CA2041312C - Catalyst containing large amounts of noble metal dispersed on hydrophobic molecular sieves - Google Patents

Abstract

A catalytic composition comprising silicalite impregnated with a noble metal at more than 8 weight percent loading and with at least 60% dispersion is obtained by pretreating the silicalite with a base and thereafter impregnating the base treated silicalite with a noble metal compound in two steps 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 between gaseous hydrogen and water vapor arising from a mass of liquid water flowing over the fixed catalyst bed.

Description

2Ci 413 "C:ATALYST CONTAINING LARGE AMOUNTS OF NOBLE NIET~I
DISPERSED ON HYDROPllOBlC MOLECULAR SIEVES"

FIELD OF THE INVENTION
The invention within relates to a method of producing hydrophobic molecular sieves which are highly loaded with large amounts of at least one noble metal a~ high dispersions. Of particular interest is the p~pa,alion of silicalite having more than 8 weight percen~ of a noble metal, or a combination 10 of noble metals, with at least 60% dispersion.
BACKGROUND OF THE INVENTION
The separation of hydrogen isotopes, especially the preparation of rela-tively pure heavy water, deuterium oxide (D2O), is of great importance to the nuclear indust~y. Historically the Girdler-Sulfide process involving isotopic ex-5 change between hydrogen sulfide and water has been by far the most impor-tant means for heavy water production. Although a process for separating hy-drogen isotopes by isotopic exchange between hydrogen and water has im-portant advantages over the Girdler-Sulfide process, the successful achieve-ment of a water-based exchange process has remained an elusive goal. Re-2t:\ cent developments have made such a process increasingly feasibls and r~-newed hopes for its eventual use. Butler and coworkers have extensively re-viewed recent progress in this area: J.P. Butler, SeparaUon Science and ~ech-nology, 15(3), 371 (1980); Canadian Patent 1,063,587; J.P. Butler, J.H. Rolston,and W. H. Stevens, "Novel Catalysts for Isotopic Exchange Between Hydrogen 25 and Liquid Water~', ACS Symposium Series, No. 68J SEPARATION OF HYDRO-GEN ISOTOPES) American Chemical Society (197~); see also J.H. Rolston et al. in Catalysis on ~he Energy Scene, S. Kaliaguine and A. Mahay, editors, Else-vier Science Publishers (1984).
The exchange between gaseous hydrogen and liquid water is known to 30 be catalyzed by many metals. The exchange rate of the overall process is lim-ited by the solubility of hydrogen in water, since the exchange rate at the inter face of phases is quite small. This solubility limitation has been circumvented by . ~

;~:04~3 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 expensh~e to be commer-cially competitive.
Using the same basio approach of metal catalyzed vapor phase isotope exchange between hydrogen and water followed by vapor-liquid exchangs be-tween water phases, the next development was that of hydrophobic catalysts.
Because of their hydrophobic character these catalysts were not as prone ~o o deactivation by liquid water as had been the prior art catalysts. The hydropho-bic catalysts could be used as a Sixed 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(tetrafluoroethylane), (PTFE), where the hydrophobic PTFE layer prevents wetting of the catalyst surface in water.
Catalysts based on platinized carbon hava the great disadvantage of being pyrophoric and combustible. What i.s needed is an active, noncom-bustible, hydrophobic, acicl 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 per-cent. 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.
4,536,488 have described platinum on silicalite catalysts for the isotope ex-change 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 sim-ilar loading owing to a relatively low platinum dispersion on the silicalite support.
This observation led the patentees to investigate different procedures for metalimpregnation and they described a procedure affording highly dispersed ~3-2~ L33 110%) platinum on silicalite with loadings at 5.9-7.4 weight percent platinum. Apeculiar trait of their method, as shown by the data in their Table 3, is tha~ plat-inum loading is virtually independent of the amount of platinum offered to the silicalite; increasing ~he amount of platinum offered by 8 fold increased the plat-5 inum loading only ~rom 5.9 to 7.4 weight percent. Their data also permU the fairinference that a loading greater than 7.5 weight percent platinum is not possible by their methocl.
The instant invention is a rnethod of preparing catalytic composites of noble metals deposited on silicalite where the composite contains at least 8 10 weight percent of a noble metal, or some combination of noble metals, with atleast 60% dispersion. The 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 ad\/ance in this art. In another aspect the invention is an improved15 isotope exchange process, where the improvement consists of the use of the catalyst of the 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 20 least 60%. An embodiment comprises soaking silicalite with a caustic solutionat 35-60~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 12Ø In a yet more 25 specific embodiment the noble metal is platinum and calcination is performed a~
300-400~C. Another aspect of the instant invention is ths 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
It has been observe~ that silicalite can be loaded with a nobie metal, such as platinum, or a mixture of noble metals, to an amount greater than 8 weigh~ percent only with great difficulty. Furthermore, when such highly loaded ; ~ .

2~L3 silicalite is prepared the platinum tends to agglomerate affording iow (under 50%) dispersion of the metallic platinum. These observations required development of new procedures which afford silicalite havin~ at least 8 wei3ht percent, preferably at least 10 weight percent, of a noble metal with at ieast 60%
5 dispersion and led to the instant invention. The features of the pres0nt invention include both a base pretreatment and a double impregnation. Thaî is, a single impregnation by a noble meta! compound was insufficient to load the requisite amount of noble metai with at least 60% dispersion. Furthermore, ~It was found necessary to calcine the material between impre~n~lions.
Silicalite is an unusual microporous crystalline silica which is hydropho-bie and has uniform pore dimensions of about 6 An~lrol" units and is de-scribed in U.S. 4,061,724. See also Flanigen et al., Nature, 271, 512 (1978). Aswas 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 the invention it is necessary to treat the silicalite witha base, in particular with a strong base. Aqueous solutions of alkali metal hy-droxides are the most convenient strong bases to use, although qualer"ary ammonium hydroxides also may be utilized but not necessarily with equivalent results. The concentration of the strong base used and base treatrnent tem-20 perature and time must be such as to not dissolve a subslar,lial amount of sili-calite. The silicalite loss due to dissolution which is acceptable is somewhat ar-bitrary, and for the purposes of this invention we place the maximum ioss 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 2 5 molar. Temperatures over 75~C must be avoided for they lead to ~issc'ution of the silicalite. Base treatment temperatures under about 10~C also are to be avoided so that base treatment times do not become too long. Within the range of about 10 through about 75~C, a base treatment temperature between about 35 and about 60~C ~enerally is preferred. The time of the base treatment will 30 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 ~0~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 35 solution of a noble metal compound or a mixture of compounds. By noble metal is meant the Group Vlll metals platinum, palladium, rhodium, ruthenium, .. . .

i~

5 2t~3 osmium, and iridium, as weil 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 0xchange 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 bo~h stable and soluble within this pH range. The prime examples of such compounds, using platinum for purposes of illustration only, include am-monia complexes such as Pt(NH3)4X2, where X is halogen, and the corre-sponding 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 abou~ 9.5 and about 11 being somewhat favorecl. For convenience, impregnation generally is conducted in the range between 30 and 75~C, more particularly between about 35 and 60~C, for a time usually on the order of 0.5-8 hours. Because impregnation is done under basic conditions temperatures in excess of abou~ 75~C are discouraged. The solution of the noble metal compound is provided in an amount c?'c~ ted to provide no more than about 7 weight percent of the noble metal based upon tha amount of sili-calite used for impregnation. Larger amounts of noble metal compound may be 2 o 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 silic~alite needs to be completely dried prior to the next impregnation. The silicalite most often is preliminarily dried, conveniently at a temperature of about 100~C for several hours. Thereafter It iscalcined at a temperature between about 250 and about 450~C for 1 to 6 hours.
Calcination temperatures above 450~C are to be avoided since they lead to ag-glomeration of the metal on the silicalite. Calcination in the range from 300 to400~C for about 1 hour represents an acceptable yet convenient c~lc;.,alion 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 atmv sphere, such as air enriched in oxygen, is quite acceptable. However, it should be apparent that air is by far the most convenient and ll ,ere~ore desirable calci-nation atmosphere.
After the first impregnation with a noble rnetal compound, the silicalite 3 5 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 sta~es.

6 ~4~3 The conditions for the second impregnation and calcination are the same a those for the initiai stages. However, the noble me~al compound may be the same or di~r~r,l from that used in the first impregnation, and the noble metal ~-seif need not be the same as was used in the first impregnation. Just as is the 5 case for the first impregnation, a mixture of noble meeal cornpounds containing renl 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 3 weight percent of10 one or more noble metals with at least 80% dispersion is a pre~er~ed embodi-ment, and silicalite with at least 10 weight percent noble metals at 80% disper-sion or greater is especially favored.
For use as a fixed bed in the isotopic exchange between gaseous hy-drogen and gaseous water as provided by the vapor pressure of a flowing iiquid 15 water stream, the platinized silicalite as prepared by the method of this invention is desirably dispersed in a matrix of poly(tetrafluoroethylene). This has be2n amply described in the prior art (see, for example, U.S. 4,536,488) and will notbe further discussed here.
The following examples merely serve to illustrate various aspects of our 20 invention. The use of Pt(NH3)4CI2 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
~5 Silicalite was commercial material taken from inventory. Telr~ m ~ ,e platinous chloride was purchased from Johnson Mat~hey Inc. Both sodium hy-droxide 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(NH3)4CI2.
3 o The final calcined composite was analyzed for platinum dispersion by hydrog~chemisorption assuming one chemisorbed hydrogen atom per surface platinum atom. See, e.g., K. Kunimori and coworkers, Applied Catalysis, 4, 67 ~1982).

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Claims (11)

1. A catalytic composite comprising a noble metal deposited on silicalite, said composite containing at least 8 weight percent of the noble metal at a dispersion of at least 60 percent, the noble metal being selected from the groupconsisting of platinum, palladium, rhodium, ruthenium, osmium, iridium, rhenium, gold, or any combination thereof.

2. The catalytic composite of Claim 1 where the noble metal constitutes at least 10 weight percent of the composite.

3. The catalytic composite of Claim 1 or 2 where the dispersion of the noble metal is at least 80 percent.

4. In a process for hydrogen isotope exchange between hydrogen and water, wherein a mass of flowing liquid water and water vapor contacts a fixed mass of a catalyst in the presence of a mass of gaseous hydrogen flowing countercurrently to the water mass, said catalyst catalyzing the isotopic exchange between gaseous hydrogen and water vapor, the improvement wherein the catalyst comprises the catalytic composite of claim 1 or 2.

5. A method of making silicalite having at least 8 weight percent noble metal thereon at a dispersion of at least 60% comprising the steps of:
a) pretreating the silicalite with a base by contacting the silicalite at a temperature from about 10 to about 75°C with an aqueous solution of a strong base; b) mixing the base pretreated silicalite with an aqueous solution of a noble metal compound at a pH from about 9.5 to about 12.0 at a temperature between about 30 and about 75°C; c) calcining the resulting noble metal impregnated silicalite at a temperature between about 250 and about 450°C for a time from 1 to about 6 hours; d) repeating steps b) and c); and recovering theresulting noble metal impregnated silicalite.

6. The method of Claim 5 where the noble metal is selected from the group consisting of platinum, palladium, rhodium, ruthenium, osmium, iridium, rhenium, gold, or any combination thereof.

7. The method of Claim 5 where the silicalite has at least 8 weight percent noble metal at a dispersion of at least 80%.

8. The method of Claim 5 where the base used in step a) is an alkali metal hydroxide at a concentration between about 0.1 to about 2.0 molar.

9. The method of Claim 5 where step a) is performed at a temperature between about 35 and about 60°C.

10. The method of Claim 1 where step b) is conducted at a pH from 9.5 to 11Ø

11. In a process for hydrogen isotope exchange between hydrogen and water, wherein a mass of flowing liquid water and water vapor contacts a fixed mass of a catalyst in the presence of a mass of gaseous hydrogen flowing countercurrently to the water mass, said catalyst catalyzing the isotopic exchange between gaseous hydrogen and water vapor, the improvement wherein the catalyst comprises the catalytic composite of claim 3.