Bi-functional heterogeneous catalysts for carbon dioxide conversion: enhanced performances at low temperature†

Liquid fuel synthesis via CO2 hydrogenation by coupling homogeneous and heterogeneous catalysis

Avoiding Sabatier’s conflict in bifunctional heterogeneous catalysts for the WGS reaction

Boosting CO2 Conversion with Terminal Alkynes by Molecular Architecture of Graphene Oxide-Supported Ag Nanoparticles

Atmospheric concentrations of carbon dioxide (CO2) are significantly increasing since the industrial revolution at an accelerating rate causing environmental impact such as global warming and climate change. Projections indicate that CO2 concentrations will continue to rise to unsustainable levels. This highlights the scale of the challenge our scientists are facing in order to reduce CO2 emissions and underpins the importance of promoting green process engineering for the utilisation of CO2 as a valuable commodity in the process industry. The transformation of CO2 to value-added chemicals such as organic carbonates provides a promising technological advancement aimed at reducing CO2 atmospheric concentrations to sustainable levels. Dimethyl carbonate (DMC) is a promising green compound that exhibits versatile and excellent chemical properties and therefore finds applications as an intermediate in the chemical and pharmaceutical industries. DMC has a high oxygen content and can be used as an oxygenate additive to gasoline to improve its performance and reduce exhaust emission. The conventional method for DMC synthesis involves the utilisation of phosgene as a toxic feedstock. Thus, greener and more sustainable synthetic processes for the synthesis of DMC are required. Recently, non-toxic synthetic routes have been explored; these include, oxidative carbonylation of carbon monoxide (CO), oxygen (O2) and MeOH, direct synthesis from MeOH and CO2 and the transesterification of cyclic carbonates and methanol (MeOH). The oxidative carbonylation route suffers from the use of expensive raw materials and corrosive reagents as well as being hazardous due to the explosive potential of CO. The direct production of DMC from MeOH and CO2 offers an attractive and green synthetic route for DMC synthesis. Also, the synthesis of DMC via the transesterification of cyclic carbonates and MeOH, where cyclic carbonates can be synthesised from their corresponding epoxides and CO2, makes the synthesis of DMC via transesterification route more environmentally friendly and desirable in terms of green chemistry and sustainable development. Therefore, in this research new greener catalytic processes for DMC synthesis via addition of MeOH to CO2 route and transesterification route have been explored. In this work, several commercially available heterogeneous catalysts such as ceria and lanthana doped zirconia (Ce–La–Zr–O), ceria doped zirconia (Ce–Zr– O), lanthana doped zirconia (La–Zr–O), lanthanum oxide (La–O) and zirconium oxide (Zr–O) have been extensively assessed for the synthesis of DMC. Strongly coupled graphene based inorganic nanocomposites represent an exciting and new class of functional materials and therefore the utilisation of graphene oxide (GO) as a suitable support for metal oxide catalysts has been explored. Ceria doped zirconia graphene nanocomposites (Ce–Zr/GO) have been synthesised using conventional wet impregnation methods. Samples of Ce–Zr/GO have been subjected to heat treatment at various temperatures (773 K, 873 K, 973 K and 1073 K) in an attempt to enhance their catalytic performance. As-prepared Ce– Zr/GO sample and the corresponding heat treated samples have been assessed for the direct synthesis for DMC from MeOH and CO2. Furthermore, a new innovative approach has been employed for synthesising advanced, highly efficient and active heterogeneous catalysts via utilisation of a continuous hydrothermal flow synthesis (CHFS) reactor. Tin doped zirconium oxide (Zr–Sn– O) and tin doped zirconia/graphene nanocomposite (Zr–Sn/GO) have been assessed as suitable heterogeneous catalysts for the synthesis of DMC via the transesterification route. The catalysts were characterised using various analytical techniques such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and Brunauer-Emmett-Teller (BET) surface area measurement. A heterogeneous catalytic process for the synthesis of DMC has been investigated using a high pressure reactor. The effect of various reaction parameters such as the reactant molar ratio, catalyst loading, reaction temperature, CO2 pressure, reaction time and the use of a dehydrating agent was studied for the optimisation of DMC synthesis. Reusability studies were conducted to evaluate the long term stability of the heterogeneous catalysts by recycling and reusing the catalyst several times for the synthesis of DMC. Tin doped zirconia graphene oxide (Sn–Zr/GO) nanocomposite catalyst has been found to be the best performed catalyst for the synthesis of DMC as compared to other catalysts evaluated in this research work. This can be attributed to the phase composition and crystallinity of the catalyst along with the defects on the graphene sheet such as, holes, acid/basic groups and presence of residual which can provide additional active catalytic sites. Catalyst reusability studies evidently showed that Sn–Zr/GO nanocomposite can be easily recovered and reused without any significant reduction in the catalytic performance. Response Surface Methodology (RSM) has track record in helping researchers in modeling and optimisation of the experimental design for various applications in food industry, catalysis and chemical reaction engineering. Therefore, it has been employed to evaluate the relationship between multiple process variables in order to optimise a specified response (i.e. yield of DMC). RSM using Box-Behneken design (BBD) was carried out for process modeling and optimisation, with an aim to better understand the relationship between five operating variables (i.e. MeOH:PC molar ratio, catalyst loading (w/w), reaction temperature, reaction time and stirring speed) and their impact on the yield of DMC. A model for the synthesis of DMC by transesterification of PC and MeOH has been developed using BBD to compare the experimental data and the predicted results by the BBD model. Furthermore, regression analysis was applied to establish the optimum reaction conditions for a maximising DMC synthesis. The BBD model predicted values are in good agreement with the experimental results.

The selective hydrogenation of alkynes to their corresponding alkenes is an important type of organic transformation, which is currently accomplished by modified palladium catalysts. Herein, we rep...

The design and synthesis of efficient heterogeneous catalysts for catalytic conversion of CO2 are an important challenge. The novel zinc porphyrin polymer reported herein, [ZnPy+CPP‐BTA]I−, is synthesized from trans‐A2B2 porphyrin precursor 5,15‐(4‐carboxyphenyl)‐10,20‐(pyridyl)‐porphyrin (PyCPP) and 1,2,4,5‐benzenetetramine tetrahydrochloride (BTA), employing a sequential post‐synthetic methylation and metalation strategy. The newly synthesized zinc porphyrin polymer was well characterized by Fourier transform infrared spectroscopy (FT‐IR), Ultraviolet–visible spectroscopy (UV–vis), scanning electron microscope (SEM), transmission electron microscope (TEM), thermal gravimetric analysis (TGA), and X‐ray photoelectron spectroscopy (XPS). As bifunctional heterogeneous catalyst incorporated zinc porphyrin and nucleophile I−, [ZnPy+CPP‐BTA]I− shows good catalytic activity and recyclability for the cycloaddition reactions of epoxides and CO2 under lower CO2 initial pressure and cocatalyst‐free conditions.

Greener synthesis of dimethyl carbonate using a novel tin-zirconia/graphene nanocomposite catalyst

The present review primarily focuses on the perspectives and state-of-the-art of heterogeneous catalysts, nanocatalysts, biocatalysts, bifunctional catalysts, metal-organic frameworks (MOF), and covalent organic frameworks (COF) for biodiesel production. The environmental concern associated with nonrenewable fossil fuels has led to finding alternative energy sources that can be used to meet global energy demands. Biofuels such as biodiesel are one of the energy sources that could replace fossil fuels. The homogeneous acid and base catalysts are generally used for commercial biodiesel production. However, homogeneous catalysts have downsides such as toxicity, corrosion, soap formation, high wastewater output, and non-reusability. Consequently, heterogeneous acid and base catalysts have been introduced that are less sensitive to moisture and free fatty acids (FFAs), easily separated and recovered, and reusable. Recently, novel catalysts such as waste biomass-derived mesoporous heterogeneous catalysts, chemically synthesized heterogeneous catalysts, metal ion-doped heterogeneous catalysts, bifunctional acid-base catalysts, and carbonaceous char-supported hetero catalysts, nanocatalysts, MOF and COF catalysts have potential to replace homogeneous base catalysts, aid in sustainable and cost-effective biodiesel production. This review provides insights into the recent advancement of various catalysts, catalyst preparation and operations, type of catalysts and suitability, catalyst efficiency, life cycle assessment, catalyst-associated challenges, and prospects for sustainable biodiesel production.

Nanosized barium titanate powders were synthesized by a hydrothermal method. The effect of titania precursors on the phase transition of BaTiO3 with respect to Ba/Ti ratio, reaction temperature, reaction time, and calcination temperature was investigated. The synthesized materials were characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. BaTiO3 in pure cubic phase with spherical morphology was observed with a lower calcination temperature, Ba/Ti ratio, reaction temperature, and time. Increase in the tetragonal phase was ascertained in treatments at higher reaction temperature with a longer reaction time. The lattice hydroxyl release is believed to be the reason for tetragonality at high reaction and calcination temperatures. To prepare tetragonal BaTiO3 using HClO4-TiO2, the optimum synthesis conditions viz., Ba/Ti ratio, reaction temperature, and reaction time, are 1.2, 160 °C, and 3 h, respectively, at a calcination temperature of 1150 °C. The reaction time and reaction temperature for the cubic−tetragonal phase transformation of BaTiO3 shifted toward shorter reaction time and lower reaction temperature when TiO2 was synthesized by hydrolysis using HClO4 as the acid catalyst.

A sustainable and efficient approach for converting carbohydrates into 5-hydroxymethylfurfural (HMF) via heterogeneous catalysis is crucial for effectively utilizing biomass. In this study, we synthesized a series of CrX-polyphenol-formaldehyde resin (PTF) catalysts, which are composites of Cr-doped phenolic-resin-based hydrothermal carbon, using a chelation-assisted multicomponent co-assembly strategy. The performance of the synthesized catalysts was assessed through various analytical techniques, including scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, X-ray photoelectron spectroscopy, pyrolysis-Fourier transform infrared spectroscopy, and Brunauer-Emmett-Teller analysis. Cr incorporation into the catalysts enhanced the total and Lewis acidities. Notably, the optimized catalyst, designated as Cr0.6-PTF, achieved an effective glucose conversion into HMF, yielding a maximum of 69.5% at 180 °C for 180 min in a saturated NaCl solution (NaClaq)/dimethyl sulfoxide (2: 18) solvent system. Furthermore, Cr0.6-PTF maintained excellent catalytic activity and a stable chemical structure after nine cyclic reactions, resulting in a 63.8% HMF yield from glucose. This study revealed an innovative approach for utilizing metal-doped phenolic resin hydrothermal carbon to transform glucose into platform chemicals.

Several heterogeneous catalysts have been investigated for solvent-free synthesis of propylene carbonate (PC) for cycloaddition reaction of propylene oxide (PO) and carbon dioxide (CO2). The characterization of different heterogeneous catalysts has been successfully carried out using Raman spectroscopy, scanning electron microscopy, and X-ray diffraction analysis. Batch cycloaddition reaction of PC and CO2 has been conducted in a high pressure reactor. The effect of various parameters that could influence the conversion of PO and the selectivity and yield of PC such as catalyst types, catalyst loading, CO2 pressure, reaction temperature, and reaction time has been studied to find the optimum conditions and the best preferred catalyst for this reaction. Ceria and lanthana doped zirconia (Ce–La–Zr–O) catalyst has been found to be the most active and selective for synthesis of PC as compared to other heterogeneous catalysts that were tested as part of this research. Catalyst reusability studies have been con...

Recent advances on surface metal hydrides studied by solid-state nuclear magnetic resonance spectroscopy

Greener synthesis of 1,2-butylene carbonate from CO2 using graphene-inorganic nanocomposite catalyst

<div class="section abstract"><div class="htmlview paragraph">This paper reports the optimisation study of a batch scaled ethanolysis conversion of waste frying oil carried out over aluminium phosphate-potassium bi-functional catalysts. All synthesised catalysts were analysed for their structural and surface chemical properties thereby following N<sub>2</sub> adsorption-desorption isotherm and CO<sub>2</sub> and NH<sub>3</sub>-temperature programmed desorption techniques respectively. X-ray diffraction and x-ray photoelectron spectroscopy were also adopted for phase identification and atomic quantification studies respectively. Ethanolysis experiments were carried out eliminating reaction rate limitations caused by solid-liquid interfacial mass transport and intraparticle diffusion. Other operating parameters were also examined in the study. These included; reaction temperature, catalyst percentage loading on support, catalyst weight and reactants molar ratio (β).</div><div class="htmlview paragraph">Results from the preliminary study revealed that 98.2% ethyl ester yield was achieved over a catalyst sample with highest K<sub>3</sub>PO<sub>4</sub> content. Even so, ethanolysis reactions carried out over such catalysts may homogeneously progress due to abnormal increased concentrations of leached K<sup>+</sup>. Homogeneous catalytic behaviour was less pronounced over those with low to medium levels of K<sub>3</sub>PO<sub>4</sub> content. The study highlights a catalyst sample with a medium level of K<sub>3</sub>PO<sub>4</sub> content as the most heterogeneous catalyst.</div><div class="htmlview paragraph">Finally, reaction governing parameters were reasonably optimised for a best catalytic performance at conditions of β=12:1, SS = 1200r.min<sup>−1</sup>, dp≤140µm, cat = 50g.L<sup>−1</sup> and were found to be optimal. The study identifies aluminium phosphate 15wt% potassium-ribbed catalyst as a quite active bi-functional catalyst for transesterification of spent frying oils.</div></div>

Acid-base bifunctional heterogeneous catalysts were prepared by the reaction of an acidic silica-alumina (SA) surface with silane-coupling reagents possessing amino functional groups. The obtained SA-supported amines (SA-NR2) were characterized by solid-state 13C and 29Si NMR spectroscopy, FT-IR spectroscopy, and elemental analysis. The solid-state NMR spectra revealed that the amines were immobilized by acid-base interactions at the SA surface. The interactions between the surface acidic sites and the immobilized basic amines were weaker than the interactions between the SA and free amines. The catalytic performances of the SA-NR2 catalysts for various carbon-carbon bond-forming reactions, such as cyano-ethoxycarbonylation, the Michael reaction, and the nitro-aldol reaction, were investigated and compared with those of homogeneous and other heterogeneous catalysts. The SA-NR2 catalysts showed much higher catalytic activities for the carbon-carbon bond-forming reactions than heterogeneous amine catalysts using other supports, such as SiO2 and Al2O3. On the other hand, homogeneous amines hardly promoted these reactions under similar reaction conditions, and the catalytic behavior of SA-NR2 was also different from that of MgO, which was employed as a typical heterogeneous base. An acid-base dual-activation mechanism for the carbon-carbon bond-forming reactions is proposed.