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Cesium salts supported heteropoly acid for oxidation of methacrolein to methacrylic acid
Abstract
A series of novel heteropoly catalysts (H4PMo11VO40/Cs4PMo11VO40) with core shell structure were designed and synthesized for effective oxidation of methacrolein to methacrylic acid. The effects of H4PMo11VO40 supporting amount on catalytic properties were investigated. With the hydrothermal treatment temperature increased from 25 to 180 °C, the surface area of Cs4PMo11VO40 decreased from 123.6 to 7.7 m² g⁻¹, while the acidity and oxidation susceptibility of Cs4PMo11VO40 were enhanced due to its re-crystallization. The XRD results showed that the crystalline form of H4PMo11VO40 changed from triclinic to cubic form because of the guidance effect of Cs4PMo11VO40. BET, NH3-TPD and XPS results indicated that compared with bulk H4PMo11VO40, surface area and oxidation susceptibility of the supported one increased significantly, and the acidity decreased. The sui thickness of H4PMo11VO40 layer on Cs4PMo11VO40 was a key point to tune the surface area, oxidation susceptibility and acidity of catalysts. At 310 °C, the methacrolein conversion and methacrylic acid selectivity on the optimum supported catalyst were more than 85% and 75%, respectively, which were much better than those on bulk H4PMo11VO40 (39% and 46%), Cs4PMo11VO40 (15.6% and 0%) and Cs2.6H1.4PMo11VO40 (99.9% and 36.5%).
... The binding energy of 517-510 eV were assigned to V 2p 3/2 and V 2p 1/2 , respectively. The peak of V 2p 3/2 could [29][30][31]. When the V/Mo ratio was 2, the peak of V 4+ and V 5+ moved to the highest binding energy (516.5-517.4 ...
V-modified phosphomolybdic acid catalysts (V-HPMo/TiO2) supported on TiO2 were developed for selective catalytic reduction of NOx with NH3 (NH3-SCR). The V-doping could promote the catalytic activity of HPMo catalysts. When the V/Mo molar ratio is 1, the NOx conversion of V-HPMo/TiO2 can reach over 99% and N2 selectivity is close to 100% in the temperature range of 200–350 °C. After introducing 200 ppm SO2 and 4 vol.% H2O in the feed gas, the catalytic activity decreased by 15% and recovered within 2 h after cutting off SO2 and H2O. V-HPMo/TiO2 catalysts exhibit outstanding redox property and resistance to SO2&H2O. V-doping can also increase the Lewis acid sites and N2 selectivity of HPMo/TiO2 catalysts. The in-situ DRIFTS results show that NH3-SCR reaction over V-HPMo/TiO2 catalysts follows L–H mechanism at low temperature (< 150 °C) and both E-R and L–H mechanisms at high temperature (> 150 °C).
Graphical Abstract
... 50 g glucose (Sinopharm Chemical Reagent Co., Ltd., AC) was loaded in a corundum crucible, and the crucible was covered by tinfoil. [23]. 5 g HPAV was dissolved into 10 mL deionized water and 1.42 g CsNO 3 was dissolved into 10 mL deionized water. ...
Decades of heavy reliance on petroleum refineries for energy have led to perpetual environmental pollution. These emissions include sulfur oxides, nitrogen oxides, carbon monoxide, and C1–C4 light hydrocarbons. Researchers worldwide strive to convert these pollutants into value-added products using catalysts. The current review provides a comprehensive overview of valorization of C4 hydrocarbons into value added chemicals, including methacrolein, methacrylic acid, or methyl methacrylate using a green solid catalyst, Keggin-type heteropolyacids. In this review, structures and properties related to catalytic activities are discussed, with the focus on the advancements of Keggin-type heteropolyacids. The influencing factors of the process design are then discussed and succeeded by mechanistic pathways. It is a timely review to place emphasis on sustainable and green chemistry resolution, meanwhile, advocating Sustainable Development Goals No. 8 Affordable and Clean Energy and No. 12 Responsible Consumption and Production.
Depletion of fossil fuels resources has triggered more research on finding potential alternative sources that are more sustainable such as biomass waste. However, deconstruction of the complex biomass waste into desired products is too challenging and usually require a strong catalyst with a great hydrolysing property. Common acids such as sulphuric acid, hydrochloric acid and nitric acid were the most studies for biomass conversion. These liquid acids catalysts suffer low recovery and recyclability that can be overcome by a solid acid catalyst. Heteropoly acid (HPA) catalyst is the preferred choice to replace these common acid catalysts as it is known to have a strong Bronsted acid site with an oxidizing property that allows this catalyst to hydrolyse and oxidize in one-step reaction. Furthermore, heteropoly acid (HPA) catalyst can be modified into heterogenous type of catalyst by solidifying HPA to increase the surface area and recyclability. Future work of this research is necessary to improvise the previous method of the catalyst preparation and to suppress the by-products after the catalytic process. In this review, we summarize the use of HPA catalyst in the complex reaction process of biomass conversion to valuable chemical products.
CsCu0.1H2.9PMo11VO40 (CsCuHPAV) with Keggin structure was successfully produced via high-shear mixer (HSM)-facilitated precipitation method and used as a catalyst for the selective catalytic oxidation of methacrolein (MAL) to methacrylic acid...
The selective oxidation of methacrolein (MAL) to high value-added methacrylic acid (MAA) using green chemical technology is of high research value. However, the process remains challenging owing to the few...
The application of heteropoly acid catalyst in methacrolein (MAL) oxidation reaction is limited for its poor thermal stability and few active centers. Here, the supported catalyst of P-Mo-V heteropoly acid (CsNH4KPMoV) active components supported on kaolin were prepared, and were characterized by FT-IR, TG/DTA, XRD, N2-adsorption and desorption, NH3-TPD, TPO-MS and XPS. The results show that kaolin was weak acidity and the negative charge on the surface of kaolin promoted the reduction of V⁵⁺ species to V⁴⁺(VO²⁺) active species. There was a strong electron interaction between kaolin and HPVA, and chemical adsorption occurred by DFT calculations. Under optimal conditions, higher MAL conversion (81.8%) and Methacrylic acid (MAA) selectivity (88.2%) were obtained by 50CsNH4KPMoV/kaolin catalyst. The Keggin structure was maintained and the acid strength and acidity did not decrease after the catalyst was used continuously for 100 h. There was no carbon deposition in the reaction process.
A strategy combing homogeneous precipitation and hydrothermal treatment was proposed to prepare Keggin-type (NH4)Cu0.2H2.8PMo11VO40 for selective oxidation of methacrolein (MAL) to methacrylic acid (MAA). The effects of the preparation conditions on the catalyst properties and catalytic performance were investigated. Compared with traditional precipitation, homogeneous precipitation could provide a homogenized supersaturation for crystal initial nucleation, while the subsequent hydrothermal treatment process under high pressure could promote crystal led self-assembly growth of rectangular sediments. Benefiting from the advantages of this new method, the as-prepared (NH4)Cu0.2H2.8PMo11VO40 with high surface area and regular morphology had a large number of active species (VO²⁺) and acidic sites, thereby enhancing the catalytic performance of the catalyst. The MAL conversion and MAA selectivity over the optimum catalysts prepared at the hydrothermal temperature of 120 ℃ and for a time of 4 hours reached above 80%. It is believed that this work offers a new strategy for the preparation of an effective catalyst for the oxidation reaction.
In this work, vanadium-doped phosphomolybdic acids were evaluated as catalysts in green oxidation routes of terpene alcohols with hydrogen peroxide. A series of phosphomolybdic acids containing a variable load of vanadium cations (i.e., V5+ ions) were synthesized, and tested as catalysts in geraniol oxidation, the model molecule selected. All the catalysts were characterized by powder X-ray diffraction, attenuated diffuse reflectance infrared spectroscopy, UV-Vis spectroscopy, thermogravimetric analysis, N2 adsorption-desorption isotherms, scanning electronic microscopy, X-ray dispersive spectroscopy, and n-butylamine potentiometric titration. Various catalysts were evaluated; phosphomolybdic acids with general formulae H3+n PMo12-n V n O40 (n = 0, 1, 2 and 3), and common Brønsted acids (i.e., H2SO4, H3PO4, and p-toluene sulfonic acid). Among them, vanadium monosubstituted phosphomolybdic acid was the most active catalyst and selective toward epoxide. The effect of main reaction variables, such as temperature, load catalyst, and reactant stoichiometry was assessed. Evaluating the effect of substrate, it was verified that only allylic alcohols such as geraniol and nerol were successfully epoxidized, demonstrating that this is a hydroxy group-assisted reaction. The effect of vanadium doping on the physicochemical properties of the phosphomolybdic acid catalysts was evaluated and used to explain their catalytic performance.
In this chapter, a number of applications of heteropoly acids are discussed. Heteropoly acids could be used as a homogeneous catalyst in many different reactions like oxidation, multicomponent reactions, and synthesis of heterocyclic compounds. From the reported results, it is identified that heteropoly acids are efficient, green, reusable, and inexpensive catalysts, and their use has been increasingly growing. In addition to catalytic applications in organic synthesis, they have intriguing applications in photochemistry. From a technological point of view, heteropoly acids may be appropriately used instead of traditional acid catalysts such as ion exchange resins, mineral acids, or zeolites in both homogeneous and heterogeneous catalytic processes. It is important to note that their use in homogeneous systems is not an obstacle to reuse. Heteropoly acids are extensively soluble in polar solvents, but they are not soluble in nonpolar solvents such as hydrocarbons. Therefore, heteropoly acids can be recovered from a polar organic solvent by precipitation using a nonpolar solvent without any neutralization.
Methacrylic acid, an important organic chemical, is commercially manufactured starting from fossil feedstock. The decarboxylation of itaconic acid derived for biomass is a green route to the synthesis of methacrylic acid. In view of the problems existing in the researches on this route such as use of noble metal catalyst, harsh reaction conditions and low desired-product yield, we prepared a series of hydroxyapatite catalysts with different Ca/P molar ratios and evaluated their catalytic performance. The results showed that the hydroxyapatite catalyst with a Ca/P molar ratio of 1.58 had the best catalytic activity. The highest yield of MAA up to 61.2% was achieved with basically complete conversion of itaconic acid under the suitable reaction conditions of 1 equivalent of NaOH, 2 MPa of N2, 250 ℃, and 2 h. On this basis, a reaction network for the decarboxylation of itaconic acid to methacrylic acid catalyzed by hydroxyapatite was established. With the aid of catalyst characterization using X-ray powder diffraction, NH3/CO2 temperature-programmed desorption, N2 physisorption, inductively coupled plasma optical emission spectrometry, and scanning electron microscopy, we found that the distribution of surface acid sites and basic sites, crystal growth orientation, texture properties and morphology of hydroxyapatite varied with the Ca/P molar ratio. Furthermore, the change of the crystal growth orientation and its influence on the surface acidity and alkalinity were clarified.
Heteropoly acids (HPAs) are a kind of important compounds possessing acidic and redox properties, but their applications are restricted by the poor stability. In this work, the effects of vanadium, cesium, and copper modification on the thermal stability of Keggin phosphomolybdates were investigated. The structure and phase transformation of HPAs and the salts were detected by XRD and DTA-TG in the thermal decomposition process. Results showed that the HPAs would decompose into oxides as a result of loss of water. The decomposition processes of the heteropoly salts showed different characteristics. The copper-doped salts would fully decompose into oxides of the corresponding elements, while the salts containing cesium would reserve the Keggin structure as neutral salts Cs3+xHPAVx and other compositions would convert into the corresponding oxides. The activation energies of the Keggin heteropoly compounds decomposition reaction were calculated with the non-isothermal kinetic study. The results indicated that the decomposition temperature and the decomposition activation energy of CsHPAV1 increased for about 20 K and 15 kJ mol−1, respectively, in comparison with those of HPAV1.
A solid catalyst, cesium salt of 2-molybdo-10-tungstophosphoric acid (Cs 2.3 H 0.7 PW 10 Mo 2 O 40) named as Cs-3, was synthesized by a simple, cheap, clean, and eco-friendly method. The physicochemical properties of the synthesized catalyst were studied via FTIR spectroscopy, XRD, EDX, ICP-AES, SEM-TEM, and BET techniques. The precursor 2-molybdo-10-tungstophosphoric acid (H 3 PW 10 Mo 2 O 40) was easily soluble in water and other polar solvents. Moreover, their cesium salts Cs x H 3Àx PW 10 Mo 2 with Cs content in the range x ¼ 2.0-2.5 were insoluble in water and other polar solvents. The surface area of the precursor (5.483 m 2 g À1) increased after partial proton exchange by Cs + ions (111.732 m 2 g À1), and all samples with x > 1 were resistant to leaching of active components and can be recycled without obvious loss of activity. This catalyst used for the synthesis of uracil derivatives via a green route under solvent free conditions at 70 C gives higher yield within a shorter reaction time. The catalyst was found to be more active and reusable over nine runs with a negligible loss of activity.
In this work, we have explored the catalytic activity of Keggin-type heteropolyanions PMo12-n V n O40 (3+n)- (n = 0, 1, 2, or 3) in the form of sodium salts in green oxidation routes of terpene alcohols with hydrogen peroxide. Nerol was the model molecule selected to assess the impacts of the main reaction parameters, such as temperature, catalyst load, and stoichiometry of reactants. The impacts of the presence of vanadium at different proportions (i.e., V1, V2, and V3 loads/per anion) in the structure of phosphomolybdate catalysts were assessed. All the catalysts were characterized by various techniques such as powder X-ray diffraction, attenuated diffuse reflectance infrared spectroscopy, ultraviolet-visible spectroscopy, thermogravimetric analysis, isotherms of adsorption-desorption of N2 measurements of surface area, scanning electronic microscopy, energy-dispersive X-ray spectroscopy, and n-butylamine potentiometric titration. Among the catalysts assessed, Na4PMo11VO40 was the most active and selective toward epoxides. The efficiency of this catalyst in the epoxidation of different terpene alcohols was investigated. Special attention was dedicated to correlating the composition and properties of the vanadium-doped phosphomolybdic catalysts with their catalytic activity.
The supported 40 wt% (NH4)3HPMo11VO40 (APMV) catalysts over two different types of supports, supports-I (Cs3PMo12O40, Cs2.5H0.5PMo12O40, Cs4PMo11VO40 and Cs3HPMo11VO40) and supports-II (CeO2, WO3/ZrO2 and S-1 molecular sieve), were synthesized by deposition–precipitation method and further evaluated in the partial oxidation of isobutane to methacrylic acid (MAA) at 340 °C under atmospheric pressure. The fresh and used catalysts were characterized by N2 adsorption/desorption, TG/DTG, XRD, FT-IR, Raman spectroscopy, pyridine- adsorption FT-IR and NH3-TPD to investigate their structure, stability and surface acidity. The better results were obtained by impregnating 40 wt% active APMV phase onto the supports-I phase with same/similar Keggin structure, and it is believed that the formation of coherent interfaces between two structural well-matched phases with the same/similar Keggin anions greatly promotes interfacial transfer abilities of electrons and lattice oxygen, which is responsible for the efficient oxygen insertion to maximize MAA selectivity and conversion of isobutane. The current work provides a concept of phase synergy to design a promising catalyst for selective oxidation of isobutane.
A series of KxH1.1-xCu0.2Cs1(NH4)1.5PVMo11O40 (KxCuCsNH4PVA) catalysts with different x values were synthesized to catalyze the selective oxidation of methacrolein (MAL) to methacrylic acid (MAA). The effects of potassium (K) ions on both the structure and catalytic activity were studied in detail. The optimum K0.6CuCsNH4PVA exhibited a large surface area, more acid sites, and abundant active species (V4+/VO2+) in the secondary structure of the Keggin structure, consequently offering good catalytic performance. Furthermore, K ions increased the MAA selectivity at the expense of carbon monoxide and carbon dioxide (together defined as COX). Additionally, several process parameters for MAL oxidation were evaluated in the processing experiments. The effects of aldehyde impurities (formaldehyde and propanal) on the catalytic performance were investigated. Possible detrimental effects (catalyst poisoning and structural damage) of aldehyde impurities were excluded. A light decrease in MAL conversion could be attributed to the competitive adsorption of aldehyde impurities and MAL on the catalyst. Hopefully, this work contributes to the design of stable and feasible catalysts for the industrial production of MAA.
Methacrylic acid (MAA) is a specialty monomer for poly methyl methacrylate (PMMA). Partial oxidation of 2–methyl–1,3–propanediol (2MDPO) to MAA is an alternative to commercial technology with fewer process steps, less cost and toxic feed stocks, and longer catalyst lifetime. Here, we evaluated the effect of 2MPDO and oxygen concentrations, reaction temperature, and contact time on product selectivity and 2MPDO conversion over VOCu0.5/Cs(NH4)2PMo12O40. Higher temperature increases selectivity and conversion; MAA selectivity reached 46~ at 69~ 2MPDO conversion. Higher 2MPDO to O2 ratio favours higher MAA selectivity due to methacrolein (intermediate) oxidation to MAA (a higher ratio favours consecutive oxidation) or lower combustion of MAA to COx. Shorter contact times decrease MAA selectivity. The Mars van Krevelen model characterizes the experimental data better than both the Langmuir‐Hinshelwood or Eley‐Rideal models; The reaction sequence involves both direct and indirect reactions in which 2MPDO reacts to methacrolein (MAC) as an intermediate and then it oxidizes toMAA(indirect) and/or 2MPDO directly oxidizes to MAA but the indirect reaction rate to MAA is 50 times faster than the direct reaction rate. The reaction is first order with respect 2MPDO and oxygen, and the rate‐limiting step is 2MPDO activation on oxidized sites.
Heteropolyacid nanoparticles (NPs) were supported on Cs-modified three-dimensionally ordered macroporous (3DOM) SiO2 and used as the catalyst for the oxidation of methacrolein (MAL) to methacrylic acid (MAA). Hydrothermal treatment and incipient wetness impregnation were employed respectively for the Cs-modification. It was found that hydrothermal Cs-modification of 3DOM SiO2 promoted the dispersion of the supported heteropolyacid, which showed an average particle size of 5.2 nm, much smaller than that (17.6 nm) on the Cs-modified 3DOM SiO2 prepared by incipient wetness impregnation. The effects of hydrothermal treatment on the structure and catalytic performance of the catalyst were investigated. Results showed that the ion exchange between Cs⁺ and the surface silanol groups on 3DOM SiO2 was promoted with the increase of the hydrothermal temperature. Meanwhile, Cs-modification helped the heteropolyacid to retain intact Keggin structure, inhibiting the formation of MoO3. Highly dispersed heteropolyacid NPs with enhanced structural stability exhibited excellent selectivity to MAA in the oxidation of MAL.
Ordered mesoporous silica, SBA-15 and MCM-41, and three-dimensionally ordered macroporous SiO 2 were used as the supports of H 4 PMo 11 VO 40 heteropolyacid for methacrolein oxidation. The dispersion and structural evolutions of the heteropolyacid along with thermal treatment were investigated. It was found that the heteropolyacid entered the one-dimensional mesoporous channels of SBA-15 and MCM-41, and the crystallization and growth were limited, leading to high dispersion of the heteropolyacid. However, the thermal stability was decreased under high dispersion. The migration of the heteropolyacid was observed to the end of the one-dimensional channels of SBA-15 and the outer surface of MCM-41 with calcination, accompanied by the decomposition of the heteropolyacid and the formation of MoO 3 . In comparison, the crystallization and growth of heteropolyacid were not limited in the open macropores of three-dimensionally ordered macroporous SiO 2 . Dispersed particles on the surface of the macropores with size of about 5 nm exhibited a higher thermal stability. The decomposition of the heteropolyacid in the SBA-15 and MCM-41 supported catalysts resulted in the loss of strong acid sites, causing low selectivity to methacrylic acid in methacrolein oxidation. High thermal stability with high exposure of the active sites in the three-dimensionally ordered macroporous SiO2 supported catalyst contributed to the enhancement in the catalytic performance.
In this work, an innovative and green oxidative process to convert renewable and abundant raw material to more value‐added fine chemicals was developed. Hydrogen peroxide, an environmentally benign oxidant, was used in Cs–exchanged lacunar Keggin heteropolyacid salts‐catalyzed oxidation reactions of the terpenic alcohols (i. e., nerol was the model molecule). The activity of most outstanding catalyst (Cs8SiW11O39) was compared with various catalysts. Epoxides and aldehydes, which are an attractive feedstock for fragrance and pharmaceutical industries were selectively obtained. Lacunar Keggin heteropolyacid salts containing cesium as the counter ion (i. e., Cs7PW11O39, Cs7PMo11O39 and Cs8SiW11O39) were easily synthesized and characterized by infrared spectroscopy, powdered X‐rays diffraction, X‐rays dispersive spectroscopy, scanning electronic microscopy, thermal analysis, porosimetry analyses, diameter and pores distribution. The influences of the main reaction variables such as catalyst concentration, the stoichiometry of the reactants and reaction temperature were evaluated. The reaction scope was successfully extended to the other terpenic alcohols. The Cs8SiW11O39 was the most active and selective lacunar salt. After five cycles of successive reuse, no loss of catalytic activity or selectivity was observed.
The involvement of water in the selective oxidation of MAL to MAA over a pure Keggin-type H3PMoO12O40 catalyst was investigated using an in-situ DRIFTS reactor coupled with a mass spectrometer for the first time to elucidate the reaction pathway associated with water. By comparing the spectra and activity data using D2O instead of H2O during transient switching experiments has allowed us to evaluate the possible active sites where D2O is activated. It has been found that, during the cycling switches of D2O in and out of MAL+O2 gas feed at 320 C, the formation of MAA-OD product is increased and decreased when D2O is added and removed, respectively. This suggests that the deuterium from D2O is involved in the production of gas phase MAA-OD. In addition, the in-situ DRIFTS-MS results obtained from the isotopic switches between D2O and H2O reveal changes in the characteristic infrared bands of Keggin unit between 1200 and 600 cm-1. It is found that the isotopic exchange possibly occurs on the bridging oxygen of MoOMo unit, where water is activated for the formation of MAA. Based on the in-situ DRIFTS-MS analysis from the transient switching experiments, the reaction mechanism associated with the effect of water on the selective oxidation of MAL to MAA over Keggin-type H3PMoO12O40 catalyst is proposed.
Molybdovanadylphosphoric acid (HPMV) was supported on a carbon nitride-modified SBA-15 (CN-SBA-15) molecular sieve to enhance its catalytic performance for oxidation of methacrolein (MAL) to methacrylic acid (MAA). HPMV/CN-SBA exhibited increased catalytic activity (20%) and five times greater MAA selectivity (98.9%) compared to bulk HPMV. HPMV supported on CN-SBA-15 exhibited much better catalytic performance as compared to that on other supports, such as KIT-6, HY zeolite, TiO2, Al2O3, SiO2, CNTs, and NH3-modified CNTs. The supported HPMV was well characterized by FT-IR, XRD, SEM, N2 physical desorption, TG-DTA, NH3-TPD, CO2-TPD, XPS, and solid-state NMR. The CN minimized the interaction between the silica support and HPMV. HPMV was successfully separated from SBA-15, which was restricted by CN to increase stability and prevent interaction between the catalysts and support that would lead to decomposition of the catalysts during calcination and reaction. HPMV reacted with amino groups on the CN, which improved MAA selectivity and enhanced the thermal stability of the supported heteropoly acid (HPA) catalysts. This work identifies a new approach to preparing highly efficient and stable supported HPA catalysts for oxidation reactions.
A series of polyaniline supported heteropoly acids were prepared through a simple method at room temperature. The obtained heterogeneous catalysts were comprehensively characterized by powder FTIR spectroscopy, UV-vis spectra, NH3 temperature programmed desorption (TPD) and scanning electron microscopy (SEM). The influence of various process parameters such as heteropoly loading (10 to 25 wt%), catalyst amount (3–5%), molar ratio of n-butanol to citric acid (3 to 5), and reaction time (3.5–12 h) have been investigated over heteropoly/polyaniline catalysts with the aim to maximize citric acid conversion and tributyl citrate selectivity. The different catalytic tests has shown that the catalyst exhibits high conversion and selectivity by using the as-prepared heteropoly/polyaniline catalysts for esterification under appropriate conditions. The present method of using 20% heteropoly/polyaniline catalyst for the synthesis of tributyl citrate would be environmentally benign in the reusability of catalyst.
An environmentally-benign, efficient and inexpensive high-surface-area barium hexa-aluminate (BaAl12O19, BHA) was developed as a catalyst for the decarboxylation of the biomass-derived itaconic acid (IA) to bio-based methacrylic acid (MAA). A maximal 50% final yield of MAA with a high product selectivity was obtained under relatively mild synthesis reaction conditions (250 °C; 20 bar N2). The reported selective MAA production was elevated, operating process characteristics were significantly less harsh, and no depleting critical raw materials were utilized when paralleled to the procedures with alkaline mineral bases, noble metal-containing heterogeneous catalysis systems and unrenewable feed resources (e.g. isobutene), applied previously. It was found that the doping of palladium on BHA support (Pd@BHA) did not improve MAA productivity. The effect of the time (25–300 min), temperature (175–275 °C), pressure (10–40 bar), reacting substrate concentration (0.10–0.19 mol L–1), metallic oxide mass (0.5–3.0 g) and type on IA conversion, MAA content MAA content and rates was determined, examining also recyclability. BHA catalyst was characterized with various structural techniques, such as energy-dispersive X-ray spectroscopy (EDS), X-ray powder diffraction (XRD), CO2 temperature-programmed desorption (TPD), scanning electron microscopy (SEM) and N2 physisorption.
Gas–phase oxidation of methacrolein to methacrylic acid was carried out over an acid-modified Cr 2 O 3 /SiO 2 catalyst. While only total oxidation occurred over bare Cr 2 O 3 /SiO 2 , the acid-modified Cr 2 O 3 /SiO 2 showed catalytic activity for the formation of methacrylic acid. In particular, H 3 PW 12 O 40 strong Brønsted acid was the most effective modifier for improving both activity and selectivity. The interface between Cr 2 O 3 and H 3 PW 12 O 40 particles on SiO 2 appears to be responsible for the formation of active sites for the selective formation of methacrylic acid. The strong Brønsted acid would help the activation of methacrolein through rendering it more electrophilic, which is a key step for the formation of methacrylic acid over the present catalyst.
Sec-amines catalyzed two-step Mannich reaction for preparation of methacrolein from propionaldehyde and formaldehyde was more attractive than the C 4 selective oxidation route. Herein, a novel continuous fixed-bed process was developed for direct preparation of methacrolein from above stocks with catalysis of sec-amine grafted D301 resin. The effect of resin, sec-amine structure and grafting percentage on catalytic performance of the modified resin were detailed investigated and demonstrated by using FT-IR, TG-DTA, titration and solid-state NMR characterization methods before the optimal conditions were obtained. The grafting percentage of amine with abundant sec-amino groups onto D301 resin could reach 93.7% at optimal condition. With this new strategy, the yield and selectivity of methacrolein could attain 40.2% and 99.3%. In addition, the Mannich reaction step with activation energy of 66.3 kJ/mol was determined as the rate-controlling step through mechanism-based kinetic calculation.
A series of valuable para‐quinones, such as 2,3‐ and 2,6‐dimethyl‐1,4‐benzoquinones (Me2BQs), were synthesized via oxidation of the corresponding 2,3‐ and 2,6‐dimethylphenols by using homogeneous high‐vanadium solutions of heteropoly acids: H11P3Mo18V8’O87 (HPA‐8’) and H17P3Mo16V10’O89 (HPA‐10’). These catalysts were shown to be capable of interdependent and reversible change of their acid and oxidative properties, the efficient regeneration of which can be performed for 25 min. The complete optimization of reaction conditions allowed one to reach the selectivity of the desired Me2BQs in 95–97% and shorten the oxidation time. The structural integrity of heteropoly anions present in the catalyst solutions was found to persist during the repeated performing substrate oxidation and catalyst regeneration. The catalysts showed the preservation of stability and efficiency for 25 cycles, demonstrating the prospectivity of high‐vanadium heteropoly acids in oxidation of organic substances.
Cu-doped three-dimensionally ordered macroporous (3DOM) SiO2 was prepared by a colloidal templating method and used as the support of heteropoly acid for the selective oxidation of methacrolein (MAL) to methacrylic acid (MAA). The doping effects of Cu on the structures, physico-chemical properties and catalytic performances of the catalysts were investigated. Results showed that Cu was well distributed in the SiO2 framework with the mole ratio of Cu/Si = 5%, and the specific surface area was much higher than that of the pure 3DOM SiO2. After supported with H4PMo11VO40 (HPVA), the specific surface areas of the as-synthesized catalysts significantly decreased, from 239 to 64 m² g⁻¹ for HPVA/3DOM (5%Cu)–SiO2 and from 141 to 32 m² g⁻¹ for HPVA/3DOM SiO2, respectively, because of the blockage of the micropores. The supported HPVA retained Keggin-type primary structure and its reducibility was obviously improved due to the existence of Cu in the supports, which thereby resulted in the great enhancement of the catalytic property. The conversion of MAL and the selectivity to MAA were respectively 36 and 47% on HPVA/3DOM SiO2. Comparatively, the conversion of MAL reached 63% with 82% MAA selectivity on HPVA/3DOM (5%Cu)–SiO2.
Graphical Abstract
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The Chitin-CsH3PMo11VO40 (Ch-CsPAV) hybrids was prepared by a facile one-pot approach and characterized by FT-IR, XRD, SEM, TG/DTA, NH3-TPD, XPS and NMR. The performance of Ch-CsPAV as a catalyst on the oxidative transformation of methacrolein (MAL) to methacrylic acid (MAA) was studied in this study. It was found that both acidity and redox property of the precusor (CsPAV) could be regulated upon introducing chitin as a component. Specifically, the resulted Ch-CsPAV with uniform microspheres had higher amount of acidic sites, better redox active sites (VO²⁺) and improved agent (NH4⁺), thereby rendering an improved catalytic performance in the reaction launched. Under optimized conditions, higher MAL conversion (80%) and MAA selectivity (94%) were attained in the presence of Ch-CsPAV.
In addition, the reaction mechanism of the conversion of MAL to MAA was proposed by in situ FT-IR. The hybrid catalyst showed good stability by the long-term stability test.
The challenges in the chemical processing industry today are environmental concerns, energy and capital costs. Catalytic distillation (CD) is a green reactor technology which combines a catalytic reaction and separation via distillation in the same distillation column. Utilization of CD in chemical process development could result in capital and energy savings, and the reduction of greenhouse gases. The efficacy of CD and the economic merits, in terms of energy and equipment savings, brought by CD for the production of biodiesel from waste oil such as yellow grease is quantified. Process flow sheets for industrial routes for an annual production of 10 million gallon ASTM purity biodiesel in a conventional process (reactor followed by distillation) and CD configurations are modeled in Aspen Plus. Material and energy flows, as well as sized unit operation blocks, are used to conduct an economic assessment of each process. Total capital investment, total operating and utility costs are calculated for each process. The waste oil feedstock is yellow grease containing both triglyceride and free fatty acid. Both transesterification and esterification reactions are considered in the process simulations. Results show a significant advantage of CD compared to a conventional biodiesel processes due to the reduction of distillation columns, waste streams and greenhouse gas emissions. The significant savings in capital and energy costs together with the reduction of greenhouse gases demonstrate that process intensification via CD is a feasible and new green process for the biodiesel production from waste oils.
Au nanoparticles supported on Ce-Zr oxides catalysts were prepared and characterized in order to study the role of support for the oxidative esterification of aldehydes in the presence of molecular oxygen. Ce-Zr solid solutions were synthesized by using (NH4)2Ce(NO3)6 as precursor, while the mixed oxides were obtained by Ce(NO3)3 precursor. The solid solutions exhibited smaller crystallite size, higher BET surface area, and larger amount of H2 consumption, acidity and basicity than the mixed oxides at the same Ce/Zr mole ratio due to the incorporation of Zr4+ into ceria lattice. The effect of the support was investigated owing to all samples presenting the similar Au particle size confirmed by HAADF-STEM study. Supports with higher reducibility showed better performances by facilitating the β-H elimination of hemiacetal to form the ester. We also found that the formation of hemiacetal was enhanced by acidic sites and basic sites of Au catalysts supported on solid solutions possessing similar reducibility. Plausible reaction mechanism for oxidative esterification of aldehydes on Ce-Zr solid solution supported Au nanoparticles was proposed. The screening catalyst was also applicable to the oxidative esterification of different benzyl aldehydes with high yields. The catalyst could be reused after a simple separation for eight times keeping high selectivity above 99%. by acidic sites and basic sites of Au catalysts supported on solid solutions possessing similar reducibility. The screening catalyst was also applicable to the oxidative esterification of different benzyl aldehydes with high yields. The catalyst could be reused after a simple separation for eight times keeping high selectivity above 99%.
A highly efficient Pd2Pb8/alumina catalyst was prepared, which provided the highest turnover number (TON) of 302 for aerobic oxidative coupling of methylacrolein with methanol. The enhanced catalytic efficiency could be attributed to the multi-scale (micron, nano and atom scales) promoting effects of the pre-loaded Pb species.
The bulk structural evolution of a vanadium-containing heteropolyoxomolybdate (HPOM), H4[PVMo11O40] × 13H2O, with vanadium substituting for Mo in the Keggin ion, was studied under reducing (propene) and partial oxidation reaction conditions (propene and oxygen) by in situ X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) combined with mass spectrometry. During treatment in propene, the loss of crystal water in the temperature range from 373 to 573 K was followed by a partial decomposition, reduction of the average Mo valence, and formation of a characteristic cubic HPOM at 573 K. This behavior is similar to the structural evolution of H3[PMo12O40] ×13H2O during treatment in propene. The formation of cubic Mox[PVMo11−xO40] with Mo centers on extra Keggin framework positions and V centers remaining in the lacunary Keggin ion coincides with the onset of catalytic activity at ∼ 573 K. Detailed investigations of the local structure around the vanadium centers in Mox[PVMo11−xO40] have made it possible to propose a model for the geometric structure of the active site in Mo- and V-containing metal oxide catalysts. The cubic Mox[PVMo11−xO40] phase prepared from H4[PVMo11O40] × 13H2O is stable in propene and oxygen up to ∼ 620 K and exhibits an onset of activity at ∼ 573 K. This onset of activity is correlated to characteristic changes in the average local Mo structure, indicating a reversible transition from the reduced state of the active site in Mox[PVMo11−xO40] to an oxidized state under propene oxidation reaction conditions.
The thermal behavior of 12-molybdophosphoric acid (PMo12H) was investigated using coupled techniques (IR and Raman spectroscopies, polarography, X-ray diffraction, and catalytic reactivity in methanol oxidation) and compared with results obtained with 12-molybdosilicic acid. The decomposition of PMo12H occurs in a wide temperature range, leading to mixtures of β- and α-MoO3. A great analogy between the P and Si compounds is evidenced outside the decomposition range. The differences inside this range (wide for P, narrow for Si compound) are discussed in terms of kinetics of decomposition, textural evolution, and capability to rebuild the Keggin unit.
The chemical and structural changes undergone by the Keggin-type compound Cs(NH4)2PMo12O40 with increasing time on stream under the conditions for methacrolein to methacrylic acid conversion have been studied by thermal analysis and in situ X-ray powder diffraction, which allowed the structure of the working catalysts and the changes involved in the catalyst ageing processes to be determined. The results show that, under the operating conditions of the catalytic reaction, restructuring of the (PMo12O40)3− Keggin anion takes place, leading to formation of the reduced heteropolyanions (PMo13O41)− and (PMo14O42)−. The six-electron reduced polyanion (PMo14O42)− represents the end member of a series of catalytically active and thermally stable species which, by further reduction, decompose into catalytically less active MoO3. Performance of catalysts derived from the above cesium-containing heteropolyacids is also compared to that of the anhydrous acid H3PMo12O40 and of the anhydride (MoOx)0.5(PMo14O42) (x≃2), for which a detailed structural analysis was carried out.
Dehydration behavior and thermal stabilities of various 12-molybdophosphates were investigated by DTA, TG and X-ray diffraction methods. The dehydration of all compounds was completed below 300°C. In addition to the three known types of crystal structures of 12-molybdophosphates, i.e., the types with high-(I), medium-(H), and low-(HI) water contents, X-ray diffraction studies of the dehydration process revealed the existence of a new low-water content type (X) and an amorphous type (A). Based on such structural information, the dehydration behavior was classified into the following three groups: Group 1 (salts of K+, Rb+, Cs+, Tl+, and NH4+) which was prepared as type HI and on heating lost its zeolitic water in one step (80~90PC); Group 2 (salts of Li+, Na+, and Ag+) which was prepared as type I and underwent dehydration in two steps (90'100°C and 150~250°C) resulting in structural changes I -∗I['Ä and Group 3 (acid and salts of multivalent cations) which underwent dehydration at two steps resulting in structural changes I ->I['>X-∗A. The thermal stabilities of 12-molybdophosphates at a high temperature were also dependent upon the counter cations. All bi- and tervalent metal salts were least stable, being decomposed at 400'450°C. The acid and NH4+ salt decomposed at 420°C and 470°C, respectively, accompanied by intramolecular desorption of water and/or ammonia. Other univalent salts were stable up to their melting points (500'700°C) which were well correlated with the radii of counter cations. The specific surface arenas of 12-molybdophosphates were found to change with a temperature of heat treatment. The behavior also deeply depended upon the kinds of counter cations, corresponding to structural and thermal stabilities of respective salts.
Heteropoly compounds with the general formula Cs1M
0.5x+H3‒0.5x
P1.2Mo11VO40 (M = Fe, Co, Ni, Cu or Zn) and Cs1Cuy
H3‒2y
P1.2Mo11VO40 (y = 0.1, 0.3 or 0.7) were synthesized and then used as catalysts for the selective oxidation of methacrolein to methacrylic acid. The effects of the transition metals on the structure and activity of the catalysts were investigated. FTIR spectra showed that the transition metal-doped catalysts maintained the Keggin structure of the undoped catalysts. X-ray diffraction results indicated that before calcination, the catalysts doped with Fe and Cu had cubic secondary structures, while the catalysts doped with Co, Ni or Zn had both triclinic and cubic phases and the Co-doped catalyst had the highest content of the triclinic form. Thermal treatment can decrease the content of the triclinic phase. NH3 temperature-programmed desorption and H2 temperature- programmed reduction results showed that the transition metals changed the acid and redox properties of the catalysts. The addition of Fe or Cu had positive effects on the activities of the catalyst which is due to the improvement of the electron transfer between the Fe or Cu and the Mo. The effects of the copper content on structure and catalytic activity were also investigated. The Cs1Cu0.3H2P1.2Mo11VO40 catalyst had the best performance for the selective oxidation of methacrolein to methacrylic acid.
The progress of the structure, performance, synthesis and characterization of the environmentally benign heteropoly compounds and their use in fine chemicals are introduced in detail. The synthesis and application of the supported heteropoly acids and the novel mixed heteropoly compounds obtained by designing molecule are also reviewed. The research for heteropoly acids (HPA) and their salts has a quite important theoretical value and realistic meanings. HPA used as excellent green catalyst will possess extensive application prospect in fine chemical synthesis.
Three Keggin-type heteropoly catalysts with different vanadyl species, H4PMo11VO40 (V existing in the primary structure), HVOPMo12O40 (V existing as VO2+ in the secondary structure), and V2O5/H3PMo12O40 (V existing in V2O5 before calcination), were prepared and studied to identify the preferable vanadyl species for oxidation of methacrolein to methacrylic acid. The catalysts were characterized by FT-IR, Raman, XRD, UV–vis, EPR, and XPS before and after calcination at different temperatures. The calcined catalysts were applied in the selective oxidation of methacrolein. The chemical state and position of the vanadyl species varied with the treatment temperature. Compared with the VO2+ species in HVOPMo12O40, the VO2+ species generated in H4PMo11VO40 and V2O5/H3PMo12O40 during calcination processes showed very different promotional effects, and the VO2+ species that interact with the Keggin structure turned to be the main active vanadyl species in all the catalysts. The active vanadyl species changed from an independent ion form to a squashed square pyramidal on Keggin structure or defect Keggin structure during the oxidation reaction. The optimum conversion of methacrolein and selectivity to methacrylic acid were obtained on the V2O5/H3PMo12O40 catalyst calcined at 350 °C, so a new means of adding vanadium in the Keggin-type heteropoly catalysts to obtain high selectivity to methacrylic acid is proposed.
Reactions involving acids and bases represent a very large and important section of modem chemistry. They are widely used in chemical synthesis and technology, and many biochemical reactions occur with the participation of acids. Charge transfer reactions occupy the central place in this area. According to Bronsted, an acid is a compound capable of donating a proton, and a base is a compound capable of withdrawing a proton. Aprotic acids (Lewis acids) exist (and are used as catalysts) along with protic acids. Lewis acid is a substance capable of being an acceptor for an electron pair of a base while lewis bases are substances that act as donors of an electron pair. The definitions of a base according to Bronsted (proton acceptor) and Lewis (electron pair donor in the reaction with an acid) virtually coincide. However, the definitions of an acid diverge, as we can see. The Bronsted acid is a proton donor, and the Lewis acid is an acceptor of a base (a substance beating an unshared electron pair). Pearson introduced a concept of hard and soft acids and bases. Hard acids form more stable compounds with hard bases, and soft acids form more stable compounds with soft bases.
Selective oxidation of methacrolein to methacrylic acid was carried out over Cu- and Fe-doped acidic cesium salts of molybdovanadophosphoric acids. The influences of preparation conditions on the structure and catalytic properties of the catalysts were investigated. Results showed that the addition method of NH4+ in the precursors greatly affected the structure of the calcined catalysts. When MoO3, V2O5, and H3PO4 were used to prepare the parent heteropoly acid, which further reacted with the mixed metal nitrates to form the Keggin-type precursor, the addition of ammonia promoted the formation of single-phase cubic secondary structure of the precursor. After being calcined at 360 °C for 12 h, ammonium ions totally decomposed, and vanadium ions were partially expelled from the primary Keggin units. The catalytically less active MoO3 phase, observed in the sample calcined in air, did not exist in the sample calcined in N2. When NH4VO3 was used to replace V2O5 and ammonia to prepare the precursor, the ammonium exhibited higher thermal stability, and the calcination atmosphere had almost no effect on the phase composition of the calcined catalysts. It is also found that Cu and Fe as counterions can improve the redox property and thereby enhance the performance of the catalyst.
Gas-phase oxidation of methacrolein to methacrylic acid has been carried out over various unsupported and supported H4PMo11VO40 catalysts. While SiO2-supported H4PMo11VO40showed low selectivity to methacrylic acid, H4PMo11VO40 supported on NH3-modified SiO2, which was prepared by using an impregnation method with acetone as the solvent, exhibited high selectivity (~ 90%) and high activity for the formation of methacrylic acid. The activity of H4PMo11VO40/NH3-modified SiO2was more than five times higher than that of the corresponding unsupported catalysts.
Supported cesium catalysts with various carriers (SiO2, Al2O3, TiO2, MgO) were prepared and characterized by X-ray diffraction, BET nitrogen adsorption–desorption, NH3 and CO2-TPD methods and thermogravimetric analysis. Experimental results showed that the Zr–Mg–Cs/SiO2 catalyst exhibited moderate activity for aldol condensation of methyl propionate with formaldehyde (FA) to produce methyl methacrylate. Though the activity of Zr–Mg–Cs/SiO2 catalyst decreased with the time-on-stream, the deactivated catalyst was completely regenerated by calcination. The catalyst was regenerated 16 times and total operation time was over 500 h, its activity was identical with that of the fresh catalyst.
Graphical Abstract
Gas aldol condensation reaction of methyl propionate with FA to produce MMA is an environmentally friendly technology; it doesn’t need toxic raw materials. Cesium-impregnated silica exhibited better catalytic activity and high selectivity for this reaction, and this catalyst had good reusability and long cycle life. So it is a promising catalyst for industrial application.
Cesium and cesium-transition metal-substituted Keggin-type phosphomolybdic acids have been prepared and characterized using several techniques. The results obtained show that all heteropolycompounds with formulae H3−xCsxPMo12O40, where 0<x<3 were composed of two phases corresponding to the hydrated acid and the pure cesium salt with the acid phase coating the salt particles. For compounds withx<2, the acid was present in large amounts and could be detected by X-ray diffraction. For compounds withx>2 the precipitation rates vary, leading to very small cesium salt particles. In this second case the acid phase also coated the particles, but it was no longer detectable by X-ray diffraction, although it could be observed by XPS or Raman spectroscopy. All the data obtained show that the transition metal cations are replacing the protons in the supported acid phase and not the cesium atoms in the salt. This replacement does not change the structure of the bulk acid phase even at high substitution levels, but it decreases the hydration rate of the supported acid.
Physicochemical and catalytic properties of heteropolyacids of the series H3 +nPVnMo12–nO40(n= 0–3), both pure and supported on the potassium salt K3PMo12O40, have been investigated. Thin acid coats formed on such a support display modified properties and enhanced thermal stability. In particular, it is postulated that the change in the acidic properties of the supported acids is a consequence of their modified hydration ability resulting from the epitaxial relationship with the support. Results of catalytic experiments for the oxidation of acrolein, methanol and alkanes are presented and compared for both series of the catalysts. Possible mechanisms of all these processes are proposed on the basis of experimental data and quantum-chemical calculations.
Proton conducting composites have been prepared by loading heteropolyacids, namely tungstophosphoric (TPA) acid and molybdophosphoric (MPA) acid, into MCM-41 molecular sieve. The synthesis procedure was optimized to ensure maximum loading of TPA and MPA with negligible leaching of solid acids. The proton conductivity of the composite powdered materials was found to depend on the loading of the heteropolyacids and strongly affected by the presence of water in the solid. The highest proton conductivity of the order of 10−2Scm−1 at room temperature was found for the TPA loaded MCM-41. The prepared materials have been characterized by FTIR, SEM and X-ray diffraction, which confirm the presence of heteropolyacids into MCM-41 molecular sieve structure. Leaching study carried out on the composite solids confirmed that the material leached out through the composite solids was negligible and hence almost complete loading of heteropolyacids into the MCM-41 structures was ensured. The new material combines the high thermal and structural stability of MCM-41 with outstanding conductivity of heteropolyacids. The high conductivity and negligible leaching of this material makes it suitable for use in the preparation of membranes for use in fuel cells and other electrochemical devices.
Microcrystallites of (NH4)(3)PW12O40 self-assembled to form symmetric dodecahedral aggregates (ca. 0.5 - 6 mu m in size) in which the microcrystallites joined together with the same crystal orientation, by controlled deposition of NH, salts from a heated aqueous solution of H3PW12O40. These aggregates were highly porous, where microcrystallites were connected each other by epitaxial interfaces.
The pore size of acidic Cs salts (CsxH3−xPW12O40) was precisely controlled by the Cs content. Cs2.2H0.8PW12O40 possesses micropores in the range from 6.2 to 7.5 Å (in diameter) and exhibits efficient shape selective catalysis toward decomposition of ester, dehydration of alcohol, and alkylation of aromatics in liquid-solid system. This is the first example of shape-selective solid superacid.
The heteropolyacids 12-tungstosilicic acid (TSA),
12-tungstophosphoric acid (TPA), 12-molybdosilicic acid (MSA) and
12-molybdophosphoric acid (MPA) were supported on SiO2 and
examined in the gas–solid phase reaction of butane to isobutane. Use
of a support was effective for TSA and TPA whereas no reaction was observed
with MSA or MPA. Effectiveness of the support for TSA was observed with
ZrO2 and Al2O3 but not with
TiO2, SnO2, or Fe2O3. The most
active catalyst was prepared by impregnating SiO2 with an
aqueous solution of 40 wt% TSA followed by water evaporation and
heat-treating in air at 300 °C (40% TSA/SiO2). The catalytic
activity was more than doubled when a physical mixture of 40%
TSA/SiO2 and Pt/ZrO2 was used.
Nitrogen-containing spherical carbon (N-SC) with a diameter of ca. 12 μm was synthesized by a hydrothermal method using melamine-formaldehyde
resin as a carbon precursor. The N-SC was then modified to have a positive charge, and thus, to provide a site for the immobilization
of H5PMo10V2O40 (PMo10V2) catalyst. The PMo10V2 catalyst was chemically immobilized on the surface-modified N-SC support by taking advantage of the overall negative charge
of [PMo10V2O40]5−. Characterization results showed that nitrogen in the N-SC support played an important role in forming a nitrogen-derived
functional group (amine group) and that PMo10V2 catalyst was chemically immobilized on the nitrogen-derived functional group of N-SC support. PMo10V2/N-SC catalyst showed a higher 2-propanol conversion than the unsupported PMo10V2 catalyst in the vapor-phase 2-propanol conversion reaction. Moreover, the PMo10V2/N-SC catalyst showed an enhanced oxidation catalytic activity (formation of acetone) and a suppressed acid catalytic activity
(formation of propylene and isopropyl ether) than the unsupported PMo10V2 catalyst. The enhanced oxidation activity of PMo10V2/N-SC catalyst was due to fine dispersion of [PMo10V2O40]5− on the N-SC support formed via chemical immobilization.
Oxidation of methacrolein over heteropoly acid catalysts (mainly 12-molybdophosphoric acid) at 300 °C was investigated by use of a flow method. Primary (Keggin) and secondary structures of the catalyst were rather stable upon heat treatment at 250–350 °C, and reproducible data of the oxidation reaction were obtained by the pretreatment at 350 °C in a N2 + O2 + H2O stream. Effects of oxygen in the feed gas demonstrated that oxygen atoms of polyanions or steam were directly involved in the reaction and the reduced polyanions were reoxidized by gaseous oxygen. Presence of steam had a remarkable effect both on the rate and selectivity. Rate equation, ESR study, and the comparison between the catalytic oxidation (flow method) and noncatalytic oxidation (pulse method) indicated that the reduction of catalysts was rate-determining in a redox mechanism and the catalysts were in a highly oxidized state under the present reaction conditions. Effects of metal salt formation and silica support were also examined. On the basis of these results, a possible reaction mechanism with an ester- or diol-type intermediate has been proposed.
Oxidative dehydrogenation of isobutyric acid to methacrylic acid has been studied using Cs salts of 12-molybdovanadophosphoric acids as catalysts (CsnH3+x−nPMo12−xVxO40, n = 0−(3+x), x = 0−2, which will be abbreviated as CsnMo12−xVx). Among them, Cs2.75Mo11V showed remarkably high performance; the highest conversion (97%) and selectivity for methacrylic acid (MAA, 78%) at 623 K. At the same reaction conditions, Mo11V (i.e. n = 0, x = 1) gave only 40% conversion and 52% selectivity. As for the series of Cs2.75Mo12−xVx, the activity was in the order of x = 1 > χ = 2 > x = 0 and the MAA selectivity, x = 1 > x = 2 > x = 0. The nature of Mo11V was confirmed by IR to be partially destroyed during pretreatment in O2 at 623 K, while Cs2.75Mo11V decomposed only slightly. This fact indicates that the thermal stability afforded this catalyst to make efficient use of the redox ability controlled by incorporation of V, and the enhanced redox ability, high surface area, and high surface acidity of Cs salts would be the main reasons for their effectiveness.
Keggin-type P/Mo polyoxometalates (POM) were dispersed in a high-surface-area silica gel by means of a co-gelation procedure, in order to prepare catalysts for the gas-phase oxidation of isobutane to methacrolein and methacrylic acid. The aim was the dilution of the active phase, in order to favor the dispersion of the considerable reaction heat and limit the oxidative degradation of the desired products. However, the diluted heteropolyacid was less stable than the undiluted one, because of the exchange between P and Si as the heteroatom. When the gel was prepared in the presence of tetrapropylammonium hydroxide, the thermal decomposition of the latter generated the ammonium ion that replaced for protons in the cationic position of the POM. This enhanced the structural stability of the diluted POM at 350°C under reaction conditions. However, no improvement of the catalytic performance was obtained for the diluted catalysts as compared to the undiluted systems, because of the combustion of methacrylic acid, of the modification of the POM redox properties and of the considerable extent of POM decomposition.
Chemical modifications of the 12-phosphomolybdic acid by substitution of a Mo atom by a V in the so-called Keggin primary structure and/or by introduction of counter-cations (NH4+, Cs+) in the secondary structure were studied. The effects of the proportion of ammonium salt in cesium–ammonium salts mixtures and of different preparation methods were also investigated. Concurrently to characterization analyses, the kinetic parameters of the Mars and Van Krevelen (MVK) model, representing well the rate of consumption of isobutane over the solids tested were determined by varying partial pressures of isobutane and oxygen. The results were used for catalysts evaluation and as a tool for the development of a new, more active and stable formulation.On the basis of the results, the role of the constituents of the catalysts are better understood. It is proposed that vanadium atoms are easily removed from the primary structure and participate in the reduction of the catalyst but have no effect on the activation of isobutane. Moreover it is shown that ammonium ions are essential to reach a high selectivity to methacrylic acid (MAA) by reducing the active phase whereas the role of cesium is to form an alkaline salt acting as a support over which the active phase is dispersed and stabilized.
The one-step selective synthesis of dimethoxymethane (DMM; CH3OCH2OCH3) was achieved by oxidation of dimethyl ether (DME) or methanol (CH3OH) with O2 at low temperatures (453-513 K) on unsupported and SiO2-supported heteropolyacids with Keggin structures (H3+nPVnMo12-nO40 (n ) 0-4)). These materials provide redox and Bronsted acid sites required for bifunctional DMM synthesis pathways. Supported structures at submonolayer coverages (0.1-0.28 Keggin units per nm2) are much more accessible than bulk structures and remove diffusional constraints. Their higher dispersions lead to marked improvements in DMM synthesis rates and selectivities and to lower COx yields using either CH3OH or DME reactants. The presence of H2O during DME oxidation increases DMM synthesis rates because of a consequent increase in the rate of DME hydrolysis reactions, which form CH3OH molecules required as intermediates in the DMM synthesis reaction sequence. Pure CH3OH reactants form DMM at much higher rates than DME reactants. The replacement of some Mo atoms in H3PMo12O40 structures with V increases DMM synthesis rates and selectivities while inhibiting the formation of COx. In fact, COx was not detected on H3+nPVnMo12-nO40 (n ) 2, 4; 0.1 KU/ nm2) even at high CH3OH conversions (50%). CH3OH converts to DMM via primary CH3OH reactions to form formaldehyde (HCHO) and subsequent secondary reactions of HCHO with CH 3OH in steps requiring both redox and acid sites; CH3OH also reacts to form DME on acid sites. These pathways are consistent with the effects of changes in residence time and of the partial removal of acidic OH groups from Keggin structures on reaction selectivities. High CH3OH pressures and conversions favor HCHO-CH3OH acetalization reactions and DMM synthesis rates and selectivities. Thermal treatments that cause dehydroxylation and loss of Bronsted acid sites without destroying the primary Keggin structures decrease DME formation rates without significant changes in DMM synthesis rates. These findings suggest that acid sites are not involved in the rate-limiting step for DMM synthesis and that much higher DMM selectivities can be achieved by further increases in the ratio of the rates of redox and acid catalysis. This study represents the first report of high DMM selectivity and yields on stable molecular oxide clusters and provides an effective approach to the rational design of oxide materials for the one-step synthesis of dimethoxymethane from either dimethyl ether or methanol.
Fundamental and superior characteristics of heteropoly
compounds (heteropolyoxometalates) in the solid state that make them
suitable for catalyst design at the atomic/molecular levels are described,
together with important principles required for the understanding and
design of solid heteropoly catalysts. First, the molecular nature of
heteropolyanions (metal oxide clusters), which can be preserved in the
solid state, enables control of the acid and redox properties over a wide
range. Second, the presence of hierarchical structures (primary, secondary
and tertiary structures) can lead to three catalysis
modes—surface-type, pseudoliquid (or bulk-type I) and bulk-type II.
Precise control of pore size is possible through the understanding of the
microstructure, which results in unique shape selectivity observed for
various reactions. Heteropoly compounds are green catalysts functioning in
a variety of reaction fields and efficient bifunctional catalysts when
combined with other components. The elucidation of catalytic processes is
also possible at the atomic/molecular level due to their molecular nature.
The positions and dynamic nature of protons as well as organic reaction
intermediates in the pseudo-liquid phase can be clarified by spectroscopic
techniques. Various reactions promoted by solid heteropoly catalysts are
collected from recent publications to illustrate the usefulness of the
above ideas and of heteropoly catalysts themselves.
While the oxidation of acrolein on mixed oxide (Mo, V, W) catalysts shows an exceptionally high selectivity with respect to acrylic acid, the selectivity of the analogous oxidation of methacrolein is only moderate. This is due to a considerably lower oxidation rate of methacrolein compared to that of acrolein, while the undesired subsequent oxidation of methacrylic acid is of the same order of magnitude as the oxidation of acrylic acid. Comprehensive kinetic studies in particular by transient experiments have led to a sound hypothesis to explain this difference and to show ways of catalyst improvement for the conversion of methacrolein to methacrylic acid. A moderate increase of methacrylic acid selectivity could be reached by variation of the Mo/V ratio of the mixed oxide. Surprisingly the addition of phosphoric acid and Cs-acetate effected a reduction of the subsequent oxidation of methacrylic acid without affecting the methacrolein oxidation, leading to a marked increase of the selectivity toward methacrylic acid. Furthermore, the comparison of methacrolein and acrolein oxidation results in a confirmation of the hypothesis that aldehyde oxidation and subsequent oxidation of acid occur on different domains of the mixed oxide catalyst.
Comprehensive kinetic studies of the oxidation of methacrolein on mixed oxide (Mo, V, W) catalysts have been undertaken in order to find ways of catalyst improvement to achieve a high selectivity toward methacrylic acid. Steady-state kinetic experiments were carried out in a differential recycle reactor that behaves like an ideal continuous stirred tank reactor (CSTR). A kinetic model based on the scheme of Mars−van Krevelen was developed for a fair representation of the reaction rates of methacrolein conversion, methacrylic acid formation, byproduct formation, and selectivity toward methacrylic acid. For a better understanding of the kinetics, transient experiments were carried out in an apparatus to render possible sorption studies as well as transient kinetic experiments, which are powerful tools to study independently both the oxidation of the aldehydes on the catalyst in the absence of oxygen and the reoxidation of the catalyst. Steady-state kinetic data and transient experiments agree well. It could be clearly shown that the conversion of methacrolein is mainly determined by the reoxidation of the catalyst and that the selective oxidation of the aldehydes and the consecutive oxidation of the acids occur on different domains of the catalyst.
The preparation of chemisorbents based on tungsto- and molybdophosphoric acids supported on two types of synthetic mesoporous carbons and two types of mesoporous silica is described. Strong solid acids with good accessibility to acid sites may potentially be effective adsorbents for the removal of basic molecular impurities, such as amines, from ultrapure manufacturing environments. Prepared materials were characterised by scanning electron microscopy, nitrogen adsorption, Fourier-transform infrared spectroscopy, powder X-ray diffraction, and equilibrium ammonia uptake. Composites of SBA-15 with heteropolyacids were synthesised. It was shown that the inclusion of HPAs into SBA-15 results in the loss of long range order. Adsorbents based on the HPAs impregnated into the supports with the open-pore morphology (Novacarb and SBA-15) were found to be promising materials. A composite of tungstophosphoric acid with sol–gel SiO2 was found to have the highest ammonia uptake.
Various compositions x in the catalyst system KxH3−xPMo12O40 · nH2O have been prepared by conventional techniques and characterized by thermal analysis, X-ray powder diffraction, 31P solid-state NMR, ESR, electron microscopy, ESCA, BET surface-area measurements, and diffuse-reflectance IR spectroscopy. Contrary to common presupposition, no significant solid-solution range was detected. The x = 3 (K3) phase is stable to 920 K; the x = 0(K0) phases lose their n water of crystallization by 450 K and their constitutional water in the temperature interval 500 < T < 700 K. Decomposition of the Keggin anion (PMO12O40)3− of the H3PMO12O40 · nH2O phase accompanies the loss of constitutional water, and an identifiable decomposition product is MoO3. Under normal calcining conditions (673 K in air for 1–5 h), the degree of decomposition of the K0 phase decreases with increasing x, little MoO3 being detectable for x ≥ 2. An epitaxial, isostructural surface layer derived from the H3PMo12O40 · nH2O appears to be stabilized on the water-insoluble K3PMo12O40 particles. Calcined catalysts in the compositional range 2 < x ≤ 3 consist of well-formed, spherical particles having surface Keggin anions; these catalysts are particularly suitable for mechanistic studies of catalytic reactions on a Keggin unit.
HPWA/MCM-41 mesoporous molecular sieves of appropriate ratios were prepared by loading HPWA on siliceous MCM-41 by the wet impregnation method. The prepared HPWA/MCM-41 materials were characterized by X-ray diffraction analysis (XRD) and BET surface area and FT-IR measurements. The morphology of mesoporous materials was studied by TEM observation. The catalytic activity of the above materials was tested for the condensation of dimedone (active methylene carbonyl compound) and various aromatic aldehyes under liquid phase conditions at 90°C. The products were confirmed by FT-IR, 1H NMR and 13C NMR studies. HPWA supported MCM-41 catalysts catalyses efficiently the condensation of dimedone and aromatic aldehydes in ethanol and other solvents under liquid phase conditions to afford the corresponding xanthenedione derivatives. Activities of the catalysts follow the order: HPWA/MCM-41(20wt.%)>HPWA/MCM-41(30wt.%)>H3PW12O40·nH2O>HPWA/MCM-41(10wt.%)>HPWA/SiO2 (20wt.%)>HM (12)>Hβ (8)>Al-MCM-41 (50). Various advantages associated with these protocols include simple workup procedure, short reaction times, high product yields and easy recovery and reusability of the catalyst.
Keggin-type phosphomolybdic acid and cesium salts with protons partially substituted by tellurium and by both tellurium and vanadium have been prepared, characterized using several techniques and tested as catalysts in the partial oxidation of isobutane into methacrylic acid (MAA). The results showed that tellurium when introduced as counter-cation was present as Te4+ capping the Keggin anion and was randomly distributed in the acid or in the cesium salt. This cation induced a positive effect on the selectivity to MAA and methacrolein (MA) without significant effect on the activity except in the acid at low loading where it also increased the activity. The co-substitution of protons by vanadyl cations had a slight effect on the selectivity but increased the activity especially at low level of substitution, which led to a very efficient catalyst. Selectivity to MAA and MA and isobutane conversion rate of 65 and 17% respectively were reached at 350 °C and were both very constant with time on stream. The catalytic results obtained in both stationary and transient conditions allowed to propose a reaction mechanism very close to one already proposed with four intermediates amongst which one is common to both MAA and MA. These results were used to understand the catalytic effect of tellurium and vanadium.
Highly ordered mesoporous silicon carbide ceramics have been successfully synthesized with yields higher than 75% via a one-step nanocasting process using commercial polycarbosilane (PCS) as a precursor and mesoporous silica as hard templates. Mesoporous SiC nanowires in two-dimensional (2D) hexagonal arrays (p6m) can be easily replicated from a mesoporous silica SBA-15 template. Small-angle X-ray diffraction (XRD) patterns and transmission electron microscopy (TEM) images show that the SiC nanowires have long-range regularity over large areas because of the interwire pillar connections. A three-dimensional (3D) bicontinuous cubic mesoporous SiC structure (Ia3d) can be fabricated using mesoporous silica KIT-6 as the mother template. The structure shows higher thermal stability than the 2D hexagonal mesoporous SiC, mostly because of the 3D network connections. The major constituent of the products is SiC, with 12% excess carbon and 14% oxygen measured by elemental analysis. The obtained mesoporous SiC ceramics are amorphous below 1200°C and are mainly composed of randomly oriented β-SiC crystallites after treatment at 1400°C. N2-sorption isotherms reveal that these ordered mesoporous SiC ceramics have high Brunauer-Emmett-Teller (BET) specific surface areas (up to 720 m2 g-1), large pore volumes (∼0.8 cm3 g-1), and narrow pore-size distributions (mean values of 2.0-3.7 nm), even upon calcination at temperatures as high as 1400°C. The rough surface and high order of the nanowire arrays result from the strong interconnections of the SiC products and are the main reasons for such high surface areas. XRD, N2-sorption, and TEM measurements show that the mesoporous SiC ceramics have ultrahigh stability even after re-treatment at 1400°C under a N2 atmosphere. Compared with 2D hexagonal SiC nanowire arrays, 3D cubic mesoporous SiC shows superior thermal stability, as well as higher surface areas (590 m2 g-1) and larger pore volumes (∼0.71 cm3 g-1).
Our recent developments in catalysis of heteropoly compounds as solid acids have been described. “Water-tolerant catalysis” is one of novel catalytic functions of an acidic Cs salt, Cs2.5H0.5PW12O40 (Cs2.5). Comparison of adsorption amounts of water and benzene led to the following order of hydrophobicity; HZSM-5 (Si/Al=628)>silica>Cs3>H-ZSM-5 (Si/Al=40)∼Cs2.5>silica-alumina>alumina. Owing to the hydrophobicity, Cs2.5 was highly active for organic reactions like hydration of alkene and hydrolysis of esters and oligosugars in water. Also, Cs2.5 was efficient for the reaction of nitrile with alcohol to N-alkylamide even in excess water. Cs2.5 was also active for solid–solid pinacol rearrangement and Beckmann rearrangement, and was reusable by washing a small amount of solvent. In a bi-phasic reaction system, H3PW12O40 (in liquid state) was exceptionally active for synthesis of diphenylmethane from formalin (aqueous formaldehyde) and benzene. As a bifunctional catalyst, Pt-Cs2.5H0.5PW12O40 showed good performance for skeletal isomerization of n-butane and hydroisomerization of benzene. Mechanism of n-butane isomerization was studied using -n-butane; the reaction proceeds mainly through monomolecular pathway over Pt-Cs2.5, while bimolecular pathway was predominant over Cs2.5. Cs2.1 and Rb2.1 possessed ultramicropores with the widths of about 0.50 and 0.60 nm, respectively, and the corresponding Pt-promoted catalysts exhibited shape selectivities for oxidation and hydrogenation.
Understanding the structural and electronic mechanisms by which catalysts activate and deactivate during use is crucial to the intelligent design of more efficient chemical syntheses. Atomic resolution electron microscopy is used in conjunction with bulk characterization tools including as X-ray Diffraction (XRD), Ultraviolet-visible Diffuse Reflectance Spectroscopy (UV-vis DRS) and X-ray Absorption Spectroscopy (XAS) to understand the activation and deactivation mechanism of dodecamolybdophosphoric acid (MPA), a promising parent material for a class of polyoxometalate catalysts useful for the direct oxidation of isobutene to methacrylic acid. These techniques show that the thermal and reactive reconstruction of MPA arises from the migration of an oxomolybdate species from the cubic form of the anhydrous MPA structure. The reconstruction continues and results in complete degradation to MoO3, which is inactive for isobutane oxidation. The mechanism by which reorganization occurs is investigated using High Resolution Transmission Electron Microscopy (HR-TEM) for the first time. These HR-TEM studies provide a picture of the atomic-scale rearrangement occurring in the catalyst. The initial structural reorganization in MPA is observed as the formation of annealing twins in the cubic form of the anhydrous polyoxometalate - this twinned structure is believed to be the active form of the catalyst. This twinning phenomenon is believed to originate from vacancies created in the MPA structure by the migration of atoms out of the primary structure. The twins then propagate across the MPA crystal and result in complete degradation of the MPA to MoO3.
Catalytic oxidation of methacrolein (MAL) to methacrylic acid (MAA) over SiO2-supported H4PMo11VO40 with different H4PMo11VO40 loadings was investigated. H4PMo11VO40/SiO2 showed high activity in comparison with unsupported H4PMo11VO40, and 3.3 mol% H4PMo11VO40/SiO2 (50 wt% H4PMo11VO40) had the highest activity, which was five-times larger than that of unsupported H4PMo11VO40 due to high dispersion of H4PMo11VO40 on SiO2, as determined by temperature-programmed desorption of benzonitrile. On the other hand, the supported catalysts were less selective towards the formation of MAA. From X-ray diffraction and Raman spectroscopy, it was determined that H4PMo11VO40 decomposed to form MoO3 on SiO2 during the catalytic reaction. Since SiO2-supported MoO3 and unsupported MoO3 had only very low selectivity toward the formation of MAA in the oxidation of MAL, it was concluded that the formation of MoO3 caused the decrease in the catalytic performance of the supported catalysts.
Acid-base catalysis II
- T Okuhara
- T Nishimura
- H Watanabe
- K Na
- M Misono
T. Okuhara, T. Nishimura, H. Watanabe, K. Na, M. Misono, Acid-base catalysis
II, in: Stud Surf. Sci. Catal., Elsevier, Amsterdam, 1994.
- N Mizuno
- M Misono
N. Mizuno, M. Misono, Chem. Rev. 98 (1998) 199-217.
- Y Li
- L Wang
- R Yan
- J Han
- S Zhang
Y. Li, L. Wang, R. Yan, J. Han, S. Zhang, Catal. Sci. Technol. 5 (2015) 3682-3692.
- Y Zheng
- H Zhang
- L Wang
- S Zhang
- S Wang
Y. Zheng, H. Zhang, L. Wang, S. Zhang, S. Wang, Front. Chem Sci. Technol. 10
(2016) 139-146.
- J Han
- S Zhang
- Y Li
- R Yan
J. Han, S. Zhang, Y. Li, R. Yan, Catal. Sci. Technol. 5 (2015) 2076-2080.
- A Gaurav
- F T T Ng
- G L Rempel
A. Gaurav, F.T.T. Ng, G.L. Rempel, Green Energy Environ. 1 (2016) 62.
- I N Staroverova
- M Kutyrev
- A Stakheev
I.N. Staroverova, M. Kutyrev, A. Stakheev, Kinet. Katal. 33 (1992) 127-138.
- U G Hong
- D R Park
- S Park
- J G Seo
- Y Bang
- S Hwang
- M H Youn
- I K Song
U.G. Hong, D.R. Park, S. Park, J.G. Seo, Y. Bang, S. Hwang, M.H. Youn, I.K. Song,
Catal Lett. 132 (2009) 377-382.
- Y Shi
- Y Meng
- D Chen
- S Cheng
- P Chen
- H Yang
- Y Wan
- D Zhao
Y. Shi, Y. Meng, D. Chen, S. Cheng, P. Chen, H. Yang, Y. Wan, D. Zhao, Adv.
Funct. Mater. 16 (2006) 561-567.
- Y Konishi
- K Sakata
- M Misono
- Y Yoneda
Y. Konishi, K. Sakata, M. Misono, Y. Yoneda, J. Catal. 77 (1982) 169-179.