Bi-reforming of methane on Ni/SBA-15 catalyst for syngas production: Influence of feed composition
Graphical abstract
Introduction
Fossil fuels, namely, crude oil, natural gas and coal are currently being exploited at an unprecedented rate to meet the demand of rising global population. In fact, approximately 80% of worldwide energy consumption is derived from petroleum-based resources [1]. The increasing depletion of crude oil resource and growing anthropogenic greenhouse gas emissions have spurred an initiative for exploring an alternative and eco-friendly energy to substitute the non-renewable fossil fuels. Methane has emerged as a promising energy source since it is highly available and can be obtained through various resources, namely, natural gas (including methane hydrate and shale gas) and biogases (viz., landfill gas and sewage gas) obtained from livestock excretion and fermented wastes [2], [3]. The conventional approach for the upgrading of methane is to transform it into synthetic gas or syngas (CO and H2 mixture) followed by Fischer-Tropsch synthesis (FTS) to generate environmentally-friendly synfuel [4], [5] for replacing crude oil.
Although the industrial syngas production currently employs steam reforming of methane (see Eq. (1)), SRM, this approach emits undesirable CO2 greenhouse gas and yields H2/CO ratio greater than 3 unsuitable for downstream production of long-chain hydrocarbons via FTS [6], [7], [8]. In order to produce syngas possessing ideal H2/CO ratio = 2 preferred for FTS, partial oxidation of methane, POM (cf. Eq. (2)) has been implemented [9]. However, several drawbacks such as highly exothermic nature, difficult control due to hot-spot formation and risk of explosions are reportedly associated with POM [9], [10]. In this regard, methane dry reforming (MDR) has lately gained substantial interest from industrial and academic research since MDR process not only alleviates emission of CO2 but also convert it to valuable products (see Eq. (3)) [10], [11], this process yields a low H2/CO ratio (less than 1) unsuitable for FTS owing to concomitant reverse water-gas shift (RWGS) side reaction [12]. However, several drawbacks are associated with catalytic stability in MDR. Indeed, carbon formation is commonly reported as one of the leading causes for catalyst deactivation [13] followed by sintering of metal particles at elevated temperature [14].
Partial oxidation of methane:
Methane dry reforming:
As a result, an integrated process of CO2 and steam reforming of methane (also known as bi-reforming of methane, BRM given in Eq. (4)) has emerged as an alluring reforming process among conventional techniques due to greenhouse gas (CO2 and CH4) mitigation, high catalytic stability using two oxidizing agents (such as CO2 and H2O) [15] and desired H2/CO ratio easily achieved via manipulation of feedstock composition [16]. The BRM also offers a direct reaction pathway to produce syngas having H2/CO ratio of 2 (known as metgas) without requiring purification and auxiliary separation of oxidative by-product [15].
Bi-reforming of methane:
However, the bibliographic reports on BRM are relatively limited in literature owing to reaction complexity involving SRM and MDR reactions as well as numerous non-coke (see Eq. (5)) and coke forming (see Eqs. (6)–(8)) parallel reactions [17].
Reverse water-gas shift reaction:
Methane decomposition:
Boudouard reaction:
Beggs reaction:
In BRM, nickel metal supported on various semiconductor oxides (such as Al2O3 [18] and ZrO2 [19] supports) or mixed metal oxides (including MgOAl2O3 [20], CeO2ZrO2 [21], and NiOB2O3 [22], [23]) has been intensively studied owing to its lower cost and great catalytic activity relatively similar to noble metals, namely, Rh [24] and Ru [25]. Olah et al. observed that 15%Ni/MgO catalyst achieved high CO2 and CH4 conversions of above 70% for BRM at 7 atm, 1103 K and CH4/CO2/H2O molar ratio as 3.0/1.2/2.4 [15]. However, supported Ni catalysts are reportedly susceptible to rapid catalytic deterioration owing to metal sintering and coke formation [18], [19], [20]. In BRM study over 10%Ni/α-Al2O3 catalyst at CH4/CO2/H2O of 0.8/1.0/0.4, Roh et al. observed catalyst deactivation within 6 h on-stream owing to severe carbon deposited on catalyst surface [21]. Although the utilization of excess steam feeding stock could effectively eliminate carbon formation during BRM, this approach would adversely increase operating cost and yield an unfavorable H2/CO ratio for FTS [26].
Recently, two different strategies were introduced for preventing carbon formation and enhancing catalytic performance for nickel-based catalysts during high temperature reforming processes [27]. The first approach implies that coke formation can be efficiently inhibited by preparing catalysts with high oxygen storage capacity or oxygen mobility on catalyst surface [28], [29]. Therefore, rare-earth oxides such as La2O3 and CeO2 have been widely employed for the oxidative removal of surface carbon species [30], [31], [32]. Despite its efficacy, the reforming reactions still suffer from carbon-induced deactivation during longevity tests. Consequently, the second strategy indicates that both coke formation and sintering can also be prevented by providing better metal dispersion, smaller particle size (<2 nm) and high metal-support interaction [33]. In this regard, the highly stable mesoporous silica scaffold such as MCM-41 and SBA-15 is one of the prevalent dispersants for active nickel particles.
In addition, the implementation of mesoporous SBA-15 support for suppressing carbon formation in SRM [7], [34] and MDR [11], [35] reactions has received significant interest owing to its high silanol group density, high surface area and uniformity of pores, enhancing active metal dispersion with small crystallite size [32]. Recently, various advanced modification techniques, namely, core-shell, skeletal structure and phyllosilicates nanotubes were used to provide nano-confinements for nickel nanoparticles [36], [37], [38]. However, a facile and industrially recognized incipient wetness impregnation technique is used in this work due to its simple execution and low-waste streams [39]. To the best of our knowledge, there is no reported investigation discussing about the effect of partial pressure for each reactant (i.e., CH4, CO2 and H2O) on the catalytic activity of SBA-15 supported nickel catalyst during BRM reaction in literature. Additionally, the influence of reactant partial pressure is an essential factor for understanding BRM kinetics and reactor design. Furthermore, the relationship between feed composition and the resulting amount of deposited carbon is an important aspect for ensuring catalytic stability but it has not been examined before. Therefore, a 10%Ni/SBA-15 catalyst prepared via incipient wetness impregnation technique was characterized and tested for BRM at different feed compositions in this work.
Section snippets
Catalyst preparation
Mesoporous silica (SBA-15) support was prepared using a facile hydrothermal route. About 3.13 g of non-ionic triblock poly (ethylene glycol)-block-poly (propylene glycol)-block-poly (ethylene glycol) referred as Pluronic® P-123 (EO20PO70EO20, Sigma-Aldrich Chemicals) was dissolved in 110 ml of fuming HCl (1.6 M) solution (supplied by Merck Millipore) with pH of about 1. The mixture was subsequently stirred for 2 h at 313 K to ensure the complete dissolution of the P-123 templating agent.
Textural properties
The N2 adsorption-desorption isotherms of SBA-15 support and fresh 10%Ni/SBA-15 catalyst are shown in Fig. 1 for confirming the mesoporous characteristic of these metal oxides. Both support and catalyst exhibit a type IV isotherm curve with a noticeable H1-type hysteresis loop at the relative pressure, P/P0 range of 0.58–0.85, which is a typical feature of the well-ordered mesoporous materials with cylindrical channels, according to IUPAC classification [8], [41]. In addition, the considerable
Conclusions
SBA-15 supported Ni catalyst has been successfully synthesized using incipient wetness impregnation of Ni(NO3)2 on SBA-15 support previously prepared by hydrothermal technique. The inevitable drop in BET surface area from 669.5 m2 g−1 (SBA-15 support) to 538.6 m2 g−1 (10%Ni/SBA-15 catalyst) was indicative of successful NiO particle penetration into the mesoporous cylindrical channels of SBA-15 support and hence pore blockage. A considerable increase in crystallite size was observed for both Ni
Acknowledgements
The authors would like to acknowledge the financial support provided by the Universiti Malaysia Pahang (UMP Research Grant Scheme, RDU170326). Doctoral Scholarship Scheme (DSS) conferred to Sharanjit Singh by UMP is also greatly appreciated.
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