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Ind. Eng. Chem. Res. 2004, 43, 1206-1210
KINETICS, CATALYSIS, AND REACTION ENGINEERING Coke Characterization on Pt/Al2O3-β-Zeolite Reforming Catalysts N. Martı´n,*,† M. Viniegra,† E. Lima,† and G. Espinosa‡ Departmento de Quı´mica, Universidad Auto´ noma MetropolitanasIztapalapa, Avenida San Rafael Atlixco 186, 09340 D.F., Me´ xico, and Instituto Mexicano del Petro´ leo, Eje Central 152, 07730 D.F., Me´ xico
The amount and nature of carbon deposits on platinum catalysts supported on a mixture of alumina-β-zeolite, non neutralized and neutralized with cesium, were investigated and compared with those of a commercial catalyst (Pt/Al2O3). The catalysts were deactivated in the methylcyclopentane reaction. Coke characterization was performed by elemental analysis, temperatureprogrammed oxidation (TPO), Fourier transform infrared (FTIR), and 13C (CP/MAS) NMR. The significant difference between Pt/Al2O3 and Pt supported on a mixture of γ-alumina and β-zeolite is the nature of the carbon deposits. Elemental analysis, TPO, FTIR, and 13C (CP/MAS) NMR pointed out different features of the deposits. The zeolitic catalyst neutralized by Cs presented similar amounts of carbon residues but was the most deactivated. The results point out acid sites with different responses to carbonaceous residues. 1. Introduction Catalyst deactivation, due to coke deposition or “fouling”, is a common problem occurring to all catalysts in hydrocarbon conversion processes. This phenomenon depends on the nature of the feed, the operating conditions, and the catalyst itself. Bifunctional catalysts used for naphtha reforming have a metallic function that is generally platinum, alone or in combination with other metals, and also an acid function that is given by chlorinated alumina. On these catalysts, coke deposits on both phases, and an increase in the temperature is needed in order to keep a proper activity. This fact results in higher yields of undesirable light compounds. In general, coke formation mechanisms consist of an initial stage, where hydrocarbon molecules are dehydrogenated, followed by the formation of unsaturated species on the metallic sites. These unsaturated species migrate by gas transport or through the adsorbed phase toward the catalyst acid sites, where dimers or polymers are formed.1 Zhorov and Panchenkov2 reported that coke is formed from 5-carbon naphthenic rings and alkylaromatics, with the former being the major contributors to coke formation at low temperatures while the latter are so at high temperatures. By incorporation of β-zeolite into a Pt/Al2O3 catalyst, a synergistic effect is shown in the selectivity to benzene (Bz) in the methylcyclopentane (MCP) reforming reaction.3 Zheng et al.4 reported that the addition of alkaline metals to a Pt catalyst supported on β-zeolite results in an important increase in the aromatization of n-hexane (n-Hex). Other studies5-7 have shown a high activity of β-zeolite in linear paraffin isomerization reactions, and this characteristic may be useful in reforming reactions. * To whom correspondence should be addressed. Tel.: (52)(55)58044666. Fax: (52)(55)58044666. E-mail: mgnc@ xanum.uam.mx. † Universidad Auto ´ noma MetropolitanasIztapalapa. ‡ Instituto Mexicano del Petro ´ leo.
Catalyst deactivation on zeolite systems depends on factors such as acidity and pore size. Several techniques have been used for coke characterization, i.e., Fourier transform infrared (FTIR)8,9 and 13C NMR.10 These techniques have proven to be helpful in providing information on coke composition, structure, and location. The most widely used technique for analysis of the reactivity of coke regarding thermal treatments is the temperature-programmed oxidation (TPO) technique. It has been used to characterize coke in several systems, such as reforming catalysts11,12 or zeolites.13 Studies characterizing coke on zeolites by 13C NMR have been reported.10,12-14 As an example, Groten et al.10 studied coke formation over USHY type zeolite, with 1-hexene as the feed and coke levels on the catalysts of around 5 wt %. Lange et al.9 used ethylene enriched with 13C to monitor polyaromatic structure formation on H-mordenite. To analyze the structure of coke, a cross-polarization (CP) technique15 can be employed in which magnetization is transferred from abundant 1H to 13C spins to improve sensitivity, resulting in a good differentiation of carbon type. Weitkamp and Maixner16 characterized coke deposited on LaY type zeolite during isobutane alkylation with butene, using CP and magic-angle spinning (MAS) 13C NMR with carbon levels of around 1 wt %, and concluded that coke is essentially paraffinic with some naphthenic multirings. Also, by 13C (CP/MAS) NMR, using experimental parameters derived from a model compound and with spectral edition techniques, detailed structural information has been obtained on the different carbonaceous residues.14 The present work deals with the characterization of coke deposited on platinum catalysts supported on a mixture of γ-alumina and β-zeolite. A comparison is made with a commercial catalyst, platinum-based, supported on chlorinated γ-alumina. Catalysts were deactivated by the MCP reaction. The techniques employed for coke characterization were
10.1021/ie034007b CCC: $27.50 © 2004 American Chemical Society Published on Web 02/05/2004
Ind. Eng. Chem. Res., Vol. 43, No. 5, 2004 1207 Table 1. Conversion (mol %) and Selectivity (%), Initial and Final, in MCP Reaction, at T ) 773 K; H2/Hc ) 2, WHSV ) 4.5 h-1 a sample Pt/A (Cf/C0 ) 0.71) C6 a
Pt/AβZCs (Cf/C0 ) 0.34)
Pt/AβZ (Cf/C0 ) 0.64)
AβZ (Cf/C0 ) 0.65)
S0
Sf
S0
Sf
S0
Sf
S0
Sf
4.4 11.4 7.2 13.6
3.3 5.5 3.6 10.7
3.9 1.7 0.8 1.9
72.6
16.5
3.0
75.1
75.9 2.0 4.7 2.5 2.4 2.9 2.0 7.6
80.9
68.5
0.9 4.7 2.8 14.1 9.2 55.0 4.8 8.3
11.5 6.3 3.1 3.4
63.6
2.4 5.4 2.9 8.0 3.5 65.2 4.7 7.8
8.5
4.2 4.7 9.9
C0; S0 ) conversion (%) and selectivity (%) at 5 min on stream. S0, Sf ) conversion (%) and selectivity (%) at 360 min on stream.
elemental analysis, TPO, FTIR, and 13C (CP/MAS) NMR. The results show the influence of the support on the nature of the deposited coke. 2. Experimental Section 2.1. Sample Preparation. From a β-zeolite sample (βZ), provided by PQ Corp. (Valfor CPE 11-75), extrudates were prepared with boehmite from Condea Chemie GMBH (99% purity, Catapal B) with 10 wt % zeolite and 90 wt % boehmite. The samples were calcined in air at 823 K for 3 h (AβZ). Another support was prepared in the same way but neutralizing first the Brønsted acid sites from the zeolite by ion exchange, using a CsOH (Aldrich) solution (with 0.82 wt % Cs). The resulting mixture was filtered and dried at 393 K and then calcined at 823 K for 2 h (AβZCs). It is important to mention that we did not perform any structural characterization; however, Ramı´rez et al.25 did some X-ray diffraction patterns of similar samples, and they reported that no important structural variations occurred in the original zeolite after the ion exchange with Cs. Platinum was incorporated by ion exchange using Pt(NH3)4(NO3)2 (Aldrich) as the precursor of 0.4 wt % Pt. The solids were filtered, washed with deionized water, dried, and calcined in air at 623 K for 3 h. Reduction of Pt was carried out with hydrogen at 773 K for 6 h. Catalysts are labeled as Pt/AβZ and Pt/AβZCs. The Brunauer-Emmett-Teller surface areas of these catalysts were 250 and 158 m2/g, respectively. As a reference, a commercial chlorinated Pt/γ-alumina catalyst (Pt/A) from Instituto Mexicano del Petro´leo (0.4 wt % Pt and 0.9 wt % chlorine; surface area of 200 m2/g; 65% D24) was used. Samples were deactivated in the MCP reaction (Aldrich 99.7%), reported in the literature as one of the most active molecules for coke formation.17 The reaction conditions were 773 K, atmospheric pressure, a H2/MCP molar ratio of 2, and weight hourly space velocity (WHSV) of 4.5 h-1. The reaction was run for 6 h. A bench-scale system was used with a fixed-bed reactor coupled to a chromatograph for online analysis of the whole gaseous effluent. 2.2. Sample Characterization. Deactivated samples covered with coke were characterized by elemental analysis using Perkin-Elmer equipment 11-CHNS/O2400. TPO experiments were carried out with an automatic reactor ISRI-RIG-100 with a thermal conductivity detector using a 5 mol % of O2 in an Ar mixture with a heating rate of 5 K/min, from room temperature to 1173 K, and a flux rate of 60 cm3/min. The spectra of 13C (CP/MAS) NMR were recorded at 75.47 MHz with
Figure 1. Conversion (mol %) versus time.
a contact time of 1 ms at 5 kHz spinning rate and 90° pulses of 4 µs using an ASX 300 Bruker spectrometer. Chemical shifts were referenced to a solid CH2 adamantane shift at 38.2 ppm relative to tetramethylsilane. For FTIR spectroscopy (Nicolet 710 FTIR), self-supported pellets were mounted on a Pyrex cell with CaF2 windows. Samples were maintained under vacuum for 2 h at 473 K prior to spectra collection. 3. Results 3.1. Catalytic Activity. Table 1 shows the activity results obtained for the MCP reaction, as initial and final conversions. Initially, the order of activity is Pt/ AβZ > Pt/A > Pt/AβZCs. The latter suffers higher deactivation (even more than AβZ), while the former is more stable (Figure 1). The selectivity data to n-Hex, n-hexene (n-Hexd), 2-methylpentane (2MP), 3-methylpentane (3MP), Bz, and methylcyclopentene (MCPd) are shown in Table 1. Bz is the major product for all Pt catalysts at the beginning of the reaction. The commercial catalyst and Pt/AβZCs have several similarities. They produce small quantities of light products and similar amounts of ringopening products; however, Pt/AβZCs produces more alkenes and MCPd, which is known to be a precursor of coke. Pt/AβZ produces more cracking products, and it is the catalyst that produces more Bz. After 6 h of reaction, the selectivity pattern of the catalysts changes. Still, some similarities are found between the neutralized zeolitic catalyst and the commercial one; for instance, Bz production decreases in a small amount,
1208 Ind. Eng. Chem. Res., Vol. 43, No. 5, 2004
Figure 2. Profiles of TPO: (A) Pt/A; (B) AβZCs; (C) Pt/AβZCs; (D) Pt/AβZ. Figure 3. 13C (CP/MAS) NMR spectra: (A) Pt/A; (B) Pt/AβZCs; (C) Pt/AβZ.
Table 2. Elemental Analysis of Coke sample wt % carbon
Pt/A
Pt/AβZCs
Pt/AβZ
AβZ
1.6
1.7
1.3
2.9
Table 3. Ratio of Areas (A1/A2) Obtained by TPO Analysisa sample Pt/A Pt/AβZCs Pt/AβZ AβZ a
T1 (°C) for T2 (°C) for T3 (°C) for T4 (°C) for peak K peak L peak M peak N A1/A2 427 225 225
500 500 450 517
610
2.4 9.0 24.0
A1 at temperatures T1 and T2, and A2 at temperatures T3 and
T4.
and the production of the alkylated products increases also in a small amount. The nonneutralized zeolitic catalyst again behaves differently: Bz production decreases markedly, and the alkylated compounds are now the major products of the reaction. For comparison, the selectivity pattern of the support (without Cs and Pt) is shown in the last column of Table 1. On this sample, a higher selectivity to light products is observed. 3.2. Coke Characterization. 3.2.1. Elemental Analysis. Table 2 shows the results from elemental analysis obtained from the deactivated samples. The amount of carbon present on the platinum catalysts is very similar. 3.2.2. TPO. TPO profiles for the three samples and the characteristic temperatures are shown in Figure 2 and Table 3. Commercial Pt catalyst presents two peaks, one at around 427 °C (peak L) and the second at 500 °C (peak M). On Pt zeolitic samples (Pt/AβZ and Pt/AβZCs), one more peak appears at 225 °C (peak K) and the peak M is shifted to higher temperatures. The TPO of AβZ (without Pt) shows a single peak at 517 °C. Literature shows that, with respect to this technique, there are three types of coke. The first one is more reactive to oxygen, appears at low temperature, and has been assigned to coke deposited on metallic centers.18 The second one is coke deposited near the metal-support interphase.19 The third one is less reactive, appears at higher temperature, and corresponds to coke deposited on the support.18 This coke deposited on the support is far from the metallic centers, which catalyze the carbon gasification.19,20
Figure 4. FTIR spectra: (A) Pt/AβZ; (B)Pt/AβZCs.
The ratio A1/A2 shown in Table 3 was calculated in order to compare the oxygen consumption for the peaks at ca. 225 °C (peak K) and 427 °C (peak L) for A1 and 500 °C (peak M) and 610 °C (peak N) for A2. 3.2.3. 13C (CP/MAS) NMR. Figure 3 shows 13C (CP/MAS) NMR spectra for the studied samples. Spectra of all samples showed both aromatic and aliphatic carbons. Peaks at 20 and 30 ppm are attributed to aliphatic carbons (sp3) and peaks at 130 and 155 ppm to aromatic carbons (sp2). 3.2.4. FTIR. FTIR spectra for Pt/AβZ and Pt/AβZCs samples are equivalent (Figure 4). The existence of four main bands at 2860, 2927, 2963, and 3047 cm-1 is observed. The first three bands are attributed to aliphatic carbons corresponding to CH stretching vibrations, νas[CH(CH3)], νas[CH(CH2)] (which is the more intense), and νs[CH(CH3)], respectively.21 The last band is attributed to aromatic -CH, and it is weak for Pt/ AβZ. In spectra of the Pt/AβZ and Pt/AβZCs, the so-called coke band at 1585 cm-1 occurs, indicating the presence of polyalkenes or polyaromatic species.22 The CH- wagging and twisting modes of polycyclic aromatics were clearly shown by the bands at ca. 1463 and 1374 cm-1.21 Those results confirmed the presence of both aromatic rings and alkyl groups in the coked samples.
Ind. Eng. Chem. Res., Vol. 43, No. 5, 2004 1209
4. Discussion Activity results obtained in the MCP reaction (Figure 1) show a fall in conversion, which is more considerable for supports having zeolite than for the commercial catalyst. The Pt/AβZCs catalyst showed the lower initial activity. In the MCP reaction, two types of active sites are involved: metallic sites, for the dehydrogenation reactions, and acid sites, responsible for bimolecular alkylation, isomerization, and ring-enlargement reactions.17 It has been reported that Pt catalysts supported on β-zeolite and alumina showed an increase in aromatics production as the zeolite/alumina ratio increased.23 Our results also show that, at the beginning of the reaction, the selectivity to Bz is favored in the nonneutralized zeolite (Table 1) and that this reaction aromatization is severely inhibited during time on stream for this catalyst. Production of significant quantities of Bz requires contribution from both metallic and acid functions. When only the latter function is present, the resulting Bz amount is very small, as in the AβZ sample. For Pt/A and Pt/AβZCs catalysts, Bz selectivity at the end is not significantly affected, but for Pt/AβZ, this value falls dramatically. This can be explained by coke deposition on the acid sites of the zeolite. Cracking reactions (
