Langmuir 1991, 7,2844-2846
2844
Highly Microporous Boron Nitride for Gas Adsorption Theodore T. Borek, William Ackerman, D. W. Hua, Robert T. Paine, and Douglas M. Smith* Department of Chemistry, Department of Chemical and Nuclear Engineering, and the UNMINSF Center for Micro-Engineered Ceramics, University of New Mexico, Albuquerque, New Mexico 87131 Received April 19,1991 Boron nitride materials with surface areas of 53-696 m2/g and pore volumes 0.036-0.390 cm3/g have been produced by vacuum pyrolysis of poly(4,6-borazinylamine)polymerized under dilute conditions. These samples exhibit type I isotherms, indicating that a majority of the porosity occurs in pores of size less than 1.0nm. The surface areas and pore volumes are maximized at pyrolysis temperatures of 800 "C, and are essentially eliminatedby 1200"C. Small-angleX-ray scattering (SAXS)measurements confirm this change in structure with pyrolysis temperature.
Introduction Boron nitride and carbon share common allotropes;there exists a hexagonal, graphitic form and a cubic, diamond form of boron nitride. The major difference between boron nitride and carbon is in the nature of the interatomic bQnding of the materials. There is a polar character to the boron-nitrogen bond that does not exist in the carboncarbon bond in graphite or diamond. Carbon materials are used extensively as gas adsorbents due to their high surface areas and the possibility of modification to introduce polar sites to the carbon surface.' Since boron nitride inherently possesses a polar character on ita surface, a synthetic scheme that produces highly microporous boron nitride is of interest for possible use as gas adsorbents. Metallurgicallyprepared boron nitride powders typically exhibit surface areas of 10-15 mz/g.z Boron nitride produced from poly(2,4,6-borazinylamine)(1) has a surface area in the range of 30-50 mz/g for powders produced at 900 0C.3 Boron nitride aerogels, which are low-density,
(1)
highly porous ceramic bodies based on critical point drying of poly(2,4,6-borazinylamine)gels, heated to 10oO "C have surface areas of -400 m2/g.4 These boron nitride aerogels have been shown to adsorb Hz,02,COZ,and CO.5 In an effort to increase the microporous surface area of the boron nitride materials by allowing greater collapse of the gel network during drying, a borazine polymer with fewer points of connectivity was designed. The candidate polymer chosen for these proposed high surface area (1) B a n d , R. C.; Donnet, J.-B.; Stoeckli, F. Active Carbon; Marcel Dekker, Inc.: New York, 1988. (2) Technical Data Sheet, Boron Nitride; ESK Engineered Ceramics Wacker Chemicals (USA) Inc. (3) Narula, C. K.; Schaeffer, R.; Paine, R. T.; Datye, A.; Hammatter, W. F. J. Am. Chem. SOC.1987,109,5556-5557. (4) Lindquist, D. A.; Borek, T.T.; Kramer,S. J.;Narula, C. K.; Johnston, G.; Schaeffer, R.; Smith, D. M.; Paine, R. T. J. Am. Ceram. SOC.1990, 73 (3), 757-760. (5) Lindquist, D. A. Ph.D. Thesis, University of New Mexico, Albuquerque, NM, 1990, pp 67-78.
0743-7463/91/2407-28444$02.50/0
materials is poly(4,6-borazinylamine) (2). Poly(4,b-borazinylamine) has two sites of cross-linking and should
r H 1 (2)
result in a more compliant (i.e., lower pore volume) polymer network after drying. It has already been demonstrated that this polymer network can be subsequently vacuum pyrolyzed at a range of temperatures to form BN6 and possibly, develop microporosity.
Experimental Section All reactions were carried out under a dry, inert atmosphere utilizing standard Schlenktechniquesor a Vacuum Atmoepherea glovebox. All solvents were dried under Nz by usual methods and vacuum distilled immediately prior to use.' 2,4,6-Trichloroborazine (HsN&Cld was synthesized as described in the literature.8 Dimethylamine [ (H&)zNH] was used as received from Matheson Co. Hexamethyldisiie and H&THF in THF were used a receivedfrom Aldrich Chemical Co. Infraredspectra were recorded from KBr pellets on a Mattson Galaxy Series 2020 FT-IR. Specific surface area, total pore volume, and pore size distributions were determinedfrom NZadsorption/condensation at 77 K. Surface area were determined by using BrunauerEmmett-Teller (BET) analysis (five points, 0.05< P/Po < 0.35) and a molecular cross-sectional area of 0.162 nm2. Micropore surfacearea and micropore pore volume were estimated by using deBoer's t-plot analysis! Small-angle X-ray scattering (SAXS) data were collected by using a Rigaku SAXS system having a Kratky U-slit. The incident beam wavelength was 1.54 A of Cu K a radiation. The intensity of the scattered X-ray was counted with an M Braun position sensitive detector system. The data were then corrected for the slit collimation, in order to evaluate the particle size and the fractal dimension by the behavior in the Guinier and Porod regions. (6) Narula, C. K.; Schaeffer, R.; Datye, A. K.; Borek, T. T.; Rapko, B. M.; Paine, R. T. Chem. Mater. 1990,2, 384-389. (7) The Chemist's Companion: A Handbook of Practical Data, Techniques, and References; Gordon, A. J., Ford, R. A., Eds.; WileyInterscience: New York, 1972. (8)Niedenzu, K.; Dawson, J. W. Znorg. Synth. 1967, 10, 139. (9) deBoer, J. H.; Linsen, B. G.; van der Plus, T.; Zondervan, G. J. J. Catal. 1965, 4 , 649.
0 1991 American Chemical Society
Langmuir, Vol. 7, No. 11, 1991 2845
Highly Microporous Boron Nitride for Gas Adsorption
N2 77K Isotherm
.
800°C Materlal
m
I
.""
-#-+I 0.0
1000°C Mater a1
0.2
0.4
0.6
0.8
1 .o
PIP0
L 4000
-
r
I
3500
3000
Figure 2. Typical nitrogen adsorption isotherms (77K)for 800 and lo00 "C BN xerogels. 2500
2000
1500
1000
500
W?JvCnUmDOPO
Surface Area
Figure 1. FTIR spectra for BN xerogel pyrolized a t 800 "C showing residual N-H. Synthesis. The syntheses of 2-(dimethylamino)-4,6-dichloroborazinelO and poly(4,6-b0razinylamine)~have been described elsewhere, but are included here due to subtle changes in the synthetic scheme that greatly influence the microporosity. 2-(Dimethylamino)-4,&dichloroborazine. A sample, 6.935 g (38.0 mmol) of 2,4,6-trichloroborazine was dissolved in 120 mL of dry diethyl ether and the solution stirred and cooled to -78 "C. To the cooled solution was added 3.402 g (5 mL, 75 mmol) of (H&)2NH. The reaction mixture was allowed to warm to room temperature overnight, and the mixture was filtered under nitrogen. The filter residue was washed with diethyl ether (2 X 50 mL) and the solvent removed by vacuum evaporation. The crude product was vacuum sublimed, extracted with hexane, and vacuum sublimed once more, resulting in a pure white product. Yield: 4.573 g (63%). Poly[ 2-(dimethylamino)-4,6-borazinylamine]. A 3.649-g (19mmol) was portion of 2-(dimethylamino)-4,6-dichloroborazine dissolved in 120 mL of CsHSCl and stirred. To this solution was added 3.060 g (4 mL, 19 mmol) of hexamethyldisilazane in one portion at room temperature. There was immediate formation of a white gel after the addition was completed. The reaction mixture was then refluxed for 12 h and cooled to room temperature, and the volatiles were removed by vacuum evaporation. Yield: 2.50 g. Poly(4,6-borazinylamine). In a typical experiment, 3.066 g (22.5 mmol) of poly[2-(dimethylamino)-4,6-borazinylamine] was suspended in 120 mL of diethyl ether, HsBeTHF (45 mL, 1.0 M solution)was added via gas-tightsyringe,and the reaction mixture was stirred for 24 h. The reaction volatiles were removed by vacuum evaporation, leaving 2.3 g of white solid. Pyrolysis. A 0.70-g portion of poly(4,6-borazinylamine)was pyrolyzed using a Lindberg Model 54032 tube furnace under vacuum in quartz tubes equipped with a valve. Samples were heated to 600,800,900,1000,and 1200 "C. The heating rate was 6 OC/min, and the duration at maximum temperature was 12 h. Typically, 0.30 g of boron nitride material was recovered.
600
2 P a
f
400
m m
In 3
200
0
Figure 3. Variation of total and micropore surface area with pyrolysis temperature.
Results and Discussion The major difference in the synthesis of poly(4,6-borazinylamine) in this work from previous studies6 is the amount of solvent used in the polymerizationstep. Using any amount less than that mentioned in the synthesis step results in boron nitride materials with little surface area and no micropore area.ll An infrared spectrum of the 800 "C pyrolysis product is shown in Figure 1. This spectrum is characteristic of amorphous boron nitride derived from preceramic polymers with absorptions at 1398 and 800
cm-l.12 There is a residual N-H stretch a t 3440 cm-l which results from incomplete transformation of the polymer to pure crystalline BN at this temperature. From our previous work with these systems, there is often little crystallinity in the boron nitride materials heated to less than 1200 "C. These materials may be considered amorphous and at this stage may closely resemble the activated carbon structure. Typical nitrogen adsorption isotherms for samples heated at several temperatures are displayed in Figure 2. The shape of the isotherm is classified as type I by Brunauer and is typical of materials with a very fine pore structure.13 Figure 3 contains a comparative graph of the total surface areas and micropore surface areas for all of the boron nitride materials in the study. Micropore surface area and pore volume are calculated using the t-plot method of d e B ~ e r . The ~ materials heated to less than 1200 "C have high total and micropore surface areas, with a maximum at 800 O C . The micropore area is 8370 of the total surface area. A comparativegraph of the totalpore and micropore volumes is shown in Figure 4. The pore volumes follow the surface area data: There is a maximum at 800 "C, and there is essentially no micropore volume at 1200 "C. Preliminary SAXS measurements show a similar trend with pyrolysis temperature. Figure 5 displays Porod plots for the 800 and 1200 "C samples. The Porod slope for all samples studied by SAXS is given in Table I. This correspondence between surface area and the magnitude of the Porod slope has been previously reported for carbon materials exhibiting a wide range of surface area (2-1500
(10) Beachley, 0. T., Jr.; Durkin, T. R. Znorg. Chem. 1974, 23 (71, 1'768-1770. (11) Borek,T.T.;Ackerman, W.;Pane,R.T.;Smith, D. M. Unpublished results.
(12) Nyquist, R. A.; Kugel, R. 0. Znfrared Spectra of Inorganic Compounds (3800-45 cm-2); Academic Press: New York, 1971. (13) Brunauer, S. Physical Adsorption of Cases and Vapors; Oxford University Press: Oxford, 1944.
2846 Langmuir, Vol. 7, No. 11, 1991
Borek et al.
Pore Volume
p f
oo-oo
-____
0.5
04
. 0 0
10.00
"03 0-
5
a
I!
$a
1200c
0.2 P 0
1 .oo 0.1
00 G'J
8CC s1-1
D
-
Rrc 5-3
I.
CIL?
3 r
1200
7
0.10
PVrolyrls Tempenlure. Dog "C
Figure 4. Variation of total and micropore pore volume with pyrolysis temperature. 0.01
1 0
q2
10.00
1 E-3
(A-2)
Figure 6. Guinier region small-angle X-ray scattering plots for 600 and 1200 "C BN xerogels. deviation from the normal Porod behavior (I(q)a q4) is
1.oo
0.10
1.00E-3
0.10
0.01
1.oo
4 Figure 5. Porod region small-angle X-ray scattering plots for 800 and 1200 "C BN xerogels. Table I. Variation of the Porod Slope with Pyrolysis Temperature temp, "C Porod slope temp, O C Porod slope 600 800
5E- 4
-3.2 f 0.2 -2.7 f 0.2
lo00 1200
-2.8 f 0.2 -3.1 f 0.2
m2/g).14 The 800 "C sample gives a maximum slope (-2.7) among all the samples. This value is the same as the Porod slope obtained for activated charcoal (-2.7), which is a typical material with very fine pore structure and large surface area.14 With increased processing temperature, t h e Porod slope decreases t o a b o u t -3.2. T h e (14)Hall, P. G.; Muller, S. A.; Williams, R. T. Carbon 1989,27,103.
probably due to the short-range density fluctuation within the solid,15 or the fractal character of the boundary separating the pore space and the solid.14J6 Figure 6 contains typical Guinier plots for these BN xerogels. The Guinier plot has a smooth changing curvature for q < 0.021 A-l, and then exhibits linearity. The shape of the Guinier plot is characteristic for a broad unimodel feature size distribution. These feature sizes are not to be confused with pore sizes. Below 1200 "C, the feature radii are estimated to be 60-65 A. A t 1200 "C, larger sizes are observed (-77 A). This increase of feature size corresponds to the formation of crystalline domains and the subsequent loss of surface area. In these preliminary results, we have shown that the pore structure of BN may be manipulated to yield a high surface area and primarily microporous solid. A number of processing parameters (drying rate, pyrolysis temperature/ time, solvent concentration, mixing two-point and three-point polymers, etc.) remain to be explored to optimize pore structure for particular adsorption applications. Also, a thorough study of the adsorption properties of these unique materials remains to be undertaken.
Acknowledgment. This work has been supported by the UNM/NSF Center for Micro-Engineered Ceramics which is funded by NSF (CDR-8803512),Sandia and Los Alamos National Laboratories, the New Mexico Research and Development Institute, and the ceramics industry. (15)Hoinkis, E.; Allen, A. J. Carbon 1991,29,81. (16) Schmidt, P. W. In The Fractal Approach to Heterogeneous Chemistry; Avinir, D., Ed.;John Wiley & Sons: Chichester,1989; p 67.
