Carbon dioxide is an odorless, non-combustible gas that could be both beneficial and threatening to the environment. This gas is inherently part of the earth’s biogeochemical cycle and is as equally important as other naturally occurring gases in the earth’s atmosphere. While carbon dioxide provides benefits to plants via photosynthesis, its effect on the atmosphere at high levels may induce modifications in the naturally occurring changes in climate all over the world. To date, carbon dioxide has been put into a spotlight as the main cause of anthropogenic climate change.
Anthropogenic emissions of carbon dioxide (CO2) have been associated to the green-house effect. Solomon and colleagues predicted that an increase in carbon dioxide levels from 385 parts per million by volume (ppmv) to 450–600 ppmv over the coming century may lead to an irreversible damage on the earth. Such damage would manifest in the irreversible dry-season along with a decreased in rainfall in some parts of the world, and sea level rise. It was predicted that a concentration of CO2 rising beyond 1000 ppmv could also result to 0.6-1.9 m sea level rise. In response to this reported threats, several approach were undertaken to challenge the increasing levels of CO2 in the atmosphere. One of the methods involved direct CO2 extraction from the surrounding air through absorption and desorption, and then processing the data under a controlled environment (Wagner et al.).
The viability of CO2 adsorption relies on the capacity of the sorbent material to address the following aspects altogether: extensive lifetime and nontoxicity, high selective for the gas species, and excellent thermodynamic cycling attributes. Some studies have demonstrated that pore-expanded mesoporous silica fixed with amines were associated to high CO2 adsorption capacity under low partial pressures stable operation and well-defined laboratory conditions. The actual potential and functionality of mesoporous CO2 adsorbent materials were also tested in the study of Wagner et al. (2013) where pure, synthetic and ambient CO2 were compared through a thermogravimetric analyses, Fourier transformed infrared (TGA-FTIR) spectroscopy and diffused reflectance infrared Fourier transformed (DRIFT) spectroscopy. Findings indicate that regardless of the nature of CO2 that is subject for functionality analysis of mesoporous adsorbents, CO2 reduction into the form of carbamate (Reaction 1) or bicarbonate (Reaction 2) as shown in the chemical reactions below is the natural fate of such absorbent materials. The differences in the chemical products produced from reacting urea to CO2 is also influenced by humidity, temperature and period of thermal desorption stage.
Reaction 1: 2RNH2 + CO2 RNHCO2–NH3R+
Reaction 2: 2RNH2 + CO2 + H2O RNH3+ HCO3
The potential of CO2 as a fuel has been explored as a response to lower the CO2 emissions in the atmosphere and the growing concern for global warming. Recent studies showed that carbon recycling has a significant effect in the energy sector. Using novel technologies such as catalytic partial oxidation technology, electrochemical conversion, and bireformation with methane, CO2 is converted into a synthesis gas such as liquid methanol and DME. Liquid methanol is a more preferred form in terms of energy storage and transportation than hydrogen. Both liquid methanol and DME make excellent transportation fuels for internal combustion engines, fuel cells, synthetic hydrocarbons and their derivatives (Olah et al.). Among the aforementioned techniques for synthesis gas production, Bharadwaj and Schmidt noted that catalytic partial oxidation is a faster and most cost effective method of converting natural gas into a synthesis gas. There are three main processes that are involved in the conversion—steam reforming, autothermal reforming and direct oxidation.
A study conducted by Chen aims to assess the potential of CO2 in the catalytic partial oxidation of methane for the conversion of synthesis gas. In this study, Chen used a rhodium-based catalyst bed for the synthesis of CO2. Taking into account the CO2/O2 ratio and O2/CH4 ratio in the feed gas, the results indicated that CO2 assumes no part during methane combustion. However, the gas delays stream reforming while intensifying the dry reforming process. As CO2 consumption increases, the CO2/O2 ratio also increases while the rate of conversion decreases. Overall, the results revealed that the increase in CO2 addition improves methane formation. However, the relationship is inversely propotional to the amount of H2 that is generated during the process of conversion. Chen also indicated that the maximum syngas production ratio is equivalent to CO2/O2 = 0.2 when the O/C ratio is at 1. Further, the maximum H2 that is produced during CO2 consumption is equivalent to O/C = 1.8 and 1.0 respectively. It should also be noted that the monotonic decrease in CO2 conversion is inversely proportional to the increase in the O/C ratio. The ratio of CO2/O2 and O/C, the yields in H2 and rate of CO2 conversion is estimated at approximately 0.42–1.34 mol(mol CH4)−1 and 10–41%, respectively. These findings indicate that there is at least 10% to 41% CO2 that could be used for synthesis gas production when using the catalytic partial oxidation of methane.
Works Cited
Bharadwaj, S.S. and L.D. Schmidt. “Catalytic Partial Oxidation of Natural Gas to Syngas.” Fuel Processing Technology, 42.2–3(1995): 109–127.
Chen, W.H. “CO2 Conversion for Syngas Production in Methane Catalytic Partial Oxidation.” Journal of CO2 Utilization, 5(2014): 1–9.
Olah, G. A., A. Goeppert and G. K. S. Prakash. “Chemical Recycling of Carbon Dioxide to Methanol and Dimethyl Ether: From Greenhouse Gas to Renewable, Environmentally Carbon Neutral Fuels and Synthetic Hydrocarbons.” Journal of Organic Chemistry, 74.2 (2009): 487–498.
Solomon, S., G. K. Plattner, R. Knutti and P. Friedlingstein. “Irreversible Climate Change due to Carbon Dioxide Emissions.” PNAS, 106.6 (2009): 1704 –1709.
Wagner, A., B. Steen, G. Johansson, E. Zanghellini, P. Jacobsson and P. Johansson. “Carbon Dioxide Capture from Ambient Air Using Amine-Grafted Mesoporous Adsorbents.” International Journal of Spectroscopy, 2013 (2013): Article ID 690186.
