Zichun Wang

Sustainable processing of waste plastics to produce high yield hydrogen-rich synthesis gas and high quality carbon nanotubes

Gasification provides a promising alternative to thermally recycle waste plastics to produce a synthesis gas. The catalytic gasification process described here can process waste plastics to produce either a high yield, hydrogen-rich synthesis gas or high value, multi-walled carbon nano-tubes; a process that can be altered to produce the desired targeted end-product.

Renewable hydrogen and carbon nanotubes from biodiesel waste glycerol

In this report, we introduce a novel and commercially viable method to recover renewable hydrogen and carbon nanotubes from waste glycerol produced in the biodiesel process. Gas-phase catalytic reforming converts glycerol to clean hydrogen fuel and by replacing the problematical coke formed on the catalyst with high value carbon nanotubes, added value can be realised. Additional benefits of around 2.8 kg CNTs from the reforming of 1 tonne of glycerol and the production of 500 Nm3 H2 could have a considerable impact on the economics of glycerol utilization. Thereby, the contribution of this research will be a significant step forward in solving a current major technical and economic challenge faced by the biofuels industry.

One‐Step Room‐Temperature Synthesis of [Al]MCM‐41 Materials for the Catalytic Conversion of Phenylglyoxal to Ethylmandelate

Mesoporous [Al]MCM‐41 materials with nSi/nAl ratios of 15 to 50 suitable for the direct catalytic conversion of phenylglyoxal to ethylmandelate have been successfully synthesized at room temperature within 1 h. The surface areas and pore sizes of the obtained [Al]MCM‐41 materials are in the ranges of 1005–1246 m2 g−1 and 3.44–3.99 nm, respectively, for the different nSi/nAl ratios. For all [Al]MCM‐41 catalysts, most of the Al species were tetrahedrally coordinated with Si in the next coordination sphere of atoms. 1H and 13C magic‐angle spinning NMR spectroscopic investigations indicated that the acid strength of the SiOH groups on these [Al]MCM‐41 catalysts and the density of these surface sites are enhanced with increasing Al content in the synthesis gels. These surface sites with enhanced acid strength were found to be catalytically active sites for phenylglyoxal conversion. The [Al]MCM‐41 material with nSi/nAl=15 showed the highest phenylglyoxal conversion (93.4 %) and selectivity to ethylmandelate (96.9 %), whereas the [Al]MCM‐41 material with nSi/nAl=50 reached the highest turnover frequency (TOF=99.3 h−1). This is a much better catalytic performance than that of a dealuminated zeolite Y (TOF=1.7 h−1) used as a reference catalyst, which is explained by lower reactant transport limitations in mesoporous materials than that in the microporous zeolite.

Brønsted acid sites based on penta-coordinated aluminum species

Zeolites and amorphous silica-alumina (ASA), which both provide Brønsted acid sites (BASs), are the most extensively used solid acid catalysts in the chemical industry. It is widely believed that BASs consist only of tetra-coordinated aluminum sites (AlIV) with bridging OH groups in zeolites or nearby silanols on ASA surfaces. Here we report the direct observation in ASA of a new type of BAS based on penta-coordinated aluminum species (AlV) by 27Al-{1H} dipolar-mediated correlation two-dimensional NMR experiments at high magnetic field under magic-angle spinning. Both BAS-AlIV and -AlV show a similar acidity to protonate probe molecular ammonia. The quantitative evaluation of 1H and 27Al sites demonstrates that BAS-AlV co-exists with BAS-AlIV rather than replaces it, which opens new avenues for strongly enhancing the acidity of these popular solid acids.

Influence of support acidity on the performance of size-confined Pt nanoparticles in the chemoselective hydrogenation of acetophenone

Size-confined Pt nanoparticles of about 1.5 nm have been introduced into [Al]MCM-41 supports with similar acid strength but various population densities of acid sites by means of wet impregnation. The Pt nanoparticles covered preferentially the surface Bronsted acid sites (BAS) of the supports or were located near acid sites rather than on the bigger free space between acid sites even at a very low acid density (6 BAS per 1000 nm2). The free BAS around the Pt particles did not interact with Pt atoms and the electronic properties of the Pt nanoparticles as probed by DRIFTS combined with CO adsorption were similar for Pt/[Al]MCM-41 with and without nearby free BAS. Ionic effects were generated by the Pt-covered acid sites, whereas the population of BAS did not contribute significantly to the ionic effects induced on the Pt nanoparticles. The coverage of BAS of similar strength by platinum nanoparticles led to similar chemoselectivity and product distribution in acetophenone (Aph) hydrogenation, though the density of BAS on the supports increased by more than 11 times. However, increasing the number of BAS on the supports significantly changed the hydrogenation rate. TOFs continuously increased from 125 h−1 up to 534 h−1, when the population of free BAS increased from 18.2 BAS per 1000 nm2 to 39.9 BAS per 1000 nm2. When the free BAS density was further increased to 70.4 BAS per 1000 nm2, the TOF then dropped to 176 h−1. The hydrogenation pathway is similar for both monofunctional (Pt covering all BAS) and bifunctional catalysts (Pt with free BAS), and the reaction was initiated on the Pt surface. This finding indicates that proper tuning of the population density of acid sites on the support can significantly improve the catalytic performance of the supported metal catalysts while keeping similar product selectivities.

Catalytic Performance of Brønsted and Lewis Acid Sites in Phenylglyoxal Conversion on Flame‐Derived Silica–Zirconia

Flame‐derived silica–zirconia has been used for the promising one‐step catalytic conversion of phenylglyoxal (PG) to mandelates, important intermediates in pharmacy and fine chemistry. In the literature it was proposed that Lewis acid sites (LAS) on solid acids promote mandelate production, whereas Brønsted acid sites (BAS) only generate acetal byproducts during PG conversion. Herein it is shown that ZrO2, which contains only LAS and no BAS, exhibits a very low activity for PG conversion to afford a yield of mandelate of only 8 % after 6 h. The activity of this ZrO2 catalyst was confirmed by the condensation of acetone, in which its LAS could activate the carbonyl groups immediately for the fast condensation of all acetone molecules. The introduction of BAS by the admixing of silica precursor to the feed in the flame‐spray pyrolysis enhanced the yield of mandelate up to 52–67 %. This indicates that BAS are mainly responsible for the PG conversion to mandelate on silica–zirconia catalysts and that their LAS are nearly inactive for this reaction.

Brønsted-Lewis Acids for Efficient Conversion of Renewables

Acid-catalyzed conversion of renewables, e.g. biomass, is a major route for the sustainable production of fuels and chemicals through a “closed carbon balance” benign to the environment. Development of solid acids offer cost-effective and clean chemical processes are considerable preferred. The performance of solid acids depends on the type (Bronsted acid vs Lewis acid), density, strength, accessibility and local environment of the active sites. Most efforts focus on four key factors for efficient catalysis, for instance, preparing catalyst with higher density and strength with one type of acid sites that are constrained by further acidity enhancement. An alternative way is to combine Bronsted and Lewis acid sites in the local structure, introducing Bronsted-Lewis cooperativity that can have synergic effects. The Bronsted-Lewis synergy effects can enhance the acid strength of Bronsted acid sites, while Bronsted-Lewis cooperativity influence the molecular adsorption and activation, resulting in Bronsted-Lewis bifunctional acid-catalysis. Bronsted-Lewis bifunctional acid-catalysts that works cooperatively during reaction rises a new research direction. The chapter focuses on the application of Bronsted-Lewis bifunctional catalysts for efficient biomass conversion emerging sustainable biorefining processes.