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Ind. Eng. Chem. Res. 2009, 48, 2878–2885
Catalyst Deactivation of Rh-Coated Foam Monolith for Catalytic Partial Oxidation of Methane Shi Ding, Yiyang Yang, Yong Jin, and Yi Cheng* Department of Chemical Engineering, Beijing Key Laboratory of Green Chemical Reaction Engineering and Technology, Tsinghua UniVersity, Beijing 100084, China
The catalyst deactivation behavior of rhodium-coated foam monolith was systematically investigated in order to understand the means to improve the durability of the rhodium catalyst applied for catalytic partial oxidation of methane (CPOM). The overall CPOM reactions on the foam structured catalyst have been acknowledged to take place first in an oxidation zone and thereafter in a reforming zone. Severe metal sintering near the entrance of the structured catalyst (i.e., in the oxidation zone) was identified to be responsible for the observed deactivation of rhodium catalyst in the course of a 1000 h time-on-stream test under quasi-adiabatic conditions. Further analyses on the deactivation process indicated that the reaction pathway in the oxidation zone near the entrance can be summarized by a mixed mechanism, that is, two oxidation reactions and one reforming reaction, where H2 is the indirect product of steam reforming of the unreacted CH4. Detailed studies on the dependence of the catalyst stability on the operating conditions and the catalyst designs showed that adding an inert gas to the reactant gases, increasing the metal loading and/or decreasing the pore size of the foamstructured catalyst in the oxidation zone, can improve the catalyst stability, while the catalyst modifications in the reforming zone has little effect on the overall behavior of the catalyst stability. 1. Introduction As the building block for many chemicals including methanol, higher hydrocarbons, and acetic acid, syngas is of great importance to the petrochemical industry from natural gas.1 Steam reforming of methane shown in eq 1, which is the main process for current syngas production, is highly endothermic, and therefore usually performed in large furnaces to supply the necessary energy.2 Alternatively, catalytic partial oxidation of methane (i.e., CPOM in eq 2), with the features of slightly exothermic reaction, nearly 100% methane conversion, more than 90% syngas yields, millisecond contact times,3,4 and feasibility of scaling down, is well suited for decentralized and small-scaled syngas production in a remote gas field.5-7 CH4 + H2O f 3H2 + CO
∆Hrθ ) 205.9 kJ/mol (1)
CH4 + 0.5O2 f CO + 2H2
∆Hrθ ) -35.6 kJ/mol (2)
Most of the catalysts applied for CPOM contain a group VIII metal as the active component (e.g., Rh, Pt, Ru, Ir, Ni, Co) on an oxide support.8 Ni and Co are highly active for syngas production, but they also promote carbon formation. Although modifications on the supports can improve the catalyst stability to some extent, catalyst deactivation is still unavoidable due to both the carbon deposition and the loss of the metal surface area.8-12 The noble metal-based catalysts with high activity and stability can accomplish the effective and fast conversion of CH4 to CO and H2 without external energy input, which is desired in the industrial process.5,7,13-15 Among them, Rh has been acknowledged as one of the best catalysts due to its high activity and selectivity, low tendency to carbon formation, and low volatility.16,17 Considering the high cost of Rh, the stability of Rh-based catalyst under extreme reaction conditions such as around 1000 °C and millisecond residence times is crucial for its practical * To whom correspondence should be addressed. E-mail: yicheng@ tsinghua.edu.cn. Tel.: 86-10-62794468. Fax: 86-10-62772051.
application in CPOM. Tavazzi et al.18 pointed out that 0.5 wt % Rh/Al2O3 catalyst suffered from significant deactivation over the course of a 150-h time-on-stream test under quasi-adiabatic conditions.18 Therefore, the appropriate understanding on the reaction mechanism and on the products profile in the catalyst bed with the deactivated catalyst would greatly benefit reactor optimization as well as the catalyst stability improvement. However, the reaction mechanism of CPOM under autothermal conditions is still an open question. Since it is difficult to investigate the products profile in situ due to the extreme reaction conditions, most studies have to be conducted either under the “unrealistic” conditions (e.g., well-defined isothermal low pressure or diluted conditions),19,20 or under the “realistic” conditions but assuming the reaction mechanism by comparing the data at the reactor exit with the numerical simulations.21-24 Depending on the experimental conditions, different product profiles are inferred and different mechanistic conclusions are drawn. The direct mechanism assumes that H2 and CO are primary reaction products formed in the oxidation zone at the catalyst entrance.20 The indirect mechanism, on the other hand, postulates a two-zone model with a combustion zone of CH4 at the entrance of the catalyst bed and a reforming zone downstream to product H2 and CO.19,23 Moreover, there is also a
Figure 1. Schematic of the millisecond reactor.
10.1021/ie801500n CCC: $40.75 2009 American Chemical Society Published on Web 02/04/2009
Ind. Eng. Chem. Res., Vol. 48, No. 6, 2009 2879
possible by Schmidt group.25,26 They assumed that there were an oxidation zone and a reforming zone in terms of the existence of O2 or not. In the oxidation zone, the consumption rate of O2 is fully controlled by the mass transfer behavior. The main products are CO, H2, and H2O. In the reforming zone, H2 is further produced due to the steam reforming of CH4. However, both the detailed reaction pathway of the oxidation zone and the products development in the catalyst bed during deactivation have not been reported in the above-mentioned researches. The present work focuses on the deactivation of the Rh-coated foam monolith catalyst for quasi-adiabatic CPOM. The 1000-h stability test and the SEM observation of the aged catalyst indicated that the observed catalyst deactivation was mainly due to the metal sintering caused by the highly exothermic reaction in the oxidation zone. A possible reaction pathway of the oxidation zone is then proposed by analyzing the catalytic performances during deactivation. Furthermore, the effects of various operation conditions and the catalyst designs on the catalyst stability are investigated, respectively. 2. Experimental Details
Figure 2. Conversion and exit temperature (a) and selectivity (b) within 1000-h stability test of CPOM using 3 wt % Rh-coated 80 ppi foam monolith. Conditions: CH4/O2 ) 2; space velocity ) 5 × 104 h-1; no inert gas.
Figure 3. SEM micrographs of the fresh catalyst (a) and the different axial positions (1, 5, and 9 mm to the entrance) of the aged catalyst (b-d) after 1000-h time-on-stream test.
mixed mechanism which suggests CO is a direct reaction product but H2 is an indirect reaction product.24 Until recently, experimental data with high resolution in space were obtained at conditions as close to the industrial ones as
2.1. Catalyst Preparation. The foam monolith purchased from Hi-Tech Ceramics was used as the catalyst support and the structured reactor as well. The foam monolith is made of R-Al2O3, specified as 10 mm in length, 15 mm in diameter, and 80 or 45 ppi (i.e., pores per linear inch). The geometric surface areas per unit volume are 6.21 × 104 and 12.40 × 104 m2/m3 for 45 and 80 ppi foams, respectively.27 For the experimental design, the foam was cut into 3, 4, or 7 mm in length as requested. All of the catalysts were prepared according to the same procedure. Briefly, RhCl3 was deposited onto R-Al2O3 via the incipient wet impregnation. The impregnated support materials were dried at 110 °C for 3-h, followed by the calcination at 600 °C in air for 6-h. 2.2. Testing Apparatus. Experiments were performed in a quartz tube containing a cylindrical R-Al2O3 foam monolith coated with rhodium (see Figure 1). The flow rates of the reactant gases were controlled by the calibrated mass flow controllers (MFCs). The gases were premixed before flowing into the quartz tube of the reactor. The product gas mixture was analyzed by a gas chromatograph (Techcomp 7890 II) with a TDX01 column and a thermal conductivity detector (TCD), excepted that the quantity of H2O was calculated by closing the mole balance of O2. Micrographs of catalysts were taken by a JSM-7401 microscope with an elemental dispersive spectrum (EDS) X-ray detector system (EDAX Inc.). All SEM micrographs were obtained using secondary electron detection. The foam monolith coated with Rh catalyst acted as the structured reactor, which was flanked with two blank foams in tandem as the front and the back heat shields. The whole sandwiched foams were wrapped tightly using alumino-silicate cloth and placed in a quartz tube (i.d. ) 18 mm). The quartz tube was accommodated inside a ceramic heater maintained at 400 °C to reduce the heat loss through the quartz tube. The cloth and the maintained oven temperature far below the reaction temperature were used to reduce the radial and axial heat losses from the reactor so as to make the process approach to the adiabatic operation. The temperatures were monitored by thermocouples at the reactor wall and the outlet. The exit temperature was measured at the point between the catalyst and the back heat shield.
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Figure 4. CH4 conversion (a), selectivity (b), O2 conversion (c), and exit temperature of the 0.3 wt % Rh-coated 45 ppi foam monolith under two conditions: (1) total flow rate of 3 L/min, CH4/O2/He ) 2:1:0; (2) before 8 h: total flow rate of 5 L/min, CH4/O2/He ) 2:1:2; after 8 h: total flow rate of 3 L/min, CH4/O2/He ) 2:1:0. Table 1. Composition of Gases in Time under Total Flow Rate of 3 L/min, CH4/O2/He)2:1:0 0h XCH4 XO2 consumed n(CH4):n(O2) CO (L/min) H2 (L/min) SCO SH2
1h
2h
4h
7h
10 h
78.30% 70.89% 67.34% 63.04% 61.12% 60.79% 100% 98.88% 97.84% 96.83% 95.92% 95.50% 1.57 1.43 1.38 1.30 1.28 1.27 1.51 2.75 96.71% 87.79%
1.36 2.33 96.16% 82.17%
1.29 2.14 95.46% 79.62%
1.20 1.91 94.81% 75.79%
1.15 1.82 94.32% 74.37%
1.14 1.80 94.38% 74.27%
3. Results and Discussion 3.1. Deactivation of the Rh-Coated Foam Monolith Catalyst. The stability of Rh catalyst (80 ppi foam monolith with 3 wt % Rh) was tested continuously during the 1000-h quasi-adiabatic CPOM process. Figure 2 shows the time-onstream CH4 conversion, selectivities to H2 and CO, and the exit temperature over the course of the stability test. It should be noted that O2 conversion would not be plotted in this and following figures if it maintains at 100% during the detected reaction time. Clear decline of CH4 conversion and H2 selectivity, coupled with increased exit temperature were observed along the reaction progress, which indicated that gradual deactivation of the Rh catalyst has occurred under this long-term quasi-adiabatic operation. This might bring negative evaluation to the practical application of Rh catalyst in CPOM process when taking its high cost into account. Therefore, it is of great importance to find out the main reason for this deactivation and seek to improve its stability. It is reported that the reaction rate of CPOM process is more dependent on the metal area on the geometric surface of the catalyst, rather than the metal surface area of the whole catalyst, because it is a millisecond-time reaction and only the catalyst at the boundary of the flowing stream causes any reaction.28 To compare the change in the catalyst surface morphology
during deactivation, both the fresh and aged catalysts were analyzed using SEM (Figure 3). The small dots (
