KLTL–MCM-41 micro–mesoporous composite as a solid base for the hydrogenation of sugars

Abstract

An approach to the synthesis of KLTL–MCM-41 micro–mesoporous composites of varying SiO₂/Al₂O₃ molar ratio (20–8) was designed by following green technology. The synthesis was based on the recycling of waste mother liquor containing preformed KLTL zeolite crystals and unutilized reagents. The micro–mesoporous composites consist of KLTL zeolite crystals preformed through hydrothermal treatment in the first step. In the second step, the siliceous mother liquor was transformed into mesoporous MCM-41. The physico-chemical properties of the KLTL–MCM-41 micro–mesoporous composites were determined by PXRD, ICP-OES, FTIR, CO₂-TPD, ²⁷Al MAS-NMR, TEM-EDX, HRTEM, and N₂ adsorption–desorption measurements. The mesopores properties of the KLTL–MCM-41 composites depended on the SiO₂/Al₂O₃ molar ratio. The characterization results led to the following conclusions with decreasing SiO₂/Al₂O₃ molar ratio: 1) decreasing order of the mesophase, 2) decreasing wall thickness of the mesopores, and 3) decreasing BET surface area and pore volume. ²⁷Al MAS-NMR spectra showed that only tetrahedrally coordinated aluminium was contained in zeolite KLTL. Moreover, the total amount and the strength of the basic sites of the KLTL–MCM-41 micro–mesoporous composites owing to the exchangeable potassium content (Al/K ratio ≈ 1) decreased in the order: 8 MMC > 10 MMC > 15 MMC > 20 MMC. The catalytic activity of the synthesized micro–mesoporous samples as a solid base was tested for the hydrogenation of xylose to sugar alcohols.