Hydrogen sorbent composition and its use

A hydrogen sorbent composition comprising a lanthanum pentanickel sorbent and a thermoplastic elastomer binder is disclosed. Also disclosed is the use of the composition in storing hydrogen and in the purification of hydrogen-containing streams.

BACKGROUND OF THE INVENTION 
Hydrogen is an important product for refinery and chemical use. Therefore, 
it has become more important to recover hydrogen from gas mixtures and 
waste gas streams. Conventional technology for the recovery of hydrogen 
from hydrocarbon mixtures is based on cryogenic fractionation. This is a 
well-developed, fully-matured technology based on the expenditure of 
energy for the generation of the low temperatures required for 
fractionation of hydrogen/light hydrocarbon mixtures. With increasing 
costs of energy, it is essential to develop purification methods which are 
potentially less energy intensive. Such techniques would entail the use of 
a separating agent along with reduced quantities of energy, or low-grade, 
inexpensive energy, as for example waste heat. 
One potential purification method is by employing a hydrogen sorbent, such 
as the sorbent alloys disclosed in British Pat. No. 1,291,976. The sorbent 
in the British patent is an alloy of elements A and D where A is calcium 
or a rare earth metal and D is nickel or cobalt. These sorbents readily 
form hydrides under appropriate conditions. Recently, alloys of the type 
RM.sub.5 where R is a rare earth metal and M is a transition metal have 
been studied for their hydrogen absorption capacity. However, one of the 
basic problems with alloys of the type RM.sub.5 is their tendency to 
pulverize upon repeated hydrogenation/dehydrogenation procedures. The 
continuously diminishing size of the alloy particles makes the use of such 
systems in either a fluidized or fixed-bed process commercially 
unacceptable due to excessively high pressure drop across the reactor bed 
resulting from the crushed particles. Further, the handling of such powder 
size alloys is dangerous since the powder form can be pyrophoric and the 
small particles can become lodged in the lungs. A means has now been found 
to reduce the attrition of a particularly desirable hydrogen sorbent 
alloy. 
SUMMARY OF THE INVENTION 
A solid hydrogen sorbent composition that combines excellent hydrogen 
sorption/desorption characteristics with an attrition resistant structure 
comprises a sorbent having greater than 50% lanthanum pentanickel alloy 
and a binder matrix selected from the group consisting of (i) 
non-hydrogenated block copolymers having at least two monoalkenyl arene 
polymer end blocks A and at least one elastomeric conjugated diene mid 
block B, and (ii) selectively hydrogenated block copolymers having at 
least two monoalkenyl arene polymer end blocks C and at least one 
substantially completely hydrogenated diene polymer mid block D. 
DETAILED DESCRIPTION OF THE INVENTION 
The principle component of the sorbent is the alloy lanthanum pentanickel 
(LaNi.sub.5). The amount of lanthanum pentanickel should be greater than 
50% by weight of the sorbent (less binder), preferably greater than 80% by 
weight. The lanthanum pentanickel can stoichiometrically absorb and desorb 
3 moles of hydrogen per mole of the alloy. 
The sorbent may also contain inert components and other hydride forming 
components. The inert components such as copper, nickel and iron are 
useful as heat sinks and heat moderators, and are also useful in 
minimizing the extent of expansion of the sorbent mass. The additional 
hydride forming components are preferably other rare earths and rare earth 
nickel compounds such as cerium, praseodymium, samarium and fluoro-nickel 
compounds. An attractive additional component is an inexpensive mixture of 
rare earth metals known as "Mischmetal" (Mm). Mischmetal is an impure 
mixture of rare earths containing about 50% w. cerium, 25% w. lanthanum 
along with other rare earths and metals such as iron, magnesium, calcium 
and the like. Mischmetal is obtained directly from the ore without 
separation and purification of the individual rare earths (R). One 
lanthanum-Mischmetal nickel system represented by the formula 
LaMmNi.sub.10 (La.sub.0.63 Co.sub.0.25 R.sub.0.12 Ni.sub.5) sorbed over 3 
moles of hydrogen per mole sorbent at an equilibrium pressure of less than 
400 psia. 
As shown in the following illustrative embodiments, the lanthanum 
pentanickel alloy is not sufficiently attrition resistant to be 
commercially acceptable in either fixed bed or fluidized bed reactors. 
Accordingly, it has been found necessary to bind the alloy in a polymeric 
matrix that not only improves the attrition characteristics of the alloy, 
but also does not diminish the hydrogen sorption/desorbtion capacity of 
the alloy. 
The binders employed in the instant invention are thermoplastic elastomers 
having a block copolymer structure. The block copolymers employed must 
have at least two monoalkenyl arene polymer end blocks A and at least one 
elastomeric conjugated diene polymer mid block B. The number of blocks in 
the block copolymer is not of special importance and the macromolecular 
configuration may be linear, graft or radial (branched) depending upon the 
method by which the block copolymer is formed. Typical block copolymers of 
the most simple configuration would have the structure 
polystyrene-polyisoprenepolystyrene and 
polystyrene-polybutadiene-polystyrene. A typical radial polymer would 
comprise one in which the diene block has three or more branches, the tip 
of each branch being connected to a polystyrene block. See U.S. Pat. No. 
3,594,452. Expressed another way, the invention also contemplates (but is 
not limited to) the use of configurations such as A--B--(B--A).sub.n where 
n varies from 1 to 5. Other useful monoalkenyl arenes from which the 
thermoplastic (non-elastomeric) blocks may be formed include alphamethyl 
styrene, tert-butyl styrene and other ring alkylated styrenes as well as 
mixtures of the same. The conjugated diene monomer preferably has 4 to 5 
carbon atoms, such as butadiene and isoprene. 
The average molecular weights of each of the blocks may be varied as 
desired. The monoalkenyl arene polymer blocks preferably have average 
molecular weights between about 5,000 and about 125,000, more preferably 
between about 15,000 and about 100,000. The elastomeric conjugated diene 
polymer blocks preferably have average molecular weights between about 
15,000 and about 250,000, more preferably between about 25,000 and about 
150,000. The average molecular weights of the polystyrene end blocks are 
determined by gel permeation chromotography, whereas the polystyrene 
content of the polymer is measured by infrared spectroscopy of the 
finished block polymer. The weight percentage of the thermoplastic 
monoalkenyl arene blocks in the finished block polymer should be between 
about 8 and 55%, preferably between about 10% and about 30% by weight. The 
general type and preparation of these block copolymers are described in 
U.S. Pat. Re. No. 28,246 and in many other U.S. and foreign patents. 
Preferably, the block copolymers employed in this invention are the block 
copolymers described above which have been hydrogenated either 
selectively, randomly or completely. Hydrogenation of the precursor block 
copolymers is preferably affected by use of a catalyst comprising the 
reaction products of an aluminum alkyl compound with nickel or cobalt 
carboxylates or alkyloxides under such conditions as to substantially 
completely hydrogenate at least 80% of the aliphatic double bonds while 
hydrogenating no more than about 25% of the alkenyl arene aromatic double 
bonds. Preferred block copolymers are those where at least 99% of the 
aliphatic double bond are hydrogenated and less than 5% of the aromatic 
double bonds are hydrogenated. See generally U.S. Pat. No. 3,595,942. When 
the diene employed is butadiene, it is preferred that polymerization 
conditions be adjusted to result in a polymer block having from about 25 
to 60% 1,2 structure. Thus, when such a block is hydrogenated, the 
resulting product is, or resembles, a regular copolymer block of ethylene 
and butene-1. If the conjugated diene employed is isoprene, the resulting 
hydrogenated product is or resembles a regular copolymer block of ethylene 
and propylene. Of course, direct synthetic preparation may be employed 
involving block polymerization of monoalkenyl arene with alpha-olefin 
mixtures resulting in block copolymers similar to those described above. 
The claims will be understood to include block copolymers prepared by this 
direct procedure as well as by the hydrogenation process. 
It is likely that when a non-hydrogenated block copolymer is employed, the 
binder will become at least partly hydrogenated during the hydrogen 
sorption cycles. It is also possible that even when employing a 
selectively hydrogenated block copolymer as a binder that the arene 
portion of the binder may become saturated during the hydrogen sorption 
cycle. However, this subsequent hydrogenation is not likely to affect the 
ability of the binder to prevent the attrition of the alloy particles. 
The binder may comprise either a neat block copolymer or a blend of block 
copolymer with other thermoplastics, resins, extending oils, and fillers. 
A preferred binder composition comprises a selectively hydrogenated block 
copolymer, polypropylene, and an extending oil. Typical blending 
components and amounts are disclosed in U.S. Pat. No. 3,639,163. While 
other elastomers, such as natural rubber, styrene-butadiene rubbers, 
polybutadiene rubber and the like might possibly be used as the binder, 
they are not as useful as the present block copolymers. 
The sorbent and the binder may be combined in any acceptable form including 
pellets and particles. The resulting composition comprises about 1-30% by 
weight binder and 99-70% by weight sorbent. Preferred amounts are 2-15% by 
weight binder and 98-85% by weight sorbent. One method comprises mixing 
the sorbent and binder, and molding the articles by heating to the 
softening point of the binder. A preferred means for forming the pellets 
is to dissolve the binder in a solvent, such as toluene, cyclohexane, 
n-butyl benzene and the like at elevated temperatures such as about 
70.degree.-130.degree. C. The solvent is then recovered at about 
130.degree.-170.degree. C. under reduced pressure. The resulting paste may 
then be pelletized at 170.degree.-200.degree. C. and 10,000 to 40,000 psig 
for about 1 to 10 minutes. If desired, the resulting pellets could be 
ground to a small particle size. It is also possible to increase the 
cohesion between the alloy and binder, and facilitate the intermixing of 
the alloy/solvent-binder system, by prewetting the alloy powder with an 
alcohol such as methanol before mixing with the binder/solvent phase. In 
addition, small amounts of silicone fluid added to the composite tend to 
reduce the tendency of small particles or pellets to become "tacky" after 
repeated cycling. Additional vacuum drying after pelletization or grinding 
also helps in that regard, by removing excess solvent. Small particles, 
rather than pellets, can be obtained by grinding (rather than pelletizing) 
the pre-frozen (liquid nitrogen) composite phase. 
It is significant that the binder employed herein does not diminish the 
capacity of the alloy sorbent to absorb and desorb hydrogen. One possible 
reason for this surprising affect may be that the block copolymer does not 
totally encapsulate the sorbent alloy particles. It is also possible, 
however, that the hydrogen can permeate the block copolymer binder. 
The capacity of the sorbent of the instant invention to absorb or render 
hydrogen gas depends upon the external hydrogen gas pressure and the 
working temperature. FIG. 1 relates to the hydrogen absorption of the 
lanthanum pentanickel alloy wherein the hydrogen gas pressure 
P.sub.H.sbsb.2 is plotted on the y axis and the absorbed quantity of 
hydrogen C.sub.H on the x axis. Each isotherm (provided it is associated 
with a temperature lying below a critical temperature T.sub.K) exhibits a 
horizontal course at a given pressure -- the so-called "plateau". At the 
plateau pressure, the material can be caused with the aid of a small 
pressure variation to absorb or release in a reversible process, 
considered volumes of hydrogen gas. The "working temperature" is the 
temperature at which a suitable plateau pressure is obtained. 
An efficient method for employing an absorption/desorption process is a 
nearly isothermal process wherein the sorbent-binder matrix is subjected 
to a hydrogen gas pressure at a given temperature, and then the stored 
hydrogen gas is released from the sorbent-binder matrix by reducing the 
hydrogen gas pressure, preferably to a value which is lower than the 
plateau pressure at essentially the same temperature. The temperature is 
kept nearly constant by removing heat during sorption and adding heat 
during desorption. Another less preferred process is a thermal-swing 
system wherein the hydrogen is absorbed at low temperature and desorbed at 
a higher temperature. However, this process has higher heating and cooling 
requirements. A third mode of operation is a modified isothermal process 
wherein a moderate thermal swing is permitted to allow hydrogen desorption 
at relatively higher pressure. 
In the hydrogen recovery and purification process, the temperature is 
typically between about 0.degree. C. and 150.degree. C., preferably 
between about 30.degree. C. and 120.degree. C. The top limit of 
150.degree. C. is necessary since the binder becomes excessively fluid at 
higher temperatures. The pressure typically varies from about 10 psia to 
about 2,000 psia. However, these pressure ranges are not limiting. The 
time required in order to reach equilibrium can easily be determined 
experimentally and is not critical to practising the instant invention. 
Illustrative but not limiting conditions for the "isothermal" mode could be 
an operating temperature of about 66.degree. C. and a pressure of the 
recovered hydrogen of about 100 psia. Illustrative but not limiting 
conditions for the "moderate thermal swing" mode could be sorption 
temperature of about 52.degree. C., desorption temperature of about 
100.degree. C., and pressure of recovered hydrogen of about 260 psia. 
There are two principle uses for the sorbent-binder matrix of the instant 
invention. First, the matrix may be employed to store hydrogen. This use 
is not a severe use since there are less frequent sorption/desorption 
cycles and since typically there are no poisons present. The second use is 
in recovering hydrogen from gaseous streams such as refinery waste gas 
streams and the like. This is a fairly severe use due to the need for 
frequent sorption/desorption cycles and the presence of poisons such as 
oxygen, water vapor, carbon monoxide, and carbon dioxide. 
When employed in gas purification processes, the sorbent-binder matrix of 
the instant invention may be employed either in a fixed bed or a fluidized 
bed. When employed in a fixed bed, the sorbent matrix is typically 
employed in pellet form. In a fluidized bed, the sorbent matrix is 
typically employed in particle form.

The invention is further illustrated by means of the following illustrative 
embodiments, which are given for the purpose of illustration only and are 
not meant to limit the invention to the particular reactants and amounts 
disclosed. 
ILLUSTRATIVE EMBODIMENT I 
An important criterion in selecting a hydrogen sorbent is the exothermic 
heat of absorption. The magnitudes of heat of absorption are as follows: 
______________________________________ 
K cal/mole H.sub.2 
FeTi -5.5 
LaNi.sub.5 -7.0 
V -9.6 
MgNi.sub.2 -15.4 
Mg -17.8 
Ca -41.7 
Li -43.3 
Ce -49.2 
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The high exothermicity of Ca, Li and Ce means that a large amount of 
thermal energy has to be fed into the hydride at high temperatures to 
bring about desorption of hydrogen. Also of consideration, Mg systems 
entail higher temperatures of sorption and desorption than the other noted 
systems. 
Further, V and LaNi.sub.5 are thermodynamically superior to FeTi for 
hydrogen recovery applications because in the FeTi system hydride 
formation at ambient temperature requires higher H.sub.2 pressures. Thus, 
at 40.degree. C., the V metal sorbs H.sub.2 at 50 psia, LaNi .sub.5 at 65 
psia, and FeTi at 110 psia. 
ILLUSTRATIVE EMBODIMENT II 
The rates of hydrogen sorption on LaNi .sub.5 powders (no binder matrix; 
powder less than 140 mesh size) at 25.degree. C. are summarized in Table 
1. In Table 1, Runs A and B were run with an initial pressure of about 300 
psia while Runs C and D were run with an intial pressure of about 150 
psia. In Runs A and C, the heat of sorption was removed by immersion in a 
water bath, whereas, in B and D, no special precautions were taken to 
remove the heat of reaction. Thus, Runs A and C are more nearly isothermal 
(at 25.degree. C.) then Runs B and D. 
The results of desorbing LaNi.sub.5 hydrides at 65.degree. C. and 
100.degree. C. are presented in Table 2. The hydrogen content at the end 
of desorption in these two runs are about 0.6 atom/mole of LaNi.sub.5. 
Table 1 
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Rates of H.sub.2 Sorption on LaNi.sub.5 Powders at 25.degree. C. 
Initial Extent of Sorption, % 
Run 
Conditions 
1 Min 
3 Min 
5 Min 
10 Min 
20 Min 
30 Min 
Final Conditions 
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(A) 
290 psia 
76.3 92.3 95.7 98.5 100 -- 62 psia 
3.21 H.sub.2 /LaNi.sub.5 
(B) 
290 psia 
76.7 90.5 92 96.3 100 -- 74 psia 
3.06 H.sub.2 /LaNi.sub.5 
(C) 
149 psia 
62 88.5 93 97.5 100 -- 80 psia 
2.7 H.sub.2 /Lani.sub.5 
(D) 
140 psia 
65 86.5 88 92 96.6 100 89 psia 
2.36 H.sub.2 /LaNi.sub.5 
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Table 2 
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Rates of Desorption at 65 and 100.degree. C. 
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Temperature 
Time (Min) 
0.5 
1 2 5 10 20 End Pressure 
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65.degree. C. 55.5 
64.3 
75 
86.7 
90.3 
91.3 
130 psia 
100.degree. C. 
82.3 
91 91 186 psia 
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ILLUSTRATIVE EMBODIMENT III 
The selectivity of LaNi.sub.5 for H.sub.2 /CH.sub.4 separations was 
determined in a static experiment at 0.degree. C. A feed mixture of 70 
psia H.sub.2 and 630 psia CH.sub.4 was contacted with LaNi.sub.5. The 
LaNi.sub.5 sorbed over 70% of the H.sub.2 present to an end pressure of 
about 20 psia H.sub.2 and 630 psia CH.sub.4. The gas composition was 
determined by GLC. No CH.sub.4 sorption could be detected. At the end of 
the experiment, the alloy contained 3 moles H.sub.2 /mole of LaNi.sub.5. 
The rates of H.sub.2 sorption were not adversely affected by the presence 
of methane. It has also been shown that the presence of H.sub.2 O, 
O.sub.2, CO.sub.2 and small amounts of CO do not hinder the hydrogen 
sorption of LaNi.sub.5. 
ILLUSTRATIVE EMBODIMENT IV 
Pellets of 97% weight LaNi.sub.5 and a 3% block copolymer composition 
binder were prepared. The binder comprised a selectively hydrogenated 
S--EB--S block copolymer also having polypropylene and oil components. 
This binder is commercially available from Shell Chemical Company under 
the tradename KRATON G Thermoplastic Rubber. About a 10% by weight 
solution of KRATON G in a n-butyl benzene solvent was added to the 
LaNi.sub.5 powder at 120.degree.-130.degree. C. The solvent was then 
recovered at 140.degree.-150.degree. C. under reduced pressure, and the 
resulting paste was pelletized at 170.degree.-180.degree. C.; 30,000 psia; 
and 5 minutes. The pellets were cylinders about 0.5 cm in diameter. These 
pellets were than made into small particles by cryogenic grinding. A 
fraction of the composite sample containing 44 to 100 .mu. (micron) 
particles was subjected to 100 hydrogenation-dehydrogenation cycles 
between 25.degree. and 120.degree. C. A size determination at the end of 
these runs showed that the &gt; 88 .mu. fraction attrited by about 
0.12%/cycle, whereas the &gt; 44 .mu. fraction attrited by 0.06%/cycle. It is 
possible that part of this attrition represents particle shrinkage due to 
loss of solvent from the polymer phase. On the other hand, free alloy 
powder samples (without binder) of a 88-100 .mu. size were reduced to less 
than 44 .mu. after less than six cycles. This shows that the reduction in 
attrition brought about by binding with the instant invention elastomeric 
compositions is substantial. Also significant, after 100 cycles, the 
capacities and rates of the composite particle samples (LaNi.sub.5 plus 
instant binder) were the same as those exhibited by fresh LaNi.sub.5 
powders. 
Other binders were considered and found to be unsatisfactory. Inorganic 
binders studied were silicic acid (commercial "water glass") and various 
low melting alloy compositions. These systems were unsatisfactory; for 
example, a pellet made by binding LaNi.sub.5 powder with 15% w. water 
glass and compressed under 40,000 psia was completely pulverized by a 
single room temperature hydrogenation. Similarly, various formulations, in 
which low melting tin or lead (20-40% w.) and their alloys were used 
either as binders of pure LaNi.sub.5 powders or as binders of LaNi.sub.5 
/Cu powder mixtures, were unsuccessful. The copper powder addition (up to 
50% w.) was tried as a means of reducing the effect of volume expansion of 
LaNi.sub.5. The resulting phases upon hydrogenation expanded over 4%, and 
after two cycles of hydrogenation they disintegrated. 
Some experiments with organic binders were made with polypropylene and 
polystyrene. These were made as follows: The polymers were dissolved in 
butylbenzene, mixed with LaNi.sub.5 powders, the solvent was removed by 
evaporation, the resulting "paste" was pelletized under pressure 
(10,000-40,000 psia) and the last traces of the solvent were recovered. 
Samples with 2-5% w. polymer were used in 95-98% w. metal. These systems 
behaved somewhat better than the inorganic-binder systems in that they did 
not disintegrate upon hydrogenation. However, the mechanical properties of 
the pellets rapidly deteriorated upon repeated cyclic sorption/desorption. 
ILLUSTRATIVE EMBODIMENT V 
In this embodiment the reactor shown in FIG. 2 is employed to selectively 
remove hydrogen from a feed gas stream. The loading in the reactor bed is 
listed in Table 3. The void refers to the portion filled with graded 
ceramic support balls. 
In case 1, the feed stream has about 73% hydrogen in the feed, whereas in 
case 2 the feed stream has about 52% hydrogen in the feed. In both cases, 
sorption and desorption are carried out at about 66.degree. C. The results 
are presented below in Table 4. In each case, the product hydrogen purity 
was about 99% at a recovery of 95%. 
Table 3 
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Sor- 
Data Type LaNi.sub.5 
Polymer bent Void Bed 
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% Weight of Sorbent- 
95 5 100 
Binder Composition 
% Volume of Sorbent- 
70 30 100 
Binder Composition 
Density, lb/ft.sup.3 
516 62.4 378 235 
Bed Volume Fraction 
0.455 0.195 0.65 0.35 1.0 
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Table 4 
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Case 1 2 
Stream Feed 
H.sub.2 
CH.sub.4 
Feed H.sub.2 
CH.sub.4 
Name Gas Product 
Product 
Gas Product 
Product 
__________________________________________________________________________ 
Component 
(moles) 
H.sub.2 
733 697 36 523 496.8 
26.3 
N.sub.2 
10 0.2 10 40 0.9 39 
CH.sub.4 
249 3.7 245 293 6 287 
C.sub.2 H.sub.6 
8 0.1 8 133 3 130 
C.sub.3 H.sub.8 
0 0 0 11 0.2 10.8 
Total 1,000 
701 299 1,000 
506.9 
493.1 
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