(744h) Enhancement of Catalytic Performance of Ordered Mesoporous “One-Pot” Fe-Al2O3 Catalysts By Ni Incorporation in Dry Reforming of Biogas | AIChE

(744h) Enhancement of Catalytic Performance of Ordered Mesoporous “One-Pot” Fe-Al2O3 Catalysts By Ni Incorporation in Dry Reforming of Biogas

Authors 

Jabbour, K. - Presenter, University of Balamand
El Hassan, N., University of Balamand
Massiani, P., Sorbone Universite
Davidson, A., Sorbone Universite
Saad, A., University of Balamand
Inati, L., University of Balamand
Introduction: Biogas, mainly composed of CH4 (50-70 vol.%) and CO2 (30-50 vol.%), obtained from anaerobic digestion of biomass is a perfect feedstock for syngas (composed mainly of H2 and CO) production via reforming of methane by CO2, known as dry reforming of methane (DRM: CH4+CO22CO+2H2,H2:CO=1) and subsequent utilization in Fisher-Tropsch synthesis [1].

Although noble metal based-catalysts (Pt, Ru) were reported as promising candidates for dry reforming [1]; their cost and low availability are drawbacks to their industrialization. From economic perspective, transition metals (Ni, Fe) particularly iron-based ones are becoming highly desirable due to extremely low prices of Fe compared to noble or non-noble metals. Nevertheless, supported Ni-catalysts are most considered owing to their good intrinsic activity levels. Yet, the main issue bounded with such metals arises from their deactivation by nickel particles agglomeration and/or coke C(s) deposition.

The coupling of both Ni and Fe species has been viewed as a tactic for improvement against C(s) encapsulation due to a synergetic effect between both metals [2]. Still, such observations await additional explanation. Therefore, the present work aimed at investigating different effects (optimum Ni:Fe molar ratio, types and quantity of carbonaceous residues) along with the combination of the special advantages of the simple “one-pot” technique yielding ordered mesoporous metal-alumina matrices. In our recent work [3], this preparation method was proven efficient for designing stable reforming catalysts. As far as we know, such approach coupling (i) structuration of porous Al2O3 support and (ii) “one-pot” active phase (Fe, Ni) insertion within catalyst matrix was not yet considered under drastic biogas dry reforming conditions (CH4/CO2= 2:1 (excess methane), T=700°C).

Experimental part: Mesoporous alumina-based materials were synthesized according to an evaporation-induced-self-assembly procedure [4]. After calcination (air, 600°C/5h), monometallic Ni5%, Fe10% and bi-metallic Fex:2.5%-7.5%Ni(1-x) alumina xerogels were obtained using P123 as templating agent.

Prior to reaction, powder (100 mg) undergoes in-situ reduction (5% H2/Ar, 700°C/2h) inside a vertical Hastelloy-X fixed-bed flow reactor. Reactivity (CH4 and CO2 conversions, H2 and CO productions) is then measured at 700°C/12h for (i) a molar ratio of CH4/CO2 of 1.8 and a (ii) space velocity of 32 L.gcat-1.h-1. Thermodynamic values were estimated using the HSC 7.1 simulation software.

In depth characterization of calcined, in-situ reduced and spent solids were performed applying several techniques namely: N2 sorption, XRD (wide and small angle), TPR/TPD, SEM, TEM, Raman spectroscopy and thermogravimetric (TGA/TDA/MS) analyses.

Results and discussion: Successful synthesis of ordered-mesoporous structures is confirmed for all mono- and bi-metallic samples, even after high temperature in-situ reduction (700°C/2h) by the type IV N2-sorption isotherms, H1-shaped hysteresis loops and steep capillary condensation steps at P/P0= 0.6-0.8. The 2D-hexagonal structure with parallel and cylindrical channels (p6 mm symmetry) is attested by SAXRD patterns displaying, independently on catalyst composition, two distinct peaks at circa 0.94° and 1.5° characteristic of the [100] and [110] plane reflections, respectively.

Catalytic results (CH4 and CO2 conversions and product ratios) show that Ni is extremely more active and more stable than Fe, even for a content that is twice less (Ni5%Al2O3 compared to Fe10%Al2O3). Intrinsic methane conversion over the Fe-free samples was 39% compared to only 12% for the Ni-free one. This latter Fe10%Al2O3 catalyst deactivates on stream compared to a stable performance for the Ni5%Al2O3 catalyst, and its deactivation is attributed in part to segregated metallic Fe0 species (confirmed by XRD) due to absence of strongly bounded Fe with Al2O3 matrix (detected from TPR profiles). In addition to sintering, TGA data reveal a total loss of 5.5 wt.% of C(s) being the highest amongst all spent catalysts. Indeed, typical SEM micrographs of spent Fe10%Al2O3 catalyst confirm the presence of carbon deposits that appear covering parts of the external surface of alumina grains.

The incorporation of a small content of Ni (Fe7.5%Ni2.5%Al2O3) promoted the reactivity and stability of Fe species and the behaviour was significantly noted when an equimolar Fe:Ni= 1 was present. Catalytic performances for the Fe5%Ni5%Al2O3 sample were the highest and the closest to the thermodynamic expected ones (XCH4= 47%, XCO2= 87%, H2:CO= 1.0) Additional increase in Ni content to 7.5 wt% induced an opposite behaviour where: conversions of CH4 and CO2 were lower (XCH4= 41%, XCO2= 75%) than those recorded over Fe5%Ni5%Al2O3 catalyst along with a higher C(s) content (4.5 wt.% of C(s)) and a predominance of poisonous graphitic (hardly oxidizable) Cγ species (DTA profiles). The small amount of carbon deposits (around 2.5 wt.%) over the most reactive equimolar catalyst is confirmed by SEM micrograph of the spent catalyst. On SEM figures, C(s) nanotubes are rarely present and their formation is totally hindered inside Al2O3 pores due to steric constraints and to the stabilization of tiny Ni0 nanoparticles that remain strongly attached and confined within the matrix (confirmed by TEM). The best performance obtained with the optimum equimolar Fe-Ni alloy is a result of an enhancement in the interaction of Fe and Ni alloys with Al2O3 (TPR profiles) upon shifting their reduction temperatures to intermediate values between weakly bounded Fe-species and hardly reducible Ni-Al2O3 alloys. Further characterizations of reduced and spent catalysts by high resolution TEM are in progress and will be presented during the conference.

Conclusions: One-pot synthesis of ordered mesoporous iron, nickel and mixed iron-nickel alumina materials is an easy, effective route for preparing biogas reforming catalysts with stable performances on stream. Ni is a more efficient active phase than Fe for biogas reforming to synthesis gas and the addition of Ni to Fe-based catalysts has a significant effect on reactivity and selectivity. An equimolar Fe-Ni alloy is optimum for enhanced catalytic reactivity and low carbon content (even in presence of higher amount of CH4 than CO2) due to an enhancement in metal-support interaction along with the beneficial effect of Ni and Fe confinement within alumina framework.

Acknowledgements: We acknowledge the SOL-CARE project funding via EranetMED ENERG-065, 2016-2019.

References:

[1] D. Pakhare, D. Spivey. Chem. Soc. Rev. 43.22 (2014) 7813-7837.

[2] S.A. Theofanidis, G.Marin. ACS Catal. 5 (2015) 3028-3039.

[3] K. Jabbour, P. Massiani, N. El Hassan. Appl. Catal. B 201 (2017) 527-542.

[4] Q. Yuan, A. X. Yin, C. H. Yan. J.Am. Chem. Soc. 130 (2008) 3465-3472.

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