Density Of Basic Sites Research Articles

Zn,Al-catalysts for heterogeneous biodiesel production: Basicity and process optimization

Methyl esters produced by transesterification of vegetable oils are the main components of biodiesel, an alternative, biodegradable, non-toxic biofuel produced from renewable resources. The use of solid catalysts allows the development of environmentally friendly processes to produce biodiesel. In this work, Zn,Al-mixed oxides with different Al/(Al + Zn) molar ratios were studied as catalysts for the transesterification of soybean oil with methanol. TPD-CO2 (temperature programmed desorption of CO2) and model reactions such as retroaldolization of diacetone alcohol and 2-propanol transformation were employed to determine the basic properties of the catalysts. The catalytic activity was influenced by the chemical composition of the catalyst and showed a good correlation with basic site density determined by TPD-CO2. The RSM (response surface methodology) was used to select experimental conditions such as alcohol/oil molar ratio, reaction temperature, and catalyst loading that maximize fatty acid methyl esters yield. The best experimental conditions indicated by RSM were reaction temperature of 182.5 °C, catalyst loading of 5.0 wt.%, and alcohol/oil molar ratio of 45, that provided a biodiesel yield of 86% without significant leaching of the catalyst.

Magnesia-supported gold nanoparticles as efficient catalysts for oxidative esterification of aldehydes or alcohols with methanol to methyl esters

Magnesia-supported gold nanoparticles were found to be highly efficient catalysts for the oxidative esterification of methacrolein (MAL) with methanol in the presence of molecular oxygen into methyl methacrylate (MMA) under liquid base-free conditions. MAL conversion of 98% and MMA selectivity of 99% were obtained over the Au/MgO catalyst at 343K after a 2h reaction. Besides the Au nanoparticles, the support also played pivotal roles in the oxidative esterification of MAL. The support with higher density of basic sites, particularly stronger basic sites, showed better performances for the formation of MMA. The enhancement of the intermediate formation by the basic sites is proposed to be the key reason for the superior activity of the Au/MgO catalyst. Our studies on the size effect of Au nanoparticles reveal that smaller Au nanoparticles favor the transformation of MAL, and the turnover frequency increases with decreasing mean size of Au nanoparticles. This suggests that the Au-catalyzed oxidative esterification of MAL is a structure-sensitive reaction. We have demonstrated that the Au/MgO catalyst is also applicable to the oxidative esterification of different aldehydes and alcohols.

Effect of Cu content on the surface and catalytic properties of Cu/ZrO2 catalyst for ethanol dehydrogenation

ZrO2-supported Cu catalysts with different Cu content (5–30wt.%) were prepared by impregnation method. The effect of Cu content on the structure, surface and catalytic properties of Cu/ZrO2 catalysts in the reaction of ethanol conversion was studied. The physicochemical characterization of the calcined and reduced samples was carried out by: N2 adsorption–desorption isotherms, N2O titration, XRD, XPS, TPR and DRFTS of CO adsorption. It was observed that the increase of Cu content leads to decrease of the apparent copper metal dispersion caused by the strong agglomeration of the metal particles. The selectivity to different reaction products was connected with the electronic properties of the catalysts defined by the copper particle size and the interface at metal-oxide support. The highest selectivity to ethyl acetate over samples with Cu content ≥10wt.% was assigned to the high density of basic sites of O2− ions and more heterogeneous distribution of copper species (Cu0/Cu+) defined by DRIFTS of CO adsorption and XPS.

Effect of the ZrO2 phase on the structure and behavior of supported Cu catalysts for ethanol conversion

The effect of amorphous (am-), monoclinic (m-), and tetragonal (t-) ZrO2 phase on the physicochemical and catalytic properties of supported Cu catalysts for ethanol conversion was studied. The electronic parameters of Cu/ZrO2 were determined by in situ XAS, and the surface properties of Cu/ZrO2 were defined by XPS and DRIFTS of CO-adsorbed. The results demonstrated that the kind of ZrO2 phase plays a key role in the determination of structure and catalytic properties of Cu/ZrO2 catalysts predetermined by the interface at Cu/ZrO2. The electron transfer between support and Cu surface, caused by the oxygen vacancies at m-ZrO2 and am-ZrO2, is responsible for the active sites for acetaldehyde and ethyl acetate formation. The highest selectivity to ethyl acetate for Cu/m-ZrO2 catalyst up to 513K was caused by the optimal ratio of Cu0/Cu+ species and the high density of basic sites (O2−) associated with the oxygen mobility from the bulk m-ZrO2.

Synthesis of glycerol carbonate by transesterification of glycerol with dimethyl carbonate over MgAl mixed oxide catalysts

A series of hydrotalcite-like layered double hydroxides (LDHx) with different Mg/Al atomic ratios (x=2–6) were prepared by using the co-precipitation method. Further calcination yields mixed oxides with tunable basicity. The basicity of the calcined LDHx (LDOx) strongly depends on the Mg/Al ratio and the calcination temperature. The resulting LDOx materials were used as solid base catalysts and evaluated in the transesterification between glycerol and dimethyl carbonate without use of organic solvent. The correlation between the basic properties of the solid catalysts and the catalytic performance was investigated. The activity of the LDOx catalysts was demonstrated to be proportional to the surface density of basic sites. LDO2 calcined at 600°C exhibited maximum activity for the transesterification reaction. The beneficial effect of the optimum ratio of Mg/Al=2 is related to its high total basicity. The LDO2 catalyst can be readily recycled while maintaining high catalytic activity and selectivity of glycerol carbonate.

Promotional effect of transition metal doping on the basicity and activity of calcined hydrotalcite catalysts for glycerol carbonate synthesis

A series of transition metal doped hydrotalcites (HT-M) were prepared by using the “memory effect” of hydrotalcites (HT). Further calcination yields mixed oxides with more open structure and tunable basicity. The basicity of the calcined hydrotalcites (HTC-M) strongly depends on the type of transition metal and calcination temperature. The resulting HTC-M materials were used as solid base catalysts and evaluated in the transesterification between glycerol and dimethyl carbonate without using organic solvent. The correlation between the basic properties of the solid catalysts and the catalytic performance was investigated. The activity of HTC-M catalysts was demonstrated to be proportional to the surface density of basic sites. The HTC-Ni calcined at 500°C exhibited maximum activity, which is about 10 times higher than uncalcined HT precursor for the transesterification reaction. The promotional effect of Ni2+ doping could be attributed to the enhancement of the base strength of all three types of basic sites of the calcined hydrotalcite. The HTC-Ni catalyst can be readily recycled while maintaining high catalytic activity and selectivity of glycerol carbonate.

Sodium modification of zirconia catalysts for ethanol coupling to 1-butanol

The influences of adding sodium to zirconia on the acid-base properties of the surface and on the catalytic conversion of ethanol and acetone were investigated. The rates of ethanol dehydration, dehydrogenation and coupling were evaluated in a fixed-bed flow reactor operating at temperatures from 613 to 673 K. The rate of acetone condensation was evaluated in the same reactor operating at 473–573 K. Addition of 1.0 wt% Na to ZrO2 decreased the rate of ethanol dehydration by more than an order of magnitude, which was consistent with a neutralization of acid sites evaluated by ammonia adsorption microcalorimetry. Addition of 1.0 wt% Na to ZrO2 also increased the base site density quantified by carbon dioxide adsorption microcalorimetry and the rate of acetone condensation. Although the rate of ethanol coupling was not increased by the addition of Na, the overall selectivity of ethanol to butanol was improved over the 1.0 wt% Na/ZrO2 sample because of the significant inhibition of ethanol dehydration.

1-Butanol synthesis from ethanol over strontium phosphate hydroxyapatite catalysts with various Sr/P ratios

Strontium phosphate hydroxyapatites (Sr-HAP) with various Sr/P molar ratios were synthesized under hydrothermal conditions. X-ray photoelectron spectroscopy revealed a close correlation between the near-surface and bulk Sr/P molar ratios of catalyst samples. Sr-HAP catalysts show high 1-butanol selectivity in the gas-phase conversion of ethanol. Sr-HAP catalysts with higher Sr/P molar ratios showed higher catalytic activity and 1-butanol selectivity. The density of both relatively strong basic sites and acidic sites was seen to increase with increasing Sr/P molar ratios, though the basic site density was significantly higher than the acidic site density. The rate-determining step in ethanol conversion over Sr-HAP catalysts was considered to be the dimerization process, as aldol condensation is mainly accelerated by base catalysis, which would explain why the Sr-HAP catalysts with higher basic site density showed higher catalytic activity and 1-butanol selectivity.

Acid–base properties of γ-Al2O3 and MgO–Al2O3 supported gold nanoparticles

Gold nanoparticles supported on γ-alumina and magnesia-doped γ-alumina were synthesized by co-precipitation via a modified sol–gel method. The syntheses were found to produce catalysts with controllable acidity; the presence of either gold or magnesia produced catalyst possessing unbalanced density of acidic and basic sites, while the addition of both gold and magnesia produced catalyst possessing balanced density of relatively stronger acidic and basic sites. The variation of the acid–base strengths of the synthesized catalysts was assessed by using NH3-TPD, CO2-TPD, and infrared analysis of adsorbed CO2 and correlated to their catalytic activity in 2-propanol decomposition probe reaction. Catalysts containing magnesia and gold were found to exhibit base catalytic properties through their greater selectivity for acetone formation, while plane γ-alumina and γ-alumina doped with magnesia or gold exhibited acid catalytic properties through their selectivity for propene formation.

Cu-TDPAT, an rht-Type Dual-Functional Metal–Organic Framework Offering Significant Potential for Use in H2 and Natural Gas Purification Processes Operating at High Pressures

The separations of CO2/CO/CH4/H2, CO2/H2, CH4/H2, and CO2/CH4 mixtures at pressures ranging to 7 MPa are important in a variety of contexts, including H2 production, natural gas purification, and fuel-gas processing. The primary objective of this study is to demonstrate the selective adsorption potential of an rht-type metal–organic framework [Cu3(TDPAT)(H2O)3]·10H2O·5DMA (Cu-TDPAT), possessing a high density of both open metal sites and Lewis basic sites. Experimental high pressure pure component isotherm data for CO2, CO, CH4, and H2 are combined with the Ideal Adsorbed Solution Theory (IAST) for estimation of mixture adsorption equilibrium. The separation performance of Cu-TDPAT is compared with four other microporous materials, specifically chosen in order to span a wide range of physicochemical characteristics: MgMOF-74, MIL-101, LTA-5A, and NaX. For all mixtures investigated, the capacity of Cu-TDPAT to produce the desired product, H2 or CH4, satisfying stringent purity requirements, in a fixed bed ...

How surface and textural properties affect the behaviour of Mn-based catalysts during transesterification reaction to produce biodiesel

This work was aimed at investigating the catalytic behaviour of MnCeOx and MnZrOx systems in biodiesel production by sunflower oil transesterification reaction with methanol. Surface and textural properties were deeply investigated by different techniques whereas CH3OH surface reactivity was evaluated by FT-IR and MeOH-TPSR measurements, in order to disclose the nature of the active sites and to provide evidences of a complex reaction mechanism, in which a synergic role of Mn and carrier significantly affects the methyl esters productivity. Direct relationship among catalytic activity, density of strong basic sites and amount of adsorbed bidentate methoxy species were found. The doubly-bridging coordinated methoxy species were recognized as the ones mainly involved in the transesterification reaction. Anyway, even if the basic strength resulted to be considered the key factor for MeOH activation, only the proper balance between basic and acid sites controls catalyst behaviour. The linear dependence of activity with catalyst porosity clearly demonstrated that a wide porous structure is required to enhance the sites accessibility to large triglycerides (TGs) molecules.

Synthesis of Glycerol Carbonate from Biomass Glycerol: Focus on the Calcination Temperature

We carried out a reaction in which glycerol carbonate was synthesized by using glycerol and urea. The physical properties of the prepared catalysts were investigated by BET surface area, X-ray diffraction, temperature programmed desorption, and Field-emission scanning electron microcopy. In addition, we confirmed the conversion of the glycerol and the yield of the glycerol carbonate according to calcination temperature. From XRD results, the crystalline of prepared ZnAl2O4 was slowly increased with an increasing of the calcination temperature. When the calcination temperature of ZnAl2O4 was 500 , the conversion of the glycerol was 74.4% and the yield of the glycerol carbonate was 73.3%. The conversion of the glycerol and the yield of the glycerol carbonate can be obtained higher when acid/base ratio is approx. 1. From this result, we may conclude that the acid & base site density and ratio of catalysts were very important parameters in the synthesis of glycerol carbonate from urea and glycerol.

Tunable basic and textural properties of hydrotalcite derived materials for transesterification of glycerol

Glycerol carbonate and glycerol dicarbonate were synthesised as glycerol value-added products via a transesterification reaction using diethyl carbonate as co-substrate. This green, efficient and selective process involved catalysis with different hydrotalcite-like compounds containing a Mg/Al molar ratio between 2 and 4 that were activated by calcination, by calcination followed by rehydration under ultrasound or by anion exchange (F−, Cl−, CO32−). The correlation between the basic properties of the solid catalysts and the activity in the glycolysis reaction was investigated. The catalytic activity shown by the different activated Mg/Al solids was proportional to the surface density of basic sites (number of medium and strong-strength basic sites presented in the catalyst per square meter). The highest activity was found for the rehydrated hydrotalcite catalysts, which showed basic Brönsted sites and which can be reused several times without significant loss of activity. Tuning the basic properties of hydrotalcites by means of changing the nature of the anion also offered interesting changes in the catalytic performance.

Industrial eggshell wastes as the heterogeneous catalysts for microwave-assisted biodiesel production

Active biodiesel production catalysts were derived from waste eggshells by simple calcination in air. The physicochemical properties of the activated catalysts were characterized by XRD, N2 sorption, CO2-TPD, TGA–DTG, XRF, and SEM, while the catalytic activity was tested in producing biodiesel via transesterification on palm oil with methanol under microwave conditions. The effect of microwave power, reaction time, methanol-to-oil ratio, and catalyst loading was investigated. The experimental results revealed that the catalysts exhibited a high content of CaO (99.2wt%) with a high density of strong base sites. The catalytic testing demonstrated a remarkable enhancement for biodiesel production using microwaves compared to conventional heating. The maximum yield of fatty acid methyl esters reached 96.7% under the optimal condition of reaction time of 4min with 900W microwave power, methanol-to-oil ratio of 18:1, and catalyst loading of 15%. The results indicated that the CaO catalysts derived from eggshells showed good reusability and had high potential to be used as biodiesel production catalysts under microwave-assisted transesterification of palm oil.

Enhanced Binding Affinity, Remarkable Selectivity, and High Capacity of CO2 by Dual Functionalization of a rht‐Type Metal–Organic Framework

Open and friendly: The smallest member of the rht-type metal–organic frameworks (MOFs, see picture) constructed by a hexacarboxylate ligand with a nitrogen-rich imino triazine backbone shows a significantly enhanced gas binding affinity relative to all other isoreticular rht-type MOFs. The high adsorption capacity and remarkable selectivity of CO2 are attributed to the high density of open metal and Lewis basic sites in the framework.

Sr–Mg Mixed Oxides as Biodiesel Production Catalysts

AbstractThe Sr–Mg basic catalysts were developed for biodiesel production through transesterification of palm oil with methanol. The evidence for synergistic effects between active Sr and Mg species was clearly demonstrated by transesterification tests over a series of Sr–Mg catalysts with varied Sr‐to‐Mg molar ratios. The catalyst properties were characterised by means of temperature‐programmed desorption of CO2 (CO2‐TPD), N2 sorption, XRD, SEM and TEM experiments. The super strong basic site was formed in Sr–Mg catalysts through a partial solid state reaction induced by thermal treatment at 600 °C. The new basic site, together with the original basic site from SrO (average basic site density=757 μmol m−2), effectively catalysed a transesterification reaction at mild conditions. Biodiesel containing 96 % methyl esters was obtained at reaction conditions of 75 min, 60 °C, 0.1 MPa, 3 wt. % catalyst loading and a methanol‐to‐oil ratio of 6:1, and the catalyst exhibited good reusability. Improved surface areas and porosities were also achieved compared to the unsupported SrO. Density functional theory (DFT) calculations of methylpropanoate adsorption on SrO, MgO and Sr/MgO showed that the adsorption energies of all adsorbate–surface complexes corresponded well with the experimental catalytic activity, which increased in the order MgO<SrO<Sr/MgO.

A designed organic–zeolite hybrid acid–base catalyst

An organic–zeolite hybrid catalyst was synthesised via solid-state impregnation of the zeolite with an amount of melamine corresponding to 20mol% of the aluminium content. A high density of basic sites is formed in the zeolite. From infrared spectroscopy and TGA measurements we infer that melaminium cations are formed in the zeolite which are highly thermally stable. IR spectroscopic and TGA measurements showed the presence of melaminium after pre-treatment in vacuum at 450°C. The catalyst exhibited activity in the Knoevenagel condensation reaction between benzaldehyde and ethyl cyanoacetate which is suggested to be promoted by the presence of acid and base sites. The catalyst activity was compared with other known base catalysts including hydrotalcites and magnesium oxide on alumina support.

Platinum-Based Catalyst for Diesel Hydrocarbon Oxidation

The catalytic performance of Pt-based catalysts for the total oxidation of hydrocarbons was investigated. The activity of supported Pt catalysts (Pt/Al 2 O 3 , Pt/ZrO 2 , Pt/TiO 2 , and Pt/H-ZSM-5) depends on the metal oxide support. Pt/Al 2 O 3 showed the highest catalytic activity when the catalysts were aged at 750°C for 50 h in air. The activity of Pt/Al 2 O 3 was dependent on the valence state of the Pt surface. Pt/Al 2 O 3 with the Pt surface in the metallic state was more active than with the surface in the cationic state. The surface density of acid and basic sites on the Al 2 O 3 support controlled the valence state of the Pt surface and stability of the Pt particles in the highly dispersed state, respectively. Pt/Al 2 O 3 that has the Pt surface in the metallic state was more active than with the surface in the cationic state. An Al 2 O 3 support with a high surface density of acid and basic sites is important.

Effect of MgO activation conditions on its catalytic properties for base-catalyzed reactions

The effect of the MgO calcination temperature on its basicity and catalytic properties was studied. Three MgO samples calcined at 673, 773 and 873 K (samples MgO-673, MgO-773 and MgO-873) were characterized by different physical and spectroscopic techniques. The surface base properties were probed by temperature-programmed desorption of CO 2 and infrared spectroscopy after CO 2 adsorption at 298 K and sequential evacuation at increasing temperatures. The dimensions of face-centered cubic unit cell for MgO samples decreased while crystallinity and mean crystallite size increased with calcination temperature. MgO samples contained surface sites of strong (low coordination O 2− anions), medium (oxygen in Mg 2+–O 2− pairs) and weak (OH − groups) basicity. The density of strong basic sites was predominant on MgO-673, but decreased with the calcination temperature together with the density of OH − groups; on the contrary, the density of Mg 2+–O 2− pair sites increased with calcination temperature. The catalytic properties of MgO samples were explored for the cross-aldol condensation of citral with acetone to obtain pseudoionones (PS), the transesterification of methyl oleate with glycerol to yield monoglycerides (MG), and the gas-phase hydrogen transfer reaction of mesityl oxide with 2-propanol to form 4-methyl-3-penten-2ol (UOL). The initial PS and MG formation rates decreased with calcination temperature following a trend similar to the density of strong basic sites which suggested that the rate limiting steps for both reactions involve coordinatively unsaturated O 2− active sites. In contrast, the initial UOL formation rate in mesityl oxide/2-propanol reaction increased with MgO calcination temperature following the same trend as medium-strength basic sites, thereby indicating that Mg 2+–O 2− pairs promote the formation of the six-atom cyclic intermediate needed in the Meerwein–Ponndorf–Verley mechanism.

MgO-based catalysts for monoglyceride synthesis from methyl oleate and glycerol: Effect of Li promotion

The synthesis of monoglycerides (glyceryl monooleates) by heterogeneously catalyzed glycerolysis of an unsaturated fatty acid methyl ester (methyl oleate) was studied on MgO and Li-promoted MgO catalysts. Several MgO-based catalysts with different Li loadings were prepared by incipient wetness impregnation and characterized by XRD, N 2 physisorption, and FTIR and TPD of CO 2 among other techniques. Promotion of MgO with lithium, a basic promoter, affected the textural and structural properties of the resulting oxides so that more crystalline MgO phases with decreased surface area were obtained at increasing Li contents. Furthermore, the addition of Li generated new strong base sites because of formation of dispersed surface Li 2O species, and thereby increased the total base site density of parent MgO. Li-containing MgO catalysts efficiently promoted the glycerolysis reaction, achieving high monoglyceride yields (70–73%) at 493 K. The initial monoglyceride formation rate increased linearly with the Li content on the sample following the enhanced overall catalyst base strength. Although conversions at the end of the run were ≈100% for all the catalysts, the monoglyceride selectivity slightly decreased with the Li loading, probably as a consequence of the less surface affinity for glycerol adsorption that facilitates competing monoglyceride re-adsorption and transformation to diglycerides by consecutive glycerolysis or disproportionation reactions.