Proof.
Part (1) means \mathit{QCoh}(\mathcal{O}_ X) (a) has all colimits, (b) filtered colimits are exact, and (c) has a generator, see Injectives, Section 19.10. By Schemes, Section 26.24 colimits in \mathit{QCoh}(\mathcal{O}_ X) exist and agree with colimits in \textit{Mod}(\mathcal{O}_ X). By Modules, Lemma 17.3.2 filtered colimits are exact. Hence (a) and (b) hold. To construct a generator U, pick a cardinal \kappa as in Lemma 28.23.3. Pick a collection (\mathcal{F}_ t)_{t \in T} of \kappa -generated quasi-coherent sheaves as in Lemma 28.23.2. Set U = \bigoplus _{t \in T} \mathcal{F}_ t. Since every object of \mathit{QCoh}(\mathcal{O}_ X) is a filtered colimit of \kappa -generated quasi-coherent modules, i.e., of objects isomorphic to \mathcal{F}_ t, it is clear that U is a generator. The assertions on limits and injectives hold in any Grothendieck abelian category, see Injectives, Theorem 19.11.7 and Lemma 19.13.2.
Proof of (2). To construct Q we use the following general procedure. Given an object \mathcal{F} of \textit{Mod}(\mathcal{O}_ X) we consider the functor
\mathit{QCoh}(\mathcal{O}_ X)^{opp} \longrightarrow \textit{Sets},\quad \mathcal{G} \longmapsto \mathop{\mathrm{Hom}}\nolimits _ X(\mathcal{G}, \mathcal{F})
This functor transforms colimits into limits, hence is representable, see Injectives, Lemma 19.13.1. Thus there exists a quasi-coherent sheaf Q(\mathcal{F}) and a functorial isomorphism \mathop{\mathrm{Hom}}\nolimits _ X(\mathcal{G}, \mathcal{F}) = \mathop{\mathrm{Hom}}\nolimits _ X(\mathcal{G}, Q(\mathcal{F})) for \mathcal{G} in \mathit{QCoh}(\mathcal{O}_ X). By the Yoneda lemma (Categories, Lemma 4.3.5) the construction \mathcal{F} \leadsto Q(\mathcal{F}) is functorial in \mathcal{F}. By construction Q is a right adjoint to the inclusion functor. The fact that Q(\mathcal{F}) \to \mathcal{F} is an isomorphism when \mathcal{F} is quasi-coherent is a formal consequence of the fact that the inclusion functor \mathit{QCoh}(\mathcal{O}_ X) \to \textit{Mod}(\mathcal{O}_ X) is fully faithful.
\square
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