Benzene adsorption and desorption in mordenite
ABSTRACT The zeolite-catalyzed synthesis of cumene from benzene and propene is an industrially important reaction. We used small mordenite crystals to study benzene adsorption and desorption behaviour for sodium, proton and nitric acid treated mordenite. Adsorption of benzene was for all samples fast and completed within 25 seconds at a benzene partial pressure of 0.12 bar in nitrogen at 423 K. The largest benzene uptake was found for the acid treated mordenite 4.5 wt.% followed by the sodium mordenite 4.0 wt.% and the proton mordenite 3.5wt.%. Lower uptake for the proton mordenite could be explained by the presence of minor blockades formed during the ion-exchange and calcination process. The higher uptake for the acid treated mordenite was explained by the partial removal of these pore blockades and more efficient stacking of benzene molecules due to the absence of cations. Desorption rates were very different for the three samples; with Na-MOR 60% of the benzene desorbed in 24 hours, H-MOR in 1 hour and for the acid treated mordenite within 10 minutes. Benzene adsorption isotherms were measured for proton and acid treated mordenite. A simple Langmuir model fit yielded a maximal benzene loading for proton mordenite of 3.0-3.4 wt.%, with isosteric heats of adsorption between 43 and 52 (±5 kJ/mol). For the acid treated mordenite a maximum loading of 5.5 wt.% was found at 423 K, and slightly lower heats of adsorption. The origin of the marked differences in desorption rate is not clear, as it cannot be related to large differences in adsorption strength. However, it is clear that post-synthesis modification is a strong tool to influence the desorption and diffusion behaviour in this system.
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ABSTRACT: Expression of V(D)J recombinase activity in developing lymphocytes is absolutely required for initiation of V(D)J recombination at antigen receptor loci. However, little is known about when during hematopoietic development the V(D)J recombinase is first active, nor is it known what elements activate the recombinase in multipotent hematopoietic progenitors. Using mice that express a fluorescent transgenic V(D)J recombination reporter, we show that the V(D)J recombinase is active as early as common lymphoid progenitors (CLPs) but not in the upstream progenitors that retain myeloid lineage potential. Evidence of this recombinase activity is detectable in all four progeny lineages (B, T, and NK, and DC), and rag2 levels are the highest in progenitor subsets immediately downstream of the CLP. By single cell PCR, we demonstrate that V(D)J rearrangements are detectable at IgH loci in approximately 5% of splenic natural killer cells. Finally, we show that recombinase activity in CLPs is largely controlled by the Erag enhancer. As activity of the Erag enhancer is restricted to the B cell lineage, this provides the first molecular evidence for establishment of a lineage-specific transcription program in multipotent progenitors.Journal of Experimental Medicine 03/2004; 199(4):491-502. · 13.21 Impact Factor
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ABSTRACT: Ig H chain (IgH) allelic exclusion remains a puzzling topic. Here, we address the following question: Do phenotypic IgH allelically included cells exist in normal mice and, if so, at what frequency? Sorted cells from heterozygous mice were evaluated for the expression of both IgM allotypes by double intracytoplasmic stainings. Dual expressors were found at a frequency of 1 in 104 splenic B cells. These data were confirmed by direct sequencing of IgH-rearranged alleles obtained after single cell (or clone) PCR on dual expressors. Typically, these cells have one rearranged J558 VH whereas, in the other allele, a D-proximal VH gene is used. Interestingly, dual expressors have rearranged IgH alleles with similar CDR3 lengths. These results show that, in contrast to the kappa L chain and the TCR beta-chain, IgH allelic exclusion is the result of an extremely stringent mechanism. We discuss two non-mutually exclusive scenarios for the origin of IgH dual expressors: 1) IgH allelically included cells arise when the first allele to rearrange productively is unable to form a pre-BCR; dual expressors could be a subset of this population in which, upon conventional L chain rearrangement, both IgH are expressed at the surface; and 2) synchronous rearrangement of the IgH alleles.The Journal of Immunology 02/2000; 164(2):893-9. · 5.52 Impact Factor
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ABSTRACT: Recent investigations have suggested that tissue-specific regulatory factors are required for immunoglobulin gene transcription. Cells of the mouse lymphocytoid pre-B-cell line 70Z/3 contain a constitutively rearranged immunoglobulin kappa light chain gene; the nucleotide sequence of this gene exhibits all the known properties of a functionally competent transcription unit. Nevertheless, transcripts derived from this gene are detectable only after exposure of the cells to bacterial lipopolysaccharide, implying that accurate DNA rearrangement is not sufficient to activate expression of the gene. Comparison of the sequence of the 70Z/3 kappa light chain gene with those encoding other immunoglobulin heavy and light chains has revealed that a distinctive promoter region structure is characteristic of this multigene family. The sequence A-T-T-T-G-C-A-T lies approximately 70 base pairs upstream from the site of transcriptional initiation in every light chain gene examined; in heavy chain genes, the corresponding location is occupied by the precise inverse (A-T-G-C-A-A-A-T) of this sequence. Although adjacent regions of DNA have diverged extensively in evolution, these octanucleotide sequences are stringently conserved at this location among diverse immunoglobulin genes from at least two mammalian species. The proximity of this conserved octanucleotide block to the site of transcriptional initiation suggests that it may serve as a recognition locus for factors regulating immunoglobulin gene expression in a tissue-specific fashion.Proceedings of the National Academy of Sciences 06/1984; 81(9):2650-4. · 9.81 Impact Factor