Polypropylene Degradation Catalyzed by Tantalum Hydrides Supported on Sulfated Alumina

Abstract

Ta–H+ sites supported on sulfated aluminum oxide (SAO), [Ta(H)2(O−)2][SAO] (1), catalyze the hydrogenolysis of isotactic polypropylene (iPP, M n = 13.3 kDa; Đ = 2.4; mmmm = 94%) to form low-molecular-weight branched alkanes (C11–C30) in good yields (70%). The alkanes formed lose stereochemical information originating from iPP, but residual iPP remains highly isotactic. In the presence of D2, similar mixtures of alkanes are formed containing −CH3–x D x , −CHD, and −CD–. Residual iPP maintains high tacticity and incorporates deuterium primarily into −CH3 groups of the polymer (−CH3–x D x /–CHD–/–CD– ∼10:1:0). 1 reacts with pinacolborane to form [TaH(κ2-H2BPin)(O−)2][SAO] (2). At 200 °C in an iPP melt, 2 reacts to form products indicative of C–H bond activation at −CH3 groups and internal −CH2– groups in iPP in a ∼1:3 ratio, indicating a slight kinetic preference for C–H bond activation at internal −CH2– groups. 1 is more reactive toward iPP than high-density polyethylene in hydrogenolysis reactions

A Supported Ziegler-Type Organohafnium Site Metabolizes Polypropylene

“…The β-Me elimination shown in Figure is a signature of β-alkyl elimination required for C–C hydrogenolysis shown in Figure b. Cationic hafnocenes generated in solution are known to engage in σ-bond metathesis reactions, analogous to the d 0 metal hydrides shown in Figure a . Therefore, cationic metallocenes formed during olefin polymerization reactions would be expected to show activity in reactions that digest polyolefins in the presence of H 2 .…”

“…Until this report the most common catalysts for hydrogenolysis of polyolefins were supported noble metal nanoparticles or supported d 0 metal hydrides (Figure a) . Comparisons between these disparate classes of catalysts are difficult, but 1 does appear to offer some advantage.…”

“…Treating plastic waste with H 2 and a catalyst forms low molecular weight alkanes that could be processed back to the monomer, which is a plausible strategy to a circular plastics economy. These hydrogenolysis reactions are usually catalyzed by supported nanoparticles or the heterogeneous “single-site” d 0 metal hydrides shown in Figure . The well-defined catalysts activate a C–H bond in the polymer by σ-bond metathesis and β-alkyl eliminate to form MR­(olefin) intermediates that are hydrogenated under the reaction conditions, Figure b.…”

Highly Selective and Efficient Perdeuteration of n‐Pentane via H/D Exchange Catalyzed by a Silica‐Supported Hafnium‐Iridium Bimetallic Complex

“…[1,40] The major roadblock preventing the use of 2 H 2 for alkane deuteration is the hydrogenolysis side-reaction, which emanates from CÀ C bond scission and results in a distribution of smaller alkanes. Alkane hydrogenolysis is commonplace and well documented for a range of early metal single sites [41][42][43][44][45][46][47][48][49][50][51][52] as well as late transition metal heterogeneous catalysts. [53][54][55][56][57][58][59][60][61][62][63][64][65][66][67][68][69] Competition between HIE and hydrogenolysis processes is far less scrutinized so far, [70] and selective catalysts able to label alkanes using D 2 or T 2 without promoting hydrogenolysis are lacking to our knowledge.…”

Influence of Steric and Dispersion Interactions on the Thermochemistry of Crowded (Fluoro)alkyl Compounds

“…The fact that branched (more highly alkyl-substituted) alkanes are more stable than linear or less-branched isomers has been investigated in great detail and fostered important insight into the performance of common quantum chemical methods. − Alkane branching results in spatially more compact structures with many close intramolecular contacts so that, at a specific chain length in highly branched systems, the detailed balance of attractive and repulsive interactions shifts to new, chemically unfavorable situations with significant steric strain, which may lead to dissociation, in extreme cases already at room temperature. ,,, Specifically, neighboring quaternary centers increase the strain up to a point where the weakest C–C bond of the molecule will break, thus preventing the synthesis of the molecule. Such molecular “explosions” are not only interesting for synthetic chemists to answer the question of whether such highly strained can be made but also important in polymer chemistry, e.g., in the degradation process of plastic waste . For a comprehensive overview of the general role of London dispersion and steric effects in molecular chemistry, the reader is referred to ref .…”