Is there a passive for the sentence "Be quiet."?
Solution 1:
Only transitive verbs can form passive constructions and be is not a transitive verb.
Solution 2:
Quiet is a predicate adjective, and almost all predicate adjectives are intransitive. Passive can only apply to a transitive predicate.
Be is an auxiliary verb, required to hold the tense for the predicate adjective; no auxiliary verb ever governs Passive. Indeed, the Passive construction uses be itself.
Solution 3:
It seems like "Silence would be appreciated." captures the meaning without being a command.