1) it takes co2 from the air and water to produce propane hydrocarbons (mostly theory with some promising lab results)

2) not quite. Well, it is a form of carbon capture, but with the intent of releasing again soon.

3) 4% co2 is everywhere (open a window and turn on a fan and you have almost infinite). Water is everywhere humans are. I think it is a lot more an engineering problem than a feasibility problem

As you said the benefit of combustion fuels is the energy density. Air isn’t dense. 4% of not much is… not much.

So how much air do you have to move to put a day worth of propane into a car?

It just doesn’t seem feasible to me at a home but I’m not a chemist or physicist. At industrial scale it works but involves a lot of fans and energy.

Good questions. From a quick Google search, direct air capture of carbon will supply 50kg a month from a residential site.

https://climeworks.com/subscriptions

Those supply you with up to 50kg of carbon a month from DAC (direct air capture). If I were to drive a sedan 120 miles (entire day commute) then I would probably burn 250kg of propane and only 80% of the weight of propane is carbon.Therefore, you are exactly correct that it is not feasible. However it may become feasible if I get creative: hybrid work and different fuel sources.

So for me to be perfectly carbon neutral I would need an average total daily commute of 30 miles to be able to have 100% of my fuel usage from solar powered propane.

I think this is some tech in infancy. I think I would be willing to install a o2 tank and a c3h8 tank on my car to have better efficiency if I had to produce all my own fuel.

Fun line of thinking.

That link appears to reference industrial carbon capture. I don't see a residential unit there or any claim of 50kg per month. A half acre facility isn't feasible in a residential setting. The site seems to sell carbon offsets (credits) for sequestration, which is nice but isn't residential carbon capture and fuel synthesis.

I just don't see how this is feasible at a residential scale. Certainly not with the convenience of solar plus battery storage.

Carbon capture and fuel synthesis is very exciting technology, especially when combined with solar for load shifting. Generate "clean" fuel from excess sunlight. But even then it doesn't scale as well as direct transmission. Combustion fuels will have to be reserved for those cases that really need the energy density, like aviation. Battery electric is far more practical for self-sufficient personal transportation.

It's 0.04% not 4%.