Metal-free phosphorus and boron-doped graphitic carbon nitride/zeolite hetero-linked particles for highly efficient green hydrogen production in methanol

Herein, the development of phosphorus and boron-doped graphitic carbon nitride/zeolite (P- and B-doped g-C3N4-zeolite) catalyst under three-step heating conditions was performed. The first step is to prepare g-C3N4 synthesis from urea at 500 °C. In the second step, the production of a B-doped zeolite-g-C3N4 catalyst by calcination of g-C3N4 and zeolite was obtained at a ratio of 1:1 with boric acid at 500 °C. In the third step, the obtained B-doped zeolite- g-C3N4 catalyst consists of the preparation of B- and P-doped g-C3N4-zeolite catalyst as a result of the hydrothermal method with phosphoric acid. Characterization studies of the obtained catalysts were carried out with X-ray powder diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared spectroscopy (FTIR). These obtained catalysts were used as a metal-free catalyst in the production of hydrogen (H2-P) by sodium borohydride in methanol(NaBH4-MR) for the first time in the literature. The hydrogen production rate (HGR) value for P- and B-doped g-C3N4-zeolite catalysts was 6250 ml min−1 g−1.


Introduction
Versatile carbon materials are widely used as adsorbents and catalysts in various industries (Danish et al. 2020;Liu et al. 2021a;Safa-Gamal et al. 2021). Polymeric graphite carbon nitride (g-C 3 N 4 ) has attracted great interest for use in different applications in recent years (Han et al. 2019;Wang et al. 2022b;Tang et al. 2022;Cao et al. 2022;Ye et al. 2022). However, besides the advantages of g-C 3 N 4 , there are also some limitations (Kumar et al. 2020;Liu et al. 2021b). Advances have been developed to compensate for these limitations with different methods such as elemental doping, combination with other materials, or grafting techniques (Park et al. 2019;Zhou et al. 2019;Liu et al. 2019Liu et al. , 2021bSaka 2022a, b, c;Kumar et al. 2022). Apart from single-element doping, double-element doping is also included in g-C 3 N 4 due to synergistic effects between different elements (Tuna and Simsek 2021;Pattanayak et al. 2022). Zeolite is used as a catalyst for various reactions with properties such as high surface area, pore size, and acidic regions (Merilaita et al. 2021). It is reported that zeolites can be made more effective by changing both their acidity and structure through various modification methods. The properties of the catalysts can be changed with both organic acids such as oxalic acid and tartaric acid and inorganic acids such as boric acid, hydrochloric acid, nitric acid, and phosphoric acid (Zhu et al. 2005;Zhang et al. 2013b;Jiang et al. 2014). The acid-base properties of zeolites in the crystalline aluminosilicate structure are related to the aluminium content in the frame. acidity can be regulated by crystallization, the appropriate SiO 2 /Al 2 O 3 molar ratio, other elements replacing the framework components, or modification of the zeolite (Emana and Chand 2015;Harhash et al. 2022;Yang et al. 2022). Studies show that Si − O − P bonds are formed by interaction with hydroxyl groups as a result of phosphorus doping on the zeolite. It has also been reported that phosphorus can cause alumina or react with extra-frame aluminium species to form aluminophosphates (Manrique et al. 2019). Besides, the phosphorus or boron-doped g-C 3 N 4 has also been used in application areas such as CO 2 reduction (Sagara et al. 2016), photodegradation (Yan et al. 2010), H 2 formation (Chen et al. 2018), and UO 2 2+ Responsible Editor: George Z. Kyzas reduction (Lu et al. 2016). As an alternative to fossil fuels, H 2 has an important potential in renewable energy systems with its important advantages such as non-toxicity and relatively high energy density (Liu et al. 2021c;Tayyab et al. 2022a, b, c). Safe hydrogen storage and efficient H 2 -P are important for practical applications Singla et al. 2021). Chemical hydrogen storage materials (such as NaBH 4 , LiBH 4 , and NH 3 BH 3 ) with relatively high hydrogen capacities are potential H 2 carriers (Liu et al. 2020;Saka 2022a, c). NaBH 4 is important for researchers with its important advantages such as 10.8% hydrogen density by weight, non-toxicity, high purity of the obtained H 2 , and relatively low price (Demirci and Miele 2014;Sahiner and Demirci 2017;Abdelhamid 2021). There are some limitations of the NaBH 4 hydrolysis reaction, which exhibits very little conversion ability and very slow reaction kinetics at low temperatures. As an alternative solvent, it has recently received attention for the H 2 -P from NaBH 4 -MR instead of water (Sahiner and Yasar 2016;Khan et al. 2020;Saka 2021aSaka , 2022d. Methanolysis reactions have important advantages such as higher HGR even at low temperatures and high solubility of H 2 -P from NaBH 4 -MR (Lo et al. 2007). This reaction is given below.
Different heterogeneous and homogeneous catalysts have been performed to accelerate H 2 -P from NaBH 4 -MR (Hannauer et al. 2010;Balbay and Saka 2018;Xu et al. 2019;Khan et al. 2020;Akti 2021). However, these mostly metal-containing catalyst systems have some limiting factors, such as both their high cost and harmful effects on the environment. Therefore, it is necessary to develop lowcost and environmentally friendly catalysts for the H 2 -P from NaBH 4 -MR. Today, metal-free catalysts for the H 2 -P from NaBH 4 -MR have attracted attention.
The main purpose of this study is to develop a hybrid material primarily from g-C 3 N 4 and zeolite composites, which are widely used in catalytic systems. Then, to increase the catalytic performance of the obtained hybrid material, boron and phosphorus elements are included in the structure. This work includes the development of a B-and P-doped g-C 3 N 4 -zeolite catalyst in three steps. The first step is to prepare g-C 3 N 4 synthesis from urea. In the second step, the production of a B-doped zeolite-g-C 3 N 4 catalyst by calcination of g-C 3 N 4 and zeolite was obtained at a ratio of 1:1 with boric acid at 500 °C. In the third step, the obtained B-doped zeolite-g-C 3 N 4 catalyst consists of the preparation of B and P-doped g-C 3 N 4 -zeolite catalyst as a result of the hydrothermal method with phosphoric acid. These obtained catalysts were used as a metal-free catalyst in the H 2 -P from NaBH 4 -MR for the first time in the literature. Characterization studies of the obtained catalysts were carried out with XRD, SEM-EDX, FTIR, and XPS analyses.

Materials
Urea was purchased from Tekkim (Turkey). Boric acid, NaBH 4 , phosphoric acid and methanol were obtained from Merck (Germany). These analytical-grade reagents were used without further purification.
Synthesis of g-C 3 N 4 , g-C 3 N 4 -zeolite, and Pand B-doped g-C 3 N 4 -zeolite composites g-C 3 N 4 was prepared by a copolymerization method involving calcination using urea as a precursor. Typically, 10 g of urea was taken into a crucible with a loose lid. It was then calcined in an air atmosphere for 2 h at 550 °C in a muffle furnace. Subsequently, when the furnace temperature was cooled to room temperature, a yellowish g-C 3 N 4 was obtained and stored in a closed environment for later use.
For the preparation of B-doped g-C 3 N 4 -zeolite nanocomposite, 1 g of zeolite, 1 g of g-C 3 N 4 , and 0.05 g of boric acid were placed in a crucible. Later, this mixture was crushed with the help of a pestle. Then, calcination was carried out in an air atmosphere at 500 °C for 2 h in an oven. After this process, the furnace was cooled to room temperature. The resulting sample was preserved for later use.
For the preparation of P-and B-doped g-C 3 N 4 -zeolite composite, 1 g of B doped g-C 3 N 4 -zeolite was taken into a flask. Then, 30 ml of phosphoric acid diluted 1:2 was added to this sample. Then, heating and mixing were applied at 80 °C for 4 h in a magnetic stirrer. After this process, washing with distilled water and drying in an oven at 70 °C (2 h) were carried out. The obtained P-and B-doped g-C 3 N 4zeolite composite was preserved for later use.

Characterization of catalyst composites
The synthesized metal-free catalysts were examined by X-ray diffraction (Bruker D8 diffractometer) and their crystal structures were examined by Cu-Kα radiation (λ = 1.540598Â) and a range of 10-80°.
The surface morphologies of these metal-free catalysts were investigated by SEM (Carl Zeiss EVO 18) and energydispersive X-ray spectrometry (EDX).
Surface functional groups of metal-free catalysts were investigated in the 400-4000 cm −1 frequency range using FTIR {Shimadzu 8400}.
The chemical compositions and states of metal-free catalysts were investigated by XPS (Escalab 250Xi, Thermo Fisher Co., USA).

Hydrogen measurement
The NaBH 4 -MR was measured by the water displacement method. 0.01 g of P-and B-doped g-C 3 N 4 -zeolite catalyst was placed in a dry reaction flask (50 ml). Then 0.125 g of NaBH 4 was transferred to this reaction flask. Subsequently, this reaction flask was placed in a water bath at 30 °C. The methanol (CH 3 OH) of 10 ml was injected into this reaction vial with a syringe. The reaction flask is connected via the gas outlet to a water-filled gas burette. The H 2 -P from NaBH 4 -MR pushes the water out, and the volume of H 2 produced from the burette is determined. The amount of H 2 produced is determined by obtaining the displacement volumes of the water in the burette depending on time. The HGR value was determined from the obtained H 2 volume. The scheme of the apparatus used for this reaction is given in Fig. 1.

Characterization
The crystal structure of g-C 3 N 4 , raw zeolite, g-C 3 N 4 -zeolite and P-and B-doped g-C 3 N 4 -zeolite composites were characterized by XRD (Fig. 2). Figure 1 showed diffraction peaks of the g-C 3 N 4 crystal faces at 13.2° (100) and 27.3°(002) (JCPDS 87-1526) (Cao et al. 2015). The diffraction peak at 13.2° corresponds to tri-s-triazine units, which corresponds to the typical (002) plane of conjugated aromatic systems (Wang et al. 2009). In contrast, the characteristic XRD peaks of the raw zeolite are 2θ = 9.88° (020), 11.19° (101), 13.06° (200), 17.36° (111), 19.1° (131), 22.36° (400), 26.04° (101), 26.04° (151), and 36.05° (530), which are in agreement with those of clinoptilolite crystalline structure data according to JCPDS No. 00-0025-134,911.3° (Wang et al. 2022a). After the 500 °C calcination process, XRD patterns of the g-C 3 N 4 -zeolite composites showed that the internal lattice structure of the zeolite was preserved and the crystalline degree increased significantly. It can also be stated that the characteristic peaks of g-C 3 N 4 coincide with the characteristic peaks of the raw zeolite in the g-C 3 N 4 -zeolite composite. Remarkably, the intensity of the peaks became stronger after the g-C 3 N 4 -zeolite composite interacted with boric acid and phosphoric acid. The mean crystal size of the raw zeolite, g-C 3 N 4 -zeolite, and P-and B-doped g-C 3 N 4 -zeolite composites was determined using the Debye-Scherrer equation. The results obtained are given in Table 1.
The mean crystal size of the raw zeolite, g-C 3 N 4 -zeolite, and P-and B-doped g-C 3 N 4 -zeolite composites were 0.505, 1.161, and 2.46 nm, respectively. N 2 adsorption-desorption isotherms (a) and BJH pore radius distributions (b) of raw zeolite, g-C 3 N 4 , and P-and B-doped g-C 3N4 -zeolite samples are shown in Fig. 3. Also, Table 2 gives the values obtained. BET surface area values for raw zeolite, g-C 3 N 4 , and P-and B-doped g-C 3 N 4zeolite were found to be 33.69, 51.68, and 49.38 m 2 g −1 , Fig. 1 The scheme of the apparatus used for the NaBH 4 -MR respectively. As will be understood, the surface area of the pores on the zeolite has increased due to both coating with g-C 3 N 4 and acid modifications. This result may have a positive effect on H 2 production depending on the increase in surface area. The pore radius values given in Table 2 for raw zeolite and g-C 3 N 4 vary between 1.5 and 2 nm. According to the classification accepted by the International Union of Pure and Applied Chemistry, these samples are in micropore structure according to their pore diameters. However, the pore radius value for P-and B-doped g-C 3 N 4 -zeolite was found to be 6.174 nm in the mesoporous structure with possible interactions of the acids used.
The surface morphology of g-C 3 N 4 (a), raw zeolite(b), g-C 3 N 4 -zeolite (c, d), and P-and B-doped g-C 3 N 4 -zeolite (e, f) composites were analysed using SEM (Fig. 4). In contrast, it can be seen in Fig. 4a that g-C 3 N 4 is relatively more homogeneous and porous. It has been determined that the raw zeolite has a layered structure (Fig. 4b). As shown in Fig. 4c, d, it can be stated that after the zeolite is mixed with g-C 3 N 4 and subjected to the calcination process, the porous structure seen in raw zeolite and g-C 3 N 4 decreases and a more homogeneous structure emerges. In addition, in the g-C 3 N 4 -zeolite composite sample, a structure more similar to zeolite grains emerged instead of grains corresponding to g-C 3 N 4 . This may be due to the excellent incorporation of the zeolite into the g-C 3 N 4 matrix. It can be seen from SEM micrographs of the P-and B-doped g-C 3 N 4 -zeolite composite (Figs. 4e and f) that the particles are relatively rough and have a good dispersion with a porous surface compared to the g-C 3 N 4 matrix of the zeolite. Thus, this relatively rough and porous surface will create a higher contact surface area and will further encourage the adsorption of molecules, which can positively affect the catalytic activity. Fig. 2 The crystal structure of g-C 3 N 4 , raw zeolite, g-C 3 N 4zeolite and P-and B-doped g-C 3 N 4 -zeolite composites Elemental composition of g-C 3 N 4 , g-C 3 N 4 -zeolite, and P-and B-doped g-C 3 N 4 -zeolite composites were obtained by EDX analysis (Fig. 5). O, C, N, B, P, Al, and Si elements were observed for the P-and B-doped g-C 3 N 4 -zeolite composite. EDX atomic percentage elemental analysis results of g-C 3 N 4 -zeolite and P-and B-doped g-C 3 N 4 -zeolite composites are given in Table 3. EDX analyses prove that the atomic percentage of oxygen groups is over 50% in both g-C 3 N 4 -zeolite and P-and B-doped g-C 3 N 4 -zeolite composites. It also confirms the incorporation of B and P atoms into the g-C 3 N 4 -zeolite structure with boric acid and phosphoric acid, respectively.
The functional groups of g-C 3 N 4 , raw zeolite, g-C 3 N 4zeolite, and P-and B-doped g-C 3 N 4 -zeolite composites were analysed by FTIR spectroscopy. Figure 6 shows FTIR spectroscopy for the functional groups of g-C 3 N 4 (a), raw zeolite and g-C 3 N 4 -zeolite(b) and g-C 3 N 4 -zeolite and P-and B-doped g-C 3 N 4 -zeolite(c) composites in the 650-4000 cm −1 range (a) and FTIR spectroscopy for the functional groups of g-C 3 N 4 -zeolite and P-and B-doped g-C 3 N 4 -zeolite composites in the 400-4000 cm −1 range(b). The peaks in the range of 850-900 cm −1 and 1000-1100 cm −1 for the raw zeolite are attributed to SiOH bending vibrations and strain and Si-O-Si vibrations, respectively (Fernández-Jiménez and Palomo 2005). Also, the peaks at 1070 cm −1 and 1200 cm −1 indicate the presence of Si-O Si vibrations. For g-C 3 N 4 -zeolite, there is a similar FTIR spectrum to raw zeolite. However, there is an increase in peak intensities compared to raw zeolite, possibly due to interaction with g-C 3 N 4 .In the FTIR spectrum of the zeolite calcined at 500 °C for g-C 3 N 4 -zeolite, there is dehydration of the OH groups. The zeolite crystal structure can be characterized by bands around 545 and 445 cm −1 (Zhang et al. 2012). Compared to g-C 3 N 4 -zeolite, new bands in the spectrum for P-and B-doped g-C 3 N 4 -zeolite in the 1300-1600 cm −1 range can be assigned to the absorption of stretch vibration of oxygen groups. The peak appearing at 1385 cm −1 in the FTIR spectrum(b) obtained in the region of 1300 to 2000 cm −1 for the P-and B-doped sample can be assigned to the boron atom (Balyan et al. 2020). Also, bands at 1400 cm −1 and 1519 cm −1 can be assigned to the P = O and P-OH groups, respectively, proving that H 3 PO 4 doping has been successfully performed (Ghiaci et al. 2007).
In Fig. 7, the signals of the elements C, N, and O were detected for the spectrum of g-C 3 N 4 at binding energies of 285, 395, and 529 eV. For the g-C 3 N 4 -zeolite and P-and B-doped g-C 3 N 4 -zeolite samples, signals corresponding to the elements Al, Si, C, N, P, B, and O were detected. This indicated the inclusion of P and B atoms in both the g-C 3 N 4zeolite composite material and the g-C 3 N 4 -zeolite structure. High-resolution XPS spectra for Al, Si 2p, B1s, O1s, P2P, C 1 s, and N 1 s elements were investigated for the P-and B-doped g-C 3 N 4 -zeolite sample and the results are shown in Fig. 6a-g. The peaks of the elements have been shown to vary significantly. Table 4 shows the percentages of elemental analysis by XPS analysis of g-C 3 N 4 -zeolite and P-and B-doped g-C 3 N 4zeolite catalysts. The Al and Si contents of g-C 3 N 4 -zeolite catalyst treated with boric acid and phosphoric acid were decreased compared to g-C 3 N 4 -zeolite. However, a high reduction of about 50% in alumina content was observed.
In the high-resolution spectrum of C1s (Fig. 8a), the peak at 284.04 eV is due to sp 3 C-C carbon. The peaks of the CN units at 286.0 eV and 287.50 eV are attributable to C-NH 2 and N = C-N, respectively (Wei et al. 2018;Zhao et al. 2018). A binding energy of 287.9 eV corresponds to NCO bindings. In the high-resolution spectrum of N1s (Fig. 8b), the three  peaks corresponding to 396.2 eV, 397.5 eV, and 398.9 eV are C-N = C in aromatic CN heterocycles and C-NH 2 and tertiary nitrogen N-(C) in the CN frame, respectively (Zhao et al. 2016;Devarayapalli et al. 2020). The shift of the N 1 s binding energy of the P-and B-doped g-C 3 N 4 -zeolite towards a lower binding energy than the g-C 3 N 4 -zeolite indicates that the electronic character and chemical environment of the composites can be changed by the doping of P and B (Zhou et al. 2015). For O1s, binding energies at 529.4 and 530.4 eV were associated with oxidation to oxygen. In addition, the binding energy of 530.9 eV is attributed to the water molecules adsorbed on the surface of the material (Fig. 8c) (Schoiswohl et al. 2006). The high-resolution P 2p spectrum of P-and B-doped g-C 3 N 4 -zeolite peaks at 130.0 and 131.1 eV have belonged to P2p 3/2 (P-P) and P2p 1/2 (P-P), respectively (Fig. 8d) (Zhou et al. 2021). The binding energy for P2p at 133.1 eV (Fig. 4c) can be interpreted into P-N bonds, indicating that P atoms have likely replaced C atoms in the triazine rings of g-C 3 N 4 (Zhang et al. 2013a). The high-resolution B 1 s spectrum of P-and B-doped g-C 3 N 4 -zeolite exhibits three peaks at ∼187.9,

Catalytic activities
For the H 2 -P from NaBH 4 -MR, the catalytic activities of g-C 3 N 4 , g-C 3 N 4 -zeolite, P-doped g-g-C 3 N 4 -zeolite, B-doped g-C 3 N 4 -zeolite and B-and P-doped g-C 3 N 4 -zeolite catalysts at 30 °C were performed and the results were presented in Fig. 9. The amount of catalyst was taken as 10 mg, the calcination temperature was 500 °C, and the ratio of g-C 3 N 4 and zeolite samples was taken as 1:1. HGR values for g-C 3 N 4 , g-C 3 N 4 -zeolite, P-doped g-C 3 N 4 -zeolite, B-doped g-C 3 N 4 -zeolite and B and P doped g-C 3 N 4zeolite catalysts were obtained with 2550, 2628, 2944, 4250, and 6250 ml min −1 g −1 . HGR values for the B and P doped g-C 3 N 4 -zeolite catalyst showed higher catalytic performance. The steps of H 2 -P from NaBH 4 -MR can be written as, Step 1 (2) NaBH 4 ↔ Na + + BH 4

−
Step 2 Step 3 Step 4 Step 5 The dissociation of NaBH 4 in CH 3 OH into Na + and BH 4 − ions can be considered the first reaction step. Then, BH 4 − and CH 3 OH molecules are adsorbed onto the charged B and P-doped g-C 3 N 4 -zeolite catalyst surface, forming an activated complex. This complex then dissociates into BH 3 and H 2 . At the same time, compound B (CH 3 O) 3 is formed from the reaction with the CH 3 O − and boron and H 2 are released. This reaction continues until a total of 4H 2 and NaB(OCH 3 ) 4 products are formed (Li et al. 2012) (Ali et al. 2019). It has been stated that the new metal or oxide added to the reaction medium can act as a Lewis acid or electrophilic site to increase the overall reaction rate. It is known that acids promote acidic species formation as Brønsted and Lewis sites on oxide surfaces (Kummert and Stumm 1980;Akdim et al. 2009). The catalytic properties of zeolites, which are important solid acid catalysts, are due to the strong acidity of the zeolite framework, which occurs when protons compensate for the negative charge of the zeolite framework due to the interaction of the Si 4+ lattice with Al 3+ . The acidic properties and catalytic activities of the zeolite can be strongly enhanced by incorporating extra species into the zeolite framework with various modifications (Van Speybroeck et al. 2015;Pidko 2017).
The presence of acid sites in a catalyst and the accessibility of molecules to these active sites significantly affect the catalytic behaviour. In a study on the effect of phosphorus on metal oxide surfaces, the acid properties and pore structure of the zeolite were modified with phosphorus, and the acid region distribution and strength were changed. At the same time, phosphorus can cause alumina formation in the zeolite structure and/ (3)  Like phosphoric acid, it was stated in a study that the surface was functionalized with boric acid that strong Lewis acid sites could be formed due to the electron-deficient boron atom (Ghosh and Curthoys 1983).
Therefore, both Lewis base sites and Lewis acid sites can be mentioned on this catalyst surface by heterojunction coupling and acid treatments. Lewis acid domains play an important role in catalytic reactions. The results obtained from XPS and EDX analyses proved the presence of P and B atoms in the g-C 3 N 4 and zeolite structure. Thus, on this catalyst surface, P + and B + centres can act as Lewis acid sites, and amine or imine groups in g-C 3 N 4 can act as Lewis base sites (Zhou et al. 2015). In addition, the binding of more electronegative O atoms will cause P and B atoms to be positively affected in Lewis acidity. P + and B + -centred positively charged groups can promote faster H 2 formation by providing faster adsorption of ions in CH 3 OH to the catalyst surface. Therefore, it can be said that the obtained P-and B-doped zeolite-g-C 3 N 4 plays an important role in the H 2 -P from NaBH 4 -MR.
The H 2 -P from NaBH 4 -MR with P-and B-doped zeoliteg-C 3 N 4 catalyst (10 mg) were investigated at solution temperatures ranging from 20 to 50 °C, and the results are given in Fig. 10. As expected, the H 2 production rate increased in methanol with the increase in solution temperature. For example, the HGR values obtained at temperatures between 20 and 50 °C are 2318 and 14,857 ml min −1 g −1 , respectively. It can be mentioned about a sevenfold increase in the HGR value. The activation energy (Ea) for the NaBH 4 -MR was determined by the Arrhenius equation.
where Ea, k, R, A, and T denote the activation energy, the rate constant for the reaction, the gas constant, the Arrhenius constant, and the absolute temperature, respectively. As given in Fig. 11, the Ea value for the reaction catalysed by B and P doped zeolite-g-C 3 N 4 catalyst from the slope of the Arrhenius plot was found to be 29.39 kJ mol −1 . The Ea value obtained in Table 5 is compared with the catalytic systems used for H 2 -P. It can be stated that the obtained Ea value is quite reasonable. Also, the maximum HGRs of the P-and B-doped zeolite-g-C 3 N 4 catalyst and other reported catalysts are listed in Table 5 (7) lnk = lnA − Ea∕RT Transmittance (a.u) Wavenumber (cm -1 ) g-C 3 N 4 g-C 3 N 4 -zeolite

P and B doped g-C 3 N 4 -zeolite
Raw zeolite a Fig. 6 FTIR spectroscopy for the functional groups of g-C 3 N 4 (a), raw zeolite and g-C 3 N 4 -zeolite (b) and g-C 3 N 4 -zeolite and P-and B-doped g-C 3 N 4 -zeolite (c) composites in the 650-4000 cm −1 range (a) and FTIR spectroscopy for the functional groups of g-C 3 N 4zeolite and P-and B-doped g-C 3 N 4 -zeolite composites in the 400-4000 cm. −1 range (b)   Sengel 2016, 2017;Saka et al. 2020;Wang et al. 2020a, b;Zhang et al. 2020;Saka 2021aSaka , b, c, d, e, 2022aInger et al. 2021;Dai et al. 2021;Saka and Balbay 2021;Yang et al. 2021). Reusability experiments were performed five times in succession with 10 mg of P-and B-doped zeolite-g-C 3 N 4 catalyst, 10 ml of CH 3 OH and 0.125 g of NaBH 4 at 30 °C (Fig. 12). After the release of H 2 gas in each experiment, it was continued with 0.125 g NaBH 4 again. On first use, 100% NaBH 4 conversion and H 2 release were completed in 7 min. However, on the fifth use, the H 2 release time increased to 10.5 min. Besides, 100% conversions were Fig. 9 Catalytic activities of g-C 3 N 4 , g-C 3 N 4 -zeolite, P-doped g-g-C 3 N 4 -zeolite, B-doped g-C 3 N 4 -zeolite and B-and P-doped g-C 3 N 4 -zeolite catalysts at 30 °C for the release of H 2 from NaBH 4 methanolysis obtained for each use. The possible cause of the reduction in the rate of the reaction may be the increased viscosity of the reaction solution or the effect of the reaction by-product blocking the catalyst's active sites (Gao et al. 2019). Figure 13 shows the EDS analysis (a), XRD patterns (b), and SEM analysis (c) of the catalyst obtained after NaBH 4 methanolysis reaction. According to Eq. 1, NaB(OCH 3 ) 4 by-product by the interaction between NaBH 4 and methanol is formed. EDS analysis (a) of the catalyst obtained after NaBH 4 methanolysis confirms the presence of both sodium and boron atoms on the catalyst surface. As can be understood, there is a significant increase in the peak intensity of B, C, and Na elements on the catalyst surface after the NaBH 4 methanolysis reaction. The reason for  this increase can be considered a result of the adsorption of NaBH 4 and CH 3 OH species present in the environment as a result of this reaction to the catalyst surface. At the same time, XRD pattern (b) results obtained for the related catalyst after NaBH 4 methanolysis reaction also support this conclusion. After the NaBH 4 methanolysis reaction, the peak intensities in the XRD pattern decreased considerably due to the adsorbing of the species present on the catalyst surface. SEM images obtained after NaBH 4 methanolysis (c) reaction also show that the surface of the relevant catalyst becomes more heterogeneous and rough due to the reaction.

Conclusion
In the study, the development of B and P doped g-C 3 N 4zeolite catalyst in three steps was aimed. These obtained catalysts were used as a metal-free catalyst in the H 2 -P from NaBH 4 -MR for the first time in the literature. HGR values for g-C 3 N 4 , g-C 3 N 4 -zeolite, P doped g-C 3 N 4 -zeolite, B doped g-C 3 N 4 -zeolite and B and P doped g-C 3 N 4zeolite catalysts were obtained with 2550, 2628, 2944, 4250 and 6250 ml min −1 g −1 . Characterization studies of the obtained catalysts were carried out with XRD, SEM-EDX, FTIR, and XPS analyses. Remarkably, the intensity of the XRD peaks became stronger after the g-C 3 N 4 -zeolite composite interacted with boric acid and phosphoric acid. Ea value for the reaction catalysed by B and P doped zeolite-g-C 3 N 4 catalyst was found to be 29.39 kJ mol −1 Also, the possible mechanism of B and P doped g-C 3 N 4 -zeolite catalyst for the H 2 -P from NaBH 4 -MR is considered. SEM micrographs of the Pand B-doped g-C 3 N 4 -zeolite composite shown that the particles are relatively rough and have a good dispersion with a porous surface compared to the g-C 3 N 4 matrix of the zeolite. EDX and SEM analyses prove that the atomic percentage of oxygen groups is over 50% in both g-C 3 N 4zeolite and P-and B-doped g-C 3 N 4 -zeolite composites. It also confirms the incorporation of B and P atoms into the g-C 3 N 4 -zeolite structure with boric acid and phosphoric acid, respectively. Compared to g-C 3 N 4 -zeolite, new bands in the spectrum for P-and B-doped g-C 3 N 4zeolite in the 1300-1600 cm −1 range can be assigned to the absorption of stretch vibration of oxygen groups.

Data availability
The raw/processed data required to reproduce these findings cannot be shared as the data also forms part of an ongoing study.

Declarations
Ethics approval and consent to participate Not applicable.

Competing interests
The authors declare no competing interests.