Study On The Preparation Of Ni-based Catalyst Based On Layered Hydroxide And Performance For POM Reaction

Our country is resource-rich in coal-bed methane and its reserve is 3rd of the world. However, the utilization of it is low, especially for the low concentration of coal-bed methane because of the low concentration of CH₄, which is difficult to process and utilized efficiently. It is usually evacuated directly by many coal mines, which results in waste and pollution. The present research involves POM reaction with the purpose of chang ing the coal-bed methane into synthesis gas and then through the following reaction, it can be made into high value-added chemical products.POM reaction can make use of low concentration of coal-bed methane efficiently. However, for the low activity, poor stability, easy deactivation of the catalyst, it still remains in the fundamental research stage. The reasons of catalyst deactivation are carbon deposition, sintering, loss of active components, poisoning, while the main are carbon deposition and sintering. For the supported catalyst, the poor dispersion of active ingredient will result in carbon deposition and sintering easily during the reaction. Ni-Al Layered double hydroxide（LDH） catalyst was prepared via urea decomposition-homogeneous precipitation method in the research. The uniform Ni-Al Layered double oxides（LDO） catalyst with high dispersion and small particle size was prepared using the LDH as precursor and was applied in the POM reaction. What’s more the modification of Ni-Al LDO catalyst was studied. Combined with XRD, BET, H₂-TPR, TG, TEM and other characterization methods, we examine the influence of the roasting temperature and the Ni content for the properties of catalyst, Mg and assistants of La, Ce, Yb were introduced to modified the catalyst. Lastly, effects of process conditions of reaction temperature and space velocity were investigated and the following conclusions were obtained: 1. Effects of baking temperature on Ni-Al LDO catalyst performanceNi-Al LDO catalysts were prepared under different calcination temperatures（350℃、400℃、500℃、700℃）. Then POM reactive was evaluated as well as the characterizations of XRD and BET. The results showed that the catalyst prepared under 400℃ with the features of small particle, high dispersity and large specific surface which are benefit to the dispersion of active components. During the process of POM reaction, high activity was found and the corresponding conversion of CH₄ is 93% and selectivity of CO and H₂ were 81% and 80%, respectively. 2. Effects of Ni content on Ni-Al LDO catalyst performance The size of catalyst particle increased with the Ni/Al from 2 to 4, which the opposite is true for specific surface and reduction temperature. The possible reason involves the weaken force between the active components and carriers results from aggregate occurs among Ni particle which ascribed to the high Ni content. Hence the activity was affected during the process of POM reaction. The catalyst activity was best when Ni/Al = 2 and the corresponding conversion of CH₄ is 93% and selectivity of CO and H₂ were 81% and 80%, respectively. 3. Effects of Mg content on Ni-Mg-Al LDO catalyst performanceWith the Ni/Mg from 0.33 to 3, the size of active components decreases, specific surface area, pore volume and activity during the process of POM increase firstly and then decrease, the reduction temperature as well as alkalinity increases. The reason involves the excellent dispersity of catalyst was obtained as the strengthener of force between the active component and carrier results from Mg addition. The catalyst activity was best when Mg/Ni = 1 and the corresponding conversion of CH₄ is 95% and selectivity of CO and H₂ were 84%and 81%, respectively. 4. Effects of different assistants on Ni-Mg-Al LDO catalyst performanceCharacterizations activity tests, XRD and BET were applied to examine the effect of assistants of La, Ce and Yb on catalyst performance. Results showed that catalysts prepared with assistant Yb exhibit good dispersivity and high catalytic activity. And the corresponding conversion of CH₄ is 97% and selectivity of CO and H₂ were 87% and 85%, respectively. The carbon deposition is least after reaction compared with that prepared with assistant of La and Ce. The addition assistants of Yb have contributed to the form of double mesoporous structure, which is conducive to dispersion of Ni particles. Then the stability and activity of catalyst can be improved. 5. Investigation on effects of process conditions and stability testsEffects of reaction temperature and space velocity on stability of Ni Mg Al_0.9Yb_0.1 catalyst were investigated. Results showed that the conversion of CH₄ and selectivity of CO and H₂ were increase with reaction temperature. The activity of catalyst rises fastest during the range of temperature（300-750℃） and then become stable as temperature beyond 750℃. This can be attributed to the activated reaction of CH₄ is the control step under the lower temperature, and the reaction becomes more balanced when the temperature beyond 750℃.The activity of Ni Mg Al_0.9Yb_0.1 catalyst increased firstly and then decrease with the increasing space velocity. The activity reaches the best at the velocity of 2.52×104 m L ? g^-1 ? h^-1. When the space velocity below 2.52×104 m L ? g^-1 ? h^-1, diffusion res istance increases with space velocity which increase the efficiency factors of diffus ion result in the increase in conversion of CH₄. When the space velocity beyond 2.52×104 m L ? g^-1 ? h^-1, the effective collision reduction between reactant molecules and active components of catalyst.Stability tests results shown the activity of Ni Mg Al_0.9Yb_0.1 catalyst did not decrease obviously after POM reaction of 170 h. From TEM, no aggregate occurs which indicates the high stability of Ni Mg Al_0.9Yb_0.1 catalyst.