Speculating for Praxair
MayorSlay.com says "Because of the density of our residential neighborhoods, it is going to be difficult to find a location for Praxair within the City limits. So, we will be expanding our search."
Admittedly, I don't know how much they would cost to purchase or lease, but it seems like several parcels in the Union Seventy Center - which is zoned "K - Unrestricted" would be feasible. For example, 5441 Brown Ave and several adjacent parcels just south of the railroad tracks and the Pepsi bottling plant are vacant and still owned by the developer of the business park.
Even if that's not an available site, several parcels are available within the Center, managed by Clark Properties.
While this would still be pretty far from downtown, it's not contaminated with nuclear waste like (part of) the McDonnell Blvd. site, and not flood-prone like Carondelet Coke.
Plus it's the equivalent of three blocks away from even the closest residences, across Natural Bridge in the Wells-Goodfellow neighborhood.
Why not try to find Praxair a site in an existing business/industrial park within the city limits? Maybe other (more influential) tenants would object, however.