andout-10 ourse 201N 1 st Semester 2006-2007 Inorganic hemistry Instructor: Jitendra K. Bera ontents 3. rganometallic hemistry omogeneous atalysis lefin ydrogenation; ydroformylation; Monsanto Acetic acid Process; Wacker Process eterogeneous atalysis Ziegler-Natta atalysts
omogeneous atalysis [catalyst] A + B atalysis involves the use of a catalyst that accelerates the rate of a reaction. More often than not this involves introducing new pathways with lower Gibbs Free energies of activation. A catalyst does not alter the Gibbs free energy of the overall reaction (since G is a state function which depends only on the initial state and the final state and not on the path traversed to reach the final state from the initial state). Therefore reactions that are thermodynamically not favorable cannot be made favorable by a catalyst. Another important feature is that stable catalytic intermediates do not occur in a catalytic cycle. If such a complex were formed it would not lead to a catalytic cycle. atalyst poisons usually interfere with the catalytic cycle by the formation of such complexes. In contrast to heterogeneous catalysts where the catalyst is an insoluble solid present in a different phase from the reagents, homogeneous catalysts are in the same phase as the reagents. Usually this implies that the reagents and the catalyst are dissolved in a solvent for carrying out the reaction. The advantage of such a procedure is that in principle all the catalyst used is available for the reaction. Also since the catalyst is soluble, the mechanism of the catalysis reaction can be more easily understood by the use of spectroscopic methods. This in turn allows for easier modification and modulation of catalyst design and activity. The disadvantage of the homogeneous catalyst is the separations of the catalyst form the reagents. Advantages/Disadvantages of omogeneous atalysts elative to eterogeneous atalysts Good homogeneous catalysts are: good bad generally far more selective for a single product far more active far more easily studied from chemical & mechanistic aspects far more easily modified for optimizing selectivity far more sensitive to permanent deactivation far more difficult for achieving product/catalyst separations eterogeneous catalysts dominate chemical and petrochemical industry: ~ 95% of all chemical processes use heterogenous catalysts. omogenous catalysts are used when selectivity is critical and product-catalyst separation problems can be solved.
lefin ydrogenation using Wilkinson s atalyst. Wilkinson s atalyst: hl(pph 3 ) 3 was the first highly active homogeneous hydrogenation catalyst and was discovered by Geoffrey Wilkinson (Nobel prize winner for Ferrocene) in 1964.. offey discovered it at about the same time while working for II (Imperial hemical Industries). It was very simply prepared by reacting hl 3 3 2 with excess PPh 3 in Et: hl 3 2 + xs PPh 3 hl(pph 3 ) 3 + Ph 3 P= + oxidzed solvent Wilkinson s atalyst is a h(i) complex, h(pph 3 ) 3 l containing three phosphine ligands and one chlorine. Notice that the complex is a 16e- species and the metal is in an oxidation state of +1. The first step in the reaction of this catalyst is believed to be the dissociation of one of the phosphines to afford a vacant coordination site that is probably taken up very quickly by a weakly binding solvent molecule. This species undergoes oxidative addition reaction; the coordination number changes from four to six and the oxidation state of the metal increases to +3. Also notice that the added hydrogens are in a cis orientation in the complex. This complex undergoes first a dissociation of the solvent molecule followed by the
addition of the olefin. There is no change in the oxidation state, coordination number or the total number of valence electrons. As a result of the olefin insertion (hydrogen migration) we obtain a h (III), 16e-, five coordinate species. A solvent occupies the sixth coordination site to take it to a 18e- species. eductive elimination occurs to give the hydrogenated product and the catalytically active species. Notice that the fragments involved in the reductive elimination are cis to each other. ydroformylation h or o + + 2 Aldehydes + * side reactions linear (normal) branched (iso) alkene isomerization alkene hydrogenation The reaction of an alkene with carbon monoxide and hydrogen, catalyzed by cobalt or rhodium salts to form an aldehyde is called hydroformylation. ydroformylation was discovered by tto oelen in 1938. oelen's observation that ethylene, 2 and were converted into propanal, and at higher pressures, diethyl ketone, marked the beginning of hydroformylation catalysis. obalt catalysts completely dominated industrial hydroformylation until the early 1970's when rhodium catalysts were commercialized. In 1992, ~70% of all hydroformylation processes were based on rhodium triarylphosphine catalysts, which excel with 8 or lower alkenes and where high regioselectivity to linear aldehydes is critical. Most aldehydes produced are hydrogenated to alcohols or oxidized to carboxylic acids. Esterfication of the alcohols with phthalic anhydride produces dialkyl phthalate plasticizers that are primarily used for polyvinyl chloride plastics -- the largest single end-use. Detergents and surfactants make up the next largest category, followed by solvents, lubricants and chemical intermediates. tto oelen (1897-1993)
Scheme 1. o () 2 8 + o () 2 7 eck/breslow ydroformylation Mechanism reductive elimination oxidative addition + 2 o + o - + alkene o ligand dissociation / alkene coordination o() 4 - proposed bimetallic pathway -- since shown to be of no importance under normal catalytic conditions oxidative addition o + 2 - rate determining step + o + anti-markovnikov alkene insertion addition to M- bond migratory 2.5 atm -- 1.6:1 linear to branched 90 atm -- 4.4:1 linear to branched { increasing partial pressure keeps back rxns from occuring -- this limits alkene isomerization and the corresponding opportunity for making branched aldehyde The salient features of the above mechanism are as follows. o() 4 is generated by the oxidative addition of 2 with cobalt octacarbonyl. o() 4 is a 18-e species; dissociation of is necessary to generate the coordinatively unsaturated catalytic species o() 3. This latter species can add an olefin, which undergoes hydrogen migration to afford the 16 e alkyl complex o() 3. Addition of followed by insertion affords the acyl complex ()o() 3. ydrogen adds oxidatively to the latter complex to generate a 18 e species. eductive elimination from this compound affords the aldehyde and regenerates the catalytic species o() 3.
Note that branched aldehydes are also possible; this would depend where the hydrogen migrates in the alkene complex. Branched aldehydes are not desirable. It has been found that addition of bulky phosphines can retard the branched aldehyde formation. Monsanto Acetic acid Process Methanol is an important starting material obtained from the synthesis gas ( + 2 ). This can be converted into acetic acid by reaction with in the presence of a h(i) catalyst. The active catalyst in this process is [h() 2 I 2 ] - obtained by the reaction between hl 3, iodine and. The catalytic cycle is actually two cycles. In one of these, methanol is converted into methyl iodide, which enters the catalytic cycle involving the h(i) catalyst. The rhodium metal shuttles between the oxidation states of +1 and +3. Wacker Process This is one of the earliest industrial processes developed in Germany for the conversion of ethylene into acetaldehyde. 2 2 + 1/2 2 [Pdl 4 ] 2-3 In the reaction Pd 2+ is reduced to palladium metal, which is reoxidized by u 2+. The reduced u + is oxidized back by oxygen to u 2+. The overall mechanism for this reaction is outlined below.
ligand dissociation / alkene coordination nucleophilic attack by water reductive elimination β-hydrogen transfer Wacker process is more complex than the other catalytic processes described above. The first step in this the formation of the olefin complex [l 3 Pd( 2 4 )] -. The metal activated olefin is susceptible to nucleophilic attack by water. This generates the complex [l 3 Pd- 2-2 -] 2-. β-hydrogen transfer leads to the second olefin complex The next step not understood properly, leads to the alkyl complex, where the alkyl complex has a hydroxyl substituent. This undergoes reductive elimination to afford the aldehyde, and Pd(0). Note that if there was no way of catalytically reoxidizing the Pd(0), the reaction will be dead now. Fortunately, u 2+ is a good catalyst, which can oxidize Pd(0) to Pd 2+. The u + thus generated is reoxidized by oxygen in another catalytic cycle. In this manner the expensive palladium reagent is reutilized
eterogeneous atalysis Ziegler-Natta atalysis for the Polymerization of olefins Polymers are large molecules with molecular weights in the range of 10 4 to 10 6. These consist of small building units known as monomers. For example polyethylene is made up of ethylene monomers. Poly vinylchloride is built from vinyl chloride and similarly polystyrene is made from styrene. In all of these cases a single monomer is repeated several times in the polymer chain. The number of repeating units determines the molecular weight of the polymer. When only a few alkenes couple together to make a short chain, we refer to that as oligomerization (oligomers are very short polymers). There are typically three parts to most polymerizations:
What is M w /M n? The average molecular weight of a polymer can be defined by M n and M w. M n is the simple average of total mass of the chains divided by the number of chains. The weight average molecular weight M w is the summation of the square of the molecular weights divided by the summation of the molecular weights of all the molecules present. In M w more weight is provided to the to the higher molecular weight polymers, while M n treats all of them the same. The basis for M w is that the larger molecules contribute more to the properties of the polymer so they should have more importance. M w is always greater than M n and the narrower the distribution, the closer M n and M w are. The ratio of M w to M n is a measure of the distribution of different length polymer chains. This ratio is referred to as the dispersivity. As the distribution narrows, the dispersivity approaches a minimum value of 1.0. Such a polymer referred to as mono disperse. Alternately as M w /M n for a polymer increases (10 or 20) it is a referred to as poly-disperse. atalyst The German chemist Karl Ziegler (1898-1973) discovered in 1953 that when l 3 (s) and AlEt 3 are combined together they produced an extremely active heterogeneous catalyst for the polymerization of ethylene at atmospheric pressure. Giulio Natta (1903-1979), an Italian chemist, extended the method to other olefins like propylene and developed variations of the Ziegler catalyst based on his findings on the mechanism of the polymerization reaction. The Ziegler-Natta catalyst family includes halides of titanium, chromium, vanadium, and zirconium, typically activated by alkyl aluminum compounds. Ziegler and Natta received the Nobel Prize in hemistry for their work in 1963. Mechanism The first step is the alkylation of the titanium center. Please note that since this is a heterogeneous catalyst we are looking at the surface of the catalyst. bviously this implies coordination unsaturation at titanium. In the bulk sample the titanium has a coordination number of six. owever, at the surface the coordination of titanium is incomplete. This site can be taken up by a ligand such as olefin. Subsequent steps are the olefin insertion and the creation of a long alkyl chain. The chain terminates by β- elimination.
Initiation and Propagation 2 2 2 2 4 2 2 2 2 Vacant oordination Site 2 2 2 2 2 2 2 2 Termination 2 2 2 2 n β -elimination + 2 = 2 ( 2 2 ) n igh Density Polyethylene Polypropylene & Stereochemistry The polymerization of propylene is slower and more complicated than ethylene due to increasing steric factors and the generation of stereochemistry on the polymer chain. It has been found that the Ziegler-Natta heterogeneous catalysts promote the formation of a linear chain polyethylene with a high amount of crystallinity (igh Density Poly Ethylene). Also, if one uses propylene as the monomer, isotactic polypropylene is preferred over the other two forms viz., atactic and syndiotactic.