Consecutive hydrogenation

A procedure similar to that used in the investigation of the hydro-demethylation of xylenes was also employed in a study of the consecutive hydrogenation of phenol via cyclohexanone to cyclohexanol in gaseous phase on a platinum on silica gel catalyst (p. 27) at 150°C [scheme (VI)] at this temperature both reactions were irreversible under the excess hydrogen used. 

Fig. 7. Dependence of relative molar concentrations n-Jn of reaction components on reciprocal space velocity W/F (hr kg mole-1) in the consecutive hydrogenation of phenol. Temperature 150°C, catalyst Pt-SiCh (1% wt. Pt), initial molar ratio of reactants G = 9. The curves were calculated (1—phenol, 2—cyclohexanone, 3—cyclohexanol) the points are experimental values. From Ref. (61). Reproduced by permission of the copyright owner.

Values of Constants ki (mole hr lkff ialm Jl) and of Adsorption Coefficients K j (atm-1) in Parallel-Consecutive Hydrogenation of Crotonaldehyde... 

In the consecutive hydrogenation of />,<5-diketo esters (Table 21.16), selection of the chiral ligand can determine the sense of diastereoselection, and the 3,5-syn dihydroxy product was formed predominantly upon use of a Ru-(S)-amino-phosphinephosphinite-((S)-AMPP) catalyst, although the enantioselectivity of the syn-product is poor (Table 21.16, entry 7) [103a]. Syn 3,5-diol formation... 

In the hydrogenation of diketones by Ru-binap-type catalysts, the degree of anti-selectivity is different between a-diketones and / -diketones [Eqs (13) and (14)]. A variety of /1-diketones are reduced by Ru-atropisomeric diphosphine catalysts to indicate admirable anti-selectivity, and the enantiopurity of the obtained anti-diol is almost 100% (Table 21.17) [105, 106, 110-112]. In this two-step consecutive hydrogenation of diketones, the overall stereochemical outcome is determined by both the efficiency of the chirality transfer by the catalyst (catalyst-control) and the structure of the initially formed hydroxyketones having a stereogenic center (substrate-control). The hydrogenation of monohydrogenated product ((R)-hydroxy ketone) with the antipode catalyst ((S)-binap catalyst) (mis-... 

Catalyst Activity Measurements. Activity measurements for thiophene HDS and the consecutive hydrogenation of butene were carried out in a Pyrex-glass, fixed-bed reactor at 625 K and at atmospheric pressure as described in Ref. (9). Before the measurements the catalysts were presulfided in 2% H2S in H2 at 675 K. For each catalyst conversions were measured at different space velocities of the thiophene/H2 mixture (2.5 thiophene) and the catalytic activities are here expressed as pseudo first-order rate constants as-... 

The consequence of low diffusivity can be detrimental in the (common) case of consecutive hydrogenation s A —> B —> C if the movement of desired B out of the porosity is slow, C by-product will increase, with a rapid selectivity drop in B. 

Vacuum pyrolysis of 12 has been recently studied by high-resolution mass spectrometry200. The ionization potential (IP = 9.17 0.10 eV) for silole 2 was found to be in excellent agreement with the IP (9.26 eV) calculated by the semiempirical PM3 method. Thermodynamic calculations on possible decomposition mechanisms of 12 have shown that 2 is probably formed from the primary intermediate silylene 9 via two consecutive hydrogen shifts (equation 3). 

A general scheme of hydrogenation of 1,3-butadiene, the rake mechanism, is given in Scheme 7. Two kinds of selectivity for the intermediate butenes can be defined. Selective formation of butenes can be expected if > 4 (mechanistic selectivity). On the other hand, if k /k- > kj,/k, adsorption of the diene prevents the readsorption of butenes and hence the consecutive hydrogen addition cannot take place (thermodynamic... 

Dow also developed polyurethane foams from polyols via hydroformylation of fatty acids. The foams have properties which are comparable to foams from petrochemicals in terms of density and flexibility. The advantages of using sustainable feedstocks in viscoelastic foams are increased load bearings and tensile and tear properties [39, 40]. The hydroformylation and consecutive hydrogenation of fatty acids derived from seed oil can also be used to form low viscosity polyester polyols. Therefore, fatty acid methyl esters are transesterified with diols, e.g., glycol (Scheme 12). The polymer contains chemically active hydroxy groups which can be used for polyurethanes in coating applications [41]. 

However, the sulfurization of the surface palladium has a detrimental effect on the consecutive hydrogenation (Fig. 17) the olefin yield is lower on sulfided palladium. Such a result is not in accordance with the literature cited above and this difference can be explained by differences in experimental conditions the improved consecutive selectivities were obtained with the sulfur present in the feedstock, i.e., in adsorption competition, when the detrimental effect on the consecutive hydrogenation was pointed out on partly sulfided palladium without sulfur compounds in the feedstock. The selectivity for olefin production in the hydrogenation of iso-prene is increased by the presence of sulfur in the substrate (Fig. 18). 

Mass spectra of 5-amino-3-arylamino-l,2,4-thiadiazoles exhibit a new fragmentation process involving two consecutive hydrogen shifts and ring cleavage leading to the elimination of the NHSH radical (Scheme 8) (77AJC563). 

TABLE 3.5 Rate Constants for the Consecutive Hydrogenation of Isomeric 1,5,9-CDT and Maximum Concentrations of CDD and CDE Intermediates ... 

Scheme 9.10 Consecutive hydrogenation of 2,6-dinitrotoluene to 2-amino-6-nitrotoluene and 2,6-diaminotoluene.

The differences between the TBR and the MR originate from the differences in catalyst geometry, which affect catalyst load, internal and external mass transfer resistance, contact areas, as well as pressure drop. These effects have been analyzed by Edvinsson and Cybulski [ 14,26] via computer simulations based on relatively simple mathematical models of the MR and TBR. They considered catalytic consecutive hydrogenation reactions carried out in a plug-flow reactor with cocurrent downflow of both phases, operated isothermally in a pseudo-steady state all fluctuations were modeled by a corresponding time average ... 

First, consider a cmde design of the TBR. For a selective consecutive hydrogenation of the type assumed, it is desirable to use as small particles as possible, since this improves both the effectiveness factor and selectivity. The limitation is the acceptable pressure drop. If only these three performance criteria are considered, the optimal design is a reactor that is very short and with a very large diameter. There are obviously practical limits on the ratio between the length and the diameter of the reactor For highly packed... 

Chromia pillared and pillared-delaminated clays have been synthesized from different montmori I Ionites and characterized by a variety of methods. Chromia-sulfide pillared materials show a high activity and selectivity in thiophene HDS and the consecutive hydrogenation of butene. The use of different clays as starting materials for the preparation of Cr-PILC enables control of their textural properties and chromium loading and thus to tailor the activity of these catalysts. 

Thiophene HDS was performed at 673 K in a microflow reactor with on-line gas chromatography (GC) analysis. The catalyst samples (200 mg) were pre-sulfided in situ using conditions described in the preparation section. The reaction mixture consisting of 4.0 mol% thiophene in H2 was fed through the reactor and was analyzed every 35 min (flow rate 50 ml min , 673 K, 1 bar). First order rate constants for thiophene conversion to hydrocarbons (Khds) and the consecutive hydrogenation of butene (knyo) were calculated as described elsewhere [8]. 

When the hydroformylation is carried out in an alkaline reaction medium, the aldehydes formed as primary products undergo aldolization and, consecutively, hydrogenation. Thus the addition of, e.g., KOH (2 mol/g at Co) or a tertiary alkylamine leads to 2-ethylhexanol with yields up to 85 %, starting from propene. The formation of by-products like C4/4/4-acetal can be controlled by proper choice of the alkalinity and its source [188, 189]. 

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