What is the Latin future participle for "be able (to)"?

Neither "possum, posse" nor "queo, quire" has a future participle in Latin. Nor does nequeo, nequire. I checked the Perseus Digital Library (which includes the Lewis and Short dictionary). So how would a Latin author write (say) "would (not) be able" in a sentence like "They were promising many things that they would not be able to bring about"? Multa promittebant quae efficere (poturi / quituri) non essent.