Intensification studies of heterogeneous catalysts: probing and overcoming catalyst deactivation during liquid phase operation

In addition to high levels of catalytic activity and target product selectivity, promising heterogeneous catalysts must also possess sufficient levels of stability for scale up, i.e. intensification, and industrialisation to be realised. However, this third – often determining – keystone is often overlooked, particularly in academic literature. This tutorial review therefore covers several elements related to the study of heterogeneous catalyst stability during liquid phase operation, an increasingly important area of heterogeneous catalysis. Particular emphasis is placed upon how stability data can be obtained and how the various forms of catalyst deactivation can be experimentally identified, and attention is drawn to emerging methods by which such events can be overcome or minimised. Drawing on some of our team's recent research, particular emphasis is placed on the stability studies of Lewis acidic silicates, state-of-the-art catalysts for a range of emerging liquid–solid catalytic processes. Factors related to heterogeneous nanoparticle catalysts are also covered. The purpose of this review is to draw attention to the issue of solid catalyst stability during liquid phase operation, and provide researchers with the information required to begin kinetic and spectroscopic studies of this determining catalytic event

Green chemistry emerging investigators 2017 themed issue

Green Chemistry is delighted to present the inaugural Emerging Investigators themed issue. This issue consists of articles by the rising stars of the green chemistry field. Each contributor was nominated by a member of the Editorial or Advisory Board as an outstanding researcher in the early stages of their independent career, making a significant contribution to green chemistry. Congratulations to all of those whose work is featured in the collection

Porous metallosilicates for heterogeneous, liquid-phase catalysis: perspectives and pertaining challenges

Porous silicates containing dilute amounts of tri-, tetraand penta-valent metal sites, such as TS-1, Sn-β and Fe- ZSM-5, have recently emerged as state of the art catalysts for a variety of sustainable chemical transformations. In contrast with their aluminosilicate cousins, which are widely employed throughout the refinery industry for gas-phase catalytic transformations, such metallosilicates have exhibited unprecedented levels of performance for a variety of liquidphase catalytic processes, including the conversion of biomass to chemicals, and sustainable oxidation technologies with H2O2. However, despite their unique levels of performance for these new types of chemical transformations, increased utilization of these promising materials is complicated by several factors. For example, their utilization in a liquid, and often polar, medium hinders process intensification (scaleup,catalyst deactivation). Moreover, such materials do not generally exhibit the active-site homogeneity of conventional aluminosilicates, and they typically possess a wide variety of active-site ensembles, only some of which may be directly involved in the catalytic chemistry of interest. Consequently,mechanistic understanding of these catalysts remains relatively low, and competitive reactions are commonly observed. Accordingly, unified approaches towards developing more active, selective and stable porous metallosilicates have not yet been achieved. Drawing on some of the most recent literature in the field, the purpose of this mini review is both to highlight the breakthroughs made with regard to the use of porous metallosilicates as heterogeneous catalysts for liquidphase processing, and to highlight the pertaining challenges that we, and others, aim to overcome during the forthcoming years

Intensification and deactivation of Sn-Beta investigated in the continuous regime

Despite a proliferation of research focusing on the synthesis and catalytic chemistry of Sn-containing zeolite Beta, research focusing on its intensification lacks behind, prohibting its further exploitation. In this manuscript, we investigate and optimise the continuous flow activity of Sn-β for a range of sustainable chemical transformations, including the transfer hydrogenation of model and bio-renewable substrates (furfural), and the isomerisation of glucose to fructose. Extended time-on-stream studies reveal Sn-β to be an very stable catalyst during continuous operation in an organic solvent. Spectroscopic methodologies reveal that deactivation in these cases is related to fouling of the micropores with the product and higher molecular weight carbonaceous residue. Periodic regeneration by heat treatment is found to restore full activity, allowing Sn-β to be used for over 700 h continuously with no greater than 20 % loss in activity. In contrast, operation in an aqueous media is extremely disadvantagous, as it causes total destruction of the catalyst and permanent deactivation. In these cases, however, long term activity can still be achieved by modifying the solvent chosen for reaction, with methanol appearing to be a suitable alternative. The promising results presented herein conclusively demonstrate the potential of Sn-β to operate as an industrial heterogeneous catalyst

Chemoselective Lactonization of Renewable Succinic Acid with Heterogeneous Nanoparticle Catalysts

The production of chemicals from renewable resources, resulting in the establishment of biorefineries, represents a challenge of increasing importance. Here we show that succinic acid, a C4 compound increasingly being produced on a kiloton scale by the microbial fermentation of sugar, can be selectively converted into a variety of important chemicals. Optimal performance in terms of activity, selectivity and reusability is observed with Al2O3-supported Pd nanoparticles, which mediate the selective, hydrogenative lactonization of succinic acid to γ-butyrolactone at >90% selectivity, even at high levels of conversion (<70%). Through a variety of kinetic, spectroscopic and microscopic studies, preliminary structure–activity relationships are presented, and the roles of the reaction conditions, the choice of metal and the nature of the support in terms of guiding the overall process selectivity, are also investigated. On a broader level, these studies demonstrate the suitability of succinic acid to act as a platform for renewable chemical production in future biorefineries

Chemoselective Lactonization of Renewable Succinic Acid with Heterogeneous Nanoparticle Catalysts

The production of chemicals from renewable resources, resulting in the establishment of biorefineries, represents a challenge of increasing importance. Here we show that succinic acid, a C4 compound increasingly being produced on a kiloton scale by the microbial fermentation of sugar, can be selectively converted into a variety of important chemicals. Optimal performance in terms of activity, selectivity and reusability is observed with Al2O3-supported Pd nanoparticles, which mediate the selective, hydrogenative lactonization of succinic acid to γ-butyrolactone at &gt;90% selectivity, even at high levels of conversion (&lt;70%). Through a variety of kinetic, spectroscopic and microscopic studies, preliminary structure-activity relationships are presented, and the roles of the reaction conditions, the choice of metal and the nature of the support in terms of guiding the overall process selectivity, are also investigated. On a broader level, these studies demonstrate the suitability of succinic acid to act as a platform for renewable chemical production in future biorefineries.</p

Acceptorless alcohol dehydrogenation catalysed by Pd/C

Although the selective oxidation of alcohols to carbonyl compounds is a critical reaction, it is often plagued by several challenges related to sustainability. Here, the continuous, acceptorless dehydrogenation of alcohols to carbonyl compounds over heterogeneous catalysts was demonstrated, in the absence of oxidants, bases or acceptor molecules. In addition to improving selectivity and atom efficiency, the absence of an acceptor resulted in the co‐production of molecular H2, a clean energy source, and permitted dehydrogenation to proceed at >98 % selectivity at turnover frequency values amongst the highest in the literature. Moreover, excellent durability was observed during continuous operation over 48 h, reaching space‐time yields of 0.683 g(product) mL−1 h−1, better than the state of the art by over two orders of magnitude. Alongside these breakthroughs, the basic kinetic parameters of the reaction were also determined, allowing some of the elementary reaction steps to be identified

Developing a continuous process for isosorbide production from renewable sources

Increasing demand for isosorbide has led to the search for sustainable and efficient methods for its production from sorbitol, a biomass‐derived platform molecule. However, sorbitol dehydration to isosorbide is currently performed with mineral acids, resulting in safety and toxicity issues. Although some progress has been made towards replacement of liquid acids with heterogeneous catalysts, continuous systems with good stability, selectivity and productivity remain scarce. Herein, sorbitol dehydration to isosorbide is efficiently performed in a continuous, liquid‐phase plug flow reactor, utilising an acidic zeolite (H‐β (38)) as solid catalyst. H‐β (38) is shown to catalyse the reaction without loss of activity for 55 hours on stream, achieving an isosorbide productivity of 9670 gisosorbide/kgcatalyst. This value is 4 times greater than any other continuous process reported in literature to date, even though the reaction was terminated prior to any loss of activity being detected. Diagnostic kinetic studies reveal improved operational conditions, and characterisation of the post‐reaction catalyst is provided

Polymer-supported metal catalysts for the heterogeneous polymerisation of lactones

A series of metal complexes were immobilised onto an inert poly(styrene) (PS) support and utilised in the solvent free ring-opening polymerisation (ROP) of various lactones. PS-LHZnOAc, PS-LHSnOct and PS-LClSnOct were identified as the most successful heterogeneous catalysts for the ROP of L-lactide. Investigations by in situ ATR-FT-IR revealed conversions reaching ca. 90% in 6, 2.3 hours and 55 minutes, respectively, with excellent molecular weight control and dispersities (ĐM 1.15–1.17). Catalyst loadings as low as 15 ppm metal and TOF values of up to 810 h−1 could also be achieved. Higher molecular weights could be targeted (ca. 35 kDa) whilst maintaining low dispersities in comparison to the industrial standard. Catalyst reuse was also possible, with up to 7 reuse cycles, albeit accompanied by a progressive reduction in conversion. Energy-Dispersive X-ray (EDX) spectroscopy and Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) showed low metal content in the unpurified polymer (as low as 335 ppm, similar to what is found in polymer purified by classical methods), suggesting these systems as promising reusable catalysts for the industrial production of metal-free renewable polymers

Catalytic formation of C(sp3 )–F bonds via decarboxylative fluorination with mechanochemically-prepared Ag2O/TiO2 heterogeneous catalysts

Mechanochemically-prepared, Ag2O-containing solid materials, are shown to be efficient heterogeneous catalysts for the synthesis of C(sp3)-F bonds via decarboxylative fluorination. Five catalytic cycles without loss of intrinsic activity could be performed with the optimal catalyst, composed of 1 wt% Ag2O supported on TiO2 (P25), despite the challenging conditions. The catalyst is easily prepared from the corresponding oxides in 20 minutes by simple mechanical mixing methods. In addition to ease of separation and re-use, the turnover numbers obtained over the solid catalyst are over one order of magnitude higher than those obtained with the state-of-the-art homogeneous catalyst, AgNO3, under otherwise identical conditions. To the best of our knowledge, this represents the first true heterogeneous catalyst for the selective formation of C(sp3)-F bonds with electrophilic fluorine donors, representing a major breakthrough in the field of catalytic fluorination