propane transformation on h-gazsm-5 as a function of ga content

React. Kinet. Catal. Lett., Vol. 52, No. 2, 461-466 (1994)

RKCL2390

PROPANE TRANSFORMATION ON H-GaZSM-5 AS A FUNCTION OF

Ga CONTENT

Alfredo Vargas, Oel Guzm~n, Gerardo Ferrat and

Ma.L. Guzm~n

Instituto Mexicano del Petrdleo, ICA, PO Box 14-805,

07730 M4xico, D.F., Mexico

Received August 18, 1993 Accepted November i0, 1993

Gallosilicates with different contents of gallium have

been studied in propane aromatization reaction. Cata-

lytic evaluations were carried out at atmospheric pres-

sure, varying temperature and reactant flow. The best

results in yield to BTX products were obtained for sam-

ples with gallium contents of 3-5 wt.%.

With the catalysts used initially in the conversion of

alkanes [i, 2, 33 mainly methane and ethane were obtained, but

this again limited the selectivity for aromatic products. On

the other hand, the use of catalysts of Zn- or Ga-oxide/H-ZSM-5,

enable the formation of larger amounts of aromatics [33. Simi-

lar results are obtained using gallosilicates [4].

Under the reducing conditions of paraffin conversion, zinc

is slowly eluted from the catalyst, but gallium is stable over

many redox cycles and hence is superior to zinc-promoted cata-

lysts [5].

The relative role of gallium promoter and protons, and the

catalytic active sites have not been fully elucidated, and they

are a matter of controversy in the literature [6-10].

Akad4miai Kiad6, Budapest

VARGAS et al.: PROPANE TRANSFOPMATION

However, the effect of gallium content on the activity, se-

lectivity, and stability of gallosilicates is not completely

known in the aromatization of propane [3, 5, ii, 12]. The ob-

jective of the present work is to study the effect of gallium

content in ga!losilicates on the propane aromatization reaction,

in the presence of hydrogen or nitrogen.

EXPERIMENTAL

The preparations of H-GaZSM-5 with oxide molar ratios of

25-150, were made in hydrothermal conditions, under autogenous

pressure at 170~ over 24 h, using Ga(NO3) 3. 9H20 as metal

source, silica powder, sodium hydroxide, tetrapropylammonium

bromide as template and deionized water.

The acid form of the solids was obtained by ion change be-

tween Na-GaZSM-5 and a solution of NH4NO 3 at room temperature

to obtain NH4-GaZSM-5, which after being calcined at 500~ in

air resulted in the protonic form H-GaZSM-5.

The samples synthesized were analyzed by X-ray diffraction

with a Siemens D 500 diffractometer using Ni-filtered CuK

radiation. Micrographs of the specimens were obtained with a

JEOL JSM-85 CF scanning electron microscope equipped with Si-Li

windows. The acidity of these catalysts was determined by am-

monia adsorption and temperature programmed desorption (NH3-TPD).

The quantity of ammonia adsorbed by each catalyst was measured

at 200~ and given in meqNH3/g.

The catalytic activity tests of these catalysts took place

in a conventional isothermal fixed-bed flow microreactor at at-

mospheric pressure in the temperature range of 400-550~ and at

W/F between 1-15 g h/mol. Mixtures of propane with nitrogen or

hydrogen (volume ratio i:i) were fed. Prior to use the catalysts

were activated at 500~ in hydrogen flow for one hour.

462

VARGAS et al.: PROPANE TRANSFORMATION

RESULTS AND DISCUSSION

The X-ray diffraction patterns of the resulting gallosili-

cates show that all the samples have the structure of H-ZSM-5

zeolite type. The micrographs of the resulting solids show

that all the samples consist, of crystalline aggregates of spheri-

cal form, and at the same time these aggregates are formed by

rectangular crystals, which present well defined profiles and

vertexes.

In Table 1 we can observe the gallium content and the total

acidity for the different samples. It was found that the total

acidity is higher when the gallium content increases. This

effect was expected due to the presence of gallium in the tetra-

hedral coordination position of the zeolitic structure.

Table 1

Gallium content (wt.%) and acidity (meq NH3/g)

of the samples

Sample Gallium Acidity

A 7.31 0.6126

B 5.13 0.4484

C 3.30 0.3982

D 1.57 0.1144

Ammonia desorption is a function of the temperature (heat-

ing rate=10~ which was recarded by a TCD, depicting a

qualitative distribution profile of the acid strength, is pres-

ented in Fig. i. We can observe that these distribution seems

not to be affected by the gallium content. All the patterns are

similar because of their shape and the temperature desorption

maximum range (360-420~ expresses high acid strength. Another

peak appears as a shoulder at lower temperature (about 250~

This means that acid sites of lower strength are present, which

463


O

O C

6 &)

B C

200 300 400 500 600

T t~

Fig. i. - NH 3 - TPD for A, B. C and D samples

Could be due to the presence of gallium not incorporated into

the zeolite lattice.

Table 2

Propane conversion (mol %) over different gallosilicates:

W/F in g h/mol, K a in 10 -2 mol/g h and K d in 10 -2 min -I

units

Sample W/F K a Kd* X Paraffs. Olefs. BTX C 9

A 1 1.95 0.27 3.95 10.64 72.8 16.0 0.56 B 1 2.44 0.05 4.09 12.60 67.4 19.1 0.90 C 1.5 2.05 0.00 4.01 12.22 63.3 23.3 0.18 , D 12.0 0.31 0.Ii 4.14 21.48 60.1 17.2 0.22

Reaction time=240 min; T=550~ atm:Nitrogen, C3Hs/N2=I

464


The results f~r catalytic activity are reported in Table 2,

where it is observed that the catalysts in nitrogen atmosphere

present a direct proportion between conversion and total acid-

ity, which depends on the gallium content (Table i). It is clear

that the catalyst with high content of gallium (Sample A) shows

higher selectivity to olefins. Likewise, this solid has a smaller

selectivity to BTX products. The deactivation tests show that

this catalyst exhibits the highest deactivation degree. This

could indicate that part of this gallium content is present as

free gallium oxide, probably enabling coke formation. The sam-

ple D shows a very similar behavior to sample A, with the dis-

advantage that the former presents the lowest activity constant

and a low gallium content.

The gallosilicates B and C exhibit the lowest deactivation

constant K d and they have the highest activation degree and the

formation of the largest amounts of BTX products. Recalling that

for these gallium contents the acidities were the closest to

the theroetical values, where we suppose to find less gallium

outside the zeolitic structure.

Table 3 shows the results of activity tests for the gallo-

silicates in hydrogen and nitrogen atmospheres. The conversion

degree presents the same behavior it had with nitrogen, but

this was not the case for the selectivity to BTX products, which

decreases in H 2 atmosphere. It appears to be that the B catalyst

is the most selective in hydrogen atmosphere, probably due to

the more homogeneous distribution of its acid sites.

Table 3

Conversion of propane in H 2 and N 2 atmospheres, with the

yield and X in mol %

Sample Atmosph. X C 1 C 2 BTX

A N 2 34.23 2.90 11.60 18.4 A H 2 35.13 4.63 13.99 14.0 B N 2 29.18 3.09 8.49 15.0 B H 2 31.78 4.71 11.43 13.9 C N 2 18.29 1.79 6.15 9-2 C H 2 17.78 2.47 6~97 7.2

465


It can be observed that the BTX products decrease in H2,

and this could be due to the presence of demethylation reac-

tions or to the H 2 transfer, which inhibit aromatics formation,

as was reported previously [ii]. Nevertheless, the useful life

of the catalysts was favored when hydrogen was used as diluent.

CONCLUSIONS

The experimental results of this study demostrate that in

propane conversion at 550~ the catalytic activities of a

gallosilicate series depend on the Ga content and they can be

ordered according to their performances: B > C > A > D samples.

The best results were obtained for samples synthesized with

3-5 wt.% Ga, where we may find more gallium incorporated into

the zeolitic structure.

When catalytic aromatization was carried out using hydrogen

as diluent, the conversion degree was the same as in nitrogen,

but it was not so for the selectivity to BTX products, which

increases in nitrogen atmosphere. Nevertheless, the catalyst

useful life was improved in the presence of hydrogen, because

coke formation was inhibited.

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propane transformation on h-gazsm-5 as a function of ga content

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