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How Chain Length and Branching Influence the Alkene Cracking Reactivity on H-ZSM-5.

ABSTRACT: Catalytic alkene cracking on H-ZSM-5 involves a complex reaction network with many possible reaction routes and often elusive intermediates. Herein, advanced molecular dynamics simulations at 773 K, a typical cracking temperature, are performed to clarify the nature of the intermediates and to elucidate dominant cracking pathways at operating conditions. A series of C₄-C₈ alkene intermediates are investigated to evaluate the influence of chain length and degree of branching on their stability. Our simulations reveal that linear, secondary carbenium ions are relatively unstable, although their lifetime increases with carbon number. Tertiary carbenium ions, on the other hand, are shown to be very stable, irrespective of the chain length. Highly branched carbenium ions, though, tend to rapidly rearrange into more stable cationic species, either via cracking or isomerization reactions. Dominant cracking pathways were determined by combining these insights on carbenium ion stability with intrinsic free energy barriers for various octene ?-scission reactions, determined via umbrella sampling simulations at operating temperature (773 K). Cracking modes A (3° ? 3°) and B₂ (3° ? 2°) are expected to be dominant at operating conditions, whereas modes B₁ (2° ? 3°), C (2° ? 2°), D₂ (2° ? 1°), and E₂ (3° ? 1°) are expected to be less important. All ?-scission modes in which a transition state with primary carbocation character is involved have high intrinsic free energy barriers. Reactions starting from secondary carbenium ions will contribute less as these intermediates are short living at the high cracking temperature. Our results show the importance of simulations at operating conditions to properly evaluate the carbenium ion stability for ?-scission reactions and to assess the mobility of all species in the pores of the zeolite.

SUBMITTER: Cnudde P

PROVIDER: S-EPMC6179455 | biostudies-literature | 2018 Oct

REPOSITORIES: biostudies-literature

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How Chain Length and Branching Influence the Alkene Cracking Reactivity on H-ZSM-5.

Cnudde Pieter P De Wispelaere Kristof K Vanduyfhuys Louis L Demuynck Ruben R Van der Mynsbrugge Jeroen J Waroquier Michel M Van Speybroeck Veronique V

ACS catalysis 20180905 10

Catalytic alkene cracking on H-ZSM-5 involves a complex reaction network with many possible reaction routes and often elusive intermediates. Herein, advanced molecular dynamics simulations at 773 K, a typical cracking temperature, are performed to clarify the nature of the intermediates and to elucidate dominant cracking pathways at operating conditions. A series of C<sub>4</sub>-C<sub>8</sub> alkene intermediates are investigated to evaluate the influence of chain length and degree of branching ...[more]

PMID: 30319885

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Project description:In serine proteases, Gly193 (chymotrypsin numbering) is conserved with rare exception. Mutants of blood coagulation proteases have been reported with Glu, Ala, Arg or Val substitutions for Gly193. To further understand the role of Gly193 in protease activity, we replaced it with Ala or Val in coagulation factor XIa (FXIa). For comparison to the reported FXIa Glu193 mutant, we prepared FXIa with Asp (short side chain) or Lys (opposite charge) substitutions. Binding of p-aminobenzamidine (pAB) and diisopropylfluorphosphate (DFP) were impaired 1.6-36-fold and 35-478-fold, respectively, indicating distortion of, or altered accessibility to, the S1 and oxyanion-binding sites. Val or Asp substitutions caused the most impairment. Salt bridge formation between the amino terminus of the mature protease moiety at Ile16 and Asp194, essential for catalysis, was impaired 1.4-4-fold. Mutations reduced catalytic efficiency of tripeptide substrate hydrolysis 6-280-fold, with Val or Asp causing the most impairment. Further studies were directed toward macromolecular interactions with the FXIa mutants. kcat for factor IX activation was reduced 8-fold for Ala and 400-1100-fold for other mutants, while binding of the inhibitors antithrombin and amyloid beta-precursor protein Kunitz domain (APPI) was impaired 13-2300-fold and 22-27000-fold, respectively. The data indicate that beta-branching of the side chain of residue 193 is deleterious for interactions with pAB, DFP and amidolytic substrates, situations where no S2'-P2' interactions are involved. When an S2'-P2' interaction is involved (factor IX, antithrombin, APPI), beta-branching and increased side chain length are detrimental. Molecular models indicate that the mutants have impaired S2' binding sites and that beta-branching causes steric conflicts with the FXIa 140-loop, which could perturb the local tertiary structure of the protease domain. In conclusion, enzyme activity is impaired in FXIa when Gly193 is replaced by a non-Gly residue, and residues with side chains that branch at the beta-carbon have the greatest effect on catalysis and binding of substrates.

Dataset Information

How Chain Length and Branching Influence the Alkene Cracking Reactivity on H-ZSM-5.

Publications

How Chain Length and Branching Influence the Alkene Cracking Reactivity on H-ZSM-5.

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