Heterogeneous Catalysts. Группа авторов
by oxidizing sulfur dioxide in air over platinum packed in porcelain tubes heated to “strong yellow heat”. The resultant sulfur trioxide forms sulfuric acid fume upon contact with water, hence earning its name as the Contact Process [5]. Ironically, despite the high importance of this catalytic process, not much is known about Phillips except that he was son of a tailor and was born in Bristol [1]. A large‐scale manufacturing of sulfuric acid using the Contact Process and platinum catalyst was realized many years later in 1875 by Rudolph Messel, a German‐born and naturalized English industrial chemist. Messel himself was very much involved in the studies of the kinetics as well as the problematic poisoning of platinum catalysts by arsenic trioxide. In 1913, BASF was granted patents on a new catalyst based on the more versatile supported vanadium pentoxide and alkali oxide on porous silica [6, 7]. The first manufacturing plant based on this new catalyst was commissioned in 1915. Improvement in the activity of the supported vanadium pentoxide catalyst through the addition of potassium sulfate promoter was invented in Germany and the United States between 1916 and 1919. It was only in 1988 that Haldor Topsoe and Anders Nielsen revealed that the addition of cesium or rubidium promoter, rather than potassium, was more efficient in enhancing the activity of sulfur dioxide oxidation. With a typical lifetime of up to 10 years, the industrial catalyst composition for the Contact Process has been largely unchanged even to this day [8].
Going back to 1838, just a few years after the discovery of the Contact Process, Frédéric Kuhlmann discovered the production of nitric acid from the oxidation of ammonia in air over platinum sponge at 300 °C and filed a patent on this [9]. Based on the discovery, he later founded the Etablissements Kuhlmann company, which still exists to this day as part of the Pechiney SA. Despite being an important chemical commodity for the use in fertilizers and explosives manufacturing, the interest in Kuhlmann reaction was not immediately of interest since Chile saltpetre (a naturally occurring mineral of alkali metal nitrate precursor found at the Atacama desert repository) was widely available. In his vision, Kuhlmann stated that “If in fact the transformation of ammonia to nitric acid in the presence of platinum and air is not economical, the time may come when this process will constitute a profitable industry.”
Indeed, the Kuhlmann reaction picked up interest toward the end of the century as part of the solution to “The Nitrogen Problem.” In 1901 and building on Kuhlmann's earlier findings, Wilhelm Ostwald of the University of Leipzig investigated the production of nitric acid using supported platinum on asbestos before moving to coiled platinum strips that gave higher conversion [9]. A large‐scale nitric acid manufacturing plant went into operation at Gerthe in 1908 with an output of 3 tons nitric acid per day using 50 g of corrugated platinum catalyst of 2 cm wide. Given the short catalyst lifetime of no more than six weeks, it was soon realized to be a costly operation. To tackle the problem, Karl Kaiser of Technische Hochschule, Charlottenburg, developed the platinum gauze catalyst in 1909, consisting of 0.06 mm diameter wires woven to 1050 mesh/cm2, that gave a higher surface‐to‐bulk ratio and uninterrupted production of nitric acid of up to six months [9]. But because the source of ammonia at that time was derived from gas works liquors containing impurities such as arsenic and sulfur that deactivate the platinum catalyst, the really large industrial‐scale production was only possible after the implementation of the Haber–Bosch process that provided clean ammonia. The present‐day nitric acid catalyst is based on rhodium–platinum gauze (5–10% Rh) [10].
Further advancement in the design of bulk metal catalysts was evident from the work of Murray Raney on the synthesis of skeletal nickel, which was granted US patent in 1925 [11]. The Raney catalyst was prepared by first forming a Ni–Al alloy and ground into small particles, followed by the selective leaching of Al in caustic brine (such as NaOH) to yield the skeletal structure. The resultant Raney catalyst is composed of finely divided nickel so fine that it is pyrophoric and hence requiring storage under deionized water. Initially, the Raney Ni was used as an industrial catalyst for the hydrogenation of vegetable oil (to make butter substitutes) but later proved to be useful for a range of other hydrogenation reactions. Other forms of Raney catalysts including those of metallic cobalt, copper, palladium, silver, and ruthenium were later developed and found applications in methanol synthesis, conversion of furfural into furfural alcohol, and the hydrogenation of acrolein to allyl alcohol, among others [12, 13].
1.2 The Game Changer: High‐Pressure Catalytic Reactions
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.