(526e) Fe2O3/CaO-Ca12Al14O33 Multifunctional Catalyst in Sorption Enhanced Chemical Looping Reforming of Ethanol Combined with Water Splitting for Hydrogen Production

Hydrogen is being considered as a future alternative/clean fuel due to abundant availability of hydrogen bearing substances in nature, its highest energy content (120.7 kJ/g) compared to any of the known fuels, its heating value approximately three to four folds higher than natural gas and coal, and its clean combustion without generating any environmental pollution [1-2]. Hydrogen is mainly produced via steam reforming. Ethanol is a good candidate of feedstocks due to its renewability, availability, ease of transportation, low toxicity and low catalyst poisons (e.g. sulfur) [3-5]. Producing hydrogen from steam reforming of ethanol is not only environmental friendly, but also offers great opportunities for utilization of renewable resources [6]. The conventional steam reforming (SR) of ethanol, however, suffers from several disadvantages such as complexity of process and high energy consumption. Various multifunctional reactors have been proposed to improve the process for efficient production of hydrogen. Introducing CO₂ sorbent as a sorption enhanced steam reforming (SESR) process offers H₂ production with in situ CO₂ separation in a single reactor. Additionally, shifting the equilibrium to product side and providing a self-heat supply from exothermic CO₂ adsorption would maximize the H₂ production and provide high purity H₂. Considering CO₂ sorbent, CaO with modification is widely used due to its high CO₂ adsorption capacity and excellent thermal stability in the carbonation and calcination.

Recently, chemical looping reforming (CLR) represents a promising process for H₂ production with low energy consumption. In CLR process, solid oxygen carrier (OC) is employed for supplying oxygen to fuel for partial oxidation. Furthermore, the re-oxidation of OC in an air reactor (AR) can provide heat for highly endothermic reaction in a reforming reactor (RR). For H₂ production in CLR, various oxides of Ni, Fe, Mn, Co and Cu have been considered as good candidates of oxygen carrier. Among them, iron oxide is a promising material in terms of abundant availability, reasonable price, and non-toxicity.

Integration of SESR and CLR becomes an intensified process called sorption enhanced chemical looping steam reforming (SECLR). This novel process has been proven by many researchers to provide the highest performance, compared to SR, SESR and CLR. Our previous study reported that the SECLR using CaO and NiO as CO₂ adsorbent and OCs could reduce the energy requirement and increase the net hydrogen production [7]. The SECLR process could be further improved by integrating water splitting to CLR for maximizing H₂ production. In previous literature, chemical looping process for the generation of hydrogen was introduced as the steam-iron process and nowadays, it has been further modified as chemical looping hydrogen generation (CLHG) or chemical looping water splitting (CLWS). The oxygen carrier functions in CLWS must be able to split water to produce H₂ in a steam reactor (SR). Iron oxides possess a multifunctional ability for chemical looping and water splitting.

In this study, a process of sorption enhanced chemical looping of ethanol reforming combined with water splitting (SECLR-WS) has been developed. The SECLR-WS process could simultaneously produce H₂ from both steam reforming of ethanol and water splitting of steam, as well as in situ CO₂ removal. To obtain these, multifunctional catalysts were synthesized from mixed precursors of Fe₂O₃ as oxygen carrierand modified CaO (CaO/Ca₁₂Al₁₄O₃₃) as CO₂ adsorbent. Fe₂O₃ was used as OC because it is cheap and widely available, compared to nickel. In addition, Fe₂O₃ in its reduced state has ability to split water into hydrogen which is unavailable in Ni species.

In experimental, multifunctional catalysts (Fe₂O₃/CaO-Ca₁₂Al₁₄O₃₃) containing Fe₂O₃ as OCs/water splitting catalyst, and CaO/Ca₁₂Al₁₄O₃₃ modified sorbent as CO₂ absorbent were prepared by sol-gel method. A series of catalysts was prepared with different iron contents of 5, 10 and 15wt%, denoted as 5Fe/CaAl, 10Fe/CaAl and 15Fe/CaAl, respectively. In addition, the effect of adding iron precursor before (5Fe/CaAl) and after peptised with acid (5Fe*/CaAl) was examined. The test of catalysts in SECLR-WS process was carried out at reforming temperature of 600ºC, steam to ethanol ratio of 4 and total flow rate of 50 ml/min (ethanol vapor and steam of 20 ml/min balanced with N₂). Various characterizations including XRD, TGA, SEM, and BET were performed before and after the H₂ production in order to investigate the physiochemical characteristics of catalysts such as the evolution of the iron species during the H₂ production process.

In the reforming reactor (RR), the reduction step would be proceeded. The Fe₂O₃ phase in catalyst bulk was supposed to be reduced into FeO/Fe by reforming with ethanol. After the Fe₂O₃ phase changed to FeO/Fe in the RR, the steam being introduced would subsequently react with FeO/Fe for water splitting which is called the oxidation step. Obviously, hydrogen generation was observed in the oxidation step. It confirmed that the Fe-based OCs could work well as multifunctional catalysts for reforming and water splitting in the developed SECLR-WS process. Typically, iron loading has a significant influence on the composition, particle size and CO₂ adsorption capacity of the catalyst. It was found that the lower H₂ production in pre-breakthrough and high CO₂ concentration were obtained as an increase in Fe content from 5% to 15wt%. In our preliminary results, the Fe/CaAl with 5% iron exhibited the highest performance for H₂ production.

Changing the step of adding iron precursor before (5Fe/CaAl) and after peptised with acid (5Fe*/CaAl) demonstrated a similar behavior of H₂ production with maximum productivity approximately of 65% and breakthrough time (t_b) of 30 min. The activity of Fe₂O₃/CaO-Ca₁₂Al₁₄O₃₃ catalysts was independent to the synthesis steps in sol-gel method. In the catalyst synthesis, sol-gel method provided the strong interactions between Fe and Ca, resulting in a formation of Ca₂Fe₂O₅ phase which lower Fe active sites and CaO sorbent capacity. As a result, lower H₂ production and breakthrough time were observed.

In conclusion, the SECLR-WS process was successfully developed in this study. The as-synthesized catalyst exhibited multifunctionality for maximizing H₂ production from steam reforming of ethanol and water splitting. The 5wt% Fe in the catalyst provided the highest performance in term of maximum H₂ production.

Selected References:

[1] Sharma YC, Kumar A, Prasad R, Upadhyay SN. Ethanol steam reforming for hydrogen production: latest and effective catalyst modification strategies to minimize carbonaceous deactivation. Renewable and Sustainable Energy Reviews. 2017;74:89-103.

[2] Saithong N, Authayanun S, Patcharavorachot Y, Arpornwichanop A. Thermodynamic analysis of the novel chemical looping process for two-grade hydrogen production with CO2 capture. Energy Conversion and Management. 2019;180:325-37.

[3] Mariño F, Boveri M, Baronetti G, Laborde M. Hydrogen production via catalytic gasification of ethanol. A mechanism proposal over copper–nickel catalysts. International Journal of Hydrogen Energy. 2004;29(1):67-71.

[4] Deluga G, Salge J, Schmidt L, Verykios X. Renewable hydrogen from ethanol by autothermal reforming. Science. 2004;303(5660):993-7.

[5] Fernando S, Hanna M. Development of a novel biofuel blend using ethanol− biodiesel− diesel microemulsions: EB-diesel. Energy & Fuels. 2004;18(6):1695-703.

[6] Haryanto A, Fernando S, Murali N, Adhikari S. Current status of hydrogen production techniques by steam reforming of ethanol: a review. Energy & Fuels. 2005;19(5):2098-106.

[7] Saupsor J, Kasempremchit N, Bumroongsakulsawat P, Kim-Lohsoontorn P, Wongsakulphasatch S, Kiatkittipong W, et al. Performance comparison among different multifunctional reactors operated under energy self-sufficiency for sustainable hydrogen production from ethanol. International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.090.