Board Thread:Questions and Answers/@comment-24499616-20140208162134/@comment-4957813-20140208204954

Ulquiorra was an Arrancar, which is technically a different species than a Hollow. Even if it was a Hollow, if it wasn't defeated by a Zanpakuto, it would have been obliterated. For Ichigo's Hollow form, I doubt that Kubo would've planned that far ahead for Hollow Ichigo to actually be the real Zanpakuto. So at the time of writing the fight, Ulquiorra was slain by a Hollow form.

Also, Zanpakuto only purify Hollows and send them to the afterlife. Arrancars are technically a different species and Kubo has not elaborated on what happens to them when they are slain.