Preparation And Application Of Highly Thermally Stable Syngas Methanation Catalysts

With the ever-increasing demand for natural gas and increasingly rigorous environmental requirements,it’s of great importance to identify new sources of natural gas.The energy resources in our country are featured as "rich coal and less gas".Therefore,coal-to-natural gas is one of the effective ways to achieve clean coal utilization,where methanation of syngas is a core conversion process.Methanation of syngas mainly refers to the process in which CO and CO₂ are hydrogenated to generate methane in the presence of a catalyst.It is worth noting that nickel-based catalysts are the main choice for industrial methanation catalysts because of good activity,high selectivity,and relatively low price.Because of the exothermic nature of syngas methanation reaction,the development of highly stable catalysts for the use under extreme thermal condition is the key for their industrial deployment.The culprits of catalysts deactivation mainly include formation carbonyl compounds,sulfur poisoning,carbon deposition,and nickel sintering.In order to prevent sulfur poisoning and the formation of carbonyl compounds from occuring,deep pre-desulfurization of the syngas feed and relatively high-temperatures are often used in the industrial practices.However,carbon deposition and nickel sintering at high temperature are still two challenging technical problems faced by the nickel-based catalysts in methanation process.Both the coke deposition and the sintering of nickel particles are related to the overheating of the catalyst bed caused by the strong exothermic characteristics of methanation.Therefore,it is very important to develop a new type of anti-coking and anti-sintering catalyst with excellent thermal stability.Spinel supports have the advantages of superior hydrothermal stability,and low cost.Here we prepared the spinel supports by co-precipitation method which give well-defined structure and morphology,which are beneficial to the dispersion of metals.In this thesis work,spinel-based supports are used to inhibit Ni sintering through the strong interaction between the support and metal particles.Meanwhile,good metal dispersion achieved in the spinal support can also help prevent carbon deposition.Thus a new type of anti-carbon deposition and anti-sintering methanation catalyst with high thermal stability is successfully prepared.We further optimized the conditions in the catalyst synthesis including reduction temperature,concentration,hydrothermal temperature,and Ni content,in order to develop a facile and cost-effective process to readily scale up the synthesis.The main work is as follows:1.First,we prepared Ni-based catalysts supported on Al₂O₃ and ZnAl₂O₄ with co-precipitation method.The prepared catalysts were characterized by XRD,XRF,H2-TPR,N2 isothermal adsorption and desorption.The activity and stability were tested under the appropriate reaction conditions,which are selected by theoretical calculations.It shows that both catalysts can achieve high CO conversion and CH₄ selectivity.The results from 35%Ni/Al₂O₃ showed that the CO conversion rate dropped from 89%to 70%within 95 h,while CO conversion on 35%Ni/ZnAl₂O₄ remained around 90%.Through XRD characterization,the Ni crystal size in 35%Ni/Al₂O₃ did not grow significantly,but the crystal phase of NiAl₂O₄ was found to appear instead.The size of Ni particles in 35%Ni/ZnAl₂O₄ increased from 17.7 nm to 20.2 nm though,no obvious NiAl₂O₄ phase was identified.The two catalysts were subsequently subjected to high-temperature steam aging treatment.The XRD characterization showed that the majority of 35%Ni/Al₂O₃ after hydrothermal transformed to NiAl₂O₄ phase after aging treatment,while the crystal structure of 3 5%Ni/ZnAl₂O₄ remained intact.The activity test results showed that the 35%Ni/Al₂O₃ was no longer active whereas the 35%Ni/ZnAl₂O₄ still maintained high activity.This further confirmed the stability of the 35%Ni/ZnAl₂O₄ catalyst,which can effectively inhibit Ni sintering and avoid the formation of inert NiAl₂O₄.A higher reduction temperature would lead to a larger Ni crystal size,resulting in lower reactivity.Accordingly,the reduction temperature of the catalysts was optimized to obtain smaller metal particles and more active catalysts.Through the H2-TPR characterization,the reduction temperature can be reduced from 800℃ to 520℃.The activity test showed that 35%Ni/Al₂O₃ reduced at lower temperature is no longer active,while 35%Ni/ZnAl₂O₄ still has high CO conversion and high CH₄ selectivity.2.The second chapter mainly focuses on the optimization of catalysts preparation conditions,including precursor concentration,hydrothermal temperature,Ni content and calcination temperature.The catalysts were prepared by the controlled variable method.Then,the effects of the crystal size,specific surface area,and actual content of Ni of the catalyst were systematically evaluated,which was then used to screen for better catalysts in activity tests.The results showed that 25 wt.%is the minimum Ni content to maintain activity for high temperature（800℃）reduction.Instead,to ensure the activity for catalysts reduced at a lower temperature（e.g.,520℃）,suitable preparation conditions are required,such as hydrothermal temperature higher than 200℃,concentration>0.02 mol/100 mL,Ni content>30%.Reduction at high temperatures can reduce more extent of Ni,so that the catalysts would have better activity than the counterparts reduced at lower temperature.Moreover,high concentration of the precursor（0.03 mol/100 mL）tends to form smaller particles,and lower hydrothermal temperature（below 150℃）was found to be difficult to form a stable ZnAl₂O₄ structure.The reduction temperature has little effect on the size of ZnAl₂O₄,but has more profound impact on the size of Ni.It should be noted that when Ni particles and ZnAl₂O₄ support are manipulated to have a comparable size through high temperature reduction,the catalyst was found to exhibit better anti-sintering ability.