Enhanced Lewis Acidity Research Articles

Lewis acid enhancement by hydrogen-bond donors for asymmetric catalysis.

Small-molecule dual hydrogen-bond (H-bond) donors such as ureas, thioureas, squaramides, and guanidinium ions enjoy widespread use as effective catalysts for promoting a variety of enantioselective reactions. However, these catalysts are only weakly acidic and therefore require highly reactive electrophilic substrates to be effective. We introduce here a mode of catalytic activity with chiral H-bond donors that enables enantioselective reactions of relatively unreactive electrophiles. Squaramides are shown to interact with silyl triflates by binding the triflate counterion to form a stable, yet highly Lewis acidic, complex. The silyl triflate-chiral squaramide combination promotes the generation of oxocarbenium intermediates from acetal substrates at low temperatures. Enantioselectivity in nucleophile additions to the cationic intermediates is then controlled through a network of noncovalent interactions between the squaramide catalyst and the oxocarbenium triflate.

Reversible Hydrogenation of Carbon Dioxide to Formic Acid and Methanol: Lewis Acid Enhancement of Base Metal Catalysts.

New and sustainable energy vectors are required as a consequence of the environmental issues associated with the continued use of fossil fuels. H2 is a potential clean energy source, but as a result of problems associated with its storage and transport as a gas, chemical H2 storage (CHS), which involves the dehydrogenation of small molecules, is an attractive alternative. In principle, formic acid (FA, 4.4 wt % H2) and methanol (MeOH, 12.6 wt % H2) can be obtained renewably and are excellent prospective liquid CHS materials. In addition, MeOH has considerable potential both as a direct replacement for gasoline and as a fuel cell input. The current commercial syntheses of FA and MeOH, however, use nonrenewable feedstocks and will not facilitate the use of these molecules for CHS. An appealing option for the sustainable synthesis of both FA and MeOH, which could be implemented on a large scale, is the direct metal catalyzed hydrogenation of CO2. Furthermore, given that CO2 is a readily available, nontoxic and inexpensive source of carbon, it is expected that there will be economic and environmental benefits from using CO2 as a feedstock. One strategy to facilitate both the dehydrogenation of FA and MeOH and the hydrogenation of CO2 and H2 to FA and MeOH is to utilize a homogeneous transition metal catalyst. In particular, the development of catalysts based on first row transition metals, which are cheaper, and more abundant than their precious metal counterparts, is desirable. In this Account, we describe recent advances in the development of iron and cobalt systems for the hydrogenation of CO2 to FA and MeOH and the dehydrogenation of FA and MeOH and provide a brief comparison between precious metal and base metal systems. We highlight the different ligands that have been used to stabilize first row transition metal catalysts and discuss the use of additives to promote catalytic activity. In particular, the Account focuses on the crucial role that alkali metal Lewis acid cocatalysts can play in promoting increased activity and catalyst stability for first row transition metal systems. We relate these effects to the nature of the elementary steps in the catalytic cycle and describe how the Lewis acids stabilize the crucial transition states. For all four transformations, we discuss in detail the currently proposed catalytic pathways, and throughout the Account we identify mechanistic similarities among catalysts for the four processes. The limitations of current catalytic systems are detailed, and suggestions are provided on the improvements that are likely required to develop catalysts that are more stable, active, and practical.

A Prediction of Proton-Catalyzed Hydrogenation of Ketones in Lewis Basic Solvent through Facile Splitting of Hydrogen Molecules.

A ketone's carbonyl carbon is electrophilic and harbors a part of the lowest unoccupied molecular orbital of the carbonyl group, resembling a Lewis acidic center; under the right circumstances it exhibits very useful chemical reactivity, although the natural electrophilicity of the ketone's carbonyl carbon is often not strong enough on its own to produce such reactivity. Quantum chemical calculations predict that a proton shared between a ketone and the Lewis basic solvent molecule (dioxane or THF) activates carbonyl carbon to the point of enabling a facile heterolytic splitting of H2 . Proton-catalyzed hydrogenation of a ketone in Lewis basic solvent is the result. The mechanism involves the interaction of H2 with the enhanced Lewis acidity of a carbonyl carbon and the free Lewis basic solvent molecule polarizes H2 and enables the hydride-type attack on carbonyl carbon, which is very strongly influenced by the proton shared between a ketone and solvent. The hydride-type attack on carbon is reminiscent of the splitting of H2 by singlet carbenes except that, in this case, a Lewis base from the surrounding environment (solvent) is necessary for polarization of H2 and acceptance of the proton resulting from the heterolytic splitting of H2 .

Propane dehydrogenation catalyzed by gallosilicate MFI zeolites with perturbed acidity

The propane dehydrogenation (PDH) behavior of gallosilicate MFI catalysts synthesized in the presence of 3-mercaptopropyl-trimethoxysilane (MPS) is investigated. MPS addition to the zeolite synthesis gel results in gallosilicate molecular sieves with reduced Brønsted acidity and enhanced Lewis acidity, as shown by combined NH3 temperature-programmed desorption (TPD), isopropylamine (IPA) TPD and IR analysis of pyridine-loaded catalysts. In particular, the gallosilicate MFI catalysts prepared using MPS provide a significant concentration of strong Lewis acid sites, which are important in controlling the selectivity of PDH. Enhanced PDH performance with higher propane conversion rates and improved propylene selectivity with limited cracking and aromatization products are obtained from the catalysts synthesized with MPS. The gallosilicate MFI materials prepared using MPS are compared with a benchmark chromia-alumina catalyst, and the gallosilicate materials have a lower rate of deactivation (>50% lower) but slightly inferior propylene selectivity (Ga-MFI: 75%; Chromia-Alumina: 85%).

New class of easily-synthesisable and modifiable organic materials for applications in luminescent devices

New class of cheap, easily-synthesisable and modifiable organic luminescent materials is presented. 1,2-phenylenediboronic acid reacts with 8-hydroxyquinoline in high yield, both in solution and under mechanochemical conditions, and forms a brightly luminescent complex (emission maxima c.a. 500 nm (solid state) – 525 nm (solution)). This very first luminescent boronic 8-oxoquinoline complex exhibits emission lifetime of ns order, which, together with the DFT results, indicates a singlet-singlet origin of the fluorescence. The emission quantum yield, determined in acetonitrile is significant (c.a. 15%) and may be increased by choice of solvent. Additionally, the impact of the enhanced Lewis acidity of boron centres in the acid moiety was investigated on a series of the fluoro derivatives of 1,2-phenylenediboronic acid reacted with 8-hydroxyquinoline. In general, the mono-oxyquinolinato complex is preferably formed, however, in the case of the tetra-fluorinated acid a doubly-coordinated complex is stable and favourable. Interestingly, all of the fluorinated complexes exhibit highly comparable luminescent properties to the parent complex. Importantly, the obtained complexes appeared to be electroluminescent, thus a working OLED was successfully produced and tested.

Synthesis of Functional Bis(pentafluoroethyl)silanes (C2 F5 )2 SiX2 , with X=H, F, Cl, Br, OPh, and O2 CCF3.

As recently shown, the introduction of pentafluoroethyl functionalities into silicon compounds is of general interest due to an enhanced Lewis acidity of the resulting species. By this means, the synthesis of previously inaccessible hypervalent silicon derivatives is enabled. While an easy access to tris(pentafluoroethyl)silanes has already been published, synthetic strategies for the selective preparation of bis derivatives are yet unknown. In this contribution, a convenient protocol for the synthesis of functional bis(pentafluoroethyl)silicon compounds is presented. These compounds represent precursors for the synthesis of pentafluoroethylated polysiloxanes. Furthermore, they prove to be resistant to oxonium cations, which is a key feature for the preparation of stable pentafluoroethylsilic acids. Treatment of dichlorodiphenoxysilane with in situ generated pentafluoroethyl lithium leads to the corresponding bis(pentafluoroethyl)silane in high yields. (C2 F5 )2 Si(OPh)2 serves as a starting material for further functionalized bis(pentafluoroethyl)silanes. These silanes have been isolated and their reactivity towards N bases studied. The pronounced Lewis acidity of the obtained compounds has been documented by the formation of octahedral adducts with nitrogen donors such as 1,10-phenanthroline and acetonitrile.

Lewis Acids as Activators in CBS-Catalysed Diels-Alder Reactions: Distortion Induced Lewis Acidity Enhancement of SnCl4.

The effect of several Lewis acids on the CBS catalyst (named after Corey, Bakshi and Shibata) was investigated in this study. While (2) H NMR spectroscopic measurements served as gauge for the activation capability of the Lewis acids, in situ FT-IR spectroscopy was employed to assess the catalytic activity of the Lewis acid oxazaborolidine complexes. A correlation was found between the Δδ((2) H) values and rate constants kDA , which indicates a direct translation of Lewis acidity into reactivity of the Lewis acid-CBS complexes. Unexpectedly, a significant deviation was found for SnCl4 as Lewis acid. The SnCl4 -CBS adduct was much more reactive than the Δδ((2) H) values predicted and gave similar reaction rates to those observed for the prominent AlBr3 -CBS adduct. To rationalize these results, quantum mechanical calculations were performed. The frontier molecular orbital approach was applied and a good correlation between the LUMO energies of the Lewis acid-CBS-naphthoquinone adducts and kDA could be found. For the SnCl4 -CBS-naphthoquinone adduct an unusual distortion was observed leading to an enhanced Lewis acidity. Energy decomposition analysis with natural orbitals for chemical valence (EDA-NOCV) calculations revealed the relevant interactions and activation mode of SnCl4 as Lewis acid in Diels-Alder reactions.

A New Approach to Non-Coordinating Anions: Lewis Acid Enhancement of Porphyrin Metal Centers in a Zwitterionic Metal-Organic Framework.

We describe a new strategy to generate non-coordinating anions using zwitterionic metal-organic frameworks (MOFs). By assembly of anionic inorganic secondary building blocks (SBUs) ([In(CO2)4](-)) with cationic metalloporphyrin-based organic linkers, we prepared zwitterionic MOFs in which the complete internal charge separation effectively prevents the potential binding of the counteranion to the cationic metal center. We demonstrate the enhanced Lewis acidity of Mn(III)- and Fe(III)-porphyrins in the zwitterionic MOFs in three representative electrocyclization reactions: [2 + 1] cycloisomerization of enynes, [3 + 2] cycloaddition of aziridines and alkenes, and [4 + 2] hetero-Diels-Alder cycloaddition of aldehydes with dienes. This work paves a new way to design functional MOFs for tunable chemical catalysis.

Synthesis of nanosized HZSM-5 zeolites isomorphously substituted by gallium and their catalytic performance in the aromatization

In this article, we describe a novel strategy for the seed-induced in-situ hydrothermal synthesis of nanosized HZSM-5 zeolites that are partially or completely isomorphously substituted by gallium, denoted as GaAlHNZ5 and GaHNZ5, respectively. The most noteworthy features of this strategy are that it significantly reduces organic templates in the synthesis process and effectively prevents the loss of gallium species compared to the traditional impregnation method. Thus, this new strategy can be considered an efficient green route for the synthesis of heteroatom HZSM-5 zeolites. In a preliminary attempt to correlate the structure, acidity and catalytic behavior of 1-hexene and FCC gasoline aromatization, various techniques, including XRD, SEM, N2 adsorption, Py-IR, 27Al MAS NMR, 29Si MAS NMR and 71Ga MAS NMR as well as 13C CP-MAS NMR, were used in the characterization of the catalysts. The results show that the samples prepared by Ga isomorphous substitution can facilitate the formation of both Brønsted and Lewis acid sites. In addition to this finding, the aromatics yield was observably increased with the increase of the Ga/Al ratio after Ga partial or complete substitution for Al in the zeolite framework. GaHNZ5 exhibited the highest aromatic yield of ca. 64.6% due to the larger amount of the weakened Brønsted acid sites and the enhanced Lewis acid sites, as well as the most appropriate B/L ratio of 4.4.

Mechanism for the Enhanced Excited-State Lewis Acidity of Methyl Viologen.

Aqueous solutions of methyl viologen (MV(2+)) exhibit anomalous fluorescence behavior. Although it has long fluorescence lifetimes in polar solvents such as acetonitrile, MV(2+) has a short fluorescence lifetime in water. Recent experiments by Kohler and co-workers (Henrich et al. J. Phys. Chem. B 2015, 119, 2737-2748) have implicated an excited-state acid/base reaction as the source of the nonradiative decay pathway. While many chemical species exhibit enhanced Brønsted acidity in their excited state, MV(2+) is the first example of a species with enhanced Lewis acidity. Using a complete active space configuration interaction (CASCI) approach, excited-state molecular dynamics simulations of aqueous MV(2+) are performed in order to test the hypothesis that MV(2+) acts as a Lewis photoacid and to elucidate a mechanism for this behavior. These simulations show that the Lewis acidity of MV(2+) is indeed enhanced by photoexcitation. On its S1 excited state, MV(2+) reacts with water to generate a hydronium ion approximately 1.5 ps after excitation. After the hydronium ion is produced, the corresponding hydroxide ion adds to MV(2+) to form a covalently bound photoproduct and, subsequently, evolves toward a conical intersection.

ChemInform Abstract: Molecular Assemblies Based on Strong Axial Coordination in Metal Complexes of Saddle‐Distorted Dodecaphenylporphyrins

AbstractReview: 48 refs.

Cooperative Lewis acidity in borane-substituted fluorophosphonium cations.

A series of aryl-difluorophosphoranes are converted to give fluorophosphonium salts [Ph2PF(o-C6X4BR2)](+) (X = H, F; R = Cy, Mes). The proximity of the two weakly Lewis acidic fluorophosphonium and borane moieties results in enhanced Lewis acid catalytic reactivity.

NiMo catalysts supported on graphene-modified mesoporous TiO2 toward highly efficient hydrodesulfurization of dibenzothiophene

The graphene-covered mesoporous TiO2 (SBET>90m2g−1) were successfully prepared by using graphene oxide (GO) as graphene source via one-step hydrothermal reduction, and then were prepared supported NiMo catalysts for hydrodesulfurization (HDS) of dibenzothiophene (DBT). The active species were incorporated by incipient impregnation. Compared with the unmodified NiMo/TiO2 catalyst, all modified catalysts exhibited higher HDS activity under mild conditions of 280°C, 2.0MPa, liquid hourly space velocity of 4h−1 and hydrogen/feed ratio of 400 (V/V). Among of them, the catalyst with the appropriate mass ratio of rGO/TiO2(0.5) showed the highest DBT conversion of 99.9%. The prepared catalysts were characterized by various techniques (N2 adsorption-desorption, XRD, Raman, Pyridine FT-IR, NH3-TPD, H2-TPR, CO uptake, SEM and TEM). The enhanced HDS activity can be ascribed to (i) The incorporation of rGO (reduced graphene oxide) induces proper enhanced Lewis acidity of the modified catalysts, which promotes the adsorption of DBT molecules and facilitates the hydrogenation route. (ii) The introduction of rGO changes the HDS product (H2S) desorption behaviors, accelerating H2S desorption rate from catalyst surface and promoting the efficiency of catalyst immediately.

Impacts of niobia loading on active sites and surface acidity in NbOx/CeO2–ZrO2 NH3–SCR catalysts

A series of NbOx/Ce0.75Zr0.25O2 catalysts for the selective catalytic reduction of NO with ammonia (NH3–SCR) were synthesized using a wetness impregnation method. The effect of niobia loading was studied in relation to the active sites and surface acidity. NH3/NO oxidation, X-ray diffraction, Infrared spectroscopy, ultraviolet-visible spectroscopy, X-ray photoelectron spectroscopy, H2 temperature-programmed reduction, O2/NH3 temperature-programmed desorption, and diffuse reflectance infrared Fourier transformed spectroscopy experiments were performed to correlate the catalyst structure and surface properties to catalytic performance after Nb2O5 modification. The catalyst with 15wt.% Nb2O5 loading showed high NH3–SCR activity and nearly 100% N2 selectivity within a broad operation temperature window (190–460°C) at a high space velocity (300,000h−1). On this catalyst, Nbn+ was mainly distributed in the form of typical monomeric and polymeric NbOx species, and was partially incorporated into the Ce0.75Zr0.25O2 lattice at the Nbn+OCen+ (Zrn+) interface. The electron redistribution effect arising from the occupation of cerium sites by Nbn+ ions promoted the formation of Ce3+ ions, oxygen vacancies and active oxygen species. This interaction was closely associated with the distribution of NbOx species which varied with niobia loading. NbOx themselves were acid sites and by attracting electrons they enhanced Lewis acid sites on CZ surface, which promoted the adsorption of NH3 and inhibited the unselective oxidation of NH3 to NOx. The increased amounts of active oxygen species over NbCZ catalysts promoted the adsorptive oxidation of NH3 to NH2 and NO to NO3− at low temperatures, and thus facilitated the reaction of ads-NH3 and ads-NO3−/NO2− species. This effect as well as the increased amount of acid sites led to good NH3–SCR performance of Nb15CZ in a wide temperature range.

Molecular assemblies based on strong axial coordination in metal complexes of saddle-distorted dodecaphenylporphyrins

In this mini-review, we have highlighted our works on metal complexes having saddle-distorted dodecaphenylporphyrin (DPP) and its derivative as ligands in the light of enhancement of the Lewis acidity of a metal center coordinated by the porphyrin. The important point through this mini-review is ill-overlap of the out-of-plane lone pairs of pyrrole nitrogen atoms with σ-orbitals of the metal center bound to the saddle-distorted porphyrin core. The enhanced Lewis acidity of the central metal ions enabled us to construct stable molecular complexes through axial coordination using metal–DPP (M(DPP)) moieties ( M = Mo V or Sn IV ) and molecular or ionic entities with Lewis-basic coordination sites, including Keggin-type polyoxometallates (POM), which are known to have weak Lewis basicity and thus hard to coordinate to metal ions. A discrete 1:2 complex with a Ru -substituted POM performs catalytic substrate oxidation reactions in organic solvents. A 1:1 complex between Sn IV ( DPP ) and a Keggin-type POM exhibited photoinduced electron transfer, in which the Sn IV ( DPP ) moiety acts as an electron donor and the POM as an electron acceptor. Besides POM, other electron acceptors, including μ3-oxo trinuclear Ru III clusters and anthraquinone, having carboxyl groups as a linker unit also formed stable complexes with DPP-metal complexes as axial ligands to perform photoinduced electron transfer. Successful photoreactions of the M(DPP)-acceptor complexes are mainly enabled by the enhanced Lewis acidity of the DPP-metal complexes for the stabilization of the assemblies and also by lowering the oxidation potential of the porphyrin ligand to gain larger driving force of electron transfer to form an electron-transfer state with avoiding intersystem crossing. The stability and photochemical behavior are in sharp contrast to those for metal complexes with planar porphyrins as ligands.

Lewis acidity enhancement of triarylborane by appended phosphine oxide groups.

A series of mono-, di-, and tri-phosphine oxide substituted triarylboranes, Mes2BAr (1), MesBAr2 (2), and BAr3 (3) (Ar = 4-(Ph2PO)-2,6-Me2-C6H2) were prepared to investigate the effect of a phosphine oxide group (Ph2PO) on Lewis acidity enhancement of triarylboranes. The X-ray crystal structure of 3 revealed peripheral decoration of Ph2PO groups with a C3-axis perpendicular to the trigonal boron center. UV/Vis absorption and PL spectra indicated a significant contribution of π(Mes or phenylene) → pπ(B) charge transfer in the lower-energy electronic transition. The reduction potential measured by cyclic voltammetry showed apparent LUMO stabilization by introduction of phosphine oxide groups, the extent of which gradually increased with the increasing number of phosphine oxide groups. Lewis acidity enhancement was also supported by the gradual increase in fluoride ion affinity in the order 3 > 2 > 1. Theoretical calculations suggest that introduction of a Ph2PO group into a triarylborane significantly enhances the Lewis acidity of the boron center via an inductive electron-withdrawing effect and this effect is additive for multiple phosphine oxide groups.

The effects of magnesium of Zn–Mg–Al additives on catalytic cracking of VGO and in situ reduction of sulfur in gasoline

The scope of the present study is to describe the cracking behavior of hydrocarbons and the reduction of sulfur in gasoline in fluid catalytic cracking (FCC) process using Zn–Mg–Al additives with varying the Mg/Al molar ratios. Experiments have been carried out on a micro-activity-test (MAT) reactor using high-sulfur vacuum gas oil (VGO) feed and zinc impregnated Mg–Al spinels as additive and the commercial cracking catalyst. It was found that Zn–Mg–Al additives exhibited enhanced Lewis acidity compared with the corresponding Zn-free Mg–Al spinels. The MAT results indicated that the addition of additives reduced the yields of liquid petroleum gas and coke at low Mg contents but increased the coke yield at high Mg contents. Overall, the additives improved the yields of gasoline and diesel. It has also been shown that the rich Lewis acidity had a positive effect on the conversion of aromatic sulfur species of gasoline and the maximum reduction of gasoline sulfur was achieved with Zn/Mg4.0Al2O3 due to the synergistic effect of basicity and Lewis acidity.

One-step synthesis of hierarchical Zn-ZSM-11 via a facile ZnO route

Zn containing ZSM-11 is of great importance for producing aromatics. Herein we report a facile route for synthesizing hierarchical Zn-ZSM-11 with olive-shaped intergrowth morphology by using ZnO as zinc source. This material exhibits excellent mesoporous properties and remarkably enhanced Lewis acid amount. More importantly, this sample shows good catalytic activity for methanol conversion.

Inelastic Neutron Scattering Studies of Methyl Chloride Synthesis over Alumina

Not only is alumina the most widely used catalyst support material in the world, it is also an important catalyst in its own right. One major chemical process that uses alumina in this respect is the industrial production of methyl chloride. This is a large scale process (650,000 metric tons in 2010 in the United States), and a key feedstock in the production of silicones that are widely used as household sealants. In this Account, we show how, in partnership with conventional spectroscopic and reaction testing methods, inelastic neutron scattering (INS) spectroscopy can provide additional insight into the active sites present on the catalyst, as well as the intermediates present on the catalyst surface. INS spectroscopy is a form of vibrational spectroscopy, where the spectral features are dominated by modes involving hydrogen. Because of this, most materials including alumina are largely transparent to neutrons. Advantageously, in this technique, the entire "mid-infrared", 0-4000 cm(-1), range is accessible; there is no cut-off at ~1400 cm(-1) as in infrared spectroscopy. It is also straightforward to distinguish fundamental modes from overtones and combinations. A key parameter in the catalyst's activity is the surface acidity. In infrared spectroscopy of adsorbed pyridine, the shifts in the ring stretching modes are dependent on the strength of the acid site. However, there is a very limited spectral range available. We discuss how we can observe the low energy ring deformation modes of adsorbed pyridine by INS spectroscopy. These modes can undergo shifts that are as large as those seen with infrared inspectroscopy, potentially enabling finer discrimination between acid sites. Surface hydroxyls play a key role in alumina catalysis, but in infrared spectroscopy, the presence of electrical anharmonicity complicates the interpretation of the O-H stretch region. In addition, the deformations lie below the infrared cut-off. Both of these limitations are irrelevant to INS spectroscopy, and all the modes are readily observable. When we add HCl to the catalyst surface, the acid causes changes in the spectra. We can then deduce both that the surface chlorination leads to enhanced Lewis acidity and that the hydroxyl group must be threefold coordinated. When we react η-alumina with methanol, the catalyst forms a chemisorbed methoxy species. Infrared spectroscopy clearly shows its presence but also indicates the possible coexistence of a second species. Because of INS spectroscopy's ability to discriminate between fundamental modes and combinations, we were able to unambiguously show that there is a single intermediate present on the surface of the active catalyst. This work represents a clear example where an understanding of the chemistry at the molecular level can help rationalize improvements in a large scale industrial process with both financial and environmental benefits.

Hydrothermal liquefaction of Chlorella pyrenoidosa for bio-oil production over Ce/HZSM-5

This paper investigated a novel hydrothermal liquefaction process of Chlorella pyrenoidosa catalyzed by Ce/HZSM-5. The chemical groups and components of the residues of C. pyrenoidosa were analyzed by Fourier transform infrared spectrometry and Gas Chromatograph-Mass Spectrometer. The crystal structure and micro surface topography of C. pyrenoidosa before and after catalytic liquefaction were characterized by X-ray diffraction and Scanning electron microscopy, respectively. The experimental results showed that the catalytic cracking effects of Ce/HZSM-5 were superior to that of HZSM-5 as a liquefaction catalyst of C. pyrenoidosa. Compared with HZSM-5, Ce/HZSM-5 has a significantly enhanced Lewis acid active center, smaller particle size, larger specific surface, and highly dispersed Ce4O7 with trivalent and tetravalent cerium in the zeolite skeleton channel that accelerate the catalytic liquefaction of C. pyrenoidosa. The rare earth modified zeolite Ce/HZSM-5 exhibits good potential and a beneficial nature for the preparation of bio-oil from microalgae with high efficiency.