Eddy W. Hansen

MCM-41: a model system for adsorption studies on mesoporous materials

Purely siliceous MCM-41 materials with different pore sizes were synthesised. The obtained nitrogen isotherms of these materials varied from a “type I-like” isotherm (small pore size) over a reversible type IV isotherm (intermediate pore size) to a typical type IV isotherm, which shows pore condensation with hysteresis. Similar types of transition states and hysteresis curves are observed by 1H nuclear magnetic resonance (NMR) by monitoring the proton signal intensity of the mobile water molecules confined in the pores versus temperature. The consistency observed between the two methods makes 1H NMR a valuable additional technique for characterisation of mesoporous materials.

N.m.r. characterization of polyethylene with emphasis on internal consistency of peak intensities and estimation of uncertainties in derived branch distribution numbers

A comprehensive n.m.r. characterization of a low-density polyethylene (LDPE) sample with respect to the distribution of short branches, saturated and unsaturated long branch/chain ends is presented. High precision carbon chemical shift values of all assigned resonance peaks with standard deviations of less than 0.005 ppm (at 403K and 75 MHz carbon resonance frequency) were obtained by mathematical deconvolution of the carbon spectrum, thus enabling a differentiation not only of short branches (Cn < 6) but also hexyl-, oxtyl- and longer branches). Nuclear Overhauser Enhancement (NOE) measurements revealed that less than 50% of the non-equivalent carbon nuclei experienced full NOE of 2.98 while more than 25% of the corresponding carbon nuclei showed a NOE of less than 2.75. Based on spin-lattice relaxation time (T1) and NOE measurements a statistical evaluation showed that internal quantitative consistency between peak intensities existed only under no-NOE conditions. Derived branch distribution numbers from a set of 100 synthetic carbon n.m.r. spectra of the sample, revealed that each branch could be characterized by a Gaussian or normal distribution function. Average values and standard deviations of the branch numbers are presented.

Enhanced n-hexane diffusion in partially filled MCM-41 of different surface hydrophobicity probed by NMR

The effect of pore surface hydrophilic/hydrophobic character on n-hexane diffusion within MCM-41 particles was investigated by PFG NMR at different pore fillings. Starting with totally filled pores, the n-hexane diffusivity within MCM-41 particles is significantly enhanced as the pore filling is decreased until it reaches a maximum value. The diffusivity is shown to decline again at a very low pore filling. This is rationalized in terms of a a three-component parallel diffusion mechanism, in which a surface diffusion, an interface diffusion and a liquid phase diffusion are involved. The influence of surface property variations on the molecular diffusivity in MCM-41 is enhanced in partially filled samples.

Self-diffusion coefficient of water confined in mesoporous MCM-41 materials determined by 1H nuclear magnetic resonance spin-echo measurements

Abstract 1H Nuclear magnetic resonance (NMR) spin-echo measurements at 268 K have been performed on water-saturated mesoporous MCM-41 materials. The pores have cylindrical geometry with diameters in the range 18–40 A and length of approximately 1 mm. Using appropriate model calculations the self-diffusion coefficient (D) of water within the pore was estimated from the Carr-Purcell-Meiboom-Gill (CPMG) spin-echo envelope curves. D was found to be in the range 0.17·10−6 to 2.3·10−6 cm2/s and could be expressed by an equation of the form D−1∝D∞−1+α·lc−2 where D∞ is the self-diffusion coefficient of bulk water (lc = ∞) and α = (2.68 ± 0.14)·10−5s. The low values for the diffusion coefficients—compared to the diffusion coefficient of bulk water — are related to the strong interaction of the water molecules with the pore surface. Moreover, the spin-spin relaxation rate ( 1 T 2 ) was related to lc by application of the Bloembergen-Purcell-Pound (BPP) model.

Improvement in the determination of triad distributions in ethylene-propylene copolymers by 13C nuclear magnetic resonance

Abstract A quantitative 13 C nuclear magnetic resonance analysis of an ethylene—propylene copolymer produced on a modern high-field instrument produces a spectrum with eight discrete groups of peaks. Assignment of these peaks provides eight simultaneous equations to describe the six triad distribution numbers. We have demonstrated the advantages of solving directly this over-determined set of equations. We have observed that the relative uncertainty in the different triad numbers is approximately constant, and have used this observation to discuss the uncertainties in individual triad numbers as a function of their concentration.

Pulsed field gradient NMR study of the diffusion of n-hexane confined in hydroxylated and dehydroxylated MCM-41 of various pore diameters

Abstract The pore dimension of a reference MCM-41 material was reduced in a controlled manner by successive silylation with tetrachlorosilane. Also, the surface of these materials was modified by temperature treatment, resulting in a reduction of the concentration of surface silanol groups. Pulsed field gradient (PFG) NMR diffusion measurements of n -hexane confined in these modified MCM-41 samples have been performed and the results discussed within the Gaussian phase distribution and short gradient pulse approximations. The observed increase of the mean residence time with increasing diffusion time of the confined n -hexane suggests a distribution of channel lengths to exist.

Reaction of poly(vinyl alcohol) and dialdehydes during gel formation probed by 1H n.m.r.—a kinetic study

Abstract Proton n.m.r. has been used to study the reaction between poly(vinyl alcohol) (PVA) and two different dialdehyde crosslinkers during gel formation (gels for improved oil recovery applications). The rate of formation of covalent bonds between PVA and glutaraldehyde in acidic saline water solutions was found to be of first order with respect to both the H 1 concentration and the formal dialdehyde concentration. The activation energy was found to increase with increasing dialdehyde concentration and increasing pH (3.2–4.7). The corresponding reaction rate of formation of covalent bonds between PVA and butenedial in the PVA/2,5-dimethoxy-2,5-dihydrofuran system at the same pH and temperature was found to be an order of magnitude less. Also, the activation energy was higher by approximately 50% as compared with the PVA/glutaraldehyde system. Possible reaction pathways for the generation of PVA-dialdehyde gels are discussed.

Proton spin diffusion in polyethylene as a function of magic-angle spinning rate. A phenomenological approach.

Starting from the phenomenological Bloembergen-Purcell-Pound equation a relation between magic-angle spinning (MAS) rate and spin diffusion is derived. The resulting model equation was fitted to observed spin diffusion versus MAS rate data obtained at 298 K on an high-density polyethylene sample, revealing a reduction in the effective spin diffusivity by (65 + 5)% when increasing the MAS rate from 2 to 12 kHz. The same model equation enabled the rigid-lattice diffusivity to be estimated and was found to be only slightly higher, by approximately 10%, compared to the spin diffusivity observed at the lowest MAS rate applied (2 kHz). Moreover, the model equation predicts a reduction in the effective spin diffusivity by more than 90% when increasing the MAS rate to more than 30 kHz.

Carbon NMR used in probing the exchange of ethanol with water in water-saturated cement pastes

The exchange of water by ethanol in two water-saturated cement pastes has been investigated by carbon NMR. The two cement pastes differed only in their thermal history. The diffusion of ethanol into the cement paste was shown to be described by Fickian diffusion, assuming one-dimensional diffusion under perfect sink boundary conditions. The diffusion coefficients were calculated to be (1.28 +/- 0.14) 10(-7) cm2/s for the virgin cement sample and (4.38 +/- 0.57) 10(-7) m2/s for the preheated cement sample (preheated at 105 degrees C for 12 h), respectively. The measurements indicate an extensive exchange between water and ethanol.

Probing the gelation of polyvinylalcohol-water glutaraldehyde within a porous material by 1H n.m.r. — a preliminary investigation

Abstract Proton n.m.r. relaxation times ( T 1 , T 2 ), chemical shift and line width of the solvent water protons in a polyvinylalcohol (PVA)-glutaraldehyde-water solution confined in a porous material (glass beads) revealed no significant changes during crosslinking and gel formation. Also, the self-diffusion coefficient was constant and identical to the self-diffusion coefficient of bulk water (2 × 10 −5 cm 2 s −1 ) during the reaction. Due to the smaller self-diffusion coefficient of the polymer molecules the solvent water resonance peak could be completely removed from the spectrum by applying a pulse gradient spin-echo technique, leaving only the signal from the polymer amenable for detection. In spite of the broadening effect caused by susceptibility differences between the solid porous matrix and the confined fluid, the PVA peaks were easily resolved. The observed distribution of self-diffusion coefficients of PVA could be approximated by three single diffusion coefficients ranging from 10 −6 to 10 −9 cm 2 s −1 at 25°C. The slower diffusion coefficient was found to decrease by almost an order of magnitude during the reaction with a rate of change of approximately 3 × 10 −5 s −1 at 80°C.