Influence of Ca/P ratio on the catalytic performance of hydroxyapatite for decarboxylation of itaconic acid to methacrylic acid

Abstract

Methacrylic acid, an important organic chemical, is commercially manufactured starting from fossil feedstock. The decarboxylation of itaconic acid derived for biomass is a green route to the synthesis of methacrylic acid. In view of the problems existing in the researches on this route such as use of noble metal catalyst, harsh reaction conditions and low desired-product yield, we prepared a series of hydroxyapatite catalysts with different Ca/P molar ratios and evaluated their catalytic performance. The results showed that the hydroxyapatite catalyst with a Ca/P molar ratio of 1.58 had the best catalytic activity. The highest yield of MAA up to 61.2% was achieved with basically complete conversion of itaconic acid under the suitable reaction conditions of 1 equivalent of NaOH, 2 MPa of N2, 250 ℃, and 2 h. On this basis, a reaction network for the decarboxylation of itaconic acid to methacrylic acid catalyzed by hydroxyapatite was established. With the aid of catalyst characterization using X-ray powder diffraction, NH3/CO2 temperature-programmed desorption, N2 physisorption, inductively coupled plasma optical emission spectrometry, and scanning electron microscopy, we found that the distribution of surface acid sites and basic sites, crystal growth orientation, texture properties and morphology of hydroxyapatite varied with the Ca/P molar ratio. Furthermore, the change of the crystal growth orientation and its influence on the surface acidity and alkalinity were clarified.