VBA- Retrieving data from an access database by referencing a column in excel
I would suggest that instead of building such a large WHERE clause dynamically, and running into the 64K character limitation of a query's text, that you create a table and insert into it the data from Excel. Your SQL then becomes:

SELECT Sheet2.Material, Sheet2.MPN
FROM Sheet2 
INNER JOIN TempSelection
  ON Sheet2.Material = TempSelection.Material


which will work with large amount of selection. 

To achieve this, you should create the table TempSelection in Access - it only needs one column, Material of the same data type as the Sheet2 table (is that a linked table to Excel or an actual Access table that was imported from Excel? If imported, good; otherwise, I encourage you to import data instead).

Then your For...Next loop would now look similar to this...

db.Execute "DELETE FROM TempSelection;", dbFailOnError
Set rsTemp = db.OpenRecordset("TempSelection", dbOpenDynaset)
For Each r In Range("A2:A6768")
  rsTemp.AddNew
  rsTemp.Fields("Material").Value = r.Value2 'Or, r.Value if needed
  rsTemp.Update
Next r