Zeolite carbonylation, methanol

The Kinetics of Methanol Carbonylation Over RhX, RhY and IrY zeolites Carbonylation of methanol proceeds readily at atmospheric pressure under mild temperature conditions 150°-180°C. This reaction ZCH OH + CO - CH COOCH + HjO produces mainly methyl acetate and water. Acetic acid was detected at high conversions and high temperatures. Traces of dimethyl ether could also form. In most cases the selectivity to methyl acetate was at least 90% in presence of the iodide promotor. 

Rh]-zeolites carbonylation reactions (oxo synthesis, methanol carbonylation)... 

It is well known that Rh(I) complexes can catalyze the carbonylation of methanol. A heterogenized catalyst was prepared by ion exchange of zeolite X or Y with Rh cations.126 The same catalytic cycle takes place in zeolites and in solution because the activation energy is nearly the same. The specific activity in zeolites, however, is less by an order of magnitude, suggesting that the Rh sites in the zeolite are not uniformly accessible. The oxidation of camphene was performed over zeolites exchanged with different metals (Mn, Co, Cu, Ni, and Zn).127 Cu-loaded zeolites have attracted considerable attention because of their unique properties applied in catalytic redox reactions.128-130 Four different Cu sites with defined coordinations have been found.131 It was found that the zeolitic media affects strongly the catalytic activity of the Cd2+ ion sites in Cd zeolites used to catalyze the hydration of acetylene.132... 

Catalytic Reactions. As the techniques for solid-state n.m.r. continue to improve with the simultaneous improvement in sensitivity and hence speed, there will be a growing trend to look at chemical reactions occurring on or in catalysts. There have already been a number of instances where catalytically stimulated reactions have been studied by 13C n.m.r. - the alkylation of toluene by methanol on X zeolite, for example,138 in which the influence of the cation, Na+ or Cs+, on selectivity was deduced. The adsorption binding and decomposition of various metal carbonyls on A1203 or in zeolites has been studied,139 likewise, the nature and sites of interaction of CO and C02 on X and Y zeolites.140... 

Carbonylation of organic substrates was investigated using these well defined complexes. These carbonyl compounds exhibited catalytic properties in the carbonylation of organic substrates. In particular methanol carbonylation to methyl acetate in the gas phase was successfully attempted. Mechanistic and kinetic studies of this reaction over rhodium and iridium zeolites showed the similarities between the homogeneous and the zeolite mediated reactions. Aromatic ni-tro compounds were also converted to aromatic isocyanates using similar catalytic systems. The mechanistic aspect of this reaction will be also examined. 

As in homogeneous media carbonylation of methanol exhibited a first order rate law with respect to methyl iodide and a zero order with respect to CO and CH OH when Rh-Zeolites were used. Similarly when Ir-zeolites were employed the reaction rate was first order with respect to methanol and zero order with respect to CO and CH I. 

Similarly, it was shown, though few studies were reported for iridium-zeolites, that Ir(I) tricarbonyl was formed upon reaction of CO with Ir(III)-Y zeolites at I70°C which is within the methanol carbonylation temperature range. 

Lefebvre et al. (170) have conducted the high pressure CO + H2 reaction (30 atm, 503-523 K) over Rh-NaY catalysts. Whatever the rhodium precursors [e.g., Rh -NaY and Rh (CO)2-NaY], the reaction data were similar. This is in agreement with the fact that all the precursors were ultimately converted to Rh6(CO),6 under catalytic conditions. The external Rh crystals deposited on the zeolite surface exhibit significant activity for hydrocarbons, mainly methane, whereas the carbonyl clusters gave lower conversion to hydrocarbons with a small amount of oxygenates such as methanol and ethanol. 

Acidic zeolites are most commonly used, but metal-containing zeolites also have catalytic properties. For example, ion exchange of Rh into faujasites produces methanol carbonylation catalysts the Rh complexes work in much the same way as the analogous soluble catalyst (see 14.2.3.1) . ... 

It must be noted that the phenol/aldehyde reaction can be catalyzed by Bronsted acids (protonation of the carbonyl oxygen) as well as by Lewis acids (coordination of the carbonyl oxygen). In the latter case one Lewis centre (e.g. Al ) can accommodate and activate both the phenol and the aldehyde (cq. the benzyl alcohol, in the consecutive reaction). As a consequence, ortho-substitution is favoured [14,15]. The high 2,2 -dihydroxydiphenylmethane selectivity we obtained with homogeneous Al " -catalysis and with 7-alumina is consistent with these data. Additionally, the finding that the H - US - Y catalyzed toluene/formaldehyde-condensation gives a low 2,2 -selectivity, 19% [16], compared to the 32% we obtained with phenol, also indicates the hydroxyl-group plays a role. However, transalkylation, reported to lead to ortho-substitution in condensations of phenol with methanol on both zeolite- and non-zeolite Bronsted acid catalysts [17], can t be ruled out. 

As seen from these data, the loading of parent NaY zeolite with iron oxide via trinuclear Fe carbonyl as a precursor increases greatly the total rate of methanol conversion because of the appearance of oxidative active centers. In contrast, the use of carbonyl complexes of four and more nuclearity resulted in a dramatic drop of MeOH conversion even in comparison to the starting material. It should be noted that all Fe-containing samples have the same amount of iron oxide (about 1 wt %, metal basis), and such an effect can be explained only on assuming the formation of multilayer oxide deposits that cover the outer zeolite surface including its pore mouths. This assumption was verified by measuring the surface area for this set of samples that decreases from about 700 mVg for NaY down to a few tens m /g for three last samples in Table 5. 

M. Boronat, C. Martinez-Sanchez, D. Law, A. Corma, Enzyme-like specificity in zeolites a unique site position in mordenite for selective carbonylation of methanol and dimethyl ether with CO, J. Am. Chem. Soc., 2008, 130, 16316-16323. 

Early studies by Scurrell and coll, demonstrated the use of rhodium zeolites as catalysts for the carbonylation of methanol into methyl acetate in the presence of methyl iodide (65). It was hoped that due to their electrostatic field zeolites would effect the direct carbonylation of methanol without the help of the iodide promoter. In fact, as the CH3OH/CH3I ratio increased, increasing amounts of CH4 and CO2 were produced indicating that the reaction... 

Carbonylation of methanol to yield Homogeneous H-mordenite and Both zeolite stmctures present eight-membered [70]... 

The chemistry of anionic Iridium carbonyl clusters in NaX zeolite parallels that in basic solutions and on the basic MgO surface (Fig. 4-9). In basic solutions, the reductive carbonylation of [Ir4(CO)i2] with KOH in methanol under CO initially gives [HIr4(CO) ]-, [61, 67] then [Ir8(CO)22] , [62, 67] and finally [Ir6(CO),5]. ... 

Sulfated zirconia catalyst (S-Zr02) was also used for carbonylation of methanol (76) and DME (77). It has been shown that S-Zr02 exhibits low activity and selectivity in methanol carbonylation similar to pure acidic zeolite catalysts (see later under Zeolite Catalysts). In DME carbonylation, Rh-modified sulfated zirconia is less active and selective as compared to Rh-modified cesium salts of 12-tungstophosphoric acid (see later under Heteropoly Acid Catalysts). 

The carbonylation reaction on the present system is most likely interpreted to be the carbonylation of methyl carbenium ion, as postulated for the Koch reaction (11). Copper(II) ion that was incorporated into the zeolites by ion exchange accelerates the carbonylation. The copper ion did not change the product pattern. The copper(II) ion on the zeolite might be reduced by methanol or carbon monoxide to copper(I) ion under reaction conditions. The copper(I) ion has been known to react easily with carbon monoxide to form Cu(CO)n (S < n < 4), which is an active carbonylation reagent for formaldehyde, n-olefins, and alcohols (85). 

Dimethyl Ether Carbonylation. Contrary to low activity of zeolite catalysts in methanol carbonylation, H-MOR and H-FER has been shown to catalyze DME carbonylation to methyl acetate with the stable rates and >99% selectivity at 423-463 K after an initial induction period, during which acidic protons were replaced by methyl groups and co-produced water was removed (91,92). The rate of DME carbonylation was much higher than that for similar reactions of CH3OH, at least in part because H2 0 formed in parallel CH3OH dehydration reactions inhibits carbonylation steps (Table 22). 

Carbonylation and decarbonylation reactions of alkyl complexes in catalytic cycles have been reviewed . A full account of the carbonylation and homologation of formic and other carboxylic acid esters catalysed by Ru/CO/I systems at 200 C and 150-200 atm CO/H2 has appeared. In a novel reaction, cyclobutanones are converted to disiloxycyclopentenes with hydrosilane and CO in the presence of cobalt carbonyl (reaction 4) . The oxidative addition of Mel to [Rh(CO)2l2] in aprotic solvents (MeOH, CHCI3, THF, MeOAc), the rate determining step in carbonylation of methyl acetate and methyl halides, is promoted by iodides, such as Bu jN+I", and bases (eg 1-methylimidazole) . A further kinetic study of rhodium catalysed methanol carbonylation has appeared . The carbonylation of methanol by catalysts prepared by deposition of Rh complexes on silica alumina or zeolites is comparable with the homogeneous analogue . 

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