Ind. Eng. Chem. Res. 2001, 40, 1471-1475
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The Role of Acid Site Strength in the Beckmann Rearrangement P. O’Sullivan,† L. Forni,‡ and B. K. Hodnett†,§ Department of Chemical and Environmental Sciences, University of Limerick, Limerick, Ireland, Dipartimento di Chimica Fisica ed Electrochimica, Universita di Milano, Milano, Italy, and Materials and Surface Science Institute, University of Limerick, Limerick, Ireland
A range of catalysts including β-zeolite, mordenite, ZSM-5, Y-zeolite, mesoporous alumino silicate, and amorphous silica-alumina with varying Si:Al ratios have been tested in the range 250350 °C for the Beckmann rearrangement of cyclohexanone oxime into caprolactam. All catalysts studied exhibited some level of deactivation which was characterized by a loss in conversion of cyclohexanone oxime with time on stream. There were considerable differences in resistances to deactivation through coke formation. Thus, mesoporous alumino silicate showed little decay in conversion with up to 16 wt % coke, whereas 6% coke on ZSM-5 caused a loss of 78% of the initial conversion. In general, the best catalysts were those which operated well in the presence of coke. Selectivity to caprolactam was in the range of 30-75% but did not change in a systematic fashion with time on stream. The number and strength of Bro¨nsted and Lewis acid sites on each catalyst were determined from FTIR analysis of pyridine adsorption. These data show that a low ratio of strong/weak acid sites is important in achieving a high yield of caprolactam. A low acid site strength combined with a high resistance to deactivation by coke formation are two necessary parameters for good catalyst performance. Introduction When ketoximes are treated with a strong Bro¨nsted or Lewis acid, they are converted to amides in a reaction known as the Beckmann rearrangement.1 The most commercially important Beckmann rearrangement reaction is that of cyclohexanone oxime to caprolactam. Almost all of the world’s annual production of ca. 3 × 106 tons of caprolactam is consumed as the monomer for nylon-6 fibers and plastics. There are five major commercial routes to caprolactam which are practiced today. All of these processes eventually produce the H2SO4 adduct of caprolactam, which is then treated with NH3 to coproduce ammonium sulfate and caprolactam.2 The vapor-phase Beckmann rearrangement of cyclohexanone oxime to caprolactam over solid acid catalysts has been the subject of intense research for some years. Catalysts studied include silica and borophosphoric acid,3 phosphoric acid supported on Kieselguhr,4 Keargy-type aluminum orthophosphates and γ-alumina mixtures,5 apatites, ortho-, pyro-, meta-, and triphosphates,6 boron oxide supported on alumina7-10 or silica,11 silica-supported tantalum oxide catalysts prepared by a reaction between tantalum alkoxide vapor and the surface hydroxyl groups of silica,12 and silica monolayers on γ-Al2O3, ZrO2, and TiO2.13 More recently, a good deal of attention has been focused on zeolites as solid acids for the Beckmann rearrangement. Deactivation is a commonly reported phenomenon. Landis and Venuto14 found that, over HY, cyclohexanone oxime (30 wt % dissolved in benzene) was converted at 380 °C and WHSV ) 1.2 h-1 to caprolactam with 76% selectivity and 85% conversion during the first 2 h. The principal byproduct was 5-cyanopent-1-ene † Department of Chemical and Environmental Sciences, University of Limerick. ‡ Universita di Milano. § Materials and Surface Science Institute, University of Limerick.
with traces of cyclohexanone and cyclohexanol. As the reaction continued, the overall conversion decreased to 30% after 20 h with 50% selectivity for caprolactam. Aucejo et al.15 studied a series of HNaY zeolites for this reaction and found that the formation of caprolactam was catalyzed by Bro¨nsted sites of pKa strength e 1.5 and that byproduct 5-cyanopent-1-ene was formed mainly on the Na+ ions of the zeolite. Sato et al.16 examined the same reaction over a series of HY zeolites calcined at high temperatures with a view to reducing its acidic strength. They observed that caprolactam was tightly adsorbed on strong acid sites, filling the micropores of the zeolite during the initial stages of the reaction, and that a large portion of the catalytic reaction proceeded mainly on weak acid sites located in a region near the external surface of the zeolite. Dai et al.17 reported that H-β-zeolites exhibited excellent activity and selectivity for the vapor-phase Beckmann rearrangement of cyclohexanone oxime in contrast with other 12-membered ring zeolites such as Y and mordenite, which gave rise to low selectivity for caprolactam accompanied by rapid decay in activity. Heitmann et al.18,19 reported that [Al]-β- and [B]-βzeolites showed significant loss in activity with time on stream, with the [B]-β catalyst deactivating 15 times faster than the corresponding [B]-ZSM-5 catalyst. Sato et al.20 found that ZSM-5 was highly active for the Beckmann rearrangement when it had a Si/Al ratio of 500 or more. However, percent conversion of the oxime and percent selectivity to the lactam still decreased with time in all cases. Takashi et al.21 examined the vapor-phase Beckmann rearrangement over ZSM-5 and boria-modified ZSM-5 zeolites and concluded that the production of a polymer over strong acid sites caused a decrease in caprolactam selectivity during the reaction. Thangaraj et al.22 examined ZSM-5, silicalite, and titanium silicalite (TS-1) and found that incorporation
10.1021/ie000673q CCC: $20.00 © 2001 American Chemical Society Published on Web 02/16/2001
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Table 1. Physicochemical Characterization and Performance Characteristics of Catalysts Used for Cyclohexanone Oxime Conversion to Caprolactam at 300 °C Bro¨nsted acidity, µmol g-1 sample code ASA Y-zeolite Y(2.6) Y(20) Y(40) β-zeolite B(10) B(20) B(65) mordenite MOR(20) MOR(60) MOR(80) MCM MCM-41 ZSM-5 ZSM-5(15) ZSM-5(40) ZSM-5(124)
Si/Al
SA, m2 g-1
strong (>250 °C)
Lewis acidity, µmol g-1
weak (100-250 °C)
strong (>250 °C)
weak (100-250 °C)
pore dimensions, nm
2.5
340
4
36
26
136
2.6 20 40
750 780 780
312 138 15
155 26 5
82 48 8
196 36 8
13 24 67
180 158 28
37 16 4
128 66 19
68 27 5
0.76 × 0.64 and 0.55 × 0.55
20 60 80
35 59 20
4 20 9
9 6 10
5 7 16
0.65 × 0.70 and 0.26 × 0.57
0.74
19
900
12
35
77
95
3.5
15 40 124
420 430 450
165 137 24
42 34 10
17 15 1
36 12 2
0.53 × 0.56 and 0.51 × 0.55
of Ti silicalite increased the percent selectivity and lowered the deactivation rate. This paper looks at the relationship between the acidity of a range of zeolites as measured quantitatively by pyridine adsorption, determined by FTIR analysis at a range of temperatures, the deposition of coke during the Beckmann rearrangement, and its subsequent effect on catalyst performance. Experimental Section Four zeolites, namely, β-zeolite, mordenite, ZSM-5, and zeolite Y, were used in this study. Abbreviations used, the Si/Al ratios, pore dimensions, acidity data, and surface areas are presented in Table 1. Three samples of β-zeolite with Si/Al ratios in the range of 13-67 were supplied by the Instituto Superior Techico de Lisbon. The parent zeolite was supplied in the sodium form from PQ zeolites, and its original code was CP806. The ammonium form of this zeolite was obtained by exchanging three times in 10 M ammonium nitrate at 100 °C. For this ion exchange, there was a minimum of 5 cm3 of ammonium nitrate/g of catalyst. The ammonium-exchanged zeolite was then calcined at 550 °C in air for 12 h to make the proton form of the zeolite. This zeolite was labeled as B(10) and was confirmed to have a Si/Al ratio of 13 from FTIR measurements. The other two β samples labeled as B(20) and B(65) were synthesized from B(10) by dealumination by HCl acid leaching. In the dealumination procedure, 0.2 M HCl was used to obtain B(20) and 1 M HCl for B(65). In each case there was 10 cm3 of acid/g of catalyst. The temperature of dealumination was 100 °C with a reaction time of 4 h in each case. Each dealuminated zeolite was washed with 500 cm3 of hot water/g of catalyst and dried at 110 °C for 2 h. Three samples of mordenite were supplied by the Universite de Poitiers. Two of these were dealuminated according to the same procedure as that used for the β-zeolites. Three samples of ZSM-5 zeolites in the proton form were supplied by PQ zeolites. Si/Al ratios were in the range of 15-124. Samples of Y-zeolite coded with Si/Al ratios in the range of 2.6-40 were supplied by PQ zeolites. The original sample was converted to the proton-exchanged
form by heating at 10 °C min-1 to 500 °C followed by a 6 h calcination at 500 °C in flowing dry air (20 cm3 min-1). The rate of temperature increase from room temperature to 500 °C was 10 °C min-1. Y(20) and Y(40) were dealuminated from their parent form, Y(2.6), using HCl at temperatures between 60 and 80 °C for 30 min and 2 h, respectively. A sample of MCM-41 was supplied by the Instituto de Technologia Quimica, Universidad Polytecnica de Valencia (pore volume 0.5 cm3g-1 and pore diameter 35 Å). It was activated by heating to 540 °C in N2 for 1 h followed by heating in air for 12 h. Catalyst characterization was by X-ray diffraction, nitrogen adsorption, thermogravimetric analysis, and FTIR analysis of adsorbed pyridine. The thermogravimetric analysis was performed on a Stanton Redcroft TG770 thermobalance. Appoximately, 7 mg of catalyst was heated at 10 °C min-1 in flowing air (25 mL min-1) to 900 °C. Weight loss between 300 and 500 °C was associated with coke removal.9 A sample of amorphous silica-alumina (ASA) was supplied by AKZO (code HA 500 SP). FTIR spectroscopy of adsorbed pyridine is routinely used to determine Bro¨nsted and Lewis acidity on acidic solids.23 Pyridine was the probe molecule of choice for the study because it is similar in size to the reactant, cyclohexanone oxime. The quantitative determination of Bro¨nsted and Lewis acidity by FTIR analysis was first proposed by Hughes and White24 and is based on the integrated Lambert-Beer law
A1 ) CSUP1
(1)
where A1 (cm-1) ) integrated absorbance of the band for the adsorbed probe molecule, CSUP (µequiv‚cm-2) ) molar concentration of the adsorbed probe molecule on the catalyst surface, and 1 (µequiv-1) ) integrated molar adsorption coefficient relative to Lewis or Bro¨nsted sites bands. FTIR spectra were collected using a Nicolet Magna550 FTIR instrument, controlled via an Olidata M/B 486-33 PC, using Nicolet OMNIC software. The catalyst samples were finely ground in an agate mortar and equilibrated overnight with water vapor at room temperature. The catalyst was then pressed in a 15-20 mg cm-2 self-supporting wafer in the absence of
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binder. Before starting adsorption and FTIR analysis, all samples were heat treated at 350 °C in air (300400 Torr) for 1 h and at 500 °C overnight, followed by evacuation of ∼10-5 Torr for 2 h at the same temperature. Following this pretreatment, the wafer was cooled to room temperature and a “clean sample” reference spectrum was recorded. Spectra were always collected as an average of 60 runs with a 2 cm-1 definition. The background spectrum, recorded without the sample under identical operating conditions, was always automatically subtracted. Adsorption of pyridine was carried out by equilibrating the catalyst wafer for 30 min with a 5 Torr pressure of the probe at 100 °C. The sample was then evacuated for 10 min at the same temperature and cooled to room temperature before recording the spectrum. The desorption of the probe molecule was successively monitored stepwise, by evacuating the sample for 30 min at 100, 150, 200, 250, and 300 °C and cooling it to room temperature between each step, to record the spectrum. The concentration of Bro¨nsted and Lewis acid sites referenced to a unit weight of dry sample {qH(µequiv‚g-1)} was obtained according to
qH ) A1πR2/w1
(2)
where R (cm) ) radius of the catalyst wafer and w (g) ) weight of the dry sample. A1 values were evaluated from the different spectra relative to all samples after baseline optimization. qH values were then determined via eq 2 from A1 data, obtained at 100, 150, 200, 250, and 300 °C, and 1 values relative to Bro¨nsted and Lewis sites calculated from Emeis.25 Further experimental details have been presented previously.26-28 Catalysts were tested between 250 and 350 °C in a quartz microreactor. Cyclohexanone oxime was entrained into the gas phase by flowing helium through a stainless steel tube which was filled with dry 3A molecular sieve. This tube was held at 67 °C during normal operation and generated an oxime partial pressure of 0.22 kPa. The reactor effluent was chilled in a glass U-tube, and the accumulated products were analyzed on a HP 5500 series II gas chromatograph fitted with a HP-1 capillary column and a FID detector. Results Figure 1 presents conversion levels and selectivities to caprolactam at 300 °C over β-zeolites with different Si/Al ratios. The corresponding data for all of the catalysts tested here are presented in Table 2. Selection of suitable performance characteristics for these catalysts is always somewhat arbitrary. In this study, percent conversion, selectivity, and yield after 6 h on stream were selected, and these are indicated for each catalyst in Table 2. In addition, the decline in percent conversion between 0 and 6 h on stream (percent decay) is indicated in this table. In general, initial conversions were close to 100% for all catalysts studied here, but a consistent feature was the decline in conversion as time on stream increased. This feature was not observable over MCM-41 above 250 °C and over ASA because conversion was maintained at 100%. The decline in conversion was most pronounced over Mordenite and ZSM-5 (Table 2). In general, selectivities were in the range of 15-75% depending on the catalyst, sometimes declining with time on stream and sometimes increas-
Figure 1. (A) % conversion of cyclohexanone oxime and (B) % selectivity to caprolactam over β-zeolites with Si/Al ratios of (O) 10, (0) 20, and (2) 65 (temperature ) 300 °C; W/F ) 0.06 g s mL-1; φoxime ) 0.22 kPa). Table 2. Performance Characteristics of Catalysts Used for Cyclohexanone Oxime Conversion to Caprolactam at 300 °C sample code ASA Y-zeolite Y(2.6) Y(20) Y(40) β-zeolite B(10) B(20) B(65) mordenite MOR(20) MOR(60) MOR(80) MCM MCM-41a MCM-41 MCM-41b ZSM-5 ZSM-5(15) ZSM-5(40) ZSM-5(124) a
% convn % yield % selectivity wt % coke % decay at 6 h at 6 h at 6 h at 6 h at 6 h 98
52
52
8.3
N/A
50 80 66
20 28 21
40 35 32
7.4 4.3 5.5
50 20 33
94 86 90
40 28 39
43 32 43
13.7 12.4 13.4
6 14 10
12 12 18
4 3 3
32 23 14
12.9 9.2 8.6
88 88 82
96 100 100
58 75 60
60 75 60
16.4 7.5 2.5
4 N/A N/A
33 61 22
11 21 8
34 36 34
4.3 4.2 6
66 39 78
Tested at 250 °C. b Tested at 350 °C.
ing. X-ray diffraction analyses of the zeolites used in this study before and after dealumination were identical, and changes in Brunauer-Emmett-Teller surface areas were also minimal. In general, yields were lower as time on stream increased. Increased selectivity with time on stream is associated with the preferential deposition of coke onto nonselective sites which favor coke formation over caprolactam formation. Elimination of these sites leads to increased selectivity. Coke formation was a feature of all catalysts studied. The weight percent of coke accumulated over each catalyst is presented in Table 2. There was less coke over MCM-41 at 350 °C (2.5%) compared to 250 °C
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Figure 2. Relationship between % decay and % coke formation.
(16.4%), and generally the various zeolites showed differing resistances to deactivation by coke deposition. Hence, conversion was strongly reduced over Y-zeolites and ZSM-5 with the deposition of 4.2-7.4 wt % coke (see Table 2), whereas conversion over β-zeolite was much less affected by the presence of 12.4-13.7 wt % coke. The largest percent decay was observed with mordenite, but larger coke deposits were observed for β-zeolite and MCM-41 tested at 250 °C. In general, there was no observed correlation between the percent decay and the weight percent of coke. This is illustrated in Figure 2 which plots the percent decay against the weight percent of coke. However, a useful region of this plot can be identified within which are located the better catalysts such as the β-zeolites, which show high resistance to deactivation through coke formation. MCM-41 and ASA are excluded from this figure because conversions at 300 °C were very close to 100% and may have been hypothetically above this value. However, MCM-41 tested at 250 °C did exhibit measurable but small decay (see Table 2) and might justifiably be considered also in the high coke-low decay region of Figure 2. Measurements of the amounts of Bro¨nsted and Lewis acidity classified as weak (corresponding to pyridine desorption below 250 °C) and strong (desorption above 250 °C) are presented in Table 1. An important parameter to have emerged from this work is the ratio of strong/weak Bro¨nsted and Lewis acid sites present in these catalysts. When the percent yield of caprolactam was plotted against the ratio for Bro¨nsted or Lewis acidity (see Figure 3A,B), an upper limit can be defined which clearly shows that the highest yields are associated with catalysts having the lowest ratios of strong/ weak acid sites. Discussion Coke formation was observed over all of the catalysts studied here, and the ultimate performance was determined by the ability of catalysts to perform in the presence of coke. This work identifies two factors that are important in achieving a reasonable catalyst, namely, the ability to operate in the presence of significant amounts of deposited coke and the elimination of strong acid sites (those from which pyridine does not desorb below 250 °C) from the catalyst. It is self-evident that narrow pores in zeolites will be strongly affected by coke formation, but even within the zeolite range, there does not appear to be a strong sensitivity to pore size. Mordenite with one very low pore dimension exhibited the greatest decay in conversion (>80%) over the time scale of the test, and selectiv-
Figure 3. Relationship between % yield and the ratio of strong/ weak (A) Bro¨nsted and (B) Lewis acid sites.
ity to caprolactam was low throughout. However, an argument based on the pore size alone would suggest that Y- and β-zeolite should have similar performance characteristics. However, β-zeolite exhibited less decay and a greater yield of caprolactam despite having a much higher accumulated coke content. However, a clear finding of this work is that the larger pore materials such as ASA and mesoporous alumino silicate are not as susceptible to deactivation by coke formation. Although not addressed specifically in this study, this finding is consistent with earlier studies which emphasize, for example, the role of the external surface for caprolactam synthesis over mordenite,29 Y-zeolite,16 silica monolayers on Al2O3, ZrO2, and TiO2,13 and ZSM-5.18 Other studies emphasize the low temperature required for caprolactam formation (
