Ultrafine tuning of the pore size in zeolite A for efficient propyne removal from propylene

Chaohui He, Rajamani Krishna, Yang Chen, Jiangfeng Yang, Jinping Li, Libo Li,3,*

1 College of Chemistry and Chemical Engineering, Shanxi Key Laboratory of Gas Energy Efficient and Clean Utilization, Taiyuan University of Technology, Taiyuan 030024, China

2 Van’t Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, the Netherlands

3 Key Laboratory of Coal Science and Technology, Taiyuan University of Technology, Taiyuan 030024, China

Keywords:Type-A zeolite Ion-exchange Pore tuning Propyne/propylene separation Breakthrough experiments

ABSTRACT The removal of trace propyne (C3H4) from propyne/propylene (C3H4/C3H6) mixtures is a technical and challenging task during the production of polymer-grade propylene in view of their very similar size and physical properties.While some progress has been made, it is still very challenging to use some highly stable and commercially available porous materials via an energy-efficient adsorptive separation process.Herein,we report the ultrafine tuning of the pore apertures in type-A zeolites for the highly efficient removal of trace amounts of C3H4 from C3H4/C3H6 mixtures.The resulting ion-exchanged zeolite 5A exhibits a large C3H4 adsorption capacity(2.3 mmol g-1 under 10-4 MPa)and high C3H4/C3H6 selectivity at room temperature, which were mainly attributed to the ultrafine-tuned pore size that selectively blocks C3H6 molecules,while maintaining the strong adsorption of C3H4 at low pressure region.High purity of C3H6 (＞99.9999%) can be directly obtained on this material under ambient conditions, as demonstrated by the experimental breakthrough curves obtained for both 1/99 and 0.1/99.9 (V/V) C3H4/C3H6 mixtures.

1.Introduction

Propylene (C3H6) is a key olefin raw material used for petrochemical production,second in importance to ethylene[1].During the production of C3H6, trace amounts of propyne (C3H4) (0.1% or 1%) is inevitably generated as an impurity and is highly undesirable.To meet the criterion of polymer-grade C3H6, trace amounts of the C3H4impurity must be removed to be ＜0.0005% or even 0.0001%.However, these two molecules have very close structural dimensions (C3H4: 0.416 nm × 0.401 nm × 0.651 nm and C3H6:0.416 nm×0.465 nm×0.644 nm)and similar physical properties,which makes the removal of C3H4from C3H6highly challenging[2,3].

Traditional cryogenic distillation and catalytic hydrogenation technologies[4]used to purify C3H6usually suffer from some obvious drawbacks, such as high energy consumption, low efficiency and secondary pollution.Finding an alternative method will reduce the energy needed to make the 120 million tons of propylene manufactured worldwide each year.Adsorptive separation based on porous materials is more energy-efficient [5-15].In 2017, we reported the first example of a porous material (ELM-12) used for the efficient removal of C3H4from a C3H4/C3H6mixture via an adsorptive separation method [16].Subsequently, many microporous metal-organic frameworks (MOFs), such as SIFSIX-3-Ni,ZJUT-1, ZU-62, NKMOF-Ni-1 (SIFSIX = S), have been developed as highly selective C3H4adsorbents for the highly selective capture of C3H4from C3H6[17-20].However, these MOF materials have some drawbacks, including the high energy cost of regeneration due to strong C3H4binding interactions and low structural stability upon exposure to moisture and/or sulfur compounds,especially in the presence of open metal sites that can lead to oligomerization of the olefins and ultimately block their porous channels.Therefore,it is highly urgent to develop stable and commercially available adsorbents with high C3H4/C3H6adsorption selectivity, excellent gas mixture separation performance and structural stability.

Small pore zeolites have attracted a lot of attention for gas separation/purification in recent years [21-23].Due to their porous channels, some zeolite exhibit high selectivity for some gas mixtures, including CO2/CH4, CH4/N2, ethane/ethylene and propane/propylene [24-36].However, with the exceptionally high purity requirement (C3H4＜ 0.0001%) and small molecular difference between C3H4and C3H6(kinetic diameters: C3H4, ～0.42 nm;C3H6, ～0.46 nm), the efficient removal of C3H4has been rarely reported using zeolites to date.At present, there is only one reported zeolite that can realize the separation of an equimolar C3H4/C3H6mixture by confining isolated open Ni sites in an FAU zeolite [37].However, molecular sieving and separation have not been realized because it is quite challenging to design ideal porous materials on traditional zeolites by fine-tuning the pore aperture size in 0.02-0.1 nm increments.Using an ion-exchange method to maximize the sieving effect for gas separation, it is possible to control the pore channels of traditional zeolites, thus allowing the entrance of C3H4, but hindering the diffusion of C3H6to significantly increasing the separation performance [38-40].

Herein, we report the highly efficient separation of trace amounts of C3H4from C3H6using Na-exchanged zeolite 5A.This zeolite material has a suitable and robust pore size that can efficiently capture C3H4molecules, while blocking C3H6at low pressure, and thus exhibit high C3H4/C3H6adsorption selectivity.In addition, trace amounts of C3H4can be readily removed from a C3H4/C3H6(1/99 and 0.1/99.9, V/V) mixture to produce highpurity propylene (＞99.9999%) under ambient conditions.

2.Experimental

2.1.Synthesis of materials

NaA (Ca2+), NaA and NaA (K+) used in this study are powdered 5A, 4A, and 3A zeolite provided by Sigma-Aldrich.Ion-exchange was performed by exposing the as-received commercial zeolites to an excess of aqueous sodium chloride (NaCl, Sigma Aldrich)solution.1 g of zeolite was treated with 50 ml of 1 mol·L-1solution of NaCl for 2 h with stirring at 353 K.Afterwards, the exchanged material was washed thoroughly with distilled water, filtered and dried at 423 K.5A(xNa+),5A(yNa+),and 5A(zNa+)in this work were converted from zeolite 5A via one,two,and three consecutive ion-exchange steps, respectively.

Propyne (C3H4, 99.99%), propylene (C3H6, 99.99%), helium(99.999%) and mixed gases comprised of C3H4/C3H6(1/99 and 0.1/99.9, V/V) were purchased from Beijing Special Gas Co.Ltd,China.

2.2.Characterization

Powder X-ray diffraction (PXRD) was carried out on a BRUKER D8 ADVANCE diffractometer employing Cu Kα radiation operated at 30 kV over the 2θ range of 5°-40° at a scanning rate of 1 (°)·min-1.Scanning electron microscopy (SEM) was performed using a Hitachi SU8010 scanning electron microscope.

2.3.Gas sorption measurements

An Intelligent Gravimetric Analyzer (IGA 001, Hiden, UK) was used to measure the C3H4and C3H6adsorption isotherms.The sorption isotherms of N2at 77 K and CO2at 273 K were measured on a Micromeritics ASAP 2020 analyzer.All the samples were activated at 473 K over 2 h to remove the guest molecules prior to actual measurement (see Fig.1).

Fig.1.A schematic of the fine-tuning of the pore size using ion-exchange in type-A zeolites for C3H4/C3H6 separation.

3.Results and Discussion

3.1.Characterization of ion-exchanged zeolite A

In view of the structure dimensions of C3H4and C3H6,commercially available type-A zeolite with a similar pore size was selected to be used as a platform for C3H4/C3H6separation(see Fig.1).Type-A zeolite can have different pore sizes from 0.3 to 0.5 nm via changing the cation (Na+, Ca2+and K+) [41], whose gradual reduction in pore size of ～0.1 nm is not precise enough to differentiate the small size difference between C3H4and C3H6.We speculated that there is still room for fine-tuning the pore size to further improve the C3H4/C3H6separation performance.An appropriate pore size to maximize the sieving effect could be achieved by controlling the number of Na+-exchange steps using 5A zeolite to precisely tailor the pore size with 0.01-0.02 nm increments and thus,improve the separation performance.

The pore size distributions of commercial and ion-exchanged type-A zeolite were first checked using N2sorption analysis at 77 K.Fig.2 shows that when the cation was changed from Ca2+to Na+and K+, the pore size of the zeolite was reduced to block the entrance of N2at 77 K.For the Na+-exchanged type-A zeolite(Fig.2b),N2adsorption was decreased in smaller increments when compared to the commercial zeolite, which indicates that controlling the number of Na+-exchange steps could precisely tailor the change in the pore size.In addition, all of the ion-exchanged zeolites maintained the main zeolite structure, as confirmed by PXRD analysis.

To further characterize the slight changes in the pore size during the ion-exchange process, the CO2adsorption isotherms were obtained at 273 K and used for our calculations.Fig.3 shows the pore size of zeolite 5A varied from 0.52 to 0.44 nm with the number of ion-exchange steps,which were near to the kinetic diameter of C3H4and C3H6, and may display a molecular sieving effect for gas mixture separation.

3.2.C3H6/C3H4 adsorption isotherms and selectivity of ion-exchanged zeolite A

Fig.2.N2-sorption isotherms obtained for (a) NaA, NaA (Ca2+) and NaA (K+); (b) 5A, 5A (xNa+), 5A (yNa+), and 5A (zNa+), and (c and d) their associated PXRD patterns.

Fig.3.(a)CO2 adsorption isotherms obtained for the ion-exchanged NaA zeolites at 273 K and(b)their associated pore size distributions calculated using the D-R equation.

In order to evaluate the effect of the changed pore size in zeolite A for the selective separation of C3H4from C3H6, we conducted single-component sorption tests at 298 K.Fig.4 shows the results obtained for commercial type-A zeolites (3A, 4A and 5A) with different micropore sizes to determine their basic adsorption properties for C3H4and C3H6.The relevant adsorption uptakes of C3H4and C3H6sharply decreased with the pore size of commercial type-A zeolite, which indicates that C3H4and C3H6do not diffuse into the pore channels of NaA (zeolite 4A) and NaA (K+) (zeolite 3A).However, the two C3 molecules can both be adsorbed in NaA(Ca2+) because the adsorption isotherms of C3H4and C3H6were steep and quickly reached a near-saturated adsorption capacity at low pressure(C3H4uptake reaches 2.3 mmol·g-1under 10-4MPa),indicating a possible platform for further optimization of the adsorption selectivity for C3H4/C3H6.Upon consecutive Na+-exchange in zeolite 5A, the adsorption of C3H4and C3H6the observed for the different ion-exchanged zeolites gradually changes with the reduction in the pore size and 5A(yNa+)exhibits the highest C3H4/C3H6(1/99, V/V) selectivity (43), as calculated using the ideal adsorbed solution theory (IAST) method.This indicates our strategy to precisely tune the pore size to optimize the adsorption selectivity of C3H4/C3H6is very effective.In addition,the pore size distribution further confirmed the relationship between the increase in the adsorption selectivity and pore size.5A (yNa+) with a pore size of 0.47 nm has the highest adsorption selectivity due to effectively blocking the adsorption of C3H6at low pressure, while maintaining the strong adsorption for C3H4.When comparing the adsorption selectivity with typical MOFs(Fig.S2, in Supplementary Material), 5A (yNa+) was larger than most of the previously reported MOFs (SIFSIX-2-Cu-i, UiO-66,MIL-100(Cr), Cu-BTC and ZIF-8), while lower than the benchmark MOFs(SIFSIX-3-Ni,ELM-12 and ZU-62).Given the thermal stability and production cost, 5A (yNa+) has great advantage for industrial application.In addition, the heats of adsorption were calculated using the Virial equation to evaluate the binding affinity for C3H4and C3H6(Fig.S3).The heats of adsorption for zeolite 5A (yNa+)toward C3H4was 52 kJ·mol-1, which was significantly larger than that toward C3H6(30 kJ·mol-1), which indicates the higher hostguest interactions between the framework and C3H4.Although the calculated heat of adsorption heat was slightly high, no hindrance was observed during the sorption process of C3H4on zeolite 5A (yNa+) and the adsorbent could easily be regenerated in vacuo or with a flow of helium under ambient conditions.

3.3.Breakthrough experiments

Fig.4.C3H4 (circles) and C3H6 (triangles) adsorption isotherms obtained for the ion-exchanged NaA zeolites at 298 K in the region of (a and b) 0-0.1 MPa and (d and e) 0-0.001 MPa.(c)IAST selectivity of the ion-exchanged zeolites for C3H4/C3H6(1/99,V/V)at 298 K.(f)A comparison of the IAST selectivity at 0.1 MPa,pore size distribution and degree of Ca-exchange in the zeolites.

Fig.5.(a and b)Experimental breakthrough curves obtained for 5A(yNa+)using a C3H4/C3H6 mixture(1/99 and 0.1/99.9)at a gas velocity of 2.0 ml/min at 298 K and 0.1 MPa(C3H4, red; C3H6, purple).(c) The retained time of C3H6 obtained in the cycling tests of 5A (yNa+) using 1/99 and 0.1/99.9 (V/V) C3H4/C3H6 mixtures.

To further assess the separation performance of 5A (yNa+) for actual C3H4/C3H6mixtures (1/99 and 0.1/99.9 V/V), we conducted breakthrough experiments in a homemade apparatus.Fig.5 shows 5A (yNa+) displays an excellent separation performance for C3H4/C3H6mixtures (1/99 and 0.1/99.9).C3H6was quickly eluted from the column and reaches high purity, which can be stability maintained for a prolonged period of time until an C3H4adsorption saturation is reached.The C3H4breakthrough time for the C3H4/C3H6mixtures(1/99 and 0.1/99.9 V/V)were 1000 and 1600 min,respectively.This great separation effect demonstrates that 5A (yNa+) is suitable for this separation process and proved the precise finetuning of the pore size of zeolite A is an effective strategy to boost its separation performance.Furthermore, transient breakthrough simulations of the C3H4/C3H6mixture (1/99, V/V) on 5A (yNa+) at 298 K confirmed the actual separation results.High purity C3H6can be recovered between 0-τbreak(2600 min)(Fig.S6).The breakthrough productivity of C3H6calculated from the transient breakthrough curves of the C3H4/C3H6mixture (1/99, V/V) was 71.7 mol·L-1(Table S6), which was larger than most of the typical MOFs reported to date(SIFSIX-2-Cu-i and UiO-66).In addition,the cycle life and separation stability are also important for industrial applications.We carried out four cyclic breakthrough experiments for the separation of the C3H4/C3H6mixtures(1/99 and 0.1/99.9,V/V) on 5A (yNa+) at 298 K and the breakthrough times were almost unchanged,which indicates the separation performance can be largely maintained (Figs.S8-S9).

4.Conclusions

In summary, we have demonstrated that it is possible and feasible to prepare an efficient separation material for the separation of C3H4/C3H6mixtures via ultrafine tuning of the pore size of zeolite A to selectively block C3H6,while retaining the high adsorption capacity for C3H4.The resulting Na-exchanged zeolite 5A exhibits both high selectivity and adsorption capacity for C3H4and breakthrough experiments confirmed that this material can completely remove trace amounts of C3H4from 1/99 and 0.1/99.9 (V/V)C3H4/C3H6mixtures to afford a high C3H6productivity and purity with a C3H4concentration of ＜0.0001%.This work not only reveals a pathway for industrial C3H4/C3H6separation, but also provides some guidance to facilitate the design of some practically useful materials at a reasonable cost for important hydrocarbon separation and purification processes in the near future.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We acknowledge the financial support from the National Natural Science Foundation of China(21922810,21908153,21908155)and program of Innovative Talents of Higher Education Institutions of Shanxi.We are grateful for the supported by Cultivate Scientific Research Excellence Programs of Higher Education Institutions in Shanxi (CSREP).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2020.11.037.