Synthesis, characterization, and catalytic evaluation of ni supported on dendritic fibrous type sba-15 (DFSBA-15) for co2 reforming of ch4 / Chong Chi Cheng

Faculty of Chemical & Natural Resources Engineering

Thesis (Doctor of Philosophy) -- Universiti Malaysia Pahang – 2019

Includes bibliographical references

CO2 reforming of CH4 (CRM) has been regarded as a promising approach to mitigate the CO2 and CH4 greenhouse gas emissions while evolving valuable syngas. Thus, great effort have been devoted on seeking well-suited catalysts for CRM with reduced coke formation, anti-sintering properties and high stability. In this study, mesoporous rodtyped SBA-15 was successfully modulated into a spherical shape with an additional dendrimer, namely Dendritic Fibrous SBA-15 (DFSBA-15). Meanwhile, DFSBA-15 with various physicochemical properties were tuned facilely by varying the preparation parameters, such as aging temperature, urea/TEOS ratio, co-surfactants types and aging time. The analysis results (XRD, BET, FTIR, TEM) confirmed the optimal conditions for DFSBA-15 synthesis were achieved at an aging temperature of 100 °C, urea/TEOS ratio of 0.5, n-butanol as co-surfactant and aging time of 12 h in term of the highest siliceous composition and highest fibre intensities. The as-synthesized optimized DFSBA-15 was also compared with conventional mesoporous SBA-15. As compared, DFSBA-15 rendered better accessibility to the active sites, higher basicity, acidity, richer siliceous framework and thermal stability, owing to the radially oriented pores which elongated to its outer surface from the nucleus of the sphere. The synthesized DFSBA-15 was loaded with Ni, applied in CO2 reforming of CH4 (CRM) for the first time, and catalytically compared with the conventional Ni/SBA-15. Characterization results indicated high basic sites, uniform Ni distribution, easier NiO reducibility, resulting in the enhanced catalytic activity, coking resistance and high catalytic stability with no deactivation for up to 30 h time-on-stream (TOS) for Ni/DFSBA-15. This study also explored the effect of Ni loadings (3, 5, 10 and 15 %) onto DFSBA-15 towards an efficient CRM. TEM images revealed the most homogeneous dispersion was shown by 10Ni/DFSBA-15 but metal agglomeration was observed for 15Ni/DFSBA-15. The optimal catalytic performance and stability of Ni/DFSBA-15 was achieved at Ni loading of 10 wt.%. It is noteworthy that the existence of an appropriate Ni loading in DFSBA-15 resulted in strong Si-O-Ni interaction, moderate NiO crystallite size, and homogeneous active metal dispersion which led to strong synergistic effect between Ni metal active sites and the DFSBA-15 support and also abundant active sites accessibility. Moreover, the operating conditions of Ni/DFSBA-15 over CRM were optimized using response surface methodology (RSM), followed by stability and regeneration study of the catalyst. Process parameters such as reaction temperature (X1, 700-900 °C), gas hourly space velocity (X2, 15,000-35,000 mL/g⸱h), and CH4/CO2 ratios (X3, 1-3) were studied with respect to CO2 conversion (Y1), CH4 conversion (Y2), and H2/CO ratio (Y3). The optimal reaction conditions were found at X1 = 794 °C, X2 = 23,815 mL/g⸱h, and X3 = 1.2. It was also proven that synthesized Ni/DFSBA-15 catalyst was appreciably stable for 30 h TOS and prone to be regenerated by air for catalytic CRM under the optimal conditions. In summary, Ni/DFSBA-15 was successfully produced and emerged as a high sustainability catalyst for efficient syngas production via CRM under optimal conditions generated from RSM. The in-situ FTIR studies of adsorbed CH4, CO2, and CH4 + CO2 confirmed the CRM of Ni/DFSBA-15 initiated by CO2 adsorption and dissociation reaction pathway by forming unidentate, and bidentate carbonates as well as linear carbonyl species. The 10Ni/DFSBA-15 catalyst performed the best with high stability under optimum conditions generated by RSM and proved its ability to be regenerated by air.