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π-complexation sorbents are mass separating agents that typically consist of main group transition metal complexes binding to a particular set of target molecules via π-complexation. In such complexes, the π-electrons of the target unsaturated molecule, the sorbate, are donated to an empty s-orbital on the transition metal, sorbent. The d-electrons of the transition metal is then back donated to the π*-orbital of the sorbate. π-complexation bonds are stronger than van der Waals interactions but weak enough to be reversible. As shown in the bond-energy- bond-strength diagram by Keller (Figure 1), the likely range for reversible chemical complexation occurs between 7 and 40 kJ per mole. This reversible complexation chemistry is critical for selective adsorption and separation processes.

π-complexation sorbents are useful for many inherently costly and energy intensive separation processes in the chemical and petrochemical industry. One such process is the removal of organosulfur compounds from transportation fuel to meet federal regulation of sulfur restriction on gasoline. In addition, other exemplary processes include the separation of olefins (ethylene/propylene) from paraffins (ethane/ propane), and the removal of CO2 from high pressured process gas stream to reduce the emission of greenhouse gases. Currently, the only π-complexation sorbent commercially available is CuCl/γ-Al2O3, used for the separation of synthesis gas H2 from other gases such as CO, CH4, etc. in the petroleum industry.

Many of the novel sorbent materials have been developed in the last two decades in response to the increasing demand for strong chemical adsorbents for use in energy and environmental applications. The focus here will be on fundamentals of π-complexation bonding, different kinds of π-complexation sorbents and how to prepare and process these materials. We will also summarize the current research status as well as discuss several important industrial applications of π-complexation sorbents.

Mechanism of π-Complexation
π-complexation functions via intermolecular interactions between transition metals and unsaturated compounds via Dewar–Chatt–Duncanson interactions (Figure 2). Here the π-bond of the sorbate donates electrons to the s-orbital of a transition metal atom on the sorbent, creating an σ-bonding effect. The metal atom then back-donates electron density from a d-orbital to the unfilled anti-bonding π*-orbital of the sorbate. Finally, the metal atom’s electron density can redistribute to in response to the loss in electron density from the back donation to the sorbate. Figure 2 shows the π-complexation mechanism performed on the ethylene-silver system. In general, for any molecular orbital interaction to occur, including π-complexation, the sorbate and sorbent orbitals must be matched energetically and symmetrically.

π-complexation bond strength can be associated with the degree of electron transfer between the sorbent and sorbate. Higher donated electron density transferred from the sorbate π-orbital to the sorbent s-orbital indicates a stronger σ-bonding and higher electron densities transferred from the sorbent d-orbitals to the sorbate π*-orbital indicates stronger back-donation bonding. π-complexation bond strength is crucial because products are separated based on the strength of each π-complexes. Species that bind more tightly relative to the others in a product stream are retained longer on the sorbent and separated from those that bind less strongly eventually leading to separation. The sorbent can then be thermally regenerated, releasing the products that were more tightly bound. Table 1 shows the change in electron occupancy for each orbital of the Ag atom that occurs due to π-complexation for several sorbates on an idealized AgCl sorbent. A value of 0.5 would represent a 50% increase in occupancy of the orbital while a value of -0.5 would represent a 50% decrease. This occupancy is best thought of as the statistical electron density rather than the presence or absence of individual electrons. The increase in the 5s occupancy indicates σ-donation from the ethylene π-orbital. The decrease across all of the 4d orbitals indicates the back donation of electron density from Ag to the ethylene π*-orbital and the sum of the 4d orbital occupancy changes shows the total amount of back donation of electron density. The sum of the magnitudes of 5s and 4d orbital occupancy changes (|5s|+|Σ4d|) can be used to represent the strength of the π-complexation by showing the sharing of electron density between ethylene and silver. In this case, ethylene bonds to the sorbent more strongly than O2 or N2 allowing separation of the organic if run at lower temperatures, where ethylene oxidation does not readily occur on the silver.

The metal chosen for a π-complexation sorbent has a major effect on the bonding. In practice, Ag and Cu are the most common metals used, although many metals, such as Pt4+ and Pd2+, can be used as halides or oxides. In Cu salts, the adsorption bonding is dominated by the d-π* back-donation rather than by σ-donation of electron density. Conversely, in Ag salts, adsorption bonding is dominated the by σ-donation. Compared to Ag, Cu is shown to adsorb both ethylene and carbon monoxide more strongly. Table 2 demonstrates this electronic structure-bond strength relationship. For both CuCl and AgCl, CO bonds more strongly than ethylene as seen by larger electron transfers in σ-bonding (5s increase in occupancy) and π*-back-bonding (Σ4d decreases in occupancy). For both ethylene and CO complexation, the CuCl sorbent has stronger bonds as evidenced by the larger total electron transfer (|5s|+|Σ4d|). Note that the values in this table do not match those in Tables 1 and 3 because a different method was used to calculate them, but the general trends is the same.

The bond strength of π-complexation is also controlled by the materials in contact with the “active” metals, such as anions and zeolite supports. A theoretical study of N2 complexation on AgX sorbents (where X is: F, Cl, I or a zeolite support Z) showed that the halide anions exhibit stronger interaction with the electronic structure of the Ag atom than the zeolite support (Table 3), resulting in stronger bonding. Metal halides in general exhibit bond strength trends that followed the electronegativity trend in the periodic table, with F¬- anions providing the strongest and I- the weakest bonding.

Classification
There are three different types of π-complexation sorbents: supported monolayer salts,6 ion-exchange zeolite and ion exchange resin.

Supported Monolayer Salts
Supported monolayer salts form when metal oxides or salts are dispersed on a solid, generally porous support. The solid supports commonly used are silica and alumina because they bond to the metal oxides and salts. Once anchored, the metal can then adsorb reagents capable of π-complexation. For instance, a SiO2 support with 57% AgNO3 monolayer coverage was shown to selectively bind ethylene 3.3 times more than ethane. Other examples of supported monolayer sorbents are listed in Table 4.12  Two common dispersion methods used are thermal monolayer dispersion13 and incipient wetness impregnation.14

Thermal monolayer dispersion
Thermal monolayer dispersion is a technique used to cover the surface of a support with one layer of salt using thermal treatment. Based on this technique, the surface area of the support is initially, measured with BET, in order to determine the amount of salt sufficient for monolayer coverage. Exact monolayer coverage is difficult, so in practice, supports have varied coverage. Under thermal treatment the salt diffuses through the porous support to react. The diffusion process can take a number of days. Efficient dispersion requires fine powders of the salts.9

Incipient wetness impregnation
Incipient wetness impregnation uses an aqueous or organic solution to disperse soluble salts on a support. The salt solution is then mixed and absorbed by the support. Heating the sample evaporates the solvent leaving a layer of metal salt on the substrate surface. Specific predetermined ratios of salts to substrate are required get monolayer coverage. This technique is commonly used for industrial applications.9,14

Ion exchange zeolite
Ion exchange zeolites6 form when the cations in the solution and zeolite phases are exchanged, usually in aqueous solutions. The reaction is AS+ + BZ+ ←→ AZ+ + BS+ where A and B are cations, and the subscripts S and Z refer the solution and zeolite phases respectively. The selectivity of ion exchange reaction depends on numerous factors including the size, shape and concentration of the cations in the solution, the temperature, the solvent as well as the structure of zeolite. The preference of zeolite for the incoming and outgoing cations is typically predicted using the ion-exchange isotherm curve. In general, the selectivity of ion exchange reaction favors cations with higher valence and higher atomic weight for cations of the same valence. Furthermore, from a thermodynamic view point, selectivity is also dictated by entropy and standard free energy of the ion exchange. For example, according to the thermodynamic analysis of alkaline earth ion exchange in zeolite X and Y by Sherry et al.,17Ba2+ ion exchange of Na+ successfully occurs because of an increase in entropy and negative value of Gibb free energy.

We will only discuss two types of noble metal cations exchange in X and Y zeolite, as they are commonly used in purification processes: Ag+ and Cu+ Ag+ exchange in Y zeolite: In figure 3 zeolite Na+ is shown to be strongly selective for Ag+. As a result, it can easily be exchanged under ambient condition. In a typical single ion-exchange procedure,19 excess amounts of AgNO3 are allowed to react with different types of Y zeolite at room temperature for 24 hours in order to ensure a complete exchange. The zeolite mixture is then filtered and washed with de-ionized water until all free ions are removed. The final suspension is dried at atmospheric condition.

Cu+ exchange doesn’t happen directly because cuprous salts are insoluble in water. In a typical Cu+ exchange process, Cu2+ is first exchanged with alkaline zeolites, and then Cu2+ is reduced to Cu+. This partial reduction occurs either through the mechanism which involves the use of a reducing gas, or through the auto reduction mechanism, which doesn’t involve reducing gas. Report by Rabo et al.20 showed that the most effective reducing gas composed of 3% H20 and 97% CO. Cu+ ion-exchanged zeolite is a superior sorbent for hydrocarbon separation, such as diene/olefin and thiophene/ benzene separation.21 Fixed bed adsorption22 test shows that liquid phase ion exchange Cu (I)-Y is more selective toward thiophene comparing to Ag-Y zeolite (Table 6)

Ion-Exchanged Resins
Ion-exchange resins are formed when cations in a solution exchange with cations on the resin. The reaction takes place using commercially available polymeric resins such as Amberlyst (H+) and Dowex (Na+). In a typical cationic exchange procedure, repeated addition of dilute salt solutions at room temperature is performed, followed by washing and drying at elevated temperatures. For example, Ag+ is exchanged with H+ on an Amberlyst 15 resin leading to a 9.2 selectivity of ethylene over ethane, shown in table 7. The increase in selectivity of ethylene observed in ion-exchange resins compared to supported monolayer salts (3.3 selectivity) is attributed partly to the hydrophobic and oleophobic (low affinity for hydrocarbons) of the resins.15

Carbon Monoxide separation/recovery
Carbon monoxide has strong reducing properties that make it a useful industrial gas. It is used in the production of polyurethane, polycarbonate, aldehydes, organic acids, and esters. Carbon monoxide can be recovered, for these productions, as a product or byproduct from other industrial processes, such as steam reforming, coke ovens, methanol plants, and refinery processes.16 One way of recovering CO is by using a π-complexation sorbent to selectively bind CO in the gas streams produced. A CuCl/NaY zeolite sorbent has been used for such separation and recovery. Preparation of the sorbent was carried out by ion exchange and heating to temperatures of 200 °C to 250 °C in a hydrogen atmosphere. Once prepared, CO is adsorbed by Cu+ atoms of the sorbent under high pressures in a pressure swing adsorption (PSA) process. Release of CO from the sorbent and thus recovery, is carried out by depressurization. One report by Golden et al. had 80% recovery of CO from synthesis gas.25 Commercialization of CO recovery by a π-complexation sorbent was done by Kansai Coke & Chemicals Co. (now a subsidiary of Mitsubishi Chemical Holdings Corp.) in 1989.6

Olefin/ Paraffin separation
Light olefins are the products of the crude oil refining process and can be used as refinery fuel. The need to conserve and recover olefin has become more critical in the petrochemical industry, makes the olefin/paraffin one of the most important class of separation26, especially there is a current decline in natural gas supply. At the same time, in response to a significant requirement of capital and energy investment for olefin/paraffin separation process as well as federal regulation on hydrocarbon emission from refineries, the development of low- cost and environmental benign separation technology is highly necessary. Typically, the olefin / paraffin separation is done using the most energy extensive cryogenic distillation process due to the closed proximity of the relative volatilities π-complexation sorbents provide the alternative approach in olefin / paraffin separation12,24,26,27. Separation by π-complexation in system such as Ag+ exchanged Y-zeolites, Ag+ exchange resin, and spontaneous monolayer dispersion make it possible to achieve significantly high selectivity of olefin over paraffin without the necessity to operate under harsh condition such as high temperature or pressure. We will use figure 4 to illustrate the basic chemisorption process of olefins by metal complexes. The molecular interaction between the olefin π bond and the metal  and π bond allows the adsorption of ethylene. The two most common complexing metals used in this process are Ag and Cu. In a typical adsorption process based on chemical complexation to recover olefin (Figure 5), after the feed contacts the complexing solution, the complexed olefin and non-complexed paraffin exit and flow to a paraffin recovery column. Olefin is then released from the complex by elevated temperature or gas stripping. Recent works on ethane/ethylene and propane/propylene separation28 by Yang and coworkers demonstrate that CuCl/γ-Al2O3sorbent and Ag (I) resin both shows rapid adsorption rate and high olefin selectivity. They are indeed excellent sorbent for C2H6/C2H4 separation application in cyclic adsorption process.

Desulfurization
Removal of organosulfur compounds is a key process in petroleum refining that is often operated at elevated temperature and pressures. While hydrodesulfurization process is commonly used to remove thiols, sulfides and disulfide, it is less effective for removing thiophene and thiophene derivatives such as benzothiophene and dibenzothiophene29. π-complexation sorbents can be used to effectively remove organosulfur compounds from liquid transportation fuel, such as commercial gasoline, diesel and jet fuel via direct interaction with sulfur atoms and selective adsorption of thiophene. Different highly selective transition-metal ion exchange zeolites developed for this purpose include Cu (I)-Y, AgY, CuCl/γ-Al2O3, AgNO3/SiO2, etc. Based on reversible complexation mechanism, the metal form the sigma bond with the sulfur’s s- orbital, simultaneously, d-electrons of the transition metal is then back donated to the π*-orbital of the sulfur ring (Figure6).

Based on molecular orbital calculation, Yang and coworkers30 have determined that the π-complexation bonds between the transition metal cations, such as Ag+ and Cu+, and thiophene are stronger than that with benzene. Thus, they are highly selective for sulfur removal. In addition, experiments by Hernandez- Maldonado and Yang31 have shown that Cu and Ag exchange Y- type zeolite are superior adsorbents for removing thiophene from liquid hydrocarbon mixture with the capacity to reduce sulfur content to values less than 4 ppmw. Various adsorption capacities of zeolites used in this process are also shown in Table 8.