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

• Applicant: Professorin Dr. Yvonne Traa
• Subject Area: Technical Chemistry
• Term: from 2017 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 388060787

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.

• DFG Programme: Research Grants