Development of vanadium-phosphate catalysts for methanol production by selective oxidation of methane

Abstract

This document is the seventeenth quarterly technical progress report under Contract No. DE-AC22-92PC92110 `Development of Vanadium- Phosphate Catalysts for Methanol Production by Selective Oxidation of Methane` and covers the period April-June, 1997. Vanadium phosphate, vanadyl pyrophosphate specifically, is used commercially to oxidize butane to maleic anhydride and is one of the few examples of an active and selective oxidation catalyst for alkanes. In this project we are examining this catalyst for the methane oxidation reaction. Initial process variable and kinetic studies indicated that vanadyl pyrophosphate is a reasonably active catalyst below 500{degrees}C but produces CO as the primary product, no formaldehyde or methanol were observed. A number of approaches for modification of the catalyst to improve selectivity have been tried. Results obtained earlier in this project are summarized under Project Description in the body of this report. Iron phosphate and iron phosphate supported on silica catalysts have been shown in our previous work to produce much higher yields of partial oxidation products from methane than VPO. During this quarter we have expanded these studies dramatically by detailed testing of a new silica support, by performance of detailed kinetic and product selectivity studies on the quartz form of FePO{sub 4}, both unsupported and supported on silica, by testing of a mixed valence iron phosphate Fe{sub 4}(P{sub 2}0{sub 7}), and by detailed characterization of and other materials by a number of methods including Moessbauer spectroscopy. The most selective catalyst examined to date is FePO{sub 4} supported on silica. This material has produced formaldehyde with space time yields of nearly 500 g/kg-h. Methanol yields are low but quantifiable at roughly 10 g/kg-h. Interestingly, addition of water to the feed gas produces large improvements in the formaldehyde yield by suppression of the parallel reaction to form carbon dioxide. Increasing oxygen partial pressure over this catalyst did not produce the expected drop in formaldehyde yield and in fact, formaldehyde yield actually increased. Characterization via Moessbauer spectroscopy appears to be a very valuable tool for examination of the iron phosphate system. Spectra of several fresh catalysts were obtained providing detailed information on the chemical environment of iron. While detailed analyses of these data have not yet been completed, it is apparent that iron exists in different chemical states in all of the catalytic materials being studied. Interestingly, the Fe-promoted VPO catalyst is shown to contain a small fraction of Fe {sub 2} by Moessbauer. This result is unexpected and may have important implications for the mechanism of the promoting effect as well as for the role of mixed valence species in iron phosphate methane oxidation catalysts. Additionally, in May a poster describing the results for VPO was presented at the North American Catalysis Society Meeting in Chicago. A paper describing these results has also been accepted for publication in Journal of Catalysis.