Heterogeneous catalysis Lecture 9 Zeolites in oil refinement Zeolites - synthesis •Reaction mixture: …, …, …, … • –pH adjustment, (gelation) –Hydrothermal treatment in an autoclave –… –… • •Result: H-zeolite (Brønsted acidic with H+ ions) • • • • Zeolites - synthesis •Reaction mixture: Na2SiO3, Al2O3, quarternary ammonium salt (=structure directing agent), water • –pH adjustment, (gelation) –Hydrothermal treatment in an autoclave –Ion exchange (Na+ for NH4+) –Calcination (= NH3 removal) • •Result: Crystalline H-zeolite (Brønsted acidic with H+ ions) • • • • Zeolites - synthesis •Pore size • • • • • • • • •Si/Al ratio ≥ 1 • DOI: 10.1039/c3cs60394f Zeolites - acidity •Brønsted: … • –Structure: • • • •Lewis: … –Structure: • • • • Zeolites - acidity •Brønsted: negative charge of the aluminosilicate net balanced by strongly acidic protons –Structure: • • • •Lewis: Al atoms that are not embedded in the aluminosilicate net (e.g. surface species, amorphous stuff, alumina particles) = extraframework aluminum species (EFAL) –Structure: • • • • Fluorination of the ε-Keggin Al13 polycation - Chemical Communications (RSC Publishing) H+ Zeolites - acidity •Brønsted: negative charge of the aluminosilicate net balanced by strongly acidic protons –Structure: • • • • • • • H+ DOI: 10.1039/C8CS00887F Changes in local Al structure revealed by EXAFS Zeolites - acidity •Brønsted: depends on the second coordination sphere (i.e. Si/Al ratio) • • • • • •Lewis: Extraframework aluminum species (EFAL) depends on –Si/Al ratio –Aging (time on stream, steaming) –Can be washed out (depending on pH – acid washing) • • • • Zeolites - acidity •High Si/Al ratio –Strong Brønsted acid sites –Weak Brønsted acid sites –Strong Lewis acid sites –Weak Lewis acid sites • •Low Si/Al ratio –Strong Brønsted acid sites –Weak Brønsted acid sites –Strong Lewis acid sites –Weak Lewis acid sites • • • • Zeolites - acidity Zeolites - acidity •Confinement effect • • • • • •Superacidity –Various probes at RT – acid site strength similar to 70 % H2SO4 (=NO!) –Ability to protonate hydrocarbons at working conditions (=YES!) –? • • • • Zeolites – diffusion/shape selectivity •Diffusion –Big difference between zeolites with 8 membered vs. 12 membered ring pore openings –Big difference between zeolites with 1D, 2D, and 3D-connected pore structure •Shape selectivity • • • • DOI: 10.1039/c3cs60394f Other microporous frameworks •Non-Al zeolites –Be2+, Zn2+, B3+, Ga3+, Fe3+, Ge4+, Ti4+, Sn4+ –15 % variation in radius (vs. Si4+) –±0.4 a.u. Pauling electronegativity •Aluminophosphates (AlPOs) –SiO2 and AlPO4 and isoelectronic structures –No catalytic activity •M(II) Aluminophosphates –Mild Brønsted acids and(or) redox catalysts •Silicoaluminophosphates (SAPOs) –Mild Brønsted acids Other microporous frameworks •Non-Al zeolites –Be2+, Zn2+, B3+, Ga3+, Fe3+, Ge4+, Ti4+, Sn4+ –15 % variation in radius (vs. Si4+) –±0.4 a.u. Pauling electronegativity Lewis acidity in non-Al zeolites Open vs. Closed acid sites in non-Al zeolites Other microporous frameworks •Aluminophosphates (AlPOs) –M(II) Aluminophosphates –Silicoaluminophosphates (SAPOs) = milder acidity DOI: 10.1016/j.cattod.2011.02.027 Zeolites in oil refinement •Fluid catalytic cracking • •Isobutane-butene alkylation • •Reforming (+ steam reforming) • •Hydrocracking • •Linear paraffin isomerization • • • • DOI: 10.1039/c3cs60394f Zeolites in oil refinement •Fluid catalytic cracking • • • Zeolites in oil refinement •Fluid catalytic cracking –Zeolite Y 10-50 wt% –Binders 50-90 wt% –At the beginning – AlCl3 –Addition of HZSM-5 • • • Zeolites in oil refinement •Fluid catalytic cracking –Shortening of long linear hydrocarbons –Isomerization to branched hydrocarbons –„Aromatization“ –HZSM-5 for higher propylene production • • • Zeolites in oil refinement •Fluid catalytic cracking –Protonation + protolytic cracking –H- abstraction + β scission • • • Zeolites in oil refinement •Fluid catalytic cracking –Protonation + protolytic cracking –H- abstraction + β scission • • • Zeolites in oil refinement •Fluid catalytic cracking –Protonation + protolytic cracking •We need strong Brøsted acid sites (zeolite Y) • –H- abstraction + β scission •We need strong Lewis acid sites (steamed/(acid washed) zeolite Y) • –Long linear hydrocarbons diffusion •Precracking on alumina and silica-alumina (non-innocent binders) •Hierarchical porosity in zeolites (steamed/(acid washed) zeolite Y) • • • • Zeolites in oil refinement •Isobutane-butene alkylation –We want highly branched C8 hydrocarbons (high octane number) –HF and H2SO4 catalyzed alkylation still running in industry –Large pore zeolites as a substitution • • • • Zeolites in oil refinement •Isobutane-butene alkylation –Large pore zeolites as a substitution –BUT! 2-butene dimerization…oligomerization…coking…deactivation • • • • Zeolites in oil refinement •Linear paraffin hydroisomerization –Linear C8 (C7) → branched C8 (C7) –Requires strong Brønsted acidity and hydrogenation/dehydrogenation activity (Pt(Ni) on mordenite) –Mordenite – large pore, monodirectional pores –Mordenite – dealuminated (strong H+), acid washed (low EFAL) –Protonation = carbocations –Stability of carbocations? Branched hydrocarbons? • • • • Zeolites in oil refinement •Linear paraffin hydroisomerization –Linear C8 (C7) → branched C8 (C7) –Requires strong Brønsted acidity and hydrogenation/dehydrogenation activity (Pt(Ni) on mordenite) –Mechanism??? •Dehydrogenation of n-alkane to n-alkene on Pt •Diffusion??? •Protonation of n-alkene to secondary carbenium ion on H+ zeolite •Secondary carbenium ion rearranges to tertiary (more stable) carbenium ion •Desorption from acid site produces iso-alkene, H+ is restored •Diffusion??? •Hydrogenation of iso-alkene to iso-alkane on Pt • • • • • Zeolites in oil refinement •Linear paraffin hydroisomerization –Researchers interested in the effect of “intimacy” (Pt-acid site) –Affects mainly selectivity, activity to some extent –An optimum between “too close” and “too far” Zeolites in oil refinement •Linear paraffin hydroisomerization –Researchers interested in the effect of “intimacy” (Pt-acid site) –Affects mainly selectivity, activity to some extent –An optimum between “too close” and “too far” DOI: 10.1021/acscatal.8b01461 Zeolites in oil refinement •Linear paraffin hydroisomerization –Pt(Ni) on mordenite –How do we deposit Pt on a zeolite? (Lecture 3) • • • • Zeolites in oil refinement •Linear paraffin hydroisomerization –Pt, Pd, Ni on mordenite –Electrostatic interaction = Ion exchange •Competitive ion exchange [NH4+] Zeolites in oil refinement •Linear paraffin hydroisomerization –Pt, Pd, Ni on mordenite –Electrostatic interaction = Ion exchange •Competitive ion exchange Zeolites in oil refinement •Hydrocracking (i.e. cracking in the presence of H2) –Shortening of long hydrocarbons –From linear to branched (alkylation, carbocations,…) –Hydrogenation/dehydrogenation –Pt, Pd on mordenite (also zeolite Y and β) • • • • Zeolites in oil refinement •Reforming and steam reforming –Cyclization, isomerization of cyclic compounds to cyclohexene, cyclohexene and its derivatives dehydrogenation to benzene, toluene, xylene (BTX), and other aromatics –H2 as a useful „by-product“ –Pt on high surface area support, non-acidic –Reforming in the presence of H2O = H2 production • • • • Microporosity of zeolites •+ •Regular structure •High stability •Shape selectivity •Confinement effect • − •Diffusion limitations •Coking •Fast deactivation • Solution? •Extra-large pore zeolites •Nanocrystalline zeolites •Hierarchical zeolites •Two-dimensional zeolites •MCM-41, SBA-15 and other mesoporous silica??? Microporosity of zeolites •+ •Regular structure •High stability •Shape selectivity •Confinement effect • − •Diffusion limitations •Coking •Fast deactivation • Microporosity of zeolites •Extra-large pore zeolites –Germanosilicates –New organic structure-directing agents –Ge for Al substitution –Assembly-disassembly-organization-reassembly (prof. Čejka, Prague) DOI: 10.1039/C8CS00887F Microporosity of zeolites •Nanocrystalline zeolites –Classic hydrothermal synthesis –But! Part of Si source = MeSi(OEt)3 DOI: 10.1016/j.cattod.2009.04.016 Microporosity of zeolites •Nanocrystalline zeolites –Classic hydrothermal synthesis –But! Part of Si source = MeSi(OEt)3 DOI: 10.1016/j.cattod.2009.04.016 Nano-HZSM-5 Micro-HZSM-5 Ethanol dehydration to ethylene Microporosity of zeolites •Hierarchical zeolites –Add something into the rxn mixture in order to create large pores •Hard templates (carbon,…) •Soft templates (alkoxysilanes with a long aliphatic chain) –Dealumination, desilication •HNO3, NaOH – DOI: 10.1039/B805502E Microporosity of zeolites •Hierarchical zeolites –Add something into the rxn mixture in order to create large pores –Dealumination, desilication: alkaline–acid, acid–alkaline, and fluorination–alkaline post-synthesis treatments of H-MOR (below) DOI: 10.1039/B805502E Microporosity of zeolites •Two-dimensional zeolites –Pillaring •CTMA+, sonication, surfactant removal = stacked layers –Delamination •CTMA+, sonication, TEOS hydrolysis, calcination = layered zeolites with permanently expanded interlayer spaces –