Acetylene Hydrogenation Catalyst Preparation

Front-end and tail-end catalysts are both produced by relatively simple pro-cedttres in which palladirrm is impregnated onto the outside surface of an alirmi-na support in a thin layer. Theoretically, in order to achieve the required selectivity, the support shorrld be inert and take no part in the hydrogenation process. 

Front-end catalysts were produced from a srritable a-altrmina support with carefully controlled surface area and pore volrrme. The support could influence the hydrogenation reaction to give good selectivity with almost no polymer formation. It was found that the adsorption of carbon monoxide onto the catalyst surface inhibited ethylene hydrogenation in the presence of a few ppm of acetylene. Carbon monoxide was always present in the process gas. 

Catalyst selectivity in front-end catalysts can also be controlled by the addition of a Group-IB metal when acetylene levels are high or carbon monoxide content is low. 

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A useful self-terminating catalyst system (77), employs a Pd catalyst [prepared from Pd(OAc)2, NaH, and r-AmOH in THF]. The solvent required for the hydrogenation depends on the acetylene structure monosubslituted acetylenes require solvents such as hexane or octane, whereas disubstituted acetylenes need ethanol, ethanol-hydrocarbon, or ethanol-THF mixtures. In all cases it was necessary to use quinoline as a catalyst modifier. The authors consider this system one of the best for achieving both high yield and stereoselectivity. 

Figure 1.6 Representative TEM image (a) and particle size distribution (b) obtained for a Au/Ti02 catalyst prepared by grafting of a [Au6(PPh3)6](BF4)2 complex onto Ti02 particles followed by appropriate reduction and oxidation treatments [42], The gold particles exhibit approximately spherical shapes and an average particle size of 4.7 nm.The measured Au particle sizes could be well correlated with the activity of the catalyst for carbon monoxide oxidation and acetylene hydrogenation. (Reproduced with permission from Springer.)...

The indirect cyclisation of bromoacetals via cobaloxime(I) complexes was first reported in 1985 [67], At that time the reactions were conducted in a divided cell in the presence of a base (40yo aqeous NaOH) and about 50% of chloropyridine cobaloximeflll) as catalyst precursor. It was recently found that the amount of catalyst can be reduced to 5% (turnover of ca. 50) and that the base is no longer necessary when the reactions are conducted in an undivided cell in the presence of a zinc anode [68, 69]. The method has now been applied with cobaloxime or Co[C2(DOXDOH)p ] to a variety of ethylenic and acetylenic compounds to prepare fused bicyclic derivatives (Table 7, entry 1). The cyclic product can be either saturated or unsaturated depending on the amount of catalyst used, the cathode potential, and the presence of a hydrogen donor, e.g., RSH (Table 7, entry 2). The electrochemical method was found with some model reactions to be more selective and more efficient than the chemical route using Zn as reductant [70]. 

C-E bond formation via hydroalumination, 10, 859 C-E bond formation via hydroboration, 10, 842 olefin cross-metathesis, 11, 195 terminal acetylene silylformylation, 11, 478 Chemspeed automated synthesizer, for high-throughput catalyst preparation, 1, 356 Chini complexes, characteristics, 8, 410 Chiral bisphosphanes, in hydrogenations on DIOP modification, 10, 7... 

The compound of formula (5) is next subjected to selective hydrogenation to convert the acetylenic bond to an ethylenic bond. This can be readily accomplished by a number of different catalysts, such as a nickel catalyst prepared from a nickel salt and NaBFi4, Lindlar catalyst, or 5% palladium on barium sulfate in the presence of qunioline. The reaction was run at one atmosphere. Analyses by nuclear magnetic resonance and vapor phase chromatography showed the correct structure in good quantity. The product obtained was 3,7,ll,15-tetramethylhexadeca-2,5-dien-l-acetate (6), a C2o dienolacetate. 

Acetylene hydrogenation. Selective hydrogenation of acetylene to ethylene is performed at 200°C over sulfided nickel catalysts or carbon-monoxide-poisoned palladium on alumina catalyst. Without the correct amount of poisoning, ethane would be the product. Continuous feed of sulfur or carbon monoxide must occur or too much hydrogen is chemisorbed on the catalyst surface. Complex control systems analyze the amount of acetylene in an ethylene cracker effluent and automatically adjust the poisoning level to prepare the catalyst surface for removing various quantities of acetylene with maximum selectivity. 

Table I summarizes the application of various low-valent titanium metallocenes as catalysts for olefin hydrogenation. Compounds 10 and 37 are very effective hydrogenation catalysts for C2H4 and cyclohexene. Since different researchers have used widely varying conditions, we can only estimate that the polystyrene-supported (7j-C8H8)2Ti (142) is comparable in activity to compounds 10 and 37. When one recalls that 37 was prepared by a formal oxidation of the Ti centers in 10, it is remarkable that 37 is as good a catalyst as 10. Solutions prepared by reaction of l-methyl-17-allylbiscyclopentadienyltitanium (54) with H2 do appear to be more active hydrogenation (126) catalysts than 10 and 37. The dicarbonyl complex, (17-CsH5)2Ti(CO)2 (39), has been shown to be a catalyst for the hydrogenation of acetylene at —50 atm of H2 (143). It does not catalyze the hydrogenation of simple olefins. However, Floriani and Fachinetti discovered that if...

The influence of the reduction temperature, preparation method and type of support on the textural properties and on the activity, selectivity and coking formation on Ni-Ti02-Al20j catalysts, using acetylene hydrogenation as the test reaction, is investigated. 

Grubbs and coworkers (35) while examining Rh and Co catalysts derived from 14 reported the loss of infrared CO stretches and visual darkening of the catalysts after use for hydrogenation of olefins, aldehydes or ketones, cyclohexene disproportionation to benzene and cyclohexane or the cyclotrimerization of a wide variety of acetylenes. Stille (36) using a rhodium catalyst prepared from 14 observed activity for the hydrogenation of benzene that increased with reuse, a phenomenon usually associated with metal crystallite formation. Rhodium catalysts of 15 and 16 used to hydroformylate octene-1 revealed a loss of carbonyl adsorptions and a loss in catalytic activity upon reuse (37). 

Rives et al. (427,428) studied the hydrogenation of acetylene to ethylene with multimetallic oxide catalysts prepared by calcining ZnNiAlCr-LDHs at 500°C for 3 h with an H2/N2 (50 50 vol) gas mixture. The redox property of Ni is essential for the activity and selectivity of the catalysts and the presence of ZnO decreases the coke formation. 

Selective Hydrogenation of Alkyne The SMFs modified by a layer of CNFs (CNFs/SMF) supported nano-Pd for the selective acetylene hydrogenation under continuous-flow conditions [73]. As a typical catalyst, Pd/IL/CNFs/SMF was prepared by impregnation of CNFs/SMF in an acetone solution of Pd(acac)2 (acac. 

Hydrofluorocarbons are also prepared from acetylene or olefins and hydrogen fluoride (3), or from chlorocarbons and anhydrous hydrogen fluoride in the presence of various catalysts (3,15). A commercial synthesis of 1,1-difluoroethane, a CFG alternative and an intermediate to vinyl fluoride, is conducted in the vapor phase over an aluminum fluoride catalyst. 

Vlayl fluoride [75-02-5] (VF) (fluoroethene) is a colorless gas at ambient conditions. It was first prepared by reaction of l,l-difluoro-2-bromoethane [359-07-9] with ziac (1). Most approaches to vinyl fluoride synthesis have employed reactions of acetylene [74-86-2] with hydrogen fluoride (HF) either directly (2—5) or utilizing catalysts (3,6—10). Other routes have iavolved ethylene [74-85-1] and HF (11), pyrolysis of 1,1-difluoroethane [624-72-6] (12,13) and fluorochloroethanes (14—18), reaction of 1,1-difluoroethane with acetylene (19,20), and halogen exchange of vinyl chloride [75-01-4] with HF (21—23). Physical properties of vinyl fluoride are given ia Table 1. 

Titanium tetraiodide can be prepared by direct combination of the elements at 150—200°C it can be made by reaction of gaseous hydrogen iodide with a solution of titanium tetrachloride in a suitable solvent and it can be purified by vacuum sublimation at 200°C. In the van Arkel method for the preparation of pure titanium metal, the sublimed tetraiodide is decomposed on a tungsten or titanium filament held at ca 1300°C (152). There are frequent hterature references to its use as a catalyst, eg, for the production of ethylene glycol from acetylene (153). 

Hydrochloric acid may conveniently be prepared by combustion of hydrogen with chlorine. In a typical process dry hydrogen chloride is passed into a vapour blender to be mixed with an equimolar proportion of dry acetylene. The presence of chlorine may cause an explosion and thus a device is used to detect any sudden rise in temperature. In such circumstances the hydrogen chloride is automatically diverted to the atmosphere. The mixture of gases is then led to a multi-tubular reactor, each tube of which is packed with a mercuric chloride catalyst on an activated carbon support. The reaction is initiated by heat but once it has started cooling has to be applied to control the highly exothermic reaction at about 90-100°C. In addition to the main reaction the side reactions shown in Figure 12.6 may occur. 

Highly stereospecific hydrogenations of acetylenes to cis olefins have been achieved also with nickel (P 2) catalysts in the presence of ethylenediamine as prorrtoter (37 8 55 58,72). The catalyst is prepared by reduction of nickel acetate in ethanol with sodium borohydridefi ). Despite successes (44), the use of nickel is relatively infrequent (51). 

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