Reaction Kinetics and Catalysis Letters, Vol 2, No. 4, 389-396 (1975)

INVESTIGATION OF N-HEPTANE CONVERSION OVER Pt-AL203

CATALYSTS UNDER GRADIENTLESS REFORMING CONDITIONS

V. P. Sokolov, V. I. Shport and N. M. Zaidrrmn

Imtitute of Catalysis, Novosibirsk, USSR

Received July 22, 1974

The rates of the total and individual directions of n-heptane conversion in

reforming over platinum-on-alumtua catalysts were shown to be independent

of dilution with hydrogen. They can be described by simple equations fol-

lowing from kinetic concepts in ideal a&orbed layers.

I l o x a s a H o r q T o CXOpOCTX o ~ @ r o H OTJI~bHUX Hat~paB~eHHI~ n p e m p a -

~eHHS H - r e n T a H a s UC~nOBHnX PH0Op~mHra Ha a~IoMon~iaTHHOBI~X x a T a -

J l ~ a T o p a x He ~aDHCRT OT p a S 6 U ~ e H H ~ BO~CpO3IOM H o r ~ c ~ a ~ T C g

~ P O C T ~ 4 F p a B H e H H ~ w BblTeKliJ0U~b~ HS n p e ~ r XHHeTHKH B

H ~ e a . a ~ z . ~ x a ~ c o p S H p o B a s m ~ x c . n o ~ x .

The pre~ent study makes use of the possibility of direct measurement of

the reaction rates under gradientl'r operating conditions to study the conversion

of n-hepmne in refcxming over a Pt-Al203 catalyst.

The experiments were carried out in a flow-circulation apparatus described

in P, ef. /1 /without hydrogen recirculation, at 40 atm and temperatures from 470

to 510 ~ The hy&ogen/n-heptane mole ratio was varied from 2.5 to 24. The

n-heptane dried on freshly calcined alumina contained no more than 1.2% lso-

heptanes according to chromatographic data. Electrolytic hydrogen was purified

over Ni-Cr and Pt-A1203 catalysts and dried over active alumina and zeolites.

389

SOKOLOV et al. : n-HEPTANE CONVERSION OVER l~-Al203

The catalyst samples contained no acid additives and differed in catalytic activ-

ity by a factor of about two. The catalyst particles were crushed to 0.5-i. 0 mm

grain size in order to ensure the kinetic regime of the experiments. Before the

experiments the catalysts were reduced in a hydrogen stream at 500 ~ for i0 hrs.

The chromatographic analysis of the gaseous and liquid conversion products was

made as described in ReL /i/, using an XL type chromatograph. Peak areas were

determined by means of an integrator developed by Alabuzhev/3/. The calcula-

tions were performed on a Minsk-32 computer using a special program.* The rel-

ative error of the total rate of n-heptane conversion was estimated to be 15-20%,

depending on the extent of reaction.

Among the products of n-heptane reforming over various samples of Pt-A1203

catalysts all the paraffins from C 1 to C 7 (with the exception of 2, 2-dimethylpro-

pane and 2, 2, 3-trimethylbumne), as well as toluene and benzene were isolated

and identified. The amount of two unidentified compounds did not exceed 0.1%.

No appreciable amounts of unsaturated compounds and cycloparafflns were found

among the products. On a more sensitive chromatograph, 13 more unidentified

peaks of various hydrocarbons were detected, but their total amount did not exceed

0.3% of the total amount of all hydrocarbons.

Figure I gives the yields of hydrocarbons ( s ) plotted against the total n-

heptane conversion Xto t, ranging from 15 to 80% for one of the e.~talyst samples.

As Xto t increases, the yields of all hydrocarbons increase with the exception of

the methylhexane yield, which passes through a maximum.

*The authors are grateful to V. B. Skomorokhov, N. G. Grazhdannikov and V. Ye.

Tsyganov for assistance in the programming and performing the calculations.

390

SOKOLOV c ta l . : n-HEPTANE CONVERSION OVER Pt-AI2O 3

0.4

0.2

0

0,2

0.1

0

0.04

0.02

Q ~ b , C

<,

~ f

0 20 40 60 80 Xto t ( % )

Fig. 1. Yields of products of n-heptane reforming over Pt-A1203 ( m o l / m o l of in i t ia l

n-heptane) as a function of the total n-heptane conversion Xto t. a - methane,

b - propane, c - butanes, d - ethane, e - pentanes, f - methylhexanes,

g - n - h e x a n e , h - dimethylpentanes, t r imethylhutane, ethylpentane, i - to-

luene, j - isohexanes, k - benzene

In contrast to the corresponding norrm], parafflm, the co[ltent of isocompounds

is considerably lower than the equil ibrium value, the 2 -me thy lhexane to 3 - m e t h y l -

hexane ra t io always approximates the equil ibr ium v a l u e / 4 / . The amounts of t r i -

methylbutane, e thylpentane and benezene a re smal l . Butane and propane are

present in equimolar quantities, the molar yields of m~_hane and ethane a re

higher than those of hexane and pentane, respect ively. The la t ter indicates that

hexane and pentane undergo further conversions after their formation.

The exper imenta l dam are in satisfactory agreement with those of studies con-

cemed with the mechanism of reforming reactions. I t follows from Refs. / 5 - 1 2 /

391

$OKOLOV r al. : n-HEPTANE CONVERSION OVER ~-AI203

that the isomerization of paraffins proceeds in a stepwise manner in the direction

of increasing degree of branching, the paraffins up to C 5 undergo hydrocracking.

aromatic hydrocarbons can form from all the isomers of heptane. Accerdingly

the scheme of n-heptane conversion over Pt-A1203 catalysts can be represented

as follows:

k~ ks Ks trimethylbutan heptane ~ ": methylhexanes ~ =- dimethylpentanes ~ =

k-i k-I k-a e thylpentane

(nh) ~ k (mh) ~ (dmp) ~ (ep)

pentane hexane methane aromatic

" ~ ' ~ ethane compound~ (hx) klo propane (a)

(p) ~ butane

(h)

In the scheme, k i are the rate constants for the given directions of n-heptanr

conversion.

Table 1 gives the rate equations for the practically important directions of

n-heptane conversion based on the scheme. The concentration factors of the hydro-

carbons converted are given in square brackets. The form of the concentration

factors was obtained from the data on the rate of total n-heptane conversion giv-

en in Fig. 2. At conversions from 15 to 80% and hydrogen/n-heptane mole ratios

of 5 to 18, the rate of total n-heptane conversion is described by the equation

r o = ko Pnh/~1 Pi (where k o is the observed rate constant, Pnh is the partial pres-

sure of n-heptane, ~ Pi is the total partial pressure of all hydrocarbons) and is

independent of the dilution with hydrogen. The independence of r o on dilution may

392

0

e~

!

0

!

A" II

II

I

-I-

§ i.o

It I

S O K O L O V a t a l . : n - H E P T A N E C O N V E R S I O N O V E R t ~ - A I , 2 0 8

II

+

+

~ +

!

! ~, .c~

~,~..~ ~'~ ~,~.~ ~ ~"

II 0 0 II ea , . ~ 0

~ ~

0

+ + 0

-b + ~ �9

0"~ II 0o ,, ~ ~ ~ . ~ .~

~.~ ~ ' ~

0

s 3 9 3

5OKOLOV et sl. : n-HEFrANE CONVERSION OVER Pt-AI208

0.2

0.1 7 16 6 05 17

1 2 ~ I I I I 0.2 0.4 0.6 0B

P.h

Fig. 2. Rate of total n=heptane conversion over a Pt=A1203 catalyst, ro (mol/g cat=

alyst hr), as a function of the n-heptane concentration. The numbers denote

the hydrogen/n-heptane mole ratios

be explained in the simplest way in terms of the kinetic concepts for an ideal

adsorbed layer if we assume that under reforming conditions ove~ the PZ-AI203

catalyst the amount of hydrogen adsorption is small, the adsorption of hydrocar=

bons is sufficiently high, the adsorption coefficients of all hydrocarbons are sim=

ilar, and the concentration factor [nh] = Pnh/.~ Pi characterizes the coverage

of the catalyst surface with n=heptane. Therefore, the experimental data for all

other directions of n-heptane conversion were handled proceeding from the as=

sumption that the concentration factors of all components A i are of the same form,

viz. [A t] = P i / ~ " Pi �9 The empirical equations which satisfactorily describe the

experimental data are given in the fourth column of Table 1, They are in good

agreement with the rate equations for the individual direcdom of n-heptane con-

version derived on the assumption of a steady state process.

Similar results were obtained for the reforming of n-heptane over the second

Pt-AI203 sample in the same range of conversions (15 to 80~ at hydrogen dilutions

of 2.5 to 24 and temperatures of 470, 49O and 510 ~ (Fig. 3).

394

SOKOLOV et al. : n-HEPTANE CONVEI~ION OVl~ Pt-Al203

0.6 ~" 10 L 7 o. 510"C

18 6 L~ o 24 o 490ec

02[ 7 ~ 7 " / L - 6 ~ q'4700C

0;2 0'.4 o'.6 dE P~

Fig. 3. Rate of total n-heptane conversion over a Pt-AI203 catalyst, ro (mol/g cat- alyst hr), as a function of the n-heptane concentration at different tempera-

tures. The numbers denote the hydrogen/n-heptane mole ratios

The equations found in the present study differ fundamentally from the ki-

netic equations obtained ear l iex/14-18/ for the reforming of hydrocarbons in that

here the reaction rates are independent of the dilution of the starting matexial

with hydrogen over a wide range of the mole ratios hydrogen: raw material. The

kinetics of the conversion of complex hydrocarbon mixtures over different Pt-AL203

catalysts at about 40 atm and from 470 to 510 ~ may be assumed to be based

on the same principle.

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S* 395

SOKOLOV et al... n-HEPTANE CONVERSION OVER Pt-AI203

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396