(583fj) Effect of Support and ZnO Texture On Zn-Ni Dispersion and Ni/ZnO-Based Adsorbent in Reactive Adsorption Desulfurization

The objective of the present study is to improve the reactive adsorption desulfurization(RADS) performance of the Ni/ZnO-based adsorbents for adsorption desulfurization by increasing the dispersion of nickel monoxide and zinc oxide in Ni/ZnO-based adsorbent, and get better insight into the correlationship between crystallite size of NiO, ZnO and the RADS performance of the adsorbents. Different supports (γ-Al₂O₃-diatomite (Al-diatomite),γ-Al₂O₃-clay(Al-clay), titanium dioxide(Ti), titanium dioxide-zirconia dioxide(Ti-Zr)) and ZnO precursors(zinc oxide, basic zinc carbonate(BZC), zinc chloride(ZC), zinc nitrate(ZN)) were used to increase the dispersion of nickel monoxide and zinc oxide. To assess the effect of support and ZnO precursors on adsorption desulfurization, the adsorbents were characterized by XRD, H₂-TPR and BET techniques. For a adsorbent to be successful it clearly needs to have stronger interactions with thiophenic than with nonsulfur-containing olefinic compounds, therefore, adsorptive activity for thiophene and effects of the coexisting olefins on the adsorptive performance were examined. In addition, the present study employed in situ FTIR of adsorption and reaction of thiophene over reduced Ni/ZnO-based adsorbent to probe the bonding mode of thiophene and products formed during RADS over a range of temperatures. Moreover, the X-ray powder diffraction (XRD) method was used to investigate the phase change of sulfur species in the RADS process. A good regeneration property is vital importance to adsorbents. In this work, the Ni/ZnO-based adsorbent regeneration after desulfurizing a model fuel using a two-step method was studied and the TG-DTA together with XRD was used to reveal the regeneration mechanism.

The effects of support composition and ZnO texture were investigated in order to evaluate the RADS performance of the adsorbent. The adsorptive capacity of different support adsorbents is in the following order: NiZn/Ti-Zr > NiZn/Ti > NiZn/Al-clay > NiZn/Al-diatomite. Olefins in model fuel have a strong inhibiting effect on the desulfurization performance of the Ni/ZnO-based adsorbents, probably by a competitive scramble for the activated hydrogen on the surface. The adsorptive capacity of different support adsorbents for the olefin-containing fuel is in the following order: NiZn/Ti-Zr > NiZn/Al-diatomite > NiZn/Al-clay > NiZn/Ti. The RADS capacity of the adsorbents increases in the order of NiZn(ZN)/Ti-Zr > NiZn(ZC)/Ti-Zr > NiZn(BZC)/Ti-Zr > NiZn/Ti-Zr. According to XRD patterns, the estimated ZnO crystal sizes for the modified ZnO precursor adsorbents (NiZn(BZC)/Ti-Zr, NiZn(ZC)/Ti-Zr and NiZn(ZN)/Ti-Zr) are smaller than that for the NiZn/Ti-Zr. The reactive adsorption behavior of thiophene over the reduced Ni/ZnO-based adsorbent was characterized by in situ FTIR spectroscopy. The XRD technology was used to investigate the phase change of sulfur species in the reactive adsorption desulfurization (RADS) process. The results indicated that S-M bonding of thiophene on the metallic Ni sites was first decomposed to form Ni₃S₂ while formed C₄ olefins were further saturated by hydrogen to form butane which was released back into the process stream, followed by the sulfur transfer from Ni₃S₂ to ZnO to form ZnS in the presence of hydrogen, and then the new formed Ni sites could participate in the adsorption of thiophene once again. The muticycle fixed-bed tests showed a good prospect for adsorption desulfurization over the Ni/ZnO-based adsorbents. The TG-DTA together with XRD were used to reveal the regeneration mechanism. The XRD results indicated that the formation of NiSO₄ species leaded to an increase of the amount and the strength of Lewis acid sites in the regenerated adsorbents, and thus temporarily improve the removal performance of the adsorbent for thiophene.