CTALYSIS Fundamentals Applications：催化原理及应用

1、heterogeneous catalysisan introduction paul ratnasamynational chemical laboratorypune-411008, india2why r& d in catalysis is important-27 % of gnp and 90 % of chemical industry involve products made using catalysts (food, fuels, polymers, textiles, pharma/agrochemicals,etc)-for discovery/use of

2、alternate sources of energy/fuels/ raw material for chem industry.-for pollution control-global warming.- for preparation of new materials (organic & inorganic-eg: carbon nanotubes).3catalysis is multidisciplinary (physics,chemistry & chem engg) the catalyst is an inorganic solid;catalysis i

3、s a surface phenomenon;solid state and surface structures play important roles. adsorption,desorption and reaction are subject to thermodynamic, transport and kinetic controls(chem engg); adsorbate-substrate and adsorbate - adsorbate interactions are both electrostatic and chemical(physical chemistr

4、y). the chemical reaction is organic chemistry.4 green chemistry is catalysis pollution control(air and waste streams; stationary and mobile) clean oxidation/halogenation processes using o2,h2o2(c2h4o, c3h6o, ech) avoiding toxic chemicals in industry ( hf,cocl2 etc.) fuel cells( h2 generation)5catal

5、ysis in nanotechnology methods of catalyst preparation are most suited for the preparation of nanomaterials . nano dimensions of catalysts. common prep methods. common characterization tools. catalysis in the preparation of carbon nanotubes.6hetrogeneous catalysis-milestones in evolution-1 1814- kir

6、chhoff-starch to sugar by acid. 1817-davy-coal gas(pt,pd selective but not cu,ag,au,fe) 1820s faraday h2 + o2 h2o(pt);c2h4 and s 1836- berzelius coins”catalysis”; 1860-deacons process ;2hcl+0.5o2 h2o + cl2; 1875-messel.so2 so3 (pt); 1880-mond.ch4+h2o co+3h2(ni); 1902-ostwald-2nh3+2.5o2 2no+3h2o(pt);

7、 1902-sabatier.c2h4+h2 c2h6(ni). 1905-ipatieff.clays for acid catalysed reactions; isomerisation, alkylation, polymerisation.7 milestones in evolution-2 1910-20: nh3 synthesis (haber,mittasch) ; langmuir 1920-30-methanol syn(zno-cr2o3); taylor;bet 1930-lang-hinsh &eley -rideal models ;ftsyn;eo;

8、1930-50:process engg; fcc / alkylates;acid-base catalysis;reforming and platforming. 1950-70: role of diffusion; zeolites, shape selectivity; bifunctional cata;oxdn cat-hds; syngas and h2 generation. 1970- surface science approach to catalysis(ertl) 1990 - assisted catalyst design using : -surface c

9、hem of metals/oxides, coordination chemistry - kinetics,catalytic reaction engg - novel materials(micro/mesoporous materials)8catalysis in the chemical industry hydrogen industry(coal,nh3,methanol, ft, hydrogenations/hdt,fuel cell). natural gas processing (sr,atr,wgs,pox) petroleum refining (fcc, hd

10、w,hdt,hcr,ref petrochemicals(monomers,bulk chemicals). fine chem.(pharma, agrochem, fragrance, textile,coating,surfactants,laundry etc) environmental catalysis(autoexhaust, denox, doc)9physical adsorption steps in a catalytic reaction: - diffusion of reactant (bulk, film, surface) - adsorption( phys

11、ical chemical) -surface reaction - desorption and diffusion of products physical adsorption: - van der waals forces;bet surface area pore size distribution ( wheeler, de boer, bjh) influence of pore size on reaction order, temperature coefficient, selectivity, influence of poisons 10chemisorption la

12、ngmuir isotherm; langmuir hinshelwood and eley- rideal mechanisms of surface reactions;kinetics of adsorption-elovich equation.uses of chemisorption (1)probes (h2,co,nh3, pyridine,co2) for fraction of catalytically active surface (only 0.1% in cracking);(2)do chemisorbed species actually participate

13、 in reactions(isotope exchange);(3) changes in surface structures on adsorption(s, h2, o2, h2o2).11the sabatier principle “there is an optimum of the rate of a catalytic reaction as a function of the heat of adsorption”- sabatier,1905: if the adsorption is too weak,the catalyst has little effect;if

14、too strong, the adsorbates will be unable to desorb from the surface;hence,the interaction between reactants or products with surface should be neither too strong nor too weak.12 sabatier principle -optimal basicity results in high carbonate yields (mmm 90(2006)314) 340 360 380 400 420 440 460 48020

15、406080100393 k - react. temp. chemisorbed - pri. aminechemisorbed - sec. aminechemisorbed - tert. aminephysisorbed - surfacechloropropene carbonate yield (%)co2 desorption temperature (k)13how catalysts accelerate rates of chemical reactions h2+0.5o2 h2o; g 0298 = -58 kcal/mol;in the gas phase: d(h-

16、h) = 103 and d(o-o)=117 kcal/mol; e# 10 kcal/mol for h+o2 or h2+o ho2 or h2o.hence,kinetically gas-phase reaction improbable. pt forms pt-h and pt-o bonds with e# 0;moreover,pt-h + pt-o pt-oh pt -oh2 has e# 0 .14turnover frequencies, rates and numberscatalysis is a kinetic phenomenonsequence of elem

17、entary steps in steady state: diffusion (bulk,film,surface) - adsorption-reaction-desorption-diffusiontof= number of product molecules formed per unit area per sec(molecules.cm-2.sec-1)tof= number of product molecules formed per active site per sec(molecules.sec-1) only if active site is known.tot=

18、1/tof = turnover time, time necessary to form a product molecule(sec);tor = turnover rate = tof x surface areaton= tof x total reaction time;ton=1( stoichiometry); ton must be 100 to be industrially useful.15conversions,rates and rate constants conversion = % reactant converted; reaction rate = kp x

19、 f(pi) or kc x f(ci) k = aexp(- e#/rt);a is temp independent. tofs between 0.0001 and 100 in industry; temp adjusted to get the desired rates. e# 35-45 kcal/mol for isom,cyclisation, cracking,dehydo/hydrogenolysis;hight needed. e# 6-12 kcal/mol for hydrogenation; 16the compensation effect k = a exp(

20、- e#/rt); for a given reaction, over different catalysts, a increases linearly with e# so that k remains constant: ln a = + ( e# / r ); is a constant and is the isokinetic temp,when the rates on all catalysts are equal; a plot of ln a vs e# gives a linear plot with +ve slope.17compensation effect fo

21、r the methanation reactionlogarithm of preexponential factor vs apparent activation energy18the active siteh.s.taylor,proc roy soc (london)a108(1925)105 “there will be all extremes between the case in which all the atoms in the surface are active and that in which relatively few are so active “. “th

22、e amount of surface which is catalytically active is determined by the reaction catalyzed”.19active sites-metals:structure sensitivity of catalytic reactions over metals structure sensitive if rate changes markedly when crystallite/particle size is changed; “active site” comprises ensemble of many m

23、etal atoms;steps & edges. eg:hydrogenolysis,h2-d2 exch, steam reform,coking, aromatization etc structure insensitive if rate is independent of crystallite /particle size; each surface metal atom is a potential active site; example: hydrogenation, dehydrogenation 20active sites-oxides /sulfides.c

24、atalysis by ions at surfaces bronsted & lewis acids in solution solid acid catalysts-historical(acid-washed clays for cat cracking) l acidity of ions:na+ ca 2+y3+ 1 mol / ml/sec) high accessibility;role of transport rates of mass and heat. long life time; regenerability. thermal/mechanical stren

25、gth in reaction conditions(sintering,crushing,attrition) reproducible/economic/safe manufacture.24catalyst characterization bulk physical properties bulk chemical properties surface chemical properties surface physical properties catalytic performance25bulk chemical properties elemental composition(

26、 of the final catalyst ), epma xrd,electron microscopy (sem,tem). thermal analysis(dta/tga). nmr/ir/uv-vis/ epr/ mossbauer tpr/tpo/tpd exafs26surface properties xps,auger, sims(bulk & surface structure). texture :surface area- porosity. counting “active” sites: -selective chemisorption (h2,co,o2

27、, nh3, pyridine,co2);surface reaction (n2o). spectra of adsorbed species (ir/epr/ nmr / exafs etc)27physical properties of formulated catalysts bulk density crushing strength & attrition loss (comparative) particle size distribution porosimetry( micro(35 nm) and meso.28catalyst activity testing

28、:definitions- activity activity may be expressed as: -rate constants or ton from kinetics -rates/weight -rates/volume -conversions at constant p,t,and sv. - temp required for a given conversion at constant partial & total pressures - space velocity required for a given conversion at constant pre

29、ssure and temp29catalyst activity testingdefinitions- selectivity selectivity = % concentration of product(s) among all the products excluding coke. yield = conversion x selectivity. selectivities may depend on t,p,sv,diffusion, catalyst particle size and shape , reactor geometry etc. always compare

30、 selectivities at constant t,p and most important,conversion. selectivity w.r.t. each of the reactants(h2o2).30catalyst testing- 1 what is the objective ?testing a solid for its catalytic properties in many reactions?screening for a particular reaction? exploring kinetics?industrial development? act

31、ivity;comparison at non-diffusion & non-thermodynamically limited, kinetically controlled conditions; 10-20 mesh;dreactor 10diacat(wall effects) bed length/ dreactor 5 to avoid channeling; comparison of selectivity at similar activity;31catalyst testing-2only at intermediate conversions and at l

32、ow temp can the quality of the catalyst, expressed in an optimum of kinetically controlled conversion,be analyzed.at high temp or at high conversions,all catalysts are almost equal for either slow kinetic control or thermodynamically limited conversion. 32start-up procedures affect catalyst performa

33、nceactivated rapidlyactivated as per manfacturers instruction33temperature dependence of catalytic activity34catalyst preparation & formulation -1 catalyst formulation - size and shape is a compromise between the wish to minimize pore diffusion effects( small size)and pressure drop( large size);

34、 - pelleting,extrusion,granulation,spray drying; choice depends on properties of powder, size/shape/density/ required strength of catalyst particle; -loading of graded sized pellets.35catalyst preparation & formulation-2 unsupported metals- very high activity(small area adequate )- high purity f

35、eedstockeg: nh3 no ( pt-rh gauze). ch3oh hcho (ag granules)- raney ni,co,cu for h2 ion (residual al2o3 present!).36catalyst preparation & formulation- 3 fused catalysts. eg: triply promoted fe ( + ca,k,al as oxides) catalyst for nh3 synthesis. fe3o4 + h2(n2 +h2) fe(1600c) melt the mixture at 160

36、0 c,cool,crush,size.37catalyst preparation & formulation- 4 wet methods of catalyst manufacture: (a) precipitation :ph of precipitating medium critical !(b)precipitation-deposition: texture of support important.influence of ageing,digestion; filterability;washability of salts;38the ph of precipi

37、tation affects chemical composition, particle size and other physical properties of cu/zno/al2o3 wgs shift catalyst 39catalyst preparation & formulation- 5 supported metal(especially noble metals) catalysts: used extensively in industry: -autoexhaust, diesel oxidation, denox, stationary power so

38、urces - hydrocracking,naptha reforming,xylene isom, isomerisations, hydrogenations, etc - fuel cell catalysts - major issues: high cost and loss of activity due to sintering .40why the need for high dispersion of pm pm are expensive: hence impregnation and not coprecipitation activity depends on met

39、al surface area (msa) msa increases with dispersion 41metal dispersionmetal dispersion, d = no of pt surface atoms / no of total pt atomsd is an operational definition (defined by technique used)n total= from chemical compositionn surface is obtained by physical or chemical methodsphysical methods:

40、crystallite size from xrd, sem/temchemical methods: chemisorption of h2, co, h2-o2 titrationpm distribution profilesa.uniformb.egg shellc.egg whited.egg yolk42pm distribution profilesoptimal dispersion depends on reaction kinetics and mode of catalyst poisoningattrition strength of catalystegg shell

41、 favors reactions with positive orderfast reactions- egg yolk favorsreactions with negative order-pore mouth poisoning egg white or egg yolk-low attrition strength egg white or egg yolk43factors affecting dispersion of pm -11.concentration of pma.low concentration high dispersion2.presence of compet

42、ing ions in impregnating solution increases d.a.citric acid in h2ptcl6 impregnation on al2o3 platforming)44factors influencing dispersion of pm -23. functional groups on substrate surface for binding the pm precursor point of zero charge (pzc) influences dispersion of pmanions and neutral complexes

43、disperse better on gamma al2o3 at ph8 pzc gamma alumina=8-9; sio2345factors influencing dispersion of pm -34. crystallite size of substrate al2o3, ceo2, czo, tio2 etcsmall crystallite sizes have large dispersion5. partially reducible oxide supports increase d eg pt-ceo26. ion exchange of pm increase

44、s d, eg: pt in zeolites46sintering of pm leads to lower dispersion, msa and activity increases with pm loading increases with t, tos, h2o, o2, s, cl increases with crystallite size of support increases with hydrophobicity of support (pt-sio2 sinters more than pt-al2o3) suppressed by spacers (zro2 in

45、 czo) suppressed by “binding” groups on surface (oh, cl-, so3h- etc)47reverse micro emulsion (rme) method enables use of lower amount of pt in denoxnissan wo 2005/063391a1, catalyst was first used in a nissan engine using gasoline fuel for 30 hrs at 700c.after engine durability test for 50 hrs at 70

46、c, catalyst was tested in test rig at 350c for denox activity.catalyst=100g/l in honeycomb; pt-co(ce)-al2o3at 350c after endurance test at 700c for 30 hrs0.5% pt is as effective as 3%wt pt48some developments in industrial catalysis-11900- 1920sindustrial process catalyst1900s:co + 3h2 ch4 + h2o nive

47、getable oil + h2 butter/margarine ni1910s:coal liquefaction nin2 +3 h2 2nh3 fe/knh3 no no2 hno3 pt1920s: co +2 h2 ch3oh (hp) (zncr)oxide fischer-tropsch synthesis co,fe so2 so3 h2so4 v2o549heterogeneous catalysis.some challenges ahead selective oxdn of long chain paraffins to terminal alcohols/ald/a

48、cids; ch4 ch3oh. activation of co2 & its use as raw material; co2 + h2o/ ch3oh/c2h5oh c2 + chiral catalysis with high ee. h2 generation from h2o without using hc . photocatalysis with sunlight.50industrial catalysis-21930s and 1940s1930s:cat cracking(fixed,houdry) mont.clay c2h4 c2h4o agc6h6 mal

49、eic anhydride v2o51940s:cat cracking(fluid) amorph. sial alkylation (gasoline) hf/acid- clay platforming(gasoline) pt/al2o3 c6h6 c6h12 ni 51industrial catalysis-3 1950sc2h4 polyethylene(z-n) tic2h4 polyethylene(phillips) cr-sio2 polyprop &polybutadiene(z-n) tisteam reforming ni-k- al2o3hds, hdt of naphtha (co-mo)/al2o3c10h8 phthalic anhydride (v,mo)oxidec6h6 c6h12 (ni)c6h11oh c6h10o (cu) c7h8+ h2 c6h6 +c