Methane conversion process

An improved method for converting methane to higher hydrocarbon products by contacting a hydrocarbon gas comprising methane, an oxygen-containing gas and a reducible metal oxide under synthesis conditions, the improvement which comprises contacting methane and oxygen with a contact solid which comprises at least one manganese silicate.

BACKGROUND OF THE INVENTION 
This invention relates to the synthesis of hydrocarbons from a methane 
source. A particular application of this invention is a method for 
converting natural gas to more readily transportable material. 
A major source of methane is natural gas. Other sources of methane have 
been considered for fuel supply, e.g., the methane present in coal 
deposits or formed during mining operations. Relatively small amounts of 
methane are also produced in various petroleum processes. 
The composition of natural gas at the wellhead varies but the major 
hydrocarbon present is methane. For example, the methane content of 
natural gas may vary within the range from about 40 to about 95 volume 
percent. Other constituents of natural gas include ethane, propane, 
butanes, pentane (and heavier hydrocarbons), hydrogen sulfide, carbon 
dioxide, helium and nitrogen. 
Natural gas is classified as dry or wet depending upon the amount of 
condensable hydrocarbons contained in it. Condensable hydrocarbons 
generally comprise C.sub.3 + hydrocarbons carbons although some ethane may 
be included. Gas conditioning is required to alter the composition of 
wellhead gas, processing facilities usually being located in or near the 
production fields. Conventional processing of wellhead natural gas yields 
processed natural gas containing at least a major amount of methane. 
Large scale use of natural gas often requires a sophisticated and extensive 
pipeline system. Liquefaction has also been employed as a transportation 
means, but processes for liquefying, transporting, and revaporizing 
natural gas are complex, energy-intensive and require extensive safety 
precautions. Transport of natural gas has been a continuing problem in the 
exploitation of natural gas resources. It would be extremely valuable to 
be able to convert methane (e.g., natural gas) to more readily handleable 
or transportable products. Moreover, direct conversion of olefins such as 
ethylene or propylene would be extremely valuable to the chemical 
industry. 
Recently, it has been discovered that methane may be converted to higher 
hydrocarbons (e.g., ethane, ethylene and higher homologs) by contacting 
methane with a reducible metal oxide as a selective oxygen source. As the 
methane is converted to hydrocarbon products and coproduct water, the 
active oxygen of the metal oxide is depleted, resulting in a reduced metal 
oxide. The reduced metal oxide is relatively inactive for the oxidative 
conversion of methane but active oxygen may be replaced by regenerating of 
a reducible metal oxide. Such regeneration is accomplished by reoxidation 
of the reduced metal oxide. 
Reducible oxides of several metals have been identified which are capable 
of converting methane to higher hydrocarbons. Oxides of manganese, tin, 
indium, germanium, lead, antimony and bismuth are particularly useful. See 
commonly-assigned U.S. patent application Ser. Nos. 522,925 (now U.S. Pat. 
No. 4,443,649); 522,944 (now U.S. Pat. No. 4,444,984); 522,942 (now U.S. 
Pat. No. 4,443,648); 522,905 (now U.S. Pat. No. 4,443,645); 522,877 (now 
U.S. Pat. No. 4,443,647); 522,876 (now U.S. Pat. No. 4,443,644); and 
522,906 (now U.S. Pat. No. 4,443,646), all filed Aug. 12, 1983, the entire 
contents of each being incorporated herein by reference. 
Commonly-assigned U.S. patent application Ser. No. 522,935, filed Aug. 12, 
1983, discloses and claims a process which comprises contacting methane 
with an oxidative synthesizing agent under elevated pressure (e.g., 2-100 
atmospheres) to produce greater amounts of C.sub.3 + hydrocarbon products. 
Commonly-assigned U.S. patent application Ser. No. 522,938, filed Aug. 12, 
1983, discloses and claims a process for the conversion of methane to 
higher hydrocarbons which comprises contacting methane with particles 
comprising an oxidative synthesizing agent which particles continuously 
recirculate between two physically separate zones--a methane contact zone 
and an oxygen contact zone. 
In a typical application of the foregoing processes for the oxidative 
conversion of methane, methane feed is contacted with a reducible metal 
oxide and regeneration is accomplished separately by contacting the 
reduced metal oxide with an oxygen-containing gas (e.g., air). Thus, a 
cyclic redox process results in which methane reaction and reoxidation of 
the metal oxide "reagent" are performed separately and repeatedly for a 
continuous process. 
Such a procedure presents several disadvantages for large scale continuous 
operation. One disadvantage is the large quantity of solid cycling between 
methane reaction and reoxidation in such a way that the methane and oxygen 
are not mixed. Another disadvantage is the necessity of developing a 
composition that is resistant to mechanical attrition and repeated 
exposure to reductive and oxidative environments. 
Hinsen and Baerns recently reported studies of a continuous mode for the 
oxidative coupling of methane wherein regenerating air is cofed with the 
methane feed. Hinsen, W. and Bearns, M., "Oxidative Kopplung von Methan zu 
C.sub.2 -Kohlenwasserstoffen in Gegenwart unterschiedlicher 
Katalysatoren", Chemiker-Zeitung, Vol. 107, No. 718, pp. 223-226 (1983). 
Using a catalyst based on lead oxide and gamma-alumina in a fixed bed 
reactor operated at 1 atmosphere total pressure and 
600.degree.-750.degree. C., they report results of approximately 53% 
selectivity to ethane and ethylene at 8% methane conversion for a feed 
consisting of about 50% methane, 25% air and 25% nitrogen. Other metal 
oxides studied by Hinsen and Baerns included oxides of Bi, Sb, Sn and Mn. 
SUMMARY OF THE INVENTION 
It has now been found that the conversion of methane to higher hydrocarbons 
in the presence of oxygen is improved by contacting a first, hydrocarbon 
gas comprising methane and a second, oxygen-containing gas with a contact 
solid which comprises at least one compound comprising Mn, Si and O, 
preferably at least one manganese silicate. Preferred manganese silicates 
are described by the formula Mn.sub.x SiO.sub.y wherein x is an integer 
selected within the range of 1 to 7 and y has a value which is determined 
by the valence and proportions of the other elements present in the 
compound. 
The improved process of the present invention produces higher methane 
conversion at similar hydrocarbon selectivity or increased hydrocarbon 
selectivity at similar methane conversion, as compared to prior methods 
such as that taught by Hinsen and Baerns, supra. 
DETAILED DESCRIPTION OF THE INVENTION 
In addition to methane the hydrocarbon feedstock employed in the method of 
this invention may contain other hydrocarbon or non-hydrocarbon 
components. The methane content of the feedstock, however, will typically 
be within the range of about 40 to 100 vol. %, preferably within the range 
of about 80 to 100 vol. %, more preferably within the range of about 90 to 
100 vol. %. 
The oxygen-containing gas generally comprises molecular oxygen: other gases 
such as nitrogen and carbon oxides may be present. A preferred 
oxygen-containing gas is air. 
The ratio of hydrocarbon feedstock to oxygen-containing gas is not narrowly 
critical to the present invention. Generally, it is desirable to control 
the hydrocarbon/oxygen molar ratio to avoid the formation of gaseous 
mixtures within the flammable region. It is preferred to maintain the 
volume ratio of hydrocarbon/oxygen within the range of about 0.1-100:1, 
more preferably within the range of about 1-50:1. Methane/air feed 
mixtures containing about 50 to 90 volume % methane have been found to 
comprise a desirable feedstream. Further dilution of the feedstream with 
gases such as nitrogen is not necessary. 
Manganese silicates suitable for use in the process of the present 
invention may be provided from a variety of sources. For example, the 
silicates are found in naturally-occurring minerals such as rhodonite 
(MnSiO.sub.3), pyroxmangite (MnSiO.sub.3), braunite (Mn.sub.7 SiO.sub.12) 
or tephroite (Mn.sub.2 SiO.sub.4). The silicates may also be synthesized 
by methods known in the art. 
According to one method of synthesis, salts of manganese are mixed with 
silica in amounts corresponding to the desired stoichiometry, followed by 
heating to about 1000.degree.-1200.degree. C. in air. The manganese salts 
should be ones that thermally decompose to yield manganese oxides. 
Suitable salts include the acetonates, carbonates and nitrates. 
In one preferred embodiment, the manganese silicates are synthesized by 
mixing a methanolic manganese acetate solution with tetraethoxy silane in 
amounts selected to provide the desired manganese to silica stoichiometry. 
A gel is precipitated from the resulting solution by addition of ammonium 
hydroxide. The gel is dried (e.g., at 100.degree.-120.degree. C.) and then 
heated (e.g., to 1000.degree.-1200.degree. C.) to form the manganese 
silicate. Particles comprising manganese silicate can be prepared from 
this material by standard methods. 
If desired, manganese silicates may be associated support materials such as 
silica, alumina, titamia, zirconia and the like and combinations thereof. 
Supported manganese silicates may be prepared by methods such as 
adsorption, impregnation, coprecipitation and dry mixing. 
Regardless of the particular form in which manganese silicates is provided, 
it is desirable to calcine the contact solid at elevated temperatures in 
an oxygen-containing gas (e.g., air) prior to use in the process of this 
invention. 
Preferably, methane and oxygen are contacted with solids comprising 
manganese silicate in the substantial absence of catalytically effective 
nickel, noble metals and compounds thereof. (i.e., nickel, rhodium, 
palladium, silver, osmium, iridium, platinum and gold) to minimize the 
deleterious catalytic effects thereof. These metals, when contacted with 
methane at the temperatures employed in the first step of the present 
invention, tend to promote coke formation, and the metal oxides tend to 
promote the formation of combustion products rather than the desired 
hydrocarbons. The term "catalytically effective" is used herein to 
identify that quantity of one or more of nickel and of the noble metals 
and compounds thereof which substantially changes the distribution of 
products obtained in the method of this invention relative to such 
contacting in the absence of such metals and compounds thereof. 
Operating temperatures for the method of this invention are generally 
within the range of about 300.degree. to 1200.degree. C., more preferably 
within the range of about 500.degree. to 1000.degree. C. 
Operating pressures are not critical to the presently claimed invention. 
However, both general system pressure and partial pressures of methane and 
oxygen have been found to effect overall results. Preferred operating 
pressures are within the range of about 0.1 to 30 atmospheres. 
The space velocity of the gaseous reaction streams are similarly not 
critical to the presently claimed invention, but have been found to effect 
overall results. Preferred total gas hourly space velocities are within 
the range of about 10 to 100,000 hr..sup.-1, more preferably within the 
range of about 600 to 40,000 hr..sup.-1. 
The solid which is contacted with methane and oxygen according to the 
present process has heretofore been generally referred to as an oxidative 
synthesizing agent. Oxidative synthesizing agents comprise at least one 
oxide of at least one metal, which oxides when contacted with methane at 
temperatures selected within the range of about 500.degree. to 
1000.degree. C. produce higher hydrocarbon products, coproduct water and a 
reduced metal oxide. The composition thus contains at least one reducible 
oxide of at least one metal. The term "reducible" identifies those oxides 
of metals which are reduced by the methane contact. The term "oxide(s) of 
metal(s)" includes: (1) one or more metal oxides (e.g., compounds 
described by the general formula M.sub.x O.sub.y wherein M is a metal and 
the subscripts x and y designate the relative atomic proportions of metal 
and oxide in the composition) and/or (2) one or more oxygen-containing 
metal compounds, provided that such oxides and compounds have the 
capability of performing to produce higher hydrocarbon products as 
described. 
Contacting methane and a reducible metal oxide to form higher hydrocarbons 
from methane also produces coproduct water and reduces the metal oxide. 
The exact nature of the reduced metal oxides are unknown, and so are 
referred to as "reduced metal oxides". Regeneration of reducible metal 
oxides in the method of the present invention occurs "in situ"--by contact 
of the reduced metal oxide with the oxygen cofed with methane to the 
contact zone. 
The contact solids may be maintained in the contact zone as fixed, moving, 
or fluidized beds of solids. A fixed bed of solids is currently preferred 
for the method of this invention. 
The effluent from the contact zone contains higher hydrocarbon products 
(e.g., ethylene, ethane and other light hydrocarbons), carbon oxides, 
water, unreacted hydrocarbon (e.g., methane) and oxygen, and other gases 
present in the oxygen-containing gas fed to the contact zone. Higher 
hydrocarbons may be recovered from the effluent and, if desired, subjected 
to further processing using techniques known to those skilled in the art. 
Unreacted methane may be recovered and recycled to the contact zone.