Note: Descriptions are shown in the official language in which they were submitted.
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DOPED ADSORBENT MATERIALS WITH ENHANCED ACTIVITY
FIELD OF THE INVENTION
The invention relates to materials for adsorbing selected species from
gaseous or liquid phases.
BACKGROUND OF THE INVENTION
Adsorption of solute species from a free fluid phase, i.e. from gaseous or
liquid materials hereinafter referred to as a gas-phase or a liquid-phase
respectively, finds utility in a number of different fields. Examples of
industrial
processes involving adsorption comprise, among gas-phase applications,
dehydration of gases; odour removal and toxic gas removal in ventilating
systems
or from vent gases for air-pollution control; separation of rare gases; and
among
liquid-phase applications, decolorization, drying, or degumming of petroleum
fractions; odour, taste, and colour removal from municipal water supplies;
decolorization of vegetable and animal oils; clarification of beverages and of
pharmaceutical preparations; recovery of vitamins and other products from
fermentation mixtures; purification of process effluents for control of water
pollution, etc.
Many different materials have been used and disclosed in the art as
adsorbents in the various applications listed above, typically inorganic or
carbonaceous materials such as for example activated alumina; siliceous
adsorbents comprising natural and synthetic aluminosilicates such as zeolites,
e.g. zeolite A; active silica's, silica gel, silicates, carbons, charcoal,
etc.
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Different methods are known in the art, which are aimed to the
enhancement of the adsorption properties of said inorganic adsorbent
materials,
said adsorption properties in turn comprising either the efficiency of
adsorption/removal, hereinafter referred to as efficiency of removal,
(generally
corresponding to the percent decrease in the concentration of a target
molecule,
corresponding to the solute specie, in a gas- or liquid-phase before and after
contact with the adsorbent material), or the adsorption capacity (i.e. grams
of
adsorbed target molecule per gram of adsorbent material at the saturation), or
the selectivity towards particular molecules or classes of molecules, or the
kinetics of adsorption, or combinations thereof.
Such enhancements can be obtained through suitable chemical and/or
morphological modifications of the surface of the adsorbent materials. Typical
examples of morphological modification are the various techniques known in the
art to synthesize molecular sieves, e.g. zeolites, having suitable dimensions
of
voids in their crystalline structure. A typical example of chemical
modification is
the treatment of charcoal with steam at high temperature in order to create a
new
microporous structure and to oxidize its surface. More in general, chemical
modification of the surface can consist in inserting on the surface of the
adsorbent material new chemical species (e.g. different atoms or functional
groups, and generally through chemical bonds with the substrate), which are
capable of enhancing the efficiency of removal, in particular towards certain
molecules or classes of chemical compounds. This is achieved by forming a
surface of the adsorbent material which is in general more "compatible" with
the
target molecules) (the solute species) in adsorption), or in some cases, under
particular conditions, also capable of forming preferential chemical bonds, of
various nature and stability, with the target molecule(s). Chemical reactions
between the target molecules) and the adsorbent material are however not
standard in adsorption processes.
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A peculiar technique for the chemical modification of an organic (e.g. an
acrylic polymer) or inorganic adsorbent material consists in creating, on the
surface and/on inside the pores of said organic or inorganic adsorbent
material,
so called "molecular imprints" for certain target organic molecules.
As disclosed in e.g. WO 99/65528, WO 98/56498, and in US 6,057,377,
generally such imprints onto the surface or inside the pores of the organic or
inorganic adsorbent material are obtained by methods of formation comprising
the following steps: a) chemically .reacting a target molecule to
functionalized
groups of suitable functionalizing monomers; b) further reacting said
functionalizing monomers with the molecular structure of an organic or
inorganic
adsorbent material; c) breaking by suitable chemical means (e.g. by oxidation)
the chemical bonds between the target molecule and the functionalized groups
of
the functionalizing monomers, previously reacted and chemically grafted to the
molecular structure of the organic or inorganic adsorbent material; and c)
finally
removing the target molecule (e.g. with a solvent), therefore leaving onto the
surface and/or into the pores of said organic or inorganic adsorbent material
the
functionalized groups, preferably into a spatially organized form which is
said to
help the specific recognition and adsorption of the same target molecule.
However, molecular imprinting involving chemical reactions between the
organic or inorganic adsorbent material and the chemical species introduced
therein, typically the functionalized monomers as explained above, is complex
to
achieve, involving multiple reaction steps, and also implies the use of
expensive
raw materials, for example, in case a silica modified with molecular
imprinting
according to the prior art is desired, there is a need for silicon in a
reactive form,
such as e.g. tetraethoxysilane (TEOS), and/or silico-organic compounds.
In our copending PCT patent application WO 99/40953 an odour controlling
material is disclosed for removing or reducing odours emanating from certain
gaseous or liquid compounds, which material comprises an inorganic adsorbent
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material modified by means of the so called "doping" technique. The "doping"
consists in the inclusion in the inorganic adsorbent material of one or more
chemical species, called dopant(s), during at least one step of the synthesis
of
the inorganic adsorbent material, or alternatively added to an already formed
inorganic adsorbent material as a post-synthesis treatment, without involving
chemical reactions between the dopant or dopants and the inorganic adsorbent
material to be doped. According to the above application the dopant or dopants
is/are selected from the gaseous or liquid compounds to be adsorbed, or from
derivatives thereof, or belong to the same chemical class, or have similar
functionalities. This is a doping made by molecules similar to the ones
preferably
targeted for adsorption ("homodoping"). Conventional adsorbent materials such
as silica, activated alumina, silicates and aluminosilicates can be used and
the
gaseous or liquid compounds are preferably selected from fatty acids and
derivatives thereof, amines and ammonia and salts thereof, aldehydes and
ketones and organic heterocompounds. The odour control material is suitable
for
incorporation in an absorbent article such as a pantiliner or a sanitary
napkin.
While the doped inorganic adsorbent materials of WO 99/40953 work well
in adsorbing volatile malodorous compounds, there is still a need for further
improving the performances of said doped inorganic adsorbent materials, and
more in general of inorganic adsorbent materials chemically and/or
morphologically modified by means of doping techniques and tailored for other
uses.
According to the present invention, and in the context of inorganic
adsorbent materials modified by means of doping techniques, it has been
discovered how to enhance the performances of doped inorganic adsorbent
materials in the adsorption of selected solute species.
According to the present invention, a method for increasing the adsorption
capacity of a doped inorganic adsorbent material has been discovered, which
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comprises both the identification of a number of different specific features
of the
doped inorganic adsorbent materials, or of the method for manufacturing the
doped inorganic adsorbent materials, or of combinations thereof, and also the
indication of how to act on said specific features, in order to increase the
adsorption performance of the doped inorganic adsorbent material.
One aspect of this method comprises choosing the type of the dopant or
dopants for the doped inorganic adsorbent material of the present invention.
Contrary to the teaching of our above mentioned patent application
WO 99/40953, the choice of the dopant or dopants shall also include compounds
in areas outside of the compounds which are the same as the selected solute
species to be adsorbed, or are derivatives thereof, or belong to the same
chemical class, or have similar functionalities.
According to another aspect of the method, an increased concentration of
dopant or dapants to be included in the doped inorganic adsorbent material has
to be chosen in order to increase the adsorption capacity of the doped
inorganic
adsorbent material of the present invention. By "increased", as used herein,
it is
meant a concentration for the dopant or dopants included in the doped
inorganic
adsorbent material of the present invention, which is much larger than the
values
indicated and preferred in our above mentioned patent application WO 99/40953.
A further aspect of the method consists in the selection of the molecular
dimension for the dopant or dopants in order to increase the adsorption
capacity
of the doped inorganic adsorbent material of the present invention.
A still further aspect of the method involves the achievement of a doped
inorganic adsorbent material having a selected predetermined pore structure by
only using doping techniques.
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According to the present invention, the method for increasing the adsorption
capacity of a doped inorganic adsorbent material can also comprise any
combination of the aspects referred to above.
It is therefore an object of the present invention to provide a method for
increasing the effectiveness of inorganic adsorbent materials, modified by
doping
techniques, in adsorbing selected solute species, as explained above.
It is a further object of the present invention to provide doped inorganic
adsorbent materials having an increased activity, namely in terms of an
enhanced adsorption capacity and/or efficiency of removal towards selected
solute species, comprising, but not limited to, volatile malodorous compounds,
generally from a gas-phase or from a liquid-phase, in order fio find utility
in
different fields of use, in addition to odour control. Said increased
adsorption
capacity and/or efficiency of removal can be either directed towards specific
solute species, i.e. involving a selecfiivity towards typically one specific
solute
specie among others, or alternatively towards broader ranges of more solute
species of different types.
SUMMARY OF THE INVENTION
The present invention provides a method for increasing the adsorption
capacity of a doped inorganic adsorbent material for adsorbing one or more
selected solute species from a gas-phase or from a liquid-phase, the doped
inorganic adsorbent material being doped with one or more suitable dopants.
The method consists of either:
choosing the type of the dopant or dopants, including in areas outside of
compounds which are the same as the one or more selected solute
species, or are derivatives thereof, or belong to the same chemical class,
or have similar functionalities, or
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~ choosing an increased concentration of the dopant or dopants, or
~ selecting the molecular dimension of the dopant or dopants, or
~ tailoring the pore structure of a doped inorganic adsorbent material
through the doping of the material,
~ or combinations thereof.
The method provides doped inorganic adsorbent materials having an
enhanced adsorption capacity towards selected solute species.
Different alternative preferred embodiments of the present invention are
also disclosed, according to the attached claims, which refer to improved
doped
inorganic adsorbent materials formed according to the method, and which have
an increased activity.
DETAILED DESCRIPTION OF THE INVENTION
1. Overall Characteristics of the Doped Inorganic Adsorbent Materials, and
Method for Making the Same
According to the present invention it has been surprisingly discovered that
the activity of inorganic doped adsorbent materials can be significantly
improved.
This is achieved by means of the method of the present invention, and
according
to different preferred alternative embodiments, as will be described
hereinafter.
Adsorption, as known in chemistry and as intended herein, involves
contacting a free fluid phase (a gas-phase or a liquid-phase) with a rigid and
durable particulate phase which is constituted by natural or synthetic
materials of
crystalline, microcrystalline, or amorphous structure having the property of
taking
up and storing one or more solute species originally contained in the fluid-
phase,
owing to the morphology and nature of their internal pore surfaces which are
accessible for selective combination of solid and solute.
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In adsorption said solute species typically consist in molecules, but can also
comprise in the context of the present invention more complex entities, such
as
fior example colloidal particles, micelles, and also living organisms such as
viruses and bacteria.
Adsorption involves relatively small attractive forces between the adsorbed
substance (solute) and the adsorbent material, e.g. of the order of the Van
der
Waals forces, or generally of electrostafiic interactions. More precisely,
chemical
reactions or chemical bonding between fihe substance to be adsorbed (the
solute
specie) and the adsorbent material, typically Si-C bonds, which involve a
stoichiometric mechanism and ratio, are excluded from adsorption in the
context
of the present invention.
As used herein, the "activity", also referred to as "adsorption activity", of
an
inorganic adsorbent material, either doped or undoped, corresponds to the
effectiveness in adsorbing selected solute species, and comprises the
efficiency
of removal and the adsorption capacity of the inorganic adsorbent material
towards one or more selected solute species from a free fluid phase, namely a
gas-phase or a liquid-phase. Also the selectivity of an inorganic adsorbent
material, i.e. the ability of the inorganic adsorbent material to selectively
adsorb
one particular solute specie among other species, is an important feature of
the
inorganic adsorbent material of the present invention.
As intended herein, the efficiency of removal results from comparing, as
percent change, the initial amount (or concentration) of a selected solute
specie
in the gas- or liquid-phase from which it has to be removed by the adsorbent
material and its final amount (or concentration) in the same gas or liquid-
phase
after contact with the adsorbent material at fixed conditions.
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Various specific methods can be used by the man skilled in the art in order
to evaluate this efficiency of removal according to the general principle
above,
and can be suitably chosen or adapted depending on the conditions in which
adsorption shall occur, i.e., for example, on whether the adsorption occurs
from a
gas- or from a liquid-phase, and also on the nature of the liquid-phase,
namely
an aqueous or non-aqueous liquid. An exemplary method is the Efficiency of
Removal Test Method described herein.
As intended herein, by "adsorption capacity" of an inorganic adsorbent
material, it is meant the actual amount of a selected solute specie adsorbed
by
the inorganic adsorbent material at fixed conditions (typically at
saturation).
According to the present invention, the adsorption capacity is evaluated
according to the Adsorption Capacity Test Method described herein.
According to the above methods, both the adsorption capacity and the
efficiency of removal of a given doped inorganic adsorbent material are
generally
evaluated both as absolute values, and in comparison to a respective reference
undoped inorganic adsorbent material, also referred to as "undoped reference
material".
As a general rule, in order to make a meaningful comparison in general
between two inorganic adsorbent materials and in particular between a doped
inorganic adsorbent material and the respective "reference undoped inorganic
adsorbent material", the selected parameter of each material (namely the
efficiency of removal or the adsorption capacity) must be evaluated with the
same test method, and under the same conditions.
As used herein, by "undoped reference material" it is meant an inorganic
adsorbent material synthesized from the same raw materials, and according to
the same process conditions, as the doped inorganic adsorbent material under
consideration, apart from the inclusion of any dopant or doping.
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As used herein, by the term "doping" it is meant the inclusion in the
inorganic adsorbent material or, in more general terms, the presence, in
suitable
concentration and form, during at least one step of the synthesis of said
inorganic adsorbent material, of one or more chemical species, called
dopant(s),
which are chemically different from the inorganic adsorbent material and from
the
raw materials) from which said adsorbent material is formed, in order to
obtain a
"doped" inorganic adsorbent material. According to the present invention,
"doping" is to be meant as excluding chemical reactions between the dopant or
dopants and the inorganic adsorbent material to be doped, or the raw materials
from which the inorganic adsorbent material is formed. When not present in one
step of the synthesis process of the inorganic adsorbent material, said dopant
or
dopants may be introduced into said material afterwards by a post-synthesis
treatment. The dopant or dopants may be left inside the finished, doped
inorganic adsorbent material or alternatively can be partially or totally
removed by
any suitable removal methods such as e.g. volatilization, pyrolysis, washing,
or
extraction with a suitable solvent. Therefore, according to the present
invention,
by "doped inorganic adsorbent material" it is meant an inorganic adsorbent
material which, through the doping technique as defined above, i.e. through
the
presence, at least in one step of the synthesis process or of a post-synthesis
treatment, of a dopant or of dopants, has been modified in its chemical and/or
physical-chemical and/or morphological characteristics, independently from the
fact that the dopant or the dopants are still present in the finished doped
inorganic adsorbent material or not.
The amount of the dopant or dopants in the doped inorganic adsorbent
material of the present invention is expressed as concentration of the dopant
itself (in ppm or weight percent) calculated with respect to the dry final
product
(the inorganic adsorbent material), and in the case of the preferred inorganic
adsorbent materials comprising silica's and silicates, typically corresponds
to the
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percentage calculated on the theoretical content of silicon dioxide Si02 in
the
synthesis solution.
Doped inorganic adsorbent materials according to the present invention
both include inorganic adsorbent materials doped with selected amounts of
dopants, which in turn are selected from those compounds which have to be
adsorbed, or derivatives thereof, or compounds of the same chemical class, or
having similar functionalities, in what will be called hereinafter
"homodoping" or
imprint doping, and inorganic adsorbent materials doped with dopants selected
from compounds which are instead different from the compounds to be
adsorbed. The latter will be referred to hereinafter as "heterodoping".
Doped inorganic adsorbent materials according to the present invention will
be herein described mainly with reference to materials for the adsorption of
volatile malodorous compounds from a liquid-phase. However, the present
invention broadly encompasses doped inorganic adsorbent materials which are
capable of adsorbing a broad range of solute species, comprising, but not
limited
to, volatile malodorous compounds, from a free flowing phase, namely a
gas-phase or a liquid-phase, and having an increased adsorpfiion activity.
Particularly, the doped inorganic adsorbent materials made in accordance
to the present invention can find use in a number of fields where adsorption
of
selected solute species from a gas- or a liquid-phase is useful and desirable.
By
way of example, possible alternative uses are listed in the non limiting list
below.
The doped inorganic adsorbent materials of the present invention can be
used in industrial adsorption processes from gas- and liquid-phase comprising
processes of separation, extraction, and purification, such as dehydration of
gases; toxic gas removal in ventilating systems or from vent gases for
air-pollution control; separation of rare gases; decolorization, drying, or
degumming of petroleum fractions, and more generally industrial oil refining;
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purification of process effluents for control of water pollution; catalyst or
catalyst
support, odour, taste, and colour removal from municipal water supplies;
purification of water and of other fluids; separation of natural mixtures of
substances; use as additive for paper e.g. for enhancing ink absorption; use
as
mineral filler for e.g. thermoplastic compositions; etc.
Alternative uses comprise treatment of foodstuff and beverages, such as
drinking water purification; edible animal and vegetal oils or tats
refining/treatment, e.g. decolorization, or production of cholesterol-free
oils/fats,
or regeneration of used oils/fats; de-caffeination of coffee; clarification of
beverages, e.g. haziness control of beer, wine, and fruit juices; etc. and
also
treatment of pharmaceuticals, such as purification or clarification of
pharmaceutical preparations; recovery of vitamins and of other products from
fermentation mixtures; etc.
Further alternative uses comprise delivery of actives, i.e. adsorption and
subsequent release of substances capable of providing an action, such as
perfumes, aromas, flavours, medicaments; and related uses in various articles
or
products, such as for example toothpastes, chewing gums, etc.
Alternative uses comprise treatment and purification of biological fluids,
both in vitro and in vivo, such as in dialysis, hemoperfusion, internal
anti-intoxication and anti-fermentation treatments; immobilization of
micro-organisms such as viruses and bacteria; blocking of endotoxins or
allergens; etc.
It has also been found that, by appropriately choosing the type of dopant or
dopants, andlor the concentration of dopant or dopants, the resulting doped
inorganic adsorbent material can exhibit a particular activity to entrap
and/or kill
bacteria, or to inhibit their growth, therefore having an antibacterial, or
bacteriostatic, or bactericide activity.
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The doped inorganic adsorbent materials of the present invention can be
also used in consumer goods in order to provide benefits related to their
specific
adsorption activity, such as for example in filters for smoking articles, e.g.
cigarettes, for blocking nicotine and char; in laundry articles, e.g.
detergents, for
selective dye transfer control in washing; etc.
A further alternative field of use comprises odour control in order to avoid
or
minimize detection of odours emanating from animate and inanimate sources,
such as for example in ventilation systems, room and car fresheners, animal
litters, in personal care and hygienic articles, in toothpastes or chewing
gums for
breath control, etc. Use as an odour control material can of course encompass
many of the other possible fields mentioned above.
The doped inorganic adsorbent material according to the present invention
typically comprise inorganic materials such as for example activated alumina;
siliceous adsorbents comprising natural and synthetic aluminosilicates such as
zeolites, e.g. zeolite A; active silica's, silica gel, silicates. Particularly
preferred
materials are active silica's.
With particular reference to the adsorption of volatile malodorous
compounds, which is illustrated in some of the examples, and which constitutes
a
specific, but not limiting field of application of the present invention, said
volatile
malodorous compounds broadly belong to different classes of compounds: fatty
acids and derivatives thereof; ammonia and amines and their salts; alcohols,
aldehydes and ketones; organic heterocompounds, etc.
The fatty acids are volatile fatty acids selected from straight chain and
branched chain fatty acids containing, for example, from 1 to 12 carbon atoms,
for example isovaleric acid. Another class of odorous compounds include
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ammonia and ammonium salts and amines having a boiling point of up to
170°C
at atmospheric pressure and salts thereof, e.g. triethylamine.
A further class of odorous compounds comprises alcohols, aldehydes such
as furaldehyde, and ketones having a boiling point of up to 170°C at
atmospheric
pressure.
Another class of odorous materials include organic heterocompounds
containing at least one nitrogen, sulfur or oxygen atom, preferably
heterocyclic
compounds containing one or two cyclic rings and containing one or two
heteroatoms which may be the same or different. Other compounds in this
category include mercapto- and thio-compounds and other compounds
containing at least one sulphur atom per molecule which have a boiling point
up
to 170°C at atmospheric pressure.
In general, but also with particular reference to the field of odour control,
when the adsorption occurs in a liquid-phase constituted by an aqueous liquid,
the doped inorganic adsorbent material is used at a preferred pH level which
can
be adjusted according to the polar character of the substance to be adsorbed,
e.g. a malodorous substance. It is known that a strongly acid or a strongly
basic
inorganic adsorbent material, e.g. active silica, can be prepared to show an
increased adsorbing power respectively with regard to basic and acid
substances. However, this apparently higher adsorption is not a true adsorbent
capability but is based on a chemical reaction so that the effect is
transitory i.e. it
is maintained only until the excess acidity or basicity, available for
reaction, has
been neutralized.
For example, when a doped inorganic adsorbent material according to the
present invention is used as an odour controlling material for feminine
hygiene,
for example incorporated in a pantiliner or a sanitary napkin, it is preferred
to
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provide the material with a neutral pH because malodorous compounds present
in physiological fluids may be acidic, basic or neutral.
When used in other absorbent articles where the malodours have a defined
and constant character, e.g. ammonia and amines originating from urine, the pH
can be adjusted, in this case to an acidic pH to provide supplementary odour
control to that provided by the doping impurities and/or by the morphological
and/or chemical modifications introduced by the doping technique.
The doped inorganic adsorbent material can be prepared by any convenient
known method. In particular doped inorganic adsorbent materials comprising
silica, silicagel or active silica can be synthesized via gellation by
acidification of
a water solution of soluble alkaline silicates, or precipitation of colloidal
silica, or
also by controlled hydrolysis of silico-organic compounds, e.g.
tetraethoxysilane,
wherein the selected dopant or dopants are suitably introduced in the reaction
solution at least during one step of the synthesis process or are added to an
already formed inorganic adsorbent material during a post-synthesis treatment.
Preferred doped inorganic adsorbent materials according to the present
invention
comprise inorganic adsorbent materials having a prevailing amorphous
structure,
and more in particular inorganic adsorbent materials essentially composed by
amorphous silicon dioxide.
According to the preferred embodiment of the present invention, it has been
discovered a method for increasing the adsorption capacity of a material for
adsorbing one or more selected solute species, which material comprises an
inorganic adsorbent material doped with one or more dopants, i.e. an inorganic
adsorbent material modified by means of doping as defined herein. The method
of the present invention for increasing the adsorption capacity of doped
inorganic
adsorbent materials consists in the identification of a number of relevant
features
of the doped inorganic adsorbent material, and of a method for manufacturing
the doped inorganic adsorbent material, and in the indication on how to act on
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them in order to increase the adsorption capacity of the doped inorganic
adsorbent material. These features are the type, the concentration, and the
molecular dimension of the dopant or dopants, the latter being relevant either
as
such, or in the context of a method for manufacturing the doped inorganic
adsorbent material of the present invention. According to the present
invention,
the method consists in any of the following options, or in combinations
thereof.
According to a first option, the method consists in choosing the type of the
dopant or dopants. Surprisingly, it has been discovered that this choice also
includes compounds in areas outside of compounds which are the same as the
one or more selected solute species to be adsorbed, or are derivatives
thereof,
or belong to the same chemical class, or have similar functionalities.
According to a second option, the method consists in choosing an increased
concentration of the dopant or dopants to be included in the doped inorganic
adsorbent material of the present invention, wherein "increased" is to be
meant
as explained in the Background of the Invention. Unexpectedly, the inventive
choice of these increased concentrations of the dopant or dopants has proved
to
be effective in increasing the adsorption capacity of a doped inorganic
adsorbent
material.
The third option of the method consists in selecting the molecular dimension
of the dopant or dopants of the doped inorganic adsorbent material.
The fourth option of the method of the present invention intended to
increase the adsorption capacity of a doped inorganic adsorbent material
consists in tailoring the pore structure of a doped inorganic adsorbent
material
through suitably doping the material.
Preferably, the method of the present invention is such that a doped
inorganic adsorbent material made according to the method has an adsorption
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capacity which is at least 8 mg/g, preferably at least 10 mg/g, more
preferably at
least 12 mg/g, greater than the adsorption capacity of a corresponding undoped
inorganic adsorbent material taken as a reference, or alternatively is at
least
30%, preferably at least 35%, more preferably at least 40%, greater than the
adsorption capacity of a corresponding undoped inorganic adsorbent material
taken as a reference. The adsorption capacity of both the doped inorganic
adsorbent material made according to the method of the present invention, and
the undoped reference are to be measured according to the Adsorption Capacity
Test Method described in Chapter 7 herein.
Further preferred alternative embodiments of the present invention,
corresponding to doped inorganic adsorbent materials made according to the
method of the present invention, will be hereinafter described. Ail of them
are in
the context of a general improvement of the known doping technique, intended
in
its broader meaning of chemical and/or physical-chemical and/or morphological
modification of the surface of inorganic adsorbent materials through the
presence, at least in one step of the synthesis process or as a post-synthesis
treatment of said inorganic adsorbent material, of a dopant or of dopants,
which
is/are species chemically different from the inorganic adsorbent material and
from the raw materials) from which said adsorbent is generated, and wherein
the
doping excludes chemical reactions between the dopant or dopants and the
inorganic adsorbent material to be doped, as explained above with reference to
the definition of "doping". All alternative embodiments of the present
invention
lead to doped inorganic adsorbent materials having an increased activity,
corresponding to an increased efficiency of removal and/or adsorption capacity
towards one or more selected solute species from a gas- or a liquid-phase.
This
increased activity can be either directed towards specific solute species,
typically
selectively towards one specific solute specie among other species, or
alternatively towards broader ranges of more solute species of different
types. In
the first case a doped inorganic adsorbent material according to the present
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invention can therefore also show an increased selectivity, i.e. the capacity
of
selectively adsorbing e.g. one specific solute specie among other species.
Particularly, alternative embodiments of the present invention comprise the
application of various doping techniques such as:
~ choice of an increased concentration of dopant or dopants
~ selection of the molecular dimension of the dopant or dopants
~ doping with metals in finely divided form (preferably colloidal metals)
~ doping with organo-metallic compounds and complexes
~ doping with precipitation salts
~ pore structuring by doping
either applied singularly, or in combinations, as will be explained in details
below. The doping techniques listed above are in the context of either
homodoping (imprinting doping), or heterodoping, as explained above.
2. Doped Inorganic Adsorbent Materials with Selected Concentration/Molecular
Dimension of the Dopant(s)
According to an alternative preferred embodiment of the present invention,
it has been surprisingly discovered that by choosing increased concentrations
of
dopant or dopants to be included in the doped inorganic adsorbent material of
the present invention, a material with an increased adsorption
activity/efficiency
of removal is obtained. Particularly, the dopant, or at least one of the
dopants,
must have a concentration of more than 1,000 ppm, preferably of more than
2,000 ppm, more preferably of more than 5,000 ppm. As explained above, the
dopant is not necessarily actually present in the final product. It has been
discovered that these high amounts lead to doped inorganic adsorbent materials
showing an increased efficiency of removal, specifically in the context of the
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"homodoping", as explained above, i.e., towards solute species/compounds to be
adsorbed which are the same as the respective dopant, or derivatives thereof,
or
of the same chemical class, or having similar functionalities.
Alternatively, or preferably in combination with the chosen increased
concentration of the dopant or dopants as explained above, according to a
further preferred embodiment of the present invention, the molecular dimension
of the dopant, or of at least of one of the dopants can be preferably
selected. By
"molecular dimension" it has to be meant in the context of the present
invention
the largest or major dimension of the molecule, or of the aggregate of
molecules,
of a dopant in the conditions of synthesis of the doped inorganic adsorbent
material. On one hand in fact, as it is known in chemistry, certain molecules
have
different dimensions depending on whether they are in solution or not, on the
concentration of the solution, on the nature of the solvent and on many other
physico-chemical parameters such as the pH, the temperature etc. For example,
certain long linear molecules, such as for example surfactants or esters of
fatty
acids, are straightened if they are not in solution, and therefore have a
certain
largest dimension (substantially corresponding to their length), while if they
are in
water solution they tend to curl or twist, therefore having a smaller major
dimension. Conversely, there are molecules which, in the synthesis conditions,
for example in water solution, tend to form supra-molecular aggregates such as
dimers, micelles etc. According to this preferred embodiment of the present
invention, the molecular dimension, which is relevant in this selection
criterion, is
therefore strictly linked to the synthesis conditions, as it may vary
depending on
them, and is to be meant as evaluated at the synthesis conditions of the doped
inorganic adsorbent material.
According to this preferred embodiment of the present invention, the dopant
or at least one of the dopants shall have a largest molecular dimension, as
defined above, of at least 0.5 nm, preferably of at least 0.7 nm, more
preferably
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of at least 1 nm, said dimension evaluated, with any suitable method known in
the art, at the synthesis conditions of the doped inorganic adsorbent
material.
The relevance of the molecular dimension of the dopant molecule in the
case of homodoping can be explained, without being bound to any theory, when
considered in comparison with the relevant dimension of the basic structure of
the inorganic adsorbent material to be modified by addition of said dopant or
dopants. Generally speaking, it is preferred that the major dimension of a
dopant
is greater than the relevant dimension of the basic structure of the undoped
inorganic adsorbent material, such that the molecule of dopant is actually
capable of modifying the surface of the inorganic adsorbent material through
an
"imprint" of its molecular size and shape (by chemically and/or
morphologically
modifying the surface of the inorganic adsorbent material). If for example the
inorganic adsorbent material is a silica, as it is preferred in the present
invention,
the relevant dimension of the basic structure is the side length of the
tetrahedron
Si04, which is 0.32 nm. A dopant having a major dimension in the condition of
formation as specified above, i.e. of at least 0.5 nm, preferably of at least
0.7 nm,
more preferably of at least 1 nm, has proved to be particularly effective in
this
context, in order to provide a doped inorganic adsorbent material having an
increased adsorption activity according to the present invention.
According to the present invention, the selection of the molecular dimension
of the dopant or dopants is particularly effective in the context of the
imprinting
doping, or homodoping, wherein said imprinting doping is to be meant as
explained above, preferably in combination with the choice of the increased
concentration of the dopant or dopants.
3. Doped Inorganic Adsorbent Materials with Specific Classes of Dopant(s)
According to other alternative preferred embodiments of the present
invention, doped inorganic adsorbent materials showing an increased adsorption
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activity can be prepared by suitably selecting the type of the dopant or
dopants to
be included in the inorganic adsorbent material.
A. Colloidal Metals
It has been surprisingly discovered that when said dopant, or at least one of
the dopants, included in the doped inorganic adsorbent material of the present
invention is selected among metals in a finely divided form, preferably in
colloidal
form, the resulting doped inorganic adsorbent material shows an increased
adsorption activity according to the present invention, particularly towards
volatile
inorganic or organic sulfur-containing compounds. This is an example of
heterodoping, according to the definition provided herein, in that the dopant
or
dopants are selected from compounds which are different from the compounds
to be adsorbed. .
Inorganic as well as organic sulfur-containing compounds present in a
gas-phase, e.g. in air in concentrations of over 103 ppm can be successfully
removed by a number of processes. What is often difficult to deal with is the
removal of organic sulfur-containing compounds (such as organic sulfides) at
concentration in order of 1-102 ppm, which is one of the main problems in the
field of odour control. The procedures to remove the sulfur-containing
malodorous compounds are mostly of the physical nature, especially adsorption,
absorption, or removal by chemical reaction. Suitable adsorbents for this
purpose
should have large active surface area and suitable other characteristics, such
as
chemical nature of the surface or pore size and pore volume.
According to this alternative preferred embodiment of the present invention,
a doped inorganic adsorbent material particularly targeted to the adsorption
of
volatile sulfur-containing compounds can include as a dopant or dopants one or
more metals in finely divided form, preferably colloidal metals selected form
the
group consisting of colloidal gold, silver, copper, platinum and platinum
group
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metals, zinc, cadmium, mercury, lead, arsenic, antimony, and manganese.
Colloidal gold or silver are particularly preferred.
The concentration of the colloidal metal or metals, for example gold or silver
as it is preferred, to be included as a dopant or dopants in the doped
inorganic
adsorbent material, is preferably overall limited to a concentration of
10-1,000 ppm, preferably of 100-600 ppm. The doped inorganic adsorbent
material of this preferred alternative embodiment of the present invention
therefore consists in an easily obtainable and cheap material, for example a
silica, modified with a very low amount of the selected dopant or dopants,
leading
to a doped material being highly effective in adsorption activity towards
sulfur-containing compounds, with substantially no increase in the cost of the
basic inorganic adsorbent material owing to the very low amount of dopant.
Doped inorganic adsorbent materials according to this preferred
embodiment of the present invention, particularly including colloidal silver
as a
dopant, have also shown a noticeable ability to kill bacteria and/or to
inhibit their
g rowth .
B. Oraano-Metallic Compounds and Complexes
According to a further alternative embodiment of the present invention, a
doped inorganic adsorbent material can be prepared by doping an inorganic
adsorbent material one or more organo-metallic compounds or complexes as a
dopant or dopants. Said organo-metallic compounds and complexes can for
example comprise Cu-phthalocyanines, and metallocene compounds.
It has been surprisingly discovered that this particular class of dopants
shows a double functionality. The metal functionality of the molecule or
complex
in fact provides the doped inorganic adsorbent material doped with the
selected
organo-metallic compound or complex with an increased adsorption activity
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towards volatile organic or inorganic sulfur-containing compounds, in an
example
of heterodoping. In this context, preferred concentrations of said selected
organo-metallic compounds or complexes in the inorganic adsorbent material of
this preferred embodiment of the present invention are about the same already
mentioned with reference to dopants selected from metals in finely divided
form,
i.e. in the range of 10-1,000 ppm, preferably 100-600 ppm, said concentration
to
be intended with reference to the metal content, i.e. to the metal moiety of
the
organo-metallic compound or complex.
Conversely, the other portion of the molecule or complex of the selected
organo-metallic compound or complex, other than the metal moiety, i.e. the
organic moiety, can provide an inorganic adsorbent material doped with the
same organo-metallic compound or complex with an increased adsorption
activity towards species/compounds which are derivatives of, or are of the
same
chemical class of, or have similar functionality as, this organic moiety of
the
molecule or complex, in an example of homodoping. In this context higher
concentrations of the selected organo-metallic compounds) or complexes) to be
included in the inorganic adsorbent material as a dopant or dopantscan be also
preferably selected, for example concentrations of more than 1,000 ppm,
preferably of more than 2,000 ppm, more preferably of more than 5,000 ppm,
said concentrations being evaluated with reference to the organic moiety of
the
molecule.
C. Precipitation Salts
According to another alternative preferred embodiment of the present
invention, suitable dopant or dopants to be included in a doped inorganic
adsorbent material in order to obtain a doped inorganic adsorbent material
having an increased adsorption activity according to the present invention can
be
selected among the group consisting of precipitation salts of a weak acid and
of
a strong base, or alternatively of salts of a strong acid and of weak base.
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It has been surprisingly discovered that the inclusion in the finished
inorganic adsorbent material of selected amounts of one or more precipitation
salts of a weak acid and of a strong base, or vice versa of a strong acid and
of a
weak base, as the dopant or as at least one of the dopants in a doped
inorganic
adsorbent material according to the present invention, in a suitable
concentration
and dispersion within the pores of the inorganic adsorbent material, which is
preferably a silica, provides a control of the pH level within the pores
themselves.
This adjustment of the pH level within the pores of the doped inorganic
adsorbent
material helps lower the concentration of molecules of solute species having
acid
or basic character from the free fluid phase, in turn providing corresponding
doped inorganic adsorbent materials according to this alternative preferred
embodiment of the present invention which show an increased adsorption
activity
towards respective acidic or basic solute species, particularly from a water
solution. Without being bound to any theory, a possible explanation of this
behaviour is in the following example, concerning an inorganic adsorbent
material provided with an increased pH within the pores through doping with
precipitation salts of ~ a weak acid and of a strong base, showing increased
adsorption activity, from a water solution (the liquid-phase), towards solute
species (fatty acids) having an acidic character. The concentration of fatty
acids
in the gas phase over their water solutions depends on the pH of the
respective
solution. For example, with 2 wt. % water solution of butyric acid the
decrease in
pH from 7.4 to 4.0 leads to an increase in the concentration in the gas phase
by
10%, while the increase in pH from 7.4 to 11.0 leads to a decrease in
concentration in the gas phase by about 60%. Consequently the local increase
in
pH within the pores of the silica due to the presence of basic precipitation
salts
(i.e. coming from a weak acid and a strong base) causes a decrease in the
concentration of the fatty acid in the gas phase.
Preferred concentrations of precipitation salts as disclosed above in a
doped inorganic adsorbent material in order to obtain a doped inorganic
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adsorbent material according to this alternative preferred embodiment of the
present invention are in the range of 1 % to 50% by weight on the dry basis of
the
inorganic adsorbent material, preferably of 5% to 40%, more preferably of 8%
to
35%. Most preferably, the precipitation salt or salts is/are actually present
in the
preferred concentrations in the final doped inorganic adsorbent material of
this
preferred embodiment of the present invention.
Any suitable method can be used in order to prepare a doped inorganic
adsorbent material according to this alternative preferred embodiment of the
present invention. When the starting inorganic adsorbent material to be doped
with the inclusion of the selected dopant or dopants is preferably a silica,
these
precipitation salts can be introduced either by formation and co-precipitation
with
the silica, in the course of the gellation of the silica e.g. in thestep when
a water
solution of a sodium silicate is treated by a suitable acid, or by post-
synthesis
impregnation of an already formed silica by a water solution of the suitable
salt or
salts.
More in detail, according to the first method a doped silica can be obtained
for example by gelling a silica from a solution of sodium silicate by acetic
acid
(which acts both as the gelling and the doping agent) at an increased
temperature till pH of the solution in the range of 5.0-8.0 is achieved,
whereupon
the gel formed is subjected to aging at an increased temperature and recovered
by filtration. If all the formed amount of the precipitation salts is to be
kept into
the finished adsorbent silica, the gel is dried e.g. at 190°G without
any previous
washing. Otherwise the gel is washed in the conditions and with the amount of
water suitable to leave inside the silica the desired concentration of the
precipifiation salt, e.g. according to the preferred concentrations as
explained
above. In the alternative method, an already formed silica is impregnated with
a
solution of a suitable salt (e.g. of a weak acid and a strong base) in a
suitable
concentration in order to have the preferred final concentration of the
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precipitation salt in the doped inorganic adsorbent material, and then is
dried at
an increased temperature (over 130°C) till the solvent is evaporated.
The solution according to this alternative preferred embodiment of the
present invention is advantageous in that it either involves, according to the
first
general method, the application of an easily obtainable and very cheap raw
material (typically, and preferably, a sodium silicate solution), which can be
directly formed into a highly efficient doped inorganic adsorbent material,
particularly for the removal of malodorous fatty acids, by a simple and cheap
method, or, according to the second general method, the direct modification of
commercial silica, whose impregnation with suitable precipitation salts
provides
the final material with the same increased adsorption activity.
4. Pore Structuring through Doping
According to a further alternative preferred embodiment of the present
invention, the adsorption activity of a doped inorganic adsorbent material can
be
also enhanced towards specific solute species with the provision of an
optimized
pore structure in said doped inorganic adsorbent material by means of a
specific
doping technique.
Specific tailoring of structural parameters of an inorganic adsorbent
material, e.g. an active silica, such as provision of specific surface and
pore size,
is obtainable with different methods taught in existing literature. The pore
size, or
average pore diameter, is to be intended herein in its usual meaning known in
physical chemistry, and can be evaluated with any known suitable method.
According to this further embodiment of the present invention, pore
structuring of
an inorganic adsorbent material can be achieved through a doping technique, by
means of suitably selecting the conditions of doping, and the type of the
dopant
or dopants.
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Specifically, according to this preferred embodiment of the present
invention, a doped inorganic adsorbent material, e.g. a doped active silica,
having a predetermined porosity structure and pores with a predetermined
selected size, can be obtained by synthesizing the doped inorganic adsorbent
material in the presence of a selected dopant or dopants which, in the
synthesis
conditions, is constituted by molecules or supra-molecular aggregates, e.g.
micelles, having the relevant dimension, i.e. the largest dimension, which is
about the same as the desired selected pore size in the final doped inorganic
adsorbent material. According to this alternative preferred embodiment of the
present invention the selected dopant or dopants provide the doped inorganic
adsorbent material with the desired pore structure/size, during the formation
process, and is/are then typically removed from the doped inorganic adsorbent
material, which substantially does not contain any residual dopant as a
finished
product. A specific pore size of the doped inorganic adsorbent material, as it
is
known in the art, can be selected in order to optimize the adsorption of
solute
species, e.g. target molecules, having approximately this same size.
The synthesis of a doped inorganic adsorbent material according to this
preferred embodiment of the present invention can be achieved with any known
method of preparation, provided the dopant or dopants selected according to
the
above criterion are added in the synthesis. For example, a doped inorganic
adsorbent material having an increased adsorption activity towards solute
species constituted by small volatile molecules, such as light organic amines,
can
be provided with an optimized pore structure characterized by a large surface
area and a small pore size, wherein this pore size is selected in order to
approximately match the molecular dimension of said target molecule(s),
According to this preferred embodiment of the present invention, this can be
achieved by selecting a suitable dopant or dopants to be added to the
synthesis
process of the inorganic adsorbent material. The dopant or dopants, in the
synthesis conditions, shall comprise molecules or supra-molecular aggregates
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which have the relevant, i.e., the largest dimension which is about the same
as
the desired pore size.
Preferably, the dopant or dopants according to this preferred embodiment of
the present invention is/are added in a concentration of at least 10,000 ppm,
more preferably of at least 18,000 ppm, even more preferably of at least
20,000 ppm, based on the dry amount of the final doped inorganic adsorbent
material. Even higher concentrations are also possible, for example up to
50,000 ppm, and above.
This preferred embodiment of the present invention typically constitutes an
example of heterodoping, according to the definition provided herein, in that
the
dopant or dopants are selected on the basis of their molecular dimension at
the
synthesis conditions, independently from the nature of the solute species to
be
adsorbed, and therefore can be typically different from them.
5. Combinations
According to the present invention, any possible combinations of the
alternative preferred embodiments described above are also possible, and are
considered within the scope of the present invention. Possible examples are
inorganic adsorbent materials comprising at least two different dopants
selected
from the different types described under Chapter 3 above. Any combinations
with
the pore structuring technique described in Chapter 4 above, as well as with
selected concentrations and/or molecular dimensions of the dopant or dopants
described in Chapter 2 are also possible, and within the scope of the present
invention.
For example, according to a preferred embodiment of the present invention,
a doped inorganic adsorbent material can be doped with one or more metals in
finely divided form, preferably colloidal metals, as disclosed under 3.A
above, in
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combination with one or more precipitation salts as disclosed under 3.B above.
Respective preferred concentrations and features are the same as described in
the relevant paragraphs. Such a doped inorganic adsorbent material shows an
increased adsorption activity towards different classes of solute species,
namely
organic fatty acids, sulfur-containing compounds, and organic amines.
6. Other Preferred Features of the Doped Inorganic Adsorbent Materials
In addition to the above features in the context of the doping techniques,
according to the various alternative preferred embodiments of the present
invention, different optional features known in the art can be further
included in
the doped inorganic adsorbent materials according to the present invention, in
order to further morphologically and/or chemically modify the surface of the
doped inorganic adsorbent material itself, such as for example, but without
limitation:
~ pore, i.e. pore size, pore shape, and pore volume, and other surface
morphology parameters engineering, achieved by means of known
techniques different from doping;
~ chemical hydrophobization of the surface of the doped inorganic
adsorbent material, particularly for the adsorption of hydrophobic organic
molecules from water, achieved by known techniques, e.g. by chemical
grafting of organic groups on the surface of the inorganic adsorbent
material;
~ change of the iso-electric point of the doped inorganic adsorbent material
in wafer, such as for example the increase of the iso-electric point of a
doped active silica from pH=2.9, which is the pH typical of a silica, to
higher values, in order to not impair the adsorption from wafer of
molecules (solute species) which are negatively charged.
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7. Test Methods
Adsorption Capacity Test Method.
The adsorption capacity test is intended to measure the actual amount of a
selected solute specie adsorbed by an inorganic adsorbent material, in mg/g,
at
fixed conditions of quasi-saturation. By quasi-saturation it is meant herein a
condition in which the inorganic adsorbent material is put in contact with an
amount of solute specie sufficient to exhaust its adsorbent capacity, in a
period
of time, which is representative of the actual adsorption conditions which are
encountered in common practice, as specified below. The respective amounts
adsorbed by a doped inorganic adsorbent material, and by its corresponding
undoped reference material, constituted, as explained above, by a similar
material, obtained in the same synthesis conditions, but without any doping
and
dopant, are evaluated and compared.
In order to verify the adsorption capacity of a doped inorganic adsorbent
material, a selected solute specie is to be tested according to the method.
The test is conducted at constant temperature of 25°C
Adsorption in liauid-phase.
A 5% by weight solution is prepared dissolving the selected solute specie in
a respective solvent. 0.5 g of the inorganic adsorbent material under test are
mixed with 0.5 ml of solution and put in a closed vessel, where contact is
maintained for 15 minutes. Thereafter, the residual amount of the solute
specie
in the solution, and if necessary in the gas in equilibrium with the solution,
is
measured by means of known methods, for example by gas or liquid
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chromatography. By difference, the amount actually adsorbed by the inorganic
adsorbent material is evaluated in mglg.
For a selected solute specie the test is to be repeated in liquid-phase with
the following solvents: water at three different pH levels, namely 2~0.5,
7~0.5,
and 9.5~0.5, ethanol, hexane, toluene, methylene chloride, and also in
gas-phase as explained below, if the solute specie is volatile. As intended
herein,
by "volatile" it is meant a solute specie having a boiling point of up to
170°C at
atmospheric pressure.
The test is performed both on the doped inorganic adsorbent material under
test, and on the undoped reference material for comparison.
Of course, depending on the selected solute specie, one or more of the
indicated solvents can be excluded, for example because the selected solute
specie is not soluble in the required concentration in a solvent, or also if a
solvent
actually corresponds to a selected solute specie. This can be readily
ascertained
by the man skilled in the art in performing the method.
The results obtained with the different solvents, and, if applicable, also
obtained from the adsorption in gas-phase (see below), are to be compared, and
the result showing the largest difference with the undoped reference material
has
to be taken as the value of the adsorption capacity of the doped inorganic
adsorbent material under test towards the selected solute specie.
Adsorption in aas-phase.
A 5% in volume mixture of the gaseous solute specie in air at ambient
conditions (temperature of 25°C, atmospheric pressure, and relative
humidity of
50% is kept in a closed vessel having a volume of about 200 ml. 0.5 g of the
inorganic adsorbent material under test are put in the vessel and contact with
the
gas-phase is kept for 15 minutes. Afterwards, the residual content of the
solute
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specie in air is evaluated with known means, e.g. by gas chromatography, and
by
difference the amount of solute specie actually adsorbed in the inorganic
adsorbent material is evaluated in mg/g.
If the residual amount of solute specie in the fluid-phase (either in the gas-
or in the liquid-phase, as the case will be), is substantially zero after a
contact of
15 minutes between the fluid phase and the inorganic adsorbent material under
test, this can imply that the quasi-saturation conditions between the
inorganic
adsorbent material and the solute specie have not been attained, and that
possibly the inorganic adsorbent material has adsorbed the entirety of the
solute
specie from the fluid-phase without exhausting its adsorption capacity. In
such a
case the test is repeated with the same, "partially exhausted", inorganic
adsorbent material until a residual amount of solute specie in the fluid phase
is
detected and measured with the selected known means, in order to evaluate the
overall amount of solute specie adsorbed by the inorganic adsorbent material
at
quasi-saturation conditions. In the adsorption in liquid-phase, this is done
by
suitably adding further 0.5 ml of the same solution to the inorganic adsorbent
material in the vessel. In the adsorption in gas-phase, a further suitable
amount
of the same gaseous solute specie is added in order to achieve the same 5%
volume concentration in the solute specie/air mixture.
A doped inorganic adsorbent material according to the present invention
has to show an adsorption capacity, towards at least one of the selected
solute
species, which is at least 3'mg/g, preferably at least 10 mg/g, more
preferably of
at lest 12 mg/g greater than the adsorption capacity of the reference undoped
inorganic adsorbent material. Alternatively, in relative terms, the adsorption
capacity of a doped inorganic adsorbent material according to the present
invention has to be at least 30%, preferably at least 35%, more preferably ate
least 40% greater than the adsorption capacity of the reference undoped
inorganic adsorbent material.
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Efficien ~ of Removal Test Method.
The Efficiency or Removal Test Method is intended to measure the activity
of the doped inorganic adsorbent materials of the present invention in terms
of
efficiency of removal towards specific solute species, as illustrated in the
following examples.
0.5 g of the tested inorganic adsorbent material and 0.5 g of a solution
containing
the specific solute specie are mixed in a china crucible and stirred for one
minute.
Four solutions are prepared by dissolving in distilled water the following
amounts
of solute species, according to the type of solute specie tested:
- 2% by weight of Butyric acid
- 5% by weight of Tri-methyl amine
- 2% by weight of Di-methyl sulfide
- 5% by weight of Pyridine
all available from Sigma Aldrich.
Each solution is buffered at pH 7.4 with Phosphate Buffer Saline available
from
Sigma Aldrich.
The crucible is put on the bottom of a glass vessel of total volume of about
300 cm3, which is immediately closed.
The glass vessel has on its cover one inlet and one outlet. Through the inlet
a
glass pipe enters into the vessels arriving at about 10 mm over the center of
the
surface of the tested blend of inorganic adsorbent material and solution,
inside
the china crucible.
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Nitrogen is flowing through this descending pipe at a suitable flow suitably
chosen between 20 ml/min and 50 ml/min in order to have a completion time for
the test between 10 and 20 minutes (see below). Once selected according to the
above criterion, the flow of nitrogen is then kept constant for the entire
test.
The gas escaping from the vessel through the outlet is passed through a
Draeger
tube, available from Draegerwerk Aktiengesellschaft (Germany), used to adsorb
and measure specific impurities contained as mixed gases or vapours in a main
gas stream.
The vessel is put into a thermostatic bath at 25°C at least 10 minutes
before
starting the test and kept there for the duration of the entire test, in order
to
perform the test at constant temperature.
The following tubes are used for the different solute speciess:
- Butyric Acid = Draeger tube for Acetic Acid - 5-80 ppm scale (Code 6722101 )
- Tri-methyl-amine and Pyridine = Draeger tube for Ammonia - 5-70 ppm (Code
CH 20501 )
- Di-methyl-sulfide = Draeger tube for Di-methyl sulfide 1-15 ppm (Code
6728451)
The test is completed when the color change of the Draeger tube reaches the
maximum of the scale. As mentioned above, the actual flow of Nitrogen used in
the test is preferably chosen, between 20 and 50 ml/min, in order to make this
happen in a time comprised between 10 and 20 minutes. If the time for the test
completion, even by varying the flow of Nitrogen within the entire range, is
shorter than 1 minute or longer than 30 minutes, the use of Draeger tubes with
different scales is advisable.
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If the detected concentration of solute specie is anyhow so low that the
change
of color of the Draeger tube is not complete even after 60 minutes, at that
point
the test is considered in any case as completed.
In parallel a similar test is conducted, the only difference being that the
china
crucible contains 0.5 g of the solution only, without any inorganic adsorbent
material, in order to measure the concentration of the solute specie in the
gas
over its water solution. This value is considered to correspond to the initial
concentration of the solute specie, before any adsorption by the inorganic
adsorbent material occurs.
The efficiency of removal of the inorganic adsorbent material under test is
calculated as:
Eff. % = 1 OO * (Cpp - Cp) / CPO
wherein:
CPO = Concentration of the solute specie over its water solution in the
experiment
without inorganic adsorbent material
CP = Concentration of the solute specie over its water solution in the
experiment
with inorganic adsorbent material.
Both concentrations are calculated according to the following formula:
C=N* 100*CDF/(F*T)
Wherein:
N = parameter given in the instructions of the specific Draeger tube used
("number of strokes")
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CDF = actual reading, read on the Draeger tube, at the end of the test (ppm)
F = flow of nitrogen during the test (ml/min)
T = actual test time in minutes.
8. Examples
The invention is illustrated by reference to the following Examples. The
efficiency of removal values, and the adsorption capacity values mentioned in
the
Examples were evaluated according to the Adsorption Test Method and to the
Adsorption Capacity Test Method, respectively, both methods described herein.
Example 1.
A doped active silica (Silica 11 ) was prepared by the gellation at
80°C of a
solution of sodium metasilicate (15 g of Na2Si03 dissolved in 700 ml of water)
by
a 16.3 wt. % solution of sulfuric acid to pH 7.2, and the gel was formed in
the
presence of 1,400 ppm of butyric acid. Finally the gel was dried at
190°C for 10
hours. Silica 11 was tested for the efficiency of removal towards butyric
acid, and
the result was an efficiency of 93.0%, compared to an efficiency of removal of
70% of the reference silica prepared according to the same conditions, but
without the dopant (the undoped reference material). The adsorption capacity
of
Silica 71 was 19 mg/g, while the adsorption capacity of the reference was
14 mg/g, with an increase of about 35.7%.
Example 2.
A doped active silica (Silica 38) was prepared by the gellation at
80°C of a
diluted solution of sodium silicate solution (20 ml of sodium silicate
solution,
available form Aldrich, in 680 ml of water) by a 29 wt. % solution of acetic
acid to
pH 7.1, and the gel was formed in the presence of 2,330 ppm of trimethylamine.
Finally the gel was dried at 190°C for 24 hours. The results for the
efficiency of
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removal and the adsorption capacity towards trimethylamine were 95% and
50 mg/g, compared to 81 % and 40 mg/g for the undoped reference, with an
increase in the adsorption capacity of 10 mg/g.
Both Example 1 and 2 show the increased performances of the doped
inorganic adsorbent materials of the present invention, in the context of the
improved imprinting doping or homodoping by means of chosen preferred
increased concentrations of the dopant.
Example 3.
A doped active silica (Silica G6) was prepared as follows. 30 ml of a
solution of sodium silicate (density of 1.39 g/cm3, the Si02/Na20 ratio of
3.35, the
concentration of Na20 and Si02 of 8.75 and 27.55 wt. %, respectively) was
mixed with 670 ml of distilled water and the solution obtained was heated
under
constant stirring to 80°C. Then 5.7 ml of a solution of colloidal gold,
which
contained 3.2 x 10~' gram-atom of gold in 1 ml (preparation of the solution
according to G. Brauer: Handbuch der Praparativen Anorganischen Chemie, F.
Enke Verlag Stuttgart, 1954) was added under constant stirring, whereupon the
gellation was started by the addition of a solution of 29 wt. % acetic acid.
After
reaching pH 7, the reaction mixture was heated at 80°C for 30 minutes.
After
having been aged for 30 minutes, the palely pink gel of silicic acid was
recovered
by sucking away the surplus liquid and washed once with 700 ml of distilled
water, Finally the gel was dried at 190°C for 10 hours. The
concentration of
colloidal gold in silica prepared according to the foregoing procedure
attained
36 ppm.
The efficiency of removal of the Silica G6 doped with colloidal gold towards
dimethylsulfide is 66.9%, compared to 37.7% of the corresponding reference
undoped silica, with an increase of 77.4%.
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Example 4.
A doped active silica (Silica G8) was prepared according to the same
method as explained in Example 3, except that 92 ml of the same solution of
colloidal gold were added, and that the gel was finally dried at 190°C
for 22
hours. The concentration of colloidal gold in Silica G8 was 581 ppm.
The efficiency of removal of Silica G8 towards dimethylsulfide is 93.5%,
which compared to 37.7% of the same undoped silica taken as a reference in
Example 3, showing an increase of 148.0%. The two examples above show the
effect of the doping with colloidal metals in enhancing the adsorption
activity of
doped silica's towards sulfur containing compounds. Namely Example 4 shows
the even better increase due to the higher preferred concentration of dopant.
Example 5.
A doped silica (Silica 16) was prepared by the gellation at 80°C of a
dilute
solution of Water Glass Silchem (30 ml of Water Glass Silchem in 670 ml of
water) by a 29 wt. % solution of acetic acid to pH 6.9 in the presence of
2,010 ppm of Cu-phthalocyanine. The gel was dried at 140°C for 24
hours. Silica
16 showed an efficiency of removal towards dimethylsulfide of 82%, compared to
an efficiency of 40% of the reference undoped silica, with an increase of
105%.
This result shows the beneficial effect ~of the metal moiety of the organo-
metallic
compound with respect to the activity towards sulfur containing compounds.
The Silica 16 was also tested towards pyridine, showing an efficiency of
removal of 69%, compared to the value of 17% of the undoped reference. This
further result demonstrates the effect of the organic moiety of the dopant
molecule with respect to the efficiency of removal towards pyridine, in the
context
of homodoping.
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Example 6.
A doped active silica (Silica 42) was prepared as follows. 60 ml of a solution
of sodium silicate (density of 1.39 g/cm3, the Si02/Na20 ratio of 3.35, the
concentration of Na20 and Si02 of 8.75 and 27.55 wt. %, respectively) was
mixed with 1,340 ml of distilled water and the solution obtained was heated
under
constant stirring to 80°C. Then the gellation was started by the
addition of a
29 wt. % solution of acetic acid acting both as a gelling agent and as a
dopant to
provide the precipitation salt (sodium acetate). After reaching pH 7, the
reaction
mixture was heated at 80°C for 30 minutes. After 30 minutes of aging
the gel
formed recovered by sucking away the surplus liquid and dried at 190°C
for 12
hours. The residual content of precipitation salt was 25% by weight of sodium
acetate.
The efficiency of removal of Silica 42 was tested towards butyric acid, with a
result of 98.8%.
Example 7.
As an alternative to the preparation of a silica doped with precipitation
salts
directly during the formation process, e.g. by gellation from a solution of
sodium
silicate with acetic acid as described in Example 6, three samples of doped
silica's were prepared via a post-synthesis impregnation by a water solution
of a
suitable salt of an already formed silica.
Commercial samples of silica A and B available from Merck and Grace
under the trade names Kieselge140 and Silicagel 123, respectively, and a
sample of silica C prepared by the gellation with acetic acid at the same
conditions of Example 6, but afterwards thoroughly washed three times with
1,300 ml of distilled water, were impregnated with a solution of sodium
acetate,
whose concentration was 11.6 wt. %. The efficiency of removal towards butyric
acid of the doped samples and of the respective reference undoped silica's are
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given in the following table, which shows the dramatic increase provided by
the
doping with precipitation salts.
Content of Efficiency Efficiency
sodium acetate of increase Vs.
(wt. %) removal (%) reference (I)
A - 23.3 -
B - 14.0 -
C - 47.5 -
Doped A 9.8 97.9 320.2
Doped B 9.8 95.6 582.9
Doped C 9.8 99.2 108.8
Example 8.
A doped active silica (Silica Univ 1 ) was prepared as follows. 60 ml of a
solution of sodium silicate (density of 1.39 g/cm3, the Si02/Na20 ratio of
3.35, the
concentration of Na20 and Si02 of 8.75 and 27.55 wt. %, respectively) was
mixed with 1,340 ml of distilled water and the solution obtained was heated
under
constant stirring to 80°C. Then 110 ml of a water solution of colloidal
gold was
added at vigorous stirring, which contained 66 pg of Au in 1 ml, i.e. 3.3 x 10-
7
gram atom of Au/1 ml. The preparation of the colloidal solution is described
in G.
Brauer: Handbuch der Praparativen Anorganischen Chemie, F. Enke Verlag,
Stuttgart 1954. Immediately afterwards the geilation was started by the
addition
of a 29 wt. % solution of acetic acid. After reaching pH 7.1, the reaction
mixture
was heated at 80°C for 30 minutes. After 30 minutes of aging the gel
formed was
recovered by sucking away the surplus liquid, washed with 1,300 ml of
distilled
water, sucked away and dried at 190°C for 11 hours. The content of
colloidal
gold in the silica prepared according to the described procedure was 346 ppm.
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The efficiency of removal of the above silica was tested towards butyric
acid, trimethylamine, and dimethylsulfide, giving the results of 97.3%, 93.6%,
and
96.7%, respectively. The example shows a silica doped with both a colloidal
metal (colloidal gold), and a precipitation salt (sodium acetate, provided by
the
acetic acid acting both as a gelling agent and a dopant). The combination of
different types of dopants in the same silica provides an enhanced efficiency
of
removal towards a broad range of solute species.
Example 9.
This is an example of a silica (Silica T7) modified by means of pore
structuring through doping. A mesoporous silica was prepared from 60 ml of a
sodium silicate solution (density of 1.39 g/cm3, the Si02/Na20 ratio of 3.35,
the
concentration of Na20 and Si02 of 8.75 and 27.55 wt. %, respectively), which
was mixed with 1,340 ml of distilled water and the solution obtained was
heated
to 80°C under consfant stirring. Tt~e geffation was started by tire
addition of a
29 wt. % water solution of acetic acid, after addition of 23,350 ppm of
butyric acid
as the dopant to create suitable pore dimensions. After reaching pH 7.03, the
gellation was finished. The reaction mixture was heated at 80°C for 30
minutes.
The gel formed was recovered by sucking away the surplus liquid, washed with
3,900 ml of distilled water (stirring with 1,300 ml of water for 10 minutes
followed
with filtration was repeated three times), sucked away and dried at
190°C for 22
hours. Its surface area, pore volume and mean pore size were 590 m2/g,
0.52 cm3/g and 4.1 nm, respectively.
The efficiency of removal of the silica towards trimethylamine was
evaluated, showing a value of 100%. The same silica was also tested for
efficiency of removal towards butyric acid and dimethylsulfide, showing values
of
32% and 20%, respectively. These results also indicate a high selectivity of
the
doped silica under test towards trimethylamine, compared to the lower values
of
efficiency of removal attained for the two other solute species.
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Example 10.
This is a further example of a silica (Silica 29) modified by pore structuring
through doping. A silica was prepared from 60 ml of a sodium metasilicate
solution, which was mixed with 1,340 ml of distilled water and the solution
obtained was heated to 80°C under constant stirring. The gellation was
started
by the addition of a 29 wt. % water solution of acetic acid, after addition of
10,370 ppm of hexadecylmercaptane as the dopant to create suitable pore
dimensions. After reaching pH 7.03, the gellation was finished. The reaction
mixture was heated at 80°C for 30 minutes. The gel formed was recovered
by
sucking away the surplus liquid, washed with 3,900 ml of distilled water
(stirring
with 1,300 ml of water for 10 minutes followed with filtration was repeated
three
times), sucked away and dried at 190°C for 24 hours. Its surface area
and mean
pore size were 190 m2/g, and 8.5 nm, respectively.
The efficiency of removal of the silica towards butyric acid was evaluated,
showing a value of 100%. The same silica was also tested for efficiency of
removal towards trimethylamine and dimethylsulfide, showing values of 57% and
47%, respectively. Also a good selectivity of the doped silica towards butyric
acid,
and in comparison to trimethylamine and dimethylsulfide is also evident.
Examples 9 and 10 show that by suitably selecting the type and the amount
of the dopant, in order to achieve the preferred pore structuring, a doped
silica is
obtained which is very efficient towards a specific solute specie, in terms of
efficiency of removal, and also shows a good selectivity.
42
