Simple, Rapid, and Green Cannabinoid Isolation with Solid Phase Extraction

Typical processes for obtaining CBD isolate are extremely effective; but they are also slow, require large amounts of energy, and produce large amounts of toxic solvent waste that can be harmful to the environment. 

With the rising popularity of CBD products and increasing demand for cannabinoid isolates, there is a real need for a cannabinoid isolation workflow that saves on time while still being kind to the environment.

At the Analytical Cannabis Science of Cannabis Extraction Online Symposium 2022, Dr Thomas M. Attard of RX Extraction Ltd and Dr Robert McElroy of the University of York and Starbons Ltd introduced a novel approach to solid phase extraction (SPE) which may hold the answer. This new isolation method uses a Starbon® stationary phase to selectively physisorb desirable cannabinoids and is simple, fast, and does not require the use of energy-intensive machinery.

The problem with industrial CBD isolate

The extraction of CBD and other cannabinoids from hemp biomass is a relatively straightforward procedure. However, these cannabinoids also tend to be co-extracted with a plethora of other undesirable compounds, such as chlorophyll and plant waxes, and these require extra separation and purification steps to deal with.

In a typical CBD isolation workflow, this means separating out the fats and lipids from the cannabinoids using winterization, a method whereby the crude extract is dissolved in a polar solvent at sub-zero temperatures. Wiped film molecular distillation is also a popular method for cannabinoid oil purification.

“Winterization is an exhaustive time consuming step taking between one to three days to ensure that all the waxes and lipids are separated,” Attard explained. “Furthermore, a large amount of energy is also needed in order to chill the ethanol to very low temperatures.

“Wiped film molecular distillation is poor as well with respect to sustainability, as it requires considerable energy and generates large amounts of waste solvent effluence,” he continued.

As more labs look to follow green chemistry principles, there is an additional need for alternative methods of cannabinoid isolation to be developed. In their webinar, McElroy and Attard demonstrate how the use of bio-based mesoporous Starbon materials in a cannabinoid isolation workflow could remove the need for these winterization and distillation steps.

“The method is based on solid phase extraction, and can be used to replace winterization and distillation,” Attard explained. “We use renewable materials in our process, and it can be carried out in less than an hour. And that's at room temperature, thus saving on energy with regard to the heating and cooling of solvents.”

What are Starbons?

First developed at the University of York’s Green Chemistry Centre of Excellence, the Starbon name derives from the starch that was carbonized when the material was first discovered.

“We were contacted by a company that had an excess of starch that they wanted to find some application for that wasn't animal feed,” McElroy recalled. “We took the starch granules — which is quite a dense, compact material — and we made it gel to make them swell up.”

“You heat [the scratch granules] in water and you get this expanded material. These expanded granules have got lots and lots of little holes in them, we call them pores. And these pores are really interesting because they allow compounds to come in and out of the material.”

In this way, biorenewable mesoporous materials can be made from any natural polysaccharide, including pectin or alginic acid, at a very low carbon footprint. Depending on the polysaccharide feedstock and how it is processed, it is possible to create materials with tunable surface functionalities which can be deployed for various applications.

“For every kilo of orange juice that you've produced, there's a kilo of waste. And pectin is one of those compounds that can be extracted from that waste,” McElroy noted.

The greener choice for cannabinoid isolation

In the case of cannabinoid isolation, these mesoporous materials can be used as the stationary phase in a solid phase extraction (SPE) procedure.

In principle, a crude hemp extract can be made using CO₂ extraction (or another extraction method of choice), which is then dissolved in an absorption solvent and passed through a cartridge containing a Starbon stationary phase. The cannabinoids of interest will then be selectively physisorbed into the Starbon micropores, before a desorption solvent is applied to remove the isolated cannabinoids from the Starbon surface. This also means that the Starbon cartridge can be reused over many cycles.

“Choice of solvent is very important. In terms of the absorption solvent, the cannabinoids needed to dissolve in it, however they must have a higher affinity to the Starbon material than to the solvent,” Attard explained.

“With our desorption solvent on the other hand, the cannabinoids need to have a higher affinity to the solvent than the Starbon material so that [the cannabinoids] can be removed.”

In a paper recently published in Materials Chemistry Frontiers, the University of York-led research team demonstrated the use of the alginic acid-derived Starbon A300 with hexane and ethanol solvents in CBD isolation. When using this method in a scaled-up manner to demonstrate industrial viability, CBD recoveries of up to 97% were successfully achieved.

As evidenced in analyses by both gas chromatography flame ionization detection (GC-FID) and nuclear magnetic resonance (NMR), the cannabinoids were well isolated in the desorbed product with unwanted fatty acids, terpenes, and alkanes being effectively removed. This showed that it is possible to isolate CBD and other cannabinoids from crude extract without the need for winterization or distillation steps.

Also of note in the NMR analysis results, is that this methodology actually results in the isolation of cannabidiolic acid (CBDA), not CBD. Raw cannabis material contains mostly CBDA and other acidic forms of cannabinoids, which are only decarboxylated when the cannabis is heated during consumption or processing. In recent years, scientists have discovered several interesting therapeutic uses for CBDA, making it a sought-after cannabinoid to isolate in its own right.

“What's really interesting is that the conventional processes use an awful lot of temperature, you get decarboxylation,” McElroy said. “Now, what we can show from investigation of the NMR was that we didn't have the decarboxylated form, the acid group was still in place. So it's CBDA — not CBD — that we have isolated here.”

A potential one-pot extraction and isolation system

Given the importance of solvent selection to this method, the researchers also sought to identify a potential alternative to the non-polar hexane adsorption solvent that was originally being used. While the food grade ethanol solvent used for desorption is biodegradable, hexane carries more serious environmental risks. For this reason, the researchers also conducted an experiment using supercritical CO₂ as an alternative non-polar adsorption solvent.

“By doing this, we could potentially carry out CO₂ extraction as well as the purification of cannabinoids in one process, in a single system set up,” Attard explained. “This would negate the need for post-purification steps following extraction, which would reduce energy input as well as time. Solvent usage will also be reduced as the CO₂ solvent could be recycled in a closed system industrially.”

The experiment found that the majority of unwanted volatile components were removed using this method, though some fatty acids were still retained in the final desorbed product. However, with some tweaking to the experiment parameters or the Starbon material, the researchers believe that this could be improved.

“Unfortunately, [this system] has shown greater specificity towards these fatty acids with CO₂ as the adsorption solvent than it has for the cannabinoids,” McElroy said. “But this has proven that it is possible to get a cannabinoid-enriched fraction in one single step.”

In principle, a Starbon-enable SPE method could also have other applications outside of the cannabis sector, the researchers add. Cannabinoids have such a strong affinity with the Starbon materials because their chemical structures contain many diverse functional groups. But this is also true of other active pharmaceutical ingredients (APIs), and so there is the potential for similar methods using these materials to be a viable, green, alternative isolation procedure in other industries.