Improved Efficiency for Partial Oxidation of Methane by Controlled Copper Deposition on Surface‐Modified ZSM‐5

Abstract The mono(μ‐oxo) dicopper cores present in the pores of Cu‐ZSM‐5 are active for the partial oxidation of methane to methanol. However, copper on the external surface reduces the ratio of active, selective sites to unselective sites. More efficient catalysts are obtained by controlling the copper deposition during synthesis. Herein, the external exchange sites of ZSM‐5 samples were passivated by bis(trimethylsilyl) trifluoroacetamide (BSTFA) followed by calcination, promoting selective deposition of intraporous copper during aqueous copper ion exchange. At an optimum level of 1–2 wt % SiO2, IR studies showed a 64 % relative reduction in external copper species and temperature‐programmed oxidation analysis showed an associated increase in the formation of methanol compared with unmodified Cu‐ZSM‐5 samples. It is, therefore, reported that the modified zeolites contained a significantly higher proportion of active, selective copper species than their unmodified counterparts with activity for partial methane oxidation to methanol.


Introduction
Partial oxidation of methane to methanolr epresents as ignificant challenge in modern chemistry. [1] Accomplishing thisd ifficult reactionwould facilitatethe production of valuable chemical raw materials from methane abundant in natural reserves. In addition, conversion of naturalg as into liquid form would simplify the issue of storage and transportation from remote or offshore sources. [2] Although interest in this process has been high among the scientific community for many years, the partial oxidation of methane has provend ifficult owing to the extreme stabilityo ft he methane molecule and the energy required to activatet he CÀHb ond. Additionally,a st he products of methane oxidation are inevitably more reactive than methane, total oxidation is ac oncern in any reactions ystem. Numerous examples of partial methane oxidation are available in the literature, but in many cases these requireh igh temperatures, harsh solvents, or expensive reagents to facilitate the process. Catalytic oxidationt of ormaldehyde was demonstrated by Herman et al. over V 2 O 5 /SiO 2 ,b ut with high temperature (600 8C) and low product selectivity towards CH 2 O( 16.3 %) and CH 3 OH (2.5 %). [3] Periana et al. used bipyrimidyl Pt II complexes dissolved in concentrated H 2 SO 4 to protect the product from furtheroxidation, obtaining methyl bisulfate with 81 %selectivity,y ielding methanol and regeneratingt he catalystb yh ydrolysis. [4] However,t he corrosive medium and homogeneous system remained problematic for product isolation.L ater, Schüthe tal. immobiliseds imilar platinum catalysts on ap olymer framework in ah eterogeneous system allowing simple separation of products,b ut the corrosive acid mediumw as retained. [5] Despite numerousa dvances, to date an efficient and viable method for direct conversion of methane to methanol remains elusive.
In the last decade,i nteresth as shifted towards metal-exchanged zeolites for the partial oxidation of methane to methanol, followingt he identification of ar eactive coppers pecies now characteriseda st he bent mono(m-oxo) dicopper core. [6][7][8] The core was found to facilitate methane oxidation under mild conditions and can be stabilised in the framework of ZSM-5 and other zeolites. The same copper species has been linked to the active site of the enzymep artial methane monooxygenase (pMMO), which offers an ideal mild reactionp athway for methane to methanolc onversion in nature. [9,10] Besides Cu-ZSM-5, other zeolite frameworks including FER, BEA, MOR, [11] and also artificial zeolitess uch as SSZ-13 and SAPO-34, [12] are known to be active for partial methane oxidation following coppere xchange. In particular,C u-FER and Cu-MORs how variable activity for methane oxidation at different temperatures, [11] leadingt ot he assumption that other unidentified active copperc ores may be present in variousz eolite frameworks. Cu-MORi so ne of the mostw idely studied examples owing to its relativelyh ighp roduct yield and as yet undefined reaction mechanism, [13,14] although it is also known to contain the mono(m-oxo) dicopper core. [15,16] Ag rowingb odyo fr esearch suggestsarich variety of speciest hat may be activef or partial The mono(m-oxo) dicopper cores present in the pores of Cu-ZSM-5 are active for the partial oxidation of methane to methanol. However,c opper on the externals urface reduces the ratio of active, selectives ites to unselective sites. More efficient catalysts are obtained by controlling the copperd eposition during synthesis. Herein, the external exchange sites of ZSM-5 samples were passivated by bis(trimethylsilyl) trifluoroacetamide (BSTFA) followed by calcination, promoting selective deposition of intraporous copperd uring aqueous copperi on ex-change. At an optimum level of 1-2 wt %S iO 2 ,I Rs tudies showeda64 %r elative reductioni ne xternalc opper species and temperature-programmed oxidation analysis showed an associated increase in the formation of methanol compared with unmodified Cu-ZSM-5 samples. It is, therefore, reported that the modified zeolitesc ontained as ignificantly higher proportion of active, selectivec opper species than their unmodified counterparts with activity for partial methane oxidation to methanol. methaneo xidation, demonstrating the versatility of zeolitesi n supporting and stabilizing such species. [15] In the present work, we focus on Cu-ZSM-5a st he earliest known example of ac atalystf or partial methane oxidation to methanol with ac opper-exchanged zeolite. The originalr eaction method over Cu-ZSM-5 developed by Schoonheydt et al. is well documented. [11] The zeolitei sf irst activated in O 2 at high temperature (350-650 8C);h owever,t his can also be achieved at lower temperatures by using different oxidantss uch as NO [17] or N 2 O. [11] After activation,t he reaction with methane proceeds under milder conditions at 150 8Ct of orm methanol stoichiometrically with > 98 %s electivity.T his can be partially recovered ex situ by washing the catalyst with water or in situ by using steam, allowing an entirely gas-phase process. [14,18] Following the interaction of methane with the activated catalyst, the product is thought to form initially as amethoxy intermediate, requiring ap rotic solvent in order to desorb. [8,14,15] However,l iquid-phase extraction is known to be inefficient and some product remains trapped within the zeolite structure, possibly as ar esult of strong interactions with the numerous adsorption sites present and poor diffusion thought he narrow channels of Cu-ZSM-5. [11,19] This behaviour has also been observed for Cu-MOR, [12] suggesting that the zeolite structurem ay play ar ole. [15,20] Alternatively,p roduct extraction with steam at elevated temperatures( 150-200 8C) has been shown to improvem ethanol yields in the case of Cu-ZSM-5, [17] and allows completed esorption of products in the case of Cu-MOR. [18] It should be emphasised that partial methane oxidation over copperz eolite systems is stoichiometric, [11] although batch-type systems for recurring yields of methanol have been demonstrated. [17,18] Recently,H utchingse tal. detailed both batch and continuous-flow liquid-phasep rocesses over bimetallic Fe/Cu-ZSM-5( in which iron is the active species) by using H 2 O 2 as the oxidant;h owever,t he relatively expensive oxidizing agentand low conversion are typicallyp roblematic for partial methane oxidation. [21,22] Studies of Cu-ZSM-5 involving selective adsorption of IR probe molecules have identified atl east two distinct copper environments;t hose on the externalz eolite surfacea nd those within the microporous framework. [19] Only the latter have shown activity for low temperature, selectivep artial methane oxidation, indicatingarelativelyl ow proportion of active coppers ites in the zeolites tructure, with estimates at around 5%. [11] In the case of Cu-ZSM-5, it is thought that the numerous unselective or inactivec oppers ites contribute to inefficient product extraction and are potentially active for the total oxidation of methanet oC O 2 . [7,8] Large volumes of inactive metal also complicated irect characterization by techniques such as X-ray absorption spectroscopy (XAS), which measure the bulk metal speciesa nd thus have difficulty in distinguishing between suspected active sites and inactive metal sites. [23,24] The latter point is particularly relevant given that active copper sites across ar ange of zeolite types are still being discovered and characterised. [13,14,[25][26][27] Similarly,t he precise activation mechanism of Cu-ZSM-5 with various oxidizing agents is still being debated. [15,20] Then eed for further detailed in situ or operando studies is therefore clear. This work describes efforts to reduce the proportion of either the unselective or inactive copperw ithin the structure of Cu-ZSM-5 through selectivem odification of the zeolite surface. Ichikawae tal. showed that interaction of as ilylating agent with Brønsted acid zeolite exchange sites followed by high temperature treatment leads to passivation, forming Lewis acid SiO x species and rendering the sites inactive for cation exchange. [28] Passivationo fz eolite exchange sites has already been appliedf or selectivei on exchange on an umber of metal-zeolite systems, including ZSM-5. For example, Lercher et al. showedt hat the acidity of the externals urface of ZSM-5 could be decreased by modifying the external surfaceh ydroxyl groups with bulky silylating agents. [29] Alternatively,I glesia et al. used the same technique to reduce the exchange of MoO x species on the external surface of ZSM-5, resulting in highers electivity for the desired low-order aromatics as ar esult of reactions occurring in the pores of the catalyst, as well as higherc onversion owing to improved dispersion. [30] Surfacem odification of various zeolites for selective catalysis on av arietyo fo rganic species is well established, [31][32][33] but to the besto fo ur knowledge has not previously been applied to the partial oxidation of methane to methanol.
Herein, selectivef unctionalization of externalz eolite exchange sites was performedw ith the bulky organic silylating agent bis(trimethylsilyl) trifluoroacetamide( BSTFA). The decreasei nt he total available exchange sites is intended to minimise formation of unselective coppern anoparticles on the externalz eolite surfacea sw ell as promote exchange of copper within the zeolite pores during wet ion exchange, owing to an increased concentration gradient. The modified catalysts produced werestructurally characterised and tested foractivity towards partial methane oxidation. An umber of unmodified catalysts were also prepared for comparison. The overall goal was to produce catalysts with an increased ratio of active and selectivei ntraporous coppers ites to unselective or inactive copper sites on the external zeolite surface, whiler etaining activity towards partial methane oxidation.

Results and Discussion
Diffuser eflectance infrared Fourier transform spectroscopy (DRIFTS)analysis of silylated zeolites Ar ange of modifiedN a-ZSM-5 samples were synthesised with SiO 2 loadings ranging from 0.2 to 3wt% ( Table 1). The label 'SCZ-Y'i ndicates as ilylated Na-ZSM-5z eolite with 'Y' loading of SiO 2 .F igure 1( andF igure S4.1 in the Supporting Information) show the DRIFTS spectra of the samples following silylation, drying at 80 8Ca nd calcination at 500 8C, in comparison to unmodified Na-ZSM-5.O nt reatment with BSTFAa nd drying, the carbonyl stretching band at 1745 cm À1 appeared with increasingi ntensity as silylation was increased, indicating functionalization of zeolite surfacesites with BSTFA.
Silylated Na-ZSM-5 samples weret hen calcined at 500 8Ct o remove the organic precursors and complete the surface modification process. Thermal decompositiono ft he organic species was noted by the removal of all carbonyl bands observed  Figure 1b). Copper ion exchange was then performed on the modified Na-ZSM-5 samples to form Cu-ZSM-5, followed by treatment in air at 500 8Ct or emove organic species (see Figure S4.2). Copper exchange wasa ccompanied by ac olour change in the catalyst from white to pale blue.

Structural characterization
The Cu-ZSM-5 samples werea nalysed by inductively coupled plasma (ICP) techniques, which revealed ar ange of total copperl oadings from 0.7 to 2.95 wt %C uf or the unmodified (CZ-X) series (Table 1). The silylated catalysts were prepared with equal concentrations of coppers olution to the unmodified CZ-2.65 sample and au niform reductioni nC uw t% was observed, which was generally more prominenta th igher levels of silylation.T he reduced copperl oading with increased silylationcould be expected from the decrease in the availability of zeolite exchange sites owing to surface passivation as well as decreased diffusion in the precursor.
Brunauer-Emmett-Teller (BET) analysiso ft he unmodified Cu-ZSM-5s amples showeda pproximate surfacea reas and pore volumes of 300-320 m 2 g À1 and0 .109-0.115 cm 3 g À1 ,r espectively.T his was within the expectedr ange for microporous zeolites. For the modified Cu-ZSM-5s amples, ar eduction in both surface area and pore volumew as noted at higher( > 1wt% SiO 2 )l evels of silylation( Figure 2). As no specific trend was identified fort he unmodified samples (Section S3 in the Supporting Information), the trends observed are proposed to be ar esult of surface silylation. As BSTFAi sl arger than the pores of the zeolite ( Figure S8.6), the decrease in surfacea rea combined with lower Cu loading indicates that large amounts of newly formed SiO 2 specieso nt he zeolite surfacer educe both the availability of the externale xchange sites through passivation, and possibly intraporous exchange sites through steric blocking. However,t he reduction in cumulative pore volumew ith increasing SiO 2 wt %s uggests increased exchange of copper within the zeolite channels. TEM performed on unmodified CZ-2.12 revealed the presenceo fc opperp articles, which were visibly less numerous than for modified SCZ-1 (Figure S3.1). Owing to the small channel size of ZSM-5 (5.2-5.5 ), visible particles of severaln anometres were likely indicative of externals urface species. [19] Energy-dispersive X-ray spectroscopy (EDX) analysisr evealed the presence of copper where none was physically visible, indicatingh igh dispersion of metal nanoparticles for the silylated sample.

FTIR analysis of copper distribution
As ICP results are only indicative of the total copperl oading, the relative proportion of coppers ites on the externals urface and within the pores of Cu-ZSM-5 was estimated by using transmission IR. IR studies werep erformed with sequential adsorptiono ft wo probe molecules onto Cu-ZSM-5;f irstly,p ivalonitrile (PVN), which was considered too bulky to enter the zeolite channels and therefore adsorbs onto Cu on the external surface, followed by NO, which could access the remaining availablec oppers ites in the channels. These probe molecules have previously been shown to be selectively indicative of external and intraporous copper species, respectively,w ith the [a] Labels:C Z-X, CZ = copper-exchanged zeolite, X = Cu wt %( determined by ICP);SCZ-Y,S CZ = silylated copper-exchanged zeolite, Y = SiO 2 wt % (theoretical). www.chemcatchem.org amount of intraporous Cu proportional to the amount of methanol formed. [19] Given that only copper sites in the zeolite channels were shown to be potentially active, the ratio of the integrated NO-Cu 2 + to PVN-Cu 2 + band intensities was used as an indicatoro fs electivef ormationo fi ntraporous coppers pecies, with amaximisedr atio being more favourable. On exposure to PVN,acomplex absorption feature was observed from 2210t o2 315cm À1 (Figures S6.2 and S6.3). Contributions from PVN on the zeolite support were determined by followingP VN adsorption on standard Na-ZSM-5. The PVN-Cu 2 + absorption band was therefore assigneda st hat at 2280 cm À1 in accordance with Bitter et al. [19] NO adsorption resulted in as econd complex band at approximately 1900 cm À1 ( Figure S6.4), which was isolated as above to identify the NO-Cu 2 + band as that at 1907 cm À1 .T he key absorption bands at 2280 and 1907 cm À1 were deconvoluted and integrated to quantify the surface area of copper on the externala nd intraporousz eolite exchange sites, respectively.I Ra nalysis results for the unmodified and modified catalyst series are shown in Figure 3w ith the data summarised in Ta ble 2.
Examinationo ft he PVN-Cu 2 + and NO-Cu 2 + absorption peaks for the unmodified catalysts (CZ series) as af unctiono f copperl oading (Figure 3a)i ndicates as imilarp eak area for PVN with increasing Cu loading, which implies similar volumes of Cu on the external zeolites urface, along with as ignificant increasei nN Op eak area, signifying ani ncreasei nt he intraporous copper species. It hasb een shown that deposition of Cu onto the surface of the zeoliteso ccurs rapidlyd uring ion exchange ( Figure S8.5) and so the relativelyc onstant PVN signal can be attributedt or apid saturation of the external exchange sites, with slower diffusion of Cu into the pores.T he increase in NO signal with higher Cu loading is likelyd ue to the increasedc oncentrationg radients from the more concentrated Cu solutions, allowing more Cu to diffusethrough to the interior zeolite channels. As the response factor of the probe molecules is unknown, it is unclear the extent to which the overall increasei nt he NO/PVN ratio observed is due to increased copperl oading in the zeolite channels and how much is from ag reater response factor for NO compared with PVN. Regardless, it is clear that there is an increase in the amount of Cu in the intraporous region of the zeolite with increasing Cu loading.
The modified catalyst series showedas imultaneous reduction in the PVN-Cu 2 + signal and ag eneral increase in the NO-Cu 2 + signal with increasing silylation (Figure 3b). As ac onstant copperc oncentration was used during preparation,t hese observations can therefore be attributed to the effects of silylation. Notably,atwo-fold increase in the NO/PVN ratio waso bserved for SCZ-2 in comparison with the unmodified analogue CZ-2.65. However,a st he average copper loading of the modified catalysts was 2.19 wt %C u, it is more appropriate to compare the structure and activity of the modifiedc atalysts with that of CZ-2.12, where a2 .8-fold increase in NO/PVN was observed comparedw ith SCZ-2. It is apparent, therefore, that althoughthe overall Cu loadings are similar, silylation has altered the distribution of Cu on the internal ande xternal surfaces of  www.chemcatchem.org the zeolite.A ni deal silylation level of 1t o2wt %S iO 2 is therefore suggested for this catalystf or maximizing the NO/PVN ratio to the implied optimum of active, selectivec opper sites compared with unselective copper sites.

Activitytestingfor partialm ethane oxidation
The activity of the modified Cu-ZSM-5 samples towards partial methane oxidation wasa ssessed. The catalysts were activated in oxygen and the presence of the UV/Vis band at 440 nm, representative of the bent mono(m-oxo) dicopper core, was determined. [11] Following activation and reaction with methane,t he strongly adsorbed reaction products were extracted with deionised water and analysed by GC. The results of the UV/Vis and GC analysis for both unmodified and modified catalyst series are shown in Ta ble 2a long with the peak areas (arbitrary units) for adsorbed NO and PVN from the IR analysis. Yields are presented as mmol g À1 of zeolite used, unless stated otherwise.
The 440 nm band was observedf or all modified and unmodifiedc atalysts tested following activation in O 2 at 500 8C. For the unmodified CZ series, the intensity of the core band increasedl inearly with increasing copper loading (Figure 4a), as observedb yS choonheydte tal. [11] No methanol was observed below am inimum copper loading of 0.7 wt %C u( 0.14 K-M, Figure 4b)a nd the methanoly ield reached am aximum at 2.12 wt %C u, (1.39 K-M,F igure4b) before decreasing slightly at higher copper loadings. Rathert han limited formation of methanol,t his plateau ath igh copper loadings has been attributedt ot he strong adsorption of product methanol to the catalysta nd the high concentration of adsorption sites limiting aqueous methanolextraction. [8,11] For the modified SCZ series,ageneral decrease in UV/Vis signal intensity was observed comparedw ith the unmodified CZ-2.12 sample (Figure 4a). Despite the small changes in Cu loading,astrong correlation with the methanol yield was not observed, rather ar ange of yields wereo bserved from 1.0 to 1.6 mmol g À1 .U V/Vis absorbance has previously been shown to be proportional to the amount of aqueous extracted product, [11,19] and the modified series also showeda ppropriate methanoly ields in line with expectations from UV/Vis absorbance of the unmodified catalysts (Figure 4b). This analysis shows that although the distribution of Cu changed after silylation, the modified catalysts were still active for partial methane oxidation. In addition, the ICP data indicatet hat the modified catalysts contained less Cu in total. Despite some samples giving as imilaro rg reater NO-Cu 2 + signal intensity compared with CZ-2.12, implying greater amounts of copper in the pores, the UV/Vis data may suggest al ower than expected amounto f core speciesw as formed. However,t he product extracted was in line with that expected from the amount of core speciesobservedb yU V/Vis.
Examining the aqueous product yield in relationt os pecific copper distribution ( Figure S8.1) confirmed that the externally based copper sites characterisedb yP VN adsorption have no correlation with the methanol yield fore ither unmodified [19] or modified zeolites. In contrast, intraporous coppers ites characterised by NO adsorption have ac learly defined relationship with product yield for the unmodified series. Notably,t he NO/ PVN ratio for unmodified catalysts (Figure 5a)r eflectst he relationship between the UV/Vis absorbance and yield shown previously ( Figure 4b). As the NO-Cu 2 + peak increases in intensity and the NO/PVNr atio increases to above ac riticalv alue of about 4( approximately 2.12 wt %C u), the methanol yield reachesaplateau. Again, this is possibly due to hindered product desorption or the increased number of potentiala dsorption sites. [8,11] For the modified series, no trend was observed for either PVN or NO absorption, and in turn no trend between the NO/PVN ratio and product yield (Figure 5b)w as observed.  Care must be taken when comparing the performance of the modified and unmodifieds amples using UV/Vis and GC analysis. As mentioned previously,l imitationst oa queousp roduct desorption have been observed on ZSM-5 at very high copperl oadings based on temperature-programmed oxidation (TPO) analysis, [11] and studies involving product desorption by steam. [17,18] Here, the Cu loadings of the modified catalysts are mostly above the limit where there is al inear relationship range betweenC H 3 OH extracted and Cu loading, as shown by the plateau in Figure 4. It is widely acceptedt hat at higher Cu loadings outside this range, aqueous extraction is limited and does not accurately represent the amount of product formed. [8,12,15] In addition, formationo fm ethanoli nasimilar process over zeolites other than ZSM-5 shows varying response with UV/Vis analysis. Cu-MORg ives ag reatlyr educed UV/Vis signalc ompared with the amount of methanol formed, whereas Cu-BEA and Cu-FAUshow activity for low temperature methane oxidation but no significant UV/Vis peak at all. [11,12,18] To test the validity of the trends observed with GC and UV/ Vis analysis, TPO experiments were performed to evaluate the potentiala mount of adsorbates present on the catalysts urface followinga ctivation and reaction with methane. Combustion products were observed by mass spectrometry (MS) beginning at 270-280 8Cf or all catalysts. CO 2 and CO were of particular interest in quantifying the adsorbed organic species. For the unmodified CZ-series catalysts, the CO 2 detected from blank TPO experiments (catalysts pre-treated in Ar,s ee Section S7) was subtracted from the final CO 2 yield. Given the high temperaturep re-treatment of all catalyst samples,i tw as assumed that any CO 2 or CO observed from activated catalysts was, therefore, ar esult of strongly adsorbed methanol formed duringr eactionwith methane. Figure 6s hows the amount of methanol observed by GC followinga queous extraction comparedw ith that observed by TPO-MS as evolved CO 2 .F or the unmodified series (Figure 6a), below 2.12 wt %C ut he CH 3 OH and CO 2 yields observed were very similar.H owever,a th igher copper loadings ac lear disparity was observed, whereby CO 2 evolvedw as in excess of the aqueous desorbed products.F or the modified series, as imilar increasei np roduct detected by TPO was observed compared with aqueous extraction ( Figure 6b). This confirms the limitation of aqueous product removal also for the silylated catalysts. At Cu loadings above approximately 1.9-2.1 wt %C u, the volumeo fa queous extracted product appears not to be an accurater epresentation of catalytic activity.N otably,C Oo rm ethanol desorption was not observed for any of the catalysts  www.chemcatchem.org tested, indicating complete combustion of the adsorbed products in O 2 .C O 2 and CH 3 OH yields are, therefore, considered as molar equivalents.
In Figure 6, product yields are expressed per moles of Cu present on the catalyst;t his can effectively be considered as at urnover number (TON). However,i ts hould be emphasised that the process shown here operates stoichiometrically;therefore, TON values are necessarily low and also the amount of copperd oes not representt he amount present in the active cores. However,c omparing catalysts with similar copperl oadings under this assumption permits useful comparison between the unmodified and modified series, as summarised in Ta ble 3. The modified series, therefore, exhibited am aximum three-fold increasei nT ON at an ideal silylation level of 1-2wt% SiO 2 ,a nd at wo-fold increase at 3wt% SiO 2 .T he former two catalysts outperformed even the most productive unmodified sample, CZ-2.95.C onsidering the near identicalc opper loading observed between SCZ-2 and CZ-2.12, this increase in catalystefficiency is particularly notable, indicating asignificant increaseinthe amount of active catalytic sites present.
It is also important to note that for the modifieds amples, no specific trend was observed between yield or TON and the total copper loading (Figure 6b). However,b yc onsidering the level of silylation of the zeolite (Figure 7a and Figure S8.3), it becomes clear that TON values are optimised at 1-2 wt %S iO 2 . Furthermore, this correlates well with the maximum value of NO/PVN observed through transmission FTIR analysis ( Figure 7b). As the volume of intraporous copper characterisedb y NO adsorption was found to correlate with the volume of product observed and TON ( Figure S8.2), the activity of Cu-ZSM-5 for partial methane oxidation is therefore confirmed to be dependento nly on the volumeo fc opper in the zeolite channels. In agreement with the work of Bitter et al., [19] external coppers ites were found to be inactive. It is apparent that the large increasei nT ON observed at 1-2 wt %S iO 2 occurs as ad irect result of silylation, which in turn offers an optimised amount of intraporous copper species that are potentially active.
In summary,f or Cu-ZSM-5, the amount of aqueous extracted methanold oes not accurately describe product formation above ac ertain Cu loading (2.12 wt %C uf or CZ-series) and, therefore, the UV/Vis signal intensity does not correlate with yield above this value (Figure 4b). However,f or unmodified catalysts, the amount of CO 2 evolved does correlate with the intensity of the NO and UV/Vis signal( Figures S8.2 and 8.4), indicative of active copper species. For the modified SCZ series catalysts, only the volumeo fC O 2 evolved was found to correlate with the number of intraporous coppers ites (Figure 7b and Figure S8.2). As the CO 2 evolvedw as in excess of the aqueous methanoly ield, the CH 3 OH yield was not regarded as an accurate representation of product formation for any of the samples tested, any correlation with methanol yield wast herefore not considered significant. Taking the CO 2 evolveda s being representative of the amount of product formed, the lack of correlation with UV/Vis data ( Figure S8.4) shows this as not representativeo ft he active species in the case of the modified catalysts. Examining the CO 2 evolved together with FTIR analysis thereby provides ar elationship between the intraporous Cu speciesa nd the amounto fp roduct formed, the effect of silylation was, therefore, assessed by using these parameters.
For the modified SCZ series, ac lear influence was observed for the amount of CO 2 evolved depending on silylation. At an  www.chemcatchem.org ideal level of between 1-2 wt %S iO 2 present on the zeolite surface, the CO 2 yield reached am aximum, indicating al arge amount of adsorbed methanol present within the zeolite pores, although product desorption in the aqueous phase remained significantly hindered,s imilar to the unmodified catalysts tested. The resulting modified zeolites, therefore, constitute more active catalysts than unmodified Cu-ZSM-5 based on the total copper loading present. The large increase in CO 2 yield and TON at 1-2 wt %S iO 2 is consistent with the structural characterization, which showed ag reatlyi ncreased NO/PVN ratio during gas adsorption studies, marking the point at which the maximum intraporous copper sites and minimum externalc oppers ites was observed. It should be noted that the trends observed began to reverse with extensive silylation of 3wt% SiO 2 ,which is possibly due to the maximum silylation level of the zeolite surfacealready being attained. [29] This presentsas trong case for applying silylation to selectively control copperd eposition, showingt hat catalysts produced through surfacep assivation have ag reater proportion of active, selective to non-selective coppers ites presentt han comparable unmodified samples. The increase in product yield can be directly linked to ac hange in specific copper distribution inducedb ys ilylation. The potentialv olume of product formed was easily determined by adsorption of gaseous probe molecules, ap rocess that can be flexibly applied to other supports. Ac lear advantage of the silylation process described here lies in its applicability to other zeolite materials and framework types, along with the abundance of different silylating agents available. Application of this methodt oo ther framework types with partial methane oxidationactivity,particularly those with greater pore diameters such as mordenite, may alleviate the problem of product desorption while resulting in af unctional and more efficient catalyst. The increased proportion of active ands elective copperp resent may also assist with further characterization of the active site.

Conclusions
It has been shown that ZSM-5 functionalised with the silylating agent BSTFAc an facilitate passivation of specific zeolite exchange sites, allowing selective control over coppere xchange during synthesis. Ar ange of 1-2 wt %S iO 2 was found to be ideal, effectively favouring deposition of copperw ithin the zeolite channels at the expense of the externals urfaces ites. With the observation that coppers ites based on the external surfacea re inactive for low temperature partial methane oxidation, this indicates as ignificant increasei np otentially active and selective copperw ithin Cu-ZSM-5. Modified zeolitesw ere found to be active for partial methane oxidation, showing product yields comparable with unmodified Cu-ZSM-5 samples based on UV/Vis analysis of the active coppers pecies. TPO indicatedt hat al arge amount of methanol was formed for 1-2wt% SiO 2 catalysts, in direct proportion to the number of intraporous coppers ites present. However,t he product was not directly recoverable by aqueous extraction. Considering the CO 2 yield in relation to the moles of Cu present, silylation of 1-3wt% SiO 2 provided at wo-to three-fold increase in TON over unmodified catalysts with comparable copperl oading. Althought he methane oxidation process demonstrated is stoichiometric, this indicates the increased efficiency of silylated catalysts for partial methane oxidation. The surface functionalization technique used is flexible and may be appliedt o ar ange of zeolite typesa nd silylating species, advancing the prospecto fd eveloping am aterial consisting of ah igh proportion of active metal sites. This may help to facilitate direct characterization of the core species. In addition, applicationt oz eolites with larger frameworks and pore diameters may contribute to developingm ore active and efficient catalysts fort his important chemical process.

Experimental Section
The synthetic parameters and characteristics of all zeolites produced are summarised in Ta ble 1. The suppliers and purity of reagents and precursors are detailed in Section S1 in the Supporting Information. Cu-ZSM-5 samples were prepared by as tandard wet ion exchange method. [11] NH 4 -ZSM-5 (Si/Al = 12, 1g)w as added to aqueous NaNO 3 (1 g, 150 mL) and stirred for 24 ha tr oom temperature (approximately 20 8C). Samples were filtered, washed and dried at 100 8C. Sodium exchange was performed three times to form Na-ZSM-5. Na-ZSM-5 (1 g) was added to solutions of Cu II acetate monohydrate of various concentration in deionised water (250 mL) and the mixture stirred at room temperature for 24 h. Samples were filtered, washed and treated at 500 8Cf or 3h to remove organic precursors before further use. Surface-modified Cu-ZSM-5 samples were prepared by silylation of Na-ZSM-5 followed by copper ion exchange. Na-ZSM-5 (1 g) as prepared above was added to dry hexane (25 mL, 3molecular sieve) and the mixture heated to 40 8Cu nder ac ontrolled nitrogen atmosphere. Varying concentrations of silylating agent BSTFA( 3.73 10 À4 to 3.73 10 À3 mol L À1 )i nd ry hexane were added and the mixture was heated at reflux for 1h.S ilylated samples were filtered, washed with dry hexane and dried at 80 8Cf or 1h.S amples were then treated in air at 500 8Cf or 6hto decompose the organosilane species and form silica-modified Na-ZSM-5. Copper exchange was performed on the modified Na-ZSM-5 precursors as above, aiming to give aC ul oading similar to that of CZ-2.65 (Table 1). Cu-ZSM-5 samples were treated again in air at 500 8Cf or 3h before further use. Details of calculations for metal and BSTFAc oncentrations used are shown in Section S2 in the Supporting Information.
More details on the characterization techniques used and methods of deconvolution of peaks are shown in Sections S3-S6 in the Supporting Information. The Cu content of samples was determined by inductively coupled plasma optical emission spectrometry (ICP-OES) by using aP EO ptima spectrometer.B ET surface areas and pore volumes of zeolite samples were determined by nitrogen adsorption at À196 8Cb yu sing aM icrometrics Tristar II instrument. TEM was performed by using an FEI Te cnai F20 electron microscope operated at 200 kV,f ollowing adherence of the samples to ac opper microgrid. DRIFTS was performed at various stages during the synthesis to monitor the progress of silylation:( a) fresh Na-ZSM-5 before modification by silylation, (b) modified Na-ZSM-5 after drying, (c) modified Na-ZSM-5 after calcination, (d) modified Cu-ZSM-5 after copper exchange. Spectra were recorded at ambient temperature by using aB ruker Te nsor 27 IR spectrometer at ar esolution of 4cm À1 .E ach spectrum was the result of 128 scans, with dry KBr used as the background. Spectra were baseline corrected and normalised to the zeolite overtones (1950-2050 cm À1 ). www.chemcatchem.org UV/Vis spectroscopy was used to monitor the presence of the active copper species in Cu-ZSM-5 during activation and reaction with methane. [11] Pelletized samples (0.7 g, 250-500 mmp ellets) were placed in aq uartz plug flow reactor and activated in oxygen at 500 8Co vernight. Spectra were recorded by ad iffuse reflectance fibre optic probe (Hellma 668.006-UVS) with aP erkinElmer Lambda 650S spectrometer,w ith the parent zeolite, NH 4 -ZSM-5, used as the background. Transmission FTIR was used in conjunction with the adsorption of probe molecules to monitor the distribution of copper on the samples. [19] Cu-ZSM-5 samples (20 mg) were pressed into self-supporting discs, placed in aS pecac gas exchange cell and outgassed under vacuum at 150 8Cf or 2h.T he sample discs were then exposed to PVN vapour (30 min,~8mbar) and subsequently NO (overnight, 25 mL min À1 ,1%i nA r) at 40 8C. Blank experiments were also performed by adsorption of PVN then NO to the parent zeolite Na-ZSM-5. During probe adsorption, spectra were recorded in situ in transmission mode on aB ruker Te nsor 27 IR spectrometer at ar esolution of 4cm À1 .E ach spectrum was the result of 128 scans, with dry KBr used as the background. Spectra were baseline corrected and normalised to the zeolite overtones (1950-2050 cm À1 ). Ad econvolution process was applied to isolate the absorption bands of interest (see Section S6 in the Supporting Information).
Standard and modified Cu-ZSM-5 samples were tested for activity towards partial methane oxidation (Section S7 in the Supporting Information). Cu-ZSM-5 samples (0.7 g, 250-500 mmp ellets) were placed in aq uartz plug flow reactor and activated overnight in oxygen (50 mL min À1 ,5 00 8C). Samples were cooled to room temperature, flushed with argon and then exposed to methane (50 mL min À1 ,1%inAr) at 150 8Cfor 1h(10 8Cmin À1 ). The products remained adsorbed to the catalyst surface. Twod ifferent analytical methods were used on spent samples following activation and reaction with methane:( i) reaction products were directly extracted in aqueous solution and analysed by GC, (ii)TPO was performed and combustion products monitored by MS. By the GC method, spent samples were removed from the reactor and stirred vigorously in deionised water (1.5 mL) for 24 h. The suspension was centrifuged and the supernatant liquid passed through as yringe filter (0.45 mm, nylon membrane). The product composition was determined by using aP erkinElmer Clarus 500 GC equipped with aF ID and aS upelco Carbowax Amine column (30 m530 mmI D). By the TPO method, spent samples were directly heated from 20 to 500 8C( 10 8Cmin À1 )i no xygen (40 mL min À1 )w ith krypton (10 mL min À1 )a sa ni nternal standard and combustion/desorption products CO (m/z = 28), CH 3 O + (m/z = 31) and CO 2 (m/z = 44) were monitored by using acalibrated Hiden Analytical MS HPR20.