Retrosynthesis of PHB

From a retrosynthetic point of view, propylene oxide (PO) and carbon monoxide (CO) are promising building blocks for the production of low-cost PHB (Fig. 13). 

CO is derived from a variety of feedstocks such as petroleum gas, fuel oil, coal, and biomass. The industrial scale production of PO starts from propylene, which is mainly obtained from crude oil. However, due to the high importance of this compound, many pathways from renewable sources have additionally been developed [54]. PP is converted to PO by either hydrochlorination or oxidation [55]. The use of chlorine leads to large amounts of salts as by-products, therefore oxidation methods are more important, such as the co-oxidation of PP using ethylbenzene or isobutene in the presence of air and a catalyst. However, this process is economically dependent on the market share of these by-products, thus new procedures without significant amounts of other side-products have been developed, such as the HPPO (hydrogen peroxide to propylene oxide) process in which propylene is oxidized with hydrogen peroxide to give PO and water [56, 57] (Fig. 14). 

Many researchers are investigating the conversion of these two feedstocks. It offers the possibility for a future technical process in which PHB is produced without side-products in an analogous way to the S3mthesis of poly(olefins). Moreover, the use of low-cost monomers from the established value-added chain generally avoids high-price renewable raw materials or discussion about food acreages. 

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