I don't see silica working in this application, but I may be wrong. If you can purify by ion-exchange, that might be easier. A common trick in purifying amino acids is to use Dowex-50 in the H+-form and then elute with 2-3 M ammonia. Dowex-1 in the acetate form is another possibility. I have used both, but if your compound has a large, hydrophobic portion, polystyrene-based exchangers can be problematic. There are some more exotic variations.