Comparison of various catalysts used in synthesis of 5-phenyl 1H-tetrazole from benzaldehyde, hydroxylamine hydrochloride and sodium azide.

Entry	Conditions	Yield (%)	Ref.
1a	(NH4)4Ce(SO4)4·2H2O (20 mol%), DMF, reflux, 5 h	72	10
2a	TiCl3 (20 mol%), DMF, reflux, 4 h	86	11
3a	Cu(OAc)2 (20 mol%), choline chloride–urea, 100 °C, 12 h	90	12
4b	PVA@Cu Schiff base complex (0.43 mol%), H2O, r.t., 7 min	98	13
5b	Cu-MCM-41 (30 mg mmol−1), DMF, 140 °C, 12 h	90	15
6a	P2O5 (200 mol%), DMF, reflux, 9 h	90	17
7a	Humic acid (0.1 g mmol−1), H2O, 100 °C, 4 h	92	This work

^aCommercially available catalyst.

^bCommercially unavailable catalyst.