Heterogeneous catalysis (C9981) Lecture 9 Zeolites in oil refinement styskalik@chemi.muni.cz styskalik.sci.muni.cz 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: – 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 • 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 • 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 Solution? • Extra-large pore zeolites • Nanocrystalline zeolites • Hierarchical zeolites • Two-dimensional zeolites • MCM-41, SBA-15 and other mesoporous silica??? 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