Establishing Experimental and Characterization Methodology for Analyzing the Performance of Silver Catalysts for Formalin Production

摘要

Formaldehyde is the essential component of wood adhesives, which can be used for a wide range of applications and is an important intermediate in the production of several fine chemicals. Industrially production of formaldehyde from methanol are produced from two main processes, where Dynea has identified oxidation over a silver-based catalyst in excess methanol as the favored technique for formaldehyde production. This thesis is combined with parts of the specialization project, and aims at establishing a suitable characterization procedure and experimental setup methodology for silver catalysts. The catalyst provided was a structured silver foam catalyst produced by Alantum, which is not applied in conventional production. Results from fresh and plant exposed Alantum catalyst were analyzed and compared in the characterization analyses. A wide range of characterization tools were evaluated, such as scanning electron microscopy (SEM-EDX), X-ray diffraction (XRD), X-ray fluorescence spectroscopy (XRF), temperature programmed techniques (TPX), chemisorption and nitrogen adsorption. Nitrogen adsorption was unsuccessful in calculating a surface area for the silver catalyst, due to low surface area. The low surface area influenced chemisorption, and the analysis resulted in low dispersion and metallic surface area. XRD showed that the catalyst contained cubic (FCC) silver, with a change in lattice constant for the exposed catalyst. The crystallite size was calculated in Topas, resulting in a decrease of crystallite size for exposed samples. XRF and EDX confirmed the majority of silver, and showed small traces of contaminations such as carbon formation. Redox properties of silver oxide were investigated in TGA, showing a thermodynamically favorable reduction in air/inert atmosphere at high temperatures. Temperatures favoring silver oxidation based on thermodynamics were controlled by slow kinetics. SEM micrographs were attained for both fresh and exposed samples, showing structural and morphological changes. The majority of the characterization techniques gave results, but with an indication of improvement possibilities. However, to fully understand characteristics of silver as a catalyst, advanced surface sensitive techniques should be considered. Lack of reproductions for several of the analyses are also a significant aspect.The aim of the project was originally to relate activity of the catalyst against advanced characterization results. However, the experimental setups operational functions and analyses proved to be of large interest. The project was shifted to focus more on the experimental setup, with a main goal to establish an experimental methodology for formaldehyde synthesis experiments. This was performed in collaboration with fellow student Vegard Naustdal. A conventional silver particulate catalyst produced by K.A. Rasmussen, was applied to generalize the analyses. Temperature, total flow and composition of the feed were under investigation, with an overall objective to identify variables critical to formaldehyde selectivity. A reactor was designed for the structured silver foam catalyst and an analysis mainly investigating temperature changes were performed. Making an ideal and practical setup is a challenging project, especially since the MTF synthesis is not a trivial process. This is due to fast, exothermic reactions involved and the challenges associated with maintaining product stability. Results from the experimental setup illustrated the significance of temperature and flow in both selectivity and conversion. A large temperature gradient was observed after the first analysis, indicating that an improved oven design would have positive influence. Based on former research, hypotheses regarding different oxygen species were deemed dependent on temperature and influential in reaction pathways and products. Results indicated that methanol conversion is highly dependent on oxygen concentration in the feed. Decreasing the void volume with silicon carbide resulted in the highest value of formaldehyde selectivity (82%) with meaningful conversion (90%). Hence, promoting the importance of reactor design. A reference analysis with an empty reactor was performed to further investigate gas phase reactions. Concluding that the empty reactor experiments showed negligible conversion of methanol below gas phase reaction temperatures (about 600 ᵒC). For the analysis of experimental Alantum catalyst, the structured catalyst was mounted inside the reactor when welded, with a glass tube protecting it from heat. Volume prior to the bed was decreased to reduce void space for gas phase reactions. Results showed low conversion of methanol, indicating a bypass of the feed at the edges and void volume of the catalyst itself. New reactor design and sample mounting needs further investigation.