Attaching a 2x4 to drywall without studs can be challenging because drywall alone may not have the structural integrity to support heavy loads. However, if you need to temporarily hang something lightweight like a small shelf or picture frame, there are some methods you can try:
Anchors or Toggle Bolts: You can use anchors or toggle bolts designed for drywall. Drill an appropriately sized hole into the drywall, insert the anchor or toggle bolt, and then screw the 2x4 into the anchor. Make sure to choose anchors rated for the weight you intend to hang.
Adhesive: Some heavy-duty adhesives are designed to bond to drywall and can be used to attach a 2x4. Clean the back of the 2x4 and the area of the drywall where you want to attach it, apply the adhesive following the manufacturer's instructions, and then press the 2x4 against the wall firmly for the recommended drying time.
Hollow Wall Anchors: These anchors are designed for use in hollow drywall without studs. Drill a hole, insert the anchor, and then screw the 2x4 into the anchor.
Command Strips: For very lightweight items, you can use Command Strips, which are designed for damage-free hanging. While these are not suitable for heavy loads, they work well for small items.
Keep in mind that these methods are best suited for lightweight applications. If you need to support heavier loads or want a more secure installation, it's highly recommended to locate and attach the 2x4 to studs behind the drywall. You can use a stud finder to locate the studs, and then screw or nail the 2x4 directly into the studs for a strong and stable attachment.