Weak Bronsted Acid Sites Research Articles

Design of γ-Alumina-Supported Phosphotungstic Acid-Palladium Bifunctional Catalyst for Catalytic Liquid-Phase Citral Hydrogenation

This study primarily addresses the development of dynamic, selective and economical metal–acid (bifunctional) catalysts for one-pot menthol production by citral hydrogenation. Specifically, various metals such as Pd, Pt, Ni, Cs and Sn were doped over alumina support. Additionally, bifunctional composite catalysts were also prepared with the impregnation of heteropoly acids and Pd precursors over alumina support. Analytical techniques (e.g., BET, PXRD, FT-IR, pyridine adsorption and amine titration methods) were applied for characterization of the most efficient and selective catalysts (e.g., Al2O3 and PTA-Cat-I). Similarly, most of the essential operational variables (e.g., loading rate of metal precursor, type of heteropoly acid, temperature, gas pressure and reaction time) were examined during this study. The experimental data shows that the bifunctional catalyst (PTA-Cat-I) produced 45% menthol at full citral substrate conversion (r = 0.038 mmoles.min−1) in liquid-phase citral hydrogenation (at optimized operating conditions: 70 °C, 0.5 MPa and 8 h). However, the heteropoly acid-supported bifunctional catalysts (e.g., PTA-Cat-I, PMA-Cat-I, SMA-Cat-I and STA-Cat-I) resulted in cracking and the dehydration of isopulegol/menthol by the generation of side products (e.g., 4-isopropyl-1-methyl, cyclohex-1-ane/ene); therefore, menthol yield was extensively diminished. On the other hand, non-acidic catalysts (e.g., Cat-I, Cat-II, Cat-III, Cat-IV and Cat-V) readily promoted hydrogenation reactions. The optimum menthol yield occurred due to the presence of strong Lewis and weak Bronsted acid sites. Mass transfer and reaction rate were substantially diminished due to acidity strength, heteropoly acid type and blockage of pores by the applied bifunctional catalysts.

Mesopore-selective incorporation of strong Brønsted acid catalytic sites via aluminium grafting on hierarchically porous siliceous MFI zeolite

Abstract Post-synthetic Al grafting onto the surface silanols in mesoporous MFI zeolite has been investigated in an attempt to obtain a model catalytic material that possesses strong Bronsted acid sites at mesopore surfaces of the zeolite, but not inside the microporous framework. To this end, a siliceous MFI zeolite with a mesopore/micropore hierarchy was synthesized using a meso-micro dual structure-directing diammonium surfactant. The hierarchically porous zeolite was treated with anhydrous AlCl3 for the formation of Si–O–Al bonding with silanols on the mesopore surfaces. According to solid-state 31P NMR spectroscopy of the surface-adsorbed phosphine oxides and catalytic activity for decalin cracking, the Al-grafting treatment performed in the conventional manner resulted in the formation of weak Bronsted acid sites, similar to Al grafting on SBA-15 mesoporous silica. To obtain strong Bronsted acid sites, we have developed a method of heating the AlCl3-grafted zeolite in a controlled-humidity chamber before calcination. The resultant zeolite was catalytically active for cracking of hydrocarbons requiring strong acid sites, but not for methanol-to-hydrocarbon conversion. The results support that the moisture treatment caused a local reconstruction of the initially grafted Al–O–Si framework portion to a strong Bronsted acid site. In addition, our model catalytic zeolite material unveiled the catalytic activity of Bronsted acid sites at the mesopore surfaces in hierarchically porous zeolite in n-octane cracking. Furthermore, we confirmed that this method was effective for other mesoporous MFI zeolites obtained through a post-synthetic desilication route.

Citric acid-modified beta zeolite for polyoxymethylene dimethyl ethers synthesis: The textural and acidic properties regulation

Beta zeolite exhibits satisfactory catalytic activity in the synthesis of polyoxymethylene dimethyl ethers. However, the microporous channels of beta zeolite have adverse effects on mass transfer of macromolecules. In this work, it was found that the textural and acidic properties of catalysts could be improved by citric acid treatment. Extensive characterization confirmed that citric acid induced significant pore-expanding on catalysts, and its dealumination and realumination effect could lead to reduced amount of total acid sites and increased amount of Brønsted acid sites (BAS). Meanwhile, a good positive correlation was found between amount of external weak BAS and initial reaction rate. It was speculated that BAS with relatively weak acid strength might be the primary active sites in this reaction. Compared with the blank sample, the initial reaction rate of optimal catalyst was increased by 100 %, and the conversion of trioxane was increased by 86.5 %, indicating a significantly improved catalytic activity.

The effect of different Ca precursors on the activity of manganese and cerium oxides supported on TiO2 for NO abatement

The different forms of Ca (CaCl2 and Ca(OH)2) poisoned manganese and cerium oxides supported on TiO2 catalysts were prepared by impregnation method and used for selective catalytic reduction (SCR) of NO with NH3. The Ca poisoning effects were investigated by many characterization methods including N2 adsorption/desorption, X-ray diffraction, X-ray photoelectron spectroscopy, H2-temperature programmed reduction and NH3-temperature programmed desorption techniques. The results showed that the addition of Ca led to greatly decrease of catalytic activity of catalysts and the catalytic activity was ranked in the order: CaCl2 poisoned catalyst < Ca(OH)2 poisoned catalyst < fresh catalyst, which indicated that the poisoning effect of different forms of Ca was different and the deactivation caused by CaCl2 was more serious. The characterization results demonstrated the addition of Ca greatly decreased the specific surface area of catalysts, the contents of surface active species (Mn4+ and Ce3+), the amounts of weak Bronsted acid sites and the low-temperature redox ability, especially for the CaCl2 poisoned catalyst, all of which led to the weaken of the catalytic activity of catalysts.

Guaiacol hydrodeoxygenation over Ni2P supported on 2D-zeolites

Abstract Different 2D zeolites, namely lamellar ZSM-5 (L-ZSM-5), the corresponding silica pillared ZSM-5 (PI-ZSM-5) and an MCM-22 sample, were investigated as nickel phosphide supports and evaluated as catalysts for guaiacol hydrodeoxygenation (HDO). Characterization confirmed that all loaded materials preserved a high crystallinity degree with formation of the Ni2P phase. The incorporation of Ni2P reduced adsorption capacity, affecting differently the micro- or mesoporous systems, depending on the zeolitic support. Ni2P deposition also modified the acidic properties of the zeolites due to the generation of new acid sites of moderate strength attributed to both Niδ+ species (Lewis acid sites) and the presence of residual P−OH moieties (weak Bronsted acid sites). The catalysts were tested in the HDO of guaiacol at 220 and 260 °C and 40 bar of H2. In all cases, ZSM-5 based catalysts exhibited higher conversions and HDO efficiencies than Ni2P/MCM-22, which denotes that MFI zeolitic structure favors the deoxygenation of phenolic compounds, yielding cyclohexane as major reaction product. This excellent catalytic performance is attributed to the combination of high surface area and optimal acidic properties since a synergetic effect was achieved between the acid sites of the raw zeolitic support and those provided by nickel phosphide having an enhanced accessibility. Thus, the best HDO performance was obtained over Ni2P supported on the pillared ZSM-5, showing a 78 % of guaiacol conversion and 95 % of selectivity towards deoxygenated molecules.

Isomerization of linear C5–C7 over Pt loaded on protonated fibrous silica@Y zeolite (Pt/HSi@Y)

Abstract A novel fibrous silica Y zeolite (HSi@Y) loaded with Pt has been studied based on its ability to produce protonic acid sites originating from molecular hydrogen. The Pt/HSi@Y was prepared using seed assisted crystallization followed by protonation and Pt-loading. The product formed had a spherical morphology with bicontinuous lamellar with a diameter in the range of 500–700 nm. The catalytic activity of the Pt/HSi@Y has been assessed based on light linear alkane (C5–C7) isomerization in a micro-catalytic pulse reactor at 423–623 K. A pyridine IR study confirmed that the introduction of fibrous silica on Y zeolite increased the Lewis acid sites corresponding with the formation of extra-framework Al which led to the generation of more protonic acid sites. A hydrogen adsorbed IR study showed that the protonic acid sites which act as active sites in the isomerization were formed via dissociative-adsorption of molecular hydrogen releasing electrons to the nearby Lewis acid sites. Thus, it is suggested that the presence of Pt and HSi@Y with a high number of Lewis acid as well as weak Bronsted acid sites improved the activity and stability in C5, C6 and C7 isomerization via hydrogen spill-over mechanism.

Removal of Low-concentration Ammonia from Ambient Air by Aluminophosphates

A series of aluminumphosphate materials was prepared and used as adsorbents for the removal of ammonia at low concentrations. The influence of various preparation parameters, including the pH value of sol, calcination temperature and molar ratio of P/Al, on the structure and surface properties as well as adsorption capacity were investigated. The results showed that large amount of P—OH present on the surface of aluminophosphates was suitable for the removal of ammonia. They were the major source of weak Bronsted acid sites and acted as the main active centers for capturing ammonia.

Revisiting Acidity of SnBEA Catalysts by Combined Application of FTIR Spectroscopy of Different Probe Molecules

The Lewis and Bronsted acid properties of SnBEA zeolites as well as SnO2 supported on silica BEA have been examined by means of IR spectroscopy of adsorbed pyridine, 2,6-ditertbutylpyridine, and deuterated acetonitrile. Three types of surface sites have been detected and assigned to (i) framework Sn centers possessing Lewis acid properties; (ii) weak Bronsted acid sites associated with framework tin atoms; and (iii) nonframework SnO2 particles which possess Lewis acidity. The total amount of Lewis sites can be determined using pyridine as a probe molecule, while the type of Lewis sites can be distinguished by FTIR of adsorbed acetonitrile. The band at 2316 cm–1 is attributed to strong framework sites, whereas the band at 2287 cm–1 is attributed to nonframework sites. Bronsted acid sites can be characterized using 2,6-ditertbutylpyridine (bands at 3363, 1613, and 1530 cm–1) and deuterated acetonitrile (band at 2308 cm–1). The relative amount of Lewis and Bronsted acid sites on SnBEA can be varied by treatm...

Influence of alcohol solvents on morphology of hollow silica–alumina composite spheres and their activity for hydrolytic dehydrogenation of ammonia borane

In this study, we reported effect of alcohol solvents on morphology of hollow silica–alumina composite spheres and their activity for hydrolytic dehydrogenation of ammonia borane. Silica–alumina composite shell was coated on polystyrene template particles via sol–gel reaction in various alcohol solvents. The hollow spheres were prepared in methanol, ethanol and 2-propanol for 36, 17 and 8 h, respectively. The results indicate that the order of sol–gel reaction rate was 2-propanol > ethanol > methanol. Amount of hydrogen evolution of the hollow spheres prepared in methanol and ethanol were higher than that of the hollow spheres prepared in 2-propanol. From the results of solid-state 27Al nuclear magnetic resonance spectra, we found that aluminum species of the hollow spheres prepared in methanol and ethanol were highly dispersed in their silica matrix. The results of temperature programed dispersion of ammonia showed that all the hollow spheres have weak and strong Bronsted acid sites. The amount of weak Bronsted acid sites in all the hollow spheres were of same level, while the amount of strong Bronsted acid sites increase with decrease of sol–gel reaction rate. These results indicate that the sol–gel reaction rate influences the dispersion of aluminum species, and highly dispersed aluminum species lead to increase in the amount of Bronsted acid sites.

Improved Catalytic Performance of C-axis Oriented HZSM-5 Nanobunches Synthesized by Re-aging

C-axis oriented HZSM-5 nanobunches were synthesized using the combination hydrothermal synthesis method and a re-aging procedure without any type of mesoscale template. Then, the samples were characterized by a variety of physical techniques, such as XRD, SEM, TEM, N2 absorption, FTIR and XPS. Their activity was investigated via the transformation of methanol to aromatic compounds using a fixed-bed micro-reactor at 450 °C of temperature and a WHSV of 2 h−1 under ambient pressure. Good correlation was observed between the re-aging procedure and the orientation of crystalline growth, the doping effect of Zn and the surface acid sites. The final product synthesized by the re-aging procedure exhibited loose aggregation with well-ordered nanobunches along the c-axis direction, and the crystal bunches are stacked together with intercrystalline distances of approximately 3–4 nm. More doped Zn sites and weak Bronsted acid sites existed on the surface of the bunches. Due to the shortened microporous channel, open mesopores and weak Bronsted acid sites, this sample exhibited a higher catalytic activity and much longer lifetime to endure coke deposition during the catalytic conversion of methanol to aromatic compounds compared to those of the conventional microsized ZSM-5 crystal.

Photoisomerization of n-pentane over nano ZnO/MoO3-ZrO2

A series of nano ZnO/MoO3-ZrO2 catalysts with different ZnO loading (1.0, 2.5, 5.0 wt%) were prepared by impregnation method for n-pentane photoisomerization under hydrogen or nitrogen atmosphere. The properties of the catalysts were characterized with X-ray Diffraction (XRD), Brunauer Emmett Teller (BET), Transmission Electron microscope (TEM) and FTIR. The XRD result showed that the fraction of tetragonal phase of ZnO/MoO3-ZrO2 was about 0.67 for all samples. While, the specific BET surface area was about 24 m2/g. Pyridine adsorbed FTIR results showed that all samples possessed high concentration of strong Lewis acid sites and small concentration of weak Bronsted acid sites. The interaction of hydrogen and surface samples at 298-523 K formed protonic acid sites with the concomitant of the partial elimination of Lewis acid sites. Whereas no changes of the concentration of acid sites were observed in the presence of nitrogen atmosphere. The activity of all samples in the n-pentane photoisomerization was strongly determined by the presence of hydrogen gas. In fact no activity was observed in the absence of hydrogen________________________________________GRAPHICAL ABSTRACT

Comparison of Porous Iron Trimesates Basolite F300 and MIL-100(Fe) As Heterogeneous Catalysts for Lewis Acid and Oxidation Reactions: Roles of Structural Defects and Stability

Two porous iron trimesates, namely, commercial Basolite F300 (Fe(BTC); BTC = 1,3,5-benzenetricarboxylate) with unknown structure and synthetic MIL-100(Fe) (MIL stands for Material of Institut Lavoisier) of well-defined crystalline structure, have been compared as heterogeneous catalysts for four different reactions. It was found that while for catalytic processes requiring strong Lewis acid sites, Fe(BTC) performs better, MIL-100(Fe) is the preferred catalyst for oxidation reactions. These catalytic results have been rationalized by a combined in situ infrared and 57Fe Mossbauer spectroscopic characterization. It is proposed that the presence of extra Bronsted acid sites on the Fe(BTC) and the easier redox behavior of the MIL-100(Fe) could explain these comparative catalytic performances. The results illustrate the importance of structural defects (presence of weak Bronsted acid sites) and structural stability (MIL-100(Fe) is stable upon annealing at 280 °C despite Fe3+-to-Fe2+ reduction) on the catalytic...

Tuning the catalytic performance of metal–organic frameworks in fine chemistry by active site engineering

The effect of a post-synthetic acid treatment on the catalytic performance of MOFs is evaluated for MIL-100(Fe), an iron-benzenetricarboxylate. The acid-treated frameworks are structurally robust as no differences have been found in XRD patterns after treatment. Porosity of the acid-treated MOFs gradually decreases, most probably as a consequence of anions remaining in the charged frameworks. Monitoring the modification of the MOFs by reactions of which the outcome depends on the acid properties of the catalyst suggests the presence of two types of active sites, with weak Bronsted acid sites in close vicinity to the Lewis acid open metal sites. This is supported by CO-chemisorption experiments which indicate a large increase of both Lewis and Bronsted acidity. In Diels–Alder reactions of oxygenated dienophiles with 1,3-cyclohexadiene, a strong increase of the activity is found for the acid-treated MOFs. This is explained by the enhanced activation of the dienophiles on the modified active sites.

Gas-Phase Alkylation of Pyridine and Phenol with Alcohols Over Halide Clusters of Group 5–7 Transition Metals as Solid Acid Catalysts

Pyridine is allowed to react with methanol under a hydrogen stream in the presence of (H3O)2[(W6Cl8)Cl6]·6H2O supported on silica gel. When the temperature is raised above 200 °C, the catalytic activity of the cluster appears. Methylation of pyridine proceeds yielding 2-methylpyridine in 61% selectivity at 400 °C. The corresponding hexanuclear chloride clusters of niobium, molybdenum, and tantalum also catalyze the reaction. Ethanol affords the corresponding 2-ethylpyridine. When phenol is allowed to react with methanol in the presence of (H3O)2[(Mo6Cl8)Cl6]·6H2O supported on silica gel in the same manner, selective O-methylation proceeds yielding anisole in 57% selectivity at 150–200 °C. Above 250 °C, C-methylation predominates and provides o-cresol with 67% selectivity at 300 °C. The corresponding clusters of niobium, tantalum, and tungsten also catalyze the reaction. Ethanol and 1-propanol afford the corresponding 2-alkylphenols. Alkyl cations produced over weak Bronsted acid sites (H 0 ≈ +1.3) developed on the clusters are assumed as intermediates for both reactions.

ChemInform Abstract: Alkylation of Toluene with Methanol over Alkali Exchanged ZSM-5.

Abstract The catalytic activity was investigated of a series of alkali exchanged and alkali impregnated ZSM-5 zeolites in the alkylation of toluene with methanol. The variations of the rate of the reaction were concluded to be governed mainly by the concentrations of protonated methanol molecules and of adsorbed toluene. The active sites were concluded to be weak Bronsted acid sites, present as a result of lattice defects. The alkali cations were not active in the alkylation reaction. They do, however, influence the acid strength of the weak acid sites. Because of this influence, the concentration of methoxonium ions and hence the rate of alkylation decreases from LiZSM-5 to CsZSM-5.

The nature of surface acidity and reactivity of MoO3/SiO2 and MoO3/TiO2–SiO2 for transesterification of dimethyl oxalate with phenol: A comparative investigation

This paper presents results of a comparative investigation of the transesterification of dimethyl oxalate (DMO) with phenol to produce methyl phenyl oxalate (MPO) and diphenyl oxalate (DPO) over MoO3/SiO2 and MoO3/TiO2–SiO2 catalysts. The evaluation results show that MoO3/TiO2–SiO2 is much more active and selective than MoO3/SiO2. The surface structure and acidity of MoO3/SiO2 and MoO3/TiO2–SiO2 were investigated by a series of characterization approaches. XRD and FT-IR demonstrated that the incorporation of amorphous TiO2 could not only enhance the interaction between MoO3 and SiO2, but also improve the dispersion state of MoO3 on the surface of SiO2. NH3-TPD and FT-IR of adsorbed pyridine measurements indicated that amorphous TiO2 incorporation into and further interaction with MoO3 and SiO2 formed more weak acid sites on the surface of the catalysts. However, Brönsted acid sites were also detected on the surface of the MoO3/TiO2–SiO2 catalysts, which further motivated us to study the nature of Brönsted acid sites in detail. Several conventional Brönsted acids were also tested in the transesterification. The results unexpectedly showed that conventional Brönsted acids have better reactivities than those of conventional Lewis acids from selective point of view. In addition, the combination of reactivity tests, NH3-TPD spectra, and FT-IR measurements of adsorbed pyridine strongly suggest that the strong Lewis acid is more responsible for the production of the by-product anisole than the strong Brönsted acid. The results indicated that the improvement of catalytic efficiency of MoO3/TiO2–SiO2, compared to MoO3/SiO2, could be ascribed to the increased dispersion capacity of MoO3 and the dominant weak acid sites (including the synergistic effect of the weak Lewis acid sites with the weak Brönsted acid sites). A tentative mechanism for the transesterification reaction over Brönsted acid was proposed.

Synthesis of Ti-Hβ zeolites by liquid-solid isomorphous substitution and the catalytic properties in the vapor phase Beckmann rearrangement of cyclohexanone oxime

A series of Ti-Hβ zeolites were synthesized by liquid-solid isomorphous substitution. Hβ and Ti-Hβ zeolites were characterized by BET, XPS, IR, etc., and studied in the vapor phase Beckmann rearrangement of cyclohexanone oxime. In comparison with pure Hβ, the incorporation of Ti in the Hβ framework leads to larger BET surface area and weaker Bronsted acid sites. The incorporation of Ti in the Hβ framework increases the conversion of cyclohexanone oxime and its stability. It is inferred that for the catalysts having the structure of a β zeolite, the suitable acid sites for the vapor phase Beckman rearrangement might be the weak Bronsted acid sites.

Pt-loaded P-MCM-41 as a novel bifunctional catalyst for catalytic combustion of trichloroethylene

Abstract Pt-loaded P-MCM-41 catalysts with different Si/P ratio were prepared and used as novel bifunctional catalysts for catalytic combustion of trichloroethylene (TCE). Acidic materials, ZSM-5, P-SiO 2 and Al 2 O 3 as supports in Pt-loaded catalysts were also studied for comparison. The high activity for TCE conversion and low selectivity to tetrachloroethylene were observed on Pt/P-MCM-41 catalysts, indicating that phosphorus modified MCM-41 with an amount of weak Bronsted acid sites, in combination with Pt, was effective to form a bifunctional catalyst.

2,2‘-Bipyridyl-3,3‘-diol Incorporated into AlPO4-5 Crystals and Its Spectroscopic Properties as Related to Aqueous Liquid Media

A novel fluorescent host−guest material, molecular sieves of AlPO4-5-type doped with 2,2‘-bipyridyl-3,3‘-diol, was prepared and spectroscopically characterized. The composite crystals show a pronounced optical anisotropy, indicating a high degree of alignment of the guest molecules within the zeolitic pore system. A mean tilting angle of 22° was found for the orientation of the individual dye molecules in the straight channels. The corrected fluorescence emission spectra were determined, and time-resolved fluorescence studies revealed that the dye molecules are preferentially found in three different types of microenvironment. By invoking pH-dependent studies of the dye in aqueous solution, we could trace these spectroscopic features back to two main influences, coadsorbed water within AlPO4-5 pores and guest−host interactions with a few relatively weak Bronsted acid (defect) sites of the inorganic host network.

The characterization of tungsten-oxide-modified zirconia supports for dual functional catalysis

The results of catalytic titration measurements indicated that WOx/ZrO2 catalysts prepared by coprecipitation have higher acid site density and strength, and are more active for pentane isomerization, than catalysts prepared by impregnation. The coprecipitated WOx/ZrO2 contains ∼0.004 meq-H+/g-catalyst. XPS of chemisorbed 2,6-dimethylpyridine and pyridine revealed the presence of Lewis acid sites as well as strong and weak Bronsted acid sites on the surface of these catalysts. The concentration of strong Bronsted sites was close to the concentration of Lewis sites, suggesting that the high acidity of these materials may originate from a conjugate Bronsted–Lewis site. Pt/WOx/ZrO2 undergoes a reversible loss of hydrogen chemisorption capacity with increasing hydrogen pretreatment temperature, accompanied by a reversible loss of pentane isomerization activity. This loss can be attributed to a strong Pt-reduced tungsten oxo species interaction (SMSI). Room temperature hydrogen spillover is observed in the Pt/WOx/ZrO2 (∼16% W) catalyst, consistent with the presence of bulk WO3 in this material as observed by TPR.