MCM-41 grafted with citric acid: The role of carboxylic groups in enhancing the synthesis of xanthenes and removal of heavy metal ions

Abstract

Order mesoporous MCM-41 was synthesized via a sol-gel method and then grafted by adding the appropriate amount of citric acid (CA). The structure and morphology of pure and grafted mesoporous MCM-41 were investigated by XRD, BET, TEM, and FT-IR measurements. TEM measurements confirm that MCM-41 particles display an ordered structure with uniform mesoporous arranges into a hexagonal, honeycomb-like lattice and still preserved their mesoporous structure after grafting with different weight contents of CA. The pore sizes and volume of the CA/MCM-41 samples were found to vary markedly within CA contents. The surface acidity and the type of acid sites (Brønsted and Lewis) were determined using the potentiometric titration method and the FTIR of pyridine adsorption. In this work, it was found that the textural, acidic properties and the activities of catalysts could be improved by citric acid treatment The results illustrated that the surface acidity and acidic strength of CA/MCM-41 gradually increased with increasing the CA amount up to 50 wt.%. Also, the prepared catalysts showed both Brönsted and Lewis acid sites and the number of Brönsted acid sites increased largely compared with Lewis acid sites with increasing the CA content which indicates the role of carboxylic groups in enhancing the Brönsted acid sites. The catalytic performance of the samples was tested by the synthesis of 14-Phenyl-14H-dibenzo [a, j] xanthene (xanthene). In this reaction, the sample with 50 wt.% of CA displayed the highest acidity and catalytic activity. The adsorption activity was examined for removing Cd2+ and Pb2+ from aqueous solution with varying parameters such as pH, concentration, temperature and time. The equilibrium adsorption data obtained fitted with the Langmuir model and the adsorption kinetics favored the pseudo 2nd order model. Also, the adsorption was spontaneous and exothermic.