Ind. Eng. Chem. Res. 1997, 36, 3533-3540
3533
Influence of the Preparation Method and Metal Precursor Compound on the Bifunctional Ni/HZSM-5 Catalysts Marı´a D. Romero,* Jose´ A. Calles, and Araceli Rodrı´guez Department of Chemical Engineering, Complutense University, Madrid, Spain E-28040
The influence of the preparation method and the nature of the precursor compound on the hydroisomerization activity of nickel catalysts was studied. Two preparation methods, mechanical mixing, and incipient wetness impregnation, with different precursor compounds were used to manufacture catalysts containing 2, 5, and 10 wt % of nickel. Temperature-programmed desorption of ammonia (TPDA), temperature-programmed reduction, scanning electron microscopy, X-ray diffraction line broadening, and atomic absorption spectroscopy techniques were used to characterize the catalysts. Catalytic behavior was tested by hydroisomerization of n-decane reactions. TPDA measurements revealed that acid strength distribution of the catalysts prepared by impregnation differs from that of the bare zeolite whereas the catalysts prepared by mechanical mixing showed similar acid strength distribution than zeolites. Nickel reducibility depends on the preparation method as well as the nickel precursor compound. Incipient wetness impregnation leads to higher hydroisomerization activity than mechanical mixing due to the more adequate contact between acid and metal function obtained with the impregnation method. 1. Introduction The deep conversion of residual oils to marketable products is one of the future challenges for oil refiners. Lubes and middle-distillate fuels with advanced performance, environmental benefits, and safety benefits are in increased demand. Considerable current emphasis takes place on highly paraffinic oils for these uses due to their high-oxidation stability, low volatility at a given viscosity, and high viscosity index. Hydrogenation is the key process in converting residual products into lighter ones, and it is better controlled in catalytic processes than in thermal processes. The isomerization of normal paraffins has been practiced in oil refining for years in order to upgrade low-octane number of light straight-run naphtha, to meet the increasing demand for high-octane unleaded gasoline (Kouwenhoven and van Zijll Langought, 1971; van Santen et al., 1985; Anders et al., 1990; Fujimoto et al., 1992; Campelo et al., 1995) and for improving cold flow properties such as pour point, viscosity, etc., to use the heavy and distilled fractions as lube oils (Angevine and Oleck, 1987; Zakarian et al., 1987; O’Rear and Lok, 1991; Grau and Parera, 1993; Miller, 1994; Taylor and Petty, 1994). Both processes require specific shape selective cracking and isomerization catalysts to convert the normal and slightly branched paraffins, leaving the multibranched ones unreacted. The hydroisomerization of normal paraffins requires the use of bifunctional catalysts (Wilson and Cooper, 1981) which contain both acid and hydrogenating/dehydrogenating components. The ideal catalysts for selective hydroisomerization should have both selectivity for isomerization which comes from the correct balance of acidity and hydrogenation activity (Gianetto et al., 1986; Degnan and Kennedy, 1993; Girgis and Tsao, 1996) and selectivity for reacting only with normal paraffins, which comes from size pore opening of the molecular sieve used. Zeolite ZSM-5 has been reported to exhibit unusual reaction selectivities and catalytic stabilities (Martens et al., 1991). Moreover, it is well-known that the * To whom all correspondence should be addressed. Email:
[email protected]. S0888-5885(96)00775-0 CCC: $14.00
catalytic behavior of ZSM-5 can be modified by incorporation of metals in order to obtain catalysts for selective hydrocarbon conversions (Hoang et al., 1994). The composition of the reaction products is linked to the reaction temperature and the relative strength of the hydrogenation and cracking activities in the catalysts, acid-metal balance, determined by the nature of the acid support and the hydrogenating function as well as the preparation method (Gil et al., 1994). The preparation method of bifunctional catalysts is as important as the chemical composition, setting both catalyst properties (Sivasanker and Ratnasamy, 1990). The objective of a preparation method is to distribute the active phase (metal) over the support in the most efficient way, highly dispersed, to obtain large specific surface areas and thus the maximum activity per weight of active compound over the support surface. The dispersion of the metal and its stability depend on the metal-support interaction. To manufacture catalysts in an efficient and reproducible process, it is essential to gain the control over parameters like the loading, dispersion, and location of the metal in the catalysts. Several types of preparation techniques may be used: mechanical mixing of metal compound and support, ion exchange processes, coprecipitation of metal oxides and support, impregnation processes, etc. (Foger, 1984; Stiles, 1987; Satterfield, 1991). The metal distribution is mainly determined by the preparation method, but the nature of the metal precursor compound has also an important effect over the relationship between acid and metallic functions, determining the state of the metal (Gil et al., 1994; Rynkowski et al., 1993; Bartholomew et al., 1980; Campelo et al., 1982; Satterfield, 1991). The acid and metal site density and strength distribution are both important, and their proper balance is critical in determining the reactivity and selectivity of bifunctional catalysts. In the present work, the influence of the metal incorporation technique and metal precursor nature on activity in n-decane hydroisomerization of bifunctional catalysts constituted by nickel and the ZSM-5 zeolite were studied. ZSM-5 was selected as the acid function because it exhibits the desired molecular shape selectiv© 1997 American Chemical Society
3534 Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997
ity for industrial use (O’Rear and Lok, 1991). The advantages of nickel as the hydrogenation-dehydrogenation function, in spite of its weaker activity as compared with noble metals (Pd and Pt), lie in their relatively low cost, remarkable thermal stability, and resistance to poisoning (Marinas et al., 1986), so it is widely used in industrial processes (Hoang et al., 1994; Gil et al., 1994; Welters et al., 1995b). Two different preparation techniques, mechanical mixing (MMX) and incipient wetness impregnation (IWI), were used, each one with different precursor compounds: nickel oxide and nickel chloride in the first method and nickel nitrate, nickel acetylacetonate, and (triethylenediamine)nickel nitrate in the second one. The catalysts were mainly characterized by temperature-programmed desorption of ammonia, TPDA, temperature-programmed reduction with hydrogen, TPR, and scanning electron microscopy, SEM. X-ray diffraction and atomic absorption spectroscopy were also carried out. 2. Experimental Methods 2.1. Catalysts Preparation Procedures. All the catalysts were prepared from ZSM-5 as the acid function and nickel as the metallic function. The zeolite was synthesized in sodium form by a procedure developed in our laboratory (Costa et al., 1987), with an atomic ratio Si/Al ) 29/1, crystallinity of 100%, and 8 µm medium particle size. The acid form of the zeolite was prepared by ion exchange with 50 mL/g of catalyst of 0.6 N HCl solution, in agitation for 5 h at room temperature. The nickel incorporation was carried out by two different methods: (a) Mechanical mixing: a weighted amount of HZSM-5 and a nickel precursor, NiO or NiCl2, were first ground and sieved and then calcined in an oxidizing atmosphere at 550 °C for 5 h. (b) Incipient wetness impregnation: the aqueous solution of the nickel precursor was slowly added to the support at room temperature. Next, the solid was dried at 110 °C for 14 h and calcined in an oxidizing atmosphere at 550 °C for 5 h. The volume of solution needed was calculated by using the water pore volume of the zeolite, and the concentration was adjusted to obtain the desired metal loading. Finally, the metal phase was activated by reduction of nickel compounds to the ground state, Ni, responsible for the hydrogenating-dehydrogenating function. Activation of the samples was carried out in a hydrogen stream at 450 °C for 3 h, and a hydrogen flow of 50 NmL/min/g of catalyst in a tubular fixed-bed reactor. 2.2. Characterization. Atomic absorption was used in order to determine the nickel content (wt %). The measurements were carried out in a ThermoJarrel ASH Corp. Smith Hieftje/11 spectrophotometer with simple beam and Smith Hieftje background correction. The catalysts were broke up in hydrofluoric acid and diluted to the interval of measurement with water. X-ray diffraction (XRD) was used to measure the crystallinity of the synthesized zeolites. The diffractograms were collected with a Siemens Kristalloflex D-500 diffractometer having a CuKR radiation and a Ni filter. Crystallinity was decided from the peak high at 2θ ) 23.1 using a 100% crystalline zeolite pattern as reference. A Philips X’PERT MPD diffractometer with CuKR radiation and a Ni filter was used to measure nickel oxide particle size on the calcined catalysts by X-ray diffraction line broadening, using the Debye-Scherrer method (Bermu´dez-Polonio, 1981). Computer facilities
for data collection enabled us to fit peak intensity for the (200) reflection from which the Bragg angle, 2θ, and the angular width at half maximum intensity were determined. A correction for instrument broadening under identical conditions was made using a wellcrystallite zeolite sample of 7 µm particle size. The lower limit of detection for this method is 5 nm, with the result that it gives information on the presence of nickel oxide located on the external zeolite surface in the calcined catalysts. The acidic properties of samples were studied by means of temperature-programmed desorption of ammonia (TPDA), performed with a dynamic Micromeritics TPD/TPR (temperature-programmed reduction) 2900 analyzer characterization system, provided with a thermal conductivity detector. The total acidity of the catalysts was characterized by the total ammonia desorbed during the TPDA runs by integration of the TPDA profiles. The detailed experimental proceduresoutgassing, metal reduction, and ammonia saturation and desorptionsand the acidity calculation methodology have been fully described earlier (Romero et al., 1996, 1997). TPR measurements were carried out with the same apparatus described for TPDA. First, the samples were outgassed in the flowing up to 550 °C and afterward cooled and stabilized in an Ar/H2 (85/15) flowing. The temperature and detector signals were then continuously recorded while the sample was heated at a rate of 10 °C/min from 50 °C to 550 °C. The total area under the profile was used to obtain the quantity of reduced nickel. The profile was also fitted to several peaks to determinate the different reducibilities of the catalysts. Morphology of catalysts and distribution of metallic species were revealed by scanning electron microscopy (SEM). The SEM measurements were carried out using a JEOL JSM-6400 microscope equipped with an energy dispersive X-ray analyzer. The images were taken with an emission current of 100 µA by a wolframium filament and an accelerator voltage of 20 kV. The pretreatment of the samples consisted of coating with an evaporated Au film in a Balzers SCD004 sputter coater metallizator to increase the catalysts electric conductivity. 2.3. n-Decane Conversion. The hydroisomerization of n-decane over bifunctional zeolite catalysts has been widely used for determining the pore topology of zeolites (Martens and Jacobs, 1986; Corma et al., 1994). The balance of the two catalytic functions, acid and metal, determines the reaction mechanism of skeletal hydroisomerization and hydrocracking of alkanes, and thus the hydroconversion of n-decane has also been proposed as a model to characterize bifunctional catalysts (Martens et al., 1991; Welters et al., 1995a,b; Romero et al., 1996, 1997). The conversion of n-decane was carried out in a fixed-bed stainless steel reactor under a total pressure of 5055 kPa at 300 °C. Hydrogen and n-decane were continuously fed to the reactor with a contact time, W/F, of 26.6 g‚h/mol and a hydrogen to n-decane ratio of 10.9/1. The catalyst samples were previously activated in situ by a hydrogen treatment at 450 °C for 3 h. The reaction products, mainly alkanes C1-C10, were analyzed using gas chromatography, in a Hewlett-Packard chromatograph using a Porapack Q 80/ 100 ASTM column and a thermal conductivity detector for the gas phase and a fused silica capillary column and a flame ionization detector for the liquid one.
Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 3535 Table 1. Catalysts Samples: Nickel Content (AA), Crystallite Size (XRD), Preparation Method, and Precursor Compound
a
catalysts
Ni (wt %)
dXRD (nm)
preparation method
precursor compound
MMX-Ox-2 MMX-Ox-5 MMX-Ox-10 MMX-Cl-1 MMX-Cl-2 IWI-Nt-2 IWI-Nt-5 IWI-Ac-2 IWI-Ac-5 IWI-Et-2 IWI-Et-5
2.0 5.0 10.0 1.6 2.4 2.0 5.0 2.0 5.0 2.0 5.0
22.9 25.0 25.8 a 34.1 16.0 16.8 20.0 25.1 13.8 25.8
mechanical mixing mechanical mixing mechanical mixing mechanical mixing mechanical mixing incipient wetness impregnation incipient wetness impregnation incipient wetness impregnation incipient wetness impregnation incipient wetness impregnation incipient wetness impregnation
NiO NiO NiO NiCl2 NiCl2 Ni(NO3)2 Ni(NO3)2 Ni(C5H7O2)2 Ni(C5H7O2)2 [Ni(C2H8N2)3](NO3)2 [Ni(C2H8N2)3](NO3)2
Undetectable by X-ray diffraction line broadening.
3. Results and Discussion Table 1 summarizes some characteristics of the prepared catalysts: the nickel precursor and incorporation method, nickel content determined by atomic absorption and crystallite size of nickel oxide in the calcined catalysts estimated from X-ray diffraction line broadening. 3.1. Catalysts Characterization. Acidity Measurements. Temperature-programmed desorption of ammonia was carried out to compare the acidity of the catalysts. In order to determine the acid strength distribution, the experimental profiles were fitted by Gaussian deconvolution, using two or three peaks, showing that the deconvoluted peaks appeared at the same temperatures in all the samples and, thus, were kept constant. Weak, A1, medium, A2, and strong, A3, acidities were defined as the areas under the peak at low, T1, intermediate, T2, and high, T3, temperatures, respectively. The use of these peaks to fit the profiles was not based on any peak assignment to a specific acid site (Bronsted or Lewis) but, instead, was a helpful procedure to categorize the acid strength distribution obtained by this method (Taylor and Petty, 1994; Romero et al., 1996, 1997). Figure 1 shows the desorption profiles and the Gaussian deconvoluted peaks of two catalysts prepared by mechanical mixing with NiO (b) and by incipient wetness impregnation with nickel nitrate (c), both with a 5 wt % of nickel content and the desorption profile of a sample without any nickel content (a), HZSM-5 zeolite. As it can be clearly seen, the zeolite and the catalyst prepared by mechanical mixture show only two deconvolution peaks located at 337 °C and 470 °C named as weak, A1, and strong, A3, acidity, respectively, while the catalyst prepared by incipient wetness impregnation displays three deconvolution peaks, two of them are equal to the preceding ones and a third located at an intermediate temperature, 414 °C, named as medium acidity, A2. Table 2 gives quantitative information about the total acidity as well as the acid strength distribution of the samples. This means that nickel incorporated by impregnation implies a modification of acidity strength distribution of the catalysts (three deconvoluted peaks) with respect to the zeolite while the catalysts prepared by mechanical mixing show a similar distribution than the zeolite one (two deconvoluted peaks). On the other hand, it has to be pointed out that the total acidity, AT, decreases in a noticeable way for the catalysts prepared by mechanical mixing with NiO and for the ones prepared by impregnation with nickel acetylacetonate compared to the bare zeolite, while the other ones show values of AT closer to the HZSM-5 zeolite. The metal loading seems not to have
Figure 1. TPDA curves and Gaussian deconvoluted peaks. Catalysts: (a) HZSM-5, (b) MMX-Ox-5, and (c) IWI-Ac-5. (s) TPDA profiles, (- - -) deconvoluted peaks, and (- - -) temperature.
any important effect over the total acidity apart from the catalysts prepared by impregnation with (triethylenediamine)nickel nitrate, which shows a slight decrease of the total acidity for a metal loading of 5%. It is well-known that not only Bronsted sites but also Lewis sites in solid catalysts can enter in strong interaction with gaseous bases such as ammonia (Kung, 1989). Nickel cations can be formally considered as Lewis acid centers and previous investigations on Nimodified Y zeolites have provided evidence that the high-temperature desorption features of ammonia can be ascribed to desorption of ammonia bounded to coordinately unsaturated nickel cations (Minchev et al., 1994). In this mean, the nickel incorporated by impregnation seems to be able to retain ammonia, since a new peak appears in these catalysts. However, no increase of the total acidity was found as was expected. The modifications over the acidity that occurs when a metal is incorporated to an acid support can be due
3536 Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 Table 2. Catalysts Acidity: Total Acidity and Gaussian Deconvoluted Peaks Gaussian deconvoluted peaks weak acidity a
a
catalysts
total acidity AT (mequiv NH3/g)
T1 (°C)
A1 (mequiv/g)
HZSM-5 MMX-Ox-2 MMX-Ox-5 MMX-Ox-10 MMX-Cl-1 IWI-Nt-2 IWI-Nt-5 IWI-Ac-2 IWI-Ac-5 IWI-Et-2 IWI-Et-5
0.53 0.47 0.46 0.49 0.56 0.54 0.53 0.45 0.45 0.55 0.50
337 337 337 337 337 337 337 337 337 337 337
0.17 0.12 0.14 0.16 0.17 0.17 0.20 0.16 0.20 0.19 0.18
a
medium acidity T2 (°C)
414 414 414 414 414 414
a
strong acidity a
A2 (mequiv/g)
T3 (°C)
A3 (mequiv/g)
0.09 0.07 0.09 0.09 0.06 0.09
479 479 479 479 479 479 479 479 479 479 479
0.36 0.35 0.32 0.33 0.39 0.28 0.26 0.20 0.16 0.30 0.23
Based on grams of zeolite.
to (i) blockage of the pore system and superficial sites by large size metallic particles and (ii) deactivation of acid sites and pore blockage by carbonaceous deposits produced during the precursor compound decomposition in the calcination step that hinder ammonia from reaching the acid sites. The predominant effect depends on the preparation method and precursor compound used. Thus, the reason for the acidity decrease in the catalysts prepared by mechanical mixing with nickel oxide seems to be the large size of the metal particles, while in the catalysts prepared by incipient wetness impregnation must also be considered the carbonaceous deposits formation when organic precursors are employed. This fact should be more important in the IWIAc catalysts than in the IWI-Et ones because of the larger size and the higher carbon content of the precursor. XRD Line Broadening. As can be seen in Table 1 crystallite sizes of nickel oxide in the calcined catalysts range from 13.8 to 34.1 nm, depending on the preparation method, metal precursor, and nickel loading. Thus, the catalysts prepared by mechanical mixing exhibit larger crystallite size than those prepared by impregnation for a metal content of 2%, being the IWI-Et-2 which shows the smallest crystallite size. The crystallite size grows, as expected, with metal content (Coughlan and Keane, 1991), slightly in the catalysts prepared by mechanical mixing with NiO and those prepared by impregnation with nickel nitrate and highly in the catalysts prepared by impregnation with organic precursor compounds, especially with (triethylenediamine)nickel nitrate. Hence, the catalysts IWI-Nt-5 show the smallest crystallite size for a metal content of 5%. The catalyst prepared by mechanical mixing with nickel chloride and 2.4% metal loading shows a surprising high crystallite size. Scanning Electron Microscopy. The morphology of the nickel in some of the above samples was determined by electron microscope examination. Figure 2 shows a SEM microphotograph of the following catalysts: (a) IWI-Nt-5, (b) IWI-Ac-5, (c) IWI-Et-5, and (d) MMX-Ox5. They show a clear difference between the catalysts prepared by impregnation and mechanical mixing. In the first case (a, b, and c), it can be seen that small nickel oxide particles are distributed over the zeolite surface crystals, whereas the catalyst prepared by mechanical mixing (d) shows clean surface zeolite crystals beside quite large (ca. 6 µm) agglomerates of small nickel oxide particles. Although the accuracy of the SEM measurements performed does not allow the size of the metal particles to be determined, they seem to be similar in all the catalysts, in agreement with XRD
measurements. As mentioned above the nickel oxide crystallite size of the MMX-Ox-5 catalyst is 25.0 nm, similar to those obtained for the catalyst IWI-Ac-5 and IWI-Et-5, 25.1 and 25.8 nm, respectively. However, the metal distribution in the impregnation catalysts is better than that obtained in the mechanical mixing catalyst where large agglomerates of small particles are formed. TPR Measurements. Temperature-programmed reduction of the catalysts was carried out to determine the differences in the reducibilities of these catalysts, related to the location and dispersion of nickel after the calcination step, and the metal-support interaction depending on the differences between the TPR profiles of the catalysts and the profile that corresponds to the unsupported nickel oxide. Figure 3 shows the TPR profiles for the catalysts prepared by mechanical mixing with nickel oxide and nickel chloride, MMX-Ox and MMX-Cl, and for an unsupported commercial nickel oxide. Unsupported nickel oxide (a) is reduced to metallic nickel with a simple reduction peak between 225 °C and 350 °C, with a maximum at 285 °C. In this respect, the reduction of the metal in the catalysts prepared with nickel oxide MMX-Ox (b, c, and d), occurs at the same temperature as unsupported nickel oxide, which indicate a weak metal-support interaction. However, the reduction profiles exhibit two peaks which move away and become more evident as the nickel loading increases. On the other hand, the TPR profiles of the catalysts prepared with nickel chloride (e and f) show higher reduction rate maxima than that of unsupported nickel oxide (a), which evidence stronger metal-support interactions than those shown in the MMX-Ox catalysts. This can be explained by solid ion exchange reactions between metal and zeolite acidic groups that can be produced when a mixture of zeolite, usually in acid form, and a metal precursor compound (salts or oxides) is subjected to high temperatures (Karge et al., 1988, 1992; Kucherov and Slinkin, 1986, 1987a,b; Beyer et al., 1988). It has been shown in previous studies (Wichterlova´ et al., 1989; Beran et al., 1990) that the reaction of various metal salts or oxides with the HZSM-5 zeolite depends on the nature of the anion. The highest exchange degrees were reached with metal chlorides, where hydrochloric acid is released into the gas phase under preservation of the zeolite structure. The deaggregation of oxides and incorporation of their cation into the zeolites seemed to depend strongly on their melting points (Wichterlova´ et al., 1989; Beran et al., 1990), while NiO (mp, 600 °C) did not interact with the
Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 3537
Figure 2. Scanning electron microscopy photographs. Catalysts: (a) IWI-Nt-5, (b) IWI-Ac-5, (c) IWI-Et-5, and (d) MMX-Ox-5.
HZSM-5 zeolite and Mn3O4 (mp, 1700 °C) exchanged 40% of the Mn cations in the same conditions. Figure 4 shows the TPR profiles obtained with three catalysts with 5% Ni content prepared by using different precursor compounds by incipient wetness impregnation and the TPR profile of commercial nickel oxide. As can be clearly seen, the profiles of the catalysts are quite different from that corresponding to nickel oxide, and the nickel precursor compound has a great influence over the reducibility of these catalysts. The TPR of the three catalysts is characterized by a main reduction peak that appears at lower or equal temperatures than that of unsupported NiO and by several minor reduction peaks poorly resolved at higher temperatures. The difference in the catalyst TPR profiles as compared to that of unsupported NiO indicates a bimodal nature of fixed oxidic nickel species which consists of two states (Rynkowski et al., 1993): “free nickel oxide” and nickelsupport interaction compounds. The free nickel oxide denomination was employed by Zielinski (Zielinski, 1982) to describe an amorphous overlayer of NiO interacting with the support but not chemically bound, which promotes spillover phenomenon, enhancing the reduction reaction. The stabilization of oxidized nickel, denoted by the higher reduction temperatures of the secondary peaks, should be due to interactions with HZSM-5, more intense than those corresponding to mechanical mixture catalysts. The marked influence of the nickel precursor compound on the reduction pattern of the catalysts prepared by incipient wetness impregnation is clearly observable. The order in the temperature of the majority reduction peak, TM, is as follows: TMIWI-Nt-5 < TMIWI-Et-5 < TMIWI-Ac-5 = TMNiO.
In brief, the catalysts prepared by mechanical mixing with NiO exhibit reducibility similar to the unsupported NiO, indicating weak metal-support interaction while the catalysts prepared by incipient wetness impregnation show multiple reduction peaks, evidencing the complex interaction between the nickel precursor compound and zeolite, which depends on the metal precursor compound used. Finally, the catalysts prepared by mechanical mixing with nickel chloride display the highest temperature reduction peaks, which indicate the strongest metal-support interaction, mainly due to the possible ion exchange reaction during the calcination. 3.2. Hydroisomerization of n-Decane. All the catalysts prepared were tested for their activity and selectivity in hydroisomerization of n-decane. The reaction parameters used are overall conversion, XT, isomerization ratio, XI/XC (relation between conversion toward n-decane isomers and cracking conversion), and selectivity toward total isomers, ST. Figures 5-7 show these parameters versus the nickel loading. As should be expected, the nickel incorporation method has a great significance over the activity of these bifunctional catalysts (Hoang et al., 1994; Gil et al., 1994). Catalysts prepared by mechanical mixing with both precursor compounds, nickel oxide and nickel chloride, show similar values of overall conversion than the zeolite HZSM-5 (Figure 5) while the catalysts prepared by incipient wetness impregnation exhibit higher activities than HZSM-5. Even though the larger metal particle size of the catalysts prepared by mechanical mixing, the reaction results cannot be explained only by the metal crystallite particle size. In this mean, catalysts prepared by different methods and precursor
3538 Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997
Figure 3. TPR profiles and Gaussian deconvoluted peaks. Catalysts: (a) NiO commercial, (b) MMX-Ox-2, (c) MMX-Ox-5, (d) MMX-Ox-10, (e) MMX-Cl-1, and (f) MMX-Cl-2.
compounds with the same metal content and particle size (MMX-Ox-5, IWI-Ac-5, and IWI-Et-5) show different values of the overall conversion. Close interaction between metal and support is required for high hydroisomerization activities (Sivasanker and Ratnasamy, 1990; Parton et al., 1991). The TPDA and TPR measurements have denoted the different nature of the metal-support interaction depending on the preparation method. Catalysts prepared by mechanical mixing show an acidity strength distribution similar to that of the zeolite, while those prepared by impregnation exhibit a modified acidity strength distribution. On the other hand, the TPR profiles of the catalysts prepared by mechanical mixing are similar to that of the unsupported NiO, whereas the catalysts prepared by impregnation exhibit TPR profiles which evidence the complexity of nickel-support interaction. Thus, all the above mentioned suggests that the contact between the two functions, acid and metal, is more adequate in the catalysts prepared by impregnation. The influence of the metal loading also depends on the preparation method and the metal precursor compound. Thus the overall conversion of the catalysts prepared by mechanical mixing grows slightly with metal content, while the catalysts prepared by incipient wetness impregnation show different behavior depending on the metal precursor compound used. In the catalysts prepared with nickel nitrate, the overall conversion increases with the metal content, while the catalysts prepared with the other precursors, (triethylenediamine)nickel nitrate and nickel acetylacetonate, decreases with the nickel content. These results could be explained by the effect of metal loading over the
Figure 4. TPR profiles and Gaussian deconvoluted peaks. Catalysts: (a) NiO commercial, (b) IWI-Nt-5, (c) IWI-Ac-5, and (d) IWI-Et-5.
Figure 5. Hydroisomerization of n-decane. Overall conversion: (*) HZSM-5, (0) MMX-Ox, (∆) MMX-Cl, (9) IWI-Nt, (b) IWI- Ac, and (1) IWI-Et.
metal crystallite size and the overall acidity, AT, which depend mainly on the nature of the precursor. When the precursor is inorganic, like nickel nitrate and nickel oxide, an increase of metal loading implies a slight increase of the crystallite size, maintaining that the total acidity implies higher overall conversions. On the other hand, if the precursor is organic, the increase of the metal loading promotes an increase of the crystallite size and after the calcination of catalysts an increase of the carbonaceous deposits is also expected, which involves a decrease in the activity, mainly due to the decrease of the total acidity. This fact is more relevant for the nickel acetylacetonate catalysts and may be due
Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 3539
Figure 6. Hydroisomerization of n-decane. XI/XC relation: (*) HZSM-5, (0) MMX-Ox, (∆) MMX-Cl, (9) IWI-Nt, (b) IWI-Ac, and (1) IWI-Et.
Figure 7. Hydroisomerization of n-decane. Selectivity toward total isomers: (*) HZSM-5, (0) MMX-Ox, (∆) MMX-Cl, (9) IWINt, (b) IWI-Ac, and (1) IWI-Et.
to the higher size of the precursor that hinders its access to the channel system and the coke deposits over the surface, even at low metal content (2%). The isomerization selectivity measured by the isomerization ratio (XI/XC) follows different patterns than those of the overall conversion as shown in Figure 6. The catalysts prepared by mechanical mixing with nickel chloride, MMX-Cl, have a major isomerization ratio than those prepared with nickel oxide, and moreover, the former shows a decrease in this parameter as the metal loading increases while the latter displays an opposite behavior. This fact may be due to solid ion exchange reactions between the metallic salts and zeolite which unfailingly change the metal-support interaction. Both catalysts show minor isomerization ratio than HZSM-5. On the other hand, the influence of the precursor compound on this parameter in the catalysts prepared by incipient wetness impregnation seems to be quite different depending on metal content. It has to be pointed out that, at low metal content (2%), these catalysts show quite different isomerization ratios, XI/ XC, whereas, for a metal content of 5 wt %, the values of this parameter are similar, regardless of which precursor compound is used. At low metal content (Jacobs et al., 1980; Ribeiro et al., 1982a,b; Alvarez et al., 1990) the reaction over the metal function is the rate-determining step reaction, and the influence of the different dispersions obtained in the catalysts prepared is enhanced, while the catalysts with 5 wt % of nickel
have enough metal to generate olefins and the differences in metal distribution do not have a notable effect over the isomerization ratio. As can be seen in Figure 7, the catalysts prepared by mechanical mixing with nickel chloride show high values of selectivity toward total isomers, ST, which may be due to the metal-support interaction determined by the solid state ion exchange reactions produced during the calcination step. Moreover, the catalysts prepared by incipient wetness impregnation have minor or equal values of this parameter compared to that of HZSM-5. It has to be pointed out that the selectivity toward total isomers accounts for the extension of both isomerization and cracking reactions because all the isomers of less than ten carbon atoms must be considered as cracking products. 4. Conclusions The activity of bifunctional catalysts constituted by an acidic support and a metal depends on effects like metal-support interaction nature, acidity properties, acid/metal balance, sintering resistance, reducibility of metal compounds, etc. The preparation method and the nature of the metal precursor compound have a great influence over acidity, metal-support interaction, metal reducibility, metal distribution, and particle size, as the characterization measurements show. Thus, the preparation method mainly affects the overall conversion so that the catalysts prepared by mechanical mixing are less active than those prepared by impregnation. In this mean, characterization measurements show that the incipient wetness impregnation technique allows us to obtain catalysts with some desirable features such as smaller particle size at low metal content and a closer interaction between the acid and metal function as opposed to the mechanical mixture method. On the other hand, the isomerization ratio seems to depend on the nature of the nickel precursor compound. At low metal content, 2%, the nature of the precursor has an important effect over this parameter in the catalysts prepared by impregnation, where an increase of the metal content to 5% implies similar values of the isomerization ratio, regardless of the precursor. The catalysts prepared by mechanical mixing with NiO exhibit lower isomerization ratios than those prepared with NiCl2, which displays the highest temperature reduction peaks, indicating the strongest metal-support interaction. Literature Cited Alvarez, F.; Ribeiro, F. R.; Guisnet, M. Hydroisomerization and Hydrocracking of 2-Methylhexane on PtUSHY Catalysts: Effect of Platinum Content. React. Kinet. Catal. Lett. 1990, 41 (2), 309. Anders, G.; Burkhardt, I.; Illgen, U.; Schulz, I. W.; Scheve, J. The Influence of HZSM-5 Zeolite on the Product Composition after Cracking of High Boiling Hydrocarbon Fraction. Appl. Catal. 1990, 62, 271. Angevine, P. J.; Oleck, S. M. Noble Metal-Containing Catalyst. U.S. Patent 4, 683, 214, 1987. Bartholomew, C. H.; Panell, R. B.; Butter, J. L. Support and Crystallite Size Effects in CO Hydrogenation on Nickel. J. Catal. 1980, 65, 335. Beran, S.; Wichterlova´, B.; Karge, H. G. Solid State Incorporation of Mn2+ Ions in H-ZSM-5 Zeolite. J. Chem. Soc., Faraday Trans. 1990, 86 (17), 3033. Bermu´dez-Polonio, J. Me´ todos de difraccio´ n de Rayos X; Pira´mide, S. A., Ed.; Madrid, 1981. Beyer, H. K.; Karge, H. G.; Borbe´ly, G. Solid State Ion Exchange in Zeolites. Part I: Alkaline Chlorides/ZSM-5. Zeolites 1988, 8, 79.
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Received for review December 5, 1996 Revised manuscript received May 12, 1997 Accepted May 19, 1997X IE960775+
Abstract published in Advance ACS Abstracts, August 1, 1997. X
