Degradation of a spent hydrotreating catalyst: interaction with environment

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendiomz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

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Degradation of a spent hydrotreating catalyst: interaction with environment Jiflio Carlos Afonso" , Tatiana Siqueira de Lima~ and Paula Constante Campos~ ~Departamento de Quimica Analitica - Instituto de Quimica Universidade Federal do Rio de Janeiro PO Box 68563. 21949-900, Rio de Janeiro - RJ - Brasil Samples of a spent commercial NilV[o/AI203 hydrotreating catalyst were placed in common tin boxes in the ground (30-50 cm depth). After 40-70 days, a tin box was removed in order to determine the amount and the behaviour of the organic and the inorganic species present in the catalyst. These compounds were extracted in a Soxhlet apparatus with dichloromethane, methanol and water, in this order. After 14 months, AI(III) and phosphate species became totally insoluble. Sulphur was quantitatively converted into sulphate species. Ni and Mo showed increasing solubility, reaching a quantitative level after 6 months. Iron content sharply increased due to the internal corrosion of the tin box. Water content increased progressively. After one year, the amount of soluble coke in dichloromethane was drastically reduced (from 1 wt% to about 0,04 wt%), whereas organic components soluble in methanol sharply increased. All data point to formation of oxygen compounds (mainly acids, ethers, esters and phenols) and the full consumption of almost all types of hydrocarbon structures. The oxidative degradation of coke can form polyfunctionalized organic species that are no longer soluble in dichloromethane but in more polar solvents, such as methanol. Additionally, during extraction of soluble coke a dark-brown precipitated was obtained, whose experimental data suggest that they were formed as a result of polymerisation/condensation reactions. After 6 months, some tin boxes present small holes due to corrosion and some catalyst content diffused into the ground. Diffusion of Ni, Fe, Mo and sulphate species was observed in the order Ni 2+>> SO42 ~ Fe 3§ > Mo. 1. INTRODUCTION Catalytic processing of petroleum, natural gas etc is subjected to deactivation of the catalyst by coke deposition (fouling). This implies in several regeneration steps (coke elimination by combustion) or the change of the catalyst. In the past, spent catalysts were generally discarded after their lifetime. This practice is questionable since such materials contain considerable amounts of heavy metals and carcinogenic compounds present in coke. Dumping is environmentally unacceptable. In fact, "clean catalysts means maximum metals recovery" [ 1]. Considerable amounts of spent catalysts are discarded each year (over 10.000 ton/year in the USA and Europe) and recycling level is lower than 50% [2-4]. Apparently there is no systematic study on the behaviour of the spent sample placed in environment with respect to time. Therefore, the aim of this work was to study the result of the interaction between a spent commercial catalyst and the neighbouring ground by determining the evolution with respect to time of the organic and the inorganic components present in the spent sample

2850 and the ~mmediate monitoring of spreadmg of such compounds through the ground after a possible rupture of the tin box. 2. EXPERIMENTAL 2.1 The catalyst A spent commercial deactivated NiMo/AI203 hydrotreating catalyst, whose chemical analysis data is presented in Table 1, was used in this study. It was employed in its sulfided form in hydrotreating of diesel fractions for about 3 years. It was not regenerated during its lifetime and was kept in its original form (cylinder extrudates 5mm).

Table 1: Chemical analysis data (% wt/wt) of the spent NiMo/AI203 catalyst (dry basis) Ni 2,1

Mo 9,5

Fe 0,7

AI 31,8

P 2,4

S 3,1

C 11,0

Si 2,5

2.2 Catalyst disposal It was placed in common tin boxes (catalyst content: 100g/box) that were put in the ground (30-50 cm depth) in February 1998 (it was summer in Rio de Janeiro). Samples were taken at intervals of 40-70 days and their content was determined in order to establish a relationship between catalyst degradation and time. Samples were submitted to an extraction procedure in a Soxhlet apparatus with solvents (time of extraction: 24 h per solvent) in polar rising order: dichloromethane (extraction of soluble coke), methanol (extraction of inorganic components and very polar organic compounds) and water (solubilization of inorganic compounds). Methanol was first eliminated after which the residue was dissolved in HCI 3 N and the organic compounds present were extracted with chloroform. 2.3 Analytical methods Organic compounds were characterised by means of IR, UV, elemental analysis, NMR (~H and ~3C), and CG-MS [5]. Inorganic species were characterised by means of atomic absorption, ion chromatography, X-ray fluorescence, spot-test analysis [6] and mass balance. 3. Results and discussion 3.1 General data During the first 4 months a considerable increase in the catalyst weight was observed, after which the final weight stabilised (25-30 wt% higher than the initial mass). There are three possibilities for the increasing weight of the catalyst: a) increase of water content; b) internal corrosion of tin box; c) oxidation of some components of the catalyst. The catalyst showed an increasingly wet aspect and pH showed to be very acid (1-2). Also, pH of the water extract in Soxldet apparatus was acid (2-3); both values are higher than pH of the neighbouring ground (4-5); these results can be explained if we take into account the presence of free acids (section 3.2) and transition metals in solution (Fe(III), AI (III) and Ni(II) - section 3.3) which are subjected to an extensive acid hydrolysis.

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3.2 Organic compounds Table 2 clearly shows a strong reduction in the amount of organic compounds soluble in dichloromethane. On the other hand solubility in methanol greatly increased. However, total organic soluble forms, which accounted f o r - lwt% of the spent catalyst, corresponded to only 0,2 wt% after one year. Reduction of the amount of overall soluble coke can be explained by: a) loss of volatiles by successive evaporation/condensation cycles in the internal surface of the tin box; b) insolubilization reactions (oxidation, polymerisation, condensation ete). For samples taken after 4 months, during extraction of soluble coke and inorganic components, a dark-brown precipitate was isolated. Its amount regularly increased with time. IR and NMR data showed the presence of aromatic and aliphatie structures, together with oxygen functional groups (carbonyl, hydroxyl and ether compounds). The H/C atomic ratio was about 1,2. Probably, this material was the result of polymerisation, condensation and oxidation reactions, both eatalysed by the pH of the wet catalyst (1-2) and/or the weather conditions (sunny hot days during winter/spring 1998 in Rio de Janeiro). At present, this point is under investigation. Table 2: Organic compounds soluble in CH2C12and CH3OH (base: 100g of the spent catalyst) Days 0 71 114 184 306 428

Weight (g) CH2C12 CH3OH 0,97 -4) 0,24 0,01 0,15 0,04 0,11 0,07 0,07 0,11 0,04 0,14

Wt% of carbon present in the catalyst CH2CI2 CH3OH 8,9 -4) 2,2 0,1 1,4 0,4 1,0 0,6 0,6 1,0 0,4 1,3

The H/C atomic ratio of coke soluble in CH2C12showed a constant decrease with time (Table 3). Three different reactions can account for this fact: dehydrocyclization, aromatization and oxidation. IIL, UV and NMR data suggest: a) a clear reduction of aliphatic structures in soluble coke; b) an increase in aromatic H and C atoms; c) an increase in the presence of oxygen compounds (carbonyl compounds, hydroxyl compounds and ethers). Table 3: H/C atomic ratio obtained by elemental analysis of soluble coke in CH2C12 Days 71 114 184

H/C atomic ratio Days 1,43 306 1,23 428 1,17 0 (deactivated sample)

H/C atomic ratio 1,13 1,11 1,61

The semi-quantitative composition of soluble coke in CH2CI2is presented in Table 4. From this table it is clear that hydrocarbon structures tended to disappear, whereas oxygen compounds increased fifteen fold. Soluble coke in methanol is composed only by oxygen compounds, with at least two oxygen atoms in the molecule. GC and GC-MS data show that carboxylic acids (naphtoic, phtalic, aliphatic, benzoic) are highly predominant (75 wt%), followed by ethers (15 wt%), esters (7 wt%), phenols (2 wt%) and unidentified compounds (1 wt%). This overall composition did not change very much with time.

2852 Table 4: Approximate composition of soluble coke in dichloromethane (GC and GC-MS data) Days Hydrocarbons Oxygen compounds** Days Hydrocarbons*

Oxygen compounds**

0 93,3 6,7 184 17,9 82,1 71 70,8 29,2 306 10,6 89,4 114 43,9 56,1 428 6,6 93,4 * Linear and branched alkanes, alkyl cyelohexanes/pentanes, alkyl benzenes, naphtalenes, tetralines, indanes, biphenyls, 3 and 4 tings aromatic compounds ** Phenols and naphtols, earboxylic acids (aliphatic/aromatic) and their esters, ethers (aliphatie/aromatie), alcohols and ketones. i

Depending on the type of compound, behaviour with respect to time can be quite different. In table 5 one can observe that after one year all aliphatic hydrocarbons disappeared; however, linear structures were consumed faster than branched ones. Table 5: Wt% of soluble coke (in CH2C12) corresponding to aliphatic saturated hydrocarbons Days 0 71 114

Linear alkanes 29 15 3

Branched alkanes 22 17 7

Days 184 306 428

Linear alkanes Negligible 0 0

Branched alkanes 4 1 Ne~,li~jble

Alkyl cyclohexanes, benzenes, indanes, tetralines, alkenyl benzenes, 3 and 4 rings aromatic/hydroaromatic compounds fully disappeared after 6 months without any clear preference with respect to their structure. Alkyl naphtalenes and biphenyls showed a drastic decrease (over 80 %) in soluble coke in CH2C12. On the contrary, phenols tended to increase their amount in this sample; even diphenols were detected after 6 months; also, alkyl naphtols were found after 2 months. These features point to oxidation of initial hydrocarbon structures present in the original coke. Perhaps the clearest effect of time on evolution of the organic species is the formation of a wide variety of acids as seen in Table 6. Some of them were not detected in the original soluble coke in dichloromethane. Clearly, original hydrocarbon structures were oxidised: linear/branched alkanes (aliphatic acids); alkyl/alkenyl-benzenes and cyelohexanes (benzoic acids); alkyl-naphtalenes (nafioic and phtalic acids). Table 6: Wt% of soluble coke corresponding to organic carboxylic acids Days 0 71 114 184

428

Aliphatic acids 0,5 2,5 6,5

Benzoic Acids Negligible Traces 1,5

Phtalic acids Negligible Negligible 0,5

Naphtoic acids Traces 1,5

12

3,5

5,5

4 10

14

4,5

6,5

13

2853 Formation of esters (methyl to butyl in decreasing order) of earboxylie acids was an important step during the first 4 months, after which a gradual consumption of such compounds was observed with simultaneous formation of free alcohols and ethers (aromatic, aliphatie and mixte ones). After 6 months the amount of alcohols tended to decrease whereas formation of ethers increased somewhat. Sterification and ether formation are add-base catalysed reactions. In fact, pH of the wet catalyst was acid all the time (1-2); also, the average temperature in Rio de Janeiro was higher during end winter/spring 1998 than in previous months. It is, at present, difficult to rationalise the influence of both factors but it seems dear that acidity of the catalyst itself has a great influence on the evolution of soluble coke with time.

3.3 Inorganic components General data are presented in Table 7. Table 7: Inorganic species soluble in methanol + water (wt%) Time (days)

Fe 3+

0 (deactiv~ed sample) 37 71 88 114 180" 184 234" 306 269" 428 288"

Ni 2+

Mo(VI)

66 82 90 97 96 96

82 102 97 97 97 97

Al3+

7 3 0,2 < 0,01 < 0,01 < 0.01

SO42-

PO4 3-

77 86 95 95 98 97

3 0,2 < 0,05 < 0,01 < 0,01 < 0,01

* the amount surpasses the original Fe 3§ content in the catalyst due to corrosion of tin box Behaviour of Al3+ and PO4 3" species is very similar: their solubility were already very low for the deactivate sample and rapidly became negligible after 4-6 months. It is suggested that soluble Al and P species tended to precipitate as AlPO4. Also a good parallelism between behaviour of Ni 2+, SO42 and Mo-containing species was found. Initial solubility was already considerable for the deactivated sample. It was quantitative (> 95 wt%) after 6 months. This behaviour can be understood if one take into account that the active phase of the catalyst (NiMo) was kept in sulphided state during its lifetime. After deactivation, nickel and molybdenum sulphides (NiS, Ni3S2, MoS2) were progressively oxidised. Therefore one can conclude that sulphur was quantitatively converted into sulphate species after 6 months; also, Mo was quantitatively converted from + 4 to + 6 state. These oxidising reactions help to explain the increase of the catalyst weight along the time (section 3.1). A regular increase of solubility ofNi, Mo and sulphate species in water was observed, reaching over 90 wt% after 6 months. Hydratation effects may be considered. For example, anhydrous NiSO4 is more soluble in methanol than in water, whereas hydrated forms are much more soluble in water than the corresponding anhydrous form. Iron behaviour was very different form the previous findings. The amount of soluble iron rapidly surpassed the total amount of this element in the deactivated catalyst, which clearly

2854 suggests an external source. Obviously, this source is the tin box. When it was opened, a clear corrosion of the cover and the basis was noted. Also, the lateral internal surface was extensively corroded. Incorporation of oxidised iron to the catalyst helps to explain the increase of the catalyst weight along the time. Iron was essentially soluble in water (> 95

wt%). Perhaps the most important fact was the rupture of two tin boxes, after 6 months, thus leading to a direct contact of the catalyst composite with the ground. After the spring rains in Rio de Janeiro (1998 spring has been the most rainy since 1968) during end September and October, it was estimated the diffusion of inorganic species by means of spot tests [6]. A fast Ni, Mo, Fe and sulphate species diffusion was observed in all directions, in the order Ni2+ >> SO42" ~ Fe3+ > Mo. AI, P and insoluble coke remained in the leak point. It is remarkable that Ni 2+ was found 13 cm depth whereas leak point presented only 3 mm opening. The reason to explain the fast diffusion of Ni is not clear, but it is supposed that it may form very stable and soluble complexes with humic acids. The limited diffusion of Mo can be ascribed to precipitation of molybdate-like compounds under the pH of the ground (4-5). 4. Conclusions Organic and inorganic components of a deactivated spent commercial Mo/AI203 showed a constant evolution with time, and are subjected to oxidation and insolubilization reactions. The catalyst weight increased as a result of several factors: oxidation reactions of inorganic and organic components; incorporation of oxidised iron from tin box to the catalyst; increase of water content. This work clearly shows the risk of contamination of ground, water resources and vegetable life when materials, which can liberate soluble forms of heavy metals and/or aromatic compounds, are not adequately discarded or are placed in clandestine dumps.

5. Acknowledgements We are grateful to CNPq-SR2/UF1LI for financial support. We thank Petrobrgs/CENPES for catalyst supply, FRX and AA analyses. We acknowledge LADA/DQA/IQ/UFRJ for ion chromatographic analyses. 6. References

1. Bader, N. Oil and Gas Journal, n~ especial, 18/03/96, 64-66 2. Erickson, H.; Foster, R.L. "Recovery of metals from used hydrocarbon conversion catalysts". US patent, 3.539.290, 10/11/70, App. 06/12/67, 7 pp 3. Jong, B.W.; Siemens, R.E. Recycle Second. Recovery Met., Proc. Int. Symp., 1985, 477488 (eds Taylor, P.R., Sohn, H.Y. e Jarrett, N.) Metall. Sot., Warrendale-PA/USA 4. Siemens, R.E.; Jong, B.W.; Russell, J.H. Conserv. Recyel., 1986, 9(2), 189-196 5. Afonso, J.C; Schmal, M.; Cardoso, J.N. and Frety, R. Ind. Eng. Chem. Res., 1991, 30, 2133-2137 6. Feigl, F. "Spot Tests in Inorganic Analysis". Elsevier, Amsterdam, 1958, chap. 3