We're getting feedback from our clients that it's frustrating not being able to see all requests upon logging into their portal.
Is there a way we can pre-set the requests to display open and closed requests in the portal?
Unfortunately, you cannot configure a default view or filter as you described. There is a feature request at the Atlassian public issue tracker where you can vote on and add yourself as a watcher to receive notifications from now on.
Alternatively, you can bookmark the URL. Not pretty, but it does the job. You could have something like: