This idea is reasonable, but there are a few very important caveats and notes that you must know before doing it:
-
Pressure does not significantly affect the decarboxylation reaction kinetics, so it should proceed at normal temperatures and times, even under pressure. With enough pressure, you may find you can get away with slightly lower temperatures, but there are severe limitations on that, as described below. Of course any pressure applied should be devoid of reactive compounds like oxygen, lest they react with your material, especially under pressure! Nitrogen (N2) is relatively inert and inexpensive, as it is very abundant in our atmosphere. Besides inert gases like argon, N2, (please don’t waste helium on this), the only other gases in the headspace above the reactants should be the vapors of the terpenes and other volatile compounds naturally present in the resin.
-
Pressure DOES affect other reactions! Terpene decomposition and cannabinoid isomerization are the two most important for you to know about for this application. Decomposition and isomerization (and really all reactions) have a certain activation energy that must be reached above the steady state energy of the reactant materials, before they can react and come to rest at a state of energy lower than where they started.
For example, because Δ8-THC exists with lower potential energy than Δ9, the Δ8 is the naturally preferred conformation between the two. That said, Δ10, Δ10a, CBN, and many of the isomers & derivatives in between are lower energy than Δ9 or Δ8.
So, activation energy is an energy “hill” that reactants must climb before a reaction can proceed spontaneously, sliding down the hill, as it were. This energy can be provided in the form of heat (very commonly), pressure, light energy of effective interacting wavelengths and sufficient power (amplitude), stirring (mechanical energy), and other forms, depending on the type of reaction. -
A catalyst is a chemical that can reduce the activation energy for the desired end resulting rxn. To accomplish this, the catalyst may temporarily and reversibly interact or react with a reactant or even the solvent, thereby changing the starting conditions and thus activation energy of the reaction, but without permanently changing the catalyst, itself. Unless something “poisons” your catalyst, it will not be used up in the reaction, so very small quantities are needed. However, the rate of catalysis is usually proportional to the quantity of catalyst present… so more catalyst = more reacting happening at the same time.
One example is a Brönstead acid (H+ or proton donor) is a catalyst for isomerization of Δ9-THC to Δ8-THC. This is so common that many folks believe acid is required for isomerization to occur, but it most certainly is not!
Lest we forget, many reactions that are commonly done by catalysis can still be done without a catalyst, just by applying sufficient energy. Pressure can give the resin compounds that extra boost they need to isomerize or decompose. So be wary! -
Catalysts can be used to your advantage for the decarboxylation reactions of cannabinoid acids, without affecting (or even hopefully increasing) the activation energy of undesired rxns. As long as your crude or solution thereof is totally devoid of water (anhydrous), I usually recommend dry magnesium oxide (MgO) as a decarboxylation catalyst, though other alkaline earth metal oxides and some transition metal oxides will also work, to varying degrees. Since decarboxylation is a pseudo-first-order rxn, the catalyst lowers the required temperature to achieve a given rate of decarboxylation. For example, based on my observation, just a teaspoon or less of MgO per liter of crude resin appears to let the THCa and other cannabinoid acids decarboxylate at a rate typically seen at about 280-300°F, at only 180°F.
MgO has the ancillary benefit of neutralizing acids present, even under the requisite anhydrous conditions, thereby effectively eliminating some or all of the plant acids present that could otherwise catalyze isomerization. However, that benefit uses the MgO as a reactant, not a catalyst, so if there is too much heat-ionizable acid present in your crude, it will “poison” (use up) your MgO decarboxylation catalyst.
I think you will find decarboxylation catalysis “essential” (heh) to retaining your terpenes, with or without inert gas pressure.
In any case using MgO, you should be forewarned that adding too much MgO to your crude will cause it to coalesce, turning the MgO into a blob of concrete at the bottom of your decarboxylation flask or beaker, even with stirring. So err on the side of caution and only add that maximum of 1 teaspoon per liter of crude!
@McWest I am fairly certain that for a given temperature, CO2 is more dense than N2… though I may just assume that based on the fact that CO2 is heavier than air.
Ultimately, I think catalysis to lower your decarboxylation temperatures will be the most crucial implement toward terpene retention in the resin. Helium is very inert, but it is also a valuable resource that is ever-depleting from our planet until we perfect controlled nuclear fusion of hydrogen for energy production. N2 will work just as well for this purpose.
I think a sealed atmospheric pressure blanket of N2 devoid of oxygen should suffice, since I have heard that even 10 psi can jumpstart isomerization. Any additional pressure should be provided by the partial vapor pressures of the volatile compounds in the resin at decarboxylation temperature. If you do decide to use N2 pressure above atmospheric, please let me know if it caused any isomerization… this would be indicated using HPLC standardized with Δ8-THC as well as the usual Δ9-THC, but even a “before and after” test of just the Δ9 would tell you if the amount was diminished or not. Good luck! 