Engineering of cyclodextrin glucanotransferases and the impact for biotechnological applications

Cyclodextrin glucanotransferases (CGTases) are industrially important enzymes that produce cyclic α-(1,4)-linked oligosaccharides (cyclodextrins) from starch. Cyclodextrin glucanotransferases are also applied as catalysts in the synthesis of glycosylated molecules and can act as antistaling agents in the baking industry. To improve the performance of CGTases in these various applications, protein engineers are screening for CGTase variants with higher product yields, improved CD size specificity, etc. In this review, we focus on the strategies employed in obtaining CGTases with new or enhanced enzymatic capabilities by searching for new enzymes and improving existing enzymatic activities via protein engineering

The effect of Sn on Pt/C catalysts for the methanol electro-oxidation

The effect of Sn for the methanol oxidn. in sulfuric acid is investigated using electrodeposited Pt and carbon supported Pt. The prepn. has a considerable influence, as the Sn effects range from a small increase to a decrease in methanol oxidn. activity. Sn is believed to act through the activation of H2O. The optimum Sn surface coverage is found to be low; of the order 10%. [on SciFinder (R)

Oxygen reduction catalysed by carbon supported iridium-chelates

Carbon supported iridium-octaethylporphyrin (IrOEP), iridium-tetraphenylporphyrin (IrTPP) and iridium-phthalocyanine (IrPc) were studied in acid for the oxygen redn. Both porphyrins give a four-electron redn., although a peculiar deactivation at low potentials occurs. At IrPc and heat-treated porphyrins hydrogen peroxide is formed. Results are compared with carbon monoxide oxidn. expts. at these catalysts. A single site mechanism is proposed for the oxygen redn. mechanism. [on SciFinder (R)

Characterization and thiophene hydrodesulfurization activity of amorphous-silica-alumina-supported NiW catalysts

The influence of the preparation method and sulfidation conditions on the structure and activity of ASA-supported NiW catalysts was investigated by a combination of Co-57 Mossbauer emission spectroscopy (MES), transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS), and thiophene hydrodesulfurization (HDS) activity measurements. Ni is sulfided already at low temperatures. This nickel sulfide phase redisperses at higher temperatures over the edges Of WS2 particles to form "Ni-W-S"-type phases. The formation of such highly active phases is facilitated by the partial transformation of intermediate WOxSy phases to WS2 (W L-III edge XAS) at 673 K. In addition to this "Co-Mo-S" analogue nickel sulfide particles are present in dispersed form close to an oxysulfidic tungsten phase. It is found that the sulfidation of NiW/ASA closely resembles that of NiW/Al2O3. Trends in the HDS activity as a function of catalyst pretreatment were evaluated. A higher calcination temperature (from 673 to 823 K) decreases the HDS performance stemming from a lower W sulfidation degree and a more dominant presence of small oxysulfidic tungsten particles. Increasing the sulfidation pressure from atmospheric pressure to 15 bar leads to a strong increase of the HDS activity. Whereas sulfidation at 923 K results in a well-crystallized WS2 phase (XAS), the concomitant loss in dispersion (TEM) is detrimental to its performance. Moreover, indications are found that more complete crystallization of the WS2 phase results in a lower activity. (C) 2004 Published by Elsevier Inc

On the structure and hydrotreating performance of carbon-supported CoMo- and NiMo-sulfides

The sulfiding behaviour and hydrotreating performance of CoMo and NiMo on carbon catalysts wereinvestigated, with a view to establishing the suitability of an active-carbon carrier for industrialhydrotreatment purposes. The sulfidation process (H2/H2S at 0.1 and 4 MPa) was followed with57CoMössbauer emission spectroscopy, and X-ray absorption spectroscopy (Mo- and Co-edge). It turns outthat the structural evolution of the CoMo/C catalyst is in broad terms very similar to that of a CoMo/Al2O3one, including the type I ¿ II phase transition at increased sulfiding pressure, albeit that some sinter-ing takes place concomitantly. In the hydrotreating part a NiMo/C catalyst was employed, sulfided at1 or 4 MPa, in trickle-flow conditions at 3.5 (HDS) and 6 (HDN) MPa. It transpires that the adsorptionproperties of NiMo/C are quite different from those of a NiMo/alumina, in that dibenzthiophene, quino-line, and polyaromatics are much stronger adsorbed on the carbon-supported catalyst. This leads to veryhigh dibenzothiophene HDS and quinoline HDN activities, but to a disappointing performance in thehydrotreatment of a heavy gasoil, where the polyaromatics can compete effectively for adsorption onthe active HDS/N sites

Formation of acid sites in amorphous silica-alumina

A suite of amorphous silica-aluminas (ASAs) was prepared by homogeneous deposition–precipitation (HDP) of aluminium on a silica surface followed by calcination. The HDP process was investigated in detail by 27Al NMR spectroscopy of solid and liquid aliquots of the synthesis mixture. Deposition occurs predominantly via a hydrolytic adsorption of aluminium onto the hydroxyl groups of the silica surface. Precipitation becomes more significant at higher aluminium concentration. Depending on the aluminium loading, the surface contains four- and six-coordinated aluminium as well as patches of aluminium hydroxides. Calcination results in two competing process, that is the diffusion of aluminium into the silica network and sintering of aluminium into separate patches of a phase which mainly consists of octahedral Al. These ASAs exhibit Brønsted acidity similar to industrial amorphous silica-aluminas prepared by the grafting of aluminium on very reactive silica gels. Their acidity does not vary systematically with the aluminium concentration, except below 5 wt% Al2O3. The acidity increases with the calcination temperature. The active sites form due to the diffusion of aluminium into the silica network at high temperatures, leading to Al substitutions of Si atoms. This is expected as the acidity does not correlate with anything else, viz., the amount of four-coordinated aluminium nor the presence of segregated Al or five-coordinated aluminium at the interface of these domains and the mixed silica-alumina phase. The surface of an amorphous silica-alumina consists of isolated aluminium grafted onto the silica surface (pure silica-alumina phase) with a very small amount of aluminium in the silica network, which brings about the Brønsted acidity, and small patches of aluminium oxides.status: publishe

Brønsted acid sites of zeolitic strength in amorphous silica-alumina

The most acidic OH groups in silica-aluminas (zeolites, clays, amorphous silica-aluminas) can be made to react selectively with C6D6 to give acidic OD groups; quantification by IR spectroscopy shows that differences in the overall Brønsted acidity of aluminosilicates are dominated by differences in the density of sites of similar acid strength