Controlled reduction of the acid site density of SAPO-34 molecular sieve by means of silanation and disilanation

SAPO-34 has been modified by silanation and disilanation reactions. The acidic properties of these modified SAPOs have been characterized by IR measurements. The bands of the bridging hydroxyls at 3600 and 3625 cm(-1) show a gradual decrease in intensity with increasing silane or disilane loading. The band intensity of the P-OH group at 3675 cm(-1) remains unaffected. The silanation process can be monitored (i) spectroscopically through the characteristic IR band of Si-H at 2250 cm(-1) and (ii) analytically by the changes in the H(2)/SiH(4) ratio during the modification. CD(3)CN has been used as an IR spectroscopic probe to elucidate the changes in Bronsted and Lewis acidity of the modified SAPO-34. The intensity of the C drop N absorption band at 2320 cm(-1), which is attributed to CD(3)CN adsorbed on Lewis acid sites, increases relative to the band at 2290 cm(-1) (Bronsted acidity). This indicates that Bronsted acid sites are irreversibly transformed into Lewis acid sites after silanation reactions on the SAPO-34. (1)H and (29)Si MAS NMR confirm that the silanes react with the Bronsted acid sites resulting in silicon incorporation and silanol formation. Methanol adsorption capacity measurements indicate that silanation and disilanation reactions cause a significant reduction of the void volume of the SAPO-34 cages without creating diffusion limitations. The ethylene/ethane and propylene/propane selectivity ratios in the MTO reaction (methanol-to-olefins) increase as a function of the (di)silane loading, while the amount of coke on the SAPO-34 decreases linearly with increasing (di)silane loading