Alumination Of Zeolite By NaAlO₂ Solution And Their Hydrogenation Performanec

Zeolite USY and Beta are widely used as acid catalysts in the catalytic cracking and dewaxing processes of oil and natural gas as well as aromatic alkylation due to their structural L acid sites resulting from skeleton Al atoms and B acid sites produced by ion-exchange. As a result, the acid properties of zeolite composite influence the properties of the whole catalyst and consequently have an effect on the catalytic process. Thereby, it is important to modify the acid properties of zeolite composite in the catalyst. In this paper, the acid properties of zeolite USY and Beta were modified with alkaline solution-NaAlO2, and the influence of concentration and alkalinity of NaAlO₂ solution as well as modification time and temperature on the modification process were studied in order to optimize the modification conditions and to improve the acid properties of zeolite. Then, the modified zeolite was chosen as the support of metal Ru catalysts, and their catalytic performances were studied in the liquid-phase benzene hydrogenation reaction.Firstly, series of modified HUSY samples were prepared under various modifying conditions using NaAlO₂ solution. These modifying conditions include various modifying time(6h, 12h, 24h), various modifying temperature(40℃, 60℃, 80℃) and NaAlO₂ solution with various concentration(0.01mol/L, 0.05mol/L, 0.1mol/L) and alkalinity (pH=13, pH=14). The effect of modifying conditions on the acid properties and pore structure of modified HUSY samples were studied by NH₃-TPD, IR and Pyridine-IR methods qualitatively and quantitatively. Then, modified HUSY samples were chosen as the support of metal Ru catalysts and their hydrogenation activity were observed in the 0.5wt.% benzene/hexane solution. Also, the effect of acid properties of support on the hydrogenation activity of Ru catalysts was studied. The result showed that all modifications with various conditions broadened the acidic distribution of HUSY, increased the acid amount of B acid sites and total acid sites, and improved the acid strength of B and L acid sites under the preservation of parent structure. After modification, the surface area per gram and volume of mesopore in HUSY samples increased. Ru catalysts supported on the modified HUSY showed higher hydrogenation activity than those on HUSY. The higher hydrogenation activity of Ru catalysts may be due to the combination effect of modified acid properties and pore structure in the HUSY support and diffusion between catalyst and reactants. The optimal modified HUSY sample was observed to be the one modified with 0.05mol/L NaAlO₂ solution under 80℃for 12 hours.Secondly, zeolite HBeta was modified with various concentration of NaAlO₂ solution under various modifying time and temperature. The acid properties and relative crystallinity of modified HBeta samples were characterized by XRD, N₂-Adsoption, Benzene-adsorption, Pyridine-IR, NH₃-TPD, ²⁹Si-MAS-NMR and ²⁷A1-MAS-NMR methods. Then, modified HBeta samples were chosen as the support of Ru catalysts and their liquid-phase hydrogenation activity were studied in the 0.06wt.% benzene/hexane model reactant solution. The B acid sites, L acid sites, total acid amount and relative content of Al atoms in HBeta increased under the preservation of parent structure after modification. It was found that Ru catalysts supported on modified HBeta showed higher hydrogenation activity than those on HBeta. ²⁹Si-MAS-NMR, ²⁷A1-MAS-NMR, OH-IR and Skeleton Vibration IR characterizations showed that the increase of Al content in HBeta was caused by the realumination during the modification process. The realumination of HBeta was achieved by both the replacement of skeleton Si atoms by Al atoms and desertion of Al species happened on the crystal defect sites in HBeta, where Al atoms came from modifying solution and non-skeleton Al species. Future, Ru catalysts supported on modified HBeta showed higher hydrogenation activity than those on HBeta.