NH3 sorption isotherms in ALPO-11 and its Si, Co, and Mn analogues

Equilibrium sorption capacities of different probe molecules such as water, n-hexane, cyclohexane, and ammonia were used as a tool for the characteriz ation of ALPO-11 and protonic forms of SAPO-11, CoAPO-11, and MnAPO-11. Sor ption uptake at P/P-0 = 0.8 and 298 K, 2h for water, n-hexane and cyclohexa ne, was found to be affected by incorporating Si, Co, and Mn in ALPO-11. Ge neration of Bronsted acidity is expected to increase the hydrophilicity; Si , Co, and Mn analogues have exhibited such trends depending upon the extent of substitution and characteristics of the substituted cation. The amount of ammonia retained irreversibly ton account of chemisorption) during sorpt ion measurements was found to be correlated with the intrinsic acidity poss essed by ALPO-11 and its analogues. Isotherms of ammonia sorption in ALPO-1 1, SAPO-1I, CoAPO-11, and MnAPO-11 in the temperature range 333-483 It up t o 500 Ton have been measured. The NH3 sorption capacity was also found to d ecrease in the order ALPO-11 > SAPO-11 > CoAPO-11 > MnAPO-11 . The sorption data were analyzed in terms of different models of sorption isotherm equat ions. BET, Dubinin, Langmuir, and Freundlich isotherm models could represen t the sorption data satisfactorily yielding linear plots. However, Sips equ ation failed to represent ammonia sorption data in these phosphate-containi ng molecular sieves. At constant coverage, the isosteric heat values have s hown the major contribution due to sorbate-sorbent interactions irrespectiv e of the chemical composition.