248J. Keijsper et al. / "Green" MMA; an environmentally benign and economically attractive process Recl. Trav. Chim. Pays-Bas 115,248-255 (1996) SSDI 0 1 6 5 -05 1 3 (9 6 ) O 000 2 -6 0165-0513/96/04248-08$12.00 "Green" MMA, an environmentally benign and economically attractive process for the production of methyl methacrylate Abstract. Over the last seven years Shell has developed a novel, economically attractive, and environmentally benign process for the production of methyl methacrylate (MMA). The process is based on the highly efficient Palladium-catalysed methoxycarbonylation of propyne (methylacetylene or MA) under mild, non-corrosive reaction conditions (temperature ca. 50°C, C O pressure ca. 10 bar) according to the equation:The activity (> 20 000 mol-MMA/mol-Pd.h) and selectivity ( > 99%mol) of the homogeneous Pd catalyst are very high making a once-through process with minimal waste possible. The MA feedstock for the process, together with propadiene (PD), is separated from a steamcracker C, stream by extractive distillation with DMF. Subsequently, P D is removed from this MA + PD/DMF mixture and isomerized over a heterogeneous K,O/Al,O, catalyst to yield additional MA. Crude MMA obtained from the methoxycarbonylation of MA can easily be purified by a 'topping and tailing' procedure to yield MMA of very high purity (> 99.99 w%). This article describes some of the R& D work carried out at the 'Koninklijke/Shell-Laboratorium, Amsterdam' which has enabled the design of a plant, based on this novel technology.a Nomenclature and abbreviations acrylic acid = propenoic acid ACH process = acetone-cyanohydrine process crotonic acid = (EI-but-2-enoic acid CSTR = continuously stirred tank reactor DMF = N,N-dimethylformamide FID = flame-ionization detection FPU = feed-preparation unit GLC = gas-liquid chromatography HE stream = heavy-end stream isobutene = methylpropene kta = kilo-ton per annum = 1000000 kg/year LE stream = light-end stream LHSV = liquid hourly space velocity (in l/l/h) MA = methylacetyleen = propyne MAA = methacrylic acid = 2-methyl propenoic acid MAPD=MA+PD MEHQ = methylether of hydroquinone = 4-methoxyphenol methacrolein = 2-methylpropenal MMA = methyl methacrylate = methyl 2-methylpropenoate MSA = methanesulfonic acid MTBE = methyl tert-butyl ether = 2-methoxy-2-methylpropane PD = propadiene PN = (pyrid-2-y1)diphenylphosphine tetrene = welding gas containing ca. 36 w% of MAPD wppm = weight ppm
Hx,) reacts smoothly at 90 °C with molecular hydrogen to yield the oxidative addition product H2Ru2(CO)5(R-DAB). The same product can be obtained when Ru2(CO)6(R-DAB) (R = ¿-Pr, c-Hx, f-Bu) is used as starting material. In the products, which are characterized by FD-mass, IR, and NMR spectroscopy, the R-DAB ligand is in a tie-bonding mode. The NMR spectra indicate that the two hydride atoms are bonded (presumably terminally) to the same Ru atom. Ru3(CO)8(neo-Pent-DAB) reacts with H2 to yield both known H4Ru4(CO)12 and novel H2Ru4(CO)8(neo-Pent-DAB)2. The latter compound can also be obtained from the reaction of Ru2(CO)5(neo-Pent-DAB) with H2. Possible reaction routes are discussed. An X-ray crystal structure determination established the molecular geometry of this tetranuclear Ru-a-diimine complex. Crystals of H2Ru4(CO)8(neo-Pent-DAB)2 are monoclinic of space group C2/c with cell constants = 18.399 (7) k,b = 8.725 (3) A, c = 29.177 (8) A, ß = 110.43 (4)°, and Z = 4. A total of 1318 reflections have been used in the refinement resulting in a final R value of 0.038 (Rw = 0.064). The molecule possesses a pseudo-Cy, symmetry. Surprisingly, the four Ru atoms are located in a linear chain arrangement which is very unusual. The Ru(l)-Ru(2)-Ru(2)* angle is 176.48 (6)°. The Ru-Ru distances are in the range expected for single Ru-Ru bond lengths (Ru(l)-Ru(2) = 2.806 (2) A, Ru(2)-Ru(2)* = 2.745(2) A). Ru(l) is coordinated by two terminal carbonyls and Ru(2) by one. The central Ru(2)-Ru(2)* bond is symmetrically bridged by two carbonyls. The neo-Pent-DAB ligand is -, -'-bonded to Ru(l) with equal Ru(l)-N bond lengths of 2.09 (1) A and r)2-C=N,7)2-C'=N' bonded to Ru(2) with an average Ru-(2)-ligand atom distance of 2.30 (5) A. Likewise, the C-N and C-C bond lengths within the a-diimine part are equal (1.36(3) A) which is consistent with an 8e-bonding mode. The hydride ligands could not be located directly, however; the geometry around Ru(l) and the NMR data strongly indicate a terminal (on Ru(l)) bond position, which is quite unusual in a polynuclear metal carbonyl complex. Electronically, this species can be considered as electron precise, i.e., a 66e complex having three metal-metal bonds.
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