TiO2-Modified Montmorillonite-Supported Porous Carbon-Immobilized Pd Species Nanocomposite as an Efficient Catalyst for Sonogashira Reactions

In this study, a combination of the porous carbon (PCN), montmorillonite (MMT), and TiO2 was synthesized into a composite immobilized Pd metal catalyst (TiO2-MMT/PCN@Pd) with effective synergism improvements in catalytic performance. The successful TiO2-pillaring modification for MMT, derivation of carbon from the biopolymer of chitosan, and immobilization of Pd species for the prepared TiO2-MMT/PCN@Pd0 nanocomposites were confirmed using a combined characterization with X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), N2 adsorption–desorption isotherms, high-resolution transition electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. It was shown that the combination of PCN, MMT, and TiO2 as a composite support for the stabilization of the Pd catalysts could synergistically improve the adsorption and catalytic properties. The resultant TiO2-MMT80/PCN20@Pd0 showed a high surface area of 108.9 m2/g. Furthermore, it exhibited moderate to excellent activity (59–99% yield) and high stability (recyclable 19 times) in the liquid–solid catalytic reactions, such as the Sonogashira reactions of aryl halides (I, Br) with terminal alkynes in organic solutions. The positron annihilation lifetime spectroscopy (PALS) characterization sensitively detected the development of sub-nanoscale microdefects in the catalyst after long-term recycling service. This study provided direct evidence for the formation of some larger-sized microdefects during sequential recycling, which would act as leaching channels for loaded molecules, including active Pd species.


Introduction
Pd-based catalysts play important roles in many chemical transformations. In most applications, the homogeneous Pd catalyst contains not only the active Pd species but also various necessary ligands. Such homogeneous catalysis processes often suffer the difficulties of separation of the catalysts and ligands, recovery of the catalysts, and purity of the products, which make the processes un-green and cost-effective [1][2][3][4]. Therefore, the immobilization of active Pd species on appropriate supports to develop heterogeneous catalysts has received increasing attention both in the academic and industrial fields [5,6].
favor of further improvement of the comprehensive catalytic performance [42,43]. Based on these studies, it is expected that a combination of porous carbon, TiO2, and TiO2-pillaring modified MMT should be another novel promising support for Pd species. However, to the best of our knowledge, the preparation of porous carbon modified with both pillared MMT and TiO2-supported stabilized Pd (TiO2-MMT/PCN@Pd) catalysts applied in liquid-solid heterogeneous coupling reactions has been reported in few studies.
In this study, a series of TiO2-modified MMT-supported PCN-stabilized Pd nanocomposites were synthesized (using CS as a carbon source), followed with fine microstructure characterizations and catalytic performance tests for the Sonogashira coupling reactions between aryl halides and terminal phenyl acetylene. The aim was to evaluate the synergism effects of the combination of PCN, MMT, and TiO2 as a composite support for the stabilization of Pd catalysts as applied in liquid-solid organic reactions. The correlations between the microstructure and catalytic performance of the prepared novel catalytic nanocomposites were discussed. In addition, comparisons were made of the prepared novel catalysts with other recent solid-supported palladium catalysts in the Sonogashira reaction [44].

Microstructure of the Catalytic Nanocomposites
The XRD patterns of the starting Na-MMT, TiO2-MMT, and TiO2-MMT/PCN@Pd nanocomposites are shown in Figure 1a,b. For Na-MMT, the diffraction (001) peak that is attributed to the ordering of the MMT layers locates at the 2θ around 7.35°, related to the basal space of 1.20 nm. Considering that the thickness of the MMT layer itself is about 0.96 nm [45,46], the interlayer spacing distance of the starting Na-MMT is about 0.24 nm. The pillaring process involves the cations exchange of the polyhydroxy-Ti 4+ species exchanged with Na + , which props open the silicate layers. Upon high temperature treatment, the intercalated polyhydroxy-Ti 4+ species are transformed into TiO2 nanoparticles, linking permanently with the silicate layers. As a result, for the TiO2-MMT and the TiO2-MMT/PCN@Pd nanocomposites, the characteristic (001) diffraction peaks become extremely broader and weaker, indicating almost disorderly alignment of the MMT silicate layers after the TiO2 modification. Nevertheless, the basal space of 2.05 nm and interlayer spacing distance of 1.09 nm can be derived from the weak peak at 2θ of around 4.30°. According to the Lagaly's method [47][48][49], the bilayer arrangement of the CS chains (PCN precursor) and its derived PCN species occur in the pillared silicate interlayers. As shown in Figure 1b, for Na-MMT, the diffraction peak at 2θ of 19.7 • is related to the two-dimensional hk indices of (02) and (11), the diffraction peak at 2θ of 34.8 • is related to the two-dimensional hk indices of (13) and (20), the diffraction peak at 26.6 • is attributed to quartz, and the diffraction peak at 28.7 • is attributed to the silica impurity [50]. For TiO 2 -MMT and all of the TiO 2 -MMT/PCN@Pd nanocomposites, the hk reflection peaks at 2θ of about 19.7 • are related to the two-dimensional MMT layers still present [51]. However, the relative intensity of the diffraction peak obviously decreases. This can be due to the formation of the crystalline TiO 2 nanoparticles. As shown in Figure 1b, [52], indicating the presence of mainly anatase TiO 2 together with some rutile TiO 2 on the surface of the MMT layers after the calcination process. This phenomenon is quite different from that of the Al-pillared MMT [23,32,33,42], in which most of the intercalated polyhydroxy-Al cations are converted into stable Al 2 O 3 pillars instead of Al 2 O 3 nanoparticles dispersed on the surface of the MMT layer.
Further evidence of the successful TiO 2 -pillaring modification can be found in the changes of the FTIR spectra of the starting MMT and the nanocomposites. As shown in Figure 2, after the TiO 2 pillaring, carbonization and Pd-loading steps, the preservation of characteristic FTIR bands in the region of 400-700 cm −1 (465 cm −1 assigned to Si-O-Si bending, 527 cm −1 assigned to Al-O-Si bending) [53][54][55][56] is observed, indicating that the small building units of the MMT layer are still present. For pure MMT, the peak at 915 cm −1 is assigned as the Si-OH vibration. After dehydroxylation in the high-temperature pillaring or carbonization process, the Si-OH vibration peak almost disappears and new peaks are found around 930 and 945 cm −1 , which can be assigned to the Si-O-Ti vibration, confirming the molecular combination of the TiO 2 species with the MMT frame. As shown in Figure 1b, for Na-MMT, the diffraction peak at 2θ of 19.7° is related to the two-dimensional hk indices of (02) and (11), the diffraction peak at 2θ of 34.8° is related to the two-dimensional hk indices of (13) and (20), the diffraction peak at 26.6° is attributed to quartz, and the diffraction peak at 28.7° is attributed to the silica impurity [50]. For TiO2-MMT and all of the TiO2-MMT/PCN@Pd nanocomposites, the hk reflection peaks at 2θ of about 19.7° are related to the two-dimensional MMT layers still present [51]. However, the relative intensity of the diffraction peak obviously decreases. This can be due to the formation of the crystalline TiO2 nanoparticles. As shown in Figure 1b, [52], indicating the presence of mainly anatase TiO2 together with some rutile TiO2 on the surface of the MMT layers after the calcination process. This phenomenon is quite different from that of the Al-pillared MMT [23,32,33,42], in which most of the intercalated polyhydroxy-Al cations are converted into stable Al2O3 pillars instead of Al2O3 nanoparticles dispersed on the surface of the MMT layer.
Further evidence of the successful TiO2-pillaring modification can be found in the changes of the FTIR spectra of the starting MMT and the nanocomposites. As shown in Figure 2, after the TiO2 pillaring, carbonization and Pd-loading steps, the preservation of characteristic FTIR bands in the region of 400-700 cm −1 (465 cm −1 assigned to Si-O-Si bending, 527 cm −1 assigned to Al-O-Si bending) [53][54][55][56] is observed, indicating that the small building units of the MMT layer are still present. For pure MMT, the peak at 915 cm −1 is assigned as the Si-OH vibration. After dehydroxylation in the high-temperature pillaring or carbonization process, the Si-OH vibration peak almost disappears and new peaks are found around 930 and 945 cm −1 , which can be assigned to the Si-O-Ti vibration, confirming the molecular combination of the TiO2 species with the MMT frame. On the one hand, the pillars successfully derived in the MMT interlayer space should originate from the molecular level of the TiO2 species, which are combined on the MMT layer at the molecular level by the Si-O-Ti bonds after the pillaring process (including the polycation precursor intercalation, hydrolysis, and calcination steps). On the other hand, as TiO2 also prefers to form nanoparticles with fine crystal structure after calcination at 500 °C, some of the derived TiO2 nanoparticles with a larger size than the basal spacing of the pillared MMT might be sandwiched in the multilayer space of the pillared MMT. This results in some irregular stacking of the MMT layers, while the other derived TiO2 nanoparticles should be directly dispersed on the external surface of the MMT layers.
Due to the low loading content of Pd and its fine dispersion, the characteristic Pd 0 crystal diffraction peaks do not appear distinctly. Nevertheless, the Pd content within the On the one hand, the pillars successfully derived in the MMT interlayer space should originate from the molecular level of the TiO 2 species, which are combined on the MMT layer at the molecular level by the Si-O-Ti bonds after the pillaring process (including the polycation precursor intercalation, hydrolysis, and calcination steps). On the other hand, as TiO 2 also prefers to form nanoparticles with fine crystal structure after calcination at 500 • C, some of the derived TiO 2 nanoparticles with a larger size than the basal spacing of the pillared MMT might be sandwiched in the multilayer space of the pillared MMT. This results in some irregular stacking of the MMT layers, while the other derived TiO 2 nanoparticles should be directly dispersed on the external surface of the MMT layers.
Due to the low loading content of Pd and its fine dispersion, the characteristic Pd 0 crystal diffraction peaks do not appear distinctly. Nevertheless, the Pd content within the catalyst can be supported using the ICP-AES and XPS determination. The percentage of Pd content within the TiO 2 -MMT/PCN@Pd 0 nanocomposite is determined as about 2% using ICP-AES. Meanwhile, the binding energies of Pd 3d are observed as 335.6 eV (assigned to the Pd 0 species) and 337.2 eV (assigned to the Pd 2+ species), confirming the presence of the Pd active species [57]. In addition to the Pd 3d , the XPS spectra of C 1s , O 1s , and N 1s are shown in Figure 3. For the XPS spectrum of C 1s in PCN, it can be deconvoluted into three peaks at 284.4 eV (C atoms on C=C), 285.9 eV (C atoms on C-N and/or C-O), and 288.4 eV (C atoms on C=O), respectively. For the XPS spectrum of N 1s in PCN, it can be deconvoluted into four peaks at 399.2 eV (N atoms on pyridinic-N), 400.3 eV (N atoms on pyrrolic-N), 401.6 eV (N atoms on graphitic-N), and 403.0 eV (N atoms on N-O bonds), respectively. For the XPS spectrum of O 1s , it can be deconvoluted into two peaks at 530.5 eV (O atoms on C-O) and 532.1 eV (O atoms on C=O), respectively. Clearly, the derived N-containing PCN from CS successfully supported on the TiO 2 -MMT is powerfully supported by the XPS results. The Raman shift of the prepared TiO 2 -MMT 60 /PCN 40 @Pd 0 is illustrated in Figure 4. The peaks at about 1360 cm −1 and 1590 cm −1 are attributed to disordered and ordered graphite carbon species (the so-called D band and G band), respectively. The derived PCN is mainly composed of disordered carbon species as the I D /I G is found as 4.1.
catalyst can be supported using the ICP-AES and XPS determination. The percentage of Pd content within the TiO2-MMT/PCN@Pd 0 nanocomposite is determined as about 2% using ICP-AES. Meanwhile, the binding energies of Pd3d are observed as 335.6 eV (assigned to the Pd 0 species) and 337.2 eV (assigned to the Pd 2+ species), confirming the presence of the Pd active species [57]. In addition to the Pd3d, the XPS spectra of C1s, O1s, and N1s are shown in Figure 3. For the XPS spectrum of C1s in PCN, it can be deconvoluted into three peaks at 284.4 eV (C atoms on C=C), 285.9 eV (C atoms on C-N and/or C-O), and 288.4 eV (C atoms on C=O), respectively. For the XPS spectrum of N1s in PCN, it can be deconvoluted into four peaks at 399.2 eV (N atoms on pyridinic-N), 400.3 eV (N atoms on pyrrolic-N), 401.6 eV (N atoms on graphitic-N), and 403.0 eV (N atoms on N-O bonds), respectively. For the XPS spectrum of O1s, it can be deconvoluted into two peaks at 530.5 eV (O atoms on C-O) and 532.1 eV (O atoms on C=O), respectively. Clearly, the derived N-containing PCN from CS successfully supported on the TiO2-MMT is powerfully supported by the XPS results. The Raman shift of the prepared TiO2-MMT60/PCN40@Pd 0 is illustrated in Figure 4. The peaks at about 1360 cm −1 and 1590 cm −1 are attributed to disordered and ordered graphite carbon species (the so-called D band and G band), respectively. The derived PCN is mainly composed of disordered carbon species as the ID/IG is found as 4.1.   The N2 adsorption-desorption isotherms and corresponding pore size distribution of the starting Na-MMT, TiO2-MMT, and TiO2-MMT/PCN@Pd nanocomposites are shown in Figure 5. Based on the isotherms, the extracted BET surface area (SBET), micropore area (Amic), and total pore volume (Vtot) are listed in Table 1. All of the samples show distinct The N 2 adsorption-desorption isotherms and corresponding pore size distribution of the starting Na-MMT, TiO 2 -MMT, and TiO 2 -MMT/PCN@Pd nanocomposites are shown in Figure 5. Based on the isotherms, the extracted BET surface area (S BET ), micropore area (A mic ), and total pore volume (V tot ) are listed in Table 1. All of the samples show distinct hysteresis loops at a higher P/P 0 of 0.4 and considerable adsorption amounts at a low relative pressure, suggesting type IV isotherms with fairly good adsorption capacity. For Na-MMT, the hysteresis loops are assigned to the typical H 3 type, implying poor mesopores. For the TiO 2 -MMT and TiO 2 -MMT/PCN@Pd nanocomposites, all of the hysteresis loops are assigned to the typical H 4 type, implying rich, narrow, and slit-like mesopores. The pore size distribution peaks of TiO 2 -MMT are located at about 4 nm (minor peak) and 10 nm (major peak). This suggests that TiO 2 modification leads to the effective formation of numerous mesoporous layered structures. After hybridizing with PCN and the Pd 2+ species, the pore size distribution peaks of TiO 2 -MMT/PCN show little shift, indicating that the introduction of these species has limited effects on the mesopore size of the TiO 2 -MMT matrix. However, both in the case of TiO 2 -MMT 80 /PCN 20 @Pd 0 and TiO 2 -MMT 60 /PCN 40 @Pd 0 , the pore distribution peaks become obviously broader in the range of 10-16 nm. The increase in the larger-sized mesopores is ascribed to the generation of numerous disordered mesopores due to the reduction in the Pd 2+ species to Pd 0 nanoparticles. The S BET of TiO 2 -MMT (161.1 m 2 /g) is about 13.8 times that of the starting Na-MMT (11.6 m 2 /g). The V tot of TiO 2 -MMT (0.37 cm 3 /g) is about 9.2 times that of the starting Na-MMT (0.04 cm 3 /g). This is closely related to the induction of a large number of new stable porous structures after TiO 2 pillaring, which is in good agreement with the XRD results. However, after the loading of the PCN and the Pd species, it is observed that the S BET and V tot show a reasonably decrease.  40 @Pd 0 ), respectively. Nevertheless, the adsorption capacity and porosity of such novel TiO 2 -MMT/PCN@Pd 0 is superior to the recently prepared adsorbents or heterogeneous catalysts using TiO 2 -pillared MMT or MMT/PCN composites as a matrix [35][36][37][38][39][40]. As confirmed in the adsorption tests for the rhodamine B dye at room temperature (as shown Figure 6), both TiO 2 -MMT 80 /PCN 20 @Pd 0 and TiO 2 -MMT 60 /PCN 40 -Pd 0 show fairly good adsorption removal efficiency for the dye molecules (only 20 min reaching the equilibrium adsorption removal). Obviously, the former (92.1%) is better than the latter (79.6%). This might be due to the higher surface area of TiO 2 -MMT 80 /PCN 20 @Pd 0 than TiO 2 -MMT 60 /PCN 40 -Pd 0 . The highly porous structure and high surface area performance should be in favor of excellent catalytic performance of the resultant nanocomposites.  (79.6%). This might be due to the higher surface area of TiO2-MMT80/PCN20@Pd 0 than TiO2-MMT60/PCN40-Pd 0 . The highly porous structure and high surface area performance should be in favor of excellent catalytic performance of the resultant nanocomposites.  SBET: specific surface area, using the Brunauer-Emmet-Teller method; Amic: micropore area obtained using the t-Polt method; Vtot: total volume of pores, the N2 quantity absorbed at a relative pressure of P/P0 = 0.99.   SBET: specific surface area, using the Brunauer-Emmet-Teller method; Amic: micropore area obtained using the t-Polt method; Vtot: total volume of pores, the N2 quantity absorbed at a relative pressure of P/P0 = 0.99.  Figure 7. The starting MMT exhibits a layered structure with a regular stacking and closing interlayer distance ( Figure 7A). For TiO 2 -MMT, the loaded TiO 2 exist in three forms. Firstly, increasing of the interlayer distance and contrast is observed, indicating successful pillaring of the TiO 2 species. However, the TiO 2 pillars seem difficult to identify using HRTEM, which can be due to the molecular level combing of the TiO 2 species with the MMT framework of the Si-O-Ti bonds. Similar phenomenon are also reported in other studies on pillared MMT characterized with TEM [58][59][60]. Secondly, it is observed (as marked with a blue rectangle) that some TiO 2 nanoparticles sized about 5 nm are clipped in multilayer spaces of the MMT, causing an obvious disordered stacking of the MMT layers. Thirdly, many of the other visible TiO 2 nanoparticles disperse directly on the external surface and/or edge of the MMT layers. After immobilization of the Pd species, it is observed that some additional nanoparticles (with sizes below 2 nm, as marked with red circles) are clipped in the space of the adjacent MMT layers, which can be attributed to the Pd 0 species. Unfortunately, for the low contrast, the successful loading of the PCN species is difficult to identify with the HRTEM images. Nevertheless, with the element mapping and weight ratio results from the HRTEM-EDX, the successful loading of the PCN and Pd species is further confirmed. Based on the HRTEM-EDX results, the actual chemical composition of the TiO 2 -MMT 60 /PCN 40 @Pd 0 may be estimated as 47% of MMT, 41% of TiO 2 , 11% of PCN, and 1% of Pd, respectively. As shown in Figure S1, the success incorporation of the PCN and Pd species can also be powerfully supported by the element's composition of the TiO 2 -MMT 60/ PCN 40 @Pd 0 using SEM-EDX. Clearly, the HRTEM-EDX results are consistent with the XRD, FTIR, and N 2 adsorption-desorption results.

Positron Annihilation Characteristics of the Catalytic Nanocomposites
According to the microstructure characterization results above, the textile structure of the nanocomposite can be illustrated in Scheme 1. The mesoporous structure (using the N2 adsorption-desorption isotherms and adsorption-removal test), interlayer spacing (using XRD), morphologies (using HRTEM), and compositions (using HRTEM-EDX, Raman spectroscopy, FTIR) are successfully revealed. However, the sub-nano level microstruc-

Positron Annihilation Characteristics of the Catalytic Nanocomposites
According to the microstructure characterization results above, the textile structure of the nanocomposite can be illustrated in Scheme 1. The mesoporous structure (using the N 2 adsorption-desorption isotherms and adsorption-removal test), interlayer spacing (using XRD), morphologies (using HRTEM), and compositions (using HRTEM-EDX, Raman spectroscopy, FTIR) are successfully revealed. However, the sub-nano level microstructure, such as the molecular packing information of the derived TiO 2 pillars, PCN, and active Pd species inside the TiO 2 -modified MMT layer space still lacks essential evidence. Positron annihilation lifetime spectroscopy (PALS) has recently been proven as one of the most highly sensitive methods to detect microstructure information at the sub-nano level [61][62][63][64][65].
In the interlayer space with low electron density, in addition to free positron annihilation, some of the thermalized positrons can be trapped with electrons to form a bound state called positronium (Ps), and then annihilate. Ps has two states, o-Ps (spin parallel) and p-Ps (spin antiparallel), depending on the spin orientations of the positrons and bound electrons. In a vacuum, the intrinsic lifetimes of p-Ps and o-Ps are 0.125 ns and 142 ns, respectively. In molecular solids, o-Ps will form interactions with the electrons in the surrounding medium and undergo pick-up annihilation, resulting in a reduction in the lifetime of 1 to several ns. Using a suitable quantum mechanical model [66,67], the o-Ps lifetime is correlated to the microdefect size. It can then be used as a sensitive probe to detect the local microdefect structure. In such TiO2-MMT/PCN@Pd nanocomposite systems, as shown in Scheme 1, o-Ps annihilation should mainly occur in the molecule-stacking gaps in all of the involved molecular substrates in the interlayer space of the MMT with low electron density, such as the PCN molecules, Pd species, TiO2 pillars, and MMT layers. Using the quantum mechanical model in Equation (1) [68,69], the width of the cuboidal defects size can be estimated, where τ3 refers to the longest lifetime, l refers to the width of the cuboidal defects size, and Δl (=0.17 nm) refers to the thickness of the fitted empirical electron layer. Therefore, o-Ps can be used as a highly sensitive probe to detect the molecule packing and interfacial interactions in the interlayer space of the MMT layers. The microdefects fraction, f, i.e., a combination of o-Ps intensity I3 with microdefects volume (V = l3), can be calculated according to Equation (2), where C is a constant. To simplify, the apparent microdefects fraction (fapp) is often used to determine the variation trends.
As shown in Table 2, the positron annihilation spectra of the nanocomposites are fitted well in the three-lifetime fitting with the LT-9 program. For all of the samples, the first lifetime component of τ1 and its intensity of I1 can be attributed to a combination of p-Ps In such TiO 2 -MMT/PCN@Pd nanocomposite systems, as shown in Scheme 1, o-Ps annihilation should mainly occur in the molecule-stacking gaps in all of the involved molecular substrates in the interlayer space of the MMT with low electron density, such as the PCN molecules, Pd species, TiO 2 pillars, and MMT layers. Using the quantum mechanical model in Equation (1) [68,69], the width of the cuboidal defects size can be estimated, where τ 3 refers to the longest lifetime, l refers to the width of the cuboidal defects size, and ∆l (=0.17 nm) refers to the thickness of the fitted empirical electron layer. Therefore, o-Ps can be used as a highly sensitive probe to detect the molecule packing and interfacial interactions in the interlayer space of the MMT layers. The microdefects fraction, f, i.e., a combination of o-Ps intensity I 3 with microdefects volume (V = l 3 ), can be calculated according to Equation (2), where C is a constant. To simplify, the apparent microdefects fraction (f app ) is often used to determine the variation trends.
(1) f = CVl 3 or f app = Vl 3 (2) As shown in Table 2, the positron annihilation spectra of the nanocomposites are fitted well in the three-lifetime fitting with the LT-9 program. For all of the samples, the first lifetime component of τ 1 and its intensity of I 1 can be attributed to a combination of p-Ps annihilation and free positron annihilation. The second lifetime component of τ 2 and its intensity of I 2 can be attributed to a combination of free positron annihilation and some trapped positron annihilation in the microdefects of the crystalline MMT layers. The third long lifetime component of τ 3 and its intensity of I 3 can be attributed to the o-Ps pick-off annihilation in the interlayer spaces of the TiO 2 -MMT 60 /PCN 40 @Pd 0 nanocomposites as illustrated in Scheme 1. In the confined nanospace between the neighboring layers, the o-Ps will be mainly trapped in the molecular packing gaps of the involved substrates, such as the MMT layer, PCN species, Pd species, and TiO 2 species. Using the cuboidal microdefects model in Equation (1), the size of the microdefects inside the interlayer space can be calculated. The microdefects size (l) inside the interlayer space of pure MMT can be calculated as 0.290 nm. After TiO 2 pillaring, modification, and further PCN derivation, the microdefects size of l increases to 0.307 nm (TiO 2 -MMT) and 0.311 nm (TiO 2 -MMT 60 /PCN 40 ). This increase in size can be related to the increase in interlayer space caused by the effective high-temperature process during the TiO 2 pillaring and carbonization steps. After Pd 2+ immobilization and further reduction in the nano Pd 0 species, the microdefect size of l decreases to 0.308 nm (TiO 2 -MMT 60 /PCN 40 @Pd 2+ ) and 0.299 nm (TiO 2 -MMT 60 /PCN 40 @Pd 0 ), which confirms that the interlayer space becomes more crowded after the Pd species are successfully incorporated into the interlayer space of the modified MMT. Moreover, it is observed that the both I 3 and f app of the TiO 2 -MMT 60 /PCN 40 @Pd nanocomposites are higher than the starting MMT, TiO 2 -MMT, and TiO 2 -MMT 60 /PCN 40 supports. This means that the nanocomposite still has high microdefect features though the interlayer spaces is more crowded. As microdefects can be used as active sites for catalytic reactions, a high comprehensive catalytic performance of the TiO 2 -MMT 60 /PCN 40 @Pd nanocomposites is expected.

Performances of the Catalytic Nanocomposites Applied in Sonogashira Reactions
The Sonogashira reaction, usually referring to the cross-coupling reaction of terminal alkyne with aryl halides, is an extremely valuable type of reaction to form C-C (sp-sp 2 ) bonds [70][71][72]. It has been broadly used in the field of synthesis of functional molecules of natural products, biological active features, pharmaceuticals, heterocycles, conducting polymers, and liquid polymer substrates, etc. Firstly, the catalytic performances of the TiO 2 -MMT/PCN@Pd nanocomposites are evaluated with a typical model Sonogashira reaction of iodo benzene and phenyl acetylene. The Pd 0 species supported on TiO 2 , PCN, TiO 2 /PCN, MMT/PCN, and TiO 2 -MMT are prepared and their N 2 adsorption performances are shown in Figure S2 and Table S1. As shown in Table S2, the model Sonogashira reaction is found to be difficult to perform without any Pd catalyst. However, with the presence of different Pd catalysts, such as TiO 2 @Pd 0 , PCN@Pd 0 , TiO 2 /PCN@Pd 0 , MMT/PCN@Pd 0 , and TiO 2 -MMT@Pd 0 , the reaction can be fairly well catalyzed with good yields ( Figure S3A). As shown in Figure 8A, for each reaction time interval, it is found that the reaction yields catalyzed by TiO 2 -MMT 80 /PCN 20 @Pd 0 are higher than that catalyzed by TiO 2 -MMT 60 /PCN 40 @Pd 0 , which should be mainly attributed to its higher adsorption capability [73,74]. The recyclability of the catalysts is further evaluated as applied in the model reaction. At each reaction time interval, hardly any improvements of the yields are observed after a hot filtration out of both catalysts, confirming the high heterogeneity. As shown in Figure 8B, for maintaining the yield higher than 70%, TiO 2 -MMT 80 /PCN 20 @Pd 0 and TiO 2 -MMT 60 /PCN 40 @Pd 0 can recycle for 16 runs and 19 runs, respectively. Obviously, as compared with the recyclable runs of TiO 2 -Pd 0 (six runs), PCN-Pd 0 (ten runs), TiO 2 /PCN-Pd 0 (eleven runs), MMT/PCN-Pd 0 (eight runs), and TiO 2 -MMT-Pd 0 (six runs) (as shown in Figure S3B), a combination of the PCN, MMT, and TiO 2 into a composite support Pd metal catalyst shows effective synergism improvements in the recyclability of the resultant TiO 2 -MMT/PCN@Pd 0 catalytic nanocomposites. Moreover, as shown in Figure 8C and  [75,76], MMT (K10@Pd(II)APTES or MMT/CS@Pd, Cu) [77,78], or carbon-based supports (CHT@Pd or Hal-pDA-NPC@Pd) [79,80]. The excellent chelation, stability, and adsorption of TiO 2 , MMT, and PCN have been well combined into a composite material, resulting in an effectively synergetic performance improvement in the case of the TiO 2 -MMT/PCN@Pd catalyst. The higher stability (three more recyclable runs) of TiO 2 -MMT 60 /PCN 40 @Pd 0 than TiO 2 -MMT 80 /PCN 20 @Pd 0 may be attributed to its higher content of the PCN, which has reasonably stronger chelation with the Pd species as compared with inorganic TiO 2 or MMT. Hence, further evaluation of the catalyst performance was mainly focused on the case of TiO 2 -MMT 60 /PCN 40 @Pd 0 with a higher content of PCN. As tracked using the ICP-AES assay, about 93% and 81% of the Pd species retained in the recovered TiO 2 -MMT 60 /PCN 40 @Pd 0 for five runs and ten runs, respectively. To clarify the reason for the Pd leaching, the recovered catalyst was further characterized with PALS.
As shown in Figure 8D, distinct differences are observed for the TiO 2 -MMT 60 /PCN 40 @Pd 0 after recycled for different times. As compared with the fresh TiO 2 -MMT 60 /PCN 40 @Pd 0 nanocomposite, many more counts of o-Ps annihilations with a longer lifetime (>2.5 ns) are observed for the PALS spectra of the recycled nanocomposites. This indicates that the recycled nanocomposites may contain more than one long-lifetime component as the usual molecular solids. Therefore, the positron annihilation spectra of the recycled nanocomposites have been refitted in four-lifetime rather than the usual three-lifetime fitting. The PALS spectrum of fresh TiO 2 -MMT 60 /PCN 40 @Pd 0 catalyst has been also fitted in the four-lifetime component for comparison. As shown in Table 3, for the spectrum of fresh TiO 2 -MMT 60 /PCN 40 @Pd 0 nanocomposite, we obtained τ 1 = 0.249 ns, τ 2 = 0.372 ns, τ 3 = 0.472 ns, and τ 4 = 2.11 ns with the relative intensities I 1 = 71.3%, I 2 = 8.8%, I 3 = 18.0%, and I 4 = 1.9%, exhibiting three short-lifetime components (<0.5 ns) and one long-lifetime component (>1 ns). The first two short-lifetime components of τ 1 and τ 2 can be attributed to the p-Ps decay and free positron annihilations. The third short-lifetime component of τ 3 can be attributed to free positron annihilations and some trapped positron annihilations in the microdefects of the MMT crystalline layer and the TiO 2 nanoparticle crystals. The long-lifetime component of τ 4 can be attributed to o-Ps pick-off annihilations in the molecules stacking gaps of all of the involved molecular substrates, such as the PCN molecules, Pd species, TiO 2 nanoparticles, and MMT layers. The mean microdefects size l (from τ 4 ) of the fresh TiO 2 -MMT 60 /PCN 40 @Pd 0 is estimated as 0.277 nm. Clearly, this size value is even smaller than the microdefects size l of 0.299 nm that is obtained from the three-lifetime fitting (as shown in Table 2). This indicates that the PCN and Pd species are tightly encaged in the nanospace of the TiO 2 -pillared MMT layers and threelifetime fitting is more reasonable than four-lifetime fitting for fresh TiO 2 -MMT 60 /PCN 40 @Pd 0 nanocomposites. MMT60/PCN40@Pd 0 than TiO2-MMT80/PCN20@Pd 0 may be attributed to its higher content of the PCN, which has reasonably stronger chelation with the Pd species as compared with inorganic TiO2 or MMT. Hence, further evaluation of the catalyst performance was mainly focused on the case of TiO2-MMT60/PCN40@Pd 0 with a higher content of PCN. As tracked using the ICP-AES assay, about 93% and 81% of the Pd species retained in the recovered TiO2-MMT60/PCN40@Pd 0 for five runs and ten runs, respectively. To clarify the reason for the Pd leaching, the recovered catalyst was further characterized with PALS. As shown in Figure 8D, distinct differences are observed for the TiO2-MMT60/PCN40@Pd 0 after recycled for different times. As compared with the fresh TiO2-MMT60/PCN40@Pd 0 nanocomposite, many more counts of o-Ps annihilations with a longer lifetime (>2.5 ns) are observed for the PALS spectra of the recycled nanocomposites.  Unlike the fresh catalyst, after recycled for five runs or ten runs, two long-lifetime components (>1 ns) of τ 3 and τ 4 are observed. For the recycled TiO 2 -MMT 60 /PCN 40 @Pd 0 catalytic nanocomposite recycled for five runs, besides the first long-lifetime component of τ 3 = 2.13 ns (close to the long-lifetime of τ4 of the fresh catalytic nanocomposite), the second long-lifetime of τ 4 is observed as 7.2 ns (with the relative intensities of I 4 = 0.26%). This indicates that o-Ps pick-off annihilations occur in other much larger-sized microdefects. The microdefect sizes of the five-runs recycled TiO 2 -MMT 60 /PCN 40 @Pd 0 catalyst are estimated as 0.279 nm and 0.616 nm. Similarly, for the recycled TiO 2 -MMT 60 /PCN 40 @Pd 0 catalyst recycled for ten runs, the microdefects sizes are estimated as 0.274 nm and 0.681 nm. This suggests that PALS is highly sensitive to the development of microdefects and provides direct evidence for the induction of some larger-sized microdefects during sequential recycling. Meanwhile, the relative intensities of o-Ps annihilation (I 3 + I 4 ) of the fiveruns recycled catalyst (I 3 + I 4 = 3.3%) and ten-runs recycled catalyst (I 3 + I 4 = 4.2%) are obviously higher than that of fresh catalyst (I 4 = 1.9%), indicating that more microdefects form after the continuous recycling process. Although larger-sized microdefects have low intensities, some of them may provide leaching channels for load molecules, including active Pd species.
Moreover, the novel TiO 2 -MMT 60 /PCN 40 @Pd 0 nanocomposite can be successfully extended to a range of substituted aryl halides coupling with phenyl acetylenes. As shown in Table 4, the reactions of the aryl iodides substituted with electron-donating groups of -CH 3 at ortho (entry 2), meta (entry 3), or para (entry 4) positions with phenylacetylene achieved excellent yields of 92-99%. Furthermore, the reactions of the aryl iodides substituted with electron-withdrawing groups of -F, -Br, and -Cl at ortho (entry 5), meta (entry 6), or para (entry 7) positions with phenylacetylene achieved excellent yields of 93-99%. For the entries 8-12, the reactions of the substituted aryl bromides with phenyl acetylene can be still effectively catalyzed with the novel TiO 2 -MMT 60 /PCN 40 -Pd 0 nanocomposite with desirable yields of 59-72%. In addition, the novel catalysts can efficiently catalyze the large-sized reactants for the coupling of iodo naphthalene and iodo fluorene with phenyl acetylene (entry 13,14), indicating the high feasibility of molecular size. Conclusively, the TiO 2 -MMT 60 /PCN 40 @Pd 0 nanocomposite developed in this study shows similarly high catalytic efficiency as compared with recently reported heterogeneous Pd catalysts for Sonogashira reactions with similar reaction conditions [23,[75][76][77][78][79][80]. The good dispersion and tight loading of the Pd species in the highly porous, stable, and amphiphilic TiO 2 -pillared MMT-supported PCN matrix is likely the main reason for this high catalytic efficiency. Table 4. Catalytic performance of TiO 2 -MMT 60 /PCN 40 @Pd 0 nanocomposites applied in Sonogashira coupling reactions between aromatic halides and alkynes.  achieved excellent yields of 92-99%. Furthermore, the reactions of the aryl iodides su tuted with electron-withdrawing groups of -F, -Br, and -Cl at ortho (entry 5), meta (e 6), or para (entry 7) positions with phenylacetylene achieved excellent yields of 93-9 For the entries 8-12, the reactions of the substituted aryl bromides with phenyl acety can be still effectively catalyzed with the novel TiO2-MMT60/PCN40-Pd 0 nanocompo with desirable yields of 59-72%. In addition, the novel catalysts can efficiently cata the large-sized reactants for the coupling of iodo naphthalene and iodo fluorene with p nyl acetylene (entry 13,14), indicating the high feasibility of molecular size. Conclusiv the TiO2-MMT60/PCN40@Pd 0 nanocomposite developed in this study shows similarly h catalytic efficiency as compared with recently reported heterogeneous Pd catalysts for nogashira reactions with similar reaction conditions [23,[75][76][77][78][79][80]. The good dispersion tight loading of the Pd species in the highly porous, stable, and amphiphilic TiO2-pill MMT-supported PCN matrix is likely the main reason for this high catalytic efficienc     achieved excellent yields of 92-99%. Furthermore, the reactions of the aryl iodides substituted with electron-withdrawing groups of -F, -Br, and -Cl at ortho (entry 5), meta (entry 6), or para (entry 7) positions with phenylacetylene achieved excellent yields of 93-99%. For the entries 8-12, the reactions of the substituted aryl bromides with phenyl acetylene can be still effectively catalyzed with the novel TiO2-MMT60/PCN40-Pd 0 nanocomposite with desirable yields of 59-72%. In addition, the novel catalysts can efficiently catalyze the large-sized reactants for the coupling of iodo naphthalene and iodo fluorene with phenyl acetylene (entry 13,14), indicating the high feasibility of molecular size. Conclusively, the TiO2-MMT60/PCN40@Pd 0 nanocomposite developed in this study shows similarly high catalytic efficiency as compared with recently reported heterogeneous Pd catalysts for Sonogashira reactions with similar reaction conditions [23,[75][76][77][78][79][80]. The good dispersion and tight loading of the Pd species in the highly porous, stable, and amphiphilic TiO2-pillared MMT-supported PCN matrix is likely the main reason for this high catalytic efficiency.   achieved excellent yields of 92-99%. Furthermore, the reactions of the aryl iodides substituted with electron-withdrawing groups of -F, -Br, and -Cl at ortho (entry 5), meta (entry 6), or para (entry 7) positions with phenylacetylene achieved excellent yields of 93-99%. For the entries 8-12, the reactions of the substituted aryl bromides with phenyl acetylene can be still effectively catalyzed with the novel TiO2-MMT60/PCN40-Pd 0 nanocomposite with desirable yields of 59-72%. In addition, the novel catalysts can efficiently catalyze the large-sized reactants for the coupling of iodo naphthalene and iodo fluorene with phenyl acetylene (entry 13,14), indicating the high feasibility of molecular size. Conclusively, the TiO2-MMT60/PCN40@Pd 0 nanocomposite developed in this study shows similarly high catalytic efficiency as compared with recently reported heterogeneous Pd catalysts for Sonogashira reactions with similar reaction conditions [23,[75][76][77][78][79][80]. The good dispersion and tight loading of the Pd species in the highly porous, stable, and amphiphilic TiO2-pillared MMT-supported PCN matrix is likely the main reason for this high catalytic efficiency. achieved excellent yields of 92-99%. Furthermore, the reactions of the aryl iodides su tuted with electron-withdrawing groups of -F, -Br, and -Cl at ortho (entry 5), meta (e 6), or para (entry 7) positions with phenylacetylene achieved excellent yields of 93-9 For the entries 8-12, the reactions of the substituted aryl bromides with phenyl acety can be still effectively catalyzed with the novel TiO2-MMT60/PCN40-Pd 0 nanocompo with desirable yields of 59-72%. In addition, the novel catalysts can efficiently cata the large-sized reactants for the coupling of iodo naphthalene and iodo fluorene with p nyl acetylene (entry 13,14), indicating the high feasibility of molecular size. Conclusiv the TiO2-MMT60/PCN40@Pd 0 nanocomposite developed in this study shows similarly h catalytic efficiency as compared with recently reported heterogeneous Pd catalysts for nogashira reactions with similar reaction conditions [23,[75][76][77][78][79][80]. The good dispersion tight loading of the Pd species in the highly porous, stable, and amphiphilic TiO2-pill MMT-supported PCN matrix is likely the main reason for this high catalytic efficienc achieved excellent yields of 92-99%. Furthermore, the reactions of the aryl iodides substituted with electron-withdrawing groups of -F, -Br, and -Cl at ortho (entry 5), meta (entry 6), or para (entry 7) positions with phenylacetylene achieved excellent yields of 93-99%. For the entries 8-12, the reactions of the substituted aryl bromides with phenyl acetylene can be still effectively catalyzed with the novel TiO2-MMT60/PCN40-Pd 0 nanocomposite with desirable yields of 59-72%. In addition, the novel catalysts can efficiently catalyze the large-sized reactants for the coupling of iodo naphthalene and iodo fluorene with phenyl acetylene (entry 13,14), indicating the high feasibility of molecular size. Conclusively, the TiO2-MMT60/PCN40@Pd 0 nanocomposite developed in this study shows similarly high catalytic efficiency as compared with recently reported heterogeneous Pd catalysts for Sonogashira reactions with similar reaction conditions [23,[75][76][77][78][79][80]. The good dispersion and tight loading of the Pd species in the highly porous, stable, and amphiphilic TiO2-pillared MMT-supported PCN matrix is likely the main reason for this high catalytic efficiency. achieved excellent yields of 92-99%. Furthermore, the reactions of the aryl iodides su tuted with electron-withdrawing groups of -F, -Br, and -Cl at ortho (entry 5), meta (e 6), or para (entry 7) positions with phenylacetylene achieved excellent yields of 93-For the entries 8-12, the reactions of the substituted aryl bromides with phenyl acety can be still effectively catalyzed with the novel TiO2-MMT60/PCN40-Pd 0 nanocomp with desirable yields of 59-72%. In addition, the novel catalysts can efficiently cata the large-sized reactants for the coupling of iodo naphthalene and iodo fluorene with nyl acetylene (entry 13,14), indicating the high feasibility of molecular size. Conclusi the TiO2-MMT60/PCN40@Pd 0 nanocomposite developed in this study shows similarly catalytic efficiency as compared with recently reported heterogeneous Pd catalysts fo nogashira reactions with similar reaction conditions [23,[75][76][77][78][79][80]. The good dispersion tight loading of the Pd species in the highly porous, stable, and amphiphilic TiO2-pill MMT-supported PCN matrix is likely the main reason for this high catalytic efficienc achieved excellent yields of 92-99%. Furthermore, the reactions of the aryl iodides substituted with electron-withdrawing groups of -F, -Br, and -Cl at ortho (entry 5), meta (entry 6), or para (entry 7) positions with phenylacetylene achieved excellent yields of 93-99%. For the entries 8-12, the reactions of the substituted aryl bromides with phenyl acetylene can be still effectively catalyzed with the novel TiO2-MMT60/PCN40-Pd 0 nanocomposite with desirable yields of 59-72%. In addition, the novel catalysts can efficiently catalyze the large-sized reactants for the coupling of iodo naphthalene and iodo fluorene with phenyl acetylene (entry 13,14), indicating the high feasibility of molecular size. Conclusively, the TiO2-MMT60/PCN40@Pd 0 nanocomposite developed in this study shows similarly high catalytic efficiency as compared with recently reported heterogeneous Pd catalysts for Sonogashira reactions with similar reaction conditions [23,[75][76][77][78][79][80]. The good dispersion and tight loading of the Pd species in the highly porous, stable, and amphiphilic TiO2-pillared MMT-supported PCN matrix is likely the main reason for this high catalytic efficiency. achieved excellent yields of 92-99%. Furthermore, the reactions of the aryl iodides su tuted with electron-withdrawing groups of -F, -Br, and -Cl at ortho (entry 5), meta (e 6), or para (entry 7) positions with phenylacetylene achieved excellent yields of 93-9 For the entries 8-12, the reactions of the substituted aryl bromides with phenyl acety can be still effectively catalyzed with the novel TiO2-MMT60/PCN40-Pd 0 nanocompo with desirable yields of 59-72%. In addition, the novel catalysts can efficiently cata the large-sized reactants for the coupling of iodo naphthalene and iodo fluorene with nyl acetylene (entry 13,14), indicating the high feasibility of molecular size. Conclusiv the TiO2-MMT60/PCN40@Pd 0 nanocomposite developed in this study shows similarly catalytic efficiency as compared with recently reported heterogeneous Pd catalysts fo nogashira reactions with similar reaction conditions [23,[75][76][77][78][79][80]. The good dispersion tight loading of the Pd species in the highly porous, stable, and amphiphilic TiO2-pill MMT-supported PCN matrix is likely the main reason for this high catalytic efficienc achieved excellent yields of 92-99%. Furthermore, the reactions of the aryl iodides substituted with electron-withdrawing groups of -F, -Br, and -Cl at ortho (entry 5), meta (entry 6), or para (entry 7) positions with phenylacetylene achieved excellent yields of 93-99%. For the entries 8-12, the reactions of the substituted aryl bromides with phenyl acetylene can be still effectively catalyzed with the novel TiO2-MMT60/PCN40-Pd 0 nanocomposite with desirable yields of 59-72%. In addition, the novel catalysts can efficiently catalyze the large-sized reactants for the coupling of iodo naphthalene and iodo fluorene with phenyl acetylene (entry 13,14), indicating the high feasibility of molecular size. Conclusively, the TiO2-MMT60/PCN40@Pd 0 nanocomposite developed in this study shows similarly high catalytic efficiency as compared with recently reported heterogeneous Pd catalysts for Sonogashira reactions with similar reaction conditions [23,[75][76][77][78][79][80]. The good dispersion and tight loading of the Pd species in the highly porous, stable, and amphiphilic TiO2-pillared MMT-supported PCN matrix is likely the main reason for this high catalytic efficiency.  nyl acetylene (entry 13,14), indicating the high feasibility of molecular size. Conclusively, the TiO2-MMT60/PCN40@Pd 0 nanocomposite developed in this study shows similarly high catalytic efficiency as compared with recently reported heterogeneous Pd catalysts for Sonogashira reactions with similar reaction conditions [23,[75][76][77][78][79][80]. The good dispersion and tight loading of the Pd species in the highly porous, stable, and amphiphilic TiO2-pillared MMT-supported PCN matrix is likely the main reason for this high catalytic efficiency.

Materials and Methods
Materials: G-105 type Na+-MMT clay from Nanocor Co., USA, with cationic change capacity of 145 meq/100 g, was used as the starting material. Chitosan (CS) f Zhejiang Aoxing Biotechnology Co., Ltd., viscosity molecular weight of 1.2 × 10 5 deacetylated degree of 95%, was used as the precursor of porous carbon (PCN). All o chemical reagents and reactant molecules involved in Sonogashira reactions used in study were of analytical grade without further purification.
Preparations: The TiO2-modified MMT was prepared using similar processes a recent studies [39]. TiCl4 was added dropwise into 2 mol/L HCl solution under mechan stirring and ice-water (0 °C) bath. The mixture was then diluted to reach the concentra of 0.6 mol/L of H + and 0.82 mol/L of Ti 4+ , respectively. The pillaring solution was aged 12 h at 25 °C prior to its use. The pillaring solution was added dropwise into 10 wt% N MMT clay aqueous suspension, to reach the Ti 4+ /Na + -MMT clay ratio of 20 mmol/1 g. mixed suspension was kept mechanically stirring for 6 h at 60 °C, and then it was ce fuged and washed with deionized water to neutral. After naturally drying, the mix was calcined in a tubular muffle furnace (BTF-1600C, Anhui BEQ Equipment Techno Co., Ltd., Hefei, China) for 3 h at a temperature of 500 °C in N2 atmosphere. A total o and/or 1.33 g of CS was dissolved in 200 mL of 2 wt% CH3COOH solution. An amoun 5 mL of Na2PdCl4 solution (containing 0.09 mmol of Pd) was added dropwise into the solution. A total of 2 g of the resultant TiO2-modified MMT was added into 200 mL o above CS or CS-Pd 2+ complex solution, and continuously stirred at 60 °C (water-bath h ing) for 10 h. The TiO2-MMT/CS or TiO2-MMT/CS@Pd 2+ products were separated from catalytic efficiency as compared with recently reported heterogeneous Pd catalysts for Sonogashira reactions with similar reaction conditions [23,[75][76][77][78][79][80]. The good dispersion and tight loading of the Pd species in the highly porous, stable, and amphiphilic TiO2-pillared MMT-supported PCN matrix is likely the main reason for this high catalytic efficiency.

Materials and Methods
Materials: G-105 type Na+-MMT clay from Nanocor Co., USA, with cationi change capacity of 145 meq/100 g, was used as the starting material. Chitosan (CS) Zhejiang Aoxing Biotechnology Co., Ltd., viscosity molecular weight of 1.2 × 10 5 deacetylated degree of 95%, was used as the precursor of porous carbon (PCN). All o chemical reagents and reactant molecules involved in Sonogashira reactions used in study were of analytical grade without further purification.
Preparations: The TiO2-modified MMT was prepared using similar processes recent studies [39]. TiCl4 was added dropwise into 2 mol/L HCl solution under mecha stirring and ice-water (0 °C) bath. The mixture was then diluted to reach the concentr of 0.6 mol/L of H + and 0.82 mol/L of Ti 4+ , respectively. The pillaring solution was age 12 h at 25 °C prior to its use. The pillaring solution was added dropwise into 10 wt% MMT clay aqueous suspension, to reach the Ti 4+ /Na + -MMT clay ratio of 20 mmol/1 g mixed suspension was kept mechanically stirring for 6 h at 60 °C, and then it was ce fuged and washed with deionized water to neutral. After naturally drying, the mi was calcined in a tubular muffle furnace (BTF-1600C, Anhui BEQ Equipment Techno Co., Ltd., Hefei, China) for 3 h at a temperature of 500 °C in N2 atmosphere. A total o and/or 1.33 g of CS was dissolved in 200 mL of 2 wt% CH3COOH solution. An amou 5 mL of Na2PdCl4 solution (containing 0.09 mmol of Pd) was added dropwise into th solution. A total of 2 g of the resultant TiO2-modified MMT was added into 200 mL o above CS or CS-Pd 2+ complex solution, and continuously stirred at 60 °C (water-bath ing) for 10 h. The TiO2-MMT/CS or TiO2-MMT/CS@Pd 2+ products were separated from the large-sized reactants for the coupling of iodo naphthalene and iodo fluorene with phenyl acetylene (entry 13,14), indicating the high feasibility of molecular size. Conclusively, the TiO2-MMT60/PCN40@Pd 0 nanocomposite developed in this study shows similarly high catalytic efficiency as compared with recently reported heterogeneous Pd catalysts for Sonogashira reactions with similar reaction conditions [23,[75][76][77][78][79][80]. The good dispersion and tight loading of the Pd species in the highly porous, stable, and amphiphilic TiO2-pillared MMT-supported PCN matrix is likely the main reason for this high catalytic efficiency.

Materials and Methods
Materials: G-105 type Na+-MMT clay from Nanocor Co., USA, with cationic change capacity of 145 meq/100 g, was used as the starting material. Chitosan (CS) f Zhejiang Aoxing Biotechnology Co., Ltd., viscosity molecular weight of 1.2 × 10 5 deacetylated degree of 95%, was used as the precursor of porous carbon (PCN). All o chemical reagents and reactant molecules involved in Sonogashira reactions used in study were of analytical grade without further purification.
Preparations: The TiO2-modified MMT was prepared using similar processes a recent studies [39]. TiCl4 was added dropwise into 2 mol/L HCl solution under mechan stirring and ice-water (0 °C) bath. The mixture was then diluted to reach the concentra of 0.6 mol/L of H + and 0.82 mol/L of Ti 4+ , respectively. The pillaring solution was aged 12 h at 25 °C prior to its use. The pillaring solution was added dropwise into 10 wt% MMT clay aqueous suspension, to reach the Ti 4+ /Na + -MMT clay ratio of 20 mmol/1 g. mixed suspension was kept mechanically stirring for 6 h at 60 °C, and then it was ce fuged and washed with deionized water to neutral. After naturally drying, the mix was calcined in a tubular muffle furnace (BTF-1600C, Anhui BEQ Equipment Techno Co., Ltd., Hefei, China) for 3 h at a temperature of 500 °C in N2 atmosphere. A total o and/or 1.33 g of CS was dissolved in 200 mL of 2 wt% CH3COOH solution. An amoun 5 mL of Na2PdCl4 solution (containing 0.09 mmol of Pd) was added dropwise into th solution. A total of 2 g of the resultant TiO2-modified MMT was added into 200 mL o above CS or CS-Pd 2+ complex solution, and continuously stirred at 60 °C (water-bath h ing) for 10 h. The TiO2-MMT/CS or TiO2-MMT/CS@Pd 2+ products were separated from For the entries 8-12, the reactions of the substituted aryl bromides with phenyl acetylene can be still effectively catalyzed with the novel TiO2-MMT60/PCN40-Pd 0 nanocomposite with desirable yields of 59-72%. In addition, the novel catalysts can efficiently catalyze the large-sized reactants for the coupling of iodo naphthalene and iodo fluorene with phenyl acetylene (entry 13,14), indicating the high feasibility of molecular size. Conclusively, the TiO2-MMT60/PCN40@Pd 0 nanocomposite developed in this study shows similarly high catalytic efficiency as compared with recently reported heterogeneous Pd catalysts for Sonogashira reactions with similar reaction conditions [23,[75][76][77][78][79][80]. The good dispersion and tight loading of the Pd species in the highly porous, stable, and amphiphilic TiO2-pillared MMT-supported PCN matrix is likely the main reason for this high catalytic efficiency.

Materials and Methods
Materials: G-105 type Na+-MMT clay from Nanocor Co., USA, with cationic change capacity of 145 meq/100 g, was used as the starting material. Chitosan (CS) f Zhejiang Aoxing Biotechnology Co., Ltd., viscosity molecular weight of 1.2 × 10 5 deacetylated degree of 95%, was used as the precursor of porous carbon (PCN). All o chemical reagents and reactant molecules involved in Sonogashira reactions used in study were of analytical grade without further purification.
Preparations: The TiO2-modified MMT was prepared using similar processes a recent studies [39]. TiCl4 was added dropwise into 2 mol/L HCl solution under mechan stirring and ice-water (0 °C) bath. The mixture was then diluted to reach the concentra of 0.6 mol/L of H + and 0.82 mol/L of Ti 4+ , respectively. The pillaring solution was aged 12 h at 25 °C prior to its use. The pillaring solution was added dropwise into 10 wt% MMT clay aqueous suspension, to reach the Ti 4+ /Na + -MMT clay ratio of 20 mmol/1 g. mixed suspension was kept mechanically stirring for 6 h at 60 °C, and then it was ce fuged and washed with deionized water to neutral. After naturally drying, the mix was calcined in a tubular muffle furnace (BTF-1600C, Anhui BEQ Equipment Techno Co., Ltd., Hefei, China) for 3 h at a temperature of 500 °C in N2 atmosphere. A total o and/or 1.33 g of CS was dissolved in 200 mL of 2 wt% CH3COOH solution. An amou 5 mL of Na2PdCl4 solution (containing 0.09 mmol of Pd) was added dropwise into th solution. A total of 2 g of the resultant TiO2-modified MMT was added into 200 mL o above CS or CS-Pd 2+ complex solution, and continuously stirred at 60 °C (water-bath h ing) for 10 h. The TiO2-MMT/CS or TiO2-MMT/CS@Pd 2+ products were separated from suspension by centrifugation. After washing with deionized water to neutral and n achieved excellent yields of 92-99%. Furthermore, the reactions of the aryl iodides substituted with electron-withdrawing groups of -F, -Br, and -Cl at ortho (entry 5), meta (entry 6), or para (entry 7) positions with phenylacetylene achieved excellent yields of 93-99%. For the entries 8-12, the reactions of the substituted aryl bromides with phenyl acetylene can be still effectively catalyzed with the novel TiO2-MMT60/PCN40-Pd 0 nanocomposite with desirable yields of 59-72%. In addition, the novel catalysts can efficiently catalyze the large-sized reactants for the coupling of iodo naphthalene and iodo fluorene with phenyl acetylene (entry 13,14), indicating the high feasibility of molecular size. Conclusively, the TiO2-MMT60/PCN40@Pd 0 nanocomposite developed in this study shows similarly high catalytic efficiency as compared with recently reported heterogeneous Pd catalysts for Sonogashira reactions with similar reaction conditions [23,[75][76][77][78][79][80]. The good dispersion and tight loading of the Pd species in the highly porous, stable, and amphiphilic TiO2-pillared MMT-supported PCN matrix is likely the main reason for this high catalytic efficiency.

Materials and Methods
Materials: G-105 type Na+-MMT clay from Nanocor Co., USA, with cationic change capacity of 145 meq/100 g, was used as the starting material. Chitosan (CS) f Zhejiang Aoxing Biotechnology Co., Ltd., viscosity molecular weight of 1.2 × 10 5 deacetylated degree of 95%, was used as the precursor of porous carbon (PCN). All o chemical reagents and reactant molecules involved in Sonogashira reactions used in study were of analytical grade without further purification.
Preparations: The TiO2-modified MMT was prepared using similar processes a recent studies [39]. TiCl4 was added dropwise into 2 mol/L HCl solution under mechan stirring and ice-water (0 °C) bath. The mixture was then diluted to reach the concentra of 0.6 mol/L of H + and 0.82 mol/L of Ti 4+ , respectively. The pillaring solution was aged 12 h at 25 °C prior to its use. The pillaring solution was added dropwise into 10 wt% MMT clay aqueous suspension, to reach the Ti 4+ /Na + -MMT clay ratio of 20 mmol/1 g. mixed suspension was kept mechanically stirring for 6 h at 60 °C, and then it was ce fuged and washed with deionized water to neutral. After naturally drying, the mix was calcined in a tubular muffle furnace (BTF-1600C, Anhui BEQ Equipment Techno Co., Ltd., Hefei, China) for 3 h at a temperature of 500 °C in N2 atmosphere. A total o and/or 1.33 g of CS was dissolved in 200 mL of 2 wt% CH3COOH solution. An amou 5 mL of Na2PdCl4 solution (containing 0.09 mmol of Pd) was added dropwise into th solution. A total of 2 g of the resultant TiO2-modified MMT was added into 200 mL o above CS or CS-Pd 2+ complex solution, and continuously stirred at 60 °C (water-bath h ing) for 10 h. The TiO2-MMT/CS or TiO2-MMT/CS@Pd 2+ products were separated from suspension by centrifugation. After washing with deionized water to neutral and n 2+ Molecules 2023, 28, x FOR PEER REVIEW 14 of 20 achieved excellent yields of 92-99%. Furthermore, the reactions of the aryl iodides substituted with electron-withdrawing groups of -F, -Br, and -Cl at ortho (entry 5), meta (entry 6), or para (entry 7) positions with phenylacetylene achieved excellent yields of 93-99%. For the entries 8-12, the reactions of the substituted aryl bromides with phenyl acetylene can be still effectively catalyzed with the novel TiO2-MMT60/PCN40-Pd 0 nanocomposite with desirable yields of 59-72%. In addition, the novel catalysts can efficiently catalyze the large-sized reactants for the coupling of iodo naphthalene and iodo fluorene with phenyl acetylene (entry 13,14), indicating the high feasibility of molecular size. Conclusively, the TiO2-MMT60/PCN40@Pd 0 nanocomposite developed in this study shows similarly high catalytic efficiency as compared with recently reported heterogeneous Pd catalysts for Sonogashira reactions with similar reaction conditions [23,[75][76][77][78][79][80]. The good dispersion and tight loading of the Pd species in the highly porous, stable, and amphiphilic TiO2-pillared MMT-supported PCN matrix is likely the main reason for this high catalytic efficiency.

Materials and Methods
Materials: G-105 type Na+-MMT clay from Nanocor Co., USA, with cationic exchange capacity of 145 meq/100 g, was used as the starting material. Chitosan (CS) from Zhejiang Aoxing Biotechnology Co., Ltd., viscosity molecular weight of 1.2 × 10 5 and deacetylated degree of 95%, was used as the precursor of porous carbon (PCN). All of the chemical reagents and reactant molecules involved in Sonogashira reactions used in this study were of analytical grade without further purification.
Preparations: The TiO 2 -modified MMT was prepared using similar processes as in recent studies [39]. TiCl 4 was added dropwise into 2 mol/L HCl solution under mechanical stirring and ice-water (0 • C) bath. The mixture was then diluted to reach the concentration of 0.6 mol/L of H + and 0.82 mol/L of Ti 4+ , respectively. The pillaring solution was aged for 12 h at 25 • C prior to its use. The pillaring solution was added dropwise into 10 wt% Na + -MMT clay aqueous suspension, to reach the Ti 4+ /Na + -MMT clay ratio of 20 mmol/1 g. The mixed suspension was kept mechanically stirring for 6 h at 60 • C, and then it was centrifuged and washed with deionized water to neutral. After naturally drying, the mixture was calcined in a tubular muffle furnace (BTF-1600C, Anhui BEQ Equipment Technology Co., Ltd., Hefei, China) for 3 h at a temperature of 500 • C in N2 atmosphere. A total of 0.5 and/or 1.33 g of CS was dissolved in 200 mL of 2 wt% CH 3 COOH solution. An amount of 5 mL of Na 2 PdCl 4 solution (containing 0.09 mmol of Pd) was added dropwise into the Adsorption tests: Prior to the test, a calibration curve was obtained using the standard rhodamine B solution with known concentrations of 1, 2, 4, 6, 8, 10, 12, 14, 16 mg/L determined with a UV-vis spectrophotometer (UV-754, Shanghai) at an absorbance wavelength of 554 nm. At room temperature, 0.05 g TiO 2 -MMT 80 /PCN 20 @Pd 0 or TiO 2 -MMT 60 /PCN 40 @Pd 0 nanocomposite was added into the 100 mL of 50 mg/L rhodamine B solution under stirring. After specific time intervals of 10, 20, 30, 40, 50, 60 min, the sample solutions were filtered to determine the residual concentrations with UV-vis spectrophotometer using a calibration curve. The dye removal rate at time t (%) was calculated as (C 0 -C t )/C 0 × 100%, where C 0 was the initial concentration of the dye solution, and C t was the concentration of the dye solution at time t.

Conclusions
In summary, a combination of three catalyst supports of MMT clay, TiO 2 , and PCN into a hybrid system for stabilizing Pd species achieved a synergistic improvement of the comprehensive performance of the catalyst for Sonogashira reactions. The microstructure of the TiO 2 -MMT 60 /PCN 40 @Pd 0 nanocomposite was carefully characterized using XRD, N 2 -adsorption, and TEM, etc. The TiO 2 -pillaring modification can effectively improve the mesoporous structure of MMT. In situ PCN derived from CS was well encaged in the enlarged layer spaces of the MMT. The PALS analysis sensitively detected the development of sub-nano level microdefects during the preparation and recycling process of the catalytic nanocomposites. The successful incorporation of the TiO 2 , Pd, and PCN species within the interlayer nanospace of MMT was well reflected by the changes in the microdefects' information from the PALS analysis. In addition, after continuous long-term recovery, many newly developed large-sized microdefects were sensitively detected in the PALS analysis. Correspondingly, the four-lifetime fitting was more appropriate than the usual threelifetime fitting for the analysis of the recovered catalysts. This study provided direct and instructive evidence for the decrease in the catalytic performance of recycled heterogeneous catalysts after long-term service from the aspect of sub-nano level microdefects.
Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/molecules28052399/s1, Figure S1: SEM-EDX scanning results of the TiO 2 -MMT 60 /PCN 40 @Pd 0 nanocomposites; Figure S2: N 2 adsorption-desorption isotherms of the heterogeneous catalysts; Figure S3: Performance of the heterogeneous catalysts applied in model Sonogashira reaction: A. Coupling yields vs time; B. Coupling yields vs recycling runs; Table S1: Structure parameter of heterogeneous catalysts extracted from the isotherms in Figure S2; Table S2: Model Sonogashira coupling reaction between iodo benzene and phenyl acetylene catalyzed using the heterogeneous catalysts; Table S3