How is sodium bicarbonate bonded leather

Or do you mean bound leather like in furniture?

Bound leather is ground dermis dust like sawdust, then polyurethane is added, which is applied under high pressure to artificial materials with a leather look. Nobody can tell from the outside that it is not leather. It is much cheaper than leather and is satisfactory for most purposes. You wouldn't use it as a book cover, however, as it wouldn't wear as well and would cost almost as much.

Bonded leather is made of real leather. As the name suggests, however, the cover consists of leather parts that are bonded together with adhesives, plastics (often polyurethane) or other adhesive materials. Once glued, the leather can be dyed, cut and embossed to look like real leather. The appearance is like real leather. Because it is not made from a single sheet of leather, bonded leather can be cheaper and less durable than real leather.
Bible bindings - Christianbook.com