ABSTRACT
Aromatization is an important process for catalytic conversion of light alkanes to aromatics which are essential feedstocks to petrochemical industries. A number of catalysts have been used for this transformation among which Zn/ZSM-5 emerged so far best owing to its good dehydrogenating activity, lower cost and environmental friendliness. Compared with similar metals used in aromatization reaction, zinc is known to be unstable at reaction conditions due to its low melting point and density, hence declines in alkane conversion and aromatic selectivity. Therefore, in this research, four different metals Me (where Me is iron, cobalt, nickel or copper of 1-3 wt. %) were co-impregnated with 2 wt. % zinc to form bimetallic catalysts Zn-Me/ZSM-5 in mitigating this stability challenge at reaction condition. The role of the second metal is to improve catalyst activity, stability and selectivity towards aromatics. The prepared catalysts physicochemical properties were analyzed with XRD, BET, N2-adsorption, FTIR, FTIR Pyridine, SEM, TEM, H2-TPR and XPS. The XRD and FTIR showed that the bimetallic catalysts were crystalline and possessed basic features of HZSM-5. XPS analysis confirmed the metal species had oxidation states of +2. The XPS and H2-TPRalso showed that there is synergistic metal-metal interaction between the co-impregnated metals with zinc through changes in binding energies of metals on HZSM-5 and reduction temperatures which is believed to have strengthened and stabilized the zinc metal species at the reaction temperature. Incorporation of the selected metals with the zinc on HZSM-5 showed reductions in Bronsted acidity from the FTIR-Pyridine. N2-adsorption isotherms confirmed that the metal modified catalysts were all microporous (90 % microporosity in surface area) as the parent HZSM-5. Performance test was conducted in a fixed bed reactor using gas hourly space velocity of 1,200 ml/hr. at 540 oC and atmospheric pressure. Zn-Fe/ZSM-5 showed improved propane conversion and selectivity towards aromatic products for all metal compositions except for 1 wt. % Fe, when compared with Zn/ZSM-5 and HZSM-5. Zn-Co/ZSM-5 catalysts of 1 and 3 wt. % cobalt have improved and sustained propane conversion. Zn-Ni/ZSM-5 and Zn-Cu/ZSM-5 (1-3 wt. % of Ni and Cu) also showed sustained and improved propane conversion and selectivity better than Zn/ZSM-5 and HZSM-5 catalyst. This is because the metals have the same oxidation states with zinc. Coimpregnation of Zinc with selected metal improved the catalytic activity with sustained aromatic selectivity at an average 12 hours? time on stream as compared to parent HZSM-5 (10%) and Zn/ZSM-5 (50%). The products distribution for modified bimetallic catalysts showed reduced light hydrocarbon formation but enhanced aromatics production. All metal impregnated catalysts had improved selectivity for toluene with Zn-Co/ZSM-5 (2 wt. % each) toluene selectivity between 40 to 45% of the total aromatic products. Zn-Cu/ZSM-5 was highly selective towards m and o-xylene. Zn-1Fe/ZSM-5, Zn-1Cu/ZSM-5 and Zn-3Ni/ZSM-5 catalysts were highly selective towards ethylbenzene of about 10 to 15 %. 2 and 3 wt. % loading of impregnated metals with zinc had good selectivity for benzene of about 10 to 15 % of the total aromatic products. Equal 2 wt. % of Zinc with other metal performed better among other catalysts for aromatic selectivity and stability out of which Zn-Ni/ZSM-5gave the best in propane conversion (55 %), aromatic selectivity (85 %), and stability for twelve hours? time on stream. The effect of temperature variation on the best catalyst showed that temperature increase has propane conversion and aromatic selectivity. Effect of flow rate increase on Zn-Ni/ZSM-5 (2 wt. % each) catalyst showed reduced conversion from80 to 38 %and increased selectivity from 54 to 80 %. In general, aromatic yield for the bimetallic catalysts were improved and sustained by 2 wt. % equal metal dosage with Zn over Zn/ZSM-5 and HZSM-5 catalysts. Thus, more stable bimetallic bifunctional Zn-Metal based catalysts for propane aromatization were developed.