Supercritical water gasification of phenol over Ni-Ru bimetallic catalyst: Intermediates and kinetics
Graphical abstract
Introduction
Biomass, as a renewable energy resource, has received great attention because of the depletion of fossil fuel and the environmental concerns caused by emissions of fossil fuel utilizations [1]. Supercritical water gasification (SCWG) is a technology well suited to produce H2 and/or CH4 from biomass material with high water content. SCWG process breaks biomacromolecules down into gaseous and other small molecular compounds [2] by advanced physicochemical characteristic of water exceeding thermodynamic critical point (Tc =374 °C, Pc =22.1 MPa). The generation of gases in SCWG of biomass is a result of the formation of water-soluble oligomers through hydrolysis followed by further decomposition of these oligomers [3]. In this process, a dry feedstock isn’t required because water acts as both reaction medium and reforming reactant [4]. This leads to a better economic performance because other conventional gasification processes are largely constrained by moisture content of biomass (typically <10 wt%) [5].
Phenolic structures were often found from SCWG of cellulose [6], lignin [7], glucose [8], microalgae [9], and other whole biomass [10], but also existed in a variety of industrial wastewaters due to its role as intermediates for dyes, pesticides, explosives, insecticides, etc. synthesis [11]. Besides, phenolic compound has been described as one of the most refractory intermediates in SCWG due to its inertness feature compared with other derivatives. Therefore, phenols draw much attention and are regarded as a kind of typical model compound. Huelsman and Savage [12] found formation of gaseous product rich in H2 and CH4 can only be promoted under high temperature condition (700 °C), but char yields increased as well. Their subsequent research [13] elucidated reaction pathways and developed a quantitative kinetic model predicting gaseous and primary liquid products.
The earlier work at PNNL in America [14,15] and PSI in Switzerland [16,17] did some valuable researches on metal catalysts under hydrothermal reaction condition. These results confirmed that Ni and Ru metals are active for hydrothermal gasification of biomass. And some attempts have been made to improve SCWG of phenol under moderate condition over monometallic Ni and Ru catalysts (i.e., Ni/CeO2 [18] and Ru/CeO2 [19]). Guan et al. [18] found NiCe alloy formed in Ni/CeO2 catalyst plays a significant role in promoting carbon gasification efficiencies and gas yields for SCWG of phenol. Yu et al. [20] pointed out that Ru/GCB (graphitic carbon black) has both excellent catalytic performance and higher hydrothermal stability. Similarly, in the work performed by Guan et al. [19] Ru/CeO2 showed enhancement effect on gasification and inhibition effect on the formation of dimers. Inspired by these findings, we recently performed an experimental study on SCWG of phenol over Ni catalysts incorporated with Ru [21]. And both H2 and CH4 yields increased as the Ru/Ni ratio increased. Phenol hydrogenation is a key step for aromatic ring rupture and the presence of Ru (either as pure Ru or as a Ni-Ru alloy) reduces the energy barrier for phenol hydrogenation by close to 0.2 eV relative to pure Ni, which was proven by DFT calculation.
As a following work, we continue the practice of using Ni80Ru20/Al2O3 catalyst, which provided a good gasification performance and evidence of overlaid bimetallic particles in our previous work [21]. In this paper, our focuses are elucidating reaction pathways and developing a kinetic model for SCWG of phenol over Ni-Ru bimetallic catalyst. We report herein possible reaction paths among intermediates, a quantitative kinetic model, reaction rate analysis, sensitivity analysis and model prediction based on a reaction network proposed. To our knowledge, this is the first reaction kinetic study concerning catalytic SCWG of phenol.
Section snippets
Materials
Phenol (≥ 99.0 %, Sinopharm Chemical Reagent) was used as reactant. RuCl3 (≥ 99.6 %, Sinopharm Chemical Reagent), Ni(NO3)2·6H2O (≥ 98.0 %, Sinopharm Chemical Reagent), Al2O3 (AR, Sinopharm Chemical Reagent) were used as received as metal precursors and catalyst support.
316 L stainless-steel mini batch reactors with an internal volume of 3.95 ml were used to conduct experiments and these reactors were constructed from a 10 mm port connector, a 10 mm cap, a 10 mm to 3 mm reducing union, a 200 mm
Result and discussion
In this section, we report the major liquid intermediates and gaseous products, develop a corresponding quantitative-phenomenological kinetic model based on the reaction network proposed and perform reaction rate analysis, sensitivity analysis and model prediction with the model developed.
Conclusion
This work presents the result of gasifying phenol in supercritical water over Ni-Ru bimetallic catalyst. Phenol, cyclohexanone, cyclohexanol, toluene, xylene and alkylphenol were identified in liquid phase of three reaction temperatures (400, 450 and 500 °C) while 2-methyl cyclohexanone, 2-methyl cyclopentanone, cyclopentanone and biphenyl were only detected in liquid product of 400 °C. Potential reaction paths among these liquid products were proposed in which reactions like isomerization,
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The work was supported by National Natural Science Foundation of China (51876174), Natural Science Foundation of Jiangsu Province, China (BK20191189) and Natural Science Foundation of Shaanxi Province, China (2018JM5011).
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