He wanted to use the canister as a booby trap for the Insecticons, which fits his Wreckers M.O. Or... he could have closed the container, picked it up, then Ground Bridge himself to the volcano, open the container and tip it upside down so that the Tox-En falls into the lava^magma. No need to set a booby trap since he'd be there and done long before the Insecticons could figure out where he was going and fly there. Unless he was afraid that the Insecticons could also Ground Bridge themselves after them, in which case the booby trap makes sense in terms of buying him time. But ITZTRU's idea of just holding it for a short amount of time while he dashes in and out of the Ground Bridge makes sense.