1. Field of the Invention
This invention broadly relates to a process for producing formaldehyde from a gas stream containing a mixture of hydrogen sulfide (G2S) and a carbon oxide, wherein the carbon oxide is selected from carbon monoxide (CO), carbon dioxide (CO2) and mixtures thereof. More particularly, this invention provides a method wherein a gas stream containing a carbon oxide and hydrogen sulfide is first passed in contact with a catalyst comprising a supported metal oxide of a metal selected from the group consisting of vanadium (V), niobium (Nb), molybdenum (Mo), chromium (Cr), rhenium (Re), tungsten (W), manganese (Mn), titanium (Ti), zirconium (Zr) and tantalum (Ta) and mixtures thereof to convert said carbon oxide and hydrogen sulfide to methyl mercaptans, (primarily methanethiol (CH3SH) and a small amount of dimethyl sulfide (CH3SCH3)), and the methyl mercaptans are then passed in contact with a catalyst comprising certain supported metal oxides or certain bulk metal oxides in the presence of an oxidizing agent and for a time sufficient to convert at least a portion of the methyl mercaptans to formaldehyde (CH2O) and sulfur dioxide (SO2).
2. Description of Related Art
Ratcliffe et al., U.S. Pat. No. 4,570,020 describes a catalytic process for producing methanethiol (CH3SH) from a gaseous feed comprising a mixture of carbon monoxide (CO) and hydrogen sulfide (H2S). Gases containing H2S are often considered an unwanted waste stream. According to the patent, the gaseous mixture is contacted, at a temperature of at least about 225xc2x0 C. with a catalyst comprising a metal oxide of a metal selected from the group consisting of vanadium (V), niobium (Nb), and tantalum (Ta) and mixtures thereof supported as an oxide layer on titania. The methanethiol is disclosed as being useful as an odorant or tracer for natural gas and as a raw material for making methionine, fungicides and jet fuel additives.
The art has also identified methyl mercaptans, such as methanethiol (CH3SH) and dimethyl sulfide (CH3SCH3), as hazardous pollutants, and has suggested a variety of ways for their destruction. Noncatalytic gas phase oxidation of such reduced sulfur compounds has been shown to produce primarily sulfur oxide and carbon oxide products. A. Turk et al., Envir. Sci. Technol 23:1242-1245 (1989). Investigators have observed that oxidation in the presence of single crystal metal surfaces (Mo, Ni, Fe, Cu) results in the formation of methane and ethane, nonselective decomposition to atomic carbon, gaseous hydrogen and the deposition of atomic sulfur on the metal surface via a stoichiometric reaction (See Wiegand et al., Surface Science, 279(1992): 105-112). Oxidation of higher mercaptans, e.g., propanethiol on oxygen-covered single crystal metal surfaces (Rh), produced acetone via a stoichiometric reaction at low selectivity and accompanied by sulfur deposition on the metal surface (See Bol et al., J. Am. Chem. Soc., 117(1995): 5351-5258). The deposition of sulfur on the metal surface obviously precludes continuous operation.
The art also has disclosed using catalysts comprising a two-dimensional metal oxide overlayer on titania and silica supports, e.g., vanadia on titania, for catalytically reducing NOx by ammonia to N2 and H2O in the presence of sulfur oxides. Bosch et al., Catal. Today 2:369 et seq. (1988). Thus, such catalysts are known to be resistant to poisoning by sulfur oxides. It also is known that such catalysts, as well as certain bulk metal oxides catalysts, can be used to oxidize methanol to formaldehyde selectively. Busca et al, J. Phys. Chem. 91:5263 et seq. (1987).
Applicant recently made the discovery that a supported metal oxide catalyst can be used to oxidize methyl mercaptans, such as methanethiol (CH3SH) and dimethyl sulfide (CH3SCH3), selectively to formaldehyde in a continuous, heterogenous catalytic process without being poisoned by the reduced sulfur. On the basis of that discovery, applicant has envisioned the present process as a way of converting gaseous streams containing carbon oxide and hydrogen sulfide to formaldehyde.