Cracking of n-Hexane over Ce/ZSM-5 catalysts

Abstract

ZSM-5 typed zeolite (SiO[subscript 2]/Al[subscript 2]O[subscript 3] = 40, or Si/Al = 20) was synthesized from gel having molar composition of 1SiO[subscript 2]: 0.36Na[subscript 2]O: 0.025Al[subscript 2]O[subscript 3]: 0.10TPABr: 37.2H[subscript 2]O. The gel was subsequently pH-adjusted to 10.5 using concentrated H[subscript 2]SO[subscript 4] and was crystallized at 170 ํC for 4 days. The product was obtained as white solid having characteristic diffraction peaks of an MFI phase. The synthetic method was found highly reproducible in terms of crystallinity and chemical composition (Si/Al = 21.0+-1.7, Na/Al = 1.0+-0.3). Introduction of as high as possible Ce[superscript 3+] into (Na, H)ZSM-5 was comparatively investigated by two methods, which differ in severity. Method with higher severity, i.e., high concentration of Ce[superscript 3+] in exchange solution, long reaction time, and harsh activation condition, incorporated more cerium. HCeZSM-5 (or HZSM-5) was prepared by ammonium exchange plus air-activation of CeNaZSM-5 (or (Na, H)ZSM-5). The resulting materials still possess relatively high crystallinity. Catalytic cracking of n-hexane vapor was performed over these catalysts, at 500 ํC, GHSV of 2000 h[superscript -1], and time-on-steam of 30 min. Thermal and CeO[subscript 2]-catalyzed reactions were also studied. It is found that pyrolysis of n-hexane is insignificant, and CeO[subscript 2] mainly converts n-hexane to coke. HZSM-5 gives high conversion, low coke yield, and acceptable selectivity to light olefins (ethylene, propylene, and butenes). Modification by Ce[superscript 3+] shows that activity of (Na, H)ZSM-5 is greatly increased by cerium cations. Selectivity to olefins and methane are satisfactorily increased and decreased, respectively, by CeMZSM-5(M = Na[superscript +] or H[superscript +]) better than by HZSM-5. Gas, liquid, and coke yields over CeMZSM-5 are more or less similar to those over HZSM-5. Moreover, alternation of cerium oxidation state can be monitored by color change. An oxidation state of cerium depends on an atmosphere used for activation during preparation step. It is found that n-hexane conversion increases 2 times when catalyzed by N[subscript 2]-calcined Ce*MZSM-5. Additionally, Ce*MZSM-5 gives higher liquid yield, lower gas and coke yields compared to both CeMZSM-5 and HZSM-5. An optimum light olefins yield is obtained over Ce*(40%)NaZSM-5 and HCe(62%)ZSM-5. These results are higher than those obtained over HZSM-5. Coke depositions over cerium-containing samples are (with one exception) lower than those over HZSM-5. Improvement of olefins yields by cerium ion exchange is found promising.