Total dehydrogenation

Under classical conditions, the reaction between 3 and styrene required 50 h of heating at 110 °C, and gave the dihydropyridazine adduct 10a [24], After SMWI with 30 W incident power for 5 min (Tmax = 154 °C), the adduct 10a was not detected whereas the totally dehydrogenated product, pyridazine 10b, was isolated in almost quantitative yield (Tab. 7.1, entry 8). Ethyl vinyl ether and 3 gave the same product, pyridazine 11, under both classical heating [25] and MW irradiation conditions (Tab. 7.1, entry 9). In this instance the DA adduct lost nitrogen and ethanol. 

Niobium and rhodium cluster anions have been prepared by laser vaporization and the reactions with benzene studied by FT-ICR/MS (58). The reactions of the anions and similar cations have been compared. With few exceptions the predominant reaction of the niobium cluster anions and cations was the total dehydrogenation of benzene to form the metal carbide cluster, [Nb C6]-. The Nb19 species, both anion and cation, reacted with benzene to form the coordinated species Nb 9C6I I6p as the predominant product ion. The Nb22 ions also formed some of the addition complex but the Nb2o Nb2i, and all the other higher clusters, formed the carbide ions, Nb C6. ... 

Other thermal reactions compete with dehydrogenation, such as the total dehydrogenation to carbon or some cracking processes. The importance of these reactions increases with temperature and time on stream and hence the equilibrium position (representing about 80% conversion), is seldom reached and most plants work at conversion levels in the range of 50-70%. 

Such selectivity changes by a poison are shown very nicely by the work of Madix and co-workers on Ni(lf)O) which showed that clean Ni totally dehydrogenates methanol, whereas with S poisoning the partial dehydrogenation to formaldehyde is favoured (fig. 48 Johnson and Madix, 1981). Thus the ellect of the selective poisoning is to make the Ni surface behave more like a copper surface, and reflects the kind of behaviour described above for Rh in the presence of a high field. 

Fig. 48. Effect of the presence of sulphur on the Ni(lOll) surface on meLhanol decomposition. Total dehydrogenation occurs on the untreated surface, while that with S yields the partial dehydrogenation product, formaldehyde. From Johnson and Madix (1981).

The clean Ni surface is a complete dehydrogenator, whereas the surface dosed with half a monolayer of S in an ordered structure results in a surface which is very selective to formaldehyde production, that is, the total dehydrogenation pathway is effectively blocked. 

Product of total dehydrogenation, tProduct of the elimination of the acetoxy group. 

About 35% of total U.S. LPG consumption is as chemical feedstock for petrochemicals and polymer iatermediates. The manufacture of polyethylene, polypropylene, and poly(vinyl chloride) requires huge volumes of ethylene (qv) and propylene which, ia the United States, are produced by thermal cracking/dehydrogenation of propane, butane, and ethane (see Olefin polymers Vinyl polymers). 

Worldwide, approximately 85% of acetone is produced as a coproduct with phenol. The remaining 17% is produced by on-purpose acetone processes such as the hydration of propylene to 2-propanol and the dehydrogenation of 2-propanol to acetone. The cost of production of 2-propanol sets the floor price of acetone as long as the acetone demand exceeds the coproduct acetone supply. However, there is a disparity in the growth rates of phenol and acetone, with phenol demand projected at 3.0%/yr and acetone demand at 2.0%/yr. If this continues, the coproduct supply of acetone will exceed the total acetone demand and on-purpose production of acetone will be forced to shut down the price of acetone is expected to fall below the floor price set by the on-purpose cost production. Projections indicate that such a situation might occur in the world market by 2010. To forestall such a situation, companies such as Mitsui Petrochemical and Shinnippon (Nippon Steel) have built plants without the coproduction of acetone. 

Worldwide propylene production and capacity utilization for 1992 are given in Table 6 (74). The world capacity to produce propylene reached 41.5 X 10 t in 1992 the demand for propylene amounted to 32.3 x 10 t. About 80% of propylene produced worldwide was derived from steam crackers the balance came from refinery operations and propylene dehydrogenation. The manufacture of polypropylene, a thermoplastic resin, accounted for about 45% of the total demand. Demand for other uses included manufacture of acrylonitrile (qv), oxochemicals, propylene oxide (qv), cumene (qv), isopropyl alcohol (see Propyl alcohols), and polygas chemicals. Each of these markets accounted for about 5—15% of the propylene demand in 1992 (Table 7). 

The acetone supply is strongly influenced by the production of phenol, and so the small difference between total demand and the acetone suppHed by the cumene oxidation process is made up from other sources. The largest use for acetone is in solvents although increasing amounts ate used to make bisphenol A [80-05-7] and methyl methacrylate [80-62-6]. a-Methylstyrene [98-83-9] is produced in controlled quantities from the cleavage of cumene hydroperoxide, or it can be made directly by the dehydrogenation of cumene. About 2% of the cumene produced in 1987 went to a-methylstyrene manufacture for use in poly (a-methylstyrene) and as an ingredient that imparts heat-resistant quaUties to polystyrene plastics. 

Recent investigations have been concerned with the reactivities observed with secondary silanes R2SiH2. In these cases, a dehydrogenative coupling of silanes to disilanes is observed as a side reaction of the hydrosilation. However, the hydrosilation can be totally suppressed if the olefins are omitted. The key intermediate in the coupling reaction has been identified as a silylene complex (sect. 2.5.4). 

The lower total activity for Rh electrodes may be partly due to increased CO poisoning and slower CO electro-oxidation kinetics compared with Pt electrodes, as demonstrated by the number of voltammetric cycles required to oxidize a saturated CO adlayer from Rh electrodes (see Section 6.2.2) [Housmans et al., 2004]. In addition, it is argued that the barrier to dehydrogenation is higher on Rh than on Pt, leading to a lower overall reaction rate [de Souza et al., 2002]. These effects may also explain the lower product selectivity towards acetaldehyde and acetic acid, which require the dehydrogenation of weakly adsorbed species. 

Thin-fdm was prepared from a slurry of catalyst powder which was prepared from 10 mg catalyst in 5 ml of 2-propanol. The catalyst slurry was sonicated for 30 min. and allowed to sit stagnant overnight. Before preparing the films, the slurry was sonicated for 15 min., 20 drops (0.1 ml) were added onto a ZnSe trough plate internal reflection element (022-2010-45, Pike Technologies). The solvent was allowed to evaporate, the procedure was repeated a total of five times. After drying in air at room temperature, the catalyst thin-film was ready for 2-propanol dehydrogenation studies. 

Yamada, Y., Ueda, A., Zhao, Z. et al. (2001) Rapid evaluation of oxidation catalysis by gas sensor system total oxidation, oxidative dehydrogenation, and selective oxidation over metal oxide catalysts. Catal. Today, 67, 379. 

Three syntheses of 6,7-secoberbines have been carried out. Two of them involved degradation of the protoberberine alkaloids (63,65), and the third was a total synthesis (69). Takao and Iwasa (63) applied the von Braun reaction to tetrahydrocoptisine (39) to obtain the 6,7-seco bromide 63, which on treatment with dimethylamine, followed by hydrolysis, gave tetrahydro-corydamine (64). This tetrahydrobase 64, which was also produced from 56 by zinc in hydrochloric acid (63), was dehydrogenated to corydamine (56) (Scheme 15). 

A new process for the manufacture of acetylene has been proposed. The process will involve the dehydrogenation of ethane over a suitable catalyst (yet to be found). Pure ethane will be fed to a reactor and a mixture of acetylene, hydrogen, and unreacted ethane will be withdrawn. The reactor will operate at 101.3 kPa total pressure and at some as yet unspecified temperature T. 

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