Article pubs.acs.org/IECR
1,3-Butadiene Adsorption over Transition Metal Polycation Exchanged Faujasites Albert M. Tsybulevski,*,†,§ Leonid M. Kustov,‡ Kerry C. Weston,† Alexander A. Greish,‡ Olga P. Tkachenko,‡ and Alexey V. Kucherov‡ †
Zeochem LLC, 1600 West Hill Street, Louisville, Kentucky 40210, United States N.D. Zelinsky Institute of Organic Chemistry RAS, Leninsky prospect, 47, Moscow 119991, Russia
‡
S Supporting Information *
ABSTRACT: New adsorbents for butadiene removal from liquid and gas streams, which are transition metal (Cu, Zn, Mn) polycation exchanged zeolites, were prepared on the basis of low-silica and dealuminated faujasites by ion exchange with the corresponding salt solutions at the conditions of their partial hydrolysis. Obtained samples were characterized by XANES, ESR, and DRIFT spectroscopy along with equilibrium and dynamic adsorption studies. It is shown that transition metal polycation containing faujasites significantly surpass monocation analogues with respect to butadiene adsorption capacity. It is also found that an enhanced ability to adsorb butadiene is peculiar to faujasites with strong acidity that defines the activity of the adsorbent in the oligomerization of adsorbed butadiene. The suggested adsorbents are fully regenerable at gradual elevation of regeneration temperature up to 250−350 °C.
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exchanged low-silica faujasite (LSF),15 while the authors16 relate the high catalytic activity of Ni and Co polycation exchanged ultrastable faujasite Y to π-complexation of olefin double bonds by TrM polycations. Moreover, new diene adsorbents on the basis of varied TrM polycation exchanged faujasites have recently been suggested.17 However, the nature of butadiene interaction with polycations and zeolite crystalline structure remains unanswered. What is the role of TrM polycations, their composition, zeolite acidity, π-complexation, and butadiene chemisorption? These are crucially important questions for proper butadiene adsorbent development. Therefore, the present study is the first approach to the clarification of the mechanism of butadiene adsorption over TrM polycation exchanged faujasites.
INTRODUCTION Dienes, in particular 1,3-butadiene, are common and, at the same time, highly undesirable impurities of many petrochemical streams. Accordingly, diene removal is a critical step in a variety of chemical and petrochemical technologies. As this takes place, adsorption separations with the use of synthetic zeolites, alongside diene selective hydrogenation, present the most useful commercial approach to solving this problem.1,2 The first and most popular zeolite adsorbent for butadiene recovery was the standard zeolite 13X.3 It is an inexpensive, easily accessible product; however, its insufficiently high adsorption capacity at low diene partial pressures cannot satisfy the contemporary requirements for petrochemical stock purity. Such an essential demerit of the commercial product forced immediate research efforts to find more efficient zeolite adsorbents.4,5 Meanwhile, Yang and associates6−8 achieved significant improvements of butadiene zeolite adsorbent efficacy by applying a concept of π-complexation of double bonds with Ag and Cu cations incorporated into the zeolite structure. What is more, the π-complexation concept was successfully propagated over various unsaturated compounds and for other transition and noble metals.9,10 In this connection, zeolites, especially faujasites in transition metal (TrM) cation exchanged forms, arouse interest due to their ability to form polycations of diversified compositions. Numerous confirmations of TrM polycation stabilization in faujasite intracrystalline cages have been obtained over the past 10−15 years. 11−14 Such cluster metal or metal oxide polycations have the general formula [TrMαOβ]n+, where α varies from 2 to 8, β varies from 0 to 4, and n varies from 2 to 6. Accordingly, cluster polycations have sizes in the nano- or subnanometer range of 4−16 Å or 0.4−1.6 nm that should define an enhanced chemisorption and catalytic activity of the materials. Indeed, an unusual adsorption capacity for organic sulfur compounds has been demonstrated for Zn polycation © 2012 American Chemical Society
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EXPERIMENTAL SECTION With the aim of revealing connections between the structure and physical properties of the suggested adsorbents, on one hand, and their affinity for butadiene adsorption, on the other hand, several samples of TrM polycation exchanged faujasites were prepared and analyzed by means of X-ray absorption, electron paramagnetic resonance (ESR), and diffuse reflectance Fourier transform infrared (DRIFT) spectroscopy along with the equilibrium and dynamics of butadiene adsorption per se. Adsorbent Preparation. The samples for this research were prepared on the basis of the faujasites LSF and ultrastable Y (USY) having Si/Al ratios equal to 1.01 and 3.0, respectively. The beaded 1.6 mm size Na, K form of LSF was manufactured by Zeochem, while the powder of USY was obtained from W. Received: Revised: Accepted: Published: 7073
October 31, 2011 April 14, 2012 April 15, 2012 April 15, 2012 dx.doi.org/10.1021/ie202478c | Ind. Eng. Chem. Res. 2012, 51, 7073−7080
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Table 1. Adsorbent Cation Composition ion exchange degree, % equiv. Zn
a
Mn
Cu
samples
mono
poly
mono
poly
mono
poly
Na
K
Ca
(Mn)pLSF (Cu)pCaLSF (Cu)p(Mn)pLSF (Zn)pDAY Cu(97)LSF
− − − 65.6 −
− − − 12.1 −
71 − 33 − −
5.5 − 5.0 − −
− 74 62 − 97.4
− 12 9.4 − −
33 9.5 8 35 2.2
6 9 0.8 a 0.4
1 24 25 a −
H+ = 20.3.
R. Grace. The USY sample was partially dealuminated17 to reach a Si/Al ratio of 3.8. The critical step of the sorbent preparation is the introduction of TrM polycations into the faujasite structure. The generally accepted methodologies for the synthesis of TrM polycations employ a chemical vapor deposition (CVD) technique11 or alkaline treatment of conventional exchanged zeolites.12 At the same time, it was demonstrated that Zn cluster polycations can be synthesized in the presence of partially hydrolyzed or reduced cations.15 Indeed, the hydrolytic methodology is well-founded18,19 and can be successfully expanded for other TrMs, including Cu and Mn.17 In such a manner, conditions for polycation formation are provided by maintaining the pH of exchanged solutions of TrM salts in a narrow range of partial hydrolysis of TrM cations. A comprehensive preparation procedure for Zn and Mn polycation exchanged faujasites was provided in ref 17. In the case of CuCl2 exchange, aiming at maintenance of the proper pH range and promotion of Cu salt hydrolysis, NaKLSF was first converted into calcium form. In so doing, the starting material was treated with a 1 N solution of CaCl2 at ambient temperature and continuous agitation over 3 h. The resulting CaLSF was then treated with a 1 N CuCl2 solution at room temperature over 4 h, keeping the pH of the exchanged solution in the range 5.0−5.4 by 0.05 M NaH2PO4 buffer. In addition, the obtained product was used for double TrM (Cu, Mn) polycation exchanged faujasite preparation. For this purpose, it was treated with a 1.8 N solution of MnCl2 as described above. As a rule, the cation equivalent ratio TrM/Na, K = ∼1, was held in all ion exchange operations. For comparison purposes, a typical π-complexation butadiene adsorbent, which is disclosed in ref 6, was also studied. Preparation of Cu(97)LSF sample having a high Cu2+ ion exchange degree of >97% is described in ref 17. All the finally exchanged products were properly washed, dried at 110 °C for 3−4 h, and calcined at 250 °C for 2 h. The chemical analysis of the prepared samples was carried out by inductively coupled plasma atomic emission spectroscopy (ICP). The sample cation composition is presented in Table 1. (Herein and after, polycations are marked with the subscript “p” following the corresponding cation symbol.) As follows from Table 1 data, due to TrM polycation formation, the sum of all cation equivalents incorporated into faujasites significantly exceeds 100% at the rate of monocations. Actually, it is a characteristic feature for polycation exchanged zeolites,11,14 and the excess of the total ion exchange degree is related to polycations. In the case of double Cu, Mn containing sample, TrM polycation contents are given in Table 1 in proportion to the corresponding monocation exchange degrees.
X-ray Absorption Near-Edge Spectroscopy (XANES) and ESR Study of the Samples. The pre-edge region of Xray absorption spectra may provide direct information with respect to the oxidation state of the atom of interest. XANES Cu K edge (8979 eV) and Mn K edge (6539 eV) X-ray absorption spectra were taken at the Hasylab X1 station (Hamburg, Germany) in transmission mode with the use of a Si(111) double crystal monochromator. The measurements were carried out in a vacuum of 10−5 Torr at 80 K. For energy calibration, alongside the zeolite samples, Cu and Mn metal foil spectra were simultaneously registered between the second and third ionization chambers. In the XANES region, the spectra had a constant E spacing with ΔE = 0.2 eV. In order to achieve reproducibility, all measurements were performed several times. The VIPER20 software was used in the data analysis. The ESR spectra were taken on a reflecting spectrometer equipped with a coaxial quartz Dewar. The measurements were conducted in the X-band (λ ≅ 3.2 cm) at 20 and −196 °C at the conditions of lack of saturation in the field range of 0−4500 G. DPPH was used as a reference. The original adsorbent samples were placed in identical glass ampules (2.0 mm diameter, 20 mm height), so that spectra were registered at room temperature in the presence of air. Then samples were evacuated to ∼0.1 Torr at 90 °C for ∼5 min, sealed off, and spectrum registration at 20 °C was repeated. Following those runs, the ampules were opened, the samples were loaded by distilled water, and ESR measurements were repeated once again at −196 °C. In addition, the spectra of the calcined samples at 400 °C and evacuated samples were registered at 20 °C. Characterization of the Samples by DRIFT Spectroscopy. This part of the research effort was undertaken primarily for the analysis of the acidity of the TrM polycation exchanged faujasites. The measurements were performed at room temperature employing a Nicolet 460 Protégé spectrometer equipped with a diffuse reflectance attachment.21 Preliminary training of the samples in a vacuum at 350 °C for 2 h was applied in order to remove physical adsorbed water. The spectra were measured in the range 400−6000 cm−1 with a resolution of 4 cm−1 and converted to Kubelka−Munk units. Deuterated acetonitrile had been chosen as the most appropriate probe molecule for testing Brønsted and Lewis acid centers of the exchanged faujasites. The samples were loaded by CD3CN at ambient temperature and an equilibrium pressure of 96 Torr. Butadiene Adsorption. Equilibrium, Dynamics, and DRIFT Study. Liquid phase isotherms and dynamics of 1,3butadiene removal from its solution in hexene-1 were measured at ambient temperature. Gas phase dynamics of diene adsorption were studied at propene and simulated flue gas 7074
dx.doi.org/10.1021/ie202478c | Ind. Eng. Chem. Res. 2012, 51, 7073−7080
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flow purification. The detailed experimental procedure for adsorption equilibrium and dynamics tests is presented in ref 17. The methodology for measuring DRIFT spectra of adsorbed 1,3-butadiene is very much similar to the one that is applied for the sample acidity analysis by adsorption of CD3CN, and that is described above. The samples were loaded by 1,3-butadiene at ambient temperature and an equilibrium pressure of 20 Torr. The IR analysis (transmission mode) of the probe molecule in gas phase was found in adequate compliance with the spectra that are given in the NIST Chemistry WebBook for 1,3butadiene. The DRIFT spectra of adsorbed 1,3-butadiene were taken after short-term exposure (10 min) and long-term exposure (36 h) at ambient temperature.
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RESULTS AND DISCUSSION Adsorbent Characterization. Cu K edge XANES spectra of (Cu)pCaLSF and (Cu)p(Mn)pLSF samples along with reference compounds containing copper in different oxidation states (Cu foil (0), CuO (2+)) are presented in Figure 1. The
Figure 2. ESR signals for (Mn)pLSF (a) as received; (b) after evacuation at 90 °C; (c) after impregnation by water. At (a, b) 20 and (c) −196 °C.
90 °C, and after impregnation by water. The original (Mn)pLSF sample is characterized by the presence of an intense broad ESR line (g ≅ 1.96; ΔH ≅ 570 G). Evacuation of the sample at 90 °C providing removal of physically sorbed water from zeolite cages causes a slight broadening of the line without considerable loss of signal intensity (Figure 2b). Filling the faujasite sample with liquid water also causes a minor change of the ESR signal shape (Figure 2c). In this case, an increase of the signal intensity is determined by low temperature of the signal registration (−196 °C). Summarizing the obtained results, it is reasonable to conclude that (Mn)pLSF adsorbent contains an appreciable amount of paramagnetic Mn2+ or Mn4+ ions being in mutual weak magnetic interaction and forming irregular clusters.22 These paramagnetic Mn cationic clusters exhibit a low reactivity to water molecules filling zeolite channels and cages. Likewise for Mn polycations, ESR study, as applied for Cu-containing faujasites, confirms formation of Cu aggregates (being characterized by the intense asymmetric ESR line, with g ≅ 2.15 and ΔH ≅ 210 G). The latter comprise paramagnetic Cu2+ ions that occur in mutual weak magnetic interaction.23,24 As this takes place, the data of Figure 3 show that the distribution and properties of paramagnetic Cu2+ in double polycation exchanged faujasite resemble the ones that are characteristic for single polycation adsorbent (Cu)pCaLSF. On the contrary, distribution and properties of paramagnetic Mn2+ in (Mn)pLSF and (Cu)p(Mn)pLSF differ drastically. An explanation of such significant differences in behavior of Mn and Cu cations might be related to the features of stabilization of Cu and Mn cations by the faujasite framework. Actually, paramagnetic Cu2+ is much stronger bonded in cationic positions, in particular in the case of replacement by Mn2+ and the formation of bicationic clusters. The results of XANES and ESR analysis cause a supposition that the incorporation of TrM polycations into the faujasite structures appreciably affects their acidity. The strength of acid centers can be assessed with the help of IR spectroscopy of adsorbed CD3CN. In this case, the shift of the frequency of the CN stretching vibrations in the IR spectrum of adsorbed
Figure 1. Cu K edge XANES spectra for Cu-containing faujasites and reference compounds: 1, Cu foil; 2, CuO; 3, (Cu)pCaLSF; 4, (Cu)p(Mn)pLSF.
results in Figure 1 indicate that the energetic position of Cu K edge X-ray absorption in the spectra of polycation exchanged faujasites is higher than the one that is observed for Cu foil and coincides with the energetic position of the Cu K edge which is peculiar to CuO. Besides, the shape of the absorption spectra of the faujasite samples is similar to the shape of the CuO spectrum. At the same time, the intensity of the white line in the spectra of Cu-containing faujasites is lower than the intensity of the same peak in the CuO spectrum and higher than the one in the Cu foil spectrum. Thus, it is shown that both studied faujasites contain copper species in the 2+ oxidation state. Similarly, the juxtaposition of Mn K edge XANES spectra for (Mn)pLSF and (Mn)p(Cu)pLSF samples with spectra for the reference compounds, which contain manganese atoms in various oxidation states, such as 0 for Mn foil, 2+ for Mn(NO3)2, 4+ for MnO2, and 7+ for KMnO4, points to the fact that manganese species in both (Mn) p LSF and (Mn)p(Cu)pLSF adsorbents are found in the 2+ oxidation state. In such a manner, the obtained results confirm that the “excessive” Cu and Mn atoms in the studied samples are found exclusively in the form of polycations and do not in the least yield separated hydroxides. Figure 2 demonstrates ESR signals for (Mn)pLSF adsorbent in a water vapor saturated state, after the sample evacuation at 7075
dx.doi.org/10.1021/ie202478c | Ind. Eng. Chem. Res. 2012, 51, 7073−7080
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Figure 5. DRIFT spectra of CD3CN adsorbed on (Cu)p(Mn)pLSF sample at 20 °C and saturated vapor pressure of 96 Torr.
The band at 2304 cm−1 is characteristic of CD3CN adsorption on strong Lewis acid sites. The blue shift of the CN stretching vibration is the result of CD3CN adsorption on these sites, and it comprises 51 cm−1 versus the frequency of the adsorbate in its gas phase spectrum (2253 cm−1). The band of CN stretching vibration at 2271 cm−1 belongs to CD3CN adsorption on weak Brønsted acid sites and/or to the probe molecules that are physically adsorbed by zeolite intracrystalline structure. Finally, the lower frequency bands at 2107 cm−1 are assigned to the C−D deformation vibrations in CD3 groups. The study of CD3CN adsorption over single TrM polycation exchanged faujasites (Cu)pCaLSF and (Mn)pLSF lead to the results which to a great extent resemble the previous ones for the (Cu)p(Mn)pLSF sample. It is interesting that the value of the blue shift of the CN stretching vibration in the spectra of CD3CN adsorbed by (Cu)pCaLSF is practically the same as the one that is observed for (Cu)p(Mn)pLSF, i.e., on the level of ∼51 cm−1. The similar characteristics for (Mn)pLSF are essentially lower and make up 40 cm−1. It may be reasonable to expect that the substantial difference in acidic properties of the studied faujasites affects their affinity for butadiene adsorption. Butadiene Adsorption. Isotherms of 1,3-butadiene adsorption from its solution in hexene-1 at low adsorbate concentration range (0−50 ppm) are presented in Figure 6. The data allow comparing butadiene adsorption capacities for four polycation exchanged faujasites with the capacity for monocation Cu exchanged faujasite. The latter was suggested as a highly selective adsorbent for unsaturated hydrocarbons.6
Figure 3. ESR signals for (a) (Mn)pLSF and (b) (Cu)p(Mn)pLSF at 20 °C.
CD3CN versus gas phase CD3CN (2253 cm−1) is a proper indicator of the acidity of the solid surface.25−28 The sharper the shift of the CN band toward higher frequencies (blue shift), the higher the strength of the acid centers. The DRIFT spectrum of (Cu)p(Mn)pLSF is shown in Figure 4. It is known that the frequency range lower than 2000 cm−1 is
Figure 4. OH region of DRIFT spectra for (Cu)p(Mn)pLSF sample evacuated at 350 °C.
characteristic of the intrinsic support lattice vibration, whereas the range that is higher than 3500 cm−1 belongs to OHstretching vibrations.29 It is seen that the designated vibration range for (Cu)p(Mn)pLSF sample includes the peaks at 3740, 3656, and 3614 cm−1. The band at 3740 cm−1 belongs to stretching vibrations of the O−H bond in isolated silanol ≡Si− O−H groups (weak Brønsted acid sites). The band at 3659 cm−1 relates to stretching modes of OH groups attached to extraframework aluminum =Al−O−H (relatively strong acid sites). The band at 3614 cm−1 belongs to bridged hydroxyls in Si(OH)Al (strong acid sites). Figure 5 displays the region of the DRIFT spectrum of CD3CN adsorbed on the same (Cu)p(Mn)pLSF adsorbent in the frequency range 2000−2400 cm−1 which is peculiar to the adsorbate molecules. Considering the obtained spectrum, it is obvious that CD3CN adsorption on the studied faujasites causes the appearance of four bands at 2304, 2271, 2253, and 2107 cm−1.
Figure 6. Equilibrium isotherms for butadiene adsorption from hexene-1 solution: 1, (Cu)p(Mn)pLSF; 2, (Zn)pDAY; 3, (Cu)pCaLSF; 4, (Mn)pLSF; 5, Cu(97)LSF. 7076
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for purification of hexene-1 liquid flow over four adsorbents, (Mn)pLSF, (Cu)pCaLSF, (Cu)p(Mn)pLSF, and Cu(97)LSF, are shown in Figure 7.
As can be seen, all TrM polycation containing faujasites demonstrate a superior ability to adsorb butadiene and significantly outperform the copper monocation Cu(97)LSF π-complexation adsorbent. Further, the preeminence of the suggested adsorbent over the standard 13X zeolite3 is even more impressive: an equilibrium adsorption capacity of the latter at a butadiene concentration of 20 ppm was found on the level of 0.24 wt %. Also noteworthy is the superiority of the double polycation exchanged faujasite (Cu)p(Mn)pLSF that is characterized by remarkable acidity at CD3CN adsorption. Interestingly, the Zn polycation exchanged dealuminated faujasite (Zn)pDAY also shows an enhanced efficacy in butadiene adsorption when its strong acidic properties are beyond doubt by definition.30,31 On account of the observed significant adsorption of CD3CN over TrM exchanged faujasites, it was interesting to reveal the effect of nitrile coadsorption in regard to butadiene adsorption. For this purpose, equilibrium adsorption capacities were measured at an even ratio of C4H6 and CH3CN concentrations employing the above-mentioned experimental methodology.17 The obtained values of butadiene adsorption at its initial concentration in hexene-1 solution of 25 ppm are presented in Table 2.
Figure 7. Butadiene breakthrough curves at liquid phase hexene-1 purification: 1, Cu(97)LSF; 2, (Mn)pLSF; 3, (Cu)pCaLSF; 4, (Cu)p(Mn)pLSF.
The data of Figure 7 allow evaluation of the ultimate achievable hexene-1 purity and calculate dynamic capacity of the tested adsorbents. The obtained characteristics for the studied adsorbents are presented in Table 3.
Table 2. Effect of Acetonitrile Presence on Butadiene Adsorption from Hexene-1
Table 3. Dynamic Capacity of the Adsorbents
butadiene adsorption capacity, wt % adsorbent (Mn)pLSF (Cu)pCaLSF (Cu)p(Mn)pLSF (Zn)pDAY Cu(97)LSF
plain hexene-1 3.4 3.7 4.7 4.0 2.1
with CH3CN
adsorbent
breakthrough concn, ppb
dynamic capacity, wt %
1.5 2.0 4.1 3.6 0.65
(Mn)pLSF (Cu)pCaLSF (Cu)p(Mn)pLSF Cu(97)LSF
350 470 240 4800
2.1 2.4 3.0 0.56
First and foremost, the obtained results not only corroborate the advantage of the suggested butadiene adsorbents but also persuasively establish that the TrM polycation exchanged faujasites provide a profound purity of the product that reaches an entirely distinct technical grade. Indeed, the residual butadiene content in purified streams on the ppb level provides additional avenues for significant savings and environmental benefits in subsequent steps of petroleum refining and petrochemical processes. By themselves, equilibrium butadiene adsorption values on the level of 2−6 wt %, which can be reached with the use of polycation containing faujasites at the extremely low adsorbate concentration range of 0−50 ppm, are absolutely incomparable. A further highly appreciable feature of the adsorbents lies in the fact that those outstanding adsorption capabilities are actual for dynamic conditions. This indirectly shows that incorporation of TrM polycation clusters into zeolite crystalline structure does not degrade their mass transfer properties. For all that, it must be noted that butadiene adsorption occurred at conditions of strong competition from the hexene-1 side. The mentioned merits make the nanostructured polycation adsorbents particularly valuable for numerous petrochemical applications. What is more, two adsorbent samples (Cu)p(Mn)pLSF and (Zn)pDAY, which possess a raised acidity, have exhibited superior performances in gas stream purification.17 The adsorbents provided almost complete butadiene recovery from propene and simulated flue gas flows with residual impurity content of
