Clean catalytic technologies for converting renewable feedstocks to chemicals and fuels

Concerns over dwindling fossil fuel reserves, and the impact of CO2 emissions on climate change, is driving the quest for alternative feedstocks to reduce dependence on non-renewable sources of fuels and chemicals. Biomass offers the only renewable source of organic molecules for the manufacture of bulk, fine and speciality chemicals necessary to secure the future needs of society. In this regard, conversion of biomass derived C6 sugars to 5-hydroxymethylfurfural (HMF), the latter a potential, bio-derived platform chemical, is of significant current interest. However, commercial implementation of HMF as a chemical intermediate is impeded by high production costs. A heterogeneously catalysed route to directly convert glucose into HMF in aqueous media thus remains highly sought after. In this thesis, the telescopic conversion of glucose to fructose and then HMF has been explored over a family of bifunctional sulfated zirconia catalysts possessing tuneable acid–base properties. Characterisation by acid–base titration, XPS, XRD and Raman reveal that sub-monolayer SO4 coverages offer the ideal balance of basic and Lewis– Brønsted acid sites required to respectively isomerise glucose to fructose, and subsequently dehydrate fructose to HMF. Here we demonstrate that systematic control over the Lewis–Brønsted acid and base properties of SZ enables one-pot conversion of glucose to HMF in aqueous media, employing a single bi-functional heterogeneous catalyst. Further improvements in catalytic performance have been achieved through the synthesis of monolayer grafted ZrO2/SBA-15 catalysts in which conformal layers of ZrO2 are grown from Zr propoxide. Analysis reveals 1-3ML can be achieved; subsequent sulfation yields a catalyst with 1.25 to 2 times the activity of bulk SZ. These catalysts also exhibit remarkable water tolerance with retention of pore structure upon hydrothermal treatment at 170 °C for 6 h. All catalysts find application in esterification,with optimum activity for samples treated with 0.1 M H2SO4

The catalytic cracking of sterically challenging plastic feedstocks over high acid density Al-SBA-15 catalysts

The catalytic cracking of polyolefinic waste materials over solid acid catalysts, such as zeolites, is a promising process for the production of useful fuels and chemicals. However, the inherent diffusional constraints of the microporous zeolites restrict the access of bulky polyolefin molecules to the active site, therefore limiting their effectiveness. To address this, a simple yet effective method of producing mesoporous Al-SBA-15 materials with a high density of Brønsted acid sites has been employed. These catalysts are shown to be very active for the catalytic cracking of low density polyethylene (LDPE), a common waste plastic. The acidic and textural properties of the catalysts were characterised by ICP-OES, XPS, XRD, N2 physisorption, propylamine-TPD, pyridine-FTIR and STEM and have been correlated with their catalytic activity. The product distribution from the catalytic cracking of LDPE has been shown to depend strongly on both the pore architecture and the Al content of the SBA-15 and thus the density and strength of Brønsted acid sites. Fine-tuning the Al content of the SBA-15 materials can direct the product distribution of the hydrocarbons. The Al-SBA-15 materials display increased cracking orientated towards aliphatic hydrocarbons compared to ZSM-5, attributed to the mesoporous nature of SBA-15, overcoming diffusional limitations

Microwave-assisted synthesis of levulinic acid from low-cost, sustainable feedstocks using organic acids as green catalysts

BACKGROUND: Modern day scientific endeavour strives towards global sustainability through the smart utilisation of renewable resources as base materials for chemicals. Until now, the most common commercial process to produce levulinic acid (a mass-produced platform chemical) depends on a two-stage mineral acid-catalysed reaction, which generates harmful environmental waste. In this work, an environmentally friendly levulinic acid production route using less harmful organic acids assisted by microwave heating from biomass feedstocks is reported for the first time. RESULTS: Using aluminum sulfate as a green Lewis acid catalyst and seven organic acids, levulinic acid was successfully produced from barley straw under microwave heating, with maleic acid giving the highest catalytic conversion. A Response Surface Methodology (RSM) approach was used to rapidly and effectively examine the effect of five reaction variables on the productivity of the levulinic acid. A wide range of different biomass wastes (barley straw, brewery waste, olive cake, spent tea leaves and potato, tomato, and mandarin peels) were subsequently screened to produce the levulinic acid. The highest yield of 86 wt% based on cellulose content from mandarin peel (a value comparable to a lengthier ‘non-green’ route) was achieved under the following optimized reaction conditions: 180 °C, 38 min, 2 M maleic acid concentration, 0.1 g Al 2(SO 4) 3 and 1:22 biomass: maleic acid ratio (g mL −1). CONCLUSIONS: The proposed method is a promising new route towards the green, high yield production of levulinic acid from a variety of agricultural and household lignocellulosic biomass wastes, without the need for pre-treatment

Ga/HZSM-5 Catalysed Acetic Acid Ketonisation for Upgrading of Biomass Pyrolysis Vapours

Pyrolysis bio-oils contain significant amounts of carboxylic acids which limit their utility as biofuels. Ketonisation of carboxylic acids within biomass pyrolysis vapours is a potential route to upgrade the energy content and stability of the resulting bio-oil condensate, but requires active, selective and coke-resistant solid acid catalysts. Here we explore the vapour phase ketonisation of acetic acid over Ga-doped HZSM-5. Weak Lewis acid sites were identified as the active species responsible for acetic acid ketonisation to acetone at 350 ◦C and 400 ◦C. Turnover frequencies were proportional to Ga loading, reaching ~6 min −1 at 400 ◦C for 10Ga/HZSM-5. Selectivity to the desired acetone product correlated with the weak:strong acid site ratio, being favoured over weak Lewis acid sites and reaching 30% for 10Ga/HZSM-5. Strong Brønsted acidity promoted competing unselective reactions and carbon laydown. 10Ga/HZSM-5 exhibited good stability for over 5 h on-stream acetic acid ketonisation

Advances in Sustainable γ-Valerolactone (GVL) Production via Catalytic Transfer Hydrogenation of Levulinic Acid and Its Esters

γ-Valerolactone (GVL) is a versatile chemical derived from biomass, known for its uses such as a sustainable and environmentally friendly solvent, a fuel additive, and a building block for renewable polymers and fuels. Researchers are keenly interested in the catalytic transfer hydrogenation of levulinic acid and its esters as a method to produce GVL. This approach eliminates the need for H 2 pressure and costly metal catalysts, improving the safety, cost effectiveness and environmental sustainability of the process. Our Perspective highlights recent advancements in this field, particularly with respect to catalyst development, categorizing them according to catalyst types, including zirconia-based, zeolites, precious metals, and nonprecious metal catalysts. We discuss factors such as reaction conditions, catalyst characteristics, and hydrogen donors and outline challenges and future research directions in this popular area of research

Efficient one-pot production of γ-valerolactone from xylose over Zr-Al-Beta zeolite: rational optimization of catalyst synthesis and reaction conditions

The one-pot conversion of xylose into γ-gammavalerolactone in 2-propanol over bifunctional Zr-Al-Beta zeolites, prepared via a post-synthetic route, was optimized in terms of both catalyst synthesis and reaction conditions. In the catalyst preparation, the use of Zr(NO3)4 as zirconium source as well as the tuning of the amount of water used during the impregnation had a strong impact on the activity of the Zr species due to an improved dispersion of Zr species. As for the aluminium to zirconium exchange, an optimal Al/Zr ratio of 0.20 was identified to provide a catalyst with better activity. The modelization of the catalytic system through experimental design methodology allowed to identify the optimal values of the most influential reaction conditions: temperature 190 °C, catalyst loading 15 g L−1, and starting xylose concentration 30.5 g L−1. Under these optimized reaction conditions, Zr-Al-Beta catalyst provides a GVL yield from xylose (ca. 34%) after only 10 h. The catalysts are stable and reusable after thermal regeneration at 550 °C

The Hydrogenation of Crotonaldehyde on PdCu Single Atom Alloy Catalysts

Recyclable PdCu single atom alloys supported on Al2O3 were applied to the selective hydrogenation of crotonaldehyde to elucidate the minimum number of Pd atoms required to facilitate the sustainable transformation of an α,β-unsaturated carbonyl molecule. It was found that, by diluting the Pd content of the alloy, the reaction activity of Cu nanoparticles can be accelerated, enabling more time for the cascade conversion of butanal to butanol. In addition, a significant increase in the conversion rate was observed, compared to bulk Cu/Al2O3 and Pd/Al2O3 catalysts when normalising for Cu and Pd content, respectively. The reaction selectivity over the single atom alloy catalysts was found to be primarily controlled by the Cu host surface, mainly leading to the formation of butanal but at a significantly higher rate than the monometallic Cu catalyst. Low quantities of crotyl alcohol were observed over all Cu-based catalysts but not for the Pd monometallic catalyst, suggesting that it may be a transient species converted immediately to butanol and or isomerized to butanal. These results demonstrate that fine-tuning the dilution of PdCu single atom alloy catalysts can leverage the activity and selectivity enhancement, and lead to cost-effective, sustainable, and atom-efficient alternatives to monometallic catalysts

A magnetically separable SO 4 /Fe-Al-TiO 2 solid acid catalyst for biodiesel production from waste cooking oil

A novel magnetic SO4/Fe-Al-TiO2 solid acid catalyst was synthesized for biodiesel production via the (trans)esterification of waste cooking oil (WCO). The nanocomposite catalyst was prepared by the sequential functionalisation of commercial rutile/anatase mixed phase TiO2 nanoparticles (NPs) with alumina as a buffer layer, and subsequently hematite to impart magnetic character, prior to sulfation with chlorosulfonic acid to introduce Brønsted acidity. XRD showed that the SO4/Fe-Al-TiO2 catalyst comprised titania (rutile and anatase phases), aluminium sulphate, and hematite nanoparticles, while electron microscopy revealed the layer-by-layer assembly of these components within the SO4/Fe-Al-TiO2 catalyst. FTIR confirmed the presence of surface sulphate groups SO42- and S2O72-/S3O102-, creating a predominantly Brønsted acid catalyst with high acid loading. The catalyst achieved 96 % fatty acid methyl ester (FAME) yield from WCO after 2.5 h of reaction at 90 °C, using 3 wt% of the magnetic catalyst, and a methanol:oil molar ratio of 10:1. SO4/Fe-Al-TiO2 was also effective for feedstocks containing up to 20 wt% of free fatty acid (FFA), and showed excellent stability for WCO (trans)esterification over 10 recycles

Ni-based bimetallic catalysts for hydrogen production via (sorption-enhanced) steam methane reforming

The catalytic performance of a monometallic Ni/Al2O3 and three bimetallic catalysts (Ni3M1/Al2O3, with M = Cu, Fe, and Ge) for the (sorption-enhanced) steam methane reforming reaction was evaluated. Ni3Cu1/Al2O3 was found to be the optimal catalyst in terms of methane conversion, hydrogen yield, and purity. Ge also has a promoting effect on the monometallic Ni catalyst, whereas the addition of Fe negatively influenced its performance. Physico-chemical characterization of the materials indicated the formation of alloys upon activation of the materials with hydrogen. The addition of Cu increased the surface area and metal dispersion, and improved the overall morphology of the catalyst. The experimental observations were also supported by a numerical study combining Density Functional Theory-based calculations and Microkinetic modelling of the SMR process. Ni3Cu1 and Ni3Ge1 were calculated to have a similar level of catalytic activity as Ni, whereas Ni3Fe1 was unsuitable for the reaction. The SMR reaction was further improved by adding calcium oxide as the CO2 sorbent, which increased methane conversion, CO selectivity, hydrogen yield, and hydrogen purity. The highest methane conversion of 97 % was achieved by Ni/Al2O3 and Ni3Cu1/Al2O3 at 700 °C

Acidity-reactivity relationships in catalytic esterification over ammonium sulfate-derived sulfated zirconia

New insight was gained into the acidity-reactivity relationships of sulfated zirconia (SZ) catalysts prepared via (NH4)2SO4 impregnation of Zr(OH)4 for propanoic acid esterification with methanol. A family of systematically related SZs was characterized by bulk and surface analyses including XRD, XPS, TGA-MS, N2 porosimetry, temperature-programmed propylamine decomposition, and FTIR of adsorbed pyridine, as well as methylbutynol (MBOH) as a reactive probe molecule. Increasing surface sulfation induces a transition from amphoteric character for the parent zirconia and low S loadings <1.7 wt %, evidenced by MBOH conversion to 3-hydroxy-3-methyl-2-butanone, methylbutyne and acetone, with higher S loadings resulting in strong Brønsted-Lewis acid pairs upon completion of the sulfate monolayer, which favored MBOH conversion to prenal. Catalytic activity for propanoic acid esterification directly correlated with acid strength determined from propylamine decomposition, coincident with the formation of Brønsted-Lewis acid pairs identified by MBOH reactive titration. Monodispersed bisulfate species are likely responsible for superacidity at intermediate sulfur loadings