Alkaline Earth Metal Modified NaY for Lactic Acid Dehydration to Acrylic Acid: Effect of Basic Sites on the Catalytic Performance

Abstract

Various NaY zeolites modified by alkaline earth metals (Mg, Ca, Sr, and Ba) were synthesized and used as catalysts for the dehydration of lactic acid to acrylic acid to investigate the effects of cationic species on the structure and surface acid/base distribution of the NaY zeolites. The important factors that affect their catalytic performance were also identified. The catalysts were characterized by N 2 physical adsorption/desorption, X-ray diffraction, temperature-programmed desorption of NH 3 , and temperature-programmed desorption of CO 2 . The results show that the basic sites that are induced by modified metallic species, particularly the medium basic sites, are mainly responsible for the formation of acrylic acid. The generation of acetaldehyde was affected by the acidic sites. Among the catalysts, 2%Ba/NaY showed the highest yield of acrylic acid at up to 44.6% (38% solution, LHSV = 3 h −1 , 325 °C) because it contained the most medium basic sites and a suitable Ba 2+ cluster character. 采用碱土金属 (Mg, Ca, Sr, Ba) 对 NaY 分子筛进行改性, 并采用氮气物理吸附、X 射线衍射、氨气及二氧化碳程序升温脱附等技术对样品进行了表征, 考察了阳离子物种对 NaY 结构、表面酸碱分布等的影响, 以进一步甄别影响催化剂活性的关键因素. 结果表明, 阳离子物种所引入的碱性位尤其是中强碱性位控制着丙烯酸的生成, 而表面酸性位则影响乙醛的生成. 当以钡含量为 2% 的 NaY 分子筛为催化剂, 在乳酸进料浓度为 38%, 进料空速为 3 h $ - $1 及 325 °C 反应条件下, 丙烯酸收率最高, 为 44.6%. 这归结于该催化剂表面具有最多的中强碱性位, 且表面 Ba 2+ 簇具有合适的 Ba–O 键长及碱度. The distribution of surface basic sites on NaY zeolites was effectively modified with alkaline earth metals and this is also adjustable by loading different amounts of cationic species. The medium basic sites (200–400°C) are mainly responsible for the generation of acrylic acid from lactic acid.