Immobilized TiO 2 nanoparticles on carbon nanotubes: an efficient heterogeneous catalyst for the synthesis of chromeno[ b ]pyridine derivatives under ultrasonic irradiation

-CNTs) as an efficient heterogeneous catalyst under ultrasonic irradiation in water. The sustainable and economic benefits of the protocol are the high yields of products, short reaction time, simple work-up procedure, and use of a non-toxic and reusable catalyst.