Try using the Office.BeforeDocumentCloseNotification interface which is available for Excel only. It represents a modal notification dialog that can appear when the user attempts to close a document. The document won't close until the user responds. The notification dialog will allow the user to confirm the request to close the document or cancel the request to close the document. Following that way you will be notified when the workbook is closed.
You can find the list of available for Excel on the Work with Events using the Excel JavaScript API page.
Note, you can post or vote for an existing feature request on Tech Community where they are considered when the Office dev team goes through the planning process.