Catalysis in supercritical fluids Leiv Låte Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway

Outline Background Introduction –Definition of SCF –Media used as SCF –Advantages of SCF Applications –Industrial use of SCF as reaction media –Research Conclusions

Background Global increase in the environmental awareness Chemical industry searching for new and cleaner processes One obvious target is replacement of the solvent Suitable candidates for replacement of organic solvents include SCF –scCO 2 –scH 2 O

Definition of a supercritical fluid Definition by IUPAC A mixture or element: Above its critical pressure (P c ) Above its critical temperature (T c ) Below its condensation pressure The critical point represents the highest T and P at which the substance can exist as a vapour and liquid in equilibrium

What is a supercritical fluid?

Appearance of a SCF

Characteristics of a supercritical fluid Dense gas –Densities similar to liquids –Occupies entire volume available Solubilities approaching liquid phase –Dissolve materials into their components –Completely miscible with permanent gases (N 2 / H 2 ) Diffusivities approaching gas phase –Viscosities nearer to gas –Diffusivity much higher than a liquid Density, viscosity, diffusivity and solvent power dependent on temperature and pressure

Comparison of physical properties

Which gases can be used as SCF? Any compressible gas –Possible to tune properties from gas like, through to liquid like The most common

Supercritical CO 2 Most widely used fluid Similar to nonpolar organic solvents (n-hexane) –scCO 2 only suitable as a solvent for nonpolar substances –addition of cosolvents can modify the solute Methanol Toluene –Modifier moves the scCO 2 away from the ideal Green solvent Mild critical parameters Non toxic and non-flammable Environmentally favourable Thermodynamically stable Inexpensive (plentiful)

Supercritical H 2 O Lower polarity than liquid water –Turns in to an almost non polar fluid Dielectric constant drops from about 80 to 5 Becomes miscible with organics and gases Reduced density –about 1/3 of water –Increased diffusivity Environmental favourable Non toxic and non-flammable Inexpensive (plentiful) The foremost application for scH 2 O is oxidative destruction of toxic wastes High supercritical temperature exclude scH 2 O –Limited thermal stability of organic reactants and products

Reaction solvent effects - pressure tunability Pressure tunability on density ( ), viscosity ( ) and D 11 ·

Pressure tunability

Ion product of water

Tunable density of SCF Density tuning Gain more direct information about a reacting system No need for different solvents in a study Can be used to control –Solvent polarity –Separation –Rate of reaction –Selectivity on catalytic surface reactions

Advantages of SCF There is no point in doing something in a supercritical fluid just because it is neat Val Krukonis Energy cost due to elevated pressures and temperatures –More expensive than traditional solvent systems –Safety hazards related to high pressure and temperature Using the fluids must have some real advantage Advantages fall into four categories –Environmental benefits –Health and safety benefits –Process benefits –Chemical benefits

Health, Safety and Environment benefits Replaces less green liquid organic solvents No acute toxicity (H 2 O and CO 2 ) No liquid wastes (except water) Non-carcinogenic (except C 6 H 6 ) Non toxic (except NH 3 ) Non-flammable (CO 2, H 2 O)

Chemical benefits High reaction rate due to: –Dissolving capabilities High concentration of reactant gases ( H 2 / O 2 ) Eliminating inter-phase transport limitations –Higher diffusivities than liquids –Better heat transfer than gases –Low viscosity Variable dielectric constant (polar SCF) –Adjustable solvent power Enhanced catalytic activity due to anti-coking of scCO 2 Higher solubilites than corresponding gases for heavy organics –Improved catalyst lifetime High product selectivities –Increased pressure may favour desired product selectivity

Process benefits Green chemistry –No use of organic solvents –Easier product separation Adjustable density (adjustable solvent power) –Recycling of SCF possible –Less by-products More efficient product/catalyst separation –Problem in homogeneous catalysis –No energy-intensive distillations Higher reaction rate and facile product separation –Smaller reactors Process safety Space requirements Inexpensive (CO 2, H 2 O, NH 3, Ar, Hydrocarbons)

Continous reactors Continuos reactors do not require depressurization like batch reactors Catalyst fixed in the reactor –Simpler separation of catalyst and products than batch reactor Parameters can be varied independently –Temperature, pressure, residence time, substrate flow rate Fluid properties can be tuned in real-time to optimize reaction conditions Smaller volume than batch reactors –More safe reactor Good heat and mass transfer

Applications Catalyzed reactions Alkylation Amination Cracking Esterification Fischer-Tropsch Synthesis Hydrogenation Isomerization Oxidation Polymerization

Industrial use of SCF as reaction media

Hydrogenation of organic compounds Hydrogen has low solubility in most organic solvents –Hydrogen completely miscible with SCF Reaction is not limited by mass transfer effects –High reaction rates The fluid has good thermal properties –Facilitate heat removal High degree of control over reaction parameters –Selectivity

Hydrogenation in scPropane Feed: Oil (fatty acid methyl esters), H 2 Supercritical fluid: Propane ( T c = 96.8°C, P c = 42.0 bar) Catalyst: Pd P. Møller, 3rd Int. Symp. On High Press. Chem. Eng., Zurich, 1996, Reaction rate 400 times faster than traditional techniques –Reduced mass transfer limitations of H 2 in homogeneous phase

Catalytic amination of amino-1-propanol with scNH 3 Catalyst Co-Fe (95/5) Production of 1,3-diaminopropane Tubular reactor –195°C –Feed ratio R-OH / NH 3 (1:40) –T c = 132, P c = 113 bar Fischer et.al, A. Angew. Chem., Int. Ed. Engl., submitted P C = 113 bar

Supercritical Fischer- Tropsch synthesis Classical synthesis involves an exothermic gas-phase reaction –Heat removal –Pore blocking and catalyst deactivation Liquid-phase process –Improved heat transfer –Better solubilities of higher hydrocarbons –Lower diffusivity than gas-phase reaction Mass transfer limitations Lower reaction rate –Accumulation of high molecular-weight products in the reactor New proposal –Supercritical conditions Gas-like diffusivity Liquid-like solubility

Supercritical Fischer- Tropsch synthesis High diffusivity of reactant gases –Homogeneous phase Rate of reaction and diffusion of reactants –Slightly lower than gas-phase –But significantly higher than liquid Effective removal of reaction heat In situ extraction of high molecular weight hydrocarbons (wax)

Supercritical Fischer- Tropsch synthesis The SCF was selected by the following criteria: –T c and P c slightly below reaction temperature and pressure –SCF should not poison the catalyst –SCF should be stable under the reaction conditions –SCF have high affinity for aliphatic hydrocarbons to extract wax Reaction temperature: 240°C and P tot =45 bar n-Hexane chosen SCF T c = 233.7°C P c = 30.1 bar p(CO+H 2 )=10 bar, CO:H 2 =1:2 Catalyst: Ru/Al 2 O 3 K. Yokota and K. Fujimoto, Ind. Eng. Chem. Res., 30 (1991)95

Supercritical Fischer- Tropsch synthesis K. Yokota and K. Fujimoto, Ind. Eng. Chem. Res., 30 (1991)95 Different CO-conversions due to different rates of diffusion –D GASS > D SCF > D Liquid Different Chain growth probabilities due to CO:H 2 diffusion –Similar SCF and gas diffusion inside the catalyst pores Effective molar diffusion in the supercritical phase

Distribution of hydrocarbon products in various phases Carbon Number

Supercritical Fischer- Tropsch synthesis The alkene content decreased with increased carbon number for all phases –Increase in hydrogenation rate relative to diffusion rate –Longer residence time on catalyst surface for high molecular weight hydrocarbons Higher alkene content in SCF –Alkenes were quickly extracted and transported by SCF out of the catalyst Minimizing readsorption and hydrogenation K. Yokota and K. Fujimoto, Ind. Eng. Chem. Res., 30 (1991)95

Wax production Addition of heavy alkene to the supercritical phase Catalyst: Co-La/SiO 2 Temperature: 220°C Pressure: 35 bar Supercritical fluid: n-pentane ( T c =196.6°C, P c =33.7 bar) p(CO+H 2 ) = 10 bar Studied the effect of addition of heavy alkenes –Addition: 4 mol% (based on CO) –1-tetradecene and 1-hexadecene Fujimoto et al., Topics in Catal. 1995, 2,

Wax production Addition of heavy alkene to the supercritical phase Fujimoto et al., Topics in Catal. 1995, 2, With alkene addition

Wax production Addition of heavy alkene to the supercritical phase Carbon chain growth accelerated by addition of alkenes Alkenes diffuse inside the catalyst pores to reach the metal sites –Adsorb as alkyl radicals to initiate carbon chain growth The resulting chains are indistinguishable from chains formed from synthesis gas Addition of heavy alkenes does not have any effect in gas phase reactions Fujimoto et al., Topics in Catal. 1995, 2,

Oxidation in scH 2 O (SCWO) SCWO of organic wastes –Complete oxidation to CO 2 Single fluid phase Faster reaction rates Complete miscibility of nonpolar organic with scH 2 O With or without heterogeneous catalyst Motivation for catalyst: –Reduce energy and processing costs Target: –Complete conversion at low temperatures and short residence time

t-butyl alcohol synthesis by air oxidation of supercritical isobutane TBA can be converted to isobutene by dehydration Commercial production of isobutene: Dehydrogenation –High temperatures: °C Catalyst deactivation Isobutane: T c = 135°C, P c = 36.4 bar Isobutane : air = 3 : 1 Reaction temperature: 153°C Reaction pressure: –44 bar for gas phase reaction –54 bar for supercritical phase reaction Fan et al., Appl. Catal. 1997, 158, L41-L46

t-butyl alcohol synthesis by air oxidation of supercritical isobutane Fan et al., Appl. Catal. 1997, 158, L41-L46

t-butyl alcohol synthesis by air oxidation of supercritical isobutane Catalyst: SiO 2 -TiO 2, P=54 bar

t-butyl alcohol synthesis by air oxidation of supercritical isobutane Catalyst: SiO 2 -TiO 2, P=54 bar

t-butyl alcohol synthesis by air oxidation of supercritical isobutane Catalyst: SiO 2 -TiO 2, T=153°C

t-butyl alcohol synthesis by air oxidation of supercritical isobutane Catalyst: SiO 2 -TiO 2, T=153°C

Friedel Crafts Alkylation Reactions Conventional reactions require: –Long reaction times –Low temperatures and –Use of environmentally dirty catalysts e.g. AlCl 3 or H 2 SO 4 –Separation of catalyst and solvent from the reaction mixture Using supercritical CO 2 allows reaction conditions to be tuned to get high product selectivity. –Solvent removal is also easy using supercritical CO 2

Friedel Crafts Alkylation Reactions Organic and water layers are easily separated to leave clean product

Alkylation of Mesitylene with Isopropanol in Supercritical CO 2 50% conversion of mesitylene to mono-alkylated product No di-alkylated product

Conclusions SCF (used as solvent or reactant) provides opportunities to enhance and control heterogeneous catalytic reactions: Control of phase behaviour Elimination of gas/liquid and liquid/liquid mass transfer resistance Enhanced diffusion rate in reactions Enhanced heat transfer Easier product separation Improved catalyst lifetime Tunability of solvents by pressure and cosolvents Pressure effect on rate constants Control of selectivity by solvent- reactant interaction

Conclusions Reagents, cosolvents or products can change properties of SCF –Critical point for a reaction mixture can change through the reaction –Need more research before use in organic synthesis scCO 2 only suitable as solvent for nonpolar substances High supercritical temperature exclude scH 2 O –Limited thermal stability of organic reactants and products Addition of reagents or cosolvents to SCF –Changed properties –Can interact with catalyst surface –Change surface properties of the catalyst –Makes the process less green

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