) MAS, these elements helped to reveal intimate structural features of the surface hydroxyls. Finally, the nature of a peculiar reactive hydroxyl group was demonstrated following this methodology in the case of CO2 reactivity with alumina.

Low temperature hydrogenolysis of waxes to diesel range gasoline and light alkanes: Comparison of catalytic properties of group 4, 5 and 6 metal hydrides supported on silica–alumina

A series of metal hydrides (M = Zr, Hf, Ta, W) supported on silica–alumina were studied for the first time in hydrogenolysis of light alkanes in a continuous flow reactor. It was found that there is a difference in the reaction mechanism between d0metal hydrides of group 4 and d0 ↔ d2metal hydrides of group 5 and group 6. Furthermore, the potential application of these catalysts has been demonstrated by the transformation of Fischer–Tropsch wax in a reactive distillation set-up into typical gasoline and diesel molecules in high selectivity (up to 86 wt%). Current results show that the group 4 metal hydrides have a promising yield toward liquid fuels.

Small changes have consequences: lessons from tetrabenzyltitanium and -zirconium surface organometallic chemistry.

Homoleptic benzyl derivatives of titanium and zirconium have been grafted onto silica that was dehydroxylated at 200 and 700 °C, thereby affording bi-grafted and mono-grafted single-site species, respectively, as shown by a combination of experimental techniques (IR, MAS NMR, EXAFS, and elemental analysis) and theoretical calculations. Marked differences between these compounds and their neopentyl analogues are discussed and rationalized by using DFT. These differences were assigned to the selectivity of the grafting process, which, depending on the structure of the molecular precursors, led to different outcomes in terms of the mono- versus bi-grafted species for the same surface concentration of silanol species. The benzylzirconium derivatives were active towards ethylene polymerization in the absence of an activator and the bi-grafted species displayed higher activity than their mono-grafted analogues. In contrast, the benzyltitanium and neopentylzirconium counterparts were not active under similar reaction conditions.

Design and Application of a Hybrid Material Featuring Well-Defined, Tuneable Grafting Sites for Supported Catalysis.

A new material based on amorphous silica, and featuring a well‐defined phenolic functionality was prepared in two simple steps by using commercially available, cheap reagents. Silica was first reacted with aluminum isobutyl etherate to yield an aluminum isobutyl site [(≡SiO)2Al(iBu)(Et2O)], which then selectively reacted with a hydroquinone spacer to yield [(≡SiO)2(AlOC6X4OH)(Et2O)] (X=H or F). This support material was then used to tether the organometallic tungstenocarbyne complex [W(≡CtBu)(CH2tBu)3] to yield the surface species [(≡SiO)2(AlOC6X4O‐W(≡CtBu)(CH2tBu)2(Et2O)] (X=H or F). Both H‐ and F‐containing species were fully characterized, and their activities in the self‐metathesis reaction of propene to ethylene and 2‐butenes were found to be two and three times higher, respectively, than the activity of the corresponding tungstenocarbyne complex directly grafted onto silica.

Well-Defined Molybdenum Oxo Alkyl Complex Supported on Silica by Surface Organometallic Chemistry: A Highly Active Olefin Metathesis Precatalyst

The well-defined silica-supported molybdenum oxo alkyl species (≡SiO-)MoO(CH2tBu)3 was selectively prepared by grafting of MoO(CH2tBu)3Cl onto partially dehydroxylated silica (silica700) using the surface organometallic chemistry approach. This surface species was fully characterized by elemental analysis and DRIFT, solid-state NMR, and EXAFS spectroscopy. This new material is related to the active species of industrial supported MoO3/SiO2 olefin metathesis catalysts. It displays very high activity in propene self-metathesis at mild (turnover number = 90 000 after 25 h). Remarkably, its catalytic performance outpaces those of the parent imido derivative and its tungsten oxo analogue.

Selective conversion of butane into liquid hydrocarbon fuels on alkane metathesis catalysts

We report a selective direct conversion of n-butane into higher molecular weight alkanes (C5+) by alkane metathesis reaction catalysed by silica–alumina supported tungsten or tantalum hydrides at moderate temperature and pressure. The product is unprecedented, asymmetrically distributed towards heavier alkanes.

Thermal decomposition of lignocellulosic biomass in the presence of acid catalysts

Transformation of lignocellulosic biomass to biofuels involves multiple processes, in which thermal decomposition, hydrotreatment are the most central steps. Current work focuses on the impact of several solid acids and Keggin-type heteropolyacids on the decomposition temperature (Td) of pine wood and the characterization of the resulted products. It has been observed that a mechanical mixture of solid acids with pine wood has no influence on Td, while the use of heteropolyacids lower the Td by 100°C. Moreover, the treatment of biomass with a catalytic amount of H3PW12O40 leads to formation of three fractions: solid, liquid and gas, which have been investigated by elemental analysis, TGA, FTIR, GC-MS and NMR. The use of heteropolyacid leads, at 300°C, to a selective transformation of more than 50 wt.% of the holocellulose part of the lignocellulosic biomass. Moreover, 60 wt.% of the catalyst H3PW12O40 are recovered.

Improved direct production of 2,3-dimethylbutenes and 3,3-dimethylbutene from 2-methylpropene on tungsten hydride based catalysts

2-Methylpropene in the presence of W–H/Ni1%–Al2O3-(500) is transformed in high selectivity into a mixture of 2,3-dimethylbutenes (2,3-DMBs = DMB-1 and DMB-2) and neohexene. 2,3-DMBs arise from the unfavoured 2-methylpropene self-metathesis reaction whereas the neohexene originates from a cascade reaction: 2-methylpropene dimerisation followed by cross metathesis.

Tailoring the selectivity in 2-butene conversion over supported d0 group 4, 5 and 6 metal hydrides: from dimerization to metathesis

2-Butenes are transformed in a continuous flow reactor over metal hydrides of zirconium, tantalum and tungsten supported on silica–alumina. Exceptionally high selectivity to dimeric products is obtained over supported zirconium hydride catalysts. Supported tantalum hydride gives high selectivity to dimeric products along with small amounts of metathesis products. Conversely, supported tungsten hydride leads only to metathesis products. The unprecedented selective production of mono-branched and di-branched octenes originates from the bi-functionality of the active sites, allowing isomerization of 2-butenes to 1-butene followed by fast insertion of 1-butene into the metal sec-butyl fragment.

Direct thermocatalytic transformation of pine wood into low oxygenated biofuel

Direct catalytic conversion of pine wood under H2 into an organic liquid composed of saturated alkanes and aromatics has been achieved. The resulting organic liquids are easily isolated from the aqueous phase with a yield up to 30 wt%. Importantly, the oxygen content is about 3 wt% and has a higher heating value of 41 MJ kg−1 which is very close to standard diesel (44 MJ kg−1) used in automotive fuels. The multi-functional catalysts comprise well size controlled bimetallic nanoparticles (Cu–Ru) supported on heteropolyacid salts. The residual acidic proton of the heteropolyanion salt combined with bimetallic nanoparticles produced a multifunctional catalyst, featuring depolymerisation, deoxygenation and hydrogenation in a single batch reactor. Current results present an alternative approach to transform lignocellulosic biomass (oxygen content higher than 40 wt%) directly into an organic liquid (oxygen content less than 5 wt%) suitable as additives in biofuels.