Full Length ArticlePreparation of high-dispersion Ni/C catalyst using modified lignite as carbon precursor for catalytic reforming of biomass volatiles
Graphical abstract
Introduction
Lignite has a high ability of spontaneous combustion since the rich in oxygen-containing species (OCSs) [1], [2], [3]. Due to abundance in OCSs, lignite was reported present high ion-exchanging ability with metal, such as Ni, and great dispersion of the Ni within the coal matrix [4], [5].
We previously reported a novel catalyst with dispersed metallic Ni particles in a Ni crystallite size (NCS) of 5.6 nm and a relatively high specific surface area (SSA) by loading Ni on Shenli lignite (SL) char. The catalyst that nickel-loaded on lignite was successfully utilized for volatile reforming in biomass gasification [6]. However, lignite contains a relatively high ash content, which might affect the ion-exchange capacity as well as the catalyst activity. Therefore, modified lignite provides a potential way to value-added utilization of lignite. Minerals in lignite can be divided into the minerals which exist independently (kaolin, montmorillonite, pyrite, calcite and quartz, etc) and inorganic elements associated with the organic matter. The Na, K, Mg and Ca are usually exchanged in carboxylate groups [7], which can affect the Ni loading and dispersity. Lignite has a certain catalytic activity in gasification reactions since the existence of high content of minerals [8], [9], [10]. The catalytic activity of minerals is dependent on their chemical form exists in the coal matrix, as well as the concentration and dispersion. It was proved that the alkali and alkaline earth metals in coal have a certain affect in the reactivity of lignite char [11], [12]. Ca and Mg usually exist in carbonates form and show predominantly organic affinity [7], [13]. To investigate the catalytic activity of minerals in lignite, DM and AW are need before catalyst preparation.
The OCSs, especially carboxylic groups, are able to exchange ions with external molecules and the exchanged ions are highly dispersed around the coal matrix [4]. The oxidation of lignite may play an important role in improving of SSA and microporosity and the amount of ion exchange in catalyst preparation. Potassium permanganate, peroxyacetic acid, sodium hypochlorite, oxygen and hydrogen peroxide were used for increasing OCSs content [14], [15], [16], [17], [18], [19], [20], especially for environmentally friendly oxidant of hydrogen peroxide [21], [22]. The ion exchange capacity of the lignite can be enhanced by oxidation treatment to provide the carboxyl group.
In this study, Ni/C catalysts were prepared by acid washing, demineralizing and oxidizing of lignite to study the effects of minerals and carboxyl groups on the Ni loading, NCS, Ni particle size and SSA of the catalysts. The catalytic activity of the as prepared Ni/C catalysts for cracking tarry materials from biomass pyrolysis was investigated in a two-stage fixed-bed quartz reactor.
Section snippets
Preparation of acid washed, demineralized and oxidized lignite
The demineralized lignite (DMSL) was prepared by acid leaching of SL. About 20 g SL sample was stirred with 5 M HCl for 1 h at 55 °C and filtered, then the filter cake was treated with 11 M HF for 1 h at 55 °C. After filtration, the sample was treated again with another batch of 5 M HCl for 1 h at 55 °C, followed by repeated washing with deionized water (DIW) until neutral and no Cl− can be detected. As reported previously [23], for acid washed lignite (AWSL), 20 g of lignite was treated with 5 M HCl for 2
FTIR analysis
As shown in Fig. 1, there is a transmission band at around 3428 cm−1, which can be attributed to the stretching vibration of OH from residual water. The bands at 2920 and 2850 cm−1 are due to aliphatic CH, CH2 and CH3 stretching vibrations. An apparent band appeared at 1710 cm−1, which corresponding to the stretching vibration of CO in carboxyl groups. The relatively strong band of carboxyl groups in OXSL, OXDMSL and OXAWSL are well consistent with the results of the chemical titration (Table S1).
Conclusions
The carbon skeleton of SL can be corroded seriously during DM. The demineralized sample, i.e., DMSL and OXDMSL have relatively low Ni loading and activity for biomass tar reforming in comparison to SL. Acid treatment of SL significantly reduce the content of metal cation associated with the OCSs, increase the carboxyl and hydroxyl amount and further enhance the Ni loading of AWSL via ion exchange. Oxidation with H2O2 can further increase the carboxyl amount of raw and treated lignite, and make
Acknowledgements
This work was subsidized by the Fundamental Research Funds for the Central Universities (China University of Mining & Technology, Grant 2015XKQY05), the National Natural Science Foundation of China (Grant 21676292), the Natural Science Foundation of Jiangsu Province (BK20151141), and the Priority Academic Program Development of Jiangsu Higher Education Institutions.
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