Surface functionalization of LTA zeolite with APTES: role of solvent, concentration, and thermal stability

Abstract

The growth mechanism and thermal stability of self-assembled monolayer (SAM) of 3-aminopropyltriethoxysilane (APTES) on LTA-type zeolite surfaces through liquid-phase adsorption have been investigated. The effects of solvent, APTES concentration, reaction time, as well as thermal stability were analyzed. The X-ray diffraction (XRD) study indicated that APTES functionalization does not alter the crystalline structure of the zeolite. The resulting APTES monolayer was characterized using X-ray photoelectron spectroscopy (XPS), suggesting that the adsorption of APTES onto the zeolite mainly occurs through siloxane bonds (Si–O–Si/Al), leaving their –NH2 terminal groups exposed to the surface. The effectiveness of functionalization is independent of the APTES concentration. Toluene was found to be a more effective solvent than ethanol, yielding a higher surface coverage and a higher proportion of non-protonated amines. XPS thermal desorption of the APTES layer under high vacuum conditions proposes a two-stage process: weakly bound –CH2–NH2 species desorb below 175 °C, while covalent Si–O–zeolite linkages remain stable up to 275 °C, providing thermal stability to the system. Temperature-induced changes in bonding further enhance the stability of the APTES-modified zeolite surface. Thermogravimetric analysis coupled with infrared spectroscopy confirms the cleavage of C–N and C–Si bonds above 250 °C, releasing volatile fragments.