Acid Site Density Research Articles

Effect of Fe/Zn ratio in composite catalyst for synthesizing polyoxymethylene dimethyl ethers

AbstractPolyoxymethylene dimethyl ethers (PODEn) are extremely effective diesel additives to reduce soot formation during combustion. We introduce a series of Fe‐Zn composite solid acid catalysts (SO42−/xFe2O3‐yZnO), for the condensation reaction of methanol and paraformaldehyde (PF) with a cheap and feasible route to efficiently synthesize PODEn. These catalysts were characterized by different characterization techniques, namely BET, XRD, SEM, EDS, FTIR, and NH3‐TPD and the results showed that Fe/Zn molar ratios strongly influenced the physicochemical characteristics of catalysts, thus affecting the methanol conversion and PODE1‐6 and PODE3‐6 selectivity. Accordingly, the methanol conversion was decreased and the selectivity of PODE3‐6 was increased after increasing the Zn molar content. Comparatively, SO42−/Fe2O3‐2ZnO exhibited superior catalytic activity among the various investigated catalysts due to the high acid density of strong acid sites. The optimal reaction conditions were observed to be at a 3.0 wt% catalyst loading (catalyst/reactant mass ratio), 2.5 hours ours of reaction time, a reaction temperature of 403 K, and a molar ratio of 3:1 of CH2O to methanol, achieving a high selectivity of 99.09% PODE1‐6 and 28.23% PODE3‐6 with 55.16% methanol conversion during the reaction.

Precise control of density and strength of acid sites of MFI-type zeolite nanoparticles via simultaneous isomorphous substitution by Al and Fe

The co-incorporation of Al and Fe at various relative ratios into the MFI-type zeolite framework enabled linear control of the density and strength of acid sites, and such well-tuned acidity led to a better catalyst lifetime.

The fixation of CO2 by epoxides over nickel-pyrazolate-based metal–organic frameworks

Ni-based MOFs with Lewis acid sites was developed for CO2 fixation with epoxides into cyclic carbonates, and a comparative study shows that the density and accessibility of the Lewis acid sites in the MOFs greatly influence the catalytic performance.

Conversion of Methanol to Aromatic-Rich Gasoline over High-Efficiency Bifunctional Catalysts: Green Synthesis of GaZSM-5 Zeolites via Dry-Gel Conversion Strategy

Nanoscale ZSM-5 (AlZ5-D) and Ga-substituted ZSM-5 zeolites ((Al,Ga)Z5-D and GaZ5-D) were synthesized by a green dry-gel conversion strategy. For comparison, ZSM-5 zeolite (AlZ5-T) was also prepared by traditional hydrothermal method. The effects of synthesis method and Ga introduction on the physicochemical property and catalytic behavior of methanol to gasoline (MTG) were studied in detail. The results show that the aromatics selectivity decreased while $$\rm{C}_5^+$$ alkanes selectivity increased for AlZ5-D sample compared with that of AlZ5-T, which was attributed to the significantly reduced density of Bronsted acid sites and weakened strong acidity. After Ga introduction, the acid strength of samples further weakened due to the formation of Si—OH—Ga bridge hydroxyl group by insertion of Ga into the zeolite framework, inhibiting the cracking reaction and thus increasing the gasoline selectivity. Moreover, the extra-framework Ga species, especially in the form of GaO+ in GaZ5-D sample, exhibited high dehydrogenation activity, leading to the highest selectivity of aromatics and gasoline.

A green approach for the preparation of a surfactant embedded sulfonated carbon catalyst towards glycerol acetalization reactions

A surfactant embedded carbon-based acid catalyst was preparedviasimple physical mixing and thermal treatment to establish the relationship between hydrophobicity and acidic site density for efficient glycerol acetalization reaction.

Simultaneous adsorption of quinoline and dibenzothiophene over Ni-based mesoporous materials at different Si/Al ratio

In this study, two mesoporous materials such as MCM-41 and Ni/MCM-41 were modified with Al at different Si/Al ratios. The mesoporous materials were proved as sorbents of heterocyclic nitrogen and sulfur compounds employing quinoline (Q) and dibenzothiophene (DBT), respectively, as model molecules of that precursor pollutants at the same concentrations of ppmw of nitrogen and sulfur (N:S = 1 ratio). The sample with the highest Si/Al ratio showed the highest specific surface, and at low Si/Al decreased the specific surface. With X-ray diffraction (XRD) the Ni containing in MCM-41, there is a clear trend and the highly hexagonal ordered sample is the one without Al and once the Al is added, the hexagonal ordering is highly modified. Employing NH3-TPD and NMR revealed that the samples with Si/Al = 5 and 16 have higher Lewis acid sites than another ratio of Si/Al. The modification successfully added small amounts of Ni that did not collapse the pore structure but increased the density of surface (acidic) groups that in turn favored the uptake of the sulfur compound through π-complexation adsorption mechanism. It is also possible to recognize that the samples with a Si/Al ratio of 5 and 16 revealed the presence of the highest surface density of weak acid sites for both sorbents. Adsorption experiments were carried out in a batch adsorption system at 313 K and atmospheric pressure using a model mixture from diesel containing: DBT, Q, and 40 ml of dodecane. The sulfur adsorption was substantially improved when Ni was incorporated to MCM-41; for example, at 250 ppmw of N uptake of DBT was almost twice than MCM-41 alone. This result would indicate that Ni/MCM-41 could remove both molecules but mainly the nitrogen molecule at high concentrations. The Langmuir and Freundlich adsorption isotherms were used for all materials considering Q in the presence of DBT depending on the Si/Al ratio. This behavior did not happen for the DBT whose behavior was modelled by an isotherm of the Freundlich type for Ni/MCM-41 and adsorbed in acid sites and multilayer. Lastly, experimental results indicated that MCM-41 with Si/Al = ∞ has the highest (Q/DBT) selectivity of 13,390 at 50 ppmw N compared to another Si/Al ratio, for example, at 250 ppmw N.

Carbon-derived from metal-organic framework MOF-74: A remarkable adsorbent to remove a wide range of contaminants of emerging concern from water

Five typical Zn-based metal-organic frameworks (MOFs) such as MOF-74(Zn) were pyrolyzed to obtain carbons-derived from MOFs (CDMs). The carbon-derived from MOF-74(Zn) or CDM-74 was firstly applied, together with other four CDMs and commercial activated carbon (AC), in the adsorptive removal of five representative contaminants of emerging concern (CECs) such as phthalic acid, diethyl phthalate, N,N-diethyl-3-methylbenzamide (DEET), chloroxylenol (CLXN) and oxybenzone (OXB) from water. CDM-74, with high mesopore volume and acid sites density, showed the highest capacity for the adsorption of DEET, OXB and CLXN from water among the tested and any reported adsorbent, so far. The plausible adsorption mechanism could be suggested, or H-bonding was important in the adsorption where the DEET and CDM-74 (especially phenolics) can be H-acceptor and H-donor, respectively. Based on the highest adsorption capacity for three adsorbates and facile recyclability, CDM-74 could be suggested as a potential adsorbent to eliminate a wide range of CECs from water.

Hydroxyapatite as a new support material for cobalt-based catalysts in Fischer-Tropsch synthesis

Hydroxyapatite [HAP, (Ca10(PO4)6(OH)2)] is an emerging catalytic support possessing exciting features such as high thermal and mechanical strength, chemically stable with low water solubility along with tunable porosity and acid-basic character. Despite of these interesting characteristics, it has not yet been investigated in Fischer-Tropsch (FT) synthesis. Herein, for the first time, HAP-supported cobalt catalysts (Co/HAP) prepared by conventional incipient wetness impregnation method were examined in the FT synthesis process. The catalytic performance of these catalysts was compared with alumina-supported cobalt catalysts (Co/Alumina). HAP support was found to exhibit considerably less acid-site density, consequently, reducing detrimental interactions of the support with cobalt precursors leading to hardly reducible Co species that are generally observed with its alumina counterpart. Co/HAP catalysts exhibit relatively larger Co particle sizes (~9 nm versus ~6 nm, as observed from TEM analysis) and better Co reducibility when compared to its counterparts on alumina. FT synthesis at 20 bar, 220 °C and H2/CO = 2.1 showed that the CO conversion was higher on the catalysts (10 wt% Co loading) using HAP as a support material when compared to alumina. Under different testing conditions (220 or 230 °C) using the Co/HAP catalyst, the C5+ selectivity was in the range of 82–88%, whereas the methane selectivity was about ~10%.

Effect of Fe Additives on the Catalytic Performance of Ion-Exchanged CsX Zeolites for Side-Chain Alkylation of Toluene with Methanol

The side-chain alkylation of toluene with methanol was investigated over some Fe-modified Cs ion-exchanged X zeolite (CsX) catalysts prepared via the impregnation method using different iron sources. The absorption/activation behaviors of the reactants on the surface of the catalysts were studied by in situ Fourier-transform infrared (FT-IR) spectroscopy and temperature programmed desorption (TPD) mass measurements. Modification of CsX with a small amount of FeCl3 could result in a considerable decrease in catalytic activity, due mainly to the remarkable decrease in the density of acidic and basic sites of the catalysts. Interestingly, the Fe(NO3)3-modified CsX with an optimum Fe loading of 0.15 wt.% shows improved catalytic activity and high yield compared to the side-chain alkylation products. Modification of CsX with Fe(NO3)3 could also result in a decrease in basic sites of the catalyst. However, such a change does not bring an obvious negative effect on the adsorption/activation of toluene, while it could effectively inhibit the generation of the undesired bidentate formate. Furthermore, the introduced FeOx species (derived from the decomposition of Fe(NO3)3) may also act as new Lewis acidic sites to participate in the activation of methanol and to stabilize the formed active intermediates (i.e., unidentate formate). Therefore, modification of CsX with a suitable amount of Fe(NO3)3 may adjust its adsorption/activation ability for reagents by changing the acid–base properties of the catalyst, which can finally enhance the catalytic performance for the side-chain alkylation of toluene with methanol.

Catalytic pyrolysis of rubbers and vulcanized rubbers using modified zeolites and mesoporous catalysts with Zn and Cu

The pyrolysis-catalytic upgrading of vulcanized rubbers with metal modified zeolites were performed in order to improve oil quality and decrease sulfur content in pyrolytic oils simultaneously. Three vulcanized rubbers were synthesized by incorporating sulfur into natural rubber (NR), butadiene rubber (BR) and styrene-butadiene rubber (SBR) respectively. The catalysts of Cu/HZSM-5, Zn/HZSM-5, Cu/MCM-41 and Zn/MCM-41 were prepared using the incipient wetness impregnation method and were characterized by various means. TPD-NH3 indicated that the incorporation of Cu and Zn changed the density and strength of acid sites. Strong acidic sites were formed in Cu modified zeolites due to the occurrence of copper crystallization, which was also evidenced by XRD analysis. Due to the high dehydrogenation activity of the Lewis acidic sites introduced by the transition metals, the incorporation of Zn and Cu increased gas yields and markedly enhanced H2 formation at the expense of liquid products. GC-MS analysis showed that the catalysts can effectively reduce the polar aromatics content in pyrolytic oils. The lowest contents of polar aromatics in oils were obtained using Cu/ZSM-5, whilst the Zn/MCM-41 catalyst showed the highest selectivity towards monocyclic aromatic hydrocarbons. Zeolites can promote the decomposition of sulfur-containing components in the liquid phase into gaseous products, which was enhanced by the incorporation of metals. The catalysts modified with Zn had a better removal efficiency of sulfur-containing components. XPS analysis indicated that the Cu and Zn modified catalysts achieved desulfurization by reacting with sulfur in pyrolytic volatiles to form stable CuS or ZnS.

Mesostructured CMK-3 carbon supported Ni–ZrO2 as catalysts for the hydrodeoxygenation of guaiacol

Abstract In this work, mesoporous carbon materials were obtained by using the nanocasting method starting from ordered mesoporous SBA-15 and commercial nonporous Cab-O-sil® M − 5 silica hard templates; materials were named CMK-3 and C-Cab, respectively. These carbon materials, displaying different porous structures (mesopore diameter, specific surface area and pore volume) were used to stabilize Ni and ZrO2 particles using different deposition methods for the nickel and zirconium phases. The catalysts were tested in the hydrodeoxygenation of guaiacol. The conversion varied from 40 to 100% depending on the textural properties of the carbon support and reaction time at 300 °C and under 50 bar of hydrogen pressure. The catalysts were selective toward the formation of the hydrogenation product (methoxycyclohexanol) and hydrogenated - partially deoxygenated product (cyclohexanol). The deposition of the Ni and ZrO2 precursors in mesoporous CMK-3 carbon replicas with a unimodal mesopore diameter, with respect to the use of non-ordered carbons, allows to obtain improved selectivity toward cyclohexanol and, more importantly, toward cyclohexane. Preparation using incipient-wetness impregnation favoured the formation of both monoclinic and tetragonal phases, while the deposition-precipitation method led to the formation of ZrO2 with a unique tetragonal structure. The presence of both monoclinic and tetragonal phases modified the density of catalyst acid sites, which allow to improve the selectivity toward cyclohexane and cyclohexanol while decreasing the selectivity toward the undesired methoxycyclohexanol and other oxygenated compounds.

Effect of Si/Al Ratio on the Catalytic Activity of Two‐Dimensional MFI Nanosheets in Aromatic Alkylation and Alcohol Etherification

AbstractZeolite catalysts with pronounced two‐dimensional (2D) morphologies, specifically 2D MFI nanosheets composed of multilamellar stacks of MFI nanosheets, are hypothesized to offer accessibility advantages in catalytic reactions involving large, bulky molecules. The Si/Al ratio of zeolite catalysts is an important parameter determining their performance. However, the effect of Si/Al ratio on catalytic reactivity is not well understood in 2D zeolite catalysts. To provide further insight into this issue, the catalytic performance of 2D MFI nanosheet materials with different Si/Al ratios is compared using the liquid phase Friedel‐Crafts alkylation of mesitylene with benzyl alcohol & the self‐etherification of benzyl alcohol that occurs in parallel, both of which are catalyzed by Brønsted acid sites. The turnover frequency (TOF) of the catalysts is found to decrease with decreasing Si/Al ratio, with the etherification reaction being the main contributor to this trend. When the same reaction is carried out in the presence of a bulky poison, 2,6 di‐tert‐butylpyridine (DTBP), to selectively deactivate the external acid sites, only the etherification reaction of benzyl alcohol takes place in the micropores of the 2D MFI catalyst and the effectiveness factor is found to decrease with decreasing Si/Al ratio. The single component adsorption isotherms of benzyl alcohol show that the uptake normalized per acid site decreases with decreasing Si/Al ratio. Thus, increasing the density of acid sites in the micropores by decreasing the Si/Al ratio makes it more difficult for the reactant molecules to access them, as demonstrated by the decrease in TOF and the effectiveness factor. While the micropore reactions (benzene alkylation, alcohol etherification) showed a clear trend with Si/Al ratio, the reaction catalyzed on the external surface (mesitylene alkylation) did not, showing distinct differences in reactivity.

Efficient conversion of carbohydrates into levulinic acid over chromium modified niobium phosphate catalyst

In this work, a series of mesoporous niobium phosphate material with different Cr content (Cr/NbP) were synthesized and used as catalysts for glucose conversion to levulinic acid (LA). The results showed that the acidic properties of the material, including acid density, acid amount, and type of acid sites, had the important effects on glucose conversion. And the doping of Cr into niobium phosphate enhanced the catalyst acid amount, especially for Lewis acid amount, which finally resulted in the increase of LA yield through glucose-fructose isomerization. The effect of various reaction parameters such as temperature, time, catalyst dosage, and substrate concentration on LA generation have been optimized, and among the testing, 5%Cr/NbP catalyst exhibited the highest catalytic performance with 62.4 mol% LA yield obtaining from 100 mol% glucose conversion at 180 °C for 180 min. Besides, the synthesized Cr/NbP also displayed high activity for disaccharide or polysaccharide conversion to LA. The recycling of Cr/NbP manifested that the material can be reused at least for four times with the activity loss less than 10.0 mol%. The experiments results demonstrated the potential of Cr/NbP catalyst for carbohydrates conversion to LA in aqueous media under mild process conditions.

A novel UiO-66 encapsulated 12-silicotungstic acid catalyst for dimethyl ether synthesis from syngas

Abstract Dimethyl ether (DME) has received great attention as a promising clean fuel and industrially important intermediate. Tremendous efforts have been made to develop highly active and stable catalysts for DME production. Here, we present the first example of the application of metal-organic framework (MOF) catalyst in DME synthesis from syngas. A Zr-MOF (UiO-66) was selected as the catalyst host and support owing to its exceptionally high stability and porosity. UiO-66 was firstly functionalized by silicotungstic acid (H4SiW12O40, STA) via a one-pot synthesis method and then loaded with Cu by a facile solid grinding method. The prepared catalysts were characterized using XRD, TGA, FT-IR, N2 adsorption and TEM. The high porosity and thermal stability of acidified UiO-66 were maintained after 15% wt STA incorporation without any destruction of the Keggin structure of STA. The catalyst was tested for one step DME synthesis from syngas, showing higher DME selectivity and space-time yield than control catalysts. The superior activity of Cu/STA-UiO was attributed to the uniform dispersion of active centres with close intimacy and high density of Bronsted acid sites, as well as strong metal support interaction between Cu and Zr oxide SBU evidenced by XPS analysis.

Continuous-Flow Process for Glycerol Conversion to Solketal Using a Brönsted Acid Functionalized Carbon-Based Catalyst

The acetalization of glycerol with acetone represents a strategy for its valorization into solketal as a fuel additive component. Thus, acid carbon-based structured catalyst (SO3H-C) has been prepared, characterized and tested in this reaction. The structured catalyst (L = 5 cm, d = 1 cm) showed a high surface density of acidic sites (2.9 mmol H+ g−1) and a high surface area. This catalyst is highly active and stable in the solketal reaction production in a batch reactor system and in a continuous downflow reactor, where several parameters were studied such as the variation of time of reaction, temperature, acetone/glycerol molar ratio (A/G) and weight hourly space velocity (WHSV). A complete glycerol conversion and 100% of solketal selectivity were achieved working in the continuous flow reactor equipped with distillation equipment when WHSV is 2.9 h−1, A/G = 8 at 57 °C in a co-solvent free operation. The catalyst maintained its activity under continuous flow even after 300 min of reaction.

Enhanced Catalytic Performance of CuO–ZnO–Al2O3/SAPO-5 Bifunctional Catalysts for Direct Conversion of Syngas to Light Hydrocarbons and Insights into the Role of Zeolite Acidity

Synthesis of light hydrocarbons from synthesis gas using bifunctional catalysts consisting of CuO–ZnO–Al2O3 methanol synthesis catalysts and SAPO-5 were investigated in a fixed bed reactor. The operating results showed that both the temperature and the ratio of CZA/SAPO-5 influenced the CO conversion and the selectivity of the catalysts. The effects of different dehydration component such as HZSM-5, HMOR and SAPO-5 and subsequently the impact of the zeolite acidity on the catalytic performance were also investigated. Experimental results indicated that zeolites in bifunctional catalysts played the crucial role for the distribution of hydrocarbons, and SAPO-5 was superior to the other zeolites in terms of better conversion and C3–C5 selectivity due to its suitable topology and proper acidic property. The efficiency of the CZA/SAPO-5 catalysts was found to be directly proportional to the Bronsted acid sites density of the zeolite and Bronsted acid sites are the likely zeolite active sites for DME dehydration. High time–space yield (461.6 mg mL−1 h−1) and high selectivity (88.1%) of light hydrocarbons (C3–C5) could be achieved on the CZA/SAPO-5-0.4 catalyst at 290 °C.

Origin and prevention of broad particle size distributions in carbon-supported palladium catalysts prepared by liquid-phase reduction

Carbon-supported palladium (Pd/C) catalysts often display broad or multimodal Pd particle size distributions detrimental for their performance. Therefore, the formation of large particles during preparation should be better understood and avoided. We prepared catalysts with up to 10 wt% Pd supported on oxygen-functionalized carbon nanotubes and activated carbon via liquid-phase reduction. It was inferred that small Pd particles (∼1 nm) were formed from Pd ions adsorbed on the carbon. At higher loadings, up to 65% of the Pd ended up in larger particles (>10 nm) probably formed from Pd ions in solution, lowering the Pd-normalized activity in the hydrogenation of cinnamaldehyde 4-fold compared to samples with small particles only. Three key factors were identified for preparing Pd/C catalysts with exclusively ∼1 nm Pd particles: a support with a high density of acid sites and high specific surface area, and a metal loading at molar ratio Pd/acid sites <0.5.

Porous clay heterostructures (PCHs) modified with copper ferrite spinel as catalyst for SCR of NO with C3H6

Selective catalytic reduction (SCR) of NO with C3H6 was investigated on porous clay heterostructures modified with CuFe2O4 (CuFe-PCH). The catalysts were characterized by XRD, HRTEM, H2-TPR, XPS, Py-FTIR and in situ DRIFTS technologies. CuFe-PCH showed high SCR activity at low temperature. Among all the catalysts in this study, Cu1Fe-PCH with the molar ratio of Cu:Fe = 1:1 exhibited the best catalytic performance, which reached 58.5% NO removal at 300 °C (0.1%C3H6) and 100% NO removal at 350 °C (0.3%C3H6) respectively. The active components on CuFe-PCH catalyst were mainly CuFe2O4 nanocrystal, exposing (2 2 0) and (3 1 1) planes. The CuFe2O4 species on Cu1Fe-PCH had high crystallinity, good redox ability, large proportion of lattice oxygen, high density of acid sites and strong Brönsted acidity, which were all responsible for the highest C3H6-SCR activity. By in situ DRIFTS, more isocyanates, including Cu-NCO and Fe-NCO, were found on Cu1Fe-PCH, which were beneficial to enhance C3H6-SCR activity. Finally, a possible reaction path was proposed.

Production of 5-HMF from glucose using TiO2-ZrO2 catalysts: Effect of the sol-gel synthesis additive

Different TiO2-ZrO2 (50/50 wt%) mixed metal oxide catalysts were synthesized by the sol-gel method after varying the synthesis additive (e.g., CH3COOH, HNO3, NH4OH and NaOH) to study its effect on the production yield of 5-HMF from the conversion of glucose at 175 °C and 30 bar (Ar) (batch reaction system). Various characterization techniques such as BET, XRD, FTIR, 1HNMR, NH3-TPD, TPO, SEM and TEM were employed to study the physicochemical properties of the synthesized materials and to correlate them with the reaction product yields. Highest HMF yield (76%) was obtained over the TiO2-ZrO2-C2H4O2 catalyst in a biphasic reaction system with a glucose concentration of 2 wt% and a glucose/catalyst mass ratio of 2.5. From the NH3-TPD results, highest amount of total acid sites was observed for the TiO2-ZrO2 (50/50 wt%) prepared by the sol-gel method using the acetic acid additive, and acidity value was 3 and 2.5 times higher than that of TiO2 and ZrO2 alone, respectively. It was concluded that total acid sites density is an important parameter to obtain high 5-HMF product yields. A higher number density of sites can be obtained using the complexing additive of acetic acid in the Sol-Gel method among the other additives investigated.

Preparation of Ga₂O₃ Doped Sulfonated Tin Oxides as a Highly Active and Recyclable Heterogeneous Solid Acid Catalyst for Aldol Reactions.

Ga₂O₃ doped sulfonated tin oxide catalysts were prepared via co-condensation method in ethanol solvent, followed by sulfonation and calcination. The samples were characterized by isothermal nitrogen adsorption/desorption, powder X-ray diffraction (XRD), thermal gravimetric analysis (TG), Raman spectra and DRIFT spectra. The number of acid sites on the catalysts was measured with the potentiometric titration of butyl amine. The results showed that the addition of small amounts of Ga₂O₃ to sulfonated tin oxide resulted in an enhanced acid site density, which makes Ga₂O₃ doped sulfonated tin oxide catalysts highly active for aldol reactions. The catalyst containing 1.5% Ga₂O₃ exhibited much higher activity than those of SO₄²-/SnO₂, SO₄²-/ZrO₂ and H₃PO₄ in aldol condensation of prenal and prenol for citral precursor, which is a important in fragrance industry. Besides the high activity, the catalyst also exhibited good recyclability, making 1.5% GST an efficient and promising solid catalyst for aldol reactions.