Richard J. Darton

Comparing quantum-chemical calculation methods for structural investigation of zeolite crystal structures by solid-state NMR spectroscopy†

Combining quantum‐chemical calculations and ultrahigh‐field NMR measurements of 29Si chemical shielding (CS) tensors has provided a powerful approach for probing the fine details of zeolite crystal structures. In previous work, the quantum‐chemical calculations have been performed on ‘molecular fragments’ extracted from the zeolite crystal structure using Hartree–Fock methods (as implemented in Gaussian). Using recently acquired ultrahigh‐field 29Si NMR data for the pure silica zeolite ITQ‐4, we report the results of calculations using recently developed quantum‐chemical calculation methods for periodic crystalline solids (as implemented in CAmbridge Serial Total Energy Package (CASTEP) and compare these calculations to those calculated with Gaussian. Furthermore, in the context of NMR crystallography of zeolites, we report the completion of the NMR crystallography of the zeolite ITQ‐4, which was previously solved from NMR data. We compare three options for the ‘refinement’ of zeolite crystal structures from ‘NMR‐solved’ structures: (i) a simple target‐distance based geometry optimization, (ii) refinement of atomic coordinates in which the differences between experimental and calculated 29Si CS tensors are minimized, and (iii) refinement of atomic coordinates to minimize the total energy of the lattice using CASTEP quantum‐chemical calculations. All three refinement approaches give structures that are in remarkably good agreement with the single‐crystal X‐ray diffraction structure of ITQ‐4. Copyright

A nickel doped perovskite catalyst for reforming methane rich biogas with minimal carbon deposition

A novel nickel-doped strontium zirconate perovskite catalyst for biogas reforming has been synthesised using a green, low temperature hydrothermal synthesis. The catalyst has been shown to be very efficient towards the conversion of methane-rich biogas at relatively low temperatures with high selectivity towards synthesis gas formation and extremely good resistance to carbon deposition in carbon-rich reaction mixtures. The catalyst displays very low carbon deposition which does not increase over time, and as a result shows excellent stability. The use of a catalyst produced by a low temperature hydrothermal route provides a potentially very attractive and sustainable source of useful chemicals from biogas that otherwise might be vented wastefully and detrimentally into the atmosphere.

Colossal thermal expansion and negative thermal expansion in simple halogen bonded complexes

The crystal structures of the simple monoclinic halogen-bonded complexes pyridine–ICl and pyridine–IBr have been redetermined using modern X-ray and neutron diffraction techniques. The representation quadric surfaces for the thermal expansion tensor have been found to consist of a hyperboloid of one sheet. Negative thermal expansion occurs parallel to the crystallographic b axis, whilst there is colossal thermal expansion in a direction approximately parallel to the a* direction. These effects arise because of a decrease in the strength of an already weak C–H⋯X (X = Br, Cl) hydrogen bond with increasing temperature. The decrease in strength of the hydrogen bonding originates because of the reduction in the strength of the halogen bond formed between the nitrogen atom and the IX moiety with increasing temperature. Thus since at 298 K the I–X bonds are shorter than at 110 K, there will be a smaller partial negative charge on the X halogen at 298 K, leading to a weaker C–H⋯X hydrogen bond.

Overcoming carbon deactivation in biogas reforming using a hydrothermally synthesised nickel perovskite catalyst

A hydrothermally synthesised nickel-strontium zirconate perovskite is shown to have excellent selectivity towards biogas reforming without suffering from deactivation due to carbon formation. Experiments reveal that this material is capable of very efficiently converting methane and carbon dioxide to synthesis gas, a mixture of hydrogen and carbon monoxide, at relatively low temperatures and, particularly importantly, high methane contents. Under these conditions we find that carbon production is extremely low and more importantly shows no increase over time, even after 10 days of continuous reforming activity. This conversion of a renewable product, using a catalyst prepared by low temperature hydrothermal methods, provides a route to future sustainable hydrogen, and oxygenate and higher hydrocarbon, production whilst lowering some greenhouse gas emissions.

Effect of the hybrid composition on the physicochemical properties and morphology of iron oxide–gold nanoparticles

Hybrid nanoparticles (HNPs) formed from iron oxide cores and gold nano-shells are becoming increasingly applicable in biomedicine. However, little investigation has been carried out on the effects of the constituent components on their physical characteristics. Here we determine the effect of polymer intermediate, gold nano-shell thickness and magnetic iron oxide core diameter on the morphological and physical properties of these nano-hybrids. Our findings suggest that the use of polymer intermediate directly impacts the morphology of the nanostructure formed. Here, we observed the formation of nano-sphere and nano-star structures by varying the cationic polymer intermediate. The nano-stars formed have a larger magnetic coercivity, T2 relaxivity and exhibited a unique characteristic nano-heating pattern upon laser irradiation. Increasing the iron oxide core diameter resulted in a greater T2 relaxivity enhanced and nano-heating capabilities due to increased surface area. Increasing the gold nano-shell thickness resulted in a decreased efficiency as a nano-heater along with a decrease in T2 relaxivity. These results highlight the importance of identifying the key traits required when fabricating HNPs in order to tailor them to specific applications.

Variable temperature high resolution 29Si MAS NMR of siliceous zeolite ferrierite

Variable temperature high resolution magic angle spinning 29Si NMR spectroscopy is used to study the phase transition between the low and high temperature versions of pure silica ferrierite. Spectra collected at 133–400 K indicate that the low temperature phase is probably of lower symmetry than that previously reported using single crystal X-ray diffraction results. The NMR spectra around the phase transition are consistent with the description of the high temperature phase as a dynamic average of possible low temperature structures. A comparison of the NMR chemical shifts with the results of previous X-ray diffraction experiments confirms the validity of proposed correlations and indicates how they can be of predictive value in certain cases.

Structural organization in the trimethylamine iodine monochloride complex

The structure of the complex formed between trimethylamine and iodine monochloride had been redetermined by a combination of neutron powder diffraction and X-ray single crystal diffraction techniques. The structure determined using NPD shows that the geometry of the trimethylamine moiety resembles that in trimethylammonium cations rather than in the free trimethylamine molecule. The structure shows the presence of weak C–D⋯Cl hydrogen bonds, The I–Cl bond length is in agreement with that derived from an analysis of the pKb values of a series of nitrogen bases and previously determined structures containing N⋯ICl fragments.

Microcrystal x-ray diffraction and NMR studies of thermal expansion properties of pure silica zeolites fer and a comparison with IFR

Abstract The mechanism of negative thermal expansion in a purely siliceous form of zeolite FER is compared with that of zeolite IFR. At first glance the materials seem to show vastly differeing behaviour—FER shows positive thermal expansion up to a phase transition (∼400K) above which it is a negative thermal expansion, IFR is a negative thermal expansion material at all temperatures so far measured. A comparison of the two mechanisms is made and a seires of ’rules‘ about the magnitude of negative thermal expansion is discussed.

Measuring the silion fluoride bond distance in zeolites

Abstract A common misconception is that X-ray diffraction is the best way to measure bond distances. However, in some cases where disorder is present it can yield incorrect answers. The silicon-fluoride bond distance in fluoride-containing zeolite SFF has been measured using two magic angle spinning NMR techniques. Both techniques, variable contact time cross polarization and spinning side band intensity fitting give shorter Si-F bond distances than X-ray diffraction.