Partial Oxidation of Methanol to Formaldehyde Over an Electrolytic Silver Catalyst

Abstract

The partial oxidation of methanol to formaldehyde over an electrolytic silver catalyst is a large-scale industrial process that provides the feedstock for the manufacture of synthetic resins, plastics and important chemical intermediates such as 1,4-butanediol and methylene diphenyl diisocyanate (MDI). The process is carried out at atmospheric pressure by passing CH3OH vapour, in the presence of air and steam, through a thin bed of electrolytic silver catalyst operating adiabatically at temperatures between 873-973 K. Process yields of CH2O may be as high as 88-90 %. Competing reactions lead to the formation of CO2, HCOOH and CO. The objective of this thesis was to gain a better understanding of the reaction mechanism and those factors which influence silver catalyst activity and selectivity to CH2O during CH3OH oxidation. Through the application of a variety of experimental techniques it is shown that the dissociative chemisorption of 02 activates electrolytic silver catalysts for CH3OH oxidation. Over the temperature range 448-1073 K, the oxygen inventory of silver catalysts comprises three distinct atomic oxygen species: two surface species (denote Oα and Oγ) and a bulk-dissolved species (Oβ). The surface species are distinguishable by their Ag-O bonding, thermal stability and reactivity differences. Oα is formed by the dissociative chemisorption of O2 on Ag(110) or Ag(111) planes. The species is weakly bound, possesses bridging Ag-O-Ag bonding and strong nucleophilic character, and opens reaction pathways towards CH2O, CO2 and HCOOH. Recombinative desorption of Oα as O2 commences at temperatures above 580 K. Oγ formation occurs exclusively on reconstructed Ag(111) planes. The species possesses a highly covalent Ag=O bonding interaction and exists to temperatures in excess of 923 K in the presence of gas phase O2. Oγ activates silver catalysts for the oxidative-dehydrogenation of CH3OH to CH2O + H2O, but shows no selectivity towards CO2 or HCOOH production. Bulk-dissolved oxygen (Oβ) exchanges reversibly with the Oα and Oγ species. The product distribution of CH3OH oxidation over electrolytic silver catalysts is controlled by the relative surface populations of the Oα and Oγ states. Formaldehyde selectivity and yield increase with temperature up to 923 K, CH3OH/O2 feed ratio from 1.5-2.25 and catalyst grain boundary density, all of which reflect a corresponding increase in the surface Oγ / Oα ratio on the catalysts. An optimum formaldehyde yield of 84.3 % was obtained during testing of the silver catalysts, under conditions close to those employed industrially (temperature = 923 K, molar feed composition CH3OH/O2/H2O/He = 2.25/1/1.7/20, GHSV = 1.25x105 h-1). Above 923 K, yields decreased due to the homogeneous decomposition of CH2O to CO + H2. Pronounced thermal and catalytic etching of the silver catalyst occurred during activation under conditions of industrial CH2O synthesis. Critically, structures created by these processes promote Oβ and Oγ formation, and improve catalyst performance. Results were used to develop comprehensive reaction schemes for CH3OH oxidation over electrolyic silver catalysts. The thermal decomposition of Ag1Ag111O2 in air was examined in relation to the industrial practice of adding silver oxides to silver catalyst beds to facilitate 'light off' during start up operations. Ag1AglllO2 was thermally reduced to metallic silver via two non-reversible steps, with the intermediate formation of Ag2O. The transformation of Ag1Ag111O2 to Ag2O occurred with heating in the 373-473 K region, while the product of this reaction remained stable to temperatures in excess of 623 K. Complete thermal decomposition of the Ag2O intermediate to Ag and O2 occurred at673 K. The oxidation of several silver substrates by reaction with ozone (5 mol % O3 in 02) was examined as potential route to the production of silver catalysts of high initial activity towards CH3OH oxidation. At 300 K, the Ag substrates were oxidised by O3 to yield Ag2O and monoclinic Ag1Ag111O2. The Ag1Ag111O2, formed at the gas/oxide interface, via the oxidation of Ag2O.