Firstly, note that this is not a duplicate of Acid in Imine Formation. Clayden, Warren, & Greeves (2012) states explicitly on p. 231 that imine formation requires acid catalysis, and that the reaction takes place optimally under slightly acidic conditions, close to neutrality. This makes sense as we need to form the $\ce {-OH2^+}$ allowing water to leave. However, if we look at this list of literature, we would observe that most of the imine formation reactions do not necessarily require acidic conditions and some even take place under solely basic conditions. How do we explain this?
Reference
Clayden, J., Greeves, N., & Warren, S. (2012). Organic Chemistry (2nd ed.). New York : Oxford University Press Inc.