CATALYSEUR RENFERMANT UN COMPLEXE METALLIQUE LIE A UN POLYMERE POUR LE REFORMAGE DE L'HYDROGENE A PARTIR D'ACIDE FORMIQUE

POLYMER BOUND SOLID METAL COMPLEX CATALYST FOR HYDROGEN REFORMING FROM FORMIC ACID

Abrégé français

La présente concerne un catalyseur hétérogène renfermant un complexe métallique lié à un polymère pour le reformage de lhydrogène à partir dacide formique. Ce complexe de métal noble est lié de manière permanente à une surface polymère dun ligand associé au catalyseur. Dans un cas privilégié, le métal noble est le ruthénium, les ligands sont un sel de trisodium de triphénylphosphine-3,3,3-acide trisulfonique (tppts) lié par polymère et la chaîne principale du polymère est le polystyrène. Selon un mode de réalisation privilégié, deux ligands de tppts sont liés à latome de métal central. Les deux molécules de ligands ou au moins lune dentre elles sont chimiquement liées à la chaîne principale du polymère à partir de la position para du groupe de phényle dans le tppts. En raison de cette liaison, le système devient un solide insoluble dans leau. En raison de lassociation des groupes de phényle métasulfoné dans les ligands de tppts, le complexe de ruthénium est hydrophile pour une réaction en surface efficace. Le catalyseur renfermant un complexe de ruthénium lié à un polymère est idéal pour le reformage dacide formique en hydrogène à basse température. En raison de la liaison permanente du catalyseur à la chaîne principale de polystyrène, la lixiviation du catalyseur du reformeur ne se produit pas et le catalyseur peut être utilisé en différentes formes, comme en poudre fine, en particules, en feuilles, en tiges, en flocons, en perles, en tubes, en blocs, etc. Le catalyseur est hydrophile, mais insoluble dans leau, lacide formique et dautres solvants. Il est stable à des températures élevées et aussi dans des conditions acides et basiques. En raison de sa nature solide, le catalyseur est idéal pour les reformeurs indépendants de leur orientation.

Abrégé anglais

A polymer bound heterogeneous metal complex catalyst which is capable
of reforming hydrogen from formic acid is discussed. This noble metal complex
is permanently bound to a polymer surface from a ligand which is associated
with
this catalyst. In a preferred case the noble metal is ruthenium, the ligands
are
polymer bound Triphenylphosphine-3,3',3"-trisulfonic acid trisodium salt
(TPPTS)
and the polymer backbone is polystyrene. In a preferred embodiment, two
ligands of TPPTS are connected to the central metal atom. Both ligand
molecules, or at least one of them, are chemically bonded to the polymer
backbone from the para-position of the one phenyl group in the TPPTS.
Because of this attachment the system becomes a water insoluble solid. Due to
the association of meta-sulfonated phenyl groups in TPPTS ligands resulting in
the ruthenium complex being hydrophilic for efficient surface reaction. The
polymer bound ruthenium complex catalyst is ideal to reform formic acid to
hydrogen at low temperature. Because of the permanent bonding of this catalyst
to the polystyrene backbone leaching out of the catalyst from the reformer
does
not occur, and can be used in different forms such as fine powder, particles,
sheets, rods, flakes, beads, tubes, blocks, etc. The catalyst is hydrophilic
but
insoluble in water, formic acid and other solvents. The catalyst is stable to
high
temperatures, and also stable to acidic and basic conditions. Because of the
solid nature, the catalyst is ideal for orientation independent reformers.

Revendications

What is claimed is:
1. A heterogeneous polymer bound solid metal complex catalyst for reforming
hydrogen from formic acid, comprising,
a noble metal complex comprising two or more triarylphosphine ligands, wherein
at
least one of the two or more triarylphosphine ligands is chemically bonded to
a
polymer by an aryl group of the at least one triarylphosphine ligand.
2. The solid metal complex catalyst of claim 1, wherein the two or more
triarylphosphine ligands are triphenyl phosphine ligands and at least one
phenyl
group of the two or more triphenyl phosphine ligands is sulfonated.
3. The solid metal complex catalyst of claim 2. wherein the triphenyl
phosphine ligand
chemically bonded to the polymer is triphenylphosphine-3,3',3"-trisulfonic
acid
trisodium salt (TPPTS).
4. The solid metal complex catalyst of claim 1, 2 or 3, wherein the noble
metal is
ruthenium, rhodium, palladium, silver, osmium, iridium, platinum or gold.
5. The solid metal complex catalyst of claim 1, 2 or 3, wherein said noble
metal is
ruthenium.
6. The solid metal complex catalyst of any one of claims 1 to 5, wherein said
polymer
is polystyrene.
7. The solid metal complex catalyst of claim 1, wherein the solid metal
complex catalyst
is Ru(H2O)4(PS-TPPTS)2 or Ru(H2O)4(PS-TPPTS)(TPPTS), wherein PS is
polystyrene and TPPTS is triphenylphosphine-3,3',3"-trisulfonic acid trisodium
salt.
8. The solid metal complex catalyst of claim 1, wherein the at least one of
the two or
more triarylphosphine ligands is chemically bonded to the polymer from at
least one
of the ortho, meta or para-positions of the aryl group.
9. The solid metal complex catalyst of claim 1, wherein the at least one of
the two or
more triarylphosphine ligands is chemically bonded to the polymer from a para-
position of the aryl group.
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10. The solid metal complex catalyst of claim 1, wherein said noble metal is
ruthenium
and the two or more triarylphosphine ligands are TPPTS, and wherein meta-
positions of the phenyl groups of TPPTS are sulfonated.
11. The solid metal complex catalyst of claim 10, wherein the sulfonated
phenyl groups
of TPPTS are sodium salts.
12. The solid metal complex catalyst of any one of claims 1 to 11, wherein the
solid
metal complex catalyst is insoluble in water, formic acid or both water and
formic
acid.
13. The solid metal complex catalyst of any one of claims 1 to 12, wherein the
solid
metal complex catalyst is inhibited from leaching out from a reformer or a
reactor
when wetted.
14. The solid metal complex catalyst of any one of claims 1 to 13, wherein the
solid
metal complex catalyst is formed one from molded shapes, powder, particles,
sheets, rods, flakes, beads, tubes, blended polymers or blocks.
15. The solid metal complex catalyst of any one of claims 1 to 14, further
comprising
water and formic acid.
16. The solid metal complex catalyst of any one of claims 1 to 15, wherein the
solid
metal complex catalyst is hydrophilic.
17. The solid metal complex catalyst of any one of claims 1 to 16, wherein the
solid
metal complex catalyst is a rigid solid catalyst.
18. A method of producing a heterogeneous solid ruthenium complex catalyst,
the
method comprising:
a) preparing a polystyrene bonded TPPTS by the steps of
reacting a polystyrene bonded triphenylphosphine with fuming
sulphuric acid; and
reacting with sodium hydroxide to form the polystyrene bonded TPPTS;
and
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b) reacting an aqueous solution of ruthenium (III) chloride with the
polystyrene
bonded TPPTS, forming a polystyrene bonded ruthenium/TPPTS complex; and
c) activating the catalyst by reacting the polystyrene bonded ruthenium/TPPTS
complex with a mixture of sodium formate and formic acid.
19. The method of claim 18, further comprising the step of:
a. filtering and drying the catalyst.
20. A method of reconditioning a heterogeneous polymer bound solid metal
complex
catalyst following contact and reaction with formic acid comprising the steps
of:
a) suspending the heterogeneous polymer bound solid metal complex catalyst in
water;
b) filtering the catalyst; and
c) activating the catalyst.
21. The method of claim 18, wherein in step b) the aqueous solution of
ruthenium (III)
chloride is reacted with polystyrene bonded TPPTS and TPPTS stepwise to form
the polystyrene bonded ruthenium/TPPTS complex.
22. A method of producing a heterogeneous polymer bound solid metal complex
catalyst for reforming hydrogen from formic acid, the method comprising:
a) preparing a polymer bonded triarylphosphine-sulfonic acid sodium salt by
the
steps of:
i. reacting polymer bonded triarylphosphine with fuming sulphuric acid;
and
ii. reacting a product from (i) with sodium hydroxide to form its sodium
salt; and
b) reacting a solution of noble metal reagent with the polymer bonded
triarylphosphine-sulfonic acid sodium salt to form the heterogeneous polymer
bound solid metal complex catalyst.
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23. A heterogeneous polymer bound solid metal complex catalyst, of the general
formula (1),
<IMG>
where
R1 and R2 are of the structure,
<IMG>
and each of A, B, C, D, E, F, G is hydrogen,
X are four ligands with electron donating ability,
M is a noble metal, and
Pol is a polymer attached to at least one of the ortho, meta or para positions
of at
least one phenyl group of R1 or R2.
24. The heterogeneous polymer bound solid metal complex catalyst of claim 23,
wherein Pol is polystyrene, X is H2O and M is ruthenium.
19

Description

Polymer bound solid metal complex catalyst for hydrogen reforming from
formic acid
Field of the Invention
The invention relates to materials used in the catalytic production of
hydrogen via hydrocarbon reforming. In particular, this invention relates to a
heterogeneous polymer bound solid metal complex catalyst, in which noble metal
complex is chemically bound to organic-polymer via the ligands associated with
catalyst molecules. The invention is best suited for multiple cycle portable
reformers.
Background of the Invention
Hydrogen gas, Hz, is a versatile energy carrier that can be used in energy
conversion devices such as fuel cells and combustion engines. The primary
challenges in the widespread adoption of H2 as an energy carrier lie in the
low
volumetric energy density of a gaseous fuel, especially in portable
applications
where high power density is required. The ideal solution is to utilize an
energy
dense liquid hydrocarbon fuel, and generate hydrogen on-demand in a fuel
processor via chemical reforming. Small chain alcohol and carboxylic acids
have
been widely exploited for this purpose, though formic acid in particular has
many
desirable properties, being liquid at nominal temperature and pressure, non-
toxic,
inflammable,, and derivable in a carbon-neutral process. One major drawback
with
traditional hydrocarbon reforming, however, is the high temperature (i.e. >
200
C) environment potentially required in attaining significant hydrogen
production
rates, which is problematic for safety and also inefficient, consuming a
significant
portion of the energy produced. Catalysts for high temperature reforming are
conventionally supported or stored in a solid phase in solid support such as
Zeolite or Carbon, as the catalyst coating methods and processing (sintering)
typically may require high temperatures unsuitable for polymers.
In addressing the limitations of high temperature reforming (> 200 C), a
number of recent publications have dealt with low temperature (i.e. < 150 C)
reforming of formic acid utilizing a class of ruthenium complexes in aqueous
solution. These homogeneous catalysts have several advantages for use in fuel
cell applications in particular, such as very high selectivity to H2. with ppm
levels
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of carbon monoxide (CO); rapid start-up time due to low temperature operation;
and the ability to use a wide range of formic acid concentrations (15 ¨ 98%
w/w)
as feedstock.
For reformed H2 use in hydrogen fuel cells, it is important to have a
hydrogen gas production process producing high purity hydrogen on demand at
acceptable pressures, as carbon monoxide is poisonous to the catalyst in most
fuel cells. For the case of formic acid decomposition has two paths,
dehydrogenation producing H2 and 002, or dehydration producing CO and
H20, as shown in the equations A and B.
HCOOH ---> H2 + CO2 Eq.A Dehydrogenation
HCOOH --> CO + H20 Eq.B Dehydration
Effective reforming of formic acid for fuel cells, then benefits from highly
selective dehydrogenation for increased H2 yield and very low levels of CO
poisonous to most PEM cells.
New developments in liquid phase catalysis have demonstrated the
desired selective dehydrogenation. A novel process for high selectivity and
low
temperature formic acid decomposition has been recently published and patent
pending (US publication No: U52010/0068131) using a ruthenium based metal
complex catalyst in aqueous phase. This novel process achieves rapid low
temperature decomposition at rates up to 50X faster by the addition of sodium
formate to > 95% conversion within 4 hours. Further recycling the catalyst led
to
200X faster conversion within 1 hour.
This aqueous catalyst formulation ('8131 patent) has many advantages for
reforming formic acid through selective dehydrogenation efficiently to produce
high pressure hydrogen product for use in portable fuel cells with acceptable
trace amounts of CO poison, specifically active long life and producing a
positive
pressure of hydrogen and primarily all gas product, as reviewed in the
references.
Realizing the benefits of such catalyst formulations and reactions has
many major challenges. These are addressed with respect to reforming formic
acid.
For example, use of liquid state ruthenium based metal complex catalyst
for hydrogen reforming process from formic acid is a major drawback for the
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development of orientation independent reforming. A system using aqueous
formic acid would be typically operated at high temperatures (-100 C). Because
of the high temperature and continuous gas production (H2 and CO2) this system
may tend to be over flooded with hot liquid including unreacted formic acid.
This
exhaust liquid may be harmful for the user and also damage the hydrogen fuel
cell. Therefore, a reformer that operates orientation independent is hard to
achieve with a liquid based catalyst to use in portable device. There is a
need for
a high selectivity catalyst for use in an orientation independent reformer.
Catalyst leaching out of catalyst from a reformer is another major problem
when using typical water soluble ruthenium based metal complex catalyst for
reforming hydrogen from formic acid. Leached solution may contain both formic
acid and toxic ruthenium compound that are harmful to the environment. In
addition, they may damage and poison the hydrogen fuel cell. There is a need
for
a high selectivity catalyst that limits or avoids toxic leaching.
Extended operation may be limited due to the concentration of the catalyst
in the reformer declining gradually, from gradual overflowing of the system
with
active catalyst and gradual leaching out of catalyst with exhaust gas, water
and
formic acid vapors. This will result a gradual decreasing of hydrogen
production
rate and eventually potential insufficient formation of hydrogen to obtain a
maximum output from an attached fuel cell. There is a need for a high
selectivity
solid catalyst that extends the effective operating time of the catalyst.
Typical liquid based ruthenium metal complex catalyst is challenging to be
reconditioned (reactivate, purified) and recycled after contaminated with
foreign
impurities (coming from formic acid and other construction materials). Impure
catalyst may decompose gradually and be inactive eventually. When this occurs,
the inactive catalyst mixture has to be discarded and the reformer has to be
refilled with brand new catalyst mixture or whole reformer has to be
discarded.
There is a need for a high selectivity solid catalyst that is convenient for
recycling
and safe in disposal.
Immobilization of liquid based ruthenium metal complex catalyst may be
considered, however may result in less reactivity due to several reasons.
(a) Low heat transfer throughout the system.
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(b) Less accessibility of fuel (formic acid) molecules to the active sites
of the catalyst.
(c) Degradation of associated materials within a short period of time
due to high acidity and high temperature of the system. These degraded
byproducts eventually become a liquid and cause the decomposition of the
catalyst.
(d) This immobilized system does not stop the leaching out of the
catalyst from the system.
Ruthenium based hydrogen reforming catalysts are not available for use in
the solid form. Therefore, application of these catalysts to build micro scale
reactors having orientation independence is an expensive, potentially
inefficient
and challenging as described. There is a need for a convenient high
sensitivity
catalyst in solid form suitable for molding and forming easily.
Summary
A heterogeneous polymer bound solid metal complex catalyst is provided,
for reforming, particularly of hydrogen from formic acid. This noble metal
complex
catalyst consists of triarylphosphine ligands and at least one of them is
chemically
bonded to a polymer backbone. In a preferred example, the triarylphosphine
ligands are triphenylphosphine ligands and at least one phenyl group is
sulfonated and at least one triphenylphosphine is chemically bonded to a
polymer
backbone. In a preferred example this polymer bound triphenylphosphine is
polymer bound Triphenylphosphine-3,3',3"-trisulfonic acid trisodium salt
(TPPTS).
In another preferred embodiment, the noble metal is ruthenium, the
organic-polymer is polystyrene, and the ruthenium complex is one of
Ru(H20)4(PS-TPPTS)2 and Ru(H20)4(PS-TPPTS)(TPPTS), and at least one of
the sulfonated triphenylphosphine molecules is chemically bonded to the
polymer
backbone from para-position of its associated phenyl group, to form a rigid
solid
catalyst.
The heterogeneous solid metal complex catalyst provides benefits of being
hydrophilic for efficient surface reaction, insoluble in water and formic
acid, and
chemically binding the catalyst to polymer backbone. That inhibits the
catalyst
leaching out from the reactor/reformer when wetted. An additional benefit is
the
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polymer bound solid metal complex catalyst can be conveniently formed into
designs suitable for reformers.
A method of producing a heterogeneous solid ruthenium complex catalyst
is provided, in a first step preparing polystyrene bonded TPPTS by the steps
of
reacting polystyrene bonded triphenylphosphine with fuming sulphuric acid,
then
reacting with sodium hydroxide to form polystyrene bonded TPPTS. Secondly
reacting an aqueous solution of ruthenium (iii) chloride with polystyrene
bonded
TPPTS and TPPTS to form polystyrene bonded ruthenium /TPPTS complex,
followed by a step of activation of catalyst by reacting with a mixture of
sodium
formate and formic acid.
Detailed Description
A new class of aqueous catalysts has recently been described for low
temperature hydrogen production from hydrocarbon fuels, including formic acid.
The research groups of Laurenczy [8131 patent] and Beller [Topics in
Catalysis,
Volume 53, Numbers 13-14, August 2010 , pp. 902-914(13), Catalytic generation
of hydrogen from formic acid and its derivatives] simultaneously discovered
metal
complexes reacting in liquid form and displaying unusually high activity
towards
formic acid decomposition. Common to both is that high purity hydrogen is
produced via high selectivity towards the dehydrogenation pathway (eq. A),
with
little or no CO production through dehydration (eq. B).
This quality makes such new catalyst formulations potentially suitable for
use in fuel cell applications, where CO content of the fuel gas can cause
rapid
deactivation. Specifically ruthenium formulations have achieved good yields of
H2
> 95% conversion from formic acid at fast rates suitable for use in real-time
power
applications. This catalyst composition results in the conversion of a
dehydrogenatable pre-cursor in an aqueous solution at high conversion rates of
near pure H2 for an extended period of time. However, liquid form catalyst and
associated homogenous reactions may by their structure and form, have the
described challenges and limitations of being orientation dependent, non-
hydrophilic, challenging to immobilize, degradation of catalyst concentration
over
time, leaching of toxic metals as discussed previously, that present major
barriers
to implementing in a reformer reactor system.
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A novel solid catalyst and process is provided to reform formic acid and
other hydrocarbons at low temperature via a selective reaction path similar to
the
aqueous catalyst. The solid catalyst is chemically and permanently bound to
the
polystyrene back bone and can be used in different forms such as fine powder,
particles, sheets, rods, flakes, beads, tubes, blocks, etc. Therefore,
leaching out
of the catalyst from the reformer does not occur. This compound is hydrophilic
but
insoluble in water, formic acid and other solvents. The solid catalyst is
stable to
high temperatures, and also stable to acidic and basic conditions. Because of
the
solid nature, the solid catalyst is ideal for the building of orientation
independent
reformers. Moreover, used and contaminated catalyst can be easily purified,
reconditioned and reused with its original activity.
The present embodiments are described using terms of definitions below:
"Catalysis," as the term used herein, is the acceleration of any physical or
chemical or biological reaction by a small quantity of a substance-herein
referred
to as "catalyst"-the amount and nature of which remain essentially unchanged
during the reaction. For teachings contained herein, a raw material is
considered
catalyzed by a substance into a product if the substance is a catalyst for one
or
more intermediate steps of associated physical or chemical or biological
reaction.
"Chemical transformation," as the term used herein, is the rearrangement,
change, addition, or removal of chemical bonds in any substance or substances
such as but not limiting to compounds, chemicals, materials, fuels,
pollutants,
biomaterials, biochemicals, and biologically active species. The terms also
includes bonds that some in the art prefer to not call as chemical bonds such
as
but not limiting to Van der Waals bonds and hydrogen bonds.
"Activity" of a catalyst, as the term used herein, is a measure of the rate of
conversion of the starting material by the catalyst.
"Selectivity" of a catalyst, as the term used herein, is a measure of the
relative rate of formation of each product from two or more competing
reactions.
Often, selectivity of a specific product is of interest, though multiple
products may
interest some applications.
"Stability" of a catalyst, as the term used herein, is a measure of the
catalyst's ability to retain useful life, activity and selectivity above
predetermined
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levels in presence of factors that can cause chemical, thermal, or mechanical
degradation or decomposition. Illustrative, but not limiting, factors include
coking,
poisoning, oxidation, reduction, thermal run away, expansion-contraction,
flow,
handling, and charging of catalyst.
"TPPTS" is [Triphenylphosphine-3,3',3"-trisulfonic acid trisodium salt].
Equivalently this may be called by other names such as triphenyl phosphine
trisulfonate sodium salt, trisulfonated triphenylphosphine or tris(3-
sulfophenyl)phosphine trisodium salt, P(06H4-3-S03-Na)3.
For H2 use in hydrogen fuel cells, it is important to have a hydrogen gas
production process producing high purity hydrogen on demand at acceptable
pressures, as carbon monoxide is poisonous to the catalyst in most fuel cells.
Formic acid decomposition has two paths, dehydrogenation producing H2 and
CO2, or dehydration producing CO and H20, as shown in the eq. A and B,
previously. Effective reforming of formic acid for fuel cells, then benefits
from
highly selective dehydrogenation for increased H2 yield and low levels of CO
which is poisonous to most PEM cells. Ruthenium based metal complexes in
homogenous catalysis are known to achieve fast and selective formic acid
decomposition without formation of carbon monoxide. However, known catalysts
exist only in liquid form.
It is now discovered that a polymer bound solid metal complex catalyst
having the following general structure (1) is capable of decomposing formic
acid
to H2 at low temperature,
Ri
X X
X X
R2
(1)
In this embodiment of the catalyst, R1 and R2 can be the same or different
ligands. The central Metal (M) atom is coordinated to four electron donating
molecules or atoms (X) and two molecules of substituted triaryphosphine (R1
and
R2) as a metal complex. The central atom (M) can be substituted with any noble
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metal such as rhodium, palladium, silver, osmium, iridium, platinum or gold,
and
ruthenium is preferred. X can be identical ligand or different types, but four
molecules of H20 are preferred.
The general structures of Ri and R2 are;
F DAB
Rt 40
F ) P Pol
Na002S G C S020Na
(2)
ED AB
AB
R2 F 1110 P 14110 Poi ()I. F 4.40) P 4 4110 H
N aGO2S G C $020Na Na002S G C S02ONa
2 (3)
where A, B, C, D, E, F, G can be Hydrogen or any other functional group, and
Poi is a polymer attached to either ortho, meta or para position of one phenyl
group. In one embodiment Ri and R2 are identical, but only one may be needed
to be polymer bound in a minimum example.
IR, and R2 are preferably substituted triphenylphosphine. At least IR, or R2
or both bound to a polymer structure. Triphenylphosphine can be mono, di or
tri
substituted at ortho, para or meta positions. A Polymer bound
triphenylphosphine with meta tri substituted functional groups is represented
by
the following general structure (4);
Y (00
P Y
PcI
(4)
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and where Pol represents an organic polymer preferably polystyrene (PS)
polymer backbone. Additionally, Y represents functional groups on the phenyl
groups that have an effect on the dehydrogenation of formic acid to produce
H2.
Substitutions (Y) of phosphine can be one of amines, carboxylic acids, salt of
carboxylic acid, carbonyl derivatives, hydroxyl, sulfonic acid or salt of
sulfonic acid
including lithium, sodium, potassium, rubidium and cesium salts. The preferred
substituent is meta trisulphonated sodium. The polymer bound meta-
trisulfonated
triphenylphosphine trisodium (Pol-TPPTS) is the preferred ligand. Preferably
both
ligand molecules, or at least one of them is chemically bonded to the polymer
backbone from ortho, meta or para-positions of the one phenyl group in the
substituted triphenylphosphine. The preferred bonding is para-position of a
phenyl
group in the substituted triphenyl phosphine with polymer backbone. Bonding of
the polymer and substituted triphenylphosphine molecule can be formed via C-C,
C-O-C, C-N-C or C-S-C bonds. The preferred linkage is C-C. Due to this
chemical
bond the whole molecule becomes water insoluble.
The preferred structure of polystyrene bound TPPTS (PS-TPPTS) is
represented by the following (5),
= P S020Na
PS
S020Na
(5)
where PS represents the polystyrene polymer backbone. The meta-
positions of the all phenyl groups in PS-TPPTS are sulfonated and exist as
sodium salts. Because of these sodium sulphonate functional groups, the
catalyst
molecule attached to the system is hydrophilic. This is beneficial to carry
out the
formic acid reforming reaction effectively on the surface of the solid
support.
Polystyrenes are hydrophobic, water insoluble and immobilized. Therefore, the
polymer bound catalyst system is water insoluble and immobilized.
Because of this solid nature of the polymer bound metal complex catalyst,
the reforming reaction can be performed effectively in the presence of a
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hydrocarbon fuel such as formic acid, in a reformer (not shown) under
partially
wet conditions (partially dry) in packed systems. This is a beneficial
characteristic
for the development of orientation independent reformers since there is no
overflowing of liquids. Because liquid is limited in the reformer system using
polymer bound solid metal complex catalyst.
The polystyrene polymer to which the active noble metal complex catalyst
is bonded can be any water insoluble polymer of sufficient molecular weight to
contain the levels of metal desired in the reforming reaction desired.
Representative examples of acceptable polystyrenes include styrene copolymer,
or modified styrene from Dow Chemical Company. In general, the backbone can
be comprised of any cross linked or macroreticular polymer having
triarylphosphine molecules. However, polystyrene containing triphenylphosphine
polymers are preferred.
The polymer bound metal complex catalyst is a heterogeneous catalyst,
where the phase of the catalyst (solid) is different from the phase of the
reactants
(liquid). When used in reforming of formic acid, a similar rate of production
of H2
to systems using aqueous only catalyst is observed in a similar volume, i.e.
the
activity is substantially the same, providing the high selectivity and fast
rates of
aqueous catalyst in a solid phase with improved safety and orientation free
operation.
An additional benefit of the polymer bound solid metal complex catalyst is
minimizing or avoiding catalyst leaching, and adverse effects related to the
catalyst leaching (environmental toxicity, damage and poisoning of hydrogen
fuel
cell and insufficient formation of hydrogen due to lack of enough active
catalyst).
Due to the insoluble solid nature of the polymer bound metal complex catalyst
and as the active metal complex catalyst is permanently and chemically bound
to
the polymer, the leaching out of the catalyst from the system is substantially
avoided.
Following use of the catalyst in reforming cycles, the polymer bound solid
metal complex catalyst is contaminated with impurities, requiring
conditioning.
Unlike liquid form catalyst which may require specialized disposal or chemical
processing, a benefit of the polymer bound solid metal complex catalyst is
that
the solid particles are conveniently recycled and reconditioned by suspension
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water (cleaning) followed by simple filtration of the purified solid followed
by
reactivation. The reconditioned catalysts can be reused instead of disposed.
In the preferred embodiment, using polymer back bone, the polymer bound
solid metal complex catalyst can be molded into different shapes and sizes
such
as catalyst powder, particles, sheets, rods, flakes, beads, tubes, blocks etc.
Such
structures are convenient for advanced and safer reformers.
Additionally, the polymer bound solid metal complex catalyst can be
blended with other co-polymers or can be used for coating other structures or
supports.
An alternate structure of the polymer bound solid metal complex catalyst
has mixed ligand and has the general structure (6),
RI
H20 H2 0
Ru
H20 H20
R2
(6)
where Ri is polystyrene bound TPPTS as previous and R2 is TPPTS that may be
aqueous or bound as solid, R1 and R2 having structures,
Na0025 Na002S
P SO2ON a P S020Na
PS
SO2ON a $0?0N a
R, R2
(7) (8)
and (6) is an effective solid catalyst for the dehydrogenation of formic acid
to
produce H2.
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In this alternate embodiment, one of the TPPTS ligands, R1 is chemically
bonded to the polystyrene backbone from the para-position of one phenyl group
in TPPTS. The other TPPTS ligand, R2 may be in an aqueous form during the
synthesis but is bound as a solid after synthesis.
The polymer bound solid metal complex catalyst achieves significant
benefits, particularly for reforming hydrocarbons efficiently and reliably.
First, due
to the meta-position of all associated phenyl groups of TPPTS molecule being
sulfonated, the solid ruthenium complex catalyst is hydrophilic for efficient
surface
reaction. Secondly unlike aqueous catalyst, the polymer bound solid ruthenium
complex catalyst is inhibited from leaching out from the reactor when wetted
during reforming. Third, the solid metal complex catalyst is insoluble in
water (and
formic acid), maintaining it's properties over longer term use and storage.
Compared to known high selectivity liquid form catalysts and processes for
formic
acid reforming, the polymer bound solid metal complex catalyst enables
orientation insensitive reforming when maintained wet, safe with no leaching,
and
the ability to recycle used catalyst. The catalyst is stable to high
temperatures,
and also stable to acidic and basic conditions. Because of the solid nature,
the
catalyst is ideal for orientation independent reformers.
A general method for preparing polymer bound heterogeneous solid metal
TPPTS complex catalyst, includes the chemical transformation steps of (a)
Reacting organic polymer bonded triphenyl phosphine with fuming sulphuric acid
followed by reacting with sodium hydroxide to form polymer bonded TPPTS (b)
Reacting an aqueous solution of noble metal reagent with polymer bonded
TPPTS and a second TPPTS to form polymer bound heterogeneous solid metal
complex. and (c) Activation of polymer bound heterogeneous solid metal complex
to form activated solid metal complex catalyst.
For the preferred solid polystyrene bound ruthenium complex catalyst, a
preferred method of preparing the catalyst, includes a first step of preparing
polystyrene bonded TPPTS (TPPTS-PS) as shown in Scheme 1. A portion of this
chemical transformation method is similar to that discussed for the
preparation of
TPPTS from triphenylphosphine by Hida et.al in J. Coord. Chem., 1998, Vol. 43,
345-348. The new preparation method
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Scheme 1
Na00,?S
40 (1) Fuming H2804
(2) Isiii011
S020Na
40 1110 PS 101 PS
S020Na
Polystyrene hound triphettylphosphinn
TPPTS-PS
shows the sub steps of (i) Reacting polystyrene bonded triphenylphosphine with
fuming sulphuric acid, and (ii) secondly reacting with sodium hydroxide to
form
polystyrene bonded TPPTS.
In the second step, an aqueous solution of ruthenium (iii) chloride (RuC13)
is reacted with the polystyrene bonded TPPTS and regular TPPTS stepwise to
form polystyrene bonded ruthenium/TPPTS complex. This solid metal complex is
then separated from the liquid.
In a third step the polymer bound metal complex is activated by reacting
with sodium formate and formic acid to form activated metal complex catalyst.
Finally the solid product is dried under vacuum.
An embodiment of the process is more clearly described in the example
shown below.
Example
Fuming sulfuric acid (contained 18-24% free SO3) was obtained from Alfa-Aesar.
Acetone was obtained from Aldrich and degassed prior to use. Water was
filtered
through Millipore filtration system and degassed prior to use. Sodium
hydroxide,
Polystyrene bound triphenylphosphine (contain 3 mmol/g), triphenylphosphine,
Ruthenium (iii) chloride (RuC13), and Sodium formate were obtained from
Aldrich
and used without purification. Formic acid was obtained from BASF and
distilled
before use.
TPPTS was prepared using the method described by Hide, et.al, and product was
obtained with 94% purity.
Preparation of Polystyrene bound TPPTS
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Experiment was carried out in an inert atmosphere of N2. Fuming Sulfuric
acid (100 mL) was put into a 1L three necked round bottomed flask and stirred
in
ice bath until the temperature reached 0 C. Then the solution was added
polystyrene bound triphenylphosphine (10 g) and reaction mixture was stirred
at 0
C for 30 min. Then the ice bath was removed and the temperature of the
reaction was increased to rt. The mixture was stirred at room temperature for
approximately 240 h. The mixture was cooled to 0 C and then added degassed
solution of 20% sodium hydroxide carefully until the pH of the mixture became
3Ø Then the mixture is centrifuged at 3500 rpm for 10 min to separate the
solid.
The liquid was discarded and the solid product was washed with degassed water
(2X 400 mL) followed by degassed solution of acetone (400 mL). Finally, the
solid
product was dried under vacuum to obtain the product (18 g).
Preparation Polystyrene bound Ruthenium/TPPTS catalyst
This preparation was conducted in an open atmosphere with proper
ventilation. A solution of ruthenium (iii) chloride (0.5 g) dissolved in water
(20 mL,
degassed) was added formic acid (1 mL, 25 M) followed by polystyrene bound
TPPTS (2.7 g) and stirred at 100 C for 30 min. Then the mixture was added
TPPTS (0.5 g) and stirred at 10000 for another 10 min. Then the mixture was
added slowly and portion wise a aqueous solution of sodium formate in formic
acid (2 g of sodium formate in 10 mL of 12 M formic acid solution) and
continued
heating at 100 C. Once the vigorous gas formation is ceased, the mixture was
centrifuged and the top liquid layer was discarded. The solid was washed with
water and dried under vacuum. This solid catalyst is capable of producing H2
by
the decomposition of formic acid and is water insoluble.
Additional noble metal complex catalyst formulations can be substituted
equivalently to formulate heterogeneous polymer bound water insoluble metal
complex catalyst using similar processes. An alternate embodiment has hybrids
or blends of noble metal complexes substituting for the ruthenium complex.
While the embodiments are described for use with the solid ruthenium
based metal complex catalyst with formic acid fuel, they may also be used in a
wider range of solid catalysts for reforming hydrocarbons in general. The
embodiments described herein have solved these various unmet needs in an
efficient, effective and integrated manner.
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While particular elements, embodiments and applications for the present
invention have been shown and described, it will be understood, of course,
that
the invention is not limited thereto since modifications may be made by those
skilled in the art without departing from the scope of the present disclosure,
particularly in light of the foregoing teachings.
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