I want to display entitlements name that are in a role but no longer in entitlement catalog

After the above line you can use below code snippet to check if the entitlement value is null or not null. If not null, then your entitlement exists in the system else it does not exist.

List valueList = (List) ((LeafFilter) cons).getValue();
                       
                        for(String entName:valueList)
                        {
                        	if(property!=null @and entName!=null)
                            {
                            	ManagedAttribute ent=ManagedAttributer.get(context, application, property, entName);
                            	if(ent==null)
									//Does not exist in the system;
								else
									//Exist in the system
                            }
                        }

Let me know if further help is needed.

Thanks