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Nanoporous Boron Nitride as Exceptionally Thermally Stable Adsorbent: Role in Efficient Separation of Light Hydrocarbons Dipendu Saha, Gerassimos Orkoulas, Samuel Yohannan, Hoi Chun Ho, Ercan Cakmak, Jihua Chen, and Soydan Ozcan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b01889 • Publication Date (Web): 03 Apr 2017 Downloaded from http://pubs.acs.org on April 8, 2017

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ACS Applied Materials & Interfaces

Nanoporous Boron Nitride as Exceptionally Thermally Stable Adsorbent: Role in Efficient Separation of Light Hydrocarbons

Dipendu Saha1,*, Gerassimos Orkoulas1, Samuel Yohannan1, Hoi Chun Ho2,3, Ercan Cakmak2, Jihua Chen4, Soydan Ozcan2 1

Department of Chemical Engineering, Widener University, One University Place, Chester, PA 19013 2 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 3 The Bredesen Center for Interdisciplinary Research and Graduate Education, The University of Tennessee, Knoxville, TN, 37996, USA, 4Center for Nanophase Materials Sciences, United States, Oak Ridge National Laboratory, Oak Ridge, TN 37831

*Corresponding author’s E-mail: [email protected] , Phone: +1 610 499 4056, Fax: 610 499 4059 (D. Saha) 1 ACS Paragon Plus Environment


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Abstract: In this work, nanoporous boron nitride sample was synthesized with Brunauer–Emmett– Teller (BET) surface area of 1360 m2/g and particle size 5-7 µm. The boron nitride was characterized with X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and electron microscopy (TEM and SEM). Thermogravimetric analysis (TGA) under nitrogen and air and subsequent analysis with XPS and XRD suggested that its structure is stable in air up to 800 °C and in nitrogen up to 1050 °C, which is higher than most of the common adsorbents reported so far. Nitrogen and hydrocarbon adsorption at 298 K and pressure up to 1 bar suggested that all hydrocarbon adsorption amounts were higher than that of nitrogen and the adsorbed amount of hydrocarbon increases with increase in its molecular weight. The kinetics of adsorption data suggested that adsorption becomes slower with the increase in molecular weight of hydrocarbons. The equilibrium data suggested that that boron nitride is selective to paraffins in paraffin-olefin mixture and hence may act as an “olefin generator”. The Ideal adsorbed solution theory (IAST)-based selectivity for CH4/N2, C2H6/CH4, and C3H8/C3H6 was very high and probably higher than the majority of adsorbents reported in literature. IAST-based calculations were also employed to simulate the binary mixture adsorption data for the gas pairs of CH4/N2, C2H6/CH4, C2H6/C2H4 and C3H8/C3H6. Finally a simple mathematical model was employed to simulate the breakthrough behavior of the above-mentioned four gas pairs in a dynamic column experiment. The overall results suggest that nanoporous boron nitride can be used as a potential adsorbent for light hydrocarbon separation.

Keywords: Boron nitride, hydrocarbon, adsorption, selectivity, breakthrough curve, IAST

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1. Introduction Boron nitride is a ceramic-based material and a structural analogue of graphene in twodimensional form and graphite in stacked form. In boron nitride, boron (B) and nitrogen (N) atoms are in a sp2-hybridized state and connected to each other in hexagonal fashion. Different layers of boron nitride are held together by a weak van der Waals force1 , similar to that of graphite. Despite being a structural analogue of graphite, it has some distinct features and that make it a unique material to study and employ for diverse applications. Unlike graphite, boron nitride is an insulator or dielectric material with wide band gap of 5 to 6 eV2,3,4,5,6. One of the key advantages of boron nitride is its very high thermal stability7,8,9. The other distinctive features of this material include very high resistance to chemical oxidation10,11,12, robustness and excellent corrosion

resistance,

low

density,

ultraviolet

(UV)

photoluminescence

and

cathodoluminescence 9,13,14,15, 16,17,18,19.

Although there are some rigorous research works on nano structured boron nitride, the published reports on porous boron nitride is limited. Nanoporous boron nitride is a unique form of boron nitride and can be used in different specific applications, like gas separation and storage, pollution abatement and catalysis in a very harsh environment. The dipolar field near its surface27 facilitates the physisorption 20 of guest molecules on its pores. High thermal and chemical stability of this material can also be harnessed to regenerate and recycle without losing its characteristic properties. Different approaches of synthesis of boron nitride strongly influenced its Brunauer–Emmett–Teller (BET) surface area. In fact, BET surface area as high as 1900 m2/g has been reported21. In the field of adsorptive storage of hydrogen (H2), porous boron nitride has made a significant progress and the adsorbed amount was higher than porous carbon and many other types of adsorbents13,22,23,24,25. It has been suggested that boron nitride interacts strongly with H2 through its heteropolar B-N bonds that lead to the partial chemisorption of H226,27,28 and hence can be harnessed in catalytic operations. Boron nitride powder was also used in the catalytic support in high temperature reactions29 and support for photocatalysis30. Besides H2, boron nitride was used for the adsorption of nitrogen (N2)31, carbon dioxide (CO2)32, and methane (CH4)32. Porous boron nitride has put its strong footprint on the field of water purification and wastewater treatment13,33,34. Lei et al.34 demonstrated that porous boron nitride 3 ACS Paragon Plus Environment


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can adsorb various kinds of dyes, solvents and oils. It adsorbed 33 times of is weight of oil and organic solvents while repelling water34. Porous boron nitride was also employed in the delivery of anticancer drug doxorubicin while maintaining its biocompatibility35.

Despite several advantages of boron nitride and its recent growth in scientific and engineering applications, the gas separation studies on nanoporous boron nitride are very limited. In this work, we have reported the synthesis, characterization, thermal stability and adsorption of several gases, mostly, lighter hydrocarbons onto nanoporous boron nitride at ambient temperature (298 K) and pressure upto 1 bar (760 Torr). These gases are nitrogen (N2), methane (CH4), ethane (C2H6), ethylene or ethene (C2H4), propane (C3H8) and propylene or propene (C3H6). Based on the gas adsorption isotherms on boron nitride, we have reported the selectivity of separation and simulated the breakthrough behavior through a packed bed filled with nanoporous boron nitride for the four gas pairs of CH4/N2, C2H6/CH4, C2H6/C2H4 and C3H8/C3H6. Separation of CH4 from N2 is required to enrich the natural gas and increase its fuel value. It is a part of the inert rejection in natural gas purification step. Separation of C2H6 from CH4 is needed to isolate the heavier hydrocarbons from natural gas as C2H6 is the highest fraction of hydrocarbon in natural gas stream after methane. Separation of C2H6 is a part of the operation towards isolation of natural gas liquids (NGLs). Separation of C2H4 from C2H6 and C3H6 from C3H8 are the key parts of separation needs for olefins from paraffins. The olefins (C2H4 and C3H6) are the monomers of the plastic polymers (polythene and polypropylene) and they must be separated from the paraffins in order to enrich the olefin concentration in the feed stream. Additionally, pure paraffins are used as fuel (C3H8) or as a precursor of other synthetic chemicals (C2H6). Commercially, these hydrocarbons are separated in cryogenic distillation units. However, cryogenic distillation is expensive and hazardous. A past DOE study estimated that 0.12 Quads (1 Quad= 1015 BTU=1.055X1018 J) of energy is required annually for paraffin/olefin separation36,37. A state-of-art of ethylene distillation unit would cost over $500 million. Different other hazards of this process include both cryogenic hazard and flammability hazard associated with the accumulation of large amounts of flammable liquid. Adsorptive separation process offers an alternative technique, which is a benign, sustainable, and inexpensive in nature. To the best of our knowledge, this is the first report on the adsorption and


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separation of these gas mixtures on nanoporous boron nitride and the results suggested that boron nitride could be employed as a potential adsorbent for the separation of hydrocarbons.

2. Experimental

2.1 Synthesis of boron nitride Nanoporous boron nitride was synthesized by modifying the procedures reported by Weng et alError!

Bookmark not defined.

. In a typical synthesis, 1.12 g boric acid and 4.56 g

dicyandiamide were dissolved in 300 mL DI water and then the water was boiled off to obtain a homogenous mixture of these precursors. In each time, only half of the mixture was employed to synthesize one batch of boron nitride. The mixture was put in an alumina boat and inserted within the Lindberg-Blue tube furnace. It was pyrolyzed under an ammonia flow rate of 60 cm3/min. The furnace was heated from room temperature to 1050 °C at a rate of 10 °C/min and dwelled in the final temperature for 4 hr and 45 minutes. It was then cooled down to 500 °C in the same ammonia flow and 500 °C to room temperature in N2 flow. In order to complete the process, the obtained product was further heated in air to about 600 °C in 10 °C/min and cooled down to room temperature. The yield of boron nitride is ca. 20% with respect to the boron precursor (boric acid).

2.2 Characterizations The thermal stability of boron nitride was performed in TA instruments’ SDT Q600 thermal analyzer upto 1450 °C in both nitrogen and air flow. The samples obtained at 600 °C (air), 800 °C (air), 1000 °C (air), 1050 °C (N2) and 1450 °C (N2) were named as A600, A800, A1000, N1050 and N1500 and further analyzed in X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) to investigate the structural changes. X-ray photoelectron spectroscopy (XPS) was performed with a Thermo Scientific Model K-Alpha XPS instrument. The instrument uses micro-focused, monochromatic Al Kα X-rays (1486.6 eV) which can be focused to a range of spot sizes from 30 to 400 microns. Most samples are analyzed using the 400 µm X-ray spot size for maximum signal and to obtain an average surface composition over the largest possible area. The instrument has a hemispherical electron energy analyzer equipped with a 128 multichannel electron detection system. For non-conducting samples, surface charging is avoided by 5 ACS Paragon Plus Environment


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using an integral charge compensation system consisting of low energy electrons and low energy Ar-ions. The base pressure in the analysis chamber is typically 2 x 10-9 mbar or lower. Powder samples were dispersed onto double-sided tape affixed to glass slides. Sufficient powder was used to completely hide the mounting tape. Because glass slides were used for sample mounting, the charge compensation system was used during data collection. Wide energy range survey spectra (0-1350 eV) were acquired for qualitative and quantitative analysis using an analyzer pass energy of 200 eV and an energy step size of 1 eV. Assessment of chemical bonding of the identified elements was accomplished by collecting core level spectra over a narrow energy range using an analyzer pass energy of 50 eV and 0.1 eV step size. Data were collected and processed using the Thermo Scientific Avantage XPS software package (v 4.61). When necessary, spectra are charge corrected using the C 1s core level peak set to 284.6 eV. The XRD were performed in PANalytical X’Pert with CuKa radiation. The software used is Jade (2012) version 9.4.5. The database used was ICDD, PDF-4+ 2014. The original sample which was obtained at 600 °C were further characterized in details for electron microscopy (TEM) in Carl Zeiss Libra 120 TEM operating at 120 kV. The SEM image was obtained in Evex mini-SEM II HR 3000. 2.2 Pore textural properties and gas adsorption studies The pore textural properties, including Brunauer–Emmett–Teller (BET) specific surface area of the boron nitride sample was calculated by analyzing nitrogen adsorption-desorption at 77 K in a Quantachrome’s Autosorb-iQ instrument. The combined pore size distribution data in low and high pore widths was obtained by using non-local density functional theory (NLDFT) on the nitrogen isotherm at 77 K and CO2 adsorption isotherm at 273 K in the instrument’s built-in software. Adsorption isotherms of N2 and other hydrocarbons including CH4, C2H6, C2H4, C3H8 and C3H6 at 298 K and pressure upto 760 torr (1 bar) were measured volumetrically in the same instrument with 99.99% of each of the gas (AirGas). Except for 77 K, which was maintained by liquid nitrogen, all other temperatures were maintained by external Julabo temperature controller with 1:1 mixture of propylene glycol and water as chilling fluid and with the temperature accuracy of ± 0.05 °. The kinetic data were obtained in the vector dose mode of instrument.


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3. Results and Discussions 3.1 Characterizations

The nitrogen adsorption-desorption plot at 77 K is shown in figure 1(a). It is a type-I isotherm according to IUPAC classifications signifying a primarily microporous material. The pore size distribution was obtained by NLDFT technique. The overall pore size distribution was obtained by combining individual plots that were calculated from N2 adsorption at 77 K and CO2 adsorption at 273 K and shown in figure 1(b). The adsorption isotherm of CO2 at 77 K is sown in figure S1 of supporting information. It has a distributed micropore structure in the regions of 4.8, 5.2, 5.7, 8.2, 12.3, 14.7 and 18.5 Å along with minor narrow mesopore within 23 Å. Its BET surface area is 1360 m2/ g with total pore volume 0.632 cm3/g as calculated by the NLDFT method. Within the total pore volume, micropore volume is 0.48 cm3/g and the rest of the 0.15 cm3/g belongs to mesopores. The disordered porosity of this boron nitride sample is quite similar to that of activated carbon materials.

Thermal stability of the BN powders are investigated by thermogravimetric analyzer (TGA) under both nitrogen and airflow and a temperature upto 1450 °C. (figure 2). Under nitrogen, nanoporous BN lost about 8 wt.% upto 100 °C that can be attributed to the adsorbed moisture within its pores. Further increase to 270 °C resulted in an additional weight loss of about 7-8 % that may be attributed to the loss of some chemisorbed species, like hydroxyl or amine group that was originated during the synthesis phase38. Part of chemically bonded carbon may also decompose at this stage. The weight of the sample remained almost constant upto 1120 °C with total weight loss less than 1 %. From 1120 °C, it gradually started to lose weight and lost about 8 % of weight upto 1450 °C which was the final obtainable temperature in our TGA. Most likely, bonds of oxygen atoms with boron or carbon have been broken at this stage. A different scenario was observed for thermal analysis under air. Although it went a slight weight loss at 100 °C and 270 °C similar to that of nitrogen environment was observed, it started gaining weight from about 900 °C and continued to gain till it reached a plateau at about 1150 °C. Such increase is attributed to the reaction of BN with oxygen of air to produce boron oxide (B2O3) leaving nitrogen by the following reaction: 7 ACS Paragon Plus Environment


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…..(1)

Figure 1. N2 adsorption-desorption on boron nitride 77 K (a) and pore size distribution (b). It was suggested that some oxides of nitrogen (NOx) might also evolve in the course of the reaction39. Owing to higher mass of B2O3 compared to 2BN, an increase in weight is registered in the thermogravimetric analysis. As suggested by XRD (discussed later in this section), boron oxide may further react with aerial moisture to form boric acid and result in additional increase in weight as observed in TGA. Although the past literature suggests that there could be two 8 ACS Paragon Plus Environment

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Figure 2. Thermogravimetric analysis (TGA) of boron nitride in N2 and air. Each arrow indicates the point where the sample was collected for further analysis. A/N stands for air/nitrogen. distinct regions of increase, 760 °C to 860 °C for primary BN oxidation and 860 °C to 1000 °C for active bulk BN oxidation40, our study revealed a much higher oxidation resistance. In our study, oxidation did not start before 900 °C and attained the final state at 1150 °C. The total gain in weight due to oxidation is about 10 % and 22 % compared to initial mass and mass prior to oxidation (900 °C), respectively. The oxidized mass remained constant in the plateau up to about 1220 °C, after which it started to lose weight very rapidly till the final temperature of thermal analysis (1450 °C). Past literature on the TGA of BN at this high temperature was not reported and hence a direct comparison with previous literature was not possible. Such a loss may be a combined action of further thermal decomposition of boric acid to boron oxide along with sublimation of boron oxide and its partial reaction with traces of moisture in air to form volatile HBO241. The XPS results of the BN samples are shown in figure 3. Presence of BN functionality can be detected from B-1s and N-1s peak deconvolution (fig 3(a)). Both A600 and A800 samples have a uniform quantity of B and N atom, which are 37-38 % and 26 at.%, respectively. These 9 ACS Paragon Plus Environment


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B-N

(a)

N-B

B 1s

B 1s

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(b)

N 1s A1050

B-O A800

N-O 198 194 190 186 Binding Energy (eV)

A600

(c) N1450

N 1s

A1050

N1050 198 194 190 186 408 404 400 396 408 404 400 396 Binding Energy (eV) Binding Energy (eV) Binding Energy (eV) Figure 3. B-N bonds of thermally treated boron nitride obtained from B1s and N1s spectra in XPS (a). B1s spectra of A1050 sample (b), left-over N1s spectra of A1050 (the peaks are normalized) (c). samples also have a significant amount of carbon, about 14 to 18 at.% that might have originated from dicyandiamide. A600 and A800 also have 17 and 20 at.% oxygen that are mostly chemically attached to boron and nitrogen as B-O, N-Ox or C-O functionality. The slight increase in oxygen in A800 compared to A600 may be attributed to partial onset of conversion of boron nitride to boron oxide. In A1050, there is a significant decrease in nitrogen content to 1.7 at.% (fig 3(b)) along with increase in oxygen content to 47 at.% that supports the conversion of boron nitride to boron oxide or boric acid. Part of oxygen was also bound to carbon in the form of C-O or O-C=O as observed in XPS. It was not possible to collect the samples for XPS above the temperature of 1050 °C and the final temperature of 1450 °C as it was stuck to the porcelain boat in the form of thin film which made it impossible to separate without contamination. For the BN samples under thermal treatment in N2 environment (N1050 and N1450), the B and N ratio remained fairly stable suggesting only a minimal chemical change in the B-N bond structure of samples. Only a change of 4 at.% of oxygen suggests that part of the oxygen bonds were broken 10 ACS Paragon Plus Environment

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at the high temperature. Large amount of carbon and nitrogen in this boron nitride sample (A600) suggested that dicyandiamide was not completely removed in the course of synthesis. XPS studies revealed that further heating of sample in air or nitrogen would not have created a more purified boron nitride sample. A possible course of action could be heating the reaction mixture in ammonia atmosphere in even higher temperature that what we achieved in our study. But our furnace did not allow us to reach the higher temperature and hence we could not minimize the level of impurities from boron nitride sample.

Powder X-ray diffraction patterns (PXRDs) of boron nitride samples obtained under different temperatures of nitrogen and air are shown in figure 4. The miller indices of the peaks were assigned according to the ICDD, PDF-4+ 2015, International Centre for Diffraction Data, Newtown Square, PA.

Figure 4. XRD patterns of thermally treated boron nitride samples. The grey indices in A1050 sample belongs to sassolite phase. The inset figure shows the magnified view of three samples A800, A600 and N1050. 11 ACS Paragon Plus Environment


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The diffraction patterns of A600, A800 and N1050 are very similar. In these samples, the large and board peak at about 2θ=26° was assigned to (002) reflection of graphite-like hexagonal boron nitride (h-BN) structure. The weaker peak at about 42° was ascribed to (100)/(101) reflection42,43, and similar to that of porous and disordered carbon materials44. Two very weak peaks at about 75.5° and 81.7° were attributed to (110) and (112) reflections and also observed in the literature42. In N1450, where boron nitride received the highest temperature in N2 atmosphere, the peak at (002) became quite sharper, probably caused by the better ordering of the hexagonal phase. (100) and (110) reflections also became slightly sharper, but not as sharp and as the (002) reflection. Strengthening of (002) and (100) and (110) peaks with the increase in temperature were also reported in literature42. Past studies also revealed the splitting of (100) peak into four peaks at the elevated temperature that were attributed to the onset of rhombohedral phase (r-BN)43. Such splitting was not observed in our samples suggesting that the r-BN phase was not present. A1100, which experienced the highest temperature in air, revealed a complete change in the XRD pattern and the majority of peaks were assigned to the sassolite (mineral of boric acid) phase. XRD data provides the possible information that the BN sample was stable upto 800 °C in air and 1050 °C in nitrogen. The overall results of XRD also suggest that the boron nitride sample obtained in our study is very amorphous and quite similar to that porous amorphous carbon or activated carbons.

Electron microscopic images, including both TEM and SEM are provided in figure 5. The high magnification image of BN shows the random layers of boron nitride within the matrix. Electron energy loss spectra (EELS) show a series of peak at ~195-219 eV, signifying the Kedge loss of boron. These peaks originate from π* and σ* antibonding orbitals of boron and representative of sp2-like ordered boron nitride45. The peak at ~405 eV signifies the K-edge for nitrogen. EELS spectra also reveals that the presence of K-edge peaks attributed to the π* and σ* antibonding orbitals of carbon in 286 and 298 eV, respectively. The presence of such peaks confirmed the presence of carbon in boron nitride system, which was also observed in the XPS study. As the K-edge peak belonged σ* is much stronger compared to π* peak, it can be argued that carbon present in BN system is mostly in amorphous or diamond-like structure.


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(a)

(b)

5 nm

0.5 μm

B-K σ*(C)

(d)

(c)

π*(C) N-K

50 μm

Figure 5. Electron microscopy images of BN (A600). High magnification TEM (a), Low magnification TEM (b), EELS spectra obtained from TEM (c), and SEM image (d) 3.2 Gas adsorption studies The gas adsorption isotherms of N2 and hydrocarbons at 298 K and pressure up to 760 torr are shown in figure 6. Boron nitride adsorbed larger amount of methane (CH4) compared to that of N2 and hence it is selective to CH4 for the separation of CH4/N2, which is also similar to other adsorbents. For other hydrocarbon adsorption, it is clear that the adsorbed amount increases with the increase in molecular weight of hydrocarbon and the general trend in adsorption is N2 < CH4 < C2H4 < C2H6 < C3H6 < C3H8. Increase in adsorbed amount with the increase in molecular weight may be attributed to the fact that adsorption is solely governed by the classical dispersion forces. In addition, the adsorption amounts at a given pressure increase with the boiling point of the adsorbing species: 77K (N2), 111K (CH4), 169K (C2H4), 185K (C2H6), 225K (C3H6), and 231K (C3H8) at 760 torr. This is expected since high boiling point implies that the species prefers to be in the condensed or adsorbed phase. In terms of separation of ethane from methane, it is selective to ethane, whereas in terms of separation of paraffins (C2H6/C2H4 or C3H8 /C3H6), it is 13 ACS Paragon Plus Environment


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selective to paraffins. Higher selectivity towards ethane for methane-ethane separation is similar to that other adsorbents reported in

Figure 6. Isotherms of hydrocarbon adsorption onto boron nitride at 298 K and upto 760 Torr (1 bar). Open and closed symbols represent adsorption and desorption, respectively. literature, like different activated carbons 46,47, CMK-3 48 , CMK-5 49 , silica (SBA-1548), MOF (UTSA-34-b50) mesoporous polymeric organic framework51, pillar clays52 or zeolites, like Linde 4A53 and ETS-1054. For paraffin-olefin separation, higher selectivity towards paraffin is, in fact, reverse to that of most of the adsorbents that reported more selectivity towards olefins, except ZIF-755. Different varieties of MOFs, like Co, Mg and Fe-MOF-7456, CuBTC56, UTSA-30a57, Ag (I) functionalized Porous Aromatic Framework (PAF) 58 , or some zeolites, like ETS-10 59 and CuCl-doped NaX60. The higher affinity towards olefins may be attributed to the open metal-sites or π-π complexation between π orbitals of olefins and s and d-orbitals of metal ions present in the sorbent. There are a few MOFs, including ZJU-48a61 and UTSA-33a62 that demonstrated higher 14 ACS Paragon Plus Environment

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selectivity towards ethane compared to ethylene within a certain pressure range. In activated carbon, the ethane adsorbed amount was only marginally higher than ethylene46. In terms of comparing the total adsorbed amount of the hydrocarbons under similar conditions, we also compared to the overall uptake of hydrocarbons. Although methane uptake amount is within the range of a large number of adsorbents, only a few of them possessed a higher adsorption, like Co or MgMOF-7456. Furthermore, total uptake amount of hydrocarbon increases with the increase in molecular weight of the hydrocarbons, especially for paraffins. Ethane or propane uptake amount was higher than the majority of adsorbents (MOFs), like UTSA-30a57, UTSA-33a61, porous aromatic framework58, ZIF-755 or porous carbon47. Similar to methane, ethane and propane adsorption also is lower than the two MOFs of Co and MgMOF-7456. To the best of our knowledge, the paraffin-olefin separation was not reported in any adsorbents other than metal containing ones, like zeolites or MOFs. Most likely, it is the first report on paraffin-olefin separation in non-metal containing adsorbents.

The kinetics of hydrocarbon adsorption is shown in figure 7. In this figure, mt and m∞ are the mass adsorbed at time t and saturated amount, respectively. The vector dose mode calculated the adsorbed mass of the gaseous components by the following equation according to ideal gas assumption.

Adsorbed Mass (mt ) =

Volume Adsorbed x Molecular Weight … (2) 22.414

It is observed that the kinetics of adsorption became slow with the increase in molecular weight of hydrocarbons, which essentially suggest that the olefin adsorption is faster than that of paraffins. For micropore-dominated adsorption, the following equation is commonly used63,

1−

 −π 2 Dct  mt 6 = 2 exp  m∞ π  rc 2 

…(3)



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Figure 7. Kinetics of hydrocarbon adsorption in boron nitride

Where Dc is the intracrystalline diffusivity and rc is the intracrystalline radius. The Diffusive

 m  time constant ( Dc / rc 2 ) can be calculated from the slope of ln  1− t  versus t plot (within the  m∞ 

Dc / rc 2 )i ( mt value of 60 to 90%). The kinetic selectivity (Sk) can be calculated as , i and j are m∞ ( Dc / rc2 ) j the faster and slower adsorbing component, respectively. The values of diffusion time constants and kinetic selectivity are given in table 1.


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Table 1. Values of diffusive time constants and kinetic selectivity

Diffusive

CH4

C2H6

C2H4

C3H8

C3H6

Time

0.1114

0.0072

0.0172

0.0019

0.0103

CH4/ C2H6

C2H4/C2H6

C3H6/C3H8

15.49

2.39

5.41

Constant

Dc / rc 2 , s

-1

(D / r ) Kinetic Selectivity (Sk), (D / r ) 2

c

c

c

c

i

2

j

For equilibrium adsorptive separation of two components through pressure swing adsorption (PSA), the equilibrium selectivity ( ) of component 1 (stronger adsorbate) over component 2 (weaker adsorbate) is defined as: ⁄

⁄ ……………(4)

where and are the mole fractions in adsorbed phase and bulk gas phase, respectively. It is a common practice to calculate the selectivity from pure component adsorption data (such as the pure-component adsorption isotherms shown in figure 6) by Ideal Adsorbed Solution Theory (IAST) as a function of total pressure. The simplest form of this theory assumes that the gas phase is an ideal gas and the adsorbed phase is an ideal solution. Hence, the compositions of the gas and the adsorbed phase are related with equations resembling Raoult’s law of chemical thermodynamics. Figure 8 shows the selectivity values of different gas pairs as a function of total pressure for the case for which the mole fraction of the higher adsorbing species is fixed to 0.15. Within each gas pairs designated in the figure legend, the first gas is the higher adsorbing species. As observed in the selectivity plot, the selectivity for C2H6/CH4 lies within 60 to 20, which, to the best of our knowledge, it is one of the highest reported in the literature. For the majority of adsorbents, such selectivity varies within 4 to 20, including porous carbons47,65 several types of MOFs51,56,64 (both experimental and simulated) or other porous materials52. So far, barium-doped titanosilicate-type of zeolite (ETS-10) has demonstrated the highest selectivity54; single point selectivity based on Henry’s law is 15-52, which is close to that of our selectivity values. As mentioned earlier, separation of CH4 from N2 is part of the inert rejection 17 ACS Paragon Plus Environment


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Page 18 of 35

Figure 8. Selectivity of higher adsorbing gases calculated by Ideal Adsorbed Solution Theory (IAST) as a function of pressure. In the figure legend, the first gas in each of the gas pairs is the higher adsorbing species. For the all the pairs, the mole fraction of each of the higher adsorbing gas pairs is selected as 0.15 process in the natural gas purification. According to IAST calculation, the highest CH4/N2 selectivity is about 10. Although depicted by only a marginal difference, selectivity of CH4/N2 on boron nitride is one of the highest in the literature along with UTSA-30a that also demonstrated the same highest selectivity of 1057. According to a comparative study made by Saha et al.65,66 the CH4/N2 selectivity of the other adsorbents lie within the range of 2 to 7, including different MOFs, zeolites, porous carbon and graphene. So far, the higher selectivity for CH4/N2 is shown by MOF-17767, MOF-5 and graphene, which are in the range of 6 to 7.

In Paraffin-olefin separation, boron nitride demonstrated a good selectivity for C3 hydrocarbons. As mentioned earlier, it represented a higher selectivity towards paraffin compared to olefin unlike the majority of the adsorbents reported in the literature. As the higher selectivity towards paraffin is quite rare in the literature, for convenience, we directly compared 18 ACS Paragon Plus Environment

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our paraffin selectivity values with that of olefins to evaluate the overall the separation.

The

highest selectivity for C2H6/C2H4 is about 2.2. Although the experimental as well as simulation studies on HKUST-1 reported the same selectivity68, several new varieties of MOFs have been developed that demonstrated very high selectivity of 2758. Although C2 paraffin/olefin selectivity was not good for boron nitride, it showed an excellent selectivity for C3 paraffin/olefin separation, i.e., propane/propylene. According to our results, the highest selectivity for C3H8/C3H6 is about 23. To the best of our knowledge, this is one of the highest selectivities for this pair of hydrocarbons and one order of magnitude higher compared to that of other adsorbents reported so far. For HKUST-1 (CuBTC-1), the selectivity is 2 to 5.55 69,70,71. For another MOF (Fe3OFm(OH)n(btc)2), the selectivity was less than 5.1 72 . A series of MOFs composed of 1,2,4,5-tetrakis(carboxyphenyl)benzene and trans-1,2-dipyridylethene struts 73 demonstrated the kinetic selectivity in the range of 1.4 to 12. The affinity of boron nitride towards heavier hydrocarbons may be attributed to the stronger interactions between hydrogen within hydrocarbon molecule and heteropolar boron nitride along with its hydrophobicity74 and wide energy band gaps75,76,77. Studies revealed that the depth of potential of hydrogen adsorption in boron nitride is higher than that of carbon materials. It was also demonstrated that structural defects and zigzag edge sites of boron nitride might enhance its binding energy towards hydrogen78. It is to be noted that our boron nitride sample has significant fractions of carbon and oxygen as impurities. These additional chemical functionalities may also play role in the adsorption of hydrocarbons. Although it is an arduous task to compare all the selectivity values in one picture owing to the different variables, techniques and constraints used in the calculation steps, we made an effort to put the selectivity values for C2H6/CH4, CH4/N2 and C3H8/C3H6 in one plot (figure 9) to make a comparison with literature. It is observed that for all the three gas pairs, boron nitride demonstrates a relatively higher selectivity. We did not compare such values for C2H6/C2H4 as our selectivity is quite lower than that of majority of literature values.

Using the same IAST theory, it is also possible to calculate the amounts adsorbed, q1 and q2 , in the porous material. These amounts are given as and where the total

amount adsorbed, q , is determined from the ideal solution formula

∗ ∗

(5)



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Figure 9. Comparison selectivity values for C2H6/CH4 (a), CH4/N2 (b) andC3H6/C3H8 (c) with literature. Reference notation- Fig 9(a): Ba/H-ETS-10, Ba-ETS-10, Na-ETS-10: Ref. 57; from NOTT-102 to UTSA-34a: Ref. 59; UTSA-33: Ref. 65, MesoPOF-1 to 3: Ref. 54, ZJU-48a: Ref. 64. Fig 9(b): La(COOH)3: Ref 60, Mesoporous carbon: 50; Al-PILCs: Ref. 55, Linde-4A: Ref. 56; ZIF68/69: Ref. 67; MOF-177: Ref. 70; CMK-5: Ref. 52; Fig 9(c): CuBTC (M1, M2 and –com): Ref.72; Cu3(BTC)2: 74; Fe3OFm(OH)n(btc)2: Ref. 75; DTO to ground DBTO: Ref. 76.


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where ∗ and ∗ correspond to the adsorbed amounts of the pure gases at the same temperature and spreading pressure (surface potential) as the adsorbed mixture. The values of ∗ and ∗ are obtained from the experimental pure-component adsorption isotherms shown in figure 6. To facilitate the computations, the pure component isotherms in figure 6 are fitted to SIPS types of forms. The amounts adsorbed are shown in figure 10. As seen in figure 10, the highest and lowest amounts adsorbed correspond to C3H8/C3H6 and CH4/N2, respectively. Another important parameter related to the efficiency of the materials as potentials adsorbents is the so-called breakthrough time, which is the time required to achieve separation in a column (packed bed) filled with porous adsorbent. As the multicomponent gaseous mixture flows through the bed the species diffuse and adsorb onto the porous solid at different degrees. Modeling of the breakthrough behavior of an adsorption column is a considerable complex process. Thus, certain simplifying approximations are necessary79,80,81. It is usually assumed that the column operates isothermally with negligible pressure drop and the flow pattern resembles that of plug flow. Furthermore, diffusion effects are often neglected and the gas phase, which is assumed to be an ideal gas, is thus at thermodynamic equilibrium with the adsorbed phase.

Under these simplifying approximations, the process can be described by a set of two first-order partial differential equations, namely, a mass balance for the strong adsorbate as well as an overall mass balance that can be solved by the method of characteristics. The details of the solution can be found in our previous work82. For the case for which the inlet concentrations are step functions, the outlet concentrations will also be step functions characterized by a time lag, tbreak , which defines the breakthrough time required to achieve separation. The breakthrough time

is usually expressed in dimensionless form as τ = u t break ε L where u is the inlet velocity and L and ε are the column length and porosity, respectively. The dependence of the dimensionless breakthrough time on the mole fraction of the strong adsorbate and the total gas pressure for all binary mixtures studied in this work are found in figure 11. As is evident from figure 11, the breakthrough time, τ , is a decreasing function of the mole fraction of the strong adsorbate and the total pressure. Furthermore, the highest and lowest values of τ correspond to C3H8/C3H6 and CH4/N2, respectively. Therefore, from figures 10 and 11 it can be inferred that the values of τ 21 ACS Paragon Plus Environment


correlate monotonically with the equilibrium amounts adsorbed onto the porous material.

Adsorp on amount (mmol/g)

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Page 22 of 35

0.7

3.5

0.6

3

0.5

2.5

0.4

2

0.3 0.2 0.1

C2H6 CH4 Total

1.5

CH4 N2 Total

1 0.5 0

0 0

0.2

0.4

0.6

0.8

0

1

3.5

6

3

5

2.5

0.2

0.4

0.6

0.8

1

4

2

3

C2H6 C2H4 Total

1.5 1 0.5

C3H8 C3H6 Total

2 1 0

0 0

0.2

0.4

0.6

0.8

1

0

0.2

0.4

0.6

0.8

1

Mole Frac on of Higher Adsorbing Component Figure 10. Simulated (IAST) Binary gas adsorption data for CH4/N2, C2H6/CH4, C2H6/C2H4 and C3H8/C3H6 at 298 K and 760 Torr. The higher adsorbing species is the first gas of each pair (red circles).


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Figure 11. Dependency of dimensionless breakthrough time ( ! ) on mole fraction of higher adsorbing species (a) and total gas pressure in the inlet or feed (b). In the figure legend, the first gas in each of the gas pairs is the higher adsorbing species.



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Page 24 of 35

4. Conclusions In this work, nanoporous (primarily microporous) boron nitride was synthesized with a BET surface area 1360 m2/ g. Thermogravimetric analysis along with XPS and XRD revealed that the boron nitride structure was stable up to 800 °C in air and 1050 °C in nitrogen. Nitrogen and hydrocarbon adsorption at 298 K and pressure up to 1 bar suggested that hydrocarbon adsorption was higher than nitrogen and adsorption of hydrocarbon increases as its molecular weight increases. Such a trend suggested that boron nitride is selective to the paraffins and hence may act as an “olefin generator”. The IAST selectivity for CH4/N2, C2H6/CH4, and C3H8/C3H6 was very high and probably higher than previous literature values. With the help of simple modeling, the breakthrough time was calculated and reported as function of total pressure and mole fraction of the higher adsorbing component. The overall results suggest that nanoporous boron nitride can be used as potential adsorbent for hydrocarbon separation.

Acknowledgements This work is partly supported by American Chemical Society sponsored Petroleum Research Fund (ACS-PRF, Grant Number 54205-UNI10). D. Saha also acknowledges the startup funding and faculty development award from School of Engineering (SOE), Widener University. TEM (J.C. and H.C.H.) experiments were partially conducted under the user proposal (CNMS2016-302) at the Center for Nanophase Materials Sciences, ORNL, which is a DOE Office of Science User Facility. The authors acknowledge Harry M. Meyer III, ORNL, for his assistance in XPS. D.S. acknowledges Karl A. Nelson, Widener University for assistance in capturing SEM image.


Page 25 of 35

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