Strong Bronsted Acid Research Articles

Highly Efficient Production of 5-Hydroxymethylfurfural from Fructose via a Bromine-Functionalized Porous Catalyst under Mild Conditions

Efficient 5-hydroxymethylfurfural (HMF) production remains challenging due to the limited tolerance of HMF to strong Bronsted acids. Herein, this work provides a new strategy for effective HMF prod...

Reactions of quinoline-2(6,8)-carbaldehydes with arenes by the action of various Brønsted or Lewis acids: synthesis of diarylmethylquinolines

The reactions of quinoline-2(6,8)-carbaldehydes with arenes by the action of strong Bronsted (H2SO4, CF3SO3H) or Lewis (AlCl3, AlBr3) acids led to the formation of substituted diarylmethylquinolines in high yields.

Review: cascade reactions for conversion of carbohydrates using heteropolyacids as the solid catalysts

Conversion of biomass into renewable chemicals and fuels has received significant attention as an approach to enhance global sustainability. Biomass conversion, especially carbohydrate conversion, commonly involves cascade reactions. During these cascade reactions, various activity sites on carbohydrates may have different reactivities that enable us to control the final products through adjusting these activity sites. Heteropolyacids (HPAs), one of the major solid acid catalysts, are extensively used in biomass conversion owing to their numerous excellent physicochemical properties, such as strong Bronsted acidity, oxidizability, structural versatility, and easy design. Accordingly, this review aims to outline recent breakthroughs in obtaining platform chemicals from carbohydrates by cascade reactions involving heteropolyacid catalysts. Moreover, the relationship between the final products and the structure of heteropolyacid catalysts is discussed and insights will help us to design more efficient solid acid catalysts for conversion of biomass to more sustainable chemical products.

Novel strategies for glucose production from biomass using heteropoly acid catalyst

Abstract Bioethanol and direct glucose fuel cells pledged clean energy to the world. Cellulose depolymerization for glucose production has been a successful approach in bioethanol production. Heteropoly acids (HPAs) are strong Bronsted solid acid catalysts for biomass hydrolysis. Keggin type HPAs, namely, Silicotungstic acid (HSiW), Phosphotungstic acid (HPW), and Phosphomolybdic acid (HPMo), were used for the hydrolysis of lignocellulosic biomass to glucose. Five different biomass feedstocks, namely, miscanthus, sugarcane leaves, switchgrass, sunflower seeds, and bamboo leaves, were examined for the feasibility of total reducing sugar (TRS) yield through the composition analysis and catalytic biomass hydrolysis. Sunflower seeds contained the maximum holocellulose with 90.6%, and switchgrass contained the least i.e., 77.63%. Among the five biomass tested, switchgrass resulted in the highest TRS (5.77 wt/dry wt. %) with HPMo catalyst at a catalyst to biomass ratio of 30:100 (wt./wt. %), a reaction temperature of 120 °C for 3 h. The reaction parameters for depolymerization were optimized for all three HPAs, and the optimized conditions were 3 h and 120 °C. HPMo showed maximum TRS yield (5.77 wt/dry wt.%) among the three HPAs at 30:100 catalyst to biomass ratio. However, a catalyst to biomass ratio of 20:100 (wt./wt.%) was economical (5.25 wt/dry wt.%) for commercial application.

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.

Highly Active Silver ion-Exchanged Silicotungstic Acid Catalysts for Selective Esterification of Glycerol with Lauric Acid

A series of highly active silver ion-exchanged silicotungstic acid (H4SiW12O40; STA) catalysts, AgxH4−xSiW12O40 (x = 1–4), were successfully synthesized and applied for the production of glycerol monolaurate (GML) via esterification of lauric acid (LA) with glycerol (GL). The physicochemical properties of these Ag-promoted STA were characterized by TGA, FE-SEM, XRD, and FT-IR, and their acidic properties were studied by a solid-state 31P NMR probe molecule method using trimethylphosphine oxide (TMPO) as the probe. Among various catalysts, the Ag2H2SiW12O40 catalyst showed extraordinary activity owing to the strong Bronsted acidity, Bronsted–Lewis acid synergy, as well as the unique Keggin structure possessed by the catalyst. A Box–Behnken design (BBD) based on response surface methodology (RSM) was exploited for the optimization of process variables. A maximum GML yield of 80.2% was achieved for the Ag2H2SiW12O40 catalyst under a GL/LA molar ratio of 4.4, catalyst amount of 4.6 wt%, reaction temperature of 429 K, and reaction time of 153 min, which was in conformity with those predicted by the mathematical model. Further recyclability and kinetic studies confirmed that the Ag2H2SiW12O40 catalyst was reusable, robust, and highly efficient for the synthesis of GML with an activation energy (Ea) of 38.14 kJ/mol. Due to the strong acidity and synergy of B/L acid sites, Ag ion-exchanged silicotungstic acid catalysts show superior activity and durability for the synthesis of glycerol monolaurate.

Pd/SAPO-41 Bifunctional Catalysts with Enhanced Pd Dispersion Prepared by Ultrasonic-Assisted Impregnation: High Selectivity for n-Hexadecane Hydroisomerization

The preparation of bifunctional catalysts with high catalytic selectivity for the n-alkanes hydroisomerization still remains challenging for the production of bio-diesel. Herein, two series of Pd/SAPO-41 bifunctional catalysts are prepared by the ultrasonic-assisted impregnation (xPd/S41-U) with different treating time and conventional incipient wetness impregnation methods (0.30Pd/S41-I) on the SAPO-41 molecular sieve, respectively. The physico-chemical property of the synthesized SAPO-41 and prepared catalysts were studied by XRD, SEM, ICP, N2 physical adsorption, H2 chemisorption and Py-IR measurements. The catalytic performance for the n-hexadecane hydroisomerization over all catalysts is also studied. The characteristic results indicate that the xPd/S41-U catalysts show stronger Bronsted acidity compared with the 0.30Pd/S41-I catalyst. In addition, the Pd dispersion of the xPd/S41-U catalysts is almost two times higher than that of the 0.30Pd/S41-I catalyst, which leads more Pd cluster to enter into the micropores of the SAPO-41 molecular sieve. Furthermore, the 0.30Pd/S41-U catalyst with 0.30 wt % Pd loading shows promoted catalytic performance than that of the 0.30Pd/S41-I catalyst with the same Pd loading because of the stronger metal function and more favourable metal-acid balance caused by the larger CPd/CH+ ratio. Therefore, the ultrasonic-assisted impregnation prepared Pd/SAPO-41 catalysts are potential to be widely employed for the n-alkane hydroisomerization.

Highly active Brønsted acidic silicon phosphate catalyst for direct conversion of glucose to levulinic acid in MIBK–water biphasic system

Due to depletion of fossil fuel reserves and environmental concerns demand the utilization of other renewable feedstocks such as biomass which is available abundantly in different quantities throughout the world. Catalytic transformation of this biomass or biomass-derived products such as glucose to valuable platform biochemicals viz: levulinic acid (LA) is academically as well as industrially important reaction. In the present work, Silicon phosphates (SixPO4) catalysts viz: Si10PO4, Si20PO4, Si30PO4, and Si40PO4 having solely Bronsted acidic properties were prepared by wet impregnation of H3PO4 (10–40 wt%) on fumed silica followed by calcination at 550 °C for 10 h. The prepared catalysts were characterized by various techniques such as XRD; NH3-TPD; FTIR etc. Formation of silicon phosphate structure was found to depend on % of P loading and its interaction with silica. The acidic property of SixPO4 catalyst was characterized by NH3-TPD and Pyridine-IR, confirmed that catalyst has exclusively strong Bronsted acidity. A high LA yield of 81 mol% over Si30PO4 has achieved at an optimized reaction condition of 140 °C; 10 h in MIBK–water (9:1) biphasic system.

Untangling the role of the organosilane functional groups in the synthesis of hierarchical ZSM-5 zeolite by crystallization of silanized protozeolitic units

Abstract Crystallization of hierarchical ZSM-5 zeolite has been performed under hydrothermal conditions from silanized protozeolitic units. The role of amine and phenyl groups present in the organosilanes, as well as the use of dipodal silanization agents, on the creation of mesoporosity in the zeolite samples has been investigated. The presence of amine groups and dipodal moieties in the organosilane increases their interactions with the protozeolitic nanounits in the gel, which results in hierarchical zeolites showing enhanced secondary porosity and a higher degree of modification of the textural properties. Nevertheless, dipodal organosilanes hinder strongly the aggregation of the nanounits, leading to non-completely crystalline materials. In contrast, the combination of phenyl and amine groups into a single organosilane has shown to be effective for the generation of more uniform mesopores. Thus, N-(2-N-Benzylaminoethyl)-3-aminopropyltrimethoxysilane (Ph-2A) has been found as the best silanization agent, affording a hierarchical zeolite with a narrow mesopore size distribution and a high concentration of strong Bronsted acid sites. Moreover, during LDPE catalytic cracking in TGA tests, the latter material allowed the conversion of the polymer to occur in a narrow temperature range as a consequence of its more uniform acid and textural properties. Likewise, this sample exhibited a high activity in LDPE cracking at 340 °C using a laboratory scale reaction system to produce mainly gasoline-range hydrocarbons and light olefins.

Efficient conversion of cellulose to methyl levulinate over heteropoly acid promoted by Sn-Beta zeolite

One-pot conversion of cellulose to valued-added platform chemicals such as alkyl levulinate is a promising and challenging process of biomass utilization due to the insolubility of cellulose in alcohol. In this work, commercial heteropoly acid combined with postsynthesized Sn-Beta zeolite was employed to transform cellulose to methyl levulinate (MLE) in one pot. The synergistic catalysis of the homogeneous strong Bronsted (B) acid and the solid strong Lewis (L) acid can effectively realize the depolymerization of solid cellulose and the subsequent isomerization, dehydration and hydration steps to MLE. 55% and 62% of MLE yields were obtained from α-cellulose and microcrystalline cellulose, respectively, at 160 °C for 10 h. Various carbohydrates besides cellulose can also be efficiently converted to MLE in high yields over the bifunctional catalyst system. The effects of the amount of B acid and L acid, the reaction temperature and time and the amount of cellulose on the production of MLE were investigated in detail. Finally, the reaction pathway of cellulose to MLE was revealed. Recyclability tests indicated that the binary catalyst can be reused without decrease of catalytic activity.

Effective adsorptive denitrogenation from model fuels over yttrium ion-exchanged Y zeolite

Abstract The adsorption removal of indole and quinoline in octane with and without toluene over zeolites NaY and Yttrium Ion-exchanged Y (YY) using batch adsorption experiments was studied at 25 o C and 1 atm. YY was prepared by treating NaY with Y(NO 3 ) 3 solution twice via liquid ion-exchange method. NaY and YY were both characterized by XRD, SEM, N 2 adsorption, XRF, NH 3 -TPD, and pyridine-FTIR techniques. Adsorption isotherms of indole, quinoline and toluene in octane were conducted at 25.0 o C to explain the influence of toluene on nitrogen removal over NaY and YY. The partial destruct of the crystalline structure of NaY was observed after the introduction of yttrium ion , which led to an evident decline in BET surface area and pore volume of YY. Strong Bronsted acidity and medium Lewis acidity were introduced by yttrium ion-exchange. Though the specific surface area and pore volume of YY were much lower than those of NaY, YY exhibited equivalent adsorption capacities for indole and quinoline as NaY in model fuels without toluene. In the presence of 20 vol.% toluene, however, YY exhibited much higher adsorption capacities for indole and quinoline than NaY, especially in the case of quinoline. The improved toluene-tolerant of YY was ascribed to the strong acid-base interaction between YY and quinoline and the decreased adsorption strength between YY and toluene.

Preparation of mesoporous Beta zeolite by fluoride treatment in liquid phase. Textural, acid and catalytic properties

Abstract Beta zeolite (nominal Si/Al = 19) has been treated with HF/NH4F solutions under different HF concentration, temperature and reaction time. It has been found that the crystallinity of the resulting materials is >80% for HF concentration up to 0.5 M, but it drops below this value for the most severe condition (HF = 1 M and 40 °C). The treatment dissolves selectively aluminum, in such a way that the Si/Al increases to 34 for the most attacked material. As a consequence of these treatments, the mesopore volume increases by up to 80% respect to the parent Beta for samples retaining high crystallinity, corresponding to newly created mesopores in the range of 5–20 nm. Detailed STEM-HAADF studies reveal that the chemical attack of the Beta crystals by the acidic fluoride solution progresses slowly along the crystals, leaving behind highly corroded and porous “sponge-like” crystals where the structure of the zeolite is nevertheless retained. Pyridine adsorption/desorption experiments show that a fraction of the extraframework Al present in the starting Beta is reinserted in framework sites, a process that would be mediated by aluminum-fluoride soluble species. As a consequence of this process, the Bronsted/Lewis ratio of the strong acid sites increases from 1 in the starting Beta to close to 3. The concentration of total and strong Bronsted acid sites associated to framework Al increases with the treatment. However, the activity of the samples in the isomerization/disproportionation of m-xylene is lower than that of the parent Beta zeolite, a result that could be due to the higher B/L ratio.

ZSM-5 surface modification by plasma for catalytic activity improvement in the gas phase methanol-to-dimethylether reaction

Abstract Dimethylether (DME) is nowadays considered as one of the most promising alternative fuels for the future. It has a very low climate impact, and can be produced in a renewable way, namely from the gas phase condensation of methanol over common acid catalysts such as γ-Al2O3 or zeolites. The challenge today is to switch from high reaction temperatures (300–350 °C) to lower ones (ideally 100–150 °C) in order to operate more energy-efficiently and with 100% selectivity to DMEThe methanol conversion needs to be increased below 300 °C in order to get high yields of DME even in that low temperature range. In the present paper, we show a way to achieve this in the case of the widely studied ZSM-5 zeolite. We show that submitting the zeolite to a cold plasma treatment before using it as a catalyst allows significantly increasing the yield of DME up to 300 °C. Indeed, the plasma treatment leads to a partial dealumination of the zeolite and thereby to an increase of the number of strong Bronsted acid sites which play a key role in the methanol-to-DME reaction mechanism.

1H and 93Nb Solid-State NMR and IR Study of Acidity of Nanodisperse Nb2O5·nH2O

Niobium oxide hydrate is a promising material for various heterogeneous catalytic processes due to its strong acidity and stability in aqueous medium. While different synthesis conditions may lead to various particle morphologies, the effect of morphology of Nb2O5·nH2O particles on their acidic properties is not fully understood yet. In this paper, we have successfully synthesized and characterized nanodisperse niobium oxide hydrate. Using infrared (IR) spectroscopy, we demonstrated that the sample exhibits strong Bronsted acidity close in strength to sulfuric acid. Furthermore, solid-state nuclear magnetic resonance (NMR) spectroscopy in combination with ab initio calculations gave additional insight into the nature of strong acidic sites and proved to be a useful tool for identification of acidic sites in Nb2O5·nH2O systems. Thus, we have shown that it is not necessary to follow difficult high-temperature solid-state processes or processes with ammonia, which often contaminates the material, to synthesize highly acidic nanodisperse Nb2O5·nH2O.

Insight into the Acid Sites over Modified NaY Zeolite and Their Adsorption Mechanisms for Thiophene and Benzene

Series of HY sorbents with different amounts and strength of Bronsted and Lewis acidic sites were prepared. Their adsorption performance in thiophene–benzene solution, pure benzene, and pure thiophene were evaluated, and the desulfurization mechanisms were studied. The results show that both thiophene and benzene could be effectively adsorbed through the interaction between Lewis acid sites and the conjugated π bond in these two molecules, and thus thiophene adsorption could be obviously competed by benzene on Lewis acidic sites. By contrast, on Bronsted acidic sites, thiophene can be efficiently adsorbed via H–S bond, which is stronger than the former interaction. Moreover, the middle strong Bronsted acid could cause thiophene oligomerization. Hence, for the desulfurization purpose by NaY zeolite in aromatic free system, its weak and middle strong Lewis acid should be increased, and for that in aromatic existing system, the Bronsted acid site with relative low strength should be promoted.

A Comparative Study of n-Butane Isomerization over H-Beta and H-ZSM-5 Zeolites at Low Temperatures: Effects of Acid Properties and Pore Structures

The influences of acidic properties and pore structures of H-Beta and H-ZSM-5 zeolites on the reaction properties of n-butane isomerization at low temperatures were investigated. The results showed that bimolecular pathway of n-butane conversion predominates over H-ZSM-5 zeolites, while the monomolecular and bimolecular pathways occur simultaneously over H-Beta zeolites. The conversion rate of n-butane strongly relies on the amount of strong Bronsted acid sites regardless of zeolite topology. However, the topology of zeolites crucially determines the products distribution, and the density of strong Bronsted acid sites plays a secondary role. The cavities of zeolites, formed in the intersections of channels, provide the places for the bimolecular reaction. The formation of trimethyl C8 intermediates is spatially restricted in the narrow channel intersections of H-ZSM-5 zeolites, resulting in higher contribution of n-butane disproportionation reaction. In addition, the narrow pore channels of H-ZSM-5 zeolite limit the monomolecular isomerization of n-butane molecules and affect the diffusion of heavier products (pentane) produced from bimolecular reaction, leading to the severe secondary reaction and high selectivity to propane. In contrast, the pore channels of H-Beta zeolite allow the monomolecular isomerization of n-butane and the deposition of coke. The topology of zeolites crucially determines the products distribution.

Diels-Alder cycloaddition of oxidized furans and ethylene over supported heteropolyacid catalysts for renewable terephthalic acid

Abstract The Diels-Alder reaction of biomass-derived dimethyl 2,5-furandicarboxylate (DMFDC) with ethylene followed by dehydrative aromatization is a sustainable route for the production of terephthalic acid, a monomer used for the synthesis of polyethylene terephthalate (PET). In this work, a series of heteropolyacid (HPA) catalysts with different supports, in this case SiO2, TiO2 and ZrO2, and hetero-addenda atoms, here phosphotungstic acid (HPW), silicotungstic acid (HSiW) and phosphomolybdic acid (HPMo), were used as catalysts, and their structure-activity correlation in the conversion of DMFDC to terephthalate was examined based on the density and strength of the acid sites. Characterization by XRD and Raman indicated that the Keggin structures of HPAs were well preserved on the SiO2 support compared to other supports, resulting in strong Bronsted acidity. The silica-supported HPW and HSiW catalysts exhibited significantly higher activity for terephthalate production than other supports and HPMo catalysts, but their activity were not correlated with the total acidity as measured by NH3-TPD. In contrast, the catalyst acid strength, as determined by the NH3 desorption temperature, decreased in the order HPW/SiO2 > HSiW/SiO2 >> HPW/ZrO2 > HPW/TiO2 > HPMo/SiO2 and was well correlated with the turnover rate for terephthalate production, suggesting that the higher activity of HPW/SiO2 is at least partly due to its stronger Bronsted acidity and that promoting the dehydration step of this reaction is a key to improving the selectivity to terephthalate. The effects of reaction conditions, in this case the acid loading amount, solvent used, temperature, and time, were also investigated with the most active silica-supported HPW catalysts to optimize the terephthalate yield. The 25%-HPW/SiO2 catalyst demonstrated a remarkable terephthalate yield of ∼60% with almost complete conversion under the optimum condition.

SYNTHESIS AND THEORETICAL STUDY OF HPW CATALYSTS SUPPORTED ON NIOBIA CALCINATED AT 500 AND 600 °C

Keggin heteropolyacids (HPW) supported on niobia have been considered for both homogeneous and heterogeneous catalysis due to its special features, like the strong Bronsted acidity, low volatility, high activity and high selectivity for various reactions in comparison with mineral acids. Literature reports many different preparation methods for these catalysts through the addition of HPW solutions, in either water or acid, over niobia under stirring. However not much is known about the way how HPW interacts with the support, neither how the temperature influences this interaction. In order to contribute to expand the knowledge on this field we report here a new way of preparing HPW supported catalysts over niobia calcined at 500 and 600 oC, followed by a theoretical study meant to check the influence of the temperature increase on the adsorption behavior of the catalysts over the support. Our proposed method showed to be an efficient alternative for impregnation of HPW over niobia, while the theoretical results suggest that pseudohexagonal and orthorhombic are the most likely crystalline forms for these catalysts.

Rhodium–Chitosan Composites Supported on Magnesium–HZSM-5 in the Conversion of Dimethyl Ether to Lower Olefins

The catalytic properties of composite catalyst systems based on zeolite HZSM-5 modified with magnesium, rhodium, and chitosan have been studied. The introduction of a rhodium–chitosan composite has a significant effect on the catalytic properties of the zeolite catalyst providing a significant increase in the activity and stability of the catalyst. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) has revealed that the use of chitosan leads to a decrease in the total acidity of the zeolite catalyst mostly owing to a decrease in the number of strong Bronsted acid sites. It has been shown that the pretreatment of Mg–Rh*chitosan–HZSM-5 with synthesis gas leads to an increase in the time-on-stream stability of the catalyst.

Effect of acidity and porosity of hierarchical HBEA zeolite on catalytic stability of α-methylnaphthalene isomerization

Hierarchical HBEA catalysts were fabricated through alkaline treatment and evaluated in a fixed-bed reactor. The relative crystallinity, pore property, Si/Al molar ratio, coke formation and structure of catalysts were investigated by the methods including X-ray diffraction, N2 physisorption, X-ray fluorescence, thermogravimetric analysis and transmission electron microscope, respectively. The acidity of catalysts was studied by Fourier transform infrared spectrometry using pyridine as the probe molecule. These results demonstrated that pure NaOH treatment improved the mesoporosity of catalyst, while decreased the β-methylnaphthalene selectivity and catalyst stability due to the partially destroyed framework and an increase in the concentration of Lewis acid sites. The Lewis acid sites are responsible for the hydride transfer and subsequent reaction that form coke. In contrast, the presence of tetra-propyl-ammonium hydroxide in the mixed-alkali treatment resulted in regular mesopores development, preserved intrinsic zeolite properties and Lewis sites proportion, leading to the maintained β-MN selectivity and coke deposition rate. The increased mesoporosity and strong Bronsted acid sites amount per number of coke were responsible for the improved coke tolerance of mixed-alkali treated HBEA, and thus its deactivation rate was 41.7% lower than that of HBEA catalyst.