Zeolites as recyclable adsorbents/catalysts for biogas upgrading: Removal of octamethylcyclotetrasiloxane
Graphical abstract
Introduction
Biogas production from anaerobic digestion of wastewater sludge and landfills has increased in the last decades and many sewage treatment plants are collecting their biogas mixture to obtain energy [1], [2]. To be used as fuel, even in low-quality demanding applications, sewage biogas needs to be upgraded to remove undesirable compounds such as siloxanes [3]. Siloxanes are a group of silicones containing SiO bonds with organic chains attached to the silicon atom, with low water solubility and high vapour pressure. Siloxanes are converted into micro-abrasive silica during biogas combustion leading to the abrasion of engine parts or build-up silica layers that inhibit essential heat transfer or lubrication [1]. As siloxanes are extensively used in industrial and household applications, their concentration in sewage biogases has risen in the past years, being octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5) the predominant occurring siloxanes [4].
A well-known and widespread technology that can contribute to biogas purification processes is adsorption onto porous materials, such as activated carbon (AC) which has been extensively investigated for the removal of siloxanes [5], [6]. The use of silica gel and zeolites for siloxane removal is reported in some publications [7], [8], [9], [10]. Inherent to the adsorption process, is the problem of regeneration of spent materials in order to avoid them becoming a residue. Thermal desorption of siloxanes for AC and silica gel regeneration has been studied [10], [11], [12] resulting unsuccessful due to the formation of higher molecular weight siloxanes which are hardly desorbable. Therefore, adsorbent replacement constitutes the main part of operational costs in the removal of siloxanes by adsorption [5].
Due to their efficiency and simplicity, advanced oxidation processes (AOPs) are becoming important technologies for treating resistant compounds [13], [14], [15], [16]. These processes are based on the generation of very reactive radicals, mainly hydroxyl radicals, which oxidise target organic pollutants. Ozone and hydrogen peroxide are the most often used chemicals to generate hydroxyl radicals. They are also combined with various kinds of catalysts in order to increase radical generation [17], [18].
In this study, a sequential siloxane uptake/wet oxidation process was applied in order to transform immobilised and concentrated siloxanes on zeolite surfaces into water-soluble products with low affinity toward zeolite, thus re-establishing its adsorption capacity. The use of this type of treatment has only been recently investigated for the regeneration of siloxane-exhausted activated carbons [19] in adsorption/oxidation cyclic processes. It was concluded that, although AC is an optimal adsorbent due to its high textural development, the use of AOPs was not completely successful in the recovery of AC adsorption capacity toward siloxanes. The adsorbed siloxane could be oxidized, at least partially, to soluble by-products by heterogeneous Fenton processes, as well as by ozonation, without silica (SiO2) deposition in the pores [19]. Nevertheless, AC itself remains too prone to oxidation and cannot be reused at long term because of porosity loss [20], [21].
Zeolites can be used as dual functional catalysts. Firstly, siloxanes can be adsorbed (and transformed) in their porous structure and secondly, zeolites can promote AOP reactions, without being vulnerable to oxidation. Thus, zeolites have the potential to be long-lived reusable catalysts [22], [23]. In addition, iron can be easily introduced into the structure of zeolites due to their ion-exchange capacity and the resulting Fe-containing zeolites show high catalytic activities in the Fenton-like oxidation of organic compounds with hydrogen peroxide with minimal iron leaching [13]. Isolated FeII/III ions bound to the ion exchange sites of the zeolite are considered to be responsible for the Fenton-like catalytic activity of Fe-zeolites obtained by ion-exchange procedures [22].
The first objective of this study was to select the most effective zeolite for siloxane catalytic removal from the gas phase. Subsequently, a selected zeolite was evaluated as catalysts for the oxidation of the adsorbed/transformed siloxane, whereby regeneration by a heterogeneous Fenton-like reaction or by ozone in aqueous suspension was carried out. Batch experiments were conducted in order to assay zeolite stability in several adsorption-regeneration cycles.
Section snippets
Materials
A set of six synthetic zeolites and one natural zeolite (Clinoptilolite) with different a wide range of characteristics were studied for D4 removal. Their properties are summarised in Table 1. Fe-BEA, Fe-MFI, dealuminated Y (DAY) and ultrastable Y (USY) zeolites were obtained as pellets, BEA-38 and BEA-300 in powder form and the natural Clinoptilolite as a sand. Fe-BEA and Fe-MFI are typical iron-ion exchanged zeolites where the dominant iron species are known to be isolated FeII/III ions bound
D4 uptake by zeolites
D4 dynamic adsorption breakthrough curves of all the zeolites considered are shown in Supplementary material Fig. S1. Adsorption tests were carried out using a high inlet concentration of D4 (3000 mg D4 m−3), in order to reach the breakthrough in one day experiments and reach high D4 loading, which represents severe exhaustion conditions used for testing the effectiveness of regeneration. Under such experimental conditions, zeolite D4 equilibrium uptake ranged from 11.2 to 143 mg g−1 (see Table 2).
Conclusions
The experimental results on the uptake of gaseous D4 by various zeolites lead to the conclusion that BEA zeolites, due to their high content of Brønsted and Lewis acidic sites, show the greatest catalytic activity for the siloxane ring-opening and the formation of α-ω-silanediols. Silanediols formed on the BEA surface can be detached from the acidic sites when water is available, e.g. due to the humidity content of the zeolite, and are narrow enough to diffuse into the channels, enhancing the
Acknowledgments
This work was funded by MINECO – Spain (CTQ2014-53718-R) co-founded by FEDER and the University of Girona (MPCUdG2016/137). A.CC thanks Generalitat de Catalunya DEC for her predoctoral grant (FI-DGR-2012). The technical assistance of Pol Agustí and Gemma Rustullet is very much appreciated. We also thank STR-UdG and Departament de Química UdG for their support on the analytical procedures.
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