Structure and catalytic activity of Al-MCM-48 materials synthesised at room temperature: Influence of the aluminium source and calcination conditions

Abstract

MCM-48 aluminosilicates with different aluminium contents were synthesised by a room temperature procedure using tetraethoxysilane and aluminium sulfate, isopropoxide or tert-butoxide as metal sources. The samples were characterised by X-ray diffraction, nitrogen adsorption at 77 K, and 27Al MAS NMR and the catalytic activity tested in the reaction of 1-butene double bond position isomerisation. The influence of the synthesis time and calcination conditions, such as heating rate and time at final temperature, on the structural and catalytic properties of the materials was also evaluated. Aluminium isopropoxide and sulfate allowed the preparation of well structured Al-MCM-48 materials, with high specific pore volume and uniform pore size, and with the majority of the aluminium incorporated in tetracoordinated environment after calcination, at least down to Si/Al of 15. The highest initial conversions were found for samples with Si/Al = 20 and 30 prepared with aluminium isopropoxide and calcined using a heating rate of 3 K min −1, with those of samples prepared with aluminium sulfate being slightly lower. Al-MCM-48 materials with high specific pore volume and uniform size were also synthesised using aluminium tert-butoxide, but the materials were less well ordered, presented a higher proportion of hexa- and pentacoordinated Al species and exhibited lower initial conversions, independently of the synthesis time or calcination conditions tested. It is concluded that aluminium isopropoxide and sulfate are more adequate metal sources than aluminium tert-butoxide to prepare Al-MCM-48 catalysts by this room temperature method and a calcination heating rate of 3 K min −1 instead of 1 K min −1 produces more effectively active catalysts.