Acid-base catalysts for polycondensation of acetaldehyde in flow

Thesis: S.M., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2015.Cataloged from PDF version of thesis.Includes bibliographical references (pages 71-74).Acetaldehyde is used as a bio-oil model compound in a polycondensation reaction over two acid-base catalysts, pelletized Evonik P25 TiO₂ and an activated hydrotalcite-like compound (HTlc), to produce high molecular weight molecules in the transportation fuel range. The catalytic performance of these materials is evaluated in a gas phase, atmospheric flow system with a packed bed microreactor designed to mimic process conditions in one step of the overall bio-oil upgrading scheme. The HTlc is activated through calcination at 500 °C followed by rehydration in decarbonated H₂O, generating the active acid-base hydroxyl pairs. Materials are characterized through XRD, low temperature N₂ adsorption-desorption isotherm experiments, TGA, and XPS. In initial experiments, high conversions are achieved but all converted acetaldehyde forms carbonaceous deposits on the catalyst surfaces over a range of temperatures and residence times. When the catalyst bed is reduced by 80%, decreasing both residence time and vapor-solid contact area, high conversions are maintained and the production of liquid phase condensation products is observed on the order of seconds. While yields are low, it is promising that tuning the packed bed results in decreased deposits and generation of liquid phase products. Further adjustments of reaction parameters and catalyst activity are of interest as future work, including shorter residence times and bed lengths, co-feeding a reaction inhibitor, and specific catalyst syntheses for control over active sites.by Marcella R. Lusardi.S.M

Understanding the active sites of Mg-Al oxide catalysts in the formation of aromatic platform chemicals from acetaldehyde

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2018.Cataloged from PDF version of thesis.Includes bibliographical references (pages 129-137).The aromatization of acetaldehyde derived from bio-based feedstocks offers a sustainable route to obtaining valuable BTX (benzene, toluene, and xylenes)-like platform chemicals traditionally produced from petroleum naphtha. The acid-base properties of the catalyst must be carefully designed in order to selectively promote the multiple condensation-cyclodehydration mechanisms, yet it is not clear what the active sites actually are. To investigate this, we built a microreactor/flow system with flame ionization detector (FID)-quantification sensitivities and residence times ([tau]) on the orders of 10 ppm and 1 ms, respectively. Pure and Al-substituted (molar Mg/Al = 1, 2, and 3) MgOs were synthesized using a co-precipitation-calcination technique. The physicochemical properties of the catalysts were studied using X-ray diffraction (XRD), ²⁷Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, inductively coupled plasma atomic emission spectroscopy (ICP-AES), and N₂ adsorption-desorption experiments at 77 K. The acid-base character of the MgO-based materials was analyzed using a homemade temperature programmed desorption-mass spectrometry (TPD-MS) system as well as diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) experiments. Benzene activity on these catalysts at the reaction conditions investigated (250 °C, 10 ms [tau]) is low and is attributed to strong basic sites (low-coordinated surface O₂⁻). Two isomers (ortho- and para-) of tolualdehyde, a drop-in replacement for xylene, were found to form via the self-condensation of the acetaldehyde dimer. Condensation experiments, in-situ acid-base titration studies, and CO2 DRIFTS comparisons on the spent oxides revealed that the same active site, a medium strength M-O type basic site in specific coordination, is responsible for the formation of both isomers. The amount of Al incorporation did not alter the condensation activity, despite changing the densities of acid and base site types. However, we found that, compared to pure MgO, the Al-substituted samples shifted the selectivity towards the significantly more valuable para- isomer. This is likely due to an increase in the kinetic favorability of one of the para-forming routes, induced by the carbonyl oxygen interactions with acid sites that arise from surface-exposed Al³⁺ , cations. These results can inform next generation catalyst design for improved selectivity and active site stability with time on stream.by Marcella R. Lusardi.Ph. D

Investigation of the active Bronsted acid site for the DME carbonylation reaction in chabazite-type zeolites

Chabazite-type (CHA) zeolites (SSZ-13s) are active acid catalysts in the carbonylation of di-Me ether (DME), an important C-C bond forming reaction involved in the conversion of syngas derivs. to valuable chem. intermediates. We observe a max. in the formation rate of Me acetate, the target product, as a function of Si/Al in the SSZ-13s that cannot be explained from the standpoint of total acid site d. While the active sites for this reaction have been better studied in other zeolite topologies (MOR, FER), to date, the origin of activity in CHA remains unclear. To investigate this issue, we synthesized SSZ-13s with a range of Si/Al (5 < Si/Al < 75) to vary both Bronsted acid site d. and siting (e.g., paired vs. isolated), and evaluated the impact of these parameters on the DME carbonylation activity. Further, because the CHA structure has only one T-site, differentiating Bronsted acid sites via 1H NMR may be feasible. Three possible extraframework, monovalent cation sites exist. Only two of these sites are accessible by reactants, reducing the active Bronsted acid site candidates to the proton that sits in the 6-membered ring (MR) window, and the one oriented in the 8MR window. We detect three distinct resonances in the high-spin 1H NMR spectra on dehydrated SSZ-13s. From the reactivity and NMR analyses, coupled with divalent cation exchange expts. for specific titrn. of paired (framework Al sepd. by no more than 3 Si-O units) acid sites, we developed a model for the location of the active site in CHA. Given the unique framework topol. of CHA compared to other active, DME carbonylation zeolite topologies, this investigation provides further insights into this useful C-C bond forming reaction

Investigation of the active Bronsted acid site for the DME carbonylation reaction in chabazite-type zeolites

Chabazite-type (CHA) zeolites (SSZ-13s) are active acid catalysts in the carbonylation of di-Me ether (DME), an important C-C bond forming reaction involved in the conversion of syngas derivs. to valuable chem. intermediates. We observe a max. in the formation rate of Me acetate, the target product, as a function of Si/Al in the SSZ-13s that cannot be explained from the standpoint of total acid site d. While the active sites for this reaction have been better studied in other zeolite topologies (MOR, FER), to date, the origin of activity in CHA remains unclear. To investigate this issue, we synthesized SSZ-13s with a range of Si/Al (5 < Si/Al < 75) to vary both Bronsted acid site d. and siting (e.g., paired vs. isolated), and evaluated the impact of these parameters on the DME carbonylation activity. Further, because the CHA structure has only one T-site, differentiating Bronsted acid sites via 1H NMR may be feasible. Three possible extraframework, monovalent cation sites exist. Only two of these sites are accessible by reactants, reducing the active Bronsted acid site candidates to the proton that sits in the 6-membered ring (MR) window, and the one oriented in the 8MR window. We detect three distinct resonances in the high-spin 1H NMR spectra on dehydrated SSZ-13s. From the reactivity and NMR analyses, coupled with divalent cation exchange expts. for specific titrn. of paired (framework Al sepd. by no more than 3 Si-O units) acid sites, we developed a model for the location of the active site in CHA. Given the unique framework topol. of CHA compared to other active, DME carbonylation zeolite topologies, this investigation provides further insights into this useful C-C bond forming reaction

Identifying the roles of acid–base sites in formation pathways of tolualdehydes from acetaldehyde over MgO-based catalysts

Pure and Al-substituted MgO catalysts are studied to identify the contributions of acid-base sites in the formation of two valuable xylene analogs, ortho- and para-tolualdehydes, from an ethanol derivative, acetaldehyde. The catalyst properties are characterized through XRD, ²⁷Al MAS NMR, ICP-AES, N₂ physisorption, TPD-MS, and DRIFTS experiments. Reactivity comparisons of untreated and CO₂-titrated catalysts at 250 °C, coupled with CO₂ DRIFTS studies on fresh and spent samples, indicate the formation of tolualdehydes from intermediates is initiated through deprotonation by a medium-strength basic site in a specific, metal-oxygen (M-O)-type coordination environment. Analyses of the catalytic surface properties and reactivity, pathways of formation, and natural bond orbital (NBO) charge distribution suggest C₄ + C₄ (rather than C₂ + C₆) mechanistic steps dominate tolualdehyde production over these catalysts under the investigated reaction conditions. Isomeric selectivity to ortho-tolualdehyde is 92 and 81 mol% over pure and Al-substituted MgO catalysts, respectively. We propose that the shift in isomeric selectivity towards para- upon introduction of a proximal Lewis acidic functionality (Al³⁺/MgO) to the catalyst is caused by electron redistribution in the conjugated enolate from the γ-C (forming ortho-) towards the α-C (forming para-) due to the carbonyl-O/Lewis acid coordination. This insight provides a framework for the development of next generation catalysts that give improved reactivity in cascade reactions of C₂ feedstocks to aromatics

Facile Soft-Templated Synthesis of High-Surface Area and Highly Porous Carbon Nitrides

Mesoporous carbon nitride is synthesized in a one-pot approach using different nonionic surfactants (Pluronic F-127, Pluronic P-123, and Triton X-100) and a melamine cyanurate hydrogen-bonded complex using just water as the solvent. We obtain three-dimensional assembled nanostructures from low-dimensional carbon nitride sheets by taking advantage of supramolecular assembly of melamine and cyanuric acid, moderate interactions between the surfactant and precursors, structure directing effects of the surfactants, and the good thermal stability of the melamine cyanurate sheets formed around the micelles. Different morphologies, including sheetlike, hollow spherical, and tubular or highly porous networks, result depending upon the synthesis approach and the surfactant/precursor ratio. Pseudoternary phase diagrams map the composition of the starting solution to the resultant carbon nitride morphology. Increasing the amount of surfactant leads to a higher carbon residue (C/N ∼ 1) and large BET surface areas (≤300 m²/g). Further tuning of the synthesis parameters as well as addition of HCl produces uniformly porous nanostructures with a high porosity (up to 0.8 cm³/g), a high surface area (>200 m²/g), and yet a stoichiometric C/N ratio (∼0.75). The synthesized high-surface area carbon nitrides show improved light absorption and enhanced photocatalytic activity in a rhodamine B dye degradation reaction under visible light irradiation compared to those of bulk melamine-derived carbon nitride