Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 1992. / Includes bibliographical references (p. 345-356). / by Claire Ann Chanenchuk. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2008. / Includes bibliographical references (leaves 174-184). / This thesis focused on advancing the microchemical field from single device based demonstrations to systems that can perform multi-step series and parallel synthesis. Few examples of micro-separators and micro-pumps suited for miniaturized lab-on-a-chip systems for organic syntheses exist, so the first half of this thesis developed systems for these micro-unit-operations, while the second half demonstrated multistep microchemical operations enabled by these systems. In-line continuous separation devices are developed that enabled removal of unreacted reagents/byproducts, making it possible to realize a series of reactions without leaving the microreactor environment. Differences in surface forces and preferential wettability characteristics of fluoropolymers are used for phase separation. Such microseparators are used to demonstrate 100% separation of two phase flows of hexane and water, toluene and water, dichloromethane and water, and hexane and methanol. Integrated liquid-liquid extraction devices are microfabricated that performed two -phase contacting by segmented flow, followed by separation - resulting in single stage extraction. Single stage extraction of N,N-dimethylformamide from dichloromethane to water, and from diethyl ether to water is demonstrated. The development of separators allows microreactors to be connected to microseparators to form microreactor networks enabling reactions and separations in succession. The starting reagents are loaded in syringes and syringe pumps push fluid through the train of microdevices. However, this pumping scheme is limited by pressure constraints at the pump drives as well as the microseparators. Therefore, there is a need to develop in-line pumps to sustain the microdevice network. Pressure-driven flow is employed for the operation of micropumps. An enclosure with the liquid is pressurized with helium gas, causing the liquid to flow. The dynamics of pressurizing and de-pressurizing an enclosure are modeled and confirmed by experiments. Active and passive control schemes to provide constant flowrate of the liquid are developed and implemented. Different schemes are developed to use the gas pressure to manipulate the flow path of liquids. / (cont.) In one scheme, two enclosures are used together to perform as an in-line pump. The in-line pumps also acted as a buffer to prevent any disturbance propagation, and allowed the upstream and downstream to operate at different flowrates. The pump concept is demonstrated at two scales - 1) microfabricated silicon chips of 40 microliter volume and 2) using glass shell vials of 10000 microliter volume. These pumps are used along with two microseparators to demonstrate two-stage countercurrent and cross-flow liquid-liquid extraction of N,Ndimethylformamide from dichloromethane to water starting with 4.4 mole percent mixture. The in-line pumps also allowed recirculation with a constant flowrate that enabled long residence time reactions. As an example, peptide synthesis from amino acids, using the Merrifield technique is implemented. Specifically, the pentapeptide, Leuenkephalin is synthesized on different resins simultaneously as an example. A new design for the silicon microreactor for packed bed reactions is developed to enable larger catalyst loadings and offer manageable pressure drops across the packed bed even when the solid loading increased in volume during operation, as is the case with the peptide synthesis experiments. These microchips are also used to study "click chemistry" reactions to synthesize drug-candidate molecules. The packed bed microreactor experiments give higher conversions and better selectivities than batch experiments after the same amount of reaction time as the microreactor experiments provide increased relative catalyst concentration, and reduce side reactions that otherwise reduce selectivity. As an example of multi -step synthesis involving reactions and separations, the synthesis of carbamates starting from azoyl chloride and sodium azide, using the Curtius rearrangement of isocyanates is performed. This example also demonstrates parallel synthesis of analogous carbamates by introducing branching in the synthesis sequence after the isocyanate production to form microreactor networks. The second reaction involved heat decomposition of the organic azide, and performs faster when catalyzed using solid acid zeolite catalyst in a packed bed microreactor. / (cont.) Continuous operation of the microdevice network for ~ 7-10 days at flowrates of 1-5 [mu]l/min show no change in performance. The microreactor based synthesis is run at higher temperatures than conventional batch scale reactions due to the inherent safety in microreactor based production. The multiple-carbamate-synthesis microreactor network consists of five microreactors and two separators. This demonstration is the first multi-step organic synthesis involving reactions and separations, and showcased the major contributions from this thesis. The development of micro-unit-operations in this thesis has advanced the microchemical field from single device based demonstrations to systems that can perform continuous-flow multi-step series and parallel chemical synthesis. / by Hemantkumar Sahoo. / Ph.D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemical Engineering, 2017. / Vita. Page 203 blank. Cataloged from PDF version of thesis. / Includes bibliographical references (pages 181-198). / Zeolites containing Sn, Ti, Zr, or Hf heteroatoms are versatile catalysts for the activation and conversion of oxygenated molecules owing to the unique Lewis acid character of their tetrahedral metal sites. Through fluoride-mediated synthesis, hydrophobic Lewis acid zeolites can behave as water-tolerant catalysts, which has resulted in a recent surge of experimental and computational studies in the field of biomass conversion. However, many open questions still surround these materials, especially relating to the nature of their active sites. This thesis reports on studies that aim to understand the reactivity, deactivation, and active site speciation for Lewis acid zeolites. First, Hf-, Zr- and Sn-Beta zeolites are shown to effectively catalyze the coupled transfer hydrogenation and etherification of 5-(hydroxymethyl)-furfural with primary and secondary alcohols into 2,5-bis(alkoxymethyl) furans, thus making it possible to generate renewable fuel additives without the use of external hydrogen sources or precious metals. Continuous flow experiments reveal nonuniform changes in the relative deactivation rates of the transfer hydrogenation and etherification reactions. "9Sn and 29Si magic-angle spinning nuclear magnetic resonance (NMR) studies show that this deactivation can be attributed to changes in the local environment of the metal sites. Next, Lewis acid zeolites are shown to catalyze the cross-aldol condensation of aromatic aldehydes with acetone under mild reaction conditions with near quantitative yields. NMR studies with isotopically labeled molecules confirm that acid-base pairs in the Si-O-M framework ensemble promote soft enolization through a-proton abstraction. These Lewis acidic zeolites maintain activity in the presence of water and, unlike traditional base catalysts, in acidic solutions, enabling synthesis of unsaturated dicarboxylic acid esters via aldol condensation of keto esters. Finally, Lewis acid zeolite active sites are characterized by adsorbing trimethylphosphine oxide (TMPO) and recording quantitative 31P solid-state NMR spectra of the TMPO probe molecule. The 31P spectra provide active site concentrations that can be correlated to catalytic activity. Overall, the method developed can be used to characterize and quantify the active sites of low-defect Lewis acidic zeolites regardless of heteroatom identity. / by Jennifer Danielle Lewis. / Ph. D.

Thesis (Sc. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 1990. / Includes bibliographical references. / by Olayiwola Oluwemimo Oyeleye. / Sc.D.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2000. / Includes bibliographical references (leaves 230-238) and index. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Technologies for dynamic simulation of chemical process flowsheets, as implemented in equation-based dynamic simulators,allow solution of fairly sophisticated process models, that include detailed descriptions of physical phenomena along with operating policies and discrete events. Simulation of flowsheet models with this level of detail forms the basis for a wide variety of activities, such as process optimization, startup and shutdown studies, process design, batch policy synthesis, safety interlock validation, and operator training. Technologies that make these activities possible for plant-scale models include sparse linear algebra routines, robust backward difference formula time integration methods, guaranteed state event location algorithms, generation of analytical Jacobian information via automatic differentiation, efficient algorithms for consistent initialization that may also be used to analyze the index of the model equations, automatic index reduction algorithms, and path-constrained dynamic optimization methods. An equation-based dynamic process simulator takes as input the model equations that describe process behavior, along with a description of the operating policy. The input language allows for model decomposition, inheritance, and reuse, which facilitates construction of plant-scale dynamic models. Technologies like the ones mentioned above allow the simulator to then analyze the model for inconsistencies and perform calculations based on dynamic simulation, with a minimum of intervention from the engineer. This reduces both the time and numerical expertise required to perform simulation-based activities. Results, in some cases made possible or economically feasible only by the modeling support provided by a simulator, / (cont.) have been impressive. However, these capabilities apply to flowsheet models that consist only of differential-algebraic, or lumped, unit models. Sometimes behavior in a particular unit cannot be adequately described by a lumped formulation, when variation with other independent variables like distance along a PFTR, film coordinate, or polymer chain length are important. In this case, behavior is most naturally modeled with partial differential, or distributed, unit models. Partial differential equations in network flow simulations bring an additional set of mathematical and numerical issues. For a distributed model to bema thematically well-posed, proper initial and boundary conditions must be specified. Boundary condition requirements for nonlinear unit models may change during the course of a dynamic simulation, even in the absence of discrete events. Some distributed models, due to improper formulation or simple transcription errors, may be ill-posed because they do not have a mathematical property called continuous dependence on data. Finally, the model equations must be discretized in the proper manner. This thesis contributes two new analyses of distributed unit models. The first relies on the definition of a differentiation index for partial differential equations developed in this thesis. It is by design a very natural generalization of the differentiation index of differential-algebraic equations. / by Wade Steven Martinson. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2007. / Includes bibliographical references (leaves 142-149). / Inspired by the variety of organisms that are naturally desiccation tolerant, anhydrobiotic preservation potentially furnishes a means of processing and storing mammalian cells in a state of "suspended animation" at ambient conditions in carbohydrate glasses. Although there have been promising applications of this technique, especially when employing the disaccharide trehalose, the ultimate goal of room temperature long-term storage has thus far not been achieved -- at least in part owing to an incomplete understanding of the fundamental cellular damage mechanisms. Although there have been many studies examining the thermodynamics of relevance to anhydrobiotic preservation, particularly with regard to lipid phase and the effect of carbohydrates thereupon, comparatively little attention has been paid to the effect of transport kinetics on preservation success. Further, although cells are typically dried in carbohydrate solutions on a solid support, there are few studies on the role played by the support. This work seeks to help remedy such deficiencies. First, considering damage mechanisms at the individual cell level, giant liposomes were employed as a model cell system, given that the cell membrane is a key damage site. / (cont.) The influence of solid surface - lipid bilayer interactions was investigated in the presence and absence of trehalose. Two lipids were chosen in order to determine the effect of lipid phase on surface interactions: gel-phase 1,2-distearoyl-sn -glycero-3-phosphocholine (DSPC) and liquid-crystalline phase 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC). In the absence of trehalose, DSPC liposomes adsorbed to the polystyrene support surface, producing irreversible structural changes and apparent leakage of all intravesicular solute upon drying and re-hydration. Addition of trehalose significantly reduced vesicle adsorption with only transitory intravesicular solute leakage for the re-hydrated vesicles, likely owing to a transient osmotic imbalance; however, at very low moisture contents, the vesicles underwent permanent structural changes. In contrast to the results with DSPC vesicles, DLPC vesicles largely evaded adsorption and exhibited high intravesicular solute retention when dried and re-hydrated even in the absence of trehalose, despite significant internal structural changes. Next, taking a more macroscopic view, the influence of the solid support and desiccation kinetics was analyzed at the whole droplet level. / (cont.) During desiccation, sessile droplets of glass-forming carbohydrate solutions exhibit complex dynamic phenomena, including fluid flow, droplet deformation and crack formation, all of which may alter cell preservation efficacy. Two factors were identified that strongly influenced the features of the preserved giant liposome suspension droplets: the underlying surface and the liposome concentration. In particular, the surface altered the droplet shape as well as the microflow pattern - and in turn the moisture conditions encountered by the liposomesr during desiccation. A ring deposit formed when the droplets were dried on polystyrene -- as would be expected owing to the capillary flow that generally occurs in pinned droplets. In contrast, when dried on the more hydrophilic glass slide, the resulting droplets were thinner and the liposomes accumulated near their centers -- an unexpected result likely owing to the glass-forming nature of the trehalose solutions. As might be anticipated given the variations in liposome distribution, the choice of surface also influenced crack formation upon continued drying. In addition to providing a preferential path for drying, such cracks are relevant because they could inflict mechanical damage on cells. / (cont.) Liposome concentration had an even more profound effect on crack formation; indeed, while cracks were found in all droplets containing liposomes, in their absence few of the droplets cracked at all, regardless of the surface type. Given the experimentally-determined non-uniform distribution of liposomes within the sessile droplets, a finite element method model was formulated to assess the moisture content variation within desiccating trehalose solution microdroplets - both unsupported and sessile. In the unsupported droplet, a thin glassy skin was found to form at the droplet surface, which significantly hampered further evaporation owing to the extremely low diffusivity of water in trehalose glasses. Thus, residual water was essentially trapped in the droplet core for long times, preventing a transition to the glassy state there. This is significant for anhydrobiotic preservation because most liposomes, or cells, would be located in the droplet core rather than in the thin glassy skin. The sessile droplet provided another degree of complexity in that the moisture concentration was inhomogeneous not only in the direction perpendicular to the interface, but along it as well, since the glassy skin did not form uniformly, instead progressing inward from the contact line. / (cont.) In summary, surface interactions were found to play a significant role in anhydrobiotic preservation, both at the cellular level through adsorption and at the whole droplet level through their effect on distribution of suspended liposomes (or cells) and crack formation. Further, kinetic phenomena had a strong influence, again at the cellular level through transient osmotic imbalances and at the whole droplet level in the form of inhomogeneous moisture distributions. Such effects clearly merit further investigation in the development of anhydrobiotic preservation protocols. / by Dana R. Adams. / Ph.D.

Thesis (Sc. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 1963. / Vita. / Includes bibliographical references (leaves 112-113). / by Frank R. Steward. / Sc.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, February 2008. / Includes bibliographical references. / Nucleation processes are ubiquitous in nature and technology. For instance, cloud formation in the atmosphere, the casting of metals, protein crystallization, biomineralization, the production of porous materials, and separation of pharmaceutical compounds from solution are a few examples of relevant nucleation processes. One pathway for nucleation to occur is homogeneous nucleation, in which an embryo of a more stable phase forms within an original metastable medium. Homogeneous nucleation is an activated process, meaning that a free energy barrier must be overcome for the transition to take place, and the height of the free energy barrier determines the rate at which the process will occur. Despite considerable advances in both theoretical and experimental techniques to date, determining nucleation mechanisms for real systems remains a considerable technical challenge. The aim of this thesis is therefore to apply molecular simulation techniques to elucidate nucleation mechanisms in organic crystals. Specifically, the newly developed methods of aimless shooting and likelihood maximization are applied for the first time to study nucleation processes in complex and technically relevant systems. The first portion of the thesis examines polymorphism, or the ability of a material to pack in different crystal lattices whilst retaining the same chemical composition. Transformation to a more stable polymorph can readily occur in the solid state, which has broad implications in pharmaceutical processing. To date, over 160 mechanisms have been proposed for polymorph transitions in the solid state, but none have been definitively verified. A model compound, terephthalic acid, is chosen for computational studies because it is similar in size to a small molecule therapeutic and exhibits a common bonding motif for organic crystals. Using aimless shooting and likelihood maximization, the mechanism of the solid state polymorph transformation in terephthalic acid is shown to be comer nucleation. The mechanism shows that for a given nucleus size, the interfacial area between the crystalline domains is minimized, thus reducing the unfavorable surface free energy penalty required for nucleation to occur. / (cont.) Furthermore, based on the results presented, it is anticipated that corner nucleation may be a common mechanism for many polymorph transformations in hydrogen bonded crystalline materials. The second portion of the thesis investigates the mechanism of freezing a subcooled liquid to form a crystal. This phenomenon has widespread application across many technical domains. Similar studies to date on freezing have been limited to model systems, such as Lennard-Jones particles or hard spheres. Benzene is chosen as a model compound. A periodic system is constructed and aimless shooting and likelihood maximization are applied to determine the nature of the critical nucleus. Local order analysis is implemented to distinguish among solid and liquid-like molecules. Preliminary results indicate that the critical nucleus is on the order of 200-300 molecules at 50 K subcooling. This thesis demonstrates that the complementary molecular simulation techniques of aimless shooting and likelihood maximization offer fundamental insight into nucleation mechanisms in molecular crystals. Knowledge of the mechanism from likelihood maximization is essential for accurate free energies and pathway optimization methods, and it should therefore be applied in computational studies of rare events prior to free energy or rate constant calculations. Moreover, these methods provide quantitative understanding of the important physical variables that determine experimentally observable rates and can further aid in experimental design. / by Gregg Tyler Beckham. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2009. / Includes bibliographical references (p. 124-130). / Single walled carbon nanotubes (SWNT) are cylindrical sheets of graphene whose electronic structures and diameters are determined by their chiralities. Current synthetic methods produce batches of nanotubes containing a variety of electronic properties. The separation of these mixtures into semiconductors and metals will greatly enhance their utility for nanoelectronic devices. Progress has been achieved in this area at the laboratory scale through chemical and density-based handles for viable separation. While pristine SWNT of a certain electronic type are in great demand, nanotubes functionalised with specific molecules have applications in the detection of biological analytes, gas sensing and nanoenergetics. An ongoing area of interest is the concept of nanostructure-guided chain reactions, wherein the superior thermal conductivity of a nanotube is used to anisotropically enhance the combustion velocity of the energetic moiety covalently attached to its sidewall. A molecular assembly with this property has the potential to act as a nanoscale thruster and a possible source of pulsed power. This thesis therefore tackles the two research problems stated above: (a) Understand the separation of metallic/semiconducting nanotubes from a modelling perspective to gauge the phenomena from a molecular standpoint; (b) Develop a simple coarse-grained model that describes the characteristics of one-dimensional nanoenergetic materials and extracts the properties of the thermally propagated reaction wave. / (cont.) The chemical route of metal/semiconductor separation involves the preferential reaction of a diazonium salt (4-hydroxybenzenediazonium tetrafluoroborate) with metallic nanotubes. Previous experimental work has identified electron transfer from the nanotube to the diazonium molecule as the source of this selectivity. We have used this insight to develop a rate model that extracts rate constants from photoabsorption spectra. This necessitated the deconvolution of the UV-vis-nIR absorption spectra of single-walled carbon nanotubes, recorded subsequent to the reactions, into individual contributions - a complicated procedure because nanotube transition energies are closely spaced. An algorithm has been presented in this work to convert spectral data to diazonium surface coverage on representative SWNT, which were then fitted using adsorption/reaction-based rate equations. The selective reaction of a metallic nanotube with diazonium causes an increase in the density of the latter, which can be harnessed to separate it from the unreacted - and hence, less dense - semiconductors via ultracentrifugation. It is already possible to use a centrifugal field to sort surfactant-suspended carbon nanotubes by diameter in a density gradient. Through a hydrodynamic model, we seek to understand the effects of surfactant adsorption, diazonium reaction and cylindrical geometry on the density of a single walled carbon nanotube. In order to study the propagation of energetic reactions in a nanotube scaffold, we have modelled the latter by a one-dimensional chain of oscillators. / (cont.) The characteristics of thermally initiated chain reactions - e.g., wave velocity and modes of propagation- will be explored with a simple molecular dynamics model in conjunction with a Monte Carlo kernel that simulates the stochastic nature of the system. The contribution of the force field to the properties of the reaction has also been analysed through the use of harmonic and anharmonic interactions between the set of oscillators. This conceptual system helps us in formulating design parameters for the fabrication of actual nanostructures in the laboratory. / by Nitish Nair. / Ph.D.

Thesis (Sc. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 1974. / Vita. / Bibliography: leaves 569-604. / Nikolaos Athanase Peppas. / Sc.D.