Development of a Greener Hydroformylation Process Guided by Quantitative Sustainability Assessments

Environmental impacts and economics associated with a potentially greener, Rh-catalyzed, 1-octene hydroformylation process in CO₂-expanded liquid (CXL) medium are quantitatively assessed against a conventional Co-catalyzed process. The economic analysis shows a more than 30% lower capital investment for the CXL process compared to the conventional Co-catalyzed process of similar capacity. This is due to the higher reaction and catalyst recovery efficiencies at milder reaction temperature and pressures (compared to the conventional process) used in the CXL process. The total production cost (TPC) associated with the CXL process is lower than the conventional process when the Rh makeup rate is lower than 0.94% (of the total amount of Rh in the reactor) per hour at the current Rh price ($20,800/lb). This translates to an economic viability criterion of ($makeup Rh/$TPC) being 0.042 or less. Life cycle analysis (LCA) was performed using GaBi software and an EIO-LCA method based on plant scale simulation of both the conventional and continuous CXL processes to produce 150 kton/year of nonanal. Gate-to-gate LCA projections show that the CXL process is environmentally friendlier than the conventional process in most impact categories such as ecotoxicity, greenhouse gas emissions, and smog formation. Predicted emissions for the conventional process are of the same order of magnitude as those reported from an actual plant of similar capacity. Cradle-to-gate environmental impacts are 1 to 2 orders of magnitude greater than the gate-to-gate impacts with energy usage for the production of raw materials being the major source of adverse environmental impacts. The EIO-LCA results agree with the GaBi analysis. Our results show that the environmental performance of the CXL process can be further improved with lower solvent usage, thus also providing valuable guidance for process optimization

Aqueous Phase Hydrogenation of Acetic Acid and Its Promotional Effect on <i>p</i>‑Cresol Hydrodeoxygenation

A systematic study of the comparative performances of various supported noble metal catalysts for the aqueous phase hydrogenation of acetic acid (as a model carboxylic acid in bio-oils) by itself and in combination with <i>p</i>-cresol (as a model phenolic compound in bio-oils) is presented. It was found that Ru/C catalyst shows the highest activity for acetic acid hydrogenation among the tested catalysts, followed by Ru/Al₂O₃, Pt/C, Pt/Al₂O₃, Pd/Al₂O₃, and Pd/C. CH₄ and CO₂ were observed to be the major products on all of these catalysts at typical hydroprocessing temperatures (∼300 °C). A systematic study on parametric effects with the Ru/C catalyst shows that the product distribution is dependent upon the temperature and presence of water. At low temperatures (∼150 °C), acetic acid hydrogenation is favored with higher selectivity to ethanol, while at high temperatures (∼300 °C), acetic acid decomposition and ethanol reforming/hydrogenolysis dominate with CO₂ and CH₄ as the major products. When water is replaced with <i>n</i>-heptane at otherwise similar conditions, the esterification reaction is favored over ethanol reforming/hydrogenolysis, resulting in substantial formation of ethyl acetate. With a mixed feed of acetic acid and <i>p</i>-cresol over the Ru/C catalyst, acetic acid hydrogenation is suppressed and <i>p</i>-cresol hydrodeoxygenation is favored, as inferred from the observed high selectivity to methylcyclohexane

Ultraviolet–Visible Spectroscopy and Temperature-Programmed Techniques as Tools for Structural Characterization of Cu in CuMgAlO_<i>x</i> Mixed Metal Oxides

Ultraviolet–visible (UV–vis) spectroscopy was used in combination with temperature-programmed reduction (TPR) methods to provide information about the Cu structure in CuMgAlO_<i>x</i> mixed oxides. UV–vis spectra revealed the presence of highly dispersed, isolated and oligomeric, CuO species in a distorted octahedral environment. From these spectra, optical absorption edge energies were determined and correlated with the number of nearest CuO neighbors (a measure of CuO domain size) and Cu content in the mixed oxides. Increasing the Cu content from 4 to 21 at. % increased the number of nearest CuO neighbors in oligomeric CuO from about 2 to 4 and produced materials that were more easily reducible, as inferred from TPR of untreated and N₂O-passivated CuMgAlO_<i>x</i>. A further increase in Cu content to 38 at. % increased the number of nearest CuO neighbors to 4.5 but resulted in a decrease of reducibility because of the evolution of an amorphous CuO phase in the bulk of the mixed oxides. This work represents the first demonstration of combining UV–vis spectroscopy with TPR of untreated and N₂O-passivated CuMgAlO_<i>x</i> as a relatively simple and inexpensive methodology for in-depth structural characterization of Cu in mixed metal oxides from which the composition, domain size, and relative fraction of total (surface + bulk) and surface oligomeric CuO species can be determined. A methodology that allows assessment of the extent of CuO bulk enrichment in CuMgAlO_<i>x</i> materials is also presented

Ultraviolet–Visible Spectroscopy and Temperature-Programmed Techniques as Tools for Structural Characterization of Cu in CuMgAlO_<i>x</i> Mixed Metal Oxides

Ultraviolet–visible (UV–vis) spectroscopy was used in combination with temperature-programmed reduction (TPR) methods to provide information about the Cu structure in CuMgAlO_<i>x</i> mixed oxides. UV–vis spectra revealed the presence of highly dispersed, isolated and oligomeric, CuO species in a distorted octahedral environment. From these spectra, optical absorption edge energies were determined and correlated with the number of nearest CuO neighbors (a measure of CuO domain size) and Cu content in the mixed oxides. Increasing the Cu content from 4 to 21 at. % increased the number of nearest CuO neighbors in oligomeric CuO from about 2 to 4 and produced materials that were more easily reducible, as inferred from TPR of untreated and N₂O-passivated CuMgAlO_<i>x</i>. A further increase in Cu content to 38 at. % increased the number of nearest CuO neighbors to 4.5 but resulted in a decrease of reducibility because of the evolution of an amorphous CuO phase in the bulk of the mixed oxides. This work represents the first demonstration of combining UV–vis spectroscopy with TPR of untreated and N₂O-passivated CuMgAlO_<i>x</i> as a relatively simple and inexpensive methodology for in-depth structural characterization of Cu in mixed metal oxides from which the composition, domain size, and relative fraction of total (surface + bulk) and surface oligomeric CuO species can be determined. A methodology that allows assessment of the extent of CuO bulk enrichment in CuMgAlO_<i>x</i> materials is also presented

Genesis of Strong Brønsted Acid Sites in WZr-KIT‑6 Catalysts and Enhancement of Ethanol Dehydration Activity

Using a one-pot synthesis technique, tungsten and zirconium were simultaneously incorporated into an ordered mesoporous KIT-6 framework. A series of such materials, denoted WZr-KIT-6, were synthesized with W and Zr loadings ranging from 0 to 10 mol % each. At sufficiently high Zr loadings, ssNMR spectra of neat as well as pyridine-adsorbed catalyst confirm that the fresh WZr-KIT-6 materials exhibit both Lewis and Brønsted acid sites of high strength, resulting in correspondingly high ethylene yields during ethanol dehydration. Such yields surpass those observed on W_<i>x</i>-KIT-6 and Zr_<i>y</i>-KIT-6 materials with W and Zr loadings identical with those of W_<i>x</i>Zr_<i>y</i>-KIT-6. Interestingly, air regeneration of the spent WZr-KIT-6 catalyst further enhances ethanol dehydration activity, with ethylene yields approaching those reported with HZSM-5 and SAPO-34 catalysts under similar operating conditions. This enhancement correlates with ssNMR evidence of the formation of additional strong Brønsted acid sites following the regeneration step, presumably from the water produced during combustion of the coke deposits. On the basis of ssNMR characterization of acid strength, these acidic protons are assigned to the hydroxyl groups bound to metals in W-O-Zr structures and to the stronger acidic protons on heteropolytungstate structures. The formation of strong Brønsted acid sites, comparable to those observed in H-ZSM-5 and H-Beta, in mesoporous WZr-KIT-6 materials should be particularly attractive for reactions that are prone to rapid deactivation by coking in microporous catalysts

Kinetic Investigations of <i>p</i>‑Xylene Oxidation to Terephthalic Acid with a Co/Mn/Br Catalyst in a Homogeneous Liquid Phase

Kinetic investigations of the liquid phase oxidation of <i>p</i>-xylene (<i>p</i>X) to terephthalic acid (TPA) with Co/Mn/Br catalyst were performed in a stirred 50 mL Parr reactor at 200 °C and 15 bar pressure under conditions wherein product precipitation is avoided. The oxidant (O₂) was introduced by sparging into the liquid phase at constant gas-phase O₂ partial pressure. Apparent kinetic rate constants, estimated by regressing experimental conversion data to a pseudo-first order lumped kinetic model, are at least an order of magnitude greater than those reported in the literature for similar catalytic reactions. We attribute this difference to the presence of gas–solid and liquid–solid mass transfer resistances in the previous studies wherein the TPA product precipitates as it forms, trapping intermediate products and slowing down their oxidation rates. Our results also indicate that it is not possible to completely eliminate the gas–liquid mass transfer limitations associated with the fast intermediate oxidation steps, even when operating without solids formation and at high stirrer speeds. Other types of reactor configurations are therefore needed to better overcome gas–liquid mass transfer limitations. Systematic studies of bromide concentration effects show that the observed reaction rates become zero order in bromide concentration at sufficiently high bromide levels where the elimination of intermediate 4-(bromomethyl)­benzoic acid by oxidation is favored. Further, the rate constants do not show any statistically significant dependence on <i>p</i>X concentration as suggested in other reports involving the traditional three-phase gas–liquid–solid reaction system. This again confirms that the formation of a solid phase hinders the overall oxidation rate, resulting in much smaller apparent rate constants

Criegee Intermediate Reaction with CO: Mechanism, Barriers, Conformer-Dependence, and Implications for Ozonolysis Chemistry

Density functional theory and transition state theory rate constant calculations have been performed to gain insight into the bimolecular reaction of the Criegee intermediate (CI) with carbon monoxide (CO) that is proposed to be important in both atmospheric and industrial chemistry. A new mechanism is suggested in which the CI acts as an oxidant by transferring an oxygen atom to the CO, resulting in the formation of a carbonyl compound (aldehyde or ketone depending upon the CI) and carbon dioxide. Fourteen different CIs, including ones resulting from biogenic ozonolysis, are considered. Consistent with previous reports for other CI bimolecular reactions, the <i>anti</i> conformers are found to react faster than the <i>syn</i> conformers. However, this can be attributed to steric effects and not hyperconjugation as generally invoked. The oxidation reaction is slow, with barrier heights between 6.3 and 14.7 kcal/mol and estimated reaction rate constants 6–12 orders-of-magnitude smaller than previously reported literature estimates. The reaction is thus expected to be unimportant in the context of tropospheric oxidation chemistry. However, the reaction mechanism suggests that CO could be exploited in ozonolysis to selectively obtain industrially important carbonyl compounds

Kinetic Modeling of Sorbitol Hydrogenolysis over Bimetallic RuRe/C Catalyst

Sorbitol hydrogenolysis kinetics using bimetallic RuRe catalyst is reported based on multiple experiments in parallel batch slurry reactors (H₂ pressure: 1.0–6.5 MPa, temperature: 473–503 K) to obtain concentration–time profiles. It is observed that RuRe/C bimetallic catalysts with Ca­(OH)₂ as a base promoter show significantly higher activity and selectivity toward liquid phase products such as 1,2-propanediol, lactic acid, ethylene glycol, and linear alcohols compared with monometallic Ru/C catalysts and other base promoters. It is further found that sorbitol hydrogenolysis initiates with dehydrogenation and subsequent C–C cleavage via retro-aldolization to form smaller molecules (C₂–C₄). Those smaller intermediates undergo dehydration, reorganization, and C–O cleavage to form C₂–C₃ acids, glycols, and linear alcohols as products, which are very similar to glycerol conversion chemistry. For the kinetic modeling, experimental data on concentration–time profiles were obtained using RuRe/C catalysts with Ca­(OH)₂ promoter in which H₂ pressure, catalyst loading, and temperature were varied. The analysis of kinetic models employed a batch slurry reactor model with which several rate equations based on different complex multistep reaction mechanisms were fit to the experimental data in order to gain insights into the reaction pathways and mechanisms. Activation energies for sorbitol hydrogenolysis to glycols and further conversion of glycols to corresponding alcohols are found to be in the range 38 kJ/mol to 125+ kJ/mol. The kinetic model from this work provides the framework for developing rational multiphase reactor engineering strategies for upgrading polyol mixtures (e.g., glycerol, xylitol, sorbitol, and mannitol) to value-added glycols and alcohols

Ligand Effects on the Regioselectivity of Rhodium-Catalyzed Hydroformylation: Density Functional Calculations Illuminate the Role of Long-Range Noncovalent Interactions

Density functional theory calculations have been performed to gain insight into the origin of ligand effects in rhodium (Rh)-catalyzed hydroformylation of olefins. In particular, the olefin insertion step of the Wilkinson catalytic cycle, which is commonly invoked as the regioselectivity-determining step, has been examined by considering a large variety of density functionals (e.g., B3LYP, M06-L); a range of substrates, including simple terminal (e.g., hexene, octene), heteroatom-containing (e.g., vinyl acetate), and aromatic-substituted (e.g., styrene) alkenes, and different ligand structures (e.g., monodentate PPh₃ ligands and bidentate ligands such as DIOP, DIPHOS). The calculations indicate that the M06-L functional reproduces the experimental regioselectivities with a reasonable degree of accuracy, while the commonly employed B3LYP functional fails to do so when the equatorial–equatorial arrangement of phosphine ligands around the Rh center is considered. The different behavior of the two functionals is attributed to the fact that the transition states leading to the Rh–alkyl intermediates along the pathways to isomeric aldehydes are stabilized by the medium-range correlation containing π–π (ligand–ligand) and π–CH (ligand–substrate) interactions that cannot be handled properly by the B3LYP functional due to its inability to describe nonlocal interactions. This conclusion is further validated using the B3LYP functional with Grimme’s empirical dispersion correction term: i.e., B3LYP-D3. The calculations also suggest that transition states leading to the linear Rh–alkyl intermediates are selectively stabilized by these noncovalent interactions, which gives rise to the high regioselectivities. In the cases of heteroatom- or aromatic-substituted olefins, substrate electronic effects determine the regioselectivity; however, these calculations suggest that the π–π and π–CH interactions also make an appreciable contribution. Overall, these computations show that the steric crowding-induced ligand–ligand and ligand–substrate interactions, but not intraligand interactions, influence the regioselectivity in Rh-catalyzed hydroformylation when the phosphine ligands are present in an equatorial–equatorial configuration in the Rh catalyst

Importance of Long-Range Noncovalent Interactions in the Regioselectivity of Rhodium-Xantphos-Catalyzed Hydroformylation

M06-L-based quantum chemical calculations were performed to examine two key elementary steps in rhodium (Rh)-xantphos-catalyzed hydroformylation: carbonyl ligand (CO) dissociation and the olefin insertion into the Rh–H bond. For the resting state of the Rh-xantphos catalyst, HRh­(xantphos)­(CO)₂, our M06-L calculations were able to qualitatively reproduce the correct ordering of the equatorial–equatorial (<i>ee</i>) and equatorial–axial (<i>ea</i>) conformers of the phosphorus ligands for 16 derivatives of the xantphos ligand, implying that the method is sufficiently accurate for capturing the subtle energy differences associated with various conformers involved in Rh-catalyzed hydroformylation. The calculated CO dissociation energy from the <i>ea</i> conformer (Δ<i>E</i> = 21–25 kcal/mol) was 10–12 kcal/mol lower than that from the <i>ee</i> conformer (Δ<i>E</i> = 31–34 kcal/mol), which is consistent with prior experimental and theoretical studies. The calculated regioselectivities for propene insertion into the Rh–H bond of the <i>ee</i>-HRh­(xantphos)­(propene)­(CO) complexes were in good agreement with the experimental l:b ratios. The comparative analysis of the regioselectivities for the pathways originating from the <i>ee</i>-HRh­(xantphos)­(propene)­(CO) complexes with and without diphenyl substituents yielded useful mechanistic insight into the interactions that play a key role in regioselectivity. Complementary computations featuring xantphos ligands lacking diphenyl substituents implied that the long-range noncovalent ligand–ligand and ligand–substrate interactions, but not the bite angles per se, control the regioselectivity of Rh-diphosphine-catalyzed hydroformylation of simple terminal olefins for the <i>ee</i> isomer. Additional calculations with longer chain olefins and the simplified structural models, in which the phenyl rings of the xantphos ligands were selectively removed to eliminate either substrate–ligand or ligand–ligand noncovalent interactions, suggested that ligand–substrate π-HC interactions play a more dominant role in the regioselectivity of Rh-catalyzed hydroformylation than ligand–ligand π–π interactions. The present calculations may provide foundational knowledge for the rational design of ligands aimed at optimizing hydroformylation regioselectivity