Evolution of the pore and framework structure of NaY zeolite during alkali treatment and its effect on methanol oxidative carbonylation over a CuY catalyst

Abstract

Alkali treatment is widely used on aluminosilicate zeolites with high Si/Al ratios in order to fabricate mesopores in the framework. However, for zeolites with low Si/Al ratios, the effect of alkali treatment on the pore and framework structure needed further study. In this work, Y zeolite is treated with NaOH solutions of different concentrations and is used as the support for Cu-based catalysts for oxidative carbonylation of methanol to dimethyl carbonate. The physicochemical properties of the supports and corresponding catalysts are characterized by N₂ adsorption–desorption, X-ray diffraction, X-ray fluorescence, transmission electron microscopy, inductively coupled plasma mass spectrometry, X-ray photoelectron spectroscopy, and H₂-temperature-programmed reduction analyses. The results show that no obvious mesopores are formed under alkali treatment, even at high NaOH concentration. However, amorphous species present in the micropores of Y zeolite are removed, which increases the micropore surface area as well as the crystallinity. Simultaneously, the cage structure is partially destroyed, which also leads to a slight increase of the pore volume and surface area. The altered micropore structure eventually increases the content and accessibility of the exchanged Cu species, which is beneficial to the catalytic activity. When the concentration of NaOH is 0.6 M, the space time yield of dimethyl carbonate for the corresponding catalyst was 151.4 mg g⁻¹ h⁻¹ which is 3.3-fold higher than that of the untreated-Y-zeolite-supported Cu catalyst. However, further increasing the alkali treatment strength can seriously destroy the basic aluminosilicate structure of the Y zeolite and decrease its intrinsic ion-exchange capacity. This results in the formation of agglomerated CuO on the catalyst surface, which was not conducive to catalytic activity.

Introduction

Dimethyl carbonate (DMC) is widely used in organic synthesis for carbonylation and alkylation reactions as a replacement for poisonous reagent, namely, phosgene or dimethyl sulfate, and it also has significant potential for nonreactive uses such as an additive for gasoline, as a solvent for coating, and as an electrolyte for lithium batteries. ¹ Due to its low toxicity and high biodegradability, DMC is regarded as a green reagent as required under increasing environmental protection regulations.^2,3 Among the various synthetic routes to DMC, methanol oxidative carbonylation is considered as an environmentally benign process considering its moderate reaction conditions and high atom economy.^4,5 For this reaction, CuCl and CuCl₂ are used conventionally which present high catalytic activity. However, the loss of Cl ions was inevitable during the reaction, which resulted in rapid deactivation and also the corrosion of the equipment. As a result, the development of Cl-free catalysts has attracted significant attention from researchers. ⁶

As microporous crystalline solids, zeolites have a well-defined aluminosilicate framework structure, ordered microporous channels, and large specific surface areas, and are widely applied as catalyst supports. One typical application involve active metal ions that can be easily exchanged with the framework charge-balancing cations that are well-stabilized on the surface of these zeolites, which can lead to a metal catalyst with highly dispersed active sites.^7–9 In this way, the presence of chloride ions in the catalytic system can be avoided. Recent research has confirmed that the zeolite-supported Cu catalysts prepared following the above strategy demonstrate high catalytic activity for methanol oxidative carbonylation.^1,10,11

However, the typically small micropores of zeolite also bring about diffusion limitations, ¹² which are not conducive to the accessibility of reactant molecules to the active sites in the internal bulk phase of Y zeolite, as well as sufficient exposure of the active sites. ¹³ Fabrication of mesopores in zeolites is an effective way to overcome diffusion limitations. ¹⁴ Alkali treatment is a simple method to construct the mesoporous structure by selective extraction of framework Si. However, desilication is influenced by the density of Al ions,^15–17 and for zeolites with high Si/Al ratios (Si/Al = 20–80), fabrication of mesopores is more efficient than in zeolites with low Si/Al ratios (Si/Al < 20).^15,18,19 This is due to the negative charge associated with AlO₄− units that can impede the extraction of the Si through hydrolysis of Si–O–Al bonds by negatively charged hydroxy groups. ²⁰ In addition, from the viewpoint of Cu loading, the accessibility of ion-exchange sites can be promoted by fabrication of mesopores, but under alkaline treatment, Si and Al are leached at the same time and further affect the ion-exchange capacity and Cu dispersion.^21,22

Zeolite-supported Cu catalysts such as Cu–Y,^10,23 Cu–X, ²⁴ Cu–β, ²⁵ Cu–ZSM-5,^26,27 and Cu–MOR ²⁸ have been widely used for methanol oxidative carbonylation. Among these, faujasite zeolite Y–supported catalyst showed the best activity and selectivity for DMC due to its relatively large pore diameter and unique three-dimensional channel structure. ¹ In particular, the low binding energy of CO to Cu⁺ cations in Cu–Y is conducive to methoxide displacement and subsequent generation of DMC. ²⁸ However, as a micropore zeolite with a low Si/Al ratio, zeolite Y has high framework Al density. The effect of alkali treatment on textural properties, ion-exchange capacity, as well as the dispersion and activation of exchanged Cu species needs further study. Herein, Y zeolite (Si/Al ratio = 2.93) was treated with different concentrations of NaOH solution. The CuY catalysts were prepared by using the copper–ammonia solution-exchange method. The effect of alkaline treatment on the textural properties of the supports and the dispersion and reduction of loaded Cu species were investigated using different physicochemical techniques, namely, N₂ adsorption–desorption, X-ray fluorescence (XRF), X-ray diffraction (XRD), transmission electron microscopy (TEM) and H₂-temperature-programmed reduction (H₂-TPR) analyses. The prepared catalysts were successfully used for the oxidative carbonylation of methanol to DMC.

Results and discussion

Textural properties of the supports

The N₂ adsorption–desorption isotherms (Figure 1(a)) of all the alkali-treated samples maintain type I adsorption isotherms, without any hysteresis loop, indicating that no mesoporous structure was constructed during alkali treatment and that the treated Y zeolites still maintained the characteristics of microporous materials. This demonstrates that the desilication in bulk phase of Y zeolite with a low Si/Al ratio of 2.93 is very difficult, even in NaOH solution with a high concentration of 1.8 M. During this process, negatively charged AlO₄− tetrahedra in the framework repel OH⁻ because of their same charge and thereby hindering the hydrolysis of Si–O–Al bonds that eventually restrict the formation of mesopores. ¹⁸

Figure 1.

(a) N₂ adsorption–desorption isotherms. (b) Wide-angle XRD patterns of the parent and alkaline-treated Y zeolite. (c–f) TEM images of the parent Y zeolite and alkali-treated Y zeolite (insets show the respective crystal size distribution).

The detailed textural properties of the supports and catalysts are summarized in Table 1. The external surface area and micropore surface area of the treated samples increase with increasing the concentration of the NaOH solution. When the NaOH concentration increased to 1.2 M, the external surface area reached a maximum (53 m² g⁻¹). However, as the concentration of NaOH increased to 1.8 M, the external specific surface area did not show any further increase (50 m² g⁻¹). Such changes in the external surface area may be attributed to the formation of crevices on the crystal surface. Furthermore, with increasing the concentration of the NaOH solution, the micropore surface area gradually increased from 758 to 806 m² g⁻¹, and the pore volume increased from 0.11 to 0.20 cm³ g⁻¹. It is obvious that although the restricted desilication is not conducive to mesopore formation in the Y zeolite with a low Si/Al ratio, the connectivity of the micropores is improved by the alkali treatment. The reason may be related to the removal of amorphous aluminosilicate and the partly destroyed structure of the small cage of Y zeolite which was caused by desilication. However, when the concentration of the NaOH solution was 1.8 M, some of the microporous structure collapses and the micropore surface area of Y-1.8 is reduced to 677 m² g⁻¹. Similarly, the pore volume decreased to 0.13 cm³ g⁻¹.

Table 1.

Relative crystallinity, specific surface area, and pore volume of the parent and alkali-treated Y zeolite supports and the corresponding catalysts.

Support and catalyst	Relative crystallinity (%)	Total surface area (m2 g−1)	External surface area a (m2 g−1)	Micropore surface area (m2 g−1)	Pore volume b (cm3 g−1)
Y–P	100	780	23	758	0.11
Y-0.4	112	782	24	759	0.11
Y-0.6	122	837	53	784	0.17
Y-1.2	101	859	53	806	0.20
Y-1.8	93	727	50	677	0.13
Cu–Y	–	710	15	695	0.07
CuY-0.4	–	728	23	706	0.19
CuY-0.6	–	756	23	733	0.21
CuY-1.2	–	730	21	709	0.24
CuY-1.8	–	711	23	687	0.27

a

Calculated by the t-plot method.

b

Pore volume was determined at a relative pressure (P/P₀) of 0.99.

The XRD results (Figure 1(b)) show that all the alkali-treated samples exhibit characteristic diffraction patterns the same as the parent Y zeolite, indicating the retention of the framework structure of the Y zeolite even after alkali treatment. The relative crystallinities of all the samples were calculated based on the reference to the parent Y zeolite, the crystallinity of which was regarded as 100% and listed in Table 1. The relative crystallinity increased with an increase of the concentration of the NaOH solution. When the concentration of the NaOH solution was 0.6 M, the relative crystallinity of Y-0.6 was 122%. However, with a further increase of the NaOH concentration up to 1.8 M, the relative crystallinity decreased to 93%. This is due to the fact that the removal of some amorphous species present in the Y zeolite channel during alkali treatment resulted in the increased relative crystallinity. However, the crystal structures of Y zeolite are destroyed after treated with excess NaOH, leading to a decrease of the crystallinity.

The TEM images of treated Y zeolite in Figure 1(c)–(f) further confirm that no typical mesopores were formed during the alkali treatment, which is consistent with the Brunauer–Emmett–Teller (BET) results. A closer examination revealed that with increasing NaOH concentration, the particle size of these samples gradually decreased. The statistics of the particle size distribution revealed that the average particle size of the parent Y zeolite is 625 nm. In comparison, when the concentration of NaOH was 1.8 M, the average particle size is only 361 nm. In addition, numerous small defects and disordered cracks on the crystal surface of the treated Y zeolite also verify this conclusion as shown in the magnified TEM images, which indicate that etching of Si⁻ on the crystals of Y zeolite occurred from the external area to the inner phase.

The extraction of the framework Si atoms is subject to the content and distribution of Al in the bulk phase of the zeolite. The Si/Al ratio of the bulk phase and the external surface of the parent and alkali-treated Y zeolite were analyzed using XRF and X-ray photoelectron spectroscopy (XPS), respectively (Figure 2). For Y–P, the Si/Al ratio in the external area was 2.33%, which is lower than that of the whole bulk phase. This indicates that similar to the zeolite with a high Si/Al ratio, the Al distribution gradient also exists in the Y zeolite with a low Si/Al ratio. ²⁹ As shown in Figure 2, with an increasing NaOH concentration, a decrease in the Si/Al ratio is observed on both the exterior surface and the bulk phase. Due to the enrichment of Al on the external surface, the rate of decline in the Si/Al ratio in the exterior area is lower than that of the bulk phase. As a result, extraction of Si in exterior area was difficult as compared to the bulk phase. However, at high Al density, the internal desilication and formation of mesopores was not obvious. Unlike, NaY zeolite, the alkali-treated ZSM-5 zeolite with a relatively high Si/Al ratio resulted in the formation of mesopores and a hollow structure more easily because of the rich Al exterior area and also the gradient distribution of the Al content.^30,31

Figure 2.

The bulk phase and external surface Si/Al ratio of the parent and alkali-treated Y zeolite as a function of NaOH concentration.

Textural properties of CuY catalysts

The textural properties of CuY catalysts are shown in Figure 3(a) and Table 1. All the Cu catalysts possess a characteristic micropore structure as verified by the presented type I isotherm. This indicates that no obvious mesopores are formed after exchanging with the copper–ammonia solution (pH ≈ 9.5). As shown in Table 1, the surface area of the catalysts is decreased as compared to the support, which suggests that the zeolite channels are blocked by the loaded Cu species. Simultaneously, the slightly increased pore volume may be due to further enhancement by the alkaline copper–ammonia solution. However, the alkaline copper–ammonia solution cannot impact the microporous structure of untreated zeolite framework. As a result, the pore volume of CuY–P is slightly decreased under the influence of agglomerated Cu particles.

Figure 3.

(a) N₂ adsorption–desorption isotherms. (b) Wide-angle XRD patterns of CuY catalysts.

The XRD patterns of the CuY catalysts (Figure 3(b)) show that the crystal structure of the Y zeolites is retained even after the Cu exchange process. In addition, no typical diffraction peak due to the Cu species is observed in the XRD pattern of each catalyst, indicating that the Cu species are highly dispersed in the catalysts. In general, when NaY is exchanged with a weakly acidic Cu solution, such as CuSO₄ and Cu(NO₃)₂, a single Cu²⁺ occupies two exchangeable Na⁺ sites or two surface hydroxy groups. In addition, the acidic exchange environment is not conducive to the utilization of ion-exchange sites, since H⁺ ions are also competitive ions for ion-exchange. ³² In contrast, in this work, the loading of Cu was achieved by liquid ion-exchange method by using a copper–ammonia solution at pH 9.5. In this way, the competitive exchange of H⁺ ions was avoided in the weak alkaline solution. Importantly, the Cu species were present in the form of Cu(NH₃)₄·(OH)⁺ which could exchange with one Na⁺ ion and lead to more efficient Cu exchange. Even so, the total Cu loading is just 6.29 wt%. Due to the limited Cu loading, the Cu species exist mostly in the dispersed form in each catalyst, even after high temperature activation under an N₂ atmosphere.

Discussion on the content and state of Cu

TEM images confirm the dispersion state of the Cu species as shown in Figure 4. A certain amount of black spots representing the surface CuO particles can be observed in each catalyst. In Figure 4(a), the CuO particles on CuY–P have a wide particle size distribution from 2.2 to 7.2 nm with an average diameter of 4.30 nm. In contrast, the distribution of the CuO particles is more homogeneous on the treated-Y-zeolite-supported Cu catalysts. With the increasing concentration of the NaOH solution, the particle size of CuO gradually increases from 2.8 nm for CuY-0.6 to 3.4 nm for CuY-1.2 and 3.8 nm for CuY-1.8. Besides, in view of the Cu loading, along with the increasing concentration of the NaOH solution, the size of the CuO particles gradually decreases. (Additional TEM images of the catalysts are provided in Supplemental Figure S2.)

Figure 4.

TEM images of the CuY catalysts supported by parent and alkali-treated Y zeolite and the normal distribution of CuO particle size: (a) CuY–P, (b) CuY-0.6, (c) CuY-1.2, (d) CuY-1.8.

As shown in Figure 5, the total Cu loading decreases with increasing NaOH concentration. When the concentration is 1.8 M, the total Cu loading of CuY-1.8 is 4.2 wt%, which is 32% lower than the 6.2 wt% for CuY–P. This indicates that, although the extraction of framework Al was limited during NaOH treatment, along with the increased treatment strength, the loss of framework Al brought about a negative impact on the ion-exchange capacity of the Y zeolite.

Figure 5.

The Cu loading on the surface (based on XPS) and total (based on ICP) of the catalysts.

The surface Cu content was also analyzed by XPS, which revealed that the catalyst external surface contained more Cu species than the bulk phase (Figure 5). This is attributed to the higher Al content on the external surface. Moreover, due to the increased rate of Al gradient on the external surface (Figure 2), the surface Cu loading continues to increase with increasing concentration of the NaOH solution. This is supported by an increase in the surface Cu loading of CuY-0.6% to 9.0 wt% while the surface loading became a maximum for CuY-1.8 at 10.1 wt%.

H₂-TPR characterization was used to determine the content and location of each Cu species. As shown in Figure 6, each catalyst exhibited three reduction peaks in the H₂-TPR profiles. It is suggested that the exchanged Cu species realized a two-step reduction process, Cu²⁺→Cu⁺→Cu⁰ with increasing temperature. ¹¹ According to the reducibility of different Cu species, the overlapping peak in the range of 200–217 °C belongs to the reduction of CuO to Cu⁰ and Cu²⁺ in the supercage (Cu_sup²⁺) to Cu⁺. The peak at the higher temperature range of 217–307 °C can be attributed to reduction of Cu²⁺ in the small cage (Cu_small²⁺) to Cu⁺. The reduction peak above 500 °C is due to the reduction of Cu⁺ to Cu⁰. This peak consists of two kinds of Cu⁺ species, both of which are exchanged Cu species. While one is exchanged Cu⁺ derived from self-reduced Cu²⁺, the other is due to the exchanged Cu²⁺ which was incompletely reduced by hydrogen in the low temperature range of the H₂-TPR process. In general, only exchanged Cu⁺ species in the supercage is considered as the active center of the methanol oxidative carbonylation, but the content of exchanged Cu⁺ is hard to verify due to the complexity of the Cu existence form and the reduction process.

Figure 6.

(a) H₂-TPR profiles of parent and alkali-treated-Y-zeolite-supported catalysts. (b) Gaussian fitting results of H₂-TPR in the temperature range of 60–400 °C.

The H₂ consumption of different Cu species in each catalyst was calculated by Gaussian fitting, and the results are listed in Table 2. For the CuY–P catalyst, the total H₂ consumption of CuO and Cu²⁺ in the supercage is 0.16 mmol g⁻¹. After treatment with 0.4 M and 0.6 M NaOH solution, the peak areas of CuO and Cu_sup²⁺ decreased, and the corresponding H₂ consumption dropped to 0.11 and 0.08 mmol g⁻¹, respectively. This indicates that during the moderate alkali treatment, removal of amorphous species and slight destruction of the small cage promoted the number of ion-exchange sites and the reduction ability of the Cu species. However, when the concentration of the NaOH solution was increased to 1.8 M, the destruction of framework Al became more severe with further extraction of the framework Si. This also has a negative impact on the Cu exchange, leading to increased CuO formation. In addition, as shown in Figure 2, Al are more enriched on the exterior area than the bulk phase, and it can be seen from the TEM images that the damage on alkaline treatment to the zeolite framework structure is from the outside in. The numbers of destroyed aluminum species on the exterior area of the catalyst are increased on enhanced alkali treatment, which eventually led to more serious agglomeration of CuO. As a result, with an increase in alkali strength, the H₂ consumption of CuO and Cu_sup²⁺ was increased. Besides, after gradually increasing the NaOH concentration, the temperature of the Cu²⁺ reduction peak gradually decreased from 307 °C for CuY–P to 271 °C for CuY-1.8. This suggests that alkali treatment can destroy the structure of the small cage and increase the accessibility and reduction ability of Cu²⁺, which is in accordance with the BET result. For the reduction peak in the high temperature range, with an increase of the treatment strength, the reduction peak area gradually decreases, which indicates a decrease in the exchanged Cu content. This result is consistent with the trend as shown in Figure 5. After treatment with a moderate concentration of NaOH solution (0.4 M), the reduction peak in the high temperature range increased, which illustrates that the stability of exchanged Cu⁺ can be promoted by the optimized channel structure formed by NaOH treatment (Figure 1(b)). However, the reduction peaks gradually migrate to low temperature with increasing NaOH treatment strength, which indicates that the extraction of the small cage structure could improve the reducibility of Cu⁺.

Table 2.

The H₂-TPR results and catalytic activity data for the methanol oxidative carbonylation reaction.

Catalyst	H2 consumption (mmol g−1)			Catalytic activity a
	Low temperature range		High temperature range	CMeOH (%)	SDMC (%)	STYDMC (mg g−1 h−1)
	CuO and Cu2+sup	Cu2+small	Cu+→Cu0
CuY–P	0.16	0.11	0.44	1.6	37.7	35.1
CuY-0.4	0.11	0.07	0.31	2.9	59.7	100.0
CuY-0.6	0.08	0.18	0.24	5.1	52.0	151.4
CuY-1.2	0.15	0.19	0.23	2.7	63.3	97.7
CuY-1.8	0.20	0.14	0.20	1.6	52.0	46.7

a

Reaction conditions: 0.6 g catalyst, 140 °C, molar ratio of O₂:CO: CH₃OH = 1:10:8.

To sum up, appropriate alkali treatment can improve the content of exchanged Cu²⁺ and its self-reduction. Meanwhile, the accessibility of ion-exchange sites was promoted by the opened micropore structure. However, after treatment with NaOH solution of higher concentration (1.2 and 1.8 M), the Al framework was destructed, which led to a decrease in the number of ion-exchange sites and serious CuO agglomeration.

Catalytic activity

The catalytic activity for the synthesis of DMC via oxidative carbonylation over CuY catalysts was performed using a fixed-bed reactor under similar reaction conditions. The methanol conversion (C_MeOH) is given in Figure 7(a). For the CuY–P catalyst, C_MeOH is 1.6%, and after alkali treatment, the C_MeOH of each catalyst increased. With an increase in the NaOH concentration, C_MeOH increased first and then decreased. For the 0.6 M NaOH-treated catalyst, a maximum C_MeOH of 5.1% was observed. With a further increase in the treatment strength, the C_MeOH value gradually decreased to 1.6% for CuY-1.8, which is same as that of CuY–P. The space time yield of DMC (STY_DMC) based on each catalyst is shown in Figure 7(b). On account of limited C_MeOH, CuY–P exhibits a poor DMC yield of 35.1 mg g⁻¹ h⁻¹. After alkali treatment, the increase in C_MeOH dramatically improves the STY_DMC. With an increase in the NaOH concentration, the STY_DMC value is increased initially and reached a maximum value of 151.4 mg g⁻¹ h⁻¹ which is 3.3-fold higher than that of CuY–P. On further increasing the NaOH concentration, the STY_DMC declined.

Figure 7.

Catalytic performance of parent and alkali-treated-Y-zeolite-supported Cu catalysts in the oxidative carbonylation of methanol: (a) Methanol conversion (C_MeOH) of various CuY catalysts within 10 h. (b) Space time yield of DMC (STY_DMC) over different CuY catalysts with time on stream. (Reaction conditions: 0.6 g catalyst, 140 °C, molar ratio of O₂: CO: CH₃OH = 1:10:8).

Although the C_MeOH of CuY-1.8 and CuY–P are similar, the S_DMC (Supplemental Figure S3) and STY_DMC values are relatively higher. The observed STY_DMC of CuY-1.8 was 46.7 mg g⁻¹ h⁻¹. In addition, the presence of CuO in the catalyst increased the yield of by-products dimethyl ether (DME), methyl formate (MF), and dimethoxymethane (DMM), which negatively affect the yield of DMC.^10,33 However, due to multifarious influencing factors, the effect of alkali treatment on the by-product selectivity needs to be investigated further in future work.

Discussion on the catalytic activity

The influence of the alkali treatment on the framework structure and subsequent Cu dispersion ability is shown in Figure 8. Although no mesopores were formed, the connectivity of the micropores for the supports was improved due to the removal of amorphous species and the partially destroyed cage structure. More importantly, with the optimized aforementioned microporous structure, more ion-exchange sites were available, which increased the number of exchanged Cu species even though the total Cu loading was slightly decreased. However, treatment with highly concentrated alkali solution led to the destruction of the microporous structure that seriously affected the ion-exchange capacity and eventually the Cu loading. Furthermore, after high temperature calcination, these catalysts resulted in more agglomerated CuO species.

Figure 8.

The impact of alkali treatment on the microporous structure and Cu exchange capacity of the Y zeolite.

Apparently, alkali treatment can change the microporous structure and Cu exchange capacity of the Y zeolite. The catalytic performance of the prepared catalysts depends on the pore structure of the zeolite and the dispersion of Cu on its surface. Specifically, the alkali treatment affects the textural properties of Y zeolite. As shown in Table 1, the surface area of untreated-zeolite-supported CuY–P catalyst is 710 m² g⁻¹ and resulted in the minimum C_MeOH values. After alkali treatment, the micropore surface areas of the catalysts had increased (Table 1), which leads to an obvious improvement of C_MeOH and STY_DMC (Figure 7). Owing to the well-defined microporous structure, CuY-0.6 has the highest micropore surface area and maximum relative crystallinity (Table 1). The C_MeOH and STY_DMC values over CuY-0.6 reach optimums of 5.1% and 151.4 mg g⁻¹ h⁻¹, respectively. On further increasing the concentration of the NaOH solution, the micropore surface area is decreased, resulting in the declined catalytic activity. In addition, due to the limitation of the microporous structure of Y zeolite, exchanged Cu species cannot come into contact with the reactants, especially Cu located in the small cages. ¹¹ This is another reason for the low catalytic activity of CuY–P. After treatment with a low concentration of NaOH, the increased surface Cu content (Figure 5) and decreased reduction peak temperature of Cu_small²⁺ (Figure 6) illustrate the increase in the exposed exchange sites. The optimized channels and the improved content and accessibility of exchanged Cu species are conducive to the catalytic activity.

Furthermore, alkali treatment not only improves the textural properties but also influences the valence state and distribution of loaded Cu species. Before alkali treatment, due to the confinement of microporous structure and amorphous species, some of the Cu species could not exchange with the ion-exchange sites. Although CuY–P has the highest Cu loading, the large part of that are CuO species. After alkali treatment, the ion-exchange capacity of Y zeolite is improved by the destruction of the microporous structure, which promotes an increase in exchanged Cu species. Besides, the accessibility of the exchanged Cu species is improved along with the partial desilication of the pore structure. Consequently, after alkali treatment, the C_MeOH and STY_DMC are improved significantly. However, this phenomenon only occurs in the catalysts subjected to moderate alkali treatment. Further increase in the concentration of NaOH solution led to the increase in the proportion of CuO in the catalyst (Table 2), which results in declined utilization of Cu species and which is not conducive to the reaction.

The utilization of Cu species in the reaction can be further verified by the STY_DMC based on the Cu unit loading. As shown in Figure 9, although the CuY–P catalyst has the highest Cu loading, the STY_DMC on the unit mass of Cu is lower than other catalysts (567 mg g⁻¹ h⁻¹). After alkali treatment, the STY_DMC value on the unit mass of Cu for the treated catalysts is increased. Among them, CuY-0.6 has the highest STY_DMC per unit mass of Cu (2694 mg g⁻¹ h⁻¹). Although the total Cu loading of CuY-0.6 is 90.3% of CuY–P, the unit Cu activity was 4.75 times higher than that of CuY–P. Utilization of Cu species depends strongly on their dispersion state. The CuO particles on the surface of the catalyst supported by the untreated support are seriously agglomerated (Figure 4). After alkali treatment, the CuO particles become small and uniformly distributed. Compared with the CuY–P, the CuO particle size of CuY-0.6 is reduced by 36.6%. Since the partial exchangeable copper–ammonia complex [Cu(NH₃)₄·(OH)⁺] can form CuO-like clusters after calcination, the high dispersion of CuO illustrates the greater existence of exchanged Cu species. ⁹ In CuY-0.6, the high utilization of Cu species is realized by optimized Cu dispersion. However, as the concentration of the NaOH solution continued to increase, the number of effective ion-exchange sites declined, which makes the size and content of the CuO particles increase gradually and results in declined catalytic activity.

Figure 9.

Space time yields of DMC over unit masses of Cu for the CuY catalysts.

As mentioned above, although no mesopores are formed, the aluminosilicate environment of Y zeolite is changed due to alkali treatment, which brings about an obvious impact on the Cu exchange ability. In particular, the loading, the valence state, and the presence of Cu species have a close relation with the framework Al. Consequently, for the zeolites with low Si/Al ratios, initial acid treatment can modify the framework Al and facilitate the desilication process by alkali treatment, which can improve the mesopore formation and results in further improvement of the catalytic activity.

Conclusion

In this work, evolution of the pore and framework structure of NaY zeolite with a low Si/Al ratio upon alkali treatment and its effect on the methanol oxidative carbonylation over CuY catalyst were investigated. It was found that the high Al content inhibited the formation of mesopores, even at a concentration of NaOH solution as high as 1.8 M. However, the amorphous species in the micropores of Y zeolite could be removed, and slightly opened micropores based on the destruction of the cage structure can be achieved after appropriate NaOH treatment, which increases the microporous surface area as well as the crystallinity. This positively affects the content of exchanged Cu species and also the accessibility of Cu species, which were beneficial to the catalytic activity. When the concentration of NaOH was 0.6 M, the corresponding catalyst (CuY-0.6) exhibited optimal C_MeOH of 5.1% and a space time yield (STY) of 151.4 mg g⁻¹ h⁻¹. In contrast, excessive NaOH treatment on NaY could seriously destroy the basic aluminosilicate structure and decrease its intrinsic ion-exchange capacity. After high temperature activation, the formation of agglomerated CuO on the catalyst surface was observed, which was not conducive to the catalytic activity. According to the impact of desilication on the change of the framework Al structure and subsequent ion-exchange capacity, acid treatment should be considered to control framework Al before alkali treatment. In addition, pre-dealumination is also beneficial to mesopore formation during alkali treatment.

Experimental section

Alkali treatment of Y zeolite

Commercial NaY zeolite with an Si/Al ratio = 2.93 (Catalyst plant of Nankai University) was used as the parent support. The alkali-treated samples were prepared by mixing 3 g of commercial NaY zeolite (Si/Al = 2.93) with 90 mL of preheated NaOH solution (80 °C) with different concentrations, namely, 0.4, 0.6, 1.2 and 1.8 M. After stirring for 4 h, the slurry was centrifuged at 7000 r/min and washed with deionized water until the pH of the solution turned neutral. Subsequently, the sample was dried at 100 °C for 3 h, then sealed and preserved in a plastic sample sack. According to the NaOH treatment concentration, the obtained Y zeolites were denoted as Y-0.4, Y-0.6, Y-1.2, and Y-1.8, respectively. The untreated Y zeolite was denoted as Y–P (P stands for parent).

Catalyst preparation

The CuY catalysts were prepared by ion-exchange. Five grams of NaY zeolite was added into 50 mL of 0.1 M copper–ammonia solution (pH ≈ 9.5) and the mixed solution stirred for 1 h at room temperature. The filtrate and solid were separated by centrifugation three times at 7000 r/min and dried for 4 h at 100 °C. During the first centrifugation, the mixed solution was not diluted with deionized water to avoid the formation of a Cu(OH)₂ precipitate. Finally, the precursors were calcined at 600 °C under N₂ for 4 h keeping the heating rate of 2 °C min⁻¹. After cooling, the color of all the samples changed from light blue to light green, and the obtained catalysts were denoted as CuY–P and CuY–z (where, z = 0.4, 0.6, 1.2, and 1.8).

Characterization techniques

The Si/Al ratio of the supports and Cu content in the catalysts were evaluated by the XRF (DANalytical Epsilon 3×) and inductively coupled plasma mass spectrometry (ICP) (Agilent 7700) techniques. The N₂ adsorption–desorption analysis was performed on a Beishide 3H2000PS2 apparatus, and the corresponding specific surface areas were calculated from the isotherms by the BET method. The total pore volume was obtained at P/P₀ = 0.99. TEM images were obtained using a JEOL JEM-2100 transmission electron microscope. The sample was dispersed in ethanol for 40 min with ultrasound assistance, and the final samples were placed on a carbon-coated Cu grid for the test. The XRD analysis was conducted using a Bruker D4 X-ray diffractometer in the range of 5° to 60° under an exposure angular velocity of 8° min⁻¹. The location and reduction properties of the Cu species were evaluated by H₂-TPR. The sample was first pretreated at 300 °C under an N₂ flow (30 mL min⁻¹) for 0.5 h, and after natural cooling to room temperature, the sample was treated under the condition of a constant flow of 10 vol% H₂ + 90 vol% Ar from 50 to 1000 °C at a rate of 10 °C min⁻¹. The hydrogen consumption of each catalyst was continuously monitored by thermal conductivity detector (TCD). The XPS study was performed on a Thermo Scientific ESCALAB 250Xi spectrometer equipped with an Al-Kα radiation source (1486.6 eV). The binding energy of the samples was calibrated to the C1s line (284.6 eV).

Catalytic testing

The catalytic activity of the synthesized Cu-based catalysts was evaluated in a continuous fixed-bed micro reactor (Φ 6 × 450 mm) under atmospheric pressure. Methanol was introduced using a constant-flux pump (2PB-05) and vaporized in the pre-heater at 120 °C. Simultaneously, gaseous CO and O₂ reactants were introduced into the pre-heater and mixed with the vaporized methanol. Next, the mixed reactants were fed into the reactor bed to catalyze the oxidative carbonylation of methanol. Then, 0.6 g of the catalyst was used, and the molar ratio of O₂:CO:methanol was kept at 1:10:8. The reaction temperature was kept at 140 °C, and the final products were automatically analyzed by an online GC (Agilent 6890N) after every 20 min. The corresponding craft flowchart is listed in Supplemental Figure S1. The STY_DMC per unit mass of catalyst and C_MeOH were calculated according to the molar percentage based on gas chromatographic data as shown in equations (1) and (2), respectively

$$ST{Y}_{\mathrm{DMC}}=\frac{n_{\mathrm{DMC}}\cdot M_{\mathrm{DMC}}}{m_{\mathrm{cat}}\cdot t}$$

(1)

where n_DMC is the molar quantity of DMC, M_DMC is the molar mass of DMC, m_cat is the mass of the catalyst, m_Cu is the mass of Cu species, and t is the reaction time

$$C_{\mathrm{MeOH}}=\frac{\mathrm{A}-\mathrm{B}}{\mathrm{A}}\cdot 100\%$$

(2)

where A is the molar injection quantity of methanol per unit time and B is the molar quantity of organic products per unit time.

STY for DMC based on unit catalyst and unit Cu was calculated based on the actual Cu loading, which was analyzed by ICP. The calculation method is listed in equation (3)

$$ST{Y}_{\mathrm{DMC}}=\frac{n_{\mathrm{DMC}}\cdot M_{\mathrm{DMC}}}{m_{\mathrm{Cu}}\cdot t}$$

(3)

Supplemental Material

Supporting_information__ID_CHL-19-0392._R1 – Supplemental material for Evolution of the pore and framework structure of NaY zeolite during alkali treatment and its effect on methanol oxidative carbonylation over a CuY catalyst

Supplemental material, Supporting_information__ID_CHL-19-0392._R1 for Evolution of the pore and framework structure of NaY zeolite during alkali treatment and its effect on methanol oxidative carbonylation over a CuY catalyst by Lifei Yan, Tingjun Fu, Jiajun Wang, Nilesh Narkhede and Zhong Li in Journal of Chemical Research