Heats of Adsorption of Ammonia and Correlation of Activity and Acidity in Heterogeneous Catalysis

Solid acid materials with tunable structural and acidic properties are promising heterogeneous catalysts for manipulating and/or emulating the activity and selectivity of industrially important catalytic reactions. On the other hand, the performances of acid-catalyzed reactions are mostly dictated by the acidic features, namely, type (Brønsted vs Lewis acidity), amount, strength, and local environment of acid sites. The latter is relevant to their location (intra- vs extracrystalline), and possible confinement and Brønsted-Lewis acid synergy effects that may strongly affect the host-guest interactions, reaction mechanism, and shape selectivity of the catalytic system. This account aims to highlight some important applications of state-of-the-art solid-state NMR (SSNMR) techniques for exploring the structural and acidic properties of solid acid catalysts as well as their catalytic performances and relevant reaction pathway invoked. In addition, density functional theory (DFT) calculations may be exploited in conjunction with experimental SSNMR studies to verify the structure-activity correlations of the catalytic system at a microscopic scale. We describe in this Account the developments and applications of advanced ex situ and/or in situ SSNMR techniques, such as two-dimensional (2D) double-quantum magic-angle spinning (DQ MAS) homonuclear correlation spectroscopy for structural investigation of solid acids as well as study of their acidic properties. Moreover, the energies and electronic structures of the catalysts and detailed catalytic reaction processes, including the identification of reaction species, elucidation of reaction mechanism, and verification of structure-activity correlations, made available by DFT theoretical calculations were also discussed. Relevant discussions will focus primarily on results obtained from our laboratories in the past decade, including (i) quantitative and qualitative acidity characterization utilizing assorted probe molecules, (ii) probing the spatial proximity and synergy effect of acid sites, and (iii) influence of acid features and pore confinement effect on catalytic activity, transition-state stability, reaction pathway, and product selectivity of solid acid catalysts such as zeolites, metal oxides, and heteropolyacids. It is conclusive that a synergy of acidity (local effect) and pore confinement (environmental effect) tend to strongly dictate the formations of intermediates and transition states, hence, the reaction pathways and catalytic performance of solid acid catalysts. We hope that these information can provide additional insights toward our understanding in heterogeneous catalysis, especially the roles of structural and acidic properties on catalytic performances and reaction mechanism of acid-catalyzed systems, which should be beneficial for rational design of solid acid catalysts.

Liquid fuel synthesis via CO2 hydrogenation by coupling homogeneous and heterogeneous catalysis

Crystalline zeolites have high acidity but limited utility due to microporosity, whereas mesoporous amorphous aluminosilicates offer better porosity but lack sufficient acidity. In this work, we investigated defect engineering to fine-tune the acidity of amorphous acidic aluminosilicates (AAS). Here we introduced oxygen vacancies in AAS to synthesize defective acidic aluminosilicates (D-AAS). 1H, 27Al, and 17O solid-state nuclear magnetic resonance (NMR) studies indicated that defects induced localized structural changes around the acidic sites, thereby modifying their acidity. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy studies substantiated that oxygen vacancies alter the chemical environment of Brønsted acidic sites of AAS. The effect of defect creation in AAS on its acidity and catalytic behavior was demonstrated using four different acid-catalyzed reactions namely, styrene oxide ring opening, vesidryl synthesis, Friedel-Crafts alkylation, and jasminaldehyde synthesis. The defects played a role in activating reactants during AAS-catalyzed reactions, enhancing the overall catalytic process. This was supported by in-situ FTIR, which provided insights into the molecular-level reaction mechanism and the role of defects in reactant activation. This study demonstrates defect engineering as a promising approach to fine-tune acidity in amorphous aluminosilicates, bridging the porosity and acidity gaps between mesoporous amorphous aluminosilicates and crystalline zeolites.

Catalytic activities of NiO–SiO2 for ethylene dimerization and butene isomerization run parallel when the catalysts are activated by evacuation at elevated temperatures, giving two maxima in activities. The variations in catalytic activities are closely correlated to the acidity of NiO–SiO2 catalysts. Catalytic activities of NiO–TiO2 catalysts modified with H2SO4, H3PO4, H3BO3, and H2SeO4 for ethylene dimerization and butene isomerization were examined. The order of catalytic activities for both reactions was found to be NiO–TiO2/SO42- >> NiO–TiO2/PO43-NiO–TiO2/BO33- > NiO–TiO2/SeO42-> NiO–TiO2, showing clear dependence of catalytic activity upon acid strength. The high catalytic activity of supported nickel sulfate for ethylene dimerization was related to the increase of acidity and acid strength due to the addition of NiSO4. The asymmetric stretching frequency of the S=O bonds for supported NiSO4 catalysts was related to the acidic properties and catalytic activity. That is, the higher the frequency, the larger both the acidity and catalytic activity. For NiSO4/Al2O3–ZrO2 catalyst, the addition of Al2O3 up to 5 mol% enhanced catalytic activity for ethylene dimerzation and strong acidity gradually due to the formation of Al–O–Zr bond. The active sites responsible for ethylene dimerization consist of a low-valent nickel, Ni+, and an acid, as evidenced by the IR spectra of CO adsorbed on NiSO4/γ -Al2O3 and Ni 2p XPS.

Characterization of the Acid Properties of Tungsten/Zirconia Catalysts Using Adsorption Microcalorimetry and n-Pentane Isomerization Activity

Effect of catalyst deactivation on the acid properties of zeolites used for isobutane/butene alkylation

The importance of short range interactions in the development of acidic and catalytic properties of H, Na - MoR

Heterogeneous catalysis, a field important industrially and scientifically, is increasingly seeking and refining strategies to render itself more predictable. The main issue is due to the nature and the population of catalytically active sites. Their number is generally low to very low, their "acid strengths" or " redox properties" are not homogeneous, and the material may display related yet inactive sites on the same material. In many heterogeneous catalysts, the discovery of a structure-activity reationship is at best challenging. One possible solution is to generate single-site catalysts in which most, if not all, of the sites are structurally identical. Within this context and using the right tools, the catalyst structure can be designed and well-defined, to reach a molecular understanding. It is then feasible to understand the structure-activity relationship and to develop predictable heterogeneous catalysis. Single-site well-defined heterogeneous catalysts can be prepared using concepts and tools of surface organometallic chemistry (SOMC). This approach operates by reacting organometallic compounds with surfaces of highly divided oxides (or of metal nanoparticles). This strategy has a solid track record to reveal structure-activity relationship to the extent that it is becoming now quite predictable. Almost all elements of the periodical table have been grafted on surfaces of oxides (from simple oxides such as silica or alumina to more sophisticated materials regarding composition or porosity). Considering catalytic hydrocarbon transformations, heterogeneous catalysis outcome may now be predicted based on existing mechanistic proposals and the rules of molecular chemistry (organometallic, organic) associated with some concepts of surface sciences. A thorough characterization of the grafted metal centers must be carried out using tools spanning from molecular organometallic or surface chemistry. By selection of the metal, its ligand set, and the support taken as a X, L ligands in the Green formalism, the catalyst can be designed and generated by grafting the organometallic precursor containing the functional group(s) suitable to target a given transformation (surface organometallic fragments (SOMF)). The choice of these SOMF is based on the elementary steps known in molecular chemistry applied to the desired reaction. The coordination sphere necessary for any catalytic reaction involving paraffins, olefins, and alkynes also can thus be predicted. Only their most complete understanding can allow development of catalytic reactions with the highest possible selectivity, activity, and lifetime. This Account will examine the results of SOMC for hydrocarbon transformations on oxide surfaces bearing metals of group 4-6. The silica-supported catalysts are exhibiting remarkable performances for Ziegler-Natta polymerization and depolymerization, low temperature hydrogenolysis of alkanes and waxes, metathesis of alkanes and cycloalkanes, olefins metathesis, and related reactions. In the case of reactions involving molecules that do not contain carbon (water-gas shift, NH3 synthesis, etc.) this single site approach is also valid but will be considered in a later review.

The aim of this thesis is to investigate heterogeneous catalysis for the dehydration of methanol and ethanol at a gas-solid interface over a wide range of solid Bronsted acid catalysts based on Keggin-type heteropoly acids (HPAs), focussing on the formation of dimethyl ether (DME) and diethyl ether (DEE), respectively. The dehydration of methanol to dimethyl ether (DME) was studied over a wide range of bulk and supported HPAs and was compared with the reaction over HZSM-5 zeolites (Si/Al = 10−120). Turnover rates for these catalysts were measured under zero-order reaction conditions. The HPA catalysts were demonstrated to have much higher catalytic activities than the HZSM-5 zeolites. A good correlation between the turnover rates and catalyst acid strengths, represented by the initial enthalpies of ammonia adsorption, was established. This correlation holds for the HPA and HZSM-5 catalysts studied, which indicates that the methanol-to-DME dehydration occurs via the same (or a similar) mechanism with both HPA and HZSM-5 catalysts, and that the turnover rate of methanol dehydration for both catalysts is primarily determined by the strength of catalyst acid sites, regardless of the catalyst pore geometry. Dehydration of ethanol was also studied over a wide range of solid Bronsted acid catalysts based on Keggin-type HPAs in a continuous flow fixed-bed reactor in the temperature range of 90-220 oC. The catalysts included H3PW12O40 (HPW) and H4SiW12O40 (HSiW) supported on SiO2, TiO2, Nb2O5 and ZrO2 with sub-monolayer HPA coverage, as well as bulk acidic Cs salts of HPW (Cs2.5H0.5PW12O40 and Cs2.25H0.75PW12O40) and the corresponding core-shell materials with the same total composition (15%HPW/Cs3PW12O40 and 25%HPW/Cs3PW12O40, respectively) comprising HPW supported on the neutral salt Cs3PW12O40. The ethanol-to-DEE reaction was found to be zero order in ethanol in the range of 1.5-10 kPa ethanol partial pressure. The acid strength of the catalysts was characterised by ammonia adsorption microcalorimetry. A fairly good correlation between the catalyst activity (turnover frequency) and the catalyst acid strength (initial enthalpy of ammonia adsorption) was established, which demonstrates that Bronsted acid sites play an important role in ethanol-to-DEE dehydration over HPA catalysts. The acid strength and the catalytic activity of core-shell catalysts HPW/Cs3PW12O40 did not exceed those of the corresponding bulk Cs salts of HPW with the same total composition, which contradicts the claims in the literature of the superiority of the core-shell HPA catalysts.

Surface acidities and acid strength distributions of amorphous silica–alumina, Y-type faujasite and mordenite have been examined by measuring the differential heats of adsorption of ammonia. The number of acid sites with adsorption heats higher than 70 kJ/mol increased in the order: NH4Y>NH4M>RENH4Y>silica–alumina. On the other hand, the order of the acid strength was: NH4M>RENH4Y>silica–alumina>NH4Y. The high catalytic activities of zeolites for cumene cracking and toluene disproportionation were correlated with acid strength, Bronsted acidity, and electrostatic effect.

Mechanistic study of glycerol dehydration on Brønsted acidic amorphous aluminosilicate

Controlled Assembly of Hierarchical Metal Catalysts with Enhanced Performances

The surface properties of gallium oxide and tin dioxide supported on alumina or titania have been studied by adsorption microcalorimetry. The differential heats of adsorption of various pollutant adsorbates such as sulfur dioxide, nitrogen monoxide, nitrogen dioxide and also ammonia were measured on these catalytic surfaces. NH3, SO2, NO2 are strongly adsorbed while NO is only physisorbed. The supported Ga2O3 samples show a slight decrease in acidity as probed by ammonia adsorption, compared to alumina or titania. The addition of SnO2 decreases the number of strong acid sites but creates a few weak and medium strength acid sites on alumina and does not modify the acidity of titania. In all cases, the basicity, probed by SO2 adsorption, is very strongly affected by the deposition of Ga2O3 or SnO2. The differential heats of NO2 adsorption remain nearly constant on all samples. The heats of adsorption are discussed as a function of the coverage and of the amount of guest oxide.

Observation of a potential-dependent switch of water-oxidation mechanism on Co-oxide-based catalysts

The main objective of this work was to design an innovative method to prepare heterogeneous heteropolyacid catalysts. The heteropolyacid H3PW12O40 (HPW) was modified with tin(II) by two methodologies: a conventional aqueous ion-exchange (CS) and a redox solid-state (SS). In both cases, Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) measurements evidenced that Keggin structure was preserved. All materials were active in the esterification of lactic acid with several alcohols and a mechanism was proposed. The best results were obtained for octanol and benzyl alcohol, where higher conversion values were obtained. The catalytic activity (turnover frequency, TON) showed an efficient performance for the materials prepared with 4 h of calcination (CS4h and SS4h). However, the catalyst prepared by the SS method was in accordance with the development of environmentally friendly processes.