React. Kinet. Catal. Lett., Vol. 37, No. 2, 457-462 (1988)

CATALYTIC PROPERTIES OF METALLIC AND ELECTRON-DEFICIENT

PLATINUM IN REFORMING OVER Pt/AI203 CATALYSTS

A.S. Belyi, M.D. Smolikov, N.M. Ostrovskii, Yu.N. Kolomytsev

and V.K. Duplyakin

Omsk Department of the Institute of Catalysis, Omsk 644029,

USSR

Received January 15, 1988 Accepted March 16, 1988

The activity of Pt/AIgO 3 catalysts with various con-

tents of metallic (Pt ~) and electron-deficient (Pt ~)

platinum has been examined in dehydrocyclization of

n-heptane and dehydrogenation of cyclohexane. In the

former case it is proportional to the number of sur-

face Pt a atoms and in the latter case it is propor-

tional to the BET surface of metallic platinum Pt ~

Hccne~oBaHa aKTHBHOCT5 KaTanHsaTOpOB C paanHqHNM CO-

~ep~aHHeM MeTannHqecKo~ (Pt ~ H ~neKTpOHo~e~H~HTHO~

(Pt ~ HnaTHHN B peaKuH~X ~eFH~pO~HKnH3auHH H-FenTaHa

H ~eFH~pHpOBaHMS UMKHOFeKCaHa. ~OKaBaHo, qTO akTHB--

HOCTB B ~eFH~pOHHK~H3a~HH Hponop~HoHaHbHa qHcny no~

BepxHOCTH~X aTOMOB Pt~ a B ~eFH~pHpOBaHHM - HOBepx-

HOCTH MeTa~HqeCKOH H~aTHH~ Pt O.

Dehydrogenation of C6-naphthenes (DG) and dehydrocycliza-

tion of paraffins (DGC) are known to be the main sources for

the formation of aromatic hydrocarbons during reforming. In

recent years the efforts of many research workers have been

aimed at the elucidation of the nature of active sites for these

reactions.

The concepts formulated by McHenry [I] and further deve-

loped by Bursian [2] concerning the specific activity of soluble

Akaddmiai Kiadd, Budapest

BELYI et al. : REFORMING

platinum in DGC of paraffins have not been generally accepted.

The reason is the absence of correlation between the soluble

platinum and its contribution to the aromatization of hydro-

carbons [3].

The concepts concerning the existence of electron-defi-

dent platinum in reduced Pt/AI203 catalysts have been universally

adopted now, though its concrete forms and mechanism of its

formation are treated differently.

The aim of the present study was, by applying the method of

quantitative estimation for the content of metallic (Pt ~ and

nonmetallic (Pt ~ platinum [4], to examine their contribution

to the catalytic process exemplified in DG of cvclohexane and

DGC of n-heptane.

Catalysts. Samples A-I, A-2, A-3, A-4 and A-5 were prepared

through 7-A1203 (S~ T-- = 180 m2/g, V = 0.65 cm3/g, Ref f = 7 nm)

impregnation by an aqueous solution of H2PtCl 6 without a com-

petitor (A-3), in the presence of HCI (A-4), with pretreatment

of T-AI203 by a solution of HCI (A-I~ A-2, A-5) up to I wt.%

Cl content in the catalyst.

Catalysts A-I, A-2 and A-5 after their reduction by hydro-

gen at T = 823 K were dechlorinated by solutions of NH4OH

(A-ID, A-5D) and NaOH (A-2D) without contacting with air. The

residual Cl content amounted to 0.1 and 0.02 wt.%, respectivel~

Then the adsorbed alkali was neutralized by acetic acid and

washed out by water. The samples were calcined in air at 673 K

and reduced by hydrogen at 823 K. No variations in the pore voll-

ume and content of platinum under dechlorination were observed.

A-IX was obtained through treating A-ID catalyst by di-

unloroethane vapors with air at 773 K and its subsequent reduc-

tion at TH2 = 823 K. Chlorine content was as high as 1.1 wt.%.

A-IC is the catalyst A-I reacted in naphtha reforming (a

~fraction boiling at 85-180~ at T = 753 K~ P = 1.0 MPa and

V L = 2 h -I for 6 h' Coke content was 1.7 wt.%.

B-1 was obtained through the adsorption of the carbonyl

complex [Pt(CO) 2]n/Y-AI203 in acetone like in Ref. [5]. The

conditions for its activation were like for the other catalysts~

458

BELYI et al. : REFORMING

Platinum state. Dispersion degree and state of Pt were deter-

mined using the (O2-H 2) titration method like in Ref. [6]. In

the presence of chemisorbed water, Pt a is not detected [4,6].

It permits to determine quantitatively the number of surface

Pt ~ and Pt a atoms (Tables ~ and 2).

Table I:

Dehydrogenation of cyclohexane

s o Cata- D E Dpto Pts content k (s -I) tN tN

lyst (%) (%) 1018 at.Pt/g cat 553 K .molec , (mole______cc)

lat~sSl at,Pt ~ s P t ~ P t a s

S S

A-5 75 68 8.9 3.8 4.3 14.2 20.3

A-5D 46 59 7.8 no 4.1 22.3 22.3

Table 2

Dehydrocyclization of n-heptane

Cata- Pt D[ Pt s content k A (773 K) t N

lyst (wt.%) (%) 1018 at.Pt/gcat imol toluene% molec. " ~ca-~. h ' at Pt g s

pt o pt ~ s s s

A-I 0.40 88 7.4 3.4 0.075 3.72

A-ID 0.40 75 8.2 I .0 0.025 4.00

A-IX 0.40 100 9.0 3.3 0.058 2.93

A-IC 0.40 - I .7 I .8 0.038 3.52

A-2 0.59 73 9.3 4.0 0.075 3.76

A-2D 0.59 27 4.9 no 0.01 -

A-3 0.51 59 6.1 3.2 0.06 3.39

A-4 0.39 57 3.5 3.3 0.075 3.79

B-I 0.49 75 11 .3 no 0.02 -

Deh~dro@enation of cyclohexan e. Catalysts were tested in a flow

circulation reactor like in Ref. [7]. The reaction conditions

were as follows: T = 553-613 K, P = 0.1MPa, H2/C6H12 = 4.3 and

459

BELYI et al. : REFORMING

conversion degree X = 10-80 %. A measure for the activity was

the ~ate constant according to the kinetics like in Ref. [7].

It has been shown previously [4] that the content of Pt ~

in the catalyst is primarily determined by the presence of CI.

Therefore to examine Pt a activity in DG of cyclohexane, cata-

lysts A-5 and A-5D containing 0.55 wt.% Pt and 1.1 and 0.02

wt.% CI, respectively, were used.

As is seen from Table I, dechlorination leads to the dis-

appearance of Pt a, and it is this factor that accounts for the

decrease in the total dispersion degree D E. The amount of sur-

face atoms in the metallic state Pt ~ and hence the dispersion s

degree of metallic platinum Dpt o diminish only slightly.

Since the removal of Pt ~ from the sample does not lead to

any significant variations in the rate constant k and the ato-

mic activity (t~ is the reaction turnover number) calculated

per the whole of the available platinum (Pt s) rises, it is

natural to suggest that in this reaction P~ is inactive. At O o

the same time the activity of surface Pt~ atoms (tN) remains

approximately unchanged, which testifies to the decisive con-

tribution of Pt ~ to the dehydrogenation of cyclohexane.

Dehydrocyclization of n-heptane. Catalysts were tested in a

flow isothermal reactor at T = 753-803 K, P = 1.0 MPa, H~C7H16 =

= 5 and V L = 5-25 h -I like in Ref. [8]. The activity was cha-

racterized by the rate constant of heptane aromatization accor-

ding to the scheme from Ref. [8].

Experimental results represented in Table 2 indicate the

absence of correlations between the specific activity and the

total dispersion degree of platinum D E (Table 2) and also the

content of surface atoms of metal platinum. On the contrary, as

is seen from Fig. I, the activity linearly grows with increasing

number of Pt s atoms in the catalysts. s

The point Pt~ = 0 in Fig. I corresponds to the activity of

metallic platinum (A-2D, B-I).

At T = 773 K the atomic activity of Pt ~ (t~ = 3-4 molecJa~

Pt ~ s) is an order of magnitude higher than the activity of S

Pt O (t~ = 0.3 molec./at. Pt~ s), and it is this factor that

460

BELYI et al. : REFORMING

accounts for the proportional activity increase of the catalysts

k A with growing the content of Pt a.

Thus the two forms of Pt state in reduced Pt/y-AI203 cata-

lysts act as specific active sites for the two main reforming

reactions.

Catalyst activity in dehydrogenation is proportional to

the number of surface atoms of platinum in the metallic state

pt ~ . s

Electron-deficient atoms Pt~ are the active sites for the

dehydrocyclization of paraffins, and the level of catalyst ac-

tivity in this reaction depends on their content in the catalyst.

These data offer possibilities for the optimization of

reforming catalysts to increase the aromatization rate of par-

affins whose rate is the lowest among those for the formation

of aromatic hydrocarbons.

10

8

6

e, i

4

2

0

/ ! ' I I I I

0 1 2 3 4 5

[Pts ~ ] (1018at/g cot.)

Fig. I. Activity in DGC of n-heptane of catalysts with

various content of electron-deficient platinum.

Catalysts: uA-I; mA-ID; mA-IX; mA-IC; AA-2; AA-2D;

�9 &A-4; eB-1

461

BELYI et al. : REFORMING

The authors thank I,E. Smirnova and O.V. Oshchurkova for

the assistance in the preparation of catalysts and O.B. Bogc~olova

for her participation in some dehydrogenation experiments.

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