Can Bisphenol A Be Replaced By a Derivative of Vanilla? [View all]

[NNadir]

Recently I remarked in this space that one possible route, maybe the only route, to fixing carbon dioxide from the air - something that will prove necessary for future generations since our generation did nothing whatsoever to address climate change - will involve making products from biomass that effective sequester carbon.

The difficulty of taking a lab route for usefully fixing CO2 to an industrial level.

In recent years there have been many discussions of this approach in the scientific literature, and I came across an interesting paper that puts a fun - and interesting - spin on the topic.

The paper is here: Synthesis and Characterization of Bio-based Epoxy Resins Derived from Vanillyl Alcohol (Joseph F. Stanzione III, ACS Sustainable Chem. Eng., 2016, 4 (8), pp 4328–4339)

Many people are aware that there is growing concern that many plastics, in particular polycarbonates, are co-polymerized with bis-phenol A, which can leach out of the plastics and is a known endocrine disrupting chemical owing to some structural features making it similar to some steroidal compounds in the estrogenic pathway. Polycarbonate plastics are the tough, hard plastics commonly used for water storage bottles, baby bottles, etc.

An excerpt from the text of the paper is here:

Thermosetting polymers, such as epoxy, vinyl ester (VE), and unsaturated polyester (UPE) resins, have found utility in a wide range of industrial and commercial applications including adhesives, coatings, and composites.(1, 2) Epoxy resins dominate the thermosetting polymers market making up roughly 70% of all thermosetting polymers,(3) due to their outstanding thermomechanical properties comprising high glass transition temperatures (Tg’s) and high glassy moduli (E′’s) at 25 °C as well as good chemical resistance, when polymerized with an appropriate curing agent.(1, 2, 4) Unfortunately, the majority of commercial thermosetting polymers currently being produced are synthesized from nonrenewable, petrol-based chemicals. Since the inception of the first commercial diglycidyl ethers in the 1940s, the epoxy resin industry has been dominated by the petrochemical-based diglycidyl ether of bisphenol A (DGEBA).(1, 2, 5) This bisphenol A (BPA)-based epoxy resin is found in over 90% of thermosetting epoxy resins worldwide, in a market with a global production currently exceeding 2 million tons per year.(3)

DGEBA is a product of two main reactants, BPA and epichlorohydrin, with epichlorohydrin, historically synthesized via a multistep pathway starting with propylene.(1, 2, 6, 7) There is currently no renewable source for BPA; however, Dow Glycerin to Epichlorohydrin (GTE) Technologies and Solvay Epicerol have recently reported processes for the synthesis of epichlorohydrin from bio-based glycerol, a byproduct of biodiesel production.(8, 9) BPA (4,4′-isopropylidenediphenol) is typically synthesized via an acid catalyzed electrophilic aromatic condensation of phenol and acetone with a stoichiometric ratio of 2:1, yet the process uses large excesses of phenol to reduce the formation of higher molecular weight oligomers.(10, 11) BPA is used as the base molecule in thermosetting epoxy resins, as the bisphenolic structure provides molecular rigidity to the polymer network; thus, promoting their outstanding thermomechanical properties.(11) However, the use of BPA in epoxy resins has received a great deal of scrutiny and debate, while concerns of human exposure to BPA, a known human endocrine disruptor, via leaching from resins and food and beverage can coatings are driving the search for a suitable alternative that is both renewable and nontoxic.(3, 12)

The Dow process for producing epichlorohydrin from glycerol, a generally worthless side product of the production of biodiesel and soap, is described here:

Glycerin as a Renewable Feedstock for Epichlorohydrin Production. The GTE Process (Briggs et al Clean 2008, 36 (8), 657 – 661)

The idea is to take a "generally worthless" product and make it worth something. Polymers derived from glycerol are fixed carbon that is removed from the atmosphere.

Now for the fun part, the chemical whose flavor I love, vanillin, actually the alcohol made by reducing vanillin.

The authors write:

In the present work, we report the electrophilic aromatic condensation of vanillyl alcohol (1) with guaiacol (2) to produce bisguaiacol (BG) isomers (3). The reaction is given in Scheme 2, in which the major structural bisguaiacol isomer formed was determined to be para–para. The synthesis of bisguaiacol avoids the use of carcinogenic and highly volatile molecules like formaldehyde and acetone as the hydromethyl group present on vanillyl alcohol already provides the necessary handle and reactivity for facile and desired phenolic coupling and methylene bridge formation.

To produce a bio-based epoxy, BG (3) was then reacted with epichlorohydrin (4) to produce a diglycidyl ether of bisguaiacol (DGEBG; 5) as shown in Scheme 3. To study the influence of the methoxy moiety attached to the aromatic ring on cured polymer properties, diglycidyl ether of vanillyl alcohol (DGEVA) and diglycidyl ether of gastrodigenin (DGEGD) were also synthesized in the same manner as DGEBG (Figure 1). Furthermore, as DGEVA and DGEGD can be considered useful bio-based epoxies, diglycidyl ether of hydroquinone (DGEHQ; Figure 1) was also synthesized in order to study the influence of the methylene spacer between the aromatic ring and the glycidyl ether. The epoxy resins were cured, either by themselves or with a commercial BPA-based epoxy resin, with stoichiometric equivalents of Amicure PACM (4,4′-methylenebiscyclohexanamine; Figure 1). The thermomechanical properties of the cured polymers were tested via dynamic mechanical analysis (DMA) to determine if these resins are suitable alternatives to current commercially available petroleum-based resins.

For those who are interested in organic chemistry, here is scheme 2:




Here is scheme 3, regrettably not high quality as a graphic, but if you know some organic chemistry, you can fill in the blanks:



Recall that the authors are proposing to use epichlorohydrin derived from glycerol, so the molecules in scheme 3 are entirely derived from biomass.

Vanillin is of course available from vanilla beans, but it is mostly available on an industrial scale from the hydrolysis or thermolysis of lignin, the structural component of wood and straw that is not cellulose. (Many similar compounds are also obtained from the digestion of lignin, many of which will prove to have other uses.)

The authors report that the resulting polymers have excellent properties, comparable to the properties of the petroleum based thermosetting polymers.

There's a long way between bench top chemistry and commercial applications, but, nonetheless, it's interesting I think.

I wonder if the leachates will taste good. I personally love the vanilla flavor; can't get enough of it. I'm a vanilla kind of guy.

Have a nice evening.