MODIFICATION OF ZSM-5 ZEOLITE WITH TRIMETHYL PHOSPHITE PART 1 - STRUCTURE AND ACIDITY

In the present work we have studied the structure and the acidic prope rties of HZSM-5 zeolite modified with trimethyl phosphite. The modific ations were carried out by using a chemical vapour deposition method a s well as impregnation from n-octane solution. Both modification techn iques resulted in materials with similar acidic properties. Trimethyl phosphite reacted with both Bronsted acid sites and terminal SiOH grou ps. The modifications resulted in the total elimination of strong Bron sted acid sites accompanied by the formation of new Bronsted type site s with decreased acid strength. However, the total acidity of the modi fied zeolites stayed relatively high. A possible structural transforma tion resulting from the phosphite modifications was proposed to be an insertion of phosphorus species between the structural aluminium- and silicon-oxygen tetrahedra. However, on the basis of P-31 MAS NMR studi es no evidence of the isomorphous substitution of structural T-atoms b y phosphorus was found. (C) 1998 Elsevier Science B.V.