Conversion of Olefins and Paraffins Over Novel Catalyst Composition

1. Field of the Invention

This invention relates to use of crystalline silicates having a high silica/alumina mole ratio. In one aspect of this invention a novel catalytic composition is prepared from said high silica materials by reacting the same with certain binders such as an alumina-containing binder and in another aspect of this invention conversion processes are carried out with said novel catalytic composition. In particular, the invention relates to conversion of feedstock comprising C.sub.2.sup.+ olefins and/or C.sub.2 -C.sub.7 paraffins to product comprising C.sub.5.sup.+ hydrocarbons.

2. Description Of The Prior Art

High silica/alumina mole ratio crystalline silicates are well known in the art and it is generally accepted that the ion exchange capacity of such crystalline silicates is directly dependent on the amount of metal which is tetrahedrally coordinated with the silica in the framework. Thus, for example, with regard to the most common zeolitic crystalline materials; namely, crystalline aluminosilicate zeolites, such material can be described as a rigid three dimensional network of SiO.sub.4 and AlO.sub.4 in which the tetrahedra are cross-linked by the sharing of oxygen atoms whereby the ratio of total aluminum and silicon atoms to oxygen is 1:2. The electrovalence of the tetrahedra-containing aluminum is balanced by the inclusion in the crystal of a cation. Quite obviously, the more aluminum that is present in the crystal the more cations can be introduced into the crystalline structure. Recently, the scientific and technical literature has disclosed high silica-containing zeolitic structures wherein substantially all or a portion of the aluminum present in the crystal framework has been replaced by other metals either partially or completely. Thus, for example, iron, chromium and boron are materials which have been described in the prior art as capable of being substituted for aluminum in the crystal framework and quite obviously, the ion exchange capacity of the resulting zeolitic structure will again be determined by the amount of metal which is in tetrahedral coordination with the silica. Thus, for example, the more boron there is in a crystalline structure, the more cations are required to balance the electronegativity thereof and when such cations are of the acidic type, such as hydrogen, they impart tremendous catalytic activity to the crystalline material. On the other hand, crystalline silicates having a high silica/alumina mole ratio of greater than about 1600 have many important properties and characterisitcs and have a high degree of structural stability such that they have become candidates for use in various processes, incuding catalytic processes. Materials of this type are well known in the art and include high silica-containing aluminosilicates such as ZSM-5 (U.S. Pat. No. 3,702,886), ZSM-11 (U.S. Pat. No. 3,709,979), and ZSM-12 (U.S. Pat. No. 3,832,449) to mention a few. It is also known in the art that the silica/alumina ratio of a given zeolite is often variable. For example, zeolite X can be synthesized with a silica/alumina ratio of from 2 to 3 and zeolite Y from about 3 to about 6. In some zeolites, the upper limit of silica/alumina ratio is virtually unbounded. Zeolite ZSM-5 is one such material wherein the silica/alumina ratio is at least 5. U.S. Pat. No. 3,941,871 discloses a crystalline metallo-aluminosilicate essentially free of aluminum and exhibiting an X-ray diffraction pattern characteristic of ZSM-5. U.S. Pat. Nos. 4,061,724; 4,073,865; and 4,104,294 describe microporous crystalline silicas wherein the aluminum content is present at very low levels. Because of the extremely low aluminum content of these high silica-containing zeolites their ion exchange capacity is not as great as materials with a higher aluminum content. Therefore, when these materials are contacted with an acidic solution and thereafter are processed in a conventional manner, they are not as catalytically active as their higher aluminum containing counterparts. This invention permits the preparation and use of certain high silica-containing materials which have all the desirable properties inherently possessed by such high silica materials and yet have an acid activity which has heretofore only been possible to be achieved by materials having a higher aluminum content in their "as synthesized" form or by certain activation techniques, such as treatment with metallic vapors.

In U.S. Pat. Nos. 3,960,978 and 4,021,502, conversion of C.sub.2 -C.sub.5 olefins, alone or in admixture with paraffinic components, into higher hydrocarbons over crystalline zeolites having controlled acidity is disclosed. Processing techniques for conversion of olefins to gasoline and distillate are disclosed in U.S. Pat. Nos. 4,150,062, 4,211,640 and 4,227,992. The above-identified disclosures are incorporated herein by reference.

Olefinic feedstocks may be obtained from various sources, including fossil fuel processing streams, such as gas separation units, cracking of C.sub.2.sup.+ hydrocarbons, coal byproducts, and various synthetic fuel processing streams. Cracking of ethane and conversion of conversion effluent is disclosed in U.S. Pat. No. 4,100,218 and conversion of ethane to aromatics over Ga-ZSM-5 is disclosed in U.S. Pat. No. 4,350,835. Olefinic effluent from fulidized catalytic cracking of gas oil or the like is a valuable source of olefins, mainly C.sub.3 -C.sub.4 olefins.

The present invention relates to use of a high silica crystalline material having improved acid activity such as, for example, catalytic cracking and dewaxing activity, as a result of reaction of said high silica material with an acidic inorganic oxide, e.g. alumina. The reaction requires mixing said high silica material with an acidic inorganic oxide in the presence of water, forming or shaping the mixture into a desired shape, and then calcining the formed or shaped mixture at an elevated temperature.

It has been found that it is absolutely crucial in this invention that the inorganic oxide, e.g. alumina, and the crystalline high silica material be physically admixed in the presence of water. If the materials are added in a completely dry state, absolutely no activation takes place irrespective of whether or not the later processing steps are carried out. In a particular preferred embodiment of this invention, the high silica zeolite and alumina are extruded under hydraulic pressures ranging from about 2 to about 50 tons per square inch (t/in.sup.2). For reasons which are not completely understood, it has been found that the extrusion step under high pressure imparts even a greater activation to the high silica zeolitic material.

The feedstock to the present process comprises C.sub.2 -C.sub.7 paraffins and/or olefins of at least two carbon atoms. Product of the present process comprises C.sub.5.sup.+ hydrocarbons. When the feedstock comprises paraffins, product comprises aromatics, e.g. benzene, toluene and xylenes, and conversion conditions include a temperature of from about 100.degree. C. to about 700.degree. C., a pressure of from about 10 kPa to about 11000 kPa, preferably from 10 kPa to 7000 kPa, a liquid hourly space velocity (LHSV) of from about 0.1 hr.sup.-1 to about 500 hr.sup.-1, preferably 0.5 hr.sup.-1 to 400 hr.sup.-1, and a hydrogen/hydrocarbon mole ratio of from 0 (no added hydrogen) to about 20. Under these same conversion conditions, a feedstock comprising C.sub.2 -C.sub.7 olefins is converted to product comprising aromatics, e.g. benzene, toluene and xylenes.

A feedstock to the present process may comprise primarily C.sub.2 -C.sub.7 olefins for conversion to gasoline and distillate products when the conversion conditions are tailored to be within the following ranges. In general, the temperature will be maintained at from about 190.degree. C. to about 375.degree. C., the pressure at from about 400 kPa to about 11000 kPa, preferably from 400 kPa to about 7000 kPa, and the liquid hourly space velocity (LHSV based on feedstock olefin) at from about 0.3 to about 2, preferably from 0.5 to 2 hr.sup.-1. Specifically when the present process is operated in the distillate mode, the temperature will be from about 190.degree. C. to about 315.degree. C., the pressure from about 4200 kPa to abut 11000 kPa, preferably from 4200 kPa to 7000 kPa, and the LHSV from about 0.3 to about 1.0 hr.sup.-1, preferably from 0.5 to 1.0 hr.sup.-1. When the present process is operated in the gasoline mode, the temperature will be from about 230.degree. C. to about 375.degree. C., the pressure from about 400 kPa to about 4700 kPa, preferably from 400 kPa to 3000 kPa and the LHSV from about 0.3 to about 2.0, preferably from 0.5 to 2.0 hr.sup.-1. The feedstocks, products, process conditions and other variables for conversion of olefins to higher hydrocarbons are detailed in U.S. Pat. No. 4,456,779, incorprated entirely herein by reference.

This invention is concerned with use of high silica-containing crystalline materials. The expression "high silica-containing crystalline material" is intended to define a crystalline structure which has a silica/alumina mole ratio greater than about 1600 up to and including those highly siliceous material where the silica/alumina mole ratio is infinity, or as reasonably close to infinity as practically possible. This latter group of highly siliceous materials is exemplified by U.S. Pat. Nos. 3,941,871; 4,061,724; 4,073,865 and 4,104,294 wherein the materials are prepared from reaction solutions which involve no deliberate addition of aluminum. However, trace quantities of aluminum are usually present due to the impurity of the reaction solutions. It is to be understood that the expression "high silica-containing crystalline material" also specifically includes those materials which have other metals besides aluminum associated therewith, such as boron, iron, chromium, etc. Thus, the only requirements with regard to the starting materials utilized in the novel process of this invention is that they have a silica/alumina ratio greater than about 1600 (irrespective of what other materials or metals are present in the crystal structure).

As is well known in the art, the concept of incorporating a crystalline silicate including crystalline aluminosilicate zeolites with an inorganic oxide such as alumina, silica, titania, silica-alumina, etc. is well known in the art and these materials are broadly referred to as binders or matrices. The binders and matrices serve very valuable functions, including imparting extra strength to the zeolitic catalysts, but strictly from a catalytic point of view they are not as catalytically active as the zeolite with which they are admixed so that the total effect is that the catalytic activity of the composition has been reduced. Thus, it is conventional in the prior art to use an inorganic oxide binder such as alumina with a zeolite such as ZSM-5 but the resulting composition has less catalytic activity than would be obtainable by the use of the pure zeolite alone. Thus, quite simply put, the instant invention is not concerned with those crystalline silicates which have an inherently sufficiently high acid activity such that incorporating them with alumina would result in obtaining a composition which had less activity than the silicate material itself. This invention is concerned with crystalline silicate materials which have substantially little or no acid catalytic activity as conveniently measured by the Alpha test. They have Alpha Values of less than about 5. This invention would reside in improving said acid catalytic activity.

Another way of expressing the same concept is to point out that in one embodiment, the novel process of this invention involves the following steps: (1) mixing, such as by mulling, of a high silica zeolite with an inorganic oxide, such as alumina, and water, (2) forming or shaping, and (3) air calcining (e.g. in air) at elevated temperatures.

Optionally, the formed or shaped composition of step (2) is calcined in a non-oxidizing atmosphere, e.g. ammonia, at from about 900.degree. F. to about 1200.degree. F. for at least about one hour, preferably for from about one hour to about five hours. Also optionally, the formed or shaped composition of step (2) or the product of calcination thereof in a non-oxidizing atmosphere is treated with acidic cations, especially if the sodium content of the material is greater than about 0.02 wt. %, in order to insure less than 0.02 wt. % sodium content.

In short, the following list of steps will be followed by the present method:

(1) mixing with inorganic oxide in pressure of water

(2) forming or shaping

(3) optionally calining the formed or shaped composition in a non-oxidizing atmosphere

(4) optionally treating with acidic cations

(5) calcining

It appears obvious that for certain zeolites, steps (3), (4), and (5) will result in enhancing their catalytic activity without carrying out steps (1) and (2). Thus, it is very well known in the art that base exchange of the sodium in a crystalline zeolite with acidic ions, such as ammonium ions, results in enhancing the acid activity of such material. As indicated earlier, this is because those materials contain sufficient aluminum in the framework structure such that they must be balanced by cations and base exchange with acidic ions allows the incorporation of a substantial amount of acidic cations into the crystal structure, thereby resulting in enhanced acid activity.

Quite obviously, if the steps (1) and (2) were carried out with a crystaline material having these characteristics no enhancement in acitivy would result, but rather, a dilution of activity would result merely because the inorganic oxide would dilute the acid activity of the far more active zeolitic component.

As indicated earlier, the high silica crystalline materials with which this invention is concerned contain very small amounts of aluminum, with or without iron, chromium, boron, etc. It is difficult to set exact numerical limits for the amount of such materials which the composition must contain in order to be operable within the novel process of this invention due to the fact that at such low levels analytical techniques are not necessarily accurate. However, in order to give some indication of the ranges of such content, it would be generally from about 50 to no more than about 1000 ppm, preferably no more than about 500 ppm, i.e. a silica/alumina mole ratio of greater than about 1600.

As has previously been indicated, the novel method for the preparation of catalysts having enhanced acid activity according to this invention involves carrying out at least three necessary steps. The first necessary step in the novel process of this invention involves (1) mixing, such as for example mulling, a crystalline silicate having a high silica/alumina mole ratio of greater than about 1600 and an Alpha Value of less than about 5 with an appropriate acidic inorganic oxide in the presence of water. It is absolutely crucial that there be water present during step (1) since it has been found that if the zeolite and the inorganic oxide material is mixed, e.g. mulled, in a dry state that substantially no activation will occur. The amount of water which is utilized is not narrowly critical and only enough water has to be used to ensure an adequate mixture of the inorganic oxide and the high silica material. The mixing can be carried out by hand with a mortar and pestle or commercially available mullers can be used. An example of such mullers are those manufactured by the Cincinnati Muller Company of Cincinnati, Ohio.

The acidic inorganic binder which is used in step (1) is preferably alumina or an alumina-containing material. However, other binders can be used such as titania and zirconia or mixtures of alumina, titania and zirconia. For reasons which are not completely understood, silica has essentially no effect on the acid activity of the material and, thus, cannot be used by itself.

The amount of inorganic oxide which is incorporated with the high silica crystalline material has a surprising effect on the acid activity of the resulting composition. The preferred range of inorganic oxide is, thus, 20-95 wt. % based on total composition of inorganic oxide plus high silica crystalline material.

The second necessary step in the novel process of this invention involves a forming step (2) so as to obtain discrete particles of the catalyst composite. The forming step includes simply sizing the material to any appropriate size using any appropriate die or compacting type device, including hand pelleting. The forming step also includes spray drying the step (1) mixture or using the oil-drop method. However, a preferred embodiment of this step is to use extrusion, i.e. to pass the composition of step (1) through a die at extremely high pressures, i.e. at pressures ranging from about 2 to about 50 t/in.sup.2 or even higher, preferably higher than about 5 t/in.sup.2. Typical extruders can be of the hydraulic ram type or of the bonnet auger type.

The third necessary step in the novel process of this invention involves calcination, such as in air, at elevated temperature, i.e. temperature from about 800.degree.-1500.degree. F. for periods of time ranging from about 2-5 hours. A particularly preferred embodiment would be air calcination at about 1000.degree. F. for about three hours.

An optional step in the novel process of this invention involves calcination of the step (2) product in a non-oxidizing atmosphere at temperatures from about 900.degree.-1200.degree. F. for at least about 1 hour, e.g. 1-5 hours. The non-oxidizing atmosphere is preferably ammonia, although nitrogen and/or inert gases can be used.

A further optional step involves ion exchange of the composition with hydrogen ions or ammonium ions in order to reduce the sodium content to less than about 0.02 wt. %.

Of the high silica materials advantageously treated in accordance herewith, those having the structure of zeolite ZSM-5, ZSM-5/ZSM-11 intermediate or ZSM-11 are particularly noted. ZSM-5 is described in U.S. Pat. Nos. 3,702,886 and Re. 29,948, the entire contents of each being incorporated herein by reference. ZSM-11 is described in U.S. Pat. No. 3,709,979, the teaching of which is hereby incorporated by reference. ZSM-5/ZSM-11 intermediate is described in U.S. Pat. No. 4,229,424, the entire contents thereof being incorporated herein by reference. Quite obviously, these materials must be used in the manner previously indicated, i.e. having a silica/alumina mole ratio of greater than about 1600 and an Alpha Value of less than about 5.

The following examples will illustrate the best mode contemplated for carrying out this invention.

ZSM-5 crystals were synthesized from a formulation containing tetraethylorthosilicate, sodium hydroxide, tetrapropylammonium bromide, and water at 212.degree. F. with intensive agitation. Special precautions were taken to prevent alumina contamination from the crystallization equipment and the environment.

A 50 g quantity of the material was calcined in an ammonia atmosphere at 1000.degree. F. for three hours to decompose the organic components in the zeolite. The product was pure white without any signs of a carbon residue. The NH.sub.3 calcined material was subsequently purged with nitrogen and then air for one hour to remove any absorbed NH.sub.3. The sodium content of the sample was reduced to 0.01 wt. % by treatment with a 0.1 N ammonium nitrate solution followed by hot water washing. The sample was then dried and its chemical composition was determined to be as follows:

A portion of a zeolite produced in accordance with Example 1 was calcined in a muffle furnace at 1000.degree. F. for three hours, sized to 30-60 mesh, and 2.5 g (4.8 cc) were charged to a 5/16" ID stainless steel microreactor. The ZSM-5 was treated in situ with hydrogen at 900.degree. F. for one hour. Propylene, admixed with 50 volume percent of hydrogen, was then passed over the catalyst at 500 psig (total pressure--1000 psig), 0.4 WHSV, 400.degree. F. for two hours. No C.sub.6.sup.+ liquid product was obtained.

A mixture of 1.63 g (2.9 cc) 50 ppm Al.sub.2 O.sub.3 ZSM-5 plus 0.87 g (1.8 cc) alpha alumina monohydrate, both sized to 30-60 mesh and calcined at 1000.degree. F. for three hours, was charged to the reactor and treated again in situ with hydrogen at 1000.degree. F. for one hour. Propylene was then passed over the catalyst under the same conditions as in Example 2 for successive periods of 161/2 and 221/2 hours. Again, no C.sub.6.sup.+ liquid was formed.

The 50 ppm Al.sub.2 O.sub.3 ZSM-5, as synthesized, was mulled with 35 wt. % alpha alumina monohydrate with added deionized water, extruded (1/16") at 25 t/in.sup.2 pressure, dired at 230.degree. F., precalcined in ammonia for three hours at 1000.degree. F., ammonium exchanged to insure sodium content less than 0.02 wt. %, dried at 230.degree. F., and calcined in air for three hours at 1000.degree. F.

A 2.50 g quantity of the product from Example 4, sized to 30-60 mesh, was charged to the reactor, treated with hydrogen in situ for one hour at 900.degree. F. Then propylene was passed over the catalyst under the conditions of Examples 2 and 3. Results are shown below:

The C.sub.6.sup.+ liquid from the two runs was composited and distilled to give 28.6 wt. % 330.degree. F..sup.- gasoline and 42.9 wt. % 330.degree. F..sup.+ fuel oil. The gasoline has an octane number (R+O) of 94 and the fuel oil a pour point of less than -70.degree. F., diesel index 67.

A furfural extracted, waxy heavy neutral lube stock, designated Coryton 0048 raffinate, was processed over the catalysts of Examples 1 and 4 at 1 LHSV, 400 psig, 2500 SCF hydrogen/bbl. Results are compared below with a conventional ZSM-5 catalyst (1% Ni on 70/1 SiO.sub.2 /Al.sub.2 O.sub.3 ZSM-5 with 35% alumina binder, steamed to an Alpha Value of 70).

The 50 ppm Al.sub.2 O.sub.3 catalyst had essentially no lube catalytic dewaxing activity. Addition of the alumina binder (Example 4) imparts activity with no adverse effect on the ZSM-5 shape selectivity as indicated by the pour point-viscosity index relationship. In this example it is less active but more selective than the standard conventional catalyst, i.e. gives a higher viscosity index at about the same pour point.

The following examples will illustrate that the use of zeolites having a significant amount of alumina, i.e. a silica/alumina mole ratio of 1600 or less, are not activated by the procedure of this invention, but rather, their activity is diluted.

Propylene was passed over the 70/1 SiO.sub.2 /Al.sub.2 O.sub.3 ZSM-5 catalyst, with and without 35% alumina binder under the same conditions as those used in Examples 2, 3 and 5. Results are listed below:

It can be seen that the catalyst without binder was actually more active than that with binder, i.e. the liquid product has a higher boiling range. The binder in this case "dilutes" the concentration of ZSM-5 and thus decreases the activity of a given weight of final catalyst.

In this example a ZSM-5 having a silica-to-alumina ratio of 1600:1 (about 0.10 wt. % alumina, or 1,000 ppm) was used. The waxy lube raffinate of Example 6 was processed over this 1600/1 SiO.sub.2 /Al.sub.2 O.sub.3 ZSM-5, with and without 35% alumina binder. Conditions were again the same as those used in that example.

From the above example, it can be seen that a binder is not necessary for generation of an active catalyst at this alumina level, and in fact, the presence of alumina actually dilutes the activity of the catalyst without the binder.

The procedure of Example 4 was repeated with the sole exception that no water was used in mulling the alumina with a high silica ZSM-5.

In this example alumina alone was mulled with added water (no high silica zeolite) and then the high silica zeolite of Example 2 was wetted with the extract liquid and thereafter processed in identical manner to Example 2, i.e. ammonium precalcination for three hours at 1000.degree., base exchanged with an ammonium solution to reduce the sodium content to 0.02 wt. % followed by air calcination.

This example consisted of the use of 100% Kaiser alumina binder alone in order to completely eliminate the possibility that alumina itself was responsible for any catalytic activity.

The catalysts of Examples 9, 10 and 11 were evaluated for the conversion of propylene under conditions recited in Examples 2 and 3 and the results are shown in the following table together with results from some of the previously referred to examples.

From the above table, the pure crystals, i.e. Example 2 made no liquid product. The physical mixture, i.e. Example 3, eliminated the possibility that the high silica ZSM-5 and the alumina particles as such are interacting and the dry mulling experiment, i.e. Example 9, eliminates ammonium exchange in the final calcination as being involved in the generation of active sites. The water extract experiment, i.e. Example 10, eliminates room temperature wet mulling as a possible source of dissolved alumina entering the ZSM-5 pores. The alumina only experiment, i.e. Example 11, also made no liquid product and as such this result confirms the fact that the binder as such in the wet mulled catalyst is not making liquid product at 400.degree. F.

Another series of examples were carried out in order to illustrate the criticality of the use of water during the mulling of the high silica zeolite with the alumina. In all of these examples, 65 wt. % of the ultra low alumina zeolite of Example 1 was used and the binder was 35 wt. % alpha alumina monohydrate. Each sample was treated after forming in accordance with the procedure of Example 4. Examples 12 and 14 did not use water. Example 13 did use water but no hydraulic pressure.

The compositions were evaluated together for Alpha activity.

As is known in the art, the Alpha Value is an approximate indication of the catalytic cracking activity of the catalyst compared to a standard catalyst and it gives the relative rate constant (rate of normal hexane conversion per volume of catalyst per unit time). The Alpha test is described in U.S. Pat. No. 3,354,078 and in The Journal of Catalysis, Vol. IV, pp. 522-529 (August 1965).

The results are shown in the following table along with a comparison with the catalyst of Example 4.

As can be seen, the preparation without water, i.e. Examples 12 and 14, resulted in very low Alpha Values--even though the ammonia precalcination, ammonium exchange and air calcination were carried out. However, the preparations with water gave enhanced results.

Examples 15 and 16 illustrate the effect of time and temperature of ammonia precalcination on alpha activity and a comparison is made with the catalyst of Example 4. In each of Examples 15 and 16, the procedure of Example 4 was followed.

The results and operation conditions are shown in the following table.

Examples 17 and 18 show the effect of binder concentration on activation.

In each of Examples 17 and 18 the exact procedure of Example 4 was followed with the exception of varying the binder content. Note that the binder has a surprising effect on activity.

The results and specific formualtions are shown below together with the catalyst of Example 4.

These examples show that ZrO.sub.2 and TiO.sub.2 as binder material impart significant Alpha activity compared to Al.sub.2 O.sub.3, using mortar and pestle mixing and pressing as the method of incorporation (insufficient material for hydraulic extrusion). SiO.sub.2 as a binder has essentially no effect on Alpha activity.

Details of the preparation of these materials are described below.

As synthesized ultra low Al ZSM-5 zeolite was intimately mixed with a zirconia gel of 25% solution by weight (from duPont Chemical). The weight ratio of ZSM-5 to ZrO.sub.2 was adjusted to 65/35. The mixture was then dried and calcined in a NH.sub.3 stream at 1000.degree. F. for three hours. The Na content in the material was then exchanged with 1 N NH.sub.4 /NO.sub.3 solution to reduce to less than 0.05%. The sample was then sized to 14/25 mesh size material and calcined to 1000.degree. F. in air for three hours.

Preparation procedure and catalytic test method of the catalyst are similar to that in Example 19 except that 25% titania gel (from duPont Chemical) was used instead of ZrO.sub.2 gel.

Preparation procedure and catalytic test method of the catalyst are similar to that in Example 19 except that 30% colloidal SiO.sub.2 (from duPont Chemical) was used instead of ZrO.sub.2 gel.

Same as Example 19, except Kaiser alpha alumina monohydrate was used.

The results are shown in the following table together with the results obtained with the composition of Example 1.

These examples illustrate that the mesh size of the catalyst is not critical.

In each of Examples 23-25, the catalyst prepared by the process of Example 4 was ground to different mesh size and the Alpha activity measured.

The results are shown in the following table.