Synthesis and Regeneration of Raney Catalysts by Mechanochemical Methods

G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors),Prepamtion of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands

591

SYNTHESIS AND REGENERATION OF RANEY CATALYSTS BY MECHANOCHEMICAL METHODS

A. B. FASMAN 1, G.

S. D

v .GOLUBKOVA~

.MIKHAIL E N K O ~, 0.T .KALININA'

.

, E ~ u I.V A N O V ~ ,

Institute of Organic Ca tal ysys and Electrochemistry, 142, K.Marx st., 480100,Alma-Ata,USSR 21nstitute of Solid State Chemistry, 18, Derzhavina st. 630091.Novosibirsk, U S S R

I

SUMMARY The effect of mechanical alloying (MA) on the structure of Raney catalysts and their activity and selectivity in liquid-phase hydrogenation reactions has been studied. The data illustrating a possibility of the method developed by the authors f o r mechanochemica1 (MC) regeneration of Raney catalysts irreversibly deactivated in hydrogenation process are given. INTRODUCTION Raney catalysts are widely used in industry due to their high activity, technological ability and relatively low cost (ref.1).

As

a rule they are made by leaching a non-noble component from pyrometallurgical alloys (PM). The MC method for synthesis of alloys from initial component powders traditional PM ones.

With lower

one stage to produce alloy a wider concentration range

has a number of advantages over the energy

expenses it allows within

powders that form skeleton catalysts in due to a higher efficiency while reac-

ting with alkali (ref.2). At the same time the conditions of MA become the parameters influencing the properties of catalysts. The object of the present work is to study the effect of the MA conditions on the structure of alloys and Raney Ni-catalysts made from them as well as on adsorption properties, activity and selectivity of catalysts in liquid-phase hydrogenation reactions. EFFECT OF PREPARATION CONDITION ON THE FORMATION MECHANISM OF M A ALLOYS AND THEIR STRUCTURE BEFORE AND AFTER LEACHING

MA elloy structure Initial alloys were made in a planetarium-type ball mill and a attritor as in (refs.3,4). Use was made of commercial carbonyl nickel and aluminium powders. The phase composition was analysed by the X-ray diffraction method using CuK,,~emission. It had been found before (ref.4) that in the attritor

with an

uncooled

case the MA

592

was characterized by a latent period during which local heating could result in A1 melting. Then the exothermic effect initiates a reaction which proceeds very fast. The composition of its products differ but little from the equilibrium one. At the same time the mechanism of MA alloy formation in a cooled planetarium mill is close to a diffusion type, whereas the phase composition is far from an equilibrium one. Table 1 gives phase composition of alloys produced in these mills. TABLE 1 Effect of preparation conditions on the MA alloy phase composition Charge composition

Duration MA, min

Phase composition

Planetarium ball mill Before annealing After

17 A183

30

Al+NiA13

i25A1?5

30

NiA13+Ni2A13

Ni32A168

5 30

Ni+Al Ni2A13

i35A '65

5 30

NiZA13 +Ni+Al NiAl

i42A158

20

NiAl

i52

5 20

NiAl+Ni+Al NiAl

A1+NiAlj NiAl3 Ni2A13 +NiA13 Ni2A13

NiAl NiAl

Attri tor Al+NiA13 Ni2A13 +A1 Ni+A1 N i2A13+N iA 13 Ni+A1 Ni2A13 NigAlg +NiA1 Ni+A1 NiAl

Of importance is, probably, the fact that the reaction proceeds in

an open apparatus (an attritor) and it has outside characteristics it is accompanied by a puff, and the activation was ceased right after its proceeding. A planetarium mill is a close-type apparatus so the control like that is impossible, that is why structural changes can occur after the reaction. Structures Ni2A13 and NiAl are close and the former can be produced from the latter by replacing one third of nickel atoms for definitely-ordered vacancies. Their disordering that take place with MA leads to formation of solid solutions on the NiAl basis. T h u s the NiAl homogeneity range is widened from equilibrium 45-60% to metastable 35-60% Ni. This supposition was verified by means of the X-ray method for radial distribution of atoms (ref.5). The MA Ni35Aluproduced in a planetarium mill during 60 min has a diffraction pattern that corresponds to the BCC-structure of NiA1.

593 The positions of coordination maxima on the radial distribution curve (r.d.c-1 correspond to BCC-structure of NiAl too, but there

ore deviations from theoretical values as to the distribution of their areas. Table 2 gives the relationships betwen the areas of experimental r.d.c. maxima P e and theoretical P t for a model relating to a non-distorted structure NiAl (Ni/A1=50/50) and for a model, describing a solid solution on the NiAl-basis (Ni/A1=35/65). It is seen that the second model fully corresponds to experiment. Hence MA alloys are, indeed, able to produce solid solutions on the NiAl-basis losing up to 40% of nickel atoms. TABLE 2 Structural parameters of MA Ni35A165 as compared to NiAl

structure

models

Rt, 2.50 4.08 5.78 6.29 7.08

Re, 2.50 4.05 5.72 6.32 7.00

P e / P t (Ni/Al=50/50)

Ni/A1=35/65)

P,/Pt(

1.018 1.125 1.089 1.019 0.979

0.549 0.422 0.471 0.448 0.368

Structure of Raney nickel catalysts from MA alloys Raney catalysts from Ni-A1 MA alloys possess structural peculiarities. Leaching of A1 from N i s A k 5 -Ni50A15~,as a rule does not lead to any changes of diffraction pattern though from 60 to 20% A1 is removed. It should be noted that the PM NiAl does not react with alkali. Evidently the defect structure of MA NiAl facilitates A1 extraction. From Table 3 it is seen that leaching does not result in changes of MA alloy srtuctural parameters (a borderline composition is taken as an example, Ni35A16s). TABLE 3 Structural parameters of MA Ni35A165 before and after leaching ~

~

Sample Initial alloy Catalyst

0

Lattice parameter.A 2.860-0.001 2.860-0.002

0

Particle size,A 120 110

C . 103 7.94 8.77

On the diffractogram one can see only NiAl lines and a very weak diffused maximum that canbe attributedto FCC-Ni. However, calculations of r.d.c. show that other components make contributions too. Taking account of the fact that the leaching degree was, In this

594

case, 55% by technique (ref.6).

a difference

curve

([cat.]-0.45*

*[alloy]) was drawn. The calculation results correlate tern of FCC-Ni on whose background there are maxima of Table 4 shows P e and P t of r.d.c. peaks for models supposition of 100 and 40% content of Ni in a leached second

model well agrees with experiment, though one

to the patNiO. built up in sample. The

can notice a

greater lowering of coordination numbers with an increase of R. It is, evidently. due to a high dispersity of the nickel. When use is made of the regularities established in (ref.7) then from the slope of the P,/P+=f(R)

0

the size of Ni particles as <50 A was determined.

TABLE 4 Structural parameters of Ni-Raney according to difference r.d.c. Re. 2.50 4.30 5.56 6.60 7.47

h

P t (Ni-100%)

88 176 176 353 353

P t (Ni-40%) 35.25 70.50 70.50 141.00 141.00

Pe

33.20 61.80 60 88 114.40 108.90

-

Thus, simulation of catalyst structure made it possible to establish that it is a mixture of an unleached NiAl (or a solid solution on its basis) of high-dispersion nickel and small amounts of NiO. ADSORPTION PROPERTIES, ACTIVITY AND SELECTIVITY OF MA CATALYSTS Effect of initial MA alloy structure on catalyst activity MA catalysts were studied in a model reaction of phenylacetylene hydrogenation. The charge content and the activation duration were varied in MA. Since in storage the MA alloys can relax their active state that is expressed in a lowering of their accumulated energy, studied was also the effect of MA alloy aging on the properties of catalysts. Hydrogenation was carried out in a reactor with intensive stirring in 96% ethanol at 4OoC. Catalyst activity was determined by the rate of reaction with adsorption of 25-75% of required hydrogen volume. Alloys with 25,30,35 and 40 % Ni were studied. The MA duration was 10,20,40 min. Determined was the activity of catalysts made from fresh alloys and those stored for 6 months. Fig.1 shows the dependence of catalyst activity on the factors enumerated. One must note that catalysts from MA alloys at t=40 min are more stable and 15-30% more active than PM ones (when comparison is made between catalysts of similar dispersion and in the same

595

"4

t=10 min

600

F

E \

d

-200

€

3

t=40 min

./

---

-

c

l-4

7:

t=20 min

-

40

30

20

20

40

30

20

40

30

Ni content in initial alloy, 8

Fig. 1. The relation between the catalyst activity in phenylacetylene hydrogenation and MA duration and charge composition. 0 , e - hydrogenation of >C=C< bond A,A- hydrogenation of -C=C- bond -fresh MA alloy - 6-month stored MA alloy

--

conditions).

MA

alloys

The with

Ni*A&j&atalyst To explain structure

least stable in storage were catalysts made from a

high

A1 content and t=10 min.

With all t the

was the most stable one. the observed

facts it is

both of the initial

necessary to consider the

and leached MA alloys. It was shown

previously that their phase content is determined not only charge content but by the time of MA (ref.8). activity

with

an increase

of

by

t from 10 to 20 min is due to more

complete reaction of MA and transfer of the rest of non-reacted into

leachable compound.

the

The general growth of

This is,

partly,

Ni

an explanation of the

growth of catalyst activity with an increase of t up to 40 min. But in this

case

more important

is the growth of the degree of alloy

nonequilibrium at the expense of lattice distortions.

No other fac-

tors can explain, for example, the activity growth of catalyst from MA

NiaA160 the phase

change.

content

of

which

after t=20 min does not

These defects are likely to affect the catalyst structure.

The long life-time of defects is seen from the fact that even after 6 month storage the activity of t=40 min catalysts is higher than that of catalysts made of fresh alloys with t=20 min.

596

1

100

I

I

300

I

I

500

Fig. 2. DSC-curves of MA N i X A k 1- fresh ( 4 . 4 8 kJ/mol) 2- 6-month stored (1.95 kJ/mol)

I

T,OC

The decrease of catalyst activity with alloy storage is well correlated with the results of differential scanning calorimetry. From Fig.2 it follows that the DSC-curves recorded on DSC-111 for fresh and aged alloys obey the same rules. However, in the first case the store of excess energy is much higher. Apparently, a higher activity of MA catalysts as compared to PM ones can be explained by a high degree o f disordering. With all t the catalysts from Ni35AlG5 were most active. As to PM catalysts their activity is increased up to 75% A1 (ref.9). As shown above the composition NisA165 is related by a solid solution on the NiAl-basis with a maximum possible deficit of Ni atoms. Alloys with a higher Ni content are close to equilibrium NiA1, whereas with the smaller one they have other phases (NiA13 , NizA13). Apparently, the highest activity of these catalysts is due to the leaching the largest quantity of fact that as a result of MA N&Ak most dispersive Ni is formed, which is fixed on unleached NiAl particles. The thing is that richer nickel alloys are leached to a less extent and contain less Ni skeleton phase. Whereas Ni2A13 and NiA13 leaching yields are not so fine particles of an active metal. Selectivity of MA catalysts and their adsomtion vroverties MA catalysts are more often of a higher selectivity then PM ones. In hydrogenation of phenylacetylene into styrene and of styrene into ethylbenzene,the relationship of rates varies from 1.8-2.0 for t=10 min to 2.3-3.5 for alloys with t=20 min and thus selectivity reaches 90-93%. With PM catalysts from alloys of the same dispersivity (5-8&) selectivity is 8045%. Catalyst selectivity is closely connected with adsorption properties which were studied in this work by the method of hydrogen TPD. Fig.3 presents TPD-curves of catalysts from fresh and 6-month stois the red MA alloys. The surface of a catalyst from fresh Niq~Al~jo most energy-homogeneous. It adsorbs, mainly, weakly bound hydrogen

597

139

I

300

100

11 II

I /

100

100

300

Fig. 3 . TPD-curves of MA catalysts.

--

fresh alloy 6-month stored alloy

---

that is unable to displace styrene from the surface, reason of its lowest selectivity -75%. (S=93%) is the one from fresh MA

300T,"G

which

is the

The most selective catalyst

Ni35A165 , the TPD curve of which

is moredisplaced to the high-tempereture range. that the arm in the low-temperature range

It is

belongs

of interest

only to

NiA13

catalyst, that contains nickel in a less dispersive state than that made from WiA1. Its selectivity S=87%. Fig.3 shows that storage, practically, does not change the TPDcurve of a NiqoAl60 catalyst. Its selectivity does not change either. It is of interest to compare the selectivity of catalysts MA alloys after 6-month storage in another model reaction hexenehydrogenation.

same

from

that of

Table 5 presents results for MA catalysts

for the sake of comparison for PM ones of the and of coarser

-

and

dispersiveness

dispersion. This comparison allows to find out whe-

ther the high activity of MA catalysts is due to their high dispersiveness or it is aconsequence of their microstructure.

It follows

from the table the MA catalyst activity is 2-3 times higher, which favours the second supposition. Their selectivity is also much higher than that of coarse-dispersion PM catalysts and differs but little from highly dispersed ones. The relatively low activity of the latter can be due to partial oxidation of alloys dissipated

in

an air separator. It is known that even introduction of oxide-forming additions or redox treatment (refs.lO,ll),suppressing of

the

migration

bond along the carbon chain affects but little the isomerization ability of Raney catalysts. 1n.this case the coeffi>C=C<

cient of isomerization for all samples differed little indeed.A decrease

of the migration

coefficient is

due to an increase of the

598 TABLE 5 The

activity (W ml H2/min*g) and migration (F,)

(Fc) coefficients N i content i n alloy.% 25

W 584 814 290

35

40

MA ( 5 - 8 J A ) Fm F;

bound

isomerisation

0.27

0.44

0.30

PM ( 5 - 8 f i )

W

0.63 0.65 0.67

hydrogen.

Fm

113 176

W

0.61

248

0.36

0.65

-

-

and

This is

a rise

of

verified by

TPD-curves which with PM catalysts (5-8&) range of low temperatures. Thus, a high-adsorption

potential of

their increased selectivity

in

PM ( 4 0 - 6 0 s ) Fm Fi

FL

0.25

-

surface adsorption heterogeneity strongly

and

of MA and PM catalysts in hexen-1 hydrogenation.

202 114

the

0.77 0.78

0.58

0.72 0.72 0.73

fraction of

comparison of the

have a broad arm in

the

MA catalysts influences hydrogenation of different comp-

In their turn, specific adsorption properties of M A catalysts are due to peculiarities of their structure,which can be imagined as a sinter of high-dispersion microcrystals of an active phase that are fixed on particles of an unleached initial aluminide. ounds.

REGENERATION OF SPENT CATALYSTS VIA MA The MA method can be used for making new Raney catalysts

from

spent and deactivated ones in industrial processes. Utilization and regeneration of the latter is

an important

problem in economy and

ecology which has not been settled as yet.

There are big difficul-

ties

in

associated

easy

oxidizability

the remelting of powders wich is leading

with

their

to big losses (up to 40%) of metallic

nickel (ref.12). Experiments on MA alloy preparation using

deactivated Raney Ni

and A1 powder as initial components have shown that in this case leachable alloys are formed. Table 6 presents the activity of skeleton catalysts from such alloys in hydrogenation of some of unsaturated compounds. Completely deactivated catalysts that were used for hydrogenation of some of organic compounds, were taken as Ni components in MA. A3 seen from Table 6 the regenerated catalysts are, as a rule, more active than b o t h the PM Ni-Raney and the MA ones for making of which commercial nickel was used. The nature of this effect requires further investigations; however, it is pos-

sible

to assume

that in

the

burn-out of organic residues on the

599

TABLE 6 Activity (ml H2/min-g) of Raney Nickel from MA and PM Ni35A165 in the hydrogenation of different compounds ( t=4OoC) Reaction Potassium maleate Phenylacetylene Nitrobenzene Hexen-1 ( t=20°C)

PM catalyst (5-~JA)

70 140 100 200

MA catalyst (commercial Ni) 100 480 160

aio

MA catalyst (spent Raney Ni) 160 700 220 1000

surface of catalysts they start interacting with it, modifying them in a specific way. Thus, MA can become a promising efficient method for utilization of production waste in catalytic processes and at the same time a a way to increase their activity. CONCLUSION Thus, mechanical alloying can be considered as a promising alternative for the pyrometallurgical method of producing initial alloys for Raney catalysts. The MA performance conditions made it possible to influence the structure of initial alloys and, mainly to obtain nonequilibrium solid solutions on the NiAl basis the leaching of which as compared to PM ones yields more active and selective catalysts in a number of processes. Besides, MA can be the basis of a few-operation technology for regeneration of Raney-Ni deactivated in industrial processes. The leaching of MA alloys produced from spent Raney catalysts yields catalysts that are superior in activity to those from PM and even analogous MA alloys on the basis of commercial Ni powder. ACKNOWLEDGMENTS The authors are very thankful to Dr.E.V.Leongard for carrying out experiments on the temperature-programmed desorption of hydrogen, to A.K.Dzhunusov. who took part in investigation of samples by the r.d.a. method and to Professor E.M.Moroz for useful discussion of the results of the r.d.a. experiments. REFERENCES 1

E-1-Gildebrandtand A.B.Fasman, Skeleton catalysts chemistry, Nauka. Alma-Ata, 1982 (in Russian).

in organic

E-Ivanov, T-Grigorieva, G.Golubkova, V-Boldyrev, A.B.Fasman, S.D.Mikhailenko. 0.T.Kalinina. Raney nickel catalysts from mechanical Ni-A1 alloys, Materials Letters 7(1-2) (1988) 55-56 E.Ivanov, T-Grigorieva, G.Golubkova, V-Boldyrev, A.B.Fasman, S.D.Mikhailenko, O.T.Kalinina, Synthesis of Ni aluminides by chanical alloying, Materials Letters 7(1-2) (1988) 51-54 S.D.Mikhailenko, B.F.Petrov, 0.T.Kalinina. A.B.Fasman, Nickel aluminide mechanochemical synthesis mechanism, Powder metallurgy,lO (1989) 44-48 (in Russian). K.G.Rikhter, X-ray analysis of amorphous catalysts by r.d.a. method, in: Rentgenografiya katalizatorov, Nauka, Novosibirsk, 1977,pp.5-40. E.M.Moroz, Development of X-ray methods for the investigation of fine-dispersive systems, Doctor's thesis, Novosibirsk. 1989. V.N.Kolomiichuk, On the correctness of quantitative charcteriscurves, in: tics of catalyst structure obtained from r.d.a. Rentgenografya katalizatorov, Nauka. Novosibirsk, 1977, pp.67-70 E.Yu.Ivanov. T.F.Grigorieva, G.V.Golubkova, V.V.Boldyrev,A.B.Fasman, S.D.Mikhailenko, O.T.Kalinina, Mechanochemical synthesis of nickel aluminide, Izvestiya SO AN SSSR (ser khim), 19(6) (1988) 80-83. V.I.Vorobieva, V.M.Safronov. G.A.Pushkarieva, A.B.Fasman, Phenylacetylene hydrogenations on the Raney Ni from Ni-A1 alloys of different composition and dispersion, Vestnik AN KazSSR, 4(1987) 54-58. 10 A,B.Fasman, T.A.Khodareva, S.D.Mikhailenko, E.V.Leongard, A-1-Lyashenko, The effect of preparation condition on structure All Union and properties of modified Raney Ni catalysts,Proc.II Seminar on Scientific basis of catalyst preparation, Minsk, September 25-28, 1989, Nauka, Minsk, 1989, p.295. 11 T.A.Khodareva. E.V.Leongard, S.D.Mikhailenko, Raney Ni transformation under the influence of thermal treatment in redox media, in: Science and technology problems of catalysis, Nauka. Novosibirsk,1989, p.101 (in Russian). 12 A.I.Kryagova,

A new merhod for spent Raney N i catalyst regeneration, Trudy LVMI, 5 (1956) 85-90.