Catalytic Activity Of Materials Research Articles

In-situ prepared co exsolution nano catalyst for efficient hydrogen generation via ammonia decomposition

Active and durable catalytic material for ammonia (NH3) decomposition reaction is attracting attentions for utilization of NH3 as an innovative hydrogen carrier. In this study, diverse single metal and alloy nano catalysts are prepared via in-situ exsolution method and their NH3 decomposition properties are evaluated. Transition metal cations (Ni, Co, Fe ions) are doped into the La0.43Ca0.37MxNyTi1-(x+y)O3-δ (LCMNT) perovskite oxide structure and exsolved on its surface as supported nano particles under reduction condition. The maximum doping level and chemical composition of exsolution catalysts are investigated to optimize their NH3 decomposition activity. The exsolution catalyst demonstrates improved NH3 decomposition characteristics compared to conventionally prepared infiltration catalysts, indicating higher conversion efficiency and H2 production rate. The exsolved nano catalysts also exhibit great thermochemical stability against catalyst agglomeration or surface nitriding. The results obtained in this study suggest the potential utilization of exsolution catalysts for on-site production of H2 through NH3 decomposition catalysis.

Study on dry desulfurization performance of modified HNTs

The production characteristics of coking industry determine that the coke quenching process has the characteristics of complex pollutant composition, high purification difficulty and serious environmental pollution. SO2 is one of the main pollutants in coke dry quenching tail gas, which is easy to cause serious harm. Dry desulfurization process is the use of particles or powder desulfurizer to remove SO2. The hydroxyl groups on the surface of Halloysite nanotubes make the active components of catalytic reaction fixed on the nano-scale surface or in the lumen, which can produce more selective and active catalytic materials with good catalytic performance. In this study, HNTs were modified by molten salt etching method. The functionalized halloysite and chitosan were cured by chemical precipitation method to prepare halloysite chitosan nanospheres desulfurizer, and its application performance and reaction mechanism in SO2 removal were explored.

Watching atoms at work during reactions

Abstract The development of new technologies for the current energy and environmental challenges requires the acquisition of a very fundamental knowledge about the structure and activity of catalytic materials at the nanometric scale. As a consequence, in situ and operando methodologies are blossoming, but only a fraction of them really aims at a local vision that would allow watching atoms at work during reactions. In this short report, we want to outline the merits of a new technique based on scanning tunnelling microscopy (Current-roughness electrochemical scanning tunneling microscopy, cr-EC-STM) which can visualize electrocatalytic reactions down at the single atom level. Results of two case studies in the field of hydrogen evolution reaction (HER) are briefly summarized, witnessing the capability of cr-EC-STM to provide critical information about the structure and catalytic performance of the active sites with atomic resolution.

Tandem reactions on phase separated MnO2 and C to enhance formaldehyde conversion to hydrogen

Hydrogen production from formaldehyde aqueous solution at room temperature shows a unique advantage regarding the bi-functional reaction route. MnO2 was reported to be an active catalytic material for this process, especially after coating with carbon composite for improved active site distribution. Here, we unravel that the conversion of formaldehyde can be treated as two tandem half-reactions: the oxidative dehydrogenation (ODH) of formaldehyde and the hydrogen evolution reaction (HER). β-MnO2 in a physical mixture with carbon (i.e., XC-72R, graphene) enhances both formaldehyde conversion (from 13% to 19.5%) and hydrogen productivity (from 22.35 μmol mg−1 h−1 to 33.86 μmol mg−1 h−1) by a synergistic effect of the phase separation catalyst in terms of electron redistribution and transport over the active sites. The catalyst examination supported by DFT calculation shows relatively low activation energy and band gap of β-MnO2 + C with a density of states spanning the Fermi level. The discovery of this novel synergistic catalytic effect: a phase separation catalyst with cooperative enhancement, provides new insight into the area of tandem catalysis.

Double-modified C–MgCo2O4 as a highly active catalytic material for the thermal decomposition of AP and its application for CSPs

The design and development of microscopic surface structures and surface defects of nanomaterials provide infinite possibilities for optimizing their catalytic activity. In this study, the microscopic surface structure of MgCo2O4 was modified while introducing combustible porous defective carbon (PDC) in MgCo2O4 by taking advantage of the incomplete decarbonization during solution combustion after the addition of biotemplates. The double-modified Cx-MgCo2O4 (x denotes the abbreviations of Reed, Cherry, Bougainvillea-spectabilis, Crab-apple-flower and Cotton: Re, Ch, Bs, Cf and Co) exhibited high activity in catalyzing the thermal decomposition of AP. Among them, CCo–MgCo2O4 with the highest carbon defect level (ID/IG = 4.54) significantly optimized the thermal decomposition performance of AP. The addition of CCo–MgCo2O4 ensured both the reduction of the AP thermal decomposition THTD (From 473.48 °C to 258.17 °C) and the increase of the AP heat of decomposition (ΔH, From 888.26 J/g to 3669.38 J/g). The introduction of combustible defective carbon in Cx-MgCo2O4 not only provides favorable conditions for the electron transfer process, but also generates synergistic heat during the thermal decomposition of AP and optimizes its energy release behavior. Compared with MgCo2O4, CCo–MgCo2O4 further accelerates the ignition (From 17 ms to 14 ms) of the composite solid propellants (CSPs) and ensures the energy release during combustion.

Green synthesis of the ZnO/ZSM-5 composite by using Convolvulus persicus extract: As an efficient and reusable catalyst in the Biginelli reaction

The active catalytic materials in stable inorganic supports are increasingly developing for the creation of ultra-stable catalysts with excellent performance. Here, the ZnO was supported on the surface of ZSM-5-type zeolite using Convolvulus persicus extract as a mild, efficient, cost‐effective, and eco‐friendly approach. The structure of the synthesized nanocatalyst was characterized using different techniques including UV, FT-IR, XRD, FE-SEM, EDS/Map, TGA, BET, and ICP-OES. The application of the ZnO/ZSM-5 as a catalyst was investigated in the Biginelli reaction between urea/thiourea, ethyl acetoacetate, and various aldehydes. Based on the results, ZnO/ZSM-5 exhibited good catalytic behavior, and the products were produced in excellent yields (90–95 %) in EtOH. This catalyst has notable benefits such as easy recovery and reusability (at least for five consecutive runs), simple work‐up, and high product yields.

The cactus bulb-shaped metal-S-C/N nanoparticles supported by highly conductive channels for efficient electrocatalytic hydrogen evolution reaction

Accelerating electron transfer and exposing active sites are key factors that could improve electrocatalytic hydrogen evolution reaction (HER). In this study, Co3S4-Ni3S2-C/N, the cactus bulb-shaped nanoparticles based on metal-organic frameworks (MOFs), were successfully prepared. The catalyst requires only 101 and 227 mV overpotential to reach current densities of 20 and 100 mA cm−2, respectively, in 1 M KOH. The high catalytic activity is attributed to the confinement effect achieved by introducing 2-aminoterephthalic acid as a precursor, allowing the catalyst to have a small particle size and fully exposed metal active sites. Moreover, the catalyst has a highly conductive metal-S-C/N channel that effectively reduces the intrinsic charge transfer resistance of the catalytic electrode, improves its conductivity, and modulates the electron density at the metal active site. Furthermore, compared to other crystal planes, the exposed (111) crystal plane of Co3S4 provides the most octahedral coordination of high spin state Co3+, which is highly catalytically active. This makes it the dominant crystal plane for catalysis. Through spatial confinement effect to limit metal sulfide particle size and introducing an organic backbone to create highly conductive metal-S-C/N channels, our study provides a general approach to developing highly active catalytic materials.

Using reactivity predictors for enhancing the electrocatalytic activity of MN4 molecular catalysts for the oxygen reduction reaction: The role of the N-pyridinium functional group in the porphyrazine-derivative ligands

Using reactivity predictors to enhance or control the electrocatalytic activity of materials is a fascinating concept. This is especially true for the development of alternative platinum metal group-free materials as it facilitates the rational design of active catalytic materials for the oxygen reduction reaction (ORR). In previous work, we have found that the peripheral and non-peripheral electron-withdrawing effects and the electron-pull effect from axial extraplanar ligand in iron-phthalocyanine (FePc) are key factors in improving the binding energy between the active Fe site and O2 resulting in an increase of the electrocatalytic activity of FePcs for the ORR. In this work, we have utilized fundamental principles of electrocatalysis and DFT calculations to design and synthesize FeN4 molecular catalysts to increase their catalytic performance for the ORR through the "pull" effect. To achieve this, by chemical synthesis, we have incorporated pyridinium functional groups (N+py) in peripheral and non-peripheral positions into the porphyrazine cyclic ligands. In this fashion we obtain the porphyrazinium molecular catalysts, [Fe(II)2,3-(TMe)TPyPz]4+ and [Fe(II)3,4-(TMe)TPyPz]4+. Because these new compounds are not commercially available and, to the best of our knowledge, they have not been tested for ORR. In order to determine their effectiveness, we have compared porphyrazinium with neutral analog porphyrazine compounds (Fe(II)TPyPz) and perfluorinated and perchlorinated iron phthalocyanines, which are currently the best molecular catalysts for ORR. The electrocatalytic activity was determined for each molecular catalyst deposited on the edge plane of a graphite electrode (EPG) surface in an alkaline medium. Only for the purpose of comparison we include two Fe porphyrins studied previously, which show low activity for ORR. Although the DFT theoretical analysis of porphyrazinium complexes suggests a high activity for these catalysts, our experimental findings revealed the opposite trend. Therefore, this finding makes us reconsider the interfacial effects, such as the counter-ions effects on N+py that could influence the electron-pull effect, opening new insights for designing molecular catalysts considering interface engineering. Moreover we report for the first time, the reactivity linear relationship between the metal-centered redox potential gap (E°Fe(III)/(II) – E°Fe(II)/(I))) with the electrocatalytic activity for ORR for all catalysts studied, emerging this potential gap as a possible and promising new reactivity descriptor for ORR in MN4 catalyst.

Clever Nanomaterials Fabrication Techniques Encounter Sustainable C1 Catalysis.

ConspectusC1 catalysis, which refers to the conversion of molecules with a single carbon atom, such as CO, CO2, and CH4, into clean fuels and basic building blocks for chemical industries, has built a bridge between carbon resource utilization and valuable chemical supply. With respect to the goal of carbon neutrality, C1 catalysis also plays an essential role owing to its integrated functions in the green catalytic process with fewer CO2 emissions and even direct high-value-added utilization of greenhouse gases (CO2 and CH4). However, the inert nature of the C-O or C-H bond in C1 molecules as well as uncontrollable C-C coupling render C1 catalysis challenging. The rational design of highly active catalytic materials (denoted as C1 catalysts) with strong capacities for C-O or C-H bond activation and C-C coupling by convenient nanomaterials fabrication methods to boost the catalytic performance of C1 molecule conversion, including targeted product selectivity and long-term stability, is the cornerstone of C1 catalysis.Notably, the familiar concepts in heterogeneous catalysis, such as tandem catalysis and confinement catalysis, are applicable for C1 catalysis and have been successfully used to design a C1 catalyst. Regarding the tandem catalysis concept that integrates multiple reactions in a single-pass via a bi- or multifunctional catalyst, it is promising to shed new light on the oriented conversion of C1 molecules, especially for C2+ hydrocarbon or oxygenate synthesis. The confinement effect is powerful for controlling the product distribution and enhancing activation efficiency of inert chemical bonds in C1 catalysis due to the unique reactants/intermediate adsorption and evolution behaviors on the confined catalytic interface with a special electronic environment. Moreover, metal-support interactions (MSIs), electronic properties of the active site, and catalytic engineering issues are also susceptible to the C1 molecule conversion performance. Therefore, under the guidance of basic and novel rules in heterogeneous catalysis, the innovation of catalytic materials with the aid of advanced catalytic materials fabrication techniques has always been a hot research topic in C1 catalysis.In this Account, we briefly describe the challenges in thermal-catalytic C1 molecule (mainly CO, CO2, and CH4) conversion. At the same time, the synergistic functioning of the physicochemical properties of the catalytic materials on the performance in C1 molecule conversion is highlighted. More importantly, we summarize our progress in rationally designing tailor-made C1 catalysts to enhance C1 molecule activation efficiency and targeted product selectivity via powerful nanomaterials fabrication techniques, such as traditional wet-chemistry strategies, the magnetron sputtering method, and 3D printing technology. Specifically, the ingenious capsule catalyst and ammonia pools in zeolites fabricated by a wet chemistry process possess an extraordinary effect on the transformation of CO, CO2, and CH4 molecules. Also, the sputtering method is reliable in modulating the electronic properties of metallic active sites for C1 molecule conversion, thereby tailoring the final product selectivity. Furthermore, we showcase the strong capability of metal 3D printing technology in fabricating a self-catalytic reactor, by which the functions of the reaction field and nanoscale active sites are well integrated. Finally, we predict the future research opportunities in highly efficient C1 catalyst design with the assistance of clever nanomaterials fabrication techniques.

Intrinsic spin shielding effect in platinum-rare-earth alloy boosts oxygen reduction activity.

Oxygen reduction reactions (ORRs) involve a multistep proton-coupled electron process accompanied by the conversion of the apodictic spin configuration. Understanding the role of spin configurations of metals in the adsorption and desorption of oxygen intermediates during ORRs is critical for the design of efficient ORR catalysts. Herein, a platinum-rare-earth-metal-based alloy catalyst, Pt2Gd, is introduced to reveal the role of spin configurations in the catalytic activity of materials. The catalyst exhibits a unique intrinsic spin reconfiguration because of interactions between the Gd-4f and Pt-5d orbitals. The adsorption and desorption of the oxygen species are optimized by modifying the spin symmetry and electronic structures of the material for increased ORR efficiency. The Pt2Gd alloy exhibits a half-wave potential of 0.95V and a superior mass activity of 1.5 A·mgPt-1 in a 0.1M HClO4 electrolyte, as well as higher durability than conventional Pt/C catalysts. Theoretical calculations have proven that the spin shielding effect of Gd pairs increases the spin symmetry of Pt-5d orbitals and adsorption preferences toward spin-polarized intermediates to facilitate ORR. This work clarifies the impact of modulating the intrinsic spin state of Pt through the interaction with the local high spin 4f orbital electrons in rare-earth metals, with the aim of boosting the spin-related oxygen reduction reaction, thus fundamentally contributing to the understanding of new descriptors that control ORR activity.

Wearable non-invasive glucose sensors based on metallic nanomaterials.

The development of wearable non-invasive glucose sensors provides a convenient technical means to monitor the glucose concentration of diabetes patients without discomfortability and risk of infection. Apart from enzymes as typical catalytic materials, the active catalytic materials of the glucose sensor are mainly composed of polymers, metals, alloys, metal compounds, and various metals that can undergo catalytic oxidation with glucose. Among them, metallic nanomaterials are the optimal materials applied in the field of wearable non-invasive glucose sensing due to good biocompatibility, large specific surface area, high catalytic activity, and strong adsorption capacity. This review summarizes the metallic nanomaterials used in wearable non-invasive glucose sensors including zero-dimensional (0D), one-dimensional (1D), and two-dimensional (2D) monometallic nanomaterials, bimetallic nanomaterials, metal oxide nanomaterials, etc. Besides, the applications of wearable non-invasive biosensors based on these metallic nanomaterials towards glucose detection are summarized in detail and the development trend of the wearable non-invasive glucose sensors based on metallic nanomaterials is also outlook.

Ex-Solution Hybrids Functionalized on Oxide Nanofibers for Highly Active and Durable Catalytic Materials

Ex-solution catalysts containing spontaneously formed metal nanoparticles socketed on the surface of reservoir oxides have recently been employed in various research fields including catalysis and sensing, due to the process efficiency and outstanding chemical/thermal stability. However, since the ex-solution process accompanies harsh reduction heat treatment, during which many oxides undergo phase decomposition, it restricts material selection and further advancement. Herein, we propose an elaborate design principle to uniformly functionalize ex-solution catalysts at porous oxide frameworks via an electrospinning process. As a case study, we selected the ex-solved La0.6Ca0.4Fe0.95Co0.05-xNixO3-δ (x = 0, 0.025 and 0.05) and SnO2 nanofibers as ex-solution hybrids and main frameworks, respectively. We confirmed superior dimethyl sulfide (C2H6S) gas sensing characteristics with excellent long-cycling stability. In particular, the high catalytic activities of ex-solved CoNiFe ternary nanoparticles, strongly socketed on reservoir oxide, accelerate the spillover process of O2 to dramatically enhance the response toward sulfuric analytes with exceptional tolerance. Altogether, our contribution represents an important stepping-stone to a rational design of ex-solved particle-reservoir oxide hybrids functionalized on porous oxide scaffolds for a variety of applications.

Improved heterogeneous photo-Fenton-like degradation of ofloxacin through polyvinylpyrrolidone modified CuFeO2 catalyst: Performance, DFT calculation and mechanism

The exploitation of effective modification means to get highly active and stable catalytic materials for heterogeneous photo-Fenton-like (HPF-like) process has been enormously stimulated by the globally increased demands for water purification. Oxygen vacancies (OVs) modulation is regarded as a prevalent strategy to improve the performance of heterogeneous catalysts. Herein, the polyvinylpyrrolidone (PVP) modified CuFeO2 (PVP/CFO) catalysts were prepared to degrade ofloxacin (OFX) target pollutant in HPF-like process. The modification of PVP introduced the rich OVs and more Fe2+ active sites on PVP/CFO surface, while the aggregation of CuFeO2 particles was inhibited by the modification of PVP. The PVP/CFO catalyst with the addition of 4 g PVP in the synthesis (PVP/CFO-4) possessed more superior catalytic performance for OFX degradation than other prepared catalysts in the present work. Additionally, the as-prepared PVP/CFO-4 catalyst possessed good stability for OFX degradation in HPF-like system. The introduced OVs by PVP modification and the generated photoelectron on PVP/CFO by light excitation effectively accelerated in-situ recycling of Cu2+/Cu+ and Fe3+/Fe2+, which was beneficial for •OH/•O2– formation and OFX mineralization. The quenching experiment and EPR spectra revealed that •OH, •O2- and e- were the major active species, and h+ was auxiliary species for OFX degradation. The OFX degradation pathways were proposed by density functional theory calculation and liquid chromatography mass spectrometry. The overall toxicity of intermediates was relieved after HPF-like degradation of OFX based on quantitative structure–activity relationship predictions.

Recent Advances on Fine-Tuning Engineering Strategies of CeO2-Based Nanostructured Catalysts Exemplified by CO2 Hydrogenation Processes

Ceria-based oxides have been extensively involved in a wide range of catalytic applications due to their intriguing properties, related mostly to their superior redox features in conjunction with peculiar metal-support interaction phenomena. Most importantly, the fine-tuning of key interrelated factors, such as the size, morphology and electronic state of the catalyst’s counterparts, can exert a profound influence on the intrinsic characteristics and interfacial reactivity with pronounced implications in catalysis. The present review, while also elaborating our recent efforts in the field, aims to provide key fundamental and practical aspects in relation to the rational design and functionalization strategies of ceria-based catalysts, exemplified by the CO2 hydrogenation processes, namely, CO2 methanation and reverse water–gas shift (rWGS) reactions. Firstly, a description of the most prominent catalytically relevant features of cerium oxide is provided, focusing on reducibility and metal-support interaction phenomena, followed by a brief overview of the current status of ceria-based catalysts for various energy and environmental applications. Then, the main implications of fine-tuning engineering via either appropriate synthesis routes or aliovalent doping on key activity descriptors are thoroughly discussed and exemplified by state-of-the-art ceria-based catalysts for CO2 hydrogenation. It is clearly revealed that highly active and cost-efficient ceria-based catalytic materials can be obtained on the grounds of the proposed functionalization strategy, with comparable or even superior reactivity to that of noble metal catalysts for both the studied reactions. In a nutshell, it can be postulated that the dedicated fabrication of CeO2-based systems with augmented redox capabilities and, thus, oxygen vacancies abundance can greatly enhance the activation of gas-phase CO2 towards CO or CH4. Besides, the morphology-engineering of CeO2-based catalysts can notably affect the CO2 hydrogenation performance, by means of an optimum metal-ceria interphase based on the exposed facets, whereas doping and promotion strategies can effectively shift the reaction pathway towards the selective production of either CO or CH4. The conclusions derived from the present work can provide design and fine-tuning principles for cost-efficient, highly active and earth-abundant metal oxide systems, not only for the CO2 hydrogenation process but for various other energy and environmental applications.

Theoretical Study of Hydrogen Production from Ammonia Borane Catalyzed by Metal and Non-Metal Diatom-Doped Cobalt Phosphide.

The decomposition of ammonia borane (NH3BH3) to produce hydrogen has developed a promising technology to alleviate the energy crisis. In this paper, metal and non-metal diatom-doped CoP as catalyst was applied to study hydrogen evolution from NH3BH3 by density functional theory (DFT) calculations. Herein, five catalysts were investigated in detail: pristine CoP, Ni- and N-doped CoP (CoPNi-N), Ga- and N-doped CoP (CoPGa-N), Ni- and S-doped CoP (CoPNi-S), and Zn- and S-doped CoP (CoPZn-S). Firstly, the stable adsorption structure and adsorption energy of NH3BH3 on each catalytic slab were obtained. Additionally, the charge density differences (CDD) between NH3BH3 and the five different catalysts were calculated, which revealed the interaction between the NH3BH3 and the catalytic slab. Then, four different reaction pathways were designed for the five catalysts to discuss the catalytic mechanism of hydrogen evolution. By calculating the activation energies of the control steps of the four reaction pathways, the optimal reaction pathways of each catalyst were found. For the five catalysts, the optimal reaction pathways and activation energies are different from each other. Compared with undoped CoP, it can be seen that CoPGa-N, CoPNi-S, and CoPZn-S can better contribute hydrogen evolution from NH3BH3. Finally, the band structures and density of states of the five catalysts were obtained, which manifests that CoPGa-N, CoPNi-S, and CoPZn-S have high-achieving catalytic activity and further verifies our conclusions. These results can provide theoretical references for the future study of highly active CoP catalytic materials.

Easy control of surface morphology through a natural phenomenon

Easy control of surface morphology through a natural phenomenon

Nickel Nanoparticles Immobilized on Pristine Halloysite: An Outstanding Catalyst for Hydrogenation Processes

AbstractNickel nanoparticles (NiNPs) immobilized on halloysite‐based supports were straightforwardly synthesized and fully characterized by different techniques with the purpose of evaluating the catalytic performance of these as‐prepared composite materials as catalysts in hydrogenation reactions. Thus, halloysite bearing amino (HAL‐NH2) and ammonium (HAL‐NEt3) groups led to less active catalytic materials in comparison with unfunctionalized halloysite supports, probably due to relative strong metal‐support interactions with amino and tetraalkylammonium functions, consequently hindering the interaction of unsaturated substrates at the surface of the catalytic material. Despite NiNPs supported on pristine halloysite provided merely agglomerates, NiNPs stabilized by quinidine on unfunctionalized halloysite (NiC) proved to be a highly effective and versatile catalyst operating at relative low temperature (25–100 °C) and metal loading (1–20 mol %Ni), very well‐adapted to reduce a large range of functions (alkenes, alkynes, ketones, aldehydes, nitro…). Moreover, NiC was able to selectively reduce substrates from biomass to yield high value‐added products, such as squalane, saturated fatty acids, furfuryl alcohol and γ‐valerolactone.

Rapid Plasma Exsolution from an A‐site Deficient Perovskite Oxide at Room Temperature

AbstractHigh‐performance nanoparticle platforms can drive catalysis progress to new horizons, delivering environmental and energy targets. Nanoparticle exsolution offers unprecedented opportunities that are limited by current demanding process conditions. Unraveling new exsolution pathways, particularly at low‐temperatures, represents an important milestone that will enable improved sustainable synthetic route, more control of catalysis microstructure as well as new application opportunities. Herein it is demonstrated that plasma direct exsolution at room temperature represents just such a step change in the synthesis. Moreover, the factors that most affect the exsolution process are identified. It is shown that the surface defects produced initiate exsolution under a brief ion bombardment of an argon low‐pressure and low‐temperature plasma. This results in controlled nanoparticles with diameters ≈19–22 nm with very high number densities thus creating a highly active catalytic material for CO oxidation which rivals traditionally created exsolved samples.

Investigating the impact of micropore volume of aminosilica functionalized SBA-15 on catalytic activity for amine-catalyzed reactions

Mesoporous materials such as SBA-15 are used in many applications since the materials are robust and have tunable properties. Whereas these materials are thought to be ideal, the materials are complex with the common synthesis methods producing materials that contain micropores. This work will investigate the impact of micropores on catalytic activity of aminosilica materials. For comparison, materials are formed using a standard method that produces materials with micropores (REG-SBA-15) and using a modified method that produces materials with negligible micropore volume (NMP-SBA-15). After grafting the calcined support with an aminosilane (either (N-methyl amino propyl) trimethoxy silane (denoted as 2°Am) or (N,N-diethyl-3-amino propyl) trimethoxy silane (3°Am)), catalytic testing experiments reveal that NMP-SBA-15 functionalized materials are more active than REG-SBA-15 functionalized materials for a range of reactions. The reactions include (a) glucose isomerization where amines interacting with surface silanols limit catalytic activity and (b) aldol and Knoevenagel chemistry where amine-silanol interactions increase the reaction rate. For aldol chemistry, the observed difference in rate is further investigated using site quantification experiments. Interestingly, the results reveal that three types of sites exist for each material, including sites that are (1) highly active, (2) intermediate active, and (3) inactive. NMR experiments indicate that amines immobilized on REG-SBA-15 supports are less mobile than amines on NMP-SBA-15. For REG-SBA-15, the materials are found to have aminosilanes that have limited mobility and the largest fraction of inactive sites, which suggests that the inactive catalytic sites are in the micropores. Overall, the design of highly active catalytic materials can be achieved using materials that have limited micropore volumes.

Quantifying the fraction and activity of catalytic sites at different surface densities of aminosilanes in SBA-15 for the aldol reaction and condensation

The complexity of aminosilica materials manifests as catalysts with multiple types of active sites. Whereas previous work has assumed that each site contributes equally to the observed catalytic behavior, this work rigorously investigates the catalytic activity on a per site basis using site quantification experiments. These experiments reveal that aminosilica materials have four types of catalytic sites, including high, medium, and low activity sites as well as inactive sites. The fraction of each of these sites depends on the surface density. The low activity sites are associated with amines clustered on the surface. Additional experiments reveal that the high activity sites are likely not associated with amines grafted in pairs or amines located on the external surface, leading us to speculate that the difference is amines cooperatively interacting with different types of silanols. Overall, the results demonstrate the potential to create highly active catalytic materials using cooperative interactions.