Catalysis acetylene trimerization

The NiY zeolite was also shown to be active for the cyclotrimerization of propyne with 1,2,4-trimethylbenzene being the main product. The activities of the above-mentioned transition metal ions for acetylene trimerization are not so surprising since simple salts and complexes of these metals have been known for some time to catalyze this reaction (161, 162). However, the tetramer, cyclooctatetraene, is the principal product in homogeneous catalysis, particularly when simple salts such as nickel formate and acetate are used as catalysts (161). The predominance of the trimer product, benzene, for the zeolite Y catalysts might be indicative of a stereoselective effect on product distribution, possibly due to the spatial restrictions imposed on the reaction transition-state complex inside the zeolite cages. 

There are relatively few examples of C-C bond formation on solid surfaces under UHV conditions. There are virtually no examples of catalytic C-C bond formation under such conditions. Perhaps the closest precedent for the present studies on reduced Ti02 can be found in the studies of Lambert et al. on single crystal Pd surfaces. Early UHV studies demonstrated that acetylene could be trimerized to benzene on the Pd(lll) surface in both TPD and modulated molecular beam experiments [9,10]. Subsequent studies by the same group and others [11,12] demonstrated that this reaction could be catalyzed at atmospheric pressure both by palladium single crystals and supported palladium catalysts. While it is not clear that catalysis was achieved in UHV, these and subsequent studies have provided valuable insights into the mechanism of this reaction as catalyzed by metals, including spectroscopic evidence for the hypothesized metallacyclopentadiene intermediates [10,13,14]. 

A2Acetylene oligomerization. Trimerization of acetylene into benzene has been studied using spectroscopic techniques by IMELIK and coll. (43,44). They showed that only a few cations could catalyze this reaction d Ni and Co, d and d Cr. They suggested that only those transition metal ions with partially filled orbitals and an even number of electrons are active. The reaction pathway involved, as is the case in homogeneous catalysis, the simultaneous coordination of two acetylene molecules to form a pentacycle that includes the transion ion. 

The catalysis of the cyclotrimerization of acetylenes by transition metal complexes has been extensively studied (reviews Bird, 1967 Hoogzand and HUbel, 1968 Maitlis, 1971, 1973 Heck, 1974). Various mechanistic studies have been done on this type of reaction (Blomquist and Maitlis, 1962 Meriwether et al., 1962 Schrauzer, 1964 Collman et al., 1968 Whitesides and Ehmann, 1969 Yamazaki and Hagihara, 1970 Miiller and Beissner, 1973 Gardner et a/., 1973). The iridium-promoted trimerization of dimethyl acetylenedicarboxylate has been demonstrated to occur through an iridocycle as shown in Scheme 2 (Collman and Kang, 1967 Baddley and Tupper, 1974). 

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