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1 Trabajo Fin de Máster Development of nanostructured titanosilicate catalyst layers in monoliths Autor Diego Sanz Carrillo Director Reyes Mallada Viana Universidad de Zaragoza Facultad de Ciencias Departamento de Ingeniería Química y Tecnologías del Medio Ambiente Instituto de Nanociencia de Aragón 2014 2 A scientist is happy, not in resting on his attainments but in the steady acquisition of fresh knowledge. Max Planck 3 4 ACKNOWLEDGEMENTS First and foremost, I would like to thank the Institute of Nanoscience of Aragón and the SAMCA Foundation for their trust and financial support through the corresponding research and scholarship grants. I would also like to thank Reyes Mallada, for her tutelage, and Pilar Cea, for her patience and always being willing to help with the Master’s issues and paperwork. I appreciate the help and support provided by all of INAs researchers, specially Ángela and Adrián for their valuable help with this project, and Alberto, for being a stalwart defender of order around the workbench, my first “residence” in the lab. I also appreciate César Rubio’s assistance with the JDF-L1 synthesis. Not less important than the previous acknowledgements, because they helped me keep a good state of mind during the course, was the kindness and friendship of my Master colleagues, autochthonous and foreign alike. Finally, and on a more personal note, I’ll thank my parents for their unconditional support. My thanks to my friends from the University for keeping me in a good mood during the degree and afterwards. My thanks to the “Vacallerows” for bringing a continuous stream of chaos into my life. Also thanks to Jesús, a.k.a. Chico Bellota, for helping me through Module 7 with his logos, and through life with a joke always at hand or an infinite amount of chatter. And last, but not least, to Estela, for her patience during the writing of this report, for enduring my personality and awfully bad jokes during the last five years and counting, and for being awesome altogether. 5 6 TABLE OF CONTENTS 1.- Abstract...................................................................................................................1 2.- Objectives................................................................................................................2 3.- State of the art..........................................................................................................2 3.1.- Metallic supported catalysts ..............................................................................2 3.2.- Titanosilicates as catalyst supports....................................................................2 3.2.1.- ETS-10.......................................................................................................3 3.2.2.- JDF-L1.......................................................................................................4 3.3.- Structured microreactors ...................................................................................5 3.4.- Selective CO oxidation (SELOX)......................................................................6 3.5.- Ethylene epoxidation.........................................................................................7 4.- Experimental ...........................................................................................................9 4.1.- Synthesis of catalytic powders...........................................................................9 4.1.1.- Synthesis of titanosilicate supports.............................................................9 4.1.2.- Synthesis of CoPt3 nanoparticles ..............................................................11 4.1.3.- Active phase deposition on the supports ...................................................12 4.2.- Development of structured catalytic microreactors..........................................13 4.2.1.- Ceramic monoliths ...................................................................................13 4.2.2.- Seeding procedures ..................................................................................14 4.2.3.- Growth of support layer by hydrothermal synthesis..................................15 4.2.4.- Active phase deposition on the microreactor.............................................17 4.3.- Experimental setups for testing catalytic activity.............................................18 4.3.1.- Selective CO oxidation (SELOX).............................................................18 4.3.2.- Ethylene epoxidation................................................................................20 4.4.- Characterization techniques.............................................................................21 4.4.1.- Zeta potential ...........................................................................................21 4.4.2.- Scanning Electron Microscopy (SEM) and Energy-dispersive X-ray Spectroscopy (EDX) ...........................................................................................22 4.4.3.- Transmission Electron Microscopy (TEM)...............................................23 4.4.4.- X-ray Diffraction (XRD)..........................................................................23 4.4.5.- Microwave Plasma Atomic Emission Spectroscopy (MP-AES)................23 4.4.6.- Thermogravimetric Analysis (TGA).........................................................24 5.- Results and discussion ...........................................................................................25 5.1.- Catalytic powders............................................................................................25 5.1.1.- Titanosilicate supports characterization ....................................................25 5.1.2.- CoPt3 nanoparticles characterization.........................................................30 5.1.3.- Catalytic powder characterization.............................................................33 5.2.- Structured catalytic microreactors ...................................................................34 5.2.1.- Seeding of cordierite monoliths................................................................34 5.2.2.- Titanosilicate growth on cordierite monoliths...........................................36 5.2.3.- Active phase deposition in titanosilicate layers in monoliths.....................40 5.3.- Catalytic activity testing..................................................................................41 5.3.1.- Selective CO oxidation (SELOX).............................................................41 5.3.2.- Ethylene epoxidation................................................................................46 6.- Conclusions and future work..................................................................................48 7.- References.............................................................................................................49 7 1 1.- Abstract Supported heterogeneous catalysts are an essential part of the chemical industry, and new supports or catalysts are always under research. Titanosilicates, microporous zeotype materials, are being widely investigated for catalytic support purposes. Structured microreactors, with a high surface to volume ratio and enhanced mass and heat transfer, overcome some of the limitations of fixed bed reactors. Taking into account these considerations, we proposed developing a reproducible method of synthesizing titanosilicate support layers on the channels of ceramic monoliths, for their use in catalysis. High purity crystals of two different titanosilicate phases (ETS-10 and JDF-L1) were synthesized. These crystals were used for seeding the ceramic monoliths by different methods, of which dip coating and sonication offered the most homogeneous results. Only ETS-10 could be deposited successfully; JDF-L1 crystals were too large. After the seeding, several hydrothermal synthesis methods were tested for the layer growth. The TiCl3-A method was the most reproducible and offered the most uniform layers. The grown crystal phase was not ETS-10, as the seeds, but JDF-L1, as confirmed by XRD. Catalytic powders of both titanosilicates were prepared by ion exchange or impregnation procedures for their testing in two catalytic reactions: Ethylene Epoxidation and Selective CO Oxidation (SELOX). For ethylene epoxidation, silver-based powders were prepared. The size of the silver particles obtained in these powders was generally too small for presenting catalytic activity and the catalyst testing provided very poor results or no activity whatsoever. For SELOX, platinum-based and CoPt3 nanoparticle-based powders were prepared, which offered overall very good catalytic activity and reaching total CO conversion, with CoPt3 providing better results than Pt as the active phase. Supporting the catalyst on the monolith caused an enhancement of the catalytic performance, and Pt-based monoliths provided slightly better performance than CoPt3 powders. 2 2.- Objectives The main goal of this final master project is to develop a reproducible method for synthesizing ETS-10 and JDF-L1 structured layers well anchored to the walls of monolith microreactors. Furthermore, these layers, after deposition of an active phase, will be tested in catalytic reactions such as CO selective oxidation, or ethylene epoxidation, this last reaction requiring a basic support for better performance. The main tasks to be accomplished during the project will be: -Hydrothermal synthesis by secondary growth method of ETS-10 and JDF-L1 on monolithic supports. -Deposition of silver and platinum, by conventional impregnation and ion exchange methods, on powdered titanosilicates and titanosilicate layers grown on monoliths. -Characterization of the prepared materials by SEM, TEM, XRD, EDX, MP-AES as well as catalytic test evaluations in experimental setups for each reaction. 3.- State of the art 3.1.- Metallic supported catalysts In the chemical industry, most reactions happen at the surface of an heterogeneous catalyst. Its efficiency will be defined by the quality of the materials used, the exposed surface area of the active phase and its stability. The smaller the size of the active phase particles, the higher the surface area, but too small particles cannot be used in reactors. That’s when catalyst supports enter the game. These supports are usually porous materials that allow the highest loading of highly dispersed metal particles possible. The active phase can then be synthesized in the smallest particle size available if needed[1]. Several procedures exist for attaching the active phase to the support, but that will not be covered here as some of them are explained in the experimental section 4.1.3.-. 3.2.- Titanosilicates as catalyst supports Microporous materials such as zeolites and zeotypes present a high surface area and are well known as sorbents and catalytic supports for dispersion of nanoparticles. However, most zeolitic materials present acid sites which promote combustion and should be avoided in selective oxidations, and only a few of these materials possess basic sites. In particular, titanosilicates, such as ETS-10, are characterized for a high basicity and they have been successfully tested in biodiesel production[2]. JDF-L1 is another titanosilicate closely related to ETS-10 in composition, but with a layered structure. It has been used 9 4.- Experimental 4.1.- Synthesis of catalytic powders This section will cover all procedures conducted during the experimental work of the project leading towards obtaining titanosilicate-based catalytic powders. As a first step, small crystals of the selected titanosilicates must be synthesized. The next step involves the deposition of the metallic active phase on the catalytic support. These metals are silver for ethylene epoxidation and platinum for selective CO oxidation, and can be deposited on the supports by several methods, being Ion Exchange and Incipient Wetness Impregnation the ones explored in this work. CoPt3 nanoparticles were also synthesized and deposited on the supports by means of electrostatic stabilization for their use in selective CO oxidation. 4.1.1.- Synthesis of titanosilicate supports Synthesis of ETS-10 Crystals of the titanosilicate ETS-10 were obtained following a hydrothermal synthesis procedure based on the previous works of Rocha et al.[19] and various members of the Nanoporous Films and Particles group of the Nanoscience Institute of Aragón[20], using Anatase (TiO2) as the source of Titanium. To prepare a gel of molar composition 4.8 Na2O / 1.4 K2O / TiO2 / 5.1 SiO2 / 125 H2O, deionized water (23.22 g), NaCl (Merck) (5.06 g) and KCl (99.0 wt.%, Panreac) (1.23 g) were mixed and stirred until complete dissolution of the salts. Afterwards, a sodium silicate solution (10.6 wt.% Na2O, 26.5 wt.% SiO2, Sigma-Aldrich) (17.69 g) was slowly added while increasing the stirring. The solution was kept under stirring for 1h and 30 min. After that time, KF (≥99.0 wt.%, Sigma-Aldrich) (1.57 g) was added and the solution was stirred for an additional 20 min. Finally, TiO2 (Anatase, <25 nm nanopowder, 99.7 wt.%, Sigma-Aldrich) (1.22 g) was added to the mixture to obtain approximately 50 g of a white gel which was stirred for 1h. The pH of our gel was usually around 10.8. According to Rocha et al.[19], pH is a critical value for obtaining pure titanosilicate phases. Based on his results, the optimum pH for obtaining pure ETS-10 would be pH = 10.4 ± 0.2. Thus, the pH of the gel was measured (Cyberscan pH 2100, Eutech Instruments; Hamilton Filltrode probe with Skylyte-CL electrolyte) and adjusted using concentrated HCl (37 wt.%, SigmaAldrich) to minimize the increase on the molar ratio of water. 10 After pH adjustment, the gel was transferred to a 35 mL Teflon-lined autoclave and kept 24h in an oven at 230 ºC for the static crystallization to take place. After that time, the autoclave was quenched using tap water. The contents of the autoclave were then centrifuged (10000 rpm, 10 min; Heraeus MEGAFUGE 16R Centrifuge, Thermo Scientific) and washed with deionized water several times. The recovered solid was then dried overnight at 110 ºC, finally obtaining ca. 5 g of powder. Synthesis of JDF-L1 Crystals of JDF-L1 were synthesized by unseeded hydrothermal synthesis following the procedure developed by Rubio et al.[10,21] using TiCl3 as Ti source. To prepare a gel of molar composition 2.9 Na2O / TiO2 / 4.2 SiO2 / 101 H2O, a sodium silicate solution (8 wt.% Na2O, 27 wt.% SiO2, Merck) (10.05 g) was mixed with deionized water (6.54 g) and NaOH (≥98 wt.%, pellets, Sigma-Aldrich). After adding the TiCl3 solution (20% wt.% in 3 wt.% HCl, Alfa Aesar) (8.24 g), the mixture turned into a bluish-black gel which was almost a dry solid. This gel was stirred manually for 5 min until it started turning into a liquid gel again, and then was kept stirring for 1h. Afterwards, the gel was degassed in an ultrasonic bath (JP Selecta, Ultrasons model, 40 kHz), and then transferred to a 35 mL Teflon-lined autoclave were the static crystallization took place for 93h at 230 ºC. After that time, the autoclave was quenched with tap water and the solid was filtered (2-3 µm pores, 200 µm thickness; PRAT DUMAS), washed several times with deionized water and dried overnight at 110 ºC. About 3.8 g of powder were obtained. Crystals of JDF-L1 were also obtained by seeded hydrothermal synthesis. The procedure used was exactly the same as for the unseeded synthesis, but adding 79 mg of the crystals from the previous synthesis (thoroughly grounded) to the new synthesis gel, which was kept in the oven for only 24h. Circa 4.2 g of powder were obtained. As with the synthesis of other titanosilicates, pH is a critical factor, however no adjustment of pH was necessary since the amount of reagents is optimized for obtaining a gel with a pH value around 10.4. However, this can be checked by 1/100 dilution of a fraction of the gel to measure its pH (dilution needed for an adequate contact with the pH-meter probe). A brief summary of the most relevant information of both titanosilicate syntheses is collected in Table 1. 11 Table 1.- Summary of methods for obtaining small crystals of ETS-10 and JDF-L1 by hydrothermal synthesis. For all methods: pH 10.4; Static crystallization; T=230 ºC. Method name Molar ratio of the synthesis gel Seeding Synthesis time ETS-10 TiO2 4,8 Na2O / 1,2 K2O / TiO2 / 5,1 SiO2 / 125 H2O No 24 h JDF-L1 TiCl3 2,9 Na2O / TiO2 / 4,2 SiO2 / 101 H2O No 93 h JDF-L1 TiCl3-S 2,9 Na2O / TiO2 / 4,2 SiO2 / 101 H2O Yes 24 h The yield of these syntheses was calculated with the mass of solids obtained after synthesis and the mass of reagents employed. The mass of solid, along with the molecular weight of the appropiate titanosilicate (ETS-10: Na1.6K0.4Si5TiO13·3.5H2O, MW=511.84 g/mol; JDF-L1: Na4Si8Ti2O22·4H2O, MW=836,60 g/mol) will give us the experimental moles of titanosilicate obtained. The maximum theoretical moles which could be obtained can be known by the moles of Si or Ti (whichever is the limiting reagent) that were added to the synthesis. Dividing experimental by theoretical moles, the yield of the synthesis is obtained. 4.1.2.- Synthesis of CoPt3 nanoparticles Pt-based intermetallic compounds (IMCs) may be promising catalysts for the selective oxidation of CO[16] and, in fact, CoPt3-ETS-10 catalytic powders have already been tested by a fellow researcher of the NFP group of the Nanoscience Institute of Aragón. All steps required to prepare the bimetallic nanoparticles[22] were carried out under N2 atmosphere and vigorous stirring at 0 ºC. Firstly, a NaBH4 (≥99 wt.%, Sigma-Aldrich) ethanol solution (100 mL, 0.066M) was added drop by drop to a CoCl2 (≥98 wt.%, anhydrous, Sigma-Aldrich) ethanol solution (100 mL, 3.6 mM) which also contained 0.5000 g of PVP (M.W.=10000, Sigma-Aldrich). The solution suffers a colour change from blue [Co(II)] to brown [Co(0)] indicative of the formation of cobalt nanoparticles. Then, a H2PtCl6 (~38 wt.% Pt, Sigma-Aldrich) ethanol solution (100 mL, 10.8 mM) was added drop by drop to the previous nanoparticle suspension. After 30 min of vigorous stirring, another NaBH4 ethanol solution (100 mL, 0.066M) was slowly added and the mixture was kept under stirring for 3h. Afterwards, the final suspension was subjected to several cycles of centrifugation (12000 rpm, 15 min) and ethanol washing. Previous experiences from other NFP researchers suggested that drying the nanoparticle suspension and redispersing it later when needed caused too much aggregation of the NPs, even after sonication, so the CoPt3 NPs were kept in ethanol. 12 4.1.3.- Active phase deposition on the supports Ion Exchange is a common procedure in zeolite-like materials, where ions of a certain metal are incorporated into the matrix by exchange in solution with the extra framework cations of said matrix. In the case of ETS-10 and JDF-L1 the extra framework cations are Na+ and K+. Incipient Wetness Impregnation (IWI) is another common technique for the synthesis of heterogeneous catalysts. A salt containing the metal precursor is dissolved and added to the catalyst support in a quantity approximately equal to the pore volume of this support, so the mass transfer is driven by capillary action and not diffusion (much slower). Both of these deposition techniques require afterwards a calcination step, with the dual objective of decomposing (or driving off, if volatile) other remanent chemicals of the solutions, and causing the annealing of the metal ions into active metal particles or clusters. Ionic Exchange (Ag) The Ag(I)-exchanged titanosilicate samples were obtained by adding 400 mg of ETS-10 or JDF-L1 to 100 mL of an AgNO3 (99.9999 wt.%, Sigma-Aldrich) aqueous solution (5.56 mM) and stirring the solution for 24h. Once the ion exchange was finished, the solution was centrifuged and the solid washed with water several times. Afterwards, the powder was dried in a furnace at 100 ºC for 5h and then calcined at 500 ºC for 1h. The amount of silver used in this procedure is a 15% of the mass of titanosilicate. However, since the ion exchange is a diffusion equilibrium process, such a high loading won’t be reached. The experimental metal loading of the titanosilicate crystals will be lower than a 15 wt.% Ag. Ionic Exchange (Pt) A similar procedure is conducted for obtaining Pt(II)-exchanged titanosilicate samples, adding 300 mg of titanosilicate to 100 mL of a [Pt(NH3)4](NO3)2 (99.995 wt.%, SigmaAldrich) aqueous solution (0.25 mM) and stirring for 24h, followed by several cycles of centrifugation and washing. Finally, the powder is calcined in a furnace at 350 ºC for 3h using a temperature ramp of 2 ºC/min. Due to equilibrium process, the experimental metal loading of the crystals will be lower than the theoretical 1.7 wt.% Pt. Incipient Wetness Impregnation (Ag) For this method, 0.5 mL of an Ag(NO3) aqueous solution (1.11 M) were added to 400 mg of the titanosilicate (in the case of JDF-L1, with a larger pore volume, 0.5 mL of water were also added) and the slurry was grounded for around 5-10 min. The slurry 13 was then dried for 5h at 100 ºC and calcined in a furnace at 500 ºC for 1h. The expected silver loadings are approximately 15 wt.% Ag. CoPt3 nanoparticle deposition on the supports Nanoparticle deposition on the titanosilicate supports was achieved through electrostatic stabilization, due to the negative superficial charges both the nanoparticles and the supports present in aqueous and ethanolic solutions (see Figure 18). As the first step, a polyethyleneimine (PEI) solution is prepared by dissolving 250 mg of PEI (branched, M.W.=25000, Sigma-Aldrich) in 20 g of deionized water. This PEI solution (1.4 g) is added to 500 mg of the titanosilicate support and the mixture is sonicated 15 min in an ultrasonic bath and later dried at 100 ºC for at least 1h. The suspension of CoPt3 nanoparticles in ethanol obtained before is added to the PEI-functionalized titanosilicate in a quantity such that the mass of particles is a 3% of the titanosilicate mass. The final loading of particles may be lower. (The volume of NP suspension needed can be known by drying fractions of it and measuring the solid residue to obtain an approximate concentration). The mixture is sonicated 15 min in an ultrasonic bath and later dried at 80 ºC for 3h. Afterwards, the powder is calcined in a furnace with a heating ramp of 2 ºC/min up to 500 ºC, which is maintained for 2h. 4.2.- Development of structured catalytic microreactors Another of our objectives is developing structured catalytic microreactors and testing them for the same reactions as the corresponding powders. To achieve that we seeded cordierite monoliths, conducted hydrothermal syntheses to create well-intergrown titanosilicate layers on the inner surface of the monoliths and, finally, deposited the active metals onto the support layers. 4.2.1.- Ceramic monoliths As the structural component of our catalytic microreactors we chose ceramic monoliths. Specifically, monoliths made of cordierite, a magnesium-aluminosilicate (2MgO x 2Al2O3 x 5SiO2) with a very low thermal expansion coefficient (TEC), high refractoriness and good mechanical strength among other properties[11]. The monoliths used in this work, provided by Corning, were made of synthetic cordierite and had a cell density of 400 cells/inch2. Large monolith pieces were cut down using a utility knife until they had between 1.5 and 2 cm in length and between 1.2 and 1.5 cm in diameter or width. They were cut as square-prisms for all the testing and analysis, except the 14 monoliths meant for use in the catalytic reaction setups which were cut as cylinders for a better fitting into the quartz tubes used in those setups. For all the seeding, growth and catalyst deposition steps, the monoliths were covered with teflon tape to avoid deposition or growth on their external walls. 4.2.2.- Seeding procedures Three different methodologies were considered for depositing small crystals of titanosilicates onto the internal surface of the monoliths. Sonication This method is based on immersing the pristine monolith into a suspension containing the titanosilicate seeds and then sonicating the suspension in an ultrasonic bath to favor the movement and deposition of the crystals onto the rough surface of the monolith channels, while at the same time preventing crystal sedimentation. Two different concentrations of the titanosilicate suspension were tested: ~13 wt.% seeds and ~2.5 wt.% seeds. The monoliths were subjected to 2 immersions, sonicating for 20 min each, blowing with nitrogen the excess of suspension after each immersion. The monoliths were later dried at 110 ºC for at least 3h. Layer by layer deposition by Electrostatic Stabilization (LbL by ES) Due to the repulsive nature of the interactions between titanosilicate crystals and cordierite (see Figure 18 in Section 5.1.-), the attachment of an oppositely charged polymer to one of the materials can lead to a better adhesion of the crystal seeds to the monolith surface due to electrostatic stabilization. This process consisting on depositing the polymer and then the seeds can be repeated several times and is known as layer by layer deposition[23]. Cordierite, ETS-10 and JDF-L1 have negative surface charges, so a cationic polymer is needed. Polyethyleneimine (PEI) and Poly(diallyldimethylammonium chloride) (PDDA) are two polymers which can be found in literature[23,24] being used for these purposes. The procedure is identical for both polymers. The monolith is first immersed in a 0.4 wt.% aqueous solution of PEI (branched, M.W.=25000, Sigma-Aldrich) or PDDA (20 wt.% in H2O, M.W.=100000-200000, Sigma-Aldrich) and sonicated for 10 min. The polymer is then rinsed in water and air blown. Next, it is immersed in a 2.5 wt.% suspension of the titanosilicate seeds, sonicated for 10 min, rinsed and air blown. This cycle of immersion in polymer solution and seed suspension is repeated once more. Afterwards the monolith is dried in an oven at 110 ºC and calcined in a furnace at 550 15 ºC for 3h with a ramp of 2 ºC/min, so the polymer is completely eliminated, to avoid its possible interference with the subsequent hydrothermal synthesis. Dip coating Dip coating is a process in which a substrate is immersed in a suspension of the material to be deposited and then it is taken out slowly, at a constant speed, causing its deposition (the final thickness of the deposited layer depends on the withdrawal speed). The substrate is then left to dry, so all the solvent evaporates allowing the formation of a thin film of the deposited material. For the deposition of titanosilicate seed crystals onto cordierite monoliths, a home-made dip-coating setup was employed, comprising a peristaltic pump and two wide burettes. The suspension used had a 13 wt.% of seeds and the immersion-withdrawal cycle was repeated 4 times, using a withdrawal speed of 1 cm/min. Table 2.- Main parameters of the different seeding procedures. Seeding method Polymer solution Immersions Duration Seed suspension Immersions Duration Withdraw speed Sonication (13 wt.%) - - - ~13 wt.% 2 20 min - Sonication (2.5 wt.%) - - - ~2.5 wt.% 2 20 min - LbL by ES (PEI) 0.4 wt.% 2 10 min ~2.5 wt.% 2 10 min - LbL by ES (PDDA) 0.4 wt.% 2 10 min ~2.5 wt.% 2 10 min - Dip Coating - - - ~13 wt.% 4 - 1 cm/min 4.2.3.- Growth of support layer by hydrothermal synthesis To prepare a continuous titanosilicate layer on the monolith’s inner surface, different hydrothermal synthesis gels were tested. These gels, modified from the bibliography[19,20], were initially designed for seeded and/or unseeded synthesis of free ETS-10 crystals using TiCl3 as the Ti source, with the crystals so obtained being much larger than those from the Anatase route used for seed synthesis in Section 4.1.1.-. Due to the size of the synthesized crystals, it is expected that a seeded synthesis with these gels onto a support such as the cordierite monoliths will lead to large crystals of the titanosilicate intergrowing into an homogeneous layer covering the whole internal surface, as previously reported for alumina supports and metallic microreactor channels[25]. Figure 6.- Dip coating setup. 16 The synthesis method referred to as “TiCl3 A” is explained in detail in the following pharagraphs, while the rest of the methods tested have very similar procedures and thus can be extrapolated from the information given in Table 3. “TiCl3 A” method for titanosilicate layer growth on monoliths To prepare a synthesis gel of molar composition 4.2 Na2O / 1.2 K2O / TiO2 / 5.6 SiO2 / 343 H2O, TiCl3 solution (15 wt.% in 5-10 wt.% HCl, Riedel-de Häen) (6.5g) was mixed with deionized water (27.5g). Hydrogen peroxide (30 wt.% in H2O, Sigma-Aldrich) (0.85g) was then added under constant stirring to oxidize Ti(III) into Ti(IV), causing the solution to change colour from violet to red. Then, KCl (99 wt.%, Panreac) (1.1g) and NaOH (≥98 wt.%, pellets, Sigma-Aldrich) (1.3g) were added and another colour change observed, from red to light orange, at the same time the mixture got denser. The slow addition of a sodium silicate solution (8 wt.% Na2O, 27 wt.% SiO2, Merck) (7.8g) yielded the final gel, which turned from light orange to a light yellow or cream colour. The pH was measured and adjusted to 10.4 ± 0.2. With the help of a hollow cylindrical teflon piece (Figure 7 A) to avoid channel blocking against the bottom of the autoclave, the monolith is introduced vertically into a teflon-lined 35 mL autoclave (Figure 7 B), which is then filled to ¾ it’s capacity with the previously synthesized gel. Hydrothermal synthesis is then conducted by putting the autoclave in an oven at 230 ºC for 36h. After the crystallization, the autoclaves are quenched with tap water to stop the synthesis. The monolith is taken out, washed with deionized water, air blown, sonicated in deionized water for 5-10 seconds and then washed and air blown again. It is finally dried at 110 ºC. Due to the limited accessibility of the gel into the monolith, primary nucleation always happens in the bulk of the gel outside the monolith. The gel or solids remaining inside the autoclave after removing the monolith can be recovered (by centrifugation, washing and drying, as specified previously for the ETS-10 seeds) to compare the titanosilicate phases grown inside and outside the monolith. As previously stated, Table 3 contains information on all the methods used for the titanosilicate layer growth on monoliths. Each method with a distinctive synthesis gel is given an identificative letter, while methods with small modifications such as the use of a rotatory oven, not using hydrogen peroxide, or different synthesis times, all while Figure 7.- A) Teflon piece schematic. B) Monolith position inside the autoclave. 17 maintaining the same gel molar ratio, are identified with the same letter as the parent method but adding a suffix. Table 3.- Summary of the different methodologies tested for the growth of a titanosilicate layer onto the monolith supports. For all methods: Gel pH = 10.4. Synthesis gel Hydrothermal synthesis conditions Method name Molar ratio Use of H2O2 Time Temperature Oven TiCl3-A Yes 36 h TiCl3-A* No 72 h TiCl3-A** Yes 45 h Static TiCl3-A/R 4.2 Na2O / 1.2 K2O / TiO2 / 5.6 SiO2 / 343 H2O Yes 36 h 230 ºC Rotatory TiCl3-B 2.9 Na2O / 1.8 K2O / TiO2 / 5.3 SiO2 / 280 H2O TiCl3-B* 4.7 Na2O / 1.8 K2O / TiO2 / 5.3 SiO2 / 280 H2O No 72 h 200 ºC Static TiCl3-C 4.2 Na2O / 1.2 K2O / TiO2 / 5.6 SiO2 / 202 H2O Yes 36 h 230 ºC Static TiCl3-D 4.6 Na2O / 1.5 K2O / TiO2 / 5.2 SiO2 / 124 H2O Yes 36 h 230 ºC Static TiCl3-E 36 h TiCl3-E* 24 h Static TiCl3-E/R 4.6 Na2O / 1.5 K2O / TiO2 / 5.2 SiO2 / 203 H2O Yes 36 h 230 ºC Rotatory 4.2.4.- Active phase deposition on the microreactor As with the catalytic powders, the appropiate active phases for our studied reactions can be introduced into the titanosilicate layer by ion exchange, which is followed by a thermal reduction step. Ion Exchange (Ag and Pt) The Ion Exchange procedure for the titanosilicate layers grown on monoliths is mainly the same as for the titanosilicate crystals. The same reagents, their amount, solution volume, exchange time and calcination procedure are used, for both Ag and Pt exchanges. The only difference is that, instead of suspending the powdered titanosilicate in the solution, a layer grown monolith is immersed in the solution and positioned, with the help of a teflon piece and teflon tape, in such a way that the stirring causes the solution to constantly flow in and outside of the monolith’s channels. The weight of titanosilicate inside the monoliths once the layers have been grown averaged 160 mg for the TiCl3-A method (chosen as the ideal growth method). This means that we had a higher metal precursor/titanosilicate ratio in comparison with the ion exchange procedure for the powders, so higher theoretical loadings were expected. 18 4.3.- Experimental setups for testing catalytic activity Once obtained and ready, both the catalytic powders and the structured catalyst layer microreactors were ready for their catalytic activity testing on each specific experimental setup. 4.3.1.- Selective CO oxidation (SELOX) In the SELOX reaction setup, the catalytic powders and microreactors containing Pt or bimetallic CoPt3 nanoparticles as active components were tested for catalytic activity. The reactor and experimental setup used for this work are schematized in Figure 8. The gas chromatograph was a CP-4900 Micro GC from Varian, which comprised a MS5A molecular sieve column and a PPQ column with the corresponding TCD detectors, which allow the analysis of permanent gases (O2, N2, H2 and CO), and of CO2 and H2O, respectively. Gas cylinders were provided by Praxair (H2, 99.999 %; O2, 99.99%; N2, 99.999 %; CO2, 99.995 %; H2-CO mixture, 1.3 v.% CO with H2 balance). Figure 8.- Experimental setup for catalytic activity analysis of SELOX powder and microreactor catalysts. 1Feed gas cylinders. 2Mass flow meters (5850 TR series, Brooks). 3Mass flow controller. 4Valves for operation control. 5Quartz reactor (φ = 7 mm for powders or φ = 16 mm for monoliths). 6Oven with PID Eurotherm temperature controller. 7Thermocouple with quartz sheath and temperature controller. 8Bubble-meter for inlet and outlet flow measurement. 9Gas chromatographer. 10Computer system and software. 11Portable CO sensor. For the catalytic powder testing, 200 mg of the catalyst were mixed with 300 mg of silica spheres for enhanced heat dissipation. The resulting powder was introduced into the quartz tube reactor at an intermediate height (to ensure isothermal heating from the oven) with the aid of glass wool. The catalyst bed was then compressed a little bit, 25 5.- Results and discussion 5.1.- Catalytic powders 5.1.1.- Titanosilicate supports characterization This section will cover the results observed and the data obtained from the characterization of the synthesized titanosilicate crystals. ETS-10 Several ETS-10 batches were synthesized during the course of the project which had slightly different characteristics. Table 4 collects specific data from these syntheses. Table 4.- Summary of information from the synthesized ETS-10 crystal batches. Crystal sizes were only measured for selected batches. Batch code Gel pH Yield Crystal size (µm) Crystal morphology a = 0.55 ± 0.14 ETS-10 B1 10,1 63,81% c = 0.81 ± 0.17 ETS-10 B2 10,1 60,17% - ETS-10 B3 10,4 57,09% - ETS-10 B4 10,4 45,64% - a = 0.26 ± 0.08 ETS-10 B5 10,4 72,89% c = 0.41 ± 0.09 a = 0.35 ± 0.05 ETS-10 B7 10,4 65,68% c = 0.43 ± 0.11 As can be seen in the table, the yield of the hydrothermal synthesis (method “ETS-10 TiO2”) was found to vary between 45% and 75% (yield based on Ti, the limiting agent). The variability doesn’t seem to arise from the pH adjustment of the gel, which would be the obvious choice, so we cannot assure which parameter is responsible for this. The powder diffraction patterns from the selected batches B1, B5 and B7 are shown in Figure 11. The patterns are compared with a simulation diffractogram of ETS-10[26]. For the three samples, all the peaks corresponding to the simulation can be found, thus confirming the presence of ETS-10 in the batches. The sample patterns show two wide peaks at 5.90º and 29.85º (not marked in the figure) that belong to ETS-10 despite not correlating with the ETS-10 simulation spectrum. These two peaks appear in every experimental XRD analysis we have performed on our ETS-10 samples (including TiCl3 route syntheses, which rules out anatase as the source of those peaks), and even in 26 bibliography, disregarding purity of samples[5,20]. Additionally, another two peaks can be found in all the diffractograms (marked with x in the figure). These peaks belong to quartz impurities, which was expected, since quartz has been reported previously as the main impurity appearing in TiO2-based ETS-10 syntheses[5]. It is here where the pH value plays an important role, since it has a great effect on the amount of impurities present in the obtained powder. In Figure 11 it can be observed that the intensity of the quartz peaks for B1, which was adjusted to pH 10.1, is much higher than for B5 or B7, both of them adjusted to pH 10.4. The intensity of the ETS-10 peaks for these two batches is also higher than in the B1 pattern. These experimental results indicate that batches synthesized after adjusting the pH value to 10.4 are of higher purity. This is in accordance with what was reported by Rocha et al.[19], who recommended pH around 10.4 for obtaining high purity ETS-10 crystals. An unidentified peak remains in the B1 pattern, but other impurities from the anatase synthesis were neither expected nor found, since the crystallization of ETS-4 happens at higher pH[5]. 5 10 15 20 25 30 35 40 45 x x x x x x Intensity (a.u.) 2 θ ETS-10 B1 ETS-10 B5 ETS-10 B7 ? Simulated ETS-10 Figure 11.- XRD spectra from simulated ETS-10[26] and from experimental batches B1, B5 and B7. Peaks marked with x correspond to quartz impurities[27]. The crystals of these batches were also analyzed by SEM (Figure 12 shows crystals of B1 and B5 with their corresponding histograms; Additional images for B1, B5 and B7 can be seen in the Appendix, Figure A 2 to Figure A 4). The images show that the crystals have a truncated bipyramid morphology, thus being in accordance with the 27 reported morphology of ETS-10 synthesized by the Anatase route[20]. The purity of each batch, already discussed before, is in correlation with the amount of amorphous material or impurities which can be seen in the images. B1, with pH 10.1 and a higher amount of quartz impurities (according to Figure 11) presents a “dirtier” look than B5 and B7, with pH 10.4 and higher sample purity. Average crystal sizes (shown before in Table 4) of the synthesized batches with pH 10.4 are in agreement with reported values of ~0.3 µm x ~0.4 µm (a=b x c)[20]. Batches adjusted to pH 10.1 maintained the same shape and crystal habit (as has been indicated) but showed a larger crystal size (0.55 x 0.81 µm). Figure 12.- SEM images of ETS-10 crystals from batches B1 (A) and B5 (B), accompanied by the histograms obtained from the particle size distribution analysis. Synthesis of JDF-L1 The titanosilicate JDF-L1 was synthesized both by unseeded and by seeded hydrothermal syntheses. Table 5 collects information on the synthesized batches. 28 Table 5.- Summary of information from the synthesized JDF-L1 crystal batches. Batch code Yield Crystal size (µm) Crystal morphology - JDF-L1 B1 93,78% - a = 52.87 ± 6.31 JDF-L1 B2 94,58% c = 0.65 ± 0.14 a = 13.69 ± 4.27 JDF-L1 B2-S 98,62% c = 0.50 ± 0.12 a = 5.90 ± 1.48 JDF-L1 B3-S 97,65% c = 0.21 ± 0.06 Two batches (JDF-L1 B1 and B2) were synthesized following the unseeded synthesis procedure (JDF-L1 TiCl3 method). Another three batches of the layered titanosilicate (JDF-L1 B1-S, B2-S and B3-S) were synthesized by the seeded synthesis procedure (JDF-L1 TiCl3-S method). For B1-S and B2-S, the powders obtained from B1 and B2, respectively, were grounded and used as seeds; for B3-S, seeds were provided by César Rubio from the CREG group of the Institute of Nanoscience of Aragón. The yield of both synthesis methods was very high, more so for the seeded synthesis. 5 10 15 20 25 30 35 40 45 50 JDF-L1 Batch 3 (Seeded) JDF-L1 Batch 2 Intensity (a.u.) 2 θ JDF-L1 Simulation Figure 13.- XRD spectra from simulated JDF-L1[28] and from experimental batches B2 and B3-S. No impurities identified. The powder diffraction patterns from the selected batches B2 and B3-S are shown in Figure 13. The patterns are compared with a simulation diffractogram of JDF-L1[28]. 29 This XRD analysis showed that the JDF-L1 crystals obtained in each synthesis were of high purity, since no other crystallographic phases but JDF-L1 can be identified with the spectra of B2 and B3-S. Additionally, not only peak position but also the intensity of each peak fits very well with the simulation of the material used for the identification. SEM images of the batches reveal that the crystals obtained by the unseeded synthesis are somewhat large (~50 µm) which renders them inappropiate for the monolith seeding procedures. However, smaller crystals of JDF-L1 (5-10 µm) were obtained by the seeded hydrothermal synthesis. The observed morphology of the crystals, square or rectangular sheets, is the expected for the laminated titanosilicate. The sheets from the unseeded syntheses (B2, Figure 14) are, however, densely grouped up in big bundles. The crystal sheets from the seeded syntheses (B2-S, Figure 14) are grouped in smaller bundles, mainly due to the smaller sheet size, but also the bundles are more “open” or “loose”, as seen in Figure 15 for B3-S. Figure 14.- SEM images of JDF-L1 crystals. A) JDF-L1 B2; B) JDF-L1 B2-S. 30 Figure 15.- SEM image of JDF-L1 B3-S and its size distribution histogram. The size distribution analysis performed for B2 and B3-S provided average sizes in agreement with the expected crystal size of the unseeded and seeded syntheses, according to bibliography[21]. However, B2-S crystals were bigger in size than those from B3-S synthesis. The explanation we suggest to justify this size difference is that the initial size of the seeds has a great influence on the final crystal size. For B2-S, B2 crystals were grounded as seeds, and they probably were quite large even after the grounding. For B3-S, we used 3 µm seeds provided by the CREG group, and it is reasonable that their small size meant more seeds in the same amount of powder, so a high number of crystals translates into an overall small growth. Additional images of B3-S crystals are available in the Appendix, Figure A 5. 5.1.2.- CoPt3 nanoparticles characterization The metallic particles were analyzed by XRD and the powder diffraction pattern is shown in Figure 16. The peaks in the diffractogram are very broad due to the small size of these metallic particles. It can be seen in the diffractogram that the peaks correlate reasonably to the CoPt3 metallic phase. However, peaks expected for pure Pt are also reasonably close to the particle diffraction peaks, but the possibility of having Pt and Co metallic phases and not the bimetallic compound can be discarded given that none of the peaks could be correlated with the metallic Co phase. There is another bimetallic phase, CoPt, that could have been formed in the synthesis, but this possibility is also discarded as none of its peaks are present in the sample’s diffractogram. However, the diffraction patterns present a series of very intense, narrow peaks that do not belong to any of these metals and their combined phases. These peaks have been identified as NaCl, which 31 could be formed in the synthesis since there are sodium and chlorine ions in solution (from NaBH4 and from the metal precursors, respectively). 20 30 40 50 60 70 80 90 Intensity (a.u.) 2 θ Simulation peaks CoPt 3 nanoparticles Figure 16.- XRD spectra of the CoPt3 NPs and reference peak locations (from ICDD) for NaCl (red), Pt (grey) and CoPt3 (blue). The Scherrer equation (Equation 12), was used to estimate the particle size. Taking K = 0.94 and λCu Kα = 0.154 nm, and using the peak at 2θ = 40.18º which has a broadening of 4.06º or 0.0709 rads, the particle size (L) is estimated as 2.17 nm. Equation 12: θ λ cosL K B= The CoPt3 nanoparticles were also studied in TEM to assess the morphology and particle size. A selected picture and the histogram obtained from the size distribution analysis are shown in Figure 17. Despite having kept the particles in ethanol without any drying process after the synthesis, and the vigorous sonication they were subjected to before preparing the TEM grid, images show that the nanoparticles are agglomerated. According to the size distribution analysis from the TEM images, the average particle size was 3.34 ± 0.58 nm, slighty larger than the estimation from the XRD data, but in agreement the results obtained by other authors using the same method[22]. 32 Figure 17.- TEM image of CoPt3 nanoparticles and the particle size distribution histogram. Since the deposition techniques involve electrostatic interactions, the zeta potential of the titanosilicates, cordierite and bimetallic nanoparticles were measured in water at different pH and in ethanol (see Figure 18). The titanosilicates and the bimetallic nanoparticles present negative surface charges in water or ethanol solutions. In the case of cordierite the surface is positively charged at pH=2. The interactions between any two of these samples will be repulsive (except cordierite at low pH). For this reason the surfaces of titanosilicates and cordierite were positively charge using a cationic polymer, see section 4.2.2.-. 2 4 6 8 10 12 14 -40 -20 0 20 Zeta Potential (mV) pH Cordierite ETS-10 JDF-L1 CoPt 3 NPs Aqueous media EtOH Figure 18.- Zeta potential vs pH representation of the samples. Values of zeta potential in ethanolic solution are shown in the right section of the graph. Tabulated values available in Table A 1. 33 5.1.3.- Catalytic powder characterization Once the catalytic supports were synthesized and ready, the active metal phases were deposited by different means and a variety of catalytic powders were prepared. Table 6 collects some fundamental information about these powders. Table 6.- Summary of prepared catalytic powders and their characteristics. *=Value obtained for the individual NPs in Figure 17. Catalytic powder Deposition method Theoretical loading (wt.%) Experimental loading (wt.%) Metal particle size (nm) Impregnation 15% 12.72 ± 0.10 4.59 ± 2.43 Ag-ETS-10 Ion Exchange <15% 5.86 ± 0.38 10.08 ± 3.96 Ag-JDF-L1 Impregnation 15% 13.62 ± 0.26 7.52 ± 4.40 Pt-JDF-L1 Ion Exchange <1.7% 1.14 ± 0.04 2.64 ± 0.65 CoPt3-JDF-L1 Electrostatic stabilization 3% NPs (0.27% Co; 2.73% Pt) 2.15 ± 0.26 NPs (0.18% Co; 1.97% Pt) Atomic Pt/Co ratio = 3.24 3.34 ± 0.58 * The catalytic powders’ metal contents were studied by atomic emission spectroscopy, and the experimental loadings determined in the analysis are shown in Table 6. Silver loadings for the impregnated samples were very similar to the theoretical values, as expected from this deposition method, though some of the metal mass has been lost in some of the treatment steps. For the ion-exchanged samples, due to the equilibrium process involved, loadings are much lower than the theoretical value. Platinum is more easily exchanged than silver, and this could be attributed to the smaller size of the ionic radius of Pt2+, 0.7 Å vs 1.26 Å in the case of Ag+. The procedure for depositing CoPt3 is similar to an impregnation method (see section 4.1.3.-). As a result the loading is near to the theoretical value. The difference could be attributed to an error in the calculation of the mass concentration of the solution of the NPs, since the PVP stabilizing the NPs has not been considered. The weight percentages of Co and Pt, once transformed into moles, provide an atomic Pt/Co ratio of 3.24, very close to the theoretical value of 3 expected if the metal phase is CoPt3. Particle sizes of the deposited metals were averaged from size distribution analyses of TEM images of the supported catalysts. These images and the histograms obtained from them are included in the Appendix, Figure A 6 and Figure A 7. 34 It is important to note that during calcination after depositing or exchanging the metal ions, the annealing process not only reduces the ions into metallic phase, but since it is a high temperature treatment, it also causes a migration of part of the guest metal atoms out of the internal cavities of the titanosilicate, leading to the formation of big particles of the metal on the surface of the titanosilicate crystals, as reported in various studies for the case of silver in ETS-10[29]. As a general rule (see section 3.1.-) for heterogeneous metal catalysts it is preferred that the active metal particles have the smallest size possible, for an enhanced surface/volume ratio which favours catalytic activity. In the case of the Pt-based catalyst, the particle size is 2.6 nm, small enough to achieve high catalytic activity, and no big particles were observed. However, the particle size increase is not necessarily a problem for our Ag-based catalytic powders due to the characteristics reported for silver in ethylene epoxidation[30,31]. 5.2.- Structured catalytic microreactors 5.2.1.- Seeding of cordierite monoliths Different approaches were selected for seeding the monoliths with small titanosilicate crystals (listed in Table 2). The amount of seeding achieved by the different methodologies conducted is expressed as relative weight gains ( ) [ ] oseeds MMM − 0 ·100 and the average results are collected in Table 7. The weight gain values of all the monoliths prepared are detailed in the Appendix (Table A 2). Table 7.- Average weight gain of monoliths after ETS-10 seed deposition by different methods. Seeding Method Average Gainseed (%) Sonication (13 wt.%) 8.26 ± 2.33 Sonication (2.5 wt.%) 3.34 ± 1.72 LbL by ES (PEI) 1.05 ± 0.23 LbL by ES (PDDA) 0.22 ± 0.21 Dip Coating 7.18 ± 1.83 Apart from the method itself, there are a variety of different factors that have an effect on the final loading and the coating quality, such as the properties of the slurry (given by the properties of the solids, the solvent, and the amount of solid). It is known that by varying the powder concentration in the slurry and the number of immersions, loading and film thickness can be adjusted. These parameters are also affected by the rate of blowing air used in the drying steps[23]. A comparison between the two sonications, 41 the experimental metal loadings, according to EDX, are higher than those theoretical values, so we can assume that the weight percentages for the active metals are being overestimated by the technique. The atomic percentages for the other studied elements are in fair correlation with the JDF-L1 unit formula. For example, Si/Ti ratios for the average EDX measurements are 5.15 for Ag-Monolith, and 5.86 for Pt-Monolith, whereas the JDF-L1 Si/Ti ratio is 4. The discrepancy can be attributed to Si of the cordierite below the titanosilicate layer being also measured, which can be assured due to the significant presence of Mg and Al (coming only from cordierite) in the analysis. However, specific EDX measurements of the crystals (not measuring the monolith background areas) gave Si/Ti ratios around 4.5, which is more reasonable. The images taken during the EDX analysis (Figure 25) show big metal particles (~0.36 µm Ag particles; ~0.16 µm Pt particles). However, it is expected that the titanosilicate surfaces are covered by a majority of smaller sized particles, such as the ones in the powders, but SEM-EDX does not have enough resolution for confirming this. Figure 25.- Metal particles observed on titanosilicate crystals of the ion-exchanged monoliths. Images obtained with a Backscattered electron detector. 5.3.-Catalytic activity testing 5.3.1.- Selective CO oxidation (SELOX) The reproducibility of the catalytic tests was assessed by repeating the measurements of certain experiments at each temperature while cooling the oven, not only when heating. Figure 26 and Figure 27 show the experiments for Pt-JDF-L1 powder and monolith microreactor. 42 0 50 100 150 200 250 300 0 25 50 75 100 0 25 50 75 100 Selectivity (%) Conversion (%) T (ºC) CO conversion O 2 conversion CO 2 selectivity Figure 26.- Pt-JDF-L1 Powder (1.14% Pt). WHSV = 30 mL/h·mg. Solid lines: increasing temperature. Dashed lines: decreasing temperature. 0 100 200 0 25 50 75 100 0 25 50 75 100 Selectivity (%) Conversion (%) T (ºC) CO conversion O 2 conversion CO 2 selectivity Figure 27.- Pt-JDF-L1 Monolith (5.21% Pt). WHSV = 30 mL/h·mg. Solid lines: increasing temperature. Dashed lines: decreasing temperature. 43 For the monolith, both conversions and selectivity are quite similar, but for the powder, CO2 selectivity is not reproducible since higher values are obtained when cooling than upon heating, and subsequently, CO conversion is also affected. Thus, we can conclude that the experiments are not truly reproducible. However, the lack of reproducibility is favourable to us, since the selectivity and CO conversion of the powder are increased. Several consecutive measurements should be performed, with heating measurements again after cooling, if we desire to further assess the reproducibility issue and check is the increased selectivity is maintained while heating. 0 50 100 150 200 250 300 0 25 50 75 100 0 50 100 150 200 250 300 0 25 50 75 100 Pt-ETS-10 (1.87% Pt) Pt-JDF-L1 (1.14% Pt) CO Conversion (%) T (ºC) Pt-ETS-10 (1.87% Pt) Pt-JDF-L1 (1.14% Pt) CO 2 Selectivity (%) T (ºC) Figure 28.- Catalytic performance of Pt-exchanged titanosilicate powders. WHSV = 30 mL/h·mg. If we compare the catalytic performances of Pt-ETS-10 and Pt-JDF-L1 (Figure 28) we can observe that, despite having a lower Pt content, Pt-JDF-L1 reaches higher CO conversion values than Pt-ETS-10. The ETS-10 catalytic powder we are using for comparison was already available from a previous project by a fellow researcher, but its light-off temperatures are, however, not in agreement with previous results using PtETS-10 with higher Pt loadings (7.3 wt.%) and WHSV of 120 mL/h·mg reported by Sebastian et al.[13,34] that showed T50 = 170 ºC. This Pt-ETS-10 has a much lower Pt content, and though the WHSV is lower, a T50 of 110 ºC is a big difference that makes us think that set of data may be unreliable. Those reported light-off values for Pt-ETS10 (T50 = 170 ºC; T75 = 200 ºC)[13] when compared to those of our Pt-JDF-L1 powder 44 (T50 = 144 ºC; T98 = 200 ºC) still show that JDF-L1 as catalytic support provides better catalytic performance, both in light-off temperatures, that are lower, and in maximum CO conversion values, which are higher. This could be attributed to the larger external surface of JDF-L1 crystals, due to its laminated and open morphology, compared to the small and compact crystals of ETS-10. This morphological advantage causes a higher exposure of Pt particles, granting better accessibility of the reactant gases. 0 50 100 150 200 250 300 0 25 50 75 100 0 50 100 150 200 250 300 0 25 50 75 100 Pt-JDF-L1 (1.14% Pt) CoPt3-JDF-L1 (2.15% NPs) CO Conversion (%) T (ºC) CO 2 Selectivity (%) T (ºC) Pt-JDF-L1 (1.14% Pt) CoPt3-JDF-L1 (2.15% NPs) Figure 29.- Catalytic performance of Ptand CoPt3-JDF-L1 powders. WHSV = 30 mL/h·mg. Comparing Pt-JDF-L1 with CoPt3-JDF-L1 (Figure 29), we can observe the improved performance of the powder containing cobalt. Light-off temperatures are approximately 50 ºC lower for CoPt3 than for the Pt powder (T50=114ºC and T100=150ºC for CoPt3; T50=144ºC and T98=200ºC for Pt). For CoPt3, total CO conversion is reached at 150 ºC and maintained until 175 ºC, when the conversion starts to decay, but slowly. Selectivity is higher overall for the CoPt3 powder. This improved performance is, in part, because of the higher metal loading of CoPt3JDF-L1. But it must also be a consequence of the cobalt presence in the sample. High catalytic activities of CoPt3 catalysts have been reported, and this activity appears to be related to the easier adsorption of oxygen to the particles compared to surfaces of pure Pt, and to the weaker adsorption of CO (due to geographic and electronic effects caused by the Co metal atoms on the particles)[16,35]. Our CoPt3-JDF-L1 powder results are in 45 moderate agreement with reported performances of other similar catalytst, like CoPt3SiO2 (3 wt.% NPs)[16] with T100 = 180 ºC using WHSV = 170 mL/h·mg. Our selectivity, however, is much lower. Pt-JDF-L1, as observed in the TEM images (Figure A 7), has plenty of small sized (~2.6 nm) Pt particles well distributed over the crystal surfaces. CoPt3 nanoparticles of CoPt3JDF-L1, on the other hand, are highly agglomerated and thus the distribution is very heterogeneous, but this does not appear to be detrimental to CO conversion, since the final load of metals is higher. It will be necessary to achieve a more homogeneous NP deposition and evaluate the metal dispersion using chemisorption techniques to reach further conclusions. 0 50 100 150 200 250 300 0 25 50 75 100 0 50 100 150 200 250 300 0 25 50 75 100 Pt-JDF-L1 (1.14% Pt) Pt-JDF-L1 Monolith (< 3% Pt) CO Conversion (%) T (ºC) CO 2 Selectivity (%) T (ºC) Pt-JDF-L1 (1.14% Pt) Pt-JDF-L1 Monolith (< 3% Pt) Figure 30.- Catalytic performance of Pt-JDF-L1 powder vs monolith. WHSV = 30 mL/h·mg. A comparison of the Pt-JDF-L1 powder catalytic performance with that of the Pt-JDFL1 monolith (Figure 30) reveals an enhancement of the catalytic activity when changing from fixed bed reactor to structured microreactor. Both experiments were conducted with the same WHSV and measurements taken at the same temperatures for the results to be comparable. Light-off temperatures are ~50 ºC lower for the monolith (T50=112ºC and T100=150ºC for Pt-monolith; T50=144ºC and T98=200ºC for Pt-powder), and once total CO conversion has been reached, it is maintained until 200 ºC, the point at which 46 the powder achieves its maximum conversion. After this point, the monolith conversion and selectivity values drop below those observed for the powder. The enhanced catalytic performance of the catalyst supported on the monolith can be explained by the higher surface/volume ratio characteristic of structured microreactors, and the straight-channel monolith structures coated with thin-film catalysts are known to enhance diffusion of reactants towards active sites[35]. The enhancement is so notable, that the performance of the Pt-JDF-L1 monolith equals and nearly surpasses that of the CoPt3-JDF-L1 catalytic powder. However, we also have to take into account the metal loading of the monolith. SEM-EDX analysis of the monolith produced an overestimation of the Pt loading, so we only know that the load of the monolith is 3% or lower. An accurate determination of the monolith Pt content would be needed to assure that all the catalytic enhancement is due to the catalyst being supported on the microreactor, and not because of the Pt loading. 5.3.2.-Ethylene epoxidation The testing of catalytic activity in ethylene epoxidation was performed for different silver-based catalytic powders. Both ETS-10 and JDF-L1 titanosilicates were tested as supports, and silver was deposited by impregnation and ion exchange procedures. Two different compositions of the gas stream were fed to the reactor, one with equal volumes of oxygen and ethylene, the other with excess oxygen (3:1 in volume). All the experiments were performed with WHSV = 10 mL/h·mg. The catalytic performances obtained in the experimental setup were very poor, and ethylene conversion was lower than 0.5% for all studied cases and samples. These results are tabulated in the Appendix, Table A 4. The ethylene and oxygen conversions (Equation 9 and Equation 10) and ethylene oxide selectivity (Equation 11) were defined based on the product output of the reactor, instead of defining them by input and output flows or concentrations (like for CO SELOX) because the input/output formulas presented very large errors due to the very low conversions achieved with the catalysts. For the experiments with a gas feed comprising 6% of O2 and 6% of C2H4, only the AgETS-10 (IWI) powder achieved ethylene oxide production, though with low selectivity (compared to industry) and almost null ethylene conversion values. At high temperatures the formation of acetaldehyde (AA) was observed in some of the samples, which tells us that the reaction was happening through the OMC intermediate, as explained in the introductory section 3.5.-, which is not the desired pathway. In the 47 remaining powders, only ethylene combustion was observed. For the experiments with a gas feed comprising 6% of O2 and 2% of C2H4, no catalytic activity was observed, only ethylene combustion. The observed lack of catalytic activity cannot be attributed to the silver loading, since the impregnated Ag-ETS-10 powder showed activity but the similarly loaded impregnated Ag-JDF-L1 did not. The explanation of this result lies in the particle size of the deposited silver. It is widely accepted that for silver, the catalytic effect typical of nanoparticles is observed in the range of 10-100 nm, with <10 nm particles generally considered inactive[30]. Good catalytic performance on this wide size range has been explained by different authors as arising from changes in the surface structure or particle morphology and an effect of the support nature has been ruled out. The activity of silver comes from its 100 and 001 planes, more of which are exposed in bigger crystals than in nanoparticles, with cubic shapes also promoting this exposure. The fact that remains is that the rate of ethylene epoxidation is increased 20-fold when changing the silver particle size from 30 nm to 50 nm, and the catalyst activity remains unchanged within the 50-100 nm range, as reported by different studies that also acknowledge the influence of particle shape on the catalyst performance[30,31]. Our silver particles are not that large (see Table 6), in fact, the silver exchanged powders have an average particle size on the verge of the catalytic range, and the impregnated powders have even smaller particles. However, the powders also have bigger metallic particles (from 20 to 100 nm), though they are much less frequent than the sub-20nm particles. Given all this, no activity would be expected from these powders with such small particles, and the results obtained are in correlation with this. It should not be a surprise either, that the catalytic powder for which the highest number of particles in the catalytic range were observed, Ag-ETS-10 IWI (Figure A 6), is the only catalyst that has shown any activity. However, the number of these particles is so low that the activity is almost non-existent. 48 6.-Conclusions and future work 1. Seed deposition and the subsequent titanosilicate crystal layer growth on ceramic monoliths were successfully achieved, with a homogeneous distribution of crystals and layer thicknesses of 18 µm. 2. The crystal phase obtained on the monoliths was JDF-L1 with a layered morphology that enables good accessibility of reactants. 3. In the SELOX catalytic testing, the Pt-JDF-L1 catalytic powder showed good performance (T50 = 144 ºC; T98 = 200 ºC), similar to other related catalysts (Pt-ETS10, T50 = 170 ºC; T75 = 200 ºC). 4. CoPt3-JDF-L1 catalytic powders had an increased performance compared to the Ptexchanged powders, thanks to the beneficial effect of Co on the activity (T50=114ºC and T100=150ºC). 5. Performance of Pt-JDF-L1 was enhanced when supported in monoliths (T50=112ºC and T100=150ºC), proving the advantages of structured microreactors. 6. Silver deposition on titanosilicate supports by ion exchange or impregnation yields too small metal particles, out of the catalytic range for silver in ethylene epoxidation. Future work in this project would involve: 1. Developing a CoPt3-JDF-L1 monolith, in the hopes of obtaining an even higher catalytic performance. 2. 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Applied Catalysis A: General 2010, 387, 215-223. 7 Figure A 7.- TEM images of the Pt and Pt-Co-based catalytic powders and their corresponding histograms. A) Pt-JDF-L1; B) CoPt3-JDF-L1. Figure A 8.- Additional SEM images of titanosilicate crystal layers grown by the TiCl3-A method. 8 Figure A 9.- SEM images of additional morphologies found on monoliths grown by the TiCl3-A method (M24). Table A 2.- Relative weight gain of each monolith after ETS-10 seed deposition by different methods. Seeding method Monolith Gainseed (%) Seeding method Monolith Gainseed (%) S1M1 3.99 M5 1.78 S1M2 3.16 M8 1.48 S1M3 3.68 M9 3.03 M16 8.49 M10 3.94 M17 9.66 M11 2.29 M18 4.88 M12 6.32 M19 10.08 Sonication* M13 4.51 M20 9.71 M21 6.15 Seeding method Monolith Gainseed (%) M22 8.63 M1 0.79 M24 9.35 M3 1.12 M25 11.04 LbL by ES (PEI) M6 1.24 M26 9.06 M2 0.46 M27 8.42 M4 0.13 M28 7.93 LbL by ES (PDDA) M7 0.08 M29 7.67 M30 8.90 Seeding Method Monolith Gainseed (%) M31 8.42 M14 5.88 M32 6.75 Dip Coating M15 8.47 M33 6.99 M34 11.25 M35 9.62 M36 10.52 M38 12.08 M40 8.71 Sonication M41 9.64 9 Table A 3.- Results from SELOX catalysts testing at increasing temperatures. X = Conversion; S = Selectivity. Pt-JDF-L1 (1.14% Pt) CoPt3-JDF-L1 (2.15% NPs) Pt-JDF-L1 Monolith (5.21% Pt) T (ºC) XCO (%) XO2 (%) SCO2 (%) XCO (%) XO2 (%) SCO2 (%) XCO (%) XO2 (%) SCO2 (%) 50 12.40 7.75 66.47 10.69 3.56 100.00 18.97 4.78 100.00 75 14.49 11.18 53.81 13.54 8.71 64.84 32.36 15.87 77.03 100 20.75 19.95 43.22 25.68 18.48 57.97 38.09 26.23 54.86 125 30.44 35.85 35.28 69.73 54.47 53.40 62.38 49.64 47.47 150 56.18 66.24 35.24 100.00 100.00 41.69 100.00 97.70 38.67 175 84.53 92.37 38.01 100.00 100.00 41.66 100.00 100.00 37.75 200 98.19 99.81 40.87 98.56 100.00 41.04 100.00 100.00 37.73 225 92.29 99.61 38.49 94.77 100.00 39.46 91.37 100.00 34.46 250 85.54 99.43 35.74 - - - 77.50 100.00 29.23 Table A 4.- Results from the ethylene epoxidation catalytic powder testing using different gas feeds and temperatures. X = Ethylene Conversion; S = Ethylene Oxide Selectivity. Catalytic tests with 6% O2 and 6% C2H4 in the feed. Catalytic tests with 6% O2 and 2% C2H4 in the feed Ag-ETS-10 (IWI) (12.72% Ag) Ag-ETS-10 (IE) (5.86% Ag) Ag-JDF-L1 (IWI) (13.62% Ag) Ag-ETS-10 (IWI) (12.72% Ag) Ag-ETS-10 (IE) (5.86% Ag) T (ºC) X (%) S (%) X (%) S (%) X (%) S (%) X (%) S (%) X (%) S (%) 150 0.01 0.00 0.01 0.00 0.01 0.00 - - 0.02 0.00 175 0.01 0.00 0.01 0.00 0.01 0.00 0.03 0.00 0.02 0.00 200 0.05 57.09 0.01 0.00 0.01 0.00 0.03 0.00 0.03 0.00 225 0.14 44.50 0.03 0.00 0.02 0.00 0.05 0.00 0.12 0.00 250 0.25 38.18 0.14 0.00 0.41 0.00 0.19 0.00 0.37 0.00 275 0.34 21.77 0.34 0.00 0.41 0.00 0.33 0.00 1.11 0.00 300 0.49 10.50 1.25 0.00 2.11 0.00 0.46 0.00 2.95 0.00