赵淑芳，黄 骏

（悉尼大学化学与生物分子工程学院,悉尼纳米中心,催化工程实验室,悉尼2006）

1 Introduction

Zeolites are inorganic crystallites containing well-defined pore structures with unique shape-selectivity and abundant acid sites with adjustable acidity［1—4］. They are successfully used as catalysts in a variety of in⁃dustrial transformation，including biomass conversions，crude oil refining，and petrochemical processes［4—6］.The framework of zeolite is composed of TO4tetrahedra（T=Si，Al，B，P，etc.）［7—10］. For example，the Brønsted acid sites（BASs）in zeolites is associated with the tetrahedral AlO4［11］. The acidity of zeolites has been proven to play a key role in various heterogeneously catalyzed reactions. Thus，a comprehensive study regard⁃ing the nature of acid sites（Brønsted or Lewis），their number，strength，as well as synergistic effects must be given for the design of catalysts with improving catalytic performance［10，12］. The first part of this review presents general strategies for characterizing the acidity in zeolites by solid-state nuclear magnetic resonance（ssNMR）spectroscopy with/without probe molecules. In addition，shape-selectivity as a unique zeolite feature can effec⁃tively control the selectivity towards target products［13—16］. In particular，shape-selectivity can be categorized into reactant，product，and transition-state shape-selectivity［17］. Therefore，the second part gives an overview of studies of the shape-selectivity in zeolites for heterogeneous catalytic reactions by ssNMR spectroscopy. The third part describes briefly the synergistic effect between the acidity and shape-selectivity for the better design of the zeolites.

2 Acidity in Zeolites

The acid properties of zeolites mainly involve the type of acid sites（Brønsted or Lewis），number，acid strength，and synergistic relations［10，12，18］. It is generally accepted that only tetra-coordinated framework Al（AlIV）in zeolites can contribute to the formation of BASs（bridging hydroxyls；Si—OH—Al）［4］. Lewis acid sites（LASs）is an electron acceptor that is often associated with extra-framework Al in zeolites［15］. Also，the incorporation of metal species（Zn，Ga，Sn，etc.）into zeolites can generate LASs［19—22］. The concentration of BASs of zeolite changes with different Si/Al ratios. The strength of BASs can adjust by controlling Al distribu⁃tion［23］.

Generally，the catalytic activity of the zeolite catalyst depends on their acidity. Thus a comprehensive un⁃derstanding of acidity is crucial to design better zeolite catalysts. At present，many methods have been pro⁃posed to investigate zeolite acidity，including titration，Hammett acidity function，TPD［24］，and FTIR spectros⁃copy of adsorbed probe molecules（CO，NH3，pyridine）［7，25—27］. However，obtaining a full characterization of zeolite acidity remains a great challenge. In recent years，ssNMR spectroscopy has developed into a powerful and reliable method in evaluating the acidity of zeolites［7，28］.

2.1 Direct Detection by1H,11B,27Al,29Si,17O,71Ga,and119Sn ssNMR

Direct detection of constituent atoms in zeolites by MAS ssNMR spectroscopy such as1H，17O，27Al，and29Si MAS ssNMR for aluminosilicate zeolites［3，29—31］，11B ssNMR for B-containing zeolites［9，32］，71Ga ssNMR for Ga-containing zeolites［19］，and119Sn ssNMR for Sn-containing zeolites［21，33］can gain information on the local structure and distribution of zeolite framework species.

For example，1H MAS ssNMR can provide direct information about various hydroxyl groups in zeolites［7］.The δ1Hresonances referring to the Brønsted acidic proton，AlOH，and non-acidic SiOH groups in zeolites are listed in Table 1. Their quantitative concentrations can be obtained by measuring the integrated area of1H MAS ssNMR spectroscopy.27Al MAS ssNMR gives information on the coordination state of Al species in microporous and hierarchical zeolites［4，27，31，34］. From Table 1，AlIVspecies related to BASs have the δ27Alreso⁃nance within the range of 50—65，whereas LASs（acidic extra-framework five-and six-coordinated Al species）have δ27Al＜50［4］. There are five different possible environments for a Si atom in zeolite framework：Si（OAl）4，Si（OAl）3（OSi），Si（OAl）2（OSi）2，Si（OAl）（OSi）3，and Si（OSi）4，which can be denoted as Si（nAl）with n≤4.29Si MAS ssNMR is suitable for observing the chemically distinct and the crystallographic non-equivalent Si sites in zeolites（Table 1）［35］. The17O MAS ssNMR has been used to observe the zeolite framework O species. The framework O sites（Si—O—Si and Al—O—Si）could be directly detected from17O NMRspectroscopy［3，36］. However，the O sites in Si—OH—Al groups cannot be clearly distinguished by17O NMR spectroscopy due to its relatively low concentration.

Table 1 Assignments of MAS NMR chemical shift of framework atoms in zeolites

Additionally，the71Ga MAS NMR spectroscopy of the Ga-modified zeolite shows a signal of tetrahedrally coordinated Ga species atδ71Ga150―160 and octahedrally coordinated Ga species atδ71Ga−7—12［19，37］. It’s also important to note that direct detection of67Zn ssNMR due to the low natural abundance of the NMR active isotope（4.1%，67Zn）［20］.

Although the one-dimensional（1D）MAS NMR is a well-established tool to explore the structural informa⁃tion of the acid sites on zeolites，the utilization of multi-nuclear or multi-dimensional ssNMR spectroscopy gives a more comprehensive picture. For example，the detailed extra-framework Al species such as Al（OH）3，AlOOH，AlO+，Al（OH）2+，and AlOH2+have been identified in dealuminated zeolites by1H double quantum（DQ）MAS ssNMR and27Al DQ MAS ssNMR［15］.

2.2 ssNMR Spectroscopy by Adsorbed Probe Molecule

As shown in Table 1，1D1H and27Al MAS ssNMR can provide chemical environments of the acid sites of zeolites，whereas they cannot reliably give the quantitative results of the strength of the acid sites in zeolites.ssNMR in combination with basic probe molecules have been employed to quantitatively assess the acid num⁃ber and strength of zeolites［35，38，39］. By far，1H MAS ssNMR of adsorbed acetonitrile-d3（CD3CN）and pyridined5，31P MAS ssNMR of adsorbed trimethylphosphine（TMP）and trimethylphosphine oxide（TMPO），13C MAS ssNMR of adsorbed 2-13C-acetone are usually utilized.

2.2.11H MAS ssNMR

1H MAS ssNMR spectroscopy of adsorbed CD3CN probe molecule has been used to determine the strength of BASs on zeolites［35，38，40］. Fig.1 shows the Brønsted acid strength of SiOHAl sites in dehydrated Hβ1 to Hβ4 zeolites with different Si/Al ratios. The adsorbate-induced resonance shifts of protons in SiOHAl groups（Δδ1H）indicate that Hβ4 zeolite（Δδ1H7.5）has the highest Brønsted acid strength compared to other Hβzeolites［38］.Furthermore，the liquid-phase benzylation performance was enhanced with increasing the Brønsted acid strength of Hβzeolites due to the formation of more aryl cation intermediates which can react with benzyl chloride.

Fig.1 1H MAS NMR spectra of dehydrated Hβ with different Si/Al ratios of Hβ1 to Hβ4[38]

Furthermore，deuterated pyridine-d5has been employed as an NMR probe molecule to characterize the acid strength of zeolites［20，41—43］. As shown in Fig.2，compared to the spectra of1H MAS ssNMR on H-ZSM-5，in the spectra of1H MAS ssNMR adsorbed pyridine-d5［20］，the downfield signals atδ1H15.5 and 19.3 can be assigned to pyridine-d5probe adsorbed on the acidic protons with different acid strength. The signals atδ1H8.3 and 6.7 are due to the formation of hydrogen bonds between pyridine-d5and non-acidic SiOH groups［20］. The SiOH group inaccessible to pyridine-d5can give the signal atδ1H1.9［20］. Pyridinium ion complexes are formed with1H chemical shifts of 12—20，and a smaller chemical shift corresponds to stronger acid strength of acid sites of zeolites.

Fig.2 1H MAS NMR spectra recorded at 7.05 T of H⁃ZSM⁃5,ZSM⁃5(G2),ZSM⁃5(I2),and ZSM⁃5(I6)(A)and pyridine⁃d5 adsorbed on these samples(B)[20]

Additionally，1H MAS NMR spectra of perfluorotributylamine adsorbed on zeolites were found to be a promising probe molecule for distinguishing the internal and external acidic sites in zeolites［44］.

H/D hydrogen exchange is an excellent test reaction to study the activation of C—H bonds by BASs of zeolites.In situ1H MAS ssNMR monitoring of the H/D hydrogen exchange rates and the exchangeEavalues be⁃tween C1-n-C4alkanes or alkylaromatics and BASs of zeolites can obtain the proton transfer behavior，and thereby giving acid strength of BASs［7，45，46］. As demonstrated by Krameret al.［47］，methane H/D exchange on acidic zeolites is determined by the acid strength，a stronger acidity resulting in a higher H/D exchange rate.Methane and benzene adsorbed on the studied samples with a certain quantity have been used as internal stan⁃dards for O—H group concentration determination［48］. The H/D exchange carried out between C6D6and surface BASs in amorphous silica networks was also confirmed byin situ1H ssNMR［18］.

Huanget al.［49］indicated that due to the strong correlation between H/D exchange activities and the acidi⁃ty of zeolites，deuterated alkylaromatics can be employed to evaluate the acid strengths of zeolites. From Fig.3，four signals appear in1H MAS ssNMR spec⁃tra when ethylbenzene was adsorbed on a zeolite［49］.Theδ1H7.3 is due to hydrogen atoms bound to the aromatic rings of ethylbenzene molecules，and theδ1H3.9 results from the bridging SiOHAl groups in the supercages of zeolite H，Na-Y. The intensity of the signal atδ1H7.3 increases with the decreasing of the signal atδ1H3.9. This indicates that the H/D ex⁃change occurs between BASs on zeolites and the ad⁃sorbate ethylbenzene molecules. Moreover，theδ1Hof 1.2 and 2.6 are caused by residual hydrogen at⁃oms in methyl and methylene groups，respective⁃ly［49］. The other deuterated aromatic molecules such as benzene-d6，ethylbenzene-d10，toluene-d8，andp-xylene-d10with a deuteration degree of 99.6%，98.0%，99.6%，and 99.5%can also be employed as probe［49］. Small C1-n-C4alkanes as the probe can detect the strength of BASs of metal-modified zeolites［50―53］.

2.2.231P MAS ssNMR

It is noteworthy that Deng and his co-workers have made many contributions to studying the acidic features of zeolites by31P ssNMR. At pres⁃ent，31P MAS ssNMR of adsorbed TMP［28，54，55］and TMPO have been widely used in distingui-shing the BASs and LASs of zeolites. Moreover，the influ⁃ence of Zn2+species introduced into the zeolite on the strength of BASs has been analyzed by ssNMR and FTIR techniques using TMP and CO as probe molecules［55］.

Fig.3 Stack plot of the 1H MAS NMR spectra recorded at the temperature of 358 K during H/D exchange of deuterated ethylbenzene loaded on dehydrated zeolite H⁃Y[49]

Fig.4 Proton⁃decoupled 31P MAS NMR spectra of MFI⁃2(a,b)and Zn/MFI⁃2(c,d)zeolite samples[55]

From Fig.4，when the zeolites are exposed to the TMP probe molecule，the signal atδ31P―4.5 in31P ssNMR spectra is observed［55］. It belongs to the formation of TMPH+species on the BASs of pure and Zn2+-exchanged ZSM-5 zeolites［55］. As TMP molecules bound to LASs normally result in δ31Pwithin the range of −30—−70［55］. Therefore，BASs and LASs in zeolites can be distinguished and their concentrations can be quantitatively determined by31P NMR spectroscopy of adsorbed TMP probe.

TMPO probe molecule interacts with both BASs and LASs in zeolites giving δ31Pin the range of ca. 53—86，and ca. 55—60，respectively in31P ssNMR spectroscopy［10，11］. A linear correlation between the strength of BASs and the δ31Phas been proposed［10，11］. However，the spectral assignments remain controversial when using 1D31P ssNMR for studying TMPO-loaded zeolites［10］. For instance，the peak at δ31P37 has been previously attributed either to TMPO interacting with LASs or TMPO confined within the zeolite pores. Two-dimensional（2D）1H-31P heteronuclear correlation ssNMR spectroscopy of TMPO molecule adsorbed in zeolites has been used to elucidate the nature of acid sites in ZSM-5 zeolite［10，11］. Fig.5 shows the δ31Pcorrelated with the Brønsted protons（δ1H＞10）range of 40―80. This evidence strongly supports the assignment of the δ31Pca. 86 to TMPO molecule interacting with LASs. Deng et al.［11］identified two types of tri-coordinated framework Al sites by the sensitivity-enhanced 2D31P-27Al heteronuclear correlation（HETCOR）MAS ssNMR spectroscopy of adsorbed TMPO. The synergistic effects arising from the close spatial proximity between the tri-coordinated framework Al site and the BASs can lead to the generation of super acidity in the zeolite［11］.

Fig.5 Schematic representation of TMPO⁃loaded HZSM⁃5[n(Si)/n(Al)=15] and silicalite⁃1(A, C, E), 1H⁃31P HETCOR NMR spectra of TMPO⁃lloaded silicalite⁃1,HZSM⁃5,and steam⁃HZM⁃5(B,D,F)[10]

2.2.313C MAS ssNMR

13C MAS ssNMR of adsorbed 2-13C-acetone probe molecule is frequently used as a probe for characterizing the acidic properties of BASs and LASs，which are associated in zeolites with the framework Al and extraframework Al species，respectively［12，43，56—58］. The hydrogen-bond interaction between the acidic proton and the carbonyl oxygen of adsorbed 2-13C-acetone normally causes a down-field shift of the13C NMR resonance of the carbonyl to higher13C chemical shifts. In general，the stronger the Brønsted acidity，the higher isδ13Cof adsorbed 2-13C-acetone. This approach was utilized to determine the Brønsted acid strength of dealuminated HY zeolite［57］.

As shown in Fig.6 curvea，in the13C NMR spectrum of 2-13C-acetone adsorbed on HY zeolite，only one sharp resonance atδ13Cca. 220 was observed due to unreacted acetone adsorbed on the BASs of HY zeolite［57］.From Fig.6 curvesb—e，the calcined HY zeolites exhibit several resonance signals atδ13C214，183，79，74，and 30 attributed to the products formed through aldol reaction of acetone，and atδ13C228 and 234 assigned to acetone adsorbed on BASs that are interacting with LASs［57］. This type of BASs has stronger acid strengths as compared to the BASs with the peak atδ13C220［57］. The shoulder peak atδ13C241 may due to the acetone direct⁃ly adsorbed on LASs［57］. It is generally accepted that theδ13Csignals of 213—225 are associated with BASs andδ13C233—245 are associated with LASs［12，59］. Also，Huanget al.［60］indicated that a weak signal atδ13C235 in Al/Na-ZSM-5/2 and Al/HZSM-5/2 zeolites is assigned to acetone adsorbed on LASs. As for the reaction of glycerol dehydration，it has been reported that BASs favor the reaction pathway of forming acrolein and LASs contribute to the pathway of generating acetol［60］.

Fig.6 13C CP/MAS NMR spectra of 2⁃13C⁃acetone adsorbed on parent H⁃Y(a),CAL⁃400(b),CAL⁃500(c), CAL⁃600(d), CAL⁃700(e), STY⁃350(f), STY⁃450(g), STY⁃550(h),OXA⁃0.8(i),and OXA⁃1.4(j)zeolites[57]

2.3 Brønsted/Lewis Acid Synergy

Enhancing the BAS strength in zeolites is very important in the various industrial process，such as al⁃kanes cracking［61］and dehydrogenation［62］. The strength of BASs can be increased due to the synergy between the BASs and LASs［7，57，60，61，63］. Lercheret al.［62］reported that two types of strong BASs（SBAS）coexist in steamed ZSM-5 zeolites. It can be seen that the turnover frequencies of overall pentane cracking and dehydro⁃genation on SBAS close to extra-framework Al atoms（EFAl-SBAS）are 50 and 80 times higher than on isolated SBAS（I-SBAS）. Huanget al.［18］indicated the synergy between BASs with zeolitic strength and LASs in amor⁃phous silica-aluminas can lead to the increasing of the strength of surface BASs. The synergistic effect of Brønsted/Lewis in aluminosilicate zeolites has been successfully observed using1H-1H double-quantum（DQ）MAS ssNMR and27Al-27Al DQ MAS ssNMR［57，58，63，64］.

As shown in Fig.7，27Al MAS and27Al DQ-MAS NMR spectra were used to study the spatial proximities of various Al species in parent HY and calcined HY zeolites at 500，600，and 700oC（denoted HY-500，HY-600，and HY-700，respectively）［64］. From Fig.7（B），the AlⅣspecies is close to the six-coordinate extraframework Al（OH）3species on the HY⁃500 zeolite because one cross-peak pair atδ（61，61）and（0，61）has been observed. The formation of Al（OH）2+species on HY⁃600 zeolite was confirmed by the new signal ofδ30. Fig.1（D）reveals detailed spatial correlations among various Al species in calcined HY⁃700 zeolite.

Fig.7 27Al MAS and DQ⁃MAS NMR spectra of parent HY(A), HY⁃500(B), HY⁃600(C), and HY⁃700(D)zeolites[64]

Moreover，the synergistic effect of metal active sizes and BASs in metal-zeolites，such as Zn-ZSM-5［20］，Ga-ZSM-5［22］，Sn-Beta zeolites［65］，Zn-Beta［50］，and Sn-MCM-41［39］has been proposed. The spatial proximity and synergistic effect of metal active sizes（serve as LASs）and BASs in metal-modified zeolites，such as Zn-ZSM-5［20］and Ga-ZSM-5［22］has been directly observed by1H-67Zn and1H-71Ga double-resonance ssNMR spectroscopy，respectively. The importance of the close spatial proximity between LASs（metal species）and BASs of zeolite has been widely noted in MTA reactions over metal-modified zeolites. The introduction of metals on zeolites leads to the formation of strong LASs. The enhanced acid strength of metal-containing zeo⁃lites，as revealed by1H MAS ssNMR of adsorbed pyridine-d5，could be attributed to the spatial proximity［22］.

3 Shape-selectivity in Zeolites

The concept of“shape-selective catalysis”proposed by Weisz［66］，is generally used to explain the selec⁃tive transformation behavior from reactants into products［67—69］. Zeolites are the ideal shape-selective catalyst for many catalytic reactions due to their micropore’s molecular-sized sieving［70］. The shape-selective catalysis using zeolite reduces the cost of chemical separation in the chemical industry. The shape-selectivity of zeolites with well-developed mesoporous surfaces can also be fully preserved［71］.

The shape-selectivity of the zeolites has a significant influence on the reaction routes and thus product distribution in heterogeneous catalysis processes，such as MTH reaction［67］，ethylbenzene disproportion⁃ation［72，73］，alkylation of benzene with ethylene［74］，and hydroconversion of heptane［75］. To control shape-selec⁃tivity in zeolites，the approach of choosing zeolite materials with suitable pore sizes or maximizing sinusoidal channels of HZSM-5［16］. This broad concept can be further summarized as the three well-accepted types of shape-selectivity including reactant，transition-state，and product selectivity［17］.

Direct observation of shape-selectivity in ZSM-5 zeolite by MAS ssNMR spectroscopy has been pro⁃posed［76］. More importantly，the application ofin situssNMR spectroscopy in heterogeneous catalysis has re⁃ceived widespread interest in recent years［77，78］. Among them，in situ13C MAS ssNMR spectroscopy is a power⁃ful tool to investigate the reaction pathways in shape catalysis of zeolites［79，80］. As an example，a triethyloxoni⁃um ion as a transition-state intermediate has been observed on zeolite H-ZSM-5 in the dehydration of ethanol to ethene by usingex situandin situssNMR spectroscopy［81］.

The reactant shape-selectivity is reflected in limiting the entry of bulky molecules into the micropore sys⁃tem of zeolites，and the product shape selectivity occurs when bulky products formed in the zeolite channels are prohibited from diffusing out of the pores［17，66，82，83］. For instance，1，3，5-triisopropylbenzene（TIPB）mole⁃cules are too large to enter the micropores of zeolites during the cracking process and thus represent examples of reactant shape selectivity［2］. Additionally，the product shape-selectivity has been reported in the catalytic hydroconversion of heptane to explain the distribution of di-branched isomers on MFI，MEL，and BEA zeolite catalysts［75］. The BEA-type zeolite with a relatively large pore produces mainly dimethylpentane isomers be⁃cause they can diffuse rapidly through the pores. In contrast，due to the diffusion limitation of dimethylpen⁃tane isomers within the smaller pores of MFI-type and MEL-type zeolites，most of the dimethylpentanes are re⁃tained into the zeolites and inevitably undergo consecutive cracking［75］.

The reactant and product shape selectivities originate from mass transfer effects［69］. This type of shape se⁃lectivity is closely related to the diffusion anisotropy of reactant/product molecules in zeolites when the pore size of zeolites is comparable to the kinetic diameter of reactant/product molecules［84］. The diffusion properties of the zeolites can be determined by Pulsed Field Gradient-Nuclear Magnetic Resonance（PFG-NMR）experi⁃ments with diffusion probe molecules，such asn-octane，cumene，or TIPB［4，15］.

The type of transition-state shape selectivity is more difficult to prove than the reactant shape and product shape selectivity because of the short lifetime of the reaction intermediates［85］. Recently，the transition-state shape selectivity that exists in the MTH reaction on SAPO-34 zeolite has been demonstrated by1H and13C MAS ssNMR spectroscopy［86］.

The hydrocarbon pool（HP）species serving as intermediates of the MTH process include alkenes，methyl⁃benzenes（MBs），and cyclic carbenium ions. Different HP species depend on the topology of zeolites. To gain insight into the identification of the crucial HP intermediates formed on the zeolites during the MTH process，1D13C ssNMR spectroscopy combined with UV/Vis spectroscopy，GC-MS and density functional theory（DFT）calculations have been investigation［87—91］. Several carbenium ions involved in the MTH reaction have been ob⁃served and confirmed as the active HP species in zeolites by1H and13C ssNMR spectroscopy［86］. The polymeth⁃ylcyclopentenyl cations are formed on H-ZSM-5，and heptamethylbenzenium on H-beta and H-SAPO-34）［86］.For 12-MR zeolites such as Beta，SAPO-34，and SSZ-13 with large pores or cages，the bulky MBs such as pentamethylbenzenes，hexamethylbenzenes，and the heptamethylbenzenium cations are formed.

The dynamic nuclear polarization（DNP）enhanced13C CP ssNMR spectroscopy and 2 D DNP-enhanced13C-13C ssNMR spectroscopy have been used to detect the formation of carbocations confined in zeolite catalysts during the MTH reaction［92］. Furthermore，the host-guest interaction between surface Si sites of zeolites and HP species by DNP-enhanced29Si｛13C｝REDOR NMR experiments［92］. The interactions between the HP species and the topology structure of zeolites depend on the shape-selectivity of zeolite. The distances and proximities are essential to probe the interactions between retained organic HP species and framework Al sites of zeolites by13C-｛27Al｝symmetry-based rotational-echo saturation-pulse double-resonance（S-RESPDOR）solid-state NMR experiments［67，93］. The supramolecular reaction center with higher reactivity toward methanol is related to the specific HP species strongly interacting with the Brønsted acid/base sites［93］. The shape-selec⁃tivity of the zeolites［67］determined the formation of supramolecular reaction center on ZSM-5，SSZ-13，and MOR zeolite with different shape-selectivities derived from their different pore sizes and shapes were demon⁃strated by13C-27Al double-resonance ssNMR spectroscopy［67］. The27Al nucleus is associated with acid sites of zeolites，and the13C nucleus is associated with trapped organic HP species. Over SSZ-13 and MOR zeolites，the supramolecular reaction center. The distances and proximities are essential to probe the interactions between zeolite and acetone by13C-｛27Al｝symmetry-based rotational-echo saturation-pulse double-resonance（S-RESPDOR）ssNMR experiments［94］. Very recently，Denget al.［88］detected the spatial proximities between the various HP species confined inside SSZ-13 zeolite using 2D13C-13C proton-driven spin diffusion（PDSD）ssNMR spectroscopy.

The ethylbenzene disproportionation on zeolite catalysts has also been known as a typical transition-state shape-selective reaction［72，73］. Huanget al.［72，73］provided the evidence for understanding the catalytic behavior of the ethylbenzene disproportionation caused by the transition-state shape-selectivity byin situ13C CP/MAS ssNMR experiments. As depicted in Scheme 1，on X and Y zeolites with a large-pore ofca. 0.74 nm，the bulky diphenylethane transition-state intermediates（D in Scheme 1，δ13C46 in13C MAS NMR spectra）was gen⁃eratedviaa bimolecular reaction mechanism of ethylbenzene disproportionation. In contrast，a monomolecular reaction pathway occurring in ZSM-5 zeolite with a medium-pore ofca. 0.56 nm contributed to the formation of surface ethoxy intermediates（J in Scheme 2，δ13C73 in13C CP NMR spectra）. As a result，the product ofp⁃diethylbenzene（H in Scheme 2）was formedviaethoxy-mediated intermolecular ethyl group transfer［73］. It is worth noting that bulky diphenylethane intermediates are difficult to form in ZSM-5 zeolites due to its transition-state shape-selective limitations［73］.

Scheme 1 Mechanism of the ethylbenzene disproportionation on acidic large⁃pore zeolites via formation of surface ethoxy species[73]

Scheme 2 Mechanism of the ethylbenzene disproportionation on acidic medium⁃pore zeolites via formation of surface ethoxy species[73]

4 Synergistic Effect of Acidity and Shape-selectivity of Zeolites

Not only the synergy between BASs and LASs of zeolite but also the synergistic effect of acidity and shape-selectivity of zeolite catalysts play a crucial role in heterogeneous catalysis. Zhaoet al.［95］suggested that there was a synergy between medium-strong BASs and the maximal space available of zeolites by investigating various zeolites with different topology structures，including SAPO-34，SUZ-4，ZSM-5，USY，MOR，and beta in the synthesis of polyoxymethylene dimethyl ethers from dimethoxymethane and trioxymethylene. Tanget al.［96］and Gonget al.［97］indicated that the combination of suitable acidity and hierarchical structure is neces⁃sary to design excellent ZSM-5 zeolite catalysts for the dehydration of glycerol to acrolein and MTH process.Xiaoet al.［14］ascribed the excellent catalytic performances in glycerol to aromatics（GTA）with the highest BTX aromatics yield，lowest coking rate，and longest catalyst lifetime to the synergistic effect of moderate acidity and porosity（the shape-selectivity of micropores，alleviative diffusional limitation of meso-micro hierar⁃chical structure，and size confinement of mesopores with smaller-aperture）. Xiaoet al.［98］proposed the synergistic effect between hierarchical structure and Sn species of Sn-containing ZSM-5 zeolite catalyst in glycerol aromatization reaction gives rise to a higher yield of BTX and higher stability. Denget al.［88］reported that the formation of naphthalene as a precursor to coke species is favored due to the reaction of aromatics with the nearby cyclopentenyl cations and correlates with both zeolite acid density and topology. The intermolecular spatial proximities/interactions between the HP species is related to both the pore structure and acid density of the zeolites［88］. 2D13C-13C PDSD NMR spectra recorded on ZSM-5 and SSZ-13 zeolite give powerful evidence［88］.

Very recently，Huanget al.［35］reported the cooperation between hierarchical pores and strong BASs on SAPO-34 zeolite catalysts for the glycerol dehydration to acrolein. Strong BASs can activate the center OH group of glycerol and initiate the dehydration to the acrolein production pathways. The introduction of a secondary network of mesopores can enhance the diffusion rate of reactants to active sites and the diffusion rate of products out of the pores［35］. Methods to verify the formation of more strong BASs in hierarchi⁃cal SAPO-34 zeolite include the29Si MAS ssNMR and1H MAS ssNMR of loading deuterated CD3CN probe molecule［35］.

Denget al.［67］demonstrated that closer spatial proximity and stronger interaction between MBs and Brønsted acid/base sites were observedvia13C-｛27Al｝S-RESPDOR MAS ssNMR over H-SSZ-13 and H-MOR zeolites， which facilitate aromatic-based reaction routes and rationalize the higher selectivity to ethene during the MTH reac⁃tion. As shown in Fig.8，the increased inter⁃actions between the two methyl groups（δ13C24.0 and 21.5）and active sites evidence the formation and growth of the carbonaceous spe⁃cies in the CHA cage of H-SSZ-13 zeolite.The broad distribution of the polyaromatics along the 12-MR channel of H-SSZ-13（0.067 nm×1.09 nm）would be responsible for the observation of the weak interactions with the active sites.

Fig.8 13C MAS NMR spectra of deactivated H⁃SSZ⁃13(a) and HMOR(b)at 400°C for 250 and 100 min,respectively[67]

5 Summary and Perspective

The surface structure，acidity，and their characterization of zeolites by ssNMR spectroscopy have been reviewed in this work，further effort is required to develop a better understanding and the construction of new acid sites for the outstanding performance in target reactions. Furthermore，the application of approaches for the characterization of a mechanism of the shape-selective reaction has been described in this review. The reaction environment of heterogeneous catalysis on zeolites is very complex. The nanopores of zeolites are one kind of nano-reactors：the location，type，density，and strength of acid sites have to collaborate with the shape，size，and structure of nanocages or pores of zeolites and make the reaction performance out of the limitation of diffusion and adsorption. The application and further development of ssNMR and itsin situtechnology are important to draw the whole picture of the reaction environment inside zeolite pores and guide to pre-design the high-performance zeolite catalysts.

This paper is supported by the Australian Research Council Discovery Projects（Nos. DP150103842，DP180104010），the SOAR Fellowship，and the Sydney Nano Grand Challenge from the University of Sydney.