Liquid phase phenol hydrogenation

Allied-Signal Process. Cyclohexanone [108-94-1] is produced in 98% yield at 95% conversion by liquid-phase catal57tic hydrogenation of phenol. Hydroxylamine sulfate is produced in aqueous solution by the conventional Raschig process, wherein NO from the catalytic air oxidation of ammonia is absorbed in ammonium carbonate solution as ammonium nitrite (eq. 1). The latter is reduced with sulfur dioxide to hydroxylamine disulfonate (eq. 2), which is hydrolyzed to acidic hydroxylamine sulfate solution (eq. 3). 

A route to phenol has been developed starting from cyclohexane, which is first oxidised to a mixture of cyclohexanol and cyclohexanone. In one process the oxidation is carried out in the liquid phase using cobalt naphthenate as catalyst. The cyclohexanone present may be converted to cyclohexanol, in this case the desired intermediate, by catalytic hydrogenation. The cyclohexanol is converted to phenol by a catalytic process using selenium or with palladium on charcoal. The hydrogen produced in this process may be used in the conversion of cyclohexanone to cyclohexanol. It also may be used in the conversion of benzene to cyclohexane in processes where benzene is used as the precursor of the cyclohexane. 

The kinetics of hydrogenation of phenol has already been studied in the liquid phase on Raney nickel (18). Cyclohexanone was proved to be the reaction intermediate, and the kinetics of single reactions were determined, however, by a somewhat simplified method. The description of the kinetics of the hydrogenation of phenol in gaseous phase on a supported palladium catalyst (62) was obtained by simultaneously solving a set of rate equations for the complicated reaction schemes containing six to seven constants. The same catalyst was used for a kinetic study also in the liquid phase (62a). 

One of the exciting results to come out of heterogeneous catalysis research since the early 1980s is the discovery and development of catalysts that employ hydrogen peroxide to selectively oxidize organic compounds at low temperatures in the liquid phase. These catalysts are based on titanium, and the important discovery was a way to isolate titanium in framework locations of the inner cavities of zeolites (molecular sieves). Thus, mild oxidations may be run in water or water-soluble solvents. Practicing organic chemists now have a way to catalytically oxidize benzene to phenols alkanes to alcohols and ketones primary alcohols to aldehydes, acids, esters, and acetals secondary alcohols to ketones primary amines to oximes secondary amines to hydroxyl-amines and tertiary amines to amine oxides. 

An alternate route to get cyclohexanone is sometimes used— hydrogenating phenol in the liquid phase catalyzed by palladium on carbon. The rest of the process is then the same, except that yields are in the 70% range. Since the by-product yield is so high, the process has had limited acceptance, with only about 5% of the adipic acid being made this way. 

For bipolar organic liquids, especially for hydrogen-bonding liquids such as alcohols and amines, the tendency to orient in the liquid phase, due to these highly directional intermolecular attractions, is greatly increased by this intermolecular interaction. We can see the effect of this in the significantly larger entropies of vaporization of bipolar chemicals, like aniline, phenol, benzyl alcohol, or ethanol (Table 4.2). 

The direct hydroxylation of benzene and aromatics with a mixture of 02 and H2 have been performed by simultaneously mixing benzene, oxygen and hydrogen in the liquid phase using a very complicated system containing a multi-component catalyst, a solvent and some additives. Besides the possibility of an explosive gas reaction, these hydroxylations gave only very low yields, 0.0014—0.69% of phenol and aromatic alcohols. For example, Pd-containing titanium silicalite zeolites catalyzed... 

The hydrogenation of phenol can take place either in vapor or in liquid phase. Both processes today employ palladium-based catalyst, but with different supports and activators. 

Liquid-phase hydrogenation of phenol operates at temperatures below the atmospheric boiling point, around 140-150 °C. High selectivity is claimed, over 99% at 90% conversion. In addition, the process needs less catalyst inventory and is inherently safe [2]. 

The impurities can be grouped into two categories lights (water, cyclohexene, cyclohexadiene) and heavies (phenol, dicyclohexyl-ether, cyclohexenyl- cyclohexanone). To limit their amount, the conversion is kept around 80% with a selectivity of about 98%. The hot reactor effluent is cooled in countercurrent with the feed in FEHE, and finally for phase separation in the heat exchanger (E-2) at 33 °C. The simple flash (S-2) can ensure a sharp split between hydrogen, recycled to hydrogenation reactor, and a liquid phase sent to separation. 

The proposed one-reactor flowsheet is similar to recent technologies, both in structure and economic performance. Further flowsheet simplification would be difficult to obtain. Liquid-phase hydrogenation could suppress the evaporator, but it requires a more complicated reactor technology. As a result, the availability of low-cost phenol can make this technology highly competitive with the cyclohexane oxidation. 

Production of a-methjistyrene (AMS) from cumene by dehydrogenation was practiced commercially by Dow until 1977. It is now produced as a by-product in the production of phenol and acetone from cumene. Cumene is manufactured by alkjiation of benzene with propjiene. In the phenol—acetone process, cumene is oxidized in the liquid phase thermally to cumene hydroperoxide. The hydroperoxide is split into phenol and acetone by a cleavage reaction catalyzed by sulfur dioxide. Up to 2% of the cumene is converted to a-methjistyrene. Phenol and acetone are laige-volume chemicals and the supply of the by-product a-methylstyrene is weU in excess of its demand. Producers are forced to hydrogenate it back to cumene for recycle to the phenol—acetone plant. Estimated plant capacities of the U.S. producers of OC-methylstyrene are listed in Table 13 (80). 

The Oxirane process is a mature technology that has stood the test of time. Both ARCO and Shell have been successfully operating for more than two decades. More recently a heterogeneous titanium-substituted silicalite (TS-1) catalyst was developed by Enichem [43, 44]. In contrast to the Shell Ti /Si02 catalyst, TS-1 has a hydrophobic surface and is a remarkably effective catalyst for a variety of liquid-phase oxidations with 30 % aqueous hydrogen peroxide, including epoxidation [44]. It has been commercialized for the hydroxylation of phenol to... 

In a typical synthesis, a mixture of C13H12O2 bisphenols was prepared in 80% yield by slow addition of a solution of trioxane (0.036 mole) in benzene (over 1.75 hour) to a stirred, liquid phase suspension of phenol (0.64 mole) and HY zeolite (5 gm) at 182°. The ratio of the 2,2, 2,4, and 4,4 isomers was 1.3 1.8 1.0. This technique, which afforded very high instantaneous ratios of phenol to aldehyde, prevented rapid catalyst aging. Generally, high yields were observed for carbonyl reactants with no a-hydrogens, since competitive intracrystalline aldol condensation reactions were eliminated. 

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