The physics of long range hypersonic flight requires such aircraft to operate at the very upper levels of the atmosphere. That requires specialist propellants and possibly a supply of on board oxider in addition to fuel. Basically a specialised rocket that takes off and lands horizontally. If regular flights ever do take place, does spewing thousands of tons a day of carbon dioxide, water vapour and nitrous oxides into the uppermost levels of the atmosphere seem like a good idea? That is where current technology will lead us.
An air breathing, liquid hydrogen powered vehicle might solve the most obvious pollution problems but there would still remain the problem of extreme altitude clouds forming. They might have a beneficial effect in cooling the earth slightly, but then we have no idea what other side effects might occur, especially if combined with a major volcanic eruption or two.
Perhaps it is time to take a step back and try to perfect a slower non (directly) polluting form of flight. If you can get from one side of the earth to another withing 24 hours, that should be quick enough. Anything more urgent will just have to be dealt with by a virtual presence teleconference.