The specimen that were collected during the different exploration missions are preserved in different Belgian collections. They are stored in the Museum for Central Africa, The Royal Institute for Natural Sciences and the National Botanical Gardens of Belgium, among others. At this moment there has not yet been a consorted effort to make these collections digital available.
Some of the specimen collected can be searched through the DARWIN database of the Royal Belgian Institut for Natural Sciences. You can use the Country name "congo belge" when you visit the search page http://darwin.naturalsciences.be/search/search. At this moment the search gives a result of only 434 specimen, however it is certain that a lot af specimen still need to be digitalised. Also some of the collections were given to the Royal Museum for Central Africa and can be searched through their databases. These can be found here