Al2O3: initial catalyst deactivation

applied catalysis A ELSEVIER

Applied Catalysis A: General, 108 (1994) 171-186

Hydrodemetallisation of nickel-5,10,15,20tetraphenylporphyrin over sulphided Mo/A1203: Initial catalyst deactivation R.L.C. Bonn6 a, p. van Steenderen, A.E. van Diepen, J.A. Moulijn .,b University of Amsterdam, Department of Chemical Engineering, Nieuwe Achtergracht 166, 1018 WV Amsterdam, Netherlands

(Received 6 July 1993, revised manuscript received 22 October 1993)

Abstract Hydrodemetallisation (HDM) of Ni-TPP was studied in a fixed-bed reactor at temperatures ranging from 550-615 K in the presence of a sulphided Mo/A1203 catalyst. The kinetic network of Ni-TPP HDM comprises hydrogenation steps and a lumped hydrogenolysis step. It appears that these two types of reactions occur on sites with different characteristics. Initial catalyst deactivation takes place in two distinct stages. The first deactivation stage is the result of nickel deposition and its end is marked by a relatively constant molar ratio of deposited nickel to molybdenum. Hydrogenolysis is affected to a larger extent by nickel deposition than hydrogenation. In the second stage deactivation is much slower. Deactivation follows first order kinetics in both stages. Finally, a quasi steady-state activity is reached. Despite a continuing deposition of nickel, the catalyst remains remarkably active. It is concluded that nickel is not deposited on active sites. A variety of adsorbed species was found to exist on the working catalyst. Kinetic analysis of data sets obtained from experiments at steadystate conditions revealed that product inhibition of the hydrogenation reactions is negligible. Key words." deactivation; hydrodemetallisation; molybdenum/alumina; nickel deposition; sulphidation

1. Introduction N i - T P P has been reported to demetallise via a sequential reaction m e c h a n i s m through the h y d r o g e n a t e d intermediate species N i - T P C and N i - T P i B ( f o r definitions see caption *Corresponding author. "Present adress: Unilever Research Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral L63 3JW, UK. hPresent adress: Delft University of Technology, Faculty of Chemical Technology and Materials Science, Julianalaan 136, 2628 BL Delft, the Netherlands. 0926-860X/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved S S D I O 9 2 6 - 8 6 0 X ( 9 3 ) E0221-W

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R.L.C. Bonng et al. /Appl. Catal. A 108 (1994) 171-186

Fig. 1). The reaction sequence was found to be uniquely related to the type of porphyrin and independent of the type of catalyst used or its chemical state [ 1-9 ]. Also in the absence of a catalyst, Ni-TPP was found to demetallise via this sequential mechanism [ 10]. The generalised reaction scheme for Ni-TPP is shown in Fig. 1. In this mechanism Ni-TPP is hydrogenated to the intermediate Ni-TPC which is subsequently hydrogenated to the tetrahydrogenated intermediate Ni-TPiB. Ni-TPiB is thought to react via a series of reactions, resulting in deposition of a nickel sulphide on the catalyst and fragmentation of the porphyrin macrocycle. For sulphided Mo/A1203 it was found [ 11 ] that both hydrogenation reactions are approximately first order with respect to the hydrogen concentration and independent of the H z S concentration, whereas hydrogenolysis appeared to be approximately second order with respect to hydrogen and first order with respect to H2S. Dehydrogenation reactions were concluded to be independent of hydrogen and H2S. The terminology for hydrodemetallisation (HDM) used in the literature is somewhat confusing. In the HDM process several reaction types are observed, viz. hydrogenations, dehydrogenations and a (lumped) hydrogenolysis which may comprise a sequence of hydrogenation reactions and even (thermally induced) cracking reactions. In fact, a similar reasoning applies to hydrodenitrogenation (HDN) and, to a lesser extent, hydrodesulphurisation (HDS) processes where also networks of various reaction types are operative, while the kinetic scheme used is an oversimplification. With respect to the reactants, conflicting results have been reported on the reaction order.

9

()

o

3 ) <

4

(}

6 NI-TPP

N~-TPC

N~-TPIB

NiS× d e p o s i t + ring fragments Fig. 1. Sequential reaction mechanism for HDM of Ni-TPP. The abbreviations Ni-TPP, Ni-TPC and Ni-TPiB refer to nickel-5,10,15,20-tetraphenylporphyrin, nickel-5,10,15,20-tetraphenylchlorin and nickel-5,10,15,20-tetraphenylisobacteriochlorin, respectively.

R.L.C. Bonn~ et al./Appl. Catal. A 108 (1994) 171-186

173

Chen and Massoth [7] studied the HDM mechanism of Ni-TPP over a sulphided C o M o / A1203 catalyst in a batch reactor and reported the HDM to be first order in Ni-TPP at temperatures above 625 K and smaller than first order below 625 K; the latter was ascribed to inhibition effects. Ware and Wei [ 3 ] studied the HDM kinetics of nickel porphyrins over an oxidic CoMo/A1203 catalyst in a fixed bed reactor at temperatures up to 623 K. They reported the reactions in the HDM network to be well represented by first order kinetics and found no evidence for the inhibition by porphyrinic compounds. Weitkamp et al. [2] studied the HDM of nickel-5,10,15,20-tetra(3-methylphenyl)porphyrin (Ni-T3MPP) over a sulphided CoMo/AI203 catalyst and reported the initial catalytic activity to decline in two distinct stages. In the first stage, a rapid deactivation was observed which was ascribed to the poisoning of active sites by deposition of nickel sulphides. The objective of the current study is to obtain insight in the global kinetics of Ni-TPP HDM and deactivation behaviour of a sulphided Mo/A1203 catalyst at relatively low levels of deposited nickel. The role of nickel deposition on the catalytic selectivity with respect to hydrogenation and hydrogenolysis reactions is discussed. Catalyst deactivation was studied at 613 K in several experiments in which the liquid flow-rate and amount of catalyst were varied over a wide range. A deactivation model is presented. The kinetics of the HDM of Ni-TPP were studied at quasi steady-state conditions.

2. Experimental Ni-TPP was used as a model compound. The solvent employed was o-xylene (Janssen Chimica, p.a. grade) which was degassed in vacuum, prior to use. Because of the poor solubility of the model compounds at room temperature and their reactivity towards oxygen at elevated temperatures [ 12], they were dissolved under argon in the refluxing solvent for 7.2 ks. The resulting porphyrin solutions were filtered and stored under argon. An alumina supported molybdenum catalyst ( 7 5 - 1 5 0 / ~ m particles) with a loading of 1.2 atoms M o / n m 2 (2.9 wt.-% Mo) was employed. The catalyst was prepared by dry impregnation of a wide pore T-A1203 support (Rh6ne-Poulenc SCM 99XL, specific surface area 156 m2/g, pore volume 1.2.10 - 6 m3/g, mean pore radius 13 nm) with an aqueous solution of (NH4) 6M07024 "4H20 (Merck, p.a. grade). After impregnation the catalyst was dried overnight at 373 K and subsequently calcined at 823 K (7.2 ks). Prior to the activity measurements, the catalyst was sulphided in situ in a 15 vol.-% H2S in hydrogen gas mixture (total flow-rate: 13.67/~mol/s) according to the following procedure: - - Purging with Ar at 293 K to remove air. - - Isothermal sulphiding at 293 K in HzS/H2 for 1.8 ks. - - Temperature programmed heating (0.167 K / s ) in HzS/H 2 to 650 K. - - Isothermal sulphiding at 650 K in H2S/H2 for 7.2 ks. - - Purging with Ar at 650 K for 1.8 ks. - - Cooling down to 293 K in Ar. HDM experiments were performed in a tubular reactor (SS-316 tube with gasket seal weld-fittings, 1.8.10 -3 m I.D., 0.14 m long). The catalyst bed was fixed in the reactor

174

R.L.C. Bonn~ et al./Appl. Catal. A 108 (1994) 171-186

between layers of silicon carbide. Preliminary experiments revealed that silicon carbide and the alumina carrier had no catalytic activity under the reaction conditions applied. The flow reactor setup consisted of a feed storage tank ( 1.10-3 m 3 stirred autoclave, Hofer, DR-2), an HPLC pump (Waters, 510), the reactor, a high pressure UV/VIS flowcell and a gas-liquid separator which also served as a back pressure vessel. The liquid feed was saturated with hydrogen and H2S in the feed storage autoclave at 293 K. The reactor was operated isothermally in upflow mode. On-line analysis of the reactor effluent was performed with a scanning UV/VIS spectrophotometer (Philips PU 8725). The UV/VIS detector was connected to a personal computer which controlled the spectrophotometer and stored the data. Fig. 2 shows a scheme of the flow equipment. A detailed description is given elsewhere [ 13 ]. The liquid-phase concentrations of hydrogen and HzS and the liquid density at reaction conditions were calculated with the Peng-Robinson equation of state. Flow-rates and feed concentrations of Ni-TPP were corrected for the liquid density at reaction conditions. An overview of the experimental conditions is presented in Table 1. In order to check for the presence of nickel compounds in the reactor effluent not detected by UV/VIS spectrometry, the total metal content of several samples was determined by graphite furnace atomic absorption spectrometry (GFAAS, Perkin-Elmer HGA 500/560). The amount of nickel deposited on the catalysts after long-duration runs was determined with X-ray fluorescence (Philips 1450, AHP X-ray spectrometer). Catalyst samples were fixed on indium foil wafers and the nickel content was determined by normalising the nickel signal to that of aluminium. Kinetic models were tested by evaluation of concentration versus space time data with a

i i i

/

© _1

Ar <

gas-liquid separator

feed autoclave

I I

reactor

I-ll L

_

j

Fig. 2. Schematicof the flowreactor set-up.

175

R,L.C. Bonnd et al. /Appl. Catal. A 108 (1994) 171-186

Table 1 Experimentalconditions Catalyst weight Reaction temperature Liquid flow-rate (293 K) Feed pressure (293 K) System pressure (293 K) H2 concentration(293 K) H2S concentration(293 K) Ni-TPP feed concentration(293 K)

0.027-0.108 g (8.2-32.9/~mol Mo) 548-613 K 1.7.10-9-41.7 • 10-9 m3/s 5.5 MPa 7.0 MPa 111.6 m o l / m 3

4.1 mol/m3 0.037-0.173 mol/m3

a

b

0.10

1O0

A

Ni T P P

a c 0

f

~

80

a

60

~

40

c 0

2O

o

0.05

NI T P C

f f NI-TPtB

o (J

o~

~ ~7~ x i~- Tpp

©

0.00 10

0 t~me

on

20 stream

30 (ks)

0

10 t~me

on

30

20 stream

(ks)

Fig. 3. Typical deactivationbehaviourof sulphided Mo/A1203 (613 K, 8.3- 10-9 m 3 / s , 0.108 g catalyst). (a) Concentrationplot; (b) conversionplot. non-linear regression programme (NLS). The objective function was minimised using a combination of the Simplex and Levenberg-Marquardt methods. A fourth order R u n g e Kutta routine was used to solve the sets of coupled differential equations in each kinetic model numerically. Measurements revealed that the current experimental results are not obscured by mass or heat transfer limitations or back-mixing phenomena.

3. R e s u l t s

3.1. Catalyst deactivation

A plot of concentration versus time-on-stream (613 K) at the beginning of a flow experiment is given in Fig. 3a. Fig. 3b shows the corresponding conversion of Ni-TPP (XNi-TPp) and the HDM conversion (XnDM). The conversion of Ni-TPP is defined as: Co,Ni_TPp - - CNi_TPp XNi_TP p - -

(

1)

CO,Ni-TPP

Only three porphyrin compounds were detected in the effluent, viz. Ni-TPP and its

R.L.C. Bonn(" et al./AppL Catal. A 108 (1994) 171-186

176

hydrogenated forms Ni-TPC and Ni-TPiB. Determination of the total nickel content of the effluent revealed that the total nickel concentration is at all times in good agreement with the sum of the concentrations of Ni-TPP, Ni-TPC and Ni-TPiB, within limits of experimental accuracy ( < 5%). Therefore, the amount of nickel deposited on the catalyst can be calculated directly from the HDM conversion XHDMdefined as: - - ECt ECn

(2)

XHDM = E C °

where ~Cn represents the sum of concentrations of all detected porphyrinic compounds in the feed (i.e. Ni-TPP, Ni-TPC and Ni-TPiB) and EC, the sum of the concentrations of all porphyrinic compounds at time-on-stream t. The initial activity of the catalyst declines in two distinct stages. In the first stage deactivation is fast, whereas a slow deactivation is observed in the second stage (Fig. 4a). From the experimental data the amount of nickel deposited can be calculated. In Fig. 4b these data are plotted normalised as Ni/Mo. The rapid deactivation in the first stage coincides with a high rate of nickel deposition. The duration of the first deactivation stage depends on the molar feed rate, the amount of active phase present and the conversion. Quite interestingly, at the end of this stage the amount of nickel deposited per mol molybdenum is essentially independent of the above variables, i.e. 0.06-0.1 mol Ni / mol Mo (Table 2). A lower nickel deposition rate is observed in the second stage. After the period of initial deactivation a quasi steady-state activity is reached where the molar ratios CN~-TPc/CNi-TPPand CNi_TPiB/fNi TPC as a function of time-on-stream become constant (Fig. 5). This confirms the reversibility of the hydrogenation reactions of Ni-TPP and Ni-TPC to Ni-TPC and Ni-TPiB, respectively. Although the initial nickel deposition strongly affected the catalytic activity, deposition of a large amount of nickel after long duration runs ( molar ratios Ni/Mo up to 3.8) resulted only in a limited activity decline. Two plots of the Ni-TPP and HDM conversion for long duration runs are given in Fig. 6.

0.4

10 ~

100

X....

80

!

Nl~. v~

8

o

0.3

E

10 5 60

0.2 o

40

1 0 -6

0 o

~-

:z Z

20

X~c~ 1 Q-7

0

0

10

20

time on stream (ks)

0.1

2 0.0

U

~o

3O

time

2'o

on stream

30

(ks)

Fig. 4. (a) Relation between deactivation and amount of nickel deposited on the catalyst (613 K, 8.3- 10 9 m3/ s, 0.108 g catalyst). (b) Molar ratio of deposited nickel to molybdenum. The end of the first deactivation stage is marked by the dotted line.

t78

R.L.C. Bonn~ et al. /Appl. Catal. A 108 (1994) 171-186

3.2. Effect of reaction temperature, feed concentration and liquid flow-rate at steady-state conditions

A plot of concentration levels of the various porphyrinic compounds as a function of reaction temperature is depicted in Fig. 7a; the corresponding conversions are shown in Fig. 7b. Reaction temperatures were varied at random. Ni-TPiB is only detected at the highest temperature (613 K). At lower temperatures its concentration is below the detection limit. The effect of the Ni-TPP feed concentration on the conversion at different reaction temperatures is illustrated in Fig. 8. a 593

0.10

K

613 I

K

548

b K

I

573

K

t

593 I

K

100

613

K

548

I

K

I

573

K

I

XN, T~

_s

NJ-TPP

S

0.05 "

t 50

NJ TPIB N~ TPC

o

o o

t

0

0

t-T' 60

70

~

0 80

90

1O 0

60

70

ttme on stream (ks)

BO

90

1O 0

t~rne on stream (ks)

Fig. 7. Concentration plot (a) and conversion plot (b) as a function of reaction temperature (8.3.10 0.108 g catalyst, Co,Yi_TPp= 0.103 mol/m3).

9 m3/s,

100 A

o~ 80 c o co

613 593

C

o

o

K

6O 40

573

K

K

~

~

y,q

EL

II

2O

Z

0 C 0 , N , _ T p p (mol/m s)

*

m

~

0.037

~

0.052

~

0.103

~

0.153

Fig. 8. Conversion of Ni-TPP as a function of initial concentration and reaction temperature (8.3.10 0.108 g catalyst).

-9

m3/s,

R.LC

B o n n g et a l . / A p p l .

CataL A 108 ( 1 9 9 4 ) 1 7 1 - 1 8 6

179

Table 3 Pseudo first order rate constants and apparent activation energy for the reaction of Ni-TPP to products

Temperature (K) 548 573 593 613

k.*pp (m 3 mol i s I)

Eapp ( kJ / mol)

90 _+2

(4.7 +0.2) • 10 s ( 1.06_+0.04) • 1O-4 (2.15_+0.09)-10 4 (3.73_+0.10)'10 4

Clearly, the conversion of Ni-TPP is only slightly influenced by its feed concentration and the conversion data are satisfactorily represented by pseudo first order kinetics: - - F Ni_TP P - - k app CNi-TPP

(3)

On insertion of the rate equation into that of a fixed bed reactor and subsequent integration, the following design equation is obtained: k* (

Wcat

)

app~ ] k" 0,Ni-TPP )

-- ln( 1 --XNi-TPP) C0.Ni-TPP

(4)

where ka*p is the apparent rate constant per unit active phase (m 3 mol-~ s - ~), Wcat is the amount of molybdenum in the catalyst bed (mol), F0.Ni_TPP represents the molar feed rate (mol s -~) of Ni-TPP, XN~-Tpp is the conversion of Ni-TPP and C0,Ni_TPP is the initial concentration of Ni-TPP (mol m - 3). Values for k*pp were determined at different temperatures from experiments in which both the initial concentration of Ni-TPP and the liquid flow-rate were varied over a wide range. The pseudo first-order rate constants and the corresponding apparent activation energy for the reaction of Ni-TPP to products are given in Table 3.

4. Discussion

4.1. Catalyst deactivation Catalyst deactivation takes place in two stages. A first stage of fast deactivation concurs with a high rate of nickel deposition (Fig. 4) and its duration appears to be related to the deposition ratio Ni/Mo. In the second stage the rate of deactivation is low, although a significant amount of nickel is deposited. Also Weitkamp et al. [2], in their study on the HDM of Ni-T3MPP over sulphided CoMo/A1203, observed two similar stages of catalyst deactivation. At the end of the first, a constant value of nickel deposited ( 1.8 wt.-% Ni, N i / M o = 0.3) was observed. To verify the possible role of deactivation by coking, the carbon content was determined by temperature-programmed oxidation (TPO) [ 14 ] for a catalyst which had been subjected to a long-duration experiment. The carbon content was below the limit of detection (less than 600/xmol C / g catalyst), although a significant amount of nickel was deposited on this

180

R.L.C. Bonng et al. /Appl. Catal. A 108 (1994) 171-186

catalyst ( N i / M o = 3 . 9 ) . Hence, it is concluded that catalyst deactivation is essentially caused by nickel deposition. The HDM of Ni-TPP can be represented satisfactorily by first order kinetics: Ni-TPP ~ products - rNi_TP p = kap p CNi_TP P

(5)

By introducing an empirical parameter a, the fractional activity at time t, the rate expression is often expressed as: --FtNi-TPP ----kapp CNi-Tppa w i t h : a

r -- f Ni_TPp

(6)

-- rNi_TPp

where - rNi TPP is the rate of reaction on a fresh catalyst and - r~qi_TP p is the reaction rate on a partially deactivated catalyst (a < 1 ). Since the initial d~activation is caused by nickel deposition and Ni-TPP can be regarded as a precursor of it, the deactivation will be a function of the rate at which Ni-TPP is converted. This is confirmed by the constant molar ratio of nickel to molybdenum at the end of the first deactivation stage. For first order concentration-dependent deactivation the following correlation has been proposed for a [ 15 ] : da - - ~ = kd CO.Ni_TPp a

(7)

where ka is the intrinsic deactivation rate constant. On integration and rearrangement, the following expression is obtained: a = a o e - koC,,,N,-T,'Pt

(8)

and: = kapp C N i _ T p p a o e _ r Ni_Tpp !

kdCO.Ni-TPPt

(9)

For a fixed-bed reactor (assuming plug-flow and no transport limitations), the following conversion-time relationship is obtained [ 16]: In

) X N i TPP

- " .....

~

k

a 7"

/ = ln(e ~pp o _ 1 ) - ka Co Ni vppt

(10)

-- XNi TPP} P l o t t i n g l n ( X N i _ T p p / 1 - - XNi_Tpp) versus time-on-stream (Fig. 9) yields straight lines with slope kdCo.Ni-Tppand intercept ln(exp(kappao~-- l )). Values for ka and kappao are given in Table 4. It is striking that the deactivation rate constants for the two stages differ significantly, thus supporting the conclusion that two different deactivation mechanisms are operative. Krishna and Kittrell [ 17] and Krishna [ 18] reported that the plots can be approximately linear even when diffusion limitations are present. In that case the slope of the lines decreases with increasing Thiele modulus ~. In the present study diffusion limitations are absent and, therefore, the deactivation rate constants are intrinsic parameters. It is interesting that during the first deactivation stage XHD M decreases faster than XNi_TP p. Since only compounds of porphyrinic nature are found in the reactor effluent, the difference

R.L. C. Bonn~ et al. / Appl. Catal. A 108 (1994) 171-186

181

1.4 ~a O_ I Z

X I

stage

I

0.9

[k O_ I

2

0.4

X C

0.1

I

0

10 time

on

20 stream

30 (ks)

Fig. 9. Conversion versus time-on-stream plot for the concentration-dependent first order deactivation by nickel deposition (613 K, 8.3.10 - 9 m 3 / s , 0.108 g catalyst). Table 4 Apparent first order reaction rate constants and deactivation rate constants for the two stages of initial deactivation (613 K)

kd(s i) k~opao(s i)

First stage deactivation

Second stage deactivation

(2.0+1.2).10 3 (4.7±2.8).10-5

(6.5+2.4).10 5 (2.9_+2.5).10 5

between XNi-Tppand XHI~r~is a measure for the amount of reaction intermediates present. The concentration of intermediates increases with increasing degree of deactivation, implying that the hydrogenolysis activity (ks in Fig. 1) is the parameter most affected. Hence, it may be concluded that hydrogenation and hydrogenolysis reactions occur on different sites. After the second stage of deactivation, the catalytic activity reaches a relatively constant level (Fig. 6). Notice, that even after the deposition of as much as 4 mol Ni/mol Mo the catalytic activity remains fairly constant at a relatively high level. Apparently nickel is not deposited on the active sites.

4.2. Steady-state activity Although the experimental data are represented satisfactorily by first order kinetics (Eqs. 3 and 4) a systematic deviation was observed in experiments in which either the feed concentration or the liquid flow-rate was varied. This deviation is illustrated for the highest reaction temperature in Figs. 10a and b. The solid lines were calculated according to firstorder kinetics, while the experimental data were fitted with a spline (dashed line). The slope of the dashed line increases with decreasing feed concentration of Ni-TPP and

182

g E

R.L. C. Bonn~ et al. / Appl. Catal. A 108 (1994) 171-186

b o E

5O o 613 K

40

!

6 •

+ 593 K 4

2O 10

"d I

0

613 K

8

~o

M I

10

u

~

73

573 K

X I

548 K

K

B

2 0

50

W~.,/Fo, ~-r~

100

(ks)

150

I

0

10

W=JFo. N,-r~

20

30

(ks)

Fig. 10. First order plots of the conversion of Ni-TPP as a function of reaction temperature (0.108 g catalyst). (a) Variation of initial concentration of Ni-TPP (8.3"10 -9 m3/s); (b) variation of liquid flow-rate ( CO.Ni-Tpp= 0.150 mol/ m3). decreases with decreasing liquid flow-rate. This implies that the conversion of Ni-TPP is lower than expected at the highest feed concentration (Fig. 10a) and the lowest liquid flowrate (Fig. 10b), respectively. These data suggest that the Ni-TPP conversion is slightly inhibited by reaction products. A similar effect was found by Broderick [ 19] in the desulphurisation of dibenzothiophene and was explained in terms of a Langmuir dependence of the reaction rate on dibenzothiophene concentration. Injection of a small amount (500/~1) of porphyrin-free feed into the liquid stream gave a remarkable effect. After the injected plug reached the reactor, simultaneously with an increased concentration of Ni-TPP, Ni-TPC and Ni-TPiB, a large number of unidentified VIS absorptions was observed in the effluent. Directly afterwards, a temporarily increased uptake of Ni-TPP from the feed was observed. Clearly, injection of a plug of reactant-free feed into the reactor results in a change of the existing adsorption-desorption equilibria. Anyhow, this experiment shows that under quasi steady-state conditions the catalyst surface contains reactants and intermediates, some of which only exist on the catalyst surface, indicating a high reactivity. A model for competitive adsorption of Ni-TPP and its products on the catalytic sites is illustrated in Fig. 11 (A = Ni-TPP, B = Ni-TPC, C = Ni-TPiB, P = products, H = atomic hydrogen). The unidentified surface intermediates have not been included in this model. In order to assess if the HDM reactions of Ni-TPP are inhibited by competitive adsorption, two kinetic models were tested using data sets from experiments at 613 K in which amount of catalyst, initial concentration of Ni-TPP and liquid flow-rate were varied over a wide range. It was found that hydrogenation reactions are approximately first order in hydrogen concentration and independent of the HzS concentration [ 11 ]. The rate of hydrogenolysis was found to be approximately second order in hydrogen and first order in H2S. The fractional orders in hydrogen may be due to an inhibition by hydrogen. In the present study, hydrogen inhibition is small because of the relatively low concentration (approx. 75 mol H z m - 3). Therefore, it is lumped into the rate constants of the currently evaluated kinetic models.

R.L.C. Bonn~ et al./Appl. Catal. A 108 (1994) 171-186

adSOrlOt/on A

and B

surface C

P

A, t , B , 3 , C

5 p

I

2

I

4

surfac~

react/on." 1

~H 2

IT

I

183

H

I

I

oov~rage."

IX] occupied stte

/

[ ~ vacant site

[ ~ deacbvated s~te

Fig. 11. Model for the competitive adsorption of Ni-TPP and reaction products at quasi steady-state conditions. For symplicity, no distinction is made between hydrogenation and hydrogenolysis sites. The following kinetic scheme was used [11] ® = hydrogenolysis site) : H2

(P=products;

*=hydrogenation

site;

+ 2* ~ 2 H - *

H2S + ® ,---~-H 2 S - ® Ni-TPP + * ~ Ni-TPP-* N i - T P P - * + 2/4-* ~ N i - T P C - * + 2* Ni-TPC-* ~ Ni-TPC + * N i - T P C - * + 2 H - * ~ N i - T P i B - * + 2* N i - T P i B - * ~- N i - T P i B + * N i - T P i B - * + 4 H - * + H 2 S - @ --* P - * + 4 * + @ P-*~-P+*

(11)

( a ) In the first model, although the bonding of the porphyrinic ring to the catalyst surface may increase as a result of hydrogenation [ 20,21 ], the differences in adsorption characteristics of Ni-TPP, Ni-TPC and Ni-TPiB are assumed to be negligible and their adsorption

184

R.L. C Bonng et al. / Appl. Catal. A 108 (1994) 171-186

Table 5 Evaluation of the considered kinetic models (613 K, sulphided Mo/A1203) Model 1 ( inhibition model )

SSR"

1.21.10 3

1.30.10 3

(3.6_+1.7).10 4(s ]) (9.1+5.3).10 2(molm 3s-I) ( 2 . 7 + 1 . 3 ) . 1 0 3(s i) (8.7+2.6)-10 i(mol m 3 s t) (5.9_+3.3).10 ~'(m6mol -'s i) 0.6_+0.5 (m3mol i)

kl k2 k3

k4 ks K~d~

Model 2 ( no or constant inhibition)

(2.3+_0.5)'10 5 (m3mol ] s - l ) (5.9_+2.4)'10 3(s i) (2.7+_1.2)'10 4(m3mol is ]) (8.4_+1.5)-10 2(s ]) ( 4 . 0 + 2 . 2 ) . 1 0 - 7 (m,~mol-3s t)

"Objective function, defined as: SSR = ~ ( C~.~xp- C,,c~k.)2.

constants are assumed to be identical (gads). Chen and Massoth [ 7] reported the Ni-TPP HDM reaction to be inhibited by porphyrins and products. At temperatures up to 625 K, the major reaction products were found to be dipyrroles. From kinetic analysis of the inhibition term they concluded that adsorption of porphyrins is approximately 7-14 times larger than that of the dipyrrolic products. Considering the molecular structures of porphyfins and dipyrroles this is reasonable. Hence, non-porphyrinic reaction products are presently left out of consideration. Rate equations for the inhibition model are given below. -- rNi_TPp --

-- rNi_TPC =

Kads (kl CNi-TPp CH 2 -- k2 CNi-TPC) " l -1- gads(CNi_TPp "Jr-CNi_TPC "t- CNi TPiB)

(12)

Kad~( -- kj CNim, p CH,_+ k2 CNi-Tr'C + k3 Chi TPC Cm - k4 CNi-TPiB CH 2) 1 + Kad~( CNi-TPp + CNi-Tr'C + CNi TPiB) (13)

-- rNi_TPiB

Kaa~( - k 3 CNi_TPC CH2 -I- k 4 CNi_TPiB ~- k5 CNi TPiB CH2 CH2S )

(14)

1 + Kad~( CNi TPP + CNi-TPC + CNi-TPiB)

(b) In the second model the same reaction network is proposed but the inhibition by porphyrinic compounds is assumed negligible or constant. Consequently, the inhibition term Kad~(CN~-Tpp+ CN~-Tr'C+ CN~-Tr'iB)is either much smaller than 1 or constant. Results of the evaluation of the above described models are given in Table 5 (error margins are 95% confidence intervals). The difference between both models is small, although slightly better results are obtained with the inhibition model. Consequently, it is concluded that product inhibition of the hydrogenation reactions is negligible.

5. Conclusions Ni-TPP demetallises via reversible sequential reactions in which the hydrogenated porphyrinic intermediates Ni-TPC and Ni-TPiB are formed.

R.L.C. Bonn~ et al. /Appl. Catal. A 108 (1994) 171-186

185

In the HDM of Ni-TPP over sulphided M o / A 1 2 0 3 , the fresh catalyst deactivates in two stages due to the deposition of nickel. The end of the first deactivation stage is marked by a constant molar ratio of deposited nickel to molybdenum. In both stages deactivation follows first order kinetics. After the two stages of initial deactivation a quasi steady-state activity is reached. In spite of a continuing nickel deposition (molar ratios of N i / M o up to 4) the catalyst remains remarkably active. Apparently, nickel is not deposited on active sites. At quasi steady-state conditions the HDM reactions follow pseudo first-order kinetics with respect to the reactant concentration at temperatures ranging from 550 to 615 K. The apparent activation energy for the reaction of Ni-TPP to its products is 90 kJ/mol. Hydrogenation reactions and hydrogenolysis occur on sites with different characteristics. The working catalyst contains a variety of adsorbed species, i.e. Ni-TPP, Ni-TPC, NiTPiB and a number of as yet unidentified highly reactive intermediates. The respective adsorption-desorption equilibria are established relatively fast. Inhibition of the hydrogenation reactions by porphyrinic compounds is negligible at the present reaction conditions.

Acknowledgement The authors are indebted to S. Tajik for the assistance in designing the flow reactor equipment and to Dr. F. Luck of Rh6ne-Poulenc for the kind donation of various porphyrins and aluminas. This study was supported by the Netherlands Foundation for Chemical Research (SON) with financial aid from the Netherlands Organisation for the Advancement of Scientific Research ( N W O ) and the European Economic Community (Jouf 0049).

Notation a ao

Co,i

Ci F0,Ni-TPP kapp k

a~p

kd gads - - FNi TPP - - FNi_TPC - - rNi_TPi B - - FNi TPP --T ! Ni-TPP

t

fractional activity at time t fractional activity at time t = 0 feed concentration of compound i (mol m - 3) concentration of compound i at reactor outlet (mol m - 3 ) molar feed rate of Ni-TPP (mol s - ] ) apparent rate constant (Eq. 5) (s ]) apparent rate constant per unit active phase (m 3 m o l - ~ s - ~) intrinsic deactivation rate constant (s ]) adsorption constant of porphyrinic species ( m 3 m o l - ~) reaction rate of Ni-TPP as defined in Eq. 12 (mol m - 3 S - l) reaction rate of Ni-TPC as defined in Eq. 13 (mol m 3 s J ) reaction rate of Ni-TPiB as defined in Eq. 14 (mol m - 3 s - a ) reaction rate of Ni-TPP per unit active phase ( mol m o l - ~ s - I ) Ni-TPP reaction rate, partially deactivated catalyst (mol m 3 s J ) time-on-stream (s)

186 "r

W~at XNi-TPP

XHt)M 13

R.L. C. Bonn( et al. / Appl. Catal. A 108 (1994) 171-186

space time (s) amount of active phase (Mo) in the reactor (mol) Ni-TPP conversion as defined in Eq. 1 HDM conversion as defined in Eq. 2 liquid volumetric flow-rate (m3/s)

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