Basic Sites Of Catalysts Research Articles

Insights into the synergetic mechanism of basic active site and oxygen vacancy on thermally activated MOFs for the colophony esterification: Experiments and DFT calculations

Investigating an feasible catalytic mechanism on Zn-MOF and its derivative amphoteric oxides is of great importance for esterification. Herein, activated Fe3O4-functionalized IRMOF-1 that was well retained the porosity was fabricated by encapsulating magnetic Fe3O4 in IRMOF-1 and activating thermally at 298−1273 K, which was evaluated for the esterification of colophony with glycerol. Results demonstrate that the catalytic performance of catalysts could be effectively enhance by thermal activation due to the additional basic sites and oxygen vacancies created. FT-IR and several thermal analyses reveal the unique adsorption of glycerol instead of the traditional carboxylic acids. Density functional theory (DFT) calculations suggest that the basic sites of catalysts are the crucial active sites for the glycerol hydroxyl dissociation and the formation of nucleophilic glyceroxide, and the presence of oxygen vacancy could significantly destabilize the surface proton and glyceroxide anions. A synergetic esterification mechanism of basic active site and oxygen vacancy was proposed.

Synergistic effects of gamma irradiation on the PET surface and heat treatment of hydrotalcite catalyst supported by Pt/TiO2 nanoparticles on PET depolymerization rate

Acquiring new methods to achieve the products with lesser molecular weight associated with minimum energy consumption in poly(ethylene terephthalate) (PET) recycling can improve the glycolysis reaction performance. The synergistic effects of gamma irradiation on the properties of PET wastes and heat treatment on the catalytic activity of novel hydrotalcite supported by 2‐wt% Pt/TiO2 nanoparticles (NPs) were studied. The PET samples treated by different gamma irradiation doses, γ, were degraded by glycolysis, which resulted in an increased yield of bis(2‐hydroxyethyl) terephthalate (BHET) monomer. The XPS analysis showed that the calcination temperature of the catalyst, T, affects the strength of the acid and/or basic sites of catalyst, degradation products, and reaction time. The NH3‐temperature‐programmed desorption (TPD) analysis showed the interaction between TiO2 and Pt led to a wide distribution of middle strength acid sites. Processing of PET by gamma irradiation showed that the average molecular weight of degradation products and reaction time decreased by increasing gamma dose. The composition (wt%) analysis indicated that the highest values of yield and selectivity of BHET were achieved in the optimum values of γ = 70 kGy and T = 450°C. The proposed procedure helps to attain minimize the residual oligomers and unreacted species during PET recycling.

Synthesis of HTLcs modified by K3PO4 for side chain alkylation of toluene with methanol

A series of calcined HTLcs catalysts were prepared and modified with potassium phosphate by impregnation method to clarify the influence of catalyst alkalinity on the side chain alkylation of toluene with methanol for synthesis of ethylbenzene and styrene. The catalysts were characterized by X-ray diffraction (XRD), N2 physical adsorption–desorption, Fourier-transform infrared spectroscopy (FT-IR), Scanning electron microscopy (SEM), X-ray photoelectron spectrometry (XPS), NH3 temperature-programmed desorption (NH3-TPD) and CO2 temperature-programmed desorption （CO2-TPD）. It was found that the selectivity of styrene was highest (39.25%) when the K3PO4 loading was 7.5 wt%. And the total yield of styrene and ethylbenzene could reach 65.08% with 10 wt% K3PO4 loading. This might due to the fact that the addition of K3PO4 could adjust the acid and basic sites of catalysts. In addition, appropriate strength and amount of basic sites were favorable to producing more styrene.

Tailoring the Properties of Metal Oxide Loaded/KCC-1 toward a Different Mechanism of CO2 Methanation by in Situ IR and ESR.

Nickel (Ni), cobalt (Co), and zinc (Zn) loaded on fibrous silica KCC-1 was investigated for CO2 methanation reactions. Ni/KCC-1 exhibits the highest catalyst performance with a CH4 formation rate of 33.02 × 10-2 molCH4 molmetal-1 s-1, 1.77 times higher than that of Co/KCC-1 followed by Zn/KCC-1 and finally the parent KCC-1. A pyrrole adsorption FTIR study reveals shifting of perturbed N-H stretching decreasing slightly with the addition of metal oxide, suggesting that the basic sites of catalyst were inaccessible due to metal oxide deposition. The strengths of basicity were found to follow sthe equence KCC-1, Ni/KCC-1, Zn/KCC-1, and Co/KCC-1. The data were supported by N2 adsorption desorption analysis, where Co/KCC-1 displayed the greatest reduction in total surface area whereas Ni/KCC-1 displayed the least reduction. The elucidation of difference mechanism pathways has also been studied by in situ IR spectroscopy studies to determine the role of different metal oxides in CO2 methanation. It was discovered that Ni/KCC-1 and Co/KCC-1 follow a dissociative mechanism of CO2 methanation in which the CO2 molecule was dissociated on the surface of the metal oxide before migration onto the catalyst surface. This was confirmed by the evolution of a peak corresponding to carbonyl species (COads) on a metal oxide surface in FTIR spectra. Zn/KCC-1, on the other hand, showed no such peak, indicating associative methanation pathways where a hydrogen molecule interacts with an O atom in CO2 to form COads and OH. These results offers a better understanding for catalytic studies, particularly in the field of CO2 recycling.

The occurrence of Cannizzaro reaction over Mg-Al hydrotalcites

Mg-Al mixed oxides and reconstructed hydrotalcites prepared from mixed oxides by rehydration attract much attention as solid basic catalysts for organic reactions, such as aldol condensation. Therefore, the understanding of parameters that govern their catalytic performance is of great importance. Cannizzaro reaction in the presence of mixed oxide and reconstructed HTC as basic catalysts was used as a model reaction to characterize both catalytic systems and to understand their behavior in aldol condensation. The performance of the samples was investigated in the conversion of aqueous furfural mixture using stirred batch reactor operating at T=50°C. The properties of both initial samples and catalysts after reaction were studied by different physico-chemical methods, such as XRD, DRIFT and TGA. It was shown that both mixed oxide and reconstructed HTC possessed poor activity in the conversion of dried furfural proving the importance of water presence for the reaction to take place. Addition of water to the reaction mixture resulted in growth of the yield of both furfuryl alcohol and furoyl furoate as reaction products. Furoic acid was not identified by GC analysis, but the interaction of the acid with the basic sites of catalysts was confirmed by DRIFT and TGA analyses indicating that at optimum reaction conditions, most of the basic sites were involved in the formation of furoate species. The formation of the surface furoate species influenced the behavior of the basic catalysts in aldol condensation of furfural and acetone. It allows concluding that the occurrence of Cannizzaro reaction should be taken into account when considering the behavior of basic catalysts in organic reactions with furfural as a reactant.

Novel Synthetic Strategy for Developing an Isospecific Unbridged Metallocene System for Propylene Polymerization

New unbridged zirconocenes functionalized with a Lewis base, [{1-(E-C(6)H(4))-3,4-Me(2)C(5)H(2)}(2)ZrCl(2)] (E = p-NMe(2) (3); p-OMe (4); p-SMe (5)) were prepared and their propylene polymerization behavior was examined. Under methylaluminoxane (MAO) activation at atmospheric monomer pressure, these complexes afford mixtures of polymers exhibiting multimelting transition temperatures and broad molecular weight distribution, whereas they produce completely atactic polypropylenes under [Ph(3)C][B(C(6)F(5))(4)] activation. Stepwise solvent extraction of the polymer mixtures reveals that the polymers consist of amorphous, moderately isotactic, as well as, highly isotactic portions and the weight ratio of each portion is dependent upon reaction temperature. The generation of rigid rac-like cationic active species in situ by the interaction between basic sites of catalysts and acidic sites of the [Me-MAO](-) counter anion is considered to be the origin of the observed isospecificity. Further investigation of bulk polymerization in liquid propylene shows not only a considerable increase of the isotactic portion of the obtained polypropylenes but also apparent isospecificity of 4 and 5/MAO systems even at high temperature. Variation of the Lewis basic center leads to a dramatic change in stereoselectivity of the catalyst in the decreasing order of 3>4>>5, in spite of their structural similarity.

Screening of amorphous metal–phosphate catalysts for the oxidative dehydrogenation of ethylbenzene to styrene

The gas-phase oxidative dehydrogenation of ethylbenzene to styrene was carried out by using as catalyst a series of metal phosphates (Al, Fe, Ni, Ca and Mn) and stoichiometric (Al/Fe=Al/Ca=1) mixed systems: FeAl(PO4)2 and Ca3Al3(PO4)5, that were prepared by an ammonia gelation method. Their amorphous character was determined through several physical methods: nitrogen adsorption, DRIFT and XRD patterns. These results were compared to those obtained with 24 commercial inorganic solids (several metal oxides, sulfates and phosphates). Reactions were also carried out without oxygen, under non-oxidative conditions, where the catalytic activity was always appreciably lower than under oxidative conditions. Experimental results indicated that the oxidative gas-phase dehydrogenation of ethylbenzene to styrene could be related to the total number of acid and basic sites of catalysts, so that this reaction probably needs selected acid–basic pairs for coke formation, where the oxidative dehydrogenation process is developed.The main practical conclusion of the catalyst screening was that the best results were obtained with the synthesized amorphous AlPO4, where 43% ethylbenzene conversion and 99.7% styrene selectivity were achieved. A very reduced number of commercial inorganic solids like Al2(SO4)3, Cr2(SO4)3, Fe2(SO4)3, NiSO4, Al2O3 and Fe2O3 were also able to obtain an acceptable catalytic behavior, with conversions ranging between 18 and 23% and selectivity in the 95–100% range. Among the other synthesized solids, Ni3(PO4)2-A-450 was the only metal phosphate exhibiting results in such a range. All the other catalysts studied were rather inactive and/or selective. Additional experiments carried out at longer times on stream (3.5h) and longer contact times (W/F 0.254 and 0.654) confirmed the superior catalytic behavior of amorphous AlPO4. Consequently, this solid could be a good candidate for application as a catalyst in the industrial oxydehydrogenation of ethylbenzene to styrene.

Support effect in dehydrogenation of propane in the presence of CO 2 over supported gallium oxide catalysts

Dehydrogenation of propane to propene in the absence or presence of CO 2 over different supported gallium oxide catalysts was investigated. Ga 2O 3/TiO 2, Ga 2O 3/Al 2O 3, and Ga 2O 3/ZrO 2 catalysts exhibited high activity, and Ga 2O 3/SiO 2 and Ga 2O 3/MgO were ineffective catalysts for the dehydrogenation of propane. The conversions of propane over Ga 2O 3/TiO 2, Ga 2O 3/Al 2O 3, and Ga 2O 3/ZrO 2 catalysts at 600 °C reached 23, 33, and 39%, respectively. The high dehydrogenation activities are connected with the abundant medium-strong acid sites on the catalyst surface. Carbon dioxide can promote the catalytic dehydrogenation activity of Ga 2O 3/TiO 2 but suppress those of Ga 2O 3/ZrO 2 and Ga 2O 3/Al 2O 3. The conversion of propane becomes 32, 26, and 30% in the presence of CO 2 over Ga 2O 3/TiO 2, Ga 2O 3/Al 2O 3, and Ga 2O 3/ZrO 2 catalysts, respectively. Results of pulse reaction and the H 2 and propane chemisorptions indicate that two contrary roles of CO 2 exist in the reaction: the positive role of removing dissociatively adsorbed H 2 on catalyst surface through the reverse water–gas shift reaction and the negative role of displacing the propane adsorbed on basic sites of catalyst. XPS studies show that the different behavior of the five supported Ga 2O 3 catalysts may be attributed to the different interactions between the support and the Ga 2O 3 species. The dehydrogenation reaction is suggested to proceed through a heterolytic dissociation reaction pathway.