Hey there your friendly neighborhood ref tech. R600a and R290 are straight up bad ideas ( more than a wee bit flammable (isobutan / Propane) ) and although r744 is good ( CO2 ) it's pressure are nuts. ( the whole system needs to be made of steel/iron) and would not work well with most common split systems. So ya nice idea I am not sure what's next but these are not the answers in private residents.