“…In the literature, many works have been approved, and prepared heterogeneous catalysts are used for the synthesis of pyrano-[3,2-c]chromenes under heating and in the presence of solvents notably: rare earth perfluorooctanoates [RE(PFO) 3 ] [31], MgO nanoparticle [32], High Surface Area MgO [33], H 14 [NaP 5 W 30 O 110 ], H 6 P 2 W 18 O 62 ⧠18H 2 O [34], Nano aluminum oxide, Nano aluminum hydroxide [35], nanostructured diphosphate Na 2 CaP 2 O 7 [36], α-Fe 2 O 3 nanoparticles [37], sulfonic acid functionalized silica (SiO 2 PrSO 3 H) [38], silica-bonded N-propylpiperazine sodium n-propionate (SBPPSP) [39], magnetic catalytic system ([Îł-Fe 2 O 3 @Hap-Si-(CH 2 ) 3 -AMP] [40], polymer supported sulfanilic acid [41], ZnAl 2 O 4 -Bi 2 O 3 composite nanopowder [42], Fe 3 O 4 @SiO 2imid-PMAn nanoparticles [43], CuO nanoparticles [44,45], Bi 2 O 3 nanoparticles [46], ZnO nano-particles [47], polystyrene supported 7-methyl-1,5,7-triazabicyclo [4.4.0]dec-5-ene PS-TBD [48], nano-SiO 2 [49], dehydroabietylamine-cinchonine-squaramide [50], Zn 2 SnO 4 nanoparticles [51], Zn 3 (PO 4 ) 2 â§4H 2 O [52].…”