【Haichuan Chemical Salary News】Overcoming challenges with heterogeneous catalysts to produce phenol from wood

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Phenol is widely used in production and daily life; it is an important raw material for manufacturing certain resins, fungicides, preservatives, and drugs, and is extensively applied in fields such as medicine, chemical industry, and synthetic fibers. Currently, the main methods for producing phenol include the benzene sulfonation method, the chlorobenzene method, the toluene and benzoic acid methods, and the ** method; the more direct phenol oxidation method can also be used. But almost all of them rely on benzene, a substance with high toxicity and high carcinogenicity, and are primarily produced from reformed gasoline and coking by-products. This production process causes serious pollution problems to the atmosphere, water bodies, and vegetation. Since the relevant reactions require a certain temperature, and processes such as subsequent separation also consume a large amount of energy, high pollution and high energy consumption have become serious problems in the practical production of phenol. Fortunately, such a useful phenol can now be produced in green chemical plants. So, how do green chemical plants function? Traditionally, the primary raw materials for almost all chemical plants are traditional fossil fuels such as coal and oil; whereas the raw materials for green chemical plants are one of the most common and inexpensive resources in nature – biomass, that is, various organisms produced through photosynthesis using elements like air, water, and land. Among them, lignocellulose is the most widely used. As the most abundant source of organic carbon in nature, lignocellulose can be subjected to pretreatment processes such as mechanical breakdown, steam explosion, acid-base treatment, and biological treatment. The resulting pentose or hexose sugars can then be processed using methods like hydrothermal liquefaction to yield platform compounds such as furans and furfural. Through catalytic techniques, these platform compounds can be converted into a range of products including biofuels and fine chemicals. For example, through selective hydrogenation green chemistry catalytic reactions, by using precious metal or transition metal catalysts under a hydrogen atmosphere as well as high temperature and pressure conditions, selective hydrogenation of platform compounds such as furfural, furan, and acetylpropionic acid can be successfully achieved. This approach replaces traditional resources like coal and oil, enabling the synthesis of chemicals with certain value and thus realizing the green chemistry-based production of these chemicals. However, since hydrogen is required as a hydrogen source in the preparation process, and high temperature and pressure conditions are needed, this reaction system poses certain risks. Furthermore, this reaction requires high-temperature conditions, which means that a certain amount of energy must be consumed. Therefore, an increasing number of researchers are exploring the use of renewable clean energy—light energy—as a source, along with semiconductors loaded with precious metals or transition metals that possess excellent catalytic properties, to carry out selective hydrogenation reactions on a range of platform compounds derived from biomass. With the use of heterogeneous catalysts to overcome challenges, the groundwork has been laid; now comes the main part – let’s see how rotten wood can be turned into the miraculous substance phenol. Research on the synthesis of aromatic compounds from lignocellulose has actually been conducted for some time. However, during the reaction process, the two benzene rings in the lignocellulose monomers cannot separate from each other effectively, making subsequent reactions difficult to proceed. In 2017, a team led by Researcher Wang Feng from the Dalian Institute of Chemical Physics, Chinese Academy of Sciences, utilized light energy and the heterogeneous catalyst ZnIn2S4 to catalyze this reaction, thereby overcoming this challenge through hydrolysis technology. In this reaction, the CαH-OH group containing hydroxyls in the lignocellulosic monomers provides the hydrogen required to produce phenol. When a molecule of reactant adsorbs on the catalyst surface, the O-H bond and the CαH bond in CαH-OH break apart sequentially, generating two molecules of hydrogen that are transferred to the semiconductor conduction band and stored there to form a hydrogen reservoir for subsequent use. Next, the hydrogen stored in the semiconductor conduction band hydrogen pool broke the C-O bonds of the reactants, successfully yielding the target product phenol. During the entire reaction process, pollution-free by-products are formed, but in low quantities. Upon reading this, some might ask: since there is a large amount of hydrogen on the surface of the catalyst, why isn’t a large amount of hydrogen gas produced? In fact, a small amount of hydrogen gas is indeed generated during the reaction; the reason for such a low amount is that the activation energy required to combine two hydrogen atoms into one molecule of hydrogen gas is higher than the activation energy needed to produce the desired product. For example, the reaction to synthesize phenol is as easy as crossing a low hill, while the reaction to produce hydrogen is as difficult as climbing a high mountain. If we are to ask why this is the case, it is thanks to our excellent and remarkable catalyst – in the entire reaction system, light serves as the driving force for the reaction; the reaction takes place at normal temperature and pressure, and no hydrogen is required. Ultimately, it was successful to convert wood into phenol, turning decay into something miraculous.