Hydrogenation of nickel and vanadyl tetraphenylporphyrin in absence of a catalyst

Applied Catalysis A: General 206 (2001) 171–181

Hydrogenation of nickel and vanadyl tetraphenylporphyrin in absence of a catalyst A kinetic study R.L.C. Bonné∗ , P. van Steenderen, J.A. Moulijn1 Department of Chemical Engineering, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands Received 4 January 2000; received in revised form 18 April 2000; accepted 20 April 2000

Abstract The hydrodemetallation (HDM) reactions of nickel-5,10,15,20-tetraphenylporphyrin (Ni-TPP) and vanadyl-5,10,15,20tetraphenylporphyrin (VO-TPP) were studied in the absence of a catalyst. Ni-TPP as well as VO-TPP is found to demetallate through a reversible sequential reaction mechanism in which similar hydrogenated intermediate species are formed. As compared to catalized HDM, reaction rates are low. Hydrogenation reactions of Ni-TPP are first order with respect to the liquid phase concentration of hydrogen. Hydrogen sulfide promotes the conversion of Ni-TPP. An equilibrium constant for the reversible hydrogenation of Ni-TPP to nickel-5,10,15,20-tetraphenylchlorin (Ni-TPC) has been estimated as a function of temperature. A heat of reaction of −97 kJ/mol was found for the reversible hydrogenation of Ni-TPP to Ni-TPC. Comparison of the reactivities of a metal-free porphyrin (H2 -TPP), Ni-TPP and VO-TPP revealed that the metals have a stabilizing influence towards hydrogenation and ring fragmentation of porphyrin macrocycles. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Vanadyl tetraphenylporphyrin; Nickel tetraphenylporphyrin; Catalyst; Hydrogenation

1. Introduction Petroleum contains a large variety of metal compounds with vanadium and nickel being the major metals [1]. Some 6–34% of the total vanadium and nickel content is made up out of metalloporphyrins [2], also referred to as petroporphyrins. During refining, product streams undergo several thermal and ∗ Corresponding author. Present address: Synetix, Steintor 9, D-46446 Emmerich, Germany. 1 Present address: Delft University of Technology, Faculty of Chemical Technology and Materials Science, Julianalaan 136, 2628 BL Delft, The Netherlands.

catalytic steps in which the feedstocks are exposed to H2 and H2 S (resulting from desulphurization reactions) at elevated temperatures and pressures and hydrodemetallation (HDM) reactions may occur. At present, though several studies on catalyzed demetallation reactions have been reported, the exact nature of thermal demetallation reactions and the conditions at which they occur remains to be understood. It has been recognized that the use of model compounds in HDM is an appropriate method to acquire kinetic data which are suitable for modelling of the actual process. In model studies that have been undertaken so far, nickel- and vanadyl-porphyrins are the most frequently used model compounds. It has

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been reported that in the presence of sulfided catalysts nickel-5,10,15,20-tetraphenylporphyrin (Ni-TPP), vanadyl-5,10,15,20-tetraphenylporphyrin (VO-TPP) and analogues thereof demetallate through a reversible sequential mechanism via hydrogenated intermediates [3–9]. The reaction mechanism appears to be uniquely related to the porphyrin molecule and independent of the catalyst applied and its chemical state (i.e. oxidic, sulfided or reduced) [3–5,10–14]. Rankel [15] investigated the thermal degradation of H2 -TPP, Ni-TPP, VO-TPP, Ni-OEP and mixtures of petroporphyrins in refluxing 1-methylnaphthalene (513 K) in the presence of H2 and H2 S. In the presence of both H2 and H2 S traces of hydrogenated metalloporphyrins, porphyrin type species and polypyrrolics were found. With only H2 S present polypyrrolics were formed somewhat more rapidly while in the presence of only H2 no noticeable reaction occurred. Autoclave experiments (7 MPa H2 +H2 S) up to 623 K resulted in very low conversions. At 673 K, however, 50% of the metalloporphyrins was converted to metallochlorins. When a sulfided CoMo/Al2 O3 catalyst was added to the reaction mixture it appeared that, although the rate of reaction was enhanced significantly, the total Ni or V content of the reaction mixtures remained relatively constant. Only at elevated pressures (7 MPa) and in the presence of a sulfided CoMo/Al2 O3 catalyst, demetallation was found to occur at temperatures above 473 K. In the present study, the HDM reactions of Ni-TPP and VO-TPP at temperatures ranging from 548–613 K and at pressures of 3 and 5 MPa (H2 +H2 S) in the absence of a catalyst are reported. Issues explored are (i) elucidation of the HDM reaction mechanism of Ni-TPP and VO-TPP in the absence of a catalyst, (ii) the influence of H2 and H2 S on the various reaction steps and (iii) the influence of the presence and nature of the central metal group on the HDM kinetics.

2. Experimental 5,10,15,20-tetraphenylporphyrin (H2 -TPP), nickel5,10,15,20-tetraphenylporphyrin (Ni-TPP) and vanadyl-5,10,15,20-tetraphenylporphyrin (VO-TPP) were used as model compounds. All porphyrins mentioned

before were synthetic compounds. Toluene (Merck, p.a. grade) and o-xylene (Janssen Chimica, p.a. grade) were used as solvents. They were degassed in vacuum, prior to use. Because of the relatively low solubility at room temperature and their reactivity towards oxygen at elevated temperatures [16], the porphyrins were dissolved under argon in the refluxing solvent for 7.2 ks. The resulting porphyrin solutions contained 0.2–0.5 mol/m3 H2 -TPP, Ni-TPP or VO-TPP and were either used directly or stored under Ar. Two sets of experiments were made, i.e. comparative and kinetic experiments. Comparative experiments with H2 -TPP, Ni-TPP and VO-TPP were made in a 15×10−6 m3 stirred batch autoclave at 560 K. A solution of the model compound in toluene (approximately 8×10−6 m3 ) was loaded in the reactor which, after purging with H2 and subsequent evacuation, was pressurized to 5 MPa with H2 and heated to 560 K. After 7.2 ks the reactor was cooled to room temperature and liquid samples were analyzed using UV–VIS spectrophotometry (Philips PU 8725). For reference purposes, two experiments were carried out with a sulfided alumina supported catalyst present (Ketjen CK300 ␥-Al2 O3 , 3.8 wt.% MoO3 , 8×10−3 g catalyst). Details on the preparation and activation of this catalyst are given elsewhere [17]. Kinetic experiments were done in a 2×10−4 m3 stirred batch autoclave equipped with a sampling port from which samples were taken at regular intervals. For each experiment, approximately 100 g of porphyrin solution was loaded into the reactor. After purging with H2 or Ar and subsequent evacuation, the reactor was pressurized to 0–0.3 MPa with a gas mixture containing 15 vol.% H2 S in H2 . The H2 S/H2 mixture was used as received. The reactor was further pressurized to 3 MPa with H2 . Subsequently, the autoclave was closed and heated to the desired reaction temperature while stirring its contents. Liquid phase concentrations of H2 and H2 S and the liquid density at reaction conditions were calculated using the Peng–Robinson equation of state. An overview of experimental conditions is given in Table 1. Liquid samples of approximately 0.5 g were taken at various reaction times and analyzed at line using a UV–VIS spectrometer. Concentrations of porphyrins and their hydrogenated intermediates were calculated

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Table 1 Overview of HDM experiments with Ni-TPP and VO-TPP Porphyrin

T (K)

[M-TPP]a (mol/m3 )

[H2 ]a (mol/m3 )

[H2 S]a (mol/m3 )

No. of samples

Ni-TPP Ni-TPP Ni-TPP Ni-TPP Ni-TPP Ni-TPP

548 573 593 593 603 613

0.52 0.23 0.24 0.24 0.16 0.12

332.0 382.5 464.5 457.1 488.1 594.2

14.3 13.4 13.2 0 12.9 13.3

16 12 15 9 7 9

VO-TPP VO-TPP

573 613

0.30 0.06

398.0 644.9

13.7 13.9

12 8

a

Concentration at the moment (t=0) the reaction temperature is reached (M=Ni, VO).

applying Beer’s law. The specific absorption of each compound was corrected for overlap with other absorption bands in the spectra. Characteristic absorption maxima of the porphyrins and their hydrogenated intermediates together with the corresponding extinction coefficients are given elsewhere [3–5,18–22]. In order to check for the presence in the liquid samples of metal compounds not detected by UV–VIS spectrometry, the total nickel content of some samples was determined by graphite furnace atomic absorption spectrometry (GFAAS). Kinetic parameters were obtained by evaluation of concentration versus time curves using a non-linear regression programme (NLS). The objective function, defined as the sum of squares of deviations (residuals) between experimental and calculated concentrations, was minimized using a combination of Simplex and Levenberg–Marquardt methods. Several reaction schemes were evaluated. A fourth-order Runge–Kutta method was used to numerically solve the sets of differential equations for each kinetic model. During heating of the reactor some conversion of the reactants already occurred. Therefore, reaction time was set to zero at the moment the desired reaction temperature was reached. Concentrations of reactants and intermediates in the sample taken at t=0 were used as the initial concentrations in the kinetic analysis of the experiments. Activation energies and pre-exponential factors of the HDM reactions of Ni-TPP were determined by simultaneously evaluating data sets obtained at the various reaction temperatures. Kittrell’s reparameterization method [23] was used to decouple pre-exponential factors and activation energies.

3. Results A plot of conversions of Ni-TPP, VO-TPP and H2 -TPP after 7.2 ks at 560 K (comparative experiments) is given in Fig. 1. The conversion of both Ni-TPP and VO-TPP appears to be relatively low (4 and 10%, respectively). The metal-free porphyrin is much more reactive under the current conditions: no absorption maxima were present in the VIS spectrum suggesting a complete destruction of the porphyrin macrocycle. The catalytic conversions of Ni-TPP and VO-TPP under identical reaction conditions are also shown in Fig. 1. It appears that conversions of both Ni-TPP and VO-TPP are clearly much higher (46 and 60%, respectively). A typical VIS-spectrum of a sample from an experiment with Ni-TPP at 613 K is shown in Fig. 2.

Fig. 1. Comparison of reactivities of Ni-TPP, VO-TPP and H2 -TPP (560 K, 7.2 ks, [H2 ]=340 mol/m3 ). For comparison, catalytic data (sulfided Mo/Al2 O3 ) are also included.

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Fig. 2. Typical VIS-spectrum (Ni-TPP, 613 K, without catalyst).

It is dominated by two major absorption bands, located at 527.6 (1) and 616.4 nm (2). Two shoulders are observed at 596.1 (3) and 650 nm (4). A broad absorption band with low intensity is seen at approximately 750 nm (5). Based on literature data [3–5,10,19,20] the absorption bands at 527.6 and 616.4 nm are assigned to Ni-TPP and its dehydrogenated form nickel5,10,15,20-tetraphenylchlorin (Ni-TPC), respectively. The shoulder at 596.1 nm is a characteristic absorption of nickel-5,10,15,20-tetraphenylisobacteriochlorin (Ni-TPiB) [3–5,10,24]. It has been reported that on reduction of Zn-TPP a hexahydrogenated species is formed with characteristic absorption bands at 592 and 643 nm, the latter being the most intensive [25]. In analogy with Zn-TPP, the absorption band observed at approximately 650 nm is tentatively assigned to the hexahydrogenated species nickel-5,10,15,20-tetraphenylhexahydroporphyrin (Ni-TPHP). The broad absorption band centered at approximately 750 nm could not be identified at present, but in analogy with VIS-spectra of hydrogenated metalporphyrins [26] it is tentatively suggested that this absorption band is due to the presence of nickel5,10,15,20-tetraphenylbacteriochlorin (Ni-TPB), the adjacently tetrahydrogenated analogue of Ni-TPiB. A plot of observed VIS absorptions as a function of reaction time of the experiment at 613 K is presented in Fig. 3. Similar observations were made at 548, 573, 593 and 603 K. At least five compounds were identified in the samples at 613 K. However, as the extinction coefficients of the compounds with absorptions at 650 and 750 nm were not known, it was not possible to calculate the

Fig. 3. Plot of observed VIS absorptions as a function of reaction time (Ni-TPP, 613 K).

absolute concentrations of all compounds. The total concentration of nickel as determined by GFAAS was in all cases in good agreement with the sum of the concentrations of Ni-TPP, Ni-TPC and Ni-TPiB as calculated from the VIS-spectra and it is therefore, concluded that their concentration was very low. Consequently, the compounds with absorption bands at 650 and 750 nm were not included in the kinetic analysis of the HDM reactions of Ni-TPP. A concentration versus reaction time plot of the experiment at 613 K is depicted in Fig. 4 (left hand side). For convenience, a plot of an experiment in the presence of a sulfided Mo/Al2 O3 catalyst under similar conditions has been included (right hand side; see also [17]). In both cases Ni-TPP is found to react via a sequential mechanism: its concentration decreases with reaction time while the concentrations of Ni-TPC and Ni-TPiB build up and then, after reaching a maximum, decrease. In analogy with earlier observations in catalyzed reactions [8,9,17,27], this suggests that also in the absence of a catalyst Ni-TPC and Ni-TPiB are intermediates in Ni-TPP HDM. A priori, the observed reactions might in fact be catalyzed by the reactor wall. In order to check for such a contribution an experiment was made at 613 K (not shown) in which the reactor wall had been covered with pyrex. It followed that, under the current reaction conditions, contribution of the reactor wall in the HDM reactions of Ni-TPP is negligible. Fig. 4 (see also Table 3) demonstrates the influence of a catalyst (in this case sulfided Mo/Al2 O3 ) on the reaction rates: with a catalyst present, the rates are much higher.

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Fig. 4. Concentration vs. reaction time plots (Ni-TPP, 613 K). Experimental values are given by markers. Calculated values (according to model 3 in Fig. 5) are represented by solid lines. Left-hand side: no catalyst present; right-hand side: in the presence of a sulfided Mo/Al2 O3 catalyst [17].

For the reaction in absence of a catalyst, several kinetic schemes were evaluated which were chiefly based on those depicted in Fig. 5. It appeared that the data were more or less equally well described by two models, i.e. models 2 and 3. They are both based on sequential reaction mechanisms in which Ni-TPP is reversibly hydrogenated to Ni-TPC. The reversibility of the second hydrogenation step, i.e. the reaction of Ni-TPC to Ni-TPiB, is less conclusive with the current data. This is probably due to the very low concentrations of Ni-TPiB, especially at the lower temperatures and at short reaction times. Also the overall relatively low reaction rates may play a role in this. It appeared that when model 3 was used, in which both hydrogenation steps are reversible reactions, the fit was satisfactory but found to converge to physically unrealistic activation energies. This was not the case when using model 2. This model is, in fact, a variation of

model 3 in which the second (reversible) hydrogenation step is lumped into one irreversible reaction. To emphasize this, the second hydrogenation step is represented by k30 in the reaction equations given below. It is suggested that further reaction of Ni-TPiB results in loss of the macrocycle’s aromaticity, ring destruction and demetallation. Formation of non-porphyrinic metal compounds may also occur. As the nature of these products is not clear at present, all possibly occurring reactions in the demetallation step have been lumped into one irreversible step with (lumped) rate constant k500 . It remains to be understood where the demetallation products of the non-catalytic HDM experiments are deposited, if at all. The proposed reaction mechanism for the noncatalyzed HDM of Ni-TPP is given in Fig. 6. From kinetic analysis it followed that the hydrogenation reactions (1 and 3) are first order in H2 concentration. Dehydrogenation was assumed to be independent of H2 . First-order hydrogen dependence was assumed for the reaction of Ni-TPiB to products. Rate expressions are given below. Activation energies and pre-exponential factors for the non-catalytic HDM reactions of Ni-TPP are given in Table 2. For comparison, rate constants in the absence and in the presence of a catalyst (sulfided Mo/Al2 O3 ) are collected in Table 3. −rNi-TPP = k1 CNi-TPP CH2 − k2 CNi-TPC

Fig. 5. Summary of kinetic models considered for the non-catalytic HDM of Ni-TPP.

(1)

−rNi-TPC = −k1 CNi-TPP CH2 + k2 CNi-TPC +k30 CNi-TPC CH2

(2)

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Fig. 6. Proposed reaction mechanism for Ni-TPP HDM in the absence of a catalyst.

Table 2 Activation energies and pre-exponential factors for non-catalytic HDM reaction of Ni-TPP (model 2)a Rate constants (m3 /mol s)

k1 k2 (1/s) k30 (m3 /mol s) k500 (m3 /mol s) a

A0

Ea (kJ/mol)

8.2×102 ±0.7

119±11 216±11 80±21 154±48

1.9×1014 ±0.6 3.6×102 ±2.9×102 2.3×107 ±4×105

Error margins are 95% confidence intervals.

−rNi-TPiB = −k30 CNi-TPC CH2 + k500 CNi-TPiB CH2

(3)

The effect of H2 S on Ni-TPP HDM was evaluated qualitatively at 593 K. A plot of the conversion of Ni-TPP as a function of reaction time in the absence of H2 S and with H2 S present is depicted in Fig. 7. Clearly, H2 S promotes the conversion of Ni-TPP. A typical VIS-spectrum obtained during an experiment with VO-TPP (573 K) is shown in Fig. 8. Three absorption maxima can be discerned, viz. 548.0 (1), 615.1 (3) and 632.7 nm (2). Based on

Fig. 7. Conversion of Ni-TPP in absence (a) and in the presence (b) of H2 S at 593 K.

Table 3 Comparison of rate constants for non-catalytic, Al2 O3 catalyzed and Mo/Al2 O3 catalyzed HDM of Ni-TPP (613 K) k1

k3

k5

Non-catalytic Al2 O3 catalyzed Mo/Al2 O3 catalyzed

5.9×10−8 6.1×10−5 7.3×10−4

5.5×10−5 7.7×10−5 1.1×10−3

1.7×10−6 5.5×10−4 1.9×10−5

Ratios Alumina/non-catalytic Mo catalyst/non-catalytic Mo catalyst/alumina

1029 12310 12

1.4 20 14

320 11 0 Fig. 8. Typical VIS-spectrum (VO-TPP, 573 K).

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177

Fig. 9. Concentration vs. reaction time plot (VO-TPP, 573 K). Experimental values are given by markers. Calculated values (model 3 in Fig. 5) are represented by solid lines. Left-hand side: no catalyst present; right-hand side: in the presence of a sulfided Mo/Al2 O3 catalyst.

literature data [21,22] they were assigned to vanadyl5,10,15,20-tetraphenylporphyrin (VO-TPP), vanadyl5,10,15,20-tetraphenylisobacteriochlorin (VO-TPiB) and vanadyl-5,10,15,20-tetraphenylchlorin (VO-TPC), respectively. A concentration versus reaction time plot of an experiment at 573 K is shown in Fig. 9. It can be seen that with decreasing concentration of VO-TPP, concentration levels of VO-TPC and VO-TPiB build up and, after reaching a maximum, slowly decline. Similar trends were observed at 613 K. From this, in analogy with Ni-TPP, it is concluded that VO-TPP also reacts via a sequential mechanism. Several kinetic models were tested with the VO-TPP experiments. Based on the current observations and in analogy with Ni-TPP, all models considered were based on sequential hydrogenation of VO-TPP to its intermediates VO-TPC and VO-TPiB. Since no other compounds were detected in the UV–VIS spectra, it

is assumed that the actual demetallation takes place after the formation of VO-TPiB. As with Ni-TPP, this demetallation may consist of a series of reactions, the nature of which remains to be understood. Therefore, the demetallation of VO-TPiB is currently lumped into one irreversible step. No data concerning the hydrogen dependence of the various steps in the HDM mechanism of VO-TPP are available at present. However, based on the molecular similarity of Ni-TPP and VO-TPP and on the fact that in both cases reaction intermediates of identical nature are formed, it is suggested that the hydrogen dependencies found with Ni-TPP are also valid in the HDM of VO-TPP. This was, in fact, nicely confirmed in separate studies of the HDM kinetics of Ni-TPP and VO-TPP over sulfided alumina supported vanadium catalysts [8,9]. A model for the non-catalytic HDM of VO-TPP is given in Fig. 10.

Fig. 10. Proposed reaction mechanism for the non-catalytic HDM of VO-TPP.

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4. Discussion Rankel [15] studied the thermal degradation of H2 -TPP, Ni-TPP and VO-TPP at 513 K in refluxing 1-methylnaphthalene in the presence of H2 and H2 S and reported that the reactivity follows the order: H2 -TPP>VO-TPP>Ni-TPP. Also in the current study this order of reactivity is found to apply. The very high conversion of the free-base porphyrin as compared to Ni-TPP and VO-TPP (Fig. 1) suggests that the central metal group stabilizes the molecule with respect to hydrogenation and ring fragmentation. This may be explained in terms of ␲–d interactions [28]: donation of d-orbital electrons from the central metal to the ␲-system of the porphyrin macrocycle contributes to the preservation of the aromatic character on hydrogenation of the porphyrin. Hydrogenation of vanadyl and nickel porphyrins takes place at the peripheral double bonds of the pyrrolic groups. Apparently, the central metal group has no significant influence on the reaction mechanism. The main difference of HDM reactivity of Ni-TPP and VO-TPP is found in the number of intermediate species and their concentration levels in the course of the reaction. Whereas with Ni-TPP at least two and possibly four intermediates are found, only two intermediate compounds are observed during the reaction of VO-TPP, viz. VO-TPC and VO-TPiB. Their concentration levels appear to be much higher than their nickel analogues. Also in catalytic HDM experiments with VO-TPP, the concentration of intermediates was in several cases higher than in comparable experiments with Ni-TPP [8,9]. This may be explained by a combination of factors: the VO2+ group has a stronger electron pulling capacity than Ni2+ , resulting in a less basic and, consequently, more readily reduced porphyrin macrocycle [14,29]; also a difference in the activation energies for the demetallation step may contribute to the fact that higher concentration levels of hydrogenated intermediates are observed with VO-TPP. Although the reactivity of metal porphyrins in the presence of a catalyst is several orders of magnitude higher (Fig. 1), it appears that the reaction mechanism is unaffected by a catalyst. This is in agreement with Wei and coworkers [6,10–14] who studied the kinetics of the catalytic HDM of several nickel porphyrins and reported that the individual demetallation networks are

unique for the type of porphyrins and independent of the type of catalysts applied. When comparing reaction rate constants for the HDM of Ni-TPP (613 K) for the non-catalytic case with those in case Al2 O3 or Mo/Al2 O3 [17] is present several interesting observations can be made (Table 3). First of all, for both the alumina support and the alumina supported molybdenum catalyst the rate constants for the first hydrogenation step are considerably higher than that for the non-catalytic case. In the case of the alumina this may be due to the presence of acid functions on the material supplying the protons necessary for the hydrogenation. In the second hydrogenation step, as expected, the rate constant for the molybdenum catalyzed situation is again higher. However, the difference is not that large for this reaction. Interestingly, the rate constant for the alumina catalyzed case is of the same order of magnitude than that in the absence of a catalyst. The (lumped) hydrogenolysis step has, as expected, a higher rate constant for the alumina and alumina supported molybdenum case than that for the non-catalytic case. From the fact that catalytic reaction rates are much higher it is concluded that radical reactions do not play a major role in the catalytic HDM of both Ni-TPP and VO-TPP. It is interesting that Ni-TPP and VO-TPP react via a similar sequential mechanism in which the initial porphyrin is reversibly hydrogenated to the corresponding metal-chlorin, which is then further hydrogenated to a tetrahydrogenated intermediate species, viz. metal-isobacteriochlorin. The metal-isobacteriochlorin is then assumed to react via a series of reactions, resulting in the loss of aromatic character of the porphyrin macrocycle and eventually in demetallation and ring fragmentation. Janssens, in a recent study employing GC-MS analysis and molecular modelling [30], has elucidated the complete sequence of reactions taking place after the second hydrogenation reaction. It appears that in the actual demetallation, the M-TPiB (M=Ni, VO) is subsequently hydrogenated on a pyrrole position to give M-5,10,15,20-tetraphenylhexahydroporphyrin. This intermediate is then hydrogenated on a meso-bridge position to an intermediate referred to as M-B. This intermediate is then converted by an acid attack reaction on which the porphyrin ring structure opens leading to either tolyl group elimination (bilane type

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porphyrin, M-Bil) or an immediate ring cleavage (forming pyrrolic compounds). Of course, the latter two reactions can occur in parallel. In the present study, the elementary reactions occurring after the second hydrogenation step have been lumped into one irreversible step for practical reasons: as the reactions occurring in the steps leading up to the actual metal removal are very fast, it is extremely difficult to identify, let alone quantify, the proposed intermediates. Kinetic evaluation of the experiments with Ni-TPP showed that the current data are best represented by the model as visualized in Fig. 6. The fact that from the current data the reversibility of the second hydrogenation step (Ni-TPC to Ni-TPiB) could not be established unambiguously may be due to the fact that, especially at low temperatures and relatively short reaction times, observed concentrations of Ni-TPiB are very low which may lead to errors in the determination of the reaction mechanism. In analogy with observations made in catalytic experiments with Ni-TPP and derivatives thereof [6,10–14,17] and in view of the similarity of the reaction mechanism in both cases, it is tentatively assumed that Ni-TPC is hydrogenated reversibly to Ni-TPiB. In separate studies on the catalyzed Ni-TPP HDM [17,27] it was found that the hydrogenation of Ni-TPC to Ni-TPiB could best be described as a reversible reaction. In view of literature reports that the reaction mechanism is uniquely related to the type of porphyrin and independent of the catalyst applied and its chemical state (i.e. oxidic, sulfided or reduced) [3–5,10–14] and based on the current observation that also in the absence of a catalyst Ni-TPP reacts through a sequential mechanism via identical reaction intermediates, In view of the reversibility of the hydrogenation of Ni-TPP to Ni-TPC and the low demetallation rates, equilibrium concentrations will be reached (see also Fig. 4). The hydrogenation–dehydrogenation equilibrium constant for the reaction of Ni-TPP to Ni-TPC was estimated from the corresponding intrinsic rate constants:   k1 CNi-TPC (4) K12 = k2 CNi-TPC CH2 eq By plotting the logarithm of the equilibrium constant

179

K12 versus the reciprocal temperature the heat of reaction as well as the entropy change was determined for the reaction Ni-TPPNi-TPC. The values for 1H and for 1S0 are −97.2 and −217.6 J/mol K, respectively. The proposed reaction mechanisms for the HDM of Ni-TPP and VO-TPP differ only in that VO-TPC is clearly reversibly hydrogenated to VO-TPiB. The reason for this is obvious: with VO-TPP the tetrahydrogenated intermediate is, because of a higher extinction coefficient and higher concentration levels, more easily detected than its nickel analogue. As a result, no lumping is required in the hydrogenation of VO-TPC to VO-TPiB. H2 S in the reaction mixture promotes the overall conversion of Ni-TPP (Fig. 7). The role of H2 S in the non-catalytic HDM of metalporphyrins has been pointed out briefly by Rankel [15] who reported that polypyrrolics are formed rapidly with H2 S present in the reaction mixture. With only H2 present in the reaction mixture formation of polypyrrolics was slow and probably the result of thermal degradation [15]. Also in a separate catalytic study [17] it was found that H2 S promotes the demetallation of Ni-TPiB. It appeared that the (lumped) demetallation step was first order in the H2 S concentration while the hydrogenation reactions appeared to be independent of H2 S. Although the true nature of this H2 S effect is not clear at present, it is common knowledge that the final product is a metal sulfide. Furthermore, it is known that sulphur can be strongly coordinated to metals like vanadium and nickel [31]. From this it is tentatively suggested that H2 S, by coordination with the metal in the porphyrin, is capable of weakening the covalent metal–nitrogen bonds in the molecule. As a result, the metal porphyrin may become more vulnerable to successive hydrogenation reactions and ring fragmentation by e.g. thermal cleavages. The fate of the central metal groups after demetallation under non-catalytic conditions remains obscure. In the presence of a catalyst they are deposited as metal sulfides on the catalytic surface. A priori, one might be tempted to suggest that in the absence of a catalyst deposition of the metals on the reactor wall could be one of the driving forces in the demetallation. As the influence of the reactor wall appeared to be insignificant under the current reaction conditions, it is tentatively concluded that this option can be dismissed. Alternatively, nickel and vanadium

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cations may remain in solution as coordination compounds. It could be argued, of course, that the metals in the porphyrin macrocycle act as catalytic centers for the hydrogenation in the absence of a catalyst in the reaction mixture. From the current observations, however, it can be concluded that it is more probable that metal ions stabilize rather than destabilize the porphyrin macrocycle. The fact that Ni-TPP and VO-TPP are more stable than their metal-free analogues supports this statement. It is, therefore, concluded that the rates presented in this paper are uncatalyzed.

and a stronger electron pulling capacity of the VO2+ group. From the fact that identical reaction intermediates are found and that even similar reaction mechanisms apply for Ni-TPP and VO-TPP with or without a catalyst present it is concluded that their HDM mechanisms are unique related to type of porphyrin. Reaction rates for Ni-TPP and VO-TPP HDM are much higher in the presence of a catalyst and thermal degradation reactions do not play a major role in the catalytic hydrotreating operations.

Acknowledgements 5. Conclusions Under the current reaction conditions, reactivity of tetraphenylporphyrins follows the order H2 -TPP > VO-TPP > Ni-TPP The central metal group has a stabilizing influence on the porphyrin macrocycle with respect to hydrogenation reactions and ring fragmentation. In the absence of a catalyst HDM of Ni-TPP proceeds through a sequential mechanism via hydrogenated intermediate compounds: nickel-5,10, 15,20-tetraphenylchlorin (Ni-TPC) and nickel-5,10, 15,20-tetraphenylisobacteriochlorin (Ni-TPiB). In this reaction mechanism, Ni-TPP is reversibly hydrogenated to Ni-TPC which is subsequently hydrogenated to Ni-TPiB. Ni-TPiB is then reacts via a series of fast reactions, resulting in demetallation and fragmentation of the porphyrin macrocycle. The hydrogenation reactions in the HDM of Ni-TPP are first order with respect to hydrogen concentration. H2 S has an accelerating effect on the overall conversion of Ni-TPP. This is probably due to a coordination of H2 S with the central metal group. As a result the covalent metal-nitrogen bonds are weakened and the porphyrin macrocycle may become more vulnerable to successive hydrogenation reactions and ring fragmentation by e.g. thermal cleavages. VO-TPP also demetallates through a reversible sequential reaction mechanism, via the intermediates VO-TPC and VO-TPiB. As compared to Ni-TPP, the concentrations of the hydrogenated intermediates are higher which is ascribed to a lower demetallation rate

The authors wish to thank Mr. S. Tajik for the design of the autoclaves. The experimental assistance of Mr. J.D. Jacobs and Mr. J.B.H. Machielse is acknowledged. The authors are indebted to Dr. F. Luck (Rhône-Poulenc) for the kind donation of the various porphyrins. The present study was supported by The Netherlands Foundation for Chemical Research (SON) with financial aid from the Netherlands Organization for the Advancement of Scientific Research (NWO) and the European Economic Community (contract number JOUF 0049).

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