A Novel, Highly Efficient Catalyst for Propane Dehydrogenation

A titanium tube of outer diameter 3 mm was located in the center of the reactor. ... The catalysts were screened at 600 °C, 1 bar pressure, and 690 h...
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Ind. Eng. Chem. Res. 2001, 40, 4741-4748

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A Novel, Highly Efficient Catalyst for Propane Dehydrogenation D. Akporiaye,† S. F. Jensen,‡ U. Olsbye,*,† F. Rohr,† E. Rytter,‡ M. Rønnekleiv,‡ and A. I. Spjelkavik† SINTEF Applied Chemistry, P.O. Box 124 Blindern, N-0314 Oslo, Norway, Statoil Research Center, Rotvoll, N-7005 Trondheim, Norway

Platinum and tin deposited on a calcined hydrotalcite constitute a new and highly efficient catalyst for the dehydrogenation of propane. The catalyst is superior to conventional aluminasupported systems in terms of lifetime stability, activity, and propene selectivity. The catalysts were characterized by BET, XRD, SEM, TEM, CO chemisorption, and chemical analysis. The preparation procedure used in the metal deposition step proved to be important for the performance of the catalyst. Impregnation of the calcined hydrotalcite in HCl-acidified aqueous solution with subsequent chlorine removal enhances regenerability and long-term stability of the catalyst compared to impregnation in ethanol. This is most likely due to a water-induced phase transition in the hydrated hydrotalcite. CO adsorption results indicate that activation is correlated with platinum dispersion. 1. Introduction For many years, growth in the propene market has been higher than that for ethylene. Forecasts suggest that this gap will widen even more in the coming years as the propene market is expected to grow by 9% annually compared to an estimated 5% growth for ethylene.1 The propene extracted from steam crackers or FCC units alone does not satisfy the growing demand. Consequently, there is a great interest in alternative routes to propene. Propane dehydrogenation is believed to have a great potential as a propene booster in the future. The reaction is carried out catalytically because of cracking losses under thermal dehydrogenation conditions. Originally, chromium oxide was used as the catalyst for the dehydrogenation of ethane, propane, n-butane, and isobutane.2,3 Chromium oxide supported on alumina is used for the dehydrogenation of C2-C4 hydrocarbons to the corresponding alkenes.4-6 Modified reforming catalysts consisting of platinum supported on nonacidic or alkaline alumina are used as well.7,8 UOP has commercialized a catalytic dehydrogenation process based on such a catalyst.9 The process can be applied to pure or mixed C2-C5 feedstocks.10 This paper reports on a novel, highly efficient catalyst for propane dehydrogenation, which contains platinum promoted with tin supported on a calcined hydrotalcitelike carrier material. Hydrotalcite-like double hydroxides are mixed hydroxycarbonates that exhibit the same crystal structure as hydrotalcite, Mg6Al2(OH)16CO3‚ 4H2O. The structure of hydrotalcite is similar to that of brucite (Mg(OH)2).11-13 In Mg(OH)2, the hydroxide ions are hexagonally close-packed. The magnesium ions occupy all of the octahedral interstitials in every other OH layer. This results in a layer structure with an OHMg-OH-OH-Mg-OH stacking sequence perpendicu* Author to whom correspondence should be addressed. Present adress: Department of Chemistry, University of Oslo, P.O. Box 1033, N-0315 Oslo, Norway. † SINTEF Applied Chemistry. ‡ Statoil Research Center.

lar to the (0001) plane. In hydrotalcite, a fraction of the magnesium ions are substituted by trivalent aluminum ions, with carbonate ions compensating for the surplus charge. Both carbonate and water are located between the layers and are an integral part of the crystal structure of hydrotalcite. Calcination of hydrotalcite yields a mixed Mg(Al)O oxide that can be regarded as a defect-rich, aluminum-containing magnesium oxide. The calcined hydrotalcite has a high surface area (typical 160-220 m2/g) and shows a much higher resistance to sintering under steam-rich conditions than pure MgO. These properties combined with a basic character (unlike, e.g., alumina) make Mg(Al)O an interesting material for applications as a catalyst carrier for some petrochemical processes. Compared to more conventional carrier materials such as alumina or silica, hydrotalcite materials are rather uncommon for application in catalysis. Only a limited number of examples exist in the literature. In 1970, the first patent appeared in which it was claimed that hydrotalcite-like compounds might be very good precursors for hydrogenation catalysts.14 The first papers in the open literature date back to 1971 and 1975.15,16 They explored the potential of hydrotalcite-like materials for basic catalysts and hydrogenation catalysts, respectively. Davis and Derouane reported a catalyst for the aromatization of n-hexane that utilizes calcined hydrotalcite as the carrier material.17 The authors further concluded that hydrotalcite-based catalysts have a great potential for the reforming of hydrocarbons. The catalysts described in the present paper contain calcined hydrotalcite as a support. Platinum and tin are deposited onto the support by different impregnation techniques. The catalysts are characterized with different analytical techniques (XRD, SEM, BET, chemical analysis, CO adsorption). Their catalytic performance is evaluated in screening tests. These results are used to optimize the formulation procedure and to identify the preparation conditions resulting in the best catalytic performance. Parts of the work have already been published in the patent literature.18-20 The optimization of the carrier itself is the topic of another paper.21

10.1021/ie010299+ CCC: $20.00 © 2001 American Chemical Society Published on Web 10/09/2001

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2. Experimental Section 2.1. Catalyst Preparation. The catalysts were prepared in a two-step manner. First, the hydrotalcite was prepared by precipitation from a basic solution. In a second step, the active metals were deposited onto the hydrotalcite or calcined hydrotalcite by impregnation from solution. Magnesium nitrate [Mg(NO3)2‚6H2O] and aluminum nitrate [Al(NO3)3‚9H2O] were dissolved in distilled water. The molar ratio of the two nitrates varied depending on the desired magnesium-to-aluminum ratio in the hydrotalcite. In most cases, the solution had a Mg/Al molar ratio of 10. The resulting Mg/Al ratio in the hydrotalcite was in the range of 3-5. A second solution was prepared containing the basic agent together with a carbonate in distilled water. (NH4)2CO3/ NH3 was used in some cases, whereas other catalysts were prepared using a NaOH/Na2CO3 solution in distilled water. The choice of the basic agent is of importance for the Mg/Al ratio in the hydrotalcite. For a given molar ratio of the metal nitrates, higher Mg/Al ratios are obtained with NaOH compared to (NH4)2CO3/NH3. The two solutions were slowly added dropwise over a period of 40 min to a glass flask under stirring. The pH of the solution was kept constant at approximately 8, and the temperature was held at 60 °C. The precipitate was filtered, washed with distilled water to neutrality, and dried overnight at 100 °C. In some cases, the dried hydrotalcite was then used in the impregnation step. Other samples were prepared using the calcined hydrotalcite (calcination temperatures between 500 and 800 °C, heating rate of 5 °C/min, duration of approximately 15 h). Platinum and tin were deposited onto the support by impregnation techniques using ethanol or water as the solvent. The importance of Sn in supported Pt-Sn catalysts is well described in the literature.22-24 An intimate mixture of platinum and tin on the catalyst surface even on a molecular scale was therefore considered important for optimal performance of the catalyst. It is known that the two metal precursors, SnCl2 and H2PtCl6, form various colored anionic complexes, for example, the redcolored PtCl2(SnCl3)2- species.25,26 These complexes can be obtained in aqueous solution as well as in polar organic solvents such as ethanol. These mixed tinplatinum complexes are ideal species for the metal impregnation. First, they are likely to yield the best possible molecular mixture of platinum and tin on the surface because the two elements are already mixed in the precursor. Second, under strongly acidic conditions, a negatively charged metal complex is advantageous from an electrostatic point of view because the support surface is positively charged as a result of protonation. This point is particularly important because all impregnations carried out in aqueous solution required the addition of concentrated HCl in order to fully dissolve SnCl2. Therefore, all catalysts were impregnated in one step using one single solution containing the anionic complex. The deep red color obtained after mixing the platinum and tin precursors, either in ethanol or in water, was taken as an indication of complex formation. In the initial procedure, the calcined hydrotalcite was impregnated with an ethanol solution. Ethanol was chosen because the calcined hydrotalcite maintains its structure in this solvent. In water, on the other hand, the hydrotalcite phase is re-formed.27 This water-

induced re-formation of hydrotalcite was considered disadvantageous because of the lower surface area compared to that of the calcined material. Another argument in favor of ethanol was the possible decomposition of the hydrotalcite material in an acidified aqueous solution. However, water is preferred to ethanol for the industrial-scale production of a catalyst because of the health and safety hazards associated with organic solvents. Therefore, a detailed study was launched into aqueous impregnation of the carrier material. Impregnation using ethanol was carried out as follows: Tin chloride (SnCl2‚2H2O, 0.115 g) and hexachloroplatinum (H2PtCl6‚6H2O, 0.0805 g) was dissolved in 40 mL of ethanol. Calcined hydrotalcite Mg(Al)O (10.1 g), prepared as described above, was stirred with the salt solution for 3 h at room temperature. After evaporation of the solvent, the catalyst was dried overnight at 100 °C and then calcined for 3 h at 560 °C. Some of the catalysts prepared using ethanol solution also contained Cs as a promoter. Cs was deposited onto the catalyst by incipient wetness impregnation with CsNO3. The calcined Pt,Sn/Mg(Al)O catalyst (9.65 g) was impregnated with 0.071 g of CsNO3 dissolved in 25 mL of distilled water. After being allowed to dry overnight at 100 °C, the catalyst was calcined for 3 h at 560 °C. A typical impregnation in aqueous solution is as follows: Tin chloride (SnCl2‚2H2O, 0.3398 g) was dissolved in HCl (1 M, 109 mL). Hexachloroplatinum (H2PtCl6‚6H2O, 0.1116 g) was dissolved in distilled water (40 mL). The two solutions were mixed. A Mg-Al-O carrier material (14.88 g), prepared as described above, was stirred with the salt solution for 30 min. The solution was then filtered and washed 3 times with water (approximately 750 mL). The solid material was dried (100 °C) and calcined (5 °C/min up to 560 °C, hold at final temperature for 5 h). In a few cases, the catalyst was subjected to a “wet calcination” procedure. The sample was heated in dry air to 250 °C (3 °C/min heating rate) and kept under these conditions for 1.5 h. Then, the temperature was raised to 570 °C (3 °C/min) in humid air. The air was humidified by bubbling it through warm water (70 °C). The sample was held at the final temperature for 2 h in humid air and then for 1 h in dry air. The background for using this procedure was that our group had previously experienced that wet calcination of a Pt/Al2O3 material is an efficient method for chlorine removal from that system.28 2.2. Catalyst Characterization. The catalysts were characterized by different analytical techniques. Powder X-ray diffraction measurements were performed using a Siemens D-5000 diffractometer with Cu KR radiation. The catalysts were characterized by scanning electron microscopy (SEM) using a JEOL JSM-840 apparatus. In most cases, the sample was deposited on an adhesive tape and analyzed without further pretreatment. Transmission electron microscopy (TEM) studies were performed using a JEOL 2000 F apparatus. The sample was prepared by grinding followed by dispersion in butanol. The wet sample was placed on a TEM grid with a carbon film. The butanol was allowed to vaporize (25 °C) before the TEM experiments. The specific surface area was measured by the BET method using nitrogen as the probe gas. Chemical analysis was performed using X-ray fluorescence (for Mg, Al, Pt, and Sn) and atomic absorption spectroscopy (AAS) (for Al and Cl).

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The platinum dispersion was determined by CO chemisorption measurements using an AMI-1 analysis unit from AltaMira. The following procedure was used. The solid material (approximately 0.5 g) was placed into the sample holder of the Altamira apparatus. All fresh catalysts were subject to an in situ ROR treatment (R ) reduction, O ) oxidation) prior to the pulse chemisorption sequence. The ROR treatment was chosen to match the pretreatment of catalyst screening (cf. section 2.3). Catalysts that had been used as dehydrogenation catalysts before the analysis were only subject to a R treatment prior to the pulse chemisorption sequence. The ROR pretreatment involved heating the sample (35 °C/min) to 600 °C under a H2 flow (20 mL/min). The sample was then kept at the final temperature for 120 min. In the next step, the sample was oxidized under a 5% O2/He flow (50 mL/min) at 600 °C for 60 min and under an air flow (50 mL/min) at 600 °C for another 60 min. Then, the catalyst was again reduced under a H2 flow (20 mL/min) at 600 °C for another 120 min and flushed with Ar (20 mL/min) at 600 °C for 60 min. Finally, the sample was cooled to 35 °C under an Ar flow. The pulse chemisorption experiments were performed at 35 °C. CO pulses (50 µL) were passed over the sample with Ar (20 mL/min) as the carrier gas. Fifteen pulses were introduced, with a pulse interval of 8 min. The effluent gas concentration was monitored by an internal TC detector as well as an external MS detector (Fisons Sensorlab). The CO/Pt ratio was calculated using the TCD results. The average peak area obtained when CO pulses were passed over the sample after “breakthrough” of the pulses (i.e., with no adsorption) was used as the reference area. 2.3. Catalyst Screening. The performance of the prepared samples as catalysts for propane dehydrogenation was evaluated at Statoil research center. Propane dehydrogenation tests were performed in a fixedbed titanium reactor. The inner diameter of the reactor was 9 mm. A titanium tube of outer diameter 3 mm was located in the center of the reactor. The reactor temperature was controlled by thermocouples placed in the 3-mm tube inside the reactor. The catalysts were pressed (5 tons) to tablets with a 24-mm diameter. The tablets were then crushed and sieved. Catalyst pellets in the range 0.7-1 mm were used for testing. The catalyst pellets (approximately 3 g) were placed on a titanium sinter in the reactor. The total pressure in the reactor was 1.1 bar, and the reaction temperature was 600 °C. The GHSV was 1000 h-1 based on propane, and the reaction gas contained 4.5% hydrogen, 32% propane, and the remainder steam on a mole basis. The catalyst deactivated with time on stream and was regenerated periodically. Catalyst regeneration was performed by oxidative treatment using air diluted with nitrogen. The oxygen content was initially 2% and was increased in a stepwise manner to a final level of 21% (pure air). Finally, the catalyst was reduced in flowing hydrogen. Both the oxidative treatment and the reduction step were carried out at 600 °C. The duration of one test cycle consisting of the dehydrogenation and regeneration steps was 24 h. The majority of the catalysts tested reached a stable initial activity (activity at the beginning of each cycle) after 5-10 cycles. Prior to the first test cycle, the catalyst was activated in situ through a ROR (reduction-

Table 1. Calcined Hydrotalcite Materials Used as Supports in the Preparation of the Catalysts Listed in Table 2 support

Mg/Al ratio

BET surface area (m2/g)

HT-1 HT-2

4.0 2.1

162 197

Table 2. Propane Dehydrogenation Catalysts Supported on Alumina and Calcined Hydrotalcite catalyst

support

pretreatment

Pt (wt %)

Sn (wt %)

Cs (wt %)

HT-1a HT-1aR HT-1b HT-2a HT-2b HT-2c AL-R AL

HT-1 HT-1 HT-1 HT-2 HT-2 HT-2 θ-Al2O3 θ-Al2O3

ROR R ROR ROR ROR ROR R ROR

0.3 0.3 0.3 0.3 0.3 0.3 0.7 0.7

0.6 0.6 0.6 0.6 0.9 1.2 0.5 0.5

0.5 0.5 0 0 0 0 3.9 3.9

oxidation-reduction) treatment at 600 °C (cf. section 2.2 for a detailed description of the procedure). 3. Results and Discussion This paper summarizes work that has been performed over a 10-year period as part of an ongoing catalyst development project. Two different sets of catalysts prepared at different stages of the project will be discussed. The comparison study presented in section 3.1 was carried out at an early stage of the project, whereas the samples discussed in the remainder of the text were prepared later. However, their compositions are very similar. As indicated in section 3.1, hydrotalcite is not the catalyst support itself but rather the precursor. However, for the sake of brevity the term “hydrotalcite” (abbreviated HT) will be used more loosely in the following discussion. If not stated otherwise, a “hydrotalcite-based” or “hydrotalcite-supported” catalyst denotes a catalyst that, after the final preparation step, contains the calcined hydrotalcite Mg(Al)O as the support. 3.1. Comparison with Alumina-Supported Systems. To assess the performance of the hydrotalcitebased catalysts, a comparison with conventional aluminabased catalysts was carried out. A reference catalyst was prepared according to a UOP patent.29 The catalyst contained platinum, tin, and cesium supported on θ-alumina. This sample was compared to several hydrotalcite-supported catalysts. The HT support materials were prepared as described above. Platinum, tin, and in some cases cesium were deposited onto the support by impregnation in ethanol solution as outlined above. Two HT support samples were used in the preparation of the catalysts (Table 1). The HT-1 sample was precipitated using NaOH as the basic agent, whereas (NH4)2(CO3)2/NH3 was used for the preparation of HT2. The use of the stronger base NaOH is presumably the reason for the higher Mg/Al ratio of the HT-1 sample. Table 2 shows a list of all samples prepared for the comparison of the alumina-supported and hydrotalcite-supported catalysts. The catalytic performance for propane dehydrogenation was tested in a fixed-bed reactor. The conditions differed slightly from those described in the Introduction, which were used for all other testing results reported in this paper. The catalysts were screened at 600 °C, 1 bar pressure, and 690 h-1 space velocity based on propane (NTP). The feed was propane (33%), H2

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Table 3. Conversiona and Selectivityb Results from Catalyst Testing in a Fixed-Bed Reactorc X(propane) (%)

S(propene)

catalyst

after 5h

after 25 h

after 5h

after 25 h

propene yield after 25 h (%)

HT-1a HT-1aR HT-1b HT-2a HT-2b HT-2c AL-R AL

52.8 19.3 58.7 58.8 58.0 58.6 38.0 41.4

45.6 16.7 53.0 57.5 57.8 57.5 27.0 31.0

97.5 97.8 93.3 93.0 93.9 94.9 97.0 96.4

97.8 97.3 97.3 95.9 96.1 95.9 95.0 95.9

44.6 16.2 51.6 55.1 55.5 55.1 25.7 29.7

a X ) conversion. b S ) selectivity. c Test conditions: T ) 600 °C; P ) 1 bar; GHSV ) 690 h-1 (NTP, based on propane); feed ) propane (32%), H2 (4.5%), N2 (24%), and steam (38.5%).

Table 4. List of the Hydrotalcite-Supported Catalysts Discussed in Sections 3.2-3.4 material Mg/Al Pt Sn BET code ratio (wt %) (wt %) (m2/g)a cat-A cat-B

3 3

0.3 0.3

1.2 1.2

150 162

cat-C

3

0.3

1.2

130

cat-D cat-E

5 3

0.3 0.25

1.2 0.5

156 -

cat-F

3

0.3

1.2

123

impregnation method ethanol onto calcined HT aqueous solution onto calcined HT; chlorine removal aqueous solution onto calcined HT; no chlorine removal ethanol onto uncalcined HT aqueous solution onto calcined HT; chlorine removal; supplied by catalyst manufacturer ethanol onto calcined HT

a

The BET surface area was measured on the pelletized catalysts.

(4.5%), N2 (24%), and steam (38.5%). Most of the catalysts were activated prior to the testing by an in situ reduction-oxidation-reduction (ROR) sequence carried out at 600 °C. Conversion and selectivity data from the screening test results are given in Table 3. All catalysts show a higher initial activity after a ROR treatment compared to only one initial reduction. However, this effect is much more pronounced for the hydrotalcite-based catalysts. Comparing the catalytic performance of the catalysts, the hydrotalcite-based catalysts are clearly superior in terms of activity and stability. Especially the HT-2 catalysts show very little deactivation after 25 h on stream compared to the alumina-supported catalysts. Even though the HTsupported catalysts give much higher conversion than the alumina-supported catalysts, their propene selectivity is comparable or even better. This effect becomes more evident after 25 h on stream, at which point almost all of the HT-based catalysts show a higher propene selectivity than the alumina-based samples. 3.2. Metal Deposition Parameters. Table 4 lists the catalysts that are discussed here and in the next section. They all have the composition Pt,Sn/Mg(Al)O. The BET surfaces reported in Table 4 were measured using pelletized samples. The values were some 50 m2/g higher on the powders. H2PtCl6 and SnCl2 were used as the metal precursors. SnCl2 is only soluble in acidified aqueous solution. Therefore, addition of acid was necessary. In most cases, HCl was used. Initial concerns regarding possible dissolution of the hydrotalcite in acidified aqueous solution proved to be groundless. However, chlorine is known to increase the acidity of the support, which can lead to increased coking and, in turn, render the catalyst more difficult to regenerate. Therefore, measures had to be taken to reduce the

Figure 1. X-ray diffraction data for different materials. Spectra b, e, and f were recorded with different instrument parameters. Therefore, the different signal/noise ratio of d compared to that of b, e, and f is not structurally significant. (a) Hydrotalcite, precipitated as described in the text, dried, uncalcined; (b) hydrotalcite, precipitated as described in the text, dried, calcined; (c) cat-B before the final calcination; (d) cat-B; (e) cat-A before the final calcination; (f) cat-A.

chlorine content of the catalysts after impregnation. Calcination under steam-rich conditions (wet calcination,see section 2.1) did not give satisfying results. The chlorine content of one Pt,Sn/Mg(Al)O catalyst was only reduced from 1.7 to 1.1 wt %. However, repeated washing with water proved to be very efficient. The chlorine content of cat-B was reduced from 5.4 wt % before washing to 0.25 wt % after washing. Repeated washing with water was therefore used as the standard procedure for chlorine removal. As mentioned in section 2.1, choosing water as the solvent during impregnation has important consequences for the structure of the carrier material. Figure 1 shows XRD data for different materials. After precipitation and drying, the uncalcined hydrotalcite exhibits a characteristic XRD spectrum (Figure 1a). After calcination, the XRD pattern shows MgO as the only crystalline phase (Figure 1b). However, the diffuse peaks indicate that the material has a relatively high defect density. XRD analysis of the cat-B sample after aqueous impregnation but before the final calcination step shows that the hydrotalcite structure is regenerated (Figure 1c). XRD analysis of the cat-A sample

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Figure 2. Propane conversion (circles) and propene selectivity (triangles) obtained from catalytic testing of cat-A in a fixed-bed reactor. Testing conditions: 600 °C; GHSV 1000 h-1; 4.5% H2, 32% propane, remainder steam.

Figure 4. Propane conversion (circles) and propene selectivity (triangles) obtained from catalytic testing of cat-C in a fixed-bed reactor. Testing conditions: 600 °C; GHSV 1000 h-1; 4.5% H2, 32% propane, remainder steam.

Figure 3. Propane conversion (circles) and propene selectivity (triangles) obtained from catalytic testing of cat-B in a fixed-bed reactor. Testing conditions: 600 °C; GHSV 1000 h-1; 4.5% H2, 32% propane, remainder steam.

Figure 5. Propane conversion (circles) and propene selectivity (triangles) obtained from catalytic testing of cat-D in a fixed-bed reactor. Testing conditions: 600 °C; GHSV 1000 h-1; 4.5% H2, 32% propane, remainder steam.

(prepared by impregnation of the calcined hydrotalcite with an ethanol solution) before the final calcination shows that the usual diffuse Mg(Al)O pattern is retained. This result indicates that no major structural changes take place during impregnation in ethanol (Figure 1e). After the final calcination step, both cat-A and cat-B exhibit Mg(Al)O as the only crystalline phase, regardless of whether water or ethanol was used during impregnation (Figure 1d and f). Propane dehydrogenation tests were carried out in order to rank the catalysts according to their performance. Initially, the presence of the hydrotalcite phase during metal deposition was considered disadvantageous because of the lower specific surface area compared to the calcined phase. However, testing results reveal that aqueous impregnation, in fact, improves the catalytic performance compared to the initial procedure using ethanol. Figures 2-5 and Table 5 show results from the catalytic tests for the catalysts cat-A (ethanol impregnation), cat-B (aqueous impregnation with chlorine removal), cat-C (aqueous impregnation without chlorine removal), and cat-D (ethanol impregnation on uncalcined HT). All catalysts show deactivation within each cycle mainly as a result of coke formation. As the propane conversion decreases, the selectivity to propene increases. All catalysts give similar propene selectivities ((1%) at similar propane conversions. A comparison of samples cat-A and cat-B (Figures 2 and 3, respectively) shows that aqueous impregnation with subsequent chlorine removal yields a catalyst with improved stability of the initial conversion after each regeneration compared to the sample where ethanol is used as the

Table 5. Propane Conversion Data Obtained from Catalytic Testing in a Fixed-Bed Reactor conversion (%)a cycle 1 catalyst initial

final

n.m.b 59 68 52

n.m.b 48 47 48

cat-A cat-B cat-C cat-D

cycle 3

cycle 5

initial final initial 62 57 59 53

38 45 43 45

54 55 46 n.m.b

cycle 10

final

initial

final

33 38 32 n.m.b

n.m.b 55 n.m.b 49

n.m.b 39 n.m.b 40

a Test conditions: 600 °C; GHSV 1000 h-1; 4.5% H , 32% 2 propane, remainder steam. b n.m. ) not measured.

solvent. After each regeneration cycle, the initial activity of the former reaction cycle is nearly fully restored for cat-B. The ethanol sample, on the other hand, deactivates more strongly from one cycle to the next. However, the solvent itself is probably not the reason for the differences in performance of the two catalysts but rather the different structure of the support during impregnation. Evidence for that conclusion comes from a comparison of the catalysts cat-B and cat-D. The latter catalyst is prepared by impregnation of the uncalcined hydrotalcite in ethanol solution, whereas for cat-B, the hydrotalcite phase is regenerated during aqueous impregnation (Figure 1c). Table 5 and Figures 3 and 5 show that cat-B and cat-D are comparable in their stabilities and are clearly superior to cat-A. A comparison between catalysts cat-B and cat-C (Figures 3 and 4, respectively) illustrates the importance of chlorine removal. The chlorine-rich catalyst deactivates significantly in terms of initial activity, presumably because

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Figure 6. Propane conversion (circles) and propene selectivity (triangles) obtained from catalytic testing of cat-E in a fixed-bed reactor. Testing conditions: 600 °C; GHSV 1000 h-1; 4.5% H2, 32% propane, remainder steam.

of increased coking on acidic sites. Hence, a higher chlorine content clearly reduces the lifetime stability of the catalyst. Given the encouraging results from aqueous impregnation, other preparation parameters were investigated in more detail. These parameters were the mode of addition of the impregnating solution (dropwise vs all at once), the contact time of the support and solution, and dry support vs slurried support. However, testing results showed that these parameters have no significant importance for the characteristics of the catalyst. The two most important factors are the structure of the support during impregnation and the chlorine content of the calcined catalyst. Metal deposition onto the (uncalcined) hydrotalcite and a low chlorine content on the final catalyst lead to enhanced performance compared to the initial preparation procedure (e.g., ethanol on calcined HT). Finally, Figure 6 gives an impression of the performance of the fully optimized catalyst. This state-of-theart catalyst, cat-E, was prepared on a laboratory scale by a cooperating catalyst manufacturer using the “impregnation-in-water” route described in section 2.1. After having reached a stable level, cat-E shows virtually no deactivation from cycle to cycle over a period of more than 500 h on stream. The initial activity in each cycle is actually limited by thermodynamic equilibrium. At the same time, the catalyst delivers excellent propene selectivity. In contrast to cat-A, cat-B, cat-C, and cat-D however, this catalyst increases in initial activity during the first four cycles. The occurrence of this activation phase is not predictable. Some catalysts reach their levels of stable activity by deactivation from cycle to cycle, whereas others go through activation. There are indications that the initial behavior during the first cycles might be related to the amount of platinum in the catalyst. 3.3. Characterization with SEM and TEM. The morphology of the catalysts was characterized by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM and TEM data are presented for cat-A only as the other Pt,Sn/HT catalysts exhibited comparable results. The SEM analysis shows ball-like structures (Figure 7) with diameters in the range of 1-5 µm. An image recorded higher magnification (Figure 8) shows that the balls are made up of plates with thicknesses of less than 0.1 µm. A typical TEM picture is shown in Figure 9. The picture shows small intergrown crystallites varying in size from 4 to 10 nm. This result is in line with crystal size estimations

Figure 7. SEM image of cat-A.

Figure 8. SEM image of cat-A at higher magnification.

Figure 9. TEM image of cat-A.

based on peak fitting and deconvolution of the XRD patterns with respect to a standard (NaCl). It is also in line with the results of Derouane et al.,30 who proposed that a crystallite size of 5 nm corresponds to a BET surface area of approximately 200 m2/g for a calcined hydrotalcite. EDS analysis showed that the dark spots in Figure 9 contain Pt. Pt was always detected together with Sn, whereas Sn was sometimes found without Pt. The observed Pt-containing areas (on this and other

Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001 4747 Table 6. Results from CO Chemisorption Measurements material code

pretreatment

CO/Pt ratio

HT-3 cat-F, fresh, #1 cat-F, fresh, #2 cat-F, fresh, #3 cat-F, fresh cat-F, useda cat-E, fresh

ROR ROR ROR ROR R ROR ROR

0 0.18 0.20 0.21 0.13 0.11 0.09

a Test conditions: 600 °C; GHSV 1000 h-1; 4.5% H , 32% 2 propane, remainder steam.

catalysts) are quite large, typically with diameters of 1-5 nm. 3.4. Metal Dispersion. The dispersion of the platinum metal on the surface is of great importance for the catalyst performance. In the case of platinum, some authors found meaningful and reliable dispersion results using H2 chemisorption.31 However, whereas our group found significant H2 adsorption on a Pt/Al2O3 catalyst, no H2 adsorbed on an equally active Pt/Mg(Al)O catalyst. It has been suggested that the lack of H2 adsorption on certain supported Pt catalysts could be due to a strong metal-support or metal-promoter interaction32,33 or to geometric effects.32 In our case, we instead chose CO chemisorption for Pt dispersion measurements. CO chemisorption has been applied to the determination of Pt dispersion, and good agreement between CO chemisorption results and dispersion data obtained by other methods are reported in the literature for supported Pt as well as Pt-Sn systems.33,34 CO can adsorb in two different geometries on Pt, i.e., on-top adsorption on one Pt atom or bridged adsorption on two Pt atoms. The adsorption stoichiometry (CO/Pt ratio) is therefore not well-defined and is considered to be in the range CO/Pt ) 0.7-1.0. The CO chemisorption experiments were carried out in the pulsed mode. Compared to volumetric measurements the setup is easier and the experiments are less demanding. Absolute dispersions are difficult to determine with pulsed CO chemisorption measurements. However, relative dispersions are in line with volumetric chemisorption results. Measurements were performed for one catalyst, cat-F (see also Table 4). The sample contained 0.3 wt % Pt and 1.2 wt % Sn, and the hydrotalcite had a ratio of Mg/Al )3. Analysis was performed before and after propane dehydrogenation testing under different conditions. In a preliminary experiment, CO chemisorption was carried out on the calcined hydrotalcite support alone (HT-3) to rule out any contribution of the support to the total CO uptake (see Table 6). The support material showed no CO uptake. Three consecutive tests of the cat-F catalyst showed acceptable reproducibility of the measurement. One analysis performed on cat-F pretreated with only one R step (reduction) yields a CO/ Pt ratio that is significantly lower. This clearly points out the importance of keeping the pretreatment procedure the same for all samples. CO uptake after dehydrogenation testing is significantly lower than that for the fresh catalysts. The CO/Pt ratios found were in the range 0.11-0.20 depending on the testing conditions. The CO/Pt ratio for the used cat-F given in Table 6 was measured after 126 h on stream at the testing conditions described above (600 °C; 4.5% H2, 32% propane, remainder steam). A comparison of the initial and final CO/Pt ratios with initial and final propane conversions for cat-F is shown

Table 7. Comparison between CO Chemisorption Measurements and Propane Conversion Data for Cat-E and Cat-F run no.a

catalyst

CO/Pt ratio (final/initial)

propane conversion (final/initial)

37 41 58 59 60 50

cat-F cat-F cat-F cat-F cat-F cat-E

0.6 0.6 0.9 0.6 0.8 1.0

0.5 0.4 0.8 0.5 0.8 2.5

a The run numbers signify different runs with various test durations and regeneration cycles. Test conditions: 600 °C; GHSV 1000 h-1; 4.5% H2, 32% propane, remainder steam.

in Table 7. All tests were performed under standard test conditions at 600 °C, but with different regeneration times and test cycle durations. The loss in catalytic activity with time on stream shows good correlation with the loss in Pt dispersion. CO chemisorption was further performed for cat-E before and after catalytic testing (Tables 6 and 7). The initial Pt dispersion was lower than for cat-F and was stable throughout the test. This result is in line with the lower initial activity of cat-E compared to cat-F but not in line with the observed increase in catalyst activity observed for cat-E with time on stream (Figure 6). The results presented above indicate a complex relationship between the preparation procedure, composition, and characteristics of Pt,Sn/Mg(Al)O materials as catalysts for the propane dehydrogenation reaction. This is in line with the findings of Armenda´riz et al., who very recently published a paper on the influence of the Mg/Al ratio in Pt,Sn/Mg-Al-O catalysts for the dehydrogenation of isopentane.35 A clear correlation was found between the Mg/Al ratio in the support and the selectivity and stability of the catalyst. However, they could not find a clear correlation between the platinum species (alloyed or not), platinum content, and platinum crystallite size and the observed catalytic differences between the materials. 4. Conclusions The novel catalyst for the dehydrogenation of propane presented in this paper is clearly superior to comparable alumina-supported systems. The Pt,Sn/Mg(Al)O catalyst shows substantially higher propane conversion while at the same time maintaining a high propene selectivity near the thermodynamic limit. The proper choice of the metal deposition procedure is very important with respect to the long-term stability of the catalytic performance. Impregnation of the calcined HT in aqueous solution yields a catalyst that shows better long-term stability than the corresponding catalyst prepared in ethanol solution. The reason for this effect is probably the different support phases present during impregnation. Metal deposition on the hydrated form apparently yields more stable catalysts than the mixed Mg(Al)O oxide. Removing chlorine from the catalysts impregnated in HCl-acidified aqueous solution proved to be important in obtaining stable performance. The adverse effect of chlorine is most likely due to increased coke formation on acid sites. Acknowledgment The authors thank the Norwegian research council for its financial support of this work.

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Received for review April 3, 2001 Revised manuscript received August 29, 2001 Accepted August 30, 2001 IE010299+