High Acid Site Research Articles

Study on Preparation and Catalytic Properties of Pd/γ-Al<sub>2</sub>O<sub>3</sub> Catalysts in One-Step Synthesis of Dimethyl Ether

A series of Pd/γ-Al2O3 catalysts with different additons of Pd were prepared by impregnation. The effect of calcination condition and Pd loading on catalytic performance of catalysts for one-step dimethyl ether synthesis has been investigated. The physic-chemical performance and structure of Pd/γ-Al2O3 catalysts were characterized by CO-TPD, TGA and nitrogen physisorption. The results show that the dispersion of Pd and the amount of adsorbration on the CO-bridge of Pd could be increased to by the moderate microwave heating on the catalysts, and the catalyst performance can be improved. But a large number of surface acidic sites of Pd/γ-Al2O3 are covered by highly fragmented Pd-grain, it causes DME selectivity reduced. And the excessive Pd can reduce the samples’ surface acid, decrease the dispersion of the metal Pd and block up the pore of γ-Al2O3. The CO conversion rate and DME space-time yield could reach 60.1% and 28.76 mmol·g-1·h-1 respectively at 2% Pd loading, at this time Pd/γ-Al2O3 has a high Pd activity surface and ideal acid sites.

Production of biodiesel from mixed waste vegetable oil using an aluminium hydrogen sulphate as a heterogeneous acid catalyst

Al(HSO4)3 heterogeneous acid catalyst was prepared by the sulfonation of anhydrous AlCl3. This catalyst was employed to catalyze transesterification reaction to synthesis methyl ester when a mixed waste vegetable oil was used as feedstock. The physical and chemical properties of aluminum hydrogen sulphate catalyst were characterized by scanning electron microscopy (SEM) measurements, energy dispersive X-ray (EDAX) analysis and titration method. The maximum conversion of triglyceride was achieved as 81wt.% with 50min reaction time at 220°C, 16:1molar ratio of methanol to oil and 0.5wt.% of catalyst. The high catalytic activity and stability of this catalyst was related to its high acid site density (–OH, Brönsted acid sites), hydrophobicity that prevented the hydration of –OH group, hydrophilic functional groups (–SO3H) that gave improved accessibility of methanol to the triglyceride. The fuel properties of methyl ester were analyzed. The fuel properties were found to be observed within the limits of ASTM D6751.

On the influence of pore geometry and acidity on the activity of parent and modified zeolites in the synthesis of methylenedianiline

Methylenedianiline synthesis from formaldehyde and aniline is catalyzed by solid acid catalysts, such as zeolites, which offer strong acidity and high acid site density. Critical parameters for catalyst activity were explored using materials with MFI, BEA, MOR and FAU structure. While materials with MOR and MFI structure hardly show activity, BEA and FAU type zeolites are active and selective. The higher activity for the latter materials is related to their larger pore structure. Nevertheless, the reaction is limited by pore diffusion over parent BEA- and FAU-type zeolites. Mesoporous materials, such as dealuminated FAU, offer significantly higher activity as pore diffusion limitation can be overcome. Therefore, we synthesized and tested other mesoporous aluminosilicates such as desilicated BEA and the delaminated MWW materials ITQ-2 and MCM-36. It was shown that desilication increases the activity of parent BEA type zeolites for MDA formation, yet not to the same extend as dealumination in case of FAU. ITQ-2 shows promising results, but MCM-36 surpasses ITQ-2 in terms of activity by a factor of two displaying activities similar to the dealuminated FAU materials.

Esterification of industrial-grade palm fatty acid distillate over modified ZrO 2 (with WO 3–, SO 4 –and TiO 2–): Effects of co-solvent adding and water removal

The esterification of palm fatty acid distillate (PFAD), a by-product from palm oil industry, in the presence of three modified zirconia-based catalysts i.e. SO 4–ZrO 2, WO 3–ZrO 2 and TiO 2–ZrO 2 (with several sulfur- and tungsten-loading contents, Ti/Zr molar ratios, and calcination temperatures) was studied. It was found that, among all synthesized catalysts, the reaction in the presence of SO 4–ZrO 2 and WO 3–ZrO 2 (with 1.8%SO 4 calcined at 500 °C and/or 20%WO 3 calcined at 800 °C) enhances relatively high fatty acid methyl ester (FAME) yield (84.9–93.7%), which was proven to relate with the high acid site density and specific surface area as well as the formation of tetragonal phase over these catalysts. The greater benefit of WO 3–ZrO 2 over SO 4–ZrO 2 was its high stability after several reaction cycles, whereas significant deactivation was detected over SO 4–ZrO 2 due to the leaching of sulfur from catalyst. For further improvement, the addition of toluene as co-solvent was found to increase the FAME yield along with reduce the requirement of methanol to PFAD molar ratio (while maintains the FAME yield above 90%). Furthermore, it was observed that the presence of water in the feed considerably lower the FAME yield due to the catalyst surface interfering by water and the further hydrolysis of FAME back to fatty acids. We proposed here that the negative effect can be considerably minimized by adding molecular sieve to remove water from the feed and/or during the reaction.

Synthesis of biodiesel from waste vegetable oil with large amounts of free fatty acids using a carbon-based solid acid catalyst

A carbon-based solid acid catalyst was prepared by the sulfonation of carbonized vegetable oil asphalt. This catalyst was employed to simultaneously catalyze esterification and transesterification to synthesis biodiesel when a waste vegetable oil with large amounts of free fatty acids (FFAs) was used as feedstock. The physical and chemical properties of this catalyst were characterized by a variety of techniques. The maximum conversion of triglyceride and FFA reached 80.5 wt.% and 94.8 wt.% after 4.5 h at 220 °C, when using a 16.8 M ratio of methanol to oil and 0.2 wt.% of catalyst to oil. The high catalytic activity and stability of this catalyst was related to its high acid site density (–OH, Brönsted acid sites), hydrophobicity that prevented the hydration of –OH species, hydrophilic functional groups (–SO 3H) that gave improved accessibility of methanol to the triglyceride and FFAs, and large pores that provided more acid sites for the reactants.

N-Pentane Hydroconversion Using Pt-loaded Zeolite Catalysts

Pt/H-ZSM-5 and Pt/H-MOR catalysts with different Pt contents were prepared via impregnation using H2PtCl6 · 6H2O or via exchange using Pt(NH3)4Cl2, calcination in air at 530°C and reduction in H2 at 500°C. The prepared catalysts were tested for n-pentane hydroisomerization and hydrocracking via bifunctionality at 250–500°C using a micro-catalytic pulse reactor. It is found that the dispersion of Pt-exchanged zeolites is higher than the corresponding Pt-impregnated zeolites at all Pt contents. It is also found that the dispersion of Pt/H-ZSM-5 catalysts either exchanged or impregnated are higher than the corresponding Pt/H-MOR catalysts. Temperature-programmed desorption (TPD) data showed that the impregnated catalysts possess a higher acid sites number than the exchanged catalysts; and that the Pt/H-ZSM-5 catalysts have a higher number of acid sites than do the Pt/H-MOR catalysts, whereas the latter catalysts possess higher strength of acid sites at all Pt contents. The hydroisomerization activities using Pt exchanged catalysts, supported either on H-ZSM-5 or H-MOR, are higher than the impregnated catalysts at almost all Pt contents. It is also concluded that the H-ZSM-5-supported catalysts, either exchanged or impregnated, are more active than the H-MOR supported ones. Hydrocracking is higher using all loaded H-MOR catalysts.

Synthesis of biodiesel from cottonseed oil and methanol using a carbon-based solid acid catalyst

A carbon-based solid acid catalyst was prepared by the sulfonation of carbonized vegetable oil asphalt and used to catalyze the transesterification of methanol with cottonseed oil. This catalyst was characterized by scanning electron microscopy/energy dispersive spectroscopy, BET surface area and pore size measurement, thermogravimetry analysis and Fourier transform infrared spectroscopy. The sulfonated multi-walled carbon nanotubes (s-MWCNTs) was also prepared and used to catalyze the same transesterification as the asphalt catalyst. The asphalt-based catalyst shows higher activity than the s-MWCNTs for the production of biodiesel, which may be correlated to its high acid site density, its loose irregular network and large pores can provide more acid sites for the reactants. The conversion of cottonseed oil 89.93% was obtained (using the asphalt-based catalyst) when the methanol/cottonseed oil molar ratio was 18.2, reaction temperature at 260 °C, reaction time 3.0 h and catalyst/cottonseed oil mass ratio of 0.2%. Also, it can be re-used. The sulfonated polycyclic aromatic hydrocarbons provide an electron-withdrawing function to keep the acid site stable. The catalyst can substantially reduce energy consumption and waste generation in the production of biodiesel.

IR study on methanol-to-olefin reaction over zeolites with different pore structures and acidities

The effects of the pore structure and acidity of zeolites on their product distribution and deactivation rates in the methanol-to-olefin (MTO) reaction were studied according to the different topologies of CHA, LTA, MFI, BEA, MOR and FAU zeolites. The in situ IR study of the MTO reaction over various zeolites with different pore structures and acidities confirmed the considerable effect exerted by these factors on the species of alkylaromatics formed and on the rates of further condensation of alkylaromatics to polycyclic aromatic hydrocarbons (PAHs). The nature and continuing concentration of alkylaromatics formed in the zeolite pores determined their product composition and deactivation rate in the MTO reaction. LTA and FAU zeolites with large cages and high acid site density were deactivated rapidly due to the high concentration of alkylaromatics with large substituents and their rapid condensation to PAHs. However, the small cages of CHA zeolite and the sinusoidal pores of MFI zeolite suppressed the accumulation of large hydrocarbon molecules in their pores and lengthened their catalyst lives, although the composition of the products formed over them clearly differed because of their different pore structures.

Influence of Structure Type of Al2O3 on Dehydration of Methanol for Dimethyl Ether Synthesis

This study focused on the effect of alumina properties on the catalytic performance in the dehydration of methanol to synthesize dimethyl ether. The catalysts (γ-Al2O3 and η-Al2O3) were prepared from boehmite and bayerite, respectively, via calcination at various temperatures. With heat treatment, the properties of alumina have been changed significantly. Under identical reaction conditions, γ-Al2O3 calcined at 500 °C and η-Al2O3 calcined at 600 °C showed the highest catalytic activity, because of high acid site density among the same types of alumina. Interestingly, η-Al2O3 showed higher catalytic activity than γ-Al2O3, although both catalysts possess comparable acidic properties. This means the structure type of alumina is crucial to determine catalytic performance in this reaction.

Product selectivity and catalytic deactivation of MOR zeolites with different acid site densities in methanol-to-olefin (MTO) reactions

The effects of the acid site densities of MOR zeolites on their product selectivity and deactivation were investigated in the methanol-to-olefin (MTO) reactions. The MOR zeolites showed a high conversion and high yield of the lower olefins at the initial time of the MTO reaction, regardless of their acid site density. However, the conversion and the yield of the lower olefins over them rapidly decreased in the order of MOR(103) < MOR(55) < MOR(12) < MOR(5), where the numbers in parenthesis represent their Si/Al ratios. The materials occluded on the MOR zeolites during the MTO reaction were investigated by the IR, 13C NMR and UV–vis spectroscopic methods. A large amount of polycyclic aromatic hydrocarbons (PAHs) with 3–4 fused aromatic rings was occluded on the MOR(5) zeolite with the highest acid site density. However, alkylbenzenes and alkylnaphthalenes were the major components of the materials occluded on the MOR(103) zeolite with the lowest acid site density. The IR spectra recorded in situ during the MTO reaction over the MOR zeolites clearly exhibited the differences in the accumulation rates of PAH on their pores according to the acid site density. The rapid accumulation of PAH on the MOR zeolites with high acid site densities leads to the loss of the active alkylbenzene intermediates as well as to the blockage of the pores, resulting in their rapid deactivation. The sparse distribution of acid sites in the pores of the MOR zeolites suppresses the condensation of alkylbenzenes in their pores and lengthens their catalyst lifetime.

Catalytic activity and magnetic properties of Co–ZSM-5 zeolites prepared by different methods

Abstract The selective catalytic oxidation of styrene with hydrogen peroxide was studied over Co–ZSM-5 catalysts which differ in the preparation methods. The catalysts were obtained from NH 4 + –ZSM-5 by wetness impregnation and ion-exchange techniques followed by different post-thermal treatments: oxidation and reduction. The characterization of acidic properties by FTIR of adsorbed pyridine revealed that new and strong Lewis sites (1452 cm −1 ), different from those characteristics of aluminum in the matrix (1456 cm −1 ) were generated by cobalt introduction. Styrene conversion shows an important increment over the exchanged samples as compared to the impregnated ones. This behavior may be explained by the high Lewis acid sites concentration. Benzaldehyde was the main product in all the samples under study. The samples’ magnetic properties were improved with the cobalt incorporation and the applied thermal treatments. The catalytic and magnetic properties of Co–ZSM-5 samples are discussed.

Deactivation and Coke Formation on Nickel−Tungsten Supported on Silica−Alumina Catalysts

Nickel−tungsten oxides supported on silica−alumina catalysts were submitted to the coking reaction with n-octane at 150, 250, and 300 °C. The characteristics of deposited coke with different times on stream were studied using the temperature-programmed oxidation (TPO) technique. The results revealed that the strength and density of acid sites as well as the reaction temperature affect the nature of the coke formed: at 300 °C, it was observed that for the WT7 catalyst, the most acidic solid, there were two peaks corresponding to two kinds of coke recorded in TPO profiles, while at 150 °C only one peak was observed. On the other hand, for the WT4 catalyst, the less acidic sample, only the soft coke was observed. In addition, the oxidation temperature of coke shifted to higher values with high medium acid sites concentration and high temperature values with the increase of coke deposits. Besides, coke molecules interacted preferentially with the most acidic sites, which was confirmed by the strong deactivation in the first reaction stages, especially for the WT7 catalyst. For the regeneration tests, the collected data suggested that the coked WT4 catalyst showed a good combustion behavior; it was regenerated at 500 °C for 1 h, whereas the coked WT7 could not be regenerated at this temperature.

Preparation, characterization and catalytic activity of tin oxide-supported 12-tungstophosphoric acid as a solid catalyst

A series of solid acid catalysts were synthesized by incipient wetness impregnation method by varying the wt% of 12-tungstophosphoric acid (TPA) on tin oxide. The prepared catalysts were characterized by thermal analysis, XRD, FT-IR and BET surface area. The acidity of the catalyst was measured by different techniques. The suitability of the materials was studied for acid-catalyzed esterification reaction using acetic acid and amyl alcohol. The acidity measurements showed that the total acidity increases with the rise of TPA content up to 20 wt% (monolayer coverage) and decreases thereafter. FT-IR spectra of pyridine adsorbed on the catalysts showed the presence of both Brønsted and Lewis acidities. Material with 20 wt% TPA on tin oxide having high surface area and acid sites acts as the better catalyst for esterification reaction. For the material that exhibits the optimum catalytic activity, selective poisoning of the potential catalytic centers revealed that Brønsted sites play the major role in carrying out the reaction. The straight-line plots of −ln(1-ester formed) versus catalyst weights support that the esterification reaction obeys first-order kinetics. The reusability study justifies that the catalyst is stable and active.

NiMCM-36 and NiMCM-22 catalysts for the ethylene oligomerization: Effect of zeolite texture and nickel cations/acid sites ratio

Nickel ion-exchanged MCM-36 and MCM-22 zeolites were prepared, characterized, and tested in the ethylene oligomerization reaction performed in slurry semi-batch mode (T = 70–150 °C, p = 4 MPa). The behavior of catalysts was evaluated in relation to their texture and nickel cations/acid sites ratio. NiMCM-36 catalysts, with large accessibility in the mesoporous structure and mild acidity, showed an excellent activity (46 g of oligomers/gcatalyst h) and selectivity (100% olefins with even number of carbon atoms) in ethylene oligomerization, while lower catalytic activity and selectivity were observed over NiMCM-22 possessing microporous structure and high acid sites concentration.

Silicotungstic acid supported zirconia: An effective catalyst for esterification reaction

A series of solid acid catalysts were synthesized by incipient wetness impregnation method by varying the wt% of silicotungstic acid on hydrous zirconia (ZSTA). The prepared catalysts were characterized by PXRD, FTIR, UV–vis DRS, EPMA, BET surface area, acid sites, etc. FTIR and UV–vis DRS studies indicate that the material retain the Keggin-type structure of silicotungstic acid up to 500 °C. The suitability of the materials was studied for acid catalysed esterification reactions using formic, acetic, propionic, n-butyric acid and n-butyl alcohol (NBA), isobutyl alcohol (IBA) and sec-butyl alcohol (SBA). Material with 15 wt% STA on hydrous zirconia having high surface area and acid sites acts as better catalyst for esterification reactions. The esterification of acids with NBA was found to be higher than IBA and SBA. In all the cases, the selectivity for the corresponding esters is nearly100%. The straight-line plot of −ln(1 − conversion) versus reaction time for the reactions carried out at 98 °C supports that the esterification reaction obeys first order kinetics with respect to acid concentration. The reusability study justifies that the catalyst is stable and active.

A comparative study of solid sulfonic acid catalysts based on various ordered mesoporous silica materials

Acid catalysts based on ordered mesoporous silica materials (MCM-41, HMS, and SBS-15) with covalently anchored propanesulfonic or benzenesulfonic acid groups were synthesized. Six samples were prepared by co-condensation of tetraethylorthosilicate with 3-mercaptopropyltrimethoxysilane or phenyltriethoxysilane in the presence of appropriate templating agents. Six other samples were made by post-synthesis modification of the parent silica materials. Syntheses were followed by oxidation or sulfonation to form surface sulfonic acid groups. Sample characterization was performed by physical (low-angle X-ray powder diffraction, nitrogen adsorption and desorption, diffuse reflectance IR, Raman, NMR, and X-ray photoelectron spectroscopies) and chemical (acid–base titration) methods. Spectroscopic techniques proved successful incorporation of the functional groups. Functionalization of the parent silicas, however, results in changes in structural characteristics (decreasing BET surface areas, changes in pore size distribution, decreasing long range orders). The samples exhibit high activities in various catalytic transformations. Catalysts with anchored benzenesulfonic acid groups of high acid strength and high acid site densities allow the selective synthesis of the cyclic dimer indan product in the dimerization of 2-phenylpropene. 2,3,5-Trimethylhydroquinone diacetate can also be produced in similar high yields in the rearrangement–aromatization of ketoisophorone. Characteristic differences in selectivities are interpreted by taking into account of differences in acid site densities, active site distribution, and the polarity of reactants and catalyst surface. The low activity and selectivity of samples made by grafting in the transformation of ketoisophorone are attributed to the non-random distribution of the acidic functions.

Friedel-Crafts benzylation of benzene using Zn and Cd ions exchanged clay composites

Metal ion-exchanged clay composites such as M II–Mont and M II–Mont (AT) where M = Zn and Cd, Mont = montmorillonite clay of the types Mont 1 (SWy-2) and Mont 2 (Neelkanth), and AT = acid treated have been synthesized, characterized and evaluated as catalysts for Friedel-Crafts reaction particularly for benzylation of benzene. XRD study reveals that, in general, the basal spacing ( d 0 0 1 ) increases as the interlayer cations of Na–Mont or H +–Mont are exchanged with Zn 2+ or Cd 2+ ions and interlamellar spacings are maintained in the range 3.9–5.3 and 4.3–6.0 Å, respectively, at room temperature. The basal spacing ( d 0 0 1 ) of Mont 1 supported composites are higher than Mont 2 and therefore substantiate the advantage of the former over the latter in preparing metal ion-exchanged clay composites useful as solid acid catalysts. During the initial benzylation reactions, Cd 2+-exchanged composites, in general, show higher activity but in the long run Zn 2+-exchanged clays exhibit higher catalytic activities. In general, acid treated metal ion-exchanged clay composites exhibit higher catalytic activity than non-acid treated ones because the former show higher surface area as well as higher acid sites.

Amorphous aluminium fluoride as new matrix for vanadium-containing catalysts

A new sol–gel technique was developed for preparing vanadium-containing aluminium fluoride catalysts for selective oxidation reactions. With this new synthesis, highly dispersed vanadium species can be obtained in the metal fluoride host lattice. The X-ray amorphous solids were investigated by conventional and advanced analytical methods such as for instance N2 adsorption experiments, FTIR spectroscopy, pyridine adsorption and NH3-TPD. The surface properties, acidity and catalytic performance were all dependent on the vanadium content of the samples. EDX, FTIR and Raman spectroscopy confirmed the high degree of dispersion of vanadium (oxidation state: III or IV or both) species in the aluminium fluoride matrix. The samples had relatively high BET surface areas and medium strength Lewis acid sites. Additional Brønsted acid sites were observed on samples with higher vanadium contents only. The catalytic performance of the vanadium-containing aluminium fluorides was investigated for the oxidative dehydrogenation (ODH) of propane. Depending on the vanadium content, the new catalyst system offers a surprisingly high catalytic activity and selectivity towards the desired product, propylene. The high degree of dispersion of the active vanadium species in the aluminium fluoride matrix and the type of the acid sites on the solid surface are crucial to the selectivity of the catalyst in the selective oxidation reaction.

Photo-oxidation of 4-nitrophenol in aqueous suspensions, catalysed by titania intercalated zirconium phosphate (ZrP) and titanium phosphate (TiP)

Titania pillared zirconium phosphate and titanium phosphate samples were prepared by intercalating titanium(IV) isopropoxide sol inside the phosphate layer. The sol-intercalated samples were calcined at various temperatures and characterised by different techniques. Basal spacing and surface area revealed that highest amount of pillaring in interlayer has taken place with 2 and 4 wt.% TiO 2 loading for ZrP and TiP, respectively. These pillared samples are stable up to 500 °C and exhibit highest surface area in the range of 70–172 m 2 g −1, basal spacing in the range of 15.5–25.9 Å, which is greater than that of neat ZrP and TiP. These materials also showed greater activity towards photo-oxidation of 4-nitrophenol under solar radiation. The low crystallite size, low pH pzc, high surface area, higher acid sites and larger basal spacing might be responsible for highest activity shown by 2 and 4 wt.% pillared ZrP and TiP, respectively.

On the Environment of the Active Sites in Phosphate Modified Silica–Zirconia Acid Catalysts

The acid sites of a series of phosphated silica–zirconia catalysts have been quantified by the combined use of FTIR and TGA and the data related to the activity of the samples for the isomerisation of but-1-ene. The catalyst series show a linear correlation indicative of constant TOF when the number of Bronsted acid sites is plotted against catalytic activity. However, this relationship only holds when the number of Lewis acid sites exceeds that of Bronsted acid sites suggesting that an optimal arrangement may involve both types of site. The TOF is less when the Lewis/Bronsted ratio falls below 1, a scenario that dominates at high Bronsted acid site densities. When this site arrangement prevails, a higher than expected cis/trans ratio is found in the isomerisation selectivity. These differences in the environment of the active Bronsted acid site may also be deduced from differences in the molar absorption coefficients of adsorbed basic probe molecules.