Reactivity of Gold Hydrides: O2 Insertion into the Au–H Bond

Dioxygen reacts with the gold(I) hydride (IPr)AuH under insertion to give the hydroperoxide, (IPr)AuOOH, a long-postulated reaction in gold catalysis and the first demonstration of O2 activation by Au-H in a well-defined system. Subsequent condensation gave the peroxide (IPr)Au-OO-Au(IPr) (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazole-2-ylidene). The reaction kinetics are reported, as well as the reactivity of Au(I) hydrides with radical scavengers

An element through the looking glass: Exploring the Au-C, Au-H and Au-O energy landscape

Gold, the archetypal “noble metal”, used to be considered of little interest in catalysis. It is now clear that this was a misconception, and a multitude of gold-catalysed transformations has been reported. However, one consequence of the long-held view of gold as inert metal is that its organometallic chemistry contains many “unknowns”, and catalytic cycles devised to explain gold's reactivity draw largely on analogies with other transition metals. How realistic are such mechanistic assumptions? In the last few years a number of key compound classes have been discovered that can provide some answers. This Perspective attempts to summarise these developments, with particular emphasis on recently discovered gold(III) complexes with bonds to hydrogen, oxygen, alkenes and CO ligands

Enhanced <SUP>29</SUP>Si spin-lattice relaxation and observation of three dimensional lattice connectivity in zeolites by two-dimensional <SUP>29</SUP>Si MASS NMR

It is shown that considerable sensitivity enhancement is achieved in the 29Si MASS NMR spectra of highly siliceous zeolites by pretreating the material with oxygen. The presence of adsorbed molecular oxygen in zeolite channels promotes an efficient 29Si spin lattice relaxation via a paramagnetic interaction between the lattice 29Si T-site and the adxorbed oxygen on zeolite channels. This affords an efficient 2-D data collection and leads to increased sensitivity. The utility of this method is demonstrated in a two-dimensional COSY-45 NMR experiment of a high silica zeolte ZSM-5

Influence of various catalyst factors on time-on-stream activity of Ga/H-ZSM-5 in propane aromatization

166-171Influence of various zeolite catalyst factors such as, Si/Al ratio (35-132), degree of H+ exchange (2-98%), calcination temperature (600-800°C) and Ga-loading on time-on-stream activity/selectivity of Ga/H-ZSM-5 in the propane aromatization has been investigated. The time-on-stream activity in the propane-to-aromatics conversion is found to be influenced strongly by the above zeolitic factors. The Ga/H-ZSM-5 zeolite catalysts undergo quite rapid deactivation due to coking.</span

Influence of catalyst binder on the acidity and activity/selectivity of Ga/H-ZSM-5 in propane aromatization

The acidity and initial and time-on-stream activity in propane aromatization (at 550° C, space velocity of 3100 cm3g-1 (zeolite)h-1) of Ga-impregnated H-ZSM-5 zeolite without or with binders (50 wt%), such as silica, alumina and kaolin, have been investigated. Both the acidity and catalytic activity of the zeolite are influenced by the presence of binder in the catalyst, depending upon the binder. The influence is found to be lowest for alumina and highest for kaolin. Among the binders, alumina is the most preferred binder for the zeolite catalyst

Characterization of silicon sites in monoclinic zeolite ZSM-5 using <SUP>29</SUP>Si magic angle spinning (MAS) nuclear magnetic resonance (NMR) and molecular modelling

An attempt has been made to correlate the experimentally observed <SUP>29</SUP>Si MAS NMR chemical shifts of monoclinic phase of highly siliceous ZSM-5 with their electronic properties. In order to incorporate the influence of next neighbor atoms on the <SUP>29</SUP>Si chemical shielding of central SiO<SUB>4</SUB>, a pentameric cluster model (H<SUB>12</SUB>Si<SUB>5</SUB>O<SUB>16</SUB>) has been chosen. Each of the 24 crystallographically distinct Si sites, of ZSM-5 framework has been modelled by such cluster models. Based on semi-empirical quantum chemical calculations, a multiple linear regression analysis of the various electronic properties with the <SUP>29</SUP>Si chemical shifts has been attempted. The relative difference in <SUP>29</SUP>Si chemical shifts for the Si sites in ZSM-5 is reasonably accounted, although quantitative prediction may require non-empirical quantum chemical calculations

TEOM analysis of the equilibria and kinetics of n-hexane and n-heptane adsorption on FCC catalyst/silicalite

A tapered element oscillatory microbalance is used for the measurement of uptake rates of n-hexane and n-heptane on fluid catalytic cracking catalyst and silicalite adsorbent. Investigations are considered over the temperature range of 373 to 513 K, and adsorbate partial pressures up to 0.12 bar. Based on measured adsorption isotherms, and through mechanistic descriptions of the diffusion process, a mathematical model is developed to describe the transient response of the microbalance, and subsequently the kinetics of adsorption and desorption. For example, for the hexane-silicalite system at a temperature of 473 K, the diffusion coefficient at zero-surface coverage is estimated as 2.9×10-11 m2/s, with an activation energy of 17 kJ/mol. This is consistent with published data based on other transient analysis methods. At relatively low temperatures of operation, i.e. less than 450 K, a single resistance model for mass transfer failed to accurately predict the desorption profiles for hexane on silicalite. As a means of quantifying the observed desorption rates, a dual-resistance model is introduced in which two different diffusion rates are assumed to take place above and below a threshold value of adsorbate concentration. Such a model may be used to account for silicalite phase transitions at high adsorbate loading. © 2003 Elsevier Science Ltd. All rights reserved

Acidity, catalytic activity, and deactivation of H-Gallosilicate (MFI) in propane aromatization: influence of hydrothermal pretreatments

Effect of various hydrothermal pretreatments [at different temperatures (400-800&#176;C) and concentrations of steam (13-80 mol%) or with liquid water at 150&#176;C under autogenous pressure] to H-gallosilicate (MFI) zeolite (bulk Si/Ga = 33 and Na/Ga = 0.1) on its acidity/acid strength distribution (determined by chemisorption and step-wise thermal desorption of pyridine from 100-400&#176;C), acid function (studied by acid catalyzed model reactions viz. isooctane cracking for external acidity ando-xylene isomerization and toluene disproportionation for internal or intracrystalline acidity), and deactivation due to coking in propane aromatization (at 550&#176; and time-on-stream of 8.5 &#177; 0.5) has been thoroughly investigated. With the increase in the severity of hydrothermal treatment to the zeolite, its crystallinity, framework (FW) Ga (observed by <SUP>71</SUP>Ga and <SUP>29</SUP>Si MAS NMR and FTIR), acidity (measured in terms of the pyridine chemisorbed at 400&#176;C and activity in the model reactions), and catalytic activity in the propane aromatization are decreased, but its deactivation due to coking and shape selectivity are increased appreciably. These effects are attributed to the extensive degalliation of the zeolite due its hydrothermal treatments. Its product selectivity, dehydrogenation/cracking (D/C) activity ratio, and aromatics/(methane + ethane) mass ratio in the propane aromatization are also influenced by its hydrothermal treatments, depending upon the conversion. The product and shape selectivity of the zeolite are also affected by its deactivation due to coking. The influence of hydrothermal treatments on the activity/selectivity and catalyst deactivation are attributed to a combined/complex effect produced by the decreased zeolitic acidity (i.e., FW Ga) and increased non-FW Ga oxide species in the zeolite channels, depending upon the severity of hydrothermal treatment. The hydrothermal stability of H-gallosilicate (MFI) is much lower than that H-ZSM-5

Influence of O<SUB>2</SUB> and H<SUB>2</SUB> pretreatments on acidity/acid strength distribution and acid functions of Ga/H-ZSM-5, H-GaMFI and H-GaAl MFI zeolites

H-gallosilicate (H-GaMFI), H-galloaluminosilicate (H-GaAlMFI) and Ga, impregnated H-ZSM-5 (Ga/H-ZSM-5) zeolites pretreated with O<SUB>2</SUB> and H2 (at 600° C for 10 h) have been characterized for their acidity/acid strength distribution (by chemisorption and stepwise thermal desorption of pyridine from 100° -400° C) and also for their acid functions by acid catalysed reactions [viz. isooctane cracking (at 400°C) (for characterizing external acid sites) and toluene disproportionation (at 500° C) and methanol-to-aromatics conversion (400°C)] using a pulse microreactor. The catalysts were also characterized by XPS,<SUP>29</SUP>Si,<SUP>27</SUP>Al and<SUP>71</SUP>Ga MAS NMR. The acidity/acid strength distribution, activity in the acid catalyzed reactions, frame-work Si/Ga ratio and surface Ga/Si ratio of the zeolites are significantly affected by their pretreatment by O<SUB>2</SUB> or H<SUB>2</SUB>

Initial activity/selectivity of H-Gallosilicate (MFI) in propane aromatization: influence of H<SUP>+</SUP> exchange and thermal/hydrothermal pretreatments

Initial activity/selectivity of H-gallosilicate (MFI) zeolite with different degrees of H<SUP>+</SUP> exchange and pretreated under different thermal and hydrothermal conditions in propane aromatization (at 500&#8800; C) has been determined using a pulse microreactor connected to GC. It is found to be strongly influenced by the degree of H<SUP>+</SUP> exchange, calcination temperature and hydrothermal treatment at different temperatures and concentrations of steam. There exists a close relationship between the acidity (measured in terms of pyridine chemisorbed at 400&#8800;C) of the gallosilicate and its initial propane conversion and aromatization activity. Presence of strong acidic sites (attributed to FW Ga) at high concentration is essential for the well dispersed non-FW Ga oxide species to be active for dehydrogenation in the propane aromatization over the zeolite