Professor James Economy's Group

[Activated Carbon Fibers]  [Ion Exchange Fibers]  [Chelating Fibers]  [Membranes]  [Porous Inorganic Fibers]

Activated Carbon Fibers
 

 
Society's growing concern over contamination of the environment necessitates the development of greatly improved methods for controlling the release of toxic contaminants. Industry has taken a leadership role within the past 10 years to reduce the emission of contaminants into air and water. This has led to the development of a multi-billion dollar market for commercially available technologies for purifying these media. One technology which is extremely attractive is the use of dry adsorbents such as activated carbons.

Typically, activated carbon granules (ACGs) are used as adsorbents to purify polluted waste streams because of their low cost. However, the ACGs suffer from a number of drawbacks including poor selectivity, slow kinetics, expensive containment systems, less than 100 % working capacity, and the need for a costly high temperature reactivation. Also, there is a general lack of fundamental understanding of the ACG product which sharply limits the development of a predictive capability.

In the following section, to address some of these disadvantages, we describe a number of innovations which our group has devised to further improve the selectivity and cost-effectiveness of the activated carbon system.

Early Work on the Discovery of ACF

Starting in 1970 while at the Carborundum Co.  J. Economy developed and commercialized for the first time an activated carbon fiber (ACF) using a cross-linked phenolic fiber (Kynol) as precursor. The Kynol fiber had been commercialized  in 1969 as a flame resistant fiber. The fiber form of the ACF afforded greatly improved contact efficiency, much higher capacity and the potential for a high degree of design flexibility. In 1971, a joint venture was established by Carborundum with Mitsubishi Chemical and Kanebo in Japan to manufacture and commercialize the Kynol and ACF. Both fibers have been available commercially from Nippon Kynol Inc since that time.  Subsequently, the Carborundum Co. was acquired by Kennecott  in 1977 and ceased any activity in this area.  Economy left in 1975 to join IBM research where he focused on new materials for advanced microelectronic devices.

Control of Micropore Chemistry

In 1989, Economy joined the University of Illinois (Urbana) as head of Materials Science and Engineering.  Early on, he undertook to reexamine the ACF’s commercialized two decades earlier; namely, the high cost of the ACFs (~ $100/lb), their relative fragility, and the potential to control the pore surface chemistry. Research in this last area progressed rapidly and it was found that the selectivity could be greatly improved by chemically converting the  pore surface into acidic or basic and neutral or polar surfaces (Fig.1). The nitrided system (basic) displayed sharply increased capacities for HCl gas (Fig 2),  while the oxidized system (32% oxygen and rich in carboxylic acids) displayed a 20 X improvement in breakthrough times over untreated ACF’s for ammonia removal. (Fig.3).

Figure 2. Adsorption uptake for HCl gas on ammonia-treated ACF	Figure 3. Normalized breakthrough curves for ammonia on ACFs

Nature of Micropores

Starting in the early 1990’s a program was initiated to establish the validity of  the widely held view that  the pores in activated carbon were slit shaped. As part of this program we also wanted to determine how pores begin to form in the ACFs. Applying scanning tunneling microscopy (STM) to a number of different ACF’s we were able to access both  the surface of the fiber as well as the cross-section. The fiber surface in the commercial product was highly etched displaying a relatively high concentration of mesopores with varying diameters from 20-100 Å. Examination of the fiber cross-section showed a high degree of uniformity in the dimensions of the micropores (10-18 Å) depending on the degree of activation. (Fig.4) This  uniformity extended to within 100 Å of the fiber surface where the larger pore at the etched surface necked down to the more uniform porosity in the core. From this study there was no evidence for slit shaped pore even after an examination of over 800 STM’s  taken from the cores and surfaces of a number of different ACF’s.

Figure. 4. Scanning Tunneling Microscopy (STM) of (a) a cross-section and (b) a highly etched surface of activated carbon fiber (ACF)

An insight into the origins of the microporosity was afforded by an examination of a phenolic fiber heated to 600^oC in nitrogen. It was found that a micropore structure (8-10Å) was formed with a surface area of 600 m²/g. Hence the presence of a stable cross-linked structure which persists during heating to 600^oC (Fig.5) (weight loss 30%) was sufficient to preserve the microstructure (Fig.6).  Further heating up to 800^oC using an etchant such as CO₂/ H₂O led to much higher surface areas and introduction of several percent oxygen at the surfaces of the micropores.

Figure 5. Kynol fiber was activated in N2	Figure 6. Pore size distributions for the ACFs as calculated using the DRS equation.

New Invention: ACF on Fiberglass

A major advance concerning cost  and fiber durability was made in the mid 1990’s with the discovery that the process for preparation of the ACFs, could be greatly simplified using a fiber glass substrate coated with 30-70 % phenolic resin (Figs. 7 and 8). For example, with fiber glass paper ($0.4-0.5/ lb) and a phenolic resin precursor ($0.7/ lb). The cost of ACF’s could be greatly reduced from over $100/lb to values approaching those of GAC.   In addition, the wear resistance was improved dramatically by 30 X over the ACF’s (see Table below).  Presumably the intrinsic strength of the glass fiber core was protected by the surface coating of activated carbon. As can be seen in Figs 9 and 10, a comparison of  cartridges consisting of  similar amount of granular activated carbon and ACF on glass paper showed an enormous improvement  in removal of trace contaminants such as benzene to levels  below 1 ppb. The GAC filter which is designed for use in removal of benzene was ineffective in removing benzene down to the maximum contaminant level of 5 ppb ( a value designated by USEPA). It is noteworthy that the cartridge based on ACF could be regenerated repeatedly to  its original activity by heating to190^oC for 2 hrs under vacuum (Fig.11). Typically regeneration of GAC’s is carried out by heating to 800^oC which leads to an increase in pore diameter. This requires a further step involving chemical vapor deposition of carbon to recover the original pore diameter. As a result, the GAC’s are often disposed of in land field.

Areas of opportunity where our work has demonstrated clear advantages include: 1) the recovery of ammonia and amine compounds from industrial waste streams, 2) adsorption of trichloroethylene (TCE) which is air stripped from wastewater streams, 3) adsorption of herbicides from groundwater, 4) removal of CO₂ from closed-loop anesthesia systems, 5) adsorption of hazardous pollutants from air for use in respirators and chemically protective clothing, and 6) the adsorption of SO₂ from flue gas at coal fired plants.  

Figure 7. Synthesis of Activated Carbon Fibers on Fiberglass	Figure 8. Activated Carbon Coating on Glass Fiber Substrate

Table 1 Abrasion Resistance of  ACF on Glass Substrate vs ACFs

Figure 9. Design of ACF and GAC Filters

Figure 10. Breakthrough Curves for Benzene from both ACF and GAC Filters	Figure 11. Benzene Breakthrough Curves on Regenerated ACF Filter

Chemically Activated Fibers

In the last several years  , we have found that we can use a catalyst to convert a number of polymers to high surface area fibers at relatively low temperatures. This method which is referred to as chemical activation is carried out  by coating a glass fiber with polymers such as phenolic, polyacrylonitrile (PAN), polyvinyl alcohol (PVA), and cellulose along with a catalyst such as H₃PO₄, ZnCl₂, or  NaOH.   High surface areas from 1000-2500 m²/g are achieved by heating from 250-450^oC.  As opposed to ACF’s where yield of 5-25% are obtained during activation much higher yield can be obtained by chemical activation, for example, 90% (PAN), 80% (phenolic), 60 % (PVA) and 35% (Cellulose). Some unusual pore surface chemistry can be obtained, for example with PAN we form a polyquinizarine structure (19-20% N) which has significant basic character. In the case of PVA a significant amount of oxygen persists as hydroxyl groups which imparts a strong hydrophilic character to the surface of the micro/mesopores.  Presumably, with suitable selection of the precursor polymer,  we can change the chemistry of the micropore surface without having to resort to the high temperature treatment described earlier. The polyquinizarine coated glass fiber also appears to exhibit potential for air pollution control, namely for sequestering carbon dioxide.

Table 2. ACF Made with Chemical Activation

References

1.      Lin, R.Y.; Economy, J., "The Preparation and Properties Of Activated Carbon Fibers Derived From Phenolic Precursor", Appl. Polym. Symp. 21, 143-152, 1973,

2.      Economy, J.; Lin, R.Y., "Adsorption Characteristics Of Activated Carbon Fibers", Appl. Polym. Symp. 29, 199-211,1976,

3.      J. Economy. “Now that's an interesting way to make a fiber!” CHEMTECH 10(4), 240-7. 1980.

4.      Mangun, C.L.; Daley, M.A.; Economy, J. "Chemical Modification of Activated Carbon for Enhanced Removal of Toxic Contaminants," Air & Waste Management Association, 88th Annual Meeting, San Antonio, TX, June, 1995.

5.      J. Economy, M. A. Daley, E. J. Hippo, and D. Tandon. “Elucidating the pore structure of activated carbon fibers through direct imaging using scanning tunneling microscopy (STM)”. Carbon 33(3), 344-5. 1995.

6.      Daley, M.; Tandon D.; Economy, J.; Hippo, E.J. "Elucidating the Pore Structure of Activated Carbon Fibers through Direct Imaging using Scanning Tunneling Microscopy (STM)," Carbon, 34 (10), 1191-1200,1996.

7.      Daley, M.A.; Mangun, C.L.; Economy, J. "Low-Cost Activated Carbon Fiber Assemblies for Control of Contaminants from Air/Water," Air & Waste Management Association, 89th Annual Meeting, Nashville, TN, June, 1996.

8.      Daley, M.A., C.L. Mangun and J. Economy, "Predicting adsorption properties for ACFs," Preprints of Div. of Fuel Chem., 41(1), 326-330 1996.

9.      J. Economy, M. Daley, and C.L. Mangun. “Activated carbon fibers - past, present, and future”. Preprints of Papers - American Chemical Society, Division of Fuel Chemistry 41(1), 321-5. 1996.

10.  Daley, M.A.; Mangun, C.L.; DeBarr, J.A.; Riha, S.; Lizzio, A.A.; Donnals, G.L.; Economy, J. "Adsorption of SO2 onto Oxidized and Heat Treated Activated Carbon Fibers (ACF's)," Carbon, 35(3), 411-417, 1997

11.  Mangun, C.L.; Daley, M.A.; Braatz, R.D.; Economy, J. "Effect of Pore Size on Adsorption of Hydrocarbons in Phenolic-Based Activated Carbon Fibers," Carbon, 36 (1-2), 123-131,1998.

12.  J. Economy and C. L. Mangun. “Design of a high efficiency, low cost activated carbon fiber system for removal of trace contaminants”. Preprints of Symposia - American Chemical Society, Division of Fuel Chemistry 43(4), 880-884. 1998.

13.  Mangun, C.L.; Braatz, R.; Hall, A.; Economy, J. "Fixed Bed Adsorption of Acetone and Ammonia onto Oxidized Activated Carbon Fibers," Industrial & Engineering Chemistry Research, 38 (9), pgs. 3499-3504, 1999

14.  Economy, J., C.L. Mangun, “Design of ACF for chemically protective masks/clothing,” Proc. of the 1998 ERDEC Scientific Conference on Chemical and Biological Defense Research, 365-371, 1999.

15.  Mangun, C.L., K.R. Benak, M.A. Daley, J. Economy, “Oxidation of activated carbon fibers: effect on pore size, surface chemistry, and adsorption properties,” Chemistry of Materials, 11(12), 3476-3483, 1999.

16.  Economy, J., C. Mangun, “Design of new materials for environmental control,” Macromolecular Symposia, 143, 75-80, 1999.

17.  Benak, K., L. Dominguez, J. Economy, C. Mangun, Z. Yue, “Control of Organic/Inorganic contaminants utilizing tailored ACFs,” Proc. of the Annual AWWA Conference, Denver, CO, 2000.

18.  Z. Yue, C. L. Mangun, J. Economy, P. Kemme, D. Cropek, and S. Maloney. “Removal of chemical contaminants from water to below USEPA MCL using fiber glass supported activated carbon filters”. Environmental Science and Technology 35(13), 2844-2848. 2001.

19.  Mangun, C.L., J.A. DeBarr, J. Economy, “Adsorption of sulfur dioxide on ammonia-treated activated carbon fibers,” Carbon, 39(11), 1689-1696, 2001.

20.  Mangun, C.L., K.R. Benak, J. Economy, K.L. Foster, “Surface chemistry, pore sizes and adsorption properties of activated carbon fibers and precursors treated with ammonia,” Carbon, 39(12), 1809-1820, 2001.

21.  J. Economy, L. Dominguez, and C. L. Mangun. “Polymeric ion-exchange fibers”. Industrial & Engineering Chemistry Research 41(25), 6436-6442. 2002.

22.  Economy, J., C. Mangun, “Novel fibrous systems for contaminant removal,” In Sampling and Sample Preparation for Field and Laboratory, Ed. J. Pawliszyn, Elsevier Science 2002.

23.  Z. Yue, C. L. Mangun, and J. Economy. “Preparation of fibrous porous materials by chemical activation. 1. ZnCl₂ activation of polymer-coated fibers”. Carbon 40(8), 1181-1191. 2002.

James Economy ,  Zhongren Yue