Eelco Titus Carel Vogt

Catalytic activity in individual cracking catalyst particles imaged throughout different life stages by selective staining

Fluid catalytic cracking (FCC) is the major conversion process used in oil refineries to produce valuable hydrocarbons from crude oil fractions. Because the demand for oil-based products is ever increasing, research has been ongoing to improve the performance of FCC catalyst particles, which are complex mixtures of zeolite and binder materials. Unfortunately, there is limited insight into the distribution and activity of individual zeolitic domains at different life stages. Here we introduce a staining method to visualize the structure of zeolite particulates and other FCC components. Brønsted acidity maps have been constructed at the single particle level from fluorescence microscopy images. By applying a statistical methodology to a series of catalysts deactivated via industrial protocols, a correlation is established between Brønsted acidity and cracking activity. The generally applicable method has clear potential for catalyst diagnostics, as it determines intra- and interparticle Brønsted acidity distributions for industrial FCC materials.

Preparation and performance of a silica-supported V2O5 on TiO2 catalyst for the selective reduction of NO with NH3

A preparation technique is described in which a layer of TiO2 completely covering the support can be deposited onto silica. Onto the support thus modified, V2O5 can be applied to result in a catalyst suitable for the selective catalytic reduction (SCR) of NOx with NH3. To obtain the required selectivity in the reduction of NOx the silica surface must be completely covered with TiO2. Catalysts prepared according to the above procedure exhibit good activity and a completely selective reduction of NO to N2 at temperatures to 350 °C. At higher temperatures selective oxidation of the NH3 to N2 is observed.

The reversible transition of the molecular sieve VPI-5 into ALPO−84 and the structure of ALPO−84

In this letter the authors describe the reversibility of the recrystallization of the aluminophosphate molecular sieve VPI-5 to ALPO{sub 4}{sup {minus}8}, and the structure of ALPO{sub 4}{sup {minus}8}. VPI-5 is known to contain one-dimensional circular pores circumscribed by 18 tetrahedral atoms. Upon heating in the presence of moisture it is known to recrystallize to ALPO{sub 4}{sup {minus}8}. This transition now proves to be reversible. Although phase transitions of aluminophosphate molecular sieves have been described, this is the first report of such a transition being reversible. ALPO{sub 4}{sup {minus}8} is found to contain one-dimensional elliptical pores, with free diameters of 7.5 to 9.5 {angstrom} circumscribed by 14 tetrahedral atoms. This new pore system thus contains one of the largest openings described so far, second only to VPI-5.

Structure determination and rietveld refinement of aluminophosphate molecular sieve AIPO4-8

The 18-ring pore molecular sieve VPI-5 recrystallizes to the 14-ring sieve AIPO4-8. This recrystallization appears to be reversible under proper conditions, as could be explained on the basis of the structural relationship between the two materials. VPI-5 has an 18-ring channel bounded by six 6-rings and six pairs of adjoining 4-rings. In the conversion to AIPO4-8, two-thirds of the adjoining 4-rings are converted to 6-rings and reconnected to form a smaller, 14-ring channel bounded by eight 6-rings, two pairs of adjoining 4-rings, and two single 4-rings. The space group for AIPO4-8 [Cmc21: a = 33.0894(25), b = 14.6832(13), c = 8.3630(6) A at room temperature] is a direct subgroup of that for VPI-5 [P63cm: a = 18.549(1), c = 8.404(1) A for the dehydrated form; orthohexagonal setting: a = 32.128, b = 18.549, c = 8.404 A]. Interconversion of the two structures may be facilitated by extracoordination of framework aluminum atoms with water molecules. The observed small adsorption capacity of AIPO4-8 can be explained by stacking faults, where, within a single channel, different pairs of adjoining 4-rings are left unbroken.

Solid-state n.m.r. study of the transformation of VPI-5/MCM-9 into AIPO4-8/SAPO-8

The removal of water at 100 °C (or under specific conditions) from VPI-5/MCM-9 materials results in significant structural changes. The VPI-5/MCM-9 products transform into AIPO 4 -8/SAPO-8. This transformation is partially reversible and strongly depends on the evacuation and rehydration treatments, the heating rate, and probably on lattice defect concentrations. From the knowledge of the VPI-5 structure and of the structural model recently proposed for AIPO 4 -8, we have reassigned all the 31 P n.m.r. lines characterizing both materials and explained further the effect of water on the dramatic changes occurring in their 31 P and 27 Al n.m.r. spectra. 31 P and 27 Al n.m.r. have been used to investigate systematically the transformation of VPI-5 into AIPO 4 -8 and to determine which specific T sites undergo modifications. A mechanism accounting for the transformation is proposed together with a model allowing the determination of the degree of VPI-5/AIPO 4 -8 conversion by computing the relative intensities of five specific 31 P n.m.r. lines that characterize the transition intermediate structures. 31 P n.m.r. proved more sensitive than did conventional X-ray diffraction to detect short-range structural changes in these open structures.

Molybdena on silica catalysts: selective catalytic oxidation of ammonia to nitrogen over MoO3 on siO2 catalysts

Abstract MoO3 on SiO2 catalysts are suitable for the selective oxidation of ammonia into nitrogen (N2) and water. The catalysts were prepared by homogeneous deposition precipitation using electrochemically reduced precursors. The experiments were carried out at atmospheric pressure and at temperatures ranging from 200 to 400 °C. Nitrogen and water yields of up to 100% at temperatures over 375 °C were obtained when ammonia and excess oxygen in helium were passed over a MoO3 on SiO2 catalyst. Catalysts containing less than 15 wt.% MoO3 gave rise to no conversion of ammonia. The presence of small amounts of lead in the catalyst appeared to influence the conversion of NH3 in a positive way. However, the presence of nitric oxide in the feed did not influence either the conversion of ammonia or the selectivity to nitrogen. The nitric oxide concentration remained constant during the conversion of ammonia. The results indicate that species reducible by hydrogen at 540 °C are related to the activity of the catalysts.

A consistent molecular mechanics force field for aluminophosphates

A consistent molecular mechanics force field has been developed for aluminophosphate structures including berlinite and several molecular sieve structures, which predicts the geometry, heat of formation and the vibrational frequencies reasonably well. The force field consists of a bonding term, a bending term and long-range interactions. Charges are calculated using the electronegativity equalization method of Mortier et al.(W. J. Mortier, S. K. Ghoshand and S. Shankar, J. Am. Chem. Soc., 1986, 108, 4315), modified with the relative permittivity. The parameters for oxygen are transferable with the all-silica force field recently developed. Results are given for berlinite, AlPO-5 (AFI), AlPO-8 (AET), AlPO-11 (AEL), AlPO-18 (AEI), AlPO-25 (ATV), AlPO-35 (LEV), AlPO-37 (FAU) and VPI-5 (VFI), as well as for the hydrated structures of variscite and VPI-5 (six-fold coordination around aluminium) and the structures of AlPO-18 and AlPO-21 (ATO) with extra-framework oxygen (five-fold coordinated aluminium). Atomic positions of the AlPO-35 structure (both DLS and energy minimized), which have not been reported before, are also given.

Semi-empirical atomic charges for use in computational chemistry of molecular sieves

Abstract Building on existing methods for the estimation of partial atomic charges by the use of semi-empirical density functional theory, we propose a method that is both reliable and computationally efficient when applied to solids. The modifications include the use of the s–p promotion energy for those elements where the range of charges found exceeds the boundaries of one particular valence subshell. In the case of Al, this is shown to work quite well.

Staining of Fluid‐Catalytic‐Cracking Catalysts: Localising Brønsted Acidity within a Single Catalyst Particle

A time-resolved in situ micro-spectroscopic approach has been used to investigate the Brønsted acidic properties of fluid-catalytic-cracking (FCC) catalysts at the single particle level by applying the acid-catalysed styrene oligomerisation probe reaction. The reactivity of individual FCC components (zeolite, clay, alumina and silica) was monitored by UV/Vis micro-spectroscopy and showed that only clay and zeolites (Y and ZSM-5) contain Brønsted acid sites that are strong enough to catalyse the conversion of 4-fluorostyrene into carbocationic species. By applying the same approach to complete FCC catalyst particles, it has been found that the fingerprint of the zeolitic UV/Vis spectra is clearly recognisable. This almost exclusive zeolitic activity is confirmed by the fact that hardly any reactivity is observed for FCC particles that contain no zeolite. Confocal fluorescence microscopy images of FCC catalyst particles reveal inhomogeneously distributed micron-sized zeolite domains with a highly fluorescent signal upon reaction. By examining laboratory deactivated FCC catalyst particles in a statistical approach, a clear trend of decreasing fluorescence intensity, and thus Brønsted acidity, of the zeolite domains is observed with increasing severity of the deactivation method. By comparing the average fluorescence intensities obtained with two styrenes that differ in reactivity, it has been found that the Brønsted acid site strength within FCC catalyst particles containing ZSM-5 is more uniform than within those containing zeolite Y, as confirmed with temperature-programmed desorption of ammonia.

Preparation of highly loaded vanadium oxide―silica catalysts

Abstract A technique for the application of vanadium oxide onto silica supports starting from trivalent vanadium precursors is described. At loadings up to 30 wt.%, transmission electron microscopy and X-ray diffraction show the presence of highly dispersed vanadium oxide particles in calcined samples, which is very unusual for silica-supported vanadium oxide. Combined with recorded precipitation curves and temperature-programmed reduction profiles, these results indicate a good interaction between the silica support and the vanadium oxide precursor. The kinetic parameters of the catalytic oxidation of carbon monoxide are significantly different from those of both bulk V 2 O 5 and impregnated silica-supported vanadium oxide on the one hand and from TiO 2 - and Al 2 O 3 -supported vanadium oxide on the other. X-ray photoelectron spectroscopic and IR data indicate that a considerable concentration of vanadium (IV) remains after calcination. Selectivity profiles of this catalyst in the selective catalytic reduction of nitric oxide with ammonia proved to be significantly different from those of both TiO 2 - and Al 2 O 3 -supported vanadium oxide and bulk V 2 O 5 . The typical selectivity profiles, the deviating activity in the carbon monoxide oxidation and the spectroscopic results for these catalysts all indicate a strong interaction between the active vanadium oxide and the silica support.