Surface Acidity Of Molecular Sieve Investigated By IN-SITU FT-IR Spectroscopy

Alumisilicate molecular sieve catalysts used as acid catalysts occupy an important position in industrial field,such as petroleum refining and petrochemical industry etc.In the petroleum refining process,many reactions such as catalytic cracking and isomerization of hydrocarbons are catalyzed by acidic catalyst.Lewis and Br(?)nsted acid concentration and strength on surface of different zeolite catalysts vary on their structure and composition;moreover, acidity requirements for different catalytic reactions are diverse.So study on the surface acidity of zeolite catalysts has important significance for its practical application in industry.In this paper,surface acidity of composite HS-FBZ and meosoporous super-acid material SO₄^2-/Zr-SBA-15 was characterized by using in-situ FT-IR associated with TPD technology;pyridine is employed as probe molecule during the study.The characterization results of composite HS-FBZ showed as follows:the hydroxyl peaks of composite HS-FBZ varied with increased crystallization time, FT-IR spectra of HS-FBZ-8.5 and HS-FBZ-9 samples in hydroxyl showed obvious bands around 3735cm^-1,3637cm^-1,3550cm^-1and 3610cm^-1,indicating the existence of typical hydroxyl groups of HY and HBeta zeolites on the surface of composite HS-FBZ.Subtraction FT-IR spectra of pyridine absorbed and activated samples showed typical acidic hydroxyl groups of HY and HBeta zeolites on the surface of composite HS-FBZ;meanwhile,hydroxyl groups of extra-framework aluminium and half-framwork aluminium provide parts of acid sites.The typical hydroxyl groups of HY and HBeta zeolites on the surface of composite HS-FBZ both supplied the strong acid sites.FT-IR spectra of pyridine adsorption showed that both L and B acid sites exist on the surface of composite HS-FBZ.Composite HS-FBZ has more L and B acid sites than HY(SiO₂/Al₂O₃=5.87)or HBeta(in starting mixture SiO₂/Al₂O₃=20).Moreover,the strength of L acid on HS-FBZ had been enhanced dramaticly compared to HY,HBeta and mechanical mixed Y/Beta with the same Beta/Y ratio.All these results suggest HS-FBZ is an acid catalyst with strong L acid site.Acidity of composite HS-FBZ could be controlled through control of crystallization time and Beta/Y ratio to change the surface hydroxyl structure of HS-FBZ and adjust acid concentration of L and B acid site and L/B ratio.The characterization results of mesoporous super-acid material SO₄^2/-Zr-SBA-15 were as follows:the acid sites of SO₄^2-/Zr-SBA-15 were all connected to zirconium species.The concentration and strength of L and B acid on SO₄^2-/Zr-SBA-15 varyed with the change of Zr/Si ratio,and the variation coincides with catalytic activity of n-pentane isomerization at low temperature. And,the sample when Zr/Si=1.3 having the highest catalytica activity owns the most and strongest L and B acid sites.At least two kinds of B acid sites exist on the surface of SO₄^2-/Zr-SBA-15,namely,H₂O molecule bonded to sulfated Zr species and bridge Zr-OH-Si hydroxyl groups.Most strong acid sites of SO₄^2-/Zr-SBA-15 were provided by pyrosulfated Zr species,so sample with relatively high Zr/Si ratio obtained more strong acid sites.