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Deactivation of π-Complexation Adsorbents by Hydrogen and Rejuvenation by Oxidation Ambalavanan Jayaraman and Ralph T. Yang* Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136
Curtis L. Munson and Daniel Chinn Separation Technology, Chevron Research and Technology Company, Richmond, California 94802-0627
The π-complexation sorbents are the best sorbents for bulk olefin/paraffin separation and purification of normal R-olefins. Hydrogen produced during cracking operations would remain in small or trace amounts in the process streams. The effect of hydrogen exposure on Ag+-exchanged Y zeolite (AgY) and monolayer-dispersed AgNO3/SiO2 (π-complexation sorbents) on olefin adsorption was studied by using accelerated tests. Hydrogen exposure could severely deactivate the π-complexation sorbents for adsorption of ethylene, 1,3-butadiene, and 1-butene. XPS analysis indicated that the Ag+ ion responsible for π-complexation was partially reduced, hence, the deactivation. It was found that the deactivated sorbents could be rejuvenated by mild oxidation. The reoxidation of the H2-exposed AgY samples were carried out under different oxidation conditions to locate the optimum conditions. The treatment of the H2-exposed samples to 0.13 atm of O2 in He at 350 °C for 0.5 h was the best reoxidation condition achieved in this study. Introduction Olefin/paraffin separation ranks as one of the most costly and key class of separations in the chemical and petrochemical industry. Cryogenic distillation has been used for these separations for over 6 decades.1 Because of the close relative volatilities between the olefins and paraffins, they remain the most energy-intensive distillations. Binary ethylene/ethane and propylene/propane separations are the important olefin/paraffin separations accounting for 6.3% of the total energy needed for distillation in the chemical and petrochemical industry.2 A number of alternatives to cryogenic distillation have been investigated3,4 and the most promising one appears to be π-complexation. π-Complexation-based separations are also being suggested for newer applications. Separation by π-complexation is a subgroup under chemical complexation where the mixture is contacted with a second phase containing a complexing agent.5 π-Complexation is based on chemical complexation bonds that are stronger than van der Waals interactions, yet weak enough to be reversible by slight changes in pressure and/or temperature.5 The π-complexation pertains to the d-block transition metals, that is, from Sc to Cu, Y to Ag, and La to Au in the periodic table.6 These metals or their ions can form a σ-bond to carbon and, in addition, the unique characteristics of the d orbitals in these metals or ions can form bonds with unsaturated hydrocarbons (olefins) in a process called back-donation. This type of bonding is broadly referred to as π-complexation. The π-complexation has been considered for the olefin/paraffin bulk separation and the purification of olefin streams using liquid solutions containing silver (Ag+) or cuprous (Cu+) ions.1,3,4,7-9 These earlier attempts involved gas-liquid operations. * To whom correspondence should be addressed. Tel.: (734) 936 0771. Fax: (734) 763 0459. E-mail:
[email protected].
Though gas-solid operations could be simpler and more efficient, particularly by pressure swing adsorption, the list of attempts to develop a solid π-complexation-based sorbent is a short one. More recently, several new π-complexation-based sorbents were prepared for selective olefin adsorption. These include Ag+-exchanged resins,10,11 monolayer CuCl on pillared clays,12 and monolayer AgNO3/SiO2.13-15 Hence, immense opportunities exist for developing new and better sorbents for the existing and new applications using the weak chemical bonds (e.g., chemical complexation bonds). The C4 streams from various hydrocarbon cracking operations contains numerous chemicals that lead to value-added products. The quantitative and qualitative composition of C4 streams is dependent on the type and severity of cracking and the feedstock.16 Simple distillation is ruled out for this application because of the close proximity of their boiling points. Several separation schemes have been developed for this application ranging from selective reaction of isobutene to extractive distillation of 1,3-butadiene. The remaining C4 stream can then be further separated into n-butenes and butanes by selective adsorption of n-butene on π-complexation sorbents. Normal R-olefins (NAO) are chemical intermediates used in industries to make a variety of value-added products (production of alcohols (via oxo chemistry), comonomer for polyethylene production, synthesis of poly-R-olefins, and oligomerization of n-butene to octene).17 These applications need high-purity olefins with very low levels of 1,3-butadiene (C4H6). Particularly the Ni-based catalyst used in the oligomerization of n-butenes can be poisoned by even trace amounts of C4H6 in the feed.18 Among the π-complexation sorbents, monolayer AgNO3/SiO2 is the most suitable sorbent for C2 and C3 olefin/paraffin separations. This was also shown to have excellent selectivity and capacity for 1-butene/n-butane
10.1021/ie0102753 CCC: $20.00 © 2001 American Chemical Society Published on Web 09/07/2001
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separation.19 Another key area of application for π-complexation is the purification of 1-butene by removal of the trace amounts of 1,3-butadiene. Padin et al.19 have demonstrated the effect of π-complexation on the adsorption of 1,3-butadiene and 1-butene using NaY and AgY (Ag+-exchanged Y zeolite). Because of the π-complexation ability of AgY, the threshold pressure (i.e., the pressure at which significant adsorption starts) for 1,3butadiene adsorption is found to be lower than NaY while that of 1-butene almost remains the same for both NaY and AgY. This property gives the ability for AgY to produce ultra-high-purity n-butene. The effect of silver content in AgY on 1,3-butadiene adsorption was investigated using AgY with different Si/Al ratios and Ag+-Na+ mixed ion-exchanged zeolites (AgNaY).20 AgNaY with Ag content of 34 atoms/u.c. was found to possess excellent separation characteristics for this application. During cracking operations in chemical industries, hydrogen sulfide, hydrogen, and acetylene are also produced and they may remain in the process stream. These are major impurities for π-complexation-based olefin/paraffin separations.4 The study of stability of the π-complexation-based sorbents to these gases and their effect on the π-complexation-based separation/purification is very critical. In the case of aqueous Ag+ solutions, the reduction of Ag+ followed by precipitation of metallic silver occurs upon exposure to hydrogen gas. Hydrogen sulfide and acetylene react irreversibly to precipitate silver sulfide (Ag2S) and silver acetylides (Ag2C2). Pretreatment to reduce the amount of sulfur compounds and acetylene is needed1 to mitigate the effect of these compounds on π-complexation sorbents. Small quantities of oxidizing agents like hydrogen peroxides are added to stabilize the aqueous Ag+ solution and prevent silver loss due to hydrogen. The effect of H2S exposure on adsorption of 1,3butadiene and 1-butene was examined by Takahashi et al.20 and was found to reduce the adsorption capacity of AgY due to formation of silver sulfide. However, the separation factors were still high enough for the purification application to be commercially viable. This is due to the silver in Ag2S being present in the Ag+ oxidation state, leading to π-complexation. In this paper the effect of hydrogen exposure on the π-complexation ability of AgY and AgNO3/SiO2 is studied. Adsorption of ethylene was measured before and after exposure to hydrogen. The redox reactions of AgY with hydrogen and oxygen were reported earlier.21-26 Here, the reoxidation of the hydrogen-exposed AgY was attempted and the best oxidation condition with respect to olefin adsorption was identified. The effect of these reduction reactions on adsorption of 1,3-butadiene and 1-butene on AgY was also studied. Experimental Details Materials. The Y zeolite (Strem Chemicals, Lot 12871-S) was binderless, hydrated powders and was used as received. Silica gel, 8-20 mesh (Strem Chemicals, Lot 19745-S) was used for preparing monolayer AgNO3/SiO2. Silver nitrate (AgNO3, 99.9%, Strem Chemicals) and deionized water were used for the preparation of AgNO3 solution for ion exchange and incipient wetness impregnation. Deionized water was used for sample washing and sodium chloride salt (NaCl, 99%, Strem Chemicals) was used to confirm the absence of free ions
in the washings. Helium (99.995%, Metro Welding), hydrogen (99.999%, Metro Welding), oxygen (99.6%, Metro Welding), ethylene (99.5%, Matheson Gases), ethane (99.0%, Matheson Gases), 1,3-butadiene (1.0% in helium, certified by Matheson Gases), and 1-butene (99.5%, Matheson Gases) were used without further purification. Preparation of Sorbents. (1) Silver-Exchanged Y Zeolite. AgY was prepared by two consecutive ion exchanges with a 0.05 M solution of AgNO3. Each exchange was carried with twice the required amount of cations for theoretical full exchange at room temperature with agitation27 for 24 h. The solution was decanted after the first exchange, and then fresh AgNO3 solution was added and the mixture agitated again at room temperature for 24 h. After the second exchange, the mixture was vacuum-filtered and washed in copious amounts of deionized water until no free ions were present in the filtrate (i.e., no precipitation upon treatment with NaCl). The AgY was dried at room temperature and atmospheric conditions in a dark area. (2) Monolayer Silver Nitrate on Silica Gel. The sorbent was prepared by the incipient wetness impregnation technique.28 In this technique, aqueous AgNO3 salt is dispersed on silica gel at a weight ratio of 0.33. Because AgNO3 is highly soluble in water and SiO2 is hydrophilic, proper wetting of the substrate is assured, resulting in uniform dispersion of the salt over the entire sorbent surface. H2 Exposure and Reoxidation. The hydrogen exposure and reoxidation of AgY were carried out in a tubular reactor. The AgY and AgNO3/SiO2 samples were degassed at 300 and 110 °C, respectively, with helium purge before analysis. The H2 exposure condition of 0.5 atm partial pressure of H2 in helium for 1 h at 120 °C was used for all the runs. The oxidation of the H2exposed AgY samples was carried out in an oxygen stream at various partial pressures and temperatures to locate the optimum reoxidation condition with respect to olefin adsorption. Isotherm Measurements. The ethylene and ethane isotherms were measured using a static volumetric system (Micromeritics ASAP-2010). The AgY and AgNO3/ SiO2 samples were degassed in a vacuum for several hours at 300 and 110 °C before each isotherm measurement to remove the adsorbed moisture. The 1,3-butadiene and 1-butene isotherms were measured in a Shimadzu TGA-50 automatic recording microbalance using standard gravimetric methods.29 Prior to measurement, the AgY samples were degassed at 300 °C for 2 h. The isotherms were measured before H2 exposure, after H2 exposure, and after reoxidation to study the effect of redox reaction on olefin adsorption in AgY. For the AgNO3/SiO2 samples, only the effect of H2 exposure was studied in this paper. The nitrogen isotherms at a liquid nitrogen temperature of 77 K were measured in a Micromeritics ASAP 2010 system and the Horvath-Kawazoe equation30-32 was used to calculate the pore size distribution of AgY before and after H2 exposure. XPS Analysis. X-ray photoemission spectra (XPS) of AgY was measured in a Perkin-Elmer PHI 5400 ESCA system with an Mg anode. The AgY samples (i.e., before H2 exposure, after H2 exposure, and after reoxidation) were made into thin wafers in a laboratory press. These wafers were then loaded on the sample stage in the ESCA system and degassed under high vacuum (10-8
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Table 1. Reoxidation Conditions of AgY Samples for the Various Experimental Runs run no.
degas temp. (°C)
H2 exposure condition
reoxidation conditions
1 1a 2 3 4 5 6 7 8 9 10
300 350 300 300 300 300 300 300 300 300 300
0.5 atm H2 in He for 1 h at 120 °C NaY fresh sample 0.5 atm H2 in He for 1 h at 120 °C 0.5 atm H2 in He for 1 h at 120 °C 0.5 atm H2 in He for 1 h at 120 °C 0.5 atm H2 in He for 1 h at 120 °C 0.5 atm H2 in He for 1 h at 120 °C 0.5 atm H2 in He for 1 h at 120 °C 0.5 atm H2 in He for 1 h at 120 °C
0.5 atm O2 in He for 0.5 h at 120 °C 0.5 atm O2 in He for 0.5 h at 300 °C 0.5 atm O2 in He for 0.5 h at 400 °C 0.5 atm O2 in He for 0.5 h at 350 °C 0.13 atm O2 in He for 0.5 h at 350 °C 0.13 atm O2 in He for 0.5 h at 400 °C 0.3 atm O2 in He for 0.5 h at 300 °C
Table 2. Langmuir-Freundlich Parameters for the Ethylene Adsorption on AgY Samples at 120 °C run no.a
% olefin capacity
qm (mmol/g)
B (atm-(1/n))
n
1 2 3 4 5 6 7 8 9 10
100 52
2.8555 12.5646 17.8414 6.7033 4.3329 2.9497 3.1942 2.4097 2.5719 3.2270
7.2348 0.1128 0.1074 0.3188 0.6400 1.6110 1.5258 8.7749 5.3343 1.1401
2.9051 4.2286 1.6877 5.3075 5.1115 4.3927 4.4907 2.8540 3.3508 4.9554
64 68 72 76 86 86 68
a For the sample condition refer to corresponding run no. in Table 1.
Figure 1. Ethylene adsorption isotherms at 120 °C on different AgY samplessruns 1-3 (for the specific conditions of the samples refer to inset in the figure/Table 1) fitted with LangmuirFreundlich (solid lines) isotherms.
to 10-9 Torr) at room temperature for 2 days. Then, the samples were moved to the analysis chamber and the spectroscopy was carried out at room temperature. Results and Discussion π-Complexation Ability of AgY. The equilibrium isotherm of ethylene on NaY zeolite (Si/Al ) 2.43) and AgY were measured at a temperature of 120 °C. The isotherms are shown in Figure 1. It could be seen that the ethylene adsorption is higher for the Ag+- exchanged zeolite and also the threshold pressure is lower compared to NaY. This increase in ethylene adsorption seen in AgY is due to π-complexation bonds between the olefin (ethylene) molecule and the Ag+ ion. The isotherms were measured at an elevated temperature of 120 °C so that the olefin adsorption is reversible. The measured adsorption isotherms were fitted with the Langmuir-Freundlich equation:33
q)
qmBP1/n 1 + BP1/n
(1)
The parameters are listed in Table 2 while Table 1 summarizes the reaction conditions for the different runs (1-10). The mixture adsorption isotherms could be predicted using the single-component Langmuir-Freundlich parameters in the extended Langmuir-Freundlich/Loading Ratio Correlation equation:
qi )
qmiBiPi1/n n
1+
BjPj1/n ∑ j)1
(2)
Prior to isotherm measurement, the samples were degassed for several hours in a vacuum at 300 °C. In the case of AgY, the degassing temperature is one of the key factors that influences the adsorption characteristic. Ag+ ions in silver zeolites are known to form different types of clusters.26 Other than dehydration of the zeolite samples, the migration of the cations from one site to another and autoreduction of Ag+ ions to Ag32+ clusters are known to occur during this heat treatment step.34 The effect of degassing temperature on olefin adsorption is shown in Figure 1, where the two samples degassed at 350 and 300 °C are plotted together. Effect of H2 Exposure on π-Complexation Sorbents. AgNO3/SiO2 and AgY are studied for their stability upon exposure of a hydrogen atmosphere. The effect of hydrogen exposure on these samples is studied in terms of their ability to perform π-complexation by measuring the olefin adsorption isotherms both before and after H2 exposure. (1) AgY. Figure 1 shows the effect of hydrogen exposure on olefin adsorption in AgY. After the AgY sample was exposed (degassed in a vacuum at 300 °C) to 0.5 atm hydrogen in helium for 1 h at 120 °C, the ethylene isotherms were measured. As seen in Figure 1, the hydrogen exposure has brought the capacity for ethylene adsorption down to the levels equal to that of NaY. This implies that the sorbent has lost its ability to form π-complexation bonds significantly. Upon analysis of the shape of the isotherms and the LangmuirFreundlich parameters given in Table 2, it is seen that the strength of adsorption has reduced upon exposure to hydrogen, as seen in the decrease of the parameter “B”. The threshold pressure is unaltered due to hydrogen exposure, which could be due to the presence of some unreduced silver clusters (Ag3+ clusters) due to the low temperature (
