Plants get their CO2 from the air. So, not completely impossible. There have been attempts to extract CO2 from air: https://www.theverge.com/2021/9/9/22663597/l

Plants get their CO2 from the air. So, not completely impossible. There have been attempts to extract CO2 from air: https://www.theverge.com/2021/9/9/22663597/largest-direct-air-capture-plant-c02-climeworks-iceland https://www.theverge.com/2021/9/9/22663597/largest-direct-ai... While that's a pretty expensive way of carbon capture, it does seem to work. The question is of course how much energy it costs. It's probably quite a lot. Meaning that turning CO2 into methane is probably not a very efficient process. I.e. not a great use of clean energy.