55 research outputs found

    Reaction Behaviors and Kinetics during Induction Period of Methanol Conversion on HZSM‑5 Zeolite

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    The reaction behavior in the induction period of the methanol to hydrocarbon (MTH) reaction over HZSM-5 (Si/Al = 19) zeolite has been investigated in a fixed-bed reactor. It is found that the induction period could be more than 2 h when the reaction was performed at a temperature of 255 °C and below. Meanwhile, three reaction stages can be clearly distinguished in the induction period: i.e., the initial C–C bond formation stage, the hydrocarbon pool (HCP) species formation stage, and the autocatalysis reaction stage. For each reaction stage, the kinetic parameters as well as the apparent activation energies have been evaluated. The HCP species formation stage is shown to be the rate-determining step. Addition of a ppm amount (molar) of benzene, toluene, or <i>p</i>-xylene to the methanol feed leads to a shortened induction period due to a lower energy barrier for both the HCP species formation and the autocatalysis reaction. A critical value of HCP species, [HCP]<sub>c</sub>, that is required for starting the autocatalysis reaction (the third stage) has been proposed. This critical value was measured to be 1 toluene molecule per 276 unit cells for HZSM-5 zeolite when toluene is cofed with methanol

    Forest plots of ORs for the association between the <i>MTHFD1</i> G1958A polymorphism and susceptibility to NTDs (A: A allele vs. G allele; B: AA vs. AG+GG; C: AA vs. GG; D: AA vs. AG).

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    <p>Forest plots of ORs for the association between the <i>MTHFD1</i> G1958A polymorphism and susceptibility to NTDs (A: A allele vs. G allele; B: AA vs. AG+GG; C: AA vs. GG; D: AA vs. AG).</p

    Association between <i>MTHFD1</i> G1958A Polymorphism and Neural Tube Defects Susceptibility: A Meta-Analysis

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    <div><p>Objectives</p><p>The methylenetetrahydrofolate dehydrogenase (<i>MTHFD1</i>) gene, as one of the key genes involved in the folate pathway, has been reported to play a critical role in the pathogenesis of neural tube defects (NTDs). However, the results of published studies are contradictory and inconclusive. Thus, this meta-analysis aimed to evaluate the effect of the common polymorphism in the <i>MTHFD1</i> gene, the G1958A (R653Q, dbSNP ID: rs2236225) variant, on the risk of NTDs in all eligible studies.</p><p>Methods</p><p>Relevant literature published before January 3, 2014 was retrieved from the MEDLINE, EMBASE, Cochrane Library, and CBM databases. Pooled crude odds ratios (ORs) and their corresponding 95% confidence intervals (CIs) were calculated to evaluate the association between the <i>MTHFD1</i> G1958A polymorphism and NTDs risk.</p><p>Results</p><p>We performed a meta-analysis of nine studies with a total of 4,302 NTDs patients and 4,238 healthy controls. Our results demonstrated a significant correlation between the <i>MTHFD1</i> G1958A polymorphism and NTDs in an overall meta-analysis. For family-based studies, the study subjects were classified as NTD cases, mothers with NTDs offspring, and fathers with NTDs offspring. We found no association between any of the fathers’ genotypes and NTDs, whereas there was a clear excess of the 1958A allele in the mothers of children with NTDs compared with controls individuals.</p><p>Conclusions</p><p>In summary, our meta-analysis strongly suggests that the <i>MTHFD1</i> G1958A polymorphism might be associated with maternal risk for NTDs in Caucasian populations. However, the evidence of this association should be interpreted with caution due to the selective nature of publication of genetic association studies.</p></div

    Flow diagram of studies with specific reasons for inclusion/exclusion in the present meta-analysis.

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    <p>Flow diagram of studies with specific reasons for inclusion/exclusion in the present meta-analysis.</p

    Main characteristics of the studies included in the meta-analysis.

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    <p>PCR-RFLP, Polymerase chain reaction-restriction fragment length polymorphism; PCR-SSCP, PCR-single strand conformation polymorphism; NOS, Newcastle-Ottawa scale.</p

    Forest plots of ORs for the association between the <i>MTHFD1</i> G1958A polymorphism and susceptibility to NTDs in family-based studies for each subgroup under (A: A allele vs. G allele; B: AA vs. AG+GG; C: AA vs. GG; D: AA vs. AG).

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    <p>Forest plots of ORs for the association between the <i>MTHFD1</i> G1958A polymorphism and susceptibility to NTDs in family-based studies for each subgroup under (A: A allele vs. G allele; B: AA vs. AG+GG; C: AA vs. GG; D: AA vs. AG).</p

    Surface Diffusion Barriers and Catalytic Activity Driven by Terminal Groups at Zeolite Catalysts

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    Defects that commonly exist on the surface of zeolites pose notable mass transport constraints and influence the catalytic performance. The mechanism underlying the surface defects inducing molecular transport limitations, however, is not fully understood. Herein, we use versatile spectroscopy, imaging techniques, and multiscale simulations to investigate the effect of surface defects on the molecular surface transport in zeolites, intending to establish the terminal structure–mass transport–performance relationship. Isolated silanol, which represents the foremost and eventual chemical defective accessible site at zeolite termination for guest molecules from the bulk fluid phase into zeolites or vice versa, is taken as a showcase. We demonstrate that isolated silanol at H-SAPO-34 zeolite termination not only enhances the adsorptive interaction between the polar molecules/alkenes and interface but also narrows the local 8-membered-ring pore at the external surface. The exterior surface with more isolated silanol could cause a higher diffusion barrier and hamper the accessibility of intracrystalline active sites. This work is expected to shed light on the mechanism underlying the zeolite catalyst upgrading via terminal surface modifications at zeolites

    Surface Diffusion Barriers and Catalytic Activity Driven by Terminal Groups at Zeolite Catalysts

    No full text
    Defects that commonly exist on the surface of zeolites pose notable mass transport constraints and influence the catalytic performance. The mechanism underlying the surface defects inducing molecular transport limitations, however, is not fully understood. Herein, we use versatile spectroscopy, imaging techniques, and multiscale simulations to investigate the effect of surface defects on the molecular surface transport in zeolites, intending to establish the terminal structure–mass transport–performance relationship. Isolated silanol, which represents the foremost and eventual chemical defective accessible site at zeolite termination for guest molecules from the bulk fluid phase into zeolites or vice versa, is taken as a showcase. We demonstrate that isolated silanol at H-SAPO-34 zeolite termination not only enhances the adsorptive interaction between the polar molecules/alkenes and interface but also narrows the local 8-membered-ring pore at the external surface. The exterior surface with more isolated silanol could cause a higher diffusion barrier and hamper the accessibility of intracrystalline active sites. This work is expected to shed light on the mechanism underlying the zeolite catalyst upgrading via terminal surface modifications at zeolites

    Surface Diffusion Barriers and Catalytic Activity Driven by Terminal Groups at Zeolite Catalysts

    No full text
    Defects that commonly exist on the surface of zeolites pose notable mass transport constraints and influence the catalytic performance. The mechanism underlying the surface defects inducing molecular transport limitations, however, is not fully understood. Herein, we use versatile spectroscopy, imaging techniques, and multiscale simulations to investigate the effect of surface defects on the molecular surface transport in zeolites, intending to establish the terminal structure–mass transport–performance relationship. Isolated silanol, which represents the foremost and eventual chemical defective accessible site at zeolite termination for guest molecules from the bulk fluid phase into zeolites or vice versa, is taken as a showcase. We demonstrate that isolated silanol at H-SAPO-34 zeolite termination not only enhances the adsorptive interaction between the polar molecules/alkenes and interface but also narrows the local 8-membered-ring pore at the external surface. The exterior surface with more isolated silanol could cause a higher diffusion barrier and hamper the accessibility of intracrystalline active sites. This work is expected to shed light on the mechanism underlying the zeolite catalyst upgrading via terminal surface modifications at zeolites
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