Homework 4 •Selectivity = 90 % in all cases; WHSV = 15.5 h–1 2 -2 MeOH TiO2/SiO2 Temperature [°C] Conversion sample 1 (4.2 wt% Ti) [%] Conversion sample 2 (0.9 wt% Ti) [%] 220 39 8.6 230 48 9.6 240 59 13 250 82 17 260 93 21 •Gas phase transesterification •Continuous flow fixed bed reactor •Create Arrhenius plots for both catalysts (plot ln k [molLD gcat–1 h–1] vs. 1000/T [K–1] •Estimate apparent Ea and A from Arrhenius equation •What makes the difference between these two catalysts? ACS Catal. 2018, 8, 9, 8130–8139 Heterogeneous catalysis Lecture 3 Catalysts preparation Types of catalysts •Bulk catalysts •Supported catalysts •Agglomerated catalysts Types of catalysts •Bulk catalysts •Supported catalysts •Agglomerated catalysts –Intermediate category –Catalysts obtained by mixing active species with support (powders+precursors) –Ill defined catalysts – broad application in industry Types of catalysts •Bulk catalysts •Supported catalysts •Malaxed-agglomerated catalysts Preparation of bulk catalysts •The whole body of the catalyst is made of one active substance •= It must be cheap, thermally, mechanically, and chemically stable, porous,…! –Silica-alumina (cracking) –Raney nickel, Co molybdate (hydrogenation) –Catalyst supports: silica, alumina, silica-alumina, zeolites, TiO2, MgO, carbon – Preparation of bulk catalysts •Ceramic route (heat and beat) –e.g. Molybdates Bi2O3 + MoO3 → Bi2MoO6 –Used as selective oxidation catalysts (propylene to acrolein) –Homogeneity, time, temperature •(Co-)Precipitation –E.g. NH4VO3 + Al(NO3)3 → V-Al oxynitride (used in propene ammoxidation to acrylonitrile) –change of pH leads to precipitation •Sol-gel method •Combustion synthesis •Solvothermal method –e.g. zeolites Preparation of bulk catalysts •Ceramic route, (co-)precipitation, sol-gel method, combustion synthesis, and solvothermal method: #InorgMatChem •Heat and beat quickly and effectively? •Sol-gel continuously??? • Preparation of bulk catalysts Preparation of bulk catalysts Preparation of bulk catalysts •◊ γ-AlOOH •● α-Al2O3 •Δ α-AlOOH Preparation of bulk catalysts Preparation of bulk catalysts Preparation of bulk catalysts •Flame spray pyrolysis Preparation of bulk catalysts •Flame spray pyrolysis Types of catalysts •Bulk catalysts •Supported catalysts •Malaxed-agglomerated catalysts Preparation of supported catalysts •Properties of catalyst supports –Cheap –Porous –Thermally, mechanically, and chemically stable –… •Role of catalyst supports (non-innocent) –Porosity/hydrophobicity vs. Activity/selectivity –O, H, N,…bank (MvK mechanism) –Electron donor/acceptor (Haber-Bosch) –… – Preparation of supported catalysts •Three types of interaction between support and active phase –Weak –Electrostatic –Covalent Preparation of supported catalysts •Weak interaction = Impregnation methods –Wet impregnation •Suspension of support (e.g. SiO2 in water) •Solution of metal salt (e.g. Cu(NO3)2 in water) •Mixed together (suspension) •Cu2+ cations diffuse into the pores of SiO2 support (=we need to wait for enough long time, Fick laws) •Drying, calcination (= formation of Cu2O NPs on SiO2), (reduction of Cu2O to Cu NPs) •Disadvantages: Time, homogeneity. Preparation of supported catalysts •Weak interaction = Impregnation methods –Dry impregnation (Incipient wetness impregnation) •Dry support, pores full of gas (e.g. SiO2 in ambient air) •Solution of metal salt, V = pore volume (e.g. Cu(NO3)2 in water) •Mixed together (paste) •Capillary elevation (fast) brings Cu2+ solution directly into the pores (concentration gradients might occur – pore size!) •Drying, calcination (= formation of Cu2O NPs on SiO2), (reduction of Cu2O to Cu NPs) •Solubility limits! • Preparation of supported catalysts •Weak interaction = Impregnation methods –Dry impregnation (Incipient wetness impregnation) •Big issue: Gas trapped inside the pores Pore collapse Mechanical degradation Attrition … Preparation of supported catalysts •Weak interaction = Impregnation methods –Dry impregnation (Incipient wetness impregnation) •Big issue: Gas trapped inside the pores Possible solutions: -No gas (impregnation under vacuum) -Lower surface tension (addition of a surfactant) -Wait until gas dissappears and impregnation is completed (either gas dissolves in solution or bubbles move out from the pores) Preparation of supported catalysts •Weak interaction = Impregnation methods –Dry impregnation (Incipient wetness impregnation) •Big issue: Gas trapped inside the pores Preparation of supported catalysts •Three types of interaction between support and active phase –Weak –Electrostatic –Covalent Preparation of supported catalysts •Electrostatic interaction = Ion exchange –Classic ion exchangers: clays, layered hydroxides, zeolites –Oxides with the help of pH Preparation of supported catalysts •Electrostatic interaction = Ion exchange –Zeolites, how does it work? – – – – –Cation compensating the negative charge? –Na+ from the synthetic step Preparation of supported catalysts •Electrostatic interaction = Ion exchange –Zeolites, how does it work? –Na+ → H+ directly? –Na+ → ??? → H+ Preparation of supported catalysts •Electrostatic interaction = Ion exchange –Zeolites, how does it work? –Na+ → NH4+ → H+ – –Zeolite NaY –Up to 73 % of Na+ can be exchanged at R.T. –4 consecutive steps or continuous ion exch. ΔT Preparation of supported catalysts •Electrostatic interaction = Ion exchange –Enhancement…trap for leaving ion (shifting equilibrium)! –e.g. Exchange of Ni2+ for H+ in zeolite mordenite –Ni(NO3)2 vs. Ni(OAc)2 Preparation of supported catalysts •Electrostatic interaction = Ion exchange –Trap •OAc- + H+ → AcOH (rarely dissociated) •OH- + H+ → H2O (rarely dissociated) •SCN- + Ag+ → AgSCN (rarely dissociated) Salt - trap Support Preparation of supported catalysts •Electrostatic interaction = Ion exchange –Classic ion exchangers: clays, layered hydroxides, zeolites –Oxides with the help of pH Preparation of supported catalysts •Electrostatic interaction = Ion exchange –Oxides with the help of pH Preparation of supported catalysts •Electrostatic interaction = Ion exchange –Oxides with the help of pH •Isoelectric point Preparation of supported catalysts •Electrostatic interaction = Ion exchange –Oxides with the help of pH •Exchange capacity: the farther you are from isoelectric point, the higher the exchange capacity. Preparation of supported catalysts •Electrostatic interaction = Ion exchange –Oxides with the help of pH •Stability of support at different pH! •Precursor stability at different pH! Preparation of supported catalysts •Electrostatic interaction = Ion exchange –Oxides with the help of pH •Competitive ion exchange • • • • • •Example: very high affinity between Al2O3 and Pt •Result: heterogeneous distribution of Pt (only outer surface) Preparation of supported catalysts •Electrostatic interaction = Ion exchange –Oxides with the help of pH •Competitive ion exchange • • • • • •NH4OH dissociates readily, NH4+ competes with Pt for sites (= inverse to trap) •Result: homogeneous distribution of Pt through the support 2– Preparation of supported catalysts •Electrostatic interaction = Ion exchange –Oxides with the help of pH •Competitive ion exchange [NH4+] Preparation of supported catalysts •Electrostatic interaction = Ion exchange –Oxides with the help of pH •Competitive ion exchange Preparation of supported catalysts •Electrostatic interaction = Ion exchange –Oxides with the help of pH •Competitive ion exchange Preparation of supported catalysts •Electrostatic interaction = Ion exchange –Oxides with the help of pH •Competitive ion exchange: Note •We say homogeneous Pt distribution… •But how homogeneous??? •Latest research from de Jong group (University Utrecht, HR-TEM, perfect control of Pt deposition on atomic scale) shows that uniform Pt homogeneity is not ideal for hexane isomerization! Preparation of supported catalysts •Electrostatic interaction = Ion exchange –Oxides with the help of pH •Detailed knowledge of isoelectric point, precursor behavior, control over pH: Preparation of supported catalysts •Electrostatic interaction = Ion exchange –Oxides with the help of pH •Detailed knowledge of isoelectric point, precursor behavior, control over pH: Preparation of supported catalysts –NPs by wet (dry) impregnation vs. impregnation with electrostatic interaction calcination Electrostatic interaction Uniform NPs + + + + + − − − − − Preparation of supported catalysts •Electrostatic interaction = Ion exchange –Oxides with the help of pH Preparation of supported catalysts •Three types of interaction between support and active phase –Weak –Electrostatic –Covalent Preparation of supported catalysts •Covalent bonds = grafting –Formation of metal oxide on top of catalyst support – – – – – – –Is calcination necessary? More in lecture on single site catalysts… Preparation of supported catalysts •Covalent bonds = grafting –Formation of metal NPs on top of catalyst support Preparation of supported catalysts •Covalent bonds = grafting –NPs by impregnation vs. grafting calcination Covalent bonds Uniform NPs Preparation of supported catalysts •Covalent bonds = grafting –Formation of metal NPs on top of graphene oxide – – – – – – Content of COOH groups increased by oxidation (HNO3, H2O2,O3,…) Preparation of supported catalysts •Covalent bonds = grafting –Formation of metal NPs on top of graphene oxide –Last step: Pd reduction – – – – – – Preparation of supported catalysts •Covalent bonds = Deposition/precipitation –Starts as a dry impregnation –Metal salt is then selectively precipitated at the catalyst surface –Precipitation mainly by pH change – hydrolysis/condensation – – – – –