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Feedstocks for the 21st century
If we want to seriously get away from using petroleum, there are a bunch of different hurdles we need to overcome, and I tried to explain how fundamental crude oil and its downstream products are previously in the newsletter. There are some really difficult problems in displacing petroleum centered around cost, efficiency, infrastructure, and transport as well as fundamental chemical transformations. Let’s take a minute to look at what viable alternative feedstocks we can readily access.
Fundamentally, crude oil is just really old biomass, like fossil old (get it?). Over time this biomass lost a lot of its oxygen, and we got mostly a mixture of aliphatic and aromatic hydrocarbons, and this is the crude oil we search and pump from the ground now. Theoretically, we could just bury some biomass and wait a few million years and then we’ve got more crude oil, but no one has that kind of time right now.
The real magic trick would be to use the biomass we have available to us now as opposed to waiting. What we do have in abundance right now:
Cellulose, comes with lignin and hemicellulose, and it’s the most abundant biomacromolecule on the planet.
Chitin, the stuff shrimp and lobster shell are made from, and typically comes with proteins and calcium carbonate.
Lignin, the most abundant source of aromatic (phenol/benenze type molecules not aromatic like when you are toasting spices)
Chitin, a glycoaminoglycan that is very similar to cellulose, is a topic for another day. I want to spend some time explaining cellulose and lignin and how we already have some fundamentals for how a bioeconomy might actually function.
Bioeconomy Version 1.0
Back before we had synthetic polymers we had biomass derived stuff like glue made from eggs and overcooked rice, pigments derived from red cabbage or beets, paper, leather, and bees wax. Chemicals during this era (pre-petroleum refining) were primarily used to convert biomass to usable stuff. Tanning chemicals were used to convert animal hides into leather to make things like shoes, boots, leather chaps, saddles, reins, (when is 1923 coming back?). Chemicals like “lime” or “soda” were basically crude version of sodium hydroxide and formed the basis for what we call “soda pulping” today. Essentially, cooking lignocellulose such as wood chips or straw, in an alkaline solution allowed for separation of the lignin from the cellulose. Modern wood pulp plants mostly use a modified version of soda pulping called “kraft pulping” which includes sodium sulfide. If you’ve ever wondered where the term “Kraft Paper” comes from, well don’t say I’ve never given you anything.
We still use wood pulp in abundance to make stuff like paper notebooks, toilet paper, paper towels, and the cardboard that protects your Amazon packages. Kraft pulping uses a mix of chemicals referred to as “white liquor,” to separate the cellulose, lignin, and hemicellulose. The pulp (cellulose) gets isolated, and the lignin and hemicellulose get separated in what is referred to as “black liquor.” There is about 15% organics in the black liquor with a bunch of spent pulping chemicals and water that can be recovered in what is called a “recovery boiler.” A nice breakdown on the uses of recovery boilers from Wikipedia (if the paper experts in the audience want to provide more details/facts please do so in the comments):
Combustion of organic material in black liquor to generate heat.
Reduction of inorganic sulfur compounds to sodium sulfide, which exits at the bottom as smelt
Production of molten inorganic flow of mainly sodium carbonate and sodium sulfide, which is later recycled to the digester after being re-dissolved
Recovery of inorganic dust from flue gas to save chemicals
Production of sodium fume to capture combustion residue of released sulfur compounds
I’m not here to wax poetic about recovery boilers though, but rather to point out that right now the best use for black liquor is to be burned (ideally when solids are >20%). The renewable material academics (shoutout to my ACS CELL people) have been trying to figure out better ways to utilize cellulose and lignin respectively for decades. Cellulose has some obvious structural benefits in that it gives wood strength and we build a bunch of stuff out of wood, but lignin is a tougher nut to crack so to speak.
In a sense, Origin Materials is similar to a pulp mill, but instead of isolating cellulose pulp through an alkaline process they use acid to get chloromethyl furfural. They still have a bunch of lignin type stuff left over and you can check out their recent press release on what they call hydrothermal carbon.
Cellulose, Crystalline Cash
If you are a long-time reader here then you know one issue of the bioeconomy and synthetic biology is the cost of feedstocks, mainly sugar. In order to make sugar we often have to grow either corn, sugarcane, or sugar beets. From those crops we can refine sucrose or fructose, but we need to use farmland and modern agriculture to do it, and this means:
It’s expensive and involves GMO seeds, fertilizers, pesticides, and fungicides.
Farmland gets used to make chemicals/stuff as opposed to food for people.
Cellulose, at the monomeric level, is just glucose that’s been polymerized by nature. The the sugar that makes up cellulose is difficult to obtain though and it’s been a real challenge for cellulosic ethanol producers. Cellulose is about 83% crystalline and these crystalline domains make it difficult to isolate pure glucose and this means it’s difficult to get the sugars that microbes need to make ethanol. Craig Bettenhausen has a great article that details the struggles and history for C&EN:
Despite many attempts, cellulosic ethanol, often called second-generation or 2G ethanol, has proved much more difficult to produce at scale than its first-generation cousin made from starch and sugar. In the 2010s, a handful of ambitious companies spent hundreds of millions of dollars to build cellulosic ethanol plants in the US. Today the plants are all shut down.
Two of those companies were the partners Poet, an ethanol maker, and DSM, a specialty chemical firm. They started producing ethanol from corn stover in Emmetsburg, Iowa, in 2014. The facility was designed to make 95 million L per year. But it struggled right away with handling the feedstock and preparing it for enzymatic depolymerization into fermentable five- and six-carbon sugars.
Cellulases are the enzymes most of these companies try to employ to get the glucose so they can ferment it into ethanol. Cellulase stability and activity is one obvious problem, but I think ethanol as a fuel is kind of dumb. The economics are really difficult. The better play is to make something of higher value, so the economics aren’t so difficult. I’d rather see specialty and fine chemicals being made from glucose or sugar, but I think the people at Amyris would tell you that this isn’t easy either.
The promises of cellulose as a source of sugar are abundant primarily because if we can figure out how to get the sugar, we can grow lignocellulose in places where crops cannot typically flourish, or we can use agriculture waste such as corn stover. Land that isn’t really viable for growing food can probably grow grasses or other woody biomass that can be harvested for its cellulose.
If you want to see industrial biotechnology and synthetic biology become the future, then we need abundant feedstocks that are cheap and readily accessible for these companies to utilize. The technology that made crude oil viable was fractional distillation, catalytic cracking, and steam cracking.
We need similar technologies for lignocellulose in easier separation, transport, and downstream processing of cellulose into sugar. Also, we need to figure out what to do with all that lignin. We can’t just turn it all into carbon black.
Lignin and Some Uses
I’ve previously written about trying to use lignin and some of its issues here. Let’s break down the monomers first. Lignin is essentially a thermoset (infinite molecular weight/3D networked polymer) made from three different monomers: p-coumaryl, coniferyl, and sinapyl alcohols. The picture of them is below. If you are thinking that these molecules look like viable phenols for use in synthetic polymers or chemicals then you would be right, but the problem is they are all polymerized together. Similar issue to cellulose, but this polymer isn’t exactly crystalline.
I spent a lot of my professional time trying to figure out some uses of lignin straight from a pulp mill with minimal processing. Here is one highlight from a patent of mine that got granted in 2020:
In the synthesis of phenolic resins, the use of lignin as a phenol substitute would be ideal because it would allow for a sustainable alternative source of phenol and would mitigate the volatility of the global petroleum markets. While numerous references have demonstrated that lignin substitutions into phenolic resins are possible, it is difficult to use lignin as a drop-in phenol replacement due to its inherent macromolecular structure. Lignin substitution of phenol in phenolic resins can produce non-homogenous resins, resins that are too high in viscosity, and resins with curing profiles that are difficult to predict.
It is possible for lignin to be blended into phenolic resins. However, it cannot be blended at high levels. If too much lignin is blended into a phenolic resin, the product can behave as a higher molecular weight polymer that has poor reactivity with traditional phenolic resin crosslinkers. High amounts of lignin can have a detrimental effect on processing, curing kinetics, crosslinking, and other properties of the finished product. One property that can be negatively affected is viscosity, which can result in the resin no longer being useful as a mineral wool binder, wood binder, abrasive binder, or in other high performance applications. Furthermore, different types of lignin vary greatly with regards to carbohydrate content, ash content, and inherent structural diversity depending upon the source (hardwood, softwood, species of tree, type of pulping process, etc.). Different types of lignin can further require slightly different treatments in order to be utilized and can even vary when being produced at the same pulping mill.
The issue we kept running into was the high molecular weight of the lignin and part of our solution was just to cook our lignin in either acid or base in the presence of phenol, which allowed for higher temperatures than normally possible in water. High temperatures with a strong acid or base led to depolymerization of the lignin in phenol and a better end product for phenolic resin. This process used a bunch of heat and is akin to hitting lignin repeatedly with a sledgehammer until we kind of got what we wanted.
Even with this technology the use of lignin in phenolic resins was a challenge primarily because we had to jump through a bunch of steps to get a raw material that was sort of viable. We needed to find the absolute cheapest lignin available (almost always lignosulfonate), which in turn was quite smelly, and then we needed to treat it by investing time and energy (described in the above patent). The pilot plant where I ran any pilot trial with lignin used to tell me it was the smelliest thing they had ever worked with, and they had worked with some really smelly stuff.
The magic trick we need with lignin is to make it cheap to depolymerize and enable more functionality to be unlocked. The closer we can get to a true monomeric state the more likely there will be an opportunity to use it as a chemical. Getting our lignin monomers to look more like eugenol or phenol or ferulic acid means there are more uses for it in the real world.
Next week, I’ll write about MetGen and how I think they might be the ones to bring useful lignin depolymerization to market and how their enzymes could unlock enable the second-generation bioeconomy we have been waiting on for decades.
We need abundant and readily available cheap biobased feedstocks and from there we can build the foundation of a sustainable materials future.