Preparation of Activated Carbon from the Biodegradable film for Co2 Capture Applications

Abstract In this work for the fi rst time, activated carbons were prepared from carboxymethyl fi lm (low-cost carboxymethyl fi lm waste), using chemical activation with potassium hydroxide. The samples were characterized by nitrogen adsorption-desorption at 77 K, XRD, SEM methods. The high values of the specifi c surface area and total pore volume were achieved and were equal to 2064 m2/g and 1.188 cm3/g, respectively. Waste from the fi lm can be immediately utilized without CO2 production. This is the environmentally friendly way of waste utilization. Through this process, we can protect our environment. This study showed that the activated carbon obtained from carboxymethyl fi lm waste can be used as a good adsorbent for CO2 adsorption.


INTRODUCTION
Growing global concerns on the environmental, social and economic effect of greenhouse gases (GHG) emissions motivated the development various strategies for GHG reduction 1 . The major GHG are CO 2 and CH 4 . The main sources of CO 2 are fossil fuel combustion, deforestation, cement production. The main sources of CH 4 are fossil fuel production, agriculture, landfi lls. Methane has a global warming potential 25 times 2 that of CO 2 but it is also valuable raw material mainly for syngas production. The most widely practiced production route of syngas (hydrogen and carbon oxide) is steam reforming. Unfortunately this process requires high temperatures (800-900 o C) 3 which makes it expensive. Alternatively, CO 2 can be used to produce syngas in conjunction with methane by the dry reforming 4 . This is alternative solution for CO 2 and CH 4 utilisation and generation of value-added products but it cannot be applied in industry because of fast catalysts deactivation 5 . The production of hydrogen from methane by catalytic methane decomposition 6-8 or utilizing membranes 9 especially zeolite membranes 10 was investigated deeply. Apart from hydrogen production other valuable product -carbon nanomaterials such as carbon nanotubes 7-13 , carbon nanofi bers 8 , carbon nanocapsules 14 , and metal nanowires encapsulated in carbon 15 can be obtained from methane. Studies regarding the process of methane oxidation to products different from those obtained when preparing synthesis gas were conducted already at the beginning of the 20th century. Direct methane oxidation to oxygenates such as formaldehyde 16 and methanol 17 is possible in presence of catalyst such as: niobium(V) oxide 16 , Fe-ZSM-5 18, 19 , M/SiO 2 where M= Se , Nb 20 , V 20 , Fe 20 , and Mo 21 or in presence of Methylosinus trichosporium OB3b 22 . Catalytic conversion of methane to esters in condensed phase was investigated at ambient 23, 24 and high pressure 25, 26 . As catalyst metals such as Pd 27-29 , Pt 24-31 , Ni 32 , Zn 32 and halogens 26 mainly bromine 33 and iodine 23, 24 were applied. The separation system of products obtained in condensed phase using membrane was developed 35 . Methane can be used as fuel in cars but good methane sorbent is needed. Adsorbed methane technology could allow methane consumption comparable to the other conventional petroleum-based fuels. Activated carbons can be applied as methane sorbents 36-39 .
There are many attempts for CO 2 utilization as a raw material e. g. photocatalytic reduction to methanol 40 but the effi ciency is usually very low. Carbon dioxide is applied in industry as raw material to urea production but the urea production scale is much smaller than that of synthesis gas 5, 41, 42 . The most important commercially applied technology for CO 2 removal is the absorption process in amines. One major disadvantage of amine absorption processes is the high energy consumption, arising from high energy levels required to regenerate the sorbent 41, 42 . The adsorption on solid sorbents seems more promising. Carbon materials are very good CO 2 sorbents 38 . CO 2 adsorption on commercial activated carbons 43 , modifi ed commercial activated carbons 44-47 , carbon nanosheets 48 , carbon nanotubes 49-51 , activated carbons produced from biomass 52-56 and from molasses 58 was investigated. Carbon materials have great potential. They can be applied also as sorbents of various chemicals 59-62 and hydrogen 63-67 and even catalyst 68 or catalysts supports 69-71 . They properties dependent on the carbon source and synthesis method 72-77 .
Biodegradable fi lm is environmentally friendly as it eventually degrades in the soil after about a month, and without the participation of microorganisms after year 78 . If oxygen is present, aerobic biodegradation occurs and carbon dioxide is produced 79 . If there is no oxygen, an anaerobic degradation occurs and methane is produced instead of carbon dioxide and water 80 . An application of biodegradable fi lm is obviously more advantageous than the traditional. It also has drawbacks: a) necessity for storage of the waste before decomposed; b) production of greenhouse gases (CO 2 or CH 4 ) to the atmosphere.
The goal of this work was to develop a method of preparing activated carbon using hydrophilic fi lms based on carboxymethyl starch (CMS) as a carbon precursor. Such method solves problems with storage of waste before they decompose and what more important with greenhouse gases emission. According to the our knowledge, production of activated carbon from CMS was not described up to know. In addition, activated carbons prepared by us are good CO 2 sorbents.

MATERIAL
CMS with a degree of substitution 0.8 was prepared according to the method described elsewhere by Spychaj et al. 81 . Monohydrate citric acid (CA) (p. a.), and glycerol (p.a.), potassium hydroxide (KOH) (p.a.) were delivered from Chempur (Poland).

Preparation of biodegradable CMS-based fi lm
The fi lm was prepared in accordance with the method reported by Spychaj et al. 82 namely: 3 g carboxymethyl starch, 2 g glycerol, and 1 g citric acid was introduced to 100 g of distilled water and stirred for 30 min. The fi nal mixture was poured into polytetrafl uoroethylene (PTFE) mold and dried for 48 h at 70°C. Obtained fi lm (thickness 200-300 μm) was used for the production of activated carbon.

Preparation of activated carbon
Contained in fi lm crosslinking CMS were used as a carbon precursor. CMS was crushed using an electric grinder. Chemical activation of the fi lm powdered was done with saturated solution KOH (mass ratio KOH: carbon source, 1:1) during 3 h. The mixtures were dried at 200°C for 19 h. The next step was the carbonization of materials for 1h in a horizontal tube furnace under nitrogen fl ow at range 500-700°C. Cooled samples were washed with distilled water, treated with 1 M HCl for 19 h period, and then washed with distilled water until neutral. In the end, materials were dried at 120°C. The materials were denoted as CMS500, CMS550, CMS650, CMS700 (in accordance with the carbonization temperature). Method of activated carbon preparation was described in Polish patent application 83 .

XRD
The structures of activated carbon were determined by XRD. Samples were recorded using PANalytical X-ray Empyrean diffractometer with Cu Kα radiation. The test results were analysed using the X'Pert HighStore diffraction program.

Nitrogen and sorption
The texture characterization of activated carbons was carried out by N 2 adsorption and desorption at 77 K using, a Quadrasorb automatic system (Quantachrome Instruments). Before the analysis samples were degassed overnight (16 h) under high vacuum at 250 o C. The Brunauer-Emmett-Teller (BET) equation was used to determine surface areas (SBET). The total pore volume (V tot ) was determined at the highest value relative pressure (p/p0 = 0.99). The volume of micropores (V mic ) and mesospores (V mes ), was obtained using the density functional theory (DFT).

Carbon dioxide sorption
CO 2 adsorption was provided at pressure up to 1 bar, at a temperature of 25°C. Before the analysis samples were degassed overnight (16 h) under high vacuum at 250°C.

SEM
Scanning electron microscopy (SEM) was used to investigate the morphology of the activated carbons (UHR FE-SEM Hitachi SU8020).

RESULTS AND DISCUSSION
Properties of the CMS fi lm are shown Table 1. Table 1. Useful properties of CMS-based fi lm From Fig. 2 the nitrogen isotherm of CMS500 has the shape in between type I and type II according to IUPAC classifi cation 84 . This type of isotherm is characterized by the micropore and mesopore structures. The H4-type hysteresis loop in the CMS500 material reveals the formation of narrow slit-like pores. Samples CMS550, CMS650 and CMS700 show type II isotherm with small hysteresis type H4. Table 2 shows the BET surface area, total pore volume, mesospores and micropores volume values. Note that the CMS500 sample showed the highest specifi c surface area 2064 m 2 /g and the highest micropore volume 0.417 cm 3 /g . but the process was very fast. The balance was fi xed after about a minute.
The SEM micrographs showed the presence of macropores on the surface of all tested materials. The only surface of CMS500 is shown in Fig. 5 because the micrographs of the others materials were very similar.
DFT pore size distributions and cumulative pore volume curves are shown in Table 2, Fig. 3 respectively. The total pore volumes were estimated on the basis of the volume adsorbed at a relative pressure of about 0.95. The pore size distribution and micopore volume was obtained after application of the QSDFT model to the nitrogen adsorption data and assuming a slit-shape pore model. All samples have micropores locate about 1.8 nm. Generally, the micropores result from the rapid volatilization of light organics and amorphization of carbonaceous segments during direct carbonization at a relatively high temperature of 500 o C. the occurrence of mesopores is visible for each CMS in the whole range from 2nm up to 35 nm where increasing. These pores are most demonstrating highest values in the range up to 7.5 nm. Obviously, CMS500 demonstrate a mesopore--dominant structure with a large mesopore value. Figure 4 shows the sorption capacity of samples CMS. The highest sorption capacity was observer at activated carbon obtained at the temperature of 500 o C and amounts to 3.52 mmol/g. The process of adsorption isotherms is typical for physical adsorption. Kinetics were not tested

CONCLUSION
The new carbon precursor, namely carboxymethyl starch fi lm was used for activated carbon production. Potassium hydroxide was used as an activating agent. The high values of the specifi c surface area and total pore volume were achieved and were equal to 2064 m 2 /g and 1.188 cm 3 /g, respectively. It is a good alternative to solve the problem of the foil storage until it decomposes. Waste from the fi lm can be immediately utilized without CO 2 production. This is the environmentally friendly way of waste utilization. Through this process, we can protect our environment. The activated carbons described here are good materials for removing CO 2 from the atmosphere.