Montmorillonite catalysts, palladium

Raffinate Hydrocracking with Palladium-Nickel— Containing Synthetic Mica-Montmorillonite Catalysts... 

Montmorillonite-based palladium catalysts are active in the hydrogenation of styrene to ethylbenzene under mild conditions (25 °C, 1 atmosphere H2). 

Heterogeneous catalysis is also a very attractive alternative from an industrial perspective. Lee and co-workers reported a novel montmorillonite-supported palladium catalyst for butadiene dimerization with trapping by water.t i i Simple solid-supported palla-dium(O) catalysts, such as 5% Pd/C and Pd/Al203, were modestly effective catalysts for... 

Sugar azides can be reduced directly to the corresponding acetamido-sugars with a mixture of thiolacetic acid and its potassium salt. A heterogenized homogeneous catalyst, interlamellar montmorillonite diphenylphosphine palladium(II), has been reported (two carbohydrate examples) to reduce azides selectively to amines without affecting benzyl ether or alkene moieties. ... 

Vinyl acetate reacts with the alkenyl triflate 65 at the /3-carbon to give the 1-acetoxy-1,3-diene 66[68]. However, the reaction of vinyl acetate with 5-iodo-pyrimidine affords 5-vinylpyrimidine with elimination of the acetoxy group[69]. Also stilbene (67) was obtained by the reaction of an excess of vinyl acetate with iodobenzene when interlamellar montmorillonite ethylsilyl-diphenylphosphine (L) palladium chloride was used as an active catalyst[70]. Commonly used PdCl2(Ph3P)2 does not give stilbene. 

The use of heterogeneous catalysts in this reaction has also been achieved palladium-montmorillonite clays [93] or palladium/activated carbon [94] in the presence of dppb transformed 2-allylphenols into lactones, the regiose-lectivity of the reaction being largely dependant on the nature of the support. Very recently, palladium complexes immobilized onto silica-supported (polyaminoamido)dendrimers were used as catalysts in the presence of dppb for the cyclocarbonylation of 2-allylphenols, 2-allylanilines, 2-vinylphenols, and 2-vinylanilines affording five-, six-, or seven-membered lactones and lactams. Good conversions are realized and the catalyst can be recycled 3-5 times [95]. 

Low-loaded, organophilic Pd-montmorillonites were shown to exhibit high cis-selectivity in the hydrogenation of 1-phenyl-1-alkynes working with high (=5000) substrate catalyst ratio.402 Studies with respect to the use palladium membrane catalysts403 105 and polymeric hollow fiber reactors406"408 were reported. 

The industrially important acetoxylation consists of the aerobic oxidation of ethylene into vinyl acetate in the presence of acetic acid and acetate. The catalytic cycle can be closed in the same way as with the homogeneous Wacker acetaldehyde catalyst, at least in the older liquid-phase processes (320). Current gas-phase processes invariably use promoted supported palladium particles. Related fundamental work describes the use of palladium with additional activators on a wide variety of supports, such as silica, alumina, aluminosilicates, or activated carbon (321-324). In the presence of promotors, the catalysts are stable for several years (320), but they deactivate when the palladium particles sinter and gradually lose their metal surface area. To compensate for the loss of acetate, it is continuously added to the feed. The commercially used catalysts are Pd/Cd on acid-treated bentonite (montmorillonite) and Pd/Au on silica (320). 

Styrene and 1-hexene have been selectively hydrogenated as well as substituted acetylenes, alkyne diols, stilbene and other unsaturated hydrocarbons with these palladium montmorillonites. A size selectivity was invoked to explain the enhanced hydrogenation activity of certain clay catalysts presumably due to the differences in interlamellar spacings of the clay which will depend on degree of hydration, concentration of Pd(II) complex, dielectric constant of the solvent used to disperse the reactants and other factors. 

As mentioned earlier, palladium, rhodium, and platinum catalysts lead to superior regioselectivities because they work under milder reaction conditions (20-80 °C, 0.1-1 MPa CO) [11], e.g., bimetallic catalysts based on tin(II) chloride and either platinum or palladium complexes afford linear esters in up to 98 % selectivity [12]. In addition, catalyst systems with preference for branched isomers are known. A recent example employed palladium acetate immobilized on montmorillonite in the presence of triphenylphosphine and an acid promoter for the hydroesterification of aryl olefins (eq. (3)). The reaction is totally regiospecific for the branched isomer of aromatic olefins, while aliphatic olefins afford branched chain esters only regioselectively with n/i = 1 3 [13]. 

Recently, the high activity of palladium/NHC complexes in the Heck reaction was combined with an efficient recyclability process [63]. Bis-carbene pincer complexes of palladium(II) were immobilized on montmorillonite K-10. The catalytic activity of the heterogeneous system is similar to that displayed by their homogeneous counterparts. The stability of the catalyst was tested in the reaction of phenyl iodide and styrene. The product yield decreases from 99 to 79% after ten cycles. 

Highly active immobilized palladium catalysts modified with a rigid C,N,C-pincer carbene ligand were successfully applied in the coupling of aryl halides to terminal acetylenes. The homogeneous catalyst was immobilized on three types of solids - montmorillonite K-10, bentonite A, and bentonite B - by the solvent-impregnation method. No significant differences were observed in the catalytic efficiency between the various supports. The reactivity of the system is relatively constant after as many as five cycles [68]. 

As type 2 systems are maintained by rather high levels of soluble palladium concentrations in the reaction media, palladium from any support will be depleted rapidly. Recycling of a supported catalyst will thus be difficult. Aside from stable palladium complexes, there are a few supported systems which deliver reasonable type 2 performance. One of the most active systems was introduced by Molndr and Papp [105] with palladium(II) absorbed by ion-exchange onto montmorillonite clay. The precatalyst operates at high temperatures (150-160 °C) in the presence of NaaCOs in NMP and delivers high yields of Mizoroki-Heck products with, for example, phenyl bromide, 8 and 4-chloroacetophenone (47) at 0.001-0.1 mol% catalyst loading (47 48, Scheme 2.10). With more reactive substrates (e.g. phenyl bromide) this precatalyst survives two or three reuses, which shows that palladium is extracted from the support lattice at a sparing rate. 

Palladium acetate immobilized on montmorillonite is another effective catalyst for the hydroesteriflcation of olefins to form branched-chain esters as the major products. The reaction proceeded at 600 psi of carbon monoxide, in the presence of PPhj, methanol, and an acid promoter (Eq. 14).[ ... 

Nitroaromatics have been reductively converted into aryl carbamate with methanol under CO atmosphere catalyzed by palladium(II) anchored onto montmorillonite clay. Conversely, the same carbamates can also be obtained via oxidative carbonylation of amines with carbon monoxide and oxygen in the presence of a bimetallic catalyst PdCl2-MnCl2 supported on poly( V-vinyl-2-pyrrolidone).f ... 

Interestingly, the MC approach for this reaction has been also developed using heterogeneous catalysts such as KjCoWjjO j-SH O [100], supported heteropoly acids [101], montmorillonite [102], sulfuric acid-modified PEG-6000 [103], or organometallic hollow spheres bearing bis(Ai-heterocyclic carbene)-palladium species [104], among others. 

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