Journal of Materials Chemistry A Accepted Manuscript This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains. www.rsc.org/materialsA Page 1 of 23 Journal of Materials Chemistry A Turning periodic mesoporous organosilicas selective to CO 2/CH 4 separation: deposition of aluminium oxide by atomic layer deposition Mirtha A. O. Lourenço a, Ricardo M. Silva a, Rui F. Silva a, Nicola Pinna b, Stephane Pronier c, João Pires d, José R. B. Gomes e, Moisés L. Pinto f, Paula Ferreira a* a CICECO, Department of Materials & Ceramic Engineering, University of Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal b Institut für Chemie, Humboldt-Universität zu Berlin, Brook-Taylor-Str. 2, 12489 Berlin, Germany Manuscript c Service Measures Physiques (IC2MP), University of Poitiers, CNRS, UMR7285, 4 rue Michel Brunet, 86022 Poitiers, France d Center of chemistry and biochemistry, Faculdade de Ciências, Universidade de Lisboa, 1749-016 Lisboa, Portugal e CICECO, Department of Chemistry, University of Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal f CERENA, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, n° 1, 1049-001 Lisboa, Portugal Accepted Corresponding author: Paula Ferreira, email: [email protected]; phone: +351 234401419; fax: +351 234401470 A Abstract Nowadays, CO 2/CH 4 separation is considered extremely important to turn biogas economically interesting. The search of efficient materials for biogas upgrading is at the cutting edge of research in the field of energy. Periodic mesoporous organosilicas (PMO) have high potential to be applied as selective adsorbents for CO 2 as they have concomitant Chemistry high specific surface areas and tunable surface properties. Here we describe the tuning of the surface properties of phenylene-PMO by using atomic layer deposition (ALD) to add active aluminium species to the walls of these organic-inorganic hybrid materials. The modification with aluminium oxide was attained with varying number of deposition cycles (from 2 to 100 cycles). A clear correlation between the amount of aluminium attached to Materials PMO and the number of deposition cycles is observed. Consequently, the increase in the of number of deposition cycles resulted in a reduction of the specific surface area and the pore volume of the PMO material. The variation of the number of deposition cycles to modify the surface of the PMOs yields composite materials with aluminium sites having different local coordination, but keeping intact the meso- and molecular-scale periodicity orders of the parent PMO. Adsorption results indicate that high selectivity for CO 2/CH 4 separation is Journal 1 Journal of Materials Chemistry A Page 2 of 23 obtained when pentacoordinated (Al V) and tetrahedral (Al IV ) aluminium oxide are present in the PMO. Keyword: Periodic mesoporous organosilicas, PMO, ALD, CO 2/CH 4 separation Introduction Manuscript Carbon dioxide is the major contaminant presented in biogas production and is also an important cause for global warming. Currently, the capture of CO 2 is performed by the use of monoethanol amine (MEA), which is a chemical solvent. This process demands high energy costs due to the need of solvent regeneration, and it has also associated corrosion 1,2 problems. In order to find an alternative method for CO 2 separation, researchers have Accepted devoted their attention to porous materials in the last decades.3 Activated carbon 4, zeolites 2 A 5,6 and metal organic frameworks (MOFs) , were proposed as adsorbents for selective CO 2 uptake. However the performance of these adsorbents is so far not satisfactory. The ideal adsorbent for CO 2 capture must be highly selective and possess high adsorption capacity, acceptable adsorption/desorption kinetics, keep stable after several adsorption/desorption cycles, and be thermally and mechanically stable.7 Periodic mesoporous organosilicas (PMOs) have been proposed for this aim due to their high specific surface areas, narrow Chemistry distribution of pore sizes and high pore volumes. 8–10 Furthermore, PMOs with phenylene- bridge (from now on simply denoted as PMO) can be easily modified to turn its surface 11–14 selective to CO 2. So far the enhancement on the selectivity of porous materials towards 5,15–21 CO 2 has mainly been attempted by organic modifications. Zeolites and MOFs with aluminium (Al) open centre incorporated into their matrix structure have shown to improve Materials 22–28 the adsorption of CO 2. To the best of our knowledge, the preparation of PMO/Al 2O3 composites was never tried. Here, we describe for the first time the use of atomic layer of deposition (ALD) to modify the PMO surface with Al 2O3 in order to design a novel CO 2 adsorbent. ALD comprises a sequence of self-limiting chemical reactions between gas- phase precursor molecules and the solid surface. The self-limiting nature of ALD gives rise to a conformal growth and an additional control over the total stack thickness. The film Journal 2 Page 3 of 23 Journal of Materials Chemistry A thickness in a planar substrate can be determined precisely by the number of coating cycles. 29 In this work, by varying the number of ALD deposition cycles, the quantity of aluminium oxide deposited as well as the type of aluminium species on the PMO were studied. Experimental details PMO synthesis Manuscript The mesoporous phenylene bridged PMO was synthesized according to the literature procedures with slight modifications. 8,30,31 The synthesis of PMO started with the hydrolysis and condensation of 1,4-bis(triethoxysilyl)benzene (BTEB, 4.78 g) 32 precursor in the presence of octadecyltrimethylammonium bromide surfactant template (ODTMA, 4.8 g, Aldrich, 98%) in 124 mL of distilled water with 8 mL of 6 M NaOH solution. This Accepted solution was kept for 20 minutes in an ultrasonic vessel and stirred for 24 hours at room A temperature. After 24 hours of ageing, the solution was transferred to Teflon-lined stain- steel autoclave for hydrothermal treatment. The hydrothermal treatment begins with a pre- heating of the autoclave in an oven at 200 ºC during 40 minutes, and then it was transferred to another oven at 100 ºC. After 24 hours, the solid was filter and washed. The template was extracted by an ethanol/HCl solution. Chemistry Al 2O3-PMO preparation ALD was performed with a cross flow home-made reactor working in continuous mode, using water and trimethylaluminium (Sigma Aldrich, 97%) as precursors. The precursors were alternately introduced through ALD pneumatic valves from their reservoirs, which were kept at room temperature. Alumina (Al 2O3) was deposited onto the Materials PMO at 200 °C, under 100 sccm of N 2 flow, using reactant pulse times of 0.05 s and 0.10 s of for water and trimethylaluminium, respectively, and 30 s between pulse times. The typical operating pressures varied from 1.6 to 2.0 Pa during the precursor pulses. The quantity of Al 2O3 deposited by ALD onto the PMO was determined by direct measurements performed before and after the ALD coating using an analytical balance with a precision of 0.1 mg. Journal 3 Journal of Materials Chemistry A Page 4 of 23 The resultant composite materials are denoted from now on as Al-PMO# x, where x corresponds to the number of deposition cycles. An additional sample, PMO+Al 2O3_a was prepared by physical mixture of PMO 33 and amorphous Al 2O3. The amorphous alumina was synthetized according to the literature with some experimental modifications. The aluminium isopropoxide (Al(O iPr), Aldrich, 1 g) was solubilized in ethanol (EtOH, PA, Carlo Erba Reagents, 9.75 mL). Then, 0.46g of nitric acid (HNO 3, 65 %, Panreac) was added to the solution, and the solution was vigorous stirred overnight at room temperature (RT). Then, the solution was transferred to an oven Manuscript for 2 days at 60 ºC to induce the controlled evaporation of the solvent and to obtain the dry material. The obtained material was calcined at 400 ºC during 4 h with a heating ramp of 5 ºC ⋅min -1. Materials characterization Accepted The physical, textural and chemical properties of the pristine PMO and Al-PMO A composites were evaluated by scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), powder X-ray diffraction (PXRD), low temperature (-196 ºC) nitrogen adsorption-desorption isotherms, transmission electron microscopy (TEM), Fourier transform infra-red (FTIR), 29 Si magic-angle spinning (MAS) and cross polarization (CP) MAS nuclear magnetic resonance (NMR), 13 C CP MAS and 27 Al MAS NMR spectroscopies. The thermal stability of materials was evaluated by thermogravimetry Chemistry analysis (TGA). Description of the experimental conditions is presented at the Electronic Supplementary Information (ESI). High pressure adsorption of CO 2 and CH 4 Materials Adsorption experiments of CO 2 and CH 4 on selected samples were conducted up to 1000 kPa at 25 °C, using the volumetric method. These experiments were carried out on a of lab made stainless steel volumetric apparatus, with a pressure transducer (Pfeiffer Vacuum, APR 266), and equipped with a vacuum system that allows a vacuum better than 10 -2 Pa.