The Rational Design Of Ni/ZnO-AL₂O₃ Adsorbents For Reactive Adsorption Desulfurization

The demand and use of ultra-deep desulfurized transportation energy,predominantly gasoline and diesel has attained significant consideration throughout the world and will continuously increase worldwide in the foreseeable future.In order to meet more stringent environmental protection legislations and regulations,ultra-deep desulfurization technologies and novel adsorbent development are desirable to produce ultra-low sulfur fuels.This thesis presents our efforts to develop novel adsorbents synthesis designs for reactive adsorption desulfurization（RADS）of model and commercial fuels.In first part,a series of Ni/ZnO-Al₂O₃ adsorbents were synthesized by one-pot cation-anion double hydrolysis（CADH）method.The RADS performances of these adsorbents were evaluated in a fixed bed reactor using thiophene in n-octane as a model fuel.Results showed that the thiophene conversion and sulfur capacity of adsorbents decreased with increasing the crystallization temperature.Among the tested adsorbents,sample Ni/ZnO-Al₂O₃ prepared at 28℃ presented the largest adsorption capacity and highest RADS reactivity.Textual characterization results indicated that sample Ni/ZnO-Al₂O₃（28℃）possessed relatively bigger pore size and larger pore volume than other samples,which may alleviate the pore shrinkage/blockage during the RADS process.A combination of XRD,UV-vis and H₂-TPR characterization results also demonstrate that a high crystallization temperature favors the growth of RADS’s inactive ZnAl₂O₄,crystals and induce the formation of more less reducible Ni²⁺ion,causing the loss of active ZnO phase and Ni⁰atoms,which may be the reason for the lower RADS activity of the adsorbent synthesized at higher crystallization temperatures.ZnO-Al₂O₃ mixed oxides were also synthesized with an improved structure via a freeze-drying modified cation-anion double hydrolysis（CADH）technique and used as the support for nickel loading.Freeze drying technique induces small sized ZnO and an improved pore structure compared with the normal oven drying method.Evaluation results in the RADS of a high content sulfur fuel reveals that the freeze-dried Ni/ZnO-Al₂O₃（40℃）with a crystallization temperature of 40^○C exhibits a superior RADS performance with an accumulative sulfur adsorption capacity of 90 mg S/g,which is 42%and 72%higher than those adsorbents prepared through normal oven drying and conventional kneading methods.A larger amount of small ZnO particles,improved textural properties and absence of inactive NiAl₂O₄ phase are among the factors accounting for the superior RADS performance and high RADS-regeneration stability of Ni/ZnO-Al₂O₃adsorbent prepared through freeze-drying method.The study were extended to synthesized ZnO-Al₂O₃ mixed oxides（MO）using a double hydrolysis method at different temperatures,which were used as the support for the preparation of uniformly distributed Ni on MO.Results shows that Ni/MO samples exhibited much higher RADS activity and larger accumulative breakthrough capacity than sample NZA-K prepared using the conventional kneading method.The desulfurization activity of Ni/MO adsorbents decreased with increasing the crystallization temperature of MO.As a result,sample Ni/ZnO-Al₂O₃-60℃ synthesized at 60℃ showed the best desulfurization performance among all Ni/MO adsorbents.Detailed characterization results revealed that the high dispersion of NiO and ZnO,the absence of inactive NiAl₂O₄ and high concentration of surface Lewis acid sites may account for the superior RADS performance of Ni/MOs samples compared to commercial RADS adsorbent.A series of Ni/ZnO-Al₂O₃ mixed oxide（MO）adsorbents were synthesized with a new and simple one-step homogeneous precipitation method and compared with cation-anion double hydrolysis methods for RADS activity.Results show that both Ni loading and preparation method have a significant effect on adsorbent’s RADS activities.Among the studied adsorbents,10%Ni/ZnO-Al₂O₃ prepared by the one-step urea homogeneous precipitation method showed the best RADS performance with a high thiophene conversion up to 96%and a sulfur adsorption capacity of 86 mg S/g,which is 34%larger than that of CADH adsorbents.In addition,one-step homogeneous precipitation method prepared samples shows high RADS regeneration stability with only 3%drop after five RADS cycles.Characterization results show that the one-step homogeneous precipitation method could facilitate the formation of small ZnO particles while suppressing the formation of inactive ZnAl₂O₄ and NiAl₂O₄ while inducing high concentration of Ni⁰ sites,which results high RADS performance.Lastly,micro-fiber alumina was synthesized using thermal urea precipitation technique,which was utilized as support for the ZnO and Ni precursors.The reactive adsorption desulfurization performance and effect of metallic interactions with in the desulfurization process were compared with commercially prepared alumina used as support for ZnO and Ni precursors.It was observed that higher breakthrough desulfurization activity and sulfur adsorption capacity strongly depends on the diffusion rate of molecules and extant of metallic interactions of ZnO with alumina.Results show that Ni/ZnO-Al₂O₃-Fiber adsorbent achieved highest breakthrough sulfur removal（10 ppm）up to 31.2 mL responsible for 94 mg S/g sulfur adsorption capacity where in comparison Ni/ZnO-Al₂O₃-Com adsorbent reveals a compromised RADS performance with an initial desulfurization of 700 ppm and reach to 1741 ppm at 31.2 mL with 39 mg S/g accumulative sulfur capacity.Detailed characterization results show that higher external diffusion of reactant molecules within the cress crass micro-fiber with larger small ZnO particles and Ni active sites,and lower interactions between ZnO with Al₂O₃ to formed RADS inactive ZnAl₂O₄ phases,may account for the superior RADS performance of Ni/ZnO-Al₂O₃-Fiber adsorbent.