Preparation of Prereduced Catalysts

Usually, fused iron catalyst is directly reduced in the converter of ammonia plant, while this reduction approach has some shortcomings. 

In order to overcome these difficulties, a small amount of prereduced catalyst such as the KMR type catalyst appeared on the market and was used in a few ammonia plants at the end of the 1950s. In the mid-1990s, the prereduced catalysts 

The prereduced catalyst is reduced under ideal conditions and it is then passivated during the manufacture of the catalyst. After the prereduced catalyst is loaded into the converter, only a simple reduction is needed before functioning. The preparation process of prereduction catalyst is divided into three steps  

The first step is the same as the process mentioned in Sec. 4.1. The second step is performed in a special device for the prereduction of the catalyst, in which the feed gas comes from the decomposition of ammonia or a pure fresh synthesis gas from an ammonia plant. 

14 Prereduction process flow-sheet of ammonia synthesis catalyst 

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About 15%-30% of the active a-Fe is poisoned by O2 in order to form passivation film during preparation of prereduced catalyst (this part of a-Fe is the most active site on surface and takes part in the actual catalytic activity, whereas the inside of a-Fe which is not poisoned by O2 does not work in real reaction). It is sure to lose a part of its activity when reduction is carried on again in the ammonia plant. That is to say that the activity of prereduced catalysts is lower than that of oxidation state catalysts after reduction is directed in the ammonia plant. However, when the prereduced catalyst is used in the plant, the indexes such as the net value and output of ammonia are dependent on the activity of the catalyst, and also on some other factors, which are hard to be expressed by exact amount. In point of view of economy, the use of prereduced catalyst shows some obvious economic benefits for the plant. 

Preparation of 7-amino-3-chloro-3-cephem-4-carboxylic acid To a solution of 750 mg (1 55 mmol) of p-nitrobenzyl 7-amino-3-chloro-3-cephem-4-carboxylate hydrochloride in 20 ml of tetrahydrofuran and 40 ml of methanol was added a suspension of 750 mg of prereduced 5% palladium on carbon catalyst in 20 ml of ethanol and the suspension was hydrogenated under 50 psi of hydrogen at room temperature for 45 minutes. The catalyst was filtered and washed with THF and water. The filtrate and catalyst washes were combined and evaporated to dryness. The residue was dissolved in a water-ethyl acetate mixture and the pH adjusted to pH 3. The insoluble product was filtered and triturated with acetone. The product was then dried to yield 115 mg of 7-amlno-3-chloro-3-cephem-4-carboxylic acid. 

Dimethyl cw-hexahydrophthalate also may be prepared by a similar reduction of dimethyl m-A -tetrahydrophthalate. With 0.5 g. of prereduced Adams platinum oxide catalyst, 198 g. (1 mole) of dimethyl m-A -tetrahydrophthalate was reduced to give 196 g. (98%) of dimethyl cw-hexahydrophthalate, b.p. 110-112°/5 mm., Wd 1.4570. 

Unsymmetrical secondary amines are readily prepared in good yields by the catalytic reduction of Schiff bases at moderate temperatures in high-or low-pressure equipment. Many examples have been cited. The intermediate imines are prepared from primary amines and aldehydes—very seldom from ketones—and may be used without isolation (cf. method 431). For the preparation of aliphatic amines, e.g., ethyl-w-propylamine and n-butylisoamylamine, a prereduced platinum oxide catalyst is preferred with alcohol as the solvent. Schiff bases from the condensation of aromatic aldehydes with either aromatic or aliphatic amines are more readily prepared and are reduced over a nickel catalyst. In this manner, a large number of N-alkylbenzylamines having halo, hydroxyl, or methoxyl groups on the nucleus have been made. Reductions by means of sodium and alcohol and lithium aluminum hydride have also been described,... 

The catalyst, in the form of a thin (100y) wafer held by the aluminum gasket sealing the reactor, was placed perpendicular to the IR beam. The reactants flowed along both sides of the catalyst wafer. The wafers were prepared by pressing approximately 30 mg of the prereduced catalyst powder at a pressure of about 7,000 psi. The wafers, approximately 2.5 cm dia., were reduced again in the reactor for 12 hours at 200 C. 

The industrial catalyst is prepared by fusion. The catalyst may be supplied in the unreduced state after crushing and screening to the desired particle size or the catalyst may be reduced and subsequently stabilized by controlled oxidation in the catalyst factory. Although the reduced catalyst is pyrophoric, the prereduced catalyst can be safely handled. In the ammonia synthesis plant the catalyst is activated by reduction with a mixture of hydrogen and nitrogen as the final step in the start-up procedure for the plant. The reduction of the prereduced catalyst is faster and simpler than the start-up of the unreduced catalyst. 

Catalyst Pd-15 was prepared by the method of Zhmud el al. 8), i.e., by adsorption of Pd(NH3)4++ on aerosil, filtration, careful washing and drying. After that, the catalyst was prereduced at 300°C with a hydrogen-nitrogen current containing 0.5 mole % of hydrogen. 

Pd-105 was prepared by impregnation of aerosil with HjPdCLj. The catalyst was dried and prereduced with the N2-0.5% H2 mixture at 300°C. 

These routes involve the formation of (usually) prereduced metal particles that are then adsorbed or deposited onto the support. They have the advantage that the particle size of the particles is predetermined by the chemistry of the colloids and that resulting catalysts have narrow particle size distributions. However, the colloidal particles often are surface stabilized by surfactant molecules, which can be difficult to remove once the particles are adsorbed onto the support. One further disadvantage is that the colloidal particles are prepared at high dilution (typically millimolar concentrations— for example, 0.2 g Ft 1 ), which is a disadvantage in terms of scale-up. 

The same reactions, carried out with potassium carbonate as base in place of a secondary amine, yield exocyclic dienes in good yield, although double-bond isomerization sometimes occurs (equation 38).93 Inclusion of tetra-zi-butylammonium chloride in the reaction mixture stops the double-bond isomerization. Thus, the reaction in equation (38) with the chloride yields only the bis(exomethylene) product in 45% yield in a slow reaction. Some N- and O-heterocyclic products, also, have been prepared by the intramolecular vinyl substitution reaction.94 A 16-membered ring lactone was made by the ring closure of a vinylic iodide group with a vinyl ketone group. The yield, based upon the reactant, was 55% but a stoichiometric amount of bis(acetonitrile)palladium dichloride was employed. The catalyst was prereduced with formic acid so that the reaction proceeded at 25 C (equation 39).95... 

One of the important areas of catalyst characterization is to provide information on the bulk and surface properties in relation to the preparation methods, including precursors, calcined, prereduced, or otherwise pretreated catalysts. Other, no less important areas pertain to catalysts in their active working state and to post-mortem analysis of gradually or suddenly deactivated catalysts. There is no greater incentive for a rapid catalyst characterization than a 1,000-ton reactor making a 400,000/day product going down due to a catastrophic catalyst deactivation. 

Bimetallic R-Sn/A Os catalysts were prepared either by coimpregnation (Cl) or successive impregnations (SI) of platinum and tin. In the SI method, the tin salt solution is introduced on a prereduced parent Pt/AljOs catalyst under hydrogen bubbling. SI catalysts are less sensitive than Cl catalysts to deactivation by carbon deposition for isobutane dehydrogenation and coking with cyclopentane. A more effective interaction between the two metals is responsible for the higher stability of the SI catalysts. 

Palladium hydroxide catalyst [1,782, before Pentachlorophenol]. This hydrogenation catalyst is prepared by adding a slight excess of lithium hydroxide solution to a hot aqueous solution of palladium chloride the precipitate is washed to neutrality with hot distilled water.1 Reduction of cyclohexanones in an alcohol with prereduced palladium hydroxide as catalyst gives cyclohexyl ethers in high yield.2 Thus 5a-cholestane-3-one gives /8-methoxy-5a-cholestane in 91% yield when reduced in methanol. Hydrogenation of cyclohexanol in ethanol affords ethoxycyclohexane in 96% yield. 

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