SYNGAS PRODUCTION FROM METHANE VIA DRY REFORMING USING POROUS NANOCATALYSTS

Abstract

Methane, a principal constituent of natural gas, has been categorized as a dangerous greenhouse gas. With the advent of enormous methane reserves such as methane hydrates, coal bed methane, natural gas, and shale gas around the globe, there is a need to exploit these efficiently to provide clean fuel and feedstock to produce various chemicals. The production of synthesis gas, a mixture of CO and H2, by the chemical activation of methane and CO2 can be a step forward in the direction of using methane and CO2 as chemical feedstocks as well as fuels. Different reforming processes such as steam reforming, dry reforming, partial oxidation, auto-thermal reforming have been extensively investigated around the globe for the production of syngas from methane. Catalytic dry reforming of methane (DRM) has been envisioned as a sustainable and eco-friendly technology in recent decades because this technology utilizes two greenhouse gases, i.e., methane, and CO2, as raw materials and produces syngas with an H2/CO ratio around one, which can be used in value-added chemical synthesis processes to produce long-chain hydrocarbons and oxygenates. However, the DRM process has not yet been commercialized due to the high endothermicity of the reaction and lack of a cheap, active, and stable catalyst. The literature study reveals that the Ni-based supported catalysts are the most widely investigated catalysts for the DRM process. However, catalyst deactivation by coke formation over Ni-based catalysts is still challenging. Thermal sintering is also a major issue in the development of the DRM catalyst with sustainable catalytic activity. To get a stable performance of the DRM catalyst, several parameters during and post catalyst synthesis, i.e., the structural modification of support, metal loading, surface tuning for metal-support interaction, nature of support and its porosity, catalyst particle size and calcination temperature, etc. have to be controlled and optimized. Further, the synthesis process also plays a major role in tailoring the physicochemical properties of a catalyst. Since the DRM reaction follows a bi-functional mechanism, in which both active metal as well as catalyst support actively participate in catalyzing the reactants, thereby the composition and the porous properties of catalyst support also contribute to the activity and stability of DRM catalysts. It is reported that catalysts with mesoporous and bimodal porosity shown good catalytic activity in the DRM reaction. Many supports, including SiO2, TiO2, ZrO2, CeO2, Al2O3 and MgO have been investigated for DRM reaction. It is reported that the anatase-TiO2 could be excellent support for the DRM reaction due to its ability to suppress the carbon deposition. Further, apart from the single oxide supported catalysts, mixed oxide supported catalysts like ZrO2-SiO2, Al2O3-MgO-CeO2 and TiO2-CeO2 have also been explored for DRM reaction. The optimization of catalyst recipes for better coke management might be an advance in present knowledge of experimentation in catalysis, which can help in the industrial implementation of DRM technology. In the synthesis of a heterogeneous catalyst, many synthesis variables simultaneously influence the final properties of the catalyst, and these variables are said to be mutually related by the stochastic dependencies. In a system of a large number of variables, the statistical optimization techniques such as the Taguchi method can be implemented to optimize the number of the experiment for catalyst synthesis as well as to predict the quality and quantity of catalysts. A deeper understanding of the catalyst composition and the structure performancerelationships is needed to be investigated to develop an active and stable catalyst for the DRM process. The evaluation of kinetic parameters is also important to design scale-up reactor systems for DRM. This thesis presents a systematic study on the development of a long-term active and thermally stable Ni-based catalyst for the DRM reaction. Particular emphasis has been laid on the understanding of the role of catalyst supports, pore structure, and synthesis procedure on the activity and stability of the DRM catalyst including the optimization of catalyst recipe and kinetic evaluation of the most suitable catalyst. Initially, efforts have been made to investigate the role of the porous nature of γ-Al2O3 support as well as the synthesis procedure for the deposition of Ni nanoparticles on the performance of Ni/γ-Al2O3 catalysts. It also includes optimization of synthesis produce to produce high surface area mesoporous TiO2 support as well as Ni/TiO2 catalysts and evaluation of its performance in DRM reaction. To achieve further improvement in catalyst properties the mixed oxide support TiO2 and Al2O3 containing 0-100 % Al2O3 have been utilized to produce Ni/TiO2 Al2O3 catalysts with 10 wt % of Ni loading along with their performance analysis on DRM reaction. Thereafter, the investigation has been carried to optimize a catalyst recipe for better coke management by varying seven numbers of parameters simultaneously through the design of experiment technique using Taguchi approach as well as their performance analysis in DRM process in terms of deactivation factor, initial activity (%) and catalyst productivity. Finally, the kinetic evaluation of the DRM process has also been made with Ni/γ-Al2O3 catalyst. The catalytic performance of all the catalysts was evaluated in a down-flow quartz tube reactor (ID 8mm) which was placed in the middle of an electrically operated furnace. The reactivity experiments were conducted using a feed gas mixture of CH4, CO2, and N2 in various ratios at atmospheric pressure and operating temperature between 400°C to 800°C. The physicochemical properties of the various catalysts, investigated in this study, in various stages were evaluated by means of several techniques such as ICP-MS, XRD, N2-physisorption, H2- TPR, H2-TPD, NH3-TPD, TEM, SEM, TGA/DTG, and CHN analysis. The effect of porosity of catalyst as well as the synthesis procedures on the catalytic performance of Ni nanoparticles supported on bimodal γ-Al2O3 support was evaluated by the comparison of the catalytic activity and stability of four Ni (5%)/ γ-Al2O3 bimodal pore catalysts in which Ni was introduced by the four different synthesis procedures; freeze-drying, wet impregnation, urea deposition-precipitation, and chemical vapor deposition method. The bimodal porous alumina support was prepared via the evaporation-induced self-assembly (EISA) method. Prior to catalytic activity test, the catalyst was reduced at 800°C for 2h with the flow of 20% H2 at the GHSV of 10000 mL g-1 h-1. The influence of gas hourly space velocity (GHSV) on the conversion and H2/CO molar ratio was investigated with the diluted feed at 700°C, and 1 bar pressure and the GHSV was varied from 10000-60000 ml g-1h-1. The stability of the catalyst was tested at 700 °C for 100 h at a GHSV of 10000 mL g-1 h-1 with fixed gas mixture composition (molar ratio CH4: CO2: Ar =1: 1: 8). After 100 h of time on stream study, the stable catalysts were further tested for 25 h with raw gases (molar ratio CH4: CO2 =1: 1) at GHSV of 10000 mL g-1 h-1. The initial activity of different catalysts was also evaluated under the kinetic regime of the DRM reaction. To make reactant conversions far lower than the thermodynamic conversions, initial activity experiments were carried out at a very high GHSV of 2.4x105 ml g-1h-1, 800°C, and 1 bar pressure. A Ni (5%)/γ-Al2O3 unimodal catalyst synthesized via the UD method was also compared with the performance of the bimodal catalyst. The characterization of various catalysts reveals that variation of the synthesis procedure affects the metal-support interface and the type of nickel species present on the γ-alumina support, as well as the textural properties of the catalysts. The catalytic behavior is found entirely different for each of the as-developed metal-support interface, derived by the use of different synthesis procedures. The catalyst prepared by the urea deposition-precipitation method is found to be most active and stable at 700 °C and 1bar pressure for a period of a 100 h of time on stream run with diluted gas feed and even without the dilution of feed gas for next 25h of the run. In comparison to the bimodal catalyst, a unimodal catalyst with the same metal loading exhibits the inferior catalytic activity and stability. The bimodal pore character of catalyst support prevents Ni particles from sintering during the reaction, and hence, it shows better catalytic performance and resistance to coking. It is also concluded that the metal-support interface is one of the key factors for the long-term stability of the DRM catalyst. Different physicochemical properties, in their order of the impact on the coke resisting ability of bimodal catalyst, can be arranged as; nature of metal support interface > bimodal structure of catalysts > particle size of nickel species.