Catalytic Performance Of Hydroxyapatite For Decarboxylation Of Itaconic Acid To Methacrylic Acid

Methacrylic acid is an important organic chemical.At present,methacrylic acid is produced commercially from fossil feedstock,suffering from complex process flow and environmental pollution.The decarboxylation of bio-derived itaconic acid is a green route to the synthesis of methacrylic acid.The development of highly effective catalysts in this route is of significance in both academia and application.Firstly,the catalytic performance of some solid catalysts including Ti O₂,Zr O₂,hydrotalcite,Mg O and hydroxyapatite were evaluated for the decarboxylation of itaconic acid to methacrylic acid.Among them,hydroxyapatite showed the best catalytic performance.The effect of preparation methods on the catalytic performance of hydroxyapatite was investigated and the results showed that the suitable preparation method was hydrothermal method.A series of hydroxyapatite catalysts with different Ca/P molar ratios were prepared by controlling the p H of the precursor solution and their catalytic performances were evaluated.The results showed that the hydroxyapatite catalyst with a Ca/P molar ratio of 1.58 showed the best catalytic activity.Based on the results of catalyst characterization,it is found that the increase of Ca/P mole ratio resulted in the following changes:（1）the preferred orientation of hydroxyapatite crystals gradually shifted to（211）plane from（002）plane;（2）the particle morphology changed,and the surface area and pore volume increased;（3）the density of acidic sites on the surface of hydroxyapatite gradually decreased while the density of basic sites gradually increased.The decarboxylation of itaconic acid is mainly affected by the surface acidity and alkaline of catalyst.The acidity and alkaline of hydroxyapatite surface with Ca/P ratio of 1.58 can facilitate the rapid isomerization and further reaction of itaconic acid.Meanwhile,hydroxyapatite can stabilize the monoanion of itaconic acid and improve the selectivity of methacrylic acid.The effect of reaction conditions on the catalytic performance of hydroxyapatite was investigated and the suitable reaction conditions were obtained as follows:a reaction temperature of 250℃,a reaction time of 2 h,a catalyst mass of 0.8 g,a N₂pressure of 2 MPa,and a molar ratio of Na OH to itaconic acid of 1:1.Under these conditions,the yield of methacrylic acid attained 61.2%at an itaconic acid conversion of98.7%.A possible reaction pathway to the synthesis of methacrylic acid via itaconic acid decarboxylation catalyzed by hydroxyapatite was determined.In order to get rid of the use of liquid alkali,some modified hydroxyapatite catalysts were prepared separately using ionic liquids[Bmim]Cl,[Bmim]Br and[Bmim]BF₄as a template and their catalytic performances were investigated.Based on the results of catalyst characterization and activity evaluation,it can be seen that ionic liquid[Bmim]BF₄as a template caused following changes:（1）provided fluoride ion to ion-exchange with hydroxyapatite and changed the composition of hydroxyapatite;（2）significantly improved the crystallinity of hydroxyapatite;（3）mitigated the agglomeration of hydroxyapatite and made the morphology and particle size more regular;（4）decreased the density of acidic sites and increased the density of alkaline sites,facilitating decarboxylation of itaconic acid to methacrylic acid.The effect of preparation conditions and reaction conditions on the catalytic performance of hydroxyapatite prepared using[Bmim]BF₄as a template was investigated.The suitable preparation conditions were as follows:a mass ratio of[Bmim]BF₄to calcium salt=0.2:1,a hydrothermal time of 2 h,and a hydrothermal temperature of 130℃.Without the use of liquid alkali,the conversion of itaconic acid was 97.3%and the yield of MAA was 54.7%at a reaction time of 2 h,a catalyst mass of 0.8 g,a reaction temperature of 250℃and a reaction pressure of 2 MPa N₂.