Читать книгу Heterogeneous Catalysts - Группа авторов - Страница 14

The implementation of high‐pressure reactor technologies pioneered by Robert Le Rossignol (assistant to Fritz Haber) [14] and later by Carl Bosch [15] was one of the most important milestones in the advancement of heterogeneous catalysis. Their breakthroughs enabled a series of high‐pressure catalytic reactions that include the ammonia synthesis and methanol synthesis, which to this day rank among the most important industrial catalytic reactions. High‐pressure conditions are particularly useful in overcoming reaction dilemma that under ambient pressure could obtain high selectivity but at extremely sluggish rates and vice versa at high temperatures. By carrying out the same reaction under high‐pressure conditions, one can shift the equilibrium line to higher selectivity even at high temperatures, thus allowing high yield of the desired product. Chapter 35 is devoted to this topic.

Haber in one of his earlier efforts in synthesizing ammonia by N₂ fixation (through reaction with H₂) under ambient pressure could only obtain 0.005% yield when using iron catalysts at 1000 °C [16]. A year later, in 1906, Walther Nernst at the University of Berlin reported favorable conversion at 1000 °C when using iron catalysts in a ceramic apparatus that allowed him to perform the reaction at 75 bar. Unfortunately, the reactor and the extreme condition were far too impractical for industrial‐scale implementation. Haber, who became professor at the Karlsruhe Technische Hochschule, used a steel‐based reactor but this time working with Le Rossignol (who actually built the bench‐scale high‐pressure reactor, equipped with a high‐pressure and high‐temperature valve, now known as the Le Rossignol valve). With the new reactor, they were able to screen a number of catalytic materials ranging from iron, chromium, nickel, manganese, osmium, and uranium (as uranium carbide) at 200 atm and in excess of 700 °C. Osmium and uranium catalysts were found to be active, with the former achieving a 6% conversion. Realizing that the N₂ fixation reaction is limited by its kinetics rather than equilibrium, Haber further developed the feed recycle system for which he received a patent [17]. BASF AG acquired Haber's patents on ammonia synthesis and, interestingly, also the total world supply of osmium at that time (100 kg) [2]! The amount of osmium was estimated to be capable of producing 750 tons of ammonia per year, although that amount would still be insufficient to cope with the total ammonia demand. Alwin Mittasch, who was tasked by BASF to look for more commercially feasible alternatives, together with his colleague, George Stern, screened more than 2500 catalysts and found that a magnetite (Fe₃O₄) sample taken from a Swedish mine gave very high yield. Mittasch soon realized that the presence of impurities in the sample was critical before arriving at an optimized synthetic Fe₃O₄ catalysts promoted with 2.5–4% Al₂O₃, 0.5–1.2% K₂O, 2.0–3.5% CaO, and 0.0–1.0% MgO (together with 0.2–0.5% Si present as impurity in the metal) [18, 19]. The catalyst formulation was so robust that it has not significantly changed until now.

Meanwhile, the major challenge in high‐pressure reactor design shall be described. The diffusion of hydrogen through the standard carbon steel reactor under high pressure and temperature can result in the decarbonization and formation of brittle iron hydride, thus reducing the pressure rating of the reactor [20]. As such, using such reactors would limit the standard operation of ammonia synthesis (200 atm, 500 °C) to a mere 80 hours [17]. The groundbreaking work by Bosch arrived in 1909 when he, after observing Le Rossignol's reactor design, came up with an ingenious design of using a concentric tube consisting of an inner soft (low‐carbon) steel tube encased in a pressure‐bearing carbon steel outer jacket [16]. Narrow grooves were machined on the outer wall of the inner tube to create small pockets in between the tube and the jacket. During operation, high‐pressure and high‐temperature hydrogen from the reaction in the inner tube would diffuse out through the soft steel into the pockets while experiencing rapid loss of pressure and temperature. Small holes were drilled on the outer jacket to allow continuous release of the diffused hydrogen from the pockets [21]. With the catalyst formulation and reactor design in place, a pilot test on a 4 m reactor was carried out in 1911, subsequently leading to the commissioning of a full‐scale manufacturing plant at Oppau consisting of an 8 m high reactor to produce 20 tons of ammonia per day [16], which is known now as the Haber–Bosch process.

The triumph in ammonia synthesis in Germany caught on with the industrial production of methanol (from syngas). As early as 1921, George Patas in the neighboring France patented a high‐pressure process for the synthesis of methanol using copper as well as nickel, silver, and iron catalysts [22]. BASF has again sought the help of Mittasch to search for suitable catalysts. This resulted in the discovery of zinc chromite (Cr₂O₃–ZnO) catalyst that was used in its industrial methanol production plant at Leuna in 1923. The catalytic reactor operated at 300 atm and 300–400 °C [23, 24]. Although iron‐containing (as well as nickel) catalysts also show methanol synthesis activity, they were later excluded from the catalysts screening due to the formation of iron carbonyl (from the reaction with carbon monoxide in the syngas) during the reaction that further decomposes to metallic iron (or iron carbide) [25]. Instead of catalyzing the methanol synthesis, these iron phases are more efficient at producing hydrocarbons (the basis for Fischer–Tropsch synthesis!), which is a more exothermic reaction. For the same reason, high‐pressure steel reactors were lined with copper, silver, or aluminum [26].

In 1947, Polish chemist Eugeniusz Błasiak patented a highly active methanol synthesis catalyst containing mixed copper, zinc, and aluminum prepared by coprecipitation [27]. Using the same catalyst, the Imperial Chemical Industries (ICI) developed a low‐pressure methanol synthesis process that only required operation at 30–120 atm with sufficient kinetics at 200–300 °C and selectivity of over 99.5%. The process along with the upstream high‐pressure steam reformer was patented in 1965 [28], followed closely by another landmark patent on the synthesis of mixed oxide of copper–zinc catalyst with promoter element from groups II–IV [29]. The catalytic process and catalyst formulation have remained largely unchanged.

Using Bosch's high‐pressure reactor, Franz Fischer and Hans Tropsch of Kaiser Wilhelm Institute for Coal Research (now known as Max Planck Institute of Coal Research) found the formation of high‐molecular‐weight hydrocarbons when using iron filings at 100 atm and 400 °C. As mentioned earlier, this was an undesirable reaction during the methanol synthesis, but Fischer understood the importance of this reaction. While continuing to work on this direction, they routinely assessed a range of metal oxides, hydroxides, and carbonates and in 1926 reported that reduced iron and cobalt catalysts yielded gasoline fuels from coal‐derived syngas [30, 31]. The reaction is known as the Fischer–Tropsch synthesis (FTS), which in 1935 marked the first FTS plant commissioned by Ruhrchemie using the cobalt catalyst. By 1938, there were nine such facilities within Germany with a manufacturing capacity of 600 000 tons/annum. The cobalt catalyst (100 Co/100 SiO₂/18 ThO₂) used by Ruhrchemie was developed by Fischer with Meyer and later with Koch by rapidly coprecipitating hot solutions of cobalt and thorium nitrate on SiO₂ (Kieselguhr diatomaceous earth) suspended in an ammonia‐containing solution [32, 33]. The irreducible thorium oxide restricts the crystallization of the cobalt metal to maintain a high dispersion. The slightly radioactive thoria has been replaced by zirconia, titania, or manganese oxide in the present‐day catalysts.

While cobalt is known to produce a large fraction of diesel and paraffin wax, the iron catalyst results in higher content of short‐chain olefins when carried out at high reaction temperatures (∼340 °C) or paraffin wax at much lower temperatures. As the reaction proceeds, the iron metal is gradually converted into iron carbide, which is an even more active phase [24, 34]. Compared with crude oil–derived fuels, the FTS‐derived diesel and gasolines are characterized by their exceptionally high cetane and octane ratings due to the high yields of straight‐chain paraffins for cobalt‐derived diesel and olefins/isomers in the iron‐derived gasoline, respectively. Although nickel and ruthenium catalysts are also active in FTS, they are rarely used as stand‐alone catalysts. Nickel, which forms carbonyl and decomposes to the metallic phase (like iron), has a high tendency to form methane instead of liquid fuels. Ruthenium, which is the most active FTS catalyst, is far more expensive than cobalt and iron to justify its bulk usage except as a promoter to cobalt catalysts. Incipient wetness impregnation is by far the most common technique for the synthesis of FTS catalysts [35].