4412
Ind. Eng. Chem. Res. 1996, 35, 4412-4416
Heterogeneously Catalyzed Gas-Phase Hydrogenation of cis,trans,trans-1,5,9-Cyclododecatriene on Palladium Catalysts Having Regular Pore Systems Georg Wieβmeier and Dieter Ho1 nicke* Lehrstuhl fu¨ r Technische Chemie, Technische Universita¨ t Chemnitz-Zwickau, 09107 Chemnitz, Germany
The study deals with the heterogeneously catalyzed gas-phase hydrogenation of cis,trans,trans1,5,9-cyclododecatriene on coated catalysts prepared by anodic oxidation of aluminum and a liquid-phase impregnation technique. The coat of the catalyst consists of alumina having a regular pore system and palladium as a catalytically active component which is immobilized at the pore walls. The thickness of the coat which is equal to the pore length is adjusted in each catalyst to a certain size in the range of 5-30 µm. The hydrogenation products are the intermediates cyclododecadiene and cyclododecene besides the completely saturated final product cyclododecane. The scope of the investigations is to find out the advantages of the prepared coated catalysts in comparison to a commercial hydrogenation catalyst and the interdependences between the thickness of the coat and product selectivities as a result of mass transport influences. It is shown that the formation of cyclododecene on the coated catalysts is predominant, and at conversions of more than 90%, the shorter the pore length of the coated catalysts, the higher the selectivity to cyclododecene. Introduction In the chemical industry about 80% of the reactions are catalytically performed and about 80% of that are heterogeneously catalyzed reactions (Gallei and Neumann, 1994). In general, porous materials with different shapes, for example, extrudates or pellets, are used as supports for catalytically active components. They are characterized by irregular pore systems. A special type of porous material having regular pore systems is the subject of the present investigation. The following general scheme shows a consecutive hydrogenation reaction of compound A, e.g., a triple unsaturated hydrocarbon, to the thermodynamically stable saturated product D via the intermediates B and C, which are di- and monounsaturated hydrocarbons, respectively:
AfBfCfD In some industrial hydrogenation reactions the goal is to yield the partially hydrogenated thermodynamically unstable intermediates B or C, which are valuable organic products. A prerequisite to achieve high yields of such intermediates is to carry out the reaction under kinetically controlled conditions without mass-transfer limitation. Concerning the irregular pore systems of commonly used porous materials, there is no possibility to reach a narrow residence time distribution of the molecules in the pore system due to its irregularity. As a consequence of a long residence time of the molecules in the pore system the formation of the final product D is improved and that of the intermediates B and C is diminished. From this, higher efforts are required to separate the desired intermediate product from the others, which is an important criteria related to the efficiency of industrial processes. Industrially used catalysts are mostly supported or coated catalysts. The latter consist of a thin layer of catalytically active components surrounding the unporous core. They have irregular pore systems; however, the intrinsic residence times for the reactant molecules are shorter than those in supported catalysts because S0888-5885(95)00709-3 CCC: $12.00
Figure 1. Alumina-coated catalysts formed by anodic oxidation of aluminum: (a) scheme of an alumina-coated aluminum wire; (b) structure of the alumina coat according to the model of Keller et al. (1953) and textural parameters: density np, diameter dp, and length lp of the pores; X- anion, T temperature, c concentration of the electrolyte; U anodization voltage; i current density; t time span of the anodization.
of the shorter pathways due to the shorter average pore length. This is an important advantage of coated catalysts related to the formation of intermediate products in some greater extent. Nevertheless, the wide distribution of the intrinsic residence times of the molecules due to the irregular pore systems hindering the pinpointed and even pronounced formation of intermediate products is yet a distinct disadvantage. In addition, unsufficient adhesion strengths and irregular thicknesses of the coats may be detrimental. Other drawbacks, resulting from the physical properties of the used ceramic materials as supports or cores, are the low heat conductivity and capacity and a wide pore size distribution in both types of catalysts. These drawbacks led to the development of a novel type of coated catalyst. Therefore, the preparation of catalysts having tailor-made pore systems characterized by regular and unbranched pores with uniform diameters and lengths was carried out. Thus, the possibility to control the mass transport process was given. Figure 1 shows the scheme of this type of coated catalyst without the mentioned drawbacks of the conventional © 1996 American Chemical Society
Ind. Eng. Chem. Res., Vol. 35, No. 12, 1996 4413
Figure 3. Scheme of the anodization: (a) constant voltage power supply; (b) heat exchanger; (c) electrolyte pump. Anode: aluminum wire. Cathode: aluminum plate. Figure 2. Transmission electron micrograph of an anodized aluminum foil. Horizontal raster length: 860 nm. Conditions of anodic oxidation: oxalic acid (1.5 wt %); T ) 293 K, U ) 60 V, t ) 1 h.
coated catalysts (Wieβmeier and Ho¨nicke, 1995). The catalyst support has a cylindrical shape as extrudates (Figure 1a). The core consists of aluminum and the coated layer of porous alumina. This support is prepared by anodic oxidation using an aluminum wire. The well-known structure of the oxide layer according to the model of Keller et al. (1953) is depicted schematically in Figure 1b. By anodic oxidation in aqueous acids such as sulfuric, oxalic, or phosphoric acid, a nonporous oxide layer with a thickness of less than 1 µm is formed on aluminum. At the top of this layer a porous oxide layer is formed. That layer consists of hexagonal oxide cells, and each cell contains an open concentric pore. The pores are straight lined, predominantly parallel, unbranched, and perpendicular to the metal surface. The theory of pore formation is based on different models and described in previous papers (e.g., Parkhutik and Shershulsy, 1992). The pore system is entirely characterized by three parameters, viz., pore density np, pore diameter dp, and pore length lp. These textural parameters depend on the conditions of the electrolysis used, as shown in Figure 1b. The pore density, that is the number of pores per unit of geometric surface area, is on the order of 1014 pores/m2. The size of the pore diameter is in the range between 10 and 200 nm, and the length of pores amounts from some to several hundred microns. With the choice of defined conditions in advance, there is the possibility to form tailor-made coated catalyst supports preventing the mentioned drawbacks of the conventional supports. The application of the described catalyst preparation method is designated as “pore size engineering”. A comprehensive description of such pore systems formed in coated catalysts and their usefulness in heterogeneously catalyzed reactions was previously reported in separate papers (Ho¨nicke, 1987, 1989; Wieβmeier and Ho¨nicke, 1995). In Figure 2 is shown a transmission electron micrograph as an example of a real cell structure of alumina formed by anodic oxidation of an aluminum foil in oxalic acid (Ho¨nicke, 1989). A nearly hexagonal shape of the oxide cell is recognizable. Each cell is surrounded by six other cells, which is evidence for the closest packing relating to the hexagonal shape. The dark lines, probably caused by material compression, indicate the cell boundaries. The cross sections of the real cells are deformed and are not as uniform as proposed by the model of Keller et al. (1953). The pores have an average diameter of 30 nm.
Figure 4. Scheme of the immobilization process of palladium. Palladium acetylacetonate: Pd(acac)2.
Experimental Section Catalyst Preparations. Aluminum wires (diameter: 1 mm; Johnson Matthey, purity 99%) were used as a starting material for the catalyst preparation. Before anodization the aluminum wires were treated in a degreasing solution (45 g/L of NaOH/55 g/L of NaF) for 1 min at room temperature and subsequently rinsed in deionized water. The degreased substrate was mounted on a support which was then connected to the power supply of the anodization apparatus as shown in Figure 3. Aluminum plates (20 × 20 × 0.3 cm3) were used as cathodically polarized counter electrodes. The anodization was carried out in aqueous oxalic acid (1.5 wt %) at 285 K and 50 V. To achieve the constant voltage, a special power supply Philips PE 1648 (Figure 3a) was used. To reach a constant temperature, the electrolyte was circulated through a self-made heat exchanger (Figure 3b) by a peristaltic pump Multifix MC 1000 PEC (Figure 3c). The time span of anodization was different depending on the desired thickness of the oxide layer. The supports (marked as “S”) S1, S2, and S3 were prepared during 1, 3, and 7 h of anodizing, respectively. Generally, the immobilization of the catalytically active component is a very important matter of the catalyst preparation, and the steps used in the present study are shown schematically in Figure 4. Palladium is chosen as an active component for hydrogenation reactions, and as precursor palladium acetylacetonate (Pd(acac)2) dissolved in toluene was used. Starting with evacuated pores, hydroxyl groups are present at the pore walls. The pores were first wetted with toluene and second impregnated by a liquid phase treatment in the precursor solution. During the impregnation the palladium acetylacetonate reacts with the surface hydroxyl groups and is anchored at the surface via an oxygen
4414 Ind. Eng. Chem. Res., Vol. 35, No. 12, 1996 Table 1. Experimental Data of the Immobilization Process According to the Two Methods I and II To Prepare Catalysts C1, C2, and C3 (Method I) and C4 (Method II) immobilization method I treatment step 1 2 3 4 5 6 7
t (h) thermal treatment of the support in air wetting with toluene starting at 100 Pa liquid phase impregnation in Pd(acac)2/toluene (11.5 g/L) storage drying calcination reduction in hydrogen
II T (K)
0.5 5
298 328
6 6 1
393 723 393
t (h)
T (K)
6 0.5 5 48 6 6 1
723 298 328 298 393 723 393
Table 2. Characterization Data of the Prepared Supports S1, S2, and S3 and the Corresponding Catalysts C1, C2, and C3/C4 Measured by Various Methods support corresponding catalyst time span of anodization (h) pore lengtha (µm) specific surface area (BET)b (m2/g) average pore diameterb,c (nm)
S1 C1 1 5 0.2 30
S2 C2 3 15 0.3 30
S3 C3/C4 7 30 0.6 30
a Scanning electron microscopy. b Nitrogen adsorption. c Transmission electron microscopy.
Figure 5. Reaction scheme of the hydrogenation of cis,trans,trans1,5,9-cyclododecatriene. CDT: cyclodecatriene. CDD: cyclododecadienes. CDE: cyclododecenes. CDA: cyclododecane.
bridge (Boitiaux et al., 1982), as shown in the middle of Figure 4. Drying and calcination, which was carried out as a special thermal treatment, led to the decomposition of the anchored compound forming palladium oxide at the surface (Lesage-Rosenberg et al., 1986). Finally, the last step was the conversion of palladium oxide to palladium metal by reduction in hydrogen, whereby the catalytically active component is attached to the surface of the pore walls. In the present study the immobilization of palladium on the prepared supports was carried out according to different methods, viz., I and II; their data are summarized in Table 1. According to method I the treatment steps 2, 3, and 5-7 were performed, and according to method II, steps 1-7 were performed. In step 4 the impregnated catalyst separated from the precursor solution is stored in a closed containment in order to equilibrate the concentration of the precursor in the pores. The supports S1, S2, and S3 have been prepared according to method I, which gave catalysts (marked as “C”) C1, C2, and C3, respectively. One part of support S3 has been treated according to method II, which gave catalyst C4. The completely treated aluminum wires were finally cut in pieces of 4 mm length. In order to compare the hydrogenation results obtained on these cylindrical coated catalysts having regular pores with that on a coated catalyst (peripheral impregnated) having irregular pores, a commercial spherical hydrogenation catalyst (Heraeus K-0240) was also used. Characterization Methods. BET surface areas and average pore diameters were measured by N2 adsorption at 77 K using a Fisons Sorptomatic 1900 and pore diameters proven by TEM in a Philips CM 20 microscope. Pore lengths and palladium contents as well as distributions were determined by SEM in a Cameca SX 100 which was equipped with a WDX analyzer. Hydrogenation Reactions. As a reaction of potential industrial interest, the hydrogenation of cis,trans,trans-1,5,9-cyclododecatriene was chosen. This reaction is characterized by consecutive reaction steps and is
shown in Figure 5. There is the possibility to hydrogenate the cyclododecatriene (CDT) to cyclododecadienes (CDD), cyclododecenes (CDE), and the completely saturated final product cyclododecane (CDA). However, the goal of the present investigation was to yield the thermodynamically unstable cyclododecenes with high selectivities, which might be of interest in manufacturing several products of industrial potential, e.g., nylon or polyalkenameres. The heterogeneously catalyzed gas phase hydrogenation of cis,trans,trans-1,5,9-cyclododecatriene (Fluka; 99% GC) was carried out continuously with the aid of an experimental setup including a conventional fixed-bed integral reactor made from glass having an inner diameter of 12 mm. The cylindrical and spherical catalyst particles were placed into the reactor and located on a glass frit. The maximum masses of catalysts used were 3.4 g of C1...C4 and 0.16 g of the commercial catalyst. The difference in mass is due to different Pd contents and densities of the supports. The hydrogenation was carried out at 393 K and a total pressure of 110 kPa which corresponds to 16 psi. The partial pressure of CDT was 110 Pa and that of hydrogen (Messer Griesheim; 99.999%) 330 Pa. Nitrogen (Messer Griesheim; 99.996%) was used as the carrier gas in order to adjust the total gas flow rate. Results and Discussion Catalyst Preparations. The anodic oxidation of the aluminum wires gave the supports S1, S2, and S3 having different thicknesses of the oxide layers, namely, 5, 15, and 30 µm, which are equal to the pore lengths. These pore lengths correlate with the specific surface areas (BET). The given data of the supports according to Table 2 are equal to the corresponding catalysts. The distribution and the content of palladium, aluminum, and oxygen in the oxide layer of catalyst C3 as a function of the measuring distance are shown in Figure 6. The oxide layer position is recognizable by the progress of the aluminum and oxygen curve: Starting in the aluminum core of the catalyst on the left-hand side, only aluminum is present; when reaching the oxide layer, the content of aluminum decreases and that of oxygen and palladium increase. In the oxide layer, the aluminum and oxygen contents remain constant. Palladium is distributed over the whole oxide layer but
Ind. Eng. Chem. Res., Vol. 35, No. 12, 1996 4415
Figure 6. WDX analysis: distribution and content of palladium, aluminum, and oxygen in the oxide layer of catalyst C3, i.e., contents of elements vs distance. Preparation: support S3 (see Table 2); immobilization method I (see Table 1).
Figure 7. WDX analysis: distribution and content of palladium, aluminum, and oxygen in the oxide layer of catalyst C4, i.e., contents of elements vs distance. Preparation: support S3 (see Table 2); immobilization method II (see Table 1).
remains not as constant as the other elements. The average palladium content related to the oxide layer is about 0.16 wt %. At the pore mouth, that is the range near the outer side of the oxide layer, the content of palladium rises, resulting in a nonuniform distribution. Approximately the same distributions and slightly higher palladium contents were observed for the catalysts C1 and C2. To reach a uniform palladium distribution, the conditions of the catalyst preparation (method I) have been changed (method II) to give catalyst C4. The result is depicted in Figure 7 and shows a regular distribution of palladium especially at the pore mouth. This is probably caused by a change of the surface tension of the precursor solution as a result of the thermal treatment (step 1, Table 1). This and the storage (step 4, Table 1) led to a better wetting of the pore walls and thus to a regular distribution of the active phase. The average palladium content was about 0.18 wt %. Hydrogenation Reactions. The first results of the hydrogenation of cis,trans,trans-1,5,9-cyclododecatriene (CDT) on catalysts C1, C2, and C3 having different pore lengths but nonuniform palladium distributions are illustrated in Figure 8. The conversion of CDT was in the range of 94-100% under the given reaction conditions. The conversion used in Figure 8 is the overall conversion which is in the range of 50-99% concerning both the conversion of CDT (UCDT) and of the intermediate CDD. This overall conversion was derived and was calculated as follows:
UCDT+CDD ) UCDT(1 - SCDD) The term SCDD is the selectivity to CDD. The obtained
Figure 8. Selectivities to CDE vs overall conversions on the catalysts C1, C2, and C3 and the commercial hydrogenation catalyst (K-0240). Temperature: 393 K. Total pressure: 110 kPa. Partial pressure of CDT: 0.11 kPa. Partial pressure of hydrogen: 0.33 kPa. (1) Thickness of peripheral impregnation depth.
product selectivities show the following: In the conversion range of 50-80% the selectivities to the desired CDE increased from 51 to 75% obtained on the coated catalysts C1, C2, and C3. The selectivity patterns are identical for these catalysts considering the accuracy of measurements. Higher conversions of 80% could not be reached on catalyst C1 because of the conditions applied. Using catalysts C2 and C3, the selectivity curves differ; e.g., 83% was achieved on catalyst C2 and 74% on catalyst C3 at 96% overall conversion. The corresponding selectivities to CDD were 2 and 3% and those to CDA 15 and 23%, respectively. The commercial catalyst gave distinct lower CDE selectivities of 62 and 43% at conversions between 82 and 98%, respectively. The selectivities to CDE, CDD, and CDA at 96% overall conversion were 44, 3, and 53%, respectively. These observations were attributed to the mechanism concerning adsorption, surface reaction, and mass transport, as follows: The selective formation of CDE under kinetically controlled conditions is caused by a displacement mechanism. The origin of this mechanism is the drastically higher adsorption strength of CDT and CDD on the Pd active sites related to those of CDE and CDA (Arnold, 1996). Thus, the formed CDE is always displaced from the active sites by CDT and CDD, whereby no further reaction of CDE occurs and high selectivities to CDE were observed. In the case of mass transport limitation at too low concentrations of CDT and CDD in the catalyst pores, CDE is not entirely displaced and further hydrogenation to CDA takes place. The observed low selectivities to CDE on the commercial catalyst are attributed to the mass transport limitation in the long pathways of the irregular and branched pores which diminished the CDE displacement by CDT and CDD. In comparison, the reduced mass transport limitation by the short pathways in the pores of the coated catalysts C2 and C3 led to distinguished higher CDE selectivities. The difference in CDE selectivities between C2 and C3 at conversions more than 90% is caused by higher mass transport limitation in the longer pores of catalyst C3. The decrease in selectivities to CDE at high conversions, i.e., pronounced mass transport limitation, corresponds to an increase of CDA selectivities. At lower conversion than 80% the mass transport limitation is reduced and the difference in mass transport limitation as a consequence of the different pore lengths is negligible, resulting in identical selectivity patterns. The hydrogenation of cyclododecatriene on catalyst C4 having uniform palladium distribution and a pore length of 29 µm was investigated under the same reaction conditions as previously used. The results are
4416 Ind. Eng. Chem. Res., Vol. 35, No. 12, 1996
Figure 9. Selectivities to CDE vs overall conversions on the catalyst C4. Temperature: 393 K. Total pressure: 110 kPa. Partial pressure of CDT: 0.11 kPa. Partial pressure of hydrogen: 0.33 kPa.
given in Figure 9, which shows the selectivities to CDE versus overall conversion. In the conversion range of 50 to nearly 100% the CDE selectivities increased from 57% to 88% at 96% conversion and then decreased to 80% at nearly 100% conversion. The maximum yield of CDE which is related to the conversion of CDT was 88%. The corresponding selectivities to CDD and CDA at 96% conversion were 4 and 8%, respectively. On catalyst C3 (identical support) the maximum CDE selectivity of only 79% was achieved at lower overall conversion at 87% and shows the following: The higher the mass transport limitation, the lower is the CDE selectivity, and the selectivity maximum is shifted to lower conversions. The main result of the hydrogenation on catalyst C4 is that the selectivity to CDE is higher probably due to the modified immobilization method and the uniform distribution of palladium in the catalyst pores. In comparison with the CDE yield of 88% achieved on the developed coated catalyst C4, the maximum CDE yield of 59% obtained on the commercial catalyst was obviously lower. Only one study to our knowledge describes the same heterogeneously catalyzed gas phase hydrogenation and allows one to conclude from the selectivity and conversion data a CDE yield of about 30% (Arnold, 1996). In summary, the investigation shows that the expected interdependence between the pore lengths and selectivities appeared and that very high selectivities to the desired cyclododecenes at high overall conversions on the coated catalysts having regular and unbranched pores were obtained. Outlook The application of such optimized tailor-made catalysts is not limited to their use as fixed-bed catalysts. There is also potential to build up a monolithic reactor with several hundreds of microchannels (Schubert et al., 1989; Wieβmeier, 1991; Wieβmeier and Ho¨nicke, 1996) and to treat the surface of the microchannels using the same method as described. Such a microreactor represent a wall reactor as schematically depicted in Figure 10. The microreactor contains a stack of microstructured aluminum foils, having channels with cross sections on the micron scale, coated with the catalytically active layer as previously described. The final scope is then to perform the hydrogenation reaction in the microreactor in order to increase once more the selectivities to the monoenes due to the uniform flow regime and nearly isothermal conditions in the microchannels. This work is still in progress. A variety of industrially related reactions are feasible in such microreactors
Figure 10. Scheme of a microreactor realized by a stack of microstructured and anodic coated, catalytically active aluminum foils.
because of the possibility to immobilize other catalytically active components like transition metals or metal oxides. Acknowledgment This study was financially supported by the MaxBuchner-Forschungsstiftung and the Fonds der Chemischen Industrie and is gratefully acknowledged. Literature Cited Arnold, H. Doctorate Thesis, Technische Hochschule Darmstadt, Darmstadt, Germany, 1996. Boitiaux, J. P.; Cosyns, J.; Vasudevan, S. Preparation and Characterisation of Highly Dispersed Palladium Catalysts on Low Surface Alumina. Stud. Surf. Sci. Catal. 1982, 16, 123. Gallei, E. F.; Neumann, H.-P. Development of Industrial Catalysts. Chem.-Ing.-Tech. 1994, 66, 924. Ho¨nicke, D. Partial Oxidation of 1,3-Butadiene on V2O5/Al2O3/AlCoated catalysts: Effects of Pore Lengths on Product Selectivities. J. Catal. 1987, 105, 19. Ho¨nicke, D. Pore Texture of Anodically Formed Alumina. Aluminium 1989, 65, 1154. Keller, F.; Hunter, S.; Robinson, D. L. Structural Features of Oxide Coatings on Aluminium. J. Electrochem. Soc. 1953, 100, 411. Lesage-Rosenberg, E.; Vlaic, G.; Dexpert, H.; Lagarde, P.; Freund, E. EXAFS Analysis of Low-Loaded Palladium on Alumina Catalysts. Appl. Catal. 1986, 22, 211. Parkhutik, V. P.; Shershulsy, V. I. J. Phys. D: Appl. Phys. 1992, 25, 1258. Schubert, K.; Bier, W.; Linder, G.; Seidel, D. Treaded Micro Diamonds for the Manufacture of Microstructures. Ind. Diamanten Rundschau 1989, 23, 204. Wieβmeier, G. Investigation of the Heterogeneously Catalyzed Oxidation of Propene in a Monolithic Microreactor. Diploma Thesis, Universita¨t Karlsruhe (TH), Karlsruhe, Germany, 1991. Wieβmeier, G.; Ho¨nicke, D. Heterogeneously Catalyzed Partial Hydrogenation of cis,trans,trans-1,5,9-Cyclododecatriene on a Pd/Al2O3 Coated Catalyst. Chem.-Ing.-Tech. 1995, 67, 78. Wieβmeier, G.; Ho¨nicke, D. Heterogeneously Catalyzed Reactions in a Microreactor. Microsystem Technology for Chemical and Biological Microreactors. DECHEMA Monogr. 1996, 132, 93.
Received for review November 28, 1995 Revised manuscript received August 26, 1996 Accepted August 29, 1996X IE950709S
X Abstract published in Advance ACS Abstracts, October 15, 1996.
