Hierarchization of USY zeolites with NH 4OH and its effect on catalysis

The on-purpose application of efficient catalysis is essential to obtain economically feasible and sustainable chemical processes. Zeolites are well-known catalysts in the petrochemical sector, but they also have an eminent potential for the valorization of other feedstocks such as biomass, which can be a valuable alternative for fossil carbon sources. Their conversion to chemicals, in which (part of) the original functionality is retained, is preferred. Despite the many benefits of zeolite catalysis, the microporous zeolite framework though poses a major challenge. Bulky molecules, such as typically encountered in biomass conversions, likely only reach part of the complete zeolite crystal volume due to restricted access to the small micropores or due to slow molecular transport. A solution to this problem consists in the design of hierarchical zeolites with large mesopores in or between the zeolite crystals, often resulting in an improved catalytic performance. Several synthesis methods for hierarchical zeolites have been developed, of which post-synthetic alkaline treatments of commercially available zeolites particularly have a promising potential for large-scale applications. In this respect, an important challenge though is to minimize the loss of zeolitic material obtained due to framework atom leaching. The industrially relevant USY zeolites for instance can be hierarchized by such alkaline treatments. However, their particular sensitivity to alkaline media results in difficulties to controllably form a mesoporous network without drastic amorphization, explaining the current initiatives with costly and unsustainable organic pore-directing agents. In this doctoral research, a new practical and straightforward post-synthetic alkaline treatment is developed for the design of hierarchical USY zeolites, with intrinsic potential for large-scale synthesis. More specific, a weakly alkaline NH4OH treatment is applied at mild conditions without the use of organic additives, while avoiding the need for additional ion-exchanges. The specific features of the treatment and the resulting materials have been extensively characterized. Using this NH4OH method, small mesopores (2-6 nm) are selectively formed in a controlled way, which appear to be well interconnected and largely accessible from the outside of the zeolite crystal. During the pore formation, gradual amorphization takes place, which results in varying ratios between the present micro- and mesopores. Both the treatment time and NH4OH concentration have been identified as important parameters to steer the mesopore network formation, whereby also the pore diameter can be tuned. Concurrently, no significant mass loss by leaching of framework atoms occurs, implicating an exceptionally high material yield, while the total pore volume and crystal morphology are being preserved. These findings led to the elucidation of a new pore formation mechanism through an intracrystalline transformation of the zeolite into a new phase. After extensive investigation by advanced NMR experiments, this phase was identified as a denser amorphous hydrated aluminosilicate phase. The combination of these various synthetic and material properties expressly distinguishes the densifying mild alkaline NH4OH treatment from conventional leaching methods to synthesize hierarchical USY. The catalytic potential of the novel hierarchical USY zeolites is demonstrated in two conversions of lipidic biomass to useful chemicals: the acid-catalyzed isomerization of α-pinene, and the ruthenium metal-catalyzed conjugation of safflower oil. For the α-pinene isomerization, the enhanced activity and selectivity to primary isomers after a short NH4OH treatment is ascribed to shorter microporous diffusion paths as a result of the introduced mesopores. Its associated partial amorphization hence not necessarily implicate a worse catalytic performance, as the loss of active acid sites can be overcompensated by an increased efficiency of the remaining acid sites. Besides, an enlarged indexed hierarchy factor (IHF) appears not to be a necessary prerequisite to obtain valuable hierarchical zeolites. Promisingly, a quasi-perfect reuse of the optimal shortly NH4OH treated catalyst is achieved after thermal regeneration, indicating the intrinsic stability under both the reaction and regeneration conditions. In the conjugation of more bulky triglycerides present in safflower oil, likely a combination of an enlarged amount of accessible active sites and improved mass transport in the mesopores results in the enhanced activity, stability and selectivity to primary isomers. The optimal support for highly dispersed Ru metal sites is prepared by successive alkaline NH4OH and acetate treatments, which result in a desired high mesoporosity together with a large pore diameter. Also from these results, it appears that amorphization should not be considered by definition as negative, as the optimal support is still able to highly disperse Ru metal, despite its XRD amorphous character. Eventually, this alkaline treated Ru/USY establishes the first H2-free and solventless heterogeneous conjugation process for vegetable oil, which is capable of producing exceptionally high yields of a conjugated oil.nrpages: 143status: publishe

Perspective on Overcoming Scale-Up Hurdles for the Reductive Catalytic Fractionation of Lignocellulose Biomass

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Perspective on overcoming scale-up hurdles for the reductive catalytic fractionation of lignocellulose biomass

In the last 5 years, reductive catalytic fractionation of lignocellulose biomass has emerged as a promising biorefinery concept that combines biomass fractionation with the preservation of chemical functionality in its products. Although significant efforts have been made in optimizing this technology on lab scale, the implementation on a larger (pilot) scale is still in its infancy. In our own search for the scale-up potential of this technology, we faced several fundamental and technical research questions that, to this day, remain unanswered. These fundamental questions are related to four main aspects of RCF, the lignocellulose feedstock, the operating pressure, the redox catalyst, and the solvent. In order to inspire future multidisciplinary research in the RCF community, these scale-up challenges are presented and discussed via multiple angles combining chemical process hurdles with more technical aspects, such as reactor design and process consequences

Potential of Sustainable Hierarchical Zeolites in the Valorization of alpha-Pinene

In the valorization of α-pinene, which is an important biomass intermediate derived from turpentine oil, hierarchical (mesoporous) zeolites represent a superior class of catalysts. Hierarchical USY, ZSM-5, and beta zeolites have been prepared, characterized, and catalytically evaluated, with the aim of combining the highest catalytic performance with the most sustainable synthetic protocol. These zeolites are prepared by alkaline treatment in aqueous solutions of NH4 OH, NaOH, diethylamine, and NaOH complemented with tetrapropylammonium bromide. The hierarchical USY zeolite is the most attractive catalyst of the tested series, and is able to combine an overall organic-free synthesis with an up to sixfold activity enhancement and comparable selectivity over the conventional USY zeolite. This superior performance relates to a threefold greater activity than that of the commercial standard, namely, H2 SO4 /TiO2 . Correlation of the obtained benefits to the amount of solid lost during the postsynthetic modifications highlights that the highest activity gains are obtained with minor leaching. Furthermore, a highly zeolitic character, as determined by bulk XRD, is beneficial, but not crucial, in the conversion of α-pinene. The alkaline treatments not only result in a higher overall activity, but also a more functional external surface area, attaining up to four times the pinene conversions per square nanometer. The efficiency of the hierarchical USY zeolite is concomitantly demonstrated in the conversion of limonene and turpentine oil, which emphasizes its industrial potential.status: publishe

Zeolites as sustainable catalysts for the selective synthesis of renewable bisphenols from lignin-derived monomers

Alternative biobased bisphenols from lignocellulosic biomass are not only favorable to reduce the environmental impact of current petroleum-derived plastics, but they can be simultaneously beneficial for health issues related to bisphenol A (BPA). Additionally, conventional BPA synthesis entails a large excess of unrecoverable homogeneous acid catalyst (e.g., HCl) or unrecyclable thermolabile sulfonated resins. In this report, zeolites are proposed as recoverable and thermally stable solid acids for the Brønsted-acid-catalyzed condensation between 4-methylguaiacol and formaldehyde to selectively produce renewable bisphenols. It is found that the Brønsted-acid-site density plays a pivotal role for catalyst performance. In particular, the cheap and environmentally friendly FAU 40 exhibits outstanding activity (turnover frequency of 496 h-1 ) and selectivity (>95 %), outperforming even the best benchmark catalyst. Additionally, the zeolite can be easily recycled without activity loss after regeneration by coke burn-off. The catalytic zeolite system also seems very promising for other lignin-derived alkylphenols, alkylguaiacols, and alkylsyringols.status: publishe

Reductive catalytic fractionation of pine wood: elucidating and quantifying the molecular structures in the lignin oil

In-depth structural analysis of biorefined lignin is imperative to understand its physicochemical properties, essential for its efficient valorization to renewable materials and chemicals. Up to now, research on Reductive Catalytic Fractionation (RCF) of lignocellulose biomass, an emerging biorefinery technology, has strongly focused on the formation, separation and quantitative analysis of the abundant lignin-derived phenolic monomers. However, detailed structural information on the linkages in RCF lignin oligomers, constituting up to 50 wt% of RCF lignin, and their quantification, is currently lacking. This study discloses new detailed insights into the pine wood RCF lignin oil's molecular structure through the combination of fractionation and systematic analysis, resulting in the first assignment of the major RCF-derived structural units in the 1H–13C HSQC NMR spectrum of the RCF oligomers. Specifically, β-5 γ-OH, β-5 ethyl, β-1 γ-OH, β-1 ethyl, β-β 2x γ-OH, β-β THF, and 5-5 inter-unit linkages were assigned unambiguously, resulting in the quantification of over 80% of the lignin inter-unit linkages and end-units. Detailed inspection of the native lignin inter-unit linkages and their conversion reveals the occurring hydrogenolysis chemistry and the unambiguous proof of absence of lignin fragment condensation during proper RCF processing. Overall, the study offers an advanced analytical toolbox for future RCF lignin conversion and lignin structural analysis research, and valuable insights for lignin oil valorization purposes.status: publishe

Alkane production from biomass: chemo-, bio- and integrated catalytic approaches

Linear, branched and cyclic alkanes are important intermediates and end products of the chemical industry and are nowadays mainly obtained from fossil resources. In search for alternatives, biomass feedstocks are often presented as a renewable carbon source for the production of fuels, chemicals and materials. However, providing a complete market for all these applications seems unrealistic due to both financial and logistic issues. Despite the very large scale of current alkane-based fuel applications, biomass definitely has the potential to offer a partial solution to the fuel business. For the smaller market of chemicals and materials, a transition to biomass as main carbon source is more realistic and even probably unavoidable in the long term. The appropriate use and further development of integrated chemo- and biotechnological (catalytic) process strategies will be crucial to successfully accomplish this petro-to-bio feedstock transition. Furthermore, a selection of the most promising technologies from the available chemo- and biocatalytic tool box is presented. New opportunities will certainly arise when multidisciplinary approaches are further explored in the future. In an attempt to select the most appropriate biomass sources for each specific alkane-based application, a diagram inspired by van Krevelen is applied, taking into account both the C-number and the relative functionality of the product molecules.publisher: Elsevier articletitle: Alkane production from biomass: chemo-, bio- and integrated catalytic approaches journaltitle: Current Opinion in Chemical Biology articlelink: http://dx.doi.org/10.1016/j.cbpa.2015.08.010 content_type: article copyright: Copyright © 2015 Elsevier Ltd. All rights reserved.status: publishe

Synthesis, characterisation, and catalytic evaluation of hierarchical faujasite zeolites: milestones, challenges, and future directions

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Potential and challenges of zeolite chemistry in the catalytic conversion of biomass

Increasing demand for sustainable chemicals and fuels has pushed academia and industry to search for alternative feedstocks replacing crude oil in traditional refineries. As a result, an immense academic attention has focused on the valorisation of biomass (components) and derived intermediates to generate valuable platform chemicals and fuels. Zeolite catalysis plays a distinct role in many of these biomass conversion routes. This contribution emphasizes the progress and potential in zeolite catalysed biomass conversions and relates these to concepts established in existing petrochemical processes. The application of zeolites, equipped with a variety of active sites, in Brønsted acid, Lewis acid, or multifunctional catalysed reactions is discussed and generalised to provide a comprehensive overview. In addition, the feedstock shift from crude oil to biomass involves new challenges in developing fields, like mesoporosity and pore interconnectivity of zeolites and stability of zeolites in liquid phase. Finally, the future challenges and perspectives of zeolites in the processing of biomass conversion are discussed.crosscheck: This document is CrossCheck deposited copyright_licence: The Royal Society of Chemistry has an exclusive publication licence for this journal copyright_licence: This article is freely available. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence (CC BY 3.0) history: Received 16 November 2015; Advance Article published 21 December 2015; Version of Record published 1 February 2016status: publishe