Acid-base Bifunctionality Research Articles

An Efficient Bifunctional Core-Shell ZIF-90@ZIF-67 Composite as a Stable Pickering Interfacial Catalyst for the Deacetalization-Knoevenagel Tandem Reaction.

Exploiting advanced solid particles is crucial to the construction of Pickering emulsions catalysis. Recently, metal-organic frameworks (MOFs) have been used as ideal emulsifiers stabilizing Pickering emulsions for interfacial catalysis. Although Pickering emulsions stabilized by core-shell MOFs have significant importance in practical studies, to date there have been very limited reports on this topic. Herein, ZIF-90@ZIF-67, a core-shell material with simultaneous acid-base bifunctionality, was synthesized by seeded epitaxial growth. It was firstly applied as an emulsifier in Pickering emulsions to catalyze the deacetalization-Knoevenagel tandem reaction, which exhibited excellent catalytic properties and achieved extremely high yields. Additionally, ZIF-90@ZIF-67 showed high stability and remained well repeatable after five cycles. This work provides a platform for the design of structurally and functionally diverse MOF-based Pickering emulsions interfacial catalysis.

Synthesis of ethylene urea using carbon-dioxide-adsorbed titanium–zirconium mixed oxides

To address the increasing global levels of CO2 emissions, it is necessary to develop efficient strategies that can convert CO2 into useful chemicals such as carbonates, ureas, and carbamates. Hybrid systems that combine direct capture and subsequent utilization of CO2 offer a unique emission-controlling pathway for achieving a carbon-neutral society. Ti-Zr mixed oxides could be ideal candidates for CO2 adsorption owing to the high specific surface areas pore sizes. Therefore, in this study, metal oxides with desirable acid-base sites that is, TixZr(1−x)O2 with different Ti/Zr ratios were synthesized using combined sol-gel and solvothermal methods and then utilized to adsorb and synthesize ethylene urea (EU), an important precursor of pharmaceuticals and agricultural products. The results indicated that the CO2 adsorption capacity of TiO2-ZrO2 mixed oxides were higher than that of TiO2 alone at 100 kPa and 25 °C, corresponding to their higher specific surface areas and pore volumes compared to their individual oxide counterparts. CO2-adsorbed TixZr(1−x)O2 was subsequently used as a CO2 source to generate EU through a reaction with ethylenediamine (EDA) and 2-propanol. Notably, samples containing higher proportions of Zr-Ti0.3Zr0.7O2 and ZrO2 produced EU in significant amounts, owing to their acid-base bifunctionality. Overall, EU was synthesized using CO2-adsorbed TixZr(1−x)O2 as a CO2 source and as an accelerator for reacting with EDA, without requiring high-pressure or high-purity CO2.

Selective synthesis of 1,3-butadiene by vapor-phase dehydration of 1,4-butanediol over cerium oxide catalyst

Dehydration of biomass-derived 1,4-butanediol (1,4-BDO) is an attractive approach to synthesizing 1,3-butadiene (BD), which is a useful monomer for various polymers and currently produced via naphtha cracking. Cerium oxide (CeO2) with weak acid-base bifunctionality promoted the dehydration of 1,4-BDO to BD in 66% yield with 80% selectivity under the optimized conditions. The selectivity given by the CeO2 catalyst was influenced by the calcination temperatures, since the acid-base bifunctionality and reducibility of CeO2, which were examined temperature-programmed analyses, were altered. In stark contrast, the catalysts with strong acid sites exemplified by SiO2-Al2O3 and H-type mordenite (H-MOR) were inappropriate for this reaction since they produced tetrahydrofuran as a major product. The control experiments using a set of substrates suggested that 3-butene-1-ol was the main intermediate in the 1,4-BDO-to-BD dehydration, and its isomer of 2-butene-1-ol was also involved in the reaction but rather triggered the undesired side reactions to form butyraldehyde and ketones.

Integrating Bifunctionality and Chemical Stability in Covalent Organic Frameworks via One-Pot Multicomponent Reactions for Solar-Driven H2O2 Production

Multicomponent reactions (MCRs) can be used to introduce different functionalities into highly stable covalent organic frameworks (COFs). In this work, the irreversible three-component Doebner reaction is utilized to synthesize four chemically stable quinoline-4-carboxylic acid DMCR-COFs (DMCR-1-3 and DMCR-1NH) equipped with an acid-base bifunctionality. These DMCR-COFs show superior photocatalytic H2O2 evolution (one of the most important industrial oxidants) compared to the imine COF analogue (Imine-1). This is achieved with sacrificial oxidants but also in pure water and under an oxygen or air atmosphere. Furthermore, the DMCR-COFs show high photostability, durability, and recyclability. MCR-COFs thus provide a viable materials' platform for solar to chemical energy conversion.

Recyclable Zr/Hf-Containing Acid-Base Bifunctional Catalysts for Hydrogen Transfer Upgrading of Biofuranics: A Review.

The massive burning of a large amount of fossil energy has caused a lot of serious environmental issues (e.g., air pollution and climate change), urging people to efficiently explore and valorize sustainable alternatives. Biomass is being deemed as the only organic carbon-containing renewable resource for the production of net-zero carbon emission fuels and fine chemicals. Regarding this, the selective transformation of high-oxygen biomass feedstocks by catalytic transfer hydrogenation (CTH) is a very promising strategy to realize the carbon cycle. Among them, the important Meerwein-Ponndorf-Verley (MPV) reaction is believed to be capable of replacing the traditional hydrogenation strategy which generally requires high-pressure H2 and precious metals, aiming to upgrade biomass into downstream biochemical products and fuels. Employing bifunctional heterogeneous catalysts with both acidic and basic sites is needed to catalyze the MPV reaction, which is the key point for domino/cascade reaction in one pot that can eliminate the relevant complicated separation/purification step. Zirconium (Zr) and hafnium (Hf), belonging to transition metals, rich in reserves, can demonstrate similar catalytic efficiency for MPV reaction as that of precious metals. This review introduced the application of recyclable heterogeneous non-noble Zr/Hf-containing catalysts with acid-base bifunctionality for CTH reaction using the safe liquid hydrogen donor. The corresponding catalysts were classified into different types including Zr/Hf-containing metal oxides, supported materials, zeolites, metal-organic frameworks, metal-organic hybrids, and their respective pros and cons were compared and discussed comprehensively. Emphasis was placed on evaluating the bifunctionality of catalytic material and the key role of the active site corresponding to the structure of the catalyst in the MPV reaction. Finally, a concise summary and prospect were also provided centering on the development and suggestion of Zr/Hf-containing acid-base bifunctional catalysts for CTH.

Selective synthesis of 5-hydroxymethylfurfural over natural rubber–derived carbon/silica nanocomposites with acid–base bifunctionality

5-Hydroxymethylfurfural (HMF) is an important platform molecule, derived from biomass-based carbohydrates, for the production of renewable fuel additives, liquid hydrocarbon fuels, biopolymers, and specialty chemicals. In this study, a mesoporous carbon/silica (MCS) nanocomposite, as catalyst support, was prepared using a nanocomposite of natural rubber (NR) and hexagonal mesoporous silica (HMS) as a precursor. To obtain a series of acidic, basic, and bifunctional acid-base catalysts, the MCS surface was modified using post-synthesis methods in which the carbon moieties were decorated with sulfonic acid groups, whereas the silica matrix surface was grafted with 3-aminopropyl groups. The resulting materials exhibited high surface area, large pore volume, and contain some oxygen-containing functional groups. Their acid-base properties were tunable by varying the content of sulfonic acid and aminopropyl groups. The HMF was synthesized by catalytic conversion of fructose and glucose in a biphasic medium. Fructose dehydration catalyzed by sulfonated MCS catalyst (MCS-SO3H) at 150 °C for 2 h yields 56% HMF at 81% conversion. For the glucose system, the Brønsted basic sites facilitated the glucose–fructose isomerization, however, it promoted the formation of undesired humins. The acid/base ratio of bifunctional MCS catalysts (MCS-SO3H-NH2) contributes to the HMF yield and selectivity. The HMF yield of 39.4% was obtained over the MCS-SO3H-NH2 catalyst with an acid/base ratio of 0.10 at 190 °C for 1 h. In both systems, the formation of levulinic acid, a byproduct of HMF hydrolysis, was suppressed due to the hydrophobicity of the MCS catalyst. These developed catalysts are promising for the synthesis of HMF from glucose, at high substrate concentration than those reported in the literature.

Selective synthesis of dioxolane biofuel additive via acetalization of glycerol and furfural enhanced by MCM-41-alanine bifunctional catalyst

Highly active and selective acetalization catalyst, viz. alanine functionalized MCM-41 (MCM-41-16Alanine), synthesized using post-grafting method is reported. The mesoporous solid is first reacted with (3-chloropropyl)triethoxysilane (CPTES) before alanine is covalently bonded onto the hybrid support as confirmed by TGA/DTG, IR and UV-Raman spectroscopy analyses. Furthermore, the meso-ordered structure of MCM-41 is also retained after surface modification as revealed by the X-ray powder diffraction (XRD) and transmission electron microscopy (TEM) studies. The resulting bifunctional catalyst bearing both acidity and basicity exhibits good catalytic performance in the acetalization of glycerol with furfural reaction where 90% conversion with 78% selective to (2-(furan-2-yl)-1,3-dioxolan-4-yl)methanol-a valuable dioxolane biofuel additive-are achieved under optimized reaction conditions. Furthermore, the excellent catalytic performance of MCM-41-16Alanine is demonstrated when it is compared to other reported catalysts thanks to its large surface area, acid-base bifunctionality and mesopore confinement that induce synergistic effects. The catalyst also displays high catalyst reusability and no significant deterioration in catalytic activity is shown after six consecutive runs, thus offering a green pathway for synthesizing dioxolane biofuel additive with high selectivity.

Where silylene-silicon centres matter in the activation of small molecules.

Small molecules such as H2, N2, CO, NH3, O2 are ubiquitous stable species and their activation and role in the formation of value-added products are of fundamental importance in nature and industry. The last few decades have witnessed significant advances in the chemistry of heavy low-coordinate main-group elements, with a plethora of newly synthesised functional compounds, behaving like transition-metal complexes with respect to facile activation of such small molecules. Among them, silylenes have received particular attention in this vivid area of research showing even metal-free bond activation and catalysis. Recent striking discoveries in the chemistry of silylenes take advantage of narrow HOMO-LUMO energy gap and Lewis acid-base bifunctionality of divalent Si centres. The review is devoted to recent advances of using isolable silylenes and corresponding silylene-metal complexes for the activation of fundamental but inert molecules such as H2, COx, N2O, O2, H2O, NH3, C2H4 and E4 (E = P, As).

Effects of post-thermal treatments of ball-milled boron nitrides on solid base catalysis

Abstract Ball-milled hexagonal boron nitride was investigated as a solid base catalyst. The ball-milled boron nitrides were heated in a vacuum or air and used for Knoevenagel condensation. Measurements by X-ray diffraction, nitrogen adsorption, thermogravimetry-mass spectrometry, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy indicated that the heat treatment under vacuum hardly changed the physicochemical properties including crystal structure and surface composition whereas the heat treatment in air significantly altered the properties due to the formation of B2O3. The number of basic sites increased with the increase of the evacuation temperature. The boron nitrides after evacuation gave moderate product yield for Knoevenagel condensation of benzaldehyde with malononitrile at 40 °C. The boron nitrides after calcination showed different activity, dependent on the calcination temperature. The boron nitride after ball-milling, followed by calcination at 300 °C exhibited the highest activity. The catalytic activity was not directly related to the base amount and the surface areas, suggesting that acid-base bifunctionality plays an important role in Knoevenagel condensation using the ball-milled boron nitride catalysts.

Synthesis of chromene derivatives in the presence of mordenite zeolite/MIL‐101 (Cr) metal–organic framework composite as catalyst

In the present study, the synthesis of mordenite zeolite/MIL‐101(Cr) metal–organic framework (MOF) composite [MOR/MIL‐101(Cr)] using the ship in a bottle method was suggested. The properties of prepared composite and individual MOF and MOR zeolite were characterized by X‐ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), nitrogen adsorption–desorption measurement, and thermogravimetric analysis (TGA). The XRD results indicated diffraction peaks for each compound (MOR and MOF) in composite. The SEM and TEM images showed the formation of plates MOR (with size of 2.5 × 3 μm) along with spherical particles MIL‐101. The Brunauer–Emmett–Teller results showed that the surface area of the composite was smaller than individual MOF and MOR zeolite. Based on TGA plots, the hybrid zeolite/MOF composite was more thermally stable compared with the isolated MIL‐101(Cr). The composite was functionalized by post‐synthetic modification to obtain acid–base bifunctionality (H‐MOR/MIL‐101‐ED) for the synthesis of chromene derivatives. The acidity from framework Al‐O(H)‐Si sites in MOR and basicity from amine groups in MIL‐101 were obtained by post‐synthetic modification.

Porous Zirconium-Furandicarboxylate Microspheres for Efficient Redox Conversion of Biofuranics.

Biofuranic compounds, typically derived from C5 and C6 carbohydrates, have been extensively studied as promising alternatives to chemicals based on fossil resources. The present work reports the simple assembly of biobased 2,5-furandicarboxylic acid (FDCA) with different metal ions to prepare a range of metal-FDCA hybrids under hydrothermal conditions. The hybrid materials were demonstrated to have porous structure and acid-base bifunctionality. Zr-FDCA-T, in particular, showed a microspheric structure, high thermostability (ca. 400 °C), average pore diameters of approximately 4.7 nm, large density, moderate strength of Lewis-base/acid centers (ca. 1.4 mmol g-1 ), and a small number of Brønsted-acid sites. This material afforded almost quantitative yields of biofuranic alcohols from the corresponding aldehydes under mild conditions through catalytic transfer hydrogenation (CTH). Isotopic 1 H NMR spectroscopy and kinetic studies verified that direct hydride transfer was the dominant pathway and rate-determining step of the CTH. Importantly, the Zr-FDCA-T microspheres could be recycled with no decrease in catalytic performance and little leaching of active sites. Moreover, good yields of C5 (i.e., furfural) or C4 products [i.e., maleic acid and 2(5H)-furanone] could be obtained from furfuryl alcohol without oxidation of the furan ring over these metal-FDCA hybrids. The content and ratio of Lewis-acid/base sites were demonstrated to dominantly affect the catalytic performance of these redox reactions.

Direct conversion of biomass components to the biofuel methyl levulinate catalyzed by acid-base bifunctional zirconia-zeolites

A series of metal-zeolite hybrids was prepared by impregnation and deposition-precipitation methods and charaterized with different techniques. Catalytic performance of these catalysts on microwave-assisted conversion of selected carbohydrates to the fuel component methyl levulinate (ML) in methanol was studied. It was demonstrated that metal oxide content/type and acid-base bifunctionality were closely correlated with substrate conversion and ML yield, respectively. Among various as-prepared catalysts, zirconia-zeolite hybrid ZrY6(0.5) with moderate acid-base site content (0.97 & 0.08mmolg−1), high stability and porosity (average mesopore diameter: 6.2nm) exhibited superior catalytic activity. At 180°C, around 67–73%, 78%, 53% and 27% yields of ML could be achieved from monosaccharides (e.g., glucose, mannose and galactose), sucrose, starch and cellulose, respectively. The Zr-Y6(0.5) hybrid exhibited good stability, and could be reused for five times with ML yields of ≥63% from glucose.

Direct Catalytic Transformation of Biomass Derivatives into Biofuel Component γ-Valerolactone with Magnetic Nickel-Zirconium Nanoparticles.

A series of mixed oxide nanoparticles were prepared by a coprecipitation method and characterized by many techniques. Nickel-zirconium oxide catalysts and their partially reduced magnetic counterparts were highly efficient in the direct transformation of biomass derivatives, including ethyl levulinate, fructose, glucose, cellobiose, and carboxymethyl cellulose, into γ-valerolactone (GVL) without the use of an external hydrogen source, producing a maximum GVL yield of 95.2 % at 200 °C for 3 h with hydrogen-reduced magnetic Zr5 Ni5 nanoparticles (<20 nm). The acid-base bifunctionality of these nanocatalysts plays a synergic role in the synthesis of GVL in alcohols, whereas appropriate control of the nickel/zirconium molar ratio is able to improve the selectivity towards GVL (≈98 %), along with high formation rates (up to 54.9 mmol g-1 h-1 ). Moreover, the magnetic Zr5 Ni5 nanoparticles were conveniently recovered by means of a magnet for five cycles with almost constant activity.

Solid Acid-Base Bifunctional Catalysts in Organic Transformations

The tendency in recent decades is to develop environmentally friendly heterogeneous catalytic systems for organic transformations. In particular, chemical reactions cooperatively promoted by solid acid-base bifunctional catalysts with good dispersion of active sites and high catalytic performances have aroused much attention. In this review, recent advances in preparation and application of solid acid-base bifunctional catalysts for organic transformations are described, and special emphasis is paid to understand the catalytic mechanisms of the acid-base bifunctionality. Furthermore, strategies for well positioning acidic sites and basic sites to cohabitation and cooperation are briefly discussed. Keywords: Organic transformation, heterogeneous catalysis, acid-base bifunctional catalyst, cooperative reaction, cascade reaction.

Mesoporous Borated Zirconia: A Solid Acid–Base Bifunctional Catalyst

AbstractThe development and use of reusable solid catalysts for the selective organic transformation in solvent‐free or environmentally benign solvent media is the key interest of modern frontier science. Herein a facile low temperature aqueous solution based chemical route for the synthesis of mesoporous borated zirconia, an acid–base bifunctional solid catalyst, using aqueous zirconium ammonium carbonate complex and borax in presence of cetyltrimethylammonium bromide is presented. The material has a very high surface area and acidity with weak basicity. The catalytic activity of the material was investigated for the solvent‐free Knoevenagel condensation reaction of benzaldehyde/substituted benzaldehyde and malononitrile/cyano ethylacetate to confirm the acid–base bifunctionality. High yield (&gt;90 %) of the corresponding benzylidene was obtained within 15–30 min at room temperature. The evidence that the high catalytic activity is a result of acid–base bifunctionality of the synthesized borated zirconia material was further supported by performing Claisen–Schmidt condensation of benzaldehyde and acetophenone. The methodology was also extended for targeted synthesis of cinnamyl ethyl ester and coumarin or coumarin ester; and resulted in a good yield.

Synthesis of C4 olefins from n-butane over a novel VOx/SnO2–ZrO2 catalyst using CO2 as soft oxidant

Oxidative dehydrogenation (ODH) of n-butane was investigated over ZrO2, SnO2 and SnO2–ZrO2 mixed oxide supported vanadium oxide catalysts with the aim of utilizing CO2 as the soft oxidant, and to study the effect of mixed oxide support on the ODH activity and C4 olefins selectivity. The acid–base properties, reducibility, surface area and morphology of the SnO2–ZrO2 mixed oxide and its individual component oxide supported vanadium oxide samples were thoroughly studied. The mixed oxide supported vanadia sample exhibited superior acid–base bifunctionality; in particular, more numbers of medium strength acid–base sites were observed. The reduction temperature of the vanadia was observed to decrease in the case of mixed oxide supported vanadia catalysts. The mixed oxide supported vanadia sample exhibited a high conversion and product selectivity than its individual component oxide supported vanadia samples. A significant difference in the catalyst activity was noted in the presence and absence (He) of CO2 feed gas. Among various catalysts evaluated, the VOx/SnO2–ZrO2 catalyst exhibited excellent performance (22.34 and 36.63% conversion of n-butane and selectivity of C4 olefins, respectively), which is attributed to a high specific surface area, superior acid–base bifunctionality and easily reducibility of the dispersed vanadium oxide.

Aldol Condensations Using Bio-oil Model Compounds: The Role of Acid–Base Bi-functionality

The aldol condensation of acetaldehyde, acetone, and methyl ethyl ketone, which represent model bio-oil compounds, in the condensed phase, was performed using a bi-functional aluminophosphate catalyst. It was found that increasing the number and strength of the basic sites while decreasing acid sites on the catalyst caused a reduction in yield of the condensation products. In contrast, the addition of organic acids such as acetic acid at low concentrations to the catalyst-containing reaction mixture was found to increase activity. The presence of both acid and base sites was found to be necessary and their relative number was important for both the activity and selectivity of the condensation reaction. A mechanism explaining the observed phenomena was proposed.

Oxidative dehydrogenation of ethylbenzene to styrene with CO 2 over SnO 2–ZrO 2 mixed oxide nanocomposite catalysts

SnO 2–ZrO 2 nanocomposite catalysts with different compositions ranging from 0 to 100% of SnO 2 were prepared at room temperature by co-precipitation method using aqueous ammonia as a hydrolyzing agent. X-ray diffraction, transmission electron microscopic characterization revealed the SnO 2–ZrO 2 nanocomposite behavior. Acid–base properties of these catalysts were ascertained by temperature-programmed desorption (TPD) of NH 3 and CO 2. Both acidic and basic sites distribution of the nanocomposite catalysts is quite different from those of respective single oxides (SnO 2 or ZrO 2). Catalytic activity of these nanocomposite catalysts for ethylbenzene dehydrogenation (EBD) to styrene in the presence of excess CO 2 was evaluated. The change in the acid–base bi-functionality of the nanocomposite catalysts in comparison with single oxides had profound positive influence in enhancing the catalytic activity.

The effect of acid–base pairing on catalysis: An efficient acid–base functionalized catalyst for aldol condensation

Acid–base bifunctionalized heterogeneous catalysts may be capable of exhibiting reactivity not achievable with homogeneous catalysts. We investigated the effect of p K a of the acid component of mesoporous solids containing acid–base bifunctionality on their catalytic ability toward aldol condensation between 4-nitrobenzaldehyde and acetone. We found that higher levels of aldol product conversion were obtained when weaker acid components (phosphoric, carboxylic vs sulfonic) were used, an indication of the importance of the equilibrium between free acid and free base and the resulting neutralized ion pair in the catalytic capability of acid–base functionalized materials. A catalyst containing primary amine groups in combination with carboxylic acid groups (with similar chemical functionalities to proline) was found to be a superior catalyst in this aldol reaction with a number of different electrophilic components.

Palladium-based catalysts for the synthesis of alcohols

The present work intends to illustrate the effects of precursor and support composition on the structure and performance of Pd-based catalysts for the synthesis of methanol and higher oxygenated compounds. The power of the XRD technique and the reaction itself, as characterisation tools, was evidenced. The hydrogenation of carbon monoxide has been studied over palladium supported on ZnO, Zr(OH)4 and ZrO2 using PdCl2 and Pd(NO3)2 as salt precursors. Catalysts were characterised by XRD and Raman spectroscopy. The results show that activity and selectivity are strongly dependent on the salt precursor and on the acid–basic properties of the support. As seen from the CH3OH/DME ratio, the order of acidity of the supports is: Zr(OH)4>ZrO2>ZnO independently of the salt precursor used. The Pd(NO3)2/ZnO catalyst shown the highest methanol selectivity which was related to the presence of large particles of a PdZn alloy. A double bifunctionality on the catalysts for the production of higher oxygenated seems evident. On the one hand, an acid–base bifunctionality of the support seems to be needed for the formation, stabilisation, chain growth and further reactivity of intermediates such as formate, as well as for the dissociative adsorption of CO, and on the other hand, a metal-support bifunctionality, where the palladium metal particles would play the role of hydrogenating the intermediates and possibly adsorb non-dissociatively the CO.