Chemistry of inorganic compounds – Zeolite – Isomorphic metal substitution
Reexamination Certificate
2000-12-19
2003-02-18
Sample, David (Department: 1755)
Chemistry of inorganic compounds
Zeolite
Isomorphic metal substitution
C423SDIG002, C502S079000, C502S085000, C502S086000
Reexamination Certificate
active
06521208
ABSTRACT:
FIELD OF INVENTION
This invention relates to zeolites that are useful as adsorbents or catalyst supports. In particular, it involves production of a hydrophobic zeolite.
BACKGROUND OF THE INVENTION
Most zeolites are hydrophilic (water attracting) and thus have higher preference for sorption of water than for organic materials. However, the highly siliceous zeolites tend to be hydrophobic (organic-attracting). Hydrophobic zeolites are useful in selected applications such as removal of volatile organic compounds from water-containing environments.
Hydrophobic zeolites tend to have a relatively small number of catalytically active acid sites. These low acidity zeolites are sometimes useful in catalytic processes where cracking reactions must be minimal.
In order to measure the hydrophobicity of a zeolite, we have developed a Hydrophobicity Index screening test. A Hydrophobicity Index (H) is calculated from the ratio of mass sorption of organic compound to mass sorption of water at specific partial pressures for the two adsorbates; thus H
c
=S
c
/S
w
for cyclohexane over water and H
n
=S
n
/S
w
for n-hexane over water. Highly hydrophilic zeolites will have H values of less than 1.0. Highly hydrophobic zeolites will have H values of substantially greater than 1.0. Selection of the adsorbent depends upon the pore opening of the zeolite structure of interest. It is well known that zeolites with 10-membered or less metal atoms ring openings will not adsorb substantial amounts of cyclohexane. For these zeolites, e.g. ZSM-5, ZSM-11, etc., n-hexane is much more efficacious choice for the organic adsorbent. Moreover, the partial pressure at which the adsorbtion is measured can have an effect on the absolute amount of adsorption of any component and also the hydrophobicity index value. For the purpose of defining the conditions at which the index is measured (the adsorbate and the partial pressures) we have adopted the following convention: H
c07/05
refers to an index where cyclohexane adsorption at 7 torr is referenced to water adsorption at 5 torr. Similarly, H
n07/05
refers to an index where n-hexane adsorption at 7 torr is referenced to water adsorption at 5 torr.
SUMMARY OF THE INVENTION
A hydrophobic zeolite can be prepared by calcining a precursor zeolite with silica to alumina molar ratio at least 20, under high temperature and the presence of steam and under turbulent conditions with respect to flow pattern of the zeolite. In particular, a novel hydrophobic zeolite Y is provided by this method having a Hydrophobicity Index (H
c07/05
) of greater than 20.
DETAILED DESCRIPTION OF THE INVENTION
We have found that by calcining zeolites under a turbulent condition, high temperature and in the presence of steam, a hydrophobic zeolite can be prepared. Turbulent condition arises from intimate admixture of the solid and the gas phase such that the characteristic flow pattern of the solid can be considered turbulent. These zeolites are more hydrophobic than zeolites that can be prepared by steam calcining a zeolite under non-turbolent conditions. Examples of hydrophobic zeolites that can be prepared by this method include, for example, zeolite Y, and zeolite beta. These zeolites are considered to have interconnecting pores of at least two-dimensions, preferably interconnecting two or three-dimensions, more preferably three-dimensions. The precursor (starting material) zeolites useful in preparing the hydrophobic zeolites have a silica to alumina molar ratio of at least 20, preferably from about 25, to about 150. The calcination temperature is in the range of from about 650° C., preferably from about 700° C., to 1000° C., preferably to 850° C. in the presence of steam. The steam is preferably present in an amount of at least 10% by volume.
In particular, we have found that by preparing the zeolite by calcining a zeolite having silica to alumina greater than 20, particularly stabilized zeolite Y under a turbulent condition, high temperature and in the presence of steam, a hydrophobic zeolite, particularly a stabilized zeolite Y having a Hydrophobicity Index (H
c07/05
) of greater than 20, preferably at least 25, can be prepared.
The very hydrophobic zeolite products of our invention are prepared from zeolites having the structure of zeolite Y that is stabilized. These very hydrophobic zeolites have Hydrophobicity Index (H
c07/05
) of greater than 25, preferably greater than 30: The ultrahydrophobic materials have a Hydrophobicity Index (H
c07/05
) of greater than 30, preferably equal to or greater than about 35.
It has been surprisingly found that a very hydrophobic zeolite Y material can be prepared from a precursor material with a moderate silica to alumina molar ratio (bulk silica to alumina ratio) in the range of from 25, preferably from about 40, to about 150, preferably to about 120.
It has also been surprisingly found that an ultrahydrophobic zeolite Y material can be prepared from a precursor having silica to alumina molar ratio of greater than about 60, preferably greater than about 75, preferably greater than about 85.
The hydrophobic zeolite Y material of the invention can be produced by calcining a stabilized Y zeolite having a unit cell size within the range of less than 24.40 preferably less than 24.35, more preferably less than 24.30, most preferably less than 24.27, to preferably greater than 24.15, under turbulent conditions at a temperature within the range of from about 650° C., preferably from about 700° C., to 1000° C., preferably to 850° C. in the presence of steam. The steam is preferably present in an amount of at least 10% by volume.
Turbulent condition as herein referred to is a condition in which there is sufficient mix between solid phase and gas phase in which the gas flows through the dispersed solid phase without a discernable interface. The condition is not turbulent if the gas phase flows over a stationary solid such that there is a discernable interface between the solid and the gas.
While not wishing to be bound by theory, we believe that superior contacting of the solid involved with the reactive gas atmosphere directly leads to the high hydrophobicity characteristic of the present invention. We believe that this condition is met when a substantial portion of the solid particles are continuously and completely surrounded by the reactive gas mixture. This condition can be described as a flow rate such that a significant fraction of the solid articles have reached the point where they have at least just been suspended and set in motion by the action of the gas. Such a velocity has often been described as the minimum fluidization velocity. This often occurs at Reynolds numbers (N
Re
) less than about 10 (D
p
G
mf
/&mgr;). This phenomenom has been described by the following relationship (Leva, “Fluidization,” p. 63, McGraw-Hill, New York 1959):
G
mf
=
0.0005
D
p
2
g
c
ρ
f
(
ρ
s
-
ρ
f
)
φ
s
2
ϵ
mf
3
μ
(
1
-
ϵ
mf
)
where
G
mf
=fluid superficial mass velocity for minimum fluidization, lb./(sec.)(sq.ft.)
D
p
=particle diameter, ft.
g
c
=dimensional constant, 32.17 (lb.)(ft.)/(lb.force)(sec.
2
)
&rgr;
f
=fluid density, lb./cu.ft.
&rgr;
s
=solids density, lb./cu.ft.
&PHgr;
s
=particle shape factor, dimensionless
&egr;
mf
=voidage at minimum fluidization, dimensionless
&mgr;=fluid viscosity, lb./(ft.)(sec.)
Alternately, this has been described by a similar equation (Perry, “Chemical Engineers' Handbook,” 4th Edition, p. 4-25, McGraw-Hill, New York):
G
mf
=
5.23
×
10
5
D
p
2
ρ
f
1.1
(
ρ
s
-
ρ
f
)
μ
where
G
mf
=fluid superficial mass velocity for minimum fluidization, lb./(hr.)(sq.ft.)
D
p
=particle diameter, ft.
&rgr;
f
=fluid density, lb./cu.ft.
&rgr;
s
=solids density, lb./cu.ft.
&mgr;=fluid viscosity, lb./(ft.)(sec.)
For the invention process, it is preferable to calcine under a minimum fluidization velocity through at least substantial port
Cooper David A.
Cormier William E.
Hertzenberg Elliot P.
Hinchey Richard J.
Marcus Bonita K.
PQ Corporation
Sample David
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