Is a request to book a business flight ticket for a graduate student an unreasonable one?
In Canada it is illegal to get anything but economy fare (including trains) and charge this to any one of the national tri-Council grants (there are exceptions if there is no other availability but this is rarely the case.)

The best you can do is buy an upgradable economy fare and hope for the best, or pay for an exit-row seat.

Overall your request will be perceived not only unreasonable but also blatantly unreasonable, unless you have special conditions or a rich sponsor.

I can say more.  Some years ago a candidate for a position where I work insisted on getting a business-class ticket for the interview and he was just removed from the shortlist.  

Edit: is this a direct flight?  This wiki list of long flights https://en.m.wikipedia.org/wiki/Longest_flights does not include European-Asian city pairs.