Liquid is not the way to go, period. They are pretty much all going to contain extra pollutants compared to stored energy or gas based fuels. If you lift these restrictions and just focus on what is more environmentally friendly, that can be utilized to replace liquid fuels, I can think of 3 options, but they all have trade-offs.
1 - Electric: As long as you collect your power from a force of nature such as solar, wind, hydroelectric, or geothermal, you get virtually no emissions from use. However, in most cases, electric cars are powered from the main power grid which mostly burns fossil fuels anyway and could even be more harmful since so much power is lost in the process of converting kinetic energy to electricity, then transferring it through miles of cabling, then storing it to batteries which drain a bit while waiting to be used, then back to kinetic. Batteries also make electric cars heavier meaning it takes more energy to move them. Also, the batteries are made from highly toxic materials which need to be occasionally replaced. The toxicity of battery mass production and use could in some ways be just as bad as vehicle emissions.
2- Hydrogen Fuel: This has a lot of the same advantages and disadvantages as electric. Pure Hydrogen does not naturally exist in this world. The processes to refine it require more power than you get form fueling your car with it meaning, if you don't refine it using renewable power, you're wasting your time. Also, pressurized hydrogen can be pretty dangerous stuff. The plus side is that hydrogen powered cars use less toxic materials to make than electric, but the needed materials are still rare which limits mass production.
3 - Methane Fuel: While technically this is a hydrocarbon and not a liquid fuel, this might actually be your winner. It's way more environmentally friendly than other hydrocarbons because it only burns into CO2 and H2O without releasing any of the much more harmful CO or NHx gases and it can be stored and transferred between tanks much the same way that liquid fuels are stored. Methane is also a renewable resource which can be gathered as a byproduct of agricultural waste and landfills. This means that you could renewably supply a lot of the worlds needs within the natural carbon cycle making it just as carbon neutral as solar despite having a burning emission. Once in the atmosphere it will safely decompose into CO2 and H2O just like it would have naturally if you never collected it. Also, a methane engine can be made from roughly the same materials as a traditional car meaning you can mass produce them just as cleanly. Lastly, methane does not need to be shipped or pumped. Because gasses dissipate, you can transfer them over great distances using passive forces and underground pipes. By building this infrastructure up, you could virtually eliminate the carbon footprint of distribution
As an added bonus, methane stored in a fiberglass tank is less dangerous than petroleum in an accident. Methane only burns under certain pressures and oxygen saturations meaning if the tank ruptures, the fiberglass won't fragment and the gas will dissipate too much before it mixes with enough O2 to burn, so no risk of fire or explosions.
As for tradeoffs, methane needs to be compressed first. This requires a fair amount of extra energy right now, but recent advancements in multi-stage emulator systems have shown a lot of promise in the direction of significantly reducing this overhead in future compression systems. The second concern is fracking. While methane is safe, many companies still insist on utilizing toxic chemicals to mine for it instead of utilizing the resources we already create ourselves.