We have found that (Z)-nitro olefins or mixtures of (E)- and (Z)-compounds can be converted to the pure (E)-isomers by treatment with catalytic amounts of triethylamine or polymer-bound triphenylphosphine, respectively.
Stanetty, P., & Kremslehner, M. (1998). Synthesis of (E)-nitro olefins by isomerisation of (Z)-nitro olefins with polymer-supported triphenylphosphine. Tetrahedron Letters, 39(8), 811–812. https://doi.org/10.1016/S0040-4039(97)10789-4