**1. Introduction**

Separation processes in the oil, gas and chemical industries account for up to 15% of their total energy requirements [1–4]. Light hydrocarbons (C1–C9) are vital chemical feedstocks and energy resources around the world [5]. In addition, olefin separations are critical for the chemical industry, with the greatest demand on high purity propylene (C3H6). The demand for high purity propylene has risen sharply in recent years, and the compound is now the second most widely produced hydrocarbon by volume in the world after ethylene [3,6–9]. In 2019, the production of propylene was around 145 million tons globally [5]. Propylene is an intermediate essential chemical in a large number of important chemical industries such as polypropylene based plastics, propylene oxides, isopropanol, acrylonitrile, and other copolymers [3,6,9–19]. Steam/catalytic cracking of higher chain hydrocarbons is the main method of producing propylene, although the resulting product inevitably contains amounts of propyne. Propyne (C3H4) is a common impurity that is known to cause a poisoning effect of the catalyst during the cracking process, with detrimental effect on the production of propylene [3]. To meet polymer grade propylene requirements, the content of propyne must be reduced to less than 5 ppm. It is therefore imperative to remove propyne from the propylene gas streams to produce the required propylene polymer grade gas (>99.99% purity). The separation of propane (C3H8)

F.; Walker, G. Effective Separation of Prime Olefins from Gas Stream Using Anion Pillared Metal Organic Frameworks: Ideal Adsorbed Solution Theory Studies, Cyclic Application and Stability. *Catalysts* **2021**, *11*, 510. https://doi.org/ 10.3390/catal11040510

**Citation:** Khraisheh, M.; Almomani,

Academic Editor: Jorge Bedia

Received: 7 March 2021 Accepted: 16 April 2021 Published: 18 April 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

and propylene is well reported in literature given its demanding energy requirements and the close relative volatilities of both compounds at the temperature range of operation (typically between 244–327 K) [2,7–10,13,14,17]. However, only a few studies have reported the separation of C3H4/C3H6 mixtures [4,20–22]. This separation is very challenging due to the physical and chemical similarities (molecular size C3H4: 4.16 × 4.01 × 6.51 Å [3] and C3H6: 5.25 × 4.16 × 6.44 Å [3]) between propyne and propylene compounds; more so than C2H2/C2H4 separations, for example [4]. Cryogenic distillation is the most commonly used method. Typically, it requires more than 100 theoretical stages to perform the separations of the gas fractions under low temperature and high pressure conditions [6], resulting in very high energy demands. Other technologies involve adsorption-based separations using a variety of materials such as zeolites and carbons in a variety of settings such as pressure, vacuum, or temperature swing settings [2,7,12,14,23–26]. Low separations and selectivity remain a challenge in difficult separations like those related to propyne and C3H6 separations. New developments in metal organic frameworks (MOFs) and the development of crystal engineering represent huge potential for previously difficult separations [27–29]. Metal organic frameworks, sometimes called metal coordinated polymers, are crystalline micropore materials with huge potential, owing to the flexibility in their chemical structure and intrinsic properties that can be fine-tuned to suit a given gas separation application [30–32]. Accordingly, the MOF internal aperture can be tuned for selective size or shape separation. They generally comprise organic ligands and inorganic metal clusters. Accordingly, the changes in structure pore size, one of the most appealing features of MOFs, and functional groups lends itself to many gas applications and separation processes. The structural flexibility and dynamic behavior of metal organic frameworks, such as the gate-opening effect due to variations in temperature, pressure, or other eternal stimuli, are another distinct feature of metal organic frameworks compared to traditional adsorbents such as zeolites [5]. A class of MOFs (hybrid ultra-microporous materials) are setting a new benchmark in a number of gas separations [27]. These materials are based on 3D Zn/Cu/Ni coordination networks that contain metal nodes in addition to organic linkers. Anions such as SiF6 2− (SIFSIX), NbOF5 2− (NbOFFIVE) form bridges in the MOF structure and are reported to have potential application in gas separations, such as C3H6/C3H8 and other lower alkene/alkane separations.

Gas hydrocarbons are adsorbed inside the molecule and held via van der Waals, metal-binding or hydrogen interactions, depending on the type of MOF used. To this end, equilibrium, kinetic, or molecular sieving separations can be the predominant mechanism that aids the use of a particular MOF in a given gas separation. Molecular sieving is based on size/shape exclusion, while thermodynamic equilibrium separation depends on the type and strength of the interactions and the affinity of the host-guest interactions i.e., MOF and the gas species. In equilibrium separations, there are relative thermodynamic affinities between the gas species and the MOF adsorbent via the introduction of strong interaction sites on the MOF frame. Lewis acidity related to an uncoordinated open metal site MOF is used in CO2 or olefin separations. These interactions form strong bonds which is the major challenge associated with their use in such separations due to the associated extra energy requirements related to the desorption of the gas and its subsequent release and separation, in addition to the MOF regeneration. Kinetic operations are based on the diffusivity of the gas compounds and their relative selectivity.

The use of metal organic frameworks for the separation of propyne/propylene is still in the early stages, with few reported studies in literature [4]. The first study on C3H4/C3H6 separation was reported using [Cu(4,4-bipyridine)(trifluo-romethanesulfonate (OTF)- 2](ELM-12) [21]. This material showed good potential for adsorption selectivity through the estimation of the ideal adsorbed solution theory (IAST) selectivity at a trace concentration of propyne (ratio 1/99 C3H4/C3H6), with good adoption capacity at lowpressure range. The work highlighted the potential of metal organic frameworks of flexible structures for hydrocarbon separation at low concentrations. Anion pillared metal organic frameworks with various structures were also studied, including the NbOF5 2− and SiF6 2−

pyrazine based MOF family (NbOFFIVE-1-Ni (sometimes called KAUST-7 [33]), SIFSIX-2- Cu, SIFSIX-3-Ni, SIFSIX-3-Zn) [18]. The study highlighted that the geometric disposition and pore size of SiF6 2− anions could be varied to best match the propyne molecules, resulting in very promising separation under the applied experimental conditions. The adsorption capacity reportedly reached 2.6 mmol g<sup>−</sup><sup>1</sup> for propyne concentrations of around 300 ppm. The study reported that both molecules were adsorbed, limiting productivity in separation [5]. Li et al. [4] screened 20 different MOF materials with varying structures, pore sizes, preparation methods, and functional groups for propyne/propylene separation. Their study showed that one material, called UTSA-200, exhibited grea<sup>t</sup> potential (95 cm<sup>3</sup> cm<sup>−</sup><sup>3</sup> at low pressure and 298 K) at an ultralow concentration (0.1:99.9 *v*/*v* C3H4/C3H6). The high affinity was reported to be related to the suitable pore size resulting from the rotation of the pyridine rings in the MOF with blocking propylene effect [4,5]. Recently, calcium based metal organic frameworks were investigated for C3H4/C3H6 separation [3]. An uptake gas value of 2.4 mmol g<sup>−</sup><sup>1</sup> was recorded at low pressures and trace concentrations.

In the few reported studies on propyne/propylene separation, very low concentrations were studied at pressures up to 1 bar. To establish a good trade off, high productivity and purity is required from the adsorbent material. In addition, most studies considered the adsorption capacity alone, while a few considered breakthrough analysis for a number of different cycles. There is a lack of systematic studies on this particular separation that take into account the adsorption isotherms and kinetics of the stream under consideration. Targeting materials with well-developed porous structures and stability is required to enhance the separation of propyne and propylene, to ensure the high purity of propylene, which is needed for industrial grade applications, and to reduce the energy requirements of this process. As mentioned earlier, the study of the selectivity is important to establish the type of integration between the gas and the MOF adsorbent to facilitate the understanding of the desorption and the reversibility of the interaction. Accordingly, in this study, a systematic approach was considered to study the effect of two types of pyrazine-based inorganic anions SiF6 2−, NbOF5 <sup>2</sup>−) metal organic frameworks (SIFSIX-3-Ni and NbOFFIVE-1-Ni) under a range of temperatures (300–360 K) and pressures (up to 100 kPa) for the separation of propyne/propylene in various concentrations (10/90 *v*/*v*). Adsorption capacities were analyzed using four isotherm models, i.e., single (Langmuir and Freundlich) and multi constant isotherms (Sips and Toth). No other reported studies have addressed the mathematical isotherm fitting of the experimental data for the C3H4/C3H6 system in combination with the metal organic frameworks used in this study. Such data are important in engineering calculations related to scale up and industrial applications. In addition, the selectivity was analyzed using the ideal adsorbed solution theory (IAST). Dynamic studies were conducted and breakthrough curves were established at varying numbers of cycles.

#### **2. Experimental Materials and Methods**

High purity analytical materials were supplied by Sigma-Aldrich and Buzwair Inc. Qatar. Chemical structures and characteristics of propyne and C3H6 are shown in Table 1. Gases were of 99.99% purity.


**Table 1.** Characteristic information for adsorbents and adsorbate gases.

\* D1, D2 and D3 refer to distances as illustrated in Figure 1b.

### *2.1. Adsorbent Synthesis*

The two metal organic frameworks (NbOFFIVE-1-Ni and SIFSIX-3-Ni) were prepared as detailed in our earlier work [34]. Both metal organic frameworks were pyrazine based, as detailed in Figure 1. The metal organic frameworks were architected by the bridging of the pyrazine-Ni2+ square grid layers with NbO5 2− (Figure 1). The difference between the NbO5 2− and SiF6 2− resulted in pyrazine moieties free rotation, and it affected the pore cavity which is smaller in NbO5 2− compared to that of SiF6 2− (= hexafluorosilicate) (Table 1).

**Figure 1.** (**a**) Schematic of the basic structure of the pyrazine based metal organic frameworks used in the study. (**b**) Structural representation of the metal organic frameworks and size dimensions. Values of D1, D2 and D3 are given in Table 1 (dimensions are not to scale and for illustration purposes).

### *2.2. Sample Characterization:*

2.2.1. Brunauer-Emmett-Teller (BET) Analysis

Liquid nitrogen was used for the N2 gas adsorption tests at 77 K. Micromeritics characterization was carried out using ASAP 2420 surface and porosity analyzer (Micromeritics GmbH, Unterschleißheim, Germany). Surface area was established using BET model while pore size distribution was found using the BHJ method [34]. Table 1 presents the main characteristics of the metal organic frameworks used.

#### 2.2.2. Thermogravimetric Analysis (TGA), SEM and FTIR

TGA analysis may be used to establish the weight loss of metal organic frameworks under thermal stress with high temperatures as an indication to the sample thermal stability. Tests were conducted using a Perkin Elmer Pyris 6 analyzer. Details of these tests have been reported by Khraisheh et al. [34].

Furthermore, Fourier-transform infrared spectroscopy (FTIR) (using Bruker Vertex 80) for the adsorbents was conducted in the range of 4000–400 cm<sup>−</sup>1. In addition, SEM analyses were conducted following standard protocols.
