M-keyed M.2 slots electrically support both PCIe and SATA drives. The BIOS determines whether those drives are recognised/mountable/addressable/accessible/bootable. Thus any particular M.2 slot can support either SATA drives, PCIe drives, or both — but it depends on the BIOS. What speeds can be achieved is ultimately limited by circuitry.
In this case M2_1 is hardwired to 4 dedicated/exclusive PCIe lanes, so installing a SATA drive in it would simply waste 3 lanes. Although it may be possible to put a SATA3 drive into M2_1, doing so just doesn’t make sense — it’d be like riding a scooter on the Autobahn. Maybe if you were upgrading an old motherboard and had a bunch of old SATA3 M.2 SSDs lying around… but that’s the only scenario I can think of where it could possibly make sense, and I don’t think that’s the OP’s situation. I think he’s already got something in M2_2 and is looking to populate M2_1 with something “appropriate”.