Innovative Coating Method of MgAl₂O₄ or ZnAl₂O₄ on α-Al₂O₃ Spheric Nuclei

Abstract

Structured materials composed of α-Al₂O₃ spheres covered with a porous and thin layer of MgAl₂O₄ or ZnAl₂O₄ were obtained. The innovative coating method developed consists in the formation of a precursor gel of MgAl₂O₄ or ZnAl₂O₄ by citrate-nitrate combustion method. The alumina spheres are submerged in this gel and then submitted to a drying process in vacuum and to calcinations with N₂ and N₂/O₂. SEM results showed that a very rough and porous layer uniformly covers the spheres. The thickness of the layer reaches the maximum value of 16 µm in the sample coated with two layers of ZnAl₂O₄. XRD results confirmed the MgAl₂O₄ or ZnAl₂O₄ spinel formation in the layered material. The structured materials were used as Pt catalysts supports, showing a good catalytic activity in the n-butane dehydrogenation reaction.

thin films; chemical synthesis; electron microscopy; X-ray diffraction; catalytic properties

1 Introduction

The development of new materials for petrochemical processes applications is an interesting area. The use of structured materials as catalysts supports has acquired importance in the last years due to their advantages in mass and heat transfer in fast reactions carried out at high temperatures^1,2, like catalytic dehydrogenations, partial hydrogenations, and others. The n-alkane dehydrogenation reactions used for the monoolefins production are very important in the petrochemical industry, since these olefins are used for the synthesis of alcohols, polymers, detergents, naphtha additives, etc. The use of supports with high surface area, low acidity and good interaction with metals is very important for n-alkanes dehydrogenation (endothermic reaction carried out at high temperatures) in order to avoid undesirable reactions like alkanes cracking and alkenes polymerization. Hence, an interesting alternative is the use of zinc and magnesium spinels as catalysts supports for dehydrogenation, which have very low acidity and good capacity to disperse and stabilize the supported metals^3-5. Although there is much information about coating of labs, tubes, honeycombs, etc. with metallic oxides^6,7, there is practically no bibliography related with coatings of these materials on spheres^8,9 and nothing about using MgAl₂O₄ and ZnAl₂O₄ spinels for this purpose. In this work, the method chosen for the synthesis of MgAl₂O₄ and ZnAl₂O₄ spinels was the citrate-nitrate combustion^7,10,11, since the precursor gel obtained by this technique presents suitable properties to achieve a good adherence to the α-Al₂O₃ spheres.

The objective of this work is related with the production of a uniform thin layer of MgAl₂O₄ or ZnAl₂O₄ on an inert nucleus with an adequate thickness, high specific surface area, and the proper resistance and durability under the operating conditions.

2 Experimental

For the preparation of the materials by coating, the starting material were spheres (ϕ = 2 mm) of α-Al₂O₃ (S_g = 4.7 m²g^–1) provided by SASOL, Mg(NO₃)₂. 6 H₂O (Sigma-Aldrich 99%), Zn(NO₃)₂. 6 H₂O (Sigma-Aldrich 99%), Al(NO₃)₃. 9 H₂O (Sigma-Aldrich 99%) and citric acid monohydrate (C₆H₈O₇.H₂O) (Sigma-Aldrich 99%).

2.1 Pretreatment of α-Al2O3 spheres

Spheres were treated at room temperature with 1 M solution of HCl for 24 h (ratio of 1.4 mL HCl g^–1 α-Al₂O₃) in order to develop roughness in the external surface and to improve the adherence of the coating material. Then the spheres were rinsed with distilled water until neutral pH followed by a drying step at 120 °C for 24 h.

2.2 Coating of α-Al2O3 spheres with one layer of spinels by the citrate-nitrate method

The spheres (modified with HCl) were submitted to a coating process using both spinels by the following method:

a) Preparation of the spinel precursor solution: MgAl₂O₄ : A solution of aluminium nitrate nonahydrate, magnesium nitrate hexahydrate and citric acid monohydrate (with Al/Mg molar ratio = 2, and citric acid/nitrate ions molar ratio equal to 0.5) was prepared. The final concentrations of the solutions were 0.42 M (Al(NO₃)₃. 9 H₂O), 0.21 M (Mg(NO₃)₂. 6 H₂O) and 0.84 M (C₆H₈O₇. H₂O). The ratio between the volume of the spinel precursor solution and the support weight was 60 mL/g, this being enough to cover the spheres placed in the baker. The spheres were submerged (with soft stirring) in the prepared solution, which was heated and maintained under boiling during 30 minutes forming the gel. ZnAl₂O₄ : The previous process was repeated using zinc nitrate hexahydrate instead of magnessium nitrate hexahydrate.

b) The obtained gel (with the submerged spheres) was dried for 1 h at 75 °C in vacuum furnace and then, the temperature was risen to 100 °C and maintained during 24 h.

c) The dried gel with spheres was calcined in three steps: i) Nitrogen: heating in N₂ flow (100 mLmin^–1) up to 300 °C with a heating rate of 2 °C min^–1, then the heating was continued up to 500 °C with a heating rate of 5 °Cmin^–1, and keeping at 500 °C during 2 h. ii) Mixture of nitrogen and oxygen (5 v/v % O₂-N₂): heating from 500 °C up to 700 °C (heating rate of 5 °Cmin^–1) in gas mixture flow (100 mLmin^–1) and keeping at 700 °C during 2 h, iii) Air: two different conditions were used: 1) at 700 °C in air flow (100 mLmin^–1) during 2 h, or 2) heating from 700 °C up to 800 °C in air flow (100 mLmin^–1) and keeping at 800 °C during 3 h.

Figure 1 shows the scheme of synthesis of the structured materials.

Figure 1
Scheme of synthesis of the structured materials.

2.3 Coating of α-Al2O3 spheres with two layers of spinels by the citrate-nitrate method

The process described in the previous item was repeated two times for coating spheres with two layers of MgAl₂O₄ (or ZnAl₂O₄).

2.4 Characterization of structured materials

In order to determine the thickness and uniformity of the layer deposited on spheres by the coating processes, scanning electronic microscope (SEM) (with a JSM-35C, JEOL model, equipped with SemAfore digital image acquisition system) was used. The observations were carried out under the way of images of secondary electrons using an acceleration voltage of 20 kV. The samples were adhered with silver paint on metallic slides and coated by sputtering with a thin layer of gold to give them the required conductivity before the observation.

The composition of the layer materials was analyzed by X-ray diffraction experiments performed at room temperature in a Shimadzu model XD3A instrument using CuKα radiation (λ=1,542Å), generated at 30 kV and a current of 30 mA.

The layer adherence of the coated spheres was tested. To obtain this parameter, the coated spheres were immersed in distilled water and a further treatment in ultrasonic equipment (Ultrasonic cleaner Cleanson 200 w) during 20 minutes was carried out. Then the spheres were dried in furnace during 24 h and its weight was determined in order to establish the weight loss. The Formula 1 for the calculus of the weight loss was:

$\%Lossweight=\frac{spheresweight-spheresweight(afterultrasonictest)}{spheresweight}\times 100$ (1)

2.5 Preparation of Pt catalysts and characterization

The coated spheres were used as catalytic supports. The catalysts were prepared by an excess-solution impregnation method. They were impregnated with a solution of chloroplatinic acid during 6 h at room temperature in order to obtain a catalyst with 0.3 wt % Pt. The ratio between the volume of the chloroplatinic acid solution and the support weight was 1.4 mLg^–1. The Pt concentration in the impregnating solution was 2.14 g L^–1. Then they were dried in a furnace at 100 °C and calcined during 3 h at 500 °C.

The Pt structured catalysts (coated spheres) were tested in the n-butane dehydrogenation reaction, carried out in a quartz continuous flow reactor at 530 °C for 2 h. The reactor (with a catalyst weight of 0.200 g) was fed with 18 mL min^–1 of the reactive mixture (n-butane + hydrogen, H₂/n-C₄H₁₀ molar ratio = 1.25). Before reaction, the catalysts were reduced “in situ” at 530 °C under flowing H₂ for 3 h. The reactor effluent was analyzed in a GC-FID equipment with a packed column (1/8” × 6 m, 20 wt% BMEA on Chromosorb P-AW 60/80), which was kept at 50 °C during the analysis. With this analytical device, the amounts of methane, ethane, ethylene, propane, propylene, n-butane, 1-butene, cis-2-butene, trans-2-butene and 1,3 butadiene were measured. The n-butane conversion was calculated as the sum of the percentages of the chromatographic areas of all the reaction products (except H₂) corrected by the corresponding response factor. The selectivity to the different reaction products (i) was defined as the ratio: mol of product i/Σ moles of all products (except H₂).

In order to determine the Pt dispersion of the catalysts, measurements of hydrogen chemisorption were carried out. The experiments were performed in a volumetric equipment. The sample weight used was 300-500 mg. The sample was outgassed at room temperature, heated under flowing H₂ (60 mL min^–1) from room temperature up to 500 °C, and then kept at this temperature for 2 h. Then, the sample was outgassed under vacuum (10^–4 Torr) for 2 h. After the sample was cooled down to room temperature (25 °C), hydrogen dosage was performed in the range of 25-100 Torr. The isotherms were linear in the range of used pressures. The chemisorbed hydrogen was calculated by extrapolation of the isotherm to pressure zero. From the data of chemisorbed H₂, the metallic dispersion in monometallic catalysts was calculated by using the Formula 2:

$D=\frac{n_H\cdot X\cdot M{M}_{Pt}}{W_{cat}\cdot C_{Pt}}$ (2)

$n_H$= moles of chemisorbed H₂

_$X=$ stoichiometry of chemisorbed H₂ on Pt

_{$M{M}_{Pt}=$} Molar mass of Pt

$W_{cat}$= catalyst weight

$C_{Pt}^{}=$Pt content (wt %)

3 Results and Discussion

Table 1 shows results about the increase of weight and thickness of the layers deposited on coated spheres. It is observed that the coating process produced an increase of weight up to 16%. The highest increase of weight values was presented by coating with two layers of ZnAl₂O₄. The increase of weight correlates well with the layer thickness (measured by SEM). The MgAl₂O₄ or ZnAl₂O₄ layers deposited on spheres by coating process described are shown in Figures 2 to 6. Figure 2 displays SEM microphotographs of one sphere coated with one layer of MgAl₂O₄, and a coated sphere of uniform aspect can be observed. As the magnification increases (Figures 2b, 2c and 2d), a highly rough and slightly fragmented layer adhered to the sphere surface can be appreciated. The observed fragments have 2 to 5 μm (Figure 2d). In conclusion, with this coating method, a very rough and porous layer covering uniformly the sphere is obtained.

Table 1
Increase of weight and thickness of layers in structured materials prepared by coating with MgAl₂O₄ or ZnAl₂O₄.

	Increase of weight (%)	Thickness of layer (μm)
α-Al2O3 spheres coated MgAl2O4 one layer	6.8	2
α-Al2O3 spheres coated MgAl2O4 two layers	9.4	5
α-Al2O3 spheres coated ZnAl2O4 one layer	11.1	10
α-Al2O3 spheres coated ZnAl2O4 two layers	15.9	14

Figure 2
SEM microphotographs of α-Al₂O₃ sphere coated with one layer of MgAl₂O₄: a) Nominal magnification x 48, b) Nominal magnification x 300, c) Nominal magnification x 1000, d) Nominal magnification x 3000.

Figure 6
SEM microphotographs of α-Al₂O₃ coated spheres with two layers of ZnAl₂O_4: a) view of one sphere b) view of a sector of the sphere with nominal magnification x 300, c) transverse section of a sphere with nominal magnification x 1000.

Figure 3a displays a transverse section of the sphere coated with one layer of MgAl₂O₄. A magnification of this section is observed in Figure 3b. This microphotograph shows that the layer on the sphere presents a thickness of about 2 μm.

Figure 3
SEM microphotographs of transverse section of the sphere coated with one layer of MgAl₂O₄: a) Nominal magnification x 100, b) Nominal magnification x 3000.

Spheres and transverse section of spheres coated with one layer of ZnAl₂O₄ are displayed in Figure 4. It is observed that with ZnAl₂O₄, a very rough and porous layer is also formed. As Figure 4c shows, the thickness of the layer is about 10 μm.

Figure 4
SEM microphotographs of α-Al₂O₃ coated spheres with one layer of ZnAl₂O_4: a) view of one sphere b) transverse section of a sphere with nominal magnification x 300, c) transverse section of a sphere with nominal magnification x 1000.

In order to achieve a higher thickness of the spinel layer deposited on the spheres, a modification of the original method was carried out, by adding one more layer of spinel (see experimental section). The SEM microphotographs of spheres with two layers of magnesium spinel are shown in Figure 5, where it is observed that the thickness of the layer increases up to 5 μm.

Figure 5
SEM microphotographs of α-Al₂O₃ coated spheres with two layers of MgAl₂O_4: a) view of one sphere b) transverse section of a sphere with nominal magnification x 1000, c) transverse section of a sphere with nominal magnification x 3000.

The SEM microphotographs of spheres with two layers of ZnAl₂O₄ are shown in Figure 6. In Figure 6c, a layer with a thickness of about 14 μm is observed. In conclusion, for both spinels with the modified method (two layers), an increase of the thickness of the layer deposited on the spheres could be achieved.

In order to determine if the spinel (MgAl₂O₄ or ZnAl₂O₄) are formed in the deposited layers, X-ray diffraction analysis were carried out by using powders coming from the gel with the spheres calcination step (see experimental section item 2.2), and the results are displayed in Figures 7 and 8. The diffractogram of the powder calcined at 800 °C (Figure 7b) shows the peaks corresponding to magnesium spinel, but in Figure 7a (calcination step at 700 °C), the peaks corresponding to MgAl₂O₄ (2θ = 59° and 30.6°) are not observed. This fact indicates that it is necessary to calcine the sample at 800 °C in order to complete the formation of the magnesium spinel.

Figure 7
Diffractograms of the powders obtained by the citrate-nitrate method (MgAl₂O₄) calcined at different temperatures: a) 700 °C and b) 800 °C.

Figure 8
Diffractograms of the powders obtained by the citrate-nitrate method (ZnAl₂O₄) calcined at different temperatures: a) 700 °C and b) 800 °C.

Figure 8b shows the diffraction peaks corresponding to zinc spinel, but peaks corresponding to ZnO can also be observed. The diffractogram corresponding to the sample calcined at 700 °C (Figura 8a) also displays the characteristic peaks of ZnAl₂O₄ and ZnO, however ZnO peaks present a higher intensity than the observed for the sample calcined at 800 °C. This residual zinc oxide in the layer could affect the behavior of these materials when using as catalytic support. In conclusion, XRD results indicate that a calcination step up to 800 °C has to be used.

The layer adherence test carried out with the four samples (Table 1), showed a weight loss lower than 0.5% of the total mass in all cases. These results indicate both a high stability and a good adherence of the layers to the spheres.

The two layers-structured materials (with both spinels) were used as supports of Pt catalysts for n-butane dehydrogenation reaction. The results of catalytic behavior of both samples for different reaction times are shown in Table 2. For the sake of comparison, the results corresponding to a Pt catalyst supported on structured material obtained by washcoating of γ-Al₂O₃ on α-Al₂O₃ spheres⁸ were included. As Table 2 shows, Pt/MgAl₂O₄/α-Al₂O₃ catalyst displayed a yield to butenes along the reaction time higher than that of the Pt/ZnAl₂O₄/α-Al₂O₃ catalyst. Besides, the first catalyst showed similar values of yields to butenes than those of the Pt/γ-Al₂O₃-wc/α-Al₂O₃ catalyst. The values of n-butane conversion of Pt/MgAl₂O₄/α-Al₂O₃ and Pt/γ-Al₂O₃-wc/α-Al₂O₃ catalysts were similar and high. On the other hand, the conversion values of the Pt/ZnAl₂O₄/α-Al₂O₃ catalyst were low (Table 2). The selectivity values to butenes of Pt/ZnAl₂O₄/α-Al₂O₃ catalyst were higher than those of Pt/MgAl₂O₄/α-Al₂O₃ and Pt/γ-Al₂O₃-wc/α-Al₂O₃ catalysts. The low activity of the Pt/ZnAl₂O₄/α-Al₂O₃ catalyst could be due to ZnO residues in the layer observed by XRD results. Different authors^12,13 found Pt-Zn alloys in Pt/ZnAl₂O₄ catalysts with ZnO impurities, and these alloys could cause a decrease in the catalytic activity. With respect to the high selectivity to butenes of Pt/ZnAl₂O₄/α-Al₂O₃ catalyst, it was found that the Pt particles supported on ZnAl₂O₄ spinel have a rounded shape with very low density of steps and edges¹⁴. This structure of the metallic particles would inhibit the undesirable reactions, like hydrogenolysis, thus favouring the selectivity to butenes. Note also that the Pt/MgAl₂O₄/α-Al₂O₃ catalyst showed good catalytic stability, since yield values decreased only 14% along the reaction time, whereas the decrease in the yield was 31% for the Pt/ZnAl₂O₄/α-Al₂O₃ catalyst and 33% for the Pt/γ-Al₂O₃/α-Al₂O₃ one (Table 2).

Table 2
Results of conversion of n-butane (C), selectivity to butenes (S) and yields to butenes (Y) at different reaction times for the Pt catalysts supported on structured materials.

	Y 10 min (%)	Y 60 min (%)	Y 120 min (%)	C 10 min (%)	C 60 min (%)	C 120 min (%)	S 10 min (%)	S 60 min (%)	S 120 min (%)
Pt/MgAl2O4/α-Al2O3	16.8	15.0	14.4	30.5	24.6	21.4	55.0	62.7	67.4
Pt/ZnAl2O4/α-Al2O3	14.4	11.3	9.9	14.7	11.9	10.0	98.0	95.3	98.3
Pt/γ- Al2O3 – wc/α-Al2O3 (*)	18.3	--	12.2	31.0	--	19.5	58.7	--	62.4

Yield = (% conversion of n-butane x % selectivity to butenes)/100.
• ^*
  Results extracted from the paper of Ballarini et al.8.

Pt dispersion values obtained by hydrogen chemisorption were: 82% for the Pt/MgAl₂O₄/α-Al₂O₃ catalyst and 46% for the Pt/ZnAl₂O₄/α-Al₂O₃ one. Considering that the Pt dispersion is a measurement of the surface active sites, these results are in agreement with the higher activity of the first catalyst in the n-butane dehydrogenation reaction.

4 Conclusions

An innovative method based on the deposition of a layer of MgAl₂O₄ or ZnAl₂O₄ on spherical nuclei of α-Al₂O₃, was used for the synthesis of structured materials. A very rough and porous layer of spinel that covers uniformly the spheres was obtained. However, the method need to be improved in order to eliminate the residual ZnO present in the ZnAl₂O₄ spinel. The deposition of the second layer of MgAl₂O₄ or ZnAl₂O₄ allows to increase the thickness of the layer on the spheres. Pt catalysts prepared using the structured supports with two spinels layers showed a good performance in the reaction of n-butane dehydrogenation, specially the Pt/MgAl₂O₄/α-Al₂O₃ catalyst which displayed both high yields to butenes and high stability along the reaction time. The catalytic results of the Pt/MgAl₂O₄/α-Al₂O₃ catalyst are comparable with those obtained for Pt catalysts deposited on commercial supports. These properties would turn them out to be very promising catalytic materials.

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Publication Dates

• Publication in this collection
  Mar-Apr 2015

History

• Received
  30 Oct 2014
• Reviewed
  18 Feb 2015