Industrial Chemistry and Engineering
What's New in Phenol Production?
by Jeffrey S. Plotkin
March 21, 2016
In 2015, the world demand for phenol was ≈10 million tonnes. The largest end-use for phenol is in the manufacture of bisphenol A (BPA). Although it is under regulatory pressure for health and safety reasons, BPA is the key building block for making polycarbonate and epoxy resins.
The next largest use for phenol is in the production of phenol–formaldehyde (PF) resins. PF resins are used primarily in wood adhesives, for example, for bonding the layers of plies in exterior plywood. The global demand pattern for phenol in 2015 is shown in Figure 1.
In the Hock process (Figure 2), cumene (1, made by alkylating benzene with propylene) is oxidized to cumene hydroperoxide (2), which is then cleaved to a mixture of phenol (3) and acetone (4) by treating it with H2SO4.
The good news is that for every 10 kg of phenol produced, 6.2 kg of acetone is coproduced. The bad news is that for every 10 kg of phenol produced, 6.2 kg of acetone is coproduced. The process can be good or bad, depending on the petrochemical market.
“Two-for-one” processes sound good in concept, but they only work commercially if the markets for both products are changing at about the same rate. Otherwise, one product will be in oversupply, which causes its price to fall and penalizes the economics of the entire process.
Very early in the chemical industry, phenol processes were “coproduct-free”, but these methods were deemed too expensive and have since been abandoned. The earliest phenol process, developed around the turn of the 20th century, was based on sulfonating benzene to benzenesulfonic acid, followed by fusion with NaOH. Unfortunately, large amounts of low-value Na2SO3 and NaHSO3 were coproduced.
A second phenol route was commercialized in 1924; it involved the direct chlorination of benzene to chlorobenzene, which was then hydrolyzed to the sodium salt of phenol with NaOH. This technology was last used commercially by Dow Chemical in the 1980s.
In a twist on this method, the Raschig–Hooker process, chlorobenzene was produced by the oxidative reaction of benzene with HCl. The chlorobenzene was subsequently steam hydrolyzed to give phenol and regenerate HCl, which could be recycled back to the beginning of the process. Like the original chlorobenzene method, this process is no longer in use.
DSM and Solutia
In the 1960s, DSM (then Dutch State Mines) developed a route in which toluene was oxidized to phenol. Economics of the DSM route depended more on the value of coproduced benzaldehyde and benzoic acid than on the cost of making phenol. The markets for these specialty coproducts are rather small, thus limiting the proliferation of the process. Three plants were built since the 1960s, but all have since been shuttered.
An interesting variation developed by Solutia in the 1990s was based on oxidizing benzene with N2O to give phenol and nitrogen (Figure 3).
Whereas the selectivity of the Solutia process was >95%, the difficulty was finding a low-cost route to N2O. Solutia, at the time a producer of adipic acid for making nylon 6,6, was in a unique position because N2O is a byproduct of adipic acid production. Solutia built a pilot plant to develop this route but never commercialized it. (Solutia eventually sold its nylon business to a private equity firm; in 2012, the remainder of the company was acquired by Eastman Chemical.)
More recently, ExxonMobil has been developing a three-step route from benzene to phenol that makes a coproduct, but not acetone. Instead, the ExxonMobil process coproduces cyclohexanone along with the phenol. The overall pathway is shown in Figure 4.
Step one is a unique hydroalkylation in which benzene and hydrogen are combined to give cyclohexylbenzene (CHB, 5). ExxonMobil says that this reaction proceeds via an initial hydrogenation of 1 equiv benzene to cyclohexene, which then alkylates a second equiv of benzene to CHB. Any overalkylated products such as dicyclohexylbenzene are transalkylated to give additional CHB. Any cyclohexane generated is dehydrogenated back to benzene and recycled. The reported yield is 97%.
The second step is the oxidation of CHB to phenylcyclohexyl hydroperoxide (6). The selectivity of the oxidation is improved by using the chain-propagating agent N-hydroxyphthalimide (NHPI). The third step is cleaving the hydroperoxide with H2SO4 to give equimolar amounts of phenol and cyclohexanone (7).
Cyclohexanone is a key intermediate for making nylon 6,6 and nylon 6. ExxonMobil reports that the yield of the final step is almost stoichiometric under optimal conditions. An integrated flow scheme of the three reactions is shown in Figure 5.
ExxonMobil has filed for more than 100 patents that cover improvements in all three steps. The past 2 months alone saw the issuance of six US patents or applications (see “Recent ExxonMobil Phenol–Cyclohexanone Patent Publications”). ExxonMobil has not indicated whether and when this process might be ready for commercialization.
As for the goal of an economic one-step, coproduct-free route to phenol, the search goes on.
Recent ExxonMobil Phenol–Cyclohexanone Patent Publications
Document No. Issue Date Title
US 9,233,889 Jan. 12, 2016 Hydroalkylation processes
US 9,233,890 Jan. 12, 2016 Process for producing phenol
US 9,242,918 Jan. 26, 2016 Dehydrogenation processes and phenol compositions
US 9, 260,387 Feb. 16, 2016 Process for producing phenol
US 20160001276 Jan. 7, 2016 Process for making alkylated aromatic compound
US 20160009613 Jan. 14, 2016 Process for making alkylated aromatic compound
Learn more about these patents and applications at the CAS Database.