Surface species in direct liquid phase synthesis of dimethyl carbonate from methanol and CO2: an MCR-ALS augmented ATR-IR study.

The reaction mechanism of dimethyl carbonate (DMC) production over ZrO2 from CO2 and CH3OH is well-known, but the level of understanding has not improved in the last decade. Most commonly, the reaction mechanism has been explored in the gas phase, whilst DMC production occurs in the liquid phase. To overcome this contradiction, we exploited in situ ATR-IR spectroscopy to study DMC formation over ZrO2 in the liquid phase. A multiple curve resolution-alternate least square (MCR-ALS) approach was applied to spectra collected during the CO2/CH3OH interaction with the catalyst surface, leading to the identification of five pure components with their respective concentration profiles. CO2 and CH3OH activation to carbonates and methoxide species was found to strongly depend on the reaction temperature. Low temperature prevents methanol dissociation leaving a catalyst covered with stable carbonates, whilst higher temperature decreases the stability of the carbonates and enhances the formation of methoxides. A reaction path involving the methoxide/carbonate interaction at the surface was observed at low temperature (≤50 °C). We propose that a different reaction path, independent of carbonate formation and involving the direct CO2/methoxide interplay, occurs at 70 °C.

From gaseous HCN to nucleobases at the cosmic silicate dust surface: an experimental insight into the onset of prebiotic chemistry in space.

HCN in the gas form is considered as a primary nitrogen source for the synthesis of prebiotic molecules in extraterrestrial environments. Nevertheless, the research mainly focused on the reactivity of HCN and its derivatives in aqueous systems, often using external high-energy supply in the form of cosmic rays or high energy photons. Very few studies have been devoted to the chemistry of HCN in the gas phase or at the gas/solid interphase, although they represent the more common scenarios in the outer space. In this paper we report about the reactivity of highly pure HCN in the 150-300 K range at the surface of amorphous and crystalline Mg2SiO4 (forsterite olivine), i.e. of solids among the constituents of the core of cosmic dust particles, comets, and meteorites. Amorphous silica and MgO were also studied as model representatives of Mg2SiO4 structural building blocks. IR spectroscopic results and the HR-MS analysis of the reaction products revealed Mg2+O2- acid/base pairs at the surface of Mg2SiO4 and MgO to be key in promoting the formation of HCN oligomers along with imidazole and purine compounds, already under very mild temperature and HCN pressure conditions, i.e. in the absence of external energetic triggers. Products include adenine nucleobase, a result which supports the hypothesis that prebiotic molecular building blocks can be easily formed through surface catalytic processes in the absence of high-energy supply.

A quantum mechanical study of dehydration vs. decarbonylation of formamide catalysed by amorphous silica surfaces.

Formamide is abundant in the interstellar medium and was also present during the formation of the Solar system through the accretion process of interstellar dust. Under the physicochemical conditions of primordial Earth, formamide could have undergone decomposition, either via dehydration (HCN + H2O) or via decarbonylation (CO + NH3). The first reactive channel provides HCN, which is an essential molecular building block for the formation of RNA/DNA bases, crucial for the emergence of life on Earth. In this work, we studied, at the CCSD(T)/cc-pVTZ level, the two competitive routes of formamide decomposition, i.e. dehydration and decarbonylation, either in liquid formamide (by using the polarization continuum model technique) or at the interface between liquid formamide and amorphous silica. Amorphous silica was adopted as a convenient model of the crystalline silica phases ubiquitously present in the primordial (and actual) Earth's crust, and also due to its relevance in catalysis, adsorption and chromatography. Results show that: (i) silica surface sites catalyse both decomposition channels by reducing the activation barriers by about 100 kJ mol-1 with respect to the reactions in homogeneous medium, and (ii) the dehydration channel, giving rise to HCN, is strongly favoured from a kinetic standpoint over decarbonylation, the latter being, instead, slightly favoured from a thermodynamic point of view.

Monitoring the Reactivity of Formamide on Amorphous SiO2 by In-Situ UV-Raman Spectroscopy and DFT Modeling.

Formamide has been recognized in the literature as a key species in the formation of the complex molecules of life, such as nucleobases. Furthermore, several studies reported the impact of mineral phases as catalysts for its decomposition/polymerization processes, increasing the conversion and also favoring the formation of specific products. Despite the progresses in the field, in situ studies on these mineral-catalyzed processes are missing. In this work, we present an in situ UV-Raman characterization of the chemical evolution of formamide over amorphous SiO2 samples, selected as a prototype of silicate minerals. The experiments were carried out after reaction of formamide at 160 °C on amorphous SiO2 (Aerosil OX50) either pristine or pre-calcined at 450 °C, to remove a large fraction of surface silanol groups. Our measurements, interpreted on the basis of density functional B3LYP-D3 calculations, allow to assign the spectra bands in terms of specific complex organic molecules, namely, diaminomaleonitrile (DAMN), 5-aminoimidazole (AI), and purine, showing the role of the mineral surface on the formation of relevant prebiotic molecules.

Titanium Defective Sites in TS-1: Structural Insights by Combining Spectroscopy and Simulation.

Ti silicates, and in particular, titanium silicalite-1 (TS-1), are nowadays important catalysts for several partial oxidation reactions in the presence of aqueous H2 O2 as an oxidant. Despite the numerous studies dealing with this material, some fundamental aspects are still unclear. In particular, the structure and the catalytic role of defective Ti sites, other than perfect tetrahedral sites recognized as the main active species, has not been quantitatively discussed in the literature. We assess the structural features of defective Ti sites on the basis of outcomes of electronic spectroscopies, as interpreted through quantum mechanical simulation. Strong evidence is disclosed to support the fact that the most common defective Ti sites, often reported in the TS-1 literature, are monomeric Ti centers that are embedded in the zeolite framework, and which have a distorted octahedral local symmetry.

Exact Stoichiometry of Ce xZr6- x Cornerstones in Mixed-Metal UiO-66 Metal-Organic Frameworks Revealed by Extended X-ray Absorption Fine Structure Spectroscopy.

Bimetallic Ce/Zr-UiO-66 metal-organic frameworks (MOFs) proved to be promising materials for various catalytic redox applications, representing, together with other bimetallic MOFs, a new generation of porous structures. However, no direct proof for the presence of both metals in a single cornerstone of UiO-type MOFs was reported so far. Employing element-selective X-ray absorption spectroscopy techniques herein, we demonstrate, for the first time, that our synthesis route allows obtaining Ce/Zr-UiO-66 MOFs with desired Ce content and bimetallic CeZr5 cornerstones. Performing multiple-edge extended X-ray absorption fine structure analysis, we determine the exact stoichiometry of the cornerstones, which explains the dependence of thermal and chemical stability of the materials on Ce content.

Topology-dependent hydrocarbon transformations in the methanol-to-hydrocarbons reaction studied by operando UV-Raman spectroscopy.

The methanol-to-hydrocarbons (MTH) reaction represents a versatile, industrially viable alternative to crude-oil based processes for the production of chemicals and fuels. In the MTH reaction, the shape selectivity of acidic zeolites is exploited to direct the synthesis towards the desired product. However, due to unavoidable side reactions occurring under processing conditions, all MTH catalysts suffer deactivation due to coke formation. Though it is likely that some common characteristics for carbon formation exist for all zeolite topologies, it has been proposed that the differences in shape selectivity among the different catalysts will also influence the individual deactivation mechanisms. As deactivating species are mostly aromatic compounds, highly methylated benzenes and/or polycyclic aromatic hydrocarbons (PAHs) have been discussed. In some cases, these can further grow to extended carbon structures. Here, we have investigated the hydrocarbon reactivities and carbon formation for five topologically different zeolite catalysts through an operando UV-Raman approach, taking advantage of the high sensitivity of this technique towards aromatic and other carbonaceous species. The combination of the spectroscopic tool with activity measurements allowed us to obtain valuable details and some general trends on the deactivation paths during MTH. This approach made accessible unique insight on the complex chemistry of MTH by allowing the real-time observation of hydrocarbon transformations typical for the peculiar topology of each catalyst, usually inaccessible by ex situ techniques.

Effect of Molecular Guest Binding on the d-d Transitions of Ni2+ of CPO-27-Ni: A Combined UV-Vis, Resonant-Valence-to-Core X-ray Emission Spectroscopy, and Theoretical Study.

We used Ni K-edge resonant-valence-to-core X-ray emission spectroscopy (RVtC-XES, also referred to as direct RIXS), an element-selective bulk-sensitive synchrotron-based technique, to investigate the electronic structure of the CPO-27-Ni metal-organic framework (MOF) upon molecular adsorption of significant molecular probes: H2O, CO, H2S, and NO. We compare RVtC-XES with UV-vis spectroscopy, and we show that the element selectivity of RVtC-XES is of strategic significance to observe the full set of d-d excitations in Ni2+, which are partially overshadowed by the low-energy π-π* transitions of the Ni ligands in standard diffuse-reflectance UV-vis experiments. Our combined RVtC-XES/UV-vis approach provides access to the whole set of d-d excitations, allowing us a complete discussion of the changes undergone by the electronic configuration of the Ni2+ sites hosted within the MOF upon molecular adsorption. The experimental data have been interpreted by multiplet ligand-field theory calculations based on Wannier orbitals. This study represents a step further in understanding the ability of the CPO-27-Ni MOFs in molecular sorption and separation applications.

The role of dispersive forces determining the energetics of adsorption in Ti zeolites.

Ti-zeolites are interesting materials because of their key role in partial oxidation reactions, as well as under a fundamental point of view being regarded as single site catalysts. Both experimental and computational approaches have been widely applied to the characterization of their active sites, reaching a level of knowledge unmatchable by most other important catalysts. However, several questions are still open, being a proper energetic simulation of the adsorption process of simple molecules, fitting with the experimental outcomes, still missing. The present work wants to underline the role of dispersive forces in correctly determining the adsorption energies of H2 O and NH3 in Ti chabazite: first dispersive contributions have been included through an ONIOM scheme, comparing the results from semiempirical Grimme scheme and fully ab initio MP2. Being the key contribution of dispersion proved, a fully periodic, Grimme dispersions inclusive approach has been applied, coming to results close to the experimental values. © 2016 Wiley Periodicals, Inc.

New insights into UTSA-16.

Among the metal organic framework materials proposed for CO2 separation, UTSA-16 possesses the highest CO2 volumetric density explained on the basis of favourable interactions between CO2 and structural water molecules in the material, as revealed by neutron diffraction. In this study, UTSA-16 was synthesised and extensively characterised by XRD, TEM combined with EDX analysis and DR-UV-Vis, Raman and FTIR spectroscopies, as well as by TGA measurements. The synthesised material shows XRD patterns, surface area, CO2 capacity and isosteric heat coincident to the ones reported for UTSA-16 in the original papers but a higher thermal stability and a complete removal of water upon activation under mild conditions (363 K). On the basis of EDX and IR measurements, the formula of UTSA-16 used in the present study is proposed to be K2Co3(cit)2. Infrared spectroscopy clearly shows that UTSA-16 described in this work reversibly interacts with water vapor, CO and CO2. The interaction is attributed to K(+) species, which are present as counterions in the pores. At 1 bar and 298 K a fraction of K(+) sites adsorbs 2 CO2 molecules.

Probing zeolites by vibrational spectroscopies.

This review addresses the most relevant aspects of vibrational spectroscopies (IR, Raman and INS) applied to zeolites and zeotype materials. Surface Brønsted and Lewis acidity and surface basicity are treated in detail. The role of probe molecules and the relevance of tuning both the proton affinity and the steric hindrance of the probe to fully understand and map the complex site population present inside microporous materials are critically discussed. A detailed description of the methods needed to precisely determine the IR absorption coefficients is given, making IR a quantitative technique. The thermodynamic parameters of the adsorption process that can be extracted from a variable-temperature IR study are described. Finally, cutting-edge space- and time-resolved experiments are reviewed. All aspects are discussed by reporting relevant examples. When available, the theoretical literature related to the reviewed experimental results is reported to support the interpretation of the vibrational spectra on an atomic level.

Synthesis and characterization of amine-functionalized mixed-ligand metal-organic frameworks of UiO-66 topology.

A series of amine-functionalized mixed-linker metal-organic frameworks (MOFs) of idealized structural formula Zr6O4(OH)4(BDC)(6-6X)(ABDC)6X (where BDC = benzene-1,4-dicarboxylic acid, ABDC = 2-aminobenzene-1,4-dicarboxylic acid) has been prepared by solvothermal synthesis. The materials have been characterized by thermogravimetric analysis (TGA), powder X-ray diffraction (PXRD), and Fourier transform infrared (FTIR) spectroscopy with the aim of elucidating the effect that varying the degrees of amine functionalization has on the stability (thermal and chemical) and porosity of the framework. This work includes the first application of ultraviolet-visible light (UV-vis) spectroscopy in the quantification of ABDC in mixed-linker MOFs.

Gender trends in dentistry: Dental faculty and academic leadership.

OBJECTIVES: Gender equality in the healthcare workforce has been a topic of discourse for many decades. In dental academia, women's representation of enrolled students and faculty has risen consistently since the 1980s. However, women in faculty leadership positions may still be lagging when compared to men. The purpose of this study was to evaluate the number of women who occupy the upper echelons of academic rank and title by analyzing cross-referenced data from the American Dental Association and the American Dental Education Association on women dental school graduates in relation to academic appointments. METHODS: Gender distribution in rank, title, and appointments in the decade from 2011 to 2019, as well as percentage of women graduates over the same period, were collected for descriptive statistics. Multiple linear regression analysis, Cochran Armitage, and chi-square tests were conducted to examine trends over the years and to determine significant differences in overall percentages (p < 0.05). RESULTS: The total women faculty percentages ranged from 36% to 40%. While the lower-level rank of instructor retained a higher representation of women (56%-65%), the higher rank of professor had disproportionately lower women percentages (18%-26%). Graduates, full-time faculty, lower-level academic ranks, and higher-level academic ranks for women followed similar upward trends that were statistically significant (p < 0.05). When comparing the different groups against each other, the annual increase in women DMD/DDS graduate percentage was higher than women full-time faculty (0.28%), instructor rank (0.92%), professor rank (0.50%), and department chair appointments (0.49%). CONCLUSIONS: Our data show that women are still underrepresented at higher academic ranks. However, the upward trends for professors, assistant deans, and program chairs suggest that in recent years, more women faculty may have been encouraged, mentored, or offered higher administrative positions in academic institutions.

Ten-year results of a three arms prospective cohort study on implants in periodontally compromised patients. Part 2: clinical results.

OBJECTIVES: The aim of this study was to compare long-term outcomes of implants placed both in patients treated for periodontitis and in periodontally healthy patients (PHP). MATERIAL AND METHODS: One hundred and twelve partially edentulous patients were consecutively enrolled in private specialist practice and divided into three groups according to their initial periodontal condition: PHP, moderately periodontally compromised patients (PCP) and severely PCP. Implants were placed to support fixed prostheses, after successful completion of initial periodontal therapy [full-mouth plaque score (FMPS) <25%, full-mouth bleeding score (FMBS) <25%]. At the end of active periodontal treatment (APT), patients were asked to follow an individualized supportive periodontal therapy (SPT) program. Diagnosis and treatment of peri-implant biological complications was performed according to cumulative interceptive supportive therapy. At 10 years, clinical measures were recorded by two calibrated operators, blinded to the initial patient classification, on 101 patients, as 11 were lost to follow-up. The number of sites treated according to therapy modalities C and D (antibiotics and/or surgery) during the 10 years was registered. RESULTS: Eighteen implants were removed for biological complications. Antibiotic and/or surgical therapy was performed in 10.7% of cases in PHP, in 27% of cases in moderate PCP and in 47.2% cases in severe PCP, with a statistically significant differences between PHP and severe PCP (P = 0.002). At the final examination, the percentage of implants, with at least one site which presented a PD ≥6 mm, was respectively 1.7% for PHP, 15.9% for moderate PCP and 27.2% for severe PCP, with a statistically significant difference between PHP and moderate PCP (P = 0.005) and PHP and severe PCP (P = 0.0001). CONCLUSION: Patients with a history of periodontitis presented a statistically significant higher number of sites which required additional treatment. Therefore, patients with a history of periodontitis should be informed that they are more at risk for peri-implant disease. This underlines the value of the SPT in enhancing long-term outcomes of implant therapy, particularly in subjects affected by periodontitis. Therefore, the approach for multiple preventive dental extractions and implant placement, based on the assumption the implants perform better than teeth, should be followed with extreme caution.

What is the optimal number of implants for removable reconstructions? A systematic review on implant-supported overdentures.

OBJECTIVES: The aim of this systematic review was to assess the optimal number of implants for removable reconstructions. MATERIAL AND METHODS: Medline and The Cochrane Central Register of Controlled Trials were searched and complemented by hand searching. All trials published in English to October 2011 were included, in which overdentures, supported by a various number of implants, in adult edentulous individuals were compared. Only randomized controlled trials with at least 12 months follow-up were selected. The outcomes of interest were implant loss, the amount of peri-implant bone loss, the incidence of complications and the patient satisfaction. RESULTS: No articles were found providing information regarding the maxilla. Eleven studies on the mandible were included for the final comparative analysis. It was possible to make a comparison among four categories: (i) 1 vs. 2 implants; (ii) 2 implants with ball attachments vs. 4 implants with a bar; (iii) 2 implants with a bar vs. 4 implants with a bar; (iv) 2 implants splinted with a bar vs. 2 unsplinted implants. CONCLUSION: For the maxilla there are no studies, at the present time, that can be utilized to address the question of how many implants should support an overdenture. For the mandible, it cannot be concluded that bone loss, patient satisfaction, or number of complications is significantly related to the number of implants supporting the overdenture. Furthermore, splinting two implants does not seem to offer additional value. Well conducted research is needed to identify the prognostic factors for long-term success.

Spectroscopic investigation of heterogeneous Ziegler-Natta catalysts: Ti and Mg chloride tetrahydrofuranates, their interaction compound, and the role of the activator.

X-ray powder diffraction (XRPD), Infrared, Raman, and UV/Vis spectroscopy have been used to investigate the structural, vibrational, and optical properties of Ti and Mg chloride tetrahydrofuranates as precursors of heterogeneous Ziegler-Natta catalysts for polyethylene production; as well as their interaction compound (pro-catalyst) and the final catalyst obtained after interaction with the AlR(3) activator. Although the structure of the precursors and of the pro-catalyst were well known, that of the catalyst (obtained by reaction of the pro-catalyst with AlR(3)) was not easily obtainable from XRPD data. IR and Raman spectroscopy provided important information on tetrahydrofuran (thf) coordination and on the ν(M-Cl) region; whereas UV/Vis spectroscopy gave the direct proof on both the formal oxidation state and the coordination environment of the active Ti sites. Those presented herein are among the first direct experimental data on the structure of the active Ti sites in Ziegler-Natta catalysts, and can be used to validate the many computational studies that have been increasing exponentially in the last few decades.

Surgical therapy of peri-implantitis lesions by means of a bovine-derived xenograft: comparative results of a prospective study on two different implant surfaces.

OBJECTIVES: The aim of this prospective study was to evaluate a regenerative surgical treatment modality for peri-implantitis lesions on two different implant surfaces. MATERIALS AND METHODS: Twenty-six patients with one crater-like defect, around either TPS (Control) or SLA (Test) dental implants, with a probing depth (PD) ≥6 mm and no implant mobility, were included. The implant surface was mechanically debrided and treated using a 24% EDTA gel and a 1% chlorhexidine gel. The bone defect was filled with a bovine-derived xenograft (BDX) and the flap was sutured around the non-submerged implant. RESULTS: One-year follow-up demonstrated clinical and radiographic improvements. PDs were significantly reduced by 2.1±1.2 mm in the Control implants and by 3.4±1.7 mm in the Test implants. Complete defect fill was never found around Controls, while it occurred in three out of 12 Test implants. Bleeding on probing decreased from 91.1±12.4% (Control) and 75.0±30.2% (Test) to 57.1±38.5% (p=0.004) and 14.6±16.7% (p=0.003), respectively. Several deep pockets (≥6 mm) were still present after surgical therapy around Controls. CONCLUSIONS: Surface characteristics may have an impact on the clinical outcome following surgical debridement, disinfection of the contaminated surfaces and grafting with BDX. Complete fill of the bony defect seems not to be a predictable result.

X-ray absorption spectroscopies: useful tools to understand metallorganic frameworks structure and reactivity.

The large unit cells, the enormous flexibility and variation in structural motifs of MOFs represent a big challenge in the characterization of MOF materials, particularly in cases where single crystal diffraction data are not available. In this critical review it is shown that in cases where only powder diffraction data are available additional structural information, particularly regarding local coordination within the inorganic cluster, are often mandatory in order to solve the structure. There are also cases where the inorganic cluster does not follow the symmetry of the overall structure. In such cases diffraction techniques will just "see" an average structure, missing the local structure: a lack that may be critical for understanding the specific properties of the material. In both cases, EXAFS spectroscopy is the tool that provides complementary structural information on the inorganic cluster and the way it binds to the ligand. Selected examples will show how EXAFS will be relevant in: (i) confirming the structure obtained from diffraction refinements; (ii) highlighting that the inorganic cornerstone has a lower symmetry with respect to that of the organic framework; (iii) obtaining the local structure of the inorganic cluster in the desolvated material when desolvation causes a partial loss of long range order; (iv) obtaining the local structure of the inorganic cluster in the desolvated material after coordination of a probe (or reactant) molecule, including cluster deformation upon molecule coordination and metal-molecule binding distance; (v) evidencing the presence of impurities in the form of amorphous extra-phases (339 references).

Insights on a Hierarchical MFI Zeolite: A Combined Spectroscopic and Catalytic Approach for Exploring the Multilevel Porous System Down to the Active Sites.

The hierarchization of zeolites to overcome the major drawbacks related to molecular diffusion limitation in micropores is a popular concept in heterogeneous catalysis. Despite the constant increase of new synthesis strategies to produce such hierarchical systems, the deep knowledge of their structural arrangement and how the zeolitic lattice is organized in a multilevel porous system is often missing. This information is essential to design a structure, tuning the porosity and the distribution of easily accessible active sites, and successively controlling the catalytic properties. In the present work, the synthesis of one of the most sophisticated forms of the hierarchical ZSM-5 zeolite has been reproduced, obtaining two multilevel porous materials with different crystallinity degrees, with the final aim of investigating and clarifying the finest features of their active sites. For this purpose, an extended characterization step by means of a unique multitechnique approach has been performed, thus revealing the active site nature, abundance, and distribution. IR spectroscopy with different molecular probes and a targeted catalytic test based on the hydroconversion reaction of n-decane were the toolbox for disclosing how the MFI lattice takes part in the hierarchical structure and how it, working in synergy with the mesoporous system, confers to this material a totally new shape-size selectivity. Merging the information obtained for the synthesized hierarchical zeolite with the characterization results of two reference materials (a mesoporous aluminum-containing MCM-41 and a microporous commercial ZSM-5), it was possible to define an internal and external map of the pore network of this complex and unique molecular sieve, where strong Brønsted acidic sites are located at the mouth of the MFI micropores and, at the same time, exposed at the surface of the mesoporous channels. Hence, the possibility of easily releasing bulky products is ensured and the application possibilities of the MFI lattice are expanded beyond cracking reactions.

Influence of Photon-Induced Photoacoustic Streaming (PIPS) on Root Canal Disinfection and Post-Operative Pain: A Randomized Clinical Trial.

The aim of this study was to evaluate the ability of a PIPS (photon-induced photoacoustic streaming) Er:YAG laser to reduce the root canal system bacterial count in vivo in comparison to the traditional irrigation technique. The post-operative patients' quality of life (QoL) after endodontic therapy was evaluated through a questionnaire. Fifty-four patients affected by pulp necrosis with or without apical periodontitis biofilm disease were selected for endodontic treatment and randomly assigned to Group A (n = 27) with traditional irrigation and Group B (n = 27), with PIPS irrigation applied according to the protocol. Shaping was performed with ProGlider and ProTaper Next, and irrigation was performed with 5% NaOCl and 10% EDTA. Intracanal samples for culture tests were collected before and after irrigation. The microbiological analysis was evaluated by the Kolmogorov-Smirnov normality and Mann-Whitney tests (p < 0.05). A self-assessment questionnaire was used to evaluate the QoL during the 7 days after treatment; differences were analysed with Student's t-test. Irrigation with the PIPS device was significantly effective in reducing bacterial counts, which were higher for facultative than obligate anaerobic strains, particularly for Gram-negative bacteria, without statistical significance (p > 0.05). There were no significant differences among the QoL indicators, except for the maximum pain (p = 0.02), eating difficulty (p = 0.03) and difficulty performing daily functions (p = 0.02) in the first few days post-treatment. PIPS may represent an aid to root canal disinfection not affecting the patients' QoL, particularly for the first day after treatment.