Al2O3 catalysts during propane dehydrogenation

B. Delmon and G.F. Froment (Eds.) Catalyst Deaclivation 1994 Studies in Surface Science and Catalysis, Vol. 88

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0 1994 Elsevicr Science B.V. All rights reserved.

The use of the H,-D, equilibration reaction as a probe reaction to study the deactivation on Pt/A1,0, and Pt-Sn/Al,O, catalysts during propane dehydrogenation Mikael Larssona, Bengt Anderssona, Odd Arne BariAsb and Anders Holmen' aDepartment of Chemical Reaction Engineering Chalmers University of Technology, S-412 96 Goteborg, Sweden, Fax:+46 31 772 30 35

bSINTEF,Applied Chemistry, N-7034 Trondheim, Norway 'Department of Industrial Chemistry, Norwegian Institute of Technology University of Trondheim, N-7034 Trondheim, Norway

ABSTRACT Dehydrogenation of propane was performed on platinum and platinum-tin catalysts supported on y -alumina. The dehydrogenation reaction was stopped several times during a run, the reactor was cooled to -78"C, and the H,-D, equilibration reaction was used to study the metal surface not covered by coke. The activity for the H2-D, reaction fell very fast when the dehydrogenation reaction was performed, but reached a minimum after about 10 h on stream, and was not affected by further dehydrogenation. The dehydrogenation activity, on the other hand, fell during the whole run. It was found that tin decreased the deactivation rate and increased the ability for hydrogen to remove coke from the metal. 1. INTRODUCTION

Bimetallic catalysts based on platinum and tin, supported on y-alumina have become very important commercially. Platinum-tin catalysts are widely used in the dehydrogenation of alkanes. The structure of the catalyst and the role of tin have received a lot of attention. Recently Davis [ 11 reviewed the often contradicting literature about characterization of the bimetallic system. For the dehydrogenation reactions the main purposes with adding tin to a platinum catalyst are to increase the selectivity and stability towards coke formation. In our research group we have earlier used the H2-D, equilibration reaction to study how the deactivation proceeds on a nickel catalyst used for 2-ethyl-hexenal hydrogenation [2]. In that case it was possible to study the free metal surface in-situ during the reaction. In this paper, the dehydrogenation of propane was studied at 516°C and with a catalyst that is very active for the H,-D, equilibration reaction, Because of these conditions it was not possible to use the same approach as in the former paper, without using extremely high space velocities. Here, a method will be presented, where the change in the free metal surface due to coke formation is studied by measuring the rate of the HD formation, in the H,-D, equilibration reaction, at temporary stops in the dehydrogenation reaction.

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2. EXPERIMENTAL

2.1. Preparation of catalysts The catalysts were prepared by incipient wetness impregnation from commercial y-A1 0 2. 3 support. The monometallic Pt/A1,0, and Sn/Al,03 were prepared by impregnation with aqueous solutions containing H,PtCl, and SnC1,. The catalysts were dried and calcined overnight (500°C). The bimetallic Pt-Sn/Al,O, catalyst was prepared from the Sn/Al20, catalyst by impregnation with H,PtCl, and repeating the drying and calcination procedure. Atomic Absorption Spectrometry was used to measure the metal concentrations for the samples in the range 0.05-0.14 mm, used in the experiments). By dividing the prepared catalyst into different ranges of size, and measuring the metal concentration, it was found that small particles have a higher loading than larger ones. These results may be explained by the better ability for the solutions to penetrate the small particles and adsorption on the outer parts of the particle. In all experiments, the range 0.05-0.14 mm was used (Pt-Sn: 0.74 wt% Pt and 1.53 wt% Sn; Pt: 0.85 wt% Pt). The BET surface was measured for the Pt-Sn and Pt catalyst and were found to be 172 m2/g and 162 m2/g respectively.

2.2. Catalyst characterization 2.2.1. Temperature programmed reduction (TPR) The TPR and chemisorption experiments were carried out in an apparatus described elsewhere [3] equipped with a thermal conductivity detector. The experiments were performed using a gas mixture of 7 vol% H, in Ar and a heating rate of 10"C/min. Separate TPR experiments (not shown here) indicate that the degree of reduction of tin, based on the reaction SnO, + 2H, Sn + 2H20, was about 50 % for the Pt-Sn catalyst. This indicates that most of the tin is in the Sn2+state.

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2.2.2. Hydrogen chemisorption The apparatus for hydrogen chemisorption was a conventional gas volumetric system which is described by Benson and Boudart [4]. Approximately 1 of catalyst was used, and the reduction of the samples was carried out in hydrogen, 25 cm /(min, g cat.), at 519°C for four hours before evacuation. The samples were then cooled and the adsorption experiments performed at 25°C. The amount of chemisorbed hydrogen was determined as the difference between the linear parts of the isotherms for the total and reversible adsorption, extrapolated to zero pressure. The results from the hydrogen chemisorption experiments were for WA120+ 13% dispersion and for Pt-Sn/A1,03: 29% dispersion. Recently Sgnchez et al. [5] showed that hydrogen chemisorption may give an overestimation of the dispersion of a Pt-Sn catalysts using the same procedure as here. Therefore caution should be taken in the interpretation of data based on hydrogen chemisorption (e.g. dispersion and turnover frequency).

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2.3. Catalytic activity 2.3.1. Experimental setup The apparatus used in the experiments is shown in figure 1. It is a slight modification of an apparatus designed for transient kinetic experiments [6]. The reactor was made by an Ushaped quartz tube (inner diameter 1.9 mm). In the H,-D, experiments the oven was removed and the reactor was placed in a thermos with frozen CO, mixed with acetone to obtain -78°C. An additional reactor, "the equilibrium reactor", filled with 5.6 g commercial Pt/Al,O, catalyst (EUROPT-3, CK 303), was used to achieve the equilibrium concentrations of H,, D, and HD. The GC analyses were carried out using an on-line Hewlett Packard 5880 GC equipped with a Megabore, 30 m, GS-alumina column and a flame ionization detector. H , D, and HD were measured using an on-line Balzers QMG 420 Mass Spectrometer was used. 2.3.2. Propane dehydrogenation The amounts of catalyst were about 10 mg and the pressure about 1.3 bar in all runs. The catalyst was reduced in flowing hydrogen. First during a temperature ramp from room temperature to 5 16°C.Thereafter the sample was kept at this temperature for 4 h. The reaction mixture used for the propane dehydrogenation was 20 cm3/min propane, 6 cm3/min hydrogen and 40 cm3/min nitrogen (flows at 0°C 1 atm). Three different types of runs were done: 1) A run with one H$12 experiment every hour the first few hours and at equal spaced time afterwards. 2) A run with no H$12 analyses before 120 h on stream 3) A run with many H$12 experiments during the first few hours.

sway selection valve sway selection valve

Micro needle control valve

C

Gas Chromatograph

MS MassSpectrometer PC

PressureControlier

MFC Mass Flow Controller

Figure 1 The experimental setup

Partlcie traps (2 micron)

@ MoLsieve

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Oxytrap pressure Indicator, manometer

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2.3.3. Hf12 experiments The used mixture of H,, D, and N, (16.0 cm3/min of H, and D, each and 79.2 cm3/min of N,, OT, 1 atm) was analyzed with the MS.It was possible to let the gas flow either through the reactor to the MS or directly bypassing the reactor or through the "equilibrium reactor". In this way three different gas compositions could be analyzed: 1) The gas going directly to the MS, i.e. the same as the reactor inlet, 2) The outlet of the reactor, 3) The equilibrium composition. By this method it was easy to compensate for HD in the D, feed and calculate the rate of the HD formation reaction. While bypassing the reactor with the H,-D, mixture, the reactor was flushed with nitrogen instead. Fresh catalysts were cooled in hydrogen to -78°C directly after the reduction. An H2/D2 experiment was performed followed by a rise in temperature to 516°C with hydrogen present in the feed all the time. The reaction was started by a direct switch to the reaction mixture. The H,/D, experiments, on a catalyst previously exposed to the reaction mixture, were carried out in the following way. The dehydrogenation reaction was stopped by a switch to N, from the reaction mixture. The temperature was immediately lowered to -78°C (took only a few minutes), the H2/D2 experiment was performed (a flow of H,, D, and N,), a switch back to inert gas was done, a temperature raise to 516°C (took about 15 rnin) and finally the dehydrogenation reaction was started again.

3 RESULTS AND DISCUSSION 3.1. Propane dehydrogenation The dehydrogenation reaction was run at, or close to, differential conditions. The selectivity to propene was >94 mole% for the Pt and >99.5 mole% for the Pt-Sn catalyst. Other hydrocarbons detected were methane, ethane and ethene. For the platinum catalyst the conversion to the side products was; CH,: <0.4 %, C,H& <0.3 % and C2H4: <0.1 % and for the bimetallic one; CH4: <0.04 %, C2H6: ~0.04% and C,H4: <0.004 %. Runs were also performed with the Sn/Al,03 and pure alumina catalysts. The propane conversion was <0.1 % and <0.2 % respectively. The deactivation profiles for the Pt and Pt-Sn catalysts (Figure 2) can be divided into two parts, a very fast initial deactivation, and a second region with a lower deactivation rate. The deactivation for the Pt catalyst was faster than for the bimetallic one. Two additional Y-axises are added to Figures 2 and 3 with the turnover frequency (TOF) for the propane dehydrogenation. The TOF is based on the number of hydrogen chemisorption sites on a fresh catalyst (section 3.2). If this method is used, it can be seen that TOF is higher for the monometallic than the bimetallic catalyst during the first period of the deactivation. In the second region, i.e. after about 5 hours, the TOF is about the same for both catalysts.

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3.2. H2-D2experiments The reactor was considered being a ideal plug flow reactor when the rate constant for the HD formation was calculated. Because of experimental restrictions it was difficult to determine the HD formation rate when the H,-D, reaction was close to equilibrium. This will propane conversion: 0 Pt-Sn, lype 1 - Pt-Sn, type 2

HD formation: 0 Pt-Sn, lyPe 1 0 Pt,typel

15 12

9 6 3

0 t

Time on stream, hours

Figure 2. Deactivation rofiles for experiments of type 1 and 2. Rate constants for HD formation

in moV(s, g cat., Pa)*lO- . The first GC-analyses were done 1.5 minutes after a stop and are marked with gray squares and rombs. In the end of the type 1 runs, three stops without exposure to hydrogen were performed (marked with black squares and rombs).

cause a large inaccuracy in the results from the fresh platinum catalyst. Also when the HD formation rate was low the error can be considered fairly large. The results from the H -D experiments are shown in Figures 2 and 3. In Figures 4 and 5 2 2 the propane dehydrogenation conversion just before an H,-D, experiment have been related to the HD formation rate. Experiments from all the runs were used. The TOF, based on the number of hydrogen chemisorption sites on a fresh catalyst, were calculated from the rate type 3

HD formation: 0 Pt-Sn, lype 3

04 0.

4 0

0

0

o w

0

I

0.2

0.4

0.6 0.8 Time on stream. hours

.

0

1.2

0

1.4

Figure 3. Deactivation profiles for experiments of type 3. Rate constants for HD formation in moV(s, g cat., Pa)*10b9.The first GC-analyses were done 1.5 minutes after a stop and are marked with gray squares and rombs.

.-c

-6

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1

in

0

0

B 0

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Pt, type 3

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constants, without including the reverse reaction. This will give an estimation of the hydrogen (or deuterium) adsorption rate. For the fresh catalysts the rate of the HD formation is much larger on the monometallic platinum catal st than on Pt-Sn. The TOF for the fresh Pt-Sn and Pt catalyst is estimated to 150 and 540 s- respectively.

Y

After the Pt-Sn catalyst was exposed to the reaction mixture for two minutes, the HD formation rate fell to about a third compared to the activity for the fresh catalyst. For the platinum catalyst, the activity fell to 1130th after only 15 seconds dehydrogenation reaction.

3.3. A comparison between the propane dehydrogenationand the H -D reaction 2 .2 The dehydrogenation reaction and the H2-D, reaction are proportional in activity as seen in the linear parts at higher conversions in Figures 4 and 5. During this period the coke will build up on the metal giving rise to the fast deactivation. After 10 h on stream, the activity for the HD formation was about 15% for the Pt-Sn and 0.6% for the Pt catalyst, of the original activity. The rate remained at this level even if the deactivation went on for another 110 h. These results are consistent, with those reported by Lin et al. [7] who used hydrogen chemisorption to measure the free metal area on Pt/Al,O, and Pt-Sn/A120, catalysts after deactivation. They found that the free metal surface decreased rapidly in the beginning when hydrocarbons were deposited on the catalyst, but reached a minimum level of coverage. At this level 10% and 30% of the metal remained uncovered, for the Pt and Pt-Sn catalyst

12 I

110

I

I

I

Propane conversion, %

Figure 4. Propane conversion immediately before an H -D2 experiment versus the rate constant for formation (in molls, g cat., Pa)*109, for the Pt-Sn catalyst

ID

zi

I

+., U

6

I.

n

I

Prooane conversion. %

Figure 5. hopme COnversion immediately before an H2-D2 experiment versus the rate constant for HD formation in (moVs, g cat., Pa)* for the Pt catalyst

respectively. The total amount of coke on the catalyst increased constantly caused by carbonaceous materials migrating from the metal to the support. SsUrchez et al. [ 5 ] found that hydrogen may spillover onto tin(II) sites giving an overestimation of the free platinum surface on a Pt-Sn/Al,O, catalyst. This problem, that might be of interest when the coverage of coke is measured by hydrogen chemisorption, is eliminated when the H -D reaction is used. 2 2 Two adjoined Pt atoms are necessary for hydrogen (or deuterium) to dissociate. We will expect that the H2-D, equilibration rate has a linear or second order dependence on the part of the metal uncovered by coke. A linear one if the metal is covered by coke islands, and a

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second order dependence if the coke is even distributed. The continuous decrease in the propane dehydrogenation activity, with constant I-ID formation rate can have different origins. Calculations indicate that diffusion limitations may play a role in the propane dehydrogenation, but not in the H2-D, reaction, when the amount of coke in the pores of the catalyst is high. Different structure sensitivity for the two reactions might also contribute to this effect. Somorjai [8] showed that the H,-D, reaction is structure sensitive. For the propane dehydrogenation, on the other hand, Biloen et al. [9] found that only one Pt atom is necessary for the reaction to proceed.

3.4. Influence on the deactivation profile due to the H,-D, experiments A large effect on the deactivation profile, due to the H2-D, experiments was found. During the first hours of a run, the deactivation was accelerated after each stop for an H,-D, experiment. A step down in activity directly after a stop, and then a slight increase in activity, was found. This increased deactivation is probably due to dehydrogenation of species on the surface in the absence of hydrogen. After more than 15 h on stream the conversion of propane increased immediately after each H,-D, experiment followed by a slow return to the initial deactivation profile. This activity increase was much higher for the Pt-Sn catalyst compared to the Pt catalyst. In order to investigate the nature of these phenomena, the reaction was stopped by switching to nitrogen and holding the reactor at the reaction temperature for a certain time (see the black marks at the end of the type 1 runs, Figure 2). When the Pt-Sn catalyst was exposed to the reaction mixture again, the increase in initial activity was lower but the deactivation was accelerated was considerable. For the Pt catalyst, a stop in inert or an H,-D, experiment had the same effect with only a small initial increase in the dehydrogenation conversion. The explanation could be that hydrogen (and deuterium), adsorbed at low temperature during the H -D experiment, will clean parts of the surface during the temperature rise. Tin is probably 2 .2 playing a role of making the coke precursors migrate more easily. This is consistent with experiments performed by Lieske et al. [lo] and Lin et al. [7] who found that hydrocarbons were stronger adsorbed on Pt/A1,0, than on Pt-Sn/Al,O,. 4 CONCLUSIONS

The H,-D, equilibration reaction was shown to be useful as a probe for measuring the metal area not covered by coke, on Pt/A1,0, and Pt-Sn/Al,O, catalysts deactivated during propane dehydrogenation. Problems with the method are the effect that the repeated stops have on the dehydrogenation deactivation profile, and the difficulties in correlating the HD formation rate to free metal area. The fresh Pt catalyst was found to be much more active than the bimetallic one, for the HD formation. After the catalysts were exposed to the dehydrogenation reaction mixture, the activity fell to only a fraction of the original activity. The decrease was much higher for the Pt than the Pt-Sn catalyst. A very fast initial dehydrogenation deactivation was found for both catalysts. Tin increased the stability towards coke formation and also made the coke bind less strongly to the metal.

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5 ACKNOWLEDGEMENT This work was supported by the Nordic Council, the Petroleum program. 6 REFERENCES 1. B. H. Davis, ACS Symp. Ser., 517 (Selectivity in Catalysis, 1993) 109-126 2. A. Wrammerfors and B. Andersson, J. Catal., 146 (1994) 34-39 3. E. A. Blekkan, A. Holmen and S. Vada, Acta Chemica Scandinavica 47 (1993) 275-280 4. J. E. Benson and M. Boudart, J. Catal., 4 (1965) 704-710 5 . J. Siinchez, N. Segovia, A. Moronta, A. Arteaga, G. Arteaga and E. Choren, Appl. Catal., 101 (1993) 199-206. 6. 0. A. Bariis PhD-thesis, Dept of Industrial Chemistry, NTH Trondheim, 1993 7. L. Lin, T. Zhang, J. Zang and Z. Xu, Appl. Catal., 67 (1990) 11-23 8. G. A Somorjai, J. Phys. Chem. 94 (1990) 1013-1023 9. P. Biloen, F. M. Dautzenberg and W. M. H. Sachtler, J. Catal., 50 (1977) 77-86 10. H. Lieske, A. Siirkiny and J. Volter, Appl. Catal., 30 (1987) 69-80