The Capture and Catalytic Conversion of CO2 by Dendritic Mesoporous Silica‐Based Nanoparticles

Dendritic mesoporous silica nanoparticles own three‐dimensional center‐radial channels and hierarchical pores, which endows themselves with super‐high specific surface area, extremely large pore volumes, especially accessible internal spaces, and so forth. Dissimilar guest species (such as organic groups or metal nanoparticles) could be readily decorated onto the interfaces of the channels and pores, realizing the functionalization of dendritic mesoporous silica nanoparticles for targeted applications. As adsorbents and catalysts, dendritic mesoporous silica nanoparticles‐based materials have experienced nonignorable development in CO2 capture and catalytic conversion. This comprehensive review provides a critical survey on this pregnant subject, summarizing the designed construction of novel dendritic mesoporous silica nanoparticles‐based materials, the involved chemical reactions (such as CO2 methanation, dry reforming of CH4), the value‐added chemicals from CO2 (such as cyclic carbonates, 2‐oxazolidinones, quinazoline‐2,4(1H,3H)‐diones), and so on. The adsorptive and catalytic performances have been compared with traditional silica mesoporous materials (such as SBA‐15 or MCM‐41), and the corresponding reaction mechanisms have been thoroughly revealed. It is sincerely expected that the in‐depth discussion could give materials scientists certain inspiration to design brand‐new dendritic mesoporous silica nanoparticles‐based materials with superior capabilities towards CO2 capture, utilization, and storage.

Dendritic mesoporous silica nanoparticles own three-dimensional centerradial channels and hierarchical pores, which endows themselves with superhigh specific surface area, extremely large pore volumes, especially accessible internal spaces, and so forth.Dissimilar guest species (such as organic groups or metal nanoparticles) could be readily decorated onto the interfaces of the channels and pores, realizing the functionalization of dendritic mesoporous silica nanoparticles for targeted applications.As adsorbents and catalysts, dendritic mesoporous silica nanoparticles-based materials have experienced nonignorable development in CO 2 capture and catalytic conversion.This comprehensive review provides a critical survey on this pregnant subject, summarizing the designed construction of novel dendritic mesoporous silica nanoparticles-based materials, the involved chemical reactions (such as CO 2 methanation, dry reforming of CH 4 ), the value-added chemicals from CO 2 (such as cyclic carbonates, 2-oxazolidinones, quinazoline-2,4(1H,3H)-diones), and so on.The adsorptive and catalytic performances have been compared with traditional silica mesoporous materials (such as SBA-15 or MCM-41), and the corresponding reaction mechanisms have been thoroughly revealed.It is sincerely expected that the in-depth discussion could give materials scientists certain inspiration to design brand-new dendritic mesoporous silica nanoparticles-based materials with superior capabilities towards CO 2 capture, utilization, and storage.
their shapes are irregular and their sizes can even reach as high as 33.7 nm (not the maximum), while in TEM images (Figure 1d-f), conical center-radial channels bring about internal pores.As indicated by the model in Figure 1f, it is easy to conclude that the size of each channel gradually decreases from the outside to the inside (e.g., ≤33.7 nm).These two kinds of pores can be directly identified by visual observation.[24] Earlier investigators attributed this to tiny parts of the conical center-radial channels; however, Tallarek et al. [25] and our team [26] have testified that mesopores inserted in the inter-walls (Figure 1g) are the actual components.The above three types of pores give rise to ultra-high pore volume and specific surface area that provide the affluent space for CO 2 loading.

Easily Functionalized Interfaces
For the chemical modification of silica mesoporous materials, there generally exist two approaches, being one-step co-condensation (one-pot) and two-step postgrafting (stepby-step) methods. [27,28]The functionalization of DMSNs interfaces naturally fall into the latter.[31][32] As a result, DMSNs can be easily modified by organo-silanes with desirable functional groups.[35] In particular, poly-amino groups (multiamino groups) could be introduced by step-by-step modifications.For instance, (3-glycidyloxypropyl) trimethoxysilane (GTMS) was first anchored onto DMSNs, and then tetraethylenepentamine (TEPA) covalently bonded with the subsistent epoxy groups, giving rise to poly-amino groups for CO 2 trapping. [33]On the other hand, other kind of silanes have been grafted onto DMSNs as the "linkers" to import active sites for CO 2 catalytic conversion.For example, 3-chloropropyltriethoxysilane (CPTES) and 1methylimidazole were successively anchored onto DMSNs, leading to catalyst units of ionic liquid (IL) for the synthesis of cyclic carbonate from CO 2 and epoxides. [36]

Feasible Doping with Other Elements
Directly mixing other elements during synthesis processes could also endow DMSNs with novel properties (one-step co-condensation).For example, Guo et al. dispersed vanadium (V) in the skeleton of DMSNs and the hybrid vanadium-doped composites could serve as excellent catalyst for the oxidative dehydrogenation of the propane (ODHP) reaction in the presence of CO 2 . [37]

Versatile Microemulsion as the Mother Liquor for the Synthesis of DMSNs-Based Materials
Adding a moderate amount of other species into the microemulsion utilized for DMSNs synthesis can yield a novel composite, in which these species are surrounded by dendritic silica skeletons.For example, the center-radial core-shell materials could be synthesized with the introduced substances as cores and dendritic SiO 2 as shells, such as zeolite-based TS-1@DMSNs, [38] Fe 3 O 4 @DMSNs, [39,40] and so forth.As long as the size is small enough, the extraneous specimen can serve as the building block of DMSNs framework.For instance, Jalil et al. integrated β-zeolite (BEA) of ~20 nm into the mother liquor and a cockscomb-like nanosphere with a size of ~450 nm could be obtained. [41]It should be mentioned that because BEA was encircled by SiO 2 networks instead of being anchored on the surface, we remark this . SEM (a-c) and TEM (d-f) images of a DMSNs.g) The physical reconstruction of a DMSNs with inter-wall mesopores of different sizes.Reproduced with permission. [25]Copyright 2021, Elsevier.
It should be mentioned that, for each reference, DMSNs' fundamental data have been listed individually because researchers might utilize different approaches to synthesize DMSNs with variant characters.As illustrated by previous reports, [16,17,45,46] aqueous phase system, biphase stratification system, and bicontinuous microemulsion phase could construct DMSNs with distinguishing morphologies and architectures, definitely resulting in diverse structural parameters.

Amino-Functionalized DMSNs
The capture capabilities of adsorbents can be significantly improved by the introduction of functional groups.Specifically, various amine groups possess high affinities to CO 2 .[49][50] Amino groups (-NH 2 ) can be decorated onto DMSNs with organosilane of (3-aminopropyl)triethoxysilane (APTES) (Figure 2b 1 ).Even though high loadings of the organic moieties can be achieved, there exist many drawbacks such as the damaged longevity of the materials, the degraded original topology and properties, the utilization of toxic solvents, the high preparation cost, and multistep operations.On the basis of the above considerations, Polshettiwar and Bhanage et al. functionalized DMSNs by an ammonolysis modification method under a flow of ammonia (NH 3 ) gas at a high temperature (Figure 2b 2 ). [51]he as-obtained DMSNs-N700 exhibited numerous advantages over conventional amine-grafted DMSNs-APTES (Table S1, Supporting Information), including 1) better CO 2 capture capacity, 2) faster kinetics, 3) easier regeneration and more efficient reuse, 4) higher thermal stability, and so forth.Lai's team functionalized DMSNs with TEPA by a wet impregnation method (Figure 2c). [52]DMSNs-TEPA platform merely adsorbed CO 2 at room temperature in the presence of N 2 , showing a high selectivity.As the adsorbent temperature increased from 30 °C to 75 °C, the capacity of CO 2 capture also boosted from ~3.2 to 4.5 mmol g −1 (Table S1, Supporting Information).Even though the adsorptive property was superior over DMSNs-N700 due to more active sites of amino groups in TEPA, 85% mass lose was encountered after nine cycles because of the weak linkages between DMSNs and TESPA via the probable hydrogen bonds (Figure 2c).
Polshettiwar et al. elaborately compared the adsorptive abilities of various amino-functionalized DMSNs which were modified by the covalent linkages (Figure 2d) or the physical adsorption (ads.). [33,34]he applied amine compounds are illustrated in Figure 2e, being TEPA, polyethylenimine (PEI) with low or high molecular weights (PEI-LMW or PEI-HMW), APTES, N-(3-(trimethoxysilyl)propyl) ethane-1,2-diamine (PEDATS), and N-(3-trimethoxysilylpropyl) diethylenetriamine) (PDETATS).Six samples with covalent bonds and three ones with physical absorption were contrasted as shown in Table S1, Supporting Information.It is clear that the capture capability of the sample with covalent linkages was inferior to that of the one with physisorption, due to the low N content.This can be testified by their S BET and V p values in Table S1, Supporting Information.More TEPA molecules filled the pores in DMSNs, and the resultant S BET plus V p greatly decreased.Nevertheless, two aspects should be emphasized as follows.1) Physically adsorbed materials are apt to lose TEPA during the practical adsorption, as ascertained by 19.5% loss of TEPA after 21 cycles [33] and 14% loss after nine cycles. [52]However, their adsorptive properties were remained even after several recycling owing to their enormous amounts of TEPA.2) In experimental or practical application conditions, ambient temperatures that these adsorbing materials situate do not exceed 70 °C.Bonding strength is not the primary influence factor in this situation.For instance, the ambient humidity and the adsorbent temperature strongly impact CO 2 capture. [33,34,52]ery recently, Jana et al. functionalized DMSNs with different amines (TEPA, TAEA, and PEI) through the wet impregnation method to solid adsorbents for CO 2 uptake. [35]The capture ability declined with the order of DMSNs-TEPA > DMSNs-TAEA > DMSNs-PEI (Table S1, Supporting Information).The adsorption isotherms showed that pristine DMSNs captured CO 2 only via a physisorption process, while amino-functionalized DMSNs participated in the chemisorption of CO 2 .The uptake capacities of the three materials mainly depended on the type of amine moieties, considering that the affinity of an amine compound to CO 2 follows the order of primary > secondary > tertiary amine. [53,54]Their adsorption kinetics followed Avrami's fractional order kinetic model, implying that the adsorption processes were governed by multiple reaction pathways composed of physisorption and chemisorption.The subsequent de-protonation by another amine could produce alkylammonium carbamate species under dry conditions. [55]ery recently, Bahadur's team developed novel composite microgranules composed of DMSNs and PEI by an evaporation-induced assembly approach, where PEI incorporated into the pores of DMSNs or onto their surfaces. [56]Small and large microgranules were fabricated with different porosity and interconnectivity, giving rise to significantly improved CO 2 adsorption capacity and kinetics for DMSNs-PEI (Table S2, Supporting Information).Most importantly, in situ neutron scattering as a nondestructive technique was applied for the first time to explore the kinetics of CO 2 adsorption.This technique benefits from the penetration of neutrons, and its scattering length between neutrons and atoms varies to a good approximation randomly with atomic number and has been adopted as a powerful tool to mechanism investigations. [57,58]It is proved that the fast kinetics of CO 2 adsorption was ascribed to the excellent connectivity between macropores and mesopores in the adsorbent here.

DMSNs Modified with Metal Oxides
Without amine moieties inside, bare DMSNs exhibited weak physical interactions with CO 2 , low adsorption capacity, and inferior selectivity (Table S1, Supporting Information).There exist many drawbacks in the aforementioned amine-decorated absorbents as well, such as the low operating temperature, the degradation of amine moieties, amine volatilizationbased pollution, and so forth. [59,60]In order to minimize or eliminate these limitations, metal oxides have been selected as the modifiers in view of their high alkaline active sites which can greatly enhance the CO 2 adsorption capacity and the selectivity of DMSNs-based absorbents. [61,62]hang et al. loaded MgO onto DMSNs by a simple wet impregnation-calcination method (Figure 3a). [61]The as-prepared material possessed an ideal CO 2 adsorption capacity of 1.7 mmol g −1 at a 200 °C intermediatetemperature with the excellent cyclic performances (Table S3, Supporting Information), which was applicable to precombustion CO 2 uptake in integrated gasification combined cycle (IGCC).Teh et al. fabricated DMSNs-CaO, DMSNs-MgO, and DMSNs-CeO 2 by an ultrasonic assisted impregnation method to explore the role of metal oxides on CO 2 adsorption performances. [62]DMSNs-CaO could achieve maximum physical absorption in a flow of pure CO 2 gas at room temperature and one atmospheric pressure (Table S3, Supporting Information).DMSNs-MgO showed maximum chemisorption with the help of their affluent basic active sites by the temperature programmed desorption (TPD) technique.Most importantly, in situ Fourier transform infrared (FT-IR) confirmed the emergence of carbonate species on the three DMSNs-based materials and proved the basic strength with the sequence of DMSNs-CaO > DMSNs-MgO > DMSNs-CeO 2 ≥ DMSNs, as supported by CO 2 -TPD valuation.

DMSNs-Based Composite Materials
Zeolites are aluminosilicate minerals with interconnected channels.Their specific properties of controllable crystal sizes, tunable acidities, high thermal stability, and shape selectivity make themselves extensively applied in industry.However, their microporous structural drawbacks, such as diffusion limitations and the low accessibility of active site, give rise to rapid catalyst deactivation. [63,64][67][68][69] Without regard to ZSM's catalytic performances, Teh's team loaded CuO onto dendritic hydro ZSM-5 (HZSM-5) nanospheres as novel adsorbents (Figure 3b), on the basis of that CuO has high ability to adsorb CO 2 by the formation of carbide metals (Cu-C). [65]The resultant DMSNs/ HZSM-5 and DMSNs/HZSM-5-CuO showed higher physisorption capacities than those of pure DMSNs-based materials decorated with metal oxides (Table S3, Supporting Information).With CuO content increased, the channels were gradually filled and the corresponding surface areas as well as pore volumes decreased successively (Figure 3b 2-5 and Table S3, Supporting Information), which lowered the CO 2 adsorption capacities.
The specific properties of carbon-based materials render themselves promising as CO 2 adsorbents at low temperature and moderate at partial high pressure.Furthermore, according to the principle of "like dissolves like," carbon-based materials should have a high affinity to CO 2 . [70,71]Polshettiwar et al. fabricated dendritic mesoporous carbon nanoparticles (DMCNs) by a hard template approach where DMSNs were first coated by phenol formaldehyde polymer and then the carbon composition was left by silica dissolution (Figure 3c 1 ). [72]MSNs with various architectures were utilized to generate DMCNs with different sizes, fiber density, surface area, pore volume, etc. (Figure 3c 2-6 and Table S4, Supporting Information).The difference  [65] Copyright 2020, Elsevier.c 1 ) Preparation diagram of DMCNs by a DMSNs hard template approach and c 2 -c 6 ) DMCNs absorbents with different sizes, fiber density, surface area, pore volume.Reproduced with permission. [72]Copyright 2018, John Wiley and Sons.d) The construction process of DMSNs-containing MMM and its gas pathway.
in CO 2 capture capacities depended on their textural properties and morphologies.DMCNs-5 owned the maximum CO 2 capture capacity of 1.47 mmol g −1 , whereas the others became much lower (0.47-0.97 mmol g −1 ).The highest surface area and the biggest pore volume could account for the best performances.It should also be noted that capture capacity of 1.47 mmol g −1 is far better than the bare DMSNs sorbents (1.06 mmol g −1 in Table S3, Supporting Information).

DMSNs-Based Membrane
Mixed matrix membrane (MMM), with rigid permeable or impermeable particles (e.g., zeolites, carbon molecular sieves, silica and carbon nanotubes) dispersed in their polymer-based frameworks, could greatly improve the separation properties of polymeric membranes. [73,74]Shao et al. designed a DMSNs-stimulated MMM towards superior CO 2 capture, as shown in Figure 3d. [75]In the presence of DMSNs as fillers, the CO 2 -philic membrane materials of poly(ethylene glycol)diacrylate (PEGDA) and poly(ethylene glycol monomethyl ether)acrylate (PEGMEA) were thoroughly mixed and cross-linked under the exposure of ultraviolet (UV) light.Polyethylene glycol dimethyl ether (PEGDME) was then impregnated into the as-prepared MMM by ultrasound assistance.The addition of DMSNs could greatly boost the permeability of MMM and maintain a high gas separation performance, owning to the unique radial fibrous structure that provided a relatively easy transmission path for gas molecules.With the addition of PEGDME-500, the CO 2 permeability could be highly increased and MMM's selectivity of CO 2 /N 2 could be finely maintained at the same level.Nevertheless, the selectivity of CO 2 /H 2 was significantly improved because mesopores with 2-4 nm sizes inside DMSNs allowed H 2 to diffuse more easily.The maximum CO 2 permeability reached up to 2281.1 Barrer, 361.7% higher than that of MMM (770.1 Barrer) in the absence of PEGDME-500 and 478.5% higher than that of original cross-linked PEO membrane (394.3Barrer) without DMSNs as fillers. [75]

The Catalytic Conversion of CO 2 by DMSNs-Based Materials
The center-radial nanochannels of DMSNs can be extraordinarily useful as the platforms for loading various nanoparticles (NPs) or species with diverse catalytic activities.The 3D superstructures with extremely high accessible surface areas and pore volumes can greatly enhance the accessibility of active sites, thus further improving the catalytic activities. [76,77][80][81][82][83] For instance, cyclic carbonates serve as electrolytes of lithium batteries, aprotic polar solvents in organic synthesis, degreasing agents, industrial raw materials, etc. [84] Quinazoline-2,4(1H, 3H-diones) are an important class of pharmaceutical intermediates. [85]Numerous heterogeneous and homogeneous catalysts have been developed for the conversion of CO 2 to these compounds, in pursuit of economical and environmentally friendly sustainable development.DMSNs-based catalysts have made tremendous progress in the catalytic field, [29][30][31]34,43,86,87] absolutely covering the catalytic conversion of CO 2 into high value-added chemical products. This review o longer interprets the details of organic reactions that DMSNs-based catalysts participate in, because the research approaches and contents are consistent largely.For example, 1) textural parameters of involved NPs have been compared, such as specific surface area, pore volume.2) Synthetic conditions have been discussed, such as the use of solvents, catalyst dosage, reaction temperature, reaction time, reaction pressure, catalytic efficiency, the recyclability, etc. 3) More importantly, reactive substrates with different substituent groups have been adopted to explore the catalyst's applicability.4) The plausible mechanisms have been proposed to reveal the catalytic nature.

The Synthesis of Cyclic Carbonates
Up to date, there are three ways (Schemes 1-3 in Figures 4 and 5) to synthesize cyclic carbonates from carbon dioxide by DMSNs-based catalysts as follows.1) The conversion of CO 2 to cyclic carbonates via epoxide substances (Scheme 1 in Figure 4).This reaction is 100% atom-economical.Three types of catalysts with DMSNs as supports have been designed, namely, imidazolium-and phosphotungstic heteropolyacid (HPW)-based ionic liquid (① DMSNs-IL/HPW), [36] dual methylimidazole-based dual ionic liquid (② DMSNs-DIL), [88] and sodium tripolyphosphate (STPP)-based ammonium salt (③ DMSNs-STPP), [44] which could be simply regarded as a special kind of low-cost ionic liquid.2) The synthesis of α-alkylidene cyclic carbonates by CO 2 and a propargyl alcohol of 2-methy-4-phenylbut-3-yn-2-ol by the catalyst ③ (Scheme 2 in Figure 4). [44]DMSNs-STPP displays catalytic multifunctionalities, except for the catalysis of CO 2 and epoxide substance (Scheme 1).3) Oxidation carboxylation of styrenes (a kind of olefins) with CO 2 in the presence of oxidants, as seen in Scheme 3 (Figure 5).Three types of catalysts with DMSNs as the supports have been constructed, being dual 1,4-diazabicyclo[2.2.2]octane-based ionic liquid (④ DABCO-based DMSNs-DIL), [89] a platinum complex (⑤ DMSNs-Pt (II)), [90] and a magnetic Pd-implanted Schiff base complex coupled with imidazolium-based ionic liquid (⑥ FeNi 3 @DMSNs-DIL/Pd(II)/ Salen). [91]It should be mentioned that a magnetic core was introduced before the subsequent organic modifications for FeNi 3 @DMSNs.This treatment is universal and can be feasible to generate other core-shelled magnetic DMSNs, as ascertained by scholars who manufactured Fe 3 O 4 @DMSNs. [92,93]The magnetic part could make the composite NPs recyclable more easily.All in all, magnetic core-shelled architectures combine merits of the cores and the outside functional shells for more extensive applications.
In the above six catalysts, it can be seen that cyclic carbonates could be prepared with the help of CO 2 by ionic liquids or noble metal complexes.For DMSNs-IL/HPW, researchers also studied the performances of other anion-substituted DMSNs-IL catalysts by replacing HPW with BF À 4 , PF À 6 , trifluoromethanesulfonate (OTf − ), etc.However, their catalytic activities were not as outstanding as that of HPW. [36]For methylimidazole-based DMSNs-DIL, the unit of IL was simply changed from a "first-order" state to a "second-order". [88]DMSNs-STPP exhibited multifunctionality towards cyclic carbonates with different synthetic routes, as illustrated in Schemes 1 and 2. [44] It is interesting to note that, very recently, Bhanage et al. utilized the aforementioned DMSNs-N700 as a robust and efficient catalytic system for chemical fixation of carbon dioxide to cyclic carbonates. [94]However, the addition of tetrabutylammonium iodide (TBAI) can only Energy Environ.Mater.2024, 7, e12593 proceed this reaction.DMSNs-N700 was the adsorbent rather than the catalyst here (Figure S1, Supporting Information).

The Synthesis of β-Oxopropylcarbamates
Up to now, two kinds of DMSNs-based catalysts can be utilized to synthesize β-oxopropylcarbamates from carbon dioxide (Scheme 9 in Figure 8).What they have in common is the introduction of ionic gel (IG), no matter onto DMSNs, [107] or onto FeNi 3 @DMSNs [108] substrates.Furthermore, the spatial IG-networks derived from spirulina and tripolyphosphate (TPP) could act as the cross-linkers and dispersion platforms without catalytic activity.Well-dispersed Ag(I) (⑮ DMSNs-IG-Ag(I)) ions or Pd NPs (⑯ FeNi 3 @DMSNs-IG-Pd) could not only be robustly anchored, but also fight against the severe aggregation and leaching during their catalysis processes.

The Syntheses of Other Organic Compounds
The aforementioned DMSNs/Sn-PrVO 4 composite could also catalyze CO 2 fixation to benzimidazolones (Scheme 11). [101]Actually, Sn species doped in DMSNs framework could work as photo-catalytic sites in comparison with the non-Sn modified one.Sadeghzadeh et al. further deposited CdSnO 3 onto the channels of DMSNs/Sn.The enhancement of Sn content in DMSNs/Sn-CdSnO 3 (⑲) contrasted to DMSNs/Sn-PrVO 4 made itself applicable for carbonizing o-phenylenediamine with CO 2 under UV-Vis irradiation (200 nm). [111]The yield of benzimidazolones under visible light was less than that under ultraviolet.Similarly, BaMnO 3 could decorate onto DMSNs/Sn and the resultant composite (⑳ DMSNs/Sn-BaMnO 3 ) could catalyze the N-formylation of amines by CO 2 hydrogenation (Scheme 12). [112]ollowing the same procedure DMSNs-IG in Scheme 9, Sadeghzadeh et al. constructed ionic gelation with sodium tripolyphosphate and spirulina supported on magnetic DMSNs.The as-obtained Fe 3 O 4 @DMSNs-IG (㉑) could accelerate the synthesis of N-[(2hydroxyethoxy)carbonyl]glycine from carbon dioxide, ethylene oxide, and α-amino acid (Scheme 13, Figure 9). [113]Unlike DMSNs-IG-Ag(I) and FeNi 3 @DMSNs-IG-Pd that incorporated catalytic noble metals, Fe 3 O 4 @DMSNs-IG could directly function towards this organic reaction.Just like the catalytic mechanism of DMSNs-STPP in Figure 4c, spirulina contained some amino acids and could offer amino groups (-NH 2 ), developing Lewis acidic sites (-NH þ 3 ). [44]ang et al. manufactured another type of cyclodextrin-based Au NPs catalysts for the interpolation of CO 2 into aryl alkynes followed by SN 2 coupling with allylic chlorides, [114] as exhibited in Scheme 14.Unlike the straightforward chemical bonds between DMSNs substrates and β-CD modifiers in the aforementioned DMSNs-CD-Cu(II), [99] a total different mechanism existed in DMSNs-CD-Au (㉒) herein.That is, cyclodextrins (CDs) could selectively bind different guest molecules with suitable shape and size to form inclusion complexes on the basis of their unique structures with hydrophile exterior and hydrophobic central cavity.Hydrophobic benzene ring with a smaller size could be comprised into the hydrophobic internal cavity with the size of ~0.47-0.53nm, giving rise to the host-guest complex. [115,116]Very lately, Chen et al. designed three kinds of polyoxomolybdate-incorporated DMSNs-based catalysts (Figure S2a, Supporting Information) to accelerate the reaction shown in Scheme 14. [117] Thereinto, DMSNs were modified by ILs with different alkyl groups (triethylamine, triphenylphosphine, and pyridine) and polyoxomolybdate [Mo 132 ] consecutively.Triethylamine-decorated DMSNs-IL-Mo 132 exhibited more excellent performances than the other two.Its plausible catalytic mechanism was proposed as displayed in Figure S2b, Supporting Information.
Wei et al. recently prepared a DMSNscellulose-Au (㉓) for the synthesis of thiazolidin-2-ones from arylamines, elemental sulfur (S 8 ), and CO 2 (Scheme 15). [118]The nature of the cellulose was a kind of polysaccharide which derived from glucoses.Thus, plenty of hydroxyl groups (-OH) could finely load and disperse Au NPs against its leaching and aggregation.Au NPs also might coordinate with 1,2,3-triazole moiety, forming triazole-Au (0) complexes. [119]Liu et al. designed a series of imidazole-based ionic liquids to anchor Ru (II) (㉔-㉖ in Figure S3, Supporting Information), and the resultant DMSNs-IL-Ru(II) owned high catalytic activity for αaminomethyl carboxylation of alkenes with CO 2 and amines (Scheme 16). [120]Very recently, Li et al. immobilized Ru NPs onto an IL-functionalized DMSNs heterogeneous catalyst (Figure S4, Supporting Information) for hydroformylation of alkenes with CO 2 and H 2 (Scheme 17). [121]Even though the catalytic activity and the recyclability of the as-prepared DMSNs-IL-Rh were satisfactory, the selectivity of the n-phenylpropyl and iso-phenylpropyl aldehyde was not probed into.

Discussion
The demonstrated 23 catalysts in Schemes 1-16 involve ionic liquids, ionic gels, noble metals, base metals, and composites of ionic liquids coupled with noble metals.They maintain the pristine architecture of DMSNs or possess the core-shelled structures.
As "green solvents," ILs could be designed by reasonably choosing the combinations of the cations and anions. [122,123]These changes can also alter their physical properties such as melting point, viscosity, solubility.As a result, the physicochemical performances can be tuned according to different reactions or processes.Taking DMSNs-IL/HPW [36] and urotropin-based DMSNs-IL [110] as examples, the substituted anions in their IL segments greatly affected the corresponding catalytic efficiencies.Ionic gels function as a novel class of stretchable materials where the ionic conducting liquid has been immobilized in a polymer matrix.IGs-based materials show extraordinary nature in recent years as electronic skin, energy storage devices, flexible displays, soft actuators, etc. [124] In a same way, the physicochemical performances can be controllable by adjusting the precursors, as illustrated in Fe 3 O 4 @DMSNs-IG. [113]Their networks with special affinity could load and disperse functional units, such as Ag (I) [107] and Pd [108] NPs in DMSNs-IG.Metal NPs have drawn tremendous attention due to their high efficiencies as heterogeneous catalysts in numerous liquid-phase catalytic processes.The catalytic activity of NPs greatly depends on the number of active atoms on the surface.Smaller NPs display more superior catalytic activities; however, their stabilities are confronted with a serious challenge.All in all, metal NPs in catalysts need higher loading, finer dispersion, and better resistance against leaching or aggregation during utilization process.That is the reason why researchers have adopted DMSNs as the supports, along with surface modifications (e.g., DMSNs-HPG-Au [109] ).Organometallic ionic complexes (OICs) comprise a nucleophilic portion of a halogen anion (X − ) and a Lewis acid metallic center, functioning as very impressive bifunctional catalysts, like FeNi 3 @DMSNs-DIL/Pd(II)/ Salen. [91]Suitable choice of metals and ligands is the key to success.DMSNs-STPP and DMSNs/Sn-PrVO 4 own catalytic multifunctionalities as observed in Schemes 1, 2, 5, 10 and Schemes 7, 11, respectively.These phenomena enlighten us to explore the nature of "killing two or more birds with one stone."The plausible catalytic mechanisms of some ILs- [88] and OICsbased [97] materials are represented in Figure S5, Supporting Information.Others can be found in the followings. [99,107,108,120]In summary, specifically selecting, rationally designing, and elaborately constructing DMSNs-based catalysts could successfully convert CO 2 into value-added organic compounds.

Dry Reforming of Methane
The dry reforming of methane (DRM) is deemed as a promising application for CO 2 conversion, because this reaction can efficiently convert two greenhouse gases (CO 2 and CH 4 ) into two synthesis gases (CO and H 2 ).As important building blocks, the syngas can ultimately produce a variety of value-added downstream products such as liquid fuels, lower olefins, wax, alcohols.DRM is a typical hightemperature reaction with high-energy input (>600 °C), as exhibited in Equation ( 1) from Table 1.Thus, high-temperature and lowpressure conditions are normally favorable.In general, side reactions might occur with time prolonged (Table 1).Two typical ones are CH 4 decomposition reaction (Equation 6in Table 1) and the reverse water gas shift (RWGS, Equation 4 in Table 1) reaction.The decomposition of CH 4 might even lead to severe coking.Due to these side reactions, the molar ratio of H 2 to CO, being about 1:1 theoretically in Equation (1), always deviates from this value. [43,125,126]129] Ni-based catalysts have been extensively adopted in DRM because of their specific selectivity, high activity, and low cost.Despite the environmental and economic advantages, Ni-based catalysts still face the big challenge of the fast deactivation which caused by the sintering of the catalysts and carbon deposition (coking) via side reactions at the high working temperatures.The sintering can be conquered by dispersing Ni as diffusely as possible, while the coking can be eliminated to the greatest degree by manipulating the size of Ni NPs below 5 nm during the long-time reaction. [43,125,130]Up to now, three types of Ni-containing catalysts with the dendritic topology have been explored for DRM as follows.owing to a surface spatial confinement effect. [43]Thus, the catalyst showed super-high catalytic performances close to its equilibrium conversion (76% conversion for CH 4 at 700 °C) and remained stable after 145 h time-on-stream at 700 °C without noticeable carbon deposition (Table S5, Supporting Information).As exhibited in Figure 10a, three factors constituted the surface spatial confinement, including 1) the steric effect kept active Ni NPs on DMSNs surfaces from migrating between layers, 2) Ni NPs inserted in the mesopores were also forbidden to be migrated and sintered, and 3) the reactants and products could readily diffuse into the mesopores on the layers for mass transfer.The anti-coking performance could be finely achieved when Ni NPs sizes were controlled below 5 nm.33] Zhao's team fabricated Ni-doped catalyst by the co-condensation approach (Figure 10b 1 ) and the as-prepared DMSNs/Ni could encapsulate small Ni NPs into the dendritic SiO 2 skeleton. [125]Greatly different from Ni NPs atop SiO 2 interface in DMSNs-Ni produced by the postgrafting method, the internally decorated DMSNs/Ni could catalyze DRM more effectively and sustain for much longer time up to 200 h (Table S5, Supporting Information).The authors found that Ni 55 clusters of ~3 nm and Ni NPs of ~18 nm were distributed throughout SiO 2 framework and external, respectively.The formation energies for different C clusters during coking were calculated on the Ni 55 cluster and Ni (111) surface, including C 1 , C 12 , C 20 , C 40 , and C 60 .The plausible existing configurations on these two interfaces could be accordingly confirmed (Figure 10b 2 ).Single C atom appeared on Ni 55 clusters because there existed a highest energy in C 12 , while C nanotubes formed on Ni (111) surface because of the continual decreasing energy from C 1 to C 60 .

DMSNs-Based Catalysts
Jalil et al. obtained the similar rule of catalytic abilities (Table S5, Supporting Information) by comparing Ni-decorated DMSNs-based samples of the co-condensation one (DMSNs/Ni), the postgrafting one (DMSNs-Ni), and conventional SiO 2 -supported one (SiO 2 /Ni). [134]The dispersity of Ni element could be directly observed with the help of FESEM mapping (Figure 10c).Ni NPs were more uniformly scattered on the surface of DMSNs/Ni, owing to their smaller particle size of ~11.6 nm, in contrast with DMSNs-Ni of ~12.3 nm Ni NPs.Furthermore, DMSNs frameworks functioned as the protective shield for Ni NPs sandwiched in the catalyst against carbon deposit.Only disordered amorphous carbon was found on DMSNs/Ni with negligible effects on catalyst stability.Apart from Ni, lanthanum (La) element was added into DMSNs skeleton by this group later through the in situ one-pot hydrothermal method. [135]The appearance of La 2 O 3 promoter stabilized Ni catalyst and increased the intensity of basic sites, improving the affinity to CO 2 chemisorption for the subsequent activation and conversion (Table S6, Supporting Information).
Very recently, Ge et al. highly dispersed Ni and alkaline promoters (MgO, CaO, and La 2 O 3 ) onto DMSNs by the impregnation method and the following H 2 reduction treatment for DMR (Figure 10d). [136]mong these catalysts, DMSNs-MgO/Ni exhibited stable conversions of CH 4 and CO 2 around 82% and 85% in 120 h (Table S7, Supporting Information).On the one hand, Mg 2+ ions enhanced the interaction between Ni 2+ and DMSNs substrates, leading to the high dispersion of active centers and strong "metal-support" interactions against the sintering and deactivation of Ni.On the other hand, the interface of "Ni&MgO" NPs (Figure 10d) not only boosted Ni 0 distribution and CH 4 cracking, but also activated CO 2 and eliminated carbon deposits.

Dendritic SBA-15-Based Catalysts
The performance and applicability of traditional silica mesoporous materials as the supports for metal-incorporated catalysts (e.g., SBA-15) have been impeded by the limitation in the accessibility of active sites, due to the obstruction of pores by the mass transfer of reactants, the sintering of active metals, and carbon deposition (coking). [137]Setiabudi et al. successfully transformed rod-typed SBA-15 into spherical shape with dendritic morphology (DMSNs/SBA-15) by employing microwave-assisted microemulsion system with the addition of SBA-15 seeds (Figure 10e). [138]In comparison to SBA-15, the updated DMSNs/SBA-15 was rendered with a superior accessibility to adsorption sites, a higher basicity (~86% increase), a higher acidity (~66% enhancement), more abundant siliceous frameworks, and a higher thermal stability (~19% increase).
Setiabudi's team modified DMSNs/SBA-15 with a dosage of 5 wt% Ni by an ultrasonic co-impregnation technique. [139]Compared with SBA-15-Ni of the same Ni loading, the as-obtained DMSNs/SBA-15-Ni catalyst possessed ameliorated catalytic activity, coking resistance, and high catalytic stability with no indication of deactivation after 30 h time-onstream (Table S8, Supporting Information).The superiority could be ascribed to the stronger metal-support interaction, finer Ni particle size, higher homogeneity of Ni dispersion, higher basicity, etc., which met requirements for restricting coke deposition and Ni sintering.The authors further investigated the effect of Ni loading on the catalytic performances of DMSNs/SBA-15-Ni catalyst (Table S8, Supporting Information). [140]It was found that lower loadings of Ni NPs (i.e., 3% and 5%) could not give a high metal-support interaction, while higher Ni loading of 15% could make coke deposition more easier.A 10 wt% Ni dopant was the most optimum loadings for DMSNs/SBA-15-Ni against the coking, thus enhancing the catalytic activity and stability.On the basis of this Ni content, optimal reaction conditions and yields were screened with parameters as follows: reaction temperature of 794.37 °C, weight hourly space velocity (WHSV) of 23 815.022 mL g −1 h −1 , CH 4 /CO 2 ratio of 1.199, CO 2 conversion of 95.67%, CH 4 conversion of 93.48%, and H 2 /CO ratio of 0.983. [141]The possible mechanism is illustrated in Figure 10f.DRM was initiated by the adsorption of the reactant gaseous CH 4 and CO 2 onto DMSNs/SBA-15-Ni surface.CO 2 could adsorb onto the oxygen vacancies of SiO 2 skeletons and then dissociate into CO + O. Active Ni sites could catalyze CH 4 cracking, giving rise to CH X and H species. Unidentate carbonates, bidentate carbonates, and linear carbonyls as the intermediate specimens could oxidize CH X into CO in the end.The remaining H atoms could react with each other to form H 2 .When X equals 0, coke deposition naturally took place. [141]It is interesting that the addition sequence of TEOS and SBA-15 seeds in the fabrication process could alter the shape of dendritic SBA-15, namely, rod-like DMSNs/SBA-15 with TEOS followed by SBA-15 seeds and spherical DMSNs/SBA-15 with SBA-15 seeds followed by TEOS.After Ni loading onto the two supports, spherical DMSNs/SBA-15-Ni still exhibited preferable catalytic capabilities in DMR. [142]

Dendritic ZSM-Based Catalysts
The aforementioned dendritic ZSM-based CO 2 adsorbents possessed potential catalytic ability owing to the existence of zeolites.Jalil et al. incorporated Ni and another kind of metal NPs (Mg, Ca, Ta, or Ga) onto DMSNs/ZSM-5 surfaces by an incipient wetness impregnation method. [67]As exhibited in Table S9, Supporting Information, homogenous distribution of surface acid-basic sites reduced the propensity of coke deposition.Bimetallic Ni/Ta-based catalyst (DMSNs/ZSM-5-Ni/Ta) possessed the highest CH 4 and CO 2 conversions with H 2 /CO ratio close to unity.The synergism of acid-basic and bimetallic sites amplified interactions between catalysts and reactants, thus impeding metal sintering and coking.The authors further explored the deactivation process of DMSNs/ZSM-5-based catalyst. [143]It was found that carbon on DMSNs/ZSM-5-Ni could quickly polymerized to whiskerlike species, which accelerated the driving force for agglomeration and coke formation.Ta species on DMSNs/ZSM-5-Ni/Ta could enhance the dissociation rate of reactants and accelerate carbon gasification, owing to a magnified Ni-support interaction and abundant oxygen species atop its surface.An optimum CH 4 conversion of 96.6% was realized with operating temperature of 784.15 °C, CO 2 : CH 4 ratio of 2.52, and WHSV of 33 760 mL g −1 h −1 by means of central composite design (CCD) on the basis of response surface methodology (RSM).What is more, this team probed into the effect of Ni-Ta ratio on the catalytic selectivity of DMR (Table S10, Supporting Information). [144]DMSNs/ZSM-5-Ni without Ta was more active than DMSNs/ZSM-5-Ta without Ni, but exhibited inferior stability.Ta species could prevent Ni NPs from sintering, facilitate the chemisorption of CO 2 to counteract coke deposit, and lower the reactants' activation energies.All in all, Ta-rich catalysts were not active enough, while low content of Ta could enhance catalytic performance.The addition of little Ta to DMSNs/ZSM-5-Ni catalyst could dilute Ni surface and limit the graphitization degree of carbon, which was a prerequisite for remarkable catalytic performances.
Similar to the fabrication processes of DMSNs/SBA-15 and DMSNs/ ZSM-5 where SBA-15 or ZSM-5 seeds were added into mother liquors of the micro-emulsions, commercial MFI (Mobil-type five) support was successfully surrounded by dendritic architectures.The catalytic abilities of the as-prepared DMSNs/MFI-Ni outweighed most of prevailing catalysts, but was not that excellent as DMSNs/ZSM-5-Ni. [145]

CO 2 Methanation
The reaction of CO 2 methanation (Table 1) is highly exothermic and needs high-performance catalysts to effectuate satisfactory rates and selectivity. [146,147]The synthetic efficiency of methanol is severely hampered by thermodynamics.50]

Pure DMSNs-Based Catalysts
Triwahyono et al. evaluated the catalytic performances of pure DMSNs, MCM-41, and SiO 2 materials for CO 2 methanation without the use of any metal NPs (Table S11, Supporting Information). [151]DMSNs owned more active sites of the basicity and oxygen vacancies than those of MCM-41 and SiO 2 .These specialities rendered DMSNs' activity fivefold higher than that of MCM-41, along with a 38.9% yield of CH 4 at 723 K. Linear carbonyl was revealed to be the main route for methane formation, because the catalytic activity increased with the presence of linear carbonyl.Further hydrogenation of these surface carbonyls could generate methane and water molecule. [151]As is known that amorphous silica involves three main defects (Figure 11a): being oxygendeficient centers (ODCs), nonbridging oxygen hole centers (NBOHCs, ≡Si-O⋅), and E'-centers (≡Si⋅). [151,152]Polshettiwar's team adopted a magnesiothermic reduction protocol to tune the type, concentration, and proximity of these defects. [153]The optimum concentrations of the three defect sites could work synergistically to activate CO 2 and dissociate hydrogen for satisfactory productivity and selectivity in CO 2 methanation.Activity loss in the defect-containing DMSNs catalysts was less significant and only air was required in its regeneration process.Unlike metal catalysts, their activity decreased greatly with time prolonged and hydrogen gas was required for their recovery.The best catalytic activity of defect-containing DMSNs could reach more than double the methane production rate (9569 μmol g −1 h −1 ) after eight regeneration cycles as compared to the initial one's (3810 μmol g −1 h −1 ).The catalyst kept stable for more than 200 h with a good formation rate and selectivity. [153]u et al. dispersed Ni NPs onto three kinds of supports (DMSNs, MCM-41, and commercial SiO 2 ) to investigate their capabilities for low-temperature CO 2 methanation. [154]DMSNs-Ni catalyst exhibited much higher low-temperature catalytic activities and sintering-proof property of Ni NPs than the other two (Table S11, Supporting Information), because the dendritic architecture could accommodate more uniform metallic Ni active sites.Kinetic study testified that the topology of the supports significantly influences the apparent activation energy of the reaction.The in situ diffused reflectance infrared Fourier transform spectroscopy (DRIFTS) revealed that monodentate HCOO − , monodentate CO 2À 3 , linear CO, and bridged CO were the intermediates and could be further hydrogenated to develop CH x species (Figure 11b).Figure 10.a) The three factors constituted the surface spatial confinement in DMSNs-Ni catalyst that prepared by the postgrafting method.Reproduced with permission. [43]Copyright 2019, American Chemical Society.b) Ni-doped catalyst of DMSNs/Ni with Ni NPs capsulate inside the SiO 2 skeleton by the cocondensation approach.Reproduced with permission. [125]Copyright 2018, Royal Society of Chemistry.c) The Ni dispersity of DMSNs/Ni, DMSNs-Ni, and conventional SiO 2 -supported SiO 2 /Ni.Reproduced with permission. [134]Copyright 2020, Elsevier.d) Schematic illustration of DMSNs-MgO/Ni synthesis and reaction mechanism for the DRM reaction.Reproduced with permission. [136]Copyright 2022, Tsinghua University Press.e 1,2 ) TEM images of SBA-15, e 3,4 ) TEM images of DMSNs/SBA-15 with dendritic morphology.Reproduced with permission. [138]Copyright 2020, Elsevier.f) The possible mechanism of DMSNs/ SBA-15-Ni in DRM.
Energy Environ.Mater.2024, 7, e12593 Jalil et al. loaded different metal NPs of Ni, Co, and Zn onto DMSNs for CO 2 methanation. [155]DMSNs-Ni possessed the highest CO 2 conversion, CH 4 formation rate, and turnover frequency (TOF), which were higher than that of DMSNs-Co, followed by DMSNs-Zn, and finally the parent DMSNs.However, the three DMSNs-based catalysts exhibited difference mechanism pathways originated from the roles of different metal NPs, as testified by in situ IR spectroscopy (Figure 11c).It was discovered that DMSNs-Ni and DMSNs-Co followed a dissociative mechanism where CO 2 molecule had been dissociated on metal surfaces before migrating onto the catalysts' (DMSNs') surface.DMSNs-Zn followed an associative methanation pathway in which a hydrogen molecule interacted with an O atom in CO 2 . [155]This group further fabricated V 2 O 5 -promoted DMSNs-Ni by the incipient wetness impregnation where vanadium and nickel precursors were co-added in the procedure of postmodification. [156]V 2 O 5 could not only enhance the basicity of catalyst, but also provide additional adsorption sites of CO 2 due to its amphoteric property, developing more reactive unidentate species (Table S11, Supporting Information).In contrast to DMSNs-Ni, the as-prepared DMSNs-Ni/V 2 O 5 could be more active at a lower temperature.The light-off temperatures for the above two catalysts were 423 and 473 K, respectively.DMSNs-Ni/V 2 O 5 reached up to 94.4% CO 2 conversion at 623 K, ~15% higher than that of DMSNs-Ni. [156]

Other Catalysts with the Dendritic Topology
Setiabudi et al. compared the catalytic performances of DMSNs/SBA-15-Ni and SBA-15-Ni. [157]The former had more excellent catalytic capability, higher stability, and coke resistance ability than that of the latter (Table S12, Supporting Information), due to the higher homogeneity of finer Ni NPs.Even though both the two catalysts proceeded via a CO 2 dissociation pathway, three intermediate species of linear carbonyl, unidentate, and bidentate carbonates could be found in DMSNs/SBA-15-Ni, while only bidentate carbonates could be found in SBA-15-Ni. [157]Jalil's squad implanted another two types of zeolites, that is, mordenite (MOR) [158] and BEA, [159] into DMSNs skeletons.The as-obtained metal-free DMSNs/MOR and DMSNs/BEA could be utilized for CO 2 methanation because of the catalytic nature of zeolites.DMSNs/MOR and DMSNs/BEA exhibited higher CH 4 selectivity, CO 2 conversion, thermal stability, and coke-resistance compared to commercial MOR or BEA (Table S12, Supporting Information).Carbonyls and linear CO 2 * as the intermediates were produced through an associative mechanism over DMSNs/MOR. [158]xperimental results were highly consistent with thermodynamic observations at high temperature for DMSNs/BEA. [159]This group also embedded Al 2 O 3 seeds into DMSNs and then loaded Rh NPs into the channels (Figure 11d). [160]The best CH 4 formation rate (16.1 × 10 4 mol g cat −1 s −1 ) of the as-designed DMSNs/Al 2 O 3 -Rh could be reached as Rh content equaled 1%, while the formation rate of DMSNs/Al 2 O 3 only came to 4.26 × 10 4 mol g cat −1 s −1 .The rate of CO hydrogenation was superior to that of RWGS reaction over DMSNs/Al 2 O 3 -Rh.Besides, graphitic carbon could not be detected on the spent catalyst, suggesting that most active sites were preserved because of enhanced CO 2 adsorption, which effectively removed amorphous carbon through CO 2 gasification process. [160]ery recently, Jalil et al. chemically modified DMSNs/Ni with boron (B) element through the wet impregnation method by immersing the co-condensed DMSNs/Ni into boric acid solution. [161]The as-prepared DMSNs/Ni-B (Figure 11e) could catalyze CO 2 hydrogenation to methane more efficiently than DMSNs/Ni (Table S12, Supporting Information).The role of B in the reaction is proposed in Figure 11e.The well-dispersed boron on catalyst's surface could enhance CO 2 adsorption and further accelerate the formation of such bridged and/or formats, hydrogen carbonate, as well as chelated bidentate carbonates, when reacted with the hydrogen atoms that split from nickel surfaces.The dehydration of these intermediates could generate C-H bonded species, and the subsequent serial hydrogenation could yield final methane.
Lately, Liu's team synthesized a type of yolk-shelled Ni-decorated composite catalyst with DMSNs as the core and Ni-phyllosilicate as the shell by a simple hydrothermal treatment (Figure 11f). [162]The specific radial channels of DMSNs could accelerate the mass transfer and accessibility of Ni(NO 3 ) 2 for the simultaneous growth of Ni-phyllosilicate both on the external and in the internal surfaces at the initial stage.The rapid formation of the outside Ni-phyllosilicate shell inhibited the growth of the internal in the following hydrothermal reaction, leading to Niphyllosilicate as the shell and unreacted DMSNs as the yolk (DMSNs@hollow@Ni).The hollow chamber could be further expanded by increasing the reaction time.The authors prepared DMSNs-Ni as the reference sample by the impregnated method, where DMSNs were just dissolved in Ni(NO 3 ) 2 solution without heating.The CO 2 conversion, CH 4 yield, and CH 4 selectivity of DMSNs@hollow@Ni were 70%, 61%, and 90% at 450 °C under 0.1 MPa with WHSV of 60 000 mL g cat −1h −1 , respectively.These values decreased for DMSNs-Ni with CO 2 conversion, CH 4 yield, and CH 4 selectivity of 60%, 45%, and 80%, respectively.The higher catalytic performances could be attributed to the smaller particle size and crystal size of Ni NPs in DMSNs@hollow@Ni.

The Production of Methane by Photocatalysis
Photocatalysis has been regarded as one of best solutions to remove organic pollutants from the environment, to produce hydrogen from water, etc., by means of sunlight resource. [163]Over the past decades, various photocatalysts have been developed in an attempt to apply solar energy for environmental purification and energy conversion, such as TiO 2 , CdS, Bi 2 WO 6 , Bi 2 MoO 6 , BiOX (X = Cl, Br, I, or CO 3 ), SrTiO 3 , and so on. [164,165]Some of semiconductor materials have been coupled with DMSNs to yield the novel photo-catalysts for the production of CH 4 and CO from CO 2 reduction.

TiO 2 -Modified DMSNs
Our group first constructed hybrid dendritic mesoporous silica-titania nanospheres (DMSTNs) by grafting tetrabutyl orthotitanate (TBOT) onto DMSNs.Then, hollow dendritic mesoporous silica-titania nanospheres (HDMSTNs) could be prepared just by etching the solid DMSTNs with the help of alkaline solution, as demonstrated in Figure 12. [26] DMSTNs kept the dendritic topology with spherical TiO 2 NPs uniformly scattered.HDMSTNs not only maintained the dendritic morphology, but also owned the hollow texture with TiO 2 NPs unevenly aggregated.DMSTNs and HDMSTNs exhibited different photo-catalytic selectivities and activities for the production of CO and CH 4 (Table S13, Supporting Information).The declined catalytic abilities from DMSTNs to HDMSTNs originated from the decrease of specific surface area and pore volume, the aggregation of nano-sized TiO 2 crystallite, and the loss of framework oxygen vacancies (Vo).On the basis of DMSTNs, our team facilely constructed black dendritic Energy Environ.Mater.2024, 7, e12593 mesoporous silica-titania nanospheres (b-DMSTNs) with TiO 2-X NPs dispersed in the channels by reducing DMSTNs with NaBH 4 under Ar atmosphere (Figure 12). [166]Plenty of Ti 3+ and oxygen vacancies emerged in TiO 2−X bulks, which accelerated the yield of CO and CH 4 with its particular selectivity (Table S13, Supporting Information).
Du et al. prepared a series of DMSTNs photocatalysts with tunable sizes of black TiO 2−X NPs inside by reducing TiO 2 in H 2 atmosphere (Figure 13a). [87]CO was merely yielded for b-DMSTNs with small TiO 2−X NPs of 1-3 nm, while both CO and CH 4 could be obtained for the one with larger TiO 2−X NPs of ~3-12 nm.Specifically, b-DMSTNs with 8 nm TiO 2−X had ultrahigh CH 4 production rate, moderate CO production rate, and high photocatalytic stability (Table S14, Supporting Information).The fundamental physicochemical properties gave rise to the lower recombination rate of photo-generated electrons and holes (bandgaps), as well as the improved carrier transfer and separation.That was the key to the dramatically high photoreduction activity. [87]Du and our groups further loaded Au NPs onto DMSTNs and b-DMSTNs supports for photo-catalytic reduction of CO 2 (Figure 13b). [167]The CH 4 and CO production rates of DMSTNs-Au outweighed those of b-DMSTNs-Au (Table S15, Supporting Information).Besides, the size of Au NPs significantly affected the production rate and selectivity.DMSTNs-Au not only could lower the recombination rate of photongenerated electrons and holes under UV irradiation, but also could realize the visible-light-induced reduction of CO 2 through the local surface plasmon resonance (LSPR) effect of Au NPs. [167]

Carbon Nitride-Modified DMSNs
Polshettiwar et al. mixed DMSNs and melamine in a vacuum-sealed quartz tube.Subsequent calcination brought out g-C 3 N 4 -coated DMSNsbased catalyst, namely, DMSNs-g-C 3 N 4 .The boosted light absorption in the visible region and increased lifetime of photo-generated charge carriers could attribute to the formation of interfaces between DMSNs and g-C 3 N 4 (Figure 14a).Correspondingly, the CH 4 production of DMSNs-g-C 3 N 4 was about 2.8-fold of pristine g-C 3 N 4 . [168]Along with methane, hydrogen was also detected as one of the products.However, H 2 production largely outweighed the CH 4 production.Our group coated DMSTNs with multilayer amorphous carbon nitride (ACN) nanofilm by impregnating DMSTNs with cyanamide precursor and calcining in N 2 at 550 °C for 5 h. [169]The as-prepared DMSTNs-ACN displayed greatly improved visible-light response due to the semiconductor characteristics of both TiO 2 and carbon nitride (Figure 14b).The novel catalyst could produce approximately equal CO and CH 4 in carbon dioxide reduction with its specific selectivity (Table S13, Supporting Information).

Other Types of DMSNs-Based Photocatalysts
Adopting DMSNs as the supports, Polshettiwar et al. fabricated a series of dendritic plasmonic colloidosomes (DPCs) by a cycle-by-cycle solution-phase synthetic technique via manipulating the nucleation-growth of Au NPs. [170]In spite of different inter-particle distances and particle-size distribution of Au NPs (Figure 14c), these DPCs could absorb light over the entire visible region and in the near-infrared region of the solar spectrum, which transformed gold into black gold with multifunctionality.All DPCs could play significant roles in the oxidation reaction of cinnamyl alcohol, hydrosilylation of aldehydes, temperature jump assisted protein unfolding, and seawater purification through steam generation.However, only DPCs with 55 wt% Au NPs of ~8.6 nm (DPCs-4) could photo-catalytically reduce CO 2 to CH 4 , while others could not yield neither CH 4 nor CO. [170] 5.5.The Production of Methanol by CO 2 Hydrogenation Zeng et al. synthesized Al-doped DMSNs (dendritic mesoporous silicaaluminum nanospheres, DMSANs) by adding NaAlO 2 precursor into the toluene-water microemulsion system that was also generally utilized for DMSNs. [104]The morphology and formation mechanism of the final production varied with the increase of Al/Si ratio (Figure 15a 1 ).DMSANs with a low Al/Si ratio possessed 2D hexagonal symmetry, which could be assigned to the p6mm plane group, while DMSANs with a high Al/Si ratio had cubic symmetry attributed to the Pm3n space group.The latter one exhibited higher curvature of pore walls and a more discrete pore network (Figure 15a 2 ).The Hofmeister effects of [Al(OH) 4 ] − were responsible for these changes.The cosmotropic anion functioned the dual effect of smaller surfactant packing parameter and stronger inter-micellar repulsion.By further decorating Cu/ZnO onto DMSANs, the as-prepared DMSANs-Cu/ZnO could convert CO 2 into dimethyl ether (DME), where Cu/ZnO composites catalyzed the methanol synthesis and DMSANs support functioned as a solid acid to catalyze methanol dehydration to DME. [104] Very recently, utilizing  15b). [171]Among these composite catalysts, d-Ce X Zr 1−X O 2 -PdZn displayed the highest CO 2 conversion and methanol yield than others (Table S16, Supporting Information).The authors also discussed the ratio of Ce/Zr on the catalytic performances of d-Ce X Zr 1−X O 2 -PdCu. [172]The d-Ce 0.3 Zr 0.7 O 2 -PdCu catalyst showed the highest CO 2 conversion, CH 3 OH yield, TOF, and the long-term stability than others (Table S16, Supporting Information), due to highly dispersed active metals and more oxygen vacancies.Lately, this team dispersed Pd NPs onto dendritic CeZrZnO x bulks and discussed the effect of Pd content on the performances of CO 2 hydrogenation to methanol. [173]It is found that 1) the optimized amount of Pd (2.0 wt%) and the fine dispersity played a critical role in H 2 dissociation for more spillover hydrogen to boost the CO 2 hydrogenation performance.2) The asprepared Pd/CeZrZnO x increased the ratio of Ce 3+ /Ce 4+ in CeZrZnO x support and thus yielded more oxygen vacancies to accelerate the adsorption and activation of CO 2 molecules.3) Compared with the aforementioned d-Ce X Zr 1−X O 2 and d-Ce X Zr 1−X O 2 -AB, the catalyst showed the highest CO 2 conversions of 29.1%, MeOH selectivity of 43.8%, MeOH yield of 12.7%, TOF of 87.3 h −1 and excellent 100 h long-term stability.

Others
Lang et al. introduced NH 4 VO 3 into the heterogeneous oil-water biphase stratification reaction system that was another synthetic system for DMSNs. [37]The topology and synthesis mechanism also varied with vanadium concentrations.At low concentrations, vanadium species mainly emerged in the form of HVO 2À 4 and gave rise to a single mesopore wall growth mechanism (Figure 15c 1 ).The V 2 O 4À 7 anions gradually became the major species with the increase of the vanadium concentration, which led to a multimesopore wall growth mechanism (Figure 15c 2 ).The as-prepared dendritic mesoporous silica-vanadium nanospheres (DMSVNs) showed the best catalytic performances for ODHP in the presence of CO 2 when vanadium content was 5.2 wt%.DMSVNs catalysts achieved a propane conversion of 19%, a propylene selectivity of 89%, and a propane reaction rate of 1.06 × 10 −5 mol g −1 s −1 .The corresponding TOF value equaled 10.4 × 10 −3 mol C3H6 mol VOx À1 s À1 , which was markedly higher than that of the sample synthesized with SBA-15 as the supports (7.1 × 10 −3 mol C3H6 mol VOx À1 s À1 ).The mechanism mainly involved four steps as follows (Figure 15c 3 ).It should be pointed out that CO 2 was not converted into propylene, but acted as soft oxidants for the regeneration of V(+5) active sites through the fast re-oxidation process. [37]This situation totally differed from that CO 2 was the raw material and converted into valueadded chemicals. [174,175]ery recently, Jalil et al. fabricated two kinds of TiO 2 -modified DMSNs-based catalysts for amine-assisted CO 2 photo-conversion to methanol. [176]One was DMSNs-TiO 2 synthesized by an electrolysis Energy Environ.Mater.2024, 7, e12593 method with TiO 2 NPs dispersed on the surfaces; the other was DMSNs/TiO 2 by using TiO 2 as seeds via one-pot co-condensation with TiO 2 NPs dispersed throughout the skeletons.In the absence of triethylamine (TEA), formic acid (HCOOH) could be favorably yielded by both catalysts.In the presence of TEA as a sacrificial agent, DMSNs/TiO 2 could generate more methanol than that of DMSNs-TiO 2 (3487 μmol g −1 h −1 vs 2773 μmol g −1 h −1 ).It was proposed that the unique structure of DMSNs/TiO 2 could provide more photons and absorb more CO 2 molecules to produce carbonic acid (H 2 CO 3 ) active radicals.With the help of TEA, methanol drastically produced due to the formation of more active radicals activated by the electrons.
Lately, Bahari et al. added CuO seed into the mother microemulsion of DMSNs, and the as-obtained DMSNs/CuO could photo-catalytically convert CO 2 to methanol with the help of TEA as a sacrificial reductant (Figure 15d 1 ). [177]Compared with DMSNs-CuO prepared by the incipient wetness impregnation approach (Figure 15d 2 ), DMSNs/CuO possessed more considerable surface area and pore volume than those of  c 2 ) schematic diagrams of DPCs-1 to DPCs-6 with different magnifications.Reproduced with permission. [170]Copyright 2019, Royal Society of Chemistry.

The Adsorption and Conversion of Other Reactive Gases by DMSNs-Based Materials
In practical application of DMSNs-based materials for CO 2 adsorption and conversion, other reactive gases might act as competitors to occupy the active sites of DMSNs-based adsorbents or catalysts.Thus, it is necessary to thoroughly understand their behaviors and influences.Three gases have been dealt up to now, including NO 2 , SO 2 , and CO.NO 2 and SO 2 can lead to severe respiratory problems for human beings and environment crises such as acid rain, acid smog, low visibility, and eutrophication. [178]CO can easily combine with hemoglobin and destroy the hosts' health. [179]In the view of sustainable development, it is meaningful to remove the above three detrimental gases by DMSNsbased materials.

NO 2
Shang et al. loaded Cu NPs onto DMSNs and investigated the NO 2 removal performance of the as-prepared DMSNs-Cu with different loadings (Figure 16a 1 ). [180]Pure DMSNs could negligibly remove NO 2 with a 0.07 mmol g −1 capacity, due to their unfavorable affinities.The addition of Cu NPs could significantly increase NO 2 adsorption and the best case achieved with a 10 wt% Cu content where the efficiency was boosted up to the 51-fold of the bare.The release of the by-product NO greatly decreased when pristine DMSNs were modified with Cu, indicating that copper species could prevent NO 2 dissociation to NO.The best performance was also reached by sample with 10 wt% Cu, because more loading gave rise to the aggregation and even further to pore blockages.The plausible chemical reactions are illustrated in Figure 16a 2 .DMSNs-Cu could reactively adsorb NO 2 by oxidizing Cu and Cu 2 O to form NO À 3 and NO simultaneously.Cu NPs were more suitable for NO 2 adsorption than Cu 2 O because of their high NO 2 removal and low NO release capacity. [180]2.SO 2 Ibrahim et al. developed alkali metal-modified DMSNs (DMSNs-CaO and DMSNs-Na 2 O) by the incipient wet impregnation method to remove SO 2 .[181] The sorption beds of all samples achieved breakthrough in the first 70 s with the arrival sequence of DMSNs-CaO > DMSNs-Na 2 O > DMSNs (Figure 16b 1 ).Nevertheless, a more gradual increase in the measured outlet SO 2 concentration could be observed for both modified samples after about 180 s.The removal capacities of DMSNs-Na 2 O and DMSNs-CaO were 1.88 and 1.99 times higher than the parent DMSNs, respectively (Table S18, Supporting Information).The authors further explored the effects of Ca content, calcination temperature, and calcination time on the adsorption performances of DMSNs-CaO.[182] Thereinto, it should be mentioned that DMSNs' morphology was destroyed with the increment of CaO loading, similar to most of the previous studies.Several broken spheres could be easily observed. Especally for DMSNs-CaO with the highest loading, some fibers were damaged and led to a mixture of fibrous and short hairy-like morphology (Figure 16b 2 ).The best DMSNs-CaO for SO 2 removal should load 4.93 wt % alkali metal and be calcined at 923 K for 6.5 h (Table S18, Supporting Information).[182] Very recently, this team thoroughly investigated the adsorption process of DMSNs-CaO for SO 2 removal, including isotherm, kinetics, thermodynamics, and mass transfer mechanism.[183] Freundlich isotherm model could best describe the adsorption process as a multilayer adsorption on a heterogenous surface with an exponential distribution of active sites.Avrami kinetic model proved that SO 2 molecules were adsorbed by a combination of physisorption and chemisorption.Thermodynamic studies indicated that the process was exothermic and spontaneous from an ordered stage to a random one.The mass transfer of SO 2 towards the adsorbent fell into rate limit by both film diffusion and intra-particle diffusion.The adsorption capacity was governed by the reaction temperature and inlet SO 2 concentration, where low values of both parameters could lead to the higher adsorption capacity.

CO
Tian et al. immobilized a nickel(II) dibenzotetramethyltetraaza [14]  annulene complex (Nitmtaa) onto the amino-functionalized DMSNs-APTES through Ni-NH 2 bond to develop a stable and reusable nanocatalyst (Figure 16c 1 ). [184]The as-designed DMSNs-APTES-Nitmtaa could catalyze carbonylative sonogashira coupling reactions between aryl iodides and terminal alkynes to phenylacetylenes (Figure 16c 2 ).Greatly different from Ni NPs, Ni(II) in Nitmtaa was robust to resist its leaching.Triwahyono et al. compared the catalytic performances of DMSNs/ ZSM-5 and traditional ZSM-5 for CO methanation. [185]DMSNs/ZSM-5 exhibited greatly improved rates of CO conversion and CH 4 formation than those of the conventional one (Table S19, Supporting Information), as well as high stability with no sign of deactivation up to 50 h.Oxygen vacancies should account for the remarkably catalytic performances, because these defects gave rise to the increase of inter-particle porosity, basicity, and CO/H 2 adsorption sites.
Very recently, Jalil et al. investigated the catalytic performances of DMSNs/BEA and conventional BEA for CO methanation. [186]DMSNs/ ZSM-5 possessed improved catalytic abilities due to its structural advantages, similar to that of the above DMSNs/ZSM-5 (Table S20, Supporting Information).Very lately, Peng et al. synthesized DMSNs-CuO by the impregnation method for low-temperature CO oxidation. [187]The abundant mesoporous and surface active oxygen species could make CO completely converted at 120 °C, because these advantages were beneficial to the mass transfer, CO adsorption, and the decomposition of Cu + -CO species.The morphology and formation mechanism of DMSANs with the increase of Al/Si ratio.Reproduced with permission. [104]Copyright 2015, American Chemical Society.b) Schematic diagram of preparing dendritic Ce X Zr 1 -X O 2 -AB catalysts where AB stands for PdCu, PdZn, CuZn, GaCu, or CuNi.c 1,2 ) The topology and synthesis mechanism of DMSVNs with different vanadium concentrations, c 3 ) the catalytic mechanism of DMSVNs with different vanadium concentrations for ODHP.Reproduced with permission. [37]Copyright 2017, American Chemical Society.d 1,2 ) The electron images of DMSNs/CuO and DMSNs-CuO, d 3 ) probable catalytic mechanisms of DMSNs/CuO and DMSNs-CuO.Reproduced with permission. [177]Copyright 2022, Elsevier.

Summary and Perspectives
The emergence of DMSNs as catalysts or supports has unfolded the brand-new synthetic approaches, specific components, delicate architectures, and targeted functionalization.In this review, we have focused on the progress of CO 2 capture and catalytic conversion by DMSNs-based materials.The functionalization methods have been particularly demonstrated for DMSNs-based adsorbents or catalysts.The adsorptive or catalytic performances of traditional and DMSNs-based materials have been compared.Most importantly, the catalytic conversion of CO 2 have been thoroughly analyzed, including the syntheses of organic chemicals (cyclic carbonates, 2-oxazolidinones, β-oxopropylcarbamates, quinazoline-2,4 (1H,3H)-diones, etc.), dry reforming of methane, CO 2 methanation, methane production, and so forth.The capture and catalytic conversion of other gases, such as NO 2 , have been analyzed as well.Figure 17 summarizes the most important aspects of the DMSNs for designing novel DMSNs-based materials to use CO 2 as follows.

The Choice of Synthetic Approaches
Three synthetic approaches have been developed to construct DMSNs, as summarized by our previous work, being the aqueous phase system, the biphase stratification system, and the bicontinuous microemulsion phase, respectively. [16,17]Up to date, the aqueous phase system has not been adopted to fabricate DMSNs-based materials for the removal of CO 2 .However, this route has been extensively carried out to synthesize dendritic mesoporous organosilica nanoparticles (DMONs). [16]MONs-based materials might be indispensable for CO 2 capture and conversion, because organic moieties (like benzene) in DMONs skeleton have high affinities to CO 2 and could bring about compatibility, hydrophobicity, and biodegradability.The bicontinuous microemulsion phase along with the seeds seems to be a potential way, such as the aforementioned DMSNs/SBA-15, DMSNs/ZSM, and so forth.

The Design of Architectures
This review has presented DMSNs-based materials with special architectures of the hollow (such as HDMSTNs [26] ) and core-shelled ones (such as Fe 3 O 4 @DMSNs [97] and FeNi 3 @DMSNs [91] ).In fact, other DMSNs' architectures have been explored for different application areas, such as the Janus, [188] yolk-shell, [189] shuttlecock-shaped [190] ones, and so forth.Integrating an extraordinary structure into DMSNs-based materials could be a promising research hotspot.

The Control of Interface Functionalization or the Size of Functional Units
Interface modification has become the main technique for functionalizing DMSNs-based materials, as illustrated in almost all the cited figures.
Step-by-step post-treatments can render DMSNs-based materials with desirable chemical features for expected properties, other than characteristics from element-or unit-hybrid in their skeletons, such as the import of ionic liquids, [36] Salen segments, [91] cyclodextrins, [99] and so on.The functionalized interfaces could work individually or further load other types of adsorptive and/or catalytic units, such as metal NPs (Ni, Au, Ni@Pd, [95] etc.), metallic oxides (CuO, MgO, [61] etc.), and so forth.Most importantly, it is worth noting that the size of the functional unit significantly influences the performances of the catalysts, as discussed in the above examples, such as TiO 2 NPs [87] and Au NPs. [167,170]The nanometer effect shows up and becomes more significant with the size of functional units decreased.Future investigations could concentrate on the following four parts that originate from the nanometer effect and greatly affect the materials' properties, namely, 1) quantum size effect, 2) small size effect, 3) surface effect, and 4) quantum tunnel effect. [191]

ICCC and Explorations at a Molecular Level
To our regret, almost all articles involved in this review (i.e., the capture and catalytic conversion of CO 2 by DMSNs-based materials) do not relate to ICCC.That is to say, DMSNs-based materials function as either adsorbents or catalysts for CO 2 .But to our excitement, the ICCC might be a breakthrough direction of this subject and worth of investigating in the future.Another significant research orientation should be the more profound understanding of CO 2 adsorption, activation, and catalytic conversion by DMSNs-based materials at a molecular level (Figures 10f, 11c, and 15c; Figures S1, S2, and S5, Supporting Information).This review just serves as a modest spur to induce other researchers to come forward with their valuable contributions.
With the development of this subject, the concentrations should shift stage by stage, from synthesis, architecture control, and property evaluation to integrated functions, covering chemistry, material science, catalysis, energy, environmental, and so on.Even though great progress has been made by researchers from all over the world, more works are needed to be conducted to reveal the relationships between structural advantages and the excellent performances in CO 2 capture and catalytic conversion.Particularly, DMSNs-based materials might act as ideal platforms to construct novel advanced systems with multifunctionalities by encapsulating various active species.It is also believed that these solid, hollow, core-shell, and yolk-shell ones would become significant platforms in many application fields.

Figure 2 .
Figure 2. a) A three-dimensional model of a DMSNs extracted by combining its SEM, TEM, high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM), and bright field STEM (BF-STEM).b 1 ) The functionalization of DMSNs with (3-minopropyl)triethoxysilane (APTES) by a postgrafting method via reflux reaction, b 2 ) with NH 3 by an ammonolysis modification method, c) with tetraethylenepentamine (TEPA) by a wet impregnation method, and d) with 3glycidyloxypropyltrimethoxysilane (GTMS) as a cross-linking agent to anchor amines by a ring-opening reaction.e) Chemical structures of amino-containing compounds utilized to functionalize DMSNs.

Figure 3 .
Figure 3. a) Synthesis of DMSNs-MgO composite absorbent.b 1 ) The preparation process of DMSNs/HZSM-5-CuO, b 2 -b 5 ) SEM images of the absorbents with different CuO contents.Reproduced with permission.[65]Copyright 2020, Elsevier.c 1 ) Preparation diagram of DMCNs by a DMSNs hard template approach and c 2 -c 6 ) DMCNs absorbents with different sizes, fiber density, surface area, pore volume.Reproduced with permission.[72]Copyright 2018, John Wiley and Sons.d) The construction process of DMSNs-containing MMM and its gas pathway.

Figure 4 .
Figure 4.The conversion of CO 2 to cyclic carbonates by DMSNs-based catalysts.

Figure 5 .
Figure 5.The conversion of CO 2 to cyclic carbonates by DMSNs-based catalysts.
Dai et al. utilized the postgrafting method to disperse Ni NPs into DMSNs channels and found that the resultant DMSNs-Ni catalyst could effectively inhibit the sintering and coking of active Ni NPs

Figure 13 .
Figure 13.a) Preparation schemes and catalytic mechanisms of b-DMSTNs with different sized TiO 2−X NPs.Reproduced with permission. [87]Copyright 2019, Elsevier.b) Preparation schemes and catalytic mechanisms of DMSTNs-Au plus b-DMSTNs-Au.

Table 1 .
Possible reactions involved in CO 2 methanation with their corresponding enthalpy, entropy, and Gibbs energy values at T = 25 °C and P = 0.1 MPa.