Tailored Nucleophilic and Amphoteric Zeolites without Large Cavities for the Dehydration of Lactic Acid to Acrylic Acid

阳离子交换沸石可以看作是酸碱对。阳离子作为Lewis酸，骨架氧作为Lewis碱。根据桑德森公式可以计算出骨架氧的平均电荷。因此，阳离子交换沸石可分为亲核沸石、两性沸石和亲电沸石。作为亲核和两性沸石的适宜实验反应，选择了乳酸脱水制取丙烯酸。这种反应对原材料的变化和从可再生能源中生产化学品具有重要的经济意义。由于乳酸的两个官能团，除了脱水成丙烯酸之外，还有其他可能的反应。为了避免亲电性和亲核性中心的副反应，最好是通过E2机制消除乳酸的脱水，在此过程中乳酸将同时吸附在亲电性和亲核性中心。因此，淘汰可能是一种协调一致的反应。由于主要的质子酸中心有利于乳酸的脱碳和聚合，因此乳酸脱水催化剂应该只具有Lewis酸的亲电中心。此外，乳酸的羧基应该被足够高浓度的亲核试剂所掩盖。由于催化剂的失活只能避免，如果减少乳酸、丙烯酸和开链乳酸低聚物/聚合物等酸性碳氢化合物的积累，催化剂不应该有大的空洞或大的孔。因此，筛选了所有沸石骨架类型，寻找没有大空穴的非亲电沸石。因此，重点将集中在以下不具有大空腔的富铝沸石骨架类型的钠形态上：CAN、NAT、LAU、GME、MAZ和OFF。通过与Li、K和Cs的离子交换以及改变沸石中的铝含量，可以进一步优化酸碱活性中心之间的平衡。通过这些措施，对丙烯酸的选择性应该可以达到90%或更高。这项工作将加深对沸石催化剂中亲电活性中心和亲核活性中心之间相互作用的理解，并使新的非溴酸性、富铝沸石催化剂的价态成为可能。

Cation-exchanged zeolites can be seen as acid-base pairs. The cation works as a Lewis acid, the framework oxygen as Lewis base. The average charge on the framework oxygen can be calculated after Sanderson. Thus, cation-exchanged zeolites can be classified as nucleophilic, amphoteric or electrophilic. As suitable test reaction for nucleophilic and amphoteric zeolites, the dehydration of lactic acid to acrylic acid was chosen. This reaction is of economic importance for the raw materials change and the production of chemicals from renewables. Due to the two functional groups of lactic acid, there are other reactions possible besides the dehydration to acrylic acid. In order to avoid side reactions on electrophilic and nucleophilic sites, it would be desirable, if the dehydration of lactic acid would be an elimination via the E2 mechanism during which the lactic acid would be adsorbed on an electrophilic and a nucleophilic site at the same time. Thus, the elimination could be a concerted reaction. Since mainly protic sites favor the decarbonylation and polymerization of lactic acid, the catalyst for the dehydration of lactic acid should only have Lewis-acid electrophilic sites. In addition, the carboxyl group of the lactic acid should be masked by a sufficiently high concentration of nucleophiles. Since catalyst deactivation can only be avoided, if accumulation of acid hydrocarbons such as lactic acid, acrylic acid and open-chain lactic acid oligomers/polymers is minimized, the catalyst should have neither large cavities nor large pores. Hence, all zeolite framework types were screened, looking for non-electrophilic zeolites without large cavities. Therefore, the focus will be on the sodium forms of the following aluminum-rich zeolite framework types without large cavities: CAN, NAT, LAU, GME, MAZ and OFF. The balance between acid and basic active sites can furthermore be optimized by ion exchange with Li, K and Cs as well as by a variation of the aluminum content of the zeolites. By these measures, selectivities to acrylic acid of 90% or higher should be possible. The work should increase the understanding of the interaction between electrophilic and nucleophilic active sites in zeolite catalysts and enable the valorization of new non-Brønsted-acidic, aluminum-rich zeolite catalysts.