The mechanism and kinetics of methyl isobutyl ketone synthesis from acetone over ion-exchanged hydroxyapatite

The synthesis of methyl isobutyl ketone (MIBK) can be carried out by the condensation of acetone in the presence of hydrogen over a supported metal catalyst. Previous studies have shown that hydroxyapatite is an excellent catalyst for condensation reactions. The present investigation was undertaken in order to elucidate the reaction mechanism and site requirements for acetone coupling to MIBK over a physical mixture of hydroxyapatite and Pd/SiO2. The reaction is found to proceed by consecutive aldol addition to form diacetone alcohol (DAA), dehydration of DAA to mesityl oxide (MO), and hydrogenation of MO to MIBK. The products formed by feeding DAA and MO reveal that aldol addition of acetone is rapid and reversible, and that the subsequent dehydration of DAA is rate-limiting. Pyridine and CO2 titration show that aldol dehydration occurs over basic sites via an E1cB mechanism. A series of cation-substituted hydroxyapatite samples were prepared by ion-exchange to further investigate the role of acid-base strength on catalyst performance. Characterization of these samples by PXRD, BET, ICP-OES, XPS, CO2-TPD, and Raman spectroscopy demonstrated that the exchange procedure used does not affect the bulk properties of hydroxyapatite. DFT calculations reveal that in addition to affecting the Lewis acidity/basicity of the support, the size of the cation plays a significant role in the chemistry: cations that are too large (Ba2+) or too small (Mg2+) adversely affect reaction rates due to excessive stabilization of intermediate species. Strontium-exchanged hydroxyapatite was found to be the most active catalyst because it promoted α-hydrogen abstraction and C–O bond cleavage of DAA efficiently

Comparison of the utilisation of antihypertensive drugs in a hypertension outpatient clinic in 1996 and 1998

published_or_final_versio

Plasma bradykinin level is related to angiotensin-converting enzyme inhibition and gene polymorphism in hypertensive patients

published_or_final_versio

Drug utlisation in a hypertension outpatient clinic 1996-1999

published_or_final_versio

Zeolite-Catalyzed Isobutene Amination: Mechanism and Kinetics

Amination of isobutene with NH was investigated over Brønsted acidic zeolites at 1 atm and 453-483 K. To compare catalytic activities over different zeolites, the measured reaction rates are normalized by the number of active sites determined by tert-butylamine temperature-programmed desorption (TPD). Small- A nd medium-pore zeolites with one-dimensional channels exhibit low activity because of pore blockage by adsorbed tert-butylammonium ions. However, turnover frequencies and activation energies are not sensitive to framework identity, as long as the active site is accessible to isobutene and tert-butylamine. Kinetic measurements and FTIR spectroscopy reveal that the Brønsted acid sites in MFI are covered predominantly with tert-butylammonium ions under reaction conditions. The desorption of tert-butylamine is assisted by the concurrent adsorption of isobutene. DFT simulations show that at very low tert-butylamine partial pressures, for example, at the inlet to the reactor, tert-butylamine desorption is rate-limiting. However, at sufficiently high tert-butylamine partial pressures (>0.03 kPa), protonation of isobutene to the corresponding carbenium ion limits the rate of amination.

Calcium channel blockers are overused and thiazides are underused in a hypertension outpatient clinic

published_or_final_versio

Reducing sodium intake reduces ambulatory blood pressure in hypertensive patients

published_or_final_versio

Non-Pharmacological treatment of hypertension

published_or_final_versio

Randomised controlled trial of low salt diet in the treatment of hypertension

published_or_final_versio

The prognosis of allocentric and egocentric neglect : evidence from clinical scans

We contrasted the neuroanatomical substrates of sub-acute and chronic visuospatial deficits associated with different aspects of unilateral neglect using computed tomography scans acquired as part of routine clinical diagnosis. Voxel-wise statistical analyses were conducted on a group of 160 stroke patients scanned at a sub-acute stage. Lesion-deficit relationships were assessed across the whole brain, separately for grey and white matter. We assessed lesions that were associated with behavioural performance (i) at a sub-acute stage (within 3 months of the stroke) and (ii) at a chronic stage (after 9 months post stroke). Allocentric and egocentric neglect symptoms at the sub-acute stage were associated with lesions to dissociated regions within the frontal lobe, amongst other regions. However the frontal lesions were not associated with neglect at the chronic stage. On the other hand, lesions in the angular gyrus were associated with persistent allocentric neglect. In contrast, lesions within the superior temporal gyrus extending into the supramarginal gyrus, as well as lesions within the basal ganglia and insula, were associated with persistent egocentric neglect. Damage within the temporo-parietal junction was associated with both types of neglect at the sub-acute stage and 9 months later. Furthermore, white matter disconnections resulting from damage along the superior longitudinal fasciculus were associated with both types of neglect and critically related to both sub-acute and chronic deficits. Finally, there was a significant difference in the lesion volume between patients who recovered from neglect and patients with chronic deficits. The findings presented provide evidence that (i) the lesion location and lesion size can be used to successfully predict the outcome of neglect based on clinical CT scans, (ii) lesion location alone can serve as a critical predictor for persistent neglect symptoms, (iii) wide spread lesions are associated with neglect symptoms at the sub-acute stage but only some of these are critical for predicting whether neglect will become a chronic disorder and (iv) the severity of behavioural symptoms can be a useful predictor of recovery in the absence of neuroimaging findings on clinical scans. We discuss the implications for understanding the symptoms of the neglect syndrome, the recovery of function and the use of clinical scans to predict outcome