Secondary Micropores Research Articles

Titanium dioxide bioceramics prepared by 3D printing method and its structure effect on stem cell behavior

3D printing was widely used for the modification of bioceramic structures. Here, Bioactive titanium dioxide (TiO2) ceramics with macropore structures were prepared by controlling the pore size through 3D printing, and further obtained secondary micropores by altering the sintering temperature. The structures and in vitro bioactivity of materials were characterized, and the properties of the ceramics for osteogenic differentiation of rabbits bone marrow mesenchymal stem cells (rBMSCs) were investigated. The results showed that titanium dioxide ceramics could induce apatite formation in SBF, which indicated it is bioactive. All materials exhibited porous structures, which could promote the proliferation and osteogenic differentiation of rBMSCs. Importantly, the structure with 200 μm pore size significantly promoted the proliferation and adhesion of cells. Furthermore, more secondary micropores formed on the surface of materials with the decreased sintering temperature, which promoted cell proliferation. The pore size of 400 μm significantly upregulated the expression of osteogenesis-related genes, including Col I, ALP, Runx2, and OPN. It had the strongest osteogenic ability when sintered at 1100 °C compared to other groups. This implies that the 3D-printed TiO2 ceramics have good osteogenic properties, and the hierarchical structure consisting of macroscopic pores and secondary micropores can control it well.

Lithology and Lithofacies Characteristics of Deeply Buried Clastic Reservoirs in Alternating Wet-arid Climate and its Hydrocarbon Significance

There have been series of HPHT (high temperature and high pressure) reservoirs with good quality and high productivity discovered in the Triassic Skagerrak Formation in the Central Graben of the UK North Sea. The bottlenecks for further E&P activity are the complicated depositional facies and high reservoir heterogeneity. The main reservoir space is a mixed porosity network composed of primary pores, micro-pores, secondary pores, secondary micro-pores and micro-fractures. Reservoirs of the Skagerrak Formation have greatly varied lateral connectivity as well as poor vertical connectivity. In the case of sufficiently high pressure differentials, hydrocarbon can be fed from high-potential area to low-potential area. There is no simple linear relationship between depth and reservoir quality. The primary depositional texture (grain size/sorting/clay content, etc.) linked to facies has a fundamental control on reservoir quality. Clean and well sorted cross laminated sandstone has the best reservoir quality, while the reservoir quality of planar laminated sandstone decreased due to increased degree of compaction caused by mica drapes. The reservoir quality of the fine-grained ripple cross laminated sandstone is even poorer than that of the planar laminated sandstone, due to abundant mica and clay drapes. Mottled bioturbated sandstone is fine grained with high clay content, which is more muddy because of containing burrows and insect larva. Pedoturbated sandstone has the poorest reservoir quality due to severe dolomite cementation. The dominant detrital material of the Skagerrak Formation is feldspar that provides abundant secondary porosity by alteration and dissolution. Chlorite coats are commonly developed and stained with residual oil or bitumen, which clearly inhibits quartz cementation and feldspar overgrowth. The presence of these clay coats contributes to the preservation of anomalously high porosity at depth in the cleaner and coarser grained fluvial channel facies. The Skagerrak formation facies is a dryland fluvial-lacustrine depositional system which is controlled by fluctuations of depositional base level due to salt mobilisation. The Skagerrak formation consists of several cycles that are vertically separated by lacustrine mudstone. The reservoir permeability system is composed of active fluvial channel sandstones, terminal splay sandstones, abandoned fluvial channel sandstones, minor channel sandstones connecting fluvial channel and terminal splay, as well as low-permeability faults, while the active fluvial channel sandstone is the “sweet spot”. The analysis of controls on deeply buried reservoir quality is significant to precisely predict “sweet spot” in the Skagerrak Formation.

Deformation failure characteristics of weathered phyllite tunnel and variable-stiffness support countermeasures: A case study

Extreme deformation and collapse induced by tunnelling commonly occur in weathered phyllite strata even with conservative design. Taking a highway tunnel in weathered phyllite strata as an example, the deformation characteristics, failure factors, mechanism and countermeasures were investigated. Firstly, a systematic analysis of in-situ monitoring results was conducted, indicating that the vault subsidence and peripheral convergence varied in 232–658 mm and 262–568 mm with the maximum deformation rate of 49.2 mm/day and the support structures were damaged in different modes such as steel arch distortion, shotcrete cracking and even collapse. Then, the laboratory tests were carried out to explore the failure factors and mechanism. Due to the weathering effect and groundwater erosion, the density and strength of phyllite decreased, and the water content, water absorption and porosity increased. The microstructure was also more fragmented with more secondary micropores and microcracks. The failure causes were mainly attributed to three aspects: weathered degree, groundwater and unreasonable support. Finally, the innovative concept of variable-stiffness support and corresponding countermeasures were proposed, including using the double-deck advance pipes, long-short compound steel pipe, double primary support, high-strength lining, temporary support and increasing reserved deformation value. The feedback of engineering application showed that the deformation of surrounding rock was effectively controlled, verifying the validity of the countermeasures. The results can provide a valuable reference for the prevention and control of the extreme deformation and collapse in weathered phyllite tunnels.

TS-1 with Abundant Micropore Channel-Supported Au Catalysts toward Improved Performance in Gas-Phase Epoxidation of Propylene

The gas-phase propylene epoxidation reaction catalyzed by Au/TS-1 has attracted great attention due to its clean and efficient merits. However, high conversion of propylene (>10%) and high selectivity of propylene oxide (PO) (>90%) often cannot be obtained at the same time. Although a variety of TS-1 supports with different microstructures have been developed to improve the catalytic performance, few have met the above criteria. In this work, we have reported a silane-assisted strategy to prepare TS-1-CTES zeolites containing abundant micropore channels, which showed both high propylene conversion (13.3%) and high PO selectivity (92.7%). The silane, (3-chloropropyl) triethoxysilane (CTES), not only participates in constructing the MFI framework but also serves as a spatial filling agent to build secondary micropore channels. Ar physisorption characterization exhibited the increased micropore volume and surface area of the as-synthesized TS-1-CTES, which benefit molecule diffusion. XPS results showed an enriched Ti concentration on the surface of TS-1-CTES and a decreased electron density of Ti atoms. A few more Au nanoparticles were proved to be distributed inside the TS-1-CTES crystal compared with those inside TS-1. The optimized distribution of Au and Ti active sites in Au/TS-1-CTES enhanced their synergistic catalytic effects. The PO formation rate remained relatively stable for 24 h, superior to traditional Au/TS-1. The construction of a micropore-rich TS-1 structure opens a way for the reasonable design and manufacture of the Au/TS-1 catalyst, for the gas-phase epoxidation of propylene.

Microporosity evolution and destruction in the Jurassic Arab D reservoir, Qatar

Core samples were collected from three wells, one onshore and two offshore, from Qatar’s Upper Jurassic Arab D reservoir. The samples were subjected to multiproxy petrographic and chemical analyses to identify their micro- and nanoporosity types and understand their evolution and destruction. Based on the petrographic and petrophysical properties of studied rocks, the Arab D succession was divided into seven rock types. Primary microporosity includes intergranular and interplanar, while secondary types include vuggy, intercrystalline, moldic, dissolution, pyrite displacement, microfracture, and microbial boring. Primary micropores were found mainly between the micrite grains in the lime mudstone facies, between the grains or the plates of clay minerals. Secondary micropores result from open and closed diagenetic systems. The open diagenetic system led to the development of dissolution and moldic micropores, while the closed system created pyrite displacement and boring porosity. Mechanical stress due to crystal growth or displacement generated microfractures. Micropores were destroyed either by cementation, clay minerals growth, dolomitization, or microbial pustular overgrowth. Microporosity was important in quantity and varied in nature in the mud-supported rocks. They are similar to macropores in grain-supported sediments but of less importance. The complex lithology of the studied rocks has significantly influenced the development and destruction of the porosity system of the Arab Formation.

Evaluation of the mechanisms of adsorption of microcystins and nodularin-R onto rice husk-based biochar

• Non-electrostatic and electrostatic interactions govern cyanotoxin adsorption onto RH. • Charge-neutral MC-RR displays high adsorption onto RH at different concentrations. • MC-LR and NOD-R at low concentrations experience attractive interactions with RH surface. • MCs with net negative charge and NOD-R show less adsorption onto RH than MC-RR. • Lateral electrostatic repulsions are more pronounced for MC-LA than for MC-LW and MC-LF. Adsorption mechanisms of individual and collective sorption of microcystins (MCs) and nodularin-R (NOD-R) onto heat-treated rice husk (RH) at neutral pH were evaluated. The functional groups on cyanotoxin standards and RH surface as well as their charge indicate that non-electrostatic and electrostatic interactions are present. Surface area and porosity of RH were determined indicating that MCs and NOD-R can be adsorbed in secondary micropores and mesopores of RH. Positively charged guanidinium ion in arginine-containing MCs and NOD-R created attractive electrostatic forces with the negatively charged RH surface (pH PZC ∼3.3). MCs and NOD-R experience electrostatic repulsion with the surface due to the presence of negatively charged carboxylate groups. MC-RR, even at significantly high toxin concentrations, could be removed by RH as it had a strong attraction to the surface due to two arginines and its neutral charge. Other congeners adsorbed on RH experienced lateral intermolecular repulsion and repulsion with molecules in solution. At low sorbate concentrations, MC-LR and NOD-R had electrostatic attraction to the RH surface and MC-LA, MC-LW, and MC-LF experienced electrostatic repulsion with the surface at all concentrations. Non-electrostatic forces govern the adsorption process of MC-LA, MC-LW, and MC-LF, and electrostatic forces are significant for the adsorption of MC-LR, MC-RR and NOD-R at low concentrations (1-100 ng/mL) as well as the adsorption of MC-RR at high concentrations (1000 ng/mL). The presence of 0.3 M NaCl increases the adsorption of MC-LA and MC-LF proving that those molecules undergo repulsive interactions with RH. The adsorption of MCs and NOD-R onto RH is pH dependent, and it appears to be improved at pH values < pH PZC . Overall, the adsorption of MCs and NOD-R onto RH biochar depends on congener concentration, structure, and charge, and can be modified through optimization of structural properties of RH sorbent and sorption conditions, such as ionic strength and pH.

Cryogenic 3D Printing of w/o Pickering Emulsions Containing Bifunctional Drugs for Producing Hierarchically Porous Bone Tissue Engineering Scaffolds with Antibacterial Capability

How to fabricate bone tissue engineering scaffolds with excellent antibacterial and bone regeneration ability has attracted increasing attention. Herein, we produced a hierarchical porous β-tricalcium phosphate (β-TCP)/poly(lactic-co-glycolic acid)-polycaprolactone composite bone tissue engineering scaffold containing tetracycline hydrochloride (TCH) through a micro-extrusion-based cryogenic 3D printing of Pickering emulsion inks, in which the hydrophobic silica (h-SiO2) nanoparticles were used as emulsifiers to stabilize composite Pickering emulsion inks. Hierarchically porous scaffolds with desirable antibacterial properties and bone-forming ability were obtained. Grid scaffolds with a macroscopic pore size of 250.03 ± 75.88 μm and a large number of secondary micropores with a diameter of 24.70 ± 15.56 μm can be fabricated through cryogenic 3D printing, followed by freeze-drying treatment, whereas the grid structure of scaffolds printed or dried at room temperature was discontinuous, and fewer micropores could be observed on the strut surface. Moreover, the impartment of β-TCP in scaffolds changed the shape and density of the micropores but endowed the scaffold with better osteoconductivity. Scaffolds loaded with TCH had excellent antibacterial properties and could effectively promote the adhesion, expansion, proliferation, and osteogenic differentiation of rat bone marrow-derived mesenchymal stem cells afterward. The scaffolds loaded with TCH could realize the strategy to “kill bacteria first, then induce osteogenesis”. Such hierarchically porous scaffolds with abundant micropores, excellent antibacterial property, and improved bone-forming ability display great prospects in treating bone defects with infection.

Pore structure evolution during lean-oxygen combustion of pyrolyzed residual from low-rank coal and its effect on internal oxygen diffusion mechanism

The lean-oxygen combustion of incompletely pyrolyzed char dominates at the combustion front of coalfield fires. Using non-local-density functional theory (NLDFT) and Barrette-Joynere-Halenda (BJH) analysis based on N2 and CO2 composited adsorption isotherms, the pore (0.4–300 nm) structure characteristics of pyrolyzed char during lean-oxygen combustion were obtained. The results indicate that the main pore morphology changes from irregular ink-bottle shape of raw coal to cylindrical shape of pyrolyzed char, then to slit-shaped and wedge-shaped pores under the effect of oxidative expansion, and finally coalesce into amorphous macropores during combustion. For the weak-caking coal, softened colloids diffuse and block the mesopores, leading to the reverse development of mesopores into smaller-sized pores. The pore development changes from the generation of secondary micropores in pyrolysis stage to the progressive expansion mode of mesopores in initial combustion stage, then to the free coalescence mode of macropores in late combustion stage. The dominant internal oxygen diffusion mechanism transforms from Knudsen diffusion to transition diffusion at 650 °C, and the carbon-oxygen reaction interfaces shift from the inner surface of deep micropores to the shallow mesopores and macropores. Finally, 650 °C is determined as the early-warning temperature for the accelerating spread of coalfield fires.

Petrographic characteristics of the Ordovician and Silurian deposits in the Baltic Basin (N Poland) and their relevance for unconventional hydrocarbon accumulations

The Ordovician and Silurian deposits in the Baltic Basin are represented by siltstones, mudstones, limestones, marls, sandstones and, most commonly, claystones with a markedly shaley structure. Claystones that are particularly rich in organic matter are considered as potential reservoirs for shale gas accumulations. Samples from 11 boreholes, including core samples of the Piaśnica, Słuchowo, Kopalino, Sasino, Prabuty, Jantar, Pelplin and locally Pasłęk, Kociewie and Puck formations, were analysed by optical microscopy, cathodoluminescence (CL), scanning electron microscopy (SEM) and energy-dispersive spectrometry and X-ray diffraction analyses (XRD). The Piaśnica, Sasino, Jantar, Pasłęk and Pelplin formations are characterized locally by a dominance of black bituminous claystones with a high content of organic matter, and a generally consistent clay mineral composition. Individual samples only show variations in silt fraction and carbonate content. Very important is the negligible proportion of swelling minerals in the clay fraction; the percentage of smectite in the mixed-layered illite/smectite minerals does not exceed 15%. It is important for the rocks to have adequate [SiO2 &gt;10%; Quartz (Q) + Feldspars (Fs) + Carbonates &gt;40%] brittleness. The content of quartz, feldspars and carbonates (here &gt;40%) suggests that the claystones are susceptible to hydraulic fracturing. The claystones show micropores between detrital grains, between the flakes of clay minerals, within pyrite framboids, and organic matter and secondary micropores within grains.

Evaluation of pore structures in volcanic reservoirs: a case study of the Lower Cretaceous Yingcheng Formation in the Southern Songliao Basin, NE China

Different pore types and variable pore structures can determine reservoir potentials of volcanic rocks. Therefore, it is of great importance to understand the pore structures of volcanic reservoirs. With the Lower Cretaceous Yingcheng volcanic rocks, this study attempts to investigate and evaluate the micro pore structures of a volcanic reservoir through: (1) measuring porosity and permeability of core samples; (2) identifying pore types within thin sections; (3) characterizing pore structures with pressure-controlled mercury injection. The Lower Cretaceous Yingcheng volcanic rocks exhibits low porosities (on average 7.39%) and very low permeability (on average 0.283 × 10−6 m2). Three major pore types were identified from the Lower Cretaceous Yingcheng volcanic reservoir, including primary pores, secondary pores and fractures. Additionally, the pore structures of the Lower Cretaceous Yingcheng volcanic reservoir can be classified into three types. Type I pore structure primarily occurs in the volcanic breccia, crystal tuff and tuffites. The corresponding pore system is dominated by primary intergranular pores, intragranular dissolved pores and devitrified micropores with porosity and permeability higher than 5.4% and 0.05 × 10−6 µm2, respectively. Corresponding mercury intrusion curves are characterized by low displacement pressure, high maximum mercury saturation and mercury withdrawal efficiency. Type II pore structure is commonly observed in the tuff and tuffites, while primary intergranular pores and secondary micropores dominate pore system with porosity of 2.5–5.4% and permeability of 0.012–0.05 × 10−6 m2. The mercury intrusion curves are similar to the type I. Type III pore structure is also pervasively developed in the crystal tuff and tuffites. Intragranular dissolved micropores are most important ones with porosity and permeability less than 2.5% and 0.01 × 10−6 m2, respectively. Different from type I and II pore structure, the mercury intrusion curves of the type III have high-displacement pressure, low maximum mercury saturation and mercury withdrawal efficiency, indicating poor reservoir quality.

Whole-aperture characteristics and controlling factors of pore structure in the Chang 7th continental shale of the Upper Triassic Yanchang Formation in the southeastern Ordos Basin, China

Quantitative assessment is still lacking on the pore-sizes distribution of micropores and mesopores, and the major controlling factors remain enigmatic for the continental Chang 7th shale reservoirs in southeastern Ordos Basin. In this study, scanning electron microscopy, organic geochemical analysis, low-temperature gas adsorption, high-pressure mercury injection, and X-ray diffraction analysis were conducted on 29 core samples from representative eight wells in the Xiasiwan-Yanchang area, yielding a particular investigation on characteristics and controlling factors of the structure in the Chang 7th shale. The Chang 7th shale is mainly characterized by organic and secondary micropore and mesopore, respectively, ranging the peaks of pore size from 0.35 to 0.65 nm, 0.75 to 1.00 nm, 1.10 to 1.35 nm, and from 4 to 13 nm. Whole-aperture characterization of the pore structure shows that meso- and macropores demonstrate a major contribution to total pore volume (PV), and the surface area is mainly provided by micro- and mesopores with pore radii less than 20 nm. A weak positive variation was identified between the PV of micromesopores and total organic carbon content, implying that micropores are currently in the process of developing. The micromesopore specific surface area and volume, respectively, yield a significant positive and weak variation with the quartz and clay mineral content, suggesting that siliceous minerals are favorable for micromesopore development. The infusion of siliceous biogenic debris and silica-replaced carbonate biogenic debris from the northeastern and southwestern source areas, respectively, is likely responsible for development of biogenic siliceous minerals in the Chang 7th shale. Furthermore, the hollow cavities in siliceous organisms have considerably contributed to the preservation of primary pores owing to their characteristic skeleton-supported framework.

Pore evolution in hydrocarbon-generation simulation of organic matter-rich muddy shale

For exploration and potential evaluation of deep shale reservoirs with high maturity, hydrocarbon generation and pore evolution of muddy shale in deep high evolution stage were investigated by the high-temperature high-pressure simulation experiment. Results indicated that under high pressure condition, nano-scale micropores in organic matter-rich muddy shale constantly increased as rise of temperature and pressure, leading to increase of shale porosity. However, in the high mature-overmature stage, shale porosity decreased with further increase of temperature and pressure. In contrast to micropores, micro-scale capillary pores and megapores in shale constantly decreased as rise of simulation temperature or pressure, indicating that deep-burial reservoirs was not favorable for free-gas storage; but significant increase of micropores and surface area during this stage could make up for a loss of adsorbed natural gas in shale due to decrease of adsorption capacity which was induced by increase of temperature and pressure, thus leading to high shale gas potential in deep layers. A large number of secondary micropores were developed in the simulated samples such as pyrite and dolomite, demonstrating that shale clasts and mineral matrix could also form abundant secondary micropores during the deep evolution stage; during the evolution process, shale as hydrocarbon source rock could generate a large amount of acidic fluid which was favorable for development of secondary porosity.

Tunable synthesis of hierarchical mesoporous silica via porogen-carrying organosilicates

We describe the synthesis and characterization of hierarchically porous organic-inorganic hybrid silica with periodic 2-D hexagonal order and tunable location of secondary nanopores through co-condensation of tetraethylorthosilicate (TEOS) and a new type of amphiphilic organosilicate precursors. Our amphiphilic organosilicates offer unique tunability of secondary micropores by carefully controlled bond molecular structure and number of bonds between hydrophobic adamantylphenols pore-generating moieties and siloxane precursors. Small angle X-ray scattering (SAXS) and microscopy results show a good structural order of the organic-inorganic hybrid silica. In addition, 29Si CP-MAS NMR spectra confirm the successful incorporation of organosilicate precursors into a silica framework. The primary cylindrical mesopores (∼6.4 nm) were originated from the cooperative self-assembly of Pluronic P123 triblock copolymer templates while smaller meso-/micropores were derived from adamantylphenols groups. We have previously demonstrated that adam-graft SQ had an affinity for hydrophobic PO core blocks to be located close to PO segment, which resulted in small pores next to primary mesopores and the resultant increment of mesopore size as well as wrinkled silica walls. On the contrary, the secondary small pores were mainly generated within a silica network while the size of the primary mesopores unaffected with the addition of adam-bridge SQ due to its selective incorporation into a silica framework. Moreover, the organic-inorganic hybrid silica prepared with adam-bridge SQ showed higher thermal stability than that of adam-graft SQ and SBA-15 type mesoporous silica. It is attributed to more SiOSi bonds formed from the bridged structure as well as enhanced hydrogen-bonding interaction between adam-bridge SQ hybrid silica and P123 templates.

Gyroidal nanoporous carbons - Adsorption and separation properties explored using computer simulations

Adsorption and separation properties of gyroidal nanoporous carbons (GNCs) - a new class of exotic nanocarbon materials are studied for the first time using hyper parallel tempering Monte Carlo Simulation technique. Porous structure of GNC models is evaluated by the method proposed by Bhattacharya and Gubbins. All the studied structures are strictly microporous. Next, mechanisms of Ar adsorption are described basing on the analysis of adsorption isotherms, enthalpy plots, the values of Henry's constants, $\alpha_{s}$ and adsorption potential distribution plots. It is concluded that below pore diameters ca. 0.8 nm, primary micropore filling process dominates. For structures possessing larger micropores, primary and secondary micropore filling mechanism is observed. Finally, the separation properties of GNC toward CO$_2$/CH$_4$, CO$_2$/N$_2$, and CH$_4$/N$_2$ mixtures are discussed and compared with separation properties of Virtual Porous Carbon models. GNCs may be considered as potential adsorbents for gas mixture separation, having separation efficiency similar or even higher than activated carbons with similar diameters of pores.

Anionic exchange for NOM removal and the effects on micropollutant adsorption competition on activated carbon

To efficiently adsorb micropollutants from surface water and groundwater by activated carbon, the concurrent natural organic matter (NOM) was removed by anionic exchange resin (AER) to reduce the adsorption competition. The studied AER showed a great affinity to the NOM fractions ‘humic substances’ and ‘building blocks’ measured by liquid chromatography with organic carbon detection. The overlapping adsorption isotherms of target compounds (atrazine and caffeine) in the presence of raw water and AER-treated water reflected an irrelevance of AER-removed NOM to site competition. Instead, it was likely the hydrophobic low molecular organics, refractory to AER treatment, competed with the target compounds. Microporous carbon, which contains higher amounts of small and secondary micropores, tended to have less site competition, and a lower dosage was required to achieve 90% removal of the target compounds. During fixed adsorber filtration, however, AER pretreatment slightly prolonged the breakthrough of most of the investigated micropollutants. This could be attributed to less pore blockage due to the lower contents of ‘humic substances’ and ‘building blocks’ in the background water.

Tuning the pore structure of plug-containing Al-SBA-15 by post-treatment and its selectivity for C16 olefin in ethylene oligomerization

Al-SBA-15 containing plug structures inside straight channels have been hydrothermally synthesized through a one-step synthesis strategy in an environmentally friendly acid-free medium. The effects of steaming, alkali, and acid post-treatments on the structural properties were investigated by powder X-ray diffraction (XRD), nitrogen adsorption–desorption at 77K, transmission electron microscopy (TEM), scanning electron microscopy (SEM), FT-IR spectrum, 27Al MAS NMR, and inductively coupled plasma (ICP) analyses. The presence of the plugs can significantly enhance structural stability against steaming or alkali post-treatments. Acid post-treatment can be a very simple and convenient technique for adjusting the textural properties of plug-containing Al-SBA-15, and especially for optimizing the accessibility to active sites. Acid post-treatment at the low temperatures of 313 and 323K results in the formation of a larger specific surface area (905m2/g) and pore volume (0.94cm3/g) through an “open door” effect in the plugs, without eliminating any of the plugs from the channels. In particular, nearly half of the secondary micropores on the pore walls were preserved. In contrast, raising the acid post-treatment temperature further to 333 and 343K caused a “closed doors” effect through the blockage of pores in the plugs by the extracted fragments. After loading with nickel, the catalyst exhibited a higher yield of C10+ olefins in ethylene oligomerization, and more importantly, after steaming and acid post-treatments at higher temperatures, the samples showed shape selectivity for the C16 olefin.

Analysis of uncertainties in manometric gas-adsorption measurements: II. Uncertainty in [formula omitted]-analyses and pore volumes

We describe procedures to propagate the uncertainty in adsorption data and α S -values to generate uncertainty in apparent primary, secondary, and total micropore volumes for porous activated carbons exhibiting Type I and IV character. The α S -data are interpolated from selected non-porous reference material (NPRM) adsorption isotherm data with some adsorbents exhibiting surface chemistry quite different from and some similar to that of the porous adsorbents (PA). We show that a statistically constant apparent total micropore volume can be determined independent of the NPRM surface chemistry. In contrast, NPRM surface chemistry appears to influence our ability to identify unequivocally the filling and condensation ranges of micropore filling, leading to statistically different apparent primary and secondary micropore volumes.

Bimodal pore size distributions for carbons: Experimental results and computational studies

It is well known that the bimodal shape of the pore size distribution (PSD) curves is typical for various carbonaceous materials (of different origin and/or treated thermally or chemically). A systematic investigation of this effect has been discussed using the Nguyen and Do method with proposed recently the ASA algorithm. A series of numerically generated adsorption isotherms (N 2, T = 77 K ) and experimental data were analyzed. We investigated various possible situations related to the shape of the PSD curves, i.e., the intensity of the both peaks, their mutual location and the vanishing of one of them. Moreover, the problem in the similarity of the local adsorption isotherms from the range of pore widths corresponding to the gap between peaks is discussed. The analysis of obtained results (as well as published by others) shows that the bimodal shape of the pore size distributions is a characteristic feature for many adsorbents possessing even a small amount of micropores. It is shown that this feature results from the similarity of the local adsorption isotherm in the range of the pore widths for which the gap between peaks (related to the primary and secondary micropore filling mechanism) exists.

Some remarks on the calculation of the pore size distribution function of activated carbons

Different authors investigated the effects of geometric and energetic heterogeneities on adsorption and on carbon characterization methods. In most theoretical studies carbon structure is modeled as parallel infinite graphite walls that form ideal slit-shaped pores of the fixed widths. In the literature there is the lack of systematic studies showing the influence of pore structural and Lennard-Jones (LJ) potential parameters on the pore-size distribution functions. Moreover, the parameters characterizing the properties of the adsorbed phase and the heterogeneity of the adsorbent surface should be taken into account. The Nguyen and Do method with proposed by us ASA algorithm, were utilized for the assessment of the porosity from the series of almost few thousands numerically generated local adsorption isotherms. The values of the mentioned-above parameters are varied over the wide range (ca. ±20%) of the reference ones. Different types of the theoretical and experimental adsorption isotherms (nitrogen at 77 K) were taken into account as the global ones. They were related to the mechanism of the primary, secondary or mixed micropore filling. The variations in some above-mentioned parameters have significant effects only for PSDs (and for average pore widths) corresponding to the primary micropore filling mechanism. On the other hand, for the process of the secondary micropore filling, the influence of these parameters (without the BET coefficient for adsorption on a “flat” surface, c s , B ) is rather insignificant. Nevertheless the differences between local and global adsorption isotherms (in the whole range of relative pressures) the absence of micropores having pore half width equal to ca. 1 nm on PSDs was observed for studied adsorbate–adsorbent systems with exceptions of the strictly microporous adsorbents and/or the low values of c s , B . Comparison of the experimental data with the generated theoretical isosteric enthalpy of adsorption indicates that the phenomenal uptake observed from experiment can be explained in terms of the reasonable solid–fluid interaction parameters. Therefore, we varied the heterogeneity of the adsorbent surface via the strength and the range of the solid–fluid potential and the parameter c s , B in order to reproduce the experimental data of enthalpy of adsorption. Note that similar procedure was applied by Wang and Johnson to reproduce some hydrogen adsorption data measured for carbon nanofibres. The analysis of the obtained results shows that the selection of the values of the parameters of the intermolecular interactions and the quantities characterizing the properties of the adsorbed phase and the heterogeneity of the adsorbent walls for molecular simulations should be made with care and the influence of possible errors should be considered.

Effect of dealumination severity on the pore size distribution of mordenite

Acid leaching with nitric acid was conducted for the framework dealumination of H-mordenite. The parent as well as the samples obtained after acid treatment were characterized by XRD, surface area, pore volume and pore size distribution for identifying structural changes after dealumination. For the parent (HM) and two acid leached (ATHM-1 and ATHM-3) samples studied, the surface area and micro pore volumes were increased in the order of HM < ATHM-1 < ATHM-3. Nitrogen adsorption studies at 77 K indicated that the increase in micro pore volume is not uniform for the pores of diameter 0–20 Å, where the entire range of micropores were classified into three groups, namely smaller micropores belong to side pockets, micropores belong to main channels, and the secondary micropores that are larger than main channels. The pore volume in small and larger micropores was increased, where as that of the main channels was decreased in the order of HM < ATHM-1 < ATHM-3, and the detailed analysis could provide a direct evidence for the opening of side pockets of mordenite by acid dealumination. The dealuminated samples ATHM-1 and ATHM-3 exhibited enhanced n-hexane isomerization activity.