Alumina Catalysts - American Chemical Society

Article pubs.acs.org/IECR

Drying of Ni/Alumina Catalysts: Control of the Metal Distribution Using Surfactants and the Melt Infiltration Method Xue Liu,† Johannes G. Khinast,‡ and Benjamin J. Glasser*,† †

Department of Chemical and Biochemical Engineering, Rutgers University, 98 Brett Road, Piscataway, New Jersey 08854, United States ‡ Institute for Process Engineering, Graz University of Technology, Inffeldg. 21A, A-8010 Graz, Austria ABSTRACT: Supported catalysts are used in many industrial processes. The performance of a catalytic process is intimately related to the metal profile in the support, which is determined by the preparation conditions. In this work, the impact of two surfactants on the metal distribution of Ni/alumina catalysts was investigated. It was found that the surfactants can be used to reduce the effect of drying and help maintain the metal profiles established after impregnation. This capability is related to the surfactant concentration, the surfactant distribution in the support, the interaction between the surfactant and the metal ions, and the interaction between the surfactant and the support pore surface. Three contributions of surfactants are discussed in this paper, i.e., reducing solution surface tension, capturing the metal ions at the walls, and reducing the film breakage effect during drying. We also compared two preparation methodsimpregnation and drying vs melt infiltration. Using the melt infiltration method, we can adjust the metal distribution from a significant egg-shell profile to a nearly uniform profile for low melting point metal salts under high metal loading conditions by controlling the infiltration time and the aging time.

1. INTRODUCTION Supported catalysts are essential components of many industrial processes, ranging from catalytic converters and fuel cells to the production of the newest drugs. These catalysts consist of a porous support, one or more active catalytic materials deposited on the support, and in some cases a modifier.1,2 With respect to the distribution of the active component in the support, four main categories of metal profiles can be distinguished, i.e., uniform, egg-yolk, egg-shell, and egg-white profiles.3,4 The choice of the desired metal profile is determined by the required activity and selectivity, and can be tailored for specific reactions and/or processes.5−9 It has been shown that egg-shell catalysts, where the active component layer is concentrated close to the support external surface, are advantageous in the case of fast reactions with strong diffusion restrictions,6,8,10,11 while egg-yolk catalysts, where the active component is deposited at the support center, are advantageous in the case of intense abrasion, attrition, or poisoning.5,9 Uniform and egg-white metal profiles can also be optimal for certain applications.7 The “design and controlled synthesis of catalytic structures” is one of the two grand challenges in the 2007 DOE/BES “Basic Energy Needs: Catalysis for Energy” report.12 A tremendous amount of time and effort goes into developing a catalyst for a particular process. The design of a typical supported catalyst involves a large number of quality attributes. For a catalyst pellet, this may include specified loadings of active metal(s) in the support, a specified distribution of active metal(s) from the center to the surface (e.g., egg-shell distribution), and specified dispersion (i.e., the size of catalytically active particles). Optimal catalyst quality allows one to carry out chemical reactions in the most efficient, economical, and environmentally responsible manner and thus reduce raw material usage, energy requirements, and green© 2014 American Chemical Society

house gas emissions. Optimization of manufacturing steps can reduce lot-to-lot variability and has the potential to reduce metal loads in catalystsoften a precious metal like platinumthus reducing production costs. Although the development and preparation of supported catalysts have been investigated for many years, several aspects of the various catalyst manufacturing steps are still not fully understood, and in industry, the design of catalysts is predominated by trial and error experiments, which are expensive and time-consuming, and do not always offer assurances on the final manufacturing results. The preparation of supported catalysts usually involves three steps: impregnation, drying, and reduction and calcination. This technology has been applied widely and is favored by industry due to its simplicity from a practical point of view. Metal nitrate salts are widely used as precursors because of their low cost, high solubility, and easy decomposition. Previous work has shown that the metal distribution within the support is mainly determined by the impregnation and drying steps.13−32 Therefore, to achieve an optimum metal profile, a fundamental understanding of both impregnation and drying is crucial. However, most studies on controlling the metal profiles in catalysts have focused on the impact of the impregnation step. There are still many questions regarding the impact of drying that remain unanswered. During drying, the liquid solvent is transported toward the external surface of the support due to capillary flow, and the dissolved metal precursors are transported by solvent convection and metal ion diffusion. The evolution of the Received: Revised: Accepted: Published: 5792

January 8, 2014 March 18, 2014 March 18, 2014 March 18, 2014 dx.doi.org/10.1021/ie500099c | Ind. Eng. Chem. Res. 2014, 53, 5792−5800

Industrial & Engineering Chemistry Research

Article

can be eliminated. In addition, melt infiltration prevents the competitive adsorption of solvent molecules on the support surface, and could provide better dispersion. This has been reported by the group of Zhu who examined the dispersion of copper, chromium, and iron oxide species on SAB-15 using a solvent-free method.59,60 Eggenhuisen et al. explored melt infiltration of cobalt nitrate hexahydrate in silica supports, followed by differential scanning calorimetry and XRD diffraction.61 The melting point depression of intraporous cobalt complexes was observed, and this was used to determine the metal loadings inside the support. Co-based catalysts were also examined by Iglesia et al.62 for Fischer−Tropsch synthesis. Egg-shell catalysts were prepared using molten cobalt nitrate, and the egg-shell thickness was controlled by the melt viscosity and the contact time between the melt and the support.

drying front has been experimentally observed using nuclear magnetic resonance (NMR).14,17,33,34 Recently, the groups of Lysova and Weckhuysen35−37 also developed an indirect magnetic resonance imaging method to determine the distribution of metal-ion complexes inside the catalyst support. To obtain insight into the physicochemical processes that occur during the preparation of supported catalysts, the groups of de Jong and Weckhuysen38,39 used spatially resolved Raman and UV−visible−NIR spectroscopy to study impregnation and drying of CoMo/γ-Al2O3 catalysts. They monitored the migration, disintegration, and formation of metal complexes inside the supports, and investigated the effect of the composition of the impregnation solution, the aging time, and the drying rate on the metal distribution. In a follow-up study,40 they used resolved UV−visible microspectroscopy to monitor the preparation of alumina-supported Co Fischer− Tropsch catalysts. Both homogeneous and egg-shell Co profiles were observed in their experiments. Due to the complexity of the drying step, only a few theoretical models have been reported for drying. Neimark et al.41,42 were among the first to theoretically study the metal redistribution during drying. They used a dimensionless number to characterize slow drying and fast drying regimes. This theory is in agreement with the experiments of Komiyama et al.16 who showed that a very high drying rate can result in a uniform profile, while a relatively low drying rate favors an egg-shell profile. A more sophisticated model was recently proposed and compared with experiments.22,23,25,26,32,43 This model can be applied to the different drying periods and different drying conditions. The performance of a catalytic process is a function of many parameters depending both on reactor and catalyst design, which may be further divided into subcategories according to their characteristic length scales.44,45 Three levels of research in supported catalysts have been identified. Questions concerning the catalyst on the macro-scale include mechanical strength, thermo-stability, and whether it should be used in the form of cylinders, spheres, or powders. On the microscale, the metal profile inside the support, the activity per unit surface area, and the pore structure and its effect on the metal distribution during preparation should be considered. Finally, research on the nanoscale focuses on fundamental studies of adsorption and deposition of active components on support surfaces, and catalytic reaction mechanisms. In this work, we focus on the catalyst design on the microscale level by controlling the distribution of the active metal in the solid support using surfactants, and comparing the conventional impregnation and drying method and the melt infiltration method. Surfactants have been widely used in the catalyst industry to improve catalyst carriers,46 the dispersion of active metals on supported carriers (nanoscale), and thus enhance catalyst activity.47−58 However, their impact on the metal migration inside the support pores during impregnation and drying (microscale) has not been addressed. To the best of our knowledge, this is the first study to examine the effect of surfactants on the metal distribution of supported catalysts. Melt infiltration can be used as an alternative to the impregnation and drying approach. During the process, molten metal salts penetrate inside a porous support via capillary forces. Hydrated transition metal nitrates are particularly suitable to this approach, since they have low melting points. The temperature must be carefully controlled so that the hydrated metal nitrates do not decompose during the process. Since no solvents are required in the process, the drying step

2. EXPERIMENT SETUP The system studied in this work is a nickel/alumina system. Ni/ alumina catalysts are widely used in hydrogenations, hydrodesulfurization, and steam reforming of hydrocarbons.63−68 Nickel nitrate hexahydrate powders (Sigma-Aldrich, St. Louis, MO) were used as metal precursors, and cylindrical γ-alumina pellets provided by Saint-Gobain Norpro (Stow, OH) were used as solid carriers. The cylinders are 3 mm in diameter and around 10 mm in length with a void volume of 0.3 cm3/g and a surface area of 200.7 m2/g. The impact of pellet size on the drying performance has been examined experimentally (not shown) and computationally.25 In general, the effect of drying for large supports is more pronounced than small carriers. This is because drying is much faster in small carriers so the metal ions do not have enough time to redistribute during drying. The basic experimental protocol for the impregnation and drying method includes the following steps: (1) Preheating of the solid support in an oven at 120 °C for 12 h to remove the moisture inside the pellets. (2) Immersion of the dry alumina supports in a nickel nitrate solution at room temperature for impregnation. Usually the impregnation time is more than 1 day such that a uniform metal profile can be obtained after impregnation, representing an equilibrium state. (3) The catalyst samples are dried in an oven at a constant temperature (≈80 °C) for 2 h. (4) The catalyst samples are calcined at 500 °C for 2 h. To investigate the metal distribution after drying or after calcination, we cut the catalyst samples in half in the radial direction and measure the radial nickel profile using micro Xray fluorescence spectroscopy (micro-XRF). For all cases studied in this work, the metal distribution after drying and after calcination is quite similar, indicating that the effect of calcination on the metal redistribution is not significant. This has been observed in our previous work.26 Therefore, the metal distribution after calcination is not shown in this paper, and all experimental measurements on the metal distribution were taken after the drying step. To examine the effect of surfactants on the metal distribution, surfactants are added in the solution during the impregnation step. In general, surfactants have three classifications: cationic surfactants, nonionic surfactants, and anionic surfactants. In this work, we have tested two liquid surfactants, glycolic acid ethoxylate lauryl ether (GAE) and Brij 93 (BJ). GAE is an anionic surfactant, and BJ is a nonionic surfactant. The surfactants can be easily removed during the calcination step, and thus, they do not have further impact on the catalyst performance. The second preparation method examined in this work is the melt infiltration method. The basic experimental protocol for 5793

dx.doi.org/10.1021/ie500099c | Ind. Eng. Chem. Res. 2014, 53, 5792−5800


Article

convection dominates the drying process at the early stages of drying, which transports the water and metal ions toward the external surface, leading to a pronounced egg-shell profile.22,25,26 Although at the late stages of drying diffusion may control the drying process, causing the metal to move toward the support center and the metal distribution to flatten, the final metal distribution still remains egg-shell. Film breakage is another important parameter during drying. At the beginning of drying, the liquid phase is continuously distributed through the support. As evaporation proceeds, breakage of the liquid film that goes through the pores takes place. This leads to the formation of isolated domains in the liquid phase. Finally, the liquid is only found in isolated domains.41,42 If film breakage occurs at the early stages of drying, it will reduce the liquid flux, and thus reduce the effect of convection. At the late stages of drying, diffusion may dominate the drying process and its effect is also reduced by film breakage. Therefore, film breakage at an early stage of drying suppresses the egg-shell distribution yet favors the egg-shell distribution at the late stages of drying.26 For T = 80 °C, film breakage does not occur at the very early stages of drying so its main contribution is to reduce the effect of diffusion. Once film breakage dominates the transport system, liquid flow stops and metal ions only accumulate in isolated domains. Further drying cannot change the metal distribution. For high concentrations (see 1 and 2 M, Figure 1b), the egg-shell distribution at the end of drying becomes less pronounced. This is because the melting point of nickel nitrate hexahydrate is 56 °C, which is below the drying temperature we used in the experiments. During drying, the metal precursor is dissolved or molten in the liquid phase. The volume of the nickel nitrate hexahydrate melt increases with an increase in the initial nickel nitrate hexahydrate concentration. This reduces the effect of film breakage and favors the development of uniform profiles. This has been reported in our previous work.32 In order to provide a more general view, dimensionless analysis was carried out in our previous work.22,25 We found that, for constant external conditions, metal redistribution during drying is mainly determined by three dimensionless groups, a modified Damkohler number Da (adsorption versus convective transport of metal ions), a liquid phase Peclet number Peliq (convective transport of metal ions versus diffusion of metal ions), and Φliq (liquid convection versus maximum convective flow controlled by permeability). The formulas and detailed calculations of these three dimensionless groups can be found in our previous work.22 In general, in a Φliq−Peliq regime map, the impact of drying increases with an increase in Peliq or decrease in Φliq.22 If 1% (volume fraction) of BJ is added in the system, the eggshell distribution is greatly reduced for both low concentrations and high concentrations (see Figure 2). This is due to the three contributions of surfactants. The first contribution is that surfactants have a preference to accumulate on the interface between the bulk solution and the air to reduce the solution surface tension. Surface tension is a major driving force of convection during drying. A decrease in the surface tension leads to a decrease in the Peliq number, causing a less pronounced egg-shell distribution at the early stages of drying. The second contribution is that surfactants may attach to the inside walls to hold the metal ions at the walls if the surfactant molecules are able to interact with the metal ions. This will greatly reduce the movement of metal ions during drying and can help keep the metal distribution obtained after the

melt infiltration includes the following steps: (1) Preheating of the solid support in an oven at 120 °C for 12 h to remove the moisture inside the pellets. (2) Insertion of nickel nitrate hexahydrate powders in a closed vessel, and placement of the vessel in an oven preheated at 65 °C. The nickel nitrate hexahydrate powders are completely molten in a few minutes. (3) Addition of dry alumina supports in pure nickel nitrate liquid for melt infiltration. (4) Sampling at 2, 5, 10, and 30 min. (5) Cooling down of the samples in a closed vessel at room temperature.

3. RESULTS AND DISCUSSION 3.1. Impact of Surfactants on the Metal Distribution of Ni/Alumina Catalysts. In the experiments, the solution pH is around 6.5, which is close to the value of the point of zero charge (pH ≈ 8) of the alumina support. Therefore, the support surface is weakly positively charged and adsorption of the nickel ions on the support surface is relatively weak. Previous work has shown that the drying step can significantly change the metal profile established during impregnation if adsorption between the metal component and the support is weak or moderate.22,25,26 This is because for weak adsorption the amount of metal in the solutions we examined is comparable to or greater than the amount of metal deposited on the support after impregnation, and drying can greatly change the distribution of the metal dissolved in the solvent. Therefore, for the cases studied in this work, the impact of drying on the metal distribution is significant. Figure 1 shows the metal distribution after drying without adding surfactants for a uniform initial condition for drying.

Figure 1. Metal profiles after drying at T = 80 °C without adding surfactants: (a) relatively low concentrations; (b) relatively high concentrations.

Impregnation was carried out for 0.1, 0.2, 1, and 2 M nickel nitrate solutions. The mass ratio of the metal to the support is plotted across the section of the support after drying is complete. For low concentrations (see 0.1 M, Figure 1a), a significant egg-shell distribution can be observed and this distribution is enhanced with an increase in the initial metal concentration (see 0.2 M, Figure 1a). This is due to the competition between convection and diffusion. For T = 80 °C, 5794



Article

(see Figure 3). We believe this is mainly due to the second and third contributions of surfactants. By doubling the BJ

Figure 2. Metal profiles after drying at T = 80 °C with 1% nonionic surfactant - BJ: (a) relatively low concentrations; (b) relatively high concentrations. Figure 3. Metal profiles after drying at T = 80 °C with 2% nonionic surfactant - BJ: (a) relatively low concentrations; (b) relatively high concentrations.

impregnation step. The third contribution is that the effect of film breakage during drying could be reduced due to the network structure created by surfactant molecules inside the support pores. At the late stages of drying, the liquid film may still remain connected through the surfactant network. Thus, the reduction of Φliq that would usually occur (without surfactant) at the later stages of drying may not occur (with surfactant present) and back-diffusion can occur at later times. This could also be a possible reason that the metal dispersion can be improved by adding surfactants, since once the liquid transport is dominated by film breakage large metal particles could be formed in isolated domains. For the second and third contributions, three parameters could be important: (1) the surfactant’s ability to capture the metal ions, (2) the surfactant’s ability to attach to the pore walls of the support, and (3) surfactant concentration. With increasing these three parameters, the impact of surfactants on the metal distribution is enhanced. The interaction between the surfactant molecules and the metal ions is also related to the metal ion concentration in the solution. During drying, the metal ion concentration increases due to the loss of water, and thus enhances its interaction with the surfactant molecules. Given a uniform metal distribution before drying, all three contributions help the metal profile remain uniform during drying. Therefore, it would be important to examine which one dominates the system or if all of them are equally important. For the surfactants used in the current work (BJ and GAE), the minimum surface tension of their aqueous solutions is about half of the pure water surface tension when the interface between the solution and the air is completely occupied by the surfactant molecules. Our previous work has shown that, although the egg-shell distribution can be reduced by decreasing the solution surface tension (reducing Peliq), weak or moderate egg-shell profiles could still be observed if we only reduce the solution surface tension by half.32 It is interesting to note that, if we increase the volume fraction of BJ to 2%, a nearly uniform metal distribution can be obtained after drying at 80 °C for both low concentrations and high concentrations

concentration, more metal ions can be captured by the surfactant so the movement of metal ions at the beginning of drying is reduced, causing a less pronounced egg-shell distribution generated by convection at the early stages of drying. With further drying, convection reduces due to the loss of water and back diffusion starts to dominate the system, driving the metal ions to move away from the surface. If no surfactant is added in the system, film breakage could be important at the late stages of drying. This will reduce the liquid flow and back diffusion, and help keep the egg-shell distribution generated at the early stages of drying. With surfactants, the liquid film may remain connected through the network built by the surfactant molecules at the late stages of drying (maintaining a high value of Φliq) so the effect of back diffusion is not suppressed. Without the surfactant, Φliq would start to decrease at the later stages of drying as isolated domains of liquid form. With surfactant present, Φliq does not necessarily decrease at the later stages of drying and back diffusion can still occur. Since BJ is a nonionic surfactant, the interaction between the BJ molecules and the nickel ions should be relatively weak, i.e., the surfactant’s ability to capture the metal ions is weak. If we change the surfactant from BJ to GAE, the interaction between the surfactant molecules and the nickel ions should be stronger, since GAE is negatively charged and the nickel ions are positively charged in the solution. In addition, since the support surface is weakly positively charged, at the PH of our experiments, GAE should be easier to attach to the support pore surface than BJ. Therefore, under this situation, the impact of GAE on the metal distribution could be much stronger than BJ. Our experimental results have confirmed this hypothesis. By adding only 0.5% GAE in the solution, nearly uniform profiles are obtained after drying for all cases studied (see Figure 4). In Figures 2−4, surfactants were added to the solution at the beginning of impregnation, while drying was carried out after 1 5795



Article

Figure 4. Metal profiles after drying at T = 80 °C with 0.5% anionic surfactant - GAE: (a) relatively low concentrations; (b) relatively high concentrations.

Figure 5. Metal distribution after drying at T = 80 °C with a long impregnation of Ni(NO3)2 and a relatively short impregnation of 1% GAE: (a) 0.1 M initial Ni(NO3)2 concentration; (b) 1 M initial Ni(NO3)2 concentration.

day of impregnation. Therefore, we have long impregnation times for both the metal precursor and the surfactant. After impregnation, both the metal precursor and the surfactant are uniformly distributed inside the support, and this uniform distribution is used as an initial condition for drying. To further examine the effect of the initial drying conditions on the metal distribution, we have carried out another two studies. The procedure of study I was as follows: (1) impregnation of dry alumina supports in nickel nitrate solutions without surfactants for 1 day, (2) adding 1% GAE in the solution and taking samples at 5, 10, and 30 min, and (3) drying the catalyst samples in the oven at 80 °C. In this test, we have a long impregnation time for the metal precursor and a relatively short impregnation time for the surfactant. In other words, the initial condition for drying is a uniform distribution of the metal precursor and an egg-shell distribution of the surfactant. For low concentrations, a significant egg-shell distribution can be obtained in samples with 5 min of surfactant impregnation (see Figure 5a). This is because the surfactant cannot diffuse to the support center in 5 min. After impregnation, the surfactant mainly accumulates in the area near the surface. At the beginning of drying, convection drives the metal ions to move from the support center to the surface. In this situation, some metal ions could be further captured by the surfactant near the surface, leading to a more significant egg-shell distribution. At the later stages of drying, the surfactant that was accumulated at the surface may prevent film breakage at the surface and backdiffusion will drive the metal ions to move toward the support center. Thus, the final metal distribution shows a less pronounced egg-shell profile. Therefore, we hypothesize that, for a short surfactant impregnation time, the egg-shell distribution is enhanced at the early stages of drying and suppressed at the late stages of drying due to the presence of surfactants. In Figure 5a, it is clear that the egg-shell profile is reduced with an increase in the surfactant impregnation time. This is due to the penetration of the surfactant inside the support pores during impregnation. For 30 min of surfactant impregnation, a weak egg-shell or a nearly uniform distribution

can be obtained after drying, indicating that after 30 min of impregnation the surfactant is able to penetrate to the support center. Similar trends can be observed for relatively high concentrations (see Figure 5b). The metal profiles become more uniform with increasing surfactant impregnation time. The procedure of study II was as follows: (1) impregnation of dry alumina supports in nickel nitrate solutions with 1% GAE surfactant, (2) taking samples at 5, 10, and 30 min, and (3) drying the catalyst samples in the oven at 80 °C. In this test, we have a relatively short impregnation time for both the metal precursor and the surfactant. For low concentrations, it is clear that the amount of metal deposited on the support increases with an increase in the impregnation time, while the variation of the total metal loading in the support with the impregnation time becomes much less for relatively high concentrations (see Figure 6). This is because during impregnation the concentration gradient of the metal ions from the support surface to the center is much stronger for high concentrations than low concentrations, and this gradient is the driving force for the diffusion of the metal ions inside the support. For low concentrations, it is interesting to note that the metal profile shows an “M” distribution in the samples with 5 min of impregnation, and the metal profile transitions to a significant egg-shell distribution if we increase the impregnation time to 10 min. In Figure 6b, a similar trend can also be observed for high concentrations. In Figure 6b, the metal loading in the samples with 5 min of impregnation shows an increase from the surface, reaches a maximum, and then reduces when further moving to the center. The metal distribution transitions to an egg-shell profile when we increase the impregnation to 10 min. However, if we compare Figure 6a and b, it can be seen that this transition is much more significant for low concentrations. This is because for low concentrations the high concentration ratio of the surfactant molecules to the metal ions increases the importance of the surfactant on the metal distribution. 3.2. Comparison of the Impregnation and Drying Method and the Melt Infiltration Method. In previous work, we have shown that, using the impregnation and drying 5796



Article

short impregnation time may lead to an initially high concentration but during drying the metal starts to migrate to the center of the support, reducing the extent of the egg-shell profile. This migration of metal will occur even for the case of strong adsorption of metal to the support surface, as for high metal concentrations, a significant amount of the metal remains in the liquid solution compared to the amount adsorbed. We have therefore examined a different strategy to obtain nonuniform distributions, such as an egg-shell distribution, for low melting point precursors under high concentrations. The strategy we have investigated is melt infiltration using nickel nitrate hexahydrate powders. The detailed procedure can be found in section 2. In Figure 8a, a significant egg-shell distribution can be observed after 2 and 5 min of melt infiltration. On the surface,

Figure 6. Metal distribution after drying at T = 80 °C with a relatively short impregnation of both Ni(NO3)2 and 1% GAE: (a) 0.1 M initial Ni(NO3)2 concentration; (b) 1 M initial Ni(NO3)2 concentration.

method, a nearly uniform metal distribution is obtained if a certain threshold metal loading is exceeded for low melting point metal precursors.32 In those experiments, we held the drying temperature at 65 °C, which is above the melting point of nickel nitrate hexahydrate. The metal precursor is either dissolved or molten in the liquid phase, once the temperature in the pellet is above its melting point. In this situation, if the initial concentration of nickel nitrate hexahydrate is above 2.5 M, so that the effect of the capillary force is strong enough to generate liquid flow inside the support, the metal distribution after drying will reach a nearly uniform profile.32 In this work, we observe similar behavior, since our drying temperature is above the melting point of nickel nitrate hexahydrate. In particular in Figure 7, a nearly uniform metal distribution can

Figure 8. Metal distribution using the melt infiltration method without the aging step.

the nickel loading can reach 50%. This distribution cannot be accomplished by the conventional impregnation and drying methods if the drying temperature is above the melting point of nickel nitrate hexahydrate. With a further increase in the impregnation time, the nickel nitrate liquid penetrates inside the pores and the egg-shell distribution is reduced. After 30 min of impregnation, the nickel nitrate liquid reaches the support center and a nearly uniform distribution is obtained (see Figure 8b). The melt infiltration process is highly related to the viscosity of the melt. In general, the viscosity of hydrated metal nitrate melts decreases with an increase in the temperature, and the lower the melt viscosity the better the melt flow properties. However, we need to carefully control the temperature to prevent the decomposition of the hydrated metal nitrate. If decomposition occurs, the hydrated metal nitrate will lose bonded water with a corresponding increase in the melting point. This may cause crystallization and pore blockage, leading to a high variation of the local metal loading inside the support. In Figure 8b, the local metal loading shows a high variation in the samples with the 30 min melt infiltration. We believe this is because a small amount of nickel nitrate hexahydrate decomposes to form nickel nitrate tetrahydrate when we increase the melt infiltration time. The nickel nitrate tetrahydrate has a much higher melting point69 and will

Figure 7. Metal distribution using the regular impregnation and drying method for high initial concentrations of Ni(NO3)2.

be observed after drying for an initial concentration of nickel nitrate hexahydrate of 3 and 4 M (see Figure 7). Calcination experiments were also carried out after drying, and it was found that the metal distribution changes very little after the calcination step (not shown). Therefore, the nearly uniform profiles obtained after the drying step can be used as the final metal distribution of the catalysts. At this point, we can ask the question: can we prepare nonuniform distributions, such as an egg-shell distribution, for low melting point metal precursors at high concentrations? A promising strategy could be to carry out an impregnation for a short amount of time. The problem with this strategy is that a 5797



Article

4. CONCLUSIONS In summary, the effects of a nonionic surfactant (BJ) and an anionic surfactant (GAE) on the metal distribution of Ni/ alumina catalysts were examined in this work. The surfactants can be used to reduce the metal migration during drying and help maintain the metal profiles established after impregnation. This capability is related to the surfactant concentration, the surfactant distribution in the support, the interaction between the surfactant and the metal ions, and the interaction between the surfactant and the support pore surface. Future work should include measurements of the adsorption of the metal ions in the presence of surfactants as well as the adsorption of the surfactants on the support. Therefore, we can extend our impregnation and drying models developed in previous work25,26,32 to quantitatively predict the impact of surfactants on the metal distribution during the impregnation and drying steps. In the impregnation model, the adsorption kinetics of the metal ions on the support surface can be greatly affected by the presence of surfactants. We can take into account this effect in the adsorption parameters, such as adsorption and desorption constants. The overall adsorption rate will be determined by the adsorption parameters, and the metal ion and surfactant concentrations. In the drying model, the presence of surfactants will also affect the film breakage phenomenon. This effect can be taken into account in the film breakage factors; film breakage would then be determined by liquid volume fraction, the pore network structure inside the support, the surfactant distribution, and the hydrophilic or hydrophobic properties of the solvents. It is important to note that, although the experiments and proposed modeling work focus on a Ni/alumina system, the modeling methodology is entirely general. It is not limited to specific active components and supports. Future work should also investigate whether the effect of surfactants observed experimentally for the nickel alumina system will also be observed in other metal support systems. We also compared two preparation methods: impregnation and drying versus melt infiltration. Using the melt infiltration method, we can adjust the metal distribution from a significant egg-shell profile to a nearly uniform profile for high metal loadings by controlling the infiltration time and the aging time. This cannot be obtained using the impregnation and drying method under high concentrations of Ni(NO3)2. The melt infiltration approach presented in this work can also be applied to other metal nitrate salt based catalysts, where weak adsorption of the metal complex on the support surface and high local site density requirements preclude the use of conventional impregnation and drying techniques.

precipitate on the pore surface during the melt infiltration process. The viscosity of hydrated metal nitrate melts can be controlled by adding surfactants or polymers. The added surfactants or polymers can be easily removed during the calcination process. Eggenhuisen et al. reported a melting point depression of intraporous cobalt nitrate hexahydrate in silica supports.61 The melting point of intraporous cobalt complex could be below room temperature (∼20 °C) so a metal redistribution may occur at room temperature. For our specific studies, we collected several samples for each infiltration test, and let the samples stay at room temperature for 1 day, 3 days, and 7 days before measuring their metal distribution. Similar metal profiles were observed in all of these cases. We believe this could be due to two reasons: (1) the melting point of intraporous nickel nitrate hexahydrate is above room temperature so no metal migration occurs at room temperature; (2) the melting point of intraporous nickel nitrate hexahydrate is below room temperature, but the viscosity of the melt salt is high so the metal migration inside the pores is extremely slow. It is interesting to note that if we add an aging step in the procedure the metal distribution is completely changed. In the aging step, samples were removed from the nickel nitrate melt and then kept in an oven at 65 °C for another 30 min before the cooling step. With this aging step, nearly uniform distributions can be obtained after short melt infiltration times (see Figure 9). This is because during the aging stage the

Figure 9. Metal distribution using the melt infiltration method with the aging step.

amount of the nickel nitrate melt inside the support is such that a continuous liquid phase can be maintained for all times, and the viscosity of the liquid is low at 65 °C so that the effect of the capillary force is strong enough to generate liquid flow inside the support. Our experiments indicate that, by adjusting the impregnation time and aging time, the final metal distribution can be changed from a significant egg-shell distribution to a nearly uniform distribution. This is an efficient way to control the metal distribution of low melting point metal precursors under high concentrations. If we compare Figure 8 and Figure 9 focusing on the samples with 5 and 10 min of infiltration, it can be seen that with the aging step the local variability of the metal loading inside the support increases. Instead of a smooth curve, one observes significant scatter in the data. We believe this is related to the decomposition of nickel nitrate hexahydrate during the aging stage. This causes crystallization and pore blockage. Since the chance of blocking the pores due to crystallization and precipitation increases with an increase in the average metal loading, the metal distribution in the samples with 10 min of melt infiltration shows more local variability/scatter than the samples with 5 min of melt infiltration.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: 848-445-4243. Fax: 848-445-2581. E-mail: bglasser@ rutgers.edu. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We wish to acknowledge financial support for this work from the Rutgers Catalyst Manufacturing Science and Engineering Consortium. 5798



■

Article

(25) Liu, X.; Khinast, J. G.; Glasser, B. J. A parametric investigation of impregnation and drying of supported catalysts. Chem. Eng. Sci. 2008, 63, 4517−4530. (26) Liu, X.; Khinast, J. G.; Glasser, B. J. Drying of Supported Catalysts: A Comparison of Model Predictions and Experimental Measurements of Metal Profiles. Ind. Eng. Chem. Res. 2010, 49, 2649− 2657. (27) Melo, F.; Cervello, J.; Hermana, E. Impregnation of porous supports - I Theoretical study of the impregnation of one or two active species. Chem. Eng. Sci. 1980, 35, 2165−2174. (28) Melo, F.; Cervello, J.; Hermana, E. Impregnation of porous supports - II System Ni/Ba on alumina. Chem. Eng. Sci. 1980, 35, 2175−2184. (29) Schwarz, J. A.; Heise, M. S. Preparation of metal distributions within catalyst supports IV. multicomponent effects. J. Colloid Interface Sci. 1990, 135, 461−467. (30) Uemura, Y.; Hatate, Y.; Ikari, A. Formation of nickel concentration profile in nickel/alumina catalyst during post-impregnation. J. Chem. Eng. Jpn. 1973, 6, 117−123. (31) Vincent, R. C.; Merrill, R. P. Concentration profiles in impregnation of porous catalysts. J. Catal. 1974, 35, 206−217. (32) Liu, X.; Khinast, J. G.; Glasser, B. J. Drying of supported catalysts for low melting point precursors: Impact of metal loading and drying methods on the metal distribution. Chem. Eng. Sci. 2012, 79, 187−199. (33) Koptyug, I. V.; Fenelonov, V. B.; Khitrina, L.; Sagdeev, R. Z.; Parmon, V. N. In situ NMR imaging studies of the drying kineticsw of porous catalyst support pellets. J. Phys. Chem. B 1998, 102, 3090− 3098. (34) Hollewand, M. P.; Gladden, L. F. Visualization of phases in catalyst pellets and pellet mass transfer processes using magnetic resonance imaging. Trans. IchemE 1992, 70A, 183−185. (35) Bergweff, J. A.; Lysova, A. A.; Alonso, L. E.; Koptyug, I. V.; Weckhuysen, B. M. Monitoring transport phenomena of paramagnetic metal-ion complexes inside catalyst bodies with magnetic resonance imaging. Chem.Eur. J. 2008, 14, 2363−2374. (36) Bergweff, J. A.; Lysova, A. A.; Alonso, L. E.; Koptyug, I. V.; Weckhuysen, B. M. Probing the transport of paramagnetic complexes inside catalyst bodies in a quantitative manner by magnetic resonance imaging. Angew. Chem., Int. Ed. 2007, 46, 7224−7227. (37) Lysova, A. A.; Koptyug, I. V.; Sagdeev, R. Z.; Parmon, V. N.; Bergwerff, J. A.; Weckhuysen, B. M. Noninvasive in situ visualization of supported catalyst preparations using multinuclear magnetic resonance imaging. J. Am. Chem. Soc. 2005, 127, 11916−11917. (38) van de Water, L. G. A.; Bergweff, J. A.; Leliveld, B. R. G.; Weckhuysen, B. M.; de Jong, K. P. Insights into the preparation of supported catalysts: a spatially resolved Raman and UV-Vis spectroscopic study iinto the drying process of CoMo/g-Al2O3 catalyst bodies. J. Phys. Chem. B 2005, 109, 14513−14522. (39) Bergweff, J. A.; van de Water, L. G. A.; Visser, T.; de Peinder, P.; Leliveld, B. R. G.; de Jong, K. P.; Weckhuysen, B. M. Spatially resolved Raman and UV-visible-NIR spectroscopy on the preparation of supported catalyst bodies: controlling the formation of H2PMo11CoO405- inside Al2O3 pellets during impregnation. Chem.Eur. J. 2005, 11, 4591−4601. (40) van de Water, L. G. A.; Bezemer, G. L.; Bergweff, J. A.; Helder, M. V.; Weckhuysen, B. M.; de Jong, K. P. Spatially resolved UV-vis microspectroscopy on the preparation of alumina-supported Co Fischer-Trospch catalysts: Linking activity to Co distribution and speciation. J. Catal. 2006, 242, 287−298. (41) Neimark, A. V.; Fenelonov, V. B.; Kheifets, L. I. Analysis of the drying stage in the technology of supported catalysts. React. Kinet. Catal. Lett. 1976, 5, 67−72. (42) Neimark, A. V.; Kheifez, L. I.; Fenelonov, V. B. Theory of preparation of supported catalysts. Ind. Eng. Chem. Prod. Res. Dev. 1981, 20, 439−450. (43) Lekhal, A.; Khinast, J. G.; Glasser, B. J. Predicting the effect of drying on supported coimpregnation catalysts. Ind. Eng. Chem. Res. 2001b, 40, 3989−3999.

REFERENCES

(1) Ertl, G.; Knözinger, H.; Weitkamp, J. Preparation of Solid Catalysts; Wiley-VCH: Weinheim, Germany, 1999. (2) Komiyama, M. Design and preparation of impregnated catalysts. Catal. Rev.: Sci. Eng. 1985, 27, 341−372. (3) Gavrilidis, A.; Varma, A.; Morbidelli, M. Optimal distribution of catalyst in pellets. Catal. Rev.: Sci. Eng. 1993, 35, 399−456. (4) Shyr, Y. S.; Ernst, W. Preparation of nonuniformly active catalysts. J. Catal. 1980, 63, 425−432. (5) Becker, E. R.; Wei, J. Nonuniform distribution of catalysts on supports: I. Bimolecular Langmuir reactions. J. Catal. 1977, 46, 365− 371. (6) Galarraga, C.; Peluso, E.; Lasa, H. d. Eggshell catalysts for Fischer−Tropsch synthesis Modeling catalyst impregnation. Chem. Eng. J. 2001, 82, 13−20. (7) Roth, J. F.; Reichard, T. E. Determination and effect of platinum concentration profiles in supported catalysts. J. Res. Inst. Catal., Hokkaido Univ. 1972, 20, 85−94. (8) Smith, T. G.; Carberry, J. J. On the use of partially impregnated catalysts for yield enhancement in non isothermal non adiabatic fixed bed reactors. Can. J. Chem. Eng. 1975, 53, 347−349. (9) Wei, J.; Becker, E. R. The optimum distribution of catalytic material on support layers in automotive catalysis. Adv. Chem. Ser. 1975, 143, 116−132. (10) Minhas, S.; Carberry, J. J. On the merits of partially impregnated catalysts. J. Catal. 1969, 14, 270−272. (11) Horvath, C.; Engasser, J. Pellicular heterogeneous catalysts. A theoretical study of the advantages of shell structured immobilized enzyme particles. Ind. Eng. Chem. Fundam. 1973, 12, 229−235. (12) Bell, A.; Gates, B. C.; Ray, D. Basic Energy Needs: Catalysis for Energy. U.S. DOE Basic Energy Sciences 2007. (13) Assaf, E. M.; Jesus, L. C.; Assaf, J. M. The active phase distribution in Ni/Al2O3 catalysts and mathematical modeling of the impregnation process. Chem. Eng. J. 2003, 94, 93−98. (14) Hollewand, M. P.; Gladden, L. F. Probing the structure of porous pellets: An NMR study of drying. Magn. Reson. Imaging 1994, 12, 291−294. (15) Kheifets, L. I.; Neimark, A. V.; Fenelonov, V. B. Analysis of the influence of permeation and drying stages in supported catalyst preparation on the distribution of components I. precipitation of one component from solution. Kinet. Katal. 1974, 20, 760−767. (16) Komiyama, M.; Merrill, R. P.; Harnsberger, H. F. Concentration profiles in impregnation of porous catalysts: nickel of alumina. J. Catal. 1980, 63, 35−52. (17) Koptyug, I. V.; Kabanikhin, S. I.; Iskakov, K. T.; Fenelonov, V. B.; Khitrina, L.; Sagdeev, R. Z.; Parmon, V. N. A quantitative NMR imaging study of mass transport in porous solids during drying. Chem. Eng. Sci. 2000, 55, 1559−1571. (18) Kotter, M.; Riekert, L. Impregnation type catalysts with nonuniform distribution of the active component. Part II: Preparation and properties of catalysts with different distribution of the active component on inert carriers. Chem. Eng. Fundam. 1983, 2, 31−38. (19) Kowalski, S. J. Toward a thermodynamics and mechanics of drying processes. Chem. Eng. Sci. 2000, 55, 1289−1304. (20) Lee, H. H. Catalyst preparation by impregnation and activity distribution. Chem. Eng. Sci. 1984, 39, 859−864. (21) Lee, S.-Y.; Aris, R. The distribution of active ingredients in supported catalysts prepared by impregnation. Catal. Rev.: Sci. Eng. 1985, 27, 207−340. (22) Lekhal, A.; Glasser, B. J.; Khinast, J. G. Impact of drying on the catalyst profile in supported impregnation catalysts. Chem. Eng. Sci. 2001a, 56, 4473−4487. (23) Lekhal, A.; Glasser, B. J.; Khinast, J. G. Influence of pH and ionic strength on the metal profile of impregnation catalysts. Chem. Eng. Sci. 2004, 59, 1063−1077. (24) Li, W. D.; Li, Y. W.; Qin, Z. F.; Chen, S. Y. Theoretical prediction and experimental validation of the egg-shell distribution of Ni for supported Ni=Al2O3 catalysts. Chem. Eng. Sci. 1994, 49, 4889− 4895. 5799



Article

(44) van Santen, R. A. Theoretical heterogeneous catalysis; World Scientific: Singapore, 1991. (45) Niemantsverdriet, J. W. Spectroscopy in catalysis: an introduction; Wiley-VCH: Weinheim, Germany, 2000. (46) Yu, L.; Huang, S.; Miao, S.; Zhu, X.; Zhang, S.; Liu, Z.; Xin, W.; Xie, S.; Xu, L. Influences of Si/Al Ratios on the Mesopore Distributions of Hierarchical MFI Zeolites Synthesized by Organosilane Surfactant. Ind. Eng. Chem. Res. 2014, 53, 693−700. (47) Chen, L. F.; González, G.; Wang, J. A.; Norena, L. E.; Toledo, A.; Castillo, S.; Morán-Pined, M. Surfactant-controlled synthesis of Pd/Ce0.6Zr0.4O2 catalyst for NO reduction by CO with excess oxygen. Appl. Surf. Sci. 2005, 243, 319−328. (48) Chu, Z.; Chen, H.; Yu, Y.; Wang, Q.; Fang, D. Surfactantassisted preparation of Cu/ZnO/Al2O3 catalyst for methanol synthesis from syngas. J. Mol. Catal A: Chem. 2013, 366, 48−53. (49) Fan, Y.; Bao, X.; Wang, H.; Chen, C.; Shi, G. A surfactantassisted hydrothermal deposition method for preparing highly dispersed W/γ -Al2O3 hydrodenitrogenation catalyst. J. Catal. 2007, 245, 477−481. (50) Hernández, M. L.; a, J. A. M.; Hernández, I.; Viniegra, M.; Llanos, M. E.; Garibay, V.; Ang, P. D. Effect of the surfactant on the nanostructure of mesoporous Pt/Mn/WOx/ZrO2 catalysts and their catalytic activity in the hydroisomerization of n-hexane. Microporous Mesoporous Mater. 2006, 89, 186−195. (51) Hu, Z.; Lihui, W.; Chen, S.; Dong, J.; Peng, S. External modification of zeolite by metal surfactant for methanol amination. Microporous Mesoporous Mater. 1998, 21, 7−12. (52) Jung, Y.-S.; Yoon, W.-L.; Rhee, Y.-W.; Seo, Y.-S. The surfactantassisted NieAl2O3 catalyst prepared by a homogeneous precipitation method for CH4 steam reforming. Int. J. Hydrogen Energy 2012, 37, 9340−9350. (53) Luo, M.-F.; Ma, J.-M.; Lu, J.-Q.; Song, Y.-P.; Wang, Y.-J. Highsurface area CuO−CeO2 catalysts prepared by a surfactant-templated method for low-temperature CO oxidation. J. Catal. 2007, 246, 52−59. (54) Seo, J. G.; Youn, M. H.; Park, S.; Jung, J. C.; Kim, P.; Chung, J. S.; Song, I. K. Hydrogen production by steam reforming of liquefied natural gas (LNG) over nickel catalysts supported on cationic surfactant-templated mesoporous aluminas. J. Power Sources 2009, 186, 178−184. (55) Seo, J. G.; Youn, M. H.; Song, I. K. Hydrogen production by steam reforming of liquefied natural gas (LNG) over nickel catalyst supported on mesoporous alumina prepared by a non-ionic surfactanttemplating method. Int. J. Hydrogen Energy 2009, 34, 1809−1817. (56) Wang, J. A.; Chen, L. F.; Valenzuela, M. A.; Montoya, A.; Salmones, J.; Angel, P. D. Rietveld refinement and activity of CO oxidation over Pd/Ce0.8Zr0.2O2 catalyst prepared via a surfactantassisted route. Appl. Surf. Sci. 2004, 230, 34−43. (57) Yang, H.; Xua, R.; Xuea, X.; Li, F.; Li, G. Hybrid surfactanttemplated mesoporous silica formed in ethanol and its application for heavy metal removal. J. Hazard. Mater. 2008, 152, 690−698. (58) Bikshapathi, M.; Mathur, G. N.; Sharma, A.; Verma, N. Surfactant-Enhanced Multiscale Carbon Webs Including Nanofibers and Ni-Nanoparticles for the Removal of Gaseous Persistent Organic Pollutants. Ind. Eng. Chem. Res. 2012, 51, 2104. (59) Wang, Y. M.; Wu, Z. Y.; Zhu, J. H. Surface functionalization of SBA-15 by the solvent-free method. J. Solid State Chem. 2004, 177, 3815−3823. (60) Zhou, C. F.; Wang, Y. M.; Cao, Y.; Zhuang, T. T.; Huang, W.; Chun, Y.; Zhu, J. H. Solvent-free surface functionalized SBA-15 as a versatile trap of nitrosamines. J. Mater. Chem. 2006, 16, 1520−1528. (61) Eggenhuisen, T. M.; Breejen, J. P. d.; Verdoes, D.; Jongh, P. E. d.; Jong, K. P. d. Fundamentals of Melt Infiltration for the Preparation of Supported Metal Catalysts. The Case of Co/SiO2 for Fischer− Tropsch Synthesis. J. Am. Chem. Soc. 2010, 132, 18318−18325. (62) Iglesia, E.; Soled, S. L.; Baumgartner, J. E.; Reyes, S. C. Synthesis and Catalytic Properties of Eggshell Cobalt Catalysts for the Fischer− Tropsch Synthesis. J. Catal. 1995, 153, 108−122. (63) Reinhoudt, H. R.; Troost, R.; Langeveld, A. D. v.; van, J. A. R.; Veen, S. T. S.; Moulijn, J. A. The Nature of the Active Phase in

Sulfided NiW/-Al2O3 in Relation to Its Catalytic Performance in Hydrodesulfurization Reactions. J. Catal. 2001, 203, 509−515. (64) Santos, R. M.; Lisboa, J. S.; Passos, F. B.; Noronha, F. B. Characterization of Steam-Reforming Catalysts. Braz. J. Chem. Eng. 2004, 21, 203−209. (65) Kim, H. W.; Kang, K. M.; Kwak, H. Y. Preparation of supported Ni catalysts with a core/shell structure and their catalytic tests of partial oxidation of methane. Int. J. Hydrogen Energy 2009, 34, 3351− 3359. (66) Pan, Y. X.; Liu, c. J.; Shi, P. Preparation and characterization of coke resistant Ni/SiO2 catalyst for carbon dioxide reforming of methane. J. Power Sources 2008, 176, 46−53. (67) Foo, S. Y.; Cheng, C. K.; Nguyen, T.-H.; Adesina, A. A. Oxidative CO2 Reforming of Methane on Alumina-Supported Co−Ni Catalyst. Ind. Eng. Chem. Res. 2010, 49, 10450. (68) Li, B.; Maruyama, K.; Nurunnabi, M.; Kunimori, K.; Tomishige, K. Effect of Ni Loading on Catalyst Bed Temperature in Oxidative Steam Reforming of Methane over α-Al2O3-Supported Ni Catalysts. Ind. Eng. Chem. Res. 2005, 44, 485. (69) Paulik, F.; Paulik, J.; Arnold, M. Investigation of the phase diagram for the system Ni(NO3)2-H2O and examination of the decomposition of Ni(NO3)2.6H2O. Thermochim. Acta 1987, 121, 137−149.

5800


Alumina Catalysts - American Chemical Society

Recommend Documents