Ionic liquid-assisted preparation of hydroxyapatite and its catalytic performance for decarboxylation of itaconic acid

Abstract

The synthesis of methacrylic acid from biomass-derived itaconic acid is a green route, for it can get rid of the dependence on fossil resource. In order to solve the problems on this route such as use of a precious-metal catalyst and a corrosive homogeneous alkali, we prepared a series of hydroxyapatite catalysts by an ionic liquid-assisted hydrothermal method and evaluated their catalytic performance. The results showed that the ionic liquid [Bmim]BF4 can affect the crystal growth of hydroxyapatite, provide fluoride ion for fluorination of hydroxyapatite, and adjust the surface acidity and basicity, morphology, textural properties, crystallinity, and composition of hydroxyapatite. The [Bmim]BF4 dosage and hydrothermal temperature can affect the fluoride ion concentration in the hydrothermal system, thus changing the degree of fluoridation of hydroxyapatite. High fluoride-ion concentration can lead to the formation of CaF2 and thus significantly decrease the catalytic performance of hydroxyapatite. The hydrothermal time mainly affects the growth of hydroxyapatite crystals on the c axis, leading to different catalytic performance. The suitable conditions for the preparation of this fluoridized hydroxyapatite are as follows: a mass ratio of [Bmim]BF4 to calcium salt = 0.2:1, a hydrothermal time of 12 h, and a hydrothermal temperature of 130 °C. A maximal methacrylic acid yield of 54.7% was obtained using the fluoridized hydroxyapatite under relatively mild reaction conditions (250 °C and 2 MPa of N2) in the absence of a precious-metal catalyst and a corrosive homogeneous alkali.

1. Introduction

Modern industrial production of polymers relies heavily on non-renewable resources; polymer production has consumed about 7% of the world's fossil resources up to date [1,2]. Methacrylic acid (MAA) and its derivative methyl methacrylate (MMA) are important monomers, and their price increases year by year due to the wide market demand [3]. At the present, the main commercial production of MAA relies on the acetone cyanohydrin process and the oxidation of isobutene, in which the acetone cyanohydrin process occupies the highest market share [4]. However, this process is not only based on fossil resources but also requires the use of highly toxic feedstocks and corrosive acids. At the same time, this process suffers from complex technology, expensive equipment, and generation of many low-value by-products. The gap between supply and demand of MAA and the serious defects in the production process drive the industry and academia to seek new and green synthesis processes continuously. Nowadays, developing a new process that is based on renewable resources is an effective way to achieve resource and environmental sustainability [5]. Thus the synthesis of MAA by decarboxylation of itaconic acid has aroused great interest [6].

Carlsson's group [7] achieved the decarboxylation of itaconic acid to MAA with a yield of 72%, using NaOH catalyst and water solvent at 360 °C and 34.5 MPa. However, severe reaction conditions and the use of homogeneous alkali restricted the development of this process. In order to lower the reaction conditions, Notre’ group [8] used 5% (mass) Pt/Al2O3 and NaOH as the catalyst, achieving an MAA yield of 68% at 250 °C. Although the reaction conditions were lowered, the use of precious metals significantly increased production cost, and additionally, the use of NaOH also made the post-treatment of the reaction mixture more difficult. Pirmoradi et al. [9] prepared MAA by the decarboxylation of itaconic acid, citric acid, and 2-hydroxy-isobutyric acid, using acid-base bifunctional hydrotalcite instead of homogeneous alkali without using precious metals. At the subcritical water temperature of 275 °C, the yield of MAA could reach 76% from the decarboxylation of 2-hydroxy-isobutyric acid. However, the MAA yield was only 23% under the optimum conditions as for the decarboxylation of itaconic acid. Compared to heterogeneous catalysis, homogeneous catalysis often shows higher reaction specificity and catalytic efficiency. Lansing et al. [10] used the homogeneous catalyst [Ru(CO)2(CH3CH2COO)]n to the synthesis of MAA by decarboxylation of itaconic acid with carbon tetrachloride as the cosolvent. The selectivity of MAA reached 84% at a reaction temperature of 225 °C, a reaction pressure of 2.8 MPa, and a reaction time of 1.5 h. However, the conversion of itaconic acid was only 42%, and the use of homogeneous catalyst made the separation of product difficult. Bohre et al. [11e13] reported several studies on the preparation of barium hexaaluminate catalysts.