Isomerization of n-butane over NiSO42−ZrO2

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ELSEVIER

Applied Catalysis A: General 129 (1995) 83-91

Isomerization of n-butane over Ni/SO2--ZrO2 J.C. Yori, J.M. Parera * lnstituto de lnvestigaciones en CatMisis y Petroquimica, lNCAPE-Santiago del Estero 2654, 3000 Santa Fe, Argentina

Received 3 January 1995; revised 27 March 1995; accepted 27 March 1995

Abstract n-Butane isomerization was studied on nickel supported on SO 2 -ZrO2 and compared with SO] -ZrO2 and Pt/SO 2 -ZrO2. Reaction conditions were 300°C, W H S V = 1 h ~, atmospheric pressure and several H J n - C 4 ratios. Metallic phases (Ni or Pt) do not show typical properties of supported metallic functions. The metal limits its role to the 'activation' of hydrogen. The activated hydrogen inhibits the formation of a carbonaceous deposit which affects the strong acid sites and is the cause of SO 2 -ZrO2 deactivation. Nickel has less capacity than platinum to activate hydrogen and requires greater metal loads and higher partial pressures of hydrogen in order to give stability to S O ] - - Z r O 2 for n-butane isomerization. Keywords: Butane; Isomerization; Nickel supported on sulfated zirconia; Zirconia

1. Introduction The incorporation of the metallic function onto SO42--ZrO2 produces a beneficial effect upon its catalytic performance for the isomerization of n-butane (n-Ca) [ 1 ]. Regarding the effect of platinum on Pt/SO 2 -ZrO2 two important observations were made [2]: (a) In contrast with its behaviour when supported on most other supports, platinum retains an oxidation state after a reduction treatment with hydrogen at temperatures lower than 400°C [ 3 ]. The presence of the metal, possibly in oxidized state after a reduction treatment at 300°C, is necessary to stabilize the SO]--ZrO2 catalyst for the reaction performed in the presence of hydrogen. The main role of platinum is limited to the production of 'activated' hydrogen, which inhibits the formation of coke precursors. (b) The reduction step, previous to the reaction, if accomplished at temperatures over 300°C produces a decrease in the * Corresponding author. Tel. (+54-42) 33858, fax. (+54-42) 31068 and (+54-42) 550944. (1926-860X/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI O 9 2 6 - 8 6 0 X ( 9 5 ) 0 0 0 8 6 - 0

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J. Yori, J. Parera / Applied Catalysis A: General 129 (1995) 83-91

activity of the catalysts due to a reduction in the oxidation state of S in the surface complex responsible for the superacidic properties. The isomerization activity is accordingly depleted. At reduction temperatures higher than 400°C practically all the sulfur is lost, producing a drastic deactivation. It seems evident that platinum supported over SO] -ZrO2 does not have the properties which distinguish it in many supported catalytic systems, i.e. hydrodehydrogenating capacity, H2/D 2 exchange and chemisorption of hydrogen. This effect can be attributed to a strong interaction with the support. The objective of this paper is to analyze if the effect produced by platinum is also present when other metals are deposited over SO 2 --ZrO2. Nickel will be used in this case.

2. Experimental 2.1. Catalysts SO 2 -Zr02

The first step in the preparation was the precipitation of Zr(OH)4 by hydrolysis of zirconium oxychloride (ZrOC12" 8 H 2 0 ) with an aqueous solution of ammonium hydroxide. Zr(OH)4 was dried at l l0°C and a solution of 0.5 M H2SO4 was percolated through it, as described in ref. [4] ; SO 2 - - Z r ( O H ) 4 was dried overnight at 110°C and the subsequent 3 h calcination in air in the reactor at 620°C yielded the final catalyst, SO4--ZrO2 (solid A).

Ni/SO 2 -Zr02 Ni/SO4 -ZrO2 was obtained by impregnation of SO4 - Z r ( O H ) 4 with a solution of NiSO4" 7H20. The volume used for the impregnation was 10% in excess over the pore volume (0.4 ml/g). The concentration of nickel in the impregnating solution was adjusted in order to obtain 0.1, 0.5 and 5% Ni on the catalyst (solids B, C and D). The N i / s o Z - - Z r ( O H ) 4 was then dried overnight at ll0°C and calcined in air in the reactor at 620°C for 3 h. In the case of the 5% Ni catalyst a variation in the preparation was also tried: the Zr( OH)4 was impregnated with a solution of NiSO 4 • 7H20 without performing the sulfation with H 2 S O 4 ( s o l i d E). Ni/Zr02 Ni/ZrO2 was obtained by impregnation of Zr(OH)4 with a solution of (HCOO)zNi. The volume used for the impregnation was 10% in excess over the pore volume. The concentration of nickel in the impregnating solution was adjusted in order to obtain 5% Ni on ZrO2 (solid F). Ni/Zr(OH)4 was dried overnight at 110°C and calcined in air in the reactor at 620°C for 3 h.

J. Yori, J. Parera / Applied Catalysis A." General 129 (1995) 83-91

85

Pt/SO 2 -Zr02 Pt/SO4 -ZrO2 with 0.5% Pt was obtained according to [2] (solid G). 2.2. Materials The zirconium oxychloride was Merck pro-analysi. NiSOa-7H20, HeSO 4 and N H 4 O H were Carlo Erba RPE. The gases were normal butane (99.5%) from

Matheson and pure air and hydrogen provided by AGA.

2.3. TPO and TPR analysis The diagram of the TPO and TPR units and the experimental methods are described in ref. [5].

2.4. Hydrogen adsorption and specific su.rface area Hydrogen adsorption isotherms were used to calculate nickel and platinum metallic dispersions. The samples were vacuum degassed at 300°C for 2 h. Then, at the same temperature, they were subjected to several cycles of evacuation and backfilling with hydrogen to atmospheric pressure. Finally, the isotherms of total and reversible hydrogen adsorption were measured at room temperature. For platinum the amount of chemisorbed hydrogen was obtained by subtracting the two isotherms. Nickel adsorbs hydrogen more weakly than platinum and therefore, only total hydrogen chemisorption was counted [6]. Metal dispersion was calculated assuming dissociative adsorption of hydrogen on both nickel and platinum atoms. For the measurement of specific surface areas the catalyst samples were degassed at 200°C for 2 h, and then the nitrogen adsorption isotherms were determined at liquid nitrogen temperature. Micromeritics 2100 E equipment was used for the determinations of hydrogen and nitrogen isotherms.

2.5. Catalytic test The n - C 4 isomerization was performed at atmospheric pressure using a fixed bed quartz reactor, operated under isothermal conditions, and heated by a controlled temperature electrical oven. The reactor exhaust passed through a sampling valve connected to a gas chromatograph. As quoted in Section 2.1, the catalyst samples were calcined in the reactor at 620°C before each run. Then, they were treated in a hydrogen stream at 300°C for 1 h and tested for catalytic activity. Both total conversion and selectivity to i-C4 were measured. The conditions of the catalytic test were: temperature=300°C, WHSV (referred to n-C4)= 1 h ~, total pressure = 1 kg cm 2, several H2/n-C4 ratios were used and the mass of the catalysts was 0.5 g.

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J. Yori. J. Parera /Applied Catal.wsis A: General 129 (1995) 83-91

The reaction products were analyzed by on-line gas chromatography with a FID detector and a 6 m long 1/8 in. diameter column packed with 25% dimethylsulfolane on chromosorb P. From this analysis, total conversion (fraction of n-C4 fed that was transformed into products) and selectivity (ratio of conversion to each product and total conversion) were calculated.

3. Results and discussion The properties of the samples calcined at 620°C are shown in Table 1. it can be observed that the addition of a metallic phase to SO4 -ZrO2 does not significantly modify its surface area. NiSO4-7H20 alone (solid E) and H2SO4 give catalysts with similar final sulfate concentrations if enough sulfate is used and they are calcined at the same temperature. The high SO42- concentration of solid D is due to the contribution of SO4 by both H2SO4 and NiSO4,the 3 h calcination was not enough to eliminate excess SO4 . None of the samples containing sulfur is able to chemisorb hydrogen at room temperature. This indicates the presence of an effect of the sulfate ion on the metallic phase. Solid F is the only one that chemisorbs hydrogen but it has very low adsorption. This fact should not be attributed to the method of preparation, as Martin et al. [7] postulated that when nickel is supported its total reduction is very difficult. Our TPR results confirm this behaviour because nickel reduction on Ni/ ZrO2 begins at 300°C and finishes at about 650°C. In the presence of S O ] - , hydrogen consumption begins at a higher temperature, 400°C. In Fig. 1 the values of total conversion of n-C4 and selectivity to i-C4 are displayed for some of the samples as a function of time-on-stream. To avoid blurring, values for solid D were not plotted; they practically coincide with those corresponding to solid E. In every case, a typical deactivation behaviour can be seen: conversion drops steadily with a concurrent increase in selectivity to 1-C4. Starting from the same initial conversion and selectivity values, the different samples lose their activity with a rate that depends on their nickel content. The greater the nickel load Table 1 Catalyst properties Sample

SO4 ( wt.-% )

Metal ( wt.-% )

Metallic dispersion ( 100 H / P t )

Specific surface area (m2/g)

A, SO42 -ZrO2 B, Ni/SO4 -ZrO2 C, Ni/SO~ -ZrO2 D, Ni/SO4 -ZrO2 E, Ni/SO4 - Z r O : F, N i / Z r O 2 G, Pt/SO] - Z r O 2

4.5 5.0 5.5 8.0 6.0 5.0

0.1 0.5 5.0 5.0 5.0 0.5

a " " " a 10 ~

110 115 119 110 110 30 100

a No hydrogen chemisorption at room temperature.

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Fig. 1. Total conversion of n-C4 and selectivity to i-C4 as a function of time-on-stream. Letters A, B, C and E over curves designate the catalysts whose properties are shown in Table 1. WHSV = 1 h ~, H~/n-C4 = 6, total pressure=lkgcm 2 T=300°C. on SO42--ZrQ,

the lower the observed activity drop, and the greater the level of

c a t a l y t i c a c t i v i t y at t h e e n d o f t h e run. From what has been said above, the following observations can be made: (a) The incorporation of nickel on SO 2--ZrO2

produces a beneficial effect since nickel

reduces the rate of deactivation. (b) For the values of partial pressure of hydrogen used (Hz/n-C4=6)

h i g h n i c k e l l o a d s ( > 5 % ) are n e c e s s a r y in o r d e r to k e e p t h e

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TIME, h Fig. 2. Total conversion of n-C~ and selectivity to i-C4 as a function of time-on-stream. Catalyst: 5.0% Ni/SO]-ZrOz (catalyst E). WHSV = 1 h-t, T= 300°C and variable H2/n-C4: ( • ) 10; ( • ) 15; ( • ) 20.

J. Yori, J. Parera / Applied Catalysis A: General 129 (1995) 83 91

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TIME, h Fig. 3. Tolal conversion of n-C4 and selectivity to i-C 4 as a function of time-on-stream. Letters A, C and G over curves designate the catalysts whose properties are shown in Table 1. W H S V = 1 h ~, H2/n-C4 = 6, T = 300°C.

catalyst performance stable. (c) When the precursor was NiSO4.7H20, the sulfation step with 0.5 M H2SO 4 was not necessary. In Fig. 2 the values of total conversion of n-C4 and selectivity to i-C4 are plotted against time-on-stream for the solid E with different Hz/n-C4 molar ratios ( 10, 15 and 20). If we compare the initial conversion values, it can be seen that the use of greater hydrogen partial pressures produces an inhibiting effect on the reaction. The increase in hydrogen pressure increases the catalyst stability and does not produce any change in selectivity. A H2/n-C4 molar ratio equal to 20 is necessary to stabilize the 5%Ni/SO42 -ZrO2 catalyst. SO 2 -ZrO2 without nickel is not stable at this high H2/n-C 4 ratio. Fig. 3 shows the conversion and selectivity of two samples with identical load (0.5%) and different metals, nickel (catalyst C) and platinum (catalyst G). The results corresponding to the unloaded support, SO4 -ZrO2 (catalyst A), are also included. It can be seen that for the reaction conditions tested (H2/n-C4=6) Pt/SO42 -ZrO2 does not suffer deactivation. Ni/SO4 -ZrO2 and SO4 -ZrO2 deactivate at different rates, with a concurrent increase in selectivity. The incorporation of nickel to SO42 -ZrO2 reduces the deactivation rate. The total conversion of n-C4 at 5 min time-on-stream (first effluent analysis) is the same with the three catalysts, the presence of the metals affects the catalyst stability. It seems [8,9] that the capacity of a superacid material to generate 'activated' hydrogen is a necessary condition to achieve stability. This 'activated' hydrogen modifies the reaction intermediates improving the selectivity to i-C4. In this way, the incorporation of nickel or platinum to SO2--ZrO2 under a certain partial

J. Yori, J. Parera / Applied Catalysis A: General 129 (19951 83 91

89

Table 2 Selectivity to products of the three catalysts of Fig. 3 at 5 rain (0.08 h) and 5 h time-on-stream Time (h)

Selectivity (%) SO4 -ZrO2 (Cat. A)

(I.08 5

Ni/SO4 -ZrO 2 (Cat. C)

Pt/SO4 -ZrO2 (Cat. G)

CI

C2

C~

i-C4

Cs

CI

C2

C~

J-C4

C5

CI

C2

C~

i-C4

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(/.1 1.1

0.6 1.2

14.0 6.6

75.0 85.9

1(/.3 5.2

0.3 0.5

1.5 1.7

13.6 6.5

75.4 86.0

9.2 5.3

0.5 0.5

3.4 3.6

6.6 6.(1

85.0 85.2

4.5 4.7

pressure of hydrogen 'activates' enough hydrogen so as to let SO]--ZrO2 maintain its activity for a long time. Nickel, whatever the form in which it may be present, possesses a lower capacity than platinum to 'activate' hydrogen, since it requires higher loads and hydrogen partial pressures. The latter is a great drawback if nickel is to be considered for the replacement of platinum. Although it is much cheaper than platinum, the need for great volumes of gas during the reaction involves much greater additional operational costs. In Table 2 the selectivity to the different products obtained for the three catalyst samples of Fig. 3 is detailed. Values at 5 rain (first effluent analysis, catalyst with full activity) and at 5 h (catalyst practically stabilized), are included. It is observed that at 5 h the samples have practically the same distribution of products and are stable. This indicates that the 'activated' hydrogen produces the same inhibiting effect over the reactions or products responsible for the deactivation. On the other hand, at 5 min of reaction, the distribution of products is rather different. An important production of disproportionation products (C3, Cs) can be seen on SO42--ZRO2 and on Ni/SO] -ZrO2, which decreases the selectivity to i-C4. The production of these compounds drops as the catalyst loses its activity. Kikuchi and Matsuda [ 10], when working with HY zeolite, propose that the 'spillover' of hydrogen prevents the formation of coke over the acid Lewis sites by inhibiting the disproportionation reactions. In different papers it has been postulated that on supports capable of activating hydrogen, this gas produces an inhibiting effect on certain transformations of alkanes (disproportionation, cracking and coking) [ 11-13] by means of a reduction of the concentration of the intermediate carbenium ions of these reactions. The chemical 'form' of this hydrogen is still a matter of debate. Hattori [ 11 ] proposes a mechanism for Pt/SO4 -ZrO2 in which the hydrogen is dissociatively adsorbed over the metal to form atomic hydrogen. This in turn migrates by 'spillover' to the strongly acid Lewis sites of ZrO2-SO] , where it loses an electron to form a proton. This proton is stabilized over a surface oxygen atom next to the Lewis site. With this model the results presented in Fig. 3 and Table 2 can be explained, since the formation of this proton would not produce a modification in the total number of active sites, but just a weakening of the strength of the site where it was formed. As a consequence, the total activity of the catalyst is not modified, only the selec-

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J. Yori, J. Parera/Applied Catalysis A: General 129 (1995) 83-91

tivity is. This is enhanced since the isomerization is a reaction less demanding of acid strength than disproportionation and coking. When the catalyst is not capable of activating hydrogen ( SO42--ZrO2) it is initially less selective ( greater formation of coke, C3 and C5) with the same values of conversion. Coke is produced on the stronger acid sites, deactivates the catalyst and increases the selectivity by affecting more disproportionation than isomerization. The TPO results performed on samples A and C after the catalytic test indicate that the loss of activity is due to the formation of a carbonaceous deposit. In both cases, the carbonaceous deposit is small (0.39 and 0.30% respectively). The temperature values at which the deposit is burnt off (500-550°C) indicate that it is the acid function that is affected [14,15 ]. Besides, if we take into account the former discussion over the results of Fig. 3 and Table 2 it can be concluded that the carbonaceous deposit is produced over the stronger acid sites. In the working conditions solid C does not activate enough hydrogen to maintain a constant value of activity and the deactivation by the carbonaceous deposit is even greater in catalyst A, without metal. The greater the catalyst capacity to activate hydrogen, the greater its stability and selectivity. After burning of the carbonaceous deposit with air at 500°C the catalytic activity is recovered.

4. Conclusions

(a) Metallic phases deposited over SO42 -ZrO2 display a different behaviour from the one they have over other supports. The metal limits its role only to the 'activation' of hydrogen. (b) Nickel has less capacity than platinum to activate hydrogen and it requires greater metal loads and higher partial pressures of hydrogen in order to stabilize the catalyst. (c) When NiSO4" 7H20 is used as precursor and high loads are employed, it is not necessary to perform the sulfation step with 0.5 M H2SO 4. (d) The 'activated' hydrogen generated by the metallic phase inhibits the formation of coke and the disproportionation reaction. Coke affects the strong acid sites and is the cause why SO]--ZrO2 does not maintain its activity.

References [ 1] [2] [3] [4] [5] [6] [7]

J.C. Yori, M,A. D'Amato, G. Costa and J.M. Parera, Lat. Am. Appl. Res., 24 (4) (1994) in press. J.C. Yori, M,A. D'Amato, G. Costa and J.M. Parera, J. Catal., 153 (1995) in press. K. Ebitani, H. Konno, T. Tanaka and H. Hattori, J. Catal., 143 (1993) 322. J.C. Yori, J.C. Luy and J.M. Parera, Appl. Catal., 46 (1989) 103. S.C. Fung and C.A. Querini, J. Catal., 138 (1992) 240. C.H. Bartholomew and D.G. Mustard, J. Catal., 67 ( 1981 ) 186. G.A. Martin, C. Mirodatos and H. Praliaud, Appl. Catal., 1 ( 1981 ) 367.

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K. Fujimoto, Stud. Surf. Sci. Catal., 77 (1993) 9. N.S. Gnep and M. Guisnet, Appl. Catal., I ( 1981 ) 329. E. Kikuchi and T. Matsuda, Stud. Surf. Sci. Catal., 77 ( 1993 ) 53. H. Hattori, Stud. Surf. Sci. Catal., 77 (1993) 69. N.S. Gnep, M.L. Martin de Armando, C. Marcilly, B.H. Ha and M. Guisnet, Stud. Surf. Sci. Catal., 6 (1980) 79. [ 13] B.C. Gates, Catalytic Chemistry, Wiley, New York, 1992, p. 51. [ 14] J.M. Parera and J.N. Beltramini, J. Catal., 112 (1988) 357. [ 15] C.L. Pieck, E.L. Jablonski and J.M. Parera, Appl. Catal., 70 ( 1991 ) 19.