Thesis in first link brings quite nice overview of already known possibilities. hope it can inspire you. Some other links are attached. It seem that urea or ethylenglycol are usually starting materials and catalysts are required.

https://mediatum.ub.tum.de/doc/997586/997586.pdf

http://pubs.acs.org/doi/abs/10.1021/ie0504553?journalCode=iecred

http://onlinelibrary.wiley.com/doi/10.1002/cben.201400020/abstract

ir.sxicc.ac.cn/bitstream/0/1961/1/2683-2687.pdf

thanks very much