Note: Descriptions are shown in the official language in which they were submitted.
~ 334520
DEGRADATION OF ORGANIC CHEMICALS WITH
METAL OXIDE CERAMIC MEMBRANES OF TRANSITION METALS
Field of the Invention
The present invention relates to the use of ceramic
membranes, and, in particular, relates to the reliable and
successful use of both particulate and polymeric titanium
ceramic membranes and coatings to degrade persistent organic
compounds. Broadly speaking, metal oxide ceramic membranes of
transition metals may be used for the degradation process.
~escription of the Prior Art
Ceramic membranes are used currently in industry and
science for a variety of processes and purposes, the most
common of which is separations. While organic membranes
are most often used for separation prDcesses, ceramic
membranes have had increasing popularity because of
several advantages which they offer over organic
membranes. Ceramic membranes have a greater chemical
stability since they are resistant to organic solvents,
chlorine, and extremes of pH. Ceramic membranes are also
stable at very high temperatures which allows for
20 efficient sterilization of process equipment and
pharmaceutical equipment often not possible with organic
membranes. Because ceramic membranes are inorganic they
are generally quite stable to microbial or biological
degradation which can occasionally be a problem with
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organic membranes. Ceramic membranes are also
mechanically very stable even under high pressures. The
temperature, chemical, and mechanical stability of ceramic
membranes allows them to be cleaned more effectively than
other less durable membrane compositions.
The mechanism of operation and types of separations
which can be achieved by ceramic membranes are discussed
in general by Asaeda et al., Jour. of Chem. Eng. of Japan,
19:1, 72-77 (1986). At least one line of ceramic filters
10 is currently on the market marketed under the trade-mark
"Ceraflo" by the Norton Company of Worcester,
Massachusetts. ~
While many of these characteristics seem to favor
inorganic membranes over organic membranes, the use of
15 these membranes in widespread commercial applications has
been slow because of the difficulty in producing
crack-free membranes which hav~ defined ~ore size and
distributions in desirable ranges. Some types of prior
art inorganic membranes, such as the ultra-stabilized
20 zirconia membranes made by depositing particles on a
silica support are stable but have relatively large pore
sizes which make them suitable only for very high
molecular weight separations.
Significant effort has been extended in creating metal
25 oxide membranes using aluminium. For example, it has been
demonstrated that the use of sol-gel techniques allows the
reproducible preparation of alumina ceramic membranes
which may be supported or unsupported. LeEnaars et al.,
Jour. of Membrane Science, 24, 261-270 (1985). By
30 controlling various parameters of the process, it was
demonstrated that reliable procedures can ~e developed for
creating alumina ceramic membranes having relatively fine
pores and a reliable size distri~ution of the pores.
The teachings in the art to date about the preparation
35 of titania ceramic membranes have been limited. Most of
the sol-gel teachings utilizing titanium have been aimed
at preparing very thin particulate films because of their
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optical and corrosion resistance properties. However, th~
various parameters necessary for the reproducible an~
consistent preparation of these or similar films has not
previously been rigorously described in such a fashion
that they are readily ~eplicable.
It has been recognized for some time that many toxic
organic chemicals can be degraded on suspended hydrous
oxide ~articles. Prior research has tended to focus on
easy to degrade compounds, such as acetate, and on the use
of suspended particles to degrade such compunds. There
are, ~or example, teachings in the prior art of the use of
suspensions of titanium dioxide particles for the
degradation of complex organic molecules. The use of
suspended particles for these processes is a serious
limitation, however since solid substrates are clearly
more convenient to utilize. However, completely solid
substrates do not provide enough surface area for
effective catalyzation in reasonable time periods.
Summary of the Invention
The present invention is summarized in that a process
for degrading complex organic molecules including the
steps of: positioning a porous titanium ceramic membrane
in a liquid solution containin~ the complex organic
molecules and irradiatin~ the membrane in the solution
with ultraviolet light.
Detailed Description of the Invention
The present invention is directed to the use of membranes
of titanium oxides for degradation of organic molecules. There
are two variations in methods of preparing titanium membranes.
The first variation involves the gellation of a colloidal sol.
This first variation utilizes a type of gel that is generally
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particulate but which can be formed in a coherent bulk if
the processin~ variables are controlled carefully and can
res~lt in a consistent and uniform membrane after
gellation. The second variation ~n this method involves
the hydrolysis of an organometallic titanium compound to
form a soluble intermediate compound whlch then condenses
into the inorganic titanium polymer. Since for cataLysis,
it is desired that surface area available to the substrate
be maximized, a porous or particulate titanium membrane i5
pre~erred for the process of the present invention.
The process thus includes the preparation of a
particulate gel which is then fired to achieve a ceramic
material. In this process, there are four distinct
variables which must be carefully controlled. The first
lS is the ratio of water to titanium in the colloidal sol so
that the gel is properly formed. The ratio is prefera~ly
less than about 300:1 mole-to-mole of water to titanium
atoms, the second criteria is the proper selection of an
alcohol solvent. The alcohol solvent is preferably an
alkyl alcohol different from the alkyl radical in the
titanium alkoxide used as the starting material. The
third consideration is the tight controlling of the pH of
the colloidal mixture. This control on pll limits
availability of free protons relative to titanium
molecules. The fourth consideration is an upper limit
upon the sintering temperatures to which the resultant
gels are exposed during firing. Firing temperatures in
excess of about 500C may introduce an unacceptable amount
o cracking into the resulting ceramic.
The preparation of a particulate titanium membrane
begins with a titanium alkoxide. The titanium allcoxide is
first hydrolyzed at room temperature. The typical
reaction is thus:
TiR4 + 4~I20~ Ti(OH)4 + 4R + 2H2
The R radicaL may be any alkyl, but titanium
tetraisopropoxide Ti(iso-OC3H7)4, has been found to
be a convenient starting material.
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The titanium alkoxide is first dissolved in an organic
alcohol. It has been found that the hydrolysis is best
facilitated by the use of an alkyl alcohol solvent where
the alkyl is different from the alkyl in the titanium
alkoxide, for example ethanol with titanium
tetraisopropoxide. Water is then added in increments in a
total volume of 200-300 times, mole-to-mole, of titanium
present. The resulting titanium hydroxide, Ti(OH)4 will
precipitate out of solution.
The titanium hydroxide precipitant is then peptized
with HNO3, again at room temperature. This step
converts the precipitant into a highly dispersed, stable,
colloidal solution, or sol. This suspension is maintained
by stirring and is dispersed over a time ~eriod of
about 12 hours with moderate heating (85-95C) to assist
the colloidal formation. When cooled to room temperature,
the colloid gels. The gel may be solidified onto a
support, such as glass or optical fiber, or may be
deposited in molds or layered into sheets to make
self-supporting structures. The gel is then sintered at a
firing temperature of no more than about 500C to give a
~ard dry ceramic. Higher firing temperatures may result
in cracking of the membrane. The result will be a highly
porous, continuous web of sintered particles forming a
rigid membrane.
The resulting titanium ceramic membrane functions as a
highly desirable substrate for the photo-catalyzed
degradation of organic molecules. The surface of the
membranes are highly porous, there~y readily absorbing
organic molecules. The titanium molecules are readily
available for catalytic activity. The catalysis is
actuated by UV light and broad spectrum UV radiation, even
sunlight, is usable, although intense artificial UV light
may tend to enhance the speed of the degradation.
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Example 1
a) Preparation of Particulate Membranes
Titanium tetraisopropoxide was obtained from Aldridge
Chemical Company. Water used in the reactions was
de-ionized using a Milli-~ water puri~ication system from
Milliport Corporation.
A series of hydrolysis and particle gel formation
experiments were per~ormed using a variety of pH levels
and ratio between water concentration and titanium ion
concentration. The results are summarized in Table 1
below.
Group A
H20/Ti H+/Ti TiO2 Stability Features *Weight Loss
(mole (ratio) (wt %) of Sol of Solid in Gellation
15 No. ratio) Gel
1 300 0.08 2.0 NP good
2 200 0.2 2.0 S qood 97.66%
3 200 0.4 2.0 S good 97.62%
4 200 0.7 2.0 S good 97.61%
200 1.0 2.0 S cracks 97.60%
present
6 200 1.2 2.0 NS cracks
present
Group B
1 300 0.1 1.3 S good
2 300 0.5 1.3 S good
3 300 1.0 1.3 S good
4 300 1.2 1.3 S good
S = Stable
NS = not stable, floccus appearance
NP = not peptized completely
*Weight loss from original sol to solid gel, given
as a percentage of the original qol weight.
From the above data it is evident that the stable
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titanium sols can be best achieved if the mole ratio of
free hydrogen ions (from the acid) to titanium molecules
is between 0.1 and 1Ø This range can be expanded only
in relatively dilute sol solutions such as those of Group
B on the table. The reason for this is not completely
understood but may relate to the increased interparticle
distance in the more dilute solutions making aggregation
more dif~icult than in concentrated sols. Only stable
sols could be properly then transformed by peptization
into coherent transparent gels and thereafter into
coherent oxide membranes by protolysis.
The concentration of the acid was found to effect the
gelling volume. The gelling volume goes through a minimum
when the acid concentration is about 0.4 moles of free
lS proton~ per mole of titanium. The sols need to loose at
least 4.5% of their ori~inal weight, depending upon the
electrolyte concentration, to arrive at the gelling
point. The sols must loose some ad~itional 97.6% of their
ori~inal weight in order to form a final solid gel.
Heating the final gels in the sintering process results in
a further weight loss of about 13.5~ without destroying
the internal gel structure.
b) Degradation of PCB's
An aliquot of a PCB, 3,4 ~ichloro biphenyl, was
dissolved in a non-polar solvent, alcohol or acetone. A
sintered particulate titanium ceramic membrane was then
inserted into the dissolved PC~, and allowed to absorb the
solution overnight. The membrane was then removed from
the solution and placed in distilled de-ionized water in a
Pyrex vessel in a water bath set to 50C.
The vessel was irradiated with a hiqh intensitv t~l
light source, a Xe-Hg lamp such as an LPS 200 from Photo
Technology International. Periodic samples of the water
were removed and analyzed by gas chromotography At start
of the reaction, a strong chlorine peak was apparent in
the chromotograph. The peak diminished in minutes. After
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four hours, no chlorine peak could be detected indicating
that the dechlorination of PCB's in the membrane was
completed.
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SUPPI.~:.~;N'l'ARY DISCLOSURE
This invention also encompasses a method of degrading
complex organic molecules comprising the steps of exposing the
organic molecules in solution to a ceramic porous membrane of
titanium, and irradiating the titanium membrane with
ultraviolet light wherein the step of exposing the complex
organic molecules to the membrane includes adsorbing the
molecules in the membrane.
The method of degrading the complex organic molecules may
include exposing the complex organic molecules, preferably
polychlorinated biphenyls to a porous titanium oxide ceramic
membrane by adsorbing the molecules in the membrane and
irradiating the membrane with ultraviolet light.
The method may involve adsorbing the organic molecules in
the gaseous phase into a porous titanium oxide ceramic membrane
and irradiating the membrane with ultraviolet light. The
membrane may be coated onto the exterior of the light source
specifically onto the exterior of an optical fiber carrying the
ultraviolet light. Alternatively, the molecules may be
adsorbed in the body of the porous titanium dioxide ceramic
body which body may then be irradiated.
