Practical Approaches towards NOx Emission Mitigation from Fluid Catalytic Cracking (FCC) Units

There appears to be consensus among the general public that curtailing harmful emissions resulting from industrial, petrochemical and transportation sectors is a common good. However, there is also a need for balancing operating expenditures for applying the required technical solutions and implementing advanced emission mitigation technologies to meet desired sustainability goals. The emission of NOx from Fluid Catalytic Cracking (FCC) units in refineries for petroleum processing is a major concern, especially for those units located in densely populated urban settings. In this work we strive to review options towards cost-efficient and pragmatic emissions mitigation using optimal amounts of precious metal while evaluating the potential benefits of typical promoter dopant packages. We demonstrate that at present catalyst development level the refinery is no longer forced to make a promoter selection based on preconceived notions regarding precious metal activity but can rather make decisions based on the best “total cost” financial impact to the operation without measurable loss of the CO/NOx emission selectivity

Enhancing the catalytic activity of hydronium ions through constrained environments

The dehydration of alcohols is involved in many organic conversions but has to overcome high free-energy barriers in water. Here we demonstrate that hydronium ions confined in the nanopores of zeolite HBEA catalyse aqueous phase dehydration of cyclohexanol at a rate significantly higher than hydronium ions in water. This rate enhancement is not related to a shift in mechanism; for both cases, the dehydration of cyclohexanol occurs via an E1 mechanism with the cleavage of Cβ–H bond being rate determining. The higher activity of hydronium ions in zeolites is caused by the enhanced association between the hydronium ion and the alcohol, as well as a higher intrinsic rate constant in the constrained environments compared with water. The higher rate constant is caused by a greater entropy of activation rather than a lower enthalpy of activation. These insights should allow us to understand and predict similar processes in confined spaces

Enhancing the catalytic activity of hydronium ions through constrained environments

The dehydration of alcohols is involved in many organic conversions but has to overcome high free-energy barriers in water. Here we demonstrate that hydronium ions confined in the nanopores of zeolite HBEA catalyse aqueous phase dehydration of cyclohexanol at a rate significantly higher than hydronium ions in water. This rate enhancement is not related to a shift in mechanism; for both cases, the dehydration of cyclohexanol occurs via an E1 mechanism with the cleavage of Cβ–H bond being rate determining. The higher activity of hydronium ions in zeolites is caused by the enhanced association between the hydronium ion and the alcohol, as well as a higher intrinsic rate constant in the constrained environments compared with water. The higher rate constant is caused by a greater entropy of activation rather than a lower enthalpy of activation. These insights should allow us to understand and predict similar processes in confined spaces

Al-27 MAS NMR Studies of HBEA Zeolite at Low to High Magnetic Fields

Al-27 single pulse (SP) MAS NMR :spectra,of RUA zeolites With high. Si/AL ratios of 71 and were obtained at three magnetic field strengths of 7.05, 11.75, and 19.97 T. High field Al-27 MAS NMR spectra acquired at 19.97 T show significantly improved spectral resolution, resulting in at least two well-resolved tetrahedral-Al NMR peaks. Based on, the results obtained from Al-27 MAS and MQMAS NMR, acquired at 19,97 T,four different, quadrupole peaks are used to deconvolute the Al-27 SP MAS spectra acquired at various fields by using the, same set of quadrupole coupling constants, asymmetric parameters and relative integrated peak intensities for the tetrahedral Al peaks. The line shapes of individual peaks change from typical quadrupole line shape at low field to essentially symmetrical line shapes at high field. We demonstrate,thatt for fully, hydrated HBEA zeolites, the effect of second order quadrupole interaction can be ignored, and quantitative spectral analysis can be, erformed by directly fitting the high field, spectra using mixed Gaussian/Lorentzian line shapes. Also, the analytical steps described in our work,allOw direct assignment of spectral intensity to individual Al tetrahedral sites (T-Sites) of zeolite HBEA. Finally, the proposed concept is suggested to be generally applicable to other zeolite framework types, thus allowing a direct probing of Al distributioUs by NMR spectroscopic methods in zeolites with high confidence

Impact of Aqueous Medium on Zeolite Framework Integrity

In this work, Al K-edge extended X-ray absorption fine structure and ²⁷Al MAS NMR spectroscopies in combination with DFT calculations are used to determine both qualitative and quantitative structural changes of two well-characterized samples with the BEA structure. The effects of various properties, including Al concentration, Al distribution, particle size, and structural defects, on zeolite stability are explored. As the samples are degraded by treatment in hot liquid water, the local structure about the Al T-site remains mostly intact, including the Al–O–Si angles and bond distances, while the crystalline structure as measured by XRD and STEM is disrupted. The combined data suggests the crystallinity decreases via selective hydrolysis of the T1- and T2-sites that form the 4-member rings of the zeolite framework. The hydrolysis eventually leads to the dissolution of the T-sites followed by reprecipitation on the particle surface resulting in amorphization of the sample

Elementary Steps of Faujasite Formation Followed by in Situ Spectroscopy

Ex situ and in situ spectroscopy was used to identify the kinetics of processes during the formation of the faujasite (FAU) zeolite lattice from a hydrous gel. Using solid-state ²⁷Al magic angle spinning (MAS) nuclear magnetic resonance (NMR), the autocatalytic transformation from the amorphous gel into the crystalline material was monitored. Al X-ray absorption near-edge structure shows that most Al already adopts a tetrahedral coordination in the X-ray-amorphous aluminosilicate at the beginning of the induction period, which hardly changes throughout the rest of the synthesis. Using ²³Na NMR spectroscopy, environments in the growing zeolite crystal were identified and used to define the processes in the stepwise formation of the zeolite lattice. The end of the induction period was accompanied by a narrowing of the ²⁷Al and ²³Na MAS NMR peak widths, indicating the increased level of long-range order. The experiments show conclusively that the formation of faujasite occurs via the continuous formation and subsequent condensation of intermediary sodalite-like units that constitute the key building block of the zeolite

Tracking the Chemical Transformations at the Brønsted Acid Site upon Water-Induced Deprotonation in a Zeolite Pore

The structural changes induced by reversible formation of Brønsted acidic sites and hydronium ions with water in a zeolite with MFI structure are reported as a function of temperature using a combination of physicochemical methods and theory. In the presence of an ample concentration of water, the protons are present as hydrated hydronium ions (H₃O⁺(H₂O)_<i>n</i>) that are ion-paired to the zeolite. Loss of water molecules hydrating the hydronium ions leads to an unstable free hydronium ion that dissociates to form the hydroxylated T-site. The formation of this SiOHAl species leads to the elongation of one of the four Al–O bonds and causes significant distortion of the tetrahedral symmetry about the Al atom. This distortion leads to the appearance of new pre-edge features in the Al K-edge X-ray absorption near edge structure (XANES) spectra. The pre-edge peak assignment is confirmed by time-dependent density functional theory calculation of the XANES spectrum. The XANES spectra are also sensitive to solutes or solvents that are in proximity to the T-site. As temperature increases, the minor fraction of extra-framework Al present in the sample at ambient conditions in octahedral coordination is converted to tetrahedral coordination through the decoordination of H₂O ligands

²⁷Al MAS NMR Studies of HBEA Zeolite at Low to High Magnetic Fields

²⁷Al single pulse (SP) MAS NMR spectra of HBEA zeolites with high Si/Al ratios of 71 and 75 were obtained at three magnetic field strengths of 7.05, 11.75, and 19.97 T. High field ²⁷Al MAS NMR spectra acquired at 19.97 T show significantly improved spectral resolution, resulting in at least two well-resolved tetrahedral-Al NMR peaks. Based on the results obtained from ²⁷Al MAS and MQMAS NMR acquired at 19.97 T, four different quadrupole peaks are used to deconvolute the ²⁷Al SP MAS spectra acquired at various fields by using the same set of quadrupole coupling constants, asymmetric parameters and relative integrated peak intensities for the tetrahedral Al peaks. The line shapes of individual peaks change from typical quadrupole line shape at low field to essentially symmetrical line shapes at high field. We demonstrate that, for fully hydrated HBEA zeolites, the effect of second-order quadrupole interaction can be ignored, and quantitative spectral analysis can be performed by directly fitting the high field spectra using mixed Gaussian/Lorentzian line shapes. Also, the analytical steps described in our work allow direct assignment of spectral intensity to individual Al tetrahedral sites (T-sites) of zeolite HBEA. Finally, the proposed concept is suggested to be generally applicable to other zeolite framework types, thus allowing a direct probing of Al distributions by NMR spectroscopic methods in zeolites with high confidence

Quantitatively Probing the Al Distribution in Zeolites

The degree of substitution of Si⁴⁺ by Al³⁺ in the oxygen-terminated tetrahedra (Al T-sites) of zeolites determines the concentration of ion-exchange and Brønsted acid sites. Because the location of the tetrahedra and the associated subtle variations in bond angles influence the acid strength, quantitative information about Al T-sites in the framework is critical to rationalize catalytic properties and to design new catalysts. A quantitative analysis is reported that uses a combination of extended X-ray absorption fine structure (EXAFS) analysis and ²⁷Al MAS NMR spectroscopy supported by DFT-based molecular dynamics simulations. To discriminate individual Al atoms, sets of ab initio EXAFS spectra for various T-sites are generated from DFT-based molecular dynamics simulations, allowing quantitative treatment of the EXAFS single- and multiple-photoelectron scattering processes out to 3–4 atom shells surrounding the Al absorption center. It is observed that identical zeolite types show dramatically different Al distributions. A preference of Al for T-sites that are part of one or more 4-member rings in the framework over those T-sites that are part of only 5- and 6-member rings in an HBEA150 zeolite has been determined using this analysis