Palladium catalysts fractionation

P 16] An ethanol slurry of activated carbon-supported palladium catalyst particles was introduced into a packed-bed micro reactor [36]. The fraction of 50-75 pm sized particles was used. The reaction was carried out with 1 atm hydrogen at 50 °C. 

The demethanizer, deethanizer, and debutanizer are fractionating columns that separate the lighter and heavier compounds from each other. Traces of triple bonds are removed by catalytic hydrogenation with a palladium catalyst in both the C2 and C3 stream. Cumulated double bonds are also hydrogenated in the C3 fraction. These are more reactive in hydrogenation than ethylene or propylene. The C2 and C3 splitters (Fig. 8.4) are distillation columns that can be as high as 200 ft. The mechanism of cracking was previously discussed in Chapter 7, Section 6. 

In one process, the chargestuck is nitration-grade toluene, air. hydrogen, anhydrous NHj. and H,S04. The toluene is oxidized to yield u 301 solution of benzoic acid, plus intermediates and byproducts. Pure benzoic acid, after fractionation, is hydrogenated with a palladium catalyst in stirred reactors operated at about 17o"C under a pressure of ID atmospheres. The resultant product, cyclohexanccarboxylic acid, is mixed with H,S04... 

Both of these reactions have very important industrial uses (Section 14.3.9). In order to obtain alkene streams of sufficient purity for further use, the products of steam-cracking or catalytic cracking of naphtha fractions must be treated to lower the concentration of alkynes and alkadienes to very low levels (<5ppm). For example, residual alkynes and dienes can reduce the effectiveness of alkene polymerisation catalysts, but the desired levels of impurities can be achieved by their selective hydrogenation (Scheme 9.4) with palladium catalysts, typically Pd/A Os with a low palladium content. A great deal of literature exists,13,37 particularly on the problem of hydrogenating ethyne in the presence of a large excess of... 

Catalytic experience tells us that frequently only a small fraction of the sites at the surface of the metal participate in the reaction. Recently, Krai measured the metal surface area of palladium catalysts supported on carbon, their specific activity for various hydrogenation reactions, and their poisoning by thiophene (44). By combining the classic technique of poisoning with the measurement of metal surface area. Krai was able to show that the Taylor ratio, i.e., the fraction of active sites in each particular reaction, changed from unity to about 10 . With a Taylor ratio of unity, the reaction would be called facile. Otherwise, we deal with a structure-sensitive or demanding reaction. 

This takes place in a fixed bed reactor, used in two stages at different temperatures (60°C for the first, 110 0 for the second), in the presence of 1 per cent weight of palladium catalyst on charcoal. The cooling of a fraction of the hydrogenated product, which takes place outside the reactor, removes the heat generated by the reaction and maintains the temperature at the requisite level. Diacetoxyhutane is obtained with a molar yield of 94 per cent, for virtually total conversion of diacetoxybutene. 

A besetting problem with the industrial process to remove traces of alkynes alkadienes from alkene streams using palladium catalysts has been the formation of higher hydrocarbons by oligomerisation. Although in this respect palladium is better than base metals such as nickel (which presumably explains why this cheaper metal is not used), and while the fraction of ethyne that reacts in this manner is small, nevertheless in a continuous operation these higher products accumulate, and cause problems. The carbonaceous deposits, so often mentioned, may be partly C2 species such as ethylidyne, but they also comprise adsorbed forms of oligomers in the steady state their formation is followed by release into the fluid phase. 

As an illustration, when a gaseous mixture of 2-butyne and deuterium is passed over a palladium catalyst at 14 C, the composition of the hydrocarbon fraction in the product is as shown in Table 5.2.1. In this example, Ai is2-butyne, A2 is j-2-butene-2,3-d2 and Aj is butane. The selectivity to butene is 99.9%. The stereoselectivity to cw-2-butene is 99%. The quantitative yield of butene is due to the occupancy of the surface by butyne. As long as there remains in the system enough unconverted butyne, butene has no access to the surface for its further hydrogenation to butane. Yet, all the butene will be hydrogenated readily to butane in the absence of butyne. 

The story starts with crude oil or natural gas fractions, e.g. naphtha, which are cracked to give principally ethylene. The ethylene is then reacted with acetic acid and oxygen over a supported palladium catalyst to produce the vinyl acetate (see section 12.7.4). Finally this is polymerized to polyvinyl acetate which is then mixed with the other ingredients to produce the emulsion paint. 

Kaminsky et al. succeeded in E-N copolymerizations by using a-diimine palladium catalysts [111, 112]. The Tg values of E-N copolymers produced are very high and range from 98 to 217°C [111, 112], Copolymers produced at norbomene molar fraction (xn) > 0.80, as well as homo-polynorbomenes, show no Tg values or values under 350°C, and decompose above 350°C. 

By using a supported palladium catalyst [23, 122], a-pinene can be isomerized to an equilibrium mixture containing 4% (3-pinene (31). This equilibration opens up the possibility of production of the latter from the former. The amount of (3-pinene in equilibrium with a-pinene is low, but the use of an efficient fractional distillation column with continuous processing make the process feasible albeit energy intensive. 

Figure Bl.22.1. Reflection-absorption IR spectra (RAIRS) from palladium flat surfaces in the presence of a 1 X 10 Torr 1 1 NO CO mixture at 200 K. Data are shown here for tluee different surfaces, namely, for Pd (100) (bottom) and Pd(l 11) (middle) single crystals and for palladium particles (about 500 A m diameter) deposited on a 100 A diick Si02 film grown on top of a Mo(l 10) single crystal. These experiments illustrate how RAIRS titration experiments can be used for the identification of specific surface sites in supported catalysts. On Pd(lOO) CO and NO each adsorbs on twofold sites, as indicated by their stretching bands at about 1970 and 1670 cm, respectively. On Pd(l 11), on the other hand, the main IR peaks are seen around 1745 for NO (on-top adsorption) and about 1915 for CO (tlueefold coordination). Using those two spectra as references, the data from the supported Pd system can be analysed to obtain estimates of the relative fractions of (100) and (111) planes exposed in the metal particles [26].

---