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Ind. Eng. Chem. Res. 2004, 43, 2376-2381
Effect of Nickel, Lanthanum, and Yttrium Addition to Magnesium Molybdate Catalyst on the Catalytic Activity for Oxidative Dehydrogenation of Propane S. N. Koc,*,† G. Gurdag,† S. Geissler,‡ and M. Muhler‡ Department of Chemical Engineering, Istanbul University, 34320 Avcilar, Istanbul, Turkey, and Laboratory of Industrial Chemistry, Ruhr UniversitysBochum, D-44780 Bochum, Germany
The catalytic performances of pure magnesium molybdate (MgMoO4) and MeMgMoOx (Me ) Ni, La, or Y) for oxidative dehydrogenation of propane were investigated. Catalysts were characterized by nitrogen physisorption, XRD, FT-Raman, and temperature-programmed reduction measurements. Catalytic reactions were carried out at two different temperatures, 450 and 560 °C, under atmospheric pressure. The effects of the C3H8/O2 molar ratio in the feed and bed residence time on propylene selectivity and on propane conversion were also investigated. Although the strong effect of Ni loading both on reducibility and on the catalytic activity of MgMoO4 catalyst was observed, Y and La modification did not show any significant effect as opposed to Ni. A 19.3% propylene yield was achieved over 5 mol % Ni-containing MgMoO4 catalyst at 560 °C. 1. Introduction In the past decade oxidative dehydrogenation of lower alkanes has gained great importance in natural and petroleum gas utilization.1 Oxidative dehydrogenation (ODH) of propane is an interesting alternative route to propylene production. Although there are no coking and equilibrium problems in addition to the fact that one part of the process heat can be supplied by reaction exothermicity, the improvement of low selectivity to propylene is the major goal in oxidative dehydrogenation of propane. In general, catalytic active sites which are acidic in nature are necessary for the activation of poorly reactive propane, but the reaction product propylene is also adsorbed on acidic sites and can easily convert to the complete oxidation products. Since the allylic C-H bond energy of propylene is 361 kJ/mol and the secondary C-H bond energy of propane is 421 kJ/ mol,2 propylene selectivity drastically decreases at high propane conversion. Therefore, the studies on ODH of propane focus on the activation mechanism of propane, effect of oxygen type on the product selectivity, and effect of reaction conditions and catalyst compositions on the reaction mechanism1,3 so that low propylene selectivity can be improved. Most of the catalyst systems investigated for oxidative dehydrogenation of propane were composed mainly of supported molybdenum oxide and vanadium oxide and their multicomponent mixtures.4-13 Corma et al.4 have shown that catalyst preparation methods and compositions affect catalytic activity. Similarly, Carrazan et al.5 have observed the influence of preparation conditions on the synergistic effect between different phases form* To whom correspondence should be addressed. Tel.: +90 212 591 24 79. Fax: +90 212 591 19 97. E-mail: nacik@istanbul. edu.tr. † Istanbul University. ‡ Ruhr UniversitysBochum.
ing the catalyst for oxidative dehydrogenation of propane. The findings of Stern et al.,6 Yoon et al.,7 and Courtine et al.,8 who investigated metal molybdates for oxidative dehydrogenation of propane, revealed that the presence of excess MoO3 on the surface of NiMoO4 catalyst provided higher catalytic activity in comparison to the absence of the MoO3 phase on the same catalyst. Cadus and co-workers9 have observed a similar synergistic effect over MoO3/MgMoO4 catalyst. However, catalytic activity decreases with a further increase in MoO3 loading due to less reactive Mg2Mo3O11 formation by solid-state reaction or sintering.9,10 Among the several rare-earth-metal vanadates used for oxidative dehydrogenation of propane, erbium and holmium vanadates showed better propylene yield.11 Niobium pentoxide showed good propylene selectivity, but its catalytic activity was very low.12 A multicomponent Mo-V-Nb-O system has also been studied to improve catalytic performance in ODH of propane.13 In this work, we investigated the effects of nickel, lanthanum, or yttrium loading on the catalytic activity of magnesium molybdate (MgMoO4) for oxidative dehydrogenation of propane and on the catalyst matrix. 2. Experimental Section 2.1. Catalyst Preparation. Ammonium heptamolybdate (Merck), magnesium nitrate (Merck), nickel nitrate (Merck), lanthanum chloride (BDH), yttrium chloride (BDH), and citric acid (Merck) were used for catalyst preparation as they were supplied without any treatment. Pure MgMoO4 and MeMgMoOx catalysts doped with 5, 10, and 15 mol % Me (Me ) Ni, La, Y) [Me(005), Me(010), and Me(015), respectively] were prepared by the citric acid method.14 The molar ratio of Mg to Mo in metal salts for the preparation of unloaded MgMoO4 catalyst was set to 1, and the molar ratio of Me was 5, 10, or 15 mol % of Mg for metal-doped catalyst (MeMgMoOx) preparation. Metal salts were dissolved in water, and citric acid was added to the
10.1021/ie030741j CCC: $27.50 © 2004 American Chemical Society Published on Web 04/03/2004
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solution in the molar ratio 1/4 of the total metal ion. Once a clear solution was obtained, it was heat-treated at 80 °C in an oven to obtain a viscous gel. Afterward, the gel was dried at 120 °C for 8 h and calcined at 600 °C in air for 3 h. Powder catalysts were sieved, and the fraction with a size of 250-355 µm was used both in catalytic testing and in characterization. A 0.4 g sample of catalyst diluted with 0.4 g of quartz chips was used for all catalytic experiments. 2.2. Characterization. Nitrogen physisorption measurements were carried out at -196 °C with 150 mg of sample using a Quantachrome Autosorb 1C preceded by high vacuum degassing of the catalysts at 300 °C. X-ray powder diffraction measurements were performed with a Siemens D-type diffractometer with Cu KR irradiation (λ ) 1.5404 Å). Raman spectra were recorded using a Nicolet Nexus FT-Raman spectrometer equipped with an InGaAs detector. Raman scattering was excited with a Nd:YAG laser operated between 150 and 1200 cm-1 with 4 cm-1 scans. Temperature-programmed reduction (TPR) measurements were conducted in a quartz microreactor equipped with a temperature controller using 150 mg of catalyst. A furnace was used to heat the reactor up to 850 °C at a rate of 10 °C/min under H2/Ar flow containing 4.7% H2. Water was eliminated in a cold trap, and hydrogen determination was done using an on-line Hydros 100 TCD. 2.3. Catalytic Tests. Oxidative dehydrogenation of propane was carried out in a U-type quartz reactor at 450 and 560 °C under atmospheric pressure. Oxygen and neon with 99.99% purity were supplied by MesserGriesheim, and propane with 99.95% purity was provided by Linde. The feed was a mixture of C3H8/O2 in a molar ratio of 1/1 (10.7%/10.7%), 1.5/1 (16.05%/10.7%), and 2/1 (21.4%/10.7%) with neon as the balance, and the total flow rate was varied between 37.5 and 150 mL/ min. Homogeneous temperature distribution in the reactor was provided by a fluidized bed sand bath furnace. The temperature of catalyst bed was monitored using a coaxial thermocouple installed in a quartz capillary tube. Catalysts have been diluted with quartz chips at a weight ratio of 1/1 to provide 0.8 g of catalyst charge. The dead volume of the reactor was filled with quartz chips to avoid any homogeneous reactions. Both reactant gases and reaction products were analyzed using a Satochrome on-line GC system with TCD and FID detectors using molecular sieve (for Ne, O2, and CO) and Poraplot Q (for hydrocarbons, CO2, and H2O) columns. 3. Results and Disscussion The results for BET specific surface area (SA), pore volume (Vg), and phases detected in the XRD pattern of the catalysts are given in Table 1. Nickel loading did not significantly affect the surface area of magnesium molybdate catalyst, and there is no clear difference in surface area and pore volume values in nickel-loaded catalyst compared to the parent catalyst. In contrast, in both Y- and La-loaded catalysts, surface area and pore volume values decreased with metal loading. XRD patterns of MgMoO4 and MeMgMoOx catalysts are given in Figure 1. The diffraction patterns of MgMoO4 catalyst revealed the presence of a pure MgMoO4 phase, as Miller and co-workers10 have reported the efficiency of the citric acid method to prepare pure MgMoO4 in comparison to solid-state reaction. Although neither NiMoO4 nor any other different phase was detected in
Figure 1. X-ray diffractograms of MgMoO4 and MeMgMoOx catalysts containing 15% Me: MgMoO4 (b); Y2Mo3O12 (9); Y2O3 (0); La2Mo2O7 (*); La2Mo3O12 (+).
Figure 2. Raman spectra of MgMoO4 and MeMgMoOx catalysts containing 15% Me. Table 1. Physisorption Measurement Results and XRD Phases of the Catalysts catalyst
SA (m2/g)
Vg (cm3/g)
Mg Ni(005) Ni(010) Ni(015) Y(005)
26 26 23 22 22
108 146 93 102 50
Y(010) Y(015) La(005)
16 14 18
34 41 76
La(010) La(015)
16 11
64 36
XRD phases (JCPDS files) MgMoO4 (72/2153) MgMoO4 MgMoO4, Y2Mo3O12 (30/1455), Y2O3 (44/0399) MgMoO4, La2Mo2O7 (84/1234), La2Mo3O12 (70/1382)
Ni-loaded samples, amorphous and/or highly uniform distributed NiMoO4 might have occurred probably due to solid-solution formation in the MgMoO4 matrix. In Y-promoted catalysts, Y2O3 was also observed in addition to MgMoO4 and Y2Mo3O12 phases. In Lapromoted catalysts, two different lanthanum molybdate phases, La2Mo2O7 and La2Mo3O12, and MgMoO4, were detected. Raman spectra are given in Figure 2. The peaks at 852, 908, 957, and 969 cm-1 are ascribed to the MgMoO4 phase.9 The peak at 815 cm-1 ascribed to stretching of the Mo-O-Mo bonds in MoO3 is not
2378 Ind. Eng. Chem. Res., Vol. 43, No. 10, 2004 Table 2. TPR Data of the Catalysts first peak Tmax (°C)
catalyst Mg Ni(005) Ni(010) Ni(015) La(005) La(010) La(015) Y(005) Y(010) Y(015)
second peak Tmax (°C)
H2 consumption (µmol/g)
827 689 672 668 827 831 843 827 803 788
0.992 1.990 2.628 2.365 2.185 1.910 1.900 2.155 2.290 2.173
576 573 575
590 602 610
Table 3. Catalytic Test Results of MeMgMoOx (Me ) Ni, Y, La) for ODH of Propanea T convn (%) catalyst (°C) of C3H8 C3H6 Mg
Figure 3. TPR curves of MgMoO4 and Ni-containing NiMgMoOx catalysts.
Ni(005) Ni(010) Ni(015) Y(005) Y(010) Y(015) La(005) La(010) La(015)
450 560 450 560 450 560 450 560 450 560 450 560 450 560 450 560 450 560 450 560
1.8 17.3 5.7 30.1 4.5 27.2 3.1 23.1 2.6 14.2 3.1 13.9 2.0 11.9 3.3 16.6 1.7 11.5 1.5 8.5
78.9 71.3 76.8 60.8 80.8 59.5 81.9 60.0 82.7 73.2 99.5 73.9 82.9 73.3 81.2 72.9 96.7 74.0 95.9 78.1
selectivity (%) CO
CO2
7.7 15.5 9.7 21.4 8.5 26.3 4.3 25.5 7.6 14.4 trace 14.9 6.4 14.1 9.0 13.4 trace 13.0 0.7 10.5
10.3 9.5 12.6 14.7 9.5 11.3 12.7 11.6 9.1 9.5 trace 8.6 9.4 9.2 8.5 10.8 trace 8.0 1.1 8.7
C2H4 CH2CH2CHO 0.9 1.8 0.5 1.7 0.6 1.5 0.5 1.4 0.6 1.2 0.0 0.9 0.5 0.8 0.5 1.3 1.1 2.5 1.3 1.2
2.2 1.8 0.6 1.4 0.6 1.3 0.6 1.5 0.0 1.9 0.0 1.8 0.8 2.5 0.9 1.6 2.0 1.5 1.0 1.6
a Flow conditions: C H , 10.7 kPa; O , 10.7 kPa; rest, Ne; total 3 8 2 flow rate 75 mL/min.
Figure 4. TPR curves of Y- and La-containing MeMgMoOx catalysts.
observed.9 In the Ni(015) spectrum, the 962 cm-1 peak is attributed to NiMoO4,15,16 which could not be detected in the XRD pattern, and the shoulder at 936 cm-1 in La(015) indicates the presence of lanthanum molybdate phases.17 The peak seen only in Y(015) at 839 cm-1 is probably due to O22- since it is assigned to an O-O stretching vibration.16,18 The TPR profiles of the catalysts, which are all on the same scale, TPR peak maxima, and hydrogen consumptions are given in Figures 3 and 4 and Table 2, respectively. MgMoO4 exhibits only one reduction peak at 827 °C. As seen in Figure 3, even only 5 mol % Ni loading significantly increased the reducibility of the MgMoO4 matrix. The reduction peak of MgMoO4 shifted from 827 to 689 °C in Ni(005) catalyst, and hydrogen consumption also increased. The reduction peak of low intensity at 575 °C in the TPR profiles of Ni-loaded catalysts might be due to Ni-incorporated sites since the peak intensity increases gradually with Ni ratio up to 10 mol %. However, La promotion made a significant
change neither in the reduction temperature of MgMoO4 nor in H2 consumption. In Y-modified catalysts, a small shoulder arose around 610 °C, and the major reduction peak of MgMoO4 was slightly shifted to lower temperatures upon the increase in Y loading. The reducibility of MgMoO4 by metal loading increased as follows: Ni > Y > La. The strong effect of Ni loading on the reducibility of MgMoO4 was also confirmed by high catalytic activity of nickel-loaded catalysts among the catalysts investigated in this work, which are presented in Table 3 and Figure 5. Although pure MgMoO4 shows 17.3% propane conversion at 560 °C, it significantly increased up to 30.1% even with only 5% Ni-loaded Ni(005) catalyst at the same temperature. Propane conversion over Ni-loaded magnesium molybdate catalyst at 560 °C increased with Ni loading up to 5 mol %, but a further increase in metal loading led to a slight decrease in conversion values, which are higher than those over pure MgMoO4. Selectivity to propylene decreased from 71.3% over MgMoO4 to 60.8% over Ni(005) containing 5 mol % Ni and then remained almost constant with a further increase in Ni loading. The influence of metal loading on propylene yield at 560 °C is given in Figure 6. All Ni-containing catalysts among the catalysts investigated in this work have better propylene yields, and in the case of MgMoO4 and Ni(005) catalysts, propylene yields at 560 °C are found to be 12.3% and 18.3%, respectively. Although a small increase in propylene selectivity over Y- and La-modified catalysts was observed, propylene yields over them decreased gradually with metal loading.
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Figure 5. Effect of Ni content on the MeMgMoOx catalyst on C3H8 conversion and C3H6 selectivity in ODH of propane at 560 °C. Reaction conditions: C3H8, 10.7 kPa; O2, 10.7 kPa; rest, Ne; total flow rate, 75 mL/min.
Figure 7. Effect of the C3H8/O2 ratio on C3H8 conversion and product (C3H6, CO, and CO2) selectivity in ODH of propane over MgMoO4 catalyst (total flow rate 75 mL/min).
Figure 6. Effect of Me content on the MeMgMoOx catalyst on the C3H6 yield in ODH of propane at 560 °C. Reaction conditions: C3H8, 10.7 kPa; O2, 10.7 kPa; rest, Ne; total flow rate 75 mL/min.
Oxidative dehydrogenation of propane over molybdate catalysts proceeds by a Mars-van Krevelen-type redox mechanism,19 and the oxygen depletion rate from a redox-type catalyst depends on the mobility of lattice oxygen.20 In comparison to pure MgMoO4 with Ni(005), Ni loading enhanced the reducibility of the catalyst, namely, oxygen depletion from lattice sites. MgMoO4 and NiMoO4 are semiconductor-type materials,21 and the ionic radii of Mg and Ni are very close to each other, 0.72 and 0.70 Å, respectively. For perovskite and some oxide catalyst systems,22,23 when ionic radii of the main metal and dopant are similar, the solid-solution energy and oxygen conductivity increase, but if the dopant radius is higher than that of the main metal, segregation of the dopant on the surface probably occurs. The ionic radii of Y and La are 0.90 and 1.03 Å, respectively. If there is structural similarity between catalyst phases, coherence of interfaces arises, and ions at the interfacial plane may enhance the restructuring of counterions (in this case Mg and Ni ions) and solid-solution phenomena between phases.24 It is known that only Mo-OMo bonds of MgMoO4 can be reduced in ODH of propane conditions.9 In general, the reducibility of Me-O-Mo bonds in metal molybdates is affected by metal electropositivity.25 Since Ni is less electropositive than Mg, Ni-O-Mo bonds can be reduced easier than Mg-OMo bonds. For example, Ni(005) was prepared to obtain a theoretical mixture of 5 mol % NiMoO4 and 95 mol %
Figure 8. Effect of the C3H8/O2 ratio on C3H8 conversion and product (C3H6, CO, and CO2) selectivity in ODH of propane over Ni(005) catalyst (total flow rate 75 mL/min).
MgMoO4, but under the reaction conditions local nonstoichiometry can occur due to the difference in redox behaviors of the phases; in this way oxygen diffusion into the bulk oxide is probably facilitated through an anionic lattice vacancy model. As can be seen from the TPR results of the Ni-loaded catalyst, both reduction peak maxima were decreased to lower temperatures and hydrogen consumption increased. This indicates that deeper reduction is probable in the Ni-loaded catalyst at lower temperatures than that in pure MgMoO4. A similar behavior has been observed in modified NiMoO4 catalysts.20,26 The influence of the molar ratio of C3H8 to O2 for oxidative dehydrogenation of propane over Mg and Ni(005) catalysts at 560 °C is presented in Figures 7 and 8, respectively. As the C3H8/O2 molar ratio increases, propane conversion over Mg catalyst decreases from 17.3% to 10.2%. However, Ni(005) catalyst exhibits approximately 30% propane conversion even at a C3H8/O2 molar ratio of 2 in addition to a slight increase in propylene selectivity. This situation provides a significant increase in propylene productivity per unit mass of catalyst per hour, as it given in Figure 9. Propylene productivity over Mg catalyst was almost the same for all C3H8/O2 molar ratios. However, in the case of Ni(005), it increased more than twice in comparison to that over Mg catalyst at highest propane/oxygen
2380 Ind. Eng. Chem. Res., Vol. 43, No. 10, 2004
loading. Further studies will proceed to modify the reducibility of conventional molybdate catalysts with suitable metal additives as well as to investigate their relationships with the catalytic performance for ODH of lower alkanes. Acknowledgment This work was supported by the Research Fund of Istanbul University, Project No. 1448/05052000. Literature Cited
Figure 9. Effect of the C3H8/O2 ratio on propylene productivity over MgMoO4 and Ni(005) catalysts for ODH of propane at 560 °C (total flow rate 75 mL/min). Table 4. Effect of Bed Residence Time on the Product Yield over Ni(005) Catalyst for ODH of Propanea conversion W/F (g‚s‚mL-1) (%) of C3H8 C3H6 2.64 1.32 0.66
39.3 28.9 14.5
19.3 18.0 11.3
CO 11.4 6.2 1.6
product yield (%) CO2 C2H4 CH2CH2CHO 7.1 3.5 0.9
1.0 0.6 0.2
0.6 0.5 0.3
a Reaction conditions: C H , 10.7 kPa; O , 10.7 kPa; rest, Ne; 3 8 2 T ) 560 °C.
ratios in these reaction conditions. This also indicates that Ni loading facilitates oxygen depletion from catalyst lattices. The effect of residence time on product yield over Ni005 catalyst is presented in Table 4. Yields of both propylene and secondary hydrocarbons (ethylene and acrolein) increase with residence time. It is known that a decrease in the flow rate for the same amount of catalyst causes longer contact times between the reaction gas stream and catalyst surface. Therefore, in our case both propane conversion and product yield increased. At these reaction conditions, Ni-loaded magnesium molybdate catalyst gave a 19.3% propylene yield, which can be regarded as one of the significant propylene yields obtained so far over magnesium and nickel molybdates.6,7,9,10 As expected the total COx yield is also increased because in the case of higher propane conversion, the propylene population on the catalyst surface increases and stimulates the formation of COx as well. Acrolein and ethylene are major secondary hydrocarbons, and their yields also increased from 0.5% to 1.6% in total with bed residence time. 4. Conclusion Oxidative dehydrogenation of propane is one of the interesting selective oxidation reactions. It is necessary to activate propane at low temperatures and to improve hydrocarbon product (alkene) selectivity at high propane conversions. Most probably in the case of Ni-loaded catalysts, either so-formed NiMoO4 is homogeneously distributed in MgMoO4 in the form of small particles or its solid-solution formed in the latter. Nickel loading improved the oxygen depletion/reducibility, catalytic activity, and propylene productivity of MgMoO4. A propylene yield of 19.3% and a total hydrocarbon yield of 21.9% were achieved over 5 mol % Ni-loaded catalyst. La and Y loading on MgMoO4 was not as efficient as Ni
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Received for review September 30, 2003 Revised manuscript received February 25, 2004 Accepted February 25, 2004 IE030741J
