Silica Gel vs. Alumina in Chromatography

Research will be limited to:
- Method: Column Chromatography
- Materials: Silica Gel, Alumina
- Topic: Acidity, Selectivity, Functional Groups, Compound Classes

Strategy: I will investigate the fundamental chemical differences between silica gel and alumina in column chromatography, focusing on their surface acidity/basicity, the specific mechanisms of selectivity for different functional groups, and the classes of compounds best suited for each stationary phase. This will include a comparison of Lewis acid sites and hydrogen bonding capabilities.
Queries:
- silica gel vs alumina column chromatography surface acidity selectivity
- alumina chromatography grades acidic basic neutral compound classes
- silica gel vs alumina functional group retention mechanisms
- chromatographic separation of alkaloids and steroids silica vs alumina
- Lewis acid sites alumina vs silica gel column chromatography selectivity

Executing 5 concurrent searches:
- silica gel vs alumina column chromatography surface acidity selectivity
- alumina chromatography grades acidic basic neutral compound classes
- silica gel vs alumina functional group retention mechanisms
- chromatographic separation of alkaloids and steroids silica vs alumina
- Lewis acid sites alumina vs silica gel column chromatography selectivity

Found 10 papers. Key excerpts:

Hydrolytic vs. Nonhydrolytic Sol-Gel in Preparation of Mixed Oxide Silica-Alumina Catalysts for Esterification. — Atheer Al Khudhair et al., 2022

Esterification of carboxylic acids is one of the major reactions in organic synthesis [ 1 , 2 ]. In addition to this, esterification of acetic acid with n-butyl alcohol is commercially significant because the product n-butyl acetate is widely consumed in the manufacture of artificial perfumes, photographic films, plastics, safety glasses [ 3 ], and food additives; as a solvent for coating resin; and as a pharmaceutical intermediate. In recent years, consumer demand for n-butyl acetate has grown, resulting in higher prices and profit margins. Therefore, esterification reactions have been carried out on a large scale using both homogeneous and heterogeneous catalysts. For economic purposes, homogeneous catalysts are commonly used in industries. However, these catalysts are difficult to recycle and must be neutralized, which adds cost and pollution [ 4 ]. As a result, significant research efforts have been directed to the development of heterogeneous processes to take advantage of easy separation of catalyst and products [ 5 ].
Mesoporous metal oxides and mixed oxides with acidic, basic, and/or redox properties are well-known materials used in heterogeneous catalysis, including environmental catalysis. Among these mixed oxides, silica–alumina oxides have been extensively used in esterification reactions because of their high surface area and high thermal stability [ 6 , 7 , 8 ].
Even if there is no consensus on the esterification mechanisms occurring on different (although similar) solid acid catalysts, most of the literature studies align with a mechanism that involves the Brönsted acid sites of acidic heterogeneous catalysts [ 9 ].
The strong Brönsted acidity of silica–alumina is attributed to hydroxyl groups in the “mixed phase Si-O-Al”. A debate still exists on the structure of these sites in amorphous materials ( Figure 1 ) [ 10 , 11 , 12 ]. The Si-O-Al oxo bridge is more negatively charged than the oxygen of an Al-OH or Si-OH. Species (a) and (b) are therefore likely to generate stronger acidity.
The hydrolytic sol-gel (HSG) process, based on the formation of inorganic matrices by hydrolysis and polycondensation of molecular precursors (usually silicon or metal alkoxides), has been extensively used to prepare mixed oxide catalysts with high

surface area and ordered mesoporosity [ 13 ]. This process can be defined as the conversion of a precursor solute into an inorganic solid through water-induced inorganic polymerization reactions, which can be carried out under simple experimental conditions and at low temperatures. It is one of the most popular approaches used to produce oxide materials with a high degree of control over their textural and surface properties, as well as high purity and homogeneity [ 14 ]. The nonhydrolytic sol-gel (NHSG) is another powerful technique for designing oxides and mixed oxides [ 15 ]. It is based on the reaction of chloride precursors with an oxygen donor, giving good control over the stoichiometry and homogeneity of the mixed oxide gels. The NHSG process involves condensation reactions in nonaqueous media, which significantly affects the texture, homogeneity, and surface properties of the resulting materials. It has been successfully applied to the synthesis of mixed oxides, which have shown excellent catalytic performances in various reactions [ 8 , 16 ].
In this work, we studied SiO 2 -Al 2 O 3 materials prepared via both HSG and NHSG routes. The structure, texture, and acidity of the materials obtained by these procedures were evaluated and compared. Additionally, the catalytic behaviors of the mixed oxides in the esterification reaction between acetic acid and n-butanol (n-BuOH) are discussed.
EDX analysis was used to determine the experimental compositions of the samples. As shown in Table 1 , experimental weight percentages were close to the nominal ones, based on the amounts of reactants, indicating that all of the Si and Al atoms were included in the final oxides.
It’s worth noting that similar reproducible results were observed for ternary Ni/silica–alumina mixed oxide catalysts prepared by NHSG [ 17 ].
N 2 -adsorption–desorption measurements were realized and various types of isotherms were obtained, showing that the pore structure was closely related to sample composition and sol-gel route ( Figure S1 ).
The texture of the samples depended on Si/Al ratio and on the synthetic route. As shown in Table 2 , the average pore diameter (Dp) ranged between 1.8 and 13.8 nm, demonstrating the mesoporous structure of the Si x Al y samples.
For alumina-free compositions, samples

prepared via NHSG showed higher textural properties (S BET , Vp, and Dp) than those obtained via HSG. An increase of Al loading in the composition led to a decrease in values for NHSG samples. The specific surface area, pore volume, and pore diameter of Si 75 Al 25 and Si 50 Al 50 prepared in basic HSG medium were higher than those prepared in acidic conditions. The same observation was made by Agliullin et al. when they prepared porous silica–alumina using sol-gel processes at different pH values [ 18 ]. Si 25 Al 75 and silica-free compositions prepared in acidic HSG exhibited higher textural properties than the others.
SEM images were obtained to study the morphology of the prepared materials ( Figure S2 ). After sample grinding, the grain size was heterogeneous and varied from 800 nm to 40 μm for HSG samples and from 1 to 100 µm for NHSG compositions. Independently of the sol-gel route, solids were hard and compact for high SiO 2 loadings (up to 75 wt%). 29 Si and 27 Al solid-state NMR gave information on the structure of the silica–alumina network. As shown in Figure 2 , the 29 Si CP-MAS NMR spectra of Si-containing mixed oxides showed broad resonances typical of amorphous materials [ 19 ].
The chemical shift of the Si atoms in amorphous aluminosilicates depends on the nature of the second neighbors. In amorphous silica or silica–alumina, each replacement of a (OSi) group in a Si(OSi) 4 tetrahedron by an (OAl) or an (OH) group leads to a downfield shift of about 5 to 10 ppm [ 20 ]. The spectrum of Si1 00 Al 0 shows three broad signals at ≈−110, −101, and −93 ppm, attributed to Si(OSi) 4 (Q 4 ), Si(OSi) 3 (OH) (Q 3 ), and Si(OSi) 2 (OH) 2 (Q 2 ) sites, respectively [ 21 ]. Similar 29 Si NMR spectra with a major Q 3 resonance have been reported for mesoporous silicas or silica–alumina with a low Al content [ 20 , 22 ]. A Q 4 signal was only observed in the Si 100 Al 0

Ethylene Oligomerization over Nickel Supported Silica-Alumina Catalysts with High Selectivity for C — Lei Chen et al., 2020

1. IntroductionThe oligomerization of light olefins is an important route for the production of linear and branched higher olefins, which can be used in the manufacture of detergents, petrochemicals, oil additives, high-octane ecological gasoline, etc. [1,2]. Ethylene, with huge production worldwide, is the raw material for a wide range of chemical products and intermediates. Industrial reactions of ethylene include in order of scale polymerization, oxidation, halogenation, alkylation, hydration, oligomerization and hydroformylation [3,4,5,6]. Ethylene oligomerization is of considerable academic and industrial interest because it is one of the major processes for production of linear and branched higher olefins, which are components of plastics (C4–C6 in copolymerization), plasticizers (C6–C10 through hydroformylation), lubricants (C10–C12 through oligomerization) and surfactants (C12–C16 through arylation/sulphonation) or starting materials for other important chemicals, such as propylene, alcohols, amines and acids [1,7,8,9,10,11,12,13]. Among those higher olefins products, C10+ is very desirable for jet fuel application [14,15,16].The C10+ olefins produced from ethylene can be fulfilled by using homogeneous [17,18,19,20] and heterogeneous catalysts [21,22,23] where heterogeneous catalysts have been extensively explored because of easily separation from the product and better reusability. Among those heterogeneous catalyst systems, Ni-base catalysts have attracted much attention because of high activity and selectivity towards the C10+ olefins [1,24,25,26,27,28,29,30]. The oligomerization on Ni-based catalysts coupled with co-oligomerization reactions involving the primary olefins over an acid catalyst is favorable for obtaining the olefins with C10+ chain. In order to illustrate the role of nickel sites and acid sites in this reaction, the total reaction pathways were proposed, as shown in Scheme 1 [6].The first reaction is based on the coordination chemistry on nickel sites. They act as active sites for

both the initial oligomerization of ethylene and further oligomerization reactions involving butene–ethylene coupling, leading to linear olefins of medium-chain length. The second one is based on the acid catalysis. Over acid sites, the C4 and C6 olefins can be consumed through co-oligomerization reactions (mechanism involving carbenium ions), leading to the formation of octenes or higher branched olefins, respectively. These reactions are essentially favored by a stronger acidity or/and a higher acid sites concentration and higher reaction temperatures. The same factors are responsible for the isomerization of the initial product (1-butene, 1-hexene, etc.). The C4–C10 oligomers can be involved in further acid catalyzed reactions, leading to the formation of heavy hydrocarbons which are responsible for pore blocking and catalyst deactivation. The third type of reaction, occurring under severe conditions and involving the acid sites, consists in the cracking of the primary and secondary oligomers and H transfer.In most previous studies, the active Ni species are often loaded on the different supports, such as silica17, silica–alumina [6,24] and zeolites [31,32,33,34], to improve the dispersion of Ni species. Meanwhile, the activity and selectivity towards the C10+ olefins are also affected with the change in the acid sites and the porosity of the supports. For example, Ng et al. [33] investigated the effect of NaY zeolites supports by the acid and base treatment and the calcination temperature, and found that Ni species and acid sites were necessary for the oligomerization of ethylene. Lallem and et al. [28] reported that the Ni-exchanged MCM-36 zeolite exhibited higher activity and stability than the Ni-exchanged MCM-22 zeolite because the mesoporous nature of the MCM-36 zeolite facilitates the diffusion of larger oligomers formed during the reaction.Apart from the zeolites and mesoporous silica, silica–alumina support was the earlier candidate because of their merits of easy synthesis and cheapness. Compared with zeolite materials, silica–alumina support has the moderate strength acid sites that benefit to lower the extent of over-polymerization of eth

ylene, thus inhibiting the coke formation. Moreover, silica–alumina support also has the interparticle mesopores which facilitate the larger oligomers diffusion. Heveling et al. [34] employed this catalytic system to obtain products in the C4–C20 range from ethylene and found these types of catalysts were extremely stable, showing no detectable drop in conversion after 108 days. Previous studies on Ni-loading silica–alumina mainly investigated the effect of reaction conditions [34] (reaction temperature and gas pressure) and the reactor type (fixed-bed reactor [34] and the slurry reactor [24]). In this work, we focused on the effect of the modification of the silica–alumina support in terms of aging temperature, the ratio of Si/Al and activation temperature and the C10+ yield was employed to evaluate these effects. In addition, we investigated the effects of other experimental conditions on the experimental results, such as reaction temperature, weight hourly space velocity (WHSV) and nickel precursor. 2. Results and Discussion 2.1. The Physicochemical Properties of Ni/Si-Al CatalystsFigure 1 shows the XRD patterns of as-obtained Si-Al support and Ni/Si-Al catalyst with different Si:Al ratios treated at different aging temperatures. For all samples, including Si-Al support and Ni/Si-Al catalysts, there was only a broad peak locating at around 22.5° and no sharp peaks attributed to Al2O3 were observed, indicating its amorphous structure and that they were composed of small particles with poor crystallinity. Accordingly, the ratio of Si/Al and aging temperature cannot influence the crystalline structure of Si-Al support and Ni/Si-Al catalyst under present synthesis condition.After loading Ni onto Si-Al support, there was no obvious peaks attributed to NiO in XRD patterns for all Ni/Si-Al catalysts, but XRF texts (as shown in Table 1) verified that Ni species does exist on the support surface. The diffraction peaks for Ni species were not found in XRD patterns, resulted from the lower amount of Ni loading by ion-exchange method and the well dispersity of Ni on the support surface, which is beneficial to the catalytic performance.Figure 2 shows the morphology of the Ni/Si-Al catalysts with different Si/Al ratio or aging temperature

Discovery of homogeneously dispersed pentacoordinated Al species on the surface of amorphous silica-alumina — Zichun Wang et al., 2016

The dispersion and coordination of aluminium species on the surface of silica-alumina based materials are essential for controlling their catalytic activity and selectivity. AlIV and AlVI are two common coordinations of Al species in the silica network and alumina phase, respectively. AlV is rare in nature and was found hitherto only in the alumina phase or interfaces containing alumina, a behavior which negatively affects the dispersion, population, and accessibility of AlV species on the silica-alumina surface. This constraint has limited the development of silicaalumina based catalysts, particularly because AlV had been confirmed to act as a highly active center for acid reactions and single-atom catalysts. Here, we report the direct observation of high population of homogenously dispersed AlV species in amorphous silica-alumina in the absence of any bulk alumina phase, by high resolution TEM/EDX and high magnetic-field MAS NMR. Solid-state 27Al multi-quantum MAS NMR experiments prove unambiguously that most of the AlV species formed independently from the alumina phase and are accessible on the surface for guest molecules. These species are mainly transferred to AlVI species with partial formation of AlIV species after adsorption of water. The NMR chemical shifts and their coordination transformation with and without water adsorption are matching that obtained in DFT calculations of the predicted clusters. The discovery presented in this study not only provides fundamental knowledge of the nature of aluminum coordination, but also paves the way for developing highly efficient catalysts.
Alumina and its mixed oxides are important catalytic materials both as active catalysts and as functional supports for active metal particles. The catalytic functions of these materials in chemical reactions are mainly dependent on the surface coordination of Al species due to their structure-activity relationship. Most research efforts have been focused on the tetrahedral and octahedral coordination (AlIV and AlVI), which are the most popular coordinations of Al species. Pentahedral coordination (AlV) were rarely on alumina and silica-alumina and reported to be a
transition state to octacoordinated aluminum species and generated during calcination of - alumina.1 Recently, it has been reported that AlV species on -alumina are surface active sites for stabilizing metal centers or nanoparticles to suppress sintering.2 The AlV–

metal (e.g. Ba, Cu, Ru, Au, Ag, Pt and Pd) interaction was proposed to improve the catalytic activity of metal centers in various reactions,3-6 such as CO2 reduction and deNO(x) reactions. Therefore, Al V species have recently attracted great attention, particularly, they were proposed to be coordinatively unsaturated surface centers of supports for anchoring noble metal atoms for emerging single-atom catalysts.2,7
However, the previous reports showed that AlV species are not highly populating the surface. Although a certain amount of AlV species has been found to be generated during phase transformation from -Al2O3 to -Al2O3 (up to 17 %), 1,8,9 only a very small amount of AlV species (< 2 at.% ) was stabilized on the surface of -Al2O3, when suitable hydroxides or oxides such as La2O3 and BaO were added to inhibit the phase transformation and stabilize the unsaturated Al ions. It was also reported that AlV species only exist nearby AlVI species on the surface of crystalline Al2O3 and might represent Al VI in the vicinity of an oxygen vacancy, such as the defect spinel structures of the transition aluminas.10 For silica-aluminas, AlV species were proposed to be located on the interface between alumina and silica or alumina and aluminosilicates.11,12 Therefore, AlV species on Al2O3 or corresponding interfaces were assumed to be poorly distributed on the surface and hardly available for the reactants, which reinforced the doubt that AlV species are promising active centers for building highperformance catalysts such as emerging solid acids, single-atom catalysts and spatially confined catalysts due to their poor accessibility.2,7
In this study, we discovered a new type of AlV species in amorphous silica-alumina (ASA). It is homogeneously distributed and highly populates the surface, thus providing high accessibility for guest molecules. The ASA and Al2O3 nano-particles were prepared by flame spray pyrolysis as described previously, which could offer strong acidity.13 All prepared
particles had a size around 5-10 nm and their crystalline or amorphous structure has been verified by XRD. As shown in Fig. S1, both ASA samples,

containing 10 and 30 at.% of aluminum (SA/10 and SA/30), only showed a broad reflection at 22-23o due to amorphous silica. As a reference, a pure Al2O3 sample SA/100 obtained without adding silica during synthesis has been investigated by XRD, as also shown in Fig. 1. Broad and weak signals of crystalline Al2O3 were observed for SA/100 due to its small particle size. The very fine nanoparticles with well-ordered lattice structure did not show significant diffraction and could therefore not be identified by XRD. High-resolution transmission electron microscopy (HRTEM) has also been applied to examine the existence of alumina phase domains in ASAs.1416 For SA/100, the image clearly revealed the well-ordered alumina lattice in Fig. 2a. For ASA samples, the alumina lattice disappeared, and the amorphous nature of both samples was corroborated, as shown in the HRTEM images in Fig. 2b and c. No small alumina clusters were detected on the ASA surface. As revealed by EDX atom mapping images, Al species were homogeneously distributed in the silica network of SA/10 (Fig. 2d-e) and SA/30 (Fig. 2f and g), again, no aggregated aluminum species or small alumina nanoparticles were observed on the surface.
To identify the local coordination of these homogenously dispersed Al species, solid-state 27Al MAS NMR spectroscopy combined with multiple quantum MAS (MQMAS) has been used to yield the necessary resolution for discriminating among nuclear quadrupoleinteraction-broadened signals of the different Al species.17-19 For both the SA/10 and SA/30 samples, AlV species have been clearly identified by 27Al MQMAS NMR spectroscopy. The subscript “de” and “hy” are dedicated to species present in dehydrated and hydrated state, respectively. As shown in Fig. 3a and 3c, two strong signals at (50, 55) and (23, 29) were assigned to AlIV and AlV species, respectively, indicating their predominant population in the aluminate species on both dehydrated SA/10 and SA/30 samples. Interestingly, only a small peak at (-9, 0.5) and a very weak signal at (-5

Engineering the Distinct Structure Interface of Subnano-alumina Domains on Silica for Acidic Amorphous Silica-Alumina toward Biorefining. — Zichun Wang et al., 2021

Silica–alumina
catalysts, including amorphous silica–aluminas
(ASAs) and zeolites, are the most popular emerging solid acidic catalysts
utilized in hydrocarbon transformation and biorefining. In zeolites,
the strong Brønsted acid sites (BAS) originate from protons compensating
for the negative charges caused by Al 3+ replacing framework
Si 4+ ( Scheme 1 a). 1 , 2 Enhancing the acidity of zeolites by framework
dealumination or Al exchange 3 − 5 introduces Lewis acidic extra-framework
Al (EFAl) species, which then act as bifunctional Brønsted–Lewis
acidic catalysts. These bifunctional catalysts facilitate the integration
of multiple acid-catalyzed reaction steps with high efficiency and
promote the conversion of biomass-derived sugars, alcohols, and glycerol
into valuable chemicals, 6 − 8 which have attracted great attention
recently. However, the EFAl species generated on zeolite surfaces
can easily leach out in liquid-phase reactions, resulting in severe
activity loss. 9 , 10
(a) BAS consisting of a bridging
silanol site bonded to a tetra-coordinated aluminum (Al IV ) site (Si(OH)Al) in zeolites. 2 (b) BAS
consisting of flexible coordination between silanol oxygen and neighboring
Al IV . 20 (c) BAS consisting of
a pseudo-bridging silanol (PBS) interacting with an Al IV site. 16 (d) BAS consisting of the synergy
of tetra- and penta-coordinated Al (Al IV and Al V ) sites with the same SiOH. 10 In b–d,
the dotted line does not denote a covalent bond but only the close
proximity between O and Al atoms.
Recent advances
in the sustainable production of chemicals and
fuels from biomass conversion often involve reactions with large molecules.
ASAs without diffusional constraints show superior diffusional mass
transfer properties for large molecules compared to the microporous
zeolites and are thus of great interest in acid-catalyzed conversions
of biomass and derivatives. 10 − 12 In ASAs, the formation of BAS
has been widely accepted to originate from an Al atom flexibly coordinated

to a neighboring silanol oxygen atom ( Scheme 1 b) 13 − 15 or via aluminic pseudobridging
silanol (PBS-Al, Scheme 1 c, d) at the interface between silica and alumina. 10 , 16 , 17 Zeolite-like bridging Si(OH)Al or replacement
of an Al atom by Si in Scheme 1 c (PBS-Si) are also proposed for BAS formation by theoretical
calculation studies but lack direct spectral evidence yet. 18 Moreover, alumina domains on ASAs are associated
with the formation of Lewis acid sites (LAS). 19 These properties render ASAs ideal bifunctional Brønsted–Lewis
acidic catalysts with versatile acid strengths, high stability, and
lower energy barriers compared to zeolites. 10 , 11
ASAs are generally prepared by impregnation, 21 sol–gel, 22 precipitation, 23 surface grafting, 14 , 16 , 24 and flame-spray pyrolysis (FSP) techniques. 25 , 26 Using wet-chemistry and postsynthetic modification techniques, the
dissolution and recondensation of Al or Si species with surface amorphization
result in heterogeneity of Al species inside the silica networks, 16 , 27 accounting for a wide distribution of acid strengths in ASAs. 16 , 28 , 29 Nonuniform acid strengths of
solid acids often promote side reactions resulting in lower selectivity
and even catalyst deactivation by coking. 14 , 30 , 31 Atomic layer deposition (ALD) has been proposed
to afford ASAs with uniform BAS strength via the selective reaction
of the Si precursor with surface AlOH groups on the alumina support. 14 However, the formation of BAS is strongly hampered
by the low density of AlOH groups, which can largely limit the formation
of BAS and LAS on ALD-made ASAs. Therefore, the synthesis of ASAs
with uniform acid strength and enhanced acidity remains a challenge
for both selective and bifunctional Brønsted–Lewis acid
catalysis as well as a key in tuning supported metal catalysts with
identical electronic properties for high chemoselectivity in hydrogenation
reactions. 32
Uniform acid sites generally
require an unvarying local structure
of BAS and LAS. Similar domains of alumina on silica or silica on
alum

ina can generate a uniform interface structure for the acid formation.
The FSP technique facilitates the preparation of uniform nanoparticles
with atomically mixed components or particles with segregated or embedded
components in a single step from a liquid precursor solution. 33 − 35 In contrast to the classical flame spray pyrolysis, where the precursor
solution containing all components is sprayed and dispersed into a
single flame, double-flame spray pyrolysis (DFSP) 34 − 40 uses two separate flames and the intersection zone of these flames,
where the mixing of the two separately generated aerosols occurs,
and can be controlled by proper positioning of the nozzles. This enables
the tuning of the intermixing of the components of the synthesized
materials on the micro- and/or nanoscale.
Silica–alumina
catalysts exhibit high activity in promoting
biomass conversions for sustainable production of chemicals and fuel
additives. 41 − 43 For instance, catalytic dehydration of glucose affords
5-hydroxymethylfurfural (HMF), an important building block
in the production of liquid alkanes, biofuels, and furan derivatives. 44 , 45 This production requires glucose isomerization at LAS to fructose,
followed by fructose dehydration at BAS to yield HMF, 41 , 42 , 46 which is a typical reaction for
evaluating the catalytic performance of Brønsted–Lewis
acidic catalysts with practical relevance. Dehydration of biomass-derived
cyclohexanol to cyclohexene is another example, which is a key precursor
to dicarboxylic acids for drug and resin syntheses. 47 The high selectivity to cyclohexene is sensitive to the
uniform acid strength of solid acids, which is often utilized as a
diagnostic reaction. 14 , 31
Herein, we used the DFSP
technique to tune the structure of ASAs.
We successfully synthesized ASAs made up of uniform alumina domains
and a silica–alumina interface, comprising BAS and LAS with
virtually unvarying acid strength. The structures of the as-synthesized
ASAs were characterized by EDS element mapping and 29 Si
and 27 Al MAS NMR spectroscopy, and their acidities were
determined by quantitative 1 H and

Enhanced Phenol Tert-Butylation Reaction Activity over Hierarchical Porous Silica-Alumina Materials — Ling Xu et al., 2020

1. IntroductionAs a commonly used material for adsorption separation and catalysis, molecular sieves can be simply divided into three types: micropore, mesopore, and micropore. The pores with a diameter of less than 2 nm, 2–50 nm, and more than 50 nm were called micropores, mesopores, and macropores, respectively [1,2,3]. With the advancement of research, researchers have gradually discovered that molecular sieves of three pore size have their own characteristics. However, materials with a certain pore structure applied separately in the fields of catalysis, petrochemicals, and ion exchange were usually not ideal [4,5,6,7,8]. For example, the widely used ZSM-5 molecular sieve had a strong acidity and excellent channel selectivity [9,10,11,12,13,14]. However, the narrow pore size of ZSM-5 limited the transfer and diffusion of macro reactants and products [15,16,17,18,19,20]. However, the pore walls of the mesoporous materials were amorphous, and the acidity of the materials was relatively low. Therefore, mesoporous materials had poor catalytic activity in acid-catalyzed reactions [21,22,23].Due to the shortcomings of single microporous and mesoporous materials in catalytic reactions, many researchers were committed to the composite research of microporous and mesoporous materials [24,25]. Mesoporous template was added into synthetic microporous silica-alumina precursor, which was adopted in situ synthesis method to prepare micro-mesoporous silica-alumina materials. Moreover, the pore wall of the mesoporous silica-alumina material was crystallized, which was named the post-synthesis method [26,27,28]. In situ synthesis mainly included single template synthesis and double template synthesis [29]. The method was easy to operate, but micropores and mesopores were not well compounded and were easy to separate, leading to the synthesis of a single microporous material or mesoporous material. The post-synthesis method mainly included the pore wall crystallization method, epitaxial growth method, and alkali treatment method [30]. These methods need be completed in at least two steps, so the preparation methods were

more complicated. In our previous work, we used polyethylene glycol as the mesoporous soft template to synthesize hierarchical ZSM-5 zeolites [21]. However, there were two kinds of acid sites, which would result in side reactions. In this work, we used citric acid as the mesoporous template to synthesize hierarchical ZSM-5 zeolites with main single kind of acid site, which favors improvement of the selectivity.Micro-mesoporous silica-alumina materials obtained by pre-crystallization and an in situ synthesis method can effectively avoid the defects of in situ synthesis and post-synthesis methods. The aluminum-silicon was crystallized at different times to prepare pre-crystallization product with a microporous structural unit or a microporous nanocrystal. The mesoporous pore-forming agent citric acid and the pre-crystallized product were self-assembled under hydrothermal conditions to prepare a micro-mesoporous silica-alumina material. 2. Results and DiscussionFigure 1 shows the XRD patterns of the materials. When the pre-crystallization time of the silica-alumina sol is 0 h and 4 h, a broad diffraction peak appeared at 2θ = 20–30°, indicating that the silica-alumina sol pre-crystallization time is too short to form a ZSM-5 nano-structure. When the pre-crystallization time is extended to more than 8 h, the sample exhibits characteristic diffraction peaks at 2θ = 8.0°, 8.8°, 3.2°, 24.0° and 24.5°. It is consistent with the XRD diffraction peaks of ZSM-5 zeolite in the literature [31], which indicates that the prepared materials have the crystal structure of ZSM-5 molecular sieve. With the increase of crystallization time, the intensity of ZSM-5 diffraction peaks gradually increases, suggesting that the crystallization degree of samples increases [32,33].Figure 2 shows the FT-IR spectra of the samples. The absorption peak at 465 cm−1 is ascribed to Si–O–Si (Al) variable angle bending vibration, while the absorption peak at 810 cm−1 belongs to Si–O–Si (Al) symmetric stretching vibration. Moreover,

the peak at 1080 cm−1 is due to Si–O–Si (Al) anti-symmetric stretching vibration, whereas the bands at 1639 cm−1 and 3430 cm−1 are attributed to O–H bond bending vibration of water and Si–OH or O–H bond stretching vibration of water, respectively [34]. For pre-crystallization micro-mesoporous materials at 8 h, 16 h, and 24 h, the characteristic peak at 550 cm−1 indicates MFI structure (Figure 2c–e) [34]. However, the ZSM-5 zeolites crystallized at 0 and 4 h do not have this characteristic peak, indicating that the pre-crystallization time is too short and the MFI structure of ZSM-5 is not formed. Therefore, prolonging the pre-crystallization time favors the formation of the microporous structure.Figure 3 shows the N2 adsorption-desorption isotherm curves and pore distribution of the samples. It can be seen that the N2 adsorption-desorption isotherm curves of silica-alumina material is type IV with a H2 hysteresis ring, indicating that the synthesized samples contain a mesoporous structure. With increasing pre-crystallization time, the hysteresis ring of the isothermal curves becomes smaller, indicating that the crystallinity of the material increases, and the mesoporous order becomes lower. From 0 h to 24 h, the micropore BET specific surface area of the samples gradually decreases from 547 m2 g−1 to 440 m2 g−1. With the increase of pre-crystallization time, the pore specific surface area gradually increases and the mesoporous specific surface area gradually decreases, indicating that the micropore ratio of the samples increases, which is consistent with the results of XRD and FT-IR analysis.Figure 3b exhibits the pore distribution of the silica-alumina materials. The silica-alumina materials pre-crystallized at different times contain bi-continuous micropore and mesopore distribution. The microporous distribution is mainly centered at about 1 nm, while the mesopore distribution is mainly concentrated at around 6 nm. For the samples with shorter pre-crystallization time (i.e., NM-ZSM-5-0h,

Atomic Layer Deposition of the Geometry Separated Lewis and Brønsted Acid Sites for Cascade Glucose Conversion — Wenjie Yang et al., 2023

Solid acid, as the most popular heterogeneous
catalyst, has received
much research and industrial interest in promoting sustainable hydrocarbon
transformation and biorefining. Among solid acids, due to the advantages
of tunable surface acidity of both Lewis acid site (LAS) and Brønsted
acid site (BAS), large specific surface area, adjustable porosity,
and cheap synthesis, aluminosilicate materials attracted the most
attention. 1 − 3 Generally, on crystalline aluminosilicates, such
as zeolites, the penetration of four-coordinated aluminum species
into the tetrahedral silica framework contributes to the establishment
of BAS via the formation of bridging hydroxyl structure for compensating
local negative charges, 4 whereas the dihydroxylation,
steaming, or dealumination resulting in extra-framework Al species
on zeolites are believed to be the origin of surface Lewis acidity. 5 Therefore, both BAS and LAS on zeolites contribute
to the bi-acidic catalysts, where a synergistic effect between Brønsted-Lewis
pairs has been observed. 6 , 7 The synergistic effect
gives enhancement of Brønsted acidity for the catalysis, 8 , 9 and the cooperation between both acid sites leads to the enhanced
performance of multistep catalytic conversion. 10 , 11
However, the spatially adjacent BAS-LAS pairs on zeolites
generally
contribute to the ultrastrong acidity of BAS via LAS withdrawing electron
pair from the BAS hydroxyl group. This is beneficial only for catalytic
reactions that require strong acidity. For example, with a spatial
interaction between a Lewis acidic Zn 2+ cation and Brønsted
acidic SiOHAl, the synergetic effect contributes to an ultrastrong
BAS that enhances the activation of the methane C–H bond. 12 The Ga 3+ -BAS pair results in the
enhanced acid strength of Ga-modified ZSM-5, which contributes to
an enhanced aromatic selectivity on methanol-to-aromatic conversion. 13 Caused by the polarizing effect of multivalent
extra-framework Al cations on the SiOHAl group of BAS, the BAS shows
a much stronger interaction with the basic

probe molecule of acetonitrile
(CD 3 CN) compared to that of zeolite H, Na–X and
H, Na–Y. 14 Nevertheless, the ultrastrong
acid site limits the cascade reaction on both sites and results in
lower selectivity for the target products due to side reactions such
as overoxidation. 3 , 15 , 16 Additionally, the microporous nature of zeolites restricts the entry
and diffusion of bulky biomass molecules, limiting their access to
the active sites, which further hurdles performance. 17
In this research, we develop the synthesis strategy
to separate
BAS and LAS in geometry and promote the cascade reactions on both
acid sites via atomic layer deposition (ALD) of the LAS-based domain
on BAS-based supports. ALD, as a method to grow thin films, has been
considered as a way to coat uniform conformal overlayers and tune
the surface properties of catalysts within defined overlayer regions. 18 Currently, ALD has been mainly applied in BAS
formation and the improvement of silica–alumina catalysts.
Stair et al. 19 observed the BAS formation
ability of the ALD-deposited nonacidic SiO 2 overlayer on
alumina. The formed BAS-based (AlO) 3 Si(OH) sites showed
an enhanced catalytic performance on cyclohexanol dehydration. Ardagh
et al. 20 demonstrated the criticalness
of the thickness of the overcoat layer to the formation of BAS on
the external surface. They found that the catalyst with ∼2
nm coatings of SiO x on Al 2 O 3 substrate gives the highest BAS concentration, and the prepared
BAS on the external surface leads to enhanced performance on 1,3,5-triisopropylbenzene
cracking. Similarly, coating alumina on silica also contributes to
BAS. Krishna et al. 21 identified the formation
of surface BAS between the ALD-formed alumina overlayer and the silica
substrate. With 5–10 ALD cycles, the majority of isolated
SiOH can be covered, contributing to an enhanced concentration of
BAS. Nevertheless, little research focuses on the formation and development
of LAS via ALD on silica–alumina catalysts.
Herein, via
atomic layer deposited

alumina, we develop an LAS-based
domain on the external surface of a BAS-based mesoporous silica–alumina
substrate and hide the majority of the intrinsic BAS of the substrate
inside the nanopores. Thus, BAS and LAS have been separated by the
geometry in the catalysts, which is promising for cascade reactions
with controlled diffusion. The geometry of the prepared catalysts
and the distribution of ALD alumina are characterized by high-resolution
transmission electron microscopy (HRTEM) combined with electron energy
loss spectroscopy (EELS) mapping. Multinuclear ssNMR was used to characterize
the BAS and LAS of the samples. Among biomass conversions with complex
reaction networks, 22 − 24 converting glucose into platform chemicals 5-hydroxymethylfurfural
(HMF) stands as one of the most promising routes for green chemistry. 2 , 25 Thus, the typical biorefining cascade glucose conversion is applied
as the testing reaction for the cascade performance of the prepared
catalysts.
To construct
the precisely controlled cascade architectural structure, the ALD
cycles under elevated temperature were conducted in order to make
nanolevel architectural modification on the outermost surface. The
TMA pulses led to the formation of the Al–C bonds on the outermost
surface of the substrate via the replacement of surface silanol hydroxyl
protons with Al ions, which provides the prerequisites of the formation
of the unique geometry of alumina nanolayer on the silica-based substrates. 26 The following H 2 O pulse contributed
to the replacement of the outermost surface Al–C bonds with
Al–O bonds and led to the formation of alumina nanofilm. With
more ALD cycles, the protons of surface AlOH groups can continue to
be replaced by Al ions, leading to the increase in the thickness of
the alumina nanolayer ( Figure 1 A,B).
(A) Scheme of Al ALD @high silica substrate synthetic
approach; (B) Schematic diagram of Al ALD /high silica substrate;
HRTEM images of (C) Al ALD /m-SA and (D) Al ALD /m-S; EDS mapping image of (E) Al ALD /m-SA and (F

Study of the Mineralogical Composition of an Alumina-Silica Binder System Formed by the Sol-Gel Method. — Lenka Nevřivová et al., 2023

1. IntroductionThe sol–gel bonded no-cement castable (SGBNCC) is a type of refractory castable that was developed to reduce the need for cement in the manufacturing process. The bond obtained from cement and calcium aluminate cement is connected with the development of high strength at temperatures below 1000 °C [1,2]. A general disadvantage of this is that corresponding linings need to be heated up very sensitively, especially during the initial heating. Because the cement contains calcium oxide, there is also a risk of the formation of low melting phases. The presence of calcium oxide in the cement poses a risk of the emergence of low-melting phases [3]. The SGBNCC has many advantages over traditional refractory materials. As such, it has become increasingly popular in the refractory industry and may become a major player in future applications. The use of sol–gel bonding in the preparation of a refractory castable without cement is a relatively new technique and is becoming more and more important in the construction industry due to its advantages over traditional methods.The sol–gel process is typically carried out by mixing a precursor solution, which contains nanoparticles that make up the solid material and a solvent [4,5]. The sol can then be transformed into gel, either by evaporating the solvent to leave a solid material, or by adding a chemical agent that promotes the crosslinking of the particles to form a three-dimensional network. The resulting solid material, which is made up of a network of interconnected particles, is known as a gel. This gel can be used as a binder in a no-cement refractory castable [6,7,8].In order to prepare a no-cement castable using the sol–gel method, several important materials are required. The primary inputs are refractory aggregates and a sol–gel binder. The aggregates used can vary depending on the application and the desired properties of the castable, but they are typically made of materials such as alumina, mullite, bauxite, andalusite, corundum, silicon carbide, silicon nitride, or zirconia [9]. The binder is most commonly colloidal silica or alumina; their transformation to gel leads to a “glue effect” where aggregate particles are linked together [10].The rheological behavior of the material is also important;

it should be able to flow easily and fill small voids in the mold without setting too quickly [11]. Studies have found that the addition of surfactants [12,13] or microsilica [14] can improve flowability and adjust setting time.Drying is an important step in the production of sol–gel bonded castable refractories, as it helps to remove excess water from the mixture, particularly when the gel is forming strong bonds among the particles. The removal of excess water from the mixture is beneficial, particularly when the gel is forming strong bonds among the particles.The drying process typically begins with a preliminary drying step, in which the castable is left to dry at room temperature for a period of time. The duration of this step depends on the size and shape of the castable, as well as the ambient humidity and temperature [15,16].After the preliminary drying step, the castable is typically placed in a drying oven, where it is heated at a controlled temperature and humidity. When it is used as shotcrete, the lining temperature increase must be controlled and planned properly. When it is used as a shotcrete, the temperature increase in the lining must be controlled and planned properly. The temperature and humidity used during this step depend on the composition of the castable and the desired properties of the finished product. In general, the temperature of the drying oven is kept low, between 50 and 150 °C, to prevent any chemical reaction or shrinkage in the castable. In general, a longer drying time is needed for larger and thicker pieces of castable than for smaller and thinner pieces of the castable [14,15,16]. The drying of SGBNCC is a crucial step in the production process, and care must be taken to ensure that it is performed correctly and that the finished product has the desired properties.The durability of SGBNCC is typically better than that of the traditional no-cement castable refractories. This is due to the unique properties of sol–gel binders which provide several advantages over other types of binders.One of the main advantages of sol–gel binders is that they create very strong bonds among the elements of the refractory aggregate. This improves the mechanical strength of the castable refractory, making it more resistant to abrasion and corrosion. Additionally, because sol–gel binders are made from pure oxides, they are able to withstand high temperatures and

thermal shock better than other types of binders [17,18,19,20]. This means that they are less likely to experience cracking or deterioration due to thermal cycling or rapid temperature changes.The durability of SGBNCC also depends on the composition of the refractory mixture and the operating conditions of the furnace or equipment in which they are used. The quality of the refractory aggregate, its particle size, its shape, and its chemical composition play a crucial role in determining the durability of the castable.The mineralogical composition of a sol–gel bond can also have a significant influence on its durability [21,22]. The sol–gel bond provides the castable with its strength and durability under high-temperature conditions. The mineralogical composition of the sol–gel bond can affect the properties of the castable in a number of ways, including impacting its thermal expansion coefficient, thermal conductivity, and corrosion resistance [23].One key factor that can affect the durability of refractory castable is the presence of alumina in the sol–gel bond. Alumina is a highly refractory mineral that is able to withstand very high temperatures, and it is often used in refractory castable to enhance its thermal performance. However, if the concentration of alumina in the sol–gel bond is too high, it can lead to the formation of microcracks in the concrete, which can deteriorate the material properties and reduce its durability over time [24].Another important mineral that can influence the durability of refractory concrete is silica. Silica, in the form of silica sol, is added to the sol–gel bond. Amorphous nanoparticles can react with alumina particles at elevated temperatures to form mullite, which provides excellent heat properties of the castable [25,26] and it can be obtained by several techniques [27,28,29,30]. Various types of colloidal silica sols, alumina, mullite, spinel, etc. can be used in the preparation of sol–gel bonded refractory castable. Various ions can be used to stabilize these sols [31].In our research, a silica sol stabilized with Na+ cations was selected. Al2O3 microparticles were added to the mixture as microfillers. Alumina microparticles with different mineralogical and chemical compositions can be used for research. The composition, properties and behavior of

Density functional modeling of the binding energies between aluminosilicate oligomers and different metal cations — Kai Gong et al., 2023

Interactions between negatively charged aluminosilicate species and positively charged metal cations are critical to many important engineering processes and applications, including sustainable cements and aluminosilicate glasses. In an effort to probe these interactions, here we have calculated the pair-wise interaction energies (i.e., binding energies) between aluminosilicate dimer/trimer and 17 different metal cations Mn+ (Mn+ = Li+, Na+, K+, Cu+, Cu2+, Co2+, Zn2+, Ni2+, Mg2+, Ca2+, Ti2+, Fe2+, Fe3+, Co3+, Cr3+, Ti4+ and Cr6+) using a density functional theory (DFT) approach. Analysis of the DFT-optimized structural representations for the clusters (dimer/trimer + Mn+) shows that their structural attributes (e.g., interatomic distances) are generally consistent with literature observations on aluminosilicate glasses. The DFT-derived binding energies are seen to vary considerably depending on the type of cations (i.e., charge and ionic radii) and aluminosilicate species (i.e., dimer or trimer). A survey of the literature reveals that the difference in the calculated binding energies between different Mn+ can be used to explain many literature observations associated with the impact of metal cations on materials properties (e.g., glass corrosion, mineral dissolution, and ionic transport). Analysis of all the DFT-derived binding energies reveals that the correlation between these energy values and the ionic potential and field strength of the metal cations are well captured by 2nd order polynomial functions (R2 values of 0.99-1.00 are achieved for regressions). Given that the ionic potential and field strength of a given metal cation can be readily estimated using well-tabulated ionic radii available in the literature, these simple polynomial functions would enable rapid estimation of the binding energies of a much wider range of cations with the aluminosilicate dimer/trimer, providing guidance on the design and optimization of sustainable cements and aluminosilicate glasses and their associated applications.
1 Introduction
The interactions between aluminosilicates and metal cations are important for many engineering processes and applications. One example is the formation of cementitious materials, including alkali-activated materials (AAMs) and

blended cements, which bind aggregates together to form concrete. AAM is an important sustainable material technology that is able to convert a solid precursor source (e.g., industrial wastes and calcined clay rich in amorphous aluminosilicates) to a cementitious binder [1]. The final AAM binder has many potential applications, including being used as a low-CO2 cement alternative to Portland cement (PC) [2, 3], whose production worldwide is currently responsible for ~8% of global anthropogenic CO2 emissions [4]. For geopolymers, i.e., AAMs based on low-Ca precursors (e.g., metakaolin and class F fly ash), the main binder gel responsible for most of its engineering properties is an amorphous three-dimensional alkali-alumino-silicate-hydrate (NA-S(-H), when Na is the alkali) gel mainly consisting of Q4(mAl) (m = 0, 1, 2, 3, 4) for the silica units [3]. Alkali cations, for example, Na+ and K+, charge-balance the negatively charged alumina tetrahedra (Al(O1/2)4)–1, thereby stabilizing the aluminosilicate network. This chargebalancing interaction (interaction between negatively charged alumina tetrahedra and metal cations) helps to hinder ionic transport in calcium-alumino-silicate-hydrate (C-(A)-S-H, compared with calcium-silicate-hydrate (C-S-H)) gel [5], stabilize zeolite framework structures [6] and reduce alkali leaching from geopolymer binders [3]. Furthermore, this interaction in geopolymers has been used to immobilize heavy metals [7, 8] and treat wastewater [9]. On the other hand, excess alkali metal cations beyond those needed for chargebalancing act as modifiers to depolymerize the aluminosilicate network structure, as has been shown recently for sodium-substituted calcium-alumino-silicate-hydrate (C-(N)-A-S-H) [10].
The negatively charged aluminosilicate network can also be charge-balanced by alkaline earth metal cations (e.g., Ca2+ and M

g2+). In addition to charge-balancing, these alkaline earth metal cations, beyond those required for charge balancing, are also effective network modifiers, causing the aluminosilicate gel network to depolymerize. The resulting binder gels (e.g., C-AS-H or magnesium-alumino-silicate-hydrate (M-A-S-H)) possess different atomic structures (mainly short aluminosilicate chain structure (Q2) for C-(A)-S-H [11] and plane structure (Q3) for M-A-S-H [12]), pore structures [13-16], mechanical properties [17], transport properties [14, 15, 18, 19] and chemical stability [15, 18]), compared with the three-dimensional N-A-S(H) gel.
Another important example where the interactions between metal cations and aluminosilicates are critical is aluminosilicate glass containing alkali and/or alkaline earth metal cations. Aluminosilicate glasses are ubiquitous in many important industrial applications, including nuclear waste encapsulation, high-performance glasses, ceramics, metallurgical processes, and sustainable cement [20-23]. The metal cations in these aluminosilicate glasses play two distinct roles: (i) to charge-balance the negatively charged alumina tetrahedra (i.e., (Al(O1/2)4)–1), and (ii) to depolymerize the aluminosilicate network creating non-bridging oxygen (NBO) atoms (i.e., oxygen atoms bonded to only one silica or alumina tetrahedra). Many studies have shown that the type of alkali and alkaline earth metal cations has a dramatic impact on the resulting glasses, affecting both the atomic structure [22, 24-27] and engineering properties (e.g., physical [28, 29], mechanical [28, 30], thermal [28, 31] and chemical properties [32-34]). Furthermore, the type of metal cations also has a significant impact on the dissolution of silicate minerals and glasses [22, 35], which is critical to soil fertility, transport and sequestration of contaminants, and global geochemical cycle (including the CO2 cycle) [35-37].
However, fundamental studies on the atomic scale

Cadmium and lead ions adsorption on magnetite, silica, alumina, and cellulosic materials — Surjani Wonorahardjo et al., 2023

Introduction In pragmatic modern science, the search for functional materials to solve various issues rises. Many new materials have been developed for special applications, separation processes, and controlling compound release, such as fertilizer or gas release agents 1 . New concepts for balancing environmental issues have increased interest in “green” materials and then green chemistry 2 . More physical and chemical aspects have been under investigation. Thus, the synthesis of novel materials and analysis of aspects related to environmental pollution and the impacts of chemical processes have been widely investigated 3 . Analytical chemistry and chemistry education fields have become increasingly concerned about these issues. Promoting a better understanding of our environment 4 is a new task. For this reason, ethics must be introduced to induce comprehension of natural equilibrium 2 from a chemistry point of view to a broader scale of nature. Silica and alumina materials are commonly used as adsorbents or separation agents for organic or inorganic compounds, including heavy metals. Bio-silica from plants or biomass like rice husk or dry rice straw is popular since it possesses high purity through sol–gel processing. Some applications are listed in Table 1 below. There are many adsorption stories from materials obtained, some from previous work. Table 1 Some silica/alumina/cellulosic materials and their applications. Full size table Many scientists have widely investigated adsorption phenomena. Usually, when a particular ion behavior is investigated, the other ions and their surroundings, including anions, solvent molecules, and other particles in the system, are not counted. Only some reports have considered the contribution of other ions during an analysis of the adsorption behavior of metal ions or larger molecules 19 , 20 . Several reviews of the adsorption isotherms have been provided from many perspectives, including Zhang, Liu and Jiang for mesoporous silica materials 21 , Baccar et al. 22 for activated carbon from agro-waste, and Karthikeyan et al. 23 for sawdust activated carbon. These papers also explained the adsorption phenomena in different materials, including synthetic and magnetic materials, with some theoretical consideration. Many experimental variations have been performed to study the properties of adsorption processes under many circumstances and test the matrices’ applications. Attempts to utilize agro-waste and biomass for environmental recovery by utilizing surface phenomena have been reported 24 , 25 , 26 . The same analytical methods to reveal surface potentials were discussed 27 . Cellulose acetate-polyan

iline membranes 28 and magnetic nano-adsorbent 29 were discussed as the surface part of the materials was the critical point. On the other hand, heavy metals have been a significant concern for environmentalists. Heavy metals in soil, rivers, ponds, or oceans can be indicators of environmental problems related to human activities. Cadmium is a commonly used heavy metal in many types of industrial wastes. Thus, its removal is considered mainly in wastewater treatment strategies using various adsorbents 30 and 31 . Reducing cadmium pollution by adsorption on active surfaces has been investigated as some novel separating agents were developed 32 . More sophisticated materials and composites of cellulose were invented for environmental remediation 33 , metal remover 34 and water treatment 35 . The most critical aspect of active surface adsorption is the preparation of the surface. Based on the nature of the target pollutant, surfaces can be modified for improved entrapment, adsorption, and suitable retainment properties. A promising adsorbent can be reused too. Some other efforts to reduce cadmium, cations, anions, and other pollutant molecules, like dyes 32 used magnetic adsorbents. Several magnetic materials 36 and 37 have been developed to remove metals by interacting with modified surfaces. Magnetite, Fe3O4, is a commonly used magnetic material for adsorption purposes. Other trials have used commercial silica or alumina as the main adsorbent due to their surfaces’ polar silanols and aluminols 38 . Other cadmium and heavy metal removal strategies have been reported using rice husk biomass like rice husk ash and its modifications 39 , 40 , 41 , barley hull and barley hull ash 42 . Alumina was used for a stationary phase in chromatography 43 , solid-state extractors 43 , 44 , and separation agents. These materials have different properties towards cadmium as well as lead ions. Herein, the adsorptions of cadmium ion (Cd 2+ ) by magnetite, silica, alumina, cellulose, and cellulose-acetate will be compared and discussed. Different types of surfaces were expected, giving different ways for cadmium and lead ions adsorption and desorption. The adsorbents were synthesized in the laboratory, and the adsorption patterns were examined indirectly via atomic absorption spectroscopy using the batch method. The adsorption depends on several parameters, including the tort

uosity of the materials (as modelled in Fig. 1 ), surface activity, nature, and width of the pores (pore size), pore wall, and solvents they all govern the interaction on the surface layer. These types of mechanisms are rarely discussed in describing absorption phenomena. When the adsorbates are small particles such as cadmium ions, particular dynamics occur as the surface does not firmly retain the particles. As has already been discussed regarding the microscale dynamics using relaxation and diffusion NMR 45 for fluid inside porous media 46 like silica materials, the interface or pore wall activity is crucial to adsorption behaviour. This has also been modelled with NMR methods in cement materials 47 . Different mechanisms of adsorbate interaction with the surface wall can be operative depending on the nature of both materials. An important mechanism was discovered, called reorientation mediated by translational displacement (RMTD), for water molecules in silica pores 48 , as shown in Fig. 2 . This mechanism was derived from relaxation NMR for a “clean surface” without other ions. However, other particles in this discussion may behave differently. Because of the ionic nature of cadmium ions, the surface interaction would be more robust by different polarity and dipole moments and the nature of the pores, tortuosity, and solvent molecules that facilitate the interaction of the adsorbate particles with the material. Figure 1 Schematic diagram showing the tortuosity of a porous media with narrow and wide irregular cavities. Full size image Figure 2 Description of complexity in the surface layer of an interface to model different particles near the surface. The coloured dots (arbitrarily) represent the different particles present and can represent metal ions, nitrate ions, or water molecules with hydrogen and hydroxyl ions. They are attached to the surface, detached, and adsorbed back with different-random orientations. Full size image As the pore system is formed (for example, during gelling from solution in sol–gel processing), the surface liquid is formed that differs from the bulk liquid in the pores. The dynamics of the surface liquid tend to be super-diffusive due to the fast exchange that occurs to maintain surface equilibrium. Thus, understanding the physical complexity of the surface liquid is vital for a clear description of the adsorption–desorption phenomena. Besides the above explanations, theoretical fitting using some isotherms, like Langmuir and Fre

Alumina and Silica Extraction and Byproduct Development Directly from Chemical Deashing of Coals — Lijun Zhao, 2022

1. IntroductionTraditionally, coals are mainly used as fuels for power generation, among many other utilizations [1,2]. With the global shift to a low-carbon world and the rapid development of new energy technologies, such as nuclear, wind, solar and water powers, coals as typical high-carbon fuels are expected to play a decreasing role, and new directions should be explored for the future development of the coal industry.Coals are mainly composed of carbon and inorganic minerals, which can be useful resources if separated. Ultraclean coals with the lowest amount of minerals can be used as oil substitutes, or hopefully as electrode materials in new energy technologies [1,2]. Typical mineral elements in coals include silicon, aluminum, calcium, iron and others, and the former two are dominant in most cases [3,4]. Coal ashes are contributed to by gangues mixed in coal mining, minerals carried in coal formation, and inorganic elements in coal-forming plants. Industrial physical deashing methods, such as jigging, dense media, and floatation, necessitate pretreatment of coals from large rocks to small particles, and can remove most of the gangues and some carried minerals. Inorganic elements in coal-forming plants may occur as the smallest minerals in order of 1 μm in coals, and can produce 1–2 wt% ashes or more after coal burning. Therefore, industrial physical methods can be limited by demineralization, due to carried minerals in coal formation and inorganic elements in coal-forming plants. To obtain ultraclean coals with 1 wt% ashes or less, chemical deashing can help by allowing chemicals to react with carried minerals and the smallest minerals in coals [3,4,5].By using submolten salts, an advanced deashing method has been developed [6,7]. This method treats the coals with submolten alkali solutions under the conditions of low temperatures and ambient pressures, followed by acid leaching. The method features mild treatments compared to conventional chemical methods [4,8], and can largely conserve the original structures of coals for advanced utilizations, such as electrode materials for new energy technologies, whose performance would be determined by structures [1,2]. Due to the large amount of chemicals used in deashing the coals, to eliminate secondary pollutions and reduce deashing cost, the chemicals should be recycled, and the minerals in alkali and

acid solutions recovered for the production of useful byproducts.High-ash coals are still used widely throughout the world, and the pulverized coals could have high ash of 40.8% in some power plants [9]. In this work, the method was applied to an alumina-rich coal with ash of 27.95% beneficiated by dense media, which was taken from one major coal producer of the Province of Inner Mongolia of China. The mineral elements of the coal are provided in Table 1 in the form of stable oxides, in which alumina accounts for over 51% of the total, rising to over 87% when including silica, and the rest less than 13%. The XRD diagram of alumina-rich coal is provided in Figure 1. The envelope at around 25° can be attributed to the carbon in coals, and kaolinite (PDF #782110) and boehmite (PDF #832384) can be identified as the dominant minerals [9], which give rise to the rich alumina and poor silica in coals. In this work, ultraclean coals were successfully prepared by applying the advanced deashing method to the alumina-rich coals, the deashing chemicals were regenerated and the mineral elements were recovered to produce high-purity alumina and silicate fertilizer. 2. Materials and MethodsIn chemical deashing of the coals, alkali and acid solutions were prepared. Chemical regenerations were made, and byproducts were developed. The schematic diagram of the process is provided in Figure 2, and will be described in the following sections. 2.1. Deashing Chemicals and Chemical AnalysisAlkali (NaOH) of analytic grade, concentrated H2SO4 of 98% and HCl of 37% were used or diluted as required by deionized water. The CaO in this work was prepared by the calcination of Ca(OH)2 of analytic grade at 850 °C for 180 min. The ash contents of coals were measured by proximate analyzer (5E-MAG6700II, CKIC, Changsha, China). XRF (ZSX Primus II, Rigaku, Tokyo, Japan) was used for elemental analysis (Be-U) of solid powders, which were calcinated in advance at high temperatures to eliminate element C or hydrates. ICP-OES (Spectro Arcos, Spectro

, Kleve, Germany) was used for elemental analysis of alkali and acid solutions, and XRD (D8 Advance, Bruker, Berlin, Germany) was used for phase characterizations of samples. The active silica in silicate fertilizers were measured by the silica soluble in 0.5 M HCl solutions divided by the total of the silicate fertilizer.Elements in terms of oxides can directly show how much alumina or silica were dissolved in filtrates. Since the concentrations of elements in filtrates are too high for ICP measurements, weight dilutions were precisely adopted instead of volume dilutions with possible filtrate residues during pipette transfers. Therefore, the concentrations of elements are usually given in terms of oxides with reference to the weights of filtrates. 2.2. Preparation of Alkali Solutions and Acid Solutions in Deashing the CoalsThe chemical method was applied to the coals with high ash of 27.95% given in Table 1, which was crushed to below 1 mm for deashing the coals. An alkali-to-coal ratio of 1.75 was used for the alkali treatment with little water at a temperature of 150 °C for 180 min. More water was added to leach excessive alkali and soluble ash after the reactions, and the alkali solutions were thus made, in which the concentrations of alumina (Al2O3), silica (SiO2) and Na2O were 6.63 g/kg, 1.09 g/kg and 179.82 g/kg, respectively.The chemical compositions of the ash of coal cake after alkali treatment was 28.36%, slightly higher than raw coal (27.95%). According to the chemical compositions in Table 1, alumina and silica in coals were partially dissolved in the alkali solutions, and the remainders occurred as the ash with Na2O. Subsequently, the alkali-treated coal was soaked with HCl solution of 10% at an acid-to-coal ratio of 1.5 for 60 min. The filtrates after separation were the acid solutions, the filter cake was the ultraclean coal with low ash of 0.62%, and the compositions of the ash (0.62%) are given in Table 1.By referring to the ash of the coal cake after alkali treatment in Table 1, it can be seen that almost all mineral elements in