From Science & Research
From Science & Research Catalysis: key for sustainable production Background Catalysts are indispensable in the chemical industry. In more than 80 % of all industrial chemical conversions one or more catalysts are involved. A catalyst lowers the activation energy of a reaction. In practice this means it speeds up a reaction. Therefore, catalysed reactions are more energy-efficient compared to non-catalysed reactions. Thus, catalysts contribute to the sustainable production of chemicals, materials, and fuels. Traditionally, catalysts are divided into three different categories i.e., heterogeneous catalysts (often solid materials with reactants/products in gas phase or liquid phase), homogeneous catalysts (often solutions containing both the catalysts and the reactants/products), and biocatalysts (either homogeneous or heterogeneous; they include enzymes (see bM 01/21) and microbes). All of these catalysts have their own pros and cons as summarized in Table 1. In general, the heterogeneous catalysts are very robust and can operate at high temperatures which results in high productivity per reactor volume. The homogeneous and biocatalysts are often more selective and operate at lower temperatures (which is not necessarily and advantage for exothermic reactions, see note at Table 1). A major disadvantage of homogeneous catalysts is their cumbersome separation from the reaction mixture. Often energy-intensive processes such as distillation are needed. Nevertheless, all those catalysts have their own merits and are used depending on the molecules (and their value) which need to be made. Table 1: Generalized overview of pros and cons of different types of catalysts (++: clear strength, -- clear weakness) Activity per reactor volume Activity at low temperature Heterogeneous catalyst Homogeneous catalyst Biocatalyst ++ + - -/+ + ++ Selectivity - + ++ Separation ++ - - + Note: performing reactions at low temperatures is not necessarily a holy grail. When exothermic reactions are performed at low temperatures it is very difficult to cool away the produced heat since heat exchangers are not efficient at low temperatures. Challenges for catalysis Currently, most bulk industrial processes use fossil feedstocks i.e., coal, oil, and gas to make our needed chemicals, materials, and fuels. The current catalysts are optimized for converting these feedstocks. However, for sustainability reasons, new feedstocks like biomass and recycled materials such as polymers/plastics become more important. Since these feedstocks are often more functionalized with, for example, oxygen and nitrogen functionalities new catalysts are needed to deal with these feedstocks as the traditional feedstocks contain mainly carbon and hydrogen (hydrocarbons). In addition, reactions are traditionally driven by heat input which means the burning of fossil fuels. Alternatives like renewable electricity and light as energy inputs are emerging. This also requires new catalysts that can deal with these new forms of energy input. From a chemical point of view, noble metals are preferred as catalysts since they are in general stable under different reaction conditions. However, these metals are scarce and not well available, especially when considering industrial scale productions. Therefore, readily available alternatives are sought for as catalytic active materials. Replacement of noble metals from Pd to W-carbide and Mo-carbides An example where new biobased feedstocks and new catalysts based on non-noble metals are combined is the deoxygenation of lipid-based feedstocks to alkanes and alkenes [1, 2, 3]. Especially the alkenes are interesting since they can serve as building blocks for surfactants and polymers. Already since the 70s of the last century [4], it was known that tungsten carbide has similar catalytic properties as platinum. Therefore, this carbide as well as molybdenum carbide can potentially serve as a replacement for noble metals. In the research at Wageningen University (The Netherlands) supported tungsten and molybdenum carbides were used to replace palladium (Pd) in the deoxygenation of lipid-based feedstocks. Figure 1 (top) shows a typical activity plot of a carbon-supported tungsten carbide catalyst during the deoxygenation of the model compound stearic acid. In addition, Figure 1 (bottom) shows a macroscopic and microscopic view of such a catalyst. One of the key features of this catalyst is the fact that it produces alkenes even in the presence of hydrogen. Thus, this catalyst is selective towards products, the alkenes, which cannot be made under the same conditions using noble metals. In the latter case only fully hydrogenated products, the alkanes, were observed. This shows the potential of this kind of catalyst, though further optimization and understanding of the catalyst is needed to make an industrially viable process. Stabilization of non-noble metals One of the major challenges when using non-noble metals as catalysts, especially under conditions relevant for biomass conversion i.e., in water, is the stability of the catalyst. Nonnoble metals can easily oxidize in water forming metal oxides 44 bioplastics MAGAZINE [04/21] Vol. 16
Figure 1: left: typical product distribution of stearic acid (a C18 carboxylic acid) deoxygenation over a carbon supported W2C catalyst in a batch reactor (T=350oC, 30 bar H2); middle: macroscopic view of a carbon-based catalyst; right: Transmission electromicrograph of a carbon supported W2C catalyst (dark spots are the tungsten carbide). 80 60 40 20 0 0 100 200 300 400 500 600 700 800 By: J.H.Bitter, Wageningen University, Biobased Chemistry and Technology However, it was shown [5] that the reaction conditions can have a significant stabilizing effect on the nickel particles even in aqueous conditions. Figure 2B shows that adding hydrogen to the gas phase does to a certain extent stabilize the nickel particles i.e., the surface area does not decrease, during the aqueous phase processing of ethylene glycol. This is because adding hydrogen keeps the nickel reduced and reduced metals do not dissolve as discussed above: a hydroxide or oxide is needed for that. Figure 2B also shows that adding a base to the solution also stabilizes nickel particles. This is because at higher pH levels nickel hydroxide is less soluble and therefore less leaching occurs and as a result less growth of the metal particles via Ostwald ripening is observed. This clearly shows that nonnoble metals have great potential also for aqueous phase conversions. Use of electricity With the expected increased availability of renewable electricity, the field of electrochemistry and electrocatalysis regained interest. A prime example of the use of electrocatalysis is the production of bulk chemicals and fuels from CO 2 . However, also in the field of chemicals produced from renewable or recycled feedstocks electrochemistry and electrocatalysis can play an important role. For example, Kwon et al. [6] showed that paired electrolysis From Science & Research Figure 2 A: Ni particle growth during aqueous processing; B: Stabilizing effects of H 2 in gas phase or base in solution during aqueous phase processing of ethylene glycol. and metal hydroxides which have a low though significant solubility. When that happens the heterogeneous catalyst slowly dissolves a process which is called metal leaching. While this often leads to the deactivation of the catalyst it is important to note that dissolved metals can also have catalytic activity. However, one of the major advantages of using a heterogeneous catalyst i.e., that it is easy to separate from the reaction medium is lost in that case. Therefore, catalyst leaching is undesired. Figure 2 shows electron micrographs of a nickel (Ni) on carbon catalyst before and after use in the aqueous phase conversion of a polyol (in this case ethylene glycol (EG)) to hydrogen and CO 2 (this is a way to produce biobased hydrogen). Clearly, the nickel particles were increased in size during reaction. This is what is generally observed for non-noble metals under aqueous conditions [5]. A B 100 80 60 500 nm 10 % EG / Water 230 °C 200 nm In liquid-phase reactions, the growth of metal particles often proceeds via a mechanism called Ostwald ripening. The metal dissolves from the smaller Ni particles as hydroxide and precipitates on the larger particles. In that way, the surface tension of the metal particles is decreased which is thermodynamically favourable. 40 20 0 bioplastics MAGAZINE [04/21] Vol. 16 45
