Molecular sieve abstract
A method of oxidatively coupling methane onto a manganese oxide
molecular sieve comprising: (a) passing methane through a microwave
plasma activation flow reaction zone onto a manganese oxide molecular
sieve, whereby polymer-free methane coupled products are produced;
and, (b) recovering the polymer-free methane coupled products.
Molecular sieve claims
We claim:
1. A method of oxidatively coupling methane onto a manganese molecular
sieve comprising:
(a) passing methane through a microwave plasma activation flow
reaction zone onto a manganese oxide molecular sieve catalyst to
produce polymer-free methane coupled products, the products containing
ethylene, ethane, acetylene, propane, (C.sub.4) hydrocarbons and
(C.sub.5) hydrocarbons; and,
(b) recovering said polymer-free methane coupled products.
2. The method of claim 1 wherein the methane passing through said
flow reaction zone has a flow rate of between 10 and 500 ml/min
and a pressure of between 3 and 760 torr.
3. The method of claim 1 wherein the microwave energy supplied
to the reaction zone is adjusted until the concentration of said
methane coupled products cannot be increased by further adjustment
of said microwave energy.
4. The method of claim 3 wherein the methane passing through said
flow reaction zone has a flow rate of between 10 and 500 ml/min
and a pressure of between 3 and 760 torr.
5. The method of claim 1 additionally comprising
locating the catalyst just outside the zone in which the microwave
plasma is created.
6. A method of oxidatively coupling methane onto a manganese molecular
sieve comprising:
(a) passing methane through a microwave plasma activation flow
reaction zone onto a manganese oxide molecular sieve catalyst wherein
said molecular sieve is todorokite, hollandite, or substituted octahedral
molecular sieve to produce polymer-free methane coupled products,
the products containing ethylene, ethane, acetylene, propane, (C.sub.4)
hydrocarbons and (C.sub.5) hydrocarbons; and,
(b) recovering said polymer-free methane coupled products.
7. The method of claim 6 wherein the methane passing through said
flow reaction zone has a flow rate of between 10 and 500 ml/min
and a pressure of between 3 and 760 torr.
Molecular sieve description
BACKGROUND OF THE INVENTION
This invention relates to the oxidative coupling of methane on
octahedral molecular sieves, mainly manganese oxides, equivalent
to the structures of natural minerals, e.g., todorokite and hollandite.
DISCLOSURE STATEMENT
Hollandite (BaMn.sub.8 O.sub.16), cryptomelane (KMn.sub.8 O.sub.16),
manjiroite (NaMn.sub.8 O.sub.16), and coronadite (PbMn.sub.8 O.sub.16)
are all naturally occurring manganese minerals with a 3-dimensional
framework tunnel structure. The structure consists of MnO.sub.6
octahedra which share edges to form double chains, and the octahedra
of the double chains share corners with adjacent double chains to
form a 2.times.2 tunnel structure. The size of these tunnels is
4.6 .ANG..times.4.6 .ANG.. Ba, K, Na and Pb ions are present in
the tunnels and coordinated to the oxygens of the double chains.
The identity of the tunnel cations determines the mineral species.
The minerals are members of the hollandite-romanechite family which
has a common double chain width, T(2.times.2).
Zeolites and zeolite-like materials are the well-known molecular
sieves. These materials use tetrahedral coordinated species TO.sub.4
(T.dbd.Si, Al, P, B, Be, Ga, etc.,) as the basic structure unit.
Through the secondary building units (SBU), a variety of frameworks
with different pore structures are then built. Like tetrahedra,
octahedra also can be used as the basic structural unit to form
octahedral molecular sieves (OMS).
Herein below, we will refer to the materials with a 2.times.2 tunnel
structure as hollandites, and identify each member by the identity
of the tunnel ions. Such minerals can be characterized by the general
formula:
where A is the counter ion (alkali or alkaline earth metal or Pb.sup.+2),
Mn represents Mn.sup.+4 and Mn.sup.+2 and x is 6 to 10 with y varying
from 0.1 to 1.3.
Because of their tunnel structure, the materials may be useful
as shape selective catalysts and molecular sieves. Although K-hollandite
and Ba-hollandite have reportedly been synthesized (as discussed
by Parida et al, "Chemical Composition, Microstructure and
other Characteristics of Some Synthetic MnO.sub.2 of Various Crystalline
Modifications", Electrochimica Acta, Vol. 26 pp. 435-43 (1981)
and Strobel et al, "Thermal and Physical Properties of Hollandite-Type
K.sub.1.3 Mn.sub.8 O.sub.16 and (K,H.sub.3 O).sub.x Mn.sub.8 O.sub.16
", J. Solid State Chemistry, Vol. 55 PP. 67-73 (1984). However,
these syntheses are unreliable and considerable difficulties have
been experienced by practitioners in this field.
Villa et al discussed the synthesis of oxide systems containing
Mn in combination with other elements in "Co--Mn--Ti--K OXIDE
SYSTEMS" Applied Catalysis, Vol. 26 pp. 161-173 (1986).
Torardi et al. discussed the synthesis of a hollandite-type molybdenum
compound (K.sub.2 Mo.sub.8 O.sub.16) by hydrothermal reaction of
basic K.sub.2 MoO.sub.4 solutions with Mo metal in "Hydrothermal
Synthesis of a new molybdenum hollandite," Inorganic Chemistry,
Vol. 23.
The hollandites are representative of a family of hydrous manganese
oxides with tunnel structures (also described as "framework
hydrates") in which Mn can be present as Mn.sup.+4 and other
oxidation states, the tunnels vary in size and configuration, and
various mono- or divalent cations may be present in the tunnels.
Such cations may serve to form and support the tunnels in some cases.
Clearfield describes various hydrous manganese oxides with tunnel
structures in "Role of Ion Exchange in Solid-State Chemistry,"
Chemical Reviews, Vol. 88 pp. 125-131 (1988). Pyrolusite or .beta.-MnO.sub.2
has tunnels only one MnO.sub.6 octahedron square (1.times.1), or
about 2.3 .ANG. square, while in ramsdellite, MnO.sub.2 these octahedra
form (2.times.1) tunnels, about 2.73 .ANG..times.4.6 .ANG.. Nsutite,
.gamma.-MnO2 is described as an intergrowth of pyrolusite and ramsdelite
and also has (2.times.1) tunnels. Psilomelane, Ba.sub.2 Mn.sub.5
O.sub.10 xH.sub.2 O, and romanechite (with K.sup.+2 substituted
for Ba.sup.+2 in the psilomelane formula) have (3.times.2) tunnels
parallel to the cell b axes, about 4.6 .ANG..times.6.9 .ANG.. Todorokites,
(Na,Ca,Mn) Mn.sub.3 O.sub.7 xH.sub.2 O, have (3.times.3) tunnels,
about 6.9 .ANG. square, and monoclinic cells. Todorokites and other
species are described by Turner et al. in "Todorokites: A New
Family of Naturally Occurring Manganese Oxides," Science, May
29 1981 pp. 1024-1026 in which it is noted that since todorokites
are often found in deep-sea manganese nodules containing high concentrations
of copper and nickel, "it seems probable that the smaller transition
elements substitute for Mn.sup.+2 in the octahedral framework."
The same article suggests a new partial nomenclature scheme for
such manganese oxide structures--T(m,n), in which T donates a tunnel
structure and the dimensions of the tunnels are indicated by (m,n).
In this notation, the common dimensions responsible for intergrowth
(m) is listed first, while (n) represents a variable dimension.
D. C. Golden et al., discloses the synthesis of todorokite in SCIENCE
231.717 (1986).
U.S. Pat. No. 5015349 discloses a method for cracking a hydrocarbon
material. The method includes introducing a stream including a hydrocarbon
fluid into a reaction zone. A microwave discharge plasma is continuously
maintained within the Reaction zone, and in the presence of the
hydrocarbon fluid. Reaction products of the microwave discharge
are collected downstream of the reaction zone.
The disclosure of U.S. Pat. No. 5015349 is incorporated herein
by reference.
Herein below, we will refer to the (3.times.3) tunnel structure
as OMS-1 and the (2.times.2) tunnel structure as OMS-2.
Many of these tunnel or framework hydrates in addition to the (2.times.2)
hollandites and (3.times.3) todorokites have potential for use in
separations, absorbent materials or catalyst materials. Hence, a
use of the present product as a catalyst is desired.
Thus, the object of this invention is to use the materials of this
invention as a catalyst to activate methane (CH.sub.4) onto coupled
hydrocarbons.
SUMMARY OF THE INVENTION
A method of oxidatively coupling methane (CH.sub.4) onto a manganese
molecular sieve comprising:
(a) passing methane through a microwave plasma activation flow
(quartz) reactor onto a manganese oxide catalyst sieve, whereby
polymer-free methane coupled products are produced; and
(b) recovering the polymer-free methane coupled products.
BRIEF DESCRIPTION OF THE DRAWINGS
The drawings provided for illustration are:
FIG. 1 shows the three-dimensional framework tunnel structures
of OMS-2 hollandite (2.times.2); and
FIG. 2 shows the three-dimensional framework tunnel structure of
OMS-1 todorokite (3.times.3).
DETAILED DESCRIPTION OF THE INVENTION
According to the present invention, it is possible to crack or
activate hydrocarbons such as methane, for example, by breaking
C--H bonds using the microwave plasma without a catalyst. However,
the ability to control the reaction and produce specific desired
end products is generally low in the absence of a catalyst. In other
words, the selectivity associated with the reaction is usually low
unless a catalyst is provided. Selection of an appropriate catalyst
is essential, if high selectivity of the end product and good control
of the reaction is to be obtained.
The catalyst should be positioned downstream of the reaction zone.
If the catalyst is placed within the plasma reaction zone there
is a significant danger that the surface of the catalyst may become
prematurely coked. It has been found that the best results are obtained
by locating the catalyst just outside the zone in which the microwave
plasma is created. The catalyst can be placed within the tubing
carrying gases from the reactor outlet. Alternatively, and preferably,
the catalyst may be placed within a U-tube downstream of the reactor
outlet.
Selection of the catalyst is dependent somewhat on reactants and
reaction conditions. Generally, a metal or metal oxide material
is employed as the catalyst. If methane is used as the reactant
gas, the catalyst must be a hydrogen acceptor if high selectivity
towards ethane or ethylene is to be attained. For the production
of olefins, it is necessary to use a catalyst that can adsorb hydrogen,
such that unsaturated species will result. Typically, dehydrogenation
catalysts such as nickel are used for this purpose.
Platinum catalysts are strong oxidizing catalysts. Large amounts
of CO.sub.2 are formed when Pt is used as a catalyst with the process
of the present invention. At the same time, relatively large amounts
of HCHO are formed. Conversely, nickel catalysts tend to minimize
the formation of highly oxidized species and favor methanol production
instead.
To be useful in the present invention, a catalyst should be resistant
to coking under low power microwave reaction conditions, and should
also be thermally and photochemically stable. Thermal stability
refers to the ability of the catalyst to withstand the operating
temperatures of the hydrocarbon cracking reactions carried out using
the low power microwave energy conditions of the present invention.
In general, to be useful as a catalyst element in the instant process,
a composition must withstand continuous long term exposure to temperatures
up to about 500.degree. C. Long term exposure refers to the intended
duration of operation of the reactor vessel of the invention. It
is contemplated that in commercial operation the microwave cracking
process of the invention may be conducted continuously for several
days, or more before the process is halted for cleaning the reaction
vessel. The catalyst element of the invention should be non-volatile
under operating conditions. A high catalyst surface area is desirable.
A high surface area can be attained by providing the catalyst in
a suitable shape or size, e.g. in finely divided powder form. In
an alternative arrangement, the catalyst can take the form of a
fine mesh screen or a sintered disc. In addition, the catalyst array
may be disposed on one or more silica supports that are positioned
in the reactant stream. |