Many plant materials contain the polysaccharide hemicellulose, a polymer of sugars containing five carbon atoms each. When heated with sulfuric acid, hemicellulose undergoes hydrolysis to yield these sugars, principally xylose. Under the same conditions of heat and acid, xylose and other five carbon sugars undergo dehydration, losing three water molecules to become furfural:
C5H10O5 → C5H4O2 + 3 H2O
For crop residue feedstocks, about 10% of the mass of the original plant matter can be recovered as furfural. Furfural and water evaporate together from the reaction mixture, and separate upon condensation. The global production capacity is about 450,000 tons as of 2004. China is the biggest supplier of furfural, and accounts for around half of global capacity.
In the laboratory, synthesis of furfural from corn cobs takes place by reflux with dilute sulfuric acid.