There is no firm guideline for how to treat Crohn's with any particular ASA. Pentasa tends to release mainly throughout the entire small intestine, while Asacol tends to start to release in the terminal ileum, and then continuing mainly throughout the colon.
My daughter has Crohn's confined mainly to the terminal ileum, so theoretically, either Pentasa or Asacol could treat it. She tolerated the Pentasa well, but the Asacol left a very bad taste in her mouth. She plans to try Pentasa again, some day soon, because she didn't have a chance to try it again to see how well it worked.
Lialda, like any 5 ASA, is intended mainly to treat UC but it's really a toss up which ASA can help your Crohn's. At least it is just a once a day dose, so if it works, great! This stuff is all trial and error...