Study On The Performance Of Doped ZrO₂-based Metal Oxide Catalysts For The Dehydrogenation Of Isobutane

Isobutene is an important "member" of chemical raw materials,which is widely employed in the production of various industry chemicals with high-added value.With the increasing requirement for isobutene around the world,the resources of fossil fuels,which are applied in the traditional methods for producing isobutene,have been obviously dwindled.However,a large volume of more economical shale gas as well as nature gas has beengradually exploited with the development of exploration in recent years.These factors make isobutane catalytic dehydrogenation become an efficient alternative technique for the production of isobutene.In this thesis,aimed at the direct dehydrogenation of isobutane to isobutene and based on the advantages as well as disadvantages of two types of commercial dehydrogenation catalysts,ZrO₂,which is a transition metal oxide,was chosen to be a "host oxide".And from the perspective of doped metal oxides,a series of binary ZrO₂-based nano-sized doped oxides catalysts with different metal dopants were designed and prepared.These catalysts were applied for the direct catalytic dehydrogenation of isobutane to isobutene.The physico-chemical properties of all investigated catalysts were characterized by N2 adsorption-desorption isotherms,X-ray diffraction（XRD）,transmission electron microscopy（TEM）,X-ray photoelectron spectroscopy（XPS）,NH3/CO₂/O₂ temperature-programmed desorption（NH3/CO₂/O₂-TPD）,Fourier transform infrared（FT-IR）,Pyridine-Infrared（Py-IR）,Electron paramagnetic resonance（EPR）,H2 temperature-programmed reaction（H2-TPR）,Raman spectroscopy,inductively coupled plasma（ICP）analysis,thermo gravimetric（TG）and elemental analysis.A series of experiments were designed reasonably.The effect of different metal dopants with different valence on the catalytic performances of samples were investigated.Furthermore,the types of active sites of catalysts,catalytic mechanism,the nature,formation process of of deposited carbon species and the reasons of the decline of activity were also studied.The primary research conclusions are as follows:（1）Two trivalent metal dopants,i.e.,La and Al,were selected.A series of mLaZrO and nAlZrO catalysts with different proportion were synthesized by hard template method.The crystalline structures,physico-chemical properties and catalytic performances were evaluated.The analysis results indicate that 3LaZrO possesses more active sites in the presence of H2 and exhibits higher catalytic activity;coordinatively unsaturated Zrcus 4+ sites neer oxygen vacancies（VO）are the active sites for isobutane dehydrogenation.（2）Three divalent metal dopants M,i.e.,Co,Cu and Zn,were selected.A series of MZrO-9 catalysts,where 9 stands for Zr/M atom ratio,were synthesized by hard template method.Then,the effect of Zr/M atom ratios on the crystalline structure,physico-chemical properties and dehydrogenation performances of the investigated catalysts were evaluated.The results of research reveal that Zn is the optimal metal dopant.Additionally,ZnZrO-5 catalyst prepared by hard template method are comprised of nano-sized and stacked spherical particles of<10 nm in size.The catalytic dehydrogenation activity and stability of ZnZrO-5 are higher than pure ZrO₂.The initial isobutane conversion was～39.0%with an initial isobutene selectivity of 96.6%,and then isobutane conversion gradualy dropped to 32.5%with an isobutene selectivity of 94.6%after 7 h time-on-stream.（3）The analysis results of characterization and designed experiments indicate the Lewis acid-base pairs（i.e.,Zrcus4+-VO）,are produced due to the doping of Zn,and these Lewis acid-base pairs are the active sites for isobutane dehydrogenation.Moreover,the formation of these sites can be improved by the reductive treatment.The results of research shown that deposited carbonate species on ZnZrO-5 indicate that alkanes transform into alkenes and then into graphitic species or even aromatic with time-on-stream;coke deposits and the lossing of Zn species,which make active sites decrease,are the reasons of catalyst deactivation,and the former is the main one.（4）A series of supported mesoporous yZZ-5/SiO₂ catalysts were synthesized.The analyses indicate that 1.3ZZ-5/SiO₂ catalyst possesses the maximal specific surface area but the minimal pore volume and pore diameter.This catalyst possesses maximal lattice oxygen and high-dispersed active sites,thus,it exhibits higher dehydrogenation activity than ZnZrO-5,achieving an initial isobutane conversion of 42.4%.But the stability of 1.3ZZ-5/SiO₂ catalyst is a bit lower,which is related to the amount as well as the strength of acid-sites and the amount of active sites pur unit area of 1.3ZZ-5/SiO₂.