Nanocrystals Incorporated with Mordenite Zeolite Composites with Enhanced Upconversion Emission for Cu2+ Detection

In this research, upconversion nanocrystals incorporated with MOR zeolite composites were synthesized using the desilicated MOR zeolite as a host for the in situ growth of NaREF4 (RE = Y, Gd) Yb/Er nanocrystals. The structure and morphology of the composites were studied with XRD, XPS, and TEM measurements, and the spectral studies indicated that the subsequent thermal treatment can effectively improve the upconversion emission intensity of Er3+. By using the NaYF4:Yb/Er@DSi1.0MOR-HT composite that holds the strongest upconversion emission, a probe of UCNC@DSiMOR/BPEI was constructed with the modification of branched poly ethylenimine for the detection of Cu2+. It was indicated that the integrated emission intensity of Er3+ shows a linear dependence with the logarithm value of the Cu2+ concentration ranging from 0.1 to 10 μM. This study offered a feasible method for the construction of UCNC@zeolite composites with enhanced upconversion emission, which may have a potential application as fluorescent probes for the detection of various metal ions by adjusting the doping luminescent center.


Introduction
Among various photoluminescent materials, such as quantum dots, inorganic phosphors, metal-organic frameworks, and organic dyes, the lanthanide-doped upconversion nanocrystals (UCNCs) that can convert near-infrared light into short-wavelength light in the visible range have attracted great research interest.The unique properties of large anti-Stokes shifts, narrow emission peaks, and the superior photo stability of the lanthanidedoped UCNCs enable them to be applied in sensors, detection, displays, and lasers [1][2][3].
Multiple emissions of UCNCs can be readily realized to satisfy the application requirements by selecting the rare earth (RE) activator, for example, Tm 3+ for blue emission and Er 3+ or Ho 3+ for red and green emissions [4,5].In addition, the structure and composition of the host nanocrystal are key factors to generate efficient upconversion emissions.Previous studies strongly suggested that NaYF 4 and NaGdF 4 are desirable hosts due to their lowphonon energy that can effectively suppress the non-radiative relaxation, high solubility for RE ions, and high thermal stability [6,7].Another common problem encountered by the small UCNCs is the strong surface luminescence quenching due to a high surface to volume ratio [8][9][10].The coating of the inert shell on the active core UCNCs is a generally used method to spatially separate the activators and the surface quenching centers and thus greatly improve the upconversion emission intensity [11,12].It was reported that the lanthanide-doped NaGdF 4 is wrapped with a CaF 2 shell on its surface, and the heterogeneous structure of the CaF 2 shell greatly enhances its upconversion emission [13]; the growing of an inert NaGdF4 shell can effectively improve the upconversion emission intensity of NaYF4:Yb,Er@NaGdF4:Yb,Nd core-shell UCNCs under both 808 and 980 nm excitations [14].Furthermore, the construction of UCNC-based nano-micro composites is purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China.The branched poly ethylenimine (BPEI, M.W. 25,000, 30% in water) was purchased from Tansolo Co., Ltd., Shanghai, China.All chemical reagents were used without further purification.

Desilication of Mordenite Zeolite
The mordenite zeolite was modified through desilication upon alkaline treatment in NaOH solution to increase the porosity.Each 1 g of the parent zeolite was stirred in 30 mL of NaOH aqueous solution with different concentrations of 1.0 and 2.0 mol/L at 85 • C for 2 h.After that, the obtained suspensions were cooled down to room temperature, filtered, and washed with deionized water.The final products were collected via centrifugation before drying at 60 • C for 24 h.According to the NaOH aqueous solution concentration, the alkali-treated zeolites were labeled as DSixMOR, where x = 1.0, 2.0, respectively.The purchased sodium form mordenite zeolite was labeled MOR.

Synthesis of NaREF 4 :Yb/Er (RE = Y, Gd) @DSixMOR Composites
The UCNC@DSixMOR composite was prepared via the impregnation of DSixMOR in the synthesis of NaYF 4 :Yb/Er nanocrystals with the coprecipitation method, for the growth of NaYF 4 :Yb/Er in the cages of DSixMOR.In the first step, 1 mmol of RE chlorides was mixed with 6 mL OA and 15 mL ODE in a three-necked flask.The RE chlorides included 0.18 mmol YbCl 3 •6H 2 O, 0.02 mmol ErCl 3 •6H 2 O, and 0.8 mmol YCl 3 •6H 2 O.After that, the solution was mixed for 30 min in 160 • C under an argon atmosphere to dissolve the RE chlorides.After cooling down to 40 • C, 0.25 g of DSixMOR was added into the flask and stirred for 90 min.Then, 10 mL methanol solution of NaOA (0.7611 g) and 10 mL methanol solution of NH4F (0.1482 g) were added into this solution respectively.The solution was slowly heated up to 65 • C and all of the methanol was removed.Next, the solution was heated for 30 min at 280 • C under an argon atmosphere.After the solution was cooled down to room temperature, the composites were collected via centrifugation and washed with ethanol three times before being dried at 60 • C for 24 h.Finally, the obtained composites were heat-treated at 400 • C for 150 min to obtain an improved upconversion emission from Er 3+ .The resulting composites were named as NaYF 4 :Yb/Er@DSixMOR and NaYF 4 :Yb/Er@DSixMOR-HT, respectively.NaGdF 4 :Yb/Er@DSixMOR and NaGdF 4 :Yb/Er@DSixMOR-HT were synthesized using GdCl 3 •6H 2 O instead of YCl 3 •6H 2 O at the initial synthesis step; the other synthesis conditions were the same.

Anti-Interference Test
To validate the sensitivity and selectivity for the probing of Cu 2+ in aqueous solutions, various potential interference substances, including Fe 3+ , Mn 2+ , Mg 2+ , Ca 2+ , K + , Cl − , F − , CO 3 2− , and SO 4 2− , were chosen for an anti-interference test of UCNC/BPEI.For the fluorometric assay, all the other conditions were kept the same except that the concentrations of metal ions and the anion solution were settled to 100 µM.The probe without adding any substances was set as a blank control.

Characterization
X-ray diffraction (XRD) analyses were recorded on a Rigaku Smartlab9 diffractometer with Cu-Kα radiation (λ = 1.5406Å, 40 KV/150 mA).The N 2 adsorption-desorption isotherm of the zeolite samples was measured via a Micromeritics ASAP2020, Atlanta, GA, USA, at 77 K.And the total surface area was calculated using the Brunauer-Emmett-Teller (BET) method, while the t-plot method was used to determine the surface area of micropores and mesopores.The pore size distribution was derived from the Barrett-Joyner-Halenda (BJH) model.X-ray photoelectron spectroscopy (XPS) characterization was carried out on a Thermo ESCALAB 250XI using Al Kα X (hν = 1486.6eV, 650 µm of beam spot) as the incident radiation source, and the electron flood gun was used to minimize surface charging.The ion content in the final alkaline treatment solution was determined via inductively coupled plasma optical emission spectroscopy (ICP-OES), (ICP-8300, PerkinElmer, America).Transmission electron microscope (TEM) analyses were carried out on a JEM-2100F TEM instrument.The upconversion spectra were measured using a set of well-aligned instruments (Zolix Instruments Co. Ltd.), and the 980 nm laser was used as excitation source.The UV-Vis NIR absorption spectra were recorded using a fluorescence spectrophotometer (U-4100).

Structure and Morphology
The influence of desilication on the structure of the parent MOR zeolite was firstly studied with XRD measurement and is shown in Figure 1.The XRD patterns of DSi1.0MOR and DSi2.0MOR are in accordance with that of the parent zeolite; it is noticable that the diffraction peak intensity of DSi1.0MOR is very strong, while that of DSi2.0MOR is extremely weak, which indicates that the alkali treatment at a concentration of 2.0 M had already destroyed the zeolite structure.Therefore, DSi1.0MOR was chosen as the target zeolite for the subsequent UCNC growth.DSi1.0MOR ensures a well-defined structure for the construction of composites incorporating UCNCs, while mesopores provide space for the growth of UCNCs without structural constraints.It can be seen from Table 1 that DSi1.0MOR exhibits a decreased specific surface area, but its mesopore surface area, mesopore volume, and average mesopore size are increased, which suggested the successful creation of enlarged mesopores upon alkali treatment [32,33].The removal of Si atoms from the framework led to the creation of voids and larger cavities within the zeolite structure.Compared with the parent MOR zeolite, DSi1.0MOR can offer improved accessibility for larger molecules.This is crucial for applications like adsorption and detection, where bulky reactants or products need efficient diffusion within the probe materials.
For further understanding the effect of alkali treatment on the elemental composition and chemical state of the parent MOR zeolite, the XPS measurement was carried out on MOR, DSi1.0MOR, and DSi2.0MOR, respectively.Figure 2a-f exhibit the high-resolution XPS spectra of Na 1s, Si 2p Al 2p, and O 1s, according to which the atomic percentages of each element were calculated, which are listed in Table 2.It can be seen that the atomic percentage of Si decreases in the alkali-treated zeolites and the corresponding Si/Al ratio obviously decreased in DSi1.0MOR and DSi2.0MOR, suggesting the successful removal of Si from the parent MOR zeolite.This is also consistent with the monotonously increased Si concentration in the alkaline treatment solution after a reaction measured using ICP (Table S1).Moreover, it is noticed from the high-resolution XPS spectra that the photoelectron peaks of Na 1s, Si 2p Al 2p, and O 1s all showed negative shifts in DSi1.0MOR and DSi2.0MOR compared with in the parent MOR zeolite; the binding energies of Na 1s, Si 2p Al 2p, and O 1s all shift in the same direction with changes in the Si/ Al ratio.This binding energy shift can be explained in terms of a charge transfer in the zeolite lattice.Zeolite desilication causes the removal of Si atoms and the breaking of Si-O-Si bonds and increases the Si-O-Al bond percentage of the zeolite framework.As shown in Figure 3d-f [34,35].It is estimated that the Si-O-Al percentages in MOR, DSi1.0MOR, and DSi2.0MOR are 32.6%, 45.3%, and 68.4%, respectively.In this case, the negative binding energy shifts of Na 1s, Si 2p, Al 2p, and O 1s of the desilicated MOR zeolites can be attributed to the increase in negative charge due to the increase in Si-O-Al bonds, and the lower the Si/Al ratio, the more negative the binding energy shift [36,37].For further understanding the effect of alkali treatment on the elemental composition and chemical state of the parent MOR zeolite, the XPS measurement was carried out on MOR, DSi1.0MOR, and DSi2.0MOR, respectively.Figure 2a-f exhibit the high-resolution XPS spectra of Na 1s, Si 2p Al 2p, and O 1s, according to which the atomic percentages of each element were calculated, which are listed in Table 2.It can be seen that the atomic percentage of Si decreases in the alkali-treated zeolites and the corresponding Si/Al ratio obviously decreased in DSi1.0MOR and DSi2.0MOR, suggesting the successful removal of Si from the parent MOR zeolite.This is also consistent with the monotonously increased Si concentration in the alkaline treatment solution after a reaction measured using ICP (Table S1).Moreover, it is noticed from the high-resolution XPS spectra that the photoelectron peaks of Na 1s, Si 2p Al 2p, and O 1s all showed negative shifts in DSi1.0MOR and  After the in situ growth of UCNCs in the DSi1.0MORzeolite and the subsequent thermal treatment, the structures of the compounds were firstly characterized with XRD measurements.As shown in Figure 3a, the XRD patterns of NaYF 4 :Yb/Er@DSi1.0MORand NaYF 4 :Yb/Er@DSi1.0MOR-HTboth maintain the typical diffraction peaks belong to the MOR zeolite, and meanwhile the diffraction peaks only assigned to α-NaYF 4 and assigned to both α-NaYF 4 and β-NaYF 4 nanocrystals can be observed before and after the thermal treatment, respectively, due to the heat treatment-induced phase transition [38].The XRD patterns for NaGdF 4 :Yb/Er@DSi1.0MORand NaGdF 4 :Yb/Er@DSi1.0MOR-HTcomposites show similar characteristics, which both keep the diffraction peaks belong to the MOR zeolite together with additional diffraction peaks assigned to NaGdF 4 .It is noticed that the (110) and (101) plane diffractions of NaGdF 4 can be clearly discriminated after thermal treatment, indicating an increased crystal size.The TEM images of NaYF 4 : Yb/Er @DSi1.0MOR-HTand NaGdF 4 : Yb/Er @DSi1.0MOR-HT in Figure 3c,d both show some black dots on DSi1.0MOR substrates, and no free nanoparticles can be observed.The locally magnified high-resolution TEM images reveal the fine crystalline structure of the black dots, as shown in Figure 3(c2,d2); the lattice spacing of 0.297 nm well corresponds to the (110) plane of NaYF 4 , and the lattice space of 0.297 nm is in good accordance with the (101) plane of NaGdF 4 .Moreover, the XPS measurement was carried out and the survey spectra of NaREF 4 :Yb/ Er@DSi1.0MOR(RE = Y, Gd) are shown in Figure 4a,b, from which the Na 1s, Si 2p, Al 2p, and O 1s photoelectron peaks belonging to DSi1.0MOR and the F 1s, Y 3d, and Gd 4d photoelectron peaks belonging to NaREF 4 can be observed, respectively.The high-resolution XPS spectra of Si 2p, Al 2p, and O 1s in Figure 4c-e show that the photoelectron peaks of the elements that constitute the host zeolite all positively shift in NaREF 4 :Yb/Er@DSi1.0MOR-HTcompared with those in DSi1.0MOR.The increased banding energies of Si 2p, Al 2p, and O 1s in NaREF 4 :Yb/Er@DSi1.0MOR(RE = Y, Gd) composites imply that the constitution ions of DSi1.0MOR have undergone a charge transfer with the NaREF 4 :Yb/Er nanocrystal and therefore the existing chemical complex between them [39][40][41].After the in situ growth of UCNCs in the DSi1.0MORzeolite and the subsequent thermal treatment, the structures of the compounds were firstly characterized with XRD measurements.As shown in Figure 3a, the XRD patterns of NaYF4:Yb/Er@DSi1.0MOR and NaYF4:Yb/Er@DSi1.0MOR-HTboth maintain the typical diffraction peaks belong to the MOR zeolite, and meanwhile the diffraction peaks only assigned to α-NaYF4 and assigned to both α-NaYF4 and β-NaYF4 nanocrystals can be observed before and after the thermal treatment, respectively, due to the heat treatment-induced phase transition [38].The XRD patterns for NaGdF4:Yb/Er@DSi1.0MOR and NaGdF4:Yb/Er@DSi1.0MOR-HT composites show similar characteristics, which both keep the diffraction peaks belong to the MOR zeolite together with additional diffraction peaks assigned to NaGdF4.It is noticed that the (110) and (101) plane diffractions of NaGdF4 can be clearly discriminated after thermal treatment, indicating an increased crystal size.The TEM images of NaYF4: Yb/Er @DSi1.0MOR-HTand NaGdF4: Yb/Er @DSi1.0MOR-HT in Figure 3c,d both show some black dots on DSi1.0MOR substrates, and no free nanoparticles can be observed.The locally magnified high-resolution TEM images reveal the fine crystalline structure of the black dots, as shown in Figure 3(c2,d2); the lattice spacing of 0.297 nm well corresponds to the (110) plane of NaYF4, and the lattice space of 0.297 nm is in good accordance with the (101) plane of NaGdF4.
Moreover, the XPS measurement was carried out and the survey spectra of NaREF4:Yb/Er@DSi1.0MOR (RE = Y, Gd) are shown in Figure 4a,b, from which the Na 1s,

Upconversion Properties
Under the excitation of a 980 nm laser, the upconversion luminescence spectrum of NaREF 4 :Yb/Er@DSi1.0MORexhibits a characteristic emission of Er 3+ in the visible region, in which the emission peaks located at 521, 542, and 654 nm can be well attributed to the radiative transitions of Er 3+ : 2 H 11/2 → 4 I 15/2 , 4 S 3/2 → 4 I 15/2 , and 4 F 9/2 → 4 I 15/2 , respectively, as shown in Figure 5 [42,43].After heat treatment, the upconversion emission intensity of Er 3+ is drastically enhanced; even Er 3+ : 2 H 9/2 → 4 I 15/2 radiative transition at 409 nm can be clearly observed in NaREF 4 :Yb/Er@DSi1.0MOR-HT.It is estimated from the emission spectra that the heat treatment-induced enhancements in emission intensity are 4.9 and 2.2 times for NaYF 4 :Yb/Er@DSi1.0MORand NaGdF 4 :Yb/Er@DSi1.0MOR,respectively.According to previous studies, proper thermal treatment can promote the formation of tightly combined interfaces between the nanocrystals and the host zeolite, which can effectively modify the surface defects of the nanocrystals, and thus the improved upconversion emission can be obtained [28].However, the increase in nanocrystal size after thermal treatment can also reduce the non-radiative relaxation probability of the surface RE ions and contributes to the strong upconversion emission [14,44].As shown in Figure 5, the upconversion emission intensity of Er 3+ is strongest in NaYF 4 :Yb/Er@DS1.0MOR-HTamong these four considered samples.
electron peaks of the elements that constitute the host zeolite all positively shift in NaREF4:Yb/Er@DSi1.0MOR-HT compared with those in DSi1.0MOR.The increased banding energies of Si 2p, Al 2p, and O 1s in NaREF4:Yb/Er@DSi1.0MOR (RE = Y, Gd) composites imply that the constitution ions of DSi1.0MOR have undergone a charge transfer with the NaREF4:Yb/Er nanocrystal and therefore the existing chemical complex between them [39][40][41].

Upconversion Properties
Under the excitation of a 980 nm laser, the upconversion luminescence spectrum of NaREF4:Yb/Er@DSi1.0MOR exhibits a characteristic emission of Er 3+ in the visible region, in which the emission peaks located at 521, 542, and 654 nm can be well attributed to the radiative transitions of Er 3+ : 2 H11/2→ 4 I15/2, 4 S3/2→ 4 I15/2, and 4 F9/2→ 4 I15/2, respectively, as shown in Figure 5 [42,43].After heat treatment, the upconversion emission intensity of Er 3+ is drastically enhanced; even Er 3+ : 2 H9/2→ 4 I15/2 radiative transition at 409 nm can be clearly observed in NaREF4:Yb/Er@DSi1.0MOR-HT.It is estimated from the emission spectra that the heat treatment-induced enhancements in emission intensity are 4.9 and 2.2 times for NaYF4:Yb/Er@DSi1.0MOR and NaGdF4:Yb/Er@DSi1.0MOR, respectively.According to previous studies, proper thermal treatment can promote the formation of tightly combined interfaces between the nanocrystals and the host zeolite, which can effectively modify the surface defects of the nanocrystals, and thus the improved upconversion emission can be obtained [28].However, the increase in nanocrystal size after thermal treatment can also reduce the non-radiative relaxation probability of the surface RE ions and contributes to the strong upconversion emission [14,44].As shown in Figure 5, the upconversion emission intensity of Er 3+ is strongest in NaYF4:Yb/Er@DS1.0MOR-HTamong these four considered samples.

Detection of Cu 2+ via Upconversion Emission
In the present research, we used the NaYF4:Yb/Er@DSi1.0MOR-HTcomposite that holds the strongest upconversion emission to construct an example probe of UCNC@DSi-MOR/BPEI and to demonstrate the detection of Cu 2+ .The emission spectra of UCNC@DSi-MOR/BPEI and the absorption spectra of the BPEI solution and BPEI-Cu 2+ complex solution are shown in Figure S1.After BPEI was added to the solution of Cu 2+ , the amino 2+

Detection of Cu 2+ via Upconversion Emission
In the present research, we used the NaYF 4 :Yb/Er@DSi1.0MOR-HTcomposite that holds the strongest upconversion emission to construct an example probe of UCNC@DSiMOR/ BPEI and to demonstrate the detection of Cu 2+ .The emission spectra of UCNC@DSiMOR/ BPEI and the absorption spectra of the BPEI solution and BPEI-Cu 2+ complex solution are shown in Figure S1.After BPEI was added to the solution of Cu 2+ , the amino groups of BPEI on the surface of UCNC@DSiMOR/BPEI coordinate with Cu 2+ , which shows an absorption band between 450 and 700 nm [45].Upon excitation with a 980 nm laser, UCNC@DSiMOR/BPEI exhibits three emission peaks centered at 521, 542, and 654 nm, which can be attributed to Er 3+ : 2 H 11/2 → 4 I 15/2 , 4 S 3/2 → 4 I 15/2 , and 4 F 9/2 → 4 I 15/2 transitions, respectively.According to the spectra in Figure S1, the absorption spectrum of BPEI-Cu 2+ complexes overlaps with the emission spectra of UCNC@DSiMOR.Therefore, the emission of UCNC@DSiMOR will be quenched in the presence of BPEI-Cu 2+ , especially for the red emission wavelength region.In order to obtain the relationship between the upconversion emission intensity of the UCNC@DSiMOR/BPEI probe and the Cu 2+ level in water, the upconversion emission spectra of Er 3+ were measured upon the addition of different concentrations of Cu 2+ , and the corresponding results are shown in Figure 6a.In order to reduce the errors caused by experimental and environmental factors, the spectral measurements of the samples for the detection of Cu were repeated three times using the same method, and the repeatability deviations are shown in Figure 6b.We can observe that the upconversion emission intensity monotonously decreases with increases in the Cu 2+ concentration, and, meanwhile, the integrated emission intensity shows a linear dependence with the logarithm value of the Cu 2+ concentration ranging from 0.1 to 10 µM.The limit of detection (LOD) is defined as 3 s/k, where s represents the standard deviation of the blank and k represents for the slope of the linear calibration equation.Here, the LOD of the UCNC@DSiMOR/BPEI probe was determined to be 1.507 µmol/L, which is comparable to or lower than some of the previously reported detection limits for Cu 2+ (Table S2) [46][47][48][49][50][51][52].Practical applications often contain a variety of ions and molecules that can interfere with Cu 2+ detection.In order to validate the selectivity for the detection of Cu 2+ using the UCNC@DSiMOR/BPEI as probe, various potential interference metal ions and ionic clusters, including K + , Ca 2+ , Mg 2+ , Mn 2+ , Fe 3+ , Cl − , F − , CO3 2− , and SO4 2− , were chosen for an antiinterference test, in which the concentrations of the interference ions' aqueous solutions were set to 100 µM.The corresponding integrated upconversion emission intensities upon the adding of each interference solution are shown in Figure 7.To minimize errors due to experimental and environmental factors, the spectral measurements were repeated three times using the same method, and the repeatability deviations are also shown.It can be seen that compared with Cu 2+ , the presence of the considered interference ions and ionic clusters did not lead to an obvious decrease in the upconversion emission intensity of the probe.These results demonstrated the excellent sensitivity and selectivity of UCNC@DSi- Practical applications often contain a variety of ions and molecules that can interfere with Cu 2+ detection.In order to validate the selectivity for the detection of Cu 2+ using the UCNC@DSiMOR/BPEI as probe, various potential interference metal ions and ionic clusters, including K + , Ca 2+ , Mg 2+ , Mn 2+ , Fe 3+ , Cl − , F − , CO 3 2− , and SO 4 2− , were chosen for an anti-interference test, in which the concentrations of the interference ions' aqueous solutions were set to 100 µM.The corresponding integrated upconversion emission inten-sities upon the adding of each interference solution are shown in Figure 7.To minimize errors due to experimental and environmental factors, the spectral measurements were repeated three times using the same method, and the repeatability deviations are also shown.It can be seen that compared with Cu 2+ , the presence of the considered interference ions and ionic clusters did not lead to an obvious decrease in the upconversion emission intensity of the probe.These results demonstrated the excellent sensitivity and selectivity of UCNC@DSiMOR/BPEI toward the detection of Cu 2+ .
photoelectron peaks of Na 1s, Si 2p Al 2p, and O 1s all showed negative shifts in DSi1.0MOR and DSi2.0MOR compared with in the parent MOR zeolite; the binding energies of Na 1s, Si 2p Al 2p, and O 1s all shift in the same direction with changes in the Si/ Al ratio.This binding energy shift can be explained in terms of a charge transfer in the zeolite lattice.Zeolite desilication causes the removal of Si atoms and the breaking of Si-O-Si bonds and increases the Si-O-Al bond percentage of the zeolite framework.As shown in Figure 3d-f, the high-resolution XPS spectra of O 1s can be deconvoluted into three components that can be attributed to Si-O-Si, Si-O-Al, and Si-O-H, respectively.As the concentration of the NaOH solution increases, the proportion of Si-O-Si bonds decreases and the proportion of Si-O-Al bonds increases, accompanied by a decrease in Si-O-H bonds, indicating that the removal of Si atoms lead to an increased Si-O-Al bond percentage[34,35].It is estimated that the Si-O-Al percentages in MOR, DSi1.0MOR, and DSi2.0MOR are 32.6%, 45.3%, and 68.4%, respectively.In this case, the negative binding energy shifts of Na 1s, Si 2p, Al 2p, and O 1s of the desilicated MOR zeolites can be attributed to the increase in negative charge due to the increase in Si-O-Al bonds, and the lower the Si/Al ratio, the more negative the binding energy shift[36,37].

Table 1 .
Surface area, pore volume, and average pore size for MOR and DSi1.0MOR zeolites.

Table 1 .
Surface area, pore volume, and average pore size for MOR and DSi1.0MOR zeolites.