Hydrogenation, selective

Selective hydrogenation to remove dienes and alkynes from alkene streams is needed to prevent poisoning of the polymerization catalysts. Supported 

Partial hydrogenation of cumulenes affords m-polyenes selectively in the presence of the Lindlar catalyst (a palladium-calcium catalyst deactivated by lead acetate see p. 19) hydrogenation ceases almost entirely after a rapid absorption of (n — l)/2 moles of hydrogen (n = number of double bonds). According to Kuhn and Fischer,139 tetraphenylbutatriene absorbs one equivalent of hydrogen, yielding l9l9494-tetraphenyl-l93-butadiene  

To a pre-reduced suspension of 800 mg of Lindlar catalyst in 50 ml of tetrahydrofuran (distilled over KOH) are added 500 mg of tetraphenylbutatriene, and the mixture is shaken at 21°. In 15 min 35 ml (31.5 ml at NTP) corresponding to 1 equivalent of H2 are absorbed. Only a further 3 ml are consumed in a further hour s shaking. The mixture is filtered and the solvent removed. The residue, recrystallized from cyclohexane, constitutes 420 mg (84%) of colorless rhombs, m.p. 201°. 

Tetraphenylhexapentaene absorbs 2 equivalents of hydrogen with formation of the corresponding m-polyene  

According to Natta, Rigamonti, and Tono,204 butadiene and 1,3-pentadiene are hydrogenated to the corresponding monoalkenes quantitatively in the 

owing to differing substitution, conjugated double bonds are not equivalent, the substituted double bond is more difficult to hydrogenate than the unsubstituted one 205 

So far, gold dusters have already been used to catalyze some typical reactions, including selective hydrogenation of unsaturated aldehydes and ketones selective oxidation of alkanes, alkenes, and alcohols CO oxidation and organic synthesis. 

Zhu et al. [69] have studied the catalytic performance of thiolate-protected Au25(SR)jg dusters for the selective hydrogenation of different a,p-unsaturated ketones and aldehydes to unsaturated alcohols (U As) (Table 11.2). A100% sdectivity was achieved for different reactant substrates except acrolein, for which a selectivity of 91% allylic alcohol was obtained. The core-shell structure of the Au25(SR)jg (Aujj core/AUi2 shell) and their unique electronic properties (electron-rich Auj, 

Reaction conditions substrate, 0.1 mmol solvent, 5 ml toluene and 5 ml ethanol catalyst amount, 

1 mg Au25(SR)jg (unsupported) or 100 mg supported catalyst containing lwt% Au j(SR)jg reaction temperature 0 ° C or room temperature (no difference in catalytic activity or selectivity was found). The reaction was typically allowed to proceed for 3 h under continuous flow. 

Zhu et al. [70] have also investigated the catalytic activity of thiolate-capped Au25(SR) jg in the stereoselective hydrogenation ofbicyclic ketone 7-(phenylmethyl)-3-oxa-7-azabicyclo[3.3.1]nonan-9-one. In industry, these kinds of cyclic alcohol are synthesized by the reduction ofbicyclic ketones by metal hydrides such as NaBH4. This process will yield equal amounts of two isomers. A 100% stereoselectivity to 

Nanoparticulate gold supported on oxides (SiOa, AI2O3, TiOa, ZnO, ZrOa and FeaOs) has been used for the selective catal3dic hydrogenation of several organics such as a, -unsaturated aldehydes [98,99,249,433-440], a, 3-unsaturated ketones [58, 326, 441] and unsaturated hydrocarbons [18, 437, 442-445]. 

The hydrogenation of a, -unsaturated aldehydes to give allylic alcohols is of considerable interest, since these reactions are an important step in the industrial symthesis of fine chemicals, particularly, of pharmaceuticals and cosmetics [98,99,249,433-440,446]. A simplified reaction scheme is shown in Fig. 6.18. 

Allylic alcohols were hydrogenated using f to 3 nm diameter bimetallic Pd-Au dendrimer-encapsulated catalysts (DECs) [fi8]. Both alloy and core/shell Pd-Au nanoparticles were prepared. The catalytic hydrogenation of allyl alcohol was significantly enhanced in the presence of the alloy and core/shell Pd-Au nanoparticles as compared to mixtures of single-metal nanoparticles [fi8]. 

Some time ago, selective Pt/Y zeolite catalysts were developed for the liquid-phase hydrogenation of cinnamaldehyde into cinnamyl alcohol (selectivity 96% [449]) and an organometallic derived Rh[Sn-(n-C4H9)2]Si02 catalyst for the conversion of citral to the corresponding a, (f-unsaturated alcohols, 

The selective hydrogenation of C3 and C4 a, P-unsaturated aldehydes, such as crotonaldehyde (but-2-enal) (R = CH3 in Fig. 6.19) and acrolein (R = H in Fig. 6.19), to the corresponding allylic alcohols is even more challenging [98,99,438]. 

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Liquefied gas fractions (propane, propylene, butanes, butenes) that will be able to provide feedstocks to units of MTBE, ETBE, alkylation, dimerization, polymerization after sweetening and/or selective hydrogenation. 

From acid chlorides by selective hydrogenation in the presence of a catalyst (palladium deposited upon a carrier, which is usually barium sulphate but is... 

Both 1 2 dihydronaphthalene and 1 4 dihydronaphthalene may be selectively hydrogenated to 1 2 3 4 tetrahydronaphthalene... 

Cyclohexane, produced from the partial hydrogenation of benzene [71-43-2] also can be used as the feedstock for A manufacture. Such a process involves selective hydrogenation of benzene to cyclohexene, separation of the cyclohexene from unreacted benzene and cyclohexane (produced from over-hydrogenation of the benzene), and hydration of the cyclohexane to A. Asahi has obtained numerous patents on such a process and is in the process of commercialization (85,86). Indicated reaction conditions for the partial hydrogenation are 100—200°C and 1—10 kPa (0.1—1.5 psi) with a Ru or zinc-promoted Ru catalyst (87—90). The hydration reaction uses zeotites as catalyst in a two-phase system. Cyclohexene diffuses into an aqueous phase containing the zeotites and there is hydrated to A. The A then is extracted back into the organic phase. Reaction temperature is 90—150°C and reactor residence time is 30 min (91—94). 

The ethynylation reaction takes place at 10—40°C and 2 MPa (20 atm) and hquid ammonia is the solvent. The methylbutynol is converted into methylbutenol by selective hydrogenation and then is dehydrated over alumina at 250—300°C. Polymerization-grade isoprene is obtained. 

Finally, selective hydrogenation of the olefinic bond in mesityl oxide is conducted over a fixed-bed catalyst in either the Hquid or vapor phase. In the hquid phase the reaction takes place at 150°C and 0.69 MPa, in the vapor phase the reaction can be conducted at atmospheric pressure and temperatures of 150—170°C. The reaction is highly exothermic and yields 8.37 kJ/mol (65). To prevent temperature mnaways and obtain high selectivity, the conversion per pass is limited in the Hquid phase, and in the vapor phase inert gases often are used to dilute the reactants. The catalysts employed in both vapor- and Hquid-phase processes include nickel (66—76), palladium (77—79), copper (80,81), and rhodium hydride complexes (82). Complete conversion of mesityl oxide can be obtained at selectivities of 95—98%. 

Nickel Arsenate. Nickel arsenate [7784-48-7] Ni2(As0 2 8H20, is a yellowish green powder, density 4.98 g/cm. It is highly iasoluble ia water but is soluble ia acids, and decomposes on heating to form As20 and nickel oxide. Nickel arsenate is formed by the reaction of a water solution of arsenic anhydride and nickel carbonate. Nickel arsenate is a selective hydrogenation catalyst for iaedible fats and oils (59). 

Improved feedstock pretreatment is important to minimize catalyst consumption and reduce subsequent spent-catalyst handling requirements. Selective hydrogenation of dienes can be used to reduce acid consumption, both in HF and H2SO4 alkylation (29). More effective adsorptive treating systems have been appHed to remove oxygen-containing contaminants that are frequently introduced in upstream processing steps. 

HP Alkylation Process. The most widely used technology today is based on the HE catalyst system. AH industrial units built in the free world since 1970 employ this process (78). During the mid-1960s, commercial processes were developed to selectively dehydrogenate linear paraffins to linear internal olefins (79—81). Although these linear internal olefins are of lower purity than are a olefins, they are more cost-effective because they cost less to produce. Furthermore, with improvement over the years in dehydrogenation catalysts and processes, such as selective hydrogenation of diolefins to monoolefins (82,83), the quaUty of linear internal olefins has improved. 

Homogeneous and heterogenous catalysts which selectively or partially hydrogenate fatty amines have been developed (50). Selective hydrogenation of cis and trans isomers, and partial hydrogenation of polyunsaturated moieties, such as linoleic and linolenic to oleic, is possible. 

A catalyst, usually acid, is required to promote chemoselective and regioselective reduction under mild conditions. A variety of organosilanes can be used, but triethylsilane ia the presence of trifiuoroacetic acid is the most frequendy reported. Use of this reagent enables reduction of alkenes to alkanes. Branched alkenes are reduced more readily than unbranched ones. Selective hydrogenation of branched dienes is also possible. 

Polyunsaturated fatty acids in vegetable oils, particularly finolenic esters in soybean oil, are especially sensitive to oxidation. Even a slight degree of oxidation, commonly referred to as flavor reversion, results in undesirable flavors, eg, beany, grassy, painty, or fishy. Oxidation is controlled by the exclusion of metal contaminants, eg, iron and copper addition of metal inactivators such as citric acid minimum exposure to air, protection from light, and selective hydrogenation to decrease the finolenate content to ca 3% (74). Careful quality control is essential for the production of acceptable edible soybean oil products (75). 

One of the important processes for manufacturing linalool is from the P-methylheptenone intermediate produced by the methods from petrochemical sources discussed earlier. For example, addition of sodium acetyUde to P-methylheptenone gives dehydrolinalool (4), which can be selectively hydrogenated, using a Lindlar catalyst, to produce linalool. 

Oxychlorination reactor feed purity can also contribute to by-product formation, although the problem usually is only with low levels of acetylene which are normally present in HCl from the EDC cracking process. Since any acetylene fed to the oxychlorination reactor will be converted to highly chlorinated C2 by-products, selective hydrogenation of this acetylene to ethylene and ethane is widely used as a preventive measure (78,98—102). 

By-products from EDC pyrolysis typically include acetjiene, ethylene, methyl chloride, ethyl chloride, 1,3-butadiene, vinylacetylene, benzene, chloroprene, vinyUdene chloride, 1,1-dichloroethane, chloroform, carbon tetrachloride, 1,1,1-trichloroethane [71-55-6] and other chlorinated hydrocarbons (78). Most of these impurities remain with the unconverted EDC, and are subsequendy removed in EDC purification as light and heavy ends. The lightest compounds, ethylene and acetylene, are taken off with the HCl and end up in the oxychlorination reactor feed. The acetylene can be selectively hydrogenated to ethylene. The compounds that have boiling points near that of vinyl chloride, ie, methyl chloride and 1,3-butadiene, will codistiU with the vinyl chloride product. Chlorine or carbon tetrachloride addition to the pyrolysis reactor feed has been used to suppress methyl chloride formation, whereas 1,3-butadiene, which interferes with PVC polymerization, can be removed by treatment with chlorine or HCl, or by selective hydrogenation. 

Benzylacetone, [2550-26-7] CgH CH2CH2COCH2 (bp, 233—234°C at 101.3 kPa) is produced by condensing acetone and benzaldehyde, followed by selective hydrogenation, and is used in soap perfumes. 

Selective hydrogenation of the carboxyl or ester group in preference to the olefinic unsaturation also produces unsaturated alcohols. 

For more selective hydrogenations, supported 5—10 wt % palladium on activated carbon is preferred for reductions in which ring hydrogenation is not wanted. Mild conditions, a neutral solvent, and a stoichiometric amount of hydrogen are used to avoid ring hydrogenation. There are also appHcations for 35—40 wt % cobalt on kieselguhr, copper chromite (nonpromoted or promoted with barium), 5—10 wt % platinum on activated carbon, platinum (IV) oxide (Adams catalyst), and rhenium heptasulfide. Alcohol yields can sometimes be increased by the use of nonpolar (nonacidic) solvents and small amounts of bases, such as tertiary amines, which act as catalyst inhibitors. 

Cocoa butter substitutes and equivalents differ greatly with respect to their method of manufacture, source of fats, and functionaHty they are produced by several physical and chemical processes (17,18). Cocoa butter substitutes are produced from lauric acid fats such as coconut, palm, and palm kernel oils by fractionation and hydrogenation from domestic fats such as soy, com, and cotton seed oils by selective hydrogenation or from palm kernel stearines by fractionation. Cocoa butter equivalents can be produced from palm kernel oil and other specialty fats such as shea and ilHpe by fractional crystallization from glycerol and selected fatty acids by direct chemical synthesis or from edible beef tallow by acetone crystallization. 

Hydrogenation of cinnamaldehyde has been studied extensively since selectivity has often been an issue. Under mild conditions the carbonyl group is reduced giving cinnamyl alcohol, whereas at elevated temperatures complete reduction to 3-phenylpropanol [122-97 ] results. It is possible to saturate the double bond without concomitant reduction of the carbonyl group through selective hydrogenation with a ferrous chloride-activated palladium catalyst (30), thereby producing 3-phenylpropanol [104-53-0]. 

B = B 4, 1,4-polybutadiene block Bj 2, 1,2-polybutadiene block B y, medium vinyl (35-60%) polybutadiene block) I, 1,4-polyisoprene block. Selective hydrogenation this block not hydrogenated. 

The depropanizer overhead, Cj and lighter feed is compressed to about 300 psi and then passed over a fixed bed of acetylene removal catalyst, generally palladium on alumina. Because of the very large amount of hydrogen contained in this stream, the operating conditions are critical to selectively hydrogenate the acetylene without degrading the valuable ethylene to ethane. 

Significant quantities of Cj and C, acetylenes are produced in cracking. They can be converted to olefins and paraffins. For the production of high purity ethylene and propylene, the contained Cj and C3 acetylenes and dienes are catalytically hydrogenated leaving only parts per million of acetylenes in the products. Careful operation is required to selectively hydrogenate the small concentrations of acetylenes only, and not downgrade too much of the wanted olefin products to saturates. 

The (dextrorotatory) 11,15-bis-THP ether of PGp2a was also transformed into prostaglandins of the first series by selective hydrogenation of the Z-A bond (Ref. 3). 

Just as selective oxidation can be carried out on these systems, reduction also occurs with considerable selectively. Hydrogenation of binaphthol (Pd catalyst) in glacial acetic acid at room temperature for seven days affords the octahydro (bis-tetrahydro) derivative in 92% yield with no apparent loss of optical activity when the reaction is conducted on optically pure material. The binaphthol may then be converted into the bis-binaphthyl crown in the usual fashion. 

With other acetylenes steric factors may be operative which render the selective reduction somewhat difficult. In the aldosterone intermediates (53) and (54), for instance, selective hydrogenation is obtained only with the 14 -acetylenic ether " (hydroxyl group effect). 

Photochemical substitution reactions of this type which involve selective hydrogen abstractions from intramolecular sites by the m.tt ketone oxygen, are reviewed in chapter 12. ... 

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