Formaldehyde is a large volume industrial chemical, which is employed in the manufacture of numerous products. Polycrystalline silver catalyst is widely used for the oxidation of methanol to formaldehyde. Despite the technology being 90 years old there still exist many problems concerning silver catalyst performance. At present there is no consensus of opinion as to the underlying chemical reason for this behaviour. Therefore, this thesis attempts to resolve this debate by elucidating the relationship between silver structure, oxygen chemistry, mechanism of methanol oxidation and the influence of catalyst poisons.
A variety of commercially available silver catalysts were studied in an effort to determine the critical catalytic factors. The purity of all commercial silver catalysts was found to be a minimum 99.99 % and thus purity was not deduced to be of great importance. In contrast, significant differences were recorded for the bulk packing densities for the evaluated silver catalysts. Indeed, bulk-packing densities were shown to range from values of ca. 1 g/mL to over 4 g/mL and surface areas ranged from ca. 50 cm2/g to in excess of 1000 cm2/g. The other main difference in the composition of the fresh silver catalysts was the degree of silver oxidation.
Raman spectroscopy was used to identify the nature of the oxygen species present on silver catalyst. The following assignments were made: 230-240 cm-1 (molecular and/or strong bound atomic oxygen), 320-360 cm-1 (embedded atomic oxygen v(Ag-O) stretch mode for Ag-O-Ag), 370-390 cm-1 (v(-OH) mode for OOH species), 570-600 cm-1 (librational mode for subsurface water), 630-675 cm-1 (dissolved atomic oxygen), 765- 785 cm-1 (embedded atomic oxygen (Ag-O-Ag)), (v(Ag-0)), and 855-865 cm-1 (v(-O-O) of OOH species), 950-965 cm-1 (electrophilic oxygen v(Ag=0), 1125-1135 cm-1 (OOH bending mode). Carbon containing species were detected with bands identified at 690- 710 and 1036-1045 cm-1 which are δ(C0) and v(CO) modes respectively for mono and bi-dentate carbonate species.
The nature of the interaction of oxygen with polycrystalline silver catalyst was a function of temperature. Three distinct temperature regions were determined to be of relevance: (1) 473K-573K; desorption of weakly bound species such as carbonate and the segregation of dissolved nitrate species to the catalyst surface, (2) 573-773; segregation of dissolved water to the catalyst surface and subsequent explosive desorption at 773 K. Removal of nitrate species and initiation of oxygen induced surface reconstruction to terrace and steps structure and (3) 773-873K; the widespread formation of novel embedded atomic oxygen species (ca 775 cm-1), which were found to be stable up to > 873 K.
When polycrystalline silver catalyst interacted with methanol/oxygen mixtures under industrial conditions (673-873K), the main finding was the consumption of embedded oxygen Ag-O-Ag at ~775 cm-1 with reactant methanol to produce formaldehyde. SEM micrographs revealed a widespread pinhole formation as a result of the explosive release of water, which was believed to have formed via the reaction of hydrogen product from the methanol dehydrogenation reaction process (which occurred at embedded oxygen sites) and dissolved oxygen species in the subsurface region of silver. The reaction of methanol with embedded oxygen (Ag-O-Ag) was proposed to be selective towards formaldehyde formation. In contrast, adsorbed atomic oxygen species were considered to be non selective in behaviour.
Interestingly, in every industrial catalyst run that was analysed it was found that poisons were present on the catalyst surface. Therefore, it appears that the importance of catalyst poisoning upon the methanol oxidation reaction may previously have been substantially underestimated. Iron was identified as the most common poison in formaldehyde plants. The source of the iron contaminant was usually the methanol feedstock, process water or rust that deposited on the bed as a consequence of degradation of plant components. Indeed, in several catalyst beds, iron was found to be co-present with nickel and chromium, which are typical components of steel. Iron was determined to be responsible VI for enhanced methanol combustion and decreased catalyst life due to its ability to act as a sintering promoter for the silver bed.
Calcium and magnesium were also relatively common poisons present on the catalyst beds with the obvious source of these elements being inefficiently purified process water (as these elements are common to "hard water"). Calcium and magnesium did not appear to promote any undesirable side reactions but they did poison the silver by means of physically blocking active sites on the catalyst surface. Thus, these latter poisons mainly affected the degree of methanol conversion.
Silicon and aluminium were common poisons, with two possible sources being dust and contaminated water. Aluminium appeared to promote the formation of formic acid, which is an undesired constituent for resin manufacture. Whereas, silicon appeared to be relatively inert material, which poisoned silver catalyst by means of blocking the active site of the catalyst.
Copper was also known as poison upon silver catalyst due to its low activity and physically blockage of silver active sites towards formaldehyde on methanol oxidation reaction. It is found in gasket, reactor walls/lid/flanges and gauze.
In addition, other elements were found on the catalyst surface namely zinc, sulphur, chlorine, sodium and titanium. These elements are mainly found in contaminated water and dust. The role of these elements was not totally defined but at the very least they will cause blocking of the active sites of the catalyst.