In Situ X‐Ray Diffraction Study of a Microwave‐Based Hydrothermal Synthesis of Mayenite Ca ₁₂ Al ₁₄ O ₃₃

Mayenite Ca 12 Al 14 O 33 (C12A7) is known for its unique properties, resulting from a crystal structure, which consists of a positively charged ‘antizeolite‐like’ framework [Ca 12 Al 14 O 32 ] 2+ forming cages with ‘trapped’ oxide ions. Depending on the synthesis route, different transport properties and particle morphologies can be achieved. Due to its potential applications in catalysis, hydrothermally synthesised mayenite with a high specific surface is of special interest. In this work, a new approach for the synthesis of the mayenite precursor hydrogarnet via a microwave‐assisted hydrothermal route is introduced. Various synthesis parameters were optimised to shorten the hydrothermal reaction time down to 5 min at a temperature of 80°C. The final formation of C12A7 can be achieved at a moderate temperature of 800°C. An increase of the surface area up to 70 m 2 /g was obtained. In situ X‐ray diffraction and TG‐MS measurements were performed to gain insight in the reaction mechanism. This new microwave approach offers a fast and efficient route to mayenite with advantageous properties for catalytic applications.