Dehydroxylation selective

It is therefore possible to control the number of bonds between the surface and the anchored metal by controlling the concentration of hydroxyls of the support. This can be readily achieved by selecting the right temperature of partial dehydroxylation of the support. 

Figure 1.9 TG, DTG, and DTA profiles for an amorphous catalyst precursor obtained by coprecipitation of Fe(N03)3 and Mg(N03)2 in solution [65], This precursor is heated at high temperatures to produce a MgFe204 spinel, used for the selective oxidation of styrene. The thermal analysis reported here points to four stages in this transformation, namely, the losses of adsorbed and crystal water at 110 and 220°C, respectively, the decomposition and dehydroxylation of the precursor into a mixed oxide at 390°C, and the formation of the MgFe204 spinel at 640°C. Information such as this is central in the design of preparation procedures for catalysts. (Reproduced with permission from Elsevier.)...

Glycerol can be selectively dehydroxylated to either 1,2-propanediol (1,2-PDO), a chemical that can advantageously replace ethylene glycol as anti-freezing agent, or 1,3-propanediol (1,3-PDO), which when copolymerized with terephthalic acid... 

As an aside, we should mention that the same principles apply to the formation of bimetallic clusters on a support. In the case of Pt-Re on AI2O3 it has been shown that hydroxylation of the surface favors the ability of Re ions to migrate toward the Pt nuclei and thus the formation of alloy particles, whereas fixing the Re ions onto a dehydroxylated alumina surface creates mainly separated Re particles. As catalytic activity and selectivity of the bimetallic particles differ vastly from those of a physical mixture of monometallic particles, the catalytic performance of the reduced catalyst depends significantly on the protocol used during its formation. The bimetallic Pt-Re catalysts have been identified by comparison with preparations in which gaseous Re carbonyl was decomposed on conventionally prepared Pt/Al203 catalysts. ... 

Concentrated aqueous salt solutions were used for dehydration of carbohydrates catalyzed by RuCh + Ag2S04 ( RUSO4 ) [47]. Such solvents may also help in constracting aqueous-organic biphasic media with good phase separation properties. Selective dehydroxylation of polyols and sugars was achieved in aqueous solutions with the use of anionic rathenium carbonyls, as well [48]. 

Z. f Polymere 215 57-60 Wolska, E. Schwertmann, U. (1989) Nonstoi-chiometric structures during dehydroxylation of goethite. Z. Kristallogr. 189 223—237 Wolska, E. Schwertmann, U. (1989 a) Selective X-ray line broadening in the goethite-de-rived hematite phase. Phys. Stat. Sol. A 114 K11-K16... 

The nature of the donor site D depends on the type of oxide and its pretreatment temperature for pure oxides and, additionally, on the composition in the case of mixed oxides. The radical anion formation from TCNE (electron affinity 2.89 eV) on aluminas occurs on extraordinarily coordinated hydroxide ions on hydroxyl-rich surfaces, whereas exceptionally coordinated O2" ions play the role of the donor sites on more strongly dehydroxylated surfaces (328, 330). Accordingly, as the chemical nature of the donor site changes with the degree of surface hydroxylation, the spin concentration of the anion radical passes through two maxima the first is located between 400° and 500°C (OH- donor sites), and the second (brought about by the O2 ions) is between 600° and 700°C (328, 331). Trinitrobenzene (TNB) (electron affinity 1.0 eV) is a weaker electron acceptor than TCNE and interacts only with the 02 sites (332), thus acting more selectively than TCNE. 

Deoxygenation." The 17a-hydroxy group of 17a,21-dihydroxy-20-ketosteroids is selectively reduced by ISi(CH,), in CH,CN at room temperature (equation I). A free 21-hydroxyl group is necessary for this dehydroxylation. 

The tetraallyl complex Th( -C3H5)4 supported on dehydroxylated y-alumina is an outstanding heterogeneous catalyst for arene hydrogenation that rivals the most active platinum metal catalysts in activity. Th() -C3H5)4/DA also catalyzes the rapid and selective deuteration of linear and cyclic alkanes. C-H reactivities fall in the order primary > secondary > tertiary. 

Similar behavior was discovered in subsequent studies for ZSM-5 (772,174) and ZSM-11 (173) zeolites synthesized with aluminum and boron in the zeolite lattice and for boron-synthesized ZSM-11 zeolites (173). The modification of the ZSM-5 and ZSM-11 samples produced a minor improvement in shape selectivity and a large decrease in acidity and hence activity. The initial heat for the B-ZSM-11 sample decreased from 160 kJ mol" for Al-ZSM-11 to 65 kJ mor , and the acidity decreased to 10% of the original value. The q-d curve also showed a maximum at high coverages, which was attributed to the formation of a NH NHa) complex on reacting B—OH—NH3 with NH3. Dehydroxylation at 1073 K increased the initial heat to 170 kJ mol", a value comparable to the initial heat of 185 kJ mol" on Al-ZSM-11, and it sharpened the maximum in the q-9 curve. This behavior is apparently due to the formation of a few strong Lewis acid sites. The sample synthesized with both boron and aluminum behaved differently than those with only aluminum or boron. The q-6 curve for this sample showed maxima at about 145-175 kJ mol" and at about 60-70 kJ mol for 673 and 1073 K dehydroxylation temperatures, respectively. The acidity of this sample was 30% lower than an Al-ZSM-11 sample with similar Si/Al ratio. The initial heat for the aluminum zeolite was 170 to 190 kJ mol". It was shown, with IR spectroscopy of adsorbed ammonia, that the boron-modified samples showed little or no Brpnsted acidity. 

The reductive addition of methyl radicals to Mov 0 ", followed by the subsequent reactions of methoxide ions suggests a catalytic cycle which is depicted in Scheme I. All of the steps in this cycle would be rapid except for dehydroxylation. In addition to this selective cycle one must also consider the possibility of a two-electron reaction with N2O to form oxide ions. A molecule of CH4 would then have to be used to reduce the oxide in a typical Mars-van Krevelen mechanism [Ref.22]. The two-electron transfer could be minimized, in... 

Wismeijer et al. studied the liquid phase transfer hydrogenation of 4-tert-butylcyclohexanone by 2-propanol at 83°C over activated y-Al203 as the catalyst [4]. The activity of the catalyst was found to increase with increasing activation temperature. Selective poisoning experiments indicated that coordinatively unsaturated Al " surface ions (Lewis acid sites), formed upon dehydroxylation, were essential for catalytic activity. During reaction the catalyst was found to become conditioned by irreversible alcoholysis of the initial active sites, producing less-active sites. The reaction mechanism, however, remained essentially the same as indicated by the constant ratio of c/s//rans-4-/er/-butylcyclohexanol (9/91). 

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