Molecular sieve abstract
A process for the modification of a small pore molecular sieve
catalyst to increase its selectivity to ethylene in the production
of light olefins from oxygenated compounds, particularly methanol.
The catalyst is modified with phosphorus by incorporating a phosphonitrilic
oligomer with the catalyst, and then calcining the catalyst at temperature
sufficient to decompose the phosphonitrilic oligomer, and deposit
from about 0.001 wt. % to about 50 wt. % phosphorus on the catalyst.
This modification provides a novel composition in that it increases
the ethylene selectivity of the catalyst in the production of light
olefins from oxygenates as contrasted with a small pore molecular
sieve catalyst otherwise similar except that it has not been so
treated and modified with the phosphonitrilic oligomer.
Molecular sieve claims
Having described the invention, what is claimed is:
1. A process for the modification of a small pore molecular sieve
catalyst to increase its selectivity to ethylene when used in the
production of light olefins from oxygenated compounds which comprises
contacting, and incorporating with said catalyst a phosphonitrilic
oligomer formed of phosphazo groups having the formula ##STR4##
wherein the substituent R is the same or different, and is selected
from a group consisting of halogen, alkenyl, alkynyl, alkyl, cycloalkyl,
aryl, aralkyl, alkaryl, and alkoxy .chi. ranges from about 3 to
about 100 the oligomer formed from the phosphazo groups is either
linear or cyclic, and calcining said catalyst with which the phosphonitrilic
oligomer has been incorporated at temperature sufficient to decompose
said phosphonitrilic oligomer, and deposit on the catalyst from
about 0.001 percent to about 50 percent phosphorus, based on the
total weight of the phosphorus-containing catalyst.
2. The process of claim 1 wherein the phosphonitrilic oligomer
comprises a phosphonitrilic halide trimer.
3. The process of claim 2 wherein the phosphonitrilic halide trimer
is a phosphonitrilic chloride trimer.
4. The process of claim 1 wherein from about 0.05 percent to about
40 percent phosphorus is deposited on the catalyst.
5. The process of claim 1 wherein the phosphonitrilic oligomer
is dispersed, or dissolved in a solvent, impregnated into the catalyst,
dried, and then calcined to increase the ethylene selectivity of
the catalyst.
6. The process of claim 1 wherein the small pore molecular sieve
catalyst is of pore size ranging from about 5.0 .ANG. to about 4.0
.ANG..
7. The process of claim 6 wherein the small pore molecular sieve
catalyst is of pore size ranging from about 4.8 .ANG. to about 4.4
.ANG..
8. The process of claim 6 wherein the small pore molecular sieve
catalyst is selected from the group consisting of zeolites, tetrahedral
aluminophosphates, and tetrahedral silicoalumino-phosphates.
Molecular sieve description
FIELD OF THE INVENTION
This invention relates to a process for the modification of a small
pore molecular sieve catalyst to increase its selectivity in producing
ethylene and propylene, particularly ethylene, in the production
of light olefins from oxygenates, particularly methanol. It also
relates to the modified catalysts as novel compositions of matter,
and their use in the process for selectively producing ethylene
and propylene, particularly ethylene, with lessened by-products
production.
BACKGROUND
It is known to selectively convert oxygenates, including particularly
methanol, to light olefins, viz. ethylene (C.sub.2.sup.=), propylene
(C.sub.3.sup.=), and butylene (C.sub.4.sup.=). Ethylene and propylene
are in high demand, and the need for these chemical raw materials,
particularly ethylene, continues to grow. In a current process,
methanol is reacted at elevated temperature over a fluidized bed
of a molecular sieve catalyst, e.g., ZSM-5 zeolite or SAPO-34 to
produce a reaction product from which C.sub.2 -C.sub.4 olefins are
recovered.
In producing light olefins, the selectivity to ethylene can be
increased to some extent by increasing reactor severity; but this
has its limitations because as reactor severity is increased total
olefin production is decreased. Moreover, the production of paraffins,
i.e., methane, ethane, propane, etc., aromatics and other less desirable
components will also increase. Attempts have also been made to modify
the framework structure of the catalyst to increase light olefin
production, especially the C.sub.2.sup.= /C.sub.3.sup.= ratio, while
producing as little of the paraffinic and aromatic by-products as
possible.
In U.S. Pat. No. 3911041 there is disclosed a process for converting
methanol to a reaction product containing light olefins by contact
of the methanol with a phosphorus modified zeolite. A zeolite of
intermediate pore size, such as ZSM-5 is modified by incorporating
from about 0.78 wt. % to 4.5 wt. % phosphorus bonded to its structural
framework. Typically the dry ZSM-5 zeolite is contacted with a solution
of a phosphorus-containing compound, e.g., PCl.sub.3 and heated
at elevated temperature for a time sufficient to incorporate the
phosphorus within the crystalline framework of the zeolite. In conducting
a methanol to olefins reaction with a phosphorus modified ZSM-5
catalyst, as demonstrated by the Patentee, selectivities at 325.degree.
C. to the C.sub.2 -C.sub.3 olefins are high compared to the selectivities
to the C.sub.2 -C.sub.3 paraffins, but butane production was significant.
At temperatures above 325.degree. C. selectivities to the C.sub.2
-C.sub.4 olefins were high compared to the selectivities to the
C.sub.2 -C.sub.4 paraffins. At temperatures above 500.degree. C.
the concentration of ethylene in the product increased, but propylene
remained the major component relative to ethylene. At temperatures
below 500.degree. C., at conditions which increased the ratio of
ethylene:propylene, the production of butenes, butanes, or both
butenes and butanes was increased significantly; sufficiently so
that the molar ratio of C.sub.2.sup.= /C.sub.4 <1. Changing the
severity of a reaction to change the distribution of olefins and
by-products thus has its limitations.
Other modifications of zeolite catalysts have been proposed, or
made, but there remains a pressing need for improving the performance
of this type of catalyst for selectively producing light olefins,
particularly ethylene, from oxygenates, notably alcohols, with low
by-products formation.
SUMMARY OF THE INVENTION
This invention, which meets this and other needs, relates to a
process for the modification of a small pore molecular sieve catalyst
to increase its selectivity to ethylene when used in the production
of light olefins from oxygenated compounds which comprises contacting,
and incorporating with the catalyst a phosphonitrilic oligomer formed
of phosphazo groups having the formula ##STR1## wherein the substituent
R is the same or different, and is selected from a group consisting
of halogen, alkenyl, alkynyl, alkyl, cycloalkyl, aryl, aralkyl,
alkaryl, alkoxy or the like, .chi. ranges from about 3 to about
100 the oligomer formed from the phosphazo groups is either linear
or cyclic, and calcining the catalyst with which the phosphonitrilic
oligomer has been incorporated at temperature sufficient to decompose
the phosphonitrilic oligomer, and deposit on the catalyst from about
0.001 percent to about 50 percent phosphorus, based on the total
weight of the phosphorus-containing catalyst.
DETAILED DESCRIPTION
This invention, which meets this and other needs, relates to a
process for the modification of small pore molecular sieves by contact
and treatment with phosphonitrilic oligomers (linear and cyclophosphazene
compounds), use of the catalyst in the conversion of oxygenates
to olefins, and the catalyst composition which is produced. In conducting
the process, molecular sieves of pore size ranging from about 5.0
Angstrom Units, .ANG., to about 4.0 .ANG., preferably from about
4.8 .ANG. to about 4.4 .ANG., are modified by contact, and treatment,
with a phosphonitrilic oligomer sufficient to improve its stability
and increase its selectivity in producing ethylene in the catalytic
conversion of oxygenates, particularly methanol, to light olefins.
The phosphonitrilic oligomer is contacted with the molecular sieve,
and the molecular sieve modified by such treatment, prior to or
at the time of the catalytic conversion of the oxygenate to olefins,
i.e., as by pretreatment of a fresh or regenerated catalyst, or
by the addition and contact of the phosphonitrilic oligomer with
the catalyst during catalyst regeneration, or with the oxygenate
feed. In the modification of the catalyst the phosphonitrilic oligomer
is decomposed from about 0.001 percent to about 50 percent, preferably
from about 0.05 percent to about 40 percent, phosphorus being deposited
on the molecular sieve, based on the total weight of the molecular
sieve (wt. %; dry basis). The resultant catalyst is stable at high
temperature in the presence of steam, and it exhibits good activity
maintenance during conversion of oxygenates to olefins, with low
by-products formation.
In modifying the small pore molecular sieve in accordance with
the practice of this invention, a molecular sieve of pore size less
than 5.0 .ANG., exemplary of which are ZSM-34 SAPO-17 SAPO-18
SAPO-34 erionite or the like, in particulate dry solids form is
contacted with a liquid in which the phosphonitrilic oligomer has
been dispersed, dissolved, incorporated, or added, in concentration
ranging from about 0.001 percent to about 15.0 percent, preferably
from about 0.01 percent to about 5.0 percent, based on the total
weight of solids (wt. %; dry basis), or the phosphonitrilic oligomer
in molten state is heated, or calcined at temperature above about
200.degree. C., preferably above about 300.degree. C., and more
preferably within a range of from about 350.degree. C. to about
650.degree. C., for a period sufficient to decompose the phosphonitrilic
oligomer and deposit from about 0.001 wt. % to about 50 wt. %, preferably
from about 0.05 wt. % to about 40 wt. %, phosphorus on the molecular
sieve. Temperature and time are to some extent interrelated, but
generally the time period required for adequate calcination ranges
from about 0.5 hr. to about 48 hours. The surface between the pores
is coated with the phosphorus, inclusive of the surface at the outer
perimeters of the pores such that the pore openings are partially
restricted in diameter. Partial restriction of the size of the pore
diameters permits the free ingress of methanol into the pores, reaction
of the methanol within the pores, and a relatively high rate of
egress of ethylene from the pores vis-a-vis the rate of egress of
propylene, paraffins, aromatics and the like from the pores which
are more diffusion limited.
The small pore molecular sieve employed in the reaction is of pore
size ranging between about 5.0 .ANG. and 4.0 .ANG., preferably about
4.8 .ANG. and 4.4 .ANG., and comprised of a crystalline framework
oxide component. It is preferably selected from the group consisting
of zeolites, tetrahedral aluminophosphates (ALPOs) and tetrahedral
silicoaluminophosphates (SAPOs). More preferably, the crystalline
framework oxide component is a tetrahedral silicoaluminophosphate
(SAPO), e.g., SAPO-18 SAPO-43 and SAPO44. Generally, the pore apertures
of the structure consists of from about 6 to about 10 preferably
8 membered ring structures.
These materials, modified and employed in accordance with this
invention, include modified natural and synthetic crystalline structures
with tetrahedral framework oxide components such as aluminum, silicon,
phosphorus and the like. Exemplary of small pore zeolitic catalysts
are ZSM-34 ZK4 ZK-5 zeolite A, zeolite T, chabazite, gmelinite,
clinoptilalite, erionite, ZSM-35 rho, offretite and the like; and
such non-zeolitic catalysts as ferrierite, levyne, SAPO-17 SAPO-18
SAPO-34 SAPO-43 and SAPO-44.
The small pore molecular sieve, zeolitic and non-zeolitic, is modified
by contact and treatment with a phosphonitrilic oligomer, either
a linear or cyclic phosphonitrilic oligomer, or phosphazene type
compound in which a plurality of phosphazo units, or ##STR2## groups,
are joined together to form linear chains, or rings of phosphazo
units, or groups of alternating phosphorus and nitrogen atoms which
form trimers, tetramers, pentamers, hexamers and higher, and the
phosphorus atoms are each substituted with a hydrogen reactive substituent
R. In the formula the substituents R can be the same or different,
and can be halogen, e.g., chlorine, bromine, fluorine, or the like;
alkenyl, alkynyl, alkyl, cycloalkyl, or the like, e.g., methyl,
ethyl, ethynyl, propyl, butyl or the like; an aryl, e.g., phenyl
or the like; an aralkyl or alkaryl, e.g., benzyl, toluyl or the
like; alkoxy, e.g., methoxy or the like; or other moiety which will
react with hydrogen. In the formula, in forming either a linear
or cyclic phosphonitrilic oligomer, .chi. can range up to about
100 preferably from about 3 to about 100 and more preferably from
about 3 to about 20.
The phosphonitrilic trimer is a preferred oligomer. It is a cyclotriphosphazene
type compound, in which three phosphazo groups are joined together
to form a six-membered ring, and each of the three phosphorus atoms
are substituted with a hydrogen reactive substituent, R. The phosphonitrilic
trimer is thus a compound having the formula ##STR3## wherein the
substituents R can be the same or different, and can be halogen,
e.g., chlorine, bromine, fluorine, or the like; alkenyl, alkynyl,
alkyl, cycloalkyl, or the like, e.g., methyl, ethyl, ethynyl, propyl,
butyl or the like; an aryl, e.g., phenyl or the like; an aralkyl
or alkaryl, e.g., benzyl, toluyl or the like; alkoxy, e.g., methoxy
or the like; or other moiety which will react with hydrogen. Exemplary
of such compounds are hexahalocyclotriphosphazene, hexamethylcyclotriphosphazene,
hexaphenylcyclotriphosphazene, or the like. The hexachlorocyclotriphosphazenes
have been found to be particularly effective in decomposing and
modifying both the zeolites and non-zeolite catalysts by deposition
of phosphorus onto the structures.
The phosphonitrilic oligomer, e.g., the trimer, it is believed,
is decomposed into substituted phosphazo, di- and tri-phosphazo
groups, with one or more of the R substituents thereof reacting
with surface attached hydroxyl groups to liberate the hydrogen and
form an RH compound, which escapes from the surface, while the phosphorus
atom of the mono-, di-, or tri-phosphazo group attaches through
the oxygen to the surface of the catalyst; as well as other reactions
between the phosphazo groups which link together, bridge over, and
cover the surface of the catalyst, inclusive of the areas around
the pore openings to restrict the size of the pore exits and reduce
the transport rate of exit of molecules larger than ethylene. In
other words, due to the restriction of the pore openings the rate
of exit of ethylene is increased relative to the rate of exit of
the larger C.sub.3 molecules, as a consequence of which the modification
causes an increase in the formation of ethylene vis-a-vis the C.sub.3
and higher in the conversion of methanol to light olefins. The modified
catalyst is more stable to high temperature and steam, the yield
of ethylene is increased, and the yield of propylene and paraffins
are reduced as contrasted with the concentration of these products
employing an unmodified but otherwise similar catalyst used at the
same conditions for conducting a similar reaction. |