Twin-free, directly synthesized MFI nanosheets with improved thickness uniformity and their use in membrane fabrication

Zeolite nanosheets can be used for the fabrication of low-defect-density, thin, and oriented zeolite separation membranes. However, methods for manipulating their morphology are limited, hindering progress toward improved performance. We report the direct synthesis (i.e., without using exfoliation, etching, or other top-down processing) of thin, flat MFI nanosheets and demonstrate their use as high-performance membranes for xylene isomer separations. Our MFI nanosheets were synthesized using nanosheet fragments as seeds instead of the previously used MFI nanoparticles. The obtained MFI nanosheets exhibit improved thickness uniformity and are free of rotational and MEL intergrowths as shown by transmission electron microscopy (TEM) imaging. The nanosheets can form well-packed nanosheet coatings. Upon gel-free secondary growth, the obtained zeolite MFI membranes show high separation performance for xylene isomers at elevated temperature (e.g., p-xylene flux up to 1.5 × 10^-3 mol m^-2 s^-1 and p-/o-xylene separation factor of ~600 at 250°C).

N-Aryl-linked spirocyclic polymers for membrane separations of complex hydrocarbon mixtures

The fractionation of crude-oil mixtures through distillation is a large-scale, energy-intensive process. Membrane materials can avoid phase changes in such mixtures and thereby reduce the energy intensity of these thermal separations. With this application in mind, we created spirocyclic polymers with N-aryl bonds that demonstrated noninterconnected microporosity in the absence of ladder linkages. The resulting glassy polymer membranes demonstrated nonthermal membrane fractionation of light crude oil through a combination of class- and size-based "sorting" of molecules. We observed an enrichment of molecules lighter than 170 daltons corresponding to a carbon number of 12 or a boiling point less than 200°C in the permeate. Such scalable, selective membranes offer potential for the hybridization of energy-efficient technology with conventional processes such as distillation.

One-dimensional intergrowths in two-dimensional zeolite nanosheets and their effect on ultra-selective transport

Zeolite MFI is a widely used catalyst and adsorbent that also holds promise as a thin-film membrane. The discovery of nanometre-thick two-dimensional (2D) MFI nanosheets has facilitated methods for thin-film zeolite fabrication that open new horizons for membrane science and engineering. However, the crystal structure of 2D-MFI nanosheets and their relationship to separation performance remain elusive. Using transmission electron microscopy, we find that one- to few-unit-cell-wide intergrowths of zeolite MEL exist within 2D-MFI. We identify the planar distribution of these 1D or near-1D-MEL domains, and show that a fraction of nanosheets have high (~25% by volume) MEL content while the majority of nanosheets are MEL-free. Atomistic simulations show that commensurate knitting of 1D-MEL within 2D-MFI creates more rigid and highly selective pores compared to pristine MFI nanosheets, and permeation experiments show a separation factor of 60 using an industrially relevant (undiluted 1 bar xylene mixture) feed. Confined growth in graphite is shown to increase the MEL content in MFI nanosheets. Our observation of these intergrowths suggests strategies for the development of ultra-selective zeolite membranes.

Nanoscale Control of Homoepitaxial Growth on a Two-Dimensional Zeolite

Nanoscale crystal growth control is crucial for tailoring two-dimensional (2D) zeolites (crystallites with thickness less than two unit cells) and thicker zeolite nanosheets for applications in separation membranes and as hierarchical catalysts. However, methods to control zeolite crystal growth with nanometer precision are still in their infancy. Herein, we report solution-based growth conditions leading to anisotropic epitaxial growth of 2D zeolites with rates as low as few nanometers per day. Contributions from misoriented surface nucleation and rotational intergrowths are eliminated. Growth monitoring at the single-unit-cell level reveals novel nanoscale crystal-growth phenomena associated with the lateral size and surface curvature of 2D zeolites.

Oriented MFI Membranes by Gel-Less Secondary Growth of Sub-100 nm MFI-Nanosheet Seed Layers

A zeolite membrane fabrication process combining 2D-zeolite nanosheet seeding and gel-free secondary growth is described. This process produces selective molecular sieve films that are as thin as 100 nm and exhibit record high permeances for xylene- and butane-isomers.

Zeolite membranes – a review and comparison with MOFs

The latest developments in zeolite and MOF membranes are reviewed, with an emphasis on synthesis techniques. Industrial applications, hydrothermal stability, polymer-supported and mixed matrix membranes are some of the aspects discussed.

Role of pancreatic duct stenting in the treatment of chronic pancreatitis

BACKGROUND

Endoscopic retrograde cholangiopancreatography and stent placement are relatively new alternatives to surgery for the treatment of chronic pancreatitis. The objective of this study was to determine the efficacy of pancreatic duct stent placement for the treatment of chronic pancreatitis.

METHODS

This study included 89 patients treated with pancreatic stents between 1993 and 2002. The patients were contacted via telephone for a personal interview with regard to pain, medication usage, weight loss or gain, and eating patterns. Additionally, medication usage before and after treatment was documented from the Kentucky Cabinet for Health Services' electronic reporting system for narcotic use.

RESULTS

Of the 89 patients, 9 were deceased, 5 either refused to interview or could not be contacted, and 75 were interviewed. Significant weight gain exceeding 15 lb after treatment was experienced by 22%, whereas only 4% lost weight. A majority of the patients (68%) noted that they had less severe relapses or no relapses after treatment. The patients reported a decrease in pain level on a 10-point scale from 8.7 to 4.1 (53% decrease) after treatment. A decrease in pain medication usage was reported by 47% of the patients, and 83% considered their treatment successful. The Kentucky All Schedule Prescription Electronic Report (KASPER) was obtained before and after treatment for 55 patients. According to this statewide electronic reporting system, 63% had a documented decrease in narcotic use.

CONCLUSION

The findings of this study support the use of pancreatic duct stenting as an option before surgical intervention for these difficult-to-manage patients with chronic pancreatitis.

Parathyroid cysts. Report of four cases and review of the literature

Though a rare lesion, a parathyroid cyst is of clinical significance because it mimics a thyroid cyst and can be associated with hyperparathyroidism. The cyst can be ectopic in location and therefore constitutes a differential diagnosis to a bronchial or thymic cyst. Four cases of parathyroid cyst are presented. Fine needle aspiration with parathyroid hormone assay is diagnostic. Surgical removal of the cyst is recommended.