Synthesis of biodiesel from soybean oil and methanol catalyzed by zeolite beta modified with La

La/zeolite beta was prepared by an ion exchange method and used to synthesize the biodiesel (fatty acid methyl esters, FAME). The La(NO₃)₃ was applied as the ion exchange precursor to incorporate La ion into zeolite beta. The composition of the zeolite beta before and after ion exchange was analyzed by the SEM microphotographs and EDS spectrograms, the Brønsted and Lewis acid sites were investigated by FTIR imaging. The transesterification was carried out in a batch reactor and the composition of the FAME product was determined by a potassium hydroxide saponification method. The syntheses conditions with respect to catalytic activities have been optimized individually. Results of the experiment showed that La/zeolite beta shows higher conversion and stability than zeolite beta for the production of biodiesel, which may be correlated to the higher quantity of external Brønsted acid sites available for the reactants. The product consists of a mixture of monoalkyl esters primarily, and when the methanol/ soybean oil molar ratio was 14.5, reaction temperature at 333 K, reaction time 4 h and catalyst/soybean oil mass ratio of 0.011, the conversion of triglyceride 48.9 wt% was obtained from this optimal reaction condition.     

Synthesis of ionones on solid Brønsted acid catalysts: Effect of acid site strength on ionone isomer selectivity

The effect of Brønsted acid site strength on the liquid-phase conversion of pseudoionone to ionone isomers (α-, β- and γ-ionone) was studied on resin Amberlyst 35W, silica-supported heteropolyacid (HPAS) and silica-supported triflic acid (TFAS). Catalyst acidity was probed by temperature-programmed desorption of NH₃ coupled with infrared spectra of adsorbed pyridine. The initial pseudoionone conversion rate followed the order: TFAS > Amberlyst 35W ≈ HPAS. Synthesis of the three ionone isomers occurred via a common cyclic carbocation intermediate formed from the activation of the pseudoionone molecule on Brønsted acid sites. Initial ionone mixtures containing a α:β:γ isomer distribution of about 40:20:40 were formed, irrespective of the acid site strength. But the ionone mixture composition changed with the progress of the reaction because γ-ionone was consecutively converted to α-ionone on HPAS and Amberlyst 35W, whereas the stronger acid sites of TFAS converted γ-ionone to β-ionone.     

Study of alkylation on a Lewis and Brønsted acid hybrid catalyst and its industrial test

A novel catalyst ZnBr₂ modified bentonite is prepared. Pyridine adsorbed FTIR spectra demonstrated the amount of Brønsted and Lewis acid was improved to 2 and 5 times, respectively. Thermal analysis indicated the thermal stability of ZnBr₂-bentonite could reach 350 °C. Mechanism research led to an interesting hypothesis that in the former stage, the active position is Brønsted acid sites and in the later stage, both Lewis acid and Brønsted acid sites act as active positions. Industrial test showed the ability of ZnBr₂-bentonite to remove olefins is 17 times higher than that of bentonite, and the industrial application prospect is good.     

Synthesis and application of a novel strong and stable supported ionic liquid catalyst with both Lewis and Brønsted acid sites

Poly(4-vinylpyridine)-supported ionic liquid with both Lewis and Brønsted acid sites was easily prepared from its starting materials and used as a novel and highly efficient heterogeneous catalytic system for the synthesis of biscoumarins by two-component one-pot domino Knoevenagel-type condensation/Michael reaction between various aliphatic and aromatic aldehydes with 4-hydroxycoumarin. The Lewis and Brønsted acidic sites loading in [P₄VPy-BuSO₃H]Cl-X(AlCl₃) were found to be 2.15 and 0.9 mmol per gram of catalyst, respectively. The effect of the simultaneous presence of Lewis and Brønsted acid sites was evaluated. The catalyst was characterized by Fourier transform infrared spectroscopy (FT-IR), thermal gravimetric analysis (TGA), scanning electron microscopy (SEM), elemental analysis, and atomic absorption technique. The catalyst is stable (as a bench top catalyst) and reusable.     

Enzymatic conversion of corn oil into biodiesel in a batch supercritical carbon dioxide reactor and kinetic modeling

Synthesis of fatty acid methyl esters (FAME) as biodiesel from corn oil was studied in a batch supercritical carbon dioxide (SC-CO₂) bioreactor using immobilized lipase (Novozym 435) as catalyst. Effects of reaction conditions on the contents of FAME, monoacylglycerols (MAG), diacylglycerols (DAG), and triacyglycerols (TAG) were investigated at various enzyme loads (5–15%), temperatures (40–60 °C), substrate mole ratios (corn oil:methanol; 1:3–1:9), pressures (10–30 MPa), and times (1–8 h). The highest FAME content (81.3%) was obtained at 15% enzyme load, 60 °C, 1:6 substrate mole ratio, and 10 MPa in 4 h. A reaction kinetic model was used to describe the system, and the activation energy of the system was calculated as 72.9 kJ/mol. Elimination of the use of organic solvents, chemical catalysts and wastewater as well as reasonably high yields make the enzymatic synthesis of biodiesel in SC-CO₂ a promising green alternative to conventional biodiesel process.     

Small-sized HZSM-5 zeolite as highly active catalyst for gas phase dehydration of glycerol to acrolein

The catalytic properties of nanocrystalline HZSM-5 catalysts with high Si/Al molar ratio (ca. 65) were investigated in the gas phase dehydration of aqueous glycerol. Compared with bulk HZSM-5, the small-sized catalyst exhibits greatly enhanced catalytic performance in glycerol dehydration even with very high GHSV (=1438 h^?1). Catalysts with different Si/Al ratios were studied, but it is difficult to separate the influence of Si/Al ratio from that of particle size. However, by varying the proton exchange degree for one mother batch of zeolite, a series of H_xNa_1–xZSM-5 catalysts with same particle size and different Brønsted acid site densities was prepared. The catalytic results for this series of samples show that high density of Brønsted acid sites favors the production of acrolein. Based on these results, small-sized HZSM-5 with high aluminum content appears to be most promising for gas phase dehydration of glycerol.     

Platinum nanoparticles as active sites for C–С bond activation in high-silica zeolites

Density functional theory (DFT) was applied to investigate the structure and reactivity of Pt₆ particles encapsulated in silicalite as well as in sodium and hydrogen ZSM-5 zeolites. Incorporation of a metal particle in silicalite or sodium zeolite leads to the formation of a negatively charged metal cluster with stabilization energies close to 9 and 16 kcal/mol, respectively. Interaction of a platinum cluster with hydrogen zeolite and extra-framework aluminum species is characterized by energies as high as 45–50 kcal/mol and yields an electron-deficient metal particle. This positively charged metal cluster stabilized in the zeolite channel can function as active site in alkane transformations. The formation of the active site is accompanied by suppression of both Brønsted and Lewis acidities. An alternative mechanism of alkane cracking and isomerization avoiding the direct participation of acid sites has been proposed.     

Effect of structure and acidity of acid modified clay materials on synthesis of octahydro-2H-chromen-4-ol from vanillin and isopulegol

The Prins cyclization of (−)-isopulegol with vanillin to form octahydro-2H-chromen-4-ol was studied in the presence of natural layered aluminosilicates modified by 0.5 mol/dm³ HCl, such as montmorillonite, kaolin, and metakaolin obtained by the calcination of kaolin at 650 °C. According to infrared spectroscopy using pyridine as probe molecule, the amount and strength of Brønsted acid sites depend on the type of clay and decrease in the following order HCl-montmorillonite > HCl-kaolin > HCl-metakaolin. The difference in Brønsted acidity and textural properties of clays affected the reaction rate and the selectivity towards octahydro-2H-chromen-4-ol.     

Quantitative characterization of the solid acidity of montmorillonite using combined FTIR and TPD based on the NH3 adsorption system

The complete parameters of montmorillonite solid acidity, namely amount, strength, and types of acidity, were determined and the properties of the acid sites after heating were proposed by combining the temperature-programmed desorption (TPD) and Fourier transform infrared spectroscopy (FTIR) based on the NH₃ adsorption system. The total amount of montmorillonite acid sites was 1.15 mmol/g, which was higher than the value obtained by the Hammett indicator method because of the detection of solid acid sites in the montmorillonite interlayer space. These acid sites were composed of 1.00 mmol/g Brønsted and 0.15 mmol/g Lewis acid sites. The acidity of montmorillonite was primarily derived from the interlayer polarized water, Si–OH, H₃O⁺ adsorbed by negatively charged tetrahedral AlO₄, and unsaturated Al^3 + ions, all of which were attributed to the Brønsted acid sites with the exception of the unsaturated Al^3 + ions (Lewis acid sites). Heating led to an increase in the acid strength and the acid amount and altered the type of the partial acid sites. The interlayer polarized water provided more protons after heating at 120 °C and exhibited higher acid strength than that of raw montmorillonite. After heating at 400 °C, the interlayer polarized water acted as very strong acid sites. The H₃O⁺ adsorbed by tetrahedral AlO₄ was attributed to weak-strength acid sites and transformed into Si–O(H)–Al after dehydration, while displaying strong-strength acidity. The unsaturated Al^3 + ions showed medium-strength Lewis acidity, although a portion of these ions adsorbed water molecules and exhibited weak Brønsted acidity. After dehydroxylation at 600 °C, an abundance of unsaturated Al^3 + ions appeared and the amount of Lewis acid sites increased.     

Role of acidity of catalysts on methane combustion over Pd/ZSM-5

Palladium-based catalysts were prepared by the impregnation (I) and ion-exchange method (E) with ZSM-5 and γ-Al₂O₃ as support respectively. The high activity of Pd/ZSM-5(I) and Pd-ZSM-5(E) catalysts for methane combustion was observed. The order of activity is consistent with Brønsted acidity of catalysts: Pd/ZSM-5(I) > Pd-ZSM-5(E) > Pd/Al₂O₃. It is shown by FT-IR that methane adsorbs on acidic bridging hydroxyl groups of ZSM-5-supported Pd catalysts. Symmetric v₁ C–H stretching vibrations of methane shift to low frequency due to the interaction between methane molecules and Brønsted acid sites or Pd²⁺, indicating that methane molecules can be activated.     

Surface acidity study of Mn+-montmorillonite clay catalysts by FT-IR spectroscopy: Correlation with esterification activity

A simple method for evaluating the surface acidity of different cation-exchanged montmorillonite (mont) clay catalysts, Mⁿ⁺-mont (Mⁿ⁺=Al³⁺, Fe³⁺, Cr³⁺, Zn²⁺, Ni²⁺, Cu²⁺, and H⁺), involving treatment with pyridine is described. After treating with pyridine, the samples were heated at 120 °C and the FT-IR spectra were directly recorded in the region 1650 and 1350 cm⁻¹. The data obtained show the presence of both Lewis and Brønsted acid sites. The activities of the catalysts to bring about Brønsted acid catalysed esterification of succinic acid with iso-butanol to yield di-(iso-butyl) succinate have been studied. The Brønsted acidity data obtained for Mⁿ⁺-mont correlated well with activity in the esterification reaction. The activities of the catalysts were found to decrease in the order of exchange ions Al³⁺ > Fe³⁺ > Cr³⁺ > Zn²⁺ > Ni²⁺ > Cu²⁺ > Na⁺-mont. They also correlated well with the charge to radius ratio of the cations. The catalysts exchanged with trivalent cations showed stronger absorption bands attributed to Brønsted acidity (1540 cm⁻¹) whereas those exchanged with divalent cations showed an increased Lewis acidity (1450 cm⁻¹) and reduced Brønsted acidity along with charge to radius ratio. Zn²⁺-, Cu²⁺- and Ni²⁺-exchanged clays showed an additional peak around 1605 cm⁻¹ which is attributed to the pyridine adsorption on surface sites through its π electrons. The method suggested here to evaluate the acidity is suitable for active sites which are thermally unstable such as water molecules in the hydration shell of a cation in exchanged clay.     

Zirconia-supported niobia catalyzed formation of propanol from 1,2-propanediol via dehydration and consecutive hydrogen transfer

Vapor-phase catalytic dehydration of 1,2-propanediol was investigated over Zirconia-supported niobia catalysts. The catalysts exhibit selectivity favoring propanol (approximately 39%) at 85.0% 1,2-propanediol conversion at 290 °C under 1 atm N₂. The ZrNbO catalysts were analyzed by various techniques; the results indicated that the active sites were weak Brønsted acid sites. A dehydration and hydrogen transfer mechanism was also proposed.     

Synthesis,characterization and catalytic evaluation of zirconia-pillared montmorillonite for linear alkylation of benzene

Zirconia-pillared montmorillonite was synthesized by an ultrasonication method and characterized by XRD, FT-IR of adsorbed pyridine, XPS, XRF and low temperature nitrogen adsorption techniques. The basal spacing of d(0 0 1) reflection of the pillared clay estimated by XRD was found to be 20 Å. XRD study showed well distribution of pillars without any non-pillared portion of the clay. FT-IR studies showed the formation of both Lewis and Brönsted acid sites due to pillaring. Pyridine desorption studies revealed that the Brönsted sites were stronger than Lewis acid sites. The activity of the Zr-PILC for the alkylation of benzene with long chain α-olefins [C₁₀–C₁₃] was investigated. The Brönsted acid sites played major role in this reaction.     

Monolithic acidic catalysts for the dehydration of xylose into furfural

We report the application of hierarchically porous zirconium phosphate monoliths with high surface area as acidic heterogeneous catalysts for the dehydration of xylose into furfural. Analyses by NH₃-temperature programmed desorption and ³¹P solid state NMR reveal the presence of both Lewis and Brønsted acid sites in the as-synthesized zirconium phosphate monolith and that calcined at 600 °C. High accessibility and availability of the acidic sites and easy separation of the monolith from the liquid medium result in good catalytic activity (initial reaction rate for furfural production as 8.7 mmol g_cat^− 1 h^− 1) with easy handling of the catalyst.     

Catalytic hydrogenation of alkenes on acidic zeolites: Mechanistic connections to monomolecular alkane dehydrogenation reactions

Brønsted acid sites in zeolites (H-FER, H-MFI, H-MOR) selectively hydrogenate alkenes in excess H₂ at high temperatures (>700 K) and at rates proportional to alkene and H₂ pressures. This kinetic behavior and the De Donder equations for non-equilibrium thermodynamics show that, even away from equilibrium, alkene hydrogenation and monomolecular alkane dehydrogenation occur on predominantly uncovered surfaces via microscopically reverse elementary steps, which involve kinetically-relevant (C–H–H)⁺ carbonium-ion-like transition states in both directions. As a result, rate constants, activation energies and activation entropies for these two reactions are related by the thermodynamics of the overall stoichiometric gas-phase reaction. The ratios of rate constants for hydrogenation and dehydrogenation reactions do not depend on the identity or reactivity of active sites; thus, sites within different zeolite structures (or at different locations within a given zeolite) that favor alkane dehydrogenation reactions, because of their ability to stabilize the required transition states, also favor alkene hydrogenation reactions to the exact same extent. These concepts and conclusions also apply to monomolecular alkane cracking and bimolecular alkane–alkene reaction paths on Brønsted acids and, more generally, to any forward and reverse reactions that proceed via the same kinetically-relevant step on vacant surfaces in the two directions, even away from equilibrium. The evidence shown here for the sole involvement of Brønsted acids in the hydrogenation of alkoxides with H₂ is unprecedented in its mechanistic clarity and thermodynamic rigor. The scavenging of alkoxides via direct H-transfer from H₂ indicates that H₂ can be used to control the growth of chains and the formation of unreactive deposits in alkylation, oligomerization, cracking and other acid-catalyzed reactions.     

Conversion of carbohydrate biomass to methyl levulinate with Al2(SO4)3 as a simple,cheap and efficient catalyst

Al₂(SO₄)₃ was developed as a simple and efficient catalyst for the synthesis of methyl levulinate (MLE) from carbohydrate biomass including fructose, glucose, mannose, sucrose, cellobiose, starch, and cellulose. The maximum MLE yield of 64% from glucose could be obtained at 160 °C for 150 min. Important roles of Al^3 + and Brønsted acid sites generated by the hydrolysis/methanolysis of Al^3 + were elucidated. The reaction process was revealed using in situ real-time attenuated total reflection infrared spectroscopy. Al₂(SO₄)₃ can be regenerated easily and reused at least four times without the loss of activity.     

The effect of Ga and Zn over HZSM-5 on the transformation of palm fatty acid distillate (PFAD) to aromatics

The catalytic conversion of PFAD (palm fatty acid distillate) to aromatics has been studied over HZSM-5, Ga/HZSM-5, and Zn/HZSM-5 catalysts. The presence of both Ga and Zn promoted the aromatization of PFAD. The higher aromatics yield of Zn/HZSM-5 was achieved by the presence of two zinc species; exchanged Zn^2 + promoting the dehydrogenation of paraffins and ZnO promoting the decarboxylation of oxygenates. The shifting from decarbonylation over the Brønsted acid site of the parent HZSM-5 to decarboxylation over ZnO preserved the Brønsted acid site for aromatization, thus increasing the aromatics yield.     

Microwave assisted solvent free synthesis of α,ά-bis (arylidene) cycloalkanones by sulfated zirconia catalyzed cross aldol condensation of aromatic aldehydes and cycloalkanones

The catalytic application of sulfated zirconia as solid Brønsted acid catalyst was explored for cross aldol condensation reactions (Claisen Schmidt reaction). The synthesized catalyst was highly active for solvent free synthesis of α,?-bis(arylidene)cycloalkanones by cross aldol condensation of aromatic aldehydes with cycloalkanones. The microwave assisted synthesis resulted increased yields of the products (79–99%) at significantly lower reaction temperature (120–140 °C) and reaction time (20 min) as compared to the synthesis by thermal heating (63–96% yield at 170 °C after 4 h). The microwave irradiation afforded selectively cross aldol products. The catalyst could be easily regenerated and reused several times with similar efficiency.     

Variable temperature FTIR study on the surface acidity of variously treated sulfated zirconias

The acidity of three related sulfated zirconias has been compared by IR spectroscopy using CO and H₂ as probe molecules. The parent material (SZ) was obtained by calcination of a commercial sulfated zirconium hydroxide. The other two samples, SZ-WW and SZ-SO₃, were obtained from SZ by water washing and by sulfation with gaseous SO₃, respectively. The labile sulfate groups responsible for alkane activation and associated with an IR band at 1406 cm⁻¹ are removed by water washing and increased by SO₃-sulfation. No remarkable differences in the strength of either Lewis or Brønsted acid sites have been observed between SZ and SZ-SO₃. Water washing strongly weakens Brønsted acidity but only slightly weakens Lewis acidity.     

Highly selective olefin hydrogenation: Refinery oil upgrading over bifunctional PdOx/H-ZSM-5 catalyst

A novel strategy is reported in this study to design a novel bifunctional PdO_x/H-ZSM-5 catalyst for selective hydrogenation of olefin. PdO_x species (basic oxides) anchored on the strong acidic sites (Brönsted acid sites) of zeolite support could not only generate new acidic sites with moderate acidic strength but also exhibit good capability for H₂ activation and dissociation. The conversion of 1-decene is 100% and total selectivity of methyl-nonane is 98.4 ± 0.4% at 350 °C. Our research might provide a valuable approach to design a highly efficient bifunctional catalyst for olefin upgrading.