(1) A: Where are you from? B: I'm from the U.S.
(2) A: When do you leave? B: I leave a week from tomorrow/July 7th/this coming Sunday.
Traditional grammar classifies where and when in (1) and (2) as adverbs but some modern grammars (such as The Cambridge Grammar of the English Language and Oxford Modern English Grammar) classify them as prepositions.
I was wondering if it would be possible to classify them as pronouns, in part because, as shown above, they seem to correspond to noun phrases such as the U.S., a week from tomorrow, July 7th, this coming Sunday, etc.
Hence the question: Does any grammar (book or theory) classify 'where' and 'when' as pronouns?
Although the above examples are interrogative clauses, relative clauses can also be discussed.
Traditional grammar's classification of where and when as adverbs is problematic because too many words whose POS are hard to classify are lumped together as adverbs. By the same token, I believe that CGEL and OMEG's classification of where and when as prepositions is also problematic because too many words whose POS are hard to classify are lumped together as prepositions.
So there may well be some grammarians/linguists who are trying to find the middle ground between the two extremes, hopefully.