Porosity and Pore Size Distribution of Native and Delignified Beech Wood Determined by Mercury Intrusion Porosimetry.

The complex hierarchical structures of biological materials in combination with outstanding property profiles are great sources of inspiration for material scientists. Based on these characteristic features, the structure of wood has been increasingly exploited to fabricate novel hierarchical and functional materials. With delignification treatments, the density and chemistry of wood can be altered, resulting in hierarchical cellulose scaffolds with enhanced porosity for the fabrication of novel hybrid materials. In the present study, focusing on acidic delignification of beech wood and its influence on porosity, we report on a structural characterization and qualitative assessment of the cellulose scaffolds using mercury intrusion porosimetry (MIP). To account for the effect of water removal from the hygroscopic structure, different drying methods-e.g., standard oven and freeze-drying-were applied. While native beech wood is characterized by the presence of macro, meso and micro pores, delignification altered the porosity, increasing the importance of the macropores in the pore size distribution. Furthermore, we showed that the final porosity obtained in the material is strongly dependent on the applied drying process. Samples delignified under harsh conditions at high temperature (mass loss of ~35%) show a 13% higher porosity after freeze-drying compared to oven-dried samples. The obtained results contribute to a better understanding of the impact of the delignification and drying processes on the porosity of cellulose scaffolds, which is of high relevance for subsequent modification and functionalization treatments.

Facilitated delignification in CAD deficient transgenic poplar studied by confocal Raman spectroscopy imaging.

Lignocellulosic biomass represents the only renewable carbon resource which is available in sufficient amounts to be considered as an alternative for our fossil-based carbon economy. However, an efficient biochemical conversion of lignocellulosic feedstocks is hindered by the natural recalcitrance of the biomass as a result of a dense network of cellulose, hemicelluloses, and lignin. These polymeric interconnections make a pretreatment of the biomass necessary in order to enhance the susceptibility of the polysaccharides. Here, we report on a detailed analysis of the favourable influence of genetic engineering for two common delignification protocols for lignocellulosic biomass, namely acidic bleaching and soda pulping, on the example of CAD deficient poplar. The altered lignin structure of the transgenic poplar results in a significantly accelerated and more complete lignin removal at lower temperatures and shorter reaction times compared to wildtype poplar. To monitor the induced chemical and structural alterations at the tissue level, confocal Raman spectroscopy imaging, FT-IR spectroscopy, and X-ray diffraction were used.

Tracking of Short Distance Transport Pathways in Biological Tissues by Ultra-Small Nanoparticles.

In this work, ultra-small europium-doped HfO2 nanoparticles were infiltrated into native wood and used as trackers for studying penetrability and diffusion pathways in the hierarchical wood structure. The high electron density, laser induced luminescence, and crystallinity of these particles allowed for a complementary detection of the particles in the cellular tissue. Confocal Raman microscopy and high-resolution synchrotron scanning wide-angle X-ray scattering (WAXS) measurements were used to detect the infiltrated particles in the native wood cell walls. This approach allows for simultaneously obtaining chemical information of the probed biological tissue and the spatial distribution of the integrated particles. The in-depth information about particle distribution in the complex wood structure can be used for revealing transport pathways in plant tissues, but also for gaining better understanding of modification treatments of plant scaffolds aiming at novel functionalized materials.

Characterization of Wood Derived Hierarchical Cellulose Scaffolds for Multifunctional Applications.

Functional materials of high porosity and hierarchical structure, based on renewable building blocks, are highly demanded for material applications. In this regard, substantial progress has been made by functionalizing micro- and nano-sized cellulose followed by its reassembly via bottom-up approaches. However, bottom-up assembly processes are still limited in terms of upscaling and the utilization of these building blocks presupposes the disassembly of the plant feedstock inherit hierarchical cellulose scaffold. To maintain the three-dimensional structure, delignification processes from pulp and paper production were recently adapted for the treatment of bulk wood. Yet, a detailed chemical analysis and the determination of macroscopic swelling/shrinkage parameters for the scaffolds, necessary for a systematic design of cellulose scaffold based materials, are still missing. Here, acidic bleaching and soda pulping were used for producing cellulose scaffolds, for functional materials under retention of their inherent hierarchical structure. Spatially resolved chemical investigations on thin sections by Raman microscopy provided detailed information on the induced alterations at the cell wall level, revealing significant differences in dependence of the chemistry of the pre-treatment. An adaption to bulk wood samples proved the applicability of these treatments at larger scales and volumetric alterations at different atmospheric conditions indicated the effect of the altered porosity of the scaffolds on their hygroscopic behaviour.

Delignified and Densified Cellulose Bulk Materials with Excellent Tensile Properties for Sustainable Engineering.

Today's materials research aims at excellent mechanical performance in combination with advanced functionality. In this regard, great progress has been made in tailoring the materials by assembly processes in bottom-up approaches. In the field of wood-derived materials, nanocellulose research has gained increasing attention, and materials with advanced properties were developed. However, there are still unresolved issues concerning upscaling for large-scale applications. Alternatively, the sophisticated hierarchical scaffold of wood can be utilized in a top-down approach to upscale functionalization, and one can profit at the same time from its renewable nature, CO2 storing capacity, light weight, and good mechanical performance. Nevertheless, for bulk wood materials, a wider multipurpose industrial use is so far impeded by concerns regarding durability, natural heterogeneity as well as limitations in terms of functionalization, processing, and shaping. Here, we present a novel cellulose bulk material concept based on delignification and densification of wood resulting in a high-performance material. A delignification process using hydrogen peroxide and acetic acid was optimized to delignify the entire bulk wooden blocks and to retain the highly beneficial structural directionality of wood. In a subsequent step, these cellulosic blocks were densified in a process combining compression and lateral shear to gain a very compact cellulosic material with entangled fibers while retaining unidirectional fiber orientation. The cellulose bulk materials obtained by different densification protocols were structurally, chemically, and mechanically characterized revealing superior tensile properties compared to native wood. Furthermore, after delignification, the cellulose bulk material can be easily formed into different shapes, and the delignification facilitates functionalization of the bioscaffold.