C Dussenne

T Delaunay

V Wiatz

H Wyart

I Suisse

M Sauthier

Synthesis of Isosorbide: Overview of challenging reactions

Isosorbide is a diol derived from sorbitol and obtained through dehydration reactions that has raised much interest in the literature over the past decades. Thus, this platform chemical is a biobased alternative to a number of petrosourced molecules that can find applications in a large number of technical specialty fields, such as plasticizer, monomer, solvent or pharmaceutical. The synthesis of isosorbide is still a technical challenge, as several competitive reactions must be simultaneously handled to promote a high molar yield and avoid side reactions, like degradation and polymerization. In this purpose, many studies have proposed innovative and varied methods with promising results. This review gives an overview of the synthetical strategies and catalysts developed to access this very attractive molecule, pointing out both the results obtained and the remaining issues connected to isosorbide synthesis.

Introduction

Isosorbide is a 1,4:3,6-dianhydrohexitol industrially produced from the double dehydration of D-sorbitol. It is noteworthy that D-sorbitol is obtained from the hydrogenation of glucose, a readily accessible starting material obtained from the hydrolysis of starch, and in a lesser extent, sucrose or cellulose. This molecule has thus a high production potential in comparison to isomannide and isoidide, two 1,4:3,6dianhydrohexitol isomers that are obtained from D-mannitol and L-iditol (Scheme 1). Isosorbide is a chiral renewable diol with a V-shaped backbone composed by two fused furan units. The chirality arises from the asymmetric carbons of the Dsorbitol molecule whose chirality is retained during the dehydration steps. The two hydroxyl groups are not equivalent and bear somewhat different reactivities. The hydroxyl group in the endo position is sterically shielded and more acidic than the second less sterically hindered hydroxyl groups on the exoposition. 1 The difference of acidity is explained by the intramolecular hydrogen-bonding between the endo-hydroxyl group and the closely located furanic oxygen. [2][3][4][5][6] This difference between the two hydroxyl groups of isosorbide allows the selective monofunctionalization of the diol. Isosorbide can be selectively monotosylated 6 , alkylated 7, 8 or acetylated [9][10][11] . Because of the fused skeleton structure composed of the two furan units, the overall structure is rigid and this particular feature is at the source of its originality in comparison to more flexible diols such as glycols, 1,3-propanediol or 1,4-butanediol that are actually essentially obtained from non-renewable resources. Of strong significance in a context of green chemistry, isosorbide is a biobased platform molecule highlighted as an alternative to a number of petrosourced compounds. Up to now, isosorbide has been used to access a large panel of molecules with relevant applicative properties and industrial reality (Scheme 2). 12 Isosorbide dinitrate is used since several decades as vasodilator. 13, 14 The dimethyl isosorbide is for example used as solvent in cosmetics 15-17 and isosorbide diesters [18][19][20][21][22] are actually industrially produced and commercialized as surfactants [START_REF] Milstein | Improved esterification of oxyhydrocarbon polyols and ethers thereof, and products therefrom[END_REF][START_REF] Giacometti | [END_REF][START_REF] Ellis | Manufacture of fatty acid esters of sorbitan as surfactants[END_REF][26][27] and PVC plasticizer 28, [START_REF] Freres | POLYSORB ID 37 plasticizer[END_REF] . The rigid scaffold associated to the bifunctionality of the molecule has attracted a strong interest in the field of polymers chemistry. Isosorbide and derivatives have thus been shown as suitable monomers for the industrial production of polycarbonates [START_REF] Chatti | [END_REF]31 , polyesters 32-41 or polyamides [42][43][44] , with attractive applicative properties. For example, isosorbide allows the increase of Tg, improves the scratch resistance and gives excellent optical properties to polymers. Polyesters and polycarbonates containing isosorbide have now commercial developments in food packaging, spray container, automotive, material for electronic devices … . As isosorbide production should largely increase in the future due to its actual and potent industrial applications, efficient synthesis is a real challenge. Taking into account the large Please do not adjust margins Please do not adjust margins industrial production of this molecule, further yields improvements would be clearly economically relevant. This review describes all the achievements in isosorbide synthesis.

The first section will first focus on the mechanism of dehydration from sorbitol to isosorbide. This part emphasizes the selectivity issues associated to the double dehydration of sorbitol. The following sections describe the homogeneous catalysts and heterogeneous catalysts that have been developed and evaluated in sorbitol dehydration. A last section gives the most recent results concerning the direct access to isosorbide from glucose and cellulose.

Mechanism of the dehydration reaction of sorbitol

The synthesis of isosorbide is a two-steps reaction process from glucose. Glucose is first hydrogenated into sorbitol according to very large scale processes that are well established and cost-effective. [45][46][47] The key step to isosorbide is then the double dehydration of sorbitol that requires complete conversion and selectivity. This dehydration process is promoted by acids that are discussed in the following sections. However the reaction is well known to lead to the formation of several side-products that are due to chemo-and regioselectivities issues. The former is generally produced as the main reaction intermediate and its formation is the first step of the synthetic pathway that leads to isosorbide formation after the internal second dehydration step. The latter is generally observed in very low amount. In comparison to the first step, the second step of 1,4monoanhydrosorbitol dehydration requires higher reaction temperatures and reaction times at such point that under soft reaction conditions, the monoanhydrosorbitol isomers (also called sorbitans) can be isolated as main reaction products. 48 Kinetics have been measured for the dehydration of sorbitol by three different groups and the reaction rates have been measured for the dehydration of sorbitol with sulfuric acid (Table 1 and Scheme 3). [49][50][51] From these values, it appears that 1,4-sorbitan is formed more rapidly and dehydrated slowly whereas 3,6-sorbitan is formed

Sorbitol dehydration via homogeneous catalysis 3.1 Brønsted acids

Strong acids are needed to efficiently protonate hydroxyl groups of sorbitol and thus generate water as leaving group in the nucleophilic substitution. Mineral acids have been very early used in the synthesis of isosorbide. The parent substrate, isomannide obtained from mannitol has for example been synthesized in 1884 by Fauconnier from the use of sulfuric acid. 52 The synthesis of isosorbide with sulfuric acid has been early patented in 1930. [START_REF] Muller | Dehydration product of sorbitol and the process of making it[END_REF] Other mineral acids were compared with sulfuric acid in 1986. 54 Hydrochloric acid, whose use has been elsewhere patented [START_REF] Salzburg | Process for the preparation of 1,4-3,6dianhydro-hexitols[END_REF] , showed a comparable isosorbide yield to sulfuric acid after 20 h of reaction at 135 °C but it should be noticed that this acid is in general less suitable as it readily distils with water under low pressure reactions.

Phosphoric acid did not lead to a complete conversion of sorbitol under the same reaction conditions. Dabbawala has confirmed this first work by comparing the turn over frequencies of acids that bear various acid strengths (Scheme 4). 56 Mineral and organic acids were both used in this study. PTSA and trifluoromethane sulfonic acid gave turnover frequencies that compared well with those obtained with sulfuric acid. Strong acids with pKa below -1.7 showed similar reactivities whereas weaker acids with higher pKa values showed lower. Most of the evaluated acids did not show any catalytic activity with pKa values below 3. It is however noteworthy that in the presence of a stoichiometric amount of chloropyridinium salt (pKa = 5.2), the reaction proceeds and leads to a mixture of sorbitans and isosorbide after 20 h at 160 °C. 57 Weaker acids can also be used providing that higher reaction temperatures and microwaves are used to promote the reaction. It has been for example shown that at 210 °C under microwave irradiation in a water/ dioxane solution with NaHSO4 (HSO4 -/ SO4 2-, pKa = 1.9) as acid, sorbitol is readily converted into isosorbide after 90 min. 58 Mixtures of acids have also been used to promote the Please do not adjust margins Please do not adjust margins synthesis of isosorbide from sorbitol: Organic acids such as formic acid or acetic acid have been used in combination with fluorohydric acid HF at room temperature. [START_REF] Defaye | Process for preparing anhydrides of hexitols, hexonolactones, hexoses and hexosides[END_REF][START_REF] Defaye | [END_REF] The yield determined by 13 C analysis of the crude evidenced the full conversion of sorbitol in isosorbide (95 %) and the product was finally isolated with 57 % yield. Very interestingly, this example shows a very different selectivity in sorbitans after the first dehydration step. With this cocktail of acids, 3,6-sorbitan is intermediately formed as main sorbitan whereas with more conventional acids, this isomer is generally not observed. This observation is explained by the intermediate formation, with acetic acid in anhydrous HF, of acetoxonium ions observed by 13 C NMR (Scheme 5). Nevertheless, as several acetoxonium ions are formed in the case of sorbitol, this unusual selectivity could not be clearly explained. The use of leaving groups has been tackled by different research groups. The dehydration of sorbitol diesters, obtained from the reaction between sorbitol and an acetylating reagent such as acetic acid, acetyl chloride or acetic anhydride, has been patented. [START_REF] Hajek | Process for the production of dianhydrohexitol mixtures and liquid mixture of dianhydrohexitols[END_REF] The procedures directly led to the isolation of anhydrohexitol diesters that can be further hydrolysed. The mixture is finally composed of mainly isosorbide (ca 80 %) along with 7-12 % of dianhydromannitol and 6-9 % dianhydroiditol. Numerous patents applications using Bronsted acids for isosorbide production are regularly submitted. Most recent describe more complex catalytic systems using mixture of acids. [START_REF] Stensrud | Dehydration of a sugar alcohol with mixed combination of acid catalysts[END_REF] Methods and processes are also very challenging in that field in order to reduce production costs. Recent advances describe dehydration continuous process using hot steam [START_REF] Son | Method of Producing Anhydrosugar Alcohol Using Steam[END_REF] or a two-step decomposed reaction. 64

Scheme 6 DMC pathway.

Interestingly, the combination of sorbitol with dimethylcarbonate DMC (transcarbonatation) followed by an intramolecular nucleophilic substitution under basic conditions at reflux in methanol led to the formation of isosorbide with high yields (Scheme 6). 65,66 High conversions with low amounts of polymers have been patented with organic acids under hydrogenation conditions (P H2 and metallic hydrogenation catalysts) at 270°C. Although complete conversion of sorbitol led to yields in isosorbide lower than 50 %, it is noteworthy that low contents of oligomeric / polymeric by-products have been obtained. [START_REF] Bourrel | Process for the production of five-membered or six-membered cyclic ethers, in particular of anhydropolyols[END_REF] The reductive conditions with H2 are often shown as suitable to reduce the colour of the reaction crude.

Lewis acids

Lewis acids are widely used as catalysts in organic synthesis. Fleche et al. early used Lewis acids such as AlCl3 and SnCl4 to convert sorbitol into isosorbide. 54 The Lewis acids with chlorides as counter anions however showed lower reactivities compared to sulfuric acid. Among the Lewis acids used in organic synthesis, metal triflates, in particularly those with rare earth metals, are interesting candidates even in the presence of water (Table 2). 68,69 Bismuth (III) triflate (3.6 mol %) has been efficiently used to promote the synthesis of isosorbide from sorbitol at 150 °C for 16 h with 85 % yield. 70 This work has been shortly followed by a patent that gives a comparison between the catalytic activities obtained with various Lewis acids in comparison to sulfuric acid. [START_REF] Stensrud | Process for acid dehydration of sugar alcohols[END_REF] The reactions were performed at 140 °C for 2 h with 0.1 mol % and 1 h at 150°C with 0.01 mol % of catalyst. With these conditions, the conversions of sorbitol were higher with Ga(OTf)3, Sc(OTf)3, Al(OTf)3, In(OTf)3, Bi(OTf)3, Al(OTf)3, Sn(OTf)3, than with sulfuric acid.

Synthesis of isosorbide in water or ionic liquids

The dehydration reactions are classically performed under solvent free conditions as sorbitol melts at 95 °C and water is continuously removed through a distillation at low temperature. An alternative approach involves the use of water as reaction solvent. As water is also the product of reaction, the use of aqueous media to perform the reaction often leads to limited reaction rates. However, it has been shown that under high temperature conditions, the selfionization of water is sufficient to promote the dehydration process. In that case, no additional acid is needed thus avoiding, during a workup, the use of bases to neutralize the reaction medium prior to isosorbide isolation. 72,73 The yield of isosorbide reaches 57 % yield at 315°C after 2 hours of reaction ([sorbitol] = 1 mol.L -1 ) and the addition of CO2 of at least 300 MPa could accelerate the reaction rate. 74 In that latter case, carbonic acid probably acts as a weak acid. Please do not adjust margins

Please do not adjust margins

As an alternative to melted sorbitol or water, ionic liquids (Scheme 7) have also been used as solvents of reaction. Ionic solvents, due to their high polarity, are suitable to solubilize hydrophilic polyols such as sorbitol. ZnCl2 molten salt hydrate medium allows an efficient solubilisation and conversion of sorbitol into isosorbide without any additional co-catalyst. [START_REF] Makkee | Process for converting polysaccharides in an inorganic molten salt hydrate[END_REF]76 The yield determined by HPLC after 270 min at 200 °C (1:12 weight of sorbitol in 70% weight of ZnCl2 in water) reaches 85%. The reaction was in that case strongly favoured, according to molecular modelling, 77 by the zinc centre that a) enhances the leaving group ability of the primary OH-group through coordination of sorbitol and b) coordinates the released molecule of water. Hydrophobic ionic liquids have also been used as reaction media. 78 The reactions were promoted by 5% of PTSA at 180°C under microwave irradiation and afforded 50-60 % yields depending on the ionic liquid Interestingly, the hydrophobic ionic liquids could be reused thanks to an extraction with dichloromethane (100 mL for 2.5 g of ionic liquid) while isosorbide was readily solubilized in water. More recently, new catalytic systems using Bronsted acidic ionic liquids have been developed for isosorbide synthesis from sorbitol. 79 These compounds have shown very interesting results with a 75% isosorbide yield during 5 consecutive runs. These results show interesting perspectives in order to develop a continuous process for isosorbide production.

Sorbitol dehydration via heterogeneous catalysis

The recycling of the catalyst is a key issue in dehydration reactions. Acidic homogeneous catalysts tend to be corrosive for the reactors and have to be ultimately separated from isosorbide. Taking into account these drawbacks, several heterogeneous alternatives have been employed to promote the dehydration reaction. Heterogeneous catalysts advantageously allow the separation of catalysts from the products through a simple filtration. Moreover, the filtrated catalyst can be reused in a further dehydration reaction or continuously used.

In the presence of acid resins

Early article from Goodwin reports attempts of dehydration of sorbitol with Amberlite IR-120 (33 % weight / sorbitol). 80 An organic co-solvent was used and isosorbide was obtained, at reflux of dioxane/ethyl acetate (1/1: v/v) for 24 h, with 39 % yield after distillation. 57 % of isolated yield was later obtained with similar catalyst loading by using solvent free conditions and harsher reaction conditions (170°C for 2 h under 10 mmHg). 50 These results have rapidly attracted a strong interest concerning applicative synthetic approaches as shown by the patents literature. Batch reactors [START_REF] Moore | Process for the production of anhydrosugar alcohols[END_REF] and fixed bed reactors [START_REF] Hu | Method of performing sugar dehydration and catalyst treatment[END_REF][START_REF] Moore | Process for the production of anhydrosugar alcohols[END_REF] have been in this context used. 74 % yield of isosorbide has been reported with Amberlyst 35 after 2 h at 135 °C under 10 mm Hg. Clearly, it turns out from the reported results that higher acid loadings are needed if sulfonic resins are used. More activating conditions based on the use of microwave irradiation have thus been investigated. In sorbitol medium with 2% of Amberlyst 15 system, the microwave power is efficiently converted into heat, thus allowing energy saving in comparison to thermal heating. 84 Moreover, it has been observed that performing the reaction under microwaves allows enhancement of the reaction rates of a factor 20 to 34 in comparison to simple thermal activation. 85 The selectivity of the reaction toward isosorbide remains however limited to similar values whatever the heating system used. Catalysts recycling or continuous processes are limited by catalysts poisoning. One expects the oligomeric / polymeric by-products formed in the course of the reaction obstruct the pores of the solid material. Accordingly, washing cycles with organic solvents is used for the regeneration of the catalyst. [START_REF] Hu | Method of performing sugar dehydration and catalyst treatment[END_REF] New hydrophilic sulfonic acid-functionalized micro-bead silica catalysts named SA-SiO2 gave complete sorbitol conversions and high isosorbide selectivities (up to 84%, determined by HPLC). 86 The catalyst was reused 10 times without noticeable loss of activity and selectivity. The excellent performance of this catalyst was attributed to its suitable large pore diameter and high surface hydrophilicity.

The commercially available sulfonic resins are hydrophilic. It has elsewhere been postulated that more hydrophobic resins, with lowered charges of sulfonic groups, would favour the dehydration step (Scheme 8). 87 Such materials have been synthesized according to a solvo-thermal co-polymerization of styrene and divinylbenzene followed by ion-exchange with an aqueous solution of H2SO4. The catalytic activities were higher Please do not adjust margins Please do not adjust margins with this material in comparison to the results obtained with Amberlyst 15 or sulfuric acid. Moreover, the hydrophobic resin turned out to be more recyclable. More recently, other commercial Purolite and Amberlyst type resins were evaluated for isosorbide production from sorbitol. 88 Authors have shown isosorbide yields of up to 75% with almost no loss of activity after 4 catalytic runs. Isosorbide esters synthesis starting from sorbitol and using sulfonic resins have been recently reported with interesting yields. 89 Authors noticed a 83% isosorbide yield with Nafion-Type resins under mild conditions (1%mol catalyst, 130°C for 24h under 0.3 mbar vacuum). This system is reported to promote both dehydration and esterification reactions. Very recently, propyl sulfonic acid functionalized mesostructured SBA-15 silica have been used to promote the dehydration of sorbitol to isosorbide, in melted phase, under atmospheric pressure. 90 These materials were prepared with different acid capacities and modified with different types of methylsilane functionalities, leading to isosorbide productions, over 70% in the optimum case. Nevertheless, catalyst recycling led to a loss of catalytic activity even after regeneration of the catalyst by washing procedures. Magnetic sulfonic acids are accessible from the reaction between glucose, FeCl3 and PTSA followed by calcination at 140°C. The acidic material baptized Glu-Fe3O4-SO3H has been used in several acid catalysed reactions including the dehydration of sorbitol. 91 According to the authors, the isolated yield of isosorbide reached 94 % after 1 hour at 140 °C with 20 wt% of catalyst. The purity of the product was attested by 13 C NMR. This analytical technic doesn't however take into account the formation of oligomeric / polymeric byproducts that can be elsewhere obtained. Interestingly, the catalyst could be separated using external magnetic force and reused up to five times without any significant loss in yields of the products after every recycling step.

Zeolites as catalysts

Zeolites are aluminosilicates that have found applications as acid catalysts in petrochemical processes. Zeolites are for example used as dehydration catalysts for the synthesis of olefins [92][93][94][95][96][97] or ethers 96,97 from alcohols. Recently, 3 Å molecular sieves have been used to promote dehydration reactions of tetritols 98 and hexitols 99 . Reaction temperature as high as 290 °C under argon was necessary to obtain isosorbide with 45 % yield after 1 hour with 40 % weight of molecular sieves. A larger scope of zeolite type catalysts has been later evaluated at 280 °C under hydrogen thus yielding isosorbide with yields lower than 60 %. 100 It is noteworthy that 38 % yield of isosorbide was obtained with H-beta (Si/Al ratio of 12.5) after 12 h at rather lower reaction temperature (150 °C). This latter study has been recently pushed further by varying the Si/Al ratio in a larger extent (Scheme 9). 101 Interestingly, it has been shown that an optimum Si/Al ratio of 75 allows the highest reaction rates (76 % yield of isosorbide at 127°C for 1 hour with 27 % weight of zeolite HB75). Similar results have been obtained from the group of Yokoi that obtained 80 % by using beta zeolite with a Si/Al ratio of 75. 102 The catalyst was recycled up to four times providing calcination at 550 °C was performed between each step. The study evidences the importance of framework structure (large pores into zeolites are more efficient), composition (i.e. Al content) and hydrothermal stability.

As observed with sulfonic resins, the catalyst is rapidly deactivated. In the case of zeolites, the deactivation was attributed to cocking and this is very likely related to oligomeric / polymeric by-products deposition in the solid catalyst. Analysis of the zeolites after the first catalytic run evidenced lower surface area and micropore volumes. A calcination of the zeolite at 823 K for 8 hours allowed recovering the initial values as well as high yields in isosorbide. According to this protocol, the zeolite could be recycled up to five times. Very recently, a detailed kinetic study on the behaviour of several zeolites for isosorbide synthesis was published by Morales et al. 103 Authors have confronted results obtained from commercially available zeolites and acidic resins such as Amberlyst 70 to conclude on the increased selectivity to isosorbide using H-Beta type zeolites despite a lower reaction rate. It is described that this unusual behaviour is due to catalyst textural properties.

Supported metal oxides

Nickel oxides, elsewhere used as weak acid catalysts in dehydration reactions, 104 have been evaluated in the context of sorbitol dehydration. Nickel oxide deposited on activated carbon (1 weight %) allowed the synthesis of sorbitans and isosorbide at 250 °C under H2 pressure. The highest yield of isosorbide is 28 % after 6 hours of reaction whereas no isosorbide is formed with activated carbon alone. 105 Bifunctional catalytic systems using noble metals (Pt, Pd and Ir) on zirconia in mechanical mixture with tungstated titania have also been developed to promote hydrocarbons production from sorbitol. 106,107 As intermediate, isosorbide was obtained with interesting yields up to almost 80% (determined by GC analysis) at 240°C in the presence of Pd based catalysts.

Metal phosphates

Metal phosphates are since a long time recognized as useful acid heterogeneous catalysts in alcohols dehydrations 108 or Please do not adjust margins Please do not adjust margins syntheses of cyclic ethers from diols. 109, 110 Gu et al. have synthesized and characterized tin (IV), zirconium (IV) and titanium (IV) phosphates. 111 The heterogeneous catalysts showed interesting activities toward sorbitol dehydration at a reaction temperature of 300 °C. The reactions were performed in a fixed bed reactor with a continuous feeding of vaporized 10% aqueous solution of sorbitol carried out by nitrogen. Tin phosphate showed the highest selectivity toward isosorbide whereas titanium phosphate proved to be the most active. As observed with other heterogeneous catalysts, the system showed a decay of activity with time of reaction. However, catalysts could be regenerated by calcination at 773 K thus allowing initiating another run. More recently, B-, Al-, Fe-Ceand La-trivalent cations containing phosphates as well as tetravalent Zr-phosphates (noted MP) have been prepared and evaluated as heterogeneous catalysts for the dehydration of sorbitol to isosorbide under batch conditions in the presence of water (70 wt.% sorbitol was used). The conversion of sorbitol measured after 2h at 200 °C varies as follows : AlP < ZrP < FeP < LaP < CeP < BP. With the most active catalyst, boron(III) phosphate (BP), the yield in isosorbide starting from aqueous 70 wt.% of sorbitol reaches 79.9 mol% (determined by quantitative GC with ethylene glycol as internal standard). 112

Sulfated and phosphate metal oxides

Metal sulfates are heterogeneous catalysts that bear large amounts of moderate strength acidic sites. Hence, they have found numerous uses as promoters of acid catalysed transformation such as dehydration of ethanol to ethylene or cyclododecanol to cyclododecene. 113-115 Sulfated metal oxides have been only recently used as catalysts for the dehydration of sorbitol. 116 The catalysts were prepared by calcination of NiSO4•6H2O, CuSO4•5H2O and Al2(SO4)3•18H2O at temperatures ranging from 300 °C to 650 °C under flowed air. The best selectivities in isosorbide were obtained at 200 °C with the copper based catalyst obtained from the calcination at 650 °C (99.7 % conversion of sorbitol and 67.5 % selectivity). This study evidences the importance of the calcination temperature that leads to crucial changes in surface properties and the crystalline structure of the catalysts. Sulfated zirconia obtained from zirconium (IV) hydroxide and sulfuric acid followed by calcination at 600 °C have been evaluated in batch conditions (three necked balloon at 0.3 bar) at 210 °C. 117 Selectivities in isosorbide of up to 61 % have been obtained and the catalyst could be recycled 4 times without loss of selectivity. Later on, sulfated titanium 118 and sulfated tin oxides 119 have been shown as efficient and recyclable catalysts providing that a regeneration step is processed on the used catalyst (washings with water and ethanol followed by calcination). According to a similar synthetical approach, phosphoric acid modified niobium oxide (Nb2O5) 120 and tantalum oxides 121 (Ta2O5•nH2O) have been prepared by reaction with aqueous phosphoric acids followed by calcination. Both families of catalysts have been evaluated under continuous conditions. The niobium based catalysts obtained after calcination at 400 °C showed the highest efficiency and isosorbide was obtained with 62.5 % selectivity at 225 °C. The tantalum oxide based catalysts showed a similar behaviour. At the same optimum reaction temperature, 48.3 % of selectivity in isosorbide (determined by HPLC) was obtained.

In both cases, the phosphorylation affords more active catalysts compared to the free metal oxide.

Supported HPA 122

Tungstophosphoric acids (PW) have been supported on metal oxides (SiO2, γ-Al2O3, TiO2, ZrO2 and CeO2) and used as catalysts for sorbitol dehydration to isosorbide. The catalysts were prepared from the impregnation of HPA solutions on the supports followed by calcination at 300 °C. All the catalysts prepared from this impregnation procedure showed higher catalytic activities than the free support. The activity obtained for the supported catalysts varies with the nature of the support. The activities (based on sorbitol conversion) obtained with different PW supported catalyst varies according to the nature of the support and decreases as follows: SiO2>γ-Al2O3>TiO2≈ZrO2>CeO2 which is in agreement with the order of acidity of the catalysts measured by NH3 temperatureprogrammed desorption. The PW supported on silica showed the highest catalytic activity and selectivity in isosorbide determined by HPLC. The selectivity reached 60 % at 250 °C and the catalytic efficiency could be retained after regeneration of the catalyst through washings with dichloromethane. A very recent work has also explored the use of a cellulose derived solid catalyst for isosorbide production. 123 Catalysts were synthesized by hydrothermal carbonization and sulfonation of microcrystalline cellulose. Isosorbide was obtained with a moderate yield of 67% but the catalytic system did not show any activity loss after 4 runs and was easily recoverable.

One step synthesis from Glucose and cellulose

The synthesis of isosorbide can be directly performed from glucose by combining a hydrogenation catalyst with an acid. Heterogeneous bifunctional catalysts comprising 0.2% (w/w) ruthenium onto a Brønsted acid solid support (Ru@Dowex-H) have been prepared and evaluated in one-stage conversion of glucose into isosorbide with no need for organic solvent, high metal loadings, strong soluble acid additives or purification steps. 124 approaches using homogeneous or heterogeneous catalysts. Glucose can then be hydrogenated to sorbitol which led after the two steps of dehydration to isosorbide (Scheme 10). Thus, the synthesis of isosorbide from cellulose through a direct conversion process could then be an interesting orientation for future, based on several major challenges to be worked out. Design of a relevant polyfunctional catalyst is probably the cornerstone of such a development; the targeted catalyst needing to perform cellulose destructuration to glucose, chemical reduction to sorbitol and dehydration to obtain isosorbide with high yield and selectivity. For a few years, many researchers focused theirs efforts towards this process and a few publications have appeared in the literature.

Makkee first proposed an integrated process involving the dissolution of cellulose and their chemical conversion into isosorbide in ZnCl2 hydrate as molten salt. 126 Isosorbide can be separated from the reaction mixture by extraction followed by crystallization. The three steps are described in Scheme 11. As each different step appeared to be highly selective into the desired product, including the first sorbitol dehydration step to 1,4-sorbitan, the authors report that it is feasible to convert 85 % of the carbon present in cellulose to isosorbide. The selectivity arises from the sorbitol conformation in the medium due to the hydroxyl groups interacting with the ZnCl2 salt hydrate ionic species.

Zhao reported the selective conversion of concentrated microcrystalline cellulose to isosorbide over Ru/C catalysts in acidic medium with yields of 35-50%. 127 The reaction conditions are important factors to control the reaction rate and products distribution. The maximum yield of isosorbide was obtained after 6h of reaction at 215°C, under 6 MPa H2 and with 0.

Conclusions

Isosorbide is a versatile platform molecule that shows key features to make it a credible alternative to petro-based products. The molecule is already available on large industrial scale (tens of thousands tons per years), which allows its development in commercial products such as active pharma ingredient, additive for cosmetic, speciality chemicals and polymers (ex: polycarbonates -polyesters). The development of more selective and higher yields syntheses of isosorbide are greatly needed to consolidate isosorbide production in view of a large expansion of its uses. Sorbitol conversion to isosorbide, relying on a starch route, is already a tough challenge. In a farther future, development of a credible path to isosorbide relying on cellulose source could even be thought of, provided that very versatile innovative catalysts will be developed in the next years. In all cases, a key issue is to develop catalysts that will avoid the massive production of "oligomeric/polymeric" by-products in order to access more sustainable processes by limiting the amounts of wastes produced during the synthesis. For this purpose, more selective homogeneous catalysts than the conventional Brønsted acids or alternative reaction conditions would be of strong interest. Selective and recyclable heterogeneous catalysts would be even more profitable as they would allow the continuous production of catalyst free isosorbide. This latter approach faces strong limitations due to the need of high reaction temperatures that often result in high amounts of side-products and the need of frequent and often tedious catalyst regeneration. Innovation concerning isosorbide synthesis is still an open field on which the design of efficient and robust catalysts, either homogeneous or heterogeneous, is a key issue. Such developments would pave the way to high scale effective processes considering altogether synthesis and purification of isosorbide. This journal is © The Royal Society of Chemistry 20xx The review gives an overview of the catalysts and technologies developed for the synthesis of Isosorbide, a platform molecule derived from biomass (sorbitol, cellulose).

Scheme 1

 1 Scheme 1 Structures of dianhydrohexitols.

Scheme 3

 3 Scheme 3 Isosorbide production from sorbitol, synthesis and limitations. Note that intermolecular dehydrations can occur at any step to form "humins" compounds.

Scheme 4 Scheme 5

 45 Scheme 4 TOF calculated during dehydration of sorbitol into isosorbide with various Brønsted acids.

Scheme 8 Scheme 7 2

 872 Scheme 8Water droplet contact angle on hydrophobic P-SO3H and Amberlyst 15.

Scheme 9

 9 Scheme 9Influence of the Si/Al ratio in dehydration of sorbitol to isosorbide in the presence of beta zeolite.

Table 1 :

 1 Measured rate constants (h -1 ) during dehydration of D-sorbitol.

	k0	kS-1,4S	kS-3,6S	kS-2,6I	kS-2,5M	k1,4S-I	k3,6S-I
	0.109	0.092	0.008	0.007	0.002	0.0059	0.136

Conditions: Sorbitol (2.9 g) in 3 M sulfuric acid (6.6 mL) at 104°C k0 = rate constant for the disappearance of D-Sorbitol (h -1 )

Table 2

 2 Dehydration of sorbitol in presence of metal triflate salts.

	Acid	Sorbitol		Yield (mol%)
		Conversion (%)	Isosorbide	1,4-Sorbitan	2,5-Sorbitan
	Bi(OTf)3	99	25	60	9
	In(OTf)3	100	31	56	10
	Sc(OTf)3	97	15	68	8
	Ga(OTf)3	100	71	8	6
	Sn(OTf)3	86	11	69	7
	Al(OTf)3	100	26	59	9
	H2SO4	69	4	54	6
	Reaction conditions : 100 g sorbitol, 0.05 mol% triflate salt, 160°C, 5 mbar, 1h

  1 M HCl as catalyst. By using Ru/C in the presence of heteropoly acids as H4SiW12O40, Sels et al. converted microcrystalline Avicel PH-101 cellulose to isosorbide under H2 pressure and high temperature.128 High cellulose and SiW concentrations promote fast and selective formation of isosorbide with up to 50% yield. The optimal reaction conditions were used to process crude or pre-treated lignocellulosic biomass feedstocks highlighting the importance of fractionation and purification to increase reactivity and convertibility of the cellulose feedstocks. MPa H2.131 Ruthenium based catalysts proved to be the most efficient and isosorbide yield increased with the loading of ruthenium on carbon support to reach a maximal value of 56% with 4% of Ru loaded on carbon. The same authors have extended their work to the one-pot conversion of lignocellulosic biomass to isosorbide in the same catalytic conditions.132 For example, isosorbide yields from eucalyptus and bagasse were 8.3% and 12.8% respectively. Catalytic valorization of cellulose is a topic of intense research and efficient one-pot conversion of cellulose to isosorbide is Please do not adjust margins Please do not adjust margins challenging. It is then necessary to develop new very specific and robust catalysts with dual activity of hydrogenation and dehydration. Efforts should be focused on this purpose.

					H
	O H O O H O H O O H	O O O O	O H H O H H	O O O O H	O O O O	O H H O H H	H O O H O O	O O O O	H O H H O H	O O H H O O	O O Cellulose O H O O O H	Depolymerisation
					OH
		HO	HO HO OH HO HO OH OH OH O O HO	Li et al. have developed bifunctional and recyclable solid ruthenium catalysts for an efficient transformation of cellulose to isosorbide without the addition of any soluble acid. 129 Catalysts were prepared from Ru NPs supported on mesoporous niobium phosphate to afford Ru@mNbPO with strong acidity, large surface area and adequate pores size that promotes direct production of isosorbide (50% yield) from cellulose (100% conversion) under 6 MPa H2 and at 220°C. Moreover, the catalyst could be reused up to six times without loss of activity. In a same way, Wang reported a two-steps sequential process where cellulose is firstly depolymerized with 5% Ru/NbOPO4-pH2 catalyst via hydrolysis and hydrogenation and then the resultant sorbitol and sorbitans are dehydrated to isosorbide in presence of NbOPO4-pH2. 56.7% isosorbide yield can be obtained under the optimum conditions with excellent cycling stability. Authors demonstrated that the total acidity (Brønsted and Lewis) of the catalyst is the key factor for the dehydration reaction. 130 Supported metal catalysts coupled with resins have recently been reported. Yamaguchi proposed sorbitol dehydration on supported Pt/C or Ru/C and Amberlyst 70 in water at 463K under 5 O OH OH OH OH O OH Glucose Sorbitol Sorbitans Isosorbide Hydrogenation Dehydration OH Dehydration

This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 5

This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 9

This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 11

Acknowledgements

We are grateful to the Institut Français des Matériaux Agrosourcés (IFMAS) for C. Dussenne fellowship. This project is supported by the State as part of the "Programme d'Investissement d'Avenir", under reference ANR-10-IEED-0004-01.

Notes and references


Joan Perez
email: joan.perez@univ-cotedazur.fr

Araldi Alessandro

Boyer Thomas

Bridier Sébastien

Decoupiny Fabrice

Fusco Giovanni

Trémélo Laperrière Vincent

Vincent Laperrière

Trémélo Marie-Laure

Sustainable Aging in Aix-Marseille-Provence Metropolis: Assessment Indicators and Interactive Visualizations for Policy Making

Keywords:

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

As the world's population ages and more people choose to live in urban areas, the focus of urban sustainability has evolved beyond environment issues to also encompass the needs and well-being of older adults. This evolution has been driven by a growing recognition of the important role that urban environments play in promoting healthy aging and social inclusion, as highlighted by academics and intergovernmental organizations [START_REF] Verderber | Sprawling Cities and Our Endangered Public Health[END_REF][START_REF] Buffel | Ageing in urban environments: developing 'age-friendly' cities[END_REF]. The locations are significantly impacting the experiences and possibilities of all city dwellers, and especially of older adults, and from this perspective certain neighborhoods are presenting greater challenges than others. This is for example the case of car-dependent suburbs made of single-family homes, hard to walk areas because of obstacles or steep slopes, or the presence/absence of close-by shops and services which leads to different degrees of vitality for neighborhoods [START_REF] Wood | The role of urban environments in promoting active and healthy aging: a systematic scoping review of citizen science approaches[END_REF]. The challenge for cities and policy makers is to ensure that older adults are, regardless of their residential location, fully integrated into the urban environment. In recent years, cities around the world have launched initiatives aimed at improving urban sustainability for older adults. These initiatives have included everything from redesigning public spaces to better meet the needs of older adults [START_REF] Hanson | The inclusive city: delivering a more accessible urban environment through inclusive design[END_REF], to providing more opportunities for social interaction and community engagement [START_REF] Phillipson | Developing age-friendly urban communities: critical issues for public policy[END_REF]. To summarize, the aim is to make urban spaces more livable, sustainable, and resilient for generations to come.

Built environments yet require improvements and modifications to varying degrees of urgency, depending of course on the type of issues at hand, but also on the requirements of the inhabitants living in different locations. From this perspective, the collection, analysis, and interpretation of open-access data related to urban environments can help identify the level of immediacy to which different neighborhoods, associated to different ratio of older adults, are exposed to sustainability and walkability issues. Urban planners and policy makers can for example use the output of a different range of assessment indicators to design, implement and modify urban locations within the aim of improving the overall quality of life for aging residents. However, to reach that aim, it is important for assessment indicators to be designed to address specific issues and for their results to be easily comprehensible and accessible at a metropolitan scale. This paper thus focuses on a test metropolitan area, the Aix-Marseille-Provence metropolis in southern France, and shows that a wide range of assessment indicators focusing on the link between locations and aging can be calculated using specific data in open-access. The range of indicators addressed in this paper focuses on the ratio of older adults, walkability issues (steep slopes and accessibility to amenities) as well as on the physical features of urban spaces (morphotypes), as they could be perceived by people. By presenting data in a visual and interactive way, planners and makers can then quickly identify patterns and trends that might not be immediately apparent.

This paper is organized as follows. Section 2 introduces the case study Aix-Marseille-Provence Metropolis and the different datasets gathered for this study. Section 3 presents the assessment indicators that have been calculated. Section 4 presents preliminary results obtained by cross-analyzing the different indicators presented in Sect. 3. Section 5 introduces an interactive platform to visualize and explore the spatial results calculated in the previous sections. Section 6 concludes the paper with a discussion on future applications. intercommunal organization. The urban agglomeration, defined by the French Statistical Institute INSEE as the continuously built-up area with gaps up to 200 m, covers 1,758 km 2 , with a population of 1,614,500 in 2019. Spatially, the urban agglomeration includes the city of Aix-en-Provence and the industrial and logistic area around the coastal lagoon of Berre, and covers most of the Bouche-du-Rhône administrative department (5,087 km 2 and 2,034,000 inhabitants in 2019). The «Logements, individus, activité, mobilités scolaires et professionnelles, migrations résidentielles» database of the French National Institute of Statistics and Economic Studies (INSEE) [START_REF]Logements, individus, activité, mobilités scolaires et professionnelles, migrations résidentielles[END_REF] provides variables and information related to the French housing units and the households occupying them. Over the last decade (2008-2018) and across the entire metropolitan area, the population of people aged over 80 has grown by 25%, while the total population has only grown by 3.5%. However, the growth of the elderly population is significantly more pronounced in the least densely populated municipalities on the outskirts of urban areas, while in the center of the Marseille and Aix agglomerations, the trend is towards stagnation or even a decrease in the proportion of people aged over 80.

Across the Aix-Marseille-Provence metropolitan area, households consisting of a single senior, known as "isolated seniors," represent 52.3% (68,782 out of 131,522) of the households possessing at least one senior aged 75 or older. 43% of the households with at least one senior aged over 75 live in single-family houses, 90% of which owneroccupied. Among the remaining 57% living in apartments, only 53% of which are owner-occupied, and half of the remaining 47% are tenants living in social housing and the other half in private rental apartments. In the metropolitan area as a whole, 50% of apartments that elderly people occupy lack elevators, making it difficult for those living on higher floors. This lack of elevator access is equally prevalent among isolated seniors.

Data collection

With the availability of census data accessible through open data access, it is possible to delve deeper and to conduct a more comprehensive analysis of spatial disparities associated with aging. This section provides a list of all the datasets, in open access, that have been used for the calculation of assessment indicators (Sect. 3) focusing on walkability and readability of urban spaces.

The « localised disposable income system (Filosofi®)» of INSEE in 2017 [START_REF]INSEE : Localised disposable income system[END_REF]. The scale of data collection corresponds to a square tessellation of the French territory with tiles measuring of 200 m on each side. This file gathers 35 variables related to the structure and characteristics of both households (size, age, etc.) and housings (sqm of housing units, owner, etc.). The «Permanent database of facilities (BPE®)» of INSEE in 2019 [START_REF]INSEE : Permanent database of facilities[END_REF]. This dataset gathers all the amenities in France with their geolocation. Amenities are divided into several categories and sub-categories such as service, commerce, education, health and social services, transportation and mobility, sports-leisure-culture and tourism. The «Topographic dataset (BD TOPO®)» from the French National Geographic Institute (IGN) in 2019 [START_REF]Base de données topographique[END_REF]. BD TOPO® are GIS layers containing, amongst other things, building footprints (polygons on a two-dimensional plane) where buildings are digitized as single-part polygons and roads (polylines on a two-dimensional plane). The 30-m resolution digital elevation model (DEM) of the Shuttle Radar Topography Mission of the NASA Earthdata program [START_REF]NASA Earthdata program 30-Meter SRTM Tile Downloader[END_REF].

As the next sections will show, municipalities and policymakers can gain insights that were previously unavailable by leveraging and cross analyzing the aforementioned data sources.

Indicators

It is important for assessment indicators to be designed to address specific issues in order to maximize their impact and ensures their comprehensibility. Within this research, we put the focus on walkability as well as on the physical features of urban spaces as they could be perceived by an aging population. Walkability typically refers to various factors that make an environment conductive to walking. As discussed in Lo [START_REF] Lo | Walkability: what is it? J. Urbanism[END_REF] different pedestrian performance metrics have been developed within various disciplines such as traffic engineering, transportation planning, urban design, public health or sociology, and all these fields have varying definitions and methods on how to assess walkability. In the context aging and urban planning, walkability mostly refers to creating a resilient and healthy city [START_REF] Abastante | Supporting resilient urban planning through walkability assessment[END_REF] while the physical features of urban spaces refer to morphotypes around street segments.

• • • •

Share of Individuals Aged 65 Years or Older

The concept of the third age is a term that is often used interchangeably with old age. The third age is generally associated with individuals who have retired from full-time work and are no longer raising children, but are not yet considered to be in the final stages of life, also referred to as the fourth age. People belonging to the third and fourth age are commonly referred to as "seniors", "elderly persons" or "older adults" and are often recognized as a distinct demographic group with specific needs and challenges. The age structure of the population is available within the 2017 « localised disposable income system (Filosofi®)» database of INSEE [START_REF]INSEE : Localised disposable income system[END_REF]. First, the population aged 65 years or older, which roughly corresponds to both the third and the fourth age, has thus been extracted and divided by the total population per tile to obtain the share of older adults per location. On a given tile, if the number of households is less than 11, the data are imputed in order to respect the confidentiality threshold set by the French government. Overall, most of the tiles (79%) contain less than 11 households but they only account for 20% of the population.

Slope Averages

Older adults have weaker muscles and joints, making it more difficult for them to navigate steep slopes. This can lead to increased fatigue and discomfort, which can further increase the risk of falls. Moreover, walking on a steep slope enhances the intensity of physical activity, resulting in an elevation of heart rate, which in turn increases the risk of cardiovascular complications for older adults [START_REF] Alves | Walkability index for elderly health: a proposal[END_REF]. When considering the needs of aging populations, it's important to consider the slope of the terrain, above all in the case of Aix-Marseille-Provence metropolis, which includes a contrasted topography. Even if an area has pedestrian-friendly amenities, steep slopes can create difficulties for older adults who may have limited mobility. For this project, for the second assessment indicator, we thus computed the slope average using the 30-m resolution elevation data from the Shuttle Radar Topography Mission (SRTM) of the NASA Earthdata program [START_REF]NASA Earthdata program 30-Meter SRTM Tile Downloader[END_REF], at the very same scale than the previous indicator i.e., the tiles measuring 200 m on each side of the Filosofi® database. The indicator has been named "slope average".

Accessibility to Amenities

The concept of the "15-min walk city" or of hyper-proximity has gained traction in urban planning in the last decade. The idea is that cities should be designed so that residents can access most of their daily needs within a 15-min walk, bike ride, or public transit trip, thus contributing to more vibrant, compact and sustainable communities [START_REF] Moreno | Introducing the "15-minute city": sustainability, resilience and place identity in future post-pandemic cities[END_REF][START_REF] Speck | WALKABLE CITY: How Downtown Can Save America, One Step at a Time[END_REF]. Studies have also highlighted an association between a lack of shops and weak walkability scores, within neighborhoods that could be described as food deserts [START_REF] Calise | Food access and its relationship to perceived walkability, safety, and social cohesion[END_REF]. Given both these rationales, the number of accessible amenities has been calculated in a queen's topological space of contiguity from the scale of the 200-m tiles of the Filosofi® dataset [START_REF]INSEE : Localised disposable income system[END_REF] using the permanent database of facilities of INSEE [START_REF]INSEE : Permanent database of facilities[END_REF]. A queen's topological space of contiguity corresponds to a maximum of 600 m of walk from edge to edge, or of 300 m from the center to an edge, and thus look in perfect adequation to the concept of the "15-min walk city". From this perspective, SSi (Shops and Services) expresses the sum of amenities accessible within a given tile i based on the occurrences within the said tile and within the contiguous tiles. Amenities within the queen's neighborhood of i have been weighted according to the slope average. In other words, the steeper the slope, the less the amenities are contributing to the final score of SSi. The permanent database of facilities of INSEE contains hundreds of categories of shops and services. For this research, we operated a subset on categories of shops and services that could be used by the elders, such as health services, convenience stores, supermarkets, post offices, etc. A specific list of amenities used by older adults does not stand out in the academic literature and thus, the detail of the retained categories we decided to select for the research has been made available in Appendix A. From now on, this subset will be referred to as amenities.

Given i the number of amenities within a tile, Lj (low slope) are the values observed in the queen's neighborhood of i where the slope average is <5%, Mj (medium slope) are the values observed in the queen's neighborhood of i where the slope average is between 5% and 8%, and Hj (high slope) are the values observed in the queen's neighborhood of i where the slope is > 8%.

Morphotypes Around Street Segments

The urban landscape of Aix-Marseille-Provence Metropolis is made of a patchwork of very diverse forms of the urban fabric. Within this project, a first analysis of the diverse urban fabric of Marseille was carried out [START_REF] Fusco | The city and the metropolis: urban form through multiple fabric assessment in Marseille, France[END_REF]. The Multiple Fabric Assessment protocol [START_REF] Araldi | From the street to the metropolitan region: pedestrian perspective in urban fabric analysis[END_REF] was used to identify and characterize morphotypes around street segments, i.e. typical forms of the urban fabric, using street-based morphometrics of buildings, streets and plots, and their mutual relations, as they can be perceived by pedestrian moving on the street using the BD TOPO® from the French National Geographic Institute (IGN) in 2019 [START_REF]Base de données topographique[END_REF] as the primary source of data. A new version of the MFA protocol implements several improvements related to precision, robustness and scalability to analyse larger study areas, and compare them between one another. The new protocol was applied to the whole department of Bouches-du-Rhône, roughly corresponding to the Aix-Marseille-Provence Metropolis, and allowed detecting and characterizing 19 different morphotypes (Fig. 2).

Five morphotypes make up the compact urbanization in around the old city centres: UF1. Old compact village centres and densified faubourgs, organic or planned, made of townhouses and small adjoining buildings (2.4% of the street segments of the study area) UF15. Compact grid-based heterogeneous urban fabric, with varied adjoining buildings aligned on regularly meshed networks, including some specialised buildings (1% of the street segments). UF16. Compact and dense fabric of small-to-mid-sized adjoining buildings and townhouses aligned on meshed and irregular networks, with strong presence of specialised buildings (1%). UF9. Densified urban subdivisions of Two morphotypes characterize modernist urbanization, together with specialized space: UF11. Modernised mid-rise continuous/discontinuous fabric alternating fragments of buildings and houses aligned on irregularly meshed networks (1%). UF10. Discontinuous modernist fabrics of elongated apartment blocks and towers with setback on irregularly meshed networks (3.2%). UF0. Specialized space is a specific morphotype characterized by the over-representation of specialized buildings (factories, commercial buildings, logistic buildings, hospitals, university campuses) (5.1%).

Seven morphotypes make up the vast low-rise peripheral and suburban urbanization: UF6. Fabric of urban subdivisions of aligned houses (1.1%). UF7. Suburban residential fabric of homogeneous subdivisions of houses and villas (or houses) with setback on organic or tree-like networks (4.7%). UF4. Subdivisions of partially aligned houses and 1-storey villas with setback and partially aligned on meshed/tree-like and irregular network (1.6%). UF5. Heterogeneous suburban fabric of houses and villas moderately aligned on tree-like and/or sloped networks (1%). UF2. Heterogeneous suburban fringe houses, villas and specialised buildings differently positioned with refence to irregularly meshed or tree-like networks (3.7%). UF12. Heterogeneous and discontinuous suburban fabrics alternating fragments with houses/villas and fragments with buildings, setback on tree-like networks (5.4%). UF3. Heterogeneous ex-urban fabric of houses, villas and specialized low-rise buildings, differently positioned on tree-like networks (14.4%).

Several residual morphotypes characterize less developed land around the urbanisation: UF14. Suburban/exurban fabric interfacing with undeveloped land (10.4%). UF13. Empty connective fragments (negligible). UF8. Rural space with scattered buildings (17.2%). of the study area. UF18-UF19 are a single morphotype of undeveloped land interfaced with suburban and exurban fabric (10.2%). 12% of the study area are undeveloped land and 1.8% are highway segments, and both were not processed by the protocol.

Exploratory Analysis

The outskirts of towns often trade off pedestrian-friendliness in favor of car mobility. However, as people age, driving may become more challenging. Therefore, neighborhoods that are for example lacking nearby shops and services, or that are constructed on steep slopes may become problematic in the long run regarding the level of autonomy of older adults. This section thus aims at exploring the vulnerability and autonomy levels of the elderly population within the Aix-Marseille-Provence Metropolis based on three indicators calculated within the previous section: the share of individuals aged 65 years or older which provides information about intergenerational diversity within the neighborhoods, the slope average to evaluate walkability and accessibility to amenities. The scale of analysis is once again the 200-m tiles gridded data of the Filosofi® census in 2017 [START_REF]INSEE : Localised disposable income system[END_REF]. or older been divided into three categories using the following thresholds: less than 25% (S-), between 25 and 40% (S), and more than 40% (S+ for very aged population). This segmentation relies on the analysis of the density distribution of this indicator. Second, the slope average has also been divided into three categories. According to Alves et al., [START_REF] Alves | Walkability index for elderly health: a proposal[END_REF] normal heartbeats are generally between 60 and 90 bpm, while a measurement of 120 'bpm is the outcome of great efforts for an elderly person. The authors then deduce the following categories related to the difficulty of a senior's walk according to the slope: <5%-suitable, 5% < x < 8%-acceptable and >8%-inappropriate. We thus used the very same thresholds to evaluate walkability (W+ and W ) within a neighborhood. Lastly, using SSi (Sect. 3), we operated a division into two categories: presence and lack of amenities. The threshold between these two categories has been set to 23.7. Despites many studies on accessibilities to shops and services, as well as on the concept of walkability and walkability for the older adults [START_REF] Musselwhite | Mobility, accessibility and quality of later life[END_REF][START_REF] Diyanah | Implications of walkability towards promoting sustainable urban neighbourhood[END_REF], a single and clear threshold of a given number of reachable amenities for an urbanized area to be considered as properly equipped does not stands out from the academic literature. As a result, we decided to pick three peripheral locations where amenities are present: Septèmes-les-Vallons, Gardanne and Fuveaux. The mean of SSi of these three locations allowed defining 23.7 as a threshold for the following two categories: properly equipped in amenities (A+) and lack or few amenities (A ). The tree-like classificatory model using these thresholds allowed obtaining 12 different class labels (Fig. 3). Several sub-categories and spatial clusters emerge (Fig. 4), based on how the proportion of seniors, walkability and accessibility to amenities are combined. As summarized in Table 1, the statistical results of this application allow locating and quantifying the number of seniors concerned by walkability and accessibility to amenities issues. It is for example interesting to note that the worst-case scenario (S++W A : significant proportion of older adults, a bad score of walkability and a lack of amenities) only concerns

Open Access to Data, Code, and Interactive Visualizations

All four indicators discussed in Sect. 3 as well as the results of the exploratory analysis that involves classification in Sect. 4 have been compiled into a geopackage file. Furthermore, an HTML-based interactive visualization platform using these indicators has been set up using the package mapview on R [START_REF] Appelhans | mapview: Interactive Viewing of Spatial Data in R. Package on CRAN[END_REF]. The Aix-Marseille-Provence Metropolis uses the same kind of HTML-based interactive visualization within the open-access project MData [START_REF]MData: GEO-visualization platform of Aix-Marseille-Provence Metropolis[END_REF] (e.g. Fig. 1). Launched in 2021, this service allows accessing and visualizing data at the scale of the metropolis, related for example to urban planning, water management, road traffic and their regulation, sport, sanitation, waste management, construction. Within this research, all indicators have been calculated at the scale of the 200-m tiles, except for the urban fabric typology which is at the scale of the street segments. The geopackage, the version 1.0 of the platform, as well as an R code have been made available (Appendix B). The R code provides information on the session, load the packages and the layers from the geopackage, simplify and prepare the color ramps for each layer, and finally produce the platform. In this version of the platform (v1.0), the urban fabric indicator has been filtered according to the tiles possessing population, and the segments below 10 m of length have been removed. This was done with the intention of reducing the platform's size, which now stands at 93 megabytes. The HTML-based platform can be easily accessed on any computer, making it simple to engage and solicit feedback from the public. The panels on the left side allow activating-deactivating the different layers, as well as zooming in and out any neighborhood part of Aix-Marseille-Provence Metropolis. Figure 5

Conclusion and Discussions

The aim of this paper was to show that it was possible to produce detailed new information about aging for the case of a metropolitan area such as Aix-Marseille-Provence Metropolis based only on open-source data. Within this study, we decided to put the focus on two dimensions: walkability and urban form. However, the challenge for us as researchers was not only to produce and analyze new data, but also to effectively communicate our findings to a broader audience. We recognized that sharing complex research results in a clear and concise manner is essential for policymakers, urban planners, and the public to make informed decisions about the future of their communities. To overcome this challenge, in addition to sharing our data, we integrated our results into an interactive viewing platform to spatially visualize and share our data in a user-friendly manner with interactive maps. Thanks to the MData initiative of Aix-Marseille-Provence Metropolis, data access and visualization has become a tool to help people in making strategic decisions about the future of the metropolis. The data format of the indicators calculated within the research project is compliant with the MData initiative, and we are thus in contact with the metropolis to add our results to the official platform.

From a thematic point of view, a future perspective for this project is to combine the identified morphotypes within the Aix-Marseille-Provence Metropolis with sociodemographic indicators of the resident population. We will thus be able to identify the urban forms which are more often associated with the different age classes within the resident population and/or with trends of aging or rejuvenation. The distribution of services can also be assessed with respect to the morphotypes where they are located. Aging also relates to certain concepts about urban form, such as readability or legibility, which refers to how easily people can and navigate the physical layout of a city [START_REF] Lynch | The Image of a City[END_REF], or intelligibility which is the degree to which the spatial layout of an urban environment can be understood by its inhabitants [START_REF] Hillier | The Social Logic of Space[END_REF]. An intelligible urban environment is likely to be accessible and efficient, while less intelligible environments can lead to confusion, disorientation, or even social isolation for older adults. As MFA focuses on perceptible elements of urban form, this perspective could well be carried out integrating our outcomes with surveys of intelligibility-readability by seniors within different morphotypes.

Finally, our study does not consider other factors besides slope, accessibility to amenities, and physical features of urban spaces. Yet, many other factors can make an environment conducive to walking, especially for elder people, and we plan to investigate other factors in future research.

Fig. 1 .

 1 Fig. 1. Administrative boundaries of the 92 communes belonging to Aix-Marseille-Provence Metropolis [6].

Fig. 2 .

 2 Fig. 2. The 19 morphotypes around segments identified by the Multiple Fabric Assessment protocol in Aix-Marseille-Provence Metropolis

Fig. 3 .

 3 Fig. 3. A tree-like classificatory model to evaluate the degree of vulnerability/autonomy of the elderly population

Fig. 4 .

 4 Fig. 4. Sample of the classificatory results in Marseille (source: HTML-based platform, see Sect. 5)

  displays a selection of the assessment indicators calculated within this research accessible in the version 1.0 of the platform.

5 .

 5 v1.0 HTML-based platform related to Aging in Aix-Marseille-Provence Metropolis; top left: morphotypes around street segments in 2019, top right: share of 65-year-old or more in 2017; bottom left: average slopes; bottom right: accessible amenities in 2017.

1 .

 1 Statistical results by class label 74% of the total of 65-year-old or more within the Aix-Marseille-Provence metropolitan area for 1.66% of the total urbanized surface. In contrast, the best-case scenario (S++W+A : Significant proportion of older adults, suitable walkability, presence of amenities) concerns 0.40% of the total population of 65-year-old or more for 0.28% of the total urbanized surface. Yet, those two examples are not sub-spaces where intergenerational diversity prevails, but rather areas where the older adults are overrepresented. Intergenerational diversity is better within the S and S sub-categories which represent together 86.2% of the 65-year-old or more living in Aix-Marseille-Provence Metropolis.

	1.
	Label	Description +	% of Tot Pop	% of 65 yo+	% of urbanized surface
	S++W+A-	Significant proportion of	0.28	0.66	0.51
		seniors, suitable
		walkability, lack of
		amenities		-
	S++W+A+	Significant proportion of	0.40	0.93	0.28
		seniors, suitable
		walkability, presence of
		amenities
	S+A+	Large to significant	4.00	6.36	1.56
		proportion of seniors,
		presence of amenities
	S+A-	Large to significant	2.19	3.65	5.21
		proportion of seniors, lack
		of amenities
	S++W-A-	Significant proportion of	0.72	1.74	1.66
		seniors, inappropriate
		walkability, lack of
		amenities
	S++W-A+	Significant proportion of	0.20	0.45	0.12
		seniors, inappropriate
		walkability, presence of
		amenities
	SW+A-	Large proportion of	2.09	3.34	5.34
		seniors, suitable
		walkability, lack of
		amenities
	SW+A+	Large proportion of	3.99	6.22	1.88
		seniors, suitable
		walkability, presence of
		amenities
	SW-A-	Large proportion of	4.83	7.70	12.48
		seniors, inappropriate
		walkability, lack of
		amenities
	SW-A+	Large proportion of	3.38	5.16	1.08
		seniors, inappropriate
		walkability, presence of
		amenities
	S-A-	Small proportion of	29.42	23.00	56.89
		seniors, lack of amenities
	S-A+	Small proportion of	48.49	40.78	12.98
		seniors, presence of
		amenities

Case Study and Data Presentation2.1 Aix-Marseille-Provence MetropolisThe Aix-Marseille-Provence Metropolis is an intercommunal organization made of 92 communes in southern France (Fig.1). Its area is about

3,150 km 2 for a population of 1,9 million inhabitants in 2019. Marseille and Aix-en-Provence are the main cities of this

Acknowledgment. This research was funded by a grant from Région Sud, Provence-Alpes-Côte d'Azur, France (ForVie).

Appendix A: List of Retained Categories from the 2019 BPE® Dataset

A203, A101, A104, A206, A207, A208, A501, A502, A504, B101, B102, B201, B202, B203, B204, B205, B206, B301, B302, B303, B304, B305, B306, B307, B311, B312, B313, B315, D106, D108, D112, D113, D201, D202, D203, D206, D207, D208, D209, D211, D212, D213, D214, D221, D232, D233, D235, D236, D237, D238, D239, D240, D241, D242, D301, D302, D303, D305, D401, D402, D403, D404, E101, E107, E108, E109, F304, F101, F102, F108, F109, F113, F120, F201, F303, F306.

Full list of categories: https://www.insee.fr/fr/metadonnees/source/fichier/BPE19_Liste_%C3%A9quip ements_insee-fr.pdf Appendix B: Files Upload https://zenodo.org/record/7818160

This upload contains a geopackage file, R code, Readme file and an HTML-based platform with interactive maps created using the geopackage and R code. This upload presents various indicators and a HTML-based platform with interactive maps to help planners and policymakers to quickly identify patterns and trends related to aging within Aix-Marseille metropolis, France.

Files.

'AGING_AMP_INDICATORS.gpkg': The geopackage file contains spatial data used in the R code to create the maps. • 'code_R_platform_v1.0_ForVie.R': The R code used to create the interactive maps.