Noble metal exchange of hydrophobic molecular sieves

Normal methods of impregnating silicalite with noble metals leads either to metal loadings under 8% at high (greater than 60%) dispersion or loading of over 8% noble metal at low dispersion. To obtain silicalite impregnated with a noble metal at more than 8 weight percent loading and with at least 60% dispersion it has been found necessary to pretreat the silicalite with a base and to impregnate the base treated silicalite with a noble metal compound in two stages separated by calcination. Platinized silicalite so prepared may be dispersed in a poly(tetrafluoroethylene) matrix and used as a fixed bed to catalyze isotopic exchange gaseous hydrogen and water vapor arising from a mass of liquid water flowing over the fixed catalyst bed.

FIELD OF THE INVENTION 
The invention within relates to a method of producing hydrophobic molecular 
sieves which are highly loaded with at least one noble metal at high 
dispersions. Of particular interest is the preparation of silicalite 
having more than 8 weight percent of a noble metal, or a combination of 
noble metals, with at least 60% dispersion. 
BACKGROUND OF THE INVENTION 
The separation of hydrogen isotopes, especially the preparation of 
relatively pure heavy water, deuterium oxide (D.sub.2 O), is of great 
importance to the nuclear industry. Historically the Girdler-Sulfide 
process involving isotopic exchange between hydrogen sulfide and water has 
been by far the most important means for heavy water production. Although 
a process for separating hydrogen isotopes by isotopic exchange between 
hydrogen and water has important advantages over the Girdler-Sulfide 
process, the successful achievement of a water-based exchange process has 
remained an elusive goal. Recent developments have made such a process 
increasingly feasible and renewed hopes for its eventual use. Butler and 
coworkers have extensively reviewed recent progress in this area: J. P. 
Butler, Separation Science and Technology, 15(3), 371 (1980); Canadian 
Patent No. 1,063,587; J. P. Butler, J. H. Rolston, and W. H. Stevens, 
"Novel Catalysts for Isotopic Exchange Between Hydrogen and Liquid Water", 
ACS Symposium Series, No. 68, SEATION OF HYDROGEN ISOTOPES, American 
Chemical Society (1978); see also J. H. Rolston et al. in Catalysis on the 
Energy Scene, S. Kaliaguine and A. Mahay, editors, Elsevier Science 
Publishers (1984). Although we provide an abbreviated summary below, the 
interested reader should consult these references for more detailed 
information. 
The exchange between gaseous hydrogen and liquid water is known to be 
catalyzed by many metals. The exchange rate of the overall process is 
limited by the solubility of hydrogen in water, since the exchange rate at 
the interface of phases is quite small. This solubility limitation has 
been circumvented by metal catalyzed vapor phase isotope exchange between 
hydrogen and water followed by vapor-liquid exchange between water where 
the two stages are physically separate to ameliorate the rapid 
deactivation of metal catalysts by liquid water, but the resulting process 
remained too expensive to be commercially competitive. 
Using the same basic approach of metal catalyzed vapor phase isotope 
exchange between hydrogen and water followed by vapor-liquid exchange 
between water phases, the next development was that of hydrophobic 
catalysts. Because of their hydrophobic character these catalysts were not 
as prone to deactivation by liquid water as had been the prior art 
catalysts. The hydrophobic catalysts could be used as a fixed bed in a 
trickle bed operation with liquid water and the gaseous hydrogen flowing 
through the bed countercurrently, where isotope exchange occurred between 
hydrogen and the water vapor arising from the partial pressure of liquid 
water at the exchange temperature. Continued research at the Chalk River 
Nuclear Laboratory of Atomic Energy of Canada Limited led to successive 
improvements culminating in a catalyst of platinum and carbon "wetproofed" 
by bonding to poly(tetrafluoroethylene), (PTFE), where the hydrophobic 
PTFE layer prevents wetting of the catalyst surface in water. 
Catalysts based on platinized carbon have the great disadvantage of being 
pyrophoric and combustible. What is needed is an active, noncombustible, 
hydrophobic, acid stable catalyst support with good thermal stability. 
Especially for the separation of hydrogen isotopes in the trickle bed 
process previously referred to, it is desirable that metal loading be at 
least 8 weight percent. However, merely having a high metal loading by 
itself is insufficient, for it is necessary to have good platinum 
dispersion, preferably as a monolayer (100% dispersion), but with at least 
60% dispersion. 
Silicalite is a hydrophobic molecular sieve having properties as a support 
quite well suited to the process under consideration. Wanke et al. in U.S. 
Pat. No. 4,536,488 have described platinum on silicalite catalysts for the 
isotope exchange in question and made several significant observations. 
Although they were able to prepare highly loaded (12%) platinum on 
silicalite, the exchange rates using this catalyst were significantly 
lower than platinized carbon with similar loading owing to a relatively 
low platinum dispersion on the silicalite support. This observation led 
the patentees to investigate different procedures for metal impregnation 
and they described a procedure affording highly dispersed (93-110%) 
platinum on silicalite with loadings at 5.9-7.4 weight percent platinum. A 
peculiar trait of their method, as shown by the data in their Table 3, is 
that platinum loading is virtually independent of the amount of platinum 
offered to the silicalite; increasing the amount of platinum offered by 8 
fold increased the platinum loading only from 5.9 to 7.4 weight percent. 
Their data also permit the fair inference that a loading greater than 7.5 
weight percent platinum is not possible by their method. 
Our invention is a method of preparing catalytic composites of noble metals 
deposited on silicalite where the composite contains at least 8 weight 
percent of a noble metal, or some combination of noble metals, with at 
least 60% dispersion. Our invention affords a catalyst which is quite 
active in the aforementioned isotope exchange process and which has a high 
useful lifetime without being combustible or pyrophoric, thereby 
representing a significant advance in this art. In another aspect our 
invention is an improved isotope exchange process, where the improvement 
consists of the use of the catalyst of our invention. 
SUMMARY OF THE INVENTION 
The purpose of this invention is to prepare silicalite containing a noble 
metal at a concentration of at least 8 weight percent and with a 
dispersion of at least 60%. An embodiment comprises soaking silicalite 
with a caustic solution at 35.degree.-60.degree. C., impregnating the 
base-pretreated silicalite under basic conditions with a soluble noble 
metal compound, calcining the impregnated silicalite, and repeating the 
impregnation-calcination stages. In a more specific embodiment 
impregnation is performed at a pH between about 9.5 and 12.0. In a yet 
more specific embodiment the noble metal is platinum and calcination is 
performed at 300.degree.-400.degree. C. Another aspect of our invention is 
the use of a silicalite containing at least 8 weight percent platinum at a 
dispersion of at least 60% in a hydrogen isotope exchange process between 
hydrogen and water. Other embodiments will be apparent from the ensuing 
description.

DESCRIPTION OF THE INVENTION 
We have observed that silicalite can be loaded with a noble metal, such as 
platinum, or a mixture of noble metals, to an amount greater than 8 weight 
percent only with great difficulty. Furthermore, when such highly loaded 
silicalite is prepared the platinum tends to agglomerate affording low 
(under 50%) dispersion of the metallic platinum. These observations 
required development of new procedures which afford silicalite having at 
least 8 weight percent, preferably at least 10 weight percent, of a noble 
metal with at least 60% dispersion and led to the instant invention. The 
features of our invention include both a base pretreatment and a double 
impregnation. That is, a single impregnation by a noble metal compound was 
insufficient to load the requisite amount of noble metal with at least 60% 
dispersion. Furthermore, it was found necessary to calcine the material 
between impregnations. 
Silicalite is an unusual microporous crystalline silica which is 
hydrophobic and has uniform pore dimensions of about 6 Angstrom units and 
is described in U.S. Pat. No. 4,061,724. See also Flanigen et al., Nature, 
271, 512 (1978). As was remarked upon previously, silicalite is highly 
desirable as a catalyst in the isotopic exchange of hydrogen in the 
hydrogen-water system. 
As the first stage in our invention it is necessary to treat the silicalite 
with a base, in particular with a strong base. Aqueous solutions of alkali 
metal hydroxides are the most convenient strong bases to use, although 
quaternary ammonium hydroxides also may be utilized but not necessarily 
with equivalent results. The concentration of the strong base used and 
base treatment temperature and time must be such as to not dissolve a 
substantial amount of silicalite. The silicalite loss due to dissolution 
which is acceptable is somewhat arbitrary, and for the purposes of this 
invention we place the maximum loss at 30 percent. In practice, this means 
that aqueous solutions of strong base will be used that are between about 
0.1 and 2 molar, most usually between 0.2 and 1.2 molar. Temperatures over 
75.degree. C. must be avoided for they lead to dissolution of the 
silicalite. Base treatment temperatures under about 10.degree. C. also are 
to be avoided so that base treatment times do not become too long. Within 
the range of about 10.degree. through about 75.degree. C., a base 
treatment temperature between about 35.degree. and about 60.degree. C. 
generally is preferred. The time of the base treatment will depend on the 
concentration of the base solution as well as on the treatment 
temperature, and may range from several minutes to several hours. As a 
benchmark, when a 0.5 molar solution of sodium hydroxide is used at 
50.degree. C. it has been found that a treatment time of 1 hour is 
adequate. 
After the silicalite has been treated with base it is mixed with an aqueous 
solution of a noble metal compound or a mixture of compounds. By noble 
metal is meant the Group VIII metals platinum, palladium, rhodium, 
ruthenium, osmium, and iridium, as well as rhenium gold, and any 
combination thereof. Among these the platinum group metals, which consist 
of platinum, palladium, rhodium, and ruthenium, are of particular 
interest, and for the isotopic hydrogen exchange process platinum is of 
especially high value. 
The silicalite must be impregnated with the noble metal compound at a pH 
between 9.5 and 12. Therefore, one must use a noble metal compound which 
is both stable and soluble within this pH range. The prime examples of 
such compounds, using platinum for purposes of illustration only, include 
ammonia complexes such as Pt(NH.sub.3).sub.4 X.sub.2, where X is halogen, 
and the corresponding amine complexes. As previously stated, impregnation 
is conducted such that the pH is between about 9.5 and 12 throughout the 
impregnation, with the pH range between about 9.5 and about 11 being 
somewhat favored. For convenience, impregnation generally is conducted in 
the range between 30.degree. and 75.degree. C., more particularly between 
about 35.degree. and 60.degree. C., for a time usually on the order of 
0.5-8 hours. Because impregnation is done under basic conditions 
temperatures in excess of about 75.degree. C. are discouraged. The 
solution of the noble metal compound is provided in an amount calculated 
to provide no more than about 7 weight percent of the noble metal based 
upon the amount of silicalite used for impregnation. Larger amounts of 
noble metal compound may be used but are without benefit, since no more 
than about 7 weight percent noble metal can be loaded onto the silicalite 
during the first impregnation. 
After impregnation is complete the silicalite needs to be completely dried 
prior to the next impregnation. The silicalite most often is preliminarily 
dried, conveniently at a temperature of about 100.degree. C. for several 
hours. Thereafter it is calcined at a temperature between about 
250.degree. and about 450.degree. C. for 1 to 6 hours. Calcination 
temperatures above 450.degree. C. are to be avoided since they lead to 
agglomeration of the metal on the silicalite. Calcination in the range 
from 300.degree. to 400.degree. C. for about 1 hour represents an 
acceptable yet convenient calcination condition. Calcination is not to be 
performed in a reducing atmosphere. Most often calcination is done in air, 
but an inert atmosphere and an oxidizing atmosphere, such as air enriched 
in oxygen, is quite acceptable. However, it should be apparent that air is 
by far the most convenient and therefore desirable calcination atmosphere. 
After the first impregnation with a noble metal compound, the silicalite 
may contain up to about 7 weight percent of the noble metal. In order to 
obtain higher loading it is necessary to repeat the impregnation and 
calcination stages. The conditions for the second impregnation and 
calcination are the same as those for the initial stages. However, the 
noble metal compound may be the same or different from that used in the 
first impregnation, and the noble metal itself need not be the same as was 
used in the first impregnation. Just as is the case for the first 
impregnation, a mixture of noble metal compounds containing different 
noble metals also may be used in the practice of this invention. 
Although the method of our invention may be used to prepare silicalite 
having one or more noble metals at a loading of at least 8 weight percent 
with at least 60% dispersion, its use in preparing silicalite having 8 
weight percent of one or more noble metals with at least 80% dispersion is 
a preferred embodiment, and silicalite with at least 10 weight percent 
noble metals at 80% dispersion or greater is especially favored. 
For use as a fixed bed in the isotopic exchange between gaseous hydrogen 
and gaseous water as provided by the vapor pressure of a flowing liquid 
water stream, the platinized silicalite as prepared by the method of this 
invention is desirably dispersed in a matrix of poly(tetrafluoroethylene). 
This has been amply described in the prior art (see, for example, U.S. 
Pat. No. 4,536,488) and will not be further discussed here. 
The following examples merely serve to illustrate various aspects of our 
invention. The use of Pt(NH.sub.3).sub.4 Cl.sub.2 in these examples is for 
convenience only and is representative of the platinum compounds which may 
be used and is also representative of the compounds of the noble metals 
which may be used, and must not be taken to restrict this invention in any 
way. 
EXAMPLES 
Silicalite was commercial material taken from inventory. Tetrammine 
platinous chloride was purchased from Johnson Matthey Inc. Both sodium 
hydroxide and ammonium hydroxide were reagent grade materials. The results 
of experiments and the value of variables used is indicated in the 
following table. All impregnations were performed using aqueous solutions 
of Pt(NH.sub.3).sub.4 Cl.sub.2. The final calcined composite was analyzed 
for platinum dispersion by hydrogen chemisorption assuming one chemisorbed 
hydrogen atom per surface platinum atom. See, e.g., K. Kunimori and 
coworkers, Applied Catalysis, 4, 67 (1982). 
TABLE 1 
__________________________________________________________________________ 
Summary of Preparations of Platinized Silicalite.sup.a 
__________________________________________________________________________ 
Base Pretreatment Impregnation.sup.d 
Platinum Loading 
No. 
Base Conc..sup.b 
Temp 
Time.sup.c 
Amt.sup.e 
ph Temp 
Time.sup.c 
Weight Percent 
Dispersion.sup.f 
__________________________________________________________________________ 
1 none 9 -- 22 -- 8.0 6.6 
2 NaOH 0.1 50 0.5 14 9.4 
50 1.0 4.5 72 
3 NaOH 0.1 50 0.5 14 9.4 
50 0.5 4.3 76 
4 NaOH 0.1 50 1.0 7 9.4 
50 1.0 4.6 70.2 
5 NaOH 0.1 50 1.0 7 9.4 
50 0.5 4.1 82.9 
6.sup.g 
NH.sub.4 OH 
-- -- -- 8 11.6 
22 1.0 0.52 49 
7.sup.g 
NH.sub.4 OH 
-- -- -- 8 10.6 
50 1.0 0.72 57.4 
8 NH.sub.4 OH 
30 50 1.0 14 9.9 
50 3.5 1.2 65.1 
9.sup.h 
NaOH -- -- -- 8 13 22 1.0 0.81 69.1 
10.sup.h 
NaOH -- -- -- 8 12.1 
50 1.0 2.10 80.7 
11 NaOH 0.25 
50 1.0 14 9.9 
50 3.5 3.1 74.3 
12 NaOH 0.5 50 1.0 14 9.7 
50 3.5 5.7 81.7 
15 NaOH 1.0 22 1.0 14 10.5 
50 3.5 3.9 -- 
16 NaOH 1.0 50 1.0 14 10.3 
50 3.5 5.5 83.3 
17 NaOH 2.0 22 1.0 14 10.1 
50 3.5 4.8 84.6 
18 NaOH 2.0 50 1.0 14 10.2 
50 3.5 5.7 89.5 
19.sup.i 
NaOH 2.0 22 1.0 7 10.2 
50 3.5 4.5 91 
19.sup.j 
NaOH 7 10 50 3.5 5.9 84.4 
__________________________________________________________________________ 
Base Pretreatment Impregnation.sup.c 
Platinum Loading 
No. 
Base Conc..sup.a 
Temp 
Time.sup.b 
Amt.sup.d 
pH Temp 
Time.sup.b 
Weight Percent 
Dispersion.sup.e 
__________________________________________________________________________ 
20.sup.i 
NaOH 2.0 50 1.0 7 10.2 
50 3.5 4.8 94.3 
20.sup.j 
NaOH 7 9.95 
50 3.5 6.4 90.9 
21.sup.i 
NaOH 1.0 50 1.0 7 10.0 
50 3.5 5.1 91.6 
21.sup.j 
NaOH 7 9.7 
50 3.5 8.6 83.9 
22.sup.i 
NaOH 0.5 50 1.0 7 10.5 
50 3.5 6.4 58.1 
22.sup.j 
NaOH 7 10.5 
50 3.5 11.6 60.3 
__________________________________________________________________________ 
.sup.a Except for sample 1, all samples were calcined by heating to 
300.degree. C. over 4 hours, then holding at 300.degree. C. for 1 hour. 
For sample 1 heating was to 400.degree. C. 
.sup.b Concentration in molarity. 
.sup.c Time is in hours. 
.sup.d Impregnation was performed using aqueous solutions of 
Pt(NH.sub.3).sub.4 Cl.sub.2. 
.sup.e Assuming 100% absorbtion by silicalite, enough Pt is in solution t 
provide this weight percent Pt on silicalite. 
.sup.f By chemisorbtion. 
.sup.g Samples were exchanged and metal loaded in one step using NH.sub.4 
OH for exchange and pH adjustment. 
.sup.h Samples were exchanged and metal loaded in one step using NaOH for 
exchange and pH adjustment. 
.sup.i First impregnation. 
.sup.j Second impregnation.