Polymerization and Copolymerization of Olefins on Chromium Oxide

25 Polymerization and Copolymerization of Olefins on Chromium Oxide Catalysts ALFRED CLARK

Downloaded by CORNELL UNIV on October 7, 2016 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0091.ch025

Phillips Petroleum Co., Bartlesville, Okla. 74003

Fundamental

aspects of polymerization

tion of olefins on supported

and

copolymeriza-

chromium oxide catalysts are

discussed. Evidence is presented showing that hexavalent chromium is stabilized by the support. The reaction is believed to proceed through a Langmuir-Hinshelwood nism on tetrahedral

chromium

sites. Chain

mecha-

termination

occurs predominantly by transfer with monomer. The active site may be formed by interaction of ethylene or other agents such as CO with the tetrahedral chromium site. As chromium content is increased, the efficiency of reaction per chromium atom decreases. The rate-temperature

curve for

polymerization of ethylene goes through a maximum. Homopolymerization of 1-butene proceeds at one-tenth the rate of its copolymerization

with ethylene. The broad

of molecular weights obtained

distribution

is attributed to the broad

distribution of adsorption site energies.

Cince the discovery (6) of supported chromium oxide catalysts for ^ polymerization and copolymerization of olefins, many fundamental studies of these systems have been reported. Early studies by Topchiev et al. (18) deal with the effects of catalyst and reaction variables on the over-all kinetics. More recent studies stress the nature of the catalytically active species (1, 2, 9,13, 14,16,19). Using ESR techniques, evidence is developed which indicates that the active species are Cr ions in tetrahedral environment. Other recent work presents a more detailed look at the reaction kinetics. For example, Yermakov and co-workers (12) provide evidence which suggests that chain termination in the polymerization of ethylene on the catalyst surface takes place predominantly by transfer with monomer, and Clark and Bailey (3, 4) give evidence that chain growth occurs through a Langmuir-Hinshelwood mechanism. 387 Platzer; Addition and Condensation Polymerization Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

388

ADDITION

AND

CONDENSATION

POLYMERIZATION

PROCESSES

This chapter reviews and extends some of the fundamental aspects concerned with the nature of the catalyst, the reaction, and the product.


The

Process

There are at least two ways of carrying out the process: the solution process and the slurry process. In the solution process, the reaction is carried out in the presence of an inert hydrocarbon which dissolves the polymer as it is formed. The solvent may contain a portion of cycloparaffin. Both monomer and polymer remain in solution during the reaction while the catalyst is maintained in suspension by agitation. Reaction temperatures range from about 1 2 5 ° - 1 7 5 ° C . and reaction pressures from 20-30 atm. The reactor product is withdrawn, and monomer is flashed off and recycled. Suspended catalyst is then removed by filtration, and solvent is flashed from the filtrate with steam. In the slurry process, the reaction is carried out in a liquid dispersant (paraffinic in nature), in which catalyst and polymer remain in suspension. Reaction temperature is held below 1 1 0 ° C . to prevent dissolution of the polymer. Catalyst does not necessarily remain in the middle of a polymer particle but spalls and is scattered throughout the polymer. The slurry of polymer and hydrocarbon is withdrawn from the reactor and flashed to remove diluent and unreacted olefin for recycle. Because of the high productivities obtained in this process, it is unnecessary to remove catalyst for many polymer applications. The

Catalyst

The catalyst may be prepared by impregnating a silica-aluminum support with an aqueous solution of chromium trioxide. After drying, the catalyst is usually activated in a stream of dry air at temperatures ranging from ca. 4 0 0 ° - 8 0 0 ° C . Under these conditions catalysts can be prepared with a major percentage of chromium in the form of chromium trioxide. Evidently, the support is not just an inert diluent for chromium oxide by itself is stable only as C r 2 0 3 at these temperatures. There must be interaction between chromium trioxide and the support which stabilizes the former. Other evidence of stabilization exists. For example, if a catalyst is prepared by activating the support alone and mixing it with dried, powdered chromium trioxide, no polymerization activity is developed until after the catalyst has been heated to approximately the melting point of chromium trioxide (—197 ° C ) . A slow increase in the polymerization rate occurs as the mixture is heated at temperatures up to

Platzer; Addition and Condensation Polymerization Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

25.

Chromium

CLARK

Oxide

389

Catalysts


4 0 0 ° C ; at higher temperatures, the increase i n polymerization rate is more rapid. These results indicate that high temperature activation is important not only for removing water from the support but also for inducing the stabilizing interaction between the support and chromium trioxide. Further evidence for stabilization is obtained from reduction profiles of catalysts (11). These profiles were determined from changes in the reduction rate as the temperature of a test sample i n contact with hydrogen was increased at a steady rate from 1 4 5 ° - 5 5 0 ° C . Hydrogen was circulated continuously over the catalyst, passing through a drying agent on each cycle. Reduction rates, ΔΡ/Δ£, were determined by measur ing pressure drops in small time intervals, and they were plotted against temperature. The heating rate selected was 1.5°C./min. When samples are the same size and similar heating schedules are used, the areas under the curves measure the total amount of reduction. In Figure 1, the profiles represent the rates of reduction of samples containing different amounts of total chromium from 0.25% to 3.19%. It is evident that reduction starts at a lower temperature the higher the chromium content. The results clearly show that the lower the chromium content, the more resistant the chromium is to reduction. 60 50 40 REDUCTION RATE. Λ ,ΛΙ

M

M

3 0

*P/At'STn

20 10 0 200

300

400

500

TEMPERATURE ,°C Figure

1.

Reduction

profiles for air-activated mium content

catalysts with varying

chro

It is interesting to speculate how the support stabilizes the chro mium ion against reduction. A reasonable explanation may be based on the views of van Reijen and Cossee (17), who speculate that C r 0 4


390

ADDITION

AND CONDENSATION

POLYMERIZATION

PROCESSES

tetrahedra are linked to the silica network by sharing corners or edges with S1O4 tetrahedra. The amount of chromium so stabilized w i l l be limited, and, as found experimentally, the stability of the high oxidation states of chromium decreases with increasing concentration of chromium on the support not only under reducing conditions but also under oxidizing conditions as in catalyst activation.


The

Reaction

Many of the kinetic studies have been done at low pressures and temperatures because accurate kinetic measurements are more difficult to make at high pressures. Those measurements that have been made at high pressures (,—500 p.s.i.g.) confirm the approximate first-order relationships obtained at atmospheric pressure and below. Therefore, we believe that the kinetic information obtained at low pressures has general significance. In spite of the indicated first-order behavior, the reaction is undoubtedly complex, and the relative rates of the individual steps may change drastically with pressure. Yet there is no reason to believe that totally different mechanisms operate in the various pressure ranges. Reaction rates at atmospheric pressure and below have been determined by three different techniques: (1) Quartz vacuum microbalance, i n which the rate of reaction is determined by weighing the polymer produced as a function of time. (2) Static system, i n which rate of reaction is determined by pressure drop of monomer as a function of time. (3) In a flow system using helium diluent. A l l three methods gave consistent results. Figure 2 gives a typical low pressure first-order plot. In a previous publication (4) it was shown that at pressures above 500 p.s.i.g., the rate of reaction ultimately levels out, approaching zero order. This behavior offers evidence that polymerization occurs by reaction of an adsorbed monomer molecule with an adjacently adsorbed monomer molecule or growing polymer chain (Langmuir-Hinshelwood mechanism). If polymerization occurred by reaction of monomer in the gas phase with adsorbed monomer or adsorbed growing polymer chains (Rideal mechanism), the reaction rate should increase without limit as pressure is increased. The details of the mechanism are not well understood yet. Reasonable speculations based on evidence from E S R measurements have been published by van Reijen and Cossee (17) and by Pecherskaya and Kazanskii (15). V a n Reijen and Cossee speculate that the active site i n a chromium oxide—silica catalyst is a tetrahedrally coordinated chromium ion. They picture the C r 0 4 tetrahedron linked to the S i 0 2 network by


25.

CLARK

Chromium

Oxide

391

Catalysts

0.6 0.5 0.4 In Po/P

0.3


0.2 0.1 0 0

2

4

6

8

10

12

14

t, sec Figure 2. Typical first-order plot polymenzation of ethylene over Cr-Si-Al catalyst (low temperature and pressure)

sharing corners or edges with S i 0 4 tetrahedra. Four different ligands of the C r ion may be distinguished as follows: (1) O 2 " ions shared by a C r and a Si ion, (2) O 2 ' ions pointing outward, (3) O H " ions pointing outward, and (4) H 2 0 molecules pointing outward. The detailed symmetry of the ligand field w i l l be determined chiefly by the polarization of the oxygen bonds. The O 2 " ion pointing outward w i l l form the most polarized bond; the H 2 0 molecule pointing outward w i l l form the least polarized bond, and the other two types of bonds w i l l represent intermediate degrees of polarization. It is possible to form ( 1 ) a tetrahedron with a predominantly trigonal symmetry formed from one strongly and three intermediately polarized ligands, (2) a tetrahedron with predominantly a mirror symmetry formed by three differently polarized ligands. Pecherskaya and Kazanskii (15) describe the initiation of a polymeric chain reaction as occurring by interaction between the center and a molecule of ethylene, leading to alkylation of the center, and changing from tetrahedral to square pyramidal coordination. The square pyramidal configuration is considered to possess a free ligand which, if filled, produces an octahedral configuration. The free ligand of the square pyramidal structure may adsorb a second ethylene molecule (15) which then inserts itself into the C r - C bond of the first adsorbed ethylene in a manner similar to that proposed by Cossee (17) i n chain propagation for complex Ziegler-Natta catalysts. The work of Yermakov and co-workers (12) presents strong evidence showing that chain termination occurs chiefly by transfer with monomer.



392

ADDITION A N D

CONDENSATION

POLYMERIZATION

PROCESSES

The behavior of the reaction rate as a function of temperature dispels any notion that the reaction is simple. Figure 3 shows that there is a maximum in the first-order rate constant-temperature curve at approxi mately 80 ° C . A t such a low temperature, the rate-temperature maxi mum cannot be explained by depolymerization, nor can it be explained by deactivation of the catalyst as a result of more rapid polymer accumu lation on the catalyst at higher temperatures since the maximum is ob tained for initial rates measured as a function of temperature. Theoretical considerations predict that a maximum i n the rate-temperature curve may be expected from the Langmuir-Hinshelwood model for polymeriza tion on solid surfaces but not from the Rideal model (5). The rate of reaction for the Langmuir-Hinshelwood model is given by: R = N / M i ( N M i + 2kd) /kd 2

(1)

0

where N0 is the number of polymerization sites per unit catalyst surface; 0i, the fraction of sites covered with monomer; Κ = Are~Q,RT, the reaction Q/RT velocity constant; kd — Ade~ , the desorption velocity constant. The activation energy in kr for the surface reaction is assumed to be the energy required to remove an adsorbed monomer molecule laterally from its position and is taken equal to the heat of adsorption Q (4). 160 C

1.0

120

80

60

40

0

1 ST ORDER RATE CONSTANT, SEC^XIO .1 4

.03 .0020

Figure 3.

.0025

.0030 l/T

Effect of temperature on polymerization

.0035

activity

The monomer material balance on the surface at steady state is: fc20i3 + c M j 2 - 2fc0i2 + c0x + 0! - c = 0,


(2)

25.

Chromium

CLARK

Oxide

393

Catalysts

where b = krN /k , c = k P/k , and k is the velocity constant of adsorp tion in which the activation energy is assumed to be zero. Using the Arrhenius form of k andfcd i n Equation 2, we solve for 0

d

a

d

a

r

Q/RT.

e

v»T

e

+ |

= A

d

0X* +

/kj>(l

- θ

λ

(3)

-

where Κ = A /A N . Substituting Equation 3 into Equation 1, the result is: d


R=

r

0

[^PArN0VAd]

+ 2K) (Κ - Κθ - β1^)/(β1

Taking the derivative with respect to temperature equal to zero gives: Oim

4

+ \ κβ1η?

+ Κ)2].

1

+1

m

l

m

(4)

and setting it

- κ» = o,

(5)

where 0 i m is the value of 0i at the rate maximum for given K. It is inter esting to note that 0 i m is independent of T m a x , the temperature at which a rate maximum is observed. T m a x is a function of pressure. In Equation 5, when 6 is zero, Κ is zero; 0 i m approaches the value 1/2 asymptotically as Κ increases. This means that for this model rate maxima are not possible when the surface is more than one-half covered with monomer. lm

If values of 0im and Κ from Equation 5 are substituted into the monomer balance Equation 2, the values of T m a x as a function of pressure are obtained. Since e ^ 1 ( T and Q always positive), it is possible to calculate, for each pair of values of 0lm and K, the minimum value of A /k P below which a rate maximum is not possible. It may be seen from Table I that after passing through a maximum, (A /k P) min ap proaches unity asymptotically with increasing K. Q/BT

d

a

d

a

Apparent activation energies have been measured for various chro mium contents and supports at low pressures and temperatures far below the rate—temperature maximum. In the temperature range — 4 0 ° - 0 ° C . Table I.

Criteria for Existence of a Rate Maximum

Κ 1000 100 50 10 1 0.1 0.01 0.001 0.0001

0Jm 0.5 0.5 0.49 0.43 0.28 0.14 0.045 0.015 0.0045

(Ad/kaP)w{n 1.00 1.00 1.01 1.18 1.40 1.24 0.55 0.19 0.087



394

ADDITION

A N D CONDENSATION

POLYMERIZATION

PROCESSES

on catalysts containing from 0.5 to 2.5% chromium on silica-alumina (85-15), alumina, and silica, activation energies calculated from firstorder rate constants remained practically constant at 7-8 kcal. There is danger i n trying to interpret activation energies of complex reactions. Possibly the activation energy is associated with the energy of mobility required to move monomer adsorbed on the support to the chromium atom polymerization site. Such energies should correspond roughly with the average energies of adsorption of ethylene on the supports. It is interesting to note the effect of chromium content on reaction rate at high pressures ( ^ 5 0 0 p.s.i.g.). Experiments (5) were carried out with normal air-activated catalysts (Figure 4 ) . Catalysts were used with chromium contents ranging from 0.7 to 0.0005 wt. % of the total catalyst. Results of one-hour ethylene polymerization tests at 132 ° C . and 450 p.s.i.g. with these catalysts, activated at 5 0 0 ° C , are given. As the concentration of chromium was decreased, catalyst charge was increased to compensate for poisoning of catalyst sites b y trace impurities and to keep total rate of production about constant.

YIELD. G./G. CATALYST

YIELD, G./G. Cr

10 h

I

i

Polymerization and Copolymerization of Olefins on Chromium Oxide

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