Ind. Eng. Chem. Prod. Res. Dev. 1985, 2 4 , 531-539
Society, for support of this research. We also wish to acknowledge the support of the National Science Foundation, which provided funds for the purchase of the F’TIR spectrophotometer used in this research under Grant CHE-8216482.
531
Hanson, D. L.;Katzer, J. R.; Gates, E. C.; Schuit, C. G. A,; Harnsberger, H. F. J . Catal. 1974, 32, 204. Madix, R. J. A&. Catal. 1@80,29, 1. McCabe, R. W.; DiMaggio, C. L.; Madlx, R. J. J. Phys. Chem. 1985, 89, 854. McCabe, R. W.; Mitchell, P. J. Ind. Eng. Chem. Prod. Res. Dev. 1983, 22,
212. McCabe, R. W.; Mitchell, P. J. Ind. Eng. Chem. Prod. Res. Dev. 1984, 23,
196.
Registry No. CH3CH0,75-07-0; COz, 124-38-9;CH3COOH, 64-19-7; H,, 1333-74-0; HZPtCl,, 16941-12-1; HZPt(NH3),, 15651-37-3;ethanol, 64-17-5; ethyl acetate, 141-78-6.
Literature Cited Canning, N. D. S.;Madix, R. J. J. Phys. Chem. 1984, 88, 2437. Fukushima, T.; Arakawa, H.; Ichikawa, M. J. Chem. Soc., Chem. Commun. 1985, in press. Gonzalez, R. D.; Nagai. M. Appl. Catal. 1985, in press.
Miura, H.; Gonzalez, R. D. J. Phys. E 1982, 15, 373. Morikawa, K.; Shirasaki, T.; Okada, M. Adv. Catal. 1969, 2 0 , 97. Mukherji, R. Ph.D. Thesis, University of R h d e Island, 1976. Sarkany, J.; Gonzalez, R. D. J. Catal. 1982a, 7 6 , 75. Sarkany, J.; Gonzalez, R. D. Appl. Catal. l982b, 4 , 53. Sarkany, J.; Gonzalez, R. D. Appl. Catal. 1983, 5 , 85. Sarkany, J.; Eartok, M.; Gonzalez, R. D. J. Catal. 1983, 81, 347. Vannice, M. A.; Twu, C. C. J. Catal. 1983, 8.7, 213.
Received for reuiew December 17, 1984 Accepted June 12,1985
Coke Deposition on Silica-Alumina Catalysts in Dehydration Reactions Javler Bllbao,’ Andr6s T. Aguayo, and Jos6 M. Arandes Departamento de Qdmica Tgcnlca, Universidad del Pals Vasco, 48080 Bilbao, Spain
A study of the structural and kinetic aspects of deactivation by coke deposition on silica-alumina catalysts in dehydration reactions has been carried out. A reflection is made on the possible structure of coke and its evolution on the basis of the results of the analysis of the coke fractions extracted with solvents. The reactions studied are dimerization of acetaldehyde to crotonaldehyde, dehydration of 1-butanol and dehydration of 2-ethylhexanol. Relations between the catalysts’ properties and the coke deposition have been established from the differences among the physical properties and surface acidity of the fresh catalysts and from the study of their evolution with time.
Introduction Coke deposition is the main reason for catalyst deactivation in organic reactions, and due to its importance it has been subject matter for many works in recent years. The word “coke”, as used in the literature, includes all the carbonaceous material that remains on the catalyst after the reaction. Coke is not a single component, but a material that evolves to higher condensation grades. The general composition of coke seems to be (CH,),, at first rich in hydrogen with a value of 2 for x , but it polymerizes with time so that n increases and x decreases. Because of this there is not a simple definition of coke, and it is believed the intermediate compounds and the final carbonaceous product block active sites and contribute to catalyst deactivation (Wukasch and Rase, 1982; Corella and Asiia, 1982). There are different opinions about the mechanism of coke formation on acid catalysts such as silica-alumina and zeolites. In one of the earlier works (Voge et al., 1951) a direct relation between the amount of deposited coke and the content of polynuclear aromatics in the feed was observed. Later (Gladrow and Kimberlin, 1960; Appleby et al., 1960) a correlation between the coke content and the basicity of the aromatics in feed was established. Appleby et al. (1962) attribute coke formation to reactions of aromatic structures initially present in the feedstock (or formed as intermediates) with formation of intermediate 0196-4321/85/1224-0531$01.50/0
carbenium ions. Langner (1980) determined that the coke precursors adsorbed on the catalyst have a cyclic structure, identifying by GC/MS the presence of alkylated cyclopentanes, cyclohexanes, etc. Fajula and Gault (1981a-c) support the same opinion, also based on the presence of aromatics and cyclic compounds in the product stream. However, it has been proved (Rollmann, 1977; Walsh and Rollmann, 1977, 1979) that under certain reacting conditions paraffins and aromatics can contribute, almost equally, to coke formation, and such equal participation depends on the reaction temperature and percentage of A1203.These authors conclude that the initial stage of coke formation is aromatic alkylation and that hydrogen transfer plays a direct role in the evolution of the carbonaceous deposit. The role of hydride transfer in coke formation from olefins on silica-alumina catalysts was earlier postulated by Thomas (1944,1949). Langner and Meyer (1980) developed a mechanism for coke formation from butadiene on Y-zeolites by Diels-Alder additions catalyzed by Lewis sites, while Brcansted sites participate in reactions of hydride transfer. The problem of deactivation by coke deposition is more widespread than the subject of mechanisms of coke formation, and papers dealing with it spread over other fields, such as the analysis of coke and its precursors, the relation between coke and reactor operating conditions, and the 0 1985 American Chemical Society
532
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 4, 1985
Table I. Properties of the Fresh Catalysts acidity, mg of n- butylamine/g msf> /g 257 275
V cm{;g 0.52 0.88
PI?
Pa,
catalyst A B
g/cm3 2.33 1.96
g/cm3 1.05 0.72
AS, pK = 3.3 20.2 14.5
pK = 6.8 64.3 50.0
C
216
1.06
2.56
0.69
10.7
31.4
D
228
0.64
2.60
0.98
17.8
25.9
relation between coke and catalyst properties. Currently, particular and isolated aspects of these subjects are studied, which does not allow us to relate the different aspects of the problem. In this work, we have attempted to carry out the study of the above three fields for the same group of silica-alumina catalysts used in dehydration reactions and in this way obtain a set of conclusions about the general characteristics of coke deposition on acid catalysts.
Catalysts and Reaction Conditions We have used four silica-alumina catalysts, prepared by impregnation. In earlier works we studied their preparation and its effect on physical properties and superficial acidity (Aguayo, 1981; Corella et al., 1981; Romero et al., 1982, 1983). In Table I some properties of the catalysts studied are shown. Physical properties were measured from nitrogen adsorption-desorption isotherms and Hg pycnometry. The superficial acidity was measured by titration with n-butylamine (Benesi, 1956,1957;Matsuzaki et al., 1969;Deeba et al., 1979), employing neutral red (pK = 6.8) and p (dimethy1amino)azobenzene(pK = 3.3) as indicators. The reaction equipment consisted of a continuous feed system of reactant, a preheater-reactor of Pyrex (17-mm i.d.) and an apparatus to condense and analyze products. The following reaction conditions were used: Dimerization of acetaldehyde to crotonaldehyde (Arandes, 1982; Romero et al., 1985). Temperature, 180-200 "C. Feed acetaldehyde %, 100. Space time, 1-10 g of catalyst h/mol. Time on stream, 2.5-300 min. Gas velocity, 3 cm/s. Catalyst particle size, 1.25-2.00 mm. Dehydration of 1-butanol (Aguayo et al., 1984a,b). Temperature, catalyst B 270-310 "C, catalyst C 290 "C. Feed 1-butanol %, 100. Space time, catalyst B 1-6 g of catalyst h/mol, catalyst C 2 g of catalyst h/mol. Time on stream, 0.5-6 h. Gas velocity, 6 cm/s. Catalyst particle size, 0.4-0.5 mm. Dehydration of 2-ethylhexanol (Sancho, 1977; Aguayo et al., 1984a,b; Corella et al., 1982). Temperature, catalyst B 240 "C, catalyst C 240 "C, catalyst D 230-250 "C. Feed 2-ethylhexanol %, 100. Space time, catalyst B 2 g of catalyst h/mol, catalyst C 2 g of catalyst h/mol, catalyst D 0.5-6 g of catalyst h/mol. Time on stream, catalyst B 0.5-6 h, catalyst C 0.5-6 h, catalyst D 2 h. Gas velocity, 6 cm/s. Catalyst particle size, 0.4-0.5 mm. Conversion was measured for the three reactions by gas chromatography (Perkin-Elmer, Sigma 3). The following conditions for the analyses were used: Dimerization of acetaldehyde to crotonaldehyde. Column, stainless steel 2 m X 1/8 in. 0.d. of 15% LB-550-X on Chromosorb W 80-100 mesh. T,,,,, 95 "C (isothermal). Tde@cbr, 150 "C. Flow of Nz(carrier gas), 28 cm3/min. Dehydration of 1-butanol. Column, stainless steel 2 m X 1/8 in. 0.d. of 2.5% SE 30 on Chromosorb GAW-DMCS. Tovent 100 "C (isothermal). Tdetector, 150 "C. Flow of N2, 30 cm3/min.
AT,
reaction dimerization of acetaldehyde dehydration of 1-butanol dehydration of 2-ethylhexanol dehydration of 1-butanol dehydration of 2-ethylhexanol dehydration of 2-ethylhexanol
Table 11. Extraction of Coke with Solventsn solvent catalyst A 10 benzene ether-phenol-n-hexane 17 nitrobenzene 21 crotonaldehyde 23 1,2,3,4-tetrahydronaphthalene 23 pyridine 27
catalyst B 6 7 12 15 14
16
'Percentage of extracted coke.
Dehydration of 2-ethylhexanol. Same column as for 1-butanol. Tcolumn, 130 "C. Tdetectort 250 "c. Flow of N2, 30 cm3/min.
Analysis of Coke Due to the structure of coke not being well-defined and not being constant with time, definite conditions for its analysis cannot be recommended. We have started from the well-known fact that a set of intermediate compounds are involved in the coke formation. These compounds can be stripped with the reaction products, according to its surface adsorption equilibrium, so they will be collected with these products, and in part they will remain on the catalyst bed in the carbonaceous material that is generically termed "coke". The great number of involved compounds, their instability, and their low concentration make quantitative analysis of these intermediate compounds cumbersome. Furimsky (1979), in order to remove soluble fractions of coke, proposes to subject the deactivated catalyst, after reaction, to one or more of the following procedures: heating under vacuum (290 "C and 0.7 mm Hg), soxhlet extraction with benzene, soxhlet extraction with pyridine, heating to 530 "C in a stream of hydrogen at atmospheric pressure. With these treatments the lighter components of coke are removed. Their final analysis allows us to identify the precursors of coke and intermediate compounds that lead to formation of an insoluble coke, which can only be eliminated by combustion in the catalyst regeneration. This nonsoluble coke has a structure of a turbostratic, random-layer lattice formed by fused aromatic rings, as was determined by X-ray diffraction (Haldeman and Botty, 1959; Appleby et al., 1962) and infrared spectroscopy (Eberly et al., 1966). However, the analysis of soluble fractions of coke using very different treatments has been reported in the literature. In this way different results were obtained for different reactants, catalysts, and operating conditions. The coke analysis carried out in this work corresponds to the following reactions on silica-alumina catalysts: (A) dimerization of acetaldehyde to crotonaldehyde and (B) dehydration of 1-butanol. After the run the catalyst was subjected to a stream of Nz during 30 min in the reactor a t reacting temperature. After, soxhlet extractions were done using different solvents. In Table I1 is shown the weight percent of coke extracted with solvents of different
Ind. Eng. Chem. Prod. Res. Dev.. Vol. 24, No. 4, 1985 533 Table 111. Molecular Weights and H/C Ratios of the Extracted Fractions from Coke catalyst A catalyst B solvent H/C MW MW H/C 340 1.26 310 1.31 benzene 850 1.04 pyridine 770 1.16 ~~
I
Benzene extract
1
1
1 1
I
I
I
I
1
I
I
0
10
20
30
LO
50
60
tk"
Figure 2. Chromatograms (GC) of coke extracted from catalyst B (dehydration of n-butyl alcohol).
i
~
J
U L L/,l -
1
1
~,'JL,%ddL-,-L-*-L-LL----10
20
1
30
I
LO
I
I
50
60
_
tb",
Figure 1. Chromatograms (GC)of coke extracted from catalyst A.
polarity and molecular symmetry. Data correspond to independent extractions. It is observed that the asymmetry of solvent is the determinant factor of the extracting power for the coke fractions. The total percentage of coke extracted with the more effective solvent, pyridine, is 27 wt % for catalyst A and 16 wt % for B. The coke fraction extracted from catalyst A is higher than from B. The extracting power of pyridine for the lighter fractions of coke was also found by Furimsky (1979). The molecular weight (osmometric) and H/C ratio of the fractions of coke extracted with benzene and pyridine have been determined. Results are shown in Table 111. The differences among the molecular weights and H/C ratios of the extracted fractions for the two catalysts are due to these catalysts having been used in two different reactions with different reaction conditions (200 "C and 1.5 h of reaction time for the dimerization of acetaldehyde and 290 OC and 2 h for the dehydration of 1-butanol). In an earlier analysis of coke, Venuto and Hamilton (1967), in the alkylation of benzene with ethylene on zeolites, determined a molecular weight of 250 for the complex mixture of liquid products of higher boiling point. These authors found a molecular weight of 485 and a H/C ratio of 1.26 for the coke fraction extracted with a mixture of chloroform and 2 N HCl. Furimsky (1983) found a volatile fraction entrapped in the catalyst pores that had a molecular weight of 400. The H/C ratios in the literature (Appleby et al., 1962; Venuto and Hamilton, 1967; Kubota and Tashima, 1974; Langner, 1981; Wukasch and Rase, 1982; Stiegel et al., 1982; Wolf and Alfani, 1982; Furimsky, 1983) are influenced by the reaction system, temperature, time, and porous structure of the catalyst. Thus as temperature and reaction time increase, condensation of coke is favored (Appleby et al., 1962; Kubota and Tashima, 1974; Langner, 1981). Stiegel et al. (1982) have determined that as the diffusional resistance in pores increases, the H/C ratio decreases. The benzene and pyridine extracts from coke generated from catalysts A (dimerization of acetaldehyde) and B
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