Aromatic formation in the polymerization of ethene with heteropoly acid catalysts

AROMATIC FORMATION IN THE

POLYMERIZATION OF ETHENE WITH

HETEROPOLY ACID CATALYSTS

PROEFSCHRIFT

TER VERKRIJGING VAN DE GRAAD VAN DOCTOR IN DE TECHNISCHE WETENSCHAP AAN DE TECHNISCHE HOGESCHOOL TE DELFT, OP GEZAG VAN DE RECTOR MAGNI-FICUS DR. O . B O T T E M A , HOOGLERAAR IN DE AFDELING DER ALGEMENE WETENSCHAP-PEN, VOOR EEN COMMISSIE UIT DE SENAAT

TE VERDEDIGEN OP WOENSDAG 10 APRIL 1957 DES NAMIDDAGS TE 4 UUR

DOOR

JAN WILLEM KLINKENBERG

SCHEIKUNDIG INGENIEUR GEBOREN TE FORMEREND

I

Aan de nagedachtenis van mijn Moeder Aan mijn Vader

N. V. de Bataaf sche Petroleum M i j . , The Hague. Thanks a r e due to the management for their kind permission to publish the r e sults, as well as to the Koninklijke/shellLaboratorium, A m s t e r -dam, for their assistance in solving some of the analytical prob-l e m s .

Mr. J . M . Oelderik's s h a r e in the experimental work is gratefully acknowledged.

T A B L E O F C O N T E N T S

Introduction 3 Chapter 1. Literature 11

§ 1. Ethene 11 § 2. The polymerization of gaseous olefins . . . 13

§ 3. Aromatization 22 Chapter 2. The polymerization of ethene in batch 29

§ 1. Raw materials 29 § 2. The activity of various catalysts 30

§ 3. Summary of r e s u l t s 43 Chapter 3. The continuous polymerization of ethene . . . . 45

§ 1. Apparatus 45 § 2. Experimental part 48

§ 3. Discussion of r e s u l t s 56

Chapter 4. Reaction products 59 § 1. Ethene 59 § 2. Propene 68 § 3. Wax cracking olefins 70

Chapter 5. Reaction mechanism 72 § 1. Acid polymerization mechanism 72

§ 2. Aromatization mechanism 77 § 3. Application to heteropoly acid

polymeriza-tion 78

Summary 83 Samenvatting (Summary in Dutch) 86

I N T R O D U C T I O N

A striking feature of the petroleum industry is its great a c -tivity in the fields of r e s e a r c h and development. This continuous s e a r c h leads to a wider and better application of the type of com-pounds forming the raw material of this industry: hydrocarbons. The many by-products originating from this development, though creating a great many fresh problems, a r e at the same time a driving force behind new r e s e a r c h p r o g r a m s .

Basically, the formation of byproducts in chemical r e a c -tions is undesirable. However, complete selectivity in most c a s e s still being an unattainable goal, the problem of the utilization of byproducts is as r e a l as ever, especially in the p e t r o -leum industry, where starting materials a r e nearly always com-plex mixtures. Suitable outlets for by-products may be internal consumption, further processing to marketable products or the opening up of new m a r k e t s .

In the picture outlined above olefins play an important part. They a r e the connecting link between the relatively unreactive paraffins and the majority of petrochemicals. Gaseous olefins a r e produced on a large scale as a result of the big expansion p r o -grams of many oil companies, specifically in the field of cata-lytic cracking. Extensive r e s e a r c h has created new outlets for these olefin's, mainly in the petrochemical field.

Another source of olefins in petroleum refining is the wax cracking p r o c e s s , which has gained in importance by a steadily increasing demand for synthetic detergents.

As there is a growing demand for aromatics, economically one of the most attractive reactions is the aromatization of non-aromatic material. Well-known p r o c e s s e s in this field a r e "Hy-droforming" and "Platforming", which a r e finding or have found wide applications in the upgrading of gasolines and the production of aromatics.

If the molecule of the starting material contains fewer than six C atoms, polymerization must precede or accompany a r o m a -tization in order to yield benzenoid hydrocarbons. Such polymeri-zations (called conjunct polymeripolymeri-zations by Ipatieff) may produce considerable amounts of aromatics. In our case this aromatizing polymerization is carried out with heteropoly acids as catalysts. Ethene is the most important olefin in this respect, but some at-tention is paid also to propene and other olefins.

The first chapter gives a brief account of the importance of ethene, its production and application, and a literature survey on polymerization and aromatization reactions.

The next two chapters deal with catalyst preparation, the in-fluence of different c a r r i e r s and the aromatization experiments in batch as well as on a continuous scale.

The last two chapters are devoted to product composition and reaction mechanism.

C h a p t e r 1 L I T E R A T U R E

§ 1. Ethene

Ethene, today, is one of the most important raw m a t e r i a l s for the chemical industry. Its main source is the oil industry, where the different cracking p r o c e s s e s supply an abundance of gaseous paraffins and olefins.

Table 1 gives the compositions of various cracking gases: Table 1 (1) *

Composition of petroleum refinery gases (in % v)

Hydrogen Methane Ethene Ethane Propene Propane Isobutane n-Butane Butenes Butadienes Mixed-phase cracking 3 35 3 20 7 15 2 8 7 - Reform-ing 7 40 4 18 6 10 3 7 5 - Vapor-phase cracking 7 30 23 12 14 4 1 2 6 1 Poly- form-ing 7 50 8 25 3 6 -1 -Catal. cracking 7 18 5 9 16 14 16 5 10 -It is seen that the percentage by volume of ethene is rather low, except in the gas resulting from vapor-phase cracking. However, it should be remembered, that the principal method of producing ethene is the pyrolysis of ethane, propane and butanes. The potential yield of ethene from cracking gases, therefore, is high. This is villustrated by the figures of Table 2.

Depending on p r o c e s s and reaction conditions, a catalytic cracking unit will yield from 10-20% w of C4 and lighter hydro-carbons, based on charging stock. Considering that the larger catalytic cracking units consume several thousand metric tons of feed daily, it is easy to see that they can be formidable producers of gaseous olefins.

Another producer of ethene is the coal industry, though the * Literature references on p. 89.

Table 2 (1) Potential yield of C2H4 kg/m^ cracking gas Mixed-phase cracking 0.50 Reform-ing 0.45 Vapor-phase cracking 0.60 Poly- form-ing 0.40 Catalytic cracking 0.60 gases from this source contain far less ethene than those from petroleum refining, as is shown in Table 3.

Table 3 (1)

Ethene sources other than refinery gases (% v)

Methane Ethene CO CO2 Hydrogen Oxygen Nitrogen Coal gas 34.0 6.6 9.0 1.1 47.0 -2.3 Coke oven 28.5 2.9 5.1 1.4 57.4 0.5 4.2 Oil gas 27.0 2.7 1C.6 2.8 53.5 -3.4 Producer gas 2.6 0.4 22.0 5.7 10.5 -58.8

Ethene is also prepared by dehydrating ethanol. This may seem odd because ethanol is produced in great quantities not only by fermentation but also by hydration of ethene.

Here we hit upon one of the difficulties in ethene production: the degree of purity. For some purposes, notably for polythene production, an ethene of hi-h purity (99.9%) is essential and one of the best methods of obtaining this - r a d e is the dehydration of ethanol (2).

Another reason for producing ethene from ethanol was a. purely economical one. Because of the hi^h cost of shipping ethene it was at one time cheaper in the U. S. to hydrate ethene in Texas, ship it to the East coast and dehydrate it again.

The methods of ethene production may be summarized as follows:

a. Separation from refinery and coke oven gases. b. Cracking of ethane, propane (butane).

c. P a r t i a l oxidation of ethane. d. Dehydration of ethanol.

The following methods of separation a r e used to obtain a 90-95% pure ethene:

a. Fractional distillation. b . Absorption in oil.

c. Adsorption (active charcoal or silica gel).

The wide application of ethene in the chemical industry is illustrated by the flowsheet in Figure 1 *.

Some physical data on ethene (5): Molecular weight : 28.0

Critical temperature: 9.8°C Critical p r e s s u r e : 51 atm. Critical density : 0.22 g/ml. Boiling point : -103.7^0.

§ 2. The polymerization of gaseous olefins

The products resulting from the polymerization of gaseous olefins may be divided into three groups:

a. gasoline type products

b. synthetic oils (lubricating oils, etc.) c. solids (e.g. polythene).

In the present report only the p r o c e s s leading to products of group a will be considered.

Although in the literature p r o c e s s e s are described in which electric discharges, a-particles or photosensitized polymeriza-tion a r e employed to produce gasolines from olefins, these have never found application. This leaves:

A. thermal polymerizations, B. catalytic polymerizations,

of which only the latter will be discussed. C a t a l y t i c p o l y m e r i z a t i o n

This survey of the literature on the catalytic polymerization of gaseous olefins will be restricted to p r o c e s s e s involving „acid" catalysts. No mention will be made of those initiated by free r a d -icals or using AICI3, BF3 and similar Friedel-Crafts catalysts, though the latter might be called acid catalysts, because the presence of free acid (added as such or formed by the presence of water) seems to play an important part in these reactions (8) (9) (10) (11).

On the other hand, silica-alumina catalysts will be included, as there is evidence that they are acidic and may furnish protons for the acid polymerization of olefins (12) (13).

In connection with our own study, we will give a brief d e -scription of the polymerization of gaseous olefins by the following acids (groups of acids or acidic substances):

1. Sulfuric acid. 2. Phosphoric acid.

3. Acids containing fluorine. 4. Silica-alumina catalysts.

5. Heteropoly acids (this group will be dealt with extensive-ly).

* Courtesy of Petroleum Refiner and D r . J . P . Cunningham, Shell Chemical Corp.

ERYTHRITOL ACETIC ACID

n-BUTANOL AND ESTERS

BUTYRIC ACID

AND ESTERS n- BUTYRALDEHYDE f

2-ETHYLHEXAmL AND ESTERS STYRENE Polymers (Rubber) (Plos/ics) ets CHLORAL HYDRATE DDT ETHYL-BENZENE - » Esters ACETALDEHYDE ETHYL ALCOHOL Ethyl ''Ether ETHYLENE ETHYL-CHLORIDE Ethyl Fluid POLYMERS (Polyethylenesj ETHYLENE DIBROMIDE ETHYLENE OXIDE ETHYLENE CHLOROHYDRIN ETHYLENE DICHLORIDE VINYL CHLORIDE FIG.1 "Ethyl cellulose ETHYLENE

GLYCOL •Polyesters (Dacronj

POLYGLYCOLS AND ETHERS - ACRYLONITRILE ALKYLOLAMINES - f^lymers (Fibers) (Rubbers) (Soil Conditioners) SURFACTANTS

Moreover, the addition of metals or metal oxides to some of these groups will be mentioned

1. S u l f u r i c a c i d

Polymerization with sulfuric acid is restricted to propene and the butenes. The first experimenter to study the behavior of this acid in butene polymerization was Butlerov (14), who found that isobutene polymerized more easily than the other butenes. The reaction products were mainly d i m e r s and t r i m e r s . But-l e r o v ' s r e s u But-l t s were confirmed by the experiments of Lebedev and Koblyanski (15) in 1930. At about the same time sulfuric acid polymerization received much attention in the U. S., where the "cold" and "hot" acid p r o c e s s e s were developed by Shell. The "cold" acid p r o c e s s , based on 70% sulfuric acid, selectively ab-sorbs isobutene from C4 mixtures at 20-35°C, which is polymer-ized to the dimer at 90-100°C; the "hot" acid process employs a temperature of 80-90°C and copolymerizes all C4-olefins, also predominantly to the dimers (called "Hydrocodimer" after hydro-genation). Both products were used in aviation gasolines. Full details on procedure and operating conditions are given by Mc-Allister (16). The influence of temperature, concentration and quantity of sulfuric acid on the polymerization of olefins is d i s -cussed by Ipatieff and Pines (17); together with Friedman (18) these investigators made an extensive study of the co-polymeri-zation of various olefins by sulfuric acid.

Besides the acid itself, the bisulfates of the alkali metals have also found application in the polymerization of isobutene, but at higher temperatures (above 100°C) (19).

2. P h o s p h o r i c a c i d

Phosphoric acid is undoubtedly the most successful acid po-lymerization catalyst. This is clearly illustrated by the fact that in 1951 more than 150 plants used the Universal Oil Products "solid phosphoric acid" catalyst (20).

Before entering into its commercial development, however, let us first briefly review the history of phosphoric acid as a po-lymerization catalyst. In preparing ethene from ethanol by dehy-dration with phosphoric acid, Newth (21) observed the formation of small amounts of an "oily" substance, a fact reported before by Pelouze (22). The nature of this oil was investigated by De MontmoUin (23), who, after dehydrating ethanol with phosphoric acid at 210-240OC, succeeded in demonstrating the presence of paraffins, olefins, naphthenes and aromatics.

The development of phosphoric acid into a technically appli-cable catalyst was undertaken by Ipatieff, first as a liquid and later on a solid absorbent (24) (25). The latter is the so-called "solid phosphoric acid" catalyst, which finds wide application on a commercial scale. It consists of a mixture of phosphoric acid and kieselguhr (or other absorbents of this type), which is

calcin-ed at 200-300°C and is uscalcin-ed in a pelletcalcin-ed form. Three types of commercial units using the UOP catalyst (26) were developed: the low p r e s s u r e regenerative type, the small high p r e s s u r e chamber type and the tubular reactor unit. The low p r e s s u r e units were operated at about 10-13 atm. and the catalyst was regenerated; they were replaced by high p r e s s u r e chamber and reactor type units, where no catalyst regeneration is necessary. Like sulfuric acid, the UOP catalyst may be used either for selective or for non-selective polymerization of C3 and C4 olefins, depending on reactions conditions, which a r e listed in Table 4.

Table 4 (26)

Reaction conditions of UOP. phosphoric acid p r o c e s s e s P r o c e s s Selective C4 Non-selective C4 Non-selective C3 Non-selective C3+ C4 T e m p e r a t u r e , ^ c 150-175 190-210 230-250 200-250 P r e s s u r e , atm.

40-50 1

Whereas in a selective p r o c e s s conversion is sacrificed partly in order to obtain a certain product composition (dimeriza-tion in the case of C4 olefins), a non-selective p r o c e s s is operat-ed under maximum yield conditions. The type of feoperat-ed also influ-ences polymer composition and olefin conversions; detailed infor-mation on this subject is given by Ipatieff, Corson and Egloff (27).

Catalyst life depends on reaction conditions and the type of unit used. In the older regenerative type units about 600650 l i -t e r s of polymer could be produced wi-th 1kg of ca-talys-t (regenera-tions included), while for modern reactor type units this figure may run as high as 1000 1. per kg of catalyst (without r e g e n e r a -tion). As mentioned above, these production figures a r e strongly influenced by reaction conditions; rigid temperature control is necessary, because at temperatures of 260*^0 and higher forma-tion of tar and carbon will have a deleterious effect on the cata-lyst (26). An extremely important variable in the UOP solid phos-phoric acid process is the water content of the feed: a dry feed causes dehydration of the orthophosphoric acid to the far l e s s active meta-acid (26) (28), whereas overhydration may cause plugging of the reactor by catalyst that has lost mechanical strength. Hence, the water content of the feed must be controlled within narrow limits, which is considered a drawback of this p r o c e s s .

Besides the one of UOP, two other p r o c e s s e s based on phosphoric acid (phosphates) have been developed in the U.S.; one u s -ing Cu-pyrophosphate as a catalyst, and another worked out more recently, in which the catalytic medium is a phosphoric acid film on a non-porous, inert material, such a s quartz.

The Cu-pyrophosphate p r o c e s s is based on patents of M.W. Kellogg Co. (29). Reaction conditions a r e comparable to those of the UOP p r o c e s s and complete descriptions can be found in ref-erences (30) and (31).

The catalyst for the phosphoric acid film p r o c e s s is d e s c r i b -ed in U.S. Patent 2.579.433 (32), while detail-ed information can be obtained from an article by Langlois and Walkey (33). In gen-eral the operating conditions of this p r o c e s s do not differ much from those of the two p r o c e s s e s mentioned previously.

From the survey given above it should by no means be con-cluded that phosphoric acid polymerization has not been studied outside the U.S., though commercial development took place mainly in that country.

Shortly before the UOP catalyst was introduced the I. G. F a r -benindustrie worked out a similar p r o c e s s in Germany, but it has found no widespread technical application. They used active c h a r -coal as the catalyst support instead of kieselguhr (which has the disadvantage of not always having a constant composition) and their main source of olefins was the F i s c h e r - T r o p s c h p r o c e s s . A comparison between the I.G. Farben and the UOP p r o c e s s is made in an article by Sapper (34).

The polymerization of ethene to motor fuels has never been c a r r i e d out on a technical scale, either with phosphoric acid or with any other acid catalyst. Although some feedstocks used in the technical phosphoric acid p r o c e s s did contain ethene (27), it never contributed substantially to the liquid yields. It may be said that in general reaction conditions a r e too severe and yields too low to make the polymerization of ethene by means of phosphoric acid an economically attractive proposition.

Attempts to realize it all the same have been made by Ipatieff et al. (35) (36), who operated at t e m p e r a t u r e s of 250-330OC in autoclaves and (at a p r e s s u r e of 30 atm.) in a semi-technical con-tinuous installation. What makes this ethene polymerization so interesting i s the composition of the resulting liquids, which con-tain paraffins, olefins, naphthenes and aromatics. C3-C4 poly-merization products, on the other hand, a r e predominantly olef-inic in nature. The investigations of Ipatieff et al. will be dealt with in Chapter 4 (product composition), where a comparison will be drawn between Ipatieff's r e s u l t s and our own.

Further r e s e a r c h done by Universal Oil Prod. Co. i s d e -scribed in articles written by Ipatieff and co-workers on a comparison between liquid phosphoric acid catalyzed and thermal p o lymerization of propene (37), the crosspolymerization of p r o -pene and isobutene (38) and the mixed polymerization of C4 olef-ins to octenes (39).

The use of phosphorus pentoxide instead of phosphoric acid has also been reported in the l i t e r a t u r e (40), e. g. a s a mixture of P2O5 and carbon black for the polymerization of ethene between 100 and 260OC at p r e s s u r e s of 100-200 atm. As the author states that water seems to play an important p a r t in this reaction, the pentoxide apparently has no specific advantages over the acid.

To conclude this brief literature survey on phosphoric acid a s a polymerization catalyst, we would mention the addition d e -scribed in a number of patents — of certain metals or their oxides to phosphoric acid catalysts. The exact function of these additions is not altogether clear. French Patent 821.136 (41) deals with the

addition of the oxides (sulfides) of Cr, W, Mo, Fe, Ni or Co to a solid phosphoric acid catalyst. They are claimed to have a hydro-genating effect (in the presence of H2). On the other hand, some of these metals or their oxides a r e known to be active polymeri-zation agents themselves (42) (43) (44) (45), while in the case of WO3 or M0O3 (compare also U.S. Patent 2.446.619 (46) a hetero-poly acid s t r u c t u r e is possible in combination with phosphoric acid, which may promote the polymerization activity of the cata-lyst.

3. A c i d s c o n t a i n i n g f l u o r i n e

Hydrofluoric acid, though belonging to the same class as sul-phuric and phosphoric acid, has never been used to any marked extent in the polymerization of gaseous olefins; it is better known as an alkylation catalyst. Yet, in some patents it is claimed to act as a polymerization agent, either alone or in combination with fluorides. Ethene is not mentioned in these patents, but C3 and C4 olefins have been polymerized to motor fuels in the presence of gaseous HF (47) (48). In general temperatures of some 200°C and lower were employed. Two other patents (49) (50) describe the use of complex compounds consisting of HF and an alkali metal fluoride in about the same temperature range.

A group of complex fluoric acids like fluosilicic, -boric and -titanic acids has been used in the solid form, viz. on c a r r i e r s like activated alumina, silica, bauxite, charcoal or fuller's earth (51). Polymerization temperatures were between 250 and 300^0, 15-50 atm. being considered as a suitable p r e s s u r e range.

According to Brooks (52), dihydroxyfluoric acid can be used as a catalyst in the copolymerization of C3 and (or) C4 olefins at far lower temperatures (0-40^0) and at p r e s s u r e s ranging-from 3.4 to 8.5 atm. In this way heptenes a r e obtained from a mixed C3-C4 polymerization and octenes from a refinery C4 cut.

4. S i l i c a - a l u m i n a c a t a l y s t s

Both synthetic and natural silica-alumina catalysts have been used in polymerization experiments with gaseous olefins. The first investigator to experience the catalytic activity of a natural alumina ("Tonerde") in the polymerization of ethene was Ipatieff (116), who obtained products ranging from gasolines to high-boil-ing fractions (above 180°C). Gurvich (53) mentioned the catalytic properties of Floridin or Florida earth (a natural silica-alumina) as early as 1915. Floridin, which shows an outstanding activity among catalysts of this type, was studied extensively in later y e a r s . It is capable of polymerizing isobutene at temperatures as low as -100°C (54) (55), but, as is to be expected, high molecular weight products are obtained at this temperature. That at higher t e m p e r a t u r e s lower boiling products can be formed was demon-strated by Lebedev and Philonenko (56), who obtained d i m e r s and t r i m e r s from isobutene at 300°C. These authors paid particular

attention to the dehydration of Floridin in relation to its activity. (Compare also (15).) Gayer (57) used Floridin in the polymeriza-tion of propene at 350OC and atmospheric p r e s s u r e ; he found that its activity could be greatly increased by acid treatment (HCl), a phenomenon often made use of in acid catalysis nowadays. Gay-er also seems to have been the first to introduce synthetic catalysts of this type, which showed a far higher activity than F l o r i -din. A catalyst prepared by the hydrolytic adsorption of alumina on silica (1% w alumina), used at 340<^C and atmospheric p r e s s u r e , produced a liquid polymer that distilled mainly in the g a s -oline range. Some aromatic material was found in the residue (boiling above 200°C at 3 mm Hg). In these experiments olefin conversions remained rather low (up to 32% on a once-through basis), which was caused partly by the fact that they were carried out at atmospheric p r e s s u r e .

The polymerization activity of synthetic cracking catalysts was studied by Thomas (58), who used alumina, silica-zirconia, silica-alumina-thoria and silica-alumina-zirconia mix-t u r e s in C3 and C4 olefin polymerizamix-tion on a laboramix-tory scale as well as in pilot plant runs. Thus, propene was polymerized at 250-300OC and 50 a t m . , and C4 olefins at HS^C and 50 a t m . , the reaction products being of the gasoline type. These catalysts rapidly lost their activity, but could be regenerated. F r o m this work it may be concluded that polymerization is a secondary r e -action in catalytic cracking. As such it has been studied by Greensfelder et al. (59).

An important factor in the polymerization of gaseous olefins by silica-alumina catalysts is the moisture content. Lebedev (56) and Gayer (57) reported the influence of hydration on the activity of Floridin. Another catalyst whose activity is greatly affected by the amount of residual water is bauxite. Its behavior was studied by Heineman, Lalande J r . and McCarter (60), who used it as a catalyst in the polymerization of C4 olefins and employed an acti-vation temperature of 650-750°C (0.2% residual water). The op-timum temperature range for polymerization was 100-150°C, both at atmospheric and elevated p r e s s u r e s (20-35 a t m . ) , while conversions as high as 90% were obtained for isobutene.

It must be borne in mind, however, that silica-alumina catalysts show rather low yields in comparison with acids like s u l -furic acid, phosphoric acid and some heteropoly acids. In general their activity d e c r e a s e s rapidly and rather severe conditions must be applied. They a r e frequently used as catalyst supports, also in acid polymerization (kieselguhr in the UOP catalyst), and in com-bination with a " t r u e " acid they form a catalyst of superior quali-ties in many c a s e s . Another drawback of the natural silica-alu-mina mixtures like kieselguhr and bauxite, is the frequent differ-ence in composition, which depends on their origin. It is known that Floridin contains iron and magnesium compounds (58), and the presence of iron in bauxite and kieselguhr is also well known; these metal oxides may influence polymerizations carried outwith natural silica-alumina catalysts and this also holds for the syn-thetic cracking catalysts used by Thomas (58). Some of these

con-tained Th02 or Z r 0 2 , which oxides a r e known to promote the polymerization of olefins (42).

An excellent example in this field i s found in a recent article by Hogan et al. (45), who used a NiO-silica-alumina catalyst in the polymerization of the lower olefins. The catalyst, which is also described in some patents (61), contains a synthetic silica-alumina mixture, which is prepared by impregnation of silica hydrogel with an aqueous solution of an aluminum salt, followed by washing, drying and heating in air to 400-500^0. The alumina content ranges from 2-10% w (synthetic cracking catalyst may also be used). This support is impregnated with an aqueous solu-tion of NiN03. 6 aq. and the catalyst is activated in a s t r e a m of dry air at 500^0. Rapid deactivation took place when a s little a s 0.5% w moisture was absorbed. The most active catalysts con-tained about 4% w Ni.

In a temperature range of 40-95<^C this catalyst showed a good activity for the polymerization of ethene and propene from dilute feed s t r e a m s . P r e s s u r e s of about 40 atm. were used for C3 polymerization and about 20 atm. for C2 (in the presence of hydrogen). The reaction products consisted mainly of d i m e r s , though products up to the pentamers were also formed in appre-ciable quantities.

The most remarkable phenomenon in this polymerization i s the decreasing order of the reaction r a t e s for the normal olefins, which is in contrast to that observed for the usual acid type

cata-lysts; for the NiO-silica-alumina catalyst this order was found to be: ethene, propene, 2-butene, 1-butene (compare Chapter 5).

5. H e t e r o p o l y a c i d s

A heteropoly acid, in the widest sense of the word, is a com-plex acid which may be formed from two (or more) different in-organic acids by the splitting off of one or more molecules of water from either of these acids. The group of heteropoly acids important for the polymerization of gaseous olefins is formed by the combination of acids containing tungsten, molybdenum or vanadium as the sole metallic element with acids derived from the metalloids silicon, boron or phosphorus. An atom of one of the latter elements then becomes the central atom in the hetero-poly acid.

As an illustration we give the composition of a silicotungstic acid, which is by far the most important heteropoly acid in our own study: 2 H2O. Si02.12 WO3. x H2O. This acid is obtained by treating Na2Si03 and NaW04 with concentrated HCl, as will be discussed in detail in Chapter 2.

In general, heteropoly acids are strong acids, comparable to H2SO4 and HCl. The scanty literature data on their use a s catar-lysts in the polymerization of olefins a r e mainly confined to pat-ents. Only recently a study on the catalytic properties of some heteropoly acids (mainly silicotungstic acid) was undertaken by Verstappen (62), whose work can be considered a p r e c u r s o r of our own investigation.

Brit. Pat. 480,756(63) and U.S. Pat. 2,301,966 (64) describe the polymerization of olefins, preferably those with more than 3 C atoms, under the influence of acids like phosphotungstic acid, phosphomolybdic acid (63) and of silicotungstic, silicomolybdic and borotungstic acids, etc. (64). Instead of the acids, their acid salts may be used and it is stated in (64) that ethene and propene a r e among the olefins that can be polymerized. According to these patents the reaction may be promoted by the addition of a heavy-metal salt (Cu, Ag, Ni). Both patents claim good r e s u l t s in a temperature range of 100-200^0 (preferably 130-200OC), the use of increased p r e s s u r e being favorable.

An example describes the polymerization of isobutene at 175°C in a copper autoclave ( 1 | hours); 1000 p a r t s by weight of isobutene yielded 500 p a r t s of a dimer fraction (boiling range lOO-llO^C) and 300 p a r t s of a t r i m e r fraction (185-190OC). The use of a catalyst support is not mentioned in these two patents.

U.S. Patent 2,608,534 (65) extensively describes the p r e p a -ration of heteropoly acids (compare Chapter 2) and their s a l t s , and their use on catalyst supports in different hydrocarbon con-version p r o c e s s e s (including polymerization). The main object of this patent i s the preparation of a heteropoly acid catalyst that contains the same metal oxide(s) as the conventional hydrocarbon conversion catalyst, but is more active and has a longer life. An example is a conventional hydroforming catalyst, which contains M0O3 as the active constituent; U.S. Patent 2,608,534 (65) claims that a better catalyst is obtained when M0O3 i s replaced by a heteropoly acid containing M0O3, such as phosphomolybdic or silicomolybdic acid. The following explanation is offered. It seems that M0O3 has an adverse effect on the properties (mainly the surface area) of the c a r r i e r when the catalyst is subjected to high temperatures (e.g. the operating temperature of the process) and thus d e c r e a s e s its activity. When the M0O3 is present in the form of a heteropoly acid, this deteriorating effect is prevented or at least reduced.

Whereas we have no doubt that heteropoly acid catalysts do possess these qualities, it is impossible that the acid itself should be capable of preventing tliis adverse effect at high t e m p e r a t u r e s , as claimed in the patent referred to (Tables 1 and 2; pp. 20 and 21). It is stated tliat the heteropoly acid catalysts were subjected to a heat treatment at 800°C before being tested as hydroforming catalysts, but from Verstappen's work (62) we may conclude that at this temperature the acid no longer exists, but only a mixture of SiOa and WO3 is left; silicomolybdic acid is even less stable than silicotungstic acid, as we observed several t i m e s . This is illustrated by Fig. 2 *.

At 450-500^0 the chemically bound water is driven out and a mixture of WO3 and Si02 is left, which is also demonstrated by the fact that at these temperatures the catalyst completely loses its polymerization activity (ref. (62) and our own observations). We a r e of opinion that active catalysts can be prepared a c -cording to U.S. Patent 2,608,534, but that their activity is not * From (62), p. 41.

WATER LOSS mg/g ACID 70r o /lZW0yhSi02 60- / 50- f 12 WO3 SIO2.2HgO 40- \ 30- I 20-0-^ (l2W0j. Si02 2H20) 5H2O I I I I 100 zoo 300 400 500 TEMP. °C Figure 2

due to the heteropoly acids if these catalysts a r e subjected to rigorous heat treatments.

The properties of silicotungstic acid as a polymerization agent for propene were studied extensively by Verstappen (62). Some t e s t s were also made with silicomolybdic acid, but the tungstic acid was preferred because of its great stability. Excel-lent results were obtained with a catalyst consisting of 20% w of silicotungstic acid and 80% w of granulated bauxite. In continuous experiments conversions as high as 90-95% were reached when polymerizing propene from a C3 fraction of a catalytic cracking unit (60% V C3H6, 40% V CjHs) at 150-160°C and 50-120 a t m . . 80-85% V cf the polymers distilled in the gasoline range and they were monoolefinic in nature, though some saturated material wss

also present.

This heteropoly acid has the advantage that no water need be injected into the feed, while polymerization temperatures a r e about 75°C lower than for phosphoric acid. As many details of Verstappen's work will be the subject of discussion in other chapters no further information will be given here, and we will herewith conclude our brief description of acid polymerization. § 3. Aromatization

The term "aromatization" covers a wide field of chemical reactions, which can be effected by both thermal and catalytic p r o c e s s e s . Aromatization may mean a complicated set of r e a c -tions, including isomerization, cyclization and dehydrogenation, like the conversion of paraffins and olefins into aromatics, or it may refer to the relatively simple dehydrogenation of a naph-thene. When a mixture of hydrocarbons is subjected to

aromati-zation, all reactions will occur simultaneously, as in the upgrad-ing of low-octane gasolines.

The present brief literature review will deal only with the catalytic dehydrogenation and cyclization of nonaromatic m a t e -rial in the vapor phase, a p r o c e s s usually referred to as "dehy-drocyclization'' (66). Thermal reactions will not be considered and the review does not include any patent l i t e r a t u r e . Information about patents in this field can be gained from an article by Foster (67).

The catalysts used in aromatization reactions may be divided into two distinct groups: one comprising some metals, which have long been known as hydrogenation catalysts, such a s Pt, Ni and Pd; the other being a large group of metal oxides, the most i m -portant of which are those of the metals, which occur in different states of valency, like Cr, Mo, V or W. Only this latter group is of interest in connection with our own work and will be briefly dealt with.

The l i t e r a t u r e gives an abundance of articles about the metal oxide catalysts. When in the late thirties the " c l a s s i c " source of aromatics, coal tar, seemed no longer capable of meeting the demand and the production of aromatics from petroleum became economically important, numerous investigators in different p a r t s of the world, like Russia, Britain, the United States and the Neth-erlands, were attracted by the properties of these catalysts. Among them the oxides of chromium and molybdenum are the most important, though others a r e known to p o s s e s s similar p r o p e r t i e s .

The first to notice the behavior of Cr203 were Lazier and Vaughen (68), who prepared this oxide from chromium hydroxide gels and succeeded in dehydrogenating cyclohexane in a t e m p e r a -ture range of 410-430°C.

The earliest publications or\ the aromatization of paraffins and olefins with 6 or more C atoms came from Russian investi-gators. Moldavsky et al. (69) (70) made an extensive study on the activity of Cr203, M0O2, ZnO, Ti02 and also M0S2 at tempera-t u r e s above 400°C. These oxides were also used on catempera-talystempera-t supports like AI2O3, activated carbon and silica gel. The best r e -sults were obtained with chromium oxide; at 360OC n-octane could be converted into a liquid containing 63% v aromatics (predomi-nantly o-xylene but also small amounts of ethylbenzene). In the same temperature range n-heptane was converted into toluene and n-hexane into benzene, though the yields were considerably lower than the yield of aromatics from n-octane. With olefins s i m i l a r r e s u l t s were obtained, but they aromatized more easily than paraffins. Aliphatic side chains of alkylbenzenes were also attacked during this treatment, e. g. n-butylbenzene yielded naph-thalene.

Moldavsky et al. treated not only pure compounds but also Grozny gasoline fractions with a chromium oxide catalyst. This resulted in considerable formation of aromatics, mainly from the dehydrogenation of naphthenes but also from the cyclization of non-cyclic material.

Uni-v e r s a l Oil Prod. Cy. searched this field, and obtained a number of patents. The oxides of metals in groups IV, V and VI of the periodic system, like Cr203, Mo203, V2O3, Ti02 and Ce02, were mentioned in these patents and reviewed in an article by Grosse, Morrell and Mattox (71). According to this article the activity of the catalyst depends greatly on its method of p r e p a r a -tion and the surface a r e a of the catalyst supports (alumina e t c . ) . The catalysts a r e used at temperatures between 465 and 500OC and the cyclization of n-heptane to toluene is described. The cata-lysts of Grosse et al. were also used by Koch (72) in the cycliza-tion of an aliphatic C7 fraccycliza-tion (heptenes and heptanes) with a boiling range of 95-165°C and a heavier fraction (boiling range 1 5 0 - 3 0 0 ^ 0 at 500-5200c. Using Cr203 and V2O3 catalysts, a maximum of 50% v of aromatics in the liquid products was obtained from the C 7 fraction, a value which lies considerably b e -low that reached by the UOP investigators. Koch confirmed the observation of the Russian authors that olefins a r e more easily aromatized than paraffins.

An extensive study on cyclization with a pelleted Cr203 catalyst has been made by Hoog, Verheus and Zuiderweg (73). V a r i -ous hydrocarbons (paraffins, cycloolefins and aliphatic olefins) were studied at a temperature of 465^0 and the influence of dif-ferent carbon skeletons on aromatization yields was investigated. In those cases where the carbon skeleton readily p e r m i t s the for-mation of six-membered ring compounds, as with n-heptane etc. aromatics a r e formed in rather good yields. But a compound like 2, 2,4-trimethylpentane shows hardly any aromatization because it must be preceded by isomerization. The fact that small amounts of aromatics were found is proof that isomerization actually took place. Pitkethly and Steiner (84) also studied the Cr203 catalyst,

and converted n-heptane partly into olefins and toluene at 475^0. No cycloparaffins or cycloolefins were found, probable be-cause their dehydrogenation proceeds much faster than their for-mation. In Chapter 5, which deals with reaction mechanism prob-l e m s , there wiprob-lprob-l be an opportunity of entering into this matter further.

Numerous investigations on the activity of metal oxide cata-lysts have been carried out by Taylor and his collaborators at Princeton University. F r o m their work the following facts emerge: in agreement with the observations of the Russian inves-tigators, Taylor and Turkevich (75) found that olefins a r e more readily aromatized than paraffins in the temperature range of 425-520°C, using a chromia-alumina catalyst. Especially the alkenes-1 gave high conversions. The chromia-alumina catalyst (Cr203 from Cr(OH)3 gel) soon lost its activity due to carbon-aceous deposits from side reactions.

A Cr203 catalyst was also used by Goldwasser and Taylor (76), who determined the effects of temperature and feed r a t e s in the aromatization of various compounds. Above 395°C cyclo-hexene was completely converted into benzene; below this tem-p e r a t u r e benzene and cyclohexane were formed by a hydrogen transfer reaction. Cyclohexadiene dehydrogenated even more readily. At 468°C n-heptane yielded 100% toluene when using

con-tact times of 3 minutes and longer. They also discovered that at high feed r a t e s heptene-1 gives less toluene than does n-heptane, contrary to what is found at lower r a t e s . Branched olefins a r o -matized less easily, as had been stated before by Hoog et al. (73).

A study on isomerization by cyclization catalysts was under-taken by Turkevich and Young (77). Using various Cr2C)s and V2O3 catalysts they found no formation of aromatics in the t r e a t ment of 2, 2, 4trimethylpentane at 475^0, contrary to what is r e -ported by Hoog et al. and Obolentsev and Usov (78); M02O3, on the other hand, yielded small amounts of aromatics under similar conditions.

The method of preparing a Cr203 catalyst has a great influ-ence on its activity (compare Grosse et al. (71)), a phenomenon which was studied by Turkevich, F e h r e r and Taylor (79). They prepared Cr203 catalysts by gel precipitation from chromium nitrate and chromium acetate and by reduction of chromic acid with alcohol, sugar and oxalic acid. In a standard procedure these catalysts were tested in the dehydrocyclization of n-heptane to toluene. The best results were obtained with catalysts prepared by reduction of chromic acid with alcohol and oxalic acid. Both promoting and poisoning effects of Cr203-gel catalysts have been studied by F e h r e r and Taylor (80) (81). They added metals like Cu, Ni and Pd to Cr203-gels and also oxides of Mn, Zn, Mo, Si, Zr and Sn. When these catalysts were applied to n-heptane, the best r e s u l t s were obtained with ZnO, M0O2, Zr02 and Sn02 as p r o m o t e r s . M0O2 and Z r 0 2 are active aromatization catalysts themselves, which may account for their promoting activity. On addition of Zr02 and Sn02 a longer catalyst life was realized than with Cr203 alone. The aromatizing properties of Zr02 also show up in catalytic cracking p r o c e s s e s where the formation of a r o -matics is a secondary reaction when using Zr02-containing cata-lysts (59).

Ethene has a poisoning effect on dehydrocyclization catalysts (81); the highest resistance to its influence was shown by vana-dium oxides.

Another contribution to the knowledge of CrgOs-alumina cat-alysts was made by Oblad et al. (82), who also obtained small amounts of aromatics from 2,2,4-trimethylpentane. At 480OC they determined the influence of space velocity on the cyclization of n-heptane to toluene and of octenes to xylenes. Considerable carbon formation on the catalyst was observed by these authors. Good results were obtained by Green (83) with Cr203 and espe-cially with M02O3, though we must take into account that his ex-periments with the M0203-alumina catalyst were c a r r i e d out at

5500c; 25% of aromatics (based on charge) were obtained from 2,2,4-trimethylpentane (xylenes e . g . ) ; with a Cr203-alumina catalyst 1.7% of aromatics (based on charge) were formed at 450 o c . At 500^0 the liquid reaction products obtained from n-hep-tane contained 60% toluene when using a Cr203-alumina catalyst.

It is obvious from many experiments that the isomerization of paraffins precedes cyclization in those cases where no direct ring formation is possible. Another question is whether hydro-carbons containing a quaternary carbon atom, like 2,

2-dimethyl-hexane, 2, 2-dimethylhexene-3 and 2,2-dimethylpentane, can be aromatized. Their carbon skeleton permits direct formation of a naphthene ring with 6 carbon atoms, but their structure might "prohibit" dehydrogenation to aromatics. These questions were answered by Komarewsky and Shand (84), who subjected the above-mentioned hydrocarbons to treatment with a Cr203-alumina catalyst at 4650C. In every case considerable amounts of aromat-ics were found (mainly m-xylene) besides olefins, which leads to the conclusion that somewhere along the path isomerization oc-c u r s . Naphthenes oc-could not be deteoc-cted in the reaoc-ction mixture.

The presence of olefins besides aromatics in the reaction products of a n-heptane dehydrocyclization indicates that unsatu-rated compounds are intermediate products in this reaction. T e m p e r a t u r e and space velocity have a great influence on the heptene-toluene ratio in the liquid products, as was shown by Mattox (85) in experiments with a catalyst consisting of 8% Cr203 on alumina. More aromatics a r e produced after long contact times and at increased t e m p e r a t u r e s . Water r e t a r d s the formation of aromatics and should be absent in dehydrocyclizaformation r e actions. The calcination temperatures of these catalysts are i m -portant in this respect, but also in connection with the surface a r e a of the c a r r i e r and the stability of the metal oxide. The im-portance of these factors for molybdena catalysts was outlined by Russell and Stokes J r . (86). Calcination should be carried out below 700OC as a d e c r e a s e in activity results at higher tempera-t u r e s . Moreover, heatempera-t stempera-tabilitempera-ty d e c r e a s e s witempera-th increasing Mo concentration.

Of interest is the work of Obolentsev and Usov (87), who studied the aromatization of numerous hydrocarbons under standard conditions using a Cr203 catalyst, nHeptane, 2, 2, 4 t r i -methylpentane, heptene-1, 2-methylheptene-2, 3-methylheptene-3 and binary mixtures of these compounds were investigated in this way. In general olefins gave higher yields of aromatics, though liquid yields were lower; in binary mixtures they found the yields of aromatics to be strictly additive. Olefins produced more c a r -nonaceous material on the catalyst surface.

Besides metal oxides, Karshev et al. (88) also added phos-phoric acid to some catalysts, thus introducing a polymerization and alkylation agent. The following catalysts were used in the aromatization of aromatic-free Grozny gasoline fractions: Cr -i-H3PO4, Cr203 + CuO activated with BaO, Cr -^ Cu + -i-H3PO4, Cr + Mo-oxide, Cr + CuO + H3PO4.

The most active catalysts were Cr + Mo-oxide and Cr -H Cu -1-H3PO4. In all cases paraffins were partly aromatized, besides dehydrogenation of naphthenes, in a temperature range of

500-550OC.

An extensive study on the effect of various substances in p r o -moting the dehydrocyclization of paraffins by a Cr203-alumina catalyst was undertalten by Archibald and Greensfelder (89). Their article, in which they discuss several methods of preparing a Cr203-alumina catalyst, indicates that the most active catalysts a r e prepared from chromic acid, while the presence of SO4" and c r should be avoided, these ions having a deleterious effect.

Ad-ditions of 0.1% Pd and Pt were studied for their promoting effect, but after 5 hours the initial improvement in activity had consid-erably decreased. K' is a strong promoter within a certain concentration range; Ce02 also has a strong promoting effect, e s pecially as it shows no induction period. The catalysts were t e s t -ed at 490OC both for pure compounds like n-heptane and methyl-cyclohexane, and for gasolines. According to Fisher et al. (90) Sb204 is another promoter for this catalyst.

The question whether the fluid bed technique might offer ad-vantages over the fixed bed principle, was studied by Guillemin and Vincent-Genod (91). They draw a comparison between the results obtained with a semi-technical fluid bed reactor and a fixed bed p r o c e s s in the aromatization of a Middle East gasoline with a molybdena-alumina catalyst at 500-510°C. Their conclusion was that using a fluid catalyst is disadvantageous, as it r e -duces aromatization, while coke formation is considerable. The liquid yields were the same as when employing a fixed bed. They also compared the activities of Cr203, M0O3 and V2O5 supported on two different kinds of activated alumina. The one with the largest surface area gave better r e s u l t s ; molybdena catalysts a r e to be preferred. KVO3 was successfully tested as a promoter.

It will be clear from the literature review given above that metal oxide catalysts offer possibilities for industrial applica-tions, such as the improvement of low-octane gasolines and the production of certain aromatics when using a selected feedstock. Two p r o c e s s e s using metal oxide catalysts have been developed, viz. a low p r e s s u r e "aromatization" process using a chromia-alumina catalyst for the catalytic reforming of naphthas and the so-called "Hydroforming" p r o c e s s , employing a molybdena-alumina catalyst, also developed for the upgrading of gasolines. During World War II the "Hydroforming" p r o c e s s was mainly

employed for the production of toluene and aviation gasoline blending components. In post-war y e a r s a Hydroforming unit was used for the production of o-xylene, an intermediate in phthalic anhydride manufacture (92).

The main difference between "aromatization" and "Hydroforming" is that in the latter process a hydrogenrich gas is r e -cycled in the r e a c t o r s , a p r e s s u r e of 10-20 atm. being main-tained. The former operates without recycle gas and at p r e s s u r e s ranging from atmospheric to 5 atm. Both p r o c e s s e s a r e mention-ed in an article by Greensfelder et al. (93), which also deals with the main reactions occurring in catalytic reforming.

The principal reactions of the "Hydroforming" p r o c e s s a r e : 1. dehydrogenation of naphthenes,

2. dehydrocyclization of paraffins,

3. isomerization of cyclopentane derivatives to s i x m e m b e r -ed ring compounds and of n-paraffins to isoparaffins, 4. decomposition of sulfur compounds.

The molybda-alumina catalyst is more suitable for this process than chromia-alumina, as it shows a better activity under siderable hydrogen partial p r e s s u r e , gives higher aromatic con-tents in converted naphthas and causes stronger branching of n-paraffins. Moreover it is more sulfur-resistant.

Detailed descriptions of this p r o c e s s were given by Armistead J r . (94) and Hill et al. (95). "Hydroforming" is a r e g e n e r -ative p r o c e s s , operating at 10-20 atm. and 480-540OC, using a H2-rich recycle gas (60-80% H2). In general a unit consists of four r e a c t o r s , two of which a r e in reaction and two in different stages of regeneration. The cycle from reaction through regener-ation back to reaction is performed in 8-16 hours. Regenerregener-ation is necessary because of coke deposition, sulfiding of the catalyst and partial reduction of the metal oxide. The carbon is burned and the catalyst reoxidized in a s t r e a m of air diluted with cooled flue gas. The products a r e fractionated into recycle gas, a highly aromatic fraction boiling above lOS^C and a depropanized gaso-line of low olefin and sulfur content. The liquid hourly space velocity employed runs from 0.6 - 1.0 vol. /vol. cat., hour.

The literature review given in this section shows that metal oxides are important cyclization catalysts and that in particular chromia and molybdena catalysts have found wide application. As the heteropoly acids also contain oxides of transition metals, we were interested in a comparison between the two groups of cata-lyst (see Chapter 5) and for this reason a review on investigations into metal oxide catalysts was included.

C h a p t e r 2

T H E P O L Y M E R I Z A T I O N O F E T H E N E IN B A T C H

§ 1. Raw materials

a. S i l i c o t u n g s t i c a c i d

This heteropoly acid was prepaired by a well-known method given by Booth (96) and also described by Verstappen (62). In view of the importance of silicotungstic acid in our catalytic ex-periments the method of preparation is repeated below:

500 g of Na2Wo4. 2 H2O is dissolved in 1 liter of water and 37.5 g of a technical Na2Si03 solution (water glass, density 1.375) is added. This mixture is briskly s t i r r e d (mechanically) while heating it to its boiling point; 300 ml of concentrated HCl (38%, density 1.19) is added dropwise at this temperature (the first 100 ml of hydrochloric acid must be added very slowly to prevent the precipitation of yellow WO3). The hot heteropoly acid solution is filtered by suction in order to remove excess Si02 and after cooling to room temperature an additional 200 ml of concentrated hydrochloric acid is poured in. (This addition can be carried out much faster than that of the first batch of HCl.)

The acid solution is treated with a small excess of diethyl ether and the oily bottom layer, consisting of the silicotungstic acid - ether complex, is drawn off. After evaporation of the ether on a water bath, the heteropoly acid is redissolved in a 3 N HCl solution which is saturated with sodium chloride. Again the hete-ropoly acid i s removed by extraction with ether in a separatory fvmnel and the ether is evaporated on a water bath. The silico-tungstic acid is dissolved in as little water as possible and the water is evaporated on a water bath. This water treatment is repeated two more times and finally the acid is dried in a hot stove at 110-1200c.

Numerous portions of silicotungstic acid were prepared in this way with an average yield.of 80%; no difference in activity was ever noticed when the acid was tested by a standard p r o c e -dure, which will be described in this chapter.

Recently a different method of preparing heteropoly acids was described in U.S. Patent 2,608,534 (65), in which the sub-limed metal oxide was used instead of the sodium salt of the cor-responding acid.

An example gives the preparation of phosphomolybdic acid by treating sublimed M0O3 with phosphoric acid, which results in a p u r e r heteropoly acid.

and has the formula: H^SiWi2O40. x H2O or 2 H2O. SiOz. 12 WO3. X H2C. At a drying temperature of 110-120OC the acid contains 5 moles of hydration water (62) (97). The molecular weight of this acid is 2970. Silicotungstic acid is very stable and can be heated to 300-350°C before showing any signs of decomposition. More-over, the acid is not hygroscopic and can be stored for a long time without cieteriorating effects.

b. O t h e r h e t e r o p o l y a c i d s

Besides silicotungstic acid two other heteropoly acids were used in our batch experiments, viz. phosphotungstic acid and silicotungsticmolybdic acid (containing 6 WO3 and 6 M0O3 instead of 12 WO3). The latter acid is prepared in exactly the same way as silicotungstic acid, except that half of the sodiumtungstate is replaced by the equivalent amount of sodiummolybdate. Phos-photungstic acid is prepared in a similar manner by the reaction of Na2W04 and Na2HP04 in a HCl solution (98).

Whereas phosphotungstic acid p o s s e s s e s much the same properties as silicotungstic acid as far as catalytic activity, sta-bility and decomposition temperature are concerned, the hetero-poly acids containing M0O3 are inferior in stability, which easily results in a decreased catalytic activity. This instability is caus-ed by the fact that M0O3 is more readily rcaus-educcaus-ed than WO3, for instance by dust particles. We therefore preferred the use of a heteropoly acid containing tungstic trioxide.

c. E t h en e

The batch experiments were carried out with two different grades of ethene. One was prepared in tiiis laboratory on a s e m i -technical scale by the dehydration of ethanol. Analysis of this gas gave the following composition:

C2H4 95.0% V H2 3.5% V N2 1.2% V O2 0.3% V.

The other grade was purchased from Loos & Co., Amsterdam, and contained 95.0% v ethene, the r e s t being ethane and methane.

As the two gases gave identical results when polymerized with the same catalyst they were further used indiscriminately. § 2 . The activity of various catalysts

a. C a t a l y s t p r e p a r a t i o n

Though some catalytic agents a r e used as such, it is in most cases advisable to use them in a diluted form. One of the best known applications in this field is the use of a suitable c a r r i e r or catalyst support. Frequently such use shows several advantages

over that of the pure catalytic agent. In general we may say that the application of a c a r r i e r s e r v e s the following purposes:

1. Lower catalyst costs. 2. Improved activity.

3. Greater mechanical strength.

All three advantages a r e realized in the case of heteropoly acids, as may be concluded from an observation by Verstappen (62), who used bauxite as a catalyst support in the polymerization of p r o -pene with silicotungstic acid.

An extensive study has been made by us of the use of various c a r r i e r s in the polymerization of ethene with silicotungstic acid. Generally silicotungstic acid c a r r i e r combinations were p r e -pared as follows:

The heteropoly acid was dissolved in a quantity of water suf-ficient to i m m e r s e the proper amount of c a r r i e r . While s t i r r i n g occasionally the water was evaporated in a hot stove and the cat-alyst was subsequently dried at the desired temperature.

In those cases where we departed from this procedure this will be stated in detail.

b . S t a n d a r d p r o c e d u r e f o r t e s t i n g t h e a c t i v i t y of a c a t a l y s t

A 250 ml steel autoclave was charged with the proper amount of catalyst and evacuated to a p r e s s u r e of 15 mm Hg before introducing 40 g of ethene (pressure ca 55 atm. at room t e m p e r a -ture). The rotating autoclave was then heated by gas to 2500C (heating-up period 20-25 minutes) and kept at this temperature for exactly one hour, followed by cooling to room temperature in a s t r e a m of cold air (time required about l | hours).

After letting off the unconverted ethene, the autoclave was weighed and the conversion calculated as the ratio between ethene polymerized and ethene originally introduced. Temperature and p r e s s u r e were recorded every five minutes whereby detailed in-formation was obtained on the course of the polymerization.

It must be realized that in calculating conversions according to this procedure the C4 fraction (if present) was not accounted for, while some volatile product (C5, Cg) escaped in the exit gas, so that actually the conversions must have been slightly higher. Moreover, the presence of impurities in the ethene was d i s r e -garded.

c. T h e behavior of various carriers

The selection of the proper c a r r i e r for a catalytic agent is not a simple task. Actually one should be guided in this s e a r c h by theoretical evidence about the behavior of c a t a l y s t - c a r r i e r combinations in different types of reactions. But nowadays even relatively simple reactions like the hydrogenation of a double bond still offer so many problems as to the activity of catalysts on c a r r i e r s that such a selection is made mainly along empirical lines.

Natural and synthetic oxides of silicon and aluminum, both in admixture and alone, found wide application as catalyst sup-p o r t s in a variety of reactions.

Besides these oxides carbon is also well known in this field, e.g. carbon black, charcoal or coke.

Our own r e s e a r c h was also carried out in this "conventional" way, but was partly based on r e s u l t s obtained by Verstappen (62). The suitability of various c a r r i e r s was investigated and the r e s u l t s will be given in the following order:

1. Bauxite.

2. Aluminum oxide.

3. Pumice, cracking catalyst, coke. 4. Silica gel.

5. Addition of metal oxides to some c a r r i e r s .

The c a r r i e r s of these five groups were all tested in combi-nation with silicotungstic acid, mainly in a ratio of 1 part by weight of the acid to 4 p a r t s by weight of c a r r i e r . Two other heteropoly acids were tested on silica gel.

The effect of acid treatment on catalyst activity was inves-tigated in most c a s e s .

All the conversions listed in the tables on the following pages a r e average values from two or more experiments. The r e p r o -ducibility of these experiments was good; never did conversion figures differ by more than 3% in their absolute values.

In order to be able to compare the activity of silicotungstic acid on c a r r i e r s with that of the acid alone, this latter activity was determined first. In all further experiments the same amount of acid as tested alone was distended on the c a r r i e r , thereby maintaining a fixed acid-to-ethene ratio in every activity test. A 1:10 ratio was chosen, so that to 40 g of ethene (see Standard Procedure p. 31) 4 g of silicotungstic acid was present. In the description of the experiments which had to be performed at a different acid-to-olefin ratio it will be stated in detail to what ex-tent this method was departed from.

E x p e r i m e n t a l p a r t

4 g of silicotungstic acid was subjected to the standard poly-merization procedure. The average conversion was 36%. This figure will s e r v e as a reference for all conversion values of silicotungstic acid - c a r r i e r combinations.

1. Bauxite *

Catalyst A: 4 g silicotungstic acid

16 g bauxite * drying temperature 150^0.

Catalyst B: 4 g silicotungstic acid

16 g bauxite (pretreated with 4 N HCl) drying temperature 150^0. * Granulated, white; from Dutch Guyana.

The acid pretreatment of bauxite and other c a r r i e r s (further call-ed acid-treatcall-ed c a r r i e r s ) was c a r r i e d out as follows: the c a r r i e r was immersed in 4 N HCl at room temperature for 4-5 hours, while s t i r r i n g occasionally. After thorough rinsing (approx. 30 min.) with cold water the c a r r i e r was dried at 150-160'^C.

The conversion values of silicotungstic acid alone and of bauxite catalysts A and B a r e collected in Table 5.

Table 5

Activity of silicotungstic acid on bauxite Catalyst Si-W acid Bauxite A Bauxite Conversion % 36 45 55

The course of the polymerization is represented by the p r e s s u r e -time curves of Fig. 3.

SILICOTUNGSTIC ACID BAUXITE A BAUXITE B 10 20 30 40 50 60 70 80 TIME min. Figure 3 2. Aluminum oxide

Two different grades of aluminum oxide were tested for their activity:

a. AI2O3 technical grade, powder. b. Activated alumina (Peter Spence).

a. A 1 ^ 3 technical grade, powder

X-ray analysis proved this AI2O3 to have the structure of corund *.

Catalyst A: 4 g silicotungstic acid 1 6 g A 1 ^ 3

drying temperature ISO^C. Catalyst B: 4 g silicotungstic acid

16 g AI2O3 (acid-treated)

drying temperature ISO^C.

The conversions, compared with that for the pure acid, a r e given in Table 6.

Table 6

Activity of silicotungstic acid on technical AIP3 powder Catalyst Silicotungstic acid Catalyst A Catalyst B Conversion % 36 50 57

The p r e s s u r e - t i m e curves of these ejqperiments a r e collected in Fig. 4. PRESSURE 0 160 140 130 100 80 60 40 rrn. ^^r>-^ ' / ' / /

- ,7 /

'1'

i/

The behavior of activated alumina as a catalyst support for * Carried out by D r . J . J.Steggerda.

silicotungstic acid is rather complicated, as reported before by Verstappen (62). This investigator indicated the favorable effect of a higher drying temperature on the activity of silicotungstic acid - activated alumina catalysts. When the drying temperature was raised from 120^0 to 300^0, conversion jumped from 0% to 74% in the polymerization of propene under standardized condi-tions.

It is probable that the amount of water, which may be either chemically bound (hydrates of A1^3) and (or) physically bound, plays an important part in the activity of catalysts prepared with activated alumina. An attempt was made to obtain some informa-tion on this behavior. In the first place it makes a difference whether we use the activated alumina as such or after reactivation by heating it to 550600^0 for 24 hours, as this heat t r e a t -ment results in a weight loss of 9.35%, which is due to removal of water. This fact clearly shows that the activated alumina as such contains a considerable amount of water. This may well affect the activity of catalysts prepared with this activated alu-mina. Here we must refer to our method of preparing the cata-lyst, for this method involves the use of the apparently important substance H2O. The question a r i s e s whether in preparing the cat-alyst with an aqueous solution of silicotungstic acid the effect of heating to 550-600Oc is not fully (or partly) counteracted by this method of preparation. In an attempt to elucidate these questions we carried out the following experiments (the term activated alu-mina r e f e r s to the product received and used without further treatment; the word "preheated" will be used in those cases where the c a r r i e r was preheated to 550-600°C):

Catalyst A: 4 g silicotungstic acid 16 g activated alumina

drying temperature 150^0. Catalyst B: 4 g silicotungstic acid

16 g activated alumina

drying temperature 200OC. Catalyst C: 4 g silicotungstic acid

16 g activated alumina

drying temperature SOO^C. Catalyst D: 4 g silicotungstic acid

16 g activated alumina; acid-treated with. 4 N HCl drying temperature 150OC.

Catalyst F : 4 g silicotungstic acid

16 g preheated activated alumina drying temperature ISO^C Catalyst F: 4 g silicotungstic acid

16 g preheated activated alumina. This catalyst was prepared as follows:

A silicotungstic acid - ether complex was made by dissolving the acid in sodium-dried diethyl ether which was acidified with dry gaseous HCl; 4.1 g of the oily complex was distended on 16 g of preheated c a r r i e r and the resulting mixture was heated to 1500c for 1 hour.

The conversion values for these catalysts a r e recorded in Table 7.

Table 7

Activated alumina and silicotungstic acid Catalyst Catalyst A Catalyst B Catalyst C Catalyst D Catalyst E Catalyst F Conversion %

5 1

These figures give r i s e to the following r e m a r k s :

A higher drying temperature of the catalyst has a favorable effect on its activity, though this activity remains low if com-pared to that of AI2O3 powder, technical grade (Table 6). Unlike many other c a r r i e r m a t e r i a l s , activated alumina show-ed no increase in activity after a treatment with cold HCL It is possible, however, that the activated alumina had been treated with acid during its preparation.

The r e s u l t s obtained with catalysts A and E indicate that p r e -heating of the c a r r i e r to 550-600OC gives a greater activity; together with the result of catalyst F , which was prepared from the dry ether complex, this leads to the conclusion that the presence of relatively large amounts of water, or the use of aqueous heteropoly acid solutions in the preparation of cat-alysts with activated alumina has a deleterious effect on the catalyst.

3. Pumice, synthetic cracking catalyst, coke

The following catalysts were prepared with pumice *: Catalyst A: 4 g silicotungstic acid

16 g pumice

drying temperature 150Oc. Catalyst B: 4 g silicotungstic acid

16 g acid-treated pumice 1 drying temperature 150Oc.

The cracking catalyst used was Cyanamid cracking catalyst; both fresh catalyst and "equilibrium" catalyst from Shell P e r n i s Cata-lytic Cracking Unit were tested **:

Catalyst C: 4 g silicotungstic acid 16 g fresh cracking catalyst

drying temperature 150Oc. * Granulated, 4-7 mm.

Catalyst D: 4 g silicotungstic acid

16 g equilibrium cracking catalyst drying t e m p e r a t u r e 150Oc. Two catalysts were prepared with coke: Catalyst E: 4 g silicotungstic acid

16 g coke

drying temperature 150OC. Catalyst F : 4 g silicotungstic acid

16 g acid-treated coke

drying temperature 150OC.

The conversions obtained with these six catalysts (A-F) a r e shown in Table 8.

Table 8

Pumice, cracking catalyst and coke with silicotungstic acid Catalyst

Pumice Catalyst A Pumice Catalyst B (Cracking catalyst) Catalyst C (Cracking catalyst) Catalyst D Coke Catalyst E Coke Catalyst F Conversion % 26 48 11 32 16 35

The effect of acid treatment is pronounced for both pumice and coke, but the activities of all the catalysts of Table 8 remain rather poor.

4. Silica gel

Two different grades of silica gel * were used in combination with silicotimgstic acid:

a. Silica gel; powder.

b. Silica gel; 2-5 mm, macroporous.

The following catalysts were prepared with the various silica gels:

Catalyst A: 4 g silicotungstic acid 16 g silica gel, powder

drying temperature 150Oc. Catalyst B: 4 g silicotungstic acid

16 g acid-treated silica gel, powder drying temperature 1500C.

* Both were purchased from: "Droogtechniek en Luchtbehande-ling N.V.", Rotterdam.

Catalyst C: 4 g silicotungstic acid

16 g silica gel, 2-5 mm, macroporous drying temperature 150Oc.

Catalyst D: 4 g silicotungstic acid

16 g acid-treated silica gel, 2-5 mm, macroporous drying temperature 150Oc.

The activities of these catalysts, expressed again in percent conversion under standard conditions, a r e collected in Table 9, together with the reference conversion of pure silicotungstic acid.

Table 9

Silica gel and silicotungstic acid Catalyst Catalyst A Catalyst B Catalyst C Catalyst D Silicotungstic acid Conversion % 0 63 76 36

The p r e s s u r e - t i m e curves for the experiments with catalysts B and D are given in Fig. 5.

PRESSURE " 160 140 120 100 80 60 ' " •

r

- (L-'\

While the favorable effect of HCl treatment of a c a r r i e r was mentioned before in this chapter, the differences in activity be-tween the catalysts were in no case so striking as with the silica gels. Both with the silica gel powder and t h e macroporous gel