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Hydrofining activity and acid–base properties of nickel–molybdenum catalysts incorporated on sodium and magnesium ions-modified alumina
Applied Catalysis A: General 168 (1998) 179±185
Hydro®ning activity and acid±base properties of nickel±molybdenum catalysts incorporated on sodium and magnesium ions-modi®ed alumina M. Lewandowskia, Z. Sarbakb,* a
Institute of Coal Chemistry, Polish Academy of Sciences, SowinÂskiego 5, 44-102 Gliwice, Poland b Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 PoznanÂ, Poland
Received 21 May 1997; received in revised form 25 November 1997; accepted 27 November 1997
Abstract In the study of a series of NiMo catalysts incorporated on alumina modi®ed with sodium and magnesium ions, decomposition of isopropyl or diacetone alcohol and ethylbenzene were used as model reactions. Hydro®ning of coal liquid was performed at the pressure of 9.0 MPa and at 4008C. Modi®cation of alumina with sodium and magnesium ions leads to a decrease in the acidity of nickel±molybdenum catalysts, and by the same token increases their basicity. Another effect of modi®cation with sodium and magnesium ions is dehydrogenation of catalysts. Introduction of sodium ions results in a distinct decrease of hydrodenitrogenation and hydrodesulfurization activity. On the other hand, magnesium ions decrease the hydrodenitrogenation activity, and insigni®cantly decrease the hydrodesulfurization activity of catalysts. # 1998 Elsevier Science B.V. Keywords: Catalyst NiMo; Modi®cation; Hydrotreating; HDS; HDN; Acidity; Coal liquid; 2-propanol; Ethylbenzene; Diacetone alcohol
1. Introduction Catalytic hydro®ning involves, in most general terms, the reactions inducing a removal of nitrogen, sulphur, oxygen heteroatoms and metals. These processes are accompanied by hydrocracking and hydrogenation. The applied catalysts of hydro®ning, among them the most often applied Ni±Mo and Co±Mo systems incorporated on alumina, do not reveal a
*Corresponding author. 0926-860X/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0926-860X(97)00354-2
suf®ciently high activity in the hydro®ning of coal liquids. The character of coal liquids-different from that of petroleum-derivative materials, higher aromaticity and higher content of sulphur, nitrogen, and oxygen atoms and a signi®cantly lower content of hydrogen atoms as well as a high molecular weight are the factors that signi®cantly contribute to a rapid deactivation of catalysts. It is known that coke (carbon deposit) is formed as a result of the reactions of carbonium ion taking place on acid sites of catalysts or through a thermal reaction on the surface of a
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catalyst evoked by a presence of free radicals [1]. Lewis acidity is related to formation of coke as a result of the reaction of Lewis center with a basic nitrogen of a heterocyclic compound [2]. BroÈnsted acid sites are also claimed responsible for the coking of catalysts [3], though there are also opinions they decrease the coking by transforming the coking precursors into volatile products [4,5]. This is why an important factor leading to a decrease in the amount of coke on the catalyst is a change of its properties, especially its acidity. The change of properties of hydro®ning catalysts may be induced by modi®cation of acid±base properties of a support through incorporation of sodium or magnesium ions. Addition of sodium ions at low (0.5% NO2O) and high (5% Na2O) concentrations to CoMo/Al2O3 catalyst considerably reduces hydrodesulphurization activity of thiophene [6]. Magnesium ions, on the other hand, added at the amount of 0.5%, do not affect HDS activity of cobalt±molybdenum catalyst, while its amount of 5% leads to a considerable decrease of this activity [6]. Jiratova and Kraus [7] did not ®nd any effect of sodium and magnesium ions on HDS activity of thiophene and cyclohexene hydrogenation on NiMo/ Al2O3 catalyst. Kelly and Ternan [8] proved that sodium ions only insigni®cantly increase HDS activity and decrease HDN activity of CoMo/ Al2O3 catalyst. Kovach et al. [9], on the other hand, observed a decrease of hydrogenation activity of CoMo/Al2O3 catalyst applied in hydro®ning of coal liquid. Besides the reaction of hydrogenolysis of C±N, C±O, C±S bonds, the process of hydro®ning also involves hydrogenation and hydrocracking. The hydrocracking is responsible for formation of a large amount of light gases and it favors polymerization processes and coke formation on the catalyst surface. As regards hydrogenation, it increases consumption of expensive hydrogen. Proportions between the functions involved in the process of hydro®ning depend on the composition of a catalyst, carrier, and kind of additives, for example, sodium or magnesium ions. The present paper reports results of the studies on the hydro®ning activity of coal liquid and changes of acid±base properties of nickel±molybdenum catalyst incorporated on alumina modi®ed with sodium and magnesium ions.
2. Experimental 2.1. Support and NiMo preparation The g- Al2O3 support containing NiMo/Al2O3 catalysts and their preparation have been described in detail elsewhere [10]. Brie¯y, alumina was prepared from aluminum isopropanolate (ALICO Great Britain) which was hydrolyzed in distilled water at 608C. Thus formed aluminum hydroxide was left for one week, and then submitted to ®ltering, drying and calcinating at 5508C for 24 h. Preparation of 4% NiO and 11% MoO3 (by weight) sample support on ion modi®ed aluminas was carried out by two steps incipient wetness impregnations on the modi®ed aluminas with (NH4)6Mo7O244H2O (sigma) at pH 6.5 and then with Ni(NO3)2 followed by drying in air for 4 h at 1208C and calcination in air for 5508C after each step. 2.1.1. Support modification The obtained alumina was modi®ed with sodium and magnesium ions. In the modi®cation process either sodium nitrate or magnesium nitrate were added dropwise to 40 g of alumina upon a constant stirring of the solution. The amount of solutions used for impregnation and selected on the basis of a speci®c volume of the carrier was equal to 15 cm3. Sodium and magnesium ions were incorporated at four different concentrations, corresponding to the following atomic ratios of the modifying ion to alumina atoms of the support: 1:20, 2:20, 3:20, 4:20. After impregnation, the samples were dried at the temperature of 1208C for 2 h, and then calcined for 6 h at 5508C in the air ¯ow. 2.1.2. Catalysts labelling A catalyst incorporated on an unmodi®ed alumina was labelled as NiMo. Sodium and magnesium modi®cations were labelled with an appropriate letter or digital symbol. Digits from 1 to 4 correspond to concentrations of the modi®er expressed by a ratio of the number of modi®er's atoms per 20 atoms of support alumina: 1±1:20, 2±2:20, 3±3:20, 4±4:20. Sodium modi®cations were labelled as Nal, Na2, Na3, Na4, and the magnesium ones as Mgl, Mg2, Mg3, Mg4.
M. Lewandowski, Z. Sarbak / Applied Catalysis A: General 168 (1998) 179±185
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2.2. Model catalytic studies
3. Results and Discussion
The model reactions of decomposition of isopropyl alcohol (2-propanol) (at temperature 3508C) and, diacetone alcohol (4-hydroxy-4-methyl-2 pentanone) at (708C) and ethylbenzene (at 4608C) were conducted in microreactor (of the length 100 mm, diameter 8 mm). The calculated contact time for these conditions expressed by a ratio W/F (W-weight of the catalyst, F-molar ¯ow rate) was equal, respectively: 109 ghmoleÿ1 for isopropyl alcohol, to 123 gh moleÿ1 for ethylbenzene and 119 ghmoleÿ1 for diacetone alcohol. More details are described in [11].
Modi®cation of support properties by adding sodium and magnesium ions has a signi®cant effect on the activity of nickel±molybdenum catalysts in the applied reactions of coal liquid hydro®ning. Results of catalytic activity of the discussed modi®cations in model reactions of isopropyl and diacetone alcohol decomposition are presented in Table 1, and that of ethylbenzene decomposition in Table 2. The effect of modi®cation of the carrier with sodium and magnesium ions on the activity of nickel±molybdenum catalysts is different. Catalysts modi®ed with sodium ions revealed low dehydration activity and a high dehydrogenation activity when compared to NiMo catalyst, which is evidenced by high yields of acetone both in the reaction of isopropanol and diacetyl alcohol decomposition. Nal catalyst had the highest yield of propene of all those reported for sodium modi®cations in the reaction of isopropanol decomposition. Increase in the concentration of sodium ions resulted in a distinct decrease of the yield of propene for catalysts Na2, Na3, and Na4. The latter did not practically reveal any dehydrogenation activity. Yield of acetone both in the reaction of isopropanol and diacetone alcohol decomposition increased with increasing concentration of sodium ions and in the two cases it reached the maximum value for Na3 catalyst. Na4 catalyst was characterized by a clearly lower yield of acetone in isopropanol decomposition in comparison with Na3. On the other hand, yield of
2.3. Hydrofining studies Hydro®ning was carried out at 4008C under hydrogen pressure of 9.0 MPa (90 atm) and at the volume rate of the material 1 hÿ1. The size of the samples of the studied catalysts was 0.8± 1.0 mm. The studies material was coal liquid which was a hydrogenation product from a uncatalytic process of liquidation of coal from the Mine "Janina" in Libiaz near Chrzanowo (Poland) from an installation applied in the Main Mining Institute in TychyWyry. The content of nitrogen in the hydrogenation product was equal to 0.56% wt. (5600 ppm), while that of sulfur 2.5% wt. (25000 ppm), carbon 88.5% and oxygen 0.90%. More details were described in [11].
Table 1 Activity of NiMo catalyst and catalysts modified with sodium and magnesium ions in the reactions of isopropyl and diacetone alcohol decomposition a
Catalyst
Isopropyl alcohol Yield of propene [%]
Yield of acetone [%]
Selectivity to propene [%]
DAA Yield of acetone [%]
NiMo Nal Na2 Na3 Na4 Mgl Mg2 Mg3 Mg4
33.9 18.2 2.5 1.4 2.8 27.1 25.8 26.2 14.9
2.3 33.0 43.4 49.2 37.9 5.5 7.2 14.2 31.1
93.6 35.5 5.4 2.8 6.9 83.1 78.2 64.9 32.4
0.84 3.90 4.50 5.10 4.10 1.65 1.97 2.90 4.70
a
diacetone alcohol.
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Table 2 Activity of NiMo catalyst and catalysts modified with sodium and magnesium ions in the reactions of ethylbenzene decomposition Catalyst
Yield of styrene [%]
Yield of benzene [%]
Selectivity to styrene [%]
NiMo Nal Na2 Na3 Na4 Mgl Mg2 Mg3 Mg4
10.0 6.4 8.5 10.7 16.1 8.8 9.1 11.8 13.1
3.0 0.7 0.4 0.0 0.0 0.7 0.0 0.0 0.0
76.9 90.1 95.5 100.0 100.0 92.6 100.0 100.0 100.0
propene for sample Na3 was two times as high as for sample Na4. In the reaction of ethylbenzene dehydration to styrene, ®rst the incorporation of sodium ions caused decrease of styrene yield which decreased slightly for catalysts Nal and Na2 in comparison with NiMo. Na3 catalyst, on the other hand, had a similar yield of styrene as NiMo catalyst. Further increase in sodium ions concentrations led to a signi®cant increase of styrene yield which from 10.7% for Na3 grew up to 16.1% for Na4. A different situation was observed for support modi®cation with magnesium ions. Magnesium ions are also characterized by lower dehydration and higher dehydrogenation activity, however the decrease in yield of propene in isopropanol decomposition is not as large as in the case of sodium modi®cations. Yield of propene decreased for catalysts Mgl, Mg2, and Mg3 and it was practically independent of concentration of magnesium ions. It was only for Mg4
catalyst that the obtained yield of propene was two times smaller in comparison to that obtained for the three previous catalysts. It should be mentioned, however, that Mg4 had the highest dehydrogenation activity in the reaction of isopropanol and diacetone alcohol decomposition. Yield of acetone increased with increasing concentration of magnesium ions. A distinct increase in the yield of acetone in isopropanol decomposition was observed for catalysts Mg3 and Mg4, and it was respectively two-or fourfold in comparison with Mg2 catalyst. In the reaction of ethylbenzene decomposition to styrene, the effect of magnesium ions on the dehydrogenation was similar to that of sodium ions, as in the beginning the modi®cation caused a slight decrease of the yield of styrene. Catalysts Mg3 and Mg4, on the other hand, revealed a higher yield of styrene in comparison with the NiMo sample. Sodium and magnesium modi®cations of nickel± molybdenum catalysts revealed a very low hydro®n-
Table 3 Hydrodenitrogenation and hydrodesulfurization activity of NiMo catalyst and catalysts modified with sodium and magnesium ions Catalyst
Concentration of N [ppm]
S [ppm]
Degree of HDN [%]
Degree of HDS [%]
NiMo a S-227 Nal Na2 Na3 Na4 Mgl Mg2 Mg3 Mg4
1350 1340 4270 4490 5310 5551 2250 2730 2800 3350
630 1450 2210 4750 9300 16300 1350 1580 1490 2450
75.9 76.1 23.7 19.8 5.2 0.9 59.8 51.2 50.0 41.2
97.5 94.2 91.2 81.0 62.8 34.8 94.6 93.7 94.0 90.2
a
Commercial HDS catalyst (Shell).
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ing activity, the results of which are compiled in Table 3. For catalysts with sodium ions-modi®ed support, the degree of hydrodenitrogenation and hydrodesulfurization was rather low, the former being however much smaller. From a practical point of view, catalysts Na3 and Na4 did not show any hydrodenitrogenation activity. As for HDS of coal liquid, only Nal catalyst showed a high degree (over 90%) of hydrodesulfurization. Increasing concentration of sodium ions entails a decrease of the HDS degree. As follows from the obtained results, sodium ions cause a considerable decrease of HDS activity especially negative is their in¯uence on HDN activity. The effect of magnesium ions on the hydro®ning activity is slightly different than that of sodium ions. Catalysts Mgl and Mg4 show a lower degree of HDN than NiMo catalyst and slightly lower degrees of hydrodenitrogenation. For catalysts Mg2 and Mg3 the degree of HDN was the same, i.e. ca. 51%. In the case of HDS activity, the situation was similar: its value for catalysts Mgl, Mg2, Mg3 was almost the same i.e., ca. 94%; Mg4 showed a lower HDS activity. However, in comparison with catalysts from Nal to Na4, both the hydrodenitrogenation and hydrodesulfurization activity of magnesium modi®cations was higher. Modi®cation of the support with sodium ions decreases acidity of the catalysts, which is the reason why the yields of propene in isopropanol decomposition are very low. On the other hand, increase in the yield of acetone in decomposition of isopropyl and diacetone alcohols indicates an increase of the basicity of catalyst surface. In the process of alumina modi®cation, sodium ions react with the surface OH groups, causing a decrease of alumina acidity [12± 14]; they also react with Lewis centers [15]. In their study on isopropanol dehydration on alumina modi®ed with sodium ions, Fiedorow et al. [16] found that for the content of NaOH > 0.5 wt.%, a signi®cant decrease in propene yield takes place and the acid centers present are very weak. Concentrations of sodium ions applied in this work are higher, and therefore, the obtained yields of propene in the tested sodium modi®cations of NiMo catalysts are very small. This provides grounds for a conclusion that a change of acid±base properties of a support causes similar changes in the acidic and basic properties of
183
the surface of catalysts obtained on the basis of thus modi®ed support. Baker et al. [17] also reported a decrease in the acidity of NiMo catalyst modi®ed with sodium ions and a fall of its hydro®ning activity. Results obtained in the present paper are in agreement with those achieved by Baker et al. [17]. According to Jiratova and Kraus [7], support modi®cation with sodium ions leads to a change of HDS activity of thiophene, yet they used only 0.5 wt. % of this modi®er. Hydro®ning activity (in particular HDN activity) considerably decreased. The main reason for the decrease in HDN activity, is decreasing acidity of the catalysts surface, since the process of C±N bond hydrogenolysis proceeds on acid centers of the support [18]. However, the decrease in hydro®ning activity of the catalyst should not be attributed only to decreasing acidity of the catalyst, but also to structural changes brought about by the modi®cation. It is worth noticing that HDS activity of magnesium modi®cations (Table 3) was high, even at low acidity of the catalyst. The value of the HDS degree was sometimes even higher than 90%. In the case of sodium modi®cations (except for catalyst Nal) the degrees of HDS were much lower than 90%. Sodium ions are responsible for the decrease of reducibility and sulfurization of catalyst and, consequently, for the formation of a smaller number of Mo5 ions on its surface [19]. Lycourghiotis et al. [20] found that modi®cation with sodium ions causes transformation of Co3O4 into a surface spinal CoAl2O4. By analogy, a similar situation may occur in the case of nickel ions. Muralidhar et al. [6] also suggest the possibility of sodium molybdate formation, yet the conducted FT-IR and derivatographic studies did not reveal a presence of this type of compound [21]. This however, does not mean that such a compound cannot be formed under different preparation conditions. A presence of sodium ions induces a change of coordination of molybdenum ions from octahedral to tetrahedral in the oxide of this compound as this form is more resistant to undergo sul®ding and reduction [22]. A result of the above presented factors is formation of a smaller amount of the sul®ded active form, and consequently, a fall of the catalyst activity. The obtained low degrees of hydrodesulfurization seem to support the above discussed dependencies. Among other factors responsible for the drop of the hydro®ning activity of sodium modi®ca-
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tions is their small speci®c surface area [21] and higher susceptibility to deactivation caused by formation of carbon deposit of catalysts modi®ed with sodium ions [23]. The latter factor is especially important in hydro®ning of coal liquid due to its high aromaticity which may easily bring about the coking of a catalyst. For sample Na4, a clear increase of the yield of styrene was found, which is indicative of better hydrating±dehydrating properties which should also improve the hydrodenitrogenating properties. However, under a high hydrogen pressure (9.0 MPa) the stage hindering the HDN process is C±N bond hydrogenolysis. This is why the increase of hydrogenating properties is of secondary importance. Catalysts modi®ed with magnesium ions revealed lower acidity than the unmodi®ed catalyst, but, on the other hand, their basicity increased. In the reaction of isopropanol decomposition, this was manifested by lower yields of propene (decrease of acidity) and a simultaneous increase of acetone yield (increase of basicity) in the reaction of diacetone alcohol decomposition. Magnesium oxide as a basic oxide easily forms a surface spinel of MgAl2O4 with alumina oxide, and for higher concentrations of Mg (>50% mol of MgO) it may even form a separate oxide phase [24]. In such a system two types of Lewis acid centers may be distinguished: cations Al3, Mg2 ± as coordinately unsaturated centers and OH acid groups as BroÈnsted sites and only one type of Lewis basic centers ± oxide anions and basic OH groups. Acidity of OH groups decreases with an increasing content of magnesium ions [24]. This is why thus modi®ed alumina oxide reveals a lower acidity [14]. It can be supposed therefore that decreasing acidity of nickel±molybdenum catalysts modi®ed with magnesium ions is related to a decreasing acidity of the support after modi®cation with Mg2 ions. Jiratova and Kraus [7] also found that the acidity of catalysts modi®ed with magnesium ions decreases, yet the drop of acidity of Mgl catalysts to Mg3 is not so big as it was in the case of sodium catalysts. The only exception was catalyst Mg4 whose acidity was markedly lower in comparison with that of the catalysts Mgl to Mg3. Thus, it can be supposed that on the surface of Mg4 catalyst, magnesium ions form not only a surface spinel with alumina oxide, but also a separate phase MgO [24] which is responsible for such a considerable decrease of acidity of catalyst Mg4
relative to Mg3. However, at the same time a two-fold increase of the basicity of sample Mg4 takes place, which is con®rmed by the obtained values of the degrees of hydrodesulfurization of catalysts Mgl to Mg3, which are practically the same and do not depend on the concentration of magnesium ions. Hence, it can be assumed that the interaction of magnesium ions with compounds constituting an active phase is slight, while the hydro®ning activity (in particular HDN activity) of catalyst Mg4, on the other hand, is signi®cantly lower. This might lead to a conclusion that in the latter case the active phase might interact with the phase of magnesium oxide present on the support surface, yielding magnesium molybdate [6], which was the reason for such a considerable drop of the hydro®ning activity of sample Mg4. Similar dependencies have been observed by Muralidhar et al. [6] and Saini et al. [25]. The fall of HDS activity might be induced by a weaker promoting electron effect of Ni2 brought about by modi®cation with magnesium ions [26]. According to Saini et al. [25], in the process of sulfurization and reduction of molybdenum catalyst, magnesium ions in the spinel MgAl2O4 do not undergo sulfurization, and the MoS2 phase contains plates of this compound bigger in size and with a smaller number of anion vacancies, which leads to a decrease of HDS and dehydrogenation activity of a molybdenum catalyst. Nonetheless, magnesium ions have the greatest effect on the fall of HDN activity which is mainly caused by an increase of the basicity of the catalyst. Hillerova et al. [27] claimed that NiMo catalyst supported only by magnesium oxide revealed a very low HDN activity though HDS activity of such a catalyst was high. Decreasing acidity of magnesium modi®cations involves a fall of hydrodenitrogenation activity. In the two cases the dependence of a decrease in both acidity and HDN degree of the discussed catalyst on the concentration of magnesium ions is alike, i.e. increase in their concentration results in a decrease of the two values. This dependence can be well observed on the comparison of the dehydration and HDN activity of catalysts Mg3 and Mg4. For the Mg4 sample the yield of propene was twice smaller, which resulted in a 10% drop of the degree of hydrodenitrogenation of catalyst Mg4 when compared with the Mg3 sample, which is illustrated in Fig. 1.
M. Lewandowski, Z. Sarbak / Applied Catalysis A: General 168 (1998) 179±185
The obtained results describing the in¯uence of magnesium ions on the hydro®ning activity are corroborated by the studies of Hillerova et al. [27] discussed above. The NiMo/MgO catalyst applied by the authors showed a very low HDN activity. Application of basic oxide as a support did not have a decisive effect on HDS activity. Another factor responsible for the decrease of hydro®ning activity of the discussed catalysts is a decrease of their speci®c surface area [21]. 4. Conclusions Modi®cation of alumina with sodium and magnesium ions decreases the acidity of nickel±molybdenum catalysts, thus increasing their basicity. Sodium ions in the applied concentrations lead practically to elimination of acidity of catalyst, yet considerably increase its basicity. Modi®cation with sodium and magnesium ions increases the dehydrogenation activity of catalysts. Incorporation of sodium ions considerably decreases the hydrodenitrogenation and hydrodesulfurization activity of coal liquids. Magnesium ions, on the other hand, decrease the hydrodenitrogenation activity and have a slight effect on the decrease in hydrodesulfurization activity of nickel±molybdenum catalysts. References [1] D.L. Trimm, Appl. Catal. 5 (1983) 263. [2] A.W. Scaroni, R.G. Jenkins, J.R. Utrilla, P.L. Walker, Fuel Process. Tech. 9 (1983) 103. [3] K. Tanabe, Solid Acids and Bases, Academic Press, New York, 1970, p. 125. [4] L. Brunn, A.A. Montagna, J.A. Paraskos, Preprint-Am. Chem. Soc. Div. Pet. Chem. 21 (1976) 173.
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[5] E. Furimsky ErdoÈl und Kohle, Erdgas. Petrochem. 35 (1982) 455. [6] G. Muralidhar, F.E. Massoth, J. Shabtai, J. Catal. 85 (1984) 44. [7] K. Jiratova, M. Kraus, Appl. Catal. 27 (1986) 21. [8] J.F. Kelly, M. Ternan, Can. J. Chem. Eng. 57 (1979) 726. [9] S.M. Kovach, L.J. Castle, J.V. Bennett, Ind. Eng. Chem. Prod. Res. Dev. 17 (1978) 62. [10] M. Lewandowski, Z. Sarbak, Cryst. Res. Technol. 31 (1996) 773. [11] M. Lewandowski, Z. Sarbak, Appl. Catal. 156 (1997) 181. [12] L. Vordonis, P.G. Koutsoukos, A. Lycourghiotis, J. Catal. 98 (1986) 296. [13] L. Vordonis, P.G. Koutsoukos, A. Lycourghiotis, Colloids and Surface 50 (1990) 353. [14] E.A. Paukshtis, P.J. Soltanov, E.N. Yurchenko, K. Jiratova, Collect. Czech. Chem. Commun. 47 (1982) 2044. [15] R. Fiedorow, I.G. Dalla Lana, J. Phys. Chem. 84 (1980) 2779. [16] R. Fiedorow, W. Przystajko, M. Sopa, J.G. Dalla Lana, J. Catal. 68 (1981) 33. [17] J.R. Baker, R.L. McCormick, H.W. Haynes Jr., Ind. Eng. Chem. Res. 26 (1987) 1895. [18] J.M. Lewis, R.A. Kydd, P.M. Boorman, P.H. van Rhyn, Appl. Catal. A 84 (1992) 103. [19] A.W. Ramaswamy, S. Sivasanker, P. Ratnasamy, J. Catal. 42 (1976) 107. [20] B.A. Lycourghiotis, C. Defosse, F. Delannay, J. Lemaitre, B. Delmon, J.C.S. Faraday I 76 (1980) 1677. [21] M. Lewandowski and Z. Sarbak, (submitted to Cryst. Res. Technol.). [22] M. Kantschewa, F. Delannay, H. Jeziorowski, E. Delgado, S. Eder, G. Ertl, H. KnoÈzinger, J. Catal. 87 (1984) 482. [23] P. Pomonis, S. Skaribas, A. Licourghiotis, C. Kordoulis, Catalyst Deactivation 1991, C.H. Bartholomew, I.B. Butt (Eds.), vol. 60, Elsevier, Amsterdam, 1991, s. 479. [24] J.A. Lercher, Ch. Colombier, H. Vinek, H. Noller, Catalysis by Acids and Bases, B. Imielik (Eds.), vol. 20, Elsevier, Amsterdam, 1985, s. 25. [25] A.R. Saini, B.G. Johnson, E.F. Massoth, Appl. Catal. 40 (1988) 157. [26] R.R. Chianelli, M. Daage, Hydrotreating Catalysts, M.L. Occelli, R.G. Anthony (Eds.), vol. 50, Elsevier, Amsterdam, 1989, p. 1. [27] E. Hillerova, Z. Vit, M. Zdrazil, Appl. Catal. 118 (1994) 111.
Hydro®ning activity and acid±base properties of nickel±molybdenum catalysts incorporated on sodium and magnesium ions-modi®ed alumina M. Lewandowskia, Z. Sarbakb,* a
Institute of Coal Chemistry, Polish Academy of Sciences, SowinÂskiego 5, 44-102 Gliwice, Poland b Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 PoznanÂ, Poland
Received 21 May 1997; received in revised form 25 November 1997; accepted 27 November 1997
Abstract In the study of a series of NiMo catalysts incorporated on alumina modi®ed with sodium and magnesium ions, decomposition of isopropyl or diacetone alcohol and ethylbenzene were used as model reactions. Hydro®ning of coal liquid was performed at the pressure of 9.0 MPa and at 4008C. Modi®cation of alumina with sodium and magnesium ions leads to a decrease in the acidity of nickel±molybdenum catalysts, and by the same token increases their basicity. Another effect of modi®cation with sodium and magnesium ions is dehydrogenation of catalysts. Introduction of sodium ions results in a distinct decrease of hydrodenitrogenation and hydrodesulfurization activity. On the other hand, magnesium ions decrease the hydrodenitrogenation activity, and insigni®cantly decrease the hydrodesulfurization activity of catalysts. # 1998 Elsevier Science B.V. Keywords: Catalyst NiMo; Modi®cation; Hydrotreating; HDS; HDN; Acidity; Coal liquid; 2-propanol; Ethylbenzene; Diacetone alcohol
1. Introduction Catalytic hydro®ning involves, in most general terms, the reactions inducing a removal of nitrogen, sulphur, oxygen heteroatoms and metals. These processes are accompanied by hydrocracking and hydrogenation. The applied catalysts of hydro®ning, among them the most often applied Ni±Mo and Co±Mo systems incorporated on alumina, do not reveal a
*Corresponding author. 0926-860X/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0926-860X(97)00354-2
suf®ciently high activity in the hydro®ning of coal liquids. The character of coal liquids-different from that of petroleum-derivative materials, higher aromaticity and higher content of sulphur, nitrogen, and oxygen atoms and a signi®cantly lower content of hydrogen atoms as well as a high molecular weight are the factors that signi®cantly contribute to a rapid deactivation of catalysts. It is known that coke (carbon deposit) is formed as a result of the reactions of carbonium ion taking place on acid sites of catalysts or through a thermal reaction on the surface of a
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catalyst evoked by a presence of free radicals [1]. Lewis acidity is related to formation of coke as a result of the reaction of Lewis center with a basic nitrogen of a heterocyclic compound [2]. BroÈnsted acid sites are also claimed responsible for the coking of catalysts [3], though there are also opinions they decrease the coking by transforming the coking precursors into volatile products [4,5]. This is why an important factor leading to a decrease in the amount of coke on the catalyst is a change of its properties, especially its acidity. The change of properties of hydro®ning catalysts may be induced by modi®cation of acid±base properties of a support through incorporation of sodium or magnesium ions. Addition of sodium ions at low (0.5% NO2O) and high (5% Na2O) concentrations to CoMo/Al2O3 catalyst considerably reduces hydrodesulphurization activity of thiophene [6]. Magnesium ions, on the other hand, added at the amount of 0.5%, do not affect HDS activity of cobalt±molybdenum catalyst, while its amount of 5% leads to a considerable decrease of this activity [6]. Jiratova and Kraus [7] did not ®nd any effect of sodium and magnesium ions on HDS activity of thiophene and cyclohexene hydrogenation on NiMo/ Al2O3 catalyst. Kelly and Ternan [8] proved that sodium ions only insigni®cantly increase HDS activity and decrease HDN activity of CoMo/ Al2O3 catalyst. Kovach et al. [9], on the other hand, observed a decrease of hydrogenation activity of CoMo/Al2O3 catalyst applied in hydro®ning of coal liquid. Besides the reaction of hydrogenolysis of C±N, C±O, C±S bonds, the process of hydro®ning also involves hydrogenation and hydrocracking. The hydrocracking is responsible for formation of a large amount of light gases and it favors polymerization processes and coke formation on the catalyst surface. As regards hydrogenation, it increases consumption of expensive hydrogen. Proportions between the functions involved in the process of hydro®ning depend on the composition of a catalyst, carrier, and kind of additives, for example, sodium or magnesium ions. The present paper reports results of the studies on the hydro®ning activity of coal liquid and changes of acid±base properties of nickel±molybdenum catalyst incorporated on alumina modi®ed with sodium and magnesium ions.
2. Experimental 2.1. Support and NiMo preparation The g- Al2O3 support containing NiMo/Al2O3 catalysts and their preparation have been described in detail elsewhere [10]. Brie¯y, alumina was prepared from aluminum isopropanolate (ALICO Great Britain) which was hydrolyzed in distilled water at 608C. Thus formed aluminum hydroxide was left for one week, and then submitted to ®ltering, drying and calcinating at 5508C for 24 h. Preparation of 4% NiO and 11% MoO3 (by weight) sample support on ion modi®ed aluminas was carried out by two steps incipient wetness impregnations on the modi®ed aluminas with (NH4)6Mo7O244H2O (sigma) at pH 6.5 and then with Ni(NO3)2 followed by drying in air for 4 h at 1208C and calcination in air for 5508C after each step. 2.1.1. Support modification The obtained alumina was modi®ed with sodium and magnesium ions. In the modi®cation process either sodium nitrate or magnesium nitrate were added dropwise to 40 g of alumina upon a constant stirring of the solution. The amount of solutions used for impregnation and selected on the basis of a speci®c volume of the carrier was equal to 15 cm3. Sodium and magnesium ions were incorporated at four different concentrations, corresponding to the following atomic ratios of the modifying ion to alumina atoms of the support: 1:20, 2:20, 3:20, 4:20. After impregnation, the samples were dried at the temperature of 1208C for 2 h, and then calcined for 6 h at 5508C in the air ¯ow. 2.1.2. Catalysts labelling A catalyst incorporated on an unmodi®ed alumina was labelled as NiMo. Sodium and magnesium modi®cations were labelled with an appropriate letter or digital symbol. Digits from 1 to 4 correspond to concentrations of the modi®er expressed by a ratio of the number of modi®er's atoms per 20 atoms of support alumina: 1±1:20, 2±2:20, 3±3:20, 4±4:20. Sodium modi®cations were labelled as Nal, Na2, Na3, Na4, and the magnesium ones as Mgl, Mg2, Mg3, Mg4.
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2.2. Model catalytic studies
3. Results and Discussion
The model reactions of decomposition of isopropyl alcohol (2-propanol) (at temperature 3508C) and, diacetone alcohol (4-hydroxy-4-methyl-2 pentanone) at (708C) and ethylbenzene (at 4608C) were conducted in microreactor (of the length 100 mm, diameter 8 mm). The calculated contact time for these conditions expressed by a ratio W/F (W-weight of the catalyst, F-molar ¯ow rate) was equal, respectively: 109 ghmoleÿ1 for isopropyl alcohol, to 123 gh moleÿ1 for ethylbenzene and 119 ghmoleÿ1 for diacetone alcohol. More details are described in [11].
Modi®cation of support properties by adding sodium and magnesium ions has a signi®cant effect on the activity of nickel±molybdenum catalysts in the applied reactions of coal liquid hydro®ning. Results of catalytic activity of the discussed modi®cations in model reactions of isopropyl and diacetone alcohol decomposition are presented in Table 1, and that of ethylbenzene decomposition in Table 2. The effect of modi®cation of the carrier with sodium and magnesium ions on the activity of nickel±molybdenum catalysts is different. Catalysts modi®ed with sodium ions revealed low dehydration activity and a high dehydrogenation activity when compared to NiMo catalyst, which is evidenced by high yields of acetone both in the reaction of isopropanol and diacetyl alcohol decomposition. Nal catalyst had the highest yield of propene of all those reported for sodium modi®cations in the reaction of isopropanol decomposition. Increase in the concentration of sodium ions resulted in a distinct decrease of the yield of propene for catalysts Na2, Na3, and Na4. The latter did not practically reveal any dehydrogenation activity. Yield of acetone both in the reaction of isopropanol and diacetone alcohol decomposition increased with increasing concentration of sodium ions and in the two cases it reached the maximum value for Na3 catalyst. Na4 catalyst was characterized by a clearly lower yield of acetone in isopropanol decomposition in comparison with Na3. On the other hand, yield of
2.3. Hydrofining studies Hydro®ning was carried out at 4008C under hydrogen pressure of 9.0 MPa (90 atm) and at the volume rate of the material 1 hÿ1. The size of the samples of the studied catalysts was 0.8± 1.0 mm. The studies material was coal liquid which was a hydrogenation product from a uncatalytic process of liquidation of coal from the Mine "Janina" in Libiaz near Chrzanowo (Poland) from an installation applied in the Main Mining Institute in TychyWyry. The content of nitrogen in the hydrogenation product was equal to 0.56% wt. (5600 ppm), while that of sulfur 2.5% wt. (25000 ppm), carbon 88.5% and oxygen 0.90%. More details were described in [11].
Table 1 Activity of NiMo catalyst and catalysts modified with sodium and magnesium ions in the reactions of isopropyl and diacetone alcohol decomposition a
Catalyst
Isopropyl alcohol Yield of propene [%]
Yield of acetone [%]
Selectivity to propene [%]
DAA Yield of acetone [%]
NiMo Nal Na2 Na3 Na4 Mgl Mg2 Mg3 Mg4
33.9 18.2 2.5 1.4 2.8 27.1 25.8 26.2 14.9
2.3 33.0 43.4 49.2 37.9 5.5 7.2 14.2 31.1
93.6 35.5 5.4 2.8 6.9 83.1 78.2 64.9 32.4
0.84 3.90 4.50 5.10 4.10 1.65 1.97 2.90 4.70
a
diacetone alcohol.
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Table 2 Activity of NiMo catalyst and catalysts modified with sodium and magnesium ions in the reactions of ethylbenzene decomposition Catalyst
Yield of styrene [%]
Yield of benzene [%]
Selectivity to styrene [%]
NiMo Nal Na2 Na3 Na4 Mgl Mg2 Mg3 Mg4
10.0 6.4 8.5 10.7 16.1 8.8 9.1 11.8 13.1
3.0 0.7 0.4 0.0 0.0 0.7 0.0 0.0 0.0
76.9 90.1 95.5 100.0 100.0 92.6 100.0 100.0 100.0
propene for sample Na3 was two times as high as for sample Na4. In the reaction of ethylbenzene dehydration to styrene, ®rst the incorporation of sodium ions caused decrease of styrene yield which decreased slightly for catalysts Nal and Na2 in comparison with NiMo. Na3 catalyst, on the other hand, had a similar yield of styrene as NiMo catalyst. Further increase in sodium ions concentrations led to a signi®cant increase of styrene yield which from 10.7% for Na3 grew up to 16.1% for Na4. A different situation was observed for support modi®cation with magnesium ions. Magnesium ions are also characterized by lower dehydration and higher dehydrogenation activity, however the decrease in yield of propene in isopropanol decomposition is not as large as in the case of sodium modi®cations. Yield of propene decreased for catalysts Mgl, Mg2, and Mg3 and it was practically independent of concentration of magnesium ions. It was only for Mg4
catalyst that the obtained yield of propene was two times smaller in comparison to that obtained for the three previous catalysts. It should be mentioned, however, that Mg4 had the highest dehydrogenation activity in the reaction of isopropanol and diacetone alcohol decomposition. Yield of acetone increased with increasing concentration of magnesium ions. A distinct increase in the yield of acetone in isopropanol decomposition was observed for catalysts Mg3 and Mg4, and it was respectively two-or fourfold in comparison with Mg2 catalyst. In the reaction of ethylbenzene decomposition to styrene, the effect of magnesium ions on the dehydrogenation was similar to that of sodium ions, as in the beginning the modi®cation caused a slight decrease of the yield of styrene. Catalysts Mg3 and Mg4, on the other hand, revealed a higher yield of styrene in comparison with the NiMo sample. Sodium and magnesium modi®cations of nickel± molybdenum catalysts revealed a very low hydro®n-
Table 3 Hydrodenitrogenation and hydrodesulfurization activity of NiMo catalyst and catalysts modified with sodium and magnesium ions Catalyst
Concentration of N [ppm]
S [ppm]
Degree of HDN [%]
Degree of HDS [%]
NiMo a S-227 Nal Na2 Na3 Na4 Mgl Mg2 Mg3 Mg4
1350 1340 4270 4490 5310 5551 2250 2730 2800 3350
630 1450 2210 4750 9300 16300 1350 1580 1490 2450
75.9 76.1 23.7 19.8 5.2 0.9 59.8 51.2 50.0 41.2
97.5 94.2 91.2 81.0 62.8 34.8 94.6 93.7 94.0 90.2
a
Commercial HDS catalyst (Shell).
M. Lewandowski, Z. Sarbak / Applied Catalysis A: General 168 (1998) 179±185
ing activity, the results of which are compiled in Table 3. For catalysts with sodium ions-modi®ed support, the degree of hydrodenitrogenation and hydrodesulfurization was rather low, the former being however much smaller. From a practical point of view, catalysts Na3 and Na4 did not show any hydrodenitrogenation activity. As for HDS of coal liquid, only Nal catalyst showed a high degree (over 90%) of hydrodesulfurization. Increasing concentration of sodium ions entails a decrease of the HDS degree. As follows from the obtained results, sodium ions cause a considerable decrease of HDS activity especially negative is their in¯uence on HDN activity. The effect of magnesium ions on the hydro®ning activity is slightly different than that of sodium ions. Catalysts Mgl and Mg4 show a lower degree of HDN than NiMo catalyst and slightly lower degrees of hydrodenitrogenation. For catalysts Mg2 and Mg3 the degree of HDN was the same, i.e. ca. 51%. In the case of HDS activity, the situation was similar: its value for catalysts Mgl, Mg2, Mg3 was almost the same i.e., ca. 94%; Mg4 showed a lower HDS activity. However, in comparison with catalysts from Nal to Na4, both the hydrodenitrogenation and hydrodesulfurization activity of magnesium modi®cations was higher. Modi®cation of the support with sodium ions decreases acidity of the catalysts, which is the reason why the yields of propene in isopropanol decomposition are very low. On the other hand, increase in the yield of acetone in decomposition of isopropyl and diacetone alcohols indicates an increase of the basicity of catalyst surface. In the process of alumina modi®cation, sodium ions react with the surface OH groups, causing a decrease of alumina acidity [12± 14]; they also react with Lewis centers [15]. In their study on isopropanol dehydration on alumina modi®ed with sodium ions, Fiedorow et al. [16] found that for the content of NaOH > 0.5 wt.%, a signi®cant decrease in propene yield takes place and the acid centers present are very weak. Concentrations of sodium ions applied in this work are higher, and therefore, the obtained yields of propene in the tested sodium modi®cations of NiMo catalysts are very small. This provides grounds for a conclusion that a change of acid±base properties of a support causes similar changes in the acidic and basic properties of
183
the surface of catalysts obtained on the basis of thus modi®ed support. Baker et al. [17] also reported a decrease in the acidity of NiMo catalyst modi®ed with sodium ions and a fall of its hydro®ning activity. Results obtained in the present paper are in agreement with those achieved by Baker et al. [17]. According to Jiratova and Kraus [7], support modi®cation with sodium ions leads to a change of HDS activity of thiophene, yet they used only 0.5 wt. % of this modi®er. Hydro®ning activity (in particular HDN activity) considerably decreased. The main reason for the decrease in HDN activity, is decreasing acidity of the catalysts surface, since the process of C±N bond hydrogenolysis proceeds on acid centers of the support [18]. However, the decrease in hydro®ning activity of the catalyst should not be attributed only to decreasing acidity of the catalyst, but also to structural changes brought about by the modi®cation. It is worth noticing that HDS activity of magnesium modi®cations (Table 3) was high, even at low acidity of the catalyst. The value of the HDS degree was sometimes even higher than 90%. In the case of sodium modi®cations (except for catalyst Nal) the degrees of HDS were much lower than 90%. Sodium ions are responsible for the decrease of reducibility and sulfurization of catalyst and, consequently, for the formation of a smaller number of Mo5 ions on its surface [19]. Lycourghiotis et al. [20] found that modi®cation with sodium ions causes transformation of Co3O4 into a surface spinal CoAl2O4. By analogy, a similar situation may occur in the case of nickel ions. Muralidhar et al. [6] also suggest the possibility of sodium molybdate formation, yet the conducted FT-IR and derivatographic studies did not reveal a presence of this type of compound [21]. This however, does not mean that such a compound cannot be formed under different preparation conditions. A presence of sodium ions induces a change of coordination of molybdenum ions from octahedral to tetrahedral in the oxide of this compound as this form is more resistant to undergo sul®ding and reduction [22]. A result of the above presented factors is formation of a smaller amount of the sul®ded active form, and consequently, a fall of the catalyst activity. The obtained low degrees of hydrodesulfurization seem to support the above discussed dependencies. Among other factors responsible for the drop of the hydro®ning activity of sodium modi®ca-
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tions is their small speci®c surface area [21] and higher susceptibility to deactivation caused by formation of carbon deposit of catalysts modi®ed with sodium ions [23]. The latter factor is especially important in hydro®ning of coal liquid due to its high aromaticity which may easily bring about the coking of a catalyst. For sample Na4, a clear increase of the yield of styrene was found, which is indicative of better hydrating±dehydrating properties which should also improve the hydrodenitrogenating properties. However, under a high hydrogen pressure (9.0 MPa) the stage hindering the HDN process is C±N bond hydrogenolysis. This is why the increase of hydrogenating properties is of secondary importance. Catalysts modi®ed with magnesium ions revealed lower acidity than the unmodi®ed catalyst, but, on the other hand, their basicity increased. In the reaction of isopropanol decomposition, this was manifested by lower yields of propene (decrease of acidity) and a simultaneous increase of acetone yield (increase of basicity) in the reaction of diacetone alcohol decomposition. Magnesium oxide as a basic oxide easily forms a surface spinel of MgAl2O4 with alumina oxide, and for higher concentrations of Mg (>50% mol of MgO) it may even form a separate oxide phase [24]. In such a system two types of Lewis acid centers may be distinguished: cations Al3, Mg2 ± as coordinately unsaturated centers and OH acid groups as BroÈnsted sites and only one type of Lewis basic centers ± oxide anions and basic OH groups. Acidity of OH groups decreases with an increasing content of magnesium ions [24]. This is why thus modi®ed alumina oxide reveals a lower acidity [14]. It can be supposed therefore that decreasing acidity of nickel±molybdenum catalysts modi®ed with magnesium ions is related to a decreasing acidity of the support after modi®cation with Mg2 ions. Jiratova and Kraus [7] also found that the acidity of catalysts modi®ed with magnesium ions decreases, yet the drop of acidity of Mgl catalysts to Mg3 is not so big as it was in the case of sodium catalysts. The only exception was catalyst Mg4 whose acidity was markedly lower in comparison with that of the catalysts Mgl to Mg3. Thus, it can be supposed that on the surface of Mg4 catalyst, magnesium ions form not only a surface spinel with alumina oxide, but also a separate phase MgO [24] which is responsible for such a considerable decrease of acidity of catalyst Mg4
relative to Mg3. However, at the same time a two-fold increase of the basicity of sample Mg4 takes place, which is con®rmed by the obtained values of the degrees of hydrodesulfurization of catalysts Mgl to Mg3, which are practically the same and do not depend on the concentration of magnesium ions. Hence, it can be assumed that the interaction of magnesium ions with compounds constituting an active phase is slight, while the hydro®ning activity (in particular HDN activity) of catalyst Mg4, on the other hand, is signi®cantly lower. This might lead to a conclusion that in the latter case the active phase might interact with the phase of magnesium oxide present on the support surface, yielding magnesium molybdate [6], which was the reason for such a considerable drop of the hydro®ning activity of sample Mg4. Similar dependencies have been observed by Muralidhar et al. [6] and Saini et al. [25]. The fall of HDS activity might be induced by a weaker promoting electron effect of Ni2 brought about by modi®cation with magnesium ions [26]. According to Saini et al. [25], in the process of sulfurization and reduction of molybdenum catalyst, magnesium ions in the spinel MgAl2O4 do not undergo sulfurization, and the MoS2 phase contains plates of this compound bigger in size and with a smaller number of anion vacancies, which leads to a decrease of HDS and dehydrogenation activity of a molybdenum catalyst. Nonetheless, magnesium ions have the greatest effect on the fall of HDN activity which is mainly caused by an increase of the basicity of the catalyst. Hillerova et al. [27] claimed that NiMo catalyst supported only by magnesium oxide revealed a very low HDN activity though HDS activity of such a catalyst was high. Decreasing acidity of magnesium modi®cations involves a fall of hydrodenitrogenation activity. In the two cases the dependence of a decrease in both acidity and HDN degree of the discussed catalyst on the concentration of magnesium ions is alike, i.e. increase in their concentration results in a decrease of the two values. This dependence can be well observed on the comparison of the dehydration and HDN activity of catalysts Mg3 and Mg4. For the Mg4 sample the yield of propene was twice smaller, which resulted in a 10% drop of the degree of hydrodenitrogenation of catalyst Mg4 when compared with the Mg3 sample, which is illustrated in Fig. 1.
M. Lewandowski, Z. Sarbak / Applied Catalysis A: General 168 (1998) 179±185
The obtained results describing the in¯uence of magnesium ions on the hydro®ning activity are corroborated by the studies of Hillerova et al. [27] discussed above. The NiMo/MgO catalyst applied by the authors showed a very low HDN activity. Application of basic oxide as a support did not have a decisive effect on HDS activity. Another factor responsible for the decrease of hydro®ning activity of the discussed catalysts is a decrease of their speci®c surface area [21]. 4. Conclusions Modi®cation of alumina with sodium and magnesium ions decreases the acidity of nickel±molybdenum catalysts, thus increasing their basicity. Sodium ions in the applied concentrations lead practically to elimination of acidity of catalyst, yet considerably increase its basicity. Modi®cation with sodium and magnesium ions increases the dehydrogenation activity of catalysts. Incorporation of sodium ions considerably decreases the hydrodenitrogenation and hydrodesulfurization activity of coal liquids. Magnesium ions, on the other hand, decrease the hydrodenitrogenation activity and have a slight effect on the decrease in hydrodesulfurization activity of nickel±molybdenum catalysts. References [1] D.L. Trimm, Appl. Catal. 5 (1983) 263. [2] A.W. Scaroni, R.G. Jenkins, J.R. Utrilla, P.L. Walker, Fuel Process. Tech. 9 (1983) 103. [3] K. Tanabe, Solid Acids and Bases, Academic Press, New York, 1970, p. 125. [4] L. Brunn, A.A. Montagna, J.A. Paraskos, Preprint-Am. Chem. Soc. Div. Pet. Chem. 21 (1976) 173.
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