This is a 'preproof' accepted article for Clay Minerals. This version may be subject to change during the production process. DOI: 10.1180/clm.2020.40 Structural transformation of kaolin as an active matrix for the in-situ synthesis of zeolite Y Jessyka Padilla 1 , Alexander Guzman 2 , Daniel Molina 1 , Juan Carlos Poveda-Jaramillo 1 1 Universidad Industrial de Santander, Carrera 27 calle 9 Bucaramanga - Colombia 2 ECOPETROL S.A. Instituto Colombiano del Petróleo, Piedecuesta – Colombia Running head: In-situ crystallization of Y zeolite on treated kaolin microspheres Submitted: 10 August 2020. Revised: 14 December 2020. Assoc. Editor: A. Turkmenoglu DOI: E-mail: jessyka.padilla@correo.uis.edu.co jessykpadilla@hotmail.com ABSTRACT: To produce an optimized matrix for the in-situ crystallization of zeolite Y, a commercial kaolin chemically treated with NaOH solution at 97 °C for 24 h and thermally transformed from 750 to 1100 °C was studied. The kaolin calcined at 750 °C has 20 % more reactive tetrahedral aluminum species for the synthesis of zeolite Y than kaolin calcined at 865 °C. The kaolin calcined at 1000 °C has amorphous silica environments that can be labile to extraction with caustic solution, which increases the BET area more than 16 times and generates mesopores ~5 nm in diameter. These structural changes in the calcined and treated kaolins were combined to prepare microspheres of the mesoporous matrix, upon which well- dispersed crystals of zeolite Y crystallized.