稀土元素(Y、Nd、Ce、La)改性NaY分子筛催化乳酸脱水制丙烯酸

Research on molecular sieves NaY modified by rare earth element (Y,Nd,Ce,La) and their catalytic performance for lactic acid dehydration to acrylic acid

采用浸渍法以稀土元素(Y、Nd、Ce、La)对NaY分子筛进行改性,并以改性后的分子筛为催化剂,考察催化乳酸脱水制丙烯酸反应活性。通过NH3-TPD、CO2-TPD和XRD等对催化剂进行表征,结果表明,稀土元素进入NaY分子筛骨架,其中,La对分子筛晶体结构影响最小;随着负载稀土元素离子半径的增大(Y3+<Nd3+<Ce3+<La3+),总酸性位数量减少,除了重稀土元素Y,弱酸性位比例和中等强度碱性位数量增多,La改性最有利于丙烯酸的生成。稀土元素改性的NaY分子筛在一定程度上提高丙烯酸收率,抑制乙醛生成。以质量分数38%的乳酸为原料,在空速3 h-1、反应温度325℃和2%La/NaY分子筛为催化剂时,丙烯酸收率为54.2%,而未改性NaY分子筛上丙烯酸收率仅为34.7%。

NaY molecular sieves modified by rare earth metals （lanthanum, cerium, neodymium and yttrium） were synthesized by impregnation method and used as the catalysts for the dehydration of lactic acid to acrylic acid. The catalysts were characterized by NH3-TPD, CO2-TPD and XRD. The results demonstrated that the rare earth metals were introduced into NaY molecular sieve framework and La had the least influence on the crystal structure of the catalyst. The amounts of total acidic sites of RE-modified samples decreased,but the ratios of weak acidic sites and the amounts of medium basic sites increased with the increase of ionic semidiameter of loading rare earth elements （ y3 ＋〈 Nd3 ＋〈 Ce3 ＋〈 La3 ＋ ） except the element Y. La modification was beneficial to higher selectivity to acrylic acid. RE-modified samples enhanced the yield of acrylic acid and inhibited the formation of acetaldehyde in a certain degree. Under the reaction condition of 38% lactic acid as raw material, space velocity 3 h-1, and reaction temperature 325 ℃, the highest yield of acrylic acid reached 54.2% over 2% La/NaY molecular sieve catalyst while the yield of acrylic acid over unmodified NaY molecular sieve catalyst was only 34.7%.