Does Pelletizing Catalysts Influence the Efficiency Number of Activity Measurements? Spectrochemical Engineering Considerations for an Accurate Operando Study

Porosity is a factor affecting catalyst efficiency in pelletized form. This implies that care should be taken with uncritically relating activity measurements from transmission operando FTIR to final catalyst performance. If the pelletizing pressure is excessive, a destruction of the pore structure of, for example, support oxides might take place, which in turn affects the pore size distribution and the porosity of the catalyst, leading to the observation of lower activity values due to decreased catalyst efficiency. This phenomenon can also apply to conventional activity measurements, in the cases that pelletizing and recrushing of samples are performed to obtain adequate particle size fractions for the catalytic bed. A case study of an operando investigation of a V₂O₅-WO₃/TiO₂-sepiolite catalyst is used as an example, and simple calculations of the influence of catalyst activity and internal pore diffusion properties are considered in this paper for the evaluation of catalyst performance in, for example, operando reactors. Thus, it is demonstrated that with a pelletizing pressure of <1–2 ton/cm², the pore structure is only negligibly altered, and small deteriorations of estimated catalyst efficiencies are observed for first-order kinetic constants lower than 100 mL/gs. However, if the operando study deals with highly active catalysts, it is necessary to consider efficiency losses. A simple procedure for evaluating efficiencies based on pellet dimensions and solid phase characteristics is proposed. The Thiele modulus is directly proportional to the thickness of the pellet, and, thus, inversely related to the catalyst efficiency. As a rule of thumb, we found that for catalytic constants below 100 mL/gs, the maximum thickness of the pellet pressed at 2 tons/cm² has to be as low as 80 μm to exhibit catalyst efficiencies above 90%. For catalysts with <i>k</i>′ = 10 mL/gs, the value is 260 μm. This strongly underlines the importance of taking internal diffusion limitations into account when working with highly active catalysts

Zeolite MCM-22 Modified with Au and Cu for Catalytic Total Oxidation of Methanol and Carbon Monoxide

The goal of this work was to use MCM-22 zeolites for preparation of monometallic (Cu or Au) and bimetallic (Cu and Au) catalysts for oxidation reactions. The focus was on precise determination of the nature of gold and copper species and their activity in the oxidation processes. For that purpose several characterization techniques were applied (XRD, N₂ adsorption/desorption, TEM, SEM, UV–vis, H₂-TPR, ²⁷Al MAS NMR, FT-IR with the adsorption of pyridine, NO, and CO, ESR spectroscopy). They allowed us to define the following species formed on MCM-22 surface: metallic gold particles (XRD, UV–vis), isolated Cu²⁺ with octahedral coordination (UV–vis, ESR), square planar Cu²⁺ cations (ESR, IR), Cu⁺ species (ESR+NO, FTIR+CO, and FTIR+NO), and oligonuclear clusters (UV–vis) as well as CuO-like species (H₂-TPR). The presence of gold on the MCM-22 surface modified further by copper species caused the interaction between two modifiers leading to much easier reduction of CuO-like species and higher mobility of oxygen-promoting oxidative properties. The bimetallic catalyst was highly active in total oxidation of methanol and CO in the temperature range 523–623 K. Cu/Au-MCM-22 zeolite appeared useful for simultaneous removal of CO and methanol (by total oxidation) from gases emitted from automotive devices and during a variety of industrial process operations

Infrared Spectroscopy Investigation of the Acid Sites in the Metal–Organic Framework Aluminum Trimesate MIL-100(Al)

Infrared spectra of MIL-100­(Al) have been recorded after evacuation from room temperature up to 623 K. In addition to adsorbed water molecules characterized by specific (ν+δ)­H₂O combination bands at about 5300 cm^–1, spectra analysis shows the presence of impurities like carboxylic acid and nitrates resulting from the synthesis step, explaining the low amount of Al–OH groups detected. The Lewis acidity has been characterized by CO [ν­(CO) at 2183 cm^–1], pyridine [ν8a band estimated at 1618 cm^–1], and CD₃CN [ν­(CN) at 2326 cm^–1] adsorption on the activated sample. The acidity is strong as revealed by the ν­(CN) wavenumber. Interestingly, CO gives rise to an interaction weaker than that expected from pyridine and CD₃CN results. Quantitative results relative to the number of Al³⁺_5c sites are in full agreement with those reported elsewhere from ²⁷Al NMR experiments. The Brønsted acidity mainly results from the presence of coordinated water species in the nonfully dehydrated sample and not from the structural Al–OH groups

Sr₂₁Bi₈Cu₂(CO₃)₂O₄₁, a Bi⁵⁺ Oxycarbonate with an Original 10L Structure

The layered structure of Sr₂₁Bi₈Cu₂(CO₃)₂O₄₁ (<i>Z</i> = 2) was determined by transmission electron microscopy, infrared spectroscopy, and powder X-ray diffraction refinement in space group <i>P</i>6₃/<i>mcm</i> (No. 194), with <i>a</i> = 10.0966(3)­Å and <i>c</i> = 26.3762(5)­Å. This original 10L-type structure is built from two structural blocks, namely, [Sr₁₅Bi₆Cu₂(CO₃)­O₂₉] and [Sr₆Bi₂(CO₃)­O₁₂]. The Bi⁵⁺ cations form [Bi₂O₁₀] dimers, whereas the Cu²⁺ and C atoms occupy infinite tunnels running along <i>c⃗</i>. The nature of the different blocks and layers is discussed with regard to the existing hexagonal layered compounds. Sr₂₁Bi₈Cu₂(CO₃)₂O₄₁ is insulating and paramagnetic

Dynamics of CrO₃–Fe₂O₃ Catalysts during the High-Temperature Water-Gas Shift Reaction: Molecular Structures and Reactivity

A series of supported CrO₃/Fe₂O₃ catalysts were investigated for the high-temperature water-gas shift (WGS) and reverse-WGS reactions and extensively characterized using in situ and operando IR, Raman, and XAS spectroscopy during the high-temperature WGS/RWGS reactions. The in situ spectroscopy examinations reveal that the initial oxidized catalysts contain surface dioxo (O)₂Cr⁶⁺O₂ species and a bulk Fe₂O₃ phase containing some Cr³⁺ substituted into the iron oxide bulk lattice. Operando spectroscopy studies during the high-temperature WGS/RWGS reactions show that the catalyst transforms during the reaction. The crystalline Fe₂O₃ bulk phase becomes Fe₃O₄ ,and surface dioxo (O)₂Cr⁶⁺O₂ species are reduced and mostly dissolve into the iron oxide bulk lattice. Consequently, the chromium–iron oxide catalyst surface is dominated by FeO_<i>x</i> sites, but some minor reduced surface chromia sites are also retained. The Fe_{3–<i>‑</i>x}Cr_<i>x</i>O₄ solid solution stabilizes the iron oxide phase from reducing to metallic Fe⁰ and imparts an enhanced surface area to the catalyst. Isotopic exchange studies with C¹⁶O₂/H₂ → C¹⁸O₂/H₂ isotopic switch directly show that the RWGS reaction proceeds via the redox mechanism and only O* sites from the surface region of the chromium–iron oxide catalysts are involved in the RWGS reaction. The number of redox O* sites was quantitatively determined with the isotope exchange measurements under appropriate WGS conditions and demonstrated that previous methods have undercounted the number of sites by nearly 1 order of magnitude. The TOF values suggest that only the redox O* sites affiliated with iron oxide are catalytic active sites for WGS/RWGS, though a carbonate oxygen exchange mechanism was demonstrated to exist, and that chromia is only a textural promoter that increases the number of catalytic active sites without any chemical promotion effect

Comparison of Porous Iron Trimesates Basolite F300 and MIL-100(Fe) As Heterogeneous Catalysts for Lewis Acid and Oxidation Reactions: Roles of Structural Defects and Stability

Two porous iron trimesates, namely, commercial Basolite F300 (Fe­(BTC); BTC = 1,3,5-benzenetricarboxylate) with unknown structure and synthetic MIL-100­(Fe) (MIL stands for Material of Institut Lavoisier) of well-defined crystalline structure, have been compared as heterogeneous catalysts for four different reactions. It was found that while for catalytic processes requiring strong Lewis acid sites, Fe­(BTC) performs better, MIL-100­(Fe) is the preferred catalyst for oxidation reactions. These catalytic results have been rationalized by a combined in situ infrared and ⁵⁷Fe Mössbauer spectroscopic characterization. It is proposed that the presence of extra Brønsted acid sites on the Fe­(BTC) and the easier redox behavior of the MIL-100­(Fe) could explain these comparative catalytic performances. The results illustrate the importance of structural defects (presence of weak Brønsted acid sites) and structural stability (MIL-100­(Fe) is stable upon annealing at 280 °C despite Fe³⁺-to-Fe²⁺ reduction) on the catalytic activity of these two solids, depending on the reaction type

Isomorphous Substitution in a Flexible Metal–Organic Framework: Mixed-Metal, Mixed-Valent MIL-53 Type Materials

Mixed-metal iron–vanadium analogues of the 1,4-benzenedicarboxylate (BDC) metal–organic framework MIL-53 have been synthesized solvothermally in <i>N</i>,<i>N</i>′-dimethylformamide (DMF) from metal chlorides using initial Fe:V ratios of 2:1 and 1:1. At 200 °C and short reaction time (1 h), materials (Fe,V)^II/IIIBDC­(DMF_1–<i>x</i>F_<i>x</i>) crystallize directly, whereas the use of longer reaction times (3 days) at 170 °C yields phases of composition [(Fe,V)^III_0.5(Fe,V)_0.5^II(BDC)­(OH,F)]^0.5–·0.5DMA⁺ (DMA = dimethylammonium). The identity of the materials is confirmed using high-resolution powder X-ray diffraction, with refined unit cell parameters compared to known pure iron analogues of the same phases. The oxidation states of iron and vanadium in all samples are verified using X-ray absorption near edge structure (XANES) spectroscopy at the metal K-edges. This shows that in the two sets of materials each of the vanadium and the iron centers are present in both +2 and +3 oxidation states. The local environment and oxidation state of iron is confirmed by ⁵⁷Fe Mössbauer spectrometry. Infrared and Raman spectroscopies as a function of temperature allowed the conditions for removal of extra-framework species to be identified, and the evolution of μ₂-hydroxyls to be monitored. Thus calcination of the mixed-valent, mixed-metal phases [(Fe,V)^III_0.5(Fe,V)_0.5^II(BDC)­(OH,F)]^0.5–·0.5DMA⁺ yields single-phase MIL-53-type materials, (Fe,V)^III(BDC)­(OH,F). The iron-rich, mixed-metal MIL-53 shows structural flexibility that is distinct from either the pure Fe material or the pure V material, with a thermally induced pore opening upon heating that is reversible upon cooling. In contrast, the material with a Fe:V content of 1:1 shows an irreversible expansion upon heating, akin to the pure vanadium analogue, suggesting the presence of some domains of vanadium-rich regions that can be permanently oxidized to V­(IV)

Evaluation of MIL-47(V) for CO₂‑Related Applications

Carbon dioxide and methane adsorption has been carried out up to 50 bar on the MIL-47­(V) metal–organic framework (MOF) at 303 K. The so-obtained performance has been compared with other well-known MOFs, an activated carbon, and the zeolite NaY both in amount and volume adsorbed scales. In the latter scale, which would be of interest for real applications, the MIL-47­(V) shows promising results similar to those of Cu₃­(BTC)₂ or HKUST-1. Operando Infrared experiments have been employed to characterize the strength of the interactions in play. Finally, the CO₂/CH₄ separation properties have been further predicted using a combination of macroscopic and microscopic modeling approaches. This body of results suggests that this material should be considered for gas separation

Synthesis Modulation as a Tool To Increase the Catalytic Activity of Metal–Organic Frameworks: The Unique Case of UiO-66(Zr)

The catalytic activity of the zirconium terephthalate UiO-66­(Zr) can be drastically increased by using a modulation approach. The combined use of trifluoroacetic acid and HCl during the synthesis results in a highly crystalline material, with partial substitution of terephthalates by trifluoroacetate. Thermal activation of the material leads not only to dehydroxylation of the hexanuclear Zr cluster but also to post-synthetic removal of the trifluoroacetate groups, resulting in a more open framework with a large number of open sites. Consequently, the material is a highly active catalyst for several Lewis acid catalyzed reactions

N/S-Heterocyclic Contaminant Removal from Fuels by the Mesoporous Metal–Organic Framework MIL-100: The Role of the Metal Ion

The influence of the metal ion in the mesoporous metal trimesate MIL-100­(Al³⁺, Cr³⁺, Fe³⁺, V³⁺) on the adsorptive removal of N/S-heterocyclic molecules from fuels has been investigated by combining isotherms for adsorption from a model fuel solution with microcalorimetric and IR spectroscopic characterizations. The results show a clear influence of the different metals (Al, Fe, Cr, V) on the affinity for the heterocyclic compounds, on the integral adsorption enthalpies, and on the uptake capacities. Among several factors, the availability of coordinatively unsaturated sites and the presence of basic sites next to the coordinative vacancies are important factors contributing to the observed affinity differences for N-heterocyclic compounds. These trends were deduced from IR spectroscopic observation of adsorbed indole molecules, which can be chemisorbed coordinatively or by formation of hydrogen bonded species. On the basis of our results we are able to propose an optimized adsorbent for the deep and selective removal of nitrogen contaminants out of fuel feeds, namely MIL-100­(V)